TWI722377B - Fertilizer stainless steel - Google Patents

Abstract

The present invention provides a ferrous iron-based stainless steel with excellent creep resistance and thermal fatigue properties.

The ferrous iron-based stainless steel of the present invention has the following composition, that is, in terms of mass%, it contains C: 0.020% or less, Si: 0.1 to 1.0%, Mn: 0.05 to 0.60%, P: 0.050% or less, and S: 0.008% or less, Ni: 0.02~0.60%, Al: 0.001~0.25%, Cr: 18.0~20.0%, Nb: 0.30~0.80%, Mo: 1.80~2.50%, N: 0.015% or less, and Sb: 0.002~ 0.50%, and satisfies the following formula (1), the remainder contains Fe and unavoidable impurities.

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

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

Description

Fertilizer stainless steel

The present invention relates to a ferrite-based stainless steel, especially to an exhaust pipe or converter box suitable for use in automobiles or locomotives, exhaust ducts of thermal power stations, etc., which have exhaust system components used at high temperatures. Fertile iron-based stainless steel with excellent creep resistance and thermal fatigue properties.

Exhaust system components such as exhaust manifolds or exhaust pipes, converter boxes, and mufflers of automobiles require excellent heat resistance. There are several types of heat resistance, including thermal fatigue characteristics, high temperature fatigue characteristics, high temperature strength (high temperature resistance), oxidation resistance, creep characteristics, high temperature salt damage corrosion characteristics, and the like. Among them, thermal fatigue properties 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 with the surrounding parts, so thermal expansion and contraction are limited, and the material itself generates thermal strain. The phenomenon of low-cycle fatigue that causes damage due to repeated exposure to this thermal strain is called thermal fatigue.

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

As a material that can cope with this problem, we have developed, for example, the addition of Nb and Fertilized iron-based stainless steel with improved high-temperature endurance by Mo, that is, SUS444 (19%Cr-0.5%Nb-2%Mo) specified in JIS G4305, or ferrous iron-based stainless steel with addition of Nb, Mo and W, etc. ( 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 exhaust gas restriction enhancement or to improve fuel efficiency, there is a tendency for the exhaust gas temperature to become higher. In SUS444, there are also insufficient heat resistance, especially thermal fatigue characteristics. In addition, if the exhaust gas temperature exceeds 900°C and the temperature increases, stainless steel tends to creep and deform, so creep resistance characteristics are also necessary.

SUS444 has the highest level of heat resistance among fat-grained iron-based stainless steels. However, when the exhaust gas temperature rises due to the strengthening of exhaust gas restriction and the improvement of fuel efficiency in recent years, it may not be said to have sufficient heat resistance. As the exhaust temperature increases, the thermal expansion of exhaust system components increases when the temperature rises. Therefore, due to the addition of more severe thermal strain, the ferrous iron-based stainless steel used for exhaust system components becomes prone to thermal fatigue damage. Furthermore, when kept in a high temperature range for a long time, the ferrous iron-based stainless steel is prone to creep deformation. If creep deformation occurs, the part where the thickness becomes thin due to the creep deformation is the starting point and breaks. The creep resistance characteristics must be improved.

As such, in the conventional technology including SUS444, it is impossible to obtain a ferrite-based stainless steel that has sufficient thermal fatigue characteristics when the exhaust gas temperature increases. In addition, it is not possible to sufficiently evaluate creep resistance characteristics that are particularly required when the exhaust gas temperature exceeds 900°C.

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

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

In order to develop a ferrous iron-based stainless steel that has better creep resistance and thermal fatigue properties than SUS444, the inventors have repeatedly studied the effects of various elements on creep resistance and thermal fatigue properties.

As a result, it was found that by containing 0.30 to 0.80% of Nb, 1.80 to 2.50% of Mo, and a total content of Nb and Mo of 2.3 to 3.0% in terms of mass%, the high-temperature strength increases in a wide temperature range. Improved thermal fatigue characteristics. Furthermore, it was discovered that the creep resistance property is improved by containing Sb in the range of 0.002 to 0.50 mass%.

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 more 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 the following as its gist.

[1] A ferrous iron-based stainless steel having the following compositional composition, that is, 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: 0.02~0.60%, Al: 0.001~0.25%, Cr: 18.0~20.0%, Nb: 0.30~0.80%, Mo: 1.80~2.50%, N: 0.015% or less, and Sb : 0.002~0.50%, and satisfies the following formula (1), The remainder contains Fe and unavoidable impurities, Nb+Mo: 2.3~3.0%...(1)

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

[2] The ferrous iron-based stainless steel as in [1], wherein the above-mentioned component composition is calculated by mass%, and further contains selected from Ti: 0.01~0.16%, Zr: 0.01~0.50%, Co: 0.01~0.50%, B: One or more of 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 as in [1] or [2], wherein the above-mentioned component composition is calculated by mass %, and further contains one selected from Ca: 0.0002~0.0050% and Mg: 0.0002~0.0050% Or 2 kinds.

[4] The ferrous iron-based stainless steel as in any one of [1] to [3], which is used in an exhaust manifold whose temperature rises above 700°C due to exhaust from the engine.

According to the present invention, it is possible to provide a ferrous iron-based stainless steel with creep resistance and thermal fatigue characteristics superior to that of SUS444 (JIS G4305). Therefore, the ferrous iron-based stainless steel of the present invention can be suitably used for exhaust system components of automobiles and the like.

Figure 1 is a diagram illustrating the creep test piece.

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

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

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

The ferrous iron-based stainless steel of the present invention 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, Ni: 0.02% by mass%. 0.60%, Al: 0.001~0.25%, Cr: 18.0~20.0%, Nb: 0.30~0.80%, Mo: 1.80~2.50%, N: 0.015% or less, and Sb: 0.002~0.50%, 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 (mass%) of each element).

In the present invention, the balance of the component composition is very important. By forming the above-mentioned component composition combination, a ferrous iron-based stainless steel with creep resistance and thermal fatigue properties superior to that of SUS444 can be obtained. As long as the content range of one essential element (C, Si, Mn, Ni, Al, Cr, Nb, Mo, N, Sb) deviates from the above composition, the required creep resistance and thermal fatigue properties cannot be obtained .

Next, the composition of the ferrous iron-based stainless steel of the present invention will be described. Hereinafter, unless otherwise specified, the unit% of the content of the ingredients means mass %.

C: Below 0.020%

C is an element that can effectively increase the strength of steel, but if it contains more than 0.020% of C, the decrease in toughness and formability becomes significant. In addition, the amount of carbides produced by combining with Nb, which is important in the present invention, increases, and this leads to a decrease in the effect of the following Nb in improving thermal fatigue properties and creep resistance properties. Therefore, the C content is set to 0.020% or less. Furthermore, from the viewpoint of ensuring formability, the C content is preferably set to 0.010% or less. More preferably, the C content is set to 0.008% or less. In addition, 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 oxidation resistance. In order to ensure the oxidation resistance in the high-temperature exhaust, it is necessary to contain more than 0.1% of Si. On the other hand, if excess Si is contained in excess of 1.0%, the workability at room temperature will decrease, so the upper limit of the Si content is set to 1.0%. Preferably, the Si content is set to 0.20% or more. More preferably, the Si content is set to 0.30% or more. More preferably, the Si content is set to 0.40% or more. Furthermore, it is preferable that the Si content is set to 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 properties by improving the peeling resistance of the oxide scale. In order to obtain these effects, it must contain 0.05% or more of Mn. On the other hand, when Mn is contained excessively in excess of 0.60%, the γ phase is likely to be generated at a high temperature and the heat resistance is lowered. Therefore, the Mn content is set to 0.05% or more and 0.60% or less. Preferably, the Mn content is set to 0.10% or more. More preferably, the Mn content is set to 0.15% or more. Furthermore, it is preferable that the Mn content is set to 0.50% or less. More preferably, the Mn content is set to 0.40% or less.

P: Below 0.050%

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

S: less than 0.008%

S is also a harmful element, which reduces the elongation or r value, and has a bad influence on the formability. It also reduces the corrosion resistance, which is the basic characteristic of stainless steel, so it is better 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: 0.02~0.60%

Ni is an element that improves the toughness and oxidation resistance of steel. In order to obtain these effects, the Ni content is set to 0.02% or more. If the oxidation resistance is insufficient, the increase in the amount of scale produced will reduce the cross-sectional area of the material, or the peeling of the scale will reduce the thermal fatigue properties. On the other hand, Ni is a strong γ-phase forming element, so if Ni is contained excessively, γ-phase is generated at high temperature, oxidation resistance is reduced, and thermal expansion coefficient is increased, thereby degrading 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. Furthermore, it is preferable that the Ni content is 0.40% or less. More preferably, the Ni content is 0.30% or less.

Al: 0.001~0.25%

Al is an element that has the effect of improving oxidation resistance. In order to obtain this effect, it must contain 0.001% or more of Al. On the other hand, Al is also an element that increases the coefficient of thermal expansion. If the coefficient of thermal expansion increases, the thermal fatigue characteristics decrease. Furthermore, steel is significantly hardened, resulting in a decrease in workability. Therefore, the Al content is set to 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 more than 0.020%. Furthermore, it is preferable that the Al content is 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 that are the characteristics of stainless steel. However, when the Cr content is less than 18.0%, sufficient oxidation resistance cannot be obtained in the high temperature range of over 900°C. If the oxidation resistance is not sufficient, the amount of scale produced will increase, and the thermal fatigue characteristics will also decrease as the cross-sectional area of the material decreases. On the other hand, Cr is an element for solid solution strengthening, hardening and low 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. Therefore, the Cr content is reduced. The upper limit is set at 20.0%. Preferably, the Cr content is 18.5% or more. Furthermore, it is preferable that the Cr content is 19.5% or less.

Nb: 0.30~0.80%

Nb is an important element for the present invention that increases high-temperature strength and improves thermal fatigue properties and creep resistance properties. This effect can be seen when Nb is more than 0.30%. 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 cannot be obtained. However, when the Nb content exceeds 0.80%, Laves phase (Fe ₂ Nb), which is an intermetallic compound, is easily precipitated, and the high-temperature strength decreases. Not only does the thermal fatigue property and the creep resistance property decrease, but it also promotes embrittlement. Therefore, the Nb content is set to 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%. Furthermore, it is preferable that the Nb content is 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 the steel. This effect will appear when Mo contains more than 1.80%. When the Mo content is less than 1.80%, the high temperature strength is insufficient, and excellent thermal fatigue properties and creep resistance properties cannot be obtained. On the other hand, when excessive Mo is contained, it not only hardens the steel and lowers the workability, but also precipitates as the Laves phase (Fe ₂ Mo) like Nb, reducing the amount of dissolved Mo in the steel, resulting in thermal fatigue characteristics Instead, it decreases. In addition, in the thermal fatigue test, the coarse σ phase precipitates and becomes the starting point of failure, resulting in a decrease in thermal fatigue properties. Therefore, the upper limit of the Mo content is set to 2.50%. Preferably, the Mo content is 1.90% or more. More preferably, the Mo content is more than 2.00%. Furthermore, it is preferable that 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 reduces the toughness and formability of steel. If it contains more than 0.015% of steel, not only the reduction in toughness and formability becomes significant, but also the formation of Nb nitrides causes a decrease in the amount of solid solution Nb, which makes it resistant to potential Variation characteristics and thermal fatigue characteristics are reduced. Therefore, the N content is set to 0.015% or less. Furthermore, 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 creep resistance in the present invention. Sb is dissolved in the steel to inhibit the creep deformation of the steel at high temperature. Sb will not precipitate as carbonitrides or Laves phases in the high temperature range, and will still dissolve in steel after long-term use, inhibiting creep deformation, so the creep resistance characteristics can be improved. This effect can be obtained when the Sb content is more than 0.002%. On the other hand, when excessive Sb is contained, the toughness and hot workability of the steel are reduced, so not only cracks are likely to occur during manufacturing, but also the thermal fatigue properties are also reduced due to the reduced thermal ductility. Therefore, the upper limit of the Sb content is set to 0.50%. Preferably, the Sb content is 0.005% or more. More preferably, it is 0.020% or more. Furthermore, it is preferable that the Sb content is 0.30% or less. more Preferably, the Sb content is 0.10% or less.

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

As mentioned above, Nb and Mo are elements that can effectively improve thermal fatigue properties and creep resistance properties. The effect can be seen when Nb and Mo contain 0.30% or more and 1.80% or more, respectively. However, in order to cope with the high temperature of exhaust gas, the thermal fatigue life of repeated heating and cooling between 200~950℃ is better than the thermal fatigue characteristics and creep resistance characteristics of SUS444. The two elements must be contained in the predetermined range. , And must satisfy at least Nb+Mo≧2.3%, that is, make the amount of Nb+Mo (the total content of Nb and Mo) be 2.3% or more. When this condition is not met, even if a given amount of Sb is added, excellent creep resistance properties 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 properties and creep resistance properties cannot be obtained. Therefore, the upper limit of the amount of Nb+Mo is set to 3.0%. Preferably, the amount of Nb+Mo is 2.7% or less.

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

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

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

Ti: 0.01~0.16%

Ti is an element that fixes C and N, improves corrosion resistance or formability, and prevents grain boundary corrosion in the welded portion. In the present invention, Ti may be contained as needed. Since Ti has priority over Nb and C and N Therefore, by containing Ti, the amount of solid solution Nb in the steel that is effective for high-temperature strength can be ensured, and the heat resistance can also be effectively improved. These effects can be obtained when the Ti content is more than 0.01%. On the other hand, when Ti is contained in excess of more than 0.16%, the toughness decreases. For example, the hot-rolled sheet annealing line is repeatedly subjected to bending and springback, which causes breakage, which adversely affects manufacturability. In addition, it is easy to precipitate Nb carbonitrides with Ti carbonitrides as the core, so on the contrary, the amount of solid solution Nb in steel that is effective for high-temperature strength is reduced, resulting in a decrease in thermal fatigue properties and creep resistance properties. Therefore, when Ti is contained, the Ti content is set to 0.01 to 0.16%. Preferably, the Ti content is 0.03% or more. Furthermore, it is preferable that the Ti content is 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 may be contained as needed. This effect can be obtained when Zr is contained more than 0.01%. However, if the Zr content exceeds 0.50%, Zr intermetallic compounds are precipitated and the steel is embrittled. Therefore, when Zr is contained, the Zr content is set to 0.01 to 0.50%. Preferably, the Zr content is 0.03% or more. More preferably, the Zr content is 0.05% or more. Furthermore, it is preferable that the Zr content is 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 contains more than 0.01%. On the other hand, when excessive Co is contained, the toughness of the steel will decrease instead, 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%. Preferably, the Co content is 0.03% or more. Furthermore, it is preferable that the Co content is 0.30% or less.

B: 0.0002~0.0050%

B series can effectively improve the workability of steel, especially the element of secondary workability. This effect can be obtained when B contains more than 0.0002%. On the other hand, when excessive B is contained, BN is generated and the 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. Furthermore, it is preferable that the B content is 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 it is also an element that can effectively improve the oxidation resistance. These effects become remarkable when the V content is 0.01% or more. However, when the excessive V content exceeds 1.0%, coarse V (C, N) will precipitate, which not only reduces the toughness, but also reduces 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. Furthermore, it is preferable that the V content is 0.50% or less. More preferably, the V content is 0.20% or less.

W: 0.01~5.0%

W, like Mo, is an element that greatly improves high-temperature strength through solid solution strengthening. This effect can be obtained when W contains 0.01% or more. On the other hand, when excessive W is contained, not only will the steel be significantly hardened, but also a firm scale will be produced during the annealing step during manufacturing, so it becomes difficult to remove rust during pickling. Therefore, when W is contained, W The content is set to 0.01~5.0%. Preferably, the W content is 0.05% or more. Moreover, it is preferable that the W content is 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 that has the effect of improving the corrosion resistance of steel, and is contained when corrosion resistance is required. This effect can be obtained when Cu contains 0.01% or more. On the other hand, if Cu is contained in excess of 0.40%, the scale will be easily peeled off and the anti-repetitive oxidation properties will decrease. Therefore, when Cu is contained, the Cu content is set to 0.01 to 0.40%. Preferably, the Cu content is 0.03% or more. More preferably, the Cu content is 0.06% or more. Furthermore, it is preferable that the Cu content is 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 can be obtained when it contains more than 0.001% of Sn. On the other hand, when excessive Sn is contained, the embrittlement of steel will degrade 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 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 can effectively prevent nozzle clogging caused by precipitation of Ti-based intermediary substances that are easily produced during continuous casting. This effect can be obtained when Ca contains 0.0002% or more. On the other hand, in order not to produce surface defects and obtain good surface properties, the Ca content must be Set it to 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. Furthermore, it is preferable that the Ca content is 0.0030% or less. More preferably, the Ca content is 0.0020% or less.

Mg: 0.0002~0.0050%

Mg is an element that can effectively increase the equiaxed crystal ratio of the billet and increase the workability or toughness. In the steel containing Nb or Ti as in the present invention, Mg also has the effect of suppressing the coarsening of carbonitrides of Nb or Ti. This effect can be obtained when it contains 0.0002% or more of Mg. If the Ti carbonitride becomes coarser, it becomes the starting point of brittle fracture, so the toughness is greatly reduced. If the Nb carbonitrides become coarser, the amount of Nb solid solution in the steel decreases, resulting in a decrease in thermal fatigue properties. On the other hand, if the Mg content exceeds 0.0050%, the surface properties of the steel deteriorate. 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. Furthermore, it is preferable that the Mg content is 0.0030% or less. More preferably, the Mg content is 0.0020% or less.

The remainder is Fe and unavoidable impurities. In the case of containing the above-mentioned optional components below the above lower limit, the optional components contained in the content below the lower limit are included as unavoidable impurities.

Next, the manufacturing method of the ferrous iron-based stainless steel of the present invention will be described.

The manufacturing method of the stainless steel of the present invention can be suitably adopted as long as it is a normal manufacturing method of ferrite-based stainless steel, and is not particularly limited.

For example, it can be manufactured by the following manufacturing steps, that is, melting steel in a known melting furnace such as a converter or an electric furnace, or further undergoing secondary refining such as ladle refining or vacuum refining. Refining to form steel with the above-mentioned composition of the present invention, using continuous casting method or block-block rolling method to form steel sheet (steel billet), after which it undergoes hot rolling, hot-rolled sheet annealing, pickling, and cold rolling , Final annealing and pickling and other steps to form cold-rolled annealed sheets. The above-mentioned cold rolling may be performed once or two or more times of cold rolling via intermediate annealing, and the steps of cold rolling, final annealing, and pickling may be repeated. Furthermore, the annealing of the hot-rolled sheet may be omitted, and when the surface gloss or roughness adjustment of the steel sheet is required, the temper rolling may be performed after cold rolling or final annealing.

The preferred manufacturing conditions in the above manufacturing method will be described.

The steel making step of melting steel is preferably performed by vacuum oxygen decarburization (VOD, vacuum oxygen decarburization) method or argon oxygen decarburization (AOD, argon oxygen decarburization) method on the steel melted in a converter or electric furnace. Sub-refining to form steel containing the above-mentioned essential 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. After that, the steel material is preferably heated to 1050 to 1250°C and hot rolled to form a hot-rolled sheet of the desired thickness. In terms of manufacturing, the thickness of the hot-rolled sheet is preferably less than 5mm. Of course, thermal processing other than the sheet can also be performed. Preferably, the above-mentioned hot-rolled sheet is subsequently subjected to continuous annealing at a temperature of 900 to 1150°C, or batch annealed at a temperature of 700 to 900°C, and then rust-removed by pickling or grinding. Hot rolled products. Furthermore, if necessary, rust can be removed by shot blasting before pickling.

Furthermore, the above-mentioned hot-rolled product (hot-rolled and 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 one time, but from the viewpoint of productivity or required quality, it may be cold rolling two or more times via intermediate annealing. The total reduction ratio of the cold rolling performed once or more than twice is preferably 60% or more, more preferably 70% or more. Thereafter, the cold-rolled steel sheet is preferably, preferably at 900~1200°C, more preferably Continuous annealing (final annealing) at a temperature of 1000~1150℃, and pickling or grinding to form cold-rolled products (cold-rolled annealed sheets). The final annealing can also be performed in a reducing gas environment. In this case, the pickling or polishing after the final annealing can also be omitted. Furthermore, depending on the application, temper rolling and the like may be performed after the final annealing to adjust the shape, surface roughness, and material of the steel sheet.

After that, the hot-rolled products or cold-rolled products obtained in the above manner are subjected to cutting or bending, stretching, and drawing processing according to their respective uses, and are formed into exhaust pipes and catalysts for automobiles or motorcycles. Tubes, exhaust ducts of thermal power plants, or fuel cell-related components, such as spacers, interconnecting connectors or reformers, etc. Among them, the ferrite-based stainless steel of the present invention is suitable for exhaust system components such as exhaust manifolds or exhaust pipes, converter boxes, and mufflers. Especially when the temperature rises to 700°C or higher by exhaust from the engine during use, an exhaust manifold with excellent durability can be obtained as one of the characteristics.

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

[Example]

Hereinafter, the present invention will be explained in more detail with examples.

Melt and cast steel with the composition of No. 1~41, 43, 45~47 shown in Table 1 in a vacuum melting furnace to form a 50kg steel block. After heating at 1170°C, it is formed by hot rolling 35mm thick plate. Divide the plate into two parts, heat one of the steel blocks to 1100°C, and then perform hot rolling to form a hot-rolled plate with a plate thickness of 5mm. After annealing at a temperature in the range of 1000~1150°C, it is ground and ground. Hot rolled annealing board. Then, cold-rolled with a reduction ratio of 70%, final annealed at a temperature of 1000~1150℃, and then pickled or polished to remove rust to form a cold-rolled annealed sheet with a thickness of 1.5mm for creep test . Furthermore, for reference, for SUS444 (Conventional Example No. 28), cold-rolled annealed sheets were also produced in the same manner as above for creep test. Regarding the annealing temperature, confirm the structure within the above-mentioned temperature range and determine the temperature for each steel.

<Creep Change Test>

A test piece of the shape shown in Fig. 1 was cut from each cold-rolled and annealed sheet obtained in the above manner, and a creep test with a stress of 15 MPa was carried out at 900°C. Based on the time taken before fracture, the evaluation is performed in the following manner. Regarding SUS444 (Conventional Example No. 28), which was performed as a comparison, the time taken before breaking was 5.5 hr.

◎: Breaking time≧10hr

○: 6hr≦break time<10hr

×: Breaking time<6hr

In the above evaluation, ⊚ and ◯ are regarded as pass, and × is regarded as unacceptable. The obtained results are shown in Table 1 (refer to the creep change of 900°C in Table 1).

Next, using the remaining one of the above-mentioned two-part plate, heated to 1100°C, hot forged to form each rod of 30mm square. Secondly, after annealing at a temperature of 1000~1150℃, mechanical processing is performed to process the thermal fatigue test piece with the shape and size shown in Figure 2 for the following thermal fatigue test. The annealing temperature is set to the temperature after confirming the structure for each component and completing recrystallization. Furthermore, for reference, regarding the steel with the composition of SUS444 (Conventional Example No. 28), test pieces were also made in the same manner as the above, and the heat fatigue test was performed.

<Thermal Fatigue Test>

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

Regarding the thermal fatigue life, the stress is calculated by dividing the load detected at 200°C by the cross-sectional area of the parallel portion of the test piece (refer to Figure 2), and set the stress value to the initial cycle (the first stable test) 5 cycles) The number of cycles at which the stress value is reduced to 75% is evaluated in the following manner. Regarding SUS444 (Conventional Example No. 28), which was performed as a comparison, the thermal fatigue life was 650 cycles.

◎: More than 1000 cycles (qualified)

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

×: less than 800 cycles (unqualified)

In the above evaluation, ⊚ and ◯ were regarded as pass, and × was regarded as unacceptable. The results obtained are shown in Table 1 (refer to the thermal fatigue life in Table 1 at 950°C).

According to Table 1, it is shown that the ferrous iron-based stainless steels of No. 1 to 27 of the present invention (hereinafter, the ferrous iron-based stainless steel is abbreviated as steel) are superior to SUS444 in the creep test and the thermal fatigue test (the conventional example) No. 28 steel) characteristics.

In No.29 steel, the Nb+Mo content is less than 2.3% by mass, and the creep rupture time and thermal fatigue life are unqualified. In No. 30 steel, the Ni content exceeds 0.60 mass%, and the thermal fatigue life is unqualified. In the No. 31 steel, the Cr content is less than 18.0% by mass, and the thermal fatigue life is unqualified. In No.32 steel, the Mo content is less than 1.80% by mass, and the creep rupture time and thermal fatigue life are unqualified. In No. 33 steel, the Nb content is less than 0.30% by mass, and the creep rupture time and thermal fatigue life are all unqualified. In the No. 34 steel, the Si content is less than 0.1% by mass, and oxidation is notably found in any of the creep test and thermal fatigue test, and the creep rupture time and thermal fatigue life are all unqualified. In No. 35 steel, the Ti content exceeds 0.16 mass%, and the creep rupture time and thermal fatigue life are all unqualified. In the No. 36 steel, the Cr content exceeds 20.0% by mass, and the thermal fatigue life becomes unacceptable due to the embrittlement of the steel. In No. 37 steel, the Mn content is less than 0.05% by mass, and peeling of oxide scale occurred in the thermal fatigue test, and the thermal fatigue life was unqualified. In the No. 38 steel, the C content exceeds 0.020% by mass. As the amount of Nb in the steel decreases, the creep rupture time and thermal fatigue life become unacceptable. In the steel of No. 39, the N content exceeds 0.015 mass%. As the Nb content in the steel decreases due to the precipitation of Nb nitrides, the creep rupture time and thermal fatigue life become unacceptable. In the steel of No. 40, the Sb content exceeds 0.50% by mass, and the thermal fatigue life becomes unacceptable due to the decrease in hot rolling ductility. In No.41 steel, the Mo content exceeds 2.50% by mass, the coarse σ phase (Fe-Cr series intermetallic compound) precipitates in the thermal fatigue test, and the thermal fatigue life is unqualified. In addition, the creep rupture time is also unqualified. In No. 43 steel, the Sn content exceeds 0.005 mass%, and the thermal fatigue life is unqualified. No.45 steel does not contain Sb, and the creep rupture time and thermal fatigue life are all unqualified. In No.46 steel, the Nb content exceeds 0.80 Mass%, creep rupture time and thermal fatigue life are all unqualified. In No.47 steel, the Nb+Mo content exceeds 3.0%, and the creep rupture time and thermal fatigue life are all unqualified.

(Industrial availability)

The ferrous iron-based stainless steel of the present invention is not only suitable for exhaust system components of automobiles, but also suitable for exhaust system components of thermal power generation systems requiring the same characteristics or components for solid oxide fuel cells.

Claims (5)

A ferrous iron-based stainless steel, which has the following composition, that is, in terms of mass%, it contains C: 0.020% or less, Si: 0.1 to 1.0%, Mn: 0.05 to 0.60%, P: 0.050% or less, and S: 0.008% or less, Ni: 0.02~0.60%, Al: 0.001~0.25%, Cr: 18.0~20.0%, Nb: 0.30~0.80%, Mo: 1.80~2.50%, N: 0.015% or less, Sb: 0.002~0.50 %, and satisfies the following formula (1), the remainder contains Fe and unavoidable impurities, Nb+Mo: more than 2.5% and 2.73% or less... (1) (Nb and Mo in formula (1) represent each element The content (mass%)). For example, the ferrous iron-based stainless steel of claim 1, wherein the above-mentioned component composition is calculated by mass%, and further contains selected from Ti: 0.01~0.16%, Zr: 0.01~0.50%, Co: 0.01~0.50%, B: 0.0002 ~0.0050%, One or more of V: 0.01~1.0%, W: 0.01~5.0%, Cu: 0.01~0.40%, Sn: 0.001~0.005%. For example, the ferrous iron-based stainless steel of claim 1 or 2, wherein the above-mentioned component composition is calculated by mass%, and further contains one or two selected from Ca: 0.0002 to 0.0050% and Mg: 0.0002 to 0.0050%. For example, the ferrous iron-based stainless steel of claim 1 or 2 is used in exhaust manifolds that heat up to 700°C or higher due to exhaust from the engine. Such as the ferrous iron-based stainless steel of claim 3, which is used for exhaust manifolds that heat up to 700°C or higher due to exhaust from the engine.

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