WO2020080402A1 - 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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- WO2020080402A1 WO2020080402A1 PCT/JP2019/040663 JP2019040663W WO2020080402A1 WO 2020080402 A1 WO2020080402 A1 WO 2020080402A1 JP 2019040663 W JP2019040663 W JP 2019040663W WO 2020080402 A1 WO2020080402 A1 WO 2020080402A1
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Definitions
- the present invention relates to a steel sheet and a manufacturing method thereof, which can be preferably applied to press forming used in 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 technology for improving the ductility of the steel sheet.
- Patent Document 1 discloses that a steel containing C: 0.10 to 0.45%, S: 0.5 to 1.8% and 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 (tensile strength) of 80 kgf / mm 2 or more by holding for 1 to 30 minutes to generate residual ⁇ . There is.
- Patent Document 2 discloses that steel containing C: 0.10 to 0.25%, Si: 1.0 to 2.0%, and Mn: 1.5 to 3.0% is annealed at 10 ° C./s or more after annealing. By cooling to 450-300 °C for 180-600 seconds and controlling the space factor of retained austenite to 5% or more, bainitic ferrite to 60% or more, and polygonal ferrite to 20% or less, ductility : El and stretch flange formability: It is disclosed that a steel sheet excellent in ⁇ can be obtained.
- Patent Document 3 a steel sheet having a specific component composition is annealed, cooled to a temperature range of 150 to 350 ° C., and then reheated to about 400 ° C. and held, thereby removing ferrite, tempered martensite, and retained austenite. It is disclosed that a structure containing it can be obtained, and high ductility and high stretch flange formability can be imparted to the steel sheet. This is a so-called Q & P in which the temperature is once cooled 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 reheated and held to stabilize the 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 processing is improved. That is, steel having a specific component composition is annealed at a temperature of Ae3-10 ° C. or more in order to reduce the amount of polygonal ferrite to 5% or less, and then cooled at a relatively high temperature of Ms-10 ° C. to Ms-100 ° C. Is stopped to generate upper bainite when reheated to around 400 ° C. to obtain high ductility and high stretch flange formability.
- Patent Document 5 discloses a method of obtaining a steel sheet excellent in ductility and low temperature toughness by utilizing bainite produced at low temperature and bainite produced at high temperature. That is, after annealing a steel containing C: 0.10 to 0.5%, it 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 seconds to obtain a low temperature range. Of bainite is produced, reheated to a temperature range of more than 400 ° C. and 540 ° C. or less and held for 50 seconds or longer to form bainite in a high temperature region, and to obtain a steel sheet excellent in ductility and low temperature toughness.
- Patent Document 1 has a problem in that although the El is excellent, the stretch flange formability is extremely low.
- bainitic ferrite is mainly used as a microstructure, and ferrite is suppressed to a small amount, so the stretch flange formability is excellent, but the ductility is not necessarily high. Therefore, further improvement in ductility has been required in consideration of application to difficult-to-mold parts.
- Patent Document 3 achieves relatively high ductility and high stretch flange formability compared to conventional TRIP steel and steel utilizing bainitic ferrite.
- breakage was observed in molding with difficult-to-mold parts such as center pillars, and further improvement in ductility was required.
- the local ductility (L.El) which is an index of ductility when locally deformed at the flange end face and the like, is recognized to be deteriorated in the conventional TRIP steel, and it is necessary to improve it.
- the amount of polygonal ferrite produced is reduced to reduce massive martensite, and sufficient ductility cannot be ensured.
- the cooling stop temperature is set relatively high in order to improve El, and a large amount of untransformed ⁇ remains at the time of cooling stop, so that massive martensite is likely to remain.
- the conventional technology has not been able to obtain a steel sheet that has sufficiently high ductility and high stretch flange formability.
- the present invention has been made to solve such a problem, and provides a steel sheet having extremely high ductility and excellent stretch flange formability even when it has a tensile strength of 780 to 1450 MPa class, and a manufacturing method thereof. It is what
- steel sheet here also includes galvanized steel sheet whose surface is galvanized.
- the inventors of the present invention have earnestly studied means for providing extremely high ductility and excellent stretch flange formability, and have reached the following conclusions.
- (1) a cause of insufficient stretch flange formability in TRIP steel subjected to austempering, and (2) a cause of insufficient ductility in steel using Q & P were examined.
- the cause of (1) is considered as follows. Once the alms were TRIP steels austempering, carbon during austempering around 400 ° C. is diffused into untransformed austenite bainite, the amount of carbon in austenite in the free energy is equal T 0 the composition of the bcc phase and the fcc phase approaches The bainite transformation slows down. Due to this stagnation of transformation, hard martensite in which carbon is concentrated only up to around T 0 composition and a lumpy structure composed of residual ⁇ remain.
- the cause of (2) is considered as follows. In the steel using Q & P, the bulk structure can be reduced by sufficiently lowering the cooling stop temperature, but the precipitation of carbides in martensite and the stabilization of carbon hinder the supply of carbon to the austenite phase, and ⁇ is not sufficiently stabilized.
- cooling is performed from 310 ° C. to a cooling stop temperature in the range of 220 ° C. or more and less than 255 ° C., and the remaining untransformed ⁇ region is divided by martensitic transformation or lower bainite transformation to reduce a lumpy structure.
- the ductility index El is set to TS ⁇ El ⁇ 17,000 MPa%, more preferably TS ⁇ El ⁇ 18000 MPa%, further preferably TS ⁇ El ⁇ 19000 MPa%, and stretch flange forming is further performed.
- the upper bainite transformation is utilized before the martensitic transformation, and the two-stage cooling treatment of controlling the residual amount of the remaining massive structure by the Q & P treatment is performed, and the softening is performed in the temperature range of 310 to 255 ° C.
- the two-stage cooling treatment of controlling the residual amount of the remaining massive structure by the Q & P treatment
- the softening is performed in the temperature range of 310 to 255 ° C.
- “high ductility and extremely excellent stretch flange formability are compatible” means TS ⁇ El ⁇ 17,000 MPa%, ⁇ ⁇ 50% or more at TS: 780 to 1319 MPa, ⁇ ⁇ 40% at TS: 1320 to 1450 MPa.
- “higher strength” means TS ⁇ 780 MPa.
- the present invention was made based on the above findings, and specifically provides the following. [1] C: 0.06 to 0.25%, Si: 0.6 to 2.5%, Mn: 2.3 to 3.5%, P: 0.02% or less, S: 0.01% or less, sol.
- S ⁇ Fine is from 0.4 to 5.0% equivalent circle particles
- S ⁇ Block is 4% The following (including 0%) steel sheet.
- the steel sheet according to [5], wherein the C concentration is 0.6 to 1.3% and the C concentration of the adjacent region is 0.07% or less is residual ⁇ UB particles.
- the component composition further contains, in mass%, one or two selected from Ti: 0.002 to 0.1% and B: 0.0002 to 0.01% [1. ]
- the composition of the components is such that, in mass%, Cu: 0.005 to 1%, Ni: 0.01 to 1%, Cr: 0.01 to 1.0%, Mo: 0.01 to 0.
- the steel sheet according to any one of [1] to [8], which contains one or more selected types.
- the composition of the components is, in mass%, Ca: 0.0002 to 0.0040%, Ce: 0.0002 to 0.0040%, La: 0.0002 to 0.0040%, Mg: 0. 0.002 to 0.0030%, Sb: 0.002 to 0.1%, Sn: 0.002 to 0.1%, and one or more selected from [1] to [9]
- the steel plate according to any one of 1.
- Annealing temperature of °C °C then cooling the temperature range of 810 ⁇ 700 °C average cooling rate: 1 ⁇ 2000 °C / s, further 700 ⁇ 490 °C average cooling rate: 10 ⁇ 2000 °C / s
- After cooling at 490 to 405 ° C for 10 to 200 seconds further cooling the temperature range of 405 to 310 ° C at an average cooling rate of 10 to 100 ° C / s, and then the temperature range of 310 to 255 ° C. Is cooled at an average cooling rate of 0.4 ° C./s or more and less than 20 ° C./s.
- a cooling stop temperature in the range of 255 ° C.
- FIG. 1 is a diagram showing an example of an SEM image.
- FIG. 2 is a diagram for explaining the aspect ratio, particle width, and particle length.
- FIG. 3 is a diagram showing an example of a graph showing the relationship between the C concentration and the analysis length.
- the steel sheet of the present invention has a specific composition and a specific steel structure. Therefore, the steel sheet of the present invention will be described in the order of composition and steel structure.
- the steel sheet of the present invention contains the following components.
- “%”, which is a unit of the content of components, means “mass%”.
- C 0.06 to 0.25%
- C is a viewpoint that the area ratio of the tempered martensite is secured and a predetermined strength is secured, a volume ratio of the residual ⁇ is secured and the ductility is improved, and it is concentrated in the residual ⁇ to stabilize the residual ⁇ . It is contained from the viewpoint of improving the 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 sufficiently secured, so the lower limit is made 0.06%. It is preferably 0.09% or more, and more preferably 0.11% or more.
- 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 to increase strength, suppressing the formation of carbides in martensite and bainite, and improving the stability of residual ⁇ to improve ductility.
- the Si content is set to 0.6% or more.
- the Si content is preferably 0.8% or more. It is more preferably 1.1% or more.
- the Si content exceeds 2.5%, the rolling load becomes extremely high, which makes it difficult to manufacture a thin plate. Further, the chemical conversion processability and the toughness of the welded portion are deteriorated. Therefore, the Si content is 2.5% or less.
- the Si content is preferably less than 2.0% from the viewpoint of chemical conversion treatability and securing the toughness of the material and the 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 2.3-3.5%
- Mn is the same as Si, from the viewpoint of securing tempering martensite and / or bainite having a predetermined area ratio to secure the strength, from the viewpoint of stabilizing the residual ⁇ by improving the Ms point of the residual ⁇ and improving the ductility. It is an important element from the viewpoint of suppressing the formation of carbide in bainite to improve the ductility, and from the viewpoint of increasing the volume ratio of residual ⁇ to improve the ductility. 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. It is preferably 2.6% or more, more preferably 2.7% or more.
- the Mn content is set to 3.5% or less. From the viewpoint of promoting the bainite transformation and ensuring high ductility, the Mn content is preferably 3.2% or less. It is more preferably 3.1% or less.
- P 0.02% or less
- P is an element that strengthens steel, but if its content is large, spot weldability deteriorates. Therefore, P is 0.02% or less. From the viewpoint of improving spot weldability, P is preferably 0.01% or less. Although P may not be contained, 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 that has. In order to reduce these adverse effects, at least S is 0.01% or less.
- S since the contents of C, Si and Mn are very high, the spot weldability is likely to deteriorate, and S is preferably 0.0020% or less from the viewpoint of improving the spot weldability, and further 0.0010%. More preferably, it is less than%.
- S may not be contained, the S content is preferably 0.0001% or more from the viewpoint of manufacturing cost. It is more preferably 0.0005% or more.
- sol. Al less than 0.50% Al is contained for deoxidation or for the purpose of stabilizing residual ⁇ as a substitute for Si. sol.
- the lower limit of Al is not specified, but 0.01% or more is desirable for stable deoxidation.
- Al is less than 0.50%.
- sol. Al is more preferably less than 0.20%, and even 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 reduces the hot ductility of steel and the surface quality. In addition, in the steel containing B, there is an adverse effect that the effect of B disappears through the formation of BN. When the N content is 0.015% or more, the surface quality is significantly deteriorated. Therefore, the N content is less than 0.015%. It is preferably 0.010% or less. Although N may not be contained, the N content is preferably 0.0001% or more from the viewpoint of manufacturing cost. More preferably, it is 0.001% or more.
- the component composition of the steel sheet of the present invention can appropriately contain the following optional elements in addition to the above components.
- B 0.0002 to 0.01%
- B is an element that improves the hardenability of steel, and has an advantage that it is easy to form tempered martensite and / or bainite having a predetermined area ratio. Further, the residual solid solution B improves the delayed fracture resistance.
- 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. It is more preferably 0.0050% or less. More preferably, it is 0.0030% or less.
- Cu 0.005-1%
- Cu improves the corrosion resistance in the usage environment of the automobile.
- the corrosion product of Cu coats the surface of the steel sheet and has the effect of suppressing hydrogen intrusion into the steel sheet.
- Cu is an element that is mixed when scrap is used as a raw material, and by allowing the mixing of Cu, the recycled material can be used as a raw material and the manufacturing cost can be reduced. From such a viewpoint, it is preferable to contain Cu in an amount of 0.005% or more, and it is more preferable to contain Cu in an amount of 0.05% or more from the viewpoint of improving delayed fracture resistance. More preferably, it is 0.10% or more. However, if the Cu content is too high, surface defects will occur, so the Cu content is preferably 1% or less. It is more preferably 0.4% or less, still more preferably 0.2% or less.
- Ni 0.01-1% Like Cu, Ni is also an element that has the effect of improving corrosion resistance. In addition, Ni has an action of suppressing the occurrence of surface defects that are likely to occur when Cu is contained. Therefore, it is desirable to contain Ni in an amount of 0.01% or more. It is more preferably 0.04% or more, still more preferably 0.06% or more. However, if the Ni content is too high, the scale generation in the heating furnace becomes non-uniform, which rather causes the generation of surface defects. In addition, the cost also increases. Therefore, the Ni content is 1% or less. It is more preferably 0.4% or less, still more preferably 0.2% or less.
- Cr 0.01-1.0% Cr can be contained because of 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 preferably 0.01% or more. It is more preferably 0.03% or more, still more preferably 0.06% or more.
- the Cr content is 1.0% or less. It is more preferably 0.8% or less, still more preferably 0.4% or less.
- Mo 0.01-0.5% Mo can be contained from the effect of improving the hardenability of steel and the effect of suppressing the formation of martensite and carbides in upper / lower bainite.
- the Mo content is preferably 0.01% or more. It is more preferably 0.03% or more, still more preferably 0.06% or more.
- Mo significantly deteriorates the chemical conversion treatment property of the cold rolled steel sheet, its content is preferably 0.5% or less. From the viewpoint of improving chemical conversion treatability, Mo is more preferably 0.15% or less.
- V 0.003 to 0.5%
- V is contained from the effect of improving the hardenability of steel, the effect of suppressing the formation of carbides in martensite and upper / lower bainite, the effect of refining the structure, and the effect of precipitating carbides and improving the delayed fracture resistance. Can be done.
- the V content is preferably 0.003% or more. It is more preferably 0.005% or more, still more preferably 0.010% or more.
- the V content is preferably 0.5% or less. It is more preferably 0.3% or less, still more preferably 0.1% or less.
- Nb 0.002-0.1%
- Nb can be contained from the effect of refining and strengthening the steel structure, the effect of promoting bainite transformation through grain refinement, the effect of improving bendability, and the effect of improving delayed fracture resistance.
- the Nb content is preferably 0.002% or more. It is more preferably 0.004% or more, still more preferably 0.010% or more.
- the Nb content is preferably 0.1% or less. It is more preferably 0.05% or less, still more preferably 0.03% or less.
- Zr 0.005-0.2%
- 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 delayed fracture resistance.
- the Zr content is preferably 0.005% or more. It is more preferably 0.008% or more, still more preferably 0.010% or more.
- the Zr content is preferably 0.2% or less. It is more preferably 0.15% or less, still more preferably 0.08% or less.
- W 0.005-0.2% W can be contained from the effect of improving the hardenability of steel, the effect of suppressing the formation of carbide in bainite, the effect of refining the structure, and the effect of precipitating carbide and improving the delayed fracture resistance.
- the W content is preferably 0.005% or more. It is more preferably 0.008% or more, still more preferably 0.010% or more.
- the W content is preferably 0.2% or less. It is more preferably 0.15% or less, still more preferably 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. Therefore, the Ca content is preferably 0.0002% or more. It is more preferably 0.0005% or more, still more preferably 0.0010% or more. However, if a large amount of Ca is added, the surface quality and bendability are deteriorated, so the Ca content is preferably 0.0040% or less. It is more preferably 0.0035% or less, still more preferably 0.0020% or less.
- Ce 0.0002 to 0.0040%
- the Ce content is preferably 0.0002% or more. It is more preferably 0.0004% or more, still more preferably 0.0006% or more. However, if a large amount of Ce is added, the surface quality and bendability deteriorate, so the Ce content is preferably 0.0040% or less. It is more preferably 0.0035% or less, still more preferably 0.0020% or less.
- La 0.0002 to 0.0040% Like Ca, La also fixes S and contributes to improvement in bendability and delayed fracture resistance. Therefore, the La content is preferably 0.0002% or more. It is more preferably 0.0004% or more, still more preferably 0.0006% or more. However, if a large amount of La is added, the surface quality and bendability deteriorate, so the La content is preferably 0.0040% or less. It is more preferably 0.0035% or less, still more preferably 0.0020% or less.
- Mg 0.0002 to 0.0030% Mg fixes O as MgO and contributes to the improvement of delayed fracture resistance. Therefore, the Mg content is preferably 0.0002% or more. It is more preferably 0.0004% or more, still more preferably 0.0006% or more. However, if a large amount of Mg is added, the surface quality and bendability deteriorate, so the Mg content is preferably 0.0030% or less. It is more preferably 0.0025% or less, still more preferably 0.0010% or less.
- Sb 0.002-0.1% Sb suppresses the oxidation and nitridation of the surface layer of the steel sheet and suppresses the reduction of the content of C and B in the surface layer thereof. Further, by suppressing the above-mentioned reduction of the contents of C and B, it is possible to suppress the generation of ferrite in the surface layer of the steel sheet, increase the strength, and improve the delayed fracture resistance. From such a viewpoint, the Sb content is preferably 0.002% or more. It is more preferably 0.004% or more, still more preferably 0.006% or more.
- the Sb content is preferably 0.1% or less. It is more preferably 0.04% or less, still more preferably 0.03% or less.
- Sn 0.002-0.1% Sn suppresses oxidation and nitridation of the surface layer of the steel sheet, and suppresses the reduction of the content of C and B in the surface layer due to the oxidation and nitridation. Further, by suppressing the above-mentioned reduction of the contents of C and B, it is possible to suppress the generation of ferrite in the surface layer of the steel sheet, increase the strength, and improve the delayed fracture resistance. From such a viewpoint, the Sn content is preferably 0.002% or more. It is more preferably 0.004% or more, still more preferably 0.006% or more. However, if the Sn content exceeds 0.1%, the castability deteriorates.
- the Sn content is preferably 0.1% or less. It is more preferably 0.04% or less, still more preferably 0.03% or less.
- the steel sheet according to the present embodiment contains the above component composition, and the balance other than the above component composition contains Fe (iron) and inevitable impurities.
- the balance is preferably Fe and inevitable impurities.
- Ferrite 5% or less In order to secure a high ⁇ , ferrite has an area ratio of 5% or less. It is more preferably 4% or less, still more preferably 2% or less.
- ferrite refers to polygonal ferrite.
- Microstructure consisting of one or more of upper bainite, fresh martensite, tempered martensite, lower bainite, residual ⁇ : 95 to 100%
- the total area ratio of the upper bainite, the fresh martensite, the tempered martensite, the lower bainite, and the residual ⁇ of the balance other than polygonal ferrite is 95 to 100% in order to secure the predetermined strength, ductility, and stretch flange formability.
- the lower limit is more preferably 96% or more, further preferably 98% or more.
- the area ratios of upper bainite, fresh martensite, tempered martensite, lower bainite, and residual ⁇ were observed by SEM photographs. It is considered that the content of each tissue is often in the following range.
- the upper bainite has an area ratio of 1 to 30%.
- the area ratio of fresh martensite is 0 to 20%.
- the tempered martensite has an area ratio of 3 to 40%.
- the lower bainite has an area ratio of 5 to 70%.
- Residual ⁇ 4-15%
- the residual ⁇ is 4% or more in terms of volume ratio with respect to the entire steel structure. It is more preferably at least 5% and even more preferably at least 7%.
- This residual ⁇ amount includes both the residual ⁇ formed adjacent to the upper bainite and the residual ⁇ formed adjacent to martensite or the lower bainite. If the amount of residual ⁇ is excessively increased, the strength, the stretch flange formability, and the delayed fracture resistance deteriorate. Therefore, the volume ratio of the residual ⁇ is set to 15% or less. It is more preferably 13% or less, and the “volume ratio” can be regarded as the “area ratio”.
- the effect is the area ratio of residual ⁇ UB: S ⁇ UB is obtained by being secured at least 0.2%. Therefore, S ⁇ UB is set to 0.2% or more.
- S ⁇ UB is 0.3% or more, the ductility is remarkably increased. Therefore, it is more preferable that S ⁇ UB is 0.3% or more. It is more preferably 0.4% or more.
- the form of residual ⁇ UB is preferably such that the particle width is 0.25 to 0.60 ⁇ m, the particle length is 1.5 to 15 ⁇ m, and the aspect ratio is 4 to 25.
- S ⁇ UB is set to 7.0% or less. It is more preferably 5.0% or less, still more preferably 4.0% or less.
- the above area ratio means the area ratio in the entire 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.
- the area ratio of ferrite or upper bainite adjacent residual gamma UB: S UB and S ratio GanmaUB is S UB / S ⁇ UB ⁇ 3.5
- Ductility improvement effect of the residual gamma UB can be improved by controlling the area ratio of ferrite or upper bainite generates adjacent the residual gamma UB.
- S UB / S ⁇ UB in order to ensure a high ductility, it is desirable to 3.5 or more. From the viewpoint of improving ductility, the more preferable range of S UB / S ⁇ UB is 4.0 or more.
- the upper limit is not particularly specified, but in the case of this heat history, it is preferably 15 or less.
- N ⁇ LB is 10 to 120 per 100 ⁇ m 2.
- the film-like residual ⁇ LB particles are particles having a particle width of 0.08 to 0.24 ⁇ m, a particle length of 0.6 to 15 ⁇ m, and an aspect ratio of 4 to 40.
- N ⁇ LB is a 100 [mu] m 2 per 10 or more. From the viewpoint of improving ductility, N ⁇ LB is preferably 20 or more, and more preferably 30 or more, per 100 ⁇ m 2 . If N ⁇ LB exceeds 120 per 100 ⁇ m 2 , it is hardened too much and ductility decreases, so N ⁇ LB is set to 120 or less per 100 ⁇ m 2 . From the viewpoint of improving ductility, N ⁇ LB is preferably 100 or less per 100 ⁇ m 2 , and more preferably 80 or less.
- particles with a particle width of 0.25 ⁇ m or more are plate-shaped. Further, it is assumed that the particles having a particle width of 0.24 ⁇ m or less are in a film form.
- S ⁇ Fine is 0.4 to 5.0% Fine fresh martensite particles having a circle-equivalent particle diameter of 0.4 to 1.0 ⁇ m and an aspect ratio of 3 or less and residual ⁇ particles (sometimes referred to as residual ⁇ ) are ⁇ and L.
- the effect of lowering El is small, and the effect of increasing El is large.
- the total area ratio of fresh martensite having a circle-equivalent particle diameter of 0.4 to 1.0 ⁇ m and an aspect ratio of 3 or less and residual ⁇ particles S ⁇ Fine is 0.4% or more. From the viewpoint of improving ductility, S ⁇ Fine is preferably 0.7% or more. If S ⁇ Fine increases too much, it will cause ⁇ to decrease, so these area ratios are made 5.0% or less. From the viewpoint of improving ⁇ , the total area ratio of these is more preferably 4.0% or less.
- S ⁇ Block 4% or less Conventionally, when trying to cause a lot of bainite transformation in the final tempering step, a large amount of massive martensite or massive residual ⁇ remains. Therefore, conventionally, in order to prevent this, a method of reducing Mn to 2% or less and promoting bainite transformation has been used. However, when the Mn content is reduced, the effect of stabilizing the residual ⁇ and the effect of increasing the volume ratio are lost, so that the ductility is impaired.
- the lumpy structure which adversely affects the stretch flange formability includes fresh martensite having a circle equivalent particle diameter of 1.2 to 20 ⁇ m and an aspect ratio of 3 or less and a circle equivalent particle diameter of 1.2 to 20 ⁇ m and an aspect ratio of 3 It is the following residual ⁇ particles, and by reducing the total area ratio: S ⁇ Block to 4% or less, excellent stretch flangeability molding and local ductility can be secured. S ⁇ Block is more preferably 3% or less in order to secure excellent stretch flangeability molding and local ductility.
- S ⁇ Block may be 0%. Note that only one of fresh martensite having a circle-equivalent particle diameter of 1.2 to 20 ⁇ m and an aspect ratio of 3 or less, and residual ⁇ particles having a circle-equivalent particle diameter of 1.2 to 20 ⁇ m and an aspect ratio of 3 or less When included, the area ratio of the included products shall be the total area ratio.
- the total area ratio of the regions in which the C concentration is 0.6 to 1.3% and the C concentration in the adjacent region is 0.07% or less: S C concentration is 0.1 to 5%
- the ductility can be improved by adjusting the area ratio of the region where the C concentration is higher than the surroundings. Specifically, the total area ratio of the regions in which the C concentration is 0.6 to 1.3% and the C concentration in the adjacent region is 0.07% or less: S C concentration is 0.1 to 5% By doing so, ductility is enhanced.
- the adjacent region means a region adjacent to a region having a C concentration of 0.6 to 1.3% and a C concentration of 0.07% or less.
- the region having a C concentration of 0.6 to 1.3% and the C concentration of the adjacent region being 0.07% or less is the residual ⁇ , and the residual ⁇ UB particles ( It may be referred to as residual ⁇ UB .) Is more preferable. Further, it is preferable that a part or all of the adjacent region contains upper bainite. Therefore, a case where the C concentration is 0.6 to 1.3%, the region where the C concentration in the adjacent region is 0.07% or less is the residual ⁇ UB , and the adjacent region is the upper bainite will be described below.
- the S C concentration when the above region is the residual ⁇ UB and the adjacent region is the upper bainite is referred to as S ⁇ UB * .
- the residual ⁇ UB formed adjacent to the upper bainite tends to have a very low C content on at least one side of the particle. That is, the desorption of C into austenite easily progresses from bainite (bainitic ferrite) generated at a high temperature of 405 to 490 ° C., and C efficiently concentrates in plate-like residual ⁇ UB . As a result, the C content of the plate-shaped residual ⁇ UB becomes 0.6 to 1.3%, which contributes to the improvement of ductility. Further, the amount of C in the upper bainite region around it decreases to 0.07% or less.
- the area S ⁇ UB * of the residual ⁇ having such a C distribution state in an area ratio of 0.1 to 5%. Since ductility is significantly increased by the S ⁇ UB * 0.2% or more, S ⁇ UB * further preferably set to 0.2% or more.
- the upper limit is more preferably 4% or less, further preferably 3% or less.
- the area ratio of ferrite is measured by cutting out a plate thickness cross section parallel to the rolling direction, mirror-polishing it, then corroding it with 3% Nital, and observing 10 fields of view with a SEM at 5000 times at 1/4 thickness position. went.
- the ferrite was a polygonal ferrite that is relatively equiaxed with almost no carbide inside. In SEM, it is the region that looks the most black. If it is difficult to identify whether the structure on both sides of the plate-like residual ⁇ UB is upper bainite or ferrite, the area of polygonal ferrite with aspect ratio ⁇ 2.0 is regarded as ferrite and the area with aspect ratio> 2.0.
- the aspect ratio as shown in FIG. 2, the major axis length a at which the particle length is the longest is determined, and the particle length when traversing the particle the longest in the direction perpendicular thereto is defined as the minor axis length b. And a / b is the aspect ratio.
- the size of each particle is measured by dividing at a position of a broken line shown in FIG.
- the area ratio of the structure consisting of one or more of upper bainite, fresh martensite, tempered martensite, lower bainite, and residual ⁇ was measured by the same method as for ferrite.
- the area ratio is the area ratio of the region other than the above ferrite.
- the area ratio of carbides is very small, it was included in the above area ratio.
- the volume ratio of residual ⁇ was obtained by X-ray diffraction after chemical polishing of the 1/4 thickness position from the surface layer.
- a Co-K ⁇ radiation source was used for the incident X-ray, and the area of the residual austenite was calculated from the intensity ratio of the (200), (211), (220) planes of ferrite and the (200), (220), (311) planes of austenite. The rate was calculated.
- the volume ratio of the residual ⁇ obtained by X-ray diffraction is equal to the area ratio of the residual ⁇ in the steel structure.
- the shape and area ratio of the plate-shaped residual ⁇ UB generated adjacent to the upper bainite are calculated by electropolishing the plate thickness cross section parallel to the rolling direction of the steel plate at the plate thickness 1/4 depth position, and using EBSD Map data was obtained and measured for tissues with fcc structure.
- the measurement area was 30 ⁇ m ⁇ 30 ⁇ m, and measurement was performed on 10 fields of view separated from each other by 50 ⁇ m or more.
- the particle length (major axis length), particle width (minor axis length), and aspect ratio were determined by employing the above-described particle size and aspect ratio measuring methods.
- Particle width is 0.25 to 0.60 ⁇ m
- particle length is 1.0 to 15 ⁇ m
- aspect ratio is 3.1 to 25
- particle width is 0.25 to 0.60 ⁇ m
- particle length is 1.5 to
- S ⁇ UB The area ratio of ⁇ grains having a size of 15 ⁇ m and an aspect ratio of 4 to 25 was determined as S ⁇ UB . Further, the same visual field was etched with 3% nital, and the total area ratio of ferrite or bainite existing adjacent to one side or both sides of the plate-like residual ⁇ UB : SUB was obtained.
- the volume ratio of residual ⁇ is the volume ratio of the entire steel sheet
- S ⁇ UB , S ⁇ Fine , and S ⁇ Block are the area ratios of the entire microstructure region
- N ⁇ LB is the upper bainite, fresh martensite, tempered martensite, lower bainite, and residual ⁇ . Represents the density of the number of distributions in the area (excluding ferrite).
- circle-equivalent particle diameter (circle-equivalent particle diameter)
- individual particles were observed with an SEM, the area ratio was calculated, and the circle-equivalent diameter was calculated to be the circle-equivalent particle diameter.
- the C concentration of 0.6 to 1.3%, the C concentration (% by mass) of the region where the C concentration of the adjacent region is 0.07% or less, and the C concentration (mass%) of the adjacent region are measured.
- FE-EPMA JEOL field emission electron analyzer
- JXA-8500F manufactured by JEOL at a plate thickness 1/4 position parallel to the rolling direction
- an acceleration voltage of 6 kV and an irradiation current of 7 ⁇ 10 ⁇ 8 A the beam diameter was set to the minimum, and the line analysis was performed.
- the analysis length was 6 ⁇ m, and profile data of C was randomly collected at 20 points 10 ⁇ m or more apart in order to obtain average information of the microstructure.
- 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 metal. That is, if the average value of the measured carbon content is greater than the carbon content of the base metal, the increase is considered to be contamination, and the value obtained by uniformly subtracting the increase from the analysis value at each position is the true value at each position.
- the C content of The total area ratio S C concentration of the region having C concentration of 0.07% or less adjacent to each other and C: 0.6 to 1.3% is such that the C amount at the skirt portion of the C peak is 0.
- the area ratio of C: 0.6 to 1.3% in the line analysis result was defined as the area ratio, assuming that the distribution state of the above areas was random for areas of 0.7% or less.
- FIG. 3 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 0.6 to 1.3% and the C concentration of the adjacent region is 0.07% or less is S C enrichment-1 .
- the graph as shown in FIG. 3 is derived at 30 points to obtain the total area ratio S C enrichment of S C enrichment ⁇ 1 .
- the morphology of the structure (plate-like residual ⁇ , film-like residual ⁇ ) to which “*” is added in FIG. 3 is judged using the SEM photograph.
- the amount of C enrichment of the plate-like residual ⁇ UB can be measured by the above-mentioned analytical method, in the characteristic evaluation, when the amount of C enrichment is 0.6 to 1.3%, The metal phase having the C concentration amount may be evaluated as a plate-like residual ⁇ UB .
- FIG. 1 An example of the SEM photograph is shown in Fig. 1.
- the steel sheet used for the observation in FIG. 1 was 0.18% C-1.5% Si-2.8% Mn steel, which was annealed at 840 ° C. to form a ⁇ single phase, and then at 810 to 700 ° C. at 20 ° C./s. After cooling, 700-490 ° C. at 20 ° C./s, 490-450 ° C. at 20 ° C./s, and isothermal holding at 450 ° C. for 30 sec, 450-310 ° C.
- Upper bainite, fresh martensite, tempered martensite, lower bainite, and residual ⁇ are separated and evaluated by SEM photographs.
- the upper bainite (a) contains almost no carbides, almost no streaky strain (lath interface) can be seen inside, and has a black minor axis width of 0.4 ⁇ m or more, which is almost the same as ferrite.
- the tempered martensite (c) is a region containing 2.0 to 20 fine carbides per 1 ⁇ m 2 having an aspect ratio of 4 or less and a circle equivalent diameter of 0.03 to 0.3 ⁇ m in the structure.
- the lower bainite (d) has a film width of 0.08 to 0.24 ⁇ m, a particle length of 0.6 ⁇ m to 15 ⁇ m, and an aspect ratio of 4 to 40 in the form of a film-like residual ⁇ (e) per 1 ⁇ m 2. It is a region containing 0.1 to 4 fine carbides having an aspect ratio of 4 or less and a circle equivalent diameter of 0.03 to 0.3 ⁇ m per 1 ⁇ m 2 .
- the tensile strength of the steel sheet of the present invention is preferably 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 secured at TS: 780 to 1319 MPa, ⁇ ⁇ 50% or more, preferably 55% or more, and TS: 1320 to 1450 MPa, ⁇ ⁇ 40% or more, preferably 45% or more.
- the molding stability is greatly improved.
- Hot rolling Steel slabs are hot-rolled by heating the slab after rolling, directly rolling the slab after continuous casting without heating, or rolling the slab after continuous casting by heat treatment for a short time.
- the hot rolling may be carried out according to a conventional method, for example, the slab heating temperature is 1100 to 1300 ° C., the soaking temperature is 20 to 300 min, the finish rolling temperature is Ar 3 transformation point to Ar 3 transformation point + 200 ° C.
- the taking temperature may be 400 to 720 ° C.
- the winding temperature is preferably 430 to 530 ° C. from the viewpoint of suppressing the plate thickness variation and stably ensuring high strength.
- the rolling rate may be 30 to 85%. From the viewpoint of stably securing high strength and reducing anisotropy, the rolling rate is preferably 45 to 85%.
- the rolling load is high, it is possible to perform softening annealing treatment at 450 to 730 ° C. in CAL (continuous annealing line) and BAF (box annealing furnace).
- Annealing A steel slab having a predetermined composition is hot-rolled and cold-rolled, and then annealed under the following prescribed conditions.
- the annealing equipment is not particularly limited, but from the viewpoint of ensuring the productivity and the desired heating rate and cooling rate, it is preferable to perform it on a continuous annealing line (CAL) or a continuous hot dip galvanizing line (CGL).
- CAL continuous annealing line
- CGL continuous hot dip galvanizing line
- Annealing temperature 810 to 900 ° C
- the annealing temperature is set to 810 to 900 ° C. in order to secure tempered martensite and / or bainite having a predetermined area ratio and residual ⁇ having a predetermined volume ratio.
- the annealing temperature is adjusted so that the ⁇ single-phase region is annealed so that the amount of polygonal ferrite is 5% or less. It is preferably 815 ° C or higher, and preferably 880 ° C or lower.
- the temperature range of 810 to 700 ° C. is cooled at an average cooling rate of 1 to 2000 ° C./s. If the average cooling rate is slower than 1 ° C./s, a large amount of ferrite is generated, resulting in a decrease in strength and a decrease in ⁇ . It is more preferably 3 ° C./s or more.
- the average cooling rate becomes too fast the plate shape deteriorates, so it is set to 2000 ° C./s or less. It is preferably 100 ° C / s or less, more preferably less than 30 ° C / s.
- Average cooling rate in the temperature range of 700 to 490 ° C 10 to 2000 ° C / s
- the temperature range of 700 to 490 ° C. is cooling at 10 ° C./s or more. If the average cooling rate is slower than 10 ° C./s, a large amount of ferrite is generated, resulting in a decrease in strength and a decrease in ⁇ . It is more preferably 15 ° C./s or more. On the other hand, if the average cooling rate becomes too fast, the plate shape deteriorates, so it is set to 2000 ° C./s or less. It is preferably 10065 ° C / s or less, more preferably less than 30 ° C / s.
- the plate shape can be made to a good level (the plate warpage described in Examples described later is 15 mm or less). Furthermore, by setting the average cooling rate to 14 ° C./s or less, the plate shape can be set to a better level (the plate warpage described in Examples described later is 10 mm or less), which is more preferable.
- Holding time in the temperature range of 490 to 405 ° C .: 10 to 200 sec By maintaining this temperature range for a predetermined time, it is possible to generate upper bainite that hardly causes carbide precipitation, and adjacent to it, it is possible to generate plate-like residual ⁇ UB having a high C enrichment amount. I can.
- the ratio S UB / S ⁇ UB of the area ratios of both tissues can be controlled within a predetermined range by holding in this temperature range. From these viewpoints, the temperature is maintained at 490 to 405 ° C. for 10 seconds or longer. From the viewpoint of generating plate-like residual ⁇ UB and improving the ductility, the holding time in this temperature range is more preferably 14 sec or more.
- the holding time in the temperature range of 490 to 405 ° C. is set to 10 to 200 sec.
- the holding time in the temperature range of 490 to 405 ° C. is preferably 40 sec or less. Holding in this temperature range corresponds to reducing the average cooling rate in this temperature range to 9 ° C / s or less.
- the temperature range to be held is preferably 410 ° C or higher, more preferably 420 ° C or higher. Further, it is preferably 470 ° C. or lower, more preferably 460 ° C. or lower.
- Average cooling rate in the temperature range of 405 to 310 ° C 10 to 100 ° C / s After holding at 405 to 490 ° C., it is necessary to quickly cool to 310 ° C. so that carbon is not concentrated to ⁇ too much. When staying at a temperature higher than 310 ° C., carbon is concentrated into massive untransformed ⁇ , bainite transformation in the subsequent cooling step and tempering step is suppressed, and the amount of massive martensite or residual ⁇ increases. As a result, ⁇ decreases. From the viewpoint of improving ⁇ , the average cooling rate in the temperature range of 405 to 310 ° C. is 10 ° C./s or more.
- the cooling rate in this temperature range is more preferably 12 ° C./s or higher, and even more preferably 15 ° C./s or higher. If the cooling rate in this temperature range is too high, the plate shape deteriorates, so the cooling rate in this temperature range is 100 ° C./s or less. It is more preferably less than 30 ° C / s, and even more preferably less than 20 ° C / s.
- the range of 310 ° C. to 255 ° C. is gently cooled to perform the second holding.
- carbon can be concentrated in adjacent ⁇ at the same time as martensite and lower bainite are formed, and a film-like residual ⁇ LB formed adjacent to martensite and lower bainite is generated. This improves ductility.
- the agglomerate structure is reduced, and fine martensite having a circle-equivalent particle diameter of 0.4 to 1.0 ⁇ m or residual ⁇ is formed.
- the average cooling rate in this temperature range is 0.4 ° C./s or more and less than 20 ° C./s. From the viewpoint of increasing the amount of film-like residual ⁇ LB produced and improving the ductility, the average cooling rate in this temperature range is preferably less than 15 ° C./s, more preferably less than 10 ° C./s. preferable. It is particularly preferable to set it to 7 ° C./s or less.
- the greatest effect of reducing the cooling rate is in the range of 310 ° C. to 301 ° C., and this temperature range may be set to the above cooling rate range, preferable cooling rate range, and further preferable cooling rate range. Especially important.
- This temperature range has the effect of inhibiting the concentration of carbon in the residual ⁇ due to the precipitation of carbides, so it is necessary to cool it quickly.
- the average cooling rate in this temperature range needs to be 2 ° C./s or more and less than 30 ° C./s.
- the average cooling rate in this temperature range is preferably 3 ° C./s or more, more preferably 5 ° C./s or more. If the cooling rate in this temperature range is too fast, the distribution of carbon during bainite formation will be insufficient, the film-like residual ⁇ LB will be reduced and ductility will be reduced, and the second phase will be hardened and ⁇ will be reduced. Therefore, the cooling rate in this temperature range is preferably 15 ° C./s or less, and more preferably less than 10 ° C./s.
- Cooling stop temperature Tsq 220 ° C. or more and 254 ° C. or less
- the cooling stop temperature: Tsq needs to be in the range of 220 ° C. or higher and 254 ° C. or lower.
- the cooling stop temperature is lower than 220 ° C, fine martensite and fine residual ⁇ are reduced.
- carbide precipitation occurs in martensite and lower bainite, resulting in residual ⁇ . Carbon distribution is suppressed.
- the cooling stop temperature is 220 ° C. or higher. More preferably, it is 230 ° C. or higher. If the cooling stop temperature exceeds 254 ° C., a lumpy structure remains and stable high ⁇ cannot be obtained. Therefore, the cooling stop temperature is set to 254 ° C or lower. More preferably, it is 250 ° C. or lower.
- Average heating rate in the temperature range from the cooling stop temperature Tsq to 350 ° C .: 2 ° C./s or more By further heating the temperature range from the cooling stop temperature to 350 ° C. in a short time, carbide precipitation is suppressed and high ductility is secured. You can Further, when the martensite or the lower bainite produced by cooling is reheated to 350 ° C. or higher by the nucleus, the upper bainite is produced. If the average heating rate up to 350 ° C. is slow, these effects cannot be obtained. As a result, the amount of residual ⁇ decreases and ductility decreases. Therefore, the average heating rate in the temperature range from the cooling stop temperature to 350 ° C is 2 ° C / s or more.
- the average heating rate is preferably 5 ° C./s or more, and more preferably 10 ° C./s or more from the viewpoint of suppressing the precipitation of carbides and the viewpoint of forming upper bainite during reheating.
- the upper limit of the average heating rate is not particularly limited, but is preferably 50 ° C / s or less, more preferably 30 ° C / s or less.
- the holding time at 350 to 550 ° C. to 60 to 3000 sec, the total area ratio of the regions where the C concentration is 0.6 to 1.3% and the C concentration of the adjacent region is 0.07% or less.
- the SC concentration becomes 0.1 to 5%, and the ductility is further improved.
- the C concentration is 0.6 to 1.3%
- the total area ratio S C concentration of the region where the C concentration of the adjacent region is 0.07% or less becomes 0.2 to 5%
- holding in the temperature range of 350 ° C to 550 ° C may also serve as hot dip galvanizing treatment.
- a galvanizing bath having an Al content of 0.10% or more and 0.22% or less it is preferable to use a galvanizing bath having an Al content of 0.10% or more and 0.22% or less.
- an alloying treatment of zinc plating can be performed after the hot dip galvanizing treatment.
- the galvanizing alloying treatment is performed, it is preferably performed in a temperature range of 470 ° C. or higher and 550 ° C. or lower.
- the steel sheet can be skin-pass rolled.
- the skin pass extension ratio is preferably 0.1 to 0.5%.
- the plate shape can be flattened by a leveler. It is more preferably 5 ° C./s or higher, and preferably 100 ° C./s or lower.
- low temperature heat treatment at 100 to 300 ° C for 30 seconds to 10 days after the above heat treatment or after skin pass rolling.
- Low temperature heat treatment can reduce hydrogen to less than 0.1 ppm.
- electroplating After the electroplating, it is preferable to perform the above-mentioned low temperature heat treatment from the viewpoint of reducing hydrogen in the steel.
- TS: 780 to 1319 MPa ⁇ ⁇ 50% or more, and more preferably ⁇ ⁇
- the steel of the present invention exhibits a high local ductility (L.El) of 6.0% or more in the 780 to 1179 MPa class and 5.0% or more in the 1180 to 1450 MPa class.
- a cold-rolled steel sheet having a thickness of 1.2 mm and having the composition shown in Table 1 was treated under the annealing conditions shown in Table 2-1 to produce the steel sheet of the present invention and the steel sheet of Comparative Example.
- the steel sheets were further subjected to hot dip galvanizing treatment to obtain hot dip galvanized steel sheets (GI).
- GI hot dip galvanized steel sheets
- the steel sheet was immersed in a galvanizing bath at 440 ° C. or more and 500 ° C. or less for hot dip galvanizing treatment, and then the amount of coating adhered was adjusted by gas wiping or the like.
- the hot dip galvanizing used a galvanizing bath having an Al content of 0.10% or more and 0.22% or less.
- some of the hot-dip galvanized steel sheets were subjected to galvanizing alloying treatment after the hot-dip galvanizing treatment to obtain alloyed hot-dip galvanized steel sheets (GA).
- the galvanizing alloying treatment was performed in a temperature range of 470 ° C. or higher and 550 ° C. or lower.
- some steel plates (cold rolled steel plates) were electroplated to obtain electrogalvanized steel plates (EG).
- the steel structure was measured by the above method. The measurement results are shown in Table 2-2.
- the area ratio of the plate-like residual ⁇ UB generated adjacent to the upper bainite is such that the particle width is 0.25 to 0.60 ⁇ m, the particle length is 1.5 to 15 ⁇ m, and the aspect ratio is 4 to 25.
- the area ratio of the ⁇ -grains was determined as S ⁇ UB .
- a JIS No. 5 tensile test piece was sampled from the obtained steel sheet and a tensile test (in accordance with JIS Z2241) was carried out.
- TS and El, L.L. El is shown in Table 2-2.
- the stretch flange formability was evaluated by a hole expanding test in accordance with the Japan Iron and Steel Federation Standard JFST1001. That is, after punching a 100 mm ⁇ 100 mm square sample with a punching tool having a punch diameter of 10 mm and a die diameter of 10.3 mm (clearance 13%), a conical punch having an apex angle of 60 ° is used to form a punched hole. The holes were expanded until the burrs that occurred were on the outside and cracks that penetrated the plate thickness occurred.
- d 0 initial hole diameter (mm)
- d hole diameter at the time of crack occurrence (mm)
- hole expansion rate ⁇ (%) ⁇ (d ⁇ d 0 ) / d 0 ⁇ ⁇ 100 .
- examples of 1, 7, 8, 9, 10, 14, 15, 16, 20, 21, 24, 27, 29, 30, 32, 33, TS ⁇ El ⁇ 17000 MPa% and 1180 MPa class are 50% or more. While the hole expandability ( ⁇ ) is satisfied, any of the comparative examples is inferior.
- the plate warpage measured by the following method was a good level of 11 to 15 mm. 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 more favorable level.
- the above-mentioned warpage for evaluating the plate shape was obtained by cutting a 1500 mm-long cut sample from the annealed steel plate, and placing the sample on a horizontal flat plate to measure the warp height of four sides. It was evaluated by a method of measuring the maximum value (unit: mm) of the height. In addition, when the cut sample is cut in the longitudinal direction, the clearance of the blade of the shearing machine is 4% (the upper limit of the control range is 10%).
- a cold rolled steel sheet having a thickness of 1.2 mm and having the composition shown in Table 1 was processed under the annealing conditions shown in Table 3-1 to produce the steel sheet of the present invention and the steel sheet of the comparative example.
- the steel structure of the obtained steel sheet was measured and the mechanical properties were evaluated in the same manner as above, and the results are shown in Table 3-2.
- the area ratio of the plate-like residual ⁇ UB formed adjacent to the upper bainite has a particle width of 0.25 to 0.60 ⁇ m, a particle length of 1.0 to 15 ⁇ m, and an aspect ratio of 3.1 to 25.
- the area ratio of ⁇ -grains satisfying the above condition was determined as S ⁇ UB .
- inventive examples 1, 2, 3, 4, 5, and 9 satisfy the hole expansivity ( ⁇ ) of 50% or more in the TS ⁇ El ⁇ 17000 MPa%, 1180 MPa class, while any of the comparative examples.
- ⁇ hole expansivity
- the present invention has extremely high ductility and excellent stretch flange formability, and can be preferably applied to press forming used in automobiles, home appliances, etc. through the press forming process.
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Abstract
Description
[1] 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、鋼組織は、面積率でフェライト:5%以下、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:95~100%、体積率で残留γ:4~15%を含み、粒子幅が0.25~0.60μm、粒子長さが1.0~15μm、アスペクト比が3.1~25である残留γUBの面積率:SγUBが0.2~7.0%であり、粒子幅が0.08~0.24μm、粒子長さが0.6~15μm、アスペクト比が4~40である残留γLBの分布個数:NγLBが100μm2あたり10~120個であり、円相当粒子直径が0.4~1.0μm、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0.4~1.0μm、アスペクト比が3以下の残留γ粒子の合計面積率:SγFineが0.4~5.0%であり、円相当粒子直径が1.2~20μm、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1.2~20μm、アスペクト比が3以下の残留γ粒子の合計面積率:SγBlockが4%以下(0%を含む)である鋼板。
[2] 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、鋼組織は、面積率でフェライト:5%以下、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:95~100%、体積率で残留γ:4~15%を含み、粒子幅が0.25~0.60μm、粒子長さが1.5~15μm、アスペクト比が4~25である残留γUBの面積率:SγUBが0.2~7.0%であり、粒子幅が0.08~0.24μm、粒子長さが0.6~15μm、アスペクト比が4~40である残留γLBの分布個数:NγLBが100μm2あたり10~120個であり、円相当粒子直径が0.4~1.0μm、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0.4~1.0μm、アスペクト比が3以下の残留γ粒子の合計面積率:SγFineが0.4~5.0%であり、円相当粒子直径が1.2~20μm、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1.2~20μm、アスペクト比が3以下の残留γ粒子の合計面積率:SγBlockが4%以下(0%を含む)である鋼板。
[3] 前記残留γUBに隣接するフェライトもしくは上部ベイナイトの面積率:SUBとSγUBの比がSUB/SγUB≧3.5を満たす[1]または[2]に記載の鋼板。
[4] 前記組織において、C濃度が0.6~1.3%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0.1~5%である[1]~[3]のいずれかに記載の鋼板。
[5] C濃度が0.6~1.3%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[4]に記載の鋼板。
[6] C濃度が0.6~1.3%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[5]に記載の鋼板。
[7] 前記隣接領域が上部ベイナイトを含む[3]~[6]のいずれかに記載の鋼板。
[8] 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種を含有する[1]~[7]のいずれかに記載の鋼板。
[9] 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[1]~[8]のいずれかに記載の鋼板。
[10] 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[1]~[9]のいずれかに記載の鋼板。
[11] 引張強度が780MPa以上1450MPa以下である[1]~[10]のいずれかに記載の鋼板。
[12] 表面に亜鉛めっき層を有する[1]~[11]のいずれかに記載の鋼板。
[13] [1]、[2]、[8]~[10]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、810~900℃の焼鈍温度で焼鈍し、次いで810~700℃の温度範囲を平均冷却速度:1~2000℃/sで冷却し、さらに700~490℃の温度範囲を平均冷却速度:10~2000℃/sで冷却した後、490~405℃の温度範囲で10~200sec保持し、さらに405~310℃の温度範囲を平均冷却速度:10~100℃/sで冷却した後、310~255℃の温度範囲を平均冷却速度:0.4℃/s以上20℃/s未満で冷却し、さらに255℃から254~220℃の範囲の冷却停止温度:Tsqまでの温度範囲を平均冷却速度:2℃/s以上30℃/s未満の冷却速度で冷却し、Tsqから350℃までの温度範囲を平均加熱速度:2℃/s以上で加熱し、350~550℃で20~3000sec保持した後、350~50℃以下の温度まで0.1℃/s以上の平均冷却速度で冷却する鋼板の製造方法。
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、穴広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1.1%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、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の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2.7%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は熱間圧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、穴広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。より好ましくは0.0005%以上である。
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を及ぼすので、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。好ましくは0.010%以下である。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。より好ましくは0.001%以上である。
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。さらに好ましくは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λを確保するために、フェライトは面積率で5%以下とする。より好ましくは4%以下、さらに好ましくは2%以下である。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、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の合計面積率は95~100%とする。下限についてより好ましくは96%以上、さらに好ましくは98%以上である。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写真で観察した。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1~30%である。フレッシュマルテンサイトは面積率で0~20%である。焼戻しマルテンサイトは面積率で3~40%である。下部ベイナイトは面積率で5~70%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4%以上とする。より好ましくは5%以上、さらに好ましくは7%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15%以下とする。より好ましくは13%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490~405℃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。粒子幅が0.25~0.60μm、粒子長さが1.0~15μm、アスペクト比が3.1~25である残留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。より高い延性を確保するため、残留γUBの形態は、粒子幅が0.25~0.60μm、粒子長さが1.5~15μm、アスペクト比が4~25であることが好ましい。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7.0%以下とする。より好ましくは5.0%以下、さらに好ましくは4.0%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。
残留γUBの延性向上効果は、残留γUBに隣接して生成するフェライトもしくは上部ベイナイトとの面積比率を制御することで向上できる。高い延性を確保するためにSUB/SγUBは3.5以上とすることが望ましい。延性向上の観点から、より好ましいSUB/SγUBの範囲は4.0以上である。上限は特に規定しないが、本熱履歴の場合、15以下が好ましい。
後述する製造方法において、冷却過程の310~255℃の温度範囲で冷却速度を遅くする第2の中間保持を設けることで、マルテンサイトと下部ベイナイトに隣接して生成するフィルム状の残留γLB(残留γLB粒子と称する場合もある。)を得ることができる。このフィルム状の残留γLB粒子は、粒子幅が0.08~0.24μm、粒子長さが0.6~15μm、アスペクト比が4~40の粒子である。この粒子は主に残留γからなるが、一部に炭化物やマルテンサイトも含む。ここではSEM写真における形態でフィルム状の残留γLB粒子を識別した。延性向上の観点から残留γLB粒子の分布個数:NγLBは100μm2あたり10個以上とする。延性向上の観点からNγLBは100μm2あたり20個以上であることが好ましく、30個以上であることがさらに好ましい。NγLBは100μm2あたり120個超えとなると硬質化しすぎて延性が低下するので、NγLBは100μm2あたり120個以下とする。延性向上の観点からは、NγLBは100μm2あたり100個以下であることが好ましく、80個以下であることがさらに好ましい。
円相当粒子直径が0.4~1.0μm、アスペクト比が3以下の微細なフレッシュマルテンサイトや残留γ粒子(残留γと称する場合もある。)は、λやL.Elを低下させる作用が小さく、Elを増加させる作用が大きい。したがって、円相当粒子直径が0.4~1.0μm、アスペクト比が3以下のフレッシュマルテンサイトと残留γ粒子の合計面積率:SγFineは0.4%以上とする。延性向上の観点からSγFineは0.7%以上とすることが好ましい。SγFineが増加しすぎると、λを低下させる要因になるので、これらの面積率は5.0%以下とする。λ向上の観点からこれらの合計面積率は4.0%以下とすることがより好ましい。
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する手法が用いられていた。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延性が損なわれていた。これに対して、Mnを多く含む鋼板に適切な冷却処理を施す本発明では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1.2~20μm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1.2~20μm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4%以下に低減することで優れた伸びフランジ性成形や局部延性を確保できる。優れた伸びフランジ性成形および局部延性を確保するためにSγBlockは3%以下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1.2~20μm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1.2~20μm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。
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0.6~1.3%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0.1~5%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0.6~1.3%であり、C濃度が0.07%以下である領域と隣合う領域を意味する。
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らは、430~530℃とするのが好ましい。
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(連続焼鈍ライン)、BAF(箱焼鈍炉)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、以下に規定の条件で焼鈍を施す。焼鈍設備は特に限定されないが、生産性、および所望の加熱速度および冷却速度を確保する観点から、連続焼鈍ライン(CAL)または連続溶融亜鉛めっきライン(CGL)で実施することが好ましい。
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810~900℃とする。ポリゴナルなフェライトを5%以下とするために、焼鈍温度はγ単相域焼鈍となるように調整する。好ましくは815℃以上であり、好ましくは880℃以下である。
焼鈍後、810~700℃の温度範囲を平均冷却速度:1~2000℃/sで冷却する。平均冷却速度が1℃/sより遅いと、フェライトが多量に生成し、強度低下、λの低下を招く。より好ましくは3℃/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、さらに好ましくは30℃/s未満である。
700~490℃の温度範囲は10℃/s以上で冷却する。平均冷却速度が10℃/sより遅いと、フェライトが多量に生成し、強度低下、λの低下を招く。より好ましくは15℃/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10065℃/s以下、さらに好ましくは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。また、この温度域での保持により両組織の面積率の比SUB/SγUBを所定範囲に制御することができる。これらの観点から490~405℃の温度範囲で10sec以上保持する。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は、この温度域での保持時間は、14sec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残留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、490~405℃の温度範囲での保持時間は10~200secとする。伸びフランジ成形性を向上させる観点からは、490~405℃の温度範囲での保持時間は40sec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9℃/s以下に低減することに対応する。延性向上の観点からは、保持する温度域は、410℃以上が好ましく、420℃以上がさらに好ましい。また、470℃以下が好ましく、460℃以下がさらに好ましい。
405~490℃で保持した後、炭素のγへの濃化が進行しすぎないように速やかに310℃まで冷却する必要がある。310℃より高い温度で滞留すると、炭素が塊状の未変態γへ濃化して、引き続く冷却工程や焼き戻し工程でのベイナイト変態が抑制され、塊状のマルテンサイトもしくは残留γの量が増大する。その結果、λが低下する。λを向上させる観点から405~310℃の温度範囲の平均冷却速度は10℃/s以上とする。より好ましくは12℃/s以上、さらに好ましくは15℃/s以上である。この温度範囲の冷却速度が大きくなりすぎると、板形状が劣化するので、この温度範囲の冷却速度は100℃/s以下とする。より好ましくは30℃/s未満、さらに好ましくは20℃/s未満である。
310℃から255℃の範囲を緩やかに冷却して第2の保持を行う。これにより、マルテンサイトや下部ベイナイトの生成と同時に炭素を隣接したγに濃化させることが出来、マルテンサイトや下部ベイナイトに隣接して生成するフィルム状の残留γLBを生成させる。これにより延性が向上する。また、低温で保持することで塊状組織を低減し、円相当粒子径が0.4~1.0μmの微細なマルテンサイトもしくは残留γを形成させる。延性向上の観点からは、この温度範囲の平均冷却速度は0.4℃/s以上20℃/s未満とする。フィルム状の残留γLBの生成量を高くして延性を向上させる観点からは、この温度範囲の平均冷却速度は15℃/s未満とすることが望ましく、10℃/s未満とすることがさらに好ましい。7℃/s以下とすることが特に好ましい。
さらに、255℃から220℃以上254℃以下の範囲の冷却停止温度:Tsqまでの温度範囲を速やかに冷却して塊状組織をさらに低減する。この温度範囲は炭化物が析出して残留γへの炭素の濃化を阻害する作用があるので、速やかに冷却する必要がある。延性の低下を防ぎ、塊状組織を低減してλを向上させる観点からは、この温度範囲の平均冷却速度は2℃/s以上30℃/s未満とする必要がある。炭化物の生成を抑制する観点からは、この温度範囲の平均冷却速度は3℃/s以上とすることが望ましく、5℃/s以上とすることがさらに好ましい。この温度範囲の冷却速度が速すぎるとベイナイト生成中の炭素の分配が不十分となり、フィルム状の残留γLBが少なくなり延性が低下するとともに、第二相が硬質化してλが低下する。したがって、この温度範囲の冷却速度は15℃/s以下とすることが望ましく、10℃/s未満とすることがさらに好ましい。
円相当粒子直径が0.4~1.0μm、アスペクト比が3以下の微細なフレッシュマルテンサイトまたは微細な残留γを分散させて高い延性を確保するため、残留γ量を確保するためには、冷却停止温度:Tsqは220℃以上254℃以下の範囲とする必要がある。冷却停止温度が220℃未満になると微細なマルテンサイトや微細な残留γが減少するのに加え、わずかな保持時間でも220℃未満ではマルテンサイトや下部ベイナイト内部での炭化物析出が生じて残留γへの炭素の分配が抑制される。したがって、冷却停止温度は220℃以上とする。より好ましくは230℃以上である。冷却停止温度が254℃超えでは塊状組織が残存して安定して高いλを得ることが出来なくなる。このため、冷却停止温度は254℃以下とする。より好ましくは250℃以下である。
さらに冷却停止温度から350℃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350℃以上に再加熱した際に上部ベイナイトが生成する。350℃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350℃までの温度範囲の平均加熱速度は2℃/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の残留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の残留γLB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、350~550℃の温度域で20~3000sec保持する。
また、一部の鋼板(冷延鋼板)は、電気めっきを施し、電気亜鉛めっき鋼板(EG)とした。
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、
鋼組織は、面積率でフェライト:5%以下、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:95~100%、体積率で残留γ:4~15%を含み、
粒子幅が0.25~0.60μm、粒子長さが1.0~15μm、アスペクト比が3.1~25である残留γUBの面積率:SγUBが0.2~7.0%であり、
粒子幅が0.08~0.24μm、粒子長さが0.6~15μm、アスペクト比が4~40である残留γLBの分布個数:NγLBが100μm2あたり10~120個であり、
円相当粒子直径が0.4~1.0μm、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0.4~1.0μm、アスペクト比が3以下の残留γ粒子の合計面積率:SγFineが0.4~5.0%であり、
円相当粒子直径が1.2~20μm、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1.2~20μm、アスペクト比が3以下の残留γ粒子の合計面積率:SγBlockが4%以下(0%を含む)である鋼板。 - 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、
鋼組織は、面積率でフェライト:5%以下、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:95~100%、体積率で残留γ:4~15%を含み、
粒子幅が0.25~0.60μm、粒子長さが1.5~15μm、アスペクト比が4~25である残留γUBの面積率:SγUBが0.2~7.0%であり、
粒子幅が0.08~0.24μm、粒子長さが0.6~15μm、アスペクト比が4~40である残留γLBの分布個数:NγLBが100μm2あたり10~120個であり、
円相当粒子直径が0.4~1.0μm、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0.4~1.0μm、アスペクト比が3以下の残留γ粒子の合計面積率:SγFineが0.4~5.0%であり、
円相当粒子直径が1.2~20μm、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1.2~20μm、アスペクト比が3以下の残留γ粒子の合計面積率:SγBlockが4%以下(0%を含む)である鋼板。 - 前記残留γUBに隣接するフェライトもしくは上部ベイナイトの面積率:SUBとSγUBの比がSUB/SγUB≧3.5を満たす請求項1または2に記載の鋼板。
- 前記組織において、C濃度が0.6~1.3%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0.1~5%である請求項1~3のいずれかに記載の鋼板。
- C濃度が0.6~1.3%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4に記載の鋼板。
- C濃度が0.6~1.3%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5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3~6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。 - 引張強度が780MPa以上1450MPa以下である請求項1~10のいずれかに記載の鋼板。
- 表面に亜鉛めっき層を有する請求項1~11のいずれかに記載の鋼板。
- 請求項1、2、8~10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、810~900℃の焼鈍温度で焼鈍し、
次いで810~700℃の温度範囲を平均冷却速度:1~2000℃/sで冷却し、さらに700~490℃の温度範囲を平均冷却速度:10~2000℃/sで冷却した後、
490~405℃の温度範囲で10~200sec保持し、
さらに405~310℃の温度範囲を平均冷却速度:10~100℃/sで冷却した後、
310~255℃の温度範囲を平均冷却速度:0.4℃/s以上20℃/s未満で冷却し、
さらに255℃から254~220℃の範囲の冷却停止温度:Tsqまでの温度範囲を平均冷却速度:2℃/s以上30℃/s未満の冷却速度で冷却し、
Tsqから350℃までの温度範囲を平均加熱速度:2℃/s以上で加熱し、350~550℃で20~3000sec保持した後、
350~50℃以下の温度まで0.1℃/s以上の平均冷却速度で冷却する鋼板の製造方法。
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