JP5152387B2 - Ferritic stainless steel with excellent heat resistance and workability - Google Patents

Description

  The present invention has high heat resistance (preferably used for exhaust system members used in high temperature environments such as exhaust pipes of automobiles and motorcycles, catalyst outer cylinder materials (also referred to as converter cases) and exhaust ducts of thermal power plants. The present invention relates to a ferritic stainless steel having both thermal fatigue properties, oxidation resistance, and high temperature fatigue properties) and workability.

  Exhaust manifolds, exhaust pipes, converter cases, mufflers and other exhaust system components used in automobile exhaust system environments have thermal fatigue characteristics, high temperature fatigue characteristics, and oxidation resistance (hereinafter collectively referred to as “heat resistance”). It is required to be excellent. Exhaust manifolds, etc. are heated and cooled by repeatedly starting and stopping the engine, but they are constrained in relation to surrounding parts, so the thermal expansion and contraction of the material itself is limited and thermal distortion occurs. . The fatigue phenomenon resulting from this thermal strain is thermal fatigue. On the other hand, while the engine is starting, it continues to receive vibration in a heated state. The fatigue phenomenon resulting from the accumulation of strain due to this vibration is high temperature fatigue. The former is low cycle fatigue and the latter is high cycle fatigue, which are completely different fatigue phenomena.

  In applications where such heat resistance is required, Cr-containing steels such as Cr-containing steels with a composite addition of Nb and Si (for example, JFE429EX 14Cr-0.9Si-0.4Nb steel) are currently widely used. Yes. However, when the exhaust gas temperature rises to a temperature exceeding 900 ° C. as the engine performance improves, the Nb—Si composite added steel has insufficient thermal fatigue characteristics.

  For this problem, Cr-containing steel with improved high-temperature proof stress by adding Nb and Mo, SUS444 (19Cr-0.5Nb-2Mo) specified in JIS G4305, the content of Cr is lowered, Ferritic stainless steel to which Nb, Mo, W is added has been developed (for example, see Patent Document 1). However, due to the recent abnormal rise of rare metal raw materials such as Mo and W, development of materials having equivalent heat resistance using inexpensive raw materials has been required.

  As materials excellent in heat resistance without using expensive elements such as Mo and W, for example, those disclosed in Patent Documents 2 to 4 are known. Patent Document 2 discloses an automobile exhaust gas flow channel in which Nb: 0.50 mass% or less, Cu: 0.8-2.0 mass%, and V: 0.03-0.20 mass% are added to 10-20 mass% Cr steel. Ferritic stainless steel for members is disclosed. In Patent Document 3, 10-20 mass% Cr steel, Ti: 0.05-0.30 mass%, Nb: 0.10-0.60 mass%, Cu: 0.8-2.0 mass%, B: 0 Ferritic stainless steels having excellent thermal fatigue properties with addition of .0005 to 0.02 mass% are disclosed. Patent Document 4 discloses a ferritic stainless steel for automobile exhaust system parts in which Cu: 1 to 3 mass% is added to 15 to 25 mass% Cr steel. All of the steels disclosed therein are characterized in that the thermal fatigue properties are improved by adding Cu.

JP 2004-018921 A International Publication 2003/004714 Pamphlet JP 2006-117985 A JP 2000-297355 A

  However, according to the researches of the inventors, when Cu is added as in the techniques disclosed in Patent Documents 2 to 4, the thermal fatigue characteristics are improved, but the oxidation resistance of the steel itself is decreased. On the whole, however, it has become clear that the heat resistance deteriorates.

  In addition, with the reduction in the weight of automobile bodies, the space that the exhaust manifold can occupy in the engine space has become smaller, so that the exhaust manifold is required to be processed into a complicated shape.

  The present invention has been made in view of such circumstances, and prevents heat resistance (oxidation resistance, thermal fatigue) without adding an expensive element such as Mo or W while preventing a decrease in oxidation resistance due to Cu. It is an object of the present invention to provide a ferritic stainless steel having excellent properties and high temperature fatigue properties) and workability.

  In the present invention, “excellent in heat resistance” means that the oxidation resistance, thermal fatigue characteristics and high temperature fatigue characteristics are equal to or higher than those of SUS444. Specifically, the oxidation resistance at 950 ° C. is equivalent to or better than SUS444 for oxidation resistance, and the thermal fatigue characteristic between 100-850 ° C. is more than equivalent to SUS444 for thermal fatigue characteristics. The high temperature fatigue property means that the high temperature fatigue property at 850 ° C. is equal to or higher than that of SUS444. In the present invention, “excellent in workability” means that the average elongation in three directions at room temperature is 36% or more.

  The inventors have developed a ferritic stainless steel that has both oxidation resistance and thermal fatigue characteristics without adding any expensive elements such as Mo and W, preventing the decrease in oxidation resistance due to Cu, which is the conventional technology. We studied as hard as possible. As a result, by combining Nb in the range of 0.3 to 0.65 mass% and Cu in the range of 1.0 to 2.5 mass%, high high-temperature strength can be obtained in a wide temperature range, and thermal fatigue can be obtained. It is possible to prevent the deterioration of the oxidation resistance due to the improvement of the characteristics and the inclusion of Cu by adding an appropriate amount of Al (0.2 to 1.0 mass%), and thus Nb, It was found for the first time by controlling Cu and Al within the proper range that heat resistance (thermal fatigue characteristics, oxidation resistance) equal to or higher than that of SUS444 can be obtained without adding Mo or W. Further, as a result of intensive investigations on means for improving oxidation resistance in an environment containing water vapor as expected when actually used as an exhaust manifold or the like, the amount of Si is optimized (0.4 to 1.. 0 mass%), it was found that the oxidation resistance in a steam atmosphere (hereinafter referred to as “steam oxidation resistance”) is also equal to or higher than that of SUS444.

  Further, in an exhaust system member of an automobile such as an exhaust manifold, characteristics against fatigue due to vibration during use are also important. Accordingly, the inventors diligently studied the means for improving the high temperature fatigue characteristics, and found that, by optimizing the balance between the Si content and the Al content (Si ≧ Al), the high temperature fatigue characteristics are equal to or higher than that of SUS444.

  Furthermore, the inventors have intensively studied the influence of Cr content on workability and oxidation resistance. As a result, workability can be improved by decreasing the Cr content, and this greatly affects the oxidation resistance. Clarified not to.

  Although it has been conventionally known that workability is improved by reducing the amount of Cr, oxidation resistance is lowered only by reducing the amount of Cr. The addition of Mo and W to the surface compensates for the decrease in oxidation resistance. On the other hand, by adding an appropriate amount of Al, it has been found that excellent oxidation resistance and workability can be achieved even if the amount of Cr is reduced without adding expensive elements such as Mo and W.

  The present invention has been completed based on the above findings of the present inventors.

That is, the present invention is mass%, C: 0.015% or less, Si: 0.4 to 1.0%, Mn: 1.0% or less, P: 0.040% or less, S: 0.010 % or less, Cr: less than 12% or more 16%, N: 0.015% or less, Nb: 0.3~0.65%, C u : 1.0~2.5%, Al: 0.2~1 0.0% , Ti, Mo and W are respectively regulated to Ti: 0.01% or less, Mo: 0.1% or less, W: 0.1% or less, and satisfying Si ≧ Al , and the balance The present invention provides a ferritic stainless steel excellent in heat resistance and workability, characterized by comprising Fe and inevitable impurities.

  Further, the present invention further includes mass%, B: 0.003% or less, REM: 0.08% or less, Zr: 0.5% or less, V: 0.5% or less, Co: 0.5% Provided is a ferritic stainless steel excellent in heat resistance and workability, characterized by containing one or more selected from the following and Ni: 0.5% or less.

  According to the present invention, ferritic stainless steel having heat resistance (thermal fatigue characteristics, oxidation resistance, high temperature fatigue characteristics) equal to or better than SUS444 (JIS G4305) and excellent workability without adding expensive Mo or W. Steel can be obtained at low cost. Therefore, the steel of the present invention is suitable for automobile exhaust system members.

It is a figure explaining a thermal fatigue test piece. It is a figure explaining the temperature in a thermal fatigue test, and constraint conditions. It is a figure explaining a high temperature fatigue test piece. It is a graph which shows the influence of Cu content which acts on a thermal fatigue characteristic. It is a graph which shows the influence of Al content which affects oxidation resistance (oxidation increase). It is a graph which shows the influence of Si content which acts on steam oxidation resistance (oxidation increase). It is a graph which shows the influence of Si content-Al content (Si-Al) which has on high temperature fatigue characteristics. It is a graph which shows the influence of Cr content which acts on steam oxidation resistance (oxidation increase). It is a graph which shows the influence of Cr content which acts on the three-way average elongation at room temperature.

First, the basic experiment that led to the completion of the present invention will be described. In the following description, all percentages in the components are mass%.
C: 0.005-0.007%, N: 0.004-0.006%, P: 0.02-0.03%, S: 0.002-0.004%, Si: 0.85% , Mn: 0.4%, Cr: 14%, Nb: 0.45%, Al: 0.35%, Ti: 0.007%, Mo: 0.01 to 0.03%, W: 0.01 A steel with a component composition of ~ 0.03% as the base and the Cu content changed within the range of 0 to 3% is melted in the laboratory to form a 50kg steel ingot, and this steel ingot is forged. A heat fatigue test piece having a dimension as shown in FIG. 1 was produced from the steel material by heat treatment to obtain a steel material having a cross-sectional area of 35 mm × 35 mm. Then, the heat fatigue life was measured by repeatedly applying heat treatment for heating / cooling between 100 ° C. and 850 ° C. at a constraint ratio of 0.30 as shown in FIG. The thermal fatigue life is calculated by dividing the load detected at 100 ° C. by the cross-sectional area of the test piece soaking parallel section shown in FIG. The minimum number of cycles when the stress began to decrease was taken. This corresponds to the number of cycles in which a crack occurred in the test piece. For comparison, the same test was performed for SUS444 (Cr: 19% -Mo: 2% -Nb: 0.5% steel).

  FIG. 4 shows the influence of the Cu content on the thermal fatigue life in the thermal fatigue test. From this figure, by setting the Cu content to 1.0% or more, it is possible to obtain a thermal fatigue life equal to or greater than that of SUS444 (about 1350 cycles), and therefore to improve the thermal fatigue characteristics. It can be seen that it is effective to set the Cu content to 1.0% or more.

  Next, C: 0.006%, N: 0.007%, P: 0.02-0.03%, S: 0.002-0.004%, Mn: 0.2%, Si: 0.00. 85%, Cr: 14%, Nb: 0.49%, Cu: 1.5%, Ti: 0.007%, Mo: 0.01-0.03%, W: 0.01-0.03% Based on the component composition of the above, the steel in which the Al content was changed within the range of 0 to 2% was melted in the laboratory to form a 50 kg steel ingot, which was hot-rolled and hot rolled. Sheet annealing, cold rolling, and finish annealing were performed to obtain a cold-rolled annealing sheet having a thickness of 2 mm. A 30 mm × 20 mm test piece was cut out from the cold-rolled steel sheet obtained as described above, a 4 mmφ hole was drilled on the top of the test piece, the surface and end face were polished with # 320 emery paper, degreased, and the following atmosphere It was subjected to a medium continuous oxidation test.

<Atmospheric continuous oxidation test>
The test piece is held in an atmospheric furnace heated to 950 ° C. for 200 hours, the difference in the mass of the test piece before and after the heating test is measured, and the increase in oxidation per unit area (g / m ² ) is obtained. It was.

FIG. 5 shows the influence of the Al content on the increase in oxidation in the atmospheric continuous oxidation test. From this figure, it can be seen that when the Al content is 0.2% or more, oxidation resistance equal to or higher than that of SUS444 (oxidation increase: 19 g / m ² or less) can be obtained.

  Next, C: 0.006%, N: 0.007%, P: 0.02-0.03%, S: 0.002-0.004%, Mn: 0.2%, Al: 0.00. 45%, Cr: 14%, Nb: 0.49%, Cu: 1.5%, Ti: 0.007%, Mo: 0.01-0.03%, W: 0.01-0.03% Based on the component composition of the above, steel with various Si contents varied in the laboratory to produce a 50kg ingot, which was hot-rolled, hot-rolled and annealed, cold-rolled Then, finish annealing was performed to obtain a cold-rolled annealing plate having a thickness of 2 mm. A test piece of 30 mm × 20 mm was cut out from the cold-rolled steel sheet obtained as described above, a hole of 4 mmφ was made in the upper part of the test piece, the surface and the end surface were polished with # 320 emery paper, degreased, and the following water vapor It was subjected to an atmospheric continuous oxidation test.

<Continuous oxidation test in steam atmosphere>
10 vol% CO ₂ -20 vol% H ₂ O-5 vol% O ₂ -bal. Heated to 950 ° C. using the above test piece. N ₂ gas was allowed to flow at 0.5 L / min, held in a furnace heated to 950 ° C. in a steam atmosphere for 200 hours, the difference in the mass of the test piece before and after the heating test was measured, and the oxidation increase per unit area ( g / m ² ).

FIG. 6 shows the influence of the Si content on the oxidation increase in the steam oxidation test. From this figure, it can be seen that unless the Si content is 0.4% or more, oxidation resistance equivalent to SUS444 (oxidation increase: 37 g / m ² or less) cannot be obtained.

  Next, C: 0.006%, N: 0.007%, P: 0.02-0.03%, S: 0.002-0.004%, Mn: 0.2%, Cr: 14% Nb: 0.49%, Cu: 1.5%, Ti: 0.007%, Mo: 0.01-0.03%, W: 0.01-0.03% In this, steel with various contents of Si and Al was melted in a laboratory to form a 50 kg steel ingot, which was hot-rolled, hot-rolled and annealed, cold-rolled, Finish annealing was performed to obtain a cold-rolled annealing plate having a thickness of 2 mm. A fatigue test piece having a shape as shown in FIG. 3 was produced from the cold-rolled steel sheet obtained as described above, and was subjected to the following high-temperature fatigue test.

<High temperature fatigue test>
Evaluation was performed by shaking the steel plate at 1300 rpm at 850 ° C. using a Schenck fatigue tester using the above test piece. During the test, a bending stress of 70 MPa was applied to the steel sheet surface. High temperature fatigue properties were evaluated by the number of fatigue times (cycles) until breakage.

FIG. 7 shows the influence of Si—Al on the number of fatigue times (cycles) in the high temperature fatigue test. From this figure, it can be seen that in order to obtain a high temperature fatigue life (24 × 10 ⁵ cycles) equal to or greater than that of SUS444, it is necessary to satisfy Si ≧ Al.

  Next, C: 0.006%, N: 0.007%, P: 0.02-0.03%, S: 0.002-0.004%, Mn: 0.2%, Si: 0.00. 85%, Al: 0.45%, Nb: 0.49%, Cu: 1.5%, Ti: 0.007%, Mo: 0.01-0.03%, W: 0.01-0. A steel having a Cr content of 3% and a Cr content varied in a laboratory is made into a 50 kg steel ingot, which is hot-rolled, hot-rolled, annealed, and cooled. Cold rolling and finish annealing were performed to obtain a cold-rolled annealed sheet having a thickness of 2 mm. A 30 mm × 20 mm test piece was cut out from the cold-rolled steel sheet obtained as described above, a hole of 4 mmφ was made in the upper part of the test piece, the surface and the end surface were polished with # 320 emery paper, degreased, and then steam-oxidized. It used for the test.

FIG. 8 shows the influence of the Cr content on the oxidation increase in the steam oxidation test. From this figure, it can be seen that if the Cr content is 12% or more, oxidation resistance equivalent to SUS444 (oxidation increase: 37 g / m ² or less) can be obtained.

Also, JIS13B tensile test specimens are prepared from these cold-rolled annealed sheets, with the rolling direction (L direction), the perpendicular direction to the rolling direction (C direction), and the 45 ° direction (D direction) as the rolling direction. Then, a tensile test was performed at room temperature. Tensile tests in each direction were performed at room temperature to measure the elongation at break, and the average elongation El was determined from the following formula.
Average elongation El (%) = (E _L + 2E _D + E _C ) / 4
Here, E _L : El (%) in the L direction, E _D : El (%) in the D direction, E _C : El (%) in the C direction

  FIG. 9 shows the influence of the Cr content on the average elongation value in the three directions (L, C, D direction). As shown in this figure, it can be seen that when the Cr content is less than 16%, good workability with an average elongation of 36% or more in the three directions (L, C, and D directions) can be obtained.

  The present invention has been completed as a result of further studies based on the results of the basic experiment as described above.

Hereinafter, the ferritic stainless steel according to the present invention will be described in detail.
First, the component composition of the present invention will be described.

C: 0.015% or less C is an element effective for increasing the strength of steel, but if it exceeds 0.015%, the toughness and formability are significantly reduced. Therefore, in the present invention, the C content is set to 0.015% or less. In addition, from the viewpoint of ensuring moldability, the lower the C content, the more preferable, and 0.008% or less is desirable. On the other hand, in order to ensure the strength as an exhaust system member, the C content is preferably 0.001% or more, and more preferably in the range of 0.002 to 0.008%.

Si: 0.4 to 1.0%
Si is an important element for improving the oxidation resistance in a water vapor atmosphere. As shown in FIG. 6, in order to obtain the steam oxidation resistance equivalent to SUS444, it is necessary to contain 0.4% or more. On the other hand, if the Si content exceeds 1.0%, the workability is significantly reduced. For this reason, Si content shall be 0.4 to 1.0% of range. More preferably, it is 0.4 to 0.8% of range. Although the detailed mechanism for improving the steam oxidation resistance by making the Si content 0.4% or more is not necessarily clear, the Si oxide layer dense on the steel sheet surface by making Si 0.4% or more. It is considered that the steam oxidation resistance is improved by continuously generating and suppressing the invasion of gas components from the outside. In the case where oxidation resistance under a more severe environment is required, the Si content is preferably 0.5% or more.

Mn: 1.0% or less Mn is an element that increases the strength of steel and also has a function as a deoxidizer. However, if contained excessively, a γ phase is easily generated at a high temperature, and heat resistance is lowered. For this reason, Mn content shall be 1.0% or less. Preferably, it is 0.7% or less. Further, in order to obtain the effect of increasing the strength and the deoxidation effect, 0.05% or more is preferable.

P: 0.040% or less P is a harmful element that lowers toughness, and is desirably reduced as much as possible. For this reason, the P content is set to 0.040% or less. Preferably, it is 0.030% or less.

S: 0.010% or less S is a harmful element that lowers elongation and r value, adversely affects formability, and lowers corrosion resistance, which is a basic characteristic of stainless steel. Therefore, it is desirable to reduce S as much as possible. For this reason, S content shall be 0.010% or less. Preferably, it is 0.005% or less.

Cr: 12% or more and less than 16% Cr is an important element effective for improving the corrosion resistance and oxidation resistance, which are the characteristics of stainless steel. However, if its content is less than 12%, sufficient oxidation resistance is obtained. I can't get it. On the other hand, Cr is an element that solidifies and strengthens steel at room temperature to harden and lower the ductility. Particularly, when the content thereof is 16% or more, the above-described adverse effect becomes remarkable. For this reason, Cr content shall be 12% or more and less than 16% of range. More preferably, it is 12 to 15% of range.

N: 0.015% or less N is an element that decreases the toughness and formability of steel. When the content exceeds 0.015%, the above-described decrease becomes significant. For this reason, N content shall be 0.015% or less. Note that N is preferably reduced as much as possible from the viewpoint of securing toughness and moldability, and is preferably less than 0.010%.

Nb: 0.3 to 0.65%
Nb forms and fixes C, N and carbides, nitrides or carbonitrides, and has the effect of enhancing corrosion resistance, formability, and intergranular corrosion resistance of welds, and increases high-temperature strength to cause thermal fatigue. It is an element having the effect of improving the characteristics. Such an effect is recognized by containing 0.3% or more. On the other hand, if the content exceeds 0.65%, the Laves phase (Fe ₂ Nb), which is an intermetallic compound of Fe and Nb, is likely to precipitate, and embrittlement is promoted. For this reason, Nb content is taken as 0.3 to 0.65% of range. Preferably, it is 0.4 to 0.55% of range.

Mo: 0.1% or less Mo is an expensive element and is not actively added for the purpose of the present invention. However, it may be mixed in the range of 0.1% or less from the raw material scrap or the like. For this reason, Mo content is made into 0.1% or less.

W: 0.1% or less W is an expensive element like Mo, and is not actively added for the purpose of the present invention. However, it may be mixed in the range of 0.1% or less from the raw material scrap or the like. For this reason, W content shall be 0.1% or less.

Cu: 1.0 to 2.5%
Cu is an extremely effective element for improving thermal fatigue characteristics. As shown in FIG. 3, in order to obtain a thermal fatigue characteristic equal to or higher than that of SUS444, the Cu content needs to be 1.0% or higher. However, if its content exceeds 2.5%, ε-Cu precipitates during cooling after heat treatment, and the steel becomes extremely hard, and embrittlement tends to occur during hot working. More importantly, the inclusion of Cu improves the thermal fatigue properties, but the oxidation resistance of the steel itself decreases, and the overall heat resistance decreases. The cause of this is not necessarily clarified, but Cu is concentrated in the generated Cr-depleted layer immediately below the scale to suppress the re-diffusion of Cr, which is an element that improves the original oxidation resistance of stainless steel. it is conceivable that. For this reason, Cu content is taken as 1.0 to 2.5% of range. More preferably, it is 1.1 to 1.8% of range.

Ti: 0.15% or less Ti, like Nb, fixes C and N, and has an effect of improving the corrosion resistance, formability, and intergranular corrosion of the welded portion. However, such effects are saturated in the component system of the present invention containing Nb when the content exceeds 0.15%, and the steel is hardened by solid solution hardening. For this reason, Ti content shall be 0.15% or less. Ti is easier to bond with N than Nb, and it is easy to form coarse TiN. Coarse TiN tends to be the starting point of cracks and lowers the toughness. Therefore, when the toughness of the hot-rolled sheet is required, the content is preferably 0.01% or less. In the present invention, Ti does not need to be positively contained, and therefore the lower limit includes 0%.

Al: 0.2 to 1.0%
As shown in FIG. 5, Al is an indispensable element for improving the oxidation resistance of the Cu-added steel. In addition, Al acts as a solid solution strengthening element when dissolved in steel, and particularly has the effect of increasing the high-temperature strength at a temperature exceeding 800 ° C. Therefore, it is important for improving high-temperature fatigue properties in the present invention. Element. In order to obtain an oxidation resistance equal to or higher than that of SUS444, which is the object of the present invention, Al needs to be contained in an amount of 0.2% or more. On the other hand, if the content exceeds 1.0%, the steel becomes hard and workability decreases. Therefore, the Al content is set to a range of 0.2 to 1.0%. More preferably, it is 0.3 to 1.0% of range. More preferably, it is 0.3 to 0.5% of range.

Si ≧ Al
As described above, Al acts as a solid solution strengthening element when dissolved in steel, and has the effect of increasing the high temperature strength particularly at temperatures exceeding 800 ° C. Therefore, Si is an important element for effectively utilizing such a solid solution strengthening action of Al. When the amount of Si is less than the amount of Al, Al preferentially forms oxides and nitrides at a high temperature, and the amount of solute Al decreases, so that Al does not contribute to strengthening. On the other hand, if the amount of Si is larger than the amount of Al, Si is preferentially oxidized, and a dense oxide layer is continuously formed on the steel sheet surface. This oxide layer acts as a barrier to oxygen and nitrogen diffusion and suppresses the diffusion of oxygen and nitrogen from the outside, so that Al remains in a solid solution state without being oxidized or nitrided, and strengthens the steel by solid solution strengthening. Thus, the high temperature fatigue characteristics can be improved. For this reason, in order to obtain high temperature fatigue characteristics equivalent to or higher than those of SUS444, it is necessary to satisfy Si ≧ Al.

  In addition to the essential components, the ferritic stainless steel of the present invention further contains one or more selected from B, REM, Zr, V, Co and Ni in the following range. Also good.

B: 0.003% or less B is an element effective for improving workability, particularly secondary workability. However, when the content exceeds 0.0030%, BN is generated and workability is lowered. For this reason, when it contains B, the content shall be 0.0030% or less. Since the said effect is exhibited effectively at 0.0004% or more, the range of 0.0004 to 0.0030% is more preferable.

REM: 0.08% or less, Zr: 0.5% or less Each of REM (rare earth element) and Zr is an element that improves oxidation resistance, and can be contained as necessary in the present invention. However, if the REM content exceeds 0.080%, the steel becomes brittle, and if the Zr content exceeds 0.50%, the Zr intermetallic compound precipitates and the steel becomes brittle. For this reason, when REM is contained, the content is 0.080% or less, and when Zr is contained, the content is 0.50% or less. The above effect is effectively exhibited when the REM is 0.01% or more and the Zr is 0.005% or more. Therefore, the REM content is 0.01 to 0.080%, and the Zr content is 0.005 to 0.00. A range of 50% is preferred.

V: 0.5% or less V is an element effective for improving workability and oxidation resistance. However, when the content exceeds 0.50%, coarse V (C, N) is precipitated, and the surface properties are deteriorated. For this reason, when it contains V, the content shall be 0.50% or less. Since the effect of improving workability and oxidation resistance is effectively exhibited at 0.15% or more, 0.15 to 0.50% is preferable. More preferably, it is 0.15 to 0.4% of range.

Co: 0.5% or less Co is an element effective for improving toughness and an element for increasing high-temperature strength. However, Co is an expensive element, and the above effect is saturated even if its content exceeds 0.5%. For this reason, when it contains Co, the content shall be 0.5% or less. Since the said effect is exhibited effectively at 0.02% or more, 0.02 to 0.5% of range is preferable. More preferably, it is 0.02 to 0.2% of range.

Ni: 0.5% or less Ni is an element that improves toughness. However, since Ni is expensive and is a strong γ-phase forming element, it generates a γ-phase at a high temperature, and if its content exceeds 0.5%, the oxidation resistance is lowered. For this reason, when it contains Ni, the content shall be 0.5% or less. Since the above effect is effectively exhibited at 0.05% or more, the range of 0.05 to 0.5% is preferable. More preferably, it is 0.05 to 0.4% of range.

The balance is Fe and inevitable impurities. Of the inevitable impurities, O is preferably 0.010% or less, Sn is 0.005% or less, Mg is 0.005% or less, and Ca is preferably 0.005% or less. More preferably, O is 0.005% or less, Sn is 0.003% or less, Mg is 0.003% or less, and Ca is 0.003% or less.

Next, the manufacturing method of the ferritic stainless steel of this invention is demonstrated.
The stainless steel of this invention can be manufactured with the normal manufacturing method of ferritic stainless steel, The manufacturing conditions are not specifically limited. For example, steel is produced in a known melting furnace such as a converter or an electric furnace, or further subjected to secondary refining such as ladle refining or vacuum refining to obtain steel having the above-described component composition of the present invention, and then continuously It is made into a steel slab (slab) by the casting method or ingot-bundling rolling method, and then cold-rolled annealing through each process of hot rolling, hot-rolled sheet annealing, pickling, cold rolling, finish annealing, pickling, etc. A method for producing a plate can be mentioned as a preferred production method. In addition, the said cold rolling may perform cold rolling of 2 times or more on both sides of intermediate annealing, and each process of cold rolling, finish annealing, and pickling may be performed repeatedly. . Further, depending on the case, the hot-rolled sheet annealing may be omitted, and when the gloss of the steel sheet surface is required, a skin pass may be applied after cold rolling or after finish annealing.

More preferable production conditions include the following.
It is preferable that a specific condition is a partial condition in the hot rolling process and the cold rolling process. Moreover, in steelmaking, it is preferable to melt the molten steel containing the essential components and components to be contained as necessary in a converter or an electric furnace and perform secondary refining by the VOD method. The molten steel can be made into a steel material according to a known production method, but from the viewpoint of productivity and quality, it is preferable to use a continuous casting method. The steel material obtained by continuous casting is heated to 1000 to 1250 ° C., for example, and is hot rolled into a desired thickness by hot rolling. Of course, it can be processed as other than the plate material. The hot-rolled sheet is subjected to batch-type annealing at 600 to 800 ° C. or continuous annealing at 900 to 1100 ° C. as necessary, and then descaled by pickling or the like to obtain a hot-rolled sheet product. If necessary, the scale may be removed by shot blasting before pickling.

  Furthermore, in order to obtain a cold-rolled annealed plate, the hot-rolled annealed plate obtained above is made a cold-rolled plate through a cold rolling process. In this cold rolling process, two or more cold rollings including intermediate annealing may be performed as necessary for the convenience of production. The total rolling reduction of the cold rolling process comprising one or more cold rollings is set to 60% or more, preferably 70% or more. The cold-rolled sheet is subjected to continuous annealing (finish annealing) at 900 to 1150 ° C., more preferably from 950 to 1120 ° C., and then pickled to form a cold-rolled annealed sheet. Depending on the application, the shape and quality of the steel sheet can be adjusted by applying mild rolling (skin pass rolling or the like) after cold rolling annealing.

  Using hot-rolled sheet products or cold-rolled annealed sheet products obtained by such a manufacturing method, bending processing is performed according to each application, exhaust pipes for automobiles and motorcycles, catalyst outer cylinder materials, and thermal power plants It is formed into an exhaust duct or a fuel cell-related member (for example, a separator, an interconnector, a reformer, etc.). The welding method for welding these members is not particularly limited, and is a normal arc welding method such as MIG (Metal Inert Gas), MAG (Metal Active Gas), TIG (Tungsten Inert Gas), or spot welding. High-frequency resistance welding and high-frequency induction welding such as resistance welding methods such as seam welding and electric resistance welding methods can be applied.

[Example 1]
No. having the component composition shown in Table 1. Steels 1 to 23 were melted in a vacuum melting furnace, cast into a 50 kg steel ingot, and forged into two parts. Thereafter, one of the two steel ingots was heated to 1170 ° C., and then hot-rolled to form a hot-rolled sheet having a thickness of 5 mm, hot-rolled sheet annealed at a temperature of 1020 ° C., pickled, and a reduction rate of 60%. Was cold-rolled, finish-annealed at 1040 ° C., cooled at an average cooling rate of 5 ° C./sec, and pickled to obtain a cold-rolled annealed plate having a thickness of 2 mm. No. 1 to 11 are examples of the present invention, No. 1 within the scope of the present invention. Nos. 12 to 23 are comparative examples outside the scope of the present invention. Of the comparative examples, No. No. 19 corresponds to the composition of JFE429EX as an example of Nb—Si composite added steel. No. 20 corresponds to the composition of SUS444. 21, 22, and 23 correspond to the compositions of Invention Example 3 of Patent Document 2, Invention Example 3 of Patent Document 3, and Invention Example 5 of Patent Document 4, respectively.

  No. obtained as described above. The 1 to 23 cold-rolled annealed plates were subjected to the following two types of oxidation resistance tests, high temperature fatigue tests, and room temperature tensile tests.

<Atmospheric continuous oxidation test>
Cut out a 30mm x 20mm sample from the various cold-rolled annealed plates obtained as described above, drill a 4mmφ hole at the top of the sample, polish the surface and end face with # 320 emery paper, degrease, and heat to 950 ° C It was suspended in a furnace in a maintained atmospheric atmosphere and held for 200 hours. After the test, the mass of the sample was measured, the difference from the pre-measured mass before the test was determined, and the increase in oxidation (g / m ² ) was calculated. In addition, the test was implemented twice and the continuous oxidation resistance was evaluated by the average value.

<Continuous oxidation test in steam atmosphere>
A 30 mm × 20 mm sample was cut out from the various cold-rolled annealed plates obtained as described above, a 4 mmφ hole was made in the upper part of the sample, and the surface and end face were polished with # 320 emery paper for degreasing. Then _{10vol% CO 2 -20vol% H 2} O-5vol% O 2 -bal. N ₂ gas was flowed at 0.5 L / min to make a water vapor atmosphere, and kept in a furnace heated to 950 ° C. for 200 hours. After the test, the mass of the sample was measured, and the pre-test before measurement was measured in advance. The difference from the mass was determined, and the increase in oxidation (g / m ² ) was calculated.

<High temperature fatigue test>
A test piece having a shape as shown in FIG. 3 was cut out from the various cold-rolled annealed plates obtained as described above, and the steel plates were shaken at 1300 rpm at 850 ° C. using a Schenck fatigue tester. During the test, a bending stress of 70 MPa was applied to the steel sheet surface, and the number of cycles until rupture was evaluated.

<Room temperature tensile test>
From the cold-rolled annealed plate, a rolling direction (L direction), a direction perpendicular to the rolling direction (C direction), and a 45 ° direction (D direction) in the rolling direction are each prepared as a JIS 13B tensile test piece, Tensile tests in each direction were performed at room temperature to measure the elongation at break, and the average elongation El was determined from the following formula.
Average elongation El (%) = (E _L + 2E _D + E _C ) / 4
Here, E _L : El (%) in the L direction, E _D : El (%) in the D direction, E _C : El (%) in the C direction

[Example 2]
The remaining steel ingot of the 50 kg steel ingot divided into two in Example 1 was heated to 1170 ° C. and hot-rolled to obtain a sheet bar having a thickness of 30 mm × width: 150 mm. Thereafter, this sheet bar was forged into a 35 mm square bar, annealed at 1040 ° C., machined, processed into a thermal fatigue test piece having the dimensions shown in FIG. 1, and subjected to the thermal fatigue test shown below.

<Thermal fatigue test>
In the thermal fatigue test, the thermal fatigue life was measured by repeatedly raising and lowering the temperature between 100 ° C. and 850 ° C. with a restraint ratio of 0.30. At this time, the heating rate and the cooling rate were 10 ° C./sec, the holding time at 100 ° C. was 2 min, and the holding time at 850 ° C. was 5 min. The thermal fatigue life is calculated by dividing the load detected at 100 ° C by the cross-sectional area of the test piece soaking parallel part, and when the stress begins to decrease continuously with respect to the stress of the previous cycle. The minimum number of cycles.

  Table 2 summarizes the results of the continuous oxidation test in the atmosphere of Example 1, the continuous oxidation test in a steam atmosphere, the high temperature fatigue test, the room temperature tensile test, and the thermal fatigue test of Example 2. As is apparent from Table 2, the steels of the examples of the present invention within the scope of the present invention all have heat resistance (oxidation resistance, thermal fatigue characteristics, high temperature fatigue characteristics) equal to or higher than SUS444 and three directions at room temperature ( (L, C, D direction) excellent workability with an average elongation of 36% or more, and it was confirmed that the objective of the present invention was satisfied. On the other hand, the steel of the comparative example outside the scope of the present invention is inferior in any of oxidation resistance, thermal fatigue characteristics, high temperature fatigue characteristics, and workability, and the target of the present invention is not achieved. Was confirmed.

  The steel of the present invention is not only suitable for exhaust system members such as automobiles, but also suitably used as exhaust system members for thermal power generation systems and solid oxide fuel cell members that require similar characteristics. be able to.

Claims (2)

In mass%, C: 0.015% or less, Si: 0.4 to 1.0%, Mn: 1.0% or less, P: 0.040% or less, S: 0.010% or less, Cr: 12 % or more but less than 16%, N: 0.015% or less, Nb: 0.3~0.65%, C u : 1.0~2.5%, Al: it contains 0.2% to 1.0% Ti, Mo, and W are respectively regulated to Ti: 0.01% or less, Mo: 0.1% or less, and W: 0.1% or less, satisfying Si ≧ Al, the balance being Fe and inevitable impurities Ferritic stainless steel with excellent heat resistance and workability characterized by comprising   Furthermore, in mass%, B: 0.003% or less, REM: 0.08% or less, Zr: 0.5% or less, V: 0.5% or less, Co: 0.5% or less, and Ni: 0.00. The ferritic stainless steel excellent in heat resistance and workability according to claim 1, comprising one or more selected from 5% or less.

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