High bendability 1300 mpa grade cold rolled martensitic steel and method of making

Abstract

This application relates to a high-flexural-performance 1300MPa grade cold-rolled martensitic steel and its preparation method. The method includes: obtaining a chilled sheet with a predetermined chemical composition; continuously annealing the chilled sheet to obtain the high-flexural-performance 1300MPa grade cold-rolled martensitic steel, including: first heating the chilled sheet to a first temperature, and pre-oxidizing the surface of the chilled sheet at the first temperature under a first dew point condition, and performing a first heat treatment at the first temperature to obtain a first annealed sheet; second heating the first annealed sheet to a second temperature, and performing a second heat treatment at the second temperature, followed by slow cooling and rapid cooling to obtain a second annealed sheet; third heating the second annealed sheet to a third temperature, and performing over-aging under a first predetermined time condition to obtain the high-flexural-performance 1300MPa grade cold-rolled martensitic steel. This application improves the bending performance of 1300MPa grade cold-rolled martensitic steel.

Description

Technical Field

[0001] This application relates to the field of steel preparation technology, and in particular to a high bending performance 1300MPa grade cold-rolled martensitic steel and its preparation method. Background Technology

[0002] With the rapid development of the global automotive industry, lightweighting is of great significance for energy conservation and emission reduction in automobiles. Furthermore, for safety reasons, the automotive industry uses a large amount of high-strength steel. Cold-rolled high-strength steel mainly includes DP steel, DH steel, TRIP steel, CP steel, and MS steel, which are mainly achieved by adding alloying elements, increasing rapid cooling speed, and utilizing phase transformation strengthening. Cold-rolled martensitic steel is currently the highest commercially available advanced high-strength cold-rolled steel, ranging from a minimum of 900MPa to a maximum of 1700MPa. It possesses ultra-high strength and excellent bending properties and is commonly used in automotive safety components such as anti-collision beams and bumpers. In recent years, the proportion of high-strength steel used in automobiles has been increasing year by year, and the application of thin-gauge high-strength martensitic steel products has provided the automotive industry with ample opportunities for lightweighting, reducing energy consumption, and improving fuel economy.

[0003] Cold-rolled martensitic steel is usually rolled into corresponding parts with relatively simple cross-sections, but it has high requirements for bending performance. Therefore, whether it can have good bending performance is the key to whether martensitic steel can be promoted and applied in large quantities. Summary of the Invention

[0004] This application provides a high-bending-performance 1300MPa grade cold-rolled martensitic steel and its preparation method to solve the following technical problem: how to improve the bending performance of 1300MPa grade cold-rolled martensitic steel.

[0005] In a first aspect, this application provides a method for preparing a 1300MPa grade cold-rolled martensitic steel with high bending performance, the method comprising:

[0006] A cold-hardened plate with a set chemical composition is obtained;

[0007] The cold-rolled martensitic steel with high bending performance of 1300MPa was obtained by continuous annealing of the cold-rolled sheet.

[0008] The process of continuously annealing the cold-rolled sheet to obtain a high-flexural-performance 1300MPa grade cold-rolled martensitic steel includes:

[0009] The cold-hardened plate is first heated to a first temperature, and the surface of the cold-hardened plate at the first temperature is pre-oxidized under the condition of a first dew point, and then subjected to a first heat preservation under the condition of the first temperature to obtain a first annealed plate.

[0010] The first annealed plate is heated to a second temperature and held at the second temperature for a second time, followed by slow cooling and rapid cooling to obtain the second annealed plate.

[0011] The second annealed plate is heated to a third temperature and aged under the condition of a first set time to obtain a cold-rolled martensitic steel with high bending performance of 1300MPa.

[0012] Optionally, the first temperature is 640℃~680℃, the first heat preservation time is 30s~180s, and the first dew point is (-30)℃~(-20)℃.

[0013] Optionally, the second temperature is (Ac3+10)℃~(Ac3+30)℃, and the second heat preservation time is 180s~360s.

[0014] Optionally, the third temperature is 200℃~280℃, and the first set time is 300s~600s.

[0015] Optionally, the slow cooling process parameters include: a cooling rate of 1℃ / s to 15℃ / s, and an endpoint temperature of 700℃ to 800℃; and / or,

[0016] The rapid cooling process parameters include: the cooling medium is a gas containing 50% hydrogen by volume, and the cooling rate is...

[0017] ≥45℃ / s, with an endpoint temperature of (Ms-20)℃~(Ms-10)℃.

[0018] Optionally, the heating rate of the first heating is 2℃ / s to 5℃ / s; and / or,

[0019] The second heating rate is 3°C / s to 8°C / s; and / or,

[0020] The heating rate of the third heating is ≥2℃ / s.

[0021] Optionally, the specified chemical composition includes:

[0022] C, Si, Mn, P, S, Cr, B, N, and Fe; where, by mass fraction,

[0023] The content of C is 0.12%–0.18%, the content of Si is 0.2%–0.8%, the content of Mn is 1.5%–2.5%, the content of P is ≤0.015%, the content of S is ≤0.008%, the content of Cr is 0.3%–0.6%, the content of B is 0.001%–0.002%, and the content of N is 0.003%–0.005%.

[0024] It also includes at least one of the following chemical components: Nb, Ti, V; wherein,

[0025] The Nb content is 0.01%–0.04%, the Ti content is 0.01%–0.04%, and the V content is 0.01%–0.04%.

[0026] Optionally, obtaining a cold-hardened plate with a set chemical composition includes:

[0027] Molten steel is continuously cast to obtain slabs;

[0028] A slab with the specified chemical composition is heated and rolled, and then coiled to obtain a hot-rolled coil.

[0029] The hot-rolled coil is cold-rolled to obtain a chilled rigid sheet with a specified chemical composition; wherein,

[0030] The continuous casting speed is 4 m / min to 7 m / min, and the slab thickness is 110 mm to 125 mm; and / or,

[0031] The heating temperature is 1120℃~1220℃; and / or,

[0032] The final rolling temperature is 890℃~920℃; and / or,

[0033] The winding temperature is 550℃~600℃.

[0034] Secondly, this application provides a high-bending-performance 1300MPa grade cold-rolled martensitic steel prepared by the method described in the first aspect.

[0035] Optionally, the cold-rolled martensitic steel meets the following mechanical properties: tensile strength > 1300 MPa, yield strength 1030 MPa~1300 MPa, elongation A80 ≥ 3%, and bending performance meets the requirement of not cracking when bent 180° for 2t parallel to the rolling direction.

[0036] The microstructure of the cold-rolled martensitic steel comprises lath martensite and ferrite; wherein,

[0037] The volume fraction of the lath martensite is 92%–96%, and the volume fraction of the ferrite is 4%–8%.

[0038] The technical solutions provided in this application have the following advantages compared with the prior art:

[0039] The method for preparing the high bending performance 1300MPa grade cold-rolled martensitic steel provided in this application includes: obtaining a cold-hardened sheet with a set chemical composition; continuously annealing the cold-hardened sheet, including: first heating the cold-hardened sheet to a first temperature, and pre-oxidizing the surface layer of the cold-hardened sheet at the first temperature under a first dew point condition, and performing a first heat preservation under the first temperature condition to obtain a first annealed sheet; second heating the first annealed sheet to a second temperature, and performing a second heat preservation under the second temperature condition, followed by slow cooling and rapid cooling to obtain a second annealed sheet; third heating the second annealed sheet to a third temperature, and performing over-aging under a first set time condition to obtain the high bending performance 1300MPa grade cold-rolled martensitic steel. A cold-hardened steel sheet with a predetermined chemical composition is first heated to a first temperature and then held at that temperature for a first time. This allows for sufficient tempering of the sheet, resulting in a uniform microstructure. Consequently, ferrite grains in different parts of the sheet undergo sufficient recrystallization, reducing grain size differences. Simultaneously, martensite undergoes sufficient tempering, transforming into equiaxed ferrite and granular cementite. Pearlite also exhibits significant spheroidization. Since grain boundaries are rapid diffusion channels for carbon atoms, carbon atoms gradually diffuse into the previously carbon-depleted ferrite regions during the first holding period, achieving a uniform distribution of carbide particles. Matching the first temperature with the first dew point allows for the formation of a decarburized layer on the shallow surface of the steel sheet, and a ferrite microstructure on both the inner and outer surfaces, enabling it to withstand greater deformation. The process involves three steps: first, heating to a second temperature and holding for a second time; second, austenitizing the microstructure of the cold-rolled plate; third, rapid cooling to a third temperature and aging for a first set time; and fourth, improving the bending performance of 1300MPa grade cold-rolled martensitic steel. These steps involve: first, heating to a second temperature and holding for a second time; second, austenitizing the fibrous microstructure; and fifth, rapid cooling to promote the transformation of austenite to martensite, resulting in a suitable amount of martensite and ensuring a tensile strength of over 1300MPa. Finally, rapid cooling to a third temperature and aging for a first set time promotes the tempering of martensite, further enhancing its ductility and toughness, thus improving bending performance. In summary, the synergistic effect of chemical composition design and controlled continuous annealing process parameters improves the bending performance of 1300MPa grade cold-rolled martensitic steel. Attached Figure Description

[0040] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0041] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0042] Figure 1 This is a schematic flowchart illustrating a method for preparing a high-bending-performance 1300MPa grade cold-rolled martensitic steel according to some embodiments of this application.

[0043] Figure 2 Microstructure of a high-bending-performance 1300MPa grade cold-rolled martensitic steel provided according to some embodiments of this application;

[0044] Figure 3 Images showing the bending test results of a 1300MPa grade cold-rolled martensitic steel with high bending performance, provided according to some embodiments of this application. Detailed Implementation

[0045] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0046] 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 merely for convenience and brevity and should not be construed as a hard limitation on the scope of this application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical 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 the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Furthermore, whenever a numerical range is referred to herein, it means including any referenced number (fraction or integer) within the referred range.

[0047] In this application, unless otherwise stated, directional terms such as "upper" and "lower" specifically refer to the drawing directions in the accompanying drawings. Furthermore, in the description of this application, terms such as "comprising" and "including" mean "including but not limited to." In this document, relational terms such as "first" and "second" are used merely 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 document, "and / or" describes the relationship between related objects, indicating that three relationships can exist; for example, A and / or B can represent: A alone, A and B simultaneously, or B alone. A and B can be singular or plural. In this document, "at least one" means one or more, and "more than one" means two or more. "At least one," "at least one of the following," or similar expressions refer to any combination of these items, including any combination of single or plural items. For example, "at least one of a, b, or c" or "at least one of a, b, and c" can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be a single or multiple.

[0048] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0049] Figure 1 This is a schematic flowchart illustrating a method for preparing a high-bending-performance 1300MPa grade cold-rolled martensitic steel according to some embodiments of this application; please refer to [link / reference]. Figure 1 This application provides a method for preparing a 1300MPa grade cold-rolled martensitic steel with high bending performance, the method comprising:

[0050] S1. Obtain a cold-hardened plate with a set chemical composition;

[0051] In some embodiments, the specified chemical composition includes:

[0052] C, Si, Mn, P, S, Cr, B, N, and Fe; where, by mass fraction,

[0053] The content of C is 0.12%–0.18%, the content of Si is 0.2%–0.8%, the content of Mn is 1.5%–2.5%, the content of P is ≤0.015%, the content of S is ≤0.008%, the content of Cr is 0.3%–0.6%, the content of B is 0.001%–0.002%, and the content of N is 0.003%–0.005%.

[0054] It also includes at least one of the following chemical components: Nb, Ti, V; wherein,

[0055] The Nb content is 0.01%–0.04%, the Ti content is 0.01%–0.04%, and the V content is 0.01%–0.04%.

[0056] In this embodiment, carbon (C) is the most important solid solution strengthening element and austenite hardenability improving element in martensitic steel. To obtain sufficient martensite during cooling to ensure a tensile strength of over 1300 MPa, and to refine the grains by forming carbonitrides with microalloyed Nb and Ti during heat treatment, thus contributing to a yield strength of over 1030 MPa, and avoiding excessive C content which degrades weldability, the C content can be exemplarily 0.12%, 0.13%, 0.14%, 0.15%, 0.16%, 0.17%, 0.18%, etc.

[0057] Si is also an important solid solution strengthening element. Simultaneously, Si effectively promotes the enrichment of carbon (C) into austenite, improving the hardenability of austenite while purifying the ferrite phase and improving elongation. Si also helps inhibit carbide formation during the tempering process after cooling, thereby improving the bending properties of the steel plate. However, Si is a significant element contributing to the formation of iron oxide scale on the surface of hot-rolled plates. Excessive Si content can lead to residual iron oxide scale after pickling, which deteriorates the surface quality of the continuously annealed plate. For example, the Si content can be 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, etc.

[0058] Manganese (Mn) is an austenite stabilizing element. During annealing, it diffuses from ferrite to austenite, improving austenite stability and hardenability. Additionally, Mn can enhance martensitic strength through solid solution strengthening, ensuring high yield strengths above 1030 MPa and high tensile strengths above 1300 MPa. However, Mn increases bending performance, and the formation of MnS inclusions often becomes the initiation point for cracks during bending; therefore, the manganese content should not be too high. Furthermore, excessive Mn content easily causes microstructure segregation, leading to bending cracks and deteriorating the overall properties of the steel. It also tends to accumulate on the surface during annealing; therefore, the Mn content should not be too high. For example, the aforementioned Mn contents can be 1.5%, 1.7%, 1.9%, 2.1%, 2.3%, 2.5%, etc.

[0059] Cr can improve the hardenability of austenite, thereby obtaining a sufficient amount of martensite to achieve a tensile strength of over 1300 MPa while ensuring a sufficiently low alloy manufacturing cost. However, Cr is also a ferrite region-expanding element; excessive Cr can lead to a shrinkage of the two-phase region and hinder bainite transformation. For example, the Cr content mentioned above can be 0.3%, 0.4%, 0.5%, 0.6%, etc.

[0060] As an interstitial solid solution atom, phosphorus (P) can appropriately improve the strength of steel plates, but it is also prone to segregation at grain boundaries, which can deteriorate plasticity and formability. For example, the content of P can be 0.015%, 0.013%, 0.010%, etc.

[0061] S readily combines with Mn to form coarse MnS inclusions, which deteriorate the bending properties of the steel plate. For example, the content of S can be 0.008%, 0.007%, 0.006%, etc.

[0062] Boron (B) plays a crucial role in improving the hardenability of steel plates during the cooling process. When the B content is too low, grain boundary strengthening is difficult to achieve, necessitating it to obtain excellent resistance to delayed fracture. Furthermore, B diffuses to grain boundaries significantly faster than phosphorus (P), preventing the adverse effects of P segregation on these boundaries, which deteriorates the bending properties of steel. However, excessively high B content easily leads to the formation of carborides, which is detrimental to performance improvement. For example, the aforementioned B content can be 0.001%, 0.0013%, 0.0015%, 0.0018%, 0.002%, etc.

[0063] Nitrogen (N) can reduce nitride precipitates in steel, thereby reducing the formation of irreversible hydrogen-trapped second-phase particles. It also coarsens the original austenite grain boundaries, reducing their number and increasing crack propagation tendency, ultimately leading to deterioration of bending properties. However, when the N content is too high, the nitrides in the steel coarsen, thus reducing grain pinning and causing further deterioration of the steel's bending properties. For example, the N content can be 0.003%, 0.0035%, 0.004%, 0.0045%, 0.005%, etc.

[0064] Nitrogen (Nb) not only has a significant grain-refining effect but is also a strong carbide-forming element, which can improve the yield strength of martensitic steel through precipitation strengthening. However, Nb significantly inhibits recrystallization; excessive Nb content can lead to the entry of the non-recrystallized zone during annealing, resulting in the formation of deformation bands along the rolling direction and deteriorating bending properties. Like Nb, titanium (Ti) is also a strong carbide-forming element, which can improve yield strength through precipitation strengthening. Simultaneously, Ti is beneficial for grain refinement, resulting in a uniform equiaxed microstructure. Furthermore, Ti is less expensive than Nb. However, excessive Ti addition increases cost. V has a similar effect to Nb and Ti, forming carbides or carbonitrides to promote microstructure refinement and enhance steel strength. For example, the chemical composition may contain one or more of Nb, Ti, and V; the contents of Nb, Ti, and V can all be 0.01%, 0.02%, 0.03%, 0.04%, etc.

[0065] In some embodiments, obtaining a cold-hardened sheet having a predetermined chemical composition includes:

[0066] Molten steel is continuously cast to obtain slabs;

[0067] A slab with the specified chemical composition is heated and rolled, and then coiled to obtain a hot-rolled coil.

[0068] The hot-rolled coil is cold-rolled to obtain a chilled rigid sheet with a specified chemical composition; wherein,

[0069] The continuous casting speed is 4 m / min to 7 m / min, and the slab thickness is 110 mm to 125 mm; and / or,

[0070] The heating temperature is 1120℃~1220℃; and / or,

[0071] The final rolling temperature is 890℃~920℃; and / or,

[0072] The winding temperature is 550℃~600℃.

[0073] In the embodiments of this application, the above rolling process specifically includes: rough descaling, rough rolling, induction heating, fine descaling, fine rolling, and laminar flow cooling; the hot-rolled coil can be air-cooled and then cold-rolled; the above fine rolling adopts any one of single-slab rolling, semi-endless rolling, and endless rolling, and high bending performance 1300MPa grade cold-rolled martensitic steel is prepared based on a multi-mode thin slab continuous casting and rolling production line.

[0074] Limiting the casting speed and thickness of continuous casting ensures production rhythm while avoiding steel leakage during endless rolling. For example, the casting speed can be 4m / min, 5m / min, 6m / min, 7m / min, etc.; the slab thickness can be 110mm, 115mm, 120mm, 125mm, etc.

[0075] Limiting the heating temperature can homogenize the alloying elements in the slab. Limiting the final rolling temperature and the coiling temperature can homogenize the microstructure of the heated slab. For example, the heating temperature can be 1120℃, 1130℃, 1140℃, 1150℃, 1160℃, 1170℃, 1180℃, 1190℃, 1200℃, 1210℃, 1220℃, etc.; the final rolling temperature can be 890℃, 900℃, 910℃, 920℃, etc.; and the coiling temperature can be 550℃, 560℃, 570℃, 580℃, 590℃, 600℃, etc.

[0076] S2. The cold-rolled hardened sheet is continuously annealed to obtain a cold-rolled martensitic steel with high bending performance of 1300MPa.

[0077] The process of continuously annealing the cold-rolled sheet to obtain a high-flexural-performance 1300MPa grade cold-rolled martensitic steel includes:

[0078] The cold-hardened plate is first heated to a first temperature, and the surface of the cold-hardened plate at the first temperature is pre-oxidized under the condition of a first dew point, and then subjected to a first heat preservation under the condition of the first temperature to obtain a first annealed plate.

[0079] The first annealed plate is heated to a second temperature and held at the second temperature for a second time, followed by slow cooling and rapid cooling to obtain the second annealed plate.

[0080] The second annealed plate is heated to a third temperature and aged under the condition of a first set time to obtain a cold-rolled martensitic steel with high bending performance of 1300MPa.

[0081] In some embodiments, the first temperature is 640℃~680℃, the first heat preservation time is 30s~180s, and the first dew point is (-30)℃~(-20)℃.

[0082] In some embodiments, the heating rate of the first heating is 2°C / s to 5°C / s; and / or,

[0083] The second heating rate is 3°C / s to 8°C / s; and / or,

[0084] The heating rate of the third heating is ≥2℃ / s.

[0085] In this embodiment, by controlling the process parameters during continuous annealing based on the aforementioned chemical composition, the target microstructure can be optimized. Limiting the heating rate of the first heating balances the residence time of the chilled plate in the continuous annealing furnace, ensuring sufficient diffusion of C / Mn elements, adequate recrystallization, and homogenization of the microstructure. For example, the heating rate of the first heating can be 2℃ / s, 3℃ / s, 4℃ / s, 5℃ / s, etc.

[0086] The first holding is performed at a first temperature (below Ac1) to further temper the cold-rolled sheet at high temperature, achieving the goal of homogenization of the microstructure. This allows for more thorough recrystallization of ferrite grains in different locations, reducing grain size differences. Simultaneously, martensite undergoes more thorough tempering, transforming into equiaxed ferrite and granular cementite; pearlite also undergoes significant spheroidization. Since grain boundaries are rapid diffusion channels for carbon atoms, carbon atoms gradually diffuse into the previously carbon-depleted ferrite region during the first holding, achieving a uniform distribution of carbide particles. This pre-oxidation serves two purposes: firstly, it inhibits the external oxidation of elements such as Si, achieving good phosphating performance; secondly, it facilitates the formation of a decarburized layer on the shallow surface of the steel sheet, improving bending performance. During cold bending, the deformation is greatest on the inner and outer surfaces of the bending sample, with the outer surface being stretched and the inner surface compressed. If surface decarburization occurs, a ferrite microstructure will form on both the inner and outer surfaces. Compared to hard-phase martensite, soft-phase ferrite has better plasticity and can withstand greater deformation, thus improving the bending performance of the martensitic steel. If the first dew point is too low, the oxygen partial pressure will be insufficient, which is not conducive to obtaining a decarburized layer; if the first dew point is too high, decarburization will be too severe, the surface hardness of the steel plate will decrease, and the formability of the steel plate will also be unfavorable. For example, the first temperature can be 640℃, 650℃, 660℃, 670℃, 680℃, etc., the first holding time can be 30s, 50s, 70s, 100s, 120s, 150s, 170s, 180s, etc., and the first dew point can be -20℃, -22℃, -24℃, -25℃, -26℃, -28℃, -30℃, etc.

[0087] In some embodiments, the second temperature is (Ac3+10)℃~(Ac3+30)℃, and the second heat preservation time is 180s~360s.

[0088] By limiting the heating rate of the second heating stage, and heating at higher temperatures, a further increase in the heating rate is needed to heat the steel plate to the second temperature in a short time. After heating above point Ac1, the reverse-transformed austenite will be uniformly distributed in the matrix, thereby eliminating the inherited unevenness of the original microstructure and improving the bending performance of the finished product. For example, the heating rate of the second heating stage can be 6℃ / s, 3℃ / s, 4℃ / s, 5℃ / s, 7℃ / s, 8℃ / s, etc.

[0089] Performing a second heat treatment (soaking stage) at a second temperature allows for complete austenitization of the microstructure. The fibrous microstructure completes the recovery and recrystallization process and is completely transformed into austenite. At the same time, the residual cementite is fully dissolved, and the components and microstructure in the austenite are fully homogenized. The carbides of Nb and Ti precipitate sufficiently, effectively improving the bending performance of the finished product while ensuring a tensile strength of over 1300 MPa. For example, the second temperature can be (Ac3+10)℃, (Ac3+12)℃, (Ac3+14)℃, (Ac3+16)℃, (Ac3+180℃, (Ac3+20)℃, (Ac3+25)℃, (Ac3+23)℃, (Ac3+28)℃, (Ac3+30)℃, etc., and the second holding time can be 180s, 200s, 220s, 240s, 260s, 280s, 300s, 320s, 340s, 360s, etc.

[0090] In some embodiments, the third temperature is 200°C to 280°C, and the first set time is 300s to 600s.

[0091] In the embodiments of this application, during cold bending deformation, cracks in the steel plate are most likely to occur at the interface between two phases with a large difference in hardness. Cold bending performance is closely related to the uniformity of the microstructure, which mainly includes the distribution and hardness of inclusions and martensite phases in the cold-rolled martensitic steel. Low-temperature over-aging in the over-aging stage has virtually no effect on the morphology and distribution of martensite, only its hardness. Therefore, appropriately increasing the over-aging temperature can reduce the hardness of martensite and the hardness difference between the martensite and ferrite phases, which is beneficial to improving the cold bending performance of martensitic steel. If the over-aging temperature is too low or the over-aging time is too short, it is not conducive to martensite tempering, resulting in poor martensite plasticity and deteriorating bending performance; if the over-aging temperature is too high or the over-aging time is too long, it will promote cementite precipitation, deteriorating bending formability. Limiting the third heating rate is beneficial to improving the plasticity of martensite. For example, the third temperature can be 200℃, 220℃, 240℃, 260℃, 270℃, 280℃, etc., the first set time can be 300s, 350s, 400s, 450s, 500s, 550s, 600s, etc., and the third heating rate can be 2℃ / s, 2.5℃ / s, 3℃ / s, etc.

[0092] In some embodiments, the slow cooling process parameters include: a cooling rate of 1°C / s to 15°C / s, and an endpoint temperature of 700°C to 800°C; and / or,

[0093] The rapid cooling process parameters include: the cooling medium is a gas containing 50% hydrogen by volume, and the cooling rate is...

[0094] ≥45℃ / s, with an endpoint temperature of (Ms-20)℃~(Ms-10)℃.

[0095] In this embodiment, the process parameters for slow cooling are defined to ensure that the formation of new ferrite is suppressed during the initial cooling process. This not only avoids the evolution of inhomogeneous microstructure but also prevents the reduction of austenite hardenability due to carbon enrichment into austenite. The process parameters for rapid cooling are also defined, with the endpoint temperature below the martensitic transformation temperature. Combined with the rapid cooling medium and cooling rate, this promotes the transformation of austenite to martensite, resulting in a microstructure of 92-96% martensite and 4-8% ferrite. For example, in the aforementioned slow cooling process parameters, the cooling rate can be 1℃ / s, 3℃ / s, 5℃ / s, 7℃ / s, 9℃ / s, 11℃ / s, 13℃ / s, 15℃ / s, etc.; the endpoint temperature can be 700℃, 720℃, 740℃, 760℃, 780℃, 800℃, etc. In the above-mentioned rapid cooling process parameters, the cooling rate can be 45℃ / s, 46℃ / s, 47℃ / s, 48℃ / s, 49℃ / s, etc.; the final temperature can be (Ms-20)℃, (Ms-10)℃, (Ms-15)℃, (Ms-13)℃, (Ms-18)℃, etc.

[0096] Secondly, this application provides a high-bending-performance 1300MPa grade cold-rolled martensitic steel prepared by the method described in the first aspect.

[0097] In some embodiments, the cold-rolled martensitic steel meets the following mechanical properties: tensile strength > 1300 MPa, yield strength 1030 MPa ~ 1300 MPa, elongation A80 ≥ 3%, and bending performance that meets the requirement of not cracking when bent 180° for 2t parallel to the rolling direction.

[0098] The microstructure of the cold-rolled martensitic steel includes lath martensite and ferrite. Figure 2 For a microstructure diagram of a high-bending-performance 1300MPa grade cold-rolled martensitic steel provided according to some embodiments of this application, please refer to [link to relevant documentation]. Figure 2 ;in,

[0099] The volume fraction of the lath martensite is 92%–96%, and the volume fraction of the ferrite is 4%–8%.

[0100] In the embodiments of this application, the 1300MPa grade cold-rolled martensitic steel has excellent strength and bending properties. The volume fraction of the lath martensite can be 92%, 93%, 94%, 95%, 96%, etc., and the volume fraction of the ferrite can be 4%, 5%, 6%, 7%, 8%, etc.

[0101] The high bending performance 1300MPa grade cold-rolled martensitic steel is realized based on the above-mentioned preparation method of the high bending performance 1300MPa grade cold-rolled martensitic steel. The specific steps of the preparation method of the high bending performance 1300MPa grade cold-rolled martensitic steel can be referred to the above embodiments. Since the high bending performance 1300MPa grade cold-rolled martensitic steel adopts some or all of the technical solutions of the above embodiments, it has at least all the beneficial effects brought by the technical solutions of the above embodiments, which will not be repeated here.

[0102] The present application is further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the application. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to national standards. If there is no corresponding national standard, then general international standards, conventional conditions, or conditions recommended by the manufacturer are followed.

[0103] This application provides a high-bending-performance 1300MPa grade cold-rolled martensitic steel. Please refer to Table 1 for the chemical composition of the high-bending-performance 1300MPa grade cold-rolled martensitic steel, Table 2 for the preparation process parameters of the cold-rolled sheet, Table 3 for the continuous annealing process parameters of the cold-rolled sheet, and Table 4 for the performance results of the high-bending-performance 1300MPa grade cold-rolled martensitic steel.

[0104] Table 1. Chemical composition (wt%) of high bending performance 1300MPa grade cold-rolled martensitic steel

[0105]

[0106] Table 2. Manufacturing process parameters of cold-rolled steel sheets

[0107]

[0108] Table 3 Process parameters for continuous annealing of cold-rolled steel sheets

[0109]

[0110]

[0111] The high bending performance 1300MPa grade cold-rolled martensitic steels prepared in Examples 1-6 and Comparative Examples 1-4 were subjected to performance tests, as shown in Table 4.

[0112] The tensile strength Rm, yield strength Rp0.2, and A80 in Table 4 were evaluated using GB / T 228.1-2021 Metallic materials, tensile testing, Part 1: Test method at room temperature.

[0113] The bending test was carried out in accordance with the method of GBT 232-2010 Metallic materials - Bend test. Here, R is the radius of the bend core, t is the thickness of the steel plate, and it is considered qualified if there are no visible cracks on the outer surface after bending.

[0114] Table 4 shows the performance results of the cold-rolled martensitic steel with high bending performance at the 1300 MPa level.

[0115]

[0116]

[0117] In summary, the cold-rolled martensitic steel with high bending performance at the 1300 MPa level prepared by the embodiments of this application meets the following mechanical properties: tensile strength > 1300 MPa, yield strength 1030 MPa - 1300 MPa, elongation A80 ≥ 3%, and the bending performance meets the requirement of no cracking when bending 180° with 2t parallel to the rolling direction. At the same time, it has excellent strength performance and bending performance. Figure 3 For the pictures of the bending test results of the cold-rolled martensitic steel with high bending performance at the 1300 MPa level provided by some embodiments of this application, please refer to Figure 3 , which shows that the cold-rolled martensitic steel at the 1300 MPa level provided by the embodiments of this application has excellent bending performance. In Comparative Example 1, the second temperature is too low; in Comparative Example 2, it is directly heated to the second temperature; in Comparative Example 3, over-aging is carried out at the final cooling temperature of rapid cooling, and the over-aging temperature is too low; in Comparative Example 4, the C content is too low, which to some extent makes the performance of the cold-rolled martensitic steel inferior to that of the embodiments.

[0118] The above are only the specific implementation manners of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be obvious to those skilled in the art. The general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application will not be limited to the embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method of manufacturing a high bend performance 1300 MPa grade cold rolled martensitic steel, characterized in that, The method includes: A cold-hardened plate with a predetermined chemical composition is obtained, the predetermined chemical composition comprising, by mass fraction: C: 0.12%~0.18%, Si: 0.2%~0.8%, Mn: 1.5%~2.5%, P≤0.015%, S≤0.008%, Cr: 0.5%~0.6%, B: 0.001%~0.002%, N: 0.003%~0.005%; and at least one of the following chemical components: Nb, Ti, V; wherein Nb: 0.01%~0.04%, Ti: 0.01%~0.04%, V: 0.01%~0.04%, and the balance being Fe and unavoidable impurities; The cold-rolled martensitic steel with high bending performance of 1300MPa was obtained by continuous annealing of the cold-rolled sheet. The continuous annealing includes: The cold-hardened plate is first heated to a first temperature of 640°C to 680°C at a heating rate of 2°C / s to 5°C / s, and the surface of the cold-hardened plate at the first temperature is pre-oxidized under a first dew point condition of -30°C to -20°C, and then held at the first temperature for 30s to 180s to obtain a first annealed plate. The first annealed plate is heated to a second temperature at a heating rate of 3℃ / s to 8℃ / s, and then held at the second temperature for 180s to 360s. After that, it is slowly cooled and then rapidly cooled to obtain the second annealed plate. The second temperature is Ac3+10℃ to Ac3+30℃. The second annealing plate is heated to a third temperature of 200℃~280℃ at a heating rate of ≥2℃ / s, and then aged for a first set time of 300s~600s. The cold-rolled martensitic steel meets the following mechanical properties: tensile strength > 1300 MPa, yield strength 1030 MPa~1300 MPa, elongation A80 ≥ 3%, and bending performance meets the requirement of not cracking when bent 180° for 2t parallel to the rolling direction; The microstructure of the cold-rolled martensitic steel comprises lath martensite and ferrite; wherein, The volume fraction of the lath martensite is 92% to 96%, and the volume fraction of the ferrite is 4% to 8%.

2. The method of claim 1, wherein, The slow cooling process parameters include: a cooling rate of 1℃ / s to 15℃ / s, and an endpoint temperature of 700℃ to 800℃; and / or, The rapid cooling process parameters include: the cooling medium is a gas containing 50% hydrogen by volume, and the cooling rate is... The speed is ≥45℃ / s, and the final temperature is (Ms-20)℃~(Ms-10)℃.

3. The method of claim 1, wherein, The process of obtaining a cold-hardened plate with a specified chemical composition includes: Molten steel is continuously cast to obtain slabs; A slab with the specified chemical composition is heated and rolled, and then coiled to obtain a hot-rolled coil. The hot-rolled coil is cold-rolled to obtain a chilled rigid sheet with a specified chemical composition; wherein, The continuous casting speed is 4 m / min to 7 m / min, and the slab thickness is 110 mm to 125 mm; and / or, The heating temperature is 1120℃~1220℃; and / or, The final rolling temperature is 890℃~920℃; and / or, The winding temperature is 550℃~600℃.

4. A cold-rolled martensitic steel with high bending performance of 1300MPa, prepared by the method according to any one of claims 1 to 3.

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