JP2825674B2 - Galvannealed steel sheet with excellent low-temperature chipping properties - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a galvannealed steel sheet having excellent low-temperature chipping properties.

[0002] An alloyed hot-dip galvanized steel sheet is formed by heating a hot-dip galvanized steel sheet to diffuse iron of a base steel sheet into a plating layer to form an iron-zinc alloy. Because of the excellent corrosion resistance after,
It is used in many fields such as automobiles, building materials and home appliances.

[0003]

2. Description of the Related Art Various types of galvanized steel sheets, such as galvannealed steel sheets, have been used to solve the problem of corrosion of automobile bodies due to rock salts for preventing road freeze in winter in cold regions.

[0004] Although alloyed hot-dip galvanized steel sheets have excellent corrosion resistance, when used on the outer surface of a vehicle body, there arises a problem that the adhesion of plating to the steel sheet base material is reduced. Normally, the outer surface of the vehicle is coated with cationic electrodeposition, middle coat, and top coat with a total film thickness of about 100μ (three coats).
During the baking of these coatings, the shrinkage stress of the coating acts on the plating layer to lower the adhesion to the substrate. In particular, in cold regions in winter, the temperature drops to about -20 ° C,
Since the shrinkage of the coating film progresses, the stress acting on the plating layer is also increased, and the adhesion to the substrate is much lower.
Thus, when the vehicle is running on a road with low adhesion to the base material, if the splashed pebbles or the scattered rock hits (this phenomenon is called chipping), a phenomenon occurs in which the plating layer is separated from the ground iron. As a result, water from the snowmelt and the scattered salt act on the detached portion to promote corrosion. Such a problem of low-temperature chipping is remarkable in a zinc-based alloy electroplated steel sheet or a galvannealed steel sheet in which the internal stress of the plating film is higher than that of the zinc-coated steel sheet.

In the case of a zinc-based alloy electroplated steel sheet, for example, as disclosed in JP-A-59-200789, Cr, M
n, Fe, Co, Ni, Cu, In, Zn, Cd, S
A method has been proposed in which a coating layer composed of one or more of n and Pb is provided between a zinc-based alloy coating and a base steel sheet. However, in the case of an alloyed hot-dip galvanized steel sheet, even if these coating layers are provided, they are diffused during heating, and thus cannot be an effective countermeasure.

[0006]

Means for Solving the Problems The present inventors have developed an alloyed hot-dip galvanized coating exhibiting good substrate adhesion (low-temperature chipping) even when subjected to low-temperature chipping in a state where a three-coat coating for automobiles has been applied. Various experiments were conducted for the purpose of obtaining a steel sheet. As a result, they have found that an alloyed hot-dip galvanized steel sheet having a controlled plating composition exhibits good low-temperature chipping properties. That is, in the present invention, the plating composition is Fe 6 to
11%, Al 0.05-0.5%, Si 0.005
It is an alloyed hot-dip galvanized steel sheet having excellent low-temperature chipping properties, characterized by being composed of about 0.5%, the balance being Zn and unavoidable impurities.

[0007]

Hereinafter, the present invention will be described in detail. FIG. 1 is a schematic diagram of the present invention, and FIG. 2 is a schematic diagram of a distribution state of an iron-zinc alloy phase in a plating layer when a plating composition of a comparative example is used.

In the examples of the present invention, the soft 混 在 phase is mixed in the δ ₁ phase, and the hard and brittle Γ phase is extremely thin and discontinuous. For this reason, even if stress is applied to the plating layer at a low temperature, the stress is dispersed, and the plating does not peel off.

On the other hand, in the comparative example, the hard and brittle Γ phase is thick,
Continuously formed on the upper layer of the steel sheet, and δ
_One phase is formed in layers. For this reason, the stress concentrates on the Γ phase, and the plating separates from the base iron.

As a result, the low-temperature chipping property of the galvannealed steel sheet of the present invention is improved.

In order to make the distribution state of the iron-zinc alloy phase in the plating layer of the present invention, the plating composition is Fe 6 to 11.
%, Al 0.05-0.5%, Si 0.005-
Essential conditions are 0.5%, the balance being Zn and unavoidable impurities. If Fe is less than 6%, alloying failure occurs partially. When Fe exceeds 12%, Γ
Since the phase is formed thick, the low-temperature chipping property is not good. Similarly, A1 is more than 0.5% or Si is 0.1%.
If it is more than 5%, alloying failure occurs, and A1
Is less than 0.05% or Si is less than 0.005%, the low-temperature chipping property decreases.

In order to obtain the plating composition of the present invention, a steel sheet to be plated is plated using a plating bath containing Si and Al in a usual Sendzimer hot-dip galvanizing line, and then the plated steel sheet is alloyed by heating. A method or a method of pre-plating a Si layer on the surface of the steel sheet to be plated, followed by annealing, hot-dip galvanizing, and heat alloying may be used.

[0013] Even if unavoidable impurities such as Pb, Cd, and Sb added to the plating bath as a spangle adjusting element for plating are contained in the plating layer in a total amount of less than 0.3%, the present invention is not limited thereto. It does not show any hindrance to the effects of the invention.

[0014] The plating adhesion amount is 20 to 100 g / m
A range of ² is preferred. If it is less than 20 g / m ² , the corrosion resistance, which is the original purpose of the alloyed hot-dip galvanized steel sheet, is insufficient.
If it exceeds 0 g / m ² , the alloying treatment time will be prolonged and productivity will be impaired, which is not preferable.

[0015]

EXAMPLE A low-carbon Ti-added steel cold-rolled to a thickness of 0.8 mm and a width of 1200 mm was annealed in a Sendzimer hot-dip galvanizing line, and then immersed in a hot-dip galvanizing bath.
Immediately after plating to a predetermined amount, the plate temperature is 450 to 60
The alloy was heated and alloyed at 0 ° C. for a holding time of 4 to 20 seconds to produce galvannealed steel sheets having various plating compositions.

The plating composition of an alloyed hot-dip galvanized steel sheet is determined by dissolving the plating layer with a solution comprising hydrochloric acid, nitric acid, and hydrofluoric acid, and measuring the amounts of Zn, Al, Fe, and Si by an atomic absorption method. Was.

The low-temperature chipping property was evaluated by treating the test piece under the conditions shown in Table 1.

Table 2 shows the test results together with comparative examples.
Symbols 1 to 10 in Table 2 are examples of the present invention, and symbols 11 to 13 are comparative examples. In Comparative Example 11, the amount of Si in the plating layer was 0.1.
6% and Fe is as low as 5.5%, so that the alloy is partially unalloyed. In Comparative Examples 12 and 13, the Si content in the plating layer was as small as 0.001% and 0%, respectively, so that the low-temperature chipping property was not good. In contrast, the examples of the present invention have good levels of appearance, low-temperature chipping properties, pressability, and weldability.

[0019]

[Table 1]

[0020]

[Table 2]

[0021]

As described above, the alloyed hot-dip galvanized steel sheet of the present invention is a high-performance steel sheet having excellent low-temperature chipping properties, and has great industrial effects.

[Brief description of the drawings]

FIG. 1 is a schematic view showing a distribution state of an iron-zinc alloy layer in a plating layer of an example of the present invention.

FIG. 2 is a schematic diagram of a distribution state of an iron-zinc alloy layer in a plating layer of a comparative example.

Claims (1)

(57) [Claims] 1. A plating composition comprising Fe 6 to 11% Al
An alloyed hot-dip galvanized steel sheet having excellent low-temperature chipping properties, comprising 0.05 to 0.5%, 0.005 to 0.5% of Si, balance Zn and unavoidable impurities.