JPH06122952A - Production of galvanized steel sheet excellent in powdering resistance - Google Patents

Abstract

PURPOSE:To produce a galvanized steel sheet excellent in powdering resistance by applying plating to the steel sheet to be plated in a galvanizing bath contg. specified amounts of Al, B, Mn, Si, Na or the like and thereafter executing alloying treatment. CONSTITUTION:Plating is applied to the steel sheet to be plated in a galvanizing bath contg. 0.05 to 0.2% Al and 0.0002 to 0.004% B in a coexistent manner and furthermore contg. one or more kinds among 0.01 to 1.0% Mn, 0.001 to 0.1% Si and 0.0001 to 0.07% Na, and the balance Zn with inevitable impurities. Next, the coating weight of the plating is regulated, and after that, the plated sheet is heated to about 500 to 530 deg.C and is subjected to alloying treatment. In this way, the galvanized steel sheet excellent in powdering resistance even to formation into thick plating can be 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 galvannealed steel sheet having excellent powdering resistance.

[0002]

2. Description of the Related Art Alloyed hot-dip galvanized steel sheets are widely used in home appliances and automobile materials because of their excellent paintability and weldability. Such alloyed hot-dip galvanized steel sheets are disclosed, for example, in JP-B-61-113009 and JP-B-57-
As disclosed in many patent publications such as Japanese Patent No. 15665, generally, a plating base plate (hot rolled steel plate or cold rolled steel plate) is surface-soiled in an oxidizing furnace or a non-oxidizing furnace, and rolling oil and the like are oxidatively burned and removed. The surface oxide film is reduced by heating in a reducing atmosphere and burned, then cooled to a temperature suitable for plating, immersed in a plating bath for plating, and pulled up to adjust the amount of plating adhesion. Then, it is introduced into an alloying furnace, and the plated layer and the base iron are interdiffused by heating to alloy the plated layer and then cooled and wound.

Thus, the plated layer of the galvannealed steel sheet is an Fe-Zn intermetallic compound, and has a hard and brittle property, so that when subjected to severe processing such as deep drawing, the plated layer becomes powdery. That is, the so-called powdering easily peels off the plating. This powdering becomes remarkable when the Fe concentration in the plating layer is high, and alloying is insufficient, so Fe
If the concentration is too low, plating peeling is likely to occur. For this reason, the degree of alloying is generally controlled so that the Fe concentration in the plated layer is within a certain range during the production of the galvannealed steel sheet. In addition, the powdering phenomenon of the plating layer during processing is influenced by the composition of the plating bath, the components of the plating original plate, the thickness of the plating layer, and the like, and these are adjusted to appropriate conditions. For example, as in JP-A-63-157848, B is added to the plating bath to improve the powdering resistance.

[0004]

Various factors influence the powdering resistance of the galvannealed steel sheet, as described above. Therefore, the plating conditions and the degree of alloying are properly adjusted. When subjected to extremely severe processing, powdering is likely to occur. In particular, in recent years, as an anticorrosive steel sheet for automobiles, the aim is to thicken the plate from the viewpoint of prolonging its life, and if the plating thickness becomes thick, the powdering resistance deteriorates. The present invention provides a method for producing a galvannealed steel sheet having excellent powdering resistance.

[0005]

DISCLOSURE OF THE INVENTION The present inventors have conducted various searches for the purpose of developing an alloyed hot dip galvanized steel sheet excellent in powdering resistance, and as a result, in the galvanizing bath, B,
It has been found that the powdering resistance is remarkably improved by the combined addition of one or more of Mn, Si and Na. That is, the gist of the present invention is that in the hot dip galvanizing by the Zenzimer method or the non-oxidizing furnace method, the steel sheet to be plated has A1: 0.05-0.2% and B:
0.0002 to 0.0040%, and Mn0.0
1-1.0%, Si 0.001-0.1%, Na0.0
001 to 0.07% of 1 type or 2 types or more, and the balance Zn
And a galvanizing bath containing other unavoidable impurities to adjust the required amount of plating, and then alloying the galvanized steel sheet and cooling it.

Generally, an alloyed hot-dip galvanized steel sheet is plated by a Zenzimer method or a non-oxidizing furnace method, and Al is kept in the plating bath at about 0.05 to 0.2%. When an alloy plated with such an Al-containing bath is alloyed by heating, the main phase forming the plating layer is the δ ₁ phase (FeZn ₇ ), and if the heating temperature during the alloying treatment is low, the ζ phase (FeZn ₁₃ ) Exists in the surface layer of the plating, and if the degree of alloying progresses, the Fe concentration in the δ ₁ phase increases, and at the same time, a Γ layer (Fe ₃ Zn ₁₀ ) is formed at the base iron interface. The Fe content in the plating layer can be used as an index for the alloying degree, and is usually controlled to 7 to 11% from the viewpoint of powdering resistance.

If the amount of Al in the plating bath is less than 0.05%, the growth of the alloy layer in the plating bath becomes remarkable and it becomes difficult to control the coating amount of the plating. Especially, it is necessary to adjust the coating amount to a low coating amount. Becomes difficult and the powdering resistance is also unfavorable. Further, when the amount of Al in the plating bath increases, the amount of Fe—Al—Zn ternary reaction suppressing layer that controls alloying increases, which requires higher temperature and longer treatment for alloying, which impedes productivity. . Actually, when the amount of Al in the plating bath exceeds 0.2%, a partially incompletely alloyed portion is likely to occur, which makes alloying treatment in a continuous line difficult. However, the larger the amount of Al in the plating bath, the better the powdering resistance in general. Therefore, the amount of Al in the plating bath should be kept within a narrow range that secures the powdering resistance and does not hinder the productivity so much. There is. Therefore, there is a great possibility that the powdering resistance may be deteriorated by a slight change in the conditions.

On the other hand, B, Mn, and S are contained in the plating bath.
It has been found that the powdering resistance is remarkably improved by the combined addition of one or two or more of i and Na. When the content of B added in the plating bath is as small as less than 0.0002%, There is little effect of improving the powdering property, and at least in the plating bath B for improving the powdering resistance.
The amount is required to be 0.0002% or more, and even if added in excess of 0.004%, the effect of improving the powdering resistance is saturated, so 0.004% is sufficient. Therefore, in the present invention, the addition of B in the plating bath is properly in the range of 0.0002 to 0.004%.

In order to further improve the powdering resistance, M
Selectively add n, Si and Na. Addition of Mn is an effective component that significantly improves powdering resistance. If the Mn content in the plating bath is less than 0.01%, the powdering resistance improving effect is small, and at least 0.01% or more is necessary for the improvement, and if it exceeds 1.0%, the effect is saturated. This causes a significant amount of dust and 1.0%, which is disadvantageous in terms of plating operation. Therefore, in the present invention, the Mn content in the plating bath is set in the range of 0.01 to 1.0%. The addition of Si to the plating bath is also an effective component for improving the powdering resistance, and if the Si content in the plating bath is less than 0.001%, the effect of improving the powdering resistance is small and at least 0.0
If it is required to be 01% or more, and if it is added in excess of 0.1%, the temperature of the plating bath rises, the operability is lowered, and the plating layer becomes hard and the powdering resistance is lowered. Therefore, in the present invention, the Si content in the plating bath is set in the range of 0.001 to 0.1%.

Addition of Na to the plating bath is also an effective component for improving the powdering resistance, and if the content of Na in the bath is less than 0.0001%, the powdering resistance improving effect is small, and for improving the powdering resistance. Na needs to be 0.0001% or more. Moreover, even if added in excess of 0.07%, the effect is saturated, and significant oxides and dross are generated on the plating bath surface, resulting in deterioration of surface quality due to adhesion to the plating surface, and plating operability due to dross treatment. There is a decrease and it is a problem. Therefore, the proper content of Na is 0.0001 to 0.07%.

In the present invention, the mechanism by which B, Mn, Si, and Na are added within the above range to improve the powdering resistance is not clear, but it is dense in the initial reaction in the plating bath. It is considered that this is effective in forming a uniform initial alloy layer, and further, the Fe—Zn mixed phase is formed by the alloying treatment, and the weak bonding force between the phases is relaxed. In addition, it is considered that the effect of relieving the stress caused by working is enhanced and the powdering resistance is improved.

In the present invention, B is added to the galvanizing bath by a method of forming an Al-B alloy and adding it because zinc and B do not react at all. Therefore, in the present invention, coexistence of Al is an essential condition for dissolving B in the plating bath, and Pb in the plating bath is preferably 0.1% or less from the viewpoint of powdering resistance. There are Sb, Fe, Cl, etc. as the mixed elements, but these have not been found to have any particular effect on the powdering resistance.
Examples of the present invention will be described below.

[0013]

[Embodiment] Plates of steel components shown in Table 1 (plate thickness 0.8
(mm) in the hot dip galvanizing line by the non-oxidizing furnace method, Al and B are added to the plating bath using Al-4% B metal, and each element is adjusted to the target concentration, and then hot dip galvanizing is performed. After the same amount was adjusted to 60 g / m ² per side, the alloy was introduced into an alloying furnace and heated to about 500 to 530 ° C. for alloying, and after cooling, the winding performance was evaluated. As can be seen from the performance evaluation results in Table 2, B, Mn, S in the plating bath
The addition of i and Na has a remarkable effect on improving the powdering resistance.

[0014]

[Table 1]

[0015]

[Table 2]

Powdering resistance test method: A punch tip was bent at a radius of 0.5 mm and then flattened, and the inner side of the bend was taped.

Powdering resistance rating Score Rating powdering resistance judgment criteria ◎ Almost no peeling ○ Powdering is slight △ Powdering is somewhat marked × Powdering is marked

[0018]

INDUSTRIAL APPLICABILITY As described above, the present invention makes it possible to produce an alloyed hot-dip galvanized steel sheet with significantly improved powdering resistance as compared with the conventional alloyed hot-dip galvanized steel sheet. It plays.

Front page continuation (72) Inventor Makoto Yoshida 1-1 No. 1 Tobita-cho, Tobata-ku, Kitakyushu-shi, Fukuoka New Nippon Steel Corporation Yawata Works

Claims (1)

[Claims] 1. A steel plate to be plated is Al0.05-0.2.
%, B 0.0002 to 0.004%, and Mn
0.01-1.0%, Si 0.001-0.1%, Na
Powdering resistance, characterized by containing 0.0001 to 0.07% of one or more, and the balance being alloyed after plating in a hot dip galvanizing bath consisting of Zn and other unavoidable impurities. A method of manufacturing hot-dip galvanized steel sheets with excellent properties.