JPH0797616A - Production of cold rolled steel sheet excellent in chemical convertibility - Google Patents

Abstract

PURPOSE:To produce a cold rolled steel sheet excellent in chemical convertibility. CONSTITUTION:At the time of subjecting a steel sheet after cold rolling to batch annealing, the atmospheric gas of the batch annealing is constituted of a reducing mixed gas of >=80% hydrogen concn. and <=100ppm oxygen concn. The hatched part of >=80% hydrogen concn. and <=100ppm oxygen concn. is a zone in which the color developing ratio is regulated to <=3%, and high chemical convertibility is shown. Thus, the good cold rolled steel sheet excellent in chemical convertibility can be obtd. without being influenced by the chemical conversion method.

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 cold rolled steel sheet having excellent chemical conversion treatability.

[0002]

2. Description of the Related Art When cold-rolled steel sheets are batch-annealed to produce cold-rolled steel sheets, atmospheric gases such as N ₂ , AX (ammonia decomposition gas), H ₂ and Ar are unavoidable. In order to suppress the oxide film that is generated spontaneously, the atmosphere gas usually uses a non-oxidizing protective gas in which hydrogen and nitrogen are mixed. Usually, as the protective gas, a mixed gas of 10% or less of hydrogen and the balance of nitrogen is used.

[0003]

However, the cold-rolled steel sheet manufactured as described above is subjected to chemical conversion treatment by a user by a chemical conversion treatment method such as a dip type or a spray type, and then a treatment such as painting is performed. There is. In this case, there may be a large difference in the chemical conversion treatment film due to the difference in the chemical conversion treatment method and the like, which has a problem of affecting the processing such as coating in the next step. It is considered that such a difference in the chemical conversion coating is due to the surface properties of the cold rolled steel sheet, and it is strongly desired to improve the surface cleanliness of the cold rolled steel sheet.

The present invention has been made to solve the above problems, and provides a method for producing a cold rolled steel sheet having excellent chemical conversion treatability, which is not affected by differences in chemical conversion treatment methods. The purpose is to

[0005]

Means and Method for Solving the Problems The present inventors have found that the chemical conversion reaction is carried out through a micro local battery on the surface of the steel sheet, and the formation of this local battery is caused by the influence of the oxide film on the surface of the steel sheet. Therefore, various studies have been made to achieve the present invention in view of the possibility that the above phenomenon is suppressed and the chemical conversion treatment is defective.

According to the present invention, when batch-annealing a steel sheet after cold rolling, the atmosphere gas for the batch annealing is set to a hydrogen concentration of 8
A method for producing a cold-rolled steel sheet having excellent chemical conversion treatability, which is characterized in that a mixed gas of 0% or more and an oxygen concentration of 100 ppm or less is used.

Here, the hydrogen concentration of the atmospheric gas in the batch annealing is set to 80% or more according to the experimental result described later, but if the hydrogen concentration is less than 80%, the oxide film may not be sufficiently reduced. Therefore, when the chemical conversion treatment method is different, a stable chemical conversion treatment film cannot be obtained.

Further, the oxygen concentration of the atmosphere gas is 100 pp.
The reason why m is set to m or less is based on the experimental result described later, but when the oxygen concentration exceeds 100 ppm, the oxide film may not be sufficiently reduced even if the hydrogen concentration is set to 80% or more. When the chemical conversion treatment method is different, a stable chemical conversion treatment film cannot be obtained. The batch annealing includes single stack, multi-stack (box) annealing, and coiled continuous annealing.

[0009]

EXAMPLES Examples of the present invention will be described in detail below. The steels of the three kinds of components A, B, and C shown in Table 1 were formed into slabs by continuous casting after tapping, and heated to a heating temperature of 1250 ° C., a finishing temperature of 870 ° C., and a winding temperature of 530 ° C. to a plate thickness of 3.4 mm. Rolled, pickled and cold rolled to a plate thickness of 0.8 mm and washed,
After batch (box) annealing at 650 to 800 ° C., tempered steel sheet having an elongation of 1.0% was used as a test piece.

[0010]

[Table 1]

In this step, the hydrogen concentration during batch annealing was changed from 0 to 100% depending on the mixing ratio of hydrogen gas and nitrogen gas. The oxygen concentration was changed to 10-200 ppm. The test piece was subjected to a phosphate treatment (two kinds of dipping method and spraying method), and then a ferro test was performed.

The ferrotest is a method for detecting defects in a phosphate-treated film. Filter paper dipped in a potassium ferrocyanide solution is applied to a chemical conversion-treated steel sheet and eluted from the phosphate-treated portion. By reacting with iron ions, a blue color is produced, and if the color development ratio is 3% or less, the chemical conversion treatment is considered to be good. Although a significant difference was observed for both the dip method and the spray method, the spray method had a higher coloring rate than the dip method, and a significant difference appeared.

Therefore, FIG. 1 shows the relationship between hydrogen concentration and chemical conversion treatability (coloring ratio) by the spray method. in this case,
The oxygen concentration was 100 ppm. O indicates A steel, Δ indicates B steel, and □ indicates C steel. As is clear from FIG. 1, in the region where the hydrogen concentration in the atmosphere gas is 80% or more, the color forming ratio is 3% or less, which shows good chemical conversion treatability.

In FIG. 1, for steel A having the worst chemical conversion treatability, the hydrogen concentration and oxygen concentration in the atmosphere were changed to
The results of examining the coloring ratio are shown in FIG. ● indicates a coloring ratio of 3% or less, ▲ indicates more than 3% to less than 4%, and ■ indicates 4% or more. As is clear from FIG. 2, the shaded area with a hydrogen concentration of 80% or more and an oxygen concentration of 100 ppm or less has a coloring ratio of 3%.
The following is a region showing good chemical conversion treatability.

As shown in the examples, the cold-rolled steel sheet produced by the method of the present invention shows good chemical conversion treatability even if it is subsequently subjected to chemical conversion treatment such as a dipping method or a spray method. It was

[0016]

As described above, according to the present invention, it is possible to obtain a cold-rolled steel sheet having excellent chemical conversion treatability, which is not affected by the chemical conversion treatment method.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a hydrogen concentration of an atmospheric gas and a color development ratio in a ferrotest for a spray type phosphate treatment of a cold rolled steel sheet manufactured according to the present invention.

FIG. 2 is a diagram showing a relationship between a hydrogen concentration and an oxygen concentration of an atmosphere gas and a coloring ratio in a ferrotest for a phosphate treatment of a cold rolling steel sheet manufactured by the present invention by a spray method.

Claims (1)

[Claims] 1. When batch-annealing a steel sheet after cold rolling, an atmosphere gas of the batch annealing is a mixed gas having a hydrogen concentration of 80% or more and an oxygen concentration of 100 ppm or less, which is excellent in chemical conversion treatability. Manufacturing method of cold rolled steel sheet.