WO2018074164A1 - Hot-rolled and annealed ferritic stainless steel sheet and method for producing same - Google Patents

Hot-rolled and annealed ferritic stainless steel sheet and method for producing same Download PDF

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Publication number
WO2018074164A1
WO2018074164A1 PCT/JP2017/034949 JP2017034949W WO2018074164A1 WO 2018074164 A1 WO2018074164 A1 WO 2018074164A1 JP 2017034949 W JP2017034949 W JP 2017034949W WO 2018074164 A1 WO2018074164 A1 WO 2018074164A1
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Prior art keywords
hot
rolling
rolled
content
steel sheet
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PCT/JP2017/034949
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French (fr)
Japanese (ja)
Inventor
正崇 吉野
光幸 藤澤
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Ｊｆｅスチール株式会社
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Application filed by Ｊｆｅスチール株式会社 filed Critical Ｊｆｅスチール株式会社
Priority to ES17862905T priority Critical patent/ES2831841T3/en
Priority to US16/327,988 priority patent/US20190226045A1/en
Priority to CN201780051736.3A priority patent/CN109642286B/en
Priority to EP17862905.1A priority patent/EP3486347B1/en
Priority to JP2017567271A priority patent/JP6304469B1/en
Priority to KR1020197005080A priority patent/KR102201004B1/en
Publication of WO2018074164A1 publication Critical patent/WO2018074164A1/en

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    • 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
    • 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
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • 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
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • 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
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • 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
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • 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
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • 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
    • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
    • 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
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/26—Methods of annealing
    • 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
    • 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

Definitions

  • the present invention relates to a ferritic stainless hot-rolled annealed steel sheet excellent in workability suitable for application to a flange or the like and a method for producing the same.
  • an exhaust gas recirculation (EGR) system in which exhaust gas generated from an automobile engine is used again as intake air for the engine is being applied. Exhaust gas generated from the engine is supplied to the engine again after passing through an EGR cooler for lowering the gas temperature.
  • EGR exhaust gas recirculation
  • each exhaust system component is fastened via a flange to prevent the exhaust gas from leaking.
  • the flange applied to such an exhaust system part needs to have sufficient rigidity. For this reason, a thick flange (for example, a plate thickness of 5 mm or more) is applied to such an exhaust system component.
  • Patent Document 1 in mass%, C: 0.015% or less, Si: 0.01 to 0.4%, Mn: 0.01 to 0.8%, P: 0.04% or less, S: 0.01% or less, Cr: 14.0 to less than 18.0%, Ni: 0.05 to 1%, Nb: 0.3 to 0.6%, Ti: 0.05% or less, N: 0.020% or less, Al: 0.10% or less, B: 0.0002 to 0.0020%, the balance being Fe and inevitable impurities, Nb, C and A ferritic stainless hot rolled steel sheet having a N content satisfying Nb / (C + N) ⁇ 16, a Charpy impact value at 0 ° C. of 10 J / cm 2 or more, and a plate thickness of 5.0 to 9.0 mm is disclosed. Has been.
  • the present invention provides a ferritic stainless steel hot-rolled annealed steel sheet that can solve such problems, has sufficient corrosion resistance, and can suppress cracking when punching into a thick flange, and a method for producing the same. Objective.
  • the inventors of the present invention have made a detailed study in order to solve the problem.
  • the workability is the Charpy impact value that has been used conventionally.
  • a Threshold Stress Intensity Factor K IC which is a toughness evaluation index in the field of thick plates. This is because with thin steel plates with a thickness of less than 5.0 mm, the plastic deformation region near the punched end surface during processing is larger than the plate thickness, so the fracture phenomenon associated with forming is uniquely handled by fracture mechanics.
  • the plastic deformation region near the punched end surface during processing sufficiently satisfies the small-scale yield state where the thickness is sufficiently small relative to the plate thickness. Therefore, it can be considered that the fracture phenomenon associated with the predetermined processing can be handled by the stress intensity factor, which is a quantitative index of fracture mechanics, and particularly the critical value, that is, the critical stress intensity factor K IC can be accurately evaluated. .
  • the present inventors have investigated in detail the relationship between the presence or absence of cracking and the limit stress intensity factor K IC when processing into a flange of a predetermined shape.
  • the critical stress intensity factor K IC 20 MPa ⁇ m 1/2 or more, it is possible to effectively suppress the occurrence of cracks in the burring part when processing into a thick flange having a burring part. It has been found that it can be sufficiently put into practical use for a thick flange having a burring portion.
  • the critical stress intensity factor K IC is improved. I found out.
  • component composition in mass%, Cu: 0.01 to 1.00%, Mo: 0.01 to 2.00%, W: 0.01 to 0.20%, Co: 0.01 The ferritic stainless steel hot-rolled annealed steel sheet according to the above [1], containing one or more selected from ⁇ 0.20%.
  • component composition in mass%, V: 0.01 to 0.20%, Nb: 0.01 to 0.10%, Zr: 0.01 to 0.20%, REM: 0.00.
  • a hot-rolled steel sheet is obtained by setting the pass to a temperature range of 800 to 1100 ° C. and the cumulative rolling reduction ratio of the final three passes to 25% or more, and the hot-rolled steel plate is further annealed at 800 to 1100 ° C.
  • the critical stress intensity factor K IC is taken from the center of the plate width from a CT (Compact Tension) test piece according to ASTM E399 so that the fatigue precrack is in the direction perpendicular to the rolling and the stress axis is in the direction parallel to the rolling. It refers to the stress intensity factor obtained by testing in accordance with ASTM E399.
  • a ferritic stainless steel hot-rolled annealed steel sheet having sufficient corrosion resistance and excellent workability capable of suppressing cracking when punching into a thick flange is obtained.
  • sufficient corrosion resistance in the present invention refers to a salt spray cycle test (salt spray (5 mass% NaCl) specified in JIS H8502 on a steel plate whose end face is sealed after polishing the surface to be evaluated with # 600 emery paper. , 35 ° C., spraying 2 hr) ⁇ drying (60 ° C., 4 hr, relative humidity 40%) ⁇ wet (50 ° C., 2 hr, relative humidity
  • the ferritic stainless steel hot-rolled annealed steel sheet of the present invention is, in mass%, C: 0.001 to 0.020%, Si: 0.05 to 1.00%, Mn: 0.05 to 1.00%, P : 0.04% or less, S: 0.01% or less, Al: 0.001 to 0.100%, Cr: 10.0 to 24.0%, Ni: 0.01 to 0.60%, Ti: 0.10 to 0.40%, N: 0.001 to 0.020%, the balance is composed of Fe and inevitable impurities, and the critical stress intensity factor K IC is 20 MPa ⁇ m 1 / 2 or more.
  • the critical stress intensity factor K IC was obtained by collecting CT specimens in accordance with ASTM E399 from the center of the plate width so that the fatigue precrack is in the direction perpendicular to the rolling and the stress axis is in the direction parallel to the rolling, and in accordance with ASTM E399. Refers to the stress intensity factor obtained by testing.
  • the present inventors examined in detail the relationship between the remarkable progress of the microcracks and the material properties. As a result, progress of microcracks have found that there is a tendency to occur as the critical stress intensity factor K IC of the steel sheet is small. Therefore, as a result of attempts to form the flange using various ferritic stainless steel hot-rolled annealed steel plates (thickness 5.0 mm), cracks due to the development of microcracks are limited stress intensity factors obtained by a predetermined measurement method. It has been found that K IC is particularly likely to occur in a steel sheet having a value of less than 20 MPa ⁇ m 1/2 .
  • the present inventors investigated in detail the crack part of said steel plate in order to clarify the cause of the small critical stress intensity factor K IC of the steel plate in which cracking occurred during forming on the flange. As a result, it was found that in the steel plate in which cracks occurred, the cracks generated in the vicinity of the center portion of the punched end face significantly progressed at the grain boundary in the vicinity of the center portion of the plate thickness.
  • the crystal grains of the portion where the cracks remarkably progressed are independent crystal grains, It was ascertained that so-called colonies (groups of crystal grains having similar crystal orientations) having substantially the same crystal orientation as adjacent crystal grains were formed.
  • a crystal grain has a crystal orientation different from that of an adjacent crystal grain, and when a crack propagates on the grain boundary, a grain boundary having a different orientation functions as an obstacle to crack propagation.
  • the present inventors have intensively studied a method for improving the critical stress intensity factor K IC in a ferritic stainless hot-rolled annealed steel sheet.
  • the final three passes in the hot rolling process in which finish rolling consisting of multiple passes is performed in the temperature range of 800 to 1100 ° C., and the cumulative reduction ratio of the final three passes.
  • the thickness of the ferritic stainless steel hot-rolled annealed steel sheet according to the present invention is not particularly limited. However, since it is desirable that the thickness be applicable to a thick flange, 5.0 mm or more is preferable, and 7.0 mm or more is preferable. More preferred. Moreover, the said plate
  • ferritic stainless steel hardly undergoes dynamic recrystallization (referred to as recrystallization during deformation) during hot rolling, and tends to cause recovery of processing strain due to rolling. Therefore, in the hot rolling according to the conventional technique, excessive recovery of the working strain introduced by rolling occurs, and the working strain cannot be effectively maintained until after hot rolling. As a result, the recrystallization sites become insufficient, the colonies are not effectively destroyed in the subsequent hot-rolled sheet annealing, and the predetermined critical stress intensity factor K IC cannot be obtained.
  • recrystallization during deformation dynamic recrystallization
  • the present inventors diligently studied a method for effectively and sufficiently introducing rolling distortions over the entire thickness of the steel sheet in the hot rolling process.
  • the final three passes of finish hot rolling are controlled within an appropriate temperature range, and rolling is performed at a large cumulative reduction rate, thereby suppressing the recovery of rolling distortion and reducing the rolling distortion to the center of the plate thickness.
  • Hot-rolled sheet annealing is a process of recrystallizing a processed structure formed by hot rolling. Therefore, it is necessary to perform annealing at a temperature at which sufficient recrystallization occurs.
  • hot-rolled sheet annealing is performed at an excessively high temperature, recrystallization occurs but recrystallized grains become extremely coarse. This remarkably coarse recrystallized grain is an independent single crystal grain, but the grain boundary length is remarkably long, so that the effect of suppressing crack growth by grain boundaries with different orientations is the same as when colonies existed. It was found that the predetermined critical stress intensity factor K IC could not be obtained.
  • the present inventors investigated in detail the relationship between the grain size of recrystallized grains and the annealing temperature. As a result, it has been found that by suppressing the hot-rolled sheet annealing temperature to 1100 ° C. or lower, it is possible to suppress the generation of coarse recrystallized grains so that the critical stress intensity factor K IC is significantly reduced.
  • component composition of the ferritic stainless steel hot-rolled annealed steel sheet of the present invention will be described.
  • % indicating the component composition means mass%.
  • the C content is in the range of 0.001 to 0.020%.
  • the C content is preferably 0.003% or more, and more preferably 0.004% or more. Further, the C content is preferably 0.015% or less, and more preferably 0.012% or less.
  • Si 0.05 to 1.00%
  • Si has an effect of concentrating on an oxide film formed at the time of welding to improve the corrosion resistance of the welded portion, and is also an element useful as a deoxidizing element in the steel making process. These effects are obtained by containing 0.05% or more of Si, and the effect increases as the content increases.
  • Si is contained in excess of 1.00%, the rolling load increases in the hot rolling process and a significant scale is generated.
  • the pickling property decreases due to the formation of the Si concentrated layer on the steel sheet surface layer.
  • the Si content is set to 0.05 to 1.00%.
  • the Si content is preferably 0.10% or more.
  • Si content becomes like this. Preferably it is 0.60% or less, More preferably, it is 0.40% or less.
  • Mn 0.05 to 1.00% Mn has the effect of increasing the strength of the steel and also acts as a deoxidizer. In order to obtain the effect, it is necessary to contain 0.05% or more of Mn. However, if the Mn content exceeds 1.00%, the generation of MnS that is the starting point of corrosion is promoted, and the corrosion resistance is lowered. Therefore, the Mn content is set to 0.05 to 1.00%.
  • the Mn content is preferably 0.10% or more. Further, the Mn content is preferably 0.60% or less, more preferably 0.30% or less.
  • P 0.04% or less
  • P is an element inevitably contained in steel. However, it is preferably reduced as much as possible because it is an element harmful to corrosion resistance and workability. In particular, when the P content exceeds 0.04%, workability is remarkably lowered due to solid solution strengthening. Therefore, the P content is 0.04% or less. Preferably, the P content is 0.03% or less.
  • S 0.01% or less S is an element inevitably contained in steel like P. However, it is preferably reduced as much as possible because it is an element harmful to corrosion resistance and workability. In particular, when the S content exceeds 0.01%, the corrosion resistance significantly decreases. Therefore, the S content is 0.01% or less. Preferably, the S content is 0.008% or less. More preferably, the S content is 0.003% or less.
  • Al 0.001 to 0.100%
  • Al is an effective deoxidizer. Furthermore, since Al has a stronger affinity for nitrogen than Cr, when nitrogen penetrates into the weld zone, it has the effect of precipitating nitrogen by precipitating nitrogen as Al nitride instead of Cr nitride. These effects can be obtained by containing 0.001% or more of Al. However, it is not preferable to contain Al exceeding 0.100% because the penetration property during welding is lowered and the welding workability is lowered. Therefore, the Al content is in the range of 0.001 to 0.100%. The Al content is preferably 0.005% or more, and more preferably 0.010% or more. Moreover, Al content becomes like this. Preferably it is 0.060% or less, More preferably, it is 0.040% or less.
  • Cr 10.0-24.0% Cr is the most important element for ensuring the corrosion resistance of stainless steel. If the content is less than 10.0%, sufficient corrosion resistance cannot be obtained in an automobile exhaust gas atmosphere. On the other hand, if the Cr content exceeds 24.0%, the toughness is remarkably reduced due to the formation of the ⁇ (sigma) phase, and in the present invention, the predetermined critical stress intensity factor K IC cannot be obtained. Therefore, the Cr content is in the range of 10.0 to 24.0%.
  • the Cr content is preferably 14.0% or more, more preferably 16.0% or more, and further preferably 17.0% or more. Moreover, Cr content becomes like this. Preferably it is 21.5% or less, More preferably, it is 19.5% or less, More preferably, it is 18.5% or less.
  • Ni 0.01 to 0.60%
  • Ni is an element that improves the corrosion resistance of stainless steel, and is an element that suppresses the progress of corrosion in a corrosive environment where a passive film is not formed and active dissolution occurs.
  • Ni is a strong austenite generating element, and has the effect of suppressing ferrite formation at the weld and suppressing sensitization due to precipitation of Cr carbonitride. This effect is obtained by containing 0.01% or more of Ni, and increases as the Ni content increases. However, when the Ni content exceeds 0.60%, workability is lowered and stress corrosion cracking is likely to occur. Furthermore, since Ni is an expensive element, an increase in the content of Ni causes an increase in manufacturing cost, which is not preferable. Therefore, the Ni content is set to 0.01 to 0.60%.
  • the Ni content is preferably 0.10% or more. Moreover, Ni content becomes like this. Preferably it is 0.50% or less, More preferably, it is 0.40% or less.
  • Ti 0.10 to 0.40%
  • Ti is an extremely important element. Ti preferentially binds to C and N, suppresses the precipitation of Cr carbonitride, lowers the recrystallization temperature, and suppresses the decrease in corrosion resistance due to sensitization due to the precipitation of Cr carbonitride There is. In order to obtain these effects, it is necessary to contain 0.10% or more of Ti. However, if the Ti content exceeds 0.40%, the solid solution Ti amount increases excessively, so the recrystallization temperature rises conversely, and the technique of the present invention cannot be applied. In addition, if Ti content exceeds 0.40%, coarse Ti carbonitrides are produced in the casting process and cause surface defects, which is not preferable in production.
  • the Ti content is set to 0.10 to 0.40%.
  • the Ti content is preferably 0.15% or more, and more preferably 0.20% or more. Moreover, Ti content becomes like this. Preferably it is 0.35% or less, More preferably, it is 0.30% or less. From the viewpoint of corrosion resistance of the weld zone, the Ti content satisfies the formula: Ti / (C + N) ⁇ 8 (in the formula, Ti, C, and N are the contents (mass%) of each element). It is preferable.
  • N 0.001 to 0.020%
  • the workability and the corrosion resistance of the welded portion are significantly reduced. From the viewpoint of corrosion resistance, the lower the N content, the better.
  • reducing the N content to less than 0.001% requires refining for a long time, which is not preferable because it causes an increase in manufacturing cost and a decrease in productivity. . Therefore, the N content is in the range of 0.001 to 0.020%.
  • the N content is preferably 0.005% or more, and more preferably 0.007% or more.
  • N content becomes like this. Preferably it is 0.015% or less, More preferably, it is 0.012% or less.
  • the present invention is a ferritic stainless steel characterized in that it contains the above-mentioned essential components and the balance consists of Fe and inevitable impurities. Furthermore, as required, one or more selected from Cu, Mo, W and Co, or / or one selected from V, Nb, Zr, REM, B, Mg and Ca. Or 2 or more types can be contained in the following range.
  • Cu 0.01 to 1.00%
  • Cu is an element particularly effective for improving the corrosion resistance of the base material and the welded part when an aqueous solution or weakly acidic water droplets adhere. This effect is obtained when the content is 0.01% or more, and the effect increases as the Cu content increases. However, when Cu is contained exceeding 1.00%, hot workability may be reduced and surface defects may be induced. In addition, descaling after annealing may be difficult. Therefore, when Cu is contained, the Cu content is preferably in the range of 0.01 to 1.00%.
  • the Cu content is more preferably 0.10% or more, and further preferably 0.30% or more. Further, the Cu content is more preferably 0.60% or less, and further preferably 0.45% or less.
  • Mo 0.01-2.00%
  • Mo is an element that remarkably improves the corrosion resistance of stainless steel. This effect is obtained when the content is 0.01% or more, and the effect improves as the content increases. However, if the Mo content exceeds 2.00%, the rolling load at the time of hot rolling increases, and the manufacturability may decrease, or the steel sheet strength may increase excessively. Moreover, since Mo is an expensive element, a large content increases the manufacturing cost. Therefore, when Mo is contained, the Mo content is preferably 0.01 to 2.00%.
  • the Mo content is more preferably 0.10% or more, and further preferably 0.30% or more. Moreover, Mo content becomes like this. More preferably, it is 1.40% or less, More preferably, it is 0.90% or less.
  • W 0.01-0.20% W, like Mo, has the effect of improving corrosion resistance. This effect is obtained by containing 0.01% or more of W. However, if it exceeds 0.20% and W is contained, the strength increases, and the productivity may decrease due to an increase in rolling load. Therefore, when W is contained, the W content is preferably in the range of 0.01 to 0.20%. The W content is more preferably 0.05% or more. Further, the W content is more preferably 0.15% or less.
  • Co 0.01-0.20%
  • Co is an element that improves toughness. This effect is obtained by containing 0.01% or more of Co.
  • the Co content exceeds 0.20%, workability may be reduced. Therefore, when Co is contained, the Co content is preferably in the range of 0.01 to 0.20%.
  • the Co content is more preferably 0.10% or less.
  • V 0.01-0.20%
  • V forms carbonitride with C and N, suppresses sensitization during welding and improves the corrosion resistance of the weld. This effect is obtained when the V content is 0.01% or more.
  • the V content is preferably 0.01 to 0.20%.
  • the V content is more preferably 0.03% or more. Further, the V content is more preferably 0.10% or less, and even more preferably 0.05% or less.
  • Nb 0.01 to 0.10%
  • Nb has the effect of improving the toughness of the steel sheet by refining crystal grains and dissolving in the matrix. These effects are obtained when the Nb content is 0.01% or more.
  • Nb also has an effect of increasing the recrystallization temperature. When the Nb content exceeds 0.10%, the annealing temperature necessary for causing sufficient recrystallization by hot-rolled sheet annealing becomes excessively high. As a result, recrystallized grains become extremely coarse as the crystal grain size reaches 300 ⁇ m or more during annealing, and a predetermined critical stress intensity factor K IC may not be obtained. Therefore, when Nb is contained, the Nb content is preferably in the range of 0.01 to 0.10%. The Nb content is more preferably 0.02% or more. Further, the Nb content is more preferably 0.05% or less.
  • Zr 0.01-0.20%
  • Zr has the effect of binding to C and N to suppress sensitization. This effect is obtained by containing 0.01% or more of Zr.
  • the Zr content exceeds 0.20%, the workability may be significantly reduced. Therefore, when Zr is contained, the Zr content is preferably in the range of 0.01 to 0.20%.
  • the Zr content is more preferably 0.02% or more. Further, the Zr content is more preferably 0.10% or less, and even more preferably 0.05% or less.
  • REM 0.001 to 0.100% REM (Rare Earth Metals) has an effect of improving the oxidation resistance, and suppresses formation of a Cr-deficient region immediately below the oxide film by suppressing formation of an oxide film (weld temper color) in the welded portion. This effect is acquired by containing REM 0.001% or more.
  • productivity such as pickling at the time of cold rolling annealing, may be reduced. Therefore, when REM is contained, the REM content is preferably in the range of 0.001 to 0.100%.
  • the REM content is more preferably 0.010% or more.
  • the REM content is more preferably 0.050% or less.
  • B 0.0002 to 0.0025%
  • B is an element effective for improving the secondary work brittleness resistance after molding. This effect is obtained by making the B content 0.0002% or more. On the other hand, if the B content exceeds 0.0025%, workability and toughness may be reduced. Therefore, when B is contained, the B content is preferably in the range of 0.0002 to 0.0025%. The B content is more preferably 0.0003% or more. Further, the B content is more preferably 0.0006% or less.
  • Mg 0.0005 to 0.0030%
  • Mg is an element that improves the equiaxed crystal ratio of the slab and is effective in improving workability and toughness. Further, in the steel containing Ti as in the present invention, when Ti carbonitride becomes coarser, the toughness decreases, but Mg also has an effect of suppressing the coarsening of Ti carbonitride. These effects can be obtained by containing 0.0005% or more of Mg. On the other hand, if the Mg content exceeds 0.0030%, the surface properties of the steel may be deteriorated. Therefore, when Mg is contained, the Mg content is preferably in the range of 0.0005 to 0.0030%. The Mg content is more preferably 0.0010% or more. The Mg content is more preferably 0.0020% or less.
  • Ca 0.0003 to 0.0030%
  • Ca is an effective component for preventing nozzle clogging due to crystallization of Ti-based inclusions that are likely to occur during continuous casting. The effect is acquired by containing 0.0003% or more of Ca. However, if the Ca content exceeds 0.0030%, the corrosion resistance may decrease due to the formation of CaS. Therefore, when Ca is contained, the Ca content is preferably in the range of 0.0003 to 0.0030%. The Ca content is more preferably 0.0005% or more. Moreover, Ca content becomes like this. More preferably, it is 0.0015% or less, More preferably, it is 0.0010% or less.
  • Limit stress intensity factor K IC 20 MPa ⁇ m 1/2 or more
  • the ferritic stainless steel hot rolled annealed steel sheet of the present invention has a limit stress intensity factor K IC of 20 MPa ⁇ m 1/2 or more, so that it becomes a thick flange. It is possible to suppress cracks during the punching process.
  • the critical stress intensity factor K IC is preferably 25 MPa ⁇ m 1/2 or more, more preferably 30 MPa ⁇ m 1/2 or more.
  • the thick flange is not particularly limited, and examples thereof include a flange having a thickness of 5.0 mm or more. As the flange, for example, a flange having a plate thickness of 5.0 to 15.0 mm is preferable, and a flange having a plate thickness of 5.0 to 10.0 mm is more preferable.
  • the temperature is the surface temperature measured with a surface thermometer such as a steel slab or hot-rolled steel sheet.
  • the ferritic stainless steel hot-rolled annealed steel sheet of the present invention uses a steel slab having the above composition, and in the hot rolling consisting of rough rolling and finishing rolling of 3 or more passes, the final 3 passes of finishing rolling are performed in a temperature range of 800. It is obtained by obtaining a hot-rolled steel sheet at a temperature of ⁇ 1100 ° C. and a cumulative reduction ratio of 25% or more in the final three passes, and subjecting the hot-rolled steel sheet to further hot-rolled sheet annealing at 800 to 1100 ° C.
  • the molten steel having the above component composition is melted by a known method such as a converter, electric furnace, vacuum melting furnace or the like, and is made into a steel material (slab) by a continuous casting method or an ingot-bundling method.
  • the slab is heated at 1100 to 1250 ° C. for 1 to 24 hours, or subjected to hot rolling at a stage where the temperature reaches 1100 to 1250 ° C. after casting without heating.
  • there is no particular limitation for rough rolling if the cast structure is effectively destroyed before finish hot rolling, it is superior to refinement of crystal grains in subsequent finish hot rolling.
  • Rolling temperature range for the final three passes of finish hot rolling 800-1100 ° C
  • Cumulative rolling reduction in the final three passes of finish hot rolling 25% or more
  • K IC critical stress intensity factor
  • the cumulative reduction ratio of the final three passes of finish hot rolling is set to 25% or more.
  • the cumulative rolling reduction is 30% or more. More preferably, the cumulative rolling reduction is 35% or more.
  • the upper limit of the cumulative rolling reduction is not particularly limited, but if the cumulative rolling reduction is excessively increased, the rolling load increases and the productivity decreases, and surface roughness may occur after rolling. It is preferable to do.
  • the rolling temperature in the final three passes of finish hot rolling is less than 800 ° C.
  • the rolling load increases remarkably as the steel plate temperature decreases, which is not preferable for production. Further, rolling at a low temperature may cause the surface roughness of the steel sheet to deteriorate the surface quality.
  • the rolling temperature of the final three passes exceeds 1100 ° C.
  • recovery of strain imparted by rolling occurs, and the recrystallization sites after the hot-rolled sheet annealing in the next step are insufficient.
  • the predetermined limit stress intensity factor K IC cannot be obtained. Therefore, the rolling temperature for the final three passes is in the range of 800 to 1100 ° C.
  • the rolling temperature in the final three passes is in the range of 800 to 1050 ° C. More preferably, the rolling temperature in the final three passes is in the range of 850 to 1000 ° C.
  • the rolling temperature range of the first pass among the final three passes is 950 to 1100 ° C., and this first pass
  • the rolling temperature range for the second pass to be performed next is preferably 925 to 1075 ° C.
  • the rolling temperature range for the third pass to be performed next to the second pass is preferably 875 to 1050 ° C.
  • the method for producing a ferritic stainless steel hot-rolled annealed steel sheet according to the present invention is characterized in that a large reduction is applied after controlling the temperature range in the final three passes of finishing hot rolling consisting of three or more passes. If rolling with a large reduction is performed over the final four passes or more, even if the cumulative reduction rate is the same, the reduction rate will be distributed to each pass, so the strain applied to the center of the plate thickness will be insufficient, and each pass Since the accumulated conveyance time increases, recovery during conveyance between each pass is promoted, and the effect of imparting strain is reduced.
  • the rolling temperature and the cumulative reduction ratio of the finish rolling are controlled to the final two passes or less, the rolling load is significantly increased and the productivity is lowered because the large reduction with the cumulative reduction ratio of 25% or more is performed in two passes. This is not preferable. Therefore, in the method for producing a ferritic stainless steel hot-rolled steel sheet according to the present invention, the rolling temperature and cumulative rolling reduction of the final three passes of finish rolling are controlled.
  • the manufacturing method of the ferritic stainless steel hot-rolled steel sheet of the present invention it is important to control the final three-pass rolling temperature and cumulative rolling reduction of the finish hot rolling, and if it is a finish rolling of three or more passes Any number of finishing rolls may be performed, but if the maximum number of passes is greater than 15 passes, the steel plate temperature is likely to decrease due to an increase in the number of contacts with the rolling roll, and the steel plate temperature is kept within a predetermined temperature range.
  • the maximum number of passes is preferably 15 passes or less because it may lead to a decrease in manufacturability or an increase in manufacturing costs, such as heating from the outside required for maintenance. More preferably, the maximum number of paths is 10 paths or less.
  • the steel sheet After finishing hot rolling, the steel sheet is cooled, and then the steel sheet is wound to form a hot-rolled steel strip.
  • the coiling temperature is not particularly limited, but when the coiling temperature is more than 450 ° C. to less than 500 ° C., embrittlement due to 475 ° C. embrittlement may occur. Therefore, the winding temperature is preferably 450 ° C. or lower or 500 ° C. or higher.
  • Hot-rolled sheet annealing temperature 800-1100 ° C
  • hot-rolled sheet annealing is performed after the hot rolling step.
  • the rolled structure formed in the hot rolling process is recrystallized.
  • the rolling strain is effectively applied in the hot rolling process, and the recrystallization sites are increased, thereby promoting the destruction of colonies in the hot-rolled sheet annealing.
  • the annealing temperature is less than 800 ° C., recrystallization is insufficient, and a predetermined critical stress intensity factor K IC cannot be obtained.
  • the hot-rolled sheet annealing temperature is in the range of 800 to 1100 ° C.
  • the hot-rolled steel sheet subjected to such hot-rolled sheet annealing has the above-described component composition, and has a critical stress intensity factor K IC of 20 MPa ⁇ m 1/2 or more.
  • the hot-rolled sheet annealing temperature is in the range of 800 to 1050 ° C.
  • the hot-rolled sheet annealing temperature is in the range of 850 to 1000 ° C.
  • maintenance time and method of hot-rolled sheet annealing You may implement by any of box annealing (batch annealing) and continuous annealing.
  • the obtained hot-rolled annealed steel sheet may be descaled by shot blasting or pickling as necessary. Furthermore, in order to improve the surface properties, grinding or polishing may be performed. In addition, the hot-rolled annealed steel sheet provided by the present invention may be subsequently subjected to cold rolling and cold-rolled sheet annealing.
  • a molten stainless steel having the chemical composition shown in Table 1 is melted by refining a converter with a capacity of 150 ton and strong stirring and vacuum oxygen decarburization (SS-VOD), and a steel slab having a width of 1000 mm and a thickness of 200 mm by continuous casting.
  • SS-VOD strong stirring and vacuum oxygen decarburization
  • the slab was heated at 1200 ° C. for 1 h, and then hot-rolled by reverse rough rolling using a three-stage stand to obtain a steel plate of about 40 mm, and then the final three passes of final rolling consisting of 7 passes ( The fifth pass, the sixth pass, and the seventh pass) were performed under the conditions shown in Table 2 to obtain hot-rolled steel sheets.
  • No. No. 31 was subjected to hot rolling after heating the slab at 1300 ° C. for 1 h.
  • the obtained hot-rolled steel sheet was subjected to hot-rolled sheet annealing by box annealing under the conditions shown in Table 2 to obtain a hot-rolled anne
  • the obtained hot-rolled annealed steel sheet was evaluated as follows.
  • Photograph the evaluation surface of the test piece after 5 cycles of the salt spray cycle test measure the rusting area of the evaluation surface of the test piece by image analysis, and determine the rusting rate ((test Rust area in the piece / total area of the test piece) ⁇ 100 [%]) was calculated.
  • a rusting rate of 10% or less was evaluated as being particularly excellent with respect to corrosion resistance ()), more than 10% being 25% or less, passing (O), and more than 25% being rejecting (X).
  • test results are shown in Table 2 together with hot rolling and hot rolled sheet annealing conditions.
  • No. No. 31 is an example in which the slab was heated at 1300 ° C. for 1 h and then subjected to hot rolling, and the rolling temperature range in the final three passes of finish hot rolling was all over 1100 ° C. No. In No. 31, the recovery of excessive working strain occurred during the rolling of the final three passes and the recrystallization sites became insufficient, so that colonies remained after hot-rolled sheet annealing, and the predetermined critical stress intensity factor K IC was It was not obtained.
  • No. of the rolling temperature range for the final 3 passes is less than the range of the present invention for all 3 passes.
  • the rolling load increased remarkably, and the rolling exceeded the allowable range during the rolling of the final third pass, so that the rolling could not be completed and the predetermined evaluation could not be performed.
  • the ferritic stainless steel hot-rolled annealed steel sheet obtained by the present invention is particularly suitable for applications that require high workability and corrosion resistance, such as a flange having a burring portion.

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Abstract

Provided are: a hot-rolled and annealed ferritic stainless steel sheet that has sufficient corrosion resistance and that can be suppressed from cracking when punched into a thick-walled flange; and a method for producing the same. This hot-rolled and annealed ferritic stainless steel sheet is characterized by: having a component composition comprising, in mass%, 0.001 to 0.020% C, 0.05 to 1.00% Si, 0.05 to 1.00% Mn, at most 0.04% P, at most 0.01% S, 0.001 to 0.100% Al, 10.0 to 24.0% Cr, 0.01 to 0.60% Ni, 0.10 to 0.40% Ti, and 0.001 to 0.020% N, with the remainder comprising Fe and inevitable impurities; and having a threshold stress intensity factor KIC of at least 20 MPa·m1/2.

Description

フェライト系ステンレス熱延焼鈍鋼板およびその製造方法Ferritic stainless steel hot rolled annealed steel sheet and method for producing the same
　本発明は、フランジ等への適用に好適な加工性に優れたフェライト系ステンレス熱延焼鈍鋼板およびその製造方法に関するものである。 The present invention relates to a ferritic stainless hot-rolled annealed steel sheet excellent in workability suitable for application to a flange or the like and a method for producing the same.
　近年、自動車における排気ガスに関する法規制の強化が進んでおり、燃費の向上が急務となっている。そこで、自動車エンジンから生じた排気ガスを再度エンジンの吸気として用いる排気ガス再循環（Ｅｘｈａｕｓｔ　Ｇａｓ　Ｒｅｃｉｒｃｕｌａｔｉｏｎ、ＥＧＲ）システムの適用が進んでいる。エンジンから生じた排気ガスは、ガス温度を下げるためのＥＧＲクーラーを通過した後に再度エンジンに供給される。排気ガスを循環させるにあたって、各排気系部品は排気ガスの漏洩を防ぐためにフランジを介して締結される。このような排気系部品に適用されるフランジは十分な剛性を有する必要がある。このことから、このような排気系部品には厚肉（例えば板厚で５ｍｍ以上）のフランジが適用されている。 In recent years, regulations on exhaust gas in automobiles have been strengthened, and improvement in fuel efficiency has become an urgent issue. Therefore, an exhaust gas recirculation (EGR) system in which exhaust gas generated from an automobile engine is used again as intake air for the engine is being applied. Exhaust gas generated from the engine is supplied to the engine again after passing through an EGR cooler for lowering the gas temperature. When the exhaust gas is circulated, each exhaust system component is fastened via a flange to prevent the exhaust gas from leaking. The flange applied to such an exhaust system part needs to have sufficient rigidity. For this reason, a thick flange (for example, a plate thickness of 5 mm or more) is applied to such an exhaust system component.
　従来、厚肉のフランジには普通鋼が用いられてきた。しかし、ＥＧＲシステムのような高温の排気ガスが通過する部品に適用するフランジには十分な耐食性が求められる。そのため、普通鋼に比べて耐食性に優れるステンレス鋼、特に熱膨張率が比較的小さく熱応力が発生しにくいフェライト系ステンレス鋼の適用が検討されており、厚肉のフランジに適用可能な板厚の大きい（例えば板厚で５ｍｍ以上）フェライト系ステンレス鋼板が強く求められている。 Conventionally, plain steel has been used for thick flanges. However, a flange applied to a part through which high-temperature exhaust gas passes, such as an EGR system, is required to have sufficient corrosion resistance. Therefore, the application of stainless steel, which has superior corrosion resistance compared to ordinary steel, especially ferritic stainless steel, which has a relatively low coefficient of thermal expansion and is unlikely to generate thermal stress, has been studied. There is a strong demand for a ferritic stainless steel sheet having a large thickness (for example, a thickness of 5 mm or more).
　このような市場要求に対し、例えば、特許文献１には、質量％で、Ｃ：０．０１５％以下、Ｓｉ：０．０１～０．４％、Ｍｎ：０．０１～０．８％、Ｐ：０．０４％以下、Ｓ：０．０１％以下、Ｃｒ：１４．０～１８．０％未満、Ｎｉ：０．０５～１％、Ｎｂ：０．３～０．６％、Ｔｉ：０．０５％以下、Ｎ：０．０２０％以下、Ａｌ：０．１０％以下、Ｂ：０．０００２～０．００２０％を含有し、残部がＦｅ及び不可避的不純物であり、Ｎｂ、ＣおよびＮの含有量がＮｂ／（Ｃ＋Ｎ）≧１６を満たし、０℃におけるシャルピー衝撃値が１０Ｊ／ｃｍ２以上であり、板厚が５．０～９．０ｍｍであるフェライト系ステンレス熱延鋼板が開示されている。 In response to such market demand, for example, in Patent Document 1, in mass%, C: 0.015% or less, Si: 0.01 to 0.4%, Mn: 0.01 to 0.8%, P: 0.04% or less, S: 0.01% or less, Cr: 14.0 to less than 18.0%, Ni: 0.05 to 1%, Nb: 0.3 to 0.6%, Ti: 0.05% or less, N: 0.020% or less, Al: 0.10% or less, B: 0.0002 to 0.0020%, the balance being Fe and inevitable impurities, Nb, C and A ferritic stainless hot rolled steel sheet having a N content satisfying Nb / (C + N) ≧ 16, a Charpy impact value at 0 ° C. of 10 J / cm 2 or more, and a plate thickness of 5.0 to 9.0 mm is disclosed. Has been.
国際公開第２０１４／１５７５７６号International Publication No. 2014/157576
　しかし、本発明者らが特許文献１に記載されるフェライト系ステンレス熱延鋼板を用いてバーリング加工部を有する厚肉のフランジ形状への加工を試みたところ、上記の鋼板が十分なシャルピー衝撃値を有していたにも関わらず、バーリング加工部の特に板厚中央部に割れが生じ、所定のフランジ形状を得ることができない場合があり、厚肉のフランジに適用するには十分ではないことが明らかとなった。 However, when the present inventors tried processing into a thick flange shape having a burring portion using the ferritic stainless hot-rolled steel sheet described in Patent Document 1, the above steel sheet has a sufficient Charpy impact value. In spite of the fact that the burring part has cracks, especially in the central part of the plate thickness, it may not be possible to obtain the prescribed flange shape, which is not sufficient for application to thick flanges. Became clear.
　本発明は、かかる課題を解決し、十分な耐食性を有するとともに、厚肉のフランジへの打ち抜き加工をする際の割れを抑制可能なフェライト系ステンレス熱延焼鈍鋼板およびその製造方法を提供することを目的とする。 The present invention provides a ferritic stainless steel hot-rolled annealed steel sheet that can solve such problems, has sufficient corrosion resistance, and can suppress cracking when punching into a thick flange, and a method for producing the same. Objective.
　本発明者らは、課題を解決するために詳細な検討を行った。その結果、板厚が５．０ｍｍを超える厚肉の鋼板を、割れを発生させることなくバーリング加工部を有する厚肉のフランジへ成形する場合、その加工性は、従来用いられてきたシャルピー衝撃値では正確な評価ができないが、厚板分野の靭性評価指標である限界応力拡大係数(Threshold Stress Intensity Factor)ＫＩＣで正確に評価できることを見出した。これは、板厚が５．０ｍｍ未満の薄鋼板では、加工時の打ち抜き端面部近傍の塑性変形領域が板厚に対して大きいために、成形に伴う破壊現象を破壊力学的な取り扱いで一義的に整理できないのに対し、板厚が５．０ｍｍ以上の厚肉の鋼板では、加工時の打ち抜き端面近傍部の塑性変形領域が板厚に対して十分に小さくなる小規模降伏状態を十分に満足するために、所定の加工に伴う破壊現象を破壊力学的な定量指標である応力拡大係数で扱うことができ、特にその限界値、すなわち限界応力拡大係数ＫＩＣで正確に評価できるためと考えられる。 The inventors of the present invention have made a detailed study in order to solve the problem. As a result, when a thick steel plate with a thickness exceeding 5.0 mm is formed into a thick flange having a burring portion without causing cracks, the workability is the Charpy impact value that has been used conventionally. However, it has been found that although it cannot be accurately evaluated, it can be accurately evaluated by a Threshold Stress Intensity Factor K IC which is a toughness evaluation index in the field of thick plates. This is because with thin steel plates with a thickness of less than 5.0 mm, the plastic deformation region near the punched end surface during processing is larger than the plate thickness, so the fracture phenomenon associated with forming is uniquely handled by fracture mechanics. On the other hand, in the case of thick steel plates with a thickness of 5.0 mm or more, the plastic deformation region near the punched end surface during processing sufficiently satisfies the small-scale yield state where the thickness is sufficiently small relative to the plate thickness. Therefore, it can be considered that the fracture phenomenon associated with the predetermined processing can be handled by the stress intensity factor, which is a quantitative index of fracture mechanics, and particularly the critical value, that is, the critical stress intensity factor K IC can be accurately evaluated. .
　以上のことから、本発明者らは所定形状のフランジへ加工した場合の割れの発生有無と限界応力拡大係数ＫＩＣとの関係を詳細に調査した。その結果、限界応力拡大係数ＫＩＣを２０ＭＰａ・ｍ１／２以上にすることで、バーリング加工部を有する厚肉フランジへ加工する際のバーリング加工部での割れの発生を効果的に抑制することができ、バーリング加工部を有する厚肉のフランジへ十分に実用化できることを知見した。 From the above, the present inventors have investigated in detail the relationship between the presence or absence of cracking and the limit stress intensity factor K IC when processing into a flange of a predetermined shape. As a result, by making the critical stress intensity factor K IC 20 MPa · m 1/2 or more, it is possible to effectively suppress the occurrence of cracks in the burring part when processing into a thick flange having a burring part. It has been found that it can be sufficiently put into practical use for a thick flange having a burring portion.
　そして、適切な成分のフェライト系ステンレス鋼に対して、特に３パス以上の多パスからなる仕上熱間圧延工程における最終３パスの累積圧下率（＝１００－（最終板厚／最終３パスの圧延開始前の板厚）×１００［％］）を適切に制御して得られた熱延鋼板に対して、適切な温度で熱延板焼鈍を行うことにより、限界応力拡大係数ＫＩＣが向上することを知見した。 For the ferritic stainless steel of an appropriate component, the cumulative reduction ratio of the final three passes (= 100− (final plate thickness / final three-pass rolling) in the finishing hot rolling process, particularly comprising three or more passes. By performing hot-rolled sheet annealing at an appropriate temperature on a hot-rolled steel sheet obtained by appropriately controlling the sheet thickness before start) × 100 [%]), the critical stress intensity factor K IC is improved. I found out.
　本発明は以上の知見に基づいてなされたものであり、以下を要旨とするものである。
［１］質量％で、Ｃ：０．００１～０．０２０％、Ｓｉ：０．０５～１．００％、Ｍｎ：０．０５～１．００％、Ｐ：０．０４％以下、Ｓ：０．０１％以下、Ａｌ：０．００１～０．１００％、Ｃｒ：１０．０～２４．０％、Ｎｉ：０．０１～０．６０％、Ｔｉ：０．１０～０．４０％、Ｎ：０．００１～０．０２０％を含有し、残部がＦｅおよび不可避的不純物からなる成分組成を有し、
　限界応力拡大係数ＫＩＣが２０ＭＰａ・ｍ１／２以上であることを特徴とするフェライト系ステンレス熱延焼鈍鋼板。
［２］成分組成として、質量％で、さらにＣｕ：０．０１～１．００％、Ｍｏ：０．０１～２．００％、Ｗ：０．０１～０．２０％、Ｃｏ：０．０１～０．２０％のうちから選ばれる１種または２種以上を含有することを特徴とする前記［１］に記載のフェライト系ステンレス熱延焼鈍鋼板。
［３］成分組成として、質量％で、さらに、Ｖ：０．０１～０．２０％、Ｎｂ：０．０１～０．１０％、Ｚｒ：０．０１～０．２０％、ＲＥＭ：０．００１～０．１００％、Ｂ：０．０００２～０．００２５％、Ｍｇ：０．０００５～０．００３０％、Ｃａ：０．０００３～０．００３０％のうちから選ばれる１種または２種以上を含有することを特徴とする前記［１］または［２］に記載のフェライト系ステンレス熱延焼鈍鋼板。
［４］前記［１］～［３］のいずれかに記載のフェライト系ステンレス熱延焼鈍鋼板の製造方法であって、３パス以上の仕上圧延を行う熱間圧延工程で、仕上圧延の最終３パスを温度範囲８００～１１００℃、且つ前記最終３パスの累積圧下率を２５％以上として熱延鋼板を得て、該熱延鋼板に対してさらに８００～１１００℃で熱延板焼鈍を行うことを特徴とするフェライト系ステンレス熱延焼鈍鋼板の製造方法。 This invention is made | formed based on the above knowledge, and makes the following a summary.
[1] By mass%, C: 0.001 to 0.020%, Si: 0.05 to 1.00%, Mn: 0.05 to 1.00%, P: 0.04% or less, S: 0.01% or less, Al: 0.001 to 0.100%, Cr: 10.0 to 24.0%, Ni: 0.01 to 0.60%, Ti: 0.10 to 0.40%, N: 0.001 to 0.020% is contained, and the balance has a component composition consisting of Fe and inevitable impurities,
A ferritic stainless steel hot-rolled annealed steel sheet having a critical stress intensity factor K IC of 20 MPa · m 1/2 or more.
[2] As component composition, in mass%, Cu: 0.01 to 1.00%, Mo: 0.01 to 2.00%, W: 0.01 to 0.20%, Co: 0.01 The ferritic stainless steel hot-rolled annealed steel sheet according to the above [1], containing one or more selected from ˜0.20%.
[3] As component composition, in mass%, V: 0.01 to 0.20%, Nb: 0.01 to 0.10%, Zr: 0.01 to 0.20%, REM: 0.00. One or more selected from 001 to 0.100%, B: 0.0002 to 0.0025%, Mg: 0.0005 to 0.0030%, Ca: 0.0003 to 0.0030% The ferritic stainless steel hot-rolled annealed steel sheet according to [1] or [2], characterized in that
[4] The method for producing a ferritic stainless steel hot-rolled annealed steel sheet according to any one of [1] to [3], wherein in the hot rolling process in which finish rolling is performed for 3 passes or more, the final 3 of finish rolling is performed. A hot-rolled steel sheet is obtained by setting the pass to a temperature range of 800 to 1100 ° C. and the cumulative rolling reduction ratio of the final three passes to 25% or more, and the hot-rolled steel plate is further annealed at 800 to 1100 ° C. A method for producing a ferritic stainless hot-rolled annealed steel sheet.
　ここで、限界応力拡大係数ＫＩＣは、板幅中央部からＡＳＴＭ　Ｅ３９９に準拠したＣＴ(Compact Tension)試験片を、疲労予き裂が圧延直角方向、応力軸が圧延平行方向となるように採取し、ＡＳＴＭ　Ｅ３９９に準拠して試験することにより得られる応力拡大係数のことを指す。 Here, the critical stress intensity factor K IC is taken from the center of the plate width from a CT (Compact Tension) test piece according to ASTM E399 so that the fatigue precrack is in the direction perpendicular to the rolling and the stress axis is in the direction parallel to the rolling. It refers to the stress intensity factor obtained by testing in accordance with ASTM E399.
　本発明によれば、十分な耐食性を有するとともに、厚肉のフランジへの打ち抜き加工をする際の割れを抑制可能な加工性に優れるフェライト系ステンレス熱延焼鈍鋼板が得られる。 According to the present invention, a ferritic stainless steel hot-rolled annealed steel sheet having sufficient corrosion resistance and excellent workability capable of suppressing cracking when punching into a thick flange is obtained.
　なお、本発明における十分な耐食性とは、評価する表面を＃６００エメリーペーパーにより研磨仕上げした後に端面部をシールした鋼板にＪＩＳ　Ｈ　８５０２に規定された塩水噴霧サイクル試験（塩水噴霧（５質量％ＮａＣｌ、３５℃、噴霧２ｈｒ）→乾燥（６０℃、４ｈｒ、相対湿度４０％）→湿潤（５０℃、２ｈｒ、相対湿度≧９５％））を１サイクルとする試験）を５サイクル行った場合の鋼板の評価面における発錆面積率（＝発錆面積／鋼板全面積×１００［％］）が２５％以下であることを意味する。 In addition, sufficient corrosion resistance in the present invention refers to a salt spray cycle test (salt spray (5 mass% NaCl) specified in JIS H8502 on a steel plate whose end face is sealed after polishing the surface to be evaluated with # 600 emery paper. , 35 ° C., spraying 2 hr) → drying (60 ° C., 4 hr, relative humidity 40%) → wet (50 ° C., 2 hr, relative humidity ≧ 95%)))) This means that the rust area ratio (= rust area / total area of steel plate × 100 [%]) on the evaluation surface is 25% or less.
　また、厚肉のフランジへの打ち抜き加工をする際の割れを抑制可能な加工性に優れるとは、板幅中央部から、ＡＳＴＭ　Ｅ３９９に準拠したＣＴ試験片を、疲労予き裂が圧延直角方向、応力軸が圧延平行方向となるように採取し、ＡＳＴＭ　Ｅ３９９に準拠して試験することにより得られる限界応力拡大係数ＫＩＣが２０ＭＰａ・ｍ１／２以上であることを指す。 In addition, it is said that it is excellent in workability capable of suppressing cracking when punching into a thick-walled flange. From the center of the plate width, a CT test piece in accordance with ASTM E399 is used, and the fatigue precrack is perpendicular to the rolling direction. The critical stress intensity factor K IC obtained by taking the stress axis in the rolling parallel direction and testing in accordance with ASTM E399 is 20 MPa · m 1/2 or more.
　本発明のフェライト系ステンレス熱延焼鈍鋼板は、質量％で、Ｃ：０．００１～０．０２０％、Ｓｉ：０．０５～１．００％、Ｍｎ：０．０５～１．００％、Ｐ：０．０４％以下、Ｓ：０．０１％以下、Ａｌ：０．００１～０．１００％、Ｃｒ：１０．０～２４．０％、Ｎｉ：０．０１～０．６０％、Ｔｉ：０．１０～０．４０％、Ｎ：０．００１～０．０２０％を含有し、残部がＦｅおよび不可避的不純物からなる成分組成を有し、限界応力拡大係数ＫＩＣが２０ＭＰａ・ｍ１／２以上である。 The ferritic stainless steel hot-rolled annealed steel sheet of the present invention is, in mass%, C: 0.001 to 0.020%, Si: 0.05 to 1.00%, Mn: 0.05 to 1.00%, P : 0.04% or less, S: 0.01% or less, Al: 0.001 to 0.100%, Cr: 10.0 to 24.0%, Ni: 0.01 to 0.60%, Ti: 0.10 to 0.40%, N: 0.001 to 0.020%, the balance is composed of Fe and inevitable impurities, and the critical stress intensity factor K IC is 20 MPa · m 1 / 2 or more.
　限界応力拡大係数ＫＩＣは、板幅中央部からＡＳＴＭ　Ｅ３９９に準拠したＣＴ試験片を、疲労予き裂が圧延直角方向、応力軸が圧延平行方向となるように採取し、ＡＳＴＭ　Ｅ３９９に準拠して試験することにより得られる応力拡大係数のことを指す。 The critical stress intensity factor K IC was obtained by collecting CT specimens in accordance with ASTM E399 from the center of the plate width so that the fatigue precrack is in the direction perpendicular to the rolling and the stress axis is in the direction parallel to the rolling, and in accordance with ASTM E399. Refers to the stress intensity factor obtained by testing.
　以下、本発明を詳細に説明する。 Hereinafter, the present invention will be described in detail.
　本発明者らは、板厚５．０ｍｍの各種フェライト系ステンレス鋼板を用いて、３０ｍｍφのフランジ孔部をブランクまま（打ち抜いたまま）の鋼板表面から１０ｍｍ持ち上げるバーリング加工部を有するフランジへ成形した際に割れが発生した原因について詳細に検討した。その結果、割れが発生した上記の鋼板では、打ち抜き端面の板厚中央部近傍に生じた微小亀裂がバーリング加工において著しく進展したために割れが生じていることを突き止めた。 When the present inventors formed various ferritic stainless steel plates having a thickness of 5.0 mm into a flange having a burring portion that lifts a 30 mmφ flange hole from the steel plate surface as blank (punched) 10 mm. The cause of the occurrence of cracks was examined in detail. As a result, it was found that in the above-described steel plate in which cracking occurred, the micro crack generated in the vicinity of the central portion of the punched end face significantly progressed in the burring process, so that cracking occurred.
　本発明者らは、この微小亀裂の著しい進展と材料特性の関係を詳細に検討した。その結果、微小亀裂の進展は鋼板の限界応力拡大係数ＫＩＣが小さいほど生じやすい傾向があることを突き止めた。そこで、種々のフェライト系ステンレス熱延焼鈍鋼板（板厚５.０ｍｍ）を用いて該フランジへの成形を試みた結果、微小亀裂の進展による割れは、所定の測定方法で得られる限界応力拡大係数ＫＩＣが２０ＭＰａ・ｍ１／２を下回った鋼板で特に生じやすいことを知見した。 The present inventors examined in detail the relationship between the remarkable progress of the microcracks and the material properties. As a result, progress of microcracks have found that there is a tendency to occur as the critical stress intensity factor K IC of the steel sheet is small. Therefore, as a result of attempts to form the flange using various ferritic stainless steel hot-rolled annealed steel plates (thickness 5.0 mm), cracks due to the development of microcracks are limited stress intensity factors obtained by a predetermined measurement method. It has been found that K IC is particularly likely to occur in a steel sheet having a value of less than 20 MPa · m 1/2 .
　さらに、本発明者らは、該フランジへの成形時に割れが生じた鋼板の限界応力拡大係数ＫＩＣが小さい原因を明らかにするため、上記の鋼板の割れ部を詳細に調査した。その結果、割れが生じた鋼板では、打ち抜き端面の板厚中央部近傍に発生した亀裂が板厚中央部近傍の結晶粒界において著しく進展していることを突き止めた。 Furthermore, the present inventors investigated in detail the crack part of said steel plate in order to clarify the cause of the small critical stress intensity factor K IC of the steel plate in which cracking occurred during forming on the flange. As a result, it was found that in the steel plate in which cracks occurred, the cracks generated in the vicinity of the center portion of the punched end face significantly progressed at the grain boundary in the vicinity of the center portion of the plate thickness.
　そして、上記の鋼板の組織をＳＥＭ(Scanning Electron Microscopy)／ＥＢＳＤ(Electron Backscatter Diffraction)法により調査および解析した結果、亀裂が著しく進展した部位の結晶粒は、それぞれが独立した結晶粒ではあるものの、隣接する結晶粒とほぼ同じ結晶方位を有する、いわゆるコロニー（類似結晶方位を有する結晶粒群）を形成していることを突き止めた。一般に、結晶粒は隣接する結晶粒と異なる結晶方位を有しており、亀裂が粒界上を進展するに際して方位の異なる粒界が亀裂進展の障害として機能する。しかし、コロニーでは隣接する結晶粒の結晶方位がほぼ等しいために、コロニー内の各結晶粒間の粒界による亀裂進展の抑制効果が小さくなることで、コロニーが形成されている鋼板では限界応力拡大係数ＫＩＣが低下し、該フランジへの成形時に割れが生じたことを突き止めた。 And, as a result of investigating and analyzing the structure of the steel sheet by SEM (Scanning Electron Microscopy) / EBSD (Electron Backscatter Diffraction) method, the crystal grains of the portion where the cracks remarkably progressed are independent crystal grains, It was ascertained that so-called colonies (groups of crystal grains having similar crystal orientations) having substantially the same crystal orientation as adjacent crystal grains were formed. Generally, a crystal grain has a crystal orientation different from that of an adjacent crystal grain, and when a crack propagates on the grain boundary, a grain boundary having a different orientation functions as an obstacle to crack propagation. However, since the crystal orientation of adjacent crystal grains in a colony is almost equal, the effect of suppressing crack growth due to the grain boundary between each crystal grain in the colony is reduced, and the steel sheet with a colony has an increased critical stress. The coefficient K IC decreased, and it was ascertained that cracks occurred during molding on the flange.
　そこで、本発明者らは、フェライト系ステンレス熱延焼鈍鋼板において限界応力拡大係数ＫＩＣを向上させる手法について鋭意検討した。その結果、適切な成分のフェライト系ステンレス鋼に対して、特に多パスからなる仕上圧延を行う熱間圧延工程の最終３パスを８００～１１００℃の温度範囲で、かつ最終３パスの累積圧下率（＝１００－（最終板厚／最終３パスの圧延開始前の板厚）×１００［％］）が２５％以上となるように適切に制御して得られた熱延鋼板に対して、８００～１１００℃で熱延板焼鈍を行うことにより、コロニーが効果的に破壊され、２０ＭＰａ・ｍ１／２以上の限界応力拡大係数ＫＩＣが得られることを知見した。 Therefore, the present inventors have intensively studied a method for improving the critical stress intensity factor K IC in a ferritic stainless hot-rolled annealed steel sheet. As a result, for the ferritic stainless steel having an appropriate component, the final three passes in the hot rolling process in which finish rolling consisting of multiple passes is performed in the temperature range of 800 to 1100 ° C., and the cumulative reduction ratio of the final three passes. (= 100− (final plate thickness / plate thickness before starting the final 3 pass rolling) × 100 [%]) is appropriately controlled to be 25% or more, with respect to a hot-rolled steel plate obtained by appropriately controlling 800 It was found that by performing hot-rolled sheet annealing at ˜1100 ° C., colonies were effectively destroyed and a critical stress intensity factor K IC of 20 MPa · m 1/2 or more was obtained.
　なお、本発明のフェライト系ステンレス熱延焼鈍鋼板の板厚は、特に限定されないが、厚肉のフランジに適用できる板厚であることが望ましいため、５．０ｍｍ以上が好ましく、７．０ｍｍ以上がより好ましい。また、前記板厚は、特に限定されないが、１５．０ｍｍ以下が好ましく、１０．０ｍｍ以下がより好ましい。 The thickness of the ferritic stainless steel hot-rolled annealed steel sheet according to the present invention is not particularly limited. However, since it is desirable that the thickness be applicable to a thick flange, 5.0 mm or more is preferable, and 7.0 mm or more is preferable. More preferred. Moreover, the said plate | board thickness is although it does not specifically limit, 15.0 mm or less is preferable and 10.0 mm or less is more preferable.
　上記手法によりコロニーの破壊が促進される理由について以下に説明する。 The reason why destruction of colonies is promoted by the above method will be described below.
　フェライト系ステンレス鋼の熱間圧延前のスラブの板厚中央部には、粗大かつ展伸したコロニー（類似結晶方位を有する結晶粒群）が鋳造方向に沿って連なって分布している。一方、鋼板を圧延した場合、鋼板は表層部から変形して伸長する。そのため、圧下率が小さい場合には板厚中央部の変形量が小さくなり、板厚中央部に圧延ひずみがほとんど導入されない。 In the central portion of the slab before hot rolling of ferritic stainless steel, coarse and expanded colonies (a group of crystal grains having similar crystal orientations) are continuously distributed along the casting direction. On the other hand, when a steel plate is rolled, the steel plate deforms from the surface layer portion and extends. For this reason, when the rolling reduction is small, the amount of deformation in the central portion of the plate thickness is small, and almost no rolling strain is introduced into the central portion of the plate thickness.
　そのため、従来技術による熱間圧延では、鋼板の板厚中央部の展伸粒に圧延ひずみが十分に導入されず、その後の熱延板焼鈍における再結晶サイトが不足し、熱延板焼鈍時に板厚中央付近では再結晶は生じるもののコロニーが分断されずに残存しやすく、本発明が必要とする２０ＭＰａ・ｍ１／２以上の限界応力拡大係数ＫＩＣが得られない。 Therefore, in the hot rolling according to the prior art, rolling strain is not sufficiently introduced into the expanded grain at the center of the plate thickness of the steel sheet, and there are insufficient recrystallization sites in the subsequent hot-rolled sheet annealing. Although recrystallization occurs in the vicinity of the thickness center, colonies tend to remain without being divided, and the critical stress intensity factor K IC of 20 MPa · m 1/2 or more required by the present invention cannot be obtained.
　さらに、フェライト系ステンレス鋼は熱間圧延において動的再結晶（加工変形中における再結晶を言う）がほとんど生じず、圧延による加工ひずみの回復が生じやすい傾向がある。そのため、従来技術による熱間圧延では圧延によって導入された加工ひずみの過度な回復が生じて加工ひずみを熱間圧延後まで効果的に維持することができない。その結果、再結晶サイトが不十分となり次工程の熱延板焼鈍においてコロニーが効果的に破壊されず、所定の限界応力拡大係数ＫＩＣが得られない。 Further, ferritic stainless steel hardly undergoes dynamic recrystallization (referred to as recrystallization during deformation) during hot rolling, and tends to cause recovery of processing strain due to rolling. Therefore, in the hot rolling according to the conventional technique, excessive recovery of the working strain introduced by rolling occurs, and the working strain cannot be effectively maintained until after hot rolling. As a result, the recrystallization sites become insufficient, the colonies are not effectively destroyed in the subsequent hot-rolled sheet annealing, and the predetermined critical stress intensity factor K IC cannot be obtained.
　そこで本発明者らは、熱間圧延工程において鋼板の全厚にわたって圧延加工ひずみを効果的にかつ十分に導入する手法について鋭意検討した。その結果、仕上熱間圧延の最終３パスを適切な温度範囲に管理したうえで大きな累積圧下率で圧延を行うことにより、圧延加工ひずみの回復を抑制しつつ、圧延加工ひずみが板厚の中央部まで十分にかつ効果的に導入され、次工程の熱延板焼鈍における再結晶サイトとなる圧延加工ひずみを十分に残した熱延板組織を得ることができ、次工程の熱延板焼鈍においてコロニーが効果的に破壊されることを知見した。 Therefore, the present inventors diligently studied a method for effectively and sufficiently introducing rolling distortions over the entire thickness of the steel sheet in the hot rolling process. As a result, the final three passes of finish hot rolling are controlled within an appropriate temperature range, and rolling is performed at a large cumulative reduction rate, thereby suppressing the recovery of rolling distortion and reducing the rolling distortion to the center of the plate thickness. Can be obtained effectively and effectively to the part, and a hot-rolled sheet structure can be obtained in which a sufficient amount of rolling processing strain remains as a recrystallization site in the next-stage hot-rolled sheet annealing. It was found that colonies were effectively destroyed.
　具体的には、３パス以上からなる仕上熱間圧延工程の最終３パスを８００～１１００℃の温度範囲で、かつ最終３パスの累積圧下率（＝１００－（最終板厚／最終３パスの圧延開始前の板厚）×１００［％］）が２５％以上となるように適切に制御して熱間圧延を行うことを考案した。 Specifically, the final 3 passes of the finishing hot rolling process consisting of 3 passes or more are performed in a temperature range of 800 to 1100 ° C., and the cumulative reduction ratio of the final 3 passes (= 100− (final plate thickness / final 3 passes). It has been devised that hot rolling is performed with appropriate control so that the sheet thickness before rolling starts) × 100 [%]) is 25% or more.
　また、本発明者らは、次工程の熱延板焼鈍の好適な条件についても鋭意検討した。熱延板焼鈍は熱間圧延によって形成された加工組織を再結晶させる工程である。そのため、十分な再結晶が生じる温度で焼鈍を行う必要がある。しかし、過度な高温で熱延板焼鈍を行った場合、再結晶は生じるものの再結晶粒の著しい粗大化が生じる。この著しく粗大な再結晶粒は独立した単一の結晶粒であるが、粒界長さが著しく長くなるため、コロニーが存在していた場合と同様に方位の異なる粒界による亀裂進展の抑制効果が低下し、所定の限界応力拡大係数ＫＩＣが得られなくなることを知見した。 In addition, the present inventors have also intensively studied suitable conditions for the hot-rolled sheet annealing in the next step. Hot-rolled sheet annealing is a process of recrystallizing a processed structure formed by hot rolling. Therefore, it is necessary to perform annealing at a temperature at which sufficient recrystallization occurs. However, when hot-rolled sheet annealing is performed at an excessively high temperature, recrystallization occurs but recrystallized grains become extremely coarse. This remarkably coarse recrystallized grain is an independent single crystal grain, but the grain boundary length is remarkably long, so that the effect of suppressing crack growth by grain boundaries with different orientations is the same as when colonies existed. It was found that the predetermined critical stress intensity factor K IC could not be obtained.
　そこで本発明者らは、再結晶粒の粒径と焼鈍温度の関係について詳細に調査した。その結果、熱延板焼鈍温度を１１００℃以下に抑えることによって、限界応力拡大係数ＫＩＣが顕著に低下するほどの粗大な再結晶粒の生成を抑制できることを見出した。 Therefore, the present inventors investigated in detail the relationship between the grain size of recrystallized grains and the annealing temperature. As a result, it has been found that by suppressing the hot-rolled sheet annealing temperature to 1100 ° C. or lower, it is possible to suppress the generation of coarse recrystallized grains so that the critical stress intensity factor K IC is significantly reduced.
　次に、本発明のフェライト系ステンレス熱延焼鈍鋼板の成分組成について説明する。
以下、特に断らない限り、成分組成を示す％は質量％を意味する。 Next, the component composition of the ferritic stainless steel hot-rolled annealed steel sheet of the present invention will be described.
Hereinafter, unless otherwise specified,% indicating the component composition means mass%.
　Ｃ：０．００１～０．０２０％
　Ｃを０．０２０％超えて含有すると、加工性の低下および溶接部の耐食性低下が顕著になる。Ｃ含有量が少ないほど耐食性および加工性の観点では好ましいが、Ｃ含有量を０．００１％未満にするためには精錬に時間がかかり製造上好ましくない。そのため、Ｃ含有量は０．００１～０．０２０％の範囲とする。Ｃ含有量は、好ましくは０．００３％以上であり、より好ましくは０．００４％以上である。また、Ｃ含有量は、好ましくは０．０１５％以下であり、より好ましくは０．０１２％以下である。 C: 0.001 to 0.020%
When C is contained in excess of 0.020%, the workability and the corrosion resistance of the welded portion are significantly reduced. The smaller the C content, the better from the viewpoint of corrosion resistance and workability. However, in order to make the C content less than 0.001%, it takes time for refining, which is not preferable in production. Therefore, the C content is in the range of 0.001 to 0.020%. The C content is preferably 0.003% or more, and more preferably 0.004% or more. Further, the C content is preferably 0.015% or less, and more preferably 0.012% or less.
　Ｓｉ：０．０５～１．００％
　Ｓｉは溶接時に形成される酸化皮膜に濃縮して溶接部の耐食性を向上させる効果があるとともに、製鋼工程における脱酸元素としても有用な元素である。これらの効果は０．０５％以上のＳｉの含有により得られ、含有量が多いほどその効果は大きくなる。しかし、１．００％を超えてＳｉを含有すると、熱間圧延工程における圧延荷重の増大と顕著なスケールの生成、焼鈍工程においては鋼板表層でのＳｉ濃化層の形成による酸洗性の低下がそれぞれ生じ、表面欠陥の増加や製造コストの上昇を誘引するため好ましくない。そのため、Ｓｉ含有量は０．０５～１．００％とする。Ｓｉ含有量は、好ましくは０．１０％以上である。また、Ｓｉ含有量は、好ましくは０．６０％以下であり、より好ましくは０．４０％以下である。 Si: 0.05 to 1.00%
Si has an effect of concentrating on an oxide film formed at the time of welding to improve the corrosion resistance of the welded portion, and is also an element useful as a deoxidizing element in the steel making process. These effects are obtained by containing 0.05% or more of Si, and the effect increases as the content increases. However, if Si is contained in excess of 1.00%, the rolling load increases in the hot rolling process and a significant scale is generated. In the annealing process, the pickling property decreases due to the formation of the Si concentrated layer on the steel sheet surface layer. Respectively, which causes an increase in surface defects and an increase in manufacturing cost. Therefore, the Si content is set to 0.05 to 1.00%. The Si content is preferably 0.10% or more. Moreover, Si content becomes like this. Preferably it is 0.60% or less, More preferably, it is 0.40% or less.
　Ｍｎ：０．０５～１．００％
　Ｍｎは鋼の強度を高める効果があり、また、脱酸剤としての作用もある。その効果を得るためには０．０５％以上のＭｎの含有が必要である。しかし、Ｍｎ含有量が１．００％を超えると、腐食の起点となるＭｎＳの生成が促進され、耐食性が低下する。そのため、Ｍｎ含有量は０．０５～１．００％とする。Ｍｎ含有量は、好ましくは０．１０％以上である。また、Ｍｎ含有量は、好ましくは０．６０％以下であり、より好ましくは０．３０％以下である。 Mn: 0.05 to 1.00%
Mn has the effect of increasing the strength of the steel and also acts as a deoxidizer. In order to obtain the effect, it is necessary to contain 0.05% or more of Mn. However, if the Mn content exceeds 1.00%, the generation of MnS that is the starting point of corrosion is promoted, and the corrosion resistance is lowered. Therefore, the Mn content is set to 0.05 to 1.00%. The Mn content is preferably 0.10% or more. Further, the Mn content is preferably 0.60% or less, more preferably 0.30% or less.
　Ｐ：０．０４％以下
　Ｐは鋼に不可避的に含まれる元素であるが、耐食性および加工性に対して有害な元素であるので可能な限り低減することが好ましい。特に、Ｐ含有量が０．０４％を超えると固溶強化により加工性が顕著に低下する。よって、Ｐ含有量は０．０４％以下とする。好ましくは、Ｐ含有量は０．０３％以下である。 P: 0.04% or less P is an element inevitably contained in steel. However, it is preferably reduced as much as possible because it is an element harmful to corrosion resistance and workability. In particular, when the P content exceeds 0.04%, workability is remarkably lowered due to solid solution strengthening. Therefore, the P content is 0.04% or less. Preferably, the P content is 0.03% or less.
　Ｓ：０．０１％以下
　ＳもＰと同様に鋼に不可避的に含まれる元素であるが、耐食性および加工性に対して有害な元素であるので可能な限り低減するのが好ましい。特に、Ｓ含有量が０．０１％を超えると耐食性が顕著に低下する。よって、Ｓ含有量は０．０１％以下とする。好ましくは、Ｓ含有量は０．００８％以下である。より好ましくは、Ｓ含有量は０．００３％以下である。 S: 0.01% or less S is an element inevitably contained in steel like P. However, it is preferably reduced as much as possible because it is an element harmful to corrosion resistance and workability. In particular, when the S content exceeds 0.01%, the corrosion resistance significantly decreases. Therefore, the S content is 0.01% or less. Preferably, the S content is 0.008% or less. More preferably, the S content is 0.003% or less.
　Ａｌ：０．００１～０．１００％
　Ａｌは有効な脱酸剤である。さらに、Ａｌは窒素との親和力がＣｒよりも強いため、溶接部に窒素が侵入した場合に、窒素をＣｒ窒化物ではなくＡｌ窒化物として析出させて、鋭敏化を抑制する効果がある。これらの効果は、Ａｌを０．００１％以上含有することで得られる。しかし、０．１００％を超えるＡｌを含有すると、溶接時の溶け込み性が低下して溶接作業性が低下するので好ましくない。そのため、Ａｌ含有量は０．００１～０．１００％の範囲とする。Ａｌ含有量は、好ましくは０．００５％以上であり、より好ましくは０．０１０％以上である。また、Ａｌ含有量は、好ましくは０．０６０％以下であり、より好ましくは０．０４０％以下である。 Al: 0.001 to 0.100%
Al is an effective deoxidizer. Furthermore, since Al has a stronger affinity for nitrogen than Cr, when nitrogen penetrates into the weld zone, it has the effect of precipitating nitrogen by precipitating nitrogen as Al nitride instead of Cr nitride. These effects can be obtained by containing 0.001% or more of Al. However, it is not preferable to contain Al exceeding 0.100% because the penetration property during welding is lowered and the welding workability is lowered. Therefore, the Al content is in the range of 0.001 to 0.100%. The Al content is preferably 0.005% or more, and more preferably 0.010% or more. Moreover, Al content becomes like this. Preferably it is 0.060% or less, More preferably, it is 0.040% or less.
　Ｃｒ：１０．０～２４．０％
　Ｃｒはステンレス鋼の耐食性を確保するために最も重要な元素である。その含有量が１０．０％未満では、自動車排気ガス雰囲気において十分な耐食性が得られない。一方、２４．０％を超えてＣｒを含有すると、σ（シグマ）相の生成により靭性が著しく低下し、本発明では、所定の限界応力拡大係数ＫＩＣを得ることができない。そのため、Ｃｒ含有量は１０．０～２４．０％の範囲とする。Ｃｒ含有量は、好ましくは１４．０％以上であり、より好ましくは１６．０％以上であり、さらに好ましくは１７．０％以上である。また、Ｃｒ含有量は、好ましくは２１．５％以下であり、より好ましくは１９．５％以下であり、さらに好ましくは１８．５％以下である。 Cr: 10.0-24.0%
Cr is the most important element for ensuring the corrosion resistance of stainless steel. If the content is less than 10.0%, sufficient corrosion resistance cannot be obtained in an automobile exhaust gas atmosphere. On the other hand, if the Cr content exceeds 24.0%, the toughness is remarkably reduced due to the formation of the σ (sigma) phase, and in the present invention, the predetermined critical stress intensity factor K IC cannot be obtained. Therefore, the Cr content is in the range of 10.0 to 24.0%. The Cr content is preferably 14.0% or more, more preferably 16.0% or more, and further preferably 17.0% or more. Moreover, Cr content becomes like this. Preferably it is 21.5% or less, More preferably, it is 19.5% or less, More preferably, it is 18.5% or less.
　Ｎｉ：０．０１～０．６０％
　Ｎｉはステンレス鋼の耐食性を向上させる元素であり、不動態皮膜が形成されず活性溶解が生じる腐食環境において腐食の進行を抑制する元素である。また、Ｎｉは強いオーステナイト生成元素であり、溶接部でのフェライト生成を抑制し、Ｃｒ炭窒化物の析出による鋭敏化を抑制する効果がある。この効果は、Ｎｉを０．０１％以上含有することで得られ、Ｎｉの含有量が多いほど高くなる。しかし、Ｎｉ含有量が０．６０％を超えると、加工性が低下することに加えて、応力腐食割れが発生しやすくなる。さらには、Ｎｉは高価な元素であるため、Ｎｉの含有量の増大は製造コストの増大を招くため好ましくない。そのため、Ｎｉ含有量は０．０１～０．６０％とする。Ｎｉ含有量は、好ましくは０．１０％以上である。また、Ｎｉ含有量は、好ましくは０．５０％以下であり、より好ましくは０．４０％以下である。 Ni: 0.01 to 0.60%
Ni is an element that improves the corrosion resistance of stainless steel, and is an element that suppresses the progress of corrosion in a corrosive environment where a passive film is not formed and active dissolution occurs. Ni is a strong austenite generating element, and has the effect of suppressing ferrite formation at the weld and suppressing sensitization due to precipitation of Cr carbonitride. This effect is obtained by containing 0.01% or more of Ni, and increases as the Ni content increases. However, when the Ni content exceeds 0.60%, workability is lowered and stress corrosion cracking is likely to occur. Furthermore, since Ni is an expensive element, an increase in the content of Ni causes an increase in manufacturing cost, which is not preferable. Therefore, the Ni content is set to 0.01 to 0.60%. The Ni content is preferably 0.10% or more. Moreover, Ni content becomes like this. Preferably it is 0.50% or less, More preferably, it is 0.40% or less.
　Ｔｉ：０．１０～０．４０％
　本発明においてＴｉは極めて重要な元素である。Ｔｉは、ＣおよびＮと優先的に結合して、Ｃｒ炭窒化物の析出を抑制し、再結晶温度を低下させるとともにＣｒ炭窒化物の析出による鋭敏化に起因した耐食性の低下を抑制する効果がある。これらの効果を得るためには０．１０％以上のＴｉの含有が必要である。しかし、Ｔｉ含有量が０．４０％を超えると固溶Ｔｉ量が過度に増加するために再結晶温度が逆に上昇してしまい、本発明の技術を適用することができない。また、０．４０％超のＴｉの含有は、鋳造工程において粗大なＴｉ炭窒化物が生成し、表面欠陥を引き起こすため製造上も好ましくない。そのため、Ｔｉ含有量は０．１０～０．４０％とする。Ｔｉ含有量は、好ましくは０．１５％以上であり、より好ましくは０．２０％以上である。また、Ｔｉ含有量は、好ましくは０．３５％以下であり、より好ましくは０．３０％以下である。なお、溶接部耐食性の観点では式：Ｔｉ／（Ｃ＋Ｎ）≧８（なお、該式中、Ｔｉ、Ｃ、Ｎは各元素の含有量（質量％）である。）を満たすＴｉ含有量とすることが好ましい。 Ti: 0.10 to 0.40%
In the present invention, Ti is an extremely important element. Ti preferentially binds to C and N, suppresses the precipitation of Cr carbonitride, lowers the recrystallization temperature, and suppresses the decrease in corrosion resistance due to sensitization due to the precipitation of Cr carbonitride There is. In order to obtain these effects, it is necessary to contain 0.10% or more of Ti. However, if the Ti content exceeds 0.40%, the solid solution Ti amount increases excessively, so the recrystallization temperature rises conversely, and the technique of the present invention cannot be applied. In addition, if Ti content exceeds 0.40%, coarse Ti carbonitrides are produced in the casting process and cause surface defects, which is not preferable in production. Therefore, the Ti content is set to 0.10 to 0.40%. The Ti content is preferably 0.15% or more, and more preferably 0.20% or more. Moreover, Ti content becomes like this. Preferably it is 0.35% or less, More preferably, it is 0.30% or less. From the viewpoint of corrosion resistance of the weld zone, the Ti content satisfies the formula: Ti / (C + N) ≧ 8 (in the formula, Ti, C, and N are the contents (mass%) of each element). It is preferable.
　Ｎ：０．００１～０．０２０％
　Ｎ含有量が０．０２０％を超えると、加工性の低下および溶接部の耐食性の低下が顕著になる。耐食性の観点からＮ含有量は低いほど好ましいが、Ｎ含有量を０．００１％未満にまで低減するには長時間の精錬が必要となり、製造コストの上昇および生産性の低下を招くため好ましくない。よって、Ｎ含有量は０．００１～０．０２０％の範囲とする。Ｎ含有量は、好ましくは０．００５％以上であり、より好ましくは０．００７％以上である。また、Ｎ含有量は、好ましくは０．０１５％以下であり、より好ましくは０．０１２％以下である。 N: 0.001 to 0.020%
When the N content exceeds 0.020%, the workability and the corrosion resistance of the welded portion are significantly reduced. From the viewpoint of corrosion resistance, the lower the N content, the better. However, reducing the N content to less than 0.001% requires refining for a long time, which is not preferable because it causes an increase in manufacturing cost and a decrease in productivity. . Therefore, the N content is in the range of 0.001 to 0.020%. The N content is preferably 0.005% or more, and more preferably 0.007% or more. Moreover, N content becomes like this. Preferably it is 0.015% or less, More preferably, it is 0.012% or less.
　本発明は、上記必須成分を含有し残部がＦｅおよび不可避的不純物からなることを特徴とするフェライト系ステンレス鋼である。さらに、必要に応じて、Ｃｕ、Ｍｏ、ＷおよびＣｏのうちから選ばれる１種または２種以上、あるいは／さらに、Ｖ、Ｎｂ、Ｚｒ、ＲＥＭ、Ｂ、ＭｇおよびＣａのうちから選ばれる１種または２種以上を、下記の範囲で含有することができる。 The present invention is a ferritic stainless steel characterized in that it contains the above-mentioned essential components and the balance consists of Fe and inevitable impurities. Furthermore, as required, one or more selected from Cu, Mo, W and Co, or / or one selected from V, Nb, Zr, REM, B, Mg and Ca. Or 2 or more types can be contained in the following range.
　Ｃｕ：０．０１～１．００％
　Ｃｕは水溶液中や弱酸性の水滴が付着した場合の母材および溶接部の耐食性を向上させるのに特に有効な元素である。この効果は０．０１％以上の含有により得られ、その効果はＣｕ含有量が多いほど高くなる。しかし、１．００％を超えてＣｕを含有すると、熱間加工性が低下して表面欠陥を誘引する場合がある。さらには焼鈍後の脱スケールが困難となる場合もある。そのため、Ｃｕを含有する場合は、Ｃｕ含有量は０．０１～１．００％の範囲とすることが好ましい。Ｃｕ含有量は、より好ましくは０．１０％以上であり、さらに好ましくは０．３０％以上である。また、Ｃｕ含有量は、より好ましくは０．６０％以下であり、さらに好ましくは０．４５％以下である。 Cu: 0.01 to 1.00%
Cu is an element particularly effective for improving the corrosion resistance of the base material and the welded part when an aqueous solution or weakly acidic water droplets adhere. This effect is obtained when the content is 0.01% or more, and the effect increases as the Cu content increases. However, when Cu is contained exceeding 1.00%, hot workability may be reduced and surface defects may be induced. In addition, descaling after annealing may be difficult. Therefore, when Cu is contained, the Cu content is preferably in the range of 0.01 to 1.00%. The Cu content is more preferably 0.10% or more, and further preferably 0.30% or more. Further, the Cu content is more preferably 0.60% or less, and further preferably 0.45% or less.
　Ｍｏ：０．０１～２．００％
　Ｍｏはステンレス鋼の耐食性を顕著に向上させる元素である。この効果は０．０１％以上の含有によって得られ、その効果は含有量が多いほど向上する。しかし、Ｍｏ含有量が２．００％を超えると、熱間圧延時の圧延負荷が大きくなり製造性が低下したり、鋼板強度の過度な上昇が生じたりする場合がある。また、Ｍｏは高価な元素であることから、多量の含有は製造コストを増大させる。そのため、Ｍｏを含有する場合は、Ｍｏ含有量は０．０１～２．００％とすることが好ましい。Ｍｏ含有量は、より好ましくは０．１０％以上であり、さらに好ましくは０．３０％以上である。また、Ｍｏ含有量は、より好ましくは１．４０％以下であり、さらに好ましくは０．９０％以下である。 Mo: 0.01-2.00%
Mo is an element that remarkably improves the corrosion resistance of stainless steel. This effect is obtained when the content is 0.01% or more, and the effect improves as the content increases. However, if the Mo content exceeds 2.00%, the rolling load at the time of hot rolling increases, and the manufacturability may decrease, or the steel sheet strength may increase excessively. Moreover, since Mo is an expensive element, a large content increases the manufacturing cost. Therefore, when Mo is contained, the Mo content is preferably 0.01 to 2.00%. The Mo content is more preferably 0.10% or more, and further preferably 0.30% or more. Moreover, Mo content becomes like this. More preferably, it is 1.40% or less, More preferably, it is 0.90% or less.
　Ｗ：０．０１～０．２０％
　ＷはＭｏと同様に耐食性を向上させる効果がある。この効果は０．０１％以上のＷの含有により得られる。しかし、０．２０％を超えてＷを含有すると強度が上昇し、圧延荷重の増大等による製造性の低下を招く場合がある。そのため、Ｗを含有する場合は、Ｗ含有量は０．０１～０．２０％の範囲とすることが好ましい。Ｗ含有量は、より好ましくは０．０５％以上である。また、Ｗ含有量は、より好ましくは０．１５％以下である。 W: 0.01-0.20%
W, like Mo, has the effect of improving corrosion resistance. This effect is obtained by containing 0.01% or more of W. However, if it exceeds 0.20% and W is contained, the strength increases, and the productivity may decrease due to an increase in rolling load. Therefore, when W is contained, the W content is preferably in the range of 0.01 to 0.20%. The W content is more preferably 0.05% or more. Further, the W content is more preferably 0.15% or less.
　Ｃｏ：０．０１～０．２０％
　Ｃｏは靭性を向上させる元素である。この効果は０．０１％以上のＣｏの含有によって得られる。一方、Ｃｏ含有量が０．２０％を超えると加工性が低下する場合がある。そのため、Ｃｏを含有する場合は、Ｃｏ含有量は０．０１～０．２０％の範囲とすることが好ましい。Ｃｏ含有量は、より好ましくは０．１０％以下である。  Co: 0.01-0.20%
Co is an element that improves toughness. This effect is obtained by containing 0.01% or more of Co. On the other hand, if the Co content exceeds 0.20%, workability may be reduced. Therefore, when Co is contained, the Co content is preferably in the range of 0.01 to 0.20%. The Co content is more preferably 0.10% or less.
　Ｖ：０．０１～０．２０％
　ＶはＣ、Ｎと炭窒化物を形成し、溶接時の鋭敏化を抑制して溶接部の耐食性を向上させる。この効果はＶ含有量が０．０１％以上で得られる。一方、Ｖ含有量が０．２０％を超えると加工性および靭性が顕著に低下する場合がある。そのため、Ｖ含有量は０．０１～０．２０％とすることが好ましい。Ｖ含有量は、より好ましくは０．０３％以上である。また、Ｖ含有量は、より好ましくは０．１０％以下であり、さらにより好ましくは０．０５％以下である。 V: 0.01-0.20%
V forms carbonitride with C and N, suppresses sensitization during welding and improves the corrosion resistance of the weld. This effect is obtained when the V content is 0.01% or more. On the other hand, if the V content exceeds 0.20%, workability and toughness may be significantly reduced. Therefore, the V content is preferably 0.01 to 0.20%. The V content is more preferably 0.03% or more. Further, the V content is more preferably 0.10% or less, and even more preferably 0.05% or less.
　Ｎｂ：０．０１～０．１０％
　Ｎｂは結晶粒を微細化させるとともに、母相中に固溶することにより鋼板の靭性を向上させる効果がある。これらの効果は０．０１％以上のＮｂの含有で得られる。一方、Ｎｂは再結晶温度を上昇させる効果もあり、Ｎｂ含有量が０．１０％を超えると熱延板焼鈍にて十分な再結晶を生じさせるために必要な焼鈍温度が過度に高温となって、焼鈍中に結晶粒径が最大で３００μｍ以上となるほどの再結晶粒の著しい粗大化が生じ、所定の限界応力拡大係数ＫＩＣを得ることができなくなる場合がある。そのため、Ｎｂを含有させる場合には、Ｎｂ含有量は０．０１～０．１０％の範囲とすることが好ましい。Ｎｂ含有量は、より好ましくは０．０２％以上である。また、Ｎｂ含有量は、より好ましくは０．０５％以下である。 Nb: 0.01 to 0.10%
Nb has the effect of improving the toughness of the steel sheet by refining crystal grains and dissolving in the matrix. These effects are obtained when the Nb content is 0.01% or more. On the other hand, Nb also has an effect of increasing the recrystallization temperature. When the Nb content exceeds 0.10%, the annealing temperature necessary for causing sufficient recrystallization by hot-rolled sheet annealing becomes excessively high. As a result, recrystallized grains become extremely coarse as the crystal grain size reaches 300 μm or more during annealing, and a predetermined critical stress intensity factor K IC may not be obtained. Therefore, when Nb is contained, the Nb content is preferably in the range of 0.01 to 0.10%. The Nb content is more preferably 0.02% or more. Further, the Nb content is more preferably 0.05% or less.
　Ｚｒ：０．０１～０．２０％
　Ｚｒは、ＣおよびＮと結合して鋭敏化を抑制する効果がある。この効果は０．０１％以上のＺｒの含有により得られる。一方、０．２０％を超えてＺｒを含有すると加工性が顕著に低下する場合がある。そのため、Ｚｒを含有する場合、Ｚｒ含有量は０．０１～０．２０％の範囲とすることが好ましい。Ｚｒ含有量は、より好ましくは０．０２％以上である。また、Ｚｒ含有量は、より好ましくは０．１０％以下であり、さらにより好ましくは０．０５％以下である。 Zr: 0.01-0.20%
Zr has the effect of binding to C and N to suppress sensitization. This effect is obtained by containing 0.01% or more of Zr. On the other hand, if the Zr content exceeds 0.20%, the workability may be significantly reduced. Therefore, when Zr is contained, the Zr content is preferably in the range of 0.01 to 0.20%. The Zr content is more preferably 0.02% or more. Further, the Zr content is more preferably 0.10% or less, and even more preferably 0.05% or less.
　ＲＥＭ：０．００１～０．１００％
　ＲＥＭ（Ｒａｒｅ　Ｅａｒｔｈ　Ｍｅｔａｌｓ：希土類金属）は耐酸化性を向上させる効果があり、溶接部の酸化皮膜（溶接テンパーカラー）形成を抑制して酸化皮膜直下におけるＣｒ欠乏領域の形成を抑制する。この効果は、ＲＥＭを０．００１％以上含有することで得られる。一方、０．１００％を超えてＲＥＭを含有すると冷延焼鈍時の酸洗性などの製造性を低下させる場合がある。そのため、ＲＥＭを含有する場合、ＲＥＭ含有量は０．００１～０．１００％の範囲とすることが好ましい。ＲＥＭ含有量は、より好ましくは０．０１０％以上である。また、ＲＥＭ含有量は、より好ましくは０．０５０％以下である。 REM: 0.001 to 0.100%
REM (Rare Earth Metals) has an effect of improving the oxidation resistance, and suppresses formation of a Cr-deficient region immediately below the oxide film by suppressing formation of an oxide film (weld temper color) in the welded portion. This effect is acquired by containing REM 0.001% or more. On the other hand, when it contains REM exceeding 0.100%, productivity, such as pickling at the time of cold rolling annealing, may be reduced. Therefore, when REM is contained, the REM content is preferably in the range of 0.001 to 0.100%. The REM content is more preferably 0.010% or more. The REM content is more preferably 0.050% or less.
　Ｂ：０．０００２～０．００２５％
　Ｂは成形後の耐二次加工脆性を改善するために有効な元素である。この効果はＢの含有量を０．０００２％以上にすることで得られる。一方、０．００２５％を超えてＢを含有すると加工性と靭性が低下する場合がある。そのため、Ｂを含有する場合、Ｂ含有量は０．０００２～０．００２５％の範囲とすることが好ましい。Ｂ含有量は、より好ましくは０．０００３％以上である。また、Ｂ含有量は、より好ましくは０．０００６％以下である。 B: 0.0002 to 0.0025%
B is an element effective for improving the secondary work brittleness resistance after molding. This effect is obtained by making the B content 0.0002% or more. On the other hand, if the B content exceeds 0.0025%, workability and toughness may be reduced. Therefore, when B is contained, the B content is preferably in the range of 0.0002 to 0.0025%. The B content is more preferably 0.0003% or more. Further, the B content is more preferably 0.0006% or less.
　Ｍｇ：０．０００５～０．００３０％
　Ｍｇはスラブの等軸晶率を向上させ、加工性や靭性の向上に有効な元素である。さらに、本発明のようにＴｉを含有する鋼においては、Ｔｉ炭窒化物が粗大化すると靭性が低下するが、ＭｇはＴｉ炭窒化物の粗大化を抑制する効果も有する。これらの効果は、０．０００５％以上のＭｇを含有することで得られる。一方で、Ｍｇ含有量が０．００３０％を超えると、鋼の表面性状を悪化させてしまう場合がある。したがって、Ｍｇを含有する場合、Ｍｇ含有量は０．０００５～０．００３０％の範囲とすることが好ましい。Ｍｇ含有量は、より好ましくは０．００１０％以上である。また、Ｍｇ含有量は、より好ましくは０．００２０％以下である。 Mg: 0.0005 to 0.0030%
Mg is an element that improves the equiaxed crystal ratio of the slab and is effective in improving workability and toughness. Further, in the steel containing Ti as in the present invention, when Ti carbonitride becomes coarser, the toughness decreases, but Mg also has an effect of suppressing the coarsening of Ti carbonitride. These effects can be obtained by containing 0.0005% or more of Mg. On the other hand, if the Mg content exceeds 0.0030%, the surface properties of the steel may be deteriorated. Therefore, when Mg is contained, the Mg content is preferably in the range of 0.0005 to 0.0030%. The Mg content is more preferably 0.0010% or more. The Mg content is more preferably 0.0020% or less.
　Ｃａ：０．０００３～０．００３０％
　Ｃａは、連続鋳造の際に発生しやすいＴｉ系介在物の晶出によるノズルの閉塞を防止するのに有効な成分である。その効果は０．０００３％以上のＣａを含有することで得られる。しかし、０．００３０％を超えてＣａを含有すると、ＣａＳの生成により耐食性が低下する場合がある。従って、Ｃａを含有する場合、Ｃａ含有量は０．０００３～０．００３０％の範囲とすることが好ましい。Ｃａ含有量は、より好ましくは０．０００５％以上である。また、Ｃａ含有量は、より好ましくは０．００１５％以下であり、さらに好ましくは０．００１０％以下である。 Ca: 0.0003 to 0.0030%
Ca is an effective component for preventing nozzle clogging due to crystallization of Ti-based inclusions that are likely to occur during continuous casting. The effect is acquired by containing 0.0003% or more of Ca. However, if the Ca content exceeds 0.0030%, the corrosion resistance may decrease due to the formation of CaS. Therefore, when Ca is contained, the Ca content is preferably in the range of 0.0003 to 0.0030%. The Ca content is more preferably 0.0005% or more. Moreover, Ca content becomes like this. More preferably, it is 0.0015% or less, More preferably, it is 0.0010% or less.
　限界応力拡大係数ＫＩＣ：２０ＭＰａ・ｍ１／２以上
　本発明のフェライト系ステンレス熱延焼鈍鋼板は、限界応力拡大係数ＫＩＣが２０ＭＰａ・ｍ１／２以上であることで、厚肉のフランジへの打ち抜き加工をする際の割れを抑制することができる。限界応力拡大係数ＫＩＣは、好ましくは２５ＭＰａ・ｍ１／２以上、さらに好ましくは３０ＭＰａ・ｍ１／２以上である。なお、厚肉のフランジとは、特に限定されないが、例えば板厚５.０ｍｍ以上のフランジが挙げられる。前記フランジとしては、例えば板厚５．０～１５．０ｍｍのフランジが好ましく、板厚５．０～１０．０ｍｍのフランジがより好ましい。 Limit stress intensity factor K IC : 20 MPa · m 1/2 or more The ferritic stainless steel hot rolled annealed steel sheet of the present invention has a limit stress intensity factor K IC of 20 MPa · m 1/2 or more, so that it becomes a thick flange. It is possible to suppress cracks during the punching process. The critical stress intensity factor K IC is preferably 25 MPa · m 1/2 or more, more preferably 30 MPa · m 1/2 or more. The thick flange is not particularly limited, and examples thereof include a flange having a thickness of 5.0 mm or more. As the flange, for example, a flange having a plate thickness of 5.0 to 15.0 mm is preferable, and a flange having a plate thickness of 5.0 to 10.0 mm is more preferable.
　次に、本発明のフェライト系ステンレス熱延焼鈍鋼板の製造方法について説明する。なお、以下の説明において、特に断らない限り、温度は、鋼スラブ、熱延鋼板等の表面温度計等で測定した表面温度とする。 Next, a method for producing the ferritic stainless steel hot-rolled annealed steel sheet according to the present invention will be described. In the following description, unless otherwise specified, the temperature is the surface temperature measured with a surface thermometer such as a steel slab or hot-rolled steel sheet.
　本発明のフェライト系ステンレス熱延焼鈍鋼板は、上記成分組成を有する鋼スラブを用い、粗圧延および３パス以上の仕上げ圧延からなる熱間圧延において、仕上圧延の最終３パスの圧延を温度範囲８００～１１００℃、且つ最終３パスの累積圧下率２５％以上として熱延鋼板を得て、該熱延鋼板に対してさらに８００～１１００℃で熱延板焼鈍を行うことによって得られる。 The ferritic stainless steel hot-rolled annealed steel sheet of the present invention uses a steel slab having the above composition, and in the hot rolling consisting of rough rolling and finishing rolling of 3 or more passes, the final 3 passes of finishing rolling are performed in a temperature range of 800. It is obtained by obtaining a hot-rolled steel sheet at a temperature of ˜1100 ° C. and a cumulative reduction ratio of 25% or more in the final three passes, and subjecting the hot-rolled steel sheet to further hot-rolled sheet annealing at 800 to 1100 ° C.
　まずは、上記した成分組成からなる溶鋼を、転炉、電気炉、真空溶解炉等の公知の方法で溶製し、連続鋳造法あるいは造塊－分塊法により鋼素材（スラブ）とする。 First, the molten steel having the above component composition is melted by a known method such as a converter, electric furnace, vacuum melting furnace or the like, and is made into a steel material (slab) by a continuous casting method or an ingot-bundling method.
　このスラブを、１１００～１２５０℃で１～２４時間加熱するか、あるいは加熱することなく鋳造後１１００～１２５０℃の温度になった段階で、熱間圧延に供する。本発明では粗圧延については特に限定すべき点はないが、仕上熱間圧延前に鋳造組織を効果的に破壊しておいた場合、その後の仕上熱間圧延における結晶粒の微細化に優位に働くため、粗圧延における累積圧下率を６５％以上とすることが好ましい。その後、仕上熱間圧延により所定板厚まで圧延するが、仕上圧延の最終３パスの圧延を８００～１１００℃の温度範囲とし、累積圧下率を２５％以上として行う。 The slab is heated at 1100 to 1250 ° C. for 1 to 24 hours, or subjected to hot rolling at a stage where the temperature reaches 1100 to 1250 ° C. after casting without heating. In the present invention, there is no particular limitation for rough rolling. However, if the cast structure is effectively destroyed before finish hot rolling, it is superior to refinement of crystal grains in subsequent finish hot rolling. In order to work, it is preferable to set the cumulative rolling reduction in rough rolling to 65% or more. Thereafter, the sheet is rolled to a predetermined plate thickness by finish hot rolling, but the final three passes of finish rolling are performed in a temperature range of 800 to 1100 ° C., and the cumulative reduction ratio is 25% or more.
　仕上熱間圧延の最終３パスの圧延温度範囲：８００～１１００℃
　仕上熱間圧延の最終３パスの累積圧下率：２５％以上
　仕上げ圧延前の粗圧延において粗大な鋳造組織は破壊されているが、当該組織における結晶粒は著しく粗大である。熱延板焼鈍後に所定の限界応力拡大係数ＫＩＣを得るためには、仕上熱間圧延の最終３パスの圧延の温度および累積圧下率を適切に制御することによって、圧延中の過度の回復を抑制しつつ、特に板厚中央部へ圧延ひずみを効果的に付与する必要がある。 Rolling temperature range for the final three passes of finish hot rolling: 800-1100 ° C
Cumulative rolling reduction in the final three passes of finish hot rolling: 25% or more In the rough rolling before finish rolling, the coarse cast structure is destroyed, but the crystal grains in the structure are remarkably coarse. In order to obtain a predetermined critical stress intensity factor K IC after hot-rolled sheet annealing, excessive recovery during rolling can be achieved by appropriately controlling the temperature and cumulative rolling reduction of the final three passes of finish hot rolling. In particular, it is necessary to effectively apply a rolling strain to the central portion of the plate thickness while suppressing it.
　次工程である熱延板焼鈍において所定の金属組織を得るために十分な再結晶サイトを導入するためには、仕上熱間圧延の最終３パスの圧延温度を８００～１１００℃の範囲とし、かつ最終３パスの累積圧下率（＝１００－（最終板厚／最終３パスの圧延開始前の板厚）×１００［％］）を２５％以上として、最終３パスによって付与される圧延ひずみが回復によって解消されることを防ぎつつ、圧延ひずみを板厚中央に効果的に付与することが必要である。 In order to introduce sufficient recrystallization sites to obtain a predetermined metal structure in the subsequent hot-rolled sheet annealing, the rolling temperature in the final three passes of finish hot rolling is set in the range of 800 to 1100 ° C., and Rolling strain applied by the final three passes is recovered by setting the cumulative reduction ratio of the final three passes (= 100- (final plate thickness / plate thickness before the start of rolling of the final three passes) × 100 [%]) to 25% or more. It is necessary to effectively apply the rolling strain to the center of the plate thickness while preventing it from being eliminated by.
　仕上熱間圧延の最終３パスの累積圧下率が２５％未満では、板厚中央への圧延ひずみが効果的に付与されないため、次工程の熱延板焼鈍でコロニーが残存してしまい、所定の限界応力拡大係数ＫＩＣを得ることができない。そのため、最終３パスの累積圧下率を２５％以上とする。好ましくは、累積圧下率は３０％以上である。さらに好ましくは、累積圧下率は３５％以上である。なお、累積圧下率の上限は特に限定されないが、累積圧下率を過度に大きくすると圧延負荷が上昇して製造性が低下するとともに、圧延後に表面肌荒れが発生する場合があるため、６０％以下とすることが好ましい。 If the cumulative reduction ratio of the final three passes of finish hot rolling is less than 25%, the rolling strain to the center of the sheet thickness is not effectively applied, so that colonies remain in the hot-rolled sheet annealing in the next process, and the predetermined reduction The critical stress intensity factor K IC cannot be obtained. Therefore, the cumulative reduction ratio of the final three passes is set to 25% or more. Preferably, the cumulative rolling reduction is 30% or more. More preferably, the cumulative rolling reduction is 35% or more. The upper limit of the cumulative rolling reduction is not particularly limited, but if the cumulative rolling reduction is excessively increased, the rolling load increases and the productivity decreases, and surface roughness may occur after rolling. It is preferable to do.
　仕上熱間圧延の最終３パスの圧延温度を８００℃未満とした場合、鋼板温度の低下に伴って圧延荷重が著しく上昇するため製造上好ましくない。また、低温での圧延により鋼板表面の肌荒れが発生して表面品質が低下する場合がある。一方、最終３パスの圧延温度が１１００℃を超えると、圧延によって付与したひずみの回復が生じて、次工程の熱延板焼鈍後における再結晶サイトが不足するため、熱延板焼鈍後にコロニーが残存してしまい、所定の限界応力拡大係数ＫＩＣを得ることができない。そのため、最終３パスの圧延温度は８００～１１００℃の範囲とする。好ましくは、最終３パスの圧延温度は８００～１０５０℃の範囲とする。より好ましくは、最終３パスの圧延温度は８５０～１０００℃の範囲とする。 If the rolling temperature in the final three passes of finish hot rolling is less than 800 ° C., the rolling load increases remarkably as the steel plate temperature decreases, which is not preferable for production. Further, rolling at a low temperature may cause the surface roughness of the steel sheet to deteriorate the surface quality. On the other hand, when the rolling temperature of the final three passes exceeds 1100 ° C., recovery of strain imparted by rolling occurs, and the recrystallization sites after the hot-rolled sheet annealing in the next step are insufficient. The predetermined limit stress intensity factor K IC cannot be obtained. Therefore, the rolling temperature for the final three passes is in the range of 800 to 1100 ° C. Preferably, the rolling temperature in the final three passes is in the range of 800 to 1050 ° C. More preferably, the rolling temperature in the final three passes is in the range of 850 to 1000 ° C.
　なお、仕上熱間圧延の最終３パスにおける特定パスで過度の圧延負荷がかかることを防ぐため、最終３パスのうち、第１パス目の圧延温度範囲を９５０～１１００℃、この第１パスの次に行われる第２パス目の圧延温度範囲を９２５～１０７５℃、この第２パス目の次に行われる第３パス目の圧延温度範囲を８７５～１０５０℃とすることが好ましい。 In order to prevent an excessive rolling load from being applied in a specific pass in the final three passes of finish hot rolling, the rolling temperature range of the first pass among the final three passes is 950 to 1100 ° C., and this first pass The rolling temperature range for the second pass to be performed next is preferably 925 to 1075 ° C., and the rolling temperature range for the third pass to be performed next to the second pass is preferably 875 to 1050 ° C.
　また、本発明のフェライト系ステンレス熱延焼鈍鋼板の製造方法では、３パス以上からなる仕上熱間圧延の最終３パスにおいて温度範囲を制御したうえで大きな圧下を加えることを特徴としている。大きな圧下を加える圧延を最終の４パス以上にわたって行うと、同じ累積圧下率であっても圧下率が各パスに分散されてしまうため板厚中央へのひずみ付与が不十分になるとともに、各パス間の累積搬送時間が増加するために、各パス間を搬送している間の回復が助長され、ひずみ付与の効果が低下する。また、仕上げ圧延の圧延温度および累積圧下率の制御を最終の２パス以下とすると、２パスで累積圧下率２５％以上の大圧下を行うために圧延負荷が著しく上昇し製造性が低下する場合があるため好ましくない。よって、本発明のフェライト系ステンレス熱延鋼板の製造方法では、仕上げ圧延の最終の３パスの圧延温度および累積圧下率を制御する。 In addition, the method for producing a ferritic stainless steel hot-rolled annealed steel sheet according to the present invention is characterized in that a large reduction is applied after controlling the temperature range in the final three passes of finishing hot rolling consisting of three or more passes. If rolling with a large reduction is performed over the final four passes or more, even if the cumulative reduction rate is the same, the reduction rate will be distributed to each pass, so the strain applied to the center of the plate thickness will be insufficient, and each pass Since the accumulated conveyance time increases, recovery during conveyance between each pass is promoted, and the effect of imparting strain is reduced. In addition, when the rolling temperature and the cumulative reduction ratio of the finish rolling are controlled to the final two passes or less, the rolling load is significantly increased and the productivity is lowered because the large reduction with the cumulative reduction ratio of 25% or more is performed in two passes. This is not preferable. Therefore, in the method for producing a ferritic stainless steel hot-rolled steel sheet according to the present invention, the rolling temperature and cumulative rolling reduction of the final three passes of finish rolling are controlled.
　なお、本発明のフェライト系ステンレス熱延鋼板の製造方法では、仕上熱間圧延の最終の３パスの圧延温度および累積圧下率を制御することが肝要であり、３パス以上の仕上げ圧延であれば、何パスの仕上げ圧延を行ってもよいが、最大パス数が１５パスよりも多くなると、圧延ロールとの接触回数の増加による鋼板温度の低下が生じやすくなり、鋼板温度を所定温度範囲内に維持するために外部からの加熱が必要になる等の製造性の低下または製造コストの増加を招く場合があるため、最大パス数は１５パス以下とすることが好ましい。より好ましくは、最大パス数は１０パス以下である。 In addition, in the manufacturing method of the ferritic stainless steel hot-rolled steel sheet of the present invention, it is important to control the final three-pass rolling temperature and cumulative rolling reduction of the finish hot rolling, and if it is a finish rolling of three or more passes Any number of finishing rolls may be performed, but if the maximum number of passes is greater than 15 passes, the steel plate temperature is likely to decrease due to an increase in the number of contacts with the rolling roll, and the steel plate temperature is kept within a predetermined temperature range. The maximum number of passes is preferably 15 passes or less because it may lead to a decrease in manufacturability or an increase in manufacturing costs, such as heating from the outside required for maintenance. More preferably, the maximum number of paths is 10 paths or less.
　仕上熱間圧延後は鋼板の冷却を行い、ついで鋼板の巻取処理を行い熱延鋼帯とする。本発明において巻取温度は特に限定されないが、巻取温度を４５０℃超～５００℃未満とした場合、４７５℃脆化に起因した脆化が生じる場合がある。そのため、巻取温度は４５０℃以下もしくは５００℃以上とすることが好ましい。 After finishing hot rolling, the steel sheet is cooled, and then the steel sheet is wound to form a hot-rolled steel strip. In the present invention, the coiling temperature is not particularly limited, but when the coiling temperature is more than 450 ° C. to less than 500 ° C., embrittlement due to 475 ° C. embrittlement may occur. Therefore, the winding temperature is preferably 450 ° C. or lower or 500 ° C. or higher.
　熱延板焼鈍温度：８００～１１００℃
　本発明では上記熱間圧延工程終了後に熱延板焼鈍を行う。熱延板焼鈍において、熱間圧延工程で形成させた圧延加工組織を再結晶させる。本発明では熱間圧延工程において効果的に圧延ひずみを付与し、再結晶サイトを増加させることによって熱延板焼鈍におけるコロニーの破壊を促進させる。この効果を得るためには熱延板焼鈍を８００～１１００℃の範囲で行う必要がある。焼鈍温度が８００℃未満では再結晶が不十分となり、所定の限界応力拡大係数ＫＩＣを得ることができない。一方、焼鈍温度が１１００℃を超えると、再結晶粒は、その結晶粒径が最大で３００μｍ以上となるほどの著しい粗大化が生じ、所定の限界応力拡大係数ＫＩＣを得ることができない。そのため、熱延板焼鈍温度は８００～１１００℃の範囲とする。かかる熱延板焼鈍がされた熱延鋼板は、上述の成分組成を有し、２０ＭＰａ・ｍ１／２以上の限界応力拡大係数ＫＩＣを有する。好ましくは、熱延板焼鈍温度は８００～１０５０℃の範囲である。さらに好ましくは、熱延板焼鈍温度は８５０～１０００℃の範囲である。なお、熱延板焼鈍の保持時間および手法に特に限定はなく、箱焼鈍（バッチ焼鈍）、連続焼鈍のいずれで実施してもかまわない。 Hot-rolled sheet annealing temperature: 800-1100 ° C
In the present invention, hot-rolled sheet annealing is performed after the hot rolling step. In hot-rolled sheet annealing, the rolled structure formed in the hot rolling process is recrystallized. In the present invention, the rolling strain is effectively applied in the hot rolling process, and the recrystallization sites are increased, thereby promoting the destruction of colonies in the hot-rolled sheet annealing. In order to obtain this effect, it is necessary to perform hot-rolled sheet annealing in the range of 800 to 1100 ° C. When the annealing temperature is less than 800 ° C., recrystallization is insufficient, and a predetermined critical stress intensity factor K IC cannot be obtained. On the other hand, when the annealing temperature exceeds 1100 ° C., the recrystallized grains are markedly coarsened so that the crystal grain size becomes 300 μm or more at the maximum, and a predetermined limit stress intensity factor K IC cannot be obtained. Therefore, the hot-rolled sheet annealing temperature is in the range of 800 to 1100 ° C. The hot-rolled steel sheet subjected to such hot-rolled sheet annealing has the above-described component composition, and has a critical stress intensity factor K IC of 20 MPa · m 1/2 or more. Preferably, the hot-rolled sheet annealing temperature is in the range of 800 to 1050 ° C. More preferably, the hot-rolled sheet annealing temperature is in the range of 850 to 1000 ° C. In addition, there is no limitation in particular in the holding | maintenance time and method of hot-rolled sheet annealing, You may implement by any of box annealing (batch annealing) and continuous annealing.
　得られた熱延焼鈍鋼板には、必要に応じてショットブラストや酸洗による脱スケール処理を行ってもよい。さらに、表面性状を向上させるために、研削や研磨等を施してもよい。また、本発明が提供する熱延焼鈍鋼板はその後、冷間圧延および冷延板焼鈍を行ってもよい。 The obtained hot-rolled annealed steel sheet may be descaled by shot blasting or pickling as necessary. Furthermore, in order to improve the surface properties, grinding or polishing may be performed. In addition, the hot-rolled annealed steel sheet provided by the present invention may be subsequently subjected to cold rolling and cold-rolled sheet annealing.
　以下、本発明を実施例により詳細に説明する。 Hereinafter, the present invention will be described in detail with reference to examples.
　表１に示す化学組成を有するステンレス溶鋼を容量１５０ｔｏｎの転炉と強攪拌・真空酸素脱炭処理（ＳＳ－ＶＯＤ）の精錬で溶製し、連続鋳造により幅１０００ｍｍ、厚さ２００ｍｍの鋼スラブとした。Ｎｏ．３１以外は該スラブを１２００℃で１ｈ加熱後に、熱間圧延として３段のスタンドを用いたリバース式の粗圧延を行って約４０ｍｍの鋼板とし、ついで７パスからなる仕上げ圧延の最終３パス（５パス目、６パス目、７パス目）を表２に記載の条件で行い熱延鋼板とした。Ｎｏ．３１は該スラブを１３００℃で１ｈ加熱した後に熱間圧延に供した。得られた熱延鋼板について同じく表２に記載の条件で箱焼鈍による熱延板焼鈍を行い、熱延焼鈍板を得た。 A molten stainless steel having the chemical composition shown in Table 1 is melted by refining a converter with a capacity of 150 ton and strong stirring and vacuum oxygen decarburization (SS-VOD), and a steel slab having a width of 1000 mm and a thickness of 200 mm by continuous casting. did. No. Except for 31, the slab was heated at 1200 ° C. for 1 h, and then hot-rolled by reverse rough rolling using a three-stage stand to obtain a steel plate of about 40 mm, and then the final three passes of final rolling consisting of 7 passes ( The fifth pass, the sixth pass, and the seventh pass) were performed under the conditions shown in Table 2 to obtain hot-rolled steel sheets. No. No. 31 was subjected to hot rolling after heating the slab at 1300 ° C. for 1 h. Similarly, the obtained hot-rolled steel sheet was subjected to hot-rolled sheet annealing by box annealing under the conditions shown in Table 2 to obtain a hot-rolled annealed sheet.
　得られた熱延焼鈍鋼板について、以下の評価を行った。 The obtained hot-rolled annealed steel sheet was evaluated as follows.
　（１）限界応力拡大係数ＫＩＣの評価
　板幅中央部から、ＡＳＴＭ　Ｅ３９９に準拠したＣＴ試験片を、疲労予き裂が圧延直角方向、応力軸が圧延平行方向となるように採取した。該試験片について、ＡＳＴＭ　Ｅ３９９に準拠して限界応力拡大係数ＫＩＣを求めた。限界応力拡大係数ＫＩＣが２０ＭＰａ・ｍ１／２以上を合格、２０ＭＰａ・ｍ１／２未満を不合格とした。 (1) Evaluation of critical stress intensity factor K IC From the central part of the plate width, a CT specimen according to ASTM E399 was sampled so that the fatigue precrack was in the direction perpendicular to the rolling and the stress axis was in the direction parallel to the rolling. The critical stress intensity factor K IC was determined for the test piece in accordance with ASTM E399. A critical stress intensity factor K IC of 20 MPa · m 1/2 or more was accepted and less than 20 MPa · m 1/2 was rejected.
　（２）耐食性の評価
　熱延焼鈍鋼板から、６０×１００ｍｍの試験片を採取し、評価する表面を＃６００エメリーペーパーにより研磨仕上げした後に端面部をシールした試験片を作製し、ＪＩＳ　Ｈ　８５０２に規定された塩水噴霧サイクル試験に供した。塩水噴霧サイクル試験は、塩水噴霧（５質量％ＮａＣｌ、３５℃、噴霧２ｈr）→乾燥（６０℃、４ｈr、相対湿度４０％）→湿潤（５０℃、２ｈr、相対湿度≧９５％）を１サイクルとして、５サイクル行った。塩水噴霧サイクル試験を５サイクル実施後の試験片の評価面を写真撮影し、画像解析により試験片の評価面の発錆面積を測定し、試験片全面積との比率から発錆率（（試験片中の発錆面積／試験片全面積）×１００　［％］）を算出した。発錆率１０％以下を特に優れた耐食性で合格（◎）、１０％超２５％以下を合格（○）、２５％超を不合格（×）とした。 (2) Evaluation of corrosion resistance A 60 × 100 mm test piece was sampled from a hot-rolled annealed steel sheet, and a test piece was prepared by polishing the surface to be evaluated with # 600 emery paper and sealing the end face. Subjected to the prescribed salt spray cycle test. In the salt spray cycle test, salt spray (5 mass% NaCl, 35 ° C., spray 2 hr) → dry (60 ° C., 4 hr, relative humidity 40%) → wet (50 ° C., 2 hr, relative humidity ≧ 95%) is one cycle. As a result, 5 cycles were performed. Photograph the evaluation surface of the test piece after 5 cycles of the salt spray cycle test, measure the rusting area of the evaluation surface of the test piece by image analysis, and determine the rusting rate ((test Rust area in the piece / total area of the test piece) × 100 [%]) was calculated. A rusting rate of 10% or less was evaluated as being particularly excellent with respect to corrosion resistance (）), more than 10% being 25% or less, passing (O), and more than 25% being rejecting (X).
　試験結果を熱間圧延および熱延板焼鈍条件と併せて表２に示す。 The test results are shown in Table 2 together with hot rolling and hot rolled sheet annealing conditions.
Figure JPOXMLDOC01-appb-T000001
  
Figure JPOXMLDOC01-appb-T000001
  
Figure JPOXMLDOC01-appb-T000002
  
Figure JPOXMLDOC01-appb-T000002
  
　鋼成分、熱間圧延条件および熱延板焼鈍条件が本発明の範囲を満たすＮｏ．１～２６は、所定の熱間圧延および熱延板焼鈍によってコロニーが効果的に破壊された結果、所定の限界応力拡大係数ＫＩＣが得られていた。さらに得られた熱延焼鈍板の耐食性を評価した結果、いずれも発錆率は２５％であり十分な耐食性も有していることが確認された。 Steel components, hot rolling conditions and hot-rolled sheet annealing conditions satisfy the scope of the present invention. In Nos. 1 to 26, as a result of the colonies being effectively destroyed by predetermined hot rolling and hot-rolled sheet annealing, a predetermined critical stress intensity factor K IC was obtained. Furthermore, as a result of evaluating the corrosion resistance of the obtained hot-rolled annealed plates, it was confirmed that all had a rusting rate of 25% and sufficient corrosion resistance.
　特に、Ｍｏを含有させた鋼Ｅ、Ｆ、Ｇ、Ｊを用いたＮｏ．５～７とＮｏ．１０、およびＣｕを含有させた鋼ＨとＩを用いたＮｏ．８および９では発錆率が１０％以下（◎）と一層優れた耐食性が得られた。 Especially, No. using steel E, F, G, J containing Mo. 5-7 and no. No. 10 and No. 10 using steels H and I containing Cu. In 8 and 9, rusting rate was 10% or less (◎), and further excellent corrosion resistance was obtained.
　最終３パスの圧延温度が本発明の範囲を上回るＮｏ．２７では、所定の累積圧下率で圧延したものの、圧延温度が過度に高温であったために加工ひずみの回復が生じて再結晶サイトが不十分となったために熱延板焼鈍後にもコロニーが残存し、所定の限界応力拡大係数ＫＩＣが得られなかった。 No. 3 in which the rolling temperature in the final three passes exceeds the range of the present invention. In No. 27, although rolled at a predetermined cumulative rolling reduction, since the rolling temperature was excessively high, recovery of processing strain occurred and the recrystallization sites became insufficient, and colonies remained after hot-rolled sheet annealing. The predetermined critical stress intensity factor K IC could not be obtained.
　最終３パスの累積圧下率が本発明の範囲を下回るＮｏ．２８では、板厚中央部への圧延加工ひずみの導入が不十分であったために、熱延板焼鈍後にも板厚中央部にコロニーが残存した結果、所定の限界応力拡大係数ＫＩＣが得られなかった。 No. in which the cumulative rolling reduction of the final three passes is below the range of the present invention. In No. 28, since the introduction of rolling strain into the center portion of the plate thickness was insufficient, colonies remained in the center portion of the plate thickness even after hot-rolled sheet annealing, and as a result, a predetermined critical stress intensity factor K IC was obtained. There wasn't.
　熱延板焼鈍温度が本発明の範囲を上回るＮｏ．２９では、生成した再結晶粒の著しい粗大化が生じた結果、所定の限界応力拡大係数ＫＩＣが得られなかった。 No. of hot-rolled sheet annealing temperature exceeding the range of the present invention. In No. 29, as a result of remarkable coarsening of the generated recrystallized grains, a predetermined critical stress intensity factor K IC could not be obtained.
　熱延板焼鈍温度が本発明の範囲を下回るＮｏ．３０では、再結晶が不十分であったためにコロニーが破壊されずに残存した結果、所定の限界応力拡大係数ＫＩＣが得られなかった。 No. of hot-rolled sheet annealing temperature is below the range of the present invention In 30, the recrystallization was insufficient and the colony remained without being destroyed. As a result, the predetermined critical stress intensity factor K IC could not be obtained.
　Ｎｏ．３１はスラブを１３００℃で１ｈ加熱した後に熱間圧延に供し、仕上げ熱間圧延の最終３パスの圧延温度範囲をいずれも１１００℃超えとした例である。Ｎｏ．３１では、最終３パスの圧延実施中に過度の加工ひずみの回復が生じて再結晶サイトが不十分となったために熱延板焼鈍後にもコロニーが残存し、所定の限界応力拡大係数ＫＩＣが得られなかった。 No. No. 31 is an example in which the slab was heated at 1300 ° C. for 1 h and then subjected to hot rolling, and the rolling temperature range in the final three passes of finish hot rolling was all over 1100 ° C. No. In No. 31, the recovery of excessive working strain occurred during the rolling of the final three passes and the recrystallization sites became insufficient, so that colonies remained after hot-rolled sheet annealing, and the predetermined critical stress intensity factor K IC was It was not obtained.
　最終３パスの圧延温度範囲が３パスともに本発明の範囲を下回るＮｏ．３２では、圧延荷重が著しく上昇し、最終３パス目の圧延実施時に荷重が装置許容範囲を超過したために圧延を完了することができず、所定の評価を行うことができなかった。 No. of the rolling temperature range for the final 3 passes is less than the range of the present invention for all 3 passes. In No. 32, the rolling load increased remarkably, and the rolling exceeded the allowable range during the rolling of the final third pass, so that the rolling could not be completed and the predetermined evaluation could not be performed.
　Ｔｉ含有量が本発明の範囲を上回る鋼Ｖを用いたＮｏ．３３では、過剰なＴｉ含有によって再結晶温度が上昇し、所定の熱延板焼鈍を行っても十分な再結晶が生じずにコロニーが残存した結果、所定の限界応力拡大係数ＫＩＣが得られなかった。一方、Ｔｉ含有量が本発明の範囲を下回る鋼Ｗを用いたＮｏ．３４では、熱延板焼鈍時にＣｒ炭窒化物が多量に析出したことによる鋭敏化が生じ、所定の耐食性を得ることができなかった。また、Ｔｉ含有量が本発明の範囲を下回り、かつＮｂ含有量が本発明の範囲を上回る鋼Ｚを用いたＮｏ．３５では、Ｎｂ含有量が過剰であったために、熱延板焼鈍において十分な再結晶組織を得るために過度の高温焼鈍が必要となった結果、熱延板焼鈍によって生成した再結晶粒の著しい粗大化に起因した著しい靭性低下が生じたために所定の限界応力拡大係数ＫＩＣが得られなかった。さらにＴｉ含有量が不十分であったために、熱延板焼鈍時にＣｒ炭窒化物が多量に析出したことによる鋭敏化が生じ、所定の耐食性を得ることもできなかった。 No. using steel V with Ti content exceeding the range of the present invention. In No. 33, the recrystallization temperature rises due to the excessive Ti content, and even when the predetermined hot-rolled sheet annealing is performed, sufficient recrystallization does not occur and colonies remain. As a result, a predetermined critical stress intensity factor K IC is obtained. There wasn't. On the other hand, No. using steel W whose Ti content is below the range of the present invention. In No. 34, sensitization due to the precipitation of a large amount of Cr carbonitride during hot-rolled sheet annealing occurred, and the predetermined corrosion resistance could not be obtained. Further, No. 1 using steel Z in which the Ti content falls below the range of the present invention and the Nb content exceeds the range of the present invention. In No. 35, since the Nb content was excessive, excessive high-temperature annealing was necessary to obtain a sufficient recrystallization structure in the hot-rolled sheet annealing, and as a result, the recrystallized grains generated by the hot-rolled sheet annealing were marked. The predetermined critical stress intensity factor K IC could not be obtained because of a significant decrease in toughness due to coarsening. Further, since the Ti content was insufficient, sensitization occurred due to the precipitation of a large amount of Cr carbonitride during the hot-rolled sheet annealing, and the predetermined corrosion resistance could not be obtained.
　本発明で得られるフェライト系ステンレス熱延焼鈍鋼板は、高い加工性と耐食性が要求される用途、例えばバーリング加工部を有するフランジ等への適用に特に好適である。 The ferritic stainless steel hot-rolled annealed steel sheet obtained by the present invention is particularly suitable for applications that require high workability and corrosion resistance, such as a flange having a burring portion.

Claims (4)

  1. 　質量％で、
    Ｃ：０．００１～０．０２０％、
    Ｓｉ：０．０５～１．００％、
    Ｍｎ：０．０５～１．００％、
    Ｐ：０．０４％以下、
    Ｓ：０．０１％以下、
    Ａｌ：０．００１～０．１００％、
    Ｃｒ：１０．０～２４．０％、
    Ｎｉ：０．０１～０．６０％、
    Ｔｉ：０．１０～０．４０％、
    Ｎ：０．００１～０．０２０％を含有し、残部がＦｅおよび不可避的不純物からなる成分組成を有し、
    限界応力拡大係数ＫＩＣが２０ＭＰａ・ｍ１／２以上であることを特徴とするフェライト系ステンレス熱延焼鈍鋼板。     % By mass
    C: 0.001 to 0.020%,
    Si: 0.05 to 1.00%,
    Mn: 0.05 to 1.00%
    P: 0.04% or less,
    S: 0.01% or less,
    Al: 0.001 to 0.100%,
    Cr: 10.0 to 24.0%,
    Ni: 0.01 to 0.60%,
    Ti: 0.10 to 0.40%,
    N: 0.001 to 0.020% is contained, and the balance has a component composition consisting of Fe and inevitable impurities,
    A ferritic stainless steel hot-rolled annealed steel sheet having a critical stress intensity factor K IC of 20 MPa · m 1/2 or more.
  2. 　成分組成として、質量％で、さらに
    Ｃｕ：０．０１～１．００％、
    Ｍｏ：０．０１～２．００％、
    Ｗ：０．０１～０．２０％、
    Ｃｏ：０．０１～０．２０％のうちから選ばれる１種または２種以上を含有することを特徴とする請求項１に記載のフェライト系ステンレス熱延焼鈍鋼板。     As a component composition, in mass%, Cu: 0.01 to 1.00%,
    Mo: 0.01 to 2.00%
    W: 0.01-0.20%,
    The ferritic stainless steel hot-rolled annealed steel sheet according to claim 1, comprising one or more selected from Co: 0.01 to 0.20%.
  3. 　成分組成として、質量％で、さらに、
    Ｖ：０．０１～０．２０％、
    Ｎｂ：０．０１～０．１０％、
    Ｚｒ：０．０１～０．２０％、
    ＲＥＭ：０．００１～０．１００％、
    Ｂ：０．０００２～０．００２５％、
    Ｍｇ：０．０００５～０．００３０％、
    Ｃａ：０．０００３～０．００３０％のうちから選ばれる１種または２種以上を含有することを特徴とする請求項１または２に記載のフェライト系ステンレス熱延焼鈍鋼板。     As a component composition, in mass%,
    V: 0.01-0.20%,
    Nb: 0.01 to 0.10%,
    Zr: 0.01 to 0.20%,
    REM: 0.001 to 0.100%,
    B: 0.0002 to 0.0025%,
    Mg: 0.0005 to 0.0030%,
    3. The ferritic stainless steel hot-rolled annealed steel sheet according to claim 1 or 2, characterized by containing one or more selected from Ca: 0.0003 to 0.0030%.
  4. 　請求項１～３のいずれかに記載のフェライト系ステンレス熱延焼鈍鋼板の製造方法であって、
    ３パス以上の仕上圧延を行う熱間圧延工程で、仕上圧延の最終３パスを温度範囲８００～１１００℃、且つ前記最終３パスの累積圧下率を２５％以上として熱延鋼板を得て、
    該熱延鋼板に対してさらに８００～１１００℃で熱延板焼鈍を行うことを特徴とするフェライト系ステンレス熱延焼鈍鋼板の製造方法。     A method for producing a ferritic stainless steel hot-rolled annealed steel sheet according to any one of claims 1 to 3,
    In a hot rolling process in which finishing rolling of 3 passes or more is performed, a hot rolled steel sheet is obtained by setting the final 3 passes of finish rolling to a temperature range of 800 to 1100 ° C. and the cumulative rolling reduction of the final 3 passes to 25% or more,
    A method for producing a ferritic stainless steel hot-rolled annealed steel sheet, characterized by further performing hot-rolled sheet anneal on the hot-rolled steel sheet at 800 to 1100 ° C.
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