EP3517647A1 - Acier inoxydable ferritique - Google Patents

Acier inoxydable ferritique Download PDF

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EP3517647A1
EP3517647A1 EP17883821.5A EP17883821A EP3517647A1 EP 3517647 A1 EP3517647 A1 EP 3517647A1 EP 17883821 A EP17883821 A EP 17883821A EP 3517647 A1 EP3517647 A1 EP 3517647A1
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case
thermal fatigue
steel
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German (de)
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EP3517647A4 (fr
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Tetsuyuki Nakamura
Shin Ishikawa
Reiko Sugihara
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JFE Steel Corp
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JFE Steel Corp
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/007—Heat treatment of ferrous alloys containing Co
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/02—Hardening by precipitation
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron

Definitions

  • the present invention relates to ferritic stainless steel excellent in adhesion of scale, thermal fatigue resistance, and corrosion resistance to condensed water.
  • an exhaust manifold which is disposed on the upstream side and also directly connected to an engine, is used in a severe environment in which the maximum operating temperature reaches 800°C to 900°C. Therefore, since a material for such a member is required to have excellent thermal fatigue resistance, ferritic stainless steel containing Nb is mainly used.
  • Nb which is added to ferritic stainless steel improves thermal fatigue resistance by improving high-temperature strength by solid solution in the steel.
  • Nb tends to combine with C and N in the steel to form carbonitrides
  • the formation of Nb carbonitrides is prevented by adding Ti, which is more likely than Nb to combine with C and N, in combination with Nb so that C and N are used to form Ti carbonitrides.
  • the representative example of steel containing a combination of Nb and Ti is ferritic stainless steel Type 441 (18%Cr-0.5%Nb-0.2%Ti) (EN 10088-2: EN 1.4509), and this steel is widely used for, for example, the exhaust manifold of an automobile.
  • An exhaust manifold is used in a severe cyclically oxidizing environment in which heating and rapid cooling are alternately occurred when an engine is alternately started and stopped. Therefore, in the case where spalling of scale occurs, since base steel is directly exposed to high-temperature exhaust gas, there is a decrease in the wall thickness of the manifold due to the progress of oxidation, which may result in a hole formation or deformation occurring in the wall of the manifold. Therefore, ferritic stainless steel containing a combination of Nb and Ti which is used for the exhaust manifold of an automobile is also required to have excellent adhesion of scale so that spalling of scale does not occur.
  • Patent Literature 1 and Patent Literature 2 disclose methods in which Mo is added.
  • Patent Literature 3 through Patent Literature 5 disclose methods in which Mo, Cu, and W are added.
  • Patent Literature 3 discloses a method in which REM, Ca, Y, and Zr are added.
  • Patent Literature 5 discloses a method in which REM and Ca are added.
  • Patent Literature 6 discloses ferritic stainless steel containing a combination of Nb and Ti whose adhesion of scale and thermal fatigue resistance are improved by adding Co and Ni.
  • ferritic stainless steel containing Ti and Mo is used for such members.
  • ferritic stainless steel examples include SUS436L (18%Cr-0.2%Ti-1%Mo) and SUS430LX (18%Cr-0.2%Ti) prescribed in JIS G 4305.
  • the present invention has been completed to solve the problems described above, and an object of the present invention is to provide ferritic stainless steel excellent not only in adhesion of scale and thermal fatigue resistance but also in corrosion resistance to condensed water.
  • the expression "excellent in adhesion of scale” in the present invention refers to a case where, after performing a cyclic oxidation test, in which holding at a temperature of 1000°C for 20 minutes and holding at a temperature of 100°C for 1 minute are alternately performed 400 times each in air (at a heating rate of 5°C/sec and a cooling rate of 1.5°C/sec) on a polished cold-rolled and annealed steel sheet, the ratio of an area in which scale is separated to the total area of the surface of a test piece is less than 5%.
  • the expression "excellent in thermal fatigue resistance” refers to a case where, when strain is cyclically applied with a restraint ratio of 0.6 while heating and cooling is alternately performed in a temperature range of 200°C to 900°C in accordance with JSMS-SD-7-03, the number of cycles (thermal fatigue life) at which a value (stress) calculated by dividing a load determined at a temperature of 200°C by the cross-sectional area of the gauged portion of a test piece is 75% of the stress at the 5th cycle is 660 or more.
  • the expression "excellent in corrosion resistance to condensed water” refers to a case where, after a test has been repeated for 30 cycles, where, in one cycle of the test, a polished cold-rolled and annealed steel sheet is immersed in a thermostatic bath containing Cl - : 500 ppm and SO 4 2- : 1000 ppm and having a pH of 4 and a temperature of 80°C for 2 hours and then dried for 6 hours, a decrease in weight due to corrosion is 10 g/m 2 or less.
  • the present inventors conducted investigations regarding the influence of the amount of (C + N) on the thermal fatigue resistance of ferritic stainless steel containing a combination of Nb, Ti, Co, and Ni and found that it is possible to achieve more excellent thermal fatigue resistance by appropriately controlling the amounts of (C + N) and Ti in steel containing Ti.
  • the present inventors conducted investigations regarding the corrosion resistance to condensed water of ferritic stainless steel containing a combination of Nb, Ti, Co, and Ni and found that it is possible to improve corrosion resistance to condensed water and to use the steel for members on the downstream side such as a muffler by containing both Mo and Cu in appropriate amounts.
  • ferritic stainless steel excellent in adhesion of scale, thermal fatigue resistance, and corrosion resistance to condensed water. Since the ferritic stainless steel according to the present invention is excellent in both heat resistance (adhesion of scale and thermal fatigue resistance) and corrosion resistance to condensed water, the steel can preferably be used for members on both of the upstream and downstream sides of the exhaust system of an automobile.
  • the ferritic stainless steel according to the present invention has a chemical composition containing, by 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, and the balance being Fe and inevitable impurities, in which expression (1) below is satisfied, and the steel is excellent in adhesion of scale, thermal fatigue resistance, and corrosion resistance to condensed water.
  • C ⁇ % + N ⁇ % : 0.018 ⁇ % or less In expression (1), C% and N% respectively denote
  • the C is an element which is effective for improving the strength of steel, and it is possible to realize such an effect in the case where the C content is 0.001% or more. Therefore, it is preferable that the C content be 0.001% or more.
  • the C content is set to be 0.010% or less.
  • the C content be as low as possible from the viewpoint of achieving satisfactory toughness and workability and preventing a deterioration in thermal fatigue resistance due to a decrease in the amount of solid solution Nb in steel as a result of coarsening of NbC and an increase in amount of NbC precipitated. Therefore, it is preferable that the C content be 0.008% or less. It is more preferable that the C content be 0.005% or more.
  • Si is an element which is effective for improving oxidation resistance, and it is possible to realize such an effect in the case where the Si content is 0.01% or more. Therefore, it is preferable that the Si content be 0.01% or more.
  • the Si content is set to be 1.0% or less. It is more preferable that the Si content be 0.20% or more or even more preferably 0.30% or more. In particular, in the case where the Ni content is 0.20% or more and the Si content is 0.30% or more, the adhesion of scale is particularly excellent.
  • the Si content be 1.00% or less, more preferably 0.50% or less, or even more preferably 0.40% or less.
  • Mn is an element which improves the strength of steel and which functions as a deoxidizing agent. Since it is possible to realize such effects in the case where the Mn content is 0.01% or more, it is preferable that the Mn content be 0.01% or more. On the other hand, since there is a deterioration in oxidation resistance due to a significant increase in weight caused by oxidation in the case where the Mn content is more than 1.0%, the Mn content is set to be 1.0% or less. It is more preferable that the Mn content be 0.20% or more or even more preferably 0.30% or more. In addition, it is preferable that the Mn content be 1.00% or less, more preferably 0.60% or less, or even more preferably 0.50% or less.
  • the P content is set to be 0.040% or less, preferably 0.035% or less, or more preferably 0.030% or less.
  • the S content be as low as possible. Therefore, the S content is set to be 0.030% or less. It is preferable that the S content be 0.006% or less or more preferably 0.003% or less.
  • Cr is an element which is necessary for improving corrosion resistance and oxidation resistance, and it is necessary that the Cr content be 17.0% or more to achieve good corrosion resistance and oxidation resistance.
  • the Cr content is less than 17.0%, there is a deterioration in adhesion of scale due to a tendency for the amount of oxide scale to increase, and there may also be a deterioration in thermal fatigue resistance. Moreover, it is not possible to achieve sufficient corrosion resistance to condensed water.
  • the Cr content is set to be 18.5% or less. It is preferable that the Cr content be 17.5% or more and 18.5% or less.
  • the N content is set to be 0.015% or less, preferably 0.012% or less, or more preferably 0.010% or less.
  • Nb 0.40% or more and 0.80% or less
  • Nb is an element which is effective for improving thermal fatigue resistance by significantly improving high-temperature strength as a result of solid solution in steel. It is possible to realize such an effect in the case where the Nb content is 0.40% or more.
  • the Nb content is set to be 0.80% or less. It is preferable that the Nb content be 0.43% or more or more preferably 0.45% or more. In addition, it is preferable that the Nb content be 0.60% or less or more preferably 0.50% or less.
  • Ti prevents the generation of Nb carbonitrides, improves corrosion resistance and formability, and improves grain-boundary corrosion resistance in a weld as a result of being more likely than other elements to combine with C and N to generate carbonitrides. It is necessary that the Ti content be 0.10% or more to realize such effects. In the case where the Ti content is less than 0.10%, since it is not possible to completely consume C and N by forming Ti carbonitrides, Nb carbonitrides are formed, which results in a deterioration in thermal fatigue resistance due to a decrease in the amount of solid solution Nb.
  • the Ti content is set to be 0.40% or less. It is preferable that the Ti content be 0.15% or more. In addition, it is preferable that the Ti content be 0.30% or less or more preferably 0.25% or less.
  • Al is an element which is effective for deoxidation, and it is possible to realize such an effect in the case where the Al content is 0.01% or more. Therefore, it is preferable that the Al content be 0.01% or more.
  • the Al content is set to be 0.20% or less. It is more preferable that the Al content be 0.02% or more. In addition, it is preferable that the Al content be 0.10% or less or more preferably 0.06% or less.
  • Ni 0.05% or more and 0.40% or less
  • Ni is an element which is important for achieving satisfactory adhesion of scale in the present invention, and it is necessary that the Ni content be 0.05% or more to realize such an effect.
  • the thermal expansion coefficient is decreased by containing an appropriate amount of Co, and it is possible to realize the effect described above with less Ni content than in the case of steel containing no Co or an insufficient amount of Co.
  • Ni is an expensive element, and there is conversely a deterioration in adhesion of scale as a result of the generation of a ⁇ phase at a high temperature in the case where the Ni content is more than 0.40%. Therefore, the Ni content is set to be 0.05% or more and 0.40% or less. It is preferable that the Ni content be 0.10% or more or more preferably 0.20% or more. In addition, it is preferable that the Ni content be 0.30% or less or more preferably 0.25% or less.
  • Co 0.01% or more and 0.30% or less
  • Co is an element which is important in the present invention.
  • Co is an element which is necessary for improving thermal fatigue resistance, and it is necessary that the Co content be 0.01% or more for this purpose. Since Co decreases the amount of thermal expansion when heating is performed by decreasing the thermal expansion coefficient of steel, there is a decrease in the amount of strain generated when heating and cooling are performed, which results in an improvement in thermal fatigue resistance. Moreover, since there is a decrease in the difference in the thermal expansion coefficient between steel and scale due to a decrease in the thermal expansion coefficient of steel, spalling of scale becomes less likely to occur when cooling is performed. Therefore, there is an advantage in that it is possible to prevent spalling of scale from occurring with less Ni content.
  • the Co content is set to be 0.01% or more and 0.30% or less. It is preferable that the Co content be 0.02% or more or more preferably 0.03% or more. In addition, it is preferable that the Co content be 0.10% or less.
  • Mo is an element which improves thermal fatigue resistance by improving the strength of steel through solid solution strengthening and improves corrosion resistance to condensed water by improving salt corrosion resistance, and it is possible to realize such effects in the case where the Mo content is 0.02% or more.
  • Mo is an expensive element.
  • the Mo content is set to be 0.02% or more and 0.30% or less. It is preferable that the Mo content be 0.04% or more. In addition, it is preferable that the Mo content be 0.10% or less.
  • Cu is effective for improving thermal fatigue resistance by strengthening steel as a result of being precipitated in the form of refined ⁇ -Cu and for improving corrosion resistance to condensed water by improving sulfuric acid corrosion resistance. It is necessary that the Cu content be 0.02% or more to realize such effects. On the other hand, in the case where the Cu content is more than 0.40%, there is a deterioration in cyclic oxidation resistance due to a deterioration in adhesion of oxide scale. Moreover, since Cu tends to be precipitated in the generation of coarse ⁇ -Cu, there is also a deterioration in corrosion resistance to condensed water. Therefore, the Cu content is set to be 0.40% or less. Therefore, the Cu content is set to be 0.02% or more and 0.40% or less. It is preferable that the Cu content be 0.04% or more. In addition, it is preferable that the Cu content be 0.10% or less.
  • the contents of C and N are set to be 0.010% or less and 0.015% or less respectively from the viewpoint of toughness, workability, and adhesion of scale.
  • (C% + N%) is limited to be 0.018% or less from the viewpoint of thermal fatigue resistance as indicated in expression (1) above.
  • (C% + N%) is more than 0.018%, a high amount of coarse Ti nitride (TiN) is generated, which is accompanied by the precipitation of NbC around the TiN, resulting in an increase in the amount of NbC precipitated.
  • (C% + N%) is set to be 0.018% or less to realize sufficient effect of solid solution strengthening caused by Nb. It is preferable that (C% + N%) be 0.015% or less.
  • the present invention is ferritic stainless steel which is excellent in adhesion of scale, thermal fatigue resistance, and corrosion resistance to condensed water and which is characterized by having a chemical composition containing the above-described indispensable constituents and the balance being Fe and inevitable impurities. Moreover, one, two, or all selected from Ca, Mg, and B and/or one, two, or all selected from V, W, and Zr may be contained as needed in the amounts described below.
  • Ca is an element which is effective for preventing nozzle clogging caused by the precipitation of Ti-based inclusions which tend to be generated when continuous casting is performed. It is possible to realize such an effect in the case where the Ca content is 0.0005% or more. On the other hand, it is preferable that the Ca content be 0.0030% or less to achieve good surface quality without the occurrence of surface defects. Therefore, in the case where Ca is contained, it is preferable that the Ca content be 0.0005% or more and 0.0030% or less, more preferably 0.0005% or more and 0.0020% or less, or even more preferably 0.0005% or more and 0.0015% or less.
  • Mg 0.0002% or more and 0.0020% or less
  • Mg is an element which is effective for improving workability and toughness. Moreover, Mg is an element which is effective for inhibiting coarsening of the carbonitrides of Nb and Ti. In the case where Ti carbonitrides are coarsened, since brittle fracturing starts from the Ti carbonitrides, there is a deterioration in toughness. In addition, in the case where Nb carbonitrides are coarsened, since there is a decrease in the amount of solid solution Nb in steel, there is a deterioration in thermal fatigue resistance. It is possible to realize the above-described effects of improving workability and toughness and of inhibiting coarsening of the carbonitrides of Ti and Nb in the case where the Mg content is 0.0002% or more.
  • the Mg content is more than 0.0020%, there may be a deterioration in the surface quality of steel. Therefore, in the case where Mg is contained, it is preferable that the Mg content be 0.0002% or more and 0.0020% or less. It is more preferable that the Mg content be 0.0004% or more. In addition, it is more preferable that the Mg content be 0.0015% or less or even more preferably 0.0010% or less.
  • B is an element which is effective for improving workability, in particular, secondary workability. It is possible to realize such effects in the case where the B content is 0.0002% or more. On the other hand, since there may be a deterioration in the workability and toughness of steel in the case where the B content is more than 0.0020%, the B content is set to be 0.0020% or less. Therefore, in the case where B is added, it is preferable that the B content be 0.0002% or more and 0.0020% or less. It is more preferable that the B content be 0.0003% or more. In addition, it is more preferable that the B content be 0.0010% or less.
  • V 0.01% or more and 0.50% or less
  • V is an element which is effective for improving high-temperature strength and which is effective for inhibiting coarsening of the carbonitrides of Ti and Nb. It is possible to realize such effects in the case where the V content is 0.01% or more. On the other hand, in the case where the V content is more than 0.50%, since coarse V(C, N) is precipitated, there may be a deterioration in toughness. Therefore, in the case where V is contained, it is preferable that the V content be 0.01% or more and 0.50% or less. It is more preferable that the V content be 0.02% or more. In addition, it is more preferable that the V content be 0.20% or less.
  • W is, like Mo, an element which improves the strength of steel through solid solution strengthening, and it is possible to realize such an effect in the case where the W content is 0.02% or more.
  • W is an expensive element, and, in the case where the W content is high, surface defects occur, and there is a significant deterioration in workability due to, for example, a deterioration in toughness. It is preferable that the W content be 0.30% or less to achieve good surface quality. Therefore, in the case where W is contained, it is preferable that the W content be 0.02% or more and 0.30% or less.
  • Zr is an element which improves oxidation resistance. It is preferable that the Zr content be 0.005% or more to realize such an effect. On the other hand, in the case where the Zr content is more than 0.50%, since Zr-based intermetallic compounds are precipitated, there is a tendency for embrittlement to occur in steel. Therefore, in the case where Zr is contained, it is preferable that the Zr content be 0.005% or more and 0.50% or less.
  • the ferritic stainless steel according to the present invention may be manufactured by using an ordinary method for manufacturing stainless steel.
  • Molten steel having the chemical composition described above is prepared by using a melting furnace such as a converter or an electric furnace, subjected to secondary refining by using a method such as a ladle refining method or a vacuum refining method, made into a steel piece (slab) by using a continuous casting method or a ingot casting-slabbing method, and made into a hot-rolled, annealed, and pickled steel sheet by performing hot rolling, hot-rolled-sheet annealing, and pickling. It is recommended that processes such as a cold rolling process, a finish annealing process, a pickling process, and so forth be performed to obtain a cold-rolled and annealed steel sheet.
  • a method is as follows.
  • Molten steel having the chemical composition described above is prepared by using, for example, a converter or an electric furnace, subjected to secondary refining by using an AOD method or a VOD method, and made into a slab by using a continuous casting method.
  • This slab is heated to a temperature of 1000°C to 1250°C and subjected to hot rolling to obtain a hot-rolled steel sheet having a desired thickness.
  • This hot-rolled steel sheet is subjected to continuous annealing at a temperature of 900°C to 1100°C and subjected to descaling by performing shot blasting and pickling to obtain a hot-rolled, annealed, and pickled steel sheet.
  • cold rolling, annealing, and pickling may further be performed to obtain a cold-rolled, annealed, and pickled steel sheet.
  • cold rolling with process annealing may be performed two or more times as needed.
  • the total rolling reduction ratio in the cold rolling process, in which cold rolling is performed once, twice, or more, is set to be 60% or more or preferably 70% or more.
  • the cold-rolled-sheet annealing temperature is set to be 900°C to 1150°C or preferably 950°C to 1100°C.
  • the shape and properties of the steel sheet may be controlled by performing light-reduction rolling (such as skin pass rolling) after the pickling has been performed.
  • annealing may be performed in a reducing atmosphere containing hydrogen to obtain a bright annealed steel sheet without performing pickling.
  • the product sheet is formed into the exhaust pipe or catalyst outer cylinder of an automobile or a motorcycle, the exhaust air duct of a thermal power generation plant, or a fuel cell-related member.
  • an arc welding method such as TIG, MIG, or MAG
  • a resistance welding method such as a spot welding method or a seam welding method
  • a high-frequency resistance welding method such as an electric resistance welding method, or a high-frequency induction welding method may be used.
  • Molten steel Nos. 1 through 40 having the chemical compositions given in Table 1 were prepared and cast into steel ingots having a weight of 30 kg by using a vacuum melting furnace. Subsequently, the ingots were heated to a temperature of 1170°C and subjected to hot rolling to obtain sheet bars having a thickness of 35 mm and a width of 150 mm. Each of these sheet bars was divided into two pieces. One of the two pieces was subjected to forging to obtain a square bar having a cross section of 30 mm ⁇ 30 mm. The square bar was annealed at a temperature of 950°C to 1050°C and machined to obtain a thermal fatigue test piece illustrated in Fig. 1 . The thermal fatigue test described below was performed on the test piece. The annealing temperature was controlled in the temperature range of 950°C to 1050°C in accordance with the chemical composition while the microstructure was checked. The same applies to the annealing described below.
  • the other half of the two pieces described above was heated to a temperature of 1050°C and subjected to hot rolling to obtain a hot-rolled steel sheet having a thickness of 5 mm. Subsequently, the steel sheet was subjected to hot-rolled-sheet annealing in a temperature range of 900°C to 1050°C and pickled to obtain a hot-rolled, annealed, and pickled steel sheet. At this stage, the surface quality of the steel sheet was visually inspected. The steel sheet was subjected to cold rolling to a thickness of 2 mm and subjected to finish annealing in a temperature range of 900°C to 1050°C to obtain a cold-rolled and annealed steel sheet. The steel sheet was subjected to the cyclic oxidation test and the condensed water immersion test described below.
  • a test piece having a width of 20 mm and a length of 30 mm was taken from the cold-rolled and annealed steel sheet described above, the entire 6 surfaces of the test piece were polished by using #320 emery paper, and the polished test piece was subjected to the test.
  • the oxidation test holding at a temperature of 1000°C for 20 minutes and holding at a temperature of 100°C for 1 minute were alternately performed 400 times each in air. Heating and cooling were performed respectively at a heating rate of 5°C/sec and at a cooling rate of 1.5°C/sec. After the test, by performing visual observation to determine whether spalling of scale occurred or not, adhesion of scale was evaluated. The obtained results are given in Table 1.
  • the thermal fatigue life of the thermal fatigue test piece described above was evaluated by cyclically applying strain with a restraint ratio of 0.6 as illustrated in Fig. 2 while heating and cooling was alternately performed in a temperature range of 200°C to 900°C. The determination was performed in accordance with the "Standard for high temperature low cycle fatigue testing" (JSMS-SD, 7-03) published by the Society of Material Science, Japan .
  • the stress of each of the cycles was defined as a value calculated by dividing a load determined at a temperature of 200°C by the cross-sectional area (50.3 mm 2 ) of the gauged portion of the test piece illustrated in Fig. 1 .
  • the thermal fatigue life of the test piece was defined as the number of cycles at which the stress was 75% of the stress at the 5th cycle, at which the behavior becomes stable. Thermal fatigue resistance was evaluated on the basis of the fatigue life. The obtained results are given in Table 1.
  • free thermal expansion strain refers to strain generated when heating is performed with no mechanical stress being applied
  • controlled strain refers to strain with respect to a state in which no stress is applied at room temperature. Substantial strain generated in the material due to restraint is equal to (free thermal expansion strain - controlled strain), that is, strain with respect to the free thermal expansion strain.
  • test piece having a width of 60 mm and a length of 80 mm was taken from the cold-rolled and annealed steel sheet obtained as described above, the entire 6 surfaces of the test piece were polished by using #320 emery paper, and the polished test piece was subjected to the test. At the time of the test, the end surfaces of the test pieces were covered with a protection tape.
  • the testing solution that is, simulated condensed water, contained Cl - : 500 ppm and SO 4 2- : 1000 ppm and had a pH of 4. The solution was held in a thermostatic bath so that the temperature of the solution was 80°C. The test was repeated 30 cycles, where, in one cycle of the test, the test piece was immersed in the solution for 2 hours and then dried for 6 hours. After the test had been performed, corrosion product was removed, and a decrease in weight due to corrosion was calculated from the weight of the test piece determined before and after the test.
  • Nos. 1 through 20 and Nos. 36 through 40 were all excellent in adhesion of scale, thermal fatigue resistance, and corrosion resistance to condensed water.
  • Nos. 2 through 4, 6, 9, 10, 12, 14 through 16, 19, 20, and 36 through 40 which were the examples of the present invention, and in which the contents of Si and Ni were within the preferable ranges (Si ⁇ 0.30% and Ni ⁇ 0.20%), were particularly excellent in adhesion of scale. Nos.
  • comparative example Nos. 21 and 24 in which the contents of Mo and Cu were both less than the lower limits of the present invention
  • comparative example No. 22 in which the Cu content was less than the lower limit of the present invention
  • comparative example No. 23 in which the Mo content was less than the lower limit of the present invention
  • Comparative example No. 25 in which (C + N) was more than the upper limit of the present invention, was unsatisfactory in thermal fatigue resistance. Comparative example No. 26, in which the Co content was less than the lower limit of the present invention, was unsatisfactory in thermal fatigue resistance. Comparative example No. 27, in which the Ni content was less than the lower limit of the present invention, was unsatisfactory in adhesion of scale and thermal fatigue resistance.
  • Comparative example No. 28 in which the contents of Ni and Co were both less than the lower limit of the present invention, was unsatisfactory in adhesion of scale and thermal fatigue resistance.
  • Comparative example No. 30 in which the Ti content was more than the upper limit of the present invention, was unsatisfactory in all of adhesion of scale, thermal fatigue resistance, and corrosion resistance to condensed water. Comparative example No. 31, in which the C content was more than the upper limit of the present invention, was unsatisfactory in adhesion of scale and thermal fatigue resistance. Comparative example No. 32, in which the N content was more than the upper limit of the present invention, was unsatisfactory in adhesion of scale and thermal fatigue resistance.
  • Comparative example No. 33 in which the Cr content was less than the lower limit of the present invention, was unsatisfactory in all of adhesion of scale, thermal fatigue resistance, and corrosion resistance to condensed water.
  • Comparative example No. 34 in which the Nb content was less than the lower limit of the present invention, and comparative example No. 35, in which the Ti content was less than the lower limit of the present invention, were both unsatisfactory in thermal fatigue resistance.
  • the steels within the range of the present invention were excellent in all of adhesion of scale, thermal fatigue resistance, and corrosion resistance to condensed water.
  • the ferritic stainless steel sheet according to the present invention is excellent in all of adhesion of scale, thermal fatigue resistance, and corrosion resistance to condensed water
  • the steel sheets can preferably be used for all the exhaust system members of an automobile or the like such as exhaust manifolds, various kinds of exhaust pipes, flanges, converter cases, and mufflers, and, since it is possible to form all the exhaust pipe members by using one steel grade, there is an improvement in efficiency from the viewpoint of the stable availability and weldability of a steel material.
  • the steel can preferably be used for the exhaust system members of a thermal power generation system and the members of a fuel cell.

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