Super high strength cold rolled automotive sheet with excellent painting performance and method for manufacturing the same

Abstract

The present application relates to the technical field of cold-rolled high-strength automobile steel, in particular to an ultra-high-strength cold-rolled automobile plate with excellent coating performance and a preparation method thereof.The chemical composition of the cold-rolled automobile plate is as follows in terms of mass percentage: C: 0.2%-0.3%, Si: 2.0%-2.2%, Mn: 2.5%-3.0%, Cr: 0.5%-0.6%, Al: 0.1%-0.2%, B: 0.01%-0.03%, Nb: 0.005%-0.010%, and the balance is iron and inevitable impurities.The present application solves a series of problems such as poor phosphating coverage, coarse phosphating crystal size, and unsatisfactory coating quality and corrosion resistance of the existing ultra-high-strength cold-rolled automobile plate by controlling the internal and external oxidation processes of the ultra-high-strength cold-rolled automobile plate, preparing a decarburized layer, and electroplating nickel on the automobile plate after cold rolling, and has very significant social and economic benefits.

Description

Technical Field

[0001] This invention relates to the field of cold-rolled high-strength automotive steel technology, and more particularly to ultra-high-strength cold-rolled automotive steel sheets with excellent coating performance and their preparation methods. Background Technology

[0002] Ultra-high strength steel plates have high work hardening capacity and strong energy absorption capacity, and are widely used in structural beam parts.

[0003] Ultra-high strength steel plates typically refer to steel with a yield strength of 780 MPa or higher. As material strength increases, its formability decreases sharply, leading to increasingly prominent cracking problems during stamping. To improve the strength and formability of steel plates, a certain amount of C, Si, and Mn are usually added. The C content is closely related to the quality of phosphating coating. Not only does increasing the carbon content in steel hinder the iron dissolution reaction and affect phosphating performance, but during continuous annealing, high-Si and high-Mn steel plates experience selective oxidation of Si and Mn elements on the steel plate surface. Selective oxidation is divided into internal oxidation and external oxidation. Internal oxidation occurs when alloying elements diffuse below the steel plate surface, while external oxidation occurs when oxygen reacts with alloying elements on the steel plate surface. External oxidation affects the iron dissolution reaction during phosphating, severely deteriorating the phosphating performance of the steel plate, leading to poor phosphating coverage, coarse phosphating crystal size, and substandard coating quality and corrosion resistance, severely limiting the application of ultra-high strength steel plates in automobiles. Therefore, improving the phosphate coating performance of high-strength cold-rolled steel sheets during use has always been a major issue. Summary of the Invention

[0004] To address a series of problems in existing ultra-high strength cold-rolled automotive steel sheets, such as poor phosphating coverage, large phosphating crystal size, and inadequate coating quality and corrosion resistance, this invention provides a method for preparing ultra-high strength cold-rolled automotive steel sheets with excellent phosphating coating performance, which has broad application prospects in the automotive steel sheet industry.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows:

[0006] This invention provides an ultra-high strength cold-rolled automotive sheet with excellent coating performance. The chemical composition of the cold-rolled automotive sheet, by mass percentage, is: C: 0.2%-0.3%, Si: 2.0%-2.2%, Mn: 2.5%-3.0%, Cr: 0.5%-0.6%, Al: 0.1%-0.2%, B: 0.01%-0.03%, Nb 0.005%-0.010%, with the balance being iron and unavoidable impurities.

[0007] In the above technical solution, the tensile strength of the cold-rolled automotive sheet is ≥1450MPa, the yield strength is ≥1260MPa, and the elongation A80 is ≥18%.

[0008] Another aspect of the present invention provides a method for preparing the above-mentioned ultra-high strength cold-rolled automotive sheet with excellent coating performance, comprising the steps of hot rolling, coiling, pickling, continuous annealing, leveling, degreasing, cleaning, and electroplating, with specific control parameters as follows:

[0009] Hot rolling: The initial rolling temperature of hot rolling is 1200-1300℃, and the final rolling temperature of finishing rolling is 850-950℃;

[0010] Winding: After rolling, the coil is wound at a temperature of 550℃-620℃;

[0011] Pickling and rolling: Total reduction during pickling and rolling is 40%-60%;

[0012] Continuous cooling: the temperature of the heat-soaking section is 800-850℃, the heat-soaking time is 30-50s, the temperature of the slow cooling section is 700℃, the cooling rate of the slow cooling section is 2-5℃ / s, the temperature of the rapid cooling section is 260-300℃, the cooling rate of the rapid cooling section is 15-25℃ / s, and the temperature of the over-aging section is 230℃.

[0013] Leveling: After leveling, the surface roughness Ra value of the steel strip is controlled at 0.6-1.0μm, and the number of peaks Rpc value is controlled at 100-120;

[0014] Degreasing and cleaning: The reflectivity of the strip steel after degreasing and cleaning is ≥80%;

[0015] Electroplating: The plating solution temperature is 60-70℃, and the electroplating time is 10-20s.

[0016] In the above technical solution, further, in the continuous annealing process, the atmosphere inside the annealing furnace is 98%-98.2% N2 and 1.8%-2.0% H2 by volume, the oxygen content in each section of the furnace is ≤30ppm, and the dew point value is 0-10℃.

[0017] In the above technical solution, further, in the continuous annealing process, the average decarburization layer thickness on one side of the plate after continuous annealing is 20-50μm, the thickness of the outer oxide layer on the surface is ≤16nm, the proportion of Fe(1 1 1) crystal plane is ≥70%, and the Fe(1 1 1) crystal plane dissolves fastest in the phosphating solution.

[0018] In the above technical solution, further, in the electroplating process, the current density is 3-4 A / dm³. 2 The pH value is 3-5.

[0019] In the above technical solution, further, in the electroplating process, the plating solution is 60-80g / L nickel chloride, 150-200g / L potassium chloride, and 20-30g / L additives.

[0020] The beneficial effects of this invention are as follows:

[0021] This invention provides an ultra-high strength cold-rolled automotive sheet by controlling the internal and external oxidation process of ultra-high strength cold-rolled automotive sheet, pre-fabricating a decarburized layer, and preparing a method for electroplating nickel on the cold-rolled automotive sheet. This invention solves a series of problems in existing ultra-high strength cold-rolled automotive sheets, such as poor phosphating coverage, large phosphating crystal size, and inadequate coating quality and corrosion resistance. It has significant social and economic benefits. Attached Figure Description

[0022] Figure 1 The image shows a comparison of the phosphating quality of different high-strength cold-rolled automotive steel sheets. a represents DP 1180, and b represents Example 1. Detailed Implementation

[0023] The following examples are intended to enable those skilled in the art to more fully understand the present invention, but do not limit the invention in any way.

[0024] Example 1

[0025] A method for preparing ultra-high strength cold-rolled automotive steel sheet with excellent coating performance, specifically including the following steps:

[0026] (1) The steel material is slab made after smelting and casting. The chemical composition of the raw steel is shown in Table 1.

[0027] Table 1 Chemical composition (wt%) of raw steel in Example 1

[0028] 0.2 2.0 2.5 0.5 0.1 0.01 0.005

[0029] (2) Heat the slab at 1350℃ for 1 hour;

[0030] (3) The initial rolling temperature of hot rolling is 1250℃, and the finishing rolling is completed at a final rolling temperature of 850℃. The thickness of the hot-rolled steel plate is 2.5mm. The hot-rolled steel plate is coiled at 550℃.

[0031] (4) The pickling reduction is 52%, and the final thickness of the hardened strip is 1.2 mm;

[0032] (5) After uncoiling and cleaning the obtained hardened strip steel, it is subjected to continuous annealing. The temperature of the soaking section in the annealing furnace is 800℃, the soaking time is 30s, the temperature of the slow cooling section is 700℃, the cooling rate of the slow cooling section is 2℃ / s, the temperature of the fast cooling section is 300℃, the cooling rate of the fast cooling section is 15℃ / s, the temperature of the over-aging section is 230℃, the atmosphere in the annealing furnace is 2% hydrogen volume content and 98% nitrogen volume content, the oxygen content in each section of the furnace is 30ppm, the dew point is 0℃, and the high-strength steel obtained has a tensile strength of 1450MPa, a yield strength of 1260MPa, an elongation of 22%, an average decarburization layer thickness of 30μm on one side of the plate, an outer oxide layer thickness of 14nm, and an Fe(1 11) crystal plane ratio of 72%.

[0033] (6) The surface roughness Ra value of the flattened steel strip is controlled at 0.6μm, and the number of peaks Rpc value is controlled at 120;

[0034] (7) The reflectivity of the strip steel after degreasing and cleaning is 80%;

[0035] (8) Electroplating is performed on the degreased and cleaned steel strip. The plating bath temperature is 60℃, the plating bath consists of 60g / L nickel chloride, 150g / L potassium chloride, and 20g / L additives, the electroplating time is 10s, and the current density is 3A / dm³. 2 With a pH value of 4, high-strength cold-rolled automotive steel sheets were obtained.

[0036] The high-strength cold-rolled automotive sheet was subjected to phosphating treatment, and the weight of the phosphating film was 2.2 g / m³. 2 The phosphating film has a grain size of 5μm and a phosphorus content (P) of over 85%, meeting the requirements for automotive sheet metal production. A comparison of phosphating quality with existing DP1180 products is shown below. Figure 1 .

[0037] Example 2

[0038] (1) The steel material is slab made after smelting and casting. The chemical composition of the raw steel is shown in Table 2.

[0039] Table 2 Chemical composition (wt%) of raw steel in Example 2

[0040] 0.3 2.2 3.0 0.6 0.2 0.03 0.01

[0041] (2) Heat the slab at 1350℃ for 1 hour;

[0042] (3) The initial rolling temperature of hot rolling is 1250℃, the finishing rolling is completed at a final rolling temperature of 900℃, and the coiling temperature is 620℃.

[0043] (4) The total reduction of pickling and rolling is 45%, and the final thickness of the hardened strip is 1.5 mm;

[0044] (5) After uncoiling and cleaning the obtained hardened strip steel, it is subjected to continuous annealing. The temperature of the soaking section in the annealing furnace is 850℃, the soaking time is 50s, the temperature of the slow cooling section is 700℃, the cooling rate of the slow cooling section is 5℃ / s, the temperature of the fast cooling section is 260℃, the cooling rate of the fast cooling section is 25 / s, the temperature of the over-aging section is 230℃, the atmosphere in the annealing furnace is 5% hydrogen volume content and 95% nitrogen volume content, the oxygen content in each section of the furnace is 30ppm, the dew point value is 10℃, the tensile strength of the obtained high-strength steel is 1600MPa, the yield strength is 1350MPa, the elongation A80 is 18%, the average decarburization layer thickness on one side of the plate is 50μm, the outer oxide layer thickness is 8nm, and the Fe(1 1 1) crystal plane ratio is 75%.

[0045] (6) The surface roughness Ra value of the flattened steel strip is controlled to be 1.0 μm, and the number of peaks Rpc value is controlled to be 100;

[0046] (7) The reflectivity of the strip steel after degreasing and cleaning is 85%;

[0047] (8) Electroplating is performed on the degreased and cleaned steel strip. The plating bath temperature is 70℃, the plating bath consists of 80g / L nickel chloride, 200g / L potassium chloride, and 30g / L additives, the electroplating time is 10s, and the current density is 3A / dm³. 2 With a pH value of 4, high-strength cold-rolled automotive steel sheets were obtained.

[0048] The high-strength cold-rolled automotive sheet was subjected to phosphating treatment, and the phosphating film weight was 2.5 g / m³. 2 The phosphate film has a grain size of 3μm and a phosphorus content of over 85%, which meets the requirements for automotive sheet production.

[0049] The above embodiments are merely preferred embodiments of the present invention and are not intended to limit the implementation. The scope of protection of the present invention should be determined by the scope defined in the claims. Other variations or modifications can be made based on the above description. Obvious variations or modifications derived therefrom are still within the scope of protection of the present invention.

Claims

1. A method for preparing ultra-high strength cold-rolled automotive steel sheet with excellent coating performance, characterized in that, The chemical composition of the cold-rolled automotive sheet, by mass percentage, is: C: 0.2%-0.3%, Si: 2.0%-2.2%, Mn: 2.5%-3.0%, Cr: 0.5%-0.6%, Al: 0.1%-0.2%, B: 0.01%-0.03%, Nb: 0.005%-0.010%, with the balance being iron and unavoidable impurities; The method for preparing the cold-rolled automotive sheet includes the following steps: hot rolling, coiling, pickling, continuous annealing, leveling, degreasing, cleaning, and electroplating. Specific control parameters are as follows: Hot rolling: The initial rolling temperature of hot rolling is 1200℃-1300℃, and the final rolling temperature of finishing rolling is 850℃-950℃; Winding: After rolling, the coil is wound at a temperature of 550℃-620℃; Pickling and rolling: Total reduction in pickling and rolling is 40%-60%; Continuous cooling: The temperature of the heat-spreading section is 800-850℃, the heat-spreading time is 30-50s, the temperature of the slow cooling section is 600-700℃, the cooling rate of the slow cooling section is 2-5℃ / s, the temperature of the fast cooling section is 260-300℃, the cooling rate of the fast cooling section is 15-25℃ / s, and the temperature of the over-aging section is 200-230℃. Leveling: After leveling, the surface roughness Ra value of the steel strip is controlled at 0.6-1.0µm, and the number of crests Rpc value is controlled at 100-120; Degreasing and cleaning: The reflectivity of the strip steel after degreasing and cleaning is ≥80%; Electroplating: The plating solution temperature is 60-70℃, and the electroplating time is 10-20s; In the continuous annealing process, the average decarburized layer thickness on one side of the plate after continuous annealing is 20-50µm, the outer oxide layer thickness is ≤16nm, and the Fe(1 1 1) crystal plane ratio is ≥70%; In the electroplating process, the current density is 3-4 A / dm². 2 pH is 3-5; In the continuous annealing process, the atmosphere inside the annealing furnace is 95%-98% N2 and 2%-5% H2 by volume, the oxygen content in each section of the furnace is ≤30ppm, and the dew point value is 0-10℃.

2. The preparation method according to claim 1, characterized in that, The cold-rolled automotive sheet has a tensile strength ≥1450MPa, a yield strength ≥1260MPa, and an elongation A80 ≥18%.

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