JP2525165B2 - Method for manufacturing high strength galvanized steel sheet - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a plated steel sheet having high plating adhesion when performing high-strength steel containing Si by vapor deposition zinc plating.

<Prior art and problems> C, Mn and Si are well known as constituent elements for increasing the strength of steel sheet. Among them, Si increases the strength with a relatively small addition and the addition amount increases. Even so, there is an advantage that the ductility does not extremely decrease.

However, after recrystallizing and heating the rolled steel strip in a reducing atmosphere,
In a plating line for continuous plating, when Si-added steel is plated, Si diffuses and oxidizes on the surface of the steel strip during heating, resulting in a marked decrease in platability.

For example, in a hot-dip galvanizing line in which a cold-rolled steel strip is heated to 700 to 800 ° C. in a 50% H ₂ —N ₂ atmosphere and then continuously immersed in a molten Zn bath (0.2% Al—Zn), 0.6 wt% ( When hot-dip Zn plating is applied to a steel strip containing Si (% below), local non-plating occurs and the product does not become a product. In order to avoid such localized non-plating, there is also a method of raising the temperature of the plating bath or raising the temperature of the steel strip immediately before being immersed in the plating bath during the hot dip Zn plating, but either method, This time, the local Fe near the interface between the steel plate surface and the galvanized layer
-There is a problem that the Zn alloy layer develops abnormally and the plating adhesion decreases.

In electroplating, since the SiO _{2 on the} surface of the steel strip is cleaned in the process of pickling after annealing, the above problems do not occur,
In a vapor deposition plating line in which a rolled steel strip is recrystallized and heated in a reducing atmosphere and then continuously plated, the same problem as hot dipping occurs.

<Findings for Solving Problems> Generally, in vapor deposition zinc plating, the higher the strip temperature is within a temperature range in which vapor deposition zinc and Fe in steel do not form an intermetallic compound, the better the strip adhesion is. Can be obtained. The inventors of the present invention, for Si-added steel, even if Si is diffused on the steel surface by reduction heating immediately before plating, if the steel strip is heated according to a predetermined control formula according to the Si content, good plating adhesion It has been found that a vapor-deposited zinc steel strip having

<Structure of the Invention> According to the present invention, when the high-strength steel strip containing Si in the range of 0.2 to 0.8% by weight is subjected to reduction heating annealing and continuously vapor-deposited zinc-plated, the steel strip temperature T ₀ immediately before plating is _0. Manufacture of high-strength galvanized steel sheet characterized by performing vapor-deposition galvanizing by setting (° C) in the range of 350 ≥ T ₀ ≥ 250 x (Si weight%) + 150 according to the Si content. A method is provided.

The present invention uses a high-strength steel strip containing Si as a plating base material. Mainly, C: 0.08% or less, Si: 0.20 to 0.80%, Mn:
0.15-0.85% Si with the balance Fe and unavoidable impurities
For added steel, Al, Ti, Z in addition to the above elements
It is also possible to use Si-added steel containing 0.1% or less of r, V, and Nb.

Among the constituent elements of the steel, if the C content is increased, the steel strength is increased, but the toughness is reduced, the low temperature brittleness becomes remarkable, and weld cracking is likely to occur. Therefore, the C content is preferably 0.08% or less. Si
If the amount is less than 0.20%, the effect of increasing the strength of the steel is not sufficient, and if it exceeds 0.80%, there is almost no range where the adhesion of the plating layer is good in the temperature region shown.

Mn is required to be 0.15% or more for deoxidation reaction and sulfide formation in the steelmaking process. However, if Mn exceeds 0.85%, the base material becomes too hard and cracks may occur in the plated steel sheet during processing, so the upper limit is made 0.85%.

Al, Ti, Zr, V and Nb are elements that improve the press formability of the plated steel sheet, so it is desirable to add up to 0.1% depending on the application.

 The Si-containing high-strength steel strip is subjected to vapor deposition zinc plating.

The conditions for the vapor deposition zinc plating are the steel strip temperature T ₀ immediately before plating.
Except the above, the usual vapor deposition plating conditions may be used. Incidentally, an example of vapor deposition zinc plating conditions is gradually shown.

Plate thickness: 0.3-1.2 mm, Line speed: 80-160 m / min Plating surface: One side or both sides Plating amount: 10-100 g / m ² Deposition chamber vacuum degree: 0.05-0.01 Torr Zinc bath temperature: 530 ° C During vapor deposition zinc plating, the steel strip temperature T ₀ immediately before plating is kept within the range of the following formula.

350 ≧ T ₀ ≧ 250 × (Si%) + 150 In vapor deposition galvanizing, if the steel strip temperature T ₀ is 180 ° C or less, the bond between the zinc vapor flying to the steel strip surface in the vacuum deposition chamber and the steel strip surface The energy is insufficient and the plating adhesion of the zinc plating layer is significantly reduced, making it unusable for practical use. Usually, the steel strip temperature T ₀ is preferably 200 ° C. or higher. In the present invention, the steel strip temperature T ₀ is adjusted according to the Si content. Since a steel strip having a Si content of 0.2 to 0.8% is mainly used as a plating base material, the temperature range is adjusted to T ₀ ≧ 250 × Si% + 150.

On the other hand, when the steel strip temperature T ₀ is too high, an alloy layer develops between the steel strip surface and the galvanized layer, which causes the plating adhesion to be impaired. Usually, in vapor deposition galvanizing, when the steel strip temperature T ₀ exceeds 350 ° C., the influence of the alloy layer becomes large. Therefore, the steel strip temperature T ₀ needs to be 350 ≦ T ₀ .

Example An Si-added steel strip was subjected to continuous vapor deposition zinc plating under the conditions shown below, and the plating adhesion was examined.

B) Steel plate Si produced by the steps of steelmaking, continuous casting, hot rolling, pickling and cold rolling
Cold-rolled steel strips (0.8 mmt × 300 mm × C) having different contents were used as plating base materials. Table 1 shows the results of component analysis of the base material.

B) Vapor-deposited galvanizing After the above Si-added steel strip is recrystallized and heated in the pretreatment furnace of the continuous vapor-deposition plating line (at 750 ° C for about 30 seconds), the steel is continuously passed through the gas jet cooler and pressure chamber. By adjusting the strip temperature, the vapor-deposited galvanized steel strip having a steel strip temperature immediately before vapor deposition of 180 ° C to 370 ° C was prepared. The vapor deposition zinc plating conditions are as shown in Table 2.

C) Plating Adhesion Test A 0t adhesion bending cellophane tape peeling test was performed on the vapor-deposited galvanized steel sheet thus prepared, and the plating adhesion was evaluated.

 The results obtained are shown in Table 3.

From the table, it can be seen that as the Si content in the steel increases, good plating adhesion cannot be obtained unless the steel strip temperature before vapor deposition is increased. The temperature of the steel strip before vapor deposition is 360 ° C.
Reaches the local Fe-Zn at the galvanized layer-steel plate interface.
The intermetallic compound grows and the plating adhesion decreases with any steel type.

The above results are shown in the graph of FIG. As shown in the figure, when the steel strip temperature T ₀ before vapor deposition is controlled within the range of the following equation, vapor-deposited galvanized steel sheet having good plating adhesion even if the Si content in the steel is 0.2 wt% or more. It can be seen that 350 ≧ T ₀ ≧ 250 × [Si%] + 150 can be manufactured, and the above range is a region having good plating adhesion.

<Effects of the Invention> According to the present invention, it is possible to easily produce a vapor-deposited galvanized steel sheet having excellent plating adhesion even with respect to Si-containing high-strength steels that are conventionally prone to problems with plating adhesion.

Further, the method of the present invention is to control the steel strip temperature immediately before plating within a predetermined range corresponding to the Si content.
The steel strip having a content can be widely and easily dealt with.

[Brief description of drawings]

 FIG. 1 is a graph showing the results of the examples of the present invention.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yusuke Hirose 5 Ishizu Nishimachi, Sakai City, Nisshin Steel Co., Ltd., Hanshin Research Center (56) References JP-A-57-152465 (JP, A) JP-A-57-131361 (JP, A) JP 59-83765 (JP, A) JP 52-36962 (JP, B1)

Claims (2)

(57) [Claims] 1. When a high-strength steel strip containing Si in a range of 0.2 to 0.8% by weight is subjected to reduction heating annealing and continuously vapor-deposited zinc plating, the steel strip temperature T ₀ (° C.) immediately before plating is A method for producing a high-strength galvanized steel sheet, characterized by performing vapor-deposited zinc plating in the range of 350 ≧ T ₀ ≧ 250 × (Si weight%) + 150 according to the Si content. 2. The manufacturing method according to claim 1, wherein the high-strength steel strip has a C content in the steel of 0.08% by weight or less and an Mn content in the steel of 0.15 to 0.85% by weight. .