JPH012705A - Cold-rolled steel sheet with good die galling resistance and its manufacturing method - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention] (Industrial application field) Formability, which influences press formability, together with properties such as spot weldability, chemical conversion treatment properties, and sharpness after painting, which are required in subsequent processes. This is particularly important for the quality of cold-rolled steel sheets. A cold-rolled steel sheet with good galling resistance and a method for manufacturing the same that can advantageously have all of these characteristics will be disclosed below.

Although galling resistance and image clarity after coating are contradictory characteristics, this invention achieves both of the above conflicting conditions by particularly controlling the surface roughness pattern and shape of the rolling whirl roll. Not only can this be achieved, but also the spot weldability and chemical conversion treatment properties of cold rolled steel sheets can be improved.

Now, cold-rolled steel sheets for drawing used in automobile panels, electrical appliances, kitchen appliances, etc. are required to have excellent deep drawability, and in order to improve deep drawability, the mechanical properties of the steel sheet Basically, high ductility (εβ) and high Lankford (r value) are required.

In addition, the work hardening index (n) is also important, especially in actual drawing forming such as automobile panel forming, which is often combined with pultrusion forming.

In addition, since automotive steel sheets are usually subjected to spot welding as they are pressed parts, spot weldability is also required, and furthermore, phosphate coating crystals are prevented in the chemical conversion treatment with zinc phosphate applied as a base treatment for painting. It is also necessary to satisfy the chemical conversion treatment property in which areas where phosphate crystals do not precipitate or where phosphate crystals do not precipitate, that is, defects such as the occurrence of scratches and resulting poor corrosion resistance after painting can be completely eliminated.

Particularly in the case of steel sheets for automobile exterior panels, the appearance of the finished coating film after painting is one of the important quality items for automobile users, which is directly connected to the overall quality of the automobile.

By the way, research on deep drawing to date has been carried out from both the side of the plate, which is the material, and the forming technology. However, as products become more precise and complex, the properties required for steel sheets also become more sophisticated and diversified. Currently, vehicle bodies are assembled by spot welding a large number of pre-formed press parts, but there is a strong demand for larger press parts or for reducing the number of spot welding steps by making a single press part.

On the other hand, in order to meet diversifying needs, the design of automobile bodies has become more complex, making press forming more difficult and running counter to the demand for larger and more compact press parts.

In other words, the basic characteristic of a cold-rolled copper plate is that it has even higher press formability.

Next, in recent years, there has been a strong demand among automobile manufacturers to improve the quality of their paint finishes. In addition, there is also the "feeling of texture", which is determined visually by the way light is reflected on the coating surface. The impressions of ``feeling of richness'' and ``feeling of depth'' were also added to the evaluation items.

Among these characteristics, image clarity, which is a combination of gloss and image clarity, is greatly affected by the painting method and characteristics of the paint, but it is also affected by the surface roughness of the underlying steel plate. .

In response to such demands, in actual press forming, the conventionally used (regional properties (r value, εβ to n value, etc.) of steel sheets are insufficient as evaluation criteria, and, for example, A wide range of considerations must be given to improving press formability, including surface roughness and lubricating oil used for processing.

(Prior Art) Several known technologies have been disclosed that focus on the influence of surface roughness of a steel plate on press formability.

For example, "Plasticity and Processing" Vol. 3, No. 14
(1962-3) discloses that when a high viscosity lubricating oil is used, the drawability is most improved by setting the center line average roughness Ra to a surface roughness of about several μm.

On the other hand, in JP-A No. 59-34441, the surface roughness (Ra) and the number of peaks (PPI) of a rolling work roll are
and Ra=2.8 (μm) and PPI=226, respectively.
It has been shown that by temper rolling with dull rolls, the appearance properties and press workability after painting are excellent.

These known techniques are excellent in terms of improving press formability, but all require strict control of surface roughness.

In other words, the above-mentioned known technology has nothing to do with cold-rolled steel sheets and their five structures, which are superior in press formability, spot weldability, chemical conversion treatment Vb, and sharpness after painting under arbitrary surface roughness (Ra). I can't even give you any suggestions.'J)

(Problems to be solved by the invention) As a result of intensive research, the inventors have found that press formability and
Sharpness after painting is a mutually contradictory condition, but in addition to achieving both these characteristics, spot weldability,
The first purpose of this study is to elucidate the surface roughness profile of steel sheets that can satisfy chemical conversion treatment properties, and based on this knowledge, propose cold-rolled steel sheets with good galling resistance and their manufacturing method. .

(Means for solving the problem) The above object is to have a large number of convex portions protruding and scattered in a columnar or truncated conical shape on the finished surface of the steel plate, and the finished surface is formed by the following formula: xl: The regularity parameter of the dispersion of convex portions defined by the distance between convex peaks is S≦0.25 in at least one direction along the finished surface, and the ratio of X to the average diameter of the bottom surface of each convex portion (□) The value of is 1
．． It is characterized by being in the range of 1 to 3.0, and that the remaining area excluding the convex portions forms a flat valley extending smoothly through the adjacent convex portions, including the sawdust of the convex portions. A cold-rolled steel sheet (first invention) with good galling resistance is used, and the finished surface has a center line average roughness Ra of 0.4 to 1.6 μm. is the percentage of the total cross-sectional average area of the convex parts on this roughness center plane and the projected area of the center plane within the roughness center plane theorem is a plane that includes the center line of each roughness curve in different directions along it. The average area ratio SSr of the convex portions is 40% or more, and the finished surface is on the roughness center plane defined by the plane including the center line of each roughness curve in a different direction along the convex portion 1. Individual win:)
The average area SGr of is 30000 μm2 or less,
As an embodiment of the first invention, in addition to contributing to improvements in sharpness, spot weldability, and chemical conversion treatment properties after painting, it is also possible to apply a coating to a depth of 2 μm or more on the surface of a work roll for finish rolling of cold-rolled steel sheets. The number of recesses present in ζ is expressed by the following formula
．． 25, and the ratio of T to the average opening diameter D' of the recess is 1.
．． After drilling in the range of 1 to 3.0 D', using a high-density energy source to locally melt the work roll surface and at the same time spraying auxiliary gas there, the work roll surface other than the recesses is mechanically ground to make it smooth. The method for producing a cold-rolled steel sheet with good galling resistance (second invention) is characterized in that the cold-rolled steel sheet is finish-rolled with a dull roll formed by hardening treatment or Cr plating. Stable productivity of rolled steel sheets can be advantageously ensured.

Now, FIG. 1 partially illustrates the finished surface of a cold-rolled steel sheet with good galling resistance according to the present invention. 1 in the figure is
The convex portions 2, which are in the shape of a right cylinder or a truncated cone and are protruding and scattered on the surface, spread out in a long and clear manner between the convex portions and the eyes. Shows a flat cleavage. Here, galling resistance refers to the prevention of the so-called seizure phenomenon, which causes fusion between the material and the die or punch at the center of the process, resulting in peeling of the material or mold surface, or product flaws caused by this phenomenon. refers to the effective surface properties of steel sheets.

Now, I will explain the background and results of the experiment that formed the basis of this invention.

As shown by the broken line in FIG. 2, the surface of the rolling work roll 3 is subjected to, for example, laser processing using a high-density energy source, so that the periphery of the concave portion 4 has a diameter D' and a laser irradiation interval X'.
Then, the regularity (S') of the scattering of the recesses 3 (details will be described later) is slightly different (E, after the unevenness is provided by laser processing, the protuberances 5 that inevitably occur around the recesses 4 are mechanically scraped and flattened. Using a rolling work roll 3 with a smooth surface and a surface texture as shown by the solid line in the same figure, a cold rolled steel sheet with the characteristics shown in Table 1 was rolled at a different rolling reduction rate, and a cold rolled steel sheet with a surface roughness profile as shown in Table 2 was applied. Rolled steel plate (center surface average roughness (Ra) is 0
．． 2-2.0 μm).

FIG. 3 schematically shows how the surface pattern of the rolling work roll is transferred to the cold rolled steel sheet 6 during this rolling.

Although the convex portions and concave portions are illustrated as being circular, they are not necessarily circular, and may be oval, square, or the like.

In addition, as comparison materials, a rolling roll in which the protuberances 5 were not scraped off after laser processing and a John de blast roll were also used to produce a comparative steel plate.

Regarding the press formability of these steels,
The image clarity after 3-coat painting was also investigated.

To determine the mold-stick property, first add 10 g of lubricating oil for the press.
A cold-rolled steel plate 6 with a width of 50 mm+ and a length of 250 mm, coated with oil, was placed in a die 8 with a round bead 7 of 14 φ mm as shown in Fig. The punch 10 was pushed up under 500 kg/cm2 to draw it to a height of 60 mm.Next, a cold-rolled steel plate similar to the above without oiling and in a degreased state was used as a sample and continuously drawn. The changes in the load that occur on the repeating punch 10 and the occurrence of mold galling were verbally investigated.

The state of occurrence of galling was evaluated by dividing the area ratio of mold galling to the total area into the following five stages and evaluating the average of n-2 in one pass.

0 to 20%; 1 21 to 40%; 2 41 to 60%; 3 61 to 80%; 4 81 to 100%: 5 On the other hand, the sharpness after painting was evaluated directly with nor H, and here Dot The values were measured using a Dorigonmeter manufactured by Hunter Associates Laboratory. That is, as shown in Fig. 5, light is incident on the sample at an incident angle of 30°, and the specular reflection light intensity Rs and the specular reflection are ±
It is calculated using the following formula using the value of the scattered light intensity R8,3 at 0.32.

1 (S)Although the limit drawing ratio (L, D, R) test is not shown, it is generally expressed as the ratio of the limit blanking diameter that causes cracking during the drawing test to the punch diameter, as shown in the following equation.

However, D, R= - O In this experiment, the punch diameter of 0° was the 33φ mark, the wrinkle presser pressure was 500 kgf/am2, and the purpose of the surface condition was to check the galling property, so it was carried out without oiling. 1 piece.

The above results are shown in Table 2 and FIGS. 6 to 10.

First, in Fig. 6, the regularity parameter: When S'≦0.25, the steel plate (L/D) glue produced with laser processed rolls has high D and R, but when S>0.25, the glue sharply stiffens. , D, and R decreased, and no significant difference was observed compared to the steel plate rolled with shot blasting rolls (S/B).

In Fig. 7, even in the L/D steel plate, the ratio (X/D) of the average convex spacing X to the average convex diameter is within the range of 1.1 to 3.0.
It was found that D and R were good, and when X/D was less than 1.1 and more than 30, D and R were decreased.

This tendency is probably due to the fact that when X/D is less than 1.1, the area ratio that occupies the flat area of the steel plate surface is too large, and the contact area with the mold becomes large, making it easy for mold galling to occur. Therefore, it is thought that it became difficult to narrow down.

On the other hand, when X/'D exceeds 10, the distance between the protrusions is long and the apparent volume is small, so the protrusions cannot withstand the creasing pressure and collapse, resulting in X/D is 1
．． It is thought that, contrary to the case where the value was less than 1, mold galling was more likely to occur in the valleys, making drawing difficult.

The conventional L/D I board (marked with △), which is a comparative material, has an X/D of 1.
．． Although it still showed D and R peaks in the vicinity of 0, it was not as high as the one according to the present invention, and D and R were not as high.

This is probably because the surface area of the convex portions is relatively large, which makes it more likely to cause cracking than the steel sheet of the present invention.

On the other hand, if it is less than 1.0, the gap between the valleys becomes narrower, so there is less occurrence of mold galling, making it easier to draw, and it is presumed that D and R tend to improve.

In addition, as shown in Figure 8, the galling resistance is determined by the L/D steel plate, which is obtained by using a rolling work roll whose raised part has been shaved off after laser machining, so that mold galling does not occur even when the number of sheets drawn increases. The increase in punch load was also small.

Steel plates rolled using rolling rolls using only laser processing and shot blasting were susceptible to die galling, and fracture occurred when the number of sheets drawn was around 10.

FIG. 9 shows the relationship between sharpness and regularity parameter 2S. When the regularity parameter S≦0.25, the steel plate (L/D) manufactured by laser processing roll has poor image clarity (
001 value) was found to be high.

However, in the case of S>0.25, no significant difference was observed compared to the steel plate (S/B) manufactured using a work roll for rolling by ordinary shot blasting.

Figure 10 shows the effect of R on image clarity (001 value).

As we compared the influence of R8 of S/B steel plate is 0.6μ
001>90 cannot be satisfied unless R is 1.6 μm or less, but it has been found that the L/D steel plate can be a high-definition coated steel sheet with Dot>90 even when R is as high as 1.6 μm.

Next, a cold-rolled steel plate made of steel (A) shown in Table 3 was given a symbol 0 as shown in Table 4. Temper rolling was performed at a rolling reduction ratio of 0.8% using a rolling work roll having a surface roughness of P and Q with two rollers.

Then their steel plate surface profile: SS.

(Area ratio (%) of convex portions on the center plane of the surface roughness of the steel plate
) and spot weldability.

Here, the spot welding conditions were a welding time of 10 seconds, a pressure crab of 200 kg, and a welding current of 8000 A.

Spot weldability is evaluated by the tensile shear strength of the welded part.
Figure 1 shows the results.

Symbol 0 in the same figure. Regarding P steel, it has been shown that the tensile shear strength increases rapidly when SS is 40% or more. However, with the symbol Q steel, the tensile shear strength gradually increases as SS increases, but the tensile shear strength with the symbol 0. It did not show particularly high values compared to P@.

The reason why these results were obtained is that the area ratio of the convex parts is larger than that of the valleys, and the contact between the steel plates 'JfF+
This is thought to be due to the fact that fsi has become wider.

However, in a range where SSr is less than 40%, the contact between the steel plates becomes close to point contact, which is considered to be the cause of the poor adhesion between the steel plates.

In addition, the symbol Q steel is a steel plate manufactured by a conventional method, and as shown in Fig. 12, the top of the convex portion is quite sharp, so even if the SSr is high (40% or more), the symbol Q is
It is presumed that the increase in tensile shear strength of the welded part, that is, the improvement in spot weldability, could not be achieved as with P steel.

Next, the cold-rolled steel sheet of steel B shown in Table 3 was subjected to skin pass rolling at a reduction rate of 1.0% using rolling rolls having surface roughness profiles of symbols R, S, and T shown in Table 5. The relationship between the surface profile of these steel sheets: 5Gr (average image density (μm2) per convex portion in the center plane of surface roughness) and chemical conversion treatment properties was investigated.

The results are shown in FIG.

Chemical conversion treatment was performed by immersing the steel plate in a commercially available chemical treatment solution for 2 minutes (2. In addition to examining the average particle size of phosphate crystals generated on the surface of the steel sheet,
The oxygen reduction current value was measured and evaluated in the a leaf solution (pH=11.6).

As a result, for symbols R and S, SGr is 30000 un
In the range of 2 or less, the average crystal grain size of phosphate becomes small,
Furthermore, the oxygen reduction current also decreased, and the chemical conversion treatment properties tended to be good. However, the influence of SGr on the chemical conversion treatability of the designated material shown as a comparison material was smaller than that of the designated R and S materials.

For materials with symbols R and S, the area of the convex part 1 on the steel plate surface with a low degree of working is small, and on the contrary, the area occupied by the valley 2, which is subjected to strong working, is large, so it is a precipitation site of phosphate crystals. It is thought that this is because the number of dislocations on the surface of the steel sheet increases, and phosphate crystals precipitate densely, improving chemical conversion treatment properties. On the other hand, in the case of the symbol T shown as a comparative 1-year-old, the area of the valley 2 may be reduced depending on the serum of the annular groove portion 11 which is subjected to excessively strong processing as shown in Fig. 14. Because the transition degree was small and coarse crystals, which are the precipitation sites of phosphate crystals, were coarsely distributed, chemical conversion treatment, ￥I
It is considered that the patient's life did not improve.

((乍用)) Next, the reason why the surface roughness profile /lz of the steel plate and the manufacturing means are limited in this invention will be explained.

T! Regularity of convex portions related to the surface roughness of I-saka and concave portions related to the surface roughness of rolling work rolls: S, S' If s, s' exceeds 0.25, the sharpness after painting deteriorates. The upper limit in this invention is set to 0.25.

The distance between the peaks of the convex portions related to the surface roughness of the steel plate and the concave portions related to the surface roughness of the rolling work roll 1Jffi (X, X'
) and the average diameter (D, D') of the bottom surface of the convex part and the opening of the concave part: (X, /D, X'/D') If the value of this ratio is less than 1.1, the convex parts will be continuous. Therefore, although the sharpness of the steel plate after painting is good, the average area ratio at the top of the convex portion of the steel plate is excessive, and mold galling is likely to occur. In addition, if it exceeds 3.0, the average area ratio of the valleys increases and the image clarity improves, but as the area of the valleys that undergo work hardening increases, the material quality, especially the YD, increases.
In this invention, since it causes a decrease in El and decreases the press formability of the steel sheet itself, (X/D,
/'D') was set to 1.1 to 3.0.

Steel plate surface roughness center line average roughness; Ra is 0.
If the thickness is less than 4 μm, the difference in height between the convex and convex portions will be negligible, and mold galling will easily occur.If the thickness exceeds 1.6 μm, the concave and convex portions will remain on the surface even after coating, resulting in poor image clarity, which is not preferable.

Center surface area ratio of steel plate surface roughness: Regarding SSr, when the steel plate has a surface roughness pattern like the steel plate of this invention, the surface roughness pattern of the conventional steel plate (shot blasting or a pattern having an annular groove around a convex part) )
In this invention, SSr4 has better spot weldability when SSr is 40% or more, and fluctuations in tensile shear stress due to differences in pressure during spot welding become smaller.
0% or more is recommended.

Average area of each convex portion in the center plane of the surface roughness of the steel plate: Regarding SGr, the portions forming the valleys in the steel plate surface shape of this invention undergo considerable hard working, and the dislocation density inside the steel plate, especially near the surface layer, increases, resulting in chemical formation. During treatment, phosphate crystals are densely precipitated on the surface of the steel sheet, improving chemical conversion treatment properties. However, SGr is 3
If the thickness exceeds 0,000 μm2, phosphate crystals will precipitate sparsely and chemical conversion treatment properties will deteriorate, so a thickness of 30,000 μm2 or less is recommended.

Regarding the depth of the surface recesses of the rolling work roll: The reason why the load of the recesses on the surface of the rolling roll is set to 2μm or more is 2μm.
If it is less than 2 μm, it will be difficult to adjust the center line average roughness Ra of the uneven portions formed on the steel plate surface to 0.4 to 1.6 μm during rolling, but if it is 2 μm or more, there is no need to worry about this. be.

In addition, the reason why a high-density energy source is used on the surface of the rolling work roll to locally melt the surface and at the same time blow an auxiliary gas thereon is that the surface of the rolling work roll is formed by using a high-density energy source such as a laser or an electron beam. This is because by spraying an auxiliary gas such as 02 gas onto a minute local molten pool, the molten material can be blown away and melting can be promoted, making it possible to form a deeper recess.

Furthermore, after that, the parts other than the recesses are mechanically ground to make them smooth by grinding away the bulges near the periphery of the recesses created by the blowing of auxiliary gas, and during rolling, the uneven recesses that are not flat on the steel plate side are removed. This is to prevent the formation of irregular recesses on the steel plate side, since the sharpness of the image after painting will deteriorate if such irregular recesses are formed on the steel plate side.

The hardening treatment and Cr plating of the roll must be done in order to strengthen the parts that have been melted by laser, etc., and the parts that have been mechanically flattened, as these parts are extremely abrasive. .

(Example) Next, examples will be described below along with comparisons.

Cold-rolled steel sheets with a thickness of 0.8r+un shown in Table 6 were tempered at rolling reductions of 0.5%, 0.8%, and 1.5% using rolling rolls with different surface roughness patterns shown in Table 7. Rolling was applied.

Table 8 shows the results of investigating the surface roughness profiles and materials of these steel plates.

After that, drawing processability (
L, D, R), presence or absence of mold galling, image clarity after painting (001 value), tensile rupture strength of spot welds, chemical conversion treatment properties (oxygen reduction current) were investigated, and these results were evaluated. Table 8 shows the surface roughness profile and material.

As a result, in each case of the steel plate surface shape according to the present invention with symbols A99, C, T, N, W, C, T, and S, no mold galling occurred and the drawing formability was good. The image clarity was also good.

However, in addition to symbols 2 and S, regularity parameters of the convex portions on the surface of the steel sheet, such as symbols Hol, Ru, He, and O, using a work roll dulled by conventional laser processing only or shot blasting. When S was large, mold galling occurred or the Dot value was low. The Nafu symbol Y had an excessive Ra, so no galling occurred, but the image clarity was poor, and conversely, the Ra of the Nafu symbol Y had a small Ra and galling occurred.

On the other hand, the steel plate symbol 1 is a steel plate that has been rolled for 5 times using a roll whose surface has only been roughened by ordinary laser processing. Compared to , the depth (height) of the unevenness is −
The sharpness of the image was not good, perhaps because it was not very clear.

Symbol y had a large X/D, galling was likely to occur, and the tensile shear strength of the spot weld was also relatively low.

In addition, the symbolic force was small in SSr, and the tensile shear strength of the welded part was very low compared to other steel plates.

Further, the sample with the symbol Q had a large SGr, and accordingly had poor chemical conversion properties (oxygen reduction current).

(Effects of the Invention) The cold-rolled steel sheet according to the first invention satisfies mold galling resistance and image clarity, and can be used for automobile exterior panels, etc., and is particularly effective against "sagging" that occurs during painting. “Uneven”
It is also possible to prevent this.

According to the second invention, since the rolling rolls used therein do not have convex portions, there is no fear that the shape transferred to the steel plate will change due to wear as the rolling length increases, and the rolling roll is stable over a long period of time. It becomes possible to manufacture the above-mentioned cold-rolled steel sheet.

[Brief explanation of the drawing]

Fig. 1 is a partial perspective view showing the surface shape of a cold rolled steel plate according to the present invention, Fig. 2 is a partial sectional view showing the shape of the surface layer of a rolling roll, and Fig. 3 is a cross section of the rolling roll and the surface layer of the steel plate during rolling. Fig. 4 is an explanatory drawing showing the outline of the mold galling test, Fig. 5 is an illustration showing the principle of measuring sharpness (Dot value), Fig. 6 is drawing property (L, D, R ) and the regularity (S) of the convex portions of the steel plate surface roughness. Figure 7 is a graph showing the relationship between drawability (L, D, R) and X/D (average distance of protrusions in the surface roughness of the steel plate/average diameter of the protrusions), and Figure 8 is a graph showing the relationship between drawability (L, D, R) and A graph showing the relationship between changes in load, galling evaluation, and the number of drawn sheets. Figure 9 is a graph showing the relationship between image clarity (DDIIm) and regularity (S) of the surface roughness of the steel plate. Figure 10 is a graph showing the relationship between the sharpness Figure 11 is a graph showing the relationship between the surface roughness of the steel plate and the center line average roughness (Ra) of the surface of the steel plate. Figure 12 is a cross-sectional diagram of the surface shape of a steel plate rolled by a conventional method (laser processing only); Figure 13 is a graph showing the relationship between oxygen reduction current, phosphate average grain size, and surface roughness. Figure 14 is a graph showing the relationship between the average area (SGr) per convex part in the center plane of the steel plate, and Figure 14 is a schematic plan view of the surface shape of a steel plate rolled by a rolling roll using the conventional method (rede processing only). is an explanatory diagram of SGr and SSr in the center plane of the steel sheet surface roughness. Fig. 1 1'a) 2 valleys Fig. 4 (a) Bar 0,3 Fig. 9 @) Surface grain of ash, tenoyoshi U++, 1, raw; 5L, D
, R L, D, R Fig. 10 Ra (μm) Fig. 11 tll! Average area on the surface of 1st floor txt in 3rd and 1st q curved convex platform: 5Sr(?A) Fig. 13

Claims (1)

[Claims] 1. On the finished surface of a steel plate, there are many protruding and scattered protrusions in the shape of a column or a truncated cone, and the finished surface has:
The following formula S=1/nΣ^n_i_=_1|■−X_i|/■In the above formula; ■=1/nΣ^n_i_=_1 is S≦0.25 in at least one direction along the finished surface, and the ratio of ■ to the average diameter D of the bottom of each convex portion (■/D) is in the range of 1.1 to 3.0, and A cold-rolled steel sheet with good die galling resistance, characterized in that the remaining area after removing the convex part forms a flat valley that extends smoothly through the adjacent convex parts, including the circumference of the hem of the convex part. . 2. The finished surface has a center line average roughness Ra of 0.4 to 1.
1. Steel plate according to 1, having a surface roughness of 6 μm. 3. Within the roughness center plane where the finished surface is defined as a plane including the center line of each roughness curve in different directions along it, the total cross-sectional average area of the convex parts on this roughness center plane and its center plane projection Average area ratio SS of convex portions expressed as a percentage of the area
The steel plate described in 1 or 2, in which r accounts for 40% or more. 4. Convex portion 1 on the roughness center plane defined by the plane along which the finished surface includes the center line of each roughness curve in different directions
3. The steel plate according to 3, wherein the average area SGr per piece is 30000 μm^2 or less. 5. On the surface of the work roll for finish rolling of cold rolled steel plate,
A large number of scattered recesses with a depth of 2 μm or more can be calculated using the following formula S'=1/nΣ^n_i_=_1|■'-X_i'|/
■In the above formula;■'=1/nΣ^n_i_=_1X_i',
X_i': The regularity parameter of the scattering of the recesses, expressed by the distance between the centers of the recesses, satisfies S'≦0 in at least one direction along the work roll surface.
25 and the ratio ■/D' of ■' to the average opening diameter D' of the recesses is in the range of 1.1 to 3.0, the work roll surface is locally melted using a high-density energy source, and at the same time, the work roll surface is melted locally. After the drilling process in which auxiliary gas is sprayed, the surface of the work roll other than the recesses is mechanically ground to make it smooth, and then the cold rolled steel plate is finished rolled using a dull roll formed by hardening treatment or Cr plating. A method for producing cold-rolled steel sheets with good die galling resistance.