JPH05306446A - Manufacture of high strength galvannealed steel sheet - Google Patents

Abstract

PURPOSE:To stably manufacture a high strength galvannealed steel sheet with good productivity at the time of plating a steel sheet contg. P having specified concn. by continuous galvanizing device with an oxidation free furnace by controlling the combustion air ratio of the oxidation free furnace and the dew point of the atmosphere of a reduction furnace. CONSTITUTION:By continuous galvanizing device having the oxidation free furnace, a steel sheet having >=0.03% P content is plated to manufacture a high strength galvannealed steel sheet. At this time, the combustion air ratio of the oxidation free furnace is controlled to about 0.9 to 1.2 to regulate the thickness of an Fe oxide film on the surface of the steel sheet to 100 to 1000Angstrom , gaseous N2 contg. about 1 to 70% H2 is used as the atmosphere of a reduction furnace, water vapor is introduced into the furnace to control the dew point and the film thickness of an Fe oxide film on the outlet side of the reduction furnace is regulated to <=200Angstrom . In such a manner, the objective high strength galvannealed steel sheet excellent in alloying reactivity is obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing high strength galvannealed steel sheet using a continuous hot dip galvanizing facility.

[0002]

2. Description of the Related Art In recent years, the demand for alloyed hot-dip galvanized steel sheets has been increasing and diversifying their uses. Among them, steel sheets used for automobiles are required to have improved mechanical properties such as strength and workability of raw materials for weight reduction of vehicle bodies. As a method of improving the strength of the material, it is effective to add P to the steel. However, the steel sheet containing P has a problem that P is concentrated on the surface of the steel sheet during the annealing process during alloying hot dip galvanizing using a continuous hot dip galvanizing facility, which significantly reduces the alloying reaction rate.

As a conventional technique for solving this problem, for example, according to JP-A-55-122865, after forming a thick oxide film on the steel surface in an oxidation-free furnace, annealing is performed in an atmosphere containing hydrogen. Have proposed a method of suppressing the surface concentration of elements such as Si that hinder the plating property. It is possible to apply this method to suppress P concentration. According to this method, the surface concentration of Si or the like can be suppressed by the oxide film of Fe and the generation of Si oxide or the like which hinders the plating property can be suppressed, so that the plating adhesion can be improved. However, the conventional technique has a problem that the reduction reaction of the oxide film cannot be properly controlled.

[0004]

In the method for producing a high-strength galvanized steel sheet, the prior art is to reduce a thick Fe oxide film on the surface of the base steel sheet in the annealing process, and then reduce it immediately before immersion in the plating bath. To end. This is F on the steel plate surface.
The fact that the surface concentration of the Si oxide or the like can be suppressed while the e oxide film is present is utilized, and the surface of the Si oxide or the like is suppressed and the plating property is improved. But F
Since there is no means for controlling the reduction reaction of the e oxide film, it is difficult to properly control the Fe oxide film, and when the reduction is completed early, the Si oxide or the like is concentrated on the surface and is not completed. However, the Fe oxide film remained, and in the end, it was impossible to stably produce a high-strength galvanized steel sheet without causing a poor plating property in actual operation. Further, since a thick Fe oxide film is formed on the surface of the steel sheet in the non-oxidizing furnace, there arises a problem that the surface of the steel sheet is picked up by a roll in the annealing furnace and scratched. Therefore, even if an attempt was made to apply this prior art to the surface concentration suppression of P, which is an element that lowers the alloying reactivity, the above-mentioned problems occurred and could not be applied. As a result of earnestly studying such a problem, the following solution has been found, which will be described.

[0005]

The gist of the present invention is as follows. In a method for producing a high-strength galvannealed steel sheet having a P concentration of 0.03% or more in a continuous hot-dip galvanizing facility having an oxidation-free furnace, a combustion air ratio of the oxidation-free furnace and a dew point of an atmosphere of the reduction furnace Is controlled to control the thickness of the oxide film on the surface of the steel sheet.

[0006]

The present invention will be described in detail below. In the high-strength hot-dip galvanized steel sheet, the addition level of P in the steel which is the subject of the method of the present invention is 0.03% or more. If it is less than 0.03%, the effect of improving the mechanical strength of the steel sheet cannot be obtained. The upper limit is not particularly specified, but if it exceeds 0.1%, a phenomenon such as grain boundary of the base steel sheet is caused, and mechanical properties are likely to be deteriorated. Therefore, the range is usually 0.03% or more and 0.1% or less.

The Fe oxide film thickness on the steel sheet surface is controlled by adjusting the combustion air ratio of the non-oxidizing furnace and the dew point of the atmosphere of the reducing furnace. The reason for this is that the proper F on the steel plate surface in the annealing furnace.
This is because the presence of the oxide film suppresses the surface concentration of P and prevents the alloying reaction rate from decreasing. In the non-oxidizing furnace, the combustion air ratio is usually adjusted within the range of 0.9 to 1.2. If it is 0.9 or less, a sufficient Fe oxide film thickness cannot be obtained, and if it exceeds 1.2, the burning efficiency is lowered, which is not preferable. The thickness of the Fe oxide film on the surface of the steel sheet is not particularly limited, but is preferably 100A to 1000A. If it is 100 A or less, the effect of suppressing the surface concentration of P is insufficient, and if it exceeds 1000 A, there arises a problem that the Fe oxide film is picked up by the roll in the annealing furnace.

The combustion air ratio is adjusted so as to obtain an appropriate Fe oxide film thickness by measuring the oxide film thickness on the surface of the steel sheet on the outlet side of the non-oxidizing furnace. If the oxide film thickness on the steel sheet surface cannot be measured,
The Fe oxide film thickness may be calculated by simulation from data such as the plate temperature in the non-oxidizing furnace, the in-furnace time of the steel plate, the atmosphere, and the components in the steel, and the burning air ratio may be adjusted to be appropriate. The function of adjusting the dew point of the atmosphere is provided in the reduction furnace in order to properly control the reduction reaction of the Fe oxide film.
As the composition of the atmosphere, N ₂ gas containing H ₂ in the range of 1 to 70% is used. The dew point is operated by introducing steam into the furnace. The Fe oxide film thickness on the outlet side of the reduction furnace is not particularly limited, but usually 200 A or less is preferable. When it exceeds 200 A, the plating property is deteriorated. The dew point is adjusted to be appropriate by measuring the oxide film thickness on the surface of the steel sheet on the exit side of the reduction furnace. If the oxide film thickness on the surface of the steel sheet cannot be measured, the Fe oxide film thickness may be calculated and adjusted by simulation as in the case of the non-oxidizing furnace.

[0009]

[Examples] High-strength galvannealed steel sheets with various P concentrations in steel were produced using a continuous hot-dip galvanizing facility having an oxidation-free furnace. At that time, the deoxidizing point was adjusted variously by blowing steam into the non-oxidizing furnace with a direct-fired heating system, the outlet plate temperature of 650 to 700 ° C., the reducing furnace with a radiant tube heating system, and an atmosphere of H ₂ 10% -N ₂ . After the dip plating in the hot dip bath, the steel sheet was heated and alloyed. At that time, the alloying reactivity was evaluated by Fe% in the plating layer. The coating amount was 35 g / m ² .

Examples 1 to 3 of the present invention in Table 1 are those in which the Fe oxide film thickness is properly controlled by adjusting the combustion air ratio of the non-oxidizing furnace and the dew point of the reducing furnace for materials having different P concentrations in steel. In each case, the alloying reactivity is good. On the other hand, Comparative Example 1 was manufactured by the current normal process, and since the dew point of the reducing furnace was not adjusted, P was concentrated in the surface layer in the reducing furnace and the alloying reactivity became poor.

[0011]

[Table 1]

[0012]

As described above, by applying the method of the present invention, a high strength galvannealed steel sheet having a P concentration in the steel of 0.03% or more can be stably produced without lowering the productivity. It is possible to produce a great effect industrially.

Claims (1)

[Claims] 1. A method of producing a high-strength galvannealed steel sheet having a P concentration of 0.03% or more in a continuous hot-dip galvanizing facility having an oxidation-free furnace, wherein the combustion air ratio and reduction of the oxidation-free furnace are used. Method for producing high strength galvannealed steel sheet characterized by controlling oxide film thickness on steel sheet surface by adjusting dew point of furnace atmosphere