WO2015159793A1 - H形鋼及びその製造方法 - Google Patents
H形鋼及びその製造方法 Download PDFInfo
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- WO2015159793A1 WO2015159793A1 PCT/JP2015/061107 JP2015061107W WO2015159793A1 WO 2015159793 A1 WO2015159793 A1 WO 2015159793A1 JP 2015061107 W JP2015061107 W JP 2015061107W WO 2015159793 A1 WO2015159793 A1 WO 2015159793A1
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- 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
- C21D9/0068—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for particular articles not mentioned below
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- 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
- C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
- C21D8/08—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires for concrete reinforcement
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B22—CASTING; POWDER METALLURGY
- B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
- B22D25/00—Special casting characterised by the nature of the product
- B22D25/02—Special casting characterised by the nature of the product by its peculiarity of shape; of works of art
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
- C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
- C21C7/0006—Adding metallic additives
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21C—PROCESSING OF PIG-IRON, e.g. REFINING, MANUFACTURE OF WROUGHT-IRON OR STEEL; TREATMENT IN MOLTEN STATE OF FERROUS ALLOYS
- C21C7/00—Treating molten ferrous alloys, e.g. steel, not covered by groups C21C1/00 - C21C5/00
- C21C7/04—Removing impurities by adding a treating agent
- C21C7/06—Deoxidising, e.g. killing
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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
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- 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
- C21D8/005—Modifying the physical properties by deformation combined with, or followed by, heat treatment of ferrous alloys
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C33/00—Making ferrous alloys
- C22C33/04—Making ferrous alloys by melting
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- 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
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- 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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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- 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
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- 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
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- 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
- C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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- 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
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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- 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
- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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- 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
- C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
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- 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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- 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
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
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- 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
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- 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
- C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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- E—FIXED CONSTRUCTIONS
- E04—BUILDING
- E04C—STRUCTURAL ELEMENTS; BUILDING MATERIALS
- E04C3/00—Structural elongated elements designed for load-supporting
- E04C3/02—Joists; Girders, trusses, or trusslike structures, e.g. prefabricated; Lintels; Transoms; Braces
- E04C3/04—Joists; Girders, trusses, or trusslike structures, e.g. prefabricated; Lintels; Transoms; Braces of metal
- E04C3/06—Joists; Girders, trusses, or trusslike structures, e.g. prefabricated; Lintels; Transoms; Braces of metal with substantially solid, i.e. unapertured, web
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- E—FIXED CONSTRUCTIONS
- E04—BUILDING
- E04C—STRUCTURAL ELEMENTS; BUILDING MATERIALS
- E04C3/00—Structural elongated elements designed for load-supporting
- E04C3/02—Joists; Girders, trusses, or trusslike structures, e.g. prefabricated; Lintels; Transoms; Braces
- E04C3/04—Joists; Girders, trusses, or trusslike structures, e.g. prefabricated; Lintels; Transoms; Braces of metal
- E04C2003/0404—Joists; Girders, trusses, or trusslike structures, e.g. prefabricated; Lintels; Transoms; Braces of metal beams, girders, or joists characterised by cross-sectional aspects
- E04C2003/0408—Joists; Girders, trusses, or trusslike structures, e.g. prefabricated; Lintels; Transoms; Braces of metal beams, girders, or joists characterised by cross-sectional aspects characterised by assembly or the cross-section
- E04C2003/0421—Joists; Girders, trusses, or trusslike structures, e.g. prefabricated; Lintels; Transoms; Braces of metal beams, girders, or joists characterised by cross-sectional aspects characterised by assembly or the cross-section comprising one single unitary part
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- E—FIXED CONSTRUCTIONS
- E04—BUILDING
- E04C—STRUCTURAL ELEMENTS; BUILDING MATERIALS
- E04C3/00—Structural elongated elements designed for load-supporting
- E04C3/02—Joists; Girders, trusses, or trusslike structures, e.g. prefabricated; Lintels; Transoms; Braces
- E04C3/04—Joists; Girders, trusses, or trusslike structures, e.g. prefabricated; Lintels; Transoms; Braces of metal
- E04C2003/0404—Joists; Girders, trusses, or trusslike structures, e.g. prefabricated; Lintels; Transoms; Braces of metal beams, girders, or joists characterised by cross-sectional aspects
- E04C2003/0443—Joists; Girders, trusses, or trusslike structures, e.g. prefabricated; Lintels; Transoms; Braces of metal beams, girders, or joists characterised by cross-sectional aspects characterised by substantial shape of the cross-section
- E04C2003/0452—H- or I-shaped
Definitions
- the present invention relates to a high-strength ultrathick H-section steel excellent in toughness suitable for a structural member of a building.
- an H-section steel having a flange thickness of 100 mm or more (hereinafter referred to as an extremely thick H-section steel) for a super high-rise building.
- steel materials tend to have lower toughness as strength increases or product thickness increases. Therefore, it is difficult to ensure the toughness of a high strength and thick steel material.
- H-shaped steel has a unique shape. Therefore, it is preferable to manufacture the H-section steel by universal rolling.
- the rolling conditions temperature, rolling reduction
- the rolling conditions are limited in universal rolling. For this reason, particularly in the production of ultra-thick H-section steel, there are large differences in the temperature history during rolling, the rolling reduction, the cooling rate during accelerated cooling, and the like at each part of the web, flange, and fillet.
- extremely thick H-section steel manufactured by rolling has large differences in strength, ductility, and toughness in its cross section.
- Patent Documents 1 and 2 disclose that the Ti-based oxide is dispersed in the steel and the grain is refined by promoting the formation of intragranular ferrite by this Ti oxide. A method has been proposed. Further, Patent Document 3 discloses rolling with high strength and excellent toughness by dispersing Ti oxide as ferrite-forming nuclei in steel to refine ferrite grains and performing temperature-controlled rolling and accelerated cooling. A method of manufacturing a shape steel has been proposed.
- Patent Document 4 discloses a high strength and toughness with a prior austenite grain size of 40 ⁇ m or less, by refinement of the structure by fine dispersion of Mg-based composite oxide and TiN and fine bainite structure by accelerated cooling type controlled rolling. A method for providing rolled steel is disclosed. Further, Patent Document 5 proposes a method of refining crystal grains by promoting the formation of intragranular ferrite by dispersing 20 or more Mg-based oxides of 1 ⁇ m or more in size / mm 2 in steel. Has been.
- Patent Document 6 discloses that Mg-based oxides are dispersed in steel by including 20 / mm 2 or more of Mg-containing oxides of 3 ⁇ m or less in the slab, and temperature-controlled rolling and acceleration are performed on this steel. It is disclosed that a rolled steel having high strength and excellent toughness can be produced by cooling, with Mg-containing oxide acting as a ferrite transformation nucleus in the prior austenite grains.
- Patent Document 1 has a bainite fraction in the structure of 40% or less and contains a large amount of ferrite. Therefore, although it is considered advantageous for securing toughness, it is necessary to add a large amount of alloying elements such as Ni, Cu, Nb, and V in order to secure the corresponding strength, which is extremely disadvantageous in terms of cost. is there.
- the cooling rate of an example in which reheating is not performed after accelerated cooling is as low as 1 ° C./second or less, and it is necessary to add a large amount of an alloy such as Mn, Ni, or Cu in order to ensure strength. Therefore, it is disadvantageous in terms of cost.
- Patent Document 3 an ingredient designed so that the structure can be bainite even when the cooling rate is low does not cause intragranular ferrite generation from Ti oxide, and thus cannot be applied to such a component steel material.
- Patent Document 4 when the prior austenite grain size is 40 ⁇ m or less, an extremely thick H-section steel that has a cooling rate of less than 10 ° C./s even when accelerated cooling is applied is insufficient in hardenability and has sufficient strength. It is thought that it cannot be obtained.
- Patent Document 4 also discloses a technique of performing water cooling / rolling cycle one or more times in which the flange surface of the shaped steel is cooled to 700 ° C. or less in the rolling process and rolled in the recuperation process.
- This invention is made
- the H-section steel of the present invention is not a build-up H-section steel formed by welding steel sheets, but a non-tempered rolled H-section steel that is formed by hot rolling and does not require a tempering treatment.
- the present inventors examined the temperature difference between the surface and the inside of the ultra-thick H-section steel during rolling by computer simulation. As a result, for example, when manufacturing an H-section steel with a flange thickness of 125 mm, it was clarified that the temperature difference between the surface and the interior reaches 200 ° C. In such a case, for example, even if rolling is finished at a temperature close to the ferrite transformation start temperature (Ar 3 point) on the steel material surface, the rolling finish temperature inside the steel material becomes 1000 ° C. or higher. For this reason, the austenite grains become coarser in the steel material than the surface, and the toughness tends to decrease.
- austenite grains are preferably refined in order to increase the toughness of the H-section steel.
- excessively fine austenite grain size is not preferable for increasing the strength.
- the present inventors appropriately control chemical components such as Si, Mn, V, Ti and C eq , and finely disperse Mg-containing oxides in the steel material, and to the steel material It was newly found that when the austenite grain size is controlled by increasing the finishing temperature and performing hot rolling, an extremely thick H-section steel having excellent strength and toughness can be obtained.
- the oxide containing Mg is finely dispersed in the steel, and then controlled rolling is performed to set the austenite grain size at the site where the strength is evaluated to 70 ⁇ m or more, and the site where the toughness is evaluated. It has been clarified that both the strength and toughness can be secured in the ultra-thick H-section steel by controlling the austenite grain size at 200 ⁇ m or less and controlling the subsequent cooling.
- the inventors of the present invention have an extremely thick H-section steel having the above-described structure and a high toughness having a strength of 550 MPa or more and an absorbed energy of Charpy impact test at a test temperature of 21 ° C. of 100 J or more. It revealed that.
- the oxide containing Mg may be included in TiN precipitates.
- the gist of the present invention is as follows.
- the H-section steel according to one aspect of the present invention has a chemical composition of mass%, C: 0.05 to 0.16%, Si: 0.01 to 0.50%, Mn: 0.70. To 2.00%, V: 0.01 to 0.20%, Al: 0.0001 to 0.10%, Ti: 0.003 to 0.030%, N: 0.0010 to 0.0200%, O: 0.0001 to 0.0100%, Mg: 0.0003 to 0.0050%, Ni: 0 to 0.50%, Cr: 0 to 0.50%, Cu: 0 to 0.50%, Mo : 0 to 0.30%, Nb: 0 to 0.010%, B: 0 to 0.0020%, Ca: 0 to 0.0050%, the balance consisting of Fe and impurities; carbon obtained equivalent C eq is from 0.30 to 0.50%; the Mg-containing oxide of 0.005 ⁇ 0.5 [mu] m in circle equivalent diameter, a total of 10 ⁇ 5000 / mm 2 comprising: flange thickness be
- C eq C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 Formula (a)
- C, Mn, Cr, Mo, V, Ni, and Cu are contents in mass% of each element, and are 0 when not contained.
- the chemical components are in mass%, Ni: 0.01 to 0.50%, Cr: 0.01 to 0.50%, Cu: 0.00. 01 to 0.50%, Mo: 0.001 to 0.30%, Nb: 0.001 to 0.010%, B: 0.0001 to 0.0020%, Ca: 0.0001 to 0.0050% Of these, one or more may be contained.
- the H-section steel described in (1) or (2) above has a yield strength or 0.2% proof stress of 450 MPa or more at room temperature and a tensile strength of 550 MPa or more at the strength evaluation site.
- the Charpy absorbed energy at a test temperature of 21 ° C. at the toughness evaluation site may be 100 J or more.
- Ti, Al, and Mg are sequentially added after deoxidizing so that the oxygen concentration in the molten steel is 0.0020 to 0.0100%.
- the chemical composition of the molten steel is, in mass%, C: 0.05 to 0.16%, Si: 0.01 to 0.50%, Mn: 0.70 to 2.00%, V: 0.01-0.20%, Al: 0.0001-0.10%, Ti: 0.003-0.030%, N: 0.0010-0.0200%, O: 0.0001-0.
- a cooling step of cooling the H-shaped steel with water wherein in the cooling step, a position 1/6 from the surface in the length direction of the flange and a position 1/4 from the surface in the thickness direction of the flange.
- Water cooling conditions such that the cooling rate in the range from 800 ° C. to 600 ° C. is 2.2 ° C./s or more, and the surface temperature is reheated within the temperature range of 300 to 700 ° C. after the water cooling is stopped. To control.
- C eq C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 Formula b
- C, Mn, Cr, Mo, V, Ni, and Cu are contents in mass% of each element, and are 0 when not contained.
- the chemical component is, by mass, Ni: 0.01 to 0.50%, Cr: 0.01 to 0.50%, Cu : 0.01 to 0.50%, Mo: 0.001 to 0.30%, Nb: 0.001 to 0.010%, B: 0.0001 to 0.0020%, Ca: 0.0001 to 0
- One or more of 0050% may be contained.
- the high-strength ultra-thick H-section steel of the above aspect of the present invention can be produced without adding a large amount of alloy or reducing the carbon to a very low load. Therefore, a significant cost reduction can be achieved by reducing the manufacturing cost and shortening the construction period. Therefore, the present invention makes a significant contribution to the industry, such as improving the reliability of large buildings without impairing economics.
- H-section steel according to an embodiment of the present invention (sometimes referred to as an H-section steel according to the present embodiment) and a manufacturing method thereof will be described.
- component range chemical component
- % for each element means mass%.
- C 0.05 to 0.16%
- C is an element effective for increasing the strength of steel.
- the lower limit of the C content is set to 0.05%.
- the lower limit of the preferred C content is 0.08%.
- the upper limit of C content is 0.16%.
- the upper limit of the C content is preferably set to 0.13%.
- Si 0.01 to 0.50%
- Si is a deoxidizing element and contributes to the improvement of the strength of steel.
- the lower limit of the Si content is set to 0.01%. Preferably, it is 0.10%.
- the upper limit of Si content is 0.50%. When improving toughness more, it is preferable to make the upper limit of Si content into 0.40%, and it is more preferable to set it as 0.30%.
- Mn 0.70 to 2.00% Mn increases the hardenability of the steel and promotes the formation of bainite, and suppresses the formation of ferrite from the prior austenite grain boundaries, thereby contributing to the improvement of strength.
- the lower limit of the Mn content is set to 0.70%.
- the lower limit of the Mn content is preferably 1.00%, more preferably 1.30%.
- the upper limit of the Mn content is 2.00%.
- the upper limit with preferable Mn content is 1.80%, and a more preferable upper limit is 1.60%.
- V 0.01-0.20%
- V contributes to the improvement of the hardenability of the steel.
- V also forms carbonitrides in steel and contributes to refinement of the structure and precipitation strengthening.
- the lower limit of the V content is set to 0.01%.
- the lower limit for the V content is 0.04%.
- the upper limit of V content is 0.20%.
- the upper limit of V content is 0.08%.
- Al 0.0001 to 0.10%
- Al is a deoxidizing element.
- the lower limit of the Al content is set to 0.0001%.
- Al may be contained in the Mg-containing oxide, and if the Al content in the steel is excessive, the Mg-containing oxide becomes coarse. When the Mg-containing oxide becomes coarse, it becomes a starting point for brittle fracture of the steel material, so that the toughness is lowered. Therefore, the upper limit of the Al content is 0.10%.
- the upper limit of the Al content is 0.050%, more preferably 0.020%.
- Ti is an element that combines with N to form TiN.
- TiN has the effect of refining austenite by the pinning effect and the effect of improving the pinning effect by depositing around the Mg-containing oxide. Therefore, Ti is an effective element.
- the lower limit of the Ti content is set to 0.003%.
- Ti can form TiN and can fix N.
- the lower limit of the Ti content is preferably 0.010% in order to secure the solid solution B amount.
- the upper limit of Ti content is 0.030%.
- the upper limit of the Ti content is 0.020%.
- N 0.0010 to 0.0200%
- N combines with Ti and V to form TiN and VN, and is an element that contributes to refinement of the structure and precipitation strengthening.
- the lower limit of the N content is set to 0.0010%.
- the upper limit of N content is 0.0200%.
- the upper limit of the N content is 0.0100%.
- O 0.0001 to 0.0100%
- O is an element that forms an oxide containing Mg and is necessary for refining austenite by the pinning effect, and is an especially important element in the H-section steel according to the present embodiment.
- the lower limit of the preferable O content is 0.0005%.
- the upper limit of the O content is 0.0100%.
- the upper limit of the O content is 0.0050%.
- Mg 0.0003 to 0.0050%
- Mg is an element that forms an oxide and is necessary for refining austenite by the pinning effect, and is an especially important element in the H-section steel according to the present embodiment. In order to acquire the said effect, it is necessary to make the minimum of Mg content into 0.0003%.
- the lower limit of the preferable Mg content is 0.0005%, and the lower limit of the more preferable Mg content is 0.0010%.
- the upper limit of the Mg content is set to 0.0050%.
- the upper limit of Mg content is 0.0040%.
- P and S are impurities, and the content may not be particularly limited. However, since P and S cause weld cracking and toughness reduction due to solidification segregation, the content is preferably low.
- the P content is preferably limited to 0.03% or less, and more preferably limited to 0.01% or less. Further, the S content is preferably limited to 0.02% or less.
- the H-section steel according to the present embodiment basically contains the chemical components described above, with the balance being Fe and impurities.
- the balance being Fe and impurities.
- one or more selected from Ni, Cr, Cu, Mo, Nb, B, and Ca are used in the following ranges. You may make it contain. However, since these elements do not necessarily need to be contained, the lower limit is 0%.
- an impurity means the component mixed by raw materials, such as an ore and a scrap, and other factors, when manufacturing steel materials industrially.
- Ni 0.01 to 0.50%
- Ni is an extremely effective element for increasing the strength and toughness of steel.
- the Ni content is preferably 0.01% or more.
- the Ni content is preferably 0.10% or more.
- the upper limit of the Ni content is preferably 0.50%.
- a more preferable upper limit of the Ni content is 0.30%.
- Cr 0.01 to 0.50% Cr is an element that improves the hardenability of steel and contributes to the improvement of strength.
- the Cr content is preferably 0.01% or more. More preferably, it is 0.10% or more.
- the upper limit of Cr content is preferably 0.50%. A more preferable upper limit of the Cr amount is 0.30%.
- Cu 0.01 to 0.50%
- Cu is an element that contributes to increasing the strength of a steel material by improving the hardenability of the steel and / or by precipitation strengthening.
- the Cu content is preferably 0.01% or more. More preferably, it is 0.10% or more.
- the upper limit of the Cu content is preferably 0.50%. A more preferable upper limit of the Cu content is 0.30%, and a still more preferable upper limit is 0.20%.
- Mo 0.001 to 0.30%
- Mo is an element that dissolves in steel and enhances hardenability, and contributes to improvement in strength.
- the Mo content is preferably 0.001% or more. More preferably, it is 0.01% or more.
- the Mo content exceeds 0.30%, the formation of MA is promoted and the toughness may be lowered. Therefore, even when Mo is contained, the upper limit of the Mo content is preferably set to 0.30%.
- Nb 0.001 to 0.010%
- Nb is an element that enhances hardenability and contributes to improvement in strength.
- the Nb content is preferably 0.001% or more. More preferably, it is 0.003% or more.
- the upper limit of the Nb content is preferably 0.010%. A more preferable upper limit of the Nb content is 0.007%.
- B 0.0001 to 0.0020%
- B is an element that greatly enhances the hardenability of the steel in a small amount, and is effective in suppressing the ferrite transformation from the austenite grain boundary and improving the strength.
- the B content is preferably 0.0001% or more. More preferably, it is 0.0003% or more, More preferably, it is 0.0010% or more.
- the upper limit of the B content is preferably 0.0020%, and more preferably 0.0015%.
- Ca 0.0001 to 0.0050%
- the Ca content is preferably 0.0001% or more. More preferably, it is 0.0010% or more.
- the upper limit of the Ca content is preferably 0.0050%, and more preferably 0.0030%.
- C eq 0.30 to 0.50%
- the carbon equivalent C eq calculated by the following formula (1) is 0.30 to 0.3 in order to improve hardenability and generate bainite. It needs to be 0.50%.
- the lower limit of C eq is set to 0.30%.
- a preferable lower limit of C eq is 0.35%.
- the upper limit of C eq is set to 0.50%.
- a preferable upper limit of C eq is 0.45%, and a more preferable upper limit of C eq is 0.43%.
- C eq is a carbon equivalent serving as an index of hardenability, and is obtained by the following formula (1).
- C, Mn, Cr, Mo, V, Ni and Cu in the formula are the contents in mass% of each element in the steel, and the elements not contained are calculated as 0.
- C eq C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 Formula (1)
- the H-section steel according to the present embodiment includes a total of 100 to 5000 oxides / mm 2 of oxide containing Mg having an equivalent circle diameter of 0.005 to 0.5 ⁇ m (Mg-containing oxide). . Further, at a position 1/6 from the surface in the length direction of the flange and 1/4 from the surface in the thickness direction, the bainite fraction in the steel structure is 80% or more and the prior austenite grain size averages 70 ⁇ m. That's it. Further, the prior austenite grain size in the steel structure is an average of 200 ⁇ m or less at a position 1/2 of the surface in the length direction of the flange and 3/4 of the surface in the thickness direction.
- the position of 1/6 from the surface in the length direction of the flange and the position of 1/4 from the surface in the thickness direction are sites where an average structure can be obtained. . Therefore, this part is defined as a strength evaluation part, a sample is taken from this part, and the strength of the H-section steel can be evaluated by observing the microstructure and measuring the fraction of bainite. As shown in FIG. 1, the strength evaluation site 7 is a position 1/6 from the surface in the length direction of the flange and a position 1/4 from the surface in the thickness direction.
- the strength evaluation portion 7 has an austenite grain size (old austenite grain size) of 70 ⁇ m or more in average and the steel material structure contains bainite in a fraction (area ratio) of 80% or more. .
- the austenite particle size is less than 70 ⁇ m on average, the hardenability is lowered and the fraction of bainite is lowered. If the bainite fraction is less than 80%, sufficient strength cannot be obtained.
- the remainder of the structure is one or more of ferrite, pearlite, and MA. Since the increase in the bainite fraction contributes to the improvement in strength, the upper limit of the bainite fraction is not particularly defined and may be 100%.
- the microstructure of the steel material can be determined by observation with an optical microscope.
- the measurement points are arranged in a lattice shape with a side of 50 ⁇ m using a structure photograph taken with an optical microscope taken at 200 times, and the structure is formed at 400 measurement points. It can be determined and calculated as a ratio of the number of grains in each structure.
- the austenite grains are likely to be coarse at positions far from the center of the flange thickness or the surface of the fillet or the like because the rolling finishing temperature is high. That is, in the case of extremely thick H-section steel, the austenite grains become fine near the surface because the rolling finishing temperature is low. On the other hand, inside, the rolling finishing temperature becomes high and austenite grains become coarse.
- the position where the toughness is most deteriorated is a position 1 ⁇ 2 from the surface in the length direction of the flange and a position 3/4 from the surface in the thickness direction. .
- this part is defined as a toughness evaluation part, the microstructure is observed at this part, the particle size of the prior austenite is evaluated, and a sample is taken from the same part to evaluate toughness.
- the toughness evaluation site 8 is a position 1/2 of the surface in the length direction of the flange and 3/4 of the surface in the thickness direction.
- the present inventors have observed the microstructure in the toughness evaluation region 8 and evaluated the particle size of the prior austenite. In order to ensure the toughness, it is necessary to control the prior austenite particle size to an average of 200 ⁇ m or less. I found out that there was. Although it is not necessary to limit the lower limit of the prior austenite grain size in the toughness evaluation site 8, it is difficult to make the average prior austenite grain size in the toughness assessment site smaller than the average prior austenite grain size in the strength assessment site. May be 70 ⁇ m.
- the average prior austenite grain size in the above-mentioned strength evaluation site and toughness evaluation site was measured using a micrograph taken with an optical microscope taken at 50 times or an electron beam backscatter diffraction pattern (EBSP) observation image measured at 70 times.
- the average prior austenite grain size is 1 by counting the number of prior austenite grains in the field of view using an optical micrograph or EBSP observation image of a field of view of 1 mm ⁇ 1 mm or more, and dividing the field area by this number. It is measured by calculating the area of perished austenite grains and converting to the diameter of a circle of the same area. The number of old austenite grains on the boundary of the field of view is counted as 1/2.
- the Mg-containing oxide is an oxide mainly containing Mg, and includes those included in TiN precipitates.
- the inclusion of the Mg-containing oxide in the TiN precipitate means a state in which TiN is precipitated around the Mg-containing oxide. That is, when the Mg-containing oxide is observed with a transmission electron microscope (TEM), the Mg-containing oxide may be observed alone or a TiN precipitate may be observed around the Mg-containing oxide. Further, the Mg-containing oxide in the present embodiment may contain Al.
- the prior austenite grain size at the strength evaluation site is large in order to ensure hardenability, and the prior austenite grain size at the toughness assessment site is preferably small in order to improve toughness.
- the austenite grain size of the toughness evaluation part where the rolling finishing temperature is higher than the strength evaluation part is likely to be coarser, the old austenite grain size in the toughness evaluation part is increased while increasing the old austenite grain size in the strength evaluation part. Is difficult to reduce. That is, ensuring the strength at the strength evaluation site and ensuring the toughness at the toughness evaluation site are difficult to achieve at the same time.
- the austenite grain size at the strength evaluation site and the toughness evaluation site depends on the respective rolling conditions. It was clarified that it depends on the effect of rolling recrystallization. Further, in order to increase the average austenite grain size of the strength evaluation portion to 70 ⁇ m or more on average, the rolling finishing temperature (temperature at the end of hot rolling) must be increased to 850 ° C. or more at the surface temperature. Then, the prior austenite grain size at the toughness evaluation site reached an average of 300 ⁇ m or more, and it was clarified that the toughness at the toughness evaluation site was insufficient.
- the present inventors appropriately dispersed the Mg-containing oxide in the steel and optimized the rolling conditions without excessively refining the prior austenite grain size at the strength evaluation site.
- the method of reducing the prior austenite grain size at the toughness evaluation site was examined.
- Mg-containing oxides are appropriately dispersed as pinning particles in a steel slab, and the steel slab is rolled at a high rolling temperature, whereby the average austenite grain size of the strength evaluation site is 70 ⁇ m.
- a method was considered in which the average grain size of the prior austenite grains at the toughness evaluation site was 200 ⁇ m or less.
- the effect of refining by rolling recrystallization is stronger than the effect of pinning, and the austenite grain size is almost determined by the effect of rolling recrystallization. It was clarified by experiment and analysis that the grain refinement effect by pinning became stronger and the austenite grain size was determined by the effect of pinning.
- the prior austenite grains In order to secure the strength at the strength evaluation site 7, it is necessary to make the prior austenite grains an average of 70 ⁇ m or more.
- the upper limit of the prior austenite grain size at the strength evaluation site may be an average of 200 ⁇ m or an average of 150 ⁇ m.
- the average grain size of the prior austenite grains must be 200 ⁇ m or less.
- the present inventors examined the influence of the size and number density of the Mg-containing oxide in order to realize a pinning effect in an appropriate range.
- the size of the oxide containing Mg is 0.005 to 0.5 ⁇ m in terms of the equivalent circle diameter, and it is necessary that the total number of oxides is 100 / mm 2 or more and 5000 / mm 2 or less. And found by experiment. When the number density is less than 100 / mm 2 , a sufficient pinning effect cannot be obtained at the toughness evaluation site.
- the effect of pinning is too strong, and not only the toughness evaluation part but also the strength evaluation part is finely divided more than necessary, and the strength may decrease.
- the size of the Mg-containing oxide is small, there is no effect, but if it is less than 0.005 ⁇ m in equivalent circle diameter, it becomes difficult to observe with a transmission electron microscope, so it is defined by the H-section steel according to this embodiment.
- the lower limit of the equivalent circle diameter of the Mg-containing oxide was set to 0.005 ⁇ m.
- the upper limit was set to 0.5 ⁇ m.
- an oxide exceeding 0.5 ⁇ m becomes a starting point for brittle fracture.
- the number of oxides exceeding 0.5 ⁇ m is preferably 50 / mm 2 or less.
- the Mg-containing oxide is uniformly dispersed in the steel, but in the H-section steel according to the present embodiment, the number density at the toughness evaluation site is particularly important. Therefore, in this embodiment, the number density of Mg-containing oxide particles was calculated by preparing an extraction replica from the position of the toughness evaluation site of the manufactured H-shaped steel and observing it with an electron microscope. The composition of the oxide was identified using an energy dispersive X-ray spectrometer (EDS) attached to an electron microscope.
- EDS energy dispersive X-ray spectrometer
- the thickness of the flange of the H-section steel according to this embodiment is 100 to 150 mm. This is because, for example, a strength member having a flange thickness of 100 mm or more is required for an H-shaped steel used for a high-rise building structure. However, if the thickness of the flange exceeds 150 mm, a sufficient cooling rate cannot be obtained, and it is difficult to ensure both strength and toughness, so the upper limit is set to 150 mm.
- the thickness of the H-shaped steel web is not particularly limited, but is preferably 50 to 150 mm.
- the thickness ratio between the flange and the web is preferably set to 0.5 to 2.0 assuming that the H-section steel is manufactured by hot rolling.
- the flange thickness / web thickness exceeds 2.0, the web may be deformed into a wavy shape.
- the flange thickness / web thickness is less than 0.5, the flange may be deformed into a wavy shape.
- the H-shaped steel according to this embodiment has, as its mechanical properties, a yield strength at normal temperature or a 0.2% proof stress of 450 MPa or more and a tensile strength of 550 MPa or more. Moreover, the Charpy absorbed energy at 21 ° C. is 100 J or more. If the strength is too high, the toughness may be impaired. Therefore, the yield strength at normal temperature or the 0.2% yield strength is preferably 550 MPa or less, and the tensile strength is preferably 680 MPa or less.
- a deoxidation method in the steel making process is important.
- the dissolved oxygen concentration is adjusted within a range of 0.0020 to 0.0100% by primary deoxidation after the steel from the converter. Thereafter, Ti, Al and Mg are added in this order (in order of Ti ⁇ Al ⁇ Mg). Moreover, it adjusts so that the chemical component of molten steel may become the range mentioned above after that (refining process).
- Mg is likely to form a sulfide (MgS) instead of an oxide, and a Mg-containing oxide having a predetermined equivalent circle diameter can be sufficiently obtained. Absent. On the other hand, if the dissolved oxygen concentration exceeds 0.0100%, the Mg-containing oxide becomes excessively coarse, or a large amount of dissolved oxygen remains in the steel, so that the toughness is remarkably lowered. Further, unless Ti, Al, and Mg are added in this order, an Mg-containing oxide having a desired size and number density cannot be obtained.
- the casting is preferably continuous casting from the viewpoint of productivity, but may be a beam blank having a shape close to the H-shaped steel to be manufactured.
- the thickness of the steel slab is preferably 200 mm or more from the viewpoint of productivity, and is preferably 350 mm or less in consideration of reduction of segregation, uniformity of heating temperature in hot rolling, and the like.
- the toughness evaluation site corresponds to the center segregation position of the slab, and it is preferable to perform a process for reducing the center segregation in order to further suppress a decrease in toughness.
- Center segregation can be reduced by light reduction or homogenization heat treatment during continuous casting.
- the steel slab is heated (heating process), and hot rolling is performed on the heated steel slab (hot rolling process).
- the heating temperature of the steel slab is less than 1100 ° C., the deformation resistance at the time of finish rolling is increased, so that it is 1100 ° C. or higher.
- the heating temperature is preferably 1150 ° C. or higher in order to sufficiently dissolve elements that form carbides and nitrides such as Nb and Ti.
- the heating temperature is higher than 1350 ° C., the scale of the surface of the steel slab, which is a raw material, may be liquefied and hinder manufacturing. Therefore, the upper limit of the heating temperature of the steel slab is set to 1350 ° C.
- the austenite grain size at the toughness evaluation site 8 is mainly determined by the pinning effect by the oxide particles, but the austenite grain size at the strength assessment site is mainly determined by the rolling temperature. . Therefore, it is preferable that the rolling temperature is high in order to ensure the strength at the strength evaluation site. In order to ensure strength through improvement of hardenability, it is necessary to make the austenite particle size at the strength evaluation site an average of 70 ⁇ m or more. In order to make the austenite particle size an average of 70 ⁇ m or more, the rolling finishing temperature is It shall be 850 degreeC or more on the steel material surface.
- the hot rolling step may be a so-called two-heat rolling process, in which primary rolling is performed and cooled to 500 ° C. or lower, and then heated again to 1100 to 1350 ° C. to perform secondary rolling.
- two-heat rolling the amount of plastic deformation in the hot rolling is small, and the temperature decrease in the rolling process is also small, so the second heating temperature can be lowered.
- the flanges and webs are water cooled (cooling process).
- Water cooling can be performed by spraying water with a spray or immersion water cooling in a water tank.
- accelerated cooling by water cooling is performed, the formation of ferrite that transforms from the austenite grain boundaries is suppressed, and the fraction of bainite at a position 1/6 from the surface in the length direction of the flange and 1/4 from the surface in the thickness direction.
- the rate becomes 80% or more, and the strength can be secured.
- the cooling rate from 800 ° C. to 600 ° C.
- the upper limit need not be particularly limited. However, since the upper limit of the normal cooling rate by water cooling with a very thick material is 20 ° C./s, the upper limit may be 20 ° C./s.
- Extreme thickness H-section steel has a large difference in cooling rate between the surface and the inside, and is difficult to control at the surface temperature. That is, the surface temperature is cooled to 200 ° C. or less in a short time after the start of cooling, but the internal cooling rate is small, and even after the surface temperature decreases, the internal temperature decreases according to the water cooling time. Therefore, the internal temperature cannot be evaluated at the surface temperature. Therefore, in this embodiment, the internal temperature is controlled and managed by the water cooling time and the water cooling start temperature. If the relationship between the cooling rate and the cooling time and the recuperation temperature is measured in advance, the cooling rate and the recuperation temperature at the strength evaluation portion 7 and the toughness evaluation portion 8 can be controlled.
- Ti ⁇ Al + Mg indicates that Al and Mg were added almost simultaneously (addition interval was less than 1 minute) after addition of Ti.
- the obtained steel slab was heated and subjected to hot rolling to produce an H-shaped steel.
- the components shown in Table 1 were obtained by chemical analysis of a sample collected from the H-shaped steel after production.
- the manufacturing process of H-section steel is shown in FIG.
- the steel slab heated in the heating furnace 1 is subjected to a universal rolling apparatus row including a rough rolling mill 2a, an intermediate rolling mill 2b, and a finishing rolling mill 2c, and finish rolling is performed by a finishing universal rolling mill (finishing rolling mill) 2c.
- a cooling device water cooling device 3b installed on the rear surface.
- the hot rolling is the water cooling between passes
- the outer surface of the flange is sprayed while performing the reverse rolling using the water cooling device 3a provided on the front and rear surfaces of the intermediate universal rolling mill (intermediate rolling mill) 2b. Water cooling between rolling passes was performed by water cooling by cooling.
- Table 2 shows the manufacturing conditions such as the heating temperature, hot rolling, and accelerated cooling of the steel slab during manufacturing.
- the cooling rate in Table 2 is the cooling rate at 1/6 position from the surface in the length direction of the flange and 1/4 position from the surface in the thickness direction. Based on the results of measurement with the thermocouple attached to the corresponding part during measurement by offline heating of the same size and the prediction by computer simulation, the calculation was made from the start temperature of water cooling, the stop temperature, and the water cooling application time.
- the test piece for a tensile test and the sample used for the measurement of a prior-austenite particle size and a structure fraction were extract
- the yield strength and the tensile strength were evaluated, and the prior austenite grain size and bainite fraction were measured using the measurement sample.
- a specimen for Charpy test and a sample for tissue observation were collected from the toughness evaluation site 8 shown in FIG. Using this Charpy test specimen, the toughness was evaluated, and the prior austenite particle size was measured using the measurement sample.
- t 1 is the thickness of the web
- t 2 is the thickness of the flange
- F is the length of the flange
- H is the height.
- the tensile test was performed in accordance with JIS Z 2241. When the yield behavior was exhibited, the yield point was obtained, and when the yield behavior was not exhibited, the 0.2% yield strength was obtained and designated as YS.
- the Charpy impact test was performed at a test temperature of 21 ° C. in accordance with JIS Z 2242.
- the prior austenite grain size and the structure fraction were measured by observing the microstructure with an optical microscope or EBSP.
- the fraction (area ratio) of each structure in the microstructure is determined by arranging the measurement points in a grid with a side of 50 ⁇ m using a structure photograph taken with an optical microscope taken at 200 times, and discriminating the structure at 400 measurement points. Calculated as a ratio of the number of grains in each structure.
- the average prior austenite grain size is obtained by counting the number of prior austenite grains in the field of view using an optical micrograph or EBSP observation image of a field of view of 1 mm ⁇ 1 mm or more, and dividing the field area by this number to obtain a prior austenite grain per grain.
- the area was calculated and converted into the diameter of a circle having the same area.
- the number of prior austenite grains on the boundary of the field of view was 1 ⁇ 2.
- an extraction replica is prepared from the toughness evaluation site 8, the composition of oxide and precipitate is confirmed by an electron microscope and EDS, and the number density of the Mg-containing oxide having an equivalent circle diameter of 0.005 to 0.5 ⁇ m is obtained. It was.
- the Mg-containing oxide includes TiN precipitates that contain the Mg-containing oxide.
- Number density of Mg-containing oxide, yield strength (YS) of tensile strength evaluation site, tensile strength (TS), prior austenite grain size (old ⁇ grain size) and bainite fraction, Charpy absorbed energy at 21 ° C. of toughness assessment site ( vE 21 ) and prior austenite particle size (old ⁇ particle size) are shown in Table 3.
- the target values of mechanical properties are that yield strength at room temperature or 0.2% yield strength (YS) is 450 MPa or more, and tensile strength (TS) is 550 MPa or more.
- the Charpy absorbed energy (vE 21 ) at 21 ° C. is 100 J or more.
- the production number of the present invention example In 1 to 5, 10 to 15, and 20 to 25, YS and TS satisfied the target of 450 MPa or more and 550 MPa or more, respectively. Furthermore, the Charpy absorbed energy at 21 ° C. was 100 J or more, which sufficiently satisfied the target.
- the production No. in Table 3 6 to 9, 16 to 19, and 26 to 37 are the chemical composition, the manufacturing method, the bainite fraction at the strength evaluation site, the austenite grain size at the strength assessment site, the austenite grain size at the toughness assessment site, and the density of the Mg-containing oxide. Any one or more are outside the scope of the present invention. Therefore, any one or more of YS, TS, or Charpy absorbed energy at 21 ° C. did not satisfy the above target.
- the present invention it is possible to obtain a high-strength ultra-thick H-section steel having a flange thickness of 100 to 150 mm and excellent in toughness.
- This high-strength ultra-thick H-shaped steel has excellent toughness and high strength, with yield strength or 0.2% yield strength of 450 MPa or more, tensile strength of 550 MPa or more, and Charpy absorbed energy at 21 ° C. of 100 J or more. .
- the high-strength ultra-thick H-shaped steel of the present invention can be produced without adding a large amount of alloy or reducing the carbon to a very low carbon load. Therefore, a significant cost reduction can be achieved by reducing the manufacturing cost and shortening the construction period. Therefore, the present invention makes a significant contribution to the industry, such as improving the reliability of large buildings without impairing economics.
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Abstract
Description
本願は、2014年04月15日に、日本に出願された特願2014-084017号に基づき優先権を主張し、その内容をここに援用する。
特許文献5では、1μm以上の粗大な酸化物を多数含むので、この酸化物が脆性破壊の起点となり、靭性値にばらつきが出るという問題がある。特許文献6に関しては、特許文献3と同様に、冷却速度が遅くても組織をベイナイト化できるように設計された成分では、Mg含有酸化物からの粒内フェライト生成は起きないので、そのような成分系の鋼材には適用できない。
本発明のH形鋼は、鋼板を溶接して形成されるビルドアップH形鋼ではなく、熱間圧延によって成形され、焼戻し処理を必要としない、非調質の圧延H形鋼である。
このように、鋼材表面に近い部位での強度確保と鋼材内部での靭性確保との両立は、一般的に行われる圧延温度の制御によるオーステナイト粒径の制御という手法のみでは達成が困難である。
本発明者らは、Si、Mn、V、Ti等の化学成分やCeqを適正に制御した上で、Mgを含有する酸化物を鋼材中に微細に分散させて、かつその鋼材に対して仕上温度を高くして熱間圧延を行うことによってオーステナイト粒径を制御した場合に、強度及び靭性に優れた極厚H形鋼が得られることを新たに見出した。
具体的には、Mgを含有する酸化物を鋼材中に微細に分散させた上で、制御圧延を行うことによって、強度を評価する部位でのオーステナイト粒径を70μm以上とし、靭性を評価する部位でのオーステナイト粒径を平均200μm以下とし、その後の冷却を制御すれば、極厚H形鋼において、強度及び靭性の両方を確保できることを明らかにした。本発明者らは、上記の組織を有する極厚H形鋼において、550MPa以上の強度を有して、かつ、試験温度が21℃でのシャルピー衝撃試験の吸収エネルギーが100J以上という高い靭性を示すことを明らかにした。
上記のMgを含有する酸化物は、TiN析出物に内包される場合がある。
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15 ・・・式(a)
ここで、C、Mn、Cr、Mo、V、Ni、Cuは各元素の質量%での含有量であり、含有されない場合は0とする。
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15・・・式b
ここで、C、Mn、Cr、Mo、V、Ni、Cuは各元素の質量%での含有量で、含有されない場合は0とする。
また、本発明の上記態様の高強度極厚H形鋼は、多量の合金の添加や製鋼負荷の大きい極低炭素化を行わずに、製造することが可能である。そのため、製造コストの低減及び工期の短縮による、大幅なコスト削減を図ることができる。したがって、本発明は、経済性を損なうことなく、大型建造物の信頼性を向上させることができるなど、産業上の貢献が極めて顕著である。
Cは、鋼の高強度化に有効な元素である。この効果を得るため、C含有量の下限を0.05%とする。好ましいC含有量の下限は、0.08%である。一方、C含有量が0.16%を超えると炭化物の生成量が過剰となり靭性が低下する。そのため、C含有量の上限を0.16%とする。靭性をより向上させるためには、C含有量の上限を0.13%とすることが好ましい。
Siは、脱酸元素であり、鋼の強度の向上にも寄与する。これらの効果を得るため、Si含有量の下限を0.01%とする。好ましくは、0.10%である。一方、Si含有量が過剰であると、マルテンサイト-オーステナイト混合物(MAという場合がある。)の生成が助長され、靭性が劣化する。そのため、Si含有量の上限を0.50%とする。靭性をより向上させる場合、Si含有量の上限は0.40%とすることが好ましく、0.30%とすることがより好ましい。
Mnは、鋼の焼入れ性を高めてベイナイトの生成を促進するとともに、旧オーステナイト粒界からのフェライト生成を抑制して、強度の向上に寄与する。この効果を得るため、Mn含有量の下限を0.70%とする。強度をさらに高めるには、Mn含有量の下限を1.00%にすることが好ましく、1.30%にすることが更に好ましい。一方、Mn含有量が2.00%を超えると、MAの生成が助長され、靭性が損なわれる。そのため、Mn含有量の上限を2.00%とする。Mn含有量の好ましい上限は1.80%であり、より好ましい上限は、1.60%である。
Vは、鋼の焼入れ性の向上に寄与する。また、Vは、鋼中で炭窒化物を形成し、組織の微細化及び析出強化にも寄与する。これらの効果を得るため、V含有量の下限を0.01%とする。好ましくは、V含有量の下限は0.04%である。一方、V含有量が過剰になると、析出物の粗大化に起因して靭性が損なわれる。そのため、V含有量の上限を0.20%とする。好ましくは、V含有量の上限は0.08%である。
Alは脱酸元素である。脱酸を目的として、Al含有量の下限を0.0001%とする。一方、AlはMg含有酸化物の中にも含有される場合があり、鋼中のAl含有量が過剰であると、Mg含有酸化物が粗大化する。Mg含有酸化物が粗大化すると鋼材の脆性破壊の起点となるので、靭性が低下する。そのため、Al含有量の上限を0.10%とする。好ましくは、Al含有量の上限を0.050%とし、より好ましくは0.020%とする。
Tiは、Nと結合してTiNを形成する元素である。TiNは、ピニング効果によってオーステナイトを細粒化する効果、及び、Mg含有酸化物の周囲に析出してピニング効果を向上させる効果を有する。そのため、Tiは有効な元素である。これらの効果を得るため、Ti含有量の下限を0.003%とする。
また、鋼がTiとともにBを含有する場合には、Tiは、TiNを形成してNを固定することができる。NがTiNとして固定されると、鋼中のBが固溶Bとなるので、鋼の焼入れ性が高まる。そのため、鋼がBを含有する場合には、固溶B量の確保のため、Ti含有量の下限を0.010%とすることが好ましい。
一方、Ti量が0.030%を超えると、粗大なTiNが生成し、靭性が損なわれる。そのため、Ti含有量の上限を0.030%とする。好ましくは、Ti含有量の上限を0.020%とする。
Nは、TiやVと結合してTiNやVNを形成し、組織の細粒化や析出強化に寄与する元素である。この効果を得るため、N含有量の下限を0.0010%とする。一方、N含有量が過剰になると、母材の靭性が低下するとともに、鋳造時の表面割れや製造された鋼材の歪時効による材質不良の原因となる。そのため、N含有量の上限を0.0200%とする。好ましくは、N含有量の上限を0.0100%とする。
Oは、Mgを含む酸化物を形成し、ピニング効果によるオーステナイトの細粒化に必要な元素であり、本実施形態に係るH形鋼において特に重要な元素である。上記効果を得るため、O含有量の下限を0.0001%とする必要がある。好ましいO含有量の下限は0.0005%である。一方、O含有量が過剰になると、固溶Oの影響や酸化物粒子の粗大化によって靭性が低下する。そのため、O含有量の上限を0.0100%とする。好ましくはO含有量の上限を0.0050%とする。
Mgは、酸化物を形成し、ピニング効果によるオーステナイトの細粒化に必要な元素であり、本実施形態に係るH形鋼において特に重要な元素である。上記効果を得るため、Mg含有量の下限を0.0003%とする必要がある。好ましいMg含有量の下限は0.0005%であり、より好ましいMg含有量の下限は0.0010%である。一方、Mg含有量が過剰になると、酸化物粒子の粗大化によって靭性が低下する。そのため、Mg含有量の上限を0.0050%とする。好ましくは、Mg含有量の上限を0.0040%とする。
ここで、不純物とは、鋼材を工業的に製造する際に、鉱石、スクラップ等の原料、その他の要因により混入する成分を意味する。
Niは、鋼の強度及び靭性を高めるために、極めて有効な元素である。強度を向上させるためにはNi含有量を0.01%以上とすることが好ましい。また、靭性を高めるためにはNi含有量を、0.10%以上とすることが好ましい。一方、Ni含有量が0.50%超となると、合金コストが著しく上昇する。そのため、Niを含有させる場合でも、Ni含有量の上限を0.50%とすることが好ましい。より好ましいNi含有量の上限は0.30%である。
Crは、鋼の焼入れ性を向上させる元素であり、強度の向上に寄与する。焼入れ性の向上のためには、Cr含有量を0.01%以上とすることが好ましい。より好ましくは0.10%以上である。一方、Cr含有量が0.50%を超えると、MAの生成が助長されたり、Cr炭化物が粗大化したりして、靭性が低下することがある。そのため、Crを含有させる場合でも、Cr含有量の上限を0.50%とすることが好ましい。より好ましいCr量の上限は0.30%である。
Cuは、鋼の焼入れ性を向上させることによって及び/又は析出強化によって、鋼材の高強度化に寄与する元素である。これらの効果を得る場合、Cu含有量を0.01%以上とすることが好ましい。より好ましくは、0.10%以上である。一方、Cu含有量が過剰になると、MAの生成が助長されたり、強度が過剰になったりして、靭性が低下することがある。そのため、Cuを含有させる場合でも、Cu含有量の上限を0.50%とすることが好ましい。より好ましいCu含有量の上限は0.30%であり、更に好ましい上限は、0.20%である。
Moは、鋼中に固溶して焼入れ性を高める元素であり、強度の向上に寄与する。特に、MoとともにBを含有させた場合には、焼入れ性に関するBとMoとの相乗効果は顕著である。この効果を得る場合、Mo含有量を0.001%以上とすることが好ましい。より好ましくは、0.01%以上である。一方、Mo含有量が0.30%超となると、MAの生成が助長され靭性が低下することがある。そのため、Moを含有させる場合でも、Mo含有量の上限を0.30%とすることが好ましい。
Nbは、Moと同様、焼入れ性を高める元素であり、強度の向上に寄与する。強度向上の効果を得るためには、Nb含有量を0.001%以上とすることが好ましい。より好ましくは、0.003%以上である。一方、Nb含有量が過剰になると、Nb炭化物が形成され、靭性が低下することがある。そのため、Nbを含有させる場合でも、Nb含有量の上限を0.010%とすることが好ましい。より、好ましいNb含有量の上限は、0.007%である。
Bは、微量で鋼の焼入性を大きく高める元素であり、オーステナイト粒界からのフェライト変態を抑制し、強度を向上させるのに有効である。この効果を得る場合、B含有量を0.0001%以上とすることが好ましい。より好ましくは0.0003%以上であり、更に好ましくは、0.0010%以上である。一方、B含有量が0.0020%を超えると、MAの生成が助長され、靭性が低下することがある。そのため、Bを含有させる場合でも、B含有量の上限を0.0020%とすることが好ましく、0.0015%とすることがより好ましい。
Caは、Mg含有酸化物に含まれると、Mg含有酸化物の熱的安定性を高め、Mg含有酸化物の微細化と個数密度の増加とをもたらす効果を有する。この効果を得る場合、Ca含有量を0.0001%以上とすることが好ましい。より好ましくは0.0010%以上である。一方、Ca含有量が0.0050%を超えると、酸化物が粗大化し脆性破壊の起点となり靭性が低下することがある。そのため、Caを含有させる場合でも、Ca含有量の上限を0.0050%とすることが好ましく、0.0030%とすることがより好ましい。
本実施形態に係るH形鋼では、上述の各化学成分の規定に加え、焼入れ性を高めて、ベイナイトを生成させるために、下記式(1)で求められる炭素当量Ceqを0.30~0.50%とする必要がある。Ceqが0.30%未満であるとベイナイトの生成が不十分になり、強度が低下する。そのため、Ceqの下限を0.30%とする。好ましいCeqの下限は0.35%である。一方、Ceqが0.50%を超えると、強度が高くなりすぎて、靭性が低下する。そのため、Ceqの上限0.50%とする。好ましいCeqの上限は、0.45%であり、より好ましいCeqの上限は、0.43%である。
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15 ・・・式(1)
本実施形態に係るH形鋼において、フランジの長さ方向で表面から1/6の位置かつ厚さ方向で表面から1/4の位置は、平均的な組織が得られると考えられる部位である。そのため、この部位を強度評価部位と定義し、この部位から試料を採取し、ミクロ組織の観察、及びベイナイトの分率の測定を行うことで、H形鋼の強度を評価することができる。図1に示すように、強度評価部位7は、フランジの長さ方向で表面から1/6の位置、厚さ方向で表面から1/4の位置である。
オーステナイト粒径が平均70μm未満であると、焼入れ性が低下し、ベイナイトの分率が低下する。ベイナイト分率が80%未満では十分な強度が得られない。組織の残部は、フェライト、パーライト、MAの1種又は2種以上である。ベイナイト分率の増加は強度の向上に寄与するため、ベイナイト分率の上限は特に規定せず、100%でも良い。
本実施形態に係るH形鋼においては、フランジの長さ方向で表面から1/2の位置、かつ厚さ方向で表面から3/4の位置が、靭性が最も低下する部位であると考えられる。そのため、この部位を靭性評価部位と定義して、この部位でミクロ組織を観察し、旧オーステナイトの粒径の評価を行うとともに、同じ部位から試料を採取して靭性を評価する。図1に示すように、靭性評価部位8は、フランジの長さ方向で表面から1/2の位置、かつ厚さ方向で表面から3/4の位置である。
本実施形態において、Mg含有酸化物とは、Mgを主に含有する酸化物であり、TiN析出物に内包されるものを含む。Mg含有酸化物がTiN析出物に内包されるとは、Mgを含む酸化物の周囲にTiNが析出物した状態をいう。すなわち、Mg含有酸化物は、透過型電子顕微鏡(TEM)で観察すると、単独で観察される場合と、Mg含有酸化物の周囲にTiN析出物が観察される場合とがある。また、本実施形態におけるMg含有酸化物は、Alを含んでいてもよい。
強度評価部位7で強度を確保するためには旧オーステナイト粒を平均70μm以上とすることが必要である。旧オーステナイト粒径は、大きい方が焼入れ性が高くなり強度が増すので、上限を規定する必要はない。しかしながら、強度評価部位の旧オーステナイト粒径は、靭性評価部位の旧オーステナイト粒径よりも小さくなると考えられる。そのため、強度評価部位の旧オーステナイト粒径の上限を平均200μmとしてもよく、平均150μmとしてもよい。
また、Mg含有酸化物のサイズは小さくても影響はないが、円相当径で0.005μm未満より小さくなると透過型電子顕微鏡で観察し難くなるため、本実施形態に係るH形鋼で規定するMg含有酸化物の円相当径の下限を0.005μmとした。一方、円相当径で0.5μmを超えるサイズのMg含有酸化物の数は少なく、影響が小さいと考えられるため、上限を0.5μmとした。しかしながら、0.5μmを超える酸化物は脆性破壊の起点となる。また、0.5μmを超える酸化物が多くなると、ピニングに有効な0.005~0.5μmのMg含有酸化物を所定の個数確保できなくなる。そのため、0.5μmを超える酸化物の個数は、50個/mm2以下であることが好ましい。
Tiを添加する前の溶存酸素濃度が0.0020%未満では、Mgが酸化物ではなく硫化物(MgS)を形成しやすくなり、所定の円相当径を有するMg含有酸化物が十分に得られない。また、溶存酸素濃度が、0.0100%を超えるとMg含有酸化物が過剰に粗大になったり鋼中に固溶酸素が多量に残ることにより、靭性が著しく低下する。
また、Ti、Al、Mgをこの順で添加しないと、所望のサイズ、個数密度のMg含有酸化物が得られない。例えばTi、Al、Mgの内、Mgを最初に添加した場合にはMgが強く酸素と結び付いて粗大化してしまい、その後にTiとAlとを添加しても微細な酸化物が得られない。従って、脱酸力の弱い順番であるTi、Al、Mgの順に溶鋼中にこれら元素を添加する必要がある。この順番で添加すると、溶鋼中で酸素原子がTi、Al、Mgと分離・結合を繰り返す事で酸化物の粗大化が抑制され、最終的にMgを含む微細な酸化物が得られる。
また、Ti、Al、Mgを順に添加する際、Al、Mgは、前の元素を添加してから、1分以上経過してから添加する。その理由としては、溶鋼中にTi、Al、Mgが均一に分散するための時間を確保するためである。
焼入れ性の向上を通じた強度確保のためには、強度評価部位でのオーステナイト粒径を平均70μm以上にする事が必要であり、オーステナイト粒径を平均70μm以上にするために、圧延仕上温度は、鋼材表面で850℃以上とする。
水冷による加速冷却を施すと、オーステナイト粒界から変態するフェライトの生成が抑制され、フランジの長さ方向で表面から1/6の位置、厚さ方向で表面から1/4の位置におけるベイナイトの分率が80%以上となり、強度を確保できる。
冷却工程においては、フランジの長さ方向で表面から1/6の位置、かつ厚さ方向で表面から1/4の位置(強度評価部位)において800℃から600℃までの冷却速度が2.2℃/s以上となるように水冷を行う必要がある。強度評価部位での冷却速度が2.2℃/s未満の場合、必要な焼入れ組織が得られない。強度を確保するためには、冷却速度大きい方が好ましく、上限は特に限定する必要がない。しかしながら、極厚材での水冷による通常の冷却速度は20℃/sが上限であるので、上限を20℃/sとしてもよい。
得られた鋼片を加熱し、熱間圧延を行い、H形鋼を製造した。表1に示した成分は、製造後のH形鋼から採取した試料を化学分析して求めた。
ここで、熱間圧延をパス間水冷圧延とする場合には、中間ユニバーサル圧延機(中間圧延機)2bの前後面に設けた水冷装置3aを用い、リバース圧延を行いながら、フランジ外側面をスプレー冷却により水冷することによって、圧延パス間の水冷を行った。
また、図1に示す靭性評価部位8から、シャルピー試験用試験片及び組織観察用の試料を採取した。このシャルピー試験用試験片を用いて、靭性を評価し、測定用試料を用いて、旧オーステナイト粒径を測定した。図1においてt1はウェブの厚み、t2はフランジの厚み、Fはフランジの長さ、Hは高さである。
また、旧オーステナイト粒径、組織の分率は、光学顕微鏡又はEBSPでミクロ組織の観察を行って測定した。ミクロ組織における各組織の分率(面積率)は、200倍で撮影した光学顕微鏡による組織写真を用いて、一辺が50μmの格子状に測定点を配置し、400の測定点で組織を判別し、各組織の粒の数の割合として算出した。平均旧オーステナイト粒径は、1mm×1mm以上の視野の光学顕微鏡写真またはEBSP観察像を用いて視野内の旧オーステナイト粒の個数を数え、視野面積をこの個数で割って1個当たりの旧オーステナイト粒の面積を算出し、同面積の円の直径に換算することにより測定した。視野の境界にかかっている旧オーステナイト粒は1/2個とした。
更に、靭性評価部位8から抽出レプリカを作製し、電子顕微鏡及びEDSにより酸化物及び析出物の組成を確認し、円相当径が0.005~0.5μmのMg含有酸化物の個数密度を求めた。Mg含有酸化物には、Mg含有酸化物を内包するTiN析出物も含まれている。
2a 粗圧延機
2b 中間圧延機
2c 仕上圧延機
3a 中間圧延機前後面の水冷装置
3b 仕上圧延機後面の水冷装置
4 H形鋼
5 フランジ
6 ウェブ
7 強度評価部位
8 靭性評価部位
F フランジ長さ全長
H 高さ
t1 ウェブの厚み
t2 フランジの厚み
Claims (5)
- 化学成分が、質量%で、
C:0.05~0.16%、
Si:0.01~0.50%、
Mn:0.70~2.00%、
V:0.01~0.20%、
Al:0.0001~0.10%、
Ti:0.003~0.030%、
N:0.0010~0.0200%、
O:0.0001~0.0100%、
Mg:0.0003~0.0050%、
Ni:0~0.50%、
Cr:0~0.50%、
Cu:0~0.50%、
Mo:0~0.30%、
Nb:0~0.010%、
B:0~0.0020%、
Ca:0~0.0050%
を含有し、残部がFe及び不純物からなり;
下記式1によって求められる炭素当量Ceqが0.30~0.50%であり;
円相当径で0.005~0.5μmのMg含有酸化物を、合計で100~5000個/mm2含み;
フランジの厚みが100~150mmであり;
前記フランジの長さ方向で表面から1/6の位置かつ前記フランジの厚さ方向で表面から1/4の位置である強度評価部位において、鋼材組織におけるベイナイト分率が80%以上であり、かつ旧オーステナイト粒径が平均70μm以上であり;
前記フランジの前記長さ方向で前記表面から1/2の位置かつ前記フランジの厚さ方向で前記表面から3/4の位置である靭性評価部位において、鋼材組織における旧オーステナイト粒径が平均200μm以下である
ことを特徴とするH形鋼。
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15 ・・・式(1)
ここで、C、Mn、Cr、Mo、V、Ni、Cuは各元素の質量%での含有量であり、含有されない場合は0とする。 - 前記化学成分が、質量%で、
Ni:0.01~0.50%、
Cr:0.01~0.50%、
Cu:0.01~0.50%、
Mo:0.001~0.30%、
Nb:0.001~0.010%、
B:0.0001~0.0020%、
Ca:0.0001~0.0050%
のうち、1種以上を含有することを特徴とする請求項1に記載のH形鋼。 - 前記強度評価部位における、常温での、降伏強度又は0.2%耐力が450MPa以上であり、引張強度が550MPa以上であり;
前記靭性評価部位における試験温度21℃でのシャルピー吸収エネルギーが100J以上である
ことを特徴とする請求項1または2に記載のH形鋼。 - 溶鋼中の酸素濃度が0.0020~0.0100%になるように脱酸した後、Ti、Al及びMgを順に添加し、更に、前記溶鋼の化学成分を、質量%で、C:0.05~0.16%、Si:0.01~0.50%、Mn:0.70~2.00%、V:0.01~0.20%、Al:0.0001~0.10%、Ti:0.003~0.030%、N:0.0010~0.0200%、O:0.0001~0.0100%、Mg:0.0003~0.0050%、Ni:0~0.50%、Cr:0~0.50%、Cu:0~0.50%、Mo:0~0.30%、Nb:0~0.010%、B:0~0.0020%、Ca:0~0.0050%を含有し、残部がFe及び不純物からなり、下記式2によって求められる炭素当量Ceqが0.30~0.50%となるように調整する精錬工程と;
前記溶鋼を鋳造して鋼片を得る鋳造工程と;
前記鋼片を1100~1350℃に加熱する加熱工程と;
加熱された前記鋼片を、圧延終了時の表面温度が850℃以上となるように圧延を行ってH形鋼を得る熱間圧延工程と;
前記熱間圧延工程後の前記H形鋼を、水冷する冷却工程と;
を有し、
前記冷却工程では、フランジの長さ方向で表面から1/6の位置かつ前記フランジの厚さ方向で表面から1/4の位置において800℃から600℃までの範囲の冷却速度が2.2℃/s以上となるように、かつ、水冷停止後に表面温度が300~700℃の温度範囲内に復熱するように、水冷条件を制御する
ことを特徴とするH形鋼の製造方法。
Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15・・・式2
ここで、C、Mn、Cr、Mo、V、Ni、Cuは各元素の質量%での含有量で、含有されない場合は0とする。 - 前記化学成分が、質量%で、
Ni:0.01~0.50%、
Cr:0.01~0.50%、
Cu:0.01~0.50%、
Mo:0.001~0.30%、
Nb:0.001~0.010%、
B:0.0001~0.0020%、
Ca:0.0001~0.0050%
のうち、1種以上を含有することを特徴とする請求項4に記載のH形鋼の製造方法。
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JP6421907B1 (ja) * | 2018-03-23 | 2018-11-14 | 新日鐵住金株式会社 | 圧延h形鋼及びその製造方法 |
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WO2018117228A1 (ja) * | 2016-12-21 | 2018-06-28 | 新日鐵住金株式会社 | H形鋼及びその製造方法 |
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