Cold-rolled high-strength automobile sheet with excellent phosphating performance and preparation method thereof

Abstract

The invention relates to a cold-rolled high-strength automobile sheet with excellent phosphating performance and a preparation method thereof, and belongs to the technical field of metallogy and metal processes. The automobile sheet comprises the following chemical components in percentage by weight: 0.2%-0.3% of C, 1.5%-2.5% of Si, 2.5%-3.5% of Mn, less than or equal to 0.01% of P, less than or equal to 0.003% of S, 0.2%-0.5% of Cr, 0.02%-0.1% of Ti, less than or equal to 0.1% of N and the balance of Fe and inevitable impurity elements. The preparation method of the automobile sheet comprises the working procedures of smelting, casting, hot rolling, acid rolling and continuous annealing. By regulating and controlling the hot rolling coiling temperature and the steel plate structure, the external oxidation process of alloy elements on the surface of the high-strength cold-rolled automobile steel in the continuous annealing process is inhibited, the one-step heat treatment process is adopted, and the developed cold-rolled high-strength automobile plate is low in production cost and short in production period and has excellent phosphorization performance.

Description

Technical Field

[0001] The invention relates to a cold-rolled high-strength automobile plate with excellent phosphating performance and a preparation method thereof, and belongs to the technical field of metallurgy and metal processing. Background Art

[0002] As one of the important directions for the development of the automotive industry, high-strength steel plays a key role in the lightweight of automobiles. High-strength steel has high strength and good plasticity, which can reduce the weight of automobiles while improving their safety and fuel economy. High-strength steel for automobiles is divided into two categories: traditional high-strength steel and advanced high-strength steel. Traditional high-strength steel includes bake hardening (BH) steel, phosphorus-containing high-strength steel, interstitial-free (IF) steel and high-strength low-alloy (HSLA) steel. The plasticity of traditional high-strength steel decreases significantly with the increase of strength, and the strength-plasticity product is relatively small, which can no longer meet the continuous development of the automotive industry. Compared with traditional high-strength steel, the strength-plasticity product of advanced high-strength steel is significantly improved, and it has good formability and collision energy absorption effect. More alloy elements are added to advanced high-strength steel. The surface enrichment and oxidation of alloy elements in advanced high-strength steel will cause a series of problems such as poor phosphating coverage, coarse phosphating crystal size, and poor adhesion. At present, the preparation methods of cold-rolled ultra-high strength automobile plates at home and abroad generally adopt: (1) two-step Q&P heat treatment process: remove the outer oxide layer of alloy elements through the pickling process, and (2) increase the dew point temperature in the annealing furnace: make the alloy elements on the surface of high-strength automobile steel tend to undergo internal oxidation, avoid the selective oxidation of alloy elements affecting the phosphating quality. Therefore, the existing cold-rolled ultra-high strength steel has high production costs and high requirements for production equipment. Not only does it need to be equipped with corresponding humidification devices, but the annealing furnace also needs good sealing.

[0003] However, the method of increasing the dew point temperature to promote the internal oxidation of alloy elements is complicated, and the dew point temperature, heating rate, annealing time, annealing temperature, etc. in the annealing furnace need to be strictly controlled in relation to the alloy element content and material properties. If the dew point temperature is too high during the cooling process, iron oxidation is likely to occur on the surface, affecting the surface quality of the product. In addition, most of the domestic continuous annealing furnaces do not have a humidification function, so it is impossible to control the selective oxidation process of alloy elements on the surface of high-strength automotive steel by regulating the atmosphere in the annealing furnace, resulting in poor phosphating performance of cold-rolled high-strength automotive plates. Summary of the invention

[0004] The purpose of the present invention is to provide a cold-rolled high-strength automobile plate with excellent phosphating performance and a preparation method thereof. By regulating the hot rolling coiling temperature and the steel plate microstructure, the external oxidation process of alloy elements on the surface of high-strength cold-rolled automobile steel during the continuous annealing process is suppressed, and a one-step heat treatment process is adopted. The developed cold-rolled high-strength automobile plate has low production cost and short production cycle, and at the same time has excellent phosphating performance.

[0005] In order to achieve the above object, the technical solution of the present invention is as follows:

[0006] On one hand, the present invention provides a cold-rolled high-strength automobile sheet with excellent phosphating performance. The chemical composition of the cold-rolled high-strength automobile sheet is calculated by weight percentage: C 0.2%-0.3%, Si 1.5%-2.5%, Mn: 2.5%-3.5%, P≤0.01%, S≤0.003%, Cr 0.2%-0.5%, Ti 0.02%-0.1%, N≤0.1%, and the rest is Fe and unavoidable impurity elements.

[0007] In the above technical solution, further, the tensile strength of the cold-rolled high-strength automobile sheet is ≥980MP, the elongation is ≥14%, and the internal oxidation depth is 5-20μm.

[0008] Another aspect of the present invention provides a method for preparing the above-mentioned cold-rolled high-strength automobile sheet with excellent phosphating performance, comprising smelting, casting, hot rolling, pickling, and continuous annealing steps, and the specific control parameters are as follows:

[0009] Smelting and casting: The chemical composition of the raw steel is calculated by weight percentage: C 0.2%~0.3%, Si 1.5%~2.5%, Mn: 2.5%~3.5%, P≤0.01%, S≤0.003%, Cr 0.2%~0.5%, Ti 0.02%~0.1%, N≤0.1%, and the rest are Fe and unavoidable impurity elements. After smelting and casting, it is made into slabs;

[0010] Hot rolling: Coil the strip after hot rolling. U-type coiling is adopted. The temperature of the 60m section of the strip head and tail is 30-80℃ higher than the target coiling temperature. The target coiling temperature is controlled to be 580-650℃. After coiling, the strip is kept warm and slowly cooled.

[0011] Pickling: unwind the hot rolled coil, pickle it and cold roll it to obtain hardened strip steel;

[0012] Continuous annealing process: the hardened strip is subjected to continuous annealing treatment, the soaking section temperature is 800-880°C, the soaking time is 200-260s, the slow cooling section temperature is 700-780°C, and then rapidly cooled to the over-aging temperature, the over-aging section temperature is 350-420°C, the over-aging time is 1200-1800s, and the cold-rolled high-strength automobile sheet is obtained after cooling.

[0013] In the above technical solution, further, the starting rolling temperature of the hot rolling rough rolling is 1200-1250°C, the final rolling temperature is 1100-1150°C, the starting rolling temperature of the fine rolling is 950-1000°C, and the final rolling temperature is 850-900°C.

[0014] In the above technical solution, further, the pickling speed is ≤100m / min, and the cold rolling reduction is 40% to 80%.

[0015] In the above technical solution, further, the cooling rate of the slow cooling section is 2-5°C / s; and the cooling rate of the rapid cooling section is ≥30°C / s.

[0016] The beneficial effects of the present invention are:

[0017] The present invention suppresses the external oxidation process of alloy elements on the surface of high-strength cold-rolled automobile steel during the continuous annealing process by regulating the hot rolling coiling temperature and the steel plate structure. The cold-rolled high-strength automobile plate developed by the one-step heat treatment process has low production cost, short production cycle and excellent phosphating performance. Compared with the existing method of increasing the dew point temperature in the annealing furnace to convert external oxidation to internal oxidation, the cold-rolled ultra-high strength steel provided by the present invention has low production cost and does not require a corresponding humidification device during the heat treatment process.

[0018] The present invention adopts a U-shaped coiling temperature design to avoid the problems of performance fluctuation and excessive difference in the length direction of the hot-rolled strip caused by uneven heat dissipation after the hot-rolled strip is coiled; the hot-rolled coil adopts a high-temperature coiling + slow cooling method to cause internal oxidation of Mn and Si alloy elements to form stable oxides with an internal oxidation depth of 5 to 20 μm, thereby avoiding the diffusion of Si and Mn elements in the sub-surface layer during the continuous annealing process.

[0019] In summary, the present invention has low production cost, short production cycle, excellent phosphating performance, avoids a series of problems such as poor phosphating coverage caused by surface enrichment and oxidation of alloy elements, coarse phosphating crystal size, poor coating adhesion, etc., provides guidance for the development of third-generation cold-rolled ultra-high strength for automobiles, and has broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is the cross-sectional morphology of the hot-rolled high-strength automobile plate in Example 1;

[0021] Figure 2 This is a comparison of the quality of the cold-rolled high-strength automobile sheet phosphating film produced by the traditional one-step heat treatment process and Example 1, where a is the traditional process and b is Example 1. DETAILED DESCRIPTION

[0022] The following examples may enable those skilled in the art to more fully understand the present invention, but are not intended to limit the present invention in any way.

[0023] Example 1

[0024] (1) Smelting and casting: The chemical composition of the raw steel is as follows by weight: C 0.2%, Si 1.5%, Mn 2.5%, P0.01%, S 0.002%, Cr 0.2%, Ti 0.02%, N 0.05%, and the rest is Fe and unavoidable impurity elements. After smelting and casting, it is made into slabs;

[0025] (2) Hot rolling: The starting rolling temperature of the rough rolling is 1200°C, the final rolling temperature is 1100°C, the starting rolling temperature of the finishing rolling is 950°C, and the final rolling temperature is 850°C. During the coiling process, the temperature of the strip at the head and tail 60m is 650°C, and the coiling temperature of the remaining positions is 600°C. After coiling, the strip is placed in the hot rolling finishing warehouse for heat preservation and slow cooling, and the thickness of the oxide layer in the hot rolled plate is 10 μm;

[0026] (3) Pickling: The hot rolled coil is unrolled, pickled and cold rolled at a pickling speed of 80 m / min and a cold rolling reduction of 40% to obtain a hardened strip;

[0027] (4) Continuous annealing process: The hardened strip is subjected to continuous annealing treatment, with a soaking section temperature of 850°C, a soaking time of 200s, a slow cooling section temperature of 700°C, a cooling rate of 5°C / s, and then rapidly cooled to the over-aging temperature, with a rapid cooling rate of 30°C / s, an over-aging section temperature of 350°C, and an over-aging time of 1200s. After cooling, a cold-rolled ultra-high strength automotive sheet with excellent phosphating performance is obtained.

[0028] The cold-rolled ultra-high strength automobile sheet prepared in Example 1 has a tensile strength of 1100 MPa, an elongation of 18%, and an inner oxide layer thickness of 10 μm.

[0029] The automobile plate prepared in Example 1 was phosphated using the improved low-zinc phosphating solution Gardobond 24TA, wherein the phosphating temperature was 40°C, the phosphating time was 2 min, the accelerator point was 4 Pt, the total acidity was 22 Pt, the free acidity was 1 Pt, and the phosphating film quality was Figure 2 .

[0030] Example 2

[0031] (1) The chemical composition of the raw steel is calculated by weight percentage: C 0.3%, Si 2.5%, Mn 3.5%, P0.008%, S0.003%, Cr 0.5%, Ti 0.1%, N 0.1%, and the rest is Fe and inevitable impurity elements, which is made into slabs after smelting and casting;

[0032] (2) The slab is hot rolled, the starting rolling temperature of hot rolling is 1250℃, the final rolling temperature is 1150℃, the starting rolling temperature of finishing rolling is 1000℃, and the final rolling temperature is 900℃. During the coiling process, the temperature of the 60m at the head and tail of the strip is 660℃, and the coiling temperature of the remaining positions is 620℃ to avoid the performance fluctuation and excessive difference in the length direction of the strip caused by uneven heat dissipation after the hot rolled strip is coiled. After the hot coil is coiled, it needs to be placed in the hot rolling finishing warehouse for insulation and slow cooling;

[0033] (3) uncoiling the hot-rolled coil, pickling and cold rolling, with the pickling speed being 60 m / min and the cold rolling reduction being 60%, to obtain a hardened steel strip;

[0034] (4) The hardened strip is subjected to continuous annealing treatment, with a soaking section temperature of 880°C, a soaking time of 260s, a slow cooling section temperature of 780°C, a cooling rate of 2°C / s, and then rapidly cooled to the over-aging temperature, with a rapid cooling rate of 30°C / s, an over-aging section temperature of 420°C, and an over-aging time of 1800s. After cooling, a cold-rolled ultra-high strength automotive sheet with excellent phosphating performance is obtained.

[0035] The cold-rolled ultra-high strength automobile sheet prepared in Example 2 has a tensile strength of 1300 MPa, an elongation of 15%, and an inner oxide layer thickness of 5 μm.

[0036] The above embodiments are only preferred embodiments of the present invention and are not intended to limit the implementation methods. The protection scope of the present invention shall be subject to the scope defined in the claims. Other different forms of changes or modifications may be made based on the above description. Obvious changes or modifications derived therefrom are still within the protection scope of the present invention.

Claims

1. A cold-rolled high-strength automobile sheet with excellent phosphating performance, characterized in that: The chemical composition of the automobile sheet is calculated by weight percentage: C 0.2%-0.3%, Si 1.5%-2.5%, Mn: 2.5%-3.5%, P≤0.01%, S≤0.003%, Cr 0.2%-0.5%, Ti 0.02%-0.1%, N≤0.1%, and the rest are Fe and unavoidable impurity elements.

2. The cold-rolled high-strength automobile sheet with excellent phosphating performance according to claim 1, characterized in that: The cold-rolled high-strength automobile sheet has a tensile strength of ≥980MP, an elongation of ≥14%, and an internal oxidation depth of 5 to 20 μm.

3. A method for preparing a cold-rolled high-strength automobile sheet with excellent phosphating performance according to any one of claims 1 to 2, characterized in that: Including smelting, casting, hot rolling, pickling and annealing processes, the specific control parameters are as follows: Smelting and casting: The chemical composition of the raw steel is calculated by weight percentage: C 0.2%~0.3%, Si 1.5%~2.5%, Mn: 2.5%~3.5%, P≤0.01%, S≤0.003%, Cr 0.2%~0.5%, Ti 0.02%~0.1%, N≤0.1%, and the rest are Fe and unavoidable impurity elements. After smelting and casting, it is made into slabs; Hot rolling: Coil the strip after hot rolling. U-type coiling is adopted. The temperature of the 60m section of the strip head and tail is 30-80℃ higher than the target coiling temperature. The target coiling temperature is controlled to be 580-650℃. After coiling, the strip is kept warm and slowly cooled. Pickling: unwind the hot rolled coil, pickle it and cold roll it to obtain hardened strip steel; Continuous annealing process: the hardened strip is subjected to continuous annealing treatment, the soaking section temperature is 800-880°C, the soaking time is 200-260s, the slow cooling section temperature is 700-780°C, and then rapidly cooled to the over-aging temperature, the over-aging section temperature is 350-420°C, the over-aging time is 1200-1800s, and the cold-rolled high-strength automobile sheet is obtained after cooling.

4. The preparation method according to claim 3, characterized in that: The starting rolling temperature of hot rolling rough rolling is 1200-1250°C, the final rolling temperature is 1100-1150°C, the starting rolling temperature of fine rolling is 950-1000°C, and the final rolling temperature is 850-900°C.

5. The preparation method according to claim 3, characterized in that: The pickling speed is ≤100m / min, and the cold rolling reduction is 40% to 80%.

6. The preparation method according to claim 3, characterized in that: The cooling rate of the slow cooling section is 2-5°C / s; the cooling rate of the rapid cooling section is ≥30°C / s.

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