Classifications

• • 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
  • C22F1/002—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
• • 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
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  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
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  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/005—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
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  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/008—Heat treatment of ferrous alloys containing Si
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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
• • 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
  • C21D8/0278—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular surface treatment
• • 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
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • C—CHEMISTRY; METALLURGY
  • 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 producing the same, and more particularly to a ferritic stainless steel sheet excellent in toughness and excellent in corrosion resistance, which is useful for using a flange member, and a method for producing the same.
• the exhaust gas path of a car is composed of various parts such as an exhaust manifold, a muffler, a catalyst, a flexible tube, a center pipe and a front pipe.
• fastening parts called flanges are often used.
• the flange applied to such exhaust system components needs to have sufficient rigidity. From this, a thick-walled (for example, 5 mm or more in plate thickness) flange is applied to such an exhaust system component.
• the flange is manufactured by processing such as punching other than press forming, and ordinary steel has been used.
• flange materials applied to parts exposed to high temperature exhaust gas such as an EGR (Exhaust Gas Recirculation, EGR) system are required to have sufficient corrosion resistance. Therefore, the application of stainless steel which is superior in corrosion resistance to ordinary steel, in particular, ferritic stainless steel which has a relatively small coefficient of thermal expansion and which hardly generates a thermal stress has been studied. As a result, a ferritic stainless steel plate having a large thickness (for example, 5 mm or more in thickness) applicable to a thick flange is strongly required.
• EGR exhaust Gas Recirculation
• a ferritic stainless steel having a large thickness has a problem of low temperature toughness. For example, many press cracks at the time of flange manufacture occur in winter. From these facts, there is a strong demand for improvement in toughness of a ferritic stainless steel having a large thickness.
• Patent Document 1 For such market requirements, for example, in Patent Document 1, C: 0.02% or less, N: 0.02% or less, Si: 0.005 to 1.0%, Ni: 0 in mass%. 1 to 1.0%, Mn: 0.1 to 3.0%, P: 0.04% or less, S: 0.0100% or less, Cr: 10% or more to 18% or less, and further Ti : 0.05 to 0.30%, Nb: 0.01 to 0.50%, containing one or two kinds, and the total of Ti and Nb is 8 (C + N) to 0.75%, the balance Is composed of Fe and unavoidable impurities, and has a ⁇ p of 70% or more, a ferrite grain size of 20 ⁇ m or less, and a martensite formation amount of 70% or less (a Charpy impact value at ⁇ 40 ° C.
• a stainless steel sheet excellent in 50 J / cm 2 or more) is disclosed.
• (gamma) p (%) is evaluated using following (i) Formula (In patent document 1, it describes with (1) Formula).
• ⁇ 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
• (% X) shows the mass ratio of each component X.
• An object of the present invention is to provide a ferritic stainless steel sheet which is more excellent in toughness and excellent in corrosion resistance and a method of manufacturing the same.
• more excellent toughness means that the Charpy impact value at ⁇ 50 ° C. is 100 J / cm 2 or more.
• having excellent corrosion resistance means that the rusting rate after performing three cycles of the salt spray cycle test defined in JIS H 8502 is 25% or less.
• the present inventors conducted detailed studies to solve the above problems. As a result, the following findings were obtained.
• the metal structure is refined and the Charpy impact value at -50 ° C. is 100 J / cm 2 or more.
• the average grain size of the metal structure it is possible to effectively suppress the occurrence of cracks in the burring portion when processing into a thick flange having the burring portion. It can be fully commercialized to a thick flange having a burring portion.
• Hot-rolled sheet annealing at a temperature is an effective means for refining the metal structure and obtaining a Charpy impact value of -100 J / cm 2 or more at -50 ° C.
• the present invention has been made based on the above findings, and the gist of the present invention is as follows. [1] by mass%, C: 0.001 to 0.020%, Si: 0.05 to 0.35%, Mn: 0.05 to 1.00%, P: not more than 0.04%, S: 0.01% or less, Al: 0.001 to 0.300%, Cr: 10.0 to 13.0%, Ni: 0.75 to 1.50%, Ti: 0.05 to 0.35%, N: 0.001 to 0.020%, and ⁇ I [%] of the following formula (1) is 65% or more, and the balance has a component composition consisting of Fe and unavoidable impurities, A ferritic stainless steel sheet having an average grain size of 45 ⁇ m or less in a metal structure.
• Ni, Mn, Cu, Si, Cr, and Mo in Formula (1) represent content (mass%) of each component, and let the component which is not contained be zero.
• V 0.01 to 0.20%
• Nb 0.01 to 0.10%
• Zr 0.01 to 0.20% in mass%
• REM 0.001 to 0.100%
• B 0.0002 to 0.0025%
• Mg 0.0005 to 0.0030%
• Ca 0 by mass%
• a method for producing a ferritic stainless steel sheet comprising: a hot rolling step of performing hot rolling; and a hot rolled sheet annealing step of hot rolled sheet annealing of the hot rolled steel sheet obtained in the hot rolling step at 750 to 1050 ° C.
• ferritic stainless steel sheet which is more excellent in toughness and excellent in corrosion resistance.
• the ferritic stainless steel sheet of the present invention can be suitably used for thick flanges and the like.
• the present inventors used a variety of ferritic stainless steel plates with a thickness of 5.0 mm to form flanges having a 30 mm diameter flange hole with a burred portion that lifts 10 mm from the surface of the steel plate as it is as blank (as punched out).
• the inventors examined in detail the relationship between the low toughness and the metallographic structure. As a result, it was found that the toughness was lowered as the average grain size of the steel sheet was larger. Then, forming to the above-mentioned flange was tried using various ferritic stainless steel plates (board thickness 5.0 mm). As a result, it was found that, in a steel plate having an average crystal grain size exceeding 45 ⁇ m, the toughness is lowered and a crack is easily generated. When the average crystal grain size is 45 ⁇ m or less, the toughness is excellent and the punching workability of the steel plate is good.
• the average crystal grain size is 45 ⁇ m or less, and the Charpy impact value at ⁇ 50 ° C. is 100 J / cm 2 or more.
• the said average grain size can be measured by the measuring method of the Example mentioned later.
• the Charpy impact value is a value measured in accordance with JIS Z 2242 (2005) as described later.
• the C content is in the range of 0.001% to 0.020%.
• the C content is preferably 0.003% or more, more preferably 0.004% or more. Further, the C content is preferably 0.015% or less, more preferably 0.012% or less.
• Si 0.05 to 0.35%
• Si has the effect of concentrating on the oxide film formed at the time of welding to improve the corrosion resistance of the welded portion, and is also an element useful as a deoxidizing element in the steel making process. These effects are obtained by containing Si of 0.05% or more, and the effect becomes larger as the content is larger.
• Si has the effect of promoting the formation of a ferrite phase. If Si is contained in excess of 0.35%, a predetermined amount of austenite phase is not sufficiently formed at the time of heating in the hot rolling step. The desired metallographic structure can not be obtained even if hot rolling and hot rolled sheet annealing are performed under the following conditions. Therefore, the Si content is set to 0.05% or more and 0.35% or less.
• the Si content is preferably 0.10% or more. Further, the Si content is preferably 0.30% or less.
• Mn 0.05 to 1.00% Mn has the effect of promoting the formation of the austenite phase. In order to acquire the effect, it is necessary to contain 0.05% or more of Mn. However, if the Mn content exceeds 1.00%, precipitation of MnS, which is a starting point of corrosion, is promoted, and the corrosion resistance is lowered. Therefore, the Mn content is set to 0.05% or more and 1.00% or less.
• the Mn content is preferably 0.20% or more. Also, the Mn content is preferably 0.80% or less, more preferably 0.70% or less.
• P 0.04% or less
• P is an element inevitably contained in steel and is an element harmful to corrosion resistance and workability, and therefore it is preferable to reduce as much as possible. If the P content exceeds 0.04%, the formability is markedly reduced due to solid solution strengthening. Therefore, the P content is 0.04% or less.
• the P content is preferably 0.03% or less.
• S 0.01% or less S is also an element inevitably contained in steel like P, and is an element harmful to corrosion resistance and workability, and therefore, it is preferable to reduce as much as possible. In particular, when the S content exceeds 0.01%, the corrosion resistance is significantly reduced. Therefore, the S content is 0.01% or less.
• the S content is preferably 0.008% or less, more preferably 0.003% or less.
• Al 0.001 to 0.300%
• Al is an effective deacidifying agent. Furthermore, since Al has a stronger affinity to nitrogen than Cr, when nitrogen penetrates the weld, it has the effect of precipitating nitrogen as Al nitride instead of Cr nitride to suppress sensitization. These effects are obtained by containing Al 0.001% or more. However, if the Al content exceeds 0.300%, it is not preferable because the penetration during welding decreases and the weldability decreases. Therefore, the Al content is in the range of 0.001% to 0.300%. The Al content is preferably 0.010% or more. Further, the Al content is preferably 0.200% or less, more preferably 0.100% or less, and still more preferably 0.050% or less.
• Cr 10.0 to 13.0% Cr is the most important element to ensure corrosion resistance. If the content is less than 10.0%, corrosion resistance necessary for automobile exhaust parts can not be obtained. On the other hand, when Cr is contained in excess of 13.0%, a predetermined amount of austenite phase is formed at the time of heating in the hot rolling process even if the steel component is adjusted to ⁇ I represented by predetermined formula (1) described later In order to avoid this, even if hot rolling and hot rolled sheet annealing are performed under the conditions specified by the present invention, the desired metallographic structure can not be obtained. Therefore, the Cr content is in the range of 10.0% to 13.0%. The Cr content is preferably 10.5% or more. Further, the Cr content is preferably 12.0% or less, more preferably 11.7% or less.
• Ni 0.75 to 1.50%
• Ni is an austenite-forming element, and has an effect of increasing the amount of austenite generated at the time of heating before rolling in the hot rolling process.
• a two-phase structure of a ferrite phase and an austenite phase including an austenite phase of 70% or more in volume ratio is obtained at the time of slab heating in the hot rolling process.
• the metallographic structure is a two-phase structure of a ferrite phase and an austenite phase, the heterophase interface between the ferrite phase and the austenite phase functions as an obstacle to grain growth, so that the metal structure before hot rolling is refined.
• the Ni content is set to 0.75% or more and 1.50% or less.
• the Ni content is preferably 0.80% or more. Further, the Ni content is preferably 1.20% or less, more preferably 1.00% or less.
• Ti 0.05 to 0.35%
• Ti preferentially combines with C and N to suppress the precipitation of Cr carbonitrides, and has the effect of reducing the recrystallization temperature and suppressing the drop in corrosion resistance caused by the sensitization due to the precipitation of Cr carbonitrides. is there. In order to obtain such an effect, it is necessary to contain 0.05% or more of Ti. On the other hand, if the Ti content exceeds 0.35%, the toughness is significantly reduced due to the formation of coarse TiN, and even if the technology of the present invention is applied, a predetermined toughness can not be obtained. Further, the content of Ti of more than 0.35% is not preferable in production because coarse Ti carbo-nitrides are formed in the casting process to cause surface defects. Therefore, the Ti content is set to 0.05% or more and 0.35% or less. The Ti content is preferably 0.10% or more. Further, the Ti content is preferably 0.30% or less, more preferably 0.15% or less.
• the N content is in the range of 0.001% to 0.020%.
• the N content is preferably 0.005% or more, more preferably 0.007% or more. Further, the N content is preferably 0.015% or less, more preferably 0.012% or less.
• ⁇ I [%] 65% or more
• ⁇ I [%] is determined using the following equation (1) for evaluating the stability of the austenite phase.
• ⁇ I [%] 24 Ni + 12 Mn + 6 Cu-18 Si-12 Cr-12 Mo + 188 (1)
• Ni, Mn, Cu, Si, Cr, and Mo in Formula (1) represent content (mass%) of each component, and let the component which is not contained be zero.
• the austenite-forming element has a positive coefficient
• the ferrite-forming element has a negative coefficient, and the respective values were experimentally obtained with reference to the Castro equation.
• the remainder other than the above is Fe and unavoidable impurities.
• an unavoidable impurity O (oxygen) etc. are mentioned, and if content of O is 0.01% or less, it is permissible.
• one or more groups selected from the following groups A to C can be contained.
• Group A Cu: 0.01 to 1.00%, Mo: 0.01 to 1.00%, W: 0.01 to 0.20%, Co: 0.01 to 0.20% Or two or more
• group B V: 0.01 to 0.20%, Nb: 0.01 to 0.10%, and Zr: 0.01 to 0.20% one or more
• group C REM: 0.001 to 0.100%
• B 0.0002 to 0.0025%
• Mg 0.0005 to 0.0030%
• Ca 0.0003 to 0.0030%
• Cu 0.01 to 1.00%
• Cu is an element that is particularly effective in improving the corrosion resistance in an aqueous solution or when a weakly acidic water droplet is attached. Furthermore, Cu has the effect of promoting the formation of the austenite phase. This effect is obtained by containing 0.01% or more, and the effect becomes higher as the Cu content is larger. However, if Cu is contained in excess of 1.00%, the hot workability may be reduced to induce surface defects. Furthermore, there are cases where descaling after annealing becomes difficult. Therefore, when it contains Cu, Cu content is made into the range of 0.01% or more and 1.00% or less. When Cu is contained, the Cu content is preferably 0.10% or more. When Cu is contained, the Cu content is preferably 0.50% or less.
• Mo 0.01 to 1.00%
• Mo is an element that significantly improves the corrosion resistance of stainless steel. This effect is obtained by containing 0.01% or more of Mo, and the effect improves as the content increases.
• Mo has the effect of promoting the formation of a ferrite phase, and when the Mo content exceeds 1.00%, a predetermined amount of austenite phase is not sufficiently formed at the time of heating in the hot rolling process. The desired metallographic structure can not be obtained even if hot rolling and hot rolled sheet annealing are performed under the following conditions. Therefore, when it contains Mo, Mo content is made into 0.01% or more and 1.00% or less. When Mo is contained, the Mo content is preferably 0.10% or more, more preferably 0.30% or more. Moreover, when it contains Mo, Mo content is preferably 0.80% or less, more preferably 0.50% or less.
• W 0.01 to 0.20% Like Mo, W has the effect of improving the corrosion resistance. This effect is obtained by containing 0.01% or more of W. On the other hand, if W is contained in excess of 0.20%, the strength may increase, which may lead to a decrease in manufacturability due to an increase in rolling load or the like. Therefore, when W is contained, the W content is in the range of 0.01% or more and 0.20% or less. When W is contained, the W content is preferably 0.05% or more. When W is contained, the W content is preferably 0.15% or less.
• Co 0.01 to 0.20%
• Co is an element that improves the toughness. This effect is obtained by containing 0.01% or more of Co. On the other hand, when the Co content exceeds 0.20%, the processability may be reduced. Therefore, when Co is contained, the Co content is in the range of 0.01% to 0.20%.
• V 0.01 to 0.20% V forms carbonitrides with C and N, suppresses sensitization during welding, and improves the corrosion resistance of the welded portion. This effect is obtained when the V content is 0.01% or more. On the other hand, when the V content exceeds 0.20%, the processability and the toughness may be significantly reduced. Therefore, when V is contained, V content is made into 0.01% or more and 0.20% or less. When V is contained, the V content is preferably 0.02% or more. Moreover, when V is contained, V content is preferably 0.10% or less.
• Nb 0.01 to 0.10%
• Nb has the effect of refining the crystal grains. This effect is obtained by containing 0.01% or more of Nb.
• Nb also has the effect of raising the recrystallization temperature, and if the Nb content exceeds 0.10%, the annealing temperature required to cause sufficient recrystallization in hot-rolled sheet annealing becomes excessively high. In some cases, it is not possible to obtain a metal structure having an average crystal grain size of 45 ⁇ m or less. Therefore, when Nb is contained, the Nb content is in the range of 0.01% or more and 0.10% or less. When Nb is contained, the Nb content is preferably 0.05% or less.
• Zr 0.01 to 0.20% Zr combines with C and N and has an effect of suppressing sensitization. This effect is obtained by containing 0.01% or more of Zr. On the other hand, if the content of Zr exceeds 0.20%, the workability may be significantly reduced. Therefore, when Zr is contained, the Zr content is in the range of 0.01% to 0.20%. When containing Zr, the Zr content is preferably 0.10% or less.
• REM 0.001 to 0.100% REM (Rare Earth Metals: rare earth metal) has the effect of improving the oxidation resistance, and suppresses the formation of an oxide film (welded temper collar) at the weld to suppress the formation of a Cr-deficient region immediately below the oxide film. This effect is obtained by containing 0.001% or more of REM. On the other hand, if the content of REM is more than 0.100%, the productivity such as acid washability at the time of cold rolling annealing may be reduced. Therefore, when REM is contained, the REM content is in the range of 0.001% to 0.100%. When REM is contained, the REM content is preferably 0.050% or less.
• B 0.0002 to 0.0025%
• B is an element effective to improve the secondary processing brittleness after deep drawing. This effect is obtained by setting the B content to 0.0002% or more. On the other hand, if B is contained in excess of 0.0025%, processability and toughness may be reduced. Therefore, when it contains B, B content is taken as the range of 0.0002% or more and 0.0025% or less. When B is contained, the B content is preferably 0.0003% or more. When B is contained, the B content is preferably 0.0012% or less.
• Mg 0.0005 to 0.0030%
• Mg has the effect of suppressing the coarsening of Ti carbo-nitrides. This effect is obtained by containing 0.0005% or more of Mg.
• the Mg content exceeds 0.0030%, the surface properties of the steel may be deteriorated. Therefore, when Mg is contained, the Mg content is in the 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 0.0003 to 0.0030%
• Ca is an effective component to prevent the clogging of the nozzle due to the crystallization of Ti-based inclusions that are easily generated during continuous casting. The effect is obtained by containing 0.0003% or more of Ca.
• the Ca content is more than 0.0030%, the corrosion resistance may be reduced due to the formation of CaS. Therefore, when it contains Ca, Ca content is made into the range of 0.0003% or more and 0.0030% or less.
• the Ca content is preferably 0.0005% or more.
• the Ca content is preferably 0.0015% or less, more preferably 0.0010% or less.
• the present inventors have intensively studied the method of improving the toughness in a ferritic stainless steel sheet, and preferably heat a steel slab having an appropriate steel component at preferably 1050 to 1250 ° C. and then preferably hot roll it in three or more passes.
• a metal structure having an average crystal grain size of 45 ⁇ m or less is obtained, and the Charpy impact value at 50 ° C. is 100 J
• the toughness was significantly improved to be at least 2 cm 2 .
• the desired corrosion resistance can also be obtained.
• the present inventors diligently studied, from both the steel component and the hot rolling method, an effective method for obtaining a fine structure after hot-rolled sheet annealing.
• the content of steel components in particular Si, Mn, Cr and Ni, is controlled within an appropriate range, slab heating is performed at an appropriate temperature in the hot rolling process, and austenite phase containing ferrite phase + austenite phase It turned out that it is effective to form a phase structure and perform hot rolling.
• the heterophase interface between the ferrite phase existing before heating and the austenite phase generated at the time of heating suppresses coarsening of crystal grains, so before hot rolling A fine equiaxed structure is obtained at the stage of. Then, by performing predetermined hot rolling, processing strain to be a recrystallization site is sufficiently accumulated in the hot-rolled sheet annealing in the next step, and a fine metal structure is obtained by the hot-rolled sheet annealing in the next step. Toughness can be expressed.
• Hot-rolled steel after slab heating at 1050 to 1250 ° C is adjusted for the steels adjusted so that the above-mentioned equation (1) combining the content of elements Si and Cr and negative coefficients to each of Si and Cr holds was devised to do.
• Hot-rolled sheet annealing is a process of recrystallizing the worked structure formed by hot rolling. Therefore, it is necessary to carry out annealing at a temperature at which sufficient recrystallization occurs.
• hot-rolled sheet annealing is performed at an excessively high temperature, although recrystallization occurs, significant coarsening of recrystallized grains occurs, and a predetermined fine structure can not be obtained.
• the inventors 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 suppressing the hot-rolled sheet annealing temperature to 1050 ° C. or less, it is possible to suppress the formation of coarse recrystallized grains that the toughness is reduced.
• molten steel having the above-described component composition is melted by a known method such as a converter, an electric furnace, a vacuum melting furnace or the like, and made into a steel material (slab) by a continuous casting method or an ingot-bunch method.
• Heating temperature of steel slab 1050 to 1250 ° C
• the steel slab is heated at 1050 to 1250 ° C. and subjected to hot rolling.
• the heating time at the heating temperature is not particularly limited, but heating is preferably performed for 1 to 24 hours.
• the heating temperature is less than 1050 ° C., the formation ratio of the austenite phase becomes low, and a fine metal structure can not be obtained, so that excellent toughness can not be obtained.
• the heating temperature of the steel slab is made 1250 ° C. or less.
• direct feed rolling may be performed without heating the steel material.
• the rough rolling conditions are not particularly limited. If the cast structure has been effectively broken before the finish hot rolling, it is preferable to set the cumulative rolling reduction in rough rolling to 65% or more, since the refining effect in the subsequent slab heating is further promoted. Thereafter, it is rolled to a predetermined thickness by finish hot rolling.
• Hot-rolled sheet annealing temperature 750 to 1050 ° C
• hot-rolled sheet annealing is performed after the completion of the hot rolling.
• the rolled structure formed in the hot rolling process is recrystallized.
• rolling strain is effectively applied in the hot rolling step, and coarsening of recrystallization in hot-rolled sheet annealing is suppressed by increasing recrystallization sites. In order to obtain this effect, it is necessary to carry out hot-rolled sheet annealing in the range of 750 to 1050.degree.
• the hot-rolled sheet annealing temperature is in the range of 750 ° C. or more and 1050 ° C. or less.
• the hot-rolled sheet annealing temperature is in the range of 750 ° C. or more and 900 ° C. or less.
• the ferritic stainless steel sheet obtained as described above may be subjected to a descaling treatment by shot blasting or acid washing, if necessary. Furthermore, in order to improve the surface quality, grinding, polishing or the like may be performed. After that, cold rolling and cold rolled sheet annealing may be performed.
• the metallographic structure of the ferritic stainless steel sheet obtained in the present invention is a ferrite single phase or a total of 3% or less (volume ratio) of one or both of a martensite and a retained austenite phase, and the balance is a ferrite phase.
• the ferritic stainless steel plate of the present invention has a Charpy impact value at -50 ° C. of 100 J / cm 2 or more.
• a Charpy impact value at -50 ° C. of 100 J / cm 2 or more.
• the plate thickness is not particularly limited, but is preferably 5.0 mm or more, and more preferably 8.0 mm or more, because it is desirable that the plate thickness can be applied to a thick flange. Moreover, 15.0 mm or less is preferable and, as for plate
• the molten stainless steel having the component composition shown in Table 1 was made into a 100 kg steel slab by vacuum induction melting. Subsequently, it hot-rolled on the manufacturing conditions shown in Table 2, and was set as the hot rolled sheet steel of the finish plate thickness shown in Table 2. The hot rolled steel sheet is subjected to hot rolled sheet annealing to obtain a hot rolled annealed steel sheet. In addition, hot-rolled sheet annealing was performed holding the hot-rolled sheet annealing temperature shown in Table 2 for 8 h. The following evaluation was performed about the hot-rolled annealing steel plate obtained by the above.
• the average grain size was measured by the EBSD (Electron Back Scattering Diffraction) method. The measurement conditions were set to a step of 0.4 ⁇ m at a measurement magnification of 500 times. The obtained data was defined as a grain boundary of 15 ° or more in orientation difference by OIM (Orientation Imaging Microscopy) analysis software manufactured by TSL Solutions, Inc., and the equivalent circle diameter was calculated. The value calculated from the average value of the obtained equivalent circle diameters was taken as the average crystal grain size.
• OIM Orientation Imaging Microscopy
• Salt spray cycle test 1 cycle of salt spray (5 mass% NaCl, 35 ° C, spray 2hr) ⁇ drying (60 ° C, 4hr, relative humidity 40%) ⁇ wetting (50 ° C, 2hr, relative humidity) 95%) As, went 3 cycles.
• the surface of the test piece after 3 cycles of salt spray cycle test is photographed, the rusting area of the test piece surface is measured by image analysis, and the ratio of rusting area to the area of the rusting area measurement portion Rust area / area of the rust area measurement portion) ⁇ 100 [%]) was calculated.
• the rusted area measurement portion is a portion excluding the portion of the outer periphery 15 mm of the test piece.
• the rusted area was the area of the rusted portion and the flow rusted portion.
• the rusting rate of 10% or less is regarded as pass ( ⁇ ) with particularly excellent corrosion resistance, 10% to 25% or less as pass (o), and 25% or more as rejection (x).
• the steel components, the hot rolling conditions, and the hot-rolled sheet annealing conditions satisfy the range of the present invention.
• 1 to 32 and 46 fine metal structures having an average crystal grain size of 45 ⁇ m or less were obtained, and a predetermined Charpy impact value was obtained.
• the rusting rate is 25% or less in any case and also has sufficient corrosion resistance.
• the corrosion resistance was further improved with a rusting rate of 10% or less.
• the slab heating temperature exceeds the range of the present invention. 33, and no.
• the slab heating temperature exceeds the range of the present invention. 33, and no.
• a predetermined amount of austenite phase is formed at the time of heating in the hot rolling process and rolling is performed at a predetermined cumulative reduction ratio, recovery of working strain occurs because the rolling temperature is excessively high and recrystallization site In the hot-rolled sheet annealing step, coarsening of recrystallized grains is likely to occur, and a predetermined Charpy impact value can not be obtained.
• the steel sheet A1 and the steel sheet A2 are used, and the hot-rolled sheet annealing temperature exceeds the range of the present invention. 35, and no. In No. 36, as a result of the occurrence of significant coarsening of the formed recrystallized grains, a predetermined Charpy impact value was not obtained.
• No. 40 although predetermined hot rolling and hot rolled sheet annealing were performed, as austenite phase was not sufficiently generated at the time of heating in the hot rolling process, as a result, the refining of the metal structure is sufficiently performed in the hot rolled sheet annealing process. It did not occur, and a predetermined Charpy impact value was not obtained.
• the ferritic stainless steel sheet obtained by the present invention is particularly suitable for applications where excellent toughness is required, for example, application to flanges and the like.

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• 2018
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