Production method of a steel for automobile side member without intermediate coating

Abstract

This invention belongs to the field of steel smelting technology and discloses a method for producing steel without intermediate coating for automotive side panel parts. The process flow includes: smelting billets → hot rolling → coiling → cold rolling → continuous annealing → leveling. During production, specific surface morphology requirements are specified for the cold rolling mill rolls and the leveling rolls. Specifically, the surface roughness Ra of the last cold rolling mill roll is 2.5–2.8 μm, the peak density RPc ≥ 90 particles / cm, and the waviness Wa ≤ 0.7 μm; the surface roughness Ra of the leveling roll is 1.8–2.2 μm, and the peak density RPc ≥ 110 particles / cm. This invention mainly controls two rolling processes to achieve suitable surface roughness Ra, high peak density RPc, and low waviness Wa, thereby ensuring excellent stamping and coating performance. It is mainly applied to exterior parts such as automotive side panels and can meet environmentally friendly requirements for steel without intermediate coating.

Description

Technical Field

[0001] This invention belongs to the field of steel smelting technology, and specifically relates to a method for producing steel without intermediate coating for automotive side panel parts. Background Technology

[0002] Compared to traditional coating processes, the no-intermediate-coat process reduces VOC emissions for car manufacturers by about 30%. However, due to the lack of leveling and filling of the intermediate coating layer, the outline of the electrophoretic parts is more easily revealed on the outer coating. To ensure a better appearance quality of the composite coating, higher requirements are placed on the surface morphology of the steel plate.

[0003] The main indicators for evaluating the surface morphology of steel sheets include surface roughness Ra, peak density RPc, and waviness Wa. These indicators not only affect the subsequent coating quality but also the stamping performance of the steel sheet. To achieve optimal overall performance, steel sheets used for exterior parts such as automotive side panels need to have suitable surface roughness Ra, high peak density RPc, and low waviness Wa. However, existing technologies struggle to achieve the optimal balance between these indicators, failing to meet market demands. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a production method for steel without intermediate coating for automotive side panel parts, which addresses the shortcomings of the existing technology. This method enables the product to have a high-quality surface morphology, thereby ensuring its excellent stamping and coating performance. It is mainly used in exterior parts such as automotive side panels and can meet the requirements of environmental protection without intermediate coating.

[0005] To solve the technical problem proposed in this invention, this invention provides a method for producing steel without intermediate coating for automotive side panel parts, the process flow of which includes: smelting billet → hot rolling → coiling → cold rolling → continuous annealing → leveling.

[0006] In the above scheme, the roughing temperature of the hot rolling is 1030-1060℃, and the final rolling temperature is 900-940℃.

[0007] In the above scheme, the winding temperature is 680-720℃.

[0008] In the above scheme, the total reduction rate of cold rolling is 78-81%.

[0009] In the above scheme, the surface roughness Ra of the last stand roll of the cold rolling mill is 2.5 to 2.8 μm, the peak density RPc of the roll surface is ≥90 rolls / cm, and the waviness Wa of the roll surface is ≤0.7 μm.

[0010] In the above scheme, the homogenization temperature of the continuous annealing is 810-830℃, and the strip walking speed is stabilized at a fixed value of 140-160m / min.

[0011] In the above scheme, the elongation rate of the flattening is 0.5-0.7%.

[0012] In the above scheme, the leveling process is controlled by adjusting the inlet tension and outlet tension to control the leveling rolling force to 150-200 tons.

[0013] Furthermore, the inlet tension is 15–35 kN, and the outlet tension is 20–40 kN.

[0014] Furthermore, the outlet tension is 4-6 kN greater than the inlet tension.

[0015] In the above scheme, the surface roughness Ra of the flat roll is controlled at 1.8 to 2.2 μm, and the peak density RPc of the roll surface is ≥110 particles / cm.

[0016] In the above scheme, the chemical composition and mass percentage content of the coating-free steel include: C: ≤0.001%, Si: ≤0.012%, Mn: 0.05~0.15%, P: ≤0.01%, S: ≤0.01%, Als: 0.01%~0.03%, Ti: 0.015~0.025%, N: ≤0.003%, with the balance being Fe and unavoidable impurities.

[0017] In the above scheme, the yield strength R of the steel without intermediate coating p0.2 The tensile strength is 130–190 MPa, and the tensile strength R is... m The strength is 270–330 MPa, and the elongation is A. 80mm ≥45%, plastic strain ratio r 90 ≥2.5, work hardening index n 90 ≥0.23.

[0018] In the above scheme, the surface roughness Ra of the coating-free steel is 0.5 to 1.1 μm, the peak density RPc is ≥ 75 particles / cm, and the waviness Wa is < 0.5 μm.

[0019] The technical concept of the main process parameters of the production method of this invention is as follows:

[0020] The reason why this invention controls the surface roughness Ra of steel to be 0.5-1.1 μm, the peak density RPc ≥ 75 particles / cm, and the waviness Wa < 0.5 μm is that if Ra is too high, it will result in a high Ra after coating, affecting the short-wave index of the coating, while if Ra is too low, it will be detrimental to stamping performance and affect the adhesion of the paint film. Therefore, Ra needs to be controlled within a suitable range of 0.5-1.1 μm; if Wa is too high, it will result in excessive long-wave index of the coating, which manifests as orange peel defects. Therefore, Wa is controlled to be < 0.5 μm; if RPc is low, it will reduce the fineness and uniformity of the surface. Therefore, RPc is controlled to be ≥ 75 particles / cm.

[0021] To achieve the above objectives, the main improvements to the production process in this invention are as follows:

[0022] 1) During the cold rolling process, the surface roughness Ra of the last stand roll is controlled to be 2.5-2.8 μm, the peak density RPc is ≥90 rolls / cm, and the waviness Wa is ≤0.7 μm. The cold rolling process provides initial roughness and waviness control. With this process control, the strip after the mill has Ra 0.8-0.9 μm, RPc 55-65 rolls / cm, and Wa <0.5 μm, which has a positive effect on the subsequent leveling rolling to obtain the final morphology.

[0023] 2) In the continuous annealing process, in order to promote the initial nucleation and growth of γ-oriented textured grains and obtain good deep drawing performance, it is necessary to strictly control the temperature stability during the production process. If the temperature control is not proper, the grains will be too large and the waviness will easily exceed the range. Therefore, the soaking temperature is controlled at 820~840℃ and the annealed strip speed is kept stable at a fixed value of 140~160m / min.

[0024] 3) During the leveling process, the strip stamping performance is guaranteed by the elongation rate. The rolling force is controlled by the elongation rate and the tension. Under the premise of a certain elongation rate, the required rolling force is achieved by adjusting the tension. At the same time, the surface roughness Ra and peak density RPc of the leveling roll are controlled, thereby controlling the copying coefficient of the leveling surface morphology and making the final product morphology meet the requirements.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0026] This invention uses a Ti-containing ultra-low carbon composition for smelting and hot continuous rolling of slabs. It specifies the surface morphology and process requirements for the cold rolling mill rolls and leveling rolls. By controlling the two rolling processes, the product has a suitable surface roughness Ra, a high peak density RPc, and a low waviness Wa, thereby ensuring its excellent stamping and coating performance. It is mainly used in exterior parts such as automotive side panels and can meet the requirements of environmental protection and no intermediate coating. Detailed Implementation

[0027] To better understand the present invention, the following embodiments further illustrate the content of the present invention, but the content of the present invention is not limited to the following embodiments.

[0028] Examples 1-5 and Comparative Examples 1-2

[0029] The production method of the automotive side panel parts without intermediate coating in Examples 1-5 includes the following steps:

[0030] 1) Smelt and continuously cast billets according to the chemical composition shown in Table 1;

[0031] 2) After the billet is heated, it is hot rolled. The rough rolling temperature is 1030-1060℃ and the final rolling temperature is 900-940℃.

[0032] 3) Cool to the winding temperature and then wind up; the winding temperature is 680-720℃.

[0033] 4) Cold rolling is performed with a total cold rolling reduction of 78-81%; the surface roughness Ra of the last stand roll in cold rolling is 2.5-2.8μm, the peak density RPc of the roll surface is ≥90 rolls / cm, and the waviness Wa of the roll surface is ≤0.7μm;

[0034] 5) Perform continuous annealing, with a soaking temperature of 810-830℃ and a strip walking speed of 140-160m / min. Cool to room temperature after annealing.

[0035] 6) Perform leveling with a leveling elongation of 0.5-0.7%. Control the leveling rolling force to 150-200 tons by adjusting the inlet and outlet tensions. The surface roughness Ra of the leveled rolls is controlled at 1.8-2.2μm, and the peak density RPc of the roll surface is ≥110 particles / cm.

[0036] The production steps for the automotive side panel parts without intermediate coating steel in Comparative Examples 1 and 2 are the same as those in Examples 1 to 5, except for the control of process parameters, as detailed in Tables 1 to 3.

[0037] Table 1 Chemical composition of steel

[0038]

[0039]

[0040] Table 2 Steel Production Process Parameters (I)

[0041]

[0042] Table 3 Steel Production Process Parameters (II)

[0043]

[0044] The performance of the non-coating steel used in the automotive side panel parts of Examples 1-5 and Comparative Examples 1-2 was tested, and the results are shown in Table 4.

[0045] Table 4. Performance test results of steel

[0046]

[0047] As shown in Table 4, the steels produced in Examples 1-5 possess excellent mechanical properties, deep-drawing performance, suitable surface roughness, high peak density, and low waviness. The surface roughness of Comparative Example 1 is too low, resulting in insufficient oil retention on the sheet surface, which is detrimental to stamping performance. In the coating process, excessively low roughness affects paint film adhesion and is detrimental to coating quality. Comparative Example 2, due to its high annealing temperature and excessively rough leveling roller, has an excessively high surface roughness, and both peak density and waviness fail to meet requirements.

[0048] The above embodiments are merely examples for clear illustration and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations, and any obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A production method of a primerless steel for an automobile side member, characterized by, Includes the following steps: 1) Smelting and continuous casting to form billets; The chemical composition and mass percentage content of the steel without intermediate coating include: C: ≤0.001%, Si: ≤0.012%, Mn: 0.05~0.15%, P: ≤0.01%, S: ≤0.01%, Als: 0.01%~0.03%, Ti: 0.015~0.025%, N: ≤0.003%, balance being Fe and unavoidable impurities; 2) After the billet is heated, it is hot rolled. The rough rolling temperature is 1030~1060℃ and the final rolling temperature is 900~940℃. 3) Cool to the winding temperature and then wind up; the winding temperature is 680~720℃. 4) Cold rolling is performed with a total cold rolling reduction of 78-81%; the surface roughness Ra of the last stand roll in cold rolling is 2.5-2.8μm, the peak density RPc of the roll surface is ≥90 pieces / cm, and the waviness Wa of the roll surface is ≤0.7μm; 5) Perform continuous annealing, with a soaking temperature of 810~830℃ and a strip walking speed of 140~160m / min. After annealing, cool to room temperature. 6) The roll is leveled with a leveling elongation of 0.5-0.7%; the surface roughness Ra of the leveled roll is 1.8-2.2µm, and the peak density RPc of the roll surface is ≥110 particles / cm; the surface roughness Ra of the resulting coating-free steel is 0.5-1.1µm, the peak density RPc is ≥75 particles / cm, and the waviness Wa is <0.5µm.

2. The production method of the primerless steel for automobile side members according to claim 1, characterized by, In step 6), the leveling rolling force is controlled to be 150-200 tons by adjusting the inlet tension and outlet tension.

3. The production method of the primerless steel for automobile side parts according to claim 2, characterized by, The inlet tension is 15~35kN, and the outlet tension is 20~40kN.

4. The production method of the primerless steel for automobile side members according to claim 2 or 3, characterized in that, The outlet tension is 4-6 kN greater than the inlet tension.

5. The production method of the primerless steel for automobile side parts according to claim 1, characterized by, The yield strength R of the steel without intermediate coating p0.2 The tensile strength is 130~190MPa, and the tensile strength R is... m The strength is 270~330MPa, and the elongation is A. 80mm ≥45%, plastic strain ratio r 90 ≥2.5, work hardening index n 90 ≥0.23.

Images