JP2001294990A - Ferritic stainless steel sheet excellent in ductility - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ferritic stainless steel sheet excellent in dictility in both of the rolling and width direction. SOLUTION: This ferritic stainless steel sheet excellent in ductility has a composition containing, by mass, 0.0005 to 0.03% C, 0.01 to 1% Si, 0.01 to 1% Mn, <0.04% P, 0.0001 to 0.01% S, 10 to 25% Cr, 0.01 to 0.8% Ti, 0.005 to 0.1% Al, 0.0005 to 0.03% N and 0.0005 to 0.01% Mg, also, in which Mg inclusions with the maximum size of 0.05 to 2 μm and Ti4C2S2 with the maximum size of 0.5 to 2 μm are dispersed into the steel at a density of >=3 pieces/mm2 in both cases, and further, the elongation EL of the formed product satisfies the following inequality in both directions of the rolling direction and the width direction: EL>=37-0.3×[Cr]+2.8×t, wherein [Cr]: the Cr content (mass %), and (t): the sheet thickness (mm) of the formed product.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a ferritic stainless steel sheet having excellent ductility.

[0002]

2. Description of the Related Art Ferritic stainless steel has excellent corrosion resistance and is used for many purposes. However, SUS30, which is a representative type of austenitic stainless steel,
Since the ductility was remarkably inferior to that of No. 4, there was a problem that cracks occurred during molding or molding could not be performed, and the use was sometimes limited.

[0003] Techniques for improving the ductility by defining the steel composition and the manufacturing process have been studied. Japanese Patent Publication No. 59-49301 discloses a steel with an increased amount of Al. Japanese Patent Publication No. 7-51727 discloses a method for improving characteristics such as ductility by defining hot rolling and annealing conditions after hot rolling. Also, JP-A-10-46293
In Japanese Patent Application Publication No. 2002-115, a method of ensuring ductility and workability by changing the composition of the central portion of the thickness of a steel sheet is disclosed.

[0004] All of these methods are effective methods for improving ductility in a direction parallel to the rolling direction (L direction). However, ductility in the sheet width direction (C direction) perpendicular to the rolling direction was not always sufficient. In the forming process, ductility not only in the L direction but also in the C direction is necessary. Therefore, if this is not improved, it cannot be said that sufficient ductility as a material can be secured.

[0005]

As described above, the ductility of a ferritic stainless steel sheet must satisfy both the rolling direction (L direction) and the sheet width direction perpendicular to it (C direction). An object of the present invention is to provide a ferritic stainless steel sheet having improved ductility in both the L and C directions by defining inclusions in the ferritic stainless steel sheet.

[0006]

Means for Solving the Problems The present inventors melt a ferritic stainless steel using a melting furnace in a laboratory and perform hot rolling, (annealing), cold rolling, and annealing in the L and C directions. The effects of components on ductility were investigated. As a result, it was recognized that the ductility in the L and C directions was significantly improved in the steel containing Ti and Mg. The ductility at this time means that the parallel portion 50m
JIS No. B tensile test piece of JIS Z 2201
It was evaluated by elongation at break (elongation of product) when tested by a tensile test method based on. Since the product elongation greatly changes depending on the amount of Cr and the thickness of the tensile test piece, it can be said that ductility is good when the product elongation EL in both the L and C directions satisfies the following equation (1). EL ≧ 37−0.3 × [Cr] + 2.8 × t (1) where [Cr] is the amount of Cr (mass%) and t is the product thickness (mm).

[0007] Further, the inclusions are measured by an electron microscope and an EPMA.
As a result of a detailed investigation, it was found that M
g-based inclusions and titanium carbosulfide (Ti_FourC_TwoS_Two) Is a number
There were many. The inclusions at this time are Mg-based inclusions.
Has a maximum diameter of 0.05-2 μm, Ti_FourC _TwoS_TwoThen
Large diameter is 0.5 to 2 μm, all 3 pieces / mm^TwoMore than
Existed in density. On the other hand, in a material having poor ductility, M
g Inclusions and Ti_FourC_TwoS_TwoDensity of any of the above
Full or dense but very fine
Was less than the size.

[0008] The present invention has been completed based on the above findings, and the gist thereof is as follows. (1) mass%, C: 0.0005 to 0.03%, Si: 0.01 to 1%, Mn: 0.01 to 1%, P: less than 0.04%, S: 0.0001 to 0.01%, Cr: 10 to 25%, Ti: 0.01 to 0.8%, Al: 0.005 to 0.1%, N: 0.0005 to 0.03%, Mg: 0.0005 Mg-based inclusions having a maximum diameter of 0.05 to ₂ μm, and Ti ₄ C ₂ S having a maximum diameter of 0.5 to ₂ μm. ₂ are dispersed in the steel at a density of 3 or more pieces / mm ² or more, and the product elongation EL satisfies the following expression in both the rolling direction and the sheet width direction perpendicular to the rolling direction. Excellent ferritic stainless steel sheet. EL ≧ 37−0.3 × [Cr] + 2.8 × t where [Cr]: Cr amount (mass%), t: thickness of the product (mm) (2) Mass%, B: 0.0005 to 0.0005 0.00
5%, Nb: 0.05-0.5%, Zr: 0.0005
The ferritic stainless steel sheet having excellent ductility according to the above (1), further comprising one or more of 0.5% or more. (3) In mass%, one or more of Mo: 0.1 to 2%, Ni: 0.1 to 2%, and Cu: 0.1 to 2% are further contained. The ferritic stainless steel sheet excellent in ductility according to (1) or (2).

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail. In the following description, all “%” are mass%.
Is shown. C, N: Since the ductility decreases when a large amount of C and N is added, the upper limits are each set to 0.03%. On the other hand, the lower limit is 0.0005% in consideration of the cost in the refining stage. When used for severe processing applications, both C and N are preferably set to 0.0005 to 0.015%.

Si: Si is necessary as a deoxidizing element, but since the addition of a large amount lowers the ductility, the upper limit is 1%.
And The lower limit is set to 0.01% in order to obtain a deoxidizing effect.

Mn: The upper limit of Mn is set to 1% because addition of a large amount of Mn increases the strength and lowers the ductility. The lower limit is set to 0.01% from the viewpoint of cost.

P: P is preferably low from the viewpoint of ductility, and must be less than 0.04%. The lower the value, the better the ductility. Therefore, the content is preferably 0.02% or less. The lower limit is preferably about 0.005% from the viewpoint of raw material cost.

S: If S is added in a large amount, the corrosion resistance and the workability deteriorate, so the upper limit was made 0.01%. The lower limit is 0.0001% because it can be extremely low with recent desulfurization technology.
And

Cr: If the content of Cr is less than 10%, the corrosion resistance as stainless steel is insufficient, and if it exceeds 25%, the toughness may be reduced. In addition, since the workability decreases when the Cr content increases,
For processing applications, 10 to 19% is preferred.

Ti: Ti precipitates Ti ₄ C ₂ S ₂ ,
It is an important element for improving ductility, which is the subject of the present invention. In the present invention, the effect of improving ductility is 0.01% or more, and this is defined as the lower limit. If the amount of Ti is too large, the strength increases and the ductility decreases, so the upper limit is set to 0.8%. The most preferable range is 0.08 to 0.3% from the viewpoint of productivity and the like in addition to ductility.

Al: Al is an element that fixes workability by fixing N and the like. However, since addition of a large amount lowers ductility and increases cost, the upper limit is set to 0.1%. on the other hand,
Since the deoxidation becomes difficult due to the reduction of the Al content, the lower limit is set to 0.1.
005%.

Mg: Mg is an important element that forms inclusions and improves ductility, which is an object of the present invention. Ti ₄
The lower limit for the effect of improving the ductility by precipitating (crystallizing) together with C ₂ S ₂ is 0.0005%, which was taken as the lower limit. Also, even if it is added in a large amount, the effect is saturated,
The upper limit was 0.01%.

B, Nb, Zr: These are elements for improving workability, and one or more of B, Nb, and Zr are added as needed. B: 0.000
The effect is exhibited by adding 5%, Nb: 0.05% and Zr: 0.0005% or more. However, B: 0.005
%, Nb: 0.5%, and Zr: 0.5%, the effect is saturated.

Mo, Ni, Cu: These are elements that improve corrosion resistance, and Mo, N is used in applications where corrosion resistance is a problem.
One or a combination of two or more of i and Cu is added as needed. The effect is exhibited by adding 0.1% or more of each. However, considering the workability, the upper limit is 2% in each case.

Furthermore, in the present invention, the maximum diameter of the Mg-based inclusion must be 0.05 to 2 μm. Ti ₄ C ₂
S ₂ is 0.5 to ₂ μm. The distribution density is required to be 3 pieces / mm ² or more. If less than this, the effect of improving ductility is not exhibited. In order to further improve the ductility-improving effect, it is preferable to be 30 pieces / mm ² or more. The upper limit of the density is 10,000 pieces / m in a range where the strength is not significantly increased.
m ² is desirable. In order to investigate the size of these inclusions, an extraction replica or a thin film of the inclusions is prepared at an arbitrary cross section of the steel ingot and the steel sheet, and the inspection is performed by an electron microscope. The distribution was determined by electron microscopy or EPM as described above.
The method of investigating using A is good.

Mg is an element that is also used as a deoxidizing material. If Mg is present in molten steel for a long time, it may become coarse and float, so that Mg inclusions having the above-mentioned size and distribution are required to be present. It is necessary to cast within 120 minutes after the addition of Mg. At the end of adding other component elements,
The method of adding g is preferred. In order to make Ti ₄ C ₂ S ₂ exist at the above-mentioned size and distribution density, a method of slowly cooling the cast slab or lowering the heating temperature before hot rolling (about 1160 ° C. or less) is necessary. It is valid. According to this method, the above-described size and density can be obtained because the Mg inclusions themselves also act as Ti ₄ C ₂ S ₂ precipitation nuclei.

Mg-based inclusions are inclusions containing Mg.
Is shown. Oxide (MgO, MgAl _Two0_FourEtc.) and sulfides
Any composition such as (MgS) may be used. Analyze with EDS
Indicates an inclusion in which a Mg peak is observed. Ti_Four
C_TwoS_TwoCan be identified from EDS and backscattered electron images. What
Incidentally, as far as the inventors have investigated, Mg inclusions
Ti to cover_FourC _TwoS_TwoWhere the precipitates are observed
Not in. There is a slight case where both precipitates are observed in contact with each other.
However, basically, Mg inclusions and Ti_FourC_TwoS_TwoIs
Exists independently. Therefore, the number and support
The noise should be measured.

In the above method for producing a ferritic stainless steel, a steel ingot is usually hot-rolled, annealed, cold-rolled, and annealed. It may be performed or hot rolling may be omitted.

As described above, since the product elongation EL greatly varies depending on the Cr content and the product thickness, EL ≧ 37−0.3 × [Cr] + 2.8 × t, where [Cr]: Cr content (mass%) , T: good ductility when the product thickness (mm) is satisfied. Evaluation of product elongation is based on JIS No. 13B
The tensile elongation is preferably determined by elongation at break (elongation of product) when a tensile test piece is tested by a tensile test method based on JIS Z 2201.

[0025]

The present invention will be further described below with reference to examples. Melting ferritic stainless steel shown in Table 1,
After hot rolling, cold rolling, annealing and the like to prepare a steel sheet of 0.4 to 1 mm, a JIS No. 13B tensile test piece having a parallel portion of 50 mm was sampled and subjected to a tensile test. The inclusions near the center of the thickness of the steel sheet were examined by an electron microscope, and Mg-based inclusions having a size of 0.05 to 2 μm and T
The density of i ₄ C ₂ S ₂ was measured. The density was changed depending on the constituent components and the manufacturing conditions. Table 2 shows various evaluation results.
Shown in It can be seen that the steel of the present invention has better ductility than the comparative steel.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

According to the present invention, by defining the steel components and inclusions, a ferritic stainless steel sheet excellent in ductility in both the rolling direction and the sheet width direction perpendicular thereto can be provided. Therefore, the present invention significantly improves the workability of a ferritic stainless steel sheet, and its industrial value is extremely high.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Masao Kikuchi 20-1 Shintomi, Futtsu City Nippon Steel Corporation Technology Development Headquarters (72) Inventor Masayuki Abe 1-1 1-1 Tobata-cho, Tobata-ku, Kitakyushu-shi Made in New Japan Inside Yawata Steel Works, Ltd.

Claims (3)

[Claims] 1. Mass%, C: 0.0005 to 0.03%, Si: 0.01 to 1%, Mn: 0.01 to 1%, P: less than 0.04%, S: 0. 0001 to 0.01%, Cr: 10 to 25%, Ti: 0.01 to 0.8%, Al: 0.005 to 0.1%, N: 0.0005 to 0.03%, Mg: 0 Mg-based inclusions containing 0.0005 to 0.01% and having a maximum diameter of 0.05 to ₂ μm and Ti ₄ C ₂ S ₂ having a maximum diameter of 0.5 to ₂ μm are all 3 / A ferritic stainless steel sheet excellent in ductility, wherein the ferrite stainless steel sheet is dispersed in steel at a density of not less than ² mm and the product elongation EL satisfies the following expression in both the rolling direction and the sheet width direction perpendicular thereto. EL ≧ 37−0.3 × [Cr] + 2.8 × t, where [Cr]: Cr amount (mass%), t: thickness of the product (mm) 2. The composition further comprises one or more of B: 0.0005 to 0.005%, Nb: 0.05 to 0.5%, and Zr: 0.0005 to 0.5% by mass%. The ferritic stainless steel sheet having excellent ductility according to claim 1, further comprising: 3. The composition further comprises one or more of mass%, Mo: 0.1 to 2%, Ni: 0.1 to 2%, and Cu: 0.1 to 2%. The ferritic stainless steel sheet having excellent ductility according to claim 1 or 2.