JPH08144036A - Production of galvanized steel sheet by using hot rolled steel sheet as base metal - Google Patents

Abstract

PURPOSE: To produce a galvanized steel sheet free from blisters with good productivity by subjecting a non-rolled steel sheet base metal to weak oxidizing, reducing and annealing so as to satisfy a relation consisting of a skin pass elongation, nonoxidizing preheating furnace temp. and a reducing furnace temp., then immediately subjecting the base metal to galvanizing. CONSTITUTION: The hot rolled steel sheet base metal is subjected to skin pass and pickling, then to weak oxidizing the surface thereof in a nonoxidizing preheating furnace and, thereafter, its surfaces are reduced in a gaseous hydrogen atmosphere and the base metal is subjected to annealing and galvanizing, by which the galvanized steel sheet is obtd. At this time, the steel sheet is subjected to weak oxidizing, reducing and annealing under the conditions satisfying the skin pass elongation; w(%), the nonoxidizing preheating furnace temp.; x( deg.C) and the reducing furnace temp.; y( deg.C), and y( deg.C)>=0.5x( deg.C) -100w(%)+500x( deg.C)>=450( deg.C), w>=0(%) [the skin pass is not executed at the time of w=0(%)]. Thereafter, the base metal is immediately subjected to the galvanizing. Consequently, the galvanized steel sheet which has which coating weight and is excellent in blister resistance is obtd.

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 hot-dip galvanized steel sheet, which is intended to completely prevent blisters generated when hot-dip galvanizing a hot-rolled steel sheet. is there.

[0002]

2. Description of the Related Art Conventionally, in a galvanized steel sheet produced by a continuous hot dip galvanizing line, reduction and annealing of the steel sheet surface are simultaneously performed in a hydrogen-nitrogen mixed gas as a pretreatment for a galvanizing step.

In recent years, a method of hot-dip galvanizing directly on a hot-rolled sheet has been widespread for the purpose of streamlining the manufacturing process because it has the advantage of omitting the cold-rolling step, but the hot-rolled sheet was passed through a hot-dip galvanizing line. In this case, hydrogen blown into the furnace for reduction is occluded in the steel and diffuses to the surface layer after plating,
It is well known that a cavity called a blister is formed between the plated layer and the iron base material.

As a method for preventing such blister, a number of methods have been proposed so far, as summarized below. Japanese Unexamined Patent Publication No. 49-14325 discloses a method of adding S, Se, P, As, Te, Sb, Bi, etc. to a plating bath. It is not preferable because there is a concern that it will affect the product, and it is difficult to control the concentration of such a trace amount additive component in the bath.

Japanese Patent Application Laid-Open No. 55-119159 discloses effective Al in a bath.
Although a method of operating the concentration between 0.20% and 0.25% is disclosed, this method is not preferable because there is a drawback that non-plating or the like is likely to occur as the Al concentration increases.

Further, Japanese Patent Publication No. 55-17101 discloses a method of mechanically descaling the surface of a hot-rolled steel material, oxidizing the surface and then reducing it, and plating the surface. The scale is not preferable because it requires labor and may cause defects such as flaws on the surface.

Japanese Patent Publication No. 58-53715 discloses a method of heating after plating to grow an alloy layer having a thickness of 3 to 20 μm. However, this method reduces the adhesion of the plating, Not preferred for products that are post-manufactured.

Furthermore, Al concentration in the plating bath in Japanese Patent Laid-Open No. 1-316444 (CAl (%)), bath temperature (T _B (℃)), pre-coated steel sheet temperature (T _S (℃)) and f ₁ (C _Al ) + f ₂ (T _B ) + f ₃ (T _S ) ≦ 260 However, f ₁ (C _Al ) = − 1278.2 × C _Al +304.2 f ₂ (T _B ) = 0.94 × T _B −356.2 f _Although it is disclosed that the operation of blisters can be suppressed by operating under the condition of ₃ (T _S ) = 0.41 × T _S −118.9, the inventors of the present invention have found that the non-oxidizing furnace temperature and the soaking temperature are Depending on the conditions, blisters were generated, which was not sufficient.

Further, JP-A-56-163250 discloses a method of adding B and N into a steel sheet, and JP-A-59-166692 discloses a method of adding B or Ti into a steel sheet. Japanese Unexamined Patent Publication No. 5-295483 discloses a method of adding Ti to a steel sheet, but in both cases, the addition of elements to the base material is costly in small-lot production, and lots must be aggregated. Since this is not the case, productivity is also reduced, which is not preferable.

[0010]

As described above, many proposals have been made in the past for the method of preventing the blister generated when the hot-rolled sheet is galvanized, but each of them has disadvantages and drawbacks. When a hot-rolled steel sheet was used as a plating base sheet, a galvanized steel sheet without blisters could not be produced without lowering productivity. Thus, an object of the present invention is to provide a method for producing a blister-free hot-dip galvanized steel sheet with high productivity by using a hot-rolled steel sheet as a plating base sheet.

[0011]

In order to solve the above problems, the inventors of the present invention have earnestly conducted experiments and researches, and as a result, skin blistering, preheating, and reduction under certain conditions are performed to completely eliminate blister. The present invention has been completed, knowing that a hot-dip galvanized steel sheet capable of suppressing the occurrence of the phenomenon can be manufactured.

Here, the gist of the present invention is that a hot-rolled steel sheet raw material is skin-passed, pickled, the surface is weakly oxidized in an oxidation-free preheating furnace, and then the surface is reduced and annealed in a hydrogen gas atmosphere. , A method of further hot-dip galvanizing, skin pass elongation: w (%), non-oxidizing preheating furnace temperature: x
(° C.) and reduction furnace temperature: y (° C.) y (° C.) ≧ 1/2 x (° C.)-100w (%) + 500 x ≧ 450 (° C.), w ≧ 0 (%) (where w = 0 (%), No skin pass is applied.) Manufacture of hot-rolled galvanized steel sheet characterized by performing hot-dip galvanizing immediately after performing skin pass, weak oxidation and reduction, and annealing at a temperature satisfying Is the way.

[0013]

Next, the operation of the present invention will be described in detail.
First, according to the present invention, as its premise, the hot-rolled steel sheet raw plate is not subjected to skin pass (elongation w = 0 (%)), is pickled, and the surface is weakly oxidized in a non-oxidizing preheating furnace, and then hydrogen gas is used. The surface is reduced in an atmosphere, annealed, and then hot-dip galvanizing is further performed, but the hot-dip galvanizing method of the hot-rolled steel sheet performed through such a series of treatment operations already has new contents, In the method according to the present invention, the oxidation-free preheating furnace temperature in the above series of treatment operations is:
x (° C), reduction furnace temperature: y (° C) y (° C) ≥ (1/2) x (° C) + 500 (° C), but weak oxidation and reduction at a temperature satisfying x ≥ 450 (° C) , Perform annealing.

The reason for this is that the thickness of the oxide film formed in the non-oxidizing furnace increases as the temperature of the non-oxidizing preheating furnace increases,
Since the steel sheet generally contains Al, this oxide film contains a trace amount of alumina in addition to iron oxide.
Since alumina is difficult to reduce, when the temperature of the non-oxidizing furnace is increased, the surface alumina cannot be completely removed unless the temperature of the reducing furnace is also increased. When hot dip coating is performed with alumina remaining on the surface, plating adhesion is reduced and blister occurs.

As a result of various investigations by the authors in order to solve this, when the non-oxidizing preheating furnace temperature is x (° C.) and the reducing furnace temperature is y (° C.), y (° C.) ≧ 1 / 2x ( It has been found that the occurrence of blisters can be suppressed under the condition of satisfying (° C) + 500 (° C) x ≥ 450 (° C).

However, if the temperature of the non-oxidizing furnace is 450 ° C. or less, impurities (C, etc.) on the surface cannot be completely oxidized and disappeared, resulting in defective plating. Therefore, the condition of non-oxidizing furnace temperature is x ≧ 45
0 (° C). The higher non-oxidizing furnace temperature (x) is not specified, but if the temperature is 750 ° C or higher, the thick alumina layer cannot be completely reduced, resulting in excessive scale.

On the other hand, the reduction furnace temperature is not particularly specified, but the lower limit is restricted by y ≧ (1/2) x + 500 (° C.). The upper limit is not particularly limited, but about 850 ° C. is a preferable upper limit for stable production in terms of cost and quality.

The suppression of blister is complete within the above operating range, but by applying a skin pass to the hot-rolled steel sheet raw sheet before pickling, it is possible to expand the stable production area in which blister does not occur. More preferable.

It is conventionally well known that blisters do not occur when hot-dip galvanizing a cold-rolled material. The reason is that when cold-rolled, voids are formed in the steel. It is considered that the hydrogen occluded in the hot dip galvanizing line is trapped therein and the occurrence of blisters is suppressed, but the conventional knowledge is that in order to completely suppress blisters, the elongation A reduction of 5% or more had to be applied, and the steel sheet had to be manufactured as a cold rolled steel sheet. However, according to the knowledge of the present inventors, the cold rolling step can be omitted by combining it with the conditions of the non-oxidizing preheating furnace temperature and the reducing furnace temperature, and the cold rolling manufacturing step can be performed by a skin pass after the hot rolling. It was found that by substituting in, the stable manufacturing area without blister can be expanded.

That is, from the above viewpoint, the present invention can be said to be a method of preventing the formation of blisters during hot dip galvanizing of hot-rolled steel sheets, and according to this method, a stable manufacturing area where blisters do not occur can be obtained. As for the lower limit, when a hot-rolled steel plate is skin-passed, the elongation of the skin-pass is: w (%), non-oxidizing preheating furnace temperature: x (° C), reduction furnace temperature: y (° C) y (° C) ≧ 1/2 x (° C) -100w (%) +500 x ≧ 450 (° C) w ≧ 0 (%) (However, when w = 0 (%), skin pass is not applied.) can do.

The upper limit of the rolling reduction is not particularly specified, but the upper limit is about 3% in terms of the reduction due to the skin pass. As a result, the manufacturing process under cold pressure can be omitted or a skin pass can be substituted, and significant manufacturing cost can be rationalized.

Although the steel composition of the hot-rolled steel sheet to which the present invention is applied is not limited at all, typical examples are as follows. C: 0.003 to 0.18%, Si: 1.2% or less, Mn: 2.0% or less, P: 0.12% or less, S: 0.02% or less, sol.Al: 0.1%
Hereinafter, N: 0.010% or less The reason for limiting the steel composition in this way is as follows.

C: C has the function of ensuring the strength of the steel sheet, but if the content is less than 0.003% the desired effect due to the above effects cannot be obtained, while if it exceeds 0.18% it is contained. C content is 0.003 because it causes deterioration of weldability.
0.18% is preferable.

Si: Si is a component that improves the strength and ductility of the steel sheet through solid solution strengthening. However, if the content exceeds 1.2%, the weldability deteriorates, so the Si content is
1.2% or less is preferable.

Mn: Mn is a preferable element that improves the strength and ductility of the steel sheet through solid solution strengthening. However, if the content exceeds 2.0%, the weldability is deteriorated and the workability is deteriorated. Therefore, the Mn content is preferably 2.0% or less.

P: If P is contained in excess of 0.12%, the toughness and hole expandability of the base material deteriorate. Therefore, the P content is 0.
12% or less is preferable.

S: S is an impurity element that forms MnS-based inclusions and deteriorates workability. Therefore, in order to avoid the adverse effects, the S content is preferably 0.02% or less.

Sol.Al: Al is added as a deoxidizing agent for steel, but if the content of sol.Al exceeds 0.10%, the amount of alumina inclusions increases and the workability deteriorates. Therefore, the added amount of Al is preferably 0.10% or less in terms of sol.Al content.

N: N is an element which is essentially harmful to the steel of the present invention, and is preferably 0.010% or less. Next, the function and effect of the present invention will be described more specifically by way of examples.

[0030]

EXAMPLES In this example, Table 1 descaled by hydrochloric acid pickling
After hot-rolling the hot-rolled steel sheet having the composition of No.1 (without applying some samples), pickling it, and then passing it through a non-oxidizing preheating furnace and a reducing furnace at a predetermined temperature in a Zenzimer hot-dip galvanizing line, and immediately thereafter. Hot dip galvanizing was performed in a plating bath of the component system shown in Table 2 without exposing to the atmosphere.

Next, the hot-dip galvanized steel sheet thus obtained was subjected to gas wiping in the atmosphere, the amount of zinc deposited was controlled to 100 to 200 g / m ² on one side, and water-cooled and dried to obtain a product.

The blister was evaluated by visually observing it after leaving it for about 1 week after plating. The result of omitting the skin pass after hot rolling is shown in FIG. 1, the result of reducing the skin pass having the elongation of 0.5% is shown in FIG. 2, and the result of reducing the skin pass having the elongation of 1% is shown in FIG.

As is apparent from the results of FIGS. 1 to 3, in the hot-dip galvanized steel sheet produced by the method according to the present invention, no blister is found in all the plated layers in each example, but the present invention In all areas except the examples, more or less blister formation was observed.

[0034]

[Table 1]

[0035]

[Table 2]

[0036]

As described above, according to the present invention,
The galvanized steel sheet with excellent blister resistance and thick weight can be manufactured at the continuous hot-dip galvanizing line at low cost without being restricted by the pretreatment and posttreatment conditions. A hot-dip galvanized steel sheet is obtained.

[Brief description of drawings]

FIG. 1 is a graph showing the results of Examples.

FIG. 2 is a graph showing the results of Examples.

FIG. 3 is a graph showing the results of Examples.

Claims (1)

[Claims] 1. A hot-rolled steel sheet is skin-passed, pickled, weakly oxidized in a non-oxidizing preheating furnace, then reduced and annealed in a hydrogen gas atmosphere, and then hot-dip galvanized. A method for manufacturing a galvanized steel sheet, wherein the skin pass elongation is w (%), and the oxidation-free preheating furnace temperature is x.
(° C.), reduction furnace temperature: y (° C.) y (° C.) ≧ 0.5 x (° C.)-100w (%) + 500 x ≧ 450 (° C.), w ≧ 0 (%) (where w = 0 ( %), No skin pass is applied.) A method for producing a hot-dip galvanized steel sheet, which comprises performing weak oxidation, reduction, and annealing under conditions satisfying the condition (3), and immediately performing hot-dip galvanizing.