WO2018190416A1 - 鋼板およびその製造方法 - Google Patents
鋼板およびその製造方法 Download PDFInfo
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- WO2018190416A1 WO2018190416A1 PCT/JP2018/015509 JP2018015509W WO2018190416A1 WO 2018190416 A1 WO2018190416 A1 WO 2018190416A1 JP 2018015509 W JP2018015509 W JP 2018015509W WO 2018190416 A1 WO2018190416 A1 WO 2018190416A1
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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
- 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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- C21D6/00—Heat treatment of ferrous alloys
- C21D6/005—Heat treatment of ferrous alloys containing Mn
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- 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
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0236—Cold rolling
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
- C21D8/0273—Final recrystallisation annealing
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- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- 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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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- 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
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/60—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
- C21D2211/001—Austenite
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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
- 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
- C21D2211/008—Martensite
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P10/00—Technologies related to metal processing
- Y02P10/20—Recycling
Definitions
- the present invention relates to a steel plate and a method for manufacturing the same, which can be preferably applied to press forming used through a press forming process in automobiles, home appliances, and the like.
- TRIP steel in which residual ⁇ is dispersed in the microstructure of the steel sheet has been developed as a technique for improving the ductility of the steel sheet.
- Patent Document 1 steel containing C: 0.10 to 0.45%, S: 0.5 to 1.8%, Mn: 0.5 to 3.0% is annealed at 350 to 500 ° C. It is disclosed that a steel sheet having a high ductility of TS ⁇ El ⁇ 2500 kgf / mm 2 ⁇ % can be obtained at a TS of 80 kgf / mm 2 or more by maintaining the residual ⁇ for 1 to 30 min.
- Patent Document 2 discloses that a steel containing C: 0.10 to 0.25%, Si: 1.0 to 2.0%, Mn: 1.5 to 3.0% after annealing is 10 ° C./s or more. At 450 to 300 ° C, hold for 180 to 600 seconds, and control the ductility by controlling the retained austenite to 5% or more, bainitic ferrite to 60% or more, and polygonal ferrite to 20% or less. : El and stretch flange formability: It is disclosed that a steel sheet excellent in ⁇ can be obtained.
- ferrite, tempered martensite, and retained austenite are obtained by cooling a steel plate having a specific component composition to a temperature range of 150 to 350 ° C. after annealing, and then reheating and holding near 400 ° C. It is disclosed that a containing structure is obtained and that high ductility and high stretch flangeability can be imparted to a steel sheet. This is a so-called Q & P that cools once in the cooling process to a temperature range between the martensite transformation start temperature (Ms point) and the martensite transformation completion temperature (Mf point), and then reheats and stabilizes residual ⁇ .
- Ms point martensite transformation start temperature
- Mf point martensite transformation completion temperature
- Patent Document 4 discloses a method in which the above Q & P process is improved. That is, a steel having a specific component composition is annealed at a temperature of Ae3-10 ° C. or higher in order to make polygonal ferrite 5% or less, and then cooled at a relatively high temperature of Ms-10 ° C. to Ms-100 ° C. Is stopped, and when it is reheated to around 400 ° C., upper bainite is generated to obtain high ductility and high stretch flangeability.
- Patent Document 5 discloses a technique for obtaining a steel sheet having excellent ductility and low temperature toughness by utilizing bainite produced at a low temperature and bainite produced at a high temperature. That is, after annealing a steel containing C: 0.10 to 0.5%, the steel is cooled to 150 to 400 ° C. at a cooling rate of 10 ° C./s or more, and kept in that temperature range for 10 to 200 sec. Bainite is generated, and reheated to a temperature range of 400 ° C. to 540 ° C. and held for 50 seconds or more, thereby generating a high temperature bainite and obtaining a steel sheet excellent in ductility and low temperature toughness.
- Patent Document 1 has a problem that although the El is excellent, the stretch flange formability is very low.
- the amount of polygonal ferrite produced is reduced in order to reduce massive martensite, and sufficient ductility cannot be ensured.
- the cooling stop temperature is set to be relatively high, and a large amount of untransformed ⁇ remains at the time of cooling stop, so that massive martensite tends to remain.
- the present invention has been made to solve such problems, and it is intended to provide a steel plate having extremely high ductility and excellent stretch flange formability even when it has a tensile strength of 780 to 1450 MPa, and a method for producing the same. It is what.
- the present inventors diligently studied the means for providing extremely high ductility and excellent stretch flange formability, and obtained the following conclusions.
- the cause of (1) is considered as follows.
- the bainite transformation stagnates. Due to the stagnation of the transformation, a massive structure composed of hard martensite in which carbon is concentrated to the vicinity of the T 0 composition and residual ⁇ remains.
- the cause of (2) is considered as follows.
- the massive structure can be reduced by sufficiently lowering the cooling stop temperature, but the precipitation of carbides in the martensite and the stabilization of the carbon impedes the supply of carbon to the austenite phase, which remains. ⁇ is not sufficiently stabilized.
- El is 9% or more, more preferably 10% or more in the TS: 780 to 1180 MPa class (780 to 1319 MPa in the TS range), and 8% or more in the TS: 1320 MPa class (1320 MPa or more in the TS range), more preferably 9% or more, ⁇ is 40% or more, more preferably 45% or more in TS: 780 to 1180 MPa class (780 to 1319 MPa in the TS range), and 30% or more in TS: 1320 MPa class (1320 MPa or more in the TS range). More preferably, the stability of molding is remarkably improved by securing 35% or more.
- the present invention has been made based on the above knowledge, and specifically provides the following.
- Area ratio of residual ⁇ UB of 18 to 0.60 ⁇ m, particle length of 1.7 to 7.0 ⁇ m, aspect ratio of 5 to 15: S ⁇ UB is 0.2 to 5%, and equivalent circle particle diameter is Fresh martensite and / or circle with an aspect ratio of 3 or less, 1.5 to 15 ⁇ m
- the total area ratio of those particles having a diameter 1.5 ⁇ 15 [mu] m, an aspect ratio of 3 or less of residual ⁇ grains: Sheet S GanmaBlock is that a 3% or less (including 0%).
- SC concentration is 0.2 to 5 % [1] or [2].
- the remainder is other than polygonal ferrite, upper bainite, fresh martensite, tempered martensite, lower bainite, Mn concentration in the region consisting of the residual gamma: Average Mn concentration of Mn Ganma2nd and steel: Mn Bulk ratio Mn Ganma2nd
- Mn Bulk ratio Mn Ganma2nd
- the component composition further contains one or more selected from Ti: 0.002 to 0.1% and B: 0.0002 to 0.01% by mass%.
- the steel plate according to any one of [1] to [7].
- the component composition further includes, by mass%, Cu: 0.005 to 1%, Ni: 0.01 to 1%, Cr: 0.01 to 1.0%, Mo: 0.01 to 0 .5%, V: 0.003-0.5%, Nb: 0.002-0.1%, Zr: 0.005-0.2% and W: 0.005-0.2%
- the steel sheet according to any one of [1] to [8], comprising one or more selected types.
- the component composition may further contain, by mass%, Ca: 0.0002 to 0.0040%, Ce: 0.0002 to 0.0040%, La: 0.0002 to 0.0040%, Mg: 0 [1] to [9] containing one or more selected from 0002 to 0.0030%, Sb: 0.002 to 0.1% and Sn: 0.002 to 0.1% ]
- the steel plate in any one of.
- the cold-rolled steel sheet is subjected to continuous annealing line (CAL).
- Annealing is performed at an annealing temperature of 780 to 880 ° C., and then the temperature range of 780 to 470 ° C. is cooled at an average cooling rate of 5.0 to 2000 ° C./s, and then held at a temperature range of 470 to 405 ° C. for 14 to 200 seconds. Further, the temperature range from 405 ° C.
- Tsq represented by the formula (A) is cooled at an average cooling rate: 5.0 to 80 ° C./s, and the temperature range from the cooling stop temperature to 370 ° C. Is heated at an average heating rate of 3 ° C./s or more, maintained at 300 to 550 ° C. for 35 to 3000 seconds, and then cooled to room temperature.
- a steel sheet having both extremely high ductility and excellent stretch flange formability can be obtained. Furthermore, according to the present invention, it is possible to increase the strength.
- the steel sheet of the present invention has a specific component composition and a specific steel structure. Then, the steel plate of this invention is demonstrated in order of a component composition and steel structure.
- the steel sheet of the present invention contains the following components.
- C 0.06 to 0.25%
- C is a viewpoint of securing a predetermined strength by securing an area ratio of tempered martensite, a viewpoint of securing a volume ratio of residual ⁇ and improving ductility, and a concentration in residual ⁇ to stabilize the residual ⁇ . From the viewpoint of improving ductility. If the C content is less than 0.06%, the strength of the steel sheet and the ductility of the steel sheet cannot be secured sufficiently, so the lower limit is made 0.06%. Preferably it is 0.09% or more, More preferably, it is 0.11% or more.
- the upper bainite transformation in the intermediate holding during the cooling is delayed and it becomes difficult to form a plate-like residual ⁇ UB generated adjacent to the predetermined amount of the upper bainite transformation.
- ductility is reduced.
- massive martensite or massive residual ⁇ increases, and stretch flangeability deteriorates.
- various properties such as spot weldability, bendability, and hole expandability of the steel plate are significantly deteriorated.
- the upper limit of the C content is 0.25%.
- the C content is preferably 0.22% or less.
- the C content is more preferably 0.20% or less.
- Si 0.6-2.5% Si is contained from the viewpoint of strengthening ferrite and increasing strength, suppressing the formation of carbides in martensite and bainite, improving the stability of residual ⁇ , and improving ductility.
- the Si content is set to 0.6% or more.
- the Si content is preferably 0.8% or more. More preferably, it is 0.9% or more, More preferably, it is 1.0% or more. If the Si content exceeds 2.5%, the rolling load becomes extremely high, making it difficult to produce a thin plate. Moreover, chemical conversion property and the toughness of a welding part deteriorate. For this reason, content of Si shall be 2.5% or less.
- the content of Si is preferably less than 2.0% from the viewpoint of chemical conversion property and securing the toughness of the material and welded portion. From the viewpoint of ensuring the toughness of the welded portion, the Si content is preferably 1.8% or less, more preferably 1.5% or less.
- Mn 2.3 to 3.5%
- Mn is concentrated in ⁇ during ⁇ + ⁇ 2 phase annealing, and the residual ⁇ is stabilized by lowering the Ms point of the residual ⁇ . It is an important element from the viewpoint of improving ductility, from the viewpoint of improving the ductility by suppressing the formation of carbides in bainite as with Si, and from the viewpoint of improving the ductility by increasing the volume fraction of residual ⁇ . In order to obtain these effects, the Mn content is set to 2.3% or more.
- the Mn content is preferably 2.5% or more. Preferably it is 2.6% or more, More preferably, it is 2.8% or more.
- the Mn content exceeds 3.5%, the bainite transformation is significantly delayed, so that it is difficult to ensure high ductility.
- the Mn content exceeds 3.5%, it is difficult to suppress the formation of massive ⁇ and massive martensite, and stretch flangeability deteriorates. Therefore, the Mn content is 3.5% or less.
- the Mn content is preferably 3.2% or less. More preferably, it is 3.1% or less.
- P 0.02% or less
- P is an element that strengthens steel, but if its content is large, spot weldability is deteriorated. Therefore, P is set to 0.02% or less. From the viewpoint of improving spot weldability, P is preferably 0.01% or less. P may not be contained, but the P content is preferably 0.001% or more from the viewpoint of manufacturing cost.
- S 0.01% or less S has an effect of improving scale peelability in hot rolling and an effect of suppressing nitriding during annealing, but has a great adverse effect on spot weldability, bendability, and hole expandability. It is an element. In order to reduce these adverse effects, at least S is made 0.01% or less. In the present invention, since the contents of C, Si and Mn are very high, the spot weldability is likely to deteriorate. From the viewpoint of improving the spot weldability, S is preferably 0.0020% or less, and further 0.0010 More preferably, it is less than%. Although S may not be contained, the S content is preferably 0.0001% or more from the viewpoint of manufacturing cost.
- sol. Al Less than 0.50% Al is contained for the purpose of stabilizing residual ⁇ for deoxidation or as an alternative to Si. sol.
- the lower limit of Al is not particularly specified, but is preferably 0.01% or more for stable deoxidation.
- sol. When the Al content is 0.50% or more, the strength of the material is extremely lowered and the chemical conversion property is adversely affected. Al is less than 0.50%. In order to obtain high strength, sol. Al is more preferably less than 0.20%, and still more preferably 0.10% or less.
- N Less than 0.015% N is an element that forms nitrides such as BN, AlN, and TiN in steel, and is an element that lowers the hot ductility of steel and lowers the surface quality. Moreover, in the steel containing B, there exists a bad effect which lose
- the component composition of the steel sheet of the present invention can appropriately contain the following optional elements in addition to the above components.
- Ti 0.002 to 0.1%
- Ti fixes N in steel as TiN, and has the effect of improving the hot ductility and the effect of improving the hardenability of B. Moreover, there exists an effect which refines
- the Ti content is desirably 0.002% or more. From the viewpoint of sufficiently fixing N, the Ti content is more preferably 0.008% or more. More preferably, it is 0.010% or more. On the other hand, if the Ti content exceeds 0.1%, the rolling load increases and the ductility decreases due to an increase in the precipitation strengthening amount. Therefore, the Ti content is preferably 0.1% or less. More preferably, it is 0.05% or less. In order to ensure high ductility, Ti is more preferably 0.03% or less.
- B 0.0002 to 0.01%
- B is an element that improves the hardenability of steel, and has an advantage of easily generating tempered martensite and / or bainite having a predetermined area ratio. Further, the delayed fracture resistance is improved by the solute B remaining.
- the B content is preferably 0.0002% or more. Further, the B content is more preferably 0.0005% or more. More preferably, it is 0.0010% or more.
- the B content is preferably 0.01% or less. More preferably, it is 0.0050% or less. A more preferable range is 0.0030% or less.
- Cu 0.005 to 1%
- Cu is an element mixed when scrap is used as a raw material. By allowing Cu to be mixed, recycled material can be used as a raw material and manufacturing costs can be reduced. From such a viewpoint, Cu is preferably contained in an amount of 0.005% or more, and more preferably 0.05% or more in terms of improving delayed fracture resistance. More preferably, it is 0.10% or more. However, if the Cu content is excessively large, surface defects are caused, so the Cu content is preferably 1% or less. More preferably, it is 0.4% or less, More preferably, it is 0.2% or less.
- Ni 0.01 to 1%
- Cr 0.01 to 1.0% Cr can be contained from the effect of improving the hardenability of steel and the effect of suppressing the formation of carbides in martensite and upper / lower bainite.
- the Cr content is desirably 0.01% or more. More preferably, it is 0.03% or more, More preferably, it is 0.06% or more.
- the Cr content is 1.0% or less. More preferably, it is 0.8% or less, More preferably, it is 0.4% or less.
- Mo 0.01 to 0.5% Mo can be contained from the effect of improving the hardenability of steel and the effect of suppressing the formation of carbides in martensite and upper / lower bainite.
- the Mo content is preferably 0.01% or more. More preferably, it is 0.03% or more, More preferably, it is 0.06% or more.
- the content is preferably 0.5% or less. From the viewpoint of improving chemical conversion properties, Mo is more preferably 0.15% or less.
- V 0.003-0.5%
- V is contained from the effect of improving the hardenability of steel, the effect of suppressing carbide formation in martensite and upper / lower bainite, the effect of refining the structure, and the effect of improving the delayed fracture resistance by precipitating carbides. I can do it.
- the V content is preferably 0.003% or more. More preferably, it is 0.005% or more, More preferably, it is 0.010% or more. However, if a large amount of V is contained, the castability deteriorates remarkably, so the V content is preferably 0.5% or less. More preferably, it is 0.3% or less, More preferably, it is 0.1% or less.
- Nb 0.002 to 0.1%
- Nb can be contained because of the effect of refining and strengthening the steel structure, the effect of promoting bainite transformation through fine graining, the effect of improving bendability, and the effect of improving delayed fracture resistance.
- the Nb content is preferably 0.002% or more. More preferably it is 0.004% or more, and still more preferably 0.010% or more.
- the Nb content is desirably 0.1% or less. More preferably, it is 0.05% or less, More preferably, it is 0.03% or less.
- Zr 0.005 to 0.2%
- Zr can be contained from the effect of improving the hardenability of steel, the effect of suppressing the formation of carbides in bainite, the effect of refining the structure, and the effect of precipitating carbides and improving the resistance to delayed fracture.
- the Zr content is preferably 0.005% or more. More preferably, it is 0.008% or more, More preferably, it is 0.010% or more.
- the Zr content is desirably 0.2% or less. More preferably, it is 0.15% or less, More preferably, it is 0.08% or less.
- W 0.005 to 0.2% W can be contained from the effect of improving the hardenability of steel, the effect of suppressing the formation of carbides in bainite, the effect of refining the structure, and the effect of improving the delayed fracture resistance by precipitating carbides.
- the W content is preferably 0.005% or more. More preferably, it is 0.008% or more, More preferably, it is 0.010% or more.
- the W content is desirably 0.2% or less. More preferably, it is 0.15% or less, More preferably, it is 0.08% or less.
- Ca 0.0002 to 0.0040% Ca fixes S as CaS and contributes to improvement of bendability and delayed fracture resistance. For this reason, it is preferable that Ca content shall be 0.0002% or more. More preferably, it is 0.0005% or more, More preferably, it is 0.0010% or more. However, since Ca deteriorates surface quality and bendability when Ca is added in a large amount, it is desirable that the Ca content be 0.0040% or less. More preferably, it is 0.0035% or less, More preferably, it is 0.0020% or less.
- Ce 0.0002 to 0.0040% Ce, like Ca, fixes S and contributes to improvement of bendability and delayed fracture resistance. For this reason, it is preferable that Ce content shall be 0.0002% or more. More preferably, it is 0.0004% or more, More preferably, it is 0.0006% or more. However, since the surface quality and bendability deteriorate when a large amount of Ce is added, the Ce content is preferably 0.0040% or less. More preferably, it is 0.0035% or less, More preferably, it is 0.0020% or less.
- La 0.0002 to 0.0040%
- La like Ca, fixes S and contributes to improvement of bendability and delayed fracture resistance.
- the La content is preferably 0.0040% or less. More preferably, it is 0.0035% or less, More preferably, it is 0.0020% or less.
- Mg 0.0002 to 0.0030% Mg fixes O as MgO and contributes to the improvement of delayed fracture resistance. For this reason, it is preferable that Mg content shall be 0.0002% or more. More preferably, it is 0.0004% or more, More preferably, it is 0.0006% or more. However, since the surface quality and bendability deteriorate when a large amount of Mg is added, the Mg content is preferably 0.0030% or less. More preferably, it is 0.0025% or less, More preferably, it is 0.0010% or less.
- Sb 0.002 to 0.1% Sb suppresses the oxidation and nitridation of the steel sheet surface layer portion, and thereby suppresses the reduction of the content of the C and B surface layers. Moreover, by suppressing the said reduction
- the Sb content is preferably 0.002% or more. More preferably it is 0.004% or more, and still more preferably 0.006% or more. However, when the Sb content exceeds 0.1%, the castability deteriorates and segregates at the old ⁇ grain boundary, and the delayed fracture resistance of the shear end face deteriorates. For this reason, the Sb content is desirably 0.1% or less. More preferably, it is 0.04% or less, More preferably, it is 0.03% or less.
- Sn 0.002 to 0.1% Sn suppresses oxidation and nitridation of the steel sheet surface layer portion, and thereby suppresses a reduction in the content of the C and B surface layers. Moreover, by suppressing the said reduction
- Sn content is desirably 0.1% or less. More preferably, it is 0.04% or less, More preferably, it is 0.03% or less.
- the balance other than the above is Fe and inevitable impurities. Moreover, when the said arbitrary component is included below a lower limit, the arbitrary element contained less than a lower limit does not impair the effect of this invention. Therefore, when the arbitrary element is included below the lower limit value, the arbitrary element is included as an inevitable impurity.
- the ferrite content is 6% or more. More preferably, it is 8% or more, More preferably, it is 11% or more.
- the ferrite is made 80% or less in area ratio. More preferably, it is 50% or less, more preferably less than 20%, and even more preferably less than 15%.
- ferrite refers to polygonal ferrite.
- the lower limit is more preferably 50% or more, further preferably more than 80%, and further preferably more than 85%.
- the upper limit is more preferably 92% or less, still more preferably 89% or less.
- the area ratios of upper bainite, fresh martensite, tempered martensite, lower bainite and residual ⁇ are SEM photographs, and it is considered that the content of each structure is often in the following range.
- Upper bainite has an area ratio of 3 to 20%.
- Tempered martensite has an area ratio of 5 to 80%.
- the lower bainite has an area ratio of 0 to 50%.
- Residual ⁇ 7-20%
- the residual ⁇ is 7% or more in terms of volume ratio with respect to the entire steel structure. More preferably, it is 9% or more, More preferably, it is 10% or more.
- This amount of residual ⁇ includes both residual ⁇ generated adjacent to the upper bainite and residual ⁇ generated adjacent to martensite and lower bainite. If the amount of residual ⁇ is excessively increased, the strength is lowered, the stretch flangeability is lowered, and the delayed fracture resistance is deteriorated. Therefore, the volume ratio of residual ⁇ is set to 20% or less. More preferably, it is 15% or less, and “volume ratio” can be regarded as “area ratio”.
- Area ratio of residual ⁇ UB having a particle width of 0.18 to 0.60 ⁇ m, a particle length of 1.7 to 7.0 ⁇ m, and an aspect ratio of 5 to 15: S ⁇ UB is 0.2 to 5%
- a plate-like residual ⁇ UB formed adjacent to the upper bainite (bainitic ferrite) containing almost no carbide is obtained by maintaining in the intermediate temperature range of 470 to 405 ° C. in the cooling process. be able to.
- the plate-like residual ⁇ UB particles have a particle width of 0.18 to 0.60 ⁇ m, a particle length of 1.7 to 7.0 ⁇ m, and an aspect ratio of 5 to 15.
- This organization is one of so-called MAs, and the organization defined in this specification is a stable ⁇ in which C is significantly concentrated and must be distinguished from this MA. For this reason, as will be described later, only the organization confirmed by EBSD to have the fcc structure is targeted. Further, if the plate-like residual ⁇ UB is excessively increased, the consumption of carbon is excessively increased, resulting in a significant decrease in strength. In addition, the stretch flangeability deteriorates and the delayed fracture resistance deteriorates. Therefore, S ⁇ UB is set to 5% or less. More preferably, it is 4% or less, More preferably, it is 3% or less. In addition, the said area ratio means the area ratio in the whole steel structure. The area ratio of residual ⁇ UB can be distinguished from other metal phases (bcc system) by obtaining phase map data using EBSD and measuring the structure of the fcc structure.
- particles having a particle length of 1.7 ⁇ m or more are plate-shaped.
- a particle having a particle length of less than 1.7 ⁇ m is assumed to be a film.
- S ⁇ Block is 3% or less (including 0%)
- Mn is reduced to 2% or less to promote bainite transformation, or ⁇ single phase is rapidly cooled to promote bainite transformation.
- the massive structure that adversely affects the stretch flangeability has a circle-equivalent particle diameter of 1.5 to 15 ⁇ m and a fresh martensite having an aspect ratio of 3 or less and a circle-equivalent particle diameter of 1.5 to 15 ⁇ m and an aspect ratio of 3
- S ⁇ Block can be reduced to 3% or less to ensure excellent stretch flangeability molding. In order to ensure excellent stretch flangeability molding, it is more preferable that S ⁇ Block is less than 2%. Further, S ⁇ Block may be 0%.
- the ratio of the total number N MA of 9 ⁇ m residual ⁇ particles and the total number N P of polygonal ferrite grains N MA / N P 0.3 or less
- polygonal ferrite is generated during cooling, and the incidentally distributed aspect ratio within the polygonal ferrite is 3 or less, and the equivalent-circle particle diameter is 3 or less.
- this structure does not reach the amount of C (about 1%) that sufficiently stabilizes residual ⁇ , the concentration of C and Mn that occur during slow cooling is inevitable, so the Ms point decreases, It cannot be transformed into martensite before tempering, and becomes fresh martensite at the time of final cooling or remains as a residual ⁇ with a small C concentration.
- Such a structure transforms to extremely hard martensite or extremely hard martensite after slight plastic deformation in the case of residual ⁇ , so that even if the particle diameter is less than 2 ⁇ m, ⁇ is lowered.
- N MA / N P the ratio of the total number N P of the total number N MA and polygonal ferrite grains of fresh martensite and / or residual ⁇ particles is reduced by 0.3 or less.
- N MA / N P may be 0.
- the ratio N MA / N P is 0 when fresh martensite and residual ⁇ particles are not included.
- the total number is included as the total number N MA .
- SC concentration is 0.2 to 5%
- the adjacent region means a region adjacent to a region having a C concentration of 0.7 to 1.3% and an adjacent region having a C concentration of 0.07% or less.
- the region where the C concentration is 0.7 to 1.3% and the C concentration of the adjacent region is 0.07% or less is preferably residual ⁇ , and is residual ⁇ UB . It is more preferable. Moreover, it is preferable that a part or all of an adjacent area
- Residual ⁇ UB produced adjacent to the upper bainite is characterized by a very low amount of C on at least one side of the particles.
- the bainite (bainitic ferrite) generated at a high temperature of 405 to 470 ° C.
- the separation of C into austenite proceeds easily, and C is efficiently concentrated into plate-like ⁇ UB .
- the amount of C in the plate-like residual ⁇ UB becomes 0.7 to 1.3%, which contributes to the improvement of ductility.
- C amount falls to 0.07% or less.
- S ⁇ UB * In order to further improve the ductility, it is preferable to secure the residual ⁇ region S ⁇ UB * having such a C distribution state in an area ratio of 0.2 to 5%. Since ductility is significantly increased by the S ⁇ UB * 0.3% or more, S ⁇ UB * further preferably set to 0.3% or more. About an upper limit, More preferably, it is 4% or less, More preferably, it is 3% or less.
- the effect of stabilizing the residual ⁇ by Mn concentration in the region where both the plate-like residual ⁇ UB and the film-like residual ⁇ exist is obtained. It is obtained and contributes to further improvement of ductility.
- the effect is upper bainite except polygonal ferrite, fresh martensite, tempered martensite, lower bainite, Mn concentration in the region consisting of the residual gamma: Average Mn concentration of Mn Ganma2nd and steel: Mn ratio Mn Ganma2nd / Mn Bulk of Bulk Can be obtained by setting the value to 1.1 or more. About an upper limit, 2.0 or less is preferable, More preferably, it is 1.5 or less.
- the area ratio of the ferrite was measured by cutting a plate thickness section parallel to the rolling direction, mirror polishing, corroding with 3% nital, and observing 5 fields of view at 5000 times at 1/4 thickness position. .
- the ferrite was relatively equiaxed polygonal ferrite with little carbide inside. In the SEM, this is the region that looks the most black. Ferrite inside the plate-shaped residual gamma UB is present is divided ferrite, if both sides of the tissue is difficult to distinguish whether upper bainite of the Ferrite thing is, ferrite polygonal form aspect ratio ⁇ 2.5 And the area ratio was calculated by classifying the area of aspect ratio> 2.5 as upper bainite (bainitic ferrite).
- the plate-like residual ⁇ UB may be distinguished from bainite.
- ferrite and bainite are adjacent to each other, and in an area where they cannot be distinguished, that is, in the above classification, ferrite may exist.
- the aspect ratio is obtained by obtaining the long axis length a at which the particle length is longest, and the particle length when traversing the particle longest in the direction perpendicular thereto is the short axis length b.
- a / b was the aspect ratio.
- the area ratio of the structure composed of one or more of upper bainite, fresh martensite, tempered martensite, lower bainite and residual ⁇ was measured by the same method as that for ferrite.
- the said area rate is an area rate of area
- the area ratio of the carbide is very small, it was included in the area ratio.
- the volume fraction of residual ⁇ was obtained by X-ray diffraction after chemically polishing a 1/4 thickness position from the surface layer.
- Co-K ⁇ radiation source was used for incident X-rays, and the area of residual austenite from the intensity ratio of the (200), (211), (220) faces of ferrite and the (200), (220), (311) faces of austenite.
- the rate was calculated.
- the volume ratio of the residual ⁇ obtained by X-ray diffraction becomes equal to the area ratio of the residual ⁇ in the steel structure.
- the shape and area ratio of the plate-like residual ⁇ UB formed adjacent to the upper bainite are obtained by electropolishing a plate thickness section parallel to the rolling direction of the steel plate, obtaining phase map data using EBSD, and having an fcc structure structure.
- the measurement area was 30 ⁇ m, and measurement was performed for five visual fields.
- the particle size (major axis length), particle width (minor axis length), and aspect ratio were determined by employing the particle size and aspect ratio measurement methods described above. Further, the area ratio of ⁇ grains having a particle width of 0.18 to 0.60 ⁇ m, a particle length of 1.7 to 7.0 ⁇ m, and an aspect ratio of 5 to 15 was determined as S ⁇ UB .
- the area ratio of fresh martensite with a circle-equivalent particle diameter of 1.5 to 15 ⁇ m and an aspect ratio of 3 or less, and the residual ⁇ particles with a circle-equivalent particle diameter of 1.5 to 15 ⁇ m and an aspect ratio of 3 or less are similarly SEM. I asked for it from the photo. The aspect ratio was also confirmed by the same method as that for the plate-like residual ⁇ UB .
- the equivalent circle particle diameter (circle equivalent particle diameter) was determined by observing 10 particles with an SEM, determining the area ratio of each, calculating the equivalent circle diameter, and calculating the equivalent circle diameter for each particle. did.
- the measurement of the C concentration (mass%) of the region where the C concentration is 0.7 to 1.3% and the C concentration of the adjacent region is 0.07% or less and the adjacent region is Using an electron emission electron beam microanalyzer (FE-EPMA) JXA-8500F manufactured by JEOL at a thickness 1/4 position of the thickness cross section parallel to the rolling direction, an acceleration voltage of 6 kV and an irradiation current of 7 ⁇ 10 ⁇ 8 A, Line analysis was performed with the beam diameter being minimized. The analysis length was 6 ⁇ m, and C profile data were collected at 30 locations at random in order to obtain average microstructure information.
- FE-EPMA electron emission electron beam microanalyzer
- the background was subtracted so that the average value of C obtained in each line analysis was equal to the carbon content of the base material.
- the increase is considered as contamination, and the value obtained by subtracting the increase uniformly from the analysis value at each position is the true value at each position.
- C amount A region having a C concentration of 0.07% or less is adjacent to each other, and the total area ratio S C concentration in the region of C: 0.7 to 1.3% is C.
- the area ratio of C: 0.7 to 1.3% in the line analysis result was defined as the area ratio, assuming that the distribution state of the above area was random for the area of 07% or less.
- FIG. 4 An example of a graph showing the relationship between the C concentration obtained by the above measurement and the analysis length is shown in FIG.
- the region where the C concentration is 0.7 to 1.3% and the C concentration in the adjacent region is 0.07% or less is SC concentration-1 .
- the graph as shown in FIG. 4 derives at 30 points to obtain the total area ratio S C enrichment of S C enrichment -1.
- a metal phase having a C enrichment amount may be evaluated as a plate-like residual ⁇ UB .
- Mn concentration (mass%) in the region consisting of upper bainite, fresh martensite, tempered martensite, lower bainite and residual ⁇ other than polygonal ferrite Mn ⁇ 2nd is a similar method using FE-EPMA at an acceleration voltage of 9 kV The Mn line analysis was performed, and the SEM structure was observed in the same field of view to calculate the amount of Mn in the second phase region other than ferrite. Moreover, it compared with the chemical analysis value of Mn of a base material, and calculated
- FIG. 1 An example of the SEM photograph is shown in FIG. 1
- the steel plate used for the observation in FIG. 1 was a 0.18% C-1.5% Si-2.8% Mn steel annealed in a two-phase region, cooled to 450 ° C. at 18 ° C./s, and then at 450 ° C. After being kept isothermal for 30 seconds, it was cooled to 220 ° C. at 10 ° C./s, then kept at 400 ° C. for 18 minutes, and then cooled to 100 ° C. or less at 10 ° C./s. A 1/4 thickness position of the vertical cross section in the rolling direction was polished and then corroded with 3% nital and observed by SEM.
- the upper bainite (a) has a structure having a minor axis width of 0.4 ⁇ m or more that hardly contains carbides. Adjacent to this is a plate-like residual ⁇ (b) having a particle width of 0.18 to 0.60 ⁇ m, a particle length of 1.7 to 7.0 ⁇ m, and an aspect ratio of 5 to 15.
- the adjacent structure of the plate-like residual ⁇ UB is the upper bainite. However, when the upper bainite is formed adjacent to the ferrite, the upper bainite and the ferrite are integrated with each other. The UB may appear to be adjacent to the ferrite.
- Tempered martensite (c) is a region containing 0.5 to 8 fine carbides (e) having an aspect ratio of 3 or less and an equivalent circle diameter of 0.03 to 0.2 ⁇ m per 1 ⁇ m 2 inside the structure. Adjacent to this is a film-like residual ⁇ having a particle width of 0.1 to less than 0.50 ⁇ m and a particle length of 0.5 ⁇ m or more and less than 1.7 ⁇ m (d).
- Lower bainite (f) is a structure containing an elongated film-like martensite or residual ⁇ (g) with an aspect ratio of 8 to 20 inside the structure.
- the steel sheet of the present invention preferably has a tensile strength of 780 MPa or more. More preferably, it is 980 MPa or more.
- the upper limit of the tensile strength is preferably 1450 MPa or less, more preferably 1400 MPa or less from the viewpoint of compatibility with other properties.
- the hole expansion ratio ⁇ is 40% or more, preferably 45% or more for TS: 780 to 1319 MPa class, and 30% or more, preferably 35% or more for TS: 1320 MPa or more. Is significantly improved.
- the upper limit of ⁇ is preferably 90% or less, more preferably 80% or less at any strength level from the viewpoint of compatibility with other characteristics.
- U. El is 9% or more, more preferably 10% or more in TS: 780 to 1319 MPa class, and is 8% or more, more preferably 9% or more in TS: 1320 MPa or more.
- U.S. The upper limit of El is preferably 20.0% or less and more preferably 18.0% or less at any strength level from the viewpoint of compatibility with other characteristics.
- (TS ⁇ U.El-7000) ⁇ ⁇ ⁇ 290000 is preferable. More preferably, (TS ⁇ U.E1 ⁇ 7000) ⁇ ⁇ ⁇ 291500, and further preferably (TS ⁇ U.E1 ⁇ 7000) ⁇ ⁇ ⁇ 300000.
- the upper limit is not particularly limited, but from the viewpoint of compatibility with other characteristics, (TS ⁇ U.El-7000) ⁇ ⁇ ⁇ 600,000 is preferable, and (TS ⁇ U.E1 ⁇ 7000) ⁇ ⁇ ⁇ is more preferable. 500,000.
- Hot rolling To hot-roll steel slabs, a method of rolling the slab after heating, a method of directly rolling the slab after continuous casting without heating, or rolling by subjecting the slab after continuous casting to a short heat treatment There are ways to do it.
- the hot rolling may be performed according to a conventional method.
- the slab heating temperature is 1100 to 1300 ° C.
- the soaking temperature is 20 to 300 min
- the finishing rolling temperature is Ar 3 transformation point to Ar 3 transformation point + 200 ° C.
- winding The taking temperature may be 400 to 720 ° C.
- the coiling temperature is preferably 450 to 550 ° C. from the viewpoint of suppressing fluctuations in sheet thickness and ensuring high strength stably.
- a more preferable range is 460 to 550 ° C.
- a more preferable range is 500 to 550 ° C. from the viewpoint of suppressing fluctuations in sheet thickness and ensuring high strength stably.
- the rolling rate may be 30 to 85%. From the viewpoint of stably ensuring high strength and reducing anisotropy, the rolling rate is preferably 45 to 85%.
- softening annealing can be performed by CAL and BAF at 450 to 730 ° C.
- FIG. 3 illustrates an example of manufacturing conditions.
- Annealing temperature 780-880 ° C
- the annealing temperature is set to 780 to 880 ° C.
- the annealing temperature is adjusted to be ⁇ + ⁇ two-phase region annealing according to the component. With this two-phase annealing, Mn can be uniformly concentrated in ⁇ , contributing to stabilization of ⁇ .
- Average cooling rate in the temperature range of 780 to 470 ° C . 5.0 to 2000 ° C./s
- a temperature range of 780 to 470 ° C. is cooled at an average cooling rate of 5.0 to 2000 ° C./s.
- the average cooling rate shall be 5.0 degrees C / s or more. More preferably, it is 8.0 ° C./s or more.
- the average cooling rate becomes too fast, the plate shape deteriorates.
- it is 100 degrees C / s or less.
- the plate shape is less than 30 ° C./s. More preferably, it is less than 30 ° C./s. Moreover, it is preferable to set it as 29 degrees C / s or less, since a board shape can be made into a favorable level (The board curvature as described in the Example mentioned later is 15 mm or less). Furthermore, by setting the average cooling rate to 14 ° C./s or less, it is more preferable because the plate shape can be made to a better level (the plate warpage described in Examples described later is 10 mm or less).
- Holding time in a temperature range of 470 to 405 ° C .: 14 to 200 sec By maintaining for a predetermined time in this temperature range, it is possible to generate upper bainite that hardly causes carbide precipitation, and to generate a plate-like residual ⁇ UB with a high concentration of C adjacent thereto. I can do it.
- the holding time in this temperature range is set to 14 sec or more. From the viewpoint of generating plate-like residual ⁇ UB and improving ductility, the holding time in this temperature range is more preferably 18 sec or more.
- the holding time in the temperature range of 470 to 405 ° C. is 14 to 200 sec.
- the holding time in the temperature range of 470 to 405 ° C. is preferably 100 sec or less. Note that holding in this temperature range corresponds to reducing the average cooling rate in this temperature range to 4.6 ° C./s or less.
- Average cooling rate from 405 ° C. to cooling stop temperature Tsq: 5.0-80 ° C./s
- the temperature range from 405 ° C. to the cooling stop temperature: Tsq represented by the formula (A) is cooled at an average cooling rate: 5.0 to 80 ° C./s.
- the average cooling rate in this temperature range is slow, C concentrates to untransformed ⁇ , leading to an increase in the massive structure. Further, the precipitation of carbide progresses and C is wasted, resulting in a decrease in ductility.
- the average cooling rate in this temperature range is more preferably 7.0 ° C./s or more.
- the average cooling rate in this temperature range is set to 5.0 to 80 ° C./s. From the viewpoint of promoting the diffusion of C from martensite or lower bainite during cooling into the film-like ⁇ , the average cooling rate in this temperature range is preferably 15 ° C./s or less.
- the cooling stop temperature is set to the above range from the viewpoint of suppressing the massive structure and obtaining the plate-like residual ⁇ UB .
- the average cooling rate from 780 to 470 ° C is CR1
- the average cooling rate from 470 to 405 ° C is CR2
- the average cooling rate from 405 ° C to the cooling stop temperature is CR3, CR1> CR2 and CR2 ⁇ CR3 It is extremely important to switch the cooling rate in this way.
- Average heating rate in the temperature range from the cooling stop temperature to 370 ° C . 3 ° C./s or more Further, heating in the temperature range from the cooling stop temperature to 370 ° C. in a short time suppresses carbide precipitation and ensures high ductility. I can do it. Further, when martensite or lower bainite generated by cooling is reheated to 370 ° C. or more with the core as the core, upper bainite is generated. If the average heating rate up to 370 ° C. is slow, these effects cannot be obtained. As a result, the amount of residual ⁇ decreases and ductility decreases. For this reason, the average heating rate in the temperature range from the cooling stop temperature to 370 ° C.
- the average heating rate is preferably 5 ° C./s or more, and more preferably 10 ° C./s or more.
- the upper limit of the said average heating rate is not specifically limited, 50 degrees C / s or less is preferable, More preferably, it is 30 degrees C / s or less.
- the holding time at 370 to 550 ° C. to 60 to 3000 sec, the total area ratio of the region where the C concentration is 0.7 to 1.3% and the C concentration of the adjacent region is 0.07% or less.
- the SC concentration is 0.2 to 5%, and the ductility is further improved.
- the steel sheet can be cooled to room temperature and subjected to skin pass rolling from the viewpoint of stabilizing the press formability such as adjusting the surface roughness and flattening the plate shape and increasing the YS.
- the skin pass elongation rate is preferably 0.1 to 0.5%.
- the plate shape can be flattened with a leveler.
- (TS ⁇ U.El-7000) ⁇ ⁇ ⁇ 290000 which is important as an index of formability of a component having a complicated shape in which stretch molding and stretch flange molding are mixed, is satisfied in the TS: 780 to 1319 MPa class. It is possible to satisfy (TS ⁇ U.El-7000) ⁇ ⁇ ⁇ 200000 with TS: 780 to 1319 MPa class.
- the cold-rolled steel sheet having the composition shown in Table 1 was processed under the annealing conditions shown in Table 2 to produce the steel sheet of the present invention and the steel sheet of the comparative example.
- the plate thickness of the steel plate was 1.4 mm.
- the steel structure was measured by the above method. The measurement results are shown in Table 2.
- a JIS No. 5 tensile test piece was collected from the obtained steel sheet and subjected to a tensile test (based on JIS Z2241). TS and U.S. El is shown in Table 2.
- the stretch flange formability was evaluated by a hole expansion test in accordance with the provisions of JFST1001. That is, after punching a sample of 100 mm ⁇ 100 mm square size with a punching tool having a punch diameter of 10 mm and a die diameter of 10.2 mm (clearance 13%), using a conical punch with an apex angle of 60 degrees, D 0 : Initial hole diameter (mm) and d: Hole diameter (mm) at the time of crack occurrence when hole expansion was performed until the generated burr was on the outside and a crack penetrating the plate thickness occurred
- the hole expansion ratio ⁇ (%) ⁇ (d ⁇ d 0 ) / d0 ⁇ ⁇ 100.
- TS: 780 to 1319 MPa class has an excellent uniform elongation (ductility) of 9% or more, (TS ⁇ U.El-7000) ⁇ ⁇ ⁇ 290000, and a hole expandability ( ⁇ ) of 40% or more. Satisfactory, TS: At 1320 MPa or more, excellent uniform elongation (ductility) of 8% or more, (TS ⁇ U.El-7000) ⁇ ⁇ ⁇ 200000, excellent hole expansibility ( ⁇ ) of 30% or more. On the other hand, any of the comparative examples is inferior.
- No. 1, 9, 11, 13, 19, 26, 27, 28, 29, 30, and 31 have a C concentration of 0.7 to 1.3%, and the C concentration of the adjacent region is 0.07% or less.
- the region is residual ⁇ UB , and the upper region includes upper bainite, which is particularly excellent in ductility.
- the plate warpage measured by the following method was 11 to 15 mm, which was a favorable level. Further, in the invention examples in which the average cooling rate was 5 ° C./s or more and 14 ° C./s or less, the plate warpage measured by the following method was 10 mm or less, which was a better level.
- the said board warpage for evaluating a board shape takes the cut sample of 1500 mm length from the steel plate after annealing, and the curvature height of 4 sides when the said sample is set
- the present invention has extremely high ductility and excellent stretch flange formability, and can be preferably applied to press molding used in automobiles, home appliances and the like through a press molding process.
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Abstract
Description
ここで、冷却停止温度Tsq(℃)はMs-90≧Tsq≧Ms-180・・・(A)
Ms=539-474×[%C]/(100-VF)×100-30.4×[%Mn]×1.2-12.1×[%Cr]-7.5×[%Mo]-17.7×[%Ni]・・・(B)
[%C]、[%Mn]、[%Cr]、[%Mo]、[%Ni]はC、Mn、Cr、Mo、Niそれぞれの含有量(質量%)を表し、含まない場合は0とし、VFはフェライトの面積率(%)を表す。
Cは、焼き戻しマルテンサイトの面積率を確保して所定の強度を確保する観点、残留γの体積率を確保して延性を向上させる観点、残留γ中に濃化して残留γを安定化させて延性を向上させる観点から含有する。Cの含有量が0.06%未満では鋼板の強度、鋼板の延性が十分に確保できないので、その下限は0.06%とする。好ましくは0.09%以上、より好ましくは0.11%以上である。その含有量が0.25%を超えると冷却途中の中間保持における上部ベイナイト変態が遅延して所定量の上部ベイナイト変態に隣接して生成するプレート状の残留γUBを形成することが難しくなる。その結果、延性が低下する。また、塊状のマルテンサイトもしくは塊状の残留γが増加して、伸びフランジ成形性が劣化する。さらに、鋼板のスポット溶接性、曲げ性、穴拡げ性といった諸特性が著しく劣化する。このため、C含有量の上限は0.25%とする。延性やスポット溶接性向上の観点からはC含有量は0.22%以下とすることが望ましい。延性およびスポット溶接性をさらに改善する観点からはC含有量は0.20%以下にすることがさらに望ましい。
Siは、フェライトを強化して強度を上昇させる観点、マルテンサイトやベイナイト中の炭化物生成を抑制し、残留γの安定性を向上させて延性を向上させる観点から含有する。炭化物の生成を抑制して延性を向上させる観点から、Si含有量は0.6%以上にする。延性向上の観点から、Si含有量は0.8%以上が好ましい。より好ましくは0.9%以上、さらに好ましくは1.0%以上である。Siの含有量が2.5%を超えると圧延荷重が極端に高くなり、薄板の製造が困難になる。また、化成処理性や溶接部の靭性が劣化する。このため、Siの含有量は2.5%以下とする。化成処理性や素材および溶接部の靭性確保の観点からはSiの含有量は2.0%未満とするのが好ましい。溶接部の靭性確保の観点からはSiの含有量は1.8%以下、さらには1.5%以下とするのが好ましい。
Mnは、所定の面積率の焼き戻しマルテンサイトおよび/またはベイナイトを確保して強度を確保する観点、α+γ2相域焼鈍時にγ中に濃化して残留γのMs点の低下により残留γを安定化させ延性を改善する観点、Siと同様にベイナイト中の炭化物の生成を抑制して延性を向上させる観点、残留γの体積率を増加させて延性を向上させる観点から重要な元素である。これらの効果を得るために、Mnの含有量は2.3%以上とする。従来の熱処理方法の中で、最終工程でベイナイト変態を活用する手法では、Mnを2.3%以上含有すると、硬質なマルテンサイトや残留γからなる塊状組織が多量に残存して伸びフランジ成形性が低下していた。しかし、本発明では、後述する熱処理方法を採用により得られる組織を有するので、Mnを多量に含有しても塊状組織を低減することが可能であり、Mn含有による残留γの安定化作用や体積率増加作用を享受することが出来る。残留γを安定化させて延性を向上させる観点からは、Mn含有量は2.5%以上が好ましい。好ましくは2.6%以上、より好ましくは2.8%以上である。Mnの含有量が3.5%を超えるとベイナイト変態が著しく遅延するので高い延性を確保する事が困難になる。また、Mnの含有量が3.5%を超えると、塊状の粗大γや塊状の粗大マルテンサイトの生成を抑制することは難しくなり、伸びフランジ成形性も劣化する。したがって、Mn含有量は3.5%以下とする。ベイナイト変態を促進して高い延性を確保する観点からMn含有量は3.2%以下とすることが好ましい。より好ましくは3.1%以下である。
Pは鋼を強化する元素であるが、その含有量が多いとスポット溶接性を劣化させる。したがって、Pは0.02%以下とする。スポット溶接性を改善する観点からはPは0.01%以下とすることが好ましい。なお、Pを含まなくてもよいが、P含有量は製造コストの観点から0.001%以上が好ましい。
Sは熱延でのスケール剥離性を改善する効果、焼鈍時の窒化を抑制する効果があるが、スポット溶接性、曲げ性、穴拡げ性に対して大きな悪影響を有する元素である。これらの悪影響を低減するために少なくともSは0.01%以下とする。本発明ではC、Si、Mnの含有量が非常に高いのでスポット溶接性が悪化しやすく、スポット溶接性を改善する観点からはSは0.0020%以下とすることが好ましく、さらに0.0010%未満とすることがより好ましい。なお、Sを含まなくてもよいが、S含有量は製造コストの観点から0.0001%以上が好ましい。
Alは脱酸のため、あるいはSiの代替として残留γを安定化する目的で含有する。sol.Alの下限は特に規定しないが、安定して脱酸を行うためには0.01%以上とすることが望ましい。一方、sol.Alが0.50%以上となると、素材の強度が極端に低下し、化成処理性にも悪影響するので、sol.Alは0.50%未満とする。高い強度を得るためにsol.Alは0.20%未満とすることがさらに好ましく、0.10%以下とすることがより一層好ましい。
Nは鋼中でBN、AlN、TiN等の窒化物を形成する元素であり、鋼の熱間延性を低下させ、表面品質を低下させる元素である。また、Bを含有する鋼では、BNの形成を通じてBの効果を消失させる弊害がある。N含有量が0.015%以上になると表面品質が著しく劣化する。したがって、Nの含有量は0.015%未満とする。なお、Nを含まなくてもよいが、N含有量は製造コストの点から0.0001%以上が好ましい。
Tiは鋼中のNをTiNとして固定し、熱間延性を向上させる効果やBの焼入れ性向上効果を生じさせる作用がある。また、TiCの析出により組織を微細化する効果がある。これらの効果を得るためにTi含有量を0.002%以上にすることが望ましい。Nを十分固定する観点からはTi含有量は0.008%以上がさらに好ましい。より好ましくは0.010%以上である。一方、Ti含有量が0.1%を超えると圧延負荷の増大、析出強化量の増加による延性の低下を招くので、Ti含有量は0.1%以下にすることが望ましい。より好ましくは0.05%以下である。高い延性を確保するためにTiは0.03%以下とすることがさらに好ましい。
Bは、鋼の焼入れ性を向上させる元素であり、所定の面積率の焼き戻しマルテンサイトおよび/またはベイナイトを生成させやすい利点を有する。また、固溶Bの残存により耐遅れ破壊特性は向上する。このようなBの効果を得るには、B含有量を0.0002%以上にすることが好ましい。また、B含有量は0.0005%以上がより好ましい。さらに好ましくは0.0010%以上である。一方、B含有量が0.01%を超えると、その効果が飽和するだけでなく、熱間延性の著しい低下をもたらし表面欠陥を生じさせる。したがって、B含有量は0.01%以下が好ましい。より好ましくは0.0050%以下である。さらに好ましい範囲は0.0030%以下である。
Cuは、自動車の使用環境での耐食性を向上させる。また、Cuの腐食生成物が鋼板表面を被覆して鋼板への水素侵入を抑制する効果がある。Cuは、スクラップを原料として活用するときに混入する元素であり、Cuの混入を許容することでリサイクル資材を原料資材として活用でき、製造コストを低減することができる。このような観点からCuは0.005%以上含有させることが好ましく、さらに耐遅れ破壊特性向上の観点からは、Cuは0.05%以上含有させることがより望ましい。さらに好ましくは0.10%以上である。しかしながら、Cu含有量が多くなりすぎると表面欠陥の発生を招来するので、Cu含有量は1%以下とすることが望ましい。より好ましくは0.4%以下、さらに好ましくは0.2%以下である。
Niも、Cuと同様、耐食性を向上する作用のある元素である。また、Niは、Cuを含有させる場合に生じやすい、表面欠陥の発生を抑制する作用がある。このため、Niは0.01%以上含有させることが望ましい。より好ましくは0.04%以上、さらに好ましくは0.06%以上である。しかし、Ni含有量が多くなりすぎると、加熱炉内でのスケール生成が不均一になり、却って表面欠陥を発生させる原因になる。また、コスト増も招く。このため、Ni含有量は1%以下とする。より好ましくは0.4%以下、さらに好ましくは0.2%以下である。
Crは鋼の焼入れ性を向上させる効果、マルテンサイトや上部/下部ベイナイト中の炭化物生成を抑制する効果から含有することが出来る。このような効果を得るには、Cr含有量は0.01%以上が望ましい。より好ましくは0.03%以上、さらに好ましくは0.06%以上である。ただし、Crを過剰に含有すると耐孔食性が劣化するのでCr含有量は1.0%以下とする。より好ましくは0.8%以下、さらに好ましくは0.4%以下である。
Moは鋼の焼入れ性を向上させる効果、マルテンサイトや上部/下部ベイナイト中の炭化物生成を抑制する効果から含有することが出来る。このような効果を得るには、Mo含有量は0.01%以上が好ましい。より好ましくは0.03%以上、さらに好ましくは0.06%以上である。ただし、Moは冷延鋼板の化成処理性を著しく劣化させるので、その含有量は0.5%以下とすることが好ましい。化成処理性向上の観点からはMoは0.15%以下とすることがさらに好ましい。
Vは鋼の焼入れ性を向上させる効果、マルテンサイトや上部/下部ベイナイト中の炭化物生成を抑制する効果、組織を微細化する効果、炭化物を析出させ耐遅れ破壊特性を改善する効果から含有することが出来る。その効果を得るためにはV含有量は0.003%以上が望ましい。より好ましくは0.005%以上、さらに好ましくは0.010%以上である。ただし、Vを多量に含有すると鋳造性が著しく劣化するのでV含有量は0.5%以下が望ましい。より好ましくは0.3%以下、さらに好ましくは0.1%以下である。
Nbは鋼組織を微細化し高強度化する効果、細粒化を通じてベイナイト変態を促進する効果、曲げ性を改善する効果、耐遅れ破壊特性を向上させる効果から含有することが出来る。その効果を得るためにはNb含有量は0.002%以上が望ましい。より好ましくは0.004%以上、さらに好ましくは0.010%以上である。ただし、Nbを多量に含有すると析出強化が強くなりすぎ延性が低下する。また、圧延荷重の増大、鋳造性の劣化を招く。このため、Nb含有量は0.1%以下が望ましい。より好ましくは0.05%以下、さらに好ましくは0.03%以下である。
Zrは鋼の焼入れ性の向上効果、ベイナイト中の炭化物生成を抑制する効果、組織を微細化する効果、炭化物を析出させ耐遅れ破壊特性を改善する効果から含有することができる。そのような効果を得るためにはZr含有量は0.005%以上が望ましい。より好ましくは0.008%以上、さらに好ましくは0.010%以上である。ただし、Zrを多量に含有すると、熱間圧延前のスラブ加熱時に未固溶で残存するZrNやZrSといった粗大な析出物が増加し、耐遅れ破壊特性が劣化する。このため、Zr含有量は0.2%以下が望ましい。より好ましくは0.15%以下、さらに好ましくは0.08%以下である。
Wは鋼の焼入れ性の向上効果、ベイナイト中の炭化物生成を抑制する効果、組織を微細化する効果、炭化物を析出させ耐遅れ破壊特性を改善する効果から含有することができる。そのような効果を得るためにはW含有量は0.005%以上が望ましい。より好ましくは0.008%以上、さらに好ましくは0.010%以上である。ただし、Wを多量に含有させると、熱間圧延前のスラブ加熱時に未固溶で残存するWNやWSといった粗大な析出物が増加し、耐遅れ破壊特性が劣化する。このため、W含有量は0.2%以下が望ましい。より好ましくは0.15%以下、さらに好ましくは0.08%以下である。
Caは、SをCaSとして固定し、曲げ性の改善や耐遅れ破壊特性の改善に寄与する。このため、Ca含有量は0.0002%以上とすることが好ましい。より好ましくは0.0005%以上、さらに好ましくは0.0010%以上である。ただし、Caは多量に添加すると表面品質や曲げ性を劣化させるので、Ca含有量は0.0040%以下とすることが望ましい。より好ましくは0.0035%以下、さらに好ましくは0.0020%以下である。
Ceも、Caと同様、Sを固定し、曲げ性の改善や耐遅れ破壊特性の改善に寄与する。このため、Ce含有量は0.0002%以上とすることが好ましい。より好ましくは0.0004%以上、さらに好ましくは0.0006%以上である。ただし、Ceを多量に添加すると表面品質や曲げ性が劣化するので、Ce含有量は0.0040%以下とすることが望ましい。より好ましくは0.0035%以下、さらに好ましくは0.0020%以下である。
Laも、Caと同様、Sを固定し、曲げ性の改善や耐遅れ破壊特性の改善に寄与する。このため、La含有量は0.0002%以上とすることが好ましい。より好ましくは0.0004%以上、さらに好ましくは0.0006%以上である。ただし、Laを多量に添加すると表面品質や曲げ性が劣化するので、La含有量は0.0040%以下とすることが望ましい。より好ましくは0.0035%以下、さらに好ましくは0.0020%以下である。
MgはMgOとしてOを固定し、耐遅れ破壊特性の改善に寄与する。このため、Mg含有量は0.0002%以上とすることが好ましい。より好ましくは0.0004%以上、さらに好ましくは0.0006%以上である。ただし、Mgを多量に添加すると表面品質や曲げ性が劣化するので、Mg含有量は0.0030%以下とすることが望ましい。より好ましくは0.0025%以下、さらに好ましくは0.0010%以下である。
Sbは、鋼板表層部の酸化や窒化を抑制し、それによるCやBの表層における含有量の低減を抑制する。また、CやBの含有量の上記低減が抑制されることで、鋼板表層部のフェライト生成を抑制し、高強度化するとともに、耐遅れ破壊特性が改善する。このような観点から、Sb含有量は0.002%以上が望ましい。より好ましくは0.004%以上、さらに好ましくは0.006%以上である。ただし、Sb含有量が0.1%を超えると、鋳造性が劣化し、また、旧γ粒界に偏析して、せん断端面の耐遅れ破壊特性は劣化する。このため、Sb含有量は0.1%以下が望ましい。より好ましくは0.04%以下、さらに好ましくは0.03%以下である。
Snは、鋼板表層部の酸化や窒化を抑制し、それによるCやBの表層における含有量の低減を抑制する。また、CやBの含有量の上記低減が抑制されることで、鋼板表層部のフェライト生成を抑制し、高強度化するとともに、耐遅れ破壊特性が改善する。このような観点から、Sn含有量は0.002%以上が望ましい。より好ましくは0.004%以上、さらに好ましくは0.006%以上である。ただし、Sn含有量が0.1%を超えると、鋳造性が劣化する。また、旧γ粒界にSnが偏析して、せん断端面の耐遅れ破壊特性が劣化する。このため、Sn含有量は0.1%以下が望ましい。より好ましくは0.04%以下、さらに好ましくは0.03%以下である。
高い延性を確保するために、フェライトは面積率で6%以上とする。より好ましくは8%以上、さらに好ましくは11%以上である。一方、所定の強度を得るためにフェライトは面積率で80%以下とする。より好ましくは50%以下、さらに好ましくは20%未満でさらには15%未満とするのがよい。ここで、フェライトはポリゴナルなフェライトを指す。
所定の強度、延性、伸びフランジ成形性を確保するためにこれらの面積率は20~94%とする。下限についてより好ましくは50%以上、さらに好ましくは80%超であり、さらには85%超とするのが良い。上限についてより好ましくは92%以下、さらに好ましくは89%以下である。上部ベイナイト、フレッシュマルテンサイト、焼戻しマルテンサイト、下部ベイナイト、残留γの面積率をSEM写真で、各組織の含有量は次の範囲にあることが多いと考えられる。上部ベイナイトは面積率で3~20%である。焼戻しマルテンサイトは面積率で5~80%である。下部ベイナイトは面積率で0~50%である。
高い延性を確保するために、鋼組織全体に対して残留γは体積率で7%以上とする。より好ましくは9%以上、さらに好ましくは10%以上である。この残留γ量には、上部ベイナイトに隣接して生成する残留γとマルテンサイトや下部ベイナイトに隣接して生成する残留γの両者を含む。残留γの量が増加しすぎると強度低下、伸びフランジ成形性の低下、耐遅れ破壊特性の劣化を招く。したがって、残留γの体積率は20%以下とする。より好ましくは15%以下であり、また、「体積率」は「面積率」とみなすことができる。
後述する製造方法において、冷却過程の470~405℃の中間温度域で保持することで、炭化物をほとんど含まない上部ベイナイト(ベイニティックフェライト)に隣接して生成するプレート状の残留γUBを得ることができる。このプレート状の残留γUB粒子は、粒子幅が0.18~0.60μm、粒子長さが1.7~7.0μm、アスペクト比が5~15である。この残留γUBを生成させることで、その生成量が微量であっても延性が向上する。その効果は、残留γUBの面積率:SγUBが0.2%以上確保されることで得られる。したがってSγUBは0.2%以上とする。SγUBを0.3%以上とすることで、延性は著しく上昇するので、SγUBは0.3%以上とすることがさらに望ましい。より好ましくは0.4%以上である。ここで注意すべき点は、粒子幅、粒子長さ、アスペクト比が同一の鋼組織であってもC濃化量が少ない場合は、フレッシュマルテンサイトとなり、延性の向上に対する寄与が著しく小さいばかりか伸びフランジ成形性を著しく劣化させる。この組織は所謂MAと称される組織の一つであり、本規定の組織は、Cが顕著に濃化した安定なγでありこのMAとは異なり区別しなければならない。このため、後述するように本組織はEBSDでfcc構造であることを確認したもののみを対象とする。また、このプレート状の残留γUBが多くなりすぎると、炭素の消費量が多くなりすぎ、大幅な強度低下が生じる。また、伸びフランジ成形性の低下や耐遅れ破壊特性の劣化を招く。したがって、SγUBは5%以下とする。より好ましくは4%以下、さらに好ましくは3%以下である。なお、上記面積率は、鋼組織全体における面積率を意味する。なお、残留γUBの面積率は、EBSDを用いてフェーズマップデータを得、fcc構造の組織を対象に測定し、他の金属相(bcc系)から区別しうる。
従来、最終テンパー工程でベイナイト変態を多く生じさせようとする場合、塊状のマルテンサイトもしくは塊状の残留γが多く残存する。そこで、従来、これを防ぐために、Mnを2%以下に低減してベイナイト変態を促進したり、γ単相から急冷してベイナイト変態を促進していた。しかしながら、Mn含有量を低減すると残留γの安定化効果や体積率増加効果が失われることによって、またγ単相から急冷して組織全面をベイナイト変態させるとフェライトが生成していないことによって、延性が損なわれていた。これに対して、本発明では、Mnを多く含む鋼板を2相域焼鈍した場合でもベイナイト変態の利用と塊状組織の低減の両者が可能である。この伸びフランジ成形性に悪影響する塊状組織は、円相当粒子直径が1.5~15μmでありアスペクト比が3以下のフレッシュマルテンサイトおよび円相当粒子直径が1.5~15μmでありアスペクト比が3以下の残留γ粒子であり、その合計面積率:SγBlockを3%以下に低減することで優れた伸びフランジ性成形を確保できる。優れた伸びフランジ性成形を確保するためにSγBlockは2%未満とすることが一層好ましい。また、SγBlockは0%でもよい。なお、円相当粒子直径が1.5~15μmでありアスペクト比が3以下のフレッシュマルテンサイト、円相当粒子直径が1.5~15μmでありアスペクト比が3以下の残留γ粒子のいずれか一方のみ含む場合には、その含まれるものの面積率を合計面積率とする。
例えば、ベイナイト変態を利用するためにγ単相域焼鈍を行った場合、その後に15℃/以下の緩冷却が行われると、冷却中にポリゴナルフェライトが生成し、それに付随して必然的にポリゴナルフェライト内部に分布するアスペクト比が3以下で円相当粒子直径が0.15~1.9μmの円~楕円状の微細なフレッシュマルテンサイトおよび/または残留γがフェライト粒内に生成する。この組織には残留γを十分に安定するC量(約1%)には達しないが、緩冷却中に生じる一定量のC、Mnの濃化が避けられないのでMs点が低下して、焼き戻し前までにマルテンサイト変態させることができず、最終冷却時にフレッシュマルテンサイトとなるかそのままC濃化量の少ない残留γとなる。このような組織は極めて硬質なマルテンサイトもしくは残留γの場合はわずかな塑性変形後に極めて硬質なマルテンサイトに変態するため、粒子直径が2μm未満であってもλの低下をもたらす。その影響はフレッシュマルテンサイトおよび/または残留γ粒子の合計個数NMAとポリゴナルフェライト粒の合計個数NPの比率NMA/NPが0.3以下で軽減されるので、NMA/NPはこの範囲とする。また、比率NMA/NPは0でもよい。フレッシュマルテンサイトおよび残留γ粒子が含まれない場合に比率NMA/NPは0になる。なお、アスペクト比が3以下で円相当粒子直径が0.15~1.9μmのフレッシュマルテンサイト、アスペクト比が3以下で円相当粒子直径が0.15~1.9μmの残留γ粒子のいずれか一方を含む場合には、その含むものの個数を合計個数NMAとする。
周囲よりもC濃度が高い領域の面積率を調整することで、延性を向上させることができる。具体的には、C濃度が0.7~1.3%であり、隣接領域のC濃度が0.07%以下である領域の合計面積率:SC濃化を0.2~5%とすることで延性が高められる。なお、隣接領域とは、C濃度が0.7~1.3%であり、隣接領域のC濃度が0.07%以下である領域と隣合う領域を意味する。
本発明ではα+γ2相域での焼鈍を前提としており、好ましくは、2相域焼鈍時に生じるフェライト領域からオーステナイト領域への均一なMn濃化を利用して、さらなる延性の向上を図る。このように、2相域焼鈍で均一にγ領域にMnを分配することで、プレート状の残留γUB、フィルム状の残留γ両者の存在する領域でMn濃化による残留γの安定化効果が得られ、さらなる延性の向上に寄与する。その効果は、ポリゴナルフェライト以外の上部ベイナイト、フレッシュマルテンサイト、焼戻しマルテンサイト、下部ベイナイト、残留γからなる領域のMn濃度:Mnγ2ndと鋼板の平均Mn濃度:MnBulkの比Mnγ2nd/MnBulkを1.1以上とすることで得られる。上限については、2.0以下が好ましく、より好ましくは1.5以下である。
鋼スラブを熱間圧延するには、スラブを加熱後圧延する方法、連続鋳造後のスラブを加熱することなく直接圧延する方法、連続鋳造後のスラブに短時間加熱処理を施して圧延する方法などがある。熱間圧延は、常法にしたがって実施すればよく、例えば、スラブ加熱温度は1100~1300℃、均熱温度は20~300min、仕上圧延温度はAr3変態点~Ar3変態点++200℃、巻取温度は400~720℃とすればよい。巻取温度は、板厚変動を抑制し高い強度を安定して確保する観点かは、450~550℃とするのが好ましい。板厚変動を抑制し高い強度を安定して確保する観点からより好ましい範囲は、460~550℃であり、さらに好ましい範囲は500~550℃である。
冷間圧延では、圧延率を30~85%とすればよい。高い強度を安定して確保し、異方性を小さくする観点からは、圧延率は45~85%にすることが好ましい。なお、圧延荷重が高い場合は、450~730℃でCAL、BAFにて軟質化の焼鈍処理をすることが可能である。
所定の成分組成を有する鋼スラブを、熱間圧延および冷間圧延した後、連続焼鈍ライン(CAL)において以下に規定の条件で焼鈍を施す。なお、図3は、製造条件の一例を図示したものである。
所定の面積率の焼き戻しマルテンサイトおよび/またはベイナイト、所定の体積率の残留γを確保するために、焼鈍温度は780~880℃とする。ポリゴナルなフェライトを6%以上確保するために、焼鈍温度は成分に応じてα+γの2相域焼鈍となるように調整する。この2相域焼鈍でMnをγに均一に濃化させることができ、γの安定化に寄与する。また、冷却中にMnをγに濃化させる必要が無いので、ポリゴナルフェライト内部に分布するアスペクト比が3以下で円相当粒子直径が0.15~1.9μmのフレッシュマルテンサイトまたは残留γ粒子の生成を抑制することが出来、λの向上に寄与する。
焼鈍後、780~470℃の温度範囲を平均冷却速度:5.0~2000℃/sで冷却する。平均冷却速度が5.0℃/sより遅いと、粗大なベイニティックフェライトが生成し、塊状組織の増大をもたらす。このため、5.0℃/s以上とする。より好ましくは8.0℃/s以上である。一方、平均冷却速度が速くなりすぎると、板形状が悪化するので、2000℃/s以下とする。好ましくは100℃/s以下である。より好ましくは30℃/s未満である。また、29℃/s以下とすることで、板形状を良好なレベル(後述する実施例に記載の板反りを15mm以下)とすることができるため好ましい。さらには、上記平均冷却速度を14℃/s以下とすることで板形状をより良好なレベル(後述する実施例に記載の板反りを10mm以下)とすることができるためより好ましい。
この温度域で所定時間保持することで、炭化物析出をほとんど生じない上部ベイナイトを生成させることが可能であり、それに隣接してCの濃化量の高いプレート状の残留γUBを生成させることが出来る。延性の向上に寄与するプレート状の残留γUBを所定量生成させるためにこの温度域での保持時間は14sec以上とする。プレート状の残留γUBを生成させ、延性を向上させる観点からは、この温度域での保持時間は、18sec以上とすることがさらに好ましい。一方、保持時間が200secを超えて保持してもプレート状のγUBの生成は停滞し、200secを超えて保持すると、塊状の未変態γへの炭素濃化が進行し、塊状組織の残存量の増加を招く。したがって、470~405℃の温度範囲での保持時間は14~200secとする。伸びフランジ成形性を向上させる観点からは、470~405℃の温度範囲での保持時間は100sec以下とすることが好ましい。なお、この温度域での保持は、この温度範囲での平均冷却速度を4.6℃/s以下に低減することに対応する。
さらに、405℃から(A)式で表される冷却停止温度:Tsqまでの温度範囲を平均冷却速度:5.0~80℃/sで冷却する。この温度域の平均冷却速度が遅いとCが未変態γに濃縮し、塊状組織の増加を招く。また、炭化物析出が進行してCが浪費され、延性の低下を招く。塊状組織の低減による伸びフランジ成形性の向上、炭化物析出の抑制による延性の向上の観点から、この温度域の平均冷却速度は7.0℃/s以上とすることがさらに好ましい。冷却速度が80℃/sを超えると、冷却中のマルテンサイトや下部ベイナイトからフィルム状γへのCの拡散が抑制され、その生成が抑制され、延性が低下する。このため、この温度域の平均冷却速度は5.0~80℃/sとする。冷却中のマルテンサイトや下部ベイナイトからフィルム状γへのCの拡散を促進する観点からはこの温度域の平均冷却速度は15℃/s以下とすることが望ましい。
Ms=539-474×[%C]/(100-VF)×100-30.4×[%Mn]×1.2-12.1×[%Cr]-7.5×[%Mo]-17.7×[%Ni]・・・(B)
[%C]、[%Mn]、[%Cr]、[%Mo]、[%Ni]はC、Mn、Cr、Mo、Niそれぞれの含有量(質量%)を表し、含まない場合は0とする。VFはフェライトの面積率(%)を表す。
さらに冷却停止温度から370℃までの温度範囲を短時間で加熱することで炭化物析出を抑えて高い延性を確保することが出来る。また、冷却して生成したマルテンサイトもしくは下部ベイナイトを核に370℃以上に再加熱した際に上部ベイナイトが生成する。370℃までの平均加熱速度が遅いと、これらの効果が得られなくなる。その結果、残留γ量が減少して延性が低下する。このため、冷却停止温度から370℃までの温度範囲の平均加熱速度は3℃/s以上とする。炭化物析出を抑制する観点、再加熱時に上部ベイナイトを生成させる観点からは、平均加熱速度は5℃/s以上とすることが望ましく、10℃/s以上とすることがさらに好ましい。上記平均加熱速度の上限は特に限定されないが50℃/s以下が好ましく、より好ましくは30℃/s以下である。
中間保持により生成したプレート状残留γUBやマルテンサイトや下部ベイナイトに隣接して生成したフィルム状残留γにCを分配させてこれらを安定化させる観点、未変態γとして塊状に分布している領域をベイナイト変態により細分化し、λを向上させる観点から、300~550℃の温度域で30~3000sec保持する。
Claims (13)
- 質量%で、
C:0.06~0.25%、
Si:0.6~2.5%、
Mn:2.3~3.5%、
P:0.02%以下、
S:0.01%以下、
sol.Al:0.50%未満、
N:0.015%未満を含有し、残部が鉄および不可避的不純物からなる成分組成と、
面積率でフェライト:6~80%、上部ベイナイト、フレッシュマルテンサイト、焼戻しマルテンサイト、下部ベイナイト、残留γの1種もしくは2種以上からなる組織:20~94%、体積率で残留γ:7~20%を含み、粒子幅が0.18~0.60μm、粒子長さが1.7~7.0μm、アスペクト比が5~15である残留γUBの面積率:SγUBが0.2~5%であり、円相当粒子直径が1.5~15μm、アスペクト比が3以下のフレッシュマルテンサイトおよび/または円相当粒子直径が1.5~15μm、アスペクト比が3以下の残留γ粒子の合計面積率:SγBlockが3%以下(0%を含む)である鋼板。 - ポリゴナルフェライト内部に分布し、アスペクト比が3以下で円相当粒子直径が0.15~1.9μmのフレッシュマルテンサイトおよび/またはアスペクト比が3以下で円相当粒子直径が0.15~1.9μmの残留γ粒子の合計個数NMAとポリゴナルフェライト粒の合計個数NPの比率NMA/NPが0.3以下である請求項1に記載の鋼板。
- 前記組織において、C濃度が0.7~1.3%であり、隣接領域のC濃度が0.07%以下である領域の合計面積率:SC濃化が0.2~5%である請求項1または2に記載の鋼板。
- C濃度が0.7~1.3%であり、隣接領域のC濃度が0.07%以下である前記領域は、残留γである請求項3に記載の鋼板。
- C濃度が0.7~1.3%であり、隣接領域のC濃度が0.07%以下である前記領域は、残留γUB粒子である請求項3に記載の鋼板。
- 前記隣接領域が上部ベイナイトを含む請求項3~5のいずれかに記載の鋼板。
- ポリゴナルフェライト以外である残部の、上部ベイナイト、フレッシュマルテンサイト、焼戻しマルテンサイト、下部ベイナイト、残留γからなる領域のMn濃度:Mnγ2ndと鋼板の平均Mn濃度:MnBulkの比Mnγ2nd/MnBulkが1.1以上である請求項1~6のいずれかに記載の鋼板。
- 前記成分組成が、さらに、質量%で、
Ti:0.002~0.1%、
B:0.0002~0.01%のうちから選んだ1種または2種以上を含有する請求項1~7のいずれかに記載の鋼板。 - 前記成分組成が、さらに、質量%で、
Cu:0.005~1%、
Ni:0.01~1%、
Cr:0.01~1.0%、
Mo:0.01~0.5%、
V:0.003~0.5%、
Nb:0.002~0.1%、
Zr:0.005~0.2%およびW:0.005~0.2%のうちから選んだ1種または2種以上を含有する、請求項1~8のいずれかに記載の鋼板。 - 前記成分組成が、さらに、質量%で、
Ca:0.0002~0.0040%、
Ce:0.0002~0.0040%、
La:0.0002~0.0040%、
Mg:0.0002~0.0030%、
Sb:0.002~0.1%およびSn:0.002~0.1%のうちから選んだ1種または2種以上を含有する、請求項1~9のいずれかに記載の鋼板。 - 前記成分組成が、さらに、質量%で、W:0.005~0.2%を含有する、請求項1~10のいずれかに記載の鋼板。
- 引張強度が780MPa以上1450MPa以下である請求項1~11のいずれかに記載の鋼板。
- 請求項1、8~11のいずれかに記載の成分組成を有する鋼スラブを、熱間圧延および冷間圧延した後、冷延鋼板を、連続焼鈍ライン(CAL)において780~880℃の焼鈍温度で焼鈍し、次いで780~470℃の温度範囲を平均冷却速度:5.0~2000℃/sで冷却したのち、470~405℃の温度範囲で14~200sec保持し、さらに405℃から(A)式で表される冷却停止温度:Tsqまでの温度範囲を平均冷却速度:5.0~80℃/sで冷却し、さらに冷却停止温度から370℃までの温度範囲を平均加熱速度:3℃/s以上で加熱を行い、300~550℃で30~3000sec保持した後、室温まで冷却する鋼板の製造方法。
ここで、冷却停止温度Tsq(℃)はMs-90≧Tsq≧Ms-180・・・(A)
Ms=539-474×[%C]/(100-VF)×100-30.4×[%Mn]×1.2-12.1×[%Cr]-7.5×[%Mo]-17.7×[%Ni]・・・(B)
[%C]、[%Mn]、[%Cr]、[%Mo]、[%Ni]はC、Mn、Cr、Mo、Niそれぞれの含有量(質量%)を表し、含まない場合は0とし、VFはフェライトの面積率(%)を表す。
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US16/604,398 US11390930B2 (en) | 2017-04-14 | 2018-04-13 | Steel sheet and manufacturing method therefor |
CN201880024914.8A CN110546291B (zh) | 2017-04-14 | 2018-04-13 | 钢板及其制造方法 |
EP18784256.2A EP3611285B1 (en) | 2017-04-14 | 2018-04-13 | Steel sheet and manufacturing method therefor |
MX2019012250A MX2019012250A (es) | 2017-04-14 | 2018-04-13 | Lamina de acero y metodo de fabricacion asociado. |
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CN114375343B (zh) * | 2019-09-17 | 2023-02-28 | 株式会社神户制钢所 | 高强度钢板及其制造方法 |
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KR20230084534A (ko) | 2020-11-11 | 2023-06-13 | 닛폰세이테츠 가부시키가이샤 | 강판 및 그 제조 방법 |
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KR20240121797A (ko) | 2022-01-13 | 2024-08-09 | 닛폰세이테츠 가부시키가이샤 | 용융 아연 도금 강판 및 그 제조 방법 |
Also Published As
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JPWO2018190416A1 (ja) | 2019-04-25 |
EP3611285A1 (en) | 2020-02-19 |
WO2018189950A1 (ja) | 2018-10-18 |
KR20190127831A (ko) | 2019-11-13 |
EP3611285A4 (en) | 2020-02-26 |
US20200157647A1 (en) | 2020-05-21 |
CN110546291A (zh) | 2019-12-06 |
CN110546291B (zh) | 2021-07-30 |
KR102284522B1 (ko) | 2021-07-30 |
MX2019012250A (es) | 2019-11-28 |
EP3611285B1 (en) | 2021-02-24 |
JP6439903B1 (ja) | 2018-12-19 |
US11390930B2 (en) | 2022-07-19 |
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