JP2016183415A - Ferritic stainless steel sheet excellent in carburization and oxidation resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a ferritic stainless steel sheet and a method of manufacturing the same, the ferritic stainless steel sheet being excellent in carburization and oxidation resistance, used for an automotive exhaust system or a fuel reformer that become a carburizing atmosphere.SOLUTION: There is provided a ferritic stainless steel sheet containing, by mass%, Si: 0.05-3.0%, Mn: 0.05-2.0%, Cr: 12.0-25.0%, Cu: 0.01-2.0%, Ni: 0.01-2.0%, Mo: 0.01-2.0%, Nb: 0.001-1.0%, Ti: 0.001-0.40%, Al: 0.002-0.25%, V: 0.01-0.20%, and B: 0.0002-0.0050%, satisfying formulas (i) and (ii). The formulas are: (i) . . . Cr+12Si-4Mn-14.5≥0 and (ii) . . . aCr+bSi+1.6Mn+27Ti+33Al+13.5≥0 (a, b are constants fixed by contents of Cr and Si).SELECTED DRAWING: Figure 1

Description

  The present invention relates to a heat-resistant stainless steel that is most suitable for use in an automobile exhaust system member that particularly requires high-temperature strength and oxidation resistance, and in particular, an automobile that has a carburizing atmosphere with excellent carburization resistance and oxidation resistance. The present invention relates to a ferritic stainless steel sheet for exhaust system and a manufacturing method thereof. In addition, it is used for high-temperature members of fuel cells such as reformers and heat exchangers used when reforming hydrocarbon fuels such as city gas, methane, natural gas, propane, kerosene, and gasoline into hydrogen. More particularly, the present invention relates to a ferritic stainless steel sheet for a fuel reformer, which is an atmosphere having carburizing properties excellent in carburizing resistance and oxidation resistance, and a manufacturing method thereof.

  Exhaust system members such as automobile exhaust manifolds, front pipes and center pipes pass high-temperature exhaust gas exhausted from the engine, so the materials that make up the exhaust members have various characteristics such as oxidation resistance, high-temperature strength, thermal fatigue characteristics, etc. Is required.

  Conventionally, cast iron is generally used for automobile exhaust members, but stainless steel exhaust manifolds are likely to be used from the viewpoints of strengthening exhaust gas regulations, improving engine performance, and reducing vehicle weight. Became. The exhaust gas temperature differs depending on the vehicle type, and in recent years, it is about 750 to 850 ° C., but it may reach a higher temperature. There is a demand for a material having high high-temperature strength and oxidation resistance in an environment that is used for a long time in such a temperature range.

  Among stainless steels, austenitic stainless steel has excellent heat resistance and workability, but due to its large thermal expansion coefficient, thermal fatigue failure occurs when applied to a member that repeatedly receives heating and cooling, such as an exhaust manifold. Is likely to occur.

  Ferritic stainless steel has a lower thermal expansion coefficient than austenitic stainless steel, and therefore has excellent thermal fatigue characteristics. Moreover, compared with austenitic stainless steel, since it contains almost no expensive Ni, the material cost is low and it is used for general purposes. However, since ferritic stainless steel has lower high-temperature strength than austenitic stainless steel, a technique for improving high-temperature strength has been developed.

  For example, there are SUS430J1L (Nb-added steel), Nb-Si-added steel, and SUS444 (Nb-Mo-added steel). Based on the addition of Nb, the high-temperature strength is improved by adding Si and Mo. However, according to various engine specifications, there is a demand for further reduction in cost, improvement in high temperature strength, and addition of added value to these ferritic stainless steels.

  From the viewpoint of low cost, low Cr, low Nb or no Nb addition, low Mo or no Mo addition, high C, high N, etc. can be considered.

  Cr, Nb, and Mo are important elements from the viewpoint of high temperature strength, but are also expensive elements. In addition, 90% of production of Nb is Brazil, and there is a problem that resource risk is high because supply unevenness is high. The cost can be reduced by reducing Cr, Nb and Mo to an appropriate amount according to the engine specifications, or by substituting with an inexpensive element.

  C and N are elements that are required to be reduced from the viewpoints of moldability, corrosion resistance, and high-temperature strength, but the refining cost increases at an accelerated rate as the reduction occurs. Moreover, special equipment is required to reduce C and N above a certain level, and productivity may be significantly impaired. Therefore, it is possible to avoid an increase in cost by limiting C and N according to the production system.

  Various additive elements other than Nb, Si, and Mo have been studied from the viewpoint of improving the high-temperature strength. Patent Documents 1 to 4 also disclose a Cu addition technique using Cu solid solution strengthening and precipitation strengthening by Cu precipitates (ε-Cu phase). By adding Cu, it is possible to further improve the high temperature strength or to substitute an expensive high temperature strength improving element.

  From the viewpoint of adding added value, for example, Ni addition, Ti addition, etc. are conceivable as an improvement in corrosion resistance. Ni may also be added to improve high temperature strength and toughness. Ti may be added to improve intergranular corrosion resistance and deep drawability.

  However, low Cr, low Nb or no Nb addition, low Mo or no Mo addition, high C, high N, Cu for the purpose of reducing the cost, improving the high temperature strength, and adding added value. Addition, Ni addition, and excessive addition of Ti all have a problem of reducing oxidation resistance. The oxidation resistance has two points, that is, no abnormal oxidation occurs and the oxidation increase is small, and the scale peeling resistance is good.

When stainless steel is heated, a highly protective scale mainly composed of Cr ₂ O ₃ is formed on the surface. If Cr supply from the base material is insufficient for Cr consumption necessary for maintaining a highly protective scale, Fe is oxidized. At this time, the generated oxide containing a large amount of Fe has a very high oxidation rate. As a result, the oxidation proceeds rapidly, and the base material is eroded remarkably. This is called abnormal oxidation.

  Even if a good scale that does not cause abnormal oxidation can be formed, there is a problem if the scale peels off in the cooling process of an automobile exhaust system, for example. When the scale peels off, oxygen in the atmosphere touches the steel substrate during heating, and oxidation proceeds rapidly. If scale repair is not healthy, it can cause abnormal oxidation. Moreover, if the peeled scale is scattered, there is a possibility of causing problems such as erosion of downstream equipment and blockage of the flow path due to accumulation.

  Scale peeling in an exhaust system member of an automobile is often caused by a large difference in thermal expansion between a steel substrate and an oxide or by repeated heating and cooling, and it is considered that thermal stress is a main factor. Since ferritic stainless steel has a smaller difference in thermal expansion from the scale than austenitic stainless steel, it is superior in scale peel resistance.

  Various techniques have been studied in order to improve the oxidation resistance, and a technique considered to be aimed at supplementing the reduction in oxidation resistance due to the addition of Cu is also disclosed.

Patent Document 5 discloses a technique for improving abnormal oxidation suppression and scale adhesion by adjusting the amount of Si, the amount of Mn, and the Mn / Si ratio. An increase in Si is considered to suppress abnormal oxidation because an oxide mainly composed of Cr ₂ O ₃ is formed on the surface layer. Increasing the amount of Mn produces a spinel-based oxide containing Mn having an intermediate coefficient of thermal expansion between the oxide mainly composed of Cr ₂ O ₃ and the steel substrate, and relaxes the difference in thermal expansion from the steel substrate. , Is believed to improve scale adhesion. Furthermore, even if the increase in Si decreases the scale peel resistance, and the increase in oxidation due to the increase in Mn increases, abnormal oxidation can be suppressed and the scale peel resistance can be improved by adjusting the Mn / Si ratio. However, the Cu concentration is less than 0.02 to 0.30% by mass, and it does not utilize Cu addition technology such as solid solution strengthening of Cu, precipitation strengthening by Cu precipitates (ε-Cu phase), It is hard to say that the reduction in oxidation resistance due to the addition of Cu is taken into consideration.

  In Patent Document 6, it is estimated that abnormal oxidation increases due to the addition of Cu. Cu is an austenite forming element and promotes the phase transformation of the surface layer portion from the ferrite phase to the austenite phase due to the decrease in Cr of the surface layer portion as the oxidation proceeds. Since the austenite phase has a slower Cr diffusion than the ferrite phase, the austenite phase becomes a surface layer portion, which inhibits the supply of Cr from the base material to the scale. Thereby, it is estimated that the surface layer portion is Cr-deficient and the oxidation resistance is deteriorated. For this reason, a technique for suppressing abnormal oxidation by mutually adjusting the ferrite forming element and the austenite forming element and suppressing the austenite phase is disclosed. However, there is no disclosure of the relationship between the scale characteristics and abnormal oxidation, and there is no study on the scale peeling resistance.

  Patent Document 7 discloses a technique for improving oxidation resistance by setting the V concentration to 0.15 to 0.60% by mass concentration in the range of Cu concentration to 0.8 to 1.6% by mass concentration. Is disclosed. However, it is also a technique for improving the high temperature strength by utilizing the precipitation of VN, and the N concentration is 0.15 to 0.40% by mass. Therefore, Ti, Zr, and Ta that easily form nitrides cannot be added.

  In Patent Document 8, in the range where the Cu concentration is 1.0% or less in terms of mass concentration, the Si concentration is 0.40% or more in terms of mass concentration, the Al concentration is 0.20% or more in terms of mass concentration, and Si ≧ Al By simultaneously satisfying the above, a dense Si oxide layer is continuously generated on the surface of the steel sheet, and oxygen entry from the outside is suppressed, and some oxygen that has entered the interior through the Si oxide layer is also present. A technique for improving oxidation resistance by forming an oxide in combination with Al and suppressing oxidation of Fe and Cr is disclosed. However, Si and Al need to be made extremely higher than ordinary ferritic stainless steel, which may impair workability.

  In Patent Documents 9 and 10, the Cu concentration is within the range of 0.8 to 1.5% by mass concentration, the component balance of Si and Mn is adjusted, and the Cu concentration on the steel surface layer is subjected to final annealing and pickling. The technique which improves oxidation resistance and scale peeling resistance by adjusting by this is disclosed. However, there is no disclosure of the relationship between scale characteristics and abnormal oxidation.

JP 2008-189974 A JP 2009-120893 A JP 2009-120894 A JP 2011-190468 A Japanese Patent No. 2896077 JP 2009-235555 A Japanese Patent No. 5239643 JP 2012-102376 A JP 2013-189709 A JP 2013-227659 A

  Moreover, all of the improvement of the oxidation resistance by the technique of patent documents 5-10 are evaluating the oxidation resistance in air | atmosphere, and are not evaluating supposing the exhaust gas atmosphere of a motor vehicle.

  The present inventor has discovered that oxidation resistance is lowered in an exhaust gas atmosphere having carburizing properties while examining oxidation resistance in various atmospheric compositions assuming exhaust gas of automobiles. Furthermore, for the purpose of cost reduction, high temperature strength improvement and added value addition, low Cr, low Nb or no Nb addition, low Mo or no Mo addition, high C, high N, Cu addition, Ni It has also been found that when either the addition or the excessive addition of Ti is performed, this reduction in oxidation resistance becomes significant. That is, the present inventors have found that in an atmosphere having carburizing properties, it is necessary to have carburizing resistance in addition to oxidation resistance.

  The atmosphere having a carburizing property is an atmosphere in which the C activity in the atmosphere is greater than the C activity in the steel. For example, the atmosphere contains carbon monoxide or a hydrocarbon-based gas, and all the oxygen contained in the atmosphere is present. The atmosphere has a composition in which carbon monoxide or hydrocarbon gas remains even if it reacts with carbon monoxide or hydrocarbon gas. The carburization reaction is a reaction in which carbon monoxide or hydrocarbon gas C is dissociated and enters the steel. In addition, it is considered that even an atmosphere containing a large amount of carbon dioxide in the atmosphere and assumed to use carbon dioxide as an oxygen source used for oxidation is carburizable. When carbon dioxide is used for oxidation, carbon dioxide C dissociates and enters the steel, or carbon dioxide oxygen is used in oxidation to produce carbon monoxide, and carbon monoxide C dissociates. It is thought that it penetrates into steel.

  In conventional automobile exhaust system members, carburization resistance has not been required. However, higher temperatures and thinner walls will be required for automobile exhaust systems in the future. By increasing the temperature, the rate of chemical reaction such as carburization and oxidation increases, and it is considered that the carburizability and oxidization become high. In addition, by reducing the thickness, the C concentration in the steel is likely to increase, the total amount of Cr necessary for maintaining oxidation resistance is reduced, and the material temperature is easily increased, so that high carburization and oxidation are achieved. It is considered to be sex.

  In addition, for the purpose of cost reduction, high temperature strength improvement and added value addition, low Cr, low Nb or no Nb addition, low Mo or no Mo addition, high C, high N, Cu addition, Ni If the oxidation resistance is reduced by either the addition or the excessive addition of Ti, further reduction in oxidation resistance due to carburization is considered fatal.

  In other words, for the first time by responding to the needs for higher temperatures and thinner walls required for automobile exhaust systems in the future, as well as responding to component changes of materials for the purpose of lowering costs, improving high temperature strength, and adding value. It has been found that there are uses that result in a highly carburizing atmosphere.

  The details of the mechanism by which the oxidation resistance of ferritic stainless steel decreases in a carburizing atmosphere is not necessarily clear, but can be considered as follows.

It is considered that when ferritic stainless steel is heated in a carburizing atmosphere, if Cr ₂ O ₃ is soundly formed as a scale on the surface of the base material, it is protected from carburizing. However, carburization may occur when carbon monoxide or hydrocarbon gas passes through the scale and directly contacts the base material through cracks, voids, vacancies and the like in the scale. When carburization occurs, if Cr carbide is formed, Cr necessary for maintaining Cr ₂ O ₃ is consumed, and thus abnormal oxidation is likely to occur. In addition, even when Cr carbide is not formed, when the austenite forming element C is dissolved in the surface layer of the base material and the base material surface portion is transformed from the ferrite phase to the austenite phase in which Cr diffusion is slow, the scale is changed from the base material. It is thought that the supply of Cr to the metal is hindered and abnormal oxidation is likely to occur.

Cr is necessary for maintaining Cr ₂ O ₃ . Furthermore, since Cr, Nb, and Mo are ferrite forming elements, an austenite phase is easily formed by reduction. Since C, N, Cu, and Ni are austenite forming elements, an austenite phase is easily formed by addition and increase. Therefore, low Cr, low Nb or no Nb addition, low Mo or no Mo addition, high C, high N, Cu addition, Ni addition, austenite formation by carburizing in a carburizing atmosphere It is considered that the decrease in oxidation resistance becomes remarkable. Moreover, excessive addition of Ti promotes the oxidation of Cr and promotes the reduction of Cr in the surface layer of the base material, or increases the voids and vacancies in the scale, so that the carbon monoxide or hydrocarbon gas is increased. By facilitating the permeation of the scale, it is considered that the reduction in oxidation resistance in an atmosphere having carburizing properties becomes remarkable.

  The atmosphere having carburizing properties is also an atmosphere having a low oxygen partial pressure. In an atmosphere having a low oxygen partial pressure, an element that forms an oxide having a low oxygen dissociation pressure is easily oxidized. Therefore, the scale formed in the atmosphere differs from the atmosphere having carburizing properties. Therefore, it is considered that the additive element effective for improving the oxidation resistance in the atmosphere having carburizing property and the influence of the additive element are different in the atmosphere. However, at present, there is no study on the scale formed in a carburizing atmosphere and the influence of the scale structure on the oxidation resistance.

  Based on the above consideration, the conventional knowledge for improving the oxidation resistance of automobile exhaust system members is the knowledge obtained from the evaluation of oxidation resistance in the atmosphere, and the automobile exhaust system members are exposed to the carburizing exhaust gas atmosphere. It was not a technology that considered the possibility of

  In the future, when the exhaust system of automobiles becomes hot or thin, or for the purpose of reducing costs, improving high temperature strength, and adding value, low Cr, low Nb or no Nb addition, low Mo or Mo When any of additive-free, high-C, high-N, Cu-added, Ni-added, and excessive Ti-added is performed, there is a new problem that the oxidation resistance in an atmosphere having carburizing properties is significantly reduced. I understood that.

  In other words, in addition to oxidation resistance, it is necessary to develop steel for new applications that consider carburization resistance, which had not been considered before, and also developed technology for improving oxidation resistance in an atmosphere with carburization resistance. There was a need to do.

  The present invention provides a ferritic stainless steel sheet for an automobile exhaust system that provides a carburizing atmosphere excellent in carburizing resistance and oxidation resistance, and a method for producing the same.

  In addition, reformers and heat exchangers used when reforming hydrocarbon fuels such as city gas, methane, natural gas, propane, kerosene, and gasoline into hydrogen, which also has a high temperature environment with carburizing properties. Also provided is a ferritic stainless steel sheet for a fuel reformer that is used as a fuel cell high-temperature member such as a carburizing atmosphere excellent in carburizing resistance and oxidation resistance, and a method for producing the same.

In the process of evaluating oxidation resistance in a carburizing atmosphere, the inventors have confirmed that the ratio of the Cr ₂ O ₃ layer thickness in the scale and the Si concentration between the scale and the base material are resistant to oxidation. It was found to affect carburization and oxidation resistance. Furthermore, as a result of intensive studies on the influence of various components, a ferritic stainless steel sheet excellent in carburization resistance and oxidation resistance and a method for producing the same were invented.

In order to solve the above problems, the inventors have studied in detail the relationship between the oxidation resistance of ferritic stainless steel exposed to a carburizing atmosphere at 850 ° C. and the characteristics of the scale formed by the steel. It was. Furthermore, the inventors conducted heat treatment of the ferritic stainless steel sheet with scales at 950 ° C. in the atmosphere for the adhesion of the scale formed in the carburizing atmosphere and the protection after considering long-term use. We conducted a detailed study by simulating the increase in load against scale peeling and the acceleration of scale growth. As a result, the ratio x of the Cr ₂ O ₃ layer occupying the thickness of the scale on the surface of the stainless steel is 20% or more in an atmosphere that satisfies the following formulas (i) and (ii) and further has carburizability. It is characterized by forming a scale in which the Si concentration y of the Si concentrated layer between the scale and the base material is 0.4% or more and satisfies the following formula (iii): carburization resistance and acid resistance It has been found that the chemical property is improved, the adhesion of the scale is improved, and the protective property is considered in consideration of long-term use.
Cr + 12Si-4Mn-14.5 ≧ 0 Formula (i)
aCr + bSi + 1.6Mn + 27Ti + 33Al + 13.5 ≧ 0 Formula (ii)
When Cr + Si ≧ 19.5, a = −0.8, b = −0.2
When Cr + Si <19.5 and Si ≦ 0.20, a = −1, b = 29
In the case of Cr + Si <19.5 and Si> 0.20, a = −1, b = −3.7
x + 10y / Si-50 ≧ 0 Formula (iii)
However, the element symbol in a formula means content (mass%) of the said element.

  The mechanism for improving the carburization resistance and oxidation resistance in the above-mentioned atmosphere having carburization is not necessarily clear, but is considered as follows.

Cr ₂ O ₃ is hard to permeate carbon monoxide or hydrocarbon gas that causes carburization, but is Fe oxide, Mn oxide, and spinel oxide containing a large amount of Fe and Mn (Fe, Mn, It is considered that Cr) ₃ O ₄ is easy to permeate carbon monoxide or hydrocarbon gas. In addition, when the scale has a multi-layer structure, when the ratio of the Cr ₂ O ₃ layer thickness to the thickness of the scale is low, the Cr ₂ O ₃ layer should have a few voids, cracks, or mixed portions with other phases. Therefore, it is considered that the permeation path of carbon monoxide or hydrocarbon gas reaches the base material. That is, it is considered that as the ratio of the Cr ₂ O ₃ layer thickness to the thickness of the scale is larger, the permeation of carbon monoxide or hydrocarbon gas is suppressed, carburization resistance is improved, and oxidation resistance is further improved.

Further, it is considered that Si concentrated between the scale and the base material has an effect of suppressing or delaying carburization even if carbon monoxide or hydrocarbon gas permeates Cr ₂ O ₃ for some reason. Even retarding effect, carburization resistance of Cr ₂ O ₃ CO or transmission path hydrocarbons in again if repaired Cr ₂ O ₃ is effective in the meantime. If the concentration of Si is an oxide, it acts as a protective film and has an effect of inhibiting carburization. In addition, concentration in the base material surface layer portion has an effect of delaying carburization by increasing the C activity in the base material surface layer portion due to the interaction of Si and reducing the difference in C activity from the atmosphere.

Cr also has the effect of increasing the proportion of the Cr ₂ O ₃ layer thickness in the thickness of the scale, reducing the oxidation rate, stabilizing the ferrite phase, and improving the carburization resistance and oxidation resistance. Si has an effect of concentrating Si between the scale and the base material, reducing the oxidation rate, stabilizing the ferrite phase, and improving the carburization resistance and oxidation resistance. Mn increases the proportion of the Mn oxide layer and (Fe, Mn, Cr) ₃ O ₄ layer in the thickness of the scale, increases the oxidation rate, stabilizes the austenite phase, and reduces the carburization and oxidation resistance. There is.

  The mechanism for improving the adhesion of the scale formed in the above-described atmosphere having carburizing properties is not necessarily clear, but is considered as follows.

Since Cr and Si improve the oxidation resistance, there is an effect of deteriorating the adhesion of the scale to reduce the undulations at the scale / base metal interface caused by deep oxidation at a location where the microscopic oxidation resistance is inferior. it is conceivable that. On the other hand, Cr and Si is effective to densify the _{Cr 2 O 3, Cr 2 O} 3 is less likely to crack, is considered to be effective to improve the adhesion of the scale. Further, Si has an effect of reducing the oxidation rate and thinning the scale, and is considered to have an effect of improving the adhesion of the scale. The effect on the adhesion of the Cr and Si scales varies depending on the balance of these conflicting effects. Basically, Cr and Si have a stronger effect of degrading the adhesion of the scale, but when Cr + Si ≧ 19.5, the effect of increasing the density of Cr ₂ O ₃ is increased, and the scale adhesion of Cr and Si is increased. It is considered that the effect of deteriorating the condition becomes mild. Further, when Cr + Si <19.5 and Si ≦ 0.20, the effect of thinning the scale is large, and rather Si is considered to improve the adhesion of the scale. In the case of Cr + Si <19.5 and Si> 0.20, which are intermediate between these, the effect of Cr ₂ O ₃ becoming dense and the effect of thinning the scale are small, and the effect of deteriorating the adhesion of Cr and Si scale is It is considered large. Mn thickens the scale, but the effect of increasing the ratio of the (Fe, Mn, Cr) ₃ O ₄ layer thickness to the thickness of the scale is large and improves the adhesion of the scale. This is because (Fe, Mn, Cr) ₃ O ₄ has a smaller difference in thermal expansion coefficient from that of the base material than Cr ₂ O _3, and thus reduces the thermal stress generated in the cooling process that causes scale peeling. is there. Ti and Al have the effect of improving the adhesion of the scale by internal oxidation at the surface layer of the base material. If the internal oxidation of Ti and Al forms roots from the scale, a throwing effect is generated and the adhesion of the scale is improved. Moreover, even if the internal oxidation of Ti and Al is dispersive and not connected to the scale, the internal oxide of Al alleviates the thermal shrinkage of the base material, resulting in a reduction in thermal stress, Adhesion improves.

  As a result of the examination of the effects as described above, the inventors have invented a ferritic stainless steel sheet for an automobile exhaust system or fuel reformer that has a carburizing atmosphere excellent in carburizing resistance and oxidation resistance, and a manufacturing method thereof. In the present invention, “a ferritic stainless steel sheet for an automobile exhaust system or fuel reformer that provides a carburizing atmosphere with excellent carburization resistance and oxidation resistance” means a ferritic stainless steel excellent in carburization resistance and oxidation resistance. It means a ferritic stainless steel plate that is a steel plate and is used for an automobile exhaust system that has a carburizing atmosphere or a fuel reformer that has a carburizing atmosphere.

That is, the gist of the present invention aimed at solving the above problems is as follows.
(1) In mass%,
C: 0.02% or less,
N: 0.02% or less,
Si: 0.05% or more, 3.0% or less,
Mn: 0.05% or more, 2.0% or less,
P: 0.04% or less,
S: 0.01% or less,
Cr: 12.0% or more, 25.0% or less,
Ni: 0.01% or more, 2.0% or less,
Ti: 0.001% or more, 0.40% or less,
Al: 0.002% or more, 0.25% or less,
V: 0.01% or more, 0.20% or less,
B: 0.0002% or more, 0.0050% or less,
Of the thickness of the Cr ₂ O ₃ layer in the thickness of the scale on the surface of the stainless steel containing the composition, the balance being Fe and inevitable impurities, and satisfying the following formulas (i) and (ii) Carburization resistance, wherein x is 20% or more, the Si concentration y of the Si concentrated layer between the scale and the base material is 0.4% or more, and the scale satisfies the following formula (iii): And a ferritic stainless steel sheet for an automobile exhaust system or a fuel reformer that provides a carburizing atmosphere with excellent oxidation resistance.
Cr + 12Si-4Mn-14.5 ≧ 0 Formula (i)
aCr + bSi + 1.6Mn + 27Ti + 33Al + 13.5 ≧ 0 Formula (ii)
When Cr + Si ≧ 19.5, a = −0.8, b = −0.2
When Cr + Si <19.5 and Si ≦ 0.20, a = −1, b = 29
In the case of Cr + Si <19.5 and Si> 0.20, a = −1, b = −3.7
x + 10y / Si-50 ≧ 0 Formula (iii)
However, the element symbol in a formula means content (mass%) of the said element.
(2) By mass%, further Cu: 2.0% or less,
Mo: 2.0% or less,
Nb: 1.0% or less, 1 type or 2 or more types, (1) characterized in that the car exhaust system or car exhaust system excellent in carburization resistance and oxidation resistance, or Ferritic stainless steel sheet for fuel reformers.
(3) In mass%, further Cr: less than 14.5%,
Nb: less than 0.35%,
Mo: less than 0.50%,
C + N: more than 0.020%,
Cu + 2Ni: more than 0.30%,
Ti: more than 0.20%,
Ferritic stainless steel for an automobile exhaust system or fuel reformer that satisfies the above-mentioned (1) or (2) and has an excellent carburizing resistance and oxidation resistance and an atmosphere having carburizing properties steel sheet.
(4) In mass%, Ca: 0.0002% or more, 0.0030% or less,
Zr: 0.01% or more, 0.30% or less,
Y: 0.001% or more, 0.20% or less,
Hf: 0.001% or more, 1.0% or less,
REM: 0.001% or more, 0.20% or less,
1 type or 2 types or more of these, The automobile exhaust system used as the atmosphere which has the carburizing property excellent in the carburizing resistance and oxidation resistance in any one of said (1)-(3) characterized by the above-mentioned Or a ferritic stainless steel sheet for a fuel reformer.
(5) In mass%, W: 0.01% or more, 5.0% or less,
Sn: 0.002% or more, 1.0% or less,
Mg: 0.0002% or more, 0.0030% or less,
Co: 0.01% or more, 0.30% or less,
Sb: 0.005% or more, 0.50% or less,
Bi: 0.001% or more, 1.0% or less,
Ta: 0.001% or more, 1.0% or less,
Ga: 0.0002% or more, 0.30% or less,
1 type or 2 types or more of said (1)-the automobile exhaust system used as the atmosphere which has the carburizing property excellent in carburization resistance and oxidation resistance in any one of said (4) characterized by the above-mentioned Or a ferritic stainless steel sheet for a fuel reformer.
(6) An atmosphere containing any one or more of carbon monoxide, carbon dioxide, and hydrocarbon-based gas and water vapor in the stainless steel plate having the composition according to any one of (1) to (5). In the heat treatment in the range of 600 to 1000 ° C., the ratio x of the Cr ₂ O ₃ layer thickness to the thickness of the scale is 20% or more on the surface of the stainless steel plate, and Si between the scale and the base material The concentrated layer has an Si concentration y of 0.4% or more, and forms a scale that satisfies the following formula (iii). This provides an atmosphere having carburization resistance and excellent carburization resistance and oxidation resistance. Manufacturing method of ferritic stainless steel sheet for automobile exhaust system or fuel reformer.
x + 10y / Si-50 ≧ 0 Formula (iii)
However, the element symbol in a formula means content (mass%) of the said element.

(7) By mass%
C: 0.02% or less,
N: 0.02% or less,
Si: 0.05% or more, 3.0% or less,
Mn: 0.05% or more, 2.0% or less,
P: 0.04% or less,
S: 0.01% or less,
Cr: 12.0% or more, 25.0% or less,
Ni: 0.01% or more, 2.0% or less,
Ti: 0.001% or more, 0.40% or less,
Al: 0.002% or more, 0.25% or less,
V: 0.01% or more, 0.20% or less,
B: 0.0002% or more, 0.0050% or less,
And the balance is composed of Fe and inevitable impurities, and satisfies the following formulas (i) and (ii): water vapor is 10% by volume, carbon dioxide is 10% by volume, carbon monoxide is When heated to 800 ° C. in an atmosphere of 10% by volume and the rest being nitrogen and continued for 100 hours and then cooled to room temperature, the ratio x of the Cr ₂ O ₃ layer thickness to the thickness of the scale is on the surface of the stainless steel plate. Carburization resistance, characterized by forming a scale satisfying the following formula (iii): the Si concentration y of the Si concentrated layer between the scale and the base material is 20% or more and 0.4% or more A ferritic stainless steel sheet for an automobile exhaust system or fuel reformer that provides a carburizing atmosphere with excellent oxidation resistance.
Cr + 12Si-4Mn-14.5 ≧ 0 Formula (i)
aCr + bSi + 1.6Mn + 27Ti + 33Al + 13.5 ≧ 0 Formula (ii)
When Cr + Si ≧ 19.5, a = −0.8, b = −0.2
When Cr + Si <19.5 and Si ≦ 0.20, a = −1, b = 29
In the case of Cr + Si <19.5 and Si> 0.20, a = −1, b = −3.7
x + 10y / Si-50 ≧ 0 Formula (iii)
However, the element symbol in a formula means content (mass%) of the said element.
(8) By mass%, Cu: 2.0% or less,
Mo: 2.0% or less,
Nb: 1.0% or less, 1 type or 2 types, Car exhaust system or fuel which becomes carburizing atmosphere excellent in carburizing resistance and oxidation resistance as described in (7) Ferritic stainless steel sheet for reformers.
(9) In mass%, further Cr: less than 14.5%,
Nb: less than 0.35%,
Mo: less than 0.50%,
C + N: more than 0.020%,
Cu + 2Ni: more than 0.30%,
Ti: more than 0.20%,
Or (7) or (8) satisfying one or more of the following: an automobile exhaust system that is an atmosphere having carburization resistance excellent in carburization resistance and oxidation resistance in the carburization atmosphere described in (8) or Ferritic stainless steel sheet for fuel reformers.
(10) In mass%, Ca: 0.0002% or more, 0.0030% or less,
Zr: 0.01% or more, 0.30% or less,
Y: 0.001% or more, 0.20% or less,
Hf: 0.001% or more, 1.0% or less,
REM: 0.001% or more, 0.20% or less,
(7) to (9), characterized in that it contains an carburizing resistance and an excellent carburizing atmosphere as described in (7) to (9) above. Ferritic stainless steel sheet for temperament.
(11) In mass%, W: 0.01% or more, 5.0% or less,
Sn: 0.002% or more, 1.0% or less,
Mg: 0.0002% or more, 0.0030% or less,
Co: 0.01% or more, 0.30% or less,
Sb: 0.005% or more, 0.50% or less,
Bi: 0.001% or more, 1.0% or less,
Ta: 0.001% or more, 1.0% or less,
Ga: 0.0002% or more, 0.30% or less,
1 type or 2 types or more of the following, The automobile exhaust system used as the atmosphere which has the carburizing property excellent in the carburizing resistance and oxidation resistance in any one of said (7)-(10) characterized by the above-mentioned Or a ferritic stainless steel sheet for a fuel reformer.

(12) Oxidation when heated to 850 ° C. and continued for 200 hours in an atmosphere of 10% by volume of water vapor, 10% by volume of carbon dioxide, 10% by volume of carbon monoxide, and the remainder of nitrogen, and then cooled to room temperature Heating at 850 ° C. for 200 hours in an atmosphere where the increase is 1.30 mg / cm ² or less, water vapor is 10% by volume, carbon dioxide is 10% by volume, carbon monoxide is 10% by volume, and the remainder is nitrogen. When the ferritic stainless steel plate with scale after being continued is heated to 950 ° C. for 200 hours and then cooled to room temperature, the increase in oxidation is 2.50 mg / cm ² or less, and the amount of scale peeling is 0.50 mg / cm. ² or less, the exhaust of the automobile having an atmosphere having excellent carburization resistance and oxidation resistance according to any one of (1) to (5) and (7) to (11) System or Fuel reformer ferritic stainless steel sheet.
(13) Even if all of the carbon monoxide or hydrocarbon gas contained in the atmosphere reacts with the oxygen contained in the atmosphere, the carbon monoxide or hydrocarbon gas of about 1% by volume or more remains. Automotive exhaust system members or fuel cell high temperatures that may be exposed to a carburizing atmosphere or a carburizing atmosphere containing less than 1% by volume of oxygen and 10% or more volume of carbon dioxide than oxygen An automobile exhaust system having an atmosphere having excellent carburization resistance and oxidation resistance according to any one of (1) to (5) and (7) to (12), characterized by being used as a member. Or a ferritic stainless steel sheet for a fuel reformer.

  In addition, in the present invention, those that do not define the lower limit are included to include inevitable impurity levels.

  ADVANTAGE OF THE INVENTION According to this invention, it is for car exhaust systems used as the atmosphere which has the carburization resistance excellent in the carburization resistance and oxidation resistance used as a car exhaust system member before a catalyst with high possibility of becoming the atmosphere which has carburizing property especially. A ferritic stainless steel sheet and a method for producing the same can be provided.

  In addition, according to the present invention, even when the oxidizing environment becomes severe and it is important to suppress the decrease in oxidation resistance due to carburization, it is possible to provide excellent carburization resistance and oxidation resistance, so that the oxidizing environment is severe. It is also possible to cope with higher temperatures and thinner walls.

  Further, according to the present invention, any one of low Cr, low Nb or no Nb addition, low Mo or no Mo addition, high C, high N, Cu addition, Ni addition, and excessive addition of Ti Since it can give excellent carburization resistance and oxidation resistance even when it is implemented, it can be added to automobile exhaust system members to add value such as cost reduction, high temperature strength improvement, corrosion resistance, etc. A great effect can be obtained for cost reduction of parts.

  In addition, reformers and heat exchangers used when reforming hydrocarbon fuels such as city gas, methane, natural gas, propane, kerosene, and gasoline into hydrogen, which also has a high temperature environment with carburizing properties. A ferritic stainless steel sheet for a fuel reformer that is used as a fuel cell high-temperature member such as the above and has a carburizing atmosphere excellent in carburizing resistance and oxidation resistance, and a method for producing the same.

Inventive Examples 1 to 19 and Comparative Examples 23 to 41 in Table 1 are in a carburizing atmosphere in which water vapor is 10% by volume, carbon dioxide is 10% by volume, carbon monoxide is 10% by volume, and the remainder is nitrogen. About 850 degreeC, the continuous oxidation test of 200 hours, and this invention examples 1-19 and comparative examples 32-41 of Table 1, water vapor | steam is 10 volume%, carbon dioxide is 10 volume%, carbon monoxide is 10 volume%, and the remainder 10% of water vapor is exerted on the evaluation of oxidation resistance in a continuous oxidation test at 950 ° C. for 200 hours in the air that is subsequently carried out after the continuous oxidation test at 850 ° C. for 200 hours in an atmosphere having a carburizing property of nitrogen. Scale thickness after continuous oxidation at 800 ° C. for 100 hours in a carburizing atmosphere with volume%, carbon dioxide 10 volume%, carbon monoxide 10 volume%, and the remainder being nitrogen It illustrates the effects of the present invention defined in (iii) expression using the Si concentration y of Si-concentrated layer between the Cr ₂ O ₃ layer ratio x and the scale and the base material of thickness occupied. Inventive Examples 1 to 19 and Comparative Examples 23 to 41 in Table 1 are in a carburizing atmosphere in which water vapor is 10% by volume, carbon dioxide is 10% by volume, carbon monoxide is 10% by volume, and the remainder is nitrogen. (Iii) of the present invention using the ratio x of the Cr ₂ O ₃ layer thickness to the thickness of the scale after continuous oxidation at 800 ° C. for 100 hours and the Si concentration y of the Si concentrated layer between the scale and the base material. It is the figure which showed the influence of (i) Formula prescribed | regulated to this invention which acts on Formula. Inventive Examples 1 to 19 and Comparative Examples 34 to 41 in Table 1 are in a carburizing atmosphere in which water vapor is 10% by volume, carbon dioxide is 10% by volume, carbon monoxide is 10% by volume, and the remainder is nitrogen. The figure which showed the influence of (ii) Formula of this invention prescription | regulation on evaluation of the scale adhesiveness in the continuous oxidation test of 950 degreeC and 200 hours in the air | atmosphere continued after 850 degreeC and 200 hours continuous oxidation test. It is.

  The mode for carrying out the present invention and the limiting conditions will be described in detail. In the present invention, unless otherwise noted,% described in terms of element content means mass%. The inventors have found that in the process of investigating the high temperature characteristics of ferritic stainless steel, the oxidation resistance in an atmosphere having carburizing properties varies greatly depending on slight differences in components.

(Test 1)
First, in order to investigate the scale formed in Examples 1 to 19 and Comparative Examples 20 to 41 in Table 1 in a carburizing atmosphere, water vapor is 10% by volume, carbon dioxide is 10% by volume, carbon monoxide. A continuous oxidation test was carried out at 800 ° C. for 100 hours in an atmosphere having a carburizing property with 10% by volume of nitrogen and the balance of nitrogen. None of these oxidation conditions cause abnormal oxidation, and the scale during normal oxidation can be evaluated.

The scale layer structure formed in the carburizing atmosphere was evaluated by glow discharge emission analysis (GDS). A scale layer in which Cr is detected together with O and the maximum value of Cr concentration is detected 50 mass% or more is a Cr ₂ O ₃ layer, Cr, Fe and Mn are detected together with O, and the maximum value of Cr concentration is 10 mass% or more. A scale layer in which the sum of the maximum values of Fe concentration and Mn concentration is 10% by mass or more and the sum of the maximum values of Cr concentration, Fe concentration and Mn concentration is 50% by mass or more is detected as (Fe, Mn, Cr) ₃ O _Four layers, Mn was detected together with O, the maximum value of Mn concentration was detected by 10% by mass or more, and the scale layer having a Cr concentration of less than 10% by mass was defined as a Mn oxide layer.

For example, if the Cr ₂ O ₃ layer and _{(Fe, Mn, Cr) 3} O 4 layer is adjacent, interfering peaks of mutual Cr, the low Cr concentration _{(Fe, Mn, Cr) 3} O 4 layer The peak of Cr may become unclear and the maximum value of Cr concentration may not be determined. In such a case, if the peak of Fe or Mn is clear, the Cr concentration at that position is set to the maximum value. Alternatively, the Cr concentration at the point where the absolute value of the slope of the Cr concentration curve is minimum is taken as the maximum value.

When there is a boundary between the Cr ₂ O ₃ layer and the (Fe, Mn, Cr) ₃ O ₄ layer, the boundary has the highest concentration of the higher Fe and Mn concentration in the (Fe, Mn, Cr) ₃ O ₄ layer. The position was half the value. When there was a boundary between the Cr ₂ O ₃ layer and the Mn oxide layer, the boundary was set at a position where the Mn concentration of the Mn oxide layer was half the maximum value. When there was a boundary between the (Fe, Mn, Cr) ₃ O ₄ layer and the Mn oxide layer, the boundary was set at a position where the Cr concentration of the (Fe, Mn, Cr) ₃ O ₄ layer was half the maximum value. When there was a boundary between the Cr ₂ O ₃ layer and the base material, the boundary was set at a position where the Cr concentration of the Cr ₂ O ₃ layer was half the maximum value. When there was a boundary between the (Fe, Mn, Cr) ₃ O ₄ layer and the base material, the boundary was set at a position where the Mn concentration of the (Fe, Mn, Cr) ₃ O ₄ layer was half the maximum value. When there was a boundary between the Mn oxide layer and the base material, the boundary was set at a position where the Mn concentration of the Mn oxide layer was half the maximum value. By determining the thickness from each boundary, the ratio of the Cr ₂ O ₃ layer thickness in the scale was determined. Further, the Si concentration of the Si concentrated layer between the scale and the base material was also determined from the GDS analysis result. When there is a peak of Si concentration higher than the Si concentration of the base material between the scale and the base material, the Si concentration of the peak was taken as the Si concentration of the Si concentrated layer. When there is no Si concentration peak higher than the Si concentration of the base material between the scale and the base material, and the Si concentrated layer cannot be confirmed, the Si concentration of the base material is set as the Si concentration of the Si concentrated layer. In addition, other oxides may be included in the scale, and the boundary and thickness of each layer were determined by the same means as described above using characteristic elements constituting other oxide layers.

(Test 2)
Furthermore, in order to evaluate the oxidation resistance in the carburizing atmospheres of Invention Examples 1 to 19 and Comparative Examples 20 to 41 of Table 1 that can form the scales evaluated in Test 1 above, For Invention Examples 1 to 19 and Comparative Examples 20 to 41, 850 ° C. in a carburizing atmosphere in which water vapor is 10% by volume, carbon dioxide is 10% by volume, carbon monoxide is 10% by volume, and the remainder is nitrogen, A 200 hour continuous oxidation test was performed. A test piece used for the oxidation test was subjected to # 600 polishing finish on the entire surface. In addition, the value which remove | divided the value of the weight increase of the oxidation test piece also including the peeled scale by the value of the surface area of the oxidation test piece was evaluated as the oxidation increase.

Table 2 shows the results of measurement of increase in oxidation in Test 2 above. In Comparative Examples 20 to 31 in Table 1 where the increase in oxidation in Test 2 was greater than 1.30 mg / cm ^2, nodules made of oxide containing a large amount of Fe were formed on the surface, and abnormal oxidation occurred. On the other hand, similar nodules were not observed in Invention Examples 1 to 19 and Comparative Examples 32 to 41 in Table 1. From this, when the oxidation increase was 1.30 mg / cm ² or less, it did not correspond to the abnormal oxidation state, showed good oxidation resistance, and was determined to be normally oxidized.

(Test 3)
Furthermore, in order to evaluate the adhesion of the scale formed in Test 2 and the protection after considering long-term use, the scales of Test 2 were tested for Examples 1 to 19 of the present invention and Comparative Examples 32-41 of Table 1. In the state marked with, a continuous oxidation test was conducted in the atmosphere at 950 ° C. for 200 hours. That is, 10% by volume of water vapor, 10% by volume of carbon dioxide, 10% by volume of carbon monoxide, and the remainder is nitrogen, followed by a continuous oxidation test at 850 ° C. for 200 hours in an atmosphere having a carburizing property. The continuous oxidation test was conducted at 950 ° C. for 200 hours.

  When the conditions such as 950 ° C. and 200 hours are converted to 850 ° C., the scale growth, that is, the consumption of Cr is equivalent to about 1600 hours. Further, the thermal strain generated at the time of cooling, which causes the scale peeling, is about 1.1 times, and the load on the scale peeling becomes large as the scale becomes thick and the peeling is promoted. That is, it can be considered that some overload occurs with respect to scale peeling in an actual environment, and it is possible to evaluate the protection in consideration of scale adhesion and long-term use in a simulated manner.

Table 2 shows the measurement results of the increase in oxidation and the amount of scale peeling in Test 3 above. In Comparative Examples 32 and 33 in Table 1 where the increase in oxidation in Test 3 was greater than 2.50 mg / cm ^2, nodules made of oxide containing a large amount of Fe were formed on the surface, and abnormal oxidation occurred. On the other hand, similar nodules were not observed in Invention Examples 1 to 19 and Comparative Examples 34 to 41 in Table 1. From this, when the amount of oxidation increase was 2.50 mg / cm ² or less, it did not correspond to the abnormal oxidation state, showed good oxidation resistance, and was determined to be normally oxidized.

In Comparative Examples 34 to 41 in Table 1 in which the scale peeling amount in Test 3 was larger than 0.50 mg / cm ² , the metal surface was exposed by scale peeling. On the other hand, in the inventive examples 1 to 19 in Table 1, the exposure of the metal surface was not observed. There is no practical problem unless a peeling situation is reached in which the metal surface is exposed. From this, the case where the amount of scale peeling was 0.50 mg / cm ² or less was determined as a condition excellent in the protection property in consideration of the adhesion of the scale and long-term use.

The inventors have oxidation resistance in an atmosphere having an excellent carburizing property with an oxidation increase in Test 2 of 1.30 mg / cm ² or less, and an oxidation increase in Test 3 of 2.50 mg / cm 2. the results of conditions of intensive studies of for having excellent protective properties which does not generate abnormal oxidation in consideration of the long-term use becomes ² or less shown in Fig. As a result, the ratio x of the Cr ₂ O ₃ layer thickness to the thickness of the scale in the test 1 above is 20% or more, and the Si concentration y of the Si concentrated layer between the scale and the base material is 0. It was found that the feature of forming a scale that satisfies .4% or more and satisfies the following formula (iii) improves carburization resistance and further improves oxidation resistance.
x + 10y / Si-50 ≧ 0 Formula (iii)
However, the element symbol in a formula means content (mass%) of the said element.

Desirably, the ratio x of the Cr ₂ O ₃ layer occupying the scale thickness is 30% or more on the surface of the stainless steel in Test 1 above, and the Si concentration y of the Si concentrated layer between the scale and the base material is y. Is preferably 0.6% or more and a scale satisfying the following formula (iii ′) is formed.
x + 10y / Si-60 ≧ 0 Formula (iii ′)

Further, the inventors have found that the ratio x of the Cr ₂ O ₃ layer occupying the thickness of the scale is 20% or more on the surface of the stainless steel in Test 1 above, and Si in the Si concentrated layer between the scale and the base material. FIG. 2 shows the result of intensive studies on the conditions for forming a scale where the concentration y is 0.4% or more and the above formula (iii) is satisfied. As a result, it was found that satisfying the following formula (i) improves the carburization resistance and further forms a scale necessary for improving the oxidation resistance.
Cr + 12Si-4Mn-14.5 ≧ 0 Formula (i)
However, the element symbol in a formula means content (mass%) of the said element.

Desirably, the following equation (i ′) is satisfied.
Cr + 12Si-4Mn-16.5 ≧ 0 Formula (i ′)

Furthermore, the inventors have shown the result of earnestly examining the conditions for providing the protective property after considering the adhesion of the scale and the long-term use with the scale peeling amount of the above test 3 being 0.50 mg / cm ² or less. 3 shows. As a result, it was found that the fact that the following formula (ii) is satisfied improves the carburization resistance and further improves the oxidation resistance.
aCr + bSi + 1.6Mn + 27Ti + 33Al + 13.5 ≧ 0 Formula (ii)
When Cr + Si ≧ 19.5, a = −0.8, b = −0.2
When Cr + Si <19.5 and Si ≦ 0.20, a = −1, b = 29
In the case of Cr + Si <19.5 and Si> 0.20, a = −1, b = −3.7
However, the element symbol in a formula means content (mass%) of the said element.

Desirably, the following equation (ii ′) is satisfied.
aCr + bSi + 1.6Mn + 27Ti + 33Al + 13.2 ≧ 0 Formula (ii ′)
When Cr + Si ≧ 19.5, a = −0.8, b = −0.2
When Cr + Si <19.5 and Si ≦ 0.20, a = −1, b = 29
In the case of Cr + Si <19.5 and Si> 0.20, a = −1, b = −3.7

  Furthermore, the effects of individual elements were also studied, and a ferritic stainless steel sheet excellent in oxidation resistance in an atmosphere having carburizing properties was invented.

  Hereinafter, the reason why each composition in the present invention is limited will be described.

(C: 0.02% or less)
C deteriorates moldability and corrosion resistance and brings about a decrease in high temperature strength. Therefore, it is 0.02% or less, preferably 0.015% or less. However, excessive reduction leads to an increase in refining costs, so the lower limit is preferably 0.001%. Further, even if C is not excessive, the refining cost increases at an accelerated rate as it decreases. Moreover, special equipment is required to reduce C above a certain level, and productivity may be significantly impaired. Therefore, there is a case where reduction of C is desired to reduce the cost. However, C is an austenite-forming element and promotes the phase transformation of the surface layer portion from the ferrite phase to the austenite phase due to the decrease in Cr of the surface layer portion as the oxidation proceeds. In an atmosphere having carburizing properties, in addition to C in steel, C further increases due to carburizing, which facilitates phase transformation from the ferrite phase of the surface layer portion to the austenite phase. Similarly, N is an austenite forming element and has the same effect. Therefore, considering oxidation resistance in an atmosphere having carburizing properties, it is difficult to make C + N more than 0.020%. However, C + N may be more than 0.020% within the range specified in the present invention.

(N: 0.02% or less)
N, like C, degrades moldability and corrosion resistance and causes a reduction in high-temperature strength. Therefore, it is made 0.02% or less. However, excessive reduction leads to an increase in refining costs, so the lower limit is preferably 0.003%. Further, even if N is not excessive, the refining cost increases at an accelerated rate as it decreases. Moreover, special equipment is required to reduce N above a certain level, and productivity may be significantly impaired. Therefore, there is a case where reduction of N is desired to reduce the cost. However, N is an austenite-forming element, and promotes the phase transformation from the ferrite phase to the austenite phase only in the surface layer portion due to the Cr decrease in the surface layer portion as the oxidation proceeds. In an atmosphere having carburizing properties, the carburization promotes the phase transformation from the ferrite phase of the surface layer portion to the austenite phase, so that the deterioration of oxidation resistance due to relaxation of N reduction is large. Similarly, C is an austenite forming element and has the same effect. Therefore, considering oxidation resistance in an atmosphere having carburizing properties, it is difficult to make C + N more than 0.020%. However, C + N may be more than 0.020% within the range specified in the present invention.

(Si: 0.05% or more, 3.0% or less)
Si is an element added as a deoxidizer and is an important element that improves oxidation resistance both in the atmosphere and in an atmosphere having carburizing properties. In order to maintain oxidation resistance, addition of 0.05% or more is required. In addition, as described above, in the scope of the present invention, Si addition may reduce the adhesion of the scale. Moreover, when it adds excessively, workability will fall. Therefore, it is 3.0% or less. Furthermore, excessive reduction leads to poor deoxidation and cost increase in addition to a decrease in oxidation resistance, and further considering the decrease in workability due to excessive addition, the lower limit is preferably 0.10%, the upper limit is 2.8% is desirable. More desirably, it is 0.15 to 1.2% of range.

(Mn: 0.05% or more, 2.0% or less)
Mn is an element added as a deoxidizer, and is an important element that improves the adhesion of the scale both in the atmosphere and in an atmosphere having carburizing properties. In order to maintain the adhesion of the scale, addition of 0.05% or more is required. Further, as described above, in the scope of the present invention, the addition of Mn reduces the oxidation resistance in an atmosphere having carburizing properties. Therefore, it is 2.00% or less. Furthermore, excessive reduction leads to an increase in cost, and in addition to excessive reduction of oxidation resistance, uniform elongation at room temperature decreases, and considering that corrosion resistance decreases by forming MnS, the lower limit is The upper limit is desirably 0.10%, and the upper limit is desirably 1.50%. More desirably, it is 0.15 to 1.20% of range.

(P: 0.04% or less)
P is an impurity mainly mixed from raw materials at the time of steelmaking refining, and as the content increases, the toughness and weldability decrease, so the smaller the content, the better. Therefore, it is made 0.04% or less. However, excessive reduction leads to an increase in refining costs, so the lower limit is preferably 0.01%.

(S: 0.01% or less)
S is an impurity mainly mixed from raw materials during steelmaking refining, and deteriorates the corrosion resistance. Therefore, the smaller the content, the better. Therefore, it is 0.01% or less. However, excessive reduction leads to an increase in refining costs, so the lower limit is preferably 0.0003%.

(Cr: 12.0% or more, 25.0% or less)
Cr is an extremely effective element for imparting oxidation resistance, and addition of 12.0% or more is required to maintain oxidation resistance. On the other hand, if it exceeds 25.0%, the workability is lowered and the toughness is deteriorated, so the content is made 12.0 to 25.0%. Furthermore, when considering high temperature strength, high temperature fatigue characteristics and manufacturing cost in addition to oxidation resistance, the lower limit is desirably 12.5%, and the upper limit is desirably 20%. More desirably, it is 13.0 to 18.0%. Further, since Cr is an expensive element, it may be desired to reduce it as much as possible in order to reduce the cost. However, it is an element particularly important for oxidation resistance in a carburizing atmosphere, and it is difficult to make it less than 14.5%. However, if it is within the prescribed range of the present invention, Cr can be made less than 14.5%. Furthermore, considering the improvement in workability in addition to cost reduction, Cr can be made less than 14.0%.

(Ni: 0.01% or more, 2.0% or less)
Ni is an element that improves corrosion resistance, and also has an effect of improving high-temperature strength and toughness. However, excessive addition reduces moldability. Therefore, the content is set to 0.01 to 2.0%. Furthermore, considering that Ni is expensive, the upper limit is desirably 1.0%. More preferably, it is 0.50%. Further, the effect of improving the corrosion resistance of Ni is large, and there are cases where effective use is desired in order to add the added value of corrosion resistance. However, Ni is an austenite-forming element, and promotes the phase transformation from the ferrite phase to the austenite phase only in the surface layer portion due to the decrease in Cr of the surface layer portion as the oxidation proceeds. In an atmosphere having carburizing properties, the carburization promotes the phase transformation from the ferrite phase of the surface layer portion to the austenite phase, so the deterioration of oxidation resistance due to the addition of Ni is large. Similarly, Cu is an austenite forming element, and Ni has an austenite forming ability approximately twice that of Cu. Therefore, considering oxidation resistance in an atmosphere having carburizing properties, it is difficult to make Cu + 2Ni exceed 0.30%. However, Cu + 2Ni can be more than 0.30% within the specified range of the present invention. Furthermore, in order to actively utilize the effects of these elements, it is possible to make Cu + 2Ni exceed 1.00%.

(Ti: 0.001% or more, 0.40% or less)
Ti is an element that combines with C, N, and S to improve the r value that serves as an index of corrosion resistance, intergranular corrosion resistance, and deep drawability. Further, Ti is an element that improves the scale peel resistance both in the air and in a carburizing atmosphere. However, a large amount of Ti increases the amount of solid solution Ti and decreases the uniform elongation. Therefore, it is made 0.001 to 0.40% or less. Furthermore, considering the refining cost, the lower limit is preferably 0.003%. Furthermore, considering the fact that coarse Ti-based precipitates are formed, become the starting point of cracks during hole expansion processing, and the hole expandability is deteriorated, the upper limit is preferably 0.30%. Furthermore, considering the occurrence of surface defects, the upper limit is more preferably 0.25%. Further, Ti has a large effect of improving corrosion resistance, and there are cases where effective use is desired in order to add an added value of corrosion resistance. However, if added excessively, the oxidation of Cr is promoted to promote Cr reduction in the surface layer of the base material, or the carbon monoxide or hydrocarbon gas is scaled by increasing voids and vacancies in the scale. By making it easy to permeate, oxidation resistance in an atmosphere having carburizing properties is lowered. Therefore, considering oxidation resistance in an atmosphere having carburizing properties, it is difficult to make Ti more than 0.20%. However, if it is within the prescribed range of the present invention, Ti may be more than 0.20%.

(Al: 0.002% or more, 0.25% or less)
In addition to being added as a deoxidizing element, Al is useful as a solid solution strengthening element for improving high-temperature strength. In addition, Al is an element that improves the scale peel resistance both in the air and in a carburizing atmosphere. However, excessive addition hardens and significantly reduces the uniform elongation, and toughness significantly decreases. Therefore, it is made 0.002 to 0.25%. Furthermore, considering the refining cost, the lower limit is more preferably 0.005%. Furthermore, considering the occurrence of surface flaws and weldability, the upper limit is preferably 0.20%.

(V: 0.01% or more, 0.20% or less)
V forms fine carbonitrides and causes a precipitation strengthening action, which contributes to an improvement in high temperature strength. However, excessive addition coarsens the precipitate, lowers the high-temperature strength, and reduces the thermal fatigue life. Therefore, the content is set to 0.01 to 0.20%.

(B: 0.0002% or more, 0.0050% or less)
B is an element that improves high-temperature strength and thermal fatigue characteristics. However, excessive addition reduces the hot workability and the surface properties of the steel surface. Therefore, the content is 0.0002 to 0.0050% or less.

  In addition, in the present invention, the characteristics can be further improved by adding one or more of Cu, Mo, and Nb.

(Cu: 2.0% or less)
Cu is an element effective for improving corrosion resistance. Cu is an element that improves high-temperature strength, and may be used effectively to replace Cr, Nb, and Mo, or to further improve high-temperature strength after using Cr, Nb, and Mo. is there. The high temperature strength is improved by the precipitation hardening action caused by the precipitation of ε-Cu. However, excessive addition reduces hot workability. Therefore, it is set to 2.0% or less. However, as long as Nb or Mo is within the specified range of the present invention, no addition may be performed. Further, considering that excessive addition reduces press moldability, the upper limit is desirably 1.50%. More desirably, it is 1.30%. Moreover, Cu is an austenite forming element, and promotes the phase transformation from the ferrite phase to the austenite phase only in the surface layer portion due to the decrease in Cr of the surface layer portion as the oxidation proceeds. In an atmosphere having carburizing properties, the carburization promotes the phase transformation from the ferrite phase of the surface layer portion to the austenite phase, so that the deterioration of oxidation resistance due to the addition of Cu is large. Similarly, Ni is an austenite forming element, and Ni has an austenite forming ability approximately twice that of Cu. Therefore, considering oxidation resistance in an atmosphere having carburizing properties, it is difficult to make Cu + 2Ni exceed 0.30%. However, Cu + 2Ni can be more than 0.30% within the specified range of the present invention. Furthermore, in order to actively utilize the effects of these elements, it is possible to make Cu + 2Ni exceed 1.00%. The Cu content is preferably 0.01% or more.

(Mo: 2.00% or less)
Mo is effective for improving corrosion resistance and improving high-temperature strength by solid solution strengthening. However, excessive addition reduces moldability. Therefore, it is 2.00% or less. Further, considering the manufacturability, the upper limit is desirably 1.50%. Further, since Mo is an expensive element, it may be desired to reduce or not add it in order to reduce the cost. However, Mo is a ferrite-forming element and suppresses a phase transformation from the ferrite phase to the austenite phase only in the surface layer portion due to the Cr decrease in the surface layer portion as the oxidation proceeds. In an atmosphere having carburizing properties, the carburization promotes the phase transformation from the ferrite phase of the surface layer portion to the austenite phase, so that the oxidation resistance improvement by adding Mo is great. Therefore, considering oxidation resistance in an atmosphere having carburizing properties, it is difficult to make Mo less than 0.50% or no additive. However, it is possible to make Mo less than 0.50% or no addition as long as it is within the specified range of the present invention. The Mo content is preferably 0.01% or more.

(Nb: 1.00% or less)
Nb improves the high temperature strength by solid solution strengthening and precipitate refinement strengthening, and fixes C and N as carbonitrides to improve corrosion resistance and intergranular corrosion resistance. However, excessive addition reduces the uniform elongation and deteriorates the hole expandability. Therefore, it is 1.00% or less. Further, considering the manufacturability, the upper limit is preferably 0.60%. Also, Nb is an expensive element, and 90% of production is Brazil. Since supply is highly uneven, resource risk is high, and it is desirable to reduce or add no element to reduce costs. There is a case. However, Nb is a ferrite-forming element and suppresses the phase transformation of the surface layer portion from the ferrite phase to the austenite phase only due to the decrease in Cr of the surface layer portion as the oxidation proceeds. In an atmosphere having carburizing properties, the carburization promotes the phase transformation from the ferrite phase of the surface layer portion to the austenite phase, so that the oxidation resistance is greatly improved by the addition of Nb. Further, in an atmosphere having carburizing properties, there is an effect of delaying the carburizing rate by reacting with carburizing C. Therefore, considering oxidation resistance in an atmosphere having carburizing properties, it is difficult to make Nb less than 0.35% or no additive. However, Nb may be less than 0.35% or not added as long as it is within the specified range of the present invention. The Nb content is preferably 0.001% or more.

  In addition, in the present invention, the characteristics can be further improved by adding one or more of Ca, Zr, Y, Hf, and REM.

(Ca: 0.0002% or more, 0.0030% or less)
Ca is added as necessary for desulfurization. Since this effect does not appear at less than 0.0002%, the lower limit is made 0.0002%. However, excessive addition reduces the corrosion resistance due to the formation of CaS which is a water-soluble inclusion, so the upper limit is made 0.0030%. Ca is also an element that improves oxidation resistance.

(Zr: 0.01% or more, 0.30% or less)
Zr is added in an amount of 0.01% or more as necessary to improve corrosion resistance, intergranular corrosion resistance, and high temperature strength. However, excessive addition reduces workability and manufacturability, so the upper limit is made 0.30%. Zr is also an element that improves oxidation resistance.

(Y: 0.001% or more, 0.20% or less)
Y is added in an amount of 0.001% or more as necessary in order to improve the cleanliness of the steel and improve the weather resistance and hot workability. However, excessive addition leads to an increase in alloy cost and a decrease in manufacturability, so the upper limit is made 0.20%. Y is also an element that improves oxidation resistance.

(Hf: 0.001% or more, 1.0% or less)
Hf is added in an amount of 0.001% or more as necessary to improve corrosion resistance, intergranular corrosion resistance, and high temperature strength. However, excessive addition reduces workability and manufacturability, so the upper limit is made 1.0%. Hf is also an element that improves oxidation resistance.

(REM: 0.001% or more, 0.20% or less)
REM (rare earth element) is added in an amount of 0.001% or more as necessary in order to improve the cleanliness of the steel and improve the weather resistance and hot workability. However, excessive addition leads to an increase in alloy cost and a decrease in manufacturability, so the upper limit is made 0.20%. REM is also an element that improves oxidation resistance. REM follows the general definition. It is a generic term for two elements of scandium (Sc) and yttrium (Y) and 15 elements (lanthanoid) from lanthanum (La) to lutetium (Lu). It may be added alone or as a mixture.

  In addition, in the present invention, the characteristics can be further improved by adding one or more of W, Sn, Mg, Co, Sb, Bi, Ta, and Ga.

(W: 0.01% or more, 5.0% or less)
W is added in an amount of 0.01% or more as necessary to improve corrosion resistance and high-temperature strength. However, excessive addition reduces toughness and manufacturability, so the upper limit is made 5.0%.

(Sn: 0.002% or more, 1.0% or less)
Sn is added in an amount of 0.002% or more as necessary in order to improve the corrosion resistance and the high temperature strength. However, excessive addition reduces toughness and manufacturability, so the upper limit is made 1.0%.

(Mg: 0.0002% or more, 0.0030% or less)
In addition to adding Mg as a deoxidizing element, Mg can be refined and used to improve moldability, so 0.0002% or more is added as necessary. However, excessive addition lowers the corrosion resistance, weldability, and surface quality, so the upper limit is made 0.0030%.

(Co: 0.01% or more, 0.30% or less)
Co is added in an amount of 0.01% or more as necessary to improve the high temperature strength. However, excessive addition reduces toughness and manufacturability, so the upper limit is made 0.30%.

(Sb: 0.005% or more, 0.50% or less)
Sb is added in an amount of 0.005% or more as necessary in order to improve the high temperature strength. However, excessive addition reduces weldability and toughness, so the upper limit is made 0.50%.

(Bi: 0.001% or more, 1.0% or less)
Bi is added in an amount of 0.001% or more as necessary to suppress roping that occurs during cold rolling and improve manufacturability. However, excessive addition reduces hot workability, so the upper limit is made 1.0%.

(Ta: 0.001% or more, 1.0% or less)
Ta is added in an amount of 0.001% or more as necessary in order to improve the high temperature strength. However, excessive addition reduces toughness and manufacturability, so the upper limit is made 1.0%.

(Ga: 0.0002% or more, 0.30% or less)
Ga is added in an amount of 0.0002% or more as necessary to improve corrosion resistance and hydrogen embrittlement resistance. However, excessive addition reduces workability, so the upper limit is made 0.30%.

Furthermore, the index of oxidation resistance in an atmosphere having carburizing properties is that the water vapor is heated to 850 ° C. in an atmosphere of 10% by volume, carbon dioxide of 10% by volume, carbon monoxide of 10% by volume, and the balance being nitrogen. The amount of increase in oxidation was determined after cooling to room temperature after 200 hours. When this value is 1.30 mg / cm ² or less, it does not correspond to the abnormal oxidation state, and indicates oxidation resistance in an atmosphere having good carburizing properties.

In addition, the protection index after considering the adhesion of the scale formed in the carburizing atmosphere and long-term use is 10% by volume of water vapor, 10% by volume of carbon dioxide, and 10% by volume of carbon monoxide. %, Oxidation increase and scale when cooled to room temperature after continuous oxidation test at 850 ° C. for 200 hours in the atmosphere of nitrogen at 850 ° C. for 200 hours. The amount peeled. The value of the oxidation increase is, in the case of 2.50 mg / cm ² or less, does not correspond to abnormal oxidation, not lead to a release situation where scale peeling amount to expose the metal surface as long as 0.50 mg / cm ² or less In addition, it is assumed that the protective property is taken into consideration in consideration of good scale adhesion and long-term use.

  Next, the atmosphere having carburizing properties in the present invention will be described.

  The atmosphere having a carburizing property in the present invention is an atmosphere in which the C activity in the atmosphere is larger than the C activity in the steel. For example, the atmosphere contains carbon monoxide or a hydrocarbon-based gas and is included in the atmosphere. The atmosphere has a composition in which carbon monoxide or hydrocarbon gas remains even if all oxygen reacts with carbon monoxide or hydrocarbon gas. In addition, an atmosphere that contains a large amount of carbon dioxide in the atmosphere and that carbon dioxide is assumed to be used as an oxygen source used for oxidation is also an atmosphere having carburizing properties.

  Furthermore, carburization is considered to be slow while the carbon monoxide or hydrocarbon gas in the atmosphere reacts with oxygen. Therefore, the smaller the amount of oxygen, the shorter the reaction time or frequency and the carburization. The possibility of occurrence is considered to be high. In the case of carburizing with carbon dioxide, oxygen is used for oxidation, and oxygen is deficient in a narrow range on the surface of the steel material, so carbon dioxide begins to be used for oxidation, and carburization is thought to occur accordingly. That is, it is considered that the smaller the amount of oxygen, the higher the possibility of oxidation and carburization by carbon dioxide. Therefore, the atmosphere having a carburizing property in the present invention includes carbon monoxide and hydrocarbon-based gas in a total amount of 1% by volume or more and oxygen of 1% by volume or less, and the total amount of carbon monoxide and hydrocarbon-based gas is oxygen. It is desirable to interpret it as an atmosphere containing twice or more of the amount or an atmosphere containing 5% or more of carbon dioxide and 1% by volume of oxygen.

  Furthermore, the greater the difference between the total amount of carbon monoxide and hydrocarbon gas in the atmosphere and the amount of oxygen, the greater the reaction between carbon monoxide or hydrocarbon gas and oxygen and the carburization reaction with carbon monoxide or hydrocarbon gas. It is thought that it becomes easy to generate | occur | produce simultaneously in parallel. Further, it is considered that as the amount of carbon dioxide increases, oxidation by oxygen and oxidation by carbon dioxide are more likely to occur simultaneously in parallel. Therefore, the atmosphere having carburizing properties in the present invention includes carbon monoxide and hydrocarbon-based gas in a total amount of 2% by volume or more and oxygen of 1% by volume or less, and the total of carbon monoxide and hydrocarbon-based gas is oxygen. More preferably, it is interpreted as an atmosphere containing 5 times or more, or an atmosphere containing 10% or more of carbon dioxide and 1% by volume of oxygen.

  Next, a method for producing a ferritic stainless steel sheet for an automobile exhaust system or a fuel reformer that provides an atmosphere having a carburizing property excellent in carburizing resistance and oxidation resistance in the present invention will be described.

About the manufacturing method of the steel plate of this invention, the general process which manufactures ferritic stainless steel is employable. Generally, it is made into molten steel in a converter or electric furnace, scoured in an AOD furnace or VOD furnace, and made into a steel piece by a continuous casting method or an ingot-making method, and then hot-rolled-annealed hot-rolled sheet-pickled-cooled It is manufactured through a process of hot rolling, finish annealing and pickling. If necessary, annealing of the hot-rolled sheet may be omitted, or cold rolling-finish annealing-pickling may be repeated. The conditions of these steps may be general conditions, for example, a hot rolling heating temperature of 1000 to 1300 ° C, a hot rolled sheet annealing temperature of 900 to 1200 ° C, a cold rolled sheet annealing temperature of 800 to 1200 ° C, and the like. However, the present invention is not characterized by manufacturing conditions, and the manufacturing conditions are not limited. Therefore, as long as the produced steel can achieve the effects of the present invention, hot rolling conditions, hot rolled sheet thickness, presence or absence of hot rolled sheet annealing, cold rolled conditions, hot rolled sheet and cold rolled sheet annealing temperature, atmosphere, etc. It can be selected as appropriate.
In addition, the treatment before the finish pickling may be a general treatment, for example, mechanical treatment such as shot blasting or grinding brush, chemical treatment such as molten salt treatment or neutral salt electrolysis treatment may be performed. it can. Further, temper rolling or tension leveler may be applied after cold rolling and annealing. Further, the product plate thickness may be selected according to the required member thickness. Moreover, you may manufacture as a welded pipe by the manufacturing method of the normal stainless steel pipe for exhaust system members, such as electrical resistance welding, TIG welding, and laser welding, using this steel plate as a raw material.

  Next, the use in the atmosphere which has carburizing property is demonstrated.

When a ferritic stainless steel sheet is used in an atmosphere having carburizing properties, the ratio x of the Cr ₂ O ₃ layer thickness to the thickness of the scale on the surface of the steel sheet in the atmosphere is 20% or more, between the scale and the base material. In order to have excellent carburization resistance and oxidation resistance, it is necessary to form a scale satisfying the following formula (iii) in which the Si concentration y of the Si concentrated layer is 0.4% or more.
x + 10y / Si-50 ≧ 0 Formula (iii)
However, the element symbol in a formula means content (mass%) of the said element.

  The scale necessary for having excellent carburization resistance and oxidation resistance is formed on the surface of stainless steel in a carburizing atmosphere using ferritic stainless steel having the components specified in the present invention. . In the atmosphere having carburizing properties is an atmosphere containing one or more of carbon monoxide, carbon dioxide, hydrocarbon-based gas and water vapor, and heat treatment in the range of 600 to 1000 ° C., It is formed on the surface of the ferritic stainless steel of the present invention. The atmosphere having carburizing properties may contain other gases such as nitrogen, hydrogen, argon, nitrogen oxide, and sulfur oxide.

  In addition, the scale necessary for having excellent carburization resistance and oxidation resistance may be formed on the steel plate in a carburizing atmosphere in advance, but it is formed by a trial operation at the initial stage of system operation of the final product. Alternatively, it may be formed when an atmosphere having a carburizing property is obtained during actual operation by the user. However, the scale structure formed during operation cannot be confirmed. Therefore, it is necessary to evaluate whether or not the steel sheet constituting the final product can form a scale necessary for having excellent carburization resistance and oxidation resistance.

  As a method of evaluating whether it is possible to form a scale that is necessary to have excellent carburization resistance and oxidation resistance, heat treatment is performed on a ferritic stainless steel sheet in an atmosphere having carburization property, and the formed scale is evaluated. Good.

  Although the heat treatment conditions used in evaluating whether the scale necessary for having excellent carburization resistance and oxidation resistance can be formed are not limited, the atmosphere has carburization resistance and serves as an oxidation source. It only has to contain water vapor. In order to have carburizing properties, it is sufficient that the C activity in the atmosphere is larger than the C activity in the steel. For example, the atmosphere contains carbon monoxide or a hydrocarbon-based gas, and even if all the oxygen contained in the atmosphere reacts with the carbon monoxide or hydrocarbon-based gas, the carbon monoxide or hydrocarbon-based gas is about 1% by volume or more. As long as the atmosphere has a composition in which the residual amount remains. Moreover, as a standard of C activity in atmosphere, what is necessary is just 0.00001 or more in heat processing temperature. For example, the heat treatment may be performed for 100 hours by heating to 800 ° C. in a carburizing atmosphere in which water vapor is 10% by volume, carbon dioxide is 10% by volume, carbon monoxide is 10% by volume, and the remainder is nitrogen.

  A glow discharge emission analysis (GDS) is used as a scale evaluation method for evaluating whether a scale necessary for having excellent carburization resistance and oxidation resistance can be formed. About a concrete evaluation method, it is good to carry out similarly to the said Test 1.

According to the above evaluation, the ratio x of the Cr ₂ O ₃ layer thickness to the thickness of the scale on the surface of the steel sheet is 20% or more, and the Si concentration y of the Si concentrated layer between the scale and the base material is 0.4% or more. And a ferritic stainless steel sheet forming a scale that satisfies the following formula (iii), further having the steel components specified above, and satisfying the following formulas (i) and (ii): Ferritic stainless steel sheet with excellent carburization resistance and oxidation resistance, excellent scale adhesion, and excellent protection even after considering long-term use. Can be used in environments with
Cr + 12Si-4Mn-14.5 ≧ 0 Formula (i)
aCr + bSi + 1.6Mn + 27Ti + 33Al + 13.5 ≧ 0 Formula (ii)
When Cr + Si ≧ 19.5, a = −0.8, b = −0.2
When Cr + Si <19.5 and Si ≦ 0.20, a = −1, b = 29
In the case of Cr + Si <19.5 and Si> 0.20, a = −1, b = −3.7
x + 10y / Si-50 ≧ 0 Formula (iii)
However, the element symbol in a formula means content (mass%) of the said element.

  Hereinafter, the effects of the present invention will be made clearer by examples. In addition, this invention is not limited to a following example, In the range which does not change the summary, it can change suitably and can implement.

  Test materials (invention steels 1 to 19 and comparative steels 20 to 41) having the composition shown in Table 1 were melted in a vacuum melting furnace and cast into a 30 kg ingot. The obtained ingot was a hot-rolled steel sheet having a thickness of 4.5 mm. The heating condition for hot rolling was 1200 ° C. Hot-rolled sheet annealing was set to 1000 ° C. After descaling with alumina blasting, a plate having a thickness of 1.0 mm was formed by cold rolling, and finish annealing was performed at 1100 ° C. A test piece having a thickness of 1.0 mm, a width of 20 mm, and a length of 25 mm was collected from the cold-rolled annealed plate thus obtained, and subjected to an overall # 600 polishing finish for use in the oxidation test.

(Test 1)
First, the scales formed in the carburizing atmospheres of Examples 1 to 19 and Comparative Examples 20 to 41 in Table 1 were investigated. In the oxidation test for forming a scale in an atmosphere having carburizing properties, a tubular furnace capable of controlling the atmosphere was used. After placing the test piece in the furnace, the temperature was raised to 800 ° C. in a nitrogen atmosphere. Thereafter, the atmosphere was switched to a mixed atmosphere of 10% by volume of water vapor, 10% by volume of carbon dioxide, 10% by volume of carbon monoxide, and the balance of nitrogen, and held at 800 ° C. for 100 hours. Then, it switched to nitrogen atmosphere and cooled to room temperature.

The scale layer structure formed in the carburizing atmosphere was evaluated by glow discharge emission analysis (GDS). A scale layer in which Cr is detected together with O and the maximum value of Cr concentration is detected 50 mass% or more is a Cr ₂ O ₃ layer, Cr, Fe and Mn are detected together with O, and the maximum value of Cr concentration is 10 mass% or more. A scale layer in which the sum of the maximum values of Fe concentration and Mn concentration is 10% by mass or more and the sum of the maximum values of Cr concentration, Fe concentration and Mn concentration is 50% by mass or more is detected as (Fe, Mn, Cr) ₃ O _Four layers, Mn was detected together with O, the maximum value of Mn concentration was detected by 10% by mass or more, and the scale layer having a Cr concentration of less than 10% by mass was defined as a Mn oxide layer.

For example, if the Cr ₂ O ₃ layer and _{(Fe, Mn, Cr) 3} O 4 layer is adjacent, interfering peaks of mutual Cr, the low Cr concentration _{(Fe, Mn, Cr) 3} O 4 layer The peak of Cr may become unclear and the maximum value of Cr concentration may not be determined. In such a case, if the peak of Fe or Mn is clear, the Cr concentration at that position is set to the maximum value. Alternatively, the Cr concentration at the point where the absolute value of the slope of the Cr concentration curve is minimum is taken as the maximum value.

When there is a boundary between the Cr ₂ O ₃ layer and the (Fe, Mn, Cr) ₃ O ₄ layer, the boundary has the highest concentration of the higher Fe and Mn concentration in the (Fe, Mn, Cr) ₃ O ₄ layer. The position was half the value. When there was a boundary between the Cr ₂ O ₃ layer and the Mn oxide layer, the boundary was set at a position where the Mn concentration of the Mn oxide layer was half the maximum value. When there was a boundary between the (Fe, Mn, Cr) ₃ O ₄ layer and the Mn oxide layer, the boundary was set at a position where the Cr concentration of the (Fe, Mn, Cr) ₃ O ₄ layer was half the maximum value. When there was a boundary between the Cr ₂ O ₃ layer and the base material, the boundary was set at a position where the Cr concentration of the Cr ₂ O ₃ layer was half the maximum value. When there was a boundary between the (Fe, Mn, Cr) ₃ O ₄ layer and the base material, the boundary was set at a position where the Mn concentration of the (Fe, Mn, Cr) ₃ O ₄ layer was half the maximum value. When there was a boundary between the Mn oxide layer and the base material, the boundary was set at a position where the Mn concentration of the Mn oxide layer was half the maximum value. By determining the thickness from each boundary, the ratio of the Cr ₂ O ₃ layer thickness in the scale was determined. Further, the Si concentration of the Si concentrated layer between the scale and the base material was also determined from the GDS analysis result. When there is a peak of Si concentration higher than the Si concentration of the base material between the scale and the base material, the Si concentration of the peak was taken as the Si concentration of the Si concentrated layer. When there is no Si concentration peak higher than the Si concentration of the base material between the scale and the base material, and the Si concentrated layer cannot be confirmed, the Si concentration of the base material is set as the Si concentration of the Si concentrated layer. Further, when other oxides were included in the scale, the boundary and thickness of each layer were determined by the same means as described above using characteristic elements constituting the other oxide layers.

Table 2 shows the evaluation result of the ratio x of the Cr ₂ O ₃ layer thickness to the thickness of the scale formed in the carburizing atmosphere and the Si concentration y of the Si concentrated layer between the scale and the base material.

(Test 2)
Next, the carburization resistance and oxidation resistance of Invention Examples 1 to 19 and Comparative Examples 20 to 41 in Table 1 capable of forming the scale shown in Table 2 evaluated in Test 1 were evaluated. In the oxidation test for this evaluation, a tubular furnace that can control the atmosphere as in Test 1 was used. After placing the test piece in the furnace, the temperature was raised to 850 ° C. in a nitrogen atmosphere. Thereafter, the atmosphere was switched to a carburizing atmosphere in which water vapor was 10% by volume, carbon dioxide was 10% by volume, carbon monoxide was 10% by volume, and the remainder was nitrogen, and the atmosphere was maintained at 850 ° C. for 200 hours. Then, it switched to nitrogen atmosphere and cooled to room temperature. In this oxidation test, the scale of the specimen after oxidation is hardly peeled off. However, when the scale peels off, the peeled scale is also collected, and the weight increase value of the post-oxidation specimen including the peeled scale is measured. The value obtained by dividing by the value of the surface area was defined as the amount of increase in oxidation. In such an atmosphere having carburizing properties, carburization resistance and oxidation resistance were evaluated using an increase in oxidation in a continuous oxidation test at 850 ° C. for 200 hours. If the increase in oxidation was 1.30 mg / cm ² or less, the oxidation resistance was good, and it was considered that oxidation was not accelerated by carburization, so the carburization resistance was also good.

  Table 2 shows the measurement results of the increase in oxidation in a continuous oxidation test at 850 ° C. for 200 hours in an atmosphere having carburizing properties.

(Test 3)
Furthermore, the scale formed in the atmosphere which has carburizing property about the present invention examples 1-19 and comparative examples 32-41 of Table 1 evaluated that the carburization resistance and oxidation resistance were favorable in the test 2 above. The protective properties were evaluated in consideration of the adhesion and long-term use. The oxidation test for this evaluation was a test in which an oxidation test in an atmosphere having carburizing properties and an oxidation test in the atmosphere were continuously performed. For the oxidation test in an atmosphere having carburizing properties, a tubular furnace that can control the same atmosphere as in Test 1 and Test 2 was used. After placing the test piece in the furnace, the temperature was raised to 850 ° C. in a nitrogen atmosphere. Thereafter, the atmosphere was switched to a carburizing atmosphere in which water vapor was 10% by volume, carbon dioxide was 10% by volume, carbon monoxide was 10% by volume, and the remainder was nitrogen, and the atmosphere was maintained at 850 ° C. for 200 hours. Then, it switched to nitrogen atmosphere and cooled to room temperature. Using the post-oxidation test piece in the atmosphere having the carburizing property, an oxidation test in the atmosphere was subsequently performed. A muffle furnace that performs heat treatment in a static atmosphere was used for the oxidation test in the atmosphere. After the post-oxidation test piece in an atmosphere having carburizing properties was placed in the furnace, the temperature was raised to 950 ° C. Thereafter, the mixture was kept at 950 ° C. for 200 hours and then cooled to room temperature. When the scale peeled off during the cooling process after oxidation, the peeled scale was also collected, and the value obtained by dividing the weight increase value of the post-oxidation test piece including the peeled scale by the surface area value of the test piece was taken as the oxidation increase. Moreover, the value which remove | divided the value of the weight of the peeled scale by the value of the surface area of the test piece was made into the scale peeling amount. In such an atmosphere having carburizing properties, by using an oxidation increase and a scale peeling amount in a continuous oxidation test at 850 ° C. for 200 hours and a continuous oxidation test in air at 950 ° C. for 200 hours, The protective property was evaluated in consideration of the adhesion and long-term use of the scale formed in an atmosphere having carburizing properties. The oxidation weight gain 2.50 mg / cm ² or less, the peeling of scale amount was considered good 0.50 mg / cm ² or less.

  Table 2 shows the measurement results of the amount of increase in oxidation and the amount of scale peeling in a test in which a continuous oxidation test at 850 ° C. for 200 hours in a carburizing atmosphere and a continuous oxidation test in air at 950 ° C. for 200 hours are performed. .

Inventive Examples 1 to 19 are the component composition, the ratio x of the Cr ₂ O ₃ layer thickness to the thickness of the scale formed in an atmosphere having carburizing properties, Si of the Si concentrated layer between the scale and the base material Concentration y is within the prescribed range of the present invention, and further satisfies formulas (i), (ii), and (iii), and has oxidation resistance and carburizing properties in a carburizing atmosphere. Both the adhesion of the scale formed in the atmosphere and the protective property in consideration of long-term use are good.

In Comparative Example 20, Cr is outside the lower limit of the appropriate range, x and (iii) are not satisfied, the increase in oxidation in Test 2 is greater than 1.30 mg / cm ² , and the carburization resistance and oxidation resistance are It is insufficient.

In Comparative Example 21, Si is outside the lower limit of the appropriate range, y is not satisfied, the increase in oxidation in Test 2 is greater than 1.30 mg / cm ² , and the carburization resistance and oxidation resistance are insufficient.

In Comparative Example 22, Mn is outside the upper limit of the appropriate range, x is not satisfied, the increase in oxidation in Test 2 is more than 1.30 mg / cm ² , and the carburization resistance and oxidation resistance are insufficient.

In Comparative Examples 23 to 31, the individual component compositions are within the appropriate range, but the formula (i) is outside the proper range and the component composition is out of that point, and Comparative Example 24 satisfies x and (iii). Comparative Examples 23, 25, 28 and 30 do not satisfy y and the formula (iii), Comparative Examples 26, 27, 29 and 31 do not satisfy the formula (iii), and the oxidation increase in Test 2 is 1. More than 30 mg / cm ² , and the carburization resistance and oxidation resistance are insufficient.

In Comparative Examples 32 and 33, the individual component compositions are within the proper range, but the formula (i) is slightly outside the proper range, the formula (iii) is not satisfied a little, and the oxidation increase in Test 2 is 1.30 mg. / Cm ² or less, but the amount of increase in oxidation in Test 3 is more than 2.50 mg / cm ² , and the protective property is insufficient in consideration of long-term use.

  In Comparative Examples 34 and 35, since the Si upper limit or the Mn lower limit of the component composition deviates, the amount of scale peeling is large and the protective property is insufficient.

In Comparative Examples 36 to 41, the individual component compositions and the formula (i) are within the appropriate ranges, satisfy the formulas x, y and (iii), and the oxidation increase in Test 2 is 1.30 mg / cm ² or less. The increase in oxidation in Test 3 is 2.50 mg / cm ² or less, but the formula (ii) is outside the proper range and the component composition is deviated at that point, and the scale peeling amount in Test 3 is 0.50 mg / cm. It is more than ² , and the protective property is insufficient in consideration of adhesion and long-term use of the scale formed in an atmosphere having carburizing properties.

As is clear from these, each component composition defined in the present invention has the component composition of the present invention satisfying the formulas (i) and (ii), and is formed in a carburizing atmosphere. The ratio x of the Cr ₂ O ₃ layer thickness to the scale thickness to be formed is 20% or more, the Si concentration y of the Si concentrated layer between the scale and the base material is 0.4% or more, and the formula (iii) In the present invention satisfying the above conditions, the amount of increase in oxidation in the continuous oxidation test at 850 ° C. for 200 hours in the atmosphere having carburizing properties is very small compared to the comparative example, and the carburizing resistance and oxidation resistance are excellent. , Carrying ability is very small in oxidation increase and scale peeling in continuous oxidation test at 850 ° C for 200 hours in a carburizing atmosphere and continuous oxidation test in air at 950 ° C for 200 hours. Formed in an atmosphere having It turns out that it is excellent in the protection property in consideration of the adhesion of the scale and long-term use.

  From the above, it is clear that the present invention has extremely excellent characteristics.

( 1 ) Carburizability including 1% by volume or more of carbon monoxide and hydrocarbon-based gas in total and 1% by volume or less of oxygen, and the total amount of carbon monoxide and hydrocarbon-based gas including at least twice the amount of oxygen. Or an automobile exhaust system member or a fuel cell high temperature member that may be exposed to a carburizing atmosphere containing 5% by volume or more of carbon dioxide and 1% by volume or less of carbon dioxide. ,
% By mass
C: 0.02% or less,
N: 0.02% or less,
Si: 0.05% or more, 3.0% or less,
Mn: 0.05% or more, 2.0% or less,
P: 0.04% or less,
S: 0.01% or less,
Cr: 12.0% or more, 25.0% or less,
Ni: 0.01% or more, 2.0% or less,
Ti: 0.001% or more, 0.40% or less,
Al: 0.002% or more, 0.25% or less,
V: 0.01% or more, 0.20% or less,
B: 0.0002% or more, 0.0050% or less,
And the balance is composed of Fe and inevitable impurities, and satisfies the following formulas (i) and (ii): water vapor is 10% by volume, carbon dioxide is 10% by volume, carbon monoxide is When heated to 800 ° C. in an atmosphere of 10% by volume and the rest being nitrogen and continued for 100 hours and then cooled to room temperature, the ratio x of the Cr ₂ O ₃ layer thickness to the thickness of the scale is on the surface of the stainless steel plate. Carburization resistance, characterized by forming a scale satisfying the following formula (iii): the Si concentration y of the Si concentrated layer between the scale and the base material is 20% or more and 0.4% or more Ferritic stainless steel sheet for automobile exhaust system or fuel reformer with excellent oxidation resistance.
Cr + 12Si-4Mn-14.5 ≧ 0 Formula (i)
aCr + bSi + 1.6Mn + 27Ti + 33Al + 13.5 ≧ 0 Formula (ii)
When Cr + Si ≧ 19.5, a = −0.8, b = −0.2
When Cr + Si <19.5 and Si ≦ 0.20, a = −1, b = 29
In the case of Cr + Si <19.5 and Si> 0.20, a = −1, b = −3.7
x + 10y / Si-50 ≧ 0 Formula (iii)
However, the element symbol in a formula means content (mass%) of the said element.
( 2 ) By mass%, Cu: 2.0% or less,
Mo: 2.0% or less,
Ferritic stainless steel for automobile exhaust system or fuel reformer with excellent carburization resistance and oxidation resistance as described in ( 1 ), characterized in that it contains one or more of Nb: 1.0% or less steel sheet.
( 3 ) In mass%, further Cr: less than 14.5%,
Nb: less than 0.35%,
Mo: less than 0.50%,
C + N: more than 0.020%,
Cu + 2Ni: more than 0.30%,
Ti: more than 0.20%,
Ferrite stainless steel for automobile exhaust system or fuel reformer excellent in carburization resistance and oxidation resistance in the carburizing atmosphere described in ( 1 ) or ( 2 ) satisfying one or more of steel sheet.
( 4 ) In mass%, further Ca: 0.0002% or more, 0.0030% or less,
Zr: 0.01% or more, 0.30% or less,
Y: 0.001% or more, 0.20% or less,
Hf: 0.001% or more, 1.0% or less,
REM: 0.001% or more, 0.20% or less,
1 or 2 or more types described above, ( 1 ) to ( 3 ) , the car exhaust system and the fuel reformer ferrite excellent in carburization resistance and oxidation resistance Stainless steel sheet.
( 5 ) In mass%, W: 0.01% or more, 5.0% or less,
Sn: 0.002% or more, 1.0% or less,
Mg: 0.0002% or more, 0.0030% or less,
Co: 0.01% or more, 0.30% or less,
Sb: 0.005% or more, 0.50% or less,
Bi: 0.001% or more, 1.0% or less,
Ta: 0.001% or more, 1.0% or less,
Ga: 0.0002% or more, 0.30% or less,
One or above, wherein the containing two or more (1) to (4) above automotive exhaust system or the fuel reformer ferrite having excellent carburization resistance and oxidation resistance according to any one of the Stainless steel sheet.

( 6 ) Oxidation when heated to 850 ° C. for 200 hours in an atmosphere of 10% by volume of water vapor, 10% by volume of carbon dioxide, 10% by volume of carbon monoxide, and the remainder of nitrogen, and then cooled to room temperature. Heating at 850 ° C. for 200 hours in an atmosphere where the increase is 1.30 mg / cm ² or less, water vapor is 10% by volume, carbon dioxide is 10% by volume, carbon monoxide is 10% by volume, and the remainder is nitrogen. When the ferritic stainless steel plate with scale after being continued is heated to 950 ° C. for 200 hours and then cooled to room temperature, the increase in oxidation is 2.50 mg / cm ² or less, and the amount of scale peeling is 0.50 mg / cm. above, wherein the ² or less (1) to (5) automotive exhaust system or the fuel reformer ferritic stainless steel excellent in carburization resistance and oxidation resistance according to any one of .

Furthermore, carburization is considered to be slow while the carbon monoxide or hydrocarbon gas in the atmosphere reacts with oxygen. Therefore, the smaller the amount of oxygen, the shorter the reaction time or frequency and the carburization. The possibility of occurrence is considered to be high. In the case of carburizing with carbon dioxide, oxygen is used for oxidation, and oxygen is deficient in a narrow range on the surface of the steel material, so carbon dioxide begins to be used for oxidation, and carburization is thought to occur accordingly. That is, it is considered that the smaller the amount of oxygen, the higher the possibility of oxidation and carburization by carbon dioxide. Therefore, the atmosphere having a carburizing property in the present invention includes carbon monoxide and hydrocarbon-based gas in a total amount of 1 vol% or more and oxygen of 1 vol% or less, and the total amount of carbon monoxide and hydrocarbon-based gas is oxygen It is desirable to interpret it as an atmosphere containing twice or more of the amount or an atmosphere containing 5% by volume or more of carbon dioxide and 1% by volume or less of oxygen.

Furthermore, the greater the difference between the total amount of carbon monoxide and hydrocarbon gas in the atmosphere and the amount of oxygen, the greater the reaction between carbon monoxide or hydrocarbon gas and oxygen and the carburization reaction with carbon monoxide or hydrocarbon gas. It is thought that it becomes easy to generate simultaneously in parallel. Further, it is considered that as the amount of carbon dioxide increases, oxidation by oxygen and oxidation by carbon dioxide are more likely to occur simultaneously in parallel. Therefore, the atmosphere having carburizing properties in the present invention includes carbon monoxide and hydrocarbon-based gas in a total amount of 2% by volume or more and oxygen of 1% by volume or less, and the total of carbon monoxide and hydrocarbon-based gas is oxygen. More preferably, it is interpreted as an atmosphere containing 5 times or more, or an atmosphere containing 10% by volume or more of carbon dioxide and 1% by volume or less of oxygen.

Claims (13)

% By mass
C: 0.02% or less,
N: 0.02% or less,
Si: 0.05% or more, 3.0% or less,
Mn: 0.05% or more, 2.0% or less,
P: 0.04% or less,
S: 0.01% or less,
Cr: 12.0% or more, 25.0% or less,
Ni: 0.01% or more, 2.0% or less,
Ti: 0.001% or more, 0.40% or less,
Al: 0.002% or more, 0.25% or less,
V: 0.01% or more, 0.20% or less,
B: 0.0002% or more, 0.0050% or less,
Of the thickness of the Cr ₂ O ₃ layer in the thickness of the scale on the surface of the stainless steel containing the composition, the balance being Fe and inevitable impurities, and satisfying the following formulas (i) and (ii) Carburization resistance, wherein x is 20% or more, the Si concentration y of the Si concentrated layer between the scale and the base material is 0.4% or more, and the scale satisfies the following formula (iii): And a ferritic stainless steel sheet for an automobile exhaust system or a fuel reformer that provides a carburizing atmosphere with excellent oxidation resistance.
Cr + 12Si-4Mn-14.5 ≧ 0 Formula (i)
aCr + bSi + 1.6Mn + 27Ti + 33Al + 13.5 ≧ 0 Formula (ii)
When Cr + Si ≧ 19.5, a = −0.8, b = −0.2
When Cr + Si <19.5 and Si ≦ 0.20, a = −1, b = 29
In the case of Cr + Si <19.5 and Si> 0.20, a = −1, b = −3.7
x + 10y / Si-50 ≧ 0 Formula (iii)
However, the element symbol in a formula means content (mass%) of the said element. % By mass, further Cu: 2.0% or less,
Mo: 2.0% or less,
Nb: 1.0% or less,
The ferritic stainless steel sheet for an automobile exhaust system or fuel reformer having a carburizing atmosphere excellent in carburizing resistance and oxidation resistance according to claim 1, characterized by containing one or more of . In mass%, further Cr: less than 14.5%,
Nb: less than 0.35%,
Mo: less than 0.50%,
C + N: more than 0.020%,
Cu + 2Ni: more than 0.30%,
Ti: more than 0.20%,
An automobile exhaust system or a fuel reformer that provides an atmosphere having carburization resistance and excellent carburization resistance in an atmosphere having carburization characteristics according to claim 1 or claim 2 satisfying one or more of the above. Ferritic stainless steel sheet for temperament. In mass%, further Ca: 0.0002% or more, 0.0030% or less,
Zr: 0.01% or more, 0.30% or less,
Y: 0.001% or more, 0.20% or less,
Hf: 0.001% or more, 1.0% or less,
REM: 0.001% or more, 0.20% or less,
4. An automobile exhaust system or fuel having an atmosphere having excellent carburization resistance and oxidation resistance according to any one of claims 1 to 3, characterized by containing at least one of Ferritic stainless steel sheet for reformers. In mass%, W: 0.01% or more, 5.0% or less,
Sn: 0.002% or more, 1.0% or less,
Mg: 0.0002% or more, 0.0030% or less,
Co: 0.01% or more, 0.30% or less,
Sb: 0.005% or more, 0.50% or less,
Bi: 0.001% or more, 1.0% or less,
Ta: 0.001% or more, 1.0% or less,
Ga: 0.0002% or more, 0.30% or less,
The vehicle exhaust system or fuel which becomes the atmosphere which has the carburizing property excellent in carburizing resistance and oxidation resistance according to any one of claims 1 to 4, Ferritic stainless steel sheet for reformers. The stainless steel sheet having the composition according to any one of claims 1 to 5, wherein the stainless steel sheet is 600 to 600 in an atmosphere containing one or more of carbon monoxide, carbon dioxide, and hydrocarbon gas and water vapor. By performing heat treatment in the range of 1000 ° C., the ratio x of the Cr ₂ O ₃ layer thickness to the thickness of the scale on the surface of the stainless steel plate is 20% or more, and Si of the Si concentrated layer between the scale and the base material An automobile exhaust system or fuel that has a carburizing atmosphere excellent in carburizing resistance and oxidation resistance, characterized by forming a scale that has a concentration y of 0.4% or more and that satisfies the following formula (iii): Manufacturing method of ferritic stainless steel sheet for reformer.
x + 10y / Si-50 ≧ 0 Formula (iii)
However, the element symbol in a formula means content (mass%) of the said element. % By mass
C: 0.02% or less,
N: 0.02% or less,
Si: 0.05% or more, 3.0% or less,
Mn: 0.05% or more, 2.0% or less,
P: 0.04% or less,
S: 0.01% or less,
Cr: 12.0% or more, 25.0% or less,
Ni: 0.01% or more, 2.0% or less,
Ti: 0.001% or more, 0.40% or less,
Al: 0.002% or more, 0.25% or less,
V: 0.01% or more, 0.20% or less,
B: 0.0002% or more, 0.0050% or less,
And the balance is composed of Fe and inevitable impurities, and satisfies the following formulas (i) and (ii): water vapor is 10% by volume, carbon dioxide is 10% by volume, carbon monoxide is When heated to 800 ° C. in an atmosphere of 10% by volume and the rest being nitrogen and continued for 100 hours and then cooled to room temperature, the ratio x of the Cr ₂ O ₃ layer thickness to the thickness of the scale is on the surface of the stainless steel plate. Carburization resistance, characterized by forming a scale satisfying the following formula (iii): the Si concentration y of the Si concentrated layer between the scale and the base material is 20% or more and 0.4% or more A ferritic stainless steel sheet for an automobile exhaust system or fuel reformer that provides a carburizing atmosphere with excellent oxidation resistance.
Cr + 12Si-4Mn-14.5 ≧ 0 Formula (i)
aCr + bSi + 1.6Mn + 27Ti + 33Al + 13.5 ≧ 0 Formula (ii)
When Cr + Si ≧ 19.5, a = −0.8, b = −0.2
When Cr + Si <19.5 and Si ≦ 0.20, a = −1, b = 29
In the case of Cr + Si <19.5 and Si> 0.20, a = −1, b = −3.7
x + 10y / Si-50 ≧ 0 Formula (iii)
However, the element symbol in a formula means content (mass%) of the said element. % By mass, further Cu: 2.0% or less,
Mo: 2.0% or less,
Nb: 1.0% or less, 1 type or 2 types or more, The car exhaust system which becomes the atmosphere which has the carburizing property which was excellent in carburizing resistance and oxidation resistance of claim 7 characterized by the above-mentioned Ferritic stainless steel sheet for fuel reformers. In mass%, further Cr: less than 14.5%,
Nb: less than 0.35%,
Mo: less than 0.50%,
C + N: more than 0.020%,
Cu + 2Ni: more than 0.30%,
Ti: more than 0.20%,
9. A ferritic stainless steel sheet for an automobile exhaust system or fuel reformer, which satisfies one or more of the above, and is an atmosphere having carburizing properties excellent in carburizing resistance and oxidation resistance according to claim 7 or claim 8. In mass%, further Ca: 0.0002% or more, 0.0030% or less,
Zr: 0.01% or more, 0.30% or less,
Y: 0.001% or more, 0.20% or less,
Hf: 0.001% or more, 1.0% or less,
REM: 0.001% or more, 0.20% or less,
10. An automobile exhaust system or fuel that is an atmosphere having a carburizing resistance excellent in carburizing resistance and oxidation resistance according to any one of claims 7 to 9, characterized by containing at least one of Ferritic stainless steel sheet for reformers. In mass%, W: 0.01% or more, 5.0% or less,
Sn: 0.002% or more, 1.0% or less,
Mg: 0.0002% or more, 0.0030% or less,
Co: 0.01% or more, 0.30% or less,
Sb: 0.005% or more, 0.50% or less,
Bi: 0.001% or more, 1.0% or less,
Ta: 0.001% or more, 1.0% or less,
Ga: 0.0002% or more, 0.30% or less,
11. An automobile exhaust system or a fuel having an atmosphere having excellent carburization resistance and oxidation resistance according to any one of claims 7 to 10, characterized by containing at least one of Ferritic stainless steel sheet for reformers. Heating to 850 ° C. in an atmosphere of 10% by volume of water vapor, 10% by volume of carbon dioxide, 10% by volume of carbon monoxide, and the remainder being nitrogen and continuing for 200 hours followed by an increase in oxidation when cooled to room temperature is 1 After heating to 850 ° C. for 200 hours in an atmosphere of 30 mg / cm ² or less, 10% by volume of water vapor, 10% by volume of carbon dioxide, 10% by volume of carbon monoxide, and the rest being nitrogen the ferritic stainless steel sheet with scales oxidation weight gain when cooled to room temperature after heating continued to 200 hours to the atmosphere 950 ° C. is 2.50 mg / cm ² or less, peeling of scale weight of 0.50 mg / cm ² in the following 12. An automobile exhaust system or a fuel reformer ferrule having an atmosphere having excellent carburization resistance and oxidation resistance according to any one of claims 1 to 5 and 7 to 11. System stainless steel plate.   A carburizing atmosphere having a composition in which carbon monoxide or hydrocarbon-based gas remains in an amount of about 1% by volume or more even if carbon monoxide or hydrocarbon-based gas contained in the atmosphere reacts with all oxygen contained in the atmosphere. Alternatively, it is used as an automobile exhaust system member or a fuel cell high-temperature member that may be exposed to a carburizing atmosphere that contains carbon dioxide having a volume of oxygen of 1 volume% or less and 10 or more times that of oxygen. The ferritic stainless steel sheet for an automobile exhaust system or a fuel reformer having an atmosphere having a carburizing property excellent in carburizing resistance and oxidation resistance according to any one of claims 1 to 5 and 7 to 12.

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