JPH04285149A - Method for manufacturing alloyed hot-dip galvanized steel sheet with excellent image clarity after painting - Google Patents

Abstract

PURPOSE:To manufacture a galvanized steel sheet excellent in image clarity after coating by using a steel sheet having specified roughness after cold rolling into a plated original sheet and subjecting it to galvannealing. CONSTITUTION:A steel sheet in which average waviness Wea in center line of its surface roughness after cold rolling is regulated to <=0.6mum is formed into a original sheet for plating. The above original sheet for plating can be obtd. by using a laser beam dull working roll, a shot dull roll having <=1.0mum average roughness or the like in the final stand in its cold rolling stage. This original sheet for plating is galvannealed. Furthermore, in the case high image clarity is required, the areal rate at the flat part on the surface is regulated to 30 to 90% or the average waviness Wea in center line is regulated to <=0.4mum by skin pass rolling after plating.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an alloyed hot-dip galvanized steel sheet that has excellent image clarity after painting and is used primarily for automobile exterior panels.

0002

[Prior Art] Cold-rolled steel sheets have traditionally been widely used for thin steel sheets that require a finished appearance after painting, such as the exterior panels of automobile bodies, home appliances, and sheet metal furniture, which are compatible with formability. The surface roughness is adjusted by temper rolling (skin pass). However, the use of surface-treated steel sheets is rapidly increasing, especially from the standpoint of rust prevention for automobile steel sheets, and it has become an issue to achieve both post-painting sharpness and press formability in surface-treated steel sheets.

[0003] In the case of surface-treated steel sheets with relatively thin coatings, such as electroplating, the surface roughness of the cold-rolled steel sheet as the original sheet is maintained even after surface treatment, and surface roughness control has traditionally been difficult for cold-rolled steel sheets. This is almost possible with advanced technology.

However, if further anti-corrosion measures are required, as in the case of alloyed hot-dip galvanized steel sheets, a thicker surface treatment is required, and in that case, the surface roughness is higher than that of the original cold-rolled steel sheet. There has been a problem in that the roughness varies greatly in the hot-dip galvanizing process and the post-plating alloying process.

It is known that the surface roughness of the final alloyed hot-dip galvanized steel sheet is roughened by specific fine irregularities, which adversely affects both the image clarity and press formability after painting.

[0006] Nowadays, the quality of the surface finish after painting of an automobile has recently been attracting attention as an important quality control item because it can directly appeal to customers the luxurious feel and high overall quality of the automobile. Image clarity is one of the indicators of paint finish quality, and improvements in painting technology have traditionally been made to improve image clarity. On the other hand, the surface roughness of thin steel sheets has conventionally been generally roughened by dulling for press formability. However, with the improvement of painting technology, the relationship between the surface roughness of the thin steel sheet that forms the basis of the painted surface and the surface roughness after painting has become clear, and it is now possible to improve the sharpness after painting by controlling the surface roughness of the steel sheet. It has gradually become clear that this is possible.

[0007] The surface roughness of cold-rolled steel sheets has conventionally been controlled by temper rolling using shot-dulled skin pass rolls, but the main purpose of this is to improve press formability. In order to improve the sharpness after painting, it is necessary to reduce the surface roughness of cold-rolled steel sheets.
aper Ser, No. 800208) It is also introduced in the paper.

However, even if this result is applied as is, problems remain in terms of moldability. It is impossible to achieve both formability and image clarity by controlling the average roughness as in conventional shot dull processing. JP-A-62-168602 and JP-A-62-224405 disclose surface roughness control technology for achieving both post-painting sharpness and formability in cold-rolled steel sheets. However, the applicable steel types have been limited to cold-rolled steel sheets or surface-treated steel sheets with a light weight that retains the surface roughness of the original sheet even after surface treatment.

[0009] In other words, it is considered impossible to control roughness when the surface is treated with a thick coating such as hot-dip galvanized steel sheet, or when the surface is roughened by alloying treatment.
Research on this subject has been largely ignored.

0010

[Problems to be Solved by the Invention] In the above-mentioned prior patents, the target steel types were all limited to cold-rolled steel sheets and light weight surface-treated steel sheets. The reason for this is that it is a type of steel whose surface roughness is basically determined by temper rolling, and the desired roughness control can be achieved relatively easily in this process. On the other hand, alloyed hot-dip galvanized steel sheets have fine irregularities on their surfaces, and for this reason, it has been thought that the effect of roughness control as in the case of cold-rolled steel sheets cannot be expected.

The present invention discloses a surface roughness control technology for improving the sharpness of an alloyed hot-dip galvanized steel sheet after painting to the same level as that of a cold-rolled steel sheet. The object of the present invention is to provide a method for manufacturing an alloyed galvanized steel sheet with excellent image clarity.

0012

[Means for Solving the Problems] That is, the present invention provides filtered centerline average waviness (Wca) of surface roughness after cold rolling.
The present invention provides a method for producing an alloyed hot-dip galvanized steel sheet with excellent image clarity after coating, which is characterized by applying alloyed hot-dip galvanizing to a steel sheet having a particle diameter of 0.6 μm or less as a plating base plate.

[0013] Here, at least as a roll for the final stand in the cold rolling process of the plated original plate, a roll dulled using a laser beam, a roll processed by electric discharge dulling, or a shot dull roll having an average roughness of 1.0 μm or less is used. Alternatively, it is preferable to use either Bright Roll.

[0014] Furthermore, the present invention provides alloyed hot-dip galvanizing using a steel plate having a filtered centerline average waviness (Wca) of 0.6 μm or less in surface roughness after cold rolling as a plating base plate, and further, after plating, An alloy with excellent image sharpness after coating, characterized by a flat area ratio of the plated surface of 30% or more and 90% or less, or an average undulation of the filter center line of 0.4 μm or less, by skin pass rolling. A method for manufacturing a hot-dip galvanized steel sheet is provided.

[0015] Here, in the cold rolling process of the plated original plate, at least a roll for the final stand is a roll dulled using a laser beam, a roll processed by electric discharge dulling, a shot dull roll or a bright roll having an average roughness of 1.0 μm or less. Either a roll dulled using a laser beam, a discharge dulled roll, or a shot dull roll with an average roughness of 1.0 μm or less as a skin pass roll after the alloyed hot-dip galvanizing. It is preferable to use either.

The present invention will be explained in more detail below. The surface roughness of the alloyed hot-dip galvanized steel sheet is such that the surface is roughened overall as shown in FIG. 1 due to the fine irregularities formed during the alloying process after plating as described above. On the other hand, as a result of a detailed investigation into the influence of the surface roughness of the painted base steel sheet on the sharpness after painting, it was found that among the various wavelength components that make up the surface roughness, the short wavelength components are It does not affect the sharpness of the image because it is hidden by the
It has been found that long wavelength roughness components exceeding 800 μm remain as roughness components even after coating and impair image clarity.

That is, it has been found that the surface roughness of the alloyed hot-dip galvanized steel sheet can be controlled in relation to the surface roughness of the cold-rolled steel sheet that is the original sheet. Based on this new knowledge, we have confirmed that it is possible to improve the sharpness of alloyed hot-dip galvanized steel sheets after painting, which has been a problem in the past, by controlling the roughness of the plating original sheet and temper rolling after plating. The present invention discloses the appropriate range.

Here, the important roughness parameter in the present invention is the filter center line average waviness Wca (
μm). The method for determining this average waviness is JIS B.
0601, and a high-pass cutoff filter (800
μm) and a low-pass cutoff filter (8 mm).
), it is defined by Equation 1 below.

[Equation 1] Here, L is the length of measuring the centerline waviness curve,
It is desirable that the length be three times or more the low frequency cutoff value (8 mm). Furthermore, recently, 3
The filter centerline average waviness (SWca), which can be obtained by dimensional roughness measurement, is the 3-dimensional filtering center line average waviness (SWca) obtained by measuring the conventional two-dimensional filter center waviness curve while moving at a constant pitch in a direction perpendicular to the measurement direction. Dimensional filter center curve f(x,y)
(average waviness defined by formula 2 below)

[Equation 2] can be treated similarly and can be used as a limiting parameter regarding waviness in the present invention.

In the present invention, in addition to waviness, average roughness Ra (μm) is used as a roughness limiting parameter. This parameter is also defined in the JIS standard, but the cutoff value is 8 for cross-sectional curves.
When the centerline roughness curve obtained by applying a 00 μm filter to remove long wavelength waviness components is expressed as f(x), it is defined by the following equation 3.

##EQU00003## Here, L is the length for measuring the roughness curve, and as a general rule, it is desirable that the length be at least three times the cutoff value (800 μm). In addition, as in the case of waviness, in addition to the conventional two-dimensional average roughness, three-dimensional average roughness has recently become popular, and it can be similarly used as a roughness limiting parameter in the present invention. can. The definition of three-dimensional average roughness (SRa) is that the three-dimensional roughness curve obtained by measuring the conventional two-dimensional roughness curve while moving at a constant pitch in the direction perpendicular to the measurement direction is called f(x,y). Then, it is defined by the following equation 4.

[Math 4]

Furthermore, in the present invention, the flat portion of the plating surface is defined as having an average roughness Ra (μm) of 0.6 μm or less (R
a≦0.6), and the flat area ratio is defined as the ratio of the area of the part where the average roughness Ra is 0.6 μm or less to the total area of the plated surface.

[0021] In order to establish a method for achieving the roughness conditions of the present invention described above on the surface of an alloyed hot-dip galvanized steel sheet (GA plating),
It is necessary to investigate in detail the changes in surface roughness caused by Fine irregularities increase due to alloying treatment after hot-dip galvanizing, but we investigated how the roughness component, which is actually harmful to image clarity, changes before and after alloying hot-dip galvanizing. As a result, it was found that although the fine irregularities hidden in the coating film increased with the alloying treatment, the long wavelength waviness component did not change significantly before and after plating. Based on this new knowledge, it has become possible for the first time to improve the image sharpness of the surface of an alloyed hot-dip galvanized steel sheet (GA) after painting by controlling the surface roughness of the alloyed hot-dip galvanized (GA) original sheet.

In other words, by controlling the surface roughness structure of the plated original plate so that the filter centerline average waviness (Wca) is 0.6 μm or less, the waviness component after alloyed hot-dip galvanizing (GA) is also reduced. This is effective in improving image clarity after painting. The upper limit of this average waviness, 0.6 μm, is a necessary condition to obtain the effect of improving image clarity, and it is necessary to reduce the waviness component of the plated original plate to below this value. If the above-mentioned average waviness Wca exceeds 0.6 μm, the roughness components harmful to image clarity cannot be filled in by alloyed hot-dip galvanizing or painting, and they remain as roughness components even after painting. This impairs image clarity after painting.

Furthermore, if high image clarity after painting is required, the image clarity after painting can be further improved by controlling the surface roughness after alloyed hot-dip galvanizing (GA) by skin pass rolling. . In this case, the surface roughness management index is to make the filtering center line average waviness Wca 0.4 μm or less,
Alternatively, it is important that the flat portion area ratio is 30% or more and 90% or less. The above-mentioned securing of the flat area and the reduction of the above-mentioned filtering centerline average waviness have been independently confirmed to have effects on image clarity, and the upper limit of the filtering centerline average waviness of 0.4 μm has the effect of improving image clarity. This is a necessary condition to obtain more. A higher flat area ratio of 30% or more is more advantageous for image clarity, but a high flat area ratio of over 90% is not appropriate because it significantly impairs moldability, especially mold galling resistance.

[0024] Such surface roughness control is a technology that can only be made possible by limiting the waviness component of the plated original plate, and if this technology is not applied, skin pass elongation after alloyed hot-dip galvanizing (GA) It is necessary to increase the ratio, which may lead to an increase in yield strength (YP).

[0025] The waviness component needs to be controlled from the stage of surface roughness of the roll used to roll the steel plate, but one way to achieve low waviness is to process a regular dull pattern using a laser beam. Possible methods include applying fine irregularities using a discharge duller or shot duller, or using a bright roll. The surface roughness control stage of the original sheet can be performed at the final stand of cold rolling, and the surface roughness adjustment after alloying hot-dip galvanizing can be performed by skin pass rolling. In other words, the surface roughness of the steel plate can be controlled by transferring the roll surface roughness to the steel plate surface during rolling. As a method for controlling the roll surface roughness, a cold rolling roll for controlling the surface roughness of the plated original plate is used. At least, the roll for the final stand in the cold rolling process is a dull processing method using a laser beam, a discharge dull, a shot dull or bright roll with an average roughness of 1.0 μm or less, and a bright roll is used as a roll for a skin pass after plating. 3 above except
It is effective to use any of the seed dal processing methods.

The reason for this is that when the average roughness Ra of the surface of the shot dull roll is larger than 1.0 μm, there is a correlation between Ra and the waviness (Wca), so the waviness becomes large, and the roughness limitation of the present invention is not satisfied. Due to inability to be satisfied. Further, the reason why a bright dull roll is not used as a roll for skin pass after plating is that the flatness area ratio of the surface roughness after skin pass increases and the roughness range of the present invention cannot be satisfied. The discharge dull roll makes the increase in waviness relatively small and can secure Ra, and the laser dull roll can completely resolve and control Ra and waviness Wca, so even in cold rolling of the plated original plate, there is no skin pass after plating. It can also be used.

[0027]

EXAMPLES The present invention will be specifically explained below based on examples. (Example 1) A cold-rolled steel plate with a thickness of 0.7 mm was used as a plating original plate, and the area weight was 45/45 (g/m2) on both sides of the steel plate under the same conditions.
) and then subjected to alloying treatment at 540°C for 3 seconds. The plated original plates were prepared by varying the surface roughness of cold rolling rolls to adjust the filter center line average waviness, and were used as test samples. The sample after plating was subjected to skin pass rolling, and the roll used for this was also prepared with the surface roughness adjusted in the same way.

After the skin pass, the sample was coated with 3 coats (electrodeposition: Kansai Paint's Elekron 9400 with a thickness of 20 μm, intermediate coat: TP-26 Sealer, and top coat: Amirac TM-13).
#202 (black) was coated with a thickness of 50 μm), and then the DOI value was measured as an evaluation of image clarity. The DOI value is measured using a Hunter DORIGON meter, and is the amount of specularly reflected light when irradiated with light from an angle of 30 degrees to the normal line of the sample. When the amount of is R0.3, it is given as DOI=(Rs-R0.3)/Rs×100. This evaluation method uses human visual judgment and P
It shows good correlation with conventional evaluation methods such as GD method.

Table 1 shows the relationship between the surface roughness of the original plate, the roughness after plating, and the sharpness after painting, which were tested using the above method. To control the waviness of the plated original plate, alloyed hot-dip galvanizing (G
It is found that this effect is effective in controlling waviness after plating (A), and that this effect promotes the effect of skin pass after plating.

[0030] In Table 1, the method for dulling the surface of the roll in the column for cold-rolled plated original sheet indicates the method for dulling the surface of the roll for the final stand in the cold rolling process. The method for dulling the surface of a roll in the column for galvanized (GA-plated) steel sheet indicates the method for dulling the surface of a skin pass roll after GA plating. Here, LD is a laser dull roll, EDT is a discharge dull roll, Br is a bright dull roll, S/D is a shot dull roll, and Ra is the average roughness (μm) of the roll surface.

[0031]

[Table 1]

[0032]

According to the present invention, the waviness component of the surface roughness of the plating original plate can be reduced to 0.6μ by the filter center line average waviness Wca.
m or less, by applying alloyed hot-dip galvanizing to this plated original plate, while maintaining formability,
It is possible to produce an alloyed hot-dip galvanized steel sheet with improved image clarity after painting.

Furthermore, according to the present invention, if high image sharpness after coating is required, the flat area can be adjusted to 30% or more and 90% or less by skin pass after plating, or
Alternatively, high image clarity can be achieved by setting the average filtering centerline waviness Wca to 0.4 μm or less.

Further, according to the present invention, as a roll for the final stand of cold rolling of a plated original plate, a rolling roll that has been dulled by a method using a laser beam or a method using electric discharge machining or a roll having an average roughness Ra of 1.0 μm is used. By using the following low roughness shot dull rolls or bright rolls, and by using the above rolls excluding the bright rolls as rolls for skin passes after plating, it is possible to control the waviness component of the plated original plate, and furthermore, the waviness component of the plated original plate can be controlled. The surface roughness of the plated (GA plated) steel plate can be controlled, and as described above, the image clarity after painting can be improved while maintaining the formability of the GA plated steel plate.

Furthermore, it is clear that the effects of the present invention can be similarly obtained even when two-layer plating is applied on top of hot-dip galvanizing.

[Brief explanation of the drawing]

FIG. 1 is a measurement result of the surface roughness profile of an alloyed hot-dip galvanized steel sheet. Note that the magnification is vertical and horizontal (X, Y
axis) and 500 times in the roughness (vertical Z-axis) direction.

Claims (4)

[Claims] Claim 1: A post-painting clear film characterized in that alloyed hot-dip galvanizing is applied to a steel plate having a filtered center line average waviness (Wca) of 0.6 μm or less in surface roughness after cold rolling as a plating base plate. A method for manufacturing alloyed hot-dip galvanized steel sheets with excellent properties. 2. At least as a final stand roll in the cold rolling process of the plating original plate, a roll dulled using a laser beam, a discharge dulled roll, a shot dull roll with an average roughness of 1.0 μm or less, or 2. The method for producing an alloyed hot-dip galvanized steel sheet with excellent image clarity after painting according to claim 1, wherein any one of bright rolls is used. [Claim 3] A steel plate having a filtered center line average waviness (Wca) of the surface roughness after cold rolling of 0.6 μm or less is used as a plating base plate, and alloyed hot-dip galvanizing is applied, and after plating, skin pass rolling is performed. An alloyed hot-dip galvanized steel sheet with excellent image clarity after coating, characterized by having a flat area ratio of the plated surface of 30% or more and 90% or less, or an average undulation of the filter center line of 0.4 μm or less. manufacturing method. 4. A roll dulled using a laser beam, a discharge dulled roll, a shot dull roll or a bright roll with an average roughness of 1.0 μm or less as at least a final stand roll in the cold rolling process of the plated original plate. A roll dulled using a laser beam, a discharge dulled roll, or a shot dull roll with an average roughness of 1.0 μm or less as a skin pass roll after the alloyed hot-dip galvanizing. 4. The method for producing an alloyed hot-dip galvanized steel sheet with excellent image clarity after painting according to claim 3.