阅读说明：本技术 铁素体系不锈钢板及其制造方法 (Ferritic stainless steel sheet and method for producing same ) 是由 西田修司 藤泽光幸 于 2020-03-03 设计创作，主要内容包括：本发明提供一种Cr含量小于15.0质量％,且生产率和耐腐蚀性优异、具有与AISI439同等的0.2％耐力的铁素体系不锈钢板及其制造方法。本发明的铁素体系不锈钢板,具有如下的成分组成和组织,所述成分组成是以质量％计含有C：0.004～0.020％、Si：0.05～0.90％、Mn：0.05～0.60％、P：0.050％以下、S：0.030％以下、Al：0.001～0.100％、Cr：13.0％以上且小于15.0％、Ti：0.15～0.35％、Nb：0.030～0.090％、V：0.010～0.200％以及N：0.004～0.020％％,剩余部分由Fe和不可避免的杂质构成,所述组织是晶粒的平均截面积为200～400μm～(2),L方向、D方向和C方向的0.2％耐力均为230～300MPa。(The invention provides a ferritic stainless steel sheet having a Cr content of less than 15.0 mass%, excellent productivity and corrosion resistance, and a 0.2% proof stress equivalent to AISI439, and a method for manufacturing the same. The ferritic stainless steel sheet according to the present invention has a composition and a structure, the composition containing, in mass%, C: 0.004 to 0.020%, Si: 0.05-0.90%, Mn: 0.05-0.60%, P: 0.050% or less, S: 0.030% or less, Al: 0.001-0.100%, Cr:13.0% or more and less than 15.0%, Ti: 0.15 to 0.35%, Nb: 0.030-0.090%, V: 0.010-0.200% and N: 0.004 to 0.020%, the balance being Fe and inevitable impurities, the structure being such that the average cross-sectional area of crystal grains is 200 to 400 μm 2 And the 0.2% endurance strength in the L direction, the D direction and the C direction is 230-300 MPa.)

1. A ferritic stainless steel sheet having the following composition and structure:

the composition contains, in mass%, C: 0.004 to 0.020%, Si: 0.05-0.90%, Mn: 0.05-0.60%, P: 0.050% or less, S: 0.030% or less, Al: 0.001-0.100%, Cr: 13.0% or more and less than 15.0%, Ti: 0.15 to 0.35%, Nb: 0.030-0.090%, V: 0.010-0.200% and N: 0.004 to 0.020%, the balance being Fe and unavoidable impurities,

the average cross-sectional area of the crystal grains in the structure is 200 to 400 mu m²，

The 0.2% proof stress in the L direction, the D direction and the C direction is 230-300 MPa.

2. The ferritic stainless steel sheet according to claim 1, wherein the composition further contains, in mass%, a metal selected from the group consisting of Ni: 0.01 to 0.60%, Cu: 0.01-0.80%, Co: 0.01 to 0.50%, Mo: 0.01-1.00% and W: 0.01-0.50% of 1 or more than 2.

3. The ferritic stainless steel sheet according to claim 1 or 2, wherein the composition further contains, in mass%, a metal selected from the group consisting of Zr: 0.01-0.50%, B: 0.0003-0.0030%, Mg: 0.0005 to 0.0100%, Ca: 0.0003 to 0.0030%, Y: 0.01-0.20%, REM is rare earth metal: 0.01-0.10%, Sn: 0.01-0.50% and Sb: 0.01-0.50% of 1 or more than 2.

4. The ferritic stainless steel sheet according to any one of claims 1 to 3, which is used for an automobile exhaust system component.

5. A method for producing a ferritic stainless steel sheet according to any one of claims 1 to 4, comprising:

a hot rolling step of holding the steel slab having the above composition at a temperature of 1100 to 1250 ℃ for 10 minutes or longer, hot rolling the steel slab to form a hot-rolled sheet, and then winding the hot-rolled sheet at a winding temperature of 500 to 600 ℃;

a hot-rolled sheet annealing step of subjecting the hot-rolled sheet after the hot-rolling step to hot-rolled sheet annealing at a temperature of 940 to 1000 ℃ for 5 to 180 seconds to obtain a hot-rolled annealed sheet; and

and a cold-rolled sheet annealing step of cold-rolling the hot-rolled annealed sheet after the hot-rolled sheet annealing step to form a cold-rolled sheet, and then annealing the cold-rolled sheet at 880 to 900 ℃ for 5 to 180 seconds to obtain a cold-rolled annealed sheet.

Technical Field

The present invention relates to a ferritic stainless steel sheet and a method for manufacturing the same, and more particularly, to a ferritic stainless steel sheet having excellent corrosion resistance and productivity and having a 0.2% proof stress equivalent to AISI 439.

Background

Stainless steel contains Cr, and thus a dense and chemically stable passive film is formed on the surface of the steel, resulting in excellent corrosion resistance. Among stainless steels, ferritic stainless steels are relatively inexpensive, have a small coefficient of thermal expansion, have magnetic properties, and the like because they do not contain much expensive elements as compared with austenitic stainless steels, and are therefore applicable to various applications including cooking utensils and automobile exhaust parts.

As one of typical ferritic stainless steels, there is AISI439(18 mass% Cr-0.3 mass% Ti steel). AISI439 has excellent corrosion resistance and contains Ti in steel, so that sensitization can be suppressed and the corrosion resistance of the welded portion is excellent. AISI439 is a ferritic stainless steel having a relatively low recrystallization temperature, and a relatively low-cost ordinary steel-stainless steel dual-purpose annealing line having a maximum annealing temperature of about 900 ℃ can be used instead of a special annealing line for stainless steel having a high maximum annealing temperature in a cold-rolled sheet annealing step which is one of the production steps, and this can soften the steel and improve productivity. Therefore, AISI439 is applied to a wide range of applications including automobile exhaust components.

On the other hand, in recent years, in the above-mentioned automobile exhaust system components and the like, the following examples have appeared, such as improvement of the structure of components to which steel sheets are applied: the corrosion resistance as high as that of AISI439 is not required for parts using conventional AISI 439. In these examples, SUH409L (11 mass% Cr-0.2 mass% Ti steel) was studied as a substitute for AISI 439.

SUH409L also has a lower recrystallization temperature and therefore a higher productivity, as does AISI 439. And is cheaper than AISI439 due to the low content of Cr, which causes an increase in raw material cost and manufacturing cost. However, in most cases, AISI439 cannot be replaced with SUH409L, and AISI439 can only be used continuously.

The reason why the blank material of the part using AISI439 cannot be replaced with SUH409L is mainly 2 points shown below. First, the content of Cr as an element for improving corrosion resistance in SUH409L is lower than AISI439, and thus corrosion resistance is lower than AISI 439. Although there are cases where the steel sheet does not require as high corrosion resistance as AISI439 due to optimization of the component structure or the like, the use of SUH409L may result in insufficient corrosion resistance.

Next, the content of Cr as an element for improving corrosion resistance and a solid solution strengthening element in SUH409L was lower than AISI439, and the 0.2% proof stress was also lower. The difference in the 0.2% proof stress of the steel sheet causes a change in the so-called spring back amount, which is a phenomenon that the steel sheet is slightly restored to its original shape after the steel sheet is subjected to a working such as bending. Such a difference in the spring back amount is problematic in the processing of steel sheets.

For example, in bending, the bending angle during processing is set to be larger than the target bending angle. Thus, the total of the bending angle at the time of machining and the angle recovered from the spring back amount is exactly the target bending angle, and a desired machined shape is obtained.

Therefore, when SUH409L is machined by a conventional machining method optimized for AISI439, the spring back value decreases, and the desired machined shape cannot be obtained. Since the springback value is estimated experimentally and empirically, in order to change the conventional machining method to a method suitable for SUH409L, it is necessary to review the machining method, which requires a lot of time and cost, and in some cases, it is necessary to create a new mold for machining. Therefore, in many cases, the AISI439 is not replaced with SUH 409L.

That is, a ferritic stainless steel sheet which is less expensive than AISI439, has excellent corrosion resistance than SUH409L, and has 0.2% proof stress equivalent to AISI439 is required. Therefore, the present inventors have studied ferritic stainless steel sheets having improved corrosion resistance over SUH409L and having 0.2% proof stress equivalent to AISI439 on the premise that a common steel-stainless steel dual-purpose annealing line can be used in the cold-rolled sheet annealing process similarly to SUH409L and AISI439 and that the Cr content is less than 15.0 mass% for cost reduction.

Techniques for increasing the 0.2% proof stress of ferritic stainless steel are disclosed in patent documents 1 and 2, for example.

Patent document 1 discloses a ferritic stainless steel having excellent impact resistance and excellent punching resistance, which contains C: 0.015 mass% or less, Si: 0.5 mass% or less, Cr: more than 25.0 and 35.0 mass% or less, N: 0.020% by mass or less, Ti: 0.50 mass% or less, the balance being unavoidable impurities and Fe, and the minimum value of the 0.2% proof stress in three directions being 320N/mm²The above.

Patent document 2 discloses a work-hardened raw material for a stainless steel sheet, which has a composition of C: 0.15 mass% or less, Si: 1.0 mass% or less, Mn: 1.0 mass% or less, S: 0.005 mass% or less, Cr: 10-20 mass%, Ni: 0.5 mass% or less, Al: 0.001 to 0.05 mass%, Fe: substantially the remainder, and has a dimension: al of 10 μm or less₂O₃And/or Al₂O₃Cleanliness of MgO-based inclusions: a processed ferrite structure dispersed at 0.06 or less.

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open publication No. 2007-9263

Patent document 2: international publication No. 2005/014873.

Disclosure of Invention

In the technique disclosed in patent document 1, since the crystal grains are refined for the purpose of improving the 0.2% proof stress of the steel, it is necessary to contain Cr in an amount exceeding 25.0 mass% which increases the raw material cost and the production cost, and therefore, it is expected to reduce the Cr content.

In the technique disclosed in patent document 2, a rolling process is applied to the softened steel for the purpose of improving the 0.2% proof stress of the steel. The present inventors have made and evaluated ferritic stainless steel sheets in a laboratory by using the composition and the production method disclosed in patent document 2, and have described that it is difficult to stably obtain a target 0.2% proof stress.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a ferritic stainless steel sheet having a Cr content of less than 15.0 mass%, excellent productivity and corrosion resistance, and a 0.2% proof stress equivalent to AISI439, and a method for manufacturing the same.

Here, the phrase "excellent productivity" in the present invention means that the hardness of a cold-rolled annealed sheet subjected to cold-rolled sheet annealing at 900 ℃ × 20s (20 s at 900 ℃) is reduced to satisfy formula (1) in the evaluation of the change in hardness of the cold-rolled sheet accompanying annealing described below. If the formula (1) is satisfied, cold-rolled sheet annealing can be performed at 900 ℃ for 20 seconds, and cold-rolled sheet annealing can be performed using a common steel-stainless steel combined annealing line.

The evaluation of the change in hardness of the cold-rolled sheet accompanying annealing was carried out as follows: a cold-rolled sheet obtained by cold-rolling a hot-rolled annealed sheet at a reduction ratio of 67% was subjected to comparison of the hardness a of the cold-rolled sheet (cold-rolled sheet not subjected to cold-rolled sheet annealing), the hardness b of the cold-rolled annealed sheet subjected to cold-rolled sheet annealing at 900 ℃ for 20 seconds, and the hardness c of the cold-rolled annealed sheet subjected to cold-rolled sheet annealing at 1050 ℃ for 20 seconds, which is an index for sufficient softening. In the above evaluation, 3 test pieces of 15mm in length by 20mm in width were cut out from the cold-rolled sheet obtained by the above cold rolling, and the vickers Hardness (HV) of the cross section of 1 test piece out of the test pieces was measured under the conditions of the test force of 9.8N and the holding time of 15 seconds, and was defined as the above hardness a. The remaining 2 test pieces were subjected to cold-rolled sheet annealing at 900 ℃ for 20 seconds and 1050 ℃ for 20 seconds, respectively, and then cut into a size of 15mm in length × 10mm in width, and the vickers Hardness (HV) of the cross section of the cut test piece was measured under the above conditions, and the measured values were used as the above hardnesses b and c, respectively. When the cold-rolled sheet annealing was performed, the hardness of the steel sheet was changed from a to c (softening), and 90% or more of the hardness reduction due to the softening was realized by the annealing at 900 ℃ for 20 seconds, that is, the case where the following formula (1) was satisfied was evaluated as "excellent productivity".

c+0.1×(a－c)≥b…(1)

In the present invention, "excellent corrosion resistance" means that the rust area ratio is 20% or less as a result of performing a corrosion test of 1 cycle of 5 times of cycles of spraying (2 hours, 35 ℃, 98% RH), drying (4 hours, 60 ℃, 30% RH), and wetting (2 hours, 50 ℃, 95% RH) of a 5.0 mass% NaCl aqueous solution on a corundum abrasive paper for a steel sheet after polishing to 400 # based on JASO M609-91.

In the present invention, "having a 0.2% proof stress equivalent to AISI 439" means that a B test piece of JIS13 No. B was used for a tensile test so that the steel sheet was long in the rolling direction (L direction), 45 degrees with respect to the rolling direction (D direction), and perpendicular to the rolling direction (C direction), and all 0.2% proof stresses obtained were 230MPa to 300 MPa.

The present inventors have studied a ferritic stainless steel sheet having a Cr content of less than 15.0 mass%, excellent productivity and corrosion resistance, and a 0.2% proof stress equivalent to AISI439, for the above-described problems. As a result, the following findings were obtained.

That is, a ferritic stainless steel sheet having a composition and a structure as follows, which is excellent in productivity and corrosion resistance and has a 0.2% proof stress equivalent to AISI439, can be obtained by forming a ferritic stainless steel sheet having:

the composition contains, in mass%, C: 0.004 to 0.020%, Si: 0.05-0.90%, Mn: 0.05-0.60%, P: 0.050% or less, S: 0.030% or less, Al: 0.001-0.100%, Cr: 13.0% or more and less than 15.0%, Ti: 0.15 to 0.35%, Nb: 0.030-0.090%, V: 0.010-0.200% and N: 0.004 to 0.020%, the balance being Fe and inevitable impurities, and the average cross-sectional area of crystal grains in the structure being 200 to 400 μm²And the 0.2% endurance strength in the L direction, the D direction and the C direction is 230-300 MPa.

The mechanism is considered as follows.

In the ferritic stainless steel sheet having a Cr content lower than AISI439, the 0.2% proof stress is increased and made equivalent to AISI439 by making the crystal grains of the cold-rolled annealed sheet finer and by containing an appropriate solid-solution strengthening element in the steel.

Therefore, it is effective for grain refinement of the cold-rolled annealed sheet to refine the grains of the hot-rolled annealed sheet as an intermediate material obtained during production. Further, the grain refinement of the hot-rolled annealed sheet can be achieved by optimizing the conditions of hot rolling and hot-rolled sheet annealing. Further, after cold rolling a hot-rolled annealed sheet having fine crystal grains, the finished product is annealed under appropriate conditions, whereby a cold-rolled annealed sheet having fine crystal grains can be obtained, and the 0.2% proof stress can be improved.

In addition, Nb is selected as a solid-solution strengthening element for increasing the 0.2% proof stress of the cold-rolled and annealed sheet from the viewpoint of not causing a decrease in corrosion resistance. Among them, if Nb is contained, the recrystallization temperature of the cold-rolled sheet rises. On the other hand, a method has been found in which an appropriate upper limit is set for the Nb content, and Nb and an appropriate amount of V are compositely contained in the steel to suppress the increase in recrystallization temperature. The increase in recrystallization temperature due to Nb is caused by the effect of fixing dislocations and grain boundaries, because a part of Nb precipitates as fine NbC. On the other hand, if V is contained in the steel, it is considered that the precipitated NbC is mainly precipitated as composite precipitates with coarse TiN (precipitated on the surface of coarse TiN, (Nb, V) C), and the increase in recrystallization temperature can be suppressed. By compounding the Nb with the solid solution strengthening and the above-described grain refinement, a ferritic stainless steel sheet having excellent productivity and corrosion resistance and having a 0.2% proof stress equivalent to AISI439 can be realized.

The present invention has been made in view of the above circumstances, and the gist thereof is as follows.

[1] A ferritic stainless steel sheet having the following composition and structure:

the composition contains, in mass%, C: 0.004 to 0.020%, Si: 0.05-0.90%, Mn: 0.05-0.60%, P: 0.050% or less, S: 0.030% or less, Al: 0.001-0.100%, Cr: 13.0% or more and less than 15.0%, Ti: 0.15 to 0.35%, Nb: 0.030-0.090%, V: 0.010-0.200% and N: 0.004 to 0.020%, the balance being Fe and unavoidable impurities,

of grains in said structureThe average cross-sectional area is 200 to 400 μm²，

The 0.2% proof stress in the L direction, the D direction and the C direction is 230-300 MPa.

[2] The ferritic stainless steel sheet according to [1], wherein the composition further contains, in mass%, a metal selected from the group consisting of Ni: 0.01 to 0.60%, Cu: 0.01-0.80%, Co: 0.01 to 0.50%, Mo: 0.01 to 1.00% and W: 0.01-0.50% of 1 or more than 2.

[3] The ferritic stainless steel sheet according to item [1] or [2], wherein the composition further contains, in mass%, a metal selected from the group consisting of Zr: 0.01-0.50%, B: 0.0003-0.0030%, Mg: 0.0005 to 0.0100%, Ca: 0.0003 to 0.0030%, Y: 0.01 to 0.20%, REM (rare earth metal): 0.01-0.10%, Sn: 0.01-0.50% and Sb: 0.01-0.50% of 1 or more than 2.

[4] The ferritic stainless steel sheet according to any one of [1] to [3], which is used for automobile exhaust parts.

[5] A method for producing a ferritic stainless steel sheet according to any one of the above [1] to [4], comprising:

a hot rolling step of holding the steel slab having the above composition at 1100 to 1250 ℃ for 10 minutes or longer, hot rolling the steel slab to produce a hot-rolled sheet, and then winding the hot-rolled sheet at a winding temperature of 500 to 600 ℃;

a hot-rolled sheet annealing step of subjecting the hot-rolled sheet after the hot-rolling step to hot-rolled sheet annealing at a temperature of 940 to 1000 ℃ for 5 to 180 seconds to obtain a hot-rolled annealed sheet; and

and a cold-rolled sheet annealing step of cold-rolling the hot-rolled annealed sheet after the hot-rolled sheet annealing step to form a cold-rolled sheet, and then annealing the cold-rolled sheet at 880 to 900 ℃ for 5 to 180 seconds to obtain a cold-rolled annealed sheet.

Effects of the invention

According to the present invention, a ferritic stainless steel sheet having a Cr content of less than 15.0 mass%, excellent productivity and corrosion resistance, and further having a 0.2% proof stress equivalent to AISI439, and a method for manufacturing the same can be provided.

Detailed Description

The present invention will be specifically described below.

First, the reason why the composition of the components is limited in the present invention will be described. The "%" indicating the content of the components of the steel sheet means mass% unless otherwise specified.

C：0.004～0.020％

C is an element effective for increasing the 0.2% proof stress of the steel. This effect can be obtained by setting the C content to 0.004% or more. However, if the C content exceeds 0.020%, the steel is hardened, the formability is lowered, or the corrosion resistance is lowered. Therefore, the C content is 0.004 to 0.020%. The C content is preferably 0.006% or more. More preferably, the C content is 0.008% or more. Further, the C content is preferably 0.015% or less. More preferably, the C content is 0.012% or less.

Si：0.05～0.90％

Si has a deoxidizing effect. This effect can be obtained by setting the Si content to 0.05% or more. However, if the Si content exceeds 0.90%, the steel becomes hard, and the 0.2% proof stress is excessively increased. Therefore, the Si content is 0.05 to 0.90%. The Si content is preferably 0.07% or more. More preferably, the Si content is 0.10% or more. Further preferably, the Si content is 0.15% or more. More preferably, the Si content is 0.22% or more. Further, the Si content is preferably 0.80% or less. The Si content is more preferably 0.60% or less.

Mn：0.05～0.60％

Mn has a deoxidizing effect. This effect can be obtained by setting the Mn content to 0.05% or more. However, if the Mn content exceeds 0.60%, precipitation and coarsening of MnS are promoted, and this MnS becomes a starting point of corrosion, and the corrosion resistance of the steel sheet is lowered. Therefore, the Mn content is 0.05 to 0.60%. The Mn content is preferably 0.15% or more. Further, the Mn content is preferably 0.30% or less.

P: 0.050% or less

P is an element that reduces corrosion resistance. In addition, P segregates to grain boundaries, thereby reducing hot workability. Therefore, the P content is preferably as small as possible, and is 0.050% or less. The P content is preferably 0.040% or less. The P content is more preferably 0.030% or less.

S: less than 0.030%

S forms MnS with Mn as a precipitate. This MnS becomes a starting point of corrosion, and reduces corrosion resistance. Therefore, the S content is preferably as low as possible, and is 0.030% or less. The S content is preferably 0.020% or less.

Al：0.001～0.100％

Al has a deoxidizing effect. This effect is obtained when the Al content is 0.001% or more. However, if the Al content exceeds 0.100%, the steel is hardened, the formability is lowered, and the corrosion resistance is lowered. Therefore, the Al content is 0.001 to 0.100%. The Al content is preferably 0.030% or more. Further, the Al content is preferably 0.060% or less.

Cr: more than 13.0 percent and less than 15.0 percent

Cr is an element that forms a passive film on the surface to improve corrosion resistance. If the Cr content is less than 13.0%, sufficient corrosion resistance cannot be obtained. On the other hand, if the Cr content is 15.0% or more, the raw material cost and the production cost increase. Therefore, the Cr content is 13.0% or more and less than 15.0%. The Cr content is preferably 13.5% or more. Further, the Cr content is preferably 14.5% or less. The Cr content is preferably 14.0% or less.

Ti：0.15～0.35％

Ti is an element which forms a carbonitride to immobilize C, N and suppress the occurrence of sensitization. This effect is obtained by setting the Ti content to 0.15% or more. However, if the Ti content exceeds 0.35%, the steel is hardened and formability is reduced. Therefore, the Ti content is 0.15 to 0.35%. Preferably, the Ti content is 0.20% or more. Further, the Ti content is preferably 0.30% or less.

Nb：0.030～0.090％

Nb is an element effective for improving the 0.2% proof stress of steel by being present as a solid solution in the steel of the cold-rolled annealed sheet. This effect is obtained by setting the Nb content to 0.030% or more. However, if the Nb content exceeds 0.090%, even if the recrystallization temperature of the steel rises due to the effect of suppressing the increase in recrystallization temperature by V described later, if the steel is produced using a common steel-stainless steel dual-purpose annealing line, the softening of the steel becomes insufficient, or the crystal grains become excessively fine, and the 0.2% proof stress becomes high. Therefore, the Nb content is 0.030 to 0.090%. The preferable Nb content is 0.035% or more. More preferably, the Nb content is 0.040% or more. Further, the Nb content is preferably 0.080% or less. More preferably, the Nb content is 0.070% or less.

V：0.010～0.200％

V is an element that improves productivity by suppressing the increase in the recrystallization temperature of steel due to Nb. This effect is obtained by setting the V content to 0.010% or more. On the other hand, if V is excessively contained, V carbonitrides excessively precipitate, and the recrystallization temperature rises to lower the steel productivity. Therefore, the V content is 0.010 to 0.200%. The V content is preferably 0.020% or more. More preferably, the V content is 0.030% or more. Further, the V content is preferably 0.150% or less. More preferably, the V content is 0.100% or less.

N：0.004～0.020％

N is an element effective for increasing the steel's 0.2% proof stress. This effect can be obtained by setting the N content to 0.004% or more. However, if the N content exceeds 0.020%, the steel is hardened to lower formability or the corrosion resistance is lowered. Therefore, the N content is 0.004 to 0.020%. The N content is preferably 0.005% or more. More preferably, the N content is 0.007% or more. Further, the N content is preferably 0.015% or less. More preferably, the N content is 0.012% or less.

The balance other than the above components is Fe and inevitable impurities.

In the present invention, 1 or 2 kinds selected from the following groups A and B may be contained in addition to the above-mentioned components.

(group A) is selected from Ni: 0.01 to 0.60%, Cu: 0.01-0.80%, Co: 0.01 to 0.50%, Mo: 0.01 to 1.00% and W: 0.01-0.50% of 1 or more than 2

(group B) is selected from Zr: 0.01-0.50%, B: 0.0003-0.0030%, Mg: 0.0005 to 0.0100%, Ca: 0.0003 to 0.0030%, Y: 0.01 to 0.20%, REM (rare earth metal): 0.01-0.10%, Sn: 0.01-0.50% and Sb: 0.01-0.50% of 1 or more than 2

Ni：0.01～0.60％

Ni improves corrosion resistance of steel by inhibiting active dissolution of steel in a low pH environment. On the other hand, if Ni is excessively contained, the component cost and the production cost of the steel increase, and the steel is hardened and the formability decreases. Therefore, when Ni is contained, the Ni content is set to 0.01 to 0.60%. The Ni content is preferably 0.10% or more. Further, the Ni content is preferably 0.25% or less.

Cu：0.01～0.80％

Cu is an element that improves the corrosion resistance of stainless steel. On the other hand, if Cu is excessively contained, the component cost and the production cost of the steel increase, and ∈ Cu is easily precipitated, and the corrosion resistance decreases. Therefore, when Cu is contained, the Cu content is set to 0.01 to 0.80%. The Cu content is preferably 0.30% or more. More preferably, the Cu content is 0.40% or more. Further, the Cu content is preferably 0.50% or less. More preferably, the Cu content is 0.45% or less. Further preferably, the Cu content is 0.42% or less.

Co：0.01～0.50％

Co is an element that improves the corrosion resistance of stainless steel. On the other hand, if Co is excessively contained, the steel becomes hard and the 0.2% proof stress is excessively increased. Therefore, when Co is contained, the Co content is 0.01 to 0.50%. The Co content is preferably 0.03% or more. More preferably, the Co content is 0.05% or more. Further, the Co content is preferably 0.30% or less. More preferably, the Co content is 0.10% or less.

Mo：0.01～1.00％

Mo has an effect of improving the corrosion resistance of stainless steel. On the other hand, if Mo is excessively contained, an increase in the component cost and the manufacturing cost of the steel is caused, and the steel is hardened and the 0.2% proof stress is excessively increased. Therefore, when Mo is contained, the Mo content is set to 0.01 to 1.00%. The Mo content is preferably 0.03% or more. More preferably, the Mo content is 0.05% or more. Further, the Mo content is preferably 0.50% or less. More preferably, the Mo content is 0.30% or less.

W：0.01～0.50％

W is an element for improving the corrosion resistance of stainless steel. On the other hand, if W is excessively contained, the steel becomes hard and the 0.2% proof stress is excessively increased. Therefore, when W is contained, the W content is set to 0.01 to 0.50%. The W content is preferably 0.03% or more. More preferably, the W content is 0.05% or more. Further, the W content is preferably 0.30% or less. More preferably, the W content is 0.10% or less.

Zr：0.01～0.50％

Zr is an element which fixes C, N by forming a carbonitride compound and improves the corrosion resistance of steel. On the other hand, if Zr is contained excessively, the carbonitride compound precipitates excessively, and the corrosion resistance of the steel is lowered. Therefore, when Zr is contained, the Zr content is set to 0.01 to 0.50%. Preferably, the Zr content is 0.03% or more. More preferably, the Zr content is 0.05% or more. Further, the Zr content is preferably 0.40% or less. More preferably, the Zr content is 0.30% or less.

B：0.0003～0.0030％

B has an effect of increasing the strength of the steel. On the other hand, if B is contained excessively, the steel becomes hard and the 0.2% proof stress is excessively increased. Therefore, when B is contained, the content of B is set to 0.0003 to 0.0030%. The content of B is preferably 0.0010% or more. Further, the B content is preferably 0.0025% or less.

Mg：0.0005～0.0100％

Mg functions as a deoxidizer. On the other hand, if Mg is excessively contained, surface defects increase. Therefore, when Mg is contained, the Mg content is set to 0.0005 to 0.0100%. The Mg content is preferably 0.0010% or more. Further, the Mg content is preferably 0.0050% or less. More preferably, the Mg content is 0.0030% or less.

Ca：0.0003～0.0030％

Ca functions as a deoxidizer. On the other hand, if Ca is excessively contained, surface defects increase. Therefore, when Ca is contained, the content of Ca is set to 0.0003 to 0.0030%. The Ca content is preferably 0.0005% or more. More preferably, the Ca content is 0.0007% or more. Further, the Ca content is preferably 0.0025% or less. More preferably, the Ca content is 0.0015% or less.

Y：0.01～0.20％

Y is an element for improving the cleanliness of the steel. On the other hand, if Y is excessively contained, surface defects increase. Therefore, when Y is contained, the content of Y is set to 0.01 to 0.20%. The Y content is preferably 0.03% or more. Further, the Y content is preferably 0.10% or less.

REM (Rare Earth Metals; Rare Earth Metals): 0.01 to 0.10 percent

REM (rare earth metals: elements having atomic numbers of 57 to 71 such as La, Ce, Nd, etc.) is an element for improving the cleanliness of steel. On the other hand, if REM is excessively contained, surface defects increase. Therefore, when REM is contained, the REM content is set to 0.01 to 0.10%. Preferably, the REM content is 0.02% or more. Further, the REM content is preferably 0.05% or less. Note that the REM content in the present invention is the total content of 1 or 2 or more elements selected from the REMs described above.

Sn：0.01～0.50％

Sn is an element effective for suppressing the roughness of the machined surface. On the other hand, if Sn is excessively contained, hot workability of the steel is lowered. Therefore, when Sn is contained, the Sn content is set to 0.01 to 0.50%. The Sn content is preferably 0.03% or more. Further, the Sn content is preferably 0.20% or less.

Sb：0.01～0.50％

Sb is an element effective for suppressing the surface roughness of a work, like Sn. On the other hand, if Sb is excessively contained, surface defects increase. Therefore, when Sb is contained, the Sb content is set to 0.01 to 0.50%. The Sb content is preferably 0.03% or more. Further, the Sb content is preferably 0.20% or less.

When the content of Ni, Cu, Co, Mo, W, Zr, B, Mg, Ca, Y, REM (rare earth metal), Sn, and Sb described as any of the above components is less than the lower limit, the component is included as an inevitable impurity.

Average cross-sectional area of crystal grains: 200 to 400 μm²

In the present invention, the average cross-sectional area of crystal grains of the structure is controlled to a predetermined value by controlling the contents of various elements represented by NbThe above range allows the production of a ferritic stainless steel having a 0.2% proof stress equivalent to AISI439 with excellent productivity. Here, the average sectional area of the crystal grains affects 0.2% proof stress of the steel. If the average cross-sectional area of the crystal grains is less than 200 μm²The 0.2% proof stress of the steel becomes high, and the 0.2% proof stress equivalent to AISI439 cannot be obtained. In addition, if the average cross-sectional area of the crystal grains exceeds 400 μm²The 0.2% proof stress of the steel becomes low, and the 0.2% proof stress equivalent to AISI439 cannot be obtained. Therefore, the average cross-sectional area of the crystal grains of the structure is 200 to 400 μm². The average cross-sectional area of the crystal grains is preferably 240 μm²The above. Further, it is preferable that the average cross-sectional area of the crystal grains is 360 μm²The following. The average cross-sectional area of the crystal grains can be controlled by a production method described later.

The average cross-sectional area of the crystal grains can be evaluated by the following method. A test piece for tissue observation having a width of 10mm × length of 15mm was cut out from a ferritic stainless steel plate, embedded in a resin so that a cross section in the longitudinal direction thereof became an observation surface, and the observation surface was mirror-polished. Thereafter, the observation surface was etched with picric acid hydrochloric acid solution (100mL of ethanol-1 g of picric acid-5 mL of hydrochloric acid) to form grain boundaries, and then the structure was photographed at a magnification of 500 times with an optical microscope. The observed image thus obtained was plotted in a circle of 100 μm radius (in the case of printing the observed image at 500 times magnification, the circle of 50mm radius) in an actual field of view, and the number of crystal grains completely contained in the circle was defined as n₁The number of crystal grains cut from the circumference is defined as n₂The average cross-sectional area A (. mu.m) of the obtained crystal grains was evaluated by substituting the measurement results into the following formula (2)²)。

A＝31400/(n₁+0.6×n₂)……(2)

0.2% proof stress in the L direction: 230 to 300MPa

0.2% proof stress in D direction: 230 to 300MPa

0.2% proof stress in C direction: 230 to 300MPa

In order to have a 0.2% proof stress equivalent to AISI439 and to obtain a spring back equivalent to AISI439 when processing is performed, it is necessary to set the 0.2% proof stress in each of the L direction, C direction, and D direction of the ferritic stainless steel sheet to 230 to 300 MPa. If the 0.2% proof stress in either direction is less than 230MPa, the spring back is reduced as compared with AISI439 when the steel is processed so that the direction perpendicular to the direction in which the 0.2% proof stress is less than 230MPa is a curved ridge. In addition, if the 0.2% proof stress in any direction exceeds 300MPa, if the steel is processed so that the direction perpendicular to the direction in which the 0.2% proof stress exceeds 300MPa is a curved crest line, the spring back amount becomes larger than that of AISI 439. Therefore, the 0.2% proof stress in the L direction, D direction and C direction is 230 to 300 MPa. Preferably, the 0.2% proof stress is 240MPa or more. Preferably, the 0.2% proof stress is 290MPa or less.

In order to set the springback value of the ferritic stainless steel sheet which is excellent in corrosion resistance and can be produced with high productivity to the appropriate range as described above and set the 0.2% proof stress of the steel sheet to the appropriate range as described above, it is necessary to adjust the content of each element to the above range and the average cross-sectional area of crystal grains to the below-described production method.

Next, a preferred method for producing the ferritic stainless steel sheet of the present invention will be described. After melting the steel having the above-described composition by a known method such as a converter or an electric furnace, a steel slab (steel slab) is formed by a continuous casting method or a cast-cogging method. The steel slab is held at 1100 to 1250 ℃ for 10 minutes or more, hot-rolled to obtain a hot-rolled sheet, and then the hot-rolled sheet is wound at a winding temperature of 500 to 600 ℃ to form a hot-rolled coil. In this case, the hot rolling is preferably performed so that the thickness of the hot rolled sheet becomes 2.0 to 5.0 mm. The hot rolled sheet thus produced is annealed at a temperature of 940 to 1000 ℃ for 5 to 180 seconds to form a hot-rolled annealed sheet. The atmosphere in which the hot-rolled sheet is annealed is preferably an atmospheric atmosphere. Then, acid washing is performed to remove oxide scale. Then, after cold rolling to form a cold-rolled sheet, the cold-rolled sheet is annealed at a temperature of 880 to 900 ℃ for 5 to 180 seconds to obtain a cold-rolled annealed sheet. After annealing of the cold-rolled sheet, pickling or surface grinding is performed to remove the scale. The cold-rolled annealed sheet from which the scale has been removed can be flattened. However, if the rolling reduction of the leveling is more than 2%, not only the 0.2% proof stress becomes excessively high but also the moldability is deteriorated, so that in the case of leveling, it is preferable to set the rolling reduction to 2% or less.

First, a method of controlling the average cross-sectional area of crystal grains in the above-described preferred production method will be described below. By casting the steel having the above composition, a steel slab in which carbonitride compounds such as TiN, TiC, NbC, and VC are precipitated in the steel can be obtained. By heating the steel slab before hot rolling to 1100 ℃ or higher, solid solution of TiN, TiC, NbC, and VC into steel occurs. A hot-rolled steel sheet blank is hot-rolled, then cooled, and wound at a winding temperature of 500 to 600 ℃ to form a hot-rolled coil, whereby a hot-rolled steel sheet is obtained in which hot-rolling strain remains in the steel and Cr carbonitride precipitates with less solid solution C and solid solution N. The hot-rolled sheet thus obtained has little solid solution C and N, while retaining strain in the steel, and therefore, even when the hot-rolled sheet is annealed at a relatively low temperature of 940 to 1000 ℃, recrystallization can occur. Further, by setting the annealing temperature at a relatively low temperature, a hot-rolled annealed sheet having relatively small crystal grains can be obtained.

Next, the hot-rolled annealed sheet having relatively small crystal grains is cold-rolled to form a cold-rolled sheet, and then the cold-rolled sheet is annealed at a temperature of 880 to 900 ℃, whereby a cold-rolled annealed sheet having a desired crystal grain size can be obtained.

Through the process, the average cross section area of the obtained crystal grains is 200-400 mu m²The cold-rolled annealed sheet can provide a ferritic stainless steel sheet having a desired 0.2% proof stress.

The reason why the conditions are within the above ranges in the respective steps of the above preferred production method will be described in further detail below.

A step (hot rolling step) of holding the steel slab at 1100 to 1250 ℃ for 10 minutes or more, hot rolling the steel slab to form a hot-rolled sheet, and then winding the hot-rolled sheet at a winding temperature of 500 to 600 ℃

If the heating temperature of the steel slab is less than 1100 ℃, NbC in the steel is not sufficiently dissolved, and the effect of increasing the 0.2% proof stress by Nb is obtained in the cold-rolled annealed sheet, and the 0.2% proof stress of the cold-rolled annealed sheet is lowered. Further, if the heating time of the steel slab is less than 10 minutes, NbC in the steel is not sufficiently dissolved in the solution, and the effect of increasing the 0.2% proof stress by Nb cannot be obtained in the cold-rolled and annealed sheet, and the 0.2% proof stress of the cold-rolled and annealed sheet is lowered. Further, if the heating temperature of the steel slab exceeds 1250 ℃, the steel slab is deformed, and the manufacturability of the hot-rolled sheet in the hot rolling step is lowered. Therefore, in the present invention, it is preferable that the steel slab is kept at 1100 to 1250 ℃ for 10 minutes or more and then hot-rolled to form a hot-rolled plate. More preferably, the heating temperature of the steel slab is 1150 ℃ or higher. The heating time is more preferably 30 minutes or more. Further, the heating temperature of the steel slab is more preferably 1200 ℃. Further, the heating time of the steel slab is preferably 2 hours or less because the excessive heating of the steel slab for a long time causes deformation of the steel slab and deteriorates the manufacturability of the hot-rolled sheet in the hot rolling step.

Further, if the winding temperature of the hot-rolled sheet is less than 500 ℃, precipitation of Cr carbonitride into steel becomes insufficient, and the amounts of solid solution C and solid solution N contained in the hot-rolled sheet become excessive, with the result that the recrystallization temperature of the hot-rolled sheet becomes high. In this case, even if the hot-rolled sheet is annealed at a temperature described later, the hot-rolled sheet does not recrystallize. In cold rolling of a hot-rolled sheet having a recrystallized structure, local high-strain portions in which lattice strain is locally increased are formed in the vicinity of grain boundaries, and these portions become recrystallization nuclei in the annealing process of the cold-rolled sheet, contributing to the refinement of crystal grains of the cold-rolled annealed sheet. On the other hand, if a hot-rolled annealed sheet having a non-recrystallized structure is cold-rolled, local high-strain portions that become recrystallization nuclei are less likely to be formed in the steel during annealing of the cold-rolled sheet, and as the crystal grains of the cold-rolled annealed sheet become coarse, the 0.2% proof stress of the cold-rolled annealed sheet decreases. If the winding temperature of the hot-rolled sheet exceeds 600 ℃, the strain introduced into the hot-rolled sheet in the hot-rolling step is recovered, and the recrystallization temperature of the hot-rolled sheet becomes high. In this case, even if the hot-rolled sheet is annealed at a temperature described later, the hot-rolled sheet does not recrystallize, and the crystal grains of the cold-rolled and annealed sheet accompanying this coarsen, and the 0.2% proof stress of the cold-rolled and annealed sheet decreases. Therefore, in the present invention, it is preferable that the hot-rolled sheet after hot rolling is wound at a winding temperature of 500 to 600 ℃ to form a hot-rolled sheet coil.

Annealing the hot-rolled sheet at 940-1000 ℃ for 5-180 seconds to form a hot-rolled annealed sheet (hot-rolled sheet annealing step)

If the annealing temperature of the hot-rolled sheet is less than 940 ℃, the hot-rolled sheet does not crystallize, and the grains of the cold-rolled and annealed sheet become coarse, and the 0.2% proof stress of the cold-rolled and annealed sheet decreases. If the annealing temperature of the hot-rolled sheet exceeds 1000 ℃, the crystal grains of the hot-rolled annealed sheet become coarse, the crystal grains of the cold-rolled annealed sheet become coarse, and the 0.2% proof stress of the cold-rolled annealed sheet is lowered. Further, if the holding time of the hot-rolled sheet annealing is less than 5 seconds, the hot-rolled sheet is not crystallized any more, and the crystal grains of the cold-rolled annealed sheet are coarsened, and the 0.2% proof stress of the cold-rolled annealed sheet is lowered. If the holding time of the hot rolled sheet annealing exceeds 180 seconds, the crystal grains of the hot rolled annealed sheet become coarse, the crystal grains of the cold rolled annealed sheet become coarse, and the 0.2% proof stress of the cold rolled annealed sheet is lowered. Therefore, in the present invention, it is preferable that the hot-rolled sheet is annealed at a temperature of 940 to 1000 ℃ for 5 to 180 seconds to form a hot-rolled annealed sheet. More preferably, the annealing temperature of the hot-rolled sheet is in the range of 950 to 980 ℃. The holding time is more preferably 10 seconds or more. The holding time is more preferably 30 seconds or less.

Next, the hot-rolled annealed sheet after the hot-rolled sheet annealing step is cold-rolled to form a cold-rolled sheet. The cold reduction ratio at this time is preferably 50% or more. More preferably 65% or more.

A step of annealing the cold-rolled sheet at 880 to 900 ℃ for 5 to 180 seconds to form a cold-rolled annealed sheet (cold-rolled sheet annealing step)

If the annealing temperature of the cold rolled sheet is less than 880 ℃, the crystal grains of the steel become excessively fine and the 0.2% proof stress becomes excessively high. On the other hand, cold-rolled sheet annealing at temperatures exceeding 900 ℃ cannot be performed in a common steel-stainless steel combined annealing line with high productivity. Further, if the holding time of the cold-rolled sheet annealing is less than 5 seconds, the crystal grains of the steel become excessively fine, and the 0.2% proof stress becomes excessively high. On the other hand, if the holding time of the cold rolled sheet annealing exceeds 180 seconds, the crystal grains of the steel become coarse, and the 0.2% proof stress becomes excessively low. Therefore, in the present invention, it is preferable to perform cold-rolled sheet annealing in which the cold-rolled sheet is kept at 880 to 900 ℃ for 5 to 180 seconds. More preferably, the annealing temperature of the cold-rolled sheet is 890 ℃ or higher. The holding time is more preferably 10 seconds or more. The holding time is more preferably 120 seconds or less.

Examples

[ example 1]

Ferritic stainless steel having a composition shown in Table 1-1 was melted into a 100kg steel slab (steel stock), and then hot rolled at each slab heating temperature shown in Table 1-2 for each slab heating time shown in Table 1-2 to form a hot rolled plate having a thickness of 3.0 mm. Immediately after the completion of the final pass of the hot rolling, the hot-rolled sheet was air-cooled to each coiling temperature described in tables 1 to 2, and then the hot-rolled sheet was inserted into an electric furnace, kept at each coiling temperature for 1 hour, and thereafter furnace-cooled in the electric furnace. The step of inserting the hot-rolled sheet into an electric furnace, holding the sheet at each coiling temperature for 1 hour, and thereafter cooling the sheet in the electric furnace is a step of simulating a temperature history of winding the hot-rolled sheet in a spiral shape at each coiling temperature in an actual production line and then slowly cooling the sheet.

[ tables 1-1]

The balance of the composition other than the above components is Fe and inevitable impurities.

The obtained hot-rolled sheets were kept at the respective hot-rolled sheet annealing temperatures shown in tables 1 to 2 for the respective hot-rolled sheet annealing times shown in tables 1 to 2, and then air-cooled to form hot-rolled annealed sheets. The hot-rolled annealed sheet was pickled with a sulfuric acid solution, and then pickled with a mixed solution of hydrofluoric acid and nitric acid to obtain a cold-rolled sheet blank, which was then subjected to cold rolling to a thickness of 1.0mm to obtain a cold-rolled sheet. A part of the obtained cold-rolled sheets was kept at the annealing temperatures of the cold-rolled sheets shown in tables 1 to 2 for the annealing times shown in tables 1 to 2, and then air-cooled, followed by surface grinding of the front and back surfaces to remove surface scales, thereby producing cold-rolled annealed sheets. The obtained cold-rolled sheet and cold-rolled annealed sheet were subjected to the following evaluations.

(1) Evaluation of productivity

The hardness a of the cold-rolled sheet obtained under the above-mentioned production conditions, the hardness b of the cold-rolled annealed sheet obtained by annealing the cold-rolled sheet at 900 ℃ for 20 seconds, and the hardness c of the cold-rolled annealed sheet obtained by annealing the cold-rolled sheet at 1050 ℃ for 20 seconds, which are indicators when softening is sufficiently performed, were compared, and thereby the change in hardness of the cold-rolled sheet accompanied by annealing was evaluated. Specifically, 3 test pieces of 15mm in length by 20mm in width were cut out from the cold-rolled sheet, and among them, Vickers Hardness (HV) was measured in the cross section of 1 test piece to obtain the above-mentioned hardness a. The remaining 2 test pieces were annealed at 900 ℃ for 20 seconds and 1050 ℃ for 20 seconds, cut into pieces having a length of 15mm × a width of 10mm, and the vickers Hardness (HV) of the cross section of the cut test pieces was measured to obtain the above-mentioned hardnesses b and c, respectively. After embedding the resin, the test piece was subjected to mirror polishing on the test surface and subjected to the test. Measurement of Vickers hardness the test force was 9.8N and the holding time was 15 seconds. Of the measured hardnesses a, b and c, the case where the hardness satisfies the formula (1) was evaluated as "o (pass)", and the case where the hardness does not satisfy the formula was evaluated as "a (fail)". In this evaluation, if it is O, cold-rolled sheet annealing can be performed in an annealing line which doubles as a general steel-stainless steel, and it can be evaluated that productivity is excellent.

c+0.1×(a－c)≥b……(1)

(2) Evaluation of average Cross-sectional area of Crystal grains

From the cold-rolled annealed sheet obtained under the above-mentioned production conditions, a test piece for texture observation having a width of 10mm × a length of 15mm was cut out, embedded in a resin so that a cross section in a longitudinal direction thereof became an observation surface, and then the observation surface was mirror-finishedAnd (6) grinding. Thereafter, the observation surface was etched with picric acid-hydrochloric acid solution (100mL of ethanol-1 g of picric acid-5 mL of hydrochloric acid) to form grain boundaries, and then the tissue was photographed at a magnification of 500 times with an optical microscope. The observed image thus obtained was plotted in an actual field of view as a circle of radius 100 μm (a circle of radius 50mm in the case of printing the observed image at a magnification of 500 times), and the number of crystal grains completely contained in the circle was defined as n₁N is the number of crystal grains cut from the circumference₂The average cross-sectional area A (. mu.m) of the given crystal grains was evaluated by measuring the average cross-sectional area A of the crystal grains and substituting the measurement results into the following formula (2)²)。

A＝31400/(n₁+0.6×n₂)……(2)

(3) Evaluation of 0.2% proof stress

The cold-rolled and annealed sheet obtained under the above-mentioned production conditions was subjected to a tensile test using a B test piece of JIS13 No. so that the sheet was long in the rolling direction (L direction), the rolling direction at 45 degrees (D direction) and the rolling direction at right angles (C direction). The tensile test was conducted in accordance with JIS Z2241, and the 0.2% proof stress of each test piece was evaluated.

(4) Evaluation of Corrosion resistance

From the cold-rolled annealed sheet obtained under the above-mentioned production conditions, a test piece having a length of 80mm × a width of 60mm was cut out by shearing. The surface of the test piece was polished to 400 # with emery paper, degreased with acetone, and then subjected to a corrosion test to evaluate corrosion resistance. The corrosion test was carried out based on JASO M609-91. The corrosion test was performed for 5 cycles by spraying a 5.0 mass% NaCl aqueous solution (35 ℃, relative humidity 98%) for 2h → drying (60 ℃, relative humidity 30%) for 4h → wetting (50 ℃, relative humidity 95% or more) for 2h for 1 cycle. After the test, the area ratio of rust was measured by image analysis on a 30mm × 30mm area in the center of the surface of the test piece from a photograph of the surface of the test piece. Then, the case where the rust area ratio was 20% or less was evaluated as "o (pass)", and the case where the rust area ratio exceeded 20% was evaluated as "a (fail)". In this evaluation, if it is O, the corrosion resistance is evaluated to be excellent.

The results obtained are shown in Table 1-2.

The ferritic stainless steel sheets according to the examples of the present invention (test Nos. 1-1 to 1-9) were evaluated for productivity as "O" and for average cross-sectional area of crystal grains of 200 μm²～400μm²The 0.2% proof stress in all three directions, i.e., the L direction, the D direction, and the C direction, was 230MPa to 300MPa, the corrosion resistance was evaluated as "o", and the steel plate had a 0.2% proof stress equivalent to AISI439, and was found to have excellent productivity and excellent corrosion resistance.

In the comparative examples of tests 1 to 10, the slab heating temperature was lower than the range of the present invention, and the 0.2% proof stress in the L direction and the C direction was lower than the range of the present invention.

In the comparative examples of tests 1 to 11, the slab heating time was shorter than the range of the present invention, and the 0.2% proof stress in the L direction was lower than the range of the present invention.

In the comparative examples of test nos. 1 to 12, the hot-rolled sheet winding temperature was higher than the range of the present invention, the average cross-sectional area of crystal grains was larger than the range of the present invention, and the 0.2% proof stress in the L direction and the C direction was lower than the range of the present invention.

In the comparative examples of test nos. 1 to 13, the winding temperature of the hot-rolled sheet was lower than the range of the present invention, the average cross-sectional area of crystal grains was larger than the range of the present invention, and the 0.2% proof stress was lower in all of the three directions of the L direction, the D direction and the C direction than the range of the present invention.

In the comparative examples of test nos. 1 to 14, the annealing temperature of the hot-rolled sheet was lower than the range of the present invention, the average cross-sectional area of the crystal grains was larger than the range of the present invention, and the 0.2% proof stress was lower in all of the three directions of the L direction, the D direction and the C direction than the range of the present invention.

In the comparative examples of test nos. 1 to 15, the annealing temperature of the hot-rolled sheet was higher than the range of the present invention, the average cross-sectional area of the crystal grains was larger than the range of the present invention, and the 0.2% proof stress was lower in all of the three directions of the L direction, the D direction and the C direction than the range of the present invention.

In the comparative examples of test nos. 1 to 16, the hot-rolled sheet annealing time was shorter than the range of the present invention, the average cross-sectional area of crystal grains was larger than the range of the present invention, and the 0.2% proof stress was lower in all of the three directions L, D and C than the range of the present invention.

In the comparative examples of test nos. 1 to 17, the hot-rolled sheet annealing time was longer than the range of the present invention, the average cross-sectional area of crystal grains was larger than the range of the present invention, and the 0.2% proof stress was lower in all of the three directions L, D and C than the range of the present invention.

In the comparative examples of test nos. 1 to 18, the annealing temperature of the cold-rolled sheet was lower than the range of the present invention, the average cross-sectional area of the crystal grains was smaller than the range of the present invention, and the 0.2% proof stress in all of the three directions of the L direction, the D direction and the C direction was higher than the range of the present invention.

In the comparative examples of test nos. 1 to 19, the annealing time of the cold-rolled sheet was shorter than the range of the present invention, the average cross-sectional area of the crystal grains was smaller than the range of the present invention, and the 0.2% proof stress in the D direction was higher than the range of the present invention.

In the comparative examples of test nos. 1 to 20, the annealing time of the cold-rolled sheet was longer than the range of the present invention, the average cross-sectional area of the crystal grains was larger than the range of the present invention, and the 0.2% proof stress was lower in all of the three directions of the L direction, the D direction and the C direction than the range of the present invention.

[ example 2]

Ferritic stainless steel having a composition shown in Table 2 was melted into a 100kg steel ingot (billet), and then heated at 1160 ℃ for 1 hour to be hot-rolled into a hot-rolled sheet having a thickness of 3.0 mm. Immediately after the completion of the final pass of hot rolling, the hot rolled sheet was air-cooled to 550 ℃, and then the hot rolled sheet was inserted into an electric furnace set at 550 ℃ and kept for 1 hour, and thereafter furnace cooling was performed in the electric furnace. The obtained hot rolled sheet was kept at 980 ℃ for 20 seconds, and then cooled in air to prepare a hot rolled annealed sheet. The hot-rolled annealed sheet was pickled with a sulfuric acid solution, and then pickled with a mixed solution of hydrofluoric acid and nitric acid to obtain a cold-rolled sheet, which was then cold-rolled to a sheet thickness of 1.0mm to obtain a cold-rolled sheet. After a part of the obtained cold-rolled sheet was kept at 900 ℃ for 100 seconds, air cooling was performed, and then surface grinding of the front and back surfaces was performed to remove surface scales, thereby producing a cold-rolled annealed sheet. The obtained cold-rolled sheet and cold-rolled annealed sheet were subjected to the above evaluation. Test nos. 2 to 32 and 2 to 33 are reference examples, the above test nos. 2 to 32 are constituent compositions of the SUH409L specification, and the above test nos. 2 to 33 are constituent compositions of the AISI439 specification.

The obtained results are shown in table 2.

In the ferritic stainless steel sheets (test Nos. 2-1 to 2-25) according to the examples of the present invention, the productivity was evaluated as "O" and the average cross-sectional area of crystal grains was 200 μm²～400μm²All of the three directions L, D and C were 230 to 300MPa in 0.2% proof stress, and the corrosion resistance was evaluated as "o", and the steel sheet had 0.2% proof stress equivalent to AISI439, and was found to have excellent productivity and corrosion resistance.

In the comparative examples of test Nos. 2 to 26, the Nb content was lower than the range of the components of the present invention, and the 0.2% proof stress was lower in all of the three directions L, D and C than the range of the present invention.

In the comparative examples of test nos. 2 to 27, since the content of Nb is higher than the range of the components of the present invention, productivity is deteriorated, the average cross-sectional area of crystal grains is smaller than the range of the present invention, and the 0.2% proof stress in all of the three directions of L direction, D direction and C direction is higher than the range of the present invention.

In the comparative examples of test nos. 2 to 28, since the content of V is lower than the range of the components of the present invention, the productivity is deteriorated, and the average cross-sectional area of crystal grains is smaller than the range of the present invention, the 0.2% proof stress in all of the three directions of the L direction, the D direction and the C direction is higher than the range of the present invention.

In comparative examples of test nos. 2 to 29, the content of V was higher than the range of the components of the present invention, the productivity was deteriorated, and the average cross-sectional area of crystal grains was smaller than the range of the present invention, and the 0.2% proof stress was higher in all of the three directions of the L direction, the D direction and the C direction than the range of the present invention.

In the comparative examples of test Nos. 2 to 30, the content of Si was higher than the range of the component of the present invention, and the 0.2% proof stress was higher in all of the L direction, the D direction and the C direction than in the range of the present invention.

In the comparative examples of test Nos. 2 to 31, the corrosion resistance was inferior because the Cr content was lower than the composition range of the present invention.

Test Nos. 2 to 32 are reference examples of the composition having the SUH409L standard. In test Nos. 2 to 32, the desired corrosion resistance and 0.2% proof stress could not be obtained.

Test Nos. 2 to 33 are reference examples of the composition of ingredients having AISI439 specification. Since test Nos. 2 to 33 contained Cr in an amount of 15.0 mass% or more, the raw material cost and the production cost became high.

Industrial applicability of the invention

The ferritic stainless steel sheet of the present invention is excellent in corrosion resistance and has a 0.2% proof stress equivalent to AISI439, and therefore, is suitable for automobile exhaust parts, locks, parts for home electric appliances, building materials, kitchen equipment, railway vehicles, parts for electric devices, and the like, and particularly suitable for automobile exhaust parts such as pipes for automobile exhaust, inverter cases, front hubs, intermediate hubs, mufflers, exhaust pipe throats, and the like. The ferritic stainless steel sheet of the present invention is particularly suitable as an inexpensive substitute steel for AISI 439-used parts.