KR102400403B1 - Ferritic stainless steel - Google Patents

Abstract

To provide a ferritic stainless steel sheet that has excellent scale adhesion and thermal fatigue properties, and also has excellent condensate corrosion resistance.
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: 0.20% 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 are contained, and the following formula (1) is satisfied, and the balance is made to have a composition consisting of Fe and unavoidable impurities.
C% + N%: 0.018% or less… (One)
In Formula (1), C% and N% respectively represent content (mass %) of C and N.

Description

Ferritic stainless steel {FERRITIC STAINLESS STEEL}

The present invention relates to a ferritic stainless steel excellent in scale adhesion, thermal fatigue properties and condensate corrosion resistance.

Among the exhaust system members of automobiles, the exhaust manifold directly connected to the upstream side, particularly the engine, is used in a harsh environment with a maximum operating temperature of 800 to 900°C. Therefore, excellent thermal fatigue properties are required for the material, and ferritic stainless steel to which Nb is added is mainly used.

Nb added to the ferritic stainless steel increases high-temperature strength by being dissolved in the steel to improve thermal fatigue properties. However, Nb bonds with C or N in steel to easily form carbonitrides, and the amount of dissolved Nb may decrease, resulting in a decrease in thermal fatigue properties in some cases. As a countermeasure against this, formation of Nb carbonitride is prevented by complex addition of Ti, which is more likely to bond with C or N than Nb, to generate C or N as Ti carbonitride. A typical example of this Nb-Ti composite steel is Type441 ferritic stainless steel (18%Cr-0.5%Nb-0.2%Ti) (EN10088-2: EN1.4509), which is widely used in exhaust manifolds of automobiles, etc. is being used

Since the exhaust manifold is used in a harsh, repeated oxidation environment where heating and rapid cooling are repeated every time the engine is started and stopped, when the scale is peeled off, the ground iron is directly connected to the high-temperature exhaust gas. Oxidation progresses due to exposure to exposure, and the plate thickness decreases, and in some cases, holes are drilled or deformed. For this reason, the Nb-Ti composite-added ferritic stainless steel used for exhaust manifolds of automobiles is also required to have excellent scale adhesion in which scale does not peel off.

Addition of Mo is disclosed in Patent Documents 1 and 2 as a method of improving high-temperature strength and thermal fatigue properties of Nb-Ti composite-added ferritic stainless steels. In Patent Documents 3 to 5, addition of Mo, Cu, and W is disclosed. As a method of improving scale adhesion, Patent Document 3 discloses addition of REM, Ca, Y, and Zr. In Patent Document 5, addition of REM and Ca is disclosed. Patent Document 6 discloses an Nb-Ti composite-added ferritic stainless steel in which scale adhesion and thermal fatigue properties are improved by adding Co and Ni.

On the other hand, in a muffler or pipe disposed on the downstream side of automobile exhaust pipe parts, for example, water containing snowmelt salt sprayed on the road scatters, or corrosive ions generated by cooling exhaust gas. Since it is exposed to the condensate contained therein, corrosion resistance (hereinafter, referred to as condensate corrosion resistance) is often required, and ferritic stainless steel to which Ti or Mo is added is used. As an example, SUS436L (18%Cr-0.2%Ti-1%Mo) and SUS430LX (18%Cr-0.2%Ti) prescribed|regulated to JIS G4305 are mentioned.

As described above, since the required characteristics of the exhaust manifold on the upstream side and the like on the downstream side of the muffler are different, ferritic stainless steel suitable for each use has been used, but it cannot be manufactured with a common ferritic stainless steel. If there is, the number of steel types can be reduced, the number of places where parts of different materials are welded is reduced, the manufacturability of parts is stabilized, and automobile manufacturing can be made more efficient.

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, in the method disclosed by patent documents 1-5, while Mo and W are expensive, it has the fault of reducing workability, such as toughness of a steel plate. Moreover, Cu has the drawback of not only greatly reducing the workability at room temperature, but also reducing oxidation resistance. In addition, in Patent Documents 1 to 5, there is no case where the thermal fatigue properties and oxidation resistance (scale adhesion) required for the exhaust manifold and the condensate corrosion resistance required for a muffler or the like are evaluated simultaneously. Further, when SUS436L (18%Cr-0.2%Ti-1%Mo) or SUS430LX (18%Cr-0.2%Ti) is used for the exhaust manifold, there is a problem of insufficient thermal fatigue properties.

As described above, in the conventional ferritic stainless steel, it was not yet possible to say that all the characteristics of scale adhesion, thermal fatigue characteristics, and condensate corrosion resistance were satisfactory.

The present invention has been made in order to solve the above problems, and an object of the present invention is to provide a ferritic stainless steel that is excellent in scale adhesion and thermal fatigue characteristics and also excellent in condensate corrosion resistance.

In addition, "excellent in scale adhesion" of the present invention means a repeated oxidation test (heating rate: 5) in which the polished cold-rolled annealed plate is subjected to 400 cycles of holding at 1000°C for 20 min and holding at 100°C for 1 min in the air. °C/sec, cooling rate: 1.5 °C/sec) indicates that the area where the scale peels off from the surface of the test piece is less than 5%.

In addition, "excellent in thermal fatigue properties" means that, in accordance with JSMS-SD-7-03, heating and cooling are repeated between 200 and 900°C, and deformation is repeatedly applied at a constraint factor of 0.6. Then, the value (stress) obtained by dividing the load detected at 200 °C in each cycle by the cross-sectional area of the parallel part of the crack of the specimen (stress) is the number of cycles (thermal fatigue life) that has decreased by 75% with respect to the stress at the 5th cycle It refers to more than 660 cycles.

In addition, "excellent in condensate corrosion resistance" refers to Cl-: 500 ppm, SO ₄ ^2- : 1000 ppm, pH: 4, temperature: Maintaining a polished cold ^- rolled annealing plate in a thermostat of 80 ° C. , 1 set: 30 sets of 2 hours of solution immersion and 6 hours of drying are performed, and corrosion loss indicates that it is 10 g/m 2 or less.

The present inventors examined the effect of the amount of C+N on the thermal fatigue properties of Nb—Ti—Co—Ni composite-added ferritic stainless steel, and limited the amount of C+N and Ti to appropriate amounts in steel containing Ti. It was discovered that a more excellent thermal fatigue property was obtained by doing this.

In addition, research on the condensate corrosion resistance of Nb-Ti-Co-Ni composite-added ferritic stainless steels was conducted, and the condensate corrosion resistance was improved by adding appropriate amounts of both Mo and Cu, and used for downstream components such as mufflers. found that it is possible to

This invention has been made based on the above knowledge, and the summary is as follows.

[1] 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% 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 0.30 % or less, Cu: 0.02% or more and 0.40% or less, and satisfies the following formula (1), the ferritic stainless steel having a composition in which the balance consists of Fe and unavoidable impurities.

C% + N%: 0.018% or less… (One)

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

[2] In mass%, Ca: 0.0005% or more and 0.0030% or less, Mg: 0.0002% or more and 0.0020% or less, B: 0.0002% or more and 0.0020% or less [1] The ferritic stainless steel described in ].

[3] above [1] containing one or two or more 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 less, in mass% ] or the ferritic stainless steel according to [2].

According to the present invention, it is possible to obtain a ferritic stainless steel excellent in scale adhesion, thermal fatigue characteristics and condensate corrosion resistance. Since the ferritic stainless steel of the present invention is excellent in both heat resistance (scale adhesion, thermal fatigue characteristics) and condensate corrosion resistance, it can be suitably used for both the upstream side and the downstream side of an exhaust system member of an automobile.

1 is a view for explaining a thermal fatigue test piece.
[ Fig. 2 ] Fig. 2 is a diagram for explaining the temperature and constraint conditions in a thermal fatigue test.

Hereinafter, the present invention will be described in detail.

The ferritic stainless steel of the present invention, 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 18.5% 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 satisfy the following formula (1), the balance has a composition consisting of Fe and unavoidable impurities, scale adhesion and thermal fatigue characteristics In addition to this superiority, the condensate corrosion resistance is also excellent.

C% + N%: 0.018% or less… (One)

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

Next, the reason for stipulating the component composition of the ferritic stainless steel of the present invention will be described. In addition, all of component % means mass % unless otherwise indicated.

C: 0.010% or less

C is an element effective for increasing the strength of steel, and since the effect is obtained by containing 0.001% or more of C, the C content is preferably 0.001% or more. On the other hand, when C is contained in excess of 0.010%, scale peeling occurs, so the C content is made 0.010% or less. In addition, the C content is, from the viewpoint of securing toughness and workability, and from the viewpoint that the NbC coarsens or the amount of precipitation increases, thereby reducing the amount of solid solution Nb in the steel and lowering the thermal fatigue properties, The smaller one is preferable, and it is preferable to make C content into 0.008 % or less. The C content is more preferably 0.005% or more.

Si: 1.0% or less

Si is an effective element for improving oxidation resistance, and since the effect is obtained by containing 0.01% or more of Si, it is preferable that Si content is 0.01% or more. On the other hand, since workability will fall when Si is contained exceeding 1.0 %, Si content shall be 1.0 % or less. Si content becomes like this. More preferably, it is 0.20 % or more, More preferably, it is 0.30 % or more. In particular, when the Ni content is 0.20% or more and the Si content is 0.30% or more, the scale adhesion is particularly excellent. Moreover, Si content becomes like this. Preferably it is 1.00 % or less, More preferably, it is 0.50 % or less, More preferably, it is 0.40 % or less.

Mn: 1.0% or less

Mn is an element that increases the strength of steel, and also has an action as a deoxidizer. Since the effect is obtained by containing 0.01% or more of Mn, it is preferable that the Mn content is 0.01% or more. On the other hand, when Mn is contained in excess of 1.0%, the increase in oxidation is remarkably increased and oxidation resistance is lowered. Therefore, the Mn content is made 1.0% or less. Mn content becomes like this. More preferably, it is 0.20 % or more, More preferably, it is 0.30 % or more. Moreover, Mn content becomes like this. Preferably it is 1.00 % or less, More preferably, it is 0.60 % or less, More preferably, it is 0.50 % or less.

P: 0.040% or less

P is an element which reduces toughness, and it is preferable to reduce, and P content shall be 0.040 % or less. Preferably, the P content is 0.035% or less. More preferably, the P content is 0.030% or less.

S: 0.030% or less

Since S reduces moldability and corrosion resistance, the smaller one is preferable, and S content is made into 0.030 % or less. Preferably, the S content is 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 necessary for improving corrosion resistance and oxidation resistance, and in order to obtain good corrosion resistance and oxidation resistance, it is necessary to contain 17.0% or more of Cr. When the Cr content is less than 17.0%, the oxide scale tends to increase, and not only the scale adhesion is lowered, but also the thermal fatigue property may be lowered. Also, corrosion resistance in condensed water is not sufficiently obtained. On the other hand, when Cr is contained exceeding 18.5 %, since steel hardens and manufacturability and workability fall, Cr content shall be 18.5 % or less. Preferably, the Cr content is in the range of 17.5% or more and 18.5% or less.

N: 0.015% or less

Since N decreases the toughness and workability of the steel, less is preferable. Also, when the N content is large, coarse TiN is precipitated, and NbC is precipitated in large amount as it accompanies TiN, so that the amount of solid solution Nb in the steel decreases. By doing so, the thermal fatigue characteristic falls. Moreover, since the oxide scale becomes easy to peel from coarse TiN as a starting point, and scale adhesiveness also falls, N content shall be 0.015 % or less. Preferably, the N content is 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 having an effect of improving thermal fatigue properties by being dissolved in steel to significantly increase high-temperature strength. The effect is obtained by containing 0.40% or more of Nb. On the other hand, the excessive content of Nb exceeding 0.80% not only reduces the toughness of the steel, but also forms a Laves phase (Fe ₂ Nb) at high temperature to rather lower the high-temperature strength, so the Nb content is 0.80% or less. do. Nb content becomes like this. Preferably it is 0.43 % or more, More preferably, it is 0.45 % or more. Moreover, Nb content becomes like this. Preferably it is 0.60 % or less, More preferably, it is 0.50 % or less.

Ti: 0.10% or more and 0.40% or less

Ti preferentially combines with C and N to form carbonitrides, thereby preventing the formation of Nb carbonitrides, and improving corrosion resistance, formability, and intergranular corrosion properties of welds. In order to obtain these effects, it is necessary to contain 0.10% or more of Ti. When the Ti content is less than 0.10%, C and N cannot be completely formed as Ti carbonitride, and Nb carbonitride is formed, the Nb solid capacity decreases, and thermal fatigue properties deteriorate. On the other hand, excessive Ti content exceeding 0.40% increases the precipitation amount of Ti carbonitride, and concomitantly with it, the Nb carbonitride tends to precipitate, and the Nb solid capacity decreases, so that the thermal fatigue property decreases. In addition, as the amount of Ti carbonitride is increased, the scale adhesion is also reduced, and since corrosion occurs from the coarse Ti carbonitride as a starting point, the corrosion resistance of the condensate is also reduced. For this reason, the Ti content is made 0.40% or less. Ti content becomes like this. Preferably it is 0.15 % or more. Moreover, Ti content becomes like this. Preferably it is 0.30 % or less, More preferably, it is 0.25 % or less.

Al: 0.20% or less

Al is an element effective for deoxidation, and since the effect is obtained by containing 0.01% or more, it is preferable that Al content is 0.01% or more. On the other hand, in order to harden steel and reduce workability, Al content is made into 0.20% or less. The Al content is more preferably 0.02% or more. Moreover, Al content becomes like this. Preferably it is 0.10 % or less, More preferably, it is 0.06 % or less.

Ni: 0.05% or more and 0.40% or less

Ni is an important element in order to ensure scale adhesion in this invention, and in order to acquire the effect, it is necessary to contain 0.05% or more of Ni. When Ni is less than 0.05 %, scale adhesiveness may fall, and the point where scale peeled becomes a starting point, and thermal fatigue fracture may occur. In addition, as will be described later, since the steel of the present invention has a reduced coefficient of thermal expansion by containing an appropriate amount of Co, the above effect is obtained with a smaller amount of Ni content compared to steel without Co addition or steel with insufficient Co content. . On the other hand, in addition to being an expensive element, when Ni is contained in excess of 0.40%, a γ phase is generated at a high temperature, and scale adhesion is rather reduced. Therefore, Ni content is made into the range of 0.05 % or more and 0.40 % or less. Ni content becomes like this. Preferably it is 0.10 % or more, More preferably, it is 0.20 % or more. Moreover, Ni content becomes like this. Preferably it is 0.30 % or less, More preferably, it is 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 necessary to improve thermal fatigue characteristics, and for that purpose, it is necessary to contain at least 0.01% or more of Co. Co can improve thermal fatigue characteristics by reducing the thermal expansion coefficient of steel, reducing the amount of expansion at the time of temperature rise, and making small the amount of deformation|transformation which generate|occur|produces at the time of temperature rise and cooling. Moreover, when the thermal expansion coefficient of steel is reduced, the difference between the thermal expansion coefficients of steel and scale becomes small, and it becomes difficult for a scale to peel at the time of cooling. Therefore, there is an effect that can prevent peeling of scale by containing a smaller amount of Ni. On the other hand, when Co is contained in an amount exceeding 0.30%, Co is concentrated at the interface between the oxide film and the iron, and the scale adhesion is lowered. When Co is contained exceeding 0.30%, the side effect of this interface thickening eliminates the effect of preventing scale peeling due to the reduction of the coefficient of thermal expansion described above, and the scale peels off at the time of cooling. Therefore, the Co content is made 0.01% or more and 0.30% or less. Co content becomes like this. Preferably it is 0.02 % or more, More preferably, it is 0.03 % or more. Moreover, Co content becomes like this. Preferably it is 0.10 % or less.

Mo: 0.02% or more and 0.30% or less

Mo is an element that increases the strength of steel by solid solution strengthening, improves thermal fatigue properties, and improves condensate corrosion resistance by improving salt damage corrosion resistance, and the effect is obtained by containing 0.02% or more of Mo is obtained However, while Mo is an expensive element, when Mo is contained in a large amount, not only surface defects will occur, but also workability at room temperature will fall. In order to obtain favorable surface properties without generating surface defects, the Mo content needs to be 0.30% or less. Therefore, Mo content is made into the range of 0.02 % or more and 0.30 % or less. Mo content becomes like this. Preferably it is 0.04 % or more. Moreover, Mo content becomes like this. Preferably it is 0.10 % or less.

Cu: 0.02% or more and 0.40% or less

Cu, by precipitating finely as ε-Cu, strengthens steel and improves thermal fatigue properties, and has the effect of improving condensate corrosion resistance by improving sulfuric acid corrosion resistance. In order to acquire these effects, it is necessary to contain 0.02% or more of Cu. On the other hand, when Cu is contained exceeding 0.40 %, oxidation scale adhesiveness will fall and repeated oxidation resistance will fall. In addition, ε-Cu tends to be coarsely precipitated, and the corrosion resistance of condensed water is also reduced. For this reason, Cu content is made into 0.40 % or less. Therefore, Cu content is made into the range of 0.02 % or more and 0.40 % or less. Cu content becomes like this. Preferably it is 0.04 % or more. Moreover, Cu content becomes like this. Preferably it is 0.10 % or less.

In the present invention, since Mo and Cu improve the condensate corrosion resistance by improving salt corrosion resistance and sulfuric acid corrosion resistance, respectively, sufficient condensate corrosion resistance cannot be obtained by containing Mo or Cu alone. In the present invention, since both Mo and Cu are contained in appropriate amounts, excellent corrosion resistance to condensed water is obtained.

C% + N%: 0.018% or less… (One)

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

As described above, each content of C and N is made 0.010% or less and 0.015% or less from the viewpoint of toughness, workability, and scale peelability resistance. In addition, in the present invention, from the viewpoint of thermal fatigue characteristics, C%+N% is limited to 0.018% or less as in the above formula (1). When C%+N% exceeds 0.018%, a large amount of coarse Ti nitride (TiN) is generated, and NbC is precipitated around TiN, and thus the amount of NbC precipitation increases. When the precipitation amount of NbC increases, the amount of Nb dissolved in the steel decreases and the high-temperature strength of the steel decreases, so that the effect of improving the thermal fatigue properties cannot be sufficiently obtained. Therefore, in the present invention in which Nb and Ti are compounded, C%+N% is set to 0.018% or less in order to sufficiently obtain a solid solution strengthening amount of Nb. Preferably, C%+N% is 0.015% or less. When C%+N% is 0.015% or less, TiN or NbC precipitated becomes fine, and when TiN is refined, the amount of NbC precipitated around the TiN is reduced, and the amount of solid solution Nb in steel increases. In addition, the precipitation strengthening effect is also obtained by the fine precipitation of NbC itself. By these effects, thermal fatigue characteristics are improved. More preferably, C%+N% is 0.013% or less.

The present invention is a ferritic stainless steel containing the above essential components and the balance being Fe and unavoidable impurities, characterized in that it is excellent in scale adhesion and thermal fatigue properties and also excellent in condensate corrosion resistance. Moreover, as needed, 1 type(s) or 2 or more types selected from Ca, Mg, and B, and/or 1 type(s) or 2 or more types selected from V, W, and Zr can be contained in the following range.

Ca: 0.0005% or more and 0.0030% or less

Ca is an effective component for preventing clogging of the nozzle due to the deposition of Ti-based inclusions that are likely to occur during continuous casting. The effect is obtained with Ca content of 0.0005% or more. On the other hand, in order to obtain favorable surface properties without generating surface defects, the Ca content is preferably 0.0030% or less. Accordingly, when Ca is contained, the Ca content is preferably in the range of 0.0005% or more and 0.0030% or less. More preferably, the Ca content is in the range of 0.0005% or more and 0.0020% or less. More preferably, the Ca content is in the range of 0.0005% or more and 0.0015% or less.

Mg: 0.0002% or more and 0.0020% or less

Mg is an element effective in improving workability and toughness. Moreover, Mg is an element effective in suppressing the coarsening of the carbonitride of Nb or Ti. When Ti carbonitride coarsens, since it becomes a starting point of brittle fracture, toughness will fall. Moreover, when Nb carbonitride coarsens, since the solid solution in steel will fall, it will lead to the fall of thermal fatigue characteristic. The effect of improving the said workability and toughness, or suppressing the coarsening of the carbonitride of Nb and Ti is acquired by containing 0.0002% or more of Mg. On the other hand, when Mg content exceeds 0.0020 %, the surface properties of steel may be deteriorated. Therefore, when Mg is contained, it is preferable to make Mg content into the range of 0.0002% or more and 0.0020% or less. The Mg content is more preferably 0.0004% or more. Moreover, Mg content becomes like this. More preferably, it is 0.0015 % or less, More preferably, it is 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 with the B content of 0.0002% or more. On the other hand, since the workability and toughness of steel may fall when B is contained exceeding 0.0020 %, B content shall be 0.0020 % or less. Therefore, when B is contained, it is preferable to make B content into the range of 0.0002% or more and 0.0020% or less. The B content is more preferably 0.0003% or more. Moreover, B content becomes like this. More preferably, it is 0.0010 % or less.

V: 0.01% or more and 0.50% or less

V is an element effective for the improvement of high temperature strength. Moreover, it also has the effect of suppressing that the carbonitride of Ti or Nb coarsens. The effect is obtained by containing 0.01% or more of V. On the other hand, when V is contained in an amount exceeding 0.50%, coarse V(C, N) may be precipitated and toughness may decrease. Therefore, when containing V, it is preferable to make V content into the range of 0.01 % or more and 0.50 % or less. The V content is more preferably 0.02% or more. Moreover, V content becomes like this. More preferably, it is 0.20 % or less.

W: 0.02% or more and 0.30% or less

W, like Mo, is an element that increases the strength of steel by solid solution strengthening, and the effect is obtained by containing 0.02% or more of W. However, W is an expensive element, and when W is contained in a large amount, not only surface defects will occur but also workability, such as toughness, will fall significantly. In order to obtain good surface properties, the W content is preferably 0.30% or less. Therefore, when containing W, it is preferable to make W content into the 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 which improves oxidation resistance. In order to acquire the effect, it is preferable to make Zr content into 0.005 % or more. On the other hand, when the Zr content exceeds 0.50%, a Zr intermetallic compound is precipitated and the steel tends to be brittle. Accordingly, when Zr is contained, the Zr content is preferably 0.005% or more and 0.50% or less.

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

For the ferritic stainless steel of the present invention, an ordinary stainless steel manufacturing method can be used. Steel having the above composition is melted in a melting furnace such as a converter or electric furnace, and then undergoes secondary refining such as briquette refining and vacuum refining, followed by continuous casting or ingot-ingot It is made into a steel piece (slab) by a rolling method, and hot rolling, hot-rolled sheet annealing, and pickling are performed to obtain a hot-rolled annealed pickling sheet. In addition, the method of setting it as a cold-rolled annealing plate through each process, such as cold rolling, finish annealing, and pickling, is recommended and encouraged. An example is as follows.

It is melted in a converter or electric furnace, etc., and secondary refining is performed by the AOD method or VOD method to melt the molten steel of the above composition to make a slab by the continuous casting method. This slab is heated to 1000-1250 degreeC, and it is set as the hot-rolled board of the desired plate|board thickness by hot rolling. After continuously annealing this hot-rolled sheet at a temperature of 900°C to 1100°C, it is descaled by shot blasting and pickling to obtain a hot-rolled annealing and pickling sheet. It is also possible to use this hot-rolled annealing pickling plate as it is for the purposes targeted for the present invention, such as exhaust manifolds, flanges, pipes, and mufflers, but also cold rolling, annealing and pickling can be performed to obtain a cold-rolled annealing pickling plate. there is. In this cold rolling process, you may perform cold rolling twice or more including intermediate annealing as needed. The total rolling reduction in the cold rolling process consisting of one or two or more cold rollings is 60% or more, preferably 70% or more. Cold-rolled sheet annealing temperature is 900-1150 degreeC, Preferably it is 950-1100 degreeC. In addition, depending on the use, it is also possible to apply hardness rolling (skin pass rolling, etc.) after pickling, and to adjust the shape and quality of a steel plate. Moreover, it can also be set as the BA finish which annealed in the reducing atmosphere containing hydrogen and the pickling was abbreviate|omitted.

Using the hot-rolled annealed plate product or cold-rolled annealed plate product produced in this way, bending processing according to each application is performed, and exhaust pipes of automobiles and motorcycles, catalyst outer casings, and thermal power plants are exhausted. It is molded into a duct or fuel cell related member. The welding method for welding these members is not specifically limited, Various arc welding methods, such as TIG, MIG, MAG, resistance welding methods, such as spot welding and seam welding, and high frequency resistance welding, such as an electric resistance welding method, High frequency induction welding is applicable.

Example

The steel of No. 1-40 which has the component composition shown in Table 1 was melted and cast in a vacuum melting furnace, and it was set as 30 kg of steel ingots. Then, after heating at 1170 degreeC, it hot-rolled and it was set as the sheet bar of thickness 35mm x width 150mm. This seat bar was split in two. One of them is forged into a square bar with a cross section of 30 mm × 30 mm, annealed within the range of 950 to 1050 ° C. made The thermal fatigue test mentioned later was done using this test piece. The annealing temperature was set for each component while checking the structure within a temperature range of 950 to 1050°C. The same is true for subsequent annealing.

Using the other sheet bar divided into two, after heating at 1050°C, hot rolling was performed to obtain a hot rolled sheet having a sheet thickness of 5 mm. Thereafter, hot-rolled sheet annealing was carried out in a temperature range of 900 to 1050°C, and pickling was performed to prepare a hot-rolled annealing pickling sheet. At this stage, the surface properties of the steel sheet were visually observed. The sheet thickness was 2 mm by cold rolling, and finish annealing was carried out within the temperature range of 900-1050 degreeC, and it was set as the cold-rolled annealing board. This was subjected to the following repeated oxidation test and condensate immersion test.

<Repeat oxidation test>

It was cut out from the said cold-rolled annealing board to the dimension of 20 mm width x 30 mm length, and all 6 surfaces were grind|polished with #320 emery paper, and it used for the test. For oxidation test conditions, 400 cycles of holding|maintenance for 20 min at 1000 degreeC and holding|maintenance for 1 min at 100 degreeC were repeated in air|atmosphere. The heating rate and the cooling rate were respectively 5°C/sec and 1.5°C/sec. After the test, the presence or absence of peeling of the scale was visually observed and the scale adhesion was evaluated. The obtained result serves as Table 1 and is shown.

<Thermal fatigue test>

With respect to the test piece for the thermal fatigue test, heating and cooling were repeated between 200 and 900°C, and strain was repeatedly applied at a constraint factor of 0.6 as shown in FIG. 2 , and the thermal fatigue life was measured. The measurement method was based on the standard high temperature and low cycle test method (JSMS-SD-7-03) of the Japanese Society of Materials Science and Technology. First, the load detected at 200 degreeC of each cycle was divided by the cross-sectional area (50.3 mm<2>) of the crack parallel part of the test piece shown in FIG. 1, and it was set as the stress of that cycle. The number of cycles in which the stress in that cycle decreased to 75% with respect to the stress at the 5th cycle at which the behavior was stabilized was defined as the thermal fatigue life. The thermal fatigue properties were evaluated by this number of life cycles. The obtained result is put together in Table 1, and is shown.

In addition, regarding the said constraint rate, as shown in FIG. 2, constraint rate η=a/(a+b), a is (free thermal expansion deformation amount - controlled deformation amount)/2, and b is controlled deformation amount/2. In addition, the amount of free thermal expansion deformation is the amount of deformation when the temperature is raised without applying any mechanical stress, and the amount of controlled deformation represents the amount of deformation in a state in which no stress is applied at room temperature. The actual amount of constrained deformation generated in the material by the restraint is (free thermal expansion deformation amount - controlled deformation amount), that is, the amount of deformation with respect to the free thermal expansion deformation amount.

<Condensate immersion test>

From the cold-rolled annealing plate produced above, it was cut out to a dimension of 60 mm width x 80 mm length, and all 6 sides were polished with #320 emery paper, and it used for the test. During the test, the ends were covered with a protective tape. The test solution simulated condensed water and contained Cl ⁻ : 500 ppm, SO ₄ ² −: 1000 ppm, and adjusted to pH: 4. The temperature was maintained in the thermostat to be 80°C. The test was performed for 30 sets by making one set of 2 hours of solution immersion and 6 hours of drying. After the test, the corrosion loss was calculated by removing the corrosion product and measuring the weight before and after the test.

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

(1) Scale adhesion: ◎ (Pass) when the area where scale peeled off from the surface of the test piece after repeated oxidation test was 0% (No scale peeling was observed by visual observation), and ○ (Pass) when it was more than 0% and less than 5% , 5% or more was judged as x (disqualified).

(2) Thermal fatigue characteristics: Those having a thermal fatigue life of 750 cycles or more were judged as (circle) (pass), those having 660 cycles or more and less than 750 cycles were judged as ○ (pass), and less than 660 cycles were judged as × (failed).

(3) Corrosion resistance of condensate: Those with a corrosion loss of 5 g/m or less were designated as ◎ (pass), those exceeding 5 g/m and 10 g/m or less were designated as ○ (pass), and those exceeding 10 g/m were designated as × (failed).

From Table 1, all of Nos. 1-20 and 36-40 which are examples of this invention were excellent in all of the scale adhesiveness, thermal fatigue characteristic, and condensate corrosion resistance. Inventive Examples Nos. 2 to 4, 6, 9, 10, 12, 14 to 16, 19, 20, 36 to 40 are scales in which the content of Si and Ni is in a suitable range (Si≥0.30% and Ni≥0.20%). Adhesion was particularly excellent. Inventive Example No, in which the content of C+N and Ti, Co, Mo, and Cu is in a suitable range (C+N≤0.015%, Ti≥0.15%, Co≥0.02%, Mo≥0.04%, Cu≥0.04%) .1, 2, 6 to 11, 16, and 38 were particularly excellent in thermal fatigue properties. Inventive Examples Nos. 1, 2, 6-11, 16, 18, 36-40, in which the content of Mo and Cu were in the suitable ranges (Mo ≥ 0.04% and Cu ≥ 0.04%), were particularly excellent in condensate corrosion resistance. In addition, the surface properties of all the hot-rolled annealing pickling plates of the examples of the present invention were good without any surface defects.

On the other hand, Comparative Examples No. 21 and 24, in which both Mo and Cu are less than the lower limit of the present invention range, Comparative Example No. 22 in which Cu is less than the lower limit of the present invention, Comparative Example No. 23 in which Mo is less than the lower limit of the present invention range, In all cases, the condensate corrosion resistance was rejected.

Comparative Example No. 25, in which C+N exceeded the upper limit of the range of the present invention, failed in thermal fatigue properties. Comparative Example No. 26, in which Co was less than the lower limit of the range of the present invention, failed in thermal fatigue properties. In Comparative Example No. 27 in which Ni was less than the lower limit of the range of the present invention, scale adhesion and thermal fatigue properties were disqualified.

In Comparative Example No. 28 in which Ni and Co were simultaneously less than the lower limit of the range of the present invention, scale adhesion and thermal fatigue characteristics were rejected. In Comparative Example No. 29, in which Cu exceeds the upper limit of the range of the present invention, scale adhesion and condensate corrosion resistance were disqualified.

In Comparative Example No. 30 in which Ti exceeds the upper limit of the range of the present invention, scale adhesion, thermal fatigue characteristics, and condensate corrosion resistance were all rejected. Comparative Example No. 31, in which C was greater than the upper limit of the present invention range, had scale adhesion and thermal fatigue characteristics, and Comparative Example No. 32, in which N was greater than the upper limit of the present invention range, failed in scale adhesion and thermal fatigue characteristics.

In Comparative Example No. 33 in which Cr was less than the lower limit of the range of the present invention, scale adhesion, thermal fatigue characteristics, and condensate corrosion properties were all rejected. Both Comparative Example No. 34 in which Nb is less than the lower limit of the present invention range and Comparative Example No. 35 in which Ti is less than the lower limit of the present invention range were disqualified in thermal fatigue properties.

From the above, it is clear that the steel in the range of the present invention is excellent in all of the scale adhesion, thermal fatigue properties and condensate corrosion resistance.

The ferritic stainless steel sheet of the present invention is excellent in scale adhesion, thermal fatigue characteristics, and condensate corrosion resistance, so it is suitable for exhaust manifolds, various exhaust pipes, flanges, and exhaust system parts of automobiles such as converter cases and mufflers. In addition, it is possible to configure all the exhaust pipe parts with one type of steel, so that it is possible to increase efficiency in terms of availability of steel materials and weldability. Moreover, it is suitable also as an exhaust system member of a thermal power generation system, and a member for fuel cells.

Claims (3)

in mass %,
C: 0.001% or more and 0.010% or less;
Si: 0.01% or more and 1.0% or less;
Mn: 0.01% or more and 1.0% or less;
P: greater than 0% and less than or equal to 0.040%;
S: greater than 0% and less than or equal to 0.030%;
Cr: 17.0% or more and 18.5% or less,
N: greater than 0% and less than or equal to 0.015%;
Nb: 0.40% or more and 0.80% or less;
Ti: 0.15% or more and 0.40% or less;
Al: 0.01% or more and 0.20% or less;
Ni: 0.05% or more and 0.40% or less;
Co: 0.02% or more and 0.30% or less;
Mo: 0.04% or more and 0.30% or less;
Cu: 0.04% or more and 0.40% or less;
contains, and satisfies the following formula (1), the balance has a composition consisting of Fe and unavoidable impurities,
Maintain the polished steel in a thermostat (Cl ^― : 500 ppm, SO ₄ ² ―: 1000 ppm, pH: 4, temperature: 80° C.), 1 set: solution immersion for 2 hours and drying for 6 hours , when 30 sets are performed, the corrosion loss is 5 g/m 2 or less,
According to JSMS-SD-7-03, repeated heating and cooling between 200 and 900°C, and repeatedly applying deformation at a constraint factor of 0.6, load detected at 200°C in each cycle A ferritic stainless steel with a number of cycles (thermal fatigue life) of 750 cycles or more in which the value (stress) divided by the cross-sectional area of the parallel part of the crack of the specimen is reduced to 75% with respect to the stress at the 5th cycle.
C% + N%: 0.015% or less… (One)
In Formula (1), C% and N% respectively represent content (mass %) of C and N. The method according to claim 1,
In % by mass, also,
Ca: 0.0005% or more and 0.0030% or less;
B: 0.0002% or more and 0.0020% or less;
A ferritic stainless steel containing one or two selected from among. The method according to claim 1 or 2,
In % by mass, also,
V: 0.01% or more and 0.50% or less;
Zr: 0.005% or more and 0.50% or less;
A ferritic stainless steel containing one or two selected from among.

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