Classifications

• • 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
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21B—ROLLING OF METAL
  • B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals
  • B21B3/02—Rolling special iron alloys, e.g. stainless steel
• • 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
  • C21D6/007—Heat treatment of ferrous alloys containing Co
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/008—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
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—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
  • 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
  • C21D8/0273—Final recrystallisation 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
  • 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
  • C22C38/52—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 and annealed ferritic stainless steel sheet having excellent workability and being suitable for use in flanges and the like and a method for manufacturing the same.
• an exhaust gas recirculation (EGR) system in which exhaust gas from an automobile engine is used again as intake air of the engine, has been increasingly used.
• the exhaust gas from the engine is passed through an EGR cooler for lowering the gas temperature, and then supplied again to the engine.
• EGR cooler for lowering the gas temperature
• exhaust system components are each jointed with a flange in order to prevent gas leakage.
• the flange used for such an exhaust system component is required to have sufficient rigidity. Therefore, for such an exhaust system component, a thick flange (e.g., with a sheet thickness of 5 mm or more) is used.
• Patent Literature 1 discloses a hot-rolled ferritic stainless steel sheet containing, in percent 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 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, and B: 0.0002% to 0.0020%, with the balance being Fe and unavoidable impurities, in which the contents of Nb, C, and N satisfy the formula: Nb/(C+N) ⁇ 16, and the hot-rolled ferritic stainless steel sheet has a Charpy impact value at 0° C. of 10 J/cm 2 or more and a sheet thickness of 5.0 to 9.0 mm.
• the present inventors have conducted detailed studies, and as a result, have found that by increasing a threshold stress intensity factor K IC of a steel sheet, the steel sheet can be worked into a thick flange having a burring working part without occurrence of cracks. Specifically, it has been found that, by setting the threshold stress intensity factor K IC at 35 MPa ⁇ m 1/2 or more, when a steel sheet worked into a thick flange having a burring working part, occurrence of cracks in the burring working part can be effectively prevented, and the steel sheet can be sufficiently put into practical use for a thick flange having a burring working part.
• a hot-rolled and annealed ferritic stainless steel sheet having a chemical composition containing, in percent 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 19.0%, Ni: 0.65% to 1.50%, Ti: 0.10% to 0.40%, and N: 0.001% to 0.020%, with the balance being Fe and unavoidable impurities, and having a threshold stress intensity factor K IC of 35 MPa ⁇ m 1/2 or more.
• threshold stress intensity factor K IC refers to a stress intensity factor obtained by taking a CT specimen according to ASTM E399 from the central part in the sheet width direction such 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 by conducting a test according to ASTM E399.
• the expression “excellent toughness so that cracks can be prevented during blanking into a thick flange” means that a threshold stress intensity factor K IC is 35 MPa ⁇ m 1/2 or more, the threshold stress intensity factor K IC being obtained by taking a CT specimen according to ASTM 6399 from the central part in the sheet width direction such 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 by conducting a test according to ASTM E399.
• a hot-rolled and annealed ferritic stainless steel sheet has a chemical composition containing, in percent 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 19.0%, Ni: 0.65% to 1.50%, Ti: 0.10% to 0.40%, and N: 0.001% to 0.020%, with the balance being Fe and unavoidable impurities, and has a threshold stress intensity factor K IC of 35 MPa ⁇ m 1/2 or more.
• threshold stress intensity factor K IC refers to a stress intensity factor obtained by taking a CT specimen according to ASTM E399 from the central part in the sheet width direction such 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 by conducting a test according to ASTM E399.
• the present inventors have investigated in detail the reason for the occurrence of cracks when various ferritic stainless steel sheets with a sheet thickness of 5.0 mm are each formed into a flange having a burring working part in which a flange hole (30 mm ⁇ ) is raised by 10 mm from the surface of the steel sheet as blanked.
• a flange hole (30 mm ⁇ ) is raised by 10 mm from the surface of the steel sheet as blanked.
• the present inventors have investigated in detail the relationship between the marked propagation of microcracks and material characteristics. As a result, it has been found that propagation of microcracks tends to occur as the threshold stress intensity factor of the steel sheet decreases. Accordingly, by using various hot-rolled and annealed ferritic stainless steel sheets (sheet thickness 5.0 mm), formation into the flange has been tried. As a result, it has been found that cracks due to propagation of microcracks tend to occur in particular in a steel sheet in which the threshold stress intensity factor determined by a predetermined measurement method is less than 35 MPa ⁇ m 1/2 .
• the present inventors have performed thorough studies on the technique of improving the threshold stress intensity factor in a hot-rolled and annealed ferritic stainless steel sheet.
• the sheet thickness of the hot-rolled and annealed ferritic stainless steel sheet according to aspects of the present invention is not particularly limited, but is desirably a sheet thickness that can be used for a thick flange.
• the lower limit of the sheet thickness is preferably 5.0 mm or more, and more preferably 9.0 mm or more.
• the upper limit of the sheet thickness is preferably 15.0 mm or less, and more preferably 10.0 mm or less.
• the present inventors have performed thorough studies on the effective technique of decreasing colonies remaining after hot-rolled sheet annealing from the viewpoint of both the steel composition and the hot rolling method. As a result, it has been found that it is effective to form a predetermined amount of an austenite phase in the hot rolling step by controlling the steel composition, in particular, Cr and Ni contents, to appropriate ranges and to perform rolling with a large accumulated rolling reduction while controlling the temperature of final three passes of finish hot-rolling in the hot rolling step to an appropriate range.
• the hot-rolled sheet annealing step is a step of recrystallizing the deformed microstructure 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, although recrystallization occurs, recrystallized grains are markedly coarsened.
• the markedly coarse recrystallized grains are independent single crystal grains, but the grain boundary length increases markedly. Therefore, it has been found that, as in the case where colonies are present, the effect of suppressing crack propagation due to grain boundaries having different orientations is decreased, and a predetermined threshold stress intensity factor cannot be obtained.
• the present inventors have 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 controlling the hot-rolled sheet annealing temperature to 1,100° C. or lower, formation of coarse recrystallized grains is prevented, thus making it possible to obtain a good threshold stress intensity factor.
• the C content exceeds 0.020%, workability and corrosion resistance in the weld zone noticeably deteriorate.
• a lower C content is more desirable from the viewpoint of corrosion resistance and workability.
• the C content is set in a range of 0.001% to 0.020%.
• the lower limit thereof is preferably 0.003% or more, and more preferably 0.004% or more.
• the upper limit thereof is preferably 0.015% or less, and more preferably 0.012% or less.
• Si is an element that has an effect of improving corrosion resistance of weld zone by being concentrated in an oxide layer formed during welding and is also effective as a deoxidizing element in the steelmaking process. These effects are obtained when a Si content is 0.05% or more, and increase with the increase of the Si content. However, when the Si content exceeds 1.00%, an increase in rolling load and marked formation of scales are caused in the hot rolling step, and deterioration in the pickling property due to formation of a Si concentration layer at the surface layer of the steel sheet is caused in the annealing step, inducing an increase in surface defects and a rise in production cost, all of which are undesirable. Therefore, the Si content is set at 0.05% to 1.00%.
• the lower limit thereof is preferably 0.15% or more, and more preferably 0.20% or more.
• the upper limit thereof is preferably 0.60% or less, and more preferably 0.40% or less.
• Mn has an effect of increasing the strength of steel and also acts as a deoxidizer. In order to obtain such effects, a Mn content of 0.05% or more is necessary. However, when the Mn content exceeds 1.00%, precipitation of MnS, which becomes a starting point of corrosion is promoted, resulting in deterioration in corrosion resistance. Therefore, the Mn content is set at 0.05% to 1.00%.
• the lower limit thereof is preferably 0.10% or more, and more preferably 0.20% or more.
• the upper limit thereof is preferably 0.60% or less, and more preferably 0.40% or less.
• P is an element that is unavoidably contained in steel. Since P is an element harmful to corrosion resistance and workability, it is desirable to decrease the amount of P as much as possible. In particular, when the P content exceeds 0.04%, workability is markedly deteriorated by solid solution strengthening. Therefore, the P content is set at 0.04% or less. Preferably, the P content is 0.03% or less. Since an excessive reduction in the P content requires excessive production cost, the P content is preferably 0.01% or more in consideration of production cost.
• S is also an element that is unavoidably contained in steel as in P. Since S is an element harmful to corrosion resistance and workability, it is desirable to decrease the amount of S as much as possible. In particular, when the S content exceeds 0.01%, corrosion resistance is markedly deteriorated. Therefore, the S content is set at 0.01% or less. Preferably, the S content is 0.008% or less. More preferably, the S content is 0.003% or less. Since an excessive reduction in the S content requires excessive production cost, the S content is preferably 0.001% or more in consideration of production cost.
• Al is an effective deoxidizer. Furthermore, since Al has higher affinity for nitrogen than Cr, in the case where nitrogen enters a weld zone, by precipitating nitrogen as Al nitrides instead of Cr nitrides, Al has an effect of suppressing sensitization. These effects can be obtained at an Al content of 0.001% or more. However, when the Al content exceeds 0.100%, since the penetration characteristics for welding is deteriorated, welding workability is deteriorated, which is undesirable. Therefore, the Al content is set in a range of 0.001% to 0.100%. The lower limit thereof is preferably 0.010% or more, and more preferably 0.020% or more. The upper limit thereof is preferably 0.080% or less, and more preferably 0.060% or less.
• the Cr content is set in a range of 10.0% to 19.0%.
• the lower limit thereof is preferably 10.5% or more, and more preferably 11.0% or more.
• the upper limit thereof is preferably 16.5% or less, more preferably 12.5% or less, and still more preferably 11.5% or less.
• Ni is an austenite-forming element and has an effect of increasing the amount of austenite formed during heating before rolling in the hot rolling step.
• an austenite phase is formed during heating in the hot rolling step. Owing to the formation of the austenite phase, colonies of the ferrite phase formed during casting are destroyed. Furthermore, at the heating temperature before hot rolling, the metallic microstructure is formed into a two-phase structure of ferrite phase+austenite phase.
• the metallic microstructure is formed into a two-phase structure of ferrite phase austenite phase
• the interface between different phases i.e., between the ferrite phase existing before heating and the austenite phase formed during heating, functions as an obstacle to growth of crystal grains, and therefore, the metallic microstructure before hot rolling is refined.
• the metallic microstructure after hot rolling is refined and that after the subsequent hot-rolled sheet annealing step is also refined.
• the metallic microstructure is formed into an austenite single phase.
• the metallic microstructure at the heating temperature is formed into an austenite single-phase structure, as in the above, the effect of destroying colonies due to formation of the austenite phase can be obtained.
• the metallic microstructure before hot rolling is finer than that of ferritic stainless steel based on existing techniques, and as in the above, the effect of improving toughness due to refinement of crystal grains can be obtained.
• the Ni content is set at 0.65% to 1.50%.
• the lower limit thereof is preferably 0.70% or more, and more preferably 0.75% or more.
• the upper limit thereof is preferably 1.00% or less, and more preferably, the Ni content is 0.90% or less.
• Ti is a very important element. Since Ti preferentially combines with C and N, which suppresses precipitation of Cr carbonitrides and lowers the recrystallization temperature, Ti has an effect of suppressing deterioration of corrosion resistance caused by sensitization due to precipitation of Cr carbonitrides. In order to obtain these effects, a Ti content of 0.10% or more is necessary. However, when the Ti content exceeds 0.40%, since the amount of solute Ti excessively increases, the recrystallization temperature rather rises, and the technique according to aspects of the present invention cannot be used. Furthermore, when the Ti content exceeds 0.40%, coarse Ti carbonitrides are formed in the casting step, resulting in surface defects, which is also undesirable in terms of manufacturing.
• the Ti content is set at 0.10% to 0.40%.
• the lower limit thereof is preferably 0.15% or more, more preferably 0.20% or more, and still more preferably 0.25% or more.
• the upper limit thereof is preferably 0.35% or less, and more preferably 0.30% or less.
• the Ti content is preferably set so as to satisfy the formula: Ti/(C+N) 8, where Ti, C, and N denote contents of the individual elements (percent by mass).
• the N content exceeds 0.020%, workability and corrosion resistance in the weld zone noticeably deteriorate.
• a lower N content is more desirable from the viewpoint of corrosion resistance.
• the N content is set in a range of 0.001% to 0.020%.
• the lower limit thereof is preferably 0.005% or more, and more preferably 0.007% or more.
• the upper limit thereof is preferably 0.015% or less, and more preferably 0.012% or less.
• aspects of the present invention relate to a ferritic stainless steel featured by containing the above-described essential elements, with the balance being Fe and unavoidable impurities.
• the ferritic stainless steel may contain one or two or more selected from Cu, Mo, W, and Co and/or one or two or more selected from V, Nb, Zr, REM, B, Mg, and Ca in the ranges described below.
• any range has a lower limit, even if the relevant element is contained in an amount less than the lower limit, the advantageous effects according to aspect of the present invention are not impaired. Therefore, in the case where the element is contained in an amount less than the lower limit, the element is considered as an unavoidable impurity.
• Cu is a particularly effective element in improving corrosion resistance of the base metal and weld zone in an aqueous solution or when weakly acidic water drops adhere thereto. This effect is obtained at a Cu content of 0.01% or more and increases with increasing Cu content.
• the Cu content is preferably set in a range of 0.01% to 1.00%.
• the lower limit thereof is more preferably 0.10% or more, and still more preferably 0.30% or more.
• the upper limit thereof is more preferably 0.60% or less, and still more preferably 0.45% or less.
• Mo is an element that remarkably improves the corrosion resistance of stainless steel. This effect is obtained at a Mo content of 0.01% or more and improves with increasing content.
• Mo content exceeds 2.00%, the rolling load during hot rolling increases, which may deteriorate productivity, and the strength of the steel sheet may be excessively increased in some cases.
• Mo is an expensive element, a large content of Mo increases the production cost. Therefore, when Mo is contained, the Mo content is preferably set at 0.01% to 2.00%.
• the lower limit thereof is more preferably 0.10% or more, and still more preferably 0.30% or more.
• the upper limit thereof is more preferably 1.40% or less, and still more preferably 0.90% or less.
• W has an effect of improving corrosion resistance, similarly to Mo. This effect is obtained at a W content of 0.01% or more.
• the W content is preferably set in a range of 0.01% to 0.20%.
• the lower limit thereof is more preferably 0.05% or more.
• the upper limit thereof is more preferably 0.15% or less.
• Co is an element that improves toughness. This effect is obtained at a Co content of 0.01% or more. On the other hand, when the Co content exceeds 0.20%, workability may be deteriorated in some cases. Therefore, when Co is contained; the Co content is preferably set in a range of 0.01% to 0.20%.
• V 0.01% to 0.20%
• V combines with C and N as carbonitrides and suppresses precipitation of Cr carbonitrides, V improves corrosion resistance of weld zone. This effect is obtained at a V content of 0.01% or more.
• the V content is preferably set at 0.01% to 0.20%.
• the lower limit thereof is more preferably 0.02% or more.
• the upper limit thereof is more preferably 0.10% or less.
• Nb has an effect of refining crystal grains and an effect of improving the toughness of the steel sheet by being dissolved in the matrix phase. These effects are obtained at a Nb content of 0.01% or more.
• Nb also has an effect of increasing the recrystallization temperature.
• the Nb content exceeds 0.10%, there may be a case where the annealing temperature required to cause sufficient recrystallization in hot-rolled sheet annealing becomes excessively high, recrystallized grains are markedly coarsened during annealing such that the crystal grain size is 300 ⁇ m or more at maximum, and a predetermined threshold stress intensity factor cannot be obtained. Therefore, when Nb is contained, the Nb content is preferably set in a range of 0.01% to 0.10%. The lower limit thereof is more preferably 0.02% or more. The upper limit thereof is more preferably 0.08% or less.
• the Zr content is preferably set in a range of 0.01% to 0.20%.
• the lower limit thereof is more preferably 0.03% or more.
• the upper limit thereof is more preferably 0.10% or less.
• REM Reare Earth Metals
• the REM content is preferably set in a range of 0.001% to 0.100%.
• the lower limit thereof is more preferably 0.005% or more.
• the upper limit thereof is more preferably 0.050% or less.
• B is an element effective in improving resistance to secondary work embrittlement after deep drawing. This effect is obtained at a B content of 0.0002% or more. On the other hand, when the B content exceeds 0.0025%, workability and toughness may be deteriorated in some cases. Therefore, when B is contained, the B content is preferably set in a range of 0.0002% to 0.0025%. The lower limit thereof is more preferably 0.0003% or more. The upper limit thereof is more preferably 0.0006% or less.
• Mg increases the equiaxed crystal ratio of a slab and is an element effective in improving workability and toughness. Furthermore, Mg has an effect of suppressing coarsening of Ti carbonitrides; in steel containing Ti as in accordance with aspects of the present invention, when Ti carbonitrides are coarsened, toughness deteriorates. These effects are obtained at a Mg content of 0.0005% or more. On the other hand, when the Mg content exceeds 0.0030%, surface properties of steel may be deteriorated in some cases. Therefore, when Mg is contained, the Mg content is preferably, set in a range of 0.0005% to 0.0030%. The lower limit thereof is more preferably 0.0010% or more. The upper limit thereof is more preferably 0.0020% or less.
• Ca is an element effective in preventing nozzle blockage due to crystallization of Ti-based inclusions which tends to occur during continuous casting. The effect is obtained at a Ca content of 0.0003% or more. However, when the Ca content exceeds 0.0030%, corrosion resistance may be deteriorated by formation of CaS in some cases. Therefore, when Ca is contained, the Ca content is preferably set in a range of 0.0003% to 0.0030%. The lower limit thereof is more preferably 0.0005% or more, and still more preferably 0.0006% or more. The upper limit thereof is more preferably 0.0015% or less, and still more preferably 0.0010% or less.
• a hot-rolled and annealed ferritic stainless steel sheet according to aspects of the present invention is obtained by subjecting a steel slab having the chemical composition described above to hot rolling which includes rough rolling and finish rolling with three or more passes, under the conditions that the temperature of final three passes of finish rolling is set at 800° C. to 1,100° C., and the accumulated rolling reduction of the final three passes is set at 25% or more, to obtain a hot-rolled steel sheet, and further performing hot-rolled sheet annealing on the hot-rolled steel sheet at 600° C. to 1,100° C.
• molten steel having the chemical composition described above is produced by a known method using a converter, an electric furnace, a vacuum melting furnace, or the like and is formed into a steel (slab) by a continuous casting process or an ingot casting-slabbing process.
• the slab is, after being heated at 1,050° C. to 1,250° C. for 1 to 24 hours, or without being heated, directly as cast, subjected to hot rolling.
• rough rolling is not particularly limited, however, in the case where the cast structure is effectively destroyed before finish hot-rolling, this is advantageous to refinement of crystal grains in the subsequent finish hot-rolling. Therefore, the accumulated rolling reduction in rough rolling is preferably set at 65% or more. Then, finish hot-rolling is performed until a predetermined sheet thickness is reached, in which the temperature of final three passes of finish rolling is set in a range of 800° C. to 1,100° C., and the accumulated rolling reduction of the final three passes is set at 25% or more.
• Rolling temperature range of final three passes 800° C. to 1,100° C.
• rolling strain The amount of shear strain in rolling (hereinafter, expressed as “rolling strain”) decreases from the surface layer toward the central part in the sheet thickness direction. Accordingly, when the rolling reduction is small, while a large amount of rolling strain is introduced to the surface layer and its vicinity of the steel sheet, the amount of rolling strain introduced to the central part in the sheet thickness direction is small. Rolling strain acts as recrystallization sites in the subsequent hot-rolled sheet annealing step.
• the accumulated rolling reduction of final three passes is set at 25% or more.
• the accumulated rolling reduction is preferably 30% or more, and more preferably 35% or more.
• the upper limit of the accumulated rolling reduction is not particularly limited. However, when the accumulated rolling reduction is excessively increased, the rolling load increases, resulting in deterioration of productivity, and surface roughening may occur after rolling in some cases. Therefore, the upper limit of the accumulated rolling reduction is preferably set at 60% or less.
• the rolling temperature of final three passes When the rolling temperature of final three passes is set at lower than 800° C., the rolling load markedly increases with a decrease in steel sheet temperature, which is undesirable in terms of production. Furthermore, low-temperature rolling may cause surface roughening in the steel sheet, resulting in deterioration of surface quality in some cases.
• the rolling temperature of final three passes exceeds 1,100° C., recovery of the strain introduced by rolling occurs, and the number of recrystallization sites after the subsequent hot-rolled sheet annealing step becomes insufficient. Consequently, colonies remain after hot-rolled sheet annealing, and a predetermined threshold stress intensity factor cannot be obtained. Therefore, the rolling temperatures of final three passes are set in a range of 800° C. to 1,100° C.
• the lower limit thereof is preferably 850° C. or higher.
• the upper limit thereof is preferably 1,050° C. or lower, and more preferably 1,000° C. or lower.
• the rolling temperature of the final pass means the rolling end temperature, and the rolling temperatures of the other passes mean the respective rolling start temperatures.
• the rolling temperature at the first pass, among the final three passes is set in a range of 950° C. to 1,100° C.
• the rolling temperature at the second pass performed after the first pass is set in a range of 925° C. to 1,075° C.
• the rolling temperature at the third pass performed after the second pass is set in a range of 875° C. to 1,050° C.
• the method for manufacturing a hot-rolled and annealed ferritic stainless steel sheet according to aspects of the present invention is featured by controlling the temperature range and applying a large reduction in final three passes of finish hot-rolling with three or more passes.
• rolling that applies a large reduction is performed in final four or more passes, even with the same accumulated rolling reduction as in the case of applying a large reduction in final three passes only, since the rolling reduction is distributed among the individual passes, strain is insufficiently introduced to the central part in the sheet thickness direction. Furthermore, since the accumulated transfer time for all the passes increases, recovery is promoted during the transfer period among the individual passes, and the effect of applying strain is decreased.
• the rolling temperature and the accumulated rolling reduction of finish rolling are controlled only for final two passes or less, since a large reduction with a accumulated rolling reduction of 25% or more is performed in two passes, the rolling load is markedly increased, and productivity may be deteriorated in some cases, which is undesirable. Therefore, in the method for manufacturing the hot-rolled ferritic stainless steel sheet according to aspects of the present invention, the rolling temperature and the accumulated rolling reduction of final three passes of finish rolling are controlled.
• finish rolling may be performed with any number of passes.
• the maximum number of passes exceeds 15, the steel sheet temperature tends to be decreased because of an increased number of contacts with rolls in the rolling mill, leading to deterioration, in productivity or an increase in production cost, for example, a need to perform heating from outside in order to maintain the steel sheet temperature within a predetermined temperature range. Therefore, the maximum number of passes is preferably 15 or less, and more preferably 10 or less.
• the coiling temperature is not particularly limited. However, when the coiling temperature is set at higher than 450° C. and lower than 500° C., embrittlement due to 475° C. embrittlement may occur in some cases. Therefore, the coiling temperature is preferably set at 450° C. or lower or 500° C. or higher.
• Hot-rolled sheet annealing temperature 600° C. to 1,100° C.
• hot-rolled sheet annealing is performed.
• the roll-deformed microstructure formed in the hot rolling step is recrystallized.
• by effectively introducing rolling strain in the hot rolling step so that the number of recrystallization sites is increased destruction of colonies in hot-rolled sheet annealing is promoted.
• the hot-rolled sheet annealing temperature is set in a range of 600° C. to 1,100° C.
• the lower limit thereof is preferably 650° C. or higher, and more preferably 700° C. or higher.
• the upper limit thereof is preferably 1,050° C. or lower, and more preferably 900° C. or lower.
• the holding time and the technique of hot-rolled sheet annealing are not particularly limited, and either box annealing (batch annealing) or continuous annealing may be performed.
• the resulting hot-rolled and annealed steel sheet may be subjected, as necessary, to a descaling treatment by shot blasting or pickling. Furthermore, in order to improve surface properties, the steel sheet may be subjected to grinding, polishing, or the like. Moreover, the hot-rolled and annealed steel sheet provided in accordance with aspects of the present invention may be further subjected to cold rolling and cold-rolled sheet annealing.
• a CT (compact tension) specimen according to ASTM E399 was taken from the central part in the sheet width direction of each of the hot-rolled and annealed steel sheets such that a fatigue pre-crack introduced in a direction perpendicular to the rolling direction and the stress axis was in a direction parallel to the rolling direction.
• the specimens were tested in accordance with ASTM E399 to obtain a threshold stress intensity factor K IC .
• Specimens with a threshold stress intensity factor of 35 MPa ⁇ m 1/2 or more were evaluated as “pass”, and specimens with a threshold stress intensity factor of less than 35 MPa ⁇ m 1/2 were evaluated as “rejection”.
• a specimen of 60 ⁇ 100 mm was taken from each of the hot-rolled and annealed steel sheets. After a surface to be evaluated of the specimen was polish-finished with #600 emery paper, edge surfaces and a back surface of the specimen were sealed in order to eliminate influences from the edge surfaces and the back surface. Then, the specimen was subjected to a cyclic salt spray test specified in JIS H 8502. In the cyclic salt spray test, five cycles were performed, each cycle including salt spraying (5% by mass NaCl, 35° C., spraying for 2 hours) ⁇ drying (60° C., 4 hours, relative humidity: 40%) ⁇ wetting (50° C., 2 hours, relative humidity ⁇ 95%).
• the surface to be evaluated of the specimen was photographed, and the rust area in the surface of the specimen was measured by image analysis. From the ratio of the rust area to the total area of the specimen, the rust area ratio ((rust area in specimen/total area of specimen) ⁇ 100[%]) was calculated. Specimens with a rust area ratio of 10% or less were evaluated as “pass” ( ⁇ ) with particularly excellent corrosion resistance, specimens with a rust area ratio of more than 10% and 25% or less were evaluated as “pass” ( ⁇ ), and specimens with a rust area ratio of more than 25% were evaluated as “rejection” (x).
• Example 38 9.7 28 839 22 ⁇ Comparative Example 42 9.4 31 817 21 ⁇ Comparative Example 39 10.3 28 874 22 ⁇ Comparative Example 40 10.5 26 882 23 ⁇ Comparative Example 41 9.9 28 985 16 ⁇ Comparative Example 43 10.2 27 821 36 X Comparative Example 44 9.5 29 826 16 ⁇ Comparative Example 50 10.1 29 571 18 ⁇ Comparative Example Underiined items are outside the range of the present invention.
• No. 36 is an example in which the slab was heated at 1,300° C. for one hour, and then subjected to hot rolling, in which the rolling temperature ranges of final three passes of finish hot-rolling each exceeded 1,100° C.
• the hot-rolled and annealed ferritic stainless steel sheet obtained in accordance with aspects of the present invention is suitable for application requiring high workability and corrosion resistance, for example, particularly suitable for use in a flange having a burring working part or the like.

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Heat Treatment Of Sheet Steel (AREA)
• Heat Treatment Of Steel (AREA)
• Heat Treatment Of Strip Materials And Filament Materials (AREA)

Applications Claiming Priority (3)

Publications (1)

Family

ID=63919905

Family Applications (1)

Country Status (9)

Cited By (1)

Families Citing this family (4)

Citations (1)

Family Cites Families (17)

• 2018
  • 2018-04-24 ES ES18790531T patent/ES2924685T3/es active Active
  • 2018-04-24 MX MX2019012549A patent/MX2019012549A/es unknown
  • 2018-04-24 WO PCT/JP2018/016545 patent/WO2018199062A1/fr not_active Ceased
  • 2018-04-24 EP EP18790531.0A patent/EP3587610B1/fr active Active
  • 2018-04-24 KR KR1020197030951A patent/KR20190131528A/ko not_active Ceased
  • 2018-04-24 JP JP2018540074A patent/JP6432720B1/ja active Active
  • 2018-04-24 CN CN201880027591.8A patent/CN110546294B/zh active Active
  • 2018-04-24 US US16/607,174 patent/US20200385835A1/en not_active Abandoned
  • 2018-04-26 TW TW107114282A patent/TWI685574B/zh active

Patent Citations (1)

Also Published As

Similar Documents

Legal Events