JPH04202631A - Production of galvannealed p-containing high tensile strength steel sheet - Google Patents

Abstract

PURPOSE:To obtain a galvannealed steel sheet stabilized in grade and excellent in uniform external appearance without causing uneven alloy plating by subjecting a P-containing high tensile strength steel sheet to rapid heating and oxidation under specific conditions, to annealing in a hydrogen-containing atmosphere, and then to hot dipping. CONSTITUTION:A high tensile strength steel sheet having >=0.04% P concentration in steel is rapidly oxidized under the conditions of 0.90-1.10 oxidation-zone combustion air ratio and >=30Angstrom /sec average oxidation velocity in a oxidation zone excluding a preheating zone. Subsequently, this steel sheet is annealed in an atmosphere having <=20% hydrogen concentration in a reducing zone, passed through a molten zinc bath, and then subjected to alloying treatment. Further, it is desirable that rapid oxidation is done by using a burner throwing a flame perpendicularly to the steel sheet at the time of performing heating in the above oxidation zone.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for manufacturing a P-containing high tensile strength alloyed hot-dip galvanized steel sheet, and more specifically, the present invention relates to a method for producing a P-containing high-tensile alloyed galvanized steel sheet, and more specifically, the present invention relates to a method for producing a P-containing high-tensile alloyed galvanized steel sheet, and more specifically, the present invention relates to a method for producing a P-containing high-tensile alloyed galvanized steel sheet.
Pre-annealing by rapid oxidation to ensure the properties of alloyed hot-dip galvanized steel sheets for steel sheets containing 4% or more of P;
Then, hot-dip plating is performed after annealing in a hydrogen-containing atmosphere.
A method for producing a P-containing hypertonic hot-dip galvanized steel sheet.

(Prior art) In recent years, cold-rolled steel sheets used as steel sheets for automobiles have been developed from the viewpoint of reducing the weight of car bodies, improving safety, and corrosion resistance.
In addition to excellent press workability, high strength and corrosion resistance are required, and there is a strong demand for surface treatment of supplied steel materials. Particularly recently, there has been an increasing demand for surface treatment of high-strength steel sheets. As a surface treatment method whose main purpose is to improve the rust prevention ability of high-tensile steel sheets, there is the Sendzimer hot-dip galvanizing method, which can easily increase the thickness of the galvanized steel sheet from the viewpoint of productivity. In the case of this Sendzimer hot-dip galvanizing method, the iron oxide film produced in the non-oxidizing furnace is reduced in the subsequent reduction zone, and then SL, an easily oxidizable element in steel,
Mn, P, and AQ are selectively oxidized on the surface of the steel sheet, thereby creating a concentration gradient, so that they become concentrated on the surface. Among these elements, P forms a POx film on the surface of the steel sheet and significantly impedes alloying properties (localized unevenness in alloying occurs). Conventional methods to improve this include a method of pre-plating with -Fe, Zn or N1 before annealing, and a method of removing rolling oil from the surface of the steel sheet in an oxidation furnace containing oxygen to form an appropriate oxide film. A method is already known from JP-A-55-122865, in which after reduction annealing in an atmosphere containing hydrogen, the plate temperature is adjusted in a furnace and plating is performed. In other words, in a non-oxidizing furnace method that does not contain oxygen, oil on the steel surface can be removed, but because the oxidizing atmosphere is weak, Sj, Mn, A, and Q, which are easily oxidized, are diffused and oxidized on the surface. Oxides form the steel surface. These oxides are not reduced in the reduction furnace and cause poor plating wettability and poor plating adhesion. To achieve this, the surface of the steel plate is oxidized to an oxide film thickness of 400 to 10,000 mm, then annealed in an atmosphere containing hydrogen, and then hot-dip plated.

(Problem to be solved by the invention) However, before annealing, which is the prior art, F'e.

The method of pre-plating Zn or Ni has difficulties such as the pre-plating film is unstable, the atmosphere inside the furnace is contaminated, and plating liquid must be used depending on the steel type. Regarding Publication No. 55-122865, the only finding is that good plating properties 1 can be obtained by heating with a high air ratio in a non-oxidizing furnace to generate an Fe oxide film and then heating by reduction. However, the relationship between P in the steel and the alloyed hot-dip galvanized layer is not disclosed at all. Furthermore, there is a method of alloying the entire steel plate by lowering the line speed and increasing the time in the alloying furnace. There was a problem in that the speed had to be reduced, resulting in a significant drop in productivity.

Therefore, the present invention provides a method for obtaining an alloy hot-dip galvanized steel sheet with stable quality and uniform appearance without alloy unevenness on a high productivity line using a method different from conventional methods. It's about doing.

(Means for solving the problem) In order to solve the above problems and achieve the purpose,
The gist of the present invention is to provide a method for producing P-containing high-strength hot-dip galvanized steel sheets with a P concentration of 0.04% or more in the steel, with an oxidation zone combustion air ratio of 0.95 to 1.10, and a preheating zone. After rapid oxidation with an average oxidation rate of 30 people/see or more in the oxidation zone, the process involves burning sand in an atmosphere with a hydrogen concentration of 10% or less in the reduction zone, followed by hot-dip plating. The present invention will be explained in detail below.

In the present invention, the P concentration in the steel is set to 0.04% or more because it is a desirable element because it has the effect of solid solution strengthening as a high-strength steel and refines the crystal grains of hot-rolled sheets, and is also inexpensive. It is an important element because it is particularly effective in reducing ductility deterioration associated with skin passes.To bring about this effect, 0.04% or more is required, while 0.04% or more is required.
．． If the amount exceeds 15%, an appropriate amount of P causes embrittlement and deterioration of weldability, so the optimum amount is preferably 0.04 to 0.15. On the other hand, in the case of steel containing P with a PM degree of 0.04% or more, it is generally called a dealloying material, and Si, Mn, A in the steel
Q, P, etc. are diffused into the steel plate surface layer as oxides by heating the steel plate surface, so these oxides are concentrated and form the steel surface. These oxides are not reduced even in a reducing furnace, inhibit alloying properties, and impair plating adhesion. Therefore, the present invention has made it possible to hot-dip galvanize steel materials that are difficult to alloy, with no alloying irregularities, and with excellent uniform appearance on a high-productivity line. The first condition for this was that the oxidation zone combustion air ratio was set to 0.95 to 1.10. The second is that the average oxidation rate in the oxidation zone, which is the biggest feature of the present invention, is 30 people/s.
It was set as ec or higher.

The third reason is that annealing and hot-dip plating were performed in an atmosphere with a reduction zone hydrogen concentration of 10% or less. In other words, the first condition of setting the oxidation zone combustion air ratio to 0.95 to 1.10 is because if the air ratio is less than 0.95, the oxidation behavior will change greatly due to subtle changes in the gas composition. It is not possible to adjust the oxide film of ~1000 people.

In addition, when the air ratio exceeds 1.10, an oxide film rapidly forms,
It easily exceeds the target oxide film and is difficult to adjust. Therefore, the air ratio must be within a range that can easily control the production of an oxide film, which is the optimum target, and the second condition is that the average oxidation rate in the oxidation zone, excluding the preheating zone, is 30%. Person/see or more. The reason for this is that, as shown in Fig. 1, for example, 20 people/Se
When the temperature is below C, the rate of formation of an oxide film is slow, so Si, which is easily oxidized, is removed before a suitable oxide film is formed on the steel sheet surface.
, Mn, AQ.

These oxides are formed on the steel surface because P is diffused and oxidized to the surface. Therefore, it has been discovered that rapid oxidation with short heating time is required, which does not generate these oxides. Moreover, considering the oxide film required for this,
30 people/see, which is the line B shown in FIG. 1, is required. If it is more than this, Si, Mn, which is easily oxidized,
Afl, P, etc. In particular, an Fe oxide film is formed on the iron surface before P diffuses to the surface and is not oxidized, and this Fe oxide film later becomes Si in the steel.

Prevents oxidation of Mn, AQ, P, etc. After that, this oxide film is annealed in an atmosphere with a reduction zone hydrogen concentration of 10% or less,
After reduction, it is passed through a molten zinc bath.

The amount of plating can be adjusted using air wiping. In addition, in order to perform rapid oxidation with an average oxidation rate of 30 Å/sec or more, a burner that injects flame perpendicularly to the steel plate is used.
By using a so-called direct heating burner, it is possible to control the average oxidation rate more optimally.

(Function) The present invention has the above-mentioned configuration. By rapid oxidation, only Fe oxides are generated without the oxides of P, which is an easily oxidizable element, being concentrated on the surface layer of the steel sheet.
F is easily reduced by the reduction zone and inhibits alloying properties.
e There is no oxide film, and the appearance after alloying hot-dip galvanizing can be made more uniform.

(Example) Example 1 High tension 60kg1d Krasnohyten 60 (C0,10
x.

Si0.6%, Mn 1.10%, PO,04%
, A Qo, 03%), oxidation zone combustion air ratio 1.05,
After heating and oxidizing to 500°C at an average oxidation rate of 400 persons/6 sec, the reduction zone is reduced with a hydrogen concentration of 10%, and then passed through a molten zinc bath (0.15% AQ, 450°C), and the amount of plating is reduced by air wiping. After adjusting the amount to 180 g/rrf, alloying treatment was performed. The alloying properties of the alloyed hot-dip galvanized steel sheets were investigated.

Alloying properties were evaluated using cross-sectional photographs of the plated parts.

As a result, as shown in Figure 2, the average oxidation rate was 20
When compared with those conducted under the same conditions of °C/see, according to the evaluation shown in Table 1, some alloy unevenness or alloy unevenness was observed in many cases. In contrast, according to the present invention, the appearance was uniform and no alloy unevenness was observed.

Table 1 Example 2 High tensile strength 80 kg/cJ Hi-Ten 80 (CO, 15%,
Si 1.0%, Mn 1.5%, PO, 10%
, AQo, 02%) at an oxidation zone combustion air ratio of 1.05. After heating and oxidizing to 500°C at an average oxidation rate of 500 people/12 seconds, the reduction zone was reduced with a hydrogen concentration of 5%, and then passed through a molten zinc bath (0.10AQ460°C) and air wiped to reduce the plating amount to 180 g/ After adjusting to al, alloying treatment was performed. The alloying properties of the alloyed hot-dip galvanized steel sheets were investigated. As a result, the alloying of the plating shows no alloying unevenness at all as seen from the cross-sectional photograph.

The result was 4 points.

(Effects of the Invention) As described above, by performing rapid heating and oxidation at an average oxidation rate according to the present invention, even P-containing high-strength steel sheets can be coated without unnecessarily changing the alloying hot-dip galvanizing conditions. , it is possible to obtain the same alloying properties as ordinary steel, and to efficiently obtain the performance required of an alloyed hot-dip galvanized steel sheet with stable grade and uniform appearance without causing uneven alloy plating. .

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between the thickness of the oxide film formed by the present invention and the conventional method and time, and Figure 2 is a diagram showing the relationship between the average oxidation rate in the oxidation zone excluding the preheating zone and evaluation of alloying characteristics. It is. Agent Patent Attorney Akira Shiina

Claims (2)

[Claims] (1) In the method for manufacturing high-tensile alloyed hot-dip galvanized steel sheets with a P concentration in steel of 0.04% or more, the combustion air ratio in the oxidation zone is 0.
．． 95 to 1.10 and the average oxidation rate in the oxidation zone excluding the preheating zone is 30 Å/sec or more, followed by annealing in an atmosphere with a hydrogen concentration of 10% or less in the reduction zone, followed by hot-dip plating. A method for producing a P-containing high-tensile alloyed hot-dip galvanized steel sheet. (2) The oxidation zone is rapidly oxidized using a burner that injects flame perpendicularly to the steel plate, and then the steel plate is annealed in an atmosphere with a hydrogen concentration of 10% or less, and hot-dip plated. The method for producing the P-containing high tensile strength alloyed galvanized steel sheet.