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
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • 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
  • 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
  • C22C38/005—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
• • 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 and a hot rolled and annealed ferritic stainless steel sheet which have sufficient corrosion resistance and excellent rigidity and methods for manufacturing these steel sheets.
• EGR exhaust gas recirculation
• a flange which is used in a connecting portion of a member such as an EGR cooler, which is always subjected to vibration during running of an automobile have sufficient rigidity in order to prevent gas from leaking from a gap which is formed between the parts due to the bending of the flange resulting from the vibration. Therefore, a flange having a large thickness (for example, a thickness of 6 mm or more) is used as a flange which is fitted to a member such as an EGR cooler, which is always subjected to vibration during running of an automobile.
• plain carbon steel is used for such a flange having a large thickness.
• Patent Literature 1 discloses a hot rolled ferritic stainless steel sheet having a chemical composition containing, by massa, 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. of 10 J/cm 2 or more, and a thickness of 5.0 mm to 9.0 mm.
• An object of aspects of the present invention is, by solving the problems described above, to provide a hot rolled ferritic stainless steel sheet and a hot rolled and annealed ferritic stainless steel sheet which have sufficient corrosion resistance and with which it is possible to inhibit bending and twist from occurring after forming has been performed and methods for manufacturing these steel sheets.
• the present inventors conducted close investigations in order to solve the problems and, as a result, found that a steel sheet should have decreased absolute value
• E L denotes modulus of longitudinal elasticity (GPa) in a direction parallel to the rolling direction
• E D denotes modulus of longitudinal elasticity (GPa) in a direction at an angle of 45° to the rolling direction
• E C denotes modulus of longitudinal elasticity (GPa) in a direction at a right angle to the rolling direction.
• E L , E D , and E C are respectively defined as the values of modulus of longitudinal elasticity in the rolling direction of a steel sheet, in a direction at an angle of 45° to the rolling direction, and in a direction at a right angle to the rolling direction which are measured at a temperature of 23° C. by using a transverse resonant technique prescribed in JIS Z 2280 (1993).
• a hot rolled ferritic stainless steel sheet having a chemical composition containing, by mass %, C: 0.005% to 0.060%, Si: 0.02% to 0.50%, Mn: 0.01% to 1.00%, P: 0.04% or less, S: 0.01% or less, Cr: 15.5% to 18.0%, Al: 0.001% to 0.10%, N: 0.005% to 0.100%, Ni: 0.1% to 1.0%, and the balance being Fe and inevitable impurities and an absolute value
• E L denotes modulus of longitudinal elasticity (GPa) in a direction parallel to the rolling direction
• E D denotes modulus of longitudinal elasticity (GPa) in a direction at an angle of 45° to the rolling direction
• E C denotes modulus of longitudinal elasticity (GPa) in a direction at a right angle to the rolling direction.
• a hot rolled and annealed ferritic stainless steel sheet obtained by performing hot rolled sheet annealing on the hot rolled ferritic stainless steel sheet according to any one of items [1] to [3] above.
• a method for manufacturing a hot rolled and annealed ferritic stainless steel sheet including using the method for manufacturing a hot rolled ferritic stainless steel sheet according to item [5] above, and further performing hot rolled sheet annealing at a temperature of 800° C. to 900° C. after the hot rolling process.
• a hot rolled ferritic stainless steel sheet and a hot rolled and annealed ferritic stainless steel sheet which have sufficient corrosion resistance and with which it is possible to inhibit bending and twist from occurring after forming has been performed.
• the hot rolled ferritic stainless steel sheet and the hot rolled and annealed ferritic stainless steel sheet according to aspects of the present invention have a chemical composition containing, by mass %, C: 0.005% to 0.060%, Si: 0.02% to 0.50%, Mn: 0.01% to 1.00%, P: 0.04% or less, S: 0.01% or less, Cr: 15.5% to 18.0%, Al: 0.001% to 0.10%, N: 0.005% to 0.100%, Ni: 0.1% to 1.0%, and the balance being Fe and inevitable impurities and an absolute value
• E L denotes modulus of longitudinal elasticity (GPa) in a direction parallel to the rolling direction
• E D denotes modulus of longitudinal elasticity (GPa) in a direction at an angle of 45° to the rolling direction
• E C denotes modulus of longitudinal elasticity (GPa) in a direction at a right angle to the rolling direction.
• E L , E D , and E C are respectively defined as the values of modulus of longitudinal elasticity in the rolling direction of a steel sheet, in a direction at an angle of 45° to the rolling direction, and in a direction at a right angle to the rolling direction which are measured at a temperature of 23° C. by using a transverse resonant technique prescribed in JIS Z 2280 (1993).
• the hot rolled ferritic stainless steel sheet and hot rolled and annealed ferritic stainless steel sheet according to aspects of the present invention are intended to be used mainly for a flange having a large wall thickness which is used for the EGR cooler parts of an automobile.
• the present inventors used various kinds of hot rolled ferritic stainless steel sheet for a flange having a large thickness for an EGR cooler in order to evaluate its performance in detail. As a result, it was found that large bending and twist tend to occur due to vibration during running of an automobile in the case where a hot rolled ferritic stainless steel sheet having an absolute value of planar anisotropy in terms of the modulus of longitudinal elasticity of more than 35 GPa is used.
• the present inventors diligently conducted investigations regarding a method for decreasing the degree of planar anisotropy in terms of the modulus of longitudinal elasticity of a hot rolled ferritic stainless steel sheet, in particular, focusing on the rolling temperature and rolling reduction ratio of each of multiple passes of a hot rolling process using multiple rolling stands.
• the modulus of longitudinal elasticity of a hot rolled ferritic stainless steel sheet strongly depends on the texture of the steel sheet. Since the texture of a hot rolled steel sheet is formed by repeating the application of processing strain due to rolling and recrystallization, it is possible to control such a texture by adjusting temperature at which rolling work is applied and the amount of strain applied due to rolling.
• the steel sheet is elongated with deformation starting mainly in the surface layer. Therefore, in the case of a small rolling reduction ratio, the amount of deformation is small in the central portion in the thickness direction, which results in almost no rolling strain being introduced to the central portion in the thickness direction.
• the present inventors devised a method in which rolling in the final 3 passes of finish hot rolling is performed in a temperature range of 900° C. to 1100° C. in which recrystallization actively occurs, with an accumulated rolling reduction ratio of 25% or more, which is a rolling reduction larger than that in conventional art.
• the present inventors systematically conducted investigations regarding the influences of temperature and rolling reduction ratio at which each rolling pass of finish hot rolling composed of 7 passes was performed on the degree of planar anisotropy in terms of the modulus of longitudinal elasticity of a hot rolled steel sheet manufactured.
• the degree of planar anisotropy in terms of the modulus of longitudinal elasticity of the steel sheet after hot rolling has been performed to strongly depend on the rolling temperatures and rolling reduction ratios of the final 3 passes while there is almost no influence of the temperatures and rolling reduction ratios of the first 4 passes.
• the present inventors conducted closer investigations regarding the influences of the rolling temperatures and rolling reduction ratios of the final 3 passes and the accumulated rolling reduction ratio of the final 3 passes: As a result, it was found that there is a tendency for the degree of planar anisotropy in terms of the modulus of longitudinal elasticity of a hot rolled steel sheet to significantly decrease in the case where rolling in the final 3 passes is performed in a temperature range of 900° C. to 1100° C. and that the amount of change in the degree of planar anisotropy in terms of the modulus of longitudinal elasticity of the hot rolled steel sheet in this case depends not on the rolling reduction ratio of each of the passes but on the accumulated rolling reduction ratio of the final 3 passes.
• the present inventors conducted investigations regarding the reasons why the rolling temperature and rolling reduction ratio of each of the rolling passes prior to the final 3 passes have a small influence on the planar anisotropy in terms of the modulus of longitudinal elasticity of a hot rolled steel sheet. As a result, it was found that, in the case of the rolling passes prior to the final 3 passes, since the thickness before rolling is performed is large, sufficient rolling strain is not applied to the central portion in the thickness direction even if the rolling reduction ratio is large.
• the present inventors found that, in the case where the hot rolled steel sheet according to aspects of the present invention is subjected to hot rolled sheet annealing in a temperature range of 800° C. to 900° C. in order to improve the formability of the hot rolled steel sheet, the effect of decreasing the degree of planar anisotropy in terms of modulus of longitudinal elasticity, which have been obtained through hot rolling, is maintained while the effect of improving formability of the hot rolled and annealed steel sheet is obtained.
• the thickness of the hot rolled ferritic stainless steel sheet and the hot rolled and annealed ferritic stainless steel sheet according to aspects of the present invention it is preferable that the thickness be 5.0 mm to 15.0 mm, because the steel sheet desirably has a thickness suitable for a flange having a large wall thickness.
• % used when describing a chemical composition means mass %, unless otherwise noted.
• the upper limit of the C content is set to be 0.060%.
• the lower limit of the C content is set to be 0.005%, which is at a level at which there is no significant increase in manufacturing costs in a common refining method. It is preferable that the C content be 0.010% to 0.050% from the viewpoint of the stable manufacturability in a steel-making process.
• the C content is more preferably in a range of 0.020% to 0.045%, even more preferably 0.025% to 0.040%, or even much more preferably 0.030% to 0.040%.
• Si is a chemical element which functions as a deoxidizing agent in a process for preparing molten steel. It is necessary that the Si content be 0.02% or more in order to obtain such an effect. However, it is not desirable that the Si content be more than 0.50%, because this results in a deterioration in manufacturability in a hot rolling process due to an increase in rolling load when hot rolling is performed as a result of an increase in the hardness of a steel sheet. Therefore, the Si content is set to be in a range of 0.02% to 0.50%, preferably 0.10% to 0.35%, or more preferably 0.10% to 0.30%.
• the Mn content is set to be 1.00%.
• the lower limit of the Mn content is set to be 0.01% from the viewpoint of a load placed on a refining process. It is preferable that the Mn content be in a range of 0.10% to 0.90%, or more preferably 0.45% to 0.85%.
• the P content be as small as possible, and the upper limit of the P content is set to be 0.04%. It is preferable that the P content be 0.03% or less, or more preferably 0.01% or less.
• S is a chemical element which deteriorates, for example, ductility and corrosion resistance as a result of existing in the form of sulfide-based inclusions such as MnS, and such negative effects become marked, in particular, in the case where the S content is more than 0.01%. Therefore, it is desirable that the S content be as small as possible, and the upper limit of the S content is set to be 0.01% in accordance with aspects of the present invention. It is preferable that the S content be 0.007% or less, or more preferably 0.005% or less.
• the Cr is a chemical element which is effective for improving corrosion resistance by forming a passivation film on the surface of a steel sheet. It is necessary that the Cr content be 15.5% or more in order to obtain such an effect. However, it is not desirable that the Cr content be more than 18.0%, because this results in a significant deterioration in the toughness of a steel sheet. Therefore, the Cr content is set to be in a range of 15.5% to 18.0%, preferably 16.0% to 17.0%, or more preferably 16.0% to 16.5%.
• Al is, like Si, a chemical element which functions as a deoxidizing agent. It is necessary that the Al content be 0.001% or more in order to obtain such an effect. However, in the case where the Al content is more than 0.10%, since there is an increase in the amount of Al-based inclusions such as Al 2 O 3 , there is a tendency for surface quality to deteriorate. Therefore, the Al content is set to be in a range of 0.001% to 0.10%, preferably 0.001% to 0.07%, or more preferably 0.001% to 0.05%.
• the upper limit of the N content is set to be 0.100%.
• the lower limit of the N content is set to be 0.005%, which is at a level at which there is no significant increase in manufacturing costs in a common refining method. It is preferable that the N content be 0.010% to 0.075% from the viewpoint of stable manufacturability in a steel-making process.
• the N content is more preferably in a range of 0.025% to 0.055%, or even more preferably 0.030% to 0.050%.
• Ni is a chemical element which improves corrosion resistance, and the addition of Ni is effective, in particular, in the case where high corrosion resistance is required. Such an effect becomes marked in the case where the Ni content is 0.1% or more. However, it is not desirable that the Ni content be more than 1.0%, because this results in a deterioration in formability. Therefore, the Ni content is set to be 0.1% to 1.0%. The Ni content is preferably in a range of 0.2% to 0.4%.
• the remainder is Fe and inevitable impurities.
• the chemical composition may further contain the following chemical elements in order to improve manufacturability or material properties.
• Cu is a chemical element which improves corrosion resistance, and the addition of Cu is effective, in particular, in the case where high corrosion resistance is required. Such an effect becomes marked in the case where the Cu content is 0.1% or more. However, in the case where the Cu content is more than 1.0%, there may be a deterioration in formability. Therefore, in the case where Cu is added, the Cu content is set to be 0.1% to 1.0%. The Cu content is preferably in a range of 0.2% to 0.4%.
• Mo is, like Ni and Cu, a chemical element which improves corrosion resistance, and the addition of Mo is effective, in particular, in the case where high corrosion resistance is required. Such an effect becomes marked in the case where the Mo content is 0.1% or more. However, in the case where the Mo content is more than 0.5%, there may be a deterioration in manufacturability in a hot rolling process due to an increase in rolling load when hot rolling is performed as a result of an increase in the hardness of a steel sheet. Therefore, in the case where Mo is added, the Mo content is set to be 0.1% to 0.5%. The Mo content is preferably in a range of 0.2% to 0.3%.
• Co is a chemical 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 0.5%, there may be a deterioration in formability. Therefore, in the case where Co is added, the Co content is set to be in a range of 0.01% to 0.5%.
• V ⁇ 0.0l % to 0.25%
• Ti 0.001% to 0.015%
• Nb 0.001% to 0.025%
• Mg 0.0002% to 0.0050%
• B 0.0002% to 0.0050%
• Ca 0.0002% to 0.0020%
• REM 0.01% to 0.10%
• V 0.01% to 0.25%
• V is a chemical element which forms carbonitrides more readily than Cr. V is effective for inhibiting sensitization, which is caused by the precipitation of Cr carbonitrides, by precipitating C and N in steel in the form of V-based carbonitrides when hot rolling is performed. It is necessary that the V content be 0.01% or more in order to obtain such an effect. However, in the case where the V content is more than 0.25%, there may be deterioration in workability, and there is an increase in manufacturing costs. Therefore, in the case where V is added, the V content is set to be in a range of 0.01% to 0.25%, or preferably 0.03% to 0.08%.
• Ti and Nb are, like V, chemical elements which have a high affinity for C and N and which are effective for inhibiting sensitization, which is caused by the precipitation of Cr carbonitrides, by precipitating in the form of carbides and nitrides when hot rolling is performed.
• the Ti content be 0.001% or more or that the Nb content be 0.001% or more.
• the Ti content is more than 0.015% or in the case where the Nb content is more than 0.030%, there may be a case where it is not possible to achieve good surface quality due to the precipitation of an excessive amount of TiN or NbC.
• the Ti content is set to be in a range of 0.001% to 0.015% in the case where Ti is added, and the Nb content is set to be in a range of 0.001% to 0.025% in the case where Nb is added. It is preferable that the Ti content be in a range of 0.003% to 0.010%. It is preferable that Nb content be in a range of 0.005% to 0.020%, or more preferably 0.010% to 0.015%.
• Mg is a chemical element which is effective for improving hot workability. It is necessary that the Mg content be 0.0002% or more in order to obtain such an effect. However, in the case where the Mg content is more than 0.0050%, there may be a deterioration in surface quality. Therefore, in the case where Mg is added, the Mg content is set to be in a range of 0.0002% to 0.0050%, preferably 0.0005% to 0.0035%, or more preferably 0.0005% to 0.0020%.
• B is a chemical element which is effective for preventing secondary cold work embrittlement. It is necessary that the B content be 0.0002% or more in order to obtain such an effect. However, in the case where the B content is more than 0.0050%, there may be a deterioration in hot workability. Therefore, in the case where B is added, the B content is set to be in a range of 0.0002% to 0.0050%, preferably 0.0005% to 0.0035%, or more preferably 0.0005% to 0.0020%.
• Ca is a chemical element which is effective for preventing nozzle clogging due to the precipitation of inclusions which tends to occur when continuous casting is performed. It is necessary that the Ca content be 0.0002% or more in order to obtain such an effect. However, in the case where the Ca content is more than 0.0020%, there may be a deterioration in corrosion resistance due to the formation of CaS. Therefore, in the case where Ca is added, the Ca content is set to be in a range of 0.0002% to 0.0020%, preferably 0.0005% to 0.0015%, or more preferably 0.0005% to 0.0010%.
• REM rare earth metals
• the REM content is a chemical element which improves oxidation resistance and which is effective for improving the corrosion resistance of, in particular, a weld zone by inhibiting the formation of an oxide film in the weld zone. It is necessary that the REM content be 0.01% or more in order to obtain such an effect. However, in the case where the REM content is more than 0.10%, there may be a deterioration in manufacturability such as pickling capability when cold-rolled sheet annealing is performed. In addition, since REM is an expensive chemical element, it is not preferable that the REM content be excessively large, because this results in an increase in manufacturing costs. Therefore, in the case where REM is added, the REM content is set to be in a range of 0.01% to 0.10%, or preferably 0.01% to 0.05%.
• ferritic stainless steel sheet by performing a hot rolling process involving rough rolling and finish rolling 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 900° C. to 1100° C. with an accumulated rolling reduction ratio of 25% or more.
• the maximum number of passes of finish rolling there is no particular limitation on the maximum number of passes of finish rolling from the viewpoint of achieving the specified material properties.
• the maximum number of passes is more than 15, since there is a tendency for the temperature of a steel sheet to decrease due to an increase in the number of contacts between the sheet and rolling rolls, there may be a deterioration in manufacturability or an increase in manufacturing costs, because, for example, it is necessary to heat the steel sheet from outside in order to maintain the temperature of the steel sheet within the specified temperature range. Therefore, it is preferable that the maximum number of passes be 15 or less, or more preferably 10 or less.
• 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 a steel raw material (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 a steel raw material (slab) by using a continuous casting method or an ingot casting-slabbing method.
• This slab is subjected to hot rolling after having been heated at a temperature of 1100° C. to 1250° C. for 1 hour to 24 hours or the slab as cast is directly subjected to hot rolling without having been heated.
• an accumulated rolling reduction ratio in rough rolling be 65% or more in order to effectively break a cast structure.
• the rolling temperature of the final 3 passes be higher than 1100° C., because this causes a significant increase in crystal grain size with the result that it is not possible to achieve the specified degree of planar anisotropy in terms of modulus of longitudinal elasticity and with the result that the toughness of a hot rolled steel sheet is deteriorated.
• the rolling temperature of the final 3 passes be in a range of 900° C. to 1075° C., or more preferably 930° C. to 1050° C.
• rolling in the first pass of the final 3 passes be performed in a temperature range of 950° C. to 1100° C.
• rolling in the second pass following the first pass be performed in a temperature range of 925° C. to 1075° C.
• rolling in the third pass following the second pass be performed in a temperature range of 900° C. to 1050° C.
• the accumulated rolling reduction ratio is less than 25%, since recrystallization in the central portion in the thickness direction is delayed due to an insufficient amount of rolling strain applied to the central portion in the thickness direction, it is not possible to achieve the desired degree of planar anisotropy in terms of modulus of longitudinal elasticity. Therefore, it is preferable that the accumulated rolling reduction ratio be 25% or more, more preferably 30% or more, or even more preferably 35% or more.
• the accumulated rolling reduction ratio is 60% or less.
• the accumulated rolling reduction ratio described above is expressed by the formula 100 ⁇ (the final thickness/the thickness before rolling in the final 3 passes is performed) ⁇ 100 [%].
• the method for manufacturing the hot rolled ferritic stainless steel sheet according to aspects of the present invention is characterized in that the rolling temperature and accumulated rolling reduction ratio of the final 3 passes of finish rolling are controlled.
• the control target is the rolling temperature and accumulated rolling reduction ratio of the final 4 passes or more
• control target be the rolling temperature and accumulated rolling reduction ratio of 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 performing high rolling reduction with 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 aspects of the present invention, the rolling temperature and accumulated rolling reduction ratio of the final 3 passes of finish rolling are controlled.
• the number of passes of finish rolling there is no particular limitation on the number of passes of finish rolling as long as the number is 3 or more so that the rolling temperature and accumulated rolling reduction ratio of the final 3 passes of finish rolling are controlled.
• the steel sheet is cooled and then subjected to a coiling treatment in order to obtain a hot rolled steel strip.
• a coiling temperature in the case where steel having a chemical composition with which an austenite phase is formed during hot rolling is coiled at a coiling temperature of lower than 500° C., since an austenite phase transforms into a martensite phase, there may be a deterioration in formability due to an increase in the hardness of a hot rolled steel sheet. Therefore, it is preferable that a coiling treatment be performed at a temperature of 500° C. or higher.
• a hot rolled and annealed ferritic stainless steel sheet may be manufactured by performing hot rolled sheet annealing on the hot rolled ferritic stainless steel sheet in a temperature range of 800° C. to 900° C. after the hot rolling process in order to improve formability.
• Hot rolled sheet annealing temperature 800° C. to 900° C.
• the hot rolled sheet annealing temperature is lower than 800° C., since there is insufficient recrystallization, it is not possible to obtain the effect of improving formability due to deformation microstructure formed by performing hot rolling being retained.
• the hot rolled sheet annealing temperature is higher than 900° C.
• the degree of planar anisotropy in terms of modulus of longitudinal elasticity due to the formation of an austenite phase when annealing is performed, namely, there may be a case where the specified degree of planar anisotropy in terms of modulus of longitudinal elasticity which has been obtained in the hot rolled steel sheet is lost.
• a cooling rate after hot rolled sheet annealing has been performed at a temperature of higher than 900° C.
• the annealing temperature be 800° C. to 900° C.
• the holding time and method of hot rolled sheet annealing any one of a box annealing (batch annealing) method and a continuous annealing method may be used.
• the obtained hot rolled steel sheet or steel sheet (hot rolled and annealed steel sheet) which has been subjected to hot rolled sheet annealing may be subjected to a descaling treatment such as one which utilizes shot blasting or pickling as needed. Moreover, grinding or polishing may be performed in order to improve surface quality.
• 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 (SS-VOD) method, and steel slabs having a width of 1000 mm and a thickness of 200 mm were then manufactured by using a continuous casting method.
• the obtained slabs 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 in order to obtain steel sheets having a thickness of about 40 mm and in which the final 3 passes (the fifth pass, the sixth path, and the seventh pass) of finish rolling composed of 7 passes were then performed under the conditions given in Table 2 in order to obtain hot rolled steel sheets.
• SS-VOD stirring-vacuum oxygen decarburization
• hot rolled steel sheets Nos. 25, 26, and 38 in Table 2 were subjected to hot rolled sheet annealing in which the hot rolled steel sheets were held under the conditions given in Table 2 for 8 hours after hot rolling had been performed and in which the held steel sheets were subjected furnace cooling in order to obtain hot rolled and annealed steel sheets.
• the obtained hot rolled steel sheets and hot rolled and annealed steel sheets were evaluated as described below.
• Test pieces having a length of 60 mm, a width of 10 mm, and a thickness of 2 mm whose longitudinal direction were respectively a direction parallel to the rolling direction, a direction at an angle of 45° to the rolling direction, and a direction at a right angle to the rolling direction were taken from the central portion in the thickness direction within 1 mm on both sides of the center in the thickness direction.
• the modulus of longitudinal elasticity of each of the obtained test pieces was measured at a temperature of 23° C. by using a transverse resonant technique prescribed in JIS Z 2280 (1993), and the absolute value
• E L denotes modulus of longitudinal elasticity (GPa) in a direction parallel to the rolling direction
• E D denotes modulus of longitudinal elasticity (GPa) in a direction at an angle of 45° to the rolling direction
• E C denotes modulus of longitudinal elasticity (GPa) in a direction at a right angle to the rolling direction.
• a salt spray cyclic corrosion test prescribed in JIS H 8502 was performed on a test piece having a size of 60 mm ⁇ 100 mm which had been taken from the hot rolled steel sheet, whose surface had been polished by using #600 emery paper, and whose end surfaces were sealed.
• a rust area ratio ((the rust area of the test piece/the total area of the 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 satisfactory ( ⁇ ), a case where the rust area ratio was more than 10% and 25% or less was judged as satisfactory ( ⁇ ), and a case where the rust area ratio was more than 25% was judged as unsatisfactory (x).
• the hot rolled ferritic stainless steel sheet obtained by using aspects of the present invention can particularly preferably be used for purposes which require satisfactory rigidity and corrosion resistance, for example, for the flange of an EGR cooler.

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