A method for producing a hot-rolled martensitic steel, a hot-rolled martensitic steel

Abstract

This invention provides a method for preparing hot-rolled martensitic steel and the hot-rolled martensitic steel itself, belonging to the field of steel preparation. The method includes: obtaining a billet with a predetermined chemical composition; heating and rough rolling the billet, controlling the final pass reduction rate of the rough rolling, followed by finish rolling, controlling the final pass reduction rate of the finish rolling, to obtain a hot-rolled plate; rapidly cooling the hot-rolled plate at a predetermined cooling rate, and then coiling it at a predetermined coiling temperature to obtain a hot-rolled coil; and slowly cooling the hot-rolled coil in a slow cooling pit to obtain hot-rolled martensitic steel. Through a high hardenability composition system design, combined with rapid cooling followed by slow cooling after rolling, a matrix structure of martensite + lower bainite is achieved, wherein the martensite and lower bainite content is >90%, resulting in a product with high strength and high local formability. This improves the local formability of the hot-rolled martensitic steel.

Description

Technical Field

[0001] This application relates to the field of steel preparation technology, and in particular to a method for preparing hot-rolled martensitic steel and hot-rolled martensitic steel. Background Technology

[0002] High strength has become a trend in automotive steel development in recent years. The extensive use of high-strength steel can reduce steel consumption and overall vehicle weight, making it an important way to achieve lightweighting, energy conservation, and emission reduction in automobiles. Researchers have, through years of effort, developed many methods to increase steel strength, even reaching ultra-high strength levels. However, the decrease in formability resulting from increased strength has become a bottleneck restricting the development of automotive steel.

[0003] Taking into account factors such as cost and process performance, the direction for the development of martensitic steel towards high strength and high local forming ability should be to fundamentally solve the relationship between strength and formability of martensitic steel and improve the local forming performance of the martensitic structure itself. Summary of the Invention

[0004] This application provides a method for preparing hot-rolled martensitic steel and hot-rolled martensitic steel, in order to solve the following technical problem: how to improve the local formability of hot-rolled martensitic steel.

[0005] In a first aspect, this application provides a method for preparing hot-rolled martensitic steel, the method comprising:

[0006] A cast billet with a set chemical composition is obtained;

[0007] The billet is heated and rough rolled, and the reduction rate of the last pass of the rough rolling is controlled. Then, it is finished rolled, and the reduction rate of the last pass of the finish rolling is controlled to obtain a hot-rolled plate.

[0008] The hot-rolled plate is rapidly cooled at a set cooling rate and then wound at a set coiling temperature to obtain a hot-rolled coil.

[0009] The hot-rolled coil is placed in a slow cooling pit for slow cooling to obtain hot-rolled martensitic steel.

[0010] Optionally, the specified chemical composition, by mass fraction, includes: C: 0.08%–0.18%, Si > 0 and ≤ 0.3%, Mn: 1.0%–2.5%, P ≤ 0.012%, S ≤ 0.006%, Nb: 0.01%–0.07%, Ti: 0.01%–0.05%, Cr: 0.1%–0.8%, Mo > 0 and ≤ 0.3%, N ≤ 0.005%, B > 0 and ≤ 0.003%, with the balance being Fe and unavoidable impurities.

[0011] Optionally, the final rolling temperature of the roughing mill is 1050℃~1100℃, and the final reduction rate of the roughing mill is ≥30%.

[0012] Optionally, the finishing mill adopts constant speed rolling, the finishing mill's final rolling temperature is Ar3~(Ar3+50)℃, the reduction rate of the austenite non-recrystallization zone of the finishing mill is ≥30%, the ratio of the total reduction rate of the F6 and F7 stands to the total reduction rate of the F1~F7 stands is 0.1~0.3, and the ratio of the reduction rate of the F6 stand to the reduction rate of the F7 stand is 0.5~0.7.

[0013] Optionally, the method further includes:

[0014] During the rolling process in the non-recrystallization zone of the finishing mill, rolling lubricant is introduced into the mill; the flow rate of the rolling lubricant is 80 mL / min to 120 mL / min.

[0015] Optionally, the set cooling rate of the rapid cooling is ≥30℃ / s, and during the rapid cooling process, the time to cool from the final rolling temperature of the finishing mill to (Ms+100)℃ is <10s, and the set coiling temperature is 200℃~Ms.

[0016] Optionally, the slow cooling time is ≥60h, and the time interval between winding and entering the slow cooling pit is ≤2h.

[0017] Secondly, this application provides a hot-rolled martensitic steel prepared by the method described in any embodiment of the first aspect, wherein the metallographic structure of the hot-rolled martensitic steel includes: martensite, lower bainite, granular bainite and retained austenite; wherein, by volume fraction, the sum of the contents of the martensite and the lower bainite is >90%.

[0018] Optionally, the retained austenite size is <3μm, and the martensite and lower bainite satisfy the following relationship: Hv(martensite) - Hv(lower bainite) ≤ 80.

[0019] In the formula, Hv (martensite) represents the average hardness of martensite, and Hv (lower bainite) represents the average hardness of lower bainite.

[0020] Optionally, the hot-rolled martensitic steel satisfies at least one of the following properties: yield strength ≥ 900 MPa, tensile strength ≥ 1180 MPa, and elongation A ≥ 8%.

[0021] The technical solutions provided in this application have the following advantages compared with the prior art:

[0022] This application provides a method for preparing hot-rolled martensitic steel, comprising: obtaining a billet with a set chemical composition; heating and rough rolling the billet, controlling the reduction rate of the final pass of the rough rolling, followed by finish rolling, controlling the reduction rate of the final pass of the finish rolling, to obtain a hot-rolled plate; rapidly cooling the hot-rolled plate at a set cooling rate, and then coiling it at a set coiling temperature to obtain a hot-rolled coil; and slowly cooling the hot-rolled coil in a slow cooling pit to obtain hot-rolled martensitic steel. Through a high hardenability composition system design, combined with a rapid cooling + slow cooling process after rolling, a matrix structure of martensite + lower bainite is achieved, wherein the martensite and lower bainite contents are >90%, giving the product high strength and high local forming ability; by controlling the rolling process through the reduction rate of the final pass of the rough rolling and the reduction rate of the final rolling, the average hardness Hv of the lower bainite and martensite satisfies Hvmartensite-Hvlowbainite ≤80, and the retained austenite size is <3μm. This improves the local formability of hot-rolled martensitic steel. Attached Figure Description

[0023] 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.

[0024] 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.

[0025] Figure 1 This is a schematic flowchart illustrating a method for preparing hot-rolled martensitic steel, as provided in an embodiment of this application. Detailed Implementation

[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] Figure 1 This is a schematic flowchart illustrating a method for preparing hot-rolled martensitic steel, as provided in an embodiment of this application.

[0031] Please see Figure 1 This application provides a method for preparing hot-rolled martensitic steel, the method comprising:

[0032] S1. Obtain a cast billet with a set chemical composition;

[0033] In some embodiments, prior to step S1, the method further includes: pre-treating molten iron, then smelting it in a converter, refining it in an LF furnace, refining it in an RH furnace to obtain molten steel with a set composition, and then continuously casting it to obtain a billet.

[0034] In some embodiments, the specified chemical composition, by mass fraction, includes: C: 0.08%–0.18%, Si > 0 and ≤ 0.3%, Mn: 1.0%–2.5%, P ≤ 0.012%, S ≤ 0.006%, Nb: 0.01%–0.07%, Ti: 0.01%–0.05%, Cr: 0.1%–0.8%, Mo > 0 and ≤ 0.3%, N ≤ 0.005%, B > 0 and ≤ 0.003%, with the balance being Fe and unavoidable impurities.

[0035] In this embodiment, the positive effects of controlling the C mass fraction to be 0.08%–0.18% are as follows: Within this range, C is the most important solid solution strengthening element in steel and the element that ensures the hardenability of austenite. Therefore, an appropriate C content ensures that the steel obtains sufficient martensite and lower bainite during cooling to guarantee the strength of the steel. Simultaneously, C can form carbonitrides with microalloyed Nb and Ti elements during rolling, refining the grains and strengthening the microstructure, thus improving the mechanical properties of the steel. When the mass fraction value exceeds the maximum value at the end of this range, the C content will be too high, resulting in excessively hard steel and affecting its flexibility. When the mass fraction value is less than the minimum value at the end of this range, the C content will be too low, failing to guarantee the strength of the steel. For example, the C mass fraction can be 0.08%, 0.09%, 0.10%, 0.11%, 0.12%, 0.18%, etc.

[0036] The positive effects of controlling the Si mass fraction to be Si > 0 and ≤ 0.3% are as follows: Within this range, Si is a solid solution strengthening element for steel, and it also promotes the enrichment of C into austenite, improving the hardenability of austenite. When the mass fraction exceeds the maximum value at the end of this range, the hardenability of austenite in the steel will be insufficient, and the presence of Fe2SiO4 on the hot-rolled surface will lead to a decrease in the surface quality of the pickled surface. For example, the Si mass fraction can be 0.05%, 0.10%, 0.15%, 0.20%, 0.25%, 0.29%, etc.

[0037] The positive effects of controlling the Mn mass fraction to 1.0%–2.5% are as follows: Within this range, Mn is an important element for solid solution strengthening and austenite stabilization, playing a crucial role in enhancing the mechanical properties of steel. When the mass fraction exceeds the maximum value of this range, excessive Mn content can easily lead to microstructure segregation, causing cracking during steel forming and deteriorating the steel's mechanical properties. Conversely, when the mass fraction is less than the minimum value of this range, insufficient Mn content will prevent it from effectively performing its role in solid solution strengthening and austenite stabilization. For example, the Mn mass fraction can be 1.0%, 1.2%, 1.4%, 1.6%, 1.8%, 2.0%, 2.2%, 2.5%, etc.

[0038] The positive effects of controlling the mass fraction of phosphorus (P) to ≤0.012% are as follows: Within this range, P inhibits carbide formation and ensures that the overall carbon equivalent of the steel remains within a suitable range. When the mass fraction exceeds the maximum value at the extreme end of this range, it leads to a decrease in grain boundary strength and deteriorates the material's mechanical properties. Examples of P mass fractions include 0.003%, 0.005%, 0.007%, 0.009%, 0.010%, 0.012%, and 0.012%.

[0039] The positive effects of controlling the sulfur (S) mass fraction to ≤0.006% are as follows: Within this range, S is a harmful element and will combine with Mn to produce MnS, thereby deteriorating the mechanical properties of the steel. When the mass fraction exceeds the maximum value at the end of this range, the S content will be excessive. On the one hand, this requires increasing the amount of Ti, and on the other hand, it will weaken the mechanical properties of the steel, affecting the hole-expanding performance. For example, the S mass fraction can be 0.0005%, 0.0010%, 0.002%, 0.004%, 0.005%, 0.006%, etc.

[0040] The positive effects of controlling the Nb mass fraction to be between 0.01% and 0.07% are as follows: Within this range, Nb can combine with C or N to form nano-precipitates, thereby refining the grains and promoting precipitation strengthening. This significantly improves the microstructure and yield strength, while also refining the austenite grain size during heating, ultimately achieving hard phase dispersion. When the mass fraction exceeds the maximum value of this range, the elongation of the steel decreases. When the mass fraction is less than the minimum value, insufficient Nb content prevents the formation of enough strengthening precipitates, thus failing to achieve the grain refining and precipitation strengthening effects. Examples of Nb mass fractions include 0.01%, 0.03%, 0.04%, 0.05%, and 0.07%.

[0041] The positive effects of controlling the Ti mass fraction to be 0.01%–0.05% include: within this range, Ti can combine with C or N to form nano-precipitates, thereby refining the grains and promoting precipitation strengthening. This significantly improves the microstructure and yield strength, while also refining the austenite grain size during heating, ultimately achieving hard phase dispersion, which positively impacts porosity. However, if the mass fraction exceeds the maximum value of this range, the elongation of the steel will decrease. Conversely, if the mass fraction is below the minimum value, insufficient Ti content will prevent the formation of enough strengthening phases, thus failing to achieve the grain refinement and precipitation strengthening effects. Examples of Ti mass fractions include 0.01%, 0.02%, 0.03%, 0.04%, and 0.05%.

[0042] The positive effects of controlling the Cr mass fraction to be between 0.1% and 0.8% include improved hardenability within this range. When the mass fraction exceeds the maximum value of this range, the adverse effects are higher cost and decreased surface quality. Conversely, when the mass fraction is less than the minimum value of this range, the adverse effect is insufficient hardenability. Examples of Cr mass fractions include 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, and 0.8%.

[0043] The positive effects of controlling the Mo mass fraction to be >0 and ≤0.3% include improved hardenability within this range; however, a negative effect is increased cost when the mass fraction exceeds the maximum value at the end of this range. Examples of Mo mass fractions include 0.05%, 0.10%, 0.15%, 0.20%, 0.25%, and 0.3%.

[0044] The positive effects of controlling the mass fraction of nitrogen (N) to ≤0.005% include: within this range, N forms nano-precipitates with Ti and Nb, which refines the grains and strengthens the steel, thus improving its strength. However, when the mass fraction exceeds the maximum value at the end of this range, excessive N content leads to increased precipitates and negatively impacts the steel's performance. Examples of N mass fractions include 0.002%, 0.003%, 0.004%, and 0.005%.

[0045] The positive effects of controlling the mass fraction of boron (B) to be >0 and ≤0.003% are as follows: Within this range, an appropriate amount of B can increase the binding force of grain boundaries and effectively inhibit the formation of pearlite and ferrite, thus improving hardenability. When the mass fraction exceeds the maximum value at the end of this range, it indicates that the B content is too high, which will lead to the precipitation of borides and reduce the effect of B atoms. For example, the mass fraction of B can be 0.0005%, 0.001%, 0.0015%, 0.002%, 0.0025%, 0.003%, etc.

[0046] S2. The billet is heated and rough rolled, and the reduction rate of the last pass of the rough rolling is controlled. Then, the billet is finished rolled, and the reduction rate of the last pass of the finish rolling is controlled to obtain a hot-rolled plate.

[0047] In some embodiments, the final rolling temperature of the roughing mill is 1050℃~1100℃, and the final reduction rate of the roughing mill is ≥30%.

[0048] The positive effects of controlling the final rolling temperature of roughing mill to 1050℃~1100℃ include: controlling the size of deformed austenite and improving the uniformity of the microstructure. For example, the final rolling temperature of the roughing mill is 1050℃, 1060℃, 1070℃, 1080℃, 1090℃, 1100℃, etc.

[0049] In some embodiments, the finishing mill is a constant speed rolling process, the finishing mill's final rolling temperature is Ar3 to (Ar3+50)℃, the reduction rate of the austenite non-recrystallization zone of the finishing mill is ≥30%, the ratio of the total reduction rate of the F6 and F7 stands to the total reduction rate of the F1 to F7 stands is 0.1 to 0.3, and the ratio of the reduction rate of the F6 stand to the reduction rate of the F7 stand is 0.5 to 0.7.

[0050] It should be noted that Ac3 is the final temperature at which the material transforms into austenite during heating; the finishing mill unit includes: F1 finishing mill line, F2 finishing mill line, F3 finishing mill line, F4 finishing mill line, F5 finishing mill line, F6 finishing mill line, and F7 finishing mill line. Among them, the F1 to F7 finishing mill lines adopt a four-high irreversible horizontal mill design, with a total of seven mill stands arranged continuously, named F1 to F7 stands in sequence, and each stand has a unique function and structure.

[0051] In some embodiments, the thickness of the hot-rolled plate is controlled within the range of 1.8–5 mm, and the rolling speed is controlled within the range of 5–10 m / s depending on the thickness, which can be used to improve the uniformity of the microstructure. For example, the rolling speed of the finishing mill can be 5 m / s, 6 m / s, 7 m / s, 8 m / s, 9 m / s, 10 m / s, etc.; the thickness of the hot-rolled plate can be 1.8 mm, 2 mm, 3 mm, 4 mm, 5 mm, etc.

[0052] The positive effects of controlling the ratio of the combined reduction rate of stands F6 and F7 to the combined reduction rate of stands F1 to F7 to be 0.1 to 0.3, and the ratio of the reduction rate of stand F6 to the reduction rate of stand F7 to be 0.5 to 0.7, are: to thin the austenite, increase the ferrite nucleation area, and ultimately refine the grains. For example, the ratio of the combined reduction rate of stands F6 and F7 to the combined reduction rate of stands F1 to F7 can be 0.1, 0.15, 0.2, 0.25, 0.3, etc., and the ratio of the reduction rate of stand F6 to the reduction rate of stand F7 can be 0.5, 0.55, 0.6, 0.65, 0.7, etc.

[0053] By controlling the rolling process, such as the reduction in the final pass of roughing and the reduction in the finish pass, the average hardness Hv of the lower bainite and martensite is made to satisfy Hv (马氏体) -Hv (下贝氏体) The austenite content is ≤80, and the retained austenite size is <3μm. For example, the final reduction rate of the roughing roll can be 30%, 32%, 35%, 38%, 40%, 45%, etc. The reduction rate of the austenite in the non-recrystallized zone of the finishing slag can be 30%, 32%, 35%, 38%, 40%, 45%, etc.

[0054] In some embodiments, the method further includes:

[0055] During the rolling process in the non-recrystallization zone of the finishing mill, rolling lubricant is introduced into the mill; the flow rate of the rolling lubricant is 80 mL / min to 120 mL / min.

[0056] By applying rolling lubrication through the non-recrystallization zone of the rolling mill, the uniformity of the microstructure of the hot-rolled plate is ensured, thereby improving the porosity. For example, the flow rate of the rolling lubricant can be 80 mL / min, 85 mL / min, 90 mL / min, 95 mL / min, 100 mL / min, 110 mL / min, 120 mL / min, etc.

[0057] S3. The hot-rolled plate is rapidly cooled at a set cooling rate and then wound at a set coiling temperature to obtain a hot-rolled coil.

[0058] In some embodiments, the set cooling rate of the rapid cooling is ≥30℃ / s, the time to cool from the final rolling temperature to (Ms+100)℃ during the rapid cooling process is <10s, and the set coiling temperature is 200℃~Ms.

[0059] It should be noted that rapid cooling is laminar flow cooling; the MS point is the martensitic transformation initiation temperature of steel, reflecting the highest temperature at which supercooled austenite begins to undergo martensitic transformation. It has a significant impact on the formulation of the heat treatment process and the quality and performance of the workpiece after heat treatment, and is of great importance in the preparation process. Alloying elements in steel have a significant impact on the phase transformation thermodynamics and kinetics of steel, including the MS point. It can be considered that the composition of steel determines its MS point. In this application, the value of Ms satisfies the following relationship: Ms(°C)=560-474×[C]-33×[Mn]-17×[Cr]-17×[Ni]-21×[Mo], where [C] represents the mass fraction of C, [Mn] represents the mass fraction of Mn, [Cr] represents the mass fraction of Cr, [Ni] represents the mass fraction of Ni, and [Mo] represents the mass fraction of Mo.

[0060] The positive effects of controlling the rapid cooling setting of ≥30℃ / s and cooling the slag from the final rolling temperature to (Ms+100)℃ in less than 10s are: ensuring the uniformity of the microstructure and avoiding the appearance of large-sized non-target microstructures.

[0061] The positive effects of controlling the winding temperature to 200℃~Ms: ensuring tissue uniformity and achieving the target tissue structure.

[0062] S4. The hot-rolled coil is placed into a slow cooling pit for slow cooling to obtain hot-rolled martensitic steel.

[0063] In some embodiments, the slow cooling time is ≥60h, and the time interval between the winding and the entry into the slow cooling pit is ≤2h.

[0064] The positive effects of controlling the slow cooling time to ≥60h and the time interval between take-up and entry into the slow cooling pit to ≤2h are: ensuring tissue uniformity and achieving the target tissue structure.

[0065] By designing a high-hardenability composition system and combining it with post-rolling rapid cooling and slow cooling processes, a matrix microstructure of martensite + lower bainite is achieved, with martensite and lower bainite content exceeding 90%. This results in products possessing both high strength and high local forming capability.

[0066] Based on a general inventive concept, this application provides a hot-rolled martensitic steel, the metallographic structure of which includes: martensite, lower bainite, granular bainite and retained austenite; wherein, by volume fraction, the sum of the contents of the martensite and the lower bainite is >90%.

[0067] By designing a high hardenability composition system and combining it with post-rolling rapid cooling and slow cooling processes, a matrix microstructure of martensite + lower bainite is achieved, with martensite and lower bainite content exceeding 90%. This results in products possessing both high strength and high local forming capability. For example, the sum of the martensite and lower bainite contents can be 91%, 92%, 93%, 94%, 95%, etc.

[0068] In some embodiments, the retained austenite size is <3 μm, and the martensite and lower bainite satisfy the following relationship: Hv(martensite) - Hv(lower bainite).

[0069] In the formula, Hv (martensite) represents the average hardness of martensite, and Hv (lower bainite) represents the average hardness of lower bainite.

[0070] By controlling the rolling process, such as the reduction in the final pass of roughing and the reduction in the finish pass, the average hardness Hv of the lower bainite and martensite is made to satisfy Hv (马氏体) -Hv (下贝氏体) ≤80, and the retained austenite size <3μm. For example, Hv (马氏体) -Hv (下贝氏体) The values ​​can be 60, 65, 70, 72, 75, 80, etc., and the size of the retained austenite can be 1μm, 1.5μm, 2μm, 2.5μm, 3μm, etc.

[0071] In some embodiments, the hot-rolled martensitic steel satisfies at least one of the following properties: yield strength ≥ 900 MPa, tensile strength ≥ 1180 MPa, and elongation A ≥ 8%.

[0072] The hot-rolled martensitic steel prepared in this application possesses high strength, excellent local forming properties, and formability. For example, the yield strength of this hot-rolled martensitic steel can be 900 MPa, 910 MPa, 920 MPa, 940 MPa, 960 MPa, 970 MPa, etc., the tensile strength can be 1180 MPa, 1200 MPa, 1210 MPa, 1220 MPa, 1230 MPa, etc., and the elongation A can be 8%, 8.2%, 8.4%, 8.6%, 8.8%, 9.0%, 9.5%, etc.

[0073] The hot-rolled martensitic steel is realized based on the above-described method for preparing hot-rolled martensitic steel. The specific steps of the method for preparing hot-rolled martensitic steel can be referred to the above embodiments. Since the hot-rolled 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.

[0074] 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.

[0075] Example 1

[0076] The chemical composition of a hot-rolled martensitic steel with high local forming performance is as follows: C: 0.1%, Si: 0.1%, Mn: 1.2%, P: 0.012%, S: 0.003%, Nb: 0.05%, Ti: 0.03%, Cr: 0.5%, Mo: 0.2%, N: 0.003%, B: 0.0025%, with the balance being Fe and other impurity elements.

[0077] Based on the above chemical composition, this comparative example also provides a method for preparing hot-rolled martensitic steel with high local forming performance, comprising the following steps: After pretreatment of molten iron, it is smelted in a converter, refined by LF refining, and refined by RH refining to obtain a slab. The slab is then heated and subjected to rough rolling and finish rolling to obtain a hot-rolled plate. During finish rolling, constant speed rolling is used, the hot-rolled plate thickness is 3 mm, and the rolling speed is controlled at 7 m / s. After cooling, the hot-rolled plate is coiled into a hot-rolled coil. The rough rolling termination temperature is controlled at 1070℃, and the final rough rolling reduction rate is 33%. The finishing mill's final rolling temperature is controlled at Ar3 - (Ar3 + 30℃). The reduction rate in the non-recrystallized austenite zone is 35%, and the mill in the non-recrystallized zone is lubricated with a flow rate of 100 mL / min. The ratio of the combined reduction rate of the finishing mill stands F6 and F7 to the combined reduction rate of stands F1 to F7 is 0.15, and the ratio of the reduction rate of stand F6 to stand F7 is 0.55. The rolled plate is rapidly cooled to the coiling temperature, with a target of 250℃. The time required to cool from the final rolling temperature Ar3 - (Ar3 + 30℃) to Ms(℃) + 100℃ is 8 seconds, and the average cooling rate in the layer cooling section is 45℃ / s. The hot-rolled plate is placed in the slow cooling pit within 1.5 hours after coiling and left to stand for 65 hours.

[0078] The microstructure of the hot-rolled martensitic steel prepared by this invention is martensite + lower bainite, wherein the content of martensite and lower bainite is 95%, and the remainder is granular bainite and retained austenite. The average hardness Hv of the lower bainite and martensite satisfies Hv(martensite)-Hv(lower bainite) = 65, and the size of the retained austenite is 2.5μm.

[0079] The hot-rolled martensitic steel with high local formability provided by the present invention and its preparation method, through the control of chemical composition and rolling and hot rolling process parameters, produces strip steel with a yield strength of 950MPa, a tensile strength of 1210MPa, and an elongation of A=9%, exhibiting excellent local formability and forming performance.

[0080] Example 2

[0081] The chemical composition of a hot-rolled martensitic steel with high local formability is as follows: C: 0.12%, Si: 0.13%, Mn: 1.5%, P: 0.012%, S: 0.0015%, Nb: 0.04%, Ti: 0.035%, Cr: 0.55%, Mo: 0.15%, N: 0.0028%, B: 0.0020%, with the balance being Fe and other impurity elements.

[0082] Based on the above chemical composition, this comparative example also provides a method for preparing hot-rolled martensitic steel with high local forming performance, comprising the following steps: After pretreatment of molten iron, it is smelted in a converter, refined by LF refining, and refined by RH refining to obtain a slab. The slab is then heated and subjected to rough rolling and finish rolling to obtain a hot-rolled plate. During finish rolling, constant speed rolling is used, the hot-rolled plate thickness is 2.5 mm, and the rolling speed is controlled at 8 m / s. After cooling, the hot-rolled plate is coiled into a hot-rolled coil. The rough rolling termination temperature is controlled at 1080℃, and the final rough rolling reduction rate is 35%. The finishing mill's final rolling temperature is controlled at Ar3 - (Ar3 + 25℃). The reduction rate in the non-recrystallized austenite zone is 40%, and the mill in the non-recrystallized zone is lubricated with a flow rate of 110 mL / min. The ratio of the combined reduction rate of the finishing mill stands F6 and F7 to the combined reduction rate of stands F1 to F7 is 0.18, and the ratio of the reduction rate of stand F6 to stand F7 is 0.7. The rolled plate is rapidly cooled to the coiling temperature, with a target of 280℃. The time required to cool from the final rolling temperature Ar3 - (Ar3 + 30℃) to Ms(℃) + 100℃ is 9 seconds, and the average cooling rate in the layer cooling section is 55℃ / s. The hot-rolled plate is placed in the slow cooling pit within 1 hour after coiling and left for 70 hours.

[0083] The microstructure of the hot-rolled martensitic steel prepared by this invention is martensite + lower bainite, wherein the content of martensite and lower bainite is 92%, and the remainder is granular bainite and retained austenite. The average hardness Hv of the lower bainite and martensite satisfies Hv(martensite)-Hv(lower bainite) = 70, and the size of the retained austenite is 2.8μm.

[0084] The hot-rolled martensitic steel with high local formability provided by the present invention and its preparation method, through the control of chemical composition and rolling and hot rolling process parameters, produces strip steel with a yield strength of 950MPa, a tensile strength of 1210MPa, and an elongation of A=9%, exhibiting excellent local formability and forming performance.

[0085] Example 3

[0086] The chemical composition of a hot-rolled martensitic steel with high local forming performance is as follows: C: 0.15%, Si: 0.2%, Mn: 1.8%, P: 0.012%, S: 0.0015%, Nb: 0.04%, Ti: 0.035%, Cr: 0.55%, Mo: 0.2%, N: 0.0025%, B: 0.0025%, with the balance being Fe and other impurity elements.

[0087] Based on the above chemical composition, this embodiment also provides a method for preparing hot-rolled martensitic steel with high local forming performance, including the following steps: After pretreatment of molten iron, it is smelted in a converter, refined by LF refining, and refined by RH refining to obtain a slab. The slab is then heated and subjected to rough rolling and finish rolling to obtain a hot-rolled plate. During finish rolling, constant speed rolling is used, the hot-rolled plate thickness is 3.2 mm, and the rolling speed is controlled at 9 m / s. After cooling, the hot-rolled plate is coiled into a hot-rolled coil. The rough rolling termination temperature is controlled at 1080℃, and the final rough rolling reduction rate is 42%. The finishing mill's final rolling temperature is controlled at Ar3 - (Ar3 + 25℃). The reduction rate in the non-recrystallized austenite zone is 45%, and the mill in the non-recrystallized zone is lubricated with a flow rate of 110 mL / min. The ratio of the combined reduction rate of the finishing mill stands F6 and F7 to the combined reduction rate of stands F1 to F7 is 0.18, and the ratio of the reduction rate of stand F6 to stand F7 is 0.65. The rolled plate is rapidly cooled to the coiling temperature, with a target of 280℃. The time required to cool from the final rolling temperature Ar3 - (Ar3 + 30℃) to Ms(℃) + 100℃ is 8 seconds, and the average cooling rate in the layer cooling section is 60℃ / s. The hot-rolled plate is placed in the slow cooling pit within 1 hour after coiling and left for 70 hours.

[0088] The microstructure of the hot-rolled martensitic steel prepared by this invention is martensite + lower bainite, wherein the content of martensite and lower bainite is 90%, and the remainder is granular bainite and retained austenite. The average hardness Hv of the lower bainite and martensite satisfies Hv(martensite)-Hv(lower bainite) = 65, and the size of the retained austenite is 2.5μm.

[0089] The hot-rolled martensitic steel with high local formability provided by the present invention and its preparation method, through the control of chemical composition and rolling and hot rolling process parameters, produces strip steel with a yield strength of 965MPa, a tensile strength of 1220MPa, and an elongation of A=9%, exhibiting excellent local formability and forming performance.

[0090] Comparative Example 1

[0091] The chemical composition of a hot-rolled martensitic steel with high local formability is as follows: C: 0.07%, Si: 0.1%, Mn: 1.8%, P: 0.010%, S: 0.0015%, Nb: 0.005%, Ti: 0.03%, Cr: 0.5%, Mo: 0.2%, N: 0.002%, B: 0.0035%, with the balance being Fe and other impurity elements.

[0092] Based on the above chemical composition, this comparative example also provides a method for preparing hot-rolled martensitic steel with high local forming performance, comprising the following steps: After pretreatment of molten iron, it is smelted in a converter, refined by LF refining, and refined by RH refining to obtain a slab. The slab is then heated and subjected to rough rolling and finish rolling to obtain a hot-rolled plate. During finish rolling, constant speed rolling is used, the hot-rolled plate thickness is 3.2 mm, and the rolling speed is controlled at 9 m / s. After cooling, the hot-rolled plate is coiled into a hot-rolled coil. The rough rolling termination temperature is controlled at 1080℃, and the reduction rate of the final rough rolling pass is 20%. The finishing mill's final rolling temperature is controlled at Ar3 - (Ar3 + 25℃). The reduction rate in the non-recrystallized austenite zone is 45%, and the mill in the non-recrystallized zone is lubricated with a flow rate of 110 mL / min. The ratio of the combined reduction rate of the finishing mill stands F6 and F7 to the combined reduction rate of stands F1 to F7 is 0.18, and the ratio of the reduction rate of stand F6 to stand F7 is 0.45. The rolled plate is rapidly cooled to the coiling temperature, with a target of 100℃. The time required to cool from the final rolling temperature Ar3 - (Ar3 + 30℃) to Ms(℃) + 100℃ is 8 seconds, and the average cooling rate in the layer cooling section is 60℃ / s. The hot-rolled plate is placed in the slow cooling pit within 1 hour after coiling and left for 70 hours.

[0093] The microstructure of the hot-rolled martensitic steel prepared by this invention is martensite + lower bainite, wherein the content of martensite and lower bainite is 90%, and the remainder is granular bainite and retained austenite. The average hardness Hv of the lower bainite and martensite satisfies Hv(martensite)-Hv(lower bainite) = 85, and the size of the retained austenite is 2.5μm.

[0094] This results in a strip steel with a yield strength of 850 MPa, a tensile strength of 1210 MPa, an elongation of A = 10%, and poor local formability and forming performance.

[0095] Furthermore, one or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages:

[0096] In this embodiment of the invention, a C-Mn-Cr-Nb-Ti-Mo-B composition system is designed, and hot-rolled martensitic steel with high local forming performance is obtained by controlling rolling, laminar flow cooling, and slow cooling after rolling.

[0097] In this embodiment of the invention, the hot-rolled martensitic steel prepared has a yield strength ≥900MPa, a tensile strength ≥1180MPa, and an elongation A ≥8%, exhibiting excellent local forming performance and forming properties.

[0098] 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 method for preparing hot-rolled martensitic steel, characterized in that, The method includes: A cast billet with a set chemical composition is obtained; The billet is heated and rough rolled, and the reduction rate of the last pass of the rough rolling is controlled to be ≥30%. Then, it is finished rolled, and the reduction rate of the last pass of the finish rolling is controlled to obtain a hot-rolled plate. The hot-rolled plate is rapidly cooled at a set cooling rate and then wound at a set coiling temperature to obtain a hot-rolled coil. The hot-rolled coil is placed in a slow cooling pit for slow cooling to obtain hot-rolled martensitic steel; The finishing mill is rolled at a constant speed. The final rolling temperature of the finishing mill is Ar3 ~ (Ar3+50)℃. The reduction rate of the austenite non-recrystallization zone of the finishing mill is ≥30%. The ratio of the total reduction rate of the F6 and F7 stands to the total reduction rate of the F1~F7 stands is 0.1~0.

3. The ratio of the reduction rate of the F6 stand to the reduction rate of the F7 stand is 0.5~0.

7. The rapid cooling rate is set to be ≥30℃ / s. During the rapid cooling process, the time from the final rolling temperature of the finishing mill to (Ms+100)℃ is <10s. The set coiling temperature is 200℃~Ms. The slow cooling time is ≥60h, and the time interval between the winding and the entry into the slow cooling pit is ≤2h.

2. The method according to claim 1, characterized in that, The specified chemical composition, by mass fraction, includes: C: 0.08%~0.18%, Si>0 and ≤0.3%, Mn: 1.0%~2.5%, P≤0.012%, S≤0.006%, Nb: 0.01%~0.07%, Ti: 0.01%~0.05%, Cr: 0.1%~0.8%, Mo>0 and ≤0.3%, N≤0.005%, B>0 and ≤0.003%, with the balance being Fe and unavoidable impurities.

3. The method according to claim 1, characterized in that, The final rolling temperature of the roughing mill is 1050℃~1100℃.

4. The method according to claim 1, characterized in that, The method further includes: During the rolling process in the non-recrystallization zone of the finishing mill, rolling lubricant is introduced into the mill; the flow rate of the rolling lubricant is 80 mL / min to 120 mL / min.

5. A hot-rolled martensitic steel prepared by the method according to any one of claims 1 to 4, characterized in that, The metallographic structure of the hot-rolled martensitic steel includes: martensite, lower bainite, granular bainite, and retained austenite; wherein, by volume fraction, the sum of the contents of the martensite and the lower bainite is >90%.

6. The hot-rolled martensitic steel according to claim 5, characterized in that, The retained austenite size is <3μm, and the martensite and lower bainite satisfy the following relationship: Hv 马氏体 -Hv 下贝氏体 ≤80, In the formula, Hv 马氏体 Hv represents the average hardness of martensite. 下贝氏体 This indicates the average hardness of lower bainite.

7. The hot-rolled martensitic steel according to claim 6, characterized in that, The hot-rolled martensitic steel meets at least one of the following properties: yield strength ≥ 900 MPa, tensile strength ≥ 1180 MPa, and elongation A ≥ 8%.

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