JP3277158B2 - Hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet with excellent appearance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-dip galvanized steel sheet having excellent appearance and a galvannealed steel sheet.

[0002]

2. Description of the Related Art In recent years, the range of use of surface-treated steel sheets such as hot-dip galvanized steel sheets and electroplated steel sheets has been increasingly widespread, and as a result, demands for beautiful appearance and surface uniformity of the surface-treated steel sheets have become more and more severe. Has become. On the other hand, when hot dip galvanizing is applied to a steel strip in a continuous plating line, the surface of the steel sheet of the final plated product is coated with a hot-rolled steel sheet (hereinafter also referred to as hot-rolled sheet) due to the quality of the steel sheet. Unevenness, alloying unevenness, streak-like defects, and the like often occur.

Conventionally, in order to suppress the occurrence of these defects, it is indispensable to use a hot-rolled sheet having a beautiful surface, no irregularities such as unevenness defects, distortion, and chemical composition, and a uniform appearance as a plating material. Met. That is, for example, if uneven defects mainly generated during hot rolling are present in the hot-rolled sheet, even if black scale removal or cold rolling is performed after hot rolling, the uneven defects remain, and hot-dip plating In addition, unevenness of the amount of plating and unevenness of alloying due to the uneven defects also occur during alloying by heating.

[0004] If local distortion is present in the hot-rolled sheet, it also remains after the subsequent removal of the black scale by pickling or cold rolling, and is based on the difference in the amount of iron diffusion due to the distortion during hot-dip plating. Non-uniformity in plating adhesion (plating non-uniformity) or alloying non-uniformity due to a difference in alloying speed when alloying treatment is performed. Furthermore, if there is non-uniform distribution of the chemical composition due to segregation on the steel surface after hot rolling, especially non-uniform distribution of the easily oxidizable element, then the black scale pickling removal and cold It remains after rolling, and causes plating unevenness and alloying unevenness after plating based on the distribution of the surface concentration of the chemical composition, regardless of the hot-dip plating method.

[0005] In the above phenomenon, defects caused by a hot-rolled sheet remain as they are even after cold rolling.
In addition, during hot-dip galvanizing, zinc causes a direct alloying reaction with the original sheet, and as a result, the surface state of the original sheet is significantly affected. Therefore, when using a hot rolled sheet having surface defects such as unevenness distortion and uneven distribution of chemical composition, a hot-dip galvanized steel sheet having excellent appearance, an alloyed hot-dip galvanized steel sheet (hereinafter referred to as an alloyed hot-dip It was not possible to obtain a coated steel sheet).

[0006] On the other hand, Japanese Patent Application Laid-Open No. 8-277457 discloses an alloyed hot-dip galvanized steel sheet having good appearance in which the metal structure of the surface layer of the steel sheet is adjusted. However, no matter how the original plate having the above-mentioned metal structure is used, if there are irregularities on the surface of the original plate, the irregularities cannot be eliminated by the subsequent cold rolling.

[0007] For this reason, even if the steel sheet is subjected to a pretreatment such as electrolytic degreasing or annealing after the cold rolling, and the steel sheet is hot-dip galvanized, the appearance is uneven. That is, when the hot-rolled sheet has unevenness defects, it is impossible to obtain a cold-rolled steel sheet which is a base material of a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet having excellent appearance. Further, as a method for producing a galvannealed steel sheet having good clarity after coating, JP-A-4-285149 discloses that a shot dull roll having a surface roughness of 1.0 μm or less is used as a final cold rolling stand. , So-called "filter wave center line average swell"
A method for reducing Wca to 0.6 μm or less is disclosed.

[0008] However, in the case where there is an irregularity defect in the plated original plate, the use of a roll having a small roughness makes the original plate defect more conspicuous and more conspicuous. In addition, when there is a local distortion or a local difference in chemical composition in the hot-rolled sheet, the distortion or the difference does not disappear even after cold rolling, and thereafter, electrolytic degreasing,
Even if plating is performed after pre-treatment such as annealing, appearance unevenness occurs, and it is not possible to obtain a cold-rolled steel sheet which is a base material of a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet having excellent appearance.

Further, in a normal industrial production process, the surface of a part of the hot-rolled steel strip is subjected to unevenness due to temperature unevenness, rolling unevenness, etc. during hot rolling, or after winding or unwinding. It is inevitable that irregularities such as shape defects, distortion, and chemical composition are generated. If such a steel sheet is used as a base steel sheet of a galvanized steel sheet, in a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet to be subjected to a heat alloying treatment,
It is inevitable that unevenness of plating, spot-like or streak-like alloying unevenness caused by the unevenness defect of the hot rolled sheet is caused.

As described above, conventionally, a hot-rolled steel sheet was cold-rolled, and the obtained cold-rolled steel sheet was plated. However, with such a measure, surface defects were completely eliminated. Hot-dip galvanized steel sheet with beautiful appearance and uniform surface properties,
It was impossible to obtain a cold-rolled steel sheet that served as a base for the galvannealed steel sheet.

[0011]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems of the prior art and to provide a galvanized steel sheet having excellent appearance and a galvannealed steel sheet.

[0012]

According to a first aspect of the present invention, there is provided the following formula:
The difference in surface strength ratio Δ of the base steel sheet defined by (1) is 0.4 or more, and the content of Al and Zn in the plating layer satisfies the relationship of the following formula (2). It is a hot-dip galvanized steel sheet. Difference in surface strength ratio of base steel sheet Δ = ｛(200) ² / (222)｝ _(s) − ｛(200) ² / (222)｝ _(1/4) …… (1) [Al (g / m ² )] − 0.0012 × [Zn (g / m ² )] ≧ 0.03… (2) In the above formula (1), {(200) ² / (222)} _(s) : Surface layer of the base steel sheet immediately below the plating layer The ratio of the square of the integrated X-ray diffraction intensity of the (200) plane of the ferrite to the X-ray diffraction integrated intensity of the (222) plane of the ferrite in the direction perpendicular to the plate surface ｛(200) ² / (222) ｝ _(1/4) : The X-ray diffraction integrated intensity of the (200) plane of ferrite in a direction perpendicular to the plate surface at a depth of 1/4 of the base steel plate thickness from the base steel plate surface immediately below the plating layer. Ratio of squared and integrated intensity of X-ray diffraction of (222) plane of ferrite In the above formula (2), Al (g / m ² ): Al in the plating layer per unit area of plating coating
Content (g / m ² ) Zn (g / m ² ): Zn in plating layer per unit area of plating coating
The content (g / m ² ) is shown.

[0013] In the second invention, the difference Δ in the surface strength ratio of the base steel sheet defined by the following equation (1) is 0.4 or more, and the content of Al, Zn and Fe in the plating layer is represented by the following equation (1). An alloyed hot-dip galvanized steel sheet having an excellent appearance characterized by satisfying the relationship of (2) and (3). Difference in surface strength ratio of base steel sheet Δ = ｛(200) ² / (222)｝ _(s) − ｛(200) ² / (222)｝ _(1/4) …… (1) [Al (g / m ² )] − 0.0012 × [Zn (g / m ² )] ≧ 0.03 ... (2) 14 ≧ [Fe / (Fe + Al + Zn)] × 100 ≧ 7 ... (3) In the above formula (1), ｛ (200) ² / (222)｝ _(s) : The square of the X-ray diffraction integral intensity of the (200) plane of the ferrite and the (222) of the ferrite in the direction perpendicular to the surface of the surface of the base steel sheet immediately below the plating layer. ) Ratio to the integrated X-ray diffraction intensity of the surface ｛(200) ² / (22
2)｝ _(1/4) : 1/4 of the thickness of the base steel sheet from the base steel sheet surface immediately below the plating layer, in the direction perpendicular to the plate surface.
Ratio of the square of the integrated X-ray diffraction intensity of the (200) plane of ferrite to the integrated X-ray diffraction intensity of the (222) plane of ferrite In the above formula (2), Al (g / m ² ): per unit area of plating coating In the plating layer of
Content (g / m ² ) Zn (g / m ² ): Zn in plating layer per unit area of plating coating
Content (g / m ² ) In the above formula (3), Fe, Al, Zn: The content of Fe, Al, and Zn in the plating layer (wt.
%).

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in more detail. The present inventors have conducted intensive studies with the aim of providing a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet with excellent appearance, and obtained the following findings. .

That is, in a galvanized steel sheet or a galvannealed steel sheet, (A) the following formula (1)
The difference Δ of the surface strength ratio of the base steel sheet (base steel) defined by
As described above, the content (adhesion amount) of Al and Zn in the plating layer satisfies the relationship of the following formula (2), or (B) the above two conditions are satisfied, and In the galvannealed steel sheet having an Fe, Al and Zn content (wt%) satisfying the following formula (3), the above-mentioned appearance unevenness (: plating unevenness), spotted or streaked alloy No defects such as unevenness are observed.

[0016] Difference in surface strength ratio of a base steel plate (base iron) Δ = ｛(200) ² / (222)｝ _(s) − ｛(200) ² / (222)｝ _(1/4) … (1 ) [Al (g / m ² )] − 0.0012 × [Zn (g / m ² )] ≧ 0.03 ... (2) 14 ≧ [Fe / (Fe + Al + Zn)] × 100 ≧ 7 ……… (3) In the formula (1), {(200) ² / (222)} _(s) : X-ray diffraction integrated intensity of the (200) plane of ferrite in the direction perpendicular to the sheet surface of the surface of the base steel sheet immediately below the plating layer Ratio of the square of the X-ray diffraction integrated intensity of the (222) plane of the ferrite to the square of the ferrite ｛(200) ² / (22
2)｝ _(1/4) : 1/4 of the thickness of the base steel sheet from the base steel sheet surface immediately below the plating layer, in the direction perpendicular to the plate surface.
The ratio of the square of the integrated X-ray diffraction intensity of the (200) plane of ferrite to the integrated X-ray diffraction intensity of the (222) plane of ferrite is shown.

On the other hand, when the difference Δ of the surface strength ratio of the base steel sheet defined by the above equation (1) is less than 0.4, the Al in the plating layer
When the contents of Zn and Zn do not satisfy the relationship of the above formula (2), the above-mentioned defects are extremely likely to occur in the galvanized steel sheet. Further, when the difference Δ of the surface strength ratio of the base steel sheet defined by the formula (1) is less than 0.4, when the content of Al and Zn in the plating layer does not satisfy the relationship of the formula (2),
When the contents of Fe, Al, and Zn in the plating layer do not satisfy the relationship of the above formula (3), the above-described defects are extremely likely to occur in the alloyed hot-dip galvanized steel sheet.

The surface intensity of the (200) plane and the like of the ferrite described above is measured by a symmetric reflection X-ray diffraction method and is a (200) plane density parallel to the sample plane (ratio to the same plane density of the randomly oriented sample). And the like, and the measurement results obtained by a conventional method were used. The above facts are presumed to be due to the following actions and effects. (I) Action and effect associated with the definition of the difference Δ in the surface strength ratio of the base steel sheet: Hereinafter, in the present invention, the difference Δ in the surface strength ratio of the base steel sheet defined by the following formula (1) satisfies 0.4 or more. The reason for this is described below.

The difference in the surface strength ratio of the base steel sheet Δ = ｛(200) ² / (222)｝ _(s) − ｛(200) ² / (222)｝ _(1/4) (1) 200) face (hereinafter simply referred to as (200) face)
Has a very good alloying reaction with zinc, and even without heat-alloying, if the strength of the (200) plane is large, the slight iron-zinc alloy phase generated in hot-dip galvanizing is extremely uniform. To be generated.

As a result, in the galvanized steel sheet,
When the value of (200) ² , which is the square of the (200) plane strength of the surface layer of the base steel sheet, is large, the effect of the presence of the (200) plane on the Fe-Zn alloying reaction causes uneven plating and white spots on the plated steel sheet. Defects, original plate shape that causes streak defects, chemical composition non-uniformity, distortion unevenness, etc. exist in the base steel before plating,
It is considered that since the overall reactivity is good, as a result, the occurrence of unevenness defects on the original plate, local differences in composition, and unevenness in appearance due to distortion unevenness are suppressed.

Further, the (200) plane of ferrite is considered to have not only a very good alloying reaction with zinc but also a very good reactivity with Al.
When the strength of the (200) plane is large, a slight iron-zinc alloy phase generated in hot-dip galvanizing is formed extremely uniformly, and an initial Fe-Al initial alloy phase generated in hot-dip galvanizing is sufficiently generated.

When the initial Fe-Al alloy phase is sufficiently formed, the alloying reaction between zinc and ground iron is moderately suppressed, and local abnormal occurrence of excessive alloy phase is suppressed. As a result, even if there is unevenness in plating, white spot-like defects, original sheet shape that causes streak defects, non-uniformity of chemical composition, unevenness in distortion, etc. in the base steel before plating, the base steel sheet surface layer Part of (200)
If the value of 強度 (200) ² ｝ _S , which is the square of the surface strength, is large, (20
0) The overall reactivity is good due to the synergistic effect on the Fe-Zn alloying reaction and the Fe-Al alloying reaction of the plane,
As a result, it is considered that the occurrence of unevenness defects on the original plate, local differences in composition, and unevenness in appearance (: plating unevenness, white spot-like defects, streak defects) due to distortion unevenness are suppressed.

Therefore, the (200) ² / (222) plane strength ratio of the surface layer portion of the base steel sheet immediately below the coating layer of the hot-dip galvanized steel sheet or the alloyed hot-dip galvanized steel sheet, that is, the square of the strength of the (200) plane and cold rolling When the ratio to the strength of the (222) plane, which is the highest in the steel sheet, is large, the above defects are suppressed, and a galvanized steel sheet and an alloyed hot-dip galvanized steel sheet having an extremely beautiful appearance can be obtained.

On the other hand, a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet are also required to have good workability, and therefore require a (222) plane strength with little anisotropy. Therefore, it is necessary to restrict the (200) ² / (222) plane strength ratio to a relatively low value inside the base steel sheet. In order to satisfy the above-mentioned requirements for both the plating quality of the surface of the plated steel sheet and the workability of the base steel, the (20
It is necessary that the value of the (0) ² / (222) plane strength ratio is large and the value of the (200) ² / (222) plane strength ratio inside the base steel sheet is small.

What is the difference between the above-mentioned strength ratios and whether it is possible to obtain a beautiful and excellent galvanized steel sheet and an alloyed hot-dip galvanized steel sheet, As a result of the investigation below, as shown in the first invention and the second invention, the surface strength ratio of the surface layer of the ground iron immediately below the plating layer of the hot-dip galvanized steel sheet in the direction perpendicular to the sheet surface ｛(200 ) ² / (222)｝ _(s) and 1/4 (1) of the base steel sheet thickness (t) from the base steel sheet surface just below the plating layer
= (200) ² / (22) which is the difference between the surface strength ratio (200) ² / (222)｝ _(1/4 ) of the depth of
2) It was found that｝ _(s) − ｛(200) ² / (222)｝ _(1/4) should be 0.4 or more.

(II) Actions and effects associated with the definition of the content of Al and Zn in the plating layer per unit area of the plating coating: Next, in the present invention, per unit area of the plating coating of Al and Zn in the plating layer The reason why the content of is defined so as to satisfy the relationship of the following formula (2) will be described. [Al (g / m ² )] − 0.0012 × [Zn (g / m ² )] ≧ 0.03 (2) In order to obtain a uniform and beautiful appearance, the above-mentioned control of the texture alone is not sufficient. It is necessary to form a plating layer having an optimum composition on the base steel sheet having the above-mentioned texture.

That is, it is necessary to suppress the local abnormal growth of the alloy phase by sufficiently generating the initial Fe-Al alloy phase immediately after plating under the condition that the dissolved Al is sufficiently present in the plating bath. is there. The present inventors have conducted intensive studies on the optimum composition of the above-mentioned plating layer, and as shown in Examples described later, the content of Al and Zn in the plating layer per unit area of the plating coating is expressed as [Al (g / m ² )] − 0.0012
× [Zn (g / m ² )] ≧ 0.03, the local Fe-Al alloy phase suppresses local and abnormal growth of the alloy phase, resulting in a beautiful appearance. It has been found that a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet having the following are obtained.

The value obtained by subtracting the amount β of Al in the plating layer, which is estimated from both the Al concentration in the plating bath and the amount of deposited plating, from the amount α of Al actually contained in the plating layer, is the initial Fe content. This is considered to correspond to the amount of -Al alloy phase generated. If the value of {[Al (g / m ² )] − 0.0012 × [Zn (g / m ² )]} is less than 0.03 and the amount of the initial Fe—Al alloy phase generated is too small, extremely fast Fe— The Zn alloying reaction cannot be suppressed, and as a result, local abnormal growth of the alloy phase occurs, and a hot-dip galvanized steel sheet having a beautiful appearance cannot be produced.

At a normal plating bath temperature of 470 to 490 ° C., the amount of Al dissolved in the plating bath is about 0.12 wt%.
(g / m ² )] − 0.0012 × [Zn (g / m ² )]｝ means the amount of Al taken into the plating layer over the amount of Al corresponding to the plating bath composition, and this value is specified. Doing so has the same content as specifying the amount of the Fe-Al initial alloy phase. In the present invention, in any of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet, the content per unit area of the coating layer of Al and Zn in the coating layer may satisfy the following formula (4): More preferred.

0.14 ≧ [Al (g / m ² )] − 0.0012 × [Zn (g / m ² )] ≧ 0.03 (4) This is ｛[Al (g / m ² )] − 0.0012 × [Zn (g / m ² )]｝
Even if the amount of Fe-Al initial alloy phase is increased to exceed 0.14
The effect of increasing the initial Fe-Al alloy phase is saturated in practical use, and the amount of Al used increases, which is uneconomical. In the case of an alloyed hot-dip galvanized steel sheet, Δ [Al (g / m ² )] − 0.0012 × [Zn
(g / m ² )] When the value of を exceeds 0.14, the amount of the initial Fe-Al alloy phase generated becomes excessive, and the alloying reaction does not proceed rapidly during the heat-alloying process, resulting in burn-out uneven appearance defects. cause.

In order to form a plating layer having the above-mentioned optimum composition, the plating bath at the time of hot-dip galvanizing must contain Al.
It is preferable to use a plating bath having a composition of not less than 0.08% by weight and not more than 0.20% by weight, the balance being Zn and unavoidable impurities. If the Al concentration in the bath is less than 0.08% by weight, the initial Fe-Al alloy phase is not sufficiently formed, so that the extremely fast Fe-Zn alloying reaction cannot be suppressed, resulting in local abnormal growth of the alloy phase. The hot-dip galvanized steel sheet having a beautiful appearance cannot be manufactured.

On the other hand, in the case of a hot-dip galvanized steel sheet not subjected to alloying treatment, if the Al concentration in the bath is higher than 0.20% by weight,
It is uneconomical due to the increased amount of used. Also, in the case of alloyed hot-dip galvanized steel sheet subjected to alloying treatment, if the Al concentration in the bath is higher than 0.20% by weight, the amount of initial Fe-Al alloy phase generated becomes excessive, and The reaction does not proceed promptly, resulting in a scorched uneven appearance defect.

(III) Action and effect of the content of Fe, Al and Zn in the coating layer in the galvannealed steel sheet: Next, in the present invention, in the coating layer after the alloying treatment, The reason why the contents of Fe, Al, and Zn are defined so as to satisfy the following expression (3) will be described. 14 ≧ [Fe / (Fe + Al + Zn)] × 100 ≧ 7 (3) (Fe, Al and Zn in the above formula (3) are Fe, Al and Zn in the plating layer.
Shows the Zn content (wt%). That is, when the value of {[Fe / (Fe + Al + Zn)] × 100} is less than 7 in the content of Fe, Al, and Zn in the plating layer after the alloying treatment, an unalloyed unalloyed phase remains. In addition, not only is the flaking resistance during processing (: plating peeling resistance) inferior, but it also causes a burn-out uneven appearance.

Conversely, ｛[Fe / (Fe + Al + Zn)] × 10
When the value of 0% exceeds 14, the plating adhesion at the time of processing is inferior. Therefore, the value of {[Fe / (Fe + Al + Zn)] × 100} must be 7 or more and 14 or less. ｛[Fe / (Fe +
Al + Zn)] × 100} is more preferably 8 or more and 11
It is as follows.

[Production method of hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet of the present invention] Next, a method of manufacturing the hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet of the present invention will be described. Hot-dip galvanized steel sheet of the present invention,
As the base steel sheet (base iron) of the galvannealed steel sheet, any of the steel sheets generally used for hot-dip galvanized sheets, such as so-called ultra-low carbon steel and low carbon steel, can be used. The composition of the base steel sheet is not particularly limited.

This is because setting the texture of the base steel sheet and setting the composition of the plating layer are the most important in order to improve the appearance of the plated steel sheet, and there is almost no difference depending on the steel type. The method of obtaining a base steel sheet having the above-described characteristics, that is, a method of obtaining a steel sheet having the above-described surface strength ratio is not particularly limited.However, the base steel sheet is, for example, when performing cold rolling, a large shear deformation occurs in the surface layer portion of the rolled steel sheet. By employing a cold rolling method that causes

That is, using a roll having an uneven surface layer such as a high roughness shot dull roll or a high roughness scratch dull roll, for example, a metal flow is positively induced in the surface layer of the original sheet to be rolled. By adopting the method, a ground iron surface layer having the above-described texture can be obtained. In that case, the rolling method is not particularly limited, and may be not only a normal rolling method but also another rolling method such as convergent rolling.

When rolling is performed using a roll having a surface portion having an uneven shape, remarkable shear strain is generated in the surface portion of the raw sheet to be rolled. The (200) plane orientation is easily developed. on the other hand,
Since the texture inside the base steel is affected by the rolling reduction,
The (222) plane orientation develops better than the (200) plane.

As a result, the surface strength ratio of the surface layer of the base steel immediately below the plating layer: {(200) ² / (222)} _(s), which is 1/4 of the thickness (t) of the base steel sheet from the base steel surface [: (1/4) t] Surface strength ratio of ground iron at depth: {(200) ² / (222)} Difference from _(1/4) : [｛(200) ²
/ (222)｝ _(s) − ｛(200) ² / (222)｝ _(1/4) ]. FIG. 2 shows a cold rolling method using a high-roughness work roll as an example for obtaining the above texture.

In FIG. 2, 1 is a hot-rolled steel sheet,
2 is a cold-rolled steel plate, 3 is a high roughness work roll, 4a, 4b,
4c is a bright work roll, 5 is a dulling work roll, 10 is an annealing furnace, 11 is a hot dip galvanizing equipment, 12 is an alloying furnace, F is the direction of cold rolled steel sheet passing, and f is the direction of hot rolled steel sheet. Shows the plate direction. The cold rolling equipment shown in FIG.
On the stand, the high roughness work roll 3 is placed on the first stand,
Bright work rolls 4a, 4b, 4c on the second to fourth stands
The dulling work roll 5 is arranged on the fifth stand which is the final stand.

In the cold rolling equipment shown in FIG. 2, a bright work roll may be used instead of the dull work roll in the fifth stand. The high roughness work roll 3 for obtaining the steel sheet surface having the texture according to the present invention may be either a shot dull roll or a scratch dull roll.

This is because if the surface roughness of the roll is rough, a sufficient shear strain can be imparted to the surface of the steel sheet after cold rolling. Using the above-described cold rolling equipment, the hot-rolled steel sheet after pickling was cold-rolled under various conditions while changing the surface roughness of the high-roughness work roll and the reduction ratio distribution in each stand.

The obtained cold-rolled steel sheet was subjected to continuous annealing, followed by galvanizing. Next, the plating layer of the obtained hot-dip galvanized steel sheet was peeled off, and the texture of the steel sheet immediately below the plating layer (the surface layer of the ground iron) was measured by an X-ray diffraction method. In addition, for the same sample, 1/4 of the steel sheet thickness (t) was ground and removed from the surface of the base iron, and the texture of the steel sheet at a depth of 1/4 of the steel sheet thickness (t) was removed from the base steel surface by X-ray. It was measured by a diffraction method.

Further, the contents of Al and Zn in the plating layer per unit area of the plating coating were determined from the Al content and the plating adhesion amount in the plating layer. As a result, the surface strength ratio of the surface layer of the steel immediately below the plating layer: {(200) ² / (222)} _(s), and 1/4 of the sheet thickness (t) from the surface of the steel, [: (1/4) t] is the difference from the surface intensity ratio at the depth of {｛(200) ² / (222)｝ _(1/4) [｛(20
0) ² / (222)｝ _(s) − ｛(200) ² / (222)｝ _(1/4) ) is 0.4 or more, and in the plating layer per unit area of plating coating.
When the content of Al and Zn is [Al (g / m ² )] − 0.0012 × [Zn (g /
m ² )] ≧ 0.03, a hot-dip galvanized steel sheet having an extremely beautiful appearance was obtained.

As described above, this is achieved by increasing the surface strength ratio of the surface layer of the ground iron: {(200) ² / (222)} _(s) ,
The reactivity of the surface layer of the base iron becomes good, and even if there is plating unevenness after plating, white spot-like defects, shape unevenness resulting in streak defects, chemical composition unevenness, distortion unevenness, etc. It is considered that the local difference was reduced, and as a result, a galvanized steel sheet having excellent appearance was obtained.

On the other hand, a hot-dip galvanized steel sheet is also required to have good workability, so that a (222) plane strength with little anisotropy is also required, and the plane strength ratio inside the steel sheet: ｛(200) ² / ( 222)｝
_(1/4) needs to be kept at a relatively low level. That is, the difference Δ in the surface intensity ratio described above: [{(200) ² / (222)}
_(s) − ｛(200) ² / (222)｝ _(1/4) ) is 0.4 or more,
Hot-dip galvanized steel sheet with excellent appearance and good workability,
An alloyed galvanized steel sheet is obtained.

On the other hand, the difference Δ: [｛(20
0)^Two/ (222)｝_(s)− ｛(200)^Two/ (222)｝ _(1/4)Is not 0.4
If it is full, good workability can be obtained, but
Poor appearance, uneven plating, white spot defects, streak defects
Frequently occur. Therefore, the difference Δ in the surface strength ratio must be 0.4 or more.
I have to. Further, the difference Δ in the surface intensity ratio described above:
(｛(200)^Two/ (222)｝_(s)− ｛(200)^Two/ (222)｝ _(1/4)]
Is more preferably 2.0 or less.

This is because when the difference Δ in the surface strength ratio exceeds 2.0, the effect of improving the appearance of the surface of the coated steel sheet with the increase in the difference Δ in the surface strength ratio becomes saturated in practical use, and This is because the production conditions for controlling the texture, such as the rolling conditions, become strict and are not economical. The cold rolling conditions for obtaining the above-mentioned texture of the base steel sheet according to the present invention by cold rolling are as follows: Arithmetic average roughness of the roll surface of the high roughness work roll: Ra is 0.5 μm or more; It is preferable that the reduction ratio in the work roll is 2% or more and the total reduction ratio is 75% or more.

The above-mentioned arithmetic mean roughness Ra of the roll surface is the arithmetic mean roughness specified in JIS B 0601-1994. As a plating stripping method at the time of preparing the texture measurement sample according to the present invention, an acid such as hydrochloric acid, a stripping method using an alkaline aqueous solution, a method of electrochemically dissolving and removing in a non-aqueous solvent, and the like are used. be able to.

For example, a 20 wt% sodium hydroxide aqueous solution,
An example is a method in which a plating layer is removed with an aqueous solution obtained by adding 35 wt% aqueous hydrogen peroxide to an aqueous solution in which a 10 wt% aqueous solution of triethanolamine and water are mixed at a ratio of 2: 1: 2, respectively. After removing the plating layer on the surface of the plated steel sheet by the above-mentioned method, the ferrite
By measuring the (200) plane and the (222) plane from a direction perpendicular to the plate surface by an X-ray diffraction method, the surface intensity ratio of the surface layer portion:
｛(200) ² / (222)｝ _(s) can be measured.

Further, preferably, for the sample after measuring the surface strength ratio of the above-mentioned surface layer portion, 1/4 of the steel plate thickness is removed by grinding from the surface of the ground iron, and the ferrite of the ground iron at the depth is removed.
By measuring the (200) and (222) planes in a direction perpendicular to the plate surface by X-ray diffraction, the surface intensity ratio 地 (200 ) ² / (222)｝ _(1/4) can be measured.

[0052]

EXAMPLES The present invention will be described below more specifically based on examples. Using the cold rolling equipment shown in FIG. 2 described above, a cold-rolled steel sheet was manufactured by changing the reduction ratio distribution of each stand and the surface roughness of the high roughness work roll in various ways. Table 1 shows the conditions of the cold rolling.

The arithmetic average roughness Ra of the roll surface in Table 1 is the arithmetic average roughness specified in JIS B 0601-1994. After continuous annealing of the obtained cold-rolled steel sheet, subjected to continuous hot-dip galvanizing, subsequently passed through a heating furnace and subjected to heat alloying treatment, the resulting alloyed hot-dip galvanized steel sheet was evaluated by the evaluation method described below. .

Some of the hot-dip galvanized steel sheets were not subjected to alloying treatment, and the hot-dip galvanized steel sheets were evaluated by the evaluation method described later. The conditions for hot-dip galvanizing and alloying treatment are as follows. [Hot-dip galvanizing bath:] Al: 0.07 to 0.22% by weight Bath temperature: 480 ° C [Alloying furnace:] Steel plate temperature: 490 ° C [Evaluation method of galvannealed steel sheet, hot-dip galvanized steel sheet:] ( Appearance of plating surface :) The following criteria were used to evaluate whether or not white spot-like defects or streak-like irregularities were visible by visual observation.

：: Good external appearance (: No defects, unevenness, etc. existed) ×: Defects, unevenness, etc. were visually observed. (Measurement of texture :) The plating layer was treated with an alkaline aqueous solution [;
20 wt% NaOH aqueous solution: 10 wt% triethanolamine aqueous solution: water (weight ratio) = 2: 1: 2], and the texture immediately below the plating layer was measured by X-ray diffraction. A portion having a thickness of 1/4 of the plate thickness was removed by grinding from the surface layer, and the texture was measured again by the X-ray diffraction method.

Table 2 shows alloyed galvanized steel sheets,
The measurement results of the texture of the base steel sheet (base iron) of the hot-dip galvanized steel sheet, the composition of the coating layer, and the appearance of the coated steel sheet are shown. FIG. 1 shows a difference Δ: [｛(200) ² /
(222)｝ _(s) − {(200) ² / (222)｝ _(1/4) ] and the appearance of the plated steel sheet. As shown in Table 2 and FIG. 1, an alloying treatment in which the difference Δ in the ground iron surface strength ratio is 0.4 or more and the contents of Al, Zn, and Fe in the plating layer satisfy the conditions of the present invention. The appearance of the galvanized steel sheet and the galvanized steel sheet was extremely good.

On the other hand, the above-mentioned plated steel sheet having a difference Δ in surface strength ratio of less than 0.4, and a plated steel sheet in which the contents of Al, Zn, and Fe in the plated layer do not satisfy the conditions of the present invention were all inferior in appearance. .

[0058]

[Table 1]

[0059]

[Table 2]

[0060]

As described above, according to the present invention, it is possible to provide a beautiful hot-dip galvanized steel sheet and a hot-dip galvanized steel sheet having an excellent alloy appearance.

[Brief description of the drawings]

[Fig. 1] Difference in ground surface strength ratio Δ: [｛(200) ² / (222)｝ _(s)
6 is a graph showing the relationship between − {(200) ² / (222)} _(1/4) ] and the appearance of a plated steel sheet.

FIG. 2 is an explanatory view showing one example of a cold rolling method for obtaining a texture of a base steel sheet according to the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Hot rolled steel plate (steel strip) 2 Cold rolled steel plate (steel strip) 3 High roughness work roll 4a, 4b, 4c Bright work roll 5 Dulling work roll (or bright work roll) 10 Annealing furnace 11 Hot dip galvanizing Equipment 12 Alloying furnace F Direction of passing cold-rolled steel sheet f Direction of passing hot-rolled steel sheet

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-10-18011 (JP, A) JP-A-6-2069 (JP, A) JP-A-5-33111 (JP, A) International Publication No. 92/14854 (WO, A1) (58) Field surveyed (Int. Cl. ⁷ , DB name) C23C 2/00-2/40

Claims (2)

(57) [Claims] 1. The difference Δ in the surface strength ratio of the base steel sheet defined by the following equation (1) is 0.4 or more, and the contents of Al and Zn in the plating layer satisfy the relationship of the following equation (2). Hot-dip galvanized steel sheet with excellent appearance. Difference in surface strength ratio of the base steel sheet Δ = ｛(200) ² / (222)｝ _(s) − ｛(200) ² / (222)｝ _(1/4) …… (1) [Al (g / m ² )] − 0.0012 × [Zn (g / m ² )] ≧ 0.03 ... (2) In the above formula (1), ｛(200) ² / (222)｝ _(s) : Base steel sheet immediately below the plating layer The surface of the ferrite in a direction perpendicular to the plate surface (20
0) The square of the X-ray diffraction integrated intensity of the plane and the (222)
Ratio to the X-ray diffraction integrated intensity of the surface {(200) ² / (222)} _(1/4) :
Ferrite in a direction perpendicular to the plate surface at a depth of 1/4 of the plate thickness from the base steel plate surface just below the plating layer
Square of X-ray diffraction integrated intensity of (200) plane and (22)
2) Ratio to X-ray diffraction integrated intensity of surface In the above formula (2), Al (g / m ² ): Al in the plating layer per unit area of plating coating
Content (g / m ² ) Zn (g / m ² ): Zn in plating layer per unit area of plating coating
The content (g / m ² ) is shown. 2. The method according to claim 1, wherein the difference Δ in the surface strength ratio of the base steel sheet defined by the following formula (1) is 0.4 or more and the Al, Zn and Fe
A galvannealed steel sheet having an excellent appearance, characterized by satisfying the following formulas (2) and (3). Difference in surface strength ratio of the base steel sheet Δ = ｛(200) ² / (222)｝ _(s) − ｛(200) ² / (222)｝ _(1/4) …… (1) [Al (g / m ² )] − 0.0012 × [Zn (g / m ² )] ≧ 0.03 ... (2) 14 ≧ [Fe / (Fe + Al + Zn)] × 100 ≧ 7 ... (3) In the above formula (1), ｛(200) ² / (222)｝ _(s) ： (20) of ferrite in the direction perpendicular to the surface of the surface
0) The square of the X-ray diffraction integrated intensity of the plane and the (222)
(200) ² / (222)｝ _(1/4) ： From the surface of the base steel plate just below the plating layer to the plate surface at a depth of 1/4 of the thickness of the base steel plate In the vertical direction, the ratio of the square of the integrated X-ray diffraction intensity of the (200) plane of the ferrite to the integrated X-ray diffraction intensity of the (222) plane of the ferrite In the above equation (2), Al (g / m ² ) ： Al in plating layer per plating coating unit area
Content (g / m ² ) Zn (g / m ² ): Zn in plating layer per unit area of plating coating
Content (g / m ² ) In the above formula (3), Fe, Al, Zn: The content of Fe, Al, and Zn in the plating layer (wt.
%).