JPS613898A - Manufacture of galvanized steel sheet forming hardly start mark during press working - Google Patents

Abstract

PURPOSE:To obtain easily a galvanized steel sheet reducing the formation of star marks during press forming by galvanizing one side of a steel sheet by a specified extent of deposition and carrying out skin pass rolling with dull rolls in the same line to roughen the galvanized surface. CONSTITUTION:One side of a cold rolled steel sheet is galvanized by >=40g/m<2>, and skin pass rolling is carried out with dull rolls in the same line to regulate the surface roughness of the galvanized surface to >=1.5mur.m.s. Thus, a thick galvanized steel sheet reducing the formation of star marks and scoring during press forming is obtd. The steel sheet is suitable for use as a material for an automobile or the like.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electrogalvanized steel sheet that is less susceptible to star grain and scoring during press working.

Anti-rust electroplated steel sheets are increasingly being used in automobile bodies to improve corrosion resistance and lifespan. Electroplated steel sheets used in automobile bodies are used as press-formed panel products, but because they are often used as outer panels, which are visible to the public, they have a beautiful paint finish.
A surface with a small surface roughness is required, and the surface roughness is generally 1.
．． The following are used: r, m, s. In addition, in the press working, it is particularly important to work without damaging the rolled smooth surface condition.

However, in reality, small indentations are caused on the surface after pressing due to foreign particles adhering to the punch surface or material surface, deposits on the plating film, or scratches on the punch surface, which are called star pimples, dents, etc. Various measures are taken to prevent this, such as polishing the press mold, cleaning the press mold and materials, preventing dust in the factory, and cleaning the press oil. However, it is extremely difficult to reduce this defect due to the wide variety of factors related to the star formation mechanism. If stars occur frequently, the surface of the press mold must be carefully maintained, which requires a lot of time and effort.

In addition, steel sheets with many stars on the panel surface are scrapped as defective products, reducing product yield. Furthermore, even if these defects occur in small numbers, it is necessary to polish the surface of the steel plate with a grindstone to make it smooth, which reduces the efficiency of the manufacturing process. This has become a serious problem in the press process for automobile bodies.

To solve this problem, one of the inventors of the present invention has already solved the problem by increasing the surface roughness of the galvanized surface of the galvanized steel sheet to 1.51 Lr, ts, s or more in JP-A No. 57-85989. It is proposed to achieve the above objectives.

The following are specific methods for increasing surface roughness.

(1) The surface roughness after plating is made rough by increasing the surface roughness of the cold-rolled steel sheet that is the raw material to 1.5 #1°r, m, s or more.

(2) A method of increasing the surface roughness of the plated surface by controlling the plating conditions.

Regarding (1), the surface of the rolling roll during cold rolling is roughened by shot blasting, etc., and the roughness of the surface of the steel plate after cold rolling is thereby increased.
．． 5 μr. When rolling cold-rolled steel sheets, especially high-strength steel sheets, in mass production using rolling rolls with a diameter of m, s, or higher, the wear of the roll's dull edges is significant, and the surface roughness of the steel sheets becomes small.
That's what happens. That is, the roll life is short, the frequency of shot blasting is high, and the work efficiency is poor.

Regarding (2) above, the composition of the plating solution, solution flow rate, bath pH, current density, and distance between electrodes are controlled, but since thick plating is used for automobiles, thin plating is recommended for general use. Strict control of different conditions is not suitable for mass production.

The present invention solves the problems of the prior art described above and provides a method for easily mass-producing galvanized steel sheets having a surface roughness that can minimize the occurrence of stars during press forming. The purpose is to

According to the present invention, the amount of plating deposited on one side of the copper strip is 40 g/
After electrolytic galvanizing to a surface roughness of 1.5 #Lr', m, s, or more, the surface roughness of the plated surface is 1.5 #Lr', m, s, or more by applying a skin bath using dull rolls installed in the same line. , the above objectives can be achieved.

Below, specific effects, appropriate ranges, etc. of the plated steel sheet according to the present invention will be explained in detail.

The surface roughness of the plated surface was varied by skin bathing a single-sided electrogalvanized steel sheet (thickness: 0.8 layer m, zinc coating amount: 90 g/rn') with rolls having different surface roughness. After applying press oil to the steel plate, it was press-formed into the front fender of a car under the same conditions with the cold-rolled side facing outward. Occurrence of star grains and scratches on the zinc surface (hereinafter referred to as scoring). Figure 1 shows the results of the investigation.

(1) About the star marks When the die side surface (cold rolled surface) of the steel plate after pressing is lightly rubbed with a grindstone, the star marks are slightly swollen, so only those parts are strongly polished. It will become white and shiny. The number of these white glowing dots was counted as the number of star eyes.

(2) Regarding scoring, the punch side surface (plated surface) of the steel plate after pressing was observed with the naked eye, and the number of scorings on the plated surface was counted. Note that the area counted was a 10 x 1'ocm square area at the center of the fender because unlike stars, there are many scoring rings.

From Figure 1, it is clearly seen that as the surface roughness of the galvanized steel sheet increases, the occurrence of stars decreases markedly (
As is clear from the O mark in Fig. 1), the number of scorings also decreases as the surface roughness increases, indicating that the lubricity improves as the roughness increases. It shows.

Furthermore, it is obvious that as the surface roughness of the plated surface increases, the amount of press oil attached increases.

Ru.

The substance that causes the star pattern exists at the boundary between the punch face and the steel plate surface, but its critical size is generally said to be several dozen. The reason for the decrease in star grains due to the increase in surface roughness is thought to be that the critical size of foreign particles that cause star grains is increased by improving the lubricity due to the buffering effect of the surface roughness and the increase in the oil-stopping effect.

As is clear from Figure 1, the surface roughness is 1.51Lr, m
If the surface roughness is less than 1.5 #Lr,m,s, the effect of reducing the star pattern is insufficient, and in the present invention, the surface roughness is set to 1.5 #Lr,m,s or more.

When electrogalvanized steel sheets are used as rust-preventing steel sheets for automobile bodies, single-sided plating, in which a thick coating material is mainly used on one side, is difficult to coat evenly when assembled on the car body. When press working, the plated surface will inevitably be on the back side where it will come into contact with the punch. Furthermore, when used as an exterior panel, the outer surface that is visible to the public is required to be glossy after painting, and therefore, the surface roughness of that surface is required to be small.

The object to which the present invention is applied is an electrolytic galvanized steel sheet for rust prevention used mainly for automobile bodies.

In the case of single-sided plating, the amount of zinc deposited is 40 g/m2 or more from the viewpoint of corrosion resistance, etc., so this was used as the target.

In order to reliably and continuously give a steel plate a large surface roughness on a mass production scale, if the raw steel plate is rolled with a dull finish roll as in the past, the hardness of the steel plate is 110-150 on the H scale. The wear of the dull roll is severe, but when the dull roll is skin-passed after galvanizing as in the present invention, since the hardness of zinc is soft at 40 to 50, there is almost no wear of the dull roll, and the desired roughness can be easily obtained. I can do it.

The present invention will be described below with reference to Examples and Comparative Examples.

[Example 1] Thickness 0.8 am', surface roughness 0.8 pr, ta,
The amount of zinc deposited on one side of the cold-rolled steel sheet of s is a o g/r.
Electroplating was performed in a plating bath containing zinc sulfate as a main ingredient so that n' was formed, and then skin pass was performed using a roll having a dull surface by shot blasting. The surface roughness of the plated surface is 1.
9 μr. At ts and s, the roughness level remained almost unchanged even after 500 tons of steel plates were continuously processed.

After uniformly and thinly applying press oil to both sides of this steel plate, it was pressed using an automobile front fender with the plated side facing the punch side.This mold was used to hold dozens of cold-rolled steel plates with many foreign substances attached to their surfaces. The punch surface was previously damaged by continuous pressing. After pressing, the cold-rolled surface, which is the front surface (dice contact area), was lightly rubbed with a grindstone to check the number of stars. The number of occurrences of scoring on the back side (the warm contact side of the punch) was also investigated. These results are shown in Table 1 along with other examples listed below.

[Example 2] Thickness: 0.8 ts, surface roughness: 0. ? ILr, m, s,
The amount of zinc deposited on one side of the tensile strength cold-rolled steel sheet is 90 g/rn.
Electroplating was performed in a plating bath containing zinc sulfate as the main ingredient, and then skin-passed with a roll that had been given a dull surface with a multi-layered plastic. The surface roughness of the plated surface was 2.2 r, m, s, and the roughness remained almost unchanged even after continuous treatment of 500 tons of steel plates.

This steel plate was press-formed under the same conditions as in Example 1.

[Example 3] Thickness 0.8 i*m, surface roughness 0.7 μr. ri;
One side of a cold-rolled steel sheet was electroplated in a plating bath containing zinc sulfate and zinc chloride as main ingredients so that the zinc coating amount was 110 g/m'', and then a dull surface was given by shot blasting. The surface roughness of the plated surface was 2:5 r, m, s, and the roughness level remained almost unchanged after continuous treatment of 500 tons of steel plate.

This steel plate was press-formed under the same conditions as in Example 1.

[Comparative Example 1] A cold-rolled steel plate was rolled to a surface roughness of 2. Og r,
It was rolled to obtain m, s. Initially, the desired roughness was obtained, but after 300 tons of rolling, the roughness was 1.4 r.
It decreased to S.

One side of this steel sheet was electroplated in a plating bath containing zinc sulfate as the main ingredient so that the amount of zinc deposited was 90 g/d.

The surface roughness after plating is 1.3 p-r, m, s.

Met· This steel plate was press-formed under the same conditions as in Example 1.

Chapter 1 Table

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between the surface roughness of electrogalvanized steel sheets and the number of stars and scoring. Figure 1

Claims (1)

[Claims] A single-sided electrogalvanized steel sheet with a coating weight of 40 g/m^2 or more is skin-passed using a dull roll in the plating line, and the surface roughness of the plated surface is reduced to 1.5 μr. m. s. A method for producing an electrogalvanized steel sheet that is less likely to produce stars during press working, characterized by the above.