JPH0625817A - Production of hot-dip galvanized cold rolled steel sheet having high strength and excellent in adhesion of plating film - Google Patents

Abstract

PURPOSE:To produce a hot-dip galvanized cold rolled steel sheet having high strength and excellent in plating adhesion by subjecting a steel having a specific composition consisting of C, Si, Mn, Al, and Fe to hot rolling, to cold rolling, and to specific recrystallization continuous annealing. CONSTITUTION:A steel having a composition consisting of, by weight, 0.05-0.3% C, 0.2-1.5% Si, 0.5-2.0% Mn, 0.005-0.05% sol. Al, and the balance Fe with inevitable impurities is hot-rolled, pickled, and cold-rolled. The resulting steel sheet is heated up to 750-900 deg.C in the atmosphere consisting of 0.1-1% CO, 5-10% H2, and the balance N2 and having <=-30 C dew point to undergo recrystallization continuous annealing. By this annealing decarburization at the surface of the steel sheet is prevented and Si concentration in the part right under the surface is reduced. Then hot-dip galvanizing is applied to the steel sheet. By this method, plating adhesion can be improved and the hot-dip galvanized cold rolled steel sheet having high strength and excellent in adhesion of plating film can be obtained.

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 cold-rolled steel sheet with hot dip galvanizing, which contains Si in an amount of 0.2% or more and has excellent adhesion of the galvanized coating.

[0002]

2. Description of the Related Art In recent years, cold-rolled steel sheets used for automobile members and construction are required to have high strength and corrosion resistance. In order to meet such requirements, the surface of the cold rolled steel sheet is plated with molten zinc, but when the amount of Si added exceeds 0.2%, there is a problem that the plating adhesion deteriorates. Therefore, at present, in a high-strength cold-rolled steel sheet that requires hot-dip galvanizing, strength is improved by adding Mn instead of Si, and the amount of Si is suppressed to avoid the above problems. ing.

High strength steel sheets generally have the required strength by adding Mn and Si. However, in order to attain high strength and high toughness, addition of Si gives M
In a steel sheet which is more advantageous than the addition of n and does not require plating, a large amount of Si is added. However, when plating is required, due to the above problems, since the Si content is suppressed to 0.2% or less and the strength is obtained with Mn, ductility is somewhat problematic and the annealing conditions are complicated. It is said.

[0004]

Therefore, as a result of intensive studies to solve the above problems, the present inventors have found that cold-rolled steel sheets are recrystallized and annealed in a reducing atmosphere containing carbon monoxide. By doing so, it was found that a high-strength cold-rolled steel sheet with hot-dip galvanized steel having excellent adhesion of the plating film can be obtained, and the present invention has been completed.

[0005]

According to the present invention, a method for producing a high strength cold rolled steel sheet with a hot dip galvanized steel having excellent adhesion of a galvanized coating is performed in a weight percentage of C 0.05 to 0.3%, Si Steel containing 0.2-1.5%, Mn 0.5-2.0%, and sol Al 0.005-0.05%, with the balance iron and unavoidable impurities is hot-rolled and acid-treated. After washing and cold rolling, carbon monoxide 0.1-1.
In an atmosphere of 0%, 5 to 10% hydrogen, the balance of nitrogen, and a dew point of -30 ° C or less, heating is performed at a temperature of 750 to 900 ° C, continuous recrystallization annealing is performed, and then hot dipping of molten zinc is performed. It is characterized by applying.

First, the chemical composition of steel used in the method of the present invention will be described. C is an important element that controls the tensile strength of the steel sheet, and in order to obtain a martensitic structure, it is necessary to add at least 0.05%, and it is better to increase C to increase the strength, but Since it becomes difficult to secure ductility, the upper limit of the amount added is 0.3%.

[0007] Si is an essential element for increasing the strength without deteriorating the ductility of the steel, promoting the formation of ferrite in the cooling process after heating at the Ac ₁ point or higher, and obtaining the required material. . However, if the addition amount exceeds 1.5%, the plating adhesion deteriorates, so the upper limit of the addition amount is made 1.5%. If the amount added is less than 0.2%, the required strength cannot be obtained.

[0008] Mn stabilizes the austenite phase and facilitates the formation of low temperature transformation products in the cooling process.
When it is less than 0.5%, the cooling start temperature becomes high,
There is a manufacturing problem. When it exceeds 2.0%, Si
To the austenite phase, the volume ratio of the second phase increases and the concentration of C weakens, so the addition amount is 2.0%.
Below.

Al is added for the purpose of deoxidizing molten steel. When the content of sol Al is less than 0.005%, the purpose is not achieved, while 0.05% is added. When it exceeds, the deoxidizing effect is saturated, and Al ₂ O ₃ inclusions increase to deteriorate the workability. According to the present invention, by regulating the recrystallization annealing temperature and the atmosphere composition, it is possible to manufacture a steel sheet having excellent hot-dip galvanizing adhesion. These conditions will be described in detail below.

A steel sheet having a large amount of Si has poor plating adhesion, because the Si is selectively oxidized on the surface of the steel sheet by annealing and, as a result, a concentrated layer of Si is formed, which impairs the adhesion of molten zinc plating. Has been done. According to the present invention, it is possible to reduce the Si concentration by mixing carbon monoxide in the atmosphere to prevent decarburization that normally occurs in annealing in an atmosphere of hydrogen and nitrogen. As a result, it is possible to obtain a high-strength hot-dip galvanized steel sheet having excellent plating adhesion.

In the present invention, decarburization on the surface of the steel sheet during recrystallization annealing is prevented, and a decrease in the C concentration immediately below the surface of the steel sheet is prevented, thereby suppressing diffusion of Si to the surface and reducing Si concentration. By doing so, the adhesive adhesion is improved, and the conditions of cooling after heating and overaging do not affect the adhesive adhesion at all. However, when alloying the plating layer, it is 10 to 3 at 500 to 600 ° C.
It is desirable to perform the treatment within the range of 0 seconds from the viewpoint of the plating adhesion.

Next, the composition of the atmosphere will be described. Carbon monoxide, by adding it to a conventional atmosphere,
The dew point is lowered, and further decarburization from the steel plate surface can be prevented. However, in the atmosphere, when it is less than 0.1%, its action and effect are not sufficient, while on the other hand, it is 1.0%.
If it exceeds the range, the carburizing property is too strong depending on the steel type, which causes surface contamination.

Further, the dew point of the atmosphere plays an important role with respect to the properties of the atmosphere together with the hydrogen concentration and the carbon monoxide concentration, and by making it within the range specified by the present invention, the composition steel specified by the present invention can be obtained. In contrast, the reducing property and carburizing property of the atmosphere can be maintained.

[0014]

As described above, according to the method of the present invention, after cold rolling a steel containing a predetermined chemical component, particularly 0.2% or more of Si, it contains carbon monoxide and has a predetermined dew point. Obtaining a high-strength cold-rolled steel sheet with hot-dip galvanized steel having excellent adhesion to the plating film by performing hot-dip galvanizing treatment after performing continuous recrystallization annealing in a hydrogen-nitrogen atmosphere. You can

[0015]

The present invention will be described below with reference to examples.
The present invention is not limited to these examples.

Example 1 A steel sheet having the chemical composition shown in Table 1 was finished at a finishing temperature of 800-
Thickness of 2.8mm at 900 ℃ and winding temperature of 500-600 ℃
After hot rolling, pickling, and cold rolling to a thickness of 0.8 mm. Next, after performing recrystallization annealing for 90 seconds in a temperature range of 800 to 850 ° C., forced air cooling is performed at a cooling rate of 15 ° C./second, and a temperature of 600 to 700 ° C. is set to 50 ° C. at 200 ° C./second.
It was cooled to 0 ° C. and immersed in a molten zinc bath having a bath temperature of 470 ° C. The results are shown in Table 2.

Example 2 A steel sheet having the chemical composition of steel type 3 shown in Table 1 was hot-rolled to a thickness of 2.8 mm at a finishing temperature of 800 to 900 ° C. and a winding temperature of 500 to 600 ° C. and pickled. Then, it was cold-rolled to a thickness of 0.8 mm. Next, after heating and holding in a temperature range of 800 to 850 ° C. for 90 seconds, forced air cooling is performed at a cooling rate of 10 ° C./second, and a temperature of 600 to 700 ° C. is set to 100 ° C./second for 50 seconds.
Cool to 0 ° C and immerse in a molten zinc bath with a bath temperature of 470 ° C.
Then, the zinc plating layer was alloyed at 600 ° C. for 10 seconds. The results are shown in Table 3.

In both Example 1 and Example 2, the atmosphere during recrystallization annealing was performed under any of the following five conditions. A: 10% hydrogen-nitrogen, dew point -30 ° C B: 10% hydrogen-0.05% carbon monoxide-nitrogen dew point -3
0 ° C C: 10% hydrogen-0.1% carbon monoxide-nitrogen dew point -30
C D: 10% hydrogen-0.5% carbon monoxide-nitrogen dew point -30
C E: 10% hydrogen-0.8% carbon monoxide-nitrogen dew point -30
℃ adhesiveness is measured by V-bending test, tape peeling the surface,
The amount of peeled tape was visually classified into 3 levels and evaluated.

As shown in Tables 2 and 3, according to the method of the present invention, it is possible to obtain a high strength cold-rolled steel sheet with hot dip galvanizing which is excellent in adhesion of the galling film.

[0020]

[Table 1]

[0021]

[Table 2]

[0022]

[Table 3]

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI technical display location C22C 38/06

Claims (1)

[Claims] 1. By weight%, C 0.05 to 0.3%, Si 0.2 to 1.5%, Mn 0.5 to 2.0%, and sol Al 0.005 to 0.05%. After hot rolling, steel containing the balance iron and unavoidable impurities, pickling and cold rolling, carbon monoxide 0.1 to 1.
In an atmosphere of 0%, 5 to 10% hydrogen, the balance of nitrogen, and a dew point of -30 ° C or less, heating is performed at a temperature of 750 to 900 ° C, continuous recrystallization annealing is performed, and then hot dipping of molten zinc is performed. A method for producing a high-strength cold-rolled steel sheet with hot dip galvanized, which is excellent in the adhesion of the galvanized coating.