阅读说明：本技术 具有超低铁素体含量的奥氏体不锈钢坯及其制造方法 (Austenitic stainless steel billet with ultra-low ferrite content and manufacturing method thereof ) 是由 欧阳鑫 胡昕明 王储 孙殿东 邢梦楠 颜秉宇 王勇 李广龙 贾春堂 胡海洋 于 2021-06-29 设计创作，主要内容包括：本发明提供了一种具有超低铁素体含量的奥氏体不锈钢坯及其制造方法,所述钢坯的成分按重量百分比计如下：C：0.04％～0.05％、Si：0.3％～0.5％、Mn：1.40％～1.60％、P≤0.015％、S≤0.010％、Cr：15.0％～16.0％、Ni：8.5％～9.5％、Mo：1.0％～2.0％、N：0.05％～0.07％,其余含量为Fe和不可避免的杂质。钢坯内δ-高温铁素体体积百分含量为2％～4％。制造方法主要包括电渣重熔、加热、锻造、均质化热处理；本发明通过结合电渣重熔、多向锻造开坯和高温长时均质化热处理,成功制造出200～360mm厚度以下δ-铁素体含量<4％的300系奥氏体不锈钢钢坯,用于轧制钢板内部δ-铁素体含量<1％的高端核电关键设备用奥氏体不锈钢中厚板。(The invention provides an austenitic stainless steel billet with ultra-low ferrite content and a manufacturing method thereof, wherein the components of the billet are as follows by weight percent: c: 0.04-0.05%, Si: 0.3% -0.5%, Mn: 1.40-1.60%, P is less than or equal to 0.015%, S is less than or equal to 0.010%, Cr: 15.0% -16.0%, Ni: 8.5% -9.5%, Mo: 1.0% -2.0%, N: 0.05 to 0.07 percent, and the balance of Fe and inevitable impurities. The volume percentage of delta-high temperature ferrite in the steel billet is 2 to 4 percent. The manufacturing method mainly comprises electroslag remelting, heating, forging and homogenizing heat treatment; according to the invention, by combining electroslag remelting, multidirectional forging cogging and high-temperature long-time homogenization heat treatment, a 300-series austenitic stainless steel billet with the delta-ferrite content of less than 4% and the thickness of 200-360 mm is successfully manufactured and is used for rolling an austenitic stainless steel medium plate for high-end nuclear power key equipment with the delta-ferrite content of less than 1% in the steel plate.)

1. An austenitic stainless steel blank with ultra-low ferrite content, characterized in that the composition of the blank in weight percentages is as follows: c: 0.04-0.05%, Si: 0.3% -0.5%, Mn: 1.40-1.60%, P is less than or equal to 0.015%, S is less than or equal to 0.010%, Cr: 15.0% -16.0%, Ni: 8.5% -9.5%, Mo: 1.0% -2.0%, N: 0.05 to 0.07 percent, and the balance of Fe and inevitable impurities.

2. The austenitic stainless steel blank having an ultra-low ferrite content of claim 1, wherein the volume percent ferrite in the blank is between 2% and 4%.

3. A method of manufacturing an austenitic stainless steel blank having ultra low ferrite content as claimed in claim 1 or 2, comprising electroslag remelting, heating, forging, homogenizing heat treatment; the method is characterized in that:

(1) electroslag remelting:

a) in the slagging stage, arc is automatically started, power supply parameters are gradually increased to 15000-16000A and 76V from 5000-6000A and 64V of current and voltage, and slagging time is 95-105 min; then, entering a current and voltage increasing stage, and continuously increasing the power supply parameters of current and voltage to 24000-25000A and 91V step by step;

b) a remelting stage: the target melting speed is 1000 kg/h-1100 kg/h, the remelting time is 10-15 hours, and the current and voltage are gradually reduced from 24000-25000A and 91V to 18000-19000A and 79V;

c) and (3) a cooling stage: the opening degree of a valve at the initial stage of the cooling water amount of the crystallizer is 60-70 percent; the water inlet temperature of the crystallizer is 25-30 ℃, and the water outlet temperature is 35-40 ℃; slag amount in a slagging stage: 750-800 Kg, and 190-210 mm thick slag; introducing argon after all slag is added, wherein the flow of the argon is 20-24 m³/h；

Electroslag remelting whole process time: 18-22 h, steel ingot thickness/time ratio: 20-25 mm/20min, and the weight/time ratio of steel ingot is as follows: 1.15-1.26 t/h; parameters of the heat capping stage: time: 190-210 min, the power supply parameter is current: 5000-9000A, voltage: 62-76V;

d) demolding: cooling the steel ingot in a crystallizer for 3-4 hours, then demoulding, and slowly cooling;

(2) heating:

the heating process comprises the following steps of heating at a low temperature section, a middle temperature section and a high temperature section: the specific process comprises the following steps:

the heating temperature of the low-temperature section is 400-450 ℃, and the heat preservation time is 1.5-2.5 h; heating at the medium temperature section at 850-900 ℃ and keeping the temperature for 11-12 h; the heating temperature of the high-temperature section is 1150-1200 ℃, and the heat preservation time is 14-15 h; rate of temperature rise in heating process: 35-45 ℃/h;

(3) forging:

the deformation rate of each forging pass is 3.5-4.5% of the thickness of the target steel billet, and the total deformation passes are 15-25;

(4) homogenizing and heat treating:

and (3) hot-conveying the forged steel billet to a heating furnace for high-temperature homogenization heat treatment, wherein the heat treatment heating temperature is as follows: 1210-1230 ℃, net heat preservation time: and (4) taking the product out of the furnace and cooling the product to room temperature at the air cooling rate of 40-60 ℃/min for 25-35 h.

4. The method of manufacturing an austenitic stainless steel blank having ultra-low ferrite content of claim 3, wherein: the volume percentage content of ferrite in the steel billet after electroslag remelting in the step (1) is below 13%.

5. The method of manufacturing an austenitic stainless steel blank having ultra-low ferrite content of claim 3, wherein: and (4) after forging in the step (3), the volume percentage of ferrite in the steel billet is 5-8%.

Technical Field

The invention belongs to the field of steel materials, and particularly relates to an austenitic stainless steel blank with ultra-low ferrite content and a manufacturing method thereof.

Background

For 300 series austenitic stainless steel, in order to improve the high temperature resistance and the stress corrosion resistance between crystals, the 300 series austenitic stainless steel is used for producing nuclear power key equipment, on the basis of adding a certain amount of Cr and Ni elements, a certain content of Mo element needs to be added into the steel, and compared with other austenitic stainless steels, high temperature delta-ferrite can be formed more easily, and for medium plates, the serious segregation of Cr and Mo elements can be caused due to the difference of cooling rates in the process of solidifying molten steel, so that the formation of the high temperature delta-ferrite is facilitated. In addition, the high-temperature delta-ferrite is easy to gather at the central loose position of the blank, and the central loose position is similar to a closed space, so that the deformation and the dissolution of the high-temperature ferrite are greatly limited. The manufacturing of nuclear power key materials requires that the content of high-temperature delta-ferrite of a steel plate is less than 1 percent, otherwise, the welding performance and the intergranular corrosion resistance of the steel plate are greatly influenced, and potential hazards are caused to engineering application, and because the capacity of dissolving the high-temperature delta-ferrite in the steel plate through solution heat treatment is limited, the content of the high-temperature delta-ferrite of an original billet for rolling the steel plate is required to be less than 4 percent, and at present, no smelting method for effectively controlling the content of the high-temperature delta-ferrite of the billet exists in China.

At present, the related domestic patent documents are few, and the main relevant documents related to the invention are as follows:

the patent document "a smelting process for reducing the ferrite content of nuclear-grade stainless steel castings" (application number: CN201310254833.6) discloses a method for improving the nickel equivalent, improving the strength of steel and enhancing the local corrosion resistance by using nitrogen as a violent austenite forming and stabilizing element; the key smelting process comprises the following steps: selecting furnace charge, rough smelting and fine smelting, detecting chemical components, detecting ferrite content and reducing the ferrite content, wherein the reduction of the ferrite content means that the chemical components meet the requirements, for the condition that the ferrite content exceeds the upper limit of the range specified by the standard or the procurement technical specification of owners, the ferrite content can be reduced by 0.7 to 1 percent when 0.01 to 0.015 percent of nitrogen element is added, and the nitrogen element added into stainless steel without nitrogen control can not exceed the specified upper limit within the allowable range of the content of residual elements. The method has the disadvantages that the specific ferrite content in the finished product is not clearly defined, and the ferrite content is controlled to be lower than the higher level of < 1%.

Patent document "heat treatment method for controlling ferrite content in copper-containing heat-resistant steel microstructure" (application No. CN201310703144.9) discloses a heat treatment method for controlling ferrite content in copper-containing heat-resistant steel microstructure, which mainly comprises the following steps: 1) heating Nb-containing microalloyed bainite 1.15Ni-0.65Cu-Mo-Nb steel parts for austenitizing; 2) pre-cooling the part obtained in the step 1) to 620-680 ℃, and then performing water cooling by adopting alternative quenching, wherein the water cooling time is as follows: 3s/100 mm-4 s/100mm, then air cooling is carried out, and the air cooling time is as follows: 1s/100 mm-2 s/100mm, then water cooling, wherein the water cooling time is as follows: 6s/100 mm-7 s/100mm, air cooling, and the air cooling time is as follows: 70s/100 mm-80 s/100mm to obtain a microstructure containing 40% -60% of ferrite structure on the bainite structure matrix; the invention has the disadvantages that the production cost is greatly improved from the production process perspective by adopting multi-section water cooling and air cooling heat treatment, and the ferrite content is higher; meanwhile, compared with an austenite single-phase structure, a bainite and ferrite double-phase structure has poorer intergranular corrosion resistance.

Disclosure of Invention

The invention aims to overcome the problems and the defects and provide an austenitic stainless steel blank with ultralow ferrite content and a manufacturing method thereof, wherein the austenitic stainless steel blank is used for rolling an austenitic stainless steel medium thickness section with the thickness of 10-45 mm and the delta-ferrite content of < 1% and the delta-ferrite content of < 4% and is 200-360 mm in a high-end nuclear power key equipment.

The purpose of the invention is realized as follows:

an austenitic stainless steel blank having an ultra-low ferrite content, said blank having the following composition in weight percent: c: 0.04-0.05%, Si: 0.3% -0.5%, Mn: 1.40-1.60%, P is less than or equal to 0.015%, S is less than or equal to 0.010%, Cr: 15.0% -16.0%, Ni: 8.5% -9.5%, Mo: 1.0% -2.0%, N: 0.05 to 0.07 percent, and the balance of Fe and inevitable impurities.

The ferrite in the steel billet is 2-4% by volume, and the ferrite is delta-high temperature ferrite.

The invention has the following design reasons:

c: carbon is an element that strongly forms and stabilizes austenite, and carbon easily precipitates as carbide with other alloy elements, so that an increase in the carbon content results in an increase in the strength of stainless steel, but a decrease in impact toughness. The ductile-brittle transition temperature rises, and in addition, the supersaturated carbon is precipitated in the form of carbide in the presence of carbon element, so that the adjacent area is poor in chromium, and the austenitic stainless steel has high intergranular corrosion sensitivity, therefore, the content of C in the steel is required to be controlled within the range of 0.04-0.05%.

Si: the addition of a proper amount of silicon element in the stainless steel can improve the oxidation resistance and the sulfuration resistance of the stainless steel, and endow the steel with excellent corrosion resistance in strong oxidizing media such as concentrated nitric acid, concentrated sulfuric acid and the like, which is related to the formation of a silicon-rich oxide protective film on the surface of the stainless steel by silicon. The adverse effect is that when the silicon content is less than l% and is normal in stainless steel, a higher silicon content reduces the corrosion resistance of the chromium-nickel austenitic stainless steel and significantly increases the susceptibility of the steel to solid solution intergranular corrosion. Therefore, the Si content of the alloy is controlled to be 0.3-0.5%.

Mn: in stainless steel, manganese remains in the steel as a deoxidizing element, and one of the important roles of manganese is embodied in nickel-saving stainless steel and high-nitrogen stainless steel, and the substitution of manganese for nickel saves nickel resources, and simultaneously increases the solubility of nitrogen and improves the strength. In austenitic stainless steels, manganese in a mass fraction of up to 2% has a negligible effect on hardness. Another important role of manganese is the formation of MnS, inhibiting the harmful effects of sulfur. Improve the high-temperature thermoplasticity of the high-chromium-nickel austenitic stainless steel. In terms of corrosion resistance, the increase of manganese deteriorates the corrosion resistance of stainless steel. Studies have found that as the manganese content increases from 0.25% to 1.75%. The pitting corrosion resistance of the material is reduced. An appropriate amount of manganese is beneficial, especially in combination with nitrogen, to save the rare precious metal nickel. In order to reduce the cost, however, if the amount of the additive is too large, the corrosion resistance and the ductility and toughness of the stainless steel are deteriorated. Therefore, the Mn content in the steel is required to be controlled to be 1.40-1.60%.

P: phosphorus is a harmful element in steel, increases cold brittleness of steel, deteriorates weldability, reduces plasticity, deteriorates cold bending properties, and P is also particularly sensitive to irradiation embrittlement. Therefore, the lower the P content in the steel, the better, the invention requires not more than 0.015%.

S: sulfur is a harmful element in general. S generally tends to form brittle sulfides with alloying elements in steel, to cause hot brittleness of steel, to reduce ductility and toughness of steel, and to accelerate radiation embrittlement. Therefore, the invention requires that the S content in the steel should be limited to less than 0.010%.

Cr: chromium is one of the most important elements in stainless steels, among austenitic stainless steels. The interaction of chromium and nickel forms a stable austenitic structure. In a single austenitic stainless steel, the chromium content does not have a significant effect on the mechanical properties. When a ferrite phase or a sigma phase occurs in the steel, the strength of the steel is increased and the ductility and toughness are decreased as the chromium content is increased. The content of austenite phase is gradually reduced and the content of ferrite phase is increased along with the increase of the Cr content from 24 percent to 26 percent; the yield strength and the tensile strength are increased continuously, the elongation is reduced and then increased, and the reduction of area is reduced continuously. The impact absorption power is firstly reduced and then increased, and the electrochemical corrosion resistance and the stress corrosion resistance are both enhanced. Therefore, the invention requires that the Cr content in the steel is 15.0-16.0%.

Ni: for austenitic stainless steels, the strength of the steel decreases and the plasticity increases as the nickel content increases, within the range of nickel contents where martensitic transformation is likely to occur. When the austenitic stainless steel has a stable austenitic structure, the addition of nickel can further improve the plasticity and toughness of the austenitic stainless steel, and the austenitic stainless steel has better stainless property and corrosion resistance; however, an increase in nickel content leads to an increase in the susceptibility to intergranular corrosion of austenitic stainless steels. Therefore, the invention requires that the Ni content in the steel is controlled to be 8.5-9.5%.

Mo: molybdenum is an important alloying element widely used in stainless steels. Studies have shown that in marine atmosphere, it is difficult to completely prevent the rusting of stainless steel with chromium alone, even with chromium contents as high as approximately 24%, and molybdenum must be added. However, the beneficial effect on corrosion resistance of stainless steel is premised on the fact that the steel must contain a sufficient amount of elemental chromium. Moreover, the chromium content in the steel increases. The beneficial effect of molybdenum in steel is also significantly increased. For austenitic stainless steel, molybdenum has a significant solid solution strengthening effect. Molybdenum also improves the corrosion resistance of stainless steel, but too high a content of molybdenum is detrimental to the stress corrosion resistance of austenitic stainless steel. The proper amount of molybdenum is beneficial to improving the resistance of stainless steel to stress corrosion cracking, so that the invention requires that the content of Mo in the steel is controlled as Mo: 1.0 to 2.0 percent.

N: nitrogen may partially replace nickel in austenitic stainless steels to conserve nickel. Meanwhile, the nitrogen can play a role in solid solution strengthening, the room temperature strength and the high temperature strength of the austenitic stainless steel can be obviously improved, and the nitrogen can also play a role in improving the corrosion resistance of the austenitic stainless steel. Nitrogen is combined with proper amount of chromium and molybdenum. The capability of resisting the pitting corrosion and the crevice corrosion of the austenitic stainless steel can be obviously improved, and the capability of resisting the pitting corrosion and the crevice corrosion of the austenitic stainless steel is also improved along with the increase of the nitrogen content. However, when the nitrogen content in the steel exceeds a certain amount, it will have some adverse effects on the properties of the stainless steel, and when the nitrogen content exceeds 0.12% to 0.15%, the cold and hot workability and cold formability of the steel will be reduced, so the present invention requires that the N content in the steel be controlled to be N: 0.05 to 0.07 percent of the total weight of the composition.

The second technical proposal of the invention is to provide a manufacturing method of austenitic stainless steel billet with ultra-low ferrite content, which mainly comprises electroslag remelting, heating, forging and homogenizing heat treatment;

(1) electroslag remelting:

a) in the slagging stage, arc is automatically started, power supply parameters are gradually increased to 15000-16000A and 76V from 5000-6000A and 64V of current and voltage, and slagging time is 95-105 min; then entering a current and voltage raising stage, and continuing to raise the power supply parameters to 24000-25000A and 91V step by taking the power supply parameters as current and voltage;

b) a remelting stage: the target melting speed is 1000 kg/h-1100 kg/h, the remelting time is 10-15 hours, and the power supply parameters of current and voltage are gradually reduced from 24000-25000A and 91V to 18000-19000A and 79V;

c) and (3) a cooling stage: the opening degree of a valve at the initial stage of the cooling water amount of the crystallizer is 60-70 percent; the water inlet temperature of the crystallizer is 25-30 ℃, and the water outlet temperature is 35-40 ℃; slag amount in a slagging stage: 750-800 Kg, and 190-210 mm thick slag; after all the slag is added, argon is introduced, and the flow of the argon is 20-24 m³/h；

Electroslag remelting whole process time: 18-22 h, steel ingot thickness/time ratio: 20-25 mm/20min, and the weight/time ratio of steel ingot is as follows: 1.15-1.26 t/h; parameters of the heat capping stage: time: 190-210 min, the power supply parameter is current: 5000-9000A, voltage: 62-76V;

d) and (3) demolding process: cooling the steel ingot in a crystallizer for 3-4 hours, then demoulding, sampling, and slowly cooling in a kiln;

the electroslag remelting technology has uniform and slow cooling speed, solves the problem of serious segregation of Cr and Mo elements caused by the difference of the cooling speed in the process of solidifying molten steel, and reduces the formation of delta-high-temperature ferrite; meanwhile, the central porosity grade of the blank is reduced, the deformation and dissolution of delta-high-temperature ferrite in the smelting process are promoted, and the ferrite content in the electroslag blank is successfully controlled to be below 13%, wherein the ferrite is delta-high-temperature ferrite.

(2) Heating:

heating the steel ingot before forging, wherein the heating process comprises the following steps of heating at a low temperature section, a middle temperature section and a high temperature section: the specific process comprises the following steps:

the heating temperature of the low-temperature section is 400-450 ℃, and the heat preservation time is 1.5-2.5 h; heating at the medium temperature section at 850-900 ℃ and keeping the temperature for 11-12 h; the heating temperature of the high-temperature section is 1150-1200 ℃, and the heat preservation time is 14-15 h; rate of temperature rise in heating process: 35-45 ℃/h;

(3) forging:

the deformation rate of each forging pass is 3.5% -4.5% of the thickness of a target steel billet, the directions of all forging passes are (Z-Z-Z- … Z-Y-Y-Y- … Y-X-X-X … X) in sequence, namely the length direction is forged first, then the width direction is forged, the thickness direction is forged last, after unidirectional forging is completed, the sample is rotated for 90 ℃, then the pressing deformation in the next direction is performed, the deformation in three directions is accumulated for 15-25 forging passes until the forging process is completed;

the multi-directional forging is the first important process for reducing ferrite. The cogging effect is to eliminate the central porosity of the blank through a certain degree of compression ratio, and break the core defects of the casting blank, so that the ferrite is easier to deform. In addition, the previously aggregated bulk ferrite is changed into strip ferrite through multi-direction single-pass high reduction in the cogging process, which is more beneficial to the dissolution of the ferrite. Because the blank is easy to form Cr in the ferrite during slow cooling₂₃C₆Precipitates which re-dissolve in the ferrite during subsequent heating, but the dissolved Cr elements remain inside the ferrite, which limits the ironAnd (4) diffusion of the matrix. For this reason, slow cooling of the billet should be avoided at the end of the cogging, homogenization phase. After the multidirectional forging and cogging are finished, directly and thermally conveying the steel billet to a chamber type heating furnace for carrying out homogenization heat treatment; successfully controlling the ferrite content in the steel billet to be 5-8%, wherein the ferrite is delta-high temperature ferrite.

(4) Homogenizing and heat treating:

and (3) hot-feeding the forged intermediate blank to a heating furnace for high-temperature homogenization heat treatment, wherein the heat treatment heating temperature is as follows: 1210-1230 ℃, net heat preservation time: and (4) taking the product out of the furnace and cooling the product to room temperature at the air cooling rate of 40-60 ℃/min for 25-35 h.

Rolling and solution heat treatment are auxiliary processes for reducing or eliminating ferrite and have limited capabilities. Therefore, the homogenization heat treatment is a key step for reducing or eliminating ferrite. The temperature 25-35 ℃ higher than the dissolving temperature of ferrite in the steel billet is used as the homogenization temperature, the heat preservation time is set to be 20-35 h according to the final thickness of the steel billet, the ferrite is fully dissolved, the phenomenon that the size of the blank crystal grains grows rapidly is avoided, and the size of the finished steel plate crystal grains is in a reasonable range. After the homogenization heat treatment, successfully controlling the ferrite content of the steel billet to be 2-4%, wherein the ferrite is delta-high temperature ferrite.

According to the invention, by combining electroslag remelting, multidirectional forging cogging and high-temperature long-time homogenization heat treatment, a 300-series austenitic stainless steel billet with the ferrite content of less than 4% and the thickness of 200-360 mm is successfully manufactured and is used for rolling the austenitic stainless steel medium plate for high-end nuclear power key equipment with the ferrite content of less than 1% and the thickness of 10-45 mm.

Drawings

FIG. 1 is a gold phase diagram of the microstructure of a forged billet after the homogenization heat treatment in example 2 of the present invention.

FIG. 2 is a gold phase diagram of the microstructure of a forged blank of example 2 of the present invention after non-homogenization heat treatment.

FIG. 3 is a metallographic picture of a steel ingot microstructure in example 2 of the present invention.

Detailed Description

The present invention is further illustrated by the following examples.

According to the component proportion of the technical scheme, the electroslag remelting, heating, forging and homogenizing heat treatment are carried out in the embodiment of the invention.

(1) Electroslag remelting:

a) in the slagging stage, arc is automatically started, power supply parameters are gradually increased from 5000-6000A and 64V to 15000-16000A and 76V, and slagging time is 95-105 min; then entering a current and voltage raising stage, and continuing to raise the power supply parameters to 24000-25000A and 91V step by taking the power supply parameters as current and voltage;

b) a remelting stage: the target melting speed is 1000 kg/h-1100 kg/h, the remelting time is 10-15 hours, and the power supply parameters of current and voltage are gradually reduced from 24000-25000A and 91V to 18000-19000A and 79V;

c) and (3) a cooling stage: the opening degree of a valve at the initial stage of the cooling water amount of the crystallizer is 60-70 percent; the water inlet temperature of the crystallizer is 25-30 ℃, and the water outlet temperature is 35-40 ℃; slag amount in a slagging stage: 750-800 Kg, and 190-210 mm thick slag; after all the slag is added, argon is introduced, and the flow of the argon is 20-24 m³/h；

Electroslag remelting whole process time: 18-22 h, steel ingot thickness/time ratio: 20-25 mm/20min, and the weight/time ratio of steel ingot is as follows: 1.15-1.26 t/h; parameters of the heat capping stage: time: 190-210 min, the power supply parameter is voltage: 62-76V, current: 5000-9000A;

d) demolding: cooling the steel ingot in a crystallizer for 3-4 hours, then demoulding, and slowly cooling;

the volume percentage content of ferrite in the steel billet after electroslag remelting is below 13 percent.

(2) Heating:

heating the steel ingot before forging, wherein the heating process comprises the following steps of heating at a low temperature section, a middle temperature section and a high temperature section: the specific process comprises the following steps:

the heating temperature of the low-temperature section is 400-450 ℃, and the heat preservation time is 1.5-2.5 h; heating at the medium temperature section at 850-900 ℃ and keeping the temperature for 11-12 h; the heating temperature of the high-temperature section is 1150-1200 ℃, and the heat preservation time is 14-15 h; rate of temperature rise in heating process: 35-45 ℃/h;

(3) forging:

the deformation rate of each forging pass is 3.5-4.5% of the thickness of the target steel billet, and the total deformation passes are 15-25;

after forging, the volume percentage of ferrite in the steel billet is 5 to 8 percent.

(4) Homogenizing and heat treating:

hot-feeding the forged steel billet to a heating furnace, and performing high-temperature homogenization heat treatment, wherein the heat treatment heating temperature is as follows: 1210-1230 ℃, net heat preservation time: and (4) taking the product out of the furnace and cooling the product to room temperature at the air cooling rate of 40-60 ℃/min for 25-35 h.

The ferrite is delta-high temperature ferrite.

The compositions of the steel slabs of the examples of the present invention are shown in Table 1. The main process parameters of electroslag remelting of the steel billet in the embodiment of the invention are shown in table 2. The main heating process parameters of the steel billet of the embodiment of the invention are shown in the table 3. The main forging and homogenizing heat treatment process parameters of the steel billets of the examples of the present invention are shown in table 4. The steel slabs of the examples of the invention have the steel slab structure shown in table 5.

TABLE 1 composition of steel billets of examples of the present invention (wt%)

Examples	C	Si	Mn	P	S	N	Ni	Cr	Mo
										1	0.042	0.33	1.44	0.010	0.001	0.054	8.7	15.2	1.5
2	0.045	0.41	1.59	0.009	0.002	0.061	8.8	15.5	1.6
										3	0.048	0.49	1.57	0.011	0.003	0.065	8.9	15.8	1.7
4	0.041	0.32	1.41	0.013	0.004	0.068	9.4	15.1	1.1
										5	0.049	0.38	1.48	0.012	0.001	0.057	9.1	15.4	1.9
6	0.047	0.48	1.42	0.008	0.002	0.053	9.2	15.9	1.4

TABLE 2 main process parameters of electroslag remelting of steel billets in the examples of the present invention

TABLE 3 main heating process parameters of steel billets according to the examples of the present invention

TABLE 4 Primary forging and homogenization Heat treatment Process parameters for Steel billets according to examples of the invention

TABLE 5 Steel billet texture of examples of the present invention

In order to express the present invention, the above embodiments are properly and fully described by way of examples, and the above embodiments are only used for illustrating the present invention and not for limiting the present invention, and those skilled in the relevant art can make various changes and modifications without departing from the spirit and scope of the present invention, and any modifications, equivalent substitutions, improvements, etc. made by the persons skilled in the relevant art should be included in the protection scope of the present invention, and the protection scope of the present invention should be defined by the claims.