JPH0334404B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Application Field> The present invention relates to a cold-rolled steel sheet whose press formability, spot weldability, and post-painting sharpness 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, in the case of steel plates for automobiles, it is necessary to spot weld pressed parts, so the spot weldability of the steel plate is also important. Furthermore, in the case of automobile exterior panels, the quality of the finish after painting is one of the important quality items for automobile users, which is directly connected to the overall quality of the vehicle. 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. In addition, in recent years, there has been a strong demand among automobile manufacturers to improve the quality of their paint finishes. It has been praised for its ``fleshiness,''``richness,'' and ``deepness,'' which define the sense of mass of a paint layer that can be perceived visually by the way light is reflected on the painted surface. Among these properties, image clarity, which is a combination of gloss and image clarity, is greatly affected by the painting method and the characteristics of the paint, but it is also affected by the surface roughness of the underlying steel plate. to be influenced. 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 demonstrate the influence of steel plate surface roughness on press formability and the like. 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 any surface roughness (Ra).
This paper does not provide any suggestions regarding the production of cold-rolled steel sheets which have excellent press formability, spot weldability, and sharpness after painting. <Object of the invention> The present invention solves the above-mentioned drawbacks of the prior art, gives directionality to the surface roughness pattern, and further improves the average area of the convex portions in the center plane of the surface roughness and/or the area per convex portion. The object of the present invention is to provide a cold-rolled steel sheet that can improve press formability, spot weldability, and image clarity after painting regardless of surface roughness by regulating the average area. <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 formula is S≦0.25 in at least one direction, and the area ratio SSr of the convex portions in the center plane of the surface roughness is The object of the present invention is to provide a cold-rolled steel sheet with excellent press formability and spot weldability, characterized by a 45% or more. =1/n _o 〓 ⁱ⁼¹ xi S=1/n _o 〓 ⁱ⁼¹ ｜−xi｜/x xi: Distance between peaks of convex parts on the steel plate surface The present invention also describes the rules for the steel plate surface roughness expressed by the following formula. The regularity parameter S that expresses the property is S≦0.25 in at least one direction, and the average area SGr per convex portion on the center plane of surface roughness is 2000≦
The object of the present invention is to provide a cold-rolled steel sheet that is characterized by SGr≦3000 (μm ² ) and has excellent press formability and sharpness after painting. =1/n _o 〓 ⁱ⁼¹ xi S=1/n _o 〓 ⁱ⁼¹ ｜−xi｜/x xi: Distance between peaks of convex parts on the steel plate surface The present invention also describes the rules for the steel plate surface roughness expressed by the following formula. The regularity parameter S expressing the property is S≦0.25 in at least one direction, and the area ratio SSr of the convex portions in the center plane of surface roughness is 45% or more, and the average area SGr of each convex portion is 2000≦SGr.
≦3000 (μm ² ), and has excellent press formability, spot weldability, and sharpness after painting. =1/n _o 〓 ⁱ⁼¹ xi S=1/n _o 〓 ⁱ⁼¹ |−xi|/x xi: Distance between peaks of convex portions on steel plate surface The present invention will be explained in more detail below. 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. Further, based on the first aspect of the present invention, using test steel B shown in Table 1, the relationship between the area ratio SSr (%) of convex portions in the center plane of the steel plate surface roughness after skin pass rolling and spot weldability The results of the investigation are shown in the second
As shown in the figure. At this time, the plate thickness was 0.8 mm, the skin pass reduction rate was 0.8%, and the S value was 0.15. The spot welding method strongly depends on SSr, and SSr≧45
(%), the tensile shear strength after spot welding was significantly improved. The spot welding conditions are: welding time 8 seconds, pressure 190Kg, welding current.
I went with 7800A. Further, based on the second aspect of the present invention, using test steel B shown in Table 1, the average area per convex part in the center plane of the steel sheet surface roughness after skin pass rolling is SGr.
Figure 3 shows the results of an investigation into the relationship between (μm ² ) and image clarity after painting. At this time, the skin pass reduction rate was 0.8% and the S value was 0.18. The sharpness after painting strongly depends on SGr, and SGr≧
2000 (μm ² ), the image clarity was significantly improved. As for the coating conditions, a phosphate film treatment was performed as a base treatment, and the coating processes of cationic electrodeposition coating, intermediate coating, and top coating were applied thereon.
In addition, as a method for evaluating sharpness, DOI (Distinctness
of Reflected Image). As shown in Figure 4, this method uses light incident at an angle of incidence of 30°.
When the intensity of the reflected light when reflected at a reflection angle of 30° is Rs, and the intensity of the reflected light reflected at a reflection angle of 30° ± _{0.3° is R 0.3 ,} D0I = 100 (Rs - R _0.3 )/Rs _. As a result of repeated research based on this basic data, the inventors of the present invention found that by regulating the manufacturing conditions as follows, we were able to manufacture cold-rolled steel sheets with excellent press formability, spot weldability, and image clarity after painting. We have discovered that this is possible. First, the most important thing is the steel sheet 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 area ratio of the convex portions on the center plane of the steel plate surface roughness and the average area of each convex portion are , was measured using a three-dimensional surface roughness meter. 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. Conventional cold-rolled steel sheets have an S value of about 0.3 to 0.5. Furthermore, in the first aspect of the present invention, it is essential that the area ratio SSr of the convex portions on the center plane of surface roughness is 45% or more. If SSr<45%, excellent spot weldability cannot be obtained. Furthermore, according to the second aspect of the present invention, the average area SGr per convex portion on the surface roughness center plane is
It is essential that SGr≧2000 (μm ² ).
If SGr<2000, excellent image clarity after painting cannot be obtained. On the other hand, when SGr>30000, it was confirmed that although the image clarity after painting was excellent, the press formability was conversely inferior. 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. The steel plate surface roughness pattern satisfies S≦0.25 and SSr≧45 and or 2000≦SGr≦30000 (μ
m ² ), the steel plate surface roughness, such as average surface roughness (Ra), type of lubricating oil, pressing conditions, etc. may be arbitrary. The effect of the regular surface roughness pattern in the present invention is that the lubricating oil accumulated in the recesses on the steel sheet surface is evenly supplied to the convex parts, which improves the lubrication conditions. Conceivable.
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 spot welding method, SSr≧45(%)
In this state, the adhesion between the steel plate surfaces during spot welding was good, so it is thought that the spot weldability was improved. Regarding the sharpness after painting, SGr≧2000
(μm ² ), this is thought to be due to improved light reflection after painting and reduced diffuse reflection.
Details are not clear. Furthermore, when SGr>30000 (μm ² ), the lubrication conditions for the convex portions become poor, and it is thought that press formability deteriorates. The third aspect of the present invention is a combination of the first and second aspects of the present invention, and therefore has excellent spot weldability and sharpness after painting. <Examples> The present invention will be described in more detail below based on examples. Example 1 A steel billet having 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 form a hot-rolled steel strip with a thickness of 3.2 mm. did. 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, and the parameter S value were determined. Furthermore, using a three-dimensional surface roughness meter, the area ratio of convex portions on the center surface of the surface roughness 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,
The average value in the D (45° to the rolling direction) 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. For spot welding, welding time is 8 seconds, pressure is 190 kg,
The current was 7800A. Spot weldability was evaluated based on the tensile shear strength at this time. Table 3 shows the dulling method, surface roughness, and material properties of the skin pass roll. Steel plates manufactured within the scope of the present invention exhibit superior press formability and spot weldability compared to comparative examples. Example 2 A steel billet with the chemical composition shown in Table 4 was produced by a converter-continuous casting method, heated to 1250°C, soaked, and then rough-rolled and finished-rolled to form a hot-rolled steel strip with a thickness of 3.2 mm. did. 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 steel plate surface roughness was measured in the rolling direction, and the average surface roughness Ra and S value parameters were determined. 3 more
Using a dimensional surface roughness meter, the average area of each convex portion on the center surface of the surface roughness 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,
The average value in the D (45° to the rolling direction) 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. Table 5 shows the dulling method, surface roughness, and material properties of the skin pass roll. Steel plates manufactured within the scope of the present invention exhibit superior press formability and image clarity compared to comparative examples. Example 3 A steel billet having the chemical composition shown in Table 6 was produced by a converter-continuous casting method, heated to 1250°C, then rough-rolled and finished-rolled to form a hot-rolled steel strip with a thickness of 3.2 mm. did. 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, and the parameter S value were determined. Further, using a three-dimensional surface roughness meter, the area ratio of convex portions in the center plane of surface roughness and the average area per convex portion in the center plane of surface roughness were determined. Table 7 shows the dullness method, surface roughness, and material properties of the skin pass roll. Steel sheets manufactured within the scope of the present invention exhibit excellent press formability, spot weldability, and sharpness after painting.

【table】

[Table] Note) In the remarks column, ☆ indicates a comparative example, and the others indicate examples according to the present invention.

【table】

[Table] Note) In the remarks column, ☆ indicates a comparative example, and the others indicate examples according to the present invention.
[Example 3]

【table】

[Table] <Effects of the Invention> According to the present invention, a regular roughness pattern is imparted to the surface of a steel sheet, and the average area of the convex portions and/or the area ratio of one convex portion on the center plane of the surface roughness is regulated. By doing so, the press formability, spot weldability, and sharpness after painting are significantly improved even for steel plates made of the same material, and the range of its use is expanded.
It becomes possible to produce cold-rolled steel sheets with excellent press formability, such as improved press formability regardless of surface roughness.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between the limit aperture ratio and the parameter S value. FIG. 2 is a graph showing the relationship between spot weldability and SSr. FIG. 3 is a graph showing the relationship between image clarity after painting and SGr. FIG. 4 is an explanatory diagram of numerical setting of DOI.

Claims (1)

[Claims] 1. The regularity parameter S representing the regularity of the steel plate surface roughness expressed by the following formula is S≦0.25 in at least one direction, and the area ratio SSr of the convex portions in the center plane of the surface roughness is 45%. A cold-rolled steel sheet having excellent press formability, etc., characterized by the above properties. =1/n _o 〓 ⁱ⁼¹ xi S=1/n _o 〓 ⁱ⁼¹ |-xi|/x xi: Distance between peaks of convex parts on the steel plate surface 2 Represents the regularity of the steel plate surface roughness expressed by the following formula The regularity parameter S is S≦0.25 in at least one direction, and the average area SGr per convex portion on the center plane of surface roughness is 2000≦SGr≦30000
(μm ² ) A cold-rolled steel sheet with excellent press formability, etc. =1/n _o 〓 ⁱ⁼¹ xi S=1/n _o 〓 ⁱ⁼¹ |-xi|/x xi: Distance between peaks of convex parts on the steel plate surface 3 Represents the regularity of the steel plate surface roughness expressed by the following formula The regularity parameter S is S≦0.25 in at least one direction, and the area ratio SSr of the convex portions in the center plane of surface roughness is 45% or more, and the convex portions 1
The average area SGr per piece is 2000≦SGr≦3000 (μ
A ^cold -rolled steel sheet with excellent press formability, etc., characterized by a =1/n _o 〓 ⁱ⁼¹ xi S=1/n _o 〓 ⁱ⁼¹ |−xi|/x xi: Distance between peaks of convex parts on steel plate surface