KR102590079B1 - Ferritic stainless steel sheet and manufacturing method thereof - Google Patents

Abstract

To provide a ferritic stainless steel sheet with a Cr content of less than 15.0% by mass, excellent productivity and corrosion resistance, and a 0.2% proof stress equivalent to AISI439, and a method for manufacturing the same. In mass%, C: 0.004 to 0.020%, Si: 0.05 to 0.90%, Mn: 0.05 to 0.60%, P: 0.050% or less, S: 0.030% or less, Al: 0.001 to 0.100%, Cr: 13. 0% or more 15.0 %, Ti: 0.15 to 0.35%, Nb: 0.030 to 0.090%, V: 0.010 to 0.200%, and N: 0.004 to 0.020%, with the remainder being Fe and inevitable impurities, and the crystal grains. A ferritic stainless steel sheet having a structure with an average cross-sectional area of 200 to 400 ㎛ ² and a 0.2% proof stress in the L, D, and C directions of all of 230 to 300 MPa.

Description

Ferritic stainless steel sheet and manufacturing method thereof

The present invention relates to a ferritic stainless steel sheet and a method for manufacturing the same, and in particular, to a ferritic stainless steel sheet that is excellent in corrosion resistance and productivity and has a 0.2% proof stress equivalent to AISI439.

Stainless steel contains Cr in the steel, thereby forming a dense and chemically stable passive film on the surface of the steel, and is excellent in corrosion resistance. Among stainless steels, ferritic stainless steels are relatively inexpensive compared to austenitic stainless steels because they do not contain many expensive elements, and have characteristics such as a low coefficient of thermal expansion and magnetic properties, making them suitable for cooking utensils and automobiles. It is applied to various applications including exhaust system members.

One of the representative ferritic stainless steels is AISI439 (18 mass% Cr - 0.3 mass% Ti steel). AISI439 has excellent corrosion resistance, and the inclusion of Ti in the steel suppresses the occurrence of sensitization and provides excellent corrosion resistance of the weld zone. In addition, AISI439 is a ferritic stainless steel with a relatively low recrystallization temperature, and in the cold-rolled plate annealing process, which is one of the manufacturing processes, it is not an annealing line dedicated to stainless steel with a high maximum annealing temperature, but a normal annealing line with a relatively low maximum annealing temperature of about 900°C. Steel can be softened in an annealing line for both steel and stainless steel, and productivity is high, so it is relatively inexpensive. Therefore, AISI439 is applied to a wide range of applications, including automobile exhaust system members.

Meanwhile, in recent years, in the automobile exhaust system members described above, etc., improvements have been made in the structure of members to which steel sheets are applied, and there are cases where corrosion resistance at the level of AISI439 is no longer required for members where AISI439 was conventionally used. In these cases, SUH409L (11 mass% Cr - 0.2 mass% Ti steel) was examined as a replacement for AISI439.

SUH409L, like AISI439, has a relatively low recrystallization temperature, so productivity is high. Additionally, it is cheaper than AISI439 because it has a low Cr content, which increases raw material costs and manufacturing costs. However, in many cases, AISI439 cannot be replaced by SUH409L, and AISI439 has continued to be used.

There are two main reasons why SUH409L cannot replace the material of the member where AISI439 is used, as shown below. First, SUH409L has a lower content of Cr, an element that improves corrosion resistance, than AISI439, and has lower corrosion resistance than AISI439. Due to optimization of the member structure, etc., there are cases where steel sheets do not require corrosion resistance of the level of AISI439, but there are cases where SUH409L lacks corrosion resistance.

Next, SUH409L has a lower content of Cr, which is both a corrosion resistance improving element and a solid solution strengthening element, than AISI439, and has a 0.2% lower yield strength. A difference in the 0.2% proof strength of the steel sheet causes a change in the amount of so-called springback, which is when the steel sheet returns to its original shape slightly after processing such as bending is performed on the steel sheet. This difference in springback amount becomes a problem in processing steel sheets.

For example, in bending processing, the bending angle during processing is set to be larger than the target bending angle. As a result, the sum of the bending angle during machining and the angle returned by springback becomes exactly the target bending angle, and the desired machining shape is obtained.

Here, when SUH409L is processed using a conventional processing method optimized for AISI439, the amount of springback becomes small and the desired processing shape is not obtained. Since the amount of springback is estimated experimentally and empirically, in order to change the conventional processing method to one suitable for SUH409L, a great deal of time and cost are required to reexamine the processing method, and a new mold is manufactured for processing. There are times when it is necessary to do so. Therefore, replacement of AISI439 by SUH409L is often not achieved.

That is, there was a demand for a ferritic stainless steel sheet that was cheaper than AISI439, had better corrosion resistance than SUH409L, and had a 0.2% proof strength equivalent to AISI439. Therefore, the present inventors, like SUH409L and AISI439, assume that an annealing line for both plain steel and stainless steel can be used in the cold rolled sheet annealing process and that the Cr content is set to less than 15.0 mass% to reduce costs, and SUH409L It was decided to study a ferritic stainless steel sheet that improves corrosion resistance and has a 0.2% proof strength equivalent to AISI439.

A technique for increasing the 0.2% proof strength of ferritic stainless steel is disclosed in, for example, Patent Documents 1 and 2.

Patent Document 1 includes C: 0.015 mass% or less, Si: 0.5 mass% or less, Cr: more than 25.0 to 35.0 mass%, N: 0.020 mass% or less, and Ti: 0.50 mass% or less, with the remainder being inevitable impurities. Excluding, a ferritic stainless steel having a composition of Fe and having a minimum value of 0.2% proof stress in three directions of 320 N/mm2 or more and excellent impact puncture resistance is disclosed.

In Patent Document 2, 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 to 20 mass%, Ni: 0.5 mass% or less, Al: 0.001 to 0.05% by mass, Fe: substantially the remaining composition, and a processed ferrite structure in which Al ₂ O ₃ -based and/or Al ₂ O ₃ ·MgO-based inclusions with a size of 10 ㎛ or less are dispersed with a cleanliness of 0.06 or less. Work hardening materials for stainless steel sheets are disclosed.

Japanese Patent Publication No. 2007-9263 International Publication No. 2005/014873

In the technology disclosed in Patent Document 1, the crystal grains are refined for the purpose of improving the 0.2% yield strength of the steel, so it is necessary to contain Cr in excess of 25.0 mass%, which causes an increase in raw material cost and manufacturing cost, and the Cr content is There was a desire for something to be reduced.

In the technology disclosed in Patent Document 2, rolling processing is applied to softened steel for the purpose of improving the 0.2% proof strength of the steel. The present inventors manufactured and evaluated a ferritic stainless steel sheet in a laboratory using the component composition and manufacturing method disclosed in Patent Document 2, but it was difficult to stably obtain the target 0.2% yield strength.

The present invention was developed in consideration of the above problems, and its purpose is to provide a ferritic stainless steel sheet with a Cr content of less than 15.0% by mass, excellent productivity and corrosion resistance, and a 0.2% proof stress equivalent to AISI439, and a method for manufacturing the same. It is being done.

Here, in the present invention, “excellent productivity” means that annealing of the cold-rolled sheet at 900 ° C. This means that the hardness of the cold rolled annealed plate decreases until it satisfies equation (1). If equation (1) is satisfied, cold-rolled sheet annealing is possible at 900°C

The evaluation of the change in hardness of the cold-rolled sheet due to annealing was performed on a cold-rolled sheet obtained by cold-rolling a hot-rolled annealed sheet at a reduction ratio of 67%, and the hardness a of the cold-rolled sheet (cold-rolled sheet without annealing) and 900. This is carried out by comparing the hardness b of a cold-rolled annealed sheet subjected to cold-rolled annealing at ℃ for 20 s and the hardness c of a cold-rolled annealing sheet annealed at 1050 ℃ for 20 s as an index of sufficient softness. In the above evaluation, three test pieces with a length of 15 mm Measured at , the hardness a is set as above. Additionally, for the remaining two test pieces, cold-rolled sheet annealing was performed for 20 s at 900°C and 20 s at 1050°C, respectively, and then cut into a size of 15 mm in length × 10 mm in width, and the cross section of the cut test pieces was Vickers hardness (HV) is measured under the conditions described above, and the hardnesses b and c are taken as above, respectively. By annealing a cold-rolled sheet, the hardness of the steel sheet changes from a to c (softening), but more than 90% of the decrease in hardness due to softening is achieved by annealing at 900°C for 20 s. In other words, something that satisfies the following equation (1) is evaluated as “excellent in productivity.”

c + 0.1 × (a - c) ≥ b... (One)

In addition, in the present invention, "excellent corrosion resistance" means that the steel sheet is polished up to 400 times with emery abrasive paper, and then sprayed with a 5.0% by mass NaCl aqueous solution (2 hours, 35°C, 98%) in accordance with JASO M609-91. RH), drying (4 hours, 60°C, 30%RH), and wet (2 hours, 50°C, 95%RH or more) as one cycle. As a result of conducting a corrosion test of 5 cycles, it was found that the rust area ratio was 20% or less. indicates that

In addition, in the present invention, "having a 0.2% proof stress equivalent to AISI439" means a direction from the steel sheet in the rolling direction (L direction), a direction 45 degrees to the rolling direction (D direction), and a direction perpendicular to the rolling direction (C direction). A JIS 13 No. B test piece was taken so that each of the lengths was the same, and a tensile test was performed, showing that all of the obtained 0.2% yield strength was 230 MPa or more and 300 MPa or less.

The present inventors studied a ferritic stainless steel sheet with a Cr content of less than 15.0% by mass, excellent productivity and corrosion resistance, and further having a 0.2% proof stress equivalent to AISI439 in relation to the above problem. As a result, the following knowledge was obtained.

That is, in mass%, C: 0.004 to 0.020%, Si: 0.05 to 0.90%, Mn: 0.05 to 0.60%, P: 0.050% or less, S: 0.030% or less, Al: 0.001 to 0.100%, Cr: 1 3.0% A component composition containing less than 15.0%, Ti: 0.15 to 0.35%, Nb: 0.030 to 0.090%, V: 0.010 to 0.200%, and N: 0.004 to 0.020%, with the balance consisting of Fe and inevitable impurities; By making a ferritic stainless steel sheet with a structure in which the average cross-sectional area of the crystal grains is 200 to 400 ㎛ ² and a 0.2% proof stress in the L, D, and C directions of 230 to 300 MPa, productivity and corrosion resistance are excellent, and, A ferritic stainless steel sheet having a 0.2% proof strength equivalent to AISI439 can be obtained.

The mechanism can be thought of as follows.

In a ferritic stainless steel sheet with a lower Cr content than AISI439, increasing the 0.2% proof strength and making it equivalent to AISI439 is achieved by refining the crystal grains of the cold rolled annealed sheet and including an appropriate solid solution strengthening element in the steel.

Here, crystal grain refinement of the hot-rolled annealed sheet, which is an intermediate material obtained during manufacturing, is effective in refining the crystal grains of the cold-rolled annealed sheet. In addition, grain refinement of a hot-rolled annealed sheet is achieved by appropriate conditions for hot rolling and annealing of the hot-rolled sheet. Additionally, by cold rolling a hot-rolled annealed sheet with fine grains and then performing final annealing under appropriate conditions, a cold-rolled annealing sheet with fine grains can be obtained, and the yield strength can be improved by 0.2%.

In addition, Nb was selected as a solid solution strengthening element to increase the 0.2% proof strength of the cold rolled annealed sheet from the viewpoint of not causing a decrease in corrosion resistance. However, when Nb is contained, the recrystallization temperature of the cold rolled sheet increases. In response to this, a method was discovered to suppress the rise in recrystallization temperature by establishing an appropriate upper limit on the Nb content and by containing a combination of Nb and an appropriate amount of V in the steel. The increase in recrystallization temperature due to Nb is due to some Nb precipitating as fine NbC, which causes dislocations and pinning effects on grain boundaries. On the other hand, when V is contained in the steel, the NbC that precipitates mainly precipitates as a composite precipitate with coarse TiN ((Nb, V) C precipitates on the surface of coarse TiN), and it is thought that the increase in recrystallization temperature is suppressed. . By combining solid solution strengthening by Nb and crystal grain refinement as described above, a ferritic stainless steel sheet that is excellent in productivity and corrosion resistance and has a 0.2% proof stress equivalent to AISI439 can be realized.

The present invention is based on the above knowledge, and its main structure is as follows.

[1] In mass%,

C: 0.004 to 0.020%,

Si: 0.05 to 0.90%,

Mn: 0.05 to 0.60%,

P: 0.050% or less,

S: 0.030% or less,

Al: 0.001 to 0.100%,

Cr: 13.0% or more and less than 15.0%,

Ti: 0.15 to 0.35%,

Nb: 0.030 to 0.090%,

V: 0.010 to 0.200%, and

A component composition containing N: 0.004 to 0.020%, the balance being Fe and inevitable impurities,

It has a structure where the average cross-sectional area of the crystal grains is 200 to 400 ㎛ ² ,

A ferritic stainless steel sheet with a 0.2% proof stress in the L direction, D direction, and C direction all of 230 to 300 MPa.

[2] The above component composition is further expressed in mass%,

Ni: 0.01 to 0.60%,

Cu: 0.01 to 0.80%,

Co: 0.01 to 0.50%,

Mo: 0.01 to 1.00%, and

W: 0.01 to 0.50%

The ferritic stainless steel sheet according to [1], containing one or two or more types selected from among.

[3] The above component composition is further expressed in mass%,

Zr: 0.01 to 0.50%,

B: 0.0003 to 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 to 0.10%,

Sn: 0.01 to 0.50%, and

Sb: The ferritic stainless steel sheet according to [1] or [2] containing one or two or more types selected from among 0.01 to 0.50%.

[4] The ferritic stainless steel sheet according to any one of [1] to [3], which is used as an automobile exhaust system member.

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

A hot rolling process in which the steel material having the above chemical composition is held at a temperature of 1100 to 1250°C for 10 minutes or more, then hot rolled to form a hot rolled sheet, and then coiled at a coiling temperature of 500 to 600°C;

A hot-rolled sheet annealing process of obtaining a hot-rolled annealed sheet by subjecting the hot-rolled sheet after the hot-rolling process to a hot-rolled sheet annealing held at a temperature of 940 to 1000° C. for 5 to 180 seconds;

The hot-rolled annealed sheet after the hot-rolled sheet annealing process is cold-rolled to form a cold-rolled sheet, and then the cold-rolled sheet is annealed at a temperature of 880 to 900 ° C. for 5 to 180 seconds to obtain a cold-rolled annealed sheet. Method for manufacturing ferritic stainless steel sheet.

According to the present invention, it is possible to provide a ferritic stainless steel sheet with a Cr content of less than 15.0 mass%, excellent productivity and corrosion resistance, and further having a 0.2% yield strength equivalent to AISI439, and a method for manufacturing the same.

Hereinafter, the present invention will be described in detail.

First, the reason for limiting the ingredient composition in the present invention will be explained. In addition, “%” indicating the content of the components of the steel sheet means mass% unless otherwise specified.

C: 0.004 to 0.020%

C is an element effective in increasing the 0.2% yield strength of steel. This effect is obtained by setting the C content to 0.004% or more. However, if the C content exceeds 0.020%, the steel becomes hard and formability deteriorates and corrosion resistance deteriorates. Therefore, the C content is set to 0.004 to 0.020%. Preferably, the C content is 0.006% or more. More preferably, the C content is 0.008% or more. Also, preferably, the C content is 0.015% or less. More preferably, the C content is 0.012% or less.

Si: 0.05 to 0.90%

Si has a deoxidizing effect. This effect is 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 strength increases excessively. Therefore, the Si content is set to 0.05 to 0.90%. Preferably, the Si content is 0.07% or more. More preferably, the Si content is 0.10% or more. More preferably, the Si content is 0.15% or more. Even more preferably, the Si content is 0.22% or more. Also, preferably, the Si content is 0.80% or less. More preferably, the Si content is 0.60% or less.

Mn: 0.05 to 0.60%

Mn has a deoxidizing effect. This effect is obtained by setting the Mn content to 0.05% or more. However, when the Mn content exceeds 0.60%, precipitation and coarsening of MnS are promoted, and this MnS becomes the starting point of corrosion, thereby lowering the corrosion resistance of the steel sheet. Therefore, the Mn content is set to 0.05 to 0.60%. Preferably, the Mn content is 0.15% or more. Also, preferably, the Mn content is 0.30% or less.

P: 0.050% or less

P is an element that reduces corrosion resistance. In addition, P deteriorates hot workability by segregating at grain boundaries. Therefore, it is preferable that the P content is as small as possible, and is set to 0.050% or less. Preferably, the P content is 0.040% or less. More preferably, the P content is 0.030% or less.

S: 0.030% or less

S forms MnS as a precipitate with Mn. This MnS becomes the starting point of corrosion and reduces corrosion resistance. Therefore, it is preferable that the S content is lower, and is set to 0.030% or less. Preferably, the S content is 0.020% or less.

Al: 0.001 to 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 hardens, formability deteriorates, and corrosion resistance deteriorates. Therefore, the Al content is set to 0.001 to 0.100%. Preferably, the Al content is 0.030% or more. Also, preferably, the Al content is 0.060% or less.

Cr: 13.0% or more but less than 15.0%

Cr is an element that forms a passive film on the surface and improves corrosion resistance. If the Cr content is less than 13.0%, sufficient corrosion resistance cannot be obtained. On the other hand, when the Cr content is 15.0% or more, the raw material cost and manufacturing cost increase. Therefore, the Cr content is set to be 13.0% or more and less than 15.0%. Preferably, the Cr content is 13.5% or more. Also, preferably, the Cr content is 14.5% or less. Preferably, the Cr content is 14.0% or less.

Ti: 0.15 to 0.35%

Ti is an element that fixes C and N by forming carbonitride and suppresses the occurrence of sensitization. This effect is obtained by setting the Ti content to 0.15% or more. However, when the Ti content exceeds 0.35%, the steel becomes hard and formability deteriorates. Therefore, the Ti content is set to 0.15 to 0.35%. Preferably, the Ti content is 0.20% or more. Also, preferably, the Ti content is 0.30% or less.

Nb: 0.030 to 0.090%

Nb is an element effective in increasing the 0.2% proof strength of steel by existing in 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, when the Nb content exceeds 0.090%, even if the effect of suppressing the increase in recrystallization temperature due to V, which will be described later, is obtained, the recrystallization temperature of the steel increases, and when manufactured using an annealing line for both plain steel and stainless steel, Softening of the steel becomes insufficient or the crystal grains become excessively fine, increasing the yield strength by 0.2%. Therefore, the Nb content is set to 0.030 to 0.090%. Preferably, the Nb content is 0.035% or more. More preferably, the Nb content is 0.040% or more. Also, preferably, the Nb content is 0.080% or less. More preferably, the Nb content is 0.070% or less.

V: 0.010 to 0.200%

V is an element that increases productivity by suppressing the increase in 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 contained excessively, V carbonitride precipitates excessively, the recrystallization temperature rises, and the productivity of the steel is reduced. Therefore, the V content is set to 0.010 to 0.200%. Preferably, the V content is 0.020% or more. More preferably, the V content is 0.030% or more. Also, preferably, the V content is 0.150% or less. More preferably, the V content is 0.100% or less.

N: 0.004 to 0.020%

N is an element effective in increasing the 0.2% yield strength of steel. This effect is obtained by setting the N content to 0.004% or more. However, when the N content exceeds 0.020%, the steel becomes hardened and formability decreases and corrosion resistance decreases. Therefore, the N content is set to 0.004 to 0.020%. Preferably, the N content is 0.005% or more. More preferably, the N content is 0.007% or more. Also, preferably, the N content is 0.015% or less. More preferably, the N content is 0.012% or less.

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

In the present invention, in addition to the components described above, one or two types selected from the following Group A and Group B may be contained.

(Group A) One or two or more types selected from Ni: 0.01 to 0.60%, Cu: 0.01 to 0.80%, Co: 0.01 to 0.50%, Mo: 0.01 to 1.00%, and W: 0.01 to 0.50%

(Group B) Zr: 0.01 to 0.50%, B: 0.0003 to 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 to 0.10%, Sn: 0.01 to 0.50%, and Sb: 1 or 2 or more types selected from 0.01 to 0.50%

Ni: 0.01 to 0.60%

Ni increases the corrosion resistance of steel by suppressing active dissolution of steel in a low pH environment. On the other hand, if Ni is contained excessively, the component cost and manufacturing cost of the steel increase, and the steel becomes hard and formability deteriorates. Therefore, when containing Ni, the Ni content is set to 0.01 to 0.60%. Preferably, the Ni content is 0.10% or more. Also, preferably, the Ni content is 0.25% or less.

Cu: 0.01 to 0.80%

Cu is an element that improves the corrosion resistance of stainless steel. On the other hand, if Cu is contained excessively, the component cost and manufacturing cost of the steel increase, and ε-Cu becomes prone to precipitation, thereby reducing corrosion resistance. Therefore, when it contains Cu, the Cu content is set to 0.01 to 0.80%. Preferably, the Cu content is 0.30% or more. More preferably, the Cu content is 0.40% or more. Also, preferably, the Cu content is 0.50% or less. More preferably, the Cu content is 0.45% or less. Even more preferably, the Cu content is 0.42% or less.

Co: 0.01 to 0.50%

Co is an element that improves the corrosion resistance of stainless steel. On the other hand, if Co is contained excessively, the steel becomes hard and the 0.2% yield strength increases excessively. Therefore, when it contains Co, the Co content is set to 0.01 to 0.50%. Preferably, the Co content is 0.03% or more. More preferably, the Co content is 0.05% or more. Also, preferably, the Co content is 0.30% or less. More preferably, the Co content is 0.10% or less.

Mo: 0.01 to 1.00%

Mo has the effect of improving the corrosion resistance of stainless steel. On the other hand, if Mo is contained excessively, the component cost and manufacturing cost of the steel increase, and the steel becomes hard and the 0.2% yield strength increases excessively. Therefore, when it contains Mo, the Mo content is set to 0.01 to 1.00%. Preferably, the Mo content is 0.03% or more. More preferably, the Mo content is 0.05% or more. Also, preferably, the Mo content is 0.50% or less. More preferably, the Mo content is 0.30% or less.

W: 0.01 to 0.50%

W is an element that improves the corrosion resistance of stainless steel. On the other hand, if W is contained excessively, the steel becomes hard and the 0.2% yield strength increases excessively. Therefore, when it contains W, the W content is set to 0.01 to 0.50%. Preferably, the W content is 0.03% or more. More preferably, the W content is 0.05% or more. Also, preferably, the W content is 0.30% or less. More preferably, the W content is 0.10% or less.

Zr: 0.01 to 0.50%

Zr is an element that fixes C and N by forming carbonitride and improves the corrosion resistance of steel. On the other hand, if Zr is contained excessively, carbonitride precipitates excessively and the corrosion resistance of the steel deteriorates. Therefore, when containing Zr, 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. Also, preferably, the Zr content is 0.40% or less. More preferably, the Zr content is 0.30% or less.

B: 0.0003 to 0.0030%

B has the effect of improving the strength of steel. On the other hand, if B is contained excessively, the steel becomes hard and the 0.2% yield strength increases excessively. Therefore, when it contains B, the B content is set to 0.0003 to 0.0030%. Preferably, the B content is 0.0010% or more. Also, preferably, the B content is 0.0025% or less.

Mg: 0.0005 to 0.0100%

Mg acts as a deoxidizing agent. On the other hand, if Mg is contained excessively, surface defects increase. Therefore, when it contains Mg, the Mg content is set to 0.0005 to 0.0100%. Preferably, the Mg content is 0.0010% or more. Also, preferably, the Mg content is 0.0050% or less. More preferably, the Mg content is 0.0030% or less.

Ca: 0.0003 to 0.0030%

Ca acts as a deoxidizing agent. On the other hand, if Ca is contained excessively, surface defects increase. Therefore, when it contains Ca, the Ca content is set to 0.0003 to 0.0030%. Preferably, the Ca content is 0.0005% or more. More preferably, the Ca content is 0.0007% or more. Also, preferably, the Ca content is 0.0025% or less. More preferably, the Ca content is 0.0015% or less.

Y: 0.01 to 0.20%

Y is an element that improves the cleanliness of steel. On the other hand, if Y is contained excessively, surface defects increase. Therefore, when it contains Y, the Y content is set to 0.01 to 0.20%. Preferably, the Y content is 0.03% or more. Also, preferably, the Y content is 0.10% or less.

REM (Rare Earth Metals): 0.01 to 0.10 %

REM (rare earth metals: elements with atomic numbers 57 to 71 such as La, Ce, and Nd) are elements that improve the cleanliness of steel. On the other hand, if REM is contained excessively, surface defects increase. Therefore, when containing REM, the REM content is set to 0.01 to 0.10%. Preferably, the REM content is 0.02% or more. Also, preferably, the REM content is 0.05% or less. In addition, the REM content in the present invention is the total content of one or two or more elements selected from the REM described above.

Sn: 0.01 to 0.50%

Sn is an element effective in suppressing processing surface roughness. On the other hand, if Sn is contained excessively, the hot workability of the steel decreases. Therefore, when it contains Sn, the Sn content is set to 0.01 to 0.50%. Preferably, the Sn content is 0.03% or more. Also, preferably, the Sn content is 0.20% or less.

Sb: 0.01 to 0.50%

Sb, like Sn, is an element effective in suppressing processing surface roughness. On the other hand, if Sb is contained excessively, surface defects increase. Therefore, when containing Sb, the Sb content is set to 0.01 to 0.50%. Preferably, the Sb content is 0.03% or more. Also, preferably, the Sb content is 0.20% or less.

In addition, if the content of Ni, Cu, Co, Mo, W, Zr, B, Mg, Ca, Y, REM (rare earth metal), Sn, and Sb described above as the optional components is less than the lower limit, the components are regarded as unavoidable impurities. It is assumed to be included.

Average cross-sectional area of crystal grains: 200 ~ 400 ㎛ ²

In the present invention, in addition to controlling the content of various elements including Nb, by controlling the average cross-sectional area of the crystal grains in the structure to a predetermined range, a ferrite system having a 0.2% proof strength equivalent to AISI439 can be manufactured under excellent productivity. You can get stainless steel. Here, the average cross-sectional area of the crystal grains affects the 0.2% yield strength of the steel. If the average cross-sectional area of the crystal grains is less than 200 ㎛ ² , the 0.2% yield strength of the steel becomes high, and the 0.2% yield strength equivalent to AISI439 cannot be obtained. Moreover, if the average cross-sectional area of the crystal grains exceeds 400 ㎛ ² , the 0.2% yield strength of the steel becomes low, and the 0.2% yield strength equivalent to AISI439 cannot be obtained. Therefore, the average cross-sectional area of crystal grains in the structure is set to 200 to 400 μm ² . Preferably, the average cross-sectional area of the grains is 240 ㎛ ² or more. Also, preferably, the average cross-sectional area of the crystal grains is 360 ㎛ ² or less. Additionally, the average cross-sectional area of the crystal grains can be controlled by the manufacturing method described later.

Additionally, the average cross-sectional area of crystal grains can be evaluated by the following method. A test piece for structure observation measuring 10 mm wide x 15 mm long is cut from a ferritic stainless steel plate, embedded in resin so that the longitudinal cross section serves as an observation surface, and then the observation surface is mirror polished. Thereafter, the observation surface is etched with a picric acid solution (100 ml ethanol - 1 g picric acid - 5 ml hydrochloric acid), the grain boundaries are raised, and the tissue is photographed with an optical microscope at 500 times magnification. For the obtained observation image, a circle with a radius of 100 ㎛ is drawn as the actual field (a circle with a radius of 50 mm when the observation image is printed at a magnification of 500 times), and the number of crystal grains completely contained within the circle is n ₁ , The number of crystal grains cut along the circumference is each measured as n ₂ , and the obtained measurement results are substituted into the following equation (2) to evaluate the average cross-sectional area A (μm ² ) of the given crystal grains.

A = 31400/(n ₁ + 0.6 × n ₂ ) … (2)

0.2% yield strength in L direction: 230 to 300 MPa

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

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

In order to have a 0.2% yield strength equivalent to AISI439 and to obtain a springback amount equivalent to AISI439 when processed, the 0.2% yield strength in the L direction, C direction, and D direction of the ferritic stainless steel sheet is all in the range of 230 to 300 MPa. something is needed When the 0.2% proof stress in any direction is less than 230 MPa, when the steel is processed so that the direction perpendicular to the direction in which the 0.2% proof stress is less than 230 MPa becomes a bending ridge, the amount of springback becomes smaller compared to AISI439. Additionally, when the 0.2% proof stress in any direction exceeds 300 MPa, when the steel is processed so that the direction perpendicular to the direction in which the 0.2% proof stress exceeds 300 MPa becomes a bending ridge, the amount of springback increases compared to AISI439. . Therefore, the 0.2% proof stress in the L direction, D direction, and C direction is all set to 230 to 300 MPa. Preferably, the 0.2% yield strength is all 240 MPa or more. Also, preferably, the 0.2% yield strength is all 290 MPa or less.

In order to keep the springback amount of the ferritic stainless steel sheet, which has excellent corrosion resistance and can be manufactured with higher productivity, within the appropriate range as above, and to keep the 0.2% proof strength of the steel sheet within the appropriate range as above, the content of each element must be set to the appropriate range as described above. In order to keep the average cross-sectional area of crystal grains within the range described above, it is necessary to adjust the manufacturing method as described later.

Next, a preferred manufacturing method for the ferritic stainless steel sheet of the present invention will be described. Steel having the above-described chemical composition is melted by a known method such as a converter or electric furnace, and then used as a steel material (steel slab) by continuous casting or ingot-disintegration method. After holding this steel material at a temperature of 1100 to 1250°C for 10 minutes or more, hot rolling is performed to form a hot-rolled sheet. Afterwards, the hot-rolled sheet is wound at a coiling temperature of 500°C or more and 600°C or less to form a hot-rolled sheet coil. At this time, it is preferable to hot-roll the hot-rolled sheet so that the sheet thickness is 2.0 to 5.0 mm. The hot-rolled sheet produced in this way is subjected to hot-rolled sheet annealing held at a temperature of 940 to 1000°C for 5 to 180 seconds to obtain a hot-rolled annealed sheet. The atmosphere for hot-rolled sheet annealing is preferably an atmospheric atmosphere. Next, pickling is performed to remove scale. Next, cold rolling is performed to obtain a cold rolled sheet, and then the cold rolled sheet is annealed by holding the cold rolled sheet at a temperature of 880 to 900°C for 5 to 180 seconds to obtain a cold rolled annealed sheet. After annealing the cold-rolled sheet, pickling or surface grinding is performed to remove scale. Skin pass rolling may be performed on the cold rolled annealed sheet from which the scale has been removed. However, if the reduction ratio of skin pass rolling exceeds 2%, not only does the 0.2% proof strength increase excessively, but also the formability decreases. Therefore, when skin pass rolling is performed, the reduction ratio is set to 2% or less. It is desirable.

First, the method for controlling the average cross-sectional area of crystal grains in the preferred manufacturing method described above will be described below. By casting steel of the above-mentioned composition, a steel slab in which carbonitrides 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°C or higher, solid solution of TiN, TiC, NbC, and VC into the steel progresses. After hot rolling the steel slab, the hot-rolled sheet is cooled and wound at a coiling temperature of 500 ℃ or higher and 600 ℃ or lower to form a hot-rolled sheet coil. As a result, hot rolling deformation remains in the steel and Cr carbonitride is precipitated to form solid solution C or solid solution. A hot-rolled sheet with less N is obtained. In the obtained hot-rolled sheet, strain remains in the steel and there is little dissolved C or dissolved N, so recrystallization occurs even by annealing the hot-rolled sheet at a relatively low temperature of 940°C or higher and 1000°C or lower. Additionally, by setting the annealing temperature to a relatively low temperature, a hot-rolled annealed sheet with relatively small crystal grains can be obtained.

Next, the hot-rolled annealed sheet with 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°C or more and 900°C or less, thereby obtaining a cold-rolled annealed sheet with a desired crystal grain size.

Through the above-described process, a cold-rolled annealed sheet having an average cross-sectional area of crystal grains of 200 to 400 μm ² is obtained, and a ferritic stainless steel sheet having the desired 0.2% proof stress is obtained.

Below, in each step of the preferred manufacturing method described above, the reason for setting the conditions within the range described above will be explained in more detail.

A process of holding a steel slab at a temperature of 1100°C or more and 1250°C or less for 10 minutes or more and then hot rolling it to form a hot-rolled sheet, and then coiling it at a coiling temperature of 500 to 600°C (hot rolling process).

If the heating temperature of the steel slab is less than 1100°C, NbC in the steel is not sufficiently dissolved in solid solution, the effect of increasing the 0.2% proof strength due to Nb in the cold rolled annealed sheet is not obtained, and the 0.2% proof strength of the cold rolled annealed sheet decreases. Moreover, if the heating time of the steel slab is less than 10 minutes, NbC in the steel is not sufficiently dissolved, the effect of increasing the 0.2% proof strength due to Nb in the cold rolled annealed sheet is not obtained, and the 0.2% proof strength of the cold rolled annealed sheet decreases. do. Additionally, when the heating temperature of the steel slab exceeds 1250°C, deformation of the steel slab occurs and the manufacturability of the hot rolled sheet in the hot rolling process is reduced. Therefore, in the present invention, it is preferable to keep the steel slab at 1100°C or higher and 1250°C or lower for 10 minutes or more and then hot-roll it to obtain a hot-rolled sheet. More preferably, the heating temperature of the steel slab is above 1150°C. Moreover, the above heating time is more preferably 30 minutes or more. Also, more preferably, the heating temperature of the steel slab is 1200°C or lower. In addition, heating and holding the steel slab for an excessively long time causes deformation of the steel slab and reduces the manufacturability of the hot rolled sheet in the hot rolling process, so it is preferable that the heating time of the steel slab is 2 hours or less.

Moreover, if the coiling temperature of the hot-rolled sheet is less than 500°C, precipitation of Cr carbonitride into the steel becomes insufficient, and the amount of solid solution C and solid solution N contained in the hot-rolled sheet becomes excessive, resulting in a high recrystallization temperature of the hot-rolled sheet. This happens. In that 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 with a recrystallization structure, local high-strain points where the lattice strain is locally high are formed near grain boundaries, and these become recrystallization nuclei in the annealing process of the cold-rolled sheet, forming the crystal grains of the cold-rolled annealed sheet. Contributes to miniaturization. On the other hand, when a hot-rolled annealed sheet having a non-recrystallized structure is cold-rolled, local high strain points that become recrystallization nuclei are difficult to form in the steel during the annealing process of the cold-rolled sheet, and the crystal grains of the cold-rolled annealed sheet become coarse along with this. , the yield strength of the cold rolled annealed sheet decreases by 0.2%. When the coiling temperature of the hot-rolled sheet exceeds 600°C, the strain introduced into the hot-rolled sheet during the hot rolling process is recovered, and the recrystallization temperature of the hot-rolled sheet becomes high. In that 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 annealed sheet become coarse along with this, and the 0.2% yield strength of the cold-rolled annealed sheet decreases. Therefore, in the present invention, it is preferable to coil the hot-rolled sheet after hot rolling at a coiling temperature of 500°C or more and 600°C or less to form a hot-rolled sheet coil.

A process of making a hot-rolled annealed sheet by annealing the hot-rolled sheet by holding it at a temperature of 940 ℃ or higher and 1000 ℃ or lower for 5 to 180 seconds (hot-rolled sheet annealing process).

If the hot-rolled sheet annealing temperature is less than 940°C, the hot-rolled sheet does not recrystallize, and the crystal grains of the cold-rolled annealed sheet become coarse along with this, and the 0.2% yield strength of the cold-rolled annealed sheet decreases. When the hot-rolled annealing temperature exceeds 1000°C, 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% yield strength of the cold-rolled annealed sheet decreases. Moreover, if the holding time of the hot-rolled annealing sheet is less than 5 seconds, the hot-rolled sheet does not recrystallize, and the crystal grains of the cold-rolled annealed sheet become coarse along with this, and the 0.2% yield strength of the cold-rolled annealed sheet decreases. If the holding time of hot-rolled annealing sheet 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% yield strength of the cold-rolled annealed sheet decreases. Therefore, in the present invention, it is preferable to perform hot-rolled sheet annealing by holding the hot-rolled sheet at a temperature of 940°C or more and 1000°C or less for 5 to 180 seconds to obtain a hot-rolled annealed sheet. More preferably, the annealing temperature of the hot-rolled sheet ranges from 950°C to 980°C. Moreover, the above holding time is more preferably 10 seconds or more. Moreover, the above holding time is more preferably 30 seconds or less.

Next, the hot rolled annealed sheet after the hot rolled sheet annealing process is cold rolled to obtain a cold rolled sheet. The cold rolling reduction ratio at this time is preferably 50% or more. More preferably, it is 65% or more.

A process of making a cold-rolled annealed sheet by annealing the cold-rolled sheet by maintaining it at a temperature of 880 ℃ or higher and 900 ℃ or lower for 5 to 180 seconds (cold-rolled sheet annealing process)

If the cold-rolled sheet annealing temperature is less than 880°C, the crystal grains of the steel become excessively fine and the 0.2% proof strength becomes excessively high. On the other hand, cold-rolled sheet annealing at temperatures exceeding 900°C cannot be performed on a highly productive plain steel-stainless steel annealing line. Moreover, if the cold-rolled sheet annealing holding time is less than 5 seconds, the crystal grains of the steel become excessively fine, and the 0.2% proof strength becomes excessively high. On the other hand, when the cold-rolled sheet annealing holding time exceeds 180 seconds, the crystal grains of the steel become coarse and the 0.2% proof strength becomes excessively low. Therefore, in the present invention, it is preferable to perform annealing of the cold-rolled sheet by holding the cold-rolled sheet at 880°C or higher and 900°C or lower for 5 to 180 seconds. More preferably, the annealing temperature range of the cold rolled sheet is 890°C or higher. Moreover, the above holding time is more preferably 10 seconds or more. Moreover, the above holding time is more preferably 120 seconds or less.

Example

[Example 1]

After melting the ferritic stainless steel having the composition shown in Table 1-1 into a 100 kg steel ingot (steel material), each slab heating time shown in Table 1-2 was maintained at each slab heating temperature shown in Table 1-2. Afterwards, hot rolling was performed to obtain a hot-rolled sheet with a thickness of 3.0 mm. Immediately after the final pass of hot rolling is completed, the hot-rolled sheet is air-cooled to each coiling temperature listed in Table 1-2, then the hot-rolled sheet is inserted into an electric furnace and held at each coiling temperature for 1 hour, and then furnace-cooled in the electric furnace. did. In addition, the process of inserting this hot-rolled sheet into an electric furnace, holding it at each coiling temperature for 1 hour, and then furnace cooling it in the electric furnace involves winding the hot-rolled sheet after hot rolling into a coil at each coiling temperature in the actual production line. This simulates the temperature history of then slow cooling.

[Table 1-1]

The obtained hot-rolled sheet was maintained at each hot-rolled sheet annealing temperature shown in Table 1-2 and each hot-rolled sheet annealing time shown in Table 1-2, and then air-cooled to obtain a hot-rolled annealed sheet. This hot-rolled annealed sheet was pickled with a sulfuric acid solution and then with a mixed solution of hydrofluoric acid and nitric acid to obtain a cold rolling material. Afterwards, cold rolling was performed to a sheet thickness of 1.0 mm to obtain a cold-rolled sheet. Some of the obtained cold-rolled sheets were held at the cold-rolled sheet annealing temperatures listed in Table 1-2 for each cold-rolled sheet annealing time listed in Table 1-2, then air-cooled, and then subjected to surface grinding on the front and back surfaces. The scale was removed and a cold rolled annealed plate was obtained. The obtained cold rolled sheet and cold rolled annealed sheet were subjected to the following evaluation.

(1) Evaluation of productivity

The hardness a of the cold-rolled sheet obtained under the above manufacturing conditions, the hardness b of the cold-rolled annealed sheet obtained by subjecting the cold-rolled sheet to cold-rolled annealing at 900°C for 20 s, and the hardness b of the cold-rolled annealed sheet obtained at 1050°C for 20 s as an indicator of sufficient softness. The change in hardness of the cold rolled sheet accompanying annealing was evaluated by comparing the hardness c of the cold rolled annealed sheet. Specifically, three test pieces with a length of 15 mm x a width of 20 mm were cut from the cold-rolled sheet, and the Vickers hardness (HV) of the cross section of one of them was measured, and the hardness a was set as above. Additionally, the remaining two test pieces were annealed at 900°C for 20 s and at 1050°C for 20 s, respectively, and then cut to a size of 15 mm in length x 10 mm in width. The Vickers hardness of the cross section of the cut test pieces was (HV) was measured and set as the above hardness b and c, respectively. After embedding the test piece in resin, the test surface was mirror-polished and subjected to testing. The measurement conditions for Vickers hardness were test force of 9.8 N and holding time of 15 seconds. The measured hardnesses a, b, and c were evaluated as "○ (pass)" if they satisfied equation (1), and as "▲ (fail)" if they did not satisfy the formula (1). In this evaluation, if ○, cold-rolled sheet annealing can be performed on an annealing line for both plain steel and stainless steel, and productivity can be evaluated as excellent.

c + 0.1 × (a - c) ≥ b... (One)

(2) Evaluation of the average cross-sectional area of grains

From the cold-rolled annealed plate obtained under the above manufacturing conditions, a test piece for tissue observation measuring 10 mm wide x 15 mm long was cut and embedded in resin so that the longitudinal cross-section became the observation surface, and then the observation surface was mirror-polished. After that, the observation surface was etched with a picric acid solution (100 ml ethanol - 1 g picric acid - 5 ml hydrochloric acid), the grain boundaries were made to stand out, and the tissue was photographed with an optical microscope at a magnification of 500 times. For the obtained observation image, a circle with a radius of 100 ㎛ is drawn as the actual field (a circle with a radius of 50 mm when the observation image is printed at a magnification of 500 times), and the number of crystal grains completely contained within the circle is n ₁ , The number of crystal grains cut along the circumference was measured as n ₂ , and the obtained measurement results were substituted into the following equation (2) to evaluate the average cross-sectional area A (μm ² ) of the given crystal grains.

A = 31400/(n ₁ + 0.6 × n ₂ ) … (2)

(3) Evaluation of 0.2% proof strength

From the cold-rolled annealed sheet obtained under the above manufacturing conditions, a JIS 13 standard is obtained so that the lengths are in the rolling direction (L direction), the direction 45 degrees to the rolling direction (D direction), and the direction perpendicular to the rolling direction (C direction). A test piece B was collected and a tensile test was performed. The tensile test was conducted in accordance with JIS Z 2241, and the 0.2% yield strength of each obtained test piece was evaluated.

(4) Evaluation of corrosion resistance

From the cold rolled annealed plate obtained under the above manufacturing conditions, a test piece with a length of 80 mm × width of 60 mm was cut by shearing. The surface of the test piece was polished up to 400 times with emery paper, degreased with acetone, and then subjected to a corrosion test to evaluate corrosion resistance. The corrosion test was conducted in accordance with JASO M609-91. One cycle is 5.0 mass% NaCl aqueous solution spraying (35°C, relative humidity 98%) for 2 h → drying (60°C, relative humidity 30%) for 4 h → wetting (50°C, relative humidity 95% or more) for 2 h, Five cycles of corrosion testing were performed. After the test, the water repellent area ratio was measured by image analysis in a 30 mm x 30 mm area in the center of the test piece surface from a photograph of the test piece surface. Then, those with a water repellency area ratio of 20% or less were evaluated as “○ (Pass)”, and those with a water repellency area ratio of more than 20% were evaluated as “▲ (Fail)”. If it is ○ in this evaluation, it can be evaluated that the corrosion resistance is excellent.

The obtained results are shown in Table 1-2.

[Table 1-2]

The ferritic stainless steel sheets (Test Nos. 1-1 to 1-9) of the present invention have an evaluation of productivity of "○", an average cross-sectional area of crystal grains of 200 ㎛ ² or more and 400 ㎛ ² or less, and the L and D directions. And the 0.2% proof strength in all three directions in the C direction is 230 MPa or more and 300 MPa or less, and the corrosion resistance rating is "○", which has a 0.2% proof strength equivalent to AISI439, excellent productivity, and corrosion resistance. I could see that it was excellent.

In the comparative example of Test No. 1-10, the slab heating temperature was lower than the range of the present invention, and the 0.2% yield strength in the L direction and the C direction was lower than the range of the present invention.

In the comparative example of Test No. 1-11, the slab heating time was shorter than the range of the present invention, and the 0.2% yield strength in the L direction was lower than the range of the present invention.

In the comparative example of Test No. 1-12, the hot-rolled sheet coiling temperature is higher than the range of the present invention, the average cross-sectional area of the crystal grains is larger than the range of the present invention, and the yield strength in the L direction and C direction is 0.2% compared to the range of the present invention. It was low.

In the comparative example of Test No. 1-13, the hot-rolled sheet coiling temperature 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 3 directions in the L direction, D direction, and C direction were larger than the range of the present invention. The 0.2% proof strength in all directions was low.

In the comparative example of Test No. 1-14, the hot-rolled sheet annealing temperature 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 3 directions in the L direction, D direction, and C direction were larger than the range of the present invention. The 0.2% proof strength in all directions was low.

In the comparative example of Test No. 1-15, the hot-rolled sheet annealing temperature is higher than the range of the present invention, the average cross-sectional area of the crystal grains is larger than the range of the present invention, and 3 directions in the L direction, D direction, and C direction are larger than the range of the present invention. The 0.2% proof strength in all directions was low.

In the comparative example of Test No. 1-16, the hot-rolled sheet annealing time is shorter than the range of the present invention, the average cross-sectional area of the crystal grains is larger than the range of the present invention, and 3 directions in the L direction, D direction, and C direction are larger than the range of the present invention. The 0.2% proof strength in all directions was low.

In the comparative example of Test No. 1-17, the hot-rolled sheet annealing time is longer than the range of the present invention, the average cross-sectional area of the crystal grain is larger than the range of the present invention, and 3 directions in the L direction, D direction, and C direction are larger than the range of the present invention. The 0.2% proof strength in all directions was low.

In the comparative example of Test No. 1-18, the cold-rolled sheet annealing temperature 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 3 directions in the L direction, D direction, and C direction were smaller than the range of the present invention. The 0.2% yield strength was high in all directions.

In the comparative example of Test No. 1-19, the cold-rolled sheet annealing time 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% yield strength in the D direction was higher than the range of the present invention.

In the comparative example of Test No. 1-20, the cold-rolled sheet annealing time is longer than the range of the present invention, the average cross-sectional area of the crystal grain is larger than the range of the present invention, and 3 directions in the L direction, D direction, and C direction are larger than the range of the present invention. The 0.2% proof strength in all directions was low.

[Example 2]

Ferritic stainless steel having the component composition shown in Table 2 was melted into a 100 kg steel ingot (steel material), heated at a temperature of 1160°C for 1 hour, and hot rolled to obtain a hot rolled sheet with a thickness of 3.0 mm. Immediately after the final pass of hot rolling was completed, the hot-rolled sheet was air-cooled to 550°C, then the hot-rolled sheet was inserted into an electric furnace set at 550°C, held for 1 hour, and then furnace-cooled in the electric furnace. The obtained hot-rolled sheet was held at 980°C for 20 seconds and then air-cooled to obtain a hot-rolled annealed sheet. The hot-rolled annealed sheet was pickled with a sulfuric acid solution and then with a mixed solution of hydrofluoric acid and nitric acid to obtain a material for cold rolling. Afterwards, cold rolling was performed to a sheet thickness of 1.0 mm to obtain a cold-rolled sheet. A part of the obtained cold-rolled sheet was held at 900°C for 100 seconds, then air-cooled, and then surface grinding was performed on the front and back surfaces to remove surface scale, thereby forming a cold-rolled annealed sheet. The obtained cold rolled sheet and cold rolled annealed sheet were subjected to the evaluation described above. In addition, Test No. 2-32 and 2-33 are reference examples, Test No. 2-32 is the component composition of the SUH409L standard, and Test No. 2-33 is the component composition of the AISI439 standard.

The obtained results are shown in Table 2.

[Table 2]

The ferritic stainless steel sheet (Test No. 2-1 to 2-25) of the present invention has an evaluation of productivity of "○", an average cross-sectional area of crystal grains of 200 ㎛ ² or more and 400 ㎛ ² or less, and the L direction and D direction. And the 0.2% proof strength in all three directions in the C direction is 230 MPa or more and 300 MPa or less, and the corrosion resistance rating is "○", which has a 0.2% proof strength equivalent to AISI439, excellent productivity, and corrosion resistance. I could see that it was excellent.

In the comparative example of Test No. 2-26, the Nb content was lower than the component range of the present invention, and the yield strength in all three directions of the L direction, D direction, and C direction was 0.2% lower than the range of the present invention.

In the comparative example of Test No. 2-27, since the Nb content is higher than the component range of the present invention, productivity is lowered and the average cross-sectional area of the crystal grains is smaller than the range of the present invention, and in the L direction than the range of the present invention, The 0.2% yield strength in all three directions, D and C, was high.

In the comparative example of Test No. 2-28, since the V content is lower than the component range of the present invention, productivity is lowered and the average cross-sectional area of the crystal grains is smaller than the range of the present invention, and in the L direction than the range of the present invention, The 0.2% yield strength in all three directions, D and C, was high.

In the comparative example of Test No. 2-29, the V content was higher than the component range of the present invention, the productivity was low, the average cross-sectional area of the crystal grains was smaller than the range of the present invention, and the L direction and D were larger than the range of the present invention. The yield strength was high at 0.2% in all three directions including C direction and C direction.

In the comparative example of Test No. 2-30, the Si content was higher than the component range of the present invention, and the yield strength in all three directions of the L direction, D direction, and C direction was 0.2% higher than the range of the present invention.

The comparative example of Test No. 2-31 had poor corrosion resistance because the Cr content was lower than the component range of the present invention.

Test No.2-32 is a reference example with a component composition of the SUH409L standard. In Test No. 2-32, the desired corrosion resistance and 0.2% proof strength were not obtained.

Test No. 2-33 is a reference example with a component composition according to the AISI439 standard. Since Test No. 2-33 contains 15.0% by mass or more of Cr, the raw material cost and manufacturing cost are high.

Industrial applicability

The ferritic stainless steel sheet of the present invention is excellent in corrosion resistance and has a 0.2% proof strength equivalent to AISI439, so it is suitable for automobile exhaust system members, lockers, home appliance parts, building materials, kitchen appliances, railway vehicles, and electric device parts. In particular, it is suitable for automobile exhaust system members such as automobile exhaust pipes, converter cases, front pipes, center pipes, mufflers, and muffler cutters. The ferritic stainless steel sheet of the present invention is particularly suitable as an inexpensive substitute steel for members using AISI439.

Claims (9)

In mass%,
C: 0.004 to 0.020%,
Si: 0.05 to 0.90%,
Mn: 0.05 to 0.60%,
P: 0.050% or less,
S: 0.030% or less,
Al: 0.001 to 0.100%,
Cr: 13.0% or more and less than 15.0%,
Ti: 0.15 to 0.35%,
Nb: 0.030 to 0.090%,
V: 0.010 to 0.200%, and
A component composition containing N: 0.004 to 0.020%, the balance being Fe and inevitable impurities,
It has a structure where the average cross-sectional area of the crystal grains is 200 to 400 ㎛ ² ,
The 0.2% yield strength in the L direction, D direction, and C direction is all 230 to 300 MPa,
After the steel sheet was polished up to 400 times with emery abrasive paper, sprayed with a 5.0 mass% NaCl aqueous solution at 35°C and 98%RH for 2 hours, dried at 60°C and 30%RH for 4 hours, in accordance with JASO M609-91, As a result of conducting a corrosion test of 5 cycles with 1 cycle of wetting at 50°C and 95%RH or higher for 2 hours, the rust area rate was 20% or less,
Hardness a (HV) of a cold rolled sheet obtained by cold rolling a hot rolled annealed sheet at a reduction ratio of 67%, hardness b (HV) of a cold rolled annealed sheet obtained by annealing the cold rolled sheet for 20 s at 900°C, and 20 s at 1050°C. A ferritic stainless steel sheet whose hardness c (HV) is c + 0.1 According to claim 1,
The above component composition is further expressed in mass%,
Ni: 0.01 to 0.60%,
Cu: 0.01 to 0.80%,
Co: 0.01 to 0.50%,
Mo: 0.01 to 1.00%, and
W: 0.01 to 0.50%
A ferritic stainless steel sheet containing one or two or more selected from among. According to claim 1,
The above component composition is further expressed in mass%,
Zr: 0.01 to 0.50%,
B: 0.0003 to 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 to 0.10%,
Sn: 0.01 to 0.50%, and
Sb: A ferritic stainless steel sheet containing one or two or more types selected from 0.01 to 0.50%. According to claim 2,
The above component composition is further expressed in mass%,
Zr: 0.01 to 0.50%,
B: 0.0003 to 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 to 0.10%,
Sn: 0.01 to 0.50%, and
Sb: A ferritic stainless steel sheet containing one or two or more types selected from 0.01 to 0.50%. According to claim 1,
Ferritic stainless steel sheet for automobile exhaust system components. According to claim 2,
Ferritic stainless steel sheet for automobile exhaust system components. According to claim 3,
Ferritic stainless steel sheet for automobile exhaust system components. According to claim 4,
Ferritic stainless steel sheet for automobile exhaust system components. A method for manufacturing a ferritic stainless steel sheet according to any one of claims 1 to 8, comprising:
A hot rolling process in which the steel material having the above chemical composition is held at a temperature of 1100 to 1250°C for 10 minutes or more, then hot rolled to form a hot rolled sheet, and then coiled at a coiling temperature of 500 to 600°C;
A hot-rolled sheet annealing process of obtaining a hot-rolled annealed sheet by subjecting the hot-rolled sheet after the hot-rolling process to a hot-rolled sheet annealing held at a temperature of 940 to 1000° C. for 5 to 180 seconds;
The hot-rolled annealed sheet after the hot-rolled sheet annealing process is cold-rolled to form a cold-rolled sheet, and then the cold-rolled sheet is annealed at a temperature of 880 to 900 ° C. for 5 to 180 seconds to obtain a cold-rolled annealed sheet. Method for manufacturing ferritic stainless steel sheet.