JP3198900B2 - Manufacturing method of thin galvanized steel sheet - Google Patents

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 a thin galvanized steel sheet.

[0002]

2. Description of the Related Art Conventionally, in the production of hot-dip galvanized steel sheets, the amount of plating film deposited is controlled by gas wiping immediately after the steel sheet is immersed in a zinc bath and plated.
20348) or a wiping roll (Japanese Patent Laid-Open No. 04-2775).

[0003]

In recent years, there has been a growing demand for hot-dip galvanized steel sheets to have improved weldability, improved workability, and reduced cost, so that the amount of adhesion per side is 40 g / side. m ² or less or be produced stably and with high productivity to 30 g / m ² or less of galvanized steel sheet has been desired. However, the above-described gas wiping method and wiping roll method have a problem that it is necessary to reduce the line speed or to lower the productivity in order to control the amount of coating of the plating film to be thin.

[0004] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a method capable of stably producing thin galvanized steel sheets with high productivity.

[0005]

According to a first aspect of the present invention, a steel sheet is cooled after annealing, and the cooled steel sheet is immersed in a zinc bath containing Al to form a zinc on the steel sheet surface. when attaching, by controlling the Al content of penetration temperature and zinc bath to the zinc bath of the steel sheet, the initial alloy layer formed on the surface of the steel sheet in a zinc bath with fine ζ phase, a zinc coating weight 23.4
g / m ² or less .

[0006] The second invention is a method of the present invention, wherein C: 0.001% by weight.
0.0035%, Si: 0.10% or less (0% by weight
) , Mn: 0.08 to 2.5%, P: 0.005 to
0.15%, S: 0.001 to 0.02%, Sol. Al:
0.005 to 0.1%, N: 0.0035% or less (0 times
An amount%), Ti: a steel containing 0.03 to 0.15 percent, after annealing in a continuous galvanizing line, the formula (3)
A step of cooling to an intrusion plate temperature T ° C.
A step of immersing the steel sheet cooled down to the range shown in equation (2) into a zinc bath containing Al to deposit zinc, and a step of controlling the amount of zinc deposited on the steel sheet surface. From the relationship between the infiltration plate temperature T ° C and the Al content in the bath, the equation
By setting in the range of (1), the phase of the initial alloy layer formed on the surface of the steel sheet in the zinc bath is controlled to be a fine ζ phase, and the zinc deposition amount is 2
A method for producing a thin-coated hot-dip galvanized steel sheet, wherein the thickness is 3.4 g / m ² or less .

295 + 930 × Al ≦ T ≦ 335 + 930 × Al (1) Al ≦ 0.20 (2) 350 ≦ T (3) T: penetration plate temperature (° C.), Al: Al content in bath (weight) %) In the third invention, the steel further contains 0.0035% or less (0 %).
(Including% by weight) of a hot-dip galvanized steel sheet.

The method for producing a hot-dip galvanized steel sheet according to the present invention includes a method in which the steel sheet is immersed in a zinc bath to control the amount of zinc applied, and then, if necessary, is subjected to alloying treatment or temper rolling.
The alloyed hot-dip galvanized steel sheet obtained by this method is also included.

The fine ζ phase is defined as a phase composed of か ら crystals having a major axis of less than 3 μm.測定 The method for measuring the major axis of the crystal will be described in the section of Examples. The invading plate temperature should refer to the plate temperature immediately before entering the bath, but it is difficult to measure the plate temperature due to the structure of the equipment. Therefore, in the present invention, the sheet temperature of the roll just before entering the snout (the position 12 m before the bath) was set as the intruding sheet temperature.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.
The present invention is based on new findings regarding a galvanized steel sheet as described below.

The first finding is that the form of the Fe-Al and Fe-Zn alloy layers formed at the interface between the steel sheet and the zinc bath when the steel sheet is immersed in the zinc bath, the so-called initial alloy layer, is based on the zinc bath. It depends on the Al concentration and the temperature of the steel sheet when entering the zinc bath, that is, the temperature of the entering sheet. FIG. 1 shows the results of investigation on the relationship between the Al concentration in the zinc bath, the infiltration plate temperature, and the initial alloy phase for steel having the components of the present invention (Ti-based IF steel). As shown in FIG. 1, the lower the Al concentration in the zinc bath and the higher the intruding plate temperature, the more δ
An outburst structure in which one phase is formed is formed. A fine の phase is formed in a region having a higher Al concentration and a lower penetration temperature than the region in which the outburst structure is generated. Higher A
In the region on the 1-concentration / low-invasion plate temperature side, a coarse ζ phase is formed, and in the region on the high-Al-concentration / low-invasion plate temperature side, a fine ζ phase and a FeAl alloy structure appear again.

[0012] The second finding is that the coating weight of the hot-dip galvanized steel sheet depends on the form (phase) of the initial alloy layer. When the initial alloy layer composed of the coarse ζ phase is generated and when the outburst structure is generated, the lift of the molten zinc by the steel sheet increases, and the plating adhesion after wiping increases. This is presumably because the initial alloy layer functions as an anchor for retaining the molten zinc. On the other hand, if an initial alloy layer composed of a fine ζ phase is generated, the amount of adhesion is reduced.

[0013] From the above findings, the present inventors have found that zinc bath A
By appropriately setting the l-concentration and the penetration plate temperature, it is possible to control the initial alloy layer and, consequently, control the amount of adhesion.
It has been found that if the concentration and the infiltration plate temperature are set, the initial alloy layer becomes a fine ζ phase, and the amount of adhesion can be stably and effectively reduced.

Then, Al in a zinc bath in the form of an initial alloy layer
Since the dependence on the concentration and the temperature of the intruding sheet differs depending on the steel type component of the base, in the present invention, the base steel sheet is limited to the above components.

The steel of the present invention is a TiIF steel and is characterized by having excellent deep drawability. The reasons for adding and limiting the components are described below. C: 0.001 to 0.0035%; C is a component contained in pig iron and adversely affects deep drawability, but in order to make the content less than the lower limit, advanced decarburization treatment is necessary. If it exceeds the upper limit, the production cost increases, and if it exceeds the upper limit, the deep drawability deteriorates. If the upper limit is exceeded, a large amount of Ti is required to fix C,
This also increases manufacturing costs.

Si: 0.10% or less; If Si exceeds the upper limit, the plating property of the film is degraded, so that Si is inappropriate. Mn: 0.08 to 2.50%; Mn is a component for ensuring hot workability, but if it is less than the lower limit, surface flaws due to hot embrittlement cannot be completely prevented; Deteriorate, and all are unsuitable.

P: 0.005 to 0.15%; P is a component for securing the strength of the steel sheet. However, if it is less than the lower limit, the desired effect cannot be obtained, and if it exceeds the upper limit, the deep drawability deteriorates. ,
Both are inappropriate.

S: 0.001 to 0.02%; S is contained in steel as an unavoidable impurity and causes hot brittleness.
By adding Ti, it is fixed as TiS, and such an adverse effect is suppressed. However, it is desirable that the S content be low in order to reduce the Ti addition amount. If the upper limit is exceeded, the processability deteriorates, and if it is less than the lower limit, a high desulfurization treatment is required, and the production cost is increased.

Sol. Al: 0.005 to 0.1%; Sol. A
l is a component for deoxidizing steel, but if it is less than the lower limit, the desired effect cannot be obtained, and if it exceeds the upper limit, the effect is saturated, and any of them is unsuitable.

N: 0.0035% or less; If N exceeds the upper limit, workability is deteriorated, so that N is inappropriate. Ti: 0.03 to 0.15%; Ti is N, S, C in steel
Is a component for fixing steel as TiN, TiS, and TiC, respectively, to prevent hot brittleness of steel and to improve deep drawability. If the amount is less than the lower limit, the desired effect cannot be obtained. If the amount exceeds the upper limit, the production cost is increased and the plating property is deteriorated.

B: 0.0035% or less B is a component for fixing N, S, and C in steel similarly to Ti, preventing hot brittleness of steel and improving deep drawability. Although it is added to supplement Ti, if it exceeds the upper limit, the effect is saturated, so that it is inappropriate.

Next, the reasons for limiting the manufacturing conditions will be described. The annealing temperature of the steel sheet is not more than AC3 point according to a conventional method. The penetration plate temperature T (° C.) is set in the range of Expression (3) (350 ≦ T).

Since the base steel sheet is annealed in the line, then cooled and immersed in a zinc bath, it is difficult to lower the temperature of the intruding steel sheet while maintaining high productivity. Further, when a low-temperature steel sheet is immersed in a zinc bath, the heat energy of the bath is deprived, and the production cost increases. Further, when the intrusion plate temperature is too low, the initial alloy layer is not sufficiently formed, and the quality such as film adhesion is deteriorated. The penetration plate temperature was limited to 350 ° C. or higher for the following reasons.

Further, the penetration plate temperature is set within the range of the expression (1). If it is higher than the range of the expression (1), the initial alloy layer becomes an outburst structure and the amount of adhesion increases. Also (1)
If the value is lower than the range of the expression, the initial alloy layer becomes a coarse ζ phase, and the amount of adhesion also increases. When the temperature of the intrusion plate is further reduced, a fine ζ phase appears again, but at the same time, a non-uniform Fe—Al alloy layer is also formed, which leads to deterioration in adhesion of the film and uneven alloying. Therefore, the penetration plate temperature was limited to the range of the expression (1).

In the zinc bath, the range of the formula (3) (Al ≦ 0.
In step 20), Al is added. Al concentration in zinc bath is 0.2
When the content exceeds 0% by weight, the alloying reaction of Fe-Zn hardly occurs, so that the alloying treatment becomes difficult and the production cost increases. Therefore, the Al concentration in the zinc bath is 0.2
It was limited to 0% by weight or less.

In the present invention, the temperature of the zinc bath is not particularly limited since the temperature of the intruding plate is limited, and the temperature of the zinc bath is not limited.
0 ° C to 480 ° C. After the immersion treatment, the amount of zinc adhering to the steel sheet surface is controlled. As a control method, a known method such as gas wiping or wiping roll immediately after immersing a steel sheet in a zinc bath and plating it can be applied.
The reason why the amount of adhesion is limited to 23.4 g / m ² or less per side is that the effect of the present invention is remarkably exhibited within the range .

As described above, according to the present invention, by controlling the initial alloy phase formed at the interface between the steel sheet and the plating film during wiping so as to be a fine ζ phase,
Wiping can be performed more effectively, and as a result, a thin-coated hot-dip galvanized steel sheet having an adhesion amount per side of 23.4 g / m ² or less can be stably manufactured with high productivity.

[0028]

Examples of the present invention will be described below. (1) Manufacturing conditions Table 1 shows the components of the base steel sheet used in the examples and comparative examples of the present application. After smelting the steel types listed in Table 1, hot rolling,
After being taken up, it was pickled, cold-rolled, annealed in a hot-dip galvanizing line, immersed in molten zinc, plated with zinc, and subjected to gas wiping. After that, a part is subjected to alloying treatment to make an alloyed hot-dip galvanized steel sheet,
The remainder was a hot-dip galvanized steel sheet without performing alloying treatment. Table 2 shows manufacturing conditions common to the examples and the comparative examples. Table 3 shows examples of the present invention. Tables 4 and 5 show comparative examples. Here, in the column of the phase of the initial alloy layer in Tables 3 to 5, fine ζ indicates a fine ζ phase (the major axis of the crystal is less than 3 μm), coarse ζ indicates a coarse ζ phase (the major axis of the crystal is 3 μm or more),
OB indicates an outburst tissue.

The analysis values of the steel sheet components described in Table 1 are obtained by ICP.
It is the analysis value by. The Al concentration in the bath described in Table 3, Table 4, and Table 5 is a value obtained by analyzing a test piece taken from the bath by ICP.

(2) Observation of the initial alloy layer Of the examples and comparative examples described in Tables 3, 4 and 5 where no alloying treatment was performed, the initial alloy layer was observed. The observation of the initial alloy layer was performed by SEM. Front and back at the position of 1/4, the center, and 3/4 from the end in the width direction of the prepared hot-dip galvanized steel sheet,
Test pieces were cut out from a total of six locations, the plating film was dissolved with hydrochloric acid to expose the initial alloy layer, and observed by SEM at a magnification of 1500 times directly above, and the phase occupying the largest area of this plated steel sheet was determined. It was an initial alloy phase. The longest diameter of the largest crystal in the observed range was defined as the crystal grain size of the initial alloy layer.

(3) Measurement of adhesion amount The adhesion amount was measured by dissolving the film with hydrochloric acid and measuring the weight difference before and after the dissolution. The values of the adhesion amounts described in Tables 3 and 4 and Table 5 are, respectively, the front, back, and center of the test specimen in the width direction at the quarter, center, and quarter positions, respectively.
These are the average values of a total of six locations.

(4) Comparative Example No. of Comparative Example 1,2,5,6,9,10,13,1
4,17,18,21,22,25,26,29,3
In Nos. 0, 33, and 34, since the penetration plate temperature is lower than the setting of the expression (1), the initial alloy layer becomes a coarse ζ phase and the amount of adhesion increases.

No. 43,44,47,48,51,5
2,55,56,59,60,63,64,67,6
8, 71, 72, 75, and 76 also cannot be observed in the initial alloy layer because they are alloyed after plating, but it is considered that the adhesion amount increases for the same reason.

No. 3,4,7,8,11,12,1
5,16,19,20,23,24,27,28,3
In Nos. 1, 32, 35 and 36, since the penetration plate temperature is higher than the setting of the expression (1), the initial alloy layer becomes an outburst structure and the amount of adhesion increases.

No. 45, 46, 49, 50, 53, 5
4,57,58,61,62,65,66,69,7
Also in Nos. 0, 73, 74, 77, and 78, the initial alloy layer cannot be observed because they are alloyed after plating, but it is considered that the amount of adhesion increases for the same reason. No. 37-42, 79-
In No. 84, the amount of adhesion increases because the components of the base steel sheet are outside the limited range of the present invention.

[0036]

[Table 1]

[0037]

[Table 2]

[0038]

[Table 3]

[0039]

[Table 4]

[0040]

[Table 5]

[0041]

As is evident from the above results, the penetration sheet temperature of the steel sheet in the hot-dip galvanizing line was determined by changing the Al temperature in the bath.
By controlling in accordance with the concentration, the initial alloy phase can be controlled, and thus the amount of adhesion can be controlled. In particular, when the Al concentration in the bath and the penetration plate temperature are set in a range satisfying the equations (1), (2), and (3), if the wiping conditions are the same, the Al concentration in the bath and the penetration plate temperature are changed. The amount of plating can be reduced as compared with the case where the above range is set. Therefore, according to the present invention, a thin galvanized steel sheet can be stably manufactured with high productivity.

[Brief description of the drawings]

FIG. 1 is a diagram showing the range of the Al concentration and the penetration plate temperature of a zinc bath of the present invention. The horizontal axis is the Al content (% by weight) in the bath, the vertical axis is the penetration plate temperature (° C.), and the shaded area is the range of the present invention.

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Keishi Yamashita 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (56) References JP-A-4-103749 (JP, A) JP-A-4 −103748 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C23C 2/00-2/40

Claims (3)

(57) [Claims] 1. A steel sheet is cooled after annealing, and the cooled steel sheet is
When zinc is immersed in a zinc bath containing l, the zinc is formed on the surface of the steel plate in the zinc bath by controlling the penetration temperature of the steel plate into the zinc bath and the Al content of the zinc bath. the initial alloy layer with fine ζ phase, 23.4 g of zinc deposition amount / ^{m 2}
A method for producing a thin-coated hot-dip galvanized steel sheet, characterized in that : 2. C: 0.001 to 0.003 by weight%
5%, Si: 0.10% or less (including 0% by weight) , M
n: 0.08 to 2.5%, P: 0.005 to 0.15
%, S: 0.001 to 0.02%, Sol. Al: 0.00
5 to 0.1%, N: 0.0035% or less (including 0% by weight)
E ) a steel sheet containing 0.03 to 0.15% Ti:
After annealing in a continuous hot-dip galvanizing line, a step of cooling to an intruding sheet temperature T ° C. shown in equation (3), A step of immersing the zinc in the zinc bath to deposit zinc and a step of controlling the amount of zinc deposited on the surface of the steel sheet. The phase of the initial alloy layer formed on the surface of the steel sheet in the zinc bath is controlled to a fine ζ phase by setting within the range of the formula (1), and the zinc deposition amount is 23.4 g.
/ M ² or less, a method for producing a thin-coated hot-dip galvanized steel sheet. 295 + 930 × Al ≦ T ≦ 335 + 930 × Al (1) Al ≦ 0.20 (2) 350 ≦ T (3) T: penetration plate temperature (° C.), Al: Al content in bath (% by weight) 3% or less (0% by weight ) in steel
Method for manufacturing a thin with hot-dip galvanized steel sheet according to claim 2, characterized in that the addition of B of including).