TW201934778A - Ferritic stainless steel - Google Patents

Abstract

Provided is a ferritic stainless steel having excellent creep resistance properties and thermal fatigue properties. This ferritic stainless steel has a composition containing at most 0.020 mass% of C; 0.1-1.0 mass% of Si, 0.05-0.60 mass% of Mn, at most 0.050 mass% P, at most 0.008 mass% of S, 0.02-0.60 mass% of Ni, 0.001-0.25 mass% of Al, 18.0-20.0 mass% of Cr, 0.30-0.80 mass% of Nb, 1.80-2.50 mass% of Mo, at most 0.015 mass% of N, 0.002-0.50 mass% of Sb, and the balance comprising Fe and inevitable impurities, wherein formula (1) is satisfied. (1) Nb+Mo: 2.3-3.0 mass% (in formula (1), Nb and Mo represent the content (mass%) of each element).

Description

Ferrous iron-based stainless steel

The present invention relates to a ferrous iron-based stainless steel, and more particularly to an exhaust system component suitable for an exhaust pipe or converter box of an automobile or a locomotive, an exhaust duct of a thermal power station, etc., used at high temperatures. Ferritic iron-based stainless steel with excellent creep resistance and thermal fatigue characteristics.

Exhaust system components such as exhaust manifolds, exhaust pipes, converter boxes, and mufflers of automobiles are required to have excellent heat resistance. There are several types of heat resistance, including thermal fatigue characteristics, high temperature fatigue characteristics, high temperature strength (high temperature endurance), oxidation resistance, creep characteristics, and high temperature salt damage corrosion characteristics. Among them, thermal fatigue characteristics are one of particularly important heat resistance. The exhaust system components are repeatedly heated and cooled as the engine starts and stops. At this time, the exhaust system components are connected to the surrounding parts, so thermal expansion and contraction are limited, and the material itself generates thermal strain. The phenomenon of low cycle fatigue caused by repeated thermal strains is called thermal fatigue.

As materials used in components requiring the above-mentioned thermal fatigue characteristics, currently, a ferritic iron-based stainless steel such as Type429 (14% Cr-0.9% Si-0.4% Nb) with Nb and Si added is used. However, with the improvement of engine performance, if the exhaust temperature rises to a temperature exceeding 900 ° C, the Type 429, in particular, cannot sufficiently satisfy the required thermal fatigue characteristics.

As materials that can cope with this problem, for example, Nb and Ferrous iron-based stainless steel with improved high temperature endurance by Mo, namely SUS444 (19% Cr-0.5% Nb-2% Mo) specified in JIS G4305, or ferrous iron-based stainless steel with Nb, Mo, and W added For example, refer to Patent Document 1).

[Prior technical literature] [Patent Literature]

Patent Document 1: Japanese Patent Laid-Open No. 2004-018921

In order to cope with the recent restrictions on exhaust gas exhaustion and increase fuel efficiency, there is a tendency for exhaust gas temperatures to increase in temperature. In SUS444 and the like, heat resistance, especially thermal fatigue characteristics, is insufficient. In addition, if the exhaust temperature exceeds 900 ° C. and becomes high temperature, the stainless steel is liable to undergo latent deformation, and therefore the creep resistance characteristics are also required.

SUS444 has the highest level of heat resistance among ferrous iron-based stainless steels. However, it may not be sufficient when the exhaust temperature rises with the recent increase in exhaust gas restriction and fuel efficiency. As the temperature of the exhaust gas becomes higher, the thermal expansion of the exhaust system components becomes larger at the time of heating. Therefore, due to the addition of more severe thermal strain, the ferrous iron-based stainless steel used for the exhaust system components becomes vulnerable to thermal fatigue damage. Furthermore, when it is kept in a high temperature region for a long period of time, the ferrous iron-based stainless steel is liable to undergo creep deformation. If the creep deformation occurs, damage will occur from the portion where the thickness becomes thinner due to creep deformation. Must improve creep resistance.

Thus, in the conventional technique including SUS444, it is not possible to obtain a ferrous iron-based stainless steel having sufficient thermal fatigue characteristics when the exhaust gas temperature is increased. In addition, it is not possible to sufficiently evaluate the creep resistance characteristics required particularly when the exhaust gas temperature exceeds 900 ° C.

Therefore, an object of the present invention is to solve this problem, and to provide a ferrous iron-based stainless steel excellent in creep resistance and thermal fatigue characteristics.

Furthermore, the "excellent creep resistance" of the present invention means that the breaking time is better than SUS444 when the creep test is performed at 900 ° C. In addition, "excellent thermal fatigue characteristics" means that it has characteristics superior to SUS444. Specifically, the thermal fatigue life is better than SUS444 when it is repeatedly heated and cooled between 200 and 950 ° C.

In order to develop a ferritic iron-based stainless steel having better creep resistance and thermal fatigue characteristics than SUS444, the present inventors have carefully studied the effects of various elements on creep resistance and thermal fatigue characteristics.

As a result, it was found that the high-temperature strength increases in a wide range of temperatures 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. Improved thermal fatigue characteristics. Furthermore, it was found that the Sb is contained within the range of 0.002 to 0.50% by mass to improve the creep resistance.

Based on the above findings, the present invention has been completed by forming a specific component composition containing all of Cr, Nb, Mo, and Sb in an appropriate amount. In the present invention, the above-mentioned elements are important, but in order to exert the effects of the present invention, all necessary elements must be adjusted to a predetermined content.

The present invention has as its gist the following.

[1] A ferrous iron-based stainless steel having the following composition, in terms of mass%, containing C: 0.020% or less, Si: 0.1 to 1.0%, Mn: 0.05 to 0.60%, and 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 ~ 0.50%, and satisfy the following formula (1), The residual part contains Fe and inevitable impurities, Nb + Mo: 2.3 ~ 3.0% ... (1)

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

[2] The ferrous iron-based stainless steel according to [1], wherein the above-mentioned component composition is measured by mass%, and further contains a member selected from Ti: 0.01 to 0.16%, Zr: 0.01 to 0.50%, Co: 0.01 to 0.50%, and B: 0.0002 ~ 0.0050%, V: 0.01 ~ 1.0%, W: 0.01 ~ 5.0%, Cu: 0.01 ~ 0.40%, and Sn: 0.001 ~ 0.005%.

[3] The ferrous iron-based stainless steel according to [1] or [2], in which the above-mentioned component composition is in mass% and further contains one selected from Ca: 0.0002 to 0.0050% and Mg: 0.0002 to 0.0050% Or 2 kinds.

[4] The ferritic iron-based stainless steel according to any one of [1] to [3], which is used for an exhaust manifold whose temperature rises to 700 ° C or higher due to exhaust gas from an engine.

According to the present invention, a ferrous iron-based stainless steel having creep resistance and thermal fatigue characteristics superior to SUS444 (JIS G4305) can be provided. Therefore, the ferrous iron-based stainless steel of the present invention can be suitably used as an exhaust system member for automobiles and the like.

FIG. 1 is a diagram illustrating a creep test piece.

FIG. 2 is a diagram illustrating a thermal fatigue test piece.

FIG. 3 is a diagram illustrating the temperature and constraint conditions of the thermal fatigue test.

Hereinafter, embodiments of the present invention will be described. The present invention is not limited to the following embodiments.

The ferrous iron-based stainless steel of the present invention is calculated as mass%, and contains 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, and 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 satisfy the following formula ( 1) The residue contains Fe and unavoidable impurities.

Nb + Mo: 2.3 ~ 3.0% ... (1)

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

In the present invention, the balance of the component composition is very important. By forming the combination of the component compositions as described above, a ferrous iron-based stainless steel with better creep resistance and thermal fatigue characteristics than SUS444 can be obtained. As long as the content range of one of the necessary elements (C, Si, Mn, Ni, Al, Cr, Nb, Mo, N, Sb) in the above composition deviates, the required creep resistance and thermal fatigue characteristics cannot be obtained .

Next, the component composition of the ferrous iron-based stainless steel of the present invention will be described. In the following, unless otherwise specified, the unit% of the content of the components indicates mass%.

C: 0.020% or less

C is an element that can effectively increase the strength of steel. However, if C is contained in an amount exceeding 0.020%, the decrease in toughness and formability becomes significant. In addition, the amount of carbides generated by combining with the important Nb in the present invention is increased, and as a result, the effect of improving thermal fatigue characteristics and creep resistance characteristics of Nb described below is reduced. Accordingly, the C content is set to 0.020% or less. In addition, from the viewpoint of ensuring formability, the C content is preferably 0.010% or less. More preferably, the C content is set to 0.008% or less. From the viewpoint of ensuring the strength as an exhaust system member, the C content is preferably set to 0.001% or more. More preferably, the C content is set to 0.003% or more. More preferably, the C content is set to 0.004% or more.

Si: 0.1 ~ 1.0%

Si is an important element necessary for improving the oxidation resistance. In order to ensure the oxidation resistance in the heated exhaust gas, it is necessary to contain 0.1% or more of Si. On the other hand, when excessive Si is contained in excess of 1.0%, the workability at room temperature is reduced, so the upper limit of the Si content is set to 1.0%. The Si content is preferably set to be 0.20% or more. More preferably, the Si content is 0.30% or more. More preferably, the Si content is set to 0.40% or more. The Si content is preferably 0.90% or less. More preferably, the Si content is set to 0.60% or less.

Mn: 0.05 ~ 0.60%

Mn has the effect of improving the thermal fatigue characteristics by improving the peel resistance of the oxide scale. In order to obtain these effects, it is necessary to contain Mn of 0.05% or more. On the other hand, when Mn is excessively contained in excess of 0.60%, a γ phase is liable to be generated at a high temperature and heat resistance is reduced. Therefore, the Mn content is set to be 0.05% or more and 0.60% or less. The Mn content is preferably 0.10% or more. More preferably, the Mn content is set to 0.15% or more. The Mn content is preferably 0.50% or less. More preferably, the Mn content is set to 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 set to 0.050% or less. The P content is preferably 0.040% or less. More preferably, the P content is 0.030% or less.

S: 0.008% or less

S is also a harmful element, which reduces the elongation or r value and causes a bad effect on formability. It is also desirable to reduce the corrosion resistance, which is a basic characteristic of stainless steel, as much as possible. Therefore, in the present invention, the S content is set to 0.008% or less. The S content is preferably 0.006% or less.

Ni: 0.02 ~ 0.60%

Ni is an element that improves the toughness and oxidation resistance of steel. To obtain these effects, the Ni content is set to 0.02% or more. If the oxidation resistance is insufficient, the amount of oxidized scale will increase, resulting in a reduction in the cross-sectional area of the material, or the peeling of the oxidized scale will cause a reduction in thermal fatigue characteristics. On the other hand, Ni is a strong γ-phase forming element. If Ni is contained excessively, a γ-phase is generated at a high temperature, which reduces the oxidation resistance and increases the thermal expansion coefficient, thereby reducing the thermal fatigue characteristics. Therefore, the upper limit of the Ni content is set to 0.60%. Preferably, the Ni content is 0.05% or more. More preferably, the Ni content is 0.10% or more. The Ni content is preferably 0.40% or less. More preferably, the Ni content is 0.30% or less.

Al: 0.001 ~ 0.25%

Al is an element having an effect of improving oxidation resistance. To obtain this effect, Al must be contained in an amount of 0.001% or more. On the other hand, Al is also an element that increases the coefficient of thermal expansion. As the thermal expansion coefficient becomes larger, the thermal fatigue characteristics decrease. Further, steel is significantly hardened, resulting in a decrease in workability. Therefore, the Al content is set to 0.25% or less. The Al content is preferably 0.005% or more. More preferably, the Al content is more than 0.010%. Still more preferably, the Al content is more than 0.020%. The Al content is preferably less than 0.20%. More preferably, the Al content is less than 0.08%.

Cr: 18.0 ~ 20.0%

Cr is an important element that can effectively improve the corrosion resistance and oxidation resistance of stainless steel. However, when the Cr content is less than 18.0%, sufficient oxidation resistance cannot be obtained in a high temperature region exceeding 900 ° C. If the oxidation resistance is insufficient, the amount of oxidized scale will increase, and as the cross-sectional area of the material decreases, the thermal fatigue characteristics will also decrease. On the other hand, Cr is an element that solid-solution strengthens, hardens, and reduces the ductility of steel at room temperature. If the Cr content exceeds 20.0%, the above-mentioned disadvantages become significant and the thermal fatigue characteristics are also reduced. The upper limit is set to 20.0%. The Cr content is preferably 18.5% or more. The Cr content is preferably 19.5% or less.

Nb: 0.30 ~ 0.80%

Nb is an element important to the present invention that increases high-temperature strength and improves thermal fatigue characteristics and creep resistance characteristics. This effect can be seen when Nb is contained in an amount of 0.30% or more. When the Nb content is less than 0.30%, the strength at high temperatures is insufficient, and excellent thermal fatigue characteristics and creep resistance characteristics cannot be obtained. However, when Nb is contained in an amount exceeding 0.80%, the Laves phase (Fe ₂ Nb) or the like as an intermetallic compound is easily precipitated, the high-temperature strength is reduced, and not only the thermal fatigue characteristics and creep resistance characteristics are reduced, but embrittlement is promoted. Therefore, the Nb content is set to 0.30% or more and 0.80% or less. The Nb content is preferably 0.40% or more. More preferably, the Nb content is 0.45% or more. More preferably, the Nb content is more than 0.50%. The Nb content is preferably 0.70% or less. More preferably, the Nb content is 0.60% or less.

Mo: 1.80 ~ 2.50%

Mo is an element that can effectively improve thermal fatigue properties and creep resistance properties by increasing the high temperature strength of steel due to solid solution in steel. This effect is exhibited when Mo is contained at 1.80% or more. When the Mo content is less than 1.80%, the high-temperature strength is insufficient, and excellent thermal fatigue characteristics and creep resistance characteristics cannot be obtained. On the other hand, when Mo is contained excessively, not only the steel is hardened and the workability is reduced, but also precipitated as a Laves phase (Fe ₂ Mo) like Nb, and the amount of Mo dissolved in the steel is reduced, so the thermal fatigue characteristics Instead reduced. Moreover, in the thermal fatigue test, precipitation as a coarse σ phase becomes a starting point of failure, which leads to a reduction in thermal fatigue characteristics. Therefore, the upper limit of the Mo content is set to 2.50%. The Mo content is preferably 1.90% or more. More preferably, the Mo content is more than 2.00%. 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 reduces the toughness and formability of steel. If it contains more than 0.015% of steel, not only the reduction in toughness and formability will become significant, but also the amount of solid solution Nb will be reduced due to the formation of Nb nitrides, making the latent resistance Variation characteristics and thermal fatigue characteristics are reduced. Therefore, the N content is set to 0.015% or less. From the viewpoint of ensuring toughness and formability, it is preferable to reduce N as much as possible, and the N content is more preferably less than 0.010%.

Sb: 0.002 ~ 0.50%

Sb is an important element for improving the creep resistance in the present invention. Sb is a solid solution in steel and inhibits the latent deformation of steel under high temperature. Sb does not precipitate as carbonitrides or Laves phase in high-temperature domains. It will still be dissolved in steel after long-term use, inhibiting creep deformation, so it can improve creep resistance. This effect is obtained when Sb is contained in an amount of 0.002% or more. On the other hand, when Sb is contained excessively, the toughness and hot workability of the steel are reduced, so that not only cracks are liable to occur during manufacturing, but also thermal fatigue characteristics are reduced due to a decrease in hot ductility. Therefore, the upper limit of the Sb content is set to 0.50%. The Sb content is preferably 0.005% or more. More preferably, it is 0.020% or more. The Sb content is preferably 0.30% or less. more Preferably, the Sb content is 0.10% or less.

Nb + Mo: 2.3 ~ 3.0% ... (1)

As described above, Nb and Mo are elements that can effectively improve thermal fatigue characteristics and creep resistance characteristics. The effect can be seen when Nb and Mo are contained in an amount of 0.30% or more and 1.80% or more, respectively. However, in order to meet the high temperature of the exhaust gas, the thermal fatigue life during repeated heating and cooling between 200 and 950 ° C is better than the thermal fatigue characteristics and creep resistance characteristics of SUS444. It must contain two elements in the predetermined range. Moreover, it must satisfy at least Nb + Mo ≧ 2.3%, that is, the amount of Nb + Mo (the total content of Nb and Mo) is 2.3% or more. When this condition is not satisfied, even if a predetermined amount of Sb is added, excellent creep resistance characteristics cannot be obtained. Preferably, Nb + Mo> 2.5%. On the other hand, if the amount of Nb + Mo is excessively increased, the steel becomes brittle, and excellent thermal fatigue characteristics and creep resistance characteristics cannot be obtained. Therefore, the upper limit of the amount of Nb + Mo is set to 3.0%. The amount of Nb + Mo is preferably 2.7% or less.

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

In the ferrous iron-based stainless steel of the present invention, the residual portion contains Fe and unavoidable impurities.

The ferrous iron-based stainless steel of the present invention may contain one or two or more selected from Ti, Zr, Co, B, V, W, Cu, and Sn as optional components in addition to the above-mentioned essential components.

Ti: 0.01 ~ 0.16%

Ti is an element that fixes C and N, improves corrosion resistance or formability, and prevents grain boundary corrosion of the welded portion. In the present invention, Ti may be contained as necessary. Since Ti has priority over C and N over Nb In combination, by containing Ti, the amount of solid solution Nb in steel effective for high-temperature strength can be ensured, and heat resistance can also be effectively improved. These effects can be obtained when Ti is contained at 0.01% or more. On the other hand, when Ti is contained in excess of 0.16%, the toughness is lowered, for example, breakage due to bending-rebound experienced repeatedly on the hot-rolled sheet annealing line, which adversely affects manufacturability. In addition, it is easy to precipitate the carbonitride of Nb with the carbonitride of Ti as the core, so the amount of solid solution Nb in the steel effective for high-temperature strength is reduced, and thermal fatigue characteristics and creep resistance characteristics are reduced. Therefore, when Ti is contained, the Ti content is set to 0.01 to 0.16%. The Ti content is preferably 0.03% or more. 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 ~ 0.50%

Zr is an element that improves the oxidation resistance. In the present invention, Zr is contained as necessary. This effect is obtained when Zr is contained in an amount of 0.01% or more. However, if the Zr content exceeds 0.50%, the Zr intermetallic compound is precipitated and the steel becomes brittle. Therefore, when Zr is contained, the Zr content is set to 0.01 to 0.50%. The Zr content is preferably 0.03% or more. More preferably, the Zr content is 0.05% or more. The Zr content is preferably 0.30% or less. More preferably, the Zr content is 0.10% or less.

Co: 0.01 ~ 0.50%

Co is well known as an element that can effectively improve the toughness of steel. This effect can be obtained when Co is contained at 0.01% or more. On the other hand, when Co is contained excessively, the toughness of the steel is reduced, so the upper limit of the Co content is set to 0.50%. Therefore, when Co is contained, the Co content is set to 0.01 to 0.50%. The Co content is preferably 0.03% or more. The Co content is preferably 0.30% or less.

B: 0.0002 ~ 0.0050%

B is an element that can effectively improve the workability of steel, especially the secondary workability. This effect can be obtained when B is contained in an amount of 0.0002% or more. On the other hand, when excessive B is contained, BN is generated and workability is reduced. 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. The B content is preferably 0.0030% or less. More preferably, the B content is 0.0020% or less.

V: 0.01 ~ 1.0%

V is an element that can effectively improve the workability of steel, and is an element that can also effectively improve the oxidation resistance. These effects become remarkable when the V content is 0.01% or more. However, when excessive V is contained in excess of 1.0%, coarse V (C, N) is precipitated, which reduces not only the toughness but also 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. More preferably, the V content is 0.05% or more. The V content is preferably 0.50% or less. More preferably, the V content is 0.20% or less.

W: 0.01 ~ 5.0%

W and Mo are elements that greatly increase the high temperature strength by solid solution strengthening. This effect is obtained when W is contained in an amount of 0.01% or more. On the other hand, when W is contained excessively, not only the steel is significantly hardened, but also a firm scale is generated during the annealing step during manufacturing, so that rust removal during pickling becomes difficult. Therefore, when W is included, W The content is set to 0.01 to 5.0%. The W content is preferably 0.05% or more. 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 ~ 0.40%

Cu is an element having an effect of improving the corrosion resistance of steel, and is contained when corrosion resistance is required. This effect can be obtained when Cu is contained in an amount of 0.01% or more. On the other hand, if Cu is contained in excess of 0.40%, the oxidized scale will be easily peeled, and the anti-oxidation property will be reduced. Therefore, when Cu is contained, the Cu content is set to 0.01 to 0.40%. The Cu content is preferably 0.03% or more. More preferably, the Cu content is 0.06% or more. The Cu content is preferably 0.20% or less. More preferably, the Cu content is 0.10% or less.

Sn: 0.001 ~ 0.005%

Sn is an element that can effectively improve the high temperature strength of steel. This effect is obtained when Sn is contained in an amount of 0.001% or more. On the other hand, when excessive Sn is contained, thermal fatigue characteristics are reduced with the embrittlement of the steel. Therefore, when Sn is contained, the Sn content is set to 0.001 to 0.005%. The Sn content is preferably 0.001% to 0.003%.

The ferrous iron-based stainless steel of the present invention may further contain one or two selected from Ca and Mg as optional components in the following range.

Ca: 0.0002 ~ 0.0050%

Ca is a component that can effectively prevent nozzle clogging due to precipitation of Ti-based intermediary substances that are easily generated during continuous casting. This effect is obtained when Ca is contained in an amount of 0.0002% or more. On the other hand, in order to obtain good surface properties without causing surface defects, the Ca content must be adjusted. It is set to 0.0050% or less. Therefore, when Ca is contained, the Ca content is set to 0.0002 to 0.0050%. The Ca content is preferably 0.0005% or more. The Ca content is preferably 0.0030% or less. More preferably, the Ca content is 0.0020% or less.

Mg: 0.0002 ~ 0.0050%

Mg is an element which can effectively increase the equiaxed crystal ratio of the slab and improve the workability or toughness. In steels containing Nb or Ti as in the present invention, Mg also has the effect of suppressing coarsening of carbonitrides of Nb or Ti. This effect is obtained when Mg is contained in an amount of 0.0002% or more. If the Ti carbonitride becomes coarser, it will become the starting point of brittle fracture, so the toughness will be greatly reduced. If the Nb carbonitride is coarsened, the amount of solid solution in the steel of Nb is reduced, so that the thermal fatigue characteristics are reduced. On the other hand, if 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%. The Mg content is preferably 0.0003% or more. More preferably, the Mg content is 0.0004% or more. The Mg content is preferably 0.0030% or less. More preferably, the Mg content is 0.0020% or less.

The remainder is Fe and inevitable impurities. When the above-mentioned arbitrary component is contained below the lower limit value, the arbitrary component contained in the content below the lower limit value is contained as an unavoidable impurity.

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

The manufacturing method of stainless steel of this invention can be used suitably as long as it is a normal manufacturing method of a ferrous iron-type stainless steel, It does not specifically limit.

For example, it can be manufactured by the following steps: melting steel in a well-known melting furnace such as a converter or an electric furnace, or secondary refining such as ladle refining or vacuum refining Refining to form a steel having the above-mentioned composition of the present invention, forming a steel sheet (slab) by a continuous casting method or an agglomeration-block rolling method, followed by hot rolling, hot-rolled sheet annealing, pickling, and cold rolling , Final annealing and pickling to form a cold rolled annealed sheet. The above-mentioned cold rolling may be performed once or twice or more cold rolling through intermediate annealing, and each step of cold rolling, final annealing, and pickling may be performed repeatedly. Furthermore, the hot-rolled sheet annealing may be omitted, and when the surface gloss or roughness of the steel sheet is required to be adjusted, quenched and tempered rolling may be performed after cold rolling or after final annealing.

The preferable manufacturing conditions in the said manufacturing method are demonstrated.

The steel making step of the molten steel is preferably performed by vacuum melting oxygen decarburization (VOD, vacuum oxygen decarburization) method or argon oxygen decarburization (AOD) method, etc. Sub-refining to form a steel containing the above-mentioned necessary components and optional components added as needed. The molten steel after melting can be formed into a steel material by a known method, but in terms of productivity and quality, it is preferably formed by a continuous casting method. Thereafter, the steel material is preferably heated to 1050-1250 ° C. and hot-rolled to form a desired plate thickness by hot rolling. In manufacturing, the thickness of the hot-rolled sheet is preferably 5 mm or less. Of course, it is also possible to perform heat processing other than the plate. Preferably, after the hot-rolled sheet is subjected to continuous annealing at a temperature of 900 to 1150 ° C or batch annealing at a temperature of 700 to 900 ° C as required, the hot-rolled sheet is rust-removed by pickling or grinding to form Hot rolled products. Moreover, if necessary, rust can also be removed by shot blasting before pickling.

Furthermore, the hot-rolled product (hot-rolled annealed sheet) may be subjected to steps such as cold rolling to form a cold-rolled product. In this case, the cold rolling may be performed once, but from the viewpoint of productivity or required quality, it may be cold rolling twice or more with intermediate annealing. The total rolling reduction of the cold rolling once or twice is preferably 60% or more, and more preferably 70% or more. Thereafter, the cold-rolled steel sheet is preferably, preferably at 900 to 1200 ° C, and more preferably Continuous annealing (final annealing) is performed at a temperature of 1000 to 1150 ° C, and pickling or grinding is performed to form a cold rolled product (cold rolled annealed sheet). Final annealing can also be performed in a reducing gas environment. In this case, pickling or grinding after final annealing can also be omitted. Furthermore, depending on the application, the shape, surface roughness, and material of the steel sheet may be adjusted by performing temper rolling or the like after final annealing.

Thereafter, the hot-rolled products or cold-rolled products obtained in the above manner are subjected to processing such as cutting or bending processing, drawing processing, and drawing processing according to their respective applications, and are formed into exhaust pipes and catalysts for automobiles or locomotives. Tubes, exhaust ducts of thermal power stations or related components of fuel cells, such as spacers, interconnecting connectors or reformers. Among these, the ferrous iron-based stainless steel of the present invention is suitable for exhaust system components such as an exhaust manifold or an exhaust pipe, a converter box, and a muffler. It is one of the characteristics that an exhaust manifold having excellent durability can be obtained especially when the temperature is raised to 700 ° C. or higher by exhaust gas from the engine during use.

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

[Example]

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

In a vacuum melting furnace, a steel having a composition of Nos. 1 to 41, 43, 45 to 47 shown in Table 1 was melted and cast to form a 50 kg steel block. After heating at 1170 ° C, it was formed by hot rolling. 35mm thick plate. The sheet is divided into two parts, and one of the steel blocks is heated to 1100 ° C. Next, hot rolling is performed to form a hot-rolled sheet having a thickness of 5mm. After annealing at a temperature ranging from 1000 to 1150 ° C, grinding and Hot rolling annealing board. Next, cold rolling with a rolling reduction rate of 70%, and final annealing at a temperature of 1000 to 1150 ° C, followed by pickling or grinding to remove rust to form a cold rolled annealed sheet with a thickness of 1.5 mm for the creep test. . In addition, as a reference, for SUS444 (conventional example No. 28), a cold-rolled annealed sheet was also produced for the creep test in the same manner as described above. Regarding the annealing temperature, the structure was confirmed within the above temperature range, and the temperature was determined for each steel.

<Creep Test>

From each of the cold-rolled annealed sheets obtained in the above manner, a test piece having the shape shown in FIG. 1 was cut, and a creep test under a load of 15 MPa was performed at 900 ° C. Based on the time taken before fracture, the evaluation was performed as follows. Regarding SUS444 (conventional example No. 28) performed as a comparison, the time taken before the break was 5.5 hours.

◎: Break time ≧ 10hr

○: 6hr ≦ break time <10hr

×: Breaking time <6hr

In the above evaluation, ◎ and ○ were set to pass, and X was set to fail. The obtained results are shown in Table 1 (refer to the latent 900 ° C in Table 1).

Next, the remaining one of the above-mentioned divided two plates was heated to 1100 ° C, and then hot forged to form each rod of 30 mm square. Next, after annealing at a temperature of 1000 to 1150 ° C, machining is performed to form a thermal fatigue test piece of the shape and size shown in FIG. 2 for the following thermal fatigue test. The annealing temperature is a temperature after confirming the structure for each component and completing recrystallization. In addition, as a reference, a test piece having a component composition of SUS444 (conventional example No. 28) was also prepared in the same manner as above, and subjected to a thermal fatigue test.

<Thermal fatigue test>

As shown in FIG. 3, the thermal fatigue test is performed under the condition that the test piece is constrained with a constraint rate of 0.5 on the one hand and repeatedly heated and cooled between 200 ° C and 950 ° C on the other hand. At this time, the temperature increase rate was set to 5 ° C / sec, and the temperature decrease rate was set to 2 ° C / sec. The holding times at 200 ° C and 950 ° C were 30 seconds, respectively. Furthermore, regarding the above-mentioned constraint rate, as shown in FIG. 3, the constraint rate can be expressed as η = a / (a + b), a is (free thermal expansion strain variable-control strain variable) / 2, and b is control strain variable / 2. In addition, the free thermal expansion strain amount is a strain amount when the temperature is raised without giving mechanical stress at all, and the control strain amount represents the absolute value of the strain amount generated during the test. The substantial restraint strain produced by the restraint in the material is (free thermal expansion strain-control strain).

The thermal fatigue life is calculated by dividing the load detected at 200 ° C by the cross-sectional area of the test specimen soaking parallel section (see Fig. 2), and setting the stress value to the initial cycle 5 cycles) The number of cycles where the stress value was reduced to 75% was evaluated as follows. Regarding SUS444 (conventional example No. 28) performed as a comparison, the thermal fatigue life was 650 cycles.

◎: 1000 cycles or more (pass)

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

×: Less than 800 cycles (unqualified)

In the above evaluation, ◎ and ○ were passed, and × was failed. The obtained results are shown in Table 1 (refer to the thermal fatigue life of 950 ° C in Table 1).

According to Table 1, the ferrous iron-based stainless steels No. 1 to 27 of the examples of the present invention (hereinafter, the ferrous iron-based stainless steels are simply referred to as steel) are superior to SUS444 in the creep test and thermal fatigue test (known examples). No. 28 steel).

In the steel of No. 29, the Nb + Mo content was less than 2.3% by mass, and the creep fracture time and thermal fatigue life were unqualified. In the steel of No. 30, the Ni content exceeds 0.60% by mass, and the thermal fatigue life is unqualified. In the steel of No. 31, the Cr content was less than 18.0% by mass, and the thermal fatigue life was unqualified. In the steel of No. 32, the Mo content was less than 1.80 mass%, and the creep fracture time and thermal fatigue life were unqualified. In the steel of No. 33, the Nb content was less than 0.30% by mass, and the creep rupture time and thermal fatigue life were unqualified. In the steel of No. 34, the Si content was less than 0.1% by mass. Oxidation was noticeably found in any of the creep test and thermal fatigue test, and the creep fracture time and thermal fatigue life were unqualified. In the steel of No. 35, the Ti content exceeds 0.16 mass%, and the creep fracture time and thermal fatigue life are unqualified. In the steel of No. 36, the Cr content exceeds 20.0% by mass, and the thermal fatigue life becomes unacceptable with the embrittlement of the steel. In the steel of No. 37, the Mn content was less than 0.05% by mass. In the thermal fatigue test, peeling of the oxidized scale occurred, and the thermal fatigue life was unqualified. In the steel of No. 38, the C content exceeds 0.020% by mass. With the decrease of the Nb content in the steel, both the creep rupture time and thermal fatigue life become unqualified. In the steel of No. 39, the N content exceeded 0.015% by mass, and the Nb content in the steel was reduced due to the precipitation of Nb nitrides, and the creep rupture time and thermal fatigue life became unacceptable. In the steel of No. 40, the Sb content exceeds 0.50% by mass, and as the hot rolling ductility decreases, the thermal fatigue life becomes unacceptable. In the steel of No. 41, the Mo content exceeds 2.50% by mass, coarse σ phases (Fe-Cr-based intermetallic compounds) are precipitated in the thermal fatigue test, and the thermal fatigue life is unqualified. Moreover, the creep rupture time was also unacceptable. In the steel of No. 43, the Sn content exceeds 0.005% by mass, and the thermal fatigue life is unqualified. The steel No. 45 does not contain Sb, and the creep rupture time and thermal fatigue life are unqualified. In No. 46 steel, the Nb content exceeds 0.80 The mass%, the creep breaking time and thermal fatigue life are all unqualified. In the steel of No.47, the Nb + Mo content exceeds 3.0%, and the creep fracture time and thermal fatigue life are unqualified.

(Industrial availability)

The ferrous iron-based stainless steel of the present invention is not only suitable for use as an exhaust system component of an automobile or the like, but also can be suitably used as an exhaust system component of a thermal power generation system or a solid oxide fuel cell component that requires the same characteristics.

Claims (4)

A ferrous iron-based stainless steel having the following composition, in terms of mass%, containing 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, Sb: 0.002 to 0.50 %, And satisfy the following formula (1), the residue contains Fe and unavoidable impurities, Nb + Mo: 2.3 ~ 3.0% ... (1) (Nb and Mo in formula (1) represent the content of each element ( quality%)). For example, the ferrous iron-based stainless steel according to claim 1, wherein the above-mentioned component composition, in terms of mass%, further contains a member selected from the group consisting of Ti: 0.01 to 0.16%, Zr: 0.01 to 0.50%, Co: 0.01 to 0.50%, and B: 0.0002. ~ 0.0050%, V: 0.01 to 1.0%, W: 0.01 to 5.0%, Cu: 0.01 to 0.40%, and Sn: 0.001 to 0.005%. For example, the ferritic iron-based stainless steel according to claim 1 or 2, wherein the above-mentioned component composition is based on mass% and further contains one or two selected from Ca: 0.0002 to 0.0050% and Mg: 0.0002 to 0.0050%. The ferritic iron-based stainless steel according to any one of claims 1 to 3 is used for an exhaust manifold that heats up to 700 ° C or higher due to exhaust gas from an engine.