Low-cost hot-dip galvanized high-strength steel for new energy automobile and production method thereof

Abstract

The invention discloses low-cost hot-dip galvanized high-strength steel for a new energy automobile and a production method thereof, and belongs to the technical field of automobile steel. The high-strength steel comprises the following chemical components in percentage by mass: 0.02 to 0.06 percent of C, 0.06 to 0.15 percent of Si, 0.2 to 0.5 percent of Mn, less than or equal to 0.015 percent of P, less than or equal to 0.013 percent of S, 0.02 to 0.05 percent of Alt, 0.01 to 0.03 percent of Nb, 0.02 to 0.04 percent of Ti, 0.002 to 0.003 percent of B, 0.005 to 0.015 percent of N and the balance of Fe and inevitable impurities. The production method comprises the procedures of steelmaking, hot rolling, acid rolling and galvanizing. The bake hardening value BH2 of the high-strength steel is 50-65 MPa, the geometric must dislocation GND density of a steel-based structure under a 500-time field of view is 5 * 10 < 5 >-10 < 10 > / m < 2 >, and the high-strength steel has an excellent bake hardening effect and high plasticity.

Description

Technical Field

[0001] The present invention belongs to the technical field of automotive steel, and relates to a low-cost hot-dip galvanized high-strength steel for new energy vehicles and a production method thereof. Background Art

[0002] With the continuous development of automotive lightweighting, automotive steel has gradually developed towards thinner walls and higher strength. Especially for complex-shaped outer panels, materials are required to have high formability, high strength, and good dent resistance. Currently, to address this problem, Chinese Patent CN113604637B proposes a production method for a hot-dip galvanized sheet with a tensile strength of 500 MPa. However, it has a relatively high Mn content and is prone to element enrichment on the surface layer during the galvanizing annealing process.

[0003] Research shows that the mesoscopic scale gradient heterogeneous lamellar structure promotes the gradient distribution of GND in the material along the radial direction during deformation. This structure can promote the gradient distribution of GND, thereby inducing significant strain and greatly improving the mechanical properties of the material. The patent application of Chinese Patent CN2023100617388 discloses a manufacturing method for a high-strength steel with a laminated heterogeneous structure. The chemical composition of the steel plate is: C: 0.35 - 0.45%, Mn: 9.5 - 10.5%, Al: 1.5 - 2.5%, V: 0.45 - 0.55%, Si: 0.5 - 1.5%, Mo: 0 - 0.5%. Although the provided steel plate has excellent mechanical properties, its alloy content is relatively high, especially the Mn and Al contents are relatively high, belonging to the medium manganese steel composition system. The continuous casting process is difficult, and the problems of high deformation resistance and poor weldability during the rolling process are also relatively prominent, making it not suitable for large-scale popularization and application.

[0004] How to develop more excellent automotive steel products using the mesoscopic scale gradient heterogeneous lamellar structure during the production process has also become one of the important problems faced in the development of automotive steel currently. Summary of the Invention

[0005] To solve the above problems, the present invention provides a low-cost hot-dip galvanized high-strength steel for new energy vehicles and a production method thereof. The obtained high-strength steel has excellent bake hardening effect and high strength and plasticity.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] A low-cost hot-dip galvanized high-strength steel for new energy vehicles, the chemical composition and mass percentage content thereof are: C: 0.02 - 0.06%, Si: 0.06 - 0.15%, Mn: 0.2 - 0.5%, P ≤ 0.015%, S ≤ 0.013%, Alt: 0.02 - 0.05%, Nb: 0.01 - 0.03%, Ti: 0.02 - 0.04%, B: 0.002 - 0.003%, N: 0.005 - 0.015%, and the balance is Fe and inevitable impurities;

[0008] Among them, the mass percentage contents of C, Nb, Al, B, and N satisfy: a = C - 0.13Nb, 0.016% ≤ a ≤ 0.057%; b = 10Al + 10B - 25.6N, 0 ≤ b ≤ 0.274%.

[0009] Furthermore, the thickness of the high-strength steel is 0.5 - 1.5 mm, the yield strength is 250 - 350 MPa, the tensile strength is ≥ 480 MPa, the elongation after fracture A 50mm ≥ 36%, the bake hardening value BH2 is 50 - 65 MPa, and the geometric necessary dislocation GND density of the steel matrix structure is 5 × 10 5 ~10 10 pieces / m 2 .

[0010] The production method of the above-mentioned low-cost hot-dip galvanized high-strength steel for new energy vehicles includes steelmaking, hot rolling, pickling and rolling, and galvanizing processes;

[0011] Hot rolling process: The finishing mill inlet temperature is 1000 - 1050 °C, the finishing temperature is 850 - 910 °C, and the coiling temperature is 500 - 700 °C;

[0012] Pickling and rolling process: The cold rolling reduction rate is 75 - 90%;

[0013] Galvanizing process: The steel strip is rapidly heated from room temperature to the soaking temperature, with an average heating rate of 10 - 30 °C / s during this period, the soaking temperature is 600 - 700 °C, the soaking time is 50 - 180 s, the average cooling rate is 3 - 15 °C / s, and the rapid cooling temperature is 440 - 500 °C.

[0014] Furthermore, in the hot rolling process, the heating furnace heating temperature is 1200 - 1300 °C, and the soaking time is 35 - 55 min; 3 + 3 passes of hot rolling are adopted, and after being taken off the line, the hot rolled coil is placed in a single layer in a ventilated place for ≥ 72 h.

[0015] Furthermore, in the pickling and rolling process, the tension leveling elongation is 0.9 - 2.3%.

[0016] Further, in the galvanizing process, the temperature of the strip entering the zinc pot is 460 - 480°C, the temperature of the zinc bath is 445 - 460°C, the dew point in the annealing furnace is -21 - -45°C, the skin pass elongation is 1.0 - 1.5%, and a strip with a thickness of 0.5 - 1.5 mm is obtained.

[0017] The beneficial effects of adopting the above technical solution are as follows: The hot rolling process promotes the formation of a finer matrix structure on the surface layer of the base material. Coupled with the large reduction rate in cold rolling, it provides a higher nucleation site for the recrystallization of the surface layer structure, thereby increasing the recrystallization rate of the surface layer structure. Due to the higher surface energy of the surface layer and the formation of more nucleation sites during hot rolling and cold rolling, it promotes the recrystallization and growth of the surface layer grains of the strip structure. The decrease in the recrystallization rate from the surface layer to the core and the difference in grain size promote the formation of a heterogeneous structure. During the baking process, the geometrically necessary dislocations (GND) in its heterogeneous structure will endow the material with more dislocation pinning ability, thus enabling the material to have a higher bake hardening effect.

[0018] The high-strength steel obtained by the present invention has excellent bake hardening effect and high strength and plasticity, with a yield strength of 250 - 350 MPa, a tensile strength of ≥480 MPa, and an elongation after fracture A 50mm ≥36%, a bake hardening value BH2 of 50 - 65 MPa, and the density of geometrically necessary dislocations (GND) in the steel matrix structure is 5×10 5 ~10 10 per m 2 . Description of the Drawings

[0019] Figure 1 is the micrograph of the high-strength steel in Example 1 under an optical microscope;

[0020] Figure 2 is the GND map of the structure of the high-strength steel in Example 1. Detailed Embodiments

[0021] Examples 1 - 6

[0022] A low-cost hot-dip galvanized high-strength steel for new energy vehicles, whose chemical composition is shown in Table 1;

[0023] Table 1. Chemical composition (wt / %) of the high-strength steel in each example

[0024]

[0025] In Table 1, the balance is Fe and unavoidable impurities.

[0026] The production method of the above low-cost hot-dip galvanized high-strength steel for new energy vehicles includes steelmaking, hot rolling, pickling and cold rolling, and galvanizing processes, which are specifically as follows:

[0027] (1) Steelmaking process: A slab is produced after smelting and casting according to the chemical composition described in Table 1.

[0028] (2) Hot rolling process: The heating temperature in the heating furnace is 1200 - 1300 °C, with a soaking time of 35 - 55 min; 3 + 3 passes of hot rolling are adopted. The entry temperature of the finishing mill is controlled at 1000 - 1050 °C, the finishing temperature is between 850 - 910 °C, the coiling temperature is 500 - 700 °C, and the hot coil taken off the line is placed singly in a ventilated place for slow cooling for ≥ 72 h.

[0029] The entry temperature of the finishing mill is set at 1000 - 1050 °C, and the finishing temperature is at 850 - 900 °C because plastic deformation near the Ar3 temperature is likely to form induced phase transformation, resulting in grain refinement. At the same time, due to the relatively low temperature on the surface layer of the steel plate during the hot rolling process, it is conducive to grain refinement. Also, the coiling temperature is set at 500 - 700 °C, which can promote the precipitation strengthening and precipitation hardening effects of microalloying elements, further refine grains, and improve the strength and plasticity of the material.

[0030] The parameters of the hot rolling process for each example are shown in Table 2.

[0031] Table 2. Parameters of the hot rolling process for each example

[0032]

[0033] (3) Pickling and cold rolling process: The elongation of the tension leveling is 0.9 - 2.3%, and the cold rolling reduction is 75 - 90%.

[0034] (4) Galvanizing process: The steel strip is rapidly heated from room temperature to the soaking temperature, with an average heating rate of 10 - 30 °C / s, a soaking temperature of 600 - 700 °C, a soaking time of 50 - 180 s, an average cooling rate of 3 - 15 °C / s, a rapid cooling temperature of 440 - 500 °C, the temperature of the strip entering the zinc pot is 460 - 480 °C, the temperature of the zinc bath is 445 - 460 °C, the dew point in the annealing furnace is -21 - -45 °C, and the elongation of the skin pass is 1.0 - 1.5%. Using a high heating rate increases the recrystallization temperature of the material, and a lower cooling rate is beneficial to the precipitation of microalloy carbide, further refining grains and improving the strength of the material.

[0035] The parameters of the pickling and cold rolling and galvanizing processes for each example are shown in Table 3.

[0036] Table 3. Parameters of the pickling and cold rolling and galvanizing processes for each example

[0037]

[0038] The tissue composition and mechanical property data of the high-strength steel for each example are shown in Table 4.

[0039] Table 4. Properties of the galvanized high-strength steel for each example

[0040]

[0041]

[0042] It can be seen from Figure 1 that the microstructure of the high-strength steel in Example 1 is mainly layer-banded distributed and is more obvious in the core. The decrease in the recrystallization rate and the difference in grain size from the surface layer to the core promote the formation of heterogeneous structures.

[0043] Figure 2 The GND density in the dark-colored parts is relatively high, and the density in the light-colored areas is relatively low, which helps to improve the BH value after forming and baking. Combining Figure 2 with the GND distribution map and Table 4, it can be seen that the grain orientation difference and the GND distribution further realize the effective matching of its tissue properties from the microstructure. During the baking process, the geometrically necessary dislocations GND in its heterogeneous structure will endow the material with more dislocation pinning ability, so that the material has a higher baking hardening effect, and thus further realizes the excellent strength and plasticity of the material.

[0044] The microstructures of the high-strength steels in the remaining examples are similar and will not be shown one by one.

Claims

1. A low-cost hot-dip galvanized high-strength steel for new energy vehicles, characterized in that, The chemical composition and mass percentage of the high-strength steel are as follows: C: 0.02 - 0.06%, Si: 0.06 - 0.15%, Mn: 0.2 - 0.5%, P ≤ 0.015%, S ≤ 0.013%, Alt: 0.02 - 0.05%, Nb: 0.01 - 0.03%, Ti: 0.02 - 0.04%, B: 0.002 - 0.003%, N: 0.005 - 0.015%, and the balance is Fe and inevitable impurities; Among them, the mass percentages of C, Nb, Al, B, and N satisfy: a = C - 0.13Nb, 0.016% ≤ a ≤ 0.057%; b = 10Al + 10B - 25.6N, 0 ≤ b ≤ 0.274%.

2. The low-cost hot-dip galvanized high-strength steel for new energy vehicles according to claim 2, wherein The high-strength steel has a thickness of 0.5 to 1.5 mm, a yield strength of 250 to 350 MPa, a tensile strength of ≥ 480 MPa, and an elongation after fracture A 50mm ≥ 36%, the bake hardening value BH2 is 50 to 65 MPa, and the geometric necessary dislocation GND density of the steel matrix structure is 5 × 10 5 ~10 10 per m 2 .

3. The production method of low-cost hot-dip galvanized high-strength steel for new energy vehicles according to claim 1 or 2, characterized in that, It includes steelmaking, hot rolling, pickling and cold rolling, and galvanizing processes; Hot rolling process: The entry temperature of finish rolling is 1000 - 1050 °C, the final rolling temperature is 850 - 910 °C, and the coiling temperature is 500 - 700 °C; Pickling and cold rolling process: The cold rolling reduction rate is 75 - 90%; Galvanizing process: The steel strip is rapidly heated from room temperature to the soaking temperature, with an average heating rate of 10 - 30 °C / s during this period, the soaking temperature is 600 - 700 °C, the soaking time is 50 - 180 s, the average cooling rate is 3 - 15 °C / s, and the rapid cooling temperature is 440 - 500 °C.

4. The production method of low-cost hot-dip galvanized high-strength steel for new energy vehicles according to claim 3, characterized in that, In the hot rolling process described above, the heating temperature of the heating furnace is 1200 - 1300 °C, and the soaking time is 35 - 55 min.

5. The production method of low-cost hot-dip galvanized high-strength steel for new energy vehicles according to claim 4, characterized in that In the hot rolling process described above, 3 + 3 passes of hot rolling are adopted, and after being taken off the production line, the hot rolled coil is placed singly in a ventilated place for ≥ 72 h.

6. The production method of low-cost hot-dip galvanized high-strength steel for new energy vehicles according to claim 5, characterized in that, In the pickling and cold rolling process described above, the tension leveling elongation is 0.9 - 2.3%.

7. The production method of low-cost hot-dip galvanized high-strength steel for new energy vehicles according to any one of claims 1-6, characterized in that, In the galvanizing process described above, the strip entry temperature into the zinc pot is 460 - 480 °C, the zinc bath temperature is 445 - 460 °C, and the dew point in the annealing furnace is -21 - -45 °C.

8. The production method of low-cost hot-dip galvanized high-strength steel for new energy vehicles according to claim 7, characterized in that, In the galvanizing process described above, the skin pass elongation is 1.0 - 1.5%.

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