JPH05132748A - Manufacture of galvannealed steel sheet - Google Patents

Abstract

PURPOSE:To obtain a galvannealed steel sheet excellent in corrosion resistance after coating and plating adhesion, at the time of manufacturing a galvannealed steel sheet, by progressing the mutual diffusion between Fe in a basis steel sheet and Zn in a galvanized layer at an optimum rate and securing an allaying state free from excess and deficiency. CONSTITUTION:A steel sheet contg. 0.005 to 0.10% Ti and 0.03 to 0.l5% P is used; hot-dip coating is applied by using a galvanizing bath in which the effective Al concn. satisfying the requisites in the following inequalities is regulated; and after that, alloying treatment is executed: 0.8 Y<= the effective Al concn. in the plating bathe 1.2Y; Y=-1/12X[P]+1/5X[Ti]+0.100 ([P] and [Ti] denote the content [wt.%] of P and Ti in the steel sheet.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to Fe generated between a steel sheet as a base material and a hot dip galvanized layer coated on the surface of the steel sheet.
By controlling the interdiffusion of Zn and Zn and the accompanying Fe-Zn alloying well, and ensuring an alloyed state without excess or deficiency, a galvannealed steel sheet with excellent corrosion resistance after coating and plating adhesion can be obtained. The present invention relates to a manufacturing method.

[0002]

2. Description of the Related Art Recently, the amount of various kinds of surface-treated steel sheets used has rapidly increased as a measure for strengthening rust prevention of various vehicles such as automobiles or outer panel materials for household electric appliances. Since it has excellent weldability such as welding and formability, as well as its own corrosion resistance and post-painting corrosion resistance such as organic paints, it has been put to practical use in large quantities. However, as the demand for vehicle body weight reduction or production efficiency improvement for the purpose of reducing fuel consumption and exhaust gas amount of automobiles and the like has further increased, further improvement in strength and formability has come to be required.
In order to meet such a demand, a method has been proposed in which a steel sheet to which Ti, Nb, P or other alloy elements are added alone or in combination is used as a base material and which is subjected to galvannealing.

However, when a steel sheet containing such an element is used, zinc remains unalloyed on the surface of the hot-dip galvanizing layer, or insufficient alloying occurs, or excessive alloying occurs in the galvanizing layer, resulting in brittleness. The phenomenon of excessive alloying, in which the Γ phase (Fe ₃ Zn ₁₀ ) grows thick, tends to appear, and the weldability, plating adhesion, or corrosion resistance after coating is markedly reduced. It is considered that this is because the alloying elements added to the steel material greatly affect the alloying rate of Fe and Zn. For example, T
It is said that i promotes the above-mentioned alloying and P delays the alloying on the contrary, but when these elements are added in a complex manner, the actions of both are complicatedly entangled, so that insufficient alloying or alloying occurs. It is extremely difficult to quickly and accurately grasp the optimum operating conditions that do not become excessive, and it is inevitable that the yield will decrease significantly due to excess or deficiency of alloying.

[0004]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and its object is to use a steel sheet containing Ti and P as alloying elements to form alloyed molten zinc. It is intended to establish a method capable of stably and accurately obtaining an appropriate alloying state without causing excess or deficiency of alloying in a hot-dip galvanized layer when manufacturing a plated steel sheet.

[0005]

The method for producing an alloyed hot-dip galvanized steel sheet according to the present invention, which has been able to solve the above-mentioned problems, has a Ti content of 0.005 to 0.10% by weight and a P content of 0.03.
~ 0.15% by weight of steel sheet is hot-dip galvanized using a hot dip galvanizing bath whose Al content is adjusted so that the effective Al concentration (% by weight) in the bath satisfies the requirement of the following formula [I], and then the alloy It has the gist in the place where it is processed. 0.8 x Y ≤ effective Al concentration in the bath ≤ 1.2 x Y ... [I] Y =-(1/12) x [P] + (1/5) x [Ti] + 0.100 ... [II] (however, Medium [P] and [Ti] represent P and Ti contents [% by weight] in the steel sheet)

[0006]

In the present invention, first, a steel material having a Ti content of 0.005 to 0.10% (hereinafter referred to as weight% unless otherwise specified) and a P content of 0.03 to 0.15% is used as a steel sheet. This is defined as a requirement for obtaining an alloyed hot-dip galvanized steel sheet having excellent formability and satisfying the demand for high strength. When Ti content is insufficient, satisfactory formability is improved. This is because no effect is obtained, and the effect of addition is saturated at about 0.1%, and addition of more than that is not economically wasteful, but also the tendency of strength reduction due to the impurity effect appears. Further, P is mainly added for the purpose of improving the strength of the galvannealed steel sheet, and when it is insufficient, the demand for higher strength cannot be satisfied. However, the strength improvement effect of P is about 0.15%
Is saturated, and if more is added, the strength will rather decrease due to the effect of impurities, so 0.15% was defined as the upper limit.

As described above, Ti in the steel sheet has the effect of promoting the alloying of Fe and Zn, and P has the effect of delaying the alloying of Fe and Zn. It was previously explained that it is difficult to obtain a proper alloyed state only by adding. However, the Ti and P in the steel sheet
When an appropriate amount of Al satisfying the requirements shown in the above formula [I] is contained in the hot dip galvanizing bath based on the content of Fe, the Fe in the steel sheet base material and the Zn in the hot dip galvanizing layer are intercalated with each other. It was found that the alloying by diffusion proceeds well, and a plating layer in a proper alloyed state without excess or deficiency can be obtained.

Below, the effect of Al addition and Ti, P in steel
The interaction with is explained. In the present invention, Al added to the hot dip galvanizing bath forms an Fe-Al alloy layer at the Zn plating interface by reacting with Fe in the steel sheet substrate. And Fe and Zn are alloyed with the base Fe and the plating Z.
It is believed to be initiated by breaking the Fe-Al alloy layer by interdiffusion of n. As described above, this alloying reaction is promoted by Ti in the steel and delayed by P, but Ti and P are not contained in the Fe-
It is considered that this also affects the properties of the Al alloy layer.

Al in the plating bath is Fe-A
The properties of the l-alloy layer are also greatly affected, and the barrier effect of the Fe-Al alloy layer increases as the Al concentration increases, and the Fe-Zn alloying rate decreases. Al in the hot dip galvanizing bath is divided into Al that is almost uniformly dissolved in the galvanizing bath and Al that is melted in the galvanizing bath and combined with Fe to form dross. The former Al affects the properties and is generally called effective Al. Since the latter Al is combined with Fe at a weight ratio of 1: 1, the effective Al concentration in the plating bath is
It is a value obtained by subtracting the Fe concentration in the bath from the 1 concentration. In the present invention, attention is paid to this effective Al, and the interaction between the promotion of alloying by Ti and the delay of alloying by P in the steel sheet is adjusted by the effective Al concentration to accurately control the alloying rate.

That is, the inventors of the present invention use various steel sheets having different Ti and P contents, perform a galvanizing treatment using a hot dip galvanizing bath having a different effective Al concentration, and immediately perform an alloying heat treatment. , And the alloying rate was investigated for each of them. As a result, as described above, a hot dip galvanizing bath whose effective Al concentration falls within the range of 0.8 × Y to 1.2 × Y with respect to the Y value calculated by the above formula [II] from the Ti and P concentrations in steel is used. Then, it became clear that the Fe-Zn alloying rate becomes almost constant, and an extremely accurate alloying operation without excess or deficiency of alloying can be performed.

That is, the effective Al concentration in the hot dip galvanizing bath is
When it is less than 0.8 × Y, the barrier effect of the Fe—Al alloy layer becomes insufficient and a brittle Γ phase is generated due to excessive Fe—Zn alloying, resulting in poor plating adhesion.
If it exceeds 1.2 × Y, the barrier effect of the Fe—Al alloy layer becomes too strong and alloying becomes insufficient, resulting in poor corrosion resistance and weldability after painting. If a hot dip galvanizing bath whose effective Al concentration is adjusted to fall within the above range is used, the material F
The Fe-Al alloy layer formed at the interface between the e and Zn plating layers controls the interdiffusion rate of Fe and Zn well, and a proper Fe-Zn alloyed state can be reliably obtained.

As described above, according to the present invention, the effective Al concentration in the hot dip galvanizing bath is adjusted according to the Ti and P contents in the raw steel sheet to control the alloying rate by mutual diffusion of the raw materials Fe and Zn. The hot dip plating method, the alloying heat treatment condition, etc. are not particularly limited.
It is a method of dipping and running the steel sheet to be treated in a hot dip galvanizing bath at 430 to 480 ° C (in which an appropriate amount of Al satisfying the above requirements is added), and the alloying heat treatment is performed by the above-mentioned plating treatment. Immediately after that, the plated steel plate
This is a method of heating at 700 ° C. for about 3 to 120 seconds.

[0013]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to the following examples, and the types of steel sheets, plating pretreatment methods, hot dipping conditions, and It is within the technical scope of the present invention to appropriately change the alloying heat treatment conditions and the like as necessary.

Example A steel plate having a Ti content of 0.05% and a P content of 0.10% was used as a base material, and this was immersed in various hot dip galvanizing baths (temperature: about 460 ° C.) having different effective Al concentrations. After hot-dip galvanizing (thickness: 10 μm), alloying treatment was performed at 550 ° C. for 30 seconds to obtain a galvannealed steel sheet. The obtained plated steel sheets were evaluated for corrosion resistance after coating and plating adhesion by the following methods. The results are collectively shown in Table 1.

<Corrosion resistance after coating> After each plated steel sheet is treated with phosphate, three-coat coating for automobiles (specifically, electrodeposition coating + intermediate coating + top coating three-layer coating, 150 ° C, 20 ° C)
Bake minutes). A cross cut reaching the surface of the plating layer was put into this coated steel sheet, a salt water spray test was conducted for 1000 hours, and then a three-stage evaluation was made based on the degree of swelling of the coating film from the cross cut portion. ⊚: excellent, ◯: good, ×: bad. <Plating adhesion> For each test steel plate that has undergone alloying heat treatment,
Angle 60. Was subjected to V-bending, and the amount of plating peeling when the compression side was peeled off the tape was evaluated in three levels. ⊚: excellent, ◯: good, ×: bad.

[0016]

[Table 1]

As is clear from Table 1, in the examples of the present invention in which the relationship between the effective Al concentration in the plating bath and the Y value meets the specified requirements of the present invention, both the corrosion resistance after coating and the plating adhesion are good. On the other hand, in the comparative example where the effective Al concentration is insufficient, the plating adhesion is poor, and in the comparative example where the effective Al concentration is too high, the corrosion resistance after coating is poor, and both of them satisfy the required performance of the galvanized steel sheet. I can not say.

[0018]

The present invention is constituted as described above, and by appropriately adjusting the effective Al concentration in the hot dip galvanizing bath to be used according to the contents of Ti and P in the steel sheet, By adjusting the mutual diffusion rate of Fe in the base steel sheet and Zn in the plating layer to eliminate the excess or deficiency of Fe-Zn alloying, an alloyed hot-dip galvanized steel sheet having excellent plating adhesion and corrosion resistance after painting. Will be able to provide.

Claims (1)

[Claims] 1. A steel sheet having a Ti content of 0.005 to 0.10% by weight and a P content of 0.03 to 0.15% by weight, so that the effective Al concentration (% by weight) in the bath satisfies the requirement of the following formula [I]. A method for producing an alloyed hot-dip galvanized steel sheet, which comprises hot-dip galvanizing using a hot dip galvanizing bath having an adjusted Al content, and then subjecting it to an alloying treatment. 0.8 x Y ≤ effective Al concentration in the bath ≤ 1.2 x Y ... [I] Y =-(1/12) x [P] + (1/5) x [Ti] + 0.100 ... [II] (however, Medium [P] and [Ti] represent P and Ti contents [% by weight] in the steel sheet)