P690QL2 steel plate with thickness of 90-100mm and ultrahigh low-temperature toughness and production method of P690QL2 steel plate

Abstract

The invention discloses a 90-100mm thick P690QL2 steel plate with ultrahigh low-temperature toughness, which adopts a low-carbon equivalent and high-manganese low-nickel component design, the high-manganese low-nickel component design can reduce the production cost, and a proper amount of strong nitride / oxide is introduced into the components to form an element Zr, so that the strength and low-temperature impact toughness of steel are improved; the characteristics that a water cooling die is high in cooling heat transfer efficiency, high in solidification speed and relatively light in internal segregation in the solidification process are fully utilized, a secondary finished product rolling technology is combined, large compression ratio rolling is adopted, and it is ensured that the internal quality of a steel plate reaches the forging level; and the uniformity of the mechanical property of the whole plate is improved through a stepped tempering process, the main structure of the obtained steel plate is a bainite tempered structure, non-metallic inclusions A + B + D are smaller than or equal to 1.5 grades, the yield strength is larger than or equal to 690 MPa, the tensile strength is 770-940 MPa, and the low-temperature impact value at the temperature of-60 DEG C is larger than or equal to 100 J.

Description

Technical Field

[0001] The invention belongs to the field of large thick plate production, and specifically relates to a 90-100 mm thick ultra-high low temperature toughness P690QL2 steel plate and a production method thereof. Background Art

[0002] The service characteristics of LNG ships and full-pressure storage tanks determine that this type of steel is not only required to have a yield strength of more than 690 MPa and good low-temperature toughness, but also because the storage tank is under more than 17.5 atmospheres of pressure for a long time and the unpredictable additional pressure caused by the sea wave plating, the performance requirements for the welded joints are very high. Therefore, this type of steel should have very good weldability. At the same time, under seawater conditions and wave environments, the steel should also have seawater corrosion resistance and fatigue resistance. The symmetrical structure of the liquid tank determines that the connecting thick plate between the double tanks should also maintain good resistance to lamellar tearing.

[0003] Chinese patent CN116043135B discloses an ultra-high toughness P690QL2 steel plate and a manufacturing method thereof, wherein the chemical composition by mass percentage is C: 0.06-0.12%, Si: 0.04-0.1%, Mn: 1.0-1.5%, Ni: 1.3-2.0%, Cr: 0.3-0.5%, Mo: 0.3-0.6%, V: 0.03-0.06%, Ti: 0.005-0.015, S: ≤0.002%, P: ≤0.005%, Als: 0.03-0.06%, and the remainder is Fe and unavoidable impurity elements. The performance of P690QL2 steel produced by the composition described in the document and 250-300mm cross-section continuous casting billet + two-stage rolling technology + tempering process meets the requirements, but the thickness of the produced steel plate is only 10-50mm, and the alloy cost of steel plates of the same steel grade and thickness is relatively high, which does not meet the conditions for large-scale promotion and application.

[0004] Chinese patent CN115747616B discloses a smelting method of P690QL2 marine tank steel, the chemical composition of which is C: 0.07% ~ 0.09%, Si: 0.15% ~ 0.25%, Mn: 1.0% ~ 1.3%, P ≤ 0.013%, S ≤ 0.002%, Nb: 0.010% ~ 0.020%, V: 0.040% ~ 0.050%, Ti: 0.01 0%~0.020%, Ni: 1.0%~1.30%, Cr: 0.30%~0.50%, Mo: 0.30%~0.60%, Cu≤0.20%, Al: 0.040%~0.070%, Ca: 0.00050%~0.0050%, N≤0.0050%, H≤0.0003%, B: 0.0020%~0.0030%, the remainder is Fe and unavoidable impurities. This patent is limited to a smelting method, emphasizing that the thickness of 6-180mm P690QL2 can be smelted by this method, and there is no subsequent material forming process for smelting. The maximum thickness of the steel plate that can be produced is not clearly stated in the application examples. In addition, this solution uses continuous casting billets to form materials, and the rolling compression ratio is limited, especially for the internal quality of extra-thick plates and the effect of grain refinement is limited.

[0005] In view of this, the present invention is proposed. Summary of the invention

[0006] In order to meet the above technical requirements, the purpose of the present invention is to provide a 90-100 mm thick ultra-high low-temperature toughness P690QL2 steel plate. In order to ensure high strength, high low-temperature impact toughness, weldability and other special requirements, studies have found that the reasonable ratio of Cu, Ni, Mo, and Zr elements has an important contribution to the toughness of high-strength steel. In particular, the appropriate amount of nickel and zirconium content can not only strongly improve the strength of thick steel plates, but also promote its toughness to remain at a high level; and micro-alloying elements such as Nb and V are crucial to toughness, strength and weldability, especially in the formation of various M (C, N) nano-precipitates during the heat treatment process.

[0007] Another object of the present invention is to provide a method for producing a 90-100 mm thick ultra-high low temperature toughness P690QL2 steel plate.

[0008] To achieve the above-mentioned purpose, the technical solution adopted by the present invention is: a 90-100 mm thick ultra-high low temperature toughness P690QL2 steel plate, comprising the following chemical components in mass percentage: C 0.07-0.09, Si: 0.04-0.08, Mn: 1.6-1.7, P≤0.008, S≤0.002, Als: 0.020-0.040, Nb: 0.025-0.035, V: 0.03-0.040, Ni: 0.8-1.0, Cu: 0.22-0.26, Mo: 0.40-0.50, B: 0.0008-0.0015, Zr: 0.025-0.035, and the rest are Fe and residual elements, and its carbon equivalent Ceq=0.5-0.6%.

[0009] The production method of the above-mentioned 90-100 mm thick ultra-high low temperature toughness P690QL2 steel plate includes clean steel smelting, water-cooled mold casting, blank heating and rolling, secondary heating and rolling, and heat treatment, which is specifically as follows: ① Clean steel smelting: S≤0.005%, P≤0.0050% of molten iron in converter, molten iron temperature≥1280℃, add nickel-containing scrap steel, the converter loading amount is controlled according to the remaining 6-8 tons of steel casting, the tapping temperature is ≥1600℃, 0.04%≤C≤0.07%, P≤0.007%; VD decarburization, molten steel reaches VD temperature≥1540℃, add carbon powder appropriately according to the C content of tapping steel, then cover and perform vacuum operation, under the vacuum degree of ≤67Pa, adjust the amount of argon to achieve vacuum decarburization, and keep Pressure time ≥10min, leaving station temperature ≥1535℃; LF refining, slag making is carried out according to the large slag process standard, the amount of lime added is 1000-1200 kg, the basicity is controlled at 4.0-6.0, and the white slag is required to be kept for 30-35 minutes during the refining process; VD refining, the pressure is maintained at a vacuum degree of ≤67Pa for ≥20min, the end point H content is ≤1.0PPm, zirconium iron alloy is added, rice husk is added after breaking the air, soft blowing is carried out for 5-8min, and the leaving station temperature is controlled at 1578-1583℃; ② Water-cooled mold casting: The thickness of the steel ingot is 960-990mm, and the pouring temperature is 1560-1565℃. The water-cooled mold used must ensure a taper of 30mm and an extrusion mechanism is set on the side to extrude the wide side of the ingot in the later stage of solidification to further form a dense structure. Insulation measures are taken at the ingot cap to form a shrinkage channel with a large top and a small bottom inside the ingot during the solidification process, avoiding the generation of shrinkage cavities, reducing the looseness and segregation inside the ingot, and promoting a dense internal structure and uniform composition. Cleaning should be completed within 24 hours after demoulding, and the cleaning temperature should be ≥200℃; ③ Billet heating and rolling: furnace temperature ≤750℃ when loading steel, stew steel for 2-4 hours, reduce the temperature difference between furnace temperature and ingot, reduce the stress difference caused by the temperature difference between inside and outside of ingot, insulation section temperature 1280-1300℃, insulation time 14-15 hours; rolling temperature 1050-1100℃, pass reduction 60-70mm, ensure deformation penetration to 1 / 2 of thickness, achieve loose and effective welding inside ingot, fully crush as-cast structure, promote uniform grain refinement of steel plate, release the intermediate billet when it is rolled to 350mm thick, and end rolling temperature 950-980℃; ④ Secondary heating and rolling: After the intermediate billet is cut into four pieces, it is placed in a soaking pit. The temperature of the holding section is 1220-1240℃ and the holding time is 6-7 hours. There is no need to widen it again during the secondary rolling. It can be directly extended to the finished product thickness. The reduction before airing the steel is ≥30±5mm, the start rolling temperature is ≥1020℃, and the final rolling temperature is ≤950℃. High-pressure water is used for cooling during the rolling process. Rolling can only be continued after the surface of the steel plate has fully returned to red. The total rolling time is controlled at 4-5 minutes. After rolling, ACC water cooling is turned on with a water temperature of 900-950℃ and a return to red temperature of 800-830℃. It should be noted that in order to effectively play the role of Nb, Mo, Ni, and Zr alloy elements, a sufficiently high heating temperature is required to fully dissolve them. However, too high a heating temperature will cause the austenite grains to grow, resulting in the coarsening of the ferrite grains. The coarse ferrite structure will lead to a decrease in its overall performance. Therefore, combined with the ingot shape characteristics of the water-cooled ingot mold device to produce steel ingots, the optimal heating system is formulated, using slow heating, multi-stage insulation, and turning the steel before tapping to ensure that the steel ingots are evenly burned and the surface oxide scale is easy to remove.

[0010] The secondary material forming process is adopted in the rolling process, including the "ingot to billet" (opening billet) production process and the "bill to material" production process. The first heating adopts a higher heating temperature, and a large rolling force is used in the rolling process to achieve a pass reduction of 60-70mm, which promotes effective welding of internal looseness, sufficient crushing of cast structure, and uniform grain refinement of steel plates; the heating temperature is properly controlled in the second heating, and the hard shell method is used in the rolling process to ensure that the deformation penetrates to 1 / 2 of the thickness, which promotes further grain refinement. After rolling, a rapid cooling process is used to reduce the temperature difference between the center area and the surface of the steel plate thickness, inhibit the growth and recrystallization of austenite grains, and promote grain refinement and grain uniformity.

[0011] ⑤ Heat treatment: adopt high temperature quenching + critical quenching + tempering heat treatment process, among which, the high temperature quenching heating temperature is 920~940℃, the heating time is 2.0~2.5min / mm, and the hot steel plate enters the roller quenching machine within 10 seconds after leaving the furnace and is cooled to room temperature, the water temperature is 30~40℃; the critical quenching heating temperature is 880~890℃, the heating time is 1.8~2.2min / mm, and the hot steel plate enters the roller quenching machine within 10 seconds after leaving the furnace and is cooled to room temperature, the water temperature is 15~18℃; the quenched steel plate is loaded into the roller bottom tempering furnace for tempering at 620±20℃, the furnace temperature is raised to 650℃ 30 minutes before leaving the furnace, and the furnace is air-cooled to 400℃ for warm straightening.

[0012] It should be noted that in order to ensure good welding performance, under the condition of low carbon equivalent, the heat treatment process adopts two cycles of quenching + tempering process to reduce the difference in mechanical properties of different thickness layers of steel plates. The change law of mechanical properties is closely related to the microstructural changes, carbide distribution, alloy element segregation and dissolution, and quenched matrix structure during the quenching and tempering process. Quenching is the basis of the quenching and tempering process. The quenching temperature affects the size of the original austenite grains in the steel and the dissolution and distribution state of the alloy elements, thereby changing the hardenability of the steel and the size of the lath bundle after the phase transformation, thereby affecting the mechanical properties of the steel. During the second or even multiple cycles of quenching and heating, the carbides in the steel dissolve, causing the steel plate to dissolve more alloy elements, and these alloy elements are often also hardenability elements, causing the C curve to shift to the right. The distribution of alloy elements in the steel in the original structure phases is uneven, and this uneven phenomenon remains in the austenite until the carbides are dissolved. With the first high-temperature heating and quenching and subsequent multiple heating and quenching, the carbides in the steel are redistributed, the chemical composition is more uniform, the austenite tends to be stable, the nucleation rate of the diffusion phase transformation is lower during cooling, the transformation incubation period is extended, the C curve shifts to the right, and the critical cooling is reduced. After quenching, the differential temperature tempering process is used to eliminate the residual stress in the quenched steel, improve the impact toughness and the uniformity of the strength of the whole plate.

[0013] The beneficial effects of the present invention are as follows: a low carbon equivalent and high manganese and low nickel composition design is adopted, wherein the high manganese and low nickel design can reduce production costs, an appropriate amount of hardenability elements are introduced, and the characteristics of high heat transfer efficiency, fast solidification speed, and relatively light internal segregation during the solidification process of the water-cooled mold are fully utilized, and a secondary material rolling process is combined with a large compression ratio rolling to ensure that the internal quality of the steel plate reaches the level of forgings. In the heat treatment link, the organization is refined through multiple alternating quenching, and the uniformity of the mechanical properties of the entire plate is improved through a step tempering process.

[0014] An appropriate amount of strong nitride / oxide forming element Zr is introduced into the composition design to form a beneficial fine and dispersed inoculant through oxide metallurgy, which serves as the phase transformation core, thereby improving the strength and low-temperature impact toughness of the steel.

[0015] The steel plate obtained in this application was tested for performance, and its main structure was bainite tempered structure, non-metallic inclusions A+B+D≤1.5 level, yield strength ≥690MPa, tensile strength 770~940MPa, -60℃ low temperature impact value ≥100J. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The present invention will be further described in detail below in conjunction with the accompanying drawings.

[0017] Figure 1 Schematic diagram of the metallographic structure of the steel plate cross section obtained in the example of the present invention (metallographic microscope, ×100).

[0018] Figure 2 Schematic diagram of the metallographic structure of the steel plate cross section obtained in the example of the present invention (metallographic microscope, ×200).

[0019] Figure 3 Schematic diagram of the metallographic structure of the steel plate cross section obtained in the example of the present invention (metallographic microscope, ×500). DETAILED DESCRIPTION

[0020] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the technical scheme in the embodiments of the present invention will be described clearly and completely below. If the specific conditions are not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be purchased commercially.

[0021] The features and performance of the present invention are further described in detail below in conjunction with the embodiments.

[0022] Several 90-100 mm thick ultra-high cryogenic toughness P690QL2 steel plates are produced using the following chemical compositions (wt%) and carbon equivalents: The production process of the steel includes: KR hot metal pretreatment, converter smelting, VD decarburization, LF refining, VD refining, water-cooled mold casting, ingot cleaning and coating, billet heating and rolling, secondary heating and rolling, and heat treatment, as follows: KR molten iron pretreatment: The slag must be scraped clean before the molten iron arrives at the station to ensure that the thickness of the slag layer on the liquid surface is ≤25mm. The sulfur content of the molten iron after KR treatment is controlled below 0.005%, and the desulfurization temperature drop is ≤25℃.

[0023] Converter smelting: S≤0.005%, P≤0.0050% of molten iron entering the furnace, molten iron temperature≥1280℃, scrap steel added to the converter must be dry high-quality scraps and special scrap steel containing nickel, the converter loading amount is controlled according to 6-8 tons of steel casting surplus, tapping temperature≥1600℃, 0.04%≤C≤0.07%, P≤0.007%, slag cone is used to stop slag at the end of tapping. If the slag stopping fails, the furnace must be lifted in advance to ensure that the slag thickness of the converter is controlled below 20mm to avoid the slag returning to P.

[0024] VD decarburization: Ensure that the temperature of molten steel to VD is ≥1540℃. After the molten steel is placed in the VD furnace, add carbon powder appropriately according to the C content of the steel, and then cover it for vacuum operation. During the vacuum stage of the VD furnace, increase the amount of argon gas to fully achieve the purpose of vacuum decarburization. The pressure holding time is ≥10min, and the temperature out of the station is ≥1535℃. LF refining: slag making is carried out according to the large slag volume process, with 1000-1200 kg of lime added and the alkalinity controlled at 4.0-6.0. After heating for 3 minutes, add 50 kg of calcium carbide first and then add 20-40 kg of aluminum particles. Thereafter, add 2-4 shovels of aluminum particles to the ladle with a shovel every 2 minutes to ensure that the slag turns white; in the second heating, add 10-30 kg of calcium carbide each time according to the submerged arc effect, and at the same time, add 1-3 shovels of aluminum particles to the ladle with a shovel every 2 minutes to ensure that the white slag is maintained during the entire second heating process; in the third heating, add aluminum particles according to the color of the slag to maintain the white slag; the white slag is required to be maintained for 30-35 minutes during the refining process.

[0025] VD refining: maintain the pressure for ≥20min at a vacuum degree of ≤67Pa. During the pressure maintaining process, ensure good tumbling effect of molten steel. The final H content requirement is ≤1.2PPm. Add zirconium-iron alloy, add rice husks for soft blowing for 5-8min after breaking the air, and then prepare the bag to leave the station. The temperature leaving the station is controlled at 1578-1583℃.

[0026] Water-cooled mold casting: The upper line temperature of the insulation cap is greater than 150℃, and a fixed-width and adjustable-thickness water-cooled ingot mold is used. The thickness of the ingot body is 960-990mm. The water-cooled mold used must ensure the taper (30mm) and an extrusion mechanism is set on the side to extrude the wide side of the ingot in the later stage of solidification; the pouring temperature is controlled at 1560-1565℃, and the temperature of the molten steel after arriving at the station meets the standard upper limit to the upper limit + 3℃ range. After the temperature is measured and confirmed to meet the pouring requirements, pouring is started; if the temperature of the molten steel at the station does not meet the above standards, soft argon blowing is carried out immediately, and after the argon blowing is completed, calm down for 5 minutes before pouring; after the pouring is completed, the insulation effect of the cap mouth must be ensured.

[0027] Ingot cleaning and coating: The ingot should be cleaned within 24 hours after demoulding, and the cleaning temperature should be ≥200℃. After cleaning, the ingot should be stacked in a slow cooling pit and slowly cooled to 50-100℃ before spraying anti-oxidation coating.

[0028] Ingot heating and rolling: furnace temperature ≤750℃ when loading steel, simmer steel for 2-4 hours to reduce the temperature difference between furnace temperature and ingot, and reduce stress difference caused by temperature difference inside and outside of ingot, insulation temperature 1280-1300℃, total insulation time 14-15 hours; rolling temperature 1050-1100℃, reduction per pass 60-70mm, ensure deformation penetration to 1 / 2 of thickness, release when rolling to 350mm, rolling end temperature 950-980℃.

[0029] Secondary heating and rolling: After the intermediate billet is cut into four pieces, it is placed in a soaking pit with a holding temperature of 1220-1240°C and a holding time of 6-7 hours. During the secondary rolling, it is directly extended to the finished product thickness. The reduction before airing is ≥30±5mm, the start rolling temperature is ≥1020°C, and the final rolling temperature is ≤950°C. High-pressure water is used for cooling during the rolling process. Rolling can only be started after the surface of the steel plate has fully returned to red. The total rolling time is controlled within 4-5 minutes. After rolling, ACC is turned on for two times of water cooling to quickly reduce the surface and core temperature of the steel plate to the non-recrystallized zone. The water entry temperature is 900-950°C and the return to red temperature is 800-830°C.

[0030] Heat treatment: adopt high temperature quenching + critical quenching + tempering combined process; adopt roller bottom quenching furnace, high temperature quenching heating temperature 920 ~ 940 ℃, heating time 2.4min / mm, steel plate enters quenching machine within 10 seconds after leaving the furnace and is water-cooled to room temperature, total water volume of quenching machine 7000 ~ 8000m3 / h, steel plate temperature when entering water 900 ~ 910 ℃, water temperature 30 ~ 40 ℃; critical quenching heating temperature 880 ~ 890 ℃, heating time 2min / mm, The steel plate enters the quenching machine within 10 seconds after leaving the furnace to cool to room temperature. The total water volume of the quenching machine is 9000-10000m3 / h. The temperature of the steel plate when entering the water is 860-870℃, and the water temperature is 15-18℃. After quenching, it is loaded into the roller hearth tempering furnace for tempering at 620±20℃. The furnace temperature is raised to 650℃ 30 minutes before leaving the furnace, and air-cooled to 400℃ for warm straightening after leaving the furnace.

[0031] The steel plates obtained by the above scheme were sampled and prepared according to GB / T 2975, the low temperature impact toughness was tested according to GB / T 229, and the tensile performance test was carried out according to GB / T 228. The mechanical properties and metallographic structure of the steel plates in the delivery state were comprehensively inspected. The specific performance indicators are shown in the following table:

[0032] From the above data, we can see that the steel plate has good low-temperature impact toughness, and the difference in mechanical properties between the head and tail of the steel plate is small, and the lamellar tearing resistance is good. All performance and indicators meet the requirements of ultra-high low-temperature toughness P690QL2 steel.

[0033] The metallographic structure of the 100 mm thick P690QL2 steel plate obtained in Example 3 is shown in the attached figure. Figure 1-3 shown.

[0034] Metallographic test results: (1) Inclusions A+B+D≤1.5 level; (2) Structure: bainite tempered structure; External inspection and flaw detection: The developed steel plate has a 100% genuine product rate in external inspection, and the final steel plate flaw detection meets the Level I flaw detection requirements of GB / T 2970 "Super-thick steel plate inspection method".

[0035] The above description is only a preferred embodiment of the present invention. The above specific embodiments are not limitations of the present invention. Any modifications, changes or equivalent substitutions made by ordinary technicians in the field based on the above description shall fall within the protection scope of the present invention.

Claims

1. A 90-100 mm thick ultra-high low temperature toughness P690QL2 steel plate, characterized in that: The steel plate comprises the following chemical components in mass percentage: C 0.07-0.09, Si: 0.04-0.08, Mn: 1.6-1.7, P≤0.008, S≤0.002, Als: 0.020-0.040, Nb: 0.025-0.035, V: 0.03-0.040, Ni: 0.8-1.0, Cu: 0.22-0.26, Mo: 0.40-0.50, B: 0.0008-0.0015, Zr: 0.025-0.035, and the rest are Fe and residual elements, and its carbon equivalent Ceq=0.5-0.6%; The steel plate is mainly a bainite tempered structure, non-metallic inclusions A+B+D≤1.5 level, yield strength ≥690MPa, tensile strength 770-940MPa, -60°C low temperature impact value ≥100J.

2. The method for producing a 90-100 mm thick ultra-high low temperature toughness P690QL2 steel plate according to claim 1, wherein the production of the steel comprises clean steel smelting, water-cooled mold casting, blank heating and rolling, secondary heating and rolling, and heat treatment, characterized in that The steps include: ① Clean steel smelting: S≤0.005% and P≤0.0050% of molten iron in converter, molten iron temperature≥1280℃, add nickel-containing scrap steel, the converter loading amount is controlled according to 6-8 tons of cast steel, tapping temperature≥1600℃, 0.04%≤C≤0.07%, P≤0.007%; VD decarburization, molten steel to VD temperature≥1540℃, under vacuum degree≤67Pa, pressure holding time≥10min, leaving station temperature≥1535 ℃; LF refining, slag making is carried out according to the large slag process standard, the amount of lime added is 1000-1200 kg, the basicity is controlled at 4.0-6.0, and the white slag is required to be kept for 30-35 minutes during the refining process; VD refining, the pressure is maintained at a vacuum degree of ≤67Pa for ≥20min, the end point H content is ≤1.0PPm, zirconium iron alloy is added, rice husk is added after breaking the air, soft blowing is performed for 5-8min, and the temperature leaving the station is controlled at 1578-1583℃; ② Water-cooled mold casting: Ingot thickness 960-990mm, pouring temperature 1560-1565℃, cleaning within 24 hours after demoulding, cleaning temperature ≥200℃; ③ Billet heating and rolling: furnace temperature ≤750℃ when loading steel, steel stewing for 2-4 hours, holding section temperature 1280-1300℃, holding time 14-15 hours; rolling start temperature 1050-1100℃, pass reduction 60-70mm, intermediate billet released when rolled to 350mm thick, rolling end temperature 950-980℃; ④ Secondary heating and rolling: After the intermediate billet is cut into four pieces, it is placed in a soaking pit. The temperature of the holding section is 1220-1240℃ and the holding time is 6-7 hours. There is no need to widen it again during the secondary rolling. It can be directly extended to the finished product thickness. The reduction before airing the steel is ≥30±5mm, the start rolling temperature is ≥1020℃, and the final rolling temperature is ≤950℃. High-pressure water is used for cooling during the rolling process. Rolling can only be continued after the surface of the steel plate has fully returned to red. The total rolling time is controlled at 4-5 minutes. After rolling, ACC water cooling is turned on with a water temperature of 900-950℃ and a return to red temperature of 800-830℃. ⑤ Heat treatment: adopt high temperature quenching + critical quenching + tempering heat treatment process, among which, the high temperature quenching heating temperature is 920~940℃, the heating time is 2.0~2.5min / mm, and the hot steel plate enters the roller quenching machine within 10 seconds after leaving the furnace and is cooled to room temperature, the water temperature is 30~40℃; the critical quenching heating temperature is 880~890℃, the heating time is 1.8~2.2min / mm, and the hot steel plate enters the roller quenching machine within 10 seconds after leaving the furnace and is cooled to room temperature, the water temperature is 15~18℃; the quenched steel plate is loaded into the roller bottom tempering furnace for tempering at 620±20℃, the furnace temperature is raised to 650℃ 30 minutes before leaving the furnace, and the furnace is air-cooled to 400℃ for warm straightening.

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