Short-process long-aging ultra-low carbon bake-hardening steel and production method thereof

Abstract

The invention discloses ultra-low carbon bake hardening steel with a short process and a long aging period and a production method of the ultra-low carbon bake hardening steel. The ultra-low carbon bake hardening steel comprises the following chemical components: 0.001 to 0.0035 percent of C, less than or equal to 0.03 percent of Si, 0.1 to 0.6 percent of Mn, less than or equal to 0.005 percent of S, 0.020 to 0.065 percent of Als, 0.005 to 0.02 percent of Nb, 0.0003 to 0.001 percent of B, 0.05 to 0.15 percent of Cu, less than or equal to 0.0035 percent of N, 0.0008 to 0.0015 percent of solid solution C and the balance of iron and inevitable impurities. The production method comprises the working procedures of smelting, double-roller thin strip cast rolling, cold rolling, continuous annealing and leveling. The method comprises the following steps: strictly controlling the components of molten steel, cooling and solidifying between casting rollers to form a cast strip of 3-6 mm, rolling to obtain a hot-rolled coil of 2-5 mm after online one-pass hot rolling and flattening, carrying out cold rolling to obtain a cold hard coil of 0.5-1.0 mm, and accurately controlling a continuous annealing process to obtain the bake-hardened steel of which the yield strength is 235-280 MPa, the tensile strength is 320-400 MPa, the elongation A80 is greater than or equal to 38%, the bake-hardening value BH2 is greater than or equal to 45 MPa, the r value is greater than or equal to 2.0 and the work hardening index n is greater than or equal to 0.2. In addition, the steel does not have natural aging after being stored for 6 months in summer, and shows good storage stability.

Description

TECHNICAL FIELD

[0001] The present application relates to a production method of bake hardening steel, in particular to a short process long aging ultra-low carbon bake hardening steel and a production method thereof, and belongs to the field of metal material preparation. BACKGROUND

[0002] Bake hardening steel is a kind of steel material with high strength and high formability, and is one of the main materials of modern automobile cover. The strength of the final part is obtained through work hardening in the machining process and aging phenomenon in the baking process. Under the form of global energy saving and consumption reduction, low carbon and environmental protection, the automobile industry is developing towards light weight and low pollution, especially the performance of cars is more pursuing high speed, safety, energy saving, comfort, beauty and durability. The bake hardening steel sheet has excellent stamping performance and yield strength is greatly improved during the baking process after forming, and has excellent dent resistance, so the bake hardening steel sheet is widely used in automobile panels. However, the bake hardening capacity of the steel is limited and the aging at room temperature is easy, which is the main problem in production and is widely studied by steel producers. For enterprises, improving or prolonging the aging period of bake hardening steel sheet at room temperature has important practical value for reducing transportation and storage costs.

[0003] Patent CN101497959B discloses a yield strength of 220 MPa cold rolled bake hardening steel and a production method thereof, which adds a small amount of B to inhibit the segregation of C at the grain boundary to ensure the bake hardening effect, but still cannot solve the problem of aging at room temperature.

[0004] Patent CN102400046B discloses a high-strength bake hardening steel and a preparation method thereof, which adds Cu-Ni to the Nb-Ti steel to improve the bake hardening performance, but still cannot inhibit the aging of the steel sheet at room temperature.

[0005] Patent CN106702266A discloses a bake hardening steel 220BH and a production method thereof, which adds P to the Nb-IF steel to improve the strength and uses Mo to inhibit the aging at room temperature, but the bake hardening effect will be inhibited at a carbon content of 0.001-0.003%.

[0006] In addition to the defects described above, the above-mentioned patents all have problems of complex production process, high production cost and the like. SUMMARY

[0007] The present application overcomes the deficiencies of the prior art, and provides a short process long aging ultra-low carbon bake hardening steel and a production method thereof according to the development needs of the field of automobile steel. The molten metal is directly made into a thin strip by double-roller thin strip casting, and the heating and multiple hot rolling steps are omitted, solving the problems of complex production process, multiple manufacturing processes and high energy consumption of the existing production process. The method can reduce cost, increase benefit and improve productivity.

[0008] To achieve the above-mentioned purposes, the application adopts the following solutions: In a first aspect, the application provides a short-process long-aging ultra-low-carbon bake-hardening steel, the chemical composition of the bake-hardening steel and the mass percentage thereof are as follows: C: 0.001-0.0035%, Si: ≤0.03%, Mn: 0.1-0.6%, S: ≤0.005%, Als: 0.020-0.065%, Nb: 0.005-0.02%, B: 0.0003-0.001%, Cu: 0.05-0.15%, N: ≤0.0035%, solid-solution C: 0.0008-0.0015%, and the balance being iron and inevitable impurities.

[0009] Further, the thickness of the bake-hardening steel plate is 0.5-1.0 mm, the yield strength is 235-280 MPa, the tensile strength is 320-400 MPa, the elongation A 80 ≥38%, the bake-hardening value BH2≥45 MPa, the r value≥2.0, the work-hardening index n≥0.2, and no natural aging occurs after 6 months of summer storage.

[0010] In a second aspect, the application provides a production method of a short-process long-aging ultra-low-carbon bake-hardening steel, the production method comprising the steps of smelting, double-roller thin-strip casting, cold rolling, continuous annealing, and flattening.

[0011] Further, the double-roller thin-strip casting process comprises the following steps: conveying the qualified molten steel to a pair of counter-rotating casting rollers, cooling and solidifying the molten steel on the casting surfaces of the pair of casting rollers, and forming a 3-6 mm cast strip by passing through the nip between the counter-rotating double rollers, and obtaining a hot-rolled coil with a thickness of 2-5 mm by one-pass hot-rolling and flattening the cast strip and then coiling at 650-730 ℃.

[0012] Further, the cold rolling process comprises the following steps: obtaining a cold-hard coil by cold rolling the obtained hot-rolled coil, and the thickness of the cold-hard coil is 0.5-1.0 mm and the reduction rate is 75-85%.

[0013] Further, the continuous annealing process comprises the following steps: continuously annealing the obtained cold-hard coil, controlling the soaking temperature to be 820-860 ℃ and the soaking time to be 60-180 s, slowly cooling to 670-720 ℃, rapidly cooling to 350-400 ℃, over-aging treatment, controlling the over-aging temperature to be 300-350 ℃, and controlling the over-aging time to be 180-360 s, and obtaining a continuous annealing coil.

[0014] Further, the flattening process comprises the following steps: stretching and flattening the obtained continuous annealing coil, and the flattening elongation of the steel coil is 1.0-2.2%.

[0015] In one short process long aging period ultra-low carbon bake hardening steel and its production method of the present application, the design principle of each chemical element is described as follows: C: Carbon is the most important solid solution strengthening element in steel, a certain amount of solid solution carbon can ensure the bake hardening effect. Too high carbon content is not conducive to the deep drawing performance of the steel sheet, and increases the normal temperature aging sensitivity. Too low carbon content makes it difficult to ensure the bake hardening performance. Therefore, the carbon content designed in this patent is 0.001-0.0035%.

[0016] Si: Silicon is a solid solution strengthening element, which can improve the strength of the steel sheet and increase the activity of carbon in the steel, which is beneficial to bake hardening, but too high silicon content will reduce the plasticity of the steel and cause the steel sheet to be prone to normal temperature aging. At the same time, too high silicon will worsen the surface quality of the hot-rolled steel sheet and cause defects. Therefore, the silicon content designed in this patent is ≤0.03%.

[0017] Mn: Manganese plays a role in solid solution strengthening and refining ferrite grains. However, too high manganese content can easily cause mixed crystals. Therefore, the manganese content designed in this patent is 0.1-0.6%.

[0018] S: Sulfur is a harmful element in steel, which can form manganese sulfide to reduce the performance of the steel sheet, so the less the better. Therefore, the sulfur content designed in this patent is ≤0.005%.

[0019] N: Solid solution N strongly increases the normal temperature aging sensitivity of the steel sheet, which must be reduced as much as possible considering the smelting cost. Therefore, the nitrogen content designed in this patent is ≤0.0035%.

[0020] B: Boron element is used in this application to improve the secondary processing brittleness. Impurity elements tend to segregate at grain boundaries and reduce the strength of grain boundaries, while B element precipitates at grain boundaries to increase the cohesion of grain boundaries. Therefore, in order to avoid the secondary cold deformation brittleness trend caused by impurity elements, the boron content designed in this patent is 0.0003-0.001%.

[0021] Nb: The role of niobium element is to fix C in steel, control the grain boundary segregation of C atoms during continuous annealing process, and realize "target locking" of solid solution C through the synergistic control of Nb and B elements. Therefore, the niobium content designed in this patent is 0.005-0.02%.

[0022] Cu: The high solubility of copper element in high temperature α-Fe enables it to delay the phase change process, reduce stress concentration and improve strength and toughness. When Cu content is greater than or equal to 0.05%, solid solution strengthening and corrosion resistance are activated, and when Cu content is controlled below 0.15%, the risk of hot brittleness and welding can be avoided. Therefore, the copper content designed in this patent is 0.05-0.15%.

[0023] Al: Aluminum can form A1N precipitates, fix nitrogen in steel, play a certain role in refining grains and inhibiting steel plate aging at room temperature. Therefore, the aluminum content of the patent design is 0.020-0.065%.

[0024] Solid solution C: Solid solution carbon atoms form gas clusters to pin dislocations by migrating to dislocation zones after deformation, and drive the effect of bake hardening. Therefore, the solid solution carbon content needs to be strictly controlled, too high will cause the decline of the baking performance of long-term storage, ensure BH value ≥ 45MPa; too low content weakens the hardening effect. Therefore, the solid solution C content is 0.0008~0.0015%.

[0025] The invention adopts double-roller thin strip casting process, because the liquid metal solidification speed is fast, the grain refinement, the composition segregation is reduced, the strength and toughness of the strip can be improved, through subsequent cold rolling and annealing treatment, the microstructure and texture are further controlled, thereby improving the bake hardening performance of BH steel and prolonging its aging time. The carbon content is controlled at 0.001-0.0035%, which can ensure the bake hardening effect and good aging stability. Nb fixes carbon elements, controls the segregation of carbon atoms, and B precipitates around the grain boundary or precipitates, thereby effectively improving the bake hardening performance. Cu is a high solubility element in high-temperature ferrite, which can not only delay the transformation of austenite to ferrite, but also improve the corrosion resistance. Cu has low solubility in low-temperature ferrite, and is uniformly distributed in the steel structure in the form of small particles or crystals, especially around the grain boundary or precipitates, which can effectively optimize the bake hardening performance. Therefore, the content of Cu is designed to be 0.05-0.15%, which can significantly improve the strength and toughness of the material, and successfully realize the overall improvement of the performance of ultra-low carbon bake hardening steel.

[0026] Compared with the prior art, the beneficial technical effects of the present application are: (1) The component design of the present application adopts ultra-low carbon, Mn solid solution strengthening, and adds Nb to fix part of C, avoids the secondary cold deformation embrittlement trend caused by impurity elements by adding B element, and through matching the corresponding double-roller thin strip casting, cold rolling, continuous annealing process, the obtained bake hardening steel has yield strength 235-280MPa, tensile strength 320-400MPa, elongation A 80 ≥38%, bake hardening value BH2≥45MPa, r value ≥2.0, work hardening index n≥0.2, and no natural aging occurs after 6 months of summer storage.

[0027] (2) The chemical composition and the production process matched with the chemical composition of the present application can improve the strength of the product while ensuring the deep drawing performance index, and can expand the application range of BH steel, especially in the application of automobile outer plate.

[0028] (3) The present application has three technical advantages of ultra-fast solidification, grain refinement and near-zero oxide skin by directly producing thin strips from molten metal through a twin-roll strip casting process, which eliminates the need for reheating, multiple hot rolling and pickling steps, solves the problems of complex production process, multiple manufacturing processes and high energy consumption, and significantly reduces production costs. In addition, the yield platform can be eliminated after cold rolling and leveling, reducing the risk of natural aging and surface defects caused by aging during subsequent storage or stamping.

[0029] (4) The present application makes the production process of bake-hardening steel more economical and efficient, reduces waste and energy consumption during reheating, hot rolling and pickling compared to traditional processes, has less impact on the environment, and helps improve the sustainability of the production process. Due to the more efficient production method, different specifications and grades of bake-hardening steel plates can be produced more flexibly to meet the needs of different customers and increase market share. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 ODF diagram and microstructure diagram of the annealed plate in Example 1; Figure 2 ODF diagram and microstructure diagram of the annealed plate in Example 2. DETAILED DESCRIPTION

[0031] To make the purpose, technical scheme and advantages of the present application clearer, the technical scheme in the embodiments of the present application will be described clearly and completely below in conjunction with the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all. Example 1

[0032] The present embodiment provides a short-process long-aging bake-hardening steel and a production method thereof. The chemical composition of the bake-hardening steel and the mass percentage thereof are as follows: C: 0.0015%, Si: 0.025%, Mn: 0.3, S: 0.004%, Als: 0.030%, Nb: 0.01%, B: 0.0006%, Cu: 0.07%, N: 0.003%, solid solution C: 0.001%, and the balance being iron and unavoidable impurities.

[0033] The production method includes steelmaking, twin-roll strip casting, cold rolling, annealing and leveling processes, specifically: The molten steel that meets the smelting standards is conveyed to a pair of counter-rotating casting rolls, the molten steel is cooled and solidified on the casting surface of the pair of casting rolls, and a 5mm cast strip is formed by passing through the nip between the counter-rotating rolls. After one-pass hot rolling and leveling, a 4mm hot-rolled coil is obtained at 670℃.

[0034] The obtained hot-rolled coil is cold-rolled to obtain a cold-hard coil, and the thickness of the cold-hard coil is 0.8mm.

[0035] The obtained cold hard coil is continuously annealed, the soaking temperature is controlled at 840℃, the holding time is 100s, slow cooling is performed to 710℃, then fast cooling is performed to 400℃, then overaging treatment is performed, the overaging temperature is controlled at 350℃, and the overaging time is 260s, to obtain a continuous annealing coil.

[0036] The obtained continuous annealing coil is subjected to stretch leveling, the elongation of the steel coil is 1.5%, The obtained bake hardening steel has a steel plate thickness of 0.8mm, a yield strength of 240MPa, a tensile strength of 345MPa, an elongation A 80 of 46%, a bake hardening value BH2 of 50MPa, an r value of 2.05, a work hardening index n of 0.2, and no natural aging occurs after 6 months of summer storage. The ODF diagram and the microstructure are shown in Figure 1 Example 2-6

[0037] The chemical composition of the bake hardening steel in Example 2-6 is the same as that in Example 1, and the mass percentage of each component is shown in Table 1. The production method of the bake hardening steel in Example 2-6 is basically the same as that in Example 1, and the process parameters of each step are shown in Table 2. The performance test results of the obtained bake hardening steel are shown in Table 3.

[0038] Table 1: Chemical composition of bake hardening steel in each example (mass percentage, %)

[0039] Table 2: Process parameters of each step in the production method in each example

[0040] Table 3: Performance test results of the bake hardening steel obtained in each example ​

Claims

1. A short-cycle long-aging period ultra-low carbon bake hardening steel, characterized in that, The chemical composition of the bake hardening steel and the mass percentage thereof are as follows: C: 0.001-0.0035%, Si: ≤0.03%, Mn: 0.1-0.6%, S: ≤0.005%, Als: 0.020-0.065%, Nb: 0.005-0.02%, B: 0.0003-0.001%, Cu: 0.05-0.15%, N: ≤0.0035%, solid solution C: 0.0008-0.0015%, and the balance being iron and inevitable impurities.

2. The short flow long aging period ultra-low carbon bake hardening steel according to claim 1, characterized in that, The steel sheet of the bake hardening steel has a thickness of 0.5 to 1.0 mm, a yield strength of 235 to 280 MPa, a tensile strength of 320 to 400 MPa, and an elongation A 80 ≥ 38%, a bake hardening value BH2 ≥ 45 MPa, an r value ≥ 2.0, a work hardening exponent n ≥ 0.2, and no natural aging for 6 months in summer.

3. A method for producing a short-cycle long-aging period ultra-low carbon bake-hardenable steel according to claim 1 or 2, characterized in that, The production method comprises smelting, double-roller thin strip casting, cold rolling, continuous annealing and flattening processes.

4. The method of producing a short flow long aging period ultra-low carbon bake hardening steel according to claim 3, characterized in that, The double-roller thin strip casting process comprises the following steps: conveying the qualified molten steel to a pair of counter-rotating casting rollers, cooling and solidifying the molten steel on the casting surfaces of the pair of casting rollers, and forming a 3-6 mm cast strip by passing through the nip between the counter-rotating double rollers, and winding the cast strip after one-pass hot rolling and flattening at 650-730 DEG C to obtain a hot-rolled coil with a thickness of 2-5 mm.

5. The method of producing a short flow long aging period ultra-low carbon bake hardening steel according to claim 3, characterized in that, The cold rolling process comprises the following steps: naturally cooling the hot-rolled coil, and obtaining a cold hard coil after cold rolling, wherein the thickness of the cold hard coil is 0.5-1.0 mm, and the reduction rate is 75%-85%.

6. The method of producing a short flow long aging period ultra-low carbon bake hardening steel according to claim 3, characterized in that, The continuous annealing process comprises the following steps: controlling the soaking temperature to be 820-860 DEG C, and the soaking time to be 60-180 s, slowly cooling to 670-720 DEG C, rapidly cooling to 350-400 DEG C, and then overaging treatment, wherein the overaging temperature is controlled to be 300-350 DEG C, and the overaging time is 180-360 s.

7. The method of producing a short flow long aging period ultra-low carbon bake hardening steel according to claim 3, characterized in that, The flattening process comprises the following steps: controlling the elongation of the steel coil during flattening to be 1.0-2.2%.

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