Ferritic stainless steel sheets and exhaust parts

Abstract

This ferritic stainless steel sheet has the following chemical composition in mass%: C : not more than 0.02%, Si : not more than 1.0%, Mn : not more than 1.0%, P : 0.01 to 0.10%, S : 0.0001 to 0.005%, N : not more than 0.02%, Cr : 17.0 to 20.0%, Cu : 1.0 to 1.5%, Ti : 0.05 to 0.3%, Nb : 0.005 to 0.2%, Mo : 0.02 to 0.5%, B : 0.0001 to 0.0030%, Al : 0.005 to 0.5%, Ni : 0.01 to 0.2%, V : 0.01 to 0.2%, an optional element or elements, and balance : Fe and impurities, wherein [86P + 33Nb + Mo + 4Al ≥ 5.0] is satisfied. The area ratio of Cu particles having a diameter of not more than 20 nm is not greater than 2.0%.

Description

[Technical Field]

[0001] The present disclosure relates to ferritic stainless steel sheets and exhaust components. [Background technology]

[0002] In recent years, ferritic stainless steels, which have relatively good oxidation resistance and workability, as well as a small thermal expansion coefficient, have been attracting attention as materials for automotive exhaust parts. Exhaust parts are used in environments where they are repeatedly heated to high temperatures and cooled from high temperatures. Therefore, if the thermal expansion coefficient is large, they are more likely to break due to thermal fatigue.

[0003] Thus, a small thermal expansion coefficient is an important characteristic for exhaust parts. Ferritic stainless steel, which has a small thermal expansion coefficient, is suitable for exhaust parts. However, ferritic stainless steel may have lower high-temperature strength than other materials, and there is a need to improve its high-temperature strength. Therefore, as disclosed in Patent Documents 1 to 5, ferritic stainless steels have been developed for use in exhaust parts. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] WO 03 / 4714 [Patent Document 2] Japanese Patent Application Laid-Open No. 2000-297355 [Patent Document 3] Japanese Patent Application Laid-Open No. 2000-303149 [Patent Document 4] Japanese Patent Application Laid-Open No. 2008-189974 [Patent Document 5] Japanese Patent Application Laid-Open No. 2010-248620 Summary of the Invention [Problem to be solved by the invention]

[0005] The ferritic stainless steels disclosed in Patent Documents 1 to 5 have improved high-temperature strength by containing Cu. However, in recent years, the temperature of exhaust gases has been rising due to factors such as engine downsizing. For this reason, even higher levels of high-temperature strength are required.

[0006] On the other hand, the ferritic stainless steels disclosed in Patent Documents 1 to 5 have room for further improvement in terms of high-temperature strength. In particular, since the high-temperature strength is improved by including a large amount of expensive Nb, there is room for reducing alloying. Furthermore, when attempting to improve high-temperature strength, problems such as a decrease in workability arise, and there is a problem that it is difficult to improve all properties of workability, oxidation resistance, and high-temperature strength in a balanced manner.

[0007] In light of the above, an object of the present disclosure is to solve the above problems and provide a ferritic stainless steel sheet having excellent workability, oxidation resistance, and high-temperature strength. [Means for solving the problem]

[0008] The present disclosure has been made to solve the above-mentioned problems, and relates to the following ferritic stainless steel sheet.

[0009] (1) Chemical composition, in mass%, C: 0.02% or less, Si: 1.0% or less, Mn: 1.0% or less, P: 0.01-0.10%, S: 0.0001 to 0.005%, N: 0.02% or less, Cr: 17.0~20.0%, Cu: 1.0-1.5% Ti: 0.05 to 0.3%, Nb: 0.005 to 0.2%, Mo: 0.02 to 0.5% B: 0.0001~0.0030%, Al: 0.005 to 0.5%, Ni: 0.01 to 0.2% V: 0.01 to 0.2%, W: 0-2.0%, Sn: 0 to 0.5% Mg: 0 to 0.01%, Sb: 0 to 0.5% Zr: 0-0.3% Ta: 0 to 0.3%, Hf: 0 to 0.3%, Co: 0-0.3%, Ca: 0 to 0.01%, REM: 0~0.2%, Ga: 0 to 0.3%, The balance is Fe and impurities. The following formula (i) is satisfied: A ferritic stainless steel sheet in which the area ratio of Cu particles with a diameter of 20 nm or less is 2.0% or less. 86P+33Nb+Mo+4Al≧5.0 (i) However, each element symbol in the above formula (i) represents the content (mass%) of each element contained in the steel sheet, and if no element is contained, it is set to zero.

[0010] (2) The chemical composition is in mass%: W: 0.05 to 2.0%, Sn: 0.01 to 0.5% Mg: 0.0002 to 0.01%, Sb: 0.01 to 0.5%, Zr: 0.01 to 0.3%, Ta: 0.01 to 0.3%, Hf: 0.01 to 0.3%, Co: 0.01 to 0.3%, Ca: 0.0001 to 0.01%, REM: 0.001 to 0.2%, and Ga: 0.0002 to 0.3%, The ferritic stainless steel sheet according to (1) above, containing one or more selected from the following:

[0011] (3) A ferritic stainless steel sheet according to (1) above, in which the maximum length of TiN is 15 μm or less.

[0012] (4) A ferritic stainless steel sheet according to (2) above, in which the maximum length of TiN is 15 μm or less.

[0013] (5) A ferritic stainless steel sheet according to any one of (1) to (4) above, in which the amount of precipitated Nb is 0.1% or less by mass %.

[0014] (6) An exhaust part using the ferritic stainless steel sheet according to any one of (1) to (4) above.

[0015] (7) An exhaust part using the ferritic stainless steel sheet described in (5) above. [Effects of the Invention]

[0016] According to the present disclosure, a ferritic stainless steel sheet having excellent workability, oxidation resistance, and high-temperature strength can be obtained. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a graph showing the correlation between the value of the left side of equation (i) and the 0.2% yield strength at 850°C. DETAILED DESCRIPTION OF THE INVENTION

[0018] In order to improve the high-temperature strength of ferritic stainless steel sheets, the present inventors investigated the precipitation behavior of compounds in the steel sheets at 850° C. As a result, they obtained the following findings (a) to (d).

[0019] (a) Ferritic stainless steel containing a certain amount of Cu exhibits improved high-temperature strength. This is due to the precipitation of Cu particles, such as bcc-Cu, 9R, and ε-Cu. The inventors have also found a correlation between Cu particle precipitation and Cr content. For example, in ferritic stainless steel containing 14% Cr, no Cu particle precipitation occurred at 850°C. On the other hand, in ferritic stainless steel containing 17% or more Cr, Cu particles precipitated at 850°C, improving high-temperature strength. This is thought to be due to the increase in Cu activity as the Cr content increases.

[0020] (b) The inventors have clarified that P, Nb, Mo, and Al are elements that increase strength, but also interact with Cu, affecting high-temperature strength. High-temperature strength is affected by precipitation strengthening of Cu particles, solid-solution strengthening by P, Nb, Mo, and Al, and the formation of P compounds and Laves phases. Therefore, it is effective to optimize the balance of these strengthening mechanisms.

[0021] (c) The composition of Cu particles is almost 100% Cu. Therefore, sufficient Cu diffusion is required for Cu particles to nucleate and precipitate. Furthermore, P compounds and Laves phases compete with Cu particles for precipitation. Therefore, if the precipitation of Cu particles is delayed in the operating environment, precipitation strengthening will be insufficient.

[0022] On the other hand, if the precipitation of Cu particles is too rapid, the Cu particles will grow and become coarse, resulting in insufficient precipitation strengthening. Based on the above, it is effective to maximize the precipitation strengthening of Cu particles while controlling the contents of P, Nb, Mo, and Al. As a result, a ferritic stainless steel sheet can be obtained that can be used in exhaust parts even in high-temperature regions where the exhaust gas temperature reaches 850°C.

[0023] (d) It is also effective to include Ti. This is because Ti, when included in combination with Nb, has the effect of improving high-temperature strength. On the other hand, Ti forms TiN. This TiN often has an angular shape. If such coarse, angular TiN is formed, fatigue properties at high temperatures will deteriorate. For this reason, it is preferable to limit the maximum length of the formed TiN to 15 μm or less.

[0024] One embodiment of the present disclosure has been made based on the above findings. Hereinafter, each requirement of the ferritic stainless steel sheet of this embodiment will be described in detail.

[0025] 1.Chemical composition The reasons for limiting the content of each element are as follows: In the following description, "%" in the content means "% by mass."

[0026] C: 0.02% or less C reduces room temperature ductility and workability. It also reduces corrosion resistance, high temperature strength, and oxidation resistance. In addition, it can make fine Cu particles more likely to precipitate. For this reason, the C content is set to 0.02% or less. The C content is preferably set to 0.01% or less, and more preferably set to 0.009% or less. It is desirable to reduce C as much as possible, but excessive reduction of C increases refining costs. For this reason, the C content is preferably set to 0.001% or more.

[0027] Si: 1.0% or less Si is an element used as a deoxidizer. Si also improves high-temperature strength and oxidation resistance. However, excessive Si content can lead to the precipitation of a large amount of fine Cu particles, resulting in a decrease in room-temperature ductility. Therefore, the Si content is set to 1.0% or less. The Si content is preferably set to 0.8% or less, more preferably set to 0.5% or less, and more preferably set to 0.4% or less. To achieve the above effects, the Si content is preferably set to more than 0.1%, and more preferably set to 0.2% or more. Considering pickling properties and toughness, the Si content is preferably set to a range of 0.2 to 0.4%.

[0028] Mn: 1.0% or less Mn is an element used as a deoxidizer. Furthermore, Mn has the effect of improving high-temperature strength in the medium temperature range. However, excessive Mn content causes Mn-based oxides to form on the surface at high temperatures, which increases the likelihood of scale adhesion and abnormal oxidation. As a result, oxidation resistance decreases. Furthermore, a large amount of fine Cu particles may be easily precipitated, resulting in a decrease in room-temperature ductility. Therefore, the Mn content is set to 1.0% or less. The Mn content is preferably set to 0.8% or less, more preferably 0.5% or less, and even more preferably 0.4% or less. On the other hand, to achieve the above effects, the Mn content is preferably set to 0.05% or more. Considering the pickling properties and room-temperature ductility in steel sheet production, the Mn content is preferably set to a range of 0.05 to 0.4%.

[0029] P: 0.01 to 0.10% P has the effect of improving strength through solid solution strengthening. Furthermore, P forms P compounds such as FeP, FeTiP, and FeNbP, thereby precipitation strengthening the steel sheet and improving its strength. Furthermore, reducing the P content requires the use of raw materials with a low P content. Therefore, the P content is set to 0.01% or more. The P content is preferably set to 0.02% or more, and more preferably 0.03% or more.

[0030] However, excessive P content can significantly harden the steel, potentially leading to the precipitation of large amounts of fine Cu particles, resulting in reduced room-temperature ductility and reduced oxidation resistance. Additionally, corrosion resistance, toughness, and pickling properties can be reduced. Therefore, the P content is set to 0.10% or less. The P content is preferably set to 0.08% or less, and more preferably set to 0.06% or less. Considering weldability and manufacturing costs, the P content is preferably set to a range of 0.02 to 0.06%.

[0031] S: 0.0001 to 0.005% S reduces oxidation resistance and room temperature ductility. It may also reduce corrosion resistance. For this reason, the S content is set to 0.005% or less. The S content is preferably set to 0.003% or less. It is preferable to reduce the S content as much as possible, but excessive reduction increases refining costs. For this reason, the S content is set to 0.0001% or more. The S content is preferably set to 0.0005% or more. Considering weldability and manufacturing costs, the S content is preferably set to the range of 0.0005 to 0.003%.

[0032] N: 0.02% or less Like C, N reduces room-temperature ductility. In particular, it may cause a large amount of fine Cu particles to precipitate, reducing workability. It may also reduce high-temperature strength and oxidation resistance, as well as corrosion resistance. For this reason, the N content is set to 0.02% or less. The N content is preferably set to 0.015% or less, and more preferably set to 0.01% or less. It is preferable to reduce the N content as much as possible, but excessive reduction of N increases refining costs. For this reason, the N content is preferably set to 0.003% or more.

[0033] Cr: 17.0~20.0% Cr is an element necessary for ensuring oxidation resistance and corrosion resistance in the steel sheet of this embodiment. This disclosure newly clarifies that an increase in Cr content facilitates precipitation strengthening by Cu at high temperatures. The increased Cr content increases the driving force for precipitation of Cu particles. It is believed that the formation of Cr-enriched regions around precipitated Cu particles retards their growth. As a result, precipitation strengthening is more likely to be achieved at high temperatures. The steel sheet of this embodiment is primarily intended for use at temperatures of 850°C or higher, and in consideration of the precipitation of Cu particles in this temperature range, the Cr content is set to 17.0% or more. The Cr content is preferably set to 17.2% or more.

[0034] However, excessive Cr content reduces oxidation resistance. Furthermore, a large amount of fine Cu particles may be easily precipitated, reducing room-temperature ductility and workability. Furthermore, toughness may also be reduced. Therefore, the Cr content is set to 20.0% or less. The Cr content is preferably set to 19.0% or less. Considering manufacturability and scale exfoliation, the Cr content is preferably set to a range of 17.0 to 18.0%.

[0035] Cu: 1.0 to 1.5% Cu has the effect of improving high-temperature strength through precipitation strengthening by Cu particles. As described above, the present disclosure has discovered that when the Cr content is 17.0% or more, Cu precipitation is promoted at high temperatures. In addition to the above-mentioned Cr content, in order to stably activate the precipitation strengthening of Cu particles at high temperatures of 850°C or higher, the Cu content is set to 1.0% or more. The Cu content is preferably set to 1.1% or more. However, excessive Cu content significantly reduces workability. Therefore, the Cu content is set to 1.5% or less. The Cu content is preferably set to 1.4% or less, and more preferably set to 1.3% or less. Furthermore, in consideration of high-temperature fatigue properties, manufacturability, and weldability, the Cu content is preferably set to a range of 1.1 to 1.4%. Furthermore, in consideration of pickling properties, the Cu content is preferably set to a range of 1.1 to 1.3%.

[0036] Ti: 0.05 to 0.3% Ti combines with C, N, and S to improve corrosion resistance and intergranular corrosion resistance. Ti also improves room-temperature ductility and deep drawability. In particular, the precipitation of FeTiP, a Ti-based compound, has the effect of improving room-temperature workability. For this reason, the Ti content is set to 0.05% or more. The Ti content is preferably set to 0.1% or more.

[0037] However, excessive Ti content increases the amount of dissolved Ti excessively, which not only reduces room-temperature ductility but also forms coarse Ti-based precipitates that can serve as crack initiation sites during hole expansion. This results in reduced press workability. Oxidation resistance may also be reduced. Therefore, the Ti content is set to 0.3% or less. The Ti content is preferably set to 0.25% or less, and more preferably set to 0.2% or less. Considering the occurrence of surface defects and toughness, the Ti content is preferably set to a range of 0.05 to 0.2%.

[0038] Nb: 0.005 to 0.2% Nb has the effect of solid-solution strengthening. Furthermore, Nb also strengthens by refining precipitates. These strengthening mechanisms improve high-temperature strength. Additionally, Nb fixes C and N as carbonitrides, improving the corrosion resistance of the finished steel sheet and contributing to the development of a recrystallization texture that affects the r-value. Therefore, the Nb content is set to 0.005% or more. The Nb content is preferably set to 0.01% or more, and more preferably set to 0.1% or more. As described above, the steel sheet of this embodiment utilizes Nb together with high-temperature strengthening elements such as P, Mo, and Al. However, Nb is also an expensive element. Therefore, excessive Nb content increases alloy costs. Furthermore, it increases the recrystallization temperature, thereby increasing manufacturing costs. Therefore, the Nb content is set to 0.2% or less. The Nb content is preferably set to 0.18% or less. Considering toughness and intergranular corrosion resistance of welds, the Nb content is preferably set to a range of 0.1 to 0.2%.

[0039] Mo: 0.02 to 0.5% Like Nb, Mo, when used in conjunction with high-temperature strengthening elements such as P, Nb, and Al, has the effect of improving high-temperature strength. Therefore, the Mo content is set to 0.02% or more. The Mo content is preferably set to 0.05% or more, more preferably 0.1% or more, and even more preferably 0.15% or more. However, since Mo is an expensive element, and taking into consideration the effects of other elements, the Mo content is set to 0.5% or less. The Mo content is preferably set to 0.4% or less, more preferably 0.3% or less. In consideration of oxidation resistance, workability, and manufacturability, the Mo content is preferably set to the range of 0.1 to 0.3%, and further in consideration of corrosion resistance, the Mo content is preferably set to the range of 0.15 to 0.3%.

[0040] B: 0.0001 to 0.0030% B is an element that improves secondary workability during press working. Furthermore, in the steel sheet of this embodiment, P compounds are utilized to improve room-temperature workability and high-temperature strength. In this case, the inclusion of B suppresses coarsening of P compounds in high-temperature environments. As a result, strength stability is improved when used in high-temperature environments.

[0041] This is because B segregates to grain boundaries during recrystallization in the cold-rolled sheet annealing process, and then, when exposed to high temperatures, the above-mentioned Cu precipitates are less likely to precipitate at grain boundaries. As a result, B has the effect of finely precipitating Cu precipitates within grains. In addition, it exhibits long-term stability of precipitation strengthening, suppresses strength degradation, and improves thermal fatigue life. For this reason, the B content is set to 0.0001% or more. The B content is preferably set to 0.0002% or more. However, excessive B content hardens the steel sheet. In particular, it may be prone to the precipitation of large amounts of fine Cu particles, resulting in reduced room-temperature ductility. It also reduces oxidation resistance. Other adverse effects include reduced intergranular corrosion resistance and weld cracking. For this reason, the B content is set to 0.0030% or less. The B content is preferably set to 0.0015% or less, and more preferably 0.0010% or less. In consideration of corrosion resistance and production costs, the B content is preferably in the range of 0.0002 to 0.0010%.

[0042] Al: 0.005 to 0.5% Al is added as a deoxidizing element and also improves oxidation resistance. In the steel sheet of this embodiment, Al is utilized together with high-temperature strengthening elements such as P, Nb, and Mo. Therefore, the Al content is set to 0.005% or more. The Al content is preferably set to 0.01% or more. However, excessive Al content may lead to the precipitation of a large amount of fine Cu particles, which reduces room-temperature ductility and weldability. Therefore, the Al content is set to 0.5% or less. The Al content is preferably set to 0.2% or less, and more preferably set to 0.1% or less. When considering oxidation resistance, workability, and surface defects, the Al content is preferably set to the range of 0.01 to 0.1%.

[0043] Ni: 0.01 to 0.2% Ni is an element that improves toughness. For this reason, the Ni content is set to 0.01% or more. The Ni content is preferably set to 0.05% or more. However, excessive Ni content not only reduces oxidation resistance but also increases alloy costs. Furthermore, a large amount of fine Cu particles may be easily precipitated, reducing room-temperature ductility. For this reason, the Ni content is set to 0.2% or less. The Ni content is preferably set to 0.15% or less, and more preferably set to 0.1% or less. In consideration of manufacturability and oxidation resistance, the Ni content is preferably set to the range of 0.05 to 0.1%.

[0044] V: 0.01 to 0.2% V has the effect of generating VC and improving high-temperature strength. It also has the effect of improving corrosion resistance. For this reason, the V content is set to 0.01% or more. The V content is preferably set to 0.02% or more, and more preferably set to 0.05% or more. However, excessive V content may lead to the precipitation of a large amount of fine Cu particles, which not only reduces room-temperature ductility but also increases raw material costs. For this reason, the V content is set to 0.2% or less. The V content is preferably set to 0.15% or less, and more preferably set to 0.1% or less. In consideration of production costs and manufacturability, the V content is preferably set to the range of 0.05 to 0.1%.

[0045] In addition to the above elements, one or more elements selected from W, Sn, Mg, Sb, Zr, Ta, Hf, Co, Ca, REM, and Ga may be contained within the ranges shown below. The reasons for limiting each element will be explained below.

[0046] W: 0-2.0% Like Mo, W also has the effect of solid solution strengthening. Furthermore, W also has the effect of precipitation strengthening by generating a Laves phase (Fe2W). In particular, when W is added in combination with Nb and Mo, a Laves phase of Fe2(Nb,Mo,W) is generated. Furthermore, the addition of W inhibits the coarsening of this Laves phase, improving precipitation strengthening. Furthermore, the coexistence of W with the FeP-based precipitates mentioned above tends to make the Laves phase finer. Therefore, W may be added as needed.

[0047] However, excessive W content reduces room-temperature ductility and increases alloy costs. Therefore, the W content is set to 2.0% or less. The W content is preferably set to 1.8% or less, more preferably set to 1.5% or less, and even more preferably set to 1.0% or less. On the other hand, to obtain the above effects, the W content is preferably set to 0.05% or more, more preferably set to 0.1% or more, and even more preferably set to 0.2% or more. In consideration of manufacturability, low-temperature toughness, and oxidation resistance, the W content is preferably set to the range of 0.2 to 1.5%.

[0048] Sn: 0 to 0.5% Sn has the effect of improving corrosion resistance. Sn also has the effect of improving high-temperature strength in the medium temperature range. Therefore, Sn may be contained as needed. However, excessive Sn content significantly reduces manufacturability, so the Sn content is set to 0.5% or less. The Sn content is preferably set to 0.3% or less, and more preferably set to 0.2% or less. On the other hand, in order to obtain the above effects, the Sn content is preferably set to 0.01% or more, and more preferably set to 0.05% or more. When considering oxidation resistance and manufacturability, the Sn content is preferably set to the range of 0.05 to 0.2%.

[0049] Mg: 0 to 0.01% Mg is a deoxidizing element and has the effect of refining the slab structure and improving workability and toughness. In particular, Mg oxides containing Mg serve as precipitation sites for carbonitrides such as Ti(C,N) and Nb(C,N), resulting in the finely dispersed precipitation of these compounds. Therefore, Mg may be added as needed. However, excessive Mg content reduces weldability and corrosion resistance. Therefore, the Mg content is set to 0.01% or less. The Mg content is preferably set to 0.001% or less, and more preferably set to 0.0008% or less. On the other hand, to achieve the above effects, the Mg content is preferably set to 0.0002% or more, and more preferably set to 0.0003% or more. In consideration of refining costs, the Mg content is preferably set to a range of 0.0003 to 0.001%.

[0050] Sb: 0 to 0.5% Sb has the effect of improving corrosion resistance and high-temperature strength. Therefore, it may be added as necessary. However, excessive Sb content tends to cause slab cracking and reduced ductility during steel sheet production. Therefore, the Sb content is set to 0.5% or less. The Sb content is preferably set to 0.2% or less, and more preferably set to 0.15% or less. On the other hand, in order to obtain the above effects, the Sb content is preferably set to 0.01% or more. In addition, taking into consideration refining costs and manufacturability, the Sb content is preferably set to the range of 0.01 to 0.15%.

[0051] Zr: 0 to 0.3% Like Ti and Nb, Zr is an element that forms carbonitrides and has the effect of improving corrosion resistance and deep drawability. Therefore, it may be added as needed. However, excessive Zr content reduces manufacturability. For this reason, the Zr content is set to 0.3% or less. The Zr content is preferably set to 0.2% or less. On the other hand, to obtain the above effects, the Zr content is preferably set to 0.01% or more, and more preferably 0.05% or more. In consideration of cost, surface quality, and oxidation resistance, the Zr content is preferably set to the range of 0.1 to 0.3%.

[0052] Ta: 0 to 0.3% Hf: 0 to 0.3% Ta combines with C and N to improve toughness. Therefore, it may be added as needed. However, excessive Ta content increases alloy costs and significantly reduces manufacturability. Therefore, the Ta content is set to 0.3% or less. The Ta content is preferably set to 0.2% or less, and more preferably set to 0.08% or less. On the other hand, to obtain the above effects, the Ta content is preferably set to 0.01% or more. In consideration of refining costs and manufacturability, the Ta content is preferably set to a range of 0.01 to 0.08%.

[0053] Like Ta, Hf also has the effect of improving toughness by bonding with C and N. Therefore, it may be contained as needed. However, excessive Hf content increases alloying costs and significantly reduces manufacturability. Therefore, the Hf content is set to 0.3% or less. The Hf content is preferably set to 0.2% or less, and more preferably set to 0.08% or less. On the other hand, in order to obtain the above effect, the Hf content is preferably set to 0.01% or more. In addition, taking into consideration refining costs and manufacturability, the Hf content is preferably set to the range of 0.01 to 0.08%.

[0054] Co: 0 to 0.3% Co has the effect of improving high-temperature strength. Therefore, it may be contained as necessary. However, excessive Co content increases alloy costs. Therefore, the Co content is set to 0.3% or less. The Co content is preferably set to 0.2% or less, and more preferably set to 0.1% or less. On the other hand, in order to obtain the above effect, the Co content is preferably set to 0.01% or more. In addition, taking into consideration refining costs and manufacturability, the Co content is preferably set to the range of 0.01 to 0.1%.

[0055] Ca: 0 to 0.01% Ca has a desulfurization effect. Therefore, it may be contained as necessary. However, if Ca is contained in excess, coarse CaS is formed, which reduces toughness and corrosion resistance. Therefore, the Ca content is set to 0.01% or less. The Ca content is preferably set to 0.0050% or less, and more preferably set to 0.0020% or less. On the other hand, in order to obtain the above effects, the Ca content is preferably set to 0.0001% or more, and more preferably set to 0.0003% or more. In consideration of refining costs and manufacturability, the Ca content is preferably set to the range of 0.0003 to 0.0020%.

[0056] REM: 0 to 0.2% REM has the effect of improving toughness and oxidation resistance by refining various precipitates. Therefore, it may be added as needed. However, excessive REM content significantly reduces castability and ductility. Therefore, the REM content is set to 0.2% or less. The REM content is preferably set to 0.1% or less, and more preferably set to 0.05% or less. On the other hand, to obtain the above effects, the REM content is preferably set to 0.001% or more. In consideration of refining costs and manufacturability, the REM content is preferably set to the range of 0.001 to 0.05%.

[0057] REM refers to a total of 17 elements, including Sc, Y, and lanthanides, and the REM content above refers to the total content of these elements. In industry, REM is often added in the form of misch metal.

[0058] Ga: 0 to 0.3% Ga has the effect of improving corrosion resistance. It also has the effect of suppressing hydrogen embrittlement. Therefore, it may be contained as needed. However, excessive Ga content increases alloy costs. For this reason, the Ga content is set to 0.3% or less. The Ga content is preferably set to 0.1% or less. Furthermore, from the viewpoints of manufacturability and cost, as well as ductility and toughness, the Ga content is preferably set to 0.0020% or less. On the other hand, in order to obtain the above effects, the Ga content is preferably set to 0.0002% or more.

[0059] In the chemical composition of the steel sheet of this embodiment, the balance is Fe and impurities. Here, "impurities" refers to components that are mixed in during the industrial production of ferritic stainless steel sheets due to various factors in raw materials such as ores and scraps, or in the manufacturing process, and are acceptable within a range that does not adversely affect the steel sheet of this embodiment. Possible impurities include, for example, Bi, As, and Pb.

[0060] Furthermore, the chemical composition of the steel sheet of this embodiment must satisfy the following formula (i): 86P+33Nb+Mo+4Al≧5.0 (i) However, each element symbol in the above formula (i) represents the content (mass%) of each element contained in the steel sheet, and if no element is contained, it is set to zero.

[0061] Figure 1 shows the correlation between the left-hand side of equation (i) and the 0.2% yield strength at 850°C. For the measurements shown in Figure 1, a cold-rolled sheet with a chemical composition of 17.5%Cr-0.005%C-0.2%Si-0.3%Mn-1.2%Cu-0.15%Ti-0.01%N was used. The value of the left-hand side of equation (i) was controlled by adjusting the contents of P, Nb, Mo, and Al, which are elements constituting the left-hand side of equation (i). The 0.2% yield strength at 850°C was measured by a high-temperature tensile test in accordance with JIS G 0567:2020. The rolling direction of the test specimens used in the test was the tensile direction.

[0062] As shown in Figure 1, when the value of the left side of equation (i) is 5.0 or more, the 0.2% yield strength at 850°C is 20 MPa or more. This indicates that the high-temperature strength is equivalent to that of Type 429 (14%Cr-1%Si-0.4%Nb) and Type 441 (17%Cr-0.2%Ti-0.4%Nb), which contain 0.4% or more Nb.

[0063] The reason for this improvement in high-temperature strength is believed to be as follows: High-temperature strength is affected by the balance between precipitation strengthening of Cu particles that precipitate in high-temperature environments, solid-solution strengthening of the above elements, and the formation of P compounds and Laves phases. Therefore, the inventors have found that high-temperature strength can be improved by optimizing the contents of P, Nb, Mo, and Al.

[0064] When the value of the left side of equation (i) is 5.0 or more, it is possible to reduce the Nb content while maintaining good high-temperature strength. Therefore, the value of the left side of equation (i) is set to 5.0 or more. Here, in order to achieve a 0.2% yield strength of 30 MPa or more at 850°C, the value of the left side of equation (i) is preferably set to 8.0 or more. Note that there is no particular upper limit for the value of the left side of equation (i), but if it exceeds 10.0, the effect of improving the 0.2% yield strength is reduced, so it is preferably set to 10.0.

[0065] 2.Metal structure 2-1.Cu particles In the steel sheet of this embodiment, the size of the formed Cu particles is limited. Cu particles refer to precipitates mainly composed of Cu, such as bcc-Cu, 9R, and ε-Cu. Cu particles precipitate at temperatures of 350 to 800°C and have the effect of improving high-temperature strength. However, if fine and abundant Cu particles are precipitated at room temperature, room-temperature ductility decreases, resulting in reduced workability. Therefore, it is desirable to suppress the precipitation of fine and abundant Cu particles at room temperature and maintain the state of solid-solution Cu in the matrix. It is also preferable to control the precipitation of Cu particles at high temperatures in the usage environment to improve high-temperature strength.

[0066] Therefore, the area ratio of Cu particles having a diameter of 20 nm or less (hereinafter referred to as "fine Cu particle area ratio") is set to 2.0% or less. The fine Cu particle area ratio is preferably set to 1.5% or less, more preferably to 1.0% or less, and even more preferably to 0.7% or less. This is because Cu particles having a diameter of more than 20 nm are relatively coarse and therefore do not adversely affect room temperature ductility and high temperature strength. It is desirable to reduce the fine Cu particle area ratio as much as possible, and 0% is most preferable. Note that the above Cu particles are determined to be Cu particles if they are 1 nm or larger.

[0067] Here, a method for measuring the area ratio of fine Cu particles will be described. After preparing a thin film sample by electropolishing, it is photographed at 100,000 times magnification using a transmission electron microscope (hereinafter also referred to as "TEM"). Using image analysis software (e.g., ImageJ), Cu particles with a diameter of 20 nm or less are detected by converting them into a circle-equivalent diameter, and the area ratio is calculated. This process is performed for 10 fields of view, and the total area of ​​Cu particles with a diameter of 20 nm or less is divided by the total observation area to determine the area ratio. Note that Cu particles are identified by point analysis using TEM-EDS on images presumed to be precipitates, and if Cu is detected in an amount equal to or greater than the steel content, they are determined to be Cu particles. Furthermore, the sample for observation is taken from a position 1 / 4 of the width from the center of the steel plate width and from the center to 1 / 8 of the plate thickness, so that the observation surface is parallel to the rolling direction and a cross section parallel to the plate thickness direction (hereinafter simply referred to as "L cross section").

[0068] 2-2.Maximum length of TiN By adding Ti to the steel sheet, the high-temperature strength can be improved, but on the other hand, coarse TiN may be formed, which may deteriorate the high-temperature fatigue properties. Therefore, in the steel sheet of this embodiment, the maximum length of TiN is preferably 15 μm or less. This is because if the maximum length of TiN exceeds 15 μm, the fatigue properties at high temperatures will deteriorate. For this reason, the maximum length of TiN is 15 μm or less, and preferably 10 μm or less. The lower limit of the maximum length of TiN is not particularly limited, but is often 1.0 μm in ordinary refining and the like.

[0069] Here, the method for measuring the maximum length of the TiN will be explained. A sample is taken so that the L-section serves as the observation surface. As for the L-section, as will be described later, it is preferable that the center is at 1 / 8 of the plate thickness and the L-section is at 1 / 4 to 1 / 2 in the plate width direction. Next, the obtained sample is embedded in a thermosetting resin with the observation surface facing up. The observation surface is mechanically polished to a mirror finish, and then observed and measured using a scanning electron microscope (hereinafter also simply referred to as "SEM").

[0070] In SEM / EDS analysis, the analyzed elements are Ti, N, Fe, Cr, and Nb. When the total of these elements is taken as 100 atomic %, TiN is defined as an approximately polygonal inclusion containing 40 atomic % or more of Ti. The long axis length of the TiN is taken as the maximum length and measured from the image. The long axis length of TiN is defined as the longest distance connecting two points on the periphery of the TiN, and is usually the length of a straight line connecting each corner diagonally. The measurement range is set to 10 measurement fields centered at 1 / 8 of the thickness. The size of each measurement field is 0.25 mm in the rolling direction x 0.20 mm in the thickness direction, and the center position of each field in the thickness direction is aligned with the 1 / 8 position. The total number of TiN particles measured is 50 or more across the 10 fields. The maximum long axis length of all measured TiN particles is determined and this is the maximum length of TiN.

[0071] 3. Amount of precipitated Nb In the steel sheet of this embodiment, it is desirable to improve the toughness in view of its application, as will be described later. In order to improve the toughness, it is preferable to suppress the formation of Laves phases, which are precipitates, within a certain range. If a large amount of Laves phases is formed, brittle fracture is likely to occur from the Laves phases, and as a result, the toughness is likely to decrease.

[0072] The amount of precipitated Laves phase can be confirmed by measuring the amount of precipitated Nb by electrolytic extraction residue method. Specifically, the amount of precipitated Nb is preferably 0.1% or less by mass. From the viewpoint of other properties such as high-temperature strength in addition to toughness, the amount of precipitated Nb is more preferably 0.05% or less by mass.

[0073] As mentioned above, by keeping the amount of precipitated Nb at 0.1% or less, it is possible to obtain a steel plate with good toughness, such as a ductile-brittle transition temperature of -40°C or less, when a V-notch Charpy impact test as described in JIS Z 2242:2018 is conducted on a 2 mm thick steel plate used for automobile exhaust system parts. The ductile-brittle transition temperature is the temperature at which 50% of the fracture surface after a Charpy impact test is brittle.

[0074] The amount of precipitated Nb can be measured using the electrolytic extraction residue method according to the following procedure. Specifically, using 10% acetylacetone-1% tetramethylammonium chloride / methanol, 2 Approximately 0.4 g of sample is electrolyzed at a current value of 1000 kJ / s. The electrolyzed sample solution is then filtered through a 0.2 μm filter, and the residue is decomposed with acid. The amount of Nb (mass%) analyzed as the electrolytic extraction residue using an ICP emission spectrometer is then calculated as the amount of precipitated Nb.

[0075] 4.Applications The steel sheet of this embodiment is preferably used for exhaust parts of automobiles or motorcycles. Examples of exhaust parts include exhaust manifolds, catalytic converter cases, EGR cooler cases, exhaust heat recovery devices, DPFs, GPFs, center pipes, mufflers, plungers, muffler hangers, housings that form the outer frames of turbochargers, and precision parts inside nozzle vane turbochargers (e.g., back plates, oil deflectors, compressor wheels, nozzle mounts, nozzle plates, nozzle vanes, drive rings, and drive levers). These are not limited to exhaust parts for engine-equipped vehicles, but may also be exhaust parts for electric vehicles and fuel cell vehicles.

[0076] For the above applications, the plate thickness is preferably in the range of 0.5 to 3.0 mm.

[0077] 5. Manufacturing method A preferred method for producing the steel sheet of this embodiment will now be described. The steel sheet of this embodiment can be stably produced by the following production method.

[0078] (melting process) Steel having the above chemical composition is melted in an electric furnace or converter, followed by secondary refining. During melting, the ratio of Al2O3, an Al-based inclusion, to MgO, an MgO-based inclusion (Al2O3 / MgO), is preferably 15.0 or less. This is to keep the maximum length of TiN at 15 μm or less. It is more preferable that the Al2O3 / MgO ratio be 13.0 or less. To achieve an Al2O3 / MgO ratio of 15.0 or less, the slag composition is controlled to adjust the activities of MgO and Al2O3 in the slag.

[0079] The molten steel is formed into a slab by a known casting method (such as continuous casting). To avoid excessive inclusions during continuous casting and to reduce the number of nucleation sites for TiN crystallization, a settling time of 1 minute or more, preferably 5 minutes or more, is provided before continuous casting. As a result, it becomes easier to keep the maximum length of TiN at 15 μm or less.

[0080] (Hot rolling process) The resulting slab is then hot rolled to produce a hot-rolled sheet of a predetermined thickness. The heating temperature of the slab is not particularly limited, but is usually preferably in the range of 1180 to 1250°C. Hot rolling is preferably continuous rolling, and is performed using a hot rolling mill consisting of multiple stands. The slab is then coiled. The coiling temperature is not particularly limited, but is preferably 300 to 750°C from the viewpoint of refining the structure. After hot rolling and coiling, the hot-rolled sheet may be annealed as needed. After annealing, pickling may be performed as needed.

[0081] (Cold rolling process) The hot-rolled sheet is cold-rolled to produce a cold-rolled sheet. Cold rolling is usually performed using a tandem rolling mill or a Sendzimir rolling mill. The reduction ratio during cold rolling is not particularly limited. It may be adjusted appropriately depending on the desired sheet thickness.

[0082] (Cold-rolled sheet annealing) The produced cold-rolled sheet is annealed. The cold-rolled sheet is annealed to obtain a recrystallized structure. In the chemical composition of the steel sheet of this embodiment, the recrystallization temperature is in the range of approximately 870 to 1000°C. Therefore, the annealing temperature is set within this temperature range. That is, the annealing temperature is set and maintained in the range of 870 to 1000°C.

[0083] If the annealing temperature is less than 870°C, recovery and recrystallization do not proceed sufficiently, and a recrystallized structure cannot be obtained. Therefore, the annealing temperature is preferably 870°C or higher, and more preferably 900°C or higher. On the other hand, if the annealing temperature is less than 1000°C, the crystal grains become coarse and the energy required for annealing increases, resulting in increased production costs. Therefore, the annealing temperature is preferably 1000°C or lower, and more preferably 950°C or lower.

[0084] The annealing time for the above cold-rolled sheet is not particularly limited, but is preferably in the range of 1 to 120 seconds in consideration of sufficient promotion of recrystallization and production costs. After holding the annealing temperature for the above annealing time, the sheet is cooled. To obtain a recrystallized structure, the sheet must be held in a temperature range of 870 to 1000°C. However, when cooling from this temperature range, a large amount of fine Cu particles may precipitate. This is undesirable from the viewpoint of room-temperature ductility. For this reason, the following two-stage cooling is performed.

[0085] Specifically, it is preferable to cool from the annealing temperature to 850°C at an average cooling rate of 5.0°C / s or less. The temperature range from the annealing temperature to 850°C during cooling is called the first cooling region. Setting the average cooling rate in this first cooling region to 5.0°C / s or less promotes recrystallization of the ferrite phase in the matrix and promotes the development of texture, which is effective in improving workability. If the average cooling rate in the first cooling region exceeds 5.0°C / s, for example, the fracture elongation at room temperature may be less than 30%. During cooling in the first cooling region, Cu particles may precipitate, but they are relatively coarse and have little effect on workability and yield strength. Therefore, it is important to primarily promote recrystallization in the first cooling region. It is more preferable to set the average cooling rate in the first cooling region to 4.0°C / s or less.

[0086] From the viewpoint of toughness, the average cooling rate in the first cooling region is preferably 1.5°C / s or more. If the average cooling rate in the first cooling region is slow, i.e., less than 1.5°C / s, the amount of precipitated Nb exceeds 0.1% (mass %), and a large amount of Laves phase, a coarse intermetallic compound, precipitates during the cooling process. As a result, brittle fracture is likely to occur originating from this Laves phase, making it difficult to improve toughness. In other words, the ductile-brittle transition temperature in a V-notch Charpy impact test exceeds -40°C.

[0087] Thereafter, it is preferable to cool the material to 850 to 350°C at an average cooling rate of more than 5.0°C / s. The temperature range from 850 to 350°C during cooling is called the second cooling region. In the second cooling region, the cooling rate is set to more than 5.0°C / s, thereby suppressing the precipitation of a large amount of fine Cu particles.

[0088] Considering the pickling properties in the subsequent process and the sheet shape, the average cooling rate in the second cooling zone is preferably 8.0°C / s or more. From the viewpoint of reducing yield strength, it is preferably 10°C / s or more. The upper limit of the average cooling rate in the second cooling zone is not particularly limited, but is usually 50°C / s. Furthermore, considering the sheet shape, the upper limit of the average cooling rate is more preferably 25°C / s.

[0089] By controlling the average cooling rates in the first and second cooling regions in this way, it is possible to prevent a large amount of fine Cu particles from precipitating. That is, the area ratio of Cu particles with a diameter of 20 nm or less can be set to 2.0% or less. If it is desired to keep the area ratio of fine Cu particles to 1.0% or less, it is preferable to set the cooling rate in the second cooling region to 7.0°C / s or more.

[0090] The annealing atmosphere for the cold-rolled steel sheet may be selected as needed. After annealing, temper rolling, tension leveling, etc. may be performed. After annealing and cooling, pickling is performed. The pickling conditions may be the same as those of conventional methods. The obtained cold-rolled steel sheet may be manufactured into exhaust parts according to conventional methods.

[0091] Hereinafter, the steel sheet of this embodiment will be described more specifically with reference to examples, but this embodiment is not limited to these examples. [Example]

[0092] Steels having the chemical compositions shown in Tables 1 and 2 were melted and cast into slabs. During melting, the Al2O3 / MgO ratio was set to 15.0 or less. A settling time of 1 minute or more was allowed before casting. The resulting slabs were heated to 1250°C and hot-rolled to a 5 mm thick hot-rolled sheet, which was then coiled at 450°C. The coils were then pickled and cold-rolled to 2 mm to obtain cold-rolled sheets. The cold-rolled sheets were annealed by holding them at 920°C for 120 seconds and then cooling. During cooling after holding at the annealing temperature, the cooling rate was 5.0°C / s in the temperature range from 920 to 850°C and 8.0°C / s in the temperature range from 850 to 350°C. After annealing, the sheets were pickled to obtain product sheets.

[0093] [Table 1]

[0094] [Table 2]

[0095] The above product sheet (cold-rolled sheet) was examined for fine Cu particle area ratio, high-temperature strength, oxidation resistance, and workability according to the following procedures.

[0096] (Fine Cu particle area ratio) After preparing a thin film sample by electrolytic polishing, it was photographed at 100,000 times magnification using a transmission electron microscope (hereinafter also referred to as "TEM"). The photographed image was then converted into a circle-equivalent diameter using image analysis software (e.g., ImageJ) to locate Cu particles with diameters of 20 nm or less and determine the area ratio. Ten fields of view were observed, and the total area of ​​Cu particles with diameters of 20 nm or less was divided by the total observed area to determine the area ratio. To identify Cu particles, images presumed to be precipitates were subjected to point analysis using TEM-EDS. When Cu was detected at levels greater than the steel content, it was determined to be Cu particles. The observation surface was an L-section, and the observation samples were taken from the center to 1 / 4 of the width of the steel plate and from 1 / 4 to 1 / 2 of the plate thickness.

[0097] (High temperature strength) The high-temperature strength was evaluated by the 0.2% yield strength at 850°C. The 0.2% yield strength was measured in a high-temperature tensile test at 850°C in accordance with JIS G 0567:2020. The test specimens used were taken from the center of the steel plate width. The test specimens had a gauge length of 35 mm. In the test, the rolling direction of the test specimen was the tensile direction. When the 0.2% yield strength at 850°C was 20 MPa or more, the high-temperature strength was considered to be good and recorded as "good." When the 0.2% yield strength at 850°C was 30 MPa or more, the high-temperature strength was considered to be good and recorded as "excellent." On the other hand, when the 0.2% yield strength at 850°C was less than 20 MPa, the high-temperature strength was considered to be poor and recorded as "poor."

[0098] (oxidation resistance) The oxidation resistance was evaluated by conducting a continuous oxidation test. Specifically, the continuous oxidation test was conducted in air at 950°C for 200 hours. The continuous oxidation test was conducted in accordance with JIS Z 2281:1993. If no scale spalling or abnormal oxidation occurred in the continuous oxidation test, the oxidation resistance was deemed to be good and was recorded as "good." On the other hand, if scale spalling or abnormal oxidation occurred in the continuous oxidation test, the oxidation resistance was deemed to be poor and was recorded as "poor." The test specimens were taken from near the center of the plate width.

[0099] (processability) The workability was evaluated by room-temperature ductility. Room-temperature ductility was measured by a tensile test conducted at room temperature. The tensile test was conducted in accordance with JIS Z 2241:2011. For the tensile test, JIS No. 13 B test pieces were prepared and used from near the center of the steel plate width so that the rolling direction and the tensile direction were parallel. As a result of the tensile test, specimens with a breaking elongation of 30% or more were considered to have good room-temperature ductility and were recorded as "good." On the other hand, specimens with a breaking elongation of less than 30% were considered to have poor room-temperature ductility and were recorded as "poor." The results are summarized in Table 3 below.

[0100] [Table 3]

[0101] A1 to A22, which satisfy the requirements of this embodiment, exhibited good high-temperature strength, oxidation resistance, and workability, while B1 to B19, which do not satisfy the requirements of this embodiment, exhibited poor results in at least one of high-temperature strength, oxidation resistance, and workability. [Example]

[0102] Steel having the composition of No. A1 in Table 1 was subjected to cold rolling in the same manner as in Example 1, and the annealing temperature and cooling rate of the cold-rolled sheet were changed as shown in Table 4, followed by pickling to produce cold-rolled product sheets. The holding time during annealing was the same as in Example 1. The amount of precipitated Nb and the ductility-to-brittleness transition temperature of the product sheets (cold-rolled sheets) were calculated in the following manner, and the fine Cu particle area ratio, high-temperature strength, oxidation resistance, and workability were investigated in the same manner as in Example 1. The length of TiN in the inventive examples was also measured in the same manner as in Example 3, and it was confirmed that the maximum length was 15 μm or less in all cases.

[0103] (Precipitated Nb amount) The amount of precipitated Nb was measured by the following procedure. Specifically, using 10% acetylacetone-1% tetramethylammonium chloride / methanol, 2 Approximately 0.4 g of a sample cut out from each steel sheet was electrolyzed at a current value of 1000 kJ / s. The electrolyzed sample solution was then filtered through a 0.2 μm filter, and the residue was subjected to acid decomposition. The amount of Nb (mass%) analyzed as the electrolytic extraction residue using an ICP emission spectrometer was calculated as the amount of precipitated Nb. (ductile-brittle transition temperature) Charpy test specimens were prepared from the steel plates and subjected to a V-notch Charpy impact test as described in JIS Z 2242: 2018 to measure the ductile-brittle transition temperature. The Charpy test specimens were prepared so that the rolling direction of the steel plate was the longitudinal direction of the specimen and the direction perpendicular to the rolling direction was the width direction. They measured 55 mm x 10 mm x 2 mm and had a V-notch introduced perpendicular to the rolling direction.

[0104] [Table 4]

[0105] When the area ratio of fine Cu particles satisfies the preferred range of this embodiment, the high-temperature strength is improved. Note that C7 and C8 did not satisfy the preferred manufacturing conditions of this embodiment and did not satisfy the stipulation of the area ratio of fine Cu particles, and therefore, the workability was reduced. Although C9 is an example of the present invention, the average cooling rate from the annealing temperature to 850°C was somewhat slow, so the ductile-brittle transition temperature was high and the toughness was reduced. [Example]

[0106] A steel having the composition No. A1 in Table 1 was produced. During production, the Al2O3 / MgO ratio was adjusted, and then hot rolling, coiling, cold rolling, cold-rolled sheet annealing, and pickling were performed under the same conditions as in Example 1. For this steel sheet, the maximum length of TiN was measured and high-temperature fatigue properties were investigated using the procedures described below. In addition, the fine Cu particle area fraction, high-temperature strength, oxidation resistance, and workability were calculated using the same procedures as in Example 1. In Example 3, the fine Cu particle area fraction was 2.0% or less in all cases. Furthermore, the amount of precipitated Nb in the inventive examples was measured using the same procedures as in Example 2, and it was confirmed that all values ​​were 0.1% or less by mass.

[0107] (Maximum length of TiN) Samples were taken so that the L-shaped cross section served as the observation surface. The L-shaped cross section was centered at 1 / 8 of the plate thickness and located between the center and 1 / 4 of the plate width in the plate width direction. The obtained sample was then embedded in a thermosetting resin with the observation surface facing up. The observation surface was mechanically polished to a mirror finish, and then observed and measured using a scanning electron microscope (hereinafter also simply referred to as "SEM").

[0108] In SEM / EDS analysis, the analyzed elements were Ti, N, Fe, Cr, and Nb. When the total of these elements was taken as 100 atomic %, TiN was defined as an approximately polygonal inclusion containing 40 atomic % or more of Ti. The long axis length of the TiN was taken as the maximum length and measured from the image. The long axis length of TiN was defined as the longest distance connecting two points on the periphery of the TiN, and is usually the length of a straight line connecting each corner diagonally. The measurement range was 10 measurement fields centered at 1 / 8 of the thickness. Each measurement field measured an area of ​​0.25 mm in the rolling direction x 0.20 mm in the thickness direction, with the center position of each field aligned with the 1 / 8 of the thickness. A total of 50 or more TiN particles were measured across the 10 fields. The maximum long axis length of all measured TiN particles was determined and this was taken as the maximum length of TiN.

[0109] (High temperature fatigue properties) High-temperature fatigue properties were evaluated by conducting a plane bending fatigue test at 800°C. JIS No. 1 test pieces were taken from the center of the width of the obtained product sheet so that the rolling direction was parallel to the longitudinal direction. In accordance with JIS Z 2275:1978, SN curves were taken from the test using seven test pieces to determine the fatigue limit. The fatigue limit was 1×10 7 The fatigue limit was defined as the average value of the strength that did not break and the minimum strength that did break in the cycles, and a fatigue limit of 45 MPa or more was considered good, while a fatigue limit of less than 45 MPa was considered poor. The test temperature was 800°C. The seven test specimens were taken at regular intervals to avoid any influence caused by the taking of each test specimen.

[0110] [Table 5]

[0111] When the maximum length of TiN was 15 μm or less, the high-temperature fatigue properties were improved. However, in the example of D6, where the maximum length of TiN exceeded 15 μm, the high-temperature fatigue properties were reduced.

[0112] (Addendum) (1) Chemical composition, in mass%, C: 0.02% or less, Si: 1.0% or less, Mn: 1.0% or less, P: 0.01-0.10%, S: 0.0001 to 0.005%, N: 0.02% or less, Cr: 17.0~20.0%, Cu: 1.0-1.5% Ti: 0.05 to 0.3%, Nb: 0.005 to 0.2%, Mo: 0.02 to 0.5% B: 0.0001~0.0030%, Al: 0.005 to 0.5%, Ni: 0.01 to 0.2% V: 0.01 to 0.2%, W: 0-2.0%, Sn: 0 to 0.5% Mg: 0 to 0.01%, Sb: 0 to 0.5% Zr: 0-0.3% Ta: 0 to 0.3%, Hf: 0 to 0.3%, Co: 0-0.3%, Ca: 0 to 0.01%, REM: 0~0.2%, Ga: 0 to 0.3%, The balance is Fe and impurities. The following formula (i) is satisfied: A ferritic stainless steel sheet in which the area ratio of Cu particles with a diameter of 20 nm or less is 2.0% or less. 86P+33Nb+Mo+4Al≧5.0 (i) However, each element symbol in the above formula (i) represents the content (mass%) of each element contained in the steel sheet, and if no element is contained, it is set to zero.

[0113] (2) The chemical composition is in mass%: W: 0.05 to 2.0%, Sn: 0.01 to 0.5% Mg: 0.0002 to 0.01%, Sb: 0.01 to 0.5%, Zr: 0.01 to 0.3%, Ta: 0.01 to 0.3%, Hf: 0.01 to 0.3%, Co: 0.01 to 0.3%, Ca: 0.0001 to 0.01%, REM: 0.001 to 0.2%, and Ga: 0.0002 to 0.3%, The ferritic stainless steel sheet according to (1) above, containing one or more selected from the following:

[0114] (3) A ferritic stainless steel sheet according to (1) or (2) above, in which the maximum length of TiN is 15 μm or less.

[0115] (4) A ferritic stainless steel sheet according to any one of (1) to (3) above, in which the amount of precipitated Nb is 0.1% or less by mass %.

[0116] (5) An exhaust part using the ferritic stainless steel sheet according to any one of (1) to (4) above.

Claims

1. The chemical composition, in mass%, is C: 0.02% or less, Si: 1.0% or less, Mn: 1.0% or less, P: 0.01-0.10%, S: 0.0001-0.005%, N: 0.02% or less, Cr: 17.0-20.0%, Cu: 1.0 to 1.5%, Ti: 0.05-0.3%, Nb: 0.005-0.2%, Mo: 0.02-0.5%, B: 0.0001 to 0.0030%, Al: 0.005-0.5%, Ni: 0.01-0.2%, V: 0.01-0.2%, W: 0 to 2.0%, Sn: 0 to 0.5%, Mg: 0 to 0.01%, Sb: 0 to 0.5%, Zr: 0 to 0.3%, Ta: 0-0.3%, Hf: 0-0.3%, Co: 0-0.3%, Ca: 0-0.01%, REM: 0-0.2%, Ga: 0-0.3%, The balance is Fe and impurities. The following formula (i) is satisfied: the area ratio of Cu particles having a diameter of 20 nm or less is 2.0% or less; A ferritic stainless steel plate having a maximum length of TiN of 15 μm or less. 86P+33Nb+Mo+4Al≧5.0...(i) However, each element symbol in the above formula (i) represents the content (mass %) of each element contained in the steel sheet, and if the element is not contained, it is set to zero.

2. The chemical composition is, in mass %, W: 0.05-2.0%, Sn: 0.01-0.5%, Mg: 0.0002-0.01%, Sb: 0.01 to 0.5%, Zr: 0.01 to 0.3%, Ta: 0.01-0.3%, Hf: 0.01-0.3%, Co: 0.01-0.3%, Ca: 0.0001-0.01%, REM: 0.001 to 0.2%, and Ga: 0.0002-0.3%, The ferritic stainless steel sheet according to claim 1, comprising one or more selected from the following:

3. The ferritic stainless steel sheet according to claim 1 or 2, wherein the amount of precipitated Nb is 0.1% or less by mass %.

4. An exhaust part made of the ferritic stainless steel sheet according to claim 1 or 2.

5. An exhaust part made of the ferritic stainless steel sheet according to claim 3.

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