JPH01159319A - Production of high-corrosion resistance ferritic stainless steel having excellent moldability - Google Patents

Abstract

PURPOSE:To produce a high-corrosion resistance ferritic stainless steel having excellent moldability by subjecting a high-Cr ferritic stainless steel contg. specific ratios of Nb and Ti, Zr to hot rolling, then coiling the steel sheet a a low temp. and further subjecting the same to base-sheet annealing at a specific temp. CONSTITUTION:The high-Cr ferritic stainless steel ingot contg., by weight %, 0.015-0. 03% C, 0.1-1.0% Si, <1% Mn, <0.01% S, 20-25% Cr, 0.3-1.0% Mo, 0.2-1.5% Ni, 0.004-0.5% Cu, 0.015-0.03% Ni, 8 (C+N)-20 (C+N)% Nb, and 0.02-0.1% Ti and Zr as total content of Ti+Zr/2 is made into a slab by the hot rolling. This slab is coiled at <=350 deg.C to solutionize C and N in the steel and to suppress the formation of the carbonitride of Cr. The hot rolled steel sheet which is coiled is then subjected to the base-sheet annealing in the [800+1250(Ti+Zr/2)]+ or -25 deg.C temp. region. This slab is cold rolled after picking and is annealed and pickled, by which the cold rolled steel strip of the ferritic stainless steel is produced.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a method for manufacturing ferritic stainless steel used as automobile exterior parts or various decorative materials, and particularly has excellent formability and toughness during manufacturing. The present invention relates to a method for producing highly corrosion-resistant ferritic stainless steel.

<Prior art and its problems> Ferritic stainless steels, such as 5US430, are inexpensive and have excellent stress corrosion cracking resistance, but compared to austenitic stainless steels that contain large amounts of nickel, they are less susceptible to aqueous solutions containing chloride ions. It is considerably inferior in general corrosion resistance, including rust resistance.

In recent years, with the advancement of stainless steel melting technology,
2-30715, ultra-low C%N
It has become possible to produce high Cr ferritic stainless steel, and ferritic stainless steel with significantly improved corrosion resistance has been developed.

However, in high chromium ferritic steel, C and N are Cr
and carbonitrides, resulting in a decrease in corrosion resistance, a decrease in toughness in hot-rolled annealed sheets, and an increase in strength in the final product.
This results in a decrease in r value.

Therefore, the C%N of high chromium ferritic steel is strictly limited to 0.01% or less, which increases manufacturing costs and makes the price higher than that of austenitic stainless steel.

It is desired to develop a high chromium ferritic stainless steel that has excellent corrosion resistance, toughness during manufacture, and workability even when the C%N is 0.601% or more.

<Object of the Invention> The object of the present invention is to solve the problems of the conventional technology described above, and to provide a method for producing ferritic stainless steel that has excellent workability, toughness during production, and corrosion resistance. It is something to do.

<Structure of the invention> The present invention contains C, 0,015 to 0.03% by weight, St; 0.
1-1% by weight, Mn: 1% by weight or less, S, 0.01% by weight or less, Cr: 20-25% by weight, Mo: 0.3-1.
0% by weight, Ni; 0.2-1.5% by weight, Cu;
0.04-0.5% by weight, N; 0.015-0.03% by weight, Nb, 8 (C+N)-20 (C+N)% by weight, Ti and/or Zr (Ti + Zr
A ferritic stainless steel plate containing 0.02 to 0.1% by weight as the total amount of /2), with the remainder consisting of iron and inevitable impurities is hot-rolled, then wound at 350°C or less, and then [ 800+1250x (Ti+Zr
/2)] The present invention provides a method for manufacturing highly corrosion-resistant ferritic stainless steel with excellent formability, which is characterized in that base plate annealing is performed in a temperature range of ±25°C.

“The present invention will be explained in more detail below.

The composition of the ferritic stainless steel used in the present invention is C
; 0.015 to 0.03% by weight, Si; 0.1 to 1% by weight, Mn; 1% by weight or less, S; 0.01% by weight or less, C
r; 20 to 25% by weight, Mo; 0.3 to 1.0% by weight,
Ni: 0.2-1. ．． 5% by weight, Cu; 0.04-0
．． 5% by weight, N; C1,015-0.03% by weight, Nb
; 8 (C+N) ~ 20 (C+N) 1% by weight, T
i and/or Zr is (T i +Z r / 2
);02 to 0.1% by weight, the remainder being iron and unavoidable impurities.

When C exceeds 0.03% by weight, chromium carbide is formed, which deteriorates the corrosion resistance of ferritic stainless steel and the toughness during manufacturing. When C is less than 0.015% by weight, it is expensive to melt.

When Si exceeds 1% by weight, it is harmful to moldability,
When it is less than 0.1% by weight, the deoxidizing effect is not sufficient.

Mn is an element that has desulfurization and deoxidizing effects, but when it exceeds 1% by weight, the corrosion resistance of ferritic stainless steel decreases.

When S exceeds 0.01% by weight, the corrosion resistance of the ferritic stainless steel decreases.

Cr is a central element that determines corrosion resistance, and in order to obtain corrosion resistance equivalent to that of austenitic stainless steel, the content must be 20% by weight or more. When the Cr content exceeds 25% by weight, carbonitrides of Cr are generated at grain boundaries, and the toughness during production of the resulting ferritic stainless steel decreases, making production difficult.

When Mo is less than 0.3% by weight, corrosion resistance deteriorates and 1
．． When it exceeds 0% by weight, Mo is expensive and the manufacturing cost increases.

When Ni is less than 0.2% by weight, corrosion resistance and toughness deteriorate, and when it exceeds 1.5% by weight, Ni is expensive.
Manufacturing costs increase.

When Cu is less than 0.04% by weight, the induction resistance in the atmosphere deteriorates, and when it exceeds 0.5% by weight, surface cracks occur during hot rolling.

When N is 0.03% by weight or more, Cr nitrides are produced and the toughness is reduced during manufacturing.

When it is less than 0.015% by weight, manufacturing costs increase significantly.

Nb has a strong tendency to form carbonitrides and is added to suppress the formation of carbonitrides from Cr, but if the amount of Nb is less than 8 times the total amount of C+H, the effect of suppressing the formation of carbonitrides from Cr is not sufficient. If the amount exceeds 20 times the total amount of C+H, the effect of suppressing carbonitride formation by Cr is saturated, the strength increases significantly, and formability is impaired.

Ti and Zr have a greater tendency to form carbonitrides than Nb;
It is an element that has a great effect of suppressing the carbonitride formation of Cr. When the total amount of T i + Z r /2 is less than 0.02% by weight, the above-mentioned effect of suppressing the carbonitride formation of Cr is insufficient, and the amount of 0.02% by weight is less than 0.02% by weight. When the amount of Ili exceeds %, coarse nitrides of Ti and Zr are formed, and the surface cleanliness is significantly reduced.

Since the total amount of T i + Z r /2 is limited to less than 0% by weight, the combined addition of Nb and Ti and/or Zr is essential to suppress the formation of Cr carbonitrides.

A ferritic stainless steel having the above composition can be produced under the following conditions to obtain a highly corrosion-resistant ferritic stainless steel with excellent formability.

Ferritic stainless steel with the above composition is used as a slab,
After hot rolling, winding is performed at a low temperature of 350°C or less.

By setting the winding temperature to 350°C or less, C and N are present in a solid solution state, suppressing the formation of Cr carbonitrides, and improving the corrosion resistance and toughness required during manufacturing of the resulting ferritic stainless steel. Don't lower it.

After that, the hot-rolled steel sheet that had been wound above was heated to [800+12
By annealing the mother plate in a temperature range of 50X (Ti+Zr/2)]±25°C, a highly corrosion-resistant ferritic stainless steel with excellent formability can be produced.

FIG. 1a shows the relationship between the total amount of T i + Z r /2 and the optimum annealing temperature.

Figure 2 shows No. 2 with different amounts of Ti added shown in Table 1.
．． 3.9 shows the relationship between annealing temperature and absorbed energy by i-impact test for stainless steel having a chemical composition of 3.9.

As can be seen from Figure 2, the dependence of the absorbed energy on the annealing temperature in the impact test shows that even if the annealing temperature is too high or too low, the absorbed energy will be low, and there is a moderate temperature range where the absorbed energy is high. do. The center temperature of this moderate temperature range is called the optimum annealing temperature. The temperature range with high absorbed energy is the range of the optimum annealing temperature ±25°C.

The optimal annealing temperature and the total amount of T i + Z r /2 are:
As shown in Figure 1a, there is a linear relationship, and T layer + Z r
/2 increases, the optimum annealing temperature also increases, and the optimum annealing temperature is expressed as 800+1250 (Ti+Zr/2).

The optimum annealing temperature for ferritic stainless steel is ±25°C.
By annealing the base material, the formation of Cr carbonitrides that adversely affect the toughness during manufacturing is suppressed, and the ferritic stainless steel has corrosion resistance equivalent to that of austenitic stainless steel with good workability. Can be done.

The ferritic stainless steel produced according to the present invention is pickled, cold rolled, and then annealed and pickled (or bright annealed) to be used as a cold rolled steel strip.

<Example> The present invention will be specifically explained using examples.

(Example) Steels having compositions shown in Table 1 with different amounts of Ti and Zr were melted in a vacuum melting furnace to produce slabs with a thickness of 180 mm.

No. 1 to No. 8 are steels of the present invention;
9-No, 13 is T i + Z r /
This is a comparative steel in which the total amount of 2 is outside the range of 0.02 to 0.1% by weight.

The above slab is heated to 1250°C, and the rolling end temperature is 850°C.
It was hot-rolled at ℃ to a plate thickness of 4 mm and coiled at the temperature shown in Table 2.

After that, the hot rolled steel plate was heated to [800+1250x (T
i + Z r/2) ] ±25° C. (annealing temperatures are shown in Table 2), the base material was annealed, held at the base material annealing temperature for 2 minutes, and then rapidly cooled.

A 2 mm V-notched Charpy'a impact test was conducted on the obtained hot rolled steel sheet.

The results are shown in Table 2 and Figure 1b.

The obtained hot rolled steel plate was pickled and then cold rolled to a thickness of 2 mm,
Thereafter, it was annealed under the same conditions as the base metal annealing.

Further, the cold-rolled steel sheet was pickled, cold-rolled to a thickness of 0.4 mm, annealed in the atmosphere at 950° C. for 2 minutes, and then pickled to obtain a final cold-rolled steel sheet.

A tensile test was conducted on the obtained cold-rolled steel sheet, and 0.2%
Yield strength and tensile strength were measured.

The results are shown in Table 2, Figures 3b and 3C.

Furthermore, the Lankford value (r value) of the obtained cold rolled steel sheet
was measured.

The results are shown in Table 2 and Figure 3a.

In the steel of the present invention, the 0.2% yield strength is 32 to 34 kg.
f/mm2 and tensile strength is 48-50 kg
The r value was in the range of f/mm2, and the r value was in the range of 1.9 to 2.1, and the workability was good.

In the comparative steel, when the total amount of T i + Z r /2 was less than 0.02% by weight, the 0.2% proof stress and tensile strength were high, the r value was low, and the workability was deteriorated. Ti
When the total amount of +Zr/2 exceeded 0.1% by weight, the workability was similar to the inventive example, but the surface cleanliness due to inclusions in the steel was deteriorated.

Table 2 also shows pitting potentials measured according to JIS GO577. Pitting corrosion potential of 5US304 0.32 (■
v, 5CE), all of which show higher values.

<Effects of the Invention> According to the method of the present invention, high Cr ferritic stainless steel containing an appropriate amount of Nb, Ti and/or Zr, which has a greater tendency to form carbonitrides than Cr, is hot-rolled and then rolled at a low temperature. Then, by annealing the base material at an appropriate temperature, Nb, Ti, and/or Zr generate carbonitrides, and by suppressing the formation of carbonitrides of Cr, it has corrosion resistance comparable to that of austenitic stainless steel. It is possible to obtain a ferritic stainless steel having excellent toughness during production and good workability.

[Brief explanation of the drawing]

FIG. 1a is a diagram showing the relationship between the total amount of T i + Z r /2 and the optimum annealing temperature. Figure 1b shows the T i + of the hot rolled steel plate that has been annealed in the base material.
It is a figure which shows the relationship between the total amount of Zr/2 and absorbed energy. FIG. 2 is a diagram showing the relationship between base material annealing temperature and absorbed energy of a hot rolled steel sheet subjected to base material annealing. Figure 3a shows T i + Z of the cold rolled steel sheet obtained in the example.
FIG. 2 is a diagram showing the relationship between the total amount of r/2 and the Lankford value (r value). Figure 3b shows T i + Z of the cold rolled steel sheet obtained in the example.
It is a figure showing the relationship between the total amount of r/2 and tensile strength. Figure 3c shows Ti + Z of the cold rolled steel plate obtained in the example.
It is a figure showing the relationship between the total amount of r/2 and 0.2% proof stress. FIG, 1a Ti+Zr/2 (%) FIG, 1b Ti+Zr/2 (%) FIG, 2 Smoking temperature (°C) FIG, 3a Ti+Zr/2 (shi・゛) FIG, 3b 0 0.05 0 .1
0.15Ti+Zr/2 (-/,) FIG, 3c 0 0.05 0.1
0.15Ti+Zr/2 ('10)

Claims (1)

[Claims] (1) C: 0.015 to 0.03% by weight, Si: 0.1 to 1% by weight, Mn: 1% by weight or less, S: 0.01% by weight or less, Cr: 20 to 25% by weight, Mo ; 0.3-1.0% by weight, Ni: 0.2-1.
5% by weight, Cu; 0.04 to 0.5% by weight, N; 0.015 to 0.03% by weight, Nb; 8(C+N) to 20(C+N)% by weight, T
After hot rolling a ferritic stainless steel plate containing 0.02 to 0.1% by weight of i and/or Zr as the total amount of (Ti+Zr/2), with the remainder consisting of iron and unavoidable impurities, the sheet was heated at 350°C. Wind it up below, then [8
00+1250×(Ti+Zr/2)] A method for producing highly corrosion-resistant ferritic stainless steel with excellent formability, characterized by annealing the mother plate in a temperature range of ±25°C.