JPS5849619B2 - Method for manufacturing high-strength cold-rolled steel sheet with excellent chemical conversion treatment properties - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing high-strength cold-rolled steel sheets with excellent chemical conversion treatability.

In recent years, high tensile strength cold-rolled steel sheets suitable for automobiles have been desired for the purpose of reducing the weight and improving safety of automobile bodies.

High-strength cold-rolled steel sheets contain a large amount of various alloying elements added to increase the strength of the steel, so in the case of box annealing in the steel sheet annealing process, Mn, Si, and other added alloying elements are added to the surface of the steel sheet. Uniform coloration is observed due to selective oxidation (oxidation that occurs because the affinity for oxygen is stronger than that of Fe).

In addition, in the case of conventional continuous annealing in a reducing atmosphere, coloring is unlikely to occur because the annealing time is short, but due to selective surface concentration of Mn, Si, etc. caused by heating at temperatures of 500°C or higher, the surface Reactivity deteriorates.

Therefore, when a steel plate is subjected to chemical conversion treatment, the development of good chemical crystals on the surface of the steel plate is hindered, and a good coating film cannot be obtained in subsequent painting, resulting in insufficient corrosion resistance.

In order to eliminate the above-mentioned drawbacks, the present inventors have attempted to solve the above-described Mn, Si
As a result of various experimental studies on the prevention of selective surface enrichment, it was found that during continuous annealing, the initial temperature rise was carried out in an oxidizing atmosphere to uniformly distribute Fe and various additional alloying elements contained in the steel sheet. By oxidizing the steel sheet to form an oxide film on the surface of the steel sheet, and then heating it to the recrystallization temperature in a reducing atmosphere and cooling it, the selective surface concentration of Mn, Si, etc. mentioned above can be suppressed and the steel sheet can be formed. We succeeded in obtaining a high-strength cold-rolled steel sheet with good surface properties and excellent chemical treatment properties that do not inhibit the development of chemical crystals during chemical conversion treatment.

That is, the present invention involves continuous annealing of high-strength cold-rolled steel sheets in which the temperature of the steel sheet exceeds 400°C and reaches 650°C or less in an oxidizing atmosphere to form an oxide film on the surface of the steel sheet, and then the heating and cooling are reduced. The gist is that it takes place in a sexual atmosphere.

In the present invention, the heating temperature in an oxidizing atmosphere is 400°C.
The temperature range exceeding 650℃ is limited to 400℃.
℃ or below, the oxidation rate of Fe and various alloy elements in the steel sheet is slow, and it takes a long time to oxidize these elements uniformly, and the effect of speeding up annealing by continuous annealing is lost. If the temperature exceeds 650°C, the oxidation rate increases, the oxide film becomes too thick, and the oxide film remains or peels off during subsequent annealing in a reducing atmosphere, which is undesirable since the surface quality deteriorates.

Regarding the temperature rise time in the oxidizing atmosphere, if the temperature rise is too fast, an oxide film of sufficient thickness will not be formed, and conversely, if the temperature rise rate is too slow, the oxidation time will be long. Since the effect of continuous annealing is lost, the annealing time is preferably 5 seconds to 10 minutes.

A large number of thicknesses of 0.5 mm were manufactured from steels with the compositions shown in Table 1. 8
URN's high-tensile cold-rolled steel sheet was used as the test steel sheet, and the initial temperature was raised in an oxidizing atmosphere in an experimental tubular heating furnace, and the heating temperature for the initial temperature rise and the temperature rise until reaching this heating temperature were measured. After heating at various temperature rates, the inside of the furnace was heated to n24o.
A continuous annealing test was conducted in which the temperature was raised to 8000C, soaked at 800C for 2 minutes, and then allowed to cool.

A commercially available degreasing agent, Ridrin 16 (manufactured by Nippon Paint), was sprayed onto each test steel plate thus obtained for 2 minutes, followed by degreasing and washing with water.A commercially available zinc phosphate chemical conversion treatment solution, Granozin 1
6 (manufactured by Nippon Paint) for 2 minutes, then rinse with water.
Chemical conversion treatment was performed by a drying method, and chemical treatment properties were evaluated by observing chemical crystals formed on the surface of the steel sheet using a light microscope.

The results are shown in FIG.

Figure 1 shows the heating temperature in an oxidizing atmosphere on the vertical axis, and the heating time until the heating temperature is reached on the horizontal axis, and the chemical crystals formed on the surface of the steel sheet, the heating temperature, and the temperature rise. This is a diagram showing the relationship with time. In the diagram, ○ marks indicate that the chemical crystals are fine and good, Δ marks indicate that the chemical crystals are dog-like and slightly defective, and × marks indicate that the chemical crystals are coarse and defective. be.

As shown in Figure 1, heating in an oxidizing atmosphere was performed at 400°C.
It has been found that a high tensile strength cold rolled steel sheet with good chemical treatment properties can be obtained by setting the temperature range above 650'C or less and setting the heating time to reach the heating temperature from 5 seconds to 10 minutes. Ru.

Next, the effects of the present invention will be explained with reference to Examples.

Table 2 shows the main precepts of high-tensile cold-rolled test steel sheets (width 914 mm x thickness 0.8 mm) A, B, and C used in the examples.

The heat pattern of the test steel sheets A, B, and C in Table 2 is shown by the characteristic curve P in Fig. 2, that is, the heating rate is 10'C.
/Sec. In an annealing condition cycle with a soaking temperature of 800'C, a cooling rate of 10'C/sec, and a cooling rate of 10'C/sec, the temperature was increased to 500°C in an oxidizing atmosphere (in the air), and then The method of the present invention performs heating and cooling in a mixed gas atmosphere of 40φH2 residual N2, and the entire annealing process is performed at 40mm
Continuous annealing was carried out in two ways: the conventional method was carried out in a mixed gas atmosphere of φH2 and N2.

Each of the test steel plates A, B, and C obtained by the above continuous annealing was sprayed with Ridrin 16 (manufactured by Nippon Paint) for 2 minutes, then rinsed with water, and then sprayed with Granodine 16 (manufactured by Nippon Paint) for 2 minutes. The chemical conversion treatment was strengthened by washing and drying after spraying, and the chemical conversion treatment property was evaluated by observing the chemical crystals formed on the surface of the steel sheet using a light microscope. 1000(
A corrosion resistance test was performed by electrodeposition coating (manufactured by Shinto Paint Co., Ltd.), washing with water, and baking at 170°C for 20 minutes.

Table 3 shows the test results of chemical treatment properties and corrosion resistance after electrodeposition coating of each sample steel sheet.

In the above table, for chemical conversion treatment properties, the ○ mark indicates that the chemical crystals are fine and good, the Δ mark indicates that the chemical crystals are small and slightly poor, and the x mark indicates that the chemical crystals are coarse and defective, and the corrosion resistance is as specified by JIS. Based on the salt water spray test method (JIS Z2371), evaluation was made based on the size of the blistering width of scratches (crosscuts) formed on the coating film after salt water spraying for <500 hours.

As shown in Table 3, as the amount of Si in the test steel sheet increases, the chemical crystals become larger in the conventional method, deteriorating the chemical conversion treatment properties, and the width of cross-cut blisters in the salt spray test also becomes larger, resulting in poor corrosion resistance. shows unsatisfactory performance, but
The products produced by the method of the present invention had good chemical treatment properties, had small cross-cut blisters, and had good corrosion resistance.

[Brief explanation of drawings]

Figure 1 is a chart showing the relationship between the chemical crystals formed on the surface of the steel sheet, the heating temperature in an oxidizing atmosphere, and the heating time to reach the heating temperature, and Figure 2 shows the relationship between the heat pattern of continuous annealing. 2 is a chart showing a characteristic curve of steel plate temperature. P: % sex curve.

Claims (1)

[Claims] 1. In continuous annealing of high-strength cold-rolled steel sheets, the temperature of the steel sheet is raised to over 400°C and below 650°C in an oxidizing atmosphere to form an oxide film on the surface of the steel sheet, and then to the recrystallization temperature in a reducing atmosphere. A method for producing high-strength cold-rolled steel sheets with excellent chemical conversion treatment properties, which involves heating and cooling.