Low-yield-ratio 1180MPa-grade cold-rolled martensitic steel for automobiles and preparation method thereof

Abstract

The invention belongs to the field of steel for automobiles, and particularly relates to low-yield-ratio 1180MPa-grade cold-rolled martensitic steel for automobiles and a preparation method. The steel comprises the following components in percentage by weight: 0.145%-0.175% of C, 0.20%-0.45% of Si, 1.75%-2.00% of Mn, less than or equal to 0.020% of P, less than or equal to 0.0050% of S, 0.08%-0.12% of Al, less than or equal to 0.0050% of N, 0.005%-0.008% of B, 0.03%-0.04% of Nb and the balance of Fe and other inevitable impurities. Through brand new matching design of component design and a heat treatment process, martensite serves as a matrix, a small amount of ferritic structures are introduced, the performance characteristic of the low yield ratio is successfully achieved, and the forming capacity of the 1180MPa-grade martensitic steel in the application process is effectively improved.

Description

Technical Field

[0001] This invention belongs to the field of automotive steel, specifically relating to a cold-rolled martensitic steel with a low yield strength ratio of 1180MPa for automobiles and its preparation method. Background Technology

[0002] Cold-rolled high-strength martensitic steel, as a type of automotive steel material with high yield strength ratio, is particularly suitable for automotive structural components with high strength requirements but relatively simple formability requirements. However, with the increasing competition in the automotive market, under the dual pressure of low material cost and personalized performance requirements, high-strength cold-rolled martensitic steel with low yield strength ratio is gradually being introduced and replacing traditional automotive high-strength steel.

[0003] With the deepening research and development of personalized designs for cold-rolled high-strength martensitic steel, the introduction of a small amount of ferrite into the traditional martensitic matrix to achieve a low yield strength ratio is becoming increasingly popular due to its relatively high formability, making it a sought-after product meeting the market's demand for low costs. To promote the extensive martensitic transformation of austenite into a matrix, the traditional air-cooling process for cold-rolled high-strength martensitic steel requires high hardenability due to its high cooling rate, leading to higher alloy costs. Water-cooling, with its high cooling rate and stability, significantly reduces process and alloy costs while ensuring material performance. Therefore, exploring a low yield strength ratio 1180MPa grade cold-rolled martensitic steel for automotive applications and its preparation method, focusing on the technical route of low yield strength ratio and water-cooling, is of great significance.

[0004] 1. Patent document CN110100032B, "Tempered Martensitic Steel with Low Yield Ratio and Excellent Uniform Elongation and its Manufacturing Method," discloses a tempered martensitic steel with a low yield ratio and excellent uniform elongation. By weight, the tempered martensitic steel comprises: C: 0.2–0.6%, Si: 0.01–2.2%, Mn: 0.5–3.0%, P: less than 0.015%, S: less than 0.005%, Al: 0.01–0.1%, Ti: 0. 0.01-0.1%, Cr: 0.05-0.5%, B: 0.0005-0.005%, Mo: 0.05-0.5%, N: less than 0.01%, balance Fe and unavoidable impurities, and the yield ratio is 0.4-0.6, the product of tensile strength and uniform elongation (TS*EL) is more than 10000 MPa%, and the microstructure, by area fraction, contains more than 90% tempered martensite, less than 5% ferrite and the balance bainite. Compared to the composition system designed in this invention, additional alloying elements such as Cr, B, and Mo are added. On the one hand, this significantly increases the cost of alloying elements. On the other hand, judging from the detailed technical documents, the design rationale for its alloy composition is mainly to serve the hot forming process, and the target product is 1500MPa grade 22MnB5 system hot-formed steel, which is different from the design concept of the target product 1180MPa grade cold-formed martensitic steel in this solution. At the same time, due to the difference in composition design, new process parameters are designed for hot rolling, cold rolling, and annealing processes based on composition characteristics.

[0005] 2. Patent document CN115874112B "A method for manufacturing 1300 MPa cold-rolled martensitic steel" discloses a method for manufacturing 1300 MPa cold-rolled martensitic steel, including: (1) smelting-continuous casting production process: hot metal pretreatment-converter-RH refining-casting machine; the composition of the molten steel supplied to the casting machine is C: 0.16-0.23%, Si: 0.70-1.00%, Mn: 1.80~2.20%, P: ≤0.020%, S: ≤0.010%, Cr: 0.25~0.35%, Alt: 0.030~0.050%, Ti: 0.020~0.030%, La: 20ppm; (2) hot rolling production process; (3) pickling and cold rolling process; (4) continuous annealing process. The purpose of this invention is to provide a method for manufacturing 1300MPa grade cold-rolled martensitic steel with a thickness of 1.2-1.5mm, characterized by high strength and high hardness, and whose chemical composition and mechanical properties meet user technical requirements. Compared to the composition design system of this invention, additional alloying elements including Cr and La are added, and the content of Si and Mn elements is higher than that of this invention, significantly increasing the cost of alloying elements. The heat treatment process of this invention involves air cooling (cooling rate of 50℃ / s) to the aging temperature after austenitization homogenization, which is fundamentally different from the water cooling process route of this invention.

[0006] 3. Patent document CN108977726B, "A Martensitic Ultra-High Strength Cold-Rolled Steel Strip Resistant to Delayed Cracking and Its Manufacturing Method," discloses a martensitic ultra-high strength cold-rolled steel strip resistant to delayed cracking and its manufacturing method. Its chemical composition by weight percentage is: C: 0.10-0.24%, Si+Al: 0.5-1.0%, Mn: 1.4-2.0%, W: 0.05-0.25%, Mo: 0.04-0.12%, P≤0.012%, S≤0.003%, N≤0.003%, Al: 0.05-0.3%, Ca: 0.001-0.003%, one or both of Ti and Nb, with Ti 0.02-0.04% and Nb 0.02-0.04%, and the remainder being Fe and unavoidable impurities. The steel plate of this invention has a strength of over 1180 MPa, excellent cold bending performance, and good resistance to delayed cracking. Under a pre-stress greater than or equal to one times the tensile strength, it can be immersed in 1 mol / L hydrochloric acid for over 300 hours without delayed cracking, making it particularly suitable for manufacturing automotive safety structural components. Compared to the composition design system of this invention, additional alloying elements including B, W, Mo, and Ti are added, significantly increasing the cost of alloying elements. Compared to the single-stage low-temperature tempering process of this invention, this invention employs a two-stage tempering process design in the heat treatment process. To ensure the high-temperature tempering resistance of the first stage, the Si content is designed to be significantly higher than that of this invention. Furthermore, its hot rolling process differs significantly from the design concept of this invention. Summary of the Invention

[0007] To address the aforementioned problems in the prior art, the present invention aims to provide a cold-rolled martensitic steel with a low yield strength ratio of 1180MPa for automotive applications and its preparation method. This invention fully leverages the high cooling rate advantages of the water-cooling process and transforms it into a low-alloy composition design. While ensuring the martensitic transformation of austenite after annealing, a slow cooling process is used to introduce partial ferrite microstructure, achieving personalized performance characteristics with a low yield strength ratio. Furthermore, based on a thorough consideration of composition design and microstructure properties, the process parameters for all multiple steps in the entire process are redesigned to ensure production stability, thereby achieving stable production of the 1180MPa cold-rolled martensitic steel for automotive applications and reducing production costs.

[0008] To achieve the above objectives, the present invention employs the following technical solution:

[0009] A cold-rolled martensitic steel with a low yield strength ratio of 1180MPa for automotive use has the following chemical composition by weight percentage: C: 0.145%–0.175%, Si: 0.20%–0.45%, Mn: 1.75%–2.00%, P≤0.020%, S≤0.0050%, Al: 0.08%–0.12%, N≤0.0050%, B: 0.005%–0.008%, Nb: 0.03%–0.04%, with the balance being Fe and other unavoidable impurities.

[0010] The steel has a tensile strength of ≥1180MPa, a yield strength ≤820MPa, an elongation ≥5%, and a yield strength ratio ≤0.7.

[0011] The microstructure of the steel is ≥95% martensite and ≤5% ferrite.

[0012] Explanation of the partial chemical group ratios:

[0013] Carbon (C): Carbon promotes martensitic transformation and enhances the strength of martensitic structures, making it a crucial element for achieving the required strength in martensitic steels. However, excessive carbon content can lead to carbide precipitation during aging and affect the weldability of subsequent processes. Furthermore, it easily forms carbides during subsequent aging. In this invention, to ensure a low yield strength ratio, some ferrite is introduced, which can easily cause a decrease in tensile strength. Therefore, compared to conventional high yield strength ratio products, an additional amount of carbon is added to enhance the strength of the martensitic matrix. Thus, the carbon content designed in this invention is 0.145%–0.175%.

[0014] Silicon (Si): Silicon is an important element that can effectively suppress the precipitation of large carbide particles during aging, providing a larger process window for the aging process. However, excessive silicon is detrimental to the surface quality of pickling. Considering that this invention requires a higher aging temperature to achieve the performance target of low yield strength ratio, an additional portion of silicon is added compared to the conventional high yield strength ratio product composition design. Therefore, the silicon content designed in this invention is 0.20% to 0.45%.

[0015] Manganese (Mn): Manganese can effectively improve the mechanical properties of products through solid solution strengthening and grain refinement mechanisms. Simultaneously, manganese is a beneficial element that significantly improves hardenability and promotes martensitic transformation. However, excessive manganese content can affect weldability in cold rolling processes and easily generate banded structures. Therefore, the manganese content designed in this invention is 1.75%–2.00%.

[0016] Aluminum (Al): As an element that is easily oxidized, aluminum can efficiently deoxidize molten steel during the steelmaking process. At the same time, aluminum combines with nitrogen to ensure the contribution of boron to improving hardenability. Al can also improve the tempering resistance of the martensitic matrix. Therefore, the aluminum content designed in this invention is 0.08% to 0.12%.

[0017] Boron (B): The balanced segregation of boron at austenite grain boundaries significantly inhibits ferrite nucleation and indirectly promotes martensite formation. Therefore, the boron content in this invention is designed to be 0.005% to 0.008%.

[0018] Niobium (Nb): As an important element for precipitation strengthening, niobium forms small-sized precipitates with carbon during hot rolling, promoting the formation of high-density dislocations and hindering recovery, thus contributing to a low yield strength ratio. Therefore, the niobium content designed in this invention is 0.03%–0.04%.

[0019] Phosphorus (P), Sulfur (S), and Nitrogen (N): Phosphorus, sulfur, and nitrogen are harmful elements in the product of this invention, and their content should be minimized. However, excessively low content requirements would significantly increase the cost of alloy raw materials and steelmaking processes, which is not conducive to achieving low-cost product characteristics. Therefore, the design of this invention is P≤0.020%, S≤0.0050%, and N≤0.0050%.

[0020] A method for preparing a cold-rolled martensitic steel with a low yield strength ratio of 1180MPa for automobiles includes smelting, hot rolling, cold rolling, continuous annealing and leveling processes.

[0021] The hot rolling process includes: heating the slab at a temperature between 1230 and 1270°C for 150 to 200 minutes; starting the finishing rolling at a temperature between 1040 and 1100°C; finishing the rolling at a temperature between 900 and 960°C; the temperature in the middle of the coiling section between 630 and 680°C; and the temperature within the first 100 meters of the head and tail section between 720 and 740°C.

[0022] The design of the heating temperature and heating time in hot rolling can ensure the full homogenization of alloying elements. However, higher temperatures and longer heating times are not conducive to leveraging the low-cost technical advantages of this invention. Therefore, the heating temperature of this invention is 1230-1270℃, and the heating time is 150-200min. The initial rolling temperature of finishing rolling should not be too low, as the high deformation resistance caused by lower temperatures is not conducive to the stability of hot rolling production and places higher demands on hot rolling equipment. Similarly, the final rolling temperature should not be too low, as excessively low final rolling temperatures can easily cause rolling in the two-phase region, which can easily lead to problems such as uneven grain size in hot-rolled plates. During the coiling process, the temperature in the middle is controlled at 630-680℃, and a coiling method with heating at both the head and tail can be adopted to effectively avoid the problem of flattening after coiling and improve the uniformity of the microstructure and properties at both ends.

[0023] The cold rolling process includes: laser welding of the hot-rolled plate before cold rolling to ensure continuous cold rolling production, with a laser power of 11.5-12KW, a welding speed of 3.2-3.8m / min, a front induction heating power of 10-20KW, and a rear induction heating power of 22-32KW; the pickling process adopts a shallow tank turbulent pickling process, with a tension leveling elongation of 1.0-1.4%, an acid temperature of 80-85℃, and a pickling time of 55-80s; after removing the iron oxide scale through the pickling process, the plate is cold rolled with a cold rolling reduction rate of 50%-60%.

[0024] The above welding process ensures stable rolling of the weld during cold rolling. Excessive laser power wastes energy and is detrimental to the stable operation of welding equipment, while insufficient laser power fails to guarantee weld penetration. A reasonable match between welding speed and induction heating power before and after the process facilitates control of weld microstructure and properties, ensuring no weld breakage occurs during rolling. Combined with the design of composition and hot rolling process, higher tensile elongation, acid temperature, and pickling time effectively improve the surface quality after pickling. An excessively low cold rolling reduction ratio places excessive demands on the rolling capacity of the upstream hot rolling process, while an excessively high cold rolling reduction ratio places high demands on the cold rolling equipment and easily leads to weld breakage. Therefore, the cold rolling reduction ratio is designed to be between 50% and 60%, providing reasonable deformation energy storage for annealing recrystallization to produce fine grain strengthening.

[0025] The continuous annealing process includes: heating at a temperature of 900–930°C for 185–240 seconds, followed by slow cooling at a rate of 5–10°C / s to 670–700°C, then cooling to room temperature at a rate of more than 200°C / s using a water quenching process, followed by heating to an aging temperature of 250–350°C for 230–290 seconds, and then cooling to room temperature.

[0026] After cooling to room temperature, pickling and flash nickel plating are performed. The specific process includes degreasing, pickling, water washing, flash nickel plating, and finishing.

[0027] The heating temperature is selected at 900-930℃ to ensure full austenitization, which is more conducive to the martensitic transformation in the subsequent quenching process. A slow cooling temperature of 670-700℃ is used to introduce a small amount of ferrite structure to achieve performance control of low yield strength ratio. The ultra-high cooling rate water quenching process can obtain a high-strength martensitic structure to meet the tensile strength requirements of the product. Subsequently, the strip steel is aged at an aging temperature of 250-350℃ for 230-290s to improve the deformation capacity of the product and ensure its bending performance.

[0028] This invention employs a water-cooled quenching process to achieve a product performance of 1180MPa with a low yield strength ratio while keeping alloy costs low. Furthermore, the entire production process is customized to address the characteristics of the product with a low yield strength ratio, thereby improving production stability and further reducing production costs.

[0029] Compared with the prior art, the beneficial effects of the present invention are:

[0030] 1) This invention achieves a low yield strength ratio by using a novel matching design of composition and heat treatment process, with martensite as the matrix and a small amount of ferrite structure, effectively improving the formability of 1180MPa grade martensitic steel in application.

[0031] 2) This invention utilizes a high-speed water quenching process to achieve a low-alloy product design. It fully leverages the high-speed water quenching process to produce cold-rolled high-strength martensitic steel products. It uses only a conventional carbon-silicon-manganese composition system without adding other more expensive alloying elements to meet product performance requirements, effectively reducing product production costs.

[0032] 3) This invention designs a process control scheme for each process in the entire process based on alloy composition, which avoids many quality defects in the production process and improves production stability, thus meeting the green and low-carbon development needs of steel enterprises. Attached Figure Description

[0033] Figure 1 Microstructure diagram of 1180MPa grade cold-rolled martensitic steel for automobiles produced by the manufacturing method of the present invention.

[0034] Figure 2 The diagram shows the cold bending performance of the 1180MPa grade cold-rolled martensitic steel product for automobiles produced by the manufacturing method of this invention. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of this invention clearer, the specific implementation methods of this invention will be further described below in conjunction with the embodiments. The following embodiments are used to specifically illustrate the content of this invention. These embodiments are only general descriptions of the content of this invention and do not limit the content of this invention.

[0036] Table 1 lists the chemical composition of the steels in the examples; Table 2 lists the continuous casting and hot rolling process parameters of the steels in the examples; Table 3 lists the welding process parameters of the steels in the examples; Table 4 lists the pickling and rolling process parameters of the steels in the examples; Table 5 lists the process parameters for continuous annealing; and Table 6 gives the mechanical properties of the steels in the examples.

[0037] Table 1. Chemical composition (wt%) of the steel in the examples

[0038]

[0039] Table 2 Hot rolling process of steel in the examples

[0040]

[0041] Table 3 Welding process of steel in the examples

[0042]

[0043] Table 4. Pickling and rolling processes of the steel in the examples.

[0044]

[0045] Table 5 Annealing process of steel in the examples

[0046]

[0047] Table 6 Mechanical properties of the steel in the examples

[0048]

[0049] As can be seen from the above embodiments, the automotive steel prepared by using the composition design, rolling, and continuous annealing process of the present invention has a tensile strength of over 1180MPa, a yield strength of 780-820MPa, an elongation of 5-8%, and a yield strength ratio of ≤0.7, achieving the performance characteristics of a low yield strength ratio and meeting the requirements for use of 1180MPa grade cold-rolled martensitic steel for automobiles.

[0050] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A cold-rolled martensitic steel with a low yield strength ratio of 1180 MPa for automotive applications, characterized in that, The chemical composition of the steel, by weight percentage, is as follows: C: 0.145%–0.175%, Si: 0.20%–0.45%, Mn: 1.75%–2.00%, P≤0.020%, S≤0.0050%, Al: 0.08%–0.12%, N≤0.0050%, B: 0.005%–0.008%, Nb: 0.03%–0.04%, with the balance being Fe and other unavoidable impurities.

2. The 1180MPa grade cold-rolled martensitic steel for automobiles with a low yield strength ratio according to claim 1, characterized in that, The steel has a tensile strength of ≥1180MPa, a yield strength ≤820MPa, an elongation ≥5%, and a yield strength ratio ≤0.

7.

3. The 1180MPa grade cold-rolled martensitic steel for automobiles with a low yield strength ratio according to claim 1, characterized in that, The steel has a martensite volume percentage of ≥95% and a ferrite volume percentage of ≤5%.

4. A method for preparing 1180MPa grade cold-rolled martensitic steel for automobiles with low yield strength ratio as described in any one of claims 1-3, comprising smelting, hot rolling, cold rolling, continuous annealing, and leveling processes; characterized in that, The continuous annealing process includes: heating at a temperature of 900–930°C for 185–240 seconds, followed by slow cooling at a rate of 5–10°C / s to 670–700°C, then cooling to room temperature at a rate of more than 200°C / s using a water quenching process, followed by heating to an aging temperature of 250–350°C for 230–290 seconds, and then cooling to room temperature.

5. The method for preparing 1180MPa grade cold-rolled martensitic steel for automobiles with low yield strength ratio according to claim 4, characterized in that, The hot rolling process includes: heating the slab at a temperature between 1230 and 1270°C for 150 to 200 minutes; starting the finishing rolling at a temperature between 1040 and 1100°C; finishing the rolling at a temperature between 900 and 960°C; the temperature in the middle of the coiling section between 630 and 680°C; and the temperature within the first 100 meters of the head and tail section between 720 and 740°C.

6. The method for preparing 1180MPa grade cold-rolled martensitic steel for automobiles with low yield strength ratio according to claim 4, characterized in that, The cold rolling process includes: laser welding of the hot-rolled plate before cold rolling production, with a laser power of 11.5-12KW, a welding speed of 3.2-3.8m / min, a front induction heating power of 10-20KW, and a rear induction heating power of 22-32KW; the pickling process adopts a shallow tank turbulent pickling process, with a tensile elongation of 1.0-1.4%, an acid temperature of 80-85℃, a pickling time of 55-80s, and a cold rolling reduction rate of 50%-60%.

7. The method for preparing 1180MPa grade cold-rolled martensitic steel for automobiles with low yield strength ratio according to claim 4, characterized in that, After cooling to room temperature, pickling and flash nickel plating are performed. The specific process includes degreasing, pickling, water washing, flash nickel plating, and finishing.

Images