WO2016171212A1 - 熱延鋼板、鋼材および熱延鋼板の製造方法 - Google Patents
熱延鋼板、鋼材および熱延鋼板の製造方法 Download PDFInfo
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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
- C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
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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
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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
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
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- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
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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
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- 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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- 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
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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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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—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
- C22C38/16—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
- C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
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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
- C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/003—Cementite
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/009—Pearlite
Definitions
- the present invention relates to a hot-rolled steel sheet, a steel material, and a method for producing a hot-rolled steel sheet.
- the surface of the steel sheet is hardened to improve the wear resistance and fatigue strength of steel material parts.
- curing treatment for example, heat treatment with controlled atmosphere such as carburizing treatment, nitriding treatment or soft nitriding treatment is known.
- Nb niobium carbide / Nb and carbon
- the strength of the steel plate can be increased by work hardening. Therefore, when the steel sheet to which Nb is added is cold plastically deformed to cause work hardening to increase the strength of the steel sheet, and when the steel sheet surface is hardened, the work hardening at the center of the plate thickness is softened. It is possible to cure the surface layer while suppressing the above.
- the surface when manufacturing automobile parts, the surface may be soft nitrided after cold working the steel sheet by press forming or the like.
- automobile parts since automobile parts have various shapes, when a steel plate is pressed, a part with a relatively large machining amount and a part with a relatively small machining amount are generated in one part.
- the strength at the central portion of the plate thickness may be softened at a portion where the amount of processing is relatively small due to the heat treatment during soft nitriding, which may result in insufficient component strength.
- the present invention has been made in view of the above circumstances, and the object of the present invention is to increase the strength of the central portion of the thickness of the steel sheet during heat treatment even when the amount of processing on the steel sheet is small and the work hardening rate is low.
- An object of the present invention is to provide a hot-rolled steel sheet, a steel material, and a method for producing a hot-rolled steel sheet that can prevent softening.
- Mass% as a chemical component C: 0.040 to 0.150%, Si: 0 to 0.500%, Mn: 0.10 to 1.50%, P: 0 to 0.050% S: 0 to 0.020%, Al: 0.010 to 0.050%, N: 0.0010 to 0.0060%, Nb: 0.008 to 0.035%, Cu: 0 to 0.10%, Ni: 0 to 0.10%, Cr: 0 to 0.02%, Mo: 0 to 0.020%, V: 0 to 0.020%, Ca: 0 to 0.0100%, and B: 0 to 0.0050%, Including Solid solution Nb: 0.005 to 0.030%, The balance consists of iron and impurities, Hot rolled steel sheet in which the structure of ferrite in the metal structure is 85% or more in area fraction, the remainder of the metal structure is cementite and / or pearlite structure, and the average crystal grain size of ferrite is 5 ⁇ m or more and 20 ⁇ m or less .
- Vickers hardness of The hot-rolled steel sheet according to (1) which exhibits a softening resistance of 80% or more with respect to the Vickers hardness at the center of the sheet thickness after the cold working.
- a steel material comprising the hot-rolled steel sheet according to any one of (1) to (3), The Vickers hardness at the center of the thickness when the hot-rolled steel sheet is sequentially subjected to cold working and heat treatment heated at 560 to 620 ° C. for 120 minutes, The steel material which is 80% or more with respect to the Vickers hardness of the plate
- a steel material comprising the hot-rolled steel sheet according to any one of (1) to (3), The Vickers hardness at the center of the plate thickness when the hot-rolled steel sheet is sequentially subjected to cold working in which the work hardening rate of Vickers hardness is less than 30% and heat treatment heated at 560 to 620 ° C. for 120 minutes. , The steel material which is 80% or more with respect to the Vickers hardness of the plate
- Mass% as a chemical component C: 0.040 to 0.150%, Si: 0 to 0.500%, Mn: 0.10 to 1.50%, P: 0 to 0.050% S: 0 to 0.020%, Al: 0.010 to 0.050%, N: 0.0010 to 0.0060%, Nb: 0.008 to 0.035%, Cu: 0 to 0.10%, Ni: 0 to 0.10%, Cr: 0 to 0.02%, Mo: 0 to 0.020%, V: 0 to 0.020%, Ca: 0 to 0.0100%, and B: 0 to 0.0050%, A steel slab comprising iron and impurities with the balance being heated to 1200 ° C.
- the final rolling of the finish rolling is performed at a finish rolling temperature of 860 ° C. or more and 950 ° C. or less, Cooling between the finish rolling temperature and 800 ° C. at an average cooling rate of 30 ° C./second to 100 ° C./second, Cool between 800 ° C. and the coiling temperature at an average cooling rate of 5 ° C./second to 100 ° C./second, A method for producing a hot-rolled steel sheet, which is wound at a winding temperature of 300 ° C. or higher and 600 ° C. or lower.
- NbC present in the steel is bonded to Nb and C along with plastic deformation. Is dissolved and separated into solute Nb and C and finely dispersed in the steel sheet. Furthermore, when the steel sheet after the cold working is heat-treated, the solid solution Nb and C are recombined to form NbC, and the newly formed NbC pinning action causes the growth of crystal grains at the center of the plate thickness. Is prevented and softening during heat treatment is suppressed.
- NbC particles are formed during hot rolling because the bond between Nb and C is broken.
- the relatively large ones that have been made will be the subject.
- heat treatment since the amount of dissolved Nb is small, the number of NbC particles precipitated by the heat treatment is reduced, the effect of the pinning action of newly formed NbC is reduced, and the center portion of the plate thickness during the heat treatment is reduced. It is presumed that the growth of the crystal grains cannot be prevented, and the thermal softening of the center portion of the plate thickness during the heat treatment cannot be suppressed.
- the present inventor performed heat treatment after plastic working without depending on the work hardening rate when the steel sheet was cold worked by adding a large amount of solute Nb in the steel in advance. Even in this case, it has been found that softening of the central portion of the plate thickness can be prevented.
- solute Nb has a property of generating a large amount of NbC in the vicinity of dislocations generated in steel by cold plastic working
- the steel plate of the steel plate during heat treatment is considered to be a steel plate that has been cold worked. This is advantageous in that the strength of the thickness center portion is prevented from being softened. That is, when a steel sheet having solid solution Nb in the steel is subjected to heat treatment after cold working, for example, when the temperature is raised to a soft nitriding temperature of 500 to 600 ° C., the solid solution Nb and C Combine to produce NbC.
- NbC that prevents the growth of crystal grains in the center of the plate thickness during the heat treatment
- NbC in the steel is made into solute Nb, and heat softening of the center portion of the plate thickness during heat treatment is not suppressed, but in the present invention, a hot-rolled steel plate is manufactured.
- a method for suppressing thermal softening of the central portion of the plate thickness during heat treatment was found by allowing solid solution Nb to remain in the steel.
- the amount of dislocation that is forcibly introduced to promote the generation of NbC can be expressed as the amount of Vickers hardness cured by cold working. In the present invention, it is preferable to cure 10% or more with respect to the Vickers hardness of the material dough before cold working.
- the hot-rolled steel sheet of the present invention can be particularly suitably used when heat treatment such as surface hardening such as soft nitriding is performed after cold working.
- the content rate of each component is mass%.
- the range in this specification includes an upper limit value and a lower limit value unless otherwise specified.
- C 0.040 to 0.150%)
- C is an element effective for maintaining strength.
- the C content needs to be 0.040% or more. is there.
- the amount of C exceeds 0.150%, the press workability of the hot-rolled steel sheet decreases, so 0.150% is made the upper limit.
- the amount of C is preferably 0.040 to 0.10%, more preferably 0.040 to 0.090%.
- Si is an element that increases the deoxidation and strength of steel, and is added for strength adjustment in this embodiment.
- the amount of Si shall be 0.500% or less.
- the amount of Si is preferably 0.10% or less, more preferably 0.08% or less.
- the lower limit value of the Si amount can be 0.001%.
- the amount of Si can be set to 0.090% or more, preferably 0.200% or more, for example.
- Mn is an element that improves the hardenability of the steel and improves the strength, and is added for strength adjustment in this embodiment. If the amount of Mn is less than 0.10%, embrittlement due to S in the steel tends to occur. Moreover, when the amount of Mn exceeds 1.50%, press moldability will fall.
- the amount of Mn is preferably 0.1 to 1.3%, more preferably 0.1 to 1.10%.
- S S: 0-0.020% S, like P, is likely to cause embrittlement and is preferably low in order to ensure press workability. Therefore, the upper limit of the amount of S is 0.020%.
- the amount of S is preferably 0.015% or less, more preferably 0.010% or less.
- the lower limit of the amount of S can be made 0.001%.
- the hot-rolled steel sheet of the present embodiment has solid solution Nb, so that when the temperature is raised in the soft nitriding treatment after cold working, the solid solution Nb is converted into NbC from the dislocation introduced by cold working. It is possible to preserve the work hardening caused by cold working by delaying the movement of dislocations. In order to realize this, first, 0.005% or more of solute Nb is required. In order to make solid solution Nb 0.005% or more, the amount of Nb needs to be 0.008% or more. Since the effect of solid solution Nb is saturated at 0.030%, 0.030% is made the upper limit of solid solution Nb.
- the upper limit of the Nb amount is 0.035%.
- the amount of Nb is preferably 0.010 to 0.030%, more preferably 0.010 to 0.025%.
- the amount of solute Nb is preferably 0.005 to 0.030%, more preferably 0.008 to 0.030%.
- the amount of Nb dissolved in the steel sheet can be calculated from the residue obtained by electrolytic extraction.
- Constant current electrolysis is performed in an electrolytic solution using a 1% acetylacetone-1% tetramethylammonium chloride-methanol solution.
- the residue remaining in the electrolyte after constant-current electrolysis is collected by filtration through a 0.2 ⁇ m filter, and the mass of the collected residue is measured.
- Nb in the residue is assumed to exist as a precipitate of Nb carbide or nitride, and an amount obtained by subtracting the amount of Nb in the residue from the total Nb content of the steel sheet is obtained as a solid solution Nb amount.
- Cu 0 to 0.10%
- the amount of Cu is preferably 0.01 to 0.08%, more preferably 0.02 to 0.05%, in order to increase the strength without reducing the workability.
- Ni 0-0.10% Ni is added in order to prevent embrittlement cracking during hot rolling when manufacturing steel containing Cu.
- the addition amount of Ni is preferably about half or more of the Cu amount. If the Ni content exceeds 0.10%, the workability of the steel sheet decreases, so the upper limit is made 0.10%.
- the amount of Ni is preferably 0.01 to 0.08%, more preferably 0.02 to 0.05%, in order to prevent embrittlement cracking without reducing workability.
- Cr 0-0.02%) Cr is added as needed for strength adjustment in the same manner as Cu. In order not to lower the workability, the upper limit is made 0.02%.
- the amount of Cr is preferably 0.005 to 0.020%, more preferably 0.010 to 0.015%, in order to increase the strength without reducing the workability.
- Mo 0-0.020%)
- V 0 to 0.020%
- Mo and V are added as needed for strength adjustment in the same manner as Cu.
- 0.020% is made the upper limit of each.
- the amount of Mo is preferably 0.005 to 0.020%, more preferably 0.010 to 0.018%, in order to increase the strength without reducing the workability.
- Ca 0 to 0.0100% Ca is added as necessary to prevent embrittlement due to S and to prevent local ductility deterioration due to coarsening of MnS. Since the effect is saturated at 0.0100%, this is the upper limit.
- the amount of Ca is preferably 0.002 to 0.010%, more preferably 0.002 to 0.008% in order to prevent embrittlement without reducing workability.
- B 0 to 0.0050% B is added as necessary in order to prevent aging due to N and prevent deterioration of ductility.
- the effect is saturated at 0.0050%, and since C is combined with B, the amount of NbC produced decreases, and the softening resistance during heat treatment decreases, so this is the upper limit.
- the amount of B is preferably 0.0003 to 0.0030%, more preferably 0.0004 to 0.0020% in order to prevent aging due to N without reducing the softening resistance.
- the balance of hot-rolled steel sheet is iron and impurities.
- iron is contained, for example, 97.40 to 99.84%, preferably 98.10 to 99.83%.
- the metal structure of the hot rolled steel sheet will be described.
- the ferrite structure has an area fraction of 85% or more, and the balance is cementite and / or pearlite structure.
- the average crystal grain size of ferrite is in the range of 5 ⁇ m to 20 ⁇ m.
- the area fraction of the ferrite structure is less than 85%, the workability of the steel sheet is lowered, which is not preferable.
- the area fraction of ferrite is more preferably 90% or more, and still more preferably 92% or more.
- the remaining structure is either one or both of cementite and pearlite structures. It is desirable that bainite is not included in the structure.
- the area fraction of the ferrite is determined by corroding the steel plate surface with nital and observing the white portion. In addition, the area ratio of the remaining structure is obtained by observing the steel sheet surface by corroding it with nital and obtaining the area fraction of the portion that looks black.
- the average crystal grain size of ferrite is preferably 5 ⁇ m or more and 20 ⁇ m or less. If the average crystal grain size is less than 5 ⁇ m, the strength of the steel sheet becomes excessively high, the elongation EL (%) becomes small, and the workability deteriorates. When the average crystal grain size exceeds 20 ⁇ m, the surface skin of the steel sheet after press processing becomes orange peel (skin), and the surface roughness increases.
- the average crystal grain size of ferrite is preferably 6 ⁇ m or more and 15 ⁇ m or less, more preferably 8 ⁇ m or more and 15 ⁇ m or less.
- the tensile strength TS of the hot-rolled steel sheet of this embodiment is 400 MPa or more and 640 MPa or less. Further, the elongation EL (%) is 25.0% or more. Tensile strength TS (MPa) and elongation EL (%) are based on the JIS Z 2241 (2011) metal material tensile test method.
- the height of the ear when the steel plate is formed by cylindrical deep drawing is 2 mm or less.
- the ear height was 200 mm in diameter and a steel plate cut into a circle with a thickness of 4.5 mm.
- the cylinder was deep drawn under the conditions that the punch inner diameter was 100 mm, the punch shoulder was 3 mm, and the clearance was 1.4 times the plate thickness of the steel plate.
- the difference between the maximum height and the minimum height of the cylindrical portion after deep drawing is defined as the ear height.
- the finish rolling temperature be in the range of 900 to 950 ° C.
- the hot-rolled steel sheet according to the present embodiment heats a slab (steel slab) having the above-described chemical components to 1200 ° C. or higher, performs final rolling at a finish rolling temperature of 860 ° C. or higher and 950 ° C. or lower, and finishes. Cooling is performed at an average cooling rate of 30 ° C./second or more and 100 ° C./second or less from the rolling temperature to 800 ° C., and an average cooling rate of 5 ° C./second or more and 100 ° C./second or less from 800 ° C. to the winding temperature And is wound at a winding temperature of 300 ° C. or higher and 600 ° C. or lower.
- the heating temperature of the slab may be 1200 ° C or higher, preferably 1200 ° C or higher and 1300 ° C or lower, more preferably 1220 ° C or higher and 1280 ° C or lower.
- the heating temperature here is the temperature at the center of the slab plate thickness. Since Nb is present as a compound such as NbC in the slab after casting, heating is performed at 1200 ° C. or more up to the center of the slab in order to dissolve Nb in the steel. On the other hand, if the heating temperature is too high, excessive scale is generated on the surface of the slab during heating, and wrinkles may be generated on the surface of the steel sheet after hot rolling. In addition, the yield may decrease. Therefore, the upper limit of the heating temperature is 1300 ° C.
- the finish rolling temperature in the final rolling of finish rolling is 860 ° C. or more and 950 ° C. or less.
- the finish rolling temperature is an actually measured temperature on the steel sheet surface. In order not to precipitate Nb solid-dissolved by heating as carbides, the finish rolling temperature needs to be 860 ° C. or higher. In order to develop isotropy during the hot-rolling steel sheet press work, it is desirable that the finish rolling temperature be 900 ° C. or higher. On the other hand, if the finish rolling temperature is too high, crystal grains grow too much, and anisotropy becomes significant when the hot-rolled steel sheet is pressed, so the upper limit needs to be 950 ° C. or lower.
- the average cooling rate between the finish rolling temperature and 800 ° C. is 30 ° C./second or more and 100 ° C./second or less.
- the average cooling rate is the average cooling rate at the center of the plate thickness of the steel sheet. Since the temperature range from the finish rolling temperature to 800 ° C. is a temperature range in which solute Nb is likely to precipitate in NbC, the average cooling rate between the finish rolling temperature and 800 ° C. is set in order to pass through this temperature range as soon as possible. Stipulate. If the average cooling rate in this temperature range is 30 ° C./second or more, the precipitated Nb decreases and the solid solution Nb relatively increases.
- the average cooling rate between the finish rolling temperature and 800 ° C. may be within the above-mentioned range, but is preferably 40 ° C./second or more and 100 ° C./second or less, more preferably 50 ° C./second or more and 100 ° C./second. It is as follows.
- the average cooling rate from 800 ° C. to the coiling temperature is 5 ° C./second or more and 100 ° C./second or less.
- the average cooling rate is the average cooling rate at the center of the plate thickness of the steel sheet. Since the temperature range from 800 ° C. to the coiling temperature is a temperature range where the solid solution Nb exists stably, the cooling rate may be relaxed in this temperature range as compared to the temperature range up to 800 ° C. Therefore, the average cooling rate in this temperature range is set to the above range. If the average cooling rate is 5 ° C./second or more, the steel sheet temperature can be lowered to the upper limit of the coiling temperature before the steel sheet is wound.
- the average cooling rate between 800 ° C. and the coiling temperature may be within the above-mentioned range, but is preferably 15 ° C./second or more and 100 ° C./second or less, more preferably 15 ° C./second or more and 60 ° C./second. It is as follows.
- the coiling temperature of the steel sheet after cooling is set to 300 ° C. or more and 600 ° C. or less.
- the coiling temperature is the surface temperature of the steel sheet.
- the hot-rolled steel sheet of this embodiment is wound at a low temperature, the precipitation of NbC is suppressed and Nb remains in a solid solution, and the workability is lowered, but the softening resistance during heat treatment is improved.
- the upper limit is 600 ° C. because the remaining solid solution Nb is reduced.
- coiling temperature is restrict
- the winding temperature of a steel plate should just be in the range mentioned above, Preferably it is 400 to 600 degreeC, More preferably, it is 450 to 580 degreeC. As described above, the hot-rolled steel sheet of this embodiment can be manufactured.
- the hot-rolled steel sheet of the present embodiment is formed into a predetermined part shape by cold working such as press forming, and then subjected to surface hardening treatment such as carburizing treatment, nitriding treatment, carbonitriding treatment, and soft nitriding treatment. Therefore, it is a steel material that forms automobile parts and the like.
- the surface hardening process is a process of heat-treating the hot-rolled steel sheet after cold working in a predetermined atmosphere.
- the hot-rolled steel sheet of the present embodiment has a characteristic that the amount of decrease in the Vickers hardness at the central portion of the plate thickness is small before and after the heat treatment and is not easily softened even when the heat treatment is performed after the cold working.
- the cold working may be any cold plastic working such as press working, hole expanding work, bending work or the like. Further, in the case where the degree of processing amount during cold working is expressed by work hardening rate ⁇ R (%), in this embodiment, cold working with any work hardening rate ⁇ R (%) may be applied, but ⁇ R If (%) is 10% or more, dislocations for precipitation of NbC are sufficiently introduced, and the effect of softening resistance is easily exhibited.
- the high work hardening rate means a case where ⁇ R (%) is 30% or more.
- the low work hardening rate means a case where ⁇ R (%) is less than 30%.
- the hot-rolled steel sheet of the present embodiment exhibits a characteristic that it is difficult to soften before and after heat treatment even when ⁇ R (%) is less than 10 to 30%.
- the atmosphere in the surface curing treatment is not particularly limited. As an example, an atmosphere having an NH 3 concentration of 35%, a CO 2 concentration of 5%, and an N 2 concentration of 60% can be exemplified.
- the hot-rolled steel sheet of the present embodiment exhibits sufficient softening resistance even when heat-treated at a heat treatment temperature in the range of 560 to 620 ° C. for a heat treatment time of 120 minutes.
- the temperature range applied in the actual surface hardening treatment is in the range of 500 to 600 ° C., and the heat treatment time is about 60 to 180 minutes. Even under such conditions, the hot-rolled steel sheet of the present embodiment exhibits sufficient softening resistance.
- the hot-rolled steel sheet of the present embodiment has a Vickers hardness at the center of the plate thickness when the cold processing and the heat treatment heated at 560 to 620 ° C. for 120 minutes are sequentially performed.
- the softening resistance is 80% or more with respect to the Vickers hardness.
- the Vickers hardness at the center of the plate thickness after the heat treatment is the center of the plate thickness after the cold working even when cold working is performed in which the work hardening rate of the Vickers hardness is less than 30%. It exhibits a softening resistance of 80% or more with respect to the Vickers hardness.
- the work hardening rate in this embodiment is as follows.
- the Vickers hardness at the center of the thickness of the hot-rolled steel sheet before cold working is Hv (before cold working)
- the Vickers hardness at the center of the thickness after cold working is Hv (after cold working).
- the work hardening amount ⁇ WHv is represented by the following formula ( ⁇ )
- the work hardening rate ⁇ R (%) is represented by the following formula ( ⁇ ).
- the rate of change in hardness after heat treatment is as follows.
- the heat treatment is a case where heating is performed at each heat treatment temperature for 120 minutes.
- ⁇ Hv (%) is 80% or more.
- the post-heat treatment hardening amount ⁇ THv is expressed by the following formula ( ⁇ )
- the hardness after the heat treatment is expressed by the following equation ( ⁇ ).
- the upper limit of ⁇ Hv (%) is not 100%, and includes the case where the steel plate is further hardened by heat treatment.
- solute C in steel forms NbC by heat treatment, which may increase the strength.
- a steel material manufactured by subjecting a hot-rolled steel sheet to cold working and surface hardening treatment has a hardness change rate ⁇ Hv (%) after heat treatment of 80% or more.
- the hot-rolled steel sheet of the present embodiment it is possible to prevent softening of the strength of the center portion of the steel sheet during heat treatment even when the amount of work on the steel sheet is small and the work hardening rate is low. Moreover, according to the manufacturing method of the hot-rolled steel plate of this embodiment, the hot-rolled steel plate excellent in the softening resistance at the time of heat processing can be manufactured.
- the obtained slab is heated to a predetermined heating temperature, the final rolling of the finish rolling is performed at a predetermined finish rolling temperature, and the average cooling rate between the finish rolling temperature and 800 ° C. and between 800 ° C. and the winding temperature is performed.
- the hot-rolled steel sheets S01 to S84 were manufactured by variously changing the average cooling rate and cooling, and winding at a predetermined winding temperature.
- Tables 2A to 2C show the heating temperature, finish rolling temperature, average cooling rate, and winding temperature when manufacturing the hot-rolled steel sheet. Further, the thicknesses of the obtained hot-rolled steel sheets are shown in Tables 2A to 2C.
- Tables 2A to 2C the average cooling rate between the finish rolling temperature and 800 ° C. is described as the average cooling rate I, and the average cooling rate between 800 ° C. and the coiling temperature is expressed as the average cooling rate. It was described as II.
- a press-formed product was manufactured by subjecting the obtained hot-rolled steel sheet to press working.
- the press working was performed on a hot rolled steel sheet cut into a circle having a diameter of 200 mm and a thickness of 4.5 mm, with a punch inner diameter of 100 mm, a punch shoulder R3 mm, and a clearance of 1.4 times the thickness. Under these conditions, the cylinder was deep-drawn to produce a cup-shaped press-formed product having a height of 52 mm.
- the same press work was performed also about the hot-rolled steel plate of plate
- the soft nitriding treatment atmosphere was an NH 3 concentration of 35%, a CO 2 concentration of 5%, and an N 2 concentration of 60%.
- the heating rate was 0.7 ° C./min
- the heat treatment temperature was 570 to 625 ° C.
- the heat treatment time was 120 minutes
- air cooling was performed after heating.
- the heat treatment temperatures for the soft nitriding treatment are shown in Tables 3A to 3C.
- solute Nb in the hot-rolled steel sheet was measured by the method described below.
- a 10% acetylacetone-1% tetramethylammonium chloride-methanol solution was prepared as an electrolytic solution, and the test piece was subjected to constant current electrolysis in the electrolytic solution.
- the residue remaining in the electrolyte after constant-current electrolysis was collected by filtration through a 0.2 ⁇ m filter, and the mass of the collected residue was measured.
- ICP emission spectroscopy Inductively The mass of Nb in the residue was measured by Coupled Plasma Atomic Emission Spectroscopy (ICP-AES). Assuming that Nb in the residue was present as a precipitate of Nb carbide or nitride, the amount obtained by subtracting the amount of Nb in the residue from the total Nb content of the steel sheet was defined as the solid solution Nb amount. The results are shown in Tables 2A to 2C.
- Vickers hardness at the center of the thickness of the hot-rolled steel sheet before and after pressing was measured.
- the Vickers hardness at the center of the plate thickness after press working was the Vickers hardness at the center of the plate thickness at the side surface of the cup-shaped press-formed product.
- the work hardening rate of the press-formed product varies depending on the measurement position. In order to investigate the Vickers hardness before and after the heat treatment when the work hardening rate is less than 30%, to measure the Vickers hardness before and after the heat treatment at a position of 3 to 7 mm from the bottom surface of the press-formed product and at a work hardening rate of 30% or more.
- Steels S01 to S42, S70, S72, and S73 are hot-rolled steel sheets produced by manufacturing slabs having the chemical components of the present invention under the production conditions specified in the present invention, and the rate of change in hardness after heat treatment is 80% or more. It can be seen that it has excellent softening resistance after heat treatment.
- S79 and S80 are hot-rolled steel sheets produced by producing slabs having the chemical components of the present invention under the production conditions defined in the present invention. Specifically, S79 and S03 are examples where the same steel type is hot-rolled under the same conditions, and similarly S80 and S18 are examples where the same steel type is hot-rolled under the same conditions.
- S79 and S80 since the heating temperature during soft nitriding was higher than that in S03 and S18, the rate of change in hardness after heat treatment was less than 80%. However, by setting the heating temperature during soft nitriding of these steels S79 and S80 to 620 ° C. or less, the hardness change rate after the heat treatment becomes 80% or more as shown in S18 and S03.
- Steels S43 to S54 are examples that deviate from the chemical components of the present invention. That is, steel S43 had a low C content, and the amount of NbC produced during the soft nitriding process was small, so that the hardness could not be secured. In addition, the ferrite crystal grains became coarse, resulting in rough skin. Steel S44 had excessive C content, so EL decreased and press cracks occurred. Steel S45 had excessive Si content, so EL decreased and press cracks occurred. Steel S46 had a low Mn content, and the ferrite crystal grains became coarse, resulting in rough skin. In steel S47, the amount of Mn was excessive, and the area fraction of ferrite was reduced to produce bainite, so that EL was lowered and press cracking occurred.
- steel S48 the P content was excessive, and the ferrite area fraction was reduced to produce bainite, so that EL was lowered and press cracking occurred.
- steel S49 the amount of S was excessive, so EL decreased and press cracks occurred.
- Steel S50 had a low Al content, and the ferrite crystal grains were coarsened, resulting in rough skin.
- Steel S51 since the Al amount was excessive, EL decreased and press cracks occurred.
- Steel S52 had an excessive amount of N, so EL decreased and press cracks occurred.
- Steel S53 had a low Nb content, so that the solid solution Nb was low, and the hardness after soft nitriding could not be secured.
- Steel S54 the Nb amount was excessive, and the ferrite area fraction was reduced to produce bainite, so that EL was lowered and press cracking occurred.
- Steel S58 had a high cooling rate from the end of finish rolling to winding, the winding temperature was lowered, the ferrite area fraction was reduced, bainite was generated, EL was lowered, and press cracking occurred.
- Steel S60 had a large cooling rate up to 800 ° C., a reduced area fraction of ferrite, a decrease in EL, and press cracking occurred.
- Steel S61 had a low heating temperature during hot rolling, a low solid solution Nb, and could not secure the hardness after soft nitriding.
- Steel S62 had a high finish rolling temperature, a low solute Nb, and could not secure the hardness after soft nitriding.
- steel S63 had a low finish rolling temperature, and coarse flat ferrite was generated during hot rolling. Therefore, the anisotropy at the time of press work became large and EL also decreased.
- Steel S64 had a high cooling rate up to 800 ° C., and the ferrite area fraction decreased and bainite was generated, so TS increased and EL decreased.
- steel S65 had a low cooling rate up to 800 ° C., a low solid solution Nb, and could not secure the hardness after soft nitriding.
- Steel S66 had a high cooling rate from 800 ° C. to the coiling temperature, so the area ratio of ferrite was low, EL was low, and press cracking occurred. On the other hand, steel S67 had a low cooling rate from 800 ° C. to the coiling temperature, and the solid solution Nb decreased, and the hardness after soft nitriding could not be secured.
- Steel S68 had a high coiling temperature and a low solid solution Nb, and could not secure the hardness after soft nitriding.
- the coiling temperature was low, the area fraction of ferrite was reduced, bainite was generated, EL was lowered, and press cracking occurred.
- Steel S74, Steel S75, and Steel S76 are all hot-rolled steel sheets obtained by hot rolling a slab having a low Nb content under the same conditions. These differences are examples in which the work hardening rate is changed by changing the measurement position of the Vickers hardness in the press-formed product. In either case, solid solution Nb was not sufficiently generated. For this reason, the hardness after soft nitriding could not be secured even at high machining sites like steel S74 and steel S75, and the hardness after soft nitriding could not be secured even at low machining sites like steel S76. .
- Steel S77 and steel 78 are steels with a small amount of solute Nb and a high Nb content, but when the work hardening rate is high, the hardness after soft nitriding can be secured. On the other hand, even steels with a small amount of solute Nb and a high Nb content, such as steel S59, steel S61, steel S62, steel S65, steel S67, steel S68, and steel S84, are soft when the work hardening rate is small. The hardness after nitriding cannot be secured.
- Steels S81 and S82 are examples in which a slab with a low Nb content is hot-rolled into a hot-rolled steel sheet under substantially the same conditions, pressed, and further heat-treated at a high temperature exceeding 620 ° C.
- the difference between steel S81 and S82 is an example in which the work hardening rate is changed by changing the measurement position of Vickers hardness in the press-formed product.
- the difference from S53 and S74 to S76 is that the heat treatment is performed at a high temperature exceeding 620 ° C.
- solid solution Nb was not sufficiently generated. For this reason, the hardness after soft nitriding cannot be ensured even in a high-processed part like steel S81, and the hardness after soft nitriding cannot be ensured even in a low-processed part like steel S84.
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Abstract
Description
C :0.040~0.150%、
Si:0~0.500%、
Mn:0.10~1.50%、
P :0~0.050%、
S :0~0.020%、
Al:0.010~0.050%、
N :0.0010~0.0060%、
Nb:0.008~0.035%、
Cu:0~0.10%、
Ni:0~0.10%、
Cr:0~0.02%、
Mo:0~0.020%、
V :0~0.020%、
Ca:0~0.0100%、および
B :0~0.0050%、
を含み、
固溶Nb:0.005~0.030%であり、
残部が鉄および不純物からなり、
金属組織中におけるフェライトの組織が面積分率で85%以上であり、当該金属組織の残部がセメンタイトおよび/またはパーライト組織であり、フェライトの平均結晶粒径が5μm以上20μm以下である、熱延鋼板。
(2) 前記熱延鋼板に対して冷間加工と560~620℃で120分間加熱する熱処理とを順次行った場合の板厚中心部のビッカース硬さが、
前記冷間加工後の板厚中心部のビッカース硬さに対して80%以上の耐軟化性を示すことを特徴とする(1)に記載の熱延鋼板。
(3) 前記熱延鋼板に対して、ビッカース硬さの加工硬化率が30%未満になる冷間加工と、560~620℃で120分間加熱する熱処理とを順次行った場合の板厚中心部のビッカース硬さが、
前記冷間加工後の板厚中心部のビッカース硬さに対して80%以上の耐軟化性を示すことを特徴とする(1)に記載の熱延鋼板。
(4) (1)~(3)のいずれか一項に記載の熱延鋼板からなる鋼材であり、
前記熱延鋼板に対して冷間加工と560~620℃で120分間加熱する熱処理とを順次行った場合の板厚中心部のビッカース硬さが、
前記冷間加工後の板厚中心部のビッカース硬さに対して80%以上である鋼材。
(5) (1)~(3)のいずれか一項に記載の熱延鋼板からなる鋼材であり、
前記熱延鋼板に対してビッカース硬さの加工硬化率が30%未満になる冷間加工と560~620℃で120分間加熱する熱処理とを順次行った場合の板厚中心部のビッカース硬さが、
前記冷間加工後の板厚中心部のビッカース硬さに対して80%以上である鋼材。
(6) 化学成分として質量%で、
C :0.040~0.150%、
Si:0~0.500%、
Mn:0.10~1.50%、
P :0~0.050%、
S :0~0.020%、
Al:0.010~0.050%、
N :0.0010~0.0060%、
Nb:0.008~0.035%、
Cu:0~0.10%、
Ni:0~0.10%、
Cr:0~0.02%、
Mo:0~0.020%、
V :0~0.020%、
Ca:0~0.0100%、および
B :0~0.0050%、
を含み、残部が鉄および不純物からなる鋼鋳片を1200℃以上に加熱し、
860℃以上950℃以下の仕上圧延温度で仕上圧延の最終圧延を行い、
仕上圧延温度から800℃の間を30℃/秒以上100℃/秒以下の平均冷却速度で冷却し、
800℃から巻取温度までの間を5℃/秒以上100℃/秒以下の平均冷却速度で冷却し、
300℃以上600℃以下の巻取温度で巻き取る、熱延鋼板の製造方法。
Cは、強度を保つために有効な元素である。冷間加工済みの熱延鋼板の熱処理(例えば軟窒化処理)中に、NbCを充分に生成させて板厚中心部の強度低下を防止するためには、C量が0.040%以上必要である。一方、C量が0.150%を超えると熱延鋼板のプレス加工性が低下するため0.150%を上限とする。C量は、好ましくは0.040~0.10%、より好ましくは、0.040~0.090%である。
Siは鋼の脱酸および強度を高める元素であり、本実施形態では強度調整用として添加する。Si量が高いと、熱間圧延中に鋼板表面に表面酸化物が生成して疵が発生しやすくなる。また、プレス加工性も低下する。このため、Si量は0.500%以下とする。
Si量は、好ましくは0.10%以下、より好ましくは0.08%以下である。一方で、Siは、鉄鉱石に含有されているために、通常、不可避的に存在する成分である。したがって、Si量の下限値を0.001%とすることもできる。また、鋼の脱酸および強度を高めるためには、Si量を例えば、0.090%以上、好ましくは0.200%以上とすることができる。
Mnは、鋼の焼入れ性を高めるとともに強度を向上させる元素であり、本実施形態では強度調整用として添加する。Mn量が0.10%未満では鋼中のSによる脆化が発生しやすくなる。また、Mn量が1.50%を超えるとプレス成形性が低下する。Mn量は、好ましくは0.1~1.3%、より好ましくは、0.1~1.10%である。
Pは脆化の原因となりやすく、プレス加工性を確保するためには低い方がよい。従ってP量は0.050%を上限とする。P量は、好ましくは0.03%以下、より好ましくは0.02%以下である。一方で、Pは、鉄鉱石に含有されているために、通常、不可避的に存在する成分である。したがって、P量の下限値を0.001%、より具体的には0.002%とすることもできる。
SはPと同様に脆化の原因となりやすく、プレス加工性を確保するためには低い方がよい。従ってS量は0.020%を上限とする。S量は、好ましくは0.015%以下、より好ましくは0.010%以下である。一方で、Sは、鉄鉱石に含有されているために、通常、不可避的に存在する成分である。したがって、S量の下限値を0.001%とすることもできる。
Alは、軟窒化処理において鋼板表面にAlNなる窒化物を生成して表面硬さを高める効果がある。そのため、Al量は0.010%以上必要である。一方、プレス加工性を高く保つためには0.050%を上限とする。Al量は、好ましくは0.010~0.040%、より好ましくは、0.015~0.030%である。
Nは、Alと同様に、軟窒化処理における鋼板表面のAl窒化物の生成に必要な元素であり、0.0010%以上含まれることが好ましい。一方、プレス加工前の鋼板中にNが多量に存在すると延性の低下が大きくなり、鋼板の加工性が低下する。従ってN量は少ない方が好ましく、0.0060%を上限とする。N量は、好ましくは0.0010~0.0040%、より好ましくは、0.0010~0.0030%である。
(固溶Nb:0.005~0.030%)
本実施形態の熱延鋼板は、固溶Nbを有することにより、冷間加工後の軟窒化処理において昇温された際に、冷間加工で導入された転位を起点として、固溶NbをNbCなる析出物に変化させ、転位の移動を遅延させ、冷間加工で生じた加工硬化を保存することができる。これを実現させるためには、まず0.005%以上の固溶Nbが必要である。固溶Nbを0.005%以上にするためには、Nb量は0.008%以上必要である。固溶Nbによる効果は0.030%で飽和するため、0.030%を固溶Nbの上限とする。一方、鋼中のNbが増加することによってプレス加工性が低下する。そのため、Nb量の上限は0.035%とする。Nb量は、好ましくは0.010~0.030%、より好ましくは、0.010~0.025%である。固溶Nb量は、好ましくは0.005~0.030%、より好ましくは、0.008~0.030%である。
Cuは、強度調整のために必要に応じて添加する。加工性を低下させないためには0.10%を上限とする。Cu量は、加工性を低下させずに強度を高めるためには、好ましくは0.01~0.08%、より好ましくは、0.02~0.05%である。
Niは、Cuが含有される鋼を製造する際に、熱延中の脆化割れを防止するために添加する。Niの添加量はCu量の半分以上程度が好ましい。Ni量が0.10%を超えると鋼板の加工性が低下するため、上限を0.10%とする。Ni量は、加工性を低下させずに脆化割れを防止するには、好ましくは0.01~0.08%、より好ましくは、0.02~0.05%である。
Crは、Cuと同様に強度調整のために必要に応じて添加する。加工性を低下させないためには0.02%を上限とする。Cr量は、加工性を低下させずに強度を高めるためには、好ましくは0.005~0.020%、より好ましくは、0.010~0.015%である。
(V :0~0.020%)
Mo、Vは、Cuと同様に強度調整のために必要に応じて添加する。加工性を低下させないためには0.020%をそれぞれの上限とする。Mo量は、加工性を低下させずに強度を高めるためには、好ましくは0.005~0.020%、より好ましくは、0.010~0.018%である。
CaはSによる脆化を防止するとともに、MnSの粗大化による局部延性低下を防止するために必要に応じて添加する。Caは0.0100%で効果が飽和するため、これを上限とする。Ca量は、加工性を低下させずに脆化を防止するためには、好ましくは0.002~0.010%、より好ましくは、0.002~0.008%である。
Bは、Nによる時効を防止して延性の低下を防止するために必要に応じて添加する。0.0050%で効果が飽和するうえ、CがBと結合することで、NbCの生成量が低くなり、熱処理時の耐軟化性が低下するため、これを上限とする。B量は、耐軟化性を低下させずにNによる時効を防止するためには、好ましくは0.0003~0.0030%、より好ましくは、0.0004~0.0020%である。
本実施形態の熱延鋼板の金属組織は、フェライトの組織が面積分率で85%以上であり、残部がセメンタイトおよび/またはパーライト組織である。また、フェライトの平均結晶粒径は5μm以上20μm以下の範囲である。
本実施形態の熱延鋼板は、上記記載の化学成分を有するスラブ(鋼鋳片)を1200℃以上に加熱し、860℃以上950℃以下の仕上圧延温度で仕上圧延の最終圧延を行い、仕上圧延温度から800℃の間を30℃/秒以上100℃/秒以下の平均冷却速度で冷却し、800℃から巻取温度までの間を5℃/秒以上100℃/秒以下の平均冷却速度で冷却し、300℃以上600℃以下の巻取温度で巻き取ることで製造される。
一方、仕上圧延温度が高すぎると、結晶粒が成長しすぎてしまい、熱延鋼板をプレス加工した際に異方性が顕著になることから、上限を950℃以下にする必要がある。仕上圧延の最終圧延における仕上圧延温度は、上述した範囲内であればよいが、好ましくは900℃以上940℃以下、より好ましくは900℃以上930℃以下である。
以上のようにして、本実施形態の熱延鋼板を製造することができる。
熱延鋼板の冷間加工前の板厚中心部のビッカース硬さをHv(冷間加工前)とし、冷間加工後の板厚中心部のビッカース硬さをHv(冷間加工後)としたとき、加工硬化量ΔWHvは下記(α)式で表され、加工硬化率ΔR(%)は下記(β)式で表される。
ΔR(%)=ΔWHv/Hv(冷間加工前)×100 … (β)
ΔHv(%)=ΔTHv/ΔWHv×100 … (δ)
また、本実施形態の熱延鋼板の製造方法によれば、熱処理時の耐軟化性に優れた熱延鋼板を製造できる。
転炉により鋼を溶製し、連続鋳造によりスラブを製造した。表1Aおよび表1Bにスラブの化学成分として成分1~44を示す。
得られた熱延鋼板について、断面をナイタールエッチング処理して顕微鏡観察することにより、組織形態、フェライト組織の面積分率およびフェライトの平均結晶粒径を求めた。結果を表2A~表2Cに示す。
また、熱延鋼板中の固溶Nb量を次に説明する方法により測定した。まず、巻取後室温までに冷却された熱延鋼板の板幅1/4の位置で、30mm角(30×30mm=900mm2)の大きさの試験片を採取した。次いで、電解液として10%アセチルアセトン-1%テトラメチルアンモニウムクロライド-メタノール溶液を用意し、電解液中で試験片を定電流電解させた。定電流電解後に電解液中に残った残渣を0.2μmのフィルターで濾過して採取し、採取された残渣の質量を測定するとともに、残渣を酸分解処理後、ICP発光分光分析法(Inductively
Coupled Plasma Atomic Emission Spectroscopy:ICP-AES)によって、残渣中のNbの質量を測定した。残渣中のNbは、Nbの炭化物または窒化物の析出物として存在したと仮定し、鋼板の全Nb含有量から残渣中のNbの量を差し引いた量を固溶Nb量とした。結果を表2A~表2Cに示す。
また、得られた熱延鋼板の引張強度TSおよび伸びEL(%)を求めた。引張強度TS(MPa)、伸びEL(%)は、JIS Z 2241(2011)金属材料引張試験方法により測定した。結果を表2A~表2Cに示す。TSは400~640MPaを良好とし、ELは25.0%以上を良好とした。
軟窒化処理前のプレス成形品について、割れの発生の有無をプレス割れ評価として評価した。評価結果を「E」「S」「E、S」「N」で示す。「E」~「N」の内容は以下の通りである。結果を表3A~表3Cに示す。
S:肩R部にき裂がある。
E、S:成形品の端部が割れ、肩R部にき裂がある。
N:割れなし。
軟窒化処理前のプレス成形品について、耳の発生の有無を評価した。プレス成形品の最高高さと最低高さの差を耳高さとした。評価結果を「A」「B」「C」「D」で示した。「A」~「D」の内容は以下の通りである。BとAを良好と判定した。なお、プレス割れが発生したものは、プレス耳の測定を実施しなかった。結果を表3A~表3Cに示す。
B:耳高さが1mm超2mm以下。
C:耳高さが2mm超3mm以下。
D:耳高さが3mm超。
軟窒化後のプレス成形品について、成形品の側面を400番の砥石で円周方向に擦り、スジ状疵をつけた。その際、スジ状疵が直線状に入っていれば良好と判断し、肌荒れ発生(オレンジピール発生)が無い(A)とした。一方、スジ状疵に濃淡が発生したり、分断された場合には、肌荒れ発生(オレンジピールの発生)がある(B)とした。結果を表3A~表3Cに示す。
プレス加工前後における熱延鋼板の板厚中心部のビッカース硬度を測定した。プレス加工後の板厚中心部のビッカース硬さは、カップ状のプレス成形品の側面部における板厚中心のビッカース硬さとした。プレス成形品の加工硬化率は、測定位置によって異なる。加工硬化率が30%未満における熱処理前後のビッカース硬度を調査するために、プレス成形品の底面から3~7mm位置における測定し、加工硬化率が30%以上における熱処理前後のビッカース硬度を調査するために、プレス成形品の底面から25mmおよび35mm位置における測定を行った。表3A~表3Cに、冷間加工前後の板厚中心部のビッカース硬度Hv(冷間加工前)、Hv(冷間加工後)を示す。また、冷間加工後のビッカース硬度Hv(冷間加工後)の測定位置を示す。更に、加工硬化率ΔR(%)を示す。加工硬化率ΔR(%)は、上記(α)式および(β)式により求めた。なお、プレス割れが発生したものは、硬さ測定は実施しなかった。
熱処理前後における熱延鋼板の板厚中心部のビッカース硬度を測定し、熱処理前後の加工硬化量ΔTHv、および熱処理前後の硬さ変化率ΔHvを求めた。加工硬化量ΔTHvおよび熱処理前後の硬さ変化率ΔHvは、上記(γ)式および(δ)式により求めた。
そして、ΔHvが80%以上をA、80%未満をBとした。なお、プレス割れが発生したものは、硬さ測定は実施しなかった。結果を表3A~表3Cに示す。
すなわち、鋼S43は、C含有量が少なく、軟窒化処理中にNbCの生成量が少なくなり、硬さを確保できなかった。また、フェライトの結晶粒も粗大となり、肌荒れが生じた。鋼S44は、C含有量が過剰であったため、ELが低下してプレス割れが起きた。鋼S45は、Si含有量が過剰であったため、ELが低下してプレス割れが起きた。鋼S46は、Mn含有量が少なく、フェライトの結晶粒が粗大化し、肌荒れが生じた。鋼S47は、Mn量が過剰であり、また、フェライトの面積分率が低下してベイナイトが生成したため、ELが低下してプレス割れが起きた。鋼S48は、P量が過剰であり、また、フェライトの面積分率が低下してベイナイトが生成したため、ELが低下してプレス割れが起きた。鋼S49は、S量が過剰であったため、ELが低下してプレス割れが起きた。鋼S50は、Al含有量が少なく、また、フェライトの結晶粒が粗大化し、肌荒れが生じた。鋼S51は、Al量が過剰であったため、ELが低下してプレス割れが起きた。鋼S52は、N量が過剰であったため、ELが低下してプレス割れが起きた。鋼S53は、Nb含有量が少なく、このため固溶Nbが低くなり、軟窒化後の硬さを確保できなかった。鋼S54は、Nb量が過剰であり、また、フェライトの面積分率が低下してベイナイトが生成したため、ELが低下してプレス割れが起きた。
Claims (6)
- 化学成分として質量%で、
C :0.040~0.150%、
Si:0~0.500%、
Mn:0.10~1.50%、
P :0~0.050%、
S :0~0.020%、
Al:0.010~0.050%、
N :0.0010~0.0060%、
Nb:0.008~0.035%、
Cu:0~0.10%、
Ni:0~0.10%、
Cr:0~0.02%、
Mo:0~0.020%、
V :0~0.020%、
Ca:0~0.0100%、および
B :0~0.0050%、
を含み、
固溶Nb:0.005~0.030%であり、
残部が鉄および不純物からなり、
金属組織中におけるフェライトの組織が面積分率で85%以上であり、当該金属組織の残部がセメンタイトおよび/またはパーライト組織であり、フェライトの平均結晶粒径が5μm以上20μm以下である、熱延鋼板。 - 前記熱延鋼板に対して冷間加工と560~620℃で120分間加熱する熱処理とを順次行った場合の板厚中心部のビッカース硬さが、
前記冷間加工後の板厚中心部のビッカース硬さに対して80%以上の耐軟化性を示すことを特徴とする請求項1に記載の熱延鋼板。 - 前記熱延鋼板に対して、ビッカース硬さの加工硬化率が30%未満になる冷間加工と、560~620℃で120分間加熱する熱処理とを順次行った場合の板厚中心部のビッカース硬さが、
前記冷間加工後の板厚中心部のビッカース硬さに対して80%以上の耐軟化性を示すことを特徴とする請求項1に記載の熱延鋼板。 - 請求項1~3のいずれか一項に記載の熱延鋼板からなる鋼材であり、
前記熱延鋼板に対して冷間加工と560~620℃で120分間加熱する熱処理とを順次行った場合の板厚中心部のビッカース硬さが、
前記冷間加工後の板厚中心部のビッカース硬さに対して80%以上である鋼材。 - 請求項1~3のいずれか一項に記載の熱延鋼板からなる鋼材であり、
前記熱延鋼板に対してビッカース硬さの加工硬化率が30%未満になる冷間加工と560~620℃で120分間加熱する熱処理とを順次行った場合の板厚中心部のビッカース硬さが、
前記冷間加工後の板厚中心部のビッカース硬さに対して80%以上である鋼材。 - 化学成分として質量%で、
C :0.040~0.150%、
Si:0~0.500%、
Mn:0.10~1.50%、
P :0~0.050%、
S :0~0.020%、
Al:0.010~0.050%、
N :0.0010~0.0060%、
Nb:0.008~0.035%、
Cu:0~0.10%、
Ni:0~0.10%、
Cr:0~0.02%、
Mo:0~0.020%、
V :0~0.020%、
Ca:0~0.0100%、および
B :0~0.0050%、
を含み、残部が鉄および不純物からなる鋼鋳片を1200℃以上に加熱し、
860℃以上950℃以下の仕上圧延温度で仕上圧延の最終圧延を行い、
仕上圧延温度から800℃の間を30℃/秒以上100℃/秒以下の平均冷却速度で冷却し、
800℃から巻取温度までの間を5℃/秒以上100℃/秒以下の平均冷却速度で冷却し、
300℃以上600℃以下の巻取温度で巻き取る、熱延鋼板の製造方法。
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WO2013046693A1 (ja) * | 2011-09-29 | 2013-04-04 | Jfeスチール株式会社 | 熱延鋼板およびその製造方法 |
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EP3561111A4 (en) * | 2016-12-22 | 2019-10-30 | Posco | THICK STEEL SHEET HAVING EXCELLENT CRYOGENIC IMPACT RESISTANCE AND MANUFACTURING METHOD THEREOF |
JP2020509189A (ja) * | 2016-12-22 | 2020-03-26 | ポスコPosco | 極低温衝撃靭性に優れた厚鋼板及びその製造方法 |
CN110088334B (zh) * | 2016-12-22 | 2021-06-11 | 株式会社Posco | 具有优异的低温冲击韧性的厚钢板及其制造方法 |
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KR20170117561A (ko) | 2017-10-23 |
JPWO2016171212A1 (ja) | 2017-11-09 |
CN107532263A (zh) | 2018-01-02 |
PL3260570T3 (pl) | 2021-02-08 |
US10718040B2 (en) | 2020-07-21 |
TWI597368B (zh) | 2017-09-01 |
TW201702402A (zh) | 2017-01-16 |
ES2826878T3 (es) | 2021-05-19 |
EP3260570A4 (en) | 2018-09-05 |
MX2017010031A (es) | 2017-10-27 |
CN107532263B (zh) | 2019-11-22 |
JP6497437B2 (ja) | 2019-04-10 |
BR112017021224A2 (ja) | 2018-06-26 |
EP3260570B1 (en) | 2020-09-09 |
US20180073115A1 (en) | 2018-03-15 |
EP3260570A1 (en) | 2017-12-27 |
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