JP2021195573A - Ferritic stainless steel sheet, method for producing ferritic stainless steel sheet, and automobile exhaust system parts - Google Patents

Abstract

To provide a ferritic stainless steel sheet that is excellent in workability and high temperature salt damage resistance, and to provide a method for producing ferritic stainless steel sheet and automobile exhaust system parts.SOLUTION: Provided are a ferritic stainless steel sheet that has a predetermined chemical composition and satisfies formulas (i) to (iii), and automobile exhaust system parts using the same. Al+1.7Si+0.8Mo+0.3DA-0.5≤6.1 -- formula (i) 2.6Al+4.3Si+1.5Mo≥6.00 -- formula (ii) Al/O≥200 -- formula (iii) {The element symbol in the formula means the contained amount (mass%) of the element, and DA means the average crystal grain size (mm) at the center part of the plate thickness.}.SELECTED DRAWING: Figure 1

Description

The present invention relates to a ferritic stainless steel sheet, a method for manufacturing a ferritic stainless steel sheet, and automobile exhaust system parts. INDUSTRIAL APPLICABILITY The present invention has a structure that promotes salt adhesion, which is particularly excellent in processability and high-temperature salt damage resistance, especially in heat-resistant stainless steel which is most suitable for use in automobile exhaust system members which require high-temperature strength and oxidation resistance. The present invention relates to ferritic stainless steel sheets for automobile exhaust systems, manufacturing methods of ferritic stainless steel sheets, and automobile exhaust system parts.

Exhaust system members such as automobile exhaust manifolds, front pipes and center pipes allow high-temperature exhaust gas discharged from the engine to pass through, so the materials that make up the exhaust system members are diverse, such as oxidation resistance, high-temperature strength, and thermal fatigue characteristics. Characteristics are required.

Conventionally, cast iron was generally used for automobile exhaust system members, but stainless steel exhaust manifolds are used from the viewpoints of tightening exhaust gas regulations, improving engine performance, and reducing the weight of the vehicle body. It became so. The exhaust gas temperature varies depending on the vehicle model, and in recent years, it is often about 750 to 850 ° C., but it may reach a higher temperature. There is a demand for a material having high high temperature strength and oxidation resistance in an environment where it is used for a long time in such a temperature range.

Among stainless steels, austenitic stainless steel has excellent heat resistance and workability, but because of its large coefficient of thermal expansion, when it is applied to members that are repeatedly heated and cooled, such as exhaust manifolds, it heats up. Fatigue failure is likely to occur.

Ferritic stainless steel has an excellent coefficient of thermal expansion as compared with austenitic stainless steel, and therefore has excellent thermal fatigue characteristics. Further, ferritic stainless steel contains almost no expensive Ni as compared with austenitic stainless steel, so that the material cost is low and it is widely used. However, since ferritic stainless steel has a lower high-temperature strength than austenitic stainless steel, a technique for improving the high-temperature strength has been developed. Furthermore, various technologies have been developed from the viewpoints of oxidation resistance, moldability, manufacturability, and further cost reduction.

Examples of the developed ferrite stainless steel include SUS430J1L (Nb-containing steel), Nb-Si-containing steel, and SUS444 (Nb-Mo-containing steel). These were based on the inclusion of Nb and further Nb, and the high temperature strength was improved by the inclusion of Si and Mo. On the other hand, as in Patent Document 1, a technique for improving high temperature strength and oxidation resistance by containing Al has also been developed.

On the other hand, the members such as the center pipe located at the lower part of the vehicle body have a slightly lower temperature of the exhaust gas passing through among the exhaust system members, but the snowmelt salt sprayed to prevent the road surface from freezing and the salt content derived from seawater are contained. It is easy to adhere and there is a concern about high temperature salt damage. High-temperature salt damage is a phenomenon in which high-temperature corrosion is promoted by the adhesion of salt in a high-temperature oxidizing environment. For these parts located in the lower part of the vehicle body, technologies that emphasize high temperature salt damage resistance have been developed.

For example, Patent Document 2 discloses a technique containing Al, Mo, and W to improve high-temperature oxidation resistance and high-temperature salt damage resistance. Further, Patent Document 3 discloses a technique for adjusting the contents of N, V, and Al to improve the high temperature salt damage resistance.

Japanese Unexamined Patent Publication No. 2018-168457 Japanese Unexamined Patent Publication No. 2004-18916 Japanese Unexamined Patent Publication No. 2010-53421

However, in recent years, due to changes in the installation members of the exhaust system and the addition of new parts, salt may adhere to members located at high temperatures in the exhaust system such as exhaust manifolds and front pipes. Here are two specific examples. The first is the case where the exhaust system is forcibly air-cooled by installing a fan or improving the air permeability in response to the high temperature of the exhaust gas. An example is shown in FIG. 1 (a). 1A and 1B are diagrams schematically showing an example of an automobile exhaust system component, FIG. 1A is a schematic view of an automobile exhaust system component provided with a forced cooling member, and FIG. 1B is an automobile exhaust system component provided with a heat insulating member. It is a schematic diagram of. The forced cooling member 3 that forcibly air-cools the automobile exhaust system member 1 becomes a factor that promotes the adhesion of salt to the automobile exhaust system member 1. The forced cooling member 3 may be applied mainly to a turbo-charged vehicle whose spread has been accelerating in recent years. The second is a case where an exhaust gas purification component such as a converter and a heat insulating member covering the periphery of the exhaust system through which the exhaust gas passes are provided with a heat insulating material to promote the catalytic reaction by raising the exhaust gas temperature. An example is shown in FIG. 1 (b). Wool-like ceramics or the like is used for the heat insulating material 7 that covers the automobile exhaust system member 1, and it is easy to retain water. Therefore, covering the automobile exhaust system member 1 with the heat insulating material 7 makes it easier to retain the salt content and promotes the adhesion of the salt content to the automobile exhaust system member 1. The application of the heat insulating material 7 may be mainly applied to the exhaust system of an engine that burns by HCCI (Homogeneous Charge Compression Ignition), which is expected to be widely used in the future.

Further, by using these forced cooling members or heat insulating members, the environment becomes higher in humidity than the environment in which the normal members are exposed. In an environment where the forced cooling member winds up moisture on the parts, the moisture is always sprayed onto the exhaust system. When a heat insulating member is used, the heat insulating material is covered with a cover, so that an environment in which water vapor is trapped between the exhaust system and the cover is created. Therefore, the high-temperature salt-damaged environment is an environment in which water vapor oxidation accompanies the oxidation environment together with salt adhesion, which is different from the conventional environment.

That is, when the forced cooling member or the heat insulating member is applied, not only the upstream members such as the exhaust manifold and the front pipe are required to have high temperature salt damage resistance, but also the environment with steam oxidation different from the conventional one is required. High temperature salt damage resistance is newly required. That is, the automobile exhaust system member to which the forced cooling member and the heat insulating member are applied needs to be developed as a new application different from the conventional automobile exhaust system member.

The present inventors have found that the high temperature salt damage resistance at a high temperature salt damage environment with steam oxidation was found that it is important to form the Al ₂ O ₃ on the surface layer of ferritic stainless steel. _{Stainless steel forms Cr 2} O ₃ or Al ₂ O ₃ having a low oxidation rate in a high temperature environment, and suppresses the oxidation of Fe having a high oxidation rate to obtain protection against high temperature corrosion. Al ₂ O ₃ has a _{lower oxidation rate than Cr 2} O ₃ and is excellent in protection. In an environment accompanied by water vapor oxidation, Cr ₂ O ₃ loses protection due to some mechanisms such as increased voids and defects, but Al ₂ O ₃ has a strong bond between Al and O and voids and defects are formed. hard. Further, in a high-temperature salt-damaged environment, Cr ₂ O ₃ reacts with salt to reduce the protective property, but Al ₂ O ₃ has a relatively small effect. Further, when Al ₂ O ₃ is formed, there is room for forming _{Cr 2} O ₃ even after the protective property is lowered. As a result, _{the formation of Al 2} O ₃ improves the high temperature salt damage resistance of the ferrite stainless steel.

When the ferrite stainless steel contains Al, it is _{easy to form Al 2} O ₃ at 800 ° C or higher, and it is difficult to form _{Al 2} O ₃ at lower than 800 ° C. Therefore, it is necessary to consider the high temperature salt damage resistance of the Al-containing ferritic stainless steel especially at less than 800 ° C. _{The reason why it is difficult to form Al 2} O _{3 at a} temperature lower than 800 ° C. is that the diffusion of Al from the inside of the base metal to the surface becomes more difficult at a low temperature. Therefore, in order to improve the resistance to high temperature salt damage, it is important to control not only the balance of material components but also the crystal grain size that affects the diffusion of the base metal. It is considered that the crystal grain size of the surface layer of the base metal has a particularly large influence on the crystal grain size.

In addition, many automobile exhaust system parts have complicated shapes, and workability is also required. When the ferrite stainless steel contains Al, the 0.2% proof stress at room temperature (10 to 35 ° C.) increases, and the workability is impaired. In addition to the material component balance, the crystal grain size also affects the 0.2% proof stress, and it is considered important to control these. Regarding this crystal grain size, it is considered that the influence of the crystal grain size at the center of the base plate thickness is particularly large.

Patent Document 1 is an Al-containing ferritic stainless steel, but detailed studies on both high-temperature salt damage resistance and 0.2% proof stress at room temperature are not disclosed. Further, regarding the crystal grain size, neither the surface layer portion of the base metal nor the central portion of the plate thickness is taken into consideration.

Patent Document 2 discloses a technique containing Al, Mo, and W to improve high temperature oxidation resistance and high temperature salt damage resistance. However, the high temperature salt damage resistance in the technique disclosed in Patent Document 2 does not consider the environment accompanied by steam oxidation, nor does it consider the crystal grain size of the surface layer portion of the base metal. Further, no detailed study on the 0.2% proof stress at room temperature is disclosed, and no consideration is given to the crystal grain size at the center of the plate thickness. The ferritic stainless steel disclosed in Patent Document 2 contains a large amount of Al, Mo, and W, and in particular, the W content is more than 2.0% by mass% and 5% or less, and the W content is extremely high. It is high and has an excessively high 0.2% proof stress at room temperature, which may impair workability.

Patent Document 3 discloses a technique for adjusting the contents of N, V, and Al to improve high-temperature salt damage resistance. However, the high temperature salt damage resistance in the technique disclosed in Patent Document 3 does not consider the environment accompanied by steam oxidation, nor does it consider the crystal grain size of the surface layer portion of the base metal. Further, no detailed study on the 0.2% proof stress at room temperature is disclosed, and no consideration is given to the crystal grain size at the center of the plate thickness. Further, the Al content is evaluated only in the range of less than 0.4% by mass, and the upper limit is 0.30%. Further, the ferrite-based stainless steel according to Patent Document 3 has a V concentration of 0.30 to 0.60% in mass%, which is extremely higher than that of ordinary ferrite-based stainless steel, and may impair manufacturability and the like. ..

When a forced cooling member or a heat insulating member is applied to an automobile exhaust system as described above, it is necessary not only to newly impart high temperature salt damage resistance to upstream members such as an exhaust manifold and a front pipe, but also to the conventional method. Needed high temperature salt damage resistance in the environment with different steam oxidation. High-temperature salt damage resistance is an issue for Al-containing ferritic stainless steel, and high-temperature salt damage in such an environment has not been investigated. Further, even in the conventional high-temperature salt damage environment, it is necessary to contain an extremely large amount of W, V and the like as an improvement technique. In addition, there has been insufficient study on the fact that the 0.2% proof stress at room temperature increases and the workability decreases due to the Al content.

That is, an object of the present invention is to provide a ferritic stainless steel sheet, a method for manufacturing a ferritic stainless steel sheet, and automobile exhaust system parts, which are excellent in high temperature salt damage resistance and workability in a high temperature environment accompanied by steam oxidation. In particular, it is an object of the present invention to provide a ferrite stainless steel sheet for an automobile exhaust system and an automobile exhaust system component having a structure in which salt adhesion is promoted, which is excellent in workability and high temperature salt damage resistance, in an Al-containing ferritic stainless steel.

In order to solve the above problems, the present inventors have diligently investigated the effects of various components on the high temperature salt damage resistance and processability of ferritic stainless steel, and the crystal grain size of the surface layer of the base metal and the central portion of the plate thickness. As a result, they have invented a ferritic stainless steel sheet having excellent high temperature salt damage resistance and workability. In addition, high-temperature salt damage also corresponds to the environment accompanied by steam oxidation.

That is, the gist of the present invention for solving the above problems is as follows.
(1) By mass%,
C: 0.020% or less,
N: 0.020% or less,
Si: 0.05% or more, 1.60% or less,
Mn: 0.01% or more, 1.10% or less,
P: 0.040% or less,
S: 0.0022% or less,
Cr: 10.0% or more, 15.0% or less,
Ni: 0.01% or more, 0.70% or less,
Cu: 0.001% or more, 0.80% or less,
Mo: 0.01% or more, 2.00% or less,
Ti: 0.050% or more, 0.300% or less,
Nb: Over 0.03%, 0.60% or less,
Al: Over 0.70%, 3.00% or less,
V: 0.01% or more, 0.20% or less,
B: 0.0001% or more, 0.0050% or less, and O: 0.0050% or less,
A ferritic stainless steel sheet containing Fe and impurities, and satisfying the following formulas (i) to (iii).
Al + 1.7Si + 0.8Mo + 0.3DA ^-0.5 ≤ 6.1 ... Equation (i)
2.6Al + 4.3Si + 1.5Mo ≧ 6.00 ・ ・ ・ Equation (ii)
Al / O ≧ 200 ・ ・ ・ Equation (iii)
However, the element symbol in the formula means the content (% by mass) of the element, and DA means the average crystal grain size (mm) in the center of the plate thickness.
(2) The ferrite-based stainless steel sheet according to (1), which further satisfies the following formula (iv).
900DA-1000DB ≧ -1.7 ・ ・ ・ Equation (iv)
However, DA in the formula means the average crystal grain size (mm) at the center of the plate thickness, and DB means the average crystal grain size (mm) on the plate surface.
(3) By mass%, instead of a part of Fe,
W: 0.01% or more, 0.50% or less,
Y: 0.001% or more, 0.20% or less,
REM: 0.001% or more, 0.20% or less,
Ca: 0.0001% or more, 0.0030% or less,
Zr: 0.01% or more, 0.30% or less,
Hf: 0.001% or more, 1.0% or less,
Sn: 0.001% or more, 1.0% or less,
Mg: 0.0001% or more, 0.0030% or less,
Co: 0.01% or more, 0.50% or less,
Sb: 0.001% or more, 0.50% or less,
Bi: 0.001% or more, 1.0% or less,
Ta: 0.001% or more, 1.0% or less, and Ga: 0.0001% or more, 0.30% or less,
The ferrite-based stainless steel sheet according to (1) or (2), which contains one or more of the above.
(4) The ferrite-based stainless steel sheet according to any one of (1) to (3), which is used for an automobile exhaust system member in which salt adhesion is promoted by a forced cooling member.
(5) Described in any one of (1) to (3), which is used for an automobile exhaust system member in which salt adhesion is promoted by applying a heat insulating member that covers the periphery of the automobile exhaust system member with a heat insulating material. Ferritic stainless steel plate.
(6) The method for manufacturing a ferritic stainless steel sheet according to any one of (1) to (5), wherein the holding time in the range of 750 ° C. or higher and lower than 830 ° C. in the temperature raising process of final annealing is 20 seconds. The above, or a method for producing a ferritic stainless steel sheet, characterized in that the flow velocity of the atmospheric gas in the final annealing is 0.08 m / s or more.

(7) An automobile exhaust system including an automobile exhaust system member using the ferrite stainless steel plate according to any one of (1) to (3) and a forced cooling member for forcibly cooling the automobile exhaust system member. parts.
(8) An automobile provided with an automobile exhaust system member using the ferrite stainless steel plate according to any one of (1) to (3) and a heat insulating member covering the periphery of the automobile exhaust system member with a heat insulating material. Exhaust system parts.

Further, in the present invention, those in which the lower limit is not specified are shown to include unavoidable impurity levels.

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to provide a ferrite-based stainless steel sheet, a method for manufacturing a ferrite-based stainless steel sheet, and an automobile exhaust system component, which are excellent in high-temperature salt damage resistance and workability in a high-temperature environment accompanied by steam oxidation. In particular, it is possible to provide a ferritic stainless steel sheet having excellent workability and high temperature salt damage resistance, which is used as an automobile exhaust system member to which a forced cooling member or a heat insulating member is applied to promote salt adhesion.

It is a figure which shows an example of an automobile exhaust system component schematically, (a) is a schematic diagram of an automobile exhaust system component provided with a forced cooling member, and (b) is a schematic diagram of an automobile exhaust system component provided with a heat insulating member. be.

Hereinafter, the present invention will be described in detail.

First, the reason for limiting the steel composition of the ferritic stainless steel sheet of the present invention will be described. Here, "%" for the steel composition means mass%.

(C: 0.020% or less)
Since C is an element that deteriorates moldability and corrosion resistance and causes a decrease in high-temperature strength, the C content is 0.020% or less. Further, excessive content of C lowers oxidation resistance and intergranular corrosion resistance. Considering the deterioration of oxidation resistance and intergranular corrosion resistance due to excessive C content, the C content is preferably 0.015% or less. Further, since the high temperature salt damage resistance is further improved by reducing C, the upper limit is more preferably 0.010% or less. However, since excessive reduction leads to an increase in refining cost, the C content is preferably more than 0.0010%. The C content is more preferably 0.0020% or more.

(N: 0.020% or less)
Like C, N is an element that deteriorates moldability and corrosion resistance and causes a decrease in high-temperature strength. Therefore, the N content is 0.020% or less. In addition, excessive N content lowers oxidation resistance and intergranular corrosion resistance. Considering the deterioration of oxidation resistance and intergranular corrosion resistance due to excessive N content, the N content is preferably 0.015% or less. The N content is more preferably 0.013% or less. However, since excessive reduction leads to an increase in refining cost, the N content is preferably more than 0.0030%. The N content is more preferably 0.0040% or more.

(Si: 0.05% or more, 1.60% or less)
Si is an element contained as a deoxidizing agent and also an element for improving high temperature salt damage resistance. Si forms Si oxides at high temperatures and contributes to the improvement of high temperature salt damage resistance, and the formation of Si oxides _{promotes the formation of Al 2} O ₃ to improve the high temperature salt damage resistance. In order to exhibit high temperature salt damage resistance, it is necessary to contain 0.05% or more of Si. In order to further improve the high temperature salt damage resistance, the Si content is preferably 0.30% or more. The Si content is more preferably 0.50% or more. However, the excessive Si content increases the yield strength by 0.2% at room temperature and causes a decrease in workability. Therefore, the Si content is set to 1.60% or less. In order to further improve workability, the Si content is preferably 1.20% or less. Further, in consideration of manufacturability, the Si content is more preferably 0.80% or less.

(Mn: 0.01% or more, 1.10% or less)
Mn is an element contained as a deoxidizing agent, and the Mn content is 0.01% or more. Considering the refining cost, the Mn content is more preferably 0.10% or more. The Mn content is more preferably 0.15% or more. However, excessive Mn content _{delays the formation of Al 2} O ₃ and causes a decrease in high temperature salt damage resistance, so the Mn content is set to 1.10% or less. In order to further improve the high temperature salt damage resistance, the Mn content is preferably 0.50% or less. Further, in consideration of uniform elongation, hot workability and corrosion resistance, the Mn content is more preferably 0.40% or less. The Mn content is more preferably less than 0.40%.

(P: 0.040% or less)
P is an impurity mainly mixed from the raw material during steelmaking refining, and the higher the content, the lower the toughness and weldability. Therefore, the smaller the content, the better. Therefore, the P content is 0.040% or less. Further, in consideration of manufacturability, the P content is preferably 0.035% or less. Further, since reducing P further lowers the 0.2% proof stress at room temperature and improves workability, the P content is more preferably 0.030% or less. However, since excessive reduction leads to an increase in refining cost, the P content is preferably 0.005% or more. The P content is more preferably 0.010% or more.

(S: 0.0022% or less)
S is an impurity mainly mixed from the raw material during steelmaking refining and deteriorates corrosion resistance. Further, S also lowers the high temperature salt damage resistance by lowering the scale peeling resistance. Therefore, the S content is set to 0.0022% or less. Further, in consideration of manufacturability, the S content is preferably 0.0020% or less. In order to further improve the high temperature salt damage resistance, it is more preferable that the S content is 0.0014% or less. However, since excessive reduction leads to an increase in refining cost, the S content is preferably 0.0001% or more. The S content is more preferably 0.0003% or more.

(Cr: 10.0% or more and 15.0% or less)
Cr is an element that improves corrosion resistance and is also an element that improves high-temperature salt damage resistance. Cr also promotes the formation of _{Al 2} O _3. Therefore, the Cr content is set to 10.0% or more. The Cr content is preferably 10.5% or more, more preferably 10.7% or more. However, the excessive Cr content increases the proof stress by 0.2% at room temperature and causes a decrease in workability. Therefore, the Cr content is set to 15.0% or less. Considering toughness and raw material cost, the Cr content is preferably less than 14.0%. The Cr content is more preferably less than 12.0%.

(Ni: 0.01% or more, 0.70% or less)
Ni is an element that improves corrosion resistance, and the Ni content is 0.01% or more. The Ni content is preferably 0.02% or more, more preferably 0.05% or more. However, since excessive Ni content causes deterioration of moldability, the Ni content is set to 0.70% or less. Further, since the reduction of Ni further lowers the 0.2% proof stress at room temperature and improves the workability, the Ni content is preferably 0.50% or less. Considering the raw material cost, the Ni content is more preferably 0.40% or less.

(Cu: 0.001% or more, 0.80% or less)
Cu is an element that improves corrosion resistance, and the Cu content is 0.001% or more. Considering oxidation resistance, raw material cost, and manufacturability, the Cu content is preferably 0.005% or more. The Cu content is more preferably 0.01% or more. However, since the excessive content of Cu causes a decrease in hot workability, the Cu content is set to 0.80% or less. Further, since the reduction of Cu further lowers the 0.2% proof stress at room temperature and improves the workability, the Cu content is preferably 0.30% or less. Considering oxidation resistance, raw material cost, and manufacturability, the Cu content is more preferably 0.20% or less.

(Mo: 0.01% or more, 2.00% or less)
Mo is an element that improves high-temperature strength and corrosion resistance, and is also an element that improves high-temperature salt damage resistance. Mo also enhances the protection of _{Al 2} O _3. Therefore, the Mo content is 0.01% or more. In order to further improve the resistance to high temperature salt damage, the Mo content is preferably more than 0.30%. The Mo content is more preferably 0.55% or more. However, the excessive Mo content increases the proof stress by 0.2% at room temperature and causes a decrease in processability. Therefore, the Mo content is set to 2.00% or less. Further, in consideration of manufacturability, the Mo content is preferably 1.50% or less. In order to further improve workability, the Mo content is preferably less than 1.00%. Considering the alloy cost, the Mo content is even more preferably less than 0.80%.

(Ti: 0.050% or more, 0.300% or less)
Ti is an element that binds to C, N, and S to improve corrosion resistance, intergranular corrosion resistance, and deep drawing resistance, and the Ti content is 0.050% or more. The Ti content is preferably 0.080% or more, more preferably 0.120% or more. However, the Ti content is set to 0.300% or less because excessive Ti content causes a decrease in uniform elongation and a decrease in hole-expanding workability due to the formation of coarse Ti-based precipitates. Further, considering the occurrence of surface defects and toughness, the Ti content is preferably 0.270% or less. The Ti content is more preferably 0.220% or less.

(Nb: more than 0.03%, less than 0.60%)
Nb is an element that contributes to the improvement of high temperature strength and the improvement of high temperature salt damage resistance, and the Nb content is set to more than 0.03%. The Nb content is preferably 0.05% or more, more preferably more than 0.05%. However, since Nb is an expensive element, even if it is contained, it should be 0.60% or less. The Nb content is preferably less than 0.35% in consideration of uniform elongation, hole expandability, and manufacturability. Further, since reducing Nb further lowers the 0.2% proof stress at room temperature and improves workability, it is more preferable to set the Nb content to 0.20% or less.

(Al: more than 0.70%, less than 3.00%)
Al is an element that is contained as a deoxidizing element and improves high-temperature strength, oxidation resistance, and high-temperature salt damage resistance, and has an Al content of more than 0.70%. In order to further improve the resistance to high temperature salt damage, the Al content is preferably more than 1.05%. The Al content is more preferably more than 1.55%. However, the excessive Al content increases the proof stress by 0.2% at room temperature and causes a decrease in workability. Therefore, the Al content is set to 3.00% or less. In order to further improve workability, the Al content is preferably 2.50% or less. The Al content is more preferably 2.30% or less.

(V: 0.01% or more, 0.20% or less)
V is an element that improves high temperature strength. Therefore, the V content is set to 0.01% or more. The V content is preferably 0.02% or more, and more preferably 0.03% or more. However, excessive V content causes a decrease in high-temperature strength and a decrease in thermal fatigue life due to the coarsening of the precipitate. Therefore, the V content is 0.20% or less. Further, in consideration of manufacturability, the V content is preferably less than 0.15%. The V content is more preferably 0.10% or less.

(B: 0.0001% or more, 0.0050% or less)
B is an element that improves high temperature strength and thermal fatigue characteristics. Therefore, the B content is 0.0001% or more, and the B content is preferably 0.0002% or more, more preferably 0.0003% or more. However, excessive content of B causes deterioration of hot workability and deterioration of the surface texture of the steel surface. Therefore, the B content is set to 0.0050% or less. Further, in consideration of manufacturability and moldability, the B content is preferably 0.0030% or less. The B content is more preferably 0.0015% or less.

(O: 0.0050% or less)
O is an impurity that is inevitably contained and causes surface defects due to bubbles and inclusions. Therefore, the O content is set to 0.0050% or less. Further, in consideration of manufacturability, the O content is preferably 0.0040% or less. The O content is more preferably 0.0035% or less. However, since excessive reduction of O leads to an increase in refining cost, the O content is preferably 0.0003% or more. The O content is more preferably 0.0005% or more. Here, the O content means the total content including oxygen dissolved in the steel and oxygen of the oxide intervening in the steel.

In the ferrite-based stainless steel sheet according to the present embodiment, the balance other than the above-mentioned elements is Fe and impurities. However, elements other than the above-mentioned elements can also be contained within a range that does not impair the effects of the present embodiment. The impurities referred to here are components mixed with raw materials such as ore and scrap, and various factors in the manufacturing process when the ferritic stainless steel sheet according to the present invention is industrially manufactured, and are included in the present invention. It means something that is acceptable as long as it does not have an adverse effect.

Next, the equations (i) to (iv) will be described.

In investigating various effects on the 0.2% proof stress at room temperature, the present inventors have a large effect on the crystal grain size at the center of the base plate thickness in addition to Al, Si, and Mo, and the balance between these is important. It was found that the formula (i) was obtained.
Al + 1.7Si + 0.8Mo + 0.3DA ^-0.5 ≤ 6.1 ... Equation (i)
However, the element symbol in the formula means the content (% by mass) of the element, and DA means the average crystal grain size (mm) at the center of the plate thickness. Here, the central portion of the plate thickness means a range from 2t / 5 to 4t / 5 when the plate thickness is t.
When the value on the left side of the formula (i) exceeds 6.1, the proof stress at room temperature is increased by 0.2% and the workability is deteriorated. Therefore, the Al content, Si content, Mo content and DA need to satisfy the formula (i). If the contents of Al, Si, and Mo are large and the DA is small, the cost may increase and the manufacturability may decrease. Therefore, the value on the left side of the formula (i) is preferably 5.9 or less. Further, if the contents of Al, Si, and Mo are small and the DA is large, the high temperature strength and the oxidation resistance may be deteriorated. Therefore, the value on the left side of the formula (i) is preferably 3.5 or more.

Further, in order to improve the resistance to high temperature salt damage, it is important to promote the formation of _{Al 2} O _{3 and improve its protection.} In examining various effects on high-temperature salt damage resistance, the present inventors have found that the balance of Al, Si, Mo, and O is important, and obtained the formulas (ii) and (iii).
2.6Al + 4.3Si + 1.5Mo ≧ 6.00 ・ ・ ・ Equation (ii)
Al / O ≧ 200 ・ ・ ・ Equation (iii)
However, the element symbol in the formula means the content (mass%) of the element. If the contents of Al, Si, and Mo are large, the cost may increase and the manufacturability may decrease. Therefore, the value on the left side of the formula (ii) is preferably 9.90 or less. Further, if the content of Al, Si, and Mo is small, the high temperature strength and the oxidation resistance may be lowered. Therefore, the value on the left side of the formula (ii) is preferably 6.05 or more. If the Al content is large and the O content is small, the weldability may be deteriorated. Therefore, the Al / O is preferably 3500 or less. Further, if the Al content is small and the O content is large, the manufacturability may be deteriorated. Therefore, the Al / O content is preferably 300 or more.

_{As described above, it is important to promote the formation of Al 2} O ₃ and improve its protection in order to improve the resistance to high temperature salt damage. The present inventors consider the reason why the high temperature salt damage resistance is improved by satisfying the above formula (ii) and the above formula (iii) as follows. Al is believed to contribute to both promoting the formation of _{Al 2} O _{3 and improving its protection.} It is considered that Si promotes the formation _{of Al 2} O ₃ by suppressing the oxidation of Fe and Cr contained in a large amount in the base metal. Mo is thought to enhance the protection of the _Al 2 _{O 3} by a solid solution in the crystal grain boundary of _Al 2 _{O 3.} Further, it is considered that the larger the ratio of O in the balance between Al and O, the easier it is for Al to be internally oxidized in the base metal, and the _{formation of Al 2} O _{3 on the} surface of the base metal is inhibited.

It was also found that the high temperature salt damage resistance was further improved by the balance between the crystal grain size at the center of the base metal plate thickness and the crystal grain size on the plate thickness surface, and the formula (iv) was obtained as a preferable condition.
900DA-1000DB ≧ -1.7 ・ ・ ・ Equation (iv)
However, DA means the average crystal grain size (mm) at the center of the plate thickness, and DB means the average crystal grain size (mm) at the plate surface. The value on the left side of the equation (iv) is more preferably 0.0 or more. However, if the DA is large and the DB is small, the moldability may be deteriorated. Therefore, the value on the left side of the equation (iv) is preferably 60 or less.

The present inventors have found that the formation of the diffusion and Al ₂ O ₃ of Al in the surface layer portion located between the crystal grain size finer it is the base material surface and the center of plate thickness from the center of plate thickness of the base metal surface I think that it is promoting.

However, in consideration of manufacturability, surface quality, etc., the average crystal grain size at the center of the plate thickness and the surface of the plate is preferably 0.005 to 0.150 mm. More preferably, it is 0.015 to 0.065 mm.

Regarding the measurement of the average crystal grain size at the center of the plate thickness and the plate surface, observe the structure of the plate surface and measure the crystal grain size at 5 points arbitrarily according to the cutting method of JISG0551: 2013 to obtain the average value and average. Calculate the crystal grain size.

Further, in the present invention, by selectively containing one or more of W, Y, REM, Ca, Zr, Hf, Sn, Mg, Co, Sb, Bi, Ta, and Ga as necessary. , The characteristics can be further improved. Hereinafter, these elements will be described. Since these elements do not have to be contained, the lower limit of the content of each of these elements is 0%.

(W: 0.01% or more, 0.50% or less)
W is an element that improves high-temperature strength, and is preferably contained in an amount of 0.01% or more, if necessary. Further, since W is also an element for improving corrosion resistance, the W content is more preferably 0.05% or more. However, since it is an expensive element, it is preferably 0.50% or less even if it is contained. Further, in consideration of toughness and manufacturability, the W content is more preferably 0.40% or less.

(Y: 0.001% or more, 0.20% or less)
Y is an element that improves the cleanliness of steel, improves rust resistance and hot workability, and also improves oxidation resistance, and is preferably contained in an amount of 0.001% or more, if necessary. The Y content is more preferably 0.003% or more. However, since excessive Y content may lead to an increase in raw material cost and a decrease in manufacturability, the Y content is preferably 0.20% or less. The Y content is more preferably 0.10% or less.

(REM: 0.001% or more, 0.20% or less)
REM (Rare earth element) is an element that improves the cleanliness of steel, improves rust resistance and hot workability, and also improves oxidation resistance, and is 0.001% as needed. It is preferable to contain the above. The REM content is more preferably 0.003% or more. However, excessive REM content may lead to an increase in raw material cost and a decrease in manufacturability. Therefore, the REM content is preferably 0.20% or less. The REM content is more preferably 0.10% or less. REM is a general term for scandium (Sc) and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu). As the REM, one of the above elements may be contained alone, or two or more of the above elements may be contained. When two or more of the above elements are contained as REM, the REM content is the total content of those elements.

(Ca: 0.0001% or more, 0.0030% or less)
Ca is an element that promotes desulfurization, and is preferably contained in an amount of 0.0001% or more, if necessary. The Ca content is more preferably 0.0002% or more. However, since excessive Ca content may lead to a decrease in corrosion resistance due to the formation of CaS, which is a water-soluble inclusion, the Ca content is preferably 0.0030% or less. The Ca content is more preferably 0.0020% or less.

(Zr: 0.01% or more, 0.30% or less)
Zr is an element that improves corrosion resistance, intergranular corrosion resistance, high temperature strength, and oxidation resistance, and is preferably contained in an amount of 0.01% or more, if necessary. The Zr content is more preferably 0.03% or more. However, since the excessive content of Zr may lead to a decrease in manufacturability, the Zr content is preferably 0.30% or less. The Zr content is more preferably 0.20% or less.

(Hf: 0.001% or more, 1.0% or less)
Hf is an element that improves corrosion resistance, intergranular corrosion resistance, high temperature strength, and oxidation resistance, and is preferably contained in an amount of 0.001% or more, if necessary. The Hf content is more preferably 0.003% or more. However, since the excessive content of Hf may lead to a decrease in manufacturability, the Hf content is preferably 1.0% or less. The Hf content is more preferably 0.5% or less.

(Sn: 0.001% or more, 1.0% or less)
Sn is an element that improves corrosion resistance and high-temperature strength, and is preferably contained in an amount of 0.001% or more, if necessary. The Sn content is more preferably 0.003% or more. However, since the excessive Sn content may cause a decrease in toughness and manufacturability, the Sn content is preferably 1.0% or less. The Sn content is more preferably 0.5% or less.

(Mg: 0.0001% or more, 0.0030% or less)
Mg may be contained as a deoxidizing element and is also an element for improving moldability, and is preferably contained in an amount of 0.0001% or more as needed. The Mg content is more preferably 0.0003% or more. However, since excessive content of Mg may cause deterioration of corrosion resistance, weldability and surface quality, the Mg content is preferably 0.0030% or less. The Mg content is more preferably 0.020% or less.

(Co: 0.01% or more, 0.50% or less)
Co is an element that improves high-temperature strength, and is preferably contained in an amount of 0.01% or more, if necessary. The Co content is more preferably 0.03% or more. However, since excessive Co content may cause a decrease in toughness, the Co content is preferably 0.50% or less. Considering manufacturability and processability, it is more preferable that the Co content is less than 0.3%.

(Sb: 0.001% or more, 0.50% or less)
Sb is an element that improves high-temperature strength, and is preferably contained in an amount of 0.001% or more, if necessary. The Sb content is more preferably 0.005% or more. However, since excessive Sb content may lead to deterioration of weldability and toughness, the Sb content is preferably 0.50% or less. The Sb content is more preferably 0.40% or less.

(Bi: 0.001% or more, 1.0% or less)
Bi is an element that suppresses roping that occurs during cold rolling and improves manufacturability, and is preferably contained in an amount of 0.001% or more, if necessary. The Bi content is more preferably 0.003% or more. However, since excessive Bi content may cause a decrease in hot workability, the Bi content is preferably 1.0% or less. The Bi content is more preferably 0.5% or less.

(Ta: 0.001% or more, 1.0% or less)
Ta is an element that improves high-temperature strength, and is preferably contained in an amount of 0.001% or more, if necessary. The Ta content is more preferably 0.003% or more. However, since excessive Ta content may cause deterioration of toughness and manufacturability, the Ta content is preferably 1.0% or less. The Ta content is more preferably 0.5% or less.

(Ga: 0.0001% or more, 0.30% or less)
Ga is an element that improves corrosion resistance and hydrogen embrittlement resistance, and is preferably contained in an amount of 0.0001% or more, if necessary. The Ga content is more preferably 0.0003% or more. However, since the excessive content of Ga may cause a decrease in manufacturability, the Ga content is preferably 0.30% or less. The Ga content is more preferably 0.20% or less.

Next, a method for manufacturing a ferritic stainless steel sheet according to the present invention will be described. The ferrite-based stainless steel sheet according to the present embodiment may be manufactured by any method, and for example, it can be manufactured by the following manufacturing method.

As for the method for manufacturing a ferritic stainless steel sheet of the present invention, a general process for manufacturing a ferritic stainless steel sheet can be adopted. Generally, molten steel is made in a converter or an electric furnace, refined in an AOD furnace, a VOD furnace, etc. to make steel pieces by a continuous casting method or an ingot method, and then hot-rolled-annealed hot-rolled plate-pickling-cold. Manufactured through the steps of inter-rolling-finish annealing-pickling. If necessary, annealing of the hot-rolled plate may be omitted, or cold rolling-finish annealing-pickling may be repeated. The conditions of each of these steps may be general conditions, for example, a hot-rolled plate annealing temperature of 1000 to 1300 ° C., a hot-rolled plate annealing temperature of 900 to 1200 ° C., a cold-rolled plate annealing temperature of 800 to 1200 ° C., and the like. However, the present invention is not characterized by manufacturing conditions, and the manufacturing conditions are not limited. Therefore, the hot-rolled conditions, the hot-rolled plate thickness, the presence or absence of hot-rolled plate annealing, the cold-rolled conditions, the hot-rolled and cold-rolled plate annealing temperatures, the atmosphere, and the like can be appropriately selected. In addition, the treatment before finishing pickling may be a general treatment, for example, a mechanical treatment such as shot blasting or a grinding brush, or a chemical treatment such as a melt salt treatment or a neutral salt electrolysis treatment. can. Further, temper rolling or tension leveler may be applied after cold rolling and annealing. Further, the product plate thickness may be selected according to the required member thickness. Further, this steel plate may be used as a welded pipe by a normal method for manufacturing a stainless steel pipe for an exhaust system member such as electric resistance welding, TIG welding, and laser welding.

However, in order to satisfy the formula (iv) in order to further improve the high temperature salt damage resistance, it takes 20 seconds or more to maintain the temperature of the steel sheet at 750 ° C. or higher and lower than 830 ° C. in the temperature raising process of the final annealing. There is, or the flow velocity of the atmospheric gas in the final annealing is 0.08 m / s or more. Further, in order to make the value on the left side of the equation (iv) more preferable to be 0.0 or more, the time for maintaining the temperature of the steel plate at 750 ° C. or higher and lower than 830 ° C. in the temperature raising process of the final annealing is 20 seconds or longer. And the flow velocity of the atmospheric gas in the final annealing is 0.08 m / s or more. This is because the oxide containing Al formed on the surface or surface layer of the base metal during the temperature rise process suppresses the growth of recrystallized grains on the surface layer of the base metal. If the temperature is lower than 750 ° C, the progress of oxidation is slow, and if the temperature is 830 ° C or higher, recrystallization proceeds. Therefore, it is important how much Al is oxidized at the temperature during this period. Here, the atmospheric gas is a gas that forms the atmosphere in the furnace at the time of quenching, and is a gas that flows in the furnace, a combustion gas that is generated when burner combustion is performed in the furnace, or even if it is not intended in the furnace. Existing air, etc. It should be noted that even an atmospheric gas that has a reducing atmosphere at the maximum temperature at the time of annealing contains a very small amount of gas species that are an oxidation source of Al such as oxygen and water vapor, and in the process of raising the temperature from 750 ° C. to less than 830 ° C. The plate surface can be oxidized. The flow velocity of the atmospheric gas in the final annealing affects the oxidation reaction because it contributes to the collision frequency of molecules such as oxygen and water vapor, which are the oxidation sources on the plate surface. Here, the final annealing is the final annealing performed in the manufacturing process of the steel sheet or the steel pipe. That is, the final annealing of the steel sheet is the final annealing performed in the steel sheet manufacturing process. The final annealing of the steel pipe is the final annealing performed in the steel pipe manufacturing process, or, if there is no annealing, the final annealing performed in the steel sheet pipe making process. Further, in the case of a continuous annealing line, the flow velocity of the atmospheric gas is the difference between the plate passing speed and the gas flow velocity in the furnace, and when there is no gas flowing in the furnace, the flow velocity of the atmospheric gas is the plate passing speed. However, if the temperature of the steel sheet is maintained at 750 ° C. or higher and lower than 830 ° C. in the temperature raising process of the final annealing for too long, the productivity is lowered, so the upper limit is preferably 100 seconds or less. Further, if the flow velocity of the atmospheric gas is too large, it becomes difficult to control the temperature inside the furnace, so the upper limit is preferably 10 m / s or less.

The automobile exhaust system component according to the present invention includes an automobile exhaust system member using the above-mentioned ferrite stainless steel plate and a forced cooling member for forcibly cooling the automobile exhaust system member. As shown in FIG. 1A, the automobile exhaust system component of the present invention includes, for example, an automobile exhaust system member 1 using the ferrite stainless steel plate of the present invention and a forced cooling member 3 for forcibly cooling the ferritic stainless steel plate of the present invention. I have. In FIG. 1A, the cooling air flow 11 from the air outlet 5 of the cooling fan 4 forcibly air-cools the automobile exhaust system member 1 (exhaust manifold 2). The forced cooling member 3 that forcibly air-cools the automobile exhaust system member 1 located at a high temperature in the exhaust system such as the exhaust manifold 2 and the front pipe becomes a factor that promotes salt adhesion to the automobile exhaust system member 1. .. In such automobile exhaust system parts, by using the ferrite-based stainless steel sheet of the present invention as the material of the automobile exhaust system member 1, sufficient high-temperature salt damage resistance is imparted. As described above, the above-mentioned ferritic stainless steel sheet is used for an automobile exhaust system member in which salt adhesion is promoted by the forced cooling member. The automobile exhaust system parts shown in FIG. 1 (a) are merely examples of the automobile exhaust system parts of the present invention, and the automobile exhaust system parts of the present invention are limited to the embodiment shown in FIG. 1 (a). Needless to say, it cannot be done.

The automobile exhaust system component according to the present invention also includes an automobile exhaust system member using the above-mentioned ferrite stainless steel plate and a heat insulating member that covers the periphery of the automobile exhaust system member with a heat insulating material. The automobile exhaust system component of the present invention is, for example, as shown in FIG. 1 (b), the automobile exhaust system member 1 using the ferrite stainless steel plate of the present invention and the heat insulating member 6 covering the periphery thereof with a heat insulating material 7. And have. In the heat insulating member 6 shown in FIG. 1 (b), the outer periphery of the automobile exhaust system member 1 (exhaust manifold 2) is covered with the heat insulating material 7, and the heat insulating material 7 is protected by the heat insulating material cover 8. Wool-like ceramics or the like is used for the heat insulating material 7, and it is easy to retain water. Therefore, covering the automobile exhaust system member 1 with the heat insulating material 7 makes it easier to retain the salt content and promotes the adhesion of the salt content to the automobile exhaust system member 1. In such automobile exhaust system parts, by using the ferrite-based stainless steel sheet of the present invention as the material of the automobile exhaust system member 1, sufficient high-temperature salt damage resistance is imparted. As described above, the above-mentioned ferritic stainless steel sheet is used for an automobile exhaust system member in which salt adhesion is promoted by applying a heat insulating member that covers the periphery of the automobile exhaust system member with a heat insulating material. The automobile exhaust system parts shown in FIG. 1 (b) are merely examples of the automobile exhaust system parts of the present invention, and the automobile exhaust system parts of the present invention are limited to the embodiment shown in FIG. 1 (b). Needless to say, it cannot be done.

Hereinafter, the effects of the present invention will be further clarified by examples. The present invention is not limited to the following examples, and can be appropriately modified and implemented without changing the gist thereof.

Test materials having the component compositions shown in Tables 1 and 2 (Examples A to T of the present invention, Comparative Examples a to q) are melted in a vacuum melting furnace and cast into a 150 kg ingot, and the ingot is hot-rolled 5 A hot-rolled steel plate with a thickness of 0.0 mm was used. Then, the hot-rolled steel sheet was pickled, cold-rolled to a thickness of 2.0 mm, finally annealed at 850 to 1100 ° C., which is a recrystallized structure, and then pickled to obtain a product plate. The time during which the temperature of the steel sheet was maintained at 750 ° C. or higher and lower than 830 ° C. in the temperature raising process of the final annealing and the flow velocity of the atmospheric gas in the final annealing were carried out within the scope of the present invention.

The obtained product plate was subjected to measurement of the average crystal grain size at the center of the plate thickness, evaluation of processability, and evaluation of high-temperature salt damage resistance.
Regarding the measurement of the average crystal grain size at the center of the plate thickness, the microstructure of the cross section in the direction parallel to the rolling direction at the center of the plate width was observed, and the crystal grain size was arbitrarily measured at 5 points in accordance with the cutting method of JISG0551: 2013. The average value was obtained, and the average crystal grain size was calculated.

For the evaluation of workability, JIS No. 13B test pieces were collected in a direction parallel to the rolling direction, and 0.2% proof stress at room temperature was obtained in accordance with JIS Z2241: 2011. Those with a 0.2% proof stress at room temperature of more than 400 MPa are "× (defective)", those with a proof stress of more than 390 MPa and 400 MPa or less are "● (possible)", those with a proof stress of more than 380 MPa and 390 MPa or less are "○ (good)", and those with a 380 MPa or less. The one was designated as "◎ (even better)".

For the evaluation of high temperature salt damage resistance, a test piece having a width of 20 mm and a length of 50 mm was prepared from the product plate and evaluated by a high temperature salt damage test. As a high-temperature salt damage test, the corrosion weight loss after 20 cycles of heating, cooling, salt water immersion, and drying of the test piece was evaluated. The heating conditions were a temperature of 750 ° C. and a holding time of 130 minutes. The cooling conditions were a temperature of room temperature and a holding time of 30 minutes. As the salt water immersion conditions, a saturated NaCl aqueous solution was used as the salt water, the aqueous solution temperature was normal temperature, and the immersion time was 30 minutes. That is, a 26% by mass NaCl aqueous solution was used as the salt water. The drying conditions were a temperature of 50 ° C. and a holding time of 30 minutes after taking out from the salt water. The atmosphere for heating, cooling, and drying was air with a dew point of 40 to 50 ° C. The weight difference between the test pieces before the high-temperature salt damage test and after removing the corrosion products generated in the high-temperature salt damage test was measured, and the value per surface area of the test piece before the high-temperature salt damage test was used as the corrosion weight loss. To remove the corrosion products on the surface of the test piece after the high-temperature salt damage test, the test piece is immersed in a boiling 15% by mass ammonium dihydrogen dihydrogen phosphite aqueous solution for 20 minutes, washed with water, and then brushed several times. Carried out. The corrosion resistance of the high temperature salt damage test thus obtained was used to evaluate the high temperature salt damage resistance. Corrosion weight loss of more than 150 mg / cm ² is "× (defective)", 125 mg / cm ² more than 150 mg / cm ² or less is "● (possible)", 100 mg / cm ² more than 125 mg / cm ² or less Was “○ (good)”, ^{and those with 100 mg / cm 2} or less were “◎ (even better)”.

Tables 3A and 3B show examples A to T of the present invention, average crystal grain size at the center of the plate thickness of Comparative Examples a to q, values on the left side of the formulas (i) to (iii), processability and high temperature salt damage resistance. The evaluation result is shown.

As is clear from Tables 1 to 3B, the steel sheets having the component compositions specified in the present invention and satisfying the formulas (i) to (iii) are excellent in processability and high temperature salt damage resistance as compared with the steel sheets of the comparative examples. You can see that.

Further, as described above, there is a preferable or more preferable range of Si, P, Ni, Cu, Mo, Al for further improving workability, and C, Si, Mn, for further improving high temperature salt damage resistance. There is a preferred or more preferred range of S, Mo, Al. It can be seen that the steel sheet having more elements in these ranges has a better evaluation of workability or high temperature salt damage resistance.

Further, in Table 4, steel sheets were produced under various production conditions for the material steels B, D, and E having the compositions within the scope of the present invention. Table 4 shows the time when the temperature of the steel sheet in the heating process in the final annealing is 750 ° C. or higher and lower than 830 ° C., the flow rate of the atmospheric gas, and the annealing temperature. The atmosphere gas was air. As a result of measuring the average crystal grain size of the central portion of the base metal plate thickness and the surface, the value on the left side of the formula (iv) and the result of high temperature salt damage resistance evaluation are shown.
Regarding the measurement of the average crystal grain size of the plate surface, the structure of the plate surface was observed, and the crystal grain size was arbitrarily measured at 5 points in accordance with the cutting method of JISG0551: 2013 to obtain the average value, and the average crystal grain size was obtained. Was calculated. Polishing, pickling, electrolytic polishing, and etching were appropriately performed in order to bring out the contrast of the structure of the plate surface, and the amount of reduction in the plate thickness at that time was set to be within 5% of the original plate thickness.

As is clear from Table 4, if the formula (iv) is satisfied, the high temperature salt damage resistance is improved, and if the value of the left side (900DA-1000DB) of the formula (iv) is 0.0 or more, the high temperature resistance is improved. It can be seen that the evaluation of salt damage is even better.

The steel of the present invention is resistant to high-temperature salt damage even when the heating temperature is 650 ° C, 700 ° C, 800 ° C, etc., when the salt water is a saturated CaCl ₂ aqueous solution, or when the atmosphere is a dry atmosphere. The order of high temperature salt damage was the same. From this, it is considered that the examples of the present invention exhibit excellent high-temperature salt damage resistance in various high-temperature salt damage environments.

As is clear from these, it can be seen that the steels having the individual component compositions specified in the present invention and satisfying the formulas (i) to (iii) are excellent in high temperature salt damage resistance. Further, it can be seen that steels having individual component compositions satisfying various preferable or more preferable ranges and steels satisfying the formula (iv) further improve workability or high temperature salt damage resistance.

According to the present invention, it is possible to provide a ferritic stainless steel sheet having excellent workability and high temperature salt damage resistance for applications such as an exhaust manifold and a front pipe that require workability and high temperature salt damage resistance. Specific applications include automobile exhaust system parts where salt adhesion is promoted by forced cooling members, and automobile exhaust where salt adhesion is promoted by applying heat insulating members that cover the periphery of the automobile exhaust system with heat insulating material. It is a system part. By making these parts possible, it is possible to promote the spread of turbo-equipped vehicles to which these parts are applied and engine vehicles that burn HCCI (premixed compression automatic ignition), and contribute to improving the fuel efficiency of automobiles and reducing the environmental load. ..

1 Automobile exhaust system member 2 Exhaust manifold 3 Forced cooling member 4 Cooling fan 5 Blower 6 Insulation member 7 Insulation material 8 Insulation material cover 10 Exhaust gas 11 Cooling air flow

Claims (8)

By mass%,
C: 0.020% or less,
N: 0.020% or less,
Si: 0.05% or more, 1.60% or less,
Mn: 0.01% or more, 1.10% or less,
P: 0.040% or less,
S: 0.0022% or less,
Cr: 10.0% or more, 15.0% or less,
Ni: 0.01% or more, 0.70% or less,
Cu: 0.001% or more, 0.80% or less,
Mo: 0.01% or more, 2.00% or less,
Ti: 0.050% or more, 0.300% or less,
Nb: Over 0.03%, 0.60% or less,
Al: Over 0.70%, 3.00% or less,
V: 0.01% or more, 0.20% or less,
B: 0.0001% or more, 0.0050% or less,
O: 0.0050% or less,
A ferritic stainless steel sheet containing Fe and impurities, and satisfying the following formulas (i) to (iii).
Al + 1.7Si + 0.8Mo + 0.3DA ^-0.5 ≤ 6.1 ... Equation (i)
2.6Al + 4.3Si + 1.5Mo ≧ 6.00 ・ ・ ・ Equation (ii)
Al / O ≧ 200 ・ ・ ・ Equation (iii)
However, the element symbol in the formula means the content (% by mass) of the element, and DA means the average crystal grain size (mm) at the center of the plate thickness. The ferrite-based stainless steel sheet according to claim 1, further satisfying the following formula (iv).
900DA-1000DB ≧ -1.7 ・ ・ ・ Equation (iv)
However, DA in the formula means the average crystal grain size (mm) at the center of the plate thickness, and DB means the average crystal grain size (mm) on the plate surface. By mass%, instead of a part of Fe,
W: 0.01% or more, 0.50% or less,
Y: 0.001% or more, 0.20% or less,
REM: 0.001% or more, 0.20% or less,
Ca: 0.0001% or more, 0.0030% or less,
Zr: 0.01% or more, 0.30% or less,
Hf: 0.001% or more, 1.0% or less,
Sn: 0.001% or more, 1.0% or less,
Mg: 0.0001% or more, 0.0030% or less,
Co: 0.01% or more, 0.50% or less,
Sb: 0.001% or more, 0.50% or less,
Bi: 0.001% or more, 1.0% or less,
Ta: 0.001% or more, 1.0% or less, and Ga: 0.0001% or more, 0.30% or less,
The ferrite-based stainless steel sheet according to claim 1 or 2, wherein the ferrite-based stainless steel sheet contains one or more of the above. The ferrite-based stainless steel sheet according to any one of claims 1 to 3, which is used for an automobile exhaust system member in which salt adhesion is promoted by a forced cooling member. The ferrite stainless steel according to any one of claims 1 to 3, which is used for an automobile exhaust system member in which salt adhesion is promoted by applying a heat insulating member that covers the periphery of the automobile exhaust system member with a heat insulating material. Steel plate. The method for manufacturing a ferritic stainless steel sheet according to any one of claims 1 to 5, wherein the temperature of the steel sheet is maintained at 750 ° C. or higher and lower than 830 ° C. in the temperature raising process of final annealing for 20 seconds or longer. A method for producing a ferritic stainless steel sheet, wherein the flow velocity of the atmospheric gas in the final annealing is 0.08 m / s or more. An automobile exhaust system component comprising the automobile exhaust system member using the ferrite-based stainless steel plate according to any one of claims 1 to 3 and a forced cooling member for forcibly cooling the automobile exhaust system member. An automobile exhaust system component comprising an automobile exhaust system member using the ferrite stainless steel plate according to any one of claims 1 to 3 and a heat insulating member that covers the periphery of the automobile exhaust system member with a heat insulating material.

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