High formability high manganese cold-rolled steel sheet with a tensile strength of 1000 to 1600 MPa and easy to phosphatize, and method for producing 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] The present invention belongs to the field of high-manganese cold-rolled steel, and specifically relates to a high-formability high-manganese cold-rolled steel sheet with a tensile strength of 1000 to 1600 MPa that is easy to phosphate-treat and a method for manufacturing the same.

Background Art

[0002] Under the background of increasingly strict environmental protection and carbon reduction, in the body of automobiles, ultra-high-strength steel sheets with a strength of 780 MPa or higher are widely used instead of conventional automobile steels. Reducing the thickness of the steel sheet by increasing its strength has become a technical consensus for automobiles to achieve "weight reduction, energy conservation, safety improvement, and manufacturing cost reduction". For every 10% reduction in vehicle weight, fuel consumption is saved by 5% - 8%, and accordingly, emissions of pollutants such as CO2 greenhouse gas and NO x , SO2, etc. are also reduced.

[0003] However, the conventional steel microstructure and metallurgical mechanism are difficult to meet the future needs of the automotive industry for high formability ultra-high strength steel for automobiles. Therefore, steel manufacturers have to develop various custom materials that meet strength, formability, and service performance respectively to meet the various performance requirements of vehicle body materials. As a result, the types of vehicle body materials have become complex, ranging from 340 to 1500 MPa in strength and 3 to 50% in elongation rate. The types include ferritic steel, precipitation hardening steel, martensitic steel, dual-phase steel, and composite phase steel, with dozens of different products. Both steel companies and automobile companies are faced with problems such as complex material solutions, high production management costs, and frequent switching of manufacturing processes, which have a profound impact on the production stability, production efficiency, and cost management of the company. In recent years, with the introduction of advanced metallurgical mechanisms and material designs, new steel materials with simple components and a wide range of organizational property adjustments have been developed. By adjusting the processing process, a single-component design can cover a wide range of performance requirements. This material design concept is called the integrated material solution (Uni-material), which can greatly reduce the complexity of automotive materials, not only realizing the simplification of material management and design for automobile companies, but also enabling processes such as welding and painting, which have a decisive impact on component design, to be designed and managed in a single process. At the same time, for steel companies, a relatively simple product design can achieve 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, have become one of the mainstream research themes of major steel companies worldwide. Fully austenitic steel with high C and Mn contents exhibits an elongation rate of over 50% when its tensile strength reaches 1000 MPa. However, since fully austenitic steel has no phase transformation by heat treatment, there is a problem that it is difficult to adjust the tissue characteristics, especially it is difficult to achieve higher strength. If this cannot be effectively solved, it cannot be applied to the automotive industry. In addition, this type of high-manganese fully austenitic steel has a large content of Mn, which is an easily oxidized element, and has a problem of poor plating properties due to surface oxidation.

[0005] Currently, the main methods for adjusting the properties of high-manganese steel include the addition of alloying elements such as Nb, V, Ti, Cr, Mo, etc., and there are numerous related manufacturing patents. However, the addition of these elements all has their respective problems in metallurgy. The action of V is unstable and difficult to control, which poses a major problem for industrial applications. Nb and Ti mainly improve the yield strength of the material but have little effect on the tensile strength. The action of Mo is stable but expensive, and since it significantly improves the hot strength of the material, it brings great technical difficulties to processes such as hot rolling.

[0006] European Patent EP3492618B1 discloses a high-tensile strength automotive steel with a strength level of 1500 MPa. The mass percentages of its chemical elements are C 0.1%, 0.3%, Si 0.1% - 2.0%, Mn 7.5% - 12%, Al 0.01% - 2.0%, and the balance is iron and other inevitable impurities. The microstructure of the steel of the invention is austenite + martensite + ferrite or austenite + martensite, which can reach the 1500 MPa level, and its tensile product is 30 GPa% or more. However, the austenite in the microstructure of the invention is a metastable structure, and martensite transformation occurs during the deformation process, which has an adverse effect on properties such as low-temperature toughness and shear edges. Furthermore, the steel of the invention requires a very complex and time-consuming multi-stage heat treatment, and its production efficiency and cost are very disadvantageous.

[0007] Chinese Patent CN106191404B discloses a method for manufacturing high-strength and high-ductility TWIP steel. This is a method that combines asynchronous rolling with extremely large deformation and cold rolling + annealing treatment to obtain ultrafine crystal grains with a size of less than 1 μm. By adding microalloys such as Nb and Ti, a tensile strength of 1400 MPa and an elongation rate of 7% or more can be achieved. In this invention, cold rolling needs to be carried out after warm rolling at 400 °C. Since the total deformation amount exceeds 95% and asynchronous rolling needs to be adopted, the process is complex and has a high degree of difficulty, making it impossible for large-scale industrial production.

[0008] International Patent Application WO2014097184A4 discloses a high-strength and high-ductility austenitic stainless steel with the following composition (wt.%): 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, Ti: 0.001 - 0.5%. A tensile strength of 1200 MPa and an elongation rate of 60% can be achieved. Although the performance of the material is excellent, it is necessary to add many expensive alloying elements such as Cr, Ni, Mo, and Co, and it can only be used for special purposes. For general applications such as automobiles, there is little economic viability and feasibility.

[0009] US Patent Application US20120288396(A1) discloses an ultra-high ductility austenitic steel with the following composition: Mn: 8 - 16%, Cu: ≤3%, C satisfying 33.5C + Mn ≤ 25 and 33.5 - Mn ≥ 22, and other elements such as Cr, Ti, Nb, and N may be added, with the balance 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 rate of approximately 30%. In the case of steel for automobiles, the addition of Cu is disadvantageous in terms of cost management, and an elongation rate of approximately 30% has no obvious advantage compared with conventional high-strength steels.

[0010] International Patent Application WO2009084792(A1) discloses a high-strength and delayed fracture-resistant high-Mn steel and its manufacturing method. Its components are C: 0.3 - 0.9, Mn: 15 - 25%, Si ≤ 0.1 - 2%, Al: 0.01 - 4%, Cr ≤ 10%, N ≤ 0.6%, Cu ≤ 3%. Additionally, elements such as V, Ti, Mo, Nb, Cr, and W can also be added. In this application, the tensile strength of the steel is 920 MPa or more, and the elongation rate is 55% or more. Although the steel in this application has excellent performance, the Mn and Cr contents are relatively high, which is disadvantageous in terms of cost management.

[0011] Chinese Patent Application 200810239893.X discloses a P-strengthened TWIP steel and its manufacturing method, where the components are C: 0.01 - 0.08, Mn: 15 - 35%, Si ≤ 1 - 6%, Al: 1 - 6%, P: 0.062 - 0.2%, and the balance is Fe and impurities. In this application, the tensile strength of the steel is 610 - 915 MPa, the yield strength is 225 - 610 MPa, and the elongation rate is 45 - 85.5%. It has excellent formability, but the yield strength and tensile strength are low, making it difficult to meet the requirements of future ultra-high-strength steels for automobiles. Furthermore, high-strength steel strengthened with P is also difficult to weld with other steel grades.

Summary of the Invention

Problems to be Solved by the Invention

[0012] The object of the present invention is to provide a high-formability and high-manganese cold-rolled steel sheet with a tensile strength of 1000 - 1600 MPa that is easy to phosphatize and its manufacturing method. The steel sheet has the characteristic of a wide performance adjustment range, and can achieve various combinations of performance with a yield strength (YS) of 700 - 1400 MPa, a tensile strength (TS) of 1000 - 1600 MPa, and an elongation rate (EL) of 20 - 55%. And TS 2 ×EL ≥ 49 TPa 2 %, and can achieve excellent phosphatizing coating performance and bending performance, with the bending center radius reaching 0t, and is suitable for various automotive structural parts and safety parts that require different strengths and formabilities in automobiles.

Means for Solving the Problems

[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, and 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 1000 to 1600 MPa, an elongation of 20 to 55%, and TS 2 ×EL ≧ 49 TPa 2 %.

[0014] Preferably, in the chemical composition of the matrix, the C content is 0.5 to 0.7 wt%, such as 0.55 wt%, 0.6 wt%, 0.65 wt%.

[0015] Preferably, in the chemical composition of the matrix, the Mn content is 15 to 17 wt%, such as 15.5 wt%, 16 wt%, 16.5 wt%.

[0016] Preferably, in the chemical composition of the matrix, the Al content is 1.2 to 1.5 wt%, such as 1.25 wt%, 1.3 wt%, 1.35 wt%, 1.4 wt%, 1.45 wt%.

[0017] Preferably, in the chemical composition of the matrix, the Si content is 0.2 to 0.4 wt%, such as 0.25 wt%, 0.3 wt%, 0.35 wt%.

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

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

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

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

[0022] In one or more embodiments, in the matrix, the twin density is 2×10 5 m -1 、4×10 5 m -1 、6×10 5 m -1 or 8×10 5 m -1 is.

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

[0024] In one or more embodiments, the weight percentage of the chemical components of the matrix satisfies that Mn + 25C - 1.5Al is 28 to 34%, such as 29%, 30%, 31%, 32%, 33%, 33.6%.

[0025] In one or more embodiments, the weight percentage of the chemical components of the matrix satisfies that Si + 20RE is 1.0 to 2.5%, such as 1.2%, 1.5%, 1.8%, 2.0%, 2.2%, 2.4%.

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

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

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

[0029] Preferably, the surface layer thickness of the high manganese cold-rolled steel sheet is 0.5 to 2 μm, such as 0.6 μm, 0.8 μm, 1 μm, 1.2 μm, 1.4 μm, 1.6 μm, 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 satisfy that TS 2 ×EL is 49 - 60 TPa 2 %, for example, 52 TPa 2 %, 54 TPa 2 %, 56 TPa 2 %, 58 TPa 2 %.

[0034] In the component design of the high manganese cold-rolled steel sheet described in the present invention, C: It is the most effective austenite stabilizing element in steel, which can effectively increase the stacking fault energy of the material, suppress the austenite phase transformation, and thereby improve the austenite stability. In high manganese steel, by adding an appropriate amount of C, the Mn content can be significantly reduced at the same austenite stability level, and as a result, the material cost can be reduced. However, when the C content is excessive, it not only deteriorates the welding properties of the material but also causes technical difficulties in the steelmaking and continuous casting processes. The C content in the matrix of the steel sheet of the present invention is in the range of 0.5 - 0.8 wt%.

[0035] Mn: It is an effective austenite stabilizing element. In high manganese steel, the role of Mn is similar to that of C. It can effectively increase the stacking fault energy of the material, lower the martensite transformation temperature Ms, and improve the austenite stability. Also, different from the role of Mn in ordinary carbon steel, in high manganese austenite steel, an increase in the Mn content leads to a decrease in the material strength. Therefore, on the premise of ensuring the austenite stability of the material, it is necessary to reduce the Mn content as much as possible. The Mn content in the matrix of the steel plate of the present invention ranges from 14% to 18% by weight.

[0036] Al: It can effectively improve the stress corrosion cracking resistance of the material. However, the addition of Al significantly deteriorates the steelmaking performance and continuous casting performance of the steel, and is likely to lead to the blockage of the tundish during continuous casting. Also, in the steelmaking and continuous casting processes, when a large amount of Al2O3 is generated, the fluidity of the molten steel decreases, causing problems such as slag entrainment and slab cracking. On the premise of ensuring the stress corrosion cracking resistance of the material, it is necessary to reduce the Al content as much as possible. In the matrix of the steel plate of the present invention, the Al content ranges from 1.2% to 1.8% by weight.

[0037] Mn + 25C - 1.5Al ≥ 28%: Since both C and Mn can play a role in stabilizing austenite and achieving a fully austenite structure, C and Mn can promote each other to a certain extent. However, Al has the effect of significantly reducing the stability of austenite, and there is a hedge against the effects of C / Mn. Through the analysis of a large amount of test data, the present invention has confirmed that when the addition amounts of Mn, C, and Al in the steel plate matrix satisfy the relational expression Mn + 25C - 1.5Al ≥ 28%, the austenite in the steel of the present invention can ensure sufficient stability to achieve a fully austenite room-temperature microstructure.

[0038] RE: It is generally recognized that the role of RE (rare earth element) in steel is to improve the morphology of inclusions, purify 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 one hand, the secondary cold rolling heat treatment is an effective method to improve the strength of high manganese austenitic steel. However, high manganese austenitic steel has a 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 generation of twins, thereby reducing the work hardening ability of the material in the initial stage of deformation, improving the plasticity of the material after cold working, and being beneficial to the secondary cold working production of the material. In the annealing stage, RE forms a large number of fine dispersed particles in the material, which can effectively fix the twin boundaries, improve the stability of twins during heat treatment, retain the cold deformation twins as much as possible, improve the strength of the material, and at the same time achieve the object of the present invention of not impairing the deformation ability of the material. On the other hand, RE is a good hydrogen absorption material and can react with H to form stable hydrides, so the diffusible H content in the material can be reduced, and the stress corrosion cracking resistance of the material can be improved. However, if RE is added excessively, there is a problem that it is difficult to disperse in the molten steel, a large amount of rare earth inclusions are generated, and conversely, it affects the cleanliness of the molten steel. Therefore, the designed range of RE in the matrix of the steel plate of the present invention is 0.01~0.1%.

[0039] Si: In high manganese steel, Si can effectively suppress the precipitation of cementite, improve the cleanliness of the material crystal grains, and thereby improve the plasticity of the material. However, Si reduces the stability of austenite, and excessive addition is disadvantageous for maintaining a fully austenitic structure. Therefore, in the matrix of the steel plate of the present invention, the content of Si as an alloying element to improve the plasticity of the material is limited to 0.1~0.5%, and at the same time, it is necessary to satisfy Si + 20×RE ≧ 1.0%.

[0040] P: It has a certain solid solution strengthening effect, but the addition of P significantly deteriorates the plasticity of the material and reduces the welding properties. In the matrix of the steel plate of the present invention, P is used as an impurity element and controlled at as low a level as possible.

[0041] S: As an impurity element, control its content to a relatively low level as much as possible. 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 beneficial for enhancing the stability of austenite and improving the material properties. However, excessive addition of N is likely to cause precipitation of N2, form N2 bubbles in the material, and significantly deteriorate the continuity and performance of the material. In the matrix of the steel sheet of the present invention, the N content is controlled to be 0.01 - 0.1%.

[0042] The present invention adopts the component design scheme of C, Mn, Si, Al, and RE, and without adding expensive alloying elements, it is possible to obtain high Mn cold-rolled fully austenitic steel products with low material cost, good product manufacturability, and excellent performance.

[0043] The present invention also provides a manufacturing method of a high manganese cold-rolled steel sheet with a tensile strength of 1000 - 1600 MPa of the present invention. This manufacturing method includes the following steps: 1) Smelting, billet casting Smelt according to the chemical composition of the matrix and cast it into slabs.

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

[0045] 3) Cold rolling Perform pickling and cold rolling, and the cold rolling reduction is 10 - 40%.

[0046] 4) Annealing Adopt continuous annealing for annealing. The annealing temperature T is 250 - 400 °C, the annealing time t is 120 - 180 s, and at the same time, the annealing temperature and annealing time satisfy the relationship of 1100 ≤ (T + 273) lgt ≤ 1400, whereby austenite recovery occurs and finally stabilizes to room temperature.

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

[0048] When the tensile strength is 1000 MPa or more and less than 1250 MPa, the cold rolling deformation amount is 10 to 20%, and the annealing process satisfies 1100 ≦ (T + 273) lgt ≦ 1200. When the tensile strength is 1250 MPa or more and less than 1350 MPa, the cold rolling deformation amount is 20 to 30%, and the annealing process satisfies 1200 ≦ (T + 273) lgt ≦ 1250. When the tensile strength is 1350 MPa or more and less than 1500 MPa, the cold rolling deformation amount is 30 to 35%, and the annealing process satisfies 1250 ≦ (T + 273) lgt ≦ 1350. When the tensile strength is 1500 MPa or more and 1600 MPa or less, the cold rolling deformation amount is 35 to 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 of 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 coiling temperature in step 2) is 680 °C, 700 °C, 750 °C, 800 °C or 820 °C.

[0052] In one or more embodiments, the cold rolling deformation amount 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 130 s, 140 s, 150 s, 160 s or 170 s.

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

[0056] Preferably, the steelmaking 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 adopt a thin slab continuous casting and continuous rolling process.

[0057] In the method for manufacturing a high manganese cold-rolled steel sheet described in the present invention, The steel of the present invention has a fully austenite structure, without other types of phase transformations. The role of heat preservation in the hot rolling high-temperature heating furnace is to reduce the rolling load and homogenize the components of the billet.

[0058] The reason for adopting a high coiling temperature in the present invention is to externally oxidize the steel sheet surface at a high temperature, significantly enrich the easily oxidized elements such as C, Si, Mn, etc. on the steel sheet surface, and form an element-deficient layer under the surface. Combined with the subsequent pickling process, a body-centered cubic (BCC) structure layer with element deficiency is formed on the steel sheet surface, realizing a composite structure of a surface BCC phase structure ferroalloy layer and a matrix face-centered cubic (FCC) phase structure ferroalloy layer, and significantly improving the phosphate treatment coating performance of the material.

[0059] In the recovery annealing of the steel of the present invention, increasing both the annealing temperature and the annealing time is beneficial for the diffusion of elements and can promote the recovery process of austenite. Therefore, there is a certain degree of mutual compensation between the annealing temperature and the annealing time. Through the analysis of a large amount of test data, the present invention has confirmed that when the annealing temperature T and the annealing time t satisfy the relational expression 1100 ≦ (T + 273)lgt ≦ 1400, the performance of the steel of the present invention can be ensured, and an appropriate fully austenite recovery structure can be obtained after annealing. In the annealing stage, RE improves the stability of twins during heat treatment, maintains high-density twins and low-density dislocations in the final material, and realizes a better combination performance of strength and elongation rate.

[0060] According to the strength requirements of the finished steel plate, the present invention can arbitrarily adjust the cold rolling and annealing processes to achieve performance adjustment in a wide range of tensile strengths from 1000 to 1600 MPa, and has excellent formability, meeting the performance and formability requirements of different parts of the automobile body. For example, steel plates with a tensile strength of 1000 MPa level are suitable for parts such as A, B, and C pillar inner panels, floor cross beams, and longitudinal beams; steel plates with a tensile strength of 1200 MPa level are suitable for parts such as A, B, and C pillar reinforcement plates, door sills, and door impact bars; steel plates with a tensile strength of 1500 MPa level are suitable for parts such as front and rear impact beams and door ring reinforcement plates. The details are as follows.

[0061] When the tensile strength is not less than 1000 MPa and less than 1250 MPa, the cold rolling deformation amount is 10% - 20%, and the annealing process satisfies 1100 ≦ (T + 273)lgt ≦ 1200.

[0062] When the tensile strength is not less than 1250 MPa and less than 1350 MPa, the cold rolling deformation amount is 20% - 30%, and the annealing process satisfies 1200 ≦ (T + 273)lgt ≦ 1250. When the tensile strength is not less than 1350 MPa and less than 1500 MPa, the cold rolling deformation amount is 30% - 35%, and the annealing process satisfies 1250 ≦ (T + 273)lgt ≦ 1350. When the tensile strength is 1500 MPa or more and 1600 MPa or less, the cold rolling deformation amount is 35% - 40%, and the annealing process satisfies 1350 ≦ (T + 273) lgt ≦ 1400.

[0063] Furthermore, in the present invention, continuous annealing having obvious advantages such as excellent structure and properties, high production efficiency, and energy saving is adopted, and during annealing, the high manganese steel completes the recovery process of the deformed structure.

[0064] Compared with the prior art, the beneficial effects of the present invention are as follows. The steel plate described in the present invention has a composite structure of a surface body-centered cubic (BCC) phase structure iron alloy layer and a matrix face-centered cubic (FCC) phase structure iron alloy layer, and the steel plate has the characteristic of a wide performance adjustment range, and can realize various performance combinations of yield strength (YS) of 700 - 1400 MPa, tensile strength (TS) of 1000 - 1600 MPa, and elongation (EL) of 20 - 55%. It has excellent phosphate treatment coating performance and bending performance, and is suitable for automobile structural parts and safety parts that require various strengths and formability of automobiles.

[0065] The present invention mainly utilizes the characteristics of high manganese steel that is likely to generate a large number of deformation twins during cold deformation, and realizes the coexistence of high-density twins and low-density dislocations in the final material by precisely controlling component design, cold deformation, and subsequent heat treatment, without impairing the plastic deformation ability of the material and significantly improving the strength level of the material. In particular, the addition of rare earth element RE can effectively suppress the appearance of twins during deformation, not only control the density of twins within an appropriate range, but also have no influence on the density of the formed twins, maintain the stability of twins in subsequent heat treatment, and achieve an effective reduction in dislocation density.

[0066] According to the strength requirements of the finished steel plate, the cold rolling and annealing processes can be adjusted arbitrarily, that is, by adjusting the density of twins and dislocations, a wide performance adjustment range of high manganese steel can be realized even with the same component design. The strength level covers a tensile strength (TS) of 1000 - 1600 MPa, and the elongation (EL) covers 20 - 55%. It can meet the requirements of mechanical properties and formability for different parts of the automobile white body and most parts.

[0067] By adding rare earth elements to high manganese steel, the generation of twins can be effectively delayed. Thereby, the work hardening ability of the material is reduced in the initial stage of deformation, the plasticity of the material after cold working is improved, and the recovery annealing of the material is facilitated. At the same time, by utilizing the purification, precipitation, and hydrogen storage characteristics of rare earth elements, high formability, high strength, and excellent stress corrosion cracking resistance can be obtained, and at the same time, the steelmaking and continuous casting performance of the material are greatly improved. The steel of the present invention adopts the production methods of electric furnace or converter steelmaking, conventional continuous casting or thin slab continuous casting, hot rolling, pickling cold rolling, and continuous annealing, with high production efficiency and good uniformity of product performance.

[0068] In addition, the present invention makes full use of the slow cooling stage after hot rolling and coiling, and by controlling the coiling temperature, the oxidation enrichment of easy oxidation elements such as Si and Mn on the steel plate surface is adjusted, and a BCC phase structure layer of ferroalloy lacking C, Si, and Mn with a certain thickness is formed on the steel plate surface, which can significantly improve the phosphate treatment and coating performance of the steel plate after pickling and cold rolling.

[0069] Through appropriate component design and cold rolling - continuous annealing process control, the present invention can realize the performance covering the range of tensile strength of 1000 - 1600 MPa and elongation of 20 - 55%, and can meet the performance requirements of most structural parts and safety parts of future vehicle bodies, becoming a powerful option for realizing the integrated material solution of vehicle bodies.

[0070] The steel sheet described in the present invention has good application prospects in automotive safety structural parts. In particular, it is suitable for the manufacture of vehicle structural parts and safety parts with very complex shapes and high requirements for formability, such as door impact bars, bumpers, B-pillars, etc.

Brief Description of the Drawings

[0071]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Modes for Carrying Out the Invention

[0072] Hereinafter, the present invention will be further described with reference to examples and drawings. The components of Examples 1 to 16 of the present invention are smelted, hot-rolled, cold-rolled, annealed and tempered-rolled to obtain products, and the following steps are included.

[0073] 1) Smelting, billet casting Smelt according to the components shown in Table 1 and cast into slabs.

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

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

[0076] 4) Annealing After continuous annealing, austenite recovery occurs and finally stabilizes to room temperature.

[0077] 5) Temper rolling Among them, in step 1), in Examples 2, 4, 6 to 9, 12 to 14, steelmaking was carried out in an electric furnace, and in Examples 1, 3, 5, 10, 11, 15, 16, steelmaking was carried out in a converter. In steps 1) and 2), in Examples 1 to 4, 6 to 14, conventional continuous casting + hot rolling was adopted, and in Examples 5, 15, 16, a thin slab continuous casting and continuous rolling process was adopted.

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

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

[0080] The matrix chemical components of the steel plates of Comparative Examples 1 to 4 are shown in Table 1. The product of Comparative Example 1 was manufactured according to the steps of the examples, and the manufacturing process parameters are shown in Table 3. The matrix of the steel plate 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 the matrix are shown in Table 2.

[0081] The mechanical properties of the steel plates of Comparative Examples 1 to 4 are shown in Table 4. In the present invention, for the steel plates of the above examples and comparative examples, performance tests were carried out using as indicators the components and thickness of the surface BCC layer, mechanical properties (yield strength, tensile strength, elongation), bending radius, phosphate treatment properties, twin density, dislocation density, etc.

[0082] Among these, the test method for mechanical properties referred to the American Society for Testing and Materials (ASTM) standard ASTM E8 / E8M-13, "Standard Test Methods For Tension Testing of Metallic Materials". A tensile test was conducted using an ASTM standard tensile test specimen with a gauge length of 50 mm, and the tensile direction was perpendicular to the rolling direction.

[0083] Electron backscatter diffraction (EBSD) was used to detect the twin density, and the ratio of the length of twin boundaries to the grain area within the field of view was statistically analyzed.

[0084] The detection method for dislocation density referred to "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 sample with a size of 10×20 mm was cut out from the steel plate. After surface polishing, an X-ray diffraction (XRD) pattern was measured, and the entire spectrum of the pattern was fitted and calculated using the Modified Warren-Averbach Analysis (MWAA) method to obtain the dislocation density value of the sample. Refer to Table 4 for the detection results.

[0085] The surface composition was 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 detected in accordance with GB / T232-2010, "Test Method for Bending of Metallic Materials". The phosphate treatment characteristics were detected in accordance with GB / T6807-2001, "Technical Requirements for Phosphate Treatment of Steel and Iron Workpieces before Painting".

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

[0088] As shown in FIG. 4, after cold deformation, the elongation of the steel of the present invention is significantly superior to that of the steel of the comparative example. 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 microstructure characteristics by subsequent heat treatment.

[0089] As shown in FIG. 5, in the present invention, the combination of the strength and elongation performance of the material after cold deformation and heat treatment is superior to that of the steel of the comparative example. This indicates that at the annealing stage, the present invention improves the stability of twins during heat treatment by RE, maintains high-density twins and low-density dislocations in the final material, and can achieve a better combination performance of strength and elongation.

[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 including high-density twins and low-density dislocations, and 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：0.5～0.8%； Mn: 14 to 18%; Si: 0.1 to 0.5%; RE: 0.01 to 0.10%; P：≦0.020%； S：≦0.010%； Al：1.2～1.8%； N：0.01～0.1%； 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 1000 to 1600 MPa, an elongation of 20 to 55%, and TS 2 ×EL ≧ 49 TPa 2 %, and is characterized by being a high-manganese cold-rolled steel sheet.

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

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

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

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

6. The high-manganese cold-rolled steel sheet according to any one of Claims 1 to 5, wherein the surface layer thickness of the high-manganese cold-rolled steel sheet is 0.5 to 2 μm.

7. A method for manufacturing the high-manganese cold-rolled steel sheet according to any one of Claims 1 to 6, comprising the following steps: 1) Smelting and billet casting Smelting according to the chemical composition of the matrix according to any one of Claims 1 to 5 and casting into slabs. 2) Hot rolling Heating the slabs at a heating temperature of 1170 to 1230 °C, the final rolling temperature of hot rolling is 970 to 1030 °C, and the coiling temperature is 650 to 850 °C. 3) Cold rolling Performing pickling and cold rolling, and the cold rolling reduction is 10 to 40%. 4) Annealing Annealing adopts continuous annealing, the annealing temperature T is 250 to 400 °C, the annealing time t is 120 to 180 s, and at the same time, the annealing temperature and annealing time satisfy the relationship of 1100 ≤ (T + 273) lgt ≤ 1400, and finally stabilizing to room temperature. A method for manufacturing a high-manganese cold-rolled steel sheet, characterized by including this.

8. Select the corresponding cold rolling and annealing processes according to the tensile strength of the finished steel sheet. When the tensile strength is 1000 MPa or more and less than 1250 MPa, the cold rolling deformation amount is 10 - 20%, and the annealing process satisfies 1100 ≤ (T + 273) lgt ≤ 1200, When the tensile strength is 1250 MPa or more and less than 1350 MPa, the cold rolling deformation amount is 20 - 30%, and the annealing process satisfies 1200 ≤ (T + 273) lgt ≤ 1250, When the tensile strength is 1350 MPa or more and less than 1500 MPa, the cold rolling deformation amount is 30 - 35%, and the annealing process satisfies 1250 ≤ (T + 273) lgt ≤ 1350, The 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 amount is 35 - 40%, and the annealing process satisfies 1350 ≤ (T + 273) lgt ≤ 1400.

9. The method for manufacturing a high manganese cold-rolled steel sheet according to claim 7, characterized in that the steelmaking 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) adopt conventional continuous casting + hot rolling or a thin slab continuous casting and continuous rolling process.

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