WO2019151124A1

WO2019151124A1 - Ferritic stainless steel

Info

Publication number: WO2019151124A1
Application number: PCT/JP2019/002412
Authority: WO
Inventors: 徹之中村; 石川　伸; 杉原　玲子
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2018-01-31
Filing date: 2019-01-25
Publication date: 2019-08-08
Also published as: JPWO2019151124A1; JP6624345B1

Abstract

Provided is ferritic stainless steel having both exceptional resistance to repeated oxidation and exceptional thermal fatigue characteristics. Ferritic stainless steel that contains, in terms of percent by mass, 0.020% or less of C, 0.25-1.0% of Si, 0.05-0.50% of Mn, 0.050% or less of P, 0.008% or less of S, 0.02-0.60% of Ni, 0.001-0.08% of Al, 18.0-20.0% of Cr, 0.30-0.80% of Nb, 1.80-2.50% of Mo, 0.015% or less of N, 0.01-0.40% of Zr, and 0.01-0.30% of Co, the balance being Fe and unavoidable impurities, and the ferritic stainless steel satisfying formula (1). Formula (1): Zr + Co: 0.03-0.50% (Zr and Co in formula (1) indicate the respective element contents (in terms of percent by mass) thereof.)

Description

Ferritic stainless steel

TECHNICAL FIELD The present invention relates to ferritic stainless steel, and particularly, excellent repeated oxidation resistance suitable for use in exhaust system members used at high temperatures, such as exhaust pipes and converter cases of automobiles and motorcycles, and exhaust ducts of thermal power plants. And ferritic stainless steel having thermal fatigue properties.

Excellent heat resistance is required for exhaust system members such as exhaust manifolds, exhaust pipes, converter cases, and mufflers of automobiles. There are several types of heat resistance, including thermal fatigue properties, high temperature fatigue properties, high temperature strength (high temperature strength), oxidation resistance, creep properties, and high temperature salt corrosion properties. Among them, the thermal fatigue property is one of particularly important heat resistances. The exhaust system member repeatedly receives heating and cooling as the engine is started and stopped. At this time, since the exhaust system member is connected to peripheral components, thermal expansion and contraction are limited, and thermal distortion occurs in the material itself. The low-cycle fatigue phenomenon that results in fracture due to repeated thermal strain is called thermal fatigue.

Also, the oxidation resistance includes continuous oxidation resistance and repeated oxidation resistance. The continuous oxidation resistance evaluates the presence or absence of occurrence of abnormal oxidation that significantly increases the oxide scale when held at a high temperature for a long time. On the other hand, the repeated oxidation resistance evaluates the presence or absence of peeling of the oxide scale due to the difference in thermal expansion between the steel and the oxide scale when the temperature rise and fall are repeated as in the thermal fatigue described above. When exfoliation of oxide scale occurs in an exhaust system member of an automobile, not only is the material of the exfoliated part thinned, but thermal fatigue failure is likely to occur, and the inflow of the exfoliated oxide scale into the exhaust pipe The oxidation resistance is important because it leads to engine troubles such as hindering the purification function and clogging of the exhaust pipe.

Ferritic stainless steel such as Type 429 (14% Cr-0.9% Si-0.4% Nb system) to which Nb and Si are added is currently used as a material for the above-described members that require thermal fatigue characteristics. Is often used. However, when the exhaust gas temperature rises to a temperature exceeding 900 ° C. as the engine performance is improved, Type 429 is unable to satisfy the thermal fatigue characteristics sufficiently.

As a material that can cope with this problem, for example, SUS444 (19% Cr-0.5% Nb-2%) defined in JIS G4305, which is a ferritic stainless steel in which high temperature proof stress is improved by adding Nb and Mo. Mo), or ferritic stainless steel to which Nb, Mo and W are added has been developed (see, for example, Patent Document 1). However, for the purpose of responding to the recent tightening of exhaust gas regulations and improving fuel efficiency, the exhaust gas temperature tends to increase, and even SUS444 and the like are insufficient in heat resistance. Ferritic stainless steel often has insufficient oxidation resistance, especially among heat resistance.

JP 2004-018921 A

SUS444 has the highest level of heat resistance among ferritic stainless steels, but it cannot always be said that the heat resistance is sufficient when the exhaust gas temperature rises with the recent tightening of exhaust gas regulations and improved fuel economy. As the exhaust gas temperature rises, the thermal expansion of the exhaust system member when the temperature rises increases, so that more severe thermal strain is added. Thereby, the ferritic stainless steel used for the exhaust system member is likely to be subject to thermal fatigue failure. In addition, ferritic stainless steel often lacks oxidation resistance. This is because the thermal expansion of the raw material increases due to the high temperature of the exhaust gas, thereby increasing the difference in thermal expansion from the oxide scale generated by being held at a high temperature, and therefore, the oxide scale is liable to peel off during cooling. When the oxide scale is peeled off, the thickness of the material becomes thin and it becomes easy to break down due to thermal fatigue. In addition, the peeled oxide scale may flow into the catalyst or the like in the exhaust pipe, thereby inhibiting the purification action. Therefore, it is required that the oxide scale has high peel resistance (repetitive oxidation resistance).

As described above, the conventional techniques including SUS444 have not been able to obtain a ferritic stainless steel having sufficient thermal fatigue characteristics and repeated oxidation resistance even when the exhaust gas temperature is increased.

Therefore, an object of the present invention is to solve such problems and to provide a ferritic stainless steel excellent in both repeated oxidation resistance and thermal fatigue characteristics.

In the present invention, “excellent in resistance to repeated oxidation” means repeated oxidation tests (heating rate: 5 ° C./sec, cooling) in which 400 cycles of holding at 1050 ° C. for 20 minutes and holding at 100 ° C. for 1 minute are performed in the air (Rate: 2 ° C./sec) after abnormal oxidation (oxidation increase of 50 g / m ² or more) does not occur, and the oxide scale exfoliation amount is less than 1 g / m ² .

Further, “excelling in thermal fatigue characteristics” means having characteristics superior to that of SUS444. Specifically, the thermal fatigue life when heating and cooling are repeated between 200 and 950 ° C. is superior to that of SUS444. It means that

In order to develop a ferritic stainless steel having superior resistance to repeated oxidation and thermal fatigue than SUS444, the present inventors conducted extensive studies on the effects of various elements on the resistance to repeated oxidation and thermal fatigue.

As a result, by mass containing Nb in the range of 0.30 to 0.80% and Mo in the range of 1.80 to 2.50%, the high temperature strength increases in a wide temperature range, and the thermal fatigue characteristics are improved. I found it to improve. Further, it has been found that the resistance to repeated oxidation is improved by containing both Zr and Co. Zr suppresses exfoliation of oxide scale by thinning the oxide scale generated at high temperature, and Co has an effect of making it difficult to exfoliate oxide scale generated by suppressing thermal expansion. By containing them simultaneously, excellent repeated oxidation resistance can be obtained.

Based on the above knowledge, the present invention has been completed by containing an appropriate amount of Cr, Nb, Mo, Zr and Co and adjusting the content of other essential elements. If even one of the above elements is not contained in an appropriate amount, the excellent repeated oxidation resistance and thermal fatigue properties desired by the present invention cannot be obtained.

The gist of the present invention is as follows.

[1] By mass%, C: 0.020% or less, Si: 0.25 to 1.0%, Mn: 0.05 to 0.50%, P: 0.050% or less, S: 0.008 %: Ni: 0.02 to 0.60%, Al: 0.001 to 0.08%, Cr: 18.0 to 20.0%, Nb: 0.30 to 0.80%, Mo: 1 80 to 2.50%, N: 0.015% or less, Zr: 0.01 to 0.40%, Co: 0.01 to 0.30%, and the following formula (1): A ferritic stainless steel that has a composition that is filled and the balance is composed of Fe and inevitable impurities.
Zr + Co: 0.03 to 0.50% (1)
(Zr and Co in the formula (1) indicate the content (mass%) of each element.)

[2] The component composition is in mass%, further Ti: 0.01 to 0.04%, Sb: 0.002 to 0.50%, B: 0.0002 to 0.0050%, V: 0 0.01 to 1.0%, W: 0.01 to 0.30%, Cu: 0.01 to 0.20%, Sn: 0.001 to 0.005% The ferritic stainless steel according to [1] containing the above.

[3] The component composition contains, by mass%, one or two selected from Ca: 0.0002 to 0.0050% and Mg: 0.0002 to 0.0050% [1] ] Or ferritic stainless steel according to [2].

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

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

It is a figure explaining a thermal fatigue test piece. It is a figure explaining the temperature and restraint conditions in a thermal fatigue test.

Hereinafter, the present invention will be described in detail.

The ferritic stainless steel of the present invention is, in mass%, C: 0.020% or less, Si: 0.25% or more and 1.0% or less, Mn: 0.05% or more and 0.50% or less, P: 0 0.050% or less, S: 0.008% or less, Ni: 0.02% to 0.60%, Al: 0.001% to 0.08%, Cr: 18.0% to 20.0% Hereinafter, Nb: 0.30% or more and 0.80% or less, Mo: 1.80% or more and 2.50% or less, N: 0.015% or less, Zr: 0.01% or more and 0.40% or less, Co : Ferritic stainless steel that contains 0.01% or more and 0.30% or less, satisfies the following formula (1), and the balance is composed of Fe and inevitable impurities.
Zr + Co: 0.03 to 0.50% (1)
(Zr and Co in the formula (1) indicate the content (mass%) of each element.)

In the present invention, the balance of the component composition is very important, and by using the combination of the component compositions as described above, it is possible to obtain a ferritic stainless steel superior in resistance to repeated oxidation and thermal fatigue than SUS444. . If the content of the essential elements (C, Si, Mn, Ni, Al, Cr, Nb, Mo, N, Zr, Co) in the above component composition is out of the range, the desired repeated oxidation 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 for increasing the strength of steel. However, when C is contained in excess of 0.020%, not only the deterioration of toughness and formability becomes significant, Oxidation resistance and thermal fatigue characteristics cannot be obtained sufficiently. Therefore, the C content is 0.020% or less. In addition, it is preferable that C content shall be 0.010% or less from a viewpoint of ensuring a 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 set to 0.001% or more. More preferably, the C content is 0.003% or more. More preferably, the C content is 0.006% or more.

Si: 0.25 to 1.0%
Si is an important element necessary for improving oxidation resistance. In order to ensure oxidation resistance in the exhaust gas heated to a high temperature, it is necessary to contain 0.25% or more of Si. When the Si content is less than 0.25%, the resistance to repeated oxidation is insufficient, and the thermal fatigue characteristics cannot be sufficiently obtained. On the other hand, the excessive Si content exceeding 1.0% lowers the workability at room temperature, so the upper limit of the Si content is 1.0%. On the other hand, when the Si content exceeds 1.0%, good repeated oxidation resistance and thermal fatigue life cannot be obtained. Preferably, the Si content is 0.35% or more. More preferably, the Si content is 0.45% or more. More preferably, the Si content is 0.50% or more. Preferably, the Si content is 0.90% or less. More preferably, the Si content is 0.80% or less.

Mn: 0.05 to 0.50%
Mn has the effect of increasing the resistance to repeated oxidation. In order to obtain these effects, it is necessary to contain 0.05% or more of Mn. On the other hand, when Mn is excessively contained in excess of 0.50%, a γ phase is likely to be formed at a high temperature, and on the contrary, the repeated oxidation resistance and thermal fatigue characteristics are lowered. Therefore, the Mn content is 0.05% or more and 0.50% or less. Preferably, the Mn content is 0.10% or more. More preferably, the Mn content is 0.20% or more. Preferably, the Mn content is 0.40% or less. More preferably, the Mn content is 0.30% or less.

P: 0.050% or less P is a harmful element that lowers the toughness of steel, and is desirably reduced as much as possible. Therefore, the P content is 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 lowers elongation and r value, adversely affects formability, and lowers corrosion resistance, which is a basic characteristic of stainless steel, so it is desirable to reduce it as much as possible. . Therefore, in the present invention, the S content is set to 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 0.02% or more. If the oxidation resistance is insufficient, thermal fatigue characteristics deteriorate due to a decrease in the cross-sectional area of the material due to an increase in the amount of oxide scale generated and peeling of the oxide scale. On the other hand, since Ni is a strong γ-phase-forming element, it generates a γ-phase at a high temperature and lowers the oxidation resistance, and also increases the thermal expansion coefficient and lowers the thermal fatigue characteristics. Therefore, the upper limit of the Ni content is 0.60%. Preferably, the Ni content is 0.10% or more. More preferably, the Ni content is 0.20% or more. Preferably, the Ni content is 0.40% or less. More preferably, the Ni content is less than 0.25%.

Al: 0.001 to 0.08%
Al is an element having an effect of improving oxidation resistance. In order to acquire 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 thermal expansion coefficient is increased, the thermal fatigue characteristics are degraded. Furthermore, the steel becomes extremely hard and the workability is reduced. Therefore, the Al content is set to 0.08% or less. Preferably, the Al content is 0.01% or more. More preferably, the Al content is 0.02% or more. More preferably, the Al content is more than 0.03%. Preferably, the Al content is 0.05% or less. More preferably, the Al content is 0.04% or less.

Cr: 18.0-20.0%
Cr is an important element effective in improving the corrosion resistance and oxidation resistance, which are the characteristics of stainless steel. However, if the Cr content is less than 18.0%, sufficient oxidation resistance in a high temperature range exceeding 900 ° C. Cannot be obtained. If the oxidation resistance is insufficient, the amount of oxide scale generated increases, and the thermal fatigue characteristics also decrease as the cross-sectional area of the material decreases. On the other hand, Cr is an element that solidifies and strengthens steel at room temperature, and hardens and lowers ductility. When the Cr content exceeds 20.0%, the above-described adverse effects become significant, and the thermal fatigue properties also deteriorate. Therefore, the upper limit of the Cr content is 20.0%. Preferably, the Cr content is 18.5% or more. Preferably, the Cr content is 19.5% or less.

Nb: 0.30 to 0.80%
Nb is an important element in the present invention that increases the high temperature strength and improves the thermal fatigue characteristics. Such an effect is recognized when the content of Nb is 0.30% or more. When the Nb content is less than 0.30%, the strength at high temperature is insufficient, and excellent thermal fatigue characteristics cannot be obtained. However, if the Nb content exceeds 0.80%, the Laves phase (Fe ₂ Nb), which is an intermetallic compound, is likely to precipitate, the high-temperature strength decreases, and the thermal fatigue characteristics and creep characteristics only decrease. It promotes embrittlement. Therefore, the Nb content is 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 more than 0.50%. Preferably, the Nb content is 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 thermal fatigue properties by dissolving in steel and improving the high-temperature strength of the steel. The effect appears when the Mo content is 1.80% or more. When the Mo content is less than 1.80%, the high temperature strength is insufficient, and excellent thermal fatigue characteristics cannot be obtained. On the other hand, the excessive Mo content not only hardens the steel and lowers the workability, but also precipitates as a Laves phase (Fe2Mo) in the same manner as Nb, and the amount of solute Mo in the steel is reduced. Thermal fatigue properties will be reduced. In addition, precipitation as a coarse σ phase during the thermal fatigue test serves as a starting point for fracture, and the thermal fatigue characteristics deteriorate. Therefore, the upper limit of the Mo content is 2.50%. Preferably, the Mo content is 1.90% or more. More preferably, the Mo content is over 2.00%. Preferably, the Mo content is 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. When it exceeds 0.015%, not only does the decrease in toughness and formability become remarkable, but also resistance to repeated oxidation. And thermal fatigue properties are also insufficient. Therefore, the N content is set to 0.015% or less. N is preferably reduced as much as possible from the viewpoint of securing toughness and formability, and the N content is preferably less than 0.010%.

Zr: 0.01 to 0.40%
Zr is an important element in the present invention that improves the resistance to repeated oxidation by thinning the oxide scale produced at high temperature, suppressing abnormal oxidation and suppressing exfoliation of the oxide scale. When the oxide scale becomes thick, the oxide scale tends to peel off when a temperature change occurs, particularly during cooling. When the oxide scale is peeled off, the material is thinned, so that excellent thermal fatigue characteristics cannot be obtained. In order to obtain this effect, the Zr content is set to 0.01% or more. However, if the Zr content exceeds 0.40%, a Zr intermetallic compound precipitates, embrittles the steel, and the thermal fatigue characteristics are degraded. Therefore, the Zr content is set to 0.01 to 0.40%. Preferably, the Zr content is 0.02% or more. More preferably, the Zr content is 0.03% or more. Preferably, the Zr content is not more than 0.30%. More preferably, the Zr content is 0.10% or less.

Co: 0.01 to 0.30%
Co is an important element in the present invention that improves the resistance to repeated oxidation by reducing the thermal expansion coefficient of steel and reducing the difference in thermal expansion between the oxide scale and steel. In order to obtain this effect, the Co content needs to be 0.01% or more. On the other hand, since excessive Co content lowers the toughness of the steel, the upper limit of Co content is 0.30%. Preferably, the Co content is 0.02% or more. More preferably, the Co content is 0.03% or more. Preferably, the Co content is 0.10% or less. More preferably, the Co content is 0.05% or less.

Zr + Co: 0.03 to 0.50% (1)
As described above, Zr suppresses abnormal oxidation by reducing the oxide scale and prevents exfoliation of the oxide scale, and Co suppresses exfoliation of the oxide scale by reducing the thermal expansion coefficient of the steel. Repeated oxidation resistance is greatly improved by containing both Zr and Co simultaneously. In order to obtain excellent repeated oxidation resistance in a high temperature range exceeding 900 ° C., both elements are contained within a predetermined range, and at least the amount of Zr + Co (total content of Zr and Co) is 0.03% or more. It is necessary to. When the amount of Zr + Co is less than 0.03%, the repeated oxidation resistance is insufficient, the oxide scale is peeled off, and the thermal fatigue characteristics are also lowered. Preferably, the amount of Zr + Co is 0.05% or more. On the other hand, if the amount of Zr + Co is excessive, the steel becomes brittle and the thermal fatigue properties are lowered. Therefore, the upper limit of the amount of Zr + Co is 0.50%. Preferably, the amount of Zr + Co is 0.30% or less.

In addition, Zr and Co in said formula (1) show content (mass%) of each element.

In the ferritic stainless steel of the present invention, the balance consists of Fe and inevitable impurities.

In addition to the essential components described above, the ferritic stainless steel of the present invention further includes, as an optional component, one or more selected from Ti, Sb, B, V, W, Cu, and Sn. It can be contained in a range.

Ti: 0.01 to 0.04%
Ti is an element that fixes C and N, improves corrosion resistance and formability, and prevents intergranular corrosion of the welded portion. In the present invention, Ti can be contained as necessary. By containing Ti, Ti is preferentially combined with C and N over Nb, so that it is possible to secure a solid solution Nb amount in steel effective for high-temperature strength, which is effective in improving heat resistance. This effect is obtained when the Ti content is 0.01% or more. On the other hand, if the Ti content exceeds 0.04%, it becomes easy to exfoliate the oxide scale, and not only the resistance to repeated oxidation decreases, but also the Nb carbonitride precipitates with the Ti carbonitride as the core. Since it becomes easy, the amount of solute Nb in steel effective for high-temperature strength is reduced, and thermal fatigue characteristics and creep characteristics are also deteriorated. Therefore, when Ti is contained, the Ti content is set to 0.01 to 0.04%. Preferably, the Ti content is 0.02% or less.

Sb: 0.002 to 0.50%
Sb is an element having an effect of improving creep resistance. Sb dissolves in the steel and suppresses creep deformation of the steel. Since Sb does not precipitate as a carbonitride or Laves phase even in a high temperature range and continues to dissolve in steel even after a long period of use, the creep resistance can be improved. Improvement of the creep resistance is effective for improving thermal fatigue characteristics particularly when the temperature is raised to 900 ° C. or higher. This effect is obtained when the Sb content is 0.002% or more. On the other hand, an excessive content of Sb reduces the toughness and hot workability of the steel, so that not only cracking is likely to occur during production, but also the thermal fatigue properties are reduced due to the decrease in hot ductility. Therefore, when Sb is contained, the Sb content is set to 0.002 to 0.50%. Preferably, the Sb content is 0.005% or more. More preferably, the Sb content is 0.020% or more. Moreover, Preferably Sb content is 0.30% or less. More preferably, the Sb content is 0.10% or less.

B: 0.0002 to 0.0050%
B is an element effective for improving the workability of steel, particularly the secondary workability. Such an effect can be obtained with a B content of 0.0002% or more. On the other hand, excessive B content generates BN and degrades workability. Therefore, when B is contained, the B content is set to 0.0002 to 0.0050%. Preferably, the B content is 0.0005% or more. More preferably, the B content is 0.0008% or more. Preferably, the B content is 0.0030% or less. More preferably, the B content is 0.0020% or less.

V: 0.01 to 1.0%
V is an element effective for improving the workability of steel and an element effective for improving oxidation resistance. These effects become significant when the V content is 0.01% or more. However, the excessive V content exceeding 1.0% leads to the precipitation of coarse V (C, N), not only lowering the toughness but also lowering the surface properties. Therefore, when V is contained, the V content is set to 0.01 to 1.0%. Preferably, the V content is 0.03% or more. Preferably, the V content is 0.50% or less. More preferably, the V content is 0.10% or less. More preferably, the V content is less than 0.05%.

W: 0.01 to 0.30%
W, like Mo, is an element that greatly improves high-temperature strength by solid solution strengthening. This effect is obtained with a W content of 0.01% or more. On the other hand, an excessive content not only makes the steel remarkably hard, but also produces a strong scale in the annealing process during production, so that descaling during pickling becomes difficult. Therefore, when W is contained, the W content is set to 0.01 to 0.30%. Preferably, the W content is 0.02% or more. Preferably, the W content is 0.20% or less. More preferably, the W content is less than 0.10%.

Cu: 0.01 to 0.20%
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 with a Cu content of 0.01% or more. On the other hand, when it contains Cu exceeding 0.20%, an oxide scale will peel easily and a repeated oxidation-proof characteristic will fall. If the resistance to repeated oxidation is insufficient, the oxide scale is easily peeled off, and thermal fatigue damage is also deteriorated because it becomes easy to cause thermal fatigue failure starting from the peeled portion. Therefore, when Cu is contained, the Cu content is set to 0.01 to 0.20%. Preferably, the Cu content is 0.02% or more. More preferably, the Cu content is 0.03% or more. Preferably, the Cu content is 0.10% or less. More preferably, the Cu content is 0.06% or less.

Sn: 0.001 to 0.005%
Sn is an element effective for improving the high-temperature strength of steel. This effect is obtained when the Sn content is 0.001% or more. On the other hand, the excessive Sn content embrittles the steel and decreases the thermal fatigue properties. Therefore, when Sn is contained, the Sn content is set to 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 Ca and Mg as optional components within the following range.

Ca: 0.0002 to 0.0050%
Ca is an effective component for preventing nozzle clogging due to precipitation of Ti-based inclusions that are likely to occur during continuous casting. The effect is obtained with a Ca content of 0.0002% or more. On the other hand, in order to obtain good surface properties without generating surface defects, the Ca content needs to be 0.0050% or less. Therefore, when Ca is contained, the Ca content is set to 0.0002 to 0.0050%. Preferably, the Ca content is 0.0005% or more. Preferably, the Ca content is 0.0030% or less. More preferably, the Ca content is 0.0020% or less.

Mg: 0.0002 to 0.0050%
Mg is an element that improves the equiaxed crystal ratio of the slab and is effective in improving workability and toughness. In the steel containing Nb and Ti as in the present invention, Mg also has an effect of suppressing the coarsening of Nb and Ti carbonitrides. The effect is obtained when the Mg content is 0.0002% or more. When the Ti carbonitride becomes coarse, it becomes a starting point for brittle cracking, so that the toughness is greatly reduced. When Nb carbonitrides become coarse, the amount of Nb solid solution in steel decreases, leading to a decrease in thermal fatigue characteristics. On the other hand, when the Mg content exceeds 0.0050%, the surface properties of the steel are deteriorated. Therefore, when Mg is contained, the Mg content is set to 0.0002 to 0.0050%. Preferably, the Mg content is 0.0003% or more. More preferably, the Mg content is 0.0004% or more. Preferably, the Mg content is 0.0030% or less. More preferably, the Mg content is 0.0020% or less.

The balance is Fe and inevitable impurities. When the optional component is contained below the lower limit, the optional component contained at a content below the lower limit is included as an inevitable impurity.

Next, a method for producing the ferritic stainless steel of the present invention will be described.

The manufacturing method of the ferritic stainless steel of the present invention can be suitably employed as long as it is a normal manufacturing method of ferritic stainless steel, and is not particularly 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, and the steel having the above-described component composition of the present invention. It is made into a steel slab (slab) by the ingot-bundling rolling method, and then made into a cold-rolled annealed plate through processes such as hot-rolling, hot-rolled sheet annealing, pickling, cold-rolling, finish annealing and pickling. It can be manufactured in a manufacturing process. The cold rolling may be performed once or two or more cold rolling sandwiching the intermediate annealing, and the steps of cold rolling, finish annealing, and pickling may be performed repeatedly. Furthermore, hot-rolled sheet annealing may be omitted, and skin pass rolling may be performed after cold rolling or after finish annealing when surface gloss or roughness adjustment of the steel sheet is required.

Favorable manufacturing conditions in the above manufacturing method will be described.

The steelmaking process for melting steel includes secondary refining of steel melted in a converter or electric furnace by the VOD method, AOD method, etc., and steel containing the above essential components and optional components added as necessary. It is preferable to do. Although the molten steel can be made into a steel material by a known method, it is preferable to use a continuous casting method in terms of productivity and quality. Thereafter, the steel material is preferably heated to 1050 to 1250 ° C., and hot rolled into a desired thickness by hot rolling. In production, the thickness of the hot-rolled sheet is preferably 5 mm or less. Of course, hot working can be performed in addition to the 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, It is preferable to do. If necessary, the scale may be removed by shot blasting before pickling.

Furthermore, the hot-rolled product (hot-rolled annealed plate) may be a cold-rolled product through a process such as cold rolling. In this case, the cold rolling may be performed once, but may be performed twice or more with intermediate annealing in view of productivity and required quality. The total rolling reduction of one or more cold rollings is preferably 60% or more, more preferably 70% or more. The cold-rolled steel sheet is subsequently subjected to continuous annealing (finish annealing) at a temperature of preferably 900 to 1200 ° C., more preferably 1000 to 1150 ° C., pickling or polishing, and a cold-rolled product (cold-rolled annealing plate). It is preferable to do. Finish annealing may be performed in a reducing atmosphere, and in that case, pickling or polishing after finish annealing may be omitted. Further, 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 crystal grain size after the finish annealing is desirably 100 μm or less from the viewpoint of surface properties at the time of component molding.

The hot-rolled product or cold-rolled product obtained as described above is then subjected to processing such as cutting, bending processing, overhanging processing, drawing processing, etc. according to the respective use, and exhaust pipes and catalysts for automobiles and motorcycles. It is molded into an outer cylinder material, an exhaust duct of a thermal power plant or a fuel cell-related member, such as a separator, an interconnector or a reformer. Among these, the ferritic stainless steel of the present invention is preferably used for exhaust system members such as exhaust manifolds, exhaust pipes, converter cases, and mufflers. In particular, it is one of the features 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 for welding these members is not particularly limited, and normal arc welding such as MIG (Metal Inert Gas), MAG (Metal Active Gas), TIG (Tungsten Inert Gas), spot welding, and seam welding. For example, resistance welding such as high frequency resistance welding such as electric resistance welding, high frequency induction welding, and the like can be applied.

Hereinafter, the present invention will be described in detail with reference to examples.

No. shown in Table 1. Steel having a component composition of 1-8, 10, 12-46 was melted in a vacuum melting furnace, cast into a 50 kg steel ingot, heated at 1170 ° C., and then hot rolled into a 35 mm thick sheet bar. . The sheet bar is divided into two parts, and one of the steel ingots is heated to 1100 ° C, then hot-rolled into a hot-rolled sheet having a thickness of 5 mm, annealed at a temperature in the range of 1000 to 1150 ° C, then ground and heated A fire annealed plate was used. Subsequently, after cold rolling with a rolling reduction of 70% and finish annealing at a temperature of 1000 to 1150 ° C., the scale is removed by pickling or polishing to form a cold-rolled annealed plate having a thickness of 1.5 mm. And subjected to repeated oxidation tests. For reference, SUS444 (conventional example No. 24) was prepared in the same manner as above, and a cold-rolled annealed plate was prepared and subjected to an oxidation test. About annealing temperature, temperature was determined about each steel, confirming a structure within the said temperature range.

<Repetitive oxidation test>
The cold-rolled annealed plate was cut into a dimension of 20 mm width × 30 mm length, and all six surfaces were polished with # 320 emery paper and used for the test. The oxidation test conditions were 400 cycles of holding 20 minutes at 1050 ° C. and holding 1 minute at 100 ° C. in the atmosphere. The heating rate and the cooling rate were 5 ° C./sec and 2 ° C./sec, respectively. The test was performed by placing the test piece in an alumina crucible so that the oxide scale peeled off during the test could be recovered. The increase in oxidation was calculated from the change in the total weight of the crucible and test piece before and after the test. When the increase in oxidation was 50 g / m ² or more, it was judged that abnormal oxidation had occurred. Furthermore, a value obtained by subtracting the oxidation increase obtained from the weight change of only the test piece from the increase in oxidation calculated from the total weight change of the crucible and the test piece, that is, when the oxide scale peeling amount is 1 g / m ² or more. Was judged to have peeled off. The repeated oxidation resistance was evaluated as follows. For SUS444 (conventional example No. 24), both the occurrence of abnormal oxidation and peeling of the oxide scale were observed.

◎: abnormal None oxide and the oxide scale peeling amount 0.1 g / ^{m 2} or less ○: no abnormal oxidation and oxide scale peeling amount 0.1 g / ^{m 2} exceeds 1g / ^{m 2} less ×: abnormal oxidation occurrence or oxide scale exfoliation amount 1g / M ² or more In the above evaluation, ◎ and ○ were passed, and × was rejected. The obtained results are shown in Table 1 (refer to repeated oxidation at 1050 ° C. in Table 1).

Next, after using one of the remaining sheet bars divided into two in the above, it was heated to 1100 ° C. and then hot forged into 30 mm square bars. Next, after annealing at a temperature of 1000 to 1150 ° C., it was machined, processed into a thermal fatigue test piece having the shape and dimensions shown in FIG. 1, and subjected to the following thermal fatigue test. The annealing temperature was a temperature at which recrystallization was completed after confirming the structure for each component. For reference, a test piece was prepared in the same manner as described above for a steel having a component composition of SUS444 (conventional example No. 24) and subjected to a thermal fatigue test.

<Thermal fatigue test>
As shown in FIG. 2, the thermal fatigue test was performed under the condition that the temperature rise / fall was repeated between 200 ° C. and 950 ° C. while restraining the test piece at a restraint rate of 0.5. At this time, the temperature rising rate was 5 ° C./second, and the temperature decreasing rate was 2 ° C./second. And the holding time in 200 degreeC and 950 degreeC was 30 seconds, respectively. As shown in FIG. 2, the above constraint rate can be expressed as the constraint rate η = a / (a + b), where a is (free thermal expansion strain amount−control strain amount) / 2, b Is controlled strain / 2. The free thermal expansion strain amount is the strain amount when the temperature is raised without applying any mechanical stress, and the control strain amount indicates the absolute value of the strain amount generated during the test. The substantial restraint strain amount generated in the material by restraint is (free thermal expansion strain amount−control strain amount).

The thermal fatigue life is calculated by dividing the load detected at 200 ° C. by the cross-sectional area of the test piece soaking parallel part (see FIG. 1), and calculating the stress. The number of cycles in which the stress value was reduced to 75% with respect to the stress value was evaluated as follows.

A: 1000 cycles or more (pass)
○: 800 cycles or more and less than 1000 cycles (pass)
X: Less than 800 cycles (failed)
In the above evaluation, “A” and “B” were passed, and “B” was rejected. The obtained results are shown in Table 1 (see thermal fatigue life 950 ° C. in Table 1).

From Table 1, No. of the present invention example. Ferritic stainless steels 1 to 8, 10, 12 to 23, 45 and 46 (hereinafter, ferritic stainless steel is simply referred to as steel) are all SUS444 (conventional example No. 1) in repeated oxidation tests and thermal fatigue tests. 24 steel).

No. Steel No. 25 had a Mo content of less than 1.80% by mass, and the thermal fatigue life was rejected. No. Steel No. 26 had a Zr content of less than 0.01% by mass, and both its resistance to repeated oxidation and thermal fatigue life were rejected. No. In Steel No. 27, the amount of Zr + Co was less than 0.03% by mass, and the repeated oxidation resistance and thermal fatigue life both failed. No. Steel No. 28 had a Co content of less than 0.01% by mass, and failed to undergo repeated oxidation resistance and thermal fatigue life. No. Steel No. 29 had a Ni content exceeding 0.60% by mass, and the repeated oxidation resistance and thermal fatigue life both failed. No. Steel No. 30 had an Nb content exceeding 0.80% by mass, and the thermal fatigue life was rejected. No. Steel No. 31 had a Ti content exceeding 0.04% by mass, and repeated oxidation and thermal fatigue life both failed. No. Steel No. 32 had a Mo content of more than 2.50 mass%, and the thermal fatigue life was rejected as the steel became brittle. No. Steel No. 33 had a Ni content of less than 0.02% by mass, and both its resistance to repeated oxidation and thermal fatigue life were rejected. No. In Steel No. 34, the Si content exceeded 1.0 mass%, and the repeated oxidation resistance and thermal fatigue life both failed. No. Steel No. 35 had a Cr content of less than 18.0% by mass, and both its resistance to repeated oxidation and thermal fatigue life were rejected. No. Steel No. 36 had an Nb content of less than 0.30% by mass, and the thermal fatigue life was rejected. No. In the steel No. 37, the Sn content exceeded 0.005% by mass, and the thermal fatigue life was rejected. No. Steel No. 38 had a Cr content exceeding 20.0 mass%, and the thermal fatigue life was rejected as the steel became brittle. No. In the steel No. 39, the Al content exceeded 0.08% by mass, and the thermal fatigue life was rejected. No. Steel No. 40 had a Si content of less than 0.25% by mass, and failed to be repeatedly oxidized and thermally fatigued. No. Steel No. 41 had a Mn content of more than 0.50% by mass, and both repeated oxidation resistance and thermal fatigue life were rejected. No. Steel No. 42 had a C content of more than 0.020% by mass, and both repeated oxidation resistance and thermal fatigue life were rejected. No. Steel No. 43 had an N content exceeding 0.015% by mass, and both the resistance to repeated oxidation and the thermal fatigue life were rejected. No. Steel No. 44 had a Cu content exceeding 0.20% by mass, and the repeated oxidation resistance and thermal fatigue life both failed.

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

Claims

% By mass
C: 0.020% or less,
Si: 0.25 to 1.0%,
Mn: 0.05 to 0.50%,
P: 0.050% or less,
S: 0.008% or less,
Ni: 0.02 to 0.60%,
Al: 0.001 to 0.08%,
Cr: 18.0-20.0%,
Nb: 0.30 to 0.80%,
Mo: 1.80 to 2.50%,
N: 0.015% or less,
Zr: 0.01-0.40%,
Co: 0.01 to 0.30%
And a ferritic stainless steel satisfying the following formula (1), with the balance being composed of Fe and inevitable impurities.
Zr + Co: 0.03 to 0.50% (1)
(Zr and Co in the formula (1) indicate the content (mass%) of each element.)
The component composition is mass%, and
Ti: 0.01 to 0.04%,
Sb: 0.002 to 0.50%,
B: 0.0002 to 0.0050%,
V: 0.01 to 1.0%,
W: 0.01 to 0.30%,
Cu: 0.01 to 0.20%,
Sn: 0.001 to 0.005%
The ferritic stainless steel according to claim 1, comprising one or more selected from among the above.
The component composition is mass%, and
Ca: 0.0002 to 0.0050%,
Mg: 0.0002 to 0.0050%
The ferritic stainless steel according to claim 1 or 2, comprising one or two selected from among them.
The ferritic stainless steel according to any one of claims 1 to 3, wherein the ferritic stainless steel is used in an exhaust manifold that is heated to 700 ° C or higher by exhaust gas from an engine.