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
  • 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
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  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
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  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—Heat treatment of ferrous alloys containing Mn
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  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/008—Heat treatment of ferrous alloys containing Si
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  • 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
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  • 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
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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
  • 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
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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
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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  • 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%
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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
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
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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
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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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
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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  • 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
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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
  • C22C38/18—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
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  • 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
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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
  • C22C38/18—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
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  • 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
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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  • 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 hot-rolled and annealed ferritic stainless steel sheet excellent in terms of workability which can preferably be used for, for example, a flange and to a method for manufacturing the steel sheet.
• EGR exhaust gas recirculation
• plain carbon steel is used for such thick-walled flanges.
• flanges which are used for parts such as an EGR system, through which high-temperature exhaust gas passes are required to have sufficient corrosion resistance. Therefore, consideration is being given to using stainless steel, which is superior to plain carbon steel in terms of corrosion resistance, in particular, ferritic stainless steel, with which thermal stress is less likely to be generated because of its comparatively low thermal expansion coefficient, and there is a strong demand for a ferritic stainless steel sheet having a large thickness (for example, a thickness of 5 mm or more) which can be used for thick-walled flanges.
• Patent Literature 1 discloses a hot-rolled ferritic stainless steel sheet having a chemical composition containing, by 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 18.0% (not inclusive), 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%, and the balance being Fe and inevitable impurities, in which the contents of Nb, C, and N satisfy the relationship Nb/(C + N) ⁇ 16, a Charpy impact value at a temperature of 0°C is 10 J/cm 2 or more, and a thickness is 5.0 mm to 9.0 mm.
• An object of the present invention is, by solving the problems described above, to provide a hot-rolled and annealed ferritic stainless steel sheet which has sufficient corrosion resistance and with which it is possible to inhibit a crack from occurring when punching work is performed to form a thick-walled flange and to provide a method for manufacturing the steel sheet.
• the present inventors conducted detailed investigations to solve the problems and, as a result, found that, in the case where a steel sheet having a large thickness of more than 5.0 mm is formed into a thick-walled flange having a portion subjected to burring work without a crack being generated, although it is not possible to make accurate evaluations of workability on the basis of a Charpy impact value, which has been conventionally used, it is possible to make accurate evaluations of workability on the basis of the threshold stress intensity factor K IC , which is the evaluation index of toughness in fields involving thick steel plates.
• the present inventors conducted detailed investigations regarding the relationship between crack generation and the threshold stress intensity factor K IC when flange-forming work is performed to form a specified flange shape and, as a result, found that, by controlling the threshold stress intensity factor K IC to be 20 MPa ⁇ m 1/2 or more, since it is possible to effectively inhibit a crack from occurring in a portion subjected to burring work when flange-forming work is performed to form a thick-walled flange having a portion subjected to burring work, it is possible to sufficiently put the thick steel sheet into practical use for a thick-walled flange having a portion subjected to burring work.
• threshold stress intensity factor K IC refers to the stress intensity factor obtained by performing a test in accordance with ASTM E 399 on a compact tension (CT) test piece in accordance with ASTM E 399 taken from the central portion in the width direction of a steel sheet so that the direction of a fatigue precrack is in a direction perpendicular to the rolling direction and the stress axis is in a direction parallel to the rolling direction.
• the expression "excellent workability with which it is possible to inhibit a crack from occurring when punching work is performed to form a thick-walled flange” refers to a case where, when a test is performed in accordance with ASTM E 399 on a CT test piece in accordance with ASTM E 399 taken from the central portion in the width direction of a steel sheet so that the direction of a fatigue precrack is in a direction perpendicular to the rolling direction and the stress axis is in a direction parallel to the rolling direction to obtain the threshold stress intensity factor, the obtained threshold stress intensity factor K IC is 20 MPa ⁇ m 1/2 or more.
• the hot-rolled and annealed ferritic stainless steel sheet according to the present invention has a chemical composition containing, 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%, and the balance being Fe and inevitable impurities, and a threshold stress intensity factor K IC of 20 MPa ⁇ m 1/2 or more.
• threshold stress intensity factor K IC refers to the stress intensity factor obtained by performing a test in accordance with ASTM E 399 on a CT test piece in accordance with ASTM E 399 taken from the central portion in the width direction of a steel sheet so that the direction of a fatigue precrack is in a direction perpendicular to the rolling direction and the stress axis is in a direction parallel to the rolling direction.
• the present inventors conducted detailed investigations regarding the reasons why a crack was generated when flange-forming work was performed on each of various kinds of ferritic stainless steel sheets having a thickness of 5.0 mm to form a flange having a portion subjected to burring work so that the periphery of a flange hole having a diameter of 30 mm ⁇ was raised to a height of 10 mm with respect to the surface of a blank steel sheet (in the punched state) and, as a result, found that, in the case of the steel sheet in which a crack was generated, the crack was generated due to a microcrack, which had been generated in the vicinity of the central portion in the thickness direction of the steel sheet in a punched end surface, and had grown significantly when burring work was performed.
• the present inventors conducted detailed investigations regarding the relationship between the significant growth of a microcrack and material properties and, as a result, found that, the smaller the threshold stress intensity factor K IC of a steel sheet, the more likely the microcrack is to grow significantly. Therefore, the present inventors tried to perform the above-described flange-forming work on various kinds of hot-rolled and annealed ferritic stainless steel sheets (having a thickness of 5.0 mm) and, as a result, found that a crack caused by the growth of a microcrack tends to occur in the case of a steel sheet having a threshold stress intensity factor K IC of less than 20 MPa ⁇ m 1/2 , where the threshold stress intensity factor K IC is obtained by using the specified determination method.
• the present inventors conducted detailed investigations regarding the cracks of the steel sheets described above to clarify the reasons why the threshold stress intensity factors K IC of the steel sheets, in which cracks were generated when the above-described flange-forming work was performed, was small and, as a result, found that, in the case of a steel sheet in which a crack is generated, a microcrack generated in the vicinity of the central portion in the thickness direction in a punched end surface grows significantly at grain boundaries of the crystal grains in the vicinity of the central portion in the thickness direction.
• the thickness of the hot-rolled and annealed ferritic stainless steel sheet according to the present invention since it is preferable that the thickness be that of the steel sheet to be used for a thick-walled flange, it is preferable that the thickness be 5.0 mm or more or more preferably 7.0 mm or more. In addition, although there is no particular limitation on the thickness described above, it is preferable that the thickness be 15.0 mm or less or more preferably 10.0 mm or less.
• the present inventors diligently conducted investigations regarding a method for effectively and sufficiently applying strain due to rolling work across the entire thickness of a steel sheet in a hot-rolling process and, as a result, found that, by performing rolling in the final 3 passes of finish hot-rolling in an appropriately controlled temperature range with a large accumulated rolling reduction ratio, since strain due to rolling work is sufficiently and effectively applied to the central portion in the thickness direction while the recovery of strain due to rolling work is suppressed, it is possible to form a hot-rolled steel sheet microstructure retaining sufficient strain due to rolling work which functions as recrystallization sites in a subsequent hot-rolled-sheet annealing process, resulting in colonies being effectively broken in the subsequent hot-rolled-sheet annealing process.
• a hot-rolled-sheet annealing process is a process in which a worked microstructure formed by performing hot-rolling is recrystallized. Therefore, it is necessary that annealing be performed at a temperature at which sufficient recrystallization occurs.
• hot-rolled-sheet annealing is performed at an excessively high temperature, there is a significant coarsening of recrystallized grains, although recrystallization occurs.
• the present inventors conducted detailed investigations regarding the relationship between the grain diameter of a recrystallized crystal grain and an annealing temperature and, as a result, found that, by controlling a hot-rolled-sheet annealing temperature to be 1100°C or lower, it is possible to suppress the formation of such coarse recrystallized grains that there is a significant decrease in the threshold stress intensity factor K IC .
• the C content is more than 0.020%, there is a significant deterioration in workability and in the corrosion resistance of a weld.
• the C content be as small as possible from the viewpoint of corrosion resistance and workability, it is not preferable that the C content be less than 0.001% from the viewpoint of manufacturing conditions, because this results in an increase in the time taken to perform refining. Therefore, the C content is set to be in the range of 0.001% to 0.020%. It is preferable that the C content be 0.003% or more or more preferably 0.004% or more. In addition, it is preferable that the C content be 0.015% or less or more preferably 0.012% or less.
• the Si content is 0.05% or more, and such effects increase with an increase in Si content.
• the Si content is set to be 0.05% to 1.00%. It is preferable that the Si content be 0.10% or more. In addition, it is preferable that the Si content be 0.60% or less or more preferably 0.40% or less.
• Mn is effective for improving the strength of steel and has a function as a deoxidizing agent. It is necessary that the Mn content be 0.05% or more to obtain such effects. However, in the case where the Mn content is more than 1.00%, since the formation of MnS, which becomes a starting point at which corrosion occurs, is promoted, there is a deterioration in corrosion resistance. Therefore, the Mn content is set to be 0.05% to 1.00%. It is preferable that the Mn content be 0.10% or more. In addition, it is preferable that the Mn content be 0.60% or less or more preferably 0.30% or less.
• the P content is set to be 0.04% or less or preferably 0.03% or less.
• the S content is set to be 0.01% or less, preferably 0.008% or less, or more preferably 0.003% or less.
• Al is an effective deoxidizing agent. Moreover, since Al has a higher affinity for nitrogen than Cr does, nitrogen is precipitated in the form of Al nitrides instead of Cr nitrides when nitrogen enters a weld, which results in sensitization being effectively inhibited. Such effects are obtained in the case where the Al content is 0.001% or more. However, it is not preferable that the Al content be more than 0.100%, because this results in deterioration in welding workability due to a deterioration in weld penetration capability when welding is performed. Therefore, the Al content is set to be in the range of 0.001% to 0.100%. It is preferable that the Al content be 0.005% or more or more preferably 0.010% or more. In addition, it is preferable that the Al content be 0.060% or less or more preferably 0.040% or less.
• the Cr content is set to be in the range of 10.0% to 24.0%. It is preferable that the Cr content be 14.0% or more, more preferably 16.0% or more, or even more preferably 17.0% or more. In addition, it is preferable that the Cr content be 21.5% or less, more preferably 19.5% or less, or even more preferably 18.5% or less.
• Ni is an element which improves the corrosion resistance of stainless steel and which inhibits the progress of corrosion in a corrosive environment in which active dissolution occurs due to a passivation film not being formed.
• Ni is such a strong austenite-forming element as to suppress the formation of ferrite in a weld, Ni is effective for inhibiting sensitization from occurring due to the precipitation of Cr carbonitrides.
• Such effects are obtained in the case where the Ni content is 0.01% or more, and such effects increase with an increase in Ni content.
• the Ni content is more than 0.60%, there is a deterioration in workability, and stress corrosion cracking tends to occur.
• the Ni content is set to be 0.01% to 0.60%. It is preferable that the Ni content be 0.10% or more. In addition, it is preferable that the Ni content be 0.50% or less or more preferably 0.40% or less.
• Ti is a very important element. Since Ti is more likely than other elements to combine with C and N such that the precipitation of Cr carbonitrides is inhibited, Ti is effective for decreasing a recrystallization temperature and for inhibiting a deterioration in corrosion resistance caused by sensitization due to the precipitation of Cr carbonitrides. It is necessary that the Ti content be 0.10% or more to obtain such effects. However, in the case where the Ti content is more than 0.40%, since there is an excessive increase in the amount of solid solution Ti, there is conversely an increase in recrystallization temperature, which makes it impossible to use the technique according to the present invention.
• the Ti content is set to be 0.10% to 0.40%. It is preferable that the Ti content be 0.15% or more or more preferably 0.20% or more. In addition, it is preferable that the Ti content be 0.35% or less or more preferably 0.30% or less.
• the Ti content satisfy the relational expression Ti/(C+N) ⁇ 8 (where, in the relational expression, Ti, C, and N respectively denote the content (mass%) of the corresponding elements).
• the N content is more than 0.020%, there is a significant deterioration in workability and in the corrosion resistance of a weld.
• the N content be as small as possible from the viewpoint of corrosion resistance, it is not preferable that the N content be decreased to less than 0.001%, because this results in an increase in manufacturing costs and in a decrease in productivity due to an increase in the time taken to perform refining. Therefore, the N content is set to be in the range of 0.001% to 0.020%. It is preferable that the N content be 0.005% or more or more preferably 0.007% or more. In addition, it is preferable that the N content be 0.015% or less or more preferably 0.012% or less.
• the present invention provides ferritic stainless steel having a chemical composition containing the essential elements describe above and the balance being Fe and inevitable impurities. Moreover, one, two, or more selected from Cu, Mo, W and Co and/or one, two, or more of V, Nb, Zr, REM, B, Mg, and Ca may be optionally contained within the ranges described below.
• Cu is an element which is particularly effective for improving the corrosion resistance of a base metal and a weld in an aqueous solution or in the case where weakly acidic water drops stick to them. Such an effect is obtained in the case where the Cu content is 0.01% or more, and such an effect increases with an increase in Cu content. However, in the case where the Cu content is more than 1.00%, there is a deterioration in hot workability, which may result in surface defects. Moreover, there may be a case where it is difficult to perform descaling after annealing has been performed. Therefore, in the case where Cu is contained, it is preferable that the Cu content be in the range of 0.01% to 1.00%. It is more preferable that the Cu content be 0.10% or more or even more preferably 0.30% or more. In addition, it is more preferable that the Cu content be 0.60% or less or even more preferably 0.45% or less.
• Mo is an element which significantly improves the corrosion resistance of stainless steel. Such an effect is obtained in the case where the Mo content is 0.01% or more, and such an effect increases with an increase in Mo content. However, in the case where the Mo content is more than 2.00%, there may be a deterioration in manufacturability due to an increase in rolling load when hot-rolling is performed, and there may be an excessive increase in the strength of a steel sheet. In addition, since Mo is an expensive element, there is an increase in manufacturing costs in the case where the Mo content is high. Therefore, in the case where Mo is contained, it is preferable that the Mo content be 0.01% to 2.00%. It is more preferable that the Mo content be 0.10% or more or even more preferably 0.30% or more. In addition, it is more preferable that the Mo content be 1.40% or less or even more preferably 0.90% or less.
• W is effective for improving corrosion resistance like Mo. Such an effect is obtained in the case where the W content is 0.01% or more.
• the W content is more than 0.20%, since there is an increase in strength, there may be a deterioration in manufacturability due to, for example, an increase in rolling load. Therefore, in the case where W is contained, it is preferable that the W content be in the range of 0.01% to 0.20%. It is more preferable that the W content be 0.05% or more. In addition, it is more preferable that the W content be 0.15% or less.
• Co is an element which improves toughness. Such an effect is obtained in the case where the Co content is 0.01% or more. On the other hand, in the case where the Co content is more than 0.20%, there may be a deterioration in workability. Therefore, in the case where Co is contained, it is preferable that the Co content be in the range of 0.01% to 0.20%. It is more preferable that the Co content be 0.10% or less.
• V 0.01% to 0.20%
• V improves the corrosion resistance of a weld by inhibiting sensitization from occurring when welding is performed as a result of combining with C and N to form carbonitrides. Such an effect is obtained in the case where the V content is 0.01% or more.
• the V content is more than 0.20%, there may be a significant deterioration in workability and toughness. Therefore, it is preferable that the V content be 0.01% to 0.20%. It is more preferable that the V content be 0.03% or more. In addition, it is more preferable that the V content be 0.10% or less or even more preferably 0.05% or less.
• Nb is effective for refining crystal grains and for improving the toughness of a steel sheet by forming a solid solution in a parent phase. Such effects are obtained in the case where the Nb content is 0.01% or more.
• Nb is also effective for increasing a recrystallization temperature, there is an excessive increase in annealing temperature at which sufficient recrystallization occurs in hot-rolled-sheet annealing in the case where the Nb content is more than 0.10% such that there is significant coarsening of recrystallized grains to a maximum of 300 ⁇ m or more during annealing, which may make it impossible to achieve the specified threshold stress intensity factor K IC .
• the Nb content be in the range of 0.01% to 0.10%. It is more preferable that the Nb content be 0.02% or more. In addition, it is more preferable that the Nb content be 0.05% or less.
• Zr is effective for inhibiting sensitization by combining with C and N. Such an effect is obtained in the case where the Zr content is 0.01% or more. On the other hand, in the case where the Zr content is more than 0.20%, there may be a significant deterioration in workability. Therefore, in the case where Zr is contained, it is preferable that the Zr content be in the range of 0.01% to 0.20%. It is more preferable that the Zr content be 0.02% or more. In addition, it is more preferable that the Zr content be 0.10% or less or even more preferably 0.05% or less.
• REM rare earth metals
• the REM content is 0.001% or more.
• the REM content is more than 0.100%, there may be a deterioration in manufacturability such as pickling performance when cold-rolled-sheet annealing is performed. Therefore, in the case where REM is contained, it is preferable that the REM content be in the range of 0.001% to 0.100%. It is more preferable that the REM content be 0.010% or more. In addition, it is more preferable that the REM content be 0.050% or less.
• B is an element which is effective for improving secondary work brittleness resistance after forming has been performed. Such an effect is obtained in the case where the B content is 0.0002% or more. On the other hand, in the case where the B content is more than 0.0025%, there may be a deterioration in workability and toughness. Therefore, in the case where B is contained, it is preferable that the B content be in the range of 0.0002% to 0.0025%. It is more preferable that the B content be 0.0003% or more. In addition, it is more preferable that the B content be 0.0006% or less.
• Mg is an element which is effective for improving workability and toughness by improving the equiaxial crystal ratio of a slab. Moreover, although there is a deterioration in toughness when there is coarsening of Ti carbonitrides in the case of steel containing Ti as in the case of the present invention, Mg is also effective for inhibiting coarsening of Ti carbonitrides. Such effects are obtained in the case where the Mg content is 0.0005% or more. On the other hand, in the case where the Mg content is more than 0.0030%, there may be a deterioration in the surface quality of steel. Therefore, in the case where Mg is contained, it is preferable that the Mg content be in the range of 0.0005% to 0.0030%. It is more preferable that the Mg content be 0.0010% or more. In addition, it is more preferable that the Mg content be 0.0020% or less.
• Ca is an element which is effective for preventing nozzle clogging, which tends to occur due to Ti-based inclusions being crystallized when continuous casting is performed. Such an effect is obtained in the case where the Ca content is 0.0003% or more. However, in the case where the Ca content is more than 0.0030%, there may be a deterioration in corrosion resistance due to the formation of CaS. Therefore, in the case where Ca is contained, it is preferable that the Ca content be in the range of 0.0003% to 0.0030%. It is more preferable that the Ca content be 0.0005% or more. In addition, it is more preferable that the Ca content be 0.0015% or less or even more preferably 0.0010% or less.
• Threshold stress intensity factor K IC 20 MPa ⁇ m 1/2 or more
• the threshold stress intensity factor K IC in the case of the hot-rolled and annealed ferritic stainless steel sheet according to the present invention, by controlling the threshold stress intensity factor K IC to be 20 MPa ⁇ m 1/2 or more, it is possible to inhibit a crack from occurring when punching work is performed to form a thick-walled flange. It is preferable that the threshold stress intensity factor K IC be 25 MPa ⁇ m 1/2 or more or more preferably 30 MPa ⁇ m 1/2 or more.
• the meaning of the term "thick-walled flange" includes, for example, a flange having a wall thickness of 5.0 mm or more, although there is no particular limitation on the thickness. It is preferable that the above-described flange have a wall thickness of, for example, 5.0 mm to 15.0 mm or more preferably 5.0 mm to 10.0 mm.
• the term "temperature” refers to the surface temperature of, for example, a steel slab or a hot-rolled steel sheet, which is determined by using, for example, a surface pyrometer, unless otherwise noted.
• the hot-rolled and annealed ferritic stainless steel sheet according to the present invention by performing a hot-rolling process involving rough rolling and finish rolling which is composed of 3 passes or more on a steel slab having the chemical composition described above, in which rolling in the final 3 passes of finish rolling is performed in a temperature range of 800°C to 1100°C with an accumulated rolling reduction ratio of 25% or more to obtain a hot-rolled steel sheet, and by further performing hot-rolled-sheet annealing in a temperature range of 800°C to 1100°C on the hot-rolled steel sheet.
• molten steel having the chemical composition described above is prepared by using a known method such as one which utilizes, for example, a converter, an electric furnace, or a vacuum melting furnace and made into steel (slab) by using a continuous casting method or an ingot casting-slabbing method.
• a known method such as one which utilizes, for example, a converter, an electric furnace, or a vacuum melting furnace and made into steel (slab) by using a continuous casting method or an ingot casting-slabbing method.
• This slab is subjected to the hot-rolling after having been heated at a temperature of 1100°C to 1250°C for 1 hour to 24 hours or when the slab has a temperature of 1100°C to 1250°C without having been heated after casting has been performed.
• an accumulated rolling reduction ratio in the rough rolling be 65% or more to effectively break a cast structure before the finish hot-rolling is performed, because this is effective for refining of crystal grains in the subsequent finish hot-rolling.
• rolling in the final 3 passes of the finish rolling is performed in a temperature range of 800°C to 1100°C with an accumulated rolling reduction ratio of 25% or more.
• Rolling temperature range in final 3 passes of finish hot-rolling 800°C to 1100°C
• the accumulated rolling reduction ratio in the final 3 passes of the finish hot-rolling is set to be 25% or more, preferably 30% or more, or even more preferably 35% or more.
• the accumulated rolling reduction ratio be 60% or less.
• the rolling temperature in the final 3 passes of the finish hot-rolling be lower than 800°C from the viewpoint of manufacturing conditions, because this results in a significant increase in rolling load due to a decrease in the temperature of a steel sheet.
• the rolling temperature in the final 3 passes of the finish hot-rolling is higher than 1100°C, since the recovery of strain applied by performing rolling occurs, there is an insufficient number of recrystallization sites after the subsequent hot-rolled-sheet annealing has been performed, which makes it impossible to achieve the specified threshold stress intensity factor K IC due to colonies being retained after the hot-rolled-sheet annealing has been performed. Therefore, the rolling temperature in the final 3 passes is set to be 800°C to 1100°C, preferably 800°C to 1050°C, or more preferably 850°C to 1000°C.
• the rolling temperature range of the first pass of the final 3 passes be 950°C to 1100°C, that the rolling temperature range of the second pass to be performed following the first pass be 925°C to 1075°C, and that the rolling temperature range of the third pass to be performed following the second pass be 875°C to 1050°C.
• the method for manufacturing the hot-rolled and annealed ferritic stainless steel sheet according to the present invention is characterized by performing rolling with large rolling reduction in the final 3 passes of the finish hot-rolling composed of 3 passes or more while controlling the rolling temperature range.
• rolling with large rolling reduction is performed in the final 4 passes or more, there is a decrease in the effect of applying strain, because insufficient strain is applied to the central portion in the thickness direction due to the accumulated rolling reduction ratio being divided into each of the passes even with the same accumulated rolling reduction ratio, and because recovery in the interval time between the passes is promoted due to an increase in accumulated transporting time between the passes.
• the rolling temperature and the accumulated rolling reduction ratio of the finish rolling be controlled in the final 2 passes or less, because this may result in a deterioration in manufacturability due to a significant increase in rolling load as a result of rolling being performed with such large rolling reduction as an accumulated rolling reduction ratio of 25% or more in 2 passes. Therefore, in the method for manufacturing the hot-rolled ferritic stainless steel sheet according to the present invention, the rolling temperature and the accumulated rolling reduction ratio are controlled in the final 3 passes of the finish rolling.
• the rolling temperature and the accumulated rolling reduction ratio be controlled in the final 3 passes of the finish hot-rolling
• the maximum number of passes be 15 or less or more preferably 10 or less.
• the coiling temperature there is no particular limitation on the coiling temperature, there may be a case where embrittlement occurs due to 475°C embrittlement in the case where the coiling temperature is more than 450°C to less than 500°C. Therefore, it is preferable that the coiling temperature be 450°C or lower or 500°C or higher.
• Hot-rolled-sheet annealing temperature 800°C to 1100°C
• hot-rolled-sheet annealing is performed after the above-described hot-rolling process has been performed.
• a microstructure formed by performing rolling work in the hot-rolling process is recrystallized.
• the breaking of colonies in the hot-rolled-sheet annealing is promoted. It is necessary that the hot-rolled-sheet annealing be performed at a temperature of in the range of 800°C to 1100°C to obtain such an effect.
• the hot-rolled-sheet annealing temperature is set to be 800°C to 1100°C.
• a hot-rolled steel sheet which has been subjected to such hot-rolled-sheet annealing described above has the chemical composition described above and a threshold stress intensity factor K IC of 20 MPa ⁇ m 1/2 or more.
• the hot-rolled-sheet annealing temperature be 800°C to 1050°C or more preferably 850°C to 1000°C.
• any one of box annealing (batch annealing) and continuous annealing may be used.
• the obtained hot-rolled and annealed steel sheet may be subjected to a descaling treatment such as shot blasting or pickling as needed. Moreover, grinding, polishing, or the like may be performed to improve surface quality.
• the hot-rolled and annealed steel sheet provided by the present invention may further be subjected to cold rolling and cold-rolled-sheet annealing.
• Molten stainless steels having the chemical compositions given in Table 1 were prepared by performing refining which utilized a converter having a capacity of 150 tons and a strong stirring-vacuum oxygen decarburization
• the obtained slabs other than No. 31 were heated at a temperature of 1200°C for one hour and then subjected to hot-rolling in which reverse-type rough rolling was performed by using 3 rolling stands to obtain steel plates having a thickness of about 40 mm and in which the final 3 passes of finish rolling composed of 7 passes (the fifth pass, the sixth path, and the seventh pass) were then performed under the conditions given in Table 2 to obtain hot-rolled steel sheets.
• the slab of No. 31 was subjected to heating at a temperature of 1300°C for one hour followed by the hot-rolling.
• the obtained hot-rolled steel sheets were similarly subjected to hot-rolled-sheet annealing using a box annealing method under the conditions given in Table 2 to obtain hot-rolled and annealed steel sheets.
• the obtained hot-rolled and annealed steel sheets were evaluated as described below.
• a CT test piece in accordance with ASTM E 399 was taken from the central portion in the width direction of the steel sheet so that the direction of a fatigue precrack was 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 E 399. A case where the threshold stress intensity factor K IC was 20 MPa ⁇ m 1/2 or more was judged as passed, and a case where the threshold stress intensity factor K IC was less than 20 MPa ⁇ m 1/2 was judged as failed.
• a test piece was prepared by taking a test piece having a size of 60 mm ⁇ 100 mm from the hot-rolled and annealed steel sheet, by polishing the evaluation surface thereof by using #600 emery paper, and by sealing the end surfaces thereof and subjected to a salt spray cyclic corrosion test prescribed in JIS H 8502.
• the rust area on the evaluation surface of the test piece was determined by performing image analysis on a photograph of the evaluation surface of the test piece which had been subjected to 5 cycles of the salt spray cyclic corrosion test, and a rust area ratio ((rust area of test piece/total area of test piece) ⁇ 100 [%]) was calculated as the ratio of the rust area to the total area of the test piece.
• a case where the rust area ratio was 10% or less was judged as a case of particularly excellent corrosion resistance, that is, judged as passed ( ⁇ ), a case where the rust area ratio was more than 10% and 25% or less was judged as passed ( ⁇ ), and a case where the rust area ratio was more than 25% was judged as failed ( ⁇ ).
• No. 31 is an example in which a slab was subjected to hot-rolling following heating at a temperature of 1300°C for one hour and in which the rolling temperature of the each of the final 3 passes of the finish hot-rolling was higher than 1100°C.
• No. 31 since the recovery of strain due to work excessively occurred during rolling in the final 3 passes, colonies were retained after the hot-rolled-sheet annealing had been performed due to insufficient number of recrystallization sites, which resulted in the specific threshold stress intensity factor K IC not being achieved.
• the hot-rolled and annealed ferritic stainless steel sheet obtained in the present invention can preferably be used in applications in which high workability and corrosion resistance are required, in particular, used for, for example, a flange having a portion subjected to burring work.

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• 2017
  • 2017-09-27 WO PCT/JP2017/034949 patent/WO2018074164A1/fr active Application Filing
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