阅读说明：本技术 一种可直接冷锻加工的齿轴钢的制造方法 (Method for manufacturing gear shaft steel capable of being directly subjected to cold forging ) 是由 顾铁 吴小林 白云 许晓红 官跃辉 卢明霞 曹红福 于 2020-04-01 设计创作，主要内容包括：本发明涉及可直接冷锻加工的齿轴钢的制造方法,包括步骤(1)冶炼：钢水组分为C：0.15～0.45％,Si：≤0.35％,Mn：0.60～1.40％,P：≤0.030％,S：≤0.030％,Cr：0.60～1.40％％,余量为Fe及不可避免的杂质元素；(2)浇注成方坯；(3)轧制：轧前加热使奥氏体化,出炉除鳞,并清理热坯表层,清理厚度0.5～4mm,然后采用14架二辊轧机+5架三辊轧机进行单相区轧制：轧制过程中进行控冷,中轧后、预精轧后、精轧后分别进行水冷却,终轧温度在770～850℃,精轧结束后坯料快速冷却到上冷床的温度750～830℃,降低转变前奥氏体稳定性；(4)冷却：坯料在冷床上鼓风冷却,控制平均冷速在45～70℃/min,终冷温度为300～450℃,终冷之后材料空冷至室温,得到铁素体和珠光体的微观结构。(The invention relates to a method for manufacturing gear shaft steel capable of being directly cold forged, which comprises the following steps of (1) smelting: the molten steel comprises the following components: 0.15-0.45%, Si: less than or equal to 0.35 percent, Mn: 0.60-1.40%, P: less than or equal to 0.030 percent, S: less than or equal to 0.030 percent, Cr: 0.60-1.40%, the balance being Fe and inevitable impurity elements; (2) casting into square billets; (3) rolling: heating before rolling to enable austenitization, discharging from a furnace for descaling, cleaning the surface layer of a hot blank to a thickness of 0.5-4 mm, and then carrying out single-phase region rolling by adopting 14 two-roll mills and 5 three-roll mills: controlled cooling is carried out in the rolling process, water cooling is respectively carried out after medium rolling, pre-finish rolling and finish rolling, the finish rolling temperature is 770-850 ℃, the blank is rapidly cooled to the temperature of 750-830 ℃ on an upper cooling bed after finish rolling, and the austenite stability before transformation is reduced; (4) and (3) cooling: and (3) blowing the blank on a cooling bed for cooling, controlling the average cooling speed to be 45-70 ℃/min and the final cooling temperature to be 300-450 ℃, and after final cooling, air-cooling the material to room temperature to obtain the microstructure of ferrite and pearlite.)

1. A method for manufacturing gear shaft steel capable of being directly cold forged is characterized in that: comprises the following steps

(1) Smelting, namely keeping white slag for more than or equal to 25min when molten steel is refined outside L F furnaces, keeping the total time of the refining outside L F furnaces for not less than 40min, carrying out vacuum degassing after the refining outside L F furnaces, keeping the vacuum for more than or equal to 30min, carrying out whole-process argon blowing soft stirring during the refining outside L F furnaces and the vacuum degassing, wherein the molten steel after smelting comprises, by mass, 0.15-0.45% of C, less than or equal to 0.35% of Si, 0.60-1.40% of Mn, less than or equal to 0.030% of P, less than or equal to 0.030% of S, 0.60-1.40% of Cr, and the balance of Fe and inevitable impurity elements;

(2) pouring: casting the molten steel into a steel billet;

(3) rolling: heating the steel billet to complete austenitizing, controlling the temperature of a preheating section at 750-865 ℃, the temperature of a first heating section at 950-1070 ℃, the temperature of a second heating section at 1000-1200 ℃, the temperature of a soaking section at 1070-1200 ℃, and the total heating time to be more than 3 hours;

discharging from the furnace, carrying out high-pressure water descaling, cleaning the surface layer of the hot blank to a thickness of 0.5-4 mm, and then rolling by adopting 14 two-roll mills and 5 three-roll mills: rough rolling by 4 two-roll mills, pre-finish rolling by 4 two-roll mills, finish rolling by 5 three-roll mills, controlling the maximum reduction of a single pass to be 20-26 mm, controlling cooling in the rolling process, sequentially arranging at least four spray water tanks for controlling cooling, distributing a No. 1 water tank for spray cooling after medium rolling, distributing No. 2 and No. 3 water tanks for spray cooling after pre-finish rolling, distributing a No. 4 water tank for spray cooling after finish rolling, and cooling steel alternately in the strong and weak manner in the rolling process, wherein the relative opening degree of cooling water of the No. 1 water tank is 40-65% for strong cooling, the relative opening degree of cooling water of the No. 2 and No. 3 water tanks is 10-20% for weak cooling, cooling by the No. 1, No. 2 and No. 3 water tanks to ensure that the incoming material temperature of the blank entering a finishing mill set is 780-870 ℃, the finish rolling finishing temperature is 770-850 ℃ so that finish rolling is rolled in a single-phase region, compared with the No. 2 and No. 3 water tanks, the relative opening degree of the cooling water of the No. 4 water tank is increased to 30-50%, so that the temperature of the blank after finish rolling quickly reaches the temperature requirement of 750-830 ℃ on the cooling bed, and the transfer waiting time of the blank on the cooling bed after finish rolling is saved or shortened;

(4) and (3) cooling: and after rolling, blowing the blank on a cooling bed for cooling, controlling the temperature of the blank to be 750-830 ℃ when the blank is put on the cooling bed, reducing the phase change starting temperature to reduce the stability of austenite before starting phase change, controlling the average cooling speed of the cooling bed to be 45-70 ℃/min, controlling the final cooling temperature to be 300-450 ℃, and cooling the material to room temperature after final cooling to obtain the microstructure of ferrite and pearlite.

2. The method for producing direct cold-forgeable pinion shaft steel according to claim 1, wherein: and (3) utilizing high-pressure oxygen flow to 'melt' the surface of the blank to achieve the effect of cleaning the surface layer of the hot blank.

3. The method for producing direct cold-forgeable pinion shaft steel according to claim 1, wherein: and (3) controlling the degree of superheat of pouring in the step (2) to be 10-30 ℃.

4. The method for producing direct cold-forgeable pinion shaft steel according to claim 1, wherein: the content of Si in the step (1) is less than or equal to 0.20 percent.

5. The method for producing direct cold-forgeable pinion shaft steel according to claim 4, wherein: the content of Si in the step (1) is 0.08-0.20%.

6. The method for producing direct cold-forgeable pinion shaft steel according to claim 1, wherein: the molten steel in the step (1) also contains one or more of Al, Nb, Ti and V, the total addition of Al, Nb, Ti and V is not higher than 0.16%, and the addition of Ti is not higher than 0.02%.

7. The method for producing direct cold-forgeable pinion shaft steel according to claim 1, wherein: the molten steel in the step (1) also contains one or more of Mo, Ni and B.

Technical Field

The invention relates to a gear shaft steel capable of being directly cold-forged and a manufacturing method thereof, in particular to a gear shaft steel rolling material free of heat treatment before cold forging and a manufacturing method thereof.

Background

The gearbox is widely applied to industry, and can be applied to gearboxes only with power transmission. Taking a passenger car as an example, the number of the passenger car gearboxes produced in China every year exceeds 2500 thousands. One end of the gearbox inputs power through an input shaft, changes the rotating speed after being meshed through gears with different modules, and outputs power through an output shaft. Both the input shaft and the output shaft are referred to as a pinion shaft. At present, the forming modes of a gear shaft or a gear in the gear industry are hot forging, warm forging and cold forging. The cold forging is an industry development trend due to the advantages of high dimensional precision, good surface quality, small subsequent machining allowance and the like. The pinion shaft is generally smaller than the gear forging ratio, and is therefore more suitable for cold forging.

It is known that steel materials have excellent formability, i.e., forgeability, in a heated state, particularly after austenitization. At present, the forging and forming in the industry still adopts a hot forging mode mostly, although the forgeability is good, the energy consumption is high because of heating, the decarburization degree of the steel is aggravated after heating, the surface decarburized layer becomes thick, and the surface quality is influenced. The dimensional accuracy of the hot forged part is also reduced. In contrast, hot rolled steel products are relatively poor in plasticity when directly subjected to cold forging, and in order to improve cold working plasticity, hot rolled steel products are generally subjected to heat treatment such as softening annealing, isothermal annealing, and spheroidizing annealing. And carrying out cold forging processing after annealing heat treatment. The material is subjected to heat treatment once, so that the processing period is increased, the energy consumption is improved, and the production cost is increased.

In summary, no matter the hot rolled material is subjected to hot forging or is subjected to heat treatment (annealing) and then cold forging, the hot treatment before the forming process prolongs the processing period, reduces the dimensional accuracy and increases the production cost. Therefore, a hot-rolled material capable of being directly cold-forged without heat treatment (annealing) has been developed for directly processing a pinion, which can not only save the heat treatment process but also eliminate the thickening of a decarburized layer and the deterioration of the processing accuracy due to the heat treatment.

In order to save the steps, reduce energy consumption, and reduce the decarburized layer, it is desirable to develop a steel material that can be directly cold forged.

Disclosure of Invention

The invention aims to provide the gear shaft steel with high hardenability (uniform structure and moderate hardness), shallow decarburized layer (good surface quality), low level of banded structure, high dimensional precision and better cold forging property.

The hot-rolled microstructure of the gear shaft steel is ferrite and pearlite, the average lamellar spacing of the pearlite is 0.14-0.15 mu m, the yield strength is more than or equal to 600MPa, the tensile strength is more than or equal to 880MPa, the elongation is more than or equal to 25%, the surface shrinkage is more than or equal to 50%, the hardenability J9mm (the hardness of a position 9mm away from the end in a hardenability test) is 35-40 HRC, the elongation and the surface shrinkage of the steel are high, the plasticity is strong, and the gear shaft steel can be directly subjected to cold forging processing. The steel strength and hardness interval just meets the processing requirements of the gear shaft steel.

The technical scheme of the invention is as follows: a method for manufacturing gear shaft steel capable of being directly cold forged comprises the following steps

(1) Smelting, namely keeping white slag for more than or equal to 25min when molten steel is refined outside an L F furnace, keeping total time of refining outside the L F furnace for more than or equal to 40min, carrying out vacuum degassing after refining outside the L F furnace, keeping the vacuum for more than or equal to 30min, carrying out whole-process argon blowing soft stirring during refining outside the L F furnace and vacuum degassing, wherein the flow of argon is not too small or too large, the stirring effect is not good, and the molten steel is turned over to cause secondary oxidation to generate inclusions which can cause heat treatment cracking and have the risk of malignant quality accidents.

After smelting, the molten steel comprises the following components in percentage by mass: 0.15-0.45%, Si: less than or equal to 0.35 percent, Mn: 0.60-1.40%, P: less than or equal to 0.030 percent, S: less than or equal to 0.030 percent, Cr: 0.60 to 1.40 percent, and the balance of Fe and inevitable impurity elements.

(2) Pouring: and (3) casting the molten steel into a steel blank (preferably, a continuous casting square blank is selected, the internal quality of the casting blank is controlled by adopting the process means of tail end electromagnetic stirring, light pressing and the like during continuous casting), the superheat degree is controlled at 10-30 ℃ during casting, and the molten steel is prevented from being oxidized in the whole casting process under the protection of argon.

(3) Rolling: and (3) heating the steel billet to complete austenitizing, controlling the temperature of a preheating section at 750-865 ℃, the temperature of a first heating section at 950-1070 ℃, the temperature of a second heating section at 1000-1200 ℃, the temperature of a soaking section at 1070-1200 ℃, and fully curing for more than 3 hours. And (3) discharging the blank out of the furnace, removing scale by high-pressure water, cleaning the surface layer of the hot blank to a thickness of 0.5-4 mm, removing surface defects such as cracks, pits and decarburization on the surface of the blank, and ensuring the surface quality of the rolled material, wherein the process is a key operation for ensuring the surface quality.

Then, rolling by adopting 14 two-roll mills and 5 three-roll mills: rough rolling by 4 two-roll mills, pre-finish rolling by 4 two-roll mills, finish rolling by 5 three-roll mills, controlling the maximum reduction of a single pass to be 20-26 mm, controlling cooling in the rolling process, sequentially arranging at least four spray water tanks for controlling cooling, distributing a No. 1 water tank for spray cooling after medium rolling, distributing No. 2 and No. 3 water tanks for spray cooling after pre-finish rolling, distributing a No. 4 water tank for spray cooling after finish rolling, and cooling steel alternately in the strong and weak manner in the rolling process, wherein the relative opening degree of cooling water of the No. 1 water tank is 40-65% for strong cooling, the relative opening degree of cooling water of the No. 2 and No. 3 water tanks is 10-20% for weak cooling, cooling by the No. 1, No. 2 and No. 3 water tanks to ensure that the incoming material temperature of the blank entering a finishing mill set is 780-870 ℃, the finish rolling finishing temperature is 770-850 ℃ so that finish rolling is rolled in a single-phase region, compared with the No. 2 and No. 3 water tanks, the relative opening degree of the cooling water of the No. 4 water tank is increased to 30-50%, so that the temperature of the blank after finish rolling quickly reaches the temperature requirement of 750-830 ℃ on the cooling bed, and the transfer waiting time of the blank after finish rolling on the cooling bed is saved or shortened. In addition, the finish rolling temperature of the material is 770-850 ℃, the crystalline phase still does not reach the transformation temperature, strong post-rolling water cooling is carried out through a No. 4 water tank before the finish rolling is finished and the steel is fed into a cooling bed, the temperature of the steel is rapidly adjusted to 750-830 ℃ when the steel is fed into the cooling bed, so that the austenite transformation starting temperature is reduced, the austenite stability is further reduced, a fine and uniform hot rolled tissue is easily obtained through subsequent tissue transformation, and the tissue is favorable for cold forging processing of the steel.

In the rolling mode, the cooling strength of the No. 2 and No. 3 water tanks is relatively weak to that of the No. 1 and No. 4 water tanks, the design purpose is to ensure that steel enters the finishing mill group at a preset temperature, if the cooling water of the No. 2 and No. 3 water tanks is too large, the starting temperature of the finishing mill group is not easy to control accurately, the temperature difference between the surface and the inside of the steel can be increased, the surface of the steel is sprayed by the cooling water, the surface is cooled and hardened, the load of a rolling mill is increased, and the overload of the rolling mill is easily caused due to the fact that the temperature of the surface of the steel is too low because the load capacity. And the temperature of the billet of the finishing mill group is further controlled to be 780-870 ℃, the rolling in a single-phase region is ensured during finish rolling, the banded structure of a rolled material can be effectively improved, and the generation of mixed crystals is avoided. In addition, the temperature of the finishing mill is controlled, so that the method plays a positive role in improving the hot rolling dimensional stability of the steel, and is beneficial to improving the dimensional processing precision of the steel.

(4) And (3) cooling: after rolling is finished, the blank is cooled by blowing air on a cooling bed, the temperature of the blank is controlled to be 750-830 ℃ when the blank is loaded on the cooling bed, the austenite phase transition starting temperature is reduced, the stability of austenite before cooling is reduced, the average cooling speed is controlled to be 45-70 ℃/min, the interlayer spacing of pearlite can be reduced by adopting a higher air cooling speed (the temperature is controlled to be 0.14-0.15 mu m, the strength and the hardness are improved), the final cooling temperature is 300-450 ℃, austenite is converted into ferrite and pearlite in the cooling process, the level of a banded structure can be reduced by adopting a higher cooling speed in the conversion process, the level of the banded structure is less than or equal to 2, the elongation and the face shrinkage are improved, the cold forging performance of steel is further improved, and the material is cooled to room temperature after final cooling.

The design basis of each element in the gear shaft steel is as follows:

c: 0.15 to 0.45 percent. The steel grade of the invention is directly cold forged in a hot rolling state, a ferrite plus pearlite structure is formed before forging, and the content of C directly influences the proportion of pearlite in the material, thereby influencing the strength of the material. The pearlite ratio is too low, the material lacks sufficient strength, the C content ratio is too high, the strength is improved, but the cold forgeability of the steel material is significantly reduced in order to reconcile the strength and the cold forgeability. In order to ensure the toughness of the core part of the gear shaft after final heat treatment, the C content is not easy to be too high, and a medium-low carbon design is adopted. The content of C in the steel material is set to 0.15 to 0.45%.

Si: less than or equal to 0.35 percent. Si is solid-dissolved in the ferrite phase, has a strong solid-solution strengthening effect, and can significantly improve the ferrite strength, but simultaneously reduces the plasticity and toughness of ferrite, and also promotes grain boundary segregation of the element P, S, so that grain boundaries are embrittled to some extent. In general steel grades, Si is added to steel as a deoxidizing element during steel making. The steel of the invention is used for the gear shaft steel which can be directly cold-forged, and the material is required to have better cold forging performance, namely the excellent plasticity and toughness of ferrite in the steel are required. Further, if Si is too high during the final heat treatment of the gear shaft, intergranular oxidation is likely to occur, and the performance of the gear shaft is deteriorated. Therefore, Si is set to 0.35% or less, more preferably 0.20% or less.

Mn: 0.60 to 1.40 percent. Mn is used as an element of the deoxidizer action, the strength of the steel can be improved on the basis of not obviously influencing the plasticity of the material by proper amount of Mn, and the addition of a certain amount of Mn is very important for ensuring the mechanical property of the material. In addition, Mn combines S and the like in the steel to form sulfides, and the sulfides have good plasticity, exert a notch effect during cutting and improve the cutting performance of the material. In order to fully exert the above effects, the Mn content of the steel material in the present invention is set to 0.60 to 1.40%.

P: less than or equal to 0.030 percent. P tends to undergo severe microsegregation during casting solidification and then aggregates at grain boundaries during heating, increasing the brittleness, particularly cold brittleness, of the steel, and deteriorating the cold forgeability of the material. The P content of a general steel material is desirably reduced as much as possible, and the P content of the steel material in the present invention is set to 0.030% or less, more preferably 0.020% or less.

S: less than or equal to 0.030 percent. S is a free-cutting element. When a certain amount of Mn is contained in the steel, MnS or a MnS-containing complex is easily formed, thereby improving the machinability of the material. However, sulfides generally have a low melting point and an excessively high S content, so that the material has a hot brittle effect and an increased tendency to decarburization. In order to fully exhibit the above effects, the content of S in the steel material of the present invention is set to 0.030% or less, and preferably 0.010 to 0.020%.

Cr: 0.60 to 1.40 percent. Cr element increases hardenability, thins the lamellar spacing of pearlite, and is beneficial to improving the formation proportion of a fine lamellar pearlite structure and the uniformity of a microstructure, thereby effectively improving the properties of the material such as strength, fatigue and the like. The Cr content of the steel material in the present invention is set to 0.60 to 1.40%.

In order to meet the comprehensive properties of the material, such as strength, plasticity, hardenability and the like, one or more alloy elements, such as Mo, Ni, B and the like, can be added into the tooth shaft steel designed by the invention. Meanwhile, in order to prevent the coarsening of crystal grains of the material in the final heat treatment process, one or more alloy elements such as Al, Ti, Nb, V and the like can be added.

The invention is a more typical design of medium-low carbon gear shaft steel, the gear shaft has high surface strength and hardness and good wear resistance after final heat treatment, and the core part has better toughness, thus being beneficial to prolonging the service life of a reduction gearbox. When the steel is designed into a composition, elements such as Si and P which are not favorable for cold forgeability are controlled.

Compared with the prior art, the invention has the advantages that:

the tooth shaft steel has certain tensile strength and narrow hardenability fluctuation range, and the steel is subjected to controlled rolling and controlled cooling, so that a rolled material is shallow in decarburized layer, low in banded structure level, good in surface quality, uniform in structure and moderate in hardness, a hot rolled structure is ferrite and pearlite, the interval between pearlite lamellae is small, and the tooth shaft steel is beneficial to improving steel shaping and cold processing shaping. The rolling material can be directly used for processing gear shafts such as turning, milling, hobbing and the like, and heat treatment such as softening annealing, isothermal annealing and even spheroidizing annealing can be omitted before processing.

Drawings

FIG. 1 is a 100-fold microscopic view of a gear shaft steel according to example 1 of the present invention;

FIG. 2 is a 500-fold microscopic view of a gear shaft steel according to example 1 of the present invention;

FIG. 3 is a photograph showing the structure of a pearlite layer in a pinion according to embodiment 1 of the present invention.

Detailed Description

The present invention will be described in further detail with reference to the following examples, which are illustrative of preferred embodiments of the present invention and are not intended to limit the scope of the present invention.