WO2016092733A1 - 高強度冷延鋼板及びその製造方法 - Google Patents
高強度冷延鋼板及びその製造方法 Download PDFInfo
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Definitions
- the present invention relates to a high-strength cold-rolled steel sheet and a manufacturing method thereof.
- the high-strength cold-rolled steel sheet having a tensile strength (TS) of 1180 MPa or more according to the present invention is particularly suitable as a material for structural parts such as automobiles.
- high-strength steel sheets used for automobile structural members and reinforcing members are required to have excellent formability.
- molding of a part having a complicated shape requires not only excellent individual characteristics such as elongation and hole expansibility, but also a plurality of characteristics.
- high-strength steel sheets used for automobile structural members and reinforcing members are required to have excellent impact absorption energy characteristics.
- it is effective to increase the yield ratio. If the yield ratio is increased, it is possible to efficiently absorb the collision energy even with a low deformation amount.
- a steel plate of 1180 MPa or more is required to be excellent in press formability and delayed fracture resistance.
- Patent Document 1 discloses a technique for improving the balance between elongation and stretch flangeability by controlling the distribution of cementite particles of tempered martensite.
- Patent Document 2 discloses a steel plate in which the distribution of precipitates in tempered martensite is controlled.
- a TRIP steel plate containing retained austenite can be mentioned.
- this TRIP steel sheet is deformed by processing at a temperature equal to or higher than the martensite transformation start temperature, the retained austenite is induced and transformed into martensite by stress, and a large elongation is obtained.
- this TRIP steel sheet has a defect that the retained austenite is transformed into martensite at the time of the punching process, so that a crack is generated at the interface with the ferrite and the hole expandability is inferior.
- Patent Document 3 discloses a TRIP steel sheet that has improved elongation and stretch flangeability by containing bainitic ferrite with an area ratio of 60% or more and polygonal ferrite with 20% or less.
- Patent Document 4 discloses a TRIP steel sheet having excellent hydrogen embrittlement resistance by controlling the volume fraction of ferrite, bainitic ferrite, and martensite.
- JP 2011-52295 A Japanese Patent No. 4712838 Japanese Patent No. 4411221 Japanese Patent No. 4868771
- DP steel generally has a low yield ratio due to the introduction of movable dislocations in the ferrite during martensitic transformation, resulting in low impact absorption energy characteristics.
- the hole-expanding property is enhanced by increasing the tempering temperature, but the elongation is insufficient with respect to the strength.
- the steel sheet of Patent Document 2 also has insufficient elongation with respect to strength and is inferior in formability.
- the steel plate of Patent Document 3 has low YR, and thus has low impact absorption energy characteristics, and does not have improved elongation and hole expansibility in a high strength region of 1180 MPa or more.
- the steel sheet of Patent Document 4 has insufficient elongation with respect to strength and is inferior in formability.
- the present invention has been made in order to solve the above-mentioned problems, and its purpose is to solve the problems of the above-mentioned prior art, and the above characteristics (yield ratio, strength, elongation, hole expandability, delayed fracture resistance). Is to provide a high-strength cold-rolled steel sheet and a method for producing the same.
- the inventors of the present invention made extensive studies to solve the above problems. As a result, in order to improve elongation, hole expansibility and delayed fracture resistance while maintaining a high yield ratio while having a high strength of 1180 MPa or more, ferrite, residual austenite, martensite while refining the structure It has been found that the volume fraction in the microstructure of bainite and tempered martensite may be controlled. Specifically, the present invention is based on the following knowledge.
- the present inventors have adjusted the volume fraction of the soft phase and the hard phase, which are the void generation sources, to produce tempered martensite or bainite, which is the hard intermediate phase, and further, the crystal grains It has been found that by making it finer, strength and hole-expandability can be secured while containing soft ferrite to some extent. Furthermore, the present inventors have found that the balance between strength and delayed fracture resistance is improved by including tempered martensite which is superior in delayed fracture resistance as a hard phase.
- annealing is performed at an annealing temperature in a two phase region that can contain ferrite. Furthermore, it became clear that the hole expansion property and delayed fracture resistance were improved by the effect of crystal grain refinement by making the temperature rise rate to the annealing temperature the optimum condition in order to refine crystal grains. .
- the present invention provides the following [1] to [4].
- (V1) and tempered martensite volume fraction (V2) is as follows: High-strength cold-rolled steel sheet characterized by having a satisfying microstructure of formula (1). 0.35 ⁇ V2 / V1 ⁇ 0.75 Formula (1) [2]
- the component composition is further a component composition containing one or more selected from V: 0.05% or less and Nb: 0.05% or less in mass% [1]
- the high-strength cold-rolled steel sheet as described in 1.
- the component composition further includes, by mass%, Cr: 0.50% or less, Mo: 0.50% or less, Cu: 0.50% or less, Ni: 0.50% or less, Ca: 0.00.
- a steel slab having the component composition according to any one of [1] to [3] and rolled at 1150 to 1300 ° C. under a finish rolling temperature of 850 to 950 ° C. to complete the rolling Cooling is started within 1 second after that, the first average cooling rate is 80 ° C./s or more, the first cooling stop temperature is 650 ° C. or less, and after the first cooling, the second average cooling is performed.
- Speed 5 ° C./s or more
- second cooling stop temperature a hot rolling process in which second cooling is performed under conditions of less than the first cooling stop temperature and 550 ° C.
- first average heating rate 10 ° C./s or less
- second heating is performed under the conditions of the second average heating rate: 6 to 25 ° C./s
- the second heating ultimate temperature 550 to 680 ° C.
- the third average heating rate is 10 ° C./s or less
- the third heating attainment temperature is 760 to 850 ° C.
- the first soaking temperature is 760 to 850 ° C.
- 1 soaking time First soaking under the condition of 30 seconds or more, after the first soaking, the third average cooling rate: 3 ° C./s or more, the third cooling stop temperature: 100 to 300 ° C.
- the fourth heating is performed under the condition of the fourth heating reaching temperature: 350 to 450 ° C., and after the fourth heating, the second soaking temperature: 350 to 450 ° C., the second soaking time.
- Second soaking is performed for 30 seconds or longer, and after the second soaking, the fourth cooling stop temperature is 0 to 50 ° C.
- the high-strength cold-rolled steel sheet has an extremely high tensile strength, an excellent workability based on high elongation and hole expansibility, and a high yield ratio.
- the high-strength cold-rolled steel sheet of the present invention has excellent delayed fracture resistance that hardly causes delayed fracture due to hydrogen entering from the environment even after being formed into a member.
- the high-strength cold-rolled steel sheet of the present invention is, in mass%, C: 0.15-0.25%, Si: 1.2-2.5%, Mn: 2.1-3.5%, P: 0 0.05% or less, S: 0.005% or less, Al: 0.01 to 0.08%, N: 0.010% or less, Ti: 0.002 to 0.050%, B: 0.0002 to 0 Contains 0100%.
- C 0.15-0.25%
- C is an element effective for increasing the strength of the steel sheet, and contributes to the second phase formation of bainite, tempered martensite, retained austenite and martensite in the present invention. Furthermore, C increases the hardness of martensite and tempered martensite. If the C content is less than 0.15%, it is difficult to ensure the required volume fraction of bainite, tempered martensite, retained austenite and martensite. A preferable C content is 0.17% or more. On the other hand, when C is added excessively, the hardness difference between ferrite, tempered martensite, and martensite is increased, so that the hole expandability is lowered. Therefore, the C content is 0.25% or less. A preferable C content is 0.22% or less.
- Si 1.2-2.5%
- Si strengthens the solid solution of ferrite and decreases the hardness difference between the soft phase and the hard phase, so Si increases the hole expansion rate.
- it is necessary to contain Si 1.2% or more.
- a preferable Si content is 1.3% or more.
- Si content shall be 2.5% or less.
- Mn 2.1-3.5%
- Mn is an element that contributes to increasing the strength by forming solid solution strengthening and the second phase.
- Mn is an element that stabilizes austenite, and is an element necessary for controlling the fraction of the second phase. In order to acquire the effect, it is necessary to make Mn content 2.1% or more.
- Mn content is 3.5% or less. Preferably it is 3.0% or less.
- P 0.05% or less P contributes to high strength by solid solution strengthening.
- the P content is 0.05% or less.
- it is 0.04% or less.
- the upper limit of the S content is set to 0.005%.
- it is 0.0040% or less.
- it is preferable to contain 0.0002% or more because extremely low S increases the steelmaking cost.
- Al 0.01 to 0.08%
- Al is an element necessary for deoxidation, and in order to obtain this effect, the Al content needs to be 0.01% or more. Moreover, since the effect is saturated even if the Al content exceeds 0.08%, the Al content is set to 0.08% or less. Preferably it is 0.05% or less.
- N 0.010% or less Since N forms coarse nitrides and deteriorates bendability and stretch flangeability, it is necessary to suppress the content thereof. These problems are prominent when the N content exceeds 0.010%. For this reason, N content shall be 0.010% or less. Preferably it is 0.0050% or less.
- Ti 0.002 to 0.050%
- Ti is an element that can contribute to an increase in strength by forming fine carbonitrides. Further, Ti is necessary for preventing B, which is an essential element in the present invention, from reacting with N. In order to exert such effects, the Ti content is set to 0.002% or more. Preferably it is 0.005% or more. On the other hand, when Ti is added in a large amount, the elongation is remarkably lowered, so the content is made 0.050% or less. Preferably it is 0.035% or less.
- B 0.0002% to 0.0100%
- B is an element that improves the hardenability, contributes to high strength by generating the second phase, and does not lower the martensite transformation start point while ensuring the hardenability. Furthermore, B has an effect of suppressing the formation of ferrite and pearlite when cooling after finish rolling during hot rolling. In order to exhibit this effect, it is necessary to make B content 0.0002% or more. On the other hand, even if the B content exceeds 0.0100%, the effect is saturated, so the content is made 0.0100% or less. Preferably it is 0.0050% or less.
- the high-strength cold-rolled steel sheet of the present invention may further contain one or more kinds selected from V: 0.05% or less and Nb: 0.05% or less in mass%.
- V 0.05% or less Contributing to an increase in strength by forming fine carbonitrides of V.
- the V content is preferably 0.01% or more.
- the V content is preferably 0.05% or less.
- Nb 0.05% or less
- Nb can contribute to an increase in strength by forming fine carbonitride, and can be added as necessary.
- the Nb content is preferably 0.005% or more.
- Nb when Nb is added in a large amount, the elongation is remarkably lowered. Therefore, when Nb is contained, its content is set to 0.05% or less.
- the high-strength cold-rolled steel sheet of the present invention is, in mass%, Cr: 0.50% or less, Mo: 0.50% or less, Cu: 0.50% or less, Ni: 0.50% or less, Ca: One or more selected from 0.0050% or less and REM: 0.0050% or less may be contained.
- Cr 0.50% or less Cr is an element that contributes to increasing the strength by generating the second phase, and can be added as necessary. In order to exhibit this effect, it is preferable to make Cr content 0.10% or more. On the other hand, if the Cr content exceeds 0.50%, excessive martensite is generated. Therefore, when Cr is contained, its content is 0.50% or less.
- Mo 0.50% or less
- Mo is an element that contributes to high strength by generating a second phase, and further contributes to high strength by generating a part of carbide, and may be added as necessary. it can.
- the Mo content is preferably 0.05% or more.
- the content is preferably 0.50% or less.
- Cu 0.50% or less
- Cu is an element that contributes to strengthening by solid solution strengthening and contributes to strengthening by generating a second phase, and can be added as necessary.
- the Cu content is preferably 0.05% or more.
- the Cu content is preferably 0.50% or less.
- Ni 0.50% or less
- Ni is an element that contributes to strengthening by solid solution strengthening and also contributes to strengthening by generating a second phase, and is added as necessary. be able to.
- the Ni content is preferably 0.05% or more.
- the content is preferably 0.50% or less.
- Ca 0.0050% or less
- Ca is an element that spheroidizes the shape of the sulfide and improves the adverse effect of the sulfide on the hole expandability, and can be added as necessary.
- the Ca content is preferably 0.0005% or more.
- the Ca content is set to 0.0050% or less.
- REM 0.0050% or less REM, like Ca, is an element that spheroidizes the shape of the sulfide and improves the adverse effect of the sulfide on the hole expandability, and can be added as necessary.
- the REM content is preferably 0.0005% or more.
- the content is preferably 0.0050% or less.
- Inevitable impurities include, for example, Sb, Sn, Zn, Co, etc.
- the allowable ranges of these contents are Sb: 0.01% or less, Sn: 0.1% or less, Zn: 0. 01% or less, Co: 0.1% or less.
- Sb 0.01% or less
- Sn 0.1% or less
- Zn 0. 01% or less
- Co 0.1% or less.
- this invention even if it contains Ta, Mg, and Zr within the range of a normal steel composition, the effect will not be lost.
- the microstructure of the high-strength cold-rolled steel sheet of the present invention is a composite structure containing ferrite, retained austenite, and martensite, with the balance including bainite and tempered martensite.
- ferrite has an average crystal grain size of 2 ⁇ m or less and a volume fraction of 10 to 25%
- retained austenite has a volume fraction of 5 to 20%
- martensite has an average crystal grain size of 2 ⁇ m or less.
- the volume fraction is in the range of 5 to 15%
- the balance is bainite and tempered martensite having an average crystal grain size of 5 ⁇ m or less.
- the relationship between the hard phase other than ferrite (meaning a phase other than ferrite) and the volume fraction of tempered martensite is within the range indicated by the formula (1).
- the volume fraction described here is the volume fraction with respect to the entire steel sheet, and so on.
- the volume fraction of ferrite is less than 10%, it is difficult to ensure elongation. Therefore, the lower limit of the volume fraction of ferrite is 10%. Preferably, the volume fraction of ferrite is greater than 12%. On the other hand, when the volume fraction of ferrite exceeds 25%, the amount of void generation at the time of punching increases. On the other hand, when the volume fraction of ferrite exceeds 25%, it is necessary to increase the hardness of martensite and tempered martensite in order to secure the strength, and it is difficult to achieve both strength and hole expandability. For this reason, the volume fraction of ferrite is 25% or less. Preferably it is 22% or less, More preferably, it is less than 20%.
- the average crystal grain size of ferrite exceeds 2 ⁇ m, voids generated on the punched end face during hole expansion are liable to be connected during the hole expansion, so that good hole expandability cannot be obtained. Therefore, the average grain size of ferrite is 2 ⁇ m or less.
- the volume fraction of retained austenite In order to ensure good ductility, the volume fraction of retained austenite needs to be in the range of 5 to 20%. If the volume fraction of retained austenite is less than 5%, the elongation decreases. For this reason, the volume fraction of retained austenite is 5% or more. Preferably it is 8% or more. Further, when the volume fraction of retained austenite exceeds 20%, the hole expandability deteriorates. For this reason, the volume fraction of retained austenite is 20% or less. Preferably it is 18% or less.
- Martensite (Martensite with an average grain size of 2 ⁇ m or less)
- the martensite volume fraction is set to 5 to 15% or less in order to ensure hole expansibility while ensuring desired strength and ductility.
- the volume fraction of martensite is less than 5%, since the contribution to work hardening is low, it is difficult to achieve both strength and ductility.
- it is 6% or more.
- the volume fraction of martensite exceeds 15%, voids are generated around the martensite at the time of punching, so that not only the hole expandability is deteriorated but also the yield ratio is lowered.
- the upper limit of the volume fraction of martensite is 15%.
- the upper limit is 12%.
- the average crystal grain size of martensite is 2 ⁇ m or less.
- the upper limit of the average crystal grain size of martensite is 2 ⁇ m.
- the martensite here refers to martensite that is generated when austenite that is untransformed after being held in the temperature range of 350 to 450 ° C., which is the second soaking temperature range during continuous annealing, is cooled to room temperature. That is.
- the average crystal grain size of bainite and tempered martensite is 5 ⁇ m or less.
- the upper limit of the average crystal grain size of bainite and tempered martensite is set to 5 ⁇ m.
- the volume fraction of bainite is preferably in the range of 10 to 40%, and the volume fraction of tempered martensite is preferably in the range of 20 to 60%.
- the volume fraction of bainite here is the volume fraction of bainitic ferrite (ferrite with high dislocation density) in the observation surface.
- Tempered martensite is a part of untransformed austenite that is martensitic during cooling to 100 to 300 ° C. (third cooling described later) during annealing, and after heating to a temperature range of 350 to 450 ° C., It is martensite that is tempered when held (during second soaking).
- the microstructure may contain pearlite in addition to ferrite, bainite, tempered martensite, retained austenite and martensite.
- the object of the present invention can be achieved even if pearlite is included.
- the volume fraction of pearlite is preferably 3% or less.
- the manufacturing method of the high strength cold-rolled steel sheet of the present invention includes a hot rolling process, a pickling process, a cold rolling process, and an annealing process.
- a hot rolling process (cooling start surface temperature-cooling end surface temperature) / cooling time (2)
- Average heating rate (heating end surface temperature ⁇ heating starting surface temperature) / heating time (3)
- Hot rolling process is a process in which a steel slab having the above-described composition is rolled at 1150-1300 ° C under the condition of finish rolling end temperature: 850-950 ° C, and within 1 second after the end of the rolling.
- the first average cooling rate 80 ° C./s or more
- the first cooling stop temperature 650 ° C. or less
- the first cooling to start cooling is performed
- the second average cooling rate 5 More than C / s
- second cooling stop temperature a step of performing second cooling that cools below the first cooling stop temperature and not more than 550 ° C., and winds up after the second cooling.
- the hot rolling start temperature (corresponding to the temperature of the steel slab to be rolled) is 1150 to 1300 ° C.
- the steel slab may be hot rolled at 1150 to 1300 ° C. without reheating after casting, or may be started after the slab is reheated to 1150 to 1300 ° C. That is, in the present invention, after manufacturing a steel slab, in addition to the conventional method of cooling to room temperature and then reheating, it is charged in a heating furnace as it is without cooling, or is kept warm. An energy saving process such as direct feed rolling or direct rolling in which rolling is performed immediately after casting or rolling as it is after casting can be applied without any problem.
- the steel slab is preferably manufactured by a continuous casting method in order to prevent macro segregation of components, but can also be manufactured by an ingot forming method or a thin slab casting method.
- the temperature is set to 1150 to 1300 ° C.
- Finish rolling end temperature is 850-950 ° C. Hot rolling needs to be completed in the austenite single phase region in order to improve the elongation and hole expansion property after annealing by making the structure in the steel sheet uniform and reducing the anisotropy of the material. Therefore, the finish rolling end temperature is set to 850 ° C. or higher. On the other hand, if the finish rolling end temperature exceeds 950 ° C., the hot rolled structure becomes coarse and the properties after annealing deteriorate, so the finish rolling end temperature is set to 850 to 950 ° C.
- the first cooling after the finish rolling is started within 1 second after the end of the rolling, and is performed under the conditions of the first average cooling rate: 80 ° C./s or more and the first cooling stop temperature: 650 ° C. or less. is there.
- the steel sheet After finishing rolling, the steel sheet is rapidly cooled to the temperature range where bainite transformation is performed without ferrite transformation, and the steel sheet structure of the hot rolled steel sheet is controlled.
- the steel sheet structure By controlling the steel sheet structure for homogenization, there is an effect of refining the final steel sheet structure, mainly ferrite and martensite. Therefore, cooling is started within 1 second after the finish rolling is finished, and the cooling is performed to the first cooling stop temperature: 650 ° C. or less at the first average cooling rate of 80 ° C./s or more.
- the ferrite transformation is started, so that the steel sheet structure of the hot-rolled steel sheet becomes inhomogeneous and the hole expandability after annealing decreases.
- the first cooling stop temperature exceeds 650 ° C., pearlite is excessively generated, the steel sheet structure of the hot-rolled steel sheet becomes inhomogeneous, and the hole expandability after annealing decreases. Therefore, the 1st cooling after finish rolling cools to 650 degrees C or less with the 1st average cooling rate of 80 degrees C / s or more.
- the second cooling after the first cooling is a cooling performed under conditions of a second average cooling rate: 5 ° C./s or more, a second cooling stop temperature: less than the first cooling stop temperature and 550 ° C. or less.
- the second average cooling rate is less than 5 ° C./s or the second cooling stop temperature is more than 550 ° C.
- the second average cooling rate 5 ° C./s or more
- the second cooling stop temperature less than the first cooling stop temperature, and 550 ° C. or less.
- the winding temperature at the time of winding performed after the second cooling is preferably 550 ° C. or lower. If the coiling temperature exceeds 550 ° C., ferrite and pearlite may be generated excessively. For this reason, the upper limit of the coiling temperature is preferably 550 ° C. Preferably it is 500 degrees C or less.
- the lower limit of the coiling temperature is not particularly defined, but if the coiling temperature is too low, hard martensite may be generated excessively and the cold rolling load may increase. For this reason, the lower limit of the coiling temperature is preferably 300 ° C.
- Pickling process It is preferable to carry out an acidic process after the hot rolling process to remove the scale of the surface layer of the hot rolled sheet.
- the conditions for the pickling step are not particularly limited, and may be carried out according to a conventional method.
- Cold rolling step This is a step of performing cold rolling on the hot-rolled sheet after the hot rolling step (after the pickling step when performing the pickling step).
- a cold rolling process is not specifically limited, What is necessary is just to implement by a conventional method.
- Annealing process An annealing process is implemented in order to advance recrystallization and to form bainite, a tempered martensite, a retained austenite, and a martensite in a steel plate structure
- the annealing process for that purpose includes first heating, second heating, third heating, first soaking, third cooling, fourth heating, second soaking, and fourth cooling. Specifically, it is as follows.
- the first heating is performed under the conditions of an arbitrary first average heating rate and first heating attainment temperature: 250 to 350 ° C. Specifically, the cold-rolled steel sheet at room temperature is heated from 250 to 350 ° C. at an arbitrary first average heating rate.
- the first heating is heating to a temperature of 250 to 350 ° C. at which recrystallization by annealing starts, and may be performed according to a conventional method.
- the first average heating rate is arbitrary as described above, and the value is not particularly limited, but the first average heating rate is usually 0.5 to 50 ° C./s.
- the second heating is performed after the first heating under the conditions of the second average heating rate: 6 to 25 ° C./s and the second heating reaching temperature: 550 to 680 ° C.
- the second heating is a rule that contributes to the refinement of crystal grains that is important in the present invention, and the generation rate of ferrite nuclei generated by recrystallization that appears before being heated to a temperature that becomes a two-phase region is generated. It is possible to refine crystal grains after annealing by speeding up the rate of grain growth, that is, coarsening. When heated rapidly, recrystallization hardly progresses, so that unrecrystallized remains in the final steel sheet structure, resulting in insufficient ductility.
- the upper limit of the second average heating rate is 25 ° C./s. Further, if the heating rate is too small, the ferrite phase becomes coarse and a predetermined average crystal grain size cannot be obtained, so a second average heating rate of 6 ° C./s or more is required. Preferably it is 8 degrees C / s or more.
- the third heating is performed after the second heating under conditions of a third average heating rate: 10 ° C./s or less and a third heating reaching temperature: 760 to 850 ° C. Fine ferrite is produced by the second heating temperature. Since it becomes a two-phase region at a temperature of Ac1 point or higher, austenite nucleation begins.
- the third average heating rate from the second heating attainment temperature to the third heating attainment temperature is set to 10 ° C./s or less. When the third average heating rate exceeds 10 ° C./s, nucleation of austenite is preferential, unrecrystallized remains in the final steel sheet structure, and the ductility is insufficient.
- the upper limit of the third average heating rate is 10 ° C. / S.
- the lower limit is not particularly limited, but if it is less than 0.5 ° C./s, there is a concern that the ferrite phase becomes coarse. Therefore, the third average heating rate is preferably 0.5 ° C./s or more.
- the third heating arrival temperature is the following first soaking temperature.
- the first soaking is performed under the conditions of the first soaking temperature after the third heating: 760 to 850 ° C., the first soaking time: 30 seconds or more.
- the first soaking temperature is set to a temperature range of two phases of ferrite and austenite.
- the lower limit of the first soaking temperature is 760 ° C. If the first soaking temperature is too high, annealing occurs in the austenite single phase region, and the delayed fracture resistance deteriorates, so the first soaking temperature is set to 850 ° C. or lower.
- the first soaking time needs to be maintained for 30 seconds or more in order to advance the recrystallization and partially or completely austenite transform.
- the upper limit is not particularly limited, but is preferably within 600 seconds.
- the third cooling is performed under the conditions of the third average cooling rate: 3 ° C./s or more and the third cooling stop temperature: 100 to 300 ° C. after the first soaking.
- the third cooling stop temperature 100 to 300 ° C. after the first soaking.
- the lower limit of the third cooling rate is 3 ° C./s.
- the third cooling stop temperature is less than 100 ° C.
- martensite is excessively generated at the time of cooling, so that untransformed austenite is reduced, bainite transformation and residual austenite are reduced, and elongation is lowered.
- the cooling stop temperature exceeds 300 ° C.
- the tempered martensite decreases and the hole expandability deteriorates. Therefore, the third cooling stop temperature is set to 100 to 300 ° C.
- the temperature is preferably 150 to 280 ° C.
- the fourth heating is performed after the third cooling under the fourth heating temperature: 350 to 450 ° C.
- the fourth heating is performed to heat up to the second soaking temperature.
- the second soaking is performed under the conditions of the second soaking temperature: 350 to 450 ° C. and the second soaking time: 30 seconds or more after the fourth heating.
- the purpose of the second soaking is to temper martensite generated during cooling into tempered martensite, to transform untransformed austenite to bainite and to produce bainite and retained austenite in the steel sheet structure. As done.
- the second soaking temperature is less than 350 ° C., the tempering of martensite becomes insufficient, and the difference in hardness from ferrite and martensite becomes large, so that the hole expandability deteriorates.
- the second soaking temperature exceeds 450 ° C., pearlite is excessively generated, so that the elongation decreases.
- the second soaking temperature is set to 350 to 450 ° C.
- the bainite transformation does not proceed sufficiently, so that a large amount of untransformed austenite remains, and eventually martensite is excessively generated, resulting in a decrease in hole expandability. Therefore, the second soaking time is 30 seconds or longer. Also, the second soaking time is preferably 3600 seconds or less because the volume fraction of martensite is ensured.
- the fourth cooling is performed under the condition of the fourth cooling stop temperature: 0 to 50 ° C. after the second soaking.
- the fourth cooling may be a method that does not actively cool, for example, air cooling by standing.
- Temper rolling step Temper rolling may be performed after the annealing step.
- a preferred range of elongation in temper rolling is 0.1% to 2.0%.
- hot dip galvanization may be performed to obtain a hot dip galvanized steel sheet, or after hot dip galvanization, an alloying treatment may be performed to obtain an alloyed hot dip galvanized steel sheet.
- the cold-rolled steel sheet may be electroplated to form an electroplated steel sheet.
- the obtained hot-rolled steel sheet was pickled and then cold-rolled to produce a cold-rolled steel sheet (sheet thickness: 1.4 mm). Then, 1st heating was performed on the conditions whose 1st average heating rate is 640 degreeC / s and 1st heating attainment temperature is 300 degreeC. Subsequently, it heated to 680 degreeC (2nd heating attainment temperature) with the 2nd average heating rate (C2 of Table 2) shown in Table 2. Next, heating to the first soaking temperature (also the third heating attainment temperature) at the third average heating rate (C3 in Table 2), the first soaking temperature shown in Table 2 (soaking temperature 1 in Table 2) and The first soaking was performed in the first soaking time (holding 1 in Table 2).
- a JIS No. 5 tensile test piece was taken from the direction perpendicular to the rolling direction to the longitudinal direction (tensile direction), and the tensile strength (YS), tensile strength (YS) was determined by a tensile test (JIS Z2241 (1998)).
- TS total elongation
- YR yield ratio
- the obtained steel sheet was cut into 30 mm ⁇ 100 mm with the rolling direction as the longitudinal direction, and the end face was ground and the specimen was subjected to 180 ° bending with a curvature radius of 10 mm at the punch tip. did.
- the test piece that did not crack within 100 hours was evaluated as “good”, and when a crack occurred in the test piece, it was determined as “bad”.
- the volume fraction of ferrite and martensite in the steel sheet is 2,000 times and 5,000 times magnification using SEM (scanning electron microscope) after corroding the thickness section parallel to the rolling direction of the steel sheet and corroding with 3% nital.
- the area ratio was measured by the point count method (based on ASTM E562-83 (1988)), and the area ratio was defined as the volume fraction.
- the average crystal grain size of ferrite and martensite is the area of each phase by taking a picture in which each ferrite and martensite crystal grain is identified in advance from a steel sheet structure picture using Image-Pro of Media Cybernetics. The equivalent circle diameter was calculated, and those values were obtained as the average crystal grain size (average grain size in the table).
- the volume fraction of retained austenite was determined by polishing the steel plate to a 1 ⁇ 4 plane in the thickness direction and diffracting X-ray intensity on this 1 ⁇ 4 plane.
- a K ⁇ ray of Mo as a radiation source and an acceleration voltage of 50 keV
- an X-ray diffraction method (apparatus: RINT2200 manufactured by Rigaku) and a ferrite ferrite ⁇ 200 ⁇ plane, ⁇ 211 ⁇ plane, ⁇ 220 ⁇ plane, and austenite
- the integrated intensity of X-ray diffraction lines on the ⁇ 200 ⁇ plane, ⁇ 220 ⁇ plane, and ⁇ 311 ⁇ plane is measured, and using these measured values, “X-ray diffraction handbook” (2000) Rigaku Denki Co., Ltd., p. 26, 62-64, the volume fraction of retained austenite was determined.
- the steel sheet structure was observed by SEM (scanning electron microscope), TEM (transmission electron microscope), and FE-SEM (field emission scanning electron microscope), and the types of steel structures other than ferrite, retained austenite, and martensite. It was determined.
- the average crystal grain size of the structure that is bainite and / or tempered martensite was obtained by calculating the equivalent circle diameter from the steel sheet structure photograph using the above-mentioned Image-Pro and averaging these values.
- Table 3 The measured tensile properties, hole expansion ratio, delayed fracture resistance, and steel sheet structure measurement results are shown in Table 3 (Table 3-1 and Table 3-2 are combined into Table 3).
- ferrite having an average crystal grain size of less than 2 ⁇ m is 10 to 25% in volume fraction
- residual austenite is 5 to 20%
- average crystal grain size is 2 ⁇ m.
- the following martensite has a composite structure containing bainite and tempered martensite having a volume fraction of 5 to 15% and an average grain size of 5 ⁇ m or less in the balance.
- a tensile strength of 1180 MPa or more and 70% or more Good processability with an elongation of 17.5% or more and a hole expansion rate of 40% or more was obtained while ensuring a yield ratio of 100%, and it was excellent because no fracture occurred for 100 hours in the delayed fracture property evaluation test.
- the steel sheet structure does not satisfy the scope of the present invention, and as a result, at least one of the tensile strength, yield ratio, elongation, hole expansion rate, and delayed fracture resistance is inferior.
Abstract
Description
0.35≦V2/V1≦0.75 式(1)
[2]前記成分組成は、さらに、質量%で、V:0.05%以下及びNb:0.05%以下から選択される一種以上を含有する成分組成であることを特徴とする[1]に記載の高強度冷延鋼板。
本発明の高強度冷延鋼板は、質量%で、C:0.15~0.25%、Si:1.2~2.5%、Mn:2.1~3.5%、P:0.05%以下、S:0.005%以下、Al:0.01~0.08%、N:0.010%以下、Ti:0.002~0.050%、B:0.0002~0.0100%を含有する。
Cは鋼板の高強度化に有効な元素であり、本発明におけるベイナイト、焼戻しマルテンサイト、残留オーステナイト及びマルテンサイトの第2相形成にも寄与する。さらに、Cはマルテンサイトおよび焼戻しマルテンサイトの硬度を高くする。C含有量が0.15%未満では、必要なベイナイト、焼戻しマルテンサイト、残留オーステナイト及びマルテンサイトの体積分率の確保が難しい。好ましいC含有量は0.17%以上である。一方、Cを過剰に添加するとフェライト、焼戻しマルテンサイト、マルテンサイトの硬度差が大きくなるため、穴広げ性が低下する。そこで、C含有量は0.25%以下とする。好ましいC含有量は0.22%以下である。
Siはフェライトを固溶強化し、軟質相と硬質相との硬度差を低下させるため、Siは穴広げ率を増加させる。その効果を得るためには、Siを1.2%以上の含有することが必要である。好ましいSi含有量は1.3%以上である。しかしながら、Siの過剰な添加は化成処理性を低下させる。このため、Si含有量は2.5%以下とする。好ましくは2.2%以下である。
Mnは固溶強化および第2相を生成することで高強度化に寄与する元素である。また、Mnはオーステナイトを安定化させる元素であり、第2相の分率制御に必要な元素である。その効果を得るためにはMn含有量を2.1%以上にすることが必要である。一方、過剰にMnを含有した場合、マルテンサイトの体積分率が過剰になり、さらにマルテンサイトおよび焼戻しマルテンサイトの硬度が増加してしまい、穴広げ性が低下する。また、過剰にMnを含有した場合に、水素が鋼板内に侵入すると、粒界のすべり拘束が増加し、結晶粒界でのき裂が進展しやすくなるため耐遅れ破壊特性が低下してしまう。そのため、Mn含有量は3.5%以下とする。好ましくは3.0%以下である。
Pは固溶強化により高強度化に寄与する。しかし、過剰にPを添加すると、粒界へのPの偏析が著しくなって粒界が脆化したり、溶接性が低下したりする。このため、P含有量を0.05%以下とする。好ましくは0.04%以下である。
S含有量が多い場合には、MnSなどの硫化物が多く生成し、穴広げ性に代表される局部伸びが低下する。このため、S含有量の上限を0.005%とする。好ましくは、0.0040%以下である。特に下限は無いが、極低S化は製鋼コストが上昇するため、0.0002%以上含有することが好ましい。
Alは脱酸に必要な元素であり、この効果を得るためには、Al含有量を0.01%以上とすることが必要である。また、Al含有量が0.08%を超えても効果が飽和するため、Al含有量は0.08%以下とする。好ましくは0.05%以下である。
Nは粗大な窒化物を形成し、曲げ性や伸びフランジ性を劣化させることから、その含有量を抑える必要がある。これらの問題は、N含有量が0.010%超で顕著に表れる。このため、N含有量を0.010%以下とする。好ましくは0.0050%以下である。
Tiは微細な炭窒化物を形成することで、強度上昇に寄与することができる元素である。さらに、Tiは、本発明に必須な元素であるBをNと反応させないためにも必要である。このような効果を発揮させるためには、Ti含有量を0.002%以上とする。好ましくは0.005%以上である。一方、多量にTiを添加すると、伸びが著しく低下するため、その含有量は0.050%以下とする。好ましく0.035%以下である。
Bは焼入れ性を向上させ、第2相を生成することで高強度化に寄与し、焼入れ性を確保しつつ、マルテンサイト変態開始点を低下させない元素である。さらに、Bには、熱間圧延時の仕上げ圧延後に冷却する際、フェライトやパーライトの生成を抑制する効果がある。この効果を発揮するために、B含有量を0.0002%以上とすることが必要である。一方、B含有量が0.0100%超えても効果が飽和するため、その含有量を0.0100%以下とする。好ましくは0.0050%以下である。
Vの微細な炭窒化物を形成することで、強度上昇に寄与する。このような作用を有するために、V含有量を0.01%以上にすることが好ましい。一方、多量のVを添加しても、0.05%を超えた分の強度上昇効果は小さく、そのうえ、合金コストの増加も招いてしまう。したがって、Vの含有量は0.05%以下が好ましい。
NbもVと同様に、微細な炭窒化物を形成することで、強度上昇に寄与することができるため、必要に応じて添加することができる。このような効果を発揮させるためには、Nb含有量を0.005%以上とすることが好ましい。一方、多量にNbを添加すると、伸びが著しく低下するため、Nbを含有する場合、その含有量は0.05%以下とする。
Crは第2相を生成することで高強度化に寄与する元素であり、必要に応じて添加することができる。この効果を発揮させるためには、Cr含有量を0.10%以上にすることが好ましい。一方、Cr含有量が0.50%を超えると、過剰にマルテンサイトが生成する。そこで、Crを含有する場合、その含有量は0.50%以下とする。
Moは第2相を生成することで高強度化に寄与し、さらに一部炭化物を生成して高強度化に寄与する元素であり、必要に応じて添加することができる。これら効果を発揮させるためには、Mo含有量を0.05%以上にすることが好ましい。また、Mo含有量が0.50%を超えると効果が飽和するため、その含有量は0.50%以下が好ましい。
Cuは固溶強化により高強度化に寄与して、また第2相を生成させることで高強度化に寄与する元素であり、必要に応じて添加することができる。これら効果を発揮するためにはCu含有量を0.05%以上とすることが好ましい。一方、Cu含有量が0.50%を超えても効果が飽和し、またCuに起因する表面欠陥が発生しやすくなる。そこで、Cu含有量は0.50%以下が好ましい。
NiもCuと同様、固溶強化により高強度化に寄与して、また第2相を生成させることで高強度化に寄与する元素であり、必要に応じて添加することができる。これら効果を発揮させるためには、Ni含有量を0.05%以上とすることが好ましい。また、Cuと同時に添加すると、Cu起因の表面欠陥を抑制する効果があるため、Cu添加時にNiを添加することが有効である。一方、Ni含有量が0.50%を超えても効果が飽和するため、その含有量は0.50%以下が好ましい。
Caは、硫化物の形状を球状化し穴広げ性への硫化物の悪影響を改善する元素であり、必要に応じて添加することができる。これらの効果を発揮するためにはCa含有量を0.0005%以上にすることが好ましい。一方、Ca含有量が0.0050%を超えると、Caの硫化物が曲げ性を劣化させる。そこで、Ca含有量は0.0050%以下とする。
REMもCaと同様に、硫化物の形状を球状化し穴広げ性への硫化物の悪影響を改善する元素であり、必要に応じて添加することができる。これらの効果を発揮するためにはREM含有量を0.0005%以上にすることが好ましい。一方、REM含有量が0.0050%を超えても効果が飽和するため、その含有量を0.0050%以下とすることが好ましい。
0.35≦V2/V1≦0.75 式(1)
式(1)において、フェライト以外の硬質相の体積分率がV1であり、焼戻しマルテンサイトの体積分率がV2である。
フェライトの体積分率が10%未満では、伸びの確保が困難である。そこで、フェライトの体積分率の下限は10%とする。好ましくは、フェライトの体積分率が12%超である。また、フェライトの体積分率が25%を超えると、打抜き時のボイド生成量が増加する。また、フェライトの体積分率が25%を超えると、強度確保のため、マルテンサイトや焼戻しマルテンサイトの硬度も高くする必要があり、強度と穴広げ性の両立が困難である。このためフェライトの体積分率は25%以下とする。好ましくは22%以下であり、さらに好ましくは20%未満である。
良好な延性を確保するためには、残留オーステナイトの体積分率を5~20%の範囲にすることが必要である。残留オーステナイトの体積分率が5%未満では伸びが低下する。このため、残留オーステナイトの体積分率は5%以上とする。好ましくは8%以上である。また、残留オーステナイトの体積分率が20%を超える場合、穴広げ性が劣化する。このため、残留オーステナイトの体積分率は20%以下である。好ましくは18%以下である。
所望の強度および延性を確保しつつ、穴広げ性を確保するためにマルテンサイトの体積分率を5~15%以下とする。マルテンサイトの体積分率が5%未満では、加工硬化に及ぼす寄与が低いため、強度と延性の両立が困難である。好ましくは6%以上である。また、マルテンサイトの体積分率が15%超では、打抜き時にマルテンサイト周辺にボイドが生成するため穴拡げ性が劣化するだけでなく、降伏比も低下する。このため、マルテンサイトの体積分率の上限は15%とする。好ましくは12%を上限とする。
良好な穴広げ性や高降伏比を確保するために、上記のフェライト、残留オーステナイト、マルテンサイト以外の残部には、ベイナイトおよび焼戻しマルテンサイトを含有することが必要である。ベイナイトおよび焼戻しマルテンサイトの平均結晶粒径は5μm以下とする。平均結晶粒径が5μm超では、フェライトとの界面に生成するボイドが連結しやすくなり、穴広げ性が劣化する。このため、ベイナイトおよび焼戻しマルテンサイトの平均結晶粒径の上限は5μmとする。
また、フェライト相以外の硬質相の体積分率(V1)と焼戻しマルテンサイトの体積分率(V2)において、式(1)の関係を満たすことが必要である。冷却時に生成したマルテンサイトは再加熱時およびその後の均熱保持により、焼戻されることで、焼戻しマルテンサイトとなる。この焼戻しマルテンサイトの存在により、均熱保持中のベイナイト変態が促進され、最終的に室温まで冷却した際に生成するマルテンサイトが微小になり、かつマルテンサイトの体積分率を狙いの体積分率に調整することが可能である。式(1)において、V2/V1の値が0.35未満ではその効果は薄いため、下限は0.35とする。また、V2/V1の値が0.75以上では、ベイナイト変態可能な未変態のオーステナイトが少ないため、十分な残留オーステナイトが得られず、伸びが低下する。このため、その上限は0.75とする。好ましくは0.70以下である。
0.35≦V2/V1≦0.75 式(1)
また、本発明では、フェライト、ベイナイト、焼戻しマルテンサイト、残留オーステナイトおよびマルテンサイト以外に、パーライトをミクロ組織が含む場合がある。上記のフェライト、残留オーステナイトおよびマルテンサイトの体積分率、フェライト、マルテンサイトの平均結晶粒径が満足されれば、パーライトを含んでも本発明の目的を達成できる。ただし、パーライトの体積分率は3%以下が好ましい。
次に、本発明の高強度冷延鋼板の製造法について説明する。
平均冷却速度=(冷却開始表面温度-冷却終了表面温度)/冷却時間 (2)
平均加熱速度=(加熱終了表面温度-加熱開始表面温度)/加熱時間 (3)
熱間圧延工程
熱間圧延工程とは、上記成分組成を有し1150~1300℃の鋼スラブを、仕上げ圧延終了温度:850~950℃の条件で圧延を行い、上記圧延の終了後1秒以内に、第1平均冷却速度:80℃/s以上、第1冷却停止温度:650℃以下の条件で、冷却を開始する第1冷却を行い、上記第1冷却後、第2平均冷却速度:5℃/s以上、第2冷却停止温度:第1冷却停止温度未満かつ550℃以下の条件で冷却する第2冷却を行い、上記第2冷却後に巻取る工程である。各条件の限定理由は以下の通りである。
熱間圧延工程後、酸性工程を実施し、熱延板の表層のスケールを除去するのが好ましい。酸洗工程の条件は特に限定されず、常法に従って実施すればよい。
熱間圧延工程後(酸洗工程を行う場合には酸洗工程後)に、熱延板に対して冷間圧延を施す工程である。冷間圧延工程は特に限定されず常法で実施すればよい。
焼鈍工程は、再結晶を進行させるとともに、高強度化のため鋼板組織にベイナイト、焼戻しマルテンサイト、残留オーステナイトやマルテンサイトを形成させるために実施する。そのための焼鈍工程は、第1加熱、第2加熱、第3加熱、第1均熱、第3冷却、第4加熱、第2均熱、第4冷却から構成される。具体的には以下の通りである。
焼鈍工程後に、調質圧延を実施してもよい。調質圧延における伸長率の好ましい範囲は0.1%~2.0%である。
Claims (4)
- 質量%で、C:0.15~0.25%、Si:1.2~2.5%、Mn:2.1~3.5%、P:0.05%以下、S:0.005%以下、Al:0.01~0.08%、N:0.010%以下、Ti:0.002~0.050%、B:0.0002~0.0100%を含有し、残部がFeおよび不可避的不純物からなる成分組成を有し、
平均結晶粒径が2μm以下のフェライトを体積分率で10~25%、残留オーステナイトを体積分率で5~20%、平均結晶粒径が2μm以下のマルテンサイトを体積分率で5~15%以下を含有し、残部が平均結晶粒径5μm以下のベイナイトおよび焼戻しマルテンサイトを含む複合組織であり、フェライト以外の相の体積分率(V1)と焼戻しマルテンサイトの体積分率(V2)の関係が下記の式(1)の条件を満たすミクロ組織を有することを特徴とする高強度冷延鋼板。
0.35≦V2/V1≦0.75 式(1) - 前記成分組成は、さらに、質量%で、V:0.05%以下及びNb:0.05%以下から選択される一種以上を含有する成分組成であることを特徴とする請求項1に記載の高強度冷延鋼板。
- 前記成分組成は、さらに、質量%で、Cr:0.50%以下、Mo:0.50%以下、Cu:0.50%以下、Ni:0.50%以下、Ca:0.0050%以下及びREM:0.0050%以下から選択される一種以上を含有する成分組成であることを特徴とする請求項1又は2に記載の高強度冷延鋼板。
- 請求項1~3のいずれかに記載の成分組成を有し1150~1300℃の鋼スラブを、仕上げ圧延終了温度:850~950℃の条件で圧延を行い、前記圧延の終了後1秒以内に冷却を開始する、第1平均冷却速度:80℃/s以上、第1冷却停止温度:650℃以下の条件で第1冷却を行い、前記第1冷却後、第2平均冷却速度:5℃/s以上、第2冷却停止温度:第1冷却停止温度未満かつ550℃以下の条件で冷却する第2冷却を行い、前記第2冷却後に巻取る熱間圧延工程と、
前記熱間圧延工程後に必要に応じて酸洗を行う酸洗工程と、
前記熱間圧延工程後(酸洗工程を行う場合には前記酸洗工程後)に冷間圧延を行う冷間圧延工程と、
前記冷間圧延工程後に、任意の第1平均加熱速度、第1加熱到達温度:250~350℃の条件で第1加熱を行い、前記第1加熱後に第2平均加熱速度:6~25℃/s、第2加熱到達温度:550~680℃の条件で第2加熱を行い、前記第2加熱後に第3平均加熱速度:10℃/s以下、第3加熱到達温度:760~850℃の条件で第3加熱を行い、前記第3加熱後に第1均熱温度:760~850℃、第1均熱時間:30秒以上の条件で第1均熱を行い、前記第1均熱後に第3平均冷却速度:3℃/s以上、第3冷却停止温度:100~300℃の条件で第3冷却を行い、前記第3冷却後に第4加熱到達温度:350~450℃の条件で第4加熱を行い、前記第4加熱後に第2均熱温度:350~450℃、第2均熱時間:30秒以上の条件で第2均熱を行い、前記第2均熱後に第4冷却停止温度:0~50℃の条件で第4冷却を行う焼鈍工程と、を有することを特徴とする高強度冷延鋼板の製造方法。
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EP3187613B1 (en) | 2019-09-04 |
JP5991450B1 (ja) | 2016-09-14 |
JPWO2016092733A1 (ja) | 2017-04-27 |
EP3187613A1 (en) | 2017-07-05 |
MX2017007511A (es) | 2017-08-22 |
EP3187613A4 (en) | 2017-11-15 |
US10590504B2 (en) | 2020-03-17 |
KR20170075796A (ko) | 2017-07-03 |
KR102000854B1 (ko) | 2019-07-16 |
CN107002198B (zh) | 2019-05-28 |
US20170321297A1 (en) | 2017-11-09 |
CN107002198A (zh) | 2017-08-01 |
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