KR102508125B1 - ferritic stainless steel - Google Patents

Abstract

Provided is a ferritic stainless steel having excellent creep resistance and thermal fatigue characteristics. In mass %, C: 0.020% or less, Si: 0.1 to 1.0%, Mn: 0.05 to 0.60%, P: 0.050% or less, S: 0.008% or less, Ni: 0.02 to 0.60%, Al: 0.001 to 0.25%, It contains Cr: 18.0 to 20.0%, Nb: 0.30 to 0.80%, Mo: 1.80 to 2.50%, N: 0.015% or less, and Sb: 0.002 to 0.50%, and also satisfies the following formula (1), A ferritic stainless steel having a component composition consisting of added Fe and unavoidable impurities. Nb+Mo: 2.3 to 3.0% . . . (1) (Nb and Mo in formula (1) represent the content (mass%) of each element)

Description

ferritic stainless steel

The present invention relates to a ferritic stainless steel, which has excellent creep resistance, particularly suitable for use in exhaust system members used under high temperatures, such as exhaust pipes and converter cases of automobiles and motorcycles, exhaust ducts of thermal power plants, and the like. It relates to ferritic stainless steels with creep resistance and thermal fatigue properties.

Exhaust system members such as exhaust manifolds, exhaust pipes, converter cases, and mufflers of automobiles are required to have excellent heat resistance. There are several types of heat resistance, including thermal fatigue properties, high-temperature fatigue properties, high-temperature strength (high-temperature yield strength), oxidation resistance, creep properties, high-temperature salt damage corrosion properties, and the like. Among them, thermal fatigue property is one of the particularly important heat resistance. Exhaust system members are repeatedly heated and cooled along with starting and stopping the engine. At this time, since the exhaust system member is connected to peripheral components, thermal expansion and contraction are restricted, and thermal strain occurs in the material itself. A low-cycle fatigue phenomenon that leads to destruction by repeatedly receiving this thermal strain is called thermal fatigue.

Currently, ferritic stainless steels such as Type 429 (14%Cr-0.9%Si-0.4%Nb system) to which Nb and Si are added are widely used as materials used for members requiring the above thermal fatigue properties. However, with the improvement of engine performance, when the exhaust gas temperature rises to a temperature exceeding 900°C, the Type 429 in particular cannot sufficiently satisfy the required thermal fatigue characteristics.

As a material that can deal with this problem, for example, SUS444 (19%Cr-0.5%Nb-2%Mo) specified in JIS G4305, which is a ferritic stainless steel whose high-temperature yield strength is improved by adding Nb and Mo, or Ferritic stainless steels to which Nb, Mo, and W are added have been developed (see, for example, Patent Document 1).

Japanese Unexamined Patent Publication No. 2004-018921

In response to the recent strengthening of exhaust gas regulations and for the purpose of improving fuel efficiency, the exhaust gas temperature tends to increase, and even SUS444 and the like are lacking in heat resistance, particularly thermal fatigue characteristics. In addition, since stainless steel tends to undergo creep deformation when the temperature of the exhaust gas exceeds 900°C, creep resistance is also required.

SUS444 has the highest level of heat resistance among ferritic stainless steels, but when the exhaust gas temperature rises with recent tightening of exhaust gas regulations and improvement of fuel economy, the heat resistance is not necessarily sufficient. As the temperature of the exhaust gas rises, the thermal expansion of the exhaust system members increases when the temperature rises, so the ferritic stainless steel used for the exhaust system members is easily destroyed by thermal fatigue due to the addition of more severe thermal strain. In addition, when maintained in a high temperature range for a long time, creep deformation easily occurs in ferritic stainless steel, and when creep deformation occurs, it leads to destruction starting from the portion where the thickness is reduced due to creep deformation. Improvement of creep resistance is also becoming necessary.

Thus, in the prior art including SUS444, ferritic stainless steel with sufficient thermal fatigue characteristics could not be obtained even when the temperature of the exhaust gas increased. Further, evaluation of creep resistance, which is particularly required when the exhaust gas temperature exceeds 900°C, has not been sufficiently conducted.

Therefore, an object of the present invention is to solve these problems and provide a ferritic stainless steel having excellent creep resistance and thermal fatigue properties.

In addition, the "excellent creep resistance" of the present invention means that the rupture time when performing a creep test at 900 ° C. is superior to that of SUS444. Further, "excellent in thermal fatigue properties" refers to having properties superior to SUS444, and specifically, thermal fatigue life when heating and cooling are repeated between 200 and 950 ° C. is superior to SUS444.

The inventors of the present invention, in order to develop a ferritic stainless steel superior in creep resistance and thermal fatigue characteristics to SUS444, intensively studied the effects of various elements on creep resistance and thermal fatigue characteristics.

As a result, by containing 0.30 to 0.80% of Nb, 1.80 to 2.50% of Mo, and 2.3 to 3.0% of the total content of Nb and Mo in terms of mass%, the high-temperature strength is increased over a wide temperature range, and the thermal fatigue properties are increased. I found this to improve. Moreover, it was discovered that creep resistance improved by containing Sb in the range of 0.002-0.50 mass %.

Based on the above recognition, we came to complete this invention by setting it as the specific component composition containing Cr, Nb, Mo, and all of Sb in an appropriate amount. In the present invention, the above elements are important, but all essential elements need to be adjusted to a predetermined content in order to bring about the effect of the present invention.

This invention makes the following a summary.

[1] In terms of mass%, C: 0.020% or less, Si: 0.1 to 1.0%, Mn: 0.05 to 0.60%, P: 0.050% or less, S: 0.008% or less, Ni: 0.02 to 0.60%, Al: 0.001 to 0.001% 0.25%, Cr: 18.0 to 20.0%, Nb: 0.30 to 0.80%, Mo: 1.80 to 2.50%, N: 0.015% or less, Sb: 0.002 to 0.50%, and also satisfies the following formula (1) A ferritic stainless steel having a composition with the balance being Fe and unavoidable impurities.

Nb+Mo: 2.3 to 3.0% 　　... (One)

(Nb and Mo in Formula (1) represent the content (mass %) of each element)

[2] The above component composition, in terms of mass%, further contains Ti: 0.01 to 0.16%, Zr: 0.01 to 0.50%, Co: 0.01 to 0.50%, W: 0.01 to 5.0%, Cu: 0.01 to 0.40%, Sn: The ferritic stainless steel described in [1] containing one or two or more selected from among 0.001% to 0.005%.

[3] The above component composition, in terms of mass%, further contains one or two selected from Ca: 0.0002 to 0.0050% and Mg: 0.0002 to 0.0050%, the ferrite described in [1] or [2] based stainless steel.

[4] The ferritic stainless steel according to any one of [1] to [3] used for an exhaust manifold that is heated to 700°C or higher by exhaust gas from an engine.

According to the present invention, it is possible to provide a ferritic stainless steel having better creep resistance and thermal fatigue characteristics than SUS444 (JIS G4305). Therefore, the ferritic stainless steel of the present invention can be suitably used for exhaust system members of automobiles and the like.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure explaining a creep test piece.
2 is a diagram explaining a thermal fatigue test piece.
3 is a diagram explaining temperature and constraint conditions in a thermal fatigue test.

(Mode for implementing the invention)

EMBODIMENT OF THE INVENTION Hereinafter, embodiment of this invention is described. In addition, this invention is not limited to the following embodiment.

The ferritic stainless steel of the present invention contains, in mass%, C: 0.020% or less, Si: 0.1 to 1.0%, Mn: 0.05 to 0.60%, P: 0.050% or less, S: 0.008% or less, Ni: 0.02 to 0.60 %, Al: 0.001 to 0.25%, Cr: 18.0 to 20.0%, Nb: 0.30 to 0.80%, Mo: 1.80 to 2.50%, N: 0.015% or less, and Sb: 0.002 to 0.50%, and also contains the following Formula (1) is satisfied, and the remainder consists of Fe and unavoidable impurities.

Nb+Mo: 2.3 to 3.0% 　　... (One)

(Nb and Mo in Formula (1) represent the content (mass %) of each element)

In the present invention, the balance of the component composition is very important, and by combining the component composition as described above, it is possible to obtain a ferritic stainless steel superior in creep resistance and thermal fatigue characteristics to SUS444. If the range of content of any of the essential elements (C, Si, Mn, Ni, Al, Cr, Nb, Mo, N, Sb) in the above component composition is out of range, the desired creep resistance and thermal fatigue characteristics are not obtained

Next, the component composition of the ferritic stainless steel of the present invention will be described. Hereinafter, %, which is a unit of content of components, means mass % unless otherwise specified.

C: 0.020% or less

C is an element effective in increasing the strength of steel, but when C is contained in excess of 0.020%, the toughness and formability are remarkably reduced. In addition, as the amount of carbide formed in connection with Nb, which is important in the present invention, increases, the effect of improving the thermal fatigue characteristics and creep resistance of Nb described later becomes smaller. Therefore, the C content is made 0.020% or less. Further, the C content is preferably 0.010% or less from the viewpoint of ensuring moldability. More preferably, the C content is 0.008% or less. Further, from the viewpoint of ensuring strength as an exhaust system member, the C content is preferably 0.001% or more. More preferably, the C content is 0.003% or more. More preferably, the C content is 0.004% or more.

Si: 0.1 to 1.0%

Si is an important element necessary for improving oxidation resistance. In order to ensure oxidation resistance in high-temperature exhaust gas, containing of 0.1% or more of Si is required. On the other hand, excessive Si content exceeding 1.0% deteriorates workability at room temperature, so the upper limit of the Si content is made 1.0%. Preferably, the Si content is made 0.20% or more. More preferably, the Si content is 0.30% or more. More preferably, the Si content is 0.40% or more. Also, the Si content is preferably 0.90% or less. More preferably, the Si content is 0.60% or less.

Mn: 0.05 to 0.60%

Mn has an effect of improving thermal fatigue characteristics by increasing spalling resistance of oxidized scale. In order to obtain these effects, Mn content of 0.05% or more is required. On the other hand, excessive Mn content of more than 0.60% tends to generate a γ phase at high temperatures and deteriorates heat resistance. Therefore, the Mn content is made 0.05% or more and 0.60% or less. Preferably, the Mn content is 0.10% or more. More preferably, the Mn content is 0.15% or more. Also, preferably, the Mn content is 0.50% or less. More preferably, the Mn content is 0.40% or less.

P: 0.050% or less

P is a harmful element that reduces the toughness of steel, and it is desirable to reduce it as much as possible. Therefore, the P content is made 0.050% or less. Preferably, the P content is 0.040% or less. More preferably, the P content is 0.030% or less.

S: 0.008% or less

S is a harmful element that reduces elongation and r-value, adversely affects formability, and also reduces corrosion resistance, which is a basic characteristic of stainless steel, so it is desirable to reduce it as much as possible. Therefore, in this invention, S content is made into 0.008 % or less. Preferably, the S content is 0.006% or less.

Ni: 0.02 to 0.60%

Ni is an element that improves the toughness and oxidation resistance of steel. In order to obtain these effects, the Ni content is made 0.02% or more. If the oxidation resistance is insufficient, the material cross-sectional area decreases due to the increased amount of oxide scale generated, or the thermal fatigue characteristics deteriorate due to the peeling of the oxide scale. On the other hand, since Ni is a strong γ-phase forming element, when Ni is excessively contained, γ-phase is formed at high temperature, oxidation resistance is lowered, and the thermal expansion coefficient is increased, resulting in a decrease in thermal fatigue properties. Therefore, the upper limit of the Ni content is made 0.60%. Preferably, the Ni content is 0.05% or more. More preferably, the Ni content is 0.10% or more. Also, the Ni content is preferably 0.40% or less. More preferably, the Ni content is 0.30% or less.

Al: 0.001 to 0.25%

Al is an element having an effect of improving oxidation resistance. In order to obtain the effect, Al needs to contain 0.001% or more. On the other hand, Al is also an element that increases the thermal expansion coefficient. When the coefficient of thermal expansion increases, the thermal fatigue characteristics deteriorate. In addition, steel hardens remarkably, and workability will fall. Therefore, the Al content is made 0.25% or less. Preferably, the Al content is 0.005% or more. More preferably, the Al content is more than 0.010%. More preferably, the Al content is greater than 0.020%. Also, preferably, the Al content is less than 0.20%. More preferably, the Al content is less than 0.08%.

Cr: 18.0 to 20.0%

Cr is an important element effective for improving corrosion resistance and oxidation resistance, which are characteristics of stainless steel, but if the Cr content is less than 18.0%, sufficient oxidation resistance cannot be obtained in a high temperature range exceeding 900°C. If the oxidation resistance is insufficient, the amount of oxide scale generated increases, and the thermal fatigue properties also decrease along with the decrease in the cross-sectional area of the material. On the other hand, Cr is an element that hardens and reduces ductility by solid solution strengthening of steel at room temperature, and when the Cr content exceeds 20.0%, the above-mentioned adverse effects become significant and the thermal fatigue properties also decrease. The upper limit is made into 20.0%. Preferably, the Cr content is 18.5% or more. Also, the Cr content is preferably 19.5% or less.

Nb: 0.30 to 0.80%

Nb is an important element in the present invention that increases high-temperature strength to improve thermal fatigue properties and creep resistance properties. These effects are confirmed by the content of 0.30% or more of Nb. When the Nb content is less than 0.30%, the strength at high temperature is insufficient, and excellent thermal fatigue characteristics and creep resistance characteristics are not obtained. However, when Nb is contained in an amount exceeding 0.80%, Laves phase (Fe ₂ Nb), which is an intermetallic compound, is easily precipitated, and high-temperature strength is lowered, and thermal fatigue characteristics and creep resistance are rather deteriorated. promotes embrittlement; Therefore, the Nb content is made 0.30% or more and 0.80% or less. Preferably, the Nb content is 0.40% or more. More preferably, the Nb content is 0.45% or more. More preferably, the Nb content is greater than 0.50%. Also, the Nb content is preferably 0.70% or less. More preferably, the Nb content is 0.60% or less.

Mo: 1.80 to 2.50%

Mo is an effective element that improves the thermal fatigue properties and creep resistance properties by improving the high-temperature strength of the steel by solid solution in the steel. The effect is shown by containing 1.80% or more of Mo. When the Mo content is less than 1.80%, the high-temperature strength becomes insufficient, and excellent thermal fatigue characteristics and creep resistance characteristics are not obtained. On the other hand, excessive Mo content not only hardens the steel and reduces workability, but also precipitates as a Laves phase (Fe ₂ Mo) similar to Nb, reducing the amount of solid solution Mo in the steel, so the thermal fatigue properties are rather poor. lower it In addition, by precipitating as a coarse σ phase during a thermal fatigue test, it becomes a starting point of destruction and the thermal fatigue properties are lowered. Therefore, the upper limit of Mo content is made into 2.50%. Preferably, the Mo content is 1.90% or more. More preferably, the Mo content is more than 2.00%. Also, the Mo content is preferably 2.30% or less. More preferably, the Mo content is 2.10% or less.

N: 0.015% or less

N is an element that lowers the toughness and formability of steel, and when it is contained in excess of 0.015%, not only the toughness and formability decrease significantly, but also the amount of dissolved Nb decreases due to the formation of Nb nitride, resulting in a decrease in creep resistance and heat resistance. Fatigue characteristics decrease. Therefore, the N content is made 0.015% or less. From the viewpoint of ensuring toughness and moldability, N is preferably reduced as much as possible, and the N content is preferably less than 0.010%.

Sb: 0.002 to 0.50%

Sb is an important element for improving creep resistance in the present invention. Sb is dissolved in steel and suppresses creep deformation of steel at high temperature. Sb does not precipitate as a carbonitride or Laves phase even in a high temperature region, and is dissolved in steel even after long-term use to suppress creep deformation, so that creep resistance can be improved. This effect is obtained by containing 0.002% or more of Sb. On the other hand, excessive inclusion of Sb lowers the toughness and hot workability of the steel, so that not only cracks tend to occur during manufacturing, but also the thermal fatigue properties are lowered by lowering the hot ductility. Therefore, the upper limit of Sb content is made into 0.50%. Preferably, the Sb content is 0.005% or more. More preferably, it is 0.020% or more. Also, the Sb content is preferably 0.30% or less. More preferably, the Sb content is 0.10% or less.

Nb+Mo: 2.3 to 3.0% 　　... (One)

As described above, Nb and Mo are effective elements for improving thermal fatigue properties and creep resistance properties. The effect is confirmed by containing 0.30% or more and 1.80% or more, respectively. However, in order to realize thermal fatigue characteristics and creep resistance superior to SUS444 in thermal fatigue life when the temperature is raised and lowered repeatedly between 200 and 950 ° C. to cope with the high temperature of the exhaust gas, both elements are mixed within a predetermined range. After containing, it is necessary to satisfy at least Nb+Mo≥2.3%, ie, the amount of Nb+Mo (total content of Nb and Mo) to be 2.3% or more. If this is not satisfied, even if a predetermined amount of Sb is added, excellent creep resistance cannot be obtained. Preferably, it is Nb+Mo>2.5%. On the other hand, if the amount of Nb + Mo is excessively increased, the steel will crack, and excellent thermal fatigue characteristics and creep resistance characteristics will not be obtained. Therefore, the upper limit of the amount of Nb+Mo is made into 3.0%. Preferably, the amount of Nb+Mo is 2.7% or less.

In addition, Nb and Mo in said Formula (1) represent content (mass %) of each element.

In the ferritic stainless steel of the present invention, the remainder consists of Fe and unavoidable impurities.

In addition to the above essential components, the ferritic stainless steel of the present invention further contains, as an optional component, one or more selected from among Ti, Zr, Co, W, Cu, and Sn within the following ranges. can do.

Ti: 0.01 to 0.16%

Ti is an element that fixes C and N, improves corrosion resistance and formability, and prevents intergranular corrosion of welded parts, and in the present invention, it can be contained as needed. By containing Ti, since Ti connects with C and N preferentially rather than Nb, it is possible to secure the amount of dissolved Nb in steel effective for high-temperature strength, and is also effective for improving heat resistance. These effects are obtained by containing 0.01% or more of Ti. On the other hand, excessive Ti content exceeding 0.16% causes a decrease in toughness and, for example, causes breakage by repeated bending and unbending in a hot-rolled sheet annealing line. , which adversely affects manufacturability. In addition, since carbonitrides of Nb are easily precipitated using carbonitrides of Ti as nuclei, the amount of dissolved Nb in steel effective for high-temperature strength is rather reduced, and thermal fatigue characteristics and creep resistance are reduced. Therefore, when it contains Ti, Ti content is made into 0.01 to 0.16%. Preferably, the Ti content is 0.03% or more. Also, the Ti content is preferably 0.12% or less. More preferably, the Ti content is 0.08% or less. More preferably, the Ti content is 0.05% or less.

Zr: 0.01 to 0.50%

Zr is an element that improves oxidation resistance, and in the present invention, it can be contained as needed. This effect is obtained by containing 0.01% or more of Zr. However, when the Zr content exceeds 0.50%, a Zr intermetallic compound precipitates and embrittles the steel. Therefore, when Zr is contained, the Zr content is made 0.01 to 0.50%. Preferably, the Zr content is 0.03% or more. More preferably, the Zr content is 0.05% or more. Also, preferably, the Zr content is 0.30% or less. More preferably, the Zr content is 0.10% or less.

Co: 0.01 to 0.50%

Co is known as an element effective in improving the toughness of steel. This effect is obtained by containing 0.01% or more of Co. On the other hand, since excessive Co content rather lowers the toughness of steel, the upper limit of Co content is made into 0.50%. Therefore, when Co is contained, Co content is made into 0.01 to 0.50%. Preferably, the Co content is 0.03% or more. Also, the Co content is preferably 0.30% or less.

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W: 0.01 to 5.0%

W, like Mo, is an element that greatly improves high-temperature strength by solid solution strengthening. This effect is obtained by containing 0.01% or more of W. On the other hand, excessive W content not only significantly hardens the steel, but also creates hard scale in the annealing step during production, making it difficult to descale during pickling. Therefore, when W is contained, W content is made into 0.01 to 5.0%. Preferably, the W content is 0.05% or more. Also, the W content is preferably 3.5% or less. More preferably, the W content is 1.0% or less. More preferably, the W content is less than 0.30%.

Cu: 0.01 to 0.40%

Cu is an element having an effect of improving the corrosion resistance of steel, and is contained when corrosion resistance is required. The effect is obtained by containing 0.01% or more of Cu. On the other hand, when Cu is contained in an amount exceeding 0.40%, the oxide scale is easily peeled off, and the anti-repetition oxidation characteristic is lowered. Therefore, when it contains Cu, Cu content is made into 0.01 to 0.40%. Preferably, the Cu content is 0.03% or more. More preferably, the Cu content is 0.06% or more. Also, the Cu content is preferably 0.20% or less. More preferably, the Cu content is 0.10% or less.

Sn: 0.001 to 0.005%

Sn is an element effective in improving the high-temperature strength of steel. The effect is obtained by containing 0.001% or more of Sn. On the other hand, excessive content of Sn rather deteriorates the thermal fatigue properties along with embrittlement of the steel. Therefore, when containing Sn, Sn content is made into 0.001 to 0.005%. Preferably, the Sn content is 0.001% or more and 0.003% or less.

The ferritic stainless steel of the present invention can further contain one or two selected from among Ca and Mg as an optional component within the following range.

Ca: 0.0002 to 0.0050%

Ca is an effective component for preventing clogging of nozzles due to precipitation of Ti-based inclusions, which tend to occur during continuous casting. The effect is obtained by containing 0.0002% or more of Ca. On the other hand, in order to obtain good surface properties without causing surface defects, the Ca content needs to be 0.0050% or less. Therefore, when containing Ca, Ca content is made into 0.0002 to 0.0050%. Preferably, the Ca content is 0.0005% or more. Also, the Ca content is preferably 0.0030% or less. More preferably, the Ca content is 0.0020% or less.

Mg: 0.0002 to 0.0050%

Mg is an element effective in improving workability and toughness by improving the equiaxed crystal ratio of a slab. In the steel containing Nb or Ti as in the present invention, Mg also has an effect of suppressing the coarsening of carbonitrides of Nb or Ti. The effect is obtained by containing 0.0002% or more of Mg. When Ti carbonitride coarsens, since it becomes the starting point of brittle cracking, toughness will fall significantly. When Nb carbonitride is coarsened, the amount of Nb dissolved in steel decreases, leading to a decrease in thermal fatigue properties. On the other hand, when the Mg content exceeds 0.0050%, the surface properties of the steel deteriorate. Therefore, when Mg is contained, Mg content is made into 0.0002 to 0.0050%. Preferably, the Mg content is 0.0003% or more. More preferably, the Mg content is 0.0004% or more. Also, preferably, the Mg content is 0.0030% or less. More preferably, the Mg content is 0.0020% or less.

The balance is Fe and unavoidable impurities. In the case where the above optional component is included in an amount less than the lower limit, the optional component included in an amount less than the lower limit is included as an unavoidable impurity.

Next, the manufacturing method of the ferritic stainless steel of this invention is demonstrated.

The method for producing stainless steel of the present invention can be suitably employed as long as it is a normal method for producing ferritic stainless steel, and is not particularly limited.

For example, the steel is melted in a known melting furnace such as a converter or an electric furnace, or additionally subjected to secondary refining such as ladle refining or vacuum refining, It is made into steel with a component composition, and made into steel pieces (slabs) by continuous casting or ingot casting and slabbing, then hot rolling, hot rolled sheet annealing, pickling, cold rolling, finish annealing and pickling It can manufacture by the manufacturing process used as a cold-rolled annealing board through each process, such as. The cold rolling may be performed once or twice or more with intermediate annealing interposed therebetween, and each step of cold rolling, finish annealing, and pickling may be repeated. In addition, hot-rolled sheet annealing may be omitted, and skin pass rolling may be performed after cold rolling or finish annealing when surface gloss or roughness adjustment of the steel sheet is required.

Preferred production conditions in the above production method are described.

In the steelmaking process of melting steel, the steel melted in a converter or electric furnace is subjected to secondary refining by a VOD method, an AOD method, or the like, and it is preferable to make steel containing the above essential components and optional components added as needed. The molten steel can be made into a steel material by a known method, but it is preferable to follow the continuous casting method in terms of productivity and quality. After that, the steel material is preferably heated at 1050 to 1250°C, and hot-rolled to obtain a hot-rolled sheet having a desired thickness. In terms of production, the thickness of the hot-rolled sheet is preferably 5 mm or less. Of course, hot working can also be performed other than a plate material. The hot-rolled sheet is then subjected to continuous annealing at a temperature of 900 to 1150 ° C. or batch annealing at a temperature of 700 to 900 ° C., as necessary, and then descaling by pickling or polishing to obtain a hot-rolled product. It is preferable to In addition, if necessary, you may remove scale by shot blasting before pickling.

Moreover, it is good also considering the said hot-rolled product (hot-rolled annealing board) as a cold-rolled product through processes, such as cold rolling. Cold rolling in this case may be performed once, but from the viewpoint of productivity or required quality, it may be cold rolling twice or more with intermediate annealing interposed therebetween. The total reduction ratio of one or two or more cold rolling cycles is preferably 60% or more, and more preferably 70% or more. The cold-rolled steel sheet is then subjected to continuous annealing (finish annealing) at a temperature of preferably 900 to 1200°C, more preferably 1000 to 1150°C, followed by pickling or polishing to obtain a cold-rolled product (cold-rolled annealed sheet). It is desirable to do Finish annealing may be performed in a reducing atmosphere, and in that case, pickling or polishing after finish annealing may be omitted. Furthermore, depending on the application, after finish annealing, skin pass rolling or the like may be performed to adjust the shape, surface roughness, and material of the steel sheet.

The hot-rolled product or cold-rolled product obtained as described above is then subjected to processing such as cutting, bending, bulging, and drawing according to each application, and used for exhaust pipes of automobiles and motorcycles, It is molded into a catalyst outer cylinder material, an exhaust duct of a thermal power plant, or a fuel cell-related member, such as a separator, interconnector, or reformer. Among these, the ferritic stainless steel of the present invention is suitably used for exhaust system members such as exhaust manifolds, exhaust pipes, converter cases, and mufflers. In particular, one of the characteristics is that an exhaust manifold having excellent durability can be obtained even when the temperature is raised to 700° C. or higher by exhaust gas from the engine during use.

The method of welding these members is not particularly limited, and ordinary arc welding such as MIG (Metal Inert Gas), MAG (Metal Active Gas), TIG (Tungsten Inert Gas), spot welding, seam welding, etc. Welding, high-frequency resistance welding such as electric welding, high-frequency induction welding, and the like can be applied.

Example

Hereinafter, the present invention will be described in detail by examples.

Steels having the component compositions of Nos. 1 to 15, 17, 19 to 22, 25, 28 to 41, 43, and 45 to 47 shown in Table 1 were melted in a vacuum melting furnace and cast into 50 kg ingots, After heating at 1170°C, it was made into a 35 mm thick sheet bar by hot rolling. The sheet bar is divided into two, one of the steel ingots is heated to 1100 ° C, and then hot-rolled to obtain a hot-rolled sheet having a thickness of 5 mm. After annealing at a temperature in the range of 1000 to 1150 ° C, It was ground and it was set as the hot-rolled annealing board. Next, cold rolling is performed at a reduction ratio of 70%, and after finishing annealing is performed at a temperature of 1000 to 1150 ° C., scale is removed by pickling or polishing, and a cold rolled annealed plate having a thickness of 1.5 mm is used, and a creep test is performed. provided to In addition, as a reference, also about SUS444 (Conventional Example No. 28), a cold-rolled annealed board was produced in the same manner as above and subjected to a creep test. Regarding the annealing temperature, the temperature was determined for each steel while confirming the structure within the above temperature range.

<Creep test>

A test piece having a shape shown in Fig. 1 was cut out from each cold-rolled annealing board obtained as described above, and a creep test was performed at 900°C with a load of 15 MPa of stress. Based on the time taken until fracture, it was evaluated as follows. For SUS444 (Conventional Example No. 28), which was used as a comparison, the time taken until fracture was 5.5 hr.

◎: breaking time≥10hr

○: 6hr≤ break time <10hr

×: breaking time <6hr

In the above evaluation, (circle) and (circle) were set to pass, and x was set to fail. The obtained results are shown in Table 1 (refer to creep 900°C in Table 1).

Next, using one of the remainder of the sheet bar divided into two in the above, after heating at 1100 ° C., hot forging was performed to obtain a 30 mm square bar. Next, after annealing at a temperature of 1000 to 1150 ° C., machining was performed to form a thermal fatigue test piece having the shape and dimensions shown in FIG. 2, and subjected to the thermal fatigue test described below. The annealing temperature was the temperature at which the structure was confirmed for each component and recrystallization was completed. Also, for reference, a test piece was prepared in the same manner as above for a steel having a component composition of SUS444 (Conventional Example No. 28), and subjected to a thermal fatigue test.

<Thermal fatigue test>

As shown in Fig. 3, the thermal fatigue test was conducted under conditions of repeating temperature increase and decrease between 200°C and 950°C while restraining the test piece with a restraint factor of 0.5. At this time, the temperature increase rate was 5°C/sec, and the temperature decrease rate was 2°C/sec. The holding time at 200°C and 950°C was 30 seconds, respectively. As for the above restraint factor, as shown in Fig. 3, it can be expressed as the restraint factor η=a/(a+b), where a is (free thermal expansion strain - controlled strain)/2, and b is controlled strain. is quantity/2. In addition, the amount of free thermal expansion strain is the amount of strain when the temperature is raised without applying any mechanical stress, and the amount of controlled strain represents the absolute value of the amount of strain occurring during the test. The actual amount of constraint strain generated in the material by the constraint is (amount of free thermal expansion strain - amount of controlled strain).

In addition, the thermal fatigue life is calculated by dividing the load detected at 200 ° C. by the cross-sectional area of the test piece crack parallel portion (see Fig. 2), and calculates the stress, with respect to the stress value of the initial cycle (the 5th cycle when the test is stable) The number of cycles at which the stress value decreased to 75% was evaluated as follows. For SUS444 (Conventional Example No. 28), which was used as a comparison, the thermal fatigue life was 650 cycles.

◎: 1000 cycles or more (passed)

○: 800 cycles or more and less than 1000 cycles (pass)

×: Less than 800 cycles (disqualified)

In the above evaluation, ◎ and ○ were regarded as pass, and × were regarded as disqualified. The obtained results are shown in Table 1 (see Thermal Fatigue Life 950°C in Table 1).

From Table 1, all of the ferritic stainless steels of the examples of the present invention (hereafter, ferritic stainless steels are simply referred to as steel) have properties superior to SUS444 (the steel of prior art No. 28) in creep tests and thermal fatigue tests. represents

Steel No. 29 had a Nb+Mo content of less than 2.3% by mass, and the creep rupture time and thermal fatigue life were unacceptable. The steel of No. 30 had a Ni content of more than 0.60% by mass, and the thermal fatigue life was unacceptable. The steel of No. 31 had a Cr content of less than 18.0% by mass, and the thermal fatigue life was unacceptable. In the steel of No. 32, the Mo content was less than 1.80% by mass, and the creep rupture time and thermal fatigue life were disqualified. In the steel of No. 33, the Nb content was less than 0.30% by mass, and both the creep rupture time and the thermal fatigue life were disqualified. In the steel No. 34, the Si content was less than 0.1% by mass, oxidation was significantly observed in both the creep test and the thermal fatigue test, and both the creep rupture time and the thermal fatigue life were disqualified. The steel of No. 35 had a Ti content of more than 0.16% by mass, and both the creep rupture time and the thermal fatigue life were disqualified. The steel of No. 36 had a Cr content of more than 20.0% by mass, and the thermal fatigue life was disqualified due to embrittlement of the steel. Steel No. 37 had a Mn content of less than 0.05% by mass, peeling of oxide scale occurred during the thermal fatigue test, and the thermal fatigue life was disqualified. The steel of No. 38 had a C content of more than 0.020% by mass, and both the creep rupture time and the thermal fatigue life were disqualified due to the decrease in the amount of Nb in the steel. In the steel No. 39, the N content was more than 0.015% by mass, and the creep rupture time and thermal fatigue life were unacceptable due to the decrease in the amount of Nb in the steel due to the precipitation of Nb nitride. The steel of No. 40 had a Sb content of more than 0.50% by mass, and the thermal fatigue life was unacceptable due to a decrease in hot ductility. In the steel No. 41, the Mo content exceeded 2.50% by mass, and a coarse σ phase (Fe-Cr-based intermetallic compound) precipitated during the thermal fatigue test, and the thermal fatigue life was unacceptable. Moreover, the creep rupture time also became disqualified. In the steel of No. 43, the Sn content exceeded 0.005% by mass, and the thermal fatigue life was unacceptable. The steel of No. 45 did not contain Sb, and both the creep rupture time and the thermal fatigue life were disqualified. In the steel No. 46, the Nb content exceeded 0.80% by mass, and both the creep rupture time and the thermal fatigue life were disqualified. In the steel No. 47, the Nb+Mo content exceeded 3.0%, and both the creep rupture time and the thermal fatigue life were disqualified.

The ferritic stainless steel of the present invention is not only suitable for exhaust system members of automobiles and the like, but can also be suitably used as exhaust system members of thermal power generation systems and solid oxide fuel cell members requiring the same characteristics.

Claims (5)

in mass percent,
C: more than 0% and 0.020% or less;
Si: 0.1 to 1.0%;
Mn: 0.05 to 0.60%;
P: more than 0% and 0.050% or less;
S: more than 0% and 0.008% or less;
Ni: 0.02 to 0.60%;
Al: 0.001 to 0.25%;
Cr: 18.0 to 20.0%;
Nb: 0.30 to 0.80%;
Mo: more than 2.00% and 2.50% or less;
N: more than 0% and 0.015% or less;
Sb: 0.002 to 0.50%
A ferritic stainless steel that contains, and satisfies the following formula (1), the balance being Fe and unavoidable impurities.
Nb+Mo: 2.3 to 3.0%... (One)
(Nb and Mo in Formula (1) represent the content (mass %) of each element) According to claim 1,
The component composition is, in mass%, further,
Ti: 0.01 to 0.16%;
Zr: 0.01 to 0.50%;
Co: 0.01 to 0.50%;
W: 0.01 to 5.0%;
Cu: 0.01 to 0.40%;
Sn: 0.001 to 0.005%
A ferritic stainless steel containing one or two or more selected from among. According to claim 1 or 2,
The component composition is, in mass%, further,
Ca: 0.0002% to 0.0050%;
Mg: 0.0002 to 0.0050%
A ferritic stainless steel containing one or two selected from among. According to claim 1 or 2,
Ferritic stainless steel used for exhaust manifolds that raise the temperature to 700°C or higher by the exhaust gas from the engine. According to claim 3,
Ferritic stainless steel used for exhaust manifolds that raise the temperature to 700°C or higher by the exhaust gas from the engine.

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