Highly formable, manganese cold-rolled steel sheet with tensile strength of 1000-1600 MPa, easily treatable with phosphate, and method for manufacturing the same.

Abstract

A high formability high-manganese cold-rolled steel sheet with a tensile strength of 1000 to 1600 MPa, which is easy to phosphatize, and a method for manufacturing the same. The steel sheet has a composite structure of a matrix face-centered cubic and a surface body-centered cubic. The matrix contains high-density twins (1 to 10) × 10 5 m -1 and low-density dislocations (1 to 10) × 10 13 m -1 The weight percentages of the matrix components are C 0.5 to 0.8%, Mn 12 to 20%, Si 0.1 to 0.5, Al 1.2 to 1.8%, N 0.01 to 0.1%, RE 0.01 to 0.1%. The balance contains Fe and unavoidable impurities, and Mn + 25C - 1.5Al ≧ 28%, Si + 20RE ≧ 1.0%. By selecting the cold rolling - continuous annealing process of the present invention, the steel sheet can achieve a tensile strength of 1000 to 1600 MPa and an elongation rate of 20 to 55%. It has excellent phosphatizing performance and a cold bending radius of 0t. Therefore, it is a steel for integrated material design of high-strength safety structural parts of automobiles.

Description

[Technical Field]

[0001] This invention belongs to the field of high-manganese cold-rolled steel, and more specifically, to a high-manganese cold-rolled steel sheet with a tensile strength of 1000 to 1600 MPa, high formability, and ease of phosphate treatment, and a method for manufacturing the same. [Background technology]

[0002] Against the backdrop of increasingly stringent environmental protection and decarbonization, many automobile bodies are now using ultra-high-strength steel sheets with a strength of 780 MPa or higher, replacing conventional automotive steel. Increasing the strength of the steel sheets and thus reducing their thickness has become a technological consensus for achieving "lighter weight, energy savings, improved safety, and reduced manufacturing costs" in automobiles. For every 10% reduction in vehicle weight, fuel consumption is saved by 5% to 8%, and accordingly, CO2 greenhouse gases and NOx are reduced. x This also reduces the emission of pollutants such as SO2.

[0003] However, the conventional microstructure and metallurgical mechanisms of steel make it difficult to meet the automotive industry's future needs for highly formable, ultra-high-strength steel for automobiles. Therefore, steelmakers are forced to develop various custom materials that meet the diverse performance requirements of body materials, specifically in terms of strength, formability, and usability. As a result, the types of body materials have become complex, ranging from 340 to 1500 MPa in strength and 3 to 50% in elongation, including dozens of different products such as ferritic steel, precipitation-strengthened steel, martensitic steel, duplex steel, and composite steel. Both steel companies and automotive companies face problems such as complex material schemes, high production management costs, and frequent changes in manufacturing processes, seriously impacting their production stability, efficiency, and cost management. In recent years, the introduction of advanced metallurgical mechanisms and material design has led to the development of new steel materials with simpler compositions and a wide range of microstructure properties. Adjustments to the processing process have made it possible to cover a wide range of performance requirements with single-component designs. This material design concept, called Uni-material, can significantly reduce the complexity of automotive materials, simplifying material management and design for automotive companies. Furthermore, it allows for the design and management of processes such as welding and painting, which critically impact component design, within a single process. Simultaneously, for steel companies, the relatively simple product design enables a high degree of consistency in steelmaking, continuous casting, and hot rolling processes, effectively improving efficiency, reducing costs, and enhancing the company's market competitiveness.

[0004] Among various integrated material solutions, the development and application of advanced high-strength automotive steels, including phase transformation strengthening, has become a major research theme for leading steel companies worldwide. Fully austenitic steel with high carbon and manganese content exhibits an elongation of over 50% when its tensile strength reaches 1000 MPa. However, because fully austenitic steel does not undergo phase transformation through heat treatment, it is difficult to adjust its microstructure properties, making it particularly challenging to achieve higher strengths. Unless this problem can be effectively solved, it cannot be applied to the automotive industry. Furthermore, this type of high-manganese fully austenitic steel has a high manganese content, which is an easily oxidizable element, resulting in poor plating properties due to surface oxidation.

[0005] Currently, the main methods for adjusting the properties of high-manganese steel involve the addition of alloying elements such as Nb, V, Ti, Cr, and Mo, and there are numerous related manufacturing patents. However, the addition of each of these elements presents its own metallurgical problems. The effect of V is unstable and difficult to control, posing a significant problem for industrial applications. Nb and Ti mainly improve the yield strength of the material, but have little effect on tensile strength. The effect of Mo is stable, but it is expensive and significantly increases the thermal strength of the material, creating major technical difficulties in processes such as hot rolling.

[0006] European Patent EP3492618B1 discloses a high-tensile-productivity automotive steel with a tensile strength of 1500 MPa. Its chemical elemental mass percentages are C 0.1%, 0.3%, Si 0.1%~2.0%, Mn 7.5%~12%, Al 0.01%~2.0%, with the remainder being iron and other unavoidable impurities. The microstructure of the steel of this invention is austenite + martensite + ferrite or austenite + martensite, capable of reaching a tensile strength of 1500 MPa and having a tensile strength of 30 GPa% or more. However, the austenite in the microstructure of this invention is a metastable structure, and a martensitic transformation occurs during the deformation process, negatively affecting properties such as low-temperature toughness and shear edge. Furthermore, the steel of this invention requires a very complex and time-consuming multi-stage heat treatment, resulting in very unfavorable production efficiency and cost.

[0007] Chinese patent CN106191404B discloses a method for manufacturing high-strength, high-plasticity TWIP steel, which combines asynchronous rolling and cold rolling with a very large deformation amount and annealing treatment to obtain ultrafine crystal grains of 1 μm or less. Furthermore, by adding microalloys such as Nb and Ti, it is possible to achieve a tensile strength of 1400 MPa and an elongation of more than 7%. However, this invention requires cold rolling after warm rolling at 400°C, with a total deformation amount exceeding 95%, and also requires the use of asynchronous rolling, making the process complex and difficult, and therefore impossible for large-scale industrial production.

[0008] International patent application WO2014097184A4 discloses a high-strength, highly plastic austenitic stainless steel with the following wt.% composition: C:0.01~0.50, N:0.11~0.50, Mn:6~12, Ni:0.01~6.0, Cu:0.01~6.0, Si:0.001~0.5, Al:0.001~2.0, Cr:11~20, Nb:0.001~0.5, Mo:0.01~2.0, Co:0.01~2.0, and Ti:0.001~0.5%. It can achieve a tensile strength of 1200 MPa and an elongation of 60%. Although the material has excellent performance, it requires the addition of many expensive alloying elements such as Cr, Ni, Mo, and Co, making it usable only for specialized applications and lacking economic viability and feasibility for general automotive applications.

[0009] U.S. Patent Application US20120288396(A1) discloses an ultra-high ductility austenitic steel whose composition satisfies the following conditions: Mn: 8-16%, Cu: ≤3%, C: 33.5C+Mn ≤25 and 33.5-Mn ≥22, with the addition of other elements such as Cr, Ti, Nb, and N, and the remainder being Fe and impurities. The steel in this application has an austenite fraction of 99% or more, a yield strength of 300-630 MPa, and an elongation of approximately 30%. For automotive steel, the addition of Cu is disadvantageous from a cost management perspective, and an elongation of approximately 30% does not offer a clear advantage compared to conventional high-strength steel.

[0010] International patent application WO2009084792(A1) discloses a high-strength, delayed-fracture, high-manufacturation steel and a method for manufacturing the same. Its composition is C: 0.3-0.9%, Mn: 15-25%, Si ≤ 0.1-2%, Al: 0.01-4%, Cr ≤ 10%, N ≤ 0.6%, Cu ≤ 3%, and it is also possible to add elements such as V, Ti, Mo, Nb, Cr, and W. In this application, the tensile strength of the steel is 920 MPa or higher, and the elongation is 55% or higher. Although the steel in this application has excellent performance, its relatively high Mn and Cr content is disadvantageous from a cost management perspective.

[0011] Chinese patent application 200810239893.X discloses a P-reinforced TWIP steel and a method for manufacturing the same, with components of C: 0.01~0.08%, Mn: 15~35%, Si≦1~6%, Al: 1~6%, P: 0.062~0.2%, and the remainder being Fe and impurities. In this application, the steel has a tensile strength of 610~915 MPa, a yield strength of 225~610 MPa, and an elongation of 45~85.5%. While it has excellent formability, its low yield strength and tensile strength make it difficult to meet the requirements of future ultra-high-strength steel for automobiles. Furthermore, P-reinforced high-strength steel is difficult to weld with other types of steel. [Overview of the project] [Problems that the invention aims to solve]

[0012] The object of the present invention is to provide a high-manganese cold-rolled steel sheet with a tensile strength of 1000 to 1600 MPa, high formability, and ease of phosphate treatment, and a method for manufacturing the same. The steel sheet has the characteristic of having a wide performance adjustment range, and can achieve various combinations of performance such as yield strength (YS) of 700 to 1400 MPa, tensile strength (TS) of 1000 to 1600 MPa, and elongation (EL) of 20 to 55%, and TS 2 ×EL≧49TPa 2 It meets the % requirement, achieving excellent phosphate coating performance and bending performance, with a bending center radius of 0t, making it suitable for automotive structural and safety parts that require various levels of strength and formability. [Means for solving the problem]

[0013] In order to achieve the above object, the present invention provides a high manganese cold-rolled steel sheet with a tensile strength of 1000 to 1600 MPa, which is a composite structure including a matrix and a surface layer. The matrix has a face-centered cubic phase structure containing high-density twins and low-density dislocations. The twin density is (1 to 10) × 10 5 m -1 and the dislocation density is (1 to 10) × 10 13 m -1 The weight percentages of the chemical components of the matrix are as follows: C: 0.5 to 0.8%; Mn: 14 to 18%; Si: 0.1 to 0.5%; RE: 0.01 to 0.10%; P: ≦ 0.020%; S: ≦ 0.010%; Al: 1.2 to 1.8%; N: 0.01 to 0.1%; The balance contains Fe and other inevitable impurities, and at the same time satisfies Mn + 25C - 1.5Al ≧ 28% and Si + 20RE ≧ 1.0%. The surface layer is an iron alloy layer with a body-centered cubic phase structure, and its components include C ≦ 0.03 wt%, Mn ≦ 0.5 wt%, and Al ≦ 0.1 wt%. The high manganese cold-rolled steel sheet has a yield strength of 700 to 1400 MPa, a tensile strength of 10​​​​​​​​​​​​​​​Preferably, the Al content in the chemical composition of the matrix is ​​1.2 to 1.5 wt%, for example, 1.25 wt%, 1.3 wt%, 1.35 wt%, 1.4 wt%, or 1.45 wt%.

[0017] Preferably, the Si content in the chemical composition of the matrix is ​​0.2 to 0.4 wt%, for example, 0.25 wt%, 0.3 wt%, or 0.35 wt%.

[0018] In one or more embodiments, the RE content in the chemical composition of the matrix is ​​0.02%, 0.04%, 0.06%, or 0.08%.

[0019] In one or more embodiments, the P content in the chemical composition of the matrix is ​​0 to 0.020 wt%, for example, 0.001 wt%, 0.003 wt%, 0.005 wt%, 0.010 wt%, or 0.015 wt%.

[0020] In one or more embodiments, the sulfur content in the chemical composition of the matrix is ​​0 to 0.010 wt%, for example, 0.001 wt%, 0.002 wt%, 0.003 wt%, 0.005 wt%, or 0.007 wt%.

[0021] In one or more embodiments, the N content in the chemical composition of the matrix is ​​0.02%, 0.04%, 0.06%, or 0.08%.

[0022] In one or more embodiments, the twinning density in the matrix is ​​2 × 10 5 m -1 , 4×10 5 m -1 , 6×10 5 m -1 or 8 x 10 5 m -1 That is the case.

[0023] In one or more embodiments, the dislocation density in the matrix is ​​2 × 10 13 m -1 , 4×1013 m -1 , 6×10 13 m -1 or 8 x 10 13 m -1 That is the case.

[0024] In one or more embodiments, the weight percent of the chemical composition of the matrix satisfies the requirement that Mn+25C-1.5Al is 28-34%, for example, 29%, 30%, 31%, 32%, 33%, or 33.6%.

[0025] In one or more embodiments, the weight percent of the chemical composition of the matrix satisfies the condition that Si+20RE is 1.0 to 2.5%, for example, 1.2%, 1.5%, 1.8%, 2.0%, 2.2%, or 2.4%.

[0026] In one or more embodiments, the C content in the chemical composition of the surface layer is 0 to 0.03 wt%, for example, 0.001 wt%, 0.005 wt%, 0.01 wt%, or 0.02 wt%.

[0027] In one or more embodiments, the Mn content in the chemical composition of the surface layer is 0 to 0.5 wt%, for example, 0.01 wt%, 0.02 wt%, 0.03 wt%, or 0.04 wt%.

[0028] In one or more embodiments, the Al content in the chemical composition of the surface layer is 0 to 0.1 wt%, for example, 0.01 wt%, 0.02 wt%, 0.04 wt%, 0.06 wt%, or 0.08 wt%.

[0029] Preferably, the surface thickness of the high-manganese cold-rolled steel sheet is 0.5 to 2 μm, for example, 0.6 μm, 0.8 μm, 1 μm, 1.2 μm, 1.4 μm, 1.6 μm, or 1.8 μm.

[0030] In one or more embodiments, the yield strength of the high-manganese cold-rolled steel sheet is 800 MPa, 900 MPa, 1000 MPa, 1100 MPa, 1200 MPa, or 1300 MPa.

[0031] In one or more embodiments, the tensile strength of the high-manganese cold-rolled steel sheet is 1100 MPa, 1200 MPa, 1300 MPa, 1400 MPa, or 1500 MPa.

[0032] In one or more embodiments, the elongation of the high-manganese cold-rolled steel sheet is 25%, 30%, 35%, 40%, 45%, or 50%.

[0033] In one or more embodiments, the tensile strength and elongation of the high-manganese cold-rolled steel sheet are TS 2 ×EL is 49~60TPa 2 %, for example 52TPa 2 %, 54TPa 2 %, 56TPa 2 %, 58TPa 2 It satisfies the condition of being a percentage.

[0034] In the composition design of the high-manganese cold-rolled steel sheet described in the present invention, Carbon (C) is the most effective austenite-stabilizing element in steel, effectively increasing the stacking fault energy of the material, suppressing austenite phase transformation, and thereby improving austenite stability. In high-manganese steel, adding an appropriate amount of C can significantly reduce the Mn content while maintaining the same austenite stability level, resulting in reduced material costs. However, excessive C content not only degrades the weldability of the material but also causes technical difficulties in steelmaking and continuous casting processes. In the matrix of the steel sheet of this invention, the C content is in the range of 0.5 to 0.8% by weight.

[0035] Mn is an effective austenite-stabilizing element. In high-manganese steel, the role of Mn is similar to that of C, effectively increasing the stacking fault energy of the material, lowering the martensitic transformation temperature Ms, and improving austenite stability. Furthermore, unlike the role of Mn in ordinary carbon steel, in high-manganese austenitic steel, increasing the Mn content leads to a decrease in material strength. Therefore, while ensuring the austenite stability of the material, it is necessary to reduce the Mn content as much as possible. In the matrix of the steel sheet of the present invention, the Mn content is in the range of 14 to 18% by weight.

[0036] Al can effectively improve the delayed fracture resistance of materials. However, the addition of Al significantly degrades the smelting and continuous casting performance of steel, and is highly likely to lead to gate blockage during continuous casting. Furthermore, if a large amount of Al2O3 is generated during the smelting and continuous casting processes, the fluidity of the molten steel decreases, causing problems such as slag inclusion and slab cracking. It is necessary to reduce the Al content as much as possible while ensuring the delayed fracture characteristics of the material. In the steel plate matrix of the present invention, the Al content is in the range of 1.2 to 1.8% by weight.

[0037] Mn+25C-1.5Al≧28%: Both C and Mn can play a role in stabilizing austenite and achieving a fully austenitic structure, so C and Mn can mutually promote each other to some extent. However, Al has the effect of significantly reducing austenite stability, thus hedging against the effect of C / Mn. Through the analysis of a large amount of test data, this invention has confirmed that when the amounts of Mn, C, and Al added to the steel matrix satisfy the relation Mn+25C-1.5Al≧28%, the austenite in the steel of this invention can be made sufficiently stable to achieve a fully austenitic room-temperature microstructure.

[0038] RE: It is generally recognized that the role of RE (rare earth elements) in steel is to improve the morphology of inclusions, refine the steel, and enhance the strength and formability of the material. However, in the steel of the present invention, RE plays a more important role. On the other hand, secondary cold rolling and heat treatment is an effective method for improving the strength of high manganese austenitic steel, but high manganese austenitic steel has high work hardening ability, and secondary cold rolling usually results in a significant decrease in plasticity. After cold deformation, the addition of RE can effectively delay the formation of twins, thereby reducing the work hardening ability of the material in the initial stages of deformation, improving the plasticity of the material after cold working, and being advantageous for the production of the material through secondary cold working. During the annealing stage, RE forms a large number of fine dispersed particles in the material, thereby effectively fixing the boundaries of twins and improving the stability of twins during heat treatment. This allows for the preservation of cold deformation twins as much as possible, improving the strength of the material, and simultaneously achieving the objective of the present invention, which is to not impair the deformation ability of the material. On the other hand, RE is a good hydrogen absorbent material and can react with H to form stable hydrides, thus reducing the diffusible H content in the material and improving the delayed fracture resistance of the material. However, adding RE in excess presents a problem of difficulty in dispersing in molten steel, resulting in the generation of a large amount of rare earth inclusions, which conversely affects the cleanliness of the molten steel. Therefore, the design range for RE in the steel sheet matrix of the present invention is 0.01 to 0.1%.

[0039] Si: In high-manganese steel, Si effectively suppresses cementite precipitation, improves the cleanliness of the material's crystal grains, and thereby improves the material's plasticity. However, Si reduces the stability of austenite, and excessive addition is detrimental to maintaining a perfect austenite structure. Therefore, in the steel sheet matrix of the present invention, the Si content as an alloying element to improve the material's plasticity must be limited to 0.1-0.5%, while simultaneously satisfying the condition Si + 20 × RE ≥ 1.0%.

[0040] P: While it has a certain solid solution strengthening effect, the addition of P significantly degrades the plasticity of the material and reduces its weldability. In the steel sheet matrix of this invention, P is used as an impurity element and controlled to the lowest possible level.

[0041] S: As an impurity element, its content should be controlled to the lowest possible level. N: Its role is similar to that of C, and it is an effective austenite-stabilizing element. In high-manganese steel, increasing the N content is advantageous in improving austenite stability and material properties. However, excessive N addition tends to lead to N2 precipitation, forming N2 bubbles in the material and significantly degrading the continuity and performance of the material. In the steel sheet matrix of the present invention, the N content is controlled to 0.01-0.1%.

[0042] This invention employs a compositional design scheme of C, Mn, Si, Al, and RE, enabling the production of high-Mn cold-rolled, perfectly austenitic steel products with low material costs, good manufacturability, and superior performance, without the need to add expensive alloying elements.

[0043] The present invention also provides a method for manufacturing high-manganese cold-rolled steel sheets with a tensile strength of 1000 to 1600 MPa, the manufacturing method comprising the following steps: 1) Smelting, billet casting The material is smelted according to the chemical composition of the matrix and then cast into a slab.

[0044] 2) Hot rolling The slab is heated to a heating temperature of 1170-1230°C, the final rolling temperature during hot rolling is 970-1030°C, and the coiling temperature is 650-850°C.

[0045] 3) Cold rolling The material undergoes pickling and cold rolling, with cold rolling deformation ranging from 10% to 40%.

[0046] 4) Annealing Continuous annealing is employed, with an annealing temperature T of 250-400°C and an annealing time t of 120-180 s. Simultaneously, the relationship between annealing temperature and annealing time satisfies 1100 ≤ (T + 273) l g t ≤ 1400, which leads to austenite recovery and ultimately stabilization at room temperature.

[0047] In a preferred embodiment, the corresponding cold rolling and annealing processes can be selected according to the performance range of the tensile strength of the finished steel sheet, which is 1000 to 1600 MPa.

[0048] When the tensile strength is between 1000 MPa and less than 1250 MPa, the cold rolling deformation is 10-20%, and the annealing process satisfies 1100 ≤ (T + 273) l g t ≤ 1200. When the tensile strength is between 1250 MPa and 1350 MPa, the cold rolling deformation is 20-30%, and the annealing process satisfies 1200 ≤ (T + 273) l g t ≤ 1250. When the tensile strength is between 1350 MPa and 1500 MPa, the cold rolling deformation is 30-35%, and the annealing process satisfies 1250 ≤ (T + 273) l g t ≤ 1350. When the tensile strength is between 1500 MPa and 1600 MPa, the cold rolling deformation is 35-40%, and the annealing process satisfies 1350 ≤ (T + 273) lgt ≤ 1400.

[0049] In one or more embodiments, the heating temperature of the slab in step 2) is 1180°C, 1190°C, 1200°C, 1210°C, or 1220°C.

[0050] In one or more embodiments, the final rolling temperature for the hot rolling in step 2) is 980°C, 990°C, 1000°C, 1010°C, or 1020°C.

[0051] In one or more embodiments, the winding temperature in step 2) is 680°C, 700°C, 750°C, 800°C, or 820°C.

[0052] In one or more embodiments, the amount of cold rolling deformation in step 3) is 15%, 20%, 25%, 30%, or 35%.

[0053] In one or more embodiments, the annealing temperature T in step 4) is 280°C, 300°C, 320°C, 350°C, or 380°C.

[0054] In one or more embodiments, the annealing time t in step 4) is 130s, 140s, 150s, 160s, or 170s.

[0055] In one or more embodiments, the annealing temperature and annealing time in step 4) satisfy the condition that (T+273)lgt is 1150, 1200, 1250, 1300, or 1350.

[0056] Preferably, the smelting in step 1) is carried out using an electric furnace or a converter. Preferably, steps 1) and 2) employ conventional continuous casting + hot rolling, or thin slab continuous casting and continuous rolling processes.

[0057] In the method for producing high-manganese cold-rolled steel sheets according to the present invention, The steel of this invention has a fully austenitic structure and is free from other types of phase transformations. The role of heat retention in the hot rolling high-temperature furnace is to reduce the rolling load and homogenize the composition of the billet.

[0058] The reason for employing a high winding temperature in this invention is to externally oxidize the steel sheet surface at a high temperature, thereby significantly concentrating easily oxidizable elements such as C, Si, and Mn on the steel sheet surface and forming an element-deficient layer beneath the surface. Combined with the subsequent pickling process, this forms an element-deficient body-centered cubic (BCC) structure layer on the steel sheet surface, realizing a composite structure of a surface BCC phase structure iron alloy layer and a matrix face-centered cubic (FCC) phase structure iron alloy layer, which can significantly improve the phosphate treatment coating performance of the material.

[0059] In the recovery annealing of steel according to the present invention, increasing both the annealing temperature and annealing time is beneficial for elemental diffusion and can accelerate the austenite recovery process. Therefore, there is a certain degree of mutual compensation between the annealing temperature and annealing time. Through the analysis of a large amount of test data, the present invention has confirmed that when the annealing temperature T and annealing time t satisfy the relationship 1100 ≤ (T + 273) l g t ≤ 1400, the performance of the steel according to the present invention is ensured and a suitable fully recovered austenite structure is obtained after annealing. During the annealing stage, RE improves the stability of twinning during heat treatment, maintaining high-density twinning and low-density dislocations in the final material, resulting in a better combination of strength and elongation performance.

[0060] This invention achieves performance adjustment over a wide range of tensile strengths from 1000 to 1600 MPa by arbitrarily adjusting the cold rolling and annealing processes according to the strength requirements of the finished steel sheet, while also providing excellent formability and meeting the performance and formability requirements of different parts of an automobile body. For example, a steel sheet with a tensile strength of 1000 MPa is suitable for parts such as A, B, and C pillar inner panels, floor cross beams, and longitudinal beams; a steel sheet with a tensile strength of 1200 MPa is suitable for parts such as A, B, and C pillar reinforcing plates, door sills, and door impact bars; and a steel sheet with a tensile strength of 1500 MPa is suitable for parts such as front and rear impact beams and door ring reinforcing plates. Further details are as follows.

[0061] When the tensile strength is between 1000 MPa and 1250 MPa, the cold rolling deformation is 10% to 20%, and the annealing process satisfies 1100 ≤ (T + 273) lgt ≤ 1200.

[0062] When the tensile strength is between 1250 MPa and 1350 MPa, the cold rolling deformation is 20% to 30%, and the annealing process satisfies 1200 ≤ (T + 273) l g t ≤ 1250. When the tensile strength is between 1350 MPa and 1500 MPa, the cold rolling deformation is 30% to 35%, and the annealing process satisfies 1250 ≤ (T + 273) l g t ≤ 1350. When the tensile strength is between 1500 MPa and 1600 MPa, the cold rolling deformation is 35% to 40%, and the annealing process satisfies 1350 ≤ (T + 273) lgt ≤ 1400.

[0063] Furthermore, the present invention employs continuous annealing, which has clear advantages such as excellent structure and properties, high production efficiency, and energy saving, and the high-manganese steel completes the process of recovering its deformed structure during annealing.

[0064] Compared to the conventional technology, the beneficial effects of the present invention are as follows: The steel sheet described in this invention has a composite structure of a surface-centered cubic (BCC) phase iron alloy layer and a matrix-centered cubic (FCC) phase iron alloy layer. The steel sheet has a wide range of performance adjustments and can achieve various combinations of performance, such as yield strength (YS) of 700 to 1400 MPa, tensile strength (TS) of 1000 to 1600 MPa, and elongation (EL) of 20 to 55%. It has excellent phosphate treatment coating performance and bending performance, making it suitable for automotive structural parts and safety parts that require various levels of strength and formability.

[0065] This invention primarily utilizes the properties of high-manganese steel, which tends to generate many deformation twins during cold deformation. By precisely controlling the composition design, cold deformation, and subsequent heat treatment, it achieves the coexistence of high-density twins and low-density dislocations in the final material, significantly improving the material's strength level without compromising its plastic deformation capacity. In particular, the addition of rare earth elements RE effectively suppresses the appearance of twins during deformation, controlling the twin density within an appropriate range. Furthermore, it does not affect the density of the formed twins, maintaining the stability of the twins during subsequent heat treatment and achieving an effective reduction in dislocation density.

[0066] The present invention allows for arbitrary adjustment of the cold rolling and annealing processes according to the strength requirements of the finished steel sheet. Specifically, by adjusting the density of twins and dislocations, a wide range of performance adjustments for high-manganese steel can be achieved even with the same composition design. The strength level covers a tensile strength (TS) of 1000 to 1600 MPa and an elongation (EL) of 20 to 55%, satisfying the mechanical properties and formability requirements of various parts and most components of an automobile's white body.

[0067] This invention effectively delays twin formation by adding rare earth elements to high-manganese steel, thereby reducing the work hardening ability of the material in the initial stages of deformation, improving the plasticity of the material after cold working, and facilitating recovery annealing of the material. At the same time, by utilizing the purification, precipitation, and hydrogen storage properties of rare earth elements, high formability, high strength, and excellent delayed fracture resistance are obtained, while the smelting and continuous casting performance of the material is greatly improved. The steel of this invention employs production methods such as electric furnace or converter smelting, conventional continuous casting or thin slab continuous casting, hot rolling, pickling cold rolling, and continuous annealing, resulting in high production efficiency and good uniformity of product performance.

[0068] Furthermore, by making full use of the slow cooling stage after hot rolling and coiling, and by controlling the coiling temperature, the present invention can adjust the oxidation concentration of easily oxidizable elements such as Si and Mn on the surface of the steel sheet, and form a C, Si, Mn-deficient iron alloy BCC phase structure layer of a certain thickness on the surface of the steel sheet, thereby significantly improving the phosphate treatment coating performance of the steel sheet after pickling and cold rolling.

[0069] This invention, through appropriate component design and cold rolling-continuous annealing process control, can achieve performance covering a tensile strength range of 1000-1600 MPa and an elongation of 20-55%, meeting the performance requirements of most structural and safety components of future vehicle bodies, and becoming a strong option for realizing integrated material solutions for vehicle bodies.

[0070] The steel sheet described in the present invention has good potential for application in automotive safety structural components, and is particularly suitable for the manufacture of vehicle structural and safety components with very complex shapes and high requirements for formability, such as door impact bars, bumpers, and B-pillars. [Brief explanation of the drawing]

[0071] [Figure 1] This is a schematic diagram of the multilayer structure of the high-manganese cold-rolled steel sheet according to the present invention. [Figure 2] This is a photograph of the matrix face-centered cubic (FCC) phase structure in the multilayer structure of the high-manganese cold-rolled steel sheet according to the present invention. [Figure 3] This is a photograph of the matrix RE precipitate phase in the multilayer structure of the high-manganese cold-rolled steel sheet according to the present invention. [Figure 4] This is a schematic diagram showing the elongation change data under cold rolling deformation conditions for the steel of the embodiment of the present invention and the steel of the comparative example. [Figure 5] This is a schematic diagram showing the combination of cold deformation and strength-elongation performance after heat treatment of the steel in the embodiment of the present invention and the steel in the comparative example. In Figure 5, the point where the tensile strength is 1001 MPa and the elongation is 55% corresponds to Example 14. [Modes for carrying out the invention]

[0072] The present invention will be further described below with reference to examples and drawings. The components of Examples 1 to 16 of the present invention are subjected to smelting, hot rolling, cold rolling, annealing and temper rolling to obtain the product, which includes the following steps.

[0073] 1) Smelting, billet casting The mixture is smelted according to the components shown in Table 1 and then cast into slabs.

[0074] 2) Hot rolling Slab heating, hot rolling, and winding.

[0075] 3) Cold rolling Pickling, cold rolling.

[0076] 4) Annealing After continuous annealing, austenite recovery occurs, and the material eventually stabilizes at room temperature.

[0077] 5) Temper rolling Of these, in step 1), examples 2, 4, 6-9, and 12-14 were smelted in an electric furnace, while examples 1, 3, 5, 10, 11, 15, and 16 were smelted in a converter. In steps 1) and 2), examples 1-4 and 6-14 employed conventional continuous casting + hot rolling, while examples 5, 15, and 16 employed thin slab continuous casting and continuous rolling processes.

[0078] Table 1 shows the matrix components of steel sheets 1 to 16 in the examples. The matrix has a face-centered cubic phase structure, and the surface layer has a body-centered cubic phase structure. Table 2 shows the properties of the steel sheet surface layer and matrix. Table 3 shows the manufacturing process, and Table 4 shows the mechanical properties and phosphate treatment properties.

[0079] As can be seen from Tables 1 and 2, and as shown in Figures 1 to 3, the present invention obtained a composite structure of a surface BCC phase structure iron alloy layer and a matrix FCC phase structure iron alloy layer through appropriate component design and process adjustment. In the present invention, the surface phase structure was detected by backscatter electron diffraction (EBSD), and the matrix phase structure was detected by EBSD and X-ray diffraction (XRD).

[0080] Table 1 shows the matrix chemical composition of the steel sheets of Comparative Examples 1 to 4. The product of Comparative Example 1 was manufactured according to the steps of the example, and the manufacturing process parameters are shown in Table 3. The matrix of the steel sheet of Comparative Example 1 has a face-centered cubic phase structure, and the surface layer has a body-centered cubic phase structure. The characteristics of the surface layer and matrix are shown in Table 2.

[0081] Table 4 shows the mechanical properties of the steel plates of Comparative Examples 1 to 4. In this invention, performance tests were conducted on the steel sheets of the above examples and comparative examples, using the composition and thickness of the surface BCC layer, mechanical properties (yield strength, tensile strength, elongation), bending radius, phosphate treatment characteristics, twinning density, and dislocation density as indicators.

[0082] Of these, the test method for mechanical properties followed the American Society for Testing and Materials (ASTM) standard E8 / E8M-13, "Standard Test Methods For Tension Testing of Metallic Materials." Tensile tests were performed using ASTM standard gauge length 50 mm tensile test specimens, with the tensile direction perpendicular to the rolling direction.

[0083] EBSD was used to detect the twinning density, and the ratio of the length of the twinning boundary to the grain area within the field of view was statistically analyzed.

[0084] The method for detecting dislocation density was based on "Y. Zhong, F. Yin, T. Sakaguchi, K. Nagai, K. Yang, Dislocation structure evolution and characterization in the compression deformed Mn-Cu alloy, Acta Materialia, Volume 55, Issue 8, 2007, Pages 2747-2756". Specifically, a 10 × 20 mm sample was cut from a steel plate, and after surface polishing, the XRD (X-ray diffraction) pattern was measured. The dislocation density value of the sample was obtained by fitting the entire spectrum of the pattern and performing calculations using the MWAA (Modified Warren-Averbach Analysis) method. See Table 4 for the detection results.

[0085] Surface components were detected using an energy-dispersive spectrometer (EDS). The thickness of the surface layer was measured using a scanning electron microscope (SEM).

[0086] The bending radius was determined in accordance with GB / T232-2010 "Bending Test Method for Metallic Materials". The phosphate treatment characteristics were detected in accordance with GB / T6807-2001 "Technical conditions for phosphate treatment before painting of steel workpieces".

[0087] As can be seen from Table 4, the steel of the present invention can achieve a wide range of performance adjustments under appropriate composition and process design, and ultra-high-strength cold-rolled steel sheets with a yield strength (YS) of 600-1300 MPa, a tensile strength (TS) of 1000-1600 MPa, and an elongation (EL) of 20-55% can be obtained.

[0088] As shown in Figure 4, after cold deformation, the elongation of the steel of the present invention is significantly better than that of the comparative example steel. This indicates that the addition of RE in the present invention helps to delay the decrease in the elongation of the steel sheet under cold rolling deformation, helps to maintain high formability even after secondary cold rolling, and provides good microstructural properties through subsequent heat treatment.

[0089] As shown in Figure 5, the combination of strength and elongation performance of the material after cold deformation and heat treatment in the present invention is superior to that of the comparative example steel. This indicates that, during the annealing stage, the present invention improved the stability of twinning during heat treatment by RE, maintaining high-density twinning and low-density dislocations in the final material, and achieving a superior combination of strength and elongation performance.

[0090] [Table 1]

[0091] [Table 2]

[0092] [Table 3]

[0093] [Table 4]

Claims

1. A high-manganese cold-rolled steel sheet with a tensile strength of 1000 to 1600 MPa, having a composite structure including a matrix and a surface layer. The matrix has a face-centered cubic phase structure containing high-density twins and low-density dislocations, with a twin density of (1–10) × 10⁻¹⁰ 5 m -1 Therefore, the dislocation density is (1 to 10) × 10 13 m -1 The weight percentages of the chemical components of the matrix are as follows: C：0.5～0.8%； Mn: 14–18%; Si: 0.1–0.5%; RE: 0.01–0.10%; P：≦0.020%； S：≦0.010%； Al：1.2～1.8%； N：0.01～0.1%； The remainder consists of Fe and other unavoidable impurities, while simultaneously satisfying the conditions Mn + 25C-1,5Al ≥ 28% and Si + 20RE ≥ 1.0%. The aforementioned surface layer is an iron alloy layer with a body-centered cubic phase structure, and its composition includes C ≤ 0.03 wt%, Mn ≤ 0.5 wt%, and Al ≤ 0.1 wt%. The aforementioned high-manganese cold-rolled steel sheet has a yield strength of 700 to 1400 MPa, a tensile strength of 1000 to 1600 MPa, an elongation of 20 to 55%, and TS 2 ×EL≧49TPa 2 A high-manganese cold-rolled steel sheet characterized by satisfying the % requirement.

2. The high-manganese cold-rolled steel sheet according to claim 1, characterized in that the C content in the chemical composition of the matrix is ​​0.5 to 0.7 wt%.

3. The high-manganese cold-rolled steel sheet according to claim 1, characterized in that the Mn content in the chemical composition of the matrix is ​​15 to 17 wt%.

4. The high-manganese cold-rolled steel sheet according to claim 1, characterized in that the Al content in the chemical composition of the matrix is ​​1.2 to 1.5 wt%.

5. The high-manganese cold-rolled steel sheet according to claim 1, characterized in that the Si content in the chemical composition of the matrix is ​​0.2 to 0.4 wt%.

6. The high-manganese cold-rolled steel sheet according to claim 1, characterized in that the surface thickness of the high-manganese cold-rolled steel sheet is 0.5 to 2 μm.

7. A method for manufacturing a high-manganese cold-rolled steel sheet according to any one of claims 1 to 6, comprising the following steps, namely: 1) Smelting, billet casting Smelting according to the chemical composition of the matrix described in any one of claims 1 to 5, and casting into a slab, 2) Hot rolling The slab is heated to a heating temperature of 1170-1230°C, the final rolling temperature during hot rolling is 970-1030°C, and the winding temperature is 650-850°C. 3) Cold rolling Pickling and cold rolling are performed, and the cold rolling deformation should be between 10% and 40%. 4) Annealing A method for manufacturing high-manganese cold-rolled steel sheets, characterized by employing continuous annealing, having an annealing temperature T of 250 to 400°C, an annealing time t of 120 to 180 s, and simultaneously satisfying the relationship between annealing temperature and annealing time of 1100 ≤ (T + 273) lgt ≤ 1400, and finally stabilizing down to room temperature.

8. Depending on the tensile strength of the finished steel sheet, the corresponding cold rolling and annealing processes are selected. When the tensile strength is between 1000 MPa and 1250 MPa, the cold rolling deformation is 10-20%, and the annealing process satisfies 1100 ≤ (T + 273) lgt ≤ 1200. When the tensile strength is between 1250 MPa and 1350 MPa, the cold rolling deformation is 20-30%, and the annealing process satisfies 1200 ≤ (T + 273) lgt ≤ 1250. When the tensile strength is between 1350 MPa and 1500 MPa, the cold rolling deformation is 30-35%, and the annealing process satisfies 1250 ≤ (T + 273) lgt ≤ 1350. A method for manufacturing a high-manganese cold-rolled steel sheet according to claim 7, characterized in that when the tensile strength is 1500 MPa or more and 1600 MPa or less, the cold-rolling deformation is 35 to 40%, and the annealing process satisfies 1350 ≤ (T + 273) lgt ≤ 1400.

9. The method for producing a high-manganese cold-rolled steel sheet according to claim 7, characterized in that the smelting in step 1) is carried out using an electric furnace or a converter.

10. The method for manufacturing a high-manganese cold-rolled steel sheet according to claim 7, characterized in that step 1) and step 2) employ a conventional continuous casting + hot rolling process, or employ a thin slab continuous casting and continuous rolling process.

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