JP3014529B2 - Manufacturing method of high strength galvanized steel sheet - Google Patents

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 a high-strength hot-dip galvanized steel sheet using a continuous hot-dip galvanizing equipment.

[0002]

2. Description of the Related Art In recent years, the demand for hot-dip galvanized steel sheets has increased.
There is a tendency to diversify applications. In particular, steel sheets used for automobiles are required to have improved mechanical properties such as material strength and workability in order to reduce the body weight. As a method of improving the strength of the material, it is effective to add Si into steel. However, when a steel sheet containing Si is hot-dip galvanized using a continuous hot-dip galvanizing facility, there is a problem in that the Si oxide is concentrated on the steel sheet surface during the annealing process, and the wettability of the plating is significantly reduced. As a conventional technique for solving this problem, for example, Japanese Patent Application Laid-Open No. 55-1228
According to JP-A-65-65, a method is known in which a thick oxide film is formed on a steel surface in a non-oxidizing furnace, followed by annealing in an atmosphere containing hydrogen and plating. According to this method, the surface concentration of Si can be suppressed by the oxide film of iron, and the generation of the Si oxide that inhibits the plating property can be suppressed, so that the plating adhesion can be improved. However, in the prior art, there was a problem that the reduction reaction of the iron oxide film could not be properly controlled.

[0003]

In the method for producing a high-strength hot-dip galvanized steel sheet, the prior art involves forming a thick Fe oxide film on the surface of a base steel sheet during an annealing process and then reducing the Fe oxide film immediately before immersion in a plating bath. To end. This is because F
It utilizes the fact that the surface concentration of the Si oxide can be suppressed while the e-oxide film is present, thereby suppressing the concentration of the surface of the Si oxide and improving the plating property. But,
Since there is no means for controlling the reduction reaction of the Fe oxide film, it is difficult to properly control the Fe oxide film, and when the reduction is completed early, the surface of the Si oxide is concentrated, and when the reduction is not completed, The Fe oxide film remained, and as a result, it was impossible to stably produce a high-strength hot-dip galvanized steel sheet without causing poor plating properties in actual operation. In addition, since a thick Fe oxide film is formed on the surface of the steel sheet in the non-oxidizing furnace, there is a problem that the steel sheet is picked up by a roll in the annealing furnace and scratches the surface of the steel sheet. As a result of intensive studies on such a problem, the following solution has been found and will be described.

[0004]

The gist of the present invention is as follows. Using continuous hot-dip galvanizing equipment without an oxidation-free furnace , weight% (hereinafter the same applies to chemical components)
In the method for producing a high-strength hot-dip galvanized steel sheet having a Si concentration in steel of 0.2 to 1.2% by using the method, the reduction furnace is divided into two or more zones and the dew point of the atmosphere in each zone is adjusted to oxidize the steel sheet surface Control coating thickness , then hot-dip galvanizing
A process for producing a high-strength galvanized steel sheet characterized by performing certain.

[0005]

The present invention will be described below in detail. In a high-strength hot-dip galvanized steel sheet, the addition level of Si in the steel to be subjected to the method of the present invention is 0.2 to 1.2%. If it is 0.2% or less, the effect of improving the mechanical strength of the steel sheet cannot be obtained.
If it exceeds 1.2%, the effect of improving the plating property cannot be sufficiently obtained even when the method of the present invention is applied. The reduction furnace is divided into two or more zones, and the thickness of the oxide film on the steel sheet surface is controlled by adjusting the dew point of each zone atmosphere. The reason is that the presence of a proper Fe oxide film on the steel sheet surface in the annealing furnace suppresses the formation of Si oxide and prevents poor plating wettability. As the composition of the atmosphere, an N ₂ gas containing H ₂ in a range of 1 to 70% is used. The dew point is controlled by introducing steam into the furnace. In the first zone of the reduction furnace, the dew point is adjusted to 0 ° C or higher. If the temperature is 0 ° C or less, sufficient F
e The oxide film thickness cannot be obtained. The thickness of the Fe oxide film on the surface of the steel sheet is not particularly limited, but is preferably formed at 100A to 1000A. If it is less than 100 A, the effect of suppressing the surface concentration of the Si oxide will be insufficient. If it exceeds 1000 A, there will be problems such as the Fe oxide film being picked up on the roll in the annealing furnace.

The dew point of the first zone is adjusted so that the thickness of the oxide film on the surface of the steel sheet is measured on the exit side of the first zone so that an appropriate Fe oxide film thickness is obtained. If the thickness of the oxide film on the surface of the steel sheet cannot be measured, the thickness of the Fe oxide film is calculated by simulation based on data such as the temperature of the steel sheet in the first zone, the duration of the steel sheet in the furnace, the atmosphere, and the Si concentration in the steel. The dew point may be adjusted so that The function of adjusting the dew point of the atmosphere in the second and subsequent zones is provided for appropriately controlling the reduction reaction of the Fe oxide film. The thickness of the Fe oxide film on the exit side of the second zone is not particularly limited, but is usually preferably 200 A or less. If it exceeds 200 A, the plating properties will be reduced. The dew point is adjusted to be appropriate by measuring the oxide film thickness on the steel sheet surface on the exit side of the second zone. If the oxide film thickness on the steel sheet surface cannot be measured, it may be calculated and adjusted by simulation in the same manner as in the first zone.

[0007]

EXAMPLE A high-strength hot-dip galvanized steel sheet having various Si concentrations in steel was manufactured using a continuous hot-dip galvanizing equipment having no oxidation-free furnace. At this time, the atmosphere of the reduction furnace was H ₂
Was adjusted dew point by changing the blowing amount of steam to various every zone 10% -N _2, sheet temperature in the first zone outlet side 6
It adjusted so that it might be 50-700 degreeC. The plating properties were evaluated according to the following criteria. Table 1 shows a list of manufacturing conditions and results.

<Plating Property Evaluation Criteria> Plating property was evaluated according to the following criteria. ○: No plating failure △: Pinhole-shaped plating failure ×: Plating failure of 1 mm or more in diameter

Examples 1 to 4 of the present invention in Table 1 show that the Fe oxide film thickness was appropriately controlled by adjusting the dew points of the first and second zones of the reduction furnace for materials having various Si concentrations in steel. All have good plating properties. On the other hand, when the comparative example 1 was manufactured by the current normal process, the comparative example 2
In each of the cases, the Fe oxide film was positively formed in the first zone, but the appropriate control of the oxide film was not performed in the second zone. In Comparative Example 3, although the thickness of the oxide film was controlled and the plating property was ensured, the problem of pickup to the roll in the furnace occurred because the Fe oxide film thickness on the exit side of the first zone was too thick.

[0010]

[Table 1]

[0011]

As described above, by applying the method of the present invention, a high-strength hot-dip galvanized steel sheet having a Si concentration of 0.2 to 1.2% in steel can be stably manufactured, and industrially It has a great effect.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) C23C 2/00-2/40

Claims (1)

(57) [Claims] 1. Using a continuous hot-dip galvanizing equipment without an oxidation-free furnace, the Si concentration in steel is 0.2 to 1.2 % by weight.
% Of high-strength hot-dip galvanized steel sheet, the reduction furnace is divided into two or more zones, and the dew point of the atmosphere in each zone is adjusted to control the oxide film thickness on the steel sheet surface
And then hot-dip galvanizing . A method for producing a high-strength hot-dip galvanized steel sheet.