980mpa grade martensitic steel and method of making

Abstract

The application provides a 980MPa grade martensitic steel and a preparation method thereof, and belongs to the field of steel preparation.The chemical components of the martensitic steel include C, Si, Mn, Al, P, S, Ti, Cr, N, B and Fe; wherein, the content of C is 0.04%-0.25% in mass fraction, the content of Si is 0.1%-1.0% in mass fraction, the content of Mn is 1.2%-3.0% in mass fraction, the content of Al is 0.02%-0.35% in mass fraction, the content of P is less than or equal to 0.005% in mass fraction, the content of S is less than or equal to 0.002% in mass fraction, the content of Ti is 0.010%-0.10% in mass fraction, the content of Cr is 0.1%-0.6% in mass fraction, the content of N is less than or equal to 0.004% in mass fraction, and the content of B is 0.0010%-0.0030% in mass fraction.The chemical components are reasonably designed, the C-Si-Mn-Ti-Cr-B component system is adopted, the metallographic structure of martensite + a small amount of ferrite is formed, the martensite structure is more in the structure, the strength of the martensitic steel reaches the requirement of 980MPa, and a small amount of soft phase ferrite structure is introduced into the structure, the hard phase martensite structure is coordinated in the bending deformation, and the bending performance is improved.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of steel production, in particular to a 980MPa grade martensite steel and a preparation method thereof. BACKGROUND

[0002] In recent years, automobile manufacturers have also put forward the demand of reducing carbon for steel enterprises. With the rapid development of the endless rolling technology at home and abroad, several thin slab continuous casting and rolling (ESP) production lines have been put into operation in China. Thin gauge hot-rolled strip steel can be produced, and the product specifications cover 0.8mm-4.0mm, which can replace part of the cold-rolled products. In the short process production line, high-performance hot-rolled strip steel is developed to realize "hot instead of cold", reduce the production process of steel enterprises, and achieve significant carbon reduction effect.

[0003] To realize "hot instead of cold" of 980MPa grade martensite steel, the problem of cracking during processing and forming in the use process needs to be solved first. Bending processing is the most common forming method in the manufacturing field of automobiles, engineering machinery and containers. Developing a high anti-bending performance hot-rolled thin gauge 980MPa martensite steel to meet the bending performance requirements of the product in different fields and realize "hot instead of cold" has very important significance and popularization prospect. SUMMARY

[0004] The application provides a 980MPa grade martensite steel and a preparation method thereof, to solve the technical problem that the thin gauge hot-rolled 980MPa grade martensite steel cannot realize high anti-bending performance in the prior art.

[0005] In a first aspect, the application provides a 980MPa grade martensite steel, the chemical composition of the martensite steel comprises: C, Si, Mn, Al, P, S, Ti, Cr, N, B and Fe; wherein, in terms of mass fraction,

[0006] The content of C is 0.04%-0.25%, the content of Si is 0.1%-1.0%, the content of Mn is 1.2%-3.0%, the content of Al is 0.02%-0.35%, the content of P is ≤0.005%, the content of S is ≤0.002%, the content of Ti is 0.010%-0.10%, the content of Cr is 0.1%-0.6%, the content of N is ≤0.004%, and the content of B is 0.0010%-0.0030%.

[0007] Optionally, the metallographic structure of the martensite steel comprises martensite and ferrite, the volume fraction of the martensite is 88%-95%, and the volume fraction of the ferrite is 5%-12%.

[0008] Optionally, the thickness of the martensitic steel is 0.8mm-4.0mm, and the unevenness of the steel plate of the martensitic steel is ≤5mm; and / or, the martensitic steel satisfies at least one of the following properties: yield strength >800MPa, tensile strength >1000MPa, elongation >8%, and cold bending limit R=a, 180° is qualified.

[0009] In a second aspect, the application provides a preparation method of the martensitic steel according to any one of the embodiments of the first aspect, and the method comprises:

[0010] continuously casting the molten steel to obtain a slab with the chemical composition;

[0011] preheating, rough descaling, rough rolling, induction heating, fine descaling, fine rolling, stage cooling, and coiling the slab to obtain the martensitic steel.

[0012] Optionally, the stage cooling comprises first stage cooling, second stage cooling, and third stage cooling, the cooling rate of the first stage cooling is ≥50℃ / s, and the termination temperature of the first stage cooling is 20℃-50℃ above the ferrite transformation temperature (Ar3); the second stage cooling is air cooling, and the time of the air cooling is 3s-10s; the cooling rate of the third stage cooling is ≥50℃ / s, and the termination temperature of the third stage cooling is 100℃-martensite transformation temperature (Mf).

[0013] Optionally, the mode of the fine rolling comprises single slab rolling, semi-endless rolling, and endless rolling, and the mode of the fine descaling comprises single row descaling and double row descaling; wherein, the mode of the fine rolling and the mode of the fine descaling satisfy the following relationship:

[0014] if the fine rolling is single slab rolling, then the fine descaling is double row descaling;

[0015] if the fine rolling is semi-endless rolling or endless rolling, then the fine descaling is single row descaling.

[0016] Optionally, the inlet temperature of the rough rolling is ≥1150℃, the outlet temperature of the rough rolling is 930℃-1000℃, and the pass of the rough rolling is 3 passes; and / or, the cumulative deformation of the fine rolling is 70%-80%, the final rolling temperature of the fine rolling is 850℃-900℃, and the pass of the fine rolling is 5 passes.

[0017] Optionally, the end point temperature of the preheating is 1150℃-1200℃; and / or, the end point temperature of the induction heating is 1050℃-1150℃.

[0018] Optionally, the rough descaling pressure is greater than or equal to 30 MPa, the fine descaling pressure is greater than or equal to 30 MPa, and the distance between the fine descaling nozzle and the slab is 60 mm to 110 mm.

[0019] Optionally, the casting speed is 4.0 m / min to 6.5 m / min, and the thickness of the slab is 115 mm to 123 mm.

[0020] Compared with the prior art, the above technical solution provided by the embodiments of the present application has the following advantages:

[0021] The 980 MPa grade martensitic steel provided by the present application has a reasonable chemical composition, adopts a C-Si-Mn-Ti-Cr-B component system, forms a metallographic structure of martensite + a small amount of ferrite, and has more martensite structure in the organization to ensure that the strength of the martensitic steel reaches the requirement of 980 MPa, and a small amount of soft ferrite structure is introduced into the organization to coordinate the strain with the hard phase martensite structure during bending deformation, thereby improving the bending performance. The thin-gauge hot-rolled steel obtained has a yield strength of > 800 MPa, a tensile strength of > 1000 MPa, an elongation of > 8%, a cold bending R=a, 180° of qualified, and a thickness of 0.8 mm to 4.0 mm, which can effectively solve the technical problem of cracking of the thin-gauge hot-rolled 980 MPa martensitic steel in the prior art. BRIEF DESCRIPTION OF DRAWINGS

[0022] The drawings incorporated into the specification and forming a part thereof, illustrate embodiments consistent with the present application and, together with the specification, serve to explain the principles of the present application.

[0023] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or the prior art description will be briefly introduced as follows, and obviously, other drawings can also be obtained by those skilled in the art without creative labor.

[0024] Figure 1 A flowchart of a preparation method of a martensitic steel provided by the embodiments of the present application is shown in the figure.

[0025] Figure 2 A metallographic structure diagram of the martensitic steel provided by the embodiment 1 of the present application is shown in the figure. DETAILED DESCRIPTION

[0026] 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.

[0027] 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.

[0028] Furthermore, in the description of this application, the terms "comprising," "including," etc., 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" 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.

[0029] 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.

[0030] In a first aspect, this application provides a 980MPa grade martensitic steel, wherein the chemical composition of the martensitic steel includes: C, Si, Mn, Al, P, S, Ti, Cr, N, B, and Fe; wherein, by mass fraction,

[0031] The content of C is 0.04%~0.25%, the content of Si is 0.1%~1.0%, the content of Mn is 1.2%~3.0%, the content of Al is 0.02%~0.35%, the content of P is ≤0.005%, the content of S is ≤0.002%, the content of Ti is 0.010%~0.10%, the content of Cr is 0.1%~0.6%, the content of N is ≤0.004%, and the content of B is 0.0010%~0.0030%.

[0032] The positive effects of controlling the carbon content to 0.04%~0.25%: Carbon is the most important alloying element in steel and one of the most economical elements for improving strength. C atoms are partially dissolved in γ-Fe, expanding the γ-phase region and significantly influencing microstructure and properties. It plays a major controlling role in strength and hardness. It also forms carbonitrides with microalloying elements, regulating the properties of steel over a wide range. When C > 0.25%, banded structures are easily formed during slab smelting and rolling. Severe banded structures adversely affect the cold forming and fatigue properties of the steel plate, and are also detrimental to weldability. When C < 0.04%, smelting difficulty increases. For example, the C content can be 0.04%, 0.08%, 0.12%, 0.20%, 0.25%, etc.

[0033] The positive effects of controlling the Si content to 0.1%~1.0% include: Si is a traditional solid solution strengthening element; adding higher Si content significantly improves the hardenability of the material. Furthermore, Si is inexpensive, and increasing strength with Si reduces production costs. Another important function of Si is to suppress the precipitation of cementite during phase transformation, ensuring sufficient C solid solution and thus guaranteeing martensitic hardness and material strength. However, excessive Si addition negatively impacts the material's plasticity, toughness, and surface quality. Si promotes graphitization and decarburization; higher Si content easily forms a decarburized layer on the steel plate surface, reducing surface hardness and wear resistance. For example, the Si content can be 0.1%, 0.3%, 0.6%, 0.8%, 1.0%, etc.

[0034] The positive effects of controlling the Mn content to 1.2%~3.0% include: Mn and Fe can mutually substitute for each other, acting as substitutional solute atoms in iron-based solid solutions, resulting in good solid solution strengthening properties; manganese can also refine grains and improve strength, and as an austenite stabilizing element, it expands the single austenite phase region; manganese can significantly lower the γ→α transformation temperature, thus lowering the A1 point; in thin slab continuous casting and rolling, to reduce the energy consumption of induction heating, the final rolling temperature is generally controlled at 800-860℃. If the Mn content is low, final rolling in this temperature range can easily cause the strip to enter the two-phase region, with some austenite transforming into ferrite, reducing the strip's strength. When the Mn content is >3.0%, it increases the production cost of the experimental steel. For example, the Mn content can be 1.2%, 1.6%, 2.0%, 2.4%, 3.0%, etc.

[0035] The positive effects of controlling the Al content to 0.02%~0.35% include: Al is an effective deoxidizing element and can form nitrides to refine grains. An Al content >0.35% will impair the toughness of the steel, and the toughness of the weld heat-affected zone will also deteriorate. For example, the Al content can be 0.02%, 0.025%, 0.03%, 0.35%, etc.

[0036] The positive effects of controlling the content of phosphorus (P) to ≤0.005%, sulfur (S) to ≤0.002%, and nitrogen (N) to ≤0.004% are as follows: Excessive levels of P, S, and N can negatively impact the plasticity, toughness, and fatigue properties of materials. Excessive N content makes it easier for N to combine with boron (B), forming BN precipitates at high temperatures, consuming some B and thus reducing the hardenability of the strip, preventing the formation of a complete martensitic structure, and ultimately lowering the strip's strength. For example, the P content can be 0.002%, 0.004%, or 0.005%; the S content can be 0.001% or 0.002%; and the N content can be 0.002%, 0.003%, or 0.004%.

[0037] The positive effects of controlling the Ti content to 0.010%~0.10% are as follows: Ti is a strong carbonitride forming element. At high temperatures, Ti first combines with N to form TiN precipitates. On the one hand, this consumes N, reduces the amount of N in solid solution, and improves the cleanliness of the molten steel. On the other hand, since the TiN precipitate forms earlier than BN, it avoids the combination of B and N, reducing the consumption of B in solid solution by N and ensuring the hardenability of the strip. After the molten steel solidifies, fine TiN particles precipitate. These fine TiN particles are very stable and can effectively prevent the growth of austenite grains, thereby refining the microstructure. During welding, Ti-containing second-phase particles can prevent the growth of coarse grains during welding, refine the grains, and improve the weldability of the steel. With a higher Ti content, Ti combines with C to form TiC or Ti(C,N), consuming some of the C. A lower amount of C in solid solution will reduce the strength of the strip. For example, the Ti content can be 0.010%, 0.03%, 0.06%, 0.08%, 0.10%, etc.

[0038] The positive effects of controlling the Cr content to 0.1%~0.6% are as follows: Cr is a highly hardenable element that plays a role in solid solution strengthening in the matrix. To obtain finer ferrite grains, adding an appropriate amount of Cr improves the hardenability of the material, thereby refining the grains. However, excessive Cr will deteriorate the weldability of the material. For example, the Cr content can be 0.1%, 0.2%, 0.4%, 0.5%, 0.6%, etc.

[0039] The positive effects of controlling the boron (B) content to 0.0010%~0.0030% are as follows: Boron is a highly hardenable element. Its content in steel is extremely low (not exceeding 50 ppm), and its main function is to increase the hardenability of steel. Its effect is much greater than that of Cr, Mn, and other alloying elements. Using trace amounts of B can save a significant amount of alloying elements. However, B is chemically very reactive, with a strong affinity for N and O, and can also form B4C with C. Only B existing in a solid solution state has a beneficial effect on the hardenability of steel; B existing in compounds has no effect on the hardenability. For example, the B content can be 0.0010%, 0.0015%, 0.0020%, 0.0025%, 0.0030%, etc.

[0040] In some embodiments, the microstructure of the martensitic steel includes martensite and ferrite, wherein the volume fraction of martensite is 88% to 95% and the volume fraction of ferrite is 5% to 12%.

[0041] The positive effects of controlling the volume fraction of martensite to 88%~95% and the volume fraction of ferrite to 5%~12% are as follows: This invention produces a martensitic steel with a tensile strength of 980MPa. Firstly, the higher proportion of martensite in the microstructure ensures that the martensitic steel achieves the required strength of 980MPa. Secondly, since martensite is a hard phase and relatively brittle, it is prone to cracking during bending. Therefore, introducing a small amount of soft ferrite into the microstructure allows for coordinated strain with the hard martensite during bending deformation, improving bending performance. For example, the volume fraction of martensite can be 88%, 90%, 92%, 94%, 95%, etc., and the volume fraction of ferrite can be 5%, 7%, 9%, 12%, etc.

[0042] In some embodiments, the thickness of the martensitic steel is 0.8 mm to 4.0 mm, the flatness of the martensitic steel plate is ≤ 5 mm; and / or, the martensitic steel meets at least one of the following properties: yield strength > 800 MPa, tensile strength > 1000 MPa, elongation > 8%, cold bending limit R=a, 180° qualified.

[0043] The martensitic steel obtained in this application has a metallographic structure of martensite plus a small amount of ferrite. The predominance of martensite ensures that the martensitic steel achieves the required strength of 980 MPa. Furthermore, the introduction of a small amount of soft ferrite into the microstructure allows for coordinated strain with the hard martensite during bending deformation, improving bending performance. This application also enables the production of hot-rolled martensitic steel with a diameter of 0.8 mm to 2.0 mm. For example, the thickness of the martensitic steel can be 0.8mm, 1.2mm, 1.6mm, 2.6mm, 3.6mm, 4.0mm, etc.; the flatness of the martensitic steel plate can be 1mm, 2mm, 4mm, 5mm, etc.; the yield strength of the martensitic steel can be 800MPa, 840MPa, 880MPa, 920MPa, 960MPa, 980MPa, etc.; the tensile strength of the martensitic steel can be 1001MPa, 1050MPa, 1100MPa, 1150MPa, 1200MPa, etc.; and the elongation of the martensitic steel can be 9%, 10%, 11%, 12%, 14%, etc.

[0044] Secondly, this application provides a method for preparing martensitic steel according to any embodiment of the first aspect; please refer to [link to relevant documentation]. Figure 1 The method includes:

[0045] S1. The molten steel is continuously cast to obtain a slab with the aforementioned chemical composition;

[0046] In some embodiments, prior to step S1, the process further includes: KR desulfurization, converter smelting, LF refining, and VD refining to obtain molten steel that conforms to the chemical composition.

[0047] In some embodiments, the casting speed is 4.0 m / min to 6.5 m / min, and the slab thickness is 115 mm to 123 mm.

[0048] The positive effects of controlling the casting speed of continuous casting to 4.0 m / min~6.5 m / min are: a casting speed of 4.0-6.5 m / min ensures that continuous casting matches the endless rolling production mode; when the casting speed is greater than 6.5 m / min, the adverse effect is that 6.5 m / min is the operating limit of the equipment, and excessively high casting speeds will require equipment replacement, increasing costs. When the casting speed is less than the minimum value at the end of this range, the adverse effect is that the casting speed is too low, making it impossible to guarantee the matching of continuous casting and rolling production. For example, the casting speed of continuous casting can be 4.0 m / min, 4.5 m / min, 5.0 m / min, 5.5 m / min, 6.0 m / min, 6.5 m / min, etc.

[0049] The positive effects of controlling the slab thickness in continuous casting to 115mm~123mm: The slab thickness can be selected according to the thickness of the finished strip. For finished strip thickness ≤ 4.0mm, the slab thickness can be selected as 115mm~118mm. When the finished strip thickness is 5mm~12.0mm, the finished strip thickness can be selected as 118mm~123mm. Increasing the slab thickness mainly increases the total reduction during rolling, which can refine the grains and thus improve the strength of the material. For example, the slab thickness in this continuous casting can be 115mm, 118mm, 121mm, 123mm, etc.

[0050] S2. The slab is preheated, rough descaling is performed, rough rolling is performed, induction heating is performed, fine descaling is performed, fine rolling is performed, staged cooling is performed, and coiling is performed to obtain martensitic steel.

[0051] In some embodiments, the finishing rolling mode includes single-slab rolling, semi-endless rolling, and fully endless rolling, and the finishing descaling mode includes single-row descaling and double-row descaling; wherein the finishing rolling mode and the finishing descaling mode satisfy the following relationship:

[0052] If the finishing rolling is single-slab rolling, then the finishing descaling is double-scaling.

[0053] If the finishing rolling is a semi-endless rolling or a fully endless rolling, then the finishing descaling is a single-row descaling.

[0054] It should be noted that single-slab rolling, semi-endless rolling, and fully endless rolling refer to three different rolling modes: single-slab rolling refers to the process of trimming the beginning and end of the slab during rolling. In single-slab rolling, the slab requires trimming, and one slab is rolled to produce one coil. In semi-endless rolling, the slab also requires trimming, but the slab is longer, and one slab can typically produce several coils. In fully endless rolling, the slab does not require trimming during the entire rolling process; the strip is only coiled after being coiled.

[0055] Single-row or double-row descaling refers to the number of rows of descaling manifolds. A manifold arranged in one row is called single-row descaling, and a manifold arranged in two rows is called double-row descaling.

[0056] The reason for selecting the finishing descaling mode through finishing rolling is as follows: The descaling method before finishing rolling is selected according to the rolling mode. When rolling a single billet, due to the fast rolling speed and the low entry temperature of the finishing mill, the iron oxide scale is sticky and difficult to remove completely. Therefore, double-row descaling is selected. For semi-endless and fully endless rolling, the rolling speed is slow, and the billet needs to be induction heated before finishing rolling to raise the temperature of the billet. After the temperature is raised, the stickiness of the iron oxide scale decreases and it is easy to remove completely. Therefore, single-row descaling is selected.

[0057] In some embodiments, the inlet temperature of the roughing mill is ≥1150℃, the outlet temperature of the roughing mill is 930℃~1000℃, and the roughing mill has 3 passes; and / or, the cumulative deformation of the finishing mill is 70%~80%, the finishing mill's final rolling temperature is 850℃~900℃, and the finishing mill has 5 passes.

[0058] The positive effects of controlling the inlet temperature of the roughing mill to ≥1150℃ are: within this temperature range, the temperature of the slab entering the mill can be guaranteed; when the temperature value is greater than the maximum value at the end of this range, the adverse effect is that the temperature is too high, resulting in energy waste; when the temperature value is less than the minimum value at the end of this range, the adverse effect is that the temperature is too low, resulting in rolling instability. For example, the inlet temperature of the roughing mill can be 1150℃, 1170℃, 1190℃, 1210℃, etc.

[0059] The positive effects of controlling the exit temperature of the roughing mill to 930℃~1000℃ are: within this temperature range, the temperature of the slab entering the mill can be guaranteed; when the temperature value is greater than the maximum value at the end of this range, the adverse effect is that the temperature is too high, resulting in energy waste; when the temperature value is less than the minimum value at the end of this range, the adverse effect is that the temperature is too low, resulting in rolling instability. For example, the exit temperature of the roughing mill can be 930℃, 950℃, 970℃, 1000℃, etc.

[0060] The positive effects of controlling the cumulative deformation of the finishing mill to 70%~80% are: within this deformation range, it can refine the grain structure and improve the strength and toughness of the product. When the cumulative deformation exceeds the maximum value at the end of this range, the adverse effect is that the deformation is too large, leading to excessively high product strength. Conversely, when the cumulative deformation is less than the minimum value at the end of this range, the adverse effect is that the deformation is too small, leading to insufficient product strength. For example, the cumulative deformation of the finishing mill can be 70%, 73%, 76%, 80%, etc.

[0061] The positive effects of controlling the final rolling temperature of the finishing mill to 850℃~900℃ are: within this temperature range, it ensures the uniformity of the final microstructure of the product; when the temperature value is greater than the maximum value at the end of this range, the adverse effect is that the temperature is too high, leading to rolling instability; when the temperature value is less than the minimum value at the end of this range, the adverse effect is that the temperature is too low, causing the finishing mill to roll in the two-phase region, resulting in an uneven microstructure. For example, the final rolling temperature of the finishing mill can be 850℃, 870℃, 890℃, 900℃, etc.

[0062] In some embodiments, the final temperature of the preheating is 1150°C to 1200°C; and / or, the final temperature of the induction heating is 1050°C to 1150°C.

[0063] The positive effects of controlling the final preheating temperature to 1150℃~1200℃ are: this temperature range ensures the slab enters the rolling mill at a suitable temperature, resulting in a more uniform slab transition; temperatures above 1200℃ lead to energy waste, while temperatures below 1150℃ cause rolling instability. For example, the final preheating temperature can be 1150℃, 1160℃, 1170℃, 1190℃, 1200℃, etc.

[0064] The positive effects of controlling the induction heating endpoint temperature to 1050℃~1150℃: Finishing rolling uses a 5-stand mill, resulting in a significant temperature drop during the finishing process. To ensure the final rolling temperature is between 850-900℃, the slab must have a high temperature before entering the finishing mill, ensuring it meets the final rolling temperature requirements after finishing. When using semi-endless or fully endless rolling, the rolling speed is slow, leading to a larger temperature drop during finishing, thus requiring a higher finishing entry temperature (induction heating endpoint temperature). However, with single-slab rolling, the faster finishing speed results in less temperature drop compared to semi-endless and fully endless rolling, requiring a lower finishing entry temperature, but still requiring a finishing entry temperature > 1050℃. For example, the endpoint temperature for induction heating can be 1050℃, 1070℃, 1090℃, 1120℃, 1150℃, etc.

[0065] In some embodiments, the pressure of the coarse descaling is ≥30MPa, the pressure of the fine descaling is ≥30MPa, and the distance between the nozzle of the fine descaling and the slab is 60mm~110mm.

[0066] The positive effects of controlling the coarse descaling pressure to ≥30MPa are: within this pressure range, iron oxide scale on the strip surface can be thoroughly removed; when the descaling pressure is less than the minimum value at the end of this range, the adverse effect is that iron oxide scale remains on the strip surface, thus affecting the surface quality of the strip. For example, the coarse descaling pressure can be 30MPa, 32MPa, 34MPa, 38MPa, etc.

[0067] The positive effects of controlling the descaling pressure to ≥30MPa are: within this pressure range, iron oxide scale on the strip surface can be thoroughly removed; when the descaling pressure is less than the minimum value at the end of this range, the adverse effect is that iron oxide scale remains on the strip surface, affecting the surface quality of the strip. For example, the descaling pressure can be 30MPa, 32MPa, 34MPa, 36MPa, 38MPa, etc.

[0068] The positive effects of controlling the distance between the descaling nozzle and the slab to 60mm-110mm are as follows: Maintaining the distance between the descaling nozzle and the continuously cast slab at 60-110mm ensures effective descaling of the strip. If the distance is greater than the maximum value at the end of this range, the adverse effect is that the distance is too large, resulting in poor nozzle performance and over-descaling areas. If the distance is less than the minimum value at the end of this range, the adverse effect is that the distance is too short, resulting in poor nozzle performance and undescaled areas. For example, the distance between the descaling nozzle and the slab can be 60mm, 80mm, 100mm, 110mm, etc.

[0069] In some embodiments, the staged cooling includes: a first stage cooling, a second stage cooling, and a third stage cooling. The cooling rate of the first stage cooling is ≥50℃ / s, and the termination temperature of the first stage cooling is 20℃~50℃ above the ferrite transformation temperature (Ar3). The second stage cooling is air cooling, and the air cooling time is 3s~10s. The cooling rate of the third stage cooling is ≥50℃ / s, and the termination temperature of the third stage cooling is 100℃~the martensitic transformation temperature (Mf).

[0070] The positive effects of controlling the cooling rate of the first stage cooling to ≥50℃ / s and the termination temperature of the first stage cooling to be 20℃~50℃ above the ferrite transformation temperature (Ar3) are as follows: The rapid cooling in the first stage is to refine the original austenite grain size, ultimately obtaining fine martensite and ferrite grains. Grain refinement is beneficial to improving strength and bending resistance. The termination temperature is 20℃~50℃ higher than the ferrite "nose" temperature, mainly to avoid the production of a large amount of ferrite at the ferrite transformation nose temperature, which would reduce the yield strength. For example, the cooling rate of this first stage cooling can be 50℃ / s, 55℃ / s, 60℃ / s, 65℃ / s, 70℃ / s, etc.; and the termination temperature of this first stage cooling can be 20℃, 30℃, 40℃, 50℃, etc. above the ferrite transformation temperature (Ar3).

[0071] The positive effects of controlling the air cooling time to 3-10 seconds are as follows: if the time is too short, ferrite cannot be formed, resulting in poor bending performance; if the air cooling time is too long, more ferrite is formed, resulting in lower yield strength. For example, the air cooling time can be 3 seconds, 5 seconds, 7 seconds, 10 seconds, etc.

[0072] The positive effects of controlling the cooling rate of the third stage cooling to ≥50℃ / s: a high cooling rate ensures the formation of martensitic structure and guarantees a tensile strength ≥980MPa. For example, the cooling rate of the third stage cooling can be 50℃ / s, 55℃ / s, 60℃ / s, 65℃ / s, 70℃ / s, etc.

[0073] In the above steps: Descaling is performed twice before roughing and finishing rolling to ensure the surface quality of the strip. Before finishing rolling, the intermediate billet is induction heated to dissolve the Ti carbonitrides formed during roughing, allowing Ti to precipitate again during finishing rolling, refining the austenite grains and subsequently the martensite grain size. Combined with a three-stage cooling process, the first stage cooling rate is required to be ≥50℃ / s, and the cooling stop temperature must be 20℃~50℃ higher than the "nose point" temperature of ferrite transformation. Then, air cooling is performed for 3s~10s, and an appropriate amount of [unspecified ingredient] is added. Element B delays the time for the austenite-to-ferrite phase transformation to begin, refining the ferrite grain size; the third-stage cooling rate is required to be ≥50℃ / s, directly cooling to 100℃~Mf, resulting in thin-gauge hot-rolled steel with a yield strength >800MPa, tensile strength >1000MPa, elongation >8%, and qualified cold bending R=a, 180°, with a thickness of 0.8mm~4.0mm. This effectively solves the technical problem of cracking during bending of thin-gauge hot-rolled 980MPa martensitic steel in existing technologies.

[0074] The preparation method of the 980MPa grade martensitic steel is based on the chemical composition of the 980MPa grade martensitic steel described above. The specific chemical composition of the 980MPa grade martensitic steel can be referred to in the above embodiments. Since the preparation method of the 980MPa grade martensitic steel adopts some or all of the technical solutions of the above embodiments, it has at least all the beneficial effects brought about by the technical solutions of the above embodiments, which will not be elaborated here.

[0075] 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 industry standards. If there is no corresponding industry standard, then common international standards, conventional conditions, or conditions recommended by the manufacturer are followed.

[0076] The molten steel of Examples 1-7 and Comparative Examples 1-3 was prepared and cast into slabs. The chemical composition of the slabs is shown in Table 1.

[0077] Table 1. Chemical composition of the slab, by mass percentage (wt%), with the remainder being Fe and unavoidable impurities.

[0078]

[0079] Based on the chemical composition of the above-mentioned 980MPa grade martensitic steel, this application provides a method for preparing 980MPa grade martensitic steel, the method comprising the following steps:

[0080] S11, the target molten steel is obtained through KR desulfurization, conventional converter smelting, and dual refining in an LF furnace and a VD furnace. The chemical composition of the molten steel, by mass fraction, is as follows:

[0081] C: 0.04%~0.25%, Si: 0.1%~1.0%, Mn: 1.2%~3.0%, Al: 0.02%~0.35%, P: ≤0.005%, S: ≤0.002%, Ti: 0.010%~0.10%, Cr: 0.1%~0.6%, N: ≤0.004%, B: 0.0010%~0.0030%, with the remainder being Fe and unavoidable impurities;

[0082] S21. The molten steel is continuously cast into slabs: the slab casting speed is 4.0m / min~6.5m / min, and the slab thickness is 110mm~123mm;

[0083] S31. The slab is heated in the tunnel furnace of the multi-mode thin slab continuous casting and rolling production line at a temperature of 1150℃~1200℃. Before rough rolling, rough descaling is performed with a descaling pressure ≥30MPa. The rough rolling inlet temperature is ≥1150℃. The rough rolling adopts an irreversible 3-pass rolling process. The rough rolling outlet temperature is controlled at 930℃~1000℃.

[0084] S41. Before finishing rolling, induction heating and rough descaling are performed. The outlet temperature of the induction heating is controlled at 1050℃~1250℃. The finishing rolling adopts any one of single-slab rolling, semi-endless rolling and endless rolling.

[0085] When the finishing rolling is carried out using single-slab rolling, the finishing descaling is carried out using double-layer descaling, and the descaling pressure is ≥30MPa;

[0086] When the finishing rolling adopts semi-endless rolling or endless rolling, the finishing descaling adopts single-row descaling, and the descaling pressure is ≥30MPa; in the finishing descaling, the distance between the descaling nozzle and the slab is 60mm~110mm;

[0087] S51. Finishing Rolling: The finishing rolling adopts 5 passes, with a cumulative deformation of 70% to 80%. The final rolling temperature is controlled at 850℃ to 900℃. After rolling, a three-stage cooling process is adopted. The first stage cooling rate is ≥50℃ / s, the cooling stop temperature is 20℃ to 50℃ higher than the ferrite nose temperature, and the air cooling time is 3s to 10s. The third stage cooling rate is ≥50℃ / s, the coiling temperature is 100℃ to Mf, and the obtained steel coil is air cooled to room temperature. The main process parameters are shown in Table 2.

[0088] Table 2. Main process parameters for the preparation of 980MPa grade martensitic steel

[0089]

[0090] The mechanical properties of the martensitic steels obtained in Examples 1-7 and Comparative Examples 1-3 were tested, and the results are shown in Table 3.

[0091] Table 3 Mechanical properties of martensitic steel

[0092]

[0093] Detailed analysis in Table 3:

[0094] Yield strength refers to the yield limit of steel when it yields, which is the stress that resists a small amount of plastic deformation. The higher the yield strength, the greater the yield limit of the steel when it yields.

[0095] Tensile strength refers to the maximum stress that steel can withstand before breaking. The greater the tensile strength, the higher the maximum stress value of the wire.

[0096] Elongation is the percentage of the total deformation ΔL of the gauge length after the steel breaks under tension to the original gauge length L. The higher the elongation, the stronger the tensile strength of the steel.

[0097] Transverse cold bending refers to the degree of bending that steel can withstand at room temperature. The greater the transverse cold bending, the greater the degree of bending that the steel can withstand.

[0098] From the data in Examples 1-7, it can be seen that:

[0099] This invention employs a multi-mode thin slab continuous casting and rolling production line. By adjusting the chemical composition of the steel and improving the continuous casting and rolling processes, it is possible to obtain steel with a yield strength >900 MPa, tensile strength >1000 MPa, elongation >8%, and qualified transverse cold bending R=a and 180°. Furthermore, the steel plate has good shape, high dimensional accuracy, small strength fluctuation, and the steel plate thickness can be controlled within 0.8-4.0 mm.

[0100] Appendix Figure 2 Detailed explanation:

[0101] Figure 2 The metallographic diagram of the thin-gauge hot-rolled steel provided in the embodiments of this application is by Figure 2 It can be seen that the structure is martensitic.

[0102] Furthermore, one or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages:

[0103] The high-bending-resistance thin-gauge hot-rolled 980MPa martensitic steel provided in this invention, through chemical composition design, adopts a C-Si-Mn-Ti-Cr-B composition system and combines a three-stage cooling process. The first stage cooling rate is required to be ≥50℃ / s, and the cooling stop temperature is required to be 20℃~50℃ higher than the "nose point" temperature of ferrite transformation. Then, air cooling is performed for 3s~10s. By adding an appropriate amount of B element, the austenite-to-ferrite phase transformation time is delayed, and the ferrite grain size is refined. The third stage cooling rate is required to be ≥50℃ / s, directly cooling to 100℃~Mf. The resulting thin-gauge hot-rolled steel has a yield strength >800MPa, tensile strength >1000MPa, elongation >8%, and meets the cold bending R=a and 180° requirements. The thickness can reach 0.8mm~4.0mm, which can effectively solve the technical problem of cracking during bending of thin-gauge hot-rolled 980MPa martensitic steel in the prior art.

[0104] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A 980MPa grade martensitic steel, characterized in that, The chemical composition of the martensitic steel consists of the following components: C, Si, Mn, Al, P, S, Ti, Cr, N, B, and Fe, as well as unavoidable impurities; wherein, by mass fraction, The content of C is 0.04%~0.25%, the content of Si is 0.1%~1.0%, the content of Mn is 1.2%~3.0%, the content of Al is 0.02%~0.35%, the content of P is ≤0.005%, the content of S is ≤0.002%, the content of Ti is 0.030%~0.10%, the content of Cr is 0.1%~0.6%, the content of N is ≤0.004%, and the content of B is 0.0010%~0.0030%. The microstructure of the martensitic steel consists of martensite and ferrite, with the volume fraction of martensite being 88% to 95% and the volume fraction of ferrite being 5% to 12%. The thickness of the martensitic steel is 0.8mm~4.0mm, and the flatness of the martensitic steel plate is ≤5mm; the martensitic steel meets the following properties: yield strength >800MPa, tensile strength >1000MPa, elongation >8%, cold bending limit R=a, 180° qualified. The preparation method of the 980MPa grade martensitic steel includes: Molten steel is continuously cast to obtain a slab with the aforementioned chemical composition; The slab is preheated, rough descaling is performed, rough rolling is performed, induction heating is performed, fine descaling is performed, fine rolling is performed, staged cooling is performed and coiling is performed to obtain martensitic steel. The phased cooling includes: a first-stage cooling, a second-stage cooling, and a third-stage cooling. The cooling rate of the first-stage cooling is ≥50℃ / s, and the termination temperature of the first-stage cooling is 20℃~50℃ above the ferrite transformation temperature Ar3. The second-stage cooling is air cooling, and the air cooling time is 3s~10s. The cooling rate of the third-stage cooling is ≥50℃ / s, and the termination temperature of the third-stage cooling is 100℃~the martensitic phase transformation temperature Mf.

2. A method for preparing martensitic steel according to claim 1, characterized in that, The method includes: Molten steel is continuously cast to obtain a slab with the aforementioned chemical composition; The slab is preheated, rough descaling is performed, rough rolling is performed, induction heating is performed, fine descaling is performed, fine rolling is performed, staged cooling is performed and coiling is performed to obtain martensitic steel. The phased cooling includes: a first-stage cooling, a second-stage cooling, and a third-stage cooling. The cooling rate of the first-stage cooling is ≥50℃ / s, and the termination temperature of the first-stage cooling is 20℃~50℃ above the ferrite transformation temperature Ar3. The second-stage cooling is air cooling, and the air cooling time is 3s~10s. The cooling rate of the third-stage cooling is ≥50℃ / s, and the termination temperature of the third-stage cooling is 100℃~the martensitic phase transformation temperature Mf.

3. The method according to claim 2, characterized in that, The finishing rolling modes include single-slab rolling, semi-endless rolling, and fully endless rolling; the finishing descaling modes include single-row descaling and double-row descaling; wherein, the finishing rolling modes and the finishing descaling modes satisfy the following relationship: If the finishing rolling is single-slab rolling, then the finishing descaling is double-scaling. If the finishing rolling is a semi-endless rolling or a fully endless rolling, then the finishing descaling is a single-row descaling.

4. The method according to claim 2, characterized in that, The roughing mill has an inlet temperature ≥1150℃, an outlet temperature of 930℃~1000℃, and 3 passes; and / or, the finishing mill has a cumulative deformation of 70%~80%, a finishing mill temperature of 850℃~900℃, and 5 passes.

5. The method according to claim 2, characterized in that, The final temperature of the preheating is 1150℃~1200℃; and / or, the final temperature of the induction heating is 1050℃~1150℃.

6. The method according to claim 2, characterized in that, The pressure of coarse descaling is ≥30MPa, the pressure of fine descaling is ≥30MPa, and the distance between the nozzle of fine descaling and the slab is 60mm~110mm.

7. The method according to claim 2, characterized in that, The casting speed of the continuous casting is 4.0 m / min to 6.5 m / min, and the thickness of the slab cast by continuous casting is 115 mm to 123 mm.

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