WO2020075394A1 - 高強度鋼板およびその製造方法 - Google Patents
高強度鋼板およびその製造方法 Download PDFInfo
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- WO2020075394A1 WO2020075394A1 PCT/JP2019/032513 JP2019032513W WO2020075394A1 WO 2020075394 A1 WO2020075394 A1 WO 2020075394A1 JP 2019032513 W JP2019032513 W JP 2019032513W WO 2020075394 A1 WO2020075394 A1 WO 2020075394A1
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Definitions
- the present invention relates to a high-strength steel sheet having a strength of 1180 MPa or more, which is excellent in dimensional accuracy of components, stretch flangeability, bendability, and toughness, and a manufacturing method thereof.
- the high-strength steel sheet of the present invention can be suitably used as a structural member such as an automobile part.
- the strength of thin steel sheets for automobiles is increasing with the aim of reducing CO 2 emissions by reducing the weight of vehicles and improving collision resistance by reducing the weight of vehicle bodies, and new laws and regulations are being introduced one after another. . Therefore, for the purpose of increasing the strength of the vehicle body, the number of application examples of high-strength steel sheets having a tensile strength (TS) of 1180 MPa or higher is increasing in the main structural parts forming the skeleton of an automobile cabin.
- TS tensile strength
- Patent Document 1 in the region where the tensile strength is 980 MPa or more and the 0.2% proof stress is 700 MPa or more, in addition to ductility, stretch flangeability, weldability, and bending workability. Excellent high strength cold rolled steel sheets are provided.
- Patent Document 2 provides a high-strength cold-rolled steel sheet having excellent ductility and stretch-flangeability, a high yield ratio and a tensile strength of 1180 MPa or more, and a method for producing the same.
- Patent Document 3 proposes a heat-treated steel sheet member having a tensile strength of 1.4 GPa or more, a total elongation of 8.0% or more, and excellent toughness, scale adhesion and scale peeling property, and a method for producing the same. ing.
- Patent Document 4 proposes a heat-treated steel sheet member having a tensile strength of 1.4 GPa or more, a yield ratio of 0.65 or more, and excellent toughness, scale adhesion and scale releasability, and a manufacturing method thereof. There is.
- Patent Document 5 provides a high-strength steel sheet having a tensile strength of 1320 MPa or more and excellent ductility and stretch-flangeability, and a method for producing the same.
- Patent Document 6 provides a high-strength steel sheet having a tensile strength of 1320 MPa or more, excellent ductility, stretch-flangeability and bending workability, and a method for producing the same.
- the present invention has been developed in view of such circumstances, and an object thereof is to provide a high-strength steel plate of 1180 MPa or more excellent in dimensional accuracy of components, stretch flangeability, bendability and toughness, and a manufacturing method thereof.
- the excellent dimensional accuracy of the component means that the yield ratio (YR) which is an index of the dimensional accuracy of the component is 65% or more and 85% or less.
- YR is calculated by the following equation (1).
- YR YS / TS ...
- excellent stretch flangeability means that the hole expansion ratio ( ⁇ ), which is an index of stretch flangeability, is 30% or more. The bendability was evaluated by the pass rate of the bending test, and the bending test of 5 samples was performed at the maximum R where the value R / t obtained by dividing the bending radius (R) by the plate thickness (t) was 5 or less.
- Stretch-flangeability can be achieved to 30% or more by using a structure mainly composed of a hard phase (martensite and tempered martensite).
- YR which is an index of dimensional accuracy of parts
- YR an index of dimensional accuracy of parts
- the bendability can be improved by setting the surface layer softening thickness to 10 ⁇ m or more and 100 ⁇ m or less.
- the present invention has been made based on the above findings. That is, the gist of the present invention is as follows. [1]% by mass, C: 0.09% or more and 0.37% or less, Si: more than 0.70% and 2.00% or less, Mn: 2.60% or more and 3.60% or less, P: 0.001% or more and 0.100% or less, S: 0.0200% or less, Al: 0.010% or more and 1.000% or less and N: 0.0100% or less, with the balance being Fe and inevitable impurities.
- the martensite having a carbon concentration of more than 0.7 ⁇ [% C] and less than 1.5 ⁇ [% C] is 55% or more in area ratio
- the tempered martensite having a carbon concentration of 0.7 ⁇ [% C] or less has an area ratio of 5% or more and 40% or less
- the ratio of the carbon concentration in the retained austenite to the volume ratio of the retained austenite is 0.05 or more and 0.40 or less
- the steel structure has an average crystal grain size of 5.3 ⁇ m or less for each of the martensite and the tempered martensite, A high-strength steel sheet having a tensile strength of 1180 MPa or more.
- [% C] shows the content (mass%) of the component element C in steel.
- the composition of the components is further% by mass. Ti: 0.001% or more and 0.100% or less, Nb: 0.001% or more and 0.100% or less, V: 0.001% or more and 0.100% or less, B: 0.0001% or more and 0.0100% or less, Mo: 0.010% or more and 0.500% or less, Cr: 0.01% or more and 1.00% or less, Cu: 0.01% or more and 1.00% or less, Ni: 0.01% or more and 0.50% or less, Sb: 0.001% or more and 0.200% or less, Sn: 0.001% or more and 0.200% or less, Ta: 0.001% or more and 0.100% or less, Ca: 0.0001% or more and 0.0200% or less, Mg: 0.0001% or more and 0.0200% or less, Zn: 0.00
- a method for manufacturing a high-strength steel sheet [6] The method for producing a high-strength steel sheet according to [5], wherein the oxygen concentration in the heating temperature range is 2 ppm or more and 30 ppm or less, and the dew point is ⁇ 35 ° C. or more. [7] The method for manufacturing a high-strength steel sheet according to [5] or [6], which further performs plating after the annealing.
- the present invention it is possible to obtain a high-strength steel plate having a strength of 1180 MPa or more, which is excellent in dimensional accuracy of components, stretch flangeability, bendability, and toughness. Further, by applying the high-strength steel sheet of the present invention to, for example, an automobile structural member, it is possible to improve fuel efficiency by reducing the weight of the vehicle body. Therefore, its industrial utility value is extremely high.
- C 0.09% or more and 0.37% or less
- C is one of the important basic components of steel, and particularly in the present invention, the fraction of martensite, tempered martensite and retained austenite, and the content of retained austenite. It is an important element that affects carbon concentration.
- the content of C is less than 0.09%, the fraction of martensite decreases, and it becomes difficult to realize TS of 1180 MPa or more.
- the content of C exceeds 0.37%, the fraction of tempered martensite decreases, and it becomes difficult to achieve a hole expansion rate ( ⁇ ) of 30% or more, which is an index of stretch flangeability. . Therefore, the content of C is set to 0.09% or more and 0.37% or less.
- it is 0.10% or more.
- Preferably it is 0.36% or less. More preferably, it is 0.11% or more. More preferably, it is 0.35% or less.
- Si more than 0.70% and not more than 2.00% Si suppresses the formation of carbides during continuous annealing and promotes the formation of retained austenite, and therefore affects the fraction of retained austenite and the carbon concentration in retained austenite. It is an element that does. If the Si content is 0.70% or less, retained austenite cannot be generated and YR cannot be controlled within a desired range. On the other hand, if the Si content exceeds 2.00%, the carbon concentration in the retained austenite excessively increases, and the hardness of martensite transformed from the retained austenite during punching greatly increases. The generation of voids increases and ⁇ decreases. Therefore, the Si content is set to more than 0.70% and 2.00% or less. Preferably it is 0.80% or more. Preferably it is 1.80% or less. More preferably, it is 0.90% or more. More preferably, it is 1.70% or less.
- Mn 2.60% or more and 3.60% or less
- Mn is one of the important basic components of steel, and particularly in the present invention, it is an important element that affects the fraction of martensite and tempered martensite. If the Mn content is less than 2.60%, the fraction of martensite decreases, and it becomes difficult to realize a TS of 1180 MPa or more. On the other hand, when the Mn content exceeds 3.60%, the fraction of tempered martensite decreases, and it becomes difficult to achieve ⁇ of 30% or more. Therefore, the Mn content is set to 2.60% or more and 3.60% or less. Preferably it is 2.65% or more. Preferably it is 3.50% or less. More preferably, it is 2.70% or more. More preferably, it is 3.40% or less.
- P 0.001% or more and 0.100% or less
- P is an element that has the effect of solid solution strengthening and can increase the strength of the steel sheet. In order to obtain such effects, the P content needs to be 0.001% or more.
- the content of P is set to 0.001% or more and 0.100% or less. Preferably it is 0.002% or more. Preferably it is 0.070% or less. More preferably, it is 0.003% or more. More preferably, it is 0.050% or less.
- S 0.0200% or less S exists as a sulfide and reduces the ultimate deformability of the steel, so that ⁇ decreases. In addition, bendability also decreases. Therefore, the S content needs to be 0.0200% or less. Although the lower limit of the S content is not particularly specified, it is preferable that the S content is 0.0001% or more due to restrictions in production technology. Therefore, the S content is 0.0200% or less. Preferably it is 0.0001% or more. Preferably it is 0.0050% or less.
- Al 0.010% or more and 1.000% or less
- Al suppresses carbide formation during continuous annealing and promotes the formation of retained austenite, and therefore affects the fraction of retained austenite and the carbon concentration in retained austenite. It is an element that does.
- the Al content needs to be 0.010% or more.
- the Al content is set to 0.010% or more and 1.000% or less.
- it is 0.015% or more.
- it is 0.500% or less. More preferably, it is 0.020% or more. More preferably, it is 0.100% or less.
- N 0.0100% or less
- N exists as a nitride and reduces the ultimate deformability of the steel sheet, so that ⁇ decreases. In addition, bendability also decreases. Therefore, the N content needs to be 0.0100% or less.
- the lower limit of the N content is not particularly specified, but it is preferable that the N content is 0.0005% or more due to restrictions in production technology. Therefore, the N content is 0.0100% or less. Preferably it is 0.0005% or more. Preferably it is 0.0050% or less.
- the high-strength steel sheet of the present invention is, in addition to the above-described composition, further, in mass%, Ti: 0.001% or more and 0.100% or less, Nb: 0.001% or more and 0.100% or less, V: 0.001% or more and 0.100% or less, B: 0.0001% or more and 0.0100% or less, Mo: 0.010% or more and 0.500% or less, Cr: 0.01% or more and 1.00% or less, Cu: 0.01% or more and 1.00% or less, Ni: 0.01% or more and 0.50% or less, Sb: 0.001% or more and 0.200% or less, Sn: 0.001% or more and 0.200%
- Ta 0.001% or more and 0.100% or less
- Ca 0.0001% or more and 0.0200% or less
- Mg 0.0001% or more and 0.0200% or less
- Zn 0.001% or more and 0.0. 020% or less
- Co 0.001% or more and 0.020% or less
- Ti, Nb and V increase the strength of the steel sheet by forming fine carbides, nitrides or carbonitrides during hot rolling or continuous annealing. Further, by adding Ti, Nb and V, the recrystallization temperature in the temperature rising process during continuous annealing rises and the average grain size of martensite and tempered martensite decreases, so that the toughness of the steel sheet is improved. Can be improved. In order to obtain such effects, the contents of Ti, Nb and V must be 0.001% or more. On the other hand, when the contents of Ti, Nb, and V exceed 0.100%, large amounts of coarse precipitates and inclusions are generated, and the ultimate deformability of the steel sheet is reduced, so that ⁇ is reduced. In addition, bendability also decreases. Therefore, when Ti, Nb, and V are added, their contents are 0.001% or more and 0.100% or less, respectively. Preferably it is 0.005% or more. It is preferably 0.060% or less.
- B is an element that can improve hardenability without lowering the martensitic transformation start temperature, and can suppress the formation of ferrite during the cooling process during continuous annealing. In order to obtain such effects, the B content needs to be 0.0001% or more. On the other hand, if the content of B exceeds 0.0100%, cracks occur inside the steel sheet during hot rolling, which lowers the ultimate deformability of the steel sheet, resulting in a decrease in ⁇ . In addition, bendability also decreases. Therefore, when B is added, its content is set to 0.0001% or more and 0.0100% or less. Preferably it is 0.0002% or more. Preferably it is 0.0050% or less.
- Mo is an element that improves the hardenability and is an element that is effective in producing martensite and tempered martensite. In order to obtain such effects, the Mo content needs to be 0.010% or more. On the other hand, when the Mo content exceeds 0.500%, coarse precipitates and inclusions increase and the ultimate deformability of the steel sheet decreases, so that ⁇ decreases. In addition, bendability also decreases. Therefore, when Mo is added, its content is set to 0.010% or more and 0.500% or less. Preferably it is 0.020% or more. It is preferably 0.450% or less.
- Cr and Cu not only serve as solid solution strengthening elements, but also stabilize austenite during the cooling process during continuous annealing and facilitate the formation of martensite and tempered martensite.
- the contents of Cr and Cu must be 0.01% or more.
- the contents of Cr and Cu each exceed 1.00%, a large amount of coarse precipitates and inclusions are generated and the ultimate deformability of the steel sheet is reduced, so that ⁇ is reduced.
- bendability also decreases. Therefore, when Cr and Cu are added, their contents are 0.01% or more and 1.00% or less, respectively. Preferably it is 0.02% or more. It is preferably 0.70% or less.
- Ni is an element that improves hardenability and is an element that is effective in producing martensite and tempered martensite. In order to obtain such effects, the Ni content needs to be 0.01% or more. On the other hand, when the Ni content exceeds 0.50%, coarse precipitates and inclusions increase and the ultimate deformability of the steel sheet decreases, so ⁇ decreases. In addition, bendability also decreases. Therefore, when Ni is added, its content is set to 0.01% or more and 0.50% or less. Preferably it is 0.02% or more. Preferably it is 0.45% or less.
- Sb and Sn are effective elements for controlling the softening thickness of the surface layer.
- the contents of Sb and Sn must each be 0.001% or more.
- the contents of Sb and Sn each exceed 0.200%, coarse precipitates and inclusions increase and the ultimate deformability of the steel sheet decreases, so that ⁇ decreases.
- bendability also decreases. Therefore, when Sb and Sn are added, their contents should be 0.001% or more and 0.200% or less, respectively. Preferably it is 0.005% or more. Preferably it is 0.100% or less.
- Ta like Ti, Nb and V, increases the strength of the steel sheet by forming fine carbides, nitrides or carbonitrides during hot rolling or continuous annealing.
- Ta partially dissolves in Nb carbides and Nb carbonitrides to form complex precipitates such as (Nb, Ta) (C, N), which significantly suppresses coarsening of the precipitates.
- the precipitation strengthening has an effect of stabilizing the contribution rate to the strength improvement of the steel sheet.
- the Ta content needs to be 0.001%.
- the Ta content exceeds 0.100%, a large amount of coarse precipitates and inclusions are generated, and the ultimate deformability of the steel sheet is reduced, so that ⁇ is reduced.
- bendability also decreases. Therefore, when Ta is added, its content should be 0.001% or more and 0.100% or less.
- Ca and Mg are elements used for deoxidation, and are effective elements for making the shape of sulfide spherical and improving the ultimate deformability of the steel sheet.
- the Ca and Mg contents must each be 0.0001% or more.
- the contents of Ca and Mg exceed 0.0200%, large amounts of coarse precipitates and inclusions are generated, and the ultimate deformability of the steel sheet is reduced, so that ⁇ is reduced. In addition, bendability also decreases. Therefore, when Ca and Mg are added, their contents are made 0.0001% or more and 0.0200% or less, respectively.
- Zn, Co and Zr are all effective elements for making the shape of inclusions spherical and improving the ultimate deformability of the steel sheet.
- the contents of Zn, Co and Zr must be 0.001% or more.
- the contents of Zn, Co, and Zr exceed 0.020%, large amounts of coarse precipitates and inclusions are generated and the ultimate deformability of the steel sheet is reduced, so that ⁇ is reduced.
- bendability also decreases. Therefore, when Zn, Co and Zr are added, their contents are made 0.0001% or more and 0.0200% or less, respectively.
- REM is an element effective in making the shape of inclusions spherical and improving the ultimate deformability of the steel sheet.
- the REM content needs to be 0.0001% or more.
- the content of REM exceeds 0.0200%, a large amount of coarse precipitates and inclusions are generated, and the ultimate deformability of the steel sheet is reduced, so that ⁇ is reduced.
- bendability also decreases. Therefore, when REM is added, its content should be 0.0001% or more and 0.0200% or less.
- the balance other than the above components is Fe and inevitable impurities.
- the content of the above optional components is less than the lower limit value, the effect of the present invention is not impaired. Therefore, when these optional elements are included below the lower limit values, these optional elements are included as unavoidable impurities.
- the area ratio of martensite having a carbon concentration of more than 0.7 ⁇ [% C] and less than 1.5 ⁇ [% C] needs to be 55% or more.
- the upper limit of the area ratio of martensite having a carbon concentration of more than 0.7 ⁇ [% C] and less than 1.5 ⁇ [% C] is not particularly specified, but in order to realize desired ⁇ and YR, it is 95.
- the area ratio of martensite having a carbon concentration of more than 0.7 ⁇ [% C] and less than 1.5 ⁇ [% C] is set to 55% or more. It is preferably 56% or more. It is preferably 95% or less. It is more preferably 57% or more. More preferably, it is 90% or less.
- the martensite having a carbon concentration higher than 0.7 ⁇ [% C] and lower than 1.5 ⁇ [% C] can be defined as quenched martensite.
- [% C] represents the content (mass%) of the constituent element C in the steel.
- Tempered martensite having a carbon concentration of 0.7 ⁇ [% C] or less has a carbon concentration of 0%
- the desired ⁇ and YR can be realized by adjoining to martensite larger than 0.7 ⁇ [% C] and smaller than 1.5 ⁇ [% C].
- the area ratio of tempered martensite having a carbon concentration of 0.7 ⁇ [% C] or less needs to be 5% or more.
- the tempered martensite having a carbon concentration of 0.7 ⁇ [% C] or less exceeds 40%, the martensite having a carbon concentration of more than 0.7 ⁇ [% C] and less than 1.5 ⁇ [% C] is used.
- the area ratio of the site decreases, and it becomes difficult to realize a TS of 1180 MPa or more. Therefore, the area ratio of tempered martensite having a carbon concentration of 0.7 ⁇ [% C] or less is set to 5% or more and 40% or less. It is preferably 6% or more. Preferably it is 39% or more. More preferably, it is 7% or more. Preferably it is 38% or more.
- tempered martensite having a carbon concentration of 0.7 ⁇ [% C] or less can be defined as bainite.
- [% C] represents the content (mass%) of the constituent element C in the steel.
- the area ratio of martensite having a carbon concentration of more than 0.7 ⁇ [% C] and less than 1.5 ⁇ [% C], and tempered martens having a carbon concentration of 0.7 ⁇ [% C] or less is as follows.
- the observation surface is polished with diamond paste, and then finish polishing is performed using alumina.
- EPMA electron beam microanalyzer
- 3 fields of view were measured under the conditions of an accelerating voltage of 7 kV and a measurement area of 22.5 ⁇ m ⁇ 22.5 ⁇ m, and the data after measurement was measured for carbon concentration by a calibration curve method. Converted to.
- the area of carbon concentration greater than 0.7 x [% C] and less than 1.5 x [% C] is martensite, and the carbon concentration of 0.7 x [% C] or less was defined as tempered martensite, and the area ratio of each was calculated.
- Ratio of carbon concentration in retained austenite to volume ratio of retained austenite 0.05 or more and 0.40 or less
- ratio of carbon concentration in retained austenite to volume ratio of retained austenite is a very important invention constituent element.
- a desired YR can be realized by simultaneously controlling the volume ratio of the retained austenite and the carbon concentration in the retained austenite. In order to obtain such an effect, the ratio of the carbon concentration in the retained austenite to the volume ratio of the retained austenite needs to be 0.05 or more.
- the ratio of the carbon concentration in the retained austenite to the volume ratio of the retained austenite is set to 0.05 or more and 0.40 or less. It is preferably 0.07 or more. It is preferably 0.38 or less. More preferably, it is 0.09 or more. It is preferably 0.36 or less.
- the method for measuring the ratio of the carbon concentration in the retained austenite to the volume ratio of the retained austenite is as follows.
- 0.1 mm was further polished by chemical polishing so that a 1/4 position of the plate thickness from the surface of the steel plate (a position corresponding to 1/4 of the plate thickness in the depth direction from the surface of the steel plate) was the observation surface.
- a (200) plane, a (220) plane, and a (311) plane of austenite and a (200) plane, a (211) plane of the ferrite (, 220) surface, and the volume ratio of austenite was calculated from the intensity ratio of the integrated reflection intensity from each surface of austenite to the integrated reflection intensity from each surface of ferrite, and this was taken as the volume ratio of retained austenite. .
- the carbon concentration in the retained austenite is calculated by first calculating the lattice constant of the retained austenite from the diffraction peak shift amount of the (220) plane of austenite by the formula (2), and the obtained lattice constant of the retained austenite is calculated by the formula (3).
- a 1.79021 ⁇ 2 / sin ⁇ (2)
- a 3.578 + 0.00095 [Mn] +0.022 [N] +0.0006 [Cr] +0.0031 [Mo] +0.0051 [Nb] +0.0039 [Ti] ++ 0.0056 [Al] +0.033 [ C] (3)
- a a lattice constant ( ⁇ ) of retained austenite
- ⁇ is a value (rad) obtained by dividing the diffraction peak angle of the (220) plane by 2
- [M] is a mass% of the element M in the retained austenite.
- the mass% of the element M other than C in the retained austenite is the mass% of the entire steel.
- Average crystal grain size of martensite and tempered martensite 5.3 ⁇ m or less
- the average grain size of martensite and tempered martensite is an extremely important invention constituent element.
- the toughness of the steel sheet can be improved. .
- the average crystal grain size of martensite and tempered martensite must be 5.3 ⁇ m or less.
- the lower limit of the average crystal grain size of martensite and tempered martensite is not particularly specified, but it is preferably 1.0 ⁇ m or more, and more preferably 2.0 ⁇ m or more in order to realize a desired YR. . Therefore, the average crystal grain sizes of martensite and tempered martensite are each set to 5.3 ⁇ m or less. It is preferably 1.0 ⁇ m or more.
- the thickness is preferably 5.0 ⁇ m or less. More preferably, it is 2.0 ⁇ m or more. More preferably, it is 4.9 ⁇ m or less.
- the method for measuring the average grain size of martensite and tempered martensite is as follows.
- this data is obtained by performing a clean-up process once on the original data by using the Grain Dilation method (Grain Tolerance Angle: 5, Minimum Grain Size: 2), and then setting CI (Confidence Index)> 0.05 as a threshold value. Set.
- the softening thickness of the surface layer is 10 ⁇ m or more.
- the surface layer softening thickness is 100 ⁇ m or less. Therefore, the surface softening thickness is preferably 10 ⁇ m or more and 100 ⁇ m or less. More preferably, it is 12 ⁇ m or more. More preferably, the thickness is 80 ⁇ m or less. More preferably, the thickness is 15 ⁇ m or more. More preferably, the thickness is 60 ⁇ m or less.
- the measuring method of the surface softened thickness is as follows.
- the steel structure according to the present invention in addition to the above-mentioned martensite (quenched martensite), tempered martensite (bainite), and retained austenite, ferrite, pearlite, cementite and other known carbides and other steel sheet structures are known.
- the area ratio is within the range of 3% or less, the effect of the present invention is not impaired even if it is included.
- the structure of the other steel sheet may be confirmed and determined by SEM observation, for example.
- the composition and steel structure of the high strength steel sheet of the present invention are as described above.
- the plate thickness of the high-strength steel plate is not particularly limited, but is usually 0.3 mm or more and 2.8 mm or less.
- the high-strength steel plate of the present invention may further have a plating layer on the steel plate surface.
- the type of plating layer is not particularly limited, and may be, for example, a hot dip layer or an electroplating layer. Further, the plating layer may be an alloyed plating layer.
- the plating layer is preferably a zinc plating layer.
- the galvanized layer may contain Al or Mg. Further, hot dip zinc-aluminum-magnesium alloy plating (Zn-Al-Mg plating layer) is also preferable. In this case, it is preferable that the Al content is 1% by mass or more and 22% by mass or less, the Mg content is 0.1% by mass or more and 10% by mass or less, and the balance is Zn.
- the Zn-Al-Mg plated layer in addition to Zn, Al, and Mg, one or more selected from Si, Ni, Ce, and La may be contained in a total amount of 1% by mass or less. Since the plating metal is not particularly limited, Al plating or the like may be used instead of Zn plating as described above.
- the composition of the plating layer is not particularly limited and may be a general one.
- a hot dip galvanized layer or an alloyed hot dip galvanized layer generally, Fe: 20% by mass or less, Al: 0.001% by mass or more and 1.0% by mass or less, and Pb,
- One or two or more selected from Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM are 0 mass% or more and 3.5 mass% in total.
- the composition is as follows, with the balance being Zn and inevitable impurities.
- a hot-dip galvanized layer having a coating adhesion amount of 20 to 80 g / m 2 on one side, and an alloyed hot-dip galvanized layer obtained by further alloying this.
- the galvanized layer is a hot dip galvanized layer
- the Fe content in the galvanized layer is less than 7% by mass, and in the case of the alloyed hot dip galvanized layer, the Fe content in the galvanized layer is 7 to 20 mass. %.
- the melting method of the steel material is not particularly limited, and any known melting method such as a converter or an electric furnace is suitable.
- the steel slab (slab) is preferably manufactured by a continuous casting method in order to prevent macrosegregation, but it is also possible to manufacture it by an ingot making method, a thin slab casting method, or the like.
- after manufacturing the steel slab in addition to the conventional method of once cooling to room temperature and then heating again, it is charged into the heating furnace as it is without cooling to room temperature, or a slight heat retention is performed. Energy-saving processes such as direct rolling and direct rolling, in which rolling immediately after rolling, can be applied without any problems.
- the slab heating temperature When the slab is heated, it is preferable to set the slab heating temperature to 1100 ° C. or higher from the viewpoint of melting the carbide and reducing the rolling load. Further, in order to prevent an increase in scale loss, it is preferable that the slab heating temperature is 1300 ° C or lower.
- the slab heating temperature is the temperature of the slab surface. Also, the slab is made into a sheet bar by rough rolling under normal conditions, but if the heating temperature is made low, from the viewpoint of preventing problems during hot rolling, the sheet is heated using a bar heater before finish rolling. It is preferred to heat the bar.
- Finish rolling the increase of rolling load, the rolling reduction in the unrecrystallized state of austenite is high, as a result of the development of an abnormal structure elongated in the rolling direction, it may reduce the workability of the annealed sheet, It is preferable to carry out at a finish rolling temperature not lower than the Ar 3 transformation point. Further, the coiling temperature after hot rolling is preferably 300 ° C. or higher and 700 ° C. or lower because there is a concern that the workability of the annealed plate may be deteriorated.
- rough rolling plates may be joined together during hot rolling and continuous finish rolling may be performed. Further, the rough rolled plate may be once wound. Further, in order to reduce the rolling load during hot rolling, part or all of finish rolling may be lubrication rolling. Performing the lubrication rolling is effective from the viewpoint of uniformizing the shape of the steel sheet and the material. The coefficient of friction during lubrication rolling is preferably in the range of 0.10 or more and 0.25 or less.
- the hot rolled steel sheet produced in this way is pickled. Since pickling can remove oxides on the surface of the steel sheet, it is important for ensuring good chemical conversion treatment and plating quality in the high-strength steel sheet of the final product. Further, the pickling may be performed once or may be divided into multiple times.
- cold rolling may be performed on the pickled sheet after hot rolling as it is, or it may be cooled after heat treatment. It may be subjected to hot rolling.
- the conditions for cold rolling are not particularly limited, but the rolling reduction in cold rolling is preferably 30% or more and 80% or less.
- the number of rolling passes and the rolling reduction of each pass are not particularly limited, and the effects of the present invention can be obtained.
- the annealing conditions are as follows.
- Average heating rate in the temperature range of 250 ° C. or higher and 700 ° C. or lower 10 ° C./s or higher
- the average heating rate in the temperature range of 250 ° C. or higher and 700 ° C. or lower is a very important invention constituent element.
- the average crystal grain size of martensite and tempered martensite can be controlled and desired toughness can be realized.
- the average heating rate in the temperature range of 250 ° C. or higher and 700 ° C. or lower needs to be 10 ° C./s or higher.
- the average heating rate in the temperature range from 250 ° C to 700 ° C is 10 ° C / s or more. It is preferably 12 ° C./s or more. It is preferably 50 ° C./s or less. More preferably, it is set to 14 ° C./s or more. More preferably, it is set to 40 ° C./s or less.
- Heating temperature 850 ° C. or more and 950 ° C. or less
- the heating temperature annealing temperature
- the annealing treatment is performed in the two-phase region of ferrite and austenite, and therefore a large amount of ferrite is contained after annealing, so that the desired ⁇ and It becomes difficult to realize YR.
- the heating temperature exceeds 950 ° C., the crystal grains of austenite during annealing become coarse, and the average crystal grain size of martensite and tempered martensite increases, so that the desired toughness cannot be realized. Therefore, the heating temperature is set to 850 ° C. or higher and 950 ° C. or lower.
- the temperature is preferably 860 ° C. or higher.
- the temperature is preferably 940 ° C or lower. More preferably, the temperature is 870 ° C. or higher. More preferably, the temperature is 930 ° C. or lower.
- the holding time at the above heating temperature is not particularly limited, but it is preferably 10 s or more and 600 s or less.
- the average cooling rate at a heating temperature of 400 ° C or lower is not particularly limited, but is preferably 5 ° C / s or higher and 30 ° C / s or lower.
- Oxygen concentration in heating temperature range 2 ppm or more and 30 ppm or less (suitable condition)
- decarburization proceeds via oxygen in the air, and a softened layer can be formed in the steel sheet surface layer portion, and as a result, the desired R / t can be obtained.
- the oxygen concentration in the heating temperature range is 2 ppm or more.
- the oxygen concentration in the heating temperature range be 30 ppm or less. Therefore, the oxygen concentration in the heating temperature range is preferably 2 ppm or more and 30 ppm or less.
- the temperature in the heating temperature range is based on the steel plate surface temperature. That is, when the steel plate surface temperature is in the above heating temperature range, the oxygen concentration is adjusted to the above range.
- Dew point in the heating temperature range -35 ° C or higher (preferred condition)
- the dew point in the heating temperature range is ⁇ 35 ° C. or higher.
- the upper limit of the dew point in the heating temperature range is not particularly specified, it is preferably 15 ° C or lower, and more preferably 5 ° C or lower in order to realize a desired TS. Therefore, the dew point in the heating temperature range is preferably -35 ° C or higher.
- the temperature is ⁇ 30 ° C. or higher. More preferably, it is set to 15 ° C. or lower. More preferably, the temperature is -25 ° C or higher. More preferably, the temperature is 5 ° C or lower.
- the temperature in the heating temperature range is based on the steel plate surface temperature. That is, when the steel plate surface temperature is in the above heating temperature range, the dew point is adjusted to the above range.
- Residence time in the temperature range of 50 ° C. or more and 400 ° C. or less 70 s or more and 700 s or less
- the residence time in the temperature range of 50 ° C. or more and 400 ° C. or less is a very important invention constituent element.
- the volume ratio of retained austenite and the carbon concentration in retained austenite can be controlled, and as a result, desired YR can be realized.
- the residence time in the temperature range of 50 ° C. or higher and 400 ° C. or lower needs to be 70 s or longer.
- the residence time in the temperature range of 50 ° C. or higher and 400 ° C. or lower exceeds 700 s, the carbon concentration in the retained austenite increases, and the hardness of martensite transformed from the retained austenite during punching greatly increases. Generation of voids at the time of spreading increases, and ⁇ decreases. Also, YR increases. Therefore, the residence time in the temperature range of 50 ° C. or higher and 400 ° C. or lower is 70 s or more and 700 s or less. It is preferably 75 s or more. It is preferably 500 s or less. More preferably, it is set to 80 s or more. More preferably, it is 400 s or less.
- Average cooling rate in the temperature range of 50 ° C. or higher and 250 ° C. or lower is a very important invention constituent element.
- the volume ratio of retained austenite and the carbon concentration in retained austenite can be controlled, and as a result, desired YR can be realized. it can.
- the average cooling rate in the temperature range of 50 ° C. or higher and 250 ° C. or lower needs to be 10.0 ° C./s or less.
- the lower limit of the average cooling rate in the temperature range of 50 ° C. or higher and 250 ° C. or lower is not particularly specified, but in order to achieve the desired ⁇ , it is preferably 0.5 ° C./s or higher, more preferably 1 0.0 ° C./s or more. Therefore, the average cooling rate in the temperature range of 50 ° C. or more and 250 ° C. or less is 10.0 ° C./s or less. It is preferably 0.5 ° C./s or more. It is preferably 7.0 ° C./s. More preferably, it is set to 1.0 ° C./s or more. More preferably, it is set to 5.0 ° C./s.
- Cooling below 50 ° C does not have to be specified in particular, and may be cooled to a desired temperature by any method.
- the desired temperature is preferably about room temperature.
- temper rolling may be applied to the above high strength steel plate. If the rolling reduction in skin pass rolling exceeds 1.5%, the yield stress of steel increases and YR increases, so it is preferable to set it to 1.5% or less.
- the lower limit of the rolling reduction in skin pass rolling is not particularly limited, but 0.1% or more is preferable from the viewpoint of productivity.
- the high-strength steel plate may be plated after annealing.
- a hot dip galvanizing treatment and a treatment of alloying after the hot dip galvanizing can be exemplified.
- the annealing and the galvanizing may be continuously performed in one line.
- the plating layer may be formed by electroplating such as Zn-Ni electroalloy plating, or hot-dip zinc-aluminum-magnesium alloy plating may be performed.
- the case of zinc plating has been mainly described, but the type of plating metal such as Zn plating and Al plating is not particularly limited.
- the high-strength steel sheet is immersed in a galvanizing bath at 440 ° C or higher and 500 ° C or lower to perform hot dip galvanizing, and then the amount of coating adhered is adjusted by gas wiping or the like. To do.
- a galvanizing bath having an Al content of 0.10 mass% or more and 0.23 mass% or less.
- the galvanizing alloying treatment is performed, the galvanizing alloying treatment is performed in a temperature range of 470 ° C. or more and 600 ° C. or less after the hot dip galvanizing.
- the temperature is lower than 470 ° C, the Zn-Fe alloying rate becomes excessively slow and the productivity is impaired.
- the alloying treatment is performed at a temperature higher than 600 ° C., untransformed austenite may be transformed into pearlite, and TS may decrease. Therefore, when the galvanizing alloying treatment is performed, it is preferable to perform the alloying treatment in a temperature range of 470 ° C. or higher and 600 ° C. or lower, and it is more preferable to perform the alloying treatment in a temperature range of 470 ° C. or higher and 560 ° C. or lower. preferable.
- electrogalvanizing treatment may be performed.
- the coating amount is preferably 20 to 80 g / m 2 per side (double-sided plating), and the galvannealed steel sheet (GA) has the Fe concentration in the plated layer of 7 to It is preferably 15% by mass.
- the reduction rate in skin pass rolling after plating is preferably in the range of 0.1% to 2.0%. If it is less than 0.1%, the effect is small and control is difficult, so this is the lower limit of the good range. Further, when it exceeds 2.0%, the productivity is remarkably lowered and YR is increased, so this is made the upper limit of the good range.
- the skin pass rolling may be performed online or offline. Further, the skin pass having a desired reduction rate may be performed at once, or may be performed several times.
- the conditions of the other manufacturing methods are not particularly limited, but from the viewpoint of productivity, a series of treatments such as the above-mentioned annealing, hot dip galvanizing, and galvanizing alloying treatment are CGL (Continuous Galvanizing) which is a hot dip galvanizing line. Line) is preferable. After hot dip galvanizing, wiping is possible to adjust the basis weight of plating. Conditions other than the above-mentioned conditions such as plating can be based on a conventional method of hot dip galvanizing.
- annealing treatment was performed under the conditions shown in Table 2 to obtain a high strength cold rolled steel sheet (CR). Furthermore, some high-strength cold-rolled steel sheets were subjected to plating treatment to obtain hot-dip galvanized steel sheets (GI), alloyed hot-dip galvanized steel sheets (GA), and electrogalvanized steel sheets (EG).
- GI hot-dip galvanized steel sheets
- GA alloyed hot-dip galvanized steel sheets
- EG electrogalvanized steel sheets
- the hot dip galvanizing bath in GI, a zinc bath containing Al: 0.14 to 0.19 mass% was used, and in GA, a zinc bath containing Al: 0.14 mass% was used, and the bath temperature was 470. °C was made.
- Coating weight the GI, a 45 ⁇ 72g / m 2 (two-sided plating) degree per side, also, the GA, and the degree per side 45 g / m 2 (two-sided plating).
- the Fe concentration in the plating layer was set to 9% by mass or more and 12% by mass or less.
- the Ni content in the plating layer was 9% by mass or more and 25% by mass or less.
- the tensile test was performed according to JIS Z 2241.
- a JIS No. 5 test piece was sampled from the obtained steel sheet in a direction perpendicular to the rolling direction of the steel sheet, and a tensile test was performed under the condition that the crosshead speed was 1.67 ⁇ 10 ⁇ 1 mm / s.
- TS were measured.
- TS of 1180 MPa or more was judged to be acceptable.
- the excellent dimensional accuracy of the parts was good when the yield ratio (YR), which is an index of the dimensional accuracy of the parts, was 65% or more and 85% or less. Note that YR was calculated by the calculation method described in the above formula (1).
- the hole expansion test was performed according to JIS Z 2256. After shearing to 100 mm x 100 mm from the obtained steel plate, after punching a hole having a diameter of 10 mm with a clearance of 12.5%, a die having an inner diameter of 75 mm was used to suppress wrinkle holding force of 9 tons (88.26 kN), A conical punch with an apex angle of 60 ° was pushed into the hole to measure the hole diameter at the crack initiation limit, and the limit hole expansion ratio: ⁇ (%) was calculated from the following formula, and the hole expandability was calculated from this limit hole expansion ratio value. Was evaluated.
- D f is a hole diameter (mm) when a crack is generated
- D 0 is an initial hole diameter (mm).
- the value of the hole expansion ratio ( ⁇ ) which is an index of stretch-flangeability, is 30% or more regardless of the strength of the steel sheet, the stretch-flangeability was judged to be good.
- the bending test was performed according to JIS Z 2248. A strip-shaped test piece having a width of 30 mm and a length of 100 mm was sampled from the obtained steel sheet such that the direction parallel to the rolling direction of the steel sheet was the axial direction of the bending test. After that, a 90 ° V bending test was performed under the condition that the pressing load was 100 kN and the pressing holding time was 5 seconds. In the present invention, the bendability is evaluated by the pass rate of the bending test, and the value R / t obtained by dividing the bending radius (R) by the plate thickness (t) is 5 or less.
- the bending radius is 7.0 mm
- the bending test of 5 samples is performed, and then the presence or absence of cracks at the ridge line portion of the bending apex is evaluated. Only when the rate was 100% was judged that the bendability was good.
- the presence or absence of cracks was evaluated by measuring the ridge line portion of the bending apex with a digital microscope (RH-2000: manufactured by Hylox Corporation) at a magnification of 40 times.
- the Charpy impact test was performed according to JIS Z 2242. From the obtained steel plate, the width was 10 mm, the length was 55 mm, and the notch depth was 2 mm at the center of the length so that the direction perpendicular to the rolling direction of the steel plate was the V-notching direction. The test piece provided with the V notch was sampled. Then, a Charpy impact test was performed in the test temperature range of -120 to + 120 ° C, a transition curve was obtained from the obtained brittle fracture surface ratio, and the temperature at which the brittle fracture surface ratio became 50% was determined as the brittle-ductile transition temperature. . In the present invention, the toughness was judged to be good when the brittle-ductile transition temperature obtained by the Charpy test was -40 ° C or lower.
- the area ratio of martensite and tempered martensite the ratio of the carbon concentration in the retained austenite to the volume ratio of the retained austenite, the average crystal grain size of martensite and tempered martensite, and the surface softening thickness. I asked. The remaining structure was also confirmed by structure observation.
- TS is 1180 MPa or more, and the dimensional accuracy, stretch flangeability, bendability and toughness of the parts are excellent.
- TS strength
- YR dimensional accuracy of components
- ⁇ stretch flangeability
- ⁇ bendability
- toughness one or more of strength (TS), dimensional accuracy of components (YR), stretch flangeability ( ⁇ ), bendability, and toughness are inferior.
Abstract
Description
YR=YS/TS・・・・(1)
また、伸びフランジ性に優れるとは、伸びフランジ性の指標である穴広げ率(λ)の値が30%以上を意味する。
また、曲げ性は曲げ試験の合格率で評価し、曲げ半径(R)を板厚(t)で除した値R/tが5以下となる最大のRにおいて、5サンプルの曲げ試験を実施し、次いで、曲げ頂点の稜線部における亀裂発生有無の評価を行い、5サンプルとも割れない場合、つまり、合格率100%の場合のみ、曲げ性に優れると判断した。
また、靱性に優れるとは、シャルピー衝撃試験より得られた脆性-延性遷移温度が-40℃以下とする。
(1)硬質相(マルテンサイトおよび焼戻しマルテンサイト)を主体とする組織とすることで、伸びフランジ性を30%以上に実現できる。
(2)残留オーステナイトの体積率に対する残留オーステナイト中の炭素濃度の比を0.05以上0.40以下とすることで、部品の寸法精度の指標であるYRを65%以上85%以下に実現できる。
(3)マルテンサイトおよび焼戻しマルテンサイトの平均結晶粒径を5.3μm以下とすることで、靱性の指標である脆性-延性遷移温度を-40℃以下に実現できる。
(4)さらに好ましくは、表層軟化厚みを10μm以上100μm以下とすることで、曲げ性を向上できる。
[1]質量%で、
C:0.09%以上0.37%以下、
Si:0.70%超2.00%以下、
Mn:2.60%以上3.60%以下、
P:0.001%以上0.100%以下、
S:0.0200%以下、
Al:0.010%以上1.000%以下および
N:0.0100%以下を含有し、残部がFeおよび不可避的不純物からなる成分組成であり、
炭素濃度が0.7×[%C]より大きく1.5×[%C]より小さいマルテンサイトが面積率で55%以上であり、
炭素濃度が0.7×[%C]以下である焼戻しマルテンサイトが面積率で5%以上40%以下であり、
残留オーステナイトの体積率に対する残留オーステナイト中の炭素濃度の比が0.05以上0.40以下であり、
前記マルテンサイトおよび前記焼戻しマルテンサイトの平均結晶粒径がそれぞれ5.3μm以下である鋼組織を有し、
引張強さが1180MPa以上である高強度鋼板。
なお、[%C]は、鋼中の成分元素Cの含有量(質量%)を示す。
[2]前記鋼組織は、さらに、表層軟化厚みが10μm以上100μm以下である[1]に記載の高強度鋼板。
[3]前記成分組成は、さらに、質量%で、
Ti:0.001%以上0.100%以下、
Nb:0.001%以上0.100%以下、
V:0.001%以上0.100%以下、
B:0.0001%以上0.0100%以下、
Mo:0.010%以上0.500%以下、
Cr:0.01%以上1.00%以下、
Cu:0.01%以上1.00%以下、
Ni:0.01%以上0.50%以下、
Sb:0.001%以上0.200%以下、
Sn:0.001%以上0.200%以下、
Ta:0.001%以上0.100%以下、
Ca:0.0001%以上0.0200%以下、
Mg:0.0001%以上0.0200%以下、
Zn:0.001%以上0.020%以下、
Co:0.001%以上0.020%以下、
Zr:0.001%以上0.020%以下、
REM:0.0001%以上0.0200%以下のうちから選ばれる少なくとも1種を含有する[1]または[2]に記載の高強度鋼板。
[4]さらに、鋼板表面にめっき層を有する[1]~[3]のいずれかに記載の高強度鋼板。
[5][1]~[3]のいずれかに記載の高強度鋼板の製造方法であって、熱間圧延、酸洗および冷間圧延を施し得られた冷延板を、
250℃以上700℃以下の温度域における平均加熱速度が10℃/s以上、加熱温度が850℃以上950℃以下の条件で加熱し、
次いで、50℃以上400℃以下の温度域における滞留時間が70s以上700s以下、かつ、50℃以上250℃以下の温度域における平均冷却速度が10.0℃/s以下となる条件で冷却する焼鈍を行う高強度鋼板の製造方法。
[6]前記加熱温度域での酸素濃度が2ppm以上30ppm以下、かつ、露点が-35℃以上である[5]に記載の高強度鋼板の製造方法。
[7]前記焼鈍の後に、さらにめっき処理を施す[5]または[6]に記載の高強度鋼板の製造方法。
Cは、鋼の重要な基本成分の1つであり、特に本発明では、マルテンサイト、焼戻しマルテンサイトおよび残留オーステナイトの分率や、残留オーステナイト中の炭素濃度に影響する重要な元素である。Cの含有量が0.09%未満では、マルテンサイトの分率が減少し、1180MPa以上のTSを実現することが困難になる。一方、Cの含有量が0.37%を超えると、焼戻しマルテンサイトの分率が減少し、伸びフランジ性の指標である穴広げ率(λ)を30%以上に実現することが困難になる。したがって、Cの含有量は、0.09%以上0.37%以下とする。好ましくは0.10%以上とする。好ましくは0.36%以下とする。より好ましくは0.11%以上とする。より好ましくは0.35%以下とする。
Siは、連続焼鈍中の炭化物生成を抑制し、残留オーステナイトの生成を促進することから、残留オーステナイトの分率や、残留オーステナイト中の炭素濃度に影響する元素である。Siの含有量が0.70%以下では、残留オーステナイトを生成することができず、YRを所望の範囲内に制御することができない。一方、Siの含有量が2.00%を超えると、残留オーステナイト中の炭素濃度が過度に増加し、打抜き時に残留オーステナイトから変態するマルテンサイトの硬度が大きく上昇するため、打抜きおよび穴広げ時のボイドの生成が増加してしまい、λが減少する。したがって、Siの含有量は、0.70%超2.00%以下とする。好ましくは0.80%以上とする。好ましくは1.80%以下とする。より好ましくは0.90%以上とする。より好ましくは1.70%以下とする。
Mnは、鋼の重要な基本成分の1つであり、特に本発明では、マルテンサイトおよび焼戻しマルテンサイトの分率に影響する重要な元素である。Mnの含有量が2.60%未満では、マルテンサイトの分率が減少し、1180MPa以上のTSを実現することが困難になる。一方、Mnの含有量が3.60%を超えると、焼戻しマルテンサイトの分率が減少し、λを30%以上に実現することが困難になる。したがって、Mnの含有量は、2.60%以上3.60%以下とする。好ましくは2.65%以上とする。好ましくは3.50%以下とする。より好ましくは2.70%以上とする。より好ましくは3.40%以下とする。
Pは、固溶強化の作用を有し、鋼板の強度を上昇させることができる元素である。こうした効果を得るためには、Pの含有量を0.001%以上にする必要がある。一方、Pの含有量が0.100%を超えると、旧オーステナイト粒界に偏析して粒界を脆化させるため、靱性が低下してしまい、所望の脆性-延性遷移温度を実現することができない。また、Pは鋼板の極限変形能を低下させることから、λが低下する。したがって、Pの含有量は、0.001%以上0.100%以下とする。好ましくは0.002%以上とする。好ましくは0.070%以下とする。より好ましくは0.003%以上とする。より好ましくは0.050%以下とする。
Sは、硫化物として存在し、鋼の極限変形能を低下させることから、λが低下する。また、曲げ性も低下する。そのため、Sの含有量は0.0200%以下にする必要がある。なお、Sの含有量の下限は特に規定しないが、生産技術上の制約から、Sの含有量は0.0001%以上とすることが好ましい。したがって、Sの含有量は0.0200%以下とする。好ましくは0.0001%以上とする。好ましくは0.0050%以下とする。
Alは、連続焼鈍中の炭化物生成を抑制し、残留オーステナイトの生成を促進することから、残留オーステナイトの分率や、残留オーステナイト中の炭素濃度に影響する元素である。こうした効果を得るためには、Alの含有量を0.010%以上にする必要がある。一方、Al含有量が1.000%を超えると、フェライトが生成し、YRを所望の範囲内に制御することができない。したがって、Alの含有量は、0.010%以上1.000%以下とする。好ましくは0.015%以上とする。好ましくは0.500%以下とする。より好ましくは0.020%以上とする。より好ましくは0.100%以下とする。
Nは、窒化物として存在し、鋼板の極限変形能を低下させることから、λが低下する。また、曲げ性も低下する。そのため、Nの含有量は0.0100%以下にする必要がある。なお、Nの含有量の下限は特に規定しないが、生産技術上の制約から、Nの含有量は0.0005%以上とすることが好ましい。したがって、Nの含有量は0.0100%以下とする。好ましくは0.0005%以上とする。好ましくは0.0050%以下とする。
炭素濃度が0.7×[%C]より大きく1.5×[%C]より小さいマルテンサイトを主相とすることで、1180MPa以上のTSを実現することが可能となる。こうした効果を得るためには、炭素濃度が0.7×[%C]より大きく1.5×[%C]より小さいマルテンサイトの面積率を55%以上にする必要がある。なお、炭素濃度が0.7×[%C]より大きく1.5×[%C]より小さいマルテンサイトの面積率の上限は特に規定しないが、所望のλおよびYRを実現するためには95%以下であることが好ましく、より好ましくは90%以下とする。したがって、炭素濃度が0.7×[%C]より大きく1.5×[%C]より小さいマルテンサイトの面積率は55%以上とする。好ましくは56%以上とする。好ましくは95%以下とする。より好ましくは57%以上とする。より好ましくは90%以下とする。なお、炭素濃度が0.7×[%C]より大きく1.5×[%C]より小さいマルテンサイトは焼入れマルテンサイトと定義することもできる。また、[%C]は、鋼中の成分元素Cの含有量(質量%)を示す。
炭素濃度が0.7×[%C]以下である焼戻しマルテンサイトを、炭素濃度が0.7×[%C]より大きく1.5×[%C]より小さいマルテンサイトに隣接させることで、所望のλおよびYRを実現することができる。こうした効果を得るためには、炭素濃度が0.7×[%C]以下である焼戻しマルテンサイトの面積率を5%以上にする必要がある。一方、炭素濃度が0.7×[%C]以下である焼戻しマルテンサイトが40%を超えると、炭素濃度が0.7×[%C]より大きく1.5×[%C]より小さいマルテンサイトの面積率が減少してしまい、1180MPa以上のTSを実現することが困難になる。したがって、炭素濃度が0.7×[%C]以下である焼戻しマルテンサイトの面積率は5%以上40%以下とする。好ましくは6%以上とする。好ましくは39%以上とする。より好ましくは7%以上とする。好ましくは38%以上とする。なお、炭素濃度が0.7×[%C]以下である焼戻しマルテンサイトはベイナイトと定義することもできる。また、[%C]は、鋼中の成分元素Cの含有量(質量%)を示す。
本発明において、残留オーステナイトの体積率に対する残留オーステナイト中の炭素濃度の比(残留オーステナイト中の炭素濃度[質量%]/残留オーステナイトの体積率[体積%])は、極めて重要な発明構成要件である。残留オーステナイトの体積率および残留オーステナイト中の炭素濃度を同時に制御することで、所望のYRを実現することができる。こうした効果を得るためには、残留オーステナイトの体積率に対する残留オーステナイト中の炭素濃度の比を0.05以上にする必要がある。一方、残留オーステナイトの体積率に対する残留オーステナイト中の炭素濃度の比が0.40を超えると、打抜き時に残留オーステナイトから変態するマルテンサイトの硬度が大きく上昇するため、打抜きおよび穴広げ時のボイドの生成が増加してしまい、λが減少する。また、YRが増加する。したがって、残留オーステナイトの体積率に対する残留オーステナイト中の炭素濃度の比は0.05以上0.40以下とする。好ましくは0.07以上とする。好ましくは0.38以下とする。より好ましくは0.09以上とする。好ましくは0.36以下とする。
a=1.79021√2/sinθ ・・・(2)
a=3.578+0.00095[Mn]+0.022[N]+0.0006[Cr]+0.0031[Mo]+0.0051[Nb]+0.0039[Ti]++0.0056[Al]+0.033[C] ・・・(3)
ここで、aは残留オーステナイトの格子定数(Å)、θは(220)面の回折ピーク角度を2で除した値(rad)、[M]は残留オーステナイト中の元素Mの質量%である。本発明では残留オーステナイト中のC以外の元素Mの質量%は、鋼全体に占める質量%とした。
本発明において、マルテンサイトおよび焼戻しマルテンサイトの平均結晶粒径は、極めて重要な発明構成要件である。所望の材質を得るためには、マルテンサイトおよび焼戻しマルテンサイトの組織の微細化が重要である。マルテンサイトおよび焼戻しマルテンサイトはともにオーステナイトから生成するため、マルテンサイトと焼戻しマルテンサイトの結晶粒径はともにオーステナイトの結晶粒径の影響を受ける。そのため、マルテンサイトと焼戻しマルテンサイトをそれぞれ区別し、個々の粒径を制御する必要はなく、マルテンサイトおよび焼戻しマルテンサイトの平均結晶粒径を減少させることで、鋼板の靱性を向上することができる。こうした効果を得るためには、マルテンサイトおよび焼戻しマルテンサイトの平均結晶粒径をそれぞれ5.3μm以下にする必要がある。なお、マルテンサイトおよび焼戻しマルテンサイトの平均結晶粒径の下限はそれぞれ特に規定しないが、所望のYRを実現するためには1.0μm以上であることが好ましく、より好ましくは2.0μm以上とする。したがって、マルテンサイトおよび焼戻しマルテンサイトの平均結晶粒径をそれぞれ5.3μm以下とする。好ましくは1.0μm以上とする。好ましくは5.0μm以下とする。より好ましくは2.0μm以上とする。より好ましくは4.9μm以下とする。
板厚1/4位置と比較して、鋼板の表層部を軟化させることで、所望の曲げ性を実現することができる。こうした効果を得るためには、表層軟化厚みを10μm以上にすることが好ましい。一方、所望のTSを実現するためには、表層軟化厚みが100μm以下であることが好ましい。したがって、表層軟化厚みは10μm以上100μm以下とすることが好ましい。より好ましくは12μm以上とする。より好ましくは80μm以下とする。さらに好ましくは15μm以上とする。さらに好ましくは60μm以下とする。
本発明において、250℃以上700℃以下の温度域における平均加熱速度は、極めて重要な発明構成要件である。250℃以上700℃以下の温度域における平均加熱速度を上昇することで、マルテンサイトおよび焼戻しマルテンサイトの平均結晶粒径を制御し、所望の靱性を実現することができる。こうした効果を得るためには、250℃以上700℃以下の温度域における平均加熱速度を10℃/s以上にする必要がある。なお、250℃以上700℃以下の温度域における平均加熱速度の上限は特に規定しないが、所望のYRを実現するためには50℃/s以下であることが好ましく、より好ましくは40℃/s以下とする。したがって、250℃以上700℃以下の温度域における平均加熱速度は10℃/s以上とする。好ましくは12℃/s以上とする。好ましくは50℃/s以下とする。より好ましくは14℃/s以上とする。より好ましくは40℃/s以下とする。
加熱温度(焼鈍温度)が850℃未満では、フェライトとオーステナイトの二相域での焼鈍処理になるため、焼鈍後に多量のフェライトを含有するため、所望のλおよびYRを実現することが困難になる。一方、加熱温度が950℃を超えると、焼鈍中のオーステナイトの結晶粒が粗大化してしまい、マルテンサイトおよび焼戻しマルテンサイトの平均結晶粒径が増大するため、所望の靱性を実現することができない。したがって、加熱温度は850℃以上950℃以下とする。好ましくは860℃以上とする。好ましくは940℃以下とする。より好ましくは870℃以上とする。より好ましくは930℃以下とする。
焼鈍時、加熱温度域における酸素濃度を上昇させることで、空気中の酸素を介して脱炭が進行し、鋼板表層部に軟化層が形成することができ、その結果、所望のR/tを実現することができる。こうした効果を得るためには、加熱温度域における酸素濃度を2ppm以上にすることが好ましい。一方、所望のTSを実現するためには、加熱温度域における酸素濃度を30ppm以下とすることが好ましい。したがって、加熱温度域における酸素濃度は2ppm以上30ppm以下とすることが好ましい。より好ましくは4ppm以上とする。より好ましくは28ppm以下とする。さらに好ましくは5ppm以上とする。さらに好ましくは25ppm以下とする。なお、上記加熱温度域の温度は鋼板表面温度を基準とする。即ち、鋼板表面温度が上記加熱温度域にある場合に、酸素濃度を上記範囲に調整する。
焼鈍時、加熱温度域における露点を上昇させることで、空気中の水分を介して脱炭が進行し、鋼板表層部に軟化層が形成することができ、その結果、所望のR/tを実現することができる。こうした効果を得るためには、加熱温度域における露点を-35℃以上にすることが好ましい。なお、加熱温度域における露点の上限は特に規定しないが、所望のTSを実現するためには15℃以下であることが好ましく、より好ましくは5℃以下とする。したがって、上記加熱温度域における露点は-35℃以上とすることが好ましい。より好ましくは-30℃以上とする。より好ましくは15℃以下とする。さらに好ましくは-25℃以上とする。さらに好ましくは5℃以下とする。なお、上記加熱温度域の温度は鋼板表面温度を基準とする。即ち、鋼板表面温度が上記加熱温度域にある場合に、露点を上記範囲に調整する。
本発明において、50℃以上400℃以下の温度域における滞留時間は、極めて重要な発明構成要件である。50℃以上400℃以下の温度域における滞留時間を適正に制御することで、残留オーステナイトの体積率および残留オーステナイト中の炭素濃度を制御することでき、その結果、所望のYRを実現することができる。こうした効果を得るためには、50℃以上400℃以下の温度域における滞留時間を70s以上にする必要がある。一方、50℃以上400℃以下の温度域における滞留時間が700sを超えると、残留オーステナイト中の炭素濃度が増加し、打抜き時に残留オーステナイトから変態するマルテンサイトの硬度が大きく上昇するため、打抜きおよび穴広げ時のボイドの生成が増加してしまい、λが減少する。また、YRが増加する。したがって、50℃以上400℃以下の温度域における滞留時間は70s以上700s以下とする。好ましくは75s以上とする。好ましくは500s以下とする。より好ましくは80s以上とする。より好ましくは400s以下とする。
本発明において、50℃以上250℃以下の温度域における平均冷却速度は、極めて重要な発明構成要件である。50℃以上250℃以下の温度域における平均冷却速度を適正に制御することで、残留オーステナイトの体積率および残留オーステナイト中の炭素濃度を制御することでき、その結果、所望のYRを実現することができる。こうした効果を得るためには、50℃以上250℃以下の温度域における平均冷却速度を10.0℃/s以下にする必要がある。なお、50℃以上250℃以下の温度域における平均冷却速度の下限は特に規定しないが、所望のλを実現するためには、0.5℃/s以上であることが好ましく、より好ましくは1.0℃/s以上とする。したがって、50℃以上250℃以下の温度域における平均冷却速度は10.0℃/s以下とする。好ましくは0.5℃/s以上とする。好ましくは7.0℃/sとする。より好ましくは1.0℃/s以上とする。より好ましくは5.0℃/sとする。
引張試験は、JIS Z 2241に準拠して行った。得られた鋼板より、鋼板の圧延方向に対して直角方向となるようにJIS5号試験片を採取し、クロスヘッド速度が1.67×10-1mm/sの条件で引張試験を行い、YSおよびTSを測定した。なお、本発明では、TSで1180MPa以上を合格と判断した。また、部品の寸法精度に優れるとは、部品の寸法精度の指標である降伏比(YR)が65%以上85%以下の場合を良好と判断した。なお、YRは上述の式(1)に記載の計算方法で算出した。
穴広げ試験は、JIS Z 2256に準拠して行った。得られた鋼板より、100mm×100mmに剪断後、クリアランス12.5%で直径10mmの穴を打ち抜いた後、内径75mmのダイスを用いてしわ押さえ力9ton(88.26kN)で抑えた状態で、頂角60°の円錐ポンチを穴に押し込んで亀裂発生限界における穴直径を測定し、下記の式から、限界穴広げ率:λ(%)を求め、この限界穴広げ率の値から穴広げ性を評価した。
限界穴広げ率:λ(%)={(Df-D0)/D0}×100
ただし、Dfは亀裂発生時の穴径(mm)、D0は初期穴径(mm)である。
なお、本発明では、伸びフランジ性の指標である穴広げ率(λ)の値が鋼板の強度に関係なく30%以上の場合を、伸びフランジ性が良好と判断した。
曲げ試験は、JIS Z 2248に準拠して行った。得られた鋼板より、鋼板の圧延方向に対して平行方向が曲げ試験の軸方向となるように、幅が30mm、長さが100mmの短冊状の試験片を採取した。その後、押込み荷重が100kN、押付け保持時間が5秒とする条件で、90°V曲げ試験を行った。なお、本発明では、曲げ性は曲げ試験の合格率で評価し、曲げ半径(R)を板厚(t)で除した値R/tが5以下となる最大のR(例えば、板厚が1.2mmの場合、曲げ半径は7.0mm)において、5サンプルの曲げ試験を実施し、次いで、曲げ頂点の稜線部における亀裂発生有無の評価を行い、5サンプルとも割れない場合、つまり、合格率100%の場合のみ、曲げ性が良好と判断した。ここで、亀裂発生有無は、曲げ頂点の稜線部をデジタルマイクロスコープ(RH-2000:株式会社ハイロックス製)を用いて、40倍の倍率で測定することにより評価した。
シャルピー衝撃試験は、JIS Z 2242に準拠して行った。得られた鋼板より、鋼板の圧延方向に対して直角方向がVノッチ付与方向となるように、幅が10mm、長さが55mm、長さの中央部にノッチ深さが2mmとなるよう90°のVノッチを付与した試験片を採取した。その後、-120~+120℃の試験温度域でシャルピー衝撃試験を行い、得られた脆性破面率より遷移曲線を求め、脆性破面率が50%となる温度を脆性-延性遷移温度と決定した。なお、本発明では、シャルピー試験より得られた脆性-延性遷移温度が-40℃以下の場合を、靱性が良好と判断した。
Claims (7)
- 質量%で、
C:0.09%以上0.37%以下、
Si:0.70%超2.00%以下、
Mn:2.60%以上3.60%以下、
P:0.001%以上0.100%以下、
S:0.0200%以下、
Al:0.010%以上1.000%以下および
N:0.0100%以下を含有し、残部がFeおよび不可避的不純物からなる成分組成であり、
炭素濃度が0.7×[%C]より大きく1.5×[%C]より小さいマルテンサイトが面積率で55%以上であり、
炭素濃度が0.7×[%C]以下である焼戻しマルテンサイトが面積率で5%以上40%以下であり、
残留オーステナイトの体積率に対する残留オーステナイト中の炭素濃度の比が0.05以上0.40以下であり、
前記マルテンサイトおよび前記焼戻しマルテンサイトの平均結晶粒径がそれぞれ5.3μm以下である鋼組織を有し、
引張強さが1180MPa以上である高強度鋼板。
なお、[%C]は、鋼中の成分元素Cの含有量(質量%)を示す。 - 前記鋼組織は、さらに、表層軟化厚みが10μm以上100μm以下である請求項1に記載の高強度鋼板。
- 前記成分組成は、さらに、質量%で、
Ti:0.001%以上0.100%以下、
Nb:0.001%以上0.100%以下、
V:0.001%以上0.100%以下、
B:0.0001%以上0.0100%以下、
Mo:0.010%以上0.500%以下、
Cr:0.01%以上1.00%以下、
Cu:0.01%以上1.00%以下、
Ni:0.01%以上0.50%以下、
Sb:0.001%以上0.200%以下、
Sn:0.001%以上0.200%以下、
Ta:0.001%以上0.100%以下、
Ca:0.0001%以上0.0200%以下、
Mg:0.0001%以上0.0200%以下、
Zn:0.001%以上0.020%以下、
Co:0.001%以上0.020%以下、
Zr:0.001%以上0.020%以下、
REM:0.0001%以上0.0200%以下のうちから選ばれる少なくとも1種を含有する請求項1または2に記載の高強度鋼板。 - さらに、鋼板表面にめっき層を有する請求項1~3のいずれか1項に記載の高強度鋼板。
- 請求項1~3のいずれか1項に記載の高強度鋼板の製造方法であって、熱間圧延、酸洗および冷間圧延を施し得られた冷延板を、
250℃以上700℃以下の温度域における平均加熱速度が10℃/s以上、加熱温度が850℃以上950℃以下の条件で加熱し、
次いで、50℃以上400℃以下の温度域における滞留時間が70s以上700s以下、かつ、50℃以上250℃以下の温度域における平均冷却速度が10.0℃/s以下となる条件で冷却する焼鈍を行う高強度鋼板の製造方法。 - 前記加熱温度域での酸素濃度が2ppm以上30ppm以下、かつ、露点が-35℃以上である請求項5に記載の高強度鋼板の製造方法。
- 前記焼鈍の後に、さらにめっき処理を施す請求項5または6に記載の高強度鋼板の製造方法。
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- 2019-08-20 WO PCT/JP2019/032513 patent/WO2020075394A1/ja active Application Filing
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Also Published As
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CN112823217A (zh) | 2021-05-18 |
KR102513347B1 (ko) | 2023-03-22 |
KR20210053324A (ko) | 2021-05-11 |
JP6747612B1 (ja) | 2020-08-26 |
CN112823217B (zh) | 2022-05-17 |
US20210381075A1 (en) | 2021-12-09 |
JPWO2020075394A1 (ja) | 2021-02-15 |
MX2021004073A (es) | 2021-06-04 |
EP3822382A1 (en) | 2021-05-19 |
US11939642B2 (en) | 2024-03-26 |
EP3822382A4 (en) | 2021-09-15 |
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