A method for producing bake-hardened steel with a plate width of ≥1800 mm

Abstract

A method for producing bake-hardening steel with a strip width ≥1800mm includes: conventional hot rolling and hot rolling to adjust the strip surface convexity; pickling; cold rolling using a five-stand mill, refining the group spacing and controlling the cold rolling reduction rate according to the finished product thickness; continuous annealing; and leveling. This invention not only produces strips with a width ≥1800mm, but also features a stable production process, increasing the pass rate from the current 50% to over 80%, and achieving a tensile strength ≥300MPa, meeting user performance requirements.

Description

Technical Field

[0001] The invention relates to a method for producing bake-hardened steel, in particular to a method for producing bake-hardened steel with a plate width of ≥1800mm. Background Art

[0002] With the in-depth development of the automobile market, the market requirements for automobile steel have shown a trend of differentiation and diversification. In order to improve the material utilization rate of steel plates, meet the overall forming requirements of parts, and reduce the welding cost of the original welded plates, some users require cold-rolled plates with larger plate widths.

[0003] Bake-hardened steel is a type of stamping steel for body panels. It has a low yield strength and good formability before stamping. Stamping produces strain hardening, and then paint baking produces bake hardening, resulting in good stamping formability and high dent resistance of body covering parts. Ultra-wide bake-hardened steel is mainly used for automobile door panels, side panels, fenders, fenders and large covering parts. Compared with conventional width materials, ultra-wide materials can significantly improve the material utilization rate of large-size parts, reduce the use of laser welded plates, reduce production costs and reduce carbon emissions. The integral forming of multiple parts can improve the safety performance of the body.

[0004] Because the width of ultra-wide bake-hardened steel exceeds 1800mm, it is easy to scrape the furnace wall due to deviation during production in the continuous annealing furnace, which will lead to strip breakage and shutdown. It usually takes more than 20 hours to deal with the belt breakage and shutdown accident, which seriously affects the production rhythm and causes huge losses to the company. Therefore, reducing the deviation of ultra-wide bake-hardened steel can effectively improve the production efficiency of the unit, and the most important thing to control the deviation is to control the shape of the steel plate. Secondly, ultra-wide bake-hardened steel is mainly used for automotive covering parts, which has high requirements for the stamping performance and surface quality of the material. Usually, a high-temperature annealing process route is adopted. Therefore, ultra-wide strip steel is easy to produce high-temperature thermal warping in the continuous annealing furnace, which greatly affects the surface quality of the through-plate and steel plate. The high through-plate, high surface quality and high stamping performance requirements of the material have resulted in the inability of existing technologies to efficiently and stably manufacture ultra-wide bake-hardened steel, and the production qualification rate is less than 50%.

[0005] The existing disclosed technologies only explain the production method, processing method or manufacturing method for a specific type of steel, such as patent application numbers CN201210574442.8, CN202010095552.0, CN201210205030.7, etc. There is no effective cold rolling process production control technology and technical solution for producing bake-hardened steel with good stability and high pass rate with a width of ≥1800mm at home and abroad. Summary of the invention

[0006] The present invention aims to overcome the shortcomings of the prior art and provide a method for producing bake-hardened steel with a strip width ≥1800mm, which not only has a strip width ≥1800mm, but also has stable production, a qualified rate increased from the current 50% to more than 80%, performance that meets user needs, and a tensile strength ≥300MPa.

[0007] Technical measures to achieve the above objectives:

[0008] A method for producing bake-hardened steel with a plate width of ≥1800 mm, the steps of which are as follows:

[0009] 1) Perform conventional hot rolling and hot rolled plate surface convexity: the plate surface convexity value is controlled within 30 to 90 μm, wherein: when the thickness of the hot rolled plate is ≤4.0 mm, the plate surface convexity value is controlled not to exceed 70 μm; when the thickness of the hot rolled plate is >4.0 to 6 mm, the plate surface convexity value is controlled not to exceed 90 μm;

[0010] 2) Pickling: control the acid tank temperature at 70-90°C;

[0011] The free acid concentration in the three acid tanks is controlled as follows:

[0012] No. 1 acid tank is not less than 50g / l, No. 2 acid tank is not less than 70g / l, and No. 3 acid tank is not less than 130g / l; the pickling speed is controlled at no more than 180m / min;

[0013] 3) Use five stands for cold rolling, refine the group spacing and control the cold rolling reduction rate according to the thickness of the finished product:

[0014] When the finished product thickness is 0.5mm<0.6mm, the hot rolling raw material thickness is selected to be 2.9~≤3.1mm, and the total cold rolling reduction rate is controlled at 79~84%;

[0015] When 0.6mm<finished product thickness≤0.7mm, the hot rolling raw material thickness is selected to be 3.4~≤3.6mm, and the total cold rolling reduction rate is controlled at 79~83%;

[0016] When the finished product thickness is 0.7mm<≤0.8mm, the hot rolling raw material thickness is selected to be 3.9~≤4.1mm, and the total cold rolling reduction rate is controlled at 79~83%;

[0017] When the finished product thickness is 0.8mm<≤0.9mm, the hot rolling raw material thickness is selected to be 4.4~≤4.6mm, and the total cold rolling reduction rate is controlled at 80~83%;

[0018] When the finished product thickness is 0.9mm<1.0mm, the hot rolling raw material thickness is selected to be 4.9~≤5.1mm, and the total cold rolling reduction rate is controlled at 80~82%;

[0019] When 1.0mm<finished product thickness≤1.2mm, the hot rolling raw material thickness is selected to be 5.7~≤5.9mm, and the total cold rolling reduction rate is controlled at 79~83%;

[0020] When the finished product thickness is 1.2mm<1.4mm, the hot rolling raw material thickness is selected to be 6.4~≤6.6mm, and the total cold rolling reduction rate is controlled at 78~82%;

[0021] When the finished product thickness is 1.4mm<≤2.0mm, the hot rolling raw material thickness is selected to be 6.9~≤7.1mm, and the total cold rolling reduction rate is controlled at 71~80%;

[0022] And control the emulsion temperature at 45-60°C, and the saponification value is not less than 170KOH / g;

[0023] The crown of the working rolls of each stand is controlled to be 20-40μm for the working rolls of the 1st to 4th stands, the crown of the working rolls of the 5th stand is not more than 10μm, and the inclination value of the 5th stand is controlled not to exceed 300μm. The absolute value of the tension difference on both sides of the strip at the mill exit is ≤3KN;

[0024] 4) Continuous annealing: Before annealing the bake-hardened steel with a width of ≥1800mm after cold rolling, first arrange 3-5

[0025] DC01 plain carbon steel coils with a coil width greater than the width of the bake-hardened steel to be annealed, the greater width value is not less than 5mm;

[0026] The DC01 steel and bake hardening steel adopt the same temperature and speed, and then enter the continuous annealing of bake hardening steel with plate width ≥1800mm;

[0027] Each process operation is carried out as follows:

[0028] The annealing temperature of each section is controlled as follows:

[0029] The annealing temperature in the soaking section is between 815 and 835°C;

[0030] The temperature of the slow cooling section is 620-640°C;

[0031] The temperature of the rapid cooling section is 370-420°C, and the rapid cooling rate is 35°C / s;

[0032] The temperature of the over-aging stage is 360~400℃.

[0033] The temperature of the final cooling section is 170-180°C;

[0034] Control annealing speed according to strip thickness:

[0035] When the strip thickness is greater than 0.5 to 1.0 mm, the annealing speed is controlled at 160 to 220 m / min;

[0036] When the strip thickness is greater than 1.0 to 2.0 mm, the annealing speed is controlled at 110 to 180 m / min;

[0037] Control the tension of each segment:

[0038] Heating section:

[0039] The tension of the first heating section and the second heating section is controlled at 8-11KN, and the crown of the furnace roller is controlled at 0.25-0.35mm;

[0040] The tension of the third heating section is controlled at 5-7KN;

[0041] Soaking section:

[0042] The tension of this section is controlled at 4~6KN;

[0043] Slow cooling section and fast cooling section:

[0044] The tension of the two sections is controlled at 12-16KN;

[0045] Over-aging stage and final cooling stage:

[0046] The tension of the two sections is controlled at 7-12KN;

[0047] 5) Leveling, using a constant flow wet leveling mode, using a middle roller string roller, and controlling the leveling elongation at 1.5-2.0%;

[0048] Controlling the rolling force of the skin-pass mill:

[0049] When the strip thickness is greater than 1.0 mm, the temper rolling force shall not be less than 5000 KN;

[0050] When the strip thickness is ≤1.0mm, the temper rolling force shall not be less than 6500KN;

[0051] Control the front and rear tension of the leveler:

[0052] The tension before the leveling machine is controlled at 70~100KN, the tension after the leveling machine is controlled at 80~110KN, and the tension difference between the front and rear is controlled

[0053] Within 10KN.

[0054] Furthermore: during continuous annealing, when the unit has an abnormality and needs to slow down, the heating furnace adopts a step-by-step speed reduction method, that is, the speed is reduced step by step, and the single speed reduction does not exceed 20m / min, and the minimum operating speed is controlled at no less than 80m / min.

[0055] Function and mechanism of the main process in the present invention

[0056] The reason why the present invention controls the plate convexity of the hot-rolled raw material is that the good plate shape mainly comes from the good hot-rolled raw material plate profile and the proportional extension in the width direction of the strip during the cold rolling process. The hot-rolled raw material plate profile has the most direct impact on the plate shape. The cold-rolled strip outlet convexity depends on the hot-rolled raw material convexity and the mill roll gap convexity. The size of the difference between the two determines the size of the strip wave shape. In the plate shape control of ultra-wide bake-hardened steel, because the plate width reaches the limit of the roller length of the rolling mill, conventional plate shape control methods such as bending rolls and stringing rolls cannot achieve the best plate shape control effect. Therefore, appropriate hot-rolled raw material convexity can reduce the intermediate extension degree of strip rolling and improve the strip shape. Through a large number of production results, it is found that when the hot-rolled raw material thickness of ultra-wide bake-hardened steel is ≤2.5mm, the plate convexity value is 30-70μm; when the hot-rolled raw material thickness of ultra-wide bake-hardened steel is >2.5-6mm, the plate convexity value is 30-80μm, and the cold-rolled plate shape control is better.

[0057] The present invention controls the acid tank temperature at 70-90° C., the free acid concentration in the three acid tanks, the No. 1 acid tank is not less than 50 g / l, the No. 2 acid tank is not less than 70 g / l, the No. 3 acid tank is not less than 130 g / l, and the pickling speed is controlled below 180 m / min, in order to avoid over-pickling and under-pickling of the steel coil and improve the surface quality of the product.

[0058] The reason why the present invention controls the emulsion temperature at 45-60°C and the saponification value is not less than 170KOH / g is to control the plate shape of ultra-wide bake-hardened steel and reduce residual oil and iron on the surface of the steel coil to improve the surface quality of the product.

[0059] The reason why the present invention refines the group spacing during cold rolling is that, especially for ultra-wide bake-hardened steel with a finished product thickness of less than 1.0 mm, the total reduction rate is controlled by grouping every 0.1 mm of finished product thickness. When the width exceeds 1800 mm, the increase in deformation will cause a sharp increase in the rolling force of the rolling mill, which greatly affects the adjustment effect of the bending roll and the shifting roll on the plate shape, and at the same time poses an extreme challenge to the cooling of the rolling mill, causing the rolling mill motor to overheat, which is more unfavorable for the stable control of the plate shape. With the increase in the rolling amount, the plate shape is further deteriorated. A larger reduction rate can increase the cold rolling deformation storage energy, enhance the recrystallization driving force of the steel plate during the annealing process, make recrystallization occur in advance, increase the nucleation rate of recrystallization and accelerate the grain growth rate, and make the organization size more uniform. At the same time, a larger reduction rate can increase the distortion energy and dislocation density, increase the diffusion capacity of atoms, increase the precipitation power of the second-term particles, and increase the second-phase precipitates, thereby improving the bake hardening value of the annealed plate. Therefore, taking the plate shape and performance into consideration, the total reduction rate should be appropriately increased. When the finished product thickness is ≤1.4mm, the reduction rate should be controlled at 78-83%. When the finished product thickness is greater than 1.4mm-2.0mm, the reduction rate should be controlled at 71-80%.

[0060] The present invention controls the crown of the working rolls of the 1st to 5th stands, the crown of the working rolls of the 1st to 4th stands is 20-40μm, and the crown of the working roll of the 5th stand is not more than 10μm; controls the inclination of the 5th stand, and the inclination value is controlled below 300μm to ensure that the absolute value of the tension difference on both sides of the strip at the mill outlet is ≤3KN. Reducing the tension difference on both sides of the strip can improve the plate quality, especially in the width direction, and can significantly reduce the risk of deviation of the continuous annealing unit.

[0061] The reason why the present invention arranges 3 to 5 rolls of DC01 plain carbon steel is that the width of DC01 steel needs to be greater than the width of the ultra-wide baked hardened steel of the present invention. The leading material needs to be adjusted to the production process of ultra-wide baked hardened steel in advance, in order to ensure that the ultra-wide baked hardened steel can pass smoothly after entering the annealing furnace, reduce the uneven temperature difference between the strip and the furnace roller in the length direction, and reduce the thermal stress difference in the width direction of the strip. Therefore, a certain amount of leading material is arranged for hot rolling before production to reduce the probability of strip bending and deviation.

[0062] The reason why the present invention controls the annealing temperature of different process sections is that the temperature of the soaking section is 815-835°C, the temperature of the slow cooling section is 620-640°C, the temperature of the fast cooling section is 370-420°C, the fast cooling rate reaches more than 35°C / s, the temperature of the over-aging section is 360-400°C, and the temperature of the final cooling section is 170-180°C. The high annealing temperature can decompose carbides, form a favorable {111} recrystallization texture during the cooling process, and improve the deep drawing performance and bake hardening performance of the product. However, if the annealing temperature is too high, it is easy to cause thermal buckling due to the large temperature difference between the strip and the furnace roller, affecting the quality of the ultra-wide plate and the plate surface. Appropriately reducing the temperature of the cooling section can, on the one hand, reduce the temperature difference between the strip and the furnace roller and reduce the risk of cold buckling. On the other hand, matching the annealing speed can increase the cooling rate. When the cooling rate reaches more than 35°C / s, the bake hardening value of the steel plate can stably reach more than 40MPa. At the same time, increasing the cooling rate is conducive to improving the strength of the material. Turn on the electric heating device in the aging section to improve the temperature assurance and stability of the aging section, reduce the change of roller shape, and reduce the probability of cold buckling.

[0063] The reason why the present invention controls the annealing speed is that for the strip thickness of >0.5~1.0mm, the annealing speed is 160~220m / min; for the strip thickness of >1.0~2.0mm, the annealing speed is 110~180m / min. When the cooling rate of bake-hardened steel needs to reach above 35℃ / s, the bake-hardening value of the steel plate can stably reach above 30MPa. At the same time, a certain cooling rate makes the material reach a certain strength. Therefore, it is necessary to control a suitable speed to ensure the cooling rate of the material annealing. At the same time, in order to reduce the influence of the furnace roller shape on the wide strip steel, the operating speed of the annealing furnace needs to be increased, but the speed increase is easy to cause the strip steel to deviate, and in severe cases, cause the strip to break, and the low annealing speed is easy to cause intermittent thermal buckling of the strip steel in the heating section. Set the minimum operating speed, that is, the safe operating speed of the plate. If the unit is lower than this speed, it is easy to cause serious buckling, thereby causing serious production accidents such as unit shutdown and strip breakage. In summary, the control of the annealing speed needs to be considered comprehensively.

[0064] The reason why the present invention controls the tension of each process section is mainly to control the deviation and warping of ultra-wide strip steel and reduce the fluctuation of the plate. In the first heating section and the second heating section, since the strip steel enters a higher temperature at a lower temperature, the internal stress of the strip steel is released, and the strip steel is prone to deviation. The ultra-wide baked hardened steel plate has a bad shape or the tension difference on both sides of the strip steel is too large, which will aggravate the deviation of the strip steel. Therefore, the convexity of the furnace roller in this area should be improved and controlled at 0.25-0.35mm. At the same time, the tension of the first heating section and the second heating section needs to be increased and controlled at 8-11KN. In the third heating section, since the strip temperature reaches above 815℃ and the steel is softer, the ultra-wide strip steel is prone to thermal warping, and the tension needs to be smaller and controlled at 5-7KN. In the equalizing section, since the strip steel is kept above 800℃ for a period of time and the steel is softer, the tension needs to be further reduced and controlled at 4-6KN. In the slow cooling section and the fast cooling section, since lowering the temperature in the slow cooling section and the fast cooling section is conducive to reducing the temperature difference between the strip and the furnace roller, the risk of cold buckling can be reduced. However, the temperature reduction is achieved through the cooling bellows. The wind speed of the cooling bellows is high, which can easily cause the strip to shake and cause scratches. The ultra-wide strip shakes more, causing more serious scratches. Therefore, the tension needs to be larger and controlled at 12 to 16 KN. In the over-aging section and the final cooling section, if the tension in this area is too large, the degree of cold buckling will be aggravated, and if it is too small, the degree of scratching will increase. Therefore, the setting of the tension value in this area needs to take into account both scratches and buckling, and be controlled at 7 to 12 KN.

[0065] The reason why the flattening process of the present invention adopts a constant flow wet flattening mode and uses a string of intermediate rolls is mainly to ensure the consistency of rolling force in the width direction and ensure the plate shape of the finished product of ultra-wide bake-hardened steel. Before producing ultra-wide bake-hardened steel, new working rolls are replaced to reduce the color difference of the strip edge caused by the transition from narrow specifications to wide specifications, ensuring the high surface quality of the finished product.

[0066] The reason why the flattening elongation of the present invention is controlled at 1.5-2.0% is that a large flattening elongation is beneficial to improving the baking hardening value and anti-natural aging performance of the product, and at the same time can eliminate the yield platform of the product, so that the product will not produce a stamping slip line during the stamping process. The front and rear tensions of the leveling machine are controlled mainly because the flattening elongation of the ultra-wide plate increases, which will make it difficult to control the plate shape, and the edge is prone to edge waves. Therefore, it is necessary to control and stabilize the front and rear tensions of the leveling machine. The front tension is controlled at 70-100KN, the rear tension is controlled at 80-110KN, and the rear-front tension difference is controlled within 10KN. The rolling force of the leveling machine is controlled mainly to eliminate the yield platform of the product, especially for products with a strip thickness of ≤1.0mm. When the strip flattening elongation reaches 1.5-2.0%, the plate shape control is very poor, and it is necessary to appropriately reduce the flattening elongation. At this time, it is necessary to ensure that the flattening rolling force reaches the required value.

[0067] Compared with the prior art, the present invention not only has a strip width of ≥1800mm, but also has a stable production process, a qualified rate increased from the current 50% to more than 80%, a tensile strength of ≥300MPa, and performance that meets user needs. DETAILED DESCRIPTION

[0068] The present invention is described in detail below:

[0069] Table 1 is a list of main process parameters of various embodiments and comparative examples of the present invention;

[0070] Table 2 is a table of performance test results of various embodiments of the present invention and comparative examples.

[0071] Each embodiment of the present invention is produced according to the following steps

[0072] 1) Carry out conventional hot rolling and hot rolled plate surface convexity: the plate surface convexity value is controlled at 30~90μm, among which: when the thickness of the hot rolled plate is ≤

[0073] When the thickness of the hot-rolled plate is 4.0mm, the convexity value of the plate surface is controlled not to exceed 70μm; when the thickness of the hot-rolled plate is greater than 4.0 to 6mm, the convexity value of the plate surface is controlled not to exceed 90μm;

[0074] 2) Pickling: control the acid tank temperature at 70-90°C;

[0075] The free acid concentration in the three acid tanks is controlled as follows:

[0076] No. 1 acid tank is not less than 50g / l, No. 2 acid tank is not less than 70g / l, and No. 3 acid tank is not less than 130g / l; the pickling speed is controlled at no more than 180m / min;

[0077] 3) Use five stands for cold rolling, refine the group spacing and control the cold rolling reduction rate according to the thickness of the finished product:

[0078] When the finished product thickness is 0.5mm<0.6mm, the hot rolling raw material thickness is selected to be 2.9~≤3.1mm, and the total cold rolling reduction rate is controlled at 79~84%;

[0079] When 0.6mm<finished product thickness≤0.7mm, the hot rolling raw material thickness is selected to be 3.4~≤3.6mm, and the total cold rolling reduction rate is controlled at 79~83%;

[0080] When the finished product thickness is 0.7mm<≤0.8mm, the hot rolling raw material thickness is selected to be 3.9~≤4.1mm, and the total cold rolling reduction rate is controlled at 79~83%;

[0081] When the finished product thickness is 0.8mm<≤0.9mm, the hot rolling raw material thickness is selected to be 4.4~≤4.6mm, and the total cold rolling reduction rate is controlled at 80~83%;

[0082] When the finished product thickness is 0.9mm<1.0mm, the hot rolling raw material thickness is selected to be 4.9~≤5.1mm, and the total cold rolling reduction rate is controlled at 80~82%;

[0083] When 1.0mm<finished product thickness≤1.2mm, the hot rolling raw material thickness is selected to be 5.7~≤5.9mm, and the total cold rolling reduction rate is controlled at 79~83%;

[0084] When the finished product thickness is 1.2mm<1.4mm, the hot rolling raw material thickness is selected to be 6.4~≤6.6mm, and the total cold rolling reduction rate is controlled at 78~82%;

[0085] When the finished product thickness is 1.4mm<≤2.0mm, the hot rolling raw material thickness is selected to be 6.9~≤7.1mm, and the total cold rolling reduction rate is controlled at 71~80%;

[0086] And control the emulsion temperature at 45-60°C, and the saponification value is not less than 170KOH / g;

[0087] The crown of the working rolls of each stand is controlled to be 20-40μm for the working rolls of the 1st to 4th stands, the crown of the working rolls of the 5th stand is not more than 10μm, and the inclination value of the 5th stand is controlled not to exceed 300μm. The absolute value of the tension difference on both sides of the strip at the mill exit is ≤3KN;

[0088] 4) Continuous annealing: before annealing the bake-hardened steel with a plate width of ≥1800mm after cold rolling, first arrange 3-5 rolls of DC01 plain carbon steel coils with a width greater than the width of the bake-hardened steel to be annealed, and the width greater than the width is not less than 5mm;

[0089] The DC01 steel and bake hardening steel adopt the same temperature and speed, and then enter the continuous annealing of bake hardening steel with plate width ≥1800mm;

[0090] Each process operation is carried out as follows:

[0091] The annealing temperature of each section is controlled as follows:

[0092] The annealing temperature in the soaking section is between 815 and 835°C;

[0093] The temperature of the slow cooling section is 620-640°C;

[0094] The temperature of the rapid cooling section is 370-420°C, and the rapid cooling rate is 35°C / s;

[0095] The temperature of the over-aging stage is 360~400℃.

[0096] The temperature of the final cooling section is 170-180°C;

[0097] Control annealing speed according to strip thickness:

[0098] When the strip thickness is greater than 0.5 to 1.0 mm, the annealing speed is controlled at 160 to 220 m / min;

[0099] When the strip thickness is greater than 1.0 to 2.0 mm, the annealing speed is controlled at 110 to 180 m / min;

[0100] Control the tension of each segment:

[0101] Heating section:

[0102] The tension of the first heating section and the second heating section is controlled at 8-11KN, and the crown of the furnace roller is controlled at 0.25-0.35mm;

[0103] The tension of the third heating section is controlled at 5-7KN;

[0104] Soaking section:

[0105] The tension of this section is controlled at 4~6KN;

[0106] Slow cooling section and fast cooling section:

[0107] The tension of the two sections is controlled at 12-16KN;

[0108] Over-aging stage and final cooling stage:

[0109] The tension of the two sections is controlled at 7-12KN;

[0110] 5) Leveling, using a constant flow wet leveling mode, using a middle roller string roller, and controlling the leveling elongation at 1.5-2.0%;

[0111] Controlling the rolling force of the skin-pass mill:

[0112] When the strip thickness is greater than 1.0 mm, the temper rolling force shall not be less than 5000 KN;

[0113] When the strip thickness is ≤1.0mm, the temper rolling force shall not be less than 6500KN;

[0114] Control the front and rear tension of the leveler:

[0115] The tension before the leveling machine is controlled at 70~100KN, the tension after the leveling machine is controlled at 80~110KN, and the tension difference between the front and rear is controlled

[0116] Within 10KN.

[0117] Table 1 List of main process parameters of various embodiments and comparative examples of the present invention

[0118]

[0119] Table 1

[0120]

[0121]

[0122] Table 2 Performance test results of various embodiments of the present invention

[0123]

[0124] As can be seen from Table 2, in Examples 1-9, the ultra-wide bake-hardened steel within the scope of the claims of the present invention can not only be smoothly and high-quality plate-passed and produced, but also the surface quality and mechanical properties meet the requirements, and the performance indicators meet the technical indicators of HC180B steel. The overall product qualification rate of Examples 1-9 reached 91%. In Comparative Example 1, due to the large difference in the strip tension at the exit of the rolling mill, the strip seriously deviated in the annealing furnace. The unit took emergency speed reduction measures, resulting in surface quality defects such as severe scratches on the surface of the strip. At the same time, the cooling rate was low, resulting in inconsistent product performance, and the bake-hardening value did not reach 40MPa. In Comparative Example 2, due to the excessively high heating temperature, the strip was thermally buckled. In order to prevent the thermal buckling from worsening and causing the strip to break, the unit took emergency speed reduction measures, which caused surface quality defects such as severe scratches on the surface of the strip. At the same time, the cooling rate was low, and the bake-hardening value did not reach 40MPa.

[0125] This specific implementation is only the best example and is not a restrictive implementation of the technical solution of the present invention.

Claims

1. A method for producing bake-hardened steel with a plate width of ≥1800 mm, the steps of which are as follows: 1) Carry out conventional hot rolling and control the convexity of the hot rolled plate surface: the convexity value of the plate surface is controlled at 30-90 μm, where: When the thickness of the hot-rolled plate is ≤4.0mm, the convexity value of the plate surface is controlled not to exceed 70μm; when the thickness of the hot-rolled plate is 4.0mm<≤6mm, the convexity value is controlled not to exceed 90μm; 2) Pickling: control the acid tank temperature at 70-90°C; The free acid concentration in the three acid tanks is controlled as follows: No. 1 acid tank is not less than 50g / L, No. 2 acid tank is not less than 70g / L, and No. 3 acid tank is not less than 130g / L; the pickling speed is controlled at no more than 180m / min; 3) Use five stands for cold rolling, refine the group spacing and control the cold rolling reduction rate according to the thickness of the finished product: When 0.5mm<finished product thickness≤0.6mm, the hot rolling raw material thickness is selected to be 2.9~3.1mm, and the total cold rolling reduction rate is controlled at 79~84%; When 0.6mm<finished product thickness≤0.7mm, the hot rolling raw material thickness is selected to be 3.4~3.6mm, and the total cold rolling reduction rate is controlled at 79~83%; When 0.7mm<finished product thickness≤0.8mm, the hot rolling raw material thickness is selected to be 3.9~4.1mm, and the total cold rolling reduction rate is controlled at 79~83%; When 0.8mm<finished product thickness≤0.9mm, the hot rolling raw material thickness is selected to be 4.4~4.6mm, and the total cold rolling reduction rate is controlled at 80~83%; When 0.9mm<finished product thickness≤1.0mm, the hot rolling raw material thickness is selected to be 4.9~5.1mm, and the total cold rolling reduction rate is controlled at 80~82%; When 1.0mm<finished product thickness≤1.2mm, the hot rolling raw material thickness is selected to be 5.7~5.9mm, and the total cold rolling reduction rate is controlled at 79~83%; When the finished product thickness is 1.2mm<1.4mm, the hot rolling raw material thickness is selected to be 6.4~6.6mm, and the total cold rolling reduction rate is controlled at 78~82%; The emulsion temperature is controlled at 45-60°C, and the saponification value is not less than 170 mgKOH / g; The crown of the working rolls of each stand is controlled to be 20-40μm for the working rolls of the 1st to 4th stands, the crown of the working rolls of the 5th stand is not more than 10μm, and the inclination value of the 5th stand is controlled not to exceed 300μm. The absolute value of the tension difference on both sides of the strip at the mill exit is ≤3kN; 4) Continuous annealing: Before annealing the bake-hardened steel with a plate width of ≥1800mm after cold rolling, first arrange 3-5 rolls of DC01 plain carbon steel coils with a width greater than the width of the bake-hardened steel to be annealed, and the width greater than the width is not less than 5mm; DC01 steel and bake-hardened steel use the same temperature and speed, and then enter the bake-hardened steel with a plate width of ≥1800mm for continuous annealing; Each process operation is carried out as follows: The annealing temperature of each section is controlled as follows: The annealing temperature in the soaking section is between 815 and 835°C; The temperature of the slow cooling section is 620-640°C; The temperature of the rapid cooling section is 370-420°C, and the rapid cooling rate is 35°C / s; The temperature of the over-aging stage is 368~400℃. The temperature of the final cooling section is 170-180°C; Control annealing speed according to strip thickness: 0.5<When the strip thickness is ≤1.0mm, the annealing speed is controlled at 181~220m / min; 1.0<When the strip thickness is ≤2.0mm, the annealing speed is controlled at 175~180m / min; Control the tension of each segment: Heating section: The tension of the first heating section and the second heating section is controlled at 8-11 kN, and the crown of the furnace roller is controlled at 0.25-0.35 mm; The tension of the third heating section is controlled at 5-7kN; Soaking section: The tension of this section is controlled at 4-6kN; Slow cooling section and fast cooling section: The tension of the two sections is controlled at 12-16 kN; Over-aging stage and final cooling stage: The tension of the two sections is controlled at 7-12kN; 5) For leveling, use a constant flow wet leveling mode, use an intermediate roller string roller, and control the leveling elongation at 1.5-2.0%; Controlling the rolling force of the skin-pass mill: When the strip thickness is greater than 1.0 mm, the temper rolling force shall not be less than 5000 kN; When the strip thickness is ≤1.0mm, the temper rolling force shall not be less than 6500kN; Control the front and rear tension of the leveler: The tension before the leveling machine is controlled at 70-100kN, the tension after the leveling machine is controlled at 80-110kN, and the tension difference between the front and rear is controlled Within 10kN.

2. A method for producing bake-hardened steel with a plate width of ≥ 1800 mm as claimed in claim 1, characterized in that: During continuous annealing, when the unit has an abnormality and needs to slow down, the heating furnace adopts a step-by-step speed reduction method, that is, the speed is reduced step by step, and the single speed reduction does not exceed 20m / min. At the same time, the minimum operating speed is controlled at not less than 80m / min.

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