WO2021117705A1 - 熱延鋼板 - Google Patents
熱延鋼板 Download PDFInfo
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- WO2021117705A1 WO2021117705A1 PCT/JP2020/045624 JP2020045624W WO2021117705A1 WO 2021117705 A1 WO2021117705 A1 WO 2021117705A1 JP 2020045624 W JP2020045624 W JP 2020045624W WO 2021117705 A1 WO2021117705 A1 WO 2021117705A1
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- 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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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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- 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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- 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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- 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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- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
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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
- C21D2211/002—Bainite
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- C—CHEMISTRY; METALLURGY
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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
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
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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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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
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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
Definitions
- the present invention relates to a hot-rolled steel sheet. Specifically, the present invention relates to a high-strength hot-rolled steel sheet having excellent moldability.
- the present application claims priority based on Japanese Patent Application No. 2019-222162 filed in Japan on December 9, 2019, the contents of which are incorporated herein by reference.
- the strength of steel sheets is increasing in order to ensure the collision safety of automobiles and reduce the environmental load. As the strength of the steel sheet increases, the formability decreases. Therefore, the 980 MPa class steel sheet is required to have improved formability.
- ductility, hole expandability and bendability are used as indicators of formability, but these characteristics are in a trade-off relationship, and a steel sheet having excellent ductility, hole expandability and bendability is required. ..
- a bainite phase having an area ratio of 85% or more is the main phase
- a martensite phase or a martensite-austenite mixed phase having an area ratio of 15% or less is the second phase
- the balance is a ferrite phase.
- the average particle size of the second phase is 3.0 ⁇ m or less
- the average aspect ratio of the old austenite grains is 1.3 or more and 5.0 or less
- Patent Document 2 includes a bainite phase having an area ratio of more than 90% as a main phase, or further, as a second phase, one or more of a ferrite phase, a martensite phase and a retained austenite phase.
- the average particle size of the bainite phase is 2.5 ⁇ m or less, and the interval of Fe-based carbides precipitated in the bainite ferrite grains in the bainite phase is 600 nm or less, and the tension is high.
- a high-strength hot-rolled steel sheet having a strength TS of 980 MPa or more is disclosed.
- Patent Document 1 does not consider bendability.
- the present inventors have found that in the high-strength hot-rolled steel plate disclosed in Patent Document 1, it may not be possible to obtain excellent bendability, and it is necessary to further improve the hole-expandability.
- Patent Document 2 does not consider hole expandability and bendability.
- the present inventors have found that in the high-strength hot-rolled steel sheet disclosed in Patent Document 2, excellent hole-expandability and bendability may not be obtained in some cases.
- an object of the present invention is to provide a hot-rolled steel sheet having excellent strength, ductility, bendability and hole-expanding property.
- the bendability of the hot-rolled steel sheet can be improved by controlling the texture in the surface layer (1/16 position of the plate thickness in the plate thickness direction from the surface to the surface).
- the hot-rolled steel sheet according to one aspect of the present invention has a chemical composition of mass%.
- C 0.040 to 0.150%, Si: 0.50 to 1.50%, Mn: 1.00 to 2.50%, P: 0.100% or less, S: 0.010% or less, Al: 0.010 to 0.100%, N: 0.0100% or less, Ti: 0.005 to 0.150%, B: 0.0005 to 0.0050%, Cr: 0.10 to 1.00%, Nb: 0 to 0.06%, V: 0 to 0.50%, Mo: 0 to 0.50%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Sb: 0 to 0.020%, Ca: 0 to 0.010%, REM: 0 to 0.010%, and Mg: 0 to 0.010% Containing, the balance is iron and impurities, In the metal structure at the position of 1/4 of the plate thickness in the plate thickness direction from the surface
- the average particle size of the second phase is 1.5 ⁇ m or less.
- (110) The extreme density of the ⁇ 112> orientation is 3.0 or less, and The average particle size of iron-based carbide is 0.100 ⁇ m or less, In the metal structure at the position 1/16 of the plate thickness in the plate thickness direction from the surface to the surface, the polar density in the (110) ⁇ 1-11> orientation is 3.0 or less.
- the tensile strength TS is 980 MPa or more.
- Nb 0.005 to 0.06%
- V 0.05 to 0.50%
- Mo 0.05-0.50%
- Cu 0.01-0.50%
- Ni 0.01-0.50%
- Sb 0.0002 to 0.020%
- Ca 0.0002 to 0.010%
- REM 0.0002 to 0.010%
- Mg 0.0002 to 0.010% It may contain one or more selected from the group consisting of.
- the hot-rolled steel sheet according to the present embodiment has a chemical composition of mass%, C: 0.040 to 0.150%, Si: 0.50 to 1.50%, Mn: 1.00 to 2.50%. , P: 0.100% or less, S: 0.010% or less, Al: 0.010 to 0.100%, N: 0.0100% or less, Ti: 0.005 to 0.150%, B: 0 It contains 0005 to 0.0050%, Cr: 0.10 to 1.00%, and the balance: iron and impurities.
- mass% C: 0.040 to 0.150%, Si: 0.50 to 1.50%, Mn: 1.00 to 2.50%.
- P 0.100% or less
- S 0.010% or less
- Al 0.010 to 0.100%
- N 0.0100% or less
- Ti 0.005 to 0.150%
- B 0 It contains 0005 to 0.0050%
- Cr 0.10 to 1.00%
- iron and impurities iron and impurities.
- C 0.040 to 0.150%
- C is an element that promotes the formation of bainite by improving the strength of the hot-rolled steel sheet and improving the hardenability.
- the C content is set to 0.040% or more.
- the C content is 0.050% or more, 0.060% or more, 0.070% or more.
- the C content exceeds 0.150%, it becomes difficult to control the formation of bainite, a large amount of martensite phase is formed, and the ductility and hole expansion property of the hot-rolled steel sheet are both, or one of them. Decreases. Therefore, the C content is set to 0.150% or less.
- the C content is preferably 0.140% or less, 0.120% or less, and 0.100% or less.
- Si 0.50 to 1.50%
- Si is an element that contributes to solid solution strengthening and is an element that contributes to improving the strength of hot-rolled steel sheets. Further, Si is an element that suppresses the formation of carbides in steel. By suppressing the formation of carbides during bainite transformation, a fine martensite phase is formed at the lath interface of the bainite phase. Since the martensite phase present in the baynite phase is fine, it does not deteriorate the hole expandability of the hot-rolled steel plate.
- the Si content is 0.50% or more.
- the Si content is 0.55% or more, 0.60% or more, and 0.65% or more.
- Si is an element that promotes the formation of ferrite, and when the Si content exceeds 1.50%, ferrite is formed, and the hole-expandability and strength of the hot-rolled steel sheet are lowered. Therefore, the Si content is 1.50% or less. Preferably, the Si content is 1.30% or less, 1.20% or less, and 1.00% or less.
- Mn 1.00 to 2.50% Mn dissolves in the steel and contributes to the increase in the strength of the hot-rolled steel sheet, and promotes the formation of bainite by improving the hardenability, thereby improving the hole-expanding property of the hot-rolled steel sheet.
- the Mn content is set to 1.00% or more.
- the Mn content is 1.30% or more, 1.50% or more, and 1.70% or more.
- the Mn content is set to 2.50% or less.
- the Mn content is 2.00% or less and 1.95% or less.
- P 0.100% or less
- P is an element that dissolves in steel and contributes to increasing the strength of hot-rolled steel sheets.
- P is also an element that segregates at the grain boundaries, particularly the former austenite grain boundaries, and promotes the grain boundary fracture due to the grain boundary segregation, thereby causing a decrease in ductility, bendability, and hole expansion property of the hot-rolled steel sheet.
- the P content is preferably as low as possible, but a P content of up to 0.100% is acceptable. Therefore, the P content is set to 0.100% or less.
- the P content is 0.090% or less and 0.080%.
- the P content is preferably 0%, but the P content may be 0.0001% or more because the production cost increases if the P content is reduced to less than 0.0001%.
- the P content is 0.001% or more and 0.010% or more.
- S 0.010% or less
- S is an element that adversely affects weldability and manufacturability during casting and hot rolling.
- S combines with Mn to form coarse MnS. This MnS deteriorates the bendability and hole widening property of the hot-rolled steel sheet, and promotes the occurrence of delayed fracture.
- the S content is preferably as low as possible, but the content of S up to 0.010% is acceptable. Therefore, the S content is set to 0.010% or less.
- the S content is 0.008% or less.
- the S content is preferably 0%, but if it is reduced to less than 0.0001%, the manufacturing cost increases and it is economically disadvantageous. Therefore, the S content may be 0.0001% or more.
- the S content is 0.001% or more.
- Al 0.010 to 0.100%
- Al is an element that acts as a deoxidizer and is effective in improving the cleanliness of steel.
- the Al content is 0.010% or more.
- the Al content is 0.015% or more and 0.020% or more.
- the Al content is set to 0.100% or less.
- the Al content is 0.050% or less, 0.040% or less, 0.030% or less.
- N 0.0100% or less
- N is an element that forms a coarse nitride in steel. This nitride deteriorates the bendability and hole expansion property of the hot-rolled steel sheet and also deteriorates the delayed fracture resistance. Therefore, the N content is set to 0.0100% or less. Preferably, the N content is 0.0080% or less, 0.0060% or less, 0.0050% or less. Since reducing the N content to less than 0.0001% causes a significant increase in manufacturing cost, the N content may be 0.0001% or more. Preferably, the N content is 0.0005% or more and 0.0010% or more.
- Ti 0.005 to 0.150%
- Ti is an element that forms a nitride in the austenite phase high temperature region (the high temperature region in the austenite phase region and the higher temperature region than the austenite phase region (casting stage)).
- B is in a solid solution state, so that the hardenability required for the formation of bainite can be obtained.
- the strength and hole expandability of the hot-rolled steel sheet can be improved.
- Ti forms carbides in the steel during hot rolling to suppress recrystallization of old austenite grains. In order to obtain these effects, the Ti content is set to 0.005% or more.
- the Ti content is 0.030% or more and 0.050% or more, 0.070% or more, and 0.090% or more.
- the Ti content is set to 0.150% or less.
- the Ti content is 0.130% or less and 0.120% or less.
- B 0.0005 to 0.0050%
- B is an element that segregates at the grain boundaries of the former austenite, suppresses the formation and growth of ferrite, and contributes to the improvement of the strength and hole expansion property of the hot-rolled steel sheet.
- the B content is 0.0005% or more.
- the B content is 0.0007% or more and 0.0010% or more.
- the B content is set to 0.0050% or less.
- the B content is 0.0030% or less and 0.0025% or less.
- Cr 0.10 to 1.00% Cr is an element that forms carbides in steel and contributes to increasing the strength of hot-rolled steel sheets, promotes the formation of bainite by improving hardenability, and promotes the precipitation of Fe-based carbides in bainite grains. ..
- the Cr content is set to 0.10% or more.
- the Cr content is 0.30% or more, 0.40% or more, 0.50% or more.
- the Cr content is set to 1.00% or less.
- the Cr content is 0.90% or less, 0.80% or less, 0.70% or less.
- the balance of the chemical composition of the hot-rolled steel sheet according to the present embodiment may be Fe and impurities.
- the impurities mean those mixed from ore as a raw material, scrap, manufacturing environment, etc., or those allowed within a range that does not adversely affect the characteristics of the hot-rolled steel sheet according to the present embodiment. To do.
- the hot-rolled steel sheet according to the present embodiment may contain the following elements as optional elements instead of a part of Fe.
- the lower limit of the content when the following optional elements are not contained is 0%.
- each arbitrary element will be described in detail.
- Nb 0 to 0.06%
- Nb is an element that has the effect of forming carbides during hot rolling and suppressing the recrystallization of austenite, and contributes to improving the strength of the hot-rolled steel sheet.
- the Nb content is preferably 0.005% or more.
- the Nb content is more preferably 0.02% or more.
- the Nb content is set to 0.06% or less.
- the Nb content is 0.04% or less.
- V 0 to 0.50%
- V is an element that has the effect of forming carbonitride during hot rolling and suppressing the recrystallization of austenite, and contributes to the improvement of the strength of the hot-rolled steel sheet.
- the V content is preferably 0.05% or more.
- the V content is more preferably 0.10% or more.
- the V content is set to 0.50% or less.
- the V content is 0.25% or less.
- Mo 0 to 0.50%
- Mo is an element that promotes the formation of a bainite phase by improving hardenability and contributes to improving the strength and hole expansion of hot-rolled steel sheets.
- the Mo content is preferably 0.05% or more.
- the Mo content is more preferably 0.10% or more.
- the Mo content is set to 0.50% or less.
- the Mo content is 0.30% or less.
- Cu 0 to 0.50%
- Cu is an element that dissolves in steel and contributes to increasing the strength of hot-rolled steel sheets.
- Cu is an element that promotes the formation of a bainite phase by improving the hardenability and contributes to the improvement of the strength and the hole expanding property of the hot-rolled steel sheet.
- the Cu content is preferably 0.01% or more.
- the Cu content is more preferably 0.02% or more.
- the Cu content is set to 0.50% or less.
- the Cu content is 0.20% or less.
- Ni is an element that dissolves in steel and contributes to increasing the strength of hot-rolled steel sheets. Further, Ni is an element that promotes the formation of a bainite phase by improving the hardenability and contributes to the improvement of the strength and the hole expanding property of the hot-rolled steel sheet. In order to surely obtain these effects, the Ni content is preferably 0.01% or more. The Ni content is more preferably 0.02% or more. On the other hand, when the Ni content exceeds 0.50%, the martensite phase is likely to be formed, and the bendability and / or hole expansion property of the hot-rolled steel sheet may be lowered. Therefore, the Ni content is set to 0.50% or less. Preferably, the Ni content is 0.20% or less.
- Sb 0 to 0.020%
- Sb has the effect of suppressing nitriding of the slab surface at the slab heating stage.
- the Sb content is preferably 0.0002% or more.
- the Sb content is more preferably 0.001% or more.
- the Sb content is set to 0.020% or less.
- Ca 0 to 0.010%
- Ca is an element that controls the shape of sulfide-based inclusions and improves the hole-expandability of hot-rolled steel sheets.
- the Ca content is preferably 0.0002% or more.
- the Ca content is more preferably 0.001% or more.
- the Ca content is set to 0.010% or less.
- the Ca content is 0.008% or less.
- REM 0 to 0.010%
- the REM content is an element that controls the shape of sulfide-based inclusions and improves the hole-expandability of hot-rolled steel sheets.
- the REM content is preferably 0.0002% or more.
- the REM content is more preferably 0.001% or more.
- the REM content is set to 0.010% or less.
- the REM content is 0.008% or less.
- REM refers to a total of 17 elements composed of Sc, Y and lanthanoid, and the content of REM refers to the total content of these elements.
- lanthanoids they are industrially added in the form of misch metal.
- Mg 0 to 0.010%
- Mg is an element whose morphology of sulfide can be controlled by containing it in a small amount.
- the Mg content is preferably 0.0002% or more.
- the Mg content is more preferably 0.0005% or more.
- the Mg content is set to 0.010% or less.
- the Mg content is 0.008% or less.
- the chemical composition of the hot-rolled steel sheet may be measured by a general analysis method.
- ICP-AES Inductively Coupled Plasma-Atomic Emission Spectrometry
- C and S may be measured by using the combustion-infrared absorption method
- N may be measured by using the inert gas melting-thermal conductivity method.
- the hot-rolled steel sheet according to the present embodiment is a bainite phase having a main phase of 95.00 to 98.00% in terms of area ratio in a metal structure at a position of 1/4 of the plate thickness in the plate thickness direction from the surface.
- the two phases are tempered martensite phases of 2.00 to 5.00%, the average particle size of the second phase is 1.5 ⁇ m or less, and the extreme density in the (110) ⁇ 112> orientation is 3.0.
- the average particle size of the iron-based carbide is 0.100 ⁇ m or less, and in the metal structure at the position 1/16 of the plate thickness in the plate thickness direction from the surface to the surface, the (110) ⁇ 1-11> orientation.
- the extreme density of is 3.0 or less, and the tensile strength TS is 980 MPa or more.
- the types of the main phase and the second phase at the position 1/4 of the plate thickness in the plate thickness direction from the surface, the average particle size of the second phase, and the extreme density in the (110) ⁇ 112> orientation are determined. This is specified because the metallographic structure at this position represents the typical metallic structure of the steel sheet. Further, the position defining the metal structure is preferably the center position in the plate width direction.
- each regulation will be described.
- Bainite phase (main phase): 95.00-98.00%
- the hot-rolled steel sheet according to the present embodiment has a bainite phase as the main phase.
- the area ratio of the bainite phase, which is the main phase, is 95.00% or more.
- the main phase means that the area ratio is 95.00% or more.
- the bainite phase means a structure having Fe-based carbides between and / or inside the lath-shaped bainitic ferrite and the bainitic ferrite. Unlike polygonal ferrite, bainitic ferrite has a lath-like shape and has a relatively high dislocation density inside, so it can be easily distinguished from other structures using SEM or TEM.
- the bainite phase is set to 95.00% or more. Preferably, it is 96.00% or more.
- the area ratio of the bainite phase exceeds 98.00%, the tensile strength may not be 980 MPa or more. Therefore, the area ratio of the bainite phase is set to 98.00% or less. Preferably, it is 97.50% or less and 97.00% or less.
- Tempering Martensite Phase (Phase 2): 2.00-5.00%
- the hot-rolled steel sheet according to the present embodiment has a tempered martensite phase as the second phase.
- the tempered martensite phase is a collection of lath-shaped crystal grains, and means a structure in which iron carbides have two or more elongation directions inside the crystal grains.
- the area ratio of the second phase The higher the area ratio of the second phase, the better the tensile strength of the hot-rolled steel sheet. If the area ratio of the second phase is less than 2.00%, the desired tensile strength cannot be obtained. Therefore, the area ratio of the second phase is set to 2.00% or more. Preferably, it is 3.00% or more. On the other hand, if the area ratio of the second phase exceeds 5%, the desired hole-expanding property cannot be obtained. Therefore, the area ratio of the second phase is set to 5.00% or less. Preferably, it is 4.00% or less.
- the hot-rolled steel sheet according to the present embodiment may contain 3% or less of ferrite in addition to the bainite phase and the second phase. However, since it is not always necessary to contain ferrite, the area ratio of ferrite may be 0%.
- the method for measuring the area ratio of the metal structure will be described below.
- the test piece is collected so that the region at the / 8 position, that is, the region starting from the 1/8 position in the plate thickness direction from the surface and ending at the 3/8 position in the plate thickness direction from the surface) can be observed.
- the cross section of the test piece is mirror-polished, corroded with a repera corrosive solution, and then the structure is observed using an optical microscope.
- the second phase appears as a white part with the Repeller corrosive liquid, and the other structures (bainite phase) are stained, so that they can be easily identified.
- the area ratio of the white part is calculated by binarizing the white part (bright part) and the other areas. For example, by binarizing the white portion and the other region using image analysis software such as Image-J, the area ratio of the white portion and the area ratio of the other region can be obtained.
- the observation field of view shall be three or more, and the area of each field of view shall be 300 ⁇ m ⁇ 400 ⁇ m or more.
- the area ratio of the second phase is obtained by calculating the average value of the area ratio of the white part measured in a plurality of fields of view.
- the area ratio of the bainite phase is obtained by calculating the average value of the area ratio of the region other than the white portion measured in a plurality of visual fields.
- the ferrite phase is dyed white in the same manner as the bainite phase.
- the bainite phase and the ferrite phase can be easily distinguished by observing their morphology.
- the area ratio of the bainite phase is obtained by subtracting the area ratio of the white portion determined to be the ferrite phase from the area ratio of the region other than the white portion.
- the bainite phase is observed as lath-shaped crystal grains, and the ferrite phase is observed as massive crystal grains containing no lath inside.
- Average particle size of the second phase 1.5 ⁇ m or less
- the average particle size of the second phase is small. If the average particle size of the second phase is more than 1.5 ⁇ m, the desired hole-expanding property cannot be obtained. Therefore, the average particle size of the second phase is set to 1.5 ⁇ m or less. Preferably, it is 1.4 ⁇ m or less and 1.3 ⁇ m or less. Since it is technically difficult to make the average particle size of the second phase less than 0.1 ⁇ m, the average particle size of the second phase may be 0.1 ⁇ m or more.
- the method for measuring the average particle size of the second phase will be described below.
- the hot-rolled steel sheet it is a plate thickness cross section perpendicular to the rolling direction, and is a 1/4 position of the plate thickness in the plate thickness direction from the surface (1/8 position in the plate thickness direction from the surface to 3 in the plate thickness direction from the surface).
- the test piece is collected so that the region at the / 8 position, that is, the region starting from the 1/8 position in the plate thickness direction from the surface and ending at the 3/8 position in the plate thickness direction from the surface) can be observed.
- the cross section of the test piece is mirror-polished, corroded with a repera corrosive solution, and then the structure is observed using an optical microscope.
- image analysis software (Image-J) a binarized image of the white part and the other areas is created. After that, particle analysis is performed based on the binarized image, and the area of each particle is calculated.
- the observation field of view is set to three or more, and the average particle size of the second phase is obtained by calculating the average value of the average particle size obtained in each field of view.
- the second phase having an area of less than 0.5 ⁇ m 2 is excluded from the measurement targets of the above measurement (measurement of the average particle size of the second phase) because it does not affect the hole expansion property of the hot-rolled steel sheet. To do.
- Extreme density of (110) ⁇ 112> orientation 3.0 or less
- the extreme density of (110) ⁇ 112> orientation in the metal structure at the position of 1/4 of the plate thickness in the plate thickness direction from the surface is the degree of development of the rolled texture. It is an index to evaluate. The more the polar density of the (110) ⁇ 112> orientation develops, that is, the larger the polar density of the (110) ⁇ 112> orientation, the greater the anisotropy of the structure and the lower the hole expanding property of the hot-rolled steel sheet. If the polar density of the (110) ⁇ 112> orientation exceeds 3.0, the hole expanding property is lowered. Therefore, the polar density of the (110) ⁇ 112> orientation is set to 3.0 or less. Preferably, it is 2.8 or less, 2.5 or less, and 2.3 or less.
- the lower limit may be 1.0.
- the extreme density of the ⁇ 112> orientation was measured by the EBSD (Electron Backscattering Diffraction) method using a device combining a scanning electron microscope and an EBSD analyzer and an OIM Analysis (registered trademark) manufactured by AMETEK.
- the orientation data can be obtained from the crystal orientation distribution function (ODF: Orientation Diffraction Function) that displays the three-dimensional texture calculated by calculating using the spherical harmonics.
- the measurement range is 1/4 position of the plate thickness in the plate thickness direction from the surface (1/8 position in the plate thickness direction from the surface to 3/8 position in the plate thickness direction from the surface, that is, 1 in the plate thickness direction from the surface. A region starting from the / 8 position and ending at the 3/8 position in the plate thickness direction from the surface), and a region of 400 ⁇ m in the rolling direction. It is preferable to set the measurement pitch so that the measurement pitch is 0.5 ⁇ m / step or less.
- the iron-based carbide means cementite (Fe 3 C).
- the average particle size of the iron-based carbide is set to 0.100 ⁇ m or less. It is preferably 0.080 ⁇ m or less, 0.070 ⁇ m or less, 0.060 ⁇ m or less, and 0.050 ⁇ m or less. Since it is preferable that the average particle size of the iron-based carbide is smaller in order to improve the hole-expandability, the lower limit may be 0 ⁇ m.
- the method for measuring the average particle size of iron-based carbides will be described below. It is a sheet thickness cross section perpendicular to the rolling direction from the hot-rolled steel sheet, and is at 1/4 position of the plate thickness in the plate thickness direction from the surface (1/8 position in the plate thickness direction from the surface to 3/8 in the plate thickness direction from the surface).
- the test piece is collected so that the region of the position, that is, the region starting from the 1/8 position in the plate thickness direction from the surface and ending at the 3/8 position in the plate thickness direction from the surface) can be observed.
- 10 fields of view are taken by SEM at a magnification of 5000 times.
- the interface of bainitic ferrite in the imaging field and the granular or needle-like substances dispersed in it are judged to be iron-based carbides, and the iron-based carbides are image-analyzed to calculate the equivalent diameter of the circle and one field of view.
- the average value of iron-based carbides in By calculating the average value of the iron-based carbides obtained for 10 fields of view, the average particle size of the iron-based carbides is obtained.
- the polar density of the (110) ⁇ 1-11> orientation at this position develops, that is, when the polar density of the (110) ⁇ 1-11> orientation increases, the anisotropy of the structure increases and the hot-rolled steel sheet bends. The sex is reduced. If the polar density in the (110) ⁇ 1-11> orientation exceeds 3.0, the bendability of the hot-rolled steel sheet decreases, so the polar density in the (110) ⁇ 1-11> orientation should be 3.0 or less. .. Preferably, it is 2.8 or less, 2.6 or less, 2.4 or less, and 2.2 or less.
- the extreme density of the ⁇ 1-11> orientation is 1.0 when it does not have an texture, so the lower limit may be 1.0.
- the extreme density of the ⁇ 1-11> orientation is determined by the EBSD (Electron Backscattering Diffraction) method using a device combining a scanning electron microscope and an EBSD analyzer and an OIM Analysis (registered trademark) manufactured by AMETEK.
- the measured orientation data can be obtained from the crystal orientation distribution function (ODF: Orientation Diffraction Function) that displays the three-dimensional texture calculated by calculating using the spherical harmonics.
- the measurement range is a region from the surface to the surface at a position of 1/16 of the plate thickness in the plate thickness direction (a region starting from the surface and ending at a position of 1/16 of the plate thickness in the plate thickness direction from the surface) and rolling. In the direction, a region of 400 ⁇ m or more is evaluated. It is preferable to set the measurement pitch so that the measurement pitch is 0.5 ⁇ m / step or less.
- Tensile strength TS 980 MPa or more
- Tensile strength is an index showing the strength of steel, and by using a material with high tensile strength, it is possible to make automobile parts having the same characteristics at a lighter weight.
- the tensile strength of the hot-rolled steel sheet according to this embodiment is 980 MPa or more. If the tensile strength is less than 980 MPa, the effect of reducing the weight of the vehicle body is not sufficient.
- the tensile strength is 1000 MPa or more and 1030 MPa or more. The higher the tensile strength, the more preferable, but the upper limit may be 1600 MPa or less.
- the tensile strength TS is measured by performing a tensile test using a JIS No. 5 test piece in accordance with JIS Z 2241: 2011.
- the crosshead speed is 10 mm / min.
- a preferred method for producing a hot-rolled steel sheet according to the present embodiment includes the following steps.
- a hot rolling process in which hot rolling is performed so that the hot rolling start temperature is 1050 to 1200 ° C. and the finish rolling completion temperature is more than 950 ° C. and 1050 ° C. or lower.
- a cooling step in which cooling is started within 1.0 second after the completion of the hot rolling and the cooling is performed at an average cooling rate of 30 to 150 ° C./s to a cooling stop temperature of 500 to 600 ° C.
- each step will be described in detail.
- the heating step the slab having the above-mentioned chemical composition is heated to 1100 ° C. or higher and lower than 1350 ° C. Since the coarse precipitates present at the slab stage cause cracks during rolling and deterioration of material properties, it is preferable to heat the steel material before hot rolling to dissolve the coarse carbides as a solid solution. Therefore, the heating temperature is preferably 1100 ° C. or higher. More preferably, it is 1150 ° C. or higher. On the other hand, even if the heating temperature becomes too high, the yield decreases due to the large amount of scale generated, so the heating temperature is preferably 1350 ° C. or lower. More preferably, it is 1300 ° C. or lower.
- the slab to be heated is preferably produced by continuous casting from the viewpoint of manufacturing cost, but may be produced by another casting method (for example, ingot forming method).
- Hot rolling step The temperature of the steel sheet in hot rolling affects the precipitation of carbides and nitrides of Ti and Nb in austenite. If the hot rolling start temperature is less than 1050 ° C., precipitation starts before the start of hot rolling and the precipitate becomes coarse, so that the precipitate cannot be controlled to a desired form and a homogeneous slab can be obtained. May not be possible. Therefore, the hot rolling start temperature is preferably 1050 ° C. or higher. More preferably, it is 1070 ° C. or higher. On the other hand, when the hot rolling start temperature exceeds 1200 ° C., it becomes difficult to start the precipitation of the precipitate during the hot rolling, and the precipitate may not be controlled to a desired form. Therefore, the hot rolling start temperature is preferably 1200 ° C. or lower. More preferably, it is 1170 ° C. or lower.
- the finish rolling completion temperature is a factor that affects the texture of the former austenite grains.
- the finish rolling completion temperature is preferably over 950 ° C. More preferably, it is 960 ° C. or higher.
- the finish rolling completion temperature is preferably 1050 ° C. or lower. More preferably, it is 1020 ° C. or lower.
- the slab Before hot rolling, the slab may be roughly rolled to form a rough bar and then hot rolled.
- descaling may be performed by a conventional method, for example, so that the collision pressure of the injected water is less than 3.0 MPa. If high-pressure descaling is performed in which the collision pressure of the injected water is 3.0 MPa or more, the texture in the surface layer may not be preferably controlled.
- Cooling step in order to obtain a desired metal structure, the cooling conditions after hot rolling in the cooling step, the cooling conditions after winding into a coil in the coil cooling step, and the tempering conditions in the tempering step. It is effective to control the above in a complex and indivisible manner.
- the rolling since the rolling is performed at a relatively high temperature, the coarsening of the old austenite grains tends to proceed. Therefore, it is necessary to start cooling in a short time after the completion of finish rolling to suppress the coarsening of the old austenite grains. If the time from the completion of finish rolling to the start of cooling is long, the old austenite grains may become coarse and the desired average particle size of the second phase may not be obtained.
- the earlier the cooling start time is, the better, and in the present embodiment, it is preferable to start cooling within 1.0 second after the completion of hot rolling. More preferably, it is within 0.5 seconds, and more preferably 0 seconds.
- the cooling start time referred to here refers to the elapsed time from the completion of finish rolling to the start of cooling (cooling having an average cooling rate of 30 to 150 ° C./s), which will be described later.
- Cooling after hot rolling is preferably performed at an average cooling rate of 30 to 150 ° C./s to a cooling shutdown temperature of 500 to 600 ° C. If the average cooling rate is too slow, ferrite will precipitate, making it impossible to obtain the desired amount of bainite phase, and it may not be possible to obtain the desired tensile strength and / or hole-expanding property. Further, if the average cooling rate is slow, carbide-forming elements Ti, V, Nb and the like may be bonded to carbon to form a large amount of precipitates, and the bendability of the hot-rolled steel sheet may be lowered. Therefore, the average cooling rate of cooling after the completion of hot rolling is preferably 30 ° C./s or more. The average cooling rate in cooling after hot rolling is more preferably 60 ° C./s or more.
- the average cooling rate of cooling after the completion of hot rolling is preferably 150 ° C./s or less. More preferably, it is 120 ° C./s or less, and more preferably 100 ° C./s or less.
- the average cooling rate in the present embodiment is a value obtained by dividing the temperature difference between the start point and the end point of the set range by the elapsed time from the start point to the end point.
- the cooling shutdown temperature is outside the temperature range of 500 to 600 ° C., the winding step described later cannot be performed in the desired temperature range. Further, in order to obtain a desired metal structure, it is desirable not to perform air cooling in the cooling after hot rolling.
- Winding step The winding temperature is preferably 500 to 600 ° C. in order to suppress the ferrite transformation and promote the bainite transformation, and to control the distribution, morphology, and fraction of the second phase.
- Bainite transformed at high temperature has excellent ductility. If the winding temperature is less than 500 ° C., precipitation strengthening does not work at the time of winding, so that the strength after tempering may be insufficient. Therefore, the winding temperature is preferably 500 ° C. or higher. On the other hand, if the winding temperature exceeds 600 ° C., ferrite may precipitate and the strength may decrease. Therefore, the winding temperature is preferably 600 ° C. or lower.
- Coil cooling step The cooling rate after winding into a coil affects the microstructure fraction of the second phase.
- carbon enrichment to untransformed austenite is performed.
- Unmetamorphic austenite is the tissue before metamorphosis to the "second phase (martensite phase)".
- the coil is wound and then cooled at an average cooling rate of 25 ° C./h or less, untransformed austenite may be decomposed and a desired amount of the second phase may not be obtained.
- carbon concentration to untransformed austenite progresses excessively, the hardness of the second phase becomes excessive, and the difference in hardness between the structures of the main phase and the second phase becomes large, so that the holes in the hot-rolled steel sheet become large.
- the average cooling rate is preferably more than 25 ° C./h. More preferably, it is 30 ° C./or higher.
- the average cooling rate is preferably 100 ° C./h or less. More preferably, it is 80 ° C./h or less, and even more preferably 60 ° C./h or less.
- Tempering step it is preferable to perform tempering at 350 to 600 ° C. for 30 seconds to 12 hours so that the tempering parameter LMP is 12500 to 15500.
- the tempering parameter LMP is within the above range, a desired amount of tempered martensite and an iron-based carbide having a desired average particle size can be obtained. If the tempering parameter LMP is less than 12500, the martensite phase remains, so that the desired metallographic structure cannot be obtained, and sufficient ductility and perforation may not be obtained. Therefore, the tempering parameter LMP is preferably 12500 or more. The tempering parameter LMP is more preferably 13500 or more and 14000 or more.
- the tempering parameter LMP exceeds 15,500, iron-based carbides may become coarse.
- the coarsened iron-based carbide causes stress concentration on the end face at the time of punching and tends to cause defects, and these defects reduce the hole-expandability of the hot-rolled steel sheet. Further, ferrite may be precipitated to obtain a desired metal structure, and the strength of the hot-rolled steel sheet may be lowered. Therefore, the tempering parameter LMP is preferably 15500 or less.
- the tempering parameter LMP is more preferably 15,000 or less.
- the log is a common logarithm with a base of 10.
- T is the heat treatment temperature (° C.)
- t is the heat treatment time (h).
- the literature (interpretation of the physical meaning of tempering parameters and application to continuous heating / cooling heat treatment processes, heat treatment Vol. 42 It can be calculated as an integrated tempering parameter by a method considering the heat treatment process as described in No. 3, pp. 163 to 168, June 2002).
- the integrated tempering parameter calculated based on the method described in the above document is referred to as the tempering parameter LMP.
- the tempering parameter LMP is specifically obtained by the following method.
- the time from the start of heating to the end of heating is divided by a minute time ⁇ t of a total of N.
- the average temperature of the (n-1) th section is Tn-1 (° C.)
- the average temperature of the nth section is Tn (° C.).
- the time t2 is the time required (equivalent time) for obtaining P equivalent to the integrated value of P calculated based on the heating in the section before the second section (that is, the first section) at the temperature T2.
- the heating time in the second section is the time obtained by adding the actual heating time ⁇ t to the time t2. Therefore, the integrated value P (2) of P at the time when the heating of the second section is completed can be obtained by the following formula.
- P (2) (T2 + 273) ⁇ (20 + log (t2 + ⁇ t))
- the time tn is an equivalent time for obtaining P equivalent to the integrated value of P at the time when the heating in the (n-1) th section is completed at the temperature Tn.
- the Nth tempering parameter P (n) obtained by the above method is the integrated value of P at the time when the heating of the Nth section is completed, and this is the tempering parameter LMP.
- the conditions in the examples are one condition example adopted for confirming the feasibility and effect of the present invention.
- the present invention is not limited to this one-condition example.
- the present invention may adopt various conditions as long as the gist of the present invention is not deviated and the object of the present invention is achieved.
- the microstructure fraction at the 1/4 position of the plate thickness in the plate thickness direction from the surface, the average particle size of the second phase, and the extreme density in the (110) ⁇ 112> direction are obtained by the above method.
- the average particle size of the iron-based carbides and the extreme density in the (110) ⁇ 1-11> orientation in the metal structure at 1/16 of the plate thickness in the plate thickness direction from the surface to the surface were determined.
- the results obtained are shown in Tables 5 and 6. In the case where the total area ratio of bainite and the second phase did not reach 100%, the balance of the metal structure was ferrite.
- the tensile strength TS, the total elongation El, the hole expansion ratio ⁇ , and the limit bending radius R were obtained by the method described later.
- Tensile strength TS and total elongation El were obtained by performing a tensile test using a JIS No. 5 test piece in accordance with JIS Z 2241: 2011. The crosshead speed was set to 10 mm / min. When the tensile strength TS was 980 MPa or more, it was determined to be acceptable as having excellent strength, and when it was less than 980 MPa, it was determined to be unacceptable as being inferior in strength. When the total elongation El was 13.0% or more, it was judged to be acceptable as having excellent ductility, and when it was less than 13.0%, it was judged to be rejected as being inferior in ductility.
- Hole expansion rate ⁇ The hole expandability is obtained by punching a circular hole with a diameter of 10 mm under the condition that the clearance is 12.5% using a 60 ° conical punch, and performing a hole expansion test so that the burr is on the die side. It was evaluated by the spread ratio ⁇ . For each test number, five hole expansion tests were carried out, and the average value thereof was calculated to obtain a hole expansion rate ⁇ . When the hole expanding rate was 60% or more, it was judged to be acceptable as having excellent hole expanding property, and when it was less than 60%, it was determined to be rejected as being poor in hole expanding property.
- Limit bending radius R The bendability was evaluated by the limit bending radius R obtained by performing a V-bending test.
- the limit bending radius R is V-bent using the No. 1 test piece in accordance with JIS Z 2248: 2014 so that the direction perpendicular to the rolling direction is the longitudinal direction (the bending ridge line coincides with the rolling direction). Obtained by conducting a test.
- the angle between the die and the punch was set to 60 °, and the V-bending test was performed by changing the tip radius of the punch in units of 0.1 mm to obtain the maximum value of the tip radius of the punch that could be bent without cracking. I asked.
- the maximum value of the tip radius of the punch that could be bent without cracking was defined as the limit bending radius R.
- the examples of the present invention have excellent strength, ductility, bendability and hole widening property.
- the comparative example is inferior in one or more of strength, ductility, bendability and hole widening property.
- the present invention it is possible to provide a hot-rolled steel sheet having excellent strength, ductility, bendability and hole-expanding property, and a method for producing the same.
Abstract
Description
本願は、2019年12月9日に、日本に出願された特願2019-222162号に基づき優先権を主張し、その内容をここに援用する。
(1)本発明の一態様に係る熱延鋼板は、化学組成が、質量%で、
C:0.040~0.150%、
Si:0.50~1.50%、
Mn:1.00~2.50%、
P:0.100%以下、
S:0.010%以下、
Al:0.010~0.100%、
N:0.0100%以下、
Ti:0.005~0.150%、
B:0.0005~0.0050%、
Cr:0.10~1.00%、
Nb:0~0.06%、
V:0~0.50%、
Mo:0~0.50%、
Cu:0~0.50%、
Ni:0~0.50%、
Sb:0~0.020%、
Ca:0~0.010%、
REM:0~0.010%、および
Mg:0~0.010%
を含有し、残部が鉄および不純物であり、
表面から板厚方向に板厚の1/4位置における金属組織において、
面積率で、主相が95.00~98.00%のベイナイト相であり、第2相が2.00~5.00%の焼き戻しマルテンサイト相であり、
前記第2相の平均粒径が1.5μm以下であり、
(110)<112>方位の極密度が3.0以下であり、
鉄系炭化物の平均粒径が0.100μm以下であり、
前記表面~前記表面から板厚方向に板厚の1/16位置の金属組織において、(110)<1-11>方位の極密度が3.0以下であり、
引張強さTSが980MPa以上である。
(2)上記(1)に記載の熱延鋼板は、前記化学組成が、質量%で、
Nb:0.005~0.06%、
V:0.05~0.50%、
Mo:0.05~0.50%、
Cu:0.01~0.50%、
Ni:0.01~0.50%、
Sb:0.0002~0.020%、
Ca:0.0002~0.010%、
REM:0.0002~0.010%、および
Mg:0.0002~0.010%
からなる群から選択される1種または2種以上を含有してもよい。
Cは、熱延鋼板の強度を向上させるとともに、焼入れ性を向上させることによってベイナイトの生成を促進する元素である。この効果を得るために、C含有量は0.040%以上とする。好ましくは、C含有量は0.050%以上、0.060%以上、0.070%以上である。
一方、C含有量が0.150%を超えると、ベイナイトの生成を制御することが困難となり、マルテンサイト相が多量に生成し、熱延鋼板の延性および穴広げ性の両方、またはいずれか一方が低下する。したがって、C含有量は0.150%以下とする。C含有量は、0.140%以下、0.120%以下、0.100%以下が好ましい。
Siは、固溶強化に寄与する元素であり、熱延鋼板の強度向上に寄与する元素である。また、Siは鋼中に炭化物が形成されることを抑制する元素である。ベイナイト変態時の炭化物の形成を抑制することで、ベイナイト相のラス界面に微細なマルテンサイト相が形成される。ベイナイト相中に存在するマルテンサイト相は微細であるため、熱延鋼板の穴広げ性を劣化させることはない。Si含有による上記効果を得るために、Si含有量は0.50%以上とする。好ましくは、Si含有量は0.55%以上、0.60%以上、0.65%以上である。
一方、Siはフェライト生成を促進する元素であり、Si含有量が1.50%を超えると、フェライトが生成し、熱延鋼板の穴広げ性および強度が低下する。したがって、Si含有量は1.50%以下とする。好ましくは、Si含有量は1.30%以下、1.20%以下、1.00%以下である。
Mnは、鋼中に固溶して熱延鋼板の強度増加に寄与するとともに、焼入れ性向上によってベイナイトの生成を促進し、熱延鋼板の穴広げ性を向上させる。このような効果を得るために、Mn含有量は1.00%以上とする。好ましくは、Mn含有量は1.30%以上、1.50%以上、1.70%以上である。
一方、Mn含有量が2.50%を超えると、ベイナイトの生成制御が困難となり、マルテンサイト相が増加して熱延鋼板の延性および穴広げ性の両方、またはいずれか一方が低下する。そのため、Mn含有量は2.50%以下とする。好ましくは、Mn含有量は2.00%以下、1.95%以下である。
Pは、鋼中に固溶して熱延鋼板の強度増加に寄与する元素である。しかし、Pは、粒界、特に旧オーステナイト粒界に偏析し、粒界偏析による粒界破壊を助長することで、熱延鋼板の延性、曲げ性および穴広げ性の低下を引き起こす元素でもある。P含有量は極力低くすることが好ましいが、0.100%までのPの含有は許容できる。そのため、P含有量は0.100%以下とする。好ましくは、P含有量は0.090%以下、0.080%である。
P含有量は0%とすることが好ましいが、0.0001%未満に低減すると製造コストが上昇するため、P含有量は0.0001%以上としてもよい。好ましくは、P含有量は0.001%以上、0.010%以上である。
Sは、溶接性、ならびに鋳造時および熱間圧延時の製造性に悪影響を及ぼす元素である。SはMnと結びついて粗大なMnSを形成する。このMnSは、熱延鋼板の曲げ性および穴広げ性を劣化させたり、遅れ破壊の発生を助長する。S含有量は、極力低くすることが好ましいが、0.010%までのSの含有は許容できる。そのため、S含有量は0.010%以下とする。好ましくは、S含有量は0.008%以下である。
S含有量は0%とすることが好ましいが、0.0001%未満に低減すると、製造コストが上昇して経済的に不利であることから、S含有量は0.0001%以上としてもよい。好ましくは、S含有量は0.001%以上である。
Alは、脱酸剤として作用し、鋼の清浄度を向上させるのに有効な元素である。この効果を得るために、Al含有量は0.010%以上とする。好ましくは、Al含有量は0.015%以上、0.020%以上である。
一方、Alを過剰に含有すると酸化物系介在物の増加を引き起こし、熱延鋼板の穴広げ性が低下する。そのため、Al含有量は0.100%以下とする。好ましくは、Al含有量は0.050%以下、0.040%以下、0.030%以下である。
Nは鋼中に粗大な窒化物を形成する元素である。この窒化物は、熱延鋼板の曲げ性および穴広げ性を劣化させるとともに耐遅れ破壊特性を劣化させる。そのため、N含有量は0.0100%以下とする。好ましくは、N含有量は0.0080%以下、0.0060%以下、0.0050%以下である。
N含有量を0.0001%未満に低減すると製造コストの大幅な増加を引き起こすため、N含有量は0.0001%以上としてもよい。好ましくは、N含有量は0.0005%以上、0.0010%以上である。
Tiは、オーステナイト相高温域(オーステナイト相域中の高温域およびオーステナイト相域よりも高温域(鋳造の段階))で窒化物を形成する元素である。Tiを含有させることで、BNの析出が抑制され、Bが固溶状態になることによりベイナイトの生成に必要な焼入れ性を得ることができる。結果として、熱延鋼板の強度および穴広げ性を向上することができる。また、Tiは熱間圧延時に鋼中に炭化物を形成して旧オーステナイト粒の再結晶を抑制する。これらの効果を得るために、Ti含有量は0.005%以上とする。好ましくは、Ti含有量は0.030%以上0.050%以上、0.070%以上、0.090%以上である。
一方、Ti含有量が0.150%を超えると、旧オーステナイト粒が再結晶しにくくなり、圧延集合組織が発達することで、熱延鋼板の穴広げ性が低下する。そのため、Ti含有量は0.150%以下とする。好ましくは、Ti含有量は0.130%以下、0.120%以下である。
Bは、旧オーステナイト粒界に偏析し、フェライトの生成および成長を抑制し、熱延鋼板の強度および穴広げ性向上に寄与する元素である。これらの効果を得るために、B含有量は0.0005%以上とする。好ましくは、B含有量は0.0007%以上、0.0010%以上である。
一方、0.0050%を超えてBを含有させても上記効果が飽和する。そのため、B含有量は0.0050%以下とする。好ましくは、B含有量は0.0030%以下、0.0025%以下である。
Crは、鋼中に炭化物を形成して熱延鋼板の高強度化に寄与するとともに、焼入れ性向上によってベイナイトの生成を促進し、ベイナイト粒内へのFe系炭化物の析出を促進する元素である。これらの効果を得るために、Cr含有量は0.10%以上とする。好ましくは、Cr含有量は0.30%以上、0.40%以上、0.50%以上である。
一方、Cr含有量が1.00%を超えると、マルテンサイト相が生成しやすくなり、熱延鋼板の延性および曲げ性の両方、またはいずれか一方が低下する。そのため、Cr含有量は1.00%以下とする。好ましくは、Cr含有量は0.90%以下、0.80%以下、0.70%以下である。
Nbは、熱間圧延時に炭化物を形成してオーステナイトの再結晶を抑制する効果があり、熱延鋼板の強度向上に寄与する元素である。この効果を確実に得るために、Nb含有量は0.005%以上とすることが好ましい。Nb含有量は、0.02%以上とすることがより好ましい。
一方、Nb含有量が0.06%を超えると、旧オーステナイト粒の再結晶温度が高くなりすぎて、集合組織が発達してしまい、熱延鋼板の穴広げ性が低下する場合がある。そのため、Nb含有量は0.06%以下とする。好ましくは、Nb含有量は0.04%以下である。
Vは、熱間圧延時に炭窒化物を形成してオーステナイトの再結晶を抑制する効果があり、熱延鋼板の強度向上に寄与する元素である。この効果を確実に得るために、V含有量は0.05%以上とすることが好ましい。V含有量は、0.10%以上とすることがより好ましい。
一方、V含有量が0.50%を超えると、旧オーステナイト粒の再結晶温度が高くなり、仕上圧延完了後のオーステナイト粒の再結晶温度が高くなることで、集合組織が発達し、熱延鋼板の穴広げ性が低下する場合がある。そのため、V含有量は0.50%以下とする。好ましくは、V含有量は0.25%以下である。
Moは、焼入れ性を向上することでベイナイト相の形成を促進し、熱延鋼板の強度および穴広げの向上に寄与する元素である。この効果を確実に得るために、Mo含有量は0.05%以上とすることが好ましい。Mo含有量は、0.10%以上とすることがより好ましい。
一方、Mo含有量が0.50%を超えると、マルテンサイト相が生成しやすくなり、熱延鋼板の延性および穴広げ性の両方、またはいずれか一方が低下する場合がある。そのため、Mo含有量は0.50%以下とする。好ましくは、Mo含有量は0.30%以下である。
Cuは、鋼中に固溶して熱延鋼板の強度増加に寄与する元素である。また、Cuは、焼入れ性を向上することでベイナイト相の形成を促進し、熱延鋼板の強度および穴広げ性の向上に寄与する元素である。これらの効果を確実に得るために、Cu含有量は0.01%以上とすることが好ましい。Cu含有量は、0.02%以上とすることがより好ましい。
一方、Cu含有量が0.50%を超えると熱延鋼板の表面性状の低下を引き起こす場合がある。そのため、Cu含有量は0.50%以下とする。好ましくは、Cu含有量は0.20%以下である。
Niは、鋼中に固溶して熱延鋼板の強度増加に寄与する元素である。また、Niは、焼入れ性を向上することでベイナイト相の形成を促進し、熱延鋼板の強度および穴広げ性の向上に寄与する元素である。これらの効果を確実に得るために、Ni含有量は0.01%以上とすることが好ましい。Ni含有量は、0.02%以上とすることがより好ましい。
一方、Ni含有量が0.50%を超えると、マルテンサイト相が生成しやすくなり、熱延鋼板の曲げ性および穴広げ性の両方、またはいずれか一方が低下する場合がある。そのため、Ni含有量は0.50%以下とする。好ましくは、Ni含有量は0.20%以下である。
Sbは、スラブ加熱段階でスラブ表面の窒化を抑制する効果を有する。Sbを含有することで、スラブ表層部のBNの析出が抑制される。この効果を確実に得るために、Sb含有量は0.0002%以上とすることが好ましい。Sb含有量は、0.001%以上とすることがより好ましい。
一方、0.020%を超えてSbを含有させても上記効果は飽和するため、Sb含有量は0.020%以下とする。
Caは、硫化物系の介在物の形状を制御し、熱延鋼板の穴広げ性を向上させる元素である。この効果を確実に得るために、Ca含有量は0.0002%以上とすることが好ましい。Ca含有量は0.001%以上とすることがより好ましい。
一方、Ca含有量が0.010%を超えると、熱延鋼板の表面欠陥を引き起こし、生産性が低下する場合がある。そのため、Ca含有量は0.010%以下とする。好ましくは、Ca含有量は0.008%以下である。
REMは、Caと同様、硫化物系の介在物の形状を制御し、熱延鋼板の穴広げ性を向上させる元素である。この効果を確実に得るために、REM含有量は0.0002%以上とすることが好ましい。REM含有量は、0.001%以上とすることがより好ましい。
一方、REM含有量が0.010%を超えると、鋼の清浄度が悪化し、熱延鋼板の穴広げ性および曲げ性の両方、またはいずれか一方が低下する。そのため、REM含有量は0.010%以下とする。好ましくは、REM含有量は0.008%以下である。
ここで、REMは、Sc、Yおよびランタノイドからなる合計17元素を指し、上記REMの含有量は、これらの元素の含有量の合計を指す。ランタノイドの場合、工業的にはミッシュメタルの形で添加される。
Mgは、微量に含有させることで硫化物の形態を制御できる元素である。この効果を確実に得るために、Mg含有量は0.0002%以上とすることが好ましい。Mg含有量は、0.0005%以上とすることがより好ましい。
一方、Mg含有量が0.010%を超えると、粗大な介在物の形成による冷間成形性の低下を引き起こす。そのため、Mg含有量は0.010%以下とする。好ましくは、Mg含有量は0.008%以下である。
本実施形態に係る熱延鋼板は、表面から板厚方向に板厚の1/4位置における金属組織において、面積率で、主相が95.00~98.00%のベイナイト相であり、第2相が2.00~5.00%の焼き戻しマルテンサイト相であり、前記第2相の平均粒径が1.5μm以下であり、(110)<112>方位の極密度が3.0以下であり、鉄系炭化物の平均粒径が0.100μm以下であり、前記表面~前記表面から板厚方向に板厚の1/16位置の金属組織において、(110)<1-11>方位の極密度が3.0以下であり、引張強さTSが980MPa以上である。
以下、各規定について説明する。
本実施形態に係る熱延鋼板は、ベイナイト相を主相とする。主相であるベイナイト相の面積率は95.00%以上である。なお、本実施形態において主相とは、面積率が95.00%以上であることを意味する。
一方、ベイナイト相の面積率が98.00%超では、引張強さが980MPa以上とならない場合があるため、ベイナイト相の面積率は98.00%以下とする。好ましくは、97.50%以下、97.00%以下である。
本実施形態に係る熱延鋼板は、焼き戻しマルテンサイト相を第2相とする。焼き戻しマルテンサイト相とは、ラス状の結晶粒の集合であり、結晶粒の内部に鉄炭化物の伸長方向が二つ以上である組織を意味する。
一方、第2相の面積率が5%超では、所望の穴広げ性を得ることができない。そのため、第2相の面積率は5.00%以下とする。好ましくは、4.00%以下である。
まず、熱延鋼板から、圧延方向と直行する板厚断面であり、表面から板厚方向に板厚の1/4位置(表面から板厚方向に1/8位置~表面から板厚方向に3/8位置の領域、すなわち表面から板厚方向に1/8位置を始点とし、表面から板厚方向に3/8位置を終点とする領域)を観察できるように試験片を採取する。試験片の断面を鏡面研磨し、レペラ腐食液で腐食した後、光学顕微鏡を用いて組織観察を行う。
第2相の平均粒径が大きくなるとボイドが発生しやすくなり、熱延鋼板の穴広げ性が低下する。ボイドの発生を抑制して穴広げ性を向上するためには、第2相の平均粒径は小さい程好ましい。第2相の平均粒径が1.5μm超であると、所望の穴広げ性を得ることができない。そのため、第2相の平均粒径は1.5μm以下とする。好ましくは、1.4μm以下、1.3μm以下である。
第2相の平均粒径を0.1μm未満とすることは技術的に困難なため、第2相の平均粒径は0.1μm以上としてもよい。
まず、熱延鋼板から、圧延方向と直行する板厚断面であり、表面から板厚方向に板厚の1/4位置(表面から板厚方向に1/8位置~表面から板厚方向に3/8位置の領域、すなわち表面から板厚方向に1/8位置を始点とし、表面から板厚方向に3/8位置を終点とする領域)を観察できるように試験片を採取する。試験片の断面を鏡面研磨し、レペラ腐食液で腐食した後、光学顕微鏡を用いて組織観察を行う。画像解析ソフト(Image-J)を用いて、白色部とそれ以外の領域の二値化画像を作成する。その後、二値化画像をもとに粒子解析を実施し、各々の粒子の面積を算出する。観察視野は3か所以上とし、各視野において得られた平均粒径の平均値を算出することで、第2相の平均粒径を得る。
表面から板厚方向に板厚の1/4位置における金属組織における(110)<112>方位の極密度は、圧延集合組織の発達具合を評価する指標である。(110)<112>方位の極密度が発達する程、すなわち(110)<112>方位の極密度が大きい程、組織の異方性が大きくなり、熱延鋼板の穴広げ性が低下する。(110)<112>方位の極密度が3.0を超えると、穴広げ性が低下するため、(110)<112>方位の極密度は3.0以下とする。好ましくは、2.8以下、2.5以下、2.3以下である。
(110)<112>方位の極密度は、走査電子顕微鏡とEBSD解析装置とを組み合わせた装置及びAMETEK社製のOIM Analysis(登録商標)を用いて、EBSD(Electron Back Scattering Diffraction)法で測定した方位データを、球面調和関数を用いて計算して算出した3次元集合組織を表示する結晶方位分布関数(ODF:Orientation Distribution Function)から求めることができる。測定範囲は、表面から板厚方向に板厚の1/4位置(表面から板厚方向に1/8位置~表面から板厚方向に3/8位置の領域、すなわち表面から板厚方向に1/8位置を始点とし、表面から板厚方向に3/8位置を終点とする領域)とし、圧延方向においては400μmの領域とする。測定ピッチが0.5μm/step以下になるように、測定ピッチを設定することが好ましい。
本実施形態において鉄系炭化物とは、セメンタイト(Fe3C)のことをいう。鉄系炭化物の平均粒径が粗大になると、穴広げ時のボイド発生起点となり、熱延鋼板の穴広げ性が低下する。そのため、鉄系炭化物の平均粒径は0.100μm以下とする。好ましくは0.080μm以下、0.070μm以下、0.060μm以下、0.050μm以下である。
穴広げ性向上のためには鉄系炭化物の平均粒径は小さい程好ましいため、下限は0μmとしてもよい。
熱延鋼板から、圧延方向と直行する板厚断面であり、表面から板厚方向に板厚の1/4位置(表面から板厚方向に1/8位置~表面から板厚方向に3/8位置の領域、すなわち表面から板厚方向に1/8位置を始点とし、表面から板厚方向に3/8位置を終点とする領域)を観察できるように試験片を採取する。試験片の断面をナイタール腐食した後、SEMにて倍率5000倍で10視野撮影する。撮影視野内のベイニティックフェライトの界面やその中分散している粒状あるいは針状のものを鉄系炭化物と判断し、鉄系炭化物について画像解析することで、円相当直径を算出し、1視野における鉄系炭化物の平均値を求める。10視野について得られた鉄系炭化物の平均値を算出することで、鉄系炭化物の平均粒径を得る。
表面~表面から板厚方向に板厚の1/16位置(表面を始点とし、表面から板厚方向に板厚の1/16の位置を終点とする領域)の金属組織における(110)<1-11>方位の極密度は、熱延鋼板の表層領域のせん断集合組織の発達具合を評価する指標である。この位置における(110)<1-11>方位の極密度が発達すると、すなわち(110)<1-11>方位の極密度が大きくなると、組織の異方性が大きくなり、熱延鋼板の曲げ性が低下する。(110)<1-11>方位の極密度が3.0を超えると、熱延鋼板の曲げ性が低下するため、(110)<1-11>方位の極密度は3.0以下とする。好ましくは、2.8以下、2.6以下、2.4以下、2.2以下である。
(110)<1-11>方位の極密度は、走査電子顕微鏡とEBSD解析装置とを組み合わせた装置及びAMETEK社製のOIM Analysis(登録商標)を用いて、EBSD(Electron Back Scattering Diffraction)法で測定した方位データを、球面調和関数を用いて計算して算出した3次元集合組織を表示する結晶方位分布関数(ODF:Orientation Distribution Function)から求めることができる。測定範囲は、表面~表面から板厚方向に板厚の1/16位置の領域(表面を始点とし、表面から板厚方向に板厚の1/16の位置を終点とする領域)とし、圧延方向においては400μm以上の領域を評価する。測定ピッチが0.5μm/step以下になるように、測定ピッチを設定することが好ましい。
引張強さは鋼の強度を表す指標であり、引張強さが高い素材を用いることで、同じ特性を有する自動車部品を、より軽量で作ることが可能となる。本実施形態に係る熱延鋼板の引張強さは980MPa以上である。引張強さが980MPa未満では、車体軽量化の効果が十分でない。好ましくは、引張強さは1000MPa以上、1030MPa以上である。引張強さは高いほど好ましいが、上限は1600MPa以下としてもよい。
本実施形態に係る熱延鋼板の好ましい製造方法は、以下の工程を備える。
所定の化学組成を有するスラブを1100℃以上、1350℃未満に加熱する加熱工程、
熱間圧延開始温度が1050~1200℃であり、仕上げ圧延完了温度が950℃超、1050℃以下となるように熱間圧延する熱間圧延工程、
前記熱間圧延完了後、1.0秒以内に冷却を開始し、30~150℃/sの平均冷却速度で500~600℃の冷却停止温度まで冷却する冷却工程、
前記冷却停止温度まで冷却した後、500~600℃の温度域で巻取りを行う巻取り工程、
前記巻取り後、25℃/h超、100℃/h以下の平均冷却速度で冷却するコイル冷却工程、
焼き戻しパラメーターLMPが12500~15500となるように、350~550℃で30秒~12時間の焼き戻しを行う焼き戻し工程。
以下、各工程について詳細に説明する。
加熱工程では、上述の化学組成を有するスラブを1100℃以上、1350℃未満に加熱する。スラブ段階で存在する粗大な析出物は、圧延中の割れや材料特性の低下を引き起こすため、熱間圧延前の鋼素材を加熱して、粗大な炭化物を固溶することが好ましい。そのため、加熱温度は1100℃以上とすることが好ましい。より好ましくは、1150℃以上である。一方、加熱温度が高くなりすぎても、スケール発生量が多くなることで歩留まりが低下するため、加熱温度は1350℃以下とすることが好ましい。より好ましくは、1300℃以下である。
熱間圧延における鋼板温度は、オーステナイト中のTiおよびNbの炭化物や窒化物の析出に影響を与える。熱間圧延開始温度が1050℃未満では、熱間圧延開始前に析出が開始して析出物が粗大化するため、析出物を所望の形態に制御することができず、均質なスラブを得ることができない場合がある。そのため、熱間圧延開始温度は1050℃以上とすることが好ましい。より好ましくは、1070℃以上である。
一方、熱間圧延開始温度が1200℃超では、熱間圧延中に析出物の析出を開始させることが困難となり、析出物を所望の形態に制御することができない場合がある。そのため、熱間圧延開始温度は1200℃以下とすることが好ましい。より好ましくは1170℃以下である。
一方、仕上げ圧延完了温度が高すぎると、旧オーステナイト粒の粗大化が顕著になり、第2相が粗大化することで、所望の穴広げ性を得ることができなくなる場合がある。そのため、仕上げ圧延完了温度は1050℃以下とすることが好ましい。より好ましくは、1020℃以下である。
本実施形態では、所望の金属組織を得るためには、冷却工程における熱間圧延後の冷却条件、コイル冷却工程におけるコイル状に巻取った後の冷却条件および焼き戻し工程における焼き戻し条件を複合的且つ不可分に制御することが効果的である。
巻き取り温度は、フェライト変態を抑制してベイナイト変態を進行させるために、また第2相の分布・形態・分率を制御するために500~600℃とすることが好ましい。
一方、巻き取り温度が600℃超では、フェライトが析出し、強度が低下する場合がある。そのため、巻き取り温度は600℃以下とすることが好ましい。
コイル状に巻取った後の冷却速度は、第2相の組織分率に影響を及ぼす。コイル冷却工程では、未変態オーステナイトへの炭素濃化が行われる。未変態オーステナイトは「第2相(マルテンサイト相)」に変態する前の組織である。コイル状に巻取った後に、25℃/h以下の平均冷却速度で冷却すると、未変態オーステナイトが分解し、所望量の第2相を得ることができない場合がある。また、未変態オーステナイトへの炭素濃化が過度に進行し、第2相の硬さが過剰になり、主相と第2相との組織間硬度差が大きくなることで、熱延鋼板の穴広げ性が低下する場合がある。そのため、平均冷却速度は25℃/h超とすることが好ましい。より好ましくは、30℃/以上である。
焼き戻し工程では、焼き戻しパラメーターLMPが12500~15500となるように、350~600℃で30秒~12時間の焼き戻しを行うことが好ましい。
加熱開始から加熱終了までの時間を総数Nの微小時間Δtで分割する。ここで、(n-1)番目の区間の平均温度をTn-1(℃)、n番目の区間の平均温度をTn(℃)とする。最初の微小時間(n=1の場合の区間)に対応する焼き戻しパラメーターP(1)は、以下の式により求めることができる。なお、logは底が10の常用対数を示す。
P(1)=(T1+273)×(20+log(Δt))
(T1+273)×(20+log(Δt))=(T2+273)×(20+log(t2))
P(2)=(T2+273)×(20+log(t2+Δt))
P(n)=(Tn+273)×(20+log(tn+Δt)) (4)
log(tn)=((Tn-1+273)/(Tn+273))×(20+log(tn-1))-20 (5)
得られた結果を表5および6に示す。なお、ベイナイトおよび第2相の面積率の合計が100%にならない例について、金属組織の残部はフェライトであった。
JIS Z 2241:2011に準拠して、JIS5号試験片を用いて引張試験を行うことで、引張強さTSおよび全伸びElを得た。なお、クロスヘッド速度は10mm/minとした。引張強さTSが980MPa以上である場合を、強度に優れるとして合格と判定し、980MPa未満の場合を、強度に劣るとして不合格と判定した。全伸びElが13.0%以上の場合を、延性に優れるとして合格と判定し、13.0%未満の場合を、延性に劣るとして不合格と判定した。
穴広げ性は、60°円錐ポンチを用いて、クリアランスが12.5%となる条件で直径10mmの円形穴を打ち抜き、かえりがダイ側となるようにした穴広げ試験を行って得られる、穴広げ率λで評価した。各試験番号について、5回の穴広げ試験を実施し、それらの平均値を算出することで、穴広げ率λを得た。穴広げ率が60%以上の場合を穴広げ性に優れるとして合格と判定し、60%未満の場合を穴広げ性に劣るとして不合格と判定した。
曲げ性は、V曲げ試験を行うことで得られる、限界曲げ半径Rにより評価した。限界曲げ半径Rは、圧延方向に対して垂直な方向が長手方向(曲げ稜線が圧延方向と一致)となるように、JIS Z 2248:2014に準拠して、1号試験片を用いてV曲げ試験を行うことで得た。ダイとパンチとのなす角度は60°とし、パンチの先端半径を0.1mm単位で変えてV曲げ試験を行って、亀裂が発生せずに曲げることができたパンチの先端半径の最大値を求めた。亀裂が発生せずに曲げることができたパンチの先端半径の最大値を、限界曲げ半径Rとした。限界曲げ半径Rを試験片の板厚tで除した値(R/t)が1.0以下であった場合、曲げ性に優れるとして合格と判定し、表7および8に「Good」と記載した。一方、限界曲げ半径Rを試験片の板厚tで除した値(R/t)が1.0超であった場合、曲げ性に劣るとして不合格と判定し、表7および8に「Bad」と記載した。
以上の試験結果を、表7および8に示す。
Claims (2)
- 化学組成が、質量%で、
C:0.040~0.150%、
Si:0.50~1.50%、
Mn:1.00~2.50%、
P:0.100%以下、
S:0.010%以下、
Al:0.010~0.100%、
N:0.0100%以下、
Ti:0.005~0.150%、
B:0.0005~0.0050%、
Cr:0.10~1.00%、
Nb:0~0.06%、
V:0~0.50%、
Mo:0~0.50%、
Cu:0~0.50%、
Ni:0~0.50%、
Sb:0~0.020%、
Ca:0~0.010%、
REM:0~0.010%、および
Mg:0~0.010%
を含有し、残部が鉄および不純物であり、
表面から板厚方向に板厚の1/4位置における金属組織において、
面積率で、主相が95.00~98.00%のベイナイト相であり、第2相が2.00~5.00%の焼き戻しマルテンサイト相であり、
前記第2相の平均粒径が1.5μm以下であり、
(110)<112>方位の極密度が3.0以下であり、
鉄系炭化物の平均粒径が0.100μm以下であり、
前記表面~前記表面から板厚方向に板厚の1/16位置の金属組織において、(110)<1-11>方位の極密度が3.0以下であり、
引張強さTSが980MPa以上である
ことを特徴とする熱延鋼板。 - 前記化学組成が、質量%で、
Nb:0.005~0.06%、
V:0.05~0.50%、
Mo:0.05~0.50%、
Cu:0.01~0.50%、
Ni:0.01~0.50%、
Sb:0.0002~0.020%、
Ca:0.0002~0.010%、
REM:0.0002~0.010%、および
Mg:0.0002~0.010%
からなる群から選択される1種または2種以上を含有することを特徴とする請求項1に記載の熱延鋼板。
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