WO2023241546A1 - Highly formable and easily phosphated high-manganese cold-rolled steel plate having tensile strength of 1000-1600 mpa, and manufacturing method therefor - Google Patents

Abstract

A highly formable and easily phosphated high-manganese cold-rolled steel plate having the tensile strength of 1000-1600 MPa, and a manufacturing method therefor. The steel plate is of a matrix face-centered cubic and surface body-centered cubic composite structure; a matrix comprises high-density twin crystals (1-10)*10⁵m^-1 and a low-density dislocation (1-10)*10¹³m^-1. The matrix comprises components in percentage by weight of: C: 0.5-0.8%; Mn: 12-20%; Si: 0.1-0.5%; Al: 1.2-1.8%; N: 0.01-0.1%; RE: 0.01-0.1%; and the balance being Fe and inevitable impurities, wherein Mn + 25C - 1.5Al ≥ 28%, and Si + 20Re ≥ 1.0%. According to the present invention, by means of selection of a cold rolling-continuous annealing process, a steel plate may achieve the tensile strength of 1000-1600 MPa and the elongation of 20-55%, and has excellent phosphating performance and a cold forming radius of 0t. The steel plate is steel for integrated material design for an automobile high-strength safety structural component.

Description

High formability, easily phosphated high manganese cold-rolled steel plate with tensile strength of 1000-1600MPa and manufacturing method thereof Technical field

The invention belongs to the field of high manganese cold-rolled steel, and specifically relates to a high-formability, easy-to-phosphate high-manganese cold-rolled steel plate with a tensile strength of 1000-1600 MPa and a manufacturing method thereof.

Background technique

Under the increasingly stringent environmental protection and low-carbon background, a large number of ultra-high-strength steel plates with a strength of above 780MPa are used in automobile bodies to replace traditional automobile steel. By increasing the strength of the steel plate to reduce the thickness of the steel plate, it has become an automotive reality. The technical consensus is to reduce weight, save energy, improve safety and reduce manufacturing costs. Every 10% reduction in vehicle weight can save 5% to 8% of fuel consumption, and at the same time, the emissions of CO ₂ greenhouse gases, NO _x , SO ₂ and other pollutants can be correspondingly reduced.

However, the microstructure and metallurgical mechanism of traditional steel are difficult to meet the future needs of the automotive industry for ultra-high-strength steel with high formability for automobiles. This forces steel mills to develop various personalized materials that meet strength, formability, and usability performance to meet the different performance requirements of body materials. This results in complex types of body materials, with strengths ranging from 340 to 1500 MPa. The elongation spans the range of 3 to 50%, and the types include ferritic steel, precipitation-strengthened steel, martensitic steel, dual-phase steel, and complex-phase steel, covering dozens of different products, bringing benefits to both steel and automobile companies. Problems such as complex material solutions, high production management costs, and frequent switching of manufacturing processes have seriously affected the company's production stability, production efficiency, and cost control. In recent years, through the introduction of advanced metallurgical mechanisms and material design, new steel materials with simple components and a wide range of adjustment of structural properties have been developed. By adjusting the processing technology, a single component design can be designed to cover a wide range of performance requirements. This material design idea is called Uni-material, which can greatly reduce the complexity of automobile materials. It not only simplifies the material management and design of automobile companies, but also has a decisive impact on the composition design of welding and coating. Single process design and management can also be realized in the assembly process. At the same time, for steel companies, relatively simple product design can achieve a high degree of consistency in steelmaking, continuous casting and hot rolling processes, effectively improve efficiency, reduce costs, and enhance the company's market competitiveness.

Among various integrated material solutions, the development and application of advanced high-strength automotive steel based on phase change strengthening has become one of the mainstream research topics of major steel companies in the world. Fully austenitic steel with high C and Mn content has an elongation of more than 50% when its tensile strength reaches 1000MPa. However, fully austenitic steel has no heat treatment phase transformation, so there is a problem that the structural properties are difficult to control, especially it is difficult to achieve higher strength. If it cannot be effectively solved, it will not be able to be applied in the automotive industry. Moreover, this type of high-manganese fully austenitic steel has a high content of the easily oxidizable element Mn, and has the problem of poor coating performance due to surface oxidation.

At present, the main methods to control the properties of high manganese steel include adding Nb, V, Ti, Cr, Mo and other alloying elements. There are many related manufacturing patents, but the addition of these elements has its own problems in metallurgy. The effect of V is unstable, difficult to control, and there are major problems in industrial use; Nb and Ti mainly increase the yield strength of the material, but have no obvious effect on the tensile strength; Mo has a stable effect, but is expensive and significantly increases the thermal stability of the material. Strength brings great technical difficulties to processes such as hot rolling.

European patent EP3492618B1 discloses a 1500MPa grade high-strength plastic automobile steel with mass percentages of chemical elements: C 0.1%, 0.3%, Si 0.1% ~ 2.0%, Mn 7.5% ~ 12%, Al 0.01% ~ 2.0% ; The balance is iron and other unavoidable impurities; the microstructure of the steel of the invention is austenite + martensite + ferrite or austenite + martensite, which can reach 1500MPa level, and its strong plastic volume is not Less than 30GPa%. However, the austenite in the microstructure of this invention is a metastable structure, and martensite transformation will occur during the deformation process, so the properties such as low-temperature toughness and shear edge will be adversely affected. Moreover, the invented steel requires very complex and time-consuming multi-step heat treatment, which is very detrimental to production efficiency and cost.

Chinese patent CN106191404B discloses a method for preparing high-strength and high-plasticity TWIP steel. It uses asynchronous rolling with ultra-large deformation plus cold rolling combined with annealing to obtain ultra-fine grains below 1 μm, supplemented by Nb, Ti, etc. The addition of microalloy can achieve a tensile strength of 1400MPa and an elongation of more than 7%. This invention requires warm rolling at 400°C and then cold rolling, with a total deformation exceeding 95%, and asynchronous rolling. The process is complex and difficult, and it is not feasible for large-scale industrial production.

International patent application WO2014097184A4 discloses a high-strength and high-plasticity austenitic stainless steel whose composition is (wt.%) C: 0.01-0.50, N0.11-0.50, 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%. Achievable tensile strength of 1200MPa and 60% elongation. The material has excellent performance, but requires the addition of more expensive alloy elements such as Cr, Ni, Mo, and Co. It can only be used in special applications and is basically not economical and feasible in general automobile and other applications.

US patent application US20120288396 (A1) discloses an ultra-high ductility austenitic steel whose composition is Mn: 8~16%, Cu: ≤3%, C: satisfying 33.5C+Mn≤25 and 33.5-Mn≥ 22, and other elements such as Cr, Ti, Nb, N, etc. may be added, and the remainder is Fe and impurities. The austenite fraction of the steel in this application is above 99%, the yield strength is 300-630MPa, and the elongation is about 30%. For automotive steel, the addition of Cu is unfavorable for cost control, and the elongation of about 30% does not have obvious advantages over the phase transformation of traditional high-strength steel.

International patent application WO2009084792 (A1) discloses a high-strength delayed cracking resistant high Mn steel and a manufacturing method thereof. 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%. In addition, elements such as V, Ti, Mo, Nb, Cr, and W may be added. In this application, the tensile strength of the steel is above 920MPa and the elongation is ≥55%. The steel applied for has excellent performance, but the Mn and Cr contents are relatively high, which makes cost control more disadvantageous.

Chinese patent application 200810239893. , P: 0.062~0.2%, the remainder is Fe and impurities. The tensile strength of the steel in this application is 610-915MPa, the yield strength is 225-610MPa, and the elongation is 45-85.5%. The steel has excellent formability, but the yield strength and tensile strength are low, making it difficult to meet the ultra-high strength requirements for future automobiles. steel requirements. In addition, high-strength steel reinforced with P is also difficult to weld with other steel types.

Contents of the invention

The object of the present invention is to provide a high formability, easily phosphated high manganese cold-rolled steel plate with a tensile strength of 1000-1600MPa and a manufacturing method thereof. The steel plate has the characteristics of a wide range of adjustable performance and can achieve a yield strength of ( YS) 700-1400MPa, tensile strength (TS) 1000-1600MPa, elongation (EL) 20-55% of various performance combinations, and meet TS ² × EL ≥ 49TPa ² %; excellent phosphate coating performance, bending It has excellent performance and a bending center radius of up to 0t. It is suitable for a variety of automotive structural parts and safety parts with different strength and formability requirements.

In order to achieve the above object, the present invention provides a high manganese cold-rolled steel plate with a tensile strength of 1000-1600MPa, which is a composite structure including a matrix and a surface layer;

The matrix has a face-centered cubic phase structure and contains high-density twins and low-density dislocations, where the twin density is (1～10)×10 ⁵ m ^-1 and the dislocation density is (1～10)×10 ¹³ m ^-1 ; the weight percentage of the chemical composition of the matrix is:

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 balance includes Fe and other inevitable impurities, and at the same time meets: Mn+25C-1.5Al≥28%, Si+20RE≥1.0%;

The surface layer is an iron alloy layer with a body-centered cubic phase structure, and its composition includes: C≤0.03wt%, Mn≤0.5wt%, and Al≤0.1wt%;

The high manganese cold-rolled steel plate has a yield strength of 700-1400MPa, a tensile strength of 1000-1600MPa, an elongation of 20-55%, and satisfies TS ² × EL ≥ 49TPa ² %.

Preferably, in the chemical composition of the matrix, the C content is 0.5-0.7wt%, such as 0.55wt%, 0.6wt%, or 0.65wt%.

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

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

Preferably, in the chemical composition of the matrix, the Si content is 0.2-0.4wt%, such as 0.25wt%, 0.3wt%, or 0.35wt%.

In one or more embodiments, the chemical composition of the matrix has an RE content of 0.02%, 0.04%, 0.06% or 0.08%.

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

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

In one or more embodiments, the chemical composition of the matrix has an N content of 0.02%, 0.04%, 0.06% or 0.08%.

In one or more embodiments, the twin density in the matrix is 2×10 ⁵ m ⁻¹ , 4×10 ⁵ m ⁻¹ , 6×10 ⁵ m ⁻¹ or 8×10 ⁵ m ⁻¹ .

In one or more embodiments, the dislocation density in the matrix is 2×10 ¹³ m ⁻¹ , 4×10 ¹³ m ⁻¹ , 6×10 ¹³ m ⁻¹ or 8×10 ¹³ m ⁻¹ .

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

In one or more embodiments, the weight percentage of the chemical composition of the matrix satisfies: Si+2ORE is 1.0-2.5%, such as 1.2%, 1.5%, 1.8%, 2.0%, 2.2%, 2.4%.

In one or more embodiments, in the chemical composition of the surface layer, the C content is 0-0.03wt%, such as 0.001wt%, 0.005wt%, 0.01wt%, 0.02wt%.

In one or more embodiments, in the chemical composition of the surface layer, the Mn content is 0-0.5wt%, such as 0.01wt%, 0.02wt%, 0.03wt%, 0.04wt%.

In one or more embodiments, the Al content in the chemical composition of the surface layer is 0-0.1wt%, such as 0.01wt%, 0.02wt%, 0.04wt%, 0.06wt%, 0.08wt%.

Preferably, the surface layer thickness of the high manganese cold-rolled steel plate is 0.5-2 μm, such as 0.6 μm, 0.8 μm, 1 μm, 1.2 μm, 1.4 μm, 1.6 μm, and 1.8 μm.

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

In one or more embodiments, the tensile strength of the high manganese cold rolled steel plate is 1100MPa, 1200MPa, 1300MPa, 1400MPa or 1500MPa.

In one or more embodiments, the high manganese cold rolled steel plate has an elongation of 25%, 30%, 35%, 40%, 45% or 50%.

In one or more embodiments, the tensile strength and elongation of the high manganese cold-rolled steel plate meet: TS ² × EL is 49-60TPa ² %, such as 52TPa ² %, 54TPa ² %, 56TPa ² %, 58TPa ² %.

In the composition design of the high manganese cold-rolled steel plate of the present invention:

C: It is the most effective austenite stabilizing element in steel. It can effectively increase the material's stacking fault energy, inhibit austenite phase transformation, and thereby improve austenite stability. In high manganese steel, adding an appropriate amount of C can significantly reduce the Mn content at the same austenite stability level, thereby reducing material costs. However, excessive C content not only deteriorates the welding performance of the material, but also causes technical difficulties in the steelmaking and continuous casting process. In the matrix of the steel plate of the present invention, the C content ranges from 0.5 to 0.8% by weight.

Mn: is an effective austenite stabilizing element. In high manganese steel, the role of Mn is similar to that of C, which can effectively increase the stacking fault energy of the material, reduce the martensite transformation temperature Ms, and improve the stability of austenite. In addition, unlike the role of Mn in ordinary carbon steel, in high manganese austenitic steel, an increase in Mn content will lead to a decrease in material strength. Therefore, it is necessary to reduce Mn as much as possible while ensuring the austenite stability of the material. content. In the matrix of the steel plate of the present invention, the Mn content ranges from 14 to 18% by weight.

Al: It can effectively improve the delayed cracking resistance of materials. However, the addition of Al will significantly deteriorate the smelting and continuous casting properties of steel, and can easily lead to water blockage during continuous casting. Moreover, during the smelting and continuous casting processes, the formation of a large amount of Al ₂ O ₃ will reduce the fluidity of the molten steel, leading to problems such as slag entrainment and slab cracking. On the premise of ensuring that the delayed cracking performance of the material is qualified, the Al content needs to be reduced 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.

Mn+25C-1.5Al≥28%: Since both C and Mn can stabilize austenite and achieve a full austenite structure, C and Mn can promote each other to a certain extent. However, Al has the effect of significantly reducing austenite stability, which is in conflict with the effect of C/Mn. Through a large amount of test data analysis, the present invention confirms that when the addition amounts of Mn, C and Al in the steel plate matrix satisfy the relationship formula Mn+25C-1.5Al≥28%, the austenite in the steel of the present invention can be ensured to have sufficient stability. To achieve a room temperature microstructure of full austenite.

RE: It is generally believed that the role of RE (rare earth elements) in steel is to improve the shape of inclusions, purify steel, and improve material strength and formability. But in the steel of the present invention, RE plays a more important role. On the one hand, secondary cold rolling heat treatment is an effective method to improve the strength of high manganese austenitic steel, but high manganese austenitic steel has high work hardening ability, and secondary cold rolling usually brings about a significant decrease in plasticity. After cold deformation, adding RE can effectively delay the generation of twins, thereby reducing the work hardening ability of the material in the early stage of deformation, improving the plasticity of the material after cold working, and facilitating the secondary processing of the material. Cold processing production. During the annealing stage, RE forms a large number of fine dispersed particles in the material, which can effectively pin the twin boundaries and improve the stability of the twins during the heat treatment process. This allows the present invention to retain cold deformation twins as much as possible and improve the strength of the material without sacrificing the strength of the material. The purpose is to damage the deformation ability of the material. On the other hand, RE is a good hydrogen-absorbing material and can react with H to form stable hydrides, thereby reducing the diffusible H content in the material and improving the material's resistance to delayed cracking. However, adding too much RE has the problem of difficulty in dispersing in molten steel, resulting in a large amount of rare earth inclusions, which will affect the cleanliness of the molten steel. Therefore, the designed RE range in the steel plate matrix of the present invention is 0.01 to 0.1%.

Si: In high manganese steel, Si can effectively inhibit the precipitation of cementite and improve the cleanliness in the material grains, thereby improving the shape of the material. However, Si will reduce the stability of austenite, and excessive addition is detrimental to maintaining a complete austenite structure. Therefore, in the steel plate matrix of the present invention, Si, as an alloy element that improves material shaping, is limited to 0.1 to 0.5%. , and at the same time, Si+20×RE≥1.0% must be satisfied.

P: It has a certain solid solution strengthening effect, but the addition of P will significantly deteriorate the plasticity of the material and reduce the welding performance. In the steel plate matrix of the present invention, P is used as an impurity element and is controlled to a low level as much as possible.

S: As an impurity element, its content should be kept as low as possible.

N: It has a similar effect to C and is an effective austenite stabilizing element. In high manganese steel, increasing the N content is beneficial to increasing austenite stability and improving material properties. However, adding too much N can easily cause _N2 to precipitate, forming _N2 bubbles in the material, seriously deteriorating the continuity and performance of the material. The N content in the steel plate matrix of the present invention is controlled at 0.01 to 0.1%.

The present invention adopts a composition design scheme of C, Mn, Si, Al, and RE, without adding expensive alloy elements, to obtain a high-Mn cold-rolled fully austenitic steel product with low material cost, good product manufacturability, and superior performance.

The invention also provides a method for manufacturing a high manganese cold-rolled steel plate with a tensile strength of 1000-1600MPa, which includes the following steps:

1) Smelting and casting billet

Smelted according to the chemical composition of the matrix and cast into slabs;

2) Hot rolling

Slab heating, heating temperature is 1170~1230℃; hot rolling final rolling temperature is 970~1030℃, coiling temperature is 650~850℃;

3)Cold rolling

Pickling, cold rolling, the cold rolling deformation is 10-40%;

4) Annealing

Annealing adopts continuous annealing, the annealing temperature T is 250~400℃, and the annealing time t is 120~180s. At the same time, the annealing temperature and annealing time conform to the following relationship: 1100≤(T+273)lgt≤1400, austenite recovery occurs, and finally Stable to room temperature.

In a preferred embodiment, the corresponding cold rolling and annealing processes can be selected based on the tensile strength of the finished steel plate in the performance range of 1000 to 1600MPa:

When the tensile strength is ≥1000MPa and <1250MPa, the cold rolling deformation is 10~20%, and the annealing process meets: 1100≤(T+273)lgt≤1200;

When the tensile strength is ≥1250MPa and <1350MPa, the cold rolling deformation is 20~30%, and the annealing process meets: 1200≤(T+273)lgt≤1250;

When the tensile strength is ≥1350MPa and <1500MPa, the cold rolling deformation is 30~35%, and the annealing process meets: 1250≤(T+273)lgt≤1350;

When the tensile strength is ≥1500MPa and ≤1600MPa, the cold rolling deformation is 35~40%, and the annealing process meets: 1350≤(T+273)lgt≤1400.

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

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

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

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

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

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

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

Preferably, step 1) smelting adopts electric furnace or converter smelting.

Preferably, step 1) and step 2) adopt conventional continuous casting + hot rolling, or adopt thin slab continuous casting and rolling process.

In the manufacturing method of high manganese cold-rolled steel plate of the present invention:

The steel of the present invention has a fully austenitic structure and does not have other types of phase transformations. The role of heat preservation in a hot rolling high-temperature heating furnace is to reduce the rolling load and uniformize the composition of the cast slab.

The present invention adopts a higher coiling temperature in order to cause the surface of the steel plate to undergo external oxidation at high temperature, resulting in obvious enrichment of easily oxidized elements such as C, Si, Mn, etc. on the surface of the steel plate, forming a subsurface layer of poor elements; The subsequent pickling process can form a layer of element-poor body-centered cubic (BCC) structure layer on the surface of the steel plate, achieving a composite structure of the surface BCC phase structure iron alloy layer and the matrix face-centered cubic (FCC) phase structure iron alloy layer, significantly improving Phosphating coating properties of materials.

In the recovery annealing of the steel of the present invention, increasing the annealing temperature and annealing time is beneficial to element diffusion and promotes the recovery process of austenite. Therefore, there is a certain degree of mutual compensation between annealing temperature and annealing time. Through a large amount of test data analysis, the present invention confirms that when the annealing temperature T and annealing time t satisfy the relationship 1100≤(T+273)lgt≤1400, it can ensure that a suitable all-austenite recovery structure is obtained after annealing to ensure that the present invention Properties of steel. During 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 achieves superior strength-elongation combined properties.

The invention can optionally adjust the cold rolling and annealing processes according to the strength requirements of the finished steel plate to achieve a wide range of tensile strength control performance of 1000-1600MPa, and has superior forming performance, which can meet the performance and performance requirements of different parts of the automobile body. Formability requirements. For example, steel plates with a tensile strength of 1000MPa are suitable for A, B, and C column inner panels, floor beams, longitudinal beams and other parts; tensile strength of 1200MPa are suitable for A, B, and C column reinforcement plates, thresholds, and door anti-collision Rods and other parts; the tensile strength level of 1500MPa is suitable for front and rear anti-collision beams, door knocker reinforcement plates and other parts. details as follows:

When the tensile strength is ≥1000MPa and <1250MPa, the cold rolling deformation is 10%-20%, and the annealing process meets: 1100≤(T+273)lgt≤1200;

When the tensile strength is ≥1250MPa and <1350MPa, the cold rolling deformation is 20%-30%, and the annealing process meets: 1200≤(T+273)lgt≤1250;

When the tensile strength is ≥1350MPa and <1500MPa, the cold rolling deformation is 30%-35%, and the annealing process meets: 1250≤(T+273)lgt≤1350;

When the tensile strength is ≥1500MPa and ≤1600MPa, the cold rolling deformation is 35%-40%, and the annealing process meets: 1350≤(T+273)lgt≤1400.

In addition, the present invention adopts continuous annealing because continuous annealing has obvious advantages such as superior structure, superior performance, high production efficiency, and energy saving. During the annealing process, the high manganese steel completes the recovery process of the deformed structure.

Compared with the existing technology, the beneficial effects of the present invention include:

The steel plate of the present invention is 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; the steel plate has the characteristics of a wide performance control range and can achieve a yield strength (YS) of 700 -1400MPa, tensile strength (TS) 1000-1600MPa, elongation (EL) 20-55% of various performance combinations; phosphate coating and bending properties are excellent, suitable for various strength and formability requirements on automobiles Automotive structural parts and safety parts.

This invention mainly takes advantage of the characteristics of high manganese steel that easily produces a large number of deformation twins under cold deformation. Through precise control of composition design, cold deformation and subsequent heat treatment, the coexistence of high-density twins and low-density dislocations in the final material is achieved. , which not only significantly improves the strength level of the material, but does not damage the plastic deformation ability of the material. In particular, the addition of the rare earth element RE can effectively suppress the occurrence of twins during deformation, control the twin density within an appropriate range, and maintain the stability of the twins during subsequent heat treatment to effectively reduce the dislocation density, but it does not Affects the density of formed twins.

The present invention can optionally adjust the cold rolling and annealing processes according to the strength requirements of the finished steel plate. That is, by regulating the density of twins and dislocations, it can achieve a wide range of regulation of the performance of high manganese steel designed with the same composition, covering strength levels. The tensile strength (TS) is 1000-1600MPa, and the elongation (EL) covers 20-55%, which can meet the mechanical properties and formability requirements of different parts of the automobile body-in-white and most parts.

The present invention adds rare earth elements to high manganese steel, which can effectively delay the generation of twins, thereby reducing the work hardening ability of the material in the early stage of deformation, improving the plasticity of the material after cold working, and facilitating the recovery annealing of the material; at the same time, the purification and precipitation of rare earth elements are utilized And hydrogen storage performance, while obtaining high formability, high strength and good delayed cracking resistance, the smelting and continuous casting performance of the material are significantly improved. The steel of the invention adopts electric furnace or converter smelting, conventional continuous casting or thin slab continuous casting, hot rolling, pickling cold rolling, and continuous retreat production methods, and has high production efficiency and good product performance uniformity.

In addition, the present invention makes full use of the slow cooling stage after hot rolling and coiling, and controls the coiling temperature. Adjust the oxidative enrichment of easily oxidized elements such as Si and Mn on the surface of the steel plate to form a certain thickness of C, Si, Mn-poor ferroalloy BCC phase structure layer on the surface of the steel plate, which significantly improves the phosphating coating performance of the pickled and cold-rolled steel plate.

Through appropriate component design and cold rolling-continuous retreat process control, the present invention can achieve performance covering the range of tensile strength 1000-1600MPa and elongation 20-55%, which can meet the requirements of most structural parts and safety parts on future vehicle bodies. Performance requirements are a powerful option for realizing body-integrated material solutions.

The steel plate of the present invention will have good application prospects in automobile safety structural parts, and is particularly suitable for manufacturing vehicle structural parts and safety parts with very complex shapes and high requirements on forming performance, such as door anti-collision bars, bumpers and B-pillar etc.

Description of the drawings

Figure 1 is a schematic diagram of the multi-layer structure of the high manganese cold-rolled steel plate according to the present invention;

Figure 2 is a photograph of the matrix face-centered cubic (FCC) phase structure in the multi-layer structure of the high manganese cold-rolled steel plate of the present invention;

Figure 3 is a photo of the matrix RE precipitation phase in the multi-layer structure of the high manganese cold-rolled steel plate of the present invention;

Figure 4 is a schematic diagram of the elongation change data of the example steel of the present invention and the comparative example steel under cold rolling deformation conditions;

Figure 5 is a schematic diagram of the strength-elongation performance combination after cold deformation and heat treatment of the example steel of the present invention and the comparative example steel. In Figure 5, the point where the tensile strength is 1001MPa and the elongation is 55% corresponds to Example 14 .

Detailed ways

The present invention will be further described below in conjunction with the examples and drawings.

The components of Examples 1-16 of the present invention are smelted, hot rolled, cold rolled, annealed and smoothed to obtain a product, which includes the following steps:

1) Smelting and casting billet

Smelt according to the composition shown in Table 1 and cast into slab;

2) Hot rolling

Slab heating, hot rolling, and coiling;

3)Cold rolling

Pickling, cold rolling;

4) Annealing

After continuous annealing, austenite recovery occurs and finally stabilizes to room temperature;

5) Smooth;

Among them, in step 1), Examples 2, 4, 6-9, and 12-14 are smelted with an electric furnace, and Examples 1, 3, 5, 10, 11, 15, and 16 are smelted with a converter; in steps 1) and 2) , Examples 1-4 and 6-14 adopt conventional continuous casting + hot rolling, and Examples 5, 15 and 16 adopt thin slab continuous casting and rolling process.

The matrix components of the steel plates 1-16 in Examples are as 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 surface and matrix properties of the steel plates are as shown in Table 2. The production process is as shown in Table 3. Mechanics See Table 4 for properties and phosphating properties.

It can be seen from Table 1 and Table 2 that through appropriate component design and process coordination, the present invention obtains a composite structure of a surface BCC phase structure iron alloy layer and a matrix FCC phase structure iron alloy layer, as shown in Figures 1 to 3. In the present invention, the surface phase structure is detected by backscattered electron diffraction (EBSD), and the matrix phase structure is detected by EBSD and X-ray diffraction (XRD).

The matrix chemical composition of the steel plates of Comparative Examples 1-4 is shown in Table 1.

The product of Comparative Example 1 was manufactured according to the steps of the Examples. The production process parameters are as shown in Table 3. The steel plate matrix of Comparative Example 1 has a face-centered cubic phase structure and the surface layer has a system cubic phase structure. The surface layer and matrix properties are as shown in Table 2.

The mechanical properties of the steel plates of Comparative Examples 1-4 are shown in Table 4.

The present invention performs performance testing on the steel plates of the above embodiments and comparative examples. The indicators include the composition and thickness of the surface BCC layer, mechanical properties (yield strength, tensile strength, elongation), bending radius, phosphating performance, twin density, and dislocations. density.

Among them, the testing method of mechanical properties refers to the American Society for Testing and Materials standard ASTM E8/E8M-13 "Standard Test Methods For Tension Testing of Metallic Materials". The tensile test adopts the ASTM standard 50mm gauge length. The tensile specimen is stretched in a direction perpendicular to the rolling direction.

The twin density is detected using EBSD, which counts the ratio of twin boundary length and grain area in the field of view.

Dislocation density detection method refers 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". The details are: cut a 10×20mm size sample from the steel plate, and After surface polishing, the XRD (X-ray diffraction) pattern was tested, and the MWAA (Modified Warren-Averbach Analysis) method was used to perform full spectrum fitting and calculation to obtain the dislocation density value in the sample. The test results are shown in Table 4.

Surface composition is detected using energy dispersive spectrometer (EDS)

Surface thickness was measured using scanning electron microscopy (SEM).

The bending radius is tested in accordance with the standard GB/T232-2010 "Bending Test Methods for Metal Materials".

Phosphating performance is tested in accordance with GB/T6807-2001 "Technical Conditions for Phosphating Treatment of Steel Workpieces Before Coating".

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

As shown in Figure 4, after cold deformation, the elongation of the present invention is significantly better than that of the comparative example steel. It shows that the addition of RE in the present invention helps slow down the decrease in elongation of the steel plate under cold rolling deformation, helps maintain high formability after secondary cold rolling, and provides better microstructural properties for subsequent heat treatment.

As shown in Figure 5, after cold deformation and heat treatment, the material of the present invention has a better combination of strength and elongation properties than the steel of the comparative example. It shows that during the annealing stage, the present invention improves the stability of twins during heat treatment through RE, maintains high-density twins and low-density dislocations in the final material, and achieves superior strength-elongation combined performance.

Table 1 Unit: weight percentage

Table 2

table 3

Table 4

Claims (10)

1. High manganese cold-rolled steel plate with a tensile strength of 1000-1600MPa, characterized in that it is a composite structure including a matrix and a surface layer;

   The matrix has a face-centered cubic phase structure and contains high-density twins and low-density dislocations, where the twin density is (1～10)×10 ⁵ m ^-1 and the dislocation density is (1～10)×10 ¹³ m ^-1 ; the weight percentage of the chemical composition of the matrix is:

   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 balance includes Fe and other inevitable impurities, and at the same time meets: Mn+25C-1.5Al≥28%, Si+20RE≥1.0%;

   The surface layer is an iron alloy layer with a body-centered cubic phase structure, and its composition includes: C≤0.03wt%, Mn≤0.5wt%, and Al≤0.1wt%;

   The high manganese cold-rolled steel plate has a yield strength of 700-1400MPa, a tensile strength of 1000-1600MPa, an elongation of 20-55%, and satisfies TS ² × EL ≥ 49TPa ² %.
2. The high manganese cold-rolled steel plate according to claim 1, wherein the chemical composition of the matrix has a C content of 0.5 to 0.7 wt%.
3. The high manganese cold-rolled steel plate according to claim 1, wherein the Mn content in the chemical composition of the matrix is 15 to 17 wt%.
4. The high manganese cold-rolled steel plate according to claim 1, wherein the Al content in the chemical composition of the matrix is 1.2 to 1.5 wt%.
5. The high manganese cold-rolled steel plate according to claim 1, wherein the chemical composition of the matrix has a Si content of 0.2 to 0.4 wt%.
6. The high manganese cold rolled steel plate according to any one of claims 1 to 5, characterized in that the surface layer thickness of the high manganese cold rolled steel plate is 0.5-2 μm.
7. The manufacturing method of high manganese cold-rolled steel plate according to any one of claims 1 to 6, characterized in that it includes the following steps:

   1) Smelting and casting billet

   Smelted according to the chemical composition of the substrate according to any one of claims 1 to 5, and cast into a slab;

   2) Hot rolling

   Slab heating, heating temperature is 1170~1230℃; hot rolling final rolling temperature is 970~1030℃, coiling temperature is 650~850℃;

   3)Cold rolling

   Pickling, cold rolling, the cold rolling deformation is 10-40%;

   4) Annealing

   Annealing adopts continuous annealing, the annealing temperature T is 250~400℃, and the annealing time t is 120~180s. At the same time, the annealing temperature and annealing time conform to the following relationship: 1100≤(T+273)lgt≤1400, and finally stabilize to room temperature.
8. The manufacturing method of high manganese cold-rolled steel plate as claimed in claim 7, characterized in that the corresponding cold rolling and annealing processes are selected according to the tensile strength of the finished steel plate:

   When the tensile strength is ≥1000MPa and <1250MPa, the cold rolling deformation is 10~20%, and the annealing process meets: 1100≤(T+273)lgt≤1200;

   When the tensile strength is ≥1250MPa and <1350MPa, the cold rolling deformation is 20~30%, and the annealing process meets: 1200≤(T+273)lgt≤1250;

   When the tensile strength is ≥1350MPa and <1500MPa, the cold rolling deformation is 30~35%, and the annealing process meets: 1250≤(T+273)lgt≤1350;

   When the tensile strength is ≥1500MPa and ≤1600MPa, the cold rolling deformation is 35~40%, and the annealing process meets: 1350≤(T+273)lgt≤1400.
9. The manufacturing method of high manganese cold-rolled steel plate according to claim 7, characterized in that step 1) is smelted using an electric furnace or a converter.
10. The manufacturing method of high manganese cold-rolled steel plate according to claim 7, characterized in that step 1) and step 2) adopt conventional continuous casting + hot rolling, or adopt thin slab continuous casting and rolling process.