JPH0931598A - Cold rolled steel sheet excellent in ductility and corrosion resistance - Google Patents

Abstract

(57) Abstract: [Problem] Having good ductility and corrosion resistance by making MnS, which is a precipitate in a cold-rolled steel sheet in which C, Si, Mn, P, S, Al, and N are specified, a specific size. Can be made. SOLUTION: C: 0.0060% or less by weight ratio, S
i: 0.04% or less, Mn: 0.25% or less, P: 0.
025% or less, S: 0.010% or less, Al: 0.01
0 to 0.10%, N: 0.0060% or less, the balance consisting of Fe and unavoidable impurities, and the size of MnS which is a precipitate in the steel should not exceed 0.15 μm diameter. A cold-rolled steel sheet having excellent ductility and corrosion resistance.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a cold rolled steel sheet having excellent ductility and corrosion resistance.

[0002]

2. Description of the Related Art There are various plating methods for improving the corrosion resistance of cold-rolled steel sheets for processing. However, this causes a cost increase and is expensive. In recent years, there has been a demand for a steel sheet that is inexpensive and has corrosion resistance without plating, which would increase the price.

Steel sheets developed from such a viewpoint include:
For example, as disclosed in JP-A-5-140654, there is a cold-rolled steel sheet containing a large amount of P, Cu, Cr, Mo, Ni and the like to have corrosion resistance. However, in this technique, Cu and Cr, Ni, Mo or the like is essential as an additional element. Addition of such an element raises the manufacturing cost, and further, the elongation of the steel sheet is likely to be lowered due to the increase in strength, and there is a problem that it is difficult to obtain good workability. It is possible to reduce the amount of elements such as C and Mn in order to secure the ductility of the steel sheet, but this also causes an increase in manufacturing cost. Further, a steel sheet containing Cu has a problem that cracks are likely to occur during hot rolling, and it is difficult to obtain high surface quality.

As described above, a cold rolled steel sheet containing no special element and having improved corrosion resistance without increasing the manufacturing cost has not yet been found.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a cold rolled steel sheet having high ductility and high corrosion resistance.

[0006]

Means for Solving the Problems The inventors of the present invention have conducted various studies on the composition and structure of the steel sheet, the factors affecting the generation and progress of rust, etc., and as a result, have found that a cold rolled steel sheet excellent in ductility and corrosion resistance is obtained. We have found that it can be supplied easily and economically.

In order to perform processing such as pressing, it is necessary to increase the ductility of the steel sheet, and in order to have corrosion resistance as well, the amounts and structures of various elements contained in the steel are examined, and the present invention is realized. It came to completion.

That is, the gist of the present invention is to
By weight ratio, C: 0.0060% or less, Si: 0.04
% Or less, Mn: 0.25% or less, P: 0.025% or less, S: 0.010% or less, Al: 0.010 to 0.1
0%, N: 0.0060% or less, the balance consists of Fe and unavoidable impurities, and the size of MnS which is a precipitate in steel is a size not exceeding 0.15 μm diameter. The cold-rolled steel sheet has excellent ductility and corrosion resistance.

Hereinafter, the present invention will be described in detail.

The higher the C content, the lower the ductility. Therefore, the smaller the amount of C, the better the press workability, and the upper limit was made 0.0060%.

Although a small amount of Si causes no problem, if the content of Si exceeds 0.04%, the ductility decreases. Therefore, Si
The amount should be 0.04% or less.

If Mn exceeds 0.25%, the ductility of the steel sheet deteriorates. Therefore, the Mn content is limited to 0.25% or less.

P has the effect of improving the strength, but it deteriorates the ductility, and if it is more than 0.025%, the reduction of the ductility becomes large. Therefore, the P amount is limited to 0.025% or less.

S is associated with Mn in the cold-rolled steel sheet and Mn
It exists as a precipitate called S. The present inventors have found that the size of MnS in the cold rolled steel sheet has a great influence on the corrosion resistance of the steel sheet. The larger the size of MnS in steel, the more likely it is to become the starting point for rust generation, and the corrosion resistance decreases. According to the investigation by the present inventors, it has been found that MnS must have a diameter of 0.15 μm or less in order to improve the corrosion resistance of the steel sheet.

The inventors have also found that even if the diameter of MnS is 0.15 μm or less, the corrosion resistance decreases as the number of MnS increases. Since the amount of MnS increases as the amount of S increases, the upper limit of S must be 0.010% to secure the corrosion resistance. Further, the ductility of the steel sheet decreases as the MnS number increases. In order to secure good ductility, S must be 0.010% or less.

FIG. 1 is a view of hot rolling a steel having the chemical composition shown in Table 1.
It shows the effect of the amount of soot S and the maximum size of MnS on the ductility and corrosion resistance of a cold rolled steel sheet by the steps of cold rolling and continuous annealing. As is clear from FIG. 1, when the S content is 0.010% or less, the ductility becomes good, and the S content is 0.01
It can be seen that when the MnS size is 0% or less and the MnS size is 0.15 μm or less, the maximum pitting depth becomes small and the corrosion resistance becomes good. Further, in order to secure sufficient corrosion resistance and ductility, the size of MnS is preferably 0.15 μm or less, and the amount of S is preferably 0.006% or less.

The evaluation of the corrosion resistance of the steel sheet was conducted by leaving the steel sheet in a moist environment (85% humidity, 50 ° C. × 15.5 h).
r) → drying (70 ° C. × 3 hr) → salt water immersion (50 ° C. × 2)
hr) -Left indoors (2hr) -Salt water spray (1.5h)
The cycle consisting of r)] was repeatedly applied 60 times, and the maximum pitting depth after the corrosion acceleration test was carried out. Mn
The size of S was measured by extracting a precipitate from the cold rolled steel sheet and observing it with a transmission electron microscope. Also,
It was found that similar results were obtained when the composition of the steel was changed within the scope of the present invention.

[0018]

[Table 1]

Al is an element necessary for reducing the solid solution N in steel, and at least 0.010% is necessary for precipitating N in the steel sheet as AlN. However, 0.
If it exceeds 10%, the workability is deteriorated. Therefore, the amount of Al was limited to 0.010 to 0.10%.

Further, if a large amount of precipitated AlN deteriorates the workability, it is preferable that the amount thereof be small. Therefore, the amount of N must be 0.0060% or less.

As described above, according to the present invention, the ductility and the corrosion resistance can be made excellent by making the composition of the cold rolled steel sheet and the size of MnS which is a precipitate in the steel to be specific.

Examples of the present invention will be shown below together with comparative examples.

[0023]

EXAMPLE A slab having the composition shown in Table 2 was made into a cold rolled steel sheet by hot rolling, cold rolling and continuous annealing steps. Samples 1-10
Both the composition and the MnS size are within the scope of the present invention, and in Samples 11 to 22, any one or more of the composition and the MnS size are out of the scope of the present invention. Note that Mn
The size of S was variously changed depending on the heating temperature of the slab.

The ductility of the obtained cold rolled steel sheet was measured. Furthermore, in order to evaluate the corrosion resistance in normal atmosphere, the steel sheet was left in a wet environment (humidity 85%, 5%
Corrosion acceleration test in which a cycle consisting of 0 ° C. × 15.5 hr) → drying (70 ° C. × 3 hr) → salt water immersion (50 ° C. × 2 hr) → indoor (2 hr) → salt water spray (1.5 hr) is repeated 60 times. I went. The corrosion resistance was determined by the maximum pitting depth of the steel sheet after this accelerated test.

Table 3 shows the above test results. As can be seen from Tables 2 and 3, it is clear that chemical components and MnS sizes within the scope of the present invention are required to obtain good ductility and corrosion resistance.

[0026]

[Table 2]

[0027]

[Table 3]

[0028]

As described above, the composition of steel and M
The cold-rolled steel sheet of the present invention in which the size of nS is within the specific range has good ductility and corrosion resistance.

[Brief description of drawings]

FIG. 1 is a diagram showing the influence of S amount and MnS size on ductility and corrosion resistance.

Claims (1)

[Claims] 1. By weight ratio, C: 0.0060% or less, Si: 0.04% or less, Mn: 0.25% or less, P: 0.025% or less, S: 0.010% or less, Al: 0.010 to 0.10%, N: 0.0060% or less, the balance consisting of Fe and unavoidable impurities,
Furthermore, the size of MnS, which is a precipitate in the steel, is 0.15μ.
A cold-rolled steel sheet having excellent ductility and corrosion resistance, characterized by having a size not exceeding m diameter.