Classifications

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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
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  • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
  • C22F1/002—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working by rapid cooling or quenching; cooling agents used therefor
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  • C21D6/00—Heat treatment of ferrous alloys
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  • C21D6/00—Heat treatment of ferrous alloys
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  • C21D6/00—Heat treatment of ferrous alloys
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  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/007—Heat treatment of ferrous alloys containing Co
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  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/008—Heat treatment of ferrous alloys containing Si
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  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • 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
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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
  • C21D8/0205—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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  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
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  • 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
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  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • 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
  • C21D8/0278—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular surface 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
  • 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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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
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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
  • C22C38/001—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
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  • 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
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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
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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
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • 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
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
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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
  • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
  • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
  • C22F1/08—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of copper or alloys based thereon
• • 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 ferritic stainless steel sheet and a method for manufacturing the same, and more particularly relates to a ferritic stainless steel sheet having excellent toughness and excellent corrosion resistance, which is suitable for use as a material for flanges, and a method for manufacturing the same.
• An automobile exhaust gas passage is composed of various components, such as an exhaust manifold, a muffler, a catalyst, a flexible tube, a center pipe, and a front pipe.
• fastening components called flanges are frequently used.
• Flanges used for such exhaust system components are required to have sufficient rigidity. Therefore, thick flanges (e.g., with a sheet thickness of 5 mm or more) are used for such exhaust system components.
• flanges are manufactured by press forming and blanking or the like, and plain steel has been used.
• a ferritic stainless steel sheet having a large thickness has a problem in low-temperature toughness. For example, breakage during manufacturing of flanges frequently occurs in winter. For this reason, there has been a strong demand for improvement in the toughness of a ferritic stainless steel sheet having a large thickness.
• Patent Literature 1 discloses a stainless steel sheet having excellent toughness (with a Charpy impact value of 50 J/cm 2 or more at ⁇ 40° C.), the stainless steel sheet containing, in percent by mass, C: 0.02% or less, N: 0.02% or less, Si: 0.005 to 1.0%, Ni: 0.1 to 1.0%, Mn: 0.1 to 3.0%, P: 0.04% or less, S: 0.0100% or less, Cr: 10% or more and less than 18%, and further one or two of Ti: 0.05 to 0.30% and Nb: 0.01 to 0.50%, the sum of Ti and Nb being 8(C+N) to 0.75%, with the balance being Fe and inevitable impurities, in which ⁇ p is 70% or more, the ferrite grain size is 20 ⁇ m or less, and the amount of martensite formation is 70% or less.
• ⁇ p (%) is evaluated by using the formula (i) below (in Patent Literature 1, expressed as formula (1))
• ⁇ p 420(% C)+470(% N)+23(% Ni)+9(% Cu)+7(% Mn) ⁇ 11.5(% Cr) ⁇ 11.5(% Si) ⁇ 12(% Mo) ⁇ 23(% V) ⁇ 47(% Nb) ⁇ 49(% Ti) ⁇ 52(% Al)+189 (i),
• the term “more excellent toughness” means that the Charpy impact value at ⁇ 50° C. is 100 J/cm 2 or more. Furthermore, in accordance with aspects of the present invention, the term “excellent corrosion resistance” means that, after a cyclic salt spray test specified in JIS H 8502 is performed for three cycles, the rusting area ratio is 25% or less.
• a method in which after a slab having a steel composition including appropriate steel elements, specifically, Si, Mn, Cr, Ni, and the like, that are controlled in appropriate ranges is heated at 1,050 to 1,250° C., hot rolling is performed, and hot-rolled sheet annealing is performed at an appropriate temperature, is effective in refining the metal structure and obtaining a Charpy impact value of 100 J/cm 2 or more at ⁇ 50° C.
• appropriate steel elements specifically, Si, Mn, Cr, Ni, and the like
• Ni, Mn, Cu, Si, Cr, and Mo represent contents of the respective elements (percent by mass), and an element not contained represents 0.
• ferritic stainless steel sheet having more excellent toughness and excellent corrosion resistance.
• the ferritic stainless steel sheet according to aspects of the present invention can be suitably used for thick flanges and the like.
• 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 portion in which a flange hole (30 mm ⁇ ) is raised by 10 mm from the surface of the steel sheet as blanked.
• the results have shown that cracks do not occur in steel sheets having a Charpy impact value of 100 J/cm 2 or more at ⁇ 50° C., and in steel sheets in which cracks occur, the Charpy impact value at ⁇ 50° C. is less than 100 J/cm 2 . In this way, it has been found that low toughness is a cause for cracks.
• the present inventors have investigated in detail the relationship between the low toughness and the metal structure. As a result, it has been found that as the average crystal grain size of the steel sheet increases, toughness decreases. Accordingly, by using various ferritic stainless steel sheets (sheet thickness: 5.0 mm), forming into the flange has been tried. As a result, it has been found that in steel sheets having an average crystal grain size of more than 45 ⁇ m, toughness decreases and cracks are likely to occur, and that when the average crystal grain size is 45 ⁇ m or less, toughness is excellent and blanking workability of the steel sheet is good.
• the average crystal grain size is set to be 45 ⁇ m or less, and the Charpy impact value at ⁇ 50° C. is set to be 100 J/cm 2 or more.
• the average crystal grain size can be measured by a measurement method used in examples which will be described later.
• the Charpy impact value is a value measured in accordance with JIS Z 2242 (2005) as will be described later.
• % which is the unit of measure for the content of each element, means “percent by mass”.
• the C content When the C content exceeds 0.020%, deterioration in workability and corrosion resistance becomes conspicuous. 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 C content is preferably 0.003% or more, and more preferably 0.004% or more.
• the C content 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 welds by being concentrated in an oxide film formed during welding and is also effective as a deoxidizing element in the steelmaking process. These effects are obtained at a Si content of 0.05% or more and increase with increasing its content. On the other hand, Si has an effect of accelerating ferrite phase formation. When the Si content exceeds 0.35%, a predetermined amount of austenite phase is not formed sufficiently during heating in the hot rolling process. Accordingly, even when hot rolling and hot-rolled sheet annealing are performed under the conditions specified in accordance with aspects of the present invention, a desired metal structure cannot be obtained. Therefore, the Si content is set to be 0.05% or more and 0.35% or less. The Si content is preferably 0.10% or more. Furthermore, the Si content is preferably 0.30% or less.
• Mn has an effect of accelerating austenite phase formation. In order to obtain such an effect, a Mn content of 0.05% or more is necessary. However, when the Mn content exceeds 1.00%, precipitation of MnS serving as a starting point of corrosion is accelerated, resulting in deterioration in corrosion resistance. Therefore, the Mn content is set to be 0.05% or more and 1.00% or less.
• the Mn content is preferably 0.20% or more. Furthermore, the Mn content is preferably 0.80% or less, and more preferably 0.70% or less.
• P is an element that is inevitably contained in steel. Since P is an element detrimental to corrosion resistance and workability, it is desirable to decrease the amount of P as much as possible. When the P content exceeds 0.04%, workability markedly deteriorates by solid solution strengthening. Therefore, the P content is set to be 0.04% or less. The P content is preferably 0.03% or less.
• S similar to P, is an element that is inevitably contained in steel. Since S is an element detrimental 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 markedly deteriorates. Therefore, the S content is set to be 0.01% or less.
• the S content is preferably 0.008% or less, and more preferably 0.003% or less.
• Al is an effective deoxidizer. Furthermore, since Al has higher affinity for nitrogen than Cr, in the case where nitrogen enters a weld, 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 0.300%, weld penetration deteriorates, resulting in deterioration in weldability, which is undesirable. Therefore, the Al content is set in a range of 0.001% to 0.300%. The Al content is preferably 0.010% or more. Furthermore, the Al content is preferably 0.200% or less, more preferably 0.100% or less, and still more preferably 0.050% or less.
• the Cr content is set in a range of 10.0% to 13.0%.
• the Cr content is preferably 10.5% or more.
• the Cr content is preferably 12.0% or less, and more preferably 11.7% 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 process.
• a dual-phase structure of ferrite phase+austenite phase which includes 70% or more, in volume ratio, of austenite phase, is formed during heating the slab in the hot rolling process.
• the interface between different phases i.e., between the ferrite phase and the austenite phase, functions as an obstacle to growth of crystal grains, and therefore, the metal structure before hot rolling is refined.
• the Ni content is set to be 0.75% or more and 1.50% or less.
• the Ni content is preferably 0.80% or more.
• the Ni content is preferably 1.20% or less, and more preferably 1.00% or less.
• Ti preferentially combines with C and N, suppresses precipitation of Cr carbonitrides, and lowers the recrystallization temperature. Ti also 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 0.05% or more is necessary. On the other hand, when the Ti content exceeds 0.35%, formation of coarse TiN causes marked deterioration in toughness, and even if the technique according to aspects of the present invention is applied, predetermined toughness cannot be obtained. Furthermore, when the Ti content exceeds 0.35%, coarse Ti carbonitrides are formed in the casting process, resulting in surface defects, which is undesirable in terms of manufacturing. Therefore, the Ti content is set to be 0.05% or more and 0.35% or less. The Ti content is preferably 0.10% or more. Furthermore, the Ti content is preferably 0.30% or less, and more preferably 0.15% or less.
• the N content exceeds 0.020%, deterioration in workability and corrosion resistance becomes conspicuous. A lower N content is more desirable from the viewpoint of workability and corrosion resistance.
• the N content is set in a range of 0.001% to 0.020%.
• the N content is preferably 0.005% or more, and more preferably 0.007% or more.
• the N content is preferably 0.015% or less, and more preferably 0.012% or less.
• ⁇ I represented by formula (1) below is less than 65%, because of an insufficient amount of austenite in the metal structure, a fine metal structure cannot be obtained at a slab heating temperature before starting hot rolling. Therefore, ⁇ I [%] is set to be 65% or more. Note that ⁇ I [%] is obtained by using formula (1) below, which evaluates the stability of austenite phase.
• the balance other than the above is Fe and inevitable impurities.
• the inevitable impurities include oxygen (O), and an O content of 0.01% or less is permissible.
• ferritic stainless steel sheet can further contain one group or two or more groups selected from groups A to C described below.
• Cu is a particularly effective element in improving corrosion resistance in an aqueous solution or when weakly acidic water drops adhere to the steel sheet. Furthermore, Cu has an effect of accelerating austenite phase formation. This effect can be obtained at a Cu content of 0.01% or more and increases with increasing Cu content. However, when the Cu content exceeds 1.00%, hot workability deteriorates, which may induce surface defects in some cases. Furthermore, descaling after annealing may become difficult in some cases. Therefore, when Cu is contained, the Cu content is set in a range of 0.01% to 1.00%. When Cu is contained, the Cu content is preferably 0.10% or more. Furthermore, when Cu is contained, the Cu content is preferably 0.50% or less.
• Mo is an element that markedly 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. On the other hand, Mo has an effect of accelerating ferrite phase formation. When the Mo content exceeds 1.00%, a predetermined amount of austenite phase is not formed sufficiently during heating in the hot rolling process. Accordingly, even when hot rolling and hot-rolled sheet annealing are performed under the conditions specified in accordance with aspects of the present invention, a desired metal structure cannot be obtained. Therefore, when Mo is contained, the Mo content is set to be 0.01% or more and 1.00% or less. When Mo is contained, the Mo content is preferably 0.10% or more, and more preferably 0.30% or more. Furthermore, when Mo is contained, the Mo content is preferably 0.80% or less, and more preferably 0.50% or less.
• W similar to Mo, has an effect of improving corrosion resistance. This effect is obtained at a W content of 0.01% or more.
• the W content exceeds 0.20%, strength increases, which may cause deterioration in productivity due to an increase in the rolling load and the like in some cases. Therefore, when W is contained, the W content is set in a range of 0.01% to 0.20%.
• the W content is preferably 0.05% or more.
• the W content is 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 deteriorate in some cases. Therefore, when Co is contained, the Co content is set in a range of 0.01% to 0.20%.
• V 0.01 to 0.20%
• V together with C and N, forms carbonitrides, and by suppressing sensitization during welding, improves corrosion resistance of welds. This effect is obtained at a V content of 0.01% or more.
• the V content exceeds 0.20%, workability and toughness may markedly deteriorate in some cases. Therefore, when V is contained, the V content is set to be 0.01% or more and 0.20% or less.
• the V content is preferably 0.02% or more.
• the V content is preferably 0.10% or less.
• Nb has an effect of refining crystal grains. This effect is 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, and a metal structure with an average crystal grain size of 45 ⁇ m or less cannot be obtained. Therefore, when Nb is contained, the Nb content is set in a range of 0.01% to 0.10%. When Nb is contained, the Nb content is preferably 0.05% or less.
• Zr has an effect of suppressing sensitization by combining with C and N. This effect is obtained at a Zr content of 0.01% or more. On the other hand, when the Zr content exceeds 0.20%, workability may markedly deteriorate in some cases. Therefore, when Zr is contained, the Zr content is set in a range of 0.01% to 0.20%. When Zr is contained, the Zr content is preferably 0.10% or less.
• REM Radar Earth Metals
• the REM content is set in a range of 0.001% to 0.100%.
• the REM content is 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 deteriorate in some cases. Therefore, when B is contained, the B content is set in a range of 0.0002% to 0.0025%. When B is contained, the B content is preferably 0.0003% or more. Furthermore, when B is contained, the B content is preferably 0.0012% or less.
• Mg has an effect of suppressing coarsening of Ti carbonitrides. This effect is obtained at a Mg content of 0.0005% or more.
• Mg content exceeds 0.0030%, surface properties of steel may deteriorate in some cases. Therefore, when Mg is contained, the Mg content is set in a range of 0.0005 to 0.0030%.
• the Mg content is preferably 0.0010% or more.
• the Mg content is preferably 0.0020% or less.
• Ca is an element effective in preventing nozzle blockage due to crystallization of Ti-based inclusions which is likely to occur during continuous casting. This effect is obtained at a Ca content of 0.0003% or more.
• the Ca content exceeds 0.0030%, corrosion resistance may deteriorate by formation of CaS in some cases. Therefore, when Ca is contained, the Ca content is set in a range of 0.0003% to 0.0030%.
• the Ca content is preferably 0.0005% or more.
• the Ca content is preferably 0.0015% or less, and more preferably 0.0010% or less.
• a method for manufacturing a ferritic stainless steel sheet according to aspects of the present invention will be described below.
• the present inventors have performed thorough studies on a technique of improving toughness in a ferritic stainless steel sheet. As a result, it has been found that after a steel slab having an appropriate steel composition is heated preferably at 1,050 to 1,250° C., by performing hot rolling preferably with three or more passes, and subjecting the resulting hot-rolled steel sheet to hot-rolled sheet annealing at 750 to 1,050° C., a metal structure with an average crystal grain size of 45 ⁇ m or less can be obtained, and toughness is greatly improved to a Charpy impact value of 100 J/cm 2 or more at ⁇ 50° C. Furthermore, it has been found that desired corrosion resistance can be obtained.
• the present inventors have performed thorough studies on an effective technique for obtaining a fine structure 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 control the contents of steel elements, in particular, Si, Mn, Cr, and Ni, in appropriate ranges and to perform hot rolling after performing heating of the slab at an appropriate temperature in the hot rolling process so as to form a dual-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, suppresses coarsening of crystal grains, and therefore, a fine equiaxed structure can be obtained in the stage before hot rolling.
• working strain acting as recrystallization sites in the subsequent hot-rolled sheet annealing process is sufficiently accumulated.
• a fine metal structure is obtained in the subsequent hot-rolled sheet annealing process, and excellent toughness can be exhibited.
• the hot-rolled sheet annealing process is a process of recrystallizing the worked structure formed by hot rolling. Therefore, it is necessary to perform annealing at a temperature at which sufficient recrystallization occurs.
• hot-rolled sheet annealing is performed at an excessively high temperature, although recrystallization occurs, recrystallized grains markedly coarsen. Therefore, a desired fine structure 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 1,050° C. or lower, it is possible to suppress formation of recrystallized grains that are coarse to such an extent that toughness deteriorates.
• molten steel having the composition described above is melted 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-blooming process.
• Steel slab heating temperature 1,050 to 1,250° C.
• the steel slab is heated at 1,050 to 1,250° C. and subjected to hot rolling.
• the heating time at the heating temperature is not particularly limited, but preferably, heating is performed for 1 to 24 hours.
• the heating temperature is lower than 1,050° C.
• the austenite phase formation rate decreases, a fine metal structure cannot be obtained, and thus excellent toughness cannot be obtained.
• the heating temperature increases excessively, the oxidation mass increases resulting in an increase in scale loss. Therefore, the steel slab heating temperature is set to be 1,250° C. or lower.
• the steel slab after casting is in a temperature range of 1,050° C. or higher, the steel may be, without being heated, directly subjected to rolling.
• the rough rolling conditions are not particularly limited.
• the cumulative rolling reduction in rough rolling is preferably set to be 65% or more. Then, finish hot rolling is performed until a predetermined sheet thickness is reached.
• Hot-rolled sheet annealing temperature 750 to 1,050° C.
• hot-rolled sheet annealing is performed.
• the rolled structure formed in the hot rolling process is recrystallized.
• by effectively imparting rolling strain in the hot rolling process so that the number of recrystallization sites increases coarsening of recrystallization grains in hot-rolled sheet annealing is suppressed.
• the hot-rolled sheet annealing temperature is set in a range of 750° C. to 1,050° C.
• the hot-rolled sheet annealing temperature is in a range of 750° C. to 900° C. Note that 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 ferritic stainless steel sheet thus obtained may be subjected, as necessary, to a descaling treatment by shotblasting or pickling. Furthermore, in order to improve surface properties, the steel sheet may be subjected to grinding, polishing, or the like. Moreover, the steel sheet may be further subjected to cold rolling and cold-rolled sheet annealing.
• the metal structure of the ferritic stainless steel sheet obtained in accordance with aspects of the present invention includes a ferrite single phase or includes 3% or less (in volume ratio) in total of one or both of a martensite phase and a retained austenite phase with the balance being a ferrite phase.
• the ferritic stainless steel sheet according to aspects of the present invention has a Charpy impact value of 100 J/cm 2 or more at ⁇ 50° C. Because of such excellent low-temperature toughness, occurrence of cracks in the burring portion can be effectively prevented when worked into a thick flange having a burring portion, and the steel sheet can be satisfactorily put into practical use for a thick flange having a burring portion.
• the sheet thickness is not particularly limited, but is desirably a sheet thickness that can be used for a thick flange. Therefore, the sheet thickness is preferably 5.0 mm or more, and more preferably 8.0 mm or more. Furthermore, the sheet thickness is preferably 15.0 mm or less, and more preferably 13.0 mm or less.
• Molten stainless steels having the compositions shown in Table 1 were each formed into a 100-kg steel slab by vacuum induction melting. Subsequently, by performing hot rolling under the manufacturing conditions shown in Table 2, a hot-rolled steel sheet with the finished sheet thickness shown in Table 2 was obtained. By subjecting the hot-rolled steel sheet to hot-rolled sheet annealing, a hot-rolled and annealed steel sheet was obtained. Note that hot-rolled sheet annealing was performed by holding the steel sheet at the hot-rolled sheet annealing temperature shown in Table 2 for 8 hours. The following evaluations were made on the resulting hot-rolled and annealed steel sheet.
• the average crystal grain size was measured by an EBSD (Electron Back Scattering Diffraction) method.
• the measurement conditions were as follows: a magnification, for measurement, of 500 times, with a step size of 0.4 ⁇ m.
• the obtained data were analyzed by OIM (Orientation Imaging Microscopy) analysis software developed by TSL Solutions Ltd., an orientation difference of 15° or more was defined as a grain boundary, and circle equivalent diameters were calculated. A value calculated from the average of the circle equivalent diameters was defined as an average crystal grain size.
• a V-notch Charpy specimen according to JIS Z 2242 (2005) was taken from the central part in the sheet width direction of each of the hot-rolled and annealed steel sheets, without changing the thickness of the steel sheet, such that the rolling direction corresponded to the longitudinal direction of the specimen.
• the specimen was tested in accordance with JIS Z 2242 (2005) to measure a Charpy impact value at ⁇ 50° C. Specimens with a Charpy impact value of 100 J/cm 2 or more at ⁇ 50° C. were evaluated as “pass”, and specimens with a Charpy impact value of less than 100 J/cm 2 at ⁇ 50° C. were evaluated as “rejection”.
• a specimen of 60 ⁇ 80 mm was taken from each of the hot-rolled and annealed steel sheets. After a front surface of the specimen was polish-finished with #600 emery paper, end face portions and a back surface of the specimen were sealed. Then, the specimen was subjected to a cyclic salt spray test specified in JIS H 8502. In the cyclic salt spray test, three 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 front surface of the specimen was photographed, and a rusting area in the front surface of the specimen was measured by image analysis. From the ratio of the rusting area to the area of a portion in which the rusting area is measured, the rusting area ratio (rusting area/area of portion in which rusting area is measured in specimen) ⁇ 100[%]) was calculated.
• the portion in which the rusting area is measured refers to a portion excluding an outer peripheral portion with a width of 15 mm of the specimen. Note that the rusting area includes areas of a rusting portion and a portion subjected to flow rust.
• Specimens with a rusting area ratio of 10% or less were evaluated as “pass” ( ⁇ ) with particularly excellent corrosion resistance, specimens with a rusting area ratio of more than 10% and 25% or less were evaluated as “pass” ( ⁇ ), and specimens with a rusting area ratio of more than 25% were evaluated as “rejection” (x).
• the ferritic stainless steel sheet obtained in accordance with aspects of the present invention is suitable for application requiring excellent toughness, for example, particularly suitable for use in a flange or the like.

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