Preparation method of high-surface-quality steel plate for hot forming of pure zinc coating

Abstract

The invention discloses a preparation method of a high-surface-quality steel plate for hot forming of a pure zinc coating. The preparation method comprises the procedures of steelmaking, continuous casting, hot rolling, acid pickling, cold rolling, annealing, coating and finishing. In the steelmaking process, LF + RH double refining smelting is adopted, calcium treatment is carried out after LF smelting is finished, the calcium content is controlled to be 0.004-0.005%, and the continuous casting superheat degree is controlled to be 5-10 DEG C. According to the method, the stable performance of the steel plate after hot stamping forming is guaranteed, meanwhile, the surface of the obtained steel plate is free of color difference, the number of free zinc residues is smaller than or equal to 20 / m < 2 >, the slag inclusion index is smaller than or equal to 0.01, the excellent surface effect is achieved, and the requirements of high-end users can be met.

Description

Technical Field

[0001] The invention relates to a high-surface 1500Mpa pure zinc-plated steel plate for hot forming and a preparation method thereof, which is mainly used for preparing hot stamping high-strength components with a strength level of 1500MPa for automobiles. Background Art

[0002] With the rapid development of the automobile industry, countries around the world have increasingly stringent requirements for automobile safety, energy conservation and emissions. The use of ultra-high-strength steel can not only reduce automobile weight, achieve energy conservation and emission reduction, but also improve automobile safety.

[0003] Generally, high-strength steel has more alloys added, and the impurities brought by the alloys also contaminate the molten steel, which can easily cause inclusions and slag inclusions after cold rolling. In addition, pure zinc hot-formed steel sheets are prone to surface color differences and zinc slag surface defects. After the user's hot forming process, the defects are easily magnified, causing losses to the user. There is currently no definite solution to the above surface problems, how to control them at a low level. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a method for preparing a pure zinc-coated hot-forming steel plate with high surface quality, wherein the surface slag inclusion index after cold rolling is ≤0.01.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is: A method for preparing a high-surface-quality pure zinc-coated hot-forming steel plate comprises steelmaking, continuous casting, hot rolling, pickling, cold rolling, annealing, coating and finishing processes.

[0006] Furthermore, the steelmaking process of the present invention is as follows: before smelting in the converter, the molten iron can be pretreated according to actual needs; after smelting in the converter, alloying is performed in the LF, and the molten steel is calcium treated, and the calcium content is controlled at 0.004-0.005%, and shallow treatment is performed in the RH process; the continuous casting superheat is controlled at 5-10°C, and the continuous casting billet is hot loaded into the heating furnace after being produced.

[0007] Furthermore, the hot rolling process of the present invention has the following characteristics: furnace temperature: 1300-1400°C, the opening of the vertical rolls is ≤5mm, the centering is ≤5mm, the frame is ≤3mm, the coarse roll is dephosphorized in 2 passes, the dephosphorization speed is 0.6-0.8m / s, and the roller mileage is controlled to be 3000-4000m.

[0008] Furthermore, in the pickling process of the present invention, the roller mileage is required to be 2000-5000m.

[0009] Furthermore, in the cold rolling process of the present invention, the cold rolling reduction rate is ≥50%, and the thickness of the steel strip after cold rolling is in the range of 0.5-3.0 mm.

[0010] Furthermore, the annealing process of the present invention has: annealing furnace dew point: -10 to -40°C; H content: 1-3%.

[0011] Furthermore, the coating process of the present invention is as follows: before the steel strip enters the aluminum liquid, the temperature is reduced to 430°C-440°C, the zinc liquid temperature is 430-440°C; the air knife pressure is 50-100mbar, and the air knife distance is 8-12mm.

[0012] Furthermore, in the finishing process of the present invention, the finishing elongation is 0.6-0.9.

[0013] The hot-forming galvanized steel sheet prepared by the method of the present invention has T[O]≤0.0005%, a surface slag inclusion index≤0.01 after cold rolling, and no color difference on the surface, which can meet higher product requirements.

[0014] The inventive principle of the technical solution of the present invention is: To solve the problems of slag inclusion and impurity, the present invention starts with the oxygen content of molten steel, makes full use of double refining and calcium treatment processes, controls the oxygen content at a relatively low level, and ensures that the ingot has no crack defects through extremely low superheat pouring, thereby minimizing the problems of slag inclusion and impurity after cold rolling.

[0015] In view of the surface color difference and zinc slag defects after cold rolling, we should start from the atmosphere control in the annealing furnace, reasonably control the H content, and reasonably use the skin-clearing elongation to eliminate the problem of surface color difference; and solve the zinc slag defect problem on the surface of pure zinc by starting with the zinc liquid and other parameters.

[0016] Different from other hot forming products, the present invention optimizes the surface state of the rollers by controlling the roller mileage, thereby reducing the probability of scratch defects on the steel plate surface; improves the color difference problem on the steel plate surface by controlling the atmosphere in the annealing furnace; reduces zinc slag defects by optimizing the zinc liquid temperature and air knife parameters; and optimizes the skin-finishing elongation to reduce the risk of surface defects.

[0017] By using a higher calcium content after LF, the inclusions in the molten steel are completely denatured. After light RH treatment, the cleanliness of the molten steel is improved, and the total oxygen in the molten steel is guaranteed to be within 0.0005%. At the same time, by controlling the lower superheat in the continuous casting process, the problems of dendritic segregation and edge cracks inside the ingot are improved, and the uniformity of the ingot composition is improved.

[0018] The hot rolling process uses a higher furnace temperature to ensure the uniformity of the ingot structure, and then optimizes the parameters of the opening degree, middle and frame of the large and small vertical rolls to ensure the verticality of the rolls; in addition, the coarse rolls are dephosphorized in two passes, the dephosphorization speed is optimized and the rolling roll mileage is 3000-4000m to control the iron oxide scale.

[0019] The beneficial effects of adopting the above technical solution are: The present invention controls the total oxygen content within 0.0005% through steelmaking, and ensures the uniformity of the ingot structure by continuous casting with low superheat pouring. The surface slag inclusion index is controlled within 0.01 after cold rolling. The hot rolling reduces the introduction of iron oxide scale by optimizing the parameters of the vertical rolls, the mileage of the rolls and the dephosphorization parameters. The optimization of the cold rolling annealing furnace atmosphere improves the surface color difference problem. The optimization and improvement of the skin-finishing elongation and the zinc liquid temperature air knife parameters avoid the generation of zinc slag defects, and finally obtains a steel plate with high surface quality to meet user needs. DETAILED DESCRIPTION

[0020] The present invention will be further described in detail below in conjunction with specific embodiments. Examples 1-8

[0021] A high surface quality pure zinc-coated hot forming steel sheet, the chemical composition and weight percentage of the galvanized substrate are shown in Table 1.

[0022] Table 1 Chemical composition of galvanized substrates in various embodiments (wt%)

[0023] The preparation method of the steel plate of each embodiment includes steelmaking, continuous casting, hot rolling, pickling, cold rolling, annealing, coating and skin-passing processes; the key process control is as follows: (1) Steelmaking process: LF+RH double refining smelting is adopted. Calcium treatment is carried out after LF smelting. The calcium content is controlled at 0.004-0.005%. The superheat of the continuous casting tundish is controlled at 5-10℃. After the continuous casting billet is produced, it is hot-charged into the heating furnace.

[0024] (2) Hot rolling process: furnace temperature: 1300-1400℃, vertical roll opening ≤5mm, centering ≤5mm, frame ≤3mm; coarse rolls use 2 passes for dephosphorization, the dephosphorization speed is 0.6-0.8m / s, and the roller mileage is controlled to be 3000-4000m.

[0025] The steelmaking and hot rolling process parameters of the steel plates in each embodiment are shown in Table 2.

[0026] Table 2 Parameters of steelmaking and hot rolling process in various embodiments

[0027] (3) Pickling process: The roller mileage requirement is 2000-5000m.

[0028] (4) Cold rolling process: The cold rolling reduction rate is ≥50%, and the thickness of the steel strip after cold rolling is in the range of 0.5-3.0 mm.

[0029] (5) Annealing process: Annealing furnace dew point: -10 to -40°C; H content: 1-3%.

[0030] (6) Coating process: Before the strip enters the aluminum liquid, the temperature is reduced to 430℃-440℃, the zinc liquid temperature is 430-440℃; the air knife pressure is 50-100mbar, and the air knife distance is 8-12mm.

[0031] (7) Finishing process: Finishing elongation: 0.6-0.9.

[0032] The process parameters of pickling, cold rolling, annealing, coating and finishing of the steel plates in each embodiment are shown in Table 3.

[0033] Table 3 Parameters of each steel plate pickling, cold rolling, annealing, coating, and skin-passing process

[0034] The mechanical properties and surface conditions of the steel plates of each embodiment before and after hot forming are shown in Table 4.

[0035] Table 4 Performance and surface condition of steel plates before and after hot forming in various examples

[0036] As shown in Table 4, the method of the present invention ensures the stable performance of the steel plate after hot stamping, and the surface has no color difference and the number of free zinc slag is ≤20 pieces / m 2 , slag inclusion index ≤0.01.

[0037] The above embodiments are only used to illustrate rather than limit the technical solutions of the present invention. Although the present invention is described in detail with reference to the above embodiments, those skilled in the art should understand that the present invention can still be modified or replaced by equivalents. Any modification or partial replacement that does not depart from the spirit and scope of the present invention should be included in the scope of the claims of the present invention.

Claims

1. A method for preparing a high-surface-quality pure zinc-coated hot-forming steel sheet, comprising steelmaking, continuous casting, hot rolling, pickling, cold rolling, annealing, coating and skin-finishing steps, characterized in that: The steelmaking process adopts LF+RH double refining smelting, and calcium treatment is performed after LF smelting, the calcium content is controlled at 0.004-0.005%, and the continuous casting superheat is controlled at 5-10°C.

2. The method for preparing a high-surface-quality pure zinc-coated hot-forming steel sheet according to claim 1, characterized in that: The hot rolling process: furnace temperature: 1300-1400°C, large and small vertical roll opening ≤5mm, centering ≤5mm, frame ≤3mm; coarse rolls use 2 passes for dephosphorization, the dephosphorization speed is 0.6-0.8m / s, and the roller mileage is controlled to be 3000-4000m.

3. The method for preparing a high-surface-quality pure zinc-coated hot-forming steel sheet according to claim 1, characterized in that: The pickling process: the roller mileage requirement is 2000-5000m.

4. The method for preparing a high-surface-quality pure zinc-coated hot-forming steel sheet according to claim 1, characterized in that: The cold rolling process: the cold rolling reduction rate is ≥50%, and the thickness of the steel strip after cold rolling is in the range of 0.5-3.0 mm.

5. The method for preparing a high-surface-quality pure zinc-coated hot-forming steel sheet according to claim 1, characterized in that: The annealing process: annealing furnace dew point: -10 to -40°C; H content: 1-3%.

6. The method for preparing a high-surface-quality pure zinc-coated hot-forming steel sheet according to claim 1, characterized in that: The coating process: before the steel strip enters the aluminum liquid, the temperature is reduced to 430°C to 440°C, the zinc liquid temperature is 430-440°C; the air knife pressure is 50-100mbar, and the air knife distance is 8-12mm.

7. The method for preparing a high-surface-quality pure zinc-coated hot-forming steel sheet according to claim 1, characterized in that: The finishing process: finishing elongation: 0.6-0.

9.

8. The method for preparing a high-surface-quality pure zinc-coated hot-forming steel sheet according to claim 1, characterized in that: The method can ensure that the performance of the steel plate is stable after hot stamping, and the surface of the obtained steel plate has no color difference and the number of free zinc slag is ≤20 pieces / m 2 , slag inclusion index ≤0.01.

Images