JP3142735B2 - 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 made into an Fe--Zn alloy phase by hot-diffusion after hot-dip galvanizing. It relates to a steel plate.

[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-added steel has excellent material properties and is suitable for deep drawing applications for automobiles and home appliances.
Has the effect of forming a thick Γ phase in the alloying heating process after plating, and thus has a problem of poor workability.

[0004] As a countermeasure, it has been widely practiced to use an ultra-low carbon Ti-Nb-added steel in which Nb is added instead of reducing the amount of Ti added to the steel as a base steel sheet for an alloyed hot-dip galvanized steel sheet. However, Nb has a problem that the addition cost is higher than that of Ti. Japanese Patent Application Laid-Open No. Sho 56-1 discloses a method in which a galvanizing is carried out after suppressing the amount of Al in a hot-dip galvanizing bath to a very small amount.
No. 3470, Japanese Unexamined Patent Publication No. Sho.
Although JP-A-8-104163 and JP-A-60-110859 have been proposed, 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-added ultra-low carbon steel, and the components of the plating layer are 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.
Galvannealed steel sheet near with excellent workability, characterized in that the alloy phase contained 5% or more is present locally
You.

[0007]

Next, the operation of each requirement and the reason for limiting the numerical values will be described. A Ti-B added ultra-low carbon steel is used as the base steel sheet.
Ti is the immersion type solid solution element in steel, C and N, respectively.
The steel is fixed in the form of C and TiN to improve the material properties of the steel suitable for deep drawing. Currently, Ti-Nb-added ultra-low carbon steel is generally used as a base steel sheet for alloyed hot-dip galvanized steel sheet. However, the manufacturing cost of the base steel sheet is lower when using Ti-only added steel. Benefits.

[0008] Fe: 8 to 13% as a plating layer component. 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. When the content is 0.35% or less, when the Γ phase is continuously generated in a layered manner at the interface between the plating layer and the base steel sheet, and when the plated steel sheet is processed, cracks propagate and the plating is easily peeled. If 0.6% or more, F
The alloying reaction rate of e-Zn is greatly reduced, and productivity is impaired. The plating adhesion amount is in the range of 20 to 90 g / m ² . If it is less than 20 g / m ² , the corrosion resistance is insufficient and 90
If it exceeds g / m ² , the workability is greatly reduced, and the alloying process requires a long time to impair the 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.

[0010]

[Embodiment] In a continuous hot-dip galvanizing line, a sheet thickness of 0.
After annealing the Ti-added ultra-low carbon steel cold-rolled to 8 mm, it was immersed in a hot-dip galvanizing bath, and the amount of coating was controlled by a gas wiping method. About the obtained steel plate,
Amount of plating coating, composition and Al distribution of plating layer cross section,
A powdering test was performed as workability evaluation. FIG. 1 shows the relationship between plating layer composition and workability. The steel sheet of the present invention having a high Al% 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. 3 shows a sectional structure of a 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. Powdering Test After bending and returning the steel sheet to 60 degrees V, the processed portion was subjected to cellophane tape peeling, and the plating peeling width (mm) was measured.

[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.

──────────────────────────────────────────────────続 き Continued on 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-added ultra-low carbon steel having a plating layer component of 8 to 13% by weight of Fe and Al of 0.35% by weight.
In an alloyed hot-dip galvanized steel sheet having an adhesion amount of 20 to 90 g / m ² , the Al concentration is 0.5% or more within a range of 2 μm centered on the interface between the plating layer and the base steel sheet. An alloyed hot-dip galvanized steel sheet excellent in workability, characterized in that an alloy phase exists locally.