JPH09157819A - Production of thinly coated galvanized steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method capable of stably producing a thinly coated galvanized steel sheet with a high productivity. SOLUTION: A steel sheet contg., by weight, 0.01 to 0.2% C, >=0.10% Si, 0.08 to 2.5% Mn, 0.005 to 0.15% P, 0.001 to 0.02% S and 0.005 to 0.1% Sol.Al is annealed in a continuous galvanizing line and is thereafter cooled to an impregnating sheet temp. shown in the inequality (3). This steel sheet cooled to the impregnating sheet temp. T deg.C is immersed into a zinc bath contg. Al in the range shown in the inequality (2), and the coating weight of zinc is regulated. The impregnating sheet temp. T deg.C into the zinc bath is set in the range of the inequality (1) from the relation with the content of Al in the bath, and the initial alloy layer formed on the surface of the steel sheet in the zinc bath is regulated into fine ζ phases: 335+930×Al<=T<=375+930×Al ...(1), Al<=0.20...(2), and 350<=T...(3), where T: the impregnating sheet temp. ( deg.C) and Al: the content (wt.%) of Al in the bath.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for manufacturing a hot-dip galvanized steel sheet having a light weight.

[0002]

2. Description of the Related Art Conventionally, in the production of hot-dip galvanized steel sheets, the amount of coating film deposited is controlled by gas wiping immediately after the steel sheet is immersed in a zinc bath for plating (Japanese Patent Laid-Open No. 03-1 / 03-1).
No. 20348) or a wiping roll (Japanese Patent Application Laid-Open No. 04-2757).

[0003]

In recent years, there has been an increasing demand for hot-dip galvanized steel sheets to have improved weldability, improved workability, and lower costs, so the amount of adhesion per side is 40 g / 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 gas wiping method and the wiping roll method described above have problems that the line speed needs to be reduced and the productivity becomes low in order to control the deposition amount of the plating film to be thin.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a method capable of stably producing a hot-dip galvanized steel sheet having a unit weight and with high productivity.

[0005]

The first aspect of the present invention for solving the above-mentioned problems is C: 0.01-0.
2%, Si: 0.10% or less, Mn: 0.08 to 2.5
%, P: 0.005-0.15%, S: 0.001-
After annealing a steel sheet containing 0.02% and Sol.Al: 0.005-0.1% in a continuous hot dip galvanizing line,
Cooling process to the intrusion plate temperature shown in (3) and intrusion plate temperature T ° C
The method comprises the steps of immersing the steel sheet cooled down to the range shown in equation (2) in a zinc bath containing Al to deposit zinc on the surface, and controlling the amount of zinc deposited on the surface of the steel sheet. ， Set the intrusion plate temperature T ℃ in the zinc bath within the range of formula (1) from the relationship with the Al content in the bath, and make the initial alloy layer formed on the steel plate surface in the zinc bath into a fine ζ phase. It is a method of manufacturing a hot-dip galvanized steel sheet having a controlled weight.

335 + 930 × Al ≦ T ≦ 375 + 930 × Al (1) Al ≦ 0.20 (2) 350 ≦ T (3) T: Penetration plate temperature (° C.), Al: Al content in the bath (% by weight) The second and third aspects of the present invention have a zinc adhesion amount of 40 g per side.
/ M ² or less or to 30 g / m ² or less is a manufacturing method of thin with galvanized steel sheet characterized by.

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 zinc deposition amount, and then alloying treatment or temper rolling is performed if necessary.
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 ζ crystals having a major axis of less than 3 μm. A method for measuring the major axis of the ζ crystal will be described in the section of Examples. The penetration 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 plate temperature at the roll 1 immediately before entering the snout (the position 12 m before the bath) is defined as the intrusion plate temperature.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.
The first finding is that Fe-Al and Fe-Zn formed at the interface between the steel sheet and the zinc bath when the steel sheet is penetrated into the zinc bath.
The form of the alloy layer, the so-called initial alloy layer, depends on the Al concentration of the zinc bath and the temperature of the steel plate when entering the zinc bath, that is, the penetration plate temperature. FIG. 1 shows the results of an investigation of the relationship between the Al concentration in the zinc bath, the penetration plate temperature, and the initial alloy phase for the steel (Ak-killed steel) whose components were limited in claim 1. As shown in FIG. 1, the lower the Al concentration in the zinc bath and the higher the penetration plate temperature, the more the outburst structure in which the δ1 phase was formed starting from the crystal grain boundary of the base steel sheet.
A fine ζ phase is formed in a region on the high Al concentration / low intrusion plate temperature side of the outburst structure generation region. Coarse ζ phase is formed in the region of higher Al concentration / low intrusion plate temperature side, and fine ζ phase and FeAl alloy structure appear again in the region of higher Al concentration / low intrusion plate temperature side.

The second finding is that the coating weight of the hot-dip galvanized steel sheet depends on the morphology of the initial alloy layer. When the initial alloy layer composed of the coarsened ζ phase is formed and when the outburst structure is formed, the lifted amount of the molten zinc by the steel plate is increased, and the plating adhesion amount after wiping is increased. It is presumed that this is because the initial alloy layer functions as an anchor that holds molten zinc.
On the other hand, if an initial alloy layer composed of a fine ζ phase is generated,
The adhered amount is small.

From the above findings, the present inventors have found that the zinc bath A
l By setting the concentration and intrusion plate temperature to arbitrary values,
It is possible to control the initial alloy layer and thus the deposition amount. In particular, if the Al concentration and the invasion plate temperature are set within the ranges that satisfy the equations (1), (2), and (3), the initial alloy layer becomes a fine ζ phase, and the adhesion amount is reduced stably and effectively. I found that I could do it.

Since the dependence of the form of the initial alloy layer on the Al concentration of the zinc bath and the penetration plate temperature differs depending on the steel composition of the base, the base steel sheet is limited to the above components in the present invention. The steel of the present invention is Al killed steel. The reasons for adding each component and the reasons for limitation will be described below.

C: 0.01 to 0.2%; C is a basic component for strengthening steel, but if it is less than the lower limit, the required strength cannot be secured, and if it exceeds the upper limit, deep drawability and welding are achieved. All of them are unsuitable because they deteriorate the sex.

Si: 0.10% or less; Si is unsuitable if it exceeds the upper limit because the plating property of the film is deteriorated. Mn: 0.08 to 2.5%; Mn is a component for ensuring hot workability, but if it is less than the lower limit, surface scratches due to hot brittleness cannot be completely prevented, and if it exceeds the upper limit, deep drawability is exhibited. Are deteriorated, and both are inappropriate.

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

S: 0.001 to 0.02%; S is contained in steel as an unavoidable impurity and causes hot embrittlement.
Although TiS is fixed as TiS by adding Ti and the adverse effect is suppressed, it is preferable that the S content is low in the reservoir for reducing the Ti addition amount. If it exceeds the upper limit, the workability is deteriorated, and if it is less than the lower limit, a high degree of desulfurization treatment is required, and the manufacturing cost increases, so that both are unsuitable.

Sol.Al: 0.005-0.1%; Sol.A
Although 1 is a component for deoxidizing the steel, if the amount is less than the lower limit, the desired effect cannot be obtained, and if the amount exceeds the upper limit, the effect is saturated, so that both are unsuitable.

Next, the reasons for limiting the manufacturing conditions will be described. The annealing temperature of the steel sheet is AC3 point or less according to the conventional method. When the penetration plate temperature is higher than the range of the formula (1), the initial alloy layer has an outburst structure and the amount of adhesion increases. On the other hand, if it is lower than the range of the expression (1), the initial alloy layer becomes a coarse ζ phase, and the amount of adhesion also increases. If the intrusion plate temperature becomes lower, a fine ζ phase appears again, but at the same time, a non-uniform F
Since the e-Al alloy layer is also formed, the adhesion of the film deteriorates and alloying becomes uneven. Therefore, the penetration plate temperature is (1)
Limited to the scope of the formula.

Since the base steel sheet is annealed in the line, cooled and immersed in a zinc bath, it is difficult to lower the penetration plate temperature while maintaining high productivity. Further, when a low temperature steel plate is dipped in a zinc bath, the heat energy of the bath is taken away and the manufacturing cost increases. Further, if the temperature of the penetration plate is too low, the initial alloy layer is not sufficiently formed, and the quality such as film adhesion deteriorates. For the following reason, the intrusion plate temperature is limited to 350 ° C. or higher shown in the equation (3).

If the Al concentration in the zinc bath exceeds 0.20% by weight, the Fe—Zn alloying reaction hardly occurs, the alloying process becomes difficult, and the manufacturing cost increases.
Therefore, the Al concentration in the zinc bath is limited to 0.20% by weight or less.

In the present invention, since the temperature of the penetration plate is limited, the temperature of the zinc bath is not particularly limited.
The temperature is 0 ° C to 480 ° C. After the immersion treatment, the amount of zinc deposited on the steel sheet surface is controlled. As the control method, a known method such as gas wiping or a wiping roll immediately after plating a steel sheet by immersing it in a zinc bath can be applied.
The reason why the adhesion amount is limited to 40 g / m ² or less per one surface is that the effect of the present invention is exhibited in that range, and it becomes more remarkable at 30 g / m ² or less.

In this way, by controlling the initial alloy phase formed at the interface between the steel plate and the plating film to be a fine ζ phase during wiping, wiping can be performed more effectively, and the result As a result, it becomes possible to stably produce a hot-dip galvanized steel sheet having an adhesion amount per one surface of 40 g / m ² or less or 30 g / m ² or less and with high productivity.

[0023]

Examples of the present invention will be described below. (1) Manufacturing conditions Table 1 shows the components of the base steel sheet used in Examples and Comparative Examples of the present application. After melting the steel types listed in Table 1, hot rolling,
After winding, pickling, cold rolling, annealing in a hot dip galvanizing line, dipping in hot dip zinc to plate zinc, and gas wiping were performed. After that, a part is alloyed to form a galvannealed steel sheet,
The rest was a galvanized steel sheet without alloying treatment. Table 2 shows manufacturing conditions common to Examples and Comparative Examples. Examples of the present invention are shown in Table 3. Comparative examples are shown in Tables 4 and 5. Here, the fine ζ described in the column of the phase of the initial alloy layer in Tables 3 to 5 is the fine ζ phase (the major axis of the crystal is less than 3 μm), the coarse ζ is the coarse ζ phase (the major axis of the crystal is 3 μm or more),
OB indicates an outburst structure.

The analytical values of the steel sheet components listed in Table 1 are ICP.
It is the analysis value by. The Al concentrations in the baths shown in Tables 3 and 4 and 5 are values obtained by ICP analysis of test pieces taken from the baths.

(2) Observation of Initial Alloy Layer Among the examples and comparative examples shown in Tables 3, 4 and 5, the initial alloy layer was observed for those not subjected to the alloying treatment. The observation of the initial alloy layer was performed by SEM. The prepared hot-dip galvanized steel sheet has a front surface and a back surface at a position of a quarter, a center, and a three-fourths from the end in the width direction,
Test pieces were cut out from a total of 6 places, the plating film was dissolved with hydrochloric acid to expose the initial alloy layer, and then observed with an SEM at a magnification of 1500 times from directly above, and the phase occupying the largest area of this plated steel sheet was observed. The initial alloy phase was used. Further, 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 shown in Tables 3 and 4 and 5 are the front and back of the test piece in the width direction at a position of a quarter, a center, and a third quarter, respectively.
It is the average value of 6 places in total.

(4) Comparative Example No. of the 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 intrusion plate temperature is lower than that set by the equation (1), the initial alloy layer becomes a coarse ζ phase, and the amount of adhesion increases.

No. 46, 47, 50, 51, 54, 5
5,58,59,62,63,66,67,70,7
No. 1,74,75,78,79 cannot be observed because the initial alloy layer cannot be observed 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 that set by the equation (1), the initial alloy layer has an outburst structure and the amount of adhesion increases.

No. 48, 49, 52, 53, 56, 5
7, 60, 61, 64, 65, 68, 69, 72, 7
No. 3,76,77,80,81 cannot be observed because the initial alloy layer cannot be observed because they are alloyed after plating, but it is considered that the adhesion amount increases for the same reason. No. 37-45, 82-
In No. 90, the composition of the base steel sheet is out of the limited range of the present invention, so that the adhesion amount increases.

[0031]

[Table 1]

[0032]

[Table 2]

[0033]

[Table 3]

[0034]

[Table 4]

[0035]

[Table 5]

[0036]

As is apparent from the above description, according to the present invention, the initial alloy phase can be controlled by controlling the intrusion plate temperature of the steel sheet in the hot dip galvanizing line according to the Al concentration in the bath. As a result, the amount of adhesion can be controlled. In particular, the Al concentration in the bath and the intrusion plate temperature were
When the range is set to satisfy the formula (3), if the wiping conditions are the same, the deposition amount of the plating can be reduced as compared with the case where the bath Al concentration and the penetration plate temperature are set to other ranges. it can. Therefore, according to the present invention, a hot-dip galvanized steel sheet having a light weight can be stably manufactured with high productivity.

[Brief description of the drawings]

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

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Keiji Yamashita 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Steel Pipe Co., Ltd.

Claims (3)

[Claims] 1. C: 0.01 to 0.2% by weight, S
i: 0.10% or less, Mn: 0.08 to 2.5%, P:
0.005-0.15%, S: 0.001-0.02
%, Sol.Al: 0.005-0.1%, after annealing in a continuous hot-dip galvanizing line, cooling to a penetration plate temperature shown in formula (3), and a penetration plate temperature T ° C. The steel sheet cooled to the range shown in equation (2) is immersed in a zinc bath containing Al to deposit zinc on the surface, and the step of controlling the amount of zinc deposited on the steel sheet surface is provided. Equation for the temperature of penetration plate T ° C into the zinc bath from the relationship with the Al content in the bath
A method for producing a hot-dip galvanized steel sheet having a fine weight by controlling the initial alloy layer formed on the surface of the steel sheet in a zinc bath to a fine ζ phase by setting the range of (1). 335 + 930 × Al ≦ T ≦ 375 + 930 × Al (1) Al ≦ 0.20 (2) 350 ≦ T (3) T: Penetration plate temperature (° C.), Al: Al content in the bath (% by weight) 2. The method for producing a hot-dip galvanized steel sheet according to claim 1, wherein the amount of zinc deposited is 40 g / m ² or less per side. 3. The method for producing a hot-dip galvanized steel sheet according to claim 2, wherein the amount of zinc deposited is 30 g / m ² or less per side.