JP2002012955A - Ferritic stainless steel superior in workability and corrosion resistivity with little surface deffects - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ferritic stainless steel superior in workability and corrosion resistivity with little surface defects. SOLUTION: This ferritic stainless steel has a composition which consists of, by mass, C: 0.001-0.020%, Si: 0.01-0.5%, Mn: 0.01-0.3%, P: 0.01-0.05%, S: 0.0001-0.0100%, Cr: 10-22%, Al: 0.03-0.10%, Ti: 0.05-0.50%, O: 0.0010-0.0100%, B: 0.0005-0.0050%, N: 0.005-0.020%, and one or more kinds selected from the group consisting of Nb: 0.05-0.50%, Zr: 0.05-0.50%, V: 0.05-0.50%, and Mo: 0.3-2.0% as needed, and Ti oxide of 0.1% or less in the steel, and which satisfies a condition expressed by the following equation (1). Free-Ti=Ti-4C-3.4N-1.5 S-2O>=0.05... (1).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ferritic stainless steel which is excellent in workability and corrosion resistance and has few surface defects during production.

[0002]

2. Description of the Related Art Ferritic stainless steels are widely used because they have economic advantages compared to austenitic stainless steels. They have not only high formability and high corrosion resistance but also excellent surface quality. Is required.

With the recent improvement of refining technology, extremely low carbon
Nitrogenation has become possible, and stabilizing elements such as Ti and Nb have been added to improve corrosion resistance and intergranular corrosion of a welded portion, so that higher corrosion resistance than that of austenitic stainless steel can be obtained (for example, See JP-A-61-261460 and JP-A-08-296000).

However, in the known technique, the workability, ie, r
Although the value can be improved and the corrosion resistance can be improved, in particular, in the case of a ferritic stainless steel to which Ti is added, there is a problem that a large number of surface defects occur during production and the surface quality is impaired. This is due to the T
It is caused by i-type hard inclusions, and generates barge-like flaws starting from hard inclusions existing in the surface layer during hot rolling and cold rolling.

[0005] When a large number of these barbed flaws are generated, the surface quality is deteriorated, and rust is generated from the surface quality, which causes a problem that the corrosion resistance is also reduced. Although it was possible to remove the barbed flaws by doing so, there were problems such as an increase in manufacturing cost and deterioration of the surface quality due to remaining grinding marks.

[0006] As described above, the origin of the occurrence of barbed flaws in a Ti-added ferritic stainless steel is a Ti-based hard inclusion, and specifically, a Ti-based oxide (TiO, TiO ₂ , Ti
₂ O ₃ ) and Ti-based nitride (TiN). If the amount of added Ti is reduced to reduce these, corrosion resistance and workability are deteriorated.
In addition, it was necessary to control the Ti-based inclusions that caused the barbed flaws.

[0007]

SUMMARY OF THE INVENTION It is an object of the present invention to solve the problems of the known art, in particular with respect to corrosion resistance.
i and control of precipitates of C, N, S, O, surface flaw T
It is an object of the present invention to provide a Ti-added ferritic stainless steel having excellent workability and corrosion resistance and having few surface defects with a basic technical idea of controlling i and Al oxides.

[0008]

Means for Solving the Problems To solve the above-mentioned problems, the present inventors have developed a metal composition relating to control of inclusions for reducing surface defects while improving workability and corrosion resistance of ferritic stainless steel. A detailed study was performed from a histological point of view.

The present invention is based on the result, and has the following constitution. That is, in the present invention, C: 0.001 to 0.020%, Si: 0.01 to 0.50%, Mn: 0.01 to 0.30%, P: 0.01 to 0% by mass%. 0.05%, S: 0.0001 to 0.0100%, Cr: 10 to 22%, Al: 0.03 to 0.10%, Ti: 0.05 to 0.50%, O: 0.0010 0.0100%, B: 0.0005 to 0.0050%, N: 0.005 to 0.020%, if necessary, Nb: 0.05 to 0.50%, Zr: 0.05 to 0 .50%, V: 0.05 to 0.50%, Mo: 0.3 to 2.0%, the balance consisting of Fe and unavoidable impurities,
Further, it has a composition satisfying the condition represented by the following formula (1). Further, the content of Ti oxide in steel is desirably 0.1% or less. Free · Ti = Ti-4C-3.4N-1.5S-2O ≧ 0.05 (1)

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.
First, the reasons for limitation of the present invention will be described. The main technology of the present invention is to control surface flaws by controlling Ti and Al inclusions.
In the addition of Ti-Al composite, the addition amount of Ti and Al is optimized, the generation of hard Ti, Al-based oxide and TiN causing surface flaws is controlled, and the Al-based oxide serving as crystallization nucleus of TiN Is to actively generate.

Al is sometimes added because it acts as a deoxidizer. In the present invention, it has been found that when Al is added to a Ti-added ferritic stainless steel, the composition and form of the oxide differ depending on the amount of Al added. That is, FIG. 1 shows the relationship between the amount of Al added and the Ti-based oxide. A
When the amount of l is small, a large amount of hard Ti-based oxide is generated and causes surface flaws. And 0.1% or less at which no surface flaws occur. Here, as for the amount of the Ti-based oxide, a sample was taken from a steel ingot, and the area ratio of the Ti-based oxide was measured in accordance with JIS G0555.

FIG. 2 shows the effect of Ti content and Al content on surface flaws. Thus, when the amount of Al added exceeds 0.10%, A
Since the l-based oxides aggregate and coalesce, surface flaws occur.
When the amount of Al added is small, Ti
Since the crystallization nuclei of N are small, the amount of generated TiN is small, and coarse TiN is precipitated after solidification. However, by adding an appropriate amount of Al, TiN is formed by using Al oxide generated during solidification as nuclei.
Are finely generated.

Coarse Al aggregated and coalesced with Ti-based oxide
The system oxide is hard and causes surface flaws. In addition, coarsely precipitated TiN also causes surface flaws. From FIGS. 1 and 2, it can be seen that by appropriately controlling the amounts of Ti and Al added, surface defects caused by oxides and nitrides can be prevented. Furthermore, 0.05% of Ti
If it is less than 10%, the fixation of C and N will be insufficient, and the workability and corrosion resistance of the product will be degraded. Therefore, the range of Al was set to 0.03 to 0.10%, and the range of Ti was set to 0.05 to 0.50%. Furthermore,
From the viewpoint of economy and surface properties, Al is 0.05 to 0.0
Preferably, 7% and Ti are 0.10 to 0.30%.

C deteriorates workability and corrosion resistance.
The smaller the content, the better, but an excessive decrease leads to an increase in the refining cost. Further, in consideration of economy and characteristics, 0.002 to 0.2.
010% is desirable.

Since Si is a deoxidizing element, it is added during refining. If it exceeds 0.50%, corrosion resistance and workability deteriorate, and if less than 0.01%, refining costs increase.
Therefore, the range of Si is set to 0.01 to 0.50%. Further, in consideration of the material properties, 0.10 to 0.30% is desirable.

Since the content of Mn deteriorates processability and corrosion resistance, the smaller the content, the better. However, an excessive decrease leads to an increase in refining cost and has a deoxidizing effect like Si. 0.30%. Further, in consideration of economy and characteristics, 0.10 to 0.20% is desirable.

P degrades workability and corrosion resistance.
The smaller the content, the better, but an excessive decrease leads to an increase in the refining cost, so the content was set to 0.01 to 0.05%.
Furthermore, considering the economy and characteristics, 0.02-0.04%
desirable.

S, like P, is better as the content is smaller, but excessive reduction leads to an increase in refining costs. Also, S
Forms precipitates with Ti and C and acts as a nucleus for hot recrystallization, contributing to improvement in workability. Therefore, the range of S is set to 0.0001 to 0.0100%. Further, in consideration of economy and characteristics, 0.0010 to 0.000.005.
0% is desirable.

Cr needs to be added in an amount of 10% or more in order to improve corrosion resistance and high-temperature oxidation resistance. However, if added in an amount exceeding 22%, toughness is deteriorated, and manufacturability is deteriorated. Therefore, the range of Cr is set to 10 to 22%. Further, from the viewpoint of ensuring corrosion resistance and workability, 16 to 20% is desirable.

O is preferred to be low because it forms inclusions that cause surface flaws. Since excessive deoxidation leads to an increase in refining costs, the content is set to 0.0010 to 0.0100%. Furthermore, from the viewpoint of workability and corrosion resistance, 0.0010
0.0050% is desirable.

B segregates at the crystal grain boundaries to strengthen the grain boundaries, thereby improving the secondary workability of high purity steel. The effect starts from 0.0005%, and excessive addition causes deterioration of workability.
050%. Furthermore, in consideration of economy, 0.0005
0.0020% is desirable.

N, like C, deteriorates workability and corrosion resistance, so the smaller the content, the better. However, an excessive decrease does not cause TiN to precipitate during solidification, and an equiaxed solidified structure with TiN as a nucleus. 0.005 to 0.020 because no crystallization occurs
%. Furthermore, considering economics and characteristics, 0.005
-0.010% is desirable.

Nb forms precipitates with C and N in the same manner as Ti, has the effect of reducing solid solution C and N and improving workability and corrosion resistance. . Further, excessive addition causes surface flaws due to Nb precipitates, so that 0.05 to 0.30% is desirable.

Zr also forms precipitates with C and N in the same manner as Ti and Nb, has the effect of reducing solid solution C and N and improving workability and corrosion resistance. And
Further, the content is preferably 0.05 to 0.10% in consideration of economy.

V also forms precipitates with C and N in the same manner as Ti, Nb, and Zr, reduces solid solution C and N, and has an effect of improving workability and corrosion resistance. 50%. Further, the content is preferably 0.05 to 0.10% in consideration of economy.

Mo has the effect of improving the corrosion resistance, but the effect is from 0.3% or more, and if it exceeds 2.0%, the workability is reduced. Therefore, the range of Mo is 0.
It was set to 3 to 2.0%. Further, in consideration of economy and corrosion resistance, 0.5 to 1.0% is desirable.

In order to improve the workability and corrosion resistance of the ferritic stainless steel having the above chemical components, Ti and C,
It is important to optimize the composition balance of N, S, and O. First, Ti, C and N are precipitated as TiC and TiN, respectively, and refinement of a solidification structure and development of a recrystallized texture by grain boundary purification lead to improvement of workability. Further, the suppression of the formation of Cr carbonitride leads to an improvement in corrosion resistance.

Ti and S form sulfides such as TiS and Ti ₄ C ₂ S ₂ , and O forms oxides such as TiO and TiO ₂ , so that Ti and C, N, S, O Bonding reduces the amount of Ti dissolved in the steel. In the present invention, the solid solution T
As a result of systematically evaluating the deterioration of corrosion resistance due to the amount of i (Free · Ti), it was found that the corrosion resistance was improved by securing a certain amount of solid solution Ti. FIG. 3 shows the relationship between Free · Ti and pitting potential in steel based on 16.5% Cr steel. Here, the pitting corrosion potential is determined by JI
Vc100 measured according to SG0577. From this, it can be seen that when Free.Ti is small, the corrosion resistance is inferior, and when the value is 0.05% or more, high corrosion resistance is obtained.

The steel of the present invention is produced by a general process such as continuous casting-hot rolling-annealing / pickling-cold rolling-annealing / pickling by smelting by a conventional refining method such as AOD or VOD. Just do it. In order to further improve workability,
The two-fold cold rolling method including the intermediate annealing may be adopted, and in some cases, the hot-rolled sheet annealing may be omitted.

[0030]

EXAMPLES Ferritic stainless steels having the component compositions shown in Tables 1 and 2 were melted and cast. Then, 1100-1
The sheet was heated to 200 ° C. and hot-rolled to obtain a hot-rolled sheet having a thickness of 3.8 mm. After continuous annealing and pickling, this was cold rolled to a thickness of 0.5 mm, and further subjected to continuous annealing and pickling and temper rolling to obtain a product.

From the 0.5 mm-thick product sheet obtained as described above, JIS No. 13B tensile test pieces were sampled in a direction parallel to the rolling direction, at a direction of 45 ° and at a direction of 90 °, and 15% strain was applied. Then, an average r value was calculated from the following equations (2) and (3). r = ln (W ₀ / W) / ln (t ₀ / t) (2) where W ₀ is the initial plate width, W is the plate width after tension, t ₀ is the initial plate thickness, and t is the tension. Later plate thickness. Average r value = (r ₀ + 2r ₄₅ + r ₉₀ ) / 4 (3) where r ₀ is the r value in the direction parallel to the rolling direction, r ₄₅ is the r value in the rolling direction and the 45 ° direction, and r ₉₀ is This is the r value in the direction perpendicular to the rolling direction.

A 30 × 10 mm test piece was sampled from the product plate, and the pitting potential (Vc10) was measured according to JIS G0577.
0) was measured.

Further, the state of occurrence of surface flaws on the product plate was visually observed and ranked. The flaw ranks A and B indicate that a barbed surface flaw was generated in C and D at a level that does not impair the appearance of the surface.

As is clear from Tables 1 and 2, the steel having the chemical composition specified in the present invention has higher workability (r value) and higher corrosion resistance (pitting potential Vc100) than the comparative steel. Further, it can be seen that the occurrence of surface flaws is B rank or higher, and that the workability and corrosion resistance are excellent and the number of surface flaws is small. Table 2
From the results, it can be seen that the surface flaw rank is poor in the comparative example in which the amount of Al added is small and the Ti oxide is large, and the surface flaw generation is small in the example of the present invention.

[0035]

[Table 1]

[0036]

[Table 2]

[0037]

As is apparent from the above description, according to the present invention, a ferritic stainless steel sheet excellent in workability and corrosion resistance and having few surface defects can be provided.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the amount of Al added and the amount of Ti-based oxide.

FIG. 2 is a diagram showing the relationship between Ti amount and Al amount affecting surface defects.

FIG. 3 is a diagram showing the relationship between Free-Ti and corrosion resistance.

Claims (4)

[Claims] C .: 0.001 to 0.020%, Si: 0.01 to 0.50%, Mn: 0.01 to 0.30%, P: 0.01 to 0. 05%, S: 0.0001 to 0.0100%, Cr: 10 to 22%, Al: 0.03 to 0.10%, Ti: 0.05 to 0.50%, O: 0.0010 to 0 0.0100%, B: 0.0005 to 0.0050%, N: 0.005 to 0.020%, the balance being Fe and unavoidable impurities, and satisfying the condition represented by the following formula (1). Ferritic stainless steel having excellent workability and corrosion resistance, and having few surface flaws, characterized by having the following composition. Free · Ti = Ti-4C-3.4N-1.5S-2O ≧ 0.05 (1) 2. The composition further includes one or more of Nb: 0.05 to 0.50%, Zr: 0.05 to 0.50%, and V: 0.05 to 0.50% by mass%. The ferritic stainless steel according to claim 1, which is excellent in workability and corrosion resistance and has few surface flaws. 3. The ferritic stainless steel according to claim 1, further comprising Mo: 0.3 to 2.0% by mass%. . 4. The ferritic stainless steel according to claim 1, wherein the Ti-based oxide is 0.1%.
A ferritic stainless steel having excellent workability and corrosion resistance and less surface flaws, characterized by being 1% or less.