TWI645051B - Ferrous iron-based stainless steel - Google Patents

Abstract

The present invention provides a ferrous iron-based stainless steel plate having excellent skin adhesion and thermal fatigue characteristics, and excellent corrosion resistance to condensation water.

The ferrous iron-based stainless steel plate of the present invention has the following composition, which contains C: 0.010% or less, Si: 1.0% or less, Mn: 1.0% or less, P: 0.040% or less, S: 0.030% or less, Cr: 17.0% or more and 18.5% or less, N: 0.015% or less, Nb: 0.40% or more and 0.80% or less, Ti: 0.10% or more and 0.40% or less, Al: 0.20% or less, Ni: 0.05% or more and 0.40% or less Below, Co: 0.01% or more and 0.30% or less, Mo: 0.02% or more and 0.30% or less, Cu: 0.02% or more and 0.40% or less, and satisfying the following formula (1), and the remainder contains Fe and unavoidable Impurities.

C% + N%: below 0.018% (1)

In Formula (1), C% and N% represent the content (mass%) of C and N, respectively.

Description

   Ferrous iron-based stainless steel   

The present invention relates to a ferritic iron-based stainless steel having excellent scale adhesion, thermal fatigue characteristics, and corrosion resistance to condensation water.

Among automotive exhaust system components, exhaust manifolds that are directly connected to the upstream side, especially the engine, are used in harsh environments where the maximum operating temperature also reaches 800 to 900 ° C. Therefore, the material is required to have excellent thermal fatigue characteristics, and a ferritic iron-based stainless steel to which Nb is added is mainly used.

The Nb added to the ferrous iron-based stainless steel is solid-dissolved in the steel to increase the high-temperature strength to improve the thermal fatigue characteristics. However, Nb may be easily bonded to C or N in the steel to become a carbonitride, so that the amount of solid solution Nb may be reduced and the thermal fatigue characteristics may be reduced. As a countermeasure, it is also possible to prevent the formation of Nb carbonitride by compoundly adding Ti which is more easily bonded to C or N than Nb, so that C or Ni is formed as a Ti carbonitride. The representative of this Nb-Ti composite additive steel is Type 441 ferrous iron-based stainless steel (18% Cr-0.5% Nb-0.2% Ti) (EN10088-2: EN1.4509), which is widely used in automobiles Exhaust manifold, etc.

The exhaust manifold is used under the severe repeated oxidation environment of heating and rapid cooling when the engine is started and stopped. Therefore, if the skin layer is peeled off, the ferrite will be directly exposed to the high temperature exhaust gas and oxidized. The thickness of the board is reduced, and there may be holes or deformations depending on the situation. Therefore, for the Nb-Ti compound-added ferrous iron-based stainless steel used in the exhaust manifold of automobiles, excellent skin adhesion is also required for the skin layer to not peel off.

Patent Documents 1 and 2 disclose the addition of Mo as a method for improving the high-temperature strength or thermal fatigue characteristics of Nb-Ti composite-added ferrous iron-based stainless steel. The addition of Mo, Cu, and W is disclosed in Patent Documents 3 to 5. As a method for improving the adhesion of the cortex, Patent Document 3 discloses the addition of REM, Ca, Y, and Zr. Patent Document 5 discloses addition of REM and Ca. Patent Document 6 discloses an Nb-Ti compound-added ferrous iron-based stainless steel having improved skin adhesion and thermal fatigue characteristics by adding Co and Ni.

On the other hand, for a muffler or a pipe body disposed on the downstream side of an automobile exhaust pipe part, for example, water containing snowmelt salt scattered on the road is scattered or exposed to the exhaust gas and cooled to contain corrosiveness. Ion condensed water often requires corrosion resistance (hereinafter referred to as corrosion resistance of condensed water). Therefore, a ferrous iron-based stainless steel to which Ti or Mo is added is used. As an example, SUS436L (18% Cr-0.2% Ti-1% Mo) or SUS430LX (18% Cr-0.2% Ti) specified in JIS G4305 can be mentioned.

As described above, the exhaust manifolds on the upstream side and the silencers on the downstream have different required characteristics, so although ferrous iron-based stainless steel suitable for each application is used, if common ferrous iron-based stainless steel can be used Manufacturing it can reduce the number of steel types, and reduce the number of welded parts of parts of different materials, so that the manufacturability of the parts is stable and the car can be manufactured efficiently.

    [Prior technical literature]         [Patent Literature]    

Patent Document 1: Japanese Patent Laid-Open No. 4-224657

Patent Document 2: Japanese Patent Laid-Open No. 5-70897

Patent Document 3: Japanese Patent Laid-Open No. 2004-218013

Patent Document 4: Japanese Patent Laid-Open No. 2008-240143

Patent Document 5: Japanese Patent Laid-Open No. 2009-174040

Patent Document 6: Japanese Patent No. 5505570

However, the methods disclosed in Patent Documents 1 to 5 have disadvantages in that Mo or W is expensive and reduces workability such as toughness of a steel sheet. In addition, Cu has the disadvantage that not only the workability at room temperature is greatly reduced, but also the oxidation resistance is lowered. Moreover, in Patent Documents 1 to 5, the thermal fatigue characteristics or oxidation resistance (skin adhesion) required for the exhaust manifold and the condensation water corrosion resistance required for a muffler, etc., were not evaluated at the same time. Furthermore, when SUS436L (18% Cr-0.2% Ti-1% Mo) or SUS430LX (18% Cr-0.2% Ti) is used in an exhaust manifold, there is a problem that thermal fatigue characteristics are insufficient.

As described above, the conventional ferrous iron-based stainless steels are not all good in terms of skin adhesion, thermal fatigue characteristics, and condensation water corrosion resistance.

The present invention has been made in order to solve the above-mentioned problems, and an object thereof is to provide a ferrous iron-based stainless steel which is excellent in skin adhesion and thermal fatigue characteristics, and is also excellent in corrosion resistance to condensation water.

Moreover, the so-called "excellent adhesion of the skin layer" in the present invention means that the area of the skin layer peeled off on the surface of the test piece after repeated oxidation tests (heating rate: 5 ° C / sec, cooling rate: 1.5 ° C / sec) has not reached 5%. The repeated oxidation test is performed for 400 cycles of the milled cold-rolled annealed sheet, which is maintained in the atmosphere at 1000 ° C for 20 min and at 100 ° C for 1 min.

In addition, the term "excellent thermal fatigue characteristics" means that heating and cooling are repeatedly performed between 200 and 900 ° C in accordance with JSMS-SD-7-03, and strain is repeatedly applied at a restraint rate of 0.6. The value (stress) obtained by dividing the load detected at 200 ° C by the cross-sectional area of the test specimen's soaking parallel section is reduced to 75% of the cycle number (thermal fatigue life) of 660 Loop above.

In addition, so-called "excellent corrosion resistance to cold water" refers to the milled in the cold-rolled annealed sheet held comprising _{^{Cl -: 500ppm, SO 4 2-}} : 1000ppm and pH: 4, temperature: 80 ℃ of a thermostat The solution was immersed for 2 hours and dried for 6 hours as one group to perform 30 groups, and the weight loss of corrosion was 10 g / m ² or less.

The present inventors have studied the influence of the amount of C + N on the thermal fatigue characteristics of Nb-Ti-Co-Ni compound-added ferrous iron-based stainless steel, and found that the amount of C + N, The amount of Ti is limited to an appropriate amount to obtain further excellent thermal fatigue characteristics. Furthermore, research was conducted on the corrosion resistance of condensed water of Nb-Ti-Co-Ni compound-added fertilizer-type iron-based stainless steel, and it was found that the addition of appropriate amounts of Mo and Cu can improve the corrosion resistance of condensed water, and is used in Muffler and other downstream parts.

The present invention has been completed based on the above findings, and the gist thereof is as follows.

[1] A ferrous iron-based stainless steel having a composition containing C: 0.010% or less, Si: 1.0% or less, Mn: 1.0% or less, P: 0.040% or less, and S: 0.030% by mass%. Below, Cr: 17.0% or more and 18.5% or less, N: 0.015% or less, Nb: 0.40% or more and 0.80% or less, Ti: 0.10% or more and 0.40% or less, Al: 0.20% or less, Ni: 0.05% or more and 0.40% or less, Co: 0.01% or more and 0.30% or less, Mo: 0.02% or more and 0.30% or less, Cu: 0.02% or more and 0.40% or less, and satisfying the following formula (1), and the remainder contains Fe and may not Avoid impurities.

C% + N%: below 0.018% (1)

In Formula (1), C% and N% represent the content (mass%) of C and N, respectively.

[2] The ferrous iron-based stainless steel according to the above [1], further comprising, in mass%, selected from Ca: 0.0005% or more and 0.0030% or less, Mg: 0.0002% or more and 0.0020% or less, and B: 0.0002% One or more of 0.0020% or more.

[3] The ferrous iron-based stainless steel according to the above [1] or [2], further comprising, in mass%, selected from V: 0.01% or more and 0.50% or less, W: 0.02% or more and 0.30% or less, Zr: one or more of 0.005% to 0.50%.

According to the present invention, a ferrous iron-based stainless steel excellent in skin adhesion, thermal fatigue characteristics, and resistance to condensation water corrosion can be obtained. The ferrous iron-based stainless steel of the present invention is excellent in both heat resistance (skin adhesion, thermal fatigue characteristics) and corrosion resistance of condensed water, so it can be preferably used for both the upstream side and the downstream side of an automobile exhaust system component side.

FIG. 1 is a diagram explaining a thermal fatigue test piece.

FIG. 2 is a diagram illustrating temperature and restraint conditions in a thermal fatigue test.

Hereinafter, the present invention will be described in detail. The ferrous iron-based stainless steel of the present invention has the following composition, which contains C: 0.010% or less, Si: 1.0% or less, Mn: 1.0% or less, P: 0.040% or less, S: 0.030% or less, and Cr in mass%. : 17.0% or more and 18.5% or less, N: 0.015% or less, Nb: 0.40% or more and 0.80% or less, Ti: 0.10% or more and 0.40% or less, Al: 0.20% or less, Ni: 0.05% or more and 0.40% or less , Co: 0.01% or more and 0.30% or less, Mo: 0.02% or more and 0.30% or less, Cu: 0.02% or more and 0.40% or less, and satisfying the following formula (1), and the remainder contains Fe and inevitable impurities In addition, the skin layer has excellent adhesion and thermal fatigue characteristics, and is also excellent in condensation water corrosion resistance.

C% + N%: below 0.018% (1)

In Formula (1), C% and N% represent the content (mass%) of C and N, respectively.

Next, the reason for specifying the component composition of the ferrous iron-based stainless steel of the present invention will be described. In addition, as long as there is no special explanation in advance of a component%, all means the mass%.

    C: 0.010% or less    

C is an element effective for improving the strength of steel. The effect is obtained when C is contained in an amount of 0.001% or more. Therefore, the C content is preferably 0.001% or more. On the other hand, if C is contained in an amount exceeding 0.010%, the skin layer is peeled, so the C content is set to 0.010% or less. In addition, regarding the C content, from the viewpoint of ensuring toughness and workability, and from the viewpoint that NbC coarsens or the amount of precipitation increases, the amount of solid solution Nb in the steel decreases and the thermal fatigue characteristics decrease. The C content is preferably small, and the C content is preferably set to 0.008% or less. The C content is more preferably 0.005% or more.

    Si: 1.0% or less    

Si is an element effective for improving the oxidation resistance. This effect is obtained when Si is contained in an amount of 0.01% or more. Therefore, the Si content is preferably 0.01% or more. On the other hand, if Si is contained in excess of 1.0%, the workability is lowered, so the Si content is set to 1.0% or less. The Si content is more preferably 0.20% or more, and still more preferably 0.30% or more. In particular, if the Ni content is 0.20% or more and the Si content is 0.30% or more, the skin layer adhesion is particularly excellent. The Si content is preferably 1.00% or less, more preferably 0.50% or less, and still more preferably 0.40% or less.

    Mn: 1.0% or less    

Mn is an element that increases the strength of steel, and also has a role as a deoxidizer. This effect is obtained when Mn is contained in an amount of 0.01% or more, so the Mn content is preferably 0.01% or more. On the other hand, if Mn is contained in excess of 1.0%, the oxidation increase is significantly increased and the oxidation resistance is lowered. Therefore, the Mn content is set to 1.0% or less. The Mn content is more preferably 0.20% or more, and still more preferably 0.30% or more. The Mn content is preferably 1.00% or less, more preferably 0.60% or less, and still more preferably 0.50% or less.

    P: 0.040% or less    

P is an element that reduces toughness. It is more desirable to reduce P, and the P content is set to 0.040% or less. The P content is preferably 0.035% or less. More preferably, the P content is 0.030% or less.

    S: 0.030% or less    

S is preferably less because it reduces formability and corrosion resistance, and the S content is set to 0.030% or less. The S content is preferably 0.006% or less. More preferably, the S content is 0.003% or less.

    Cr: 17.0% or more and 18.5% or less    

Cr is an element required to improve corrosion resistance and oxidation resistance. In order to obtain good corrosion resistance and oxidation resistance, it is necessary to contain 17.0% or more of Cr. If the Cr content is less than 17.0%, the oxide scale layer tends to increase, and not only the skin layer adhesion is reduced, but also the thermal fatigue characteristics may be reduced. Furthermore, the corrosion resistance in condensed water is not sufficiently obtained. On the other hand, if Cr is contained in an amount exceeding 18.5%, the steel is hardened and the manufacturability or processability is reduced. Therefore, the Cr content is set to 18.5% or less. The Cr content is preferably in a range of 17.5% to 18.5%.

    N: 0.015% or less    

N reduces the toughness and workability of the steel, so it is ideally less. Also, if the N content is large, coarse TiN precipitates, and along with TiN, NbC precipitates in large amounts, thereby reducing the amount of solid solution Nb in the steel. This reduces thermal fatigue characteristics. Furthermore, starting from the coarse TiN, the oxide scale layer is easily peeled and the skin layer adhesiveness is also reduced. Therefore, the N content is set to 0.015% or less. The N content is preferably 0.012% or less. More preferably, the N content is 0.010% or less.

    Nb: 0.40% or more and 0.80% or less    

Nb is an element that has a solid solution in steel, which significantly increases the high temperature strength and improves the thermal fatigue characteristics. This effect is obtained when Nb is contained in an amount of 0.40% or more. On the other hand, when excessive Nb is contained in excess of 0.80%, not only the toughness of the steel is reduced, but also the Laves phase (Fe ₂ Nb) is formed at a high temperature and the high-temperature strength is reduced. Therefore, the Nb content is set to 0.80% or less. The Nb content is preferably 0.43% or more, and more preferably 0.45% or more. The Nb content is preferably 0.60% or less, and more preferably 0.50% or less.

    Ti: 0.10% or more and 0.40% or less    

Ti is preferentially linked to C and N to generate carbonitrides, thereby preventing the formation of Nb carbonitrides, and improving the corrosion resistance, formability, and grain boundary corrosion of the welded portion. In order to obtain these effects, Ti must be contained in an amount of 0.10% or more. If the Ti content is less than 0.10%, C and N cannot be completely formed as Ti carbonitrides, and Nb carbonitrides are formed. As a result, the amount of Nb solid solution decreases and thermal fatigue characteristics decrease. On the other hand, when excessive Ti is contained in excess of 0.40%, the precipitation amount of Ti carbonitrides increases, and as a result, Nb carbonitrides are easily precipitated, so that the solid solution amount of Nb is reduced, and thus the thermal fatigue characteristics are reduced. Furthermore, since the amount of Ti carbonitrides is increased, the skin adhesion is also reduced, and corrosion occurs due to the coarse Ti carbonitrides as a starting point. Therefore, the corrosion resistance to condensed water is also reduced. Therefore, the Ti content is set to 0.40% or less. The Ti content is preferably 0.15% or more. The Ti content is preferably 0.30% or less, and more preferably 0.25% or less.

    Al: 0.20% or less    

Al is an element effective for deoxidation, and the effect is obtained when it contains 0.01% or more. Therefore, the Al content is preferably 0.01% or more. On the other hand, since the workability is reduced by hardening the steel, the Al content is set to 0.20% or less. The Al content is more preferably 0.02% or more. The Al content is preferably 0.10% or less, and more preferably 0.06% or less.

    Ni: 0.05% or more and 0.40% or less    

Ni is an element important in the present invention to ensure skin adhesion, and in order to obtain this effect, it is necessary to contain 0.05% or more of Ni. If the Ni content is less than 0.05%, the adhesion of the skin layer is lowered, and the point where the skin layer peels off becomes the starting point, which may cause thermal fatigue failure. Further, as described below, the steel of the present invention reduces the thermal expansion coefficient by containing an appropriate amount of Co. Therefore, the above-mentioned effect is obtained with a smaller amount of Ni content than a steel without Co addition or a steel with insufficient Co content. On the other hand, Ni is an expensive element. In addition, if Ni is contained in an amount exceeding 0.40%, a γ phase is generated at a high temperature and the skin adhesion is reduced. Therefore, the Ni content is set to a range of 0.05% to 0.40%. The Ni content is preferably 0.10% or more, and more preferably 0.20% or more. The Ni content is preferably 0.30% or less, and more preferably 0.25% or less.

    Co: 0.01% or more and 0.30% or less    

Co is an important element in the present invention. Co is an element required for improving thermal fatigue characteristics, and therefore, it must contain at least 0.01% of Co. Co system reduces the thermal expansion coefficient of steel, reduces the expansion amount during heating, and reduces the amount of strain generated during heating and cooling, thereby improving thermal fatigue characteristics. Furthermore, as the thermal expansion coefficient of the steel decreases, the difference between the thermal expansion coefficient of the steel and the skin layer becomes smaller, so that the skin layer becomes difficult to peel off during cooling. Therefore, by containing a smaller amount of Ni, there is an effect of preventing peeling of the skin layer. On the other hand, if Co is contained in excess of 0.30%, Co is concentrated at the interface between the oxide film and the ferrite, and the adhesion of the skin layer is reduced. If Co is contained in excess of 0.30%, the side effect of thickening the interface will offset the skin layer peeling prevention effect caused by the decrease in the above-mentioned thermal expansion coefficient, and the skin layer peels during cooling. Therefore, the Co content is set to 0.01% to 0.30%. The Co content is preferably 0.02% or more, and more preferably 0.03% or more. The Co content is preferably 0.10% or less.

    Mo: 0.02% or more and 0.30% or less    

Mo is an element that increases the strength of steel through solid solution strengthening, improves thermal fatigue characteristics, and improves salt corrosion resistance, thereby improving the corrosion resistance of condensed water. The effect is that Mo contains 0.02% or more Get it. However, Mo is an expensive element, and if Mo is contained in a large amount, not only surface defects are generated, but processability at room temperature is also reduced. In order to obtain good surface properties without causing surface defects, the Mo content must be 0.30% or less. Therefore, the Mo content is set to a range of 0.02% to 0.30%. The Mo content is preferably 0.04% or more. The Mo content is preferably 0.10% or less.

    Cu: 0.02% or more and 0.40% or less    

Cu has the effect of strengthening the steel by fine precipitation in the form of ε-Cu to improve the thermal fatigue characteristics and the sulfuric acid corrosion resistance, thereby improving the corrosion resistance of condensed water. In order to obtain these effects, Cu must be contained in an amount of 0.02% or more. On the other hand, if Cu is contained in an amount exceeding 0.40%, the adhesion of the oxide layer is lowered and the reoxidation resistance is lowered. Furthermore, ε-Cu is liable to precipitate coarsely, and the corrosion resistance to condensation water is also reduced. Therefore, the Cu content is set to 0.40% or less. Therefore, the Cu content is set to a range of 0.02% or more and 0.40% or less. The Cu content is preferably 0.04% or more. The Cu content is preferably 0.10% or less.

In the present invention, Mo and Cu respectively improve salt corrosion resistance and sulfuric acid corrosion resistance, thereby improving condensation water corrosion resistance. Therefore, if Mo or Cu is contained alone, sufficient condensation water corrosion resistance is not obtained. Sex. In the present invention, since both Mo and Cu are contained in appropriate amounts, excellent condensed water corrosion resistance is obtained.

    C% + N%: below 0.018% (1)    

In Formula (1), C% and N% represent the content (mass%) of C and N, respectively. As described above, from the viewpoints of toughness, processability, and peel resistance of C and N, the respective contents are set to 0.010% or less and 0.015% or less. Furthermore, in this invention, from a viewpoint of a thermal fatigue characteristic, C% + N% is limited to 0.018% or less like Formula (1) mentioned above. When C% + N% exceeds 0.018%, coarse Ti nitrides (TiN) are formed in large quantities, and along with this, NbC precipitates around TiN, so the amount of NbC precipitates increases. If the amount of NbC precipitates increases, the amount of Nb solid-dissolved in the steel decreases, and the high-temperature strength of the steel decreases, so the effect of improving the thermal fatigue characteristics will not be sufficiently obtained. Therefore, in the present invention in which Nb and Ti are added in combination, in order to sufficiently obtain the solid solution strengthening amount of Nb, C% + N% is set to 0.018% or less. The C% + N% is preferably 0.015% or less. When C% + N% is less than 0.015%, the precipitated TiN or NbC becomes finer, and then TiN is refined, so that the amount of NbC precipitated around it decreases, and the amount of solid solution Nb in steel increases . Furthermore, NbC itself is finely precipitated, thereby obtaining a precipitation strengthening effect. These effects improve the thermal fatigue characteristics. More preferably, C% + N% is set to 0.013% or less.

The ferritic iron-based stainless steel which is excellent in the skin layer's adhesiveness and thermal fatigue characteristics, and also has excellent corrosion resistance to condensed water, is characterized by containing the above-mentioned essential components, and the remainder contains Fe and inevitable impurities. Furthermore, if necessary, one or two or more kinds selected from Ca, Mg, and B, and / or one or two or more kinds selected from V, W, and Zr may be contained in the following range.

    Ca: 0.000.5% or more and 0.0030% or less    

The Ca-based component is effective for preventing clogging of the nozzle caused by the precipitation of Ti-based intermediary substances which are easily generated during continuous casting. This effect is obtained when Ca is contained in an amount of 0.0005% or more. On the other hand, in order to obtain good surface properties without causing surface defects, the Ca content is preferably set to 0.0030% or less. Therefore, when Ca is contained, the Ca content is preferably set to a range of 0.0005% to 0.0030%. The Ca content is more preferably in a range of 0.0005% to 0.0020%. The Ca content is more preferably in a range of 0.0005% to 0.0015%.

    Mg: 0.0002% or more and 0.0020% or less    

Mg is an element effective for improving workability or toughness. Furthermore, Mg is an element effective for suppressing the coarsening of carbonitrides of Nb or Ti. When the Ti carbonitride is coarsened, it becomes a starting point of brittle fracture, and thus the toughness is reduced. In addition, if the Nb carbonitride is coarsened, the solid solution amount of Nb in the steel is reduced, and thus the thermal fatigue characteristics are reduced. The aforementioned effects of improving workability and toughness, or suppressing coarsening of carbonitrides of Nb and Ti are obtained when Mg is contained in an amount of 0.0002% or more. On the other hand, if the Mg content exceeds 0.0020%, the surface properties of the steel may be deteriorated. Therefore, when Mg is contained, the Mg content is preferably set to a range of 0.0002% or more and 0.0020% or less. The Mg content is more preferably 0.0004% or more. The Mg content is more preferably 0.0015% or less, and even more preferably 0.0010% or less.

    B: 0.0002% or more and 0.0020% or less    

B is an element effective for improving workability, especially secondary workability. These effects are obtained when B is contained in an amount of 0.0002% or more. On the other hand, if B is contained in an amount exceeding 0.0020%, the workability and toughness of the steel may be reduced. Therefore, the B content is set to 0.0020% or less. Therefore, when B is contained, the B content is preferably set to a range of 0.0002% or more and 0.0020% or less. The B content is more preferably 0.0003% or more. The B content is more preferably 0.0010% or less.

    V: 0.01% or more and 0.50% or less    

V is an element effective for improving high temperature strength. It also has the effect of suppressing coarsening of carbonitrides of Ti or Nb. This effect is obtained when V is contained at 0.01% or more. On the other hand, if V is contained in excess of 0.50%, coarse V (C, N) may precipitate and the toughness may decrease. Therefore, when V is contained, the V content is preferably set to a range of 0.01% to 0.50%. The V content is more preferably 0.02% or more. The V content is more preferably 0.20% or less.

    W: 0.02% or more and 0.30% or less    

W is an element that increases the strength of steel by solid solution strengthening, similar to Mo. This effect is obtained when W is contained in an amount of 0.02% or more. However, W is an expensive element, and if W is contained in a large amount, not only surface defects occur, but also workability such as toughness is greatly reduced. In order to obtain good surface properties, the W content is preferably set to 0.30% or less. Therefore, when W is contained, the W content is preferably set to a range of 0.02% or more and 0.30% or less.

    Zr: 0.005% or more and 0.50% or less    

Zr is an element that improves oxidation resistance. To obtain this effect, the Zr content is preferably set to 0.005% or more. On the other hand, if the Zr content exceeds 0.50%, the Zr intermetallic compound precipitates and the steel becomes easily brittle. Therefore, when Zr is contained, the Zr content is preferably set to 0.005% or more and 0.50% or less.

Next, a method for manufacturing the ferrous iron-based stainless steel according to the present invention will be described.

The ferritic iron-based stainless steel of the present invention can be produced by a usual method of stainless steel. The steel containing the above-mentioned composition is melted in a melting furnace such as a converter, an electric furnace, and the like, and then refined through secondary refining such as ladle refining and vacuum refining, and a continuous casting method or an ingot-open billet rolling method is used to make steel sheets (Slab), hot-rolled, hot-rolled sheet annealing, and pickling are performed to form a hot-rolled annealed pickled sheet. Furthermore, a method for forming a cold-rolled annealed sheet through various steps such as cold rolling, finish annealing, and pickling is recommended. An example is as follows.

Melting in a converter or electric furnace, etc., secondary refining is performed by the Argon Oxygen Decarburization (AOD) method or the Vacuum Oxygen Decarburization (VOD) method, and the above components are melted. Molten steel is made into billets by continuous casting. The slab is heated to 1000 to 1250 ° C, and a hot-rolled sheet having a desired thickness is produced by hot rolling. The hot-rolled sheet was subjected to continuous annealing at a temperature of 900 ° C to 1100 ° C, and then a peeling layer was performed by shot blasting and pickling to prepare a hot-rolled annealed pickled sheet. The hot-rolled, annealed and pickled sheet can also be used directly for the purposes of the present invention, such as exhaust manifolds, flanges, tubes, or mufflers, but it can also be cold-rolled, annealed, and pickled to produce cold Roll annealed pickled plate. In this cold rolling step, if necessary, two or more cold rollings including intermediate annealing may be performed. The total reduction ratio of the cold rolling step including one or two or more cold rolling steps is set to 60% or more, and preferably 70% or more. The annealing temperature of the cold rolled sheet is 900 ~ 1150 ° C, preferably 950 ~ 1100 ° C. Depending on the application, the shape and quality of the steel sheet may be adjusted by applying mild rolling (such as temper rolling) after pickling. In addition, it may be set as Bright Annealed Finish, that is, annealing is performed in a reducing atmosphere containing hydrogen, and pickling is omitted.

Using the hot-rolled annealed plate products or cold-rolled annealed plate products manufactured in the above-mentioned manner, and performing bending processing according to their respective applications, etc., they are formed into exhaust pipes for automobiles or locomotives, outer cylinders for catalysts, and thermal power generation equipment Exhaust passage or fuel cell related components. The welding method for welding these components is not particularly limited, and tungsten inert gas arc welding (TIG), metal inert gas arc welding (MIG), or hybrid Various arc welding methods such as MAG (metal active gas arc welding), or resistance welding methods such as spot welding and seam welding, and high frequency resistance welding and high frequency induction welding such as electric seam welding.

    [Example]    

The steel having No. 1 to 40 having the component composition shown in Table 1 was melted and cast in a vacuum melting furnace to make a 30 kg steel block. Then, after heating to 1170 ° C., hot rolling was performed to produce a sheet having a thickness of 35 mm × width of 150 mm. Divide the plate into two. One of them was made into an angle rod having a cross section of 30 mm × 30 mm by forging, and was annealed in a range of 950 to 1050 ° C, and then machined to produce a thermal fatigue test piece shown in FIG. 1. Using this test piece, the following thermal fatigue test was performed. Regarding the annealing temperature, the structure is confirmed within a temperature range of 950 to 1050 ° C and is set on a component-by-component basis. The same applies to the following annealing.

The other sheet after being divided into two was heated to 1050 ° C and then hot rolled to produce a hot rolled sheet having a thickness of 5 mm. Thereafter, the hot-rolled sheet is annealed in a temperature range of 900 to 1050 ° C, and pickled to produce a hot-rolled annealed pickled sheet. At this stage, the surface properties of the steel plate were observed with the naked eye. The sheet thickness was set to 2 mm by cold rolling, and finish annealing was performed in a temperature range of 900 to 1050 ° C to produce a cold rolled annealed sheet. These were subjected to the following repeated oxidation test and condensed water immersion test.

    <Repeat oxidation test>    

A size of 20 mm wide by 30 mm long was cut out from the cold-rolled annealed sheet, and all 6 sides were polished with # 320 polishing paper (emery paper) for testing. The oxidation test conditions are in the atmosphere, and will be maintained at 1000 ° C for 20min and 100 ° C for 1min. Repeat 400 cycles. The heating rate and cooling rate were performed at 5 ° C / sec and 1.5 ° C / sec. After the test, the skin layer was visually observed for peeling, and the skin layer adhesion was evaluated. The obtained results are all shown in Table 1.

    <Thermal fatigue test>    

The test piece for the thermal fatigue test was repeatedly heated and cooled at 200 to 900 ° C, and strain was repeatedly applied at a restraint rate of 0.6 as shown in FIG. 2 to measure the thermal fatigue life. The measurement method is based on the Japanese Society of Materials Standard High Temperature and Low Cycle Test Method (JSMS-SD-7-03). First, the load detected at 200 ° C. for each cycle is divided by the cross-sectional area (50.3 mm ² ) of the soaking parallel portion of the test piece shown in FIG. 1, and the stress is set as the stress of the cycle. The number of cycles in which the stress in this cycle was reduced to 75% of the stress in the fifth cycle with stable behavior was defined as the thermal fatigue life. The number of life cycles was used to evaluate the thermal fatigue characteristics. The obtained results are all shown in Table 1.

In addition, as shown in FIG. 2, the restraint rate described above is a restraint rate η = a / (a + b), a is (free thermal expansion strain-control strain) / 2, and b is control strain / 2. In addition, the so-called free thermal expansion strain is a strain when the temperature is raised without giving mechanical stress at all, and the so-called control strain is a strain that represents a state with no stress at room temperature at all. The material restraint strain generated by the restraint on the material (free thermal expansion strain-control strain) is the strain relative to the free thermal expansion strain.

    <Condensation water immersion test>    

A 60 mm width × 80 mm length was cut out from the cold-rolled annealed sheet produced above, and all 6 sides were polished with # 320 abrasive paper for testing. The end is covered with a protective tape during the test. The test solution was simulated to condense water and adjusted to include Cl ⁻ : 500 ppm, SO ₄ ^{2 −} : 1000 ppm, and pH: 4. The temperature was maintained in a thermostatic bath so that the temperature became 80 ° C. The test was performed by immersing the solution for 2 hours and drying for 6 hours. After the test, the corrosion product was removed, and the weight before and after the test was measured to calculate the weight loss of the corrosion.

In addition, in Table 1, the determination criteria of each test are as follows.

(1) Cortex adhesion: The skin peeling area on the surface of the test piece after the repeated oxidation test is 0% (the skin peeling is not found by visual observation) is judged as ◎ (pass), and it is more than 0% and less than 5% It was judged as ○ (passed), and those with 5% or more were judged as x (unacceptable).

(2) Thermal fatigue characteristics: Those who have a thermal fatigue life of 750 cycles or more are judged as ◎ (passed), those who have 660 cycles or more and less than 750 cycles are judged as ○ (passed), and have not reached 660 cycles It was judged as x (unsatisfactory).

(3) Corrosion resistance to condensed water: Set the weight loss of corrosion to 5 g / m ² or less as ◎ (pass), set the weight loss of more than 5 g / m ² to 10 g / m ² as ○ (pass), or exceed 10 g / m ² is set to × (Failure).

According to Table 1, all the skin layers No. 1 to 20 and 36 to 40, which are examples of the present invention, are excellent in adhesiveness, thermal fatigue characteristics, and corrosion resistance to condensation water. Examples No. 2 ~ 4,6,9,10,12,14 ~ 16,19,20,36 ~ 40 of the present invention whose content of Si and Ni are in a better range (Si ≧ 0.30% and Ni ≧ 0.20%) Cortical adhesion is particularly excellent. Contents of C + N and Ti, Co, Mo, Cu in a preferred range (C + N ≦ 0.015%, Ti ≧ 0.15%, Co ≧ 0.02%, Mo ≧ 0.04%, Cu ≧ 0.04%) .1,2,6 ~ 11,16,38 series are particularly excellent in thermal fatigue characteristics. Examples No. 1, 2, 6-11, 16, 18, 36-40 of the present invention in which the content of Mo and Cu are in a preferred range (Mo ≧ 0.04% and Cu ≧ 0.04%) are particularly excellent in corrosion resistance to condensed water. In addition, the surface properties of all the hot-rolled annealed pickled plates of the examples of the present invention are good without surface defects.

On the other hand, Comparative Examples No. 21 and 24 where neither Mo nor Cu falls below the lower limit of the scope of the invention; Comparative Example No. 22 where Cu falls below the lower limit of the scope of the invention; Mo does not reach the lower limit of the invention In Comparative Example No. 23 of the lower limit, the corrosion resistance to condensation water was unacceptable.

The thermal fatigue characteristics of Comparative Example No. 25 whose C + N exceeded the upper limit of the range of the present invention became unacceptable. The thermal fatigue characteristics of Comparative Example No. 26, in which Co was below the lower limit of the scope of the present invention, became unacceptable. In Comparative Example No. 27, in which Ni did not reach the lower limit of the scope of the present invention, the adhesion and thermal fatigue characteristics of the cortical layer were unacceptable.

In Comparative Example No. 28, where both Ni and Co did not reach the lower limit of the scope of the present invention, the adhesion and thermal fatigue characteristics of the cortical layer were unacceptable. Comparative Example No. 29 based on Cu exceeding the upper limit of the range of the present invention was unsatisfactory in terms of adhesion to the skin layer and corrosion resistance to condensation water.

Comparative Example No. 30 based on Ti exceeding the upper limit of the scope of the present invention was unsatisfactory in terms of skin adhesion, thermal fatigue characteristics, and corrosion resistance to condensation water. The comparative example No. 31 based on C exceeding the upper limit of the range of the present invention and the thermal fatigue characteristics, and the comparative example No. 32 based on the upper limit of the range of the present invention and the thermal fatigue characteristics qualified.

Comparative Example No. 33, in which Cr did not reach the lower limit of the scope of the present invention, had poor adhesion, thermal fatigue characteristics, and corrosiveness to condensation water. Comparative Example No. 34 in which Nb did not reach the lower limit of the range of the present invention and Comparative Example No. 35 in which Ti did not reach the lower limit of the range of the present invention became unacceptable.

From the above, it is clear that the steel-based skin layer within the scope of the present invention is excellent in adhesion, thermal fatigue characteristics, and resistance to condensation water corrosion.

    (Industrial availability)    

The ferrous iron-based stainless steel plate of the present invention is excellent in skin adhesion, thermal fatigue characteristics, and corrosion resistance of condensed water, so it is not only applicable to exhaust manifolds, various exhaust pipes, flanges, converter cases, or converter cases. All exhaust system parts such as mufflers and other cars are preferred, and all exhaust pipe parts can also be composed of one type of steel, which can be effective in obtaining stability or weldability of steel materials. Furthermore, it is also preferable as a component of an exhaust system of a thermal power generation system or a component for a fuel cell.

Claims (3)

A ferrous iron-based stainless steel having the following composition, which is contained in mass%: C: 0.010% or less, Si: 1.0% or less, Mn: 1.0% or less, P: 0.040% or less, S: 0.030% or less, Cr: 17.0% or more and 18.5% or less, N: 0.015% or less, Nb: 0.40% or more and 0.80% or less, Ti: 0.10% or more and 0.40% or less, Al: 0.18% or less, Ni: 0.05% or more and 0.40% or less Below, Co: 0.01% or more and 0.30% or less, Mo: 0.02% or more and 0.30% or less, Cu: 0.02% or more and 0.40% or less, and satisfying the following formula (1), and the remainder contains Fe and unavoidable Impurities, C% + N%: 0.018% or less (1) In formula (1), C% and N% represent the content (mass%) of C and N, respectively. For example, the ferrous iron-based stainless steel according to claim 1 further includes, in mass%, selected from Ca: 0.0005% or more and 0.0030% or less, Mg: 0.0002% or more and 0.0020% or less, and B: 0.0002% or more and 0.0020% or less. One or more of them. For example, the ferrous iron-based stainless steel according to claim 1 or 2 further includes, in mass%, selected from V: 0.01% or more and 0.50% or less, W: 0.02% or more and 0.30% or less, and Zr: 0.005% or more and 0.50% or more. One or more of the following.