JPS62230954A - Cold-rolled steel sheet - Google Patents

Abstract

PURPOSE:To obtain a cold-rolled steel sheet having superior galling resistance at the time of press working, by regulating the center line average height of surface roughness Ra of steel sheet to the prescribed value or below and also regulating the product of the average diameter Lmp(mum) of peaks forming the roughness measured at the center line of the surface roughness and the above-mentioned Ra to a specific value or above. CONSTITUTION:The desired cold-rolled steel sheet can be obtained by specifying the center line average height Ra of at least one side of steel sheet and the value of RaXLmP(mum) to <=1.1mum and >=90mum<2>, respectively. Besides having the above characteristics, this cold-rolled steel sheet is reduced in surface roughness and is capable of improving the brightness of coated surface. Accordingly, on application to automobile body outer sheet, etc., it can provide superior high-grade impression to the coated surface. Moreover, said cold-rolled steel sheet has no possibility of deteriorating surface appearance and shape as well as of causing press-die damage due to the occurrence of galling at the time of press working.

Description

Detailed Description of the Invention Field of Industrial Application This invention relates to cold-rolled steel sheets that are used after being subjected to forming processes such as press working and painting, such as the outer panels of automobile bodies and the outer panels of household electrical appliances. It concerns surface structure.

Conventional technology The quality of the paint finish on the painted body of not only passenger cars but also light cars, wagons, and even trucks is a direct visual indicator of the overall high quality of the car to the customer. Therefore, there is a strong demand among automobile manufacturers to improve the quality of the paint finish. Surface items for paint finish quality include:
“Glossiness” defines the good reflectivity of light, and “Image clarity” defines the low distortion of the image projected on the painted surface.
Roughness includes ``feeling of fleshiness,'' ``feeling of richness,'' and ``feeling of depth,'' which define the sense of volume of a paint layer that can be visually grasped by the way light is reflected on the painted surface. Among these, gloss and image clarity are particularly important, and these gloss and image clarity are generally referred to as "image clarity."

Various methods have been developed to evaluate sharpness, but the most commonly used method is the one developed by Hunter Associates in the United States.
Laboratory (Hunter As5ociates)
Driven (DORI) manufactured by Laboratory)
Do I (D
istinctnessof Image) value is used. This DOI value is, as shown in Figure 10,
Light is incident on the sample S at an incident angle of 30', and its specular reflection light intensity R5 and scattered light intensity R at ±0.3° with respect to the specular reflection angle are
Using the values of 6 and 3, it is expressed by the following equation.

DOI value = 100X (Rs-RO, 3) /R3 It is known that the OI value has a good correlation with the image sharpness evaluation index (for example, the index by Landort CT-est) that can be seen with the naked eye. Therefore, the DOI
It can be said that the higher the value, the better the sharpness of the painted surface.Generally speaking, for paint films on the exterior of automobile bodies, a DOI value of 9 is required with 3 coats in order to provide a sufficiently luxurious look.
It is required that it be 0% or more, preferably 94% or more. In addition, even in the case of inexpensive 2-coat painting, DOII
It is said that it is more than 85 straight above 5.

It goes without saying that the sharpness mentioned above is greatly affected by the coating method and the characteristics of the paint, but it is also known that it is affected by the roughness of the surface of the steel plate that is the base for painting. There is. Conventionally, regarding the relationship between the roughness of the steel plate surface and the sharpness of the painted surface, US Steel's T,
Report by G. Milan et al. (February 1980, 7
5th SAE Congress) is necessary. According to this report, the sharpness of the image after painting is affected by the centerline average roughness (Ra) of the steel plate surface and the number of peaks forming roughness per inch (PPI: Peak perInch). has been done.

Problems to be Solved by the Invention In order to clarify the relationship between the roughness of the steel sheet surface and the sharpness of the painted surface, the present inventors investigated various factors regarding the surface roughness of the steel sheet and chemical formation before painting. After conducting a large number of coating experiments with various processing conditions, types of paint, and coating methods, we found that
The following conclusions were reached.

(i) The image clarity is strongly influenced by the coating thickness, as shown in FIG. 1, for example, and the thicker the coating, the better the image clarity.

(ii) There is a difference between the center line average roughness (Ra) of the steel plate surface and the DOI value representing image clarity, for example, as shown in Figure 2 (2 coats, coating thickness 35 units) and Figure 3 (35 coats). There is a strong correlation as shown in (in the case of coat painting), and the smaller the Ra, the better the image sharpness.
In order to obtain a DOI value of 85 or more with coatings of 0 or more and 2 coats of 1 to 1, it is necessary that Ra is 1.1 um or less.

(iii) Number of peaks per inch (PPI) on the steel plate surface
) has no particular correlation with image sharpness, as shown in FIGS. 4 and 5, for example.

As mentioned above, the research conducted by the present inventors has revealed that the recent requirements for sharpness of painted surfaces of automobile bodies (DO
In order to satisfy the I value of 90 or more and the DOll [85 or more] with two coats, it is necessary to keep Ra to 1.1 νm or less, but it has been found that there is no need to specifically regulate PPI.

Incidentally, automobile bodies are generally manufactured by pressing cold-rolled steel sheets, but during the press-forming process, the steel sheet and the press die may seize, which is the so-called gong (Gallin (1)). This may occur.

If this galling occurs, it will cause disadvantages such as damaging the mold and significantly reducing the commercial value of the product after pressing.

Generally, in press working, it is known that the smaller the surface roughness of a steel plate, the more likely the press lubricating oil will be insufficient and seizure will occur, that is, galling will occur. Therefore, as mentioned above, if the roughness of the coated surface is significantly reduced in order to improve its sharpness, there is a problem in that galling is more likely to occur during press working.

In other words, it has conventionally been difficult to achieve both improvements in the sharpness of the painted surface and improvements in galling resistance during press working.

The present invention was made against the background of the above-mentioned circumstances, and an object of the present invention is to provide a cold-rolled steel sheet that has good galling resistance during press working, has a small surface roughness, and can provide good clarity on the painted surface. The purpose is to

Means for Solving the Problems In order to achieve the above-mentioned object, the inventors of the present invention have conducted various experiments and studies on the relationship between various factors related to surface roughness and galling resistance. Ra is 1. I
Under the condition of JJm or less, that is, the image clarity is poor.
Even under microroughness conditions where the OI value is 90 or more for 3 coats and 85 or more for coats, the average diameter Lmp of the peak forming the roughness measured at the center line of the roughness
and the center line average roughness Ra (LmpXRa is 90JJ
It has been found that galling resistance is excellent when it is at least Irt, preferably at least 100IIIL, and this invention has been completed.

That is, the cold-rolled steel sheet of the present invention has a centerline average surface roughness Ra of at least one side of the steel sheet i, 1 pm or less, and an average diameter of the peak forming the roughness measured at the centerline of the roughness of that surface. It is characterized in that the product of Lmp (JJm) and center line average roughness Ra (νm) is 90 or more layers.

For production First, the knowledge that led to this invention will be explained.

The present inventors have determined that the plate thickness is 0.5# to 2. We actually conducted press working tests using cold-rolled steel sheets with various surface roughness factors of Om, and summarized the relationship between the occurrence of seizure and surface roughness factors, as shown in Figure 6 and (1) below. The product (RaXLm) of the center line average roughness Ra defined by the formula and the average peak diameter Lmt defined in FIG. 7 and the following formula (2)
If the value of p) is less than 90JJtti, galling is likely to occur, whereas if it is 90IJ/1i or more, galling is less likely to occur, that is, by satisfying the condition of formula (3) below, galling can be minimized. I discovered that it can be done.

RaxLmI]≧90 (m) ・(3)
Here, L is the measurement length, Y is the imaging width of the surface roughness, n is the diameter of the peak at the center line of the roughness, and n is the number of peaks at the center line of the roughness.

As is clear from Figure 8, the center line average roughness Ra is 1.1.
There is a region that satisfies equation (3) even below νm, so Ra is 1. IJ, 1m or less, Raxlmp is 901J
By creating a surface structure with an IA or less, it is possible to improve the sharpness of the painted surface after painting and at the same time prevent the occurrence of galling during press working.

Note that it is effective to improve the galling resistance when the Raxl-mp value is 90 layers or more, but in order to improve the galling resistance more fully, the Raxlmp value should be 1001 layers or more.
It is desirable that it is J/A or higher.

Ra is 1. The method for producing a cold-rolled steel sheet with a surface structure having a roughness of IJ, 1 m or less and a Raxlmp value of 90 layers or more is arbitrary, but for example, in the final temper rolling (skin pass rolling) in the cold-rolled steel sheet production process. This can be achieved by adjusting the surface structure of the work roll.

In other words, in temper rolling, the surface structure of the work roll is transferred to the surface of the cold-rolled steel sheet, so for example, laser machining or electric discharge machining can be used to create the required roughness profile on the work roll surface, that is, the cold rolled steel sheet. It is sufficient to form a roughness profile such that the surface roughness conditions of the rolled steel plate can satisfy the above conditions.

Note that at least one surface of the cold-rolled steel sheet may have the roughness condition as described above. In other words, in the case of general car body exterior panels, only one side, the outside of the car body, is required to have high visibility of the painted surface, so the surface roughness on the inside side of the car body can be kept considerably high. In this case, the galling resistance of the inner surface of the vehicle body can be improved even if the value of LmpXRa is not particularly restricted.

Example C0.04%, MnO, 2%, PO, 02%, 30.
0.015%, NO, 0.03%, QO. A press working test was conducted on cold rolled steel sheets to investigate the occurrence of galling.

Further, each cold-rolled steel sheet was subjected to chemical conversion treatment and three-coat painting under the following conditions.

[Chemical conversion treatment conditions] Treatment agent: fine-grained phosphate-based drug for dip treatment Dip conditions: 43°CX 120 seconds Skin Il1 heavy resistance: 2.3±0.2Cj/crtt Pretreatment:
Washing, washing with water, surface conditioning Post-treatment: Washing with water, washing with pure water, drying [3-coat painting conditions] Painting position: Horizontal painting Undercoat: Cationic ED paint 18-20JJm thick intermediate coat: Sealer 30-sob, 1m thick top coat Nidotsubu coat 30 to 35 J, 1 m thick. Sanding was not performed in each step.

The DOI value of the coated film surface after painting was measured using a DOIRGON meter.

RaXLmp
The values are shown in FIG. The Ra and Lmp of each symbol 01, e, , 2 in FIG. 9 are as shown in Table 1. In addition, each symbol (including a white background with a center line) indicates a case where no galling occurred, and a symbol with a black area indicates a case where galling occurred.

As is clear from Table 1, Figure 9, galling occurred in cold-rolled steel sheets with RaXLmp of less than 90 layers;
When the aX L mp was 90 or more layers, the occurrence of galling was reduced, and in particular, when the axlmp was 90 or more layers, galling was almost completely prevented from occurring. In addition, DOII directness, which indicates image clarity, is definitely 90 when Ra is 1.1 or less.
I was able to get more than that. Therefore, Ra is 1. IJJ
It is clear that by setting RaXLmp to 90 seats or more, preferably 100 times or more, it is possible to obtain excellent image clarity on the painted surface after painting and at the same time to obtain excellent galling resistance. be.

Effects of the Invention As is clear from the above explanation, the cold-rolled steel sheet of the present invention can obtain excellent image clarity after painting, and at the same time can effectively prevent galling during press working. Therefore, it can be applied to automobile body outer panels, etc. to give the painted surface an excellent high-class look, and there is no risk of damaging the press mold or impairing the surface appearance due to galling during press processing. .

[Brief explanation of drawings]

Figure 1 is a correlation diagram showing the relationship between the coating film thickness and the D01 value that determines the sharpness of the painted surface, and Figure 2 is a correlation diagram showing the relationship between the centerline average roughness (Ra) of the steel sheet surface and the coating in the case of two-coat painting. A correlation diagram showing the relationship with the subsequent DOI value. Figure 3 is a correlation diagram showing the relationship between Ra on the surface of the steel plate and the DOI value after painting in the case of 3-coat painting. Figure 4 is a correlation diagram showing the relationship between the DOI value after painting and 2-coat painting. Figure 5 shows the relationship between PPI on the steel plate surface and DOlliI after painting.
PPI of the steel plate surface in the case of coat painting and D after painting (]
Figure 6 shows the relationship between centerline average roughness (Ra)
Figure 7 is a diagram to explain the definition of center line average peak diameter (Lmp), Figure 8 is a diagram to explain the tendency of galling during press working on the steel plate surface. The diagram shown in FIG. 9 corresponds to Ra and Lmp, and FIG. 9 shows the DOI value after painting and the presence or absence of galling during press working of each cold-rolled steel sheet in the example, corresponding to the values of Ra and Xlmp. FIG. 10 is an explanatory diagram for explaining the method of measuring DOIfm.

Claims (1)

[Claims] The centerline average surface roughness Ra of at least one side of the steel plate is 1.
The average diameter Lmp (μ
m) and the center line average roughness Ra (μm) is 90 μm^
A cold-rolled steel sheet characterized in that it is 2 or more.