Cold rolled steel sheets having an improved press formability

Abstract

A cold rolled steel sheet or plated steel sheet having an improved press formability as well as excellent phosphatability, resistance to galling and spot weldability is produced by controlling a surface roughness pattern of the steel sheet so as to satisfy a center-line average surface roughness of 0.3-2.0 .mu.m and a regularity parameter in at least one direction of not more than 0.25 defining the regularity of surface roughness.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to cold rolled steel sheets and plated steel sheets having considerably improved press formability, phosphatability, weldability and resistance to galling by controlling surface roughness pattern of steel sheet.

2. Related Art Statement

Drawable cold rolled steel sheets used for automobile panels, electric appliances, culinary equipments and so on are required to have an excellent deep drawability. In order to enhance the deep drawability, it is necessary that the steel sheet has high ductility (E1) and Lankford value (r-value) as mechanical properties. In fact, the drawing (particularly in the formation of automobile panel) is frequently combined with the flanging, so that work hardening index (n-value) becomes also important.

In the outer panel for the automobile, the finish feeling after painting is an important item directly connecting to the quality of the automobile itself in users.

Furthermore, the pretreatment for baking or phosphatability is important in the steel sheet for automobiles. That is, when the phosphatability is not good, sufficient baking property can not be ensured.

In the steel sheet for automobiles, it is also required to subject the present part to a spot welding, so that the spot weldability of steel sheet becomes important.

In the press forming, there may be caused the seizing between the steel sheet and the press mold, or a so-called galling phenomenon. Such a galling unfavorably causes the damage of the mold, considerable degradation of commercial value of the pressed parts and the like.

In the automobile industry, one-side surface treated steel sheets have hitherto been used as a body plate of an automobile exposed to severer corrosion environment, wherein the inner surface of the steel sheet is a plated or organic coating surface and the outer surface thereof is a cold rolled surface. Even in the outer surface of the body plate, however, rusting or blistering is caused due to the collision with gravels, pebbles and so on. Therefore, both-side surface plated steel sheets have lately been used as the body plate.

Since the steel sheet for automobile is subjected to various press forming prior to the assembling into the automobile body, it is required to have an excellent deep drawability. However, the galvanized steel sheets usually used for the automobile are apt to be seized to the press mold in the press forming due to the presence of galvanized coating as compared with the usual cold rolled steel sheet, and are poor in the deep drawability.

The investigations on deep drawing are made from two viewpoints of steel sheet and drawing technique. However, requirements for the steel sheet tend to become high-grade and deversified together with the accurate increase and complication of the product. Particularly, this tendency is strong in the cold rolled steel sheet for automobile.

For instance, plural pressed parts are spot-welded in the assembling of the vehicle body at the present. Therefore, it is strongly demanded to reduce the number of spot-welded points by oversizing the pressed part or making these pressed parts into one body. On the other hand, the car design becomes more complicated in compliance with the various needs, and consequently difficult molding parts increase in the usual cold rolled steel sheet. In order to satisfy these needs, it is necessary to use cold rolled steel sheets having an improved press formability as compared with the usual cold rolled steel sheet.

In the actual press forming, the mechanical properties (r-value, E1, n-value) of steel sheet have hitherto been used as an evaluation standard of press formability, but they are not still insufficient. For instance, the press formability is also largely influenced by the surface roughness of steel sheet, the lubricating oil and the like.

There are some well-known techniques showing an influence of surface roughness of steel sheet upon press formability and so on. For example, Plasticity and Work, Vol. 3, No. 14 (1962-3) discloses that when using a high viscosity lubricating oil, the drawability is most improved at the steel sheet surface roughness of about several .mu.m. On the other hand, Japanese Patent Application Publication No. 59-34,441 discloses that the appearance after painting and press formability are more improved by subjecting a cold rolled steel sheet to a skin pass rolling through a dull roll having a center-line average surface roughness of Ra=2.8 (.mu.m) and peak number of PPI=226 as a roll surface roughness.

These well-known techniques are excellent in view of the improvement of press formability, but have a drawback that the surface roughness of steel sheet should be controlled to a certain level.

In Japanese Patent laid open No. 54-97,527 is disclosed a method wherein a cold rolled steel sheet having an improved phosphatability can be produced by subjecting to a skin pass rolling through a roll having PPI=150 as a roll surface roughness. This method provides an excellent phosphatability but does not develop an effect on press formability. In general, the phosphatability is required for the steel sheet used in automobiles, and also the press formability and distinctness of image after painting (DOI) become necessarily important.

The aforementioned conventional techniques do not teach nor suggest a method of producing cold rolled steel sheets and plated steel sheets having an optional surface roughness (Ra, PPI) and excellent press formability, phosphatability, weldability and resistance to galling.

SUMMARY OF THE INVENTION

It is therefore, an object of the invention to solve the above mentioned drawbacks of the conventional technique and to provide cold rolled steel sheets and plate steel sheets having improved press formability, phosphatability, weldability and resistance to galling by giving an orientation to a surface roughness pattern and controlling a center-line average surface roughness, a mean area ratio of convex portions measured at center plane of surface roughness, a mean area per one convex portion at center plane of surface roughness, a mean radius of convex portions measured at center plane of surface roughness and a mean radius of concave portions measured at center plane of surface roughness.

According to the invention, there are provided cold rolled steel sheets and plated steel sheets having an improved press formability, characterized in that the steel sheet has a surface roughness pattern satisfying a center-line average surface roughness (Ra, .mu.m) of 0.3-2.0 .mu.m and a regularity parameter (S) in at least one direction of not more than 0.25 showing a regularity of surface roughness represented by the following equations: ##EQU1## wherein Xi is a distance between peaks of convex portions at the surface of the steel sheet.

In a preferred embodiment of the invention, the surface roughness pattern further satisfies a mean area per one convex portion at center plane of surface roughness (SGr, .mu.m.sup.2) of 2,000-30,000.

In another embodiment of the invention, the surface roughness pattern further satisfies a mean area per one convex portion at center plane of surface roughness (SGr, .mu.m.sup.2) of 2,000-30,000 and at least one requirement selected from a product of center-line average surface roughness (Ra, .mu.m) and mean concave distance (Lmv, .mu.m) of not less than 50, a ratio of mean concave radius (Rmv, .mu.m) to mean convex radius (Rmp, .mu.m) of more than 1, and a mean area ratio of convex portions at center plane of surface roughness (SSr, %) of not less than 45.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanying drawings, wherein:

FIG. 1 is a graph showing a relation between regularity parameter (S value) in surface roughness pattern and limit drawing ratio;

FIG. 2 is a graph showing a relation between product of center-line average surface roughness (Ra) and mean concave distance (Lmv) and phosphatability;

FIG. 3 is a graph showing a relation between mean area ratio of convex portions at center plane of surface roughness (SSr) and spot weldability or tensile sharing strength;

FIG. 4 is a graph showing a relation between ratio of mean concave radius (Rmv) to mean convex radius (Rmp) at center plane of surface roughness and resistance to galling;

FIGS. 5 and 6 show surface roughness patterns of steel sheets, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will be described in detail with respect to results of studies resulting in the success of the invention.

Two cold rolled sheets of low carbon aluminum killed steel having a chemical composition shown in the following Table 1 were used as a steel to be tested.

TABLE 1______________________________________Steel C Si Mn P S N Al______________________________________A 0.032 0.02 0.21 0.013 0.008 0.0037 0.045B 0.002 0.01 0.12 0.008 0.004 0.0026 0.032______________________________________

Each of the two test sheets was subjected to a skin pass rolling through a pair of rolls, at least one of which being subjected to a dulling through a laser (hereinafter referred to as a laser dulling), at a draft of 0.8%. In this case, the surface roughness pattern of the steel sheet after skin pass rolling was changed by varying the laser dulling process. The measured results are shown in FIG. 1, wherein S value is a measured value in the rolling direction of the steel sheet.

As seen from FIG. 1, the center-line average surface roughness (Ra) was about 1.2 .mu.m, while the limit drawing ratio was strongly dependent upon S value. The press formability was considerably improved at S.ltoreq.0.25.

Further, when the mean area per one convex portion at center plane of steel sheet SGr (.mu.m.sup.2) after skin pass rolling is limited to a range of 2,000 to 30,000, the press formability is further improved and also the distinctness of image is effectively improved.

Further, a relation between a product of centerline average surface roughness (Ra, .mu.m) after the skin pass rolling of the steel sheet and mean concave distance (Lmv, .mu.m) and a phosphatability was examined with respect to the steel B of Table 1 to obtain results as shown in FIG. 2. In this case, the draft in the skin pass rolling was 0.8% and S value was 0.18.

The phosphatability was evaluated by pin hole area ratio when the steel sheet was degreased, washed with water, phosphated and subjected to a pin hole test as mentioned later. Moreover, the phosphate treatment was carried out with BT 3112 made by Japan Perkerizing K.K., by adjusting to total acidity of 14.3 and free acidity of 0.5 and then spraying for 120 seconds.

Pin hole test:

A non-covered portion of phosphate crystal coating in the surface of the steel sheet was detected by sticking a filter paper impregnated with a reagent developing a color through reaction with iron ion to the steel sheet surface, which was numeralized as a pin hole area ratio by image analysis. As a standard on the evaluation of phosphatability, 1 is a case that the pin hole area ratio is less than 0.5%, 2 is a case that the ratio is 0.5-2%, 3 is a case that the ratio is 2-9%, 4 is a case that the ratio is 9-15%, and 5 is a case that the ratio is more than 15%. 1 and 2 show evaluation causing no problem in practical use.

As seen from FIG. 2, the phosphatability is largely dependent on Ra.times.Lmv and is considerably improved at Ra.times.Lmv.gtoreq.50.

Moreover, a relation between a mean area ratio of convex portions at center plane of surface roughness (SSr, %) and a spot weldability (or tensile shearing strength) was examined with respect to the steel B of Table 1 after the skin pass rolling to obtain results as shown in FIG. 3. In this case, the sheet gauge was 0.8 mm, the draft in the skin pass rolling was 0.8% and S value was 0.15.

The spot weldability was largely dependent upon SSr. The tensile shearing strength after spot welding was remarkably improved at SSr.gtoreq.45(%). As spot welding conditions, the welding time was 8 seconds, the pressing force was 190 kg and the welding current was 7,800 A.

And also, a relation between a ratio of mean concave radius (Rmv, .mu.m) to mean convex radius (Rmp, .mu.m) at center plane of surface roughness and a resistance to galling was examined with respect to the steel B of Table 1 after the skin pass rolling to obtain results as shown in FIG. 4. In this case, the draft in the skin pass rolling was 0.8% and S value was 0.16. As seen from FIG. 4, the resistance to mold dropping off is strongly dependent upon Rmv/Rmp and is considerably improved at Rmv/Rmp>1.

The inventors have made further studies on the basis of the above fundamental data, and found that cold rolled steel sheets and plated steel sheets having improved press formability, phosphatability, spot weldability and resistance to galling can be produced by controlling the production conditions as mentioned below.

At first, the surface roughness pattern of the steel sheet is most important.

Then, the regularity parameter S showing a regularity of surface roughness in the steel sheet according to the invention can be expressed by the following equations when a distance between peaks of convex portions on the steel sheet surface is Xi; ##EQU2##

The mean concave distance Lmv is expressed by the following equation in the surface roughness pattern shown in FIG. 5: ##EQU3##

Further, the mean convex radius Rmp and mean concave radius Rmv at center plane of surface roughness are expressed by the following equations in the surface roughness pattern shown in FIG. 6, respectively: ##EQU4## wherein Sp is an area of convex portion at center plane, Sv is an area of concave portion at center plane, np is number of convex portions at center plane and nv is number of concave portions at center plane.

According to the invention, the regularity parameter S is required to satisfy S.ltoreq.0.25 in at least one direction. When S>0.25, the excellent press formability can not be obtained. In the conventional cold rolled steel sheet, S value is about 0.3-0.5.

The center-line average surface roughness (Ra) of the steel sheet is essential to be within a range of 0.3-2.0 .mu.m. When Ra<0.3 .mu.m, the excellent press formability can not be obtained, while when Ra>2.0 .mu.m, the distinctness of image substantially equal to that of the usually used steel sheet can not be obtained.

The mean area per one convex portion at center plane of surface roughness SGr (.mu.m.sup.2) is necessary to be within a range of 2,000-30,000. When SGr<2,000, the distinctness of image substantially equal to that of the usually used steel sheet can not be obtained, while when SGr>30,000, the press formability is degraded.

Further, the center-line average surface roughness Ra (.mu.m) and mean concave distance Lmv (.mu.m) are necessary to satisfy a relation of Ra.times.Lmv.gtoreq.50. When Ra.times.Lmv<50, the excellent phosphatability can not be obtained.

The mean area ratio of convex portion at center plane of surface roughness SSr(%) is necessary to be not less than 45%. When SSr<45%, the excellent spot weldability can not be obtained.

Moreover, the mean convex radius Rmp (.mu.m) and mean concave radius Rmv (.mu.m) at center plane of surface roughness are necessary to satisfy a relation of Rmv/Rmp>1. When Rmv/Rmp.ltoreq.1, the desired resistance to galling can not be obtained.

In order to provide the regular surface roughness pattern satisfying the above requirements on the steel sheet, the surface roughness pattern of skin pass roll should necessarily be regular. For this purpose, the skin pass roll is subjected to a discharge dulling process, a laser dulling process or a shot blast process using a specially formed grid.

According to the invention, the kind of lubricating oil and the pressing conditions are optional.

The effect of the regular surface roughness pattern of steel sheet according to the invention is considered to make good the lubrication condition resulted from the fact that the lubricating oil pooled in concave portions on the steel sheet surface is equally supplied to convex portions. Furthermore, it is considered that the friction state between the steel sheet and the press mold is well improved owing to the fact that metal-contacting parts of convex portions are regularly present on the steel sheet surface.

As to the phosphatability, the surface roughness pattern of the steel sheet is considered to influence the formation of phosphate crystal nucleus, the detail of which is not clear.

Further, it is considered that the spot weldability is improved at SSr.gtoreq.45% because the bonding property between steel sheet surfaces in the spot welding is good.

As regards the resistance to galling, it is considered that iron powders produced in the press working are apt to flow into concave portions at Rmv/Rmp>1 and mitigate the seizing phenomenon between the steel sheet and the press mold.

The invention will be described in detail with reference to the following examples.

A steel slab having a chemical composition as shown in the following Table 2, 5, 8, 11, 14, 17, 20, 23 or 26 was produced by a converter-continuous casting process, soaked by heating at 1,250.degree. C., and subjected to rough rolling-finish rolling to obtain a hot rolled steel sheet of 3.2 mm in thickness. The resulting steel sheet was pickled, cold rolled to a thickness of 0.8 mm, and subjected to a continuous annealing (soaking temperature: 750.degree.-850.degree. C.) and further to a skin pass rolling (draft: 0.8%).

In this case, a roll dulled by shot blast or laser process was used as a skin pass roll.

The surface roughness of the steel sheet was measured in an L-direction to obtain center-line average surface roughness Ra, ten-point average roughness Rz and regularity parameter S. Further, the mean area per one convex portion at center plane of surface roughness SGr, mean area ratio of convex portions SSr, mean convex radius Rmp and mean concave radius Rmv were determined by using a three-dimensional surface roughness measuring meter.

The tensile properties were measured by using a No. 5 test piece defined in JIS Z 2201. The Lankford value was measured by a three-point method in L-direction (rolling direction), C-direction (90.degree. to rolling direction) and D-direction (45.degree. to rolling direction) under a tensile prestrain of 15%, from which r-value was calculated according to an equation of r=(r.sub.L +r.sub.C +2r.sub.D)/4.

The limit drawing ratio (L.D.R.) was calculated according to the following equation by measuring a maximum diameter (D.sub.O max) of sheet capable of deep drawing in a mold with a punch having a diameter (dp) of 32 mm: ##EQU5## As the drawing conditions, the drawing speed was 1 mm/sec and the lubricating oil was a rust preventive oil (oil type).

Moreover, the plated steel sheet was produced by subjecting the cold rolled steel sheet to a skin pass rolling (draft: 0.8%) and further to zinc electroplating, Zn-Ni alloy electroplating or Zn-Fe alloy electroplating, or by subjecting a cold rolled steel sheet to a zinc hot dipping and further to a skin pass rolling (draft: 0.8%).

EXAMPLE 1

Table 2 shows a chemical composition of a cold rolled steel sheet used, and Table 3 shows a dulling method for skin pass roll, and surface roughness and properties of the steel sheet. As seen from Table 3, the cold rolled steel sheets according to the invention satisfying 0.3.ltoreq.Ra.ltoreq.2.0 and S.ltoreq.0.25 exhibit an excellent press formability as compared with the comparative steel sheets.

TABLE 2______________________________________Steel C Si Mn P S N Al X______________________________________C 0.035 0.02 0.18 0.012 0.007 0.0038 0.046 --D 0.002 0.01 0.11 0.007 0.004 0.0029 0.030 --E 0.002 0.01 0.12 0.007 0.005 0.0023 0.031 Ti: 0.028F 0.003 0.02 0.13 0.008 0.004 0.0026 0.033 Nb: 0.015G 0.002 0.01 0.09 0.006 0.003 0.0022 0.028 Ti: 0.014 Nb: 0.008______________________________________ TABLE 3__________________________________________________________________________ SurfaceRoll roughness Properties dulling Ra YS TS ElSteel method (.mu.m) S value (Kg/mm.sup.2) (Kg/mm.sup.2) (%) -r L.D.R. Remarks__________________________________________________________________________C shot blast 1.2 0.35 20 35 44 1.4 2.12 Comparative ExampleC shot blast 1.3 0.41 20 35 44 1.4 2.09 Comparative ExampleC laser 1.1 0.12 20 35 44 1.4 2.31 ExampleD laser 1.3 0.23 15 28 50 2.0 2.48 ExampleD shot blast 1.2 0.50 15 28 50 2.0 2.28 Comparative ExampleE laser 1.9 0.21 16 29 52 2.2 2.51 ExampleF laser 2.0 0.18 17 30 50 2.1 2.53 ExampleG shot blast 0.6 0.38 16 29 51 2.2 2.31 Comparative ExampleG laser 0.7 0.08 16 29 51 2.2 2.53 Example__________________________________________________________________________

EXAMPLE 2

Table 5 shows a dulling method for skin pass roll, kind of plating and surface roughness and properties of the plated steel sheet having a chemical composition as shown in Table 4. As seen from Table 5, the plated steel sheets according to the invention satisfying 0.3.ltoreq.Ra.ltoreq.2.0 and S.ltoreq.0.25 exhibit an excellent press formability compared with the comparative steel sheets.

TABLE 4______________________________________C Si Mn P S N Al Ti______________________________________Steel 0.002 0.01 0.09 0.007 0.008 0.0022 0.064 0.041______________________________________ TABLE 5__________________________________________________________________________ Surface rough- Deep Roll ness of plated draw- dulling steel sheet ability method Kind of plating Ra (.mu.m) S value (L.D.R.)__________________________________________________________________________Example laser Zn electroplating 1.2 0.20 2.31Example laser Zn--Ni alloy electroplating 1.5 0.15 2.33Example laser Zn--Fe alloy electroplating 1.1 0.18 2.28Example laser Zn hot dipping 0.8 0.09 2.34 (after plating)Example laser Zn electroplating 1.3 0.16 2.27Example laser Zn electroplating 1.4 0.08 2.22Comparative shot blast Zn electroplating 1.2 0.32 2.07ExampleComparative shot blast Zn--Fe alloy electroplating 1.2 0.29 2.10Example__________________________________________________________________________

EXAMPLE 3

Table 6 shows a chemical composition of a cold rolled steel sheet used, and Table 7 shows a dulling method for skin pass roll, and surface roughness and properties of the steel sheet. As seen from Table 7, the cold rolled steel sheets according to the invention satisfying 0.3.times.Ra.ltoreq.2.0, S.ltoreq.0.25 and 2,000.ltoreq.SGr.ltoreq.30,000 exhibit excellent press formability and distinctness of image after painting as compared with the comparative steel sheets.

TABLE 6______________________________________Steel C Si Mn P S N Al X______________________________________C 0.036 0.02 0.22 0.016 0.007 0.0044 0.047 --D 0.002 0.01 0.16 0.009 0.004 0.0029 0.029 --E 0.004 0.01 0.12 0.011 0.004 0.0031 0.031 Ti: 0.035F 0.003 0.01 0.15 0.012 0.003 0.0033 0.032 Nb: 0.021G 0.002 0.01 0.10 0.008 0.004 0.0028 0.028 Ti: 0.016 Nb: 0.009______________________________________ TABLE 7__________________________________________________________________________ SurfaceRoll roughness Properties dulling Ra SGr YS TS ElSteel method (.mu.m) S value (.mu.m.sup.2) (kg/mm.sup.2) (kg/mm.sup.2) (%) -r L.D.R. DOI Remarks__________________________________________________________________________C shot blast 1.2 0.33 1640 18 34 44 1.2 2.05 85 Comparative ExampleC laser 1.1 0.19 3150 18 34 44 1.2 2.31 96 ExampleC laser 1.2 0.16 2120 18 34 44 1.2 2.35 95 ExampleD laser 1.6 0.15 2530 15 28 51 1.8 2.49 95 ExampleD shot blast 1.7 0.41 1230 15 28 51 1.8 2.33 82 Comparative ExampleE shot blast 0.8 0.28 1850 16 29 52 1.9 2.34 86 Comparative ExampleE laser 0.9 0.08 2430 16 29 52 1.9 2.58 94 ExampleF laser 1.2 0.20 3960 16 28 50 2.0 2.51 97 ExampleF shot blast 1.3 0.31 1090 16 28 50 2.0 2.31 81 Comparative ExampleG shot blast 3.1 0.36 1810 17 30 49 1.9 2.29 87 Comparative ExampleG laser 2.0 0.12 2630 17 30 49 1.9 2.52 95 ExampleG laser 3.0 0.18 31510 17 30 49 1.9 2.21 88 Comparative Example__________________________________________________________________________

EXAMPLE 4

Table 9 shows a dulling method for sking pass roll, kind of plating and surface roughness and properties of the plated steel sheet having a chemical composition as shown in Table 8. As seen from Table 9, the plated steel sheets according to the invention satisfying 0.3.ltoreq.Ra.ltoreq.2.0, S.ltoreq.0.25 and 2,000.ltoreq.SGr.ltoreq.30,000 exhibit excellent press formability and distinctness of image after painting as compared with the comparative steel sheets.

TABLE 8__________________________________________________________________________C Si Mn P S N Al Ti Nb B__________________________________________________________________________Steel 0.002 0.01 0.11 0.008 0.002 0.0028 0.052 0.014 0.008 0.009__________________________________________________________________________ TABLE 9__________________________________________________________________________ Surface roughness of plated Deep Roll steel sheet draw- dulling Ra SGr ability method Kind of plating (.mu.m) S value (.mu.m.sup.2) (L.D.R.) DOI__________________________________________________________________________Example laser Zn electroplating 1.1 0.20 3210 2.30 95Example laser Zn--Ni alloy electroplating 1.6 0.16 2640 2.32 93Example laser Zn--Fe alloy electroplating 1.2 0.18 2330 2.28 96Example laser Zn hot dipping 0.9 0.09 2560 2.33 94 (after plating)Example laser Zn electroplating 1.3 0.15 15630 2.26 96Example laser Zn electroplating 1.2 0.08 28340 2.22 97Comparative shot blast Zn electroplating 1.2 0.34 1540 2.05 83ExampleComparative shot blast Zn--Ni alloy electroplating 3.0 0.33 1910 2.25 86ExampleComparative shot blast Zn--Fe alloy electroplating 1.2 0.28 1008 2.08 82ExampleComparative laser Zn hot dipping 2.9 0.21 32620 2.10 86Example (after plating)__________________________________________________________________________

EXAMPLE 5

Table 10 shows a chemical composition of a cold rolled steel sheet used, and Table 11 shows a dulling method for skin pass roll, and surface roughness and properties of the steel sheet. As seen from Table 11, the cold rolled steel sheets according to the invention satisfying 0.3.ltoreq.Ra.ltoreq.2.0, S.ltoreq.0.25, 2,000.ltoreq.SGr.ltoreq.30,000 and Ra.times.Lmv.gtoreq.50 exhibit excellent press formability, distinctness of image after painting and phosphatability as compared with the comparative steel sheets.

TABLE 10______________________________________Steel C Si Mn P S N Al X______________________________________C 0.034 0.02 0.16 0.011 0.006 0.0035 0.042 --D 0.002 0.01 0.11 0.008 0.004 0.0028 0.035 --E 0.002 0.01 0.12 0.007 0.003 0.0026 0.036 Ti: 0.022F 0.003 0.01 0.11 0.009 0.004 0.0025 0.036 Nb: 0.014G 0.002 0.01 0.08 0.008 0.005 0.0027 0.037 Ti: 0.013 Nb: 0.009______________________________________ TABLE 11__________________________________________________________________________ PropertiesRoll Surface roughness Phos- dulling Ra Lmv SGr YS TS El phat-Steel method (.mu.m) S value (.mu.m) (.mu.m.sup.2) (kg/mm.sup.2) (kg/mm.sup.2) (%) -r L.D.R. ability DOI Remarks__________________________________________________________________________C shot blast 1.4 0.36 25.6 1630 20 35 45 1.4 2.09 5 84 Comparative ExampleC shot blast 1.3 0.44 33.7 2420 20 35 45 1.4 2.06 4 88 Comparative ExampleC laser 1.5 0.20 50.6 3940 20 35 45 1.4 2.36 1 96 ExampleD laser 1.9 0.16 70.4 11420 15 28 52 1.9 2.51 1 97 ExampleD shot blast 2.3 0.35 19.6 1360 15 28 52 1.9 2.31 4 83 Comparative ExampleE shot blast 1.3 0.32 30.2 2650 16 29 52 2.1 2.33 5 88 Comparative ExampleE laser 1.2 0.06 46.6 3380 16 29 52 2.1 2.58 1 95 ExampleF laser 1.8 0.21 30.6 2160 17 30 50 2.0 2.55 1 94 ExampleF shot blast 1.7 0.41 12.1 1830 17 30 50 2.0 2.29 5 82 Comparative ExampleG shot blast 0.7 0.30 46.3 7860 16 29 51 2.2 2.30 5 86 Comparative ExampleG laser 0.8 0.19 65.4 16430 16 29 51 2.2 2.56 2 97 Example__________________________________________________________________________

EXAMPLE 6

Table 13 shows a dulling method for sking pass roll, kind of plating the surface roughness and properties of the plated steel sheet having a chemical composition as shown in Table 12. As seen from Table 13, the plated steel sheets according to the invention satisfying 0.3.ltoreq.Ra.ltoreq.2.0, S.ltoreq.0.25, 2,000.ltoreq.SGr.ltoreq.30,000 and Ra.times.Lmv.gtoreq.50 exhibit excellent press formability, distinctness of image after painting and phosphatability as compared with the comparative steel sheets.

TABLE 12______________________________________C Si Mn P S N Al Ti Nb______________________________________Steel 0.001 0.01 0.08 0.007 0.003 0.0029 0.051 0.019 0.006______________________________________ TABLE 13__________________________________________________________________________ Surface roughness of Deep Roll plated steel sheet draw- Phos- dulling Ra SGr Lmv ability phat- method Kind of plating (.mu.m) S value (.mu.m.sup.2) (.mu.m) (L.D.R.) DOI ability__________________________________________________________________________Example laser Zn electroplating 1.2 0.21 3240 51.2 2.30 95 1Example laser Zn--Ni alloy electroplating 1.6 0.15 2730 33.6 2.32 93 1Example laser Zn--Fe alloy electroplating 1.1 0.19 2310 47.4 2.28 96 1Example laser Zn hot dipping 0.8 0.09 2430 63.4 2.33 91 1 (after plating)Example laser Zn electroplating 1.4 0.16 15600 38.2 2.26 96 1Example laser Zn electroplating 1.2 0.09 27410 44.1 2.22 97 1Comparative shot blast Zn electroplating 1.1 0.34 1530 41.6 2.05 83 5ExampleComparative shot blast Zn--Ni alloy electroplating 3.1 0.33 1820 14.8 2.25 86 4ExampleComparative shot blast Zn--Fe alloy electroplating 1.3 0.29 1010 35.2 2.08 82 5Example__________________________________________________________________________

EXAMPLE 7

Table 14 shows a chemical composition of a cold rolled steel sheet used, and Table 15 shows a dulling method for skin pass roll, and surface roughness and properties of the steel sheet. As seen from Table 15, the cold rolled steel sheets according to the invention satisfying 0.3.ltoreq.Ra.ltoreq.2.0, S.ltoreq.0.25, 2,000.ltoreq.SGr.ltoreq.30,000 and Rmv/Rmp>1 exhibit excellent press formability, distinctness of image after painting and resistance to galling as compared with the comparative steel sheets.

TABLE 14______________________________________Steel C Si Mn P S N Al X______________________________________C 0.035 0.02 0.21 0.019 0.008 0.0038 0.045 --D 0.003 0.02 0.18 0.009 0.006 0.0022 0.039 B:0.002E 0.002 0.01 0.16 0.008 0.004 0.0021 0.036 Ti:0.029F 0.002 0.01 0.17 0.010 0.005 0.0019 0.032 Nb:0.012G 0.002 0.02 0.12 0.008 0.003 0.0026 0.037 Ti:0.008 Nb:0.011______________________________________ TABLE 15__________________________________________________________________________ PropertiesRoll Surface roughness Resist- dulling Ra Rmv/ SGr YS TS El ance toSteel method (.mu.m) S value Rmp (.mu.m) (kg/mm.sup.2) (kg/mm.sup.2) (%) r L.D.R. galling DOI Remarks__________________________________________________________________________C shot blast 1.2 0.36 0.83 2860 19 34 46 1.3 2.08 5 82 Comparative ExampleC shot blast 1.3 0.34 0.88 1690 19 34 46 1.3 2.06 5 83 Comparative ExampleC laser 1.2 0.19 1.54 2790 19 34 46 1.3 2.33 1 95 ExampleD shot blast 1.9 0.29 0.92 1880 15 28 52 1.8 2.32 4 84 Comparative ExampleD laser 1.8 0.20 1.23 3820 15 28 52 1.8 2.58 1 96 ExampleE laser 1.6 0.18 1.32 2820 16 29 50 2.1 2.55 1 95 ExampleE shot blast 1.5 0.39 0.73 1560 16 29 50 2.1 2.39 5 83 Comparative ExampleF shot blast 0.8 0.35 0.69 1840 17 30 51 2.0 2.33 5 84 Comparative ExampleF laser 0.9 0.22 1.12 7540 17 30 51 2.0 2.52 2 97 ExampleG laser 1.5 0.08 1.29 5520 16 29 50 2.2 2.56 1 95 ExampleG shot blast 1.6 0.30 0.82 1860 16 29 50 2.2 2.29 5 84 Comparative Example__________________________________________________________________________

EXAMPLE 8

Table 17 shows a dulling method for sking pass roll, kind of plating and surface roughness and properties of the plated steel sheet having a chemical composition as shown in Table 16. As seen from Table 17, the plated steel sheets according to the invention satisfying 0.3.ltoreq.Ra.ltoreq.2.0, S.ltoreq.0.25, 2,000.ltoreq.SGr.ltoreq.30,000 and Rmv/Rmp>1 exhibit excellent press formability, distinctness of image after painting and resistance to galling as compared with the comparative steel sheets.

TABLE 16______________________________________C Si Mn P S N Al Ti Nb______________________________________Steel 0.002 0.02 0.12 0.009 0.004 0.0019 0.061 0.026 0.011______________________________________ TABLE 17__________________________________________________________________________ Surface roughness of Deep Roll plated steel sheet draw- Resist- dulling Ra Rmv/ SGr ability ance to method Kind of plating (.mu.m) S value Rmp (.mu.m.sup.2) (L.D.R.) galling DOI__________________________________________________________________________Example laser Zn electroplating 1.2 0.15 1.51 2210 2.26 1 94Example laser Zn electroplating 1.1 0.16 1.12 2340 2.34 1 94Example laser Zn electroplating 1.2 0.19 1.65 3420 2.25 1 96Example laser Zn electroplating 1.1 0.11 1.13 3300 2.32 1 95Example laser Zn electroplating 1.3 0.20 1.61 2380 2.31 1 94Example laser Zn--Ni alloy electroplating 1.3 0.17 1.32 4210 2.28 1 94Example laser Zn--Fe alloy electroplating 1.2 0.09 1.23 3150 2.26 1 95Example laser Zn hot dipping 0.9 0.13 1.20 2440 2.33 2 95 (after plating)Comparative shot blast Zn electroplating 1.3 0.35 0.89 1890 2.16 4 75ExampleComparative shot blast Zn electroplating 1.3 0.42 0.98 2240 2.07 3 70ExampleComparative shot blast Zn electroplating 1.1 0.38 0.86 1830 2.03 5 85ExampleComparative shot blast Zn electroplating 1.1 0.29 0.75 2540 2.05 5 88ExampleComparative shot blast Zn electroplating 1.4 0.34 0.81 3220 2.05 5 81ExampleComparative shot blast Zn--Ni alloy electroplating 1.4 0.41 0.69 1850 2.01 5 80ExampleComparative shot blast Zn--Fe alloy electroplating 1.3 0.28 0.93 2110 2.00 5 82ExampleComparative shot blast Zn hot dipping 1.2 0.36 0.86 3120 2.09 5 83Example (after plating)__________________________________________________________________________

EXAMPLE 9

Table 18 shows a chemical composition of a cold rolled steel sheet used, and Table 19 shows a dulling method for skin pass roll, and surface roughness and properties of the steel sheet. As seen from Table 19, the cold rolled steel sheets according to the invention satisfying 0.3.ltoreq.Ra.ltoreq.2.0, S.ltoreq.0.25, 2,000.ltoreq.SGr.ltoreq.30,000 and SSr.gtoreq.45 exhibit excellent press formability, distinctness of image after painting and spot weldability as compared with the comparative steel sheets.

TABLE 18______________________________________Steel C Si Mn P S N Al X______________________________________C 0.033 0.02 0.20 0.013 0.007 0.0041 0.043 --D 0.002 0.01 0.09 0.009 0.005 0.0026 0.029 --E 0.003 0.01 0.12 0.011 0.003 0.0029 0.031 Ti:0.033F 0.002 0.01 0.15 0.007 0.004 0.0031 0.028 Nb:0.013G 0.004 0.01 0.13 0.010 0.003 0.0025 0.033 Ti:0.009 Nb:0.010______________________________________ TABLE 19__________________________________________________________________________ Properties TensileRoll Surface roughness shearing dulling Ra SSr SGr YS TS El forceSteel method (.mu.m) S value (%) (.mu.m.sup.2) (kg/mm.sup.2) (kg/mm.sup.2) (%) r L.D.R. (kg/mm.sup.2) DOI Remarks__________________________________________________________________________C shot blast 1.6 0.36 42 2820 19 35 45 1.3 2.08 353 81 Comparative ExampleC shot blast 1.5 0.33 33 3660 19 35 45 1.3 2.05 341 84 Comparative ExampleC laser 1.6 0.19 56 5420 19 35 45 1.3 2.35 482 95 ExampleD shot blast 1.2 0.41 39 1220 15 28 52 1.9 2.31 342 82 Comparative ExampleD laser 1.1 0.22 62 6930 15 28 52 1.9 2.51 452 96 ExampleE shot blast 0.9 0.32 41 2860 16 28 51 2.1 2.33 351 84 Comparative ExampleE laser 0.8 0.09 48 11220 16 28 51 2.1 2.59 446 97 ExampleF laser 1.2 0.11 51 1820 16 29 50 2.0 2.53 449 92 Comparative ExampleF shot blast 1.3 0.29 40 2290 16 29 50 2.0 2.35 340 83 Comparative ExampleG laser 0.5 0.23 49 2290 17 30 50 2.1 2.50 445 95 ExampleG shot blast 0.6 0.43 37 5230 17 30 50 2.1 2.31 338 84 Comparative Example__________________________________________________________________________

EXAMPLE 10

Table 21 shows a dulling method for sking pass roll, kind of plating and surface rougness and properties of the plated steel sheet having a chemical composition as shown in Table 20. As seen from Table 21, the plated steel sheets according to the invention satisfying 0.3.ltoreq.Ra.ltoreq.2.0, S.ltoreq.0.25, 2,000.ltoreq.SGr.ltoreq.30,000 and SSr.gtoreq.45 exhibit excellent press formability, distinctness of image after painting and spot weldability as compared with the comparative steel sheets.

TABLE 20______________________________________C Si Mn P S N Al Ti______________________________________Steel 0.002 0.02 0.09 0.010 0.008 0.0022 0.043 0.044______________________________________ TABLE 21__________________________________________________________________________ Surface roughness of Tensile Deep Roll plates steel sheet shearing draw- dulling Ra Rz SSr SGr force ability method Kind of plating (.mu.m) (.mu.m) S value (%) (.mu.m.sup.2) (kg/mm.sup.2) (L.D.R.) DOI Remarks__________________________________________________________________________Example laser Zn electroplating 1.9 10.0 0.21 48 2980 443 2.21 95Example laser Zn--Fe alloy electroplating 2.0 10.4 0.20 52 3380 451 2.23 94Example laser Zn electroplating 0.5 0.86 0.16 49 12500 431 2.21 97Example laser Zn electroplating 0.4 1.24 0.18 56 6850 429 2.20 95Example laser Zn electroplating 1.1 5.0 0.12 61 5120 440 2.30 95Example laser Zn--Ni alloy electroplating 1.3 5.9 0.23 48 4280 451 2.21 94Example laser Zn hot dipping 1.1 4.4 0.15 50 3860 428 2.24 93 (after plating)Comparative shot blast Zn electroplating 2.1 9.5 0.34 42 1230 382 2.05 67ExampleComparative shot blast Zn electroplating 0.2 0.8 0.30 43 2890 336 2.02 73 reskinExample pass after platingComparative shot blast Zn--Ni alloy electroplating 1.2 5.4 0.35 38 3340 354 2.03 68ExampleComparative shot blast Zn--Fe alloy electroplating 0.6 3.0 0.38 36 2140 339 2.05 68ExampleComparative shot blast Zn hot dipping 1.2 4.8 0.40 40 2290 368 2.00 65Example (after plating)__________________________________________________________________________

EXAMPLE 11

Table 22 shows a chemical composition of a cold rolled steel sheet used, and Table 23 shows a dulling method for skin pass roll, and surface roughness and properties of the steel sheet. As seen from Table 23, the cold rolled steel sheets according to the invention satisfying 0.3.ltoreq.Ra.ltoreq.2.0, S.ltoreq.0.25, 2,000.ltoreq.SGr.ltoreq.30,000, Ra.times.Lmv.gtoreq.50 and Rmv/Rmp>1 exhibit excellent press formability, distinctness of image after painting, phosphatability and resistance to galling as compared with the comparative steel sheets.

TABLE 22______________________________________Steel C Si Mn P S N Al X______________________________________C 0.035 0.03 0.24 0.019 0.009 0.0034 0.045 --D 0.003 0.02 0.18 0.009 0.006 0.0022 0.039 Ti:0.024 Nb:0.009 B:0.0009E 0.002 0.01 0.16 0.008 0.004 0.0021 0.036 Ti:0.038______________________________________ TABLE 23__________________________________________________________________________ PropertiesRoll Surface roughness Resist- Phos- dulling Ra Rmv/ Lmv SGr YS TS El ance to phat-Steel method (.mu.m) S value Rmp (.mu.m) (.mu.m.sup.2) (kg/mm.sup.2) (kg/mm.sup.2) (%) r L.D.R. galling ability DOI__________________________________________________________________________C shot blast 1.2 0.33 0.82 39.2 1790 19 35 47 1.3 2.06 5 4 84 Compar- ative ExampleC shot blast 1.3 0.31 0.86 32.1 1090 19 35 47 1.3 2.05 5 5 83 Compar- ative ExampleC laser 1.2 0.16 1.53 116.8 4580 19 35 47 1.3 2.36 1 1 97 ExampleD shot blast 0.7 0.27 0.91 14.8 210 16 29 51 1.9 2.34 4 5 82 Compar- ative ExampleD laser 0.8 0.22 1.22 66.3 2320 16 29 51 1.9 2.59 1 1 96 ExampleE laser 1.6 0.16 1.33 83.2 3070 16 28 50 2.1 2.57 1 1 97 ExampleE shot blast 1.5 0.36 0.76 18.6 470 16 28 50 2.1 2.46 5 4 82 Compar- ative Example__________________________________________________________________________

EXAMPLE 12

Table 25 shows a dulling method for sking pass roll, kind of plating and surface roughness and properties of the plated steel sheet having a chemical composition as shown in Table 24. As seen from Table 25, the plates steel sheets according to the invention satisfying 0.3.ltoreq.Ra.ltoreq.2.0, S.ltoreq.0.25, 2,000.ltoreq.SGr.ltoreq.30,000, Ra.times.Lmv.gtoreq.50 and Rmv/Rmp>1 exhibit excellent press formability, distinctness of image after painting, phosphatability and resistance to galling as compared with the comparative steel sheets.

TABLE 24______________________________________C Si Mn P S N Al Ti Nb______________________________________Steel 0.002 0.01 0.10 0.007 0.005 0.0018 0.051 0.031 0.006______________________________________ TABLE 25__________________________________________________________________________ Surface roughness of DeepRoll plated steel sheet draw- Resist- Phos-dulling Ra Lmv SGr ability ance to phat-method Kind of plating (.mu.m) S value Rmv/Rmp (.mu.m) (.mu.m.sup.2) (L.D.R.) galling DOI ability__________________________________________________________________________Example laser Zn electroplating 1.3 0.16 1.24 68.3 2380 2.26 1 93 1Example laser Zn electroplating 1.1 0.19 1.15 75.0 3340 2.25 1 96 1Example laser Zn electroplating 1.3 0.21 1.33 79.8 2830 2.31 1 94 1Example laser Zn--Fe alloy 1.1 0.09 1.23 70.2 2560 2.24 1 95 1 electroplatingCompar- shot blast Zn electroplating 1.4 0.36 0.76 33.8 1550 2.06 5 75 4ativeExampleCompar- shot blast Zn electroplating 1.2 0.38 0.86 37.2 1470 2.03 5 85 5ativeExampleCompar- shot blast Zn--Ni alloy 1.4 0.41 0.69 31.2 1610 2.05 5 80 5ative electroplatingExample__________________________________________________________________________

EXAMPLE 13

Table 26 shows a chemical composition of a cold rolled steel sheet used, and Table 27 shows a dulling method for skin pass roll, and surface roughness and properties of the steel sheet. As seen from Table 27, the cold rolled steel sheets according to the invention satisfying 0.3.ltoreq.Ra.ltoreq.2.0, S.ltoreq.0.25, 2,000.ltoreq.SGr.ltoreq.30,000, Ra.times.Lmv.gtoreq.50 and SSr.gtoreq.45 exhibit excellent press formability, distinctness of image after painting, phosphatability and spot weldability as compared with the comparative steel sheets.

TABLE 26______________________________________Steel C Si Mn P S N Al X______________________________________C 0.036 0.02 0.21 0.012 0.007 0.0038 0.046 --D 0.002 0.01 0.20 0.005 0.004 0.0029 0.030 Ti: 0.0022 Nb: 0.009 B: 0.0008E 0.002 0.01 0.12 0.004 0.005 0.0023 0.031 Ti: 0.033______________________________________ TABLE 27__________________________________________________________________________ Properties TensileRoll Surface roughness shearing Phos- dulling Ra S SSr SGr Lmv YS TS El force phat-Steel method (.mu.m) value (%) (.mu.m.sup.2) (.mu.m) (kg/mm.sup.2) (kg/mm.sup.2) (%) .sup.-r L.D.R. (kg/mm.sup.2) DOI ability Remarks__________________________________________________________________________C shot blast 1.6 0.34 41 340 30.1 19 34 45 1.3 2.02 352 82 4 Compar- ative ExampleC shot blast 1.5 0.32 44 430 29.8 19 34 45 1.3 2.03 340 80 5 Compar- ative ExampleC laser 1.6 0.19 55 2770 48.6 19 34 45 1.3 2.33 486 96 1 ExampleD shot blast 1.2 0.42 39 480 38.6 15 28 51 1.9 2.32 341 83 5 Compar- ative ExampleD laser 1.1 0.21 61 4480 48.3 15 28 51 1.9 2.53 454 97 1 ExampleE shot blast 0.9 0.33 42 1040 50.2 16 28 52 2.2 2.34 352 84 5 Compar- ative ExampleE laser 0.8 0.10 48 3580 73.1 16 28 52 2.2 2.56 446 96 1 Example__________________________________________________________________________

EXAMPLE 14

Table 29 shows a dulling method for sking pass roll, kind of plating and surface roughness and properties of the plated steel sheet having a chemical composition as shown in Table 28. As seen from Table 29 the plated steel sheets according to the invention satisfying 0.3.ltoreq.Ra.ltoreq.2.0, S.ltoreq.0.25, 2,000.ltoreq.SGr.ltoreq.30,000, Ra.times.Lmv.gtoreq.50 and SSr.gtoreq.45 exhibit excellent press formability, distinctness of image after painting, phosphatability and spot weldability as compared with the comparative steel sheets.

TABLE 28______________________________________C Si Mn P S N Al Ti Nb______________________________________Steel 0.002 0.02 0.09 0.009 0.003 0.0014 0.063 0.013 0.012______________________________________ TABLE 29__________________________________________________________________________ Surface roughness of Deep TensileRoll plated steel sheet draw- Phos- shearingdulling Ra SSr Lmv SGr ability phat forcemethod Kind of plating (.mu.m) S value (%) (.mu.m) (.mu.m.sup.2) (L.D.R.) DOI ability (kg/mm.sup.2) Remarks__________________________________________________________________________Example laser Zn electroplating 2.0 0.16 48 60.4 2440 2.29 94 1 453Example laser Zn electroplating 0.6 0.16 51 90.2 6920 2.29 97 1 482Example laser Zn electroplating 0.8 0.18 62 64.3 8640 2.35 95 2 476Example laser Zn--Ni alloy 1.4 0.24 54 45.2 2210 2.21 92 1 473 electroplatingExample laser Zn hot dipping 1.2 0.16 49 53.6 2080 2.26 93 1 452 (after plating)Compar- shot blast Zn electroplating 2.1 0.35 44 22.8 250 2.05 67 5 362ativeExampleCompar- shot blast Zn electroplating 1.2 0.33 43 38.4 660 2.02 73 4 373 reskinative passExample after platingCompar- shot blast Zn electroplating 0.8 0.41 42 58.3 1400 2.05 70 5 381 reskinative passExample after plating__________________________________________________________________________

EXAMPLE 15

Table 30 shows a chemical composition of a cold rolled steel sheet used, and Table 31 shows a dulling method for skin pass roll, and surface roughness and properties of the steel sheet. As seen from Table 31, the cold rolled steel sheets according to the invention satisfying 0.3.ltoreq.Ra.ltoreq.2.0, S.ltoreq.0.245, 2,000.ltoreq.SGr.ltoreq.30,000, Rmv/Rmp>1 and SSr.gtoreq.45 exhibit excellent press formability, distinctness of image after painting, resistance to galling and spot weldability as compared with the comparative steel sheets.

TABLE 30______________________________________Steel C Si Mn P S N Al X______________________________________C 0.033 0.02 0.22 0.013 0.006 0.0045 0.043 --D 0.002 0.01 0.09 0.009 0.005 0.0026 0.029 Ti: 0.0029 Nb: 0.011 B: 0.0012E 0.003 0.01 0.14 0.011 0.004 0.0029 0.031 Ti: 0.033______________________________________ TABLE 31__________________________________________________________________________ Properties TensileRoll Surface roughness YS TS shearing Resist- dulling Ra SSr SGr Rmv/ (kg/ (kg/ El force ance toSteel method (.mu.m) S value (%) (.mu.m.sup.2) Rmp mm.sup.2) mm.sup.2) (%) .sup.-r L.D.R. (kg/mm.sup.2) DOI galling Remarks__________________________________________________________________________C shot blast 1.5 0.33 43 1060 1.33 19 34 45 1.3 2.07 352 84 4 Compar- ative ExampleC shot blast 1.4 0.32 41 1090 1.44 19 34 45 1.3 2.06 345 83 4 Compar- ative ExampleC laser 1.5 0.21 47 2840 1.13 19 34 45 1.3 2.36 483 94 1 ExampleD shot blast 1.2 0.43 40 2120 1.50 16 28 51 1.9 2.32 341 85 3 Compar- ative ExampleD laser 1.2 0.21 48 3840 1.08 16 28 51 1.9 2.50 453 97 1 ExampleE shot blast 0.9 0.32 42 2250 1.38 15 29 52 2.1 2.34 350 84 4 Compar- ative ExampleE laser 1.0 0.08 49 4160 1.04 15 29 52 2.1 2.56 448 96 2 Example__________________________________________________________________________

EXAMPLE 16

Table 33 shows a dulling method for sking pass roll, kind of plating and surface roughness and properties of the plated steel sheet having a chemical composition as shown in Table 32. As seen from Table 33, the plated steel sheets according to the invention satisfying 0.3.ltoreq.Ra.ltoreq.2.0, S.ltoreq.0.25, 2,000.ltoreq.SGr.ltoreq.30,000, Rmv/Rmp>1 and SSr.gtoreq.45 exhibit excellent press formability, distinctness of image after painting, resistance to galling and spot weldability as compared with the comparative steel sheets.

TABLE 32______________________________________C Si Mn P S N Al Ti Nb______________________________________Steel 0.001 0.02 0.08 0.008 0.008 0.0018 0.044 0.016 0.008______________________________________ TABLE 33__________________________________________________________________________ Surface roughness of Deep TensileRoll plated steel sheet draw- Resist- shearingdulling Ra SSr SGr ability ance to forcemethod Kind of plating (.mu.m) S value Rmv/Rmp (%) (.mu.m.sup.2) (L.D.R.) galling DOI (kg/mm.sup.2)__________________________________________________________________________Example laser Zn electroplating 1.2 0.14 1.13 47 2060 2.24 1 94 438Example laser Zn electroplating 1.2 0.19 1.17 46 2850 2.26 1 96 443Example laser Zn--Ni alloy 1.4 0.16 1.08 48 3620 2.27 1 94 439 electroplatingCompar- shot blast Zn electroplating 1.3 0.38 1.38 42 1860 2.06 5 75 352ativeExampleCompar- shot blast Zn electroplating 1.1 0.38 1.44 41 2240 2.01 4 85 348ativeExampleCompar- shot blast Zn electroplating 1.3 0.34 1.27 44 3810 2.04 5 81 339ativeExample__________________________________________________________________________

EXAMPLE 17

Table 34 shows a chemical composition of a cold rolled steel sheet used, and Table 35 shows a dulling method for skin pass roll, and surface roughness and properties of the steel sheet. As seen from Table 35, the cold rolled steel sheets according to the invention satisfying 0.3.ltoreq.Ra.ltoreq.2.0, S.ltoreq.0.25, 2,000.ltoreq.SGr.ltoreq.30,000, Ra.times.Lmv.gtoreq.50, Rmv/Rmp>1 and SSr.gtoreq.45 exhibit excellent press formability, distinctness of image after painting, phosphatability, resistance to galling and spot weldability as compared with the comparative steel sheets.

TABLE 34______________________________________Steel C Si Mn P S N Al X______________________________________C 0.035 0.02 0.22 0.019 0.006 0.0041 0.045 --D 0.003 0.02 0.18 0.009 0.006 0.0022 0.039 Ti: 0.029 Nb: 0.009 B: 0.0008E 0.002 0.01 0.16 0.008 0.004 0.0021 0.036 Ti: 0.034______________________________________ TABLE 35__________________________________________________________________________ Tensile shear- Surface roughness Properties ing Roll S YS TS Resist- force Phos- dulling Ra val- Rmv/ SGr Lmv SSr (kg/ (kg/ El ance to (kg/ phat-Steel method (.mu.m) ue Rmp (.mu.m.sup.2) (.mu.m) (%) mm.sup.2) mm.sup.2) (%) .sup.-r L.D.R. galling mm.sup.2) DOI ability__________________________________________________________________________C shot 1.2 0.32 0.82 1310 33.5 55 19 34 46 1.3 2.01 5 413 82 5 Com- blast par- ative Ex- ampleC shot 1.2 0.38 0.86 1640 39.3 54 19 34 46 1.3 2.05 5 408 84 5 Com- blast par- ative Ex- ampleC laser 1.3 0.21 1.08 3220 69.2 48 19 34 46 1.3 2.32 1 451 96 2 Ex- ampleD shot 2.0 0.27 0.94 370 20.3 52 16 29 51 1.8 2.36 4 402 83 5 Com- blast par- ative Ex- ampleD laser 1.9 0.21 1.21 2170 63.6 45 16 29 51 1.8 2.59 1 421 94 1 Ex- ampleE laser 1.9 0.19 1.12 3200 71.5 47 16 29 50 2.2 2.53 1 461 97 1 Ex- ample__________________________________________________________________________

EXAMPLE 18

Table 37 shows a dulling method for sking pass roll, kind of plating and surface roughness and properties of the plates steel sheet having a chemical composition as shown in Table 36. As seen from Table 37, the plated steel sheets according to the invention satisfying 0.3.ltoreq.Ra.ltoreq.2.0, S.ltoreq.0.25, 2,000.ltoreq.SGr.ltoreq.30,000, Ra.times.Lmv.gtoreq.50, Rmv/Rmp>1 and SSr.gtoreq.45 exhibit excellent press formability, distinctness of image after painting, phosphatability, resistance to galling and spot weldability as compared with the comparative steel sheets.

TABLE 36__________________________________________________________________________C Si Mn P S N Al Ti Nb B__________________________________________________________________________Steel 0.002 0.01 0.11 0.009 0.005 0.0015 0.032 0.011 0.005 0.0011__________________________________________________________________________ TABLE 37__________________________________________________________________________ Surface roughness of Deep TensileRoll plated steel sheet draw- Resist- shearing Phos-dulling Kind of Ra Rmv/ SGr Lmv SSr ability ance to force phat-method plating (.mu.m) S value Rmp (.mu.m.sup.2) (.mu.m) (%) (L.D.R.) galling DOI (kg/mm.sup.2) ability__________________________________________________________________________Example laser Zn electro- 1.3 0.13 1.08 2440 60.4 48 2.26 1 93 481 1 platingExample laser Zn--Ni alloy 0.8 0.17 1.12 3040 70.1 47 2.21 1 96 471 1 electro- platingExample laser Zn--Fe alloy 1.9 0.18 1.20 2130 63.6 45 2.32 1 92 432 1 electro- platingCompar- shot Zn electro- 1.4 0.36 0.94 860 30.6 52 2.06 4 75 425 5ative blast platingExampleCompar- shot Zn electro- 1.1 0.38 0.86 1760 40.3 54 2.00 5 85 436 5ative blast platingExample__________________________________________________________________________

As mentioned above, according to the invention, the regular surface roughness pattern is given to the surface of the cold rolled or plated steel sheet and factors thereof are controlled to given levels, whereby cold rolled steel sheets and plated steel sheets having improved press formability as well as excellent phosphatability, resistance to galling and spot weldability can be produced.

Claims

1. A cold rolled steel sheet having an improved press formability, characterized in that said steel sheet has a surface roughness pattern satisfying a center-line average surface roughness (Ra, .mu.m) of 0.3-2.0 and a regularity parameter (S) in at least one direction of not more than 0.25 showing a regularity of surface roughness represented by the following equations: ##EQU6## wherein Xi is a distance between peaks of convex portions at the surface of the steel sheet.

2. The cold rolled steel sheet according to claim 1, wherein said surface roughness pattern further satisfies a mean area per one convex portion at center plane of surface roughness (SGr, .mu.m.sup.2) of 2,000-30,000.

3. The cold rolled steel sheet according to claim 1, wherein said surface roughness pattern further satisfies a mean area per one convex portion at center plane of surface roughness (SGr, .mu.m.sup.2) of 2,000-30,000 and a product of center-line average surface roughness (Ra, .mu.m) and mean concave distance (Lmv, .mu.m) of not less than 50.

4. The cold rolled steel sheet according to claim 1, wherein said surface roughness pattern further satisfies a mean area per one convex portion at center plane of surface roughness (SGr, .mu.m.sup.2) of 2,000-30,000 and a ratio of mean concave radius (Rmv, .mu.m) to mean convex radius (Rmp, .mu.m) of more than 1.

5. The cold rolled steel sheet according to claim 1, wherein said surface roughness pattern further satisfies a mean area per one convex portion at center plane of surface roughness (SGr, .mu.m.sup.2) of 2,000-30,000 and a mean area ratio of convex portions at center plane of surface roughness (SSr, %) of not less than 45.

6. The cold rolled steel sheet according to claim 1, wherein said surface roughness pattern further satisfies a mean area per one convex portion at center plane of surface roughness (SGr, .mu.m.sup.2) of 2,000-30,000, a product of center-line average surface roughness (Ra, .mu.m) and mean concave distance (Lmv, .mu.m) of not less than 50, and a ratio of mean concave radius (Rmv, .mu.m) to mean convex radius (Rmp, .mu.m) of more than 1.

7. The cold rolled steel sheet according to claim 1, wherein said surface roughness pattern further satisfies a mean area per one convex portion at center plane of surface roughness (SGr, .mu.m) of 2,000-30,000, a product of center-line average surface roughness (Ra, .mu.m) and mean concave distance (Lmv, .mu.m) of not less than 50, and a mean area ratio of convex portions at center plane of surface roughness (SSr, %) of not less than 45.

8. The cold rolled steel sheet according to claim 1, wherein said surface roughness pattern further satisfies a mean area per one convex portion at center plane of surface roughness (SGr, .mu.m.sup.2) of 2,000-30,000, a ratio of mean concave radius (Rmv, .mu.m) to mean convex radius (Rmp, .mu.m) of more than 1, and a mean area ratio of convex portions at center plane of surface roughness (SSr, %) of not less than 45.

9. The cold rolled steel sheet according to claim 1, wherein said surface roughness pattern further satisfies a mean area per one convex portion at center plane of surface roughness (SGr, .mu.m.sup.2) of 2,000-30,000, a product of center-line average surface roughness (Ra, .mu.m) and mean concave distance (Lmv, .mu.m) of not less than 50, a ratio of mean concave radius (Rmv, .mu.m) to mean convex radius (Rmp, .mu.m) of more than 1, and a mean area ratio of convex portions at center plane of surface roughness (SSr, %) of not less than 45.

10. A plated steel sheet having an improved press formability, characterized in that said steel sheet has a surface roughness pattern satisfying a center-line average surface roughness (Ra, .mu.m) of 0.3-2.0 and a regularity parameter (S) in at least one direction of not more than 0.25 showing a regularity of surface roughness represented by the following equations: ##EQU7## wherein Xi is a distance between peaks of convex portions at the surface of the steel sheet.

11. The plated steel sheet according to claim 10, wherein said surface roughness pattern further satisfies a mean area per one convex portion at center plane of surface roughness (SGr, .mu.m.sup.2) of 2,000-30,000.

12. The plated steel sheet according to claim 10, wherein said surface roughness pattern further satisfies a mean area per one convex portion at center plane of surface roughness (SGr, .mu.m.sup.2) of 2,000-30,000 and a product of center-line average surface roughness (Ra, .mu.m) and mean concave distance (Lmv, .mu.m) of not less than 50.

13. The plated steel sheet according to claim 10, wherein said surface roughness pattern further satisfies a mean area per one convex portion at center plane of surface roughness (SGr, .mu.m.sup.2) of 2,000-30,000 and a ratio of mean concave radius (Rmv, .mu.m) to mean convex radius (Rmp, .mu.m) of more than 1.

14. The plated steel sheet according to claim 10, wherein said surface roughness pattern further satisfies a mean area per one convex portion at center plane of surface roughness (SGr, .mu.m.sup.2) of 2,000-30,000 and a mean area ratio of convex portions at center plane of surface roughness (SSr, %) of not less than 45.

15. The plated steel sheet according to claim 10, wherein said surface roughness pattern further satisfies a mean area per one convex portion at center plane of surface roughness (SGr, .mu.m.sup.2) of 2,000-30,000, a product of center-line average surface roughness (Ra, .mu.m) and mean concave distance (Lmv, .mu.m) of not less than 50, and a ratio of mean concave radius (Rmv, .mu.m) to mean convex radius (Rmp, .mu.m) of more than 1.

16. The plated steel sheet according to claim 10, wherein said surface roughness pattern further satisfies a mean area per one convex portion at center plane of surface roughness (SGr, .mu.m) of 2,000-30,000, a product of center-line average surface roughness (Ra, .mu.m) and mean concave distance (Lmv, .mu.m) of not less than 50, and a mean area ratio of convex portions at center plane of surface roughness (SSr, %) of not less than 45.

17. The plated steel sheet according to claim 10, wherein said surface roughness pattern further satisfies a mean area per one convex portion at center plane of surface roughness (SGr, .mu.m.sup.2) of 2,000-30,000, a ratio of mean concave radius (Rmv, .mu.m) to mean convex radius (Rmp, .mu.m) of more than 1, and a mean area ratio of convex portions at center plane of surface roughness (SSr, %) of not less than 45.

18. The plated steel sheet according to claim 10, wherein said surface roughness pattern further satisfies a mean area per one convex portion at center plane of surface roughness (SGr, .mu.m.sup.2) of 2,000-30,000, a product of center-line average surface roughness (Ra, .mu.m) and mean concave distance (Lmv, .mu.m) of not less than 50, a ratio of mean concave radius (Rmv, .mu.m) to mean convex radius (Rmp, .mu.m) of more than 1, and a mean ratio of convex portions at center plane of surface roughness (SSr, %) of not less than 45.