JP2009120894A - Ferritic stainless steel material for automotive member of exhaust gas path - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ferritic stainless steel material for an automotive member of an exhaust gas path, which can be used in a high-temperature range exceeding 900°C or further exceeding 950°C. <P>SOLUTION: The ferritic stainless steel material for the automotive member of an exhaust gas path superior in heat resistance and low-temperature toughness has a composition including, by mass%, 0.03% or less C, 1% or less Si, 0.6 to 2% Mn, 3% or less Ni, 10 to 25% Cr, 0.3 to 0.7% Nb, more than 1 to 2% Cu, 1 to 2.5% Mo, 1 to 2.5% W, 0.15% or less Al, 0.03 to 0.2% V, 0.03% or less N, further B, Co, W, Ti, Zr, an REM and Ca, as needed, and the balance Fe with unavoidable impurities, while satisfying the restrictive expressions of 1.2Nb+5Mo+6Cu≥11.5 and 15Nb+2Mo+0.5Cu≥10.5. The steel material has a texture in which the total amount of Nb and Mo existing as a precipitation phase is 0.2 mass% or less. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention is a ferritic stainless steel material used for automobile exhaust gas path members, and is particularly suitable for exhaust gas path upstream members whose material temperature exceeds 900 ° C. or even 950 ° C., such as exhaust manifolds, catalytic converters, front pipes, etc. The present invention relates to a ferritic stainless steel material for an automobile exhaust gas passage member having excellent heat resistance and low temperature toughness.

  2. Description of the Related Art Conventionally, two types of ferritic steel types are widely used for exhaust gas path members of automobiles according to the operating temperature range. One is a steel type typified by SUS429 steel, which is mainly applied to members having a maximum material temperature of 750 ° C, and the other is SUS444, which is mainly applied to members having a material maximum temperature of 850 ° C. It is a steel type represented by steel.

  Recently, in order to comply with exhaust gas regulations, fuel efficiency regulations, etc., the exhaust gas temperature tends to increase, and in the future, material requirements for the upstream members of the exhaust gas path are assumed to rise to about 1000 ° C. Is expected to increase. Conventional SUS444 series steel (18Cr-2Mo-0.5Nb series) cannot be applied to members exposed to such high temperatures. In order to withstand the use at such a high temperature, it is not sufficient that the tensile strength at a high temperature is simply high, and the material has a high 0.2% proof stress as an index of stress at which the material starts plastic deformation. Is important.

  In addition, with the increase of various devices mounted in the engine room, the exhaust gas member storage space is more restricted than in the past. For this reason, the exhaust gas path upstream member is required to have excellent workability that can be molded into various shapes. In particular, not only plates but also pipes have been required to have excellent workability that can withstand severe processing into complex shapes. Further, the exhaust gas path member needs to have good low temperature toughness.

Various ferritic stainless steels having improved heat resistance as described below have been developed and put into practical use.
Patent Document 1 discloses a ferritic stainless steel in which the amount of solid solution Nb is sufficiently secured to withstand use in a temperature range exceeding 900 ° C., and the composition and structure are adjusted so that the tensile strength at 950 ° C. satisfies 20 MPa. Steel is shown. However, there is no description of 0.2% proof stress, and the durability when the material temperature actually rises to about 1000 ° C. is not certain. There is no special consideration for thermal fatigue properties and low temperature toughness.

  Patent Document 2 discloses a ferritic stainless steel having excellent high temperature strength at 900 ° C. and excellent low temperature toughness. However, there is no description of 0.2% proof stress, and it cannot be said that the measures for ensuring sufficient durability when the material temperature actually rises to about 1000 ° C. are necessarily sufficient.

  Patent Document 3 describes a ferritic stainless steel having high high-temperature strength at 950 ° C. and good workability. However, 0.2% yield strength is not shown, and it is unclear whether the material can withstand when it is actually exposed to about 1000 ° C. There is no special consideration for low temperature toughness.

  Patent Document 4 discloses an Fe—Cr alloy whose thermal expansion coefficient is reduced. However, it is not intended to improve the high-temperature strength in the temperature range of about 1000 ° C.

  Patent Document 5 describes a ferritic stainless steel having excellent thermal fatigue characteristics and good low-temperature toughness. However, the high temperature strength is evaluated with a 0.2% proof stress at 600 ° C., and the durability when the material temperature actually rises to about 1000 ° C. is not clear.

  Patent Document 6 discloses ferritic stainless steel used for an exhaust gas system member used at a temperature of 700 ° C. or higher. However, only the tensile strength at 600 ° C. and 850 ° C. is shown for the high temperature strength, and it is unclear whether the material can withstand when it is actually exposed to temperatures of about 1000 ° C. Moreover, there is no description about low temperature toughness.

Japanese Patent No. 2959934 Japanese Patent No. 2696584 Japanese Patent No. 3468156 JP 2005-206944 A JP 2006-117985 A JP 2000-303149 A

A method for stably realizing a material that exhibits excellent durability when used at a temperature exceeding 900 ° C. and also has good low-temperature toughness and workability has not been established yet (see the above-mentioned patent document). ).
The present invention is a material having a high level of 0.2% yield strength, thermal fatigue properties, low temperature toughness, and workability at a high temperature of 1000 ° C., and the material temperature actually exceeds 900 ° C. It is another object of the present invention to provide a ferritic stainless steel material for automobile exhaust gas path members that exhibits excellent durability even when used in applications reaching a high temperature range exceeding 950 ° C.

The purpose is mass%, C: 0.03% or less, Si: 1% or less, Mn: 0.6-2%, Ni: 3% or less, Cr: 10-25%, Nb: 0.3- 0.7%, Cu: more than 1 to 2%, Mo: 1 to 2.5%, W: 1 to 2.5%, Al: 0.15% or less, V: 0.03 to 0.2%, N: 0.03% or less, optionally containing at least one of Ti and Zr within a total range of less than 1%, or B: 0.02% or less, Co: 1 of 2% or less Contains at least one species, or further contains at least one type of REM (rare earth element) and Ca in a total range of 0.1% or less, the balance being Fe and inevitable impurities, and the following formula (1) and (2 ) Automotive exhaust gas route having a composition satisfying the formula and having a structure in which the total amount of Nb and Mo existing as a precipitated phase is 0.2% by mass or less and excellent in heat resistance and low temperature toughness This is achieved by using a ferritic stainless steel material.
1.2 Nb + 5Mo + 6Cu ≧ 11.5 (1)
15Nb + 2Mo + 0.5Cu ≧ 10.5 (2)

Here, the value of the content of the element expressed in mass% is substituted for the element symbol in the above formulas (1) and (2).
“Ferrite stainless steel material for automobile exhaust gas path member” means final annealing (hereinafter simply referred to as “final annealing”) that is heated to a temperature exceeding 1000 ° C. (for example, 1050 to 1100 ° C.) in the process of manufacturing the automobile exhaust gas path member. Steel) after finishing. For example, when a steel plate is welded and formed into a pipe, and then molded, and then subjected to final annealing, the pipe after the final annealing corresponds to the ferritic stainless steel material for an automobile exhaust gas path member here. When the final annealing is performed at the stage of the steel plate, the steel plate after the final annealing, and the pipes and cylinders obtained by subsequent processing correspond to the ferritic stainless steel material for the automobile exhaust gas path member.
Among the steel materials, those used for exhaust gas members whose material temperature is in the temperature range exceeding 900 ° C. or in the temperature range exceeding 950 ° C. are particularly suitable.

  According to the present invention, for an automobile exhaust gas path member having a high temperature strength that can withstand when exposed to a high temperature of 1000 ° C., good thermal fatigue properties, good workability, and good low temperature toughness at the same time. Ferritic stainless steel materials are now available. This material can cope with the recent rising trend of the exhaust gas temperature, and also increases the degree of design freedom of the exhaust gas path upstream member.

  In the present invention, it is important to improve the high temperature strength (0.2% yield strength) at the 1000 ° C. level while maintaining the high temperature strength (0.2% yield strength) at the 600 ° C. level high. Exhibiting high strength over both these temperature ranges is extremely effective for maintaining high thermal fatigue properties. As a result of various studies, the 0.2% proof stress at 600 ° C and the 0.2% proof stress at 1000 ° C are both higher than the proof value corresponding to 1.5 times the proof value of SUS444 steel at the same temperature. A high level is desirable. Specifically, the 0.2% yield strength at 600 ° C. is preferably 200 MPa or more, and the 0.2% yield strength at 1000 ° C. is preferably 15 MPa or more. It has been found that a material having such high temperature strength characteristics has practically sufficient high temperature fatigue characteristics when subjected to repeated temperature fluctuations between room temperature and about 1000 ° C. as an automobile exhaust gas path member.

In the present invention, Cu is used in order to improve the high temperature strength in a temperature range including 600 ° C. (approximately 500 to 800 ° C.). That is, by adding Cu, an ε-Cu phase is precipitated in a temperature range of about 600 ° C., and when this is finely dispersed in the matrix, a precipitation strengthening phenomenon is expressed. In order to maintain the high temperature strength (0.2% proof stress) in this temperature range about 1.5 times higher than that of SUS444 steel, in addition to precipitation of ε-Cu phase, solid solution of Nb and Mo It is necessary to take advantage of enhancements. As a result of various studies, by adjusting the components so that the contents of Nb, Mo, and Cu satisfy the formula (1), the high-temperature strength in the region of 800 ° C. or lower is increased to about 1.5 times or more of SUS444 steel. It becomes possible.
1.2 Nb + 5Mo + 6Cu ≧ 11.5 (1)

If it becomes a temperature range exceeding 800 degreeC, solid solution of an epsilon-Cu phase will advance and the improvement effect of the high temperature intensity | strength by Cu will become weak. In order to increase the high-temperature strength at 1000 ° C. (0.2% yield strength) to about 1.5 times or more that of SUS444 steel, it is important to fully utilize the solid solution strengthening of Nb and Mo. Since solute Cu is also effective in improving high-temperature strength, it is also used. As a result of various studies, it has been found that it is necessary to adjust the components so as to satisfy the expression (2).
15Nb + 2Mo + 0.5Cu ≧ 10.5 (2)
The coefficient of Nb in the formula (2) corresponds to an increase in 0.2% proof stress (MPa) at 1000 ° C. per 0.1% by mass of Nb, and the coefficients of Mo and Cu are Mo and Cu: 1 mass, respectively. This corresponds to an increase in 0.2% yield strength (MPa) at 1000 ° C. per%.

However, in order to increase the 0.2% yield strength at a high temperature of 1000 ° C. to about 1.5 times or more that of SUS444 steel, it is not sufficient to have a composition that satisfies the above formula (2). According to a detailed examination, it has been found that it is extremely important to obtain a metal structure with as few Nb and Mo precipitates as possible. Specifically, in the state after the final annealing, the structure state must be such that the total amount of Nb and Mo existing as a precipitation phase is 0.2% by mass or less.
In order to maintain not only high temperature strength but also workability, low temperature toughness, and weldability, it is extremely effective to obtain the above-described microstructure after the final annealing. When the amount of Nb or Mo added is considerably large, even if the total amount of Nb and Mo existing as a precipitation phase exceeds 0.2% by mass, the amount of solid solution Mo or solid solution Nb is sufficiently secured, and The high temperature strength at 1000 ° C. may be improved by solid solution strengthening. However, in this case, it is difficult to simultaneously improve the low temperature toughness and workability.

  “Total amount (% by mass) of Nb and Mo existing as a precipitated phase” is a quantitative analysis of elements of the residue of the precipitated phase extracted by constant potential electrolysis (SPEED method) in a non-aqueous solvent electrolyte. The total mass of Nb and Mo contained therein is divided by the total mass of the matrix dissolved by electrolysis and the extracted precipitated phase, and can be obtained by percentage display.

  In addition, in order to obtain a structure state in which the total amount of Nb and Mo existing as a precipitation phase is 0.2% by mass or less, a cooling rate from 1050 ° C. to 500 ° C. is set to 5 ° C./in the cooling process during final annealing. It is necessary to control over sec. For example, when pipes welded and piped are applied to automobile exhaust gas path members, soaking at 1050 to 1100 ° C. at least once in the stage of the steel plate before pipe making or the stage after pipe forming until it is used as a member. After heating for 0 to 10 minutes, final annealing may be performed so that the cooling rate from 1050 ° C. to 500 ° C. is 5 ° C./sec or more. In addition, once such a structural state is obtained before being used as an automobile exhaust gas route member, Nb and Mo are then used when heated and used at a temperature of about 1000 ° C. as an automobile exhaust gas route member. The precipitate phase does not occur more than necessary, and the high-temperature strength and low-temperature toughness are not impeded practically.

Hereinafter, the alloy components will be described.
C and N are generally effective elements for improving high-temperature strength such as creep strength, but if contained excessively, oxidation characteristics, workability, low-temperature toughness, and weldability deteriorate. In the present invention, both C and N are limited to 0.03 mass% or less.

  Si is effective in improving high-temperature oxidation characteristics, but if added excessively, hardness increases and workability and low-temperature toughness decrease. In the present invention, the Si content is limited to 1.0% by mass or less.

  Mn improves high temperature oxidation properties, particularly scale peel resistance. In order to sufficiently secure the high-temperature oxidation characteristics at the 1000 ° C. level, it is necessary to ensure a Mn content of 0.6% by mass or more. However, excessive addition hinders workability and weldability. Further, since Mn is an austenite stabilizing element, if added in a large amount, a martensite phase is likely to be generated, which causes a decrease in thermal fatigue characteristics and workability. Accordingly, the Mn content must be suppressed to a range of 2% by mass or less, and more preferably 1.5% by mass or less or less than 1.5% by mass.

  Ni contributes to the improvement of low temperature toughness, but excessive inclusion causes a decrease in elongation at room temperature. In the present invention, the Ni content is allowed up to 3% by mass, but it is more preferable to contain it in the range of 0.6% by mass or less.

  Cr stabilizes the ferrite phase and contributes to the improvement of oxidation resistance, which is important for high temperature materials. In order to fully exhibit its action, the present invention secures a Cr content of 15% by mass or more. However, excessive Cr content causes embrittlement and workability deterioration of the steel material, so the Cr content is 20% by mass.

  Nb also has the effect of increasing the high-temperature strength in the temperature range around 600 ° C. by solid solution strengthening, but in the present invention, Nb has a solid solution strengthening effect mainly in order to ensure high-temperature strength in the high temperature range exceeding 900 ° C. Utilize. For that purpose, it is necessary to ensure the Nb content of 0.3% by mass or more and to satisfy the above-mentioned formula (2). In addition, as described above, in the present invention, it is necessary to have a structural state in which the total amount of Nb and Mo existing as a precipitation phase is 0.2% by mass or less, but Nb has a strong affinity for C and N, and has a high temperature strength. It is easy to form precipitates that cause low temperature toughness, workability, and the like. For this reason, Nb content is restrict | limited to the range below 0.7 mass%.

  Cu is an important element in the present invention. That is, in the present invention, as described above, the strength at around 600 ° C. (approximately 500 to 850 ° C.) is increased by utilizing the fine dispersion precipitation phenomenon of the ε-Cu phase, and the thermal fatigue characteristics are improved. Moreover, in the high temperature range over 850 degreeC, it has the role which assists the improvement effect of the high temperature strength by Nb and Mo using the solid solution strengthening of Cu. As a result of various studies, in order to sufficiently bring out these effects, it is necessary to contain at least 1% by mass of Cu. However, since excessive Cu content reduces workability, low temperature toughness, and weldability, the upper limit of Cu content is limited to 2% by mass.

Mo, like Nb, has the effect of increasing the high temperature strength by solid solution strengthening. In particular, in the present invention, since it is necessary to increase the high temperature strength in a high temperature region exceeding 900 ° C., it is essential to add 1% by mass or more of Mo. Further, it is necessary to tissue condition total is not more than 0.2 mass% of Nb and Mo existing as a precipitation phase in the present invention as described above, the excess addition of Mo carbide and Laves phase (Fe ₂ Mo ) To inhibit high temperature strength and low temperature toughness. For this reason, Mo content is restrict | limited to the range of 2.5 mass% or less.

  W is an element effective for increasing the high-temperature strength in a high-temperature region exceeding 900 ° C. In the present invention, it is necessary to secure a W content of 1% by mass or more. However, since excessive W content becomes a factor that hinders workability at normal temperature, the W content needs to be 2.5% by mass or less, and more preferably 2% by mass or less.

  Al is a deoxidizer and improves high-temperature oxidation resistance. However, when Al is contained in a large amount, the surface properties, workability, weldability, and low temperature toughness are adversely affected. For this reason, Al is added in the range of 0.15% by mass or less.

  V contributes to the improvement of the high-temperature strength by the combined addition with Nb and Cu. Further, coexistence with Nb improves workability, low temperature toughness, intergranular corrosion resistance, and toughness of the heat affected zone. In order to obtain these effects sufficiently, in the present invention, V is contained in an amount of 0.03 mass% or more. However, excessive addition of V becomes a factor that causes a decrease in workability and low-temperature toughness. For this reason, V content is restrict | limited to the range below 0.2 mass%.

  Ti and Zr are effective elements for improving the high-temperature strength, and one or more of them can be added as necessary. However, since excessive addition reduces toughness, when adding one or more of Ti and Zr, the total content thereof is set to be in a range of 1% by mass or less.

  B and Co are elements that contribute to low temperature toughness like Ni, and one or two of B and Co can be added as necessary. However, excessive addition causes a decrease in elongation at room temperature, so the B content is in the range of 0.02% by mass or less and the Co content is in the range of 2% by mass or less. It is more effective to secure B in a content of 0.0005 to 0.02% by mass.

  REM (rare earth element) and Ca are elements that contribute to the improvement of high-temperature oxidation resistance, and one or more of them can be added as necessary. It is more effective to set the total content of REM and Ca to 0.001% by mass or more. However, excessive addition adversely affects manufacturability, so the total content of REM and Ca is set to be in the range of 0.1% by mass or less.

  The stainless steel material of the present invention is obtained by melting a stainless steel adjusted to the above composition by a usual method, and then converting it into a steel plate having a predetermined plate thickness by a general stainless steel plate manufacturing process, and then performing welding pipe forming and forming processing. It is manufactured by subjecting it to such processes. In that case, in the final annealing which heats to 1050-1100 degreeC, it is important to control the cooling rate from 1050 degreeC to 500 degreeC to 5 degree-C / sec or more as mentioned above. If this cooling condition is not satisfied, it becomes difficult to obtain a structural state in which the total amount of Nb and Mo existing as a precipitation phase is 0.2 mass% or less, and the high temperature strength at 1000 ° C. (0.2% proof stress) is less than that of SUS444. It becomes difficult to improve the level stably to about 1.5 times or more. Moreover, it becomes a factor which causes the fall of low-temperature toughness.

  A ferritic stainless steel shown in Table 1 was melted, and a cold-rolled annealed steel sheet having a thickness of 2 mm was prepared through the steps of hot rolling, hot-rolled sheet annealing, cold rolling, and finish annealing. Said finish annealing was performed on the conditions which simulated the final annealing as a steel material for exhaust gas path members. As for the final annealing conditions, after heating at 1050 ° C. × soaking for 1 minute, except for some comparative examples (No. 21), the average cooling rate from 1000 ° C. to 500 ° C. is 5 ° C./sec or more. Cooled down. The cooling rate was measured using a thermocouple attached to the sample surface. Various characteristics of the steel material for exhaust gas passage members were investigated using the cold-rolled annealed steel plate obtained after such final annealing as a test material.

  For the test material (after final annealing), the total amount of Nb and Mo existing as a precipitation phase (herein referred to as “precipitation Nb + precipitation Mo amount”), 0.2% proof stress at 600 ° C., 1000 ° C. The 0.2% proof stress, the low temperature toughness, and the workability at room temperature were examined as follows.

[Precipitation Nb + Precipitation Mo amount]
By the SPEED method, constant potential electrolysis was performed at a potential at which the matrix of the test material was dissolved and the precipitated phase was not dissolved, and the elements of the extracted precipitated phase residue were quantitatively analyzed. The total mass of Nb and Mo contained in the residue was divided by the total mass of the matrix dissolved by electrolysis and the extracted precipitated phase, and expressed as a percentage to determine the amount of precipitated Nb + precipitated Mo. In the SPEED method, 10% acetylacetone + 1% tetramethylammonium chloride + methyl alcohol solution was used as a nonaqueous solvent.

[0.2% yield strength at 600 ° C and 1000 ° C]
A tensile test at 600 ° C. and a tensile test at 1000 ° C. were performed in accordance with JIS G0567 using a tensile test piece having a thickness of 2 mm (the tensile direction coincided with the rolling direction). The 0.2% proof stress at 600 ° C. was evaluated as acceptable when the pressure was 200 MPa or higher, which is about 1.5 times that of SUS444 steel, and rejected when lower than that. The 0.2% proof stress at 1000 ° C. was evaluated as passing a 35 MPa or more equivalent to about 1.5 times that of SUS444 steel, and rejecting a lower one than that.

(Low temperature toughness)
A V-notch Charpy impact test piece (having a hammer striking direction parallel to the rolling direction) was prepared from a specimen having a thickness of 2 mm, and 25 ° C. in the range of −75 ° C. to 25 ° C. in accordance with JIS Z2242. A Charpy impact test was performed at the pitch to determine the ductile toughness transition temperature. Those having a transition temperature of −25 ° C. or lower were evaluated as “◯” (low temperature toughness: good), and those having a transition temperature higher than that were evaluated as “x” (low temperature toughness; poor).

[Processability at room temperature]
Three types of tensile test pieces (JIS 13B) having a tensile direction of 0 °, 45 °, and 90 ° with respect to the rolling direction were prepared from a specimen having a plate thickness of 2 mm, and the number of tests in accordance with JIS 2241, respectively. subjected to tensile tests at n = 3 to rupture, against the test piece after breaking is measured elongation at break (%), an average elongation value EL _a by the following equation (3), the subject of this EL _a The room temperature elongation value of the sample was used.
EL _A = (EL _L + EL _D + EL _T ) (3)
Here, EL _L is the elongation at break when the tensile direction is 0 ° (average value of n = 3), EL _D is the elongation at break when the tensile direction is 45 ° (average value of n = 3), and EL _T is Elongation at break when the tensile direction is 90 ° (average value of n = 3). Those having EL _A of 30% or more were evaluated as ◯ (workability at normal temperature; good), and those lower than that were evaluated as x (workability at normal temperature; poor).
The results are shown in Table 2. “Final annealing cooling rate” in Table 2 is an average cooling rate from 1050 ° C. to 500 ° C.

  As can be seen from Table 2, the steel materials of the examples of the present invention in which the composition and the amount of precipitated Nb + precipitated Mo satisfy the provisions of the present invention are 0.2% proof stress at 600 ° C and 0.2% proof stress at 1000 ° C. It is about 1.5 times higher than steel, exhibits excellent high-temperature strength in a high-temperature region exceeding 850 ° C., and has a sufficiently good thermal fatigue property. Moreover, low temperature toughness and processability at normal temperature are also good.

  On the other hand, in No. 21, although the steel composition was within the range specified in the present invention, the cooling rate from 1000 ° C. to 500 ° C. during the final annealing was slower than 5 ° C./sec. , A large amount of precipitates of Mo is generated, and the amount of precipitated Nb + precipitated Mo is too much. In this case, the high temperature strength at 1000 ° C., the low temperature toughness, and the workability at room temperature were inferior. No. 22 has a small content of Mo and Nb, and No. 23 has a low content of Cu. Since these do not satisfy the formulas (1) and (2), the temperatures are high at 600 ° C. and 1000 ° C. The strength was low. No. 24 was inferior in workability at room temperature because the W content was too high. No. 25 had a low Cu content and did not satisfy the formula (1), so the high-temperature strength at 600 ° C. was low. In No. 26, the Cu content was too high to satisfy the formula (2), and because W was not added, the high-temperature strength at 1000 ° C. was low. In No. 27, since the Mo content was too high, the Nb + precipitated Mo amount was too much, and the Nb content was too low to satisfy the formula (2). High temperature strength at 1000 ° C. and low temperature Poor toughness. Since No. 28 had a high Nb content, it was in a structural state in which the amount of precipitated Nb + precipitated Mo was too much, and the processability at room temperature was poor. No. 29 had a large amount of Mo and Nb, so that the amount of precipitated Nb + precipitated Mo was too much, and the low sound toughness was inferior. Further, since the Cu content was small, the high temperature strength at 600 ° C. was low. In No. 30, since the Mo content was too high, the amount of precipitated Nb + precipitated Mo was too much, but the high-temperature strength at 1000 ° C. was improved by solid solution Mo. However, it was inferior in low temperature toughness and workability at room temperature.

Claims (5)

In mass%, C: 0.03% or less, Si: 1% or less, Mn: 0.6-2%, Ni: 3% or less, Cr: 10-25%, Nb: 0.3-0.7% Cu: more than 1 to 2%, Mo: 1 to 2.5%, W: 1 to 2.5%, Al: 0.15% or less, V: 0.03 to 0.2%, N: 0.3. A structure having a composition satisfying the following formulas (1) and (2), and the total amount of Nb and Mo existing as a precipitated phase is 0.2% by mass or less: 03% or less, balance Fe and inevitable impurities A ferritic stainless steel material for automobile exhaust gas passage members having excellent heat resistance and low temperature toughness.
1.2 Nb + 5Mo + 6Cu ≧ 11.5 (1)
15Nb + 2Mo + 0.5Cu ≧ 10.5 (2)   Furthermore, the steel material of Claim 1 which has a composition which contains 1 or more types of Ti and Zr in the range less than 1% in total.   Furthermore, the steel material of Claim 1 or 2 which has a composition containing 1 or more types of B: 0.02% or less and Co: 2% or less.   Furthermore, the steel materials in any one of Claims 1-3 which have a composition which contains 1 or more types of REM (rare earth element) and Ca in the range of a total 0.1% or less.   The steel material in any one of Claims 1-4 used for the exhaust gas member used as the temperature range whose material temperature exceeds 900 degreeC.