JPH0334402B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Application Field> The present invention relates to a cold-rolled steel sheet whose press formability and die galling resistance are significantly improved by controlling the surface roughness pattern of the steel sheet. <Prior art and its problems> Cold-rolled steel sheets for drawing used in automobile panels, electrical appliances, kitchen appliances, etc. are required to have excellent deep drawability. In order to improve deep drawability, the mechanical properties of the steel sheet need to be high ductility (El) and high Rankford value (r value). Furthermore, in actual drawing forming (particularly for automobile panels), composite forming with stretch forming is often performed, so the work hardening index (n value) also becomes important. Furthermore, during press working, a so-called mold galling phenomenon may occur, in which the steel plate and the press mold seize. This mold galling causes damage such as damaging the mold and significantly reducing the commercial value of the product after press working. By the way, research on deep drawing forming has been conducted from both sides of the steel sheet material and forming technology. However, as products become more precise and complex, the characteristics required of steel sheets are becoming more sophisticated and diversified. This tendency is particularly strong in cold-rolled steel sheets for automobiles. For example, the current situation is that a large number of press parts are spot welded when assembling an automobile body, but there is a strong demand to reduce the number of spot welds by increasing the size and integrating these parts. Meanwhile, car designs are becoming more complex in order to meet diversifying needs, and as a result, an increasing number of parts are difficult to form using conventional steel plates. In order to meet these demands,
There is a need for cold-rolled steel sheets that have better press formability than conventional ones. By the way, in actual press forming, the conventionally used mechanical properties (r value, El, n value) of the steel plate alone are insufficient as evaluation criteria. For example, the surface roughness of the steel sheet, lubricating oil, etc. have a large effect on press formability. Several known techniques have been disclosed that show the influence of surface roughness of a steel sheet on press formability. For example, in "Plasticity and Processing" Vol. 3 No. 14 (1962-3), it is shown that in the case of high-viscosity lubricating oil, the drawability is improved most when the steel plate surface roughness is about a few μm.
On the other hand, in Japanese Patent Publication No. 59-34441, the roll surface roughness (Ra) and peak (PPI) are each Ra=2.8.
(μm), PPI = 226, which shows a temper rolling method for cold rolled steel sheets that achieves excellent appearance and press workability after painting. Although these known techniques are excellent in terms of improving press formability, they all have the drawback that the surface roughness of the steel sheet must be regulated. Furthermore, the above-mentioned known technology can be applied to arbitrary surface roughness (Ra,
There is no suggestion whatsoever regarding the production of cold-rolled steel sheets that have PPI) and have excellent press formability and die galling resistance. <Object of the invention> The present invention solves the drawbacks of the prior art described above, gives directionality to the surface roughness pattern, and further improves the average peak diameter Rmp (μm) and average valley radius Rmv in the surface roughness center plane. It is an object of the present invention to provide a cold rolled steel sheet with excellent press formability and die galling resistance, which can improve press formability and die galling resistance by regulating the surface roughness, regardless of surface roughness. <Structure of the Invention> In other words, the present invention provides a method in which the regularity parameter S representing the regularity of the steel sheet surface roughness expressed by the following equation (1) is S≦0.25 in at least one direction, and the average of the surface roughness in the center plane is The present invention provides a cold rolled steel sheet having excellent press formability and mold galling resistance, characterized in that the peak radius Rmp (μm) and the average valley radius Rmv (μm) satisfy the relationship Rmv/Rmp>1. =1/n _o 〓 ⁱ⁼¹ xi S=1/n _o 〓 ⁱ⁼¹ ｜−xi｜/x …(1) Here, xi: Distance between the peaks of the convex portions on the steel plate surface The present invention will be described in further detail below. Explain. First, the research results that form the basis of the present invention will be described. Two types of cold-rolled low carbon aluminum killed steel plates shown in Table 1 below were used as the test steels. This was skin-pass rolled at a rolling reduction of 0.8% using a skin-pass roll that had been subjected to laser dulling (hereinafter referred to as laser dulling). At this time, the surface roughness pattern of the steel sheet after skin pass rolling was changed by variously changing the laser dulling method.

[Table] Figure 1 shows the relationship between the regularity parameter S value of the steel plate surface roughness pattern and the limit drawing ratio. The S value is a value measured in the rolling direction, and the average surface roughness (Ra) is approximately 1.2 μm in both cases. The critical drawing ratio strongly depends on the S value, and press formability was significantly improved by setting S≦0.25. In addition, using test steel B shown in Table 1, the average peak radius at the center plane of the steel plate surface roughness after skin pass rolling was
Ratio of Rmp (μm) to average valley radius Rmv (μm)
Figure 2 shows the results of an investigation into the relationship between Rmv/Rmp and mold galling resistance. At this time, the skin pass reduction rate was 0.8% and the S value was 0.16. Mold galling resistance
It can be seen that mold galling resistance strongly depends on Rmv/Rmp, and mold galling resistance is significantly improved by setting Rmv/Rmp>1. As a result of repeated research based on this basic data, the inventors of the present invention have found that by regulating the manufacturing conditions as shown below, it is possible to manufacture cold rolled steel sheets with excellent press formability and die galling resistance. I found it. First, the most important thing is the steel plate surface roughness pattern. The regularity parameter S representing the regularity of the steel sheet surface roughness in the present invention can be expressed as follows, where Xi is the distance between the peaks of the convex portions on the steel sheet surface. =1/n _o 〓 ⁱ⁼¹ xi S=1/n _o 〓 ⁱ⁼¹ ｜−xi｜/x Also, the average peak radius at the surface roughness center plane Rmp
and the average valley radius Rmv in the steel plate surface roughness pattern shown in Fig. 3. It can be expressed by the formula. Here, Sp: Area of the ridges on the center plane Sv: Area of the valleys on the center plane np: Number of peaks on the center plane nv: Number of valleys on the center plane Regularity parameter S expressing the regularity of surface roughness
It is essential that S≦0.25 be satisfied in at least one direction. If S>0.25, excellent press formability cannot be obtained. In conventional cold-rolled steel sheets, S
The value is about 0.3 to 0.5. Furthermore, the average peak radius at the center plane of surface roughness
Rmp (μm) and average valley radius Rmv (μm) are Rmv/
It is essential to satisfy the relationship Rmp>1.
If Rmv/Rmp≦1, a large amount of iron powder will be generated during press working, making it impossible to obtain excellent mold galling resistance. In addition, in order to obtain such a regular steel plate surface roughness pattern, the surface roughness pattern of the skin pass roll must also be regular.
The processing method for skin pass rolls for this purpose is as follows:
Electric discharge dulling, laser dulling, or shot blasting using specially manufactured grids are suitable. Steel plate surface roughness pattern is S≦0.25 and Rmv/
As long as Rmp>1 is satisfied, the surface roughness of the steel plate, such as the average surface roughness (Ra), the number of peaks per inch (PPI), the type of lubricating oil, pressing conditions, etc. may be arbitrarily determined. The effect of the regular surface roughness pattern in the present invention is thought to be that the lubricating oil accumulated in the recesses on the steel plate surface is evenly supplied to the convex parts, resulting in better lubrication conditions. It will be done. Furthermore, it is thought that the regular presence of the metal contact portions of the convex portions changes the frictional state between the steel sheet surface and the press die. <Example> A steel billet with the chemical composition shown in Table 2 was produced by a converter-continuous casting method, heated to 1250°C, soaked, and then rough-rolled and finished-rolled to produce a hot-rolled steel strip with a thickness of 3.2 mm. And so. After pickling, it is cold rolled into a cold rolled steel strip with a plate thickness of 0.8 mm, and continuously annealed (soaking temperature 750℃~850℃).
°C), then skin pass rolling (reduction rate 0.8%)
I went there. Here, the skin pass roll used had been dulled by shot blasting and laser processing. The surface roughness of the steel plate was measured in the L direction, and the average surface roughness Ra, number of peaks per inch PPI, and S value parameters were determined. Furthermore, using a three-dimensional surface roughness meter, the average peak radius Rmp at the center surface of the surface roughness,
The average valley radius Rmv was determined. The tensile properties were determined using a JIS No. 5 test piece. value is
Apply 15% tensile prestrain, measure by three-point method, and L
(rolling) direction, C (90° to the rolling direction) direction,
Average value in direction D (45° to rolling direction) = (r _L
+r _C +2r _D )/4. The limit drawing ratio (LDR) was determined from the maximum blank diameter D ₀ max that can be deep drawn using a die with a punch diameter of 32 mm, and the ratio to the punch diameter dp. That is, LDR=D ₀ max/dp The drawing conditions were all the same, with a drawing speed of 1 mm/s and a rust preventive oil (oil type) being used as the lubricating oil. The mold galling resistance was determined by conducting a hat-shaped drawing test with a round beat, and visually inspecting the steel plate surface after the test as 1 (good).
I made a judgment of ~5 (column). Ratings 1 and 2 indicate mold-gathering properties that pose no practical problem. The drawing test conditions were as follows: the drawing speed was 1 mm/s, the lubricating oil was rust preventive oil (oil type), and the same amount of lubricating oil was applied. Table 3 shows the dulling method, surface roughness, and material properties of the skin pass roll. Steel sheets manufactured within the scope of the present invention exhibit superior press formability and die galling resistance compared to comparative examples.

【table】

[Table] <Effects of the Invention> According to the present invention, by imparting a regular roughness pattern to the surface of a steel plate and further regulating the average peak radius and average valley radius on the surface roughness center plane,
Cold rolling with excellent press formability and die galling resistance has significantly improved press forming and die galling resistance even for steel sheets made of the same material, expanding the range of its use and making it possible to perform highly difficult forming. It becomes possible to manufacture steel plates.

[Brief explanation of drawings]

FIG. 1 is a graph showing the influence of the regularity parameter S value on the limiting aperture ratio. FIG. 2 is a graph showing the influence of Rmv/Rmp on mold galling resistance. FIG. 3 is a diagram showing the steel plate surface roughness profile.

Claims (1)

[Claims] 1. The regularity parameter S representing the regularity of the steel sheet surface roughness expressed by the following equation (1) is S≦0.25 in at least one direction, and the average peak radius Rmp ( μm) and average valley radius Rmv9 (μm)
A cold-rolled steel sheet having excellent press formability and mold galling resistance, characterized in that Rmv/Rmp>1. =1/n _o 〓 ⁱ⁼¹ xi S=1/n _o 〓 ⁱ⁼¹ ｜−xi｜/x …(1) Here, Xi: distance between peaks of convex parts on the steel plate surface