Method for improving quality of steel plate for automobile

Abstract

The invention provides a method for improving the quality of a steel plate for an automobile, and belongs to the field of steel preparation. The method comprises the steps that molten steel is subjected to continuous casting, when the surface temperature of a casting blank formed through continuous casting is reduced to a first set temperature, the casting blank is subjected to ultra-fast cooling, and a first casting blank with fine grain structures on the upper surface layer and the lower surface layer is obtained; heating the first casting blank in stages to obtain a second casting blank of an austenite structure with a uniformly refined surface layer; the second casting blank is subjected to high-pressure water descaling, and a third casting blank with the set surface red bright area is obtained; and rolling the third casting blank to obtain the steel plate for the automobile. A refined solidification structure with high toughness is obtained by strongly cooling the upper surface layer and the lower surface layer of a casting blank in the continuous casting process, and the refined structure inheritance is continued to a steel plate product for an automobile, so that the performance of the steel plate for the automobile is improved.

Description

Technical Field

[0001] This application relates to the technical field of steel preparation, and particularly to a method for improving the quality of automotive steel sheets. Background Art

[0002] Automotive sheets, as important materials in automotive manufacturing, their properties are directly related to the safety, durability, and environmental friendliness of automobiles. An important property of automotive sheets is high strength. Currently, it is mainly achieved by adding alloys to steel, controlling the C and N contents in the steel, and ensuring stable rolling, coiling, annealing and other processes to control the precipitation of second-phase particles and refine grains, meeting the performance requirements of automotive steel sheets. In the current environment of low-carbon smelting, increasing the scrap ratio is an important way to reduce carbon emissions in the steel process. Due to the instability of scrap quality, increasing the scrap ratio will lead to excessive residual elements and N content in the steel, thereby reducing the tensile strength, yield strength, and tensile plastic strain ratio (r-value) of automotive steel sheets.

[0003] In the prior art, by controlling the chemical composition of automotive steel sheets, especially controlling the mass fractions of N, Al, V, and C, and ensuring sufficient precipitates and no remaining C and N in the steel during subsequent rolling, coiling, annealing and other processes, automotive steel sheets with excellent strength and formability can be obtained. Currently, there is no method to stably improve the performance of automotive steel sheets without increasing alloy costs and maintaining the stability of the original rolling process. Summary of the Invention

[0004] This application provides a method for improving the quality of automotive steel sheets to solve the following technical problems: how to improve the quality of automotive steel sheets without increasing alloy costs and maintaining the stability of the original rolling process.

[0005] The embodiments of this application provide a method for improving the quality of automotive steel sheets, and the method includes:

[0006] Continuous casting the molten steel, and when the surface temperature of the slab formed by the continuous casting drops to the first set temperature, performing ultra-fast cooling on the slab to obtain a first slab with fine-grained structures on the upper and lower surfaces;

[0007] Heating the first slab in stages to obtain a second slab with a uniformly refined austenite structure;

[0008] Performing high-pressure water descaling on the second slab to obtain a third slab with a set surface bright red area;

[0009] Rolling the third slab to obtain the automotive steel sheet.

[0010] Optionally, the first set temperature is 950°C to 1110°C.

[0011] Optionally, the ultra-fast cooling is water cooling. After the continuous casting billet is ultra-fast cooled, the method further includes:

[0012] When the surface temperature of the ultra-fast cooled continuous casting billet drops to a second set temperature, stop the water cooling to allow the continuous casting billet to cool by natural heat dissipation; the second set temperature ≤ 300 °C.

[0013] Optionally, the cooling rate of the ultra-fast cooling is 3.5 °C / s to 8.5 °C / s.

[0014] Optionally, the steel passing rate of the continuous casting is 3.0 t / min to 9.5 t / min.

[0015] Optionally, the fine grain structure on the upper and lower surfaces of the first continuous casting billet is a fine pearlite or martensite structure, and the existing regions of the fine grain structure are respectively in the intervals of 20 mm to 60 mm from the upper and lower surfaces of the continuous casting billet.

[0016] Optionally, the atmosphere for the staged heating is an inert gas protection atmosphere, the total in-furnace time for the staged heating is 130 min to 310 min, and the staged heating of the first continuous casting billet includes:

[0017] Preheat the first continuous casting billet to a first set temperature for a first set holding time;

[0018] First heat the preheated first continuous casting billet to a second set temperature for a second set holding time;

[0019] Second heat the first continuously cast billet after the first heating to a third set temperature for a third set holding time;

[0020] Perform homogenization heat treatment on the first continuously cast billet after the second heating to a fourth set temperature for a fourth set holding time.

[0021] Optionally, the first set temperature is 250 °C to 350 °C, and the first set holding time satisfies the following relationship: T1 = 0.22h - 2.3;

[0022] The second set temperature is 750 °C to 850 °C, and the second set holding time satisfies the following relationship: T2 = 0.271h + 5.2;

[0023] The third set temperature is 950 °C to 1050 °C, and the third set holding time satisfies the following relationship: T3 = 0.287h - 7.05;

[0024] The fourth set temperature is 1050 °C to 1150 °C, and the fourth set holding time satisfies the following relationship: T4 = 0.236h + 4.72;

[0025] In the formula, T1 represents the first set heat preservation time, with the unit of min; T2 represents the second set heat preservation time, with the unit of min; T3 represents the third set heat preservation time, with the unit of min; T4 represents the fourth set heat preservation time, with the unit of min; h represents the thickness of the first billet, with the unit of mm.

[0026] Optionally, the descaling water pressure of the high-pressure water descaling > 15 MPa.

[0027] Optionally, the set surface bright red area > 98%.

[0028] The above technical solutions provided by the embodiments of the present application have the following advantages compared with the prior art:

[0029] The embodiments of the present application provide a method for improving the quality of automotive steel plates. The method includes: continuously casting molten steel, and when the surface temperature of the billet formed by the continuous casting drops to the first set temperature, performing ultra-fast cooling on the billet to obtain a first billet with fine-grained structures on the upper and lower surfaces; performing staged heating on the first billet to obtain a second billet with uniformly refined austenite structure on the surface; performing high-pressure water descaling on the second billet to obtain a third billet with a set surface bright red area; and rolling the third billet to obtain the automotive steel plate. By strongly cooling the upper and lower surfaces of the billet during the continuous casting process to obtain a refined and tough solidification structure, and hereditarily continuing the refined structure into the automotive steel plate product, the performance of the automotive steel plate is improved. Description of the Drawings

[0030] The drawings here are incorporated into the description and form a part of this description, showing the embodiments that conform to the present application, and are used together with the description to explain the principles of the present application.

[0031] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, for those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0032] Figure 1 It is a schematic flow chart of a method for improving the quality of automotive steel plates provided by the embodiments of the present application. Detailed Embodiments

[0033] To make the objectives, technical solutions and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings in the embodiments of this application. Obviously, the described embodiments are some, but not all, of the embodiments of this application. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in this application without creative efforts shall fall within the scope of protection of this application.

[0034] The various embodiments of this application may exist in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity and should not be construed as a rigid limitation on the scope of this application; therefore, it should be considered that the described range description has specifically disclosed all possible sub-ranges and single values within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within that range, such as 1, 2, 3, 4, 5, and 6, and this applies regardless of the range. Additionally, whenever a numerical range is indicated herein, it means including any cited number (fraction or integer) within the indicated range.

[0035] In addition, in the description of the specification of the present application, terms such as "including" and "comprising" mean "including but not limited to". In this text, relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. In this text, "and / or" describes the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can represent: A exists alone, A and B exist simultaneously, and B exists alone. Where A and B can be singular or plural. In this text, "at least one" means one or more, and "multiple" means two or more. "At least one kind", "at least one of the following items (pieces)" or similar expressions refer to any combination of these items, including any combination of single item (piece) or plural items (pieces). For example, "at least one of a, b, or c", or "at least one of a, b, and c" can both represent: a, b, c, a - b (that is, a and b), a - c, b - c, or a - b - c, where a, b, and c can be single or multiple respectively. The "parts representation method" such as weight parts and mass parts represents the proportional relationship between each component. In the proportional relationships involved in this text, the parameters that need to be described by proportion should be understood as the antecedents of the proportion formula in the order of description, and the proportional numbers should be understood as the consequents of the proportion formula. For example, if the mass ratio of substance A, substance B, and substance C is 1:2:3, then substance A, substance B, and substance C should correspond one by one with the proportional numbers in the proportion formula in the order of description, that is, the mass of substance A: the mass of substance B: the mass of substance C = 1:2:3.

[0036] Unless otherwise specifically stated, various raw materials, reagents, instruments, equipment, etc. used in the present application can be obtained through market purchase or can be prepared by existing methods.

[0037] Figure 1 It is a schematic flow chart of a method for improving the quality of steel sheets for automobiles provided by an embodiment of the present application.

[0038] As Figure 1 shown, the present application provides a method for improving the quality of steel sheets for automobiles, and the method includes:

[0039] S1. Continuously cast the molten steel, and when the surface temperature of the slab formed by the continuous casting drops to the first set temperature, perform ultra-fast cooling on the slab to obtain a first slab with fine-grained structures on the upper and lower surfaces.

[0040] In some embodiments, the thickness of the slab is 130 mm to 300 mm, and the width is 1500 mm to 3000 mm.

[0041] In some embodiments, the first set temperature is 950°C to 1110°C.

[0042] In some embodiments, the ultra-rapid cooling uses water cooling. After the continuous casting billet is ultra-rapidly cooled, the method further includes:

[0043] When the surface temperature of the ultra-rapidly cooled continuous casting billet drops to the second set temperature, stop the water cooling to allow the continuous casting billet to cool by natural heat dissipation; the second set temperature ≤ 300°C.

[0044] In some embodiments, the cooling rate of the ultra-rapid cooling is 3.5°C / s to 8.5°C / s.

[0045] During continuous casting, in order to obtain a refined structure on the upper and lower surfaces of the continuous casting billet and avoid the precipitation of carbonitrides of microalloying elements at the phase boundaries, the continuous casting billet is rapidly cooled through the two-phase region temperature range (727°C to 850°C) of automotive steel and the temperature range (700°C to 900°C) where a large amount of carbonitrides precipitate by ultra-rapid cooling, so that the upper and lower surfaces of the continuous casting billet are cooled from 980°C ± 30°C to ≤ 300°C at the ultra-rapid cooling rate, and a continuous casting billet with a solidification structure having fine surface grains and good strength and toughness is obtained. Exemplarily, the first set temperature can be 950°C, 980°C, 1000°C, 1050°C, 1100°C, 1110°C, etc. The second set temperature can be 300°C, 290°C, 280°C, 270°C, 260°C, 250°C, etc. The cooling rate of ultra-rapid cooling can be 3.5°C / s, 4.5°C / s, 5.5°C / s, 6.5°C / s, 7.5°C / s, 8.5°C / s, etc.

[0046] In some embodiments, the steel passing rate of the continuous casting is 3.0 t / min to 9.5 t / min.

[0047] Limiting the steel passing rate of the continuous casting to 3.0 t / min to 9.5 t / min can ensure the stability and continuity of the continuous casting process, and at the same time avoid damage to the equipment caused by excessive load. Exemplarily, the steel passing rate of the continuous casting is 3.0 t / min, 4.0 t / min, 5.0 t / min, 6.0 t / min, 7.0 t / min, 8.0 t / min, 9.0 t / min, 9.5 t / min, etc.

[0048] In some embodiments, the fine-grained structure on the upper and lower surfaces of the first continuous casting billet is a fine bainite or martensite structure, and the existence regions of the fine-grained structure are respectively in the intervals of 20 mm to 60 mm from the upper and lower surfaces of the continuous casting billet.

[0049] The fine-grained structures on the upper and lower surfaces of the continuous casting billet are fine-grained bainite or martensite structures formed by ultra-fast cooling. Due to the rapid cooling, the carbonitride precipitates of micro-alloying elements change from a large amount of precipitation at the phase boundary to random and dispersed intragranular precipitation, refining the grains on the surface of the continuous casting billet. After the water cooling stops, the internal heat of the continuous casting billet returns to the surface, and the rapidly cooled structure on the surface of the continuous casting billet becomes a denser and tougher tempered structure. The region where this fine-grained layer exists is the inner arc surface and the outer arc surface of the continuous casting billet up to 20 mm to 60 mm from its surface.

[0050] S2. Heat the first continuous casting billet in stages to obtain a second continuous casting billet with a uniformly refined austenite structure on the surface.

[0051] In some embodiments, the atmosphere for the staged heating is an inert gas protection atmosphere, and the total in-furnace time for the staged heating is 130 min to 310 min. The step of heating the first continuous casting billet in stages includes:

[0052] Preheat the first continuous casting billet at a first set temperature and a first set holding time.

[0053] Heat the preheated first continuous casting billet for the first time at a second set temperature and a second set holding time.

[0054] Heat the first continuous casting billet after the first heating for the second time at a third set temperature and a third set holding time.

[0055] Perform homogenization heat treatment on the first continuous casting billet after the second heating at a fourth set temperature and a fourth set holding time.

[0056] In some embodiments, the first set temperature is 250°C to 350°C, and the first set holding time satisfies the following relationship: T1 = 0.22h - 2.3.

[0057] The second set temperature is 750°C to 850°C, and the second set holding time satisfies the following relationship: T2 = 0.271h + 5.2.

[0058] The third set temperature is 950°C to 1050°C, and the third set holding time satisfies the following relationship: T3 = 0.287h - 7.05.

[0059] The fourth set temperature is 1050°C to 1150°C, and the fourth set holding time satisfies the following relationship: T4 = 0.236h + 4.72.

[0060] Wherein, T1 represents the first set holding time, with the unit of min; T2 represents the second set holding time, with the unit of min; T3 represents the third set holding time, with the unit of min; T4 represents the fourth set holding time, with the unit of min; h represents the thickness of the first billet, with the unit of mm.

[0061] When heating a billet with a surface fine-grained structure in a heating furnace, an inert gas protective atmosphere is adopted. Adopting an inert gas protective atmosphere can effectively prevent the billet from oxidizing during the heating process, thereby protecting the surface fine-grained structure of the billet from being damaged. At the same time, the inert atmosphere also helps to reduce impurities and pollutants in the heating furnace, improving the heating quality and efficiency.

[0062] The principle of temperature setting for each stage of the heating furnace is that if the temperature is too high, the internal and external temperature difference is too large, which is likely to cause billet cracks; if the temperature is too low, it is difficult to reach the target preheating temperature within the corresponding time, increasing the heating load in the next stage.

[0063] The basis for time control of each stage is that if the heating time is too short, the billet is not heated thoroughly, and the internal and external temperature difference is large, which is likely to cause billet cracks during the rapid heating process in the next stage; if the heating time is too long, it will cause energy waste. Especially in the soaking stage, if the soaking time is too short, the internal and external temperatures of the billet are uneven; if the heating time is too long, it will cause energy waste and promote the growth of the fine grains on the upper and lower surfaces of the billet; similarly, if the soaking temperature is too high, it will also promote the growth of the fine grains on the billet surface; if the soaking temperature is too low, it is difficult to reach the target temperature within the specified time, and the alloying element dissolution effect is poor.

[0064] Exemplarily, the first set temperature can be 250°C, 270°C, 290°C, 300°C, 320°C, 340°C, 350°C, etc.; the second set temperature can be 750°C, 770°C, 790°C, 800°C, 820°C, 840°C, 850°C, etc.; the third set temperature can be 950°C, 970°C, 990°C, 1000°C, 1020°C, 1040°C, 1050°C, etc.; the fourth set temperature can be 1050°C, 1070°C, 1090°C, 1100°C, 1130°C, 1150°C, etc.

[0065] S3. Subject the second billet to high-pressure water descaling to obtain a third billet with a set surface bright red area;

[0066] In some embodiments, the descaling water pressure for the high-pressure water descaling > 15 MPa.

[0067] In some embodiments, the set surface bright red area > 98%.

[0068] During the descaling process, a large water pressure is selected to quickly and completely remove the scale on the surface of the continuous casting billet, so as to ensure that no scale is pressed into the steel plate during the rolling process, which may affect the quality of the steel plate. It is specified that the set surface bright red area > 98%, which can ensure that the surface of the continuous casting billet is fully oxidized during the heating process, forming a uniform and dense oxide layer, which is helpful for the surface quality of the steel plate during the subsequent rolling process. Exemplarily, the descaling water pressure for high-pressure water descaling can be 15.1 MPa, 16 MPa, 17 MPa, 17.5 MPa, 18 MPa, 19 MPa, etc. The surface bright red area of the third continuous casting billet can be 98.1%, 98.5%, 99%, 99.2%, 99.5%, 99.7%, etc.

[0069] In some embodiments, the surface temperature of the third continuous casting billet is 1030 - 1130 °C.

[0070] S4. Roll the third continuous casting billet to obtain the steel plate for automobiles.

[0071] After descaling, the continuous casting billet is processed by hot rolling, coiling, pickling, cold rolling, annealing and skin pass rolling according to the conventional process to obtain the steel plate for automobiles.

[0072] In summary, the method for improving the quality of the steel plate for automobiles provided by the embodiments of the present application has the following significant advantages:

[0073] (1) Refine the surface structure: Through the ultra-fast cooling treatment after continuous casting, fine bainite or martensite structures are formed on the upper and lower surfaces of the continuous casting billet. This fine grain layer can significantly improve the strength and toughness of the steel plate. At the same time, the ultra-fast cooling rate effectively inhibits the precipitation of micro-alloying element carbonitrides at the phase boundaries and promotes their dispersed precipitation within the grains, further refining the surface grains.

[0074] (2) Optimize the heating process: The staged heating strategy ensures a uniform distribution of the internal temperature of the continuous casting billet, avoiding crack problems caused by excessive internal and external temperature differences. At the same time, the inert gas protection atmosphere effectively prevents the oxidation of the continuous casting billet during the heating process, protecting the fine grain structure on the surface from being damaged and reducing impurities and pollutants in the heating furnace. In addition, precise control of the holding time not only ensures the full heating of the continuous casting billet but also avoids energy waste and the growth of fine grains.

[0075] (3) Efficient descaling: High-pressure water descaling can quickly and thoroughly remove the scale on the surface of the continuous casting billet, ensuring that no scale is pressed into the steel plate during the rolling process, thus guaranteeing the surface quality of the steel plate. At the same time, setting the surface bright red area > 98% ensures that the surface of the continuous casting billet is fully oxidized during the heating process, forming a uniform and dense oxide layer, providing a good foundation for subsequent rolling.

[0076] (4) Improve the quality of steel plates: By strongly cooling the upper and lower surfaces of the continuous casting billet during the continuous casting process, a refined and tough solidification structure is obtained, and the refined structure is genetically continued to the steel plate products for automobiles, thereby improving the performance of the steel plates for automobiles. This method is applicable to the production of steel plates with various thicknesses and widths, and has high flexibility and adaptability.

[0077] The following will further elaborate on this application in combination with specific embodiments. It should be understood that these embodiments are only used to illustrate this application and not to limit the scope of this application. The experimental methods without specific conditions noted in the following embodiments are generally determined according to industry standards. If there is no corresponding industry standard, they are carried out according to general international standards, conventional conditions, or the conditions recommended by the manufacturer.

[0078] Example 1

[0079] The thickness of the steel plate for automobiles is 0.7 mm, and the dimensions of the continuous casting billet (thickness × width) are: 130 mm × 1800 mm.

[0080] The molten steel after smelting is continuously cast. The steel passing rate during the continuous casting process is 3.2 t / min. When the surface temperature of the billet drops to 960 °C, the water-cooling rate is increased to 4.1 °C / s. When the surface temperature of the billet drops to 289 °C, the water-cooling is stopped. The high temperature at the core of the billet diffuses inward and outward arc surfaces, and a refined tempered martensite structure is formed within a range of 39 mm from the inner and outer arc surfaces of the billet.

[0081] The billet with a surface fine-grained structure is heated in a heating furnace. The atmosphere in the heating furnace is nitrogen protection. The residence time in the preheating section is 27 min, and the temperature is 300 °C; the residence time in the first heating section is 41 min, and the temperature is 800 °C; the residence time in the second heating section is 31 min, and the temperature is 1000 °C; the residence time in the soaking section is 36 min, and the temperature is 1050 °C. The total residence time in the furnace is 135 min.

[0082] The water pressure during the descaling process is 18 MPa, and the bright red surface area of the billet surface after descaling is greater than 99%.

[0083] After descaling, the billet is processed by conventional rolling and other processes to obtain the steel plate for automobiles, and its performance is shown in the following table.

[0084] Example 2

[0085] The thickness of the steel plate for automobiles is 0.8 mm, and the dimensions of the continuous casting billet (thickness × width) are: 180 mm × 2000 mm.

[0086] After the molten steel with qualified smelting components undergoes continuous casting, the steel flow rate during continuous casting is 4.1 t / min. When the surface temperature of the billet drops to 980 °C, the water-cooling rate is increased to 5.3 °C / s. When the surface temperature of the billet drops to 279 °C, the water-cooling is stopped. The high temperature at the core of the billet diffuses towards the inner and outer arc surfaces, and a refined tempered martensite structure is formed within the range of 42 mm from the upper and lower surfaces of the billet.

[0087] The billet with a surface fine-grained structure is heated in a heating furnace, and the atmosphere in the heating furnace is nitrogen protection. The residence time in the preheating section is 38 min, and the temperature is 310 °C; the residence time in the first heating section is 55 min, and the temperature is 810 °C; the residence time in the second heating section is 46 min, and the temperature is 1000 °C; the residence time in the soaking section is 48 min, and the temperature is 1050 °C. The total residence time in the furnace is 187 min.

[0088] During the descaling process, the water pressure is 18 MPa. After descaling, the bright red surface area of the billet is greater than 99%.

[0089] After descaling, the billet is processed by conventional rolling and other processes to obtain a steel plate for automobiles, and its properties are shown in the following table.

[0090] Example 3

[0091] The thickness of the steel plate for automobiles is 0.9 mm, and the dimensions of the continuous casting billet (thickness × width) are: 250 mm × 2000 mm.

[0092] After the molten steel with qualified smelting components undergoes continuous casting, the steel flow rate during continuous casting is 7.7 t / min. When the surface temperature of the billet drops to 967 °C, the water-cooling rate is increased to 6.0 °C / s. When the surface temperature of the billet drops to 262 °C, the water-cooling is stopped. The high temperature at the core of the billet diffuses towards the inner and outer arc surfaces, and a refined tempered martensite structure is formed within the range of 43 mm from the upper and lower surfaces of the billet.

[0093] The billet with a surface fine-grained structure is heated in a heating furnace, and the atmosphere in the heating furnace is nitrogen protection. The residence time in the preheating section is 54 min, and the temperature is 310 °C; the residence time in the first heating section is 74 min, and the temperature is 810 °C; the residence time in the second heating section is 65 min, and the temperature is 1000 °C; the residence time in the soaking section is 65 min, and the temperature is 1050 °C. The total residence time in the furnace is 258 min.

[0094] During the descaling process, the water pressure is 18 MPa. After descaling, the bright red surface area of the billet is greater than 99%.

[0095] After descaling, the billet is processed by conventional rolling and other processes to obtain a steel plate for automobiles, and its properties are shown in the following table.

[0096] Example 4

[0097] The thickness of the steel plate for automobiles is 1.0 mm, and the dimensions of the continuous casting billet (thickness × width) are 300 mm × 3000 mm.

[0098] After the molten steel with qualified smelting components is continuously cast, the steel flow rate during continuous casting is 9.2 t / min. When the surface temperature of the billet drops to 961 °C, the water cooling rate is increased to 8.3 °C / s. When the surface temperature of the billet drops to 254 °C, the water cooling is stopped. The high temperature in the core of the billet diffuses to the inner and outer arc surfaces, and a refined tempered martensite structure is formed within a range of 59 mm from the upper and lower surfaces of the billet.

[0099] The billet with a surface fine grain structure is heated in a heating furnace, and the atmosphere in the heating furnace is nitrogen protection. The residence time in the preheating section is 64 min, and the temperature is 310 °C; the residence time in the first heating section is 87 min, and the temperature is 810 °C; the residence time in the second heating section is 80 min, and the temperature is 1000 °C; the residence time in the soaking section is 76 min, and the temperature is 1050 °C. The total residence time in the furnace is 307 min.

[0100] During the descaling process, the water pressure is 18 MPa, and the bright red surface area of the billet after descaling is greater than 99%.

[0101] After descaling, the billet is processed by conventional rolling and other processes to obtain the steel plate for automobiles, and its properties are shown in the following table.

[0102] Comparative Example 1

[0103] Comparative Example 1 provides a method for producing automotive sheets. Referring to Example 2, the difference between Comparative Example 1 and Example 2 is that after the billet enters the heating furnace, the temperature of the heating furnace is 1200 °C, and the residence time in the furnace is 240 min, and the control of other production processes is the same as that in Example 2.

[0104] Comparative Example 2

[0105] Comparative Example 2 provides a method for producing automotive sheets. Referring to Example 3, the difference between Comparative Example 2 and Example 3 is that during continuous casting, cooling is carried out according to the traditional cooling method, without ultra-fast cooling, and there is no fine grain layer on the upper and lower surfaces of the billet, and the control of other production processes is the same as that in Example 3.

[0106] The properties of the steel plates for automobiles obtained in Examples 1 to 4 and Comparative Examples 1 to 2 were measured, and the results are shown in Table 1.

[0107] Table 1 Performance results of the steel plates for automobiles obtained in Examples 1 to 4 and Comparative Examples 1 to 2

[0108] Number Grain size Yield strength, MPa Tensile strength, MPa Elongation, A80% n value r value Example 1 Grade 12 356 482 36.2 0.25 2.23 Example 2 Grade 13 363 479 36.7 0.25 2.22 Example 3 Grade 12 367 497 37.1 0.26 2.21 Example 4 Grade 14 371 499 38.3 0.25 2.22 Comparative Example 1 Grade 10 335 439 33.8 0.19 1.86 Comparative Example 2 Grade 10 332 433 34.4 0.21 1.91

[0109] As can be seen from Table 1, the properties of the steel sheets for automobiles obtained in Examples 1 to 4 are stable, with a grain size ≥ 12 grades, a yield strength ≥ 350 MPa, a tensile strength ≥ 475 MPa, an elongation A80 ≥ 36%, an n value ≥ 0.25, and an r value ≥ 2.20. The properties of the steel sheets for automobiles in Comparative Examples 1 to 2 are all lower than those in the examples.

[0110] In addition, one or more technical solutions in the embodiments of the present application at least further have the following technical effects or advantages:

[0111] In the embodiments of the present application, by intensively cooling the upper and lower surfaces of the continuous casting billet during continuous casting, a refined and tough solidification structure is obtained, and the refined structure is genetically continued to the steel sheet product for automobiles, thereby improving the properties of the steel sheet for automobiles.

[0112] In the embodiments of the present application, the problem of unstable quality of the steel sheet for automobiles caused by high nitrogen and residual element contents in the steel can be solved, including but not limited to those caused by the increase in nitrogen and residual element contents due to a high scrap steel ratio.

[0113] In the embodiments of the present application, under the condition of not increasing the alloy cost and maintaining the stability of the original rolling process, the properties of the steel sheet for automobiles are stably improved. The obtained steel sheet for automobiles has a grain size ≥ 12 grades, a yield strength ≥ 350 MPa, a tensile strength ≥ 475 MPa, an elongation A80 ≥ 36%, an n value ≥ 0.25, and an r value ≥ 2.20.

[0114] The above are only specific implementation manners of the present application, enabling those skilled in the art to understand or implement the present application. Various modifications to these embodiments will be obvious to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to these embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for improving the quality of automobile steel sheets, the method comprising: Continuously casting the molten steel, and when the surface temperature of the ingot formed by the continuous casting drops to a first set temperature, ultra-fast cooling the ingot is performed to obtain a first ingot with fine-grained structures on the upper and lower surfaces; The first cast billet is heated in stages to obtain a second cast billet with a uniform and refined austenite structure on the surface; Descaling the second ingot with high-pressure water to obtain a third ingot with a set surface red bright area; The third cast slab is rolled to obtain the automobile steel sheet.

2. The method according to claim 1, characterized in that The first set temperature is 950°C to 1110°C.

3. The method according to claim 1, characterized in that The ultra-fast cooling is water cooling. After the ingot is ultra-fast cooled, the method further comprises: When the surface temperature of the ultra-fast cooled ingot drops to a second set temperature, water cooling is stopped to allow the ingot to dissipate heat naturally; the second set temperature is ≤300°C.

4. The method according to claim 3, characterized in that The cooling rate of the ultrafast cooling is 3.5°C / s to 8.5°C / s.

5. The method according to claim 1, characterized in that The steel throughput of the continuous casting is 3.0 t / min to 9.5 t / min.

6. The method according to claim 1, characterized in that The fine grain structure of the upper and lower surface layers of the first ingot is fine bainite or martensite structure, and the existence area of ​​the fine grain structure is respectively within the range of 20 mm to 60 mm from the upper and lower surfaces of the ingot.

7. The method according to claim 1, characterized in that The atmosphere of the staged heating is an inert gas protective atmosphere, the total furnace time of the staged heating is 130 minutes to 310 minutes, and the staged heating of the first casting comprises: preheating the first cast slab to a first set temperature and a first set holding time; The preheated first casting billet is subjected to a first heating with a second set temperature and a second set holding time; Performing a second heating on the first cast billet after the first heating with a third set temperature and a third set holding time; The first cast slab after the second heating is subjected to a soaking treatment with a fourth set temperature and a fourth set holding time.

8. The method according to claim 1, characterized in that The first set temperature is 250° C. to 350° C., and the first set insulation time satisfies the following relationship: T1=0.22h-2.3; The second set temperature is 750° C. to 850° C., and the second set holding time satisfies the following relationship: T2 = 0.271h + 5.2; The third set temperature is 950°C to 1050°C, and the third set holding time satisfies the following relationship: T3 = 0.287h-7.05; The fourth set temperature is 1050° C. to 1150° C., and the fourth set holding time satisfies the following relationship: T4 = 0.236h + 4.72; In the formula, T1 represents the first set insulation time, in min; T2 represents the second set insulation time, in min; T3 represents the third set insulation time, in min; T4 represents the fourth set insulation time, in min; h represents the thickness of the first ingot, in mm.

9. The method according to claim 1, characterized in that: The descaling water pressure of the high-pressure water descaling is greater than 15 MPa.

10. The method according to claim 1, characterized in that The set surface red bright area is >98%.

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