High Plasticity 1500 MPa Grade Ultra-High Strength Steel and Method for Producing the Same

Abstract

The composition is by weight percentage: C 0.35 - 0.40%, Si 1.0 - 1.8%, Mn 1.5 - 2.0%, Cr 0.3 - 0.6%, Al 0.02 - 0.05%, Ti 0.02 - 0.05%, B 0.002 - 0.02%, and the balance contains Fe and other inevitable impurities; and a high-plasticity 1500 MPa grade ultra-high strength steel and its manufacturing method that simultaneously satisfy the following: the carbon equivalent C in the peritectic reaction region eq1 > 0.17%, C eq1 = C - 0.03Mn - 0.06Si - 0.222S - 0.04P; the welding carbon equivalent C eq2 ≦ 0.56%, C eq2 = C + Mn / 20 + Si / 30 + 2P + 4S. The yield strength of the ultra-high strength steel according to the present invention is 1000 - 1300 MPa, the tensile strength is ≧ 1500 MPa, the elongation at break is ≧ 18%, and it has good surface quality. In particular, it is suitable for manufacturing vehicle structural parts and safety parts with complex shapes and high requirements for formability, such as A / B pillars, door crash bars, longitudinal beams, bumpers, etc.

Description

Technical Field

[0001] The present invention relates to high-strength steel manufacturing technology, and specifically to a 1500MPa grade ultra-high-strength steel with high plasticity and a manufacturing method thereof.

Background Art

[0002] In recent years, in order to achieve weight reduction of automobile bodies for the purpose of energy conservation, emission reduction, improvement of collision safety, and reduction of manufacturing costs, advanced high-strength steel for automobiles has been widely used in the automobile manufacturing industry. Advanced high-strength steel is currently the most competitive material for weight reduction of automobile bodies by increasing the strength of steel sheets while maintaining excellent formability and reducing the thickness of steel sheets.

[0003] The ultra-high-strength martensitic steel with a strength reaching the 1500MPa level, which is currently widely used, has an elongation of about 5%, and cannot meet the dual requirements for automobile safety and formability during the manufacturing process in the automobile field. Another steel that is widely used is hot stamping steel. However, in the forming process of hot stamping steel, high heating devices and cooling capabilities are required, the process is complex, and its elongation rate is basically within 10%. The third-generation high-strength steel for automobiles can achieve both high strength and high plasticity, has a low cost, and the product of strength and elongation can reach 20 - 30GPa%, so it has attracted wide attention.

[0004] In Chinese Patent Publication No. CN103667884B, a "method for manufacturing a 1400 MPa grade low yield ratio and high elongation cold-rolled ultra-high strength steel for automobiles" is disclosed. Its composition is C 0.14% - 0.16%, Si 1.31% - 1.51%, Mn 2.7% - 2.9%, S ≤ 0.005%, P ≤ 0.009%, Al 0.11% - 0.51%, RE 0.005 - 0.20%, and the balance is Fe and inevitable impurities. The composition design of the steel type according to this patent only involves C, Si, Mn, Al, and a small amount of rare earth elements. The manufacturing process is general casting - hot rolling - cold rolling - continuous annealing. In the steel structure of the finished product, martensite accounts for 70 - 85%, retained austenite accounts for 5 - 20%, and there is also a small amount of ferrite. However, the steel according to this invention has a tensile strength of 1400 MPa grade, an elongation rate of 8% or more, and a relatively low yield strength.

[0005] In Chinese Patent Publication No. CN106244918B, "1500MPa Grade High Strength and High Ductility Automotive Steel and Its Manufacturing Method" is disclosed. The composition of the steel type related to this patent is C 0.1 - 0.3%, Si 0.1 - 2.0%, Mn 7.5 - 12%, Al 0.01 - 2.0%, and its chemical elements further include at least one of Nb 0.01 - 0.07%, Ti 0.02 - 0.15%, V 0.05 - 0.20%, Cr 0.15 - 0.50%, Mo 0.10 - 0.50%, and the balance is iron and other inevitable impurities. Its manufacturing method includes the following steps: 1) Smelting and casting; 2) Hot rolling; 3) Bell-type furnace annealing, the annealing temperature is 600 - 700°C, and the annealing time is 1 - 48h; 4) Cold rolling; 5) The first annealing after cold rolling: The annealing temperature is between Ac1 and Ac3 temperatures, and the annealing time is more than 5min; 6) The second annealing after cold rolling: The annealing temperature is 750 - 850°C, and the annealing time is 1 - 10min; 7) Tempering: The tempering temperature is 200 - 300°C, and the tempering time is 3min or more. The microstructure of the 1500MPa grade high strength and high ductility automotive steel is austenite + martensite + ferrite or austenite + martensite, and its strength-ductility product is 30GPa% or more. Although this patent can obtain a good matching of strength and plasticity, the manufacturing process is extremely complicated, and the high Mn content in the composition also has an adverse effect on manufacturability.

[0006] In Chinese Patent Publication No. CN106917055B, "Third-generation high-strength and high-toughness steel for automobiles and its manufacturing method" is disclosed. Its composition is C 0.40 - 0.60%, Si 1.00 - 2.00%, Mn 1.5 - 3.0%, Ni 0 - 0.60%, Cr 0.50 - 1.50%, Mo 0.30 - 0.60%, V 0 - 0.20%, Co 0 - 1.50%, Al 0 - 1.50%, and the balance is Fe and inevitable impurities. After vacuum melting and casting, the hot-rolled plate obtained by hot rolling at a total reduction rate of 80% - 95% is heated to 900 - 970°C at a rate of 5 - 20°C / s, austenitized for 10 - 20 minutes, and then air-cooled to 250 - 320°C and held for 2 - 12 hours. The yield strength of this material exceeds 1000 MPa, the tensile strength exceeds 1500 MPa, and the elongation at break exceeds 20%. This patent mainly relates to ultra-high-strength bainite steel, but its composition is complex, containing many alloying elements. At the same time, the efficiency of the heat treatment process is low, the austenitizing temperature is too high, and the time for low-temperature treatment is too long.

[0007] In Chinese Patent Publication No. CN108018484A, "Cold-rolled high-strength steel with a tensile strength of 1500 MPa or more and excellent formability and its manufacturing method" is disclosed. The composition of the steel type related to the patent application is as follows: C 0.25 - 0.40%, Si 1.50 - 2.50%, Mn 2.0 - 3.0%, Al 0.03 - 0.06%, P ≤ 0.02%, S ≤ 0.01%, N ≤ 0.01%, and at least one of 0.1 - 1.0% of Cr and 0.1 - 0.5% of Mo. Further, it contains at least one of Nb 0.01 - 0.1%, V 0.01 - 0.2% and Ti 0.01 - 0.05%, and the balance is Fe and other inevitable impurities. Its manufacturing method includes the following steps: 1) Smelting and casting; 2) Hot rolling; 3) Pickling; 4) Cold rolling; 5) Continuous annealing: The strip steel is heated to a soaking temperature of 800 - 900°C and held for 60 s or more, then cooled to 150 - 300°C at a rate of 30 - 80°C / s, then reheated to 350 - 440°C and held for 30 - 300 s, and finally cooled to room temperature. The microstructure of the product has 5 - 20% austenite and 70 - 90% martensite, with a tensile strength of 1500 MPa or more and excellent formability.

[0008] Focusing on the field of 1500 MPa grade ultra-high strength steel in the short flow process, the related patents are relatively few.

[0009] In Chinese Patent Publication No. CN111455282B, "Tempered Partitioning Steel with Tensile Strength ≥ 1500 MPa Produced by Short Flow and Method" is disclosed. Its composition includes C 0.26 - 0.34%, Si 1.9 - 2.7%, Mn 2.6 - 3.4%, Ti 0.02 - 0.07%, Als 0.02 - 0.05%, P ≤ 0.018%, S ≤ 0.004%, N ≤ 0.006%, and O ≤ 30 ppm. Its manufacturing process includes hot metal desulfurization, steelmaking, argon injection, LF refining, soft blowing, RH vacuum treatment, continuous casting into billets, soaking of billets, scale removal, seven-pass finish rolling, laminar flow cooling, coiling, leveling, pickling, and continuous annealing. The Rel of the strip steel is 1000 - 1200 MPa, the Rm is ≥ 1500 MPa, and the elongation is ≥ 15%. It is a method for producing 1500 MPa grade tempered partitioning steel by short flow. The composition design of the steel type related to this patent only involves C, Si, Mn, Al, and Ti. Although this patent can obtain a good matching of high strength and high plasticity, due to the use of a composition system with high Si and high Mn, it causes a series of problems such as leakage of molten steel in thin slab continuous casting, slab surface cracking, and a high risk of rolling scraps in thin specifications, increasing the manufacturing difficulty and cost of short flow production of ultra-high strength steel.

[0010] In Chinese Patent Publication No. CN114012056A, "1500MPa Grade Hot Forming Steel and Its Manufacturing Method" is disclosed. Its composition is: C 0.19 - 0.26%, Si 0.05 - 1.3%, Mn 0.9 - 2.1%, P≤0.015%, S≤0.002%, Alt 0.02 - 0.12%, B 0.002 - 0.020%, Cr 0.15 - 2.0%, Ti 0.02 - 0.15%, N≤0.006%, V+Nb≤0.15%, and the balance contains Fe and inevitable impurities. After smelting and continuous casting the molten steel, homogenization heat treatment, scale removal before rough rolling, rough rolling, electromagnetic induction heating, scale removal before finish rolling, finish rolling, laminar flow cooling, coiling, air cooling to room temperature, and pickling are carried out. After obtaining the pickled sheet, the pickled sheet is hot press formed to obtain a 1500MPa grade hot forming steel with an elongation rate within 10%. This patent application is also a method for producing hot forming steel with a short flow. The composition design of its steel grade is complex, containing many alloy elements. Although the process of the manufacturing method is simple, it belongs to the field of hot forming ultra-high strength steel, and moreover, the elongation rate of the material is insufficient.

Summary of the Invention

Problems to be Solved by the Invention

[0011] Content of the Invention The object of the present invention is to provide a high-plasticity 1500MPa grade ultra-high strength steel and its manufacturing method. The ultra-high strength steel has very excellent strength and plasticity, excellent surface quality, good weldability, and a relatively simple composition, does not contain excessive alloy elements, and the manufacturing process is simple and efficient; its yield strength is 1000 - 1300MPa, the tensile strength is ≥1500MPa, and the elongation at break is ≥18%; it has good application prospects for automotive safety structure parts, especially for manufacturing vehicle structure parts and safety parts with complex shapes and high requirements for formability, such as A / B pillars, door crash bars, longitudinal beams, bumpers, etc. In this article, "1500MPa grade" and "ultra-high strength" refer to a tensile strength ≥1500MPa; "high plasticity" refers to an elongation at break ≥18%.

Means for Solving the Problems

[0012] To achieve the above object, the technical solution of the present invention is as follows. Its composition is by weight percentage, C: 0.35 - 0.40%; Si: 1.0 - 1.8%; Mn: 1.5 - 2.0%; Cr: 0.3 - 0.6%; Al: 0.02 - 0.05%; Ti: 0.02 - 0.05%; B: 0.002 - 0.02%; and the balance contains Fe and other inevitable impurities; and a high-plasticity 1500 MPa grade ultra-high-strength steel that simultaneously satisfies the following: The carbon equivalent C in the peritectic reaction region eq1 > 0.17%, C eq1 = C - 0.03Mn - 0.06Si - 0.222S - 0.04P; The welding carbon equivalent C eq2 ≤ 0.56%, C eq2 = C + Mn / 20 + Si / 30 + 2P + 4S.

[0013] Preferably, the balance is Fe and other inevitable impurities. Preferably, the C content is 0.36 - 0.38 wt%.

[0014] Preferably, the Si content is 1.4 - 1.7 wt%. Preferably, the Mn content is 1.7 - 2.0 wt%.

[0015] Preferably, the Cr content is 0.4 - 0.6 wt%. Preferably, among the other inevitable impurities, P ≤ 0.015 wt%, S ≤ 0.002 wt%, O ≤ 0.002 wt%, N ≤ 0.004 wt%.

[0016] The yield strength of the ultra-high-strength steel according to the present invention is 1000 - 1300 MPa, the tensile strength is ≥ 1500 MPa, and the elongation at break is ≥ 18%.

[0017] In some embodiments, the yield strength of the ultra-high strength steel according to the present invention is within the range of 1000 MPa, 1050 MPa, 1100 MPa, 1150 MPa, 1200 MPa, 1250 MPa, 1300 MPa, or between any two of the above values.

[0018] In some embodiments, the tensile strength of the ultra-high strength steel according to the present invention is within the range of 1500 MPa, 1520 MPa, 1540 MPa, 1560 MPa, 1580 MPa, 1600 MPa, or between any two of the above values.

[0019] In some embodiments, the elongation at break of the ultra-high strength steel according to the present invention is within the range of 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, or between any two of the above values.

[0020] In the composition design of the ultra-high strength steel according to the present invention: The present invention fully utilizes the influence law of C, Si, and Mn elements on the transformation of materials, and based on improving the stability of thin slab continuous casting and continuous rolling production, the carbon equivalent of the components is removed from the peritectic reaction region, so as to achieve the purpose of improving the surface quality of hot rolled plates. By matching low Si and low Mn with Cr and B, the strength is improved, and finally, ultra-high strength steel plate products with excellent strength, plasticity, and surface quality are realized.

[0021] C: C is the most important solid solution strengthening element and is extremely important for ensuring strength. The higher the mass percentage of the C element in the steel, the higher the fraction of retained austenite, the higher the degree of localization of C in the retained austenite during partitioning, the enhanced stability of the retained austenite, the generation of the TRIP effect, and the contribution to improving the ductility of the material. However, if the C content in the steel is too high, the weldability of the steel will decrease. When the mass percentage of C in the steel exceeds 0.40%, twins are likely to occur frequently after quenching, and the cracking sensitivity will increase. Therefore, in the present invention, the C content is controlled to be 0.35 - 0.40 wt%, for example, 0.36 wt%, 0.37 wt%, 0.38 wt%, 0.39 wt%, and preferably controlled to be 0.36 - 0.38 wt%.

[0022] Si: Si can strongly suppress the formation of cementite during the partitioning process, promote the localization of carbon in the retained austenite, and improve the stability of the retained austenite. The Si content required to effectively suppress cementite is at least 1.0%. It should be noted that if the Si element content in the steel is too high, the high-temperature plasticity of the steel will decrease, significantly increasing the risk of billet cracking and even leakage during the thin slab continuous casting process. At the same time, if the Si content is too high, stable oxides will form on the steel plate surface, adversely affecting the subsequent pickling process. Therefore, in the present invention, the Si content is controlled to be 1.0 - 1.8 wt%, for example, 1.1 wt%, 1.2 wt%, 1.3 wt%, 1.4 wt%, 1.5 wt%, 1.6 wt%, 1.7 wt%, and preferably controlled to be 1.4 - 1.7 wt%.

[0023] Mn: Mn can expand the austenite phase region, lower the Ms and Mf points, and improve the stability of austenite and the hardenability of steel. At the same time, Mn is also an important solid solution strengthening element and has a great influence on the strength of steel. However, it should be noted that if the Mn content in steel is too high, the latent heat of solidification will increase. Especially in the case of thin slab continuous casting, heat conduction is limited, the billet shell becomes too thin, and the risk of billet cracking and even leakage in the thin slab continuous casting process will increase significantly, affecting the stability of the short-flow production of ultra-high strength steel. At the same time, if the content of Mn element is too high, the corrosion resistance and weldability will deteriorate. Therefore, in the present invention, the Mn content is controlled to be 1.5 - 2.0 wt%, for example, 1.6 wt%, 1.7 wt%, 1.8 wt%, 1.9 wt%, and preferably controlled to be 1.7 - 2.0 wt%.

[0024] Cr: Cr also plays a role in supporting strength. Especially in the present invention, since it is necessary to control the contents of Si and Mn to ensure the stable progress of the thin slab continuous casting and continuous rolling process, Cr is an important strength complementary element. Cr can improve the hardenability of steel and lower the martensite transformation start temperature. At the same time, it contributes to the refinement of austenite grain size during rolling and improves the strength. When the Cr content is less than 0.3 wt%, it is difficult to ensure the strength, but when it exceeds 0.6 wt%, it will affect the weldability of the material. Therefore, in the present invention, the Cr content is controlled to be 0.3 - 0.6 wt%, for example, 0.4 wt%, 0.5 wt%, and preferably controlled to be 0.4 - 0.6 wt%.

[0025] Al: When the Al element exists in a solid solution state, it can suppress the precipitation of cementite and the transformation from γ to martensite, and improve the stability of austenite. At the same time, the Al element can also form insoluble particles that are finely and dispersedly distributed with C and N, and can refine the crystal grains. However, if the mass percentage of the Al element in steel is too high, a large amount of oxide inclusions are likely to be generated, which is disadvantageous to the cleanliness of the molten steel. Therefore, in the present invention, the Al content is controlled to be 0.02 - 0.05 wt%, for example, 0.03 wt%, 0.04 wt%.

[0026] Ti: Ti can fix nitrogen in steel to form stable compounds, improving the quality of the billet and the defects of edge cracks. It can also form fine carbides, prevent the growth of austenite crystal grains, and refine the crystal grains. However, when it exceeds 0.05 wt% limited in the present invention, it is disadvantageous for the localization of C in retained austenite and the stabilization of retained austenite. When it is below 0.02 wt% limited in the present invention, it increases the crack generation rate. Therefore, in the present invention, the Ti content is controlled to be 0.02 - 0.05 wt%, for example, 0.03 wt% or 0.04 wt%.

[0027] B: Its main function is to improve the hardenability of steel and enhance the strength of steel. B tends to be unevenly distributed at the austenite grain boundaries, delaying the transformation from austenite to ferrite. Even with a low content, it has a significant effect. At the same time, B also has a good grain boundary purification effect, inhibiting to a certain extent the uneven distribution of harmful elements at the grain boundaries and improving the deformation coordination of the material. However, if the mass percentage of B is too high, it causes an increase in the strength of the steel, which is disadvantageous for obtaining good plasticity. Therefore, in the present invention, the B content is controlled to be 0.002 - 0.02 wt%, for example, 0.005 wt%, 0.01 wt%, or 0.015 wt%.

[0028] Regarding other inevitable impurities according to the present invention, P ≤ 0.015 wt%, S ≤ 0.002 wt%, O ≤ 0.002 wt%, and N ≤ 0.004 wt%.

[0029] In the above solution, the elements P, S, O, and N are all impurity elements. However, P has a solid-solution strengthening effect, suppresses the formation of carbides, and can contribute to improving the stability of retained austenite. However, if the mass percentage of P is too high, the grain boundaries become weak, the brittleness of the material increases, and the welding performance deteriorates. That is, the positive effect of the P element is weaker than the negative effect. Therefore, it is preferable to control the mass percentage of P to P ≤ 0.015 wt%. On the other hand, regarding N, if the mass percentage of N is too high, it causes difficulties in steelmaking and continuous casting and is disadvantageous for the control of inclusions. Therefore, it is preferable to control the mass percentage of N to N ≤ 0.004 wt%. Correspondingly, if the S content in the steel is too high, the plasticity of the material is significantly deteriorated, and for ultra-high-strength steel, the S content should be controlled more strictly. Therefore, the S content is controlled to S ≤ 0.002 wt%. On the other hand, the surface streak defects and surface scale defects of ultra-high-strength steel are mainly caused by the inclusion of inclusions such as Al2O3 under the continuous casting slab skin. Therefore, in order to improve the surface quality of the finished strip steel, it is necessary to reduce the deoxidation inclusions in the steel and control the O content in the steelmaking. Therefore, in the present invention, the O content is controlled to O ≤ 0.002 wt%.

[0030] In addition, the composition content of the ultra-high-strength steel according to the present invention further needs to satisfy the following: Carbon equivalent C in the peritectic reaction region eq1 > 0.17%, C eq1 = C - 0.03Mn - 0.06Si - 0.222S - 0.04P; Welding carbon equivalent C eq2 ≤ 0.56%, C eq2 = C + Mn / 20 + Si / 30 + 2P + 4S.

[0031] The reason is that when the carbon equivalent of the molten steel enters the peritectic reaction region and the peritectic reaction occurs, the liquid phase of the molten steel reacts with the δ phase to generate the γ phase. When the molten steel solidifies in this process, a very significant volume shrinkage (about 4.7%) occurs. This characteristic makes it easier for defects such as billet surface cracks to occur in the case of thin slab continuous casting, and in severe cases, even leakage of steel may occur. Carbon equivalent C in the peritectic reaction region eq1The range is 0.08 - 0.17%, but in the case of high-carbon ultra-high-strength steel, in order to avoid the peritectic reaction, it is necessary to design the composition so that the carbon equivalent Ceq1 exceeds 0.17. At the same time, from the empirical formula, when the contents of Si and Mn elements increase, the composition is likely to enter the peritectic reaction region, which affects the surface quality of the final product.

[0032] Furthermore, through a number of studies, the present invention finds that when the mass percentages of C, Si, Mn, P, and S satisfy the welding carbon equivalent C eq2 ≤0.56%, the weldability of the obtained high-strength steel is good. At the same time, with this composition, the effects of solid solution and tissue strengthening of the material are good, and the material strength is high. However, when the welding carbon equivalent C eq2 >0.56%, it is found that the weldability of the material deteriorates significantly.

[0033] The microstructure of the ultra-high-strength steel according to the present invention is ferrite with a volume ratio of 10% - 15% + martensite with a volume ratio of 70% - 80% + retained austenite.

[0034] In some embodiments, in the microstructure of the ultra-high-strength steel according to the present invention, the volume ratio of ferrite is 10%, 11%, 12%, 13%, 14%, 15%, or within the range between any two of the above values.

[0035] In some embodiments, in the microstructure of the ultra-high-strength steel according to the present invention, the volume ratio of martensite is 70%, 72%, 74%, 76%, 78%, 80%, or within the range between any two of the above values.

[0036] In some embodiments, in the microstructure of the ultra-high-strength steel according to the present invention, the volume ratio of retained austenite is 10%, 11%, 12%, 13%, 14%, 15%, or within the range between any two of the above values.

[0037] In the ferrite of the microstructure of the ultra-high strength steel, the ratio of the number of crystal grains with a crystal grain size of ≤ 5 μm is 90% or more, and the ratio of the number of crystal grains with a crystal grain size of ≤ 3 μm is 60% or more.

[0038] The average crystal grain size of the retained austenite in the microstructure of the ultra-high strength steel is ≤ 2 μm; and / or the average C content C(ra) in the retained austenite satisfies 1.2 wt% ≤ C(ra) ≤ 2.0 wt%.

[0039] In some embodiments, in the microstructure of the ultra-high strength steel according to the present invention, the average crystal grain size of the retained austenite is ≤ 1.6 μm.

[0040] In some embodiments, in the microstructure of the ultra-high strength steel according to the present invention, the average crystal grain size of the retained austenite is within the range of 0.5 μm, 0.6 μm, 0.8 μm, 1.0 μm, 1.2 μm, 1.4 μm, 1.6 μm, 1.8 μm, 2.0 μm, or between any two of the above values.

[0041] In some embodiments, in the microstructure of the ultra-high strength steel according to the present invention, the average C content C(ra) in the retained austenite satisfies 1.2 wt% ≤ C(ra) ≤ 1.8 wt%.

[0042] The reason is that due to the presence of a predetermined content of retained austenite in the material structure, a phase transformation to martensite occurs during the deformation process, generating the TRIP effect, and it is possible to still maintain good plasticity while giving the material a tensile strength of the 1500 MPa level. Containing a predetermined amount of fine-grained ferrite can contribute to a certain extent to having better ductility among materials of the same steel grade and strength level, and at the same time, it can also contribute to a part of the material strength by the fine-grain strengthening effect. However, when the content exceeds 15%, the tensile strength of the material becomes insufficient, and when the content is less than 10%, it becomes difficult to meet the requirement of high plasticity.

[0043] Martensite exists as the main hard phase in the material to ensure strength performance. However, when its content is less than 70%, the tensile strength of the material becomes insufficient. When its content exceeds 80%, it becomes difficult to ensure sufficient retained austenite and ferrite to ensure plasticity. Regarding ferrite crystal grains, in order to fully exert the fine grain strengthening effect, it is necessary to sufficiently refine its crystal grains. At the same time, in order to avoid the appearance of abnormally large crystal grains to improve the plasticity of the material, it is also necessary to ensure the uniformity of the ferrite structure. Therefore, it is necessary to control the ratio of the number of crystal grains with a crystal grain size ≤ 5μm to be 90% or more, and control the ratio of the number of crystal grains with a crystal grain size ≤ 3μm to be 60% or more.

[0044] At the same time, regarding retained austenite crystal grains, the main factors affecting the stability of retained austenite during the material deformation process mainly include the crystal grain size of retained austenite and the average C content in retained austenite. When the crystal grain size of retained austenite is > 2μm, the size of retained austenite is relatively large, its stability is insufficient, and the TRIP effect tends to be completed at the initial stage of deformation, resulting in insufficient plasticity of the material. On the other hand, when the average C content in retained austenite is < 1.2wt%, the stability of retained austenite is insufficient, and retained austenite is prone to martensitic transformation at the initial stage of deformation. As a result, the TRIP effect is insufficient, and the formability of the steel sheet cannot be significantly improved. However, when the average C content in retained austenite is > 2wt%, retained austenite becomes too stable and martensitic transformation cannot occur during the deformation process. Also due to this, the TRIP effect is insufficient, and the formability of the steel sheet cannot be improved either.

[0045] In some embodiments, the thickness of the ultra-high strength steel according to the present invention is 0.8 - 2.0 mm.

[0046] The manufacturing method of the ultra-high strength steel according to the present invention targets the problems existing in the rolling process of current ultra-high strength steel, especially the cold rolling of 1500 MPa grade ultra-high strength steel, including the control of sheet thickness accuracy and sheet shape control of full hard plates, as well as the severe rolling force requirements imposed by the rolling of ultra-high strength steel on the rolling mill unit. Creatively, by utilizing thin slab continuous casting and continuous rolling, a hot rolled coil is directly obtained in the finished thickness from 1500 MPa grade ultra-high strength steel, and then the hot rolled coil is subjected to a heat preservation cover treatment. This significantly improves the uniformity of the tissue performance of the hot rolled coil, removes the internal stress of the material, and avoids the occurrence of brittle abnormal fracture in the subsequent process. After the heat preservation cover treatment is completed, pickling is carried out, and cold rolling, which is a bottleneck process in the conventional process, is omitted, and it directly proceeds to the final continuous annealing heat treatment; at the same time, due to the use of thin slab continuous casting technology, it has inherent advantages in terms of tissue uniformity and segregation control. Moreover, due to the characteristics of hot transfer of billets in thin slab continuous casting, a small amount of fine grain ferrite can be maintained in the tissue, and under the same strength conditions, the elongation rate of the obtained ultra-high strength steel can be significantly improved.

[0047] Specifically, the manufacturing method of the high plasticity 1500 MPa grade ultra-high strength steel according to the present invention includes the following steps: 1) Smelting and casting Smelt according to the above composition, cast into slabs, preferably utilize thin slab continuous casting, control the slab thickness at the outlet end of continuous casting to 55 - 60 mm, and control the drawing speed of continuous casting to 2 - 5 m / min; 2) Slab heating Set the heating temperature to 1200 - 1300 °C and the in-furnace time to 25 - 40 min; 3) Hot rolling and cooling First, perform high-pressure scale removal, control the rolling end temperature to 860 - 930 °C, then perform laminar flow cooling, control the cooling rate to 20 - 40 °C / s, and cool to 500 - 600 °C for coiling; 4) Slow cooling treatment After the hot rolling coil is completely wound, unwind the coil, add a heat preservation cover online for sealing, or transfer it into a sealed heat preservation cover for slow cooling treatment. When the heat preservation cover treatment reaches ≥4 hours, the cover can be opened to take out the hot rolling coil; preferably, after the hot rolling coil is completely wound, wait for ≥3 minutes with a roller and then add a heat preservation cover online for sealing, or transfer it into a sealed heat preservation cover for slow cooling treatment; 5) Pickling Control the pickling speed at 60 - 150 m / min; 6) Annealing Adopt continuous annealing, set the annealing temperature at 820 - 900 °C, slowly cool to 690 - 760 °C at a cooling rate of 3 - 10 °C / s to obtain ferrite at a predetermined ratio; then rapidly cool to 150 - 250 °C at a cooling rate of 50 - 100 °C / s to transform a part of austenite into martensite; thereafter, reheat to 360 - 460 °C, hold for 100 - 400 s, and finally cool to room temperature.

[0048] Preferably, in step 1), control the breakout rate of thin slab continuous casting at ≤1% and the crack repair rate at ≤1.2%.

[0049] Preferably, in the annealing process of step 6), set the annealing temperature at 840 - 870 °C, slowly cool to 700 - 730 °C at a cooling rate of 3 - 10 °C / s, rapidly cool to 170 - 230 °C, reheat to 400 - 430 °C after rapid cooling, and hold for 150 - 300 s.

[0050] Preferably, control the volume content of hydrogen gas in the reducing atmosphere in the continuous annealing furnace at 10 - 15%.

[0051] Preferably, when removing the high-pressure scale in step 3), control the scale removal water pressure of the first pass at 260 bar and the scale removal water pressure of the second pass at 340 bar.

[0052] Preferably, in step 3), adopt the U-type winding method, that is, control the winding temperature at 550 - 650 °C within a distance of ≤30 m from the head and tail of the strip steel.

[0053] In the method for manufacturing ultra-high strength steel according to the present invention: For the continuous casting according to the present invention, it is preferable to adopt thin slab continuous casting. Since the alloy elements in the ultra-high strength steel are relatively high, the stability of the continuous casting process is insufficient. In particular, due to the high latent heat of solidification and the thin billet shell, it is necessary to control the drawing speed of the thin slab continuous casting at a low level. When the drawing speed exceeds 5 m / min, problems such as fluctuations in the liquid level and steel leakage in the continuous casting process are likely to occur. However, the drawing speed should not be too low so as to affect the production efficiency. Therefore, it is appropriate to control it at 2 to 5 m / min.

[0054] At the same time, since thin slab continuous casting is adopted, the rough rolling process can be omitted, the hot rolling deformation amount can be reduced, and it is easier to obtain the specifications of the final product. In addition, by fully utilizing the heat of the slab in thin slab continuous casting, without waiting for the slab to completely cool down to room temperature, it can be directly loaded into the heating furnace while it is still hot, thereby reducing the energy consumption required for heating. At the same time, since there is no transformation process of rising after the temperature drops, the hot rolling high-temperature structure can be made more uniform. Also, a more uniform ferrite or ferrite + pearlite structure can be obtained after hot rolling, which contributes to maintaining a small amount of fine-grained ferrite in the microstructure of the finished product during annealing, improving the tissue uniformity, and contributing to the improvement of plasticity.

[0055] In slab heating, first, the thin slab continuous casting billet is heated at a high temperature in the full austenite region for a predetermined time to soften the material and completely and uniformly diffuse the components. Then, a treatment at a high temperature for a short time is adopted as much as possible to balance satisfying the uniformity of the composition of the ultra-high strength steel and avoiding the problem of excessive thickening of the oxide scale. Since there is a possibility of the problem of overcooking at the grain boundaries, the heating temperature should not exceed 1300°C. In this case, by shortening the heating time to 25 min, it is possible to contribute to the control of the oxide scale; on the other hand, when the heating temperature is lower than 1200°C, it takes a considerably long time to achieve the homogenization of the composition, which is also disadvantageous for the subsequent control of the oxide scale.

[0056] For scale removal of the slab, after taking out the slab from the heating furnace, first perform high-pressure water scale removal in two passes to remove as much as possible the soft oxide scale on the surface of the billet. Further roll it to the required thickness in finish rolling to form a uniform and fine recrystallized material structure. If the finish rolling end temperature is lower than 860°C, ferrite will precipitate at the end of rolling, and the strength in the hot rolling state is likely to be insufficient, which will in turn affect the subsequent cold rolling annealing performance. Considering the control of the upper limit of the slab heating temperature and the temperature drop during the rolling process, the finish rolling end temperature usually does not exceed 930°C. Controlling the cooling rate at 20 - 40°C / s during laminar flow cooling is to avoid excessive formation of bainite and then martensite during the cooling process, so as to ensure that the hot rolling coiling state structure is mainly uniform ferrite + pearlite. The hot rolling coiling temperature is one of the most important processes that affect the performance in the hot rolling state. When the coiling temperature exceeds 600°C, internal oxidation of Si and Mn is likely to occur on the surface of the steel plate, and a surface damaged layer is generated by pickling, which affects the surface quality of the final product; on the other hand, since it is necessary to maintain a fine-grained ferrite structure in the hot rolling state organization, the coiling temperature cannot be too low and needs to be controlled above 500°C. Adopting U-shaped coiling in the coiling process to increase the coiling temperature of the head and tail is mainly because the head and tail of ultra-high strength steel have a large temperature drop and are prone to transformation. As a result, the strength is higher than that at the center of the coil. At the same time, increasing the coiling temperature also contributes to the strength drop of the strip head and facilitates its coiling.

[0057] Controlling the thickness of the oxide scale on the surface of the strip after hot rolling to ≤6 μm and controlling (FeO + Fe₃O₄) in the oxide scale on the surface of the strip after hot rolling to ≤30 wt% contributes to the progress of the subsequent step (5) and can have an important impact on the performance of the steel sheet obtained after continuous annealing. The reason is that in the technical solution according to the present invention, although FeO and Fe₃O₄ are more difficult to pickling than Fe₂O₃, by controlling the thickness of the oxide scale on the surface of the strip after hot rolling and controlling (FeO + Fe₃O₄) in the oxide scale on the surface of the strip after hot rolling to ≤30 wt%, the pickling effect can be improved, a pickled plate surface that can be directly used for continuous annealing can be obtained. Also, by directly subjecting the pickled plate to continuous annealing, the amount of deformation of the hot rolling structure can be reduced, and the steel sheet structure can be mainly ferrite and pearlite or bainite, so that the material strength can be reduced and the structure can be made more uniform under the same continuous annealing conditions, and thus excellent ductility can be obtained.

[0058] After the winding is completed, waiting for the steel coil with a roller for 3 minutes or more before unwinding is mainly to complete the bainite transformation in the inner ring of the steel coil to prevent the flattening of the hot rolled coil that appears after unwinding. The reason is that for ultra-high strength steel, the temperature drop in the inner ring is faster during winding, so during the winding process, it gradually enters the bainite temperature range and bainite transformation occurs, causing volume expansion. At this time, since the temperature drop rates of the inner ring and the outer ring are different, the expansion due to bainite transformation is also different. By leaving it with a roller for 3 minutes or more to complete the bainite transformation, the volume change of the entire steel coil can be effectively equalized by the support of the roller, and the flattening of the hot rolled coil that appears after unwinding can be prevented.

[0059] By performing heat preservation cover treatment after hot rolling, the entire steel coil can be slowly cooled to improve performance and tissue uniformity. At the same time, the slow cooling process is also a process of releasing the thermal stress of the material, contributing to the stability of the subsequent material processing process and avoiding the occurrence of extreme situations such as brittle fracture. If the treatment time is less than 4 hours, the internal stress cannot be completely released, but the time should not be too long so as not to affect the production rhythm.

[0060] In the manufacturing method according to the present invention, if the pickling speed is too fast, pickling deficiency will occur, the internal oxide layer of the steel plate cannot be removed cleanly, and color difference will occur. However, if it is too slow, over-pickling will affect the surface quality of the pickled plate and also affect the production efficiency. Therefore, it is desirable to control it at 60-150 m / min.

[0061] In the manufacturing method according to the present invention, by controlling the annealing temperature at 820-900°C in continuous annealing, a uniform austenite structure or an austenite + ferrite structure can be formed; then, by slowly cooling to 690-760°C at a cooling rate of 3-10°C / s, the ferrite content in the structure is further adjusted to improve the plasticity of the material; thereafter, it is cooled at a rate of 50-100°C / s to 150-250°C (between the Ms and Mf temperatures); the reason is that in order to ensure that only martensite transformation occurs during the cooling process, the critical cooling rate needs to be 50°C / s or higher, but if the cooling rate exceeds 100°C / s, the production cost will increase significantly. At this point, most of the austenite transforms into martensite to ensure that the steel has high strength; by reheating to 360-460°C and holding for 100-400 s, carbon is distributed between martensite and austenite to form a predetermined amount of carbon-rich retained austenite and stably hold it to room temperature, and due to the TRIP effect, the work hardening ability and formability of the steel can be significantly improved, and an ultra-high strength steel sheet with excellent plasticity can be obtained. The reason for setting the above distribution process is that if the reheating temperature is lower than 360°C or the reheating time is shorter than 100 s, the stabilization process of the retained austenite in the steel will be insufficient, and finally the content of the retained austenite obtained at room temperature will be insufficient, but if the reheating temperature exceeds 460°C or the reheating time exceeds 400 s, the steel will undergo significant tempering softening, leading to a significant decrease in the final material strength.

[0062] Preferably, in the continuous annealing process of step 6) in the present invention, the annealing temperature is 840-870°C, slowly cooled to 700-730°C at a cooling rate of 3-10°C / s, quenched rapidly to 170-230°C, reheated to 400-430°C after quenching, held for 150-300 s, and the volume content of hydrogen gas in the reducing atmosphere in the continuous annealing furnace is controlled at 10-15%.

[0063] The ultra-high strength steel according to the present invention adopts a composition design of high C and alloying with Mn, Si, Cr, B and a ferrite grain refinement mechanism. Therefore, in the continuous annealing process, the nucleation points of austenite reverse transformation increase while the size is further refined, and the average grain size of the retained austenite stably held until room temperature can be ≤2 μm, and the average C content in the retained austenite is ≥1.2 wt%. Also, since the material still contains Si in a predetermined content, the martensite formed by rapid cooling is hardly decomposed during the partitioning process, thereby ensuring the martensite content in the structure and thus the strength of the steel.

[0064] Compared with the prior art, the advantages of the present invention are as follows: 1. The composition design of the ultra-high strength steel according to the present invention is unique and different from the composition design ideas of conventional patents.

[0065] In conventional patents, the composition design is generally complex, including many alloying elements (such as Mo, V, Nb, Ni, and even rare earth elements, etc.), and most of them show the composition characteristics of high Si and high Mn.

[0066] The composition design of the present invention is simple and unique. It fully utilizes the influence laws of C, Si, and Mn elements on the transformation of the material, improves the surface quality by removing the carbon equivalent of the components from the peritectic reaction region based on the improvement of the stability of thin slab continuous casting and continuous rolling production, obtains excellent welding performance by controlling the welding carbon equivalent, and ensures the strength by matching low Si and low Mn with Cr and B. Finally, an ultra-high strength steel plate product with excellent strength, plasticity, surface quality, and weldability has been realized.

[0067] 2. The manufacturing method according to the present invention is also unique and different from the manufacturing method ideas of conventional patents.

[0068] Most of the conventional patents adopt a production process of general continuous casting + hot rolling + pickling + cold rolling + continuous annealing / Bell-type annealing. However, the process flow is long, and in the case of ultra-high strength steel, there are problems with the shape and thickness accuracy of the cold-rolled sheet.

[0069] On the one hand, the present invention creatively proposes an efficient process of thin slab continuous casting + precise hot rolling + slow cooling treatment + pickling + continuous annealing, by which cold rolling, which is a bottleneck process for ultra-high strength steel, can be avoided. At the same time, it has inherent advantages in terms of tissue uniformity, segregation control, and manufacturing cost. Moreover, due to the characteristics of billet hot transfer in thin slab continuous casting, a small amount of fine-grained ferrite can be maintained in the structure, and under the same strength conditions, the elongation of the obtained ultra-high strength steel can be significantly improved. In addition, during the continuous annealing process, the nucleation points of austenite reverse transformation increase while the size further becomes finer, and finally, a very fine and uniform mixed structure of martensite, ferrite, and retained austenite is obtained. Such a structure has obvious advantages in plasticity compared with products of the same level while ensuring strength.

[0070] The ultra-high strength steel obtained by the present invention has good application prospects for automotive safety structural parts, and is particularly suitable for the manufacture of vehicle structural parts and safety parts with complex shapes and high requirements for formability, such as A / B pillars, door crash bars, longitudinal beams, bumpers, etc.

Brief Description of the Drawings

[0071]

Figure 1

Figure 2

Modes for Carrying Out the Invention

[0072] Specific Embodiments Hereinafter, the present invention will be further described based on examples and drawings.

[0073] The compositions of the steels according to the examples and comparative examples of the present invention are shown in Table 1, and the balance contains Fe and other inevitable impurities other than P, S, O, and N.

[0074] The steel manufacturing method according to the embodiment of the present invention includes the following steps: 1) Smelting and casting Smelt according to the above composition, cast into slabs, utilize thin slab continuous casting, control the slab thickness at the continuous casting outlet end to 55 - 60 mm, and control the drawing speed of continuous casting to 2 - 5 m / min; 2) Slab heating Set the heating temperature to 1200 - 1300 °C and the in - furnace time to 25 - 40 min; 3) Hot rolling and cooling First, perform high - pressure scale removal, control the rolling end temperature to 860 - 930 °C, then perform laminar flow cooling, control the cooling rate to 20 - 40 °C / s, cool to 500 - 600 °C and wind up, and adopt the U - type winding method; 4) Slow cooling treatment After the winding of the hot - rolled coil is completed, wait for ≥3 min with a roller, then unwind, transfer it into a sealed heat - preservation cover, perform slow cooling treatment, and when the heat - preservation cover treatment reaches ≥4 h or more, open the cover and take out the hot - rolled coil; 5) Pickling Control the pickling speed to 60 - 150 m / min; 6) Annealing Adopt continuous annealing, set the annealing temperature to 820 - 900 °C, slowly cool to 690 - 760 °C at a cooling rate of 3 - 10 °C / s to obtain ferrite at a predetermined ratio; further rapidly cool to 150 - 250 °C at a cooling rate of 50 - 100 °C / s to transform a part of austenite into martensite; then re - heat to 360 - 460 °C, hold for 100 - 400 s, and finally cool to room temperature.

[0075] The manufacturing process parameters of the steel according to the embodiment of the present invention and the steel according to the comparative example are shown in Table 2 and Table 3.

[0076] Comparative Examples 1 - 3 were manufactured by the steps of the manufacturing method according to the present invention, but the composition and / or manufacturing process parameters did not meet the design requirements of the present invention.

[0077] However, the C content of Comparative Example 1 exceeded the lower limit of the design range of the present invention, and at the same time, the carbon equivalent in its peritectic reaction region also entered the peritectic reaction region; both Si and Mn in Comparative Example 2 exceeded the upper limit of the design range of the present invention, and neither the carbon equivalent in its peritectic reaction region nor the welding carbon equivalent satisfied the design requirements of the present invention; although both the carbon equivalent in the peritectic reaction region and the welding carbon equivalent of Comparative Example 3 satisfied the requirements, since the contents of Si and Cr both exceeded the lower limit of the design range of the present invention, the material performance became insufficient.

[0078] In Table 2, the comparative examples are mainly different from the examples in terms of the drawing speed of continuous casting, the slab heating temperature and time, the rolling end temperature and bending temperature, and the holding cover treatment time.

[0079] In Table 3, the comparative examples are mainly different from the examples in terms of the pickling speed, the method of the annealing process, the quenching speed, the distribution temperature and time.

[0080] The measurement results of the mechanical properties of the ultra-high strength steel with excellent plasticity according to Examples 1 to 20 and the comparative steels according to Comparative Examples 1 to 3, as well as the situations of the breakout rate and crack repair rate in thin slab continuous casting under the conditions of the corresponding examples, are shown in Table 4. However, the mechanical properties were measured using the tensile test piece standard of ISO6892:1998 (Metallic materials - Tensile testing at ambient temperature), P14 (A50).

[0081] As can be seen from Table 4, the ultra-high strength steel with excellent plasticity according to Examples 1 to 20 of the present invention, while ensuring strength, also has good control over ductility, stability in the continuous process, and bit surface quality. Its yield strength YS is 1000 - 1300 MPa, the tensile strength TS is ≥1500 MPa, the elongation at break is ≥18%, and at the same time, the breakout rate in thin slab continuous casting is controlled to ≤1%, and the crack repair rate is controlled to ≤1.2%. In this text, the breakout rate and crack repair rate in continuous casting are respectively the ratio of the number of breakout slabs or cracked slabs to the total number of slabs in the batch.

[0082] The observation results of the microstructure of the ultra-high strength steel with excellent plasticity according to Examples 1 to 20 of the present invention are shown in Table 5. The specific measurement means are as follows: 1) The ratio of the retained austenite phase fraction was quantitatively measured by XRD after taking a sample with a size of 10 * 10 mm from the steel plate, polishing, and hubbing; 2) The martensite phase fraction was quantitatively measured by EBSD after taking a sample with a size of 10 * 10 mm from the steel plate, polishing, and hubbing; 3) Both the ferrite grain size ratio and the average size of retained austenite were obtained by statistical analysis when performing normalized IQ value processing during quantitative analysis by EBSD; 4) Measurement method for the C content in retained austenite: Assuming that there is no change in the Mn and Al concentrations of each constituent phase in the steel plate structure, from the diffraction peak data of retained austenite by XRD TIFF2025522611000001.tif12162

[0083] However, X C 、X Mn 、X Al represent the C, Mn, and Al concentrations in retained austenite, respectively.

[0084] As can be seen from Table 4 and Table 5, the microstructure of the 1500 MPa grade ultra-high strength steel with excellent plasticity according to Examples 1 to 20 of the present invention is ferrite with a volume ratio of 10% - 15% + martensite with a volume ratio of 70% - 80% + retained austenite. However, in ferrite, the ratio of the number of grains with a grain size ≤ 5 μm is 90% or more, the ratio of the number of grains with a grain size ≤ 3 μm is 60% or more, the average grain size of retained austenite is ≤ 2 μm, and the average C content in retained austenite satisfies 1.2 wt% ≤ C(ra) ≤ 2.0 wt%.

[0085] Thus, the 1500 MPa grade ultra-high strength steel with excellent plasticity according to each embodiment of the present invention has a predetermined amount of fine ferrite and a sufficient amount of retained austenite, and has good tissue uniformity. Therefore, it has been found that each embodiment has excellent plasticity while ensuring high strength.

[0086] FIG. 1 and FIG. 2 are respectively a typical microstructure and a phase composition EBSD photograph of Example 4 of the ultra-high strength steel according to the present invention. As can be seen from the drawings, the ultra-high strength steel according to the present invention has a uniform and fine structure and contains a large amount of finely dispersed retained austenite.

[0087] In summary, the ultra-high strength steel according to the present invention has a simple composition design. Based on carbon-silicon-manganese steel, only Cr and B are added as alloy strengthening elements, and the composition characteristics of low Si and low Mn can avoid the peritectic reaction in the continuous casting process. Therefore, the production stability in the continuous casting process can be greatly improved, and the obtained products have excellent surface quality.

[0088] At the same time, the present invention creatively proposes an efficient process of thin slab continuous casting + precise hot rolling + slow cooling treatment + pickling + continuous annealing. Thereby, cold rolling, which is a bottleneck process for ultra-high strength steel, can be avoided, and at the same time, it has inherent advantages in tissue uniformity, segregation control, and manufacturing cost. Under the same strength conditions, the elongation rate of the obtained ultra-high strength steel can be significantly improved, and it has good application prospects for automotive safety structural parts, especially for the manufacturing of vehicle structural parts and safety parts with complex shapes and high requirements for formability, such as A / B pillars, door crash bars, longitudinal beams, bumpers, etc. When the thin slab continuous casting and continuous rolling short flow technology is adopted, a hot rolled coil that meets the finished product thickness requirement (0.8 - 2.0 mm) can be directly obtained, and the cold rolling process can be correspondingly omitted, which has a very significant improvement effect on energy saving, consumption reduction, and production efficiency.

[0089] The prior art part within the scope of protection of the present invention is not limited to the embodiments described in the application documents of this application. It should be explained that all prior arts (including, but not limited to, prior patent documents, prior published publications, prior public uses, etc.) that do not conflict with the solution of the present invention are incorporated into the scope of protection of the present invention.

[0090] Moreover, the combination of each technical feature in the present invention is not limited to the combinations described in the claims of the present invention or the combinations described in the specific embodiments. As long as they do not conflict with each other, all the technical features described in the present invention can be freely combined or joined in any form.

[0091] Furthermore, it should also be noted that the above-mentioned embodiments are only specific embodiments of the present invention. The present invention is not limited to the above embodiments. It is obvious that any similar changes or modifications that those skilled in the art can directly derive from or easily conceive from the disclosed content of the present invention are included in the scope of protection of the present invention.

[0092]

Table 1

[0093]

Table 2

[0094]

Table 3

[0095]

Table 4

[0096]

Table 5

Claims

1. Its composition is by weight percentage, C: 0.35 - 0.40%; Si: 1.0 - 1.8%; Mn: 1.5 - 2.0%; Cr: 0.3 - 0.6%; Al: 0.02 - 0.05%; Ti: 0.02 - 0.05%; B: 0.002 - 0.02%; and the balance contains Fe and other inevitable impurities; and is a high - plasticity ultra - high - strength steel with a tensile strength ≥ 1500 MPa that simultaneously satisfies the following: Carbon equivalent C in the peritectic reaction region ｅｑ１ > 0.17%, C ｅｑ１ = C - 0.03Mn - 0.06Si - 0.222S - 0.04P; Welding carbon equivalent C ｅｑ２ ≤0.56%, C ｅｑ２ = C + Mn / 20 + Si / 30 + 2P + 4S.

2. The high - plasticity ultra - high - strength steel with a tensile strength ≥ 1500 MPa according to Claim 1, characterized in that the balance is Fe and other inevitable impurities.

3. The high - plasticity ultra - high - strength steel with a tensile strength ≥ 1500 MPa according to Claim 1 or 2, characterized in that the C content is 0.36 - 0.38 wt%.

4. The high - plasticity ultra - high - strength steel with a tensile strength ≥ 1500 MPa according to Claim 1 or 2, characterized in that the Si content is 1.4 - 1.7 wt%.

5. The high - plasticity ultra - high - strength steel with a tensile strength ≥ 1500 MPa according to Claim 1 or 2, characterized in that the Mn content is 1.7 - 2.0 wt%.

6. The high - plasticity ultra - high - strength steel with a tensile strength ≥ 1500 MPa according to Claim 1 or 2, characterized in that the Cr content is 0.4 - 0.6 wt%.

7. In the other inevitable impurities, P ≤ 0.015 wt%, S ≤ 0.002 wt%, O ≤ 0.002 wt%, N ≤ 0.004 wt%, the high - plasticity ultra - high - strength steel with a tensile strength ≥ 1500 MPa according to Claim 1 or 2.

8. The high - plasticity ultra - high - strength steel with a tensile strength ≥ 1500 MPa according to any one of Claims 1 - 7, characterized in that the microstructure of the ultra - high - strength steel is 10% - 15% ferrite by volume ratio + 70% - 80% martensite by volume ratio + retained austenite.

9. In the ferrite of the microstructure of the ultra - high - strength steel, the ratio of the number of crystal grains with a crystal grain size of 5 μm or less is 90% or more, and the ratio of the number of crystal grains with a crystal grain size of 3 μm or less is 60% or more, the high - plasticity ultra - high - strength steel with a tensile strength ≥ 1500 MPa according to Claim 8.

10. The average grain size of retained austenite in the microstructure of the ultra-high strength steel is ≤ 2 μm; and / or the average C content C(ra) in the retained austenite satisfies 1.2 wt% ≤ C(ra) ≤ 2.0 wt%, and the ultra-high strength steel with a tensile strength ≥ 1500 MPa according to claim 8 is characterized in that.

11. The yield strength of the ultra-high strength steel is 1000 - 1300 MPa, the tensile strength is ≥ 1500 MPa, and the elongation at break is ≥ 18%, and the ultra-high strength steel with a tensile strength ≥ 1500 MPa according to any one of claims 1 to 10 is characterized in that.

12. A method for manufacturing an ultra-high strength steel with a tensile strength ≥ 1500 MPa according to any one of claims 1 to 11, characterized by including the following steps. 1) Smelting and casting Smelt according to the composition described in claims 1 to 7, cast into slabs, preferably utilize thin slab continuous casting, control the slab thickness at the continuous casting outlet end to 55 - 60 mm, and control the drawing speed of continuous casting to 2 - 5 m / min; 2) Slab heating Set the heating temperature to 1200 - 1300 °C and the in-furnace time to 25 - 40 min; 3) Hot rolling and cooling First, perform high-pressure scale removal, control the rolling end temperature to 860 - 930 °C, then perform laminar flow cooling, control the cooling rate to 20 - 40 °C / s, cool to 500 - 600 °C and wind up; 4) Slow cooling treatment After the winding of the hot-rolled coil is completed, unwind the coil, add an in-line heat preservation cover for sealing treatment, or transfer it into a sealed heat preservation cover for slow cooling treatment. When the heat preservation cover treatment reaches ≥ 4 h or more, open the cover and take out the hot-rolled coil; preferably, after the winding of the hot-rolled coil is completed, wait for ≥ 3 min with a roller and then add an in-line heat preservation cover for sealing treatment, or transfer it into a sealed heat preservation cover for slow cooling treatment; 5) Pickling Control the pickling speed to 60 - 150 m / min; 6) Annealing Adopt continuous annealing, set the annealing temperature to 820 - 900 °C, slowly cool to 690 - 760 °C at a cooling rate of 3 - 10 °C / s; then quickly cool to 150 - 250 °C at a cooling rate of 50 - 100 °C / s; then reheat to 360 - 460 °C, keep warm for 100 - 400 s, and finally cool to room temperature.

13. In step 1), the breakout rate of thin slab continuous casting is controlled to ≤ 1%, and the crack repair rate is controlled to ≤ 1.2%, and the method for manufacturing an ultra-high strength steel with a tensile strength ≥ 1500 MPa according to claim 12 is characterized in that.

14. During the high-pressure scale removal in step 3), the scale removal water pressure of the first pass is controlled to 260 bar, and the scale removal water pressure of the second pass is controlled to 340 bar; and / or, in step 3), the U-type coiling method is adopted, that is, the coiling temperature is controlled to 550 - 650 °C within a distance of ≤ 30 m from the head and tail of the strip steel. A method for manufacturing a high-plasticity ultra-high-strength steel with a tensile strength ≥ 1500 MPa according to claim 12, characterized in that.

15. In the annealing process of step 6), the annealing temperature is 840 - 870 °C, slowly cooled to 700 - 730 °C at a cooling rate of 3 - 10 °C / s, rapidly cooled to 170 - 230 °C, reheated to 400 - 430 °C after rapid cooling, and held for 150 - 300 s; and / or, in the annealing process of step 6), the volume content of hydrogen gas in the reducing atmosphere in the continuous annealing furnace is controlled to 10 - 15%. A method for manufacturing a high-plasticity ultra-high-strength steel with a tensile strength ≥ 1500 MPa according to claim 12, characterized in that.

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