Classifications

• • 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
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  • 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
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  • 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
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  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/02—Hardening by precipitation
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • 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
  • 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
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/005—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
  • C22C38/02—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
  • 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

Definitions

• the present invention relates to a hot-rolled ferritic stainless steel sheet having excellent blanking workability and being suitable for use for a flange and the like and to a method for manufacturing the hot-rolled ferritic stainless steel sheet.
• exhaust gas recirculation (EGR) systems which are systems for reusing exhaust gases generated by an automobile engine as intake gases for the engine, are being increasingly employed.
• EGR cooler which is used to reduce the gas temperature, and are thereafter resupplied to the engine.
• Flanges used for such exhaust system parts need to have sufficient rigidity. Accordingly, thick (e.g., 5 mm or more in sheet thickness) flanges are used for such exhaust system parts.
• Patent Literature 1 discloses a hot-rolled Nb-containing ferritic stainless steel coil having a sheet thickness of 5.0 to 10.0 mm, the steel having a composition containing, in mass%, C: 0.030% or less, Si: 2.00% or less, Mn: 2.00% or less, P: 0.050% or less, S: 0.040% or less, Cr: 10.00 to 25.00%, N: 0.030% or less, and Nb: 0.01 to 0.80%, with the balance being Fe and incidental impurities.
• the coil has an adjusted hardness of 190 HV or less and an adjusted Charpy impact value at 25°C of 20 J/cm 2 or more.
• Objects of the present invention are to provide, in order to solve such problems, a hot-rolled ferritic stainless steel sheet which has sufficient corrosion resistance and which can prevented the occurrence of cracking when subjected to blanking in a crank press to be formed into a thick flange and to provide a method for manufacturing the hot-rolled ferritic stainless steel sheet.
• the present inventors performed detailed studies to solve the problems and consequently found that blanking for forming a thick flange by using a processing method with a relatively high processing speed, such as a method using a crank press, can be accomplished without causing cracking when the threshold stress intensity factor, K IC , of the steel sheet is increased. Specifically, it was found that, by ensuring that the threshold stress intensity factor K IC is 25 MPa ⁇ m 1/2 or more, the occurrence of cracking in the blanked edge surface when blanking for forming a thick flange is performed can be effectively prevented even if a processing method with a high processing speed, such as a method using a crank press, is used, and therefore practical use of the steel sheet for thick flanges can be sufficiently realized.
• the present inventors conducted detailed studies to solve the problems. As a result, it was discovered that, in the case that a thick steel sheet of more than 5.0 mm sheet thickness is to be blanked to form a thick flange by using a processing method with a high processing speed, such as a method using a crank press, without causing cracking, the workability cannot be accurately evaluated by using the Charpy impact value, which has been used in the conventional technique, but can be accurately evaluated by using the threshold stress intensity factor K IC , which is a toughness evaluation index in the field of thick steel plates. The reason for this is thought to be as follows.
• the plastic deformation region in and near the blanked edge surface resulting from the processing is large with respect to the sheet thickness, and therefore the fracture phenomenon involved in the forming cannot be uniquely defined with an approach associated with the fracture mechanics
• the plastic deformation region in and near the blanked edge surface resulting from the processing is sufficiently small with respect to the sheet thickness and thus the small scale yielding state is sufficiently satisfied, and therefore the fracture phenomenon involved in a specific processing process can be handled with the stress intensity factor, which is a quantitative index associated with the fracture mechanics, and particularly can be accurately evaluated by using the threshold value, that is, the threshold stress intensity factor K IC .
• the present inventors closely investigated the relationship between the threshold stress intensity factor K IC and the presence or absence of cracking that may occur when blanking, using a crank press, for forming a flange having a predetermined shape is performed.
• the threshold stress intensity factor K IC is 25 MPa ⁇ m 1/2 or more, the occurrence of cracking in the blanked edge surface when blanking for forming a thick flange is performed by using a crank press can be effectively prevented, and therefore practical use for thick flanges can be sufficiently realized.
• the threshold stress intensity factor K IC of the hot-rolled steel sheet is improved.
• the threshold stress intensity factor K IC is a stress intensity factor determined by cutting a CT test piece in accordance with ASTM E399 from a sheet width central portion in such a manner that a fatigue pre-crack is introduced in a direction perpendicular to the rolling direction and the stress axis is in a direction parallel to the rolling direction and testing the test piece in accordance with ASTM E399.
• the present invention makes it possible to obtain a hot-rolled ferritic stainless steel sheet which has sufficient corrosion resistance and which has excellent toughness to prevent the occurrence of cracking when subjected to blanking in a crank press to be formed into a thick flange.
• the phrase "has excellent toughness to prevented the occurrence of cracking when subjected to blanking in a crank press to be formed into a thick flange” means that the threshold stress intensity factor K IC is 25 MPa ⁇ m 1/2 or more, as determined by cutting a CT test piece in accordance with ASTM E399 from a sheet width central portion in such a manner that a fatigue pre-crack is introduced in a direction perpendicular to the rolling direction and the stress axis is in a direction parallel to the rolling direction and testing the test piece in accordance with ASTM E399.
• a hot-rolled ferritic stainless steel sheet has a chemical composition containing, 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 0.50%, N: 0.001 to 0.020%, Cr: 11.0 to 24.0%, Ni: 0.01 to 2.00%, and Nb: 0.12 to 0.80%, with the balance being Fe and incidental impurities, and the hot-rolled ferritic stainless steel sheet has a threshold stress intensity factor K IC of 25 MPa ⁇ m 1/2 or more.
• a hot-rolled ferritic stainless steel sheet of the present invention has a chemical composition containing, 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 0.50%, N: 0.001 to 0.020%, Cr: 13.0 to 24.0%, Ni: 0.01 to 0.60%, and Nb: 0.12 to 0.80%, with the balance being Fe and incidental impurities, and the hot-rolled ferritic stainless steel sheet has a threshold stress intensity factor K IC of 25 MPa ⁇ m 1/2 or more.
• the threshold stress intensity factor K IC is a stress intensity factor determined by cutting a CT test piece in accordance with ASTM E399 from a sheet width central portion in such a manner that a fatigue pre-crack is introduced in a direction perpendicular to the rolling direction and the stress axis is in a direction parallel to the rolling direction and testing the test piece in accordance with ASTM E399.
• the present inventors performed detailed studies on factors responsible for cracking that occurred when blanking was performed on various types of hot-rolled ferritic stainless steel sheets having a sheet thickness of 5.0 mm in a crank press to form a flange shaped with a hole having a diameter of 30 mm. As a result, it was found that, in the steel sheets in which cracking occurred, the cracking occurred in such a manner that microcracking generated in or near a central part in the sheet thickness of a blanked edge surface in a direction perpendicular to the blanking direction, and that the microcracks propagated.
• the present inventors performed detailed studies on the relationship between the generation and propagation of microcracks and the material properties. As a result, it was found that there is a tendency for propagation of microcracks to occur more easily as the threshold stress intensity factor of a steel sheet decreases. Accordingly, blanking for forming a flange such as that described above was attempted by using various hot-rolled ferritic stainless steel sheets (5.0 mm in sheet thickness), and as a result, it was found that no cracking occurred in steel sheets having a threshold stress intensity factor of 25 MPa ⁇ m 1/2 or more and that cracking was more likely to occur in steel sheets in which the threshold stress intensity factor was less than 25 MPa ⁇ m 1/2 , the threshold stress intensity factors being measured using a specified measurement method.
• the present inventors conducted detailed research on composition of steel and conditions for hot rolling to perform studies on techniques for improving the threshold stress intensity factor of hot-rolled ferritic stainless steel sheets. As a result, the following was found.
• the sheet thickness of hot-rolled ferritic stainless steel sheets of the present invention is not particularly limited but preferably not less than 5.0 mm because a sheet thickness suitable for a thick flange is desirable. Furthermore, the sheet thickness is not particularly limited but preferably not more than 15.0 mm and more preferably not more than 10.0 mm.
• the present inventors diligently performed studies on techniques for effectively and sufficiently introducing rolling strains to the central part in the sheet thickness in a hot rolling process, from the standpoints of both steel components and the method of hot rolling.
• the temperature range for the final three passes of finish hot rolling is controlled to be appropriate and, in such a state, rolling is performed at a high accumulated rolling reduction ratio, and as a result, rolling strains are sufficiently and effectively introduced to the central part in the sheet thickness while recovery of rolling strains is suppressed, and therefore the predetermined threshold stress intensity factor K IC can be achieved.
• C is contained in an amount more than 0.020%, workability and corrosion resistance in the weld zone noticeably deteriorate.
• a C content as low as possible is preferable from the perspectives of corrosion resistance and workability but is not preferable from the standpoint of manufacturing because obtaining a C content of less than 0.001% requires that refining be performed for a long period of time. Accordingly, the C content is within the range of 0.001 to 0.020%.
• the C content is preferably not less than 0.003% and more preferably not less than 0.004%.
• the C content is preferably not more than 0.015% and more preferably not more than 0.012%.
• Si has an effect of improving corrosion resistance in the weld zone by being concentrated in the oxide layer formed during welding.
• Si is an element also useful as a deoxidizing element in the steelmaking process. These effects are obtained when Si is contained in an amount of 0.05% or more. The effects increase as the content increases.
• the presence of Si in an amount more than 1.00% is not preferable because, in the hot rolling process, the rolling load increases and the formation of scale is noticeable, and in the annealing process, pickling properties are deteriorated because of the formation of a Si-rich layer in the surface layer of the steel sheet, and consequently, an increase in surface defects and an increase in manufacturing cost are induced.
• the Si content is 0.05 to 1.00%. It is preferable that the Si content not be less than 0.10%. Furthermore, the Si content is preferably not more than 0.60% and more preferably not more than 0.40%.
• Mn has an effect of increasing the strength of steel and also acts as a deoxidizer. To obtain the effects, Mn need to be contained in an amount of 0.05% or more. If the Mn content is more than 1.00%, however, precipitation of MnS, which acts as a corrosion initiation site, is promoted, which deteriorates corrosion resistance. Accordingly, the Mn content is 0.05 to 1.00%. It is preferable that the Mn content not be less than 0.10%. Furthermore, the Mn content is preferably not more than 0.50% and more preferably not more than 0.30%.
• P is an element inevitably present in steel, but, since P is an element harmful to corrosion resistance and workability, it is preferable that the P content be as low as possible. In particular, if the P content is more than 0.04%, workability noticeably deteriorates because of solid solution strengthening. Accordingly, the P content is not more than 0.04%. It is preferable that the P content not be more than 0.03%.
• S is an element inevitably present in steel, but, since S is an element harmful to corrosion resistance and workability, it is preferable that the P content be as low as possible. In particular, if the S content is more than 0.01%, corrosion resistance noticeably deteriorates. Accordingly, the S content is not more than 0.01%. It is preferable that the S content not be more than 0.008%. It is more preferable that the S content not be more than 0.003%.
• Al is an effective deoxidizer.
• Al has a higher affinity for N than does Cr and therefore has an effect of, if N penetrates into a weld, causing N to precipitate as an Al nitride rather than as a Cr nitride, thereby suppressing sensitization.
• These effects are obtained when Al is contained in an amount of 0.001% or more.
• the presence of Al in an amount more than 0.50% deteriorates the penetration characteristics for welding, which deteriorates welding workability, and is therefore not preferable.
• the Al content is within the range of 0.001 to 0.50%.
• the Al content is preferably not more than 0.20% and more preferably not more than 0.10%.
• the N content is more than 0.020%, workability and corrosion resistance in the weld zone noticeably deteriorate.
• a N content as low as possible is preferable from the perspective of corrosion resistance, but reducing the N content to less than 0.001% requires long-time refining, which results in an increase in the manufacturing cost and a deterioration in productivity, and is therefore not preferable.
• the N content is within the range of 0.001 to 0.020%.
• the N content is preferably not less than 0.003% and more preferably not less than 0.005%.
• the N content is preferably not more than 0.015% and more preferably not more than 0.012%.
• Cr is the most important element for ensuring the corrosion resistance of stainless steel. If the Cr content is less than 11.0%, sufficient corrosion resistance is not exhibited in an automobile exhaust gas atmosphere. On the other hand, if Cr is present in an amount more than 24.0%, toughness significantly deteriorates because of formation of the ⁇ (sigma) phase, and, in the present invention, the predetermined threshold stress intensity factor cannot be achieved. Accordingly, the Cr content is within the range of 11.0 to 24.0%.
• the Cr content is preferably not less than 13.0%, more preferably not less than 14.0%, even more preferably not less than 16.0%, and still more preferably not less than 17.0%. Furthermore, the Cr content is preferably not more than 21.5%, more preferably not more than 20.0%, and even more preferably not more than 18.5%.
• Ni is an element that improves the corrosion resistance of stainless steel and an element that suppresses the progression of corrosion in a corrosive environment in which the passivation film is not formed and active dissolution occurs. Furthermore, Ni is a strong austenite-forming element and has an effect of reducing the formation of ferrite in a weld, thereby suppressing sensitization due to precipitation of Cr carbonitrides. This effect is obtained when Ni is contained in an amount of 0.01% or more. The effect increases as the Ni content increases. If the Ni content is more than 2.00%, however, workability deteriorates, and in addition, stress corrosion cracking tends to occur. In addition, since Ni is an expensive element, increasing the Ni content increases the manufacturing cost and is therefore not preferable.
• the Ni content is 0.01 to 2.00%.
• the Ni content is preferably not less than 0.05% and more preferably not less than 0.10%.
• the Ni content is preferably not more than 1.00%, more preferably not more than 0.60%, even more preferably not more than 0.50%, and still more preferably not more than 0.45%.
• Nb In a hot rolling process, Nb combines with C and N and precipitates as Nb carbonitrides. Precipitated Nb carbonitrides have an effect of pinning the movement of dislocations and thus preventing rolling strains introduced by hot rolling from being eliminated through recovery. Consequently, recovery during hot rolling is retarded, and a decrease in the rolling strain density due to the occurrence of excessive recovery can be prevented.
• the effect described above is produced when Nb is contained in an amount of 0.12% or more. However, if the Nb content is more than 0.80%, toughness, on the contrary, may deteriorate because of the formation of a Laves phase, and a significant increase in the rolling load in hot rolling makes it difficult to employ the method of hot rolling provided by the present invention.
• the Nb content is within the range of 0.12 to 0.80%.
• the Nb content is preferably not less than 0.15% and more preferably not less than 0.20%.
• the Nb content is preferably not more than 0.75% and more preferably not more than 0.60%.
• the present invention is a ferritic stainless steel containing the above-described essential elements, with the balance being Fe and incidental impurities.
• one or more selected from Cu, Mo, W, and Co may be contained, or alternatively or additionally, one or more selected from Ti, V, Zr, REM, B, Mg, and Ca may be contained.
• the ranges of the contents are as follows.
• Cu is an element particularly effective for improving the corrosion resistance of the base metal and a weld when it is in an aqueous solution or when drops of weakly acidic water adhere thereto. This effect is obtained when Cu is contained in an amount of 0.01% or more. The effect increases as the Cu content increases. If Cu is contained in an amount more than 1.50%, however, a deterioration in hot workability may induce surface defects. In addition, in some cases, descaling after annealing is difficult. Accordingly, when Cu is to be contained, it is preferable that the Cu content be within the range of 0.01 to 1.50%. The Cu content is more preferably not less than 0.10% and even more preferably not less than 0.30%. Furthermore, the Cu content is more preferably not more than 0.60% and even more preferably not more than 0.45%.
• Mo is an element that noticeably improves the corrosion resistance of stainless steel. This effect is obtained when Mo is contained in an amount of 0.01% or more. The effect improves as the content increases. If the Mo content is more than 2.00%, however, an increase in the rolling load in hot rolling may deteriorate manufacturability, and the strength of the steel sheet may increase excessively. Furthermore, since Mo is an expensive element, the presence of Mo in large amounts increases the manufacturing cost. Accordingly, when Mo is to be contained, it is preferable that the Mo content be 0.01 to 2.00%. It is more preferable that the Mo content not be less than 0.10%. Furthermore, it is more preferable that the Mo content not be more than 1.40%.
• Mo also has an effect of deteriorating toughness, and it is therefore preferable, when Ti is contained in an amount of 0.15% or more, that the Mo content be 0.30 to not more than 1.40%. When Ti is contained in an amount of 0.15% or more, it is more preferable that the Mo content not be less than 0.40%. Furthermore, when Ti is contained in an amount of 0.15% or more, it is more preferable that the Mo content not be more than 0.90%.
• W has an effect of improving corrosion resistance. This effect is obtained when W is contained in an amount of 0.01% or more. If W is contained in an amount more than 0.20%, however, strength increases, and an increase in rolling load, for example, may deteriorate manufacturability. Accordingly, when W is to be contained, it is preferable that the W content be within the range of 0.01 to 0.20%. It is more preferable that the W content not be less than 0.05%. Furthermore, it is more preferable that the W content not be more than 0.15%.
• Co is an element that improves toughness. This effect is obtained when Co is contained in an amount of 0.01% or more. On the other hand, if the Co content is more than 0.20%, workability may deteriorate. Accordingly, when Co is to be contained, it is preferable that the Co content be within the range of 0.01 to 0.20%.
• Ti is an element having a higher affinity for C and N than does Cr and has an effect of, by precipitating as carbides or nitrides, suppressing sensitization caused by precipitation of Cr carbonitrides. To obtain this effect, Ti needs to be contained in an amount of 0.01% or more. If the Ti content is more than 0.30%, however, TiN may precipitate excessively, and consequently, good surface properties may not be obtained. Accordingly, when Ti is to be contained, it is preferable that the Ti content be within the range of 0.01 to 0.30%. The Ti content is more preferably not less than 0.03% and even more preferably not less than 0.10%. Furthermore, the Ti content is more preferably not more than 0.20% and even more preferably not more than 0.15%.
• V 0.01 to 0.20%
• V forms carbonitrides with C and N and suppresses sensitization during welding and thus improves corrosion resistance in the weld zone. This effect is obtained when the V content is 0.01% or more. On the other hand, if the V content is more than 0.20%, workability and toughness may noticeably deteriorate. Accordingly, it is preferable that the V content be 0.01 to 0.20%. It is more preferable that the V content not be less than 0.05%. Furthermore, it is more preferable that the V content not be more than 0.15%.
• Zr has an effect of, by combining with C and N, suppressing sensitization. This effect is obtained when Zr is contained in an amount of 0.01% or more. On the other hand, if Zr is contained in an amount more than 0.20%, workability may noticeably deteriorate. Accordingly, when Zr is to be contained, it is preferable that the Zr content be within the range of 0.01 to 0.20%. It is more preferable that the Zr content not be more than 0.10%.
• REM rare earth metal
• REM has an effect of improving oxidation resistance and inhibits the formation of a weld oxide layer (welding temper color) and thus suppresses the formation of a Cr depletion zone immediately below the oxide layer. This effect is obtained when one or more REM is contained in an amount of 0.001% or more.
• one or more REM is contained in an amount more than 0.100%, manufacturability, such as pickling properties for cold rolling-annealing, may deteriorate.
• the REM content be within the range of 0.001 to 0.100%. It is more preferable that the REM content not be more than 0.050%.
• B is an element effective for improving secondary working embrittlement resistance after deep drawing. This effect is obtained when the B content is 0.0002% or more. On the other hand, if B is contained in an amount more than 0.0025%, workability and toughness may deteriorate. Accordingly, when B is to be contained, it is preferable that the B content be within the range of 0.0002 to 0.0025%. It is more preferable that the B content not be less than 0.0003%. Furthermore, it is more preferable that the B content not be more than 0.0006%.
• Mg is an element that improves the equiaxed crystal ratio of a slab and is effective for improving workability and toughness. This effect is obtained when Mg is contained in an amount of 0.0005% or more. On the other hand, if the Mg content is more than 0.0030%, surface properties of the steel may deteriorate. Accordingly, when Mg is to be contained, it is preferable that the Mg content be within the range of 0.0005 to 0.0030%. It is more preferable that the Mg content not be less than 0.0010%. Furthermore, it is more preferable that the Mg content not be more than 0.0020%.
• Ca has an effect of refining inclusions formed during steelmaking and continuous casting and is a component particularly effective for preventing nozzle blockage in continuous casting.
• the effect is obtained when Ca is contained in an amount of 0.0005% or more. If Ca is contained in an amount more than 0.0030%, however, formation of CaS may decrease corrosion resistance. Accordingly, when Ca is to be contained, it is preferable that the Ca content be within the range of 0.0005 to 0.0030%.
• the Ca content is more preferably not more than 0.0015% and even more preferably not more than 0.0010%.
• Threshold stress intensity factor K IC 25 MPa ⁇ m 1/2 or more
• Hot-rolled ferritic stainless steel sheets of the present invention have a threshold stress intensity factor K IC of 25 MPa ⁇ m 1/2 or more, and as a result, when subjected to blanking in a crank press to be formed into a thick flange, the occurrence of cracking can be prevented.
• the threshold stress intensity factor K IC is preferably not less than 30 MPa ⁇ m 1/2 , more preferably not less than 35 MPa ⁇ m 1/2 , and even more preferably not less than 40 MPa ⁇ m 1/2 .
• the "thick flange" is not particularly limited but may be, for example, a flange of 5.0 mm or more sheet thickness.
• the flange is preferably a flange of 5.0 to 15.0 mm sheet thickness and more preferably a flange of 5.0 to 10.0 mm sheet thickness.
• the temperature is the surface temperature of a steel slab, a hot-rolled steel sheet, or the like measured with a surface thermometer or the like unless otherwise specified.
• the hot-rolled ferritic stainless steel sheet of the present invention can be obtained by using a steel slab having a chemical composition as described above and ensuring that, in hot rolling including rough rolling and finish rolling that includes three or more passes, the temperature range for rolling in the final three passes of the finish rolling is 800 to 1100°C and the accumulated rolling reduction ratio for the final three passes is 25% or more.
• a molten steel with a chemical composition as described above is prepared using a steelmaking process known in the art, using, for example, a converter, an electric furnace, or a vacuum melting furnace.
• the steel is subjected to a continuous casting process or an ingot casting-slabbing process to form a steel starting material (slab) .
• the slab is subjected to hot rolling after being heated at 1100 to 1250°C for 1 to 24 hours, or the as-cast slab is directly subjected to hot rolling without being heated.
• the rough rolling is not particularly limited, effectively destroying the casting structure prior to the finish hot rolling provides an advantage for grain refining in the subsequent finish hot rolling, and therefore, further improvement in toughness due to refinement of the metallurgical structure after hot rolling can be expected, and accordingly, it is preferable that the accumulated rolling reduction ratio for the rough rolling not be less than 65%.
• rolling is performed in the finish hot rolling to obtain a predetermined sheet thickness. In the final three passes of the finish rolling, rolling is performed within the temperature range of 800 to 1100°C and at the accumulated rolling reduction ratio of 25% or more.
• Rolling temperature range for final three passes of finish hot rolling 800 to 1100°C
• the threshold stress intensity factor after hot rolling is the predetermined threshold stress intensity factor
• the accumulated rolling reduction ratio for the final three passes of the finish hot rolling is less than 25%, rolling strains are not effectively introduced to the central part in the sheet thickness, and as a result, the predetermined threshold stress intensity factor cannot be achieved. Accordingly, the accumulated rolling reduction ratio for the final three passes is 25% or more. It is preferable that the accumulated rolling reduction ratio not be less than 30%. It is more preferable that the accumulated rolling reduction ratio not be less than 35%.
• the upper limit of the accumulated rolling reduction ratio is not particularly limited but is preferably not more than 60% because, if the accumulated rolling reduction ratio is excessively high, an increase in the rolling load may deteriorate manufacturability.
• the rolling temperatures for the final three passes of the finish hot rolling are lower than 800°C, the rolling load significantly increases with a decrease in the temperature of the steel sheet, and therefore such temperatures are not preferable from the standpoint of manufacturing.
• the rolling temperatures for the final three passes are higher than 1100°C, rolling strains introduced by the rolling are eliminated through excessive recovery, and the predetermined threshold stress intensity factor cannot be achieved. Accordingly, the rolling temperatures for the final three passes are within the range of 800 to 1100°C. It is preferable that the rolling temperatures for the final three passes be within a range of 800 to 1050°C. It is more preferable that the rolling temperatures for the final three passes be within a range of 850 to 1000°C.
• the rolling temperature range for the first pass be 950 to 1100°C
• the rolling temperature range for the second pass, which follows the first pass be 925 to 1075°C
• the rolling temperature range for the third pass, which follows the second pass be 875 to 1050°C.
• high-reduction rolling is applied with the temperature range being controlled. If rolling in which high-reduction rolling is applied is performed over the final four or more passes, strains cannot be sufficiently introduced to the central part in the sheet thickness even when the accumulated rolling reduction ratio is unchanged because the reduction ratio is distributed among the passes, and in addition, since the accumulated interval time between passes increases, recovery during transfer between passes is promoted, which reduces the strain introducing effect, and as a result, achieving the predetermined threshold stress intensity factor is difficult.
• the control of the rolling temperature and the accumulated rolling reduction ratio of the finish rolling be performed in the final two or less passes because, in such a case, since the high-reduction rolling at the accumulated rolling reduction ratio of 25% or more is performed in two passes, the rolling load significantly increases, which may decrease manufacturability. Accordingly, in the method for manufacturing the hot-rolled ferritic stainless steel sheet of the present invention, the rolling temperature and the accumulated rolling reduction ratio of the final three passes of the finish rolling are controlled.
• the maximum number of passes not be more than 15 passes because, if the maximum number of passes is more than 15 passes, a decrease in the temperature of the steel sheet due to increased number of contact with the rolling rolls is more likely to occur, and consequently, for example, heating of the steel sheet from the outside becomes necessary to maintain the temperature to be within the predetermined temperature range, and as a result, manufacturability may deteriorate or the manufacturing cost may increase. It is more preferable that the maximum number of passes not be more than 10 passes.
• the coiling temperature is not particularly limited, but, if the coiling temperature is between higher than 450°C and lower than 500°C, embrittlement due to 475°C embrittlement may occur. Accordingly, it is preferable that the coiling temperature not be higher than 450°C or not be lower than 500°C. Coiling may be performed at a temperature not higher than 450°C after accelerated cooling using water mist cooling or the like is performed following the final rolling. This is more preferable because elimination of rolling strains due to recovery after coiling can be further suppressed.
• the hot-rolled steel sheet obtained in accordance with the present invention may be subjected to hot-rolled-sheet annealing to form a hot-rolled and annealed steel sheet.
• the hot-rolled steel sheet provided by the present invention has excellent toughness and therefore can be subjected to hot-rolled-sheet annealing in a continuous annealing line, which is avoided in the related art because of concern over failure due to low toughness.
• the resulting hot-rolled and annealed steel sheet may be subsequently subjected to cold rolling and cold-rolled-sheet annealing.
• Molten stainless steels each having the chemical composition shown in Table 1 were prepared by steelmaking using a 150-ton capacity converter and involving refining using a strong-stirring vacuum oxygen decarburization process (SS-VOD).
• the molten stainless steel was continuously cast to form a steel slab of 1000 mm width and 200 mm thickness.
• the slab was heated at 1200°C for 1 hour and thereafter subjected to hot rolling.
• the hot rolling the slab was subjected to reverse rough rolling using three stands to form a steel sheet of approximately 40 mm in thickness, which was subsequently subjected to finish rolling including seven passes to form a hot-rolled steel sheet.
• the final three passes (fifth pass, sixth pass, and seventh pass) of the finish rolling were performed under the conditions listed in Table 2.
• the resulting hot-rolled steel sheet was evaluated as follows.
• a CT test piece in accordance with ASTM E399 was cut from a sheet width central portion in such a manner that a fatigue pre-crack was introduced in a direction perpendicular to the rolling direction and the stress axis was in a direction parallel to the rolling direction.
• the threshold stress intensity factor K IC of the test piece was determined in accordance with ASTM E399. Threshold stress intensity factors equal to or more than 25 MPa ⁇ m 1/2 were rated as "pass”, and threshold stress intensity factors less than 25 MPa ⁇ m 1/2 were rated as "fail”.
• a test piece of 60 ⁇ 100 mm was cut from the resulting hot-rolled steel sheet.
• the surface to be evaluated was polished with #600 emery paper, and thereafter the edge surface were sealed to prepare such a test piece, which was subjected to a cyclic salt spray test as specified in JIS H 8502.
• a cyclic salt spray test five cycles were conducted, with one cycle being as follows: salt spraying (5-mass% NaCl, 35°C, 2-hour spraying) ⁇ drying (60°C, 4 hours, relative humidity of 40%) ⁇ exposure to humidity (50°C, 2 hours, relative humidity ⁇ 95%).
• the evaluation surface of the test piece after conducting five cycles of the cyclic salt spray test was photographed, the rust area of the evaluation surface of the test piece was measured by image analysis, and, from the ratio of the rust area to the total area of the test piece, the rust ratio (rust area of test piece/total area of test piece) ⁇ 100 [%]) was calculated.
• Rust ratios equal to or less than 10% were rated as "pass” ( ⁇ ), as being particularly good in corrosion resistance, rust ratios more than 10% and not more than 25% were rated as "pass” ( ⁇ ), and rust ratios more than 25% were rated as "fail” ( ⁇ ).
• Hot-rolled ferritic stainless steel sheets that can be obtained in accordance with the present invention have, in particular, excellent blanking workability for crank press and are, in particular, suitable for use in, for example, forming a thick flange that is manufactured by, for example, blanking using a crank press or another technique and which needs to have high workability and corrosion resistance.

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