JP6841150B2 - Ferritic stainless steel sheet for heat-resistant members - Google Patents

Description

The present invention relates to a ferritic stainless steel sheet for heat-resistant members.

Stainless steel, which has excellent oxidation resistance and corrosion resistance at high temperatures, is used for heat-resistant members such as automobile exhaust system members (for example, exhaust manifolds and mufflers). In recent years, the temperature of exhaust gas tends to rise in order to improve the combustion efficiency of an internal combustion engine, and an exhaust system member having higher heat resistance is required.

In order to improve the heat resistance of the exhaust system member of an automobile, there are a method of improving the heat resistance of stainless steel itself, which is a material of the exhaust system member, and a method of improving the structure of the exhaust system member. In order to adapt to the high operating temperature required in recent years, it is necessary to proceed with development by combining these two methods. Therefore, the stainless steel used for the exhaust system member is required to have both higher heat resistance and high workability for expanding the degree of freedom in structural design of the exhaust system member.

Conventionally, many ferrite-based stainless steels used as heat-resistant materials improve high-temperature strength by adding Nb. However, it was difficult to obtain sufficient processability because the matrix itself was cured by the addition of Nb. Therefore, development has been promoted to improve the workability of the ferrite heat-resistant stainless steel containing Nb, particularly the elongation and the r value.

Patent Document 1 contains C: 0.005% or less and N: 0.012% or less in mass%, and either or both of Ti and Nb are C / 12 + N / 14 <Ti / 48 + Nb / 93. When ferritic stainless steel contained so as to satisfy the above conditions is hot-finished, rolling is performed in a temperature range of 1050 ° C to 900 ° C with a total rolling reduction of at least 70% or more with a friction coefficient of 0.2 or less. After finishing at a temperature of 900 ° C or higher and then winding at 750 ° C or higher, normal cold rolling and annealing are performed to obtain a ferritic stainless steel sheet for deep drawing with excellent rigging resistance and low anisotropy. The method of manufacture is disclosed.

Patent Document 2 describes a steel containing 10 to 20% by mass of Cr, with {111} oriented crystal grains and {554} in the region from the outermost layer to 1/4 of the plate thickness in the cross-sectional thickness direction. Disclosed is a ferrite-based stainless steel sheet for exhaust parts, which has excellent workability, in which the abundance ratio N1 of the azimuth crystal grains and the abundance ratio N2 of the {100} azimuth crystal grains and {110} azimuth grains satisfy N1 / N2 ≧ 3.0. ing. The {111} oriented crystal grains, {554} oriented crystal grains, {100} oriented crystal grains, and {110} oriented crystal grains are the <111> direction, <554> direction, <100> direction, and the respective crystal grains. <110> Crystal grains whose direction is perpendicular to the rolling surface and within 15 °.

Japanese Unexamined Patent Publication No. 8-31152 Japanese Unexamined Patent Publication No. 2006-23278

According to these conventional techniques, the elongation and the r-value itself can be improved to some extent. On the other hand, the anisotropy Δr of the r value can achieve the level of anisotropy Δr: 0.1 or less required for ferritic stainless steel sheets for heat-resistant members such as automobile exhaust systems in recent years whose structure has become complicated. Almost does not exist. For this reason, when a ferritic stainless steel sheet for heat-resistant members is formed into a desired shape by deep drawing or the like, it is generated due to the difference in material properties between the rolling direction and the rolling perpendicular direction of the material to be rolled, which is also called an ear (also called an earring). ) Is large, and the yield during molding decreases. For this reason, it has been difficult for conventional ferritic stainless steel sheets for heat-resistant members to have both workability at room temperature and high-temperature strength.

An object of the present invention is to provide a ferritic stainless steel sheet for heat-resistant members, which has a small r-value anisotropy Δr and can achieve both workability at room temperature and high-temperature strength at a high level.

As a result of diligent studies to solve the above problems, the present inventors have made cold products by changing the heating temperature and coil winding temperature during hot rolling and omitting the annealing of hot rolled sheets. It was found that although the annealed plate has a low r value, its anisotropy Δr is very small, and it is difficult for ears to be formed during deep drawing. The present invention is based on such novel findings and is as listed below.

(1) The chemical composition is C: 0.001 to 0.02%, Si: more than 0.2% and 3.0% or less, Mn: 0.25 to 1.0%, P: 0. 01 to 0.08%, S: 0.0001 to 0.0100%, Cr: 10 to 20%, Nb: more than 0.32% and 1.0% or less, Al: 0.0005 to 0.500%, N : 0.005 to 0.02%, Ni: 0 to 1%, Mo: 0 to 3.0%, Cu: 0 to 1%, Ti: 0 to 0.2%, V: 0 to 0.5% , B: Contains 0-0.0050%, the balance is Fe and impurities,
As a metal structure, the area ratio of ferrite crystal grains having a crystal grain size of 20 μm or less in terms of a perfect circle is 3.0 to 15.0%.
As mechanical properties, the anisotropy Δr of the r value defined by the following equation (1) is 0.1 or less, and 0.2 in a 700 ° C. tensile test using a test piece whose rolling direction is the longitudinal direction. Ferritic stainless steel sheet for heat-resistant members with a% yield strength of 75 MPa or more.
Δr = | (r ₀ + r ₉₀ ) / 2-r ₄₅ | ... (1)

In the formula (1), r ₀ , r ₄₅ and r ₉₀ are test pieces having the rolling direction of the ferrite-based stainless steel sheet for heat-resistant members, the direction of 45 ° with respect to the rolling direction, and the direction perpendicular to the rolling as the longitudinal direction, respectively. It is an r value obtained by performing a tensile test at room temperature using the product in accordance with the following equation (2) in each direction.
r = ln (W / W ₀ ) / ln (t / t ₀ ) ... (2)
In equation (2), W and t, and W ₀ and t ₀ are the width and plate thickness of the test piece after tensile plastic deformation and the width and plate thickness of the test piece before tensile plastic deformation, respectively. ..

(2) The ferrite-based stainless steel sheet for heat-resistant members according to Item 1, which contains Ni: 0.01 to 1% by mass.

(3) Item 1 or 2 containing one or more of Mo: 0.03 to 3.0%, Cu: 0.01 to 1%, and Ti: 0.01 to 0.2% in mass%. Ferritic stainless steel sheet for heat-resistant members described in.

(4) The ferrite-based stainless steel sheet for heat-resistant members according to any one of Items 1 to 3, which contains V: 0.01 to 0.5% by mass.

(5) The ferrite-based stainless steel sheet for heat-resistant members according to any one of Items 1 to 4, which contains B: 0.0001 to 0.0050% by mass.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a ferritic stainless steel sheet for heat-resistant members, which has a small r-value anisotropy Δr and has excellent workability at room temperature and high-temperature strength at a high level.

The present invention will be described. In the following description, "%" regarding the chemical composition means "mass%" unless otherwise specified.

1. 1. Ferritic stainless steel sheet for heat-resistant members according to the present invention (1) Chemical composition First, essential elements will be described.

(1-1) C: 0.001 to 0.02%
C forms a carbonitride with Nb, Ti, etc. during hot rolling, coil winding, and annealing to suppress crystal grain growth due to the pinning effect, and has a strip-shaped crystal grain size of 20 μm or less in terms of perfect circle. Contributes to the formation of fine grain structure. Therefore, the C content is 0.001% or more, preferably 0.003% or more.

On the other hand, when the C content exceeds 0.02%, the moldability and corrosion resistance of the ferrite stainless steel sheet for heat-resistant members are deteriorated, and the high-temperature strength is lowered. Therefore, the C content is 0.02% or less, preferably 0.01% or less.

(1-2) Si: More than 0.2% and 3.0% or less Si is useful as a deoxidizer in refining and improves high temperature strength and oxidation resistance of ferrite stainless steel sheets for heat-resistant members. .. The high-temperature strength of the ferrite-based stainless steel sheet for heat-resistant members around 700 ° C. improves as the Si content increases, and this effect is exhibited when the Si content exceeds 0.2%. Therefore, the Si content is more than 0.2%, and is preferably 0.5% or more in order to improve the oxidation resistance and workability at room temperature of the ferritic stainless steel sheet for heat-resistant members.

On the other hand, when the Si content exceeds 3.0%, the workability of the ferrite stainless steel sheet for heat-resistant members at room temperature is lowered. Therefore, the Si content is 3.0% or less, and is preferably 2.0% or less in order to enhance the oxidation resistance of the ferrite stainless steel sheet for heat-resistant members.

(1-3) Mn: 0.2 to 1.0%
Mn is added as a deoxidizer during refining and also contributes to an increase in high-temperature strength in a medium temperature range of a ferrite stainless steel sheet for heat-resistant members. Further, Mn forms an Mn-based oxide on the surface layer of the ferritic stainless steel sheet for heat-resistant members during long-term use, and contributes to maintaining the scale adhesion of the ferritic stainless steel sheet for heat-resistant members and suppressing abnormal oxidation. Therefore, the Mn content is 0.2% or more, and is preferably 0.22% or more in order to improve the high temperature ductility and scale adhesion of the ferrite stainless steel sheet for heat-resistant members.

On the other hand, if the Mn content exceeds 1.0%, the processability of the ferrite stainless steel sheet for heat-resistant members at room temperature is lowered, and further, the corrosion resistance is deteriorated due to the formation of MnS. Therefore, the Mn content is 1.0% or less, and preferably 0.5% or less in order to enhance the high temperature ductility of the ferrite stainless steel sheet for heat-resistant members.

(1-4) P: 0.01 to 0.08%
Like Mn and Si, P is a solid solution strengthening element, and the P content is 0.08% or less in order to ensure the workability of ferrite stainless steel sheets for heat-resistant members at room temperature, and heat resistance. Considering the corrosion resistance of the ferrite stainless steel sheet for members, it is preferably 0.06% or less.

On the other hand, excessively reducing the P content leads to an increase in refining cost and suppresses the formation of precipitates which are considered to contribute to pinning of crystal grain growth such as FeTiP. Therefore, the P content is 0.01% or more, and is preferably 0.015% or more in consideration of the manufacturing cost of the ferritic stainless steel sheet for heat-resistant members and the formation of the strip-shaped fine grain structure.

(1-5) S: 0.0001 to 0.0100%
S is an element harmful to the hot workability and corrosion resistance of the ferrite stainless steel sheet for heat-resistant members, and is an impurity inevitably contained in the raw material. Therefore, the S content is preferably low. Therefore, the S content is 0.0100% or less, and is preferably 0.0050% or less in consideration of the hot workability and corrosion resistance of the ferrite stainless steel sheet for heat-resistant members.

On the other hand, excessively reducing the S content leads to an increase in the desulfurization load in the refining step, and the refining cost increases remarkably. Therefore, the S content is 0.0001% or more, and is preferably 0.0003% or more in consideration of the manufacturing cost of the ferritic stainless steel sheet for heat-resistant members.

(1-6) Cr: 10 to 20%
The Cr content is 10% or more from the viewpoint of corrosion resistance of the ferritic stainless steel sheet for heat-resistant members, and preferably 11% or more in order to secure the oxidation resistance and high-temperature strength of the ferritic stainless steel sheet for heat-resistant members. ..

On the other hand, if the Cr content exceeds 20%, the toughness of the ferrite-based stainless steel sheet for heat-resistant members deteriorates, resulting in poor manufacturability and deterioration of the material. Therefore, the Cr content is 20% or less, and preferably 18% or less in order to ensure the workability of the ferrite stainless steel sheet for heat-resistant members at room temperature.

(1-7) Nb: More than 0.32% and 1.0% or less Nb improves the high-temperature strength of the ferritic stainless steel sheet for heat-resistant members by strengthening the solid solution of the matrix phase and strengthening the precipitation with carbonitride or the like. Further, Cr forms a carbonitride with C and N, suppresses crystal grain growth by a pinning effect, and contributes to the formation of a band-shaped fine grain structure having a crystal grain size of 20 μm or less in terms of a perfect circle. Therefore, the Nb content is more than 0.32%, and is preferably 0.35% or more in consideration of the formation of the strip-shaped fine grain structure and the high temperature strength.

On the other hand, when the Nb content exceeds 1.0%, the manufacturability of the ferrite-based stainless steel sheet for heat-resistant members and the workability at room temperature are lowered. Therefore, the Nb content is 1.0% or less, and is preferably 0.70% or less in consideration of manufacturability and processability at room temperature.

(1-8) Al: 0.0005 to 0.500%
Al may be added as a deoxidizing element and its action is manifested from a content of 0.0005%. Therefore, the Al content is 0.0005% or more, and is preferably 0.001% or more in consideration of the manufacturability of the ferritic stainless steel sheet for heat-resistant members.

On the other hand, if the Al content exceeds 0.500%, the workability of the ferrite stainless steel sheet for heat-resistant members at room temperature is deteriorated, the weldability and surface quality are deteriorated, and the oxidation resistance is deteriorated. Therefore, the Al content is 0.500% or less, and is preferably 0.2% or less in consideration of the workability of the ferrite stainless steel sheet for heat-resistant members at room temperature, the occurrence of surface defects, and the weldability.

(1-9) N: 0.005 to 0.02%
Similar to C, N forms a carbonitride with Nb, Ti, etc. during hot rolling, coil winding, and annealing to suppress grain growth by the pinning effect, and the crystal grain size is converted to a perfect circle. Contributes to the formation of strip-shaped fine grain structures of 20 μm or less. Therefore, the N content is 0.005% or more, and preferably 0.007% or more in consideration of the formation of the strip-shaped fine grain structure and the high temperature strength.

On the other hand, when the N content exceeds 0.02%, the moldability and corrosion resistance of the ferrite stainless steel sheet for heat-resistant members are deteriorated, and the high-temperature strength is lowered. Therefore, the N content is 0.02% or less, preferably 0.010% or less in consideration of moldability and corrosion resistance at room temperature.
Next, arbitrary elements that may be contained as needed will be described.

(1-10) Ni: 1% or less Ni is an element that improves the toughness and corrosion resistance of ferrite stainless steel sheets for heat-resistant members. However, if the Ni content exceeds 1%, an austenite phase is formed, and the deep drawing property of the ferritic stainless steel sheet for heat-resistant members is lowered. Therefore, the Ni content is 1% or less, preferably 0.5% or less. In order to surely obtain this effect due to the Ni content, the Ni content is preferably 0.01% or more.

(1-11) One or more selected from the group consisting of Mo: 3.0% or less, Cu: 1% or less, and Ti: 0.2% or less Mo is a ferritic stainless steel sheet for heat-resistant members. In addition to improving corrosion resistance, high temperature strength is improved by solid solution. However, if the Mo content exceeds 3.0%, a harmful compound phase is precipitated. Therefore, the Mo content is 3.0% or less, preferably 2.0% or less. In order to surely obtain this effect due to the Mo content, the Mo content is preferably 0.03% or more.

Cu improves the high temperature strength of ferritic stainless steel sheets for heat-resistant members by strengthening precipitation. However, if the Cu content exceeds 1%, the room temperature ductility and oxidation resistance of the ferrite stainless steel sheet for heat-resistant members are lowered. Therefore, the Cu content is 1% or less, preferably 0.6% or less. In order to surely obtain this effect due to the Cu content, the Cu content is preferably 0.01% or more.

Similar to Nb, Ti improves the high-temperature strength of the ferritic stainless steel sheet for heat-resistant members by strengthening the solid solution of the matrix phase and strengthening the precipitation with carbonitride or the like. Further, Ti forms a carbonitride with C and N, suppresses crystal grain growth by a pinning effect, and contributes to the formation of a band-shaped fine grain structure having a crystal grain size of 20 μm or less in terms of a perfect circle. However, when the Ti content exceeds 0.2%, the room temperature ductility of the ferritic stainless steel sheet for heat-resistant members is lowered, and coarse Ti-based precipitates are formed, which deteriorates workability. Therefore, the Ti content is 0.2% or less, preferably 0.1% or less in consideration of the occurrence of surface defects and toughness. In order to surely obtain this effect due to the Ti content, the Ti content is preferably 0.01% or more.

Mo, Cu, and Ti may be contained alone or in combination of two or more in order to increase the high temperature strength of the ferritic stainless steel sheet for heat-resistant members.

(1-12) V: 0.5% or less V forms fine carbonitrides, suppresses crystal grain growth by precipitation strengthening action and pinning effect, and has a perfect circle equivalent crystal grain size of 20 μm or less. Contributes to the formation of strip-shaped fine grain structures. However, when the V content exceeds 0.5%, the precipitate becomes coarse and the high temperature strength decreases. Therefore, the V content is 0.5% or less, and preferably 0.3% or less in consideration of manufacturing cost and manufacturability. In order to surely obtain this effect due to the V content, the V content is preferably 0.01% or more.

(1-13) B: 0.0050% or less B segregates at the grain boundaries and contributes to the improvement of the secondary workability of the ferritic stainless steel sheet for heat-resistant members. However, when the B content exceeds 0.0050%, the processability and corrosion resistance of the ferrite stainless steel sheet for heat-resistant members are lowered. Therefore, the B content is 0.0050% or less, preferably 0.0020% or less. In order to surely obtain this effect due to the B content, the B content is preferably 0.0001% or more.

(1-14) Remaining Remaining other than the above is Fe and impurities. Examples of impurities include those contained in raw materials such as ore and scrap, and those contained in the manufacturing process.

(2) Metallographic structure:
The ferrite-based stainless steel sheet for heat-resistant members according to the present invention has a metal structure in which the area ratio of ferrite crystal grains having a crystal grain size of 20 μm or less in terms of a perfect circle is 3.0 to 15.0%. This metallographic structure will be described.

In order to reduce the anisotropic Δr of the r value of the ferritic stainless steel sheet for heat-resistant members containing Nb, the present inventors have changed the metal structure by variously changing the chemical composition and manufacturing conditions for heat-resistant members. A ferritic stainless steel sheet was prototyped, and the relationship between the r-value anisotropic Δr and the metallographic structure was investigated.

As a result, in the metal structure of the ferritic stainless steel sheet for heat-resistant members containing Nb, not only the texture and particle size of the ferrite matrix, and also the precipitate, but also the crystal grain size in terms of a perfect circle is as fine as 20 μm or less. It was found that the distribution of ferritic grains in the metallographic structure was greatly affected.

That is, a strip-shaped fine grain structure extending parallel to the rolled surface in the L cross-sectional structure of the ferrite-based stainless steel sheet for heat-resistant members and having a crystal grain size of 20 μm or less in terms of a perfect circle (hereinafter referred to as “belt-shaped fine grain structure”). Is present in a certain amount or more, and when the metal structure in the other region is a ferrite grain having a crystal grain size of more than 20 μm in terms of a perfect circle, the in-plane anisotropy Δr of the r value is suppressed to 0.1 or less. To. Further, it was found that the area ratio of the strip-shaped fine grain structure correlates with the area ratio of fine ferrite grains having a crystal grain size of 20 μm or less in the metal structure in terms of a perfect circle. That is, when the area ratio of fine ferrite crystal grains having a perfect circle-equivalent crystal grain size of 20 μm or less is 3.0% or more, a strip-shaped fine grain structure is sufficiently formed.

The principle by which the band-shaped fine-grained structure reduces the in-plane anisotropy Δr of the r value is not clear, but since the band-shaped fine-grained structure has higher strength than the surrounding structure and is not easily deformed, the band-shaped fine-grained structure becomes band-shaped. Due to the distribution, when the easily deformable coarse grain portion is tensile-deformed, the strip-shaped fine grain structure suppresses the decrease in the plate width direction even if the tension is in a direction in which the decrease in the plate width direction is large. , It is estimated that the in-plane anisotropy Δr of the r value is reduced.

If the area ratio of ferrite crystal grains having a perfect circle-equivalent crystal grain size of 20 μm or less is less than 3.0%, the effect of reducing the in-plane anisotropy Δr of the r value cannot be sufficiently obtained, and this area ratio becomes If it exceeds 15.0%, the high-temperature strength of the ferrite-based stainless steel plate for heat-resistant members is insufficient, and the workability at room temperature is lowered. Therefore, the area ratio of ferrite crystal grains having a perfect circle-equivalent crystal grain size of 20 μm or less is 3.0 to 15.0%.

(3) Mechanical Properties The ferritic stainless steel sheet for heat-resistant members according to the present invention uses a test piece having an in-plane anisotropy Δr of r value of 0.1 or less and a rolling direction in the longitudinal direction 700. It has mechanical properties with a 0.2% proof stress of 75 MPa or more in a ℃ tensile test. The in-plane anisotropy Δr is defined by the above equations (1) and (2).

The ferrite-based stainless steel sheet for heat-resistant members according to the present invention has high high-temperature strength. Further, in the ferritic stainless steel sheet for heat-resistant members according to the present invention, since the in-plane anisotropy Δr of the r value is very small, it is difficult for ears to be formed during deep drawing, and the yield during molding is improved.

2. 2. Manufacturing Method The ferritic stainless steel sheet for heat-resistant members according to the present invention is hot-rolled on a steel piece having the above-mentioned chemical composition, wound at 700 to 850 ° C. after hot-rolling for finishing, and not annealed by hot-rolled sheet. It is produced by cold rolling and final annealing in a temperature range of 850 to 950 ° C. to form the above-mentioned ferritic fine grain structure.

Conventional ferritic stainless steel sheets for heat-resistant members are annealed after hot rolling and finally annealed in a temperature range of 1000 ° C. or higher after cold rolling. However, in the production method according to the present invention, hot-rolled sheet is annealed. The above-mentioned strip-shaped fine grain structure is formed on the ferritic stainless steel sheet for heat-resistant heat members by not performing the above and performing the final annealing in a lower temperature range than before.

If the take-up temperature after finishing hot rolling is less than 700 ° C or more than 850 ° C, hot rolled sheet annealing is performed, or the final annealing temperature is less than 850 ° C or more than 950 ° C, the above-mentioned strip shape Fine grain structure cannot be obtained.

The present invention will be described in more detail with reference to Examples.
A 100 mm thick 50 kg ingot having the chemical composition (mass%, balance Fe and impurities) A to V shown in Table 1 was melted, and these ingots were heated to 1150 to 1250 ° C., and then the finishing temperature was 800. It was hot-rolled to a thickness of 4.0 mm so as to be within the range of ~ 1000 ° C., water-cooled once, and subjected to atmospheric heat treatment for 1 hour in order to reproduce the heat-retaining state after winding on a coil.

Some samples were annealed on a hot-rolled plate, and then all the samples, including those not annealed, were cold-rolled to a thickness of 1.2 mm.
The obtained cold-rolled steel sheet was subjected to atmospheric heat treatment (final annealing) to obtain a ferritic stainless steel sheet for heat-resistant members, which is a product. Table 2 shows the coil winding reproduction heat treatment temperature, the hot-rolled plate annealing temperature, and the final annealing temperature.

With respect to the thin plate thus obtained, the microstructure of the L cross section (plate thickness cross section parallel to the rolling direction) was observed, and the area ratio of ferrite crystal grains having a crystal grain size of 20 μm or less in terms of a perfect circle was determined by the following method. I asked. Using an optical microscope or scanning electron microscope equipped with an image processing function, a set photograph including the total thickness in the plate thickness direction is created with a field of view of 500 times (field of view of 200 μm × 250 μm), and each ferrite crystal grain observed there is The cross-sectional area (S ₁ , S ₂ , ..., _Sn ) is measured. Crystal grain size in the case of considers each ferrite grain cross-sectional shape a perfect circle _{(2r 1, 2r 2, ···} , 2r n, in the present invention referred to as "crystal grain size of a perfect circle converted".) The obtaining from each of the cross-section area _{(2r n = 2 (S n} / π) 1/2). Crystal grains having a perfect circle-equivalent crystal grain size of 20 μm or less and crystal grains having a perfect circle-equivalent crystal grain size of more than 20 μm are selected, and the area ratio of ferrite crystal grains having a perfect circle-equivalent crystal grain size of 20 μm or less is obtained by the following formula.

Area ratio (%) of ferrite crystal grains with a perfect circle equivalent crystal grain size of 20 μm or less = (total cross-sectional area of crystal grains with a perfect circle equivalent crystal grain size of 20 μm or less) / {(round shape equivalent crystal grains) Total cross-sectional area of crystal grains with a diameter of 20 μm or less + total cross-sectional area of crystal grains with a perfect circle-equivalent crystal grain size of more than 20 μm)} × 100

Further, a JIS No. 13B tensile test piece was collected from the obtained thin plate, and the r values in the L, D, and C directions were evaluated in accordance with JIS Z 2254. Here, the r value was calculated using the above equation (2) after applying a strain of 12% to the test piece.

Further, from the measurement results of the r value in the L, D, and C directions, the anisotropy Δr of the r value was calculated using the above equation (1). If the anisotropy Δr is 0.1 or less, it is judged to be good.

Further, a high-temperature tensile test piece was cut out from this thin plate, and a high-temperature tensile test was performed at 700 ° C. in accordance with JIS G 0567 to measure 0.2% proof stress. The high temperature strength was considered to be good if the 0.2% proof stress at 700 ° C. was 75 MPa or more.

The test results are also shown in Tables 2 and 3. Underlines in Tables 1 to 3 indicate that they are outside the scope of the present invention or that the test results are not good.

The chemical compositions of steel grades A to Q in Table 1 satisfy the scope of the present invention. On the other hand, the C content of the steel type R exceeds the upper limit of the range of the present invention, the Si content of the steel type S exceeds the upper limit of the range of the present invention, and the Si content of the steel type T exceeds the lower limit of the range of the present invention. Below, the Cr content of steel type U is below the lower limit of the range of the present invention, the Cr content of steel type V is above the upper limit of the range of the present invention, and the Nb content of steel types W and X is below the lower limit of the range of the present invention. Below, the Nb content of steel type Y exceeds the upper limit of the range of the present invention.

Examples 1 to 24 of the present invention in Table 2 are examples of the present invention that satisfy all the scope of the present invention, and Comparative Examples 1 to 39 in Table 3 are comparative examples that do not satisfy the scope of the present invention.

In Examples 1 to 24 of the present invention, the area ratio (area ratio of fine grain structure) of ferrite crystal grains having a crystal grain size of 20 μm or less in terms of perfect circle is in the range of 3.0 to 15.0%, and the r value is The anisotropy Δr was 0.1 or less, specifically 0.011 to 0.099, and the 0.2% proof stress at 700 ° C. was 75 MPa or more, specifically 77 to 113 MPa. Therefore, it can be seen that the example of the present invention is a ferritic stainless steel sheet for heat-resistant members, which has a small r-value anisotropy Δr and has excellent workability at room temperature and high-temperature strength at a high level.

On the other hand, in Comparative Examples 1 to 6, since the coil winding reproduction heat treatment temperature is lower than 700 ° C., the area ratio of ferrite crystal grains having a crystal grain size of 20 μm or less in terms of perfect circle is within the range of the present invention (3.0). There were some that did not fall within the range of ~ 15.0%), had a large anisotropy Δr of the r value, and had a low 0.2% proof stress at 700 ° C.

In Comparative Examples 7 to 11, since the hot-rolled plate was annealed at 800 to 1000 ° C., the area ratio of ferrite crystal grains (area ratio of fine grain structure) having a crystal grain size of 20 μm or less in terms of perfect circle is within the scope of the present invention. It did not fall within (3.0 to 15.0%), and the anisotropy Δr of the r value was large.

In Comparative Examples 12 to 19, since the final annealing temperature is outside the temperature range of 850 to 950 ° C., the area ratio of ferrite crystal grains having a perfect circle-equivalent crystal grain size of 20 μm or less is within the range of the present invention (3.0 to 15). Some of them did not fall within 0.0%), had a large anisotropy Δr of r value, and had a low 0.2% proof stress at 700 ° C.

In Comparative Examples 20 to 24, since the C content exceeds the upper limit of the range of the present invention, the area ratio of ferrite crystal grains having a crystal grain size of 20 μm or less in terms of a perfect circle is within the range of the present invention (3.0 to 15. It did not fall within 0%), and the anisotropy Δr of the r value was large.

In Comparative Examples 25 and 26, the Si content exceeded the upper limit of the range of the present invention, and in particular, in Comparative Example 26, the coil winding reproduction heat treatment temperature exceeded the upper limit of the range of the present invention, so that the anisotropy Δr of the r value was increased. It was big.

In Comparative Examples 27 and 28, the Si content was below the lower limit of the range of the present invention, and in particular, Comparative Example 28 was annealed with a hot-rolled plate, so that Comparative Example 27 had a 0.2% proof stress at 700 ° C. In Example 28, the anisotropy Δr of the r value was large.

In Comparative Example 29, since the Cr content was below the lower limit of the range of the present invention, the 0.2% proof stress at 700 ° C. decreased.

In Comparative Examples 30 and 31, the Cr content exceeded the upper limit of the range of the present invention. In particular, in Comparative Example 29, since the hot-rolled plate was annealed, the area ratio of the ferrite crystal grains having a perfect circle-equivalent crystal grain size of 20 μm or less was performed. Was not within the range of the present invention (3.0 to 15.0%), and the anisotropy Δr of the r value was large.

In Comparative Examples 32 to 34, the Nb content was below the lower limit of the range of the present invention, and in particular, in Comparative Example 34, the final annealing temperature was below 850 ° C., so that the anisotropy Δr of the r value was large or at 700 ° C. The yield strength decreased by 0.2%.

In Comparative Examples 35 and 36, the Nb content was below the lower limit of the range of the present invention, so that the 0.2% proof stress at 700 ° C. decreased.

Further, in Comparative Examples 37 to 39, since the Nb content exceeded the upper limit of the range of the present invention, the anisotropy Δr of the r value was large.

Claims (5)

The chemical composition is mass%,
C: 0.001 to 0.02%,
Si: More than 0.2% and less than 3.0%,
Mn: 0.25 to 1.0%,
P: 0.01-0.08%,
S: 0.0001 to 0.0100%,
Cr: 10 to 20%,
Nb: More than 0.32% and less than 1.0%,
Al: 0.0005 to 0.500%,
N: 0.005 to 0.02%,
Ni: 0 to 1%,
Mo: 0-3.0%,
Cu: 0 to 1%,
Ti: 0-0.2%,
V: 0-0.5%,
B: 0 to 0.0050%
Containing, the balance is Fe and impurities,
As a metal structure, the area ratio of ferrite crystal grains having a crystal grain size of 20 μm or less in terms of a perfect circle is 3.0 to 15.0%.
As mechanical properties, the anisotropy Δr of the r value defined by the following equation (1) is 0.1 or less, and 0.2 in a 700 ° C. tensile test using a test piece whose rolling direction is the longitudinal direction. Ferritic stainless steel sheet for heat-resistant members with a% yield strength of 75 MPa or more.
Δr = | (r ₀ + r ₉₀ ) / 2-r ₄₅ | ... (1)
In the formula (1), r ₀ , r ₄₅ and r ₉₀ are test pieces having the rolling direction of the ferrite-based stainless steel sheet for heat-resistant members, the direction of 45 ° with respect to the rolling direction, and the direction perpendicular to the rolling as the longitudinal direction, respectively. It is an r value obtained by performing a tensile test at room temperature using the product in accordance with the following equation (2) in each direction.
r = ln (W / W ₀ ) / ln (t / t ₀ ) ... (2)
In equation (2), W and t, and W ₀ and t ₀ are the width and plate thickness of the test piece after tensile plastic deformation and the width and plate thickness of the test piece before tensile plastic deformation, respectively. .. The ferrite-based stainless steel sheet for heat-resistant members according to claim 1, which contains Ni: 0.01 to 1% by mass. By mass%
Mo: 0.03 to 3.0%,
Cu: 0.01 to 1%, and Ti: 0.01 to 0.2%
The ferrite-based stainless steel sheet for a heat-resistant member according to claim 1 or 2, which contains one or more of the above-mentioned. The ferrite-based stainless steel sheet for heat-resistant members according to any one of claims 1 to 3, which contains V: 0.01 to 0.5% by mass. B: The ferritic stainless steel sheet for heat-resistant members according to any one of claims 1 to 4, which contains 0.0001 to 0.0050% by mass.