780MPa grade ultra-high hole expansion performance cold-rolled complex phase steel plate for automobiles and preparation method thereof

Abstract

The present invention relates to the technical field of automobile steel manufacturing, and in particular to a 780MPa grade ultra-high hole expansion performance cold-rolled complex phase steel plate for automobiles and a preparation method thereof. The steel plate is composed of the following chemical components in weight percentage: C: 0.08% to 0.13%; Si: 0.1% to 0.5%; Mn: 1.8% to 2.5%; Cr: 0.25% to 0.6%; Mo: 0.1% to 0.5%; Ti: 0.01% to 0.04%; Nb: 0.01% to 0.04%; B: 0 to 0.0004%; P≤0.02%; S≤0.01%, and the balance is Fe and unavoidable impurities. The final structure is a ferrite + bainite + martensite complex phase structure, and the microstructure is calculated by volume fraction as follows: critical zone ferrite 10% to 20%, oriented epitaxial ferrite ≤10%, bainite 25% to 35%, martensite 35% to 52%. The yield strength of the present invention is ≥600MPa, the tensile strength is ≥780MPa, the yield ratio is relatively high, and the hole expansion rate is ≥70%, which can meet the requirements of automobile manufacturers for high flanging performance.

Description

Technical Field

[0001] The present invention relates to the technical field of automobile steel manufacturing, and in particular to a 780MPa grade ultra-high hole expansion performance cold-rolled complex phase steel plate for automobiles and a preparation method thereof. Background Art

[0002] In October 2020, the China Society of Automotive Engineering released the "Technical Route for Energy-Saving and New Energy Vehicles" Figure 2 .0”, the roadmap clearly states that in the near future, the lightweighting of automobiles will be focused on improving the application of high-strength steel. By 2035, the lightweight coefficient of fuel passenger vehicles will be reduced by 25%. At present, dual-phase steel and phase-change induced plasticity steel can achieve a combination of high plasticity and high strength, but in the application process, due to high strength, they are highly sensitive to edge fracture and have poor expansion of hole expansion, which hinders the design of parts and stamping process.

[0003] As one of the advanced high-strength steels, complex phase steel has a high yield strength ratio, high energy absorption, and good flanging performance. It is particularly suitable for the production of automobile chassis suspension parts, B-pillars, bumpers, etc. The matrix structure of complex phase steel is fine ferrite, bainite, and martensite. Through the phase transformation strengthening of bainite and martensite and the precipitation strengthening of microalloys, the strength of complex phase steel can reach 780MPa. The high-density dislocation of the bainite matrix and the small density difference between the phases make complex phase steel have good hole expansion performance.

[0004] However, the current hole expansion rate of 780MPa grade cold-rolled complex phase steel is about 50%, which is not conducive to the subsequent forming of steel plates and cannot meet the requirements of automotive structural parts for high strength, high plasticity and high flanging. Therefore, improving the hole expansion performance of complex phase steel has become a key technical issue to be solved at this stage.

[0005] CN 111041345 A discloses "an 800MPa grade vanadium-containing low-carbon bainite complex phase steel and its production method", whose main chemical components are: C: 0.10% to 0.13%; Si: 0.20% to 0.30%; Mn: 1.80% to 1.90%; V: 0.095% to 0.12%; N: 0.0110% to 0.0130%, and the rest is Fe and unavoidable impurities. The complex phase steel with a strength of 780MPa is produced by controlled rolling and controlled cooling, but the invention is a hot-rolled plate with poor surface quality, and does not mention the hole expansion performance.

[0006] CN 111394650 A discloses "a high R value 800MPa grade cold rolled steel with excellent formability", whose chemical composition is: C: 0.05% to 0.08%; Si: 0.85% to 1.25%; Mn: 2.0% to 2.5%; Nb: 0.015% to 0.025%; S≤0.004%; P≤0.006%; N≤0.005%, and the rest is Fe and unavoidable impurities. 800MPa cold rolled steel is produced by hot rolling-cold rolling-pickling-continuous annealing-aging treatment, and the R value can reach more than 0.9, but the hole expansion performance is not mentioned.

[0007] CN 108913991 A discloses "a 980MPa grade cold-rolled complex phase steel with good hole expansion performance and a preparation method", whose main chemical components are: C: 0.15% to 0.20%; Si: 0.30% to 0.80%; Mn: 1.7% to 2.3%; P≤0.015%; S≤0.008%, Als≤0.08%; Nb: 0.02% to 0.08%; Cr: 0.4% to 0.8%; Ti: 0.02% to 0.06%, the remainder is Fe and unavoidable impurities. The 980MPa grade complex phase steel is produced by hot rolling, cold rolling and continuous annealing process, and the hole expansion rate is above 15%. Although the patent mentions the hole expansion rate, the hole expansion rate is not high and cannot meet the requirements for forming special parts. Summary of the invention

[0008] In order to overcome the shortcomings of the prior art, the present invention provides a 780MPa-grade cold-rolled complex phase steel plate with ultra-high hole expansion performance for automobiles and a preparation method thereof. The steel plate has a tensile strength of more than 780MPa, a yield strength of more than 600MPa, an elongation of more than 11%, and a hole expansion rate of 70%, which meets the requirements of automobile structural parts for high strength, high plasticity, and high flanging performance.

[0009] In order to achieve the above object, the present invention adopts the following technical solutions:

[0010] A 780MPa grade ultra-high hole expansion performance cold-rolled complex phase steel for automobiles is composed of the following chemical components in weight percentage: C: 0.08%-0.13%; Si: 0.1%-0.5%; Mn: 1.8%-2.5%; Cr: 0.25%-0.6%; Mo: 0.1%-0.5%; Ti: 0.01%-0.04%; Nb: 0.01%-0.04%; B: 0-0.0004%; P≤0.02%; S≤0.01%, and the balance is Fe and unavoidable impurities.

[0011] The microstructure of the complex phase steel includes intercritical ferrite, epitaxial ferrite, bainite and martensite; the microstructure is as follows by volume fraction: intercritical ferrite 10% to 20%, epitaxial ferrite ≤10%, bainite 25% to 35%, and martensite 35% to 52%.

[0012] A 780MPa-grade ultra-high hole expansion performance cold-rolled complex phase steel plate for automobiles has a tensile strength of more than 780MPa, a yield strength of more than 600MPa, an elongation of more than 11%, and a hole expansion rate of 70%.

[0013] A method for preparing a 780MPa grade ultra-high hole expansion performance cold-rolled complex phase steel plate for automobiles, the preparation process includes converter smelting, slab continuous casting, hot rolling, pickling cold rolling, and continuous annealing, specifically comprising the following steps:

[0014] 1) Converter smelting

[0015] Through the converter, smelting is performed to obtain molten steel that meets the following composition requirements by weight percentage: C: 0.08% ~ 0.13%; Si: 0.1 ~ 0.5%; Mn: 1.8% ~ 2.5%; Cr: 0.25% ~ 0.6%; Mo: 0.1% ~ 0.5%; Ti: 0.01% ~ 0.04%; Nb: 0.01% ~ 0.04%; B: 0.0001 ~ 0.0004%; P ≤ 0.02%; S ≤ 0.01%, and the balance is Fe and unavoidable impurities. The temperature of the molten steel is 1510℃ ~ 1620℃.

[0016] 2) Slab continuous casting

[0017] The casting temperature is 1420℃~1540℃, and the thickness of the continuous casting billet is 120mm~180mm.

[0018] 3) Hot rolling

[0019] The ingot is heated to 1200℃~1280℃, isothermal for 100~130min, the starting rolling temperature is 1160℃~1250℃, the final rolling temperature is above 880℃, the coiling temperature is 600℃~700℃, and the hot rolled plate thickness is 2mm~4mm.

[0020] 4) Pickling and cold rolling

[0021] Before cold rolling, acid solution is used to remove the iron oxide scale on the surface of the steel plate, and the cold rolling reduction rate is 50% to 60%.

[0022] 5) Continuous annealing

[0023] Heating: heating to 780℃~840℃ at a rate of 5℃ / s~10℃ / s; slow cooling: slow cooling to 670℃~750℃ at a cooling rate of 0.5℃ / s~3℃ / s; over-aging: cooling to 330℃~380℃ at a cooling rate of 30~40℃ / s for over-aging treatment, keeping warm for 300s~600s.

[0024] 6) Finishing

[0025] The steel plate enters the skin-pass mill for plate shape adjustment, and the skin-pass elongation is controlled at 0.1% to 0.4%.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] 1) The present invention adopts the idea of ​​two-phase heating combined with high slow cooling to effectively control the ferrite content in the critical zone. By forming epitaxial ferrite to replace part of the ferrite in the critical zone, the hardness difference between the soft and hard phases in the steel is reduced, the coordinated deformation ability is improved, and then the hole expansion performance of the steel is improved.

[0028] 2) The chemical composition of the steel of the present invention is mainly C, Mn, Si, and trace amounts of Cr, MoTi, Nb, and B are added to ensure that the steel has a lower carbon equivalent (Ceq=C+Mn / 20+Si / 30+2p+4s<0.5) and is easy to weld.

[0029] 3) The present invention adopts a production process of converter smelting - slab continuous casting - hot rolling - pickling and cold rolling - continuous annealing. The production process is simple and can meet the needs of most domestic pickling-continuous annealing production lines. The production cost is low and the process is stable.

[0030] 4) The final structure is a ferrite + bainite + martensite complex structure, and the microstructure is calculated by volume fraction as follows: intercritical ferrite 10% to 20%, oriented epitaxial ferrite ≤ 10%, bainite 25% to 35%, martensite 35% to 52%. Yield strength ≥ 600MPa, tensile strength ≥ 780MPa, with a high yield ratio, hole expansion rate ≥ 70%, which can meet the high flanging performance requirements of automobile manufacturers. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 The microstructure of Example 2 of the present invention;

[0032] Figure 2 This is the engineering stress-strain curve of Example 2 of the present invention. DETAILED DESCRIPTION

[0033] The present invention discloses a 780MPa grade ultra-high hole expansion performance cold-rolled complex phase steel plate for automobiles and a preparation method thereof. Those skilled in the art can refer to the content of this article and appropriately improve the process parameters to achieve it. It is particularly important to point out that all similar substitutions and modifications are obvious to those skilled in the art, and they are all deemed to be included in the present invention. The method and application of the present invention have been described through preferred embodiments, and relevant personnel can obviously modify or appropriately change and combine the methods and applications described herein without departing from the content, spirit and scope of the present invention to realize and apply the technology of the present invention.

[0034] A 780MPa grade ultra-high hole expansion performance cold-rolled complex phase steel for automobiles is composed of the following chemical components in weight percentage: C: 0.08%-0.13%; Si: 0.1%-0.5%; Mn: 1.8%-2.5%; Cr: 0.25%-0.6%; Mo: 0.1%-0.5%; Ti: 0.01%-0.04%; Nb: 0.01%-0.04%; B: 0-0.0004%; P≤0.02%; S≤0.01%, and the balance is Fe and unavoidable impurities.

[0035] The microstructure of the complex phase steel includes intercritical ferrite, epitaxial ferrite, bainite and martensite; the microstructure is as follows by volume fraction: intercritical ferrite 10% to 20%, epitaxial ferrite ≤10%, bainite 25% to 35%, and martensite 35% to 52%.

[0036] The steel plate made of the above-mentioned complex phase steel has a tensile strength of more than 780 MPa, a yield strength of more than 600 MPa, an elongation of more than 11%, and a hole expansion rate of 70%.

[0037] The reasons for the alloy design of the present invention are as follows:

[0038] C: is one of the important alloying elements of the present invention. C is an important solid solution strengthening element and an element for improving the stability of austenite. During the cooling process, a certain martensite is obtained to ensure strength. When the carbon content is too high, more C is solid-dissolved in the ferrite, and a yield platform will appear on the tensile curve. Excessive carbide precipitation will reduce the elongation of the steel. Therefore, the C content needs to be controlled at 0.08% to 0.13%.

[0039] Mn: is one of the important alloying elements of the present invention. Mn itself acts as a replacement atom to strengthen the solid solution. In this study, Mn ensures the stability of austenite in the critical zone and the stability of supercooled austenite during cooling, and inhibits the formation of pearlite. The most important thing is that a sufficient amount of Mn ensures the hardenability of the steel plate, and a certain amount of martensite is generated after rapid cooling to ensure the strength of the steel plate. However, if the Mn content is too high, MnS will be generated, which is easy to cause segregation, so the Mn content needs to be controlled at 1.8% to 2.5%.

[0040] Si: is one of the important alloying elements in the present invention. Si is a solid solution strengthening element of ferrite, which expands the (α+γ) zone and widens the processing range of the critical zone; at the same time, Si inhibits the formation of cementite in steel, increases the activity of C in austenite, makes the untransformed austenite further carbon-rich, and improves its hardenability, but too high Si content will affect the surface quality of the steel plate, so the Si content needs to be controlled at 0.1-0.5%.

[0041] Cr: It is a carbide-forming element that can delay the transformation of pearlite. When its content is ≥0.25%, it can hinder the formation of ferrite and pearlite, promote the formation of bainite, refine the structure and play a strengthening effect, and avoid the formation of too much martensite to reduce elongation. Therefore, the Cr content needs to be controlled at 0.25% to 0.6%.

[0042] Ti: It is a strong carbide-forming element. Adding a certain amount of Ti helps to fix N in the steel. The generated TiN can pin the austenite grain boundaries and prevent the austenite grains from growing, thereby refining the grains. At the same time, a small amount of Ti precipitates in the continuous annealing stage to strengthen the ferrite and bainite matrix. However, excessive precipitation of Ti will occupy C atoms in the steel, so the Ti content needs to be controlled at 0.01% to 0.04%.

[0043] Nb: Significantly increases the recrystallization temperature of steel and achieves grain refinement. During the hot rolling process, the strain-induced precipitation of Nb carbides can inhibit the recovery and recrystallization of deformed austenite, and the subsequent product is relatively fine. With the addition of Nb, fine niobium carbonitrides are dispersed and precipitated during the over-aging stage of the continuous annealing process, which plays a role in precipitation strengthening. The Nb content needs to be controlled at 0.01% to 0.04%.

[0044] Mo: Improves the hardenability of austenite and obtains a certain degree of martensite strength. Excessive Mo is easy to form segregation in ferrite and deteriorate ductility. The purpose of adding Mo is to slow down the formation of pearlite and help the formation of bainite, so the Mo content needs to be controlled at 0.1% to 0.5%.

[0045] B: Trace amounts of B can significantly improve the hardenability of steel, increasing strength without sacrificing other properties; too high a B content will increase the brittleness of the steel, so the B content is controlled at 0.0001% to 0.0004%.

[0046] P: It is a harmful element in steel. The lower its content, the better. Considering the smelting cost, the P content is controlled within 0.02%.

[0047] S: It is a harmful element in steel. The lower its content, the better. Considering the smelting cost, the S content is controlled within 0.01%.

[0048] A method for preparing a 780MPa grade ultra-high hole expansion performance cold-rolled complex phase steel plate for automobiles, the preparation process includes converter smelting, slab continuous casting, hot rolling, pickling cold rolling, and continuous annealing, specifically comprising the following steps:

[0049] 1. Converter smelting: Through converter smelting, molten steel is obtained that meets the following composition requirements by mass percentage:

[0050] C: 0.08% ~ 0.13%; Si: 0.1 ~ 0.5%; Mn: 1.8% ~ 2.5%; Cr: 0.25% ~ 0.6%; Mo: 0.1% ~ 0.5%; Ti: 0.01% ~ 0.04%; Nb: 0.01% ~ 0.04%; B: 0.0001 ~ 0.0004%; P ≤ 0.02%; S ≤ 0.01%, the balance is Fe and unavoidable impurities. The temperature of the molten steel is 1510 ℃ ~ 1620 ℃.

[0051] 2. Slab continuous casting: The casting temperature is between 1420℃ and 1540℃, and the thickness of the continuous casting slab is between 120 and 180mm.

[0052] 3. Hot rolling: heat the ingot to 1200℃~1280℃, keep isothermal for 2h, start rolling temperature at 1160℃~1250℃, finish rolling temperature above 880℃, coiling temperature at 600℃~700℃, thickness of hot rolled plate at 2~4mm.

[0053] The ingot is heated to 1200℃~1280℃ and kept isothermal for 2h to ensure full solid solution of various alloys; if the heating temperature of the ingot is too high, it will easily lead to the appearance of low melting point compounds at the grain boundaries, and the heating temperature is too low to meet the rolling temperature requirements.

[0054] The starting rolling temperature is between 1160℃ and 1250℃, ensuring that the rolling is carried out in the recrystallization zone. Fine TiN or Ti(C,N) precipitates, pins the grain boundaries, and refines the original austenite grains. If the starting rolling temperature is too low, the rolling force will increase, and cracking is likely to occur during the rolling process; the final rolling temperature must be greater than 880℃. If the rolling temperature is too low, the deformation resistance of the plate will be too large, and it will be difficult to roll to the target thickness; the curling temperature is between 600℃ and 700℃. If the curling temperature is too low, bainite and martensite will appear in the structure, which will increase the mill load during subsequent cold rolling and increase the difficulty of rolling. The curling structure is: 30% to 45% pearlite + 55% to 70% ferrite.

[0055] The thickness of hot-rolled plate is 2 to 4 mm. Too thick hot-rolled coil will increase the cold rolling load, making it difficult to achieve the target thickness required for cold rolling. Too thin hot-rolled plate will result in insufficient reduction in subsequent cold rolling, so the cold rolling energy storage is insufficient and continuous annealing will result in insufficient recrystallization.

[0056] 4. Pickling cold rolling: Before cold rolling, the steel plate needs to use acid to remove the iron oxide scale on the surface, and the cold rolling reduction rate is 50% to 60%. A sufficient reduction rate provides more nucleation sites and larger deformation energy storage in the subsequent annealing process, which is conducive to the subsequent grain recrystallization.

[0057] 5. Continuous annealing: isothermal temperature is 780℃~840℃, keep warm for 60s~300s, slow cooling temperature is 670℃~750℃, slow cooling rate is 0.5~3℃ / s, then cool to over-aging temperature at a faster cooling rate, over-aging temperature is 330℃~380℃, keep warm for 300s~600s, and then cool to room temperature.

[0058] 6. Heating: Heat to 780℃~840℃ at a heating rate of 5℃~10℃, the purpose is to regulate the ferrite and austenite content in the critical zone, and then regulate the total amount of ferrite in the critical zone and subsequent bainite and martensite generated. If the heating temperature is too low, the ferrite content in the organization is high, and since ferrite is a soft phase, the tensile strength will not meet the standard; if the heating temperature is too high, the ferrite content in the organization is low, and the generated bainite and martensite content is high. Since martensite has high hardness and poor plasticity, the elongation will decrease.

[0059] 7. Slow cooling: Slow cooling to 670℃~750℃ at a cooling rate of 0.5~3℃ / s. The purpose of slow cooling is to generate epitaxial ferrite. Since epitaxial ferrite is formed based on the pre-existing critical zone ferrite / austenite interface through non-nucleation extension and is formed under continuous cooling conditions, the replacement atoms such as (Mn) can only be partially discharged into austenite. In this way, compared with critical zone ferrite, epitaxial ferrite with higher alloying elements has higher strength, can achieve high plasticity of steel while ensuring high strength, and is an important factor in improving the hole expansion rate of complex phase steel. Epitaxial ferrite can effectively increase the density of ductile dimples and promote the development of steel from cleavage fracture to ductile fracture, thereby improving the final fracture toughness.

[0060] The invention adjusts the two-phase ratio of critical zone ferrite and epitaxial ferrite by regulating the isothermal temperature and the slow cooling temperature, so that the critical zone ferrite content is 10% to 20% and the epitaxial ferrite content is ≤10%.

[0061] 8. Overaging: Cool to 330℃~380℃ at a cooling rate of 30~40℃ / s for overaging treatment, and keep warm for 300s~600s. Bainite transformation and carbonitride precipitation will occur in the overaging stage. If the overaging temperature is too high, more bainite and less martensite will be generated, resulting in insufficient tensile strength, more and larger carbides will be generated, which is easy to be the starting point of crack initiation. When the temperature is low, martensite transformation is more and the elongation is reduced.

[0062] 9. Skin finishing: The steel plate then enters the skin finishing machine for plate shape adjustment, and the skin finishing elongation is controlled at 0.1% to 0.4%.

[0063] The cold-rolled complex phase steel microstructure obtained by the above preparation method includes intercritical ferrite, epitaxial ferrite, bainite and martensite; the microstructure is as follows by volume fraction: intercritical ferrite 10% to 20%, epitaxial ferrite ≤10%, bainite 25% to 35%, martensite 35% to 52%. The yield strength of the annealed steel plate is ≥600MPa, the tensile strength is ≥780MPa, the elongation after fracture is ≥11%, and the hole expansion rate is above 70%.

[0064] [Example]

[0065] The present invention is described in more detail by way of examples. These examples are merely descriptions of the best mode for carrying out the present invention and do not limit the scope of the present invention in any way.

[0066] Table 1 lists the chemical composition of the example steel, Table 2 lists the continuous casting and hot rolling process parameters of the example steel, Table 3 lists the cold rolling and continuous annealing process parameters of the example steel, Table 4 lists the microstructure of the experimental steel, and Table 5 gives the mechanical properties of the example steel;

[0067] Figure 1 The microstructure of Example 2; Figure 2 This is the engineering stress-strain curve of Example 2.

[0068] Table 1 Chemical composition of the example steel, wt%

[0069]

[0070] Table 2 Continuous casting and hot rolling process parameters of example steel

[0071]

[0072] Table 3 Process parameters of cold rolling and continuous annealing of example steel

[0073]

[0074] Table 4 Microstructure of Example Steel (Vol.%)

[0075]

[0076] Table 5 Mechanical properties of example steel

[0077] Example YS / MPa TS / MPa YS / TS TEL / % λ / % 1 640 833 0.77 11.4 70.8 2 621 813 0.76 12.1 70.0 3 600 799 0.75 11.9 70.1 4 682 817 0.83 11.6 71.3 5 613 801 0.77 11.2 70.9 6 633 822 0.77 11.3 70.2

[0078] It can be seen from the above embodiments that by adopting the composition design and production process parameters of the present invention, a complex phase steel plate with ultra-high hole expansion performance for automobiles can be prepared, with a yield strength ≥600MPa, a tensile strength ≥780MPa, a high yield ratio, and a hole expansion rate ≥70%, which can meet the high flanging performance requirements of automobile manufacturers.

[0079] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A 780MPa grade ultra-high hole expansion performance cold-rolled complex phase steel for automobiles, characterized in that: It is composed of the following chemical components in percentage by weight: C: 0.08% ~ 0.13%; Si: 0.1 ~ 0.5%; Mn: 1.8% ~ 2.5%; Cr: 0.25% ~ 0.6%; Mo: 0.1% ~ 0.5%; Ti: 0.01% ~ 0.019%; Nb: 0.01% ~ 0.018%; B: 0.0002 ~ 0.0004%; P ≤ 0.02%; S ≤ 0.01%, the balance is Fe and unavoidable impurities; The complex phase steel microstructure includes intercritical ferrite, epitaxial ferrite, bainite, and martensite; the microstructure is as follows by volume fraction: intercritical ferrite 10% to 20%, epitaxial ferrite 5.2% to 10%, bainite 25% to 35%, and martensite 35% to 52%; The steel plate has a tensile strength of more than 780 MPa, a yield strength of more than 600 MPa, an elongation of more than 11%, and a hole expansion rate of 70%.

2. A method for preparing the 780MPa grade ultra-high hole expansion performance cold-rolled complex phase steel plate for automobiles as claimed in claim 1, wherein the preparation process comprises converter smelting, slab continuous casting, hot rolling, pickling cold rolling, and continuous annealing, characterized in that: The specific steps include: 1) Converter smelting The smelting is done through a converter, and the temperature of the molten steel is 1510℃~1620℃; 2) Slab continuous casting The casting temperature is 1420℃~1540℃, and the continuous casting billet thickness is 120mm~180mm; 3) Hot rolling The ingot is heated to 1200℃~1280℃, isothermal for 100~130min, the starting rolling temperature is 1160℃~1250℃, the final rolling temperature is above 900℃, the coiling temperature is 600℃~700℃, and the hot rolled plate thickness is 2mm~4mm; 4) Pickling and cold rolling Before cold rolling, the iron oxide scale on the surface of the steel plate is removed with acid solution, and the cold rolling reduction rate is 50% to 60%; 5) Continuous annealing Heating: Heat to 780℃~840℃ at a rate of 5℃ / s~10℃ / s, keep warm for 60s~300s; Slow cooling: Slow cooling to 670℃~750℃ at a cooling rate of 0.5℃ / s~3℃ / s; Over-aging: Cooling to 340℃~380℃ at a cooling rate of more than 40℃ / s for over-aging treatment, keeping warm for 300s~480s; 6) Skin finishing: the steel plate enters the skin finishing machine for plate shape adjustment, and the skin finishing elongation is controlled at 0.1% to 0.4%.

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