Martensitic steel with adjustable yield ratio and preparation method of martensitic steel

Abstract

The invention discloses martensitic steel with an adjustable yield ratio and a preparation method thereof.The martensitic steel comprises a martensitic matrix and manganese-rich flaky retained austenite distributed in the martensitic matrix, and the preparation method of the martensitic steel comprises the steps that a uniform hot-rolled plate of raw material steel containing carbon and manganese elements is heated to an austenite single-phase region and subjected to heat preservation; cooling to a pearlite transformation temperature interval, preserving heat, and quenching to room temperature; then the temperature is rapidly increased to a high-temperature austenite single-phase region, heat preservation is conducted, and the steel subjected to rapid short-time austenitizing is obtained; and cooling to an isothermal quenching temperature, carrying out heat preservation, carrying out primary carbon partitioning, cooling to room temperature or heating to a higher temperature, carrying out heat preservation, carrying out secondary carbon partitioning, and finally quenching to room temperature, so as to obtain the martensitic steel with the adjustable yield ratio. By adjusting the volume fraction of the retained austenite and the tempering degree of the martensite, the martensite steel with different yield ratios can be obtained on the premise of keeping high strength and high elongation of the martensite steel.

Description

Technical Field

[0001] The invention belongs to the technical field of material heat treatment, and particularly relates to the technical field of martensitic steel with adjustable yield ratio. Background Art

[0002] The yield ratio is the ratio of yield strength to tensile strength, which reflects the safety factor of the material from the beginning of yield to failure and is one of the important indicators for considering the safety performance of automobiles. When the yield ratio of martensitic steel is relatively low, there is a large difference between the yield strength and the tensile strength, which can make the steel plate obtain a wider safety range in the fracture process after reaching the yield strength, which is helpful to improve the processing performance of steel and the safety of products; at the same time, the low yield ratio of martensitic steel is conducive to improving the forming performance of parts, and is suitable for parts with complex shapes that require deep drawing or stretching. When the yield ratio of martensitic steel is relatively high, the steel can withstand higher stress before reaching the yield point, which increases the safety factor of automobile parts in actual use. For structural parts such as frames and longitudinal beams that bear large loads, steel with high yield strength is usually required to ensure the structural integrity and occupant safety of the vehicle in collisions and other situations. Therefore, low yield ratio steel has higher plasticity and toughness, which is conducive to improving the impact absorption capacity and collision safety of the car; while high yield ratio steel has higher strength and rigidity, which is helpful to improve the structural stability and occupant protection performance of the car.

[0003] Chinese patent CN103060715B discloses a method for manufacturing an ultra-high strength steel plate with a low yield ratio, by adjusting the austenitizing temperature (860-940°C) and the tempering temperature (150-350°C) to control the original austenite grain size and carbide size, and finally obtain excellent performances of tensile strength greater than 1500MPa and elongation greater than 10%, but the yield ratio of the obtained material is maintained at about 0.8. In order to further reduce the yield ratio, Chinese patent CN110100032B proposes a method of replacing the traditional quenching-tempering process, cooling the steel to a temperature range of 50°C below the martensitic transformation end (Mf) temperature and 100°C above it for a specified time, and finally obtains a tempered martensitic steel with a microstructure containing more than 90% tempered martensite, less than 5% ferrite and the remainder of bainite. Compared with the traditional quenching-tempering samples, this technology can significantly reduce the yield ratio of martensitic steel (0.4-0.6 vs. 0.7-0.95). However, due to the absence or trace amount of residual austenite, its elongation basically does not exceed 10%, which greatly limits the application of the product.

[0004] In view of the above problems, the prior art proposes to rapidly austenitize the initial structure with non-uniform manganese distribution, introduce a certain amount of manganese-rich residual austenite into the martensitic matrix after quenching, and further improve the comprehensive mechanical properties of martensitic steel, such as Chinese patents CN110468263A, CN113652612A, CN112251679A, CN116144887A and CN116949366A, etc. However, it is found in practice that the prior art cannot effectively control the yield ratio of steel. For example, the yield ratios of martensitic steels obtained in Chinese patents CN110468263A and CN113652612A are both within the range of 0.85 to 0.95, while the yield ratio of martensitic steels obtained in CN116144887A is within the range of 0.5 to 0.6. Therefore, how to effectively adjust the yield ratio of martensitic steel while maintaining high strength and high plasticity to meet the needs of different automotive structural parts has become a key technical problem to be solved. Summary of the invention

[0005] In view of the defects of the prior art, the purpose of the present invention is to provide a martensitic steel with an adjustable yield ratio and a preparation method thereof. The preparation method can obtain martensitic steels with different yield ratios to meet the needs of different automotive structural parts by simply improving the heat treatment process. At the same time, the obtained martensitic steel can maintain high strength and high elongation, solving the problem that the prior art is difficult to effectively control its yield ratio while maintaining high strength and plasticity.

[0006] The technical solution of the present invention is as follows:

[0007] A martensitic steel with an adjustable yield ratio comprises the following element components, measured in mass percentage: C: 0.2-0.5wt.%, Mn: 2.0-8.0wt.%, Si: 0-3.0wt.%, Al: 0-3.0wt.%, Cr: 0-1.5wt.%, Mo: 0-0.5wt.%, Ni: 0-2.0wt.%, Ti: 0-0.1wt.%, V: 0-0.5wt.%, Nb: 0-0.5wt.%, and the remainder is Fe and unavoidable impurities; and its microstructure comprises a tempered lamellar martensitic matrix and lamellar retained austenite distributed in the martensitic matrix, wherein the volume fraction of the retained austenite is greater than 10%.

[0008] The present invention further provides a method for preparing martensitic steel with adjustable yield ratio, comprising:

[0009] (1) forging and / or hot rolling the raw steel material after smelting and homogenization treatment to obtain a hot-rolled steel plate;

[0010] (2) heating the hot-rolled steel sheet to an austenite single-phase region and keeping the temperature to obtain a completely high-temperature austenite, then cooling the hot-rolled steel sheet to a pearlite (ferrite + cementite) transformation temperature range and keeping the temperature to obtain a pearlite structure, and then cooling the hot-rolled steel sheet to room temperature to obtain a pearlitized steel material;

[0011] (3) heating the pearlitized steel to an austenite single phase region at a heating rate of not less than 1° C. / s and keeping the temperature for a short time to obtain a fast short-time austenitized steel;

[0012] (4) cooling the rapidly short-time austenitized steel to an austempering temperature and keeping the temperature to perform a primary carbon partitioning, then cooling to room temperature or heating to a higher temperature and keeping the temperature to perform a secondary carbon partitioning, and finally quenching to room temperature to obtain a carbon partitioned steel; wherein the austempering temperature is between room temperature and the martensitic transformation start temperature (M s )between;

[0013] Wherein, the raw steel material contains the following elemental components in mass percentage: C: 0.2-0.5wt.%, Mn: 2.0-8.0wt.%, Si: 0-3.0wt.%, Al: 0-3.0wt.%, Cr: 0-1.5wt.%, Mo: 0-0.5wt.%, Ni: 0-2.0wt.%, Ti: 0-0.1wt.%, V: 0-0.5wt.%, Nb: 0-0.5wt.%, and the rest is Fe and unavoidable impurities.

[0014] According to some preferred embodiments of the present invention, the preparation method further comprises:

[0015] (5) The steel material after the primary carbon distribution is tempered.

[0016] According to some preferred embodiments of the present invention, the temperature of the homogenization treatment is 1100-1300° C., and the time is 12-48 hours.

[0017] According to some preferred embodiments of the present invention, the temperature of the austenite single-phase region is 700-900°C.

[0018] According to some preferred embodiments of the present invention, the holding time in the austenite single phase region in step (2) is 5 to 120 minutes.

[0019] According to some preferred embodiments of the present invention, the pearlite transformation temperature range is 450-700° C., and the holding time is 2-48 h.

[0020] According to some preferred embodiments of the present invention, the holding time in the austenite single phase region in step (3) is 0 to 200 seconds.

[0021] According to some preferred embodiments of the present invention, the isothermal quenching temperature is 0 to 400° C., and the holding time is 0 to 8 hours.

[0022] According to some preferred embodiments of the present invention, the temperature of the secondary carbon partitioning is 100-500° C., and the insulation time is 1-30 min.

[0023] According to some preferred embodiments of the present invention, the tempering treatment is carried out by keeping the temperature at 100-400° C. for 0 s-120 min.

[0024] According to some preferred embodiments of the present invention, the preparation method specifically comprises:

[0025] (1) subjecting the smelted raw steel ingot to homogenization treatment, and then forging and / or hot rolling to obtain a hot-rolled steel plate;

[0026] (2) heating the hot-rolled steel plate to 700-900° C. and keeping the temperature for 10-120 min to obtain a completely high-temperature austenite, then cooling the hot-rolled steel plate to 500-650° C. and keeping the temperature for 3-24 h to obtain a pearlite structure, and then quenching the hot-rolled steel plate to room temperature to obtain a pearlite steel material;

[0027] (3) heating the pearlitized steel to 700-850° C. at a heating rate of 5-100° C. / s and keeping the temperature for 0-100 seconds to obtain a fast short-time austenitized steel;

[0028] (4) cooling the rapid short-time austenitized steel to 0-250° C. and keeping the temperature for 0-3 h for primary carbon partitioning, then cooling to room temperature or heating to 100-500° C. and keeping the temperature for 0-2 h for secondary carbon partitioning, and finally quenching with water to room temperature to obtain a carbon partitioned steel;

[0029] (5) The carbon-partitioned steel is heated to 100-300° C. and kept at this temperature for 0-60 min for tempering to obtain the martensitic steel with adjustable yield ratio.

[0030] According to some preferred embodiments of the present invention, the homogenization treatment comprises: heating the ingot of the raw steel material to 1200-1250° C. and keeping the temperature for 24-36 hours.

[0031] According to some preferred embodiments of the present invention, the hot rolling comprises: hot rolling the steel after the homogenization treatment at 900-1000°C for the first time with a reduction of 50-60%, and air cooling to 800°C after rolling for hot rolling again with a reduction of 10-20%.

[0032] In the preparation method of the present invention, during the pearlite transformation process, the manganese element in the raw steel is excluded from the ferrite lamellae and is strongly enriched in the cementite lamellae. Furthermore, the inventor unexpectedly found that the diffusion of the manganese element can be restricted by rapid austenitization treatment, and the heterogeneous distribution of manganese in the pearlite is retained in the high-temperature austenite, forming a manganese-rich high-temperature austenite lamellae and a manganese-poor high-temperature austenite lamellae region; when cooled to room temperature and M s At temperatures between 200 and 250 °C, the manganese-poor austenite lamellae region preferentially undergoes martensite transformation, while the manganese-rich austenite lamellae region is retained due to its strong stability. Due to the strong interaction between manganese and carbon, the diffusion of carbon atoms from manganese-poor martensite to manganese-rich austenite in the primary carbon partitioning and / or secondary carbon partitioning stages is promoted, which is conducive to obtaining sufficient retained austenite in martensitic steel.

[0033] Based on this, the preparation method of the present invention can be specifically divided into three ways to control the yield ratio. The first is to adjust the austempering temperature to control the volume fraction of retained austenite and the tempering degree of martensite. Retained austenite is a soft phase and martensite is a hard phase. The yield ratio can be controlled by adjusting their phase ratio; at the same time, the quenching temperature can also control the tempering degree of martensite, thereby controlling the yield ratio of martensitic steel; the second is that after a carbon partitioning at the austempering temperature, part of the austenite is unstable and transformed into quenched martensite when quenched to room temperature. At this time, the microstructure contains both once tempered martensite and quenched martensite; therefore, the subsequent tempering treatment process can be adjusted so that the two martensites have different tempering degrees, thereby controlling the yield ratio of martensitic steel. The third method is to adjust the volume fraction of retained austenite and the degree of martensite tempering by adjusting the number of carbon partitioning times, thereby adjusting the yield ratio of martensitic steel; such as directly quenching to room temperature without carbon partitioning, quenching to room temperature after a carbon partitioning at an austempering temperature, and heating to a higher temperature for a secondary carbon partitioning after a carbon partitioning at an austempering temperature. It is worth noting that when the secondary carbon partitioning method is adopted, the tempering treatment is not required.

[0034] The present invention has the following beneficial effects:

[0035] (1) The preparation method of the present invention can introduce manganese-partitioned pearlite (manganese-rich cementite + manganese-poor ferrite) into the initial structure, and deliberately retain the non-uniform distribution of manganese in the subsequent austenitization stage, thereby regulating the proportion of residual austenite in the martensitic steel (more than 10%); at the same time, by adjusting the tempering degree of martensite (solid solution carbon concentration and dislocation recovery) and the volume fraction of residual austenite and its stability, martensitic steels with different yield ratios are obtained while maintaining high strength and high elongation, thereby solving the technical problem that it is difficult to effectively regulate the yield ratio of traditional martensitic steels while maintaining high strength and plasticity.

[0036] (2) This preparation method can obtain martensitic steel with different yield ratios by simply improving the heat treatment process, while maintaining high strength and high elongation, which can meet the needs of different automotive structural parts and greatly reduce production costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is a heat treatment process flow chart of an embodiment;

[0038] Figure 2 This is a SEM image of the steel after water quenching obtained in step (3) of Example 1;

[0039] Figure 3 is a SEM image of the martensitic steel obtained in step (4) of Example 1;

[0040] Figure 4 is the tensile curve of the martensitic steel obtained in Example 1;

[0041] Figure 5 is the XRD curve of the martensitic steel obtained in Example 2;

[0042] Figure 6 This is a SEM image of the water-quenched steel obtained after step (3) of Example 2;

[0043] Figure 7 This is the tensile curve of the martensitic steel obtained in Example 2. DETAILED DESCRIPTION

[0044] The present invention is described in detail below in conjunction with the embodiments and drawings, but it should be understood that the embodiments and drawings are only used to exemplify the present invention and do not constitute any limitation on the protection scope of the present invention. All reasonable changes and combinations within the scope of the inventive concept of the present invention fall within the protection scope of the present invention.

[0045] Example 1

[0046] See attached Figure 1 The raw steel material with the composition of Fe-0.39C-3.69Mn-0.3Ni-0.2Si (wt.%) is prepared by heat treatment as follows:

[0047] (1) The raw steel material is vacuum melted and forged, and then heated to 1250° C., kept at this temperature for 24 hours for homogenization treatment, and then hot rolled to a thickness of 7 mm to obtain a hot-rolled plate, wherein the first hot rolling is carried out at 900-1000° C., with a reduction of 50-60%, and after rolling, the steel is air-cooled to 800° C. and hot rolled again, with a reduction of 10-20%;

[0048] (2) The hot-rolled plate is kept in a muffle furnace at 800°C for 10 minutes for austenitization, then moved to a salt bath furnace at 570°C and kept for 6 hours for complete pearlite transformation, and then water quenched to room temperature to obtain pearlite steel;

[0049] (3) The pearlitized steel was placed in a salt bath furnace at 750°C for 30 seconds and then divided into three groups, which were respectively subjected to direct water quenching to room temperature; moved to a salt bath furnace at 110°C for 30 minutes and then water quenched to room temperature; and moved to a salt bath furnace at 200°C for 30 minutes and then water quenched to room temperature, and three water-quenched steel samples were obtained, namely, water-quenched steel sample 1, sample 2, and sample 3;

[0050] (4) The water-quenched steel samples 1, 2, and 3 were reheated to 200°C and tempered for 30 min, and then water-quenched to room temperature to obtain martensitic steel samples 1, 2, and 3, respectively.

[0051] Among them, the microstructure of steel sample 3 after water quenching is shown in the attached figure. Figure 2 As shown in FIG. 1 , it can be seen that the sample contains a pearlite lamellar structure (70%) composed of lamellar retained austenite and lath martensite overlapping each other. The microstructure of martensitic steel sample 3 after tempering in step (4) is shown in FIG. Figure 3 As shown, it can be seen that the quenched martensite is completely transformed into tempered martensite.

[0052] After testing, the volume fractions of retained austenite in steel samples 1, 2, and 3 after water quenching were 11%, 19%, and 14%, respectively. This indicates that increasing the quenching temperature can significantly increase the content of retained austenite through carbon partitioning, but as the austempering temperature further increases, the proportion of austenite in the partitioning stage increases, and accordingly, the content of martensite decreases. The carbon partitioning from martensite to austenite is not enough to stabilize all austenite, causing part of the austenite to transform into martensite when cooled to room temperature, thereby reducing the content of retained austenite and increasing the content of quenched martensite.

[0053] The mechanical properties of the obtained martensitic steel were tested, and the results are shown in the attached figure. Figure 4 The specific tensile properties are shown in Table 1 below.

[0054] Table 1

[0055]

[0056] It can be seen that the tensile strength of all martensitic steel samples is close to 2000MPa, and the elongation after fracture is more than 12%, indicating that a sufficient amount of retained austenite gradually transforms under different strains during the tensile process to improve the strain hardening ability of the sample, thereby significantly improving the plasticity of the sample. It is worth noting that by adjusting the isothermal quenching temperature during the first carbon partitioning in step (3) to control the volume fraction of retained austenite and the degree of martensite tempering, the yield ratio of martensitic steel (0.6-0.85) can be further limitedly controlled. This is because the increase in the volume fraction of retained austenite as a soft phase can effectively reduce the yield strength of martensitic steel. In addition, through the above test results, it can be found that the content of quenched martensite in sample 3 is higher than that in sample 2. After the same tempering process, the yield strength of sample 3 is also significantly higher than that of sample 2, resulting in an increase in its yield ratio.

[0057] In summary, compared with the prior art method of adjusting the yield ratio by carbide precipitation (which cannot obtain sufficient residual austenite, and the precipitation of carbides will lead to a large consumption of the C element), the preparation method of the present invention can promote the diffusion of carbon from martensite to manganese-rich austenite through the uneven distribution of manganese. The obtained product contains a large amount of lamellar residual austenite, ensuring its high plasticity. At the same time, the yield ratio is effectively regulated by adjusting the volume fraction of residual austenite and the degree of martensite tempering, thereby increasing the window for industrial application.

[0058] Example 2

[0059] See attached Figure 1 The raw steel material with the composition of Fe-0.39C-3.69Mn (wt.%) is prepared by the following heat treatment:

[0060] (1) The raw steel material is vacuum melted and then heated to 1250° C., kept at this temperature for 24 hours for homogenization treatment, and then forged and hot rolled to a thickness of 7 mm to obtain a hot rolled plate, wherein the first hot rolling is performed at 900 to 1000° C., with a reduction of 50 to 60%, and after rolling, the steel is air cooled to 800° C. and hot rolled again, with a reduction of 10 to 20%;

[0061] (2) The hot-rolled plate is kept in a muffle furnace at 800°C for 10 minutes for austenitization, then moved to a salt bath furnace at 570°C and kept for 6 hours for complete pearlite transformation, and then water quenched to room temperature to obtain pearlite steel;

[0062] (3) The pearlitized steel was placed in a salt bath furnace at 750°C for 30 seconds and then divided into three groups, which were directly quenched to room temperature; moved to a salt bath furnace at 200°C for 30 minutes and then quenched to room temperature; and moved to a salt bath furnace at 110°C for 60 seconds and then moved to a salt bath furnace at 200°C for 30 minutes. Three types of steel samples were obtained, namely, steel sample 1 after quenching, sample 2, and heat-insulated steel sample 3;

[0063] (4) The water-quenched steel samples 1 and 2 were reheated to 200°C and tempered for 30 min, and then water-quenched to room temperature to obtain martensitic steel samples 1 and 2, respectively.

[0064] XRD characterization was performed on martensitic steel samples 1 and 2 and insulation steel sample 3, as shown in the attached figure. Figure 5 As shown, the volume fractions of retained austenite are 12%, 14% and 22% respectively.

[0065] Compared with sample 1, sample 2 can retain more austenite stably to room temperature by austempering to 200℃ and then carbon partitioning. However, there is still a lot of quenched martensite in sample 2, which indicates that there is too much austenite and too little martensite when austempering to 200℃, resulting in the inability to stabilize all austenite to room temperature after partitioning.

[0066] For sample 3, it was first quenched to a lower temperature (110°C) to obtain a suitable ratio of martensite to austenite, and then the secondary carbon partitioning at 200°C was performed to stabilize almost all the austenite to room temperature. Figure 6 It is verified that there is no obvious quenched martensite in the microstructure.

[0067] The mechanical properties of the martensitic steel obtained in this embodiment were tested, and the results are shown in the attached figure. Figure 7 The specific tensile properties are shown in Table 2 below.

[0068] Table 2

[0069]

[0070] It can be seen that the tensile strength of all samples is close to 2000MPa, and the elongation after fracture exceeds 12%, especially the sample 3 with the highest volume fraction of retained austenite reaches 15%. With the increase of the volume fraction of retained austenite, the yield ratio of samples 1 to 3 is significantly reduced from 0.86 to 0.56, further effectively adjusting the yield ratio of martensitic steel.

[0071] Example 3

[0072] Refer to the attached Figure 1 The raw steel material with the composition of Fe-0.37C-3.8Mn-1.5Si-0.2Mo-0.6Ni (wt.%) is prepared by the following heat treatment:

[0073] (1) the raw steel material is vacuum melted and then heated to 1300° C., kept at this temperature for 18 hours for homogenization treatment, and then forged and hot rolled to a thickness of 7 mm to obtain a hot rolled plate, wherein the final rolling temperature of the hot rolling is greater than 800° C.;

[0074] (2) The hot-rolled plate is kept in a muffle furnace at 850°C for 5 minutes for austenitization, then moved to a salt bath furnace at 580°C and kept for 6 hours for complete pearlite transformation, and then water quenched to room temperature to obtain pearlite steel;

[0075] (3) The pearlitized steel was placed in a salt bath furnace at 770°C for 20 seconds, then divided into three groups and moved to salt bath furnaces at 70°C, 110°C and 150°C for 60 seconds, respectively. They were then moved to a salt bath furnace at 400°C for 500 seconds and then quenched with water to room temperature, thereby obtaining martensitic steel samples 1, 2 and 3, respectively.

[0076] The mechanical properties of the obtained martensitic steel were tested, and the specific tensile properties are shown in Table 3 below.

[0077] Table 3

[0078]

[0079] It can be seen that the tensile strength of all martensitic steel samples exceeds 1600MPa, and the elongation after fracture exceeds 12%, especially the sample 2 with a higher volume fraction of retained austenite and less quenched martensite structure reaches 21%. With the increase of the volume fraction of retained austenite, the yield ratio of samples 1 to 3 is significantly reduced from 0.92 to 0.73, further effectively adjusting the yield ratio of martensitic steel.

[0080] Example 4

[0081] Refer to the attached Figure 1 The raw steel material with the composition of Fe-0.3C-5Mn-0.6Si-0.8Ni (wt.%) is prepared by heat treatment as follows:

[0082] (1) The raw steel material is vacuum melted and then heated to 1200° C., kept at this temperature for 36 hours for homogenization treatment, and then forged and hot rolled to a thickness of 6 mm to obtain a hot rolled plate, wherein the final rolling temperature of the hot rolling is greater than 850° C.;

[0083] (2) The hot-rolled plate is kept in a muffle furnace at 900°C for 5 minutes for austenitization, then moved to a salt bath furnace at 590°C and kept for 12 hours for complete pearlite transformation, and then water quenched to room temperature to obtain pearlite steel;

[0084] (3) placing the pearlitized steel in a salt bath furnace at 740°C for 40 seconds, then moving it to a salt bath furnace at 200°C for 30 minutes, and then quenching it with water to room temperature to obtain water-quenched steel;

[0085] (4) The water-quenched steel was divided into two groups, which were reheated to 300°C and then kept at this temperature for 5 min and 20 min, respectively, for tempering, and then water-quenched to room temperature, to obtain martensitic steel samples 1 and 2, respectively.

[0086] The mechanical properties of the obtained martensitic steel were tested, and the specific tensile properties are shown in Table 4 below:

[0087] Table 4

[0088]

[0089] It can be seen that the tensile strength of all martensitic steel samples is around 1600MPa, and the elongation after fracture is more than 16%. With the increase of tempering time, martensite continues to recover, and its solid solution carbon concentration and dislocation density gradually decrease, resulting in a gradual decrease in solid solution strengthening and dislocation strengthening, making the yield ratio of sample 2 (0.57) significantly lower than that of sample 1 (0.76).

[0090] The above embodiments are only preferred implementations of the present invention, and the protection scope of the present invention is not limited to the above embodiments. All technical solutions under the concept of the present invention belong to the protection scope of the present invention. It should be pointed out that for ordinary technicians in this technical field, improvements and modifications without departing from the principle of the present invention should also be regarded as the protection scope of the present invention.

Claims

1. A martensitic steel with adjustable yield ratio, characterized in that: Measured in mass percentage, it comprises the following elemental components: C: 0.2-0.5wt.%, Mn: 2.0-8.0wt.%, Si: 0-3.0wt.%, Al: 0-3.0wt.%, Cr: 0-1.5wt.%, Mo: 0-0.5wt.%, Ni: 0-2.0wt.%, Ti: 0-0.1wt.%, V: 0-0.5wt.%, Nb: 0-0.5wt.%, and the rest are Fe and unavoidable impurities; and its microstructure comprises a tempered lath-like martensitic matrix and lamellar retained austenite distributed in the martensitic matrix, and the volume fraction of the retained austenite is greater than 10%.

2. A method for preparing martensitic steel with adjustable yield ratio, characterized in that: It includes: (1) forging and / or hot rolling the raw steel material after smelting and homogenization treatment to obtain a hot-rolled steel plate; (2) heating the hot-rolled steel plate to an austenite single phase region and keeping the temperature to obtain a completely high-temperature austenite, then cooling the hot-rolled steel plate to a pearlite transformation temperature range and keeping the temperature to obtain a pearlite structure, and then quenching the hot-rolled steel plate to room temperature to obtain a pearlite steel material; (3) heating the pearlitized steel to an austenite single phase region at a heating rate of not less than 1° C. / s and keeping the temperature for a short time to obtain a fast short-time austenitized steel; (4) cooling the rapid short-time austenitized steel to an austempering temperature and keeping the temperature to perform a primary carbon partitioning, then cooling to room temperature or raising the temperature to a higher temperature and keeping the temperature to perform a secondary carbon partitioning, and finally quenching to room temperature to obtain a carbon partitioned steel; wherein the austempering temperature is between room temperature and the martensitic transformation start temperature; Wherein, the raw steel material contains the following elemental components in mass percentage: C: 0.2-0.5wt.%, Mn: 2.0-8.0wt.%, Si: 0-3.0wt.%, Al: 0-3.0wt.%, Cr: 0-1.5wt.%, Mo: 0-0.5wt.%, Ni: 0-2.0wt.%, Ti: 0-0.1wt.%, V: 0-0.5wt.%, Nb: 0-0.5wt.%, and the rest is Fe and unavoidable impurities.

3. The preparation method according to claim 2, characterized in that: It also includes: (5) The steel material after the primary carbon distribution is tempered.

4. The preparation method according to claim 2, characterized in that: The temperature of the homogenization treatment is 1100-1300° C., and the time is 12-48 hours.

5. The preparation method according to claim 2, characterized in that: in, The temperature of the austenite single phase region is 700-900°C, the holding time in the austenite single phase region in step (2) is 5-120 min, and the holding time in the austenite single phase region in step (3) is 0-200 s; and / or, the pearlite transformation temperature range is 450-700°C, and the holding time is 2-48 h; and / or, the isothermal quenching temperature is 0-400°C, and the holding time is 0-8 h.

6. The preparation method according to claim 2, characterized in that: The temperature of the secondary carbon distribution is 100-500° C., and the insulation time is 1-30 minutes.

7. The preparation method according to claim 3, characterized in that: The tempering treatment is carried out by keeping the temperature at 100-400° C. for 0 s-120 min.

8. The preparation method according to claim 2, characterized in that: Specifically include: (1) subjecting the smelted raw steel ingot to homogenization treatment, and then forging and / or hot rolling to obtain a hot-rolled steel plate; (2) heating the hot-rolled steel plate to 700-900° C. and keeping the temperature for 10-120 min to obtain a completely high-temperature austenite, then cooling the hot-rolled steel plate to 500-650° C. and keeping the temperature for 3-24 h to obtain a pearlite structure, and then quenching the hot-rolled steel plate to room temperature to obtain a pearlite steel material; (3) heating the pearlitized steel to 700-850° C. at a heating rate of 5-100° C. / s and keeping the temperature for 0-100 seconds to obtain a fast short-time austenitized steel; (4) cooling the rapid short-time austenitized steel to 0-250° C. and keeping the temperature for 0-3 h for primary carbon partitioning, then cooling to room temperature or heating to 100-500° C. and keeping the temperature for 0-2 h for secondary carbon partitioning, and finally quenching with water to room temperature to obtain a carbon partitioned steel; (5) The steel material with the primary carbon partitioning is heated to 100-300° C. and kept at this temperature for 0-60 min for tempering to obtain the martensitic steel with adjustable yield ratio.

9. The preparation method according to claim 8, characterized in that: The homogenization treatment comprises: heating the ingot of the raw steel material to 1200-1250° C. and keeping the temperature for 24-36 hours.

10. The preparation method according to claim 8, characterized in that: The hot rolling comprises: performing the first hot rolling of the homogenized steel at 900-1000°C with a reduction of 50-60%, air cooling to 800°C and performing the second hot rolling with a reduction of 10-20%.

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