阅读说明：本技术 一种钢的细晶强韧化分级淬火冷却方法 (Fine-grain strengthening and toughening graded quenching cooling method for steel ) 是由 王长文 王中忞 于 2018-10-20 设计创作，主要内容包括：一种钢的细晶强韧化分级淬火冷却方法。运用“马氏体转变可逆性”规律,用热处理方法多次细化晶粒提高钢的强韧性。钢在奥氏体化过程中,为马氏体逆转变提供条件；具备马氏体逆转变条件的钢奥氏体化后,预冷后水冷绕过C-曲线不稳定区至马氏体区未冷透,马氏体晶核随温度下降逐渐长大；出水利用余热回温,温度上升,马氏体反过来又同步随温度上升而缩小,即细化晶粒。包括：钢的奥氏体化,为马氏体逆转变提供条件；钢的预冷,减小热应力；钢的控冷,进行多次“水冷-回温”,直至冷至室温。本发明能节约能源和资源,保护环境,细化晶粒提高钢的强韧性,且抗拉强度提高大于屈服强度提高,解决大截面钢件1/2T处细晶强韧化难题。(A method for cooling fine-grain strengthening and toughening graded quenching of steel. By applying the 'martensite transformation reversibility' rule, the crystal grains are refined for many times by a heat treatment method to improve the obdurability of the steel. In the austenitizing process of the steel, conditions are provided for reverse transformation of martensite; after the steel with the martensite reverse transformation condition is austenitized, precooling the steel, and then water-cooling the steel to bypass a C-curve unstable region until the martensite region is not cooled completely, wherein the martensite crystal nucleus grows gradually along with the temperature reduction; the effluent is heated by waste heat, the temperature rises, and the martensite is synchronously reduced along with the temperature rise in turn, namely, the grains are refined. The method comprises the following steps: austenitizing the steel to provide conditions for reverse transformation of martensite; precooling the steel to reduce the thermal stress; and (4) controlling the cooling of the steel, and performing water cooling-temperature returning for multiple times until the steel is cooled to room temperature. The invention can save energy and resources, protect environment, refine crystal grains, improve the obdurability of steel, improve the tensile strength more than the yield strength and solve the difficult problem of fine grain obdurability at 1/2T of the large-section steel piece.)

1. A method for the step quenching and cooling of fine-grained strengthening and toughening of steel is characterized in that the 'martensite transformation reversibility' rule is applied, and grains are refined for many times by a heat treatment method to improve the strengthening and toughening of the steel; in the austenitizing process of the steel, conditions are provided for reverse transformation of martensite; after austenitizing the steel, precooling, and then carrying out 'water cooling-temperature returning' for multiple times until the steel is cooled to room temperature, controlling the surface layer of the steel to be cooled in a NaCl aqueous solution with the mass fraction of (5-10)% to bypass a C-curve unstable region, namely the nose tip temperature, to the cold penetration degree of a martensite region for the first time, returning the temperature of the effluent water to the bainite transformation starting temperature Bs + (50-100) ° C for the first time by using waste heat, but the temperature of the effluent water is lower than the pearlite transformation temperature, namely the C-curve nose tip temperature, immediately transferring the effluent water to be cooled in the NaCl aqueous solution with the mass fraction of (5-10)% to the martensite region for multiple times until the martensite region is not cooled, and returning the temperature of the effluent water to the bainite transformation starting temperature Bs + (50-100) ° C by; before steel grading, the temperature at which the last effluent water stops rising by utilizing waste heat to return temperature, namely the higher or lower martensite transformation starting temperature Ms is the grading starting temperature for carrying out energy-free supplementary grading maintenance by utilizing the waste heat when the steel enters static air or a slow cooling box; after steel classification, cooling discharged standing air into water cooling-temperature returning for multiple times until the steel is cooled to room temperature, and immediately transferring the steel to NaCl aqueous solution with the mass fraction of 5-10% for multiple times of classification quenching and cooling to the room temperature; cooling the slowly cooling box by air cooling to room temperature; cooling the steel to room temperature, and immediately transferring to the next heat treatment process;

the method comprises the following steps:

austenitizing the steel;

pre-cooling steel;

and (III) controlled cooling of the steel.

2. Austenitizing the steel according to claim 1, characterized in that the austenitizing of the steel is:

determining the quenching heating temperature, the heat preservation time and the temperature rising speed of steel As the conditions of reducing the austenite transformation starting temperature As, increasing the martensite transformation starting temperature Ms, reducing the temperature difference between As and Ms and providing the conditions for the reverse transformation of martensite;

and (II) determining the quenching step preheating times, preheating temperature, heat preservation time and heating speed of the steel to reduce the austenite transformation starting temperature As, increase the martensite transformation starting temperature Ms, reduce the temperature difference between As and Ms and provide conditions for reverse transformation of martensite.

3. Pre-cooling of steel as claimed in claim 1, characterised in that the pre-cooling of the steel is:

determining the transformation point A of steel₁The temperature is the critical temperature of the pre-cooling consumption incubation period;

(1) determining that the precooling temperature Tn of the steel is more than or equal to A₁Temperature, no consumption of incubation period;

(2) determining the precooling temperature Tn < A of the steel₁Temperature, consumption incubation period;

after determining austenitizing of the steel, discharging the steel out of the furnace and precooling the steel in air;

and (III) determining the precooling temperature of the steel as follows:

(1) determining the precooling temperature of eutectoid steel and hypereutectoid steel as Ar₁+ (10-30) DEG C, but must be more than or equal to A₁；

(2) Determining the pre-cooling temperature of the hypoeutectoid steel as Ac₃- (10-20) DEG C, close to the upper phase transition point Ac₃(ii) temperature;

and (IV) determining the precooling temperature of the steel, and immediately transferring the steel into NaCl aqueous solution with the mass fraction of 5-10% for cooling.

4. Controlled cooling of steel according to claim 1, characterised in that the controlled cooling of steel is:

determining and applying a 'martensite transformation reversibility' rule, refining crystal grains for multiple times by a heat treatment method, and improving the strength and toughness of steel; after austenitizing the steel, precooling, and then carrying out 'water cooling-rewarming' for multiple times until the steel is cooled to room temperature, controlling the surface layer of the steel to be cooled in a NaCl aqueous solution with the mass fraction of 5-10% for the first time to bypass a C-curve unstable region, namely the nose tip temperature, to the cold penetration degree of a martensite region, and preparing for the 'water cooling-rewarming' for multiple subsequent times to reach the expected temperature of each critical point;

after austenitizing the steel, firstly cooling the steel in a NaCl aqueous solution with the mass fraction of 5-10% to bypass a C-curve unstable region, namely the nose tip temperature, to the cold penetration degree of a martensite region, namely the first cooling time is as follows: the first effluent is heated to the bainite transformation starting temperature Bs + (50-100) DEG C by using the residual heat, but the temperature of the first effluent is lower than the pearlite transformation temperature;

thirdly, determining that the first effluent of the steel is reheated to the bainite transformation starting temperature Bs + (50-100) DEG C by using the waste heat, but is lower than the pearlite transformation temperature, and immediately transferring the steel to a NaCl aqueous solution with the mass fraction of 5-10% for cooling for many times;

fourthly, determining that the steel is cooled in a NaCl water solution with the mass fraction of 5-10% for many times until the martensite region is not cooled thoroughly, and the water discharged for many times is reheated to the bainite transformation starting temperature Bs + (50-100) DEG C by using waste heat, but is lower than the pearlite transformation temperature; before steel grading, cooling the steel in a NaCl aqueous solution with the mass fraction of 5-10% for the last time until the martensite area is not cooled completely, namely the cooling time for the last time is as follows: the last time of the water outlet stops rising the temperature by utilizing the waste heat to be the grading starting temperature for the steel to enter the standing air or the slow cooling box to carry out the energy-free supplement grading maintenance by utilizing the waste heat;

(V) determining the grading starting temperature of the steel entering static air or a slow cooling box and being maintained in a grading way without energy supplement by using waste heat, namely the temperature at which the martensite transformation starting temperature Ms is higher or lower:

(1) determining the grading starting temperature of the high-carbon steel and the medium-high carbon alloy steel to be Ms + (20-40) DEG C;

(2) determining the grading starting temperature of the medium-low carbon steel and the low-carbon alloy steel as Ms- (40-60) DEG C;

(3) determining the grading starting temperature of low-temperature tempering steel of a common part to be (180-220) DEG C;

determining the steel as the grading starting temperature, immediately transferring to standing air or a slow cooling box, and carrying out grading maintenance without energy supplement by using waste heat;

(seventhly), determining the grade of the steel as;

1) determining that the steel is subjected to grading maintenance without energy supplement by using waste heat in standing air at a grading starting temperature of Ms + (20-40) DEG C or Ms- (40-60) DEG C;

2) determining that the steel is subjected to grading maintenance without energy supplement by utilizing waste heat in a slow cooling box at a grading starting temperature of 180-220 ℃;

(eighthly), determining the grading time of the steel;

(1) determining the steel as the grading starting temperature, immediately transferring to standing air or a slow cooling box to carry out grading maintenance without energy supplement by using waste heat, and starting to calculate the grading time;

(2) determining the classification time of the steel as follows:

1) determining the grading time of (90-120) s when the steel grading starting temperature is Ms + (20-40) DEG C;

2) determining the grading time (60-90) s when the steel grading starting temperature is Ms- (40-60) DEG C;

3) determining the grading time of the steel at the grading starting temperature of (180-220) DEG C for 60 min;

after steel is classified, cooling the discharged standing air for multiple times of water cooling-temperature returning until the steel is cooled to room temperature, and immediately transferring the steel to an NaCl aqueous solution with the mass fraction of 5-10% for multiple times of classified quenching and cooling to the room temperature; cooling the slowly cooling box by air cooling to room temperature;

(1) after steel classification is determined, cooling of discharged standing air is immediately transferred into NaCl aqueous solution with the mass fraction of 5-10% for multiple times, and then classified quenching cooling is carried out to room temperature;

1) after steel classification is determined, immediately transferring the cooled static air to a NaCl aqueous solution with the mass fraction of 5-10% for multiple times of reclassification quenching and cooling until a martensite area is not cooled completely, returning the temperature of the discharged water to stop rising by utilizing waste heat to be the next reclassification quenching, and utilizing the waste heat to carry out reclassification starting temperature without energy supplement classification and maintenance in the static air;

2) determining the next reclassification time of the steel to be (60-120) s;

3) after determining that the steel is classified again next time, immediately transferring the steel into a NaCl aqueous solution with the mass fraction of 5-10% for cooling until the steel is cooled to the room temperature;

(2) after steel classification is determined, cooling the steel out of the slow cooling box to room temperature by air cooling;

and (ten) determining that the steel is cooled to room temperature, and immediately transferring to the next heat treatment process.

5. The temperature of NaCl aqueous solution with the mass fraction of 5-10% is controlled at 15-38 ℃.

Technical Field

The invention relates to a heat treatment method of steel, in particular to a fine-grain strengthening and toughening graded quenching cooling method of steel.

Background

The steel in the prior art is strengthened by fine grains, the energy and resource consumption is large, the pollution is serious, the cost is high, and the technical quality index of the 1/2T part of the large-section steel piece is difficult to reach the standard; the prior art is characterized in that:

the fine crystal strengthening of the steel is to add: alloy elements for lowering critical points, such as Mn, Cr, Mo and the like; improving nucleation rate, and preventing the growth of austenite grains in hot state; so that the yield strength is improved more than the tensile strength, and the yield ratio is increased; the consistency of the fine grain strengthening effect of the steel depends on the operation of the components of micro alloy elements (Ti, Nb, V and the like);

the precooling temperature of the steel is slightly lower than Ar₁Pre-cooling in a heating furnace, heating to a temperature slightly lower than Ar₁Pre-cooled in a furnace to a temperature slightly lower than Ar₁(ii) temperature;

precooling the steel in oil, transferring to oil cooling for a few seconds to perform precooling, and then transferring to water to be cooled;

the method for improving the hardenability and the hardenability of the steel is to add enough alloy elements for stabilizing the undercooled austenite into the steel, change the shape of a C-curve, push the C-curve to the right and reduce the critical quenching speed to realize the improvement of the quenching cooling speed and the hardenability of the steel;

directly cooling the steel in a temperature-controllable molten salt bath or a hot oil groove to a grading temperature higher or lower than the martensite transformation starting temperature Ms, and carrying out grading heat preservation with energy supplement, wherein the heating temperature of the grading quenching is higher (30-80) DEG than the quenching heating temperature of the steel;

the steel is quenched in stages to form a molten salt bath or a hot oil bath, and the molten salt bath or the hot oil bath is cooled to room temperature in an air mode;

the above problems can be solved by the technical scheme of the invention.

Disclosure of Invention

In order to solve the technical problems, the invention is realized by the following technical scheme;

the invention aims to provide a fine-grain strengthening and toughening graded quenching cooling method for steel, which overcomes the defects in the prior art, and utilizes the 'martensite transformation reversibility' rule to refine grains for multiple times by a heat treatment method, thereby improving the strength and toughness of the steel.

The invention is realized as follows, which is characterized in that the method comprises the following steps:

1. a method for the step quenching and cooling of fine-grained strengthening and toughening of steel is characterized in that the 'martensite transformation reversibility' rule is applied, and grains are refined for many times by a heat treatment method to improve the strengthening and toughening of the steel; in the austenitizing process of the steel, conditions are provided for reverse transformation of martensite; after austenitizing the steel, precooling, and then carrying out 'water cooling-temperature returning' for multiple times until the steel is cooled to room temperature, controlling the surface layer of the steel to be cooled in a NaCl aqueous solution with the mass fraction of (5-10)% to bypass a C-curve unstable region, namely the nose tip temperature, to the cold penetration degree of a martensite region for the first time, returning the temperature of the effluent water to the bainite transformation starting temperature Bs + (50-100) ° C for the first time by using waste heat, but the temperature of the effluent water is lower than the pearlite transformation temperature, namely the C-curve nose tip temperature, immediately transferring the effluent water to be cooled in the NaCl aqueous solution with the mass fraction of (5-10)% to the martensite region for multiple times until the martensite region is not cooled, and returning the temperature of the effluent water to the bainite transformation starting temperature Bs + (50-100) ° C by; before steel grading, the temperature at which the last effluent water stops rising by utilizing waste heat to return temperature, namely the higher or lower martensite transformation starting temperature Ms is the grading starting temperature for carrying out energy-free supplementary grading maintenance by utilizing the waste heat when the steel enters static air or a slow cooling box; after steel classification, cooling discharged standing air into water cooling-temperature returning for multiple times until the steel is cooled to room temperature, and immediately transferring the steel to NaCl aqueous solution with the mass fraction of 5-10% for multiple times of classification quenching and cooling to the room temperature; cooling the slowly cooling box by air cooling to room temperature; cooling the steel to room temperature, and immediately transferring to the next heat treatment process;

the method comprises the following steps:

austenitizing the steel;

pre-cooling steel;

and (III) controlled cooling of the steel.

2. The austenitization of the steel is:

the quenching heating temperature, the heat preservation time and the temperature rising speed of the steel are conditions for reducing the austenite transformation starting temperature As, increasing the martensite transformation starting temperature Ms, reducing the temperature difference between As and Ms and providing conditions for the reverse transformation of martensite;

the quenching stepped preheating times, preheating temperature, heat preservation time and heating speed of the steel are conditions for reducing austenite transformation starting temperature As, increasing martensite transformation starting temperature Ms, reducing the temperature difference between As and Ms and providing conditions for martensite reverse transformation.

3. Precooling of the steel is as follows:

transformation point A of steel₁The temperature is the critical temperature of the pre-cooling consumption incubation period;

(1) precooling temperature Tn of steel is more than or equal to A₁Temperature, no consumption of incubation period;

(2) precooling temperature Tn of steel is less than A₁Temperature, consumption incubation period;

secondly, after austenitizing the steel, discharging the steel out of the furnace and precooling the steel in air;

the precooling temperature of the steel is as follows:

(1) precooling temperature of eutectoid steel and hypereutectoid steel is Ar₁+ (10-30) DEG C, but must be more than or equal to A₁；

(2) The pre-cooling temperature of the hypoeutectoid steel is Ac₃- (10-20) DEG C, close to the upper phase transition point Ac₃(ii) temperature;

and (IV) immediately transferring the steel to a NaCl aqueous solution with the mass fraction of 5-10% for cooling at a precooling temperature.

4. The controlled cooling of the steel is as follows:

firstly, refining crystal grains for multiple times by a heat treatment method by applying the 'martensite transformation reversibility' rule, and improving the strength and toughness of the steel; after austenitizing the steel, precooling, and then carrying out 'water cooling-rewarming' for multiple times until the steel is cooled to room temperature, controlling the surface layer of the steel to be cooled in a NaCl aqueous solution with the mass fraction of 5-10% for the first time to bypass a C-curve unstable region, namely the nose tip temperature, to the cold penetration degree of a martensite region, and preparing for the 'water cooling-rewarming' for multiple subsequent times to reach the expected temperature of each critical point;

after austenitizing the steel, cooling in a NaCl aqueous solution with the mass fraction of 5-10% for the first time after precooling to bypass a C-curve unstable region, namely the nose tip temperature, to the cold penetration degree of a martensite region, wherein the first cooling time is as follows: the first effluent is heated to the bainite transformation starting temperature Bs + (50-100) DEG C by using the residual heat, but the temperature of the first effluent is lower than the pearlite transformation temperature;

thirdly, the first effluent of the steel is reheated to the bainite transformation starting temperature Bs + (50-100) DEG C by using the waste heat, but is required to be lower than the pearlite transformation temperature, and is immediately transferred to a NaCl aqueous solution with the mass fraction of 5-10% for cooling for many times;

cooling the steel in a NaCl water solution with the mass fraction of 5-10% for many times until the martensite region is not cooled thoroughly, and reheating the effluent water to the bainite transformation starting temperature Bs + (50-100) DEG C by using waste heat for many times, wherein the temperature is lower than the pearlite transformation temperature; before steel grading, cooling the steel in a NaCl aqueous solution with the mass fraction of 5-10% for the last time until the martensite area is not cooled completely, namely the cooling time for the last time is as follows: the last time of the water outlet stops rising the temperature by utilizing the waste heat to be the grading starting temperature for the steel to enter the standing air or the slow cooling box to carry out the energy-free supplement grading maintenance by utilizing the waste heat;

and (V) enabling the steel to enter standing air or a slow cooling box and utilize waste heat to perform energy-free supplementary grading maintenance, wherein the grading starting temperature is higher or lower than the martensite transformation starting temperature Ms:

(1) the grading starting temperature of the high-carbon steel, the medium-carbon and high-carbon alloy steel is Ms + (20-40) DEG C;

(2) the grading starting temperature of medium, low carbon steel and low carbon alloy steel is Ms- (40-60) DEG C;

(3) the grading starting temperature of the low-temperature tempering steel of the common parts is (180-220) ° C;

sixthly, the steel is at the grading starting temperature, and immediately transferred into standing air or a slow cooling box to carry out grading maintenance without energy supplement by using waste heat;

(seventhly) grading the steel into steel;

(1) carrying out grading maintenance without energy supplement on the steel by using waste heat in standing air at a grading starting temperature of Ms + (20-40) DEG C or Ms- (40-60) DEG C;

(2) carrying out classification maintenance without energy supplement on steel by utilizing waste heat in a slow cooling box at a classification starting temperature of 180-220 ℃;

(eighthly) grading time of steel;

(1) the steel is at the grading starting temperature, and immediately transferred into static air or a slow cooling box to carry out grading maintenance without energy supplement by using waste heat, and the grading time is calculated;

(2) the steel grading time is as follows:

1) the grading time with the steel grading starting temperature of Ms + (20-40) DEG C is (90-120) s;

2) the steel grading starting temperature is Ms- (40-60) DEG C, and the grading time is (60-90) s;

3) the grading time with the steel grading starting temperature of (180-220) DEG C is 60 min;

after steel classification, cooling the discharged standing air for multiple times of water cooling-temperature returning until the steel is cooled to room temperature, namely immediately transferring the steel to an NaCl aqueous solution with the mass fraction of 5-40% for multiple times of classification quenching and cooling to the room temperature; cooling the slowly cooling box by air cooling to room temperature;

(1) after steel classification, cooling the discharged standing air, namely immediately transferring the steel into a NaCl aqueous solution with the mass fraction of 5-10% for multiple times, and then performing classification quenching and cooling to room temperature;

1) after steel classification, immediately transferring the cooled static air to a NaCl aqueous solution with the mass fraction of 5-10% for multiple times of classification quenching and cooling until a martensite area is not cooled completely, returning the temperature of the discharged water to stop rising by utilizing waste heat to be the next classification quenching, and utilizing the waste heat to carry out classification starting temperature without energy supplement in the static air;

2) the next reclassification time of the steel is (60-120) s;

3) immediately transferring the steel to a NaCl aqueous solution with the mass fraction of 5-10% for cooling after next grading until the steel is cooled to room temperature;

(2) after steel grading, taking the steel out of the slow cooling box and cooling the steel to room temperature by air;

and (ten) cooling the steel to room temperature, and immediately transferring to the next heat treatment process.

5. The temperature of NaCl aqueous solution with the mass fraction of 5-10% is controlled at 15-38 ℃.

Compared with the prior art, the invention has the beneficial effects that: the crystal grains are refined for many times by a heat treatment method to improve the obdurability of the steel, the tensile strength is improved more than the yield strength, the yield ratio is reduced, energy and resources are saved, the environment is protected, the cost is reduced, the carbon equivalent is reduced, the welding performance of the steel is improved, the working procedures are combined, the construction period is shortened, and the problem of fine grain obdurability at the 1/2T position of the large-section steel piece is solved.

Drawings

FIG. 1 is a flow chart of a method for cooling by fine-grain strengthening and toughening step quenching of steel according to the present invention, in which the abscissa represents time t; the ordinate represents temperature in deg.C.

FIG. 2 is a schematic diagram showing the influence of precooling temperature of steel on quenching results, and the abscissa represents time lg τ; the ordinate represents temperature, deg.C; phase transformation point A₁Pre-cooling critical temperature, T, of steel_PIs the critical temperature of pearlite transformation-C-curve nose tip temperature, Bs is the bainite transformation starting temperature, and Ms is the martensite transformation starting temperature; y is_S1Precooling the steel surface to a point slightly lower than A₁Temperature of (Y)_C1Precooling the steel core to a point Y which is slightly lower than A₁The temperature of (a); y is_S2Precooling the steel surface to Y point ratio Y_S1Deviation A₁Lower temperature, Y_C2Precooling the steel core to Y point ratio Y_C1Deviation A₁Lower temperature, representing Y_S2、Y_C2Ratio of each to Y_S1、Y_C1The consumed incubation period is longer; precooling temperature Y_S1And Y_C1Deviation A₁The temperature is slightly low, the consumed incubation period is slightly long, and the depth of the quenched martensite is slightly shallow; precooling temperature Y_S2And Y_C2Deviation A₁The temperature is lower, the consumed incubation period is longer, and the steel cannot be quenched into martensite; v_KThe critical cooling speed of steel quenching.

FIG. 3 is a schematic diagram of a fine-grained, tough-grained, stage-quenched steel of the present invention with the abscissa representing time lg τ; the ordinate represents temperature, deg.C; tcst (Y) precooling the steel to a point Y slightly higher than A₁Temperature (not consuming incubation period) at which quenching begins to cool; phase transformation point A₁For precooling the consumption of the inoculation period critical temperature, T, of the steel_PIs the critical temperature of pearlite transformation-C-curve nose tip temperature, Bs is the bainite transformation starting temperature, and Ms is the martensite transformation starting temperature; s is a steel surface cooling curve, and c is a steel core cooling curve; v_KThe critical cooling speed of steel quenching.

FIG. 4 staged quenching of prior art steelsIntent, abscissa represents time, lg τ; the ordinate represents temperature, deg.C; tcst is the temperature at which the steel is not precooled and begins to be cooled by stage quenching in a temperature-controllable molten salt bath or a hot oil bath; phase transformation point A₁For precooling the consumption of the inoculation period critical temperature, T, of the steel_PIs the pearlite transformation temperature-C-curve nose tip temperature, Bs is the bainite transformation starting temperature, and Ms is the martensite transformation starting temperature; v_KThe critical cooling speed of steel quenching.

Detailed Description

The present invention will be described in further detail with reference to the accompanying drawings and specific embodiments.

The present invention will now be described in detail with reference to a few preferred embodiments thereof as illustrated in the accompanying drawings, wherein the several specific details are set forth in order to provide a thorough understanding of the present invention, but it will be apparent to one skilled in the art that the present invention may be practiced without some or all of these specific details; in some instances, well known process steps have not been described in detail in order to not unnecessarily obscure the present invention.

The difficulty coefficient of heat treatment is that after austenitizing the steel (uniform internal and external temperature of the steel, uniform austenite, complete phase change, no growth of austenite grains and low cost), the center 1/2T of the steel is cooled in water to be lower than Ar₁At a certain temperature, the decomposition occurs and the transformation into martensite or lower bainite is difficult.

The heat treatment difficulty coefficient is calculated according to the right formula:

wherein N is the heat treatment difficulty coefficient (mm);

V-Cooling volume of Steel (mm)³)；

S-cooling area of steel (mm)²)；

As shown in fig. 1, the present invention is realized by the following method:

1. a method for cooling the steel by isothermal quenching for refining grains, strengthening the toughness and raising the toughness features thatThe 'martensite transformation reversibility' rule, the crystal grains are refined for many times by a heat treatment method to improve the obdurability of the steel; in the austenitizing process of the steel, conditions are provided for reverse transformation of martensite; after the steel is austenitized into EF, precooling FY, then carrying out water cooling-temperature returning for multiple times until the steel is cooled to room temperature, and controlling the YG on the surface layer of the steel to firstly cool in a NaCl aqueous solution with the mass fraction of 5-10 percent so as to bypass the unstable region of the C-curve, namely the nose tip temperature T_PPreparing for the subsequent multiple water cooling-temperature returning to reach the expected temperature of each critical point to the cold penetration degree of the martensite area; the temperature of the first effluent is returned to H which is the bainite transformation starting temperature Bs + (50-100) DEG C by utilizing the waste heat, but is lower than the pearlite transformation temperature-C-curve nose tip temperature T_PImmediately transferring the mixture to a NaCl aqueous solution with the mass fraction of 5-10% for many times to cool HJ until a martensite region is not cooled thoroughly, and returning the temperature of the effluent water to the bainite transformation starting temperature Bs + (50-100) DEG C by using residual heat for many times, wherein the temperature is lower than the pearlite transformation temperature-C-curve nose tip temperature T_P(ii) a Before steel grading, the last effluent is heated by waste heat to stop rising temperature K, namely the grading starting temperature for keeping the martensite transformation starting temperature Ms higher or lower, is the grading starting temperature for carrying out non-energy supplement grading maintenance by using the waste heat when the steel enters static air or a slow cooling box; after the steel is graded by KL, cooling the discharged standing air L for multiple times of water cooling-temperature returning until the steel is cooled to the room temperature Q', namely immediately transferring the steel to an NaCl aqueous solution with the mass fraction of 5-10% for multiple times of graded quenching and cooling to the room temperature Q "; cooling the slowly cooling box by air cooling to room temperature; cooling the steel to room temperature, and immediately transferring to the next heat treatment process;

the method comprises the following steps:

austenitizing the steel;

pre-cooling steel;

and (III) controlled cooling of the steel.

The fine-grain strengthening and toughening of the steel are strengthening modes of improving yield strength and toughness by fine-grain strengthening; controlling the ratio of the yield strength to the tensile strength, namely the yield ratio, so that the improvement of the tensile strength is greater than that of the yield strength, thereby improving the toughness, being beneficial to the safety and cold processing performance of steel and solving the problem of grain refining and strengthening at 1/2T of the large-section steel piece;

the fine grain strengthening of the steel in the prior art is to add: alloy elements for lowering critical points, such as Mn, Cr, Mo and the like; improving nucleation rate, and preventing the growth of austenite grains in hot state; the yield strength is improved more than the tensile strength, the yield ratio is increased, alloy resources are consumed, the cost and the carbon equivalent of steel are improved, the welding performance of the steel is reduced, and the safety and the cold processing performance of the steel are not good; because the yield strength is improved more than the tensile strength, the yield ratio is increased, and the safety and the cold processing performance of the steel are not good, the improvement of the yield strength and the yield ratio of the steel part are limited, the improvement of the yield strength cannot exceed 110 percent of a standard value, and the yield ratio cannot exceed 0.9;

the invention applies the 'martensite transformation reversibility' rule, refines crystal grains for many times by a heat treatment method and improves the obdurability of the steel; in the austenitizing process of the steel, the austenite transformation starting temperature As is reduced, the martensite transformation starting temperature Ms is increased, the temperature difference between As and Ms is reduced, and conditions are provided for reverse transformation of martensite; after austenitizing the steel with the martensite reverse transformation condition, water-cooling the steel after precooling to bypass the C-curve unstable region, namely the nose tip temperature T_PThe martensite transformation area is not cooled completely, and the martensite crystal nucleus grows gradually along with the temperature reduction; the effluent is reheated by waste heat, the temperature rises, martensite is synchronously shrunk along with the temperature rise in reverse, namely crystal grains are refined to improve the obdurability of the steel, and water cooling-reheating are carried out for multiple times until the temperature is cooled to room temperature, the crystal grains are refined for multiple times, so that the tensile strength is improved to be higher than the yield strength, the yield ratio is reduced, alloy resources are saved, the cost and carbon equivalent of the steel are reduced, the welding performance of the steel is improved, the toughness is also improved, the safety and cold processing performance of the steel are facilitated, and the difficulty of fine grain obdurability at the 1/2T position of the large-section steel piece is solved;

the low-carbon low-alloy ASTM A694F65 steel, carbon equivalent CE is not more than 0.43, effective thickness 160mm, under the mutual evidence of three parties of supplier, demander and third party BV, after carrying on Q + T heat treatment according to the invention and isothermal tempering cooling method of steel, all performance and organization have already reached standards by body sampling test 1/2T, performance and organization test result of 1/2T of steel are as follows:

(1) performance: sigma_s480MPa, which is 107% of the standard value (not less than 450 MPa); sigma_b605MPa, which is 113% of the standard value (not less than 530 MPa); delta is 30.5%, which is 152% of the standard value (. gtoreq.20%); -46 ℃ impact toughness value: tangential direction A_k255J (252/253/259), 637% of the standard value (average 40J or more, single 30J or more); axial direction A_k270J (274/272/264), 675% of the standard value (average 40J or more, single 30J or more);

1) the yield ratio standard value of the fine grain strengthening of the steel in the prior art is 0.85(450Mpa/530Mpa), the yield ratio value of the fine grain strengthening and toughening of the steel in the invention is 0.79(480Mpa/605Mpa), namely, the yield strength is 107 percent of the standard value of the prior art, the tensile strength is 113 percent of the standard value of the prior art, the improvement of the tensile strength is greater than the improvement of the yield strength, the yield ratio value is reduced, the toughness of the steel is improved, namely, the safety and the cold processing performance of the steel are improved;

2) the consistency of the fine grain strengthening effect in the prior art is operated by depending on the components of micro alloy elements (Ti, Nb, V and the like), the further refinement of crystal grains is not suitable for improving the production cost from the economic aspect, and the yield ratio is strictly limited from the performance aspect; the invention uses heat treatment method to refine grains and strengthen toughness for many times, without increasing production cost, the yield ratio value is reduced, the toughness tangential and axial Ak values are more than six times of the standard value, and the group deviation is not more than 10J, which is beneficial to further refining grains and improving the toughness of steel;

(2) organizing: the heating temperature of the step quenching is 10-30 ℃ higher than the original quenching heating temperature, and the grain size of a metallographic structure is 1-2 grade finer than the original grain size; the method is formed by martensite reverse transformation, when the steel has the martensite reverse transformation condition of reducing the temperature difference between As and Ms, the steel is austenitized, precooled and then passes through a C-curve unstable region for multiple times until the martensite region is not cooled completely, the martensite crystal nucleus gradually grows along with the temperature reduction, the temperature is raised by utilizing the waste heat after water is discharged, and the martensite crystal nucleus is reversely reduced along with the temperature rise synchronously, namely crystal grains are refined for multiple times.

2. The austenitization of the steel is:

the step quenching heating temperature, the heat preservation time and the temperature rise speed of the steel are that austenite phase transformation is completed, austenite grains cannot grow, the steel is thoroughly burnt and uniform, the cost is low, the heating thermal stress is reduced, the temperature distribution of the section of the steel is uniform, preparation is made for quenching water cooling, the austenite transformation starting temperature As is reduced, the martensite transformation starting temperature Ms is improved, the temperature difference between As and Ms is reduced, conditions are provided for martensite reverse transformation, and the grains are refined to improve the toughness of the steel;

(1) the heating temperature EF of the steel in the step quenching process;

1) the heating temperature EF of the low-hardenability steel in the step quenching is determined according to the following principle:

firstly, the heat treatment difficulty coefficient N of the steel is more than or equal to 30mm, and the graded quenching heating temperature EF is 10-30 ℃ higher than the quenching heating temperature of the steel;

secondly, the heat treatment difficulty coefficient N of the steel is less than 30mm, and the graded quenching heating temperature EF is 0-20 ℃ higher than the quenching heating temperature of the steel;

2) the heating temperature EF of the high-hardenability steel in the step quenching is 0-10 ℃ higher than the quenching heating temperature of the steel;

3) compared with the prior art, the heating temperature of the step quenching is reduced by (30-50) DEG C, which is due to the following steps:

firstly, As the heating temperature of the steel is increased, austenite grains grow to be large, so that the carbon content in austenite is increased, the martensite transformation starting temperature Ms is reduced, and the temperature difference between As and Ms is increased;

secondly, in the prior art, steel is directly cooled to a grading temperature in a temperature-controllable molten salt bath or a hot oil bath, the temperature of the molten salt bath or the hot oil is higher, so that the cooling capacity is weak, the heating temperature of the grading quenching is higher (30-80) DEG than the quenching heating temperature of the steel, so that a C-curve is pushed rightwards, the critical quenching speed is reduced to realize the improvement of the quenching cooling speed, and the hardenability of the steel are improved; the invention relates to a method for 'water cooling-temperature returning' of steel for multiple times, wherein before grading, the last effluent is returned to the grading starting temperature by using waste heat;

(2) quenching heating and heat preservation time E of steel₁F₁；

Quenching heating and heat preservation time E of steel₁F₁Calculated as follows:

τ_{general assembly}＝Kz+Az×D×K

In the formula tau_{General assembly}The total time of steel quenching, heating and heat preservation, unit: min;

kz-base number of heat preservation time of steel quenching, unit: min;

az-the time coefficient of steel quenching heating and heat preservation, unit: min/mm;

d-effective thickness of workpiece, unit: mm;

k is a workpiece charging correction coefficient;

(3) the quenching heating temperature rise speed DE of the steel is (150-200) DEG C/h;

secondly, the quenching stepped preheating frequency, temperature, heat preservation time and heating speed of the steel are thorough and uniform, the cost is low, the preheated thermal stress is reduced, the temperature distribution of the section of the steel is uniform, the preparation is made for quenching water cooling, the austenite transformation starting temperature As is reduced, the martensite transformation starting temperature Ms is improved, the temperature difference between As and Ms is reduced, conditions are provided for martensite reverse transformation, and the crystal grains are refined to improve the toughness of the steel;

the preheating of steel is heated gradually, causes along steel spare cross-section temperature distribution homogenization, shortens the steel and inside and outside samming required time under high temperature, increases the rate of heating, promptly: the phase transformation recrystallization heating speed of the steel is increased, the austenite forming temperature is reduced, and the austenite transformation starting temperature As is reduced; fine and uniform austenite grains are obtained, and the martensite transformation starting temperature Ms is increased; thereby reducing the temperature difference between As and Ms and providing conditions for reverse transformation of martensite;

this is because the time for the steel to pass through the austenite phase-change recrystallization is inversely proportional to the heating rate, and the heating rate is fast, the time is short, and the steel rapidly passes through Ac₁(ii) temperature; the steel has short retention time at high temperature, austenite cannot grow up, crystal grains are fine, and the amount of carbon dissolved in the austenite is small;

(1) the quenching preheating of the steel comprises two steps of preheating AB and CD;

(2) the two-step preheating temperatures AB and CD of the steel quenching are as follows:

1) when the heating temperature EF is 800 ℃, the first step preheating temperature AB and the second step preheating temperature CD are respectively 400 ℃ and 600 ℃;

2) when the heating temperature EF is 950 ℃, the first step preheating temperature AB and the second step preheating temperature CD are respectively 500 ℃ and 700 ℃;

3) when the heating temperature EF is between 800 ℃ and 950 ℃, the first step preheating temperature AB and the second step preheating temperature CD are respectively calculated according to the following formula:

①AB＝a_AB(X-800)+b_AB

in the formula: AB-first step actual preheat temperature, deg.C;

x-actual heating temperature EF, DEG C, value range: x is more than or equal to 800 and less than or equal to 950;

a_AB-the ratio of the first stepwise preheating temperature range AB to the heating temperature range EF;

b_AB——400℃；

②CD＝a_CD(X-800)+b_CD

in the formula: CD-second step actual preheat temperature, deg.C;

x-actual heating temperature EF, DEG C, value range: x is more than or equal to 800 and less than or equal to 950;

a_CD-the ratio of the second stepped pre-heating temperature range CD to the heating temperature range EF;

b_CD——600℃；

examples are: when the actual heating temperature X is 890 ℃, respectively substituting (i) and (ii) to obtain:

①

②

(3) two-step preheating and heat-preserving time A for steel quenching₁B₁And C₁D₁；

Two-step preheating and heat-preserving time A₁B₁And C₁D₁Calculated as follows:

τ＝Ay×D×K

in the formula, tau is the two-step preheating and heat preservation time, unit: min;

ay-two-step preheating and heat preservation time coefficient, unit: min/mm;

d-effective thickness of workpiece, unit: mm;

k is a workpiece charging correction coefficient;

the quenching (preheating) heat preservation time base number Kz and the heat preservation time coefficients Az and Ay of various steels are shown in a table 1; the workpiece charging correction coefficient K is shown in a table 2;

table 1: basic Kz of heat preservation time and Az and Ay values of heat preservation time coefficients of quenching (preheating) of various steels

TABLE 2 correction factor k for quenching and heating charging

The effective thickness (D) of the workpiece for heating is calculated as follows:

taking the diameter of a cylinder as an effective thickness;

the side length of the square section is taken as the effective thickness;

using the short side of the workpiece with the rectangular cross section as the effective thickness;

a plate-shaped part with an effective thickness 1.5 times the maximum thickness;

the wall thickness of the thin-wall sleeve part is taken as the effective thickness;

the diameter of the hole is less than 50mm, and the diameter of the hole is less than the diameter of the excircleThe effective thickness of the sleeve is twice of the wall thickness;

when in useThe effective thickness is calculated as follows:

wherein D-the effective thickness (mm) of the workpiece;

D₀-actual thickness (mm) of a single side of the workpiece;

Φ_{inner part}-the diameter (mm) of the inner circle of the workpiece;

Φ_{outer cover}-the diameter of the outer circle of the workpiece (mm);

examples are: workpiece OD440 × ID160 × L430mm

D₀＝(440-160)÷2＝140mm

Separating small end of conical workpieceThe diameter of (a) as an effective thickness;

and (c) calculating the maximum thickness of the part with complex shape, such as a cutter according to the section thickness of the working part.

(4) The two-step preheating heating rates OA and BC are both 100-150 ℃/h.

3. Precooling of the steel is as follows:

after austenitizing the steel, cooling the steel in water to a transformation point A₁Does not immediately undergo transformation, the austenite present at this time is called supercooled austenite; when super cooled austenite cools below Ar₁At a certain temperature, the decomposition will occur and the transformation into other tissues will occur; when the supercooled austenite cools below the transformation point A₁Temperature to Ar₁At temperature, although no transformation occurs, the incubation period, i.e. the transformation point A, is consumed₁The temperature is the critical temperature of precooling consumption incubation period in the steel quenching process, and the precooling temperature Tn deviates from A₁The lower the temperature, the longer the consumed incubation period;

(1) precooling temperature Tn of steel is more than or equal to A₁Temperature, no consumption of incubation period;

(2) precooling temperature Tn of steel is less than A₁Temperature, consumption incubation period;

after austenitizing the steel into EF, discharging the EF, and precooling FY in the air; this is due to:

the thermal stress of the steel is caused by the energy difference between the steel cooling surface and the cooling medium, the energy difference is large, the thermal stress is large, otherwise, the thermal stress is small, and in the same cooling medium, the thermal stress with high temperature of the steel cooling surface is larger than the thermal stress with low temperature of the steel cooling surface; after austenitizing the steel, reducing the temperature to the precooling temperature after precooling, immediately transferring the steel into a NaCl aqueous solution with the mass fraction of 5-10% for cooling, reducing the energy of the cooling surface of the steel, reducing the energy difference between the cooling surface of the steel and cooling medium water, namely reducing the thermal stress of the steel, improving the cooling uniformity and the cooling efficiency, obtaining uniform quenching structure, improving the performance of the quenched steel and reducing the quenching defects;

and (III) precooling the steel to the precooling temperature Tn of Y:

(1) precooling temperature Tn of eutectoid steel and hypereutectoid steel is Ar₁+ (10-30) DEG C, but must be more than or equal to A₁(ii) a This is due to:

the austenite of eutectoid and hypereutectoid steels was precooled to slightly above Ar₁Temperature, relatively stable, at critical point A₁The incubation period is not consumed at the above temperature, cementite is separated out from hypereutectoid steel, and precooling can be carried out to quench martensite and obtain precipitation strengthening;

examples are: t8 Steel A₁At a temperature of 720 ℃ Ar₁Is 700 ℃ at the temperature of the furnace,

Tn＝Ar₁+(10～30)＝700+(10～30)

＝700+10～700+30＝(710～730)℃

t8 Steel A₁The temperature is 720 ℃, and the value range of the precooling temperature Tn is (720-730);

the precooling temperature Tn of eutectoid steel and hypereutectoid steel in the prior art is slightly lower than Ar₁；

Examples are: t8 Steel A₁At a temperature of 720 ℃ Ar₁At 700 ℃ resulting in Tn < Ar₁＜A₁The long incubation period is consumed, the steel cannot be quenched into martensite, and although cementite is precipitated, the precipitation strengthening amount is small and is lower than the strength and hardness which are lost due to the fact that the steel cannot be quenched into the martensite, and the precipitation strengthening cannot offset the strength and hardness which are lost due to the fact that the steel cannot be quenched into the martensite, so that the quality technical index of quenching cannot be achieved; this is due to:

precooling temperature Tn of eutectoid steel and hypereutectoid steel in prior art is less than A₁At temperature, the incubation period is consumed and the precooling temperature Tn deviates from A₁The lower the temperature, the longer the consumed incubation period, the shallower the depth of the quenched martensite or the inability to quench into martensite; as shown in FIG. 2;

(2) the pre-cooling temperature Tn of the hypoeutectoid steel is Ac₃- (10-20) DEG C, close to the upper phase transition point Ac₃(ii) temperature; this is due to:

1) pre-cooling hypoeutectoid steel to Ac₃Temperature of (10-20) DEG C, which is still stable at critical point A₁The incubation period is not consumed at the above temperature;

2) hypoeutectoid steel in Ac₁～Ac₃The heating quenching between the temperatures is called sub-temperature quenching, and the sub-temperature quenching temperature is slightly higherFrom phase transition point Ac₃The lower the temperature, the more ferrite precipitates, the greater the decrease in the strength of the steel, and the lower the amount of ferrite precipitates, that is, the decrease in the strength of the steel, the temperature for precooling the hypoeutectoid steel is near the upper phase transition point Ac₃The sub-temperature quenching temperature of (1);

3) hypoeutectoid steel approaching upper phase transition point Ac₃The sub-temperature quenching at the temperature can refine austenite grains and improve the toughness of steel, so that harmful impurities such as phosphorus and the like are concentrated in a small amount of free dispersed ferrite grains, the notch toughness is improved, the cold-brittleness transition temperature is reduced, and the temper brittleness is reduced;

examples are: 45 steel A₁At 735 ℃ Ac₃At a temperature of 780 c,

Tn＝Ac₃-(10～20)＝780-(10～20)

＝780-10～780-20＝(760～770)℃

45 steel A₁The temperature is 735 ℃, and the value range of the precooling temperature Tn is 760-770 ℃;

the precooling temperature Tn of the hypoeutectoid steel in the prior art is slightly lower than Ar₁；

Examples are: 45 steel A₁At 735 ℃ Ac₃At 780 ℃ Ar₁At 682 ℃ resulting in Tn < Ar₁＜A₁＜Ac₃The temperature is (10-20) DEG C, a long incubation period is consumed, and the depth of the quenched martensite is shallower or the quenched martensite cannot be quenched into the martensite, which is shown in figure 2; ferrite is precipitated, and the quality technical index of quenching can not be achieved; this is due to:

1) precooling temperature Tn of hypoeutectoid steel in prior art is less than A₁At temperature, the incubation period is consumed and the precooling temperature Tn deviates from A₁The lower the temperature, the longer the consumed incubation period; the shallower the depth of the quenched martensite or the inability to quench into martensite; as shown in FIG. 2;

2) hypoeutectoid steel in Ac₁～Ac₃The heating quenching between the temperatures is called sub-temperature quenching, and the sub-temperature quenching temperature deviates from the upper phase transition point Ac₃The lower the ferrite is, the more ferrite is precipitated, the greater the strength reduction is;

fourthly, controlling the precooling degree of the steel; the precooling of the steel is insufficient, the reduction of the thermal stress is limited, the precooling is excessive, the consumed incubation period is long, the precipitation amount of the steel which can not be quenched into martensite and the precipitation amount of the hypoeutectoid steel ferrite can not reach the quality technical index of quenching, the precooling degree of the steel needs to be controlled, the precooling degree can not only reduce the thermal stress to the maximum extent, but also can not quench into the martensite and the precipitation amount of the ferrite to reach the expected precooling temperature, so as to obtain the expected structure and the performance which reach the standard;

precooling the steel to a precooling temperature Tn, immediately transferring the steel to an NaCl aqueous solution with the mass fraction of 5-10% for cooling YG for the first time, and preventing insufficient or excessive precooling;

compared with the prior art, the precooling method has the beneficial effects that:

(1) the prior art is after austenitizing of steel:

1) pre-cooling in a heating furnace, and heating to slightly lower than Ar₁Precooling in a temperature furnace to a temperature slightly below Ar₁The temperature is changed into quenching and cooling, so that energy and resources are consumed, and the heat treatment cost is increased;

2) the steel is cooled in slow cooling quenching medium oil for a few seconds to play a pre-cooling role, and then is transferred into rapid quenching medium water for cooling, so that the heat treatment cost is increased, the environment is polluted, and the steel is easy to fire and unsafe;

(2) after austenitizing the steel, precooling the steel to a precooling temperature Tn in the air, immediately transferring the steel to a NaCl aqueous solution with the mass fraction of 5-10% for cooling, and the method has the advantages of low air cost, convenient use, energy and resource conservation, cost reduction, cleanness, no pollution, environmental protection, safety, no fire, achievement of a precooling expected temperature, and achievement of a consistent expected tissue and performance standard.

4. The controlled cooling of the steel is as follows:

firstly, refining crystal grains for multiple times by a heat treatment method by applying the 'martensite transformation reversibility' rule to improve the strength and toughness of the steel; the quenching cooling speed is increased by a heat treatment method to improve the hardenability and hardenability of the steel and reduce the distortion amount and the cracking tendency; according to the austenite transformation curve of the steel, a martensite structure is obtained by quenching, and rapid cooling in the whole cooling process is not required; only the temperature T at the tip of the nose of the C-curve_PQuickly cooling at +/-50 ℃; from quenching temperature to T_PBetween +50 ℃ andT_Pno rapid cooling is needed between-50 ℃ and Ms + (10-30) DEG C (high hardenability steel) or Ms- (50-70) DEG C (low hardenability steel), and particularly no rapid cooling is needed below Ms + (10-30) DEG C (high hardenability steel) or Ms- (50-70) DEG C (low hardenability steel) accompanied with volume change, otherwise, distortion and cracking are generated; in the grading process, not only pearlite or upper bainite transformation is avoided, but also thermal stress and structural stress are reduced to the maximum extent, namely the distortion amount and the cracking tendency are reduced; the continuous cooling process is combined in the stage maintaining process; the retention time from the graded state to the room temperature is shortened, multiple times of secondary grading quenching cooling are adopted to the room temperature, the austenite heat stabilization degree of the steel is reduced, the amount of super-cooled austenite converted into martensite is increased, the amount of residual austenite is reduced, the quenching completeness is improved, and the dimensional stability is improved; after austenitizing the steel, designing a cooling process after precooling; after the steel is austenitized into EF, precooling FY and then carrying out 'water cooling-temperature returning' for many times until the temperature is cooled to room temperature, and refining crystal grains for many times to improve the toughness of the steel; controlling the cooling degree of the steel surface layer in a NaCl aqueous solution with the mass fraction of 5-10% for the first time to control the YG of the steel surface layer in the NaCl aqueous solution with the mass fraction of 5-10% for the first time to bypass the unstable region of the C-curve, namely the nose tip temperature T_PPreparing for the subsequent multiple water cooling temperature return to reach the expected temperature of each critical point to the cold penetration degree of the martensite area; as shown in FIG. 3;

secondly, after austenitizing EF, precooling FY, and cooling YG in NaCl aqueous solution with the mass fraction of 5-10% for the first time to bypass the unstable region of the C-curve, namely the nose tip temperature T_PNot cooled down to the martensite region, i.e. the first water cooling time Y₁G₁Comprises the following steps: the first effluent is heated to H of Bs + (50-100) DEG C by using the waste heat, but the temperature is lower than the pearlite transformation temperature-C-curve nose tip temperature T_P；

(1) First water cooling time Y of steel₁G₁Calculated as follows:

T＝a×N

t-water cooling time (min) of steel;

n-the heat treatment difficulty coefficient (mm) of the steel;

a-cooling coefficient of steel (min/mm);

1) the heat treatment difficulty coefficient N of the steel is more than or equal to 30mm, the a value of the carbon steel is 0.05-0.08 min/mm, and the a value of the alloy steel is 0.03-0.06 min/mm;

2) the heat treatment difficulty coefficient N of the steel is less than 30mm, the a value of the carbon steel is 0.08-0.20 min/mm, and the a value of the alloy steel is 0.06-0.15 min/mm;

the coefficient a is changed along with the influences of the temperature of water, the content of NaCl in the water, the circulation condition of the water, the chemical composition of steel, the temperature and the precooling degree of the steel, the water entering mode of the steel and the like;

the influence of various factors on the actual water cooling time seems that the value a is a variable with variable changes, and the cooling time cannot be determined; although the quenching environments of various manufacturers are different and the cooling coefficient a values are different, each manufacturer has its own production environment and stable technological process, and needs to control each main influence factor to make it stable, and the a value at this time is a constant value with little change; even if the temperature of the water changes, the rule can be found, such as the influence of the temperature of the water changes along with the change of seasons, the influence of the temperature of the water changes along with the change of the cooling frequency of the steel in the water, the change of the temperature of the water changes along with the change of the heat capacity of the water, and the like; only calculating the heat treatment difficulty coefficient N of the steel tapping, and accumulating all data of objective environmental conditions and water cooling time with the best effect during steel quenching; these data are substituted into the model formula: t ═ a × N, i.e., a ═ T/N; the value of the cooling coefficient a suitable for the factory can be calculated;

(2) the first temperature of the steel is increased to H which is Bs + (50-100) DEG C, but is lower than the pearlite transformation temperature, namely the nose tip temperature-T of the C-curve_P；

1) Examples are: the Bs of the 65Mn steel is 400 ℃ and the T_PThe temperature is 480 ℃, and the temperature H is increased;

H＝Bs+(50～100)℃＝400+50～400+100＝450～500

T_Pthe temperature range of the 65Mn steel is 480 ℃, and the temperature return temperature ranges are as follows: h is more than or equal to 450 ℃ and less than 480 ℃;

2) the first tempering temperature H of the steel is determined according to the following principle:

firstly, the heat treatment difficulty coefficient N of the steel is more than or equal to 30mm, and the temperature return temperature H is limited to the upper limit;

secondly, the heat treatment difficulty coefficient N of the steel is less than 30mm, and the lower limit of the temperature H is taken;

this is because the steel has a large heat treatment difficulty coefficient and a large cross section, whereas the steel has a small cross section; the heat treatment difficulty coefficient N is more than or equal to 30mm, the temperature H reaches the upper limit, the energy difference between the steel cooling surface and the cooling medium water is increased, the cooling speed of secondary water cooling which is higher than the heat treatment difficulty coefficient N which is less than 30mm is increased, and the heat treatment difficulty coefficient N is equivalently reduced;

(III) the steel is firstly reheated to H which is Bs + (50-100) DEG C, but is lower than pearlite transformation temperature-C-curve nose tip temperature T_PImmediately transferring the solution to a NaCl aqueous solution with the mass fraction of 5-10% for many times to cool HJ, so as to prevent pearlite and upper bainite from transforming; this is due to:

(1) the super-cooled austenite generates pearlite transformation, and the quality technical index of the steel cannot reach the standard;

(2) supercooled austenite generates upper bainite transformation, and the upper bainite is a harmful structure in steel due to coarse brittle carbides distributed among laths, harms the toughness of the steel and must be avoided;

cooling the steel in a NaCl aqueous solution with the mass fraction of 5-10% for many times until the martensite region is not cooled thoroughly, and returning the temperature of the effluent water to the bainite transformation starting temperature Bs + (50-100) DEG C by using the residual heat for many times, but the temperature is lower than the pearlite transformation temperature T_P(ii) a Before steel grading, cooling HJ in a NaCl aqueous solution with the mass fraction of 5-10% for the last time until a martensite area is not cooled completely, wherein the last water cooling time HnJn is as follows: the temperature K of the last effluent which is returned by utilizing the waste heat and stops rising is the grading starting temperature of KL (KL) which is kept by utilizing the waste heat to supplement energy and grade in a standing air or slow cooling box;

(1) the final water cooling time HnJn of the steel is determined according to the following principle:

1) the heat treatment difficulty coefficient N of the steel is more than or equal to 30mm, and the last water cooling time HnJn is the first water cooling time Y₁G₁20% -60%;

2) the heat treatment difficulty coefficient N of the steel is less than 30mm, and the last water cooling time HnJn is the first water cooling time Y₁G₁10% -50%;

3) the times of water cooling and temperature returning of the steel are more, and the last water cooling time HnJn takes the lower limit value; otherwise, taking an upper limit value;

(2) the last time of the steel is to cool HJ in a NaCl aqueous solution with the mass fraction of 5-10% again until the martensite area is not cooled completely, namely the last time of water cooling time H₂J₂Comprises the following steps: the temperature K of the last (second) effluent water which is returned by utilizing the waste heat and stops rising is the grading starting temperature of KL which is kept when the steel enters standing air or in a slow cooling box by utilizing the waste heat for energy-free supplement grading;

1) last (second) water cooling time H of steel₂J₂The method is determined according to the following principles:

firstly, the heat treatment difficulty coefficient N of the steel is more than or equal to 30mm, and the last water cooling time H₂J₂For the first water cooling time Y₁G₁20% -60% of the total weight of the composition, and taking an upper limit value;

② the heat treatment difficulty coefficient N of the steel is less than 30mm, and the last water cooling time H₂J₂For the first water cooling time Y₁G₁10% -50% of the total weight of the composition, and taking an upper limit value;

2) the temperature K at which the steel is finally (again) discharged water and is heated again by utilizing waste heat to stop rising is as follows: enabling the steel to enter standing air or a slow cooling box, and performing energy-free supplement classification by using waste heat to keep the classification starting temperature of KL;

compared with the prior art, the technical elements of the invention not only apply the 'martensite transformation reversibility' rule and use the heat treatment method to refine grains for many times to improve the toughness of the steel, but also comprise the following steps:

(1) the hardenability and the hardenability of the steel are improved;

1) in the prior art, the improvement of the hardenability and the hardenability of the steel is to add enough alloy elements for stabilizing undercooled austenite into the steel, change the C-curve shape and push the C-curve to the right, reduce the critical quenching speed to realize the improvement of the quenching cooling speed, improve the hardenability and the hardenability of the steel, consume alloy resources, improve the cost and the carbon equivalent of the steel and reduce the welding performance of the steel;

2) the invention uses heat treatment method to improve steelAfter the steel is austenitized into EF, precooling FY and then carrying out 'water cooling-temperature returning' for many times until the temperature is cooled to room temperature Q, so as to improve the hardenability and the hardenability; after austenitizing the steel into EF, precooling FY, and then cooling in a NaCl aqueous solution with the mass fraction of 5-10% for the first time to bypass a C-curve unstable region, namely the nose tip temperature T_PThe temperature of the first effluent is returned to H which is the bainite transformation starting temperature Bs + (50-100) DEG C by utilizing the residual heat until the martensite region is not cooled thoroughly, but is lower than the pearlite transformation temperature-C-curve nose tip temperature T_PImmediately transferring the steel to a water solution with the mass fraction (5-10)% of NaCl for cooling HJ again until the martensite area is not cooled thoroughly, and taking the temperature K of the discharged water which is returned by using the waste heat and stops rising as the grading starting temperature for keeping KL by using the waste heat for energy-free supplement grading when the steel enters standing air or a slow cooling box; namely, the energy of the cooling surface of the steel is improved, the energy difference between the cooling surface of the steel and cooling medium water is increased, and the cooling speed of the steel in the water is improved, so that the hardenability and the hardenability of the steel are improved; microalloying is used for replacing low alloying in the steel, low alloying is used for replacing medium alloying, medium alloying is used for replacing part of high alloying, low-price alloy elements are used for replacing high-price alloy elements, alloy resources are saved, the cost and the carbon equivalent of the steel are reduced, and the welding performance of the steel is improved; this is due to:

firstly, the cooling speed of the steel is caused by the energy difference between the cooling surface of the steel and a cooling medium, the cooling speed with large energy difference is high, and on the contrary, the cooling speed is low, so that the cooling speed with high temperature of the cooling surface of the steel is higher than the cooling speed with low temperature of the cooling surface of the steel in the same cooling medium;

secondly, the internal part of the material is changed from high energy to low energy, and the cooling of the steel in water is the process of transferring the high energy of the steel to the low energy of the steel;

after austenitizing the steel, precooling the steel, and then cooling the steel in water to a certain degree, wherein the cooling speed is attenuated continuously and tends to be gentle, and the steel is in a sub-equilibrium state; because the energy difference between the steel cooling surface and the cooling medium water is small, the high energy of the steel cooling surface is transmitted only by the high energy of the core part, and the hardenability of the steel are restricted because the mechanism restricts the cooling speed of the steel; just like a hydroelectric power station, a barrage is built, the potential energy of water is improved, namely the energy of the water is increased, then the gate is opened to generate electricity, the potential energy of the water is converted into kinetic energy, and the flow speed is accelerated;

(2) the applicable range of the effective thickness of the steel;

1) in the step quenching of the steel in the prior art, even if enough alloy elements for stabilizing undercooled austenite are added into the steel for a large-section steel part, the shape of a C-curve is changed and the C-curve is pushed to the right, and the critical quenching speed is reduced to improve the hardenability and the hardenability of the steel, but the steel is directly cooled to the temperature above or below the martensite transformation starting temperature Ms in a melting salt bath or a hot oil tank capable of controlling the temperature for step heat preservation, the temperature of the melting salt bath or the hot oil is higher, so the cooling capacity is weak, the effective thickness of the steel part is limited to a certain extent, and only the steel part with smaller effective thickness can be applied, otherwise, in the process of cooling from high temperature to the lower bainite transformation temperature at 1/2T, the pearlite and upper bainite transformation occurs due to the low cooling speed, and the quality technical index of step quenching is difficult to achieve as shown in figure;

2) the heat treatment method for the graded quenching of the steel improves the quenching cooling speed of the steel, so as to improve the hardenability and the hardenability of the steel, and after the steel is austenitized into EF, precooling FY and then carrying out water cooling-temperature returning for many times until the temperature is cooled to room temperature P, so as to improve the hardenability and the hardenability of the steel; after the steel is austenitized into EF, precooling FY, cooling YG in a NaCl aqueous solution with the mass fraction of 5-10% for the first time until the YG bypasses a C-curve unstable region and is not cooled completely to a martensite region, and returning the first effluent to H to be the bainite transformation starting temperature Bs + (50-100) DEG C by utilizing waste heat, but being lower than the pearlite transformation temperature-C-curve nose tip temperature T_PImmediately transferring the steel to a step of cooling HJ in a NaCl aqueous solution with the mass fraction of 5-10% until the HJ is not completely cooled in a martensite region, and taking the temperature K of the discharged water which is returned by using waste heat and stops rising as a grading starting temperature for enabling the steel to enter standing air or a slow cooling box to carry out non-energy supplement grading maintenance by using the waste heat; namely, the energy of the cooling surface of the steel is improved, the energy difference between the cooling surface of the steel and cooling medium water is increased, and the cooling speed of the steel in the water is improved, so that the hardenability and the hardenability of the steel are improved; the effective thickness of the steel has wide application range, and the large-section steel piece 1/2T is prevented from being cooled from high temperature to Ma' sIn the process of the bulk transformation temperature, transformation from super-cooled austenite to pearlite and upper bainite occurs to obtain a martensite structure; as shown in FIG. 3; this is due to:

firstly, the super-cooled austenite generates pearlite transformation, and the quality technical index of steel cannot reach the standard;

secondly, super-cooled austenite generates upper bainite transformation, and the upper bainite is a harmful structure in steel due to coarse brittle carbides distributed among laths, harms the toughness of the steel and must be avoided;

(3) the surface layer undercooled austenite of the steel is self-tempered by martensite which is transformed in the martensite area in advance twice;

1) reducing the amount of distortion and cracking tendency of the steel;

firstly, partially transformed martensite is transformed into tempered martensite in advance, and the structure stress is eliminated;

eliminating the thermal stress of surface tensile stress and core compressive stress caused in the initial cooling stage;

high-temperature rapid self-tempering is the same, and tempering brittleness is avoided; this is because high temperature rapid tempering does not produce temper brittleness;

fourthly, reducing the temperature difference between the exterior and the interior when the cooling is continued, namely reducing the thermal stress;

due to the fact that the cooling speed is increased after the temperature returns, the method comprises the following steps:

(i) the temperature difference between the outside and the inside is reduced, namely the thermal stress is reduced;

(ii) the time difference of the transformation of the exterior and interior tissues is shortened, namely, the tissue stress is reduced;

2) the conversion rate of super-cooled austenite is increased for the steel to be water-cooled again, the amount of residual austenite is reduced, and conditions are provided for improving the quenching completeness; the super-cooled austenite is transformed into martensite to generate volume expansion, and the non-transformed super-cooled austenite loses the growth condition and is reserved under the additional pressure of the surrounding martensite; according to Newton's ' acting force and reacting force ' theorem, multiple times of temperature return enables the super-cooled austenite to be self-tempered in the martensite partially transformed in the martensite region in advance, the martensite is transformed into the tempered martensite to generate volume contraction, the additional pressure of the surrounding tempered martensite on the non-transformed super-cooled austenite is reduced, and therefore the transformation under the condition of long and large size is provided.

Sixthly, the steel enters standing air or a slow cooling box and is subjected to energy-free supplementary grading maintenance by utilizing waste heat, and the grading starting temperature is higher or lower than the martensite transformation starting temperature Ms;

(1) the steel classification starting temperature K is determined according to the following principle:

1) the higher the grading starting temperature K of the steel is, the larger the distortion amount and the cracking tendency are reduced after the steel is subjected to the graded quenching, but the martensite is reduced, and the performance of the steel is reduced, namely the strength and the hardness of the steel are reduced; the lower the grading starting temperature K of the steel is, the smaller the distortion amount and the cracking tendency after the graded quenching are reduced, and the difference with the single medium quenching is almost the same;

2) before steel grading, the temperature K of the last effluent which stops rising by utilizing waste heat temperature return is the grading starting temperature of KL (KL) maintained by the steel entering static air or a slow cooling box by utilizing waste heat to perform energy-free supplement grading, the KL maintained by the steel in the static air or the slow cooling box by utilizing waste heat to perform energy-free supplement grading is counteracted, and the temperature is reduced with a small amount of heat loss and the grading temperature is slowly decreased;

3) the steel does not have the temperature difference to supplement the grading keeping of the energy and carry on the grading heat preservation of the energy supplement in the molten salt bath or hot oil bath with the waste heat in the air of stewing or slow cooling tank, the steel does not have the determination of the grading beginning temperature that supplements the grading keeping of the energy with the waste heat in the air of stewing or slow cooling tank, according to the grading temperature of grading heat preservation in the molten salt bath or hot oil bath plus the temperature difference; the steel is subjected to energy-free supplement grading in standing air or a slow cooling box by utilizing waste heat, the grading temperature is kept in grading time, the temperature is reduced to be not lower than the technical requirement specified by the grading temperature, and the structure and the performance of the expected quenched steel are obtained;

(2) the steel grading onset temperature K is:

1) the grading starting temperature K of the high-carbon steel, the medium-carbon and high-carbon alloy steel is Ms + (20-40) DEG C;

namely: k is Ms + (20-40) DEG C;

examples are: the Ms of the 9SiCr steel is 160 ℃,

k + Ms + (20-40) 160+ 20-160 +40 180-200

The grading starting temperature value range of the 9SiCr steel is as follows: k is more than or equal to 180 ℃ and less than or equal to 200 ℃;

2) the grading starting temperature K of the medium-carbon steel, the low-carbon steel and the low-carbon alloy steel is Ms- (40-60) DEG C;

namely: k is Ms- (40-60) DEG C;

examples are: the Ms of the 50 steel is 300 ℃,

then K is Ms- (40-60) ═ 300-40-300-60 ═ 240-260

The value range of the grading starting temperature of the 50 steel is as follows: k is more than or equal to 240 ℃ and less than or equal to 260 ℃;

3) the grading starting temperature K of the low-temperature tempering steel of the common parts is (180-220) ° C;

4) the value range of the grading starting temperature K of the steel is determined according to the following principle:

firstly, the heat treatment difficulty coefficient N of the steel is more than or equal to 30mm, and the grading starting temperature K takes the lower limit;

secondly, the heat treatment difficulty coefficient N of the steel is less than 30mm, and the grading starting temperature K is limited to the upper limit;

this is because the heat treatment difficulty coefficient of steel is large, the heat capacity is large, and conversely, the heat capacity is small;

seventhly, immediately transferring the steel to standing air or a slow cooling box to carry out classification keeping KL without energy supplement by using waste heat, wherein the steel is at a classification starting temperature K;

(VIII) the steel is graded KL;

(1) the steel is classified and KL is kept in a classification way without energy supplement by using waste heat in standing air at the classification starting temperature K of Ms + (20-40) DEG C and at the classification starting temperature Ms- (40-60) DEG C;

1) in the prior art, the steel is subjected to energy supplement grading heat preservation in a temperature-controllable molten salt bath or a hot oil groove, and only single grading transformation (the grading heat preservation line segment is a horizontal line with an Ms line above or below) exists to improve the austenite heat stabilization degree of the steel; as shown in FIG. 4;

2) the steel of the invention uses the grading starting temperature K as Ms + (20-40) DEG C or Ms- (40-60) DEG C to carry out grading maintenance KL or K 'L' without energy supplement in standing air by using waste heat, reduces the temperature with little heat loss and slowly reduces the grading temperature, has slow cooling speed, does not stop continuous cooling of super-cooled austenite, combines the two procedures of the grading maintenance procedure and the continuous cooling procedure into one procedure (the grading maintenance line segment KL and the K 'L' are oblique lines with the Ms line being above and below the line respectively), and carries out grading maintenance including grading transformation and continuous cooling transformation; as shown in FIGS. 1 and 3;

firstly, the steel super-cooled austenite is subjected to martensite transformation through graded transformation to obtain a martensite structure, and the distortion and the cracking tendency are reduced;

continuous cooling transformation reduces the austenite heat stabilization degree of steel:

(i) the difficulty of steel cold treatment and tempering treatment is reduced;

(ii) the amount of super-cooled austenite of the steel is increased and transformed into martensite, the amount of residual austenite is reduced, the quenching completeness is improved, the performance and the dimensional stability of the quenched steel are improved, and the brittle transformation temperature of the quenched steel is reduced;

this is because the supercooled austenite is stopped cooling at the classification temperature and is maintained for a certain time, the retained austenite is more retained at the temperature, and the retention time is long, and the supercooled austenite that has not been transformed becomes more stable; when the cooling is continued again, the transformation from the super-cooled austenite to the martensite is not started immediately, but can be recovered after a period of time, the transformation is carried out at a lower temperature, and the transformation quantity can not reach the transformation quantity of the continuous cooling;

(2) performing low-temperature tempering on general parts, performing grading maintenance KL without energy supplement in a slow cooling box by using waste heat at a grading starting temperature K of (180-220 ℃), and performing grading maintenance and self-tempering treatment; this is to combine three steps of the classification holding step, the continuous cooling step and the self-tempering step into one step (the classification holding line segment KL is a diagonal line) as shown in fig. 1 and 3; the staged hold includes both a staged transition, a continuous cooling transition, and an auto-tempering transition;

1) the graded transformation makes the super-cooled austenite of steel generate martensite transformation, obtains martensite structure, and reduces distortion amount and cracking tendency

2) Continuous cooling transformation reduces the degree of austenite heat stabilization of steel:

the difficulty of steel cold treatment and tempering treatment is reduced;

secondly, the transformation quantity of the super-cooled austenite of the steel is increased, the amount of the residual austenite is reduced, the quenching completeness is improved, the performance and the dimensional stability of the quenched steel are improved, and the brittle transformation temperature of the quenched steel is reduced;

this is because the supercooled austenite is stopped cooling at the classification temperature and is maintained for a certain time, the retained austenite is more retained at the temperature, and the retention time is long, and the supercooled austenite that has not been transformed becomes more stable; when the cooling is continued again, the transformation from the super-cooled austenite to the martensite is not started immediately, but can be recovered after a period of time, the transformation is carried out at a lower temperature, and the transformation quantity can not reach the transformation quantity of the continuous cooling;

3) the self-tempering transformation comprises:

firstly, transforming the residual austenite of the steel into martensite;

secondly, the martensite of the steel is transformed into tempered martensite;

4) the performance requirements of the slow cooling box are as follows:

firstly, the slow cooling box has good heat preservation performance, reduces heat loss, stores waste heat for grading maintenance without energy supplement, and once the heat is lost, the waste heat cannot be recovered, so that grading and self-tempering failure is caused;

secondly, the inlet of the slow cooling box is automatically closed immediately after the steel enters the slow cooling box to reduce the heat loss from the inlet, the residual heat is preserved to carry out grading maintenance without energy supplement, just like a thermos bottle is poured out of hot water, the thermos bottle plug is immediately covered on the bottle mouth to reduce the heat loss from the bottle mouth;

5) energy and resources are saved;

firstly, the steel in the prior art is directly cooled in a temperature-controllable molten salt bath or a hot oil tank to a grading temperature above or below the martensite transformation starting temperature Ms, grading heat preservation with energy supplement is carried out, energy and resources are consumed, the environment is polluted, and the heat treatment cost is increased;

secondly, the steel of the invention is cooled in water for multiple times and bypasses a C-curve unstable region until a martensite region is not cooled completely, and the water which is discharged for multiple times is reheated to a bainite transformation starting temperature Bs + (50-100) DEG C by using waste heat, but is lower than a pearlite transformation temperature-C-curve nose tip temperature T-_PBefore classification, the last effluent utilizes the waste heatThe temperature of stopping rising after temperature rising is the grading starting temperature, the air is immediately transferred to standing and the waste heat is utilized in a slow cooling box to carry out grading maintenance without energy supplement, so that the energy and the resource are saved, the heat treatment cost is reduced, the environment is protected, the heat treatment cost is reduced, and the construction period is advanced;

6) the cooling and classifying media used;

firstly, a cooling and grading medium used by a molten salt bath or a hot oil bath capable of controlling temperature in the prior art is the molten salt bath or the hot oil, the molten salt bath or the hot oil has high cost, the molten salt bath or the hot oil needs to be heated to a grading temperature which is slightly higher than or slightly lower than a martensite transformation starting temperature Ms during use, and the temperature is high, so that the cooling capacity is weak, the pollution is large, the fire and the splash are easy to hurt people, the safety is high, the aging phenomenon exists, and the molten salt bath or the hot oil bath needs to be replaced after being used for a certain time;

the cooling and grading media used in the invention are water and air, the water and air have low cost, the use is convenient, the cooling capacity of the water is strong, the water is clean and pollution-free, the environment is protected, the safety is realized, the fire does not occur, the aging phenomenon does not exist, the longer the clean cooling water is used, the better the clean cooling water is, the more the clean cooling water is used, the step is advanced to the real clean heat treatment of replacing oil with water, and a new way of replacing oil with water is opened up;

(nine) Steel grading time K₁L₁Is (60-120) s;

(1) the steel is at a grading starting temperature K, immediately transferred into standing air or a slow cooling box, grading is carried out without energy supplement by using waste heat to keep KL, and the grading time K is calculated₁L₁；

(2) Grading time K of Steel₁L₁Comprises the following steps:

1) the grading starting temperature K of the high-carbon steel, the medium and high-carbon alloy steel is Ms + (20-40) DEG C grading time K₁L₁Is (90-120) s;

2) the grading starting temperature K of the medium-low carbon steel and the low-carbon alloy steel is Ms- (40-60) DEG C grading time K₁L₁Is (60-90) s;

3) the grading start temperature K of the low-temperature tempering steel of the common parts is (180-220) DEG C₁L₁Is 60min, and the temperature is reduced to 160 ℃ and is not less than 60 min;

(ten) after the steel is graded by KL, taking out standing air L, cooling for multiple times of water cooling-temperature rising until the steel is cooled to room temperature, immediately transferring the steel to a NaCl aqueous solution with the mass fraction of 5-10%, and then grading, quenching and cooling to room temperature Q' for multiple times, wherein the graph is shown in figure 3; cooling the slowly cooling box L to room temperature by air cooling;

(1) after the steel is graded by KL, cooling the discharged standing air L, namely immediately transferring the steel to an NaCl aqueous solution with the mass fraction of 5-10%, and then grading, quenching and cooling the steel for multiple times until the steel is cooled to room temperature Q ";

1) after the steel is graded by KL, the cooled LM of the standing air L is immediately transferred into a NaCl aqueous solution with the mass fraction of 5-10 percent for the first time, and then graded, quenched and cooled until the martensite area is not cooled completely, namely the water cooling time L₁M₁Comprises the following steps: the temperature N of the effluent which stops rising after being reheated by waste heat is the first steel secondary classification quenching, and the effluent enters static air to be subjected to energy-free supplement and secondary classification by using the waste heat so as to keep the secondary classification starting temperature of NP;

firstly, carrying out primary reclassification quenching on steel, entering standing air, supplementing energy without using waste heat, reclassifying and keeping the reclassification starting temperature N of NP at (150-180) DEG C;

secondly, immediately transferring the steel to standing air at a reclassification starting temperature N for the first reclassification quenching, and performing reclassification without energy supplement by using waste heat to keep NP;

third, the reclassification time N of the first reclassification quenching of the steel₁P₁；

(i) The steel is the reclassification starting temperature N, the steel is immediately transferred to standing air to perform reclassification retaining NP without energy supplement by using waste heat, and the reclassification time N is calculated₁P₁；

(ii) Reclassification time N of first reclassification quenching of steel₁P₁Is (60-120) s;

fourthly, after the NP is classified again for the first time, the cooled PQ of the standing air P is immediately transferred into NaCl water solution with the mass fraction of 5-10% for the second time, and then classified, quenched and cooled until the martensite area is not cooled thoroughly;

2) after the first steel re-classification NP, the cooling PQ of the still air P is immediately shifted to the mass fraction (5)The secondary classification quenching and cooling are carried out in 10)% NaCl aqueous solution until the martensite area is not cooled completely, namely the water cooling time P₁Q₁Comprises the following steps: the temperature N ' of the effluent water which stops rising after being reheated by waste heat is used as the secondary reclassification quenching of the steel, and the effluent water enters the standing air and is reclassified by using the waste heat without energy supplement to keep the reclassification starting temperature of N ' P ';

firstly, performing secondary reclassification quenching on steel, and enabling the steel to enter standing air to perform energy-free supplement reclassification by using waste heat so as to keep the reclassification starting temperature N ' of N ' P ' at (110-140) DEG C;

secondly, immediately transferring the steel to standing air to perform energy-supply-free reclassification by using waste heat to keep N ' P ' at a reclassification starting temperature N ' of secondary reclassification quenching;

③ reclassification time N 'of secondary reclassification quenching of steel'₂P’₂；

(i) The steel is a reclassification starting temperature N ', the steel is immediately transferred to a reclassification maintaining temperature N ' P ' which is in static air and is not supplemented with energy by waste heat, and the reclassification time N ' is calculated '₂P’₂；

(ii) Reclassification time N 'of secondary reclassification quenching of steel'₂P’₂Is (60-120) s;

fourthly, after the steel is classified for the second time, the cooled P ' Q ' of the standing air P ' is immediately transferred into NaCl aqueous solution with the mass fraction of 5-10% for the third time, and then is classified, quenched and cooled until the martensite area is not cooled thoroughly;

3) after the steel is subjected to secondary reclassification N 'P', the cooled P 'Q' of the standing air P 'is immediately transferred into a NaCl aqueous solution with the mass fraction of 5-10% for secondary reclassification, and is quenched and cooled to a martensite area without thorough cooling, namely water cooling time P'₂Q’₂Comprises the following steps: the temperature N ' of the effluent water which stops rising after being returned to the temperature by utilizing the waste heat is used as the third steel secondary classification quenching, the effluent water enters the standing air and is subjected to energy-free supplement and secondary classification by utilizing the waste heat to keep the secondary classification starting temperature of N ' P ';

thirdly, performing secondary classification quenching on the steel for the third time, and enabling the steel to enter standing air to perform energy-free supplement and secondary classification by utilizing waste heat so as to keep the secondary classification starting temperature N of N 'P' at (70-100) ° C;

secondly, immediately transferring the steel to standing air to perform energy-supply-free reclassification by using waste heat to keep N ' P ' at a reclassification starting temperature N ' of the third reclassification quenching;

thirdly reclassification time N of third reclassification quenching of steel "₃P”₃；

(i) The steel is the reclassification starting temperature N ', the steel is immediately transferred to standing air to be reclassified and kept N' P 'without energy supplement by using waste heat, and the reclassification time N' is calculated "₃P”₃；

(ii) Reclassification time N of third reclassification quenching of steel "₃P”₃Is (60-120) s;

fourthly, after the steel is classified again for the third time, the cooling P 'Q' of the standing air P 'is immediately transferred into NaCl aqueous solution with the mass fraction of 5-10% to be cooled to the room temperature Q';

4) in the prior art, after steel is classified, the steel is cooled from a molten salt bath or a hot oil bath to room temperature by air cooling, the retention time from the classified steel to the room temperature is prolonged, and the thermal stability degree of austenite is improved;

5) after steel is classified, the discharged standing air is immediately transferred into NaCl aqueous solution with mass fraction of 5-10% for multiple times of reclassification, quenching and cooling until a martensite area is not cooled thoroughly, the temperature of the discharged water which stops rising due to waste heat rewarming is the reclassification starting temperature of next time in NaCl aqueous solution with mass fraction of 5-10%, the discharged water enters the standing air for carrying out energy-free supplement reclassification by using waste heat, and the discharged standing air is immediately transferred into NaCl aqueous solution with mass fraction of 5-10% for cooling to room temperature after the last reclassification; the crystal grains are refined for many times by a heat treatment method, so that the strength and toughness of the steel are improved, the retention time of the steel after grading from standing air to room temperature is shortened, the austenite heat stabilization degree of the steel is reduced, the undercooled austenite transformation amount of the steel is increased, the residual austenite amount is reduced, the quenching completeness is improved, the performance and the size stability of the quenched steel are improved, the brittleness transformation temperature of the quenched steel is reduced, and the distortion amount and the cracking tendency of the steel are also reduced; this is due to:

firstly, applying the 'martensite transformation reversibility' rule, carrying out 'water cooling-temperature returning' for multiple times until the temperature is cooled to room temperature, refining crystal grains for multiple times by using a heat treatment method to improve the toughness of steel, and carrying out fine-grain strengthening and toughening treatment;

secondly, immediately transferring the discharged standing air into a NaCl aqueous solution with the mass fraction of 5-10% for multiple times of secondary-grading quenching and cooling after steel grading until the steel is cooled to room temperature, and shortening the retention time of the discharged standing air to the room temperature after steel grading so as to reduce the austenite thermal stabilization degree of the steel;

the reason is that the supercooled austenite of the steel stops cooling at a certain temperature and is kept for a certain time, the residual austenite is more retained at the temperature, the retention time is long, and the supercooled austenite which is not transformed becomes more stable; namely, the longer the residence time, the greater the thermal stability of austenite, and the transformation from supercooled austenite to martensite does not start immediately but can be recovered after a while when cooling is continued, the transformation will be performed at a lower temperature, and the transformation amount will not reach the previous transformation amount;

thirdly, the steel does not need to be rapidly cooled under the temperature of Ms + (10-30) DEG C (high hardenability steel) or Ms- (50-70) DEG C (low hardenability steel) accompanied with volume change, otherwise, distortion and cracking are generated;

(2) after the steel is graded by KL, taking the steel out of a slow cooling box L, and cooling the steel to room temperature by air;

cooling the steel to room temperature, and immediately transferring to the next heat treatment process;

the reason is that the thermal stability of the austenite of the steel is long in retention time at a certain temperature, the thermal stability of the austenite is high, and the retention time at room temperature is shortened to reduce the thermal stability of the austenite;

5. the temperature of NaCl aqueous solution with the mass fraction of 5-10% is controlled at 15-38 ℃.