JP7479209B2 - Ferritic stainless steel sheet, method for producing the same, and automobile exhaust system part - Google Patents

Description

The present invention relates to ferritic stainless steel sheets, a manufacturing method for ferritic stainless steel sheets, and automobile exhaust system parts. The present invention relates to heat-resistant stainless steels that are particularly suitable for use in automobile exhaust system components that require high-temperature strength and oxidation resistance, and particularly to ferritic stainless steel sheets for automobile exhaust systems that have excellent workability and resistance to high-temperature salt damage and a structure that promotes salt adhesion, a manufacturing method for ferritic stainless steel sheets, and automobile exhaust system parts.

Exhaust system components such as the exhaust manifold, front pipe, and center pipe of an automobile pass high-temperature exhaust gas emitted from the engine, so the materials that make up these exhaust system components are required to have a variety of properties, including oxidation resistance, high-temperature strength, and thermal fatigue properties.

Traditionally, cast iron was generally used for automotive exhaust system components, but stainless steel exhaust manifolds have begun to be used in light of stricter exhaust gas regulations, improved engine performance, and lighter vehicle weight. Exhaust gas temperatures vary by vehicle model, and in recent years are often around 750-850°C, but can reach even higher temperatures. Materials with high high-temperature strength and oxidation resistance are required for long-term use in environments at such temperatures.

Among stainless steels, austenitic stainless steels have excellent heat resistance and workability, but because of their large thermal expansion coefficient, they are prone to thermal fatigue failure when used in components that are repeatedly heated and cooled, such as exhaust manifolds.

Ferritic stainless steel has a smaller thermal expansion coefficient than austenitic stainless steel, and therefore has excellent thermal fatigue properties. In addition, compared to austenitic stainless steel, it contains almost no expensive Ni, so the material costs are low and it is widely used. However, since ferritic stainless steel has lower high-temperature strength than austenitic stainless steel, technologies have been developed to improve high-temperature strength. Furthermore, various technologies have been developed from the perspectives of oxidation resistance, formability, manufacturability, and further cost reduction.

Ferritic stainless steels that have been developed include, for example, SUS430J1L (Nb-containing steel), Nb-Si-containing steel, and SUS444 (Nb-Mo-containing steel). These steels contain Nb, and based on the Nb content, they also contain Si and Mo to improve high-temperature strength. However, Nb has high raw material costs, and 90% of its production is in Brazil, meaning that supply is unevenly distributed, creating a high resource risk. On the other hand, technology has also been developed to improve high-temperature strength and oxidation resistance by including Al, as in Patent Document 1.

On the other hand, components such as the center pipe located under the vehicle body are exhaust system components in which the temperature of the exhaust gas passing through them is relatively low, but they are prone to adhesion of salt derived from seawater and snow-melting salt spread on road surfaces to prevent freezing, and high-temperature salt damage is a concern. High-temperature salt damage is a phenomenon in which high-temperature corrosion is accelerated by the adhesion of salt in a high-temperature oxidizing environment. For these parts located under the vehicle body, technologies have been developed that place emphasis on resistance to high-temperature salt damage.

For example, Patent Document 2 discloses a technology that improves high-temperature oxidation resistance and high-temperature salt damage resistance by incorporating Al, Mo, and W. Patent Document 3 discloses a technology that improves high-temperature salt corrosion resistance by adjusting the contents of N, V, and Al.

JP 2018-168457 A JP 2004-18916 A JP 2010-53421 A

However, in recent years, due to changes in the components installed in the exhaust system and the addition of new components, salt may adhere to components located at high temperatures in the exhaust system, such as the exhaust manifold and front pipe. Two specific examples are given below. The first is a case where the exhaust system is forced to cool by installing a fan or improving ventilation in response to the increase in exhaust gas temperature. One example is shown in FIG. 1(A). FIG. 1 is a diagram showing an example of an automobile exhaust system component, (A) is a schematic diagram of an automobile exhaust system component equipped with a forced cooling member, and (B) is a schematic diagram of an automobile exhaust system component equipped with a heat insulating member. The forced cooling member 3 that forcibly cools this automobile exhaust system component 1 is a factor that promotes salt adhesion to the automobile exhaust system component 1. The forced cooling member 3 may be applied mainly to turbo-equipped vehicles, the popularity of which has accelerated in recent years. The second is a case where a heat insulating member is provided to cover exhaust gas purification components such as a converter and the exhaust system through which exhaust gas passes with heat insulating material, thereby raising the exhaust gas temperature and promoting the catalytic reaction. One example is shown in FIG. 1(B). The insulating material 7 that covers the automobile exhaust system component 1 is made of wool-like ceramics and the like, which is easy to retain water. Therefore, covering the automobile exhaust system component 1 with the insulating material 7 also makes it easier to retain salt, promoting the adhesion of salt to the automobile exhaust system component 1. The insulating material 7 is mainly used in the exhaust systems of engines that burn fuel using homogeneous-charge compression ignition (HCCI), which is expected to become more widespread in the future.

Furthermore, the use of these forced cooling or insulating materials creates an environment with higher humidity than the environment to which normal materials are exposed. In an environment where the forced cooling materials blow moisture onto the components, moisture is constantly being sprayed onto the exhaust system. When insulating materials are used, the insulating material is covered with a cover, creating an environment in which water vapor is trapped between the exhaust system and the cover. As a result, the high-temperature salt damage environment is an environment in which not only does salt adhere to the oxidation environment, but also water vapor oxidation, making it a different environment from the conventional one.

In other words, when forced cooling or heat insulating components are used, not only do upstream components such as exhaust manifolds and front pipes need to be resistant to high temperatures and salt damage, but they also need to be resistant to high temperatures and salt damage in environments involving steam oxidation that are different from conventional ones. In other words, automobile exhaust system components to which forced cooling or heat insulating components are applied need to be developed for new applications that are different from conventional automobile exhaust system components.

The present inventors have found that it is important to form Al ₂ O ₃ in ferritic stainless steel for high-temperature salt damage resistance in a high-temperature salt damage environment accompanied by steam oxidation. Stainless steel forms Cr ₂ O ₃ or Al ₂ O ₃ , which have a low oxidation rate, in a high-temperature environment, and suppresses the oxidation of Fe, which has a high oxidation rate, thereby obtaining protection against high-temperature corrosion. Al ₂ O ₃ has a lower oxidation rate than Cr ₂ O ₃ and is more protective. In an environment accompanied by steam oxidation, the protective property of Cr ₂ O ₃ decreases due to several mechanisms such as an increase in voids and defects, but Al ₂ O ₃ has a strong bond between Al and O and is less likely to form voids and defects. In addition, in a high-temperature salt damage environment, Cr ₂ O ₃ reacts with salt and its protective property decreases, but Al ₂ O ₃ is relatively less affected. In addition, when Al ₂ O ₃ is formed, there is room for Cr ₂ O ₃ to be formed even after its protective property decreases. As a result, the formation _{of Al2O3} improves the resistance of ferritic stainless steel to high-temperature salt damage.

When ferritic stainless steel contains Al, Al ₂ O ₃ is easily formed at 800°C or higher, but is difficult to form at temperatures below 800°C. Therefore, for Al-containing ferritic stainless steel, it is necessary to consider high-temperature salt damage resistance, particularly at temperatures below 800°C. The reason why Al ₂ O ₃ is difficult to form at temperatures below ₈₀₀ ° _C is that the lower the temperature, the more difficult it is for Al to diffuse from the inside of the base material to the surface. Therefore, in order to improve high-temperature salt damage resistance, it is important to control the crystal grain size, which affects the diffusion of the base material, in addition to the balance of material components. With regard to the crystal grain size, it is considered that the crystal grain size of the surface layer of the base material has a large effect.

Furthermore, automobile exhaust system parts often have complex shapes and require workability. When ferritic stainless steel contains Al, the 0.2% yield strength at room temperature (10-35°C) increases and workability is impaired. The 0.2% yield strength is influenced by the balance of material components as well as the grain size, and control of these is considered important. With regard to grain size, it is believed that the grain size in the center of the base material plate thickness has a particularly large influence.

Patent Document 1 describes an Al-containing ferritic stainless steel, but does not disclose any detailed consideration of either high-temperature salt damage resistance or 0.2% yield strength at room temperature. In addition, no consideration is given to the grain size of either the surface layer of the base material or the center of the plate thickness.

Patent Document 2 discloses a technology that contains 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 technology disclosed in Patent Document 2 does not take into account an environment accompanied by steam oxidation, nor does it take into account the grain size of the surface layer of the base material. In addition, no detailed study is disclosed on the 0.2% yield strength at room temperature, and the grain size at the center of the plate thickness is not taken into account. The ferritic stainless steel disclosed in Patent Document 2 contains large amounts of Al, Mo, and W, and in particular, the W content is more than 2.0% and 5% or less by mass%, and the W content is extremely high, and the 0.2% yield strength at room temperature is excessively high, which may impair workability. In addition, Nb is also contained, and the raw material cost is high, including the inclusion of Mo and W.

Patent Document 3 discloses a technology for improving resistance to high-temperature salt corrosion by adjusting the contents of N, V, and Al. However, the technology disclosed in Patent Document 3 does not take into account the environment involving steam oxidation, nor does it take into account the grain size of the surface layer of the base material. In addition, no detailed study is disclosed on the 0.2% yield strength at room temperature, and the grain size of the center of the plate thickness is not taken into account. In addition, the Al content is only evaluated in the range of less than 0.4% by mass, with the upper limit set at 0.30%. In addition, the ferritic stainless steel disclosed in Patent Document 3 has a V concentration of 0.30 to 0.60% by mass, which is extremely higher than that of normal ferritic stainless steel, and may impair manufacturability. In addition, the Cr concentration is 16 to 20%, the Cu concentration is 0.8 to 1.6%, and Nb is also contained, which increases the raw material cost.

When applying forced cooling or heat insulating components to an automobile exhaust system as described above, not only does it become necessary to impart high-temperature salt damage resistance to upstream components such as exhaust manifolds and front pipes, but it also becomes necessary to provide high-temperature salt damage resistance in an environment accompanied by steam oxidation that is different from the conventional one. High-temperature salt damage resistance is an issue for Al-containing ferritic stainless steels, and high-temperature salt damage in such environments has not been considered. Furthermore, even in the conventional high-temperature salt damage environment, it has been necessary to include extremely large amounts of W, V, etc. as an improvement technology. Furthermore, there has been insufficient consideration of the fact that the inclusion of Al increases the 0.2% yield strength at room temperature and reduces workability. There is also the issue of high raw material costs due to the inclusion of expensive Nb.

That is, the object of the present invention is to provide a ferritic stainless steel sheet that has excellent resistance to high-temperature salt damage and workability in a high-temperature environment accompanied by steam oxidation, a manufacturing method for the ferritic stainless steel sheet, and an automobile exhaust system part. In particular, the object of the present invention is to provide an Al-containing ferritic stainless steel sheet for automobile exhaust systems that is a member that has excellent workability and resistance to high-temperature salt damage and promotes salt adhesion, and an automobile exhaust system part.

In order to solve the above problems, the inventors conducted extensive research into the effects of various components and the grain size at the surface and center of the base material on the high-temperature salt damage resistance and workability of ferritic stainless steel. As a result, they were able to invent a ferritic stainless steel sheet that has excellent high-temperature salt damage resistance and workability. Furthermore, high-temperature salt damage can also be achieved in an environment accompanied by steam oxidation.

That is, the gist of the present invention, which aims to solve the above problems, is as follows.
(1) In mass%,
C: 0.020% or less,
N: 0.020% or less,
Si: 0.05% or more and 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: 0.03% or less,
Al: more than 0.70% and not more than 3.00%;
V: 0.01% or more, 0.20% or less,
B: 0.0001% or more and 0.0050% or less, and O: 0.0050% or less,
and the balance consisting of Fe and impurities, and satisfying the following formulas (i) to (iii):
Al + 1.7Si + 0.8Mo + 0.3DA ^-0.5 ≦ 6.2 ... formula (i)
2.6Al+4.3Si+1.5Mo≧6.05 Formula (ii)
Al/O≧200 Formula (iii)
In the formula, the element symbol represents the content (mass%) of the element, and DA represents the average crystal grain size (mm) at the center of the sheet thickness.
(2) The ferritic stainless steel sheet according to (1), further comprising the following formula (iv):
900DA-1000DB≧-3.2 ... formula (iv)
In the formula, DA means the average crystal grain size (mm) at the center of the sheet thickness, and DB means the average crystal grain size (mm) at the sheet surface.
(3) In mass%, replacing a part of Fe,
W: 0.01% or more, 0.50% or less,
Y: 0.001% or more and 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 and 1.0% or less, and Ga: 0.0001% or more and 0.30% or less,
The ferritic stainless steel sheet according to (1) or (2), characterized in that it contains one or more of the following:
(4) A ferritic stainless steel sheet according to any one of (1) to (3), which is used as an automobile exhaust system member in which salt adhesion is promoted by a forced cooling member.
(5) A ferritic 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 applying a heat insulating member that covers the periphery of the automobile exhaust system member with a heat insulating material.
(6) A method for producing a ferritic stainless steel sheet according to any one of (1) to (5), characterized in that the holding time in the range of 750°C or more and less than 830°C during the temperature rise process of final annealing is 20 seconds or more, or the flow rate of the atmospheric gas in the final annealing is 0.08 m/s or more.

(7) An automobile exhaust system part comprising: an automobile exhaust system member using the ferritic stainless steel plate according to any one of (1) to (3); and a forced cooling member for forcedly cooling the automobile exhaust system member.
(8) An automobile exhaust system part comprising: an automobile exhaust system member using the ferritic stainless steel plate according to any one of (1) to (3); and a heat insulating member that covers the periphery of the automobile exhaust system member with a heat insulating material.

In addition, in the above invention, when no lower limit is specified, this indicates that the content includes the unavoidable impurity level.

The present invention provides a ferritic stainless steel sheet that has excellent resistance to high-temperature salt damage and workability in a high-temperature environment accompanied by steam oxidation, a manufacturing method for the ferritic stainless steel sheet, and an automobile exhaust system part. In particular, it provides a ferritic stainless steel sheet that has excellent workability and resistance to high-temperature salt damage and is used as an automobile exhaust system part where forced cooling members and heat insulating members are used and salt adhesion is promoted.

1A is a schematic diagram of an example of an automobile exhaust system part equipped with a forced cooling member, and FIG. 1B is a schematic diagram of an automobile exhaust system part equipped with a heat insulating member.

The present invention is described in detail below.

First, we will explain the reasons for limiting the steel composition of the ferritic stainless steel of the present invention. Here, "%" in the steel composition means mass %.

(C: 0.020% or less)
Since C is an element that deteriorates formability and corrosion resistance and reduces high-temperature strength, the C content is set to 0.020% or less. In addition, excessive C content reduces oxidation resistance and intergranular corrosion resistance. Considering the reduction in oxidation resistance and intergranular corrosion resistance due to excessive C content, the C content is preferably set to 0.015% or less. In addition, since high-temperature salt damage resistance is further improved by reducing C, it is more preferable to set the upper limit to 0.010% or less. However, since excessive reduction leads to an increase in refining costs, the C content is preferably set to 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 formability and corrosion resistance and reduces high-temperature strength, so the N content is set to 0.020% or less. In addition, excessive N content reduces oxidation resistance and intergranular corrosion resistance. Considering the reduction in oxidation resistance and intergranular corrosion resistance due to excessive N content, the N content is preferably set to 0.015% or less. The N content is more preferably set to 0.013% or less. However, since excessive reduction leads to an increase in refining costs, the N content is preferably set to more than 0.0030%. The N content is more preferably set to 0.0040% or more.

(Si: 0.05% or more, 1.60% or less)
Si is an element contained as a deoxidizer, and is also an element that improves high-temperature salt damage resistance. Si forms Si oxides at high temperatures, contributing to the improvement of high-temperature salt damage resistance, and the formation of Si oxides also promotes the formation of Al ₂ O ₃ , thereby improving 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 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, excessive Si content increases the 0.2% proof stress at room temperature and leads to a decrease in workability, so the Si content is 1.60% or less. In order to further improve workability, the Si content is preferably 1.20% or less. In addition, 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 deoxidizer, and the Mn content is 0.01% or more. In consideration of refining costs, the Mn content is more preferably 0.10% or more. The Mn content is even more preferably 0.15% or more. However, since excessive Mn content delays the formation of Al ₂ O ₃ and leads to a decrease in high-temperature salt damage resistance, the Mn content is 1.10% or less. In order to further improve high-temperature salt damage resistance, the Mn content is preferably 0.50% or less. In addition, in consideration of uniform elongation, hot workability and corrosion resistance, the Mn content is more preferably 0.40% or less. The Mn content is even more preferably less than 0.40%.

(P: 0.040% or less)
P is an impurity that is mainly mixed in from raw materials during steelmaking refining, and as its content increases, toughness and weldability decrease, so the lower its content, the better. Therefore, the P content is set to 0.040% or less. In addition, in consideration of manufacturability, the P content is preferably set to 0.035% or less. In addition, since reducing P further reduces the 0.2% proof stress at room temperature and improves workability, the P content is more preferably set to 0.030% or less. However, since excessive reduction leads to an increase in refining costs, the P content is preferably set to 0.005% or more. The P content is more preferably set to 0.010% or more.

(S: 0.0022% or less)
S is an impurity that is mainly mixed in from raw materials during steel refining, and deteriorates corrosion resistance. S also reduces scale spalling resistance, thereby reducing high-temperature salt damage resistance. Therefore, the S content is set to 0.0022% or less. In addition, in consideration of manufacturability, the S content is preferably set to 0.0020% or less. In order to further improve high-temperature salt damage resistance, it is more preferable to set the S content to 0.0014% or less. However, since excessive reduction leads to an increase in refining costs, the S content is preferably set to 0.0001% or more. The S content is more preferably 0.0003% or more.

(Cr: 10.0% or more, 15.0% or less)
Cr is an element that improves corrosion resistance and also improves high-temperature salt damage resistance. Cr also promotes the formation of Al ₂ O _3. Therefore, the Cr content is 10.0% or more. The Cr content is preferably 10.5% or more, and more preferably 10.7% or more. However, excessive Cr content increases the 0.2% proof stress at room temperature and reduces workability, so the Cr content is 15.0% or less. Considering toughness and raw material costs, 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 set to 0.01% or more. The Ni content is preferably 0.02% or more, and more preferably 0.05% or more. However, since excessive Ni content leads to a decrease in formability, the Ni content is set to 0.70% or less. In addition, since reducing Ni further reduces the 0.2% proof stress at room temperature and improves workability, the Ni content is preferably set to 0.50% or less. In consideration of raw material costs, 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 set to 0.001% or more. In consideration of 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 excessive Cu content leads to a decrease in hot workability, the Cu content is set to 0.80% or less. In addition, since reducing Cu further decreases the 0.2% proof stress at room temperature and improves workability, the Cu content is preferably set to 0.30% or less. In consideration of 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 also improves high-temperature salt damage resistance. Mo also enhances the protective properties of Al ₂ O _3. Therefore, the Mo content is set to 0.01% or more. In order to further improve high-temperature salt damage resistance, the Mo content is preferably set to more than 0.30%. The Mo content is more preferably set to 0.55% or more. However, since excessive Mo content increases the 0.2% proof stress at room temperature and leads to a decrease in workability, the Mo content is set to 2.00% or less. In order to further improve workability, the Mo content is preferably set to 1.50% or less. In addition, in consideration of manufacturability, it is more preferable that the Mo content is less than 1.00%. In consideration of alloy cost, it is even more preferable that the Mo content is less than 0.80%.

(Ti: 0.050% or more, 0.300% or less)
Ti is an element that combines with C, N, and S to improve corrosion resistance, intergranular corrosion resistance, and deep drawability, and the Ti content is 0.050% or more. The Ti content is preferably 0.080% or more, and more preferably 0.120% or more. However, excessive Ti content leads to a decrease in uniform elongation and a decrease in hole expansion workability due to the formation of coarse Ti-based precipitates, so the Ti content is 0.300% or less. The Ti content is preferably 0.270% or less, and more preferably 0.220% or less.

(Nb: 0.03% or less)
Nb is an element that contributes to improving high-temperature strength and high-temperature salt damage resistance. However, due to high raw material costs and resource risks, the Nb content is set to 0.03% or less. The Nb content is preferably 0.02% or less, and more preferably 0.01% or less. However, since excessive reduction leads to an increase in refining costs, the Nb content is preferably 0.001% or more. The Nb content is more preferably 0.003% or more.

(Al: more than 0.70%, 3.00% or less)
Al is contained as a deoxidizing element and is an element that improves high-temperature strength, oxidation resistance, and high-temperature salt damage resistance, and the Al content is set to more than 0.70%. In order to further improve high-temperature salt damage resistance, it is preferable that the Al content is more than 1.05%. The Al content is more preferably more than 1.55%. However, since excessive Al content increases the 0.2% proof stress at room temperature and leads to a decrease in workability, the Al content is set to 3.00% or less. In order to further improve workability, it is preferable that the Al content is 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 leads to a decrease in high-temperature strength and a decrease in thermal fatigue life due to coarsening of precipitates. Therefore, the V content is set to 0.20% or less. In addition, 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 properties. Therefore, the B content is set to 0.0001% or more, preferably 0.0002% or more, and more preferably 0.0003% or more. However, an excessive B content leads to a decrease in hot workability and a decrease in the surface properties of the steel surface. Therefore, the B content is set to 0.0050% or less. In addition, taking into consideration manufacturability and formability, the B content is preferably set to 0.0030% or less. More preferably, the B content is set to 0.0015% or less.

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

In the ferritic stainless steel sheet according to this embodiment, the remainder 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 this embodiment. Note that the impurities referred to here are components that are mixed in due to various factors in the raw materials such as ores and scraps and the manufacturing process when the ferritic stainless steel sheet according to the present invention is industrially manufactured, and are acceptable within a range that does not adversely affect the present invention.

Next, we will explain formulas (i) to (iv).

While examining various influences on the 0.2% yield strength at room temperature, the inventors discovered that in addition to Al, Si, and Mo, the crystal grain size at the center of the base material thickness has a large influence, and that the balance between these is important, and thus derived formula (i).
Al + 1.7Si + 0.8Mo + 0.3DA ^-0.5 ≦ 6.2 ... formula (i)
In the formula, the element symbol represents the content (mass%) of the element, and DA represents the average crystal grain size (mm) at the center of the sheet thickness. Here, the center of the sheet thickness refers to the range from 2t/5 to 4t/5 when the sheet thickness is t.
If the value of the left side of formula (i) exceeds 6.2, the 0.2% proof stress at room temperature increases, leading to a decrease in workability. Therefore, the Al content, the Si content, the Mo content, and the DA must satisfy 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, so the value of the left side of formula (i) is preferably 6.0 or less. In addition, if the contents of Al, Si, and Mo are small and the DA is large, the high-temperature strength and oxidation resistance may decrease, so the value of the left side of formula (i) is preferably 3.5 or more.

In addition, in order to improve resistance to high-temperature salt damage _, it is important to promote the formation of _Al2O3 and improve its protective properties. In examining various influences on resistance to high-temperature salt damage, the present inventors found that the balance of Al, Si, Mo, and O is important, and obtained formulas (ii) and (iii).
2.6Al+4.3Si+1.5Mo≧6.05 Formula (ii)
Al/O≧200 Formula (iii)
However, the element symbol in the formula means the content (mass%) of the corresponding element. Since the content of Al, Si, and Mo is large, the cost may increase and the manufacturability may decrease, so the value of the left side of formula (ii) is preferably 9.90 or less. Furthermore, since the content of Al, Si, and Mo is small, the high-temperature strength and oxidation resistance may decrease, so the value of the left side of formula (ii) is preferably 6.10 or more. Since the content of Al is large and the content of O is small, the weldability may decrease, so Al/O is preferably 3500 or less. Furthermore, since the content of Al is small and the content of O is large, the manufacturability may decrease, so Al/O is preferably 300 or more.

As described above, the promotion of the formation of Al ₂ O ₃ and the improvement of its protective properties are important for improving high-temperature salt damage resistance. The inventors believe that the reason why high-temperature salt damage resistance is improved by satisfying the above formula (ii) and the above formula (iii) is as follows. It is believed that Al contributes to both the promotion of the formation of Al ₂ O ₃ and the improvement of its protective properties. It is believed that Si promotes the formation of Al ₂ O ₃ by suppressing the oxidation of Fe and Cr, which are contained in large amounts in the base material. It is believed that Mo improves the protective properties of Al ₂ O ₃ by dissolving in the crystal grain boundaries of Al ₂ O _3. It is also believed that the greater the proportion of O in the balance between Al and O, the more easily Al is internally oxidized in the base material, and the formation of Al ₂ O ₃ on the surface of the base material is inhibited.

They also found that resistance to high-temperature salt damage can be further improved by balancing the grain size at the center of the base material thickness and the grain size at the surface of the thickness, and obtained formula (iv) as a preferable condition.
900DA-1000DB≧-3.2 ... formula (iv)
Here, DA means the average grain size (mm) at the center of the sheet thickness, and DB means the average grain size (mm) at the sheet surface. The value of the left side of formula (iv) is more preferably 0.0 or more. However, if DA is large and DB is small, it may lead to a decrease in formability, so the value of the left side of formula (iv) is preferably 60 or less.

The inventors believe that the fact that the crystal grain size at the surface of the ferritic stainless steel sheet is finer than that at the center of the sheet thickness promotes the diffusion of Al and the formation of _Al2O3 in the surface layer located between the surface of the ferritic stainless steel sheet and the center of the _sheet thickness.

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

To measure the average grain size at the center of the plate thickness and on the plate surface, the structure of the plate surface is observed, the grain size is measured at five random points in accordance with the cutting method of JIS G0551:2013, the average value is calculated, and the average grain size is calculated.

In addition, in the present invention, the properties can be further improved by selectively containing one or more of W, Y, REM, Ca, Zr, Hf, Sn, Mg, Co, Sb, Bi, Ta, and Ga as necessary. These elements are described below. Note that these elements do not necessarily have to be contained, so 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 as necessary. In addition, W is also an element that improves corrosion resistance, so the W content is more preferably 0.05% or more. However, since W is an expensive element, even if it is contained, it is preferably 0.50% or less. In addition, 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 at 0.001% or more as necessary. The Y content is more preferably 0.003% or more. However, since an excessive Y content may lead to an increase in raw material costs 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 metal; 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 preferably contained in an amount of 0.001% or more as necessary. The REM content is more preferably 0.003% or more. However, since excessive REM content may lead to an increase in raw material costs and a decrease in manufacturability, it is preferable to set the REM content to 0.20% or less. The REM content is more preferably 0.10% or less. REM refers to a collective term for 15 elements (lanthanoids) ranging from scandium (Sc) and lanthanum (La) to lutetium (Lu). As REM, one of the above elements may be contained alone, or two or more 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 at 0.0001% or more as 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 as necessary. The Zr content is more preferably 0.03% or more. However, since excessive Zr content 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 as necessary. The Hf content is more preferably 0.003% or more. However, since an excessive Hf content 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 as necessary. The Sn content is more preferably 0.003% or more. However, since 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 that improves formability, so it is preferable to contain 0.0001% or more as necessary. The Mg content is more preferably 0.0003% or more. However, since excessive Mg content may lead to deterioration of corrosion resistance, weldability, and surface quality, it is preferable to set the Mg content to 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 as 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. In consideration of manufacturability and workability, the Co content is more preferably 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 as necessary. The Sb content is more preferably 0.005% or more. However, since an excessive Sb content may cause a decrease in 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 as necessary. The Bi content is more preferably 0.003% or more. However, since an 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 as necessary. The Ta content is more preferably 0.003% or more. However, since an excessive Ta content may lead to a decrease in 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 as necessary. The Ga content is more preferably 0.0003% or more. However, since an excessive Ga content may lead to 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 the ferritic stainless steel sheet according to the present invention will be described. The ferritic stainless steel sheet according to this embodiment may be manufactured by any method, but can be manufactured, for example, by the following manufacturing method.

The manufacturing method of the ferritic stainless steel sheet of the present invention can adopt a general process for manufacturing ferritic stainless steel. In general, molten steel is produced in a converter or electric furnace, refined in an AOD furnace or VOD furnace, and made into a billet by continuous casting or ingot casting, and then the process of hot rolling - annealing the hot rolled sheet - pickling - cold rolling - finish annealing - pickling is carried out. If necessary, the annealing of the hot rolled sheet may be omitted, or the cold rolling - finish annealing - pickling may be repeated. The conditions of each of these processes may be general conditions, for example, the hot rolling heating temperature of 1000 to 1300 ° C, the hot rolled sheet annealing temperature of 900 to 1200 ° C, the cold rolled sheet annealing temperature of 800 to 1200 ° C, etc. However, the present invention is not characterized by the manufacturing conditions, and the manufacturing conditions are not limited. Therefore, the hot rolling conditions, the hot rolled sheet thickness, the presence or absence of hot rolled sheet annealing, the cold rolling conditions, the hot rolled sheet and cold rolled sheet annealing temperatures, the atmosphere, etc. can be appropriately selected. In addition, the treatment before the finish pickling may be a general treatment, for example, a mechanical treatment such as shot blasting or grinding brush, or a chemical treatment such as molten salt treatment or neutral salt electrolysis. In addition, temper rolling or tension leveling may be applied after cold rolling and annealing. Furthermore, the product plate thickness may be selected according to the required member thickness. In addition, this steel plate may be used as a material to manufacture a welded pipe by a normal manufacturing method for stainless steel pipes for exhaust system components, such as electric resistance welding, TIG welding, or laser welding.

However, in order to satisfy formula (iv) in order to further improve the high-temperature salt damage resistance, the temperature of the steel sheet is maintained at 750°C or more and less than 830°C for 20 seconds or more during the temperature rise process of the final annealing, or the flow rate of the atmospheric gas during the final annealing is 0.08 m/s or more. In order to make the value of the left side of formula (iv) 0.0 or more more preferable, the temperature of the steel sheet is maintained at 750°C or more and less than 830°C for 20 seconds or more during the temperature rise process of the final annealing, and the flow rate of the atmospheric gas during the final annealing is 0.08 m/s or more. This is because the oxides containing Al formed on the surface or surface layer of the base material during the temperature rise process suppress the growth of recrystallized grains in the surface layer of the base material. At temperatures below 750°C, oxidation proceeds slowly, and at temperatures above 830°C, recrystallization proceeds, so it is important to know how much Al oxidizes at these temperatures. Here, the atmospheric gas is a gas that forms the atmosphere in the furnace during annealing, and includes gas flowing in the furnace, combustion gas generated when burner combustion or the like is performed in the furnace, or air that is unintentionally present in the furnace. Note that even the atmospheric gas that becomes a reducing atmosphere at the highest temperature during annealing contains a very small amount of gas species that are oxidation sources of Al, such as oxygen and water vapor, and may oxidize the plate surface during the temperature rise process from 750 ° C. to less than 830 ° C. The flow rate 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 that are oxidation sources on the plate surface. Here, the final annealing is the finishing annealing performed at the end of the manufacturing process of the steel plate or steel pipe. In other words, the final annealing of the steel plate is the final annealing performed in the manufacturing process of the steel plate. The final annealing of the steel pipe is the final annealing performed in the manufacturing process of the steel pipe, or, if there is no such annealing, the final annealing performed in the pipe-making process of the steel plate. In the case of a continuous annealing line, the flow rate of the atmospheric gas is the difference between the sheet passing speed and the gas flow rate in the furnace. If there is no gas flow in the furnace, the flow rate of the atmospheric gas is the sheet passing speed. However, if the temperature of the steel sheet is maintained at 750°C or higher and lower than 830°C for too long during the temperature rise process of the final annealing, this leads to a decrease in productivity, so the upper limit is preferably 100 seconds or less. In addition, if the flow rate of the atmospheric gas is too high, it becomes difficult to control the temperature in the furnace, so the upper limit is preferably 10 m/s or less.

The automobile exhaust system part according to the present invention comprises an automobile exhaust system member using the above-mentioned ferritic stainless steel plate and a forced cooling member for forced cooling of the automobile exhaust system member. For example, as shown in FIG. 1(A), the automobile exhaust system part according to the present invention comprises an automobile exhaust system member 1 using the above-mentioned ferritic stainless steel plate of the present invention and a forced cooling member 3 for forced cooling thereof. In FIG. 1(A), a cooling air flow 11 from an air outlet 5 of a cooling fan 4 forcibly cools the automobile exhaust system member 1 (exhaust manifold 2). The forced cooling member 3 for forcibly cooling 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 is a factor in promoting salt adhesion to the automobile exhaust system member 1. In such an automobile exhaust system part, sufficient high-temperature salt damage resistance is imparted by using the above-mentioned ferritic stainless steel plate of the present invention as the material of the automobile exhaust system member 1. In this way, the above-mentioned ferritic stainless steel plate is used for an automobile exhaust system member in which salt adhesion is promoted by a forced cooling member. It should be noted that the automobile exhaust system part shown in FIG. 1(A) is merely one example of the automobile exhaust system part of the present invention, and it goes without saying that the automobile exhaust system part of the present invention is not limited to the embodiment shown in FIG. 1(A).

The automobile exhaust system part according to the present invention also comprises an automobile exhaust system member using the above-mentioned ferritic stainless steel plate, and an insulating member that covers the periphery of the automobile exhaust system member with an insulating material. For example, as shown in FIG. 1(B), the automobile exhaust system part according to the present invention comprises an automobile exhaust system member 1 using the above-mentioned ferritic stainless steel plate of the present invention, and an insulating member 6 that covers the periphery with an insulating material 7. In the insulating member 6 shown in FIG. 1(B), the outer periphery of the automobile exhaust system member 1 (exhaust manifold 2) is covered with an insulating material 7, and the insulating material 7 is protected by an insulating material cover 8. The insulating material 7 is made of wool-like ceramics or the like, which is easy to retain water. Therefore, covering the automobile exhaust system member 1 with the insulating material 7 also makes it easier to retain salt, promoting the adhesion of salt to the automobile exhaust system member 1. In such an automobile exhaust system part, by using the above-mentioned ferritic stainless steel plate of the present invention as the material for the automobile exhaust system member 1, sufficient high-temperature salt damage resistance is imparted. In this way, the above-mentioned ferritic stainless steel plate is used for an automobile exhaust system member in which salt adhesion is promoted by applying an insulating member that covers the periphery of the automobile exhaust system member with an insulating material. It should be noted that the automobile exhaust system part shown in FIG. 1(B) is merely one example of the automobile exhaust system part of the present invention, and it goes without saying that the automobile exhaust system part of the present invention is not limited to the embodiment shown in FIG. 1(B).

The following examples will make the effects of the present invention clearer. Note that the present invention is not limited to the following examples, and can be modified as appropriate without departing from the spirit of the present invention.

Test materials (Invention Examples A-S, Comparative Examples a-q) having the composition shown in Tables 1 and 2 were melted in a vacuum melting furnace and cast into 150 kg ingots, which were then hot rolled to form 5.0 mm thick hot rolled steel sheets. The hot rolled steel sheets were then pickled and cold rolled to a thickness of 2.0 mm, and finally annealed at 850-1100°C to form a recrystallized structure, after which they were pickled to form product sheets. Note that the time during which the temperature of the steel sheet was maintained at 750°C or higher and lower than 830°C during the temperature rise process of the final annealing, and the flow rate of the atmospheric gas during the final annealing, were performed within the range of the present invention.

The obtained product sheets were subjected to measurement of the average crystal grain size at the center of the sheet thickness, evaluation of workability, and evaluation of resistance to high-temperature salt damage.
To measure the average crystal grain size in the center of the sheet thickness, the structure of a cross section parallel to the rolling direction at the center of the sheet width was observed, and the crystal grain size was measured at five arbitrary points according to the cutting method of JIS G0551:2013 to obtain the average value, and the average crystal grain size was calculated.

To evaluate workability, JIS No. 13B test pieces were taken in a direction parallel to the rolling direction, and the 0.2% yield strength at room temperature was determined in accordance with JIS Z2241:2011. 0.2% yield strength at room temperature of over 400 MPa was rated as "× (poor)", over 390 MPa and up to 400 MPa was rated as "● (fair)", over 380 MPa and up to 390 MPa was rated as "〇 (good)", and 380 MPa or less was rated as "◎ (even better)".

For the evaluation of high-temperature salt damage resistance, a test piece with a width of 20 mm and a length of 50 mm was prepared from the product plate and evaluated in a high-temperature salt damage test. In the high-temperature salt damage test, the test piece was subjected to 20 cycles of heating, cooling, salt water immersion, and drying, and then the corrosion weight loss 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. The salt water immersion conditions were a saturated NaCl aqueous solution as the salt water, an aqueous solution temperature of room temperature, and an immersion time of 30 minutes. That is, a 26 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 removal 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 piece 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 taken as the corrosion weight loss. The corrosion products on the surface of the test piece after the high-temperature salt damage test were removed by immersing the test piece in a boiling 15% by mass ammonium dihydrogen citrate aqueous solution for 20 minutes, rinsing with water, and then brushing the test piece several times. The corrosion weight loss from 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/ ^cm2 was rated as "x (poor)", more than 125 mg/ ^cm2 and 150 mg/ ^cm2 or less was rated as "● (fair)", more than 100 mg/ ^cm2 and 125 mg/ ^cm2 or less was rated as "◯ (good)", and 100 mg/ ^cm2 or less was rated as "◎ (even better)".

Table 3 shows the average crystal grain size at the center of the plate thickness, the values of the left side of formulas (i) to (iii), and the evaluation results of workability and resistance to high-temperature salt damage for invention examples A to S and comparative examples a to q.

As is clear from Tables 1 to 3, the steel sheets having the composition specified in the present invention and satisfying formulas (i) to (iii) have superior workability and resistance to high-temperature salt damage compared to the steel sheets of the comparative examples.

Furthermore, as mentioned above, there are preferred or more preferred ranges for Si, P, Ni, Cu, Mo, and Al to further improve workability, and there are preferred or more preferred ranges for C, Si, Mn, S, Mo, and Al to further improve resistance to high-temperature salt damage. It can be seen that the more elements a steel plate has within these ranges, the better its evaluation of workability or resistance to high-temperature salt damage.

Table 4 also shows steel sheets manufactured under various manufacturing conditions for steel material B, D, and E having compositions within the range of the present invention. The time during which the temperature of the steel sheet during the heating process in the final annealing was 750°C or higher and lower than 830°C, the flow rate of the atmospheric gas, and the annealing temperature are shown in Table 4. The atmospheric gas was air. The results of measuring the average crystal grain size at the center of the base material thickness and on the surface, the value of the left side of formula (iv), and the results of evaluating high-temperature salt damage resistance are shown in Table 4.
In addition, the average crystal grain size of the plate surface was measured by observing the structure of the plate surface, measuring the crystal grain size at five arbitrary points according to the cutting method of JIS G0551: 2013, and calculating the average crystal grain size. In order to reveal the contrast of the structure of the plate surface, polishing, pickling, electrolytic polishing, and etching are appropriately performed, and the reduction in plate thickness at that time is within 5% of the original plate thickness.

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

The ranking of high-temperature salt damage resistance of the steels of the present invention was the same even when the heating temperature for the high-temperature salt damage resistance test was 650°C, 700°C, 800°C, etc., when the salt water was a saturated _CaCl2 aqueous solution, or when the atmosphere was dry air. From this, it is considered that the examples of the present invention show excellent high-temperature salt damage resistance in various high-temperature salt damage environments.

As is clear from these, steels that have the individual component compositions defined in the present invention and that satisfy formulas (i) to (iii) have excellent resistance to high-temperature salt damage. Furthermore, steels whose individual component compositions satisfy various preferred or more preferred ranges and steels that satisfy formula (iv) are found to have even greater improvements in workability or resistance to high-temperature salt damage.

According to the present invention, it is possible to provide a ferritic stainless steel sheet with excellent workability and resistance to high-temperature salt damage for applications such as exhaust manifolds and front pipes that require workability and resistance to high-temperature salt damage. Specific applications include automobile exhaust system parts in which salt adhesion is promoted by forced cooling members, and automobile exhaust system parts in which salt adhesion is promoted by the application of insulating members that cover the periphery of the automobile exhaust system with insulating material. By making these parts possible, it is possible to promote the spread of turbo-equipped vehicles and HCCI (homogeneous charge compression ignition) combustion engine vehicles in which these parts are used, thereby contributing to improving automobile fuel efficiency and reducing the environmental burden.

Reference Signs List 1 Automobile exhaust system member 2 Exhaust manifold 3 Forced cooling member 4 Cooling fan 5 Air outlet 6 Heat insulating member 7 Heat insulating material 8 Heat insulating material cover 10 Exhaust gas 11 Cooling air flow

Claims (8)

In mass percent,
C: 0.020% or less,
N: 0.020% or less,
Si: 0.05% or more and 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: 0.03% or less,
Al: more than 0.70% and not more than 3.00%;
V: 0.01% or more, 0.20% or less,
B: 0.0001% or more and 0.0050% or less, and O: 0.0050% or less,
and the balance consisting of Fe and impurities, and satisfying the following formulas (i) to (iii):
Al + 1.7Si + 0.8Mo + 0.3DA ^-0.5 ≦ 6.2 ... formula (i)
2.6Al+4.3Si+1.5Mo≧6.05 Formula (ii)
Al/O≧200 Formula (iii)
In the formula, the element symbol represents the content (mass%) of the element, and DA represents the average crystal grain size (mm) at the center of the sheet thickness. The ferritic stainless steel sheet according to claim 1, further satisfying the following formula (iv):
900DA-1000DB≧-3.2 ... formula (iv)
In the formula, DA means the average crystal grain size (mm) at the center of the sheet thickness, and DB means the average crystal grain size (mm) at the sheet surface. In mass%, replacing a part of Fe,
W: 0.01% or more, 0.50% or less,
Y: 0.001% or more and 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 and 1.0% or less, and Ga: 0.0001% or more and 0.30% or less,
The ferritic stainless steel sheet according to claim 1 or 2, further comprising one or more of the following: The ferritic stainless steel sheet according to any one of claims 1 to 3, which is used for automobile exhaust system components in which salt adhesion is promoted by forced cooling members. The ferritic stainless steel sheet according to any one of claims 1 to 3 is used for automobile exhaust system components in which salt adhesion is promoted by applying a heat insulating member that covers the periphery of the automobile exhaust system components with a heat insulating material. A method for producing a ferritic stainless steel sheet according to any one of claims 1 to 5, characterized in that the temperature of the steel sheet during the temperature rise process of the final annealing is maintained at 750°C or higher and lower than 830°C for 20 seconds or longer, or the flow rate of the atmospheric gas during the final annealing is 0.08 m/s or higher. An automobile exhaust system part comprising an automobile exhaust system member using the ferritic stainless steel plate according to any one of claims 1 to 3, and a forced cooling member for forcedly cooling the automobile exhaust system member. An automobile exhaust system part comprising an automobile exhaust system member using the ferritic stainless steel plate according to any one of claims 1 to 3, and an insulating member that covers the automobile exhaust system member with an insulating material.

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