EP3276028A1 - Ferrit-austenit-edelstahlblech mit hervorragender gescherter stirnseitenkorrosionsbeständigkeit - Google Patents

Ferrit-austenit-edelstahlblech mit hervorragender gescherter stirnseitenkorrosionsbeständigkeit Download PDF

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EP3276028A1
EP3276028A1 EP16768525.4A EP16768525A EP3276028A1 EP 3276028 A1 EP3276028 A1 EP 3276028A1 EP 16768525 A EP16768525 A EP 16768525A EP 3276028 A1 EP3276028 A1 EP 3276028A1
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amount
corrosion resistance
stainless steel
steel sheet
ferritic
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French (fr)
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EP3276028B1 (de
EP3276028A4 (de
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Eiichiro Ishimaru
Masatomo Kawa
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Nippon Steel Stainless Steel Corp
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Nippon Steel and Sumikin Stainless 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
    • 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
    • C22C38/002—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
    • C22C38/04—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
    • C22C38/10—Ferrous alloys, e.g. steel alloys containing 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
    • C22C38/10—Ferrous alloys, e.g. steel alloys containing cobalt
    • C22C38/105—Ferrous alloys, e.g. steel alloys containing cobalt containing Co and Ni
    • 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
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of 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
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • 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

Definitions

  • the present invention relates to a ferritic-austenitic (dual phase, duplex) stainless steel sheet with excellent sheared end face corrosion resistance (excellent corrosion resistance of a sheared end face (sheared line)).
  • the present invention relates to a ferritic-austenitic (ferrite-austenite) stainless steel sheet that is suitable for use in an atmospheric environment in a state just as a sheared state where the steel sheet is just as it is sheared and a sheared line is not subjected to a corrosion resistance treatment.
  • Ferritic-austenitic (dual phase, duplex) stainless steel is used in a wide range of applications due to excellent strength and corrosion resistance. Examples of the applications include various uses such as a solar cell frame that is not necessary to be processed, and a support component for an outdoor pipeline that is difficult to be processed.
  • the ferritic-austenitic stainless steel sheet In the process of manufacturing the ferritic-austenitic stainless steel sheet, cutting-out, shaping, and punching of the steel sheet by shearing are frequently performed in many cases due to convenience of the shearing.
  • the ferritic-austenitic stainless steel sheet is used in a state just as a sheared state where the steel sheet is just as it is sheared and a sheared line is not subjected to a corrosion resistance treatment.
  • a ferritic-austenitic stainless steel is used in a state where the ferritic-austenitic stainless steel is sheared and the end face is not subjected to a corrosion resistance treatment, corrosion of the end face (end face corrosion and end face rust) is severer than corrosion of a smooth surface.
  • the end face corrosion becomes a cause of flowed rust and another iron origned rust, and as a result, the end face corrosion becomes a cause of deteriorating the corrosion resistance of the whole steel sheet.
  • the problem related to the end face rust is not considered as an important matter in a ferritic-austenitic stainless steel because corrosion resistance is maintained to a certain degree due to passivation even in the end face of the ferritic-austenitic stainless steel that is unlike a plated steel sheet in which a base material is exposed.
  • ferritic-austenitic stainless steel sheet ferrite phases and austenite phases exist at room temperature, and the ferrite phases that exist in the sheared line become a cause of rust.
  • Patent Document 1 disclose ferritic stainless steel sheets with excellent crevice corrosion resistance.
  • ferritic stainless steel sheets have an effect on local corrosion such as the crevice corrosion. However, it cannot be said that the effect is sufficient for reducing rusting (occurrence of rust) on the sheared line, and end face corrosion occurred in some cases.
  • Patent Document 3 which focuses on burr morphologies on the end face discloses a ferritic stainless steel sheet with excellent corrosion resistance of a sheared line.
  • Patent Document 4 discloses a processing method for obtaining a favorable shape of a sheared line.
  • the ferritic-austenitic stainless steel has high-strength characteristics in comparison to the ferritic stainless steel. Therefore, properties of a sheared surface of the ferritic-austenitic stainless steel are greatly different from properties of a sheared surface of the ferritic stainless steel. In addition to a shape of the shear plane, a minute crevice shape is likely to be formed due to a different in strength between the austenite phase and the ferrite phase, and this minute crevice shape has an effect on the corrosion resistance.
  • Patent Documents mere the methods of the related art disclosed in the above-described Patent Documents are not sufficient to improve the corrosion resistance of a sheared line of the ferritic-austenitic stainless steel, and a problem related to the occurrence of rust on the sheared line still remains.
  • the present invention has been made in consideration of the above-described problem, and an object thereof is to provide a ferritic-austenitic stainless steel sheet in which corrosion resistance of a sheared line is improved with respect to the ferritic-austenitic stainless steel sheet that is used in the atmospheric environment in a state of not being subjected to a corrosion resistance treatment.
  • the present inventors have made various examinations to realize an improvement of the corrosion resistance of a sheared line of the ferritic-austenitic stainless steel sheet. Particularly, the present inventors have made a thorough observation with respect to a corrosion state of the sheared line. As a result, they found that the origin of corrosion exists on a fracture surface, and a reduction in the fracture surface and a reduction in surface roughness of the fracture surface lead to prevention of corrosion.
  • the fracture surface is one of surface states called “undercut”, “sheared surface (shear plane)", “fracture surface”, and “burr” which are confirmed when observing a processed surface after shearing the steel sheet.
  • the present inventors have made a further examination on the improvement of the corrosion resistance. As a result, they obtained the following finding. That is, when controlling crystal grain sizes of ferrite phases and austenite phases in appropriate ranges and when generating sulfides to exist appropriately in a steel, it is effective for an improvement of the fracture surface. In addition, the present inventors also obtained the following finding. When minute amounts of Co and V are added as a component that improves the corrosion resistance, the corrosion resistance of the austenite phase and the ferrite phase is improved. As a result, the corrosion resistance of a shared portion is improved.
  • First group one or more selected from, in terms of % by mass, Nb: 0.005% to 0.2%, Ti: 0.005% to 0.2%, W: 0.005% to 0.2%, and Mo: 0.01% to 1.0%.
  • Second group one or more selected from, in terms of % by mass, Sn: 0.005% to 0.2%, Sb: 0.005% to 0.2%, Ga: 0.001% to 0.05%, Zr: 0.005% to 0.5%, Ta: 0.005% to 0.1%, and B: 0.0002% to 0.0050%.
  • a ferritic-austenitic stainless steel sheet that is used mainly in an atmospheric environment in a state in which a sheared line is not subjected to a corrosion resistance treatment, it is possible to realize an improvement of the corrosion resistance of the sheared line. Therefore, it is possible to improve the corrosion resistance of the entire ferritic-austenitic stainless steel sheet. As a result, it is possible to prevent loss of aesthetic exterior appearance, a decrease in lifespan, and the like due to corrosion of the steel sheet.
  • ferritic-austenitic stainless steel sheet hereinafter, may also be referred to as “steel sheet” in brief
  • the amount of C is an element that is unavoidably mixed in steel.
  • the amount of C is more than 0.03%, Cr 23 C 6 precipitates in an austenite phase and a ferrite phase, and a crystal grain boundary is sensitized. Accordingly, the corrosion resistance deteriorates.
  • the amount of C is preferably as small as possible, and can be permitted up to 0.03%.
  • the lower limit of the amount of C is not particularly limited. From the viewpoints of productivity and cost, the amount of C is preferably 0.002% or more, and more preferably 0.008% or more.
  • the upper limit of the amount of C is preferably 0.025% or less.
  • Si is an element that is useful as a deoxidizing agent.
  • the amount of Si (content rate) is less than 0.1%, a sufficient deoxidizing effect is not obtained, and a large amount of oxides are dispersed in steel. Accordingly, an amount of origins of fracture during press working increases.
  • the amount of Si is set to be in a range of 0.1 % to 1.0%.
  • the amount of Si is preferably 0.3% or more, and is preferably set to be 0.7% or less so as to further prevent the deterioration of workability.
  • Mn has a deoxidizing effect.
  • the inventors found that when controlling a dispersion state of MnS, there is an effect of preventing an increase in surface roughness at a fracture surface portion in a sheared line. Although not clear, this mechanism is assumed as follows.
  • the amount of Mn is preferably set to be 1.0% or more from the viewpoint of preventing a decrease in surface roughness. It is preferable that the amount of Mn is set to be 4.0% or less so as to further prevent the generation of Mn oxide in the passivation film.
  • the amount of P is an element that deteriorates the corrosion resistance.
  • P segregates at a crystal grain boundary; and thereby, P deteriorates hot workability. Therefore, in the case where P is added in an excessive amount, it becomes difficult to manufacture the steel of the present embodiment. Accordingly, the amount of P is preferably as small as possible. However, the amount of P can be permitted to 0.04% or less. As a result, the amount of P is limited to 0.04% or less. The amount of P is preferably 0.03% or less.
  • Al is an effective component for deoxidation, and it is necessary for Al to be contained at an amount of 0.015% or more.
  • the amount of Al is set to be 0.015% to 0.10%.
  • the amount of Al is preferably set to be 0.02% or more from the viewpoint of sufficiently obtaining the deoxidizing effect. It is preferable that the amount of Al is set to be 0.05% or less so as to further prevent the generation of the Al-based non-metallic inclusions.
  • Cr is an important element that determines the corrosion resistance of stainless steel.
  • approximately 50% of the ferrite phases and approximately 50% of the austenite phases are mixed in a structure.
  • Cr is concentrated in the ferrite phase.
  • the amount of Cr decreases, but N that is an austenite generating element is concentrated. 19.0% or more of Cr is contained so as to secure the corrosion resistance of the austenite phase.
  • the amount of Cr is preferably 20.0% or more.
  • the amount of Cr is set to be 24.0% or less.
  • the amount of Cr is preferably 23.0% or less.
  • Cu has an effect of forming a film on a surface of stainless steel after occurrence of corrosion, and reducing dissolution of a base material due to an anodic reaction. Accordingly, Cu is an element that is also useful for an improvement of rust resistance and an improvement of crevice corrosion resistance.
  • the amount of Cu is less than 0.5%, it is difficult to expect the above-described effects.
  • the amount of Cu is more than 1.5%, embrittlement at a high temperature is promoted, and hot workability deteriorates. Accordingly, the amount of Cu is limited to a range of 0.5% to 1.5%.
  • the amount of Cu is preferably set to be 0.7% or more from the viewpoint of improving the rust resistance and the crevice corrosion resistance.
  • the amount of Cu is preferably set to be 1.2% or less so as to further prevent the deterioration of the hot workability.
  • Ni is an element that prevents an anodic reaction due to an acid, and is capable of maintaining passivation at a relatively low pH. That is, Ni is highly effective on the crevice corrosion resistance, and greatly prevent the progress of corrosion at an actively dissolved state.
  • the amount of Ni is less than 0.60%, the effect of improving the crevice corrosion resistance is not obtained, and in addition, a ratio of the austenite phases decreases. Accordingly, workability greatly deteriorates.
  • the amount of Ni is more than 2.30%, the ratio of the austenite phases increases, and the hot workability deteriorates. Accordingly, the amount of Ni is limited to a range of 0.60% to 2.30%.
  • the lower limit of the amount of Ni is preferably 1.0% or more, and more preferably 1.5% or more.
  • the upper limit of the amount of Ni is preferably 1.5% or less.
  • N is an important element that stabilizes the austenite phase, and improves the corrosion resistance.
  • the amount of N is less than 0.06%, the ratio of the austenite phases is small. Accordingly, workability deteriorates, and the corrosion resistance of the austenite phase deteriorates.
  • the amount of N is set to be 0.06% to 0.20%.
  • the amount of N is preferably set to be 0.08% or more from the viewpoint of stabilization of the austenite phase.
  • the amount of N is preferably set to be 0.17% or less so as to further prevent the deterioration of the hot workability.
  • Co is an element that exhibits the same behavior as that of Ni, and stabilizes the austenite phase. Even in the case where a slight amount of Co is added together with Ni, the effect is exhibited. However, in the case where the amount of Co is less than 0.05%, the effect is not recognized. In addition, Co stabilizes precipitation of the austenite phases in a high temperature region. Accordingly, the concentrating of N in the austenite phase is promoted, and the amount of N in the ferrite phase is greatly decreased. Accordingly, Co operates to prevent the precipitation of Cr carbonitrides (particularly, Cr nitrides).
  • the main cause of deterioration of the corrosion resistance in the steel sheet of the present embodiment is a decrease in a Cr concentration in the vicinity of the Cr carbonitrides in accordance with the precipitation of the Cr carbonitrides. Accordingly, particularly, Co operates to prevent the precipitation of the Cr carbonitrides; and thereby, Co prevents the deterioration of the corrosion resistance at a ferrite grain boundary or an interface between the ferrite phase and the austenite phase. On the other hand, an excessive amount of Co is added, a ratio of the austenite phases increases, and the hot workability deteriorates. In addition, since Co is a rare element and is expensive, the cost increases excessively when a large amount of Co is added.
  • the upper limit of the amount of Co is set to be 0.25% or less.
  • the amount of Co is preferably set to be 0.08% or more from the viewpoint of stabilizing the austenite phase.
  • the upper limit of the amount of Co is preferably 0.20% or less and more preferably 0.12% or less so as to further prevent the deterioration of hot workability.
  • V 0.01% to 0.15%
  • V is a strong carbonitride generating element.
  • the main cause of deterioration of the corrosion resistance in the steel sheet of the present embodiment is a decrease in a Cr concentration in the vicinity of the Cr carbonitrides in accordance with the precipitation of the Cr carbonitrides. Accordingly, when V carbonitrides precipitate in a high temperature region, it is possible to prevent the precipitation of Cr carbonitrides in a low temperature region. This effect is recognized when 0.01% or more of V is added. Accordingly, the lower limit of the amount of V is set to be 0.01% or more. On the other hand, in the case where an excessive amount of V is added, hardening is caused.
  • the upper limit of the amount of V is set to 0.15% or less.
  • the amount of V is preferably set to be 0.05% or more, and more preferably 0.08% or more.
  • the amount of V is preferably set to be 0.12% or less so as to further prevent the hardening.
  • the amount of V is preferably set to be less than 0.05%.
  • Ca is a component that is effective for deoxidation.
  • Ca is an element that generates sulfides.
  • Ca is an element that is effective to stabilize sulfides which contribute to satisfactory properties of a sheared fracture surface.
  • the amount of Ca is preferably set to be 0.0003% or more. However, in the case where the amount of Ca is more than 0.002%, coarse CaS is generated, and the coarse CaS becomes the origin of rust. Accordingly, the amount of Ca is set to be 0.002% or less.
  • S is an important element in the present embodiment. S forms sulfides with Mn, Ca, and the like in stainless steel. In the related art, it was recognized that it was preferable to reduce the amount of S because these sulfides became the main cause of deteriorating the corrosion resistance. However, according to research made by the present inventors, even in a case of MnS or CaS which were recognized as unfavorable sulfides, it was found that when a grain size and a dispersion state of the sulfides are appropriately controlled, it is possible to stably maintain surface properties of a sheared line in a good state, and the corrosion resistance does not deteriorate.
  • the lower limit of the amount of S is set to be 0.0002% or more.
  • the amount of S is limited to a range of 0.0002% to 0.0040%.
  • the lower limit of the amount of S is more preferably 0.0003% or more, and the upper limit of the amount of S is more preferably 0.0010% or less. Accordingly, a more preferable range of the amount of S is 0.0003% to 0.0010%.
  • the main cause of deterioration of the corrosion resistance in the steel sheet of the present embodiment is a decrease in a Cr concentration in the vicinity of the Cr carbonitrides in accordance with the precipitation of the Cr carbonitrides.
  • the lower limit of the value of Co + 0.25V is set to be 0.10 or more.
  • the value of Co + 0.25V is set to be 0.12 or more, the generated amount of Cr definitely decreases. Accordingly, the lower limit of the value of Co + 0.25V is preferably 0.12 or more.
  • the upper limit of the value of Co + 0.25V is set to be less than 0.25.
  • Co and V represent the amounts (% by mass) of Co and V, respectively.
  • Nb is an element that fixes C and N, Nb prevents sensitization due to the Cr carbonitrides, and Nb improves the corrosion resistance.
  • the amount of Nb is less than 0.005%, the effects are small.
  • the amount of Nb is more than 0.2%, the ferrite phase becomes hard due to solid-solution hardening, and workability deteriorates. Accordingly, the amount of Nb is preferably set to be in a range of 0.005% to 0.2%.
  • Ti is an element that fixes C and N, Ti prevents sensitization due to the Cr carbonitrides, and Ti improves the corrosion resistance. However, in the case where the amount of Ti is less than 0.005%, the effects are small. On the other hand, in the case where the amount of Ti is more than 0.2%, hardening of the ferrite phase is caused, and toughness decreases. In addition, a decrease in surface roughness is caused due to Ti-based precipitates. Accordingly, the amount of Ti is preferably set to be in a range of 0.005% to 0.2%.
  • W has an effect of fixing C and N, and W prevents sensitization due to the Cr carbonitrides.
  • the amount of W is less than 0.005%, the effect is not recognized.
  • the amount of W is more than 0.2%, hardening is caused, and workability deteriorates. Accordingly, the amount of W is preferably set to be in a range of 0.005% to 0.2%.
  • Mo is an element that improves the corrosion resistance. However, in the case where the amount of Mo is less than 0.01%, the effect is small. On the other hand, in the case where the amount of Mo is more than 1.0%, hardening is caused, and workability deteriorates. Accordingly, the amount of Mo is preferably set to be 0.01% to 1.0%.
  • Sn and Sb are elements which improve the corrosion resistance, and are also solid-solution strengthening elements of the ferrite phase. Accordingly, the upper limit of the amount of each of Sn and Sb is set to be 0.2%. In the case where the amount of any one of Sn and Sb is 0.005% or more, an effect of improving the corrosion resistance is exhibited. Accordingly, the amount of each of Sn and Sb is set to be 0.005% to 0.2%.
  • the lower limit of the amount of each of Sn and Sb is preferably 0.03% or more.
  • the upper limit of the amount of each of Sn and Sb is preferably 0.1% or less.
  • Ga 0.001% to 0.05%
  • Ga is an element that contributes to an improvement of the corrosion resistance. In the case where the amount of Ga is 0.001% or more, the effect is exhibited. In the case where the amount of Ga is more than 0.05%, the effect is saturated. Accordingly, Ga can be contained at an amount of 0.001% to 0.05%.
  • Zr is an element that contributes to an improvement of the corrosion resistance. In the case where the amount of Zr is 0.005% or more, the effect is exhibited. In the case where the amount of Zr is more than 0.5%, the effect is saturated. Accordingly, Zr can be contained at an amount of 0.005% to 0.5%.
  • Ta 0.005% to 0.1%
  • Ta is an element that improves the corrosion resistance by modification of inclusions, and Ta may be contained as necessary.
  • the effect is exhibited by 0.005% or more of Ta.
  • the lower limit of the amount of Ta may be set to be 0.005% or more.
  • the upper limit of the amount of Ta is preferably 0.1% or less, and more preferably 0.050% or less.
  • the amount of Ta is preferably set to be 0.020% or less.
  • B is an element that is useful to prevent secondary working embrittlement or deterioration of the hot workability, and B does not have an effect on the corrosion resistance. Accordingly, B can be contained on the condition that the lower limit of the amount of B is set to be 0.0002% or more. However, in the case where the amount of B is more than 0.0050%, the hot workability deteriorates on the contrary. Accordingly, the upper limit of the amount of B may be set to be 0.0050% or less. The upper limit of the amount of B is preferably 0.0020% or less.
  • the remainder other than the above-described elements is Fe and unavoidable impurities.
  • another element other than the above-described elements may be contained in a range not deteriorating the effect of the present embodiment.
  • the metallic structure (metallographic structure, microstructure) of the ferritic-austenitic stainless steel sheet consists of the ferrite phases and the austenite phases.
  • the crystal grain size of each of the ferrite phase and the austenite phase has a great effect on mechanical properties and surface properties of the sheared line.
  • a recrystallization temperature of the ferrite phase is different from that of the austenite phase, and grain growth of the ferrite phase occurs in a recrystallization temperature region of the austenite phase. Accordingly, the average crystal grain size of the ferrite phases becomes more than the average crystal grain size of the austenite phases.
  • the difference in the grain size between the ferrite phase and the austenite phase increases, the difference in strength becomes large (increases).
  • the difference in strength is large, fracture occurs at an interface between the ferrite phase and the austenite phase during shearing, and the fracture becomes the origin of crevice corrosion.
  • FIG. 1 is a graph showing a relationship between the average crystal grain size of the ferrite phases and the average crystal grain size of the austenite phases which have an effect on corrosion resistance after shearing. As is clear from FIG. 1 , an appropriate combination exists in the average crystal grain sizes of the ferrite phases and the austenite phases. From the results in FIG. 1 , the upper limit of the average crystal grain size of the ferrite phases is set to be 20 ⁇ m.
  • the strength is improved and burrs are less likely to be formed because recrystallization of the austenite phase is not completed.
  • an area of a fracture surface greatly increases. Accordingly, the corrosion resistance deteriorates.
  • an average crystal grain size of the austenite phases is less than 2 ⁇ m, the strength greatly increases, and the corrosion resistance deteriorates due to the same reason as described above.
  • the average crystal grain size of the austenite phases is more than 10 ⁇ m, burrs increase due to an softening effect, roughness of a fracture surface decreases, and micro-gaps are formed. In addition, coarse grains are generated at a part of the ferrite phase, and interface fracture is promoted. Accordingly, the corrosion resistance greatly deteriorates.
  • the average crystal grain size of the ferrite phases is set to be 5 to 20 ⁇ m, and the average crystal grain size of the austenite phases is set to be 2 to 10 ⁇ m.
  • the origin of corrosion on an end face subjected to shearing is a boundary portion between the shear plane and a fracture surface, and the fracture surface. Since a gap is likely to be formed at the boundary portion between the shear plane and the fracture surface, deposition of a corrosion factor is likely to occur.
  • a minute gap shape due to unevenness corresponded to a dimple fracture surface promotes lowering of pH and high salinity in an adhered solution (the gap shape lowers pH of the adhered solution and concentrates salt in the adhered solution).
  • the boundary portion and the fracture surface become the origin of corrosion.
  • the sulfides represent CaS, MnS, CrS, TiCS, CuS, and the like.
  • FIG. 2 is a graph showing a relationship between the size and the number of sulfides which have an effect on the corrosion resistance after shearing.
  • the size of the sulfides in FIG. 2 represents the maximum value of the major axes of extended sulfides.
  • the number of the sulfides in FIG. 2 represents the number (pieces per 5 mm 2 ) of sulfides having major axes of 1 to 5 ⁇ m.
  • the targeted major axis of the sulfide is set in a range of 1 to 5 ⁇ m. Therefore, in the present embodiment, sulfides having the major axes of 1 to 5 ⁇ m are set as a control target.
  • the major axis of the sulfide as a control target represents the major axis of the individual sulfide.
  • the maximum value of the major axes of sulfides is 1 to 5 ⁇ m.
  • sulfides having major axes of 1 to 5 ⁇ m are generated to exist at an amount of 5 to 20 pieces per 5 mm 2 .
  • sulfides having major axes of 1 to 5 ⁇ m are generated to exist at an amount of 6 pieces to 15 pieces per 5 mm 2 .
  • the average crystal grain sizes of the ferrite phases and the austenite phases, and the precipitation and dispersion state of the sulfides are important. Accordingly, it is important to carry out manufacturing of a steel sheet under the following conditions.
  • Rolling reductions in a hot-rolling process and a cold-rolling process are important so as to control the average crystal grain sizes of the ferrite phases and the austenite phases in the above-described ranges.
  • a reduction in at least one pass it is necessary to set a reduction in at least one pass to be 30% or more, and it is necessary to perform five passes or more of working (rolling) at a temperature of 1000°C or higher in the rough rolling process.
  • it is necessary to set a rolling reduction of cold-rolling to be 75% or more, and it is necessary to set a sheet temperature at the cold-rolling to be 150°C or higher at the time of termination of a final pass.
  • a strain that is introduced during the cold-rolling becomes a generation nucleus of a recrystallized grain.
  • high-strength steel similar to the present embodiment if work hardening proceeds, a great load is applied on the cold-rolling process. Therefore, the load is reduced by raising the sheet temperature during the cold-rolling. Thereby, not only the load in the cold-rolling process is reduced, but also the generation nuclei of recrystallized grains do not become excessive. Accordingly, it is also useful for controlling a crystal grain size.
  • the sheet temperature after the final pass can be controlled by changing a rolling reduction per one pass and a rolling speed.
  • a processing temperature in each of hot-rolled sheet annealing (process of annealing a hot-rolled sheet) and cold-rolled sheet annealing (process of annealing a cold-rolled sheet) is important to control the size and the precipitation number of the sulfides in the above-described range.
  • a hot-rolled sheet annealing temperature is set to be 1000°C to 1100°C
  • a cold-rolled sheet annealing temperature is set to be 950°C to 1050°C.
  • the ferritic-austenitic stainless steel having the above-described component composition is heated to a temperature of 1150°C to 1250°C. Then, hot-rolling is performed under conditions where a finish temperature is set to be 950°C or higher to obtain a sheet thickness of 3.0 to 6 mm. At this time, a rolling reduction of at least one pass in the rough rolling process is set to be 30% or more. In the case where cooling is performed at a typical cooling rate after finish rolling, precipitation of the austenite phase is not sufficient. Therefore, a temperature of finish rolling is set to be 950°C or higher, and a hot-rolled sheet is coiled without performing cooling positively. Mild cooling is performed up to 500°C or lower, and then, the hot-rolled sheet is put into a water bath for rapid cooling.
  • a cooling rate after coiling is not particularly defined.
  • a decrease in toughness due to so-called 475°C brittleness occurs at or in the vicinity of 475°C.
  • the cooling rate in a temperature range of 425°C to 525°C is preferably 100°C/h or higher.
  • the a hot-rolled strip that is manufactured as described above is subjected to hot-rolled sheet annealing at a temperature of 1000°C to 1100°C, and then pickling is performed.
  • ferritic-austenitic stainless steel of the present embodiment in accordance with the manufacturing method as described above; however, the present embodiment is not limited by the above-described processes and conditions.
  • the present inventors have performed a lot of experiments by variously changing shearing conditions so as to accomplish the above-described object; and as a result, it was proved that a control of a clearance is particularly effective to reduce a fracture surface ratio.
  • the clearance represents a ratio of a gap x between a blade and a stage to the thickness d of the steel sheet.
  • the clearance in the shearing is affected by an area of a fracture surface in a sheared line, and the height of burrs. From a result of examination on various clearances, it was clear that with regard to the ferritic-austenitic stainless steel of the present embodiment, in the case where the clearance is set to be 5% to 20%, the area of the fracture surface and the height of the burrs are reduced to be small values, and the corrosion resistance is improved.
  • the clearance in the shearing is preferably set to be 10% to 15%.
  • Ingots of ferritic-austenitic stainless steels making chemical compositions shown in Tables 1 and 2 were prepared. Then, each of the ingots was heated to a temperature of 1200°C, and hot-rolling was performed under conditions where a finish temperature was 980°C to obtain a hot-rolled sheet having a sheet thickness of 4 mm. A rolling reduction of at least one pass in a rough rolling process of the hot-rolling was set to be 30% or more. The hot-rolled sheet was coiled, and the hot-rolled sheet was mildly cooled down up to 500°C or lower and then was rapidly cooled down.
  • the hot-rolled sheet was annealed at an annealing temperature described in Tables 3 and 5 (hot-rolled sheet annealing), and then pickling was performed. Then, cold-rolling was performed to obtain a cold-rolled sheet having a sheet thickness of 0.6 to 1.2 mm.
  • a rolling temperature of the 1st pass was set to be 60°C, and rolling was continuously performed so that a sheet temperature was not lowered.
  • a cold-rolling reduction and a sheet temperature after a final pass (final pass temperature) were set to be values shown in Tables 3 and 5.
  • the obtained cold-rolled sheet was subjected to annealing (cold-rolled sheet annealing), and a surface thereof was adjusted by finish pickling; and thereby, a test piece was obtained.
  • the sizes and the number of sulfides in the obtained test piece were measured by an optical microscope and a SEM-EDS method.
  • the measurement method is as follows. At first, a surface of the test piece was polished with #600, and was mirror-polished. Then, a square of 5 mmx5 mm was marked on the surface of the test piece. Inclusions were observed in a marked area by using the optical microscope, and inclusions, which existed in the area and had sizes of approximately 1 ⁇ m or more, were marked. The approximate size of the inclusion was grasped through the above-described observation, and an inclusion to be measured was selected.
  • the composition of the inclusion was measured at two sites per piece by using the SEM-EDS method. In the case where a composition including 50% or more of S was confirmed at even one site, the inclusion was determined to be a sulfide.
  • the major axis of the inclusion was measured by the following method. Sulfides have relatively soft characteristics. Accordingly, a lot of sulfides exist in a state of being extended in a rolling direction. Accordingly, a length in the rolling direction was set as the major axis, and the length (maximum length) of the sulfide from a front end to a rear end was measured as the major axis. Furthermore, a measurement value of the major axis of the sulfide was calculated as an integer by rounding off the measurement value at the first decimal place (rounding off the measurement value to the nearest one). The maximum value among the obtained measurement values of the major axes is described in a column of "Major axis of sulfide" in Tables 4 and 6.
  • the number (pieces per 5 mm 2 ) of the sulfides having the major axes of 1 to 5 ⁇ m is described in "Number of sulfides" in Tables 4 and 6.
  • the ferrite phase and the austenite phase were separated from each other in accordance with a back-scattering electron beam diffraction (EBSD) method by using a field emission scanning electron microscope JSM-7000F manufactured by JEOL Ltd., and the crystal grain sizes of the ferrite phases and the austenite phases were measured.
  • EBSD back-scattering electron beam diffraction
  • an acceleration voltage was set to 25 kV
  • the step size was set to be 0.5 ⁇ m
  • a measurement position was set to the central portion of a sheet thickness on a cross-section in a rolling direction at the central position of the width of the test piece.
  • a boundary of crystal grains in which an orientation difference between adjacent crystal grains was 15° or more, was determined to be a crystal grain boundary, and the crystal grain sizes of the ferrite phases and the austenite phases were measured by using OIM software available from TSL Solutions. With regard to each of the ferrite phases and the austenite phases, the average value of the measured crystal grain sizes was calculated to obtain the average crystal grain size.
  • the average crystal grain size of the ferrite phases is described in a column of "Grain size of ferrite phase” in Tables 4 and 6.
  • the average crystal grain size of the austenite phases is described in a column of "Grain size of austenite phase” in Tables 4 and 6.
  • the clearance (shear clearance) between a male die and a female die of a punching tool was adjusted by using female dies of the punching tool which have various diameters.
  • the central portion of a sample was subjected to circular shearing processing at various shear clearances.
  • the shear clearance (%) was a value calculated by the following expression. Difference between a diameter of a male die and a diameter of a female die of a punching tool / thickness of a test piece stell sheet ⁇ 100
  • the ferritic-austenitic stainless steel sheet of the present embodiment even when being used in the atmospheric environment in a state where a sheared line is not subjected to a corrosion resistance treatment, the corrosion resistance of the sheared line is excellent. Accordingly, the ferritic-austenitic stainless steel sheet of the present embodiment can be appropriately used for various uses such as case bodies (chassis) of a power conditioner and a photovoltaic (PV) inverter, a duct hood, a frame of a solar cell, and a waste channel and a lid thereof.
  • case bodies (chassis) of a power conditioner and a photovoltaic (PV) inverter such as case bodies (chassis) of a power conditioner and a photovoltaic (PV) inverter, a duct hood, a frame of a solar cell, and a waste channel and a lid thereof.
  • PV photovoltaic

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JP7005396B2 (ja) * 2018-03-14 2022-01-21 日鉄ステンレス株式会社 自動車締結部品用フェライト・オーステナイト２相ステンレス鋼板
JP7266976B2 (ja) * 2018-07-27 2023-05-01 日鉄ステンレス株式会社 スポット溶接部の強度と耐食性に優れたフェライト・オーステナイト二相ステンレス鋼板及びその製造方法
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JP7499621B2 (ja) 2020-06-23 2024-06-14 日鉄ステンレス株式会社 二相ステンレス鋼板および二相ステンレス鋼板の製造方法
TWI774241B (zh) * 2021-02-19 2022-08-11 日商日本製鐵股份有限公司 無方向性電磁鋼板用熱軋鋼板、無方向性電磁鋼板用熱軋鋼板之製造方法、及無方向性電磁鋼板之製造方法
CN113063707B (zh) * 2021-03-12 2023-04-18 浙江美力科技股份有限公司 回火屈氏体、马氏体组织的原奥氏体晶粒度的腐蚀方法

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