CN107043895A - The composition design and production method of manganese copper bearing steel in a kind of 1500MPa grades of low-carbon - Google Patents

Abstract

The invention relates to a composition design and production method of a 1500MPa low-carbon medium-manganese copper-containing steel. The mass percentage of its chemical composition is: C: 0.20%-0.23%, Si: 0.5%-0.8%, Mn: 3.5 %～4.0%, Al: 1.2%～2.0%, Cr: 0.5%～1.0%, Cu: 0.6%～1.0%, Ni: 0.2%～0.5%, N: 0.003%～0.012%, B: 0.00051%～ 0.003%, the balance is Fe and unavoidable impurities. The present invention adds some alloy elements on the basis of traditional TRIP steel, greatly increases the manganese content to reach the range of medium manganese, replaces Si elements with Al while retaining appropriate Si elements, so that the two can be used together; adding a certain amount can precipitate The strengthened Cu element can be used together with an appropriate amount of Ni element to eliminate the "hot embrittlement" phenomenon caused by Cu during thermal processing of the material. In addition, a small amount of Cr element is also added, and an appropriate amount of N element is added to be used in combination with Al element. Through the cooperation of hot rolling and hot partitioning process, martensite + retained austenite + precipitated second element with ultra-high strength and good plasticity are obtained. Two-phase particle structure, the tensile strength exceeds 1500MPa.

Description

Composition design and production method of a 1500MPa low-carbon, medium-manganese and copper-containing steel

technical field

The invention relates to a composition design and production method of high-strength steel for automobiles, in particular to a composition design and production method of 1500MPa low-carbon medium-manganese copper-containing steel.

Background technique

At present, in order to save energy, reduce fuel consumption, and reduce exhaust emissions, the production of energy-saving and environmentally friendly vehicles has become one of the main goals of the world's auto companies. For this reason, many auto companies at home and abroad have adopted a series of measures, the most effective measures One of them is vehicle lightweighting. According to the data, if advanced high-strength steel (AHSS) plates are used, the original thickness of 1.0-1.2mm body steel plate can be reduced to 0.7-0.8mm, and the weight of the body will be reduced by 15%-20%. Fuel saving 8%~15%. So far, countries around the world have developed various advanced high-strength steel plates, such as: the first generation of advanced high-strength steel: dual-phase (DP) steel, multi-phase (CP) steel and phase transformation induced plasticity (TRIP) steel, etc., they Mainly based on ferrite, the strong plastic product (the product of tensile strength and elongation after fracture) is generally below 15GPa %; the second generation of advanced high-strength steel: twinning induced plasticity (TWIP) steel, induced plasticity light steel (L-IP) and shear band strengthening (SIP) steels, etc., which are all austenitic structures, and their strength and plasticity are often above 50GPa·%. The third generation of advanced high-strength steel: Q&P (quenching-partitioning) steel, etc., They are a kind of multi-phase with bainite, martensite or both high-strength phases as the matrix, retained austenite and a small amount of ferrite as the tough phase, and the strong-plastic product is 20-40GPa·%. Steel, with multi-phase, metastable and multi-scale fine structure.

The first generation of advanced high-strength steel is a multi-phase steel mainly composed of ferrite structure. It has low alloy content and low cost. However, due to its small strength and plasticity, it is difficult to meet the future lightweight and safety requirements of automobiles. The second generation Advanced high-strength steel has excellent mechanical properties, but because of its high alloy content, the cost is greatly increased. At the same time, the excessively high alloy content will deteriorate the welding performance and coating performance of the steel plate. Moreover, the processing of these steel plates is also very difficult. Therefore, in the case of certain defects in the first and second generation AHSS, researchers are working on the research of the third generation AHSS. The strength of this type of steel is 800~1500MPa , the strength-plastic product exceeds 30GPa·%, and the production cost is relatively low, which is easy to realize in production. Therefore, the third-generation AHSS has become a current and future research hotspot of automotive lightweight materials.

As one of the main materials supporting the lightweight concept of automobiles, AHSS plays a very important role in the future development of automobiles. The general trend of the development of AHSS for automobiles will be high-strength steel plates with good ductility and formability. With the continuous advancement of science and technology, in the near future, the development and application of AHSS with higher performance will achieve greater development.

Contents of the invention

Aiming at the above-mentioned challenges faced by automobile lightweighting, the purpose of this invention to propose a 1500MPa low-carbon, medium-manganese and copper-containing steel is to combine hot rolling + Q&P process through rational design and optimization of components to obtain a high production efficiency, Composition design and production method that are easy to produce in actual production and can greatly improve the comprehensive performance of hot stamping Q&P steel plates.

The idea of the design of the present invention is that the composition ratio of the present invention is realized on the basis of TRIP steel by changing the content of certain elements and adding some alloy elements that can strengthen and improve the mechanical properties of steel materials, and follows the " The alloying principle of "multivariate and small amount", meanwhile, the present invention obtains martensite + retained austenite + precipitated second phase particles with ultra-high strength and good plasticity through the cooperation of hot rolling and Q&P (quenching-partitioning) heat treatment process Its tensile strength exceeds 1500MPa, and it has excellent plasticity and toughness.

Its characteristics are: (1) The content of Mn is greatly increased to make it reach the manganese content in medium manganese steel.

(2) Replacing Si element with Al element and retaining appropriate Si element so that the two can be used together will help improve the material properties. If Al is used in combination with Cr and Si, it can significantly improve the high temperature affordability of the steel. The surface properties of the steel plate are improved by improving the leather properties; and the density of the steel is also reduced by the addition of Al.

(3) A certain amount of Cu element that can produce precipitation strengthening is added, and it is used together with an appropriate amount of Ni element to eliminate the "hot embrittlement" phenomenon caused by Cu during thermal processing of the material; in addition, Ni also has Improve the strength of steel and maintain good plasticity and toughness.

(4) A small amount of Cr element that can improve the comprehensive performance of steel is added.

(5) Adding an appropriate amount of N element, used in combination with Al element, can generate AlN with grain refinement and increased retained austenite content to further improve the performance of the steel.

In order to achieve the above purpose, the technical solution of the present invention is: a low-carbon medium-manganese copper-containing steel, the chemical composition content of which is: C: 0.20% to 0.23%, Si: 0.5% to 0.8%, Mn: 3.5% to 4.0% %, Al: 1.2%～2.0%, Cr: 0.5%～1.0%, Cu: 0.6%～1.0%, Ni: 0.2%～0.5%, N: 0.0035%～0.012%, B: 0.0005%～0.003%, The balance is Fe and unavoidable impurities; the tensile strength of the low-carbon, medium-manganese and copper-containing steel is above 1500 MPa.

In the preferred chemical composition of the low-carbon medium-manganese-copper-containing steel (both by weight percentage), the Si content is 0.55%-0.8%, the Al content is 1.2%-1.5%, and the Mn content is 3.5%-3.8%, The Cr content is 0.6%~0.8%, the Cu content is 0.6~0.8%, the Ni content is 0.25%~0.4%, the N content is 0.008%~0.01%, and the B content is 0.001%~0.003%.

In the composition design of the 1500MPa low-carbon medium-manganese copper-containing steel of the present invention, the design basis for the role of each element in the steel and its content is as follows: (1) Carbon: although carbon can greatly improve the strength, hardness and other properties of the material , but at the same time because it will strongly reduce the plasticity, toughness and weldability of the material, when the carbon content exceeds 0.23%, the weldability of the steel will deteriorate. Therefore, the carbon content of low carbon steel used for welding generally does not exceed 0.23%; considering the above factors, the carbon in this composition ratio is still kept within the carbon content range of low carbon steel.

(2) Manganese: Mn is a main element in TRIP steel, its content is generally between 1% and 2%, and it is usually kept at about 1.5% in actual production. It can improve the strength of the material, because in this design The strength of the steel should reach 1500MPa level. Although this strength can be achieved by increasing the carbon content and other alloying elements (such as: Cr), too high a carbon content will strongly deteriorate the formability and weldability of the steel, and the price of the alloying elements is too high. Moreover, some element reserves are very scarce, and low cost is a very important market competition factor for automotive steel, so these two approaches are not advisable; and Mn element can improve the strength of steel, and the price is low, and it can also increase the residual Austrian content. Therefore, considering performance, price, resource reserves and other factors, Mn is the most suitable choice. Although Mn will deteriorate the weldability of materials, its adverse effects are much smaller than other alloying elements. many. Therefore, its content is increased to the lower limit of the middle manganese range in this composition ratio.

(3) Silicon and aluminum: The content of Si in TRIP is basically the same as that of Mn. Si can significantly increase the elastic limit, yield point and tensile strength of steel without significantly reducing the plasticity of steel; the most important thing is that Si can Inhibit the precipitation of cementite, but too high Si content will also have an adverse effect on the weldability of the steel, and will reduce the surface coating performance of the steel plate. Therefore, the Si content is reduced in this composition ratio, and when the Si content in the steel is in the range of 0.8% to 1.0%, the plasticity and toughness of the steel will decrease significantly; therefore, the Si content should be below 0.8%, while the other Al, an alloying element, can also inhibit the precipitation of cementite. The surface quality of high Al steel is better and the coating performance is improved. However, unlike Si, Al has no solid solution strengthening effect, which will lead to a decrease in strength, but reduces the strength of Si. The strength loss caused by aluminum content or replacing silicon with aluminum can be compensated by various measures to increase strength (such as: increasing Mn content); in addition, Al also has the effect of refining grains and improving the impact toughness of steel plates. Therefore, considering factors such as the mechanical properties and physical properties of the steel plate and the fact that the process is easy to realize in actual production, the use of Al-Si is a good compromise. Therefore, the composition ratio of the present invention adopts Al to replace the part Scheme of Si element.

(4) Chromium: Cr is an alloying element widely used in actual industrial production, which can significantly improve the strength, hardness and high-temperature mechanical properties of steel, and make steel have good oxidation resistance, corrosion resistance and corrosion resistance. At the same time, the adverse effect on the plasticity and toughness of the steel plate is not obvious. Studies have shown that: when the Cr content is in the range of 0.5% to 1.65%, the steel has high strength, high wear resistance, good hardenability and fatigue resistance. From the formula for calculating the strength of low-carbon alloy steel, it can be seen that the improvement of Cr to the strength of steel is only lower than that of carbon, but the reserves of Cr in my country are small, so it should be used sparingly or replaced by other elements. Therefore, considering the strength, resource reserves and Due to factors such as cost, 0.5% to 1.0% of Cr element is added to the composition ratio of the present invention.

(5) Copper and nickel: At present, many high-strength structural steels contain copper, ranging from 0.20% to 1.50%. Cu has an enrichment effect in steel, and can be distributed in the martensite matrix with second-phase particles of a certain size. It plays the role of dispersion strengthening. Adding Cu element in low carbon steel can make the steel plate have good welding performance and corrosion resistance. The disadvantage is that when the Cu content exceeds 0.5%, the steel is prone to "hot embrittlement" during thermal processing. "Phenomenon, and when the Cu content in the steel exceeds 0.60%, Cu can be in a supersaturated state and precipitate in the form of a copper-rich phase after heat treatment, so that the steel has a second-phase particle precipitation strengthening effect. Studies have shown that when Cu increases to at least 0.75%, it is considered to have a small impact on welding performance. At the same time, in order to avoid the phenomenon of "hot embrittlement", it is necessary to add Ni element (the content ratio of Ni and Cu is 1:3 ~ 1:2 In addition, the steel containing Cu and Ni has a larger volume fraction of retained austenite and higher stability. The most important thing is that when the steel is impacted, its TRIP effect can be maintained to the high strain area , which is one of the excellent properties that advanced high-strength steel for vehicles should have; in addition, Ni will not reduce the plasticity while improving the strength of the steel, so considering the above factors, Cu is used in the composition of the present invention: Ni= 1:2~1:3 compound ratio.

(6) Nitrogen: Typical nitrogen content (about 0.003% to 0.012%) has a significant effect on strength. N can improve the strength and low temperature toughness of steel and improve weldability. The research results show that: with the increase of nitrogen content, the density of AlN precipitates increases, which delays the transformation of austenite during cooling and isothermal process, so that a large amount of retained austenite remains in the structure after quenching, and AlN has precipitation strengthening. The effect of improving the strength and plasticity of the steel plate. In addition, the presence of AlN has the effect of refining austenite, which just improves the overheating sensitivity caused by the high manganese content. Considering the above characteristics of N and the large content of Al in the composition ratio Element, it is advisable to add 0.008%~0.01% N element in the composition ratio of the present invention.

(7) Boron: The main function of B is to improve the hardenability of steel. Trace boron elements have a significant effect on improving the hardenability of products in low and medium carbon structural steels, because adding a very small amount of boron (0.0005%~0.003 %) can greatly improve the hardenability of steel, considering that when the B content is greater than 0.004%, it will strongly reduce the toughness of the steel and cause the phenomenon of "boron embrittlement", so 0.002% of the B element is added to the composition ratio of the present invention It is appropriate.

The production method of the low-carbon medium-manganese copper-containing steel involved in the present invention includes the following steps: (1) Smelting and casting process: the chemical composition content is: C: 0.20%-0.23%, Si: 0.55%-0.8%, Mn : 3.5%～3.8%, Al: 1.2%～1.5%, Cr: 0.6%～0.8%, Cu: 0.6%～0.8%, Ni: 0.2%～0.4%, N: 0.008%～0.01%, B: 0.001 % to 0.003%, the balance is Fe and unavoidable impurities mixed alloy powders are smelted in electric arc furnaces, converters, and open hearth furnaces, and then transferred to LF refining furnaces, and the nitrogen pressure required in the furnaces is maintained during the smelting period, so that the alloys Nitrogen is dissolved in the melt, and finally cast into billets or ingots.

(2) Heating and heat preservation: transfer the billet or ingot to a continuous furnace for heating and heat preservation, so that the alloying elements are evenly dissolved in the austenite, the heating temperature is kept between 1050°C and 1100°C, and the heat preservation time is 1.5 ~2 hours.

(3) Hot rolling process: After the heat preservation is completed, the cast slab or ingot is rolled, and the initial rolling temperature is kept at 1000 ° C ~ 1050 ° C, and then, when the temperature of the intermediate billet is cooled to 850 ° C ~ 950 ° C, it is further processed. The final rolling of the pass is rolled into a 1.8mm thick steel plate.

(4) Q&P heat treatment process: After the final rolling, keep the steel plate at 830°C~850°C for 120s, then cool the steel plate at a cooling rate greater than 50°C/s to 170°C~200°C, and hold it for 20s, then Immediately transfer the steel plate to the heating furnace and raise the temperature to 350°C~365°C for element distribution, keep it warm for 45s~60s, and finally water quench the steel plate to room temperature to obtain a material with martensite + retained austenite + precipitated second phase particles Microstructure; finally, the tensile strength of this low-carbon, medium-manganese and copper-containing steel plate exceeds 1500MPa, and it also has excellent plasticity and toughness.

The 1500MPa grade low-carbon medium-manganese copper-containing steel of the present invention, in the austenitizing heat preservation stage, the temperature should be below 1100°C, because the copper element is added to the composition of the present invention, and it is formed between the grain boundaries of the crystal grains. The melting point of the compound is generally about 1100°C. If the holding temperature is higher than the melting point of these compounds, it will easily cause "copper embrittlement" phenomenon; Diffusion and homogenization in densities.

For the 1500MPa grade low-carbon medium-manganese copper-containing steel of the present invention, in the heat preservation stage after rapid quenching, the heat preservation temperature (Q _T ) should be based on the martensite content to be obtained in theory before quenching, using the formula V _M =1- exp[a(Ms-Q _T )] is calculated (where V _M is the volume fraction of martensite; a is a constant, depending on the composition of the material, for carbon steel with a carbon content below 1.1%, a=-0.011; Ms is the martensitic transformation initiation temperature).

The 1500MPa grade low-carbon medium-manganese copper-containing steel of the present invention, in the stage of partitioning and heat preservation, its heat preservation temperature cannot be too low, because the present invention contains a large amount of manganese element that can stabilize residual austenite, so the heat preservation temperature is selected at Ms Above 20~40℃. Secondly, the holding temperature should not be too high, otherwise, it is easy to form cementite and consume a small amount of carbon element to reduce the stability of retained austenite. In addition, the holding temperature at this stage should neither be too long nor too short. If it is too long, it will easily form cementite and reduce the strength of martensite. If it is too short, it will not be conducive to the stabilization of retained austenite.

Description of drawings

Fig. 1 is the production process flow chart of 1500MPa level low-carbon medium-manganese copper-containing steel of the present invention.

Fig. 2 is a process diagram of hot rolling and heat treatment of the 1500MPa grade low-carbon, medium-manganese and copper-containing steel of the present invention.

detailed description

The specific implementation will be described in detail below in conjunction with the accompanying drawings and examples, as shown in accompanying drawings 1 and 2.

Example 1 The composition design and production method of a 1500MPa low-carbon medium-manganese copper-containing steel involved in the present invention includes the following steps: (1) Smelting and casting processes. The composition is: C: 0.2%, Mn: 3.5%, Si: 0.8%, Al: 1.2%, Cr: 0.8%, Cu: 0.8%, Ni: 0.4%, N: 0.008%, B: 0.002%, and The mixed alloy powder with the amount of Fe is smelted in an electric arc furnace, converter, or open hearth furnace, and then transferred to an LF refining furnace. During the smelting period, the nitrogen pressure required in the furnace is maintained to add N element, and finally cast into a slab or ingot.

(2) Heating and insulation. Transfer the billet or ingot to a continuous furnace for heating and heat preservation, so that the alloying elements are evenly dissolved in the austenite. The heating temperature is kept between 1050°C and the heat preservation time is 2 hours.

(3) Hot rolling process. After the heat preservation is completed, the cast slab or ingot is rolled, and the initial rolling temperature is kept at 1000°C, and then the intermediate billet is cooled to 870°C, and then it is subjected to multi-pass final rolling to form a 1.8mm thick steel plate.

(4) Q&P heat treatment process. After the final rolling, keep the steel plate at 830°C for 120s, then cool it to 170°C at a cooling rate of 100°C/s, and hold it for 20s, then immediately transfer the steel plate to the heating furnace to raise the temperature to 350°C, hold it for 90s, and finally The steel plate is water-quenched to room temperature to obtain a structure with martensite + retained austenite + precipitated second phase particles; finally, the tensile strength of this low-carbon medium-manganese copper-containing steel plate is 1645MPa, and it also has excellent plasticity and toughness.

Example 2 The composition design and production method of a 1500MPa low-carbon medium-manganese copper-containing steel involved in the present invention includes the following steps: (1) Smelting and casting processes. The composition is: C: 0.21%, Mn: 3.6%, Si: 0.7%, Al: 1.35%, Cr: 0.7%, Cu: 0.7%, Ni: 0.3%, N: 0.009%, B: 0.002%, and The mixed alloy powder with the amount of Fe is smelted in an electric arc furnace, converter, or open hearth furnace, and then transferred to an LF refining furnace. During the smelting period, the nitrogen pressure required in the furnace is maintained to add N element, and finally cast into a slab or ingot.

(2) Heating and insulation. Transfer the casting slab or ingot to a continuous furnace for heating and heat preservation, so that the alloying elements are uniformly dissolved in the austenite, the heating temperature is kept at 1070°C, and the heat preservation time is 1.8 hours.

(3) Hot rolling process. After the heat preservation is completed, the cast slab or ingot is rolled, and the initial rolling temperature is kept at 1020°C, and then the intermediate billet is cooled to 900°C, and then it is subjected to multi-pass final rolling to form a 1.8mm thick steel plate.

(4) Q&P heat treatment process. After the final rolling, keep the steel plate at 840°C for 120s, then cool it to 185°C at a cooling rate of 100°C/s, and keep it warm for 20s, then immediately transfer the steel plate to the heating furnace to raise the temperature to 360°C, hold it for 60s, and finally Water-quench the steel plate to room temperature to obtain a structure with martensite + retained austenite + precipitated second phase particles; finally, the tensile strength of this low-carbon medium-manganese copper-containing steel plate is 1591 MPa, and it also has excellent plasticity and toughness.

Example 3 The composition design and production method of a 1500MPa low-carbon medium-manganese copper-containing steel involved in the present invention includes the following steps: (1) Smelting and casting processes. The composition is: C: 0.23%, Mn: 3.8%, Si: 0.55%, Al: 1.5%, Cr: 0.6%, Cu: 0.6%, Ni: 0.25%, N: 0.01%, B: 0.002%, and The mixed alloy powder with the amount of Fe is smelted in an electric arc furnace, converter, or open hearth furnace, and then transferred to an LF refining furnace. During the smelting period, the nitrogen pressure required in the furnace is maintained to add N element, and finally cast into a slab or ingot.

(2) Heating and insulation. Transfer the billet or ingot to a continuous furnace for heating and heat preservation, so that the alloy elements are evenly dissolved in the austenite. The heating temperature is kept at 1100°C and the heat preservation time is 1.5 hours.

(3) Hot rolling process. After the heat preservation is completed, the cast slab or ingot is rolled, and the initial rolling temperature is kept at 1050°C, and then the intermediate billet is cooled to 940°C, and then it is subjected to multi-pass final rolling to form a 1.8mm thick steel plate.

(4) Q&P heat treatment process. After the final rolling, keep the steel plate at 850°C for 120s, then cool it to 200°C at a cooling rate of 100°C/s, and hold it for 20s, then immediately transfer the steel plate to the heating furnace to raise the temperature to 365°C, hold it for 45s, and finally Water-quench the steel plate to room temperature to obtain a structure with martensite + retained austenite + precipitated second phase particles; finally, the tensile strength of this low-carbon medium-manganese copper-containing steel plate is 1550MPa, and it also has excellent plasticity and toughness.

Claims (3)

1. The present invention relates to a composition design and production method of 1500MPa low-carbon medium-manganese copper-containing steel, which is characterized in that: the chemical composition content is: C: 0.20%-0.23%, Si: 0.5%-0.8%, Mn: 3.5%～4.0%, Al: 1.2%～2.0%, Cr: 0.5%～1.0%, Cu: 0.5%～0.8%, Ni: 0.2%～0.5%, N: 0.003%～0.012%, B: 0.0005% to 0.003%, the balance is Fe and unavoidable impurities, and the contents of the above elements are all in weight percentage. 2. The composition design and production method of a 1500MPa grade low-carbon medium-manganese copper-containing steel according to claim 1, characterized in that: the production steps include: (1) smelting and casting process: the The mixed alloy powder of said composition is smelted in electric arc furnace, converter, open hearth furnace, and then transferred to LF refining furnace, and the nitrogen pressure required in the furnace is maintained during the smelting period to dissolve nitrogen element in the alloy melt, and finally poured into casting Billet or ingot; (2) Heating and heat preservation: transfer the billet or ingot to a continuous furnace for heating and heat preservation, so that the alloy elements are evenly dissolved in the austenite, and the heating temperature is kept between 1050 ° C and 1100 ° C , and keep warm for 1.5~2 hours; (3) Hot forming process: After the heat preservation is completed, the billet or ingot is rolled, and the initial rolling temperature is kept at 1000°C~1050°C, and the temperature of the intermediate billet drops to 850°C~950°C In between, it is subjected to multi-pass final rolling, and rolled into a 1.8mm thick steel plate; (4) heat treatment process: after the final rolling, the steel plate is kept at 830°C~850°C for 120s, and then the steel plate Cool down to 170°C~200°C at a cooling rate greater than 50°C/s, and keep it warm for 20s, then immediately transfer the steel plate to a heating furnace and heat it up to 350°C~365°C for element distribution, keep it warm for 45s~90s, and finally put the steel plate Water quenching to room temperature to obtain a structure with martensite + retained austenite + precipitated second phase particles; finally, the tensile strength of this low-carbon, medium-manganese and copper-containing steel exceeds 1500MPa and it also has excellent plasticity and toughness. 3. The composition design and production method of a 1500MPa low-carbon, medium-manganese and copper-containing steel according to claim 1 or 2, characterized in that: the tensile strength of the low-carbon, medium-manganese and copper-containing steel is ≥ 1500MPa.