JPH0334403B2 - - 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 chemical conversion treatability 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, for automobile steel sheets, pretreatment for baking coating, that is, chemical conversion treatment properties, is important, and unless the chemical conversion treatment properties are good, sufficient baking coating properties cannot be ensured. 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, JP-A No. 54-97527 shows that a cold rolled steel sheet with excellent phosphate treatment properties can be produced by temper rolling using rolls having a roll surface roughness of PPI=150. However, although it has excellent phosphate treatment properties, it has no effect on press formability, and furthermore, phosphate treatment properties are generally required for automotive steel sheets, so press formability is naturally important. I'm getting old. The above-mentioned known techniques can be applied to arbitrary surface roughness (Ra,
This document does not provide any suggestions regarding the production of cold-rolled steel sheets that have PPI) and have excellent press formability and chemical conversion treatment properties. <Object of the invention> The present invention eliminates the drawbacks of the prior art described above, gives directionality to the surface roughness pattern, and further improves press formability and chemical conversion treatment by regulating the average surface roughness and average valley interval. The object of the present invention is to provide a cold rolled steel sheet having excellent press formability and chemical conversion treatment properties. <Structure of the Invention> That is, 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 surface roughness Ra (μm ) and the average valley spacing Lmv (μm) are
The present invention provides a cold-rolled steel sheet that has excellent press formability and chemical conversion treatment properties and is characterized by satisfying the relationship Ra×Lmv≧50. =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 patterns of sample steels A and B shown in Table 1 and the limit drawing ratio. The S value is a value measured in the rolling direction, and the average surface roughness (Ra) is approximately
It is 1.2 μm. The limiting drawing ratio strongly depends on the S value,
Press formability was significantly improved by setting S≦0.25. In addition, using sample steel B shown in Table 1, the relationship between the product Ra × Lmv of the average surface roughness Ra (μm) of the steel plate after skin pass rolling and the average valley spacing Lmv (μm) and chemical conversion treatability was investigated. The results are shown in Figure 2. At this time, the skin pass rolling reduction rate was 0.8% and the S value was 0.18. In addition, chemical conversion treatment property is determined by degreasing the steel plate (sample steel B).
After washing with water and treating with phosphoric acid, the pinhole area ratio was evaluated using the pinhole test described below. The phosphoric acid treatment is made by Nippon Parkerizing Co., Ltd.
Using BT3112, the total acidity was adjusted to 14.3 and free acidity to 0.5 at 55°C, and sprayed for 120 seconds. [Pinhole test] A filter paper soaked with a reagent that reacts with iron ions to form a color is brought into close contact with the test surface to detect areas where phosphoric acid crystals remain on the surface of the steel plate, and this is image analyzed to determine the pinhole area. It was quantified as a percentage. The evaluation criteria for chemical conversion treatment properties were as follows: pinhole area ratio of 0.5% or less was 1, 0.5 to 2% was 2, 2 to 9% was 3, 9 to 15% was 4, and 15% or more was 5. 1 and 2 indicate evaluations with no practical problems. As is clear from Figure 2, the chemical conversion treatment property is Ra
It strongly depends on ×Lmv, and by setting Ra×Lmv≧50, chemical conversion treatment properties were significantly improved. As a result of repeated research based on this basic data, the present inventors discovered that by regulating the manufacturing conditions as shown below, it is possible to manufacture cold-rolled steel sheets with excellent press formability and chemical conversion treatment properties. Ta. 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 In addition, the average valley interval Lmv is Lmv=1/n _o in the steel plate surface roughness pattern shown in Fig. 3. It can be expressed by the formula 〓 ⁱ⁼¹ lvi. Here, it is essential that the regularity parameter S representing the regularity of the surface roughness satisfies S≦0.25 in at least one direction. If S>0.25, excellent press formability cannot be obtained. Conventional cold-rolled steel sheets have an S value of about 0.3 to 0.5. Furthermore, average surface roughness Ra (μm) and average valley spacing
It is essential that Lmv (μm) satisfies the relationship Ra×Lmv≧50. When Ra×Lmv<50, excellent chemical conversion treatment properties cannot be obtained. 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 Ra×
As long as Lmv≧50 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 arbitrary. 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. Regarding chemical conversion treatment properties, it is thought that the steel plate surface roughness pattern influences the nucleation of phosphate crystals, but the details are not clear. <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 was cold rolled into a cold rolled steel strip with a 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 was one that had been dulled by shot blasting and laser processing. The surface roughness of the steel plate was measured in the rolling direction, and the average surface roughness Ra, the number of peaks per inch PPI, the parameter S value, and the average valley spacing Lmv were 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. Chemical conversion treatment properties were evaluated by performing the pinhole test described above, and the pinhole area ratio of 0.5% or less was evaluated as 1.
2 for 0.5-2%, 3 for 2-9%, 4 for 9-15%,
15% or more was given a score of 5. 1 and 2 indicate evaluations with no practical problems. 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 chemical conversion treatment properties compared to comparative examples.

【table】

【table】

[Table] Note) ☆ in the remarks column indicates a comparative example, and the others indicate examples according to the present invention.
<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 surface roughness and average valley interval, press forming and chemical forming are possible even in steel plates made of the same material. Processability has been significantly improved, expanding the scope of its use, and making it possible to perform highly difficult forming processes, making it possible to produce cold-rolled steel sheets with excellent press formability and chemical conversion treatment properties.

[Brief explanation of the drawing]

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 Ra×Lmv on chemical conversion treatment properties. FIG. 3 is a diagram showing a steel plate surface roughness profile.

Claims (1)

[Claims] 1. The regularity parameter S representing the regularity of the steel plate surface roughness expressed by the following equation (1) is S≦0.25 in at least one direction, and the average surface roughness Ra (μm)
A cold-rolled steel sheet with excellent press formability and chemical conversion treatment properties, characterized in that the average valley spacing Lmv (μm) satisfies the relationship Ra×Lmv≧50. =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