WO2012002566A1 - 加工性に優れた高強度鋼板およびその製造方法 - Google Patents
加工性に優れた高強度鋼板およびその製造方法 Download PDFInfo
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- WO2012002566A1 WO2012002566A1 PCT/JP2011/065415 JP2011065415W WO2012002566A1 WO 2012002566 A1 WO2012002566 A1 WO 2012002566A1 JP 2011065415 W JP2011065415 W JP 2011065415W WO 2012002566 A1 WO2012002566 A1 WO 2012002566A1
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
- C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
- C21D8/0226—Hot rolling
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/005—Ferrite
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/009—Pearlite
Definitions
- the present invention relates to a high-strength steel sheet suitable for use as a strength member for automobile parts, which requires excellent workability (stretch flangeability), and a method for producing the same.
- DP steel sheet duplex steel sheet having a two-phase structure composed of a ferrite phase and a martensite phase
- a steel sheet having a composite structure including a ferrite phase, a martensite phase, and a bainite phase Composite steel sheets have been proposed.
- Patent Document 1 C: 0.08 to 0.30%, Si: 0.1 to 2.5%, Mn: 0.5 to 2.5%, P: 0.01 to 0.15 the cold-rolled steel sheet of percent composition containing, recrystallization annealing at a c1 point or higher, then, after forced cooling to a temperature range in the range of a r1 point to 600 ° C., the cooling rate of more than 100 ° C.
- the quenching start temperature is increased to increase the volume ratio of the low-temperature transformation generation phase, and then an overaging treatment is performed at 350 to 600 ° C. to precipitate C in the ferrite, and the low temperature
- the transformation generation phase is softened to reduce Hv (L) / Hv ( ⁇ ) and improve local elongation.
- Patent Document 2 C: 0.02 to 0.25%, Si: 2.0% or less, Mn: 1.6 to 3.5%, P: 0.03 to 0.20%, S : 0.02% or less, Cu: 0.05 to 2.0%, sol.
- a steel slab containing Al: 0.005 to 0.100% and N: 0.008% or less is hot-rolled to form a hot-rolled coil. After pickling, the hot-rolled coil is 720 to 950 ° C. in a continuous annealing line. Describes a method for producing a low-yield ratio, high-tensile hot-rolled steel sheet excellent in corrosion resistance that is annealed at a temperature of 5 ° C. According to the technique described in Patent Document 2, a high-tensile hot-rolled steel sheet having a composite structure that maintains a low yield ratio, high ductility, and good hole expandability, and has excellent corrosion resistance can be manufactured.
- Patent Document 3 discloses that C: 0.03 to 0.17%, Si: 1.0% or less, Mn: 0.3 to 2.0%, P: 0.010% or less, S: 0.0.
- a high-strength cold-rolled steel sheet having a structure and satisfying (second-phase Vickers hardness) / (ferrite-phase Vickers hardness) less than 1.6 and having an excellent strength-stretch flangeability balance is described.
- the high-strength cold-rolled steel sheet described in Patent Document 3 is obtained by hot rolling a steel (slab) having the above composition, winding it at a temperature of 650 ° C. or lower, pickling it, and then cold rolling, , a 1 point or more, (a 3 point + 50 ° C.) soaking at a temperature, then slowly cooled below 20 ° C. / s until temperature T 1 of between the range of 750 ⁇ 650 ° C., then, from T 1 It is said that it can be obtained by performing an annealing treatment of cooling to 500 ° C. at a rate of 20 ° C./s or more, and subsequently overaging at a temperature of 500 to 250 ° C.
- JP-A-63-293121 JP 05-111282 A Japanese Patent Laid-Open No. 10-60593
- Patent Document 1 requires a continuous annealing facility capable of rapid cooling (quenching) after recrystallization annealing and suppresses a rapid strength decrease due to an overaging treatment at a high temperature.
- a large amount of alloying element is required.
- Patent Document 2 it is essential to add a large amount of P and Cu in combination.
- P has a strong tendency to segregate in the steel, and this segregated P has a problem of causing embrittlement of the welded part in addition to lowering the stretch flangeability of the steel sheet.
- the high-strength cold-rolled steel sheet described in Patent Document 3 is excellent in stretch flangeability, but in the case of a high strength of 540 MPa or more, the elongation is less than 26%, and the desired excellent workability can be maintained. There is a problem that sufficient growth cannot be secured.
- An object of the present invention is to solve such problems of the prior art and to provide a high-strength steel sheet having a thin plate thickness of about 1.0 to 1.8 mm and excellent workability, and a method for producing the same.
- “high strength” refers to a case where the tensile strength TS is 540 MPa or more, preferably 590 MPa or more, and “excellent workability” means elongation El: 30% or more. (When a JIS No. 5 test piece is used), the hole expansion rate ⁇ in a hole expansion test in accordance with Japan Iron and Steel Federation standard JFST 1001-1996 is 80% or more.
- the present inventors conducted extensive research on the influence of the composition and the microstructure on the strength and workability.
- the hot-rolled sheet with the alloy element amount adjusted to an appropriate range is subjected to an annealing process and an appropriate cooling process, which are heated to an appropriate two-phase temperature range, without performing cold rolling.
- the main phase and the second phase can be made mainly of fine pearlite, which can ensure the desired high strength, greatly improve the workability, and achieve the desired elongation and desired hole expansion. It was found that a high-strength steel sheet excellent in workability that combines the efficiency can be obtained.
- the present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows. (1) In mass%, C: 0.08 to 0.15%, Si: 0.5 to 1.5%, Mn: 0.5 to 1.5%, P: 0.1% or less, S: 0.01% or less, Al: 0.01 to 0.1%, N: 0.005% or less, the composition composed of the balance Fe and inevitable impurities, the ferrite phase as the main phase, and at least pearlite The ferrite phase is 75 to 90%, the pearlite is 10 to 25%, and the average particle size of the pearlite is 5 ⁇ m or less. Furthermore, the high-strength steel sheet excellent in workability, wherein the pearlite is 70% or more in terms of the area ratio with respect to the total area of the second phase.
- a method for producing a high-strength steel sheet having excellent workability characterized by performing a continuous annealing step for performing a cooling treatment with a residence time of 30 to 400 s.
- the method further comprises mass: B: 0.0003 to 0.0050%. .
- a high-strength steel sheet excellent in workability which has a high strength of tensile strength TS: 540 MPa or more, an elongation of El: 30% or more, and an elongation flangeability of ⁇ : 80% or more. It can be manufactured easily and inexpensively, and has a remarkable industrial effect.
- the present invention has an effect that cold rolling can be omitted, and the manufacturing cost can be reduced and productivity can be greatly improved.
- the steel sheet according to the present invention is applied particularly to automobile body parts, it can greatly contribute to weight reduction of the automobile body.
- C 0.08 to 0.15%
- C is an element that contributes to an increase in the strength of the steel sheet and effectively acts on the formation of a composite structure composed of a ferrite phase and a second phase other than the ferrite phase.
- the desired tensile strength In order to ensure a high strength of 540 MPa or more, it is necessary to contain 0.08% or more. On the other hand, if the content exceeds 0.15%, spot weldability is lowered, and workability such as ductility is further lowered. Therefore, C is limited to the range of 0.08 to 0.15%. Note that the content is preferably 0.10 to 0.15%.
- Si 0.5 to 1.5% Si is an element that dissolves in steel and effectively acts to strengthen the ferrite, and also contributes to the improvement of ductility.
- the content of Si is 0.8. It needs to contain 5% or more.
- an excessive content exceeding 1.5% promotes the generation of red scale and the like, lowers the surface properties of the steel sheet, and lowers the chemical conversion treatment property.
- excessive inclusion of Si is accompanied by an increase in electrical resistance during resistance welding, and impedes resistance weldability. For this reason, Si was limited to the range of 0.5 to 1.5%. In addition, Preferably it is 0.7 to 1.2%.
- Mn 0.5 to 1.5%
- Mn is an element that contributes to an increase in the strength of the steel sheet and that effectively acts in the formation of a composite structure.
- the Mn content needs to be 0.5% or more.
- a content exceeding 1.5% tends to form a martensite phase in the cooling process during annealing, and causes deterioration in workability, particularly stretch flangeability.
- Mn was limited to the range of 0.5 to 1.5%. In addition, Preferably it is 0.7 to 1.5%.
- P 0.1% or less
- P is an element that has the effect of increasing the strength of the steel sheet by solid solution in steel, but has a strong tendency to segregate to the grain boundary, lowering the bonding strength of the grain boundary, and processing
- it concentrates on the surface of the steel sheet to reduce chemical conversion properties, corrosion resistance, and the like.
- Such an adverse effect of P becomes remarkable when the content exceeds 0.1%.
- P was limited to 0.1% or less.
- P is preferably 0.1% or less and preferably reduced as much as possible.
- excessive reduction leads to an increase in manufacturing cost, so it should be about 0.001% or more. Is preferred.
- S 0.01% or less S forms sulfides (inclusions) such as MnS mainly in steel and lowers the workability of the steel sheet, particularly the local elongation. In addition, the presence of sulfide (inclusions) also reduces weldability. Such an adverse effect of S becomes remarkable when the content exceeds 0.01%. For this reason, S was limited to 0.01% or less. In order to avoid such an adverse effect of S, S is preferably 0.01% or less, and is preferably reduced as much as possible. However, excessive reduction leads to an increase in manufacturing cost, so it should be about 0.0001% or more. Is preferred.
- Al acts as a deoxidizer and is an essential element for improving the cleanliness of the steel sheet, and also effectively acts for improving the yield of carbide forming elements.
- 0.01% or more of content is required. If the content is less than 0.01%, the removal of Si-based inclusions that are the starting point of delayed fracture becomes insufficient, and the risk of delayed fracture increases. On the other hand, even if the content exceeds 0.1%, the above-described effect is saturated, an effect commensurate with the content cannot be expected, and it becomes economically disadvantageous, workability is reduced, and surface defects tend to occur. Increase. For this reason, Al was limited to the range of 0.01 to 0.1%.
- the content is preferably 0.01 to 0.05%.
- N 0.005% or less N is an element that is essentially harmful in the present invention, and it is desirable to reduce it as much as possible, but up to 0.005% is acceptable. For this reason, N was limited to 0.005% or less. In addition, since excessive reduction of N causes a rise in manufacturing cost, it is preferable to make it about 0.0001% or more.
- the above components are basic components.
- Cr 0.05 to 0.5%
- V 0.005 to 0.2%
- Mo 0.005 to One or more selected from 0.2% and / or one selected from Ti: 0.01 to 0.1% and Nb: 0.01 to 0.1% Or 2 and / or B: 0.0003 to 0.0050% and / or Ni: 0.05 to 0.5%
- Cu 0.05 to 0.5% 1 type or 2 types and / or 1 or 2 types selected from Ca: 0.001 to 0.005% and REM: 0.001 to 0.005% are selected and contained.
- Can do
- Cr 0.05 to 0.5%, V: 0.005 to 0.2%, Mo: 0.005 to 0.2% Cr, V, and Mo are Any of them is an element that increases the strength of the steel sheet and contributes to the formation of the composite structure, and can be selected as necessary and contained in one or more kinds.
- excessive content exceeding Cr: 0.5%, V: 0.2%, Mo: 0.2%, respectively makes it difficult to produce a desired amount of pearlite during the cooling treatment after the annealing treatment, A desired composite structure cannot be secured, stretch flangeability is lowered, and workability is lowered.
- it may be limited to the ranges of Cr: 0.05 to 0.5%, V: 0.005 to 0.2%, and Mo: 0.005 to 0.2%, respectively. preferable.
- Ti and Nb are both elements that increase the steel sheet strength by precipitation strengthening. These can be selected as necessary and can be contained alone or in combination. In order to obtain such an effect, it is desirable to contain Ti: 0.01% or more and Nb: 0.01% or more, respectively, but Ti: 0.1% and Nb: more than 0.1%, respectively. Containment reduces processability and shape freezing property. For this reason, when it contains, it is preferable to limit in the range of Ti: 0.01-0.1% and Nb: 0.01-0.1%, respectively.
- B 0.0003 to 0.0050%
- B is an element that segregates at the austenite grain boundary and has an action of suppressing the formation and growth of ferrite from the grain boundary, and can be contained as necessary. In order to acquire such an effect, it is desirable to contain 0.0003% or more, but inclusion exceeding 0.0050% reduces workability. For this reason, when contained, B is preferably limited to a range of 0.0003 to 0.0050%. In addition, in order to acquire the effect of B as mentioned above, it is necessary to suppress the production
- Ni and Cu both have an action of increasing the strength of the steel sheet, It is an element that also has an effect of promoting oxidation and improving plating adhesion, and can be selected and contained as necessary. In order to obtain such an effect, it is desirable to contain Ni: 0.05% or more and Cu: 0.05% or more, respectively, but Ni: 0.5% and Cu: 0.5% are exceeded. Containment makes it difficult to produce a desired amount of pearlite during the cooling treatment after the annealing treatment, making it impossible to secure a desired composite structure, lowering stretch flangeability, and lowering workability. For this reason, when it contains, it is preferable to limit to Ni: 0.05-0.5% and Cu: 0.05-0.5%.
- REM One or two selected from 0.001 to 0.005%
- Ca and REM are elements that contribute to sulfide morphology control. It has the effect of suppressing the adverse effect on the workability of sulfides, particularly the stretch flangeability, by making the shape of sulfides spherical.
- Excessive inclusion causes an increase in inclusions, resulting in frequent occurrence of surface defects and internal defects. For this reason, when it contains, it is preferable to limit to Ca: 0.001-0.005% and REM: 0.001-0.005%.
- the balance other than the components described above consists of Fe and inevitable impurities.
- the steel sheet of the present invention has the above-described composition and a structure composed of a ferrite phase as a main phase and a second phase containing at least pearlite.
- the area ratio of the ferrite phase that is the main phase is 75 to 90% in terms of the area ratio with respect to the entire structure. If the area ratio of the ferrite phase is less than 75%, the desired elongation and the desired hole expansion rate cannot be ensured, and the workability deteriorates. On the other hand, if the area ratio of the ferrite phase exceeds 90%, the area ratio of the second phase decreases, and a desired high strength cannot be ensured. For this reason, the area ratio of the ferrite phase as the main phase is limited to a range of 75 to 90%. A preferable area ratio of the ferrite phase is 80 to 90%.
- the second phase contains at least pearlite.
- the area ratio of pearlite is the area ratio with respect to the whole structure, and is 10 to 25%. If the area ratio of pearlite is less than 10%, a desired hole expansion rate cannot be ensured, the stretch flangeability is lowered, and the workability is lowered. On the other hand, when the area ratio of pearlite exceeds 25%, the interface between the ferrite phase and pearlite increases, voids are likely to be generated during processing, stretch flangeability decreases, and workability decreases.
- pearlite is a fine particle having an average particle size of 5 ⁇ m or less.
- the average grain size of pearlite exceeds 5 ⁇ m and becomes coarse, stress concentrates on the pearlite grains (interface) during the processing of the steel sheet, and microvoids are generated, so that stretch flangeability is lowered and workability is lowered.
- the average particle size of pearlite was limited to 5 ⁇ m or less.
- Preferably it is 4.0 micrometers or less.
- the second phase in the structure of the steel sheet of the present invention is a phase mainly composed of pearlite that contains at least pearlite, and the pearlite has an area ratio of 70% or more with respect to the total area of the second phase. If the pearlite is less than 70% in area ratio with respect to the total area of the second phase, the hard martensite phase, bainite phase, or residual ⁇ increases too much, and the workability tends to be lowered. For this reason, pearlite was limited to 70% or more in the area ratio with respect to the total area of a 2nd phase. Preferably, it is 75 to 100%.
- the second phase may contain bainite, martensite, retained austenite (residual ⁇ ), etc., but in particular, bainite and martensite are hard phases, and residual ⁇ is transformed during processing. It transforms into martensite, which degrades workability. For this reason, it is desirable that these bainite, martensite and retained austenite be as small as possible, and the total area ratio with respect to the entire structure is preferably 5% or less. More preferably, the total content is 3% or less.
- a steel material having the above composition is used as a starting material.
- the method for producing the steel material is not particularly limited, but the molten steel having the above composition is melted by a conventional melting method such as a converter or an electric furnace, and a slab or the like is obtained by a conventional casting method such as a continuous casting method. It is preferable to use a steel material from the viewpoint of productivity. It is also possible to apply an ingot-bundling rolling method, a thin slab casting method, or the like.
- the steel material having the above composition is subjected to a hot rolling process to obtain a hot rolled sheet.
- the steel material is heated to a temperature in the range of 1100 to 1280 ° C., and then hot rolled to a hot rolling finish temperature of 870 to 950 ° C. to form a hot rolled sheet,
- the hot-rolled sheet is preferably a step of winding at a winding temperature of 350 to 720 ° C. If the heating temperature of the steel material is less than 1100 ° C., the deformation resistance becomes too high, the rolling load becomes excessive, and hot rolling may be difficult.
- the heating temperature for hot rolling is preferably set to a temperature in the range of 1100 to 1280 ° C. More preferably, it is less than 1280 degreeC.
- the hot rolling finish temperature is less than 870 ° C.
- ferrite ( ⁇ ) and austenite ( ⁇ ) are generated during rolling, and a band-like structure is easily generated on the steel sheet.
- This band-like structure remains even after annealing, and may cause anisotropy in the obtained steel sheet characteristics or cause a decrease in workability.
- the hot rolling end temperature is preferably 870 to 950 ° C.
- the winding temperature after the hot rolling is less than 350 ° C.
- bainitic ferrite, bainite, martensite, etc. are generated, and it is easy to form a hard and non-sized hot rolled structure, and in the subsequent annealing treatment In some cases, it inherits the hot-rolled structure, tends to become a non-sized structure, and cannot secure the desired workability.
- the winding temperature is preferably set to a temperature in the range of 350 to 720 ° C. More preferably, the temperature is 500 to 680 ° C.
- the hot-rolled sheet obtained through the hot-rolling process is subjected to pickling according to a conventional method, and then cold-rolling the hot-rolled sheet.
- the continuous annealing process which performs an annealing process and a subsequent cooling process in a continuous annealing line directly is performed.
- the annealing process is a process of holding for 5 to 400 s in the first temperature range from the A c1 transformation point to the A c3 transformation point.
- the temperature (heating temperature) in the first temperature range of the annealing treatment is less than the Ac1 transformation point, or the holding time (annealing time) in the first temperature range is less than 5 s, hot rolling Since the carbide in the plate does not dissolve sufficiently, or the ⁇ ⁇ ⁇ transformation does not occur or is insufficient, the desired composite structure cannot be secured by the subsequent cooling treatment, so the desired elongation and hole expansion rate are satisfied. A steel sheet having ductility and stretch flangeability cannot be obtained.
- the heating temperature of the annealing process becomes higher than the Ac3 transformation point, coarsening of the austenite grains becomes remarkable, the structure generated by the subsequent cooling process becomes coarse, and workability may be lowered.
- the annealing treatment is limited to a treatment for holding for 5 to 400 s in the first temperature range from the A c1 transformation point to the A c3 transformation point.
- the A c1 transformation point of each steel plate is the following equation (1), and the A c3 transformation point is the value calculated by the following equation (2).
- the element is calculated as zero.
- a c1 transformation point (° C.) 723 + 29.1Si-10.7Mn-16.9Ni + 16.9Cr + 6.38W + 290As (1)
- Ac 3 transformation point (° C.) 910 ⁇ 203 ⁇ C + 44.7Si-30Mn + 700P + 400Al-15.2Ni-11Cr-20Cu + 31.5Mo + 104V + 400Ti + 13.1W + 120As (2)
- C, Si, Mn, Ni, Cr, W, As, C, P, Al, Cu, Mo, V, Ti Content of each element (mass%)
- the cooling treatment after the annealing treatment is performed by cooling from the first temperature range to 700 ° C. at an average cooling rate of 5 ° C./s or more, and further in the second temperature range of 700
- the cooling rate from the first temperature range to 700 ° C. is less than 5 ° C./s, the amount of ferrite produced increases too much, the desired composite structure cannot be obtained, the workability decreases, and the desired tensile strength is further reduced.
- the strength (540 MPa or more) may not be ensured.
- the cooling rate from the first temperature range to 700 ° C. was limited to 5 ° C./s or more on average.
- the temperature is preferably 20 ° C./s or less, more preferably 5 to 15 ° C./s.
- the residence time in the second temperature range of 700 ° C. to 400 ° C. is an important factor for the formation of pearlite contained in the second phase.
- the “residence time” means the time of staying in the above-mentioned second temperature range, and when holding at the specific temperature of the second temperature range, The case where it cools with a cooling rate and the case where it cools with the pattern which mixed them are included. If the residence time in the second temperature range is less than 30 s, pearlite transformation does not occur or the amount of pearlite produced is insufficient, so a desired composite structure cannot be ensured. On the other hand, when the residence time in the second temperature range is longer than 400 s, productivity is lowered.
- the residence time in the second temperature range is limited to the range of 30 to 400 s. In addition, Preferably it is 150 s or less.
- the cooling time in the temperature range of 700 to 550 ° C. is 10 s or more, that is, the cooling rate in the temperature range of 700 to 550 ° C. is 15 ° C./s or less on average. It is preferable for securing a desired amount of pearlite. If the cooling time in the temperature range of 700 to 550 ° C. is less than 10 s, the formation of pearlite becomes insufficient, the desired composite structure cannot be obtained, and the desired workability may not be ensured.
- Molten steel having the composition shown in Table 1 was melted and used as a steel material by a conventional method. These steel materials are hot-rolled at the heating temperature and hot rolling end temperature shown in Table 2 to form a 1.6 mm thick hot rolled sheet, and after hot rolling, coiled at the winding temperature shown in Table 2 Rolled up. Thereafter, pickling was performed. Some hot-rolled sheets (thickness: 3.2 mm) were pickled and then cold-rolled at a reduction ratio of 50% to obtain 1.6 mm-thick cold-rolled sheets as comparative examples. .
- the obtained hot-rolled sheet or cold-rolled sheet is further heated to the temperature in the first temperature range under the conditions shown in Table 2, and the annealing treatment to be held, from the temperature in the first temperature range to 700 ° C, Cooling is performed at the average cooling rate shown in Table 2, and 700 to 550 ° C. of the second temperature range is further cooled at the cooling rate (cooling time) shown in Table 2, and then the second temperature of 700 to 400 ° C.
- the region residence time is shown in Table 2 and the residence time is used. Cooling treatment is performed, and a continuous annealing step is performed to obtain an annealing plate.
- the transformation point of each steel plate shown in Table 2 is a value calculated using the above-described equations (1) and (2).
- Specimens were collected from the obtained annealed plate and subjected to a structure observation, a tensile test, and a hole expansion test.
- the test method was as follows.
- the average crystal grain size of pearlite is determined by measuring the area of each pearlite grain, calculating the equivalent circle diameter from the area, arithmetically averaging the equivalent circle diameter of each obtained grain, It was.
- the measured number of pearlite particles was 20 or more.
- the area ratio with respect to the total area of the second phase of pearlite was also calculated.
- All of the examples of the present invention have high tensile strength TS: 540 MPa or more, elongation El: high ductility of 30% or more, and excellent stretch flangeability of hole expansion ratio ⁇ : 80% or more. It is a high-strength steel sheet with excellent properties.
- the desired high strength is not obtained, the desired elongation is not obtained, or the desired hole expansion ratio ⁇ is not obtained. , Workability is degraded.
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Abstract
Description
例えば、特許文献1には、C:0.08~0.30%、Si:0.1~2.5%、Mn:0.5~2.5%、P:0.01~0.15%を含む組成の冷延鋼板を、Ac1点以上の温度にて再結晶焼鈍し、次いで、Ar1点乃至600℃の範囲の温度域まで強制空冷したのち、100℃/s以上の冷却速度で急冷し、フェライト相と低温変態生成相からなる複合組織とし、この後、所定の関係式で求められる、フェライト硬さHv(α)に対する低温変態生成相硬さHv(L)の比、Hv(L)/Hv(α)、が1.5~3.5を満足するように、350~600℃の範囲の温度にて過時効処理を行う局部延性にすぐれる高強度冷延鋼板の製造方法が記載されている。特許文献1に記載された技術では、焼入れ開始温度を高くし低温変態生成相の体積率を高め、その後、350~600℃で過時効処理を行って、フェライト中にCを析出させるとともに、低温変態生成相を軟化させて、Hv(L)/Hv(α)を小さくし、局部伸びを改善するとしている。
また、特許文献2に記載された技術では、多量のP、Cuを複合して添加することを必須としているが、Cuの多量含有は、熱間加工性を低下させ、また、Pの多量含有は、鋼を脆化させる。また、Pは、鋼中に偏析する傾向が強く、この偏析したPは、鋼板の伸びフランジ性を低下させるほか、溶接部の脆化を引き起こすという問題がある。
本発明は、かかる従来技術の問題を解決し、板厚:1.0~1.8mm程度の薄肉の、加工性に優れた高強度鋼板およびその製造方法を提供することを目的とする。なお、ここでいう「高強度」とは、引張強さTS:540MPa以上、好ましくは590MPa以上の強度を有する場合をいい、また、「加工性に優れた」とは、伸びEl:30%以上(JIS5号試験片を用いた場合)、日本鉄鋼連盟規格JFST 1001−1996に準拠した穴拡げ試験における穴拡げ率λ:80%以上である場合をいうものとする。
熱延板に、冷間圧延を施すことなく、二相温度域に加熱する焼鈍処理を施す場合は、焼鈍加熱時には、α→γ変態が生じるだけであり、新たに再結晶が生じることはない。この場合、C濃度が高い箇所で優先的にα→γ変態が生じるのみであり、より均一な組織を得ることができるうえ、拡散速度の速いCは、焼鈍処理時に平衡組成までαとγに再分配される。このため、粒界でのフィルム状セメンタイトの析出が抑制され、とくに伸びフランジ性の向上に有利に作用したと考えられる。一方、熱延板に冷間圧延を施したのちに、焼鈍処理を施す場合は、焼鈍加熱時に再結晶と、α→γ変態が競合して生じるため、不均一な組織となりやすく、大幅な加工性の向上は期待できにくい。
(1)mass%で、C:0.08~0.15%、Si:0.5~1.5%、Mn:0.5~1.5%、P:0.1%以下、S:0.01%以下、Al:0.01~0.1%、N:0.005%以下を含み、残部Feおよび不可避的不純物からなる組成と、主相であるフェライト相と、少なくともパーライトを含む第二相とからなる組織と、を有し、組織全体に対する面積率で、前記フェライト相が75~90%、前記パーライトが10~25%で、かつ該パーライトの平均粒径が5μm以下であり、さらに前記パーライトが、前記第二相の全面積に対する面積率で70%以上であることを特徴とする加工性に優れた高強度鋼板。
C:0.08~0.15%
Cは、鋼板強度の増加に寄与するとともに、組織をフェライト相とフェライト相以外の第二相とからなる複合組織の形成に有効に作用する元素であり、本発明では、所望の引張強さ:540MPa以上の高強度を確保するために、0.08%以上の含有を必要とする。一方、0.15%を超える含有は、スポット溶接性を低下させ、さらに延性等の加工性を低下させる。このため、Cは0.08~0.15%の範囲に限定した。なお、好ましくは0.10~0.15%である。
Siは、鋼中に固溶してフェライトの強化に有効に作用するとともに、延性向上にも寄与する元素であり、所望の引張強さ:540MPa以上の高強度を確保するためには、0.5%以上の含有を必要とする。一方、1.5%を超える過剰な含有は、赤スケール等の発生を促進し、鋼板の表面性状を低下させるとともに、化成処理性を低下させる。また、Siの過剰な含有は、抵抗溶接時の電気抵抗の増加を伴い、抵抗溶接性を阻害する。このため、Siは0.5~1.5%の範囲に限定した。なお、好ましくは0.7~1.2%である。
Mnは、鋼板強度の増加に寄与するとともに、複合組織の形成に有効に作用する元素であり、このような効果を得るためには、0.5%以上の含有を必要とする。一方、1.5%を超える含有は、焼鈍時の冷却過程でマルテンサイト相を形成しやすくなり、加工性、とくに伸びフランジ性の低下を招く。このため、Mnは0.5~1.5%の範囲に限定した。なお、好ましくは0.7~1.5%である。
Pは、鋼中に固溶して鋼板強度を増加させる作用を有する元素であるが、粒界へ偏析する傾向が強く、粒界の結合力を低下させて、加工性の低下を招くとともに、鋼板表面へ濃化して、化成処理性、耐食性などを低下させる。このようなPの悪影響は、0.1%を超える含有で顕著となる。このため、Pは0.1%以下に限定した。なお、このようなPの悪影響を避けるため、Pは0.1%以下で、できるだけ低減することが好ましいが、過度の低減は製造コストの高騰を招くため、0.001%程度以上とすることが好ましい。
Sは、鋼中では主としてMnS等の硫化物(介在物)を形成し、鋼板の加工性、とくに局部伸び、を低下させる。また、硫化物(介在物)の存在は、溶接性をも低下させる。このようなSの悪影響は、0.01%を超える含有で顕著となる。このため、Sは0.01%以下に限定した。なお、このようなSの悪影響を避けるため、Sは0.01%以下で、できるだけ低減することが好ましいが、過度の低減は製造コストの高騰を招くため、0.0001%程度以上とすることが好ましい。
Alは、脱酸剤として作用し鋼板の清浄度向上に必須の元素であり、さらに炭化物形成元素の歩留り向上に有効に作用する。このような効果を得るためには、0.01%以上の含有を必要とする。0.01%未満の含有では、遅れ破壊の起点となるSi系介在物の除去が不十分となり、遅れ破壊発生の危険性が増加する。一方、0.1%を超えて含有しても、上記した効果は飽和し、含有量に見合う効果が期待できなくなり経済的に不利となるとともに、加工性が低下し、表面欠陥の発生傾向が増大する。このため、Alは0.01~0.1%の範囲に限定した。なお、好ましくは0.01~0.05%である。
Nは、本発明では本質的に有害な元素として、できるだけ低減することが望ましいが、0.005%までは許容できる。このため、Nは0.005%以下に限定した。なお、過度のNの低減は、製造コストの高騰を招くため、0.0001%程度以上とすることが好ましい。
Cr、V、Moはいずれも、鋼板強度を増加させ、複合組織の形成に寄与する元素であり、必要に応じて選択して、1種または2種以上含有できる。このような効果を得るためには、Cr:0.05%以上、V:0.005%以上、Mo:0.005%以上、それぞれ含有することが望ましい。一方、Cr:0.5%、V:0.2%、Mo:0.2%、をそれぞれ超える過剰な含有は、焼鈍処理後の冷却処理中に、所望量のパーライトの生成が困難となり、所望の複合組織を確保できなくなり、伸びフランジ性が低下し、加工性が低下する。このため、含有する場合には、Cr:0.05~0.5%、V:0.005~0.2%、Mo:0.005~0.2%の範囲に、それぞれ限定することが好ましい。
Ti、Nbはいずれも、析出強化により鋼板強度を増加させる元素であり、必要に応じて選択して、1種または2種含有できる。このような効果を得るためには、Ti:0.01%以上、Nb:0.01%以上、それぞれ含有することが望ましいが、Ti:0.1%、Nb:0.1%をそれぞれ超える含有は、加工性、形状凍結性が低下する。このため、含有する場合には、Ti:0.01~0.1%、Nb:0.01~0.1%の範囲に、それぞれ限定することが好ましい。
Bは、オーステナイト粒界に偏析して、粒界からのフェライトの生成、成長を抑制する作用を有する元素であり、必要に応じて含有できる。このような効果を得るためには、0.0003%以上含有することが望ましいが、0.0050%を超える含有は、加工性を低下させる。このため、含有する場合には、Bは0.0003~0.0050%の範囲に限定することが好ましい。なお、上記したようなBの効果を得るためには、BNの生成を抑制することが必要であり、Tiとともに含有させることが好ましい。
Ni、Cuはいずれも、鋼板強度を増加させる作用を有するとともに、内部酸化を促進させめっき密着性を向上させる作用も有する元素であり、必要に応じ選択して含有できる。このような効果を得るためには、Ni:0.05%以上、Cu:0.05%以上それぞれ含有することが望ましいが、Ni:0.5%、Cu:0.5%、をそれぞれ超える含有は、焼鈍処理後の冷却処理中に、所望量のパーライトの生成が困難となり、所望の複合組織を確保できなくなり、伸びフランジ性が低下し、加工性が低下する。このため、含有する場合には、Ni:0.05~0.5%、Cu:0.05~0.5%の範囲に限定することが好ましい。
Ca、REMはいずれも、硫化物の形態制御に寄与する元素であり、硫化物の形状を球状化し、硫化物の加工性、とくに伸びフランジ性への悪影響を抑制する作用を有する。このような効果を得るためには、Ca:0.001%以上、REM:0.001%以上、それぞれ含有することが望ましいが、Ca:0.005%、REM:0.005%、をそれぞれ超える含有は、介在物の増加を招き、表面欠陥および内部欠陥の多発を招く。このため、含有する場合には、Ca:0.001~0.005%、REM:0.001~0.005%の範囲に限定することが好ましい。
本発明鋼板は、上記した組成を有するとともに、主相であるフェライト相と、少なくともパーライトを含む第二相とからなる組織を有する。
本発明鋼板では、主相であるフェライト相の面積率は、組織全体に対する面積率で、75~90%とする。フェライト相の面積率が75%未満では、所望の伸び、所望の穴拡げ率を確保できず、加工性が低下する。一方、フェライト相の面積率が90%を超えると、第二相の面積率が低下し、所望の高強度を確保できなくなる。このため、主相であるフェライト相の面積率は75~90%の範囲に限定した。なお、好ましいフェライト相の面積率は80~90%である。
上記した組成を有する鋼素材を出発素材とする。鋼素材の製造方法はとくに限定する必要はないが、上記した組成の溶鋼を転炉、電気炉等の常用の溶製方法で溶製し、連続鋳造法等の常用の鋳造方法でスラブ等の鋼素材とすることが、生産性の観点から好ましい。なお、造塊−分塊圧延法、薄スラブ鋳造法などを適用することもできる。
鋼素材の加熱温度が、1100℃未満では、変形抵抗が高くなりすぎて、圧延荷重が過大となり、熱間圧延が困難となる場合がある。一方、1280℃を超えると、結晶粒が粗大化しすぎて、熱間圧延を施しても所望の微細な鋼板組織を確保できにくくなる。このため、熱間圧延のための加熱温度は、1100~1280℃の範囲の温度とすることが好ましい。より好ましくは1280℃未満である。
焼鈍処理は、Ac1変態点~Ac3変態点の第一の温度域で5~400s間保持する処理とする。
Ac1変態点(℃)=723+29.1Si−10.7Mn−16.9Ni+16.9Cr+6.38W+290As‥‥(1)
Ac3変態点(℃)=910−203√C+44.7Si−30Mn+700P+400Al−15.2Ni−11Cr−20Cu+31.5Mo+104V+400Ti+13.1W+120As ‥‥(2)
ここで、C,Si,Mn,Ni,Cr,W,As,C,P,Al,Cu,Mo,V,Ti:各元素の含有量(mass%)
また、焼鈍処理後の冷却処理は、上記した第一の温度域から700℃までを、平均で、5℃/s以上の冷却速度で冷却し、さらに700℃~400℃の第二の温度域での滞留時間を30~400sとする処理とする。
得られた焼鈍板から、組織観察用試験片を採取し、圧延方向に平行な断面(L断面)を研磨し、ナイタール液で腐食し、走査型電子顕微鏡(倍率:3000倍)で3視野以上、組織観察し、撮像して、組織の種類、各相の組織全体に対する面積率を測定し、さらに第二相全面積の、組織全体に対する面積率を算出した。また、第二相に含まれるパーライトの平均結晶粒径も算出した。なお、パーライトの平均結晶粒径は、各パーライト粒の面積を測定し、該面積から円相当直径を算出し、得られた各粒の円相当直径を算術平均し、パーライト粒の平均結晶粒径とした。なお、測定したパーライトの粒数は20個以上とした。また、パーライトの第二相全面積に対する面積率も算出した。
得られた焼鈍板から、引張方向が、圧延方向に直角方向と一致するように、JIS5号試験片を採取し、JIS Z 2241の規定に準拠して引張試験を実施し、引張特性(降伏点YP、引張強さTS、伸びEl)を求めた。
得られた焼鈍板から、100mm角の穴拡げ試験片を採取した。そして、日本鉄鋼連盟規格JFST 1001−1996の規定に準拠して、穴拡げ試験を実施し、穴拡げ率λ(%)を求めた。
Claims (14)
- mass%で、
C :0.08~0.15%、 Si:0.5~1.5%、
Mn:0.5~1.5%、 P :0.1%以下、
S :0.01%以下、 Al:0.01~0.1%、
N :0.005%以下
を含み、残部Feおよび不可避的不純物からなる組成と、主相であるフェライト相と、少なくともパーライトを含む第二相とからなる組織と、を有し、組織全体に対する面積率で、前記フェライト相が75~90%、前記パーライトが10~25%で、かつ該パーライトの平均粒径が5μm以下であり、さらに前記パーライトが、前記第二相の全面積に対する面積率で70%以上であることを特徴とする加工性に優れた高強度鋼板。 - 前記組成に加えてさらに、mass%で、Cr:0.05~0.5%、V:0.005~0.2%、Mo:0.005~0.2%のうちから選ばれた1種または2種以上を含有することを特徴とする請求項1に記載の高強度鋼板。
- 前記組成に加えてさらに、mass%で、Ti:0.01~0.1%、Nb:0.01~0.1%のうちから選ばれた1種または2種を含有することを特徴とする請求項1または2に記載の高強度鋼板。
- 前記組成に加えてさらに、mass%で、B:0.0003~0.0050%を含有することを特徴とする請求項1ないし3のいずれかに記載の高強度鋼板。
- 前記組成に加えてさらに、mass%で、Ni:0.05~0.5%、Cu:0.05~0.5%のうちから選ばれた1種または2種を含有することを特徴とする請求項1ないし4のいずれかに記載の高強度鋼板。
- 前記組成に加えてさらに、mass%で、Ca:0.001~0.005%、REM:0.001~0.005%のうちから選ばれた1種または2種を含有することを特徴とする請求項1ないし5のいずれかに記載の高強度鋼板。
- mass%で、
C :0.08~0.15%、 Si:0.5~1.5%、
Mn:0.5~1.5%、 P :0.1%以下、
S :0.01%以下、 Al:0.01~0.1%、
N :0.005%以下
を含み、残部Feおよび不可避的不純物からなる組成を有する鋼素材に、熱間圧延を施し熱延板とする熱延工程と、前記熱延板に酸洗を施したのち、該熱延板を、連続焼鈍ラインで、Ac1変態点~Ac3変態点の第一の温度域で5~400s間保持する焼鈍処理と、該焼鈍処理後、前記第一の温度域から700℃までを、5℃/s以上の平均冷却速度で冷却し、さらに700℃~400℃までの第二の温度域での滞留時間を30~400sとする冷却処理を行う連続焼鈍工程と、を施すことを特徴とする加工性に優れた高強度鋼板の製造方法。 - 前記熱延工程が、前記鋼素材を1100~1280℃の範囲の温度に加熱したのち、熱間圧延終了温度:870~950℃とする熱間圧延を行い熱延板とし、該熱間圧延の終了後、該熱延板を、巻取り温度:350~720℃として巻き取る、工程であることを特徴とする請求項7の記載の高強度鋼板の製造方法。
- 前記第二の温度域のうち、700~550℃の温度域での冷却時間を10s以上とすることを特徴とする請求項7または8に記載の高強度鋼板の製造方法。
- 前記組成に加えてさらに、mass%で、Cr:0.05~0.5%、V:0.005~0.2%、Mo:0.005~0.2%のうちから選ばれた1種または2種以上を含有することを特徴とする請求項7ないし9のいずれかに記載の高強度鋼板の製造方法。
- 前記組成に加えてさらに、mass%で、Ti:0.01~0.1%、Nb:0.01~0.1%のうちから選ばれた1種または2種を含有することを特徴とする請求項7ないし10のいずれかに記載の高強度鋼板の製造方法。
- 前記組成に加えてさらに、mass%で、B:0.0003~0.0050%を含有することを特徴とする請求項7ないし11のいずれかに記載の高強度鋼板の製造方法。
- 前記組成に加えてさらに、mass%で、Ni:0.05~0.5%、Cu:0.05~0.5%のうちから選ばれた1種または2種を含有することを特徴とする請求項7ないし12のいずれかに記載の高強度鋼板の製造方法。
- 前記組成に加えてさらに、mass%で、Ca:0.001~0.005%、REM:0.001~0.005%のうちから選ばれた1種または2種を含有することを特徴とする請求項7ないし13のいずれかに記載の高強度鋼板の製造方法。
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US20130233453A1 (en) | 2013-09-12 |
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JP5018934B2 (ja) | 2012-09-05 |
EP2589678A1 (en) | 2013-05-08 |
TWI431124B (zh) | 2014-03-21 |
CN102971443A (zh) | 2013-03-13 |
JP2012012623A (ja) | 2012-01-19 |
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