JPH02185959A - Production of hot dip galvanized steel sheet having superior vividness - Google Patents

Abstract

PURPOSE:To ensure superior press formability and vividness by specifying a base material to be hot dip galvanized and conditions in temper rolling when a hot rolled steel sheet is cold rolled, hot dip galvanized, alloyed and temper- rolled. CONSTITUTION:A cold rolled steel sheet rolled with bright rolls or rolls machined with laser beams is used as a base material to be hot dip galvanized. The steel sheet is hot dip galvanized, alloyed and temper-rolled with rolls machined with laser beams. The vividness of the alloyed hot dip galvanized steel sheet after coating is largely affected by the surface roughness of the base material to be hot dip galvanized. Since the bright steel sheet or the laser dull steel sheet is used in place of the conventional dull steel sheet, superior vividness is ensured. Since the conventional shot dull rolls are not used in temper rolling, the vividness is not reduced. When temper rolling is carried out with bright rolls, the service life of the rolls is short and cost is increased.

Description

[Detailed Description of the Invention] (Field of Industrial Application) This invention is an alloyed hot-dip galvanized coating that has excellent corrosion resistance and sharpness after painting, and is suitable for automobile body exterior panels, household electrical appliance product exterior panels, etc. This invention relates to a method for manufacturing steel plates.

(Prior Art) Alloyed hot-dip galvanized steel sheets are widely used in many industrial fields because they have excellent corrosion resistance and can be mass-produced at low cost. The manufacturing method for this method is briefly described below. That is, hot-rolled steel sheets are cold-rolled to the required thickness using a tandem cold rolling mill, and then annealed, or they are continuously hot-dip galvanized using a hot-dip coating facility equipped with a continuous annealing furnace, and then subjected to alloying treatment. After that, temper rolling is performed.

The above-mentioned cold rolling is usually carried out using rolls whose surface has been finished by shot blasting or electrical discharge machining (hereinafter collectively referred to as dull rolls) as work rolls*? After performing fI dipping, alloying treatment is performed,
Next, temper rolling is performed to correct the flatness of 1 degree. As work rolls for temper rolling, so-called bright rolls with polished surfaces or the aforementioned dull rolls are used.

Alloyed hot-dip galvanized steel sheets are alloyed by interdiffusion of zinc and iron during the alloying process, so the surface is quite uneven and has a large roughness. The sharpness after painting is inferior to that of ordinary cold-rolled steel sheets or electroplated steel sheets with a dull finish. In addition,
Sharpness refers to the state in which a painted surface appears beautiful and clear without distortion, and it is known that this property is greatly influenced by the surface condition (roughness) of the base before painting. It is being

Recently, there has been a strong tendency to emphasize the beauty of the appearance of automobiles, home appliances, etc., and there is a growing demand for plated steel sheets for such uses that can ensure sharpness after painting. The manufacturing method for galvanized steel sheets, which particularly requires high image clarity, uses bright steel plates as the plating base material, and reduces the elongation rate of temper rolling to 4% after alloying treatment.
There are techniques that can be used before and after. However, the plated steel sheet obtained by this method has a markedly reduced press formability and cannot be applied to highly processed parts.In other words, in order to maintain press formability, it is necessary to have good mechanical properties of the base material and the use of lubricating oil. The surface of the plate needs to have a certain degree of unevenness to act as a trap site for the accumulation or fine metal powder generated during molding. In the above manufacturing method, in addition to deteriorating mechanical properties, the plate surface becomes too smooth, resulting in poor press formability.

(Problems to be Solved by the Invention) An object of the present invention is to provide a method for manufacturing an alloyed hot-dip galvanized steel sheet that has excellent image clarity after painting while maintaining press formability.

(Means for Solving the Problem) The smoother the base surface before painting, the better the sharpness of the image. Therefore, even in alloyed hot-dip galvanized steel sheets, the sharpness can be improved by increasing the rolling reduction in temper rolling, but this significantly reduces the press formability.

The present inventor has discovered that by appropriately selecting the plating base material and the conditions for temper rolling after plating, it is possible to obtain an alloyed hot-dip galvanized steel sheet that has good press formability and excellent image clarity. Knowing this, he completed this invention.

The gist of the present invention is to use cold-rolled steel sheets rolled with bright rolls or laser beam processing rolls as the plating base material, perform melt galvanizing, alloying treatment, and then temper-rolling with laser beam processing rolls. There is a method J for producing a hot-dip galvanized steel sheet with excellent image clarity, which is characterized by carrying out the following steps.

The base material used for plating is a cold-rolled steel sheet that has excellent press formability. However, in the present invention, at least in the final bath of the cold rolling, a bright roll or a laser beam processing roll is used as the work roll. A bright roll is a roll whose surface is adjusted by polishing after surface grinding, and the surface roughness is Ra (
The center line average roughness) is about 0.1 to 0.5 μm.

A laser beam processed roll is a roll whose surface is regularly patterned with minute irregularities by irradiating it with a laser beam, and is usually called a laser dull roll. The basic technology of roll processing using such a laser beam is introduced in Japanese Patent Publication No. 11922/1983.

The surface of a cold-rolled steel sheet that has been final rolled using the rolls described above is a so-called bright surface, or a dull surface on which the minute irregularities of the laser dull roll are transferred (a steel plate with such a dull surface is called a laser dull surface). Hot-dip galvanizing is applied to such a base steel plate, which is referred to as a plate.

The hot-dip galvanizing process itself may be carried out under normal conditions, that is, the base steel plate annealed offline or online is immersed in a hot-dip zinc bath, the basis weight is adjusted by gas wiping, and then the alloying treatment is performed. give

The plated steel sheet that has undergone the alloying treatment is then subjected to temper rolling. The second feature of the present invention is that a laser beam processed roll (laser dull roll) is used as the work roll for this temper rolling. The laser dull roll used here preferably has a surface roughness of about 1 to 3 μm in Ra.

Further, the degree of temper rolling is preferably about 0.1 to 2% in terms of elongation.

(Function) The image clarity after coating of an alloyed hot-dip galvanized steel sheet is greatly influenced by the surface roughness of the plating base material. Therefore, in the present invention, a bright steel plate or a laser dull steel plate is used in place of the conventional dull steel plate. It goes without saying that when a bright steel plate is used as the base material, the plated steel plate has excellent image clarity. On the other hand, even when a laser dull steel plate is used as a base material, the minute irregularities on its surface are more regular than those of a shot blast dull steel plate, and furthermore, it is easier to adjust the size and distribution density of the irregularities with laser beam processing. , it is possible to obtain a steel plate with excellent press formability without impairing image clarity.

However, when conventional shot dull rolls are used in temper rolling after plating and alloying, the surface roughness of the plating becomes irregular and rough, and the sharpness after painting is significantly reduced. On the other hand, if temper rolling is performed after alloying treatment with a bright roll, it will have the same effect as rolling with a laser dull roll, but bright rolls are prone to scratches and have a short roll life, so the rolls must be replaced frequently. This results in an increase in manufacturing costs.

In addition, it is desirable that the alloyed hot-dip galvanized steel sheet finally obtained by the method of the present invention has a surface roughness of Ra of 1.3μ or less and a Wca of 0.9μ or less. From the point of view of sharpness, the smaller both Ra and Wca, the better; however, in terms of the scratch resistance, slip resistance, pressability, etc. of the steel plate, the lower limit is about 0.3 μm for Ra and 0.2 μm for Wca. In addition, Wca is "filter center line waviness" of JIS B 0610.

FIG. 1 is a diagram showing the relationship between the surface roughness (Ra) of the plating base material and the plating alloyed material. In the figure, ○ indicates a base material rolled with a bright roll, Δ indicates a base material rolled with a laser dull roll, and × indicates a base material rolled with a shot dull roll. If a steel plate with low surface roughness is used as the plating base material, the surface roughness will be maintained below a certain level even after alloying treatment.

Figure 2 shows the results of examining changes in roughness due to plating, alloying treatment, and temper rolling using bright steel plates and shot-blasted dull steel plates as base materials. The material itself, ■ is as-alloyed, ■ is temper-rolled with a laser dull roll, and ■ is Ra = 1.4 μl.
Temper rolled with a shot dull roll, ■ is Ra
= temper rolled with a 2.7 μm shot dull roll. As shown in the figure, after the alloying treatment, the roughness becomes larger than that of the base material, but if skin pass rolling is performed with a laser dull roll, the roughness becomes smaller again. In other words, the surface roughness changes greatly depending on the rolls used for skin pass rolling, and skin pass rolling using laser dull rolls is required to reduce the roughness of the plated steel sheet to ensure sharpness after painting. be.

The surface roughness of an alloyed hot-dip galvanized steel sheet made of dull steel plate as a base material is quite high before temper rolling. Therefore, when it is rolled down with shape-correcting temper rolling rolls, the surface convexities are crushed and the surface becomes smooth. portion increases and the surface roughness Ra decreases. However, if the surface roughness of the temper rolling roll is too large, the opposite result will occur.
The surface roughness of the resulting plated steel sheet increases, which adversely affects image clarity.

If you use smooth bright steel plate or laser dull steel plate as the plating base material! Ji! The surface roughness is small even before temper rolling. Therefore, unless a temper rolling roll with a small surface roughness is used, the surface roughness will increase. The use of bright rolls reduces the surface roughness, but as mentioned above, the management of the rolls themselves is cumbersome. In the end, it is best to use a laser dull roll as the temper rolling roll, since the surface roughness can be easily adjusted and the surface of the plated steel sheet can be given an ideal roughness.

Figures 3 and 4 show that the plating base material has an Ra of 0.1 to 0.
．． Using a 4μm bright steel plate, hot-dip galvanizing and alloying treatment were performed to prepare samples with different Ra and Wca, and a 40μm steel coating with a thickness of 40μm was applied using chemical conversion treatment, cationic electrodeposition coating, and top coating. This shows the results of examining the sharpness using a PGD meter (manufactured by Japan Color Research Institute).

From FIG. 3, in terms of Ra, the image clarity is good when it is 1.3 μm or less, and in terms of Wca, the image clarity is good when it is 0.9 μm or less.

(Example) CF 0.04%, Mn: 0.17%, Pl, 01
%, S: 0.01%, Aj! : A 0.05% steel plate is cold-rolled in a tandem mill with a bright roll or laser dull roll incorporated into the work roll of the final stand to produce a bright steel strip with a surface roughness of Ra=0.26 μm.
= 0.95 μm dull steel strips were produced, and these were plated using a Sendzimer type hot-dip galvanizing equipment as a base material.
Subsequently, alloying treatment was performed in an alloying furnace. The basis weight is 45
g/m'', and the Pe content of the plating film is about 10%.

Furthermore, in-line temper rolling was performed by changing the rolling rolls so that the elongation rate was 0.1%. The UA quality rolling roll was dulled by laser beam processing and shot blasting.

The image clarity after painting was evaluated using the above steel plate.

Sharpness is determined by dipping type chemical conversion treatment, cationic electrodeposition coating (20 μm), and top coating (3
After applying two coats of 5μ+1), evaluation was made using a PGD meter from Japan Color Research Institute.

The results are shown in Table 1.

(Hereinafter, blank space) As is clear from Table 1, the sharpness of Nα2 and IO, which correspond to the examples of the present invention, are clearly superior to the others.

Note that the above effect is achieved by adding a small amount of 82, Mn to the galvanizing bath.
, Mg, Ti, etc., there is no difference at all.

(Effects of the Invention) According to the method of the present invention, an alloyed hot-dip galvanized steel sheet with excellent corrosion resistance and excellent image clarity after painting can be easily obtained. Such plated steel sheets are extremely useful as exterior materials for a wide variety of products where design is important.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between the surface roughness of a plating base material and the surface roughness after plating and alloying treatment. FIG. 2 is a diagram showing changes in surface roughness in each process from plating base material to plating, alloying treatment, and temper rolling. FIG. 3 and FIG. 4 are diagrams showing the relationship between the sharpness of the plated steel sheet after coating and the surface roughness (Ra and Wca).

Claims (1)

[Claims] Sharpness characterized by using cold-rolled steel sheets rolled with bright rolls or laser beam processing rolls as the plating base material, and performing temper rolling with laser beam processing rolls after hot-dip galvanizing and alloying treatment. A method for producing hot-dip galvanized steel sheets with excellent properties.