High cold bending performance low yield ratio 980mpa grade cold rolled dual phase steel for automobile and preparation method thereof

Abstract

The present application relates to the technical field of automobile steel, in particular to a kind of 980MPa-grade cold-rolled dual-phase steel with high cold-bending performance and low yield ratio for automobile and a preparation method thereof.The preparation method comprises: after slab hot rolling and acid rolling (reduction ratio ≥ 50%), continuous annealing is carried out, and the specific process is as follows: 840~880 ℃ soaking, slow cooling to 650~700 ℃, fast cooling to 310~340 ℃, overaging at 200~240 ℃, and finally cooling and flattening.By synergistic regulation of composition and process, a multi-phase structure of ferrite, martensite and residual austenite is obtained, the product has a yield strength of 580~760MPa, a tensile strength of ≥980MPa, a low yield ratio, an elongation after fracture of ≥12%, and a limit sharp bending angle of ≥170°, and has high strength and excellent cold-bending performance.

Description

Technical Field

[0001] This invention relates to the field of automotive steel technology, specifically to a cold-rolled dual-phase steel with high cold bending performance and low yield strength ratio of 980MPa for automobiles and its preparation method. Background Technology

[0002] With the automotive industry's increasing demands for both lightweighting and safety, reducing vehicle weight while ensuring collision safety has become a core trend in the industry. Compared to expensive aluminum alloys or carbon fiber composites, high-strength steel has become the primary material for reducing the weight of automotive body-in-white due to its excellent cost-effectiveness and mature processing system. Among them, advanced high-strength steel with a tensile strength of 980MPa, especially cold-rolled dual-phase steel, shows broad application prospects in areas such as body structural components and safety components.

[0003] Dual-phase steel, with its unique ferrite-martensite mixed structure, is advantageous for achieving good drawing properties in stamping. However, for cold-rolled dual-phase steel with a strength of 980 MPa and above, automotive manufacturing places even higher demands: parts not only require good tensile formability during manufacturing but must also meet the requirements of complex cold deformation processes such as flanging and sharp bending, i.e., excellent cold bending performance is required.

[0004] Currently, traditional cold-rolled DP980 duplex steel on the market often exhibits cracking at bends during severe bending or flanging due to issues such as the hardness and brittleness of the martensite phase, insufficient microstructure uniformity, and poor control of inclusion morphology. This results in poor cold bending performance, particularly at extreme sharp bending angles, failing to meet the manufacturing requirements of high-end parts. Furthermore, the combination of a low yield strength ratio and excellent cold bending performance has become a technical bottleneck restricting the wider application of this type of material.

[0005] While some existing research attempts to improve the properties of high-strength duplex steel by adjusting composition or processes, most focus on enhancing single properties and application indicators. They fail to simultaneously address the challenges of cold bending cracking under low yield strength ratio and cold bending performance. Summary of the Invention

[0006] In order to overcome the shortcomings of the prior art, the present invention provides a 980MPa grade cold-rolled dual-phase steel for automobiles with high cold bending performance and low yield strength ratio and its preparation method, which simultaneously solves the problem of cold bending cracking under low yield strength ratio and cold bending performance.

[0007] To achieve the above objectives, the present invention employs the following technical solution: A cold-rolled dual-phase steel for automobiles with high cold bending performance and low yield strength ratio (980MPa grade) is composed of the following chemical composition by weight percentage: C: 0.04%~0.20%, Si: 0.20%~0.5%, Mn: 1.9%~2.3%, P≤0.02%, S≤0.005%, Al: 0.02%~0.06%, Nb: 0.06%~0.08%, Ti: 0.06%~0.08%, B: 0.001%~0.003%, Mo: 0.05%~0.10%, with the balance being Fe and other unavoidable impurities.

[0008] Its microstructure contains 41% to 49% ferrite, 31% to 41% martensite and 8% to 14% retained austenite by volume.

[0009] The steel has a yield strength of 580~760MPa, a tensile strength ≥980MPa, a yield strength ratio not greater than 0.565, an elongation after fracture A50 ≥12%, and an ultimate cold bending angle ≥170°.

[0010] The preparation method of the above-mentioned 980MPa grade cold-rolled dual-phase steel for automobiles with high cold bending performance and low yield strength ratio specifically includes the following steps: 1) Hot rolling: The slab is heated to 1270~1310℃ and held at that temperature for rolling. The final rolling temperature is 870~910℃. Then, it is cooled in laminar flow to 530~620℃ and coiled to obtain a hot-rolled plate. 2) Pickling and cold rolling: The hot-rolled plate is pickled and cold-rolled, with a cold rolling reduction rate of ≥50%, to obtain a cold-rolled hard plate; 3) Continuous annealing: The cold-hardened plate is annealed at a temperature of 840~880℃, then slowly cooled to 650~700℃, then rapidly cooled to 310~340℃, then subjected to over-aging treatment at 200~240℃, and finally cooled to room temperature.

[0011] Furthermore, in step 1), the thickness of the slab is 210~240mm, and the cumulative rolling reduction rate in the austenitic region is ≥98%.

[0012] Furthermore, in step 3), the cooling to room temperature step is performed by water quenching.

[0013] Furthermore, it also includes a leveling process, in which the annealed strip is leveled using a rolling force of 550~2000kN.

[0014] Compared with the prior art, the beneficial effects of the present invention are: 1. The produced 980MPa duplex steel product has a low yield strength ratio and excellent cold bending performance: This invention lays the foundation for obtaining an ideal microstructure through precise composition design, especially the synergistic ratio of C, Si, Mn, Nb, Ti, B, and Mo. Elements such as C, Mn, B, and Mo ensure sufficient hardenability and martensitic transformation potential; Si inhibits carbide precipitation and promotes carbon enrichment into austenite; the microalloying of Nb and Ti refines the original austenite grains. These components are precisely controlled through a unique continuous annealing process: high-temperature homogenization at 840~880℃ ensures full solid solution of alloying elements and homogenization of austenite; slow cooling at 650~700℃ greatly promotes the rapid nucleation and growth of polygonal ferrite, increasing the proportion of the soft phase (ferrite), which is the direct reason for reducing yield strength and thus obtaining a low yield-to-tensile ratio; in subsequent deformation, the retained austenite undergoes the TRIP effect (transformation-induced plasticity), transforming into martensite. This process absorbs a large amount of strain energy, delaying local necking and cracking, while making the deformation more uniform. The soft ferrite matrix ensures excellent plastic deformation capacity, while the hard phase ensures tensile strength (≥980MPa). This composite structure of "soft matrix + hard phase + variable phase" is the microscopic mechanism by which stress can be redistributed during bending, avoiding stress concentration and cracking, thus achieving the outstanding performance of an extreme cold bending angle ≥170°.

[0015] 2. Stable process, uniform microstructure, and good performance consistency: In continuous annealing, precise temperature range control (soaking, slow cooling, rapid cooling, and over-aging) ensures that the nucleation and growth processes of ferrite, martensite, and retained austenite are controllable and repeatable, thus guaranteeing the final microstructure (e.g., Figure 1-4 The uniformity and stability (as shown) ensure a high degree of consistency in the product's winding performance.

[0016] 3. Significantly improves the forming and manufacturing qualification rate and safety of parts: The steel prepared by this invention, due to its low yield strength (easy to initially form) and unprecedented high cold bending performance, can perfectly adapt to the complex stamping, severe flanging and sharp angle bending processes in the manufacturing of automotive structural parts. This fundamentally avoids the cracking, wrinkling and other defects that are prone to occur in traditional DP980 steel in such processing, and reduces the scrap rate. Attached Figure Description

[0017] Figure 1 This is a SEM micrograph of the tissue from Example 1 of the present invention.

[0018] Figure 2 This is a SEM micrograph of the tissue from Embodiment 2 of the present invention.

[0019] Figure 3 This is a SEM micrograph of the tissue in Example 3 of the present invention.

[0020] Figure 4 This is a SEM micrograph of the tissue in Example 4 of the present invention. Detailed Implementation

[0021] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are merely one example of the embodiments of the present invention, and those skilled in the art can refer to the content herein and appropriately improve the process parameters to achieve the desired result. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in the present invention. The methods and applications of the present invention have been described through preferred embodiments, and those skilled in the art can obviously make modifications or appropriate changes and combinations to the methods and applications described herein without departing from the content, spirit, and scope of the present invention to realize and apply the technology of the present invention.

[0022] A cold-rolled dual-phase steel for automobiles with high cold bending performance and low yield strength ratio (980MPa grade) is composed of the following chemical composition by weight percentage: C: 0.04%~0.20%, Si: 0.20%~0.5%, Mn: 1.9%~2.3%, P≤0.02%, S≤0.005%, Al: 0.02%~0.06%, Nb: 0.06%~0.08%, Ti: 0.06%~0.08%, B: 0.001%~0.003%, Mo: 0.05%~0.10%, with the balance being Fe and other unavoidable impurities.

[0023] The preparation method of the above-mentioned 980MPa grade cold-rolled dual-phase steel for automobiles with high cold bending performance and low yield strength ratio specifically includes the following steps: 1. After LF furnace and RH vacuum refining, the molten steel is subjected to calcium treatment. Specifically, a silicon-calcium wire is fed in using a wire feeder, and the amount of calcium added is controlled to transform the inclusions into fine and dispersed spherical calcium aluminate and calcium sulfide, thereby strictly controlling the morphology and distribution of inclusions in the steel and improving the purity of the molten steel.

[0024] 2. Hot Rolling: The slab with the above chemical composition is preheated, heated, rough rolled, finish rolled, laminar cooled, and coiled to obtain hot-rolled steel plate / strip. The slab thickness is 210~240mm, the heating temperature is 1270~1310℃, the holding time is 100~150min, which is beneficial to improve cold bending performance. The cumulative rolling reduction rate in the austenitic region is ≥98%, the thickness is 1.6~4.0mm, the final rolling temperature is 870~910℃, and laminar cooling is performed to the target coiling temperature of 530~620℃.

[0025] 3. Pickling and rolling process: The reduction rate of the pickling and rolling process is ≥50%. After the hot-rolled steel coil is welded to the front and rear strips by laser welding, the strips are pickled to remove the iron oxide scale on the surface of the strips, and the hot-rolled steel plates are rolled into cold-rolled plates of corresponding thickness specifications.

[0026] 4. Continuous annealing process: After pickling and rolling, the strip steel is annealed at high temperature in an annealing furnace at 840~880℃. During the cooling process, the strip steel is uniformly distributed on the ferrite matrix. During cold bending, the soft matrix deforms and the hard phase is separated by the flowing matrix, which is beneficial to improving the cold bending performance during stamping. The slow cooling end temperature is 650~700℃. The slow cooling process accelerates the ferrite nucleation rate and increases the ferrite ratio, which is beneficial to improving the cold bending performance. It also reduces the ferrite strength. The lower slow cooling temperature also reduces the precipitation of carbonitrides, thereby reducing the ferrite strength and yield strength. However, while reducing the ferrite strength, the slow cooling rate also reduces the amount of martensite and the strengthening effect of solid solutions in the martensite, resulting in a slight decrease in tensile strength. Therefore, the rapid cooling end temperature is 310~340℃. In order to avoid the decomposition of Si and Mn solid solutions in martensite and increase tensile strength, the over-aging temperature is lowered to stabilize the residual austenite. Some of the residual austenite is retained to undergo the TRIP effect during plastic deformation. This process absorbs a large amount of strain energy, delays necking and cracking, and also increases local strength, making the deformation more uniform and improving cold bending performance. Therefore, the over-aging end temperature is 200~240℃, the final cooling temperature is 120℃, and water quenching is performed to room temperature.

[0027] 5. After the strip steel exits the furnace, it undergoes leveling treatment and is rolled using a constant rolling force of 550~2000kN. This helps to increase the work hardening degree of the material and improve its forming and cold bending performance.

[0028] The chemical composition of the steel in the embodiments of the present invention is shown in Table 1, the hot rolling process in the embodiments of the present invention is shown in Table 2, the pickling and rolling process in the embodiments of the present invention is shown in Table 3, the continuous annealing and leveling process in the embodiments of the present invention is shown in Table 4, the mechanical properties in the embodiments of the present invention are shown in Table 5, and the cold bending application performance in the embodiments of the present invention is shown in Table 6.

[0029] Table 1. Chemical composition (wt%) of steel in the embodiments of the present invention

[0030] Table 2 Hot rolling process parameters of the embodiments of the present invention

[0031] Table 3 Pickling process parameters of embodiments of the present invention

[0032] Table 4. Continuous annealing and leveling process parameters in the embodiments of the present invention

[0033] Table 5 Mechanical properties of embodiments of the present invention

[0034] Table 6 Cold bending application performance of embodiments of the present invention

[0035] Examples 1-4: Microstructure of steel plates as follows Figure 1-4 As shown, its microstructure comprises 41%–49% ferrite, 31%–41% martensite, and 8%–14% retained austenite by volume. The high proportion of ferrite in the soft matrix directly contributes to the material's low yield strength, while the martensite, as a hard phase, ensures a high tensile strength of ≥980 MPa. The excellent cold bending performance stems from the synergistic effect of the microstructure: the continuous ferrite matrix provides the basis for overall plastic deformation; during bending, the stable retained austenite undergoes the TRIP effect, absorbing energy and uniformizing stress through phase transformation; thus, together they achieve the outstanding performance of an ultimate cold bending angle ≥170°.

[0036] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing a cold-rolled dual-phase steel with high cold bending performance and low yield strength ratio (980 MPa) for automotive applications, characterized in that, Duplex steel has the following chemical composition by weight percentage. composition: C: 0.08%~0.09%, Si: 0.34%~0.38%, Mn: 1.9%~2.3%, P≤0.02%, S≤0.005%, Al: 0.02%~0.06%, Nb: 0.06%~0.08%, Ti: 0.06%~0.08%, B: 0.001%~0.003%, Mo: 0.05%~0.10%, with the balance being Fe and other unavoidable impurities; Its microstructure contains 41%–49% ferrite, 31%–41% martensite, and 8%–14% retained austenite by volume. The steel has a yield strength of 580~760MPa, a tensile strength ≥980MPa, a yield ratio of not more than 0.565, an elongation after fracture A50 ≥12%, and an ultimate cold bending angle ≥170°. Includes the following steps: 1) Hot rolling: The slab is heated to 1270~1310℃ and held at that temperature for rolling. The final rolling temperature is 870~910℃. Then, it is cooled in laminar flow to 530~620℃ and coiled to obtain a hot-rolled plate. 2) Pickling and cold rolling: The hot-rolled plate is pickled and cold-rolled, with a cold rolling reduction rate of ≥50%, to obtain a cold-rolled hard plate; 3) Continuous annealing: The cold-hardened sheet is annealed at a soaking temperature of 840~880℃, then slowly cooled to 650~700℃, then rapidly cooled to 310~340℃, then subjected to over-aging treatment at 200~240℃, and finally cooled to room temperature; the cooling to room temperature step is carried out by water quenching. 4) Leveling: The annealed strip is leveled using a rolling force of 550~2000kN.

2. The method for preparing high cold-bending performance and low yield strength ratio 980MPa grade cold-rolled duplex steel for automobiles according to claim 1, characterized in that, In step 1), the thickness of the slab is 210~240mm, and the cumulative rolling reduction rate in the austenitic region is ≥98%.

Images