Preparation method of 1500MPa-grade cold-rolled dual-phase steel with flat plate shape

Abstract

The invention relates to a preparation method of 1500MPa-grade cold-rolled dual-phase steel with a flat plate shape, and belongs to the technical field of steel manufacturing. The method comprises the following steps: obtaining molten steel with set chemical components; the molten steel is sequentially subjected to continuous casting, hot rolling and acid pickling, and an acid pickling raw material coil is obtained; the acid pickling raw material roll is subjected to cover annealing treatment, the cover annealing treatment comprises heating, soaking, first cooling, second cooling and third cooling, and a softened raw material roll is obtained; cold rolling is conducted on the softened raw material coil, the total reduction rate and rolling pass of cold rolling are controlled, so that single-pass cold deformation is reduced, and edge cracks and plate shape mutation are avoided; the softened raw material roll subjected to cold rolling is subjected to continuous annealing treatment, the 1500 MPa-grade cold-rolled dual-phase steel with the flat plate shape is obtained, and the continuous annealing treatment comprises two-stage heating and two-stage cooling. Through the process optimization, the problem that the 1500MPa-grade cold-rolled dual-phase steel plate is poor in shape is successfully solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of steel manufacturing, and particularly relates to a preparation method of 1500MPa-grade cold-rolled dual-phase steel with flat plate shape. BACKGROUND

[0002] With the intensification of global environmental challenges and the rapid development of the automobile industry, the automobile industry is facing the transformation pressure of "low carbonization, intelligentization, informatization and networking". Under such a background, the automobile lightweight technology has become the key way to achieve energy saving and emission reduction. Consumers and manufacturers are increasingly concerned about the safety, fuel economy and environmental performance of automobiles. Research shows that the fuel efficiency can be improved by 6% to 8% and the carbon dioxide emission can be reduced by about 5g per kilometer when the weight of the whole vehicle is reduced by 10%. Therefore, the key direction of the development of the automobile industry has become to achieve the thinning and weight reduction of the vehicle body by improving the strength of the steel.

[0003] At present, 1200MPa-grade dual-phase steel has been widely used in the body-in-white parts, and its production process and supporting control technology are relatively mature. However, the 1500MPa-grade dual-phase steel has poor rolling stability in the cold rolling process due to its high strength characteristics, which causes the problems of thickness fluctuation and plate shape mutation of the steel plate. In addition, it has higher requirements for the cooling control technology of the annealing production line, and the overall production is difficult and the yield is low, which to some extent restricts the sustainable and stable development of the automobile industry. Therefore, it is urgent to further improve the 1500MPa-grade cold-rolled dual-phase steel to optimize its flatness, so as to facilitate the use of downstream users. SUMMARY

[0004] The present application provides a preparation method of 1500MPa-grade cold-rolled dual-phase steel with flat plate shape to solve the technical problem of how to improve the plate shape of the existing 1500MPa-grade cold-rolled dual-phase steel.

[0005] The present application provides a preparation method of 1500MPa-grade cold-rolled dual-phase steel with flat plate shape, which comprises the following steps:

[0006] obtaining molten steel with a set chemical composition;

[0007] sequentially performing continuous casting, hot rolling and pickling on the molten steel to obtain a pickled raw material coil;

[0008] performing cover annealing treatment on the pickled raw material coil, wherein the cover annealing treatment comprises heating, soaking, first cooling, second cooling and third cooling, to obtain a softened raw material coil;

[0009] performing cold rolling on the softened raw material coil, and controlling the total reduction rate and rolling pass of the cold rolling to reduce the single-pass cold deformation and avoid edge cracks and plate shape mutation.

[0010] The softened raw material coil after cold rolling is subjected to continuous annealing treatment to obtain a 1500MPa grade cold rolled dual phase steel with flat plate shape, wherein the continuous annealing treatment comprises two-stage heating and two-stage cooling.

[0011] Optionally, the heating rate is 2-6℃ / s.

[0012] Optionally, the soaking temperature is 680-720℃, and the soaking time is 10-14h.

[0013] Optionally, the first cooling rate is 0.5-2℃ / s, and the end temperature of the first cooling is 500-600℃.

[0014] Optionally, the second cooling rate is 3-6℃ / s, and the end temperature of the second cooling is 240-280℃.

[0015] Optionally, the third cooling rate is 12-20℃ / s, and the end temperature of the third cooling is 40-70℃.

[0016] Optionally, the total reduction of the cold rolling is 41-57%, and the rolling pass is 5-7.

[0017] Optionally, the continuous annealing treatment comprises two-stage heating and two-stage cooling, comprising:

[0018] The softened raw material coil after cold rolling is heated to 700-780℃ at a heating rate of 8-12℃ / s, and then heated to 780-820℃ at a heating rate of 3-6℃ / s.

[0019] The heated softened raw material coil is cooled to 660-700℃ at a cooling rate of 6-10℃ / s, and then cooled to 120-180℃ at a cooling rate of 30-50℃ / s, using nitrogen-hydrogen mixed gas as the cooling medium.

[0020] Optionally, the volume fraction of hydrogen in the nitrogen-hydrogen mixed gas is 50-65%.

[0021] Optionally, the set chemical composition is: C: 0.12% to 0.22%, Mn: 2% to 2.6%, Si: 0.1% to 0.3%, Al: 0.02% to 0.06%, P≤0.012%, S≤0.004%, Cr: 0.4% to 0.7%, Mo: 0.05% to 0.15%, Nb: 0.01% to 0.04%, Ti: 0.02% to 0.04%, B: 0.001% to 0.003%, and the balance is Fe and inevitable impurities.

[0022] Optionally, the 1500MPa-grade cold-rolled dual-phase steel with flat shape satisfies: tensile strength≥1470MPa, yield strength≥1100MPa, and edge wave height<2mm.

[0023] Compared with the prior art, the above technical solution provided by the embodiments of the present application has the following advantages:

[0024] The embodiments of the present application provide a preparation method of 1500MPa-grade cold-rolled dual-phase steel with flat shape, the method comprising: obtaining molten steel with a set chemical composition; sequentially performing continuous casting, hot rolling and pickling on the molten steel to obtain a pickled raw material coil; performing cover annealing treatment on the pickled raw material coil, wherein the cover annealing treatment comprises heating, soaking, first cooling, second cooling and third cooling, to obtain a softened raw material coil; performing cold rolling on the softened raw material coil, and controlling the total reduction rate and rolling pass of the cold rolling to reduce single-pass cold deformation and avoid edge cracks and shape mutations; and performing continuous annealing treatment on the cold-rolled softened raw material coil to obtain 1500MPa-grade cold-rolled dual-phase steel with flat shape, wherein the continuous annealing treatment comprises two-stage heating and two-stage cooling. By combining the hot rolling-pickling-cover annealing-cold rolling-continuous annealing process on the basis of the steel material with the set chemical composition, the cover annealing controls the uniform organization performance of the three-stage cooling, the continuous annealing ensures the performance of the final product, the key control points such as the heating rate, soaking temperature and soaking time of the cover annealing, the terminal temperature and cooling rate of the three-stage cooling, the terminal temperature and heating rate of the two-stage heating of the continuous annealing, the terminal temperature and cooling rate of the two-stage cooling, and the furnace cooling gas medium are defined, the problem of poor shape of the cold-rolled 1500MPa-grade dual-phase steel plate is successfully solved, and high-strength steel material with excellent performance and flat shape is prepared. BRIEF DESCRIPTION OF DRAWINGS

[0025] The accompanying drawings, which are incorporated into and form a part of the specification, illustrate one embodiment consistent with the present application and, together with the description, serve to explain the principles of the application.

[0026] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the accompanying drawings needed to be used in the embodiments or prior art description will be briefly introduced as follows. Obviously, those skilled in the art can obtain other drawings according to these drawings without any creative effort.

[0027] Figure 1 A flowchart of a preparation method of a 1500MPa-grade cold-rolled dual-phase steel with flat plate shape provided by the embodiment of the present application is shown in the figure.

[0028] Figure 2 A 3D plate shape diagram provided by Comparative Example 1 of the present application is shown in the figure.

[0029] Figure 3 A 3D plate shape diagram provided by Example X of the present application is shown in the figure. DETAILED DESCRIPTION

[0030] In order to make the objectives, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments only represent a part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without any creative effort fall within the scope of protection of the present application.

[0031] The range descriptions described herein, such as numerical range, ratio range, etc., all include all possible sub-ranges within the range and single values, for example, the range description of "1 to 6" or "1-6" covers all sub-ranges (such as 1 to 3, 2 to 5, etc.) and single values (such as 1, 2, 3, 4, 5, 6) between 1 and 6. Unless otherwise explicitly stated, the terms "include", "contain" and the like in the present application should be understood as encompassing the meaning of "but not limited to". The relationship terms "first", "second", etc. are only used to distinguish different entities or operations, and do not imply actual sequence or relationship; "and / or" means that multiple cases can exist separately or simultaneously; "at least one", "multiple", "at least one kind" and the like refer to any combination of the corresponding objects, including single or multiple combinations of objects. The ratio relationships involved in the present application, such as mass ratio, molar ratio, etc., should be understood as the corresponding relationship between the front and rear items in the ratio according to the order of description. The raw materials, reagents, instruments and equipment used in the present application can be purchased or prepared by existing methods.

[0032] Figure 1 A flowchart of a preparation method of a 1500MPa-grade cold-rolled dual-phase steel with flat plate shape provided by the embodiment of the present application is shown in the figure.

[0033] As Figure 1As shown, the embodiment of the present application provides a preparation method of a 1500MPa grade cold rolled dual-phase steel with a flat plate shape, the method comprises:

[0034] S1, obtaining molten steel with a set chemical composition;

[0035] In some embodiments, the set chemical composition, in terms of mass fraction, is: C: 0.12% to 0.22%, Mn: 2% to 2.6%, Si: 0.1% to 0.3%, Al: 0.02% to 0.06%, P≤0.012%, S≤0.004%, Cr: 0.4% to 0.7%, Mo: 0.05% to 0.15%, Nb: 0.01% to 0.04%, Ti: 0.02% to 0.04%, B: 0.001% to 0.003%, and the balance being Fe and inevitable impurities.

[0036] The content of C (carbon) is controlled between 0.12% and 0.22%, and the carbon content in this range helps to improve the strength and hardness of the steel while maintaining good toughness and weldability. The content of Mn (manganese) is 2% to 2.6%, and manganese is an important element for strengthening steel, which can improve the hardenability and wear resistance of steel. The content of Si (silicon) is in the range of 0.1% to 0.3%, and appropriate silicon can improve the corrosion resistance and oxidation resistance of steel. The content of Al (aluminum) is controlled between 0.02% and 0.06%, which is mainly used for deoxidization and grain refinement of steel to improve the toughness of steel. The contents of P (phosphorus) and S (sulfur) are not more than 0.012% and 0.004% respectively, in order to reduce the brittleness and hot brittleness of the steel. The content of Cr (chromium) is 0.4% to 0.7%, and chromium can improve the corrosion resistance and oxidation resistance of steel, while enhancing the strength and hardness of steel. The content of Mo (molybdenum) is between 0.05% and 0.15%, and molybdenum can effectively improve the hardenability and hot strength of steel. The contents of Nb (niobium) and Ti (titanium) are 0.01% to 0.04% and 0.02% to 0.04% respectively, and these two elements are mainly used for grain refinement of steel to improve the strength and toughness of steel. The content of B (boron) is controlled between 0.001% and 0.003%, and boron can significantly improve the hardenability of steel, which helps to obtain high strength and high toughness steel. The balance is Fe (iron) and inevitable impurities, which ensures the basic composition and performance of the steel.

[0037] Fe is the base element, and the specific content / content range of Fe can be obtained by the upper and lower limit formula of the components, that is:

[0038] According to the patent examination guidelines, the sum of the content percentages of each component in a composition must be equal to 100%, and the content range of each component should meet certain conditions: the upper limit value of a certain component plus the lower limit value of other components should not exceed 100, and the lower limit value of a certain component plus the upper limit value of other components should not be less than 100.

[0039] S2, sequentially casting, hot rolling and pickling the molten steel to obtain a pickling raw coil;

[0040] In S2, the continuous casting is a key step of cooling and solidifying the high-temperature molten steel into a billet through a continuous casting machine, which ensures the stability of the internal organization and external size of the billet. Then, the hot rolling is to heat the billet obtained by continuous casting to an appropriate temperature and then pass through multiple passes on a rolling mill to roll it into a steel plate or strip with the required thickness and width. The hot rolling process not only improves the internal organizational structure of the steel, but also improves its mechanical properties. Finally, the pickling is to immerse the hot-rolled steel plate or strip into an acid solution to remove the surface scale and other impurities, obtaining a pickling raw coil with a smooth surface and good quality. This step is crucial for subsequent processing and use, as it ensures the surface quality and processing performance of the product.

[0041] S3, performing a bell annealing treatment on the pickling raw coil, wherein the bell annealing treatment comprises heating, soaking, first cooling, second cooling and third cooling, to obtain a softened raw coil;

[0042] In the bell annealing process, the heating stage is to heat the pickling raw coil to a certain temperature to eliminate internal residual stress and improve its microstructure. The soaking stage is to maintain the temperature for a period of time to ensure uniform internal temperature of the pickling raw coil, thereby further improving its organization and performance. The first cooling stage has a relatively slow cooling rate, which helps the gradual transformation of the steel strip organization. The second cooling stage has a moderate cooling rate, which promotes the transformation of the organization while ensuring the uniformity of the organization of the steel strip. Finally, the third cooling stage has a faster cooling rate, which can quickly reduce the temperature of the steel strip and lock the transformation of the organization. Through this series of complex cooling processes, the bell annealing treatment can accurately control the softening degree and microstructure of the pickling raw coil, thereby obtaining a softened raw coil with excellent flatness and flatness, laying a solid foundation for the subsequent cold rolling and dual-phase steel preparation process.

[0043] In some embodiments, the heating rate is 2-6 °C / s.

[0044] Controlling the heating rate in the range of 2-6 °C / s helps the uniform transformation of the steel organization during heating. If the heating rate is greater than 6 °C / s, it may lead to uneven transformation of the organization and local overheating; if the heating rate is less than 2 °C / s, it may prolong the production cycle and reduce the overall efficiency. Within the range of 2-6 °C / s, the internal regions of the steel can reach the required soaking temperature relatively synchronously, which is conducive to the uniform transformation of the organization in the subsequent cooling process. For example, the heating rate can be 2 °C / s, 3 °C / s, 4 °C / s, 5 °C / s, 6 °C / s, etc.

[0045] In some embodiments, the soaking temperature is 680-720℃, and the soaking time is 10-14h.

[0046] Setting the soaking temperature between 680-720℃ and maintaining for 10-14h helps to homogenize the microstructure of the strip steel, resulting in an ideal pearlite equiaxed microstructure. Through the soaking treatment, the hard phase martensite microstructure at the head and tail of the strip steel can be eliminated, reducing the internal stress caused by uneven microstructure transformation, thereby improving the overall performance of the steel plate. If the soaking temperature is too low, the uniformity of the head and tail microstructure may not be sufficient, which in turn affects the uniformity of the thickness, ultimately leading to poor plate shape. While a too high soaking temperature can cause the microstructure to be severely softened, the precipitated phase to re-melt, resulting in a significant decrease in the tensile strength of the finished steel. Therefore, the soaking temperature of 680-720℃ is the preferred range that takes into account microstructure uniformity and strength retention. After soaking at 680-720℃ for 10-14h, the plasticity and toughness of the steel are significantly improved, which not only benefits subsequent cold rolling and annealing processing, but also effectively improves the overall performance of the material. For example, the soaking temperature can be 680℃, 690℃, 700℃, 710℃, 720℃, etc. The soaking time can be 10h, 11h, 12h, 13h, 14h, etc.

[0047] In some embodiments, the first cooling has a cooling rate of 0.5-2℃ / s, and an end temperature of 500-600℃.

[0048] The first cooling has a relatively slow cooling rate of 0.5-2℃ / s, which helps to gradually transform the microstructure of the strip steel and control the phase transformation region mainly in the pearlite and bainite regions, avoiding uneven microstructure transformation caused by rapid cooling, thereby facilitating the reduction of internal stress and laying a good foundation for subsequent processing. For example, the first cooling rate can be 0.5℃ / s, 0.8℃ / s, 1.1℃ / s, 1.4℃ / s, 1.7℃ / s, 2℃ / s, etc. The end temperature of the first cooling can be 500℃, 520℃, 540℃, 560℃, 580℃, 600℃, etc.

[0049] In some embodiments, the second cooling has a cooling rate of 3-6℃ / s, and an end temperature of 240-280℃.

[0050] The cooling rate of the second-stage cooling is moderate (3-6 °C / s), which further promotes the phase transformation while ensuring the uniformity of the microstructure of the strip coil, and helps to reduce the deformation resistance in the cold rolling process and reduce the risk of shape mutation. For example, the cooling rate of the second-stage cooling can be 3 °C / s, 3.5 °C / s, 4 °C / s, 4.5 °C / s, 5 °C / s, 5.5 °C / s, 6 °C / s, etc. The end temperature of the second-stage cooling can be 240 °C, 250 °C, 260 °C, 270 °C, 280 °C, etc.

[0051] In some embodiments, the cooling rate of the third-stage cooling is 12-20 °C / s, and the end temperature of the third-stage cooling is 40-70 °C.

[0052] The cooling rate of the third-stage cooling is relatively fast (12-20 °C / s), which can rapidly reduce the temperature of the strip and lock the phase transformation, avoiding the deterioration of the microstructure and performance caused by overaging. At the same time, the cooling rate of 12-20 °C / s helps to refine the matrix microstructure and improve the strength and hardness of the steel. For example, the cooling rate of the third-stage cooling can be 12 °C / s, 13 °C / s, 14 °C / s, 15 °C / s, 16 °C / s, 17 °C / s, 18 °C / s, 19 °C / s, 20 °C / s, etc. The end temperature of the third-stage cooling can be 40 °C, 45 °C, 50 °C, 55 °C, 60 °C, 65 °C, 70 °C, etc.

[0053] In summary, the bell-type annealing process can promote the phase transformation in stages and ensure that the phase transformation in the microstructure is carried out in the ideal region, while uniformly optimizing the microstructure of the strip coil, reducing the deformation resistance in the cold rolling process, and thus significantly solving the shape mutation problem and improving the flatness of the finished steel coil.

[0054] S4, cold rolling the softened raw material coil, and controlling the total reduction rate and the rolling passes of the cold rolling to reduce the single-pass cold deformation and avoid edge cracks and shape mutation;

[0055] In some embodiments, the total reduction rate of the cold rolling is 41-57%, and the rolling passes of the cold rolling are 5-7 passes.

[0056] Reasonably distributing the total reduction rate to 5-7 passes can effectively reduce the deformation amount of each pass, thereby preventing edge cracks and shape mutation caused by excessive single-pass deformation, and thus improving the forming quality and surface finish of the steel strip. In addition, 5-7 rolling passes help the microstructure of the steel strip to gradually and uniformly transform during the rolling process, which is beneficial to refining the matrix microstructure, improving the strength and hardness of the steel, and reducing internal stress concentration, thereby improving the overall performance of the steel strip. For example, the total reduction rate of the cold rolling can be 41%, 43%, 45%, 47%, 49%, 51%, 53%, 55%, 57%, etc.

[0057] S5, continuously annealing the softened raw material coil after cold rolling to obtain a 1500MPa grade cold rolled dual phase steel with flat plate shape, wherein the continuous annealing process comprises two-stage heating and two-stage cooling.

[0058] In some embodiments, the continuous annealing process comprises two-stage heating and two-stage cooling, comprising:

[0059] heating the softened raw material coil after cold rolling to 700-780℃ at a heating rate of 8-12℃ / s, and then heating to 780-820℃ at a heating rate of 3-6℃ / s;

[0060] cooling the heated softened raw material coil to 660-700℃ at a cooling rate of 6-10℃ / s using nitrogen-hydrogen mixed gas as the cooling medium, and then cooling to 120-180℃ at a cooling rate of 30-50℃ / s.

[0061] The continuous annealing process achieves uniform austenitization of the strip's edge, middle and head / tail sections through two-stage heating. In the first stage, the softened raw material coil is heated to 700-780℃ at a heating rate of 8-12℃ / s, which helps to quickly reach the austenitization start temperature and promote microstructure transformation. The choice of heating rate is crucial, as too fast can cause stress concentration, while too slow can prolong the production cycle and increase costs. The temperature range of 700-780℃ ensures preliminary transformation of the internal microstructure of the strip without causing excessive oxidation or grain coarsening.

[0062] Next, the strip is heated to 780-820℃ at a heating rate of 3-6℃ / s. The higher heating temperature and appropriate heating rate in this stage help to further control the uniformity of austenitization and avoid uneven microstructure transformation caused by local overheating.

[0063] In the cooling stage, the heated softened raw material coil is first cooled to 660-700℃ at a cooling rate of 6-10℃ / s using nitrogen-hydrogen mixed gas as the cooling medium. The use of nitrogen-hydrogen mixed gas helps to control the atmosphere during cooling, preventing oxidation of the strip while ensuring uniform cooling rate. This stage mainly regulates the proportion of ferrite microstructure, ensuring the optimal balance of ferrite and martensite content in the steel, thereby affecting the final mechanical properties of the steel.

[0064] Subsequently, the steel strip is cooled to 120-180°C at a rapid cooling rate of 30-50°C / s. The end temperature of this stage is lower than the Ms point and the cooling rate is higher than the critical cooling rate, which ensures that the austenite in the structure is completely transformed into martensite, which is crucial for ensuring the tensile strength of the steel. At the same time, the high-speed cooling (30-50°C / s) in the second stage can quickly fix the microstructure of the steel and reduce the plate shape defects caused by uneven cooling.

[0065] In some embodiments, the volume fraction of hydrogen in the nitrogen-hydrogen mixed gas is 50-65%.

[0066] During the cooling process, the cooling medium in the furnace is a nitrogen-hydrogen mixed gas, which has a more moderate and controllable cooling rate compared to traditional water quenching. Hydrogen, as part of the cooling medium, has a high thermal conductivity that helps to quickly transfer heat, while nitrogen serves to regulate the cooling rate and stabilize the atmosphere. By adjusting the volume fraction of hydrogen, the temperature gradient during the cooling process can be precisely controlled, reducing the problem of uneven microstructure transformation and plate shape mutation caused by uneven cooling. In addition, the presence of hydrogen helps to improve the surface quality of the steel, reducing the problem of oxidation scale and rust that may occur during the cooling process, and optimizing the volume fraction of hydrogen can ensure cooling efficiency and significantly improve the quality of the finished steel product, meeting the needs of high-precision applications. For example, the volume fraction of hydrogen in the nitrogen-hydrogen mixed gas can be 50%, 53%, 56%, 59%, 62%, 65%, etc.

[0067] In some embodiments, the flatness-optimized 1500MPa-grade cold-rolled dual-phase steel satisfies: tensile strength ≥1470MPa, yield strength ≥1100MPa, and edge wave height <2mm.

[0068] In the embodiments of the present application, based on the set chemical composition of the steel, a hot rolling-pickling-bell annealing-cold rolling-continuous annealing process is combined to control the uniform microstructure performance of the three-stage cooling of the bell annealing and ensure the final product performance of the two-stage heating and two-stage cooling of the continuous annealing. The key control points such as the heating rate, soaking temperature and soaking time of the bell annealing, the end temperature and cooling rate of the three-stage cooling, the end temperature and heating rate of the two-stage heating of the continuous annealing, the end temperature and cooling rate of the two-stage cooling, and the cooling gas medium of the furnace zone are defined to produce the flatness-optimized 1500MPa-grade cold-rolled dual-phase steel, which has a tensile strength ≥1470MPa, a yield strength ≥1100MPa, and an edge wave height <2mm.

[0069] The application will be further described in conjunction with specific examples. The experimental methods in the following examples, if no specific conditions are noted, are generally determined according to national standards / industry standards; if there are no corresponding national standards / industry standards, the methods are determined according to general international standards, conventional conditions or the conditions suggested by the manufacturers.

[0070] Example 1

[0071] The cold-rolled steel coil has a specification of 1.4x1014mm, and the chemical composition of the steel coil is as follows in mass fraction: C: 0.125%, Mn: 2.44%, Si: 0.113%, Al: 0.033%, P: 0.01%, S: 0.0026%, Cr: 0.55%, Mo: 0.11%, Nb: 0.02%, Ti: 0.021%, B: 0.0024%, the cover annealing heating rate is 3.6°C / s, the soaking temperature is 714°C, the soaking time is 12.6h, the first-stage cooling end temperature is 554°C, the cooling rate is 1.7°C / s, the second-stage cooling end temperature is 267°C, the cooling rate is 4.2°C / s, the third-stage cooling end temperature is 47°C, and the cooling rate is 18.9°C / s; the cold-rolling is 5 passes with a reduction of 46%; the continuous annealing has a first-stage heating end temperature of 727°C and a heating rate of 9.6°C / s, a second-stage heating end temperature of 802°C and a heating rate of 3.2°C / s, a first-stage cooling end temperature of 685°C and a cooling rate of 7.9°C / s, and a second-stage cooling end temperature of 154°C and a cooling rate of 42°C / s; the furnace zone cooling medium is a nitrogen-hydrogen mixed gas with a hydrogen content of 57% in the mixed gas.

[0072] The obtained steel strip is subjected to mechanical property detection: tensile strength 1532MPa, yield strength 1167MPa, and elongation 6% at a gauge length of 80mm. The finished steel coil has a side wave height of 1.5mm and a middle wave height of 1mm.

[0073] Example 2

[0074] The cold-rolled steel coil has a specification of 1.15x1202mm, and the chemical composition of the steel coil is as follows in mass fraction: C: 0.188%, Mn: 2.57%, Si: 0.23%, Al: 0.027%, P: 0.009%, S: 0.0018%, Cr: 0.6%, Mo: 0.14%, Nb: 0.035%, Ti: 0.025%, B: 0.0018%, the cover annealing heating rate is 2.2°C / s, the soaking temperature is 691°C, the soaking time is 13.8h, the first stage cooling end temperature is 568°C, the cooling rate is 1.8°C / s, the second stage cooling end temperature is 244°C, the cooling rate is 5.1°C / s, the third stage cooling end temperature is 68°C, and the cooling rate is 15.4°C / s; the cold-rolling is 5 times with a reduction of 51%; the continuous annealing first stage heating end temperature is 767°C, the heating rate is 11.8°C / s, the second stage heating end temperature is 815°C, the heating rate is 3.9°C / s, the first stage cooling end temperature is 678°C, the cooling rate is 6.4°C / s, the second stage cooling end temperature is 143°C, the cooling rate is 48°C / s, and the furnace zone cooling medium is nitrogen-hydrogen mixed gas with a hydrogen content of 65% in the mixed gas.

[0075] The obtained strip steel is subjected to mechanical property detection: tensile strength 1518MPa, yield strength 1132MPa, and elongation 7% at a gauge length of 80mm. The finished steel coil has an edge wave height of 2mm and a middle wave height of 1mm.

[0076] Example 3

[0077] The cold-rolled steel coil has a specification of 1.6x1254mm, and the chemical composition of the steel coil is as follows in mass fraction: C: 0.215%, Mn: 2.34%, Si: 0.116%, Al: 0.047%, P: 0.008%, S: 0.0037%, Cr: 0.43%, Mo: 0.12%, Nb: 0.03%, Ti: 0.03%, B: 0.0018%, the cover annealing heating rate is 3.4°C / s, the soaking temperature is 708°C, the soaking time is 10.8h, the first stage cooling end temperature is 534°C, the cooling rate is 1.9°C / s, the second stage cooling end temperature is 245°C, the cooling rate is 5.2°C / s, the third stage cooling end temperature is 63°C, and the cooling rate is 15.4°C / s; the cold-rolling is 7 times with a reduction of 57%; the continuous annealing first stage heating end temperature is 754°C, the heating rate is 10.5°C / s, the second stage heating end temperature is 819°C, the heating rate is 4.8°C / s, the first stage cooling end temperature is 690°C, the cooling rate is 8.1°C / s, the second stage cooling end temperature is 172°C, the cooling rate is 39°C / s, and the furnace zone cooling medium is nitrogen-hydrogen mixed gas with a hydrogen content of 61% in the mixed gas.

[0078] The obtained strip steel was subjected to mechanical property testing: tensile strength 1489MPa, yield strength 1122MPa, elongation at a gauge length of 80mm 7.5%. The finished steel had a wave height of 1.5mm at the edge and 0.5mm at the center.

[0079] Comparative Example 1

[0080] The cold-rolled steel coil has a specification of 1.4×10¹⁴ mm. The chemical composition of the coil, by mass fraction, is: C: 0.125%, Mn: 2.44%, Si: 0.113%, Al: 0.033%, P: 0.01%, S: 0.0026%, Cr: 0.55%, Mo: 0.11%, Nb: 0.02%, Ti: 0.021%, B: 0.0024%. The bell-type annealing heating rate is 3.6℃ / s, the soaking temperature is 714℃, and the soaking time is 12.6 h. The cooling rate is 22.5℃ / s, directly cooling to 47℃; the cold rolling has a reduction rate of 46% in 5 passes; the final temperature of the first stage of continuous annealing heating is 727℃ with a heating rate of 9.6℃ / s, the final temperature of the second stage heating is 802℃ with a heating rate of 3.2℃ / s, the final temperature of the first stage cooling is 685℃ with a cooling rate of 7.9℃ / s, the final temperature of the second stage cooling is 154℃ with a cooling rate of 42℃ / s, and the cooling medium in the furnace area is a nitrogen-hydrogen mixture with a hydrogen content of 57%.

[0081] The obtained strip steel was subjected to mechanical property testing: tensile strength 1548MPa, yield strength 1169MPa, elongation at a gauge length of 80mm 5.5%. The finished steel had a wave height of 6.5mm at the edge and 4mm at the center.

[0082] Through examples and comparative examples, it was found that the method provided in this application successfully prepared cold-rolled 1500MPa grade duplex steel with flat plate shape and edge wave height within 2mm.

[0083] Appendix Figures 2-3 Detailed explanation:

[0084] Figure 2 The 3D plate shape diagram provided for Comparative Example 1 of this application; such as Figure 2 As shown, the thickness at the strip head position fluctuates significantly along the length of the strip, with the maximum approaching 30 IU.

[0085] Figure 3 This is a 3D plate shape diagram provided in Embodiment 1 of this application; as follows: Figure 3 As shown, the thickness fluctuation at the strip head along the strip length is reduced, and the strip shape is flat.

[0086] One or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages:

[0087] The embodiment of the application improves the stability of the production process, reduces the waste rate, and is beneficial to large-scale industrial production by precisely controlling various process parameters.

[0088] The above description is merely that of the specific embodiments of the application, to enable a person skilled in the art to understand or implement the application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined in the application can be implemented in other embodiments without departing from the spirit or scope of the application. Accordingly, the application is not to be restricted to these embodiments shown but is to be accorded the broadest scope consistent with the principles and novel features disclosed in the application.

Claims

1. A method for preparing a flatness-ensured 1500 MPa-grade cold-rolled dual-phase steel, the method comprising: obtaining molten steel with a set chemical composition; subjecting the molten steel to continuous casting, hot rolling and pickling in sequence to obtain a pickled raw material coil; subjecting the pickled raw material coil to a bell annealing process, wherein the bell annealing process comprises heating, soaking, first cooling, second cooling and third cooling, to obtain a softened raw material coil; subjecting the softened raw material coil to cold rolling, and controlling the total reduction rate and rolling passes of the cold rolling to reduce single-pass cold deformation and avoid edge cracks and flatness mutation; subjecting the cold-rolled softened raw material coil to a continuous annealing process to obtain a flatness-ensured 1500 MPa-grade cold-rolled dual-phase steel, wherein the continuous annealing process comprises two-stage heating and two-stage cooling.

2. The method of claim 1, wherein, the heating has a heating rate of 2-6 ℃ / s; and / or the soaking has a temperature of 680-720 ℃ and a time of 10-14 h.

3. The method of claim 1, wherein, the first cooling has a cooling rate of 0.5-2 ℃ / s and a terminal temperature of 500-600 ℃.

4. The method of claim 1, wherein, the second cooling has a cooling rate of 3-6 ℃ / s and a terminal temperature of 240-280 ℃.

5. The method of claim 1, wherein, the third cooling has a cooling rate of 12-20 ℃ / s and a terminal temperature of 40-70 ℃.

6. The method of claim 1, wherein, the cold rolling has a total reduction rate of 41-57% and a rolling pass of 5-7 passes.

7. The method of claim 1, wherein, the continuous annealing process comprises two-stage heating and two-stage cooling, comprising: heating the cold-rolled softened raw material coil to 700-780 ℃ at a heating rate of 8-12 ℃ / s, and then heating to 780-820 ℃ at a heating rate of 3-6 ℃ / s; cooling the heated softened raw material coil to 660-700 ℃ at a cooling rate of 6-10 ℃ / s using nitrogen-hydrogen mixed gas as the cooling medium, and then cooling to 120-180 ℃ at a cooling rate of 30-50 ℃ / s.

8. The method of claim 7, wherein, In the nitrogen-hydrogen mixed gas, the volume fraction of hydrogen is 50-65%.

9. The method of claim 1, wherein, The set chemical composition comprises, by mass fraction, C: 0.12-0.22%, Mn: 2-2.6%, Si: 0.1-0.3%, Al: 0.02-0.06%, P≤0.012%, S≤0.004%, Cr: 0.4-0.7%, Mo: 0.05-0.15%, Nb: 0.01-0.04%, Ti: 0.02-0.04%, B: 0.001-0.003%, and the balance being Fe and unavoidable impurities.

10. The method of claim 1, wherein, The flatness-ensured 1500 MPa-grade cold-rolled dual-phase steel satisfies: tensile strength≥1470 MPa, yield strength≥1100 MPa, and edge wave height<2 mm.

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