JP2001294977A - Hot dip galvannealed steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot dip galvannealed steel sheet in which the pattern development of the defect on a stripe is suppressed. SOLUTION: This steel sheet has a composition containing <=0.004% C, <=0.1% Si, <=0.5% Mn, <=0.025% P, <=0.025% S and 0.005 to 0.100% Al or further containing one or more kinds metals selected from 0.015 to 0.04% Ti and 0.001 to 0.010% Nb and has a hot dip galvannealing layer in which, as to the surface structure of the steel sheet by X-ray diffraction, the ratio in diffraction X-ray intensity between that from the 200} plane and that from 222} plane, i.e., I(200)/I(222) is 0.17, and also, the deviation in the sheet face is ±5 g/m2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a galvannealed steel sheet, and more particularly to a galvannealed steel sheet having excellent appearance properties.

[0002]

2. Description of the Related Art Alloyed hot-dip galvanized steel sheets have high weldability,
Excellent paintability, corrosion resistance after painting, automobile body and home appliances,
It is widely used in building materials, etc., and its production cost is lower than that of electrogalvanized steel sheets, so its share is expanding. Along with this, reductions in manufacturing costs of alloyed hot-dip galvanized steel sheets themselves are also being promoted, and cost reduction measures are being implemented in each of the processes of ironmaking, steelmaking, hot rolling, cold rolling, annealing, plating, and heat alloying. . For example, in the hot rolling process, the heating temperature is reduced to reduce the energy of the furnace for heating the steel sheet. For this reason, the hot rolling is currently performed at a finish rolling temperature slightly higher than the _Ar3 transformation point by several degrees to several tens degrees Celsius.

[0003] An alloyed hot-dip galvanized steel sheet is obtained by heating after hot-dip coating to diffuse an iron component of a mother plate into a galvanized layer, and the galvannealed layer changes depending on the surface state of the mother plate. That is easily speculated. For example, WO92-1
No. 4854 discloses that, using a steel sheet with a desired texture by controlling the cooling conditions after hot rolling, particularly the conditions of cold rolling and recrystallization, the galvanizing cold rolling is excellent in forming workability and chemical conversion treatment. A steel plate is disclosed.

[0004] However, when a hot-rolled sheet manufactured in a state where the finish rolling temperature in hot rolling is lowered is cold-rolled, and further subjected to hot-dip galvanizing and heat-alloying, streaky patterns are observed on the surface, The number of defective cases increased, which was a problem. Hereinafter, the appearance defect of the stripe pattern is simply referred to as a stripe pattern defect. Streak pattern defects have no problem in machining, welding and painting, but many consumers reject them as poor appearance.

[0005]

DISCLOSURE OF THE INVENTION The present invention advantageously solves the above-mentioned problems, and provides an alloyed hot-dip galvanized steel sheet which is free from streak pattern defects even in a state where the heat intensity of hot rolling is kept low. The purpose is to propose.

[0006]

Means for Solving the Problems The present inventors observed the surface of a galvannealed steel sheet having streak pattern defects at two locations having different surface contrasts with a scanning electron microscope and examined the surface of the plating layer. The composition was analyzed. As a result, one is a ζ phase (Zn ₁₃ Fe), the other is a more alloyed δ ₁ phase (Zn ₇ Fe) as a main constituent phase, and the ferrite crystal grains immediately below the ζ phase are smooth. , Δ ₁ phase, it was found that the ferrite crystal grains of the base plate had significant irregularities. Here, the depressions of the ferrite crystal grains of the base plate are traces of the base plate Fe diffusing into the plating layer, and it is considered that Fe of the base plate diffused into the plating layer as the irregularities increased. The (100) plane and (110) plane are developed in the smooth base plate crystal grains, and the (111) plane is developed in the base plate crystal grains with marked irregularities. The present inventors have found that the streak pattern defect is such that the orientation of the crystal grains on the surface of the mother plate is streaky, and a difference occurs in the diffusion rate of Fe to the plating layer during the alloying process. It is considered that a streak-like difference occurs in the contrast of the steel sheet surface, resulting in an appearance defect called a streak pattern defect.

That is, the present inventors have obtained a new finding that the generation of the streak pattern defect is caused by a streak-like difference in crystal grains on the surface of the mother plate. Further, as shown in FIG. 1, the present inventors measured the ratio of {200} plane intensity to {222} plane intensity, I
(200) / I (222) is, if less than 0.17, the occurrence of streaks defect is eliminated, and the X-ray diffraction intensity by a finish rolling temperature of hot rolling and A _r3 + 30 ° C. or higher ratio,
The present inventors have obtained a completely new finding that I (200) / I (222) is less than 0.17, and constituted the present invention.

[0008] That is, the present invention is an alloyed hot-dip galvanized steel sheet obtained by subjecting the surface of a cold-rolled steel sheet to alloyed hot-dip galvanizing, wherein the cold-rolled steel sheet has a C content of 0.004% or less, 0.1% or less, Mn: 0.5% or less, P: 0.025% or less, S: 0.025% or less, Al: 0.005 to 0.100%, or even Ti: 0.01 to 0.04% and Nb: 0.001 to 0.010%
Containing at least one selected from the group consisting of Fe and inevitable impurities, and having a surface roughness of 200% by X-ray diffraction.
The ratio of the diffracted X-ray intensities from the｝ plane and the {222} plane, I (200
) / I (222) is less than 0.17, and furthermore, the in-plane deviation of the coating weight of the alloyed hot-dip galvanized layer is within ± 5 g / m ^2. It is a steel plate.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The composition limitation of a cold rolled steel sheet suitable for the present invention will be described. In addition, mass% is simply described as%. C
Is detrimental to imparting workability to the cold-rolled steel sheet, and it is better to reduce it as much as possible, preferably 0.004% or less.

[0010] In addition to acting as a deoxidizing agent, Si strengthens the steel by solid solution strengthening, but if contained excessively, it impairs the workability. In addition, since Si oxide is not easily reduced, if it is present, poor plating adhesion occurs during zinc plating. Therefore, in the present invention, 0.1% or less is preferable. Mn
In addition to acting as a deoxidizing agent, it also strengthens the steel by solid solution strengthening, but if contained excessively, it impairs workability. Therefore, it is preferable that Mn is 0.5% or less.

P is a solid solution strengthening element and is effective in increasing the strength. However, if it is too large, it deteriorates the workability and further deteriorates the alloying processability after plating. For this reason, in the present invention, it is preferable to reduce as much as possible and set the upper limit to 0.025%. S forms an inclusion by impurities and lowers workability of steel, so that it is reduced as much as possible. For this reason, it is desirable that the content be 0.025% or less.

Al is used for deoxidation, but has a strong affinity for N, which is an interstitial element, and is also used for fixing N. However, if the content is too large, the workability of the steel is reduced.
5 to 0.100 wt% is preferred. Ti is added for fixing C, S, and N. However, if the amount is too large, the workability deteriorates. For this reason, Ti is preferably set to 0.01 to 0.04%.

Nb is added to obtain precipitated carbides.
Excessive addition deteriorates workability. For this reason, Nb
It is preferably in the range of 0.001 to 0.010%. The base sheet before plating, that is, the cold-rolled and annealed base sheet, had a texture of the surface of the base plate measured by X-ray inverse pole projection, which was {200} plane,
Ratio of X-ray diffraction intensity from {222} plane, I (200) / I
(222) should be less than 0.17. The diffraction X-ray intensity ratio is 0.17
In the above, the proportion of the (100) and (110) planes increases in the texture of the base plate surface, and the diffusion of Fe in the base plate to the plating layer tends to be non-uniform in the heat alloying treatment after hot-dip galvanizing. As a result, a difference occurs in the amount of plating, and thus the surface contrast becomes non-uniform, deteriorating the appearance. 0.17
If it is less than 1, non-uniformity of the surface contrast of Fe does not occur.

[0014] deposition of the galvannealed is in the range from the average value of the adhesion amount of ^{+ 5g / m 2 ~-5g} / m 2. When the variation from the average value exceeds 5 g / m ² , the surface contrast becomes non-uniform, and the appearance properties become poor. In order to produce the alloyed hot-dip galvanized steel sheet of the present invention, in hot rolling of the base sheet, it is preferable to complete the finish rolling by setting the surface temperature of the steel sheet during finish rolling to a temperature higher than the _Ar3 transformation point by 30 ° C. or more. . In addition, it is preferable to set the above-mentioned finish rolling temperature in any of the rolling direction and the sheet width direction.

[0015] Thereby, the base plate surface after cold rolling and annealing is
The (111) plane develops preferentially, and the (100) plane and (110) plane
Suppress the development of planes and reduce the amount of iron
Variation in the coating weight of the alloy plating layer due to the difference in diffusion rate
It can be made smaller and the phase of steel components and precipitates
A by difference_r3It is a measure against the rise of points. A in a small area
_r3The rise of the point is considered to be mostly within 30 ° C.
You.

When the base plate manufactured by such hot finish rolling is used and then subjected to cold rolling, annealing, hot-dip coating, and hot alloying to form a hot-dip galvanized steel sheet, the adhesion amount of the alloy plating layer Can be suppressed within a range of ± 5 g / m ^{2 in} both the rolling direction and the sheet width direction, so that generation of streak pattern defects can be prevented. When the in-plane deviation (variation) of the steel sheet in the rolling direction and the width direction of the hot rolling exceeds 30 ° C in the rolling direction and the width direction of the hot rolling, the variation in the adhesion amount of the galvannealed layer is ± 5 g / m ² . Beyond, line patterns are likely to occur. However, by joining the sheet bars before finish rolling, the inside of the finish rolling mill can be continuously rolled, the temperature variation in the rolling direction is reduced, and the deviation (variation) in the finish rolling temperature in the rolling direction is preferably within 30 ° C. Can be.

Sheet bar joining can be achieved by joining the trailing end of the preceding sheet bar to the leading end of the succeeding sheet bar after rough rolling, and the joining method may be either welding or pressure welding, and is particularly limited. Not done. In addition, by heating the width end of the steel sheet with an edge burner, the variation in the finish rolling temperature in the width direction of the steel sheet is reduced, and the deviation (variation) in the finish rolling temperature in the width direction of the steel sheet can preferably be kept within 30 ° C.

The reason why the surface temperature of the base plate at the time of finish rolling in hot rolling of the base plate is made uniform in the width direction and the rolling direction and the finish rolling is performed is because of the difference in the surface structure of the base plate in the width direction and the rolling direction. This is because the diffusion rate of Fe in the mother plate to the plating layer in the hot alloying treatment of the hot-dip galvanized layer is made uniform, thereby eliminating the difference in the amount of plating adhesion. It is preferable to install a plurality of edge burners at both ends of the width of the steel sheet, between each stand of the finishing cold rolling mill and on the exit side of the finishing cold rolling mill. After the edge burner is installed, an edge cover may be further installed for the purpose of preventing cooling of the end.

The hot-rolling heating temperature of the mother plate does not need to be particularly specified in the present invention, and is usually in a known temperature range, but is preferably set to 1250 ° C. or less in consideration of miniaturization of Ti precipitates. Is preferred. If the temperature exceeds 1250 ° C., workability deteriorates. The mother plate is subjected to cold rolling after hot rolling. In cold rolling, after hot-rolled sheet is pickled, it is rolled to a predetermined final sheet thickness by a generally known method, for example, a tandem cold rolling mill. The rolling reduction is preferably 60% or more, whereby a desired structure can be obtained by subsequent recrystallization annealing.

After cold rolling, the surface of the mother plate is subjected to galvanization, preferably by a continuous galvanizing facility. Galvanization is not necessarily limited to continuous equipment. The base plate after cold rolling is preferably annealed between 730 and 920 ° C,
Cool to a temperature suitable for plating and then immerse in a hot dip galvanizing bath to deposit zinc. Zinc bath is 450-49
The temperature is preferably set to 0 ° C. In addition, the adhesion amount of zinc is 30-70 g
/ m ² is preferable. The mother plate to which zinc is attached is immediately subjected to a heat alloying treatment. Alloying process is 450 ~ 550 ℃
It is preferred that

[0021] Incidentally, zinc coating weight after the alloying treatment is preferably in a range from the average value of the adhesion amount of ^{+ 5g / m 2 ~-5g} / m 2. When the variation from the average value exceeds 5 g / m ² , the surface contrast becomes non-uniform, and the appearance properties become poor.

[0022]

EXAMPLES Steel having the chemical composition shown in Table 1 was melted in a converter and continuously cast into slabs. The slab was heated and hot-rolled under the conditions shown in Table 1 to form a hot-rolled steel strip having a thickness of 4 mm. Next, these hot-rolled steel strips were pickled in a 5% by mass aqueous hydrochloric acid solution, and then cold-rolled to a sheet thickness of 0.8 mm. Thereafter, in a continuous hot-dip galvanizing equipment, annealing was performed at a soaking temperature of 850 ° C and a soaking time of 60 seconds, and cooled to 460 ° C at a cooling rate of 20 ° C / s.
The immersion plating was performed in a hot-dip galvanizing bath of Zn-0.14% Al at ℃. After plating, the coating weight was adjusted to 40 g / m ² by wiping, and immediately followed by heat alloying at 480 ° C. and air cooling.

The manufactured alloyed hot-dip galvanized steel sheet was visually observed and evaluated in three steps. Further, the variation in the adhesion amount of the alloyed plating layer was measured in the sheet width direction and the rolling direction. ○ indicates no line pattern, Δ indicates a line pattern with an area ratio of 1 to 10%, and X indicates a line pattern with an area ratio exceeding 10%. Table 2 also shows the results. As can be seen from Table 2, no streak pattern is generated in the alloyed hot-dip galvanized steel sheet (Example of the present invention) having a diffraction X-ray intensity ratio of less than 0.17, which is within the scope of the present invention. In addition, in the example of the present invention in which the finish rolling temperature was _Ar3 + 30 ° C. or more in both the rolling direction and the sheet width direction, the variation in the amount of plating applied was ± 5 g / m ² or less, and no streak pattern was observed. Was. In order to make the finish rolling temperature _{equal to} or higher than A _r3 + 30 ° C., the deviation of the finish rolling temperature could be easily reduced by using sheet bar joining and an edge burner.

However, No. 1 to No. 3, No. 5, No. 10, No. 1
As shown in No.3 to No.14, even if sheet bar bonding or edge heater is used, if the finish rolling temperature is less than _Ar3 + 30 ° C, the variation in the amount of coating increases to more than ± 5 g / m ² , Has occurred.

[0025]

[Table 1]

[0026]

[Table 2]

[0027]

According to the present invention, it is possible to provide an alloyed hot-dip galvanized steel sheet which has no appearance of line pattern defects and is excellent in appearance properties.

[Brief description of the drawings]

FIG. 1 shows the ratio of the intensity of diffracted X-rays on the generation of stripes, I (20
0) / I (222), a graph showing the effect of finish rolling temperature on hot rolling.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Kazuo Mochizuki, 2-3-2 Uchisaiwai-cho, Chiyoda-ku, Tokyo Kawasaki Steel Corporation (72) Inventor Yasunobu Uchida 1, Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba (72) Inventor Kazuhiro Seto 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Pref.

Claims (1)

[Claims] An alloyed hot-dip galvanized steel sheet in which the surface of a cold-rolled steel sheet is subjected to alloyed hot-dip galvanizing, wherein the cold-rolled steel sheet has a mass% of C: 0.004% or less, Si: 0.1% or less,
Mn: 0.5% or less, P: 0.025% or less, S: 0.025% or less, Al: 0.005 to 0.100% or even Ti: 0.01 to
0.04% and Nb: Contains at least one element selected from 0.001 to 0.010%, the balance being Fe and inevitable impurities, and diffraction from {200} and {222} planes of the steel sheet surface by X-ray diffraction. The ratio of the X-ray intensity, I (200) / I (222) is 0.
An alloyed hot-dip galvanized steel sheet having excellent appearance properties, wherein the deviation is less than 17 and the in-plane deviation of the adhesion amount of the alloyed hot-dip galvanized layer is within ± 5 g / m ² .