WO2019186989A1 - 鋼板 - Google Patents
鋼板 Download PDFInfo
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- WO2019186989A1 WO2019186989A1 PCT/JP2018/013554 JP2018013554W WO2019186989A1 WO 2019186989 A1 WO2019186989 A1 WO 2019186989A1 JP 2018013554 W JP2018013554 W JP 2018013554W WO 2019186989 A1 WO2019186989 A1 WO 2019186989A1
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- less
- area fraction
- ferrite
- retained austenite
- bainitic ferrite
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- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
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Definitions
- the present invention relates to a steel plate suitable for automobile parts.
- high-strength steel plates are often used for skeletal parts of vehicle bodies.
- Mechanical properties that have a significant impact on crash safety include tensile strength, ductility, ductility-brittle transition temperature, and 0.2% yield strength.
- the steel sheet used for the front side member is required to have excellent ductility.
- Patent Documents 1 and 2 propose techniques relating to improvement of collision safety or improvement of moldability.
- Patent Documents 1 and 2 propose techniques relating to improvement of collision safety or improvement of moldability.
- An object of the present invention is to provide a steel sheet capable of obtaining excellent collision safety and formability.
- the present inventors have intensively studied to solve the above problems. As a result, it has been clarified that, in a steel sheet having a tensile strength of 980 MPa or more, excellent elongation is expressed by making the area fraction and form of retained austenite and bainitic ferrite predetermined. Furthermore, when the area fraction of polygonal ferrite is low, the hardness difference in the steel sheet is small, not only excellent elongation, but also excellent hole expansibility and bendability are obtained, and resistance to embrittlement at a sufficiently low temperature It was found that properties and 0.2% yield strength were also obtained.
- the inventor of the present application has come up with the following aspects of the invention as a result of further intensive studies based on such knowledge.
- 80% or more of the bainitic ferrite has a dislocation density of a region surrounded by grain boundaries having an aspect ratio of 0.1 to 1.0 and an orientation difference angle of 15 ° or more.
- 80% or more of the retained austenite has an aspect ratio of 0.1 to 1.0, a major axis length of 1.0 ⁇ m to 28.0 ⁇ m, and a minor axis length of 0.
- the metal structure is an area fraction, Polygonal ferrite: 5% to 20% Martensite: 20% or less, Bainitic ferrite: 75% to 90%, and retained austenite: 5% to 20%.
- the steel sheet according to (1) which is represented by:
- the metal structure is an area fraction, Polygonal ferrite: more than 20% and 40% or less, Martensite: 20% or less, Bainitic ferrite: 50% to 75%, and retained austenite: 5% to 30%.
- the steel sheet according to (1) which is represented by:
- FIG. 1 is a diagram showing an example of an equivalent ellipse of retained austenite grains.
- the steel plate according to the present embodiment has an area fraction of polygonal ferrite: 40% or less, martensite: 20% or less, bainitic ferrite: 50% to 95%, and retained austenite: 5% to 50%. It has a metallographic structure represented. In terms of area fraction, 80% or more of bainitic ferrite has a dislocation density of 8 in a region surrounded by grain boundaries having an aspect ratio of 0.1 to 1.0 and an orientation difference angle of 15 ° or more. It is comprised from the bainitic ferrite grain of * 10 ⁇ 2 > (cm / cm ⁇ 3 >) or less.
- 80% or more of the retained austenite has an aspect ratio of 0.1 to 1.0, a major axis length of 1.0 ⁇ m to 28.0 ⁇ m, and a minor axis length of 0.00. It consists of residual austenite grains of 1 ⁇ m to 2.8 ⁇ m.
- Polygonal ferrite is a soft structure. For this reason, there is a large difference in hardness between polygonal ferrite and martensite, which is a hard structure, and cracks are likely to occur at the interface between them during molding. A crack may extend along this interface. When the area fraction of polygonal ferrite exceeds 40%, such cracks are likely to be generated and extended, and sufficient hole expandability, bendability, resistance to embrittlement at low temperatures and 0.2% yield strength are difficult to obtain. . Therefore, the area fraction of polygonal ferrite is 40% or less.
- the area fraction of polygonal ferrite is preferably 20% or less, and when ductility is more important than hole expandability, the area fraction of polygonal ferrite is Preferably, it is more than 20% and 40% or less. Even when hole expansibility is more important than ductility, the area fraction of polygonal ferrite is preferably 5% or more in order to ensure ductility.
- Bainitic ferrite area fraction 50% to 95%) Bainitic ferrite contains dislocations at a higher density than polygonal ferrite, and contributes to improvement in tensile strength. Since the hardness of bainitic ferrite is higher than that of polygonal ferrite and lower than that of martensite, the hardness difference between bainitic ferrite and martensite is the hardness between polygonal ferrite and martensite. Smaller than the difference. Therefore, bainitic ferrite also contributes to improvement of hole expansibility and bendability. If the area fraction of bainitic ferrite is less than 50%, sufficient tensile strength cannot be obtained. Therefore, the area fraction of bainitic ferrite is 50% or more.
- the area fraction of bainitic ferrite is preferably 75% or more.
- the area fraction of bainitic ferrite exceeds 95%, the retained austenite is insufficient and sufficient formability cannot be obtained. Therefore, the area fraction of bainitic ferrite is 95% or less.
- Martensite includes fresh martensite (non-tempered martensite) and tempered martensite. As described above, the difference in hardness between polygonal ferrite and martensite is large, and cracks are likely to occur at the interface between them during molding. A crack may extend along this interface. When the area fraction of martensite exceeds 20%, such cracks are likely to be generated and extended, and it is difficult to obtain sufficient hole expansibility, bendability, resistance to embrittlement at low temperatures, and 0.2% yield strength. Therefore, the area fraction of martensite is 20% or less.
- Retention fraction of retained austenite 5% to 50% Residual austenite contributes to improvement of formability. If the area fraction of retained austenite is less than 5%, sufficient formability cannot be obtained. On the other hand, if the area fraction of retained austenite exceeds 50%, bainitic ferrite is insufficient and sufficient tensile strength cannot be obtained. Therefore, the area fraction of retained austenite is 50% or less.
- Polygonal ferrite, bainitic ferrite, retained austenite and martensite are identified and area fractions are identified by, for example, scanning electron microscope (SEM) observation or transmission electron microscope (transmission electron microscope: TEM). This can be done by observation.
- SEM scanning electron microscope
- TEM transmission electron microscope
- a sample is corroded using a nital liquid and a repeller liquid, and a cross section parallel to the rolling direction and the thickness direction (cross section perpendicular to the width direction) and / or a cross section perpendicular to the rolling direction is used. Observe at a magnification of 1000 to 100,000.
- FE-SEM-EBSD field emission scanning electron microscope
- a cross section (cross section perpendicular to the width direction) parallel to the rolling direction and thickness direction of the steel sheet is polished and etched with a nital solution.
- the area fraction is measured by observing a region where the depth from the surface of the steel plate is 1/8 to 3/8 of the thickness of the steel plate by FE-SEM. Such observation is performed for 10 visual fields at a magnification of 5000 times, and the area fractions of polygonal ferrite and bainitic ferrite are obtained from the average value of the 10 visual fields.
- the area fraction of retained austenite can be specified by, for example, X-ray measurement.
- X-ray measurement for example, a portion from the surface of the steel plate to 1 ⁇ 4 of the thickness of the steel plate is removed by mechanical polishing and chemical polishing, and MoK ⁇ rays are used as characteristic X-rays.
- the area fraction of retained austenite is calculated using the following formula. Such observation is performed for 10 visual fields, and the area fraction of retained austenite is obtained from the average value of the 10 visual fields.
- the area fraction of martensite can be identified by, for example, field emission-scanning electron microscope (FE-SEM) observation and X-ray measurement.
- FE-SEM field emission-scanning electron microscope
- X-ray measurement a region where the depth from the surface of the steel plate is 1/8 to 3/8 of the thickness of the steel plate is an observation object, and a repelling liquid is used for corrosion. Since the structures that are not corroded by the repellent liquid are martensite and residual austenite, the martensite is obtained by subtracting the area fraction S ⁇ of the residual austenite specified by the X-ray measurement from the area fraction of the area not corroded by the repellent liquid.
- the area fraction can be specified.
- the area fraction of martensite can be specified using, for example, an electronic channeling contrast image obtained by SEM observation.
- a region having a high dislocation density and a substructure such as a block or a packet in a grain is martensite. Such observation is performed for 10 visual fields, and the area fraction of martensite is obtained from the average value of the 10 visual fields.
- a bainitic ferrite having a high dislocation density do not contribute to improvement in elongation as much as polygonal ferrite, the higher the area fraction of bainitic ferrite grains having a high dislocation density, the easier the elongation decreases.
- the area fraction of the bainitic ferrite grains in such a form is 80% or more, preferably 85% or more with respect to the entire bainitic ferrite.
- the dislocation density of bainitic ferrite can be specified by structural observation using a transmission electron microscope (TEM).
- TEM transmission electron microscope
- the dislocation density of bainitic ferrite can be specified by dividing the number of dislocation lines existing in crystal grains surrounded by a grain boundary having an orientation difference angle of 15 ° by the area of the crystal grains. .
- Residual austenite is transformed into martensite by processing-induced transformation during molding.
- the retained austenite is transformed into martensite, when this martensite is adjacent to polygonal ferrite or untransformed retained austenite, a large hardness difference is generated between them.
- a large hardness difference leads to the occurrence of cracks as described above. Such cracks are particularly likely to occur at locations where stress is concentrated, and the stress is likely to be concentrated in the vicinity of martensite transformed from retained austenite having an aspect ratio of less than 0.1.
- the aspect ratio of the retained austenite grain is a value obtained by dividing the length of the minor axis of the equivalent ellipse of the retained austenite grain by the length of the major axis.
- the aspect ratio (L2 / L1) of the retained austenite grain can be obtained from the major axis length L1 and minor axis length L2 of the equivalent ellipse 2 thereof.
- the chemical composition of the steel plate and the slab used for manufacturing the steel plate according to the embodiment of the present invention will be described.
- the steel sheet according to the embodiment of the present invention is manufactured through hot rolling, pickling, cold rolling, first annealing, second annealing, and the like. Therefore, the chemical composition of the steel plate and slab takes into account not only the properties of the steel plate but also these treatments.
- “%”, which is a unit of content of each element contained in the steel plate and slab means “mass%” unless otherwise specified.
- the steel plate according to the present embodiment and the slab used for manufacturing the steel plate are mass%, C: 0.1% to 0.5%, Si: 0.5% to 4.0%, Mn: 1.0% to 4%.
- the C content is 0.10% or more, preferably 0.15% or more.
- the C content is 0.5% or less, preferably 0.25% or less.
- Silicon (Si: 0.5% to 4.0%) contributes to the improvement of the strength of the steel or contributes to the improvement of elongation through the improvement of the stability of retained austenite. If the Si content is less than 0.5%, these effects cannot be obtained sufficiently. Therefore, the Si content is 0.5% or more, preferably 1.0% or more. On the other hand, if the Si content exceeds 4.0%, the strength of the steel becomes too high and the elongation decreases. Therefore, the Si content is 4.0% or less, preferably 2.0% or less.
- Manganese (Mn: 1.0% to 4.0%) contributes to improving the strength of the steel or suppresses the polygonal ferrite transformation that occurs during the cooling of the first annealing or the second annealing. When the hot dip galvanizing process is performed, the polygonal ferrite transformation that occurs during the cooling of the process is also suppressed. If the Mn content is less than 1.0%, these effects cannot be obtained sufficiently, or polygonal ferrite is excessively generated and the hole expandability is deteriorated. Therefore, the Mn content is 1.0% or more, preferably 2.0% or more. On the other hand, if the Mn content exceeds 4.0%, the strength of the slab and hot-rolled steel sheet becomes too high. Therefore, it is 4.0% or less, preferably 3.0% or less.
- Phosphorus (P) is not an essential element but is contained as an impurity in steel, for example. P segregates at the central portion in the thickness direction of the steel sheet to reduce toughness or embrittle the weld. For this reason, the lower the P content, the better. In particular, when the P content exceeds 0.015%, a decrease in toughness and embrittlement of weldability are remarkable. Therefore, the P content is 0.015% or less, preferably 0.010% or less. Reduction of the P content requires a cost, and if it is attempted to reduce it to less than 0.0001%, the cost increases remarkably. For this reason, the P content may be 0.0001% or more.
- S Sulfur
- S is not an essential element but is contained as an impurity in steel, for example.
- S lowers the manufacturability of casting and hot rolling, or forms coarse MnS to lower the hole expandability. For this reason, the lower the S content, the better.
- the S content is 0.050% or less, preferably 0.0050% or less. Reduction of the S content takes a cost, and if it is attempted to reduce it to less than 0.0001%, the cost increases remarkably. For this reason, S content is good also as 0.0001% or more.
- N Nitrogen (N: 0.01% or less) Nitrogen (N) is not an essential element but is contained as an impurity in steel, for example. N forms coarse nitrides and degrades bendability and hole expandability, or causes blowholes during welding. For this reason, the lower the N content, the better. In particular, when the N content exceeds 0.01%, the decrease in bendability and hole expansibility and the occurrence of blowholes are remarkable. Therefore, the N content is 0.01% or less. Reduction of the N content is costly, and if it is attempted to reduce it to less than 0.0005%, the cost increases remarkably. For this reason, the N content may be 0.0005% or more.
- Aluminum (Al: 2.0% or less) functions as a deoxidizer and suppresses precipitation of iron-based carbides in austenite, but is not an essential element. If the Al content is more than 2.0%, transformation from austenite to polygonal ferrite is promoted, and polygonal ferrite is excessively generated to deteriorate the hole expandability. Therefore, the Al content is 2.0% or less, preferably 1.0% or less. Reduction of the Al content is costly, and if it is attempted to reduce it to less than 0.001%, the cost increases remarkably. For this reason, the Al content may be 0.001% or more.
- Si and Al 0.5% to 6.0% in total
- Both Si and Al contribute to the improvement of elongation through the improvement of the stability of retained austenite. If the content of Si and Al is less than 0.5% in total, this effect cannot be obtained sufficiently. Therefore, the total content of Si and Al is 0.5% or more, preferably 1.2% or more. Only either Si or Al may be contained, and both Si and Al may be contained.
- Ti, Nb, B, Mo, Cr, V, Mg, REM, and Ca are not essential elements, but are optional elements that may be appropriately contained in steel plates and slabs up to a predetermined amount.
- Titanium (Ti: 0.00% to 0.20%) contributes to improving the strength of steel through dislocation strengthening due to precipitation strengthening and fine grain strengthening. Therefore, Ti may be contained. In order to sufficiently obtain this effect, the Ti content is preferably 0.01% or more, more preferably 0.025% or more. On the other hand, if the Ti content exceeds 0.20%, Ti carbonitrides excessively precipitate and the formability of the steel sheet decreases. Therefore, the Ti content is 0.20% or less, preferably 0.08% or less.
- Niobium (Nb) contributes to improving the strength of steel through dislocation strengthening due to precipitation strengthening and fine grain strengthening. Therefore, Nb may be contained. In order to sufficiently obtain this effect, the Nb content is preferably 0.005% or more, more preferably 0.010% or more. On the other hand, if the Nb content exceeds 0.20%, Nb carbonitrides excessively precipitate and the formability of the steel sheet decreases. Therefore, the Nb content is 0.20% or less, preferably 0.08% or less.
- B Boron (B: 0.0000% to 0.0030%) Boron (B) reinforces the grain boundaries and suppresses the polygonal ferrite transformation that occurs during the cooling of the first annealing or the second annealing.
- B may be contained.
- the B content is preferably 0.0001% or more, more preferably 0.0010% or more.
- the B content is 0.0030% or less, preferably 0.0025% or less.
- Molybdenum (Mo) contributes to strengthening of the steel or suppresses the polygonal ferrite transformation that occurs during the cooling of the first annealing or the second annealing.
- Mo may be contained.
- the Mo content is preferably 0.01% or more, more preferably 0.02% or more.
- the Mo content is 0.50% or less, preferably 0.20% or less.
- Chromium (Cr) contributes to strengthening of the steel or suppresses the polygonal ferrite transformation that occurs during the cooling of the first annealing or the second annealing.
- Cr may be contained.
- the Cr content is preferably 0.01% or more, more preferably 0.02% or more.
- the Cr content is 2.0% or less, preferably 0.10% or less.
- V Vanadium (V) contributes to the improvement of steel strength through dislocation strengthening due to precipitation strengthening and fine grain strengthening. Therefore, V may be contained. In order to sufficiently obtain this effect, the V content is preferably 0.01% or more, more preferably 0.02% or more. On the other hand, if the V content exceeds 0.50%, the carbonitride of V precipitates excessively and the formability of the steel sheet decreases. Therefore, the Nb content is 0.50% or less, preferably 0.10% or less.
- Mg 0.000% to 0.040%, REM: 0.000% to 0.040%, Ca: 0.000% to 0.040%)
- Mg, REM or Ca or any combination thereof may be contained.
- the Mg content, the REM content, and the Ca content are all preferably 0.0005% or more, and more preferably 0.0010% or more.
- the Mg content, the REM content, or the Ca content exceeds 0.040%, a coarse oxide is formed and the hole expansibility is lowered. Therefore, the Mg content, the REM content, and the Ca content are all 0.040% or less, preferably 0.010% or less.
- REM rare earth metal refers to a total of 17 elements of Sc, Y and lanthanoid, and “REM content” means the total content of these 17 elements.
- REM is added by, for example, misch metal, and misch metal may contain a lanthanoid in addition to La and Ce.
- a simple metal such as metal La or metal Ce may be used.
- impurities examples include those contained in raw materials such as ore and scrap and those contained in the manufacturing process.
- P, S, O, Sb, Sn, W, Co, As, Pb, Bi, and H are exemplified as impurities.
- O content is preferably 0.010% or less
- Sb content, Sn content, W content, Co content and As content are preferably 0.1% or less
- Pb content and Bi content are Preferably it is 0.005% or less
- H content is preferably 0.0005% or less.
- the hole expandability is 30% or more
- the ratio (R / t) of the minimum bending radius (R (mm)) to the plate thickness (t (mm)) is 0.5 or less
- the total elongation is 21% or more
- 0 Mechanical properties with a 2% proof stress of 680 MPa or more, a tensile strength of 980 MPa or more, and a ductile-brittle transition temperature of -60 ° C. or less are obtained.
- the area fraction of polygonal ferrite is 5% to 20% and the area fraction of bainitic ferrite is 75% or more, hole expandability of 50% or more is obtained, and the area fraction of polygonal ferrite is obtained.
- the rate is more than 20% and 40% or less, a total elongation of 26% or more is obtained.
- Hot rolling rough rolling, finish rolling and winding of a slab are performed.
- a slab obtained by continuous casting or a slab produced by a thin slab caster can be used.
- the slab may be supplied to a hot rolling facility while being kept at a temperature of 1000 ° C. or higher after casting, or may be heated to a hot rolling facility after being cooled to a temperature of less than 1000 ° C.
- the rolling temperature of the final pass of rough rolling is 1000 ° C to 1150 ° C, and the rolling reduction of the final pass is 40% or more.
- the rolling temperature of the final pass is set to 1000 ° C. or higher.
- the rolling temperature in the final pass is higher than 1150 ° C., the austenite grain size after finish rolling becomes excessively large. Also in this case, the uniformity of the metal structure is lowered and sufficient formability cannot be obtained.
- the rolling temperature in the final pass is 1150 ° C. or lower.
- the rolling reduction of the final pass is less than 40%, the austenite grain size after finish rolling becomes excessively large, the uniformity of the metal structure is lowered, and sufficient formability cannot be obtained. Therefore, the rolling reduction of the final pass is 40% or more.
- the rolling temperature of finish rolling is Ar 3 points or more.
- this rolling temperature is set to Ar 3 or higher.
- this rolling temperature is set to Ar 3 or higher, the rolling load during finish rolling can be relatively reduced.
- finish rolling a plurality of rough rolled plates obtained by rough rolling may be continuously rolled. After the rough rolled plate is wound up once, finish rolling may be performed while rewinding.
- the winding temperature is 750 ° C or lower.
- the coiling temperature exceeds 750 ° C., coarse ferrite or pearlite is generated in the structure of the hot-rolled steel sheet, the uniformity of the metal structure is lowered, and sufficient formability cannot be obtained.
- a thick oxide may be formed on the surface and the pickling property may be lowered.
- the winding temperature is set to 750 ° C. or lower.
- the lower limit of the winding temperature is not particularly limited, but it is difficult to wind at a temperature lower than room temperature.
- a hot rolled steel sheet coil is obtained by hot rolling of the slab.
- pickling After hot rolling, pickling is performed while rewinding the coil of the hot rolled steel sheet. Pickling is performed once or twice or more. By pickling, the oxide on the surface of the hot-rolled steel sheet is removed, and the chemical conversion treatment and plating properties are improved.
- Cold rolling is performed after pickling.
- the rolling reduction of cold rolling is 40% to 80%. If this rolling reduction is less than 40%, it may be difficult to keep the shape of the cold-rolled steel sheet flat or sufficient ductility may not be obtained. Therefore, the rolling reduction is 40% or more, preferably 50% or more. On the other hand, if the rolling reduction exceeds 80%, the rolling load becomes excessive, the recrystallization of ferrite is excessively promoted, coarse polygonal ferrite is formed, and the area fraction of polygonal ferrite exceeds 40%. Or Therefore, the rolling reduction is 80% or less, preferably 70% or less.
- the number of rolling passes and the rolling reduction per pass are not particularly limited. A cold-rolled steel sheet is obtained by cold rolling of the hot-rolled steel sheet.
- a first annealing is performed after cold rolling.
- the first heating, the first cooling, the second cooling, and the first holding of the cold-rolled steel sheet are performed.
- the first annealing can be performed, for example, in a continuous annealing line.
- the annealing temperature of the first annealing is 750 ° C to 900 ° C.
- the annealing temperature is 750 ° C. or higher, preferably 780 ° C. or higher.
- the annealing temperature exceeds 900 ° C., austenite grains become coarse, and the transformation from austenite to bainitic ferrite or tempered martensite is delayed. The area fraction of bainitic ferrite becomes too small due to this transformation delay. Therefore, the annealing temperature is 900 ° C. or lower, preferably 870 ° C. or lower.
- the annealing time is not particularly limited and is, for example, 1 second to 1000 seconds.
- the cooling stop temperature of the first cooling is 600 ° C. to 720 ° C., and the cooling rate to this cooling stop temperature is 1 ° C./second or more and less than 10 ° C./second.
- the cooling stop temperature is set to 600 ° C. or higher, preferably 620 ° C. or higher.
- the cooling stop temperature is set to 720 ° C. or lower, preferably 700 ° C. or lower.
- the cooling rate of the first cooling is less than 1.0 ° C./second, the area fraction of polygonal ferrite becomes excessive. Therefore, the cooling rate is 1.0 ° C./second or more, preferably 3 ° C./second or more. On the other hand, when the cooling rate is 10 ° C./second or more, the area fraction of retained austenite is insufficient. Therefore, the cooling rate is less than 10 ° C./second, preferably 8 ° C./second or less.
- the cooling stop temperature of the second cooling is 150 ° C. to 500 ° C., and the cooling rate to this cooling stop temperature is 10 ° C./second to 60 ° C./second.
- the cooling stop temperature of the second cooling is less than 150 ° C., the lath width of bainitic ferrite or tempered martensite becomes fine, and the residual austenite remaining between the laths becomes a fine film. As a result, the area fraction of the retained austenite grains in a predetermined form becomes too small. Therefore, the cooling stop temperature is 150 ° C. or higher, preferably 200 ° C. or higher.
- the cooling stop temperature is 500 ° C. or less, preferably 450 ° C. or less, and more preferably about room temperature. Moreover, it is preferable that this cooling stop temperature shall be below Ms point according to a composition.
- the cooling rate of the second cooling is less than 10 ° C./second, the formation of polygonal ferrite is promoted and the area fraction of polygonal ferrite becomes excessive. Therefore, the cooling rate is 10 ° C./second or more, preferably 20 ° C./second or more.
- the cooling rate exceeds 60 ° C./second, the area fraction of retained austenite becomes less than the lower limit. Therefore, the cooling rate is 60 ° C./second or less, preferably 50 ° C./second or less.
- the method of the first cooling and the second cooling is not limited, and for example, roll cooling, air cooling, water cooling, or any combination thereof can be performed.
- the cold-rolled steel sheet is held at a temperature of 150 ° C. to 500 ° C. for a time of t1 seconds to 1000 seconds determined by the following formula (1).
- This holding (first holding) is performed as it is without lowering the temperature to less than 150 ° C. after the second cooling, for example.
- T0 is a holding temperature (° C.)
- T1 is a cooling stop temperature (° C.) of the second cooling.
- t1 20 ⁇ [C] + 40 ⁇ [Mn] ⁇ 0.1 ⁇ T0 + T1-0.1 (1)
- the holding time is less than t1 seconds, C is not sufficiently concentrated in the retained austenite, and the retained austenite is transformed into martensite during the subsequent temperature drop, so that the area fraction of the retained austenite becomes too small. Accordingly, the holding time is t1 seconds or more.
- the holding time exceeds 1000 seconds, the decomposition of the retained austenite is promoted, and the area fraction of the retained austenite becomes too small. Accordingly, the holding time is 1000 seconds or less.
- An intermediate steel sheet is obtained by the first annealing of the cold-rolled steel sheet.
- the first holding may be performed by, for example, lowering the temperature to less than 150 ° C. and then reheating to a temperature of 150 ° C. to 500 ° C.
- the reheating temperature is less than 150 ° C.
- the lath width of bainitic ferrite or tempered martensite becomes fine, and the residual austenite remaining between the laths becomes a fine film.
- the reheating temperature is 150 ° C. or higher, preferably 200 ° C. or higher.
- the reheating temperature exceeds 500 ° C., the formation of polygonal ferrite is promoted, and the area fraction of polygonal ferrite becomes excessive. Therefore, the reheating temperature is set to 500 ° C. or lower, preferably 450 ° C. or lower.
- the intermediate steel sheet has, for example, an area fraction of polygonal ferrite: 40% or less, bainitic ferrite or tempered martensite, or both: 40% to 95% in total, and retained austenite: 5% to 60%, It has the metal structure represented by these. Further, for example, 80% or more of the retained austenite in terms of area fraction is composed of retained austenite grains having an aspect ratio of 0.03 to 1.00.
- a second annealing is performed after the first annealing.
- the second annealing can be performed, for example, in a continuous annealing line.
- the annealing temperature of the second annealing is 760 ° C to 800 ° C.
- the annealing temperature is less than 760 ° C., the area fraction of polygonal ferrite becomes excessive, the area fraction of bainitic ferrite grains, the area fraction of retained austenite, or both of them are too small. Therefore, the annealing temperature is 760 ° C. or higher, preferably 770 ° C. or higher.
- the annealing temperature exceeds 800 ° C., the area fraction of austenite increases with the austenite transformation, and the area fraction of bainitic ferrite becomes too small. Therefore, this annealing temperature is set to 800 ° C. or lower, preferably 790 ° C. or lower.
- the cooling stop temperature of the third cooling is 600 ° C. to 750 ° C., and the cooling rate to this cooling stop temperature is 1 ° C./second to 10 ° C./second. If the cooling stop temperature is less than 600 ° C., the area fraction of polygonal ferrite becomes excessive. Therefore, the cooling stop temperature is set to 600 ° C. or higher, preferably 630 ° C. or higher. On the other hand, when the cooling stop temperature exceeds 750 ° C., the martensite area fraction becomes excessive. Therefore, this cooling stop temperature is set to 750 ° C. or lower, preferably 730 ° C. or lower.
- the cooling rate of the third cooling is less than 1.0 ° C./second, the area fraction of polygonal ferrite becomes excessive. Therefore, the cooling rate is 1.0 ° C./second or more, preferably 3 ° C./second or more. On the other hand, when the cooling rate exceeds 10 ° C./second, the area fraction of bainitic ferrite becomes too small. Therefore, the cooling rate is 10 ° C./second or less, preferably 8 ° C./second or less.
- the cooling stop temperature is preferably 710 ° C. or higher, more preferably 720 ° C. or higher. This is because the area fraction of polygonal ferrite tends to be 20% or less. In the case where ductility is more important than hole expansibility, the cooling stop temperature is preferably less than 710 ° C., more preferably 690 ° C. or less. This is because the area fraction of polygonal ferrite tends to be more than 20% and 40% or less.
- the steel plate After the third cooling, the steel plate is cooled to a temperature of 150 ° C. to 550 ° C. and held at that temperature for 1 second or longer. During this holding (second holding), the diffusion of C into the retained austenite is promoted.
- the holding time is less than 1 second, C is not sufficiently concentrated in the retained austenite, the stability of the retained austenite is lowered, and the area fraction of the retained austenite becomes too small. Therefore, the holding time is 1 second or longer, preferably 2 seconds or longer.
- the holding temperature is 150 ° C.
- the holding temperature exceeds 550 ° C., the transformation from austenite to bainitic ferrite is delayed, so that the diffusion of C into the retained austenite does not progress, the stability of the retained austenite decreases, and the area of the retained austenite decreases. The rate is too low. Accordingly, the holding temperature is 550 ° C. or lower, preferably 500 ° C. or lower.
- the steel sheet according to the embodiment of the present invention can be manufactured.
- a part of austenite is transformed into ferrite by controlling the primary cooling rate of the first annealing to 1 ° C./s or more and less than 10 ° C./s.
- Mn diffuses and concentrates in untransformed austenite.
- An advantageous crystal orientation is preferentially generated. Therefore, the strain introduced into the bainitic ferrite is reduced, and the dislocation density can be controlled to 8 ⁇ 10 2 (cm / cm 3 ) or less.
- the mechanism of improving ductility by reducing the dislocation density of bainitic ferrite is as follows. In TRIP steel, when martensite is generated from retained austenite by work-induced transformation, dislocations are introduced into adjacent bainitic ferrite and work hardening occurs. If the dislocation density of bainitic ferrite is low, the work hardening rate can be kept high even in a region where the strain is large, so that the uniform elongation is improved.
- the steel sheet may be subjected to a plating treatment such as an electroplating treatment or a vapor deposition plating treatment, and may further be subjected to an alloying treatment after the plating treatment.
- the steel sheet may be subjected to a surface treatment such as organic film formation, film lamination, organic salt / inorganic salt treatment, or non-chromium treatment.
- the hot dip galvanizing treatment is performed on the steel plate as the plating treatment, for example, the temperature of the steel plate is heated to a temperature not lower than 40 ° C lower than the temperature of the galvanizing bath and not higher than 50 ° C higher than the temperature of the galvanizing bath. Cool and pass through galvanizing bath.
- the hot dip galvanizing treatment a steel plate having a hot dip galvanized layer on the surface, that is, a hot dip galvanized steel plate is obtained.
- the hot dip galvanized layer has, for example, a chemical composition represented by Fe: 7% by mass or more and 15% by mass or less, and the balance: Zn, Al, and impurities.
- the hot dip galvanized steel sheet is heated to a temperature of 460 ° C. or higher and 600 ° C. or lower. If this temperature is less than 460 ° C., alloying may be insufficient. If this temperature exceeds 600 ° C., alloying may be excessive and corrosion resistance may deteriorate.
- the alloying treatment a steel plate having an alloyed hot-dip galvanized layer on its surface, that is, an alloyed hot-dip galvanized steel plate is obtained.
- the slab was heated to 1100 ° C. to 1300 ° C. directly after cooling or without cooling, and hot rolling was performed under the conditions shown in Tables 4 to 7 to obtain hot rolled steel sheets. Thereafter, pickling was performed, and cold rolling was performed under the conditions shown in Tables 4 to 7 to obtain cold-rolled steel sheets.
- the underline in Tables 4 to 7 indicates that the numerical value is out of the range suitable for manufacturing the steel sheet according to the present invention.
- the second annealing of the intermediate steel plate was performed under the conditions shown in Tables 16 to 19 to obtain steel plate samples.
- Production No. 150 and No. In No. 151 after the second annealing, a plating treatment was performed, and the production No. In 151, the alloying process was performed after the plating process.
- the plating treatment hot dip galvanizing treatment was performed, and the temperature of the alloying treatment was set to 500 ° C.
- the underline in Tables 16 to 19 indicates that the numerical value is out of the range suitable for manufacturing the steel sheet according to the present invention.
- excellent elongation, 0.2% proof stress, tensile strength, hole expansion value, ratio R / t and ductile-brittle transition temperature were obtained.
- the area fraction of bainitic ferrite is insufficient, the area fraction of martensite is excessive, the proportion of residual austenite grains in a predetermined form is insufficient, and the proportion of bainitic ferrite grains in a predetermined form is In the insufficient comparative example, the elongation, the hole expansion value, and the ratio R / t were low. Production No. 30 and no. In comparative examples where the proportion of retained austenite grains in a predetermined form, such as 37, was insufficient, the elongation was low. Production No. 70 and no.
- the area fraction of bainitic ferrite, such as 85, is insufficient, the area fraction of martensite is excessive, the proportion of retained austenite grains in a predetermined form is insufficient, and the proportion of bainitic ferrite grains in a predetermined form is In the insufficient comparative example, the elongation, the hole expansion value, and the ratio R / t were low.
- the present invention can be used, for example, in industries related to steel plates suitable for automobile parts.
Abstract
Description
質量%で、
C:0.1%~0.5%、
Si:0.5%~4.0%、
Mn:1.0%~4.0%、
P:0.015%以下、
S:0.050%以下、
N:0.01%以下、
Al:2.0%以下、
Si及びAl:合計で0.5%~6.0%、
Ti:0.00%~0.20%、
Nb:0.00%~0.20%、
B:0.0000%~0.0030%、
Mo:0.00%~0.50%、
Cr:0.0%~2.0%、
V:0.00%~0.50%、
Mg:0.000%~0.040%、
REM:0.000%~0.040%、
Ca:0.000%~0.040%、かつ
残部:Fe及び不純物、
で表される化学組成を有し、
面積分率で、
ポリゴナルフェライト:40%以下、
マルテンサイト:20%以下、
ベイニティックフェライト:50%~95%、かつ
残留オーステナイト:5%~50%、
で表される金属組織を有し、
面積分率で、前記ベイニティックフェライトのうちの80%以上が、アスペクト比が0.1~1.0、かつ、方位差角が15°以上の粒界に囲まれた領域の転位密度が8×102(cm/cm3)以下のベイニティックフェライト粒から構成され、
面積分率で、前記残留オーステナイトのうちの80%以上が、アスペクト比が0.1~1.0、長軸の長さが1.0μm~28.0μm、かつ、短軸の長さが0.1μm~2.8μmの残留オーステナイト粒から構成されていることを特徴とする鋼板。
前記金属組織は、面積分率で、
ポリゴナルフェライト:5%~20%、
マルテンサイト:20%以下、
ベイニティックフェライト:75%~90%、かつ
残留オーステナイト:5%~20%、
で表されることを特徴とする(1)に記載の鋼板。
前記金属組織は、面積分率で、
ポリゴナルフェライト:20%超40%以下、
マルテンサイト:20%以下、
ベイニティックフェライト:50%~75%、かつ
残留オーステナイト:5%~30%、
で表されることを特徴とする(1)に記載の鋼板。
前記化学組成において、質量%で、
Ti:0.01%~0.20%、
Nb:0.005%~0.20%、
B:0.0001%~0.0030%、
Mo:0.01%~0.50%、
Cr:0.01%~2.0%、
V:0.01%~0.50%、
Mg:0.0005%~0.040%、
REM:0.0005%~0.040%、若しくは
Ca:0.0005%~0.040%、
又はこれらの任意の組み合わせが成り立つことを特徴とする(1)~(3)のいずれかに記載の鋼板。
表面に形成されためっき層を有することを特徴とする(1)~(4)のいずれかに記載の鋼板。
ポリゴナルフェライトは軟質な組織である。このため、ポリゴナルフェライトと硬質な組織であるマルテンサイトとの間の硬度の差が大きく、成形の際に、これらの間の界面において亀裂が発生しやすい。この界面に沿って亀裂が伸展することもある。ポリゴナルフェライトの面積分率が40%超で、このような亀裂の発生及び伸展が生じやすく、十分な穴拡げ性、曲げ性、低温での耐脆化特性及び0.2%耐力を得にくい。従って、ポリゴナルフェライトの面積分率は40%以下とする。
ベイニティックフェライトはポリゴナルフェライトよりも高密度で転位を含み、引張強度の向上に寄与する。ベイニティックフェライトの硬度は、ポリゴナルフェライトのそれより高く、マルテンサイトのそれよりも低いため、ベイニティックフェライトとマルテンサイトとの間の硬度差はポリゴナルフェライトとマルテンサイトとの間の硬度差よりも小さい。従って、ベイニティックフェライトは穴拡げ性及び曲げ性の向上にも寄与する。ベイニティックフェライトの面積分率が50%未満では、十分な引張強度が得られない。従って、ベイニティックフェライトの面積分率は50%以上とする。穴拡げ性を延性よりも重視する場合は、ベイニティックフェライトの面積分率は好ましくは75%以上とする。一方、ベイニティックフェライトの面積分率が95%超では、残留オーステナイトが不足し、十分な成形性が得られない。従って、ベイニティックフェライトの面積分率は95%以下とする。
マルテンサイトには、フレッシュマルテンサイト(焼戻していないマルテンサイト)及び焼戻しマルテンサイトが含まれる。上記のように、ポリゴナルフェライトとマルテンサイトとの間の硬度の差が大きく、成形の際に、これらの間の界面において亀裂が発生しやすい。この界面に沿って亀裂が伸展することもある。マルテンサイトの面積分率が20%超で、このような亀裂の発生及び伸展が生じやすく、十分な穴拡げ性、曲げ性、低温での耐脆化特性及び0.2%耐力を得にくい。従って、マルテンサイトの面積分率は20%以下とする。
残留オーステナイトは成形性の向上に寄与する。残留オーステナイトの面積分率が5%未満では、十分な成形性が得られない。一方、残留オーステナイトの面積分率が50%超では、ベイニティックフェライトが不足し、十分な引張強度が得られない。従って、残留オーステナイトの面積分率は50%以下とする。
Sγ=(I200f+I220f+I311f)/(I200b+I211b)×100
(Sγは残留オーステナイトの面積分率、I200f、I220f、I311fは、それぞれfcc相の(200)、(220)、(311)の回折ピークの強度、I200b、I211bは、それぞれbcc相の(200)、(211)の回折ピークの強度を示す。)
転位密度が高いベイニティックフェライト粒はポリゴナルフェライトほど伸びの向上に寄与しないため、転位密度が高いベイニティックフェライト粒の面積分率が高いほど、伸びが低下しやすい。そして、アスペクト比が0.1~1.0、かつ、方位差角が15°以上の粒界に囲まれた領域の転位密度が8×102(cm/cm3)以下のベイニティックフェライト粒の面積分率が80%未満では、十分な伸びを得にくい。従って、このような形態のベイニティックフェライト粒の面積分率はベイニティックフェライト全体に対して80%以上とし、好ましくは85%以上とする。
残留オーステナイトは、成形の際に、加工誘起変態によりマルテンサイトに変態する。残留オーステナイトがマルテンサイトへ変態すると、このマルテンサイトがポリゴナルフェライト又は未変態の残留オーステナイトと隣り合っている場合、これらの間に大きな硬度差が生じる。大きな硬度差は、上記のように、亀裂の発生に繋がる。このような亀裂は、応力が集中する箇所に特に生じやすく、応力はアスペクト比が0.1未満の残留オーステナイトから変態したマルテンサイトの近傍に集中しやすい。そして、アスペクト比が0.1~1.0、長軸の長さが1.0μm~28.0μm、かつ、短軸の長さが0.1μm~2.8μmの残留オーステナイト粒の面積分率が80%未満では、応力集中に伴う亀裂が発生しやすく、十分な伸びを得にくい。従って、このような形態の残留オーステナイト粒の面積分率は残留オーステナイト全体に対して80%以上とし、好ましくは85%以上とする。ここで、残留オーステナイト粒のアスペクト比とは、当該残留オーステナイト粒の等価楕円の短軸の長さを長軸の長さで除して得られる値である。図1に等価楕円の一例を示す。残留オーステナイト粒1が複雑な形状を有していても、その等価楕円2の長軸の長さL1及び短軸の長さL2から当該残留オーステナイト粒のアスペクト比(L2/L1)が得られる。
炭素(C)は、鋼板の強度の向上に寄与したり、残留オーステナイトの安定性の向上を通じて伸びの向上に寄与したりする。C含有量が0.10%未満では、十分な強度、例えば980MPa以上の引張強度を得ることが困難であったり、残留オーステナイトの安定性が不十分となって十分な伸びが得られなかったりする。従って、C含有量は0.10%以上とし、好ましくは0.15%以上とする。一方、C含有量が0.5%超では、オーステナイトからベイニティックフェライトへの変態が遅延するため、所定形態のベイニティックフェライト粒が不足し、十分な伸びが得られない。従って、C含有量は0.5%以下とし、好ましくは0.25%以下とする。
珪素(Si)は、鋼の強度の向上に寄与したり、残留オーステナイトの安定性の向上を通じて伸びの向上に寄与したりする。Si含有量が0.5%未満では、これらの効果を十分に得られない。従って、Si含有量は0.5%以上とし、好ましくは1.0%以上とする。一方、Si含有量が4.0%超では、鋼の強度が高くなりすぎて伸びが低下する。従って、Si含有量は4.0%以下とし、好ましくは2.0%以下とする。
マンガン(Mn)は、鋼の強度の向上に寄与したり、第1の焼鈍又は第2の焼鈍の冷却途中で生じるポリゴナルフェライト変態を抑制したりする。溶融亜鉛めっき処理が行われる場合は、当該処理の冷却途中で生じるポリゴナルフェライト変態も抑制される。Mn含有量が1.0%未満では、これらの効果を十分に得られなかったり、ポリゴナルフェライトが過剰に生成して穴拡げ性が劣化したりする。従って、Mn含有量は1.0%以上とし、好ましくは2.0%以上とする。一方、Mn含有量が4.0%超では、スラブ及び熱延鋼板の強度が高くなりすぎる。従って、4.0%以下とし、好ましくは3.0%以下とする。
りん(P)は、必須元素ではなく、例えば鋼中に不純物として含有される。Pは、鋼板の厚さ方向の中央部に偏析して靱性を低下させたり、溶接部を脆化させたりする。このため、P含有量は低ければ低いほどよい。特に、P含有量が0.015%超で、靱性の低下及び溶接性の脆化が顕著である。従って、P含有量は0.015%以下とし、好ましくは0.010%以下とする。P含有量の低減にはコストがかかり、0.0001%未満まで低減しようとすると、コストが著しく上昇する。このため、P含有量は0.0001%以上としてもよい。
硫黄(S)は、必須元素ではなく、例えば鋼中に不純物として含有される。Sは、鋳造及び熱間圧延の製造性を低下させたり、粗大なMnSを形成して穴拡げ性を低下させたりする。このため、S含有量は低ければ低いほどよい。特に、S含有量が0.050%超で、溶接性の低下、製造性の低下及び穴拡げ性の低下が顕著である。従って、S含有量は0.050%以下とし、好ましくは0.0050%以下とする。S含有量の低減にはコストがかかり、0.0001%未満まで低減しようとすると、コストが著しく上昇する。このため、S含有量は0.0001%以上としてもよい。
窒素(N)は、必須元素ではなく、例えば鋼中に不純物として含有される。Nは、粗大な窒化物を形成して、曲げ性及び穴拡げ性を劣化させたり、溶接時のブローホールの発生の原因になったりする。このため、N含有量は低ければ低いほどよい。特に、N含有量が0.01%超で、曲げ性及び穴拡げ性の低下並びにブローホールの発生が顕著である。従って、N含有量は0.01%以下とする。N含有量の低減にはコストがかかり、0.0005%未満まで低減しようとすると、コストが著しく上昇する。このため、N含有量は0.0005%以上としてもよい。
アルミニウム(Al)は、脱酸材として機能したり、オーステナイト中での鉄系炭化物の析出を抑制したりするが、必須元素ではない。Al含有量が2.0%超では、オーステナイトからポリゴナルフェライトへの変態が促進され、ポリゴナルフェライトが過剰に生成して穴拡げ性が劣化する。従って、Al含有量は2.0%以下とし、好ましくは1.0%以下とする。Al含有量の低減にはコストがかかり、0.001%未満まで低減しようとすると、コストが著しく上昇する。このため、Al含有量は0.001%以上としてもよい。
Si及びAlは、いずれも、残留オーステナイトの安定性の向上を通じて伸びの向上に寄与する。Si及びAlの含有量が合計で0.5%未満では、この効果を十分に得られない。従って、Si及びAlの含有量は合計で0.5%以上とし、好ましくは1.2%以上とする。Si又はAlのいずれかのみが含有されていてもよく、Si及びAlの両方が含有されていてもよい。
チタン(Ti)は、析出強化及び細粒強化に起因した転位強化を通じて鋼の強度の向上に寄与する。従って、Tiが含有されていてもよい。この効果を十分に得るために、Ti含有量は好ましくは0.01%以上とし、より好ましくは0.025%以上とする。一方、Ti含有量が0.20%超では、Tiの炭窒化物が過剰に析出して鋼板の成形性が低下する。従って、Ti含有量は0.20%以下とし、好ましくは0.08%以下とする。
ニオブ(Nb)は、析出強化及び細粒強化に起因した転位強化を通じて鋼の強度の向上に寄与する。従って、Nbが含有されていてもよい。この効果を十分に得るために、Nb含有量は好ましくは0.005%以上とし、より好ましくは0.010%以上とする。一方、Nb含有量が0.20%超では、Nbの炭窒化物が過剰に析出して鋼板の成形性が低下する。従って、Nb含有量は0.20%以下とし、好ましくは0.08%以下とする。
ホウ素(B)は、粒界を強化したり、第1の焼鈍又は第2の焼鈍の冷却途中で生じるポリゴナルフェライト変態を抑制したりする。溶融亜鉛めっき処理が行われる場合は、当該処理の冷却途中で生じるポリゴナルフェライト変態も抑制される。従って、Bが含有されていてもよい。この効果を十分に得るために、B含有量は好ましくは0.0001%以上とし、より好ましくは0.0010%以上とする。一方、B含有量が0.0030%超では、添加の効果が飽和したり、熱間圧延の製造性が低下したりする。従って、B含有量は0.0030%以下とし、好ましくは0.0025%以下とする。
モリブデン(Mo)は、鋼の強化に寄与したり、第1の焼鈍又は第2の焼鈍の冷却途中で生じるポリゴナルフェライト変態を抑制したりする。溶融亜鉛めっき処理が行われる場合は、当該処理の冷却途中で生じるポリゴナルフェライト変態も抑制される。従って、Moが含有されていてもよい。この効果を十分に得るために、Mo含有量は好ましくは0.01%以上とし、より好ましくは0.02%以上とする。一方、Mo含有量が0.50%超では、熱間圧延の製造性が低下する。従って、Mo含有量は0.50%以下とし、好ましくは0.20%以下とする。
クロム(Cr)は、鋼の強化に寄与したり、第1の焼鈍又は第2の焼鈍の冷却途中で生じるポリゴナルフェライト変態を抑制したりする。溶融亜鉛めっき処理が行われる場合は、当該処理の冷却途中で生じるポリゴナルフェライト変態も抑制される。従って、Crが含有されていてもよい。この効果を十分に得るために、Cr含有量は好ましくは0.01%以上とし、より好ましくは0.02%以上とする。一方、Cr含有量が2.0%超では、熱間圧延の製造性が低下する。従って、Cr含有量は2.0%以下とし、好ましくは0.10%以下とする。
バナジウム(V)は、析出強化及び細粒強化に起因した転位強化を通じて鋼の強度の向上に寄与する。従って、Vが含有されていてもよい。この効果を十分に得るために、V含有量は好ましくは0.01%以上とし、より好ましくは0.02%以上とする。一方、V含有量が0.50%超では、Vの炭窒化物が過剰に析出して鋼板の成形性が低下する。従って、Nb含有量は0.50%以下とし、好ましくは0.10%以下とする。
マグネシウム(Mg)、希土類金属(REM)及びカルシウム(Ca)は、酸化物又は硫化物として鋼中に存在し、穴拡げ性の向上に寄与する。従って、Mg、REM若しくはCa又はこれらの任意の組み合わせが含有されていてもよい。この効果を十分に得るために、Mg含有量、REM含有量及びCa含有量はいずれも、好ましくは0.0005%以上とし、より好ましくは0.0010%以上とする。一方、Mg含有量、REM含有量又はCa含有量が0.040%超では、粗大な酸化物が形成して穴拡げ性が低下する。従って、Mg含有量、REM含有量及びCa含有量はいずれも0.040%以下とし、好ましくは0.010%以下とする。
熱間圧延では、スラブの粗圧延、仕上げ圧延及び巻き取りを行う。スラブとしては、例えば、連続鋳造で得たスラブ、薄スラブキャスターで作製したスラブを用いることができる。スラブは鋳造後に1000℃以上の温度に保持したまま熱間圧延設備に供してもよく、1000℃未満の温度まで冷却した後に加熱して熱間圧延設備に供してもよい。
熱間圧延後には、熱延鋼板のコイルを巻き戻しながら酸洗を行う。酸洗は1回又は2回以上行う。酸洗により、熱延鋼板の表面の酸化物が除去され、化成処理性及びめっき性が向上する。
酸洗後に冷間圧延を行う。冷間圧延の圧下率は40%~80%とする。この圧下率が40%未満では、冷延鋼板の形状を平坦に保つことが困難であったり、十分な延性が得られなかったりすることがある。従って、この圧下率は40%以上とし、好ましくは50%以上とする。一方、この圧下率が80%超では、圧延荷重が過大になったり、フェライトの再結晶が過度に促進され、粗大なポリゴナルフェライトが形成され、ポリゴナルフェライトの面積分率が40%を超えたりする。従って、この圧下率は80%以下とし、好ましくは70%以下とする。圧延パスの回数及びパス毎の圧下率は特に限定されない。熱延鋼板の冷間圧延により冷延鋼板が得られる。
冷間圧延後に第1の焼鈍を行う。第1の焼鈍では、冷延鋼板の第1の加熱、第1の冷却、第2の冷却及び第1の保持を行う。第1の焼鈍は、例えば連続焼鈍ラインにて行うことができる。
t1=20×[C]+40×[Mn]-0.1×T0+T1-0.1 (1)
第1の焼鈍後に第2の焼鈍を行う。第2の焼鈍では、中間鋼板の第2の加熱、第3の冷却及び第2の保持を行う。第2の焼鈍は、例えば連続焼鈍ラインにて行うことができる。第2焼鈍を下記の条件で行うことにより、ベイニティックフェライトの転位密度を低下させ、転位密度が8×102(cm/cm3)以下の所定形態のベイニティックフェライト粒の面積分率を高めることができる。
第1の試験では、表1~表3に示す化学組成を有するスラブを製造した。表1~表3中の空欄は、当該元素の含有量が検出限界未満であったことを示し、残部はFe及び不純物である。表1~表3中の下線は、その数値が本発明の範囲から外れていることを示す。
Claims (5)
- 質量%で、
C:0.10%~0.5%、
Si:0.5%~4.0%、
Mn:1.0%~4.0%、
P:0.015%以下、
S:0.050%以下、
N:0.01%以下、
Al:2.0%以下、
Si及びAl:合計で0.5%~6.0%、
Ti:0.00%~0.20%、
Nb:0.00%~0.20%、
B:0.0000%~0.0030%、
Mo:0.00%~0.50%、
Cr:0.0%~2.0%、
V:0.00%~0.50%、
Mg:0.000%~0.040%、
REM:0.000%~0.040%、
Ca:0.000%~0.040%、かつ
残部:Fe及び不純物、
で表される化学組成を有し、
面積分率で、
ポリゴナルフェライト:40%以下、
マルテンサイト:20%以下、
ベイニティックフェライト:50%~95%、かつ
残留オーステナイト:5%~50%、
で表される金属組織を有し、
面積分率で、前記ベイニティックフェライトのうちの80%以上が、アスペクト比が0.1~1.0、かつ、方位差角が15°以上の粒界に囲まれた領域の転位密度が8×102(cm/cm3)以下のベイニティックフェライト粒から構成され、
面積分率で、前記残留オーステナイトのうちの80%以上が、アスペクト比が0.1~1.0、長軸の長さが1.0μm~28.0μm、かつ、短軸の長さが0.1μm~2.8μmの残留オーステナイト粒から構成されていることを特徴とする鋼板。 - 前記金属組織は、面積分率で、
ポリゴナルフェライト:5%~20%、
マルテンサイト:20%以下、
ベイニティックフェライト:75%~90%、かつ
残留オーステナイト:5%~20%、
で表されることを特徴とする請求項1に記載の鋼板。 - 前記金属組織は、面積分率で、
ポリゴナルフェライト:20%超40%以下、
マルテンサイト:20%以下、
ベイニティックフェライト:50%~75%、かつ
残留オーステナイト:5%~30%、
で表されることを特徴とする請求項1に記載の鋼板。 - 前記化学組成において、質量%で、
Ti:0.01%~0.20%、
Nb:0.005%~0.20%、
B:0.0001%~0.0030%、
Mo:0.01%~0.50%、
Cr:0.01%~2.0%、
V:0.01%~0.50%、
Mg:0.0005%~0.040%、
REM:0.0005%~0.040%、若しくは
Ca:0.0005%~0.040%、
又はこれらの任意の組み合わせが成り立つことを特徴とする請求項1乃至3のいずれか1項に記載の鋼板。 - 表面に形成されためっき層を有することを特徴とする請求項1乃至4のいずれか1項に記載の鋼板。
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