JP3142736B2 - Alloyed hot-dip galvanized steel sheet with excellent workability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an alloyed hot-dip galvanizing method which is mainly used for applications such as automobiles and home appliances, in which the coating layer is converted to an Fe--Zn alloy phase by hot diffusion treatment after hot-dip galvanizing. The present invention relates to a steel plate and a method for manufacturing the same.

[0002]

2. Description of the Related Art The so-called alloyed hot-dip galvanized steel sheet, which is hot-alloyed after being subjected to hot-dip galvanizing, is widely used in applications such as automobiles and home appliances due to its excellent paintability and weldability. Production is on the rise.
The required properties for the galvannealed steel sheet include corrosion resistance, workability, weldability, and paintability. Among these, workability is one of the particularly important characteristics. At the time of processing, if the plating layer peels (flaking, powdering), the workability decreases. For this improvement, optimization of steel type, plating pretreatment, hot-dip plating conditions, alloying heating conditions, and the like are currently being actively researched and developed.

Conventionally, it has been confirmed that the workability of an alloyed hot-dip galvanized steel sheet is deteriorated because the Γ phase having a high Fe content is hard and brittle in the Fe-Zn alloy phase. Various techniques have been proposed to take countermeasures. At present, however, no technique has been found that embodies the idea of modifying the form of the alloy phase itself to achieve high workability. In particular, ultra-low carbon Ti-Nb-added steel has excellent material properties and is suitable for deep drawing applications for automobiles and home appliances. However, since Ti has an action of forming a thick Γ phase in the alloying heating process after plating, There is a problem that workability becomes poor.

As a countermeasure against this, Al in a hot dip galvanizing bath is used.
JP-A-56-13470 in which galvanization is performed after minimizing the amount of zinc and alloying treatment is performed. 104163, JP-A-60-110
No. 859 is disclosed, but all the alloyed hot-dip galvanized steel sheets obtained by these techniques are not sufficiently effective in improving workability. Alternatively, there is a disadvantage that the production cost is increased by performing pre-plating or the like.

[0005]

The alloyed hot-dip galvanized steel sheet manufactured by the prior art has a feature that the Γ phase is formed in a layered and continuous manner at the interface between the coating layer and the base steel sheet. When the steel sheet is processed, cracks are easily generated and propagated in the Γ phase portion, and the uprooted plating layer peels off, causing practical problems such as flaking and powdering. In the present invention, in order to solve the problems of the conventional technology, the generation of the Γ phase is made into a discontinuous layer to suppress the propagation of cracks and to prevent the plating layer from peeling off.

[0006]

As a method for realizing this, a method of increasing F concentration by increasing the Al concentration in the plating phase.
A large amount of the e-Al alloy phase is generated at the interface between the base steel sheet and the plating layer to enhance the local suppressing action of the Fe-Zn alloying reaction, and the Γ phase is formed into a discontinuous layer. The configuration of the present invention will be described below. The base steel sheet is a Ti-Nb added ultra-low carbon steel, and the composition of the plating layer is 8 to 13% by weight of Fe,
l: 0.35 to 0.6%, adhesion amount 20 to 90 g / m ²
The galvannealed steel sheet,
An alloyed hot-dip galvanized steel sheet excellent in workability, characterized in that an alloy phase having an Al concentration of 0.5% or more locally exists within a range of 2 μm around the interface of the base steel sheet .
is there.

[0007]

Next, the operation of each requirement and the reason for limiting the numerical values will be described. A Ti-Nb added ultra-low carbon steel is used as the base steel sheet. Ti and Nb fix C and N, which are infiltration-type solid solution elements in steel, in the form of TiC, NbC and TiN, respectively, and improve the material properties of the steel suitable for deep drawing. F as a plating layer component
e: 8 to 13%. If it is 8% or less, the η phase of Zn remains on the plating surface layer, and deteriorates corrosion resistance and weldability after painting. On the other hand, when the content is 13% or more, the workability of the plating layer deteriorates.
Al: 0.35 to 0.6% as a plating layer component.
In the case of 0.35% or less, the interface between the plating layer and the base steel sheet
When a phase is continuously generated in layers and a plated steel sheet is processed, cracks propagate and the plating is easily peeled. 0.
If it is 6% or more, the alloying reaction rate of Fe-Zn is greatly reduced, and productivity is impaired. 20 ~
The range is 90 g / m ² . If it is less than 20 g / m ² , the corrosion resistance is insufficient, and if it is more than 90 g / m ² , the workability is significantly reduced, and the alloying process requires a long time to impair productivity.

In the sectional structure of the plating layer, the Al concentration is within a range of 2 μm around the plating layer-base steel plate interface.
An alloy phase containing 5% or more is locally present. By causing Al to be locally present near the interface, the Fe—Zn alloying reaction is locally suppressed, and the Γ phase is prevented from being continuously generated in a layered manner. This effect is not exhibited unless Al is 2 μm or less and 0.5% or more around the interface. Al
Is measured by EPMA analysis of the section of the plating layer. The frequency of the presence of the alloy phase containing 0.5% or more of the Al concentration corresponds to the size of the crystal grain boundary of the base steel sheet. In order to prevent crack propagation during processing, it is necessary to break the continuity of the Γ phase minutely. On the other hand, if the Al-concentrated region is too large, the alloying reaction is reduced as a whole, which impairs productivity.

[0009]

[Embodiment] In a continuous hot-dip galvanizing line, a sheet thickness of 0.
Annealed Ti-Nb-added ultra-low carbon steel shown in Table 1, which was cold-rolled to 8 mm, was immersed in a hot-dip galvanizing bath, and the amount of plating was controlled by gas wiping. Galvanized steel sheet was manufactured. About the obtained steel plate, a powdering test was performed as a plating adhesion amount, a component, an Al distribution state of a plating layer cross section, and workability evaluation. FIG. 1 shows the relationship between plating layer composition and workability. The steel sheet of the present invention having a high Al% in the plating layer has better workability as compared with the same Fe%. FIG. 2 shows the relationship between the amount of plating and the workability. If the amount of adhesion is high, the workability is reduced. FIG.
Shows the cross-sectional structure of the plating layer of the steel sheet of the present invention. Al is locally distributed at the interface between the plating layer and the base steel sheet, and the generation of the Γ phase in a layered manner is suppressed, and the workability of the plating layer is improved.
After bending the powdering test steel sheet by 60 degrees V, the processed portion is peeled off using cellophane tape and the plating peeling width (mm) is measured.

[0010]

[Table 1]

[0011]

As described above, the present invention significantly improves the workability by changing the structure of the coating layer of the galvannealed steel sheet. Thereby, the use of the alloyed hot-dip galvanized steel sheet is expanded, and the industrially significant effect is achieved.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between plating layer composition and workability;

FIG. 2 is a diagram showing a relationship between a coating weight and workability;

FIG. 3 is a view showing a sectional structure of a plating layer of the steel sheet of the present invention.

Continuation of front page (58) Field surveyed (Int.Cl. ⁷ , DB name) C23C 2/00-2/40

Claims (1)

(57) [Claims] 1. A base steel sheet is a Ti-Nb-added ultra-low carbon steel having a plating layer component of 8 to 13% by weight of Fe and Al:
In an alloyed hot-dip galvanized steel sheet having 0.35 to 0.6% and an adhesion amount of 20 to 90 g / m ² , the Al concentration is within a range of 2 μm around the interface between the plating layer and the base steel sheet.
An alloyed hot-dip galvanized steel sheet having excellent workability, characterized in that an alloy phase containing 5% or more is locally present.