A cold rolled martensitic steel grade 1500 mpa with resistance to delayed fracture and a method of manufacturing thereof

Abstract

The application relates to a 1500MPa-grade cold-rolled martensite steel resistant to delayed fracture and a preparation method thereof, and the method comprises the following steps: obtaining a cold hard plate with a set chemical composition; continuously annealing the cold hard plate, which comprises the following steps: first heating the cold hard plate to a first temperature, and performing first heat preservation under the condition of the first temperature to obtain a first annealed plate; second heating the first annealed plate to a second temperature, and performing second heat preservation under the condition of the second temperature, and then performing slow cooling and rapid cooling to obtain a second annealed plate with a complete martensite structure; third heating the second annealed plate to a third temperature to perform overaging, and obtaining a continuously annealed plate; and performing second annealing on the continuously annealed plate under the condition of a fourth temperature to obtain the 1500MPa-grade cold-rolled martensite steel resistant to delayed fracture. The application improves the delayed fracture resistance of the 1500MPa-grade cold-rolled martensite steel.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of steel production, in particular to an anti-delay fracture 1500MPa grade cold-rolled martensitic steel and a preparation method thereof. BACKGROUND

[0002] With the rapid development of global automobile industry, lightweight is of great significance to energy saving and emission reduction of automobiles. In addition, for safety considerations, the automobile industry uses a large number of high-strength steel plates. Cold-rolled high-strength steel mainly includes DP steel, DH steel, TRIP steel, CP steel and MS steel, etc., which are mainly realized by adding alloying elements, increasing the fast cooling rate, and utilizing phase transformation strengthening. Cold-rolled martensitic steel is a kind of cold-rolled advanced high-strength steel, with a strength from a minimum of 900MPa to a maximum of 1700MPa, and has ultra-high strength and excellent bending performance, and is usually applied to automobile bumper beams, bumpers and other parts.

[0003] However, for 1500MPa and above strength grades, high-strength special annealing production lines with water quenching function are generally required for production, but such equipment has a large investment and requires high control of process stability. If not properly controlled, it often causes serious problems such as poor plate shape and large performance fluctuations. At the same time, ultra-high strength steel with a strength of 1500MPa and above is particularly sensitive to hydrogen embrittlement, which greatly limits its application in lightweight vehicle bodies. The interaction of hydrogen with different lattice defects significantly reduces the plasticity of hydrogen embrittlement sensitive materials and leads to unpredictable failure far below the ultimate tensile strength, which makes it difficult for the product to be mass-produced. SUMMARY

[0004] The present application provides an anti-delay fracture 1500MPa grade cold-rolled martensitic steel and a preparation method thereof, to solve the technical problem of how to improve the anti-delay fracture performance of 1500MPa grade cold-rolled martensitic steel.

[0005] In a first aspect, the present application provides a preparation method of an anti-delay fracture 1500MPa grade cold-rolled martensitic steel, which comprises:

[0006] obtaining a cold hard plate with a set chemical composition;

[0007] continuously annealing the cold hard plate to obtain a continuously annealed plate;

[0008] The continuously annealing the cold hard plate to obtain a continuously annealed plate comprises: first heating the cold hard plate to a first temperature, and first holding at the first temperature to obtain a first annealed plate;

[0009] second heating the first annealed plate to a second temperature, and second holding at the second temperature, and then slow cooling and fast cooling to obtain a second annealed plate with a completely martensitic structure.

[0010] third heating the second annealed plate to a third temperature to perform overaging, to obtain a continuous annealed plate;

[0011] second annealing the continuous annealed plate at a fourth temperature to obtain a 1500MPa grade cold-rolled martensitic steel with resistance to delayed fracture.

[0012] Optionally, the first temperature is 640-680℃, and the first holding time is 30-180s.

[0013] Optionally, the second temperature is (Ac3+10)℃-(Ac3+20)℃, and the second holding time is 180-360s.

[0014] Optionally, the first heating has a heating rate of 2-5℃ / s; and / or,

[0015] the second heating has a heating rate of 3-8℃ / s.

[0016] Optionally, the third temperature is 280-320℃.

[0017] Optionally, the fourth temperature is 200-240℃.

[0018] Optionally, the process parameters of the slow cooling include: a cooling rate of 1-15℃ / s, and an end temperature of Ac1-Ac3; and / or,

[0019] the process parameters of the fast cooling include: a cooling medium containing 50% by volume of hydrogen, a cooling rate of ≥45℃ / s, and an end temperature of 200-240℃.

[0020]

[0021] Optionally, the set chemical composition includes:

[0022] C, Si, Mn, P, S, Cr, Nb, Ti, Cu, Ni, B, N, and Fe; wherein, in terms of mass fraction,

[0023] the content of C is 0.2%-0.3%, the content of Si is 0.2%-1.0%, the content of Mn is 2.0%-3.0%, 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 Nb is 0.01%-0.04%, the content of Ti is 0.01%-0.04%, the content of Cu is ≤0.03%, the content of Ni is 0.01%-0.05%, the content of B is 0.001%-0.002%, and the content of N is 0.003%-0.005%.​

[0024] Optionally, the obtaining of the cold hard plate with the set chemical composition comprises:

[0025] casting the molten steel to obtain a slab;

[0026] heating and rolling the slab with the set chemical composition, and then coiling to obtain a hot-rolled coil;

[0027] cold rolling the hot-rolled coil to obtain the cold hard plate with the set chemical composition; wherein,

[0028] the casting speed is 4 m / min to 7 m / min, and the thickness of the slab is 110 mm to 125 mm; and / or,

[0029] the heating temperature is 1120℃ to 1220℃; and / or,

[0030] the finish rolling temperature is 890℃ to 920℃; and / or,

[0031] the coiling temperature is 550℃ to 600℃.

[0032] In a second aspect, the application provides an anti-delay fracture 1500MPa-grade cold-rolled martensitic steel prepared by the method of the first aspect,

[0033] The cold-rolled martensitic steel satisfies the following mechanical properties: tensile strength > 1500MPa, yield strength

[0034] 1200MPa to 1500MPa, elongation A80≥3%, and hole expansion ratio≥50%;

[0035] The microstructure of the cold-rolled martensitic steel is all lath-shaped martensite.

[0036] The above technical solution provided by the embodiments of the application has the following advantages compared with the prior art:

[0037] The preparation method of the anti-delay fracture 1500MPa grade cold-rolled martensite steel provided by the embodiment of the present application comprises the following steps: obtaining a cold hard plate with a set chemical composition; continuously annealing the cold hard plate to obtain a continuously annealed plate; wherein the continuous annealing comprises the following steps: first heating the cold hard plate to a first temperature and performing first heat preservation at the first temperature to obtain a first annealed plate; second heating the first annealed plate to a second temperature and performing second heat preservation at the second temperature, and then performing slow cooling and rapid cooling to obtain a second annealed plate with a structure completely composed of martensite; third heating the second annealed plate to a third temperature to perform overaging; and performing second annealing on the continuously annealed plate at a fourth temperature to obtain the anti-delay fracture 1500MPa grade cold-rolled martensite steel. The cold hard plate with the set chemical composition is first heated to the first temperature and is subjected to the first heat preservation, so that the cold hard plate is fully tempered, thereby making the structure of the cold hard plate uniform, so that the ferrite grains at different parts of the cold hard plate are fully recrystallized, the grain difference of the ferrite is small, meanwhile, the martensite is fully tempered and is transformed into equiaxed ferrite and granular cementite, the pearlite is also obviously spheroidized, and due to the fact that the grain boundary is a rapid diffusion channel of carbon atoms, the carbon atoms are gradually diffused to the original ferrite area with low carbon during the first heat preservation process, so that the uniform distribution of carbide particles is realized; the second heating to the second temperature and the second heat preservation make the structure of the cold hard plate all austenitized, the fibrous microstructure completes the recovery and recrystallization process and is all transformed into austenite, meanwhile, the residual cementite is fully dissolved, the components and microstructure in the austenite are fully homogenized, and the carbide is fully analyzed, thereby improving the anti-delay fracture performance; the slow cooling can inhibit the formation of new ferrite; the rapid cooling can promote the transformation of austenite into martensite, so that a structure with a structure completely composed of martensite is obtained; the third heating to the third temperature to perform overaging can make the martensite tempered, thereby improving the formability and anti-delay fracture performance of the cold hard plate after the rapid cooling; the second annealing of the continuously annealed plate at the fourth temperature can make the hydrogen in the continuously annealed plate escape, thereby improving the anti-delay fracture performance. In summary, through the synergistic effect of the chemical composition design, the process parameter control of the continuous annealing and the temperature of the second annealing, the anti-delay fracture performance of the 1500MPa grade cold-rolled martensite steel is improved. BRIEF DESCRIPTION OF DRAWINGS

[0038] The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate the embodiments consistent with the present application and, together with the description, serve to explain the principles of the present application.

[0039] In order to more clearly illustrate the technical solutions of the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description. Obviously, for those of ordinary skill in the art, other drawings can also be obtained based on these drawings without any creative effort.

[0040] Figure 1 A flowchart of a preparation method of a 1500MPa-grade cold-rolled martensitic steel resistant to delayed fracture according to some embodiments of the present application is shown in the figure.

[0041] Figure 2 A microstructure diagram of a 1500MPa-grade cold-rolled martensitic steel resistant to delayed fracture according to some embodiments of the present application is shown in the figure.

[0042] Figure 3 A U-bend test result diagram of hydrogen embrittlement sensitivity of a 1500MPa-grade cold-rolled martensitic steel resistant to delayed fracture according to some embodiments of the present application is shown in the figure. DETAILED DESCRIPTION

[0043] In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the following will combine the drawings in the embodiments of the present application to clearly and completely describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without any creative effort fall within the scope of protection of the present application.

[0044] Various embodiments of the present application can exist in the form of a range; it should be understood that the description in the form of a range is only for the convenience and brevity, and should not be understood as a hard limit on the scope of the present application; therefore, it should be considered that the described range has specifically disclosed all possible sub-ranges and single values in the 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., as well as single numbers in the described range, such as 1, 2, 3, 4, 5 and 6, regardless of the range. In addition, whenever a numerical range is indicated in this document, it refers to any cited number (fraction or integer) in the indicated range.

[0045] In the present application, the orientation words such as "upper" and "lower" are specifically the directions of the drawing surface in the drawings unless otherwise stated. In addition, in the description of the present application, the terms "include", "contain" and the like mean "include but not limited to". In the present text, the relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between the entities or operations. In the present text, "and / or" describes the association relationship of the associated objects, which means that there can be three kinds of relationships, for example, A and / or B can represent the cases of A alone, A and B together, and B alone. Wherein A, B can be singular or plural. In the present text, "at least one" means one or more, and "multiple" means two or more. "At least one", "at least one of the following" or the like means any combination of the items, including single item or any combination of multiple items. For example, "at least one of a, b, or c", or "at least one of a, b, and c", can represent a, b, c, a-b (i.e. a and b), a-c, b-c, or a-b-c, wherein a, b, and c can be single or multiple.

[0046] Unless otherwise specified, the various raw materials, reagents, instruments and equipment used in the present application can be purchased from the market or can be prepared by existing methods.

[0047] Figure 1 A flowchart of a preparation method of a cold-rolled martensitic steel with a 1500MPa grade of anti-delay fracture according to some embodiments of the present application is shown in FIG. 1. Please refer to FIG. 1. Figure 1 The present application provides a preparation method of a cold-rolled martensitic steel with a 1500MPa grade of anti-delay fracture, which comprises the following steps:

[0048] S1, obtaining a cold hard plate with a set chemical composition;

[0049] In some embodiments, the set chemical composition comprises:

[0050] C, Si, Mn, P, S, Cr, Nb, Ti, Cu, Ni, B, N and Fe; wherein, in terms of mass fraction,

[0051] C is 0.2% to 0.3%, Si is 0.2% to 1.0%, Mn is 2.0% to 3.0%, P is ≤0.015%, S is ≤0.008%, Cr is 0.3% to 0.6%, Nb is 0.01% to 0.04%, Ti is 0.01% to 0.04%, Cu is ≤0.03%, Ni is 0.01% to 0.05%, B is 0.001% to 0.002%, and N is 0.003% to 0.005%.

[0052] In the embodiments of the present application, the element C is the most important solid solution strengthening element and the element improving the austenite hardenability in the martensitic steel. In order to obtain sufficient amount of martensite in the cooling process to ensure the tensile strength of more than 1500 MPa, and at the same time, the element C forms carbides with the micro-alloy elements Nb and Ti in the heat treatment process to refine the grains, which helps to obtain the yield strength of more than 1200 MPa, and to avoid the deterioration of the welding performance caused by the excessive content of C. Exemplarily, the content of the element C can be 0.2%, 0.22%, 0.24%, 0.26%, 0.28%, 0.3%, etc.

[0053] The element Si is also an important solid solution strengthening element, and at the same time, the element Si can effectively promote the enrichment of the element C in the austenite, improve the austenite hardenability, purify the ferrite phase, and improve the elongation. The element Si can also reduce the hydrogen diffusion kinetics and prevent the formation of H2, and inhibit the formation of carbides during the tempering process, thereby improving the delayed fracture performance of the steel plate. However, the element Si is an important element leading to the formation of iron oxide scale on the surface of the hot-rolled plate, and the excessive content of Si will cause the residual iron oxide scale after pickling to deteriorate the surface quality of the continuous annealing plate. Exemplarily, the content of the element Si can be 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, etc.

[0054] The element Mn is an austenite stabilizing element, which can diffuse from the ferrite to the austenite during the annealing process to improve the stability of the austenite and improve the hardenability. In addition, the element Mn can improve the strength of the martensite through solid solution strengthening to ensure the high yield strength of more than 1200 MPa and the high tensile strength of more than 1500 MPa. However, the element Mn increases the sensitivity to delayed fracture, and the formation of MnS inclusions often becomes the starting point of hydrogen-induced cracking, so the content of the element Mn should not be too high. In addition, the excessive content of the element Mn can easily cause the segregation of the structure, which can easily lead to the forming cracking and deteriorate the comprehensive performance of the steel, and at the same time, the element Mn can also enrich on the surface during the annealing process, so the content of the element Mn should not be too high. Exemplarily, the content of the element Mn can be 2.0%, 2.2%, 2.4%, 2.6%, 2.8%, 3.0%, etc.

[0055] The Cr element can improve the austenite hardenability in the steel, so as to obtain a sufficient amount of martensite, obtain a tensile strength of 1500 MPa or more, and ensure that the alloy manufacturing cost is low enough; but at the same time, the Cr element is a ferrite zone expansion element, and too much Cr element will cause the two-phase zone to shrink, and will also hinder the bainite transformation. Exemplarily, the content of the above-mentioned Cr can be 0.3%, 0.4%, 0.5%, 0.6%, etc.

[0056] The Nb element not only has a significant grain refinement effect, but also is a strong carbide forming element, which can improve the yield strength of the martensitic steel through precipitation strengthening. However, Nb significantly inhibits recrystallization, and too high Nb content will cause the annealing stage to enter the unrecrystallized zone, resulting in the formation of deformation bands along the rolling direction, causing uneven performance. Exemplarily, the content of the above-mentioned Nb can be 0.01%, 0.02%, 0.03%, 0.04%, etc.

[0057] Like Nb, Ti is a strong carbide forming element that can improve yield strength through precipitation strengthening. At the same time, Ti is beneficial to refining the grains and obtaining uniform equiaxed microstructure. In addition, the cost of Ti is lower than that of Nb. In addition, the addition of less than 0.01% of titanium will make the steel of the present application have low resistance to delayed fracture, and the steel of the present application will crack within less than 50 hours during the acid immersion U-bend test. Ti(C, N) precipitation promotes the capture of free H, which is beneficial to obtaining good delayed fracture resistance. By adding a certain amount of Ti / Nb micro-alloying element, controlling the addition of Ni element, and retaining a certain amount of N element, the generation of Ti(C, N) and NbC second phase particles is promoted, and irreversible hydrogen traps are obtained; at the same time, after the continuous annealing production line is produced, offline low-temperature annealing is carried out, which further promotes the escape of hydrogen atoms, and finally good resistance to delayed fracture is obtained. However, too much Ti addition will increase the cost. Exemplarily, the content of the above-mentioned Ti can be 0.01%, 0.02%, 0.03%, 0.04%, etc.

[0058] As a gap solid solution atom, P can appropriately improve the strength of the steel plate, but is also easy to segregate at the grain boundary and deteriorate the plasticity and formability. Exemplarily, the content of the above-mentioned P can be 0.015%, 0.013%, 0.010%, etc.

[0059] S is easy to combine with Mn to form coarse MnS inclusions, which deteriorates the resistance to delayed fracture of the steel plate. Exemplarily, the content of the above-mentioned S can be 0.008%, 0.007%, 0.006%, etc.

[0060] Cu can improve the strength of the steel and improve its corrosion resistance, but when its content is too high, it will have a tendency to be hot brittle. Exemplarily, the content of the above-mentioned Cu can be 0.03%, 0.02%, 0.01%, etc.

[0061] Ni has a negative effect on the resistance to delayed fracture, and an ideal steel contains less than 0.03% of Ni at the impurity level. Exemplarily, the content of the above-mentioned Ni can be 0.03%, 0.02%, 0.01%, 0.05%, 0.04%, etc.

[0062] B plays a very important role in improving the hardenability of the steel plate during cooling. When the content of B is too low, it is difficult to achieve grain boundary strengthening, which is needed to obtain excellent delayed fracture resistance. In addition, B prevents the adverse effect of P segregation on the grain boundary due to its significantly faster diffusion to the grain boundary than P, which deteriorates the delayed fracture resistance. However, when the content of B is too high, carbon borides are easily formed, which is not conducive to the improvement of the delayed fracture resistance. Exemplarily, the content of the above-mentioned B can be 0.001%, 0.0013%, 0.0015%, 0.0018%, 0.002%, etc.

[0063] N can reduce the nitride precipitates in the steel, thereby reducing the formation of irreversible hydrogen trap second phase particles, and also making the prior austenite grain boundaries coarse, reducing the number of grain boundaries, increasing the crack propagation tendency, and ultimately leading to deterioration of the delayed fracture resistance. However, when the content of N is too high, the nitrides in the steel are coarsened, thus reducing the grain pinning effect and leading to deterioration of the delayed fracture resistance of the steel. Exemplarily, the content of the above-mentioned N can be 0.003%, 0.0035%, 0.004%, 0.0045%, 0.005%, etc.

[0064] In some embodiments, the cold hard plate with the set chemical composition is obtained by:

[0065] casting the molten steel to obtain a slab;

[0066] heating and rolling the slab with the set chemical composition, and then coiling to obtain a hot-rolled coil;

[0067] cold-rolling the hot-rolled coil to obtain a cold hard plate with the set chemical composition; wherein,

[0068] the casting speed is 4 m / min to 7 m / min, and the thickness of the slab is 110 mm to 125 mm; and / or,

[0069] the heating temperature is 1120°C to 1220°C; and / or,

[0070] the finishing rolling temperature is 890°C to 920°C; and / or,

[0071] the coiling temperature is 550°C to 600°C.

[0072] In the embodiments of the present application, the rolling specifically includes rough descaling, rough rolling, induction heating, fine descaling, fine rolling, and laminar cooling; the hot-rolled coil can be air-cooled and then cold-rolled; the fine rolling adopts any one of single-billet rolling, semi-endless rolling and endless rolling, and the 1500MPa-grade cold-rolled martensitic steel with resistance to delayed fracture is prepared based on a multi-mode thin slab continuous casting and rolling production line.

[0073] The casting speed and the thickness of the slab are limited to ensure the production rhythm and avoid the occurrence of breakout in endless rolling. For example, the casting speed can be 4 m / min, 5 m / min, 6 m / min, 7 m / min, etc., and the thickness of the slab can be 110 mm, 115 mm, 120 mm, 125 mm, etc.

[0074] The temperature of heating is limited to homogenize the alloying elements of the slab. The finishing temperature of rolling and the temperature of coiling are limited to homogenize the structure of the heated slab. For example, the temperature of heating can be 1120℃, 1130℃, 1140℃, 1150℃, 1160℃, 1170℃, 1180℃, 1190℃, 1200℃, 1210℃, 1220℃, etc.; the finishing temperature of rolling can be 890℃, 900℃, 910℃, 920℃, etc., and the temperature of coiling can be 550℃, 560℃, 570℃, 580℃, 590℃, 600℃, etc.

[0075] S2, continuously annealing the cold hard slab to obtain a continuously annealed slab;

[0076] The continuously annealing the cold hard slab to obtain a continuously annealed slab includes: first heating the cold hard slab to a first temperature, and performing first heat preservation at the first temperature to obtain a first annealed slab;

[0077] Second heating the first annealed slab to a second temperature, and performing second heat preservation at the second temperature, and then performing slow cooling and rapid cooling to obtain a second annealed slab with a completely martensitic structure;

[0078] Third heating the second annealed slab to a third temperature to perform overaging, and obtaining a continuously annealed slab;

[0079] In some embodiments, the first temperature is 640℃-680℃, and the time of the first heat preservation is 30s-180s.

[0080] In some embodiments, the second temperature is (Ac3+10)℃-(Ac3+20)℃, and the time of the second heat preservation is 180s-360s.

[0081] In some embodiments, the first heating has a temperature increasing rate of 2-5 °C / s; and / or,

[0082] The second heating has a temperature increasing rate of 3-8 °C / s.

[0083] In the embodiments of the present application, based on the above-mentioned chemical composition, the process parameters in the continuous annealing are controlled, so that the target structure is optimized. The temperature increasing rate of the first heating is limited, which balances the residence time of the cold hard plate in the continuous annealing furnace, and at the same time, makes the C / Mn elements fully diffuse, and the recrystallization fully recover, and the homogenized structure. For example, the temperature increasing rate of the first heating can be 2 °C / s, 3 °C / s, 4 °C / s, 5 °C / s, etc.

[0084] The first holding is carried out at the first temperature (below Ac1), and the cold-rolled plate is further fully high-temperature tempered, so as to achieve the purpose of homogenizing the structure. In this way, the ferrite grains in different parts will be fully recrystallized, and the difference between the ferrite grains will be small. At the same time, the martensite will be fully tempered to equiaxed ferrite and granular cementite, and the pearlite will be obviously spheroidized. Since the grain boundary is a fast diffusion channel for carbon atoms, the carbon atoms will gradually diffuse to the original carbon-poor ferrite area in the first holding, so as to realize the uniform distribution of carbide particles. For example, the first temperature can be 640 °C, 650 °C, 660 °C, 670 °C, 680 °C, etc., and the first holding time can be 30 s, 50 s, 70 s, 100 s, 120 s, 150 s, 170 s, 180 s, etc.

[0085] The temperature increasing rate of the second heating is limited, and the temperature is increased at a higher temperature. In order to heat the steel plate to the second temperature in a short time, the temperature increasing rate needs to be further increased. After the temperature is increased to above the Ac1 point, the reverse transformation austenite will be uniformly distributed in the matrix, so as to eliminate the genetic inheritance of the original uneven structure. For example, the temperature increasing rate of the second heating can be 6 °C / s, 3 °C / s, 4 °C / s, 5 °C / s, 7 °C / s, 8 °C / s, etc.

[0086] The second holding (soaking stage) at the second temperature can make the tissue fully austenitized, the fibrous microstructure complete the recovery and recrystallization process, and fully transform into austenite, while the residual cementite is fully dissolved, the components and microstructure in the austenite are fully homogenized, and the carbides of Nb and Ti are fully precipitated, effectively obtaining the required resistance to delayed fracture performance while ensuring the tensile strength of 1500 MPa or more. For example, the first temperature can be (Ac3+10) °C, (Ac3+12) °C, (Ac3+14) °C, (Ac3+16) °C, (Ac3+18) °C, (Ac3+20) °C, etc., and the first holding time can be 180 s, 200 s, 220 s, 240 s, 260 s, 280 s, 300 s, 320 s, 340 s, 360 s, etc.

[0087] In some embodiments, the process parameters of the slow cooling include: a cooling rate of 1-15 °C / s, and an end temperature of Ac1-Ac3; and / or,

[0088] The process parameters of the fast cooling include: a cooling medium containing 50% hydrogen by volume, a cooling rate of

[0089] ≥45 °C / s, and an end temperature of 200-240 °C.

[0090] In the embodiments of the present application, the process parameters of the slow cooling are limited to ensure that the formation of new ferrite can be inhibited during the initial cooling process, so that not only the uneven evolution of the microstructure can be avoided, but also the decrease in the hardenability of austenite caused by the enrichment of carbon in the austenite can be avoided. The process parameters of the fast cooling are limited, the end temperature of the fast cooling is below the martensite transformation temperature, and in combination with the cooling medium and the cooling rate of the fast cooling, the transformation of austenite to martensite is promoted, and a microstructure structure of all martensite is obtained. For example, in the process parameters of the slow cooling, the cooling rate can be 1 °C / s, 3 °C / s, 5 °C / s, 7 °C / s, 9 °C / s, 11 °C / s, 13 °C / s, 15 °C / s, etc., and the end temperature can be (Ac3-60) °C, (Ac3-30) °C, (Ac3-40) °C, (Ac3-25) °C, (Ac3-48) °C, etc. In the process parameters of the fast cooling, the cooling rate can be 45 °C / s, 46 °C / s, 47 °C / s, 48 °C / s, 49 °C / s, etc., and the end temperature can be 200 °C, 210 °C, 230 °C, 240 °C, etc.

[0091] In some embodiments, the third temperature is 280-320 °C.

[0092] In the embodiments of the present application, the overaging treatment is performed at the third temperature, which is beneficial to the tempering of the martensite, enhances the plasticity of the martensite, and improves the formability and the resistance to delayed fracture of the finished product. For example, the third temperature can be 280, 290, 300, 310, 320, or the like.

[0093] S3, performing second annealing on the continuous annealing plate at a fourth temperature to obtain the cold-rolled martensitic steel with a strength of 1500 MPa and resistance to delayed fracture.

[0094] In some embodiments, the fourth temperature is 200-240℃.

[0095] In the embodiments of the present application, the second annealing is offline low-temperature annealing, which is beneficial to the escape of hydrogen and improves the resistance to delayed fracture. For example, the fourth temperature can be 200, 210, 220, 230, 240, or the like.

[0096] The present application provides a cold-rolled martensitic steel with a strength of 1500 MPa and resistance to delayed fracture prepared by the method of the first aspect,

[0097] The cold-rolled martensitic steel satisfies the following mechanical properties: tensile strength > 1500 MPa, yield strength

[0098] 1200-1500 MPa, elongation A80≥3%, and hole expansion ratio≥50%;

[0099] The cold-rolled martensitic steel has a microstructure of lath-shaped martensite. Figure 2 A microstructure diagram of a cold-rolled martensitic steel with a strength of 1500 MPa and resistance to delayed fracture provided according to some embodiments of the present application; please refer to Figure 2 .

[0100] In the embodiments of the present application, the cold-rolled martensitic steel with a strength of 1500 MPa and resistance to delayed fracture has excellent mechanical properties and does not crack after being immersed in 0.1 mol / L HCl solution for 300 h, and has good U-bend stress corrosion resistance.

[0101] The cold-rolled martensitic steel with a strength of 1500 MPa and resistance to delayed fracture is realized based on the preparation method of the cold-rolled martensitic steel with a strength of 1500 MPa and resistance to delayed fracture. The specific steps of the preparation method of the cold-rolled martensitic steel with a strength of 1500 MPa and resistance to delayed fracture can be referred to the above embodiments. Since the cold-rolled martensitic steel with a strength of 1500 MPa and resistance to delayed fracture adopts part or all of the technical solutions of the above embodiments, it at least has all the beneficial effects brought by the technical solutions of the above embodiments, which will not be repeated here.

[0102] The present application will be further described in conjunction with specific examples. It should be understood that these examples are only used to illustrate the present application and not used to limit the scope of the present application. The experimental methods in the following examples without specific conditions are generally determined according to national standards. If there is no corresponding national standard, it is determined according to the general international standard, the conventional condition, or according to the condition suggested by the manufacturer.

[0103] The present application provides a cold-rolled martensitic steel with 1500MPa grade of anti-delay fracture. Please refer to the chemical composition of the cold-rolled martensitic steel with 1500MPa grade of anti-delay fracture shown in Table 1, the preparation process parameters of the cold hard plate shown in Table 2, the process parameters of the continuous annealing and the second annealing of the cold hard plate shown in Table 3, and the performance results of the cold-rolled martensitic steel with 1500MPa grade of anti-delay fracture shown in Table 4.

[0104] Table 1 Chemical composition of the cold-rolled martensitic steel with 1500MPa grade of anti-delay fracture (wt%)

[0105]

[0106] Table 2 Preparation process parameters of the cold hard plate

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[0109] Table 3 Process parameters of the continuous annealing and the second annealing of the cold hard plate

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[0111]

[0112] The cold-rolled martensitic steel with 1500MPa grade of anti-delay fracture prepared by Examples 1-6 and Comparative Examples 1-4 was tested for performance. Please refer to Table 4.

[0113] The tensile strength Rm, yield strength Rp0.2 and A80 in Table 4 were evaluated according to GBT 228.1-2021 Metal Materials Tensile Test Part 1: Room Temperature Test Method. The anti-delay fracture test was evaluated according to SEP1970-2011, using parallel U-bend state samples, loading a certain stress, selecting 3 parallel samples, bending 180 degrees, clamping the bolt, cleaning the surface with acetone, replacing the test acid solution 0.1 mol / L HCl solution every half month, and observing the crack initiation and development.

[0114] The hole expansion ratio in the above table is measured according to the ISO 16630 standard for the test steel (HET), which is used to measure the ability of the material to resist edge cracking during the flanging, hole expansion and other processes in the stamping process. The sample size is 100 mm x 100 mm x 1.5 mm, and the initial hole diameter is 10 mm. Five groups of samples are prepared for each steel type. In order to ensure that the sample is fixed by the edge during hole expansion, the distance between the hole center of the hole expansion sample and the edge of the sample should be not less than 45 mm. In order to investigate the worst condition of the material during hole expansion, the burr direction should be consistent with the direction of the convex mold during hole expansion. This hole expansion method can cause the tear zone to deform first, causing micro-cracks to expand and crack, thereby obtaining the worst edge resistance of the material. The hole expansion ratio HER (%) is calculated using the following formula, where df is the hole diameter at the end of hole expansion, and d0 is the initial hole diameter. HER = (df - d0) / d0 x 100%

[0115] Table 4 Performance results of the delayed fracture resistant 1500 MPa grade cold rolled martensitic steel.

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[0117]

[0118] In summary, the delayed fracture resistant 1500 MPa grade cold rolled martensitic steel prepared by the embodiments of the present application meets the following mechanical properties: tensile strength > 1500 MPa, yield strength 1200 MPa ~ 1500 MPa, elongation A80≥3%, hole expansion ratio≥50%; the microstructure of the cold rolled martensitic steel is all lath martensite. And no cracking occurs after immersion in 0.1 mol / L HCl solution for 300 h, and has good U-bend stress corrosion resistance. For example, Figure 3 The U-bend test results of the hydrogen embrittlement sensitivity of a delayed fracture resistant 1500 MPa grade cold rolled martensitic steel according to some embodiments of the present application are shown in the following figure: Figure 3 However, the C content in Comparative Example 1 is too low, Comparative Example 2 is directly heated to the second temperature, the second temperature in Comparative Example 3 is too low, and Comparative Example 4 does not use the second annealing, which to some extent makes the performance of the above cold rolled martensitic steel worse than the embodiments.

[0119] The above description is only a specific embodiment of the present application, which enables those skilled in the art to understand or implement the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features applied herein.

Claims

1. A method for preparing cold-rolled martensitic steel with delayed fracture resistance of 1500 MPa, characterized in that, The method includes: obtaining a cold hard plate with a set chemical composition; continuously annealing the cold hard plate to obtain a continuously annealed plate; The continuously annealing the cold hard plate to obtain a continuously annealed plate includes: first heating the cold hard plate to a first temperature and performing first heat preservation at the first temperature to obtain a first annealed plate; second heating the first annealed plate to a second temperature and performing second heat preservation at the second temperature, followed by slow cooling and rapid cooling to obtain a second annealed plate with a fully martensitic structure; third heating the second annealed plate to a third temperature for over-aging to obtain a continuously annealed plate; performing second annealing on the continuously annealed plate at a fourth temperature to obtain a cold-rolled martensitic steel with a 1500 MPa anti-delayed fracture level; The first temperature is 640°C to 680°C, and the time of the first heat preservation is 30 s to 180 s; The second temperature is (Ac3 + 10)°C to (Ac3 + 20)°C, and the time of the second heat preservation is 180 s to 360 s; The heating rate of the first heating is 2°C / s to 5°C / s; the heating rate of the second heating is 3°C / s to 8°C / s; The third temperature is 280°C to 320°C, and the fourth temperature is 200°C to 240°C; The process parameters of the slow cooling include: the cooling rate is 1°C / s to 15°C / s, and the end temperature is Ac1 to Ac3; The process parameters of the rapid cooling include: the cooling medium is a gas containing 50% by volume of hydrogen, and the cooling rate is ≥45°C / s, and the end temperature is 200°C to 240°C.

2. The method according to claim 1, characterized in that, The set chemical composition includes: C, Si, Mn, P, S, Cr, Nb, Ti, Cu, Ni, B, N, and Fe; wherein, by mass fraction, the content of C is 0.2% to 0.3%, the content of Si is 0.2% to 1.0%, the content of Mn is 2.0% to 3.0%, the content of P is ≤0.015%, the content of S is ≤0.008%, the content of Cr is 0.3% to 0.6%, the content of Nb is 0.01% to 0.04%, the content of Ti is 0.01% to 0.04%, the content of Cu is 0 < ≤0.03%, the content of Ni is 0.01% to 0.05%, the content of B is 0.001% to 0.002%, and the content of N is 0.003% to 0.005%.

3. The method according to claim 1, characterized in that, The obtaining a cold hard plate with a set chemical composition includes: continuous casting of molten steel to obtain a slab; heating and rolling the slab with the set chemical composition, followed by coiling to obtain a hot-rolled coil; cold rolling the hot-rolled coil to obtain a cold hard plate with a set chemical composition; wherein, the casting speed of the continuous casting is 4 m / min to 7 m / min, and the thickness of the slab is 110 mm to 125 mm; and / or, the heating temperature is 1120°C to 1220°C; and / or, the final rolling temperature of the rolling is 890°C to 920°C; and / or, the coiling temperature is 550°C to 600°C.

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