Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

• the present invention relates to steel plates, parts containing the same, and methods for manufacturing steel plates.
• Patent Document 1 discloses a steel sheet having a predetermined chemical composition, wherein the microstructure in the range from a position of 1/8 of the plate thickness from the surface in the plate thickness direction to a position of 3/8 of the plate thickness from the surface in the plate thickness direction contains, by volume fraction, 10 to 75% ferrite, 20 to 90% martensite, 0 to 5% retained austenite, 0 to 5% total of bainite and bainitic ferrite, and 0 to 5% pearlite, wherein the proportion of unrecrystallized ferrite in the ferrite is 0 to 25%, cementite contained in the martensite satisfies a predetermined formula, the density of transition carbides contained in the martensite is 1.0 x 10 particles/m or more , the density of coarse inclusions having a circle equivalent diameter of 10 ⁇ m or more is 0.50 particles/mm or less, and the maximum Vickers hardness Hv in a plane parallel to the surface at a position of 1/4 of the plate thickness from
• Patent Document 1 teaches that the above configuration can provide a steel sheet that is excellent in formability, strength, and impact resistance.
• Patent Document 2 describes a dual-phase steel sheet having a predetermined chemical composition, in which the main phase of the microstructure at a thickness position of one-quarter of the plate thickness is polygonal ferrite precipitation-strengthened by Ti carbide, and the secondary phase is a dual-phase structure consisting of a plurality of dispersed low-temperature transformation products with an area fraction (fsd (%)) of 1-10%, the average crystal diameter of the low-temperature transformation products being 3-15 ⁇ m, and the average nearest neighbor distance between each low-temperature transformation product being 10-20 ⁇ m.
• fsd (%) area fraction
• Patent Document 2 also teaches that the above configuration makes it possible to obtain a high-strength dual-phase steel sheet having a tensile strength of 540 MPa or more, excellent uniform elongation, excellent burring workability, and excellent notch fatigue properties, as well as excellent surface properties.
• Patent Document 3 describes a high-strength steel sheet having a predetermined chemical composition, in which the degree of Mn segregation in a region within 100 ⁇ m from the surface in the plate thickness direction is 1.5 or less, the number of oxide-based inclusions having a particle major axis of 5 ⁇ m or more per 100 mm2 on a surface parallel to the plate surface of the steel sheet in a region within 100 ⁇ m from the surface in the plate thickness direction is 1,000 or less, and of the total number of oxide-based inclusions having a particle major axis of 5 ⁇ m or more, the number ratio of those having a composition of 50 mass% or more of alumina, 20 mass% or less of silica, and 40 mass% or less of calcia, the metallographic structure containing, by volume, 25 to 100% of the total of a martensite phase and a bainite phase, less than 75% (including 0%) of a ferrite phase, and less than 15% (including 0%) of an austenite phase, and the ten
• Patent Document 3 teaches that a high-strength steel sheet excellent in bendability (bending workability) can be obtained by reducing the number of inclusions in the surface layer of the steel sheet (a region within 100 ⁇ m from the surface of the steel sheet), controlling the composition of the inclusions within an appropriate range, and reducing the degree of Mn segregation in the surface layer of the steel sheet.
• Patent document 4 describes a high-strength hot-rolled steel sheet characterized by having a microstructure in which the total volume fraction of the ferrite phase and bainite phase in the entire structure is 95% or more, the volume fraction of the ferrite phase in the entire structure is 50 to 90%, precipitates of less than 20 nm in size containing 650 to 1100 ppm of Ti are precipitated in the ferrite phase, and the ⁇ Hv of the bainite phase (the difference between the maximum and minimum Vickers hardness values of the bainite phase measured at 1/4 of the plate thickness position in the plate thickness cross section along the rolling direction) is 150 or less.
• Patent Document 2 teaches that if a microstructure is created that is primarily composed of ferrite and bainite phases, with precipitates of less than 20 nm in size containing 650 to 1100 ppm of Ti in the ferrite phase, and the ⁇ Hv of the bainite phase is set to 150 or less, a TS of 780 MPa or more can be ensured, achieving both excellent stretch flangeability (hole expandability) and impact resistance.
• Patent Document 5 discloses a steel sheet having a predetermined chemical composition, in a cross section perpendicular to the rolling direction of the steel sheet, where W and t are the width and thickness of the steel sheet, the metallographic structure at a position of 1 ⁇ 4W or 3 ⁇ 4W from the end face of the steel sheet and at a position of 1 ⁇ 4t or 3 ⁇ 4t from the surface of the steel sheet is, in area %, ferrite: 5 to 70%, bainite: 30 to 95%, retained austenite: 2% or less, martensite: 2% or less, and pearlite: 1% or less, and the total of ferrite and bainite: 95% or more, the ferrite has precipitates containing Ti within the grains, and the number density of the precipitates containing Ti is 1.0 ⁇ 10 16 to 50.0 ⁇ 10 16 particles/cm 3 , the hot-rolled steel sheet contains TiN precipitates, the TiN precipitates have an average equivalent circle diameter of 1.0 to 10.0 ⁇ m, the average value of the shortest distance
• Patent Document 5 teaches that the above configuration makes it possible to obtain a hot-rolled steel sheet with balanced properties, such as a tensile strength (TS) of 780 MPa or more, a product of uniform elongation u-EL and tensile strength TS (TS ⁇ u-EL) of 7000 MPa % or more, and a product of hole expansion ratio ⁇ and tensile strength TS (TS ⁇ ⁇ ) of 50000 MPa % or more.
• TS tensile strength
• Patent Document 6 discloses a steel sheet having a predetermined chemical composition, in a cross section perpendicular to the rolling direction of the steel sheet, where W and t are the width and thickness of the steel sheet, the metallographic structure at a position of 1 ⁇ 4W or 3 ⁇ 4W from the end face of the steel sheet and at a position of 1 ⁇ 4t or 3 ⁇ 4t from the surface of the steel sheet is, in area %, ferrite: 5 to 70%, bainite: 30 to 95%, retained austenite: 2% or less, martensite: 2% or less, and pearlite: 1% or less, and the total of ferrite and bainite: 95% or more, the ferrite has precipitates containing Ti within its grains, and the number density of the precipitates containing Ti is 1.0 ⁇ 10 16 to 50.0 ⁇ 10 16 particles/cm 3 , the hot-rolled steel sheet contains TiN precipitates, the TiN precipitates have an average equivalent circle diameter of 1.0 to 10.0 ⁇ m, the average value of the shortest distance
• Patent Document 6 teaches that the above configuration makes it possible to obtain a hot-rolled steel sheet having a tensile strength (TS) of 780 MPa or more, a product of uniform elongation u-EL and tensile strength TS (TS ⁇ u-EL) of 7000 MPa % or more, and a product of hole expansion ratio ⁇ and tensile strength TS (TS ⁇ ⁇ ) of 50000 MPa % or more, and having well-balanced properties.
• TS tensile strength
• the present invention therefore aims to provide a steel plate that has high strength, high hole expandability, and high yield ratio, as well as a part that includes the steel plate and a method for manufacturing the steel plate.
• the inventors conducted research focusing on the metallographic structure of steel sheet, particularly hot-rolled steel sheet. Specifically, the inventors first discovered that by configuring the metallographic structure of a steel sheet having a predetermined chemical composition to primarily contain ferrite, bainite, and martensite, it is possible to improve the hole expandability and yield ratio while maintaining a relatively high level of strength of the steel sheet.
• the inventors discovered that by having TiC precipitates with an appropriate diameter present in the ferrite at a predetermined number density, the ferrite can be precipitation strengthened, thereby further increasing the strength of the steel sheet while reducing the difference in hardness between ferrite and bainite, thereby increasing hole expandability and yield ratio, and further reducing the variation in hardness between ferrite and bainite, thereby more significantly improving hole expandability, and thus completed the present invention.
• the present invention which has achieved the above object, is as follows.
• the chemical composition is, in mass%, Nb: 0.001 to 0.050%, V: 0.001-1.000%, Cr: 0.001-2.00%, Ni: 0.001 to 2.00%, Cu: 0.001 to 2.00%, Mo: 0.001 to 1.000%, B: 0.0001 to 0.0100%, Sn: 0.001 to 1.000%, Sb: 0.001 to 1.000%, Ca: 0.0001-0.0100%, Mg: 0.0001 to 0.0100%, Hf: 0.0001-0.0100%, Bi: 0.001-0.010%, REM: 0.0001-0.0100%, As: 0.001 to 0.010%, Zr: 0.001 to 0.010%, Co: 0.001 to 2.000%, Zn: 0.001 to 0.010%, and W: 0.001 to 1.000%
• the steel sheet according to (1) above characterized in that it contains at least one of the following: (3)
• a hot rolling process comprising heating a slab having the chemical composition described in (1) or (2) above and then finish rolling it, and satisfying the following conditions (a) to (e): (a) The heating temperature of the slab is 1200 to 1300°C; (b) the holding time in the temperature range of 1200 to 1300°C is 1000 to 4000 seconds; (c) The finish rolling is performed using a tandem rolling mill consisting of five or more rolling stands, and the total reduction rate in the rolling passes in the front stages other than the rear three stages is 60 to 90%; (d) the total reduction rate in the latter three rolling passes is more than 50%, and (e) the finishing temperature of finish rolling is 900 to 1000°C.
• a method for producing a steel plate comprising: an intermediate air-cooling step of primarily cooling the finish-rolled steel plate to an intermediate air-cooling temperature of 705 to 750°C at an average cooling rate of 50 to 200°C/sec, and then intermediate air-cooling for 3 to 10 seconds; and a cooling step of secondarily cooling the intermediate air-cooled steel plate at an average cooling rate of 50 to 200°C/sec, and then coiling it at a coiling temperature of 20 to 300°C.
• the present invention makes it possible to provide a steel plate having high strength, high hole expandability, and high yield ratio, as well as a part including the same and a method for manufacturing the steel plate.
• the steel sheet according to the embodiment of the present invention has a chemical composition in mass%: C: 0.03 to 0.10%, Si: 0.010-0.100%, Mn: 0.50-3.00%, Ti: 0.05-0.20%, Al: 0.20-0.40%, P: 0.100% or less, S: 0.0100% or less, N: 0.010% or less, O: 0.010% or less, Nb: 0 to 0.050%, V: 0-1.000%, Cr: 0-2.00%, Ni: 0-2.00%, Cu: 0-2.00%, Mo: 0-1.000%, B: 0 to 0.0100%, Sn: 0-1.000%, Sb: 0 to 1.000%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Hf: 0-0.0100%, Bi: 0 to 0.010%, REM: 0-0.0100%, As: 0 to 0.010%, Zr: 0 to 0.010%, Co: 0-2.000
• steel plate As the strength of steel plate increases, its workability, such as hole expandability, generally declines.
• steel plate is required that maintains high strength, for example, a tensile strength of 780 MPa or higher, which enables weight reduction, while also exhibiting excellent hole expandability.
• a tensile strength of 780 MPa or higher which enables weight reduction, while also exhibiting excellent hole expandability.
• the yield ratio which is the ratio of yield strength to tensile strength. Therefore, there is a high demand for materials that maintain sufficient strength while improving hole expandability and, from the perspective of crashworthiness, etc., of steel plate.
• the inventors conducted research, focusing particularly on the metallurgical structure of the steel plate. To explain in more detail, the inventors first discovered that by configuring the metal structure of a steel plate having a specified chemical composition to primarily contain ferrite, bainite, and martensite, and more specifically by configuring it to contain, by area percentage, 40-80% ferrite, 15-55% bainite, and 5-20% martensite, it is possible to increase the hole expandability and yield ratio while maintaining a relatively high level of strength for the steel plate.
• the inventors have investigated further improvements in hole expandability and realization of a high yield ratio from the perspective of reducing the difference in hardness between each phase in a metal structure containing such a three-phase structure.
• precipitation strengthening of ferrite, the softest of the three-phase structure more specifically, by causing TiC precipitates having a diameter of 2.0 to 8.0 nm to exist in the ferrite at a number density of 1.0 ⁇ 10 16 precipitates/cm 3 or more, not only contributes to improving the strength of the steel sheet as a whole, but also sufficiently reduces the difference in hardness between bainite, which is relatively abundant in the hard structure, and ferrite, the softest of the three-phase structure.
• the chemical composition of the steel sheet must be appropriate.
• Si and Al contained in the steel have the effect of suppressing cementite precipitation. Therefore, by adding these elements in a predetermined amount or more in the steel, more specifically, by adding 0.010 mass % or more of Si and 0.20 mass % or more of Al, respectively, it is possible to suppress the consumption of C in the steel for forming cementite, thereby making it possible to promote the formation of TiC precipitates during cooling after hot rolling.
• the inventors have found that, even though the metallographic structure is composed of a three-phase structure that contains relatively large amounts of bainite and martensite to achieve high strength and therefore tends to have a relatively large difference in hardness, by increasing the hardness of ferrite through precipitation strengthening using TiC precipitates of a specific diameter and number density, it is possible to obtain a steel sheet with further improved hole expandability and yield ratio.
• a steel sheet with significantly improved hole expandability can be obtained by reducing the variation in the hardness of ferrite and bainite within a specified range, more specifically, by controlling the standard deviation in the hardness of ferrite and bainite to 0.40 GPa or less.
• the steel sheet according to the embodiment of the present invention is particularly useful in the automotive field, as it can be effectively used in parts that require both the contradictory properties of high strength and excellent workability, and also require impact resistance.
• C is an element effective in increasing the strength of steel sheet. Furthermore, C forms carbides and/or carbonitrides with Ti and Nb in steel, contributing to precipitation strengthening based on the formed precipitates and to refinement of the structure due to the pinning effect of the precipitates. To fully obtain these effects, the C content is set to 0.03% or more. The C content may be 0.04% or more, 0.05% or more, or 0.06% or more. On the other hand, excessive C content may result in a decrease in hole expandability and yield ratio due to the formation of cementite. Therefore, the C content is set to 0.10% or less. The C content may be 0.09% or less, 0.08% or less, or 0.07% or less.
• Si is an effective solid-solution strengthening element for increasing strength. Si also inhibits cementite precipitation. Therefore, the inclusion of Si can inhibit the consumption of C in the steel for the formation of cementite, thereby promoting the formation of TiC precipitates during cooling after hot rolling. To fully achieve these effects, the Si content is set to 0.010% or more. The Si content may be 0.020% or more, 0.030% or more, or 0.040% or more. On the other hand, excessive Si content may cause surface quality defects known as Si scale. Therefore, the Si content is set to 0.100% or less. The Si content may be 0.090% or less, 0.080% or less, 0.070% or less, 0.060% or less, or 0.050% or less.
• Mn is an element that is effective in increasing hardenability and strength as a solid solution strengthening element. To fully obtain these effects, the Mn content is set to 0.50% or more. The Mn content may be 0.60% or more, 0.70% or more, 0.80% or more, 1.00% or more, 1.20% or more, or 1.50% or more. On the other hand, excessive Mn content may result in the formation of large amounts of MnS, which may reduce toughness. Therefore, the Mn content is set to 3.00% or less. The Mn content may be 2.80% or less, 2.50% or less, 2.20% or less, or 2.00% or less.
• Ti is an element that precipitates finely in steel as carbide (TiC), improving the strength of steel through precipitation strengthening and increasing the hardness of ferrite. Ti also forms carbides to fix C and suppress the formation of cementite, which is harmful to hole expandability. To fully achieve these effects, the Ti content is set to 0.05% or more. The Ti content may be 0.08% or more, 0.10% or more, 0.12% or more, or 0.14% or more. On the other hand, excessive Ti content may cause the carbides to become coarse, making it impossible to achieve the desired precipitation strengthening in ferrite.
• the Ti content is set to 0.20% or less.
• the Ti content may be 0.18% or less, 0.17% or less, 0.16% or less, or 0.15% or less.
• Al 0.20-0.40%
• Al is an element that acts as a deoxidizer for molten steel.
• Al also has the effect of suppressing cementite precipitation. Therefore, the inclusion of Al can suppress the consumption of C in the steel to form cementite, thereby promoting the formation of TiC precipitates during cooling after hot rolling.
• the Al content is set to 0.20% or more.
• the Al content may be 0.22% or more, 0.25% or more, or 0.28% or more.
• excessive Al content may form coarse oxides, resulting in reduced toughness and ductility. Therefore, the Al content is set to 0.40% or less.
• the Al content may be 0.38% or less, 0.35% or less, or 0.32% or less.
• the P content is set to 0.100% or less.
• the P content may be 0.080% or less, 0.050% or less, 0.030% or less, or 0.020% or less.
• the lower limit of the P content is not particularly limited and may be 0%, but excessive reduction will increase costs. Therefore, the P content may be 0.0001% or more, 0.001% or more, or 0.005% or more.
• the Si content is set to 0.0100% or less.
• the S content may be 0.0050% or less, 0.0030% or less, or 0.0020% or less.
• the lower limit of the S content is not particularly limited and may be 0%, but excessive reduction will increase costs. Therefore, the S content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more.
• N 0.010% or less
• Excessive N content may form coarse nitrides and reduce toughness. Therefore, the N content is set to 0.010% or less.
• the N content may be 0.008% or less, 0.005% or less, or 0.003% or less.
• the lower limit of the N content is not particularly limited and may be 0%, but excessive reduction will increase costs. Therefore, the N content may be 0.0001% or more, 0.0005% or more, or 0.001% or more.
• O is an element that is mixed in during the manufacturing process. Excessive O content may form coarse inclusions, reducing the toughness of the steel plate. Therefore, the O content is set to 0.010% or less.
• the O content may be 0.008% or less, 0.006% or less, or 0.004% or less.
• the lower limit of the O content is not particularly limited and may be 0%, but reducing the O content to less than 0.0001% requires a long refining time, resulting in reduced productivity. Therefore, the O content may be 0.0001% or more or 0.0005% or more.
• the basic chemical composition of the steel sheet according to an embodiment of the present invention is as described above. Furthermore, if necessary, the steel sheet may contain at least one of the following optional elements in place of a portion of the remaining Fe.
• Nb is an element that forms carbides, nitrides, and/or carbonitrides in steel, contributing to the refinement of the structure through a pinning effect, thereby increasing the strength of the steel sheet.
• the Nb content may be 0%, but to achieve this effect, the Nb content is preferably 0.001% or more.
• the Nb content may be 0.005% or more, 0.010% or more, 0.012% or more, 0.015% or more, or 0.020% or more.
• excessive Nb content may cause the formation of coarse carbides in the steel, resulting in a decrease in the ductility of the steel sheet. Therefore, the Nb content is preferably 0.050% or less.
• the Nb content may be 0.040% or less, 0.030% or less, or 0.025% or less.
• V is an element that contributes to improving strength through precipitation strengthening and the like.
• the V content may be 0%, but to obtain such an effect, the V content is preferably 0.001% or more.
• the V content may be 0.010% or more, 0.030% or more, or 0.050% or more.
• the V content is preferably 1.000% or less.
• the V content may be 0.500% or less, 0.200% or less, 0.100% or less, or 0.080% or less.
• Cr is an element that improves the hardenability of steel and contributes to improving its strength.
• the Cr content may be 0%, but to obtain this effect, the Cr content is preferably 0.001% or more.
• the Cr content may be 0.01% or more, 0.03% or more, or 0.05% or more.
• the Cr content is preferably 2.00% or less.
• the Cr content may be 1.50% or less, 1.00% or less, 0.90% or less, 0.80% or less, 0.70% or less, 0.60% or less, 0.50% or less, 0.40% or less, 0.30% or less, 0.15% or less, or 0.10% or less.
• Ni and Cu are elements that contribute to improving hardenability or strength by solid solution strengthening.
• the Ni and Cu contents may be 0%, but to achieve such effects, the contents of these elements are preferably 0.001% or more, and may be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, excessive inclusion of these elements may saturate the effects and increase manufacturing costs.
• the Ni and Cu contents are preferably 2.00% or less, and may be 1.50% or less, 1.00% or less, 0.90% or less, 0.80% or less, 0.70% or less, 0.60% or less, 0.50% or less, 0.40% or less, 0.30% or less, 0.15% or less, or 0.10% or less.
• Mo is an element that improves the hardenability of steel and contributes to improving its strength.
• the Mo content may be 0%, but to achieve this effect, the Mo content is preferably 0.001% or more.
• the Mo content may be 0.010% or more, 0.020% or more, or 0.050% or more.
• excessive Mo content may increase deformation resistance during hot working and increase equipment load. Therefore, the Mo content is preferably 1.000% or less.
• the Mo content may be 0.900% or less, 0.800% or less, 0.700% or less, 0.600% or less, 0.500% or less, 0.400% or less, 0.300% or less, 0.200% or less, 0.100% or less, or 0.080% or less.
• B segregates at grain boundaries to increase grain boundary strength, thereby improving low-temperature toughness.
• the B content may be 0%, but to achieve this effect, the B content is preferably 0.0001% or more.
• the B content may be 0.0002% or more, 0.0003% or more, or 0.0005% or more.
• excessive B content may saturate the effect and increase manufacturing costs. Therefore, the B content is preferably 0.0100% or less.
• the B content may be 0.0050% or less, 0.0040% or less, 0.0030% or less, 0.0020% or less, 0.0015% or less, 0.0012% or less, 0.0010% or less, 0.0009% or less, 0.0008% or less, 0.0007% or less, or 0.0006% or less.
• Sn and Sb are elements effective in improving corrosion resistance.
• the Sn and Sb contents may be 0%, but to obtain such effects, the contents of these elements are preferably 0.001% or more, and may be 0.010% or more, 0.020% or more, or 0.050% or more. On the other hand, excessive inclusion of these elements may result in a decrease in toughness. Therefore, the Sn and Sb contents are preferably 1.000% or less, and may be 0.800% or less, 0.500% or less, 0.300% or less, 0.100% or less, or 0.080% or less.
• Ca, Mg, and Hf are elements that can control the morphology of nonmetallic inclusions.
• the Ca, Mg, and Hf contents may be 0%, but to obtain such effects, the contents of these elements are preferably 0.0001% or more, and may be 0.0005% or more, or 0.0010% or more.
• the Ca, Mg, and Hf contents are preferably 0.0100% or less, and may be 0.0050% or less, 0.0030% or less, or 0.0020% or less.
• Bi is an element effective in improving corrosion resistance.
• the Bi content may be 0%, but to obtain this effect, the Bi content is preferably 0.001% or more.
• the Bi content may be 0.002% or more or 0.003% or more.
• the Bi content is preferably 0.010% or less.
• the Bi content may be 0.005% or less or 0.004% or less.
• REM is an element that can control the morphology of nonmetallic inclusions.
• the REM content may be 0%, but to achieve this effect, the REM content is preferably 0.0001% or more.
• the REM content may be 0.0005% or more or 0.0010% or more.
• the REM content is preferably 0.0100% or less.
• the REM content may be 0.0050% or less, 0.0030% or less, or 0.0020% or less.
• REM is a collective term for 17 elements: scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides lanthanum (La) with atomic numbers 57 to lutetium (Lu) with atomic numbers 71.
• the REM content is the total content of these elements.
• the As content may be 0%, but to obtain this effect, the As content is preferably 0.001% or more.
• the As content may be 0.002% or more or 0.003% or more.
• the As content is preferably 0.010% or less.
• the As content may be 0.008% or less or 0.005% or less.
• Zr is an element that can control the morphology of nonmetallic inclusions.
• the Zr content may be 0%, but to obtain this effect, the Zr content is preferably 0.001% or more.
• the Zr content may be 0.002% or more or 0.003% or more.
• the Zr content is preferably 0.010% or less.
• the Zr content may be 0.008% or less or 0.005% or less.
• Co is an element that contributes to improving hardenability and/or heat resistance.
• the Co content may be 0%, but to obtain these effects, the Co content is preferably 0.001% or more.
• the Co content may be 0.010% or more, 0.050% or more, or 0.100% or more.
• the Co content is preferably 2.000% or less.
• the Co content may be 1.000% or less, 0.500% or less, 0.300% or less, or 0.200% or less.
• Zn is an element that can be contained in steel sheets when scrap or the like is used as a steel raw material. Therefore, the Zn content is preferably 0.010% or less, and may be 0.008% or less, or 0.005% or less. The Zn content may be 0%, but reducing it to less than 0.001% requires a long refining time, resulting in a decrease in productivity. Therefore, the Zn content may be 0.001% or more, 0.002% or more, or 0.003% or more.
• W is an element that improves the hardenability of steel and contributes to improving its strength.
• the W content may be 0%, but to obtain such effects, the W content is preferably 0.001% or more.
• the W content may be 0.010% or more, 0.050% or more, or 0.100% or more.
• excessive W content may reduce weldability. Therefore, the W content is preferably 1.000% or less.
• the W content may be 0.800% or less, 0.500% or less, 0.300% or less, or 0.200% or less.
• the remainder other than the above elements consists of Fe and impurities.
• Impurities are components that are mixed in during the industrial production of steel plate due to various factors in the manufacturing process, including raw materials such as ore and scrap.
• the chemical composition of the steel sheet according to the embodiment of the present invention may be measured using a common analytical method.
• the chemical composition of the steel sheet may be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES).
• C and S may be measured using the combustion-infrared absorption method
• N may be measured using the inert gas fusion-thermal conductivity method
• O may be measured using the inert gas fusion-non-dispersive infrared absorption method.
• the metallographic structure of the steel sheet according to the present invention includes, in area percentages, 40-80% ferrite, 15-55% bainite, and 5-20% martensite.
• the metallographic structure of the steel sheet includes, in area percentages, 40-80% ferrite, 15-55% bainite, and 5-20% martensite.
• the ferrite area percentage is low, the proportion of hard phases bainite and martensite, particularly the proportion of bainite, will be high, and ferrite precipitation strengthening may not be able to appropriately reduce the hardness difference within the metallographic structure, more specifically, the hardness difference between ferrite and bainite. In such cases, the desired hole expandability and/or yield ratio cannot be achieved. Therefore, the ferrite area percentage must be 40% or more, and may be, for example, 45% or more, 50% or more, or 55% or more.
• the area fraction of ferrite is set to 80% or less, and may be, for example, 75% or less, 70% or less, 65% or less, or 60% or less.
• the area fractions of the hard phases, bainite and martensite are high.
• the area fraction of bainite may be 18% or more, 20% or more, 22% or more, 25% or more, 28% or more, 30% or more, 32% or more, or 35% or more.
• the area fraction of martensite may be 8% or more, 10% or more, or 12% or more.
• the area fractions of bainite and martensite are low.
• the area fraction of bainite may be 52% or less, 50% or less, 48% or less, 45% or less, 42% or less, 40% or less, or 38% or less.
• the area fraction of martensite may be 18% or less, 16% or less, or 14% or less.
• the metal structure of the steel plate according to an embodiment of the present invention includes ferrite, bainite, and martensite, and may also include other residual structures.
• the area ratio of the residual structure is preferably small and may even be 0%.
• the area ratio of the residual structure is not particularly limited, and may be, for example, 0-5%, 0-4%, or 0-3%.
• the total area ratio of ferrite, bainite, and martensite may be, for example, 95-100%, 96-100%, or 97-100%.
• the lower limit of the residual structure may be 1% or 2%. If a residual structure is present, the residual structure may include at least one of pearlite and retained austenite, or may be at least one of them.
• Structural observation is performed using a scanning electron microscope. Prior to observation, the sample for structural observation is wet-polished with emery paper and diamond abrasives with an average particle size of 1 ⁇ m, and the observation surface is mirror-finished. The structure is then etched with a 3% nitric acid alcohol solution. The magnification for observation is 2000x, and 10 random images are taken of a 30 ⁇ m x 40 ⁇ m field of view at a position 1/4 of the plate thickness from the surface. The structural ratio is determined using the point counting method. A total of 225 lattice points, spaced 3 ⁇ m vertically and 4 ⁇ m horizontally, are defined for the obtained structural image.
• Ferrite is a massive crystal grain that does not contain iron-based carbides with a major axis of 100 nm or more.
• Bainite is a collection of lath-shaped crystal grains that either does not contain iron-based carbides with a major axis of 20 nm or more, or contains iron-based carbides with a major axis of 20 nm or more, where the carbides belong to a single variant, i.e., a group of iron-based carbides elongated in the same direction.
• a group of iron-based carbides elongated in the same direction refers to iron-based carbides whose elongation directions differ by no more than 5°. Bainite grains surrounded by grain boundaries with a misorientation of 15° or more are counted as one bainite grain. Furthermore, martensite, which contains a large amount of solute carbon, is brighter and appears whiter than other structures, making it distinguishable from other structures. When structures other than ferrite, bainite, and martensite are present, the area fraction of the remaining structure is determined by subtracting the total area fraction of ferrite, bainite, and martensite from 100%.
• the pearlite has a unique structure in which cementite precipitates in a lamellar form, and therefore can be identified using a scanning electron microscope.
• the volume fraction of the retained austenite can be calculated by X-ray diffraction measurement, and since the volume fraction of the retained austenite is equivalent to the area fraction, this can be used as the area fraction of the retained austenite.
• TiC precipitates having a diameter of 2.0 to 8.0 nm are present in the ferrite at a number density of 1.0 x 10 16 particles/cm 3 or more.
• TiC precipitates include not only TiC but also composite carbides containing Ti and elements other than Ti, such as V or Nb. Methods for identifying TiC precipitates having a diameter of 2.0 to 8.0 nm in the ferrite and methods for measuring their number density will be described later.
• the hardness of the ferrite can be increased through precipitation strengthening. More specifically, by increasing the hardness of ferrite and reducing the difference in hardness with bainite, a hard phase that is relatively abundant in the metal structure, the difference in hardness in the metal structure primarily composed of ferrite, bainite, and martensite can be reduced. As a result, the hole expandability and yield ratio of the steel sheet can be further improved. Naturally, precipitation strengthening by TiC precipitates also contributes to improving the overall strength of the steel sheet.
• the diameter of the TiC precipitates is smaller than 2.0 nm, the TiC precipitates cannot sufficiently function as obstacles to dislocation motion, and therefore the effect of improving the hardness of ferrite due to precipitation strengthening cannot be fully obtained. In addition, the effect of improving the strength of the steel sheet may not be fully exerted. On the other hand, if the diameter of the TiC precipitates is too large, the desired precipitation strengthening in ferrite may not be obtained.
• TiC precipitates having such a diameter In order to increase the hardness of ferrite to a desired level through precipitation strengthening, it is important to control the number density of TiC precipitates having such a diameter within a predetermined range. From this viewpoint, TiC precipitates having a diameter of 2.0 to 8.0 nm need to be present in the ferrite at a number density of 1.0 ⁇ 10 16 particles/cm 3 or more, as described above.
• the higher the number density the more preferable it is, and it may be, for example, 1.2 ⁇ 10 16 particles/cm 3 or more, 1.5 ⁇ 10 16 particles/cm 3 or more, 2.0 ⁇ 10 16 particles/cm 3 or more, 5.0 ⁇ 10 16 particles/cm 3 or more, or 10.0 ⁇ 10 16 particles/cm 3 or more.
• the contents of C and Ti which are the supply sources of TiC precipitates, if the number density becomes too high, it may become difficult to control the diameter of the TiC precipitates within the desired range.
• the number density is not particularly limited as long as the diameter requirement of 2.0 to 8.0 nm is satisfied, but may be, for example, 75.0 x 10 particles/cm or less, 50.0 x 10 particles/cm or less, 30.0 x 10 particles/cm or less, or 20.0 x 10 particles/cm or less.
• the steel sheet according to the embodiment of the present invention when measured by a three-dimensional atom probe measurement method described in detail later, it is sufficient that TiC precipitates having a diameter of 2.0 to 8.0 nm are present in ferrite at a number density of 1.0 x 10 particles/cm or more, and therefore, as long as the above diameter and number density requirements are satisfied, for example, coarse TiC precipitates may be present in ferrite.
• the diameter and number density of TiC precipitates are calculated using a three-dimensional atom probe measurement method as follows. First, a needle-shaped sample is prepared from the sample to be measured by cutting and electropolishing, optionally using a focused ion beam (FIB) processing method in combination with electropolishing, with at least one end of the rod-shaped sample being sharpened to a point. The needle-shaped sample is prepared using the method used to identify ferrite in the metallographic structure identification and area fraction calculation described above, so that ferrite falls within the range of the three-dimensional atom probe measurement.
• FIB focused ion beam
• the number density of TiC precipitates with a diameter of 2.0 to 8.0 nm is calculated from the average of the measurement results for three samples closest to the average of the five measurement results.
• the accumulated data can be reconstructed to obtain an image of the actual atomic distribution in real space.
• the precipitate is determined to be a TiC precipitate.
• the diameter of the TiC precipitate is the circle-equivalent diameter calculated from the number of Ti atoms constituting the observed TiC precipitate and the lattice constant of the TiC precipitate, assuming that the TiC precipitate is spherical.
• the following describes a method for determining the diameter (circle-equivalent diameter) R of the TiC precipitate using the number of Ti atoms in the TiC precipitate obtained by three-dimensional atom probe measurement. Although the three-dimensional atom probe measurement method measures the number N of all atoms in the target sample, in reality, it is not possible to detect all atoms N in the target sample using the three-dimensional atom probe measurement method.
• a TiC precipitate with a Na—Cl structure has eight Ti atoms in its unit lattice.
• the number density of TiC precipitates with a diameter of 2.0 to 8.0 nm is calculated for each of the five samples, using the measurement field of view (the range in which atoms are ionized from the sample surface during three-dimensional atom probe measurement) as the denominator and the number of TiC precipitates with a diameter in the range of 2.0 to 8.0 nm as the numerator.
• the average value of the measurement results (number density of TiC precipitates with a diameter of 2.0 to 8.0 nm) for the three samples closest to the average value of the measurement results (number density of TiC precipitates with a diameter of 2.0 to 8.0 nm) for the five samples is taken as the number density of TiC precipitates with a diameter of 2.0 to 8.0 nm.
• the standard deviation in the hardness of ferrite and bainite is controlled to 0.40 GPa or less.
• the lower the standard deviation in the hardness of ferrite and bainite the more preferable it may be, for example, 0.38 GPa or less, 0.35 GPa or less, 0.32 GPa or less, 0.30 GPa or less, 0.28 GPa or less, or 0.26 GPa or less.
• the lower limit is not particularly limited, but for example, the standard deviation of the hardness of ferrite and bainite may be 0.05 GPa or more, 0.10 GPa or more, or 0.15 GPa or more.
• the standard deviation in the hardness of ferrite and bainite is determined as follows. First, a sample is cut out from the steel plate so that a cross section perpendicular to the surface can be observed. The cross section of the sample is wet-polished with emery paper and polished to a mirror finish with diamond abrasive grains having an average particle size of 1 ⁇ m. For the mirror-finished cross section, a test conforming to ISO 14577-1:2015 is performed in a 30 ⁇ m ⁇ 30 ⁇ m region including ferrite and bainite at a depth of 1/4 of the plate thickness from the surface using a microhardness tester.
• an indentation is made with a triangular pyramidal indenter at a load of 1000 ⁇ N, and nanoindentation hardness is measured, resulting in a total of 25 measured values.
• an indentation is made with a triangular pyramidal indenter at a load of 1000 ⁇ N using a microhardness tester, resulting in a total of 25 measured values.
• the standard deviation of the hardness of ferrite and bainite is determined based on the nanoindentation hardness obtained for a total of 50 samples.
• the 30 ⁇ m ⁇ 30 ⁇ m region containing ferrite and bainite can be identified by measuring the same sample in advance using a scanning electron microscope (SEM). Specifically, in SEM observation, martensite containing a large amount of solute carbon appears brighter and whiter than other structures. Therefore, a 30 ⁇ m ⁇ 30 ⁇ m region that does not include such a brighter, whiter region can be identified in advance using SEM observation, and this region can then be measured using the microhardness tester described above.
• SEM scanning electron microscope
• the average grain size of ferrite is preferably 5.0 ⁇ m or less.
• the average grain size of ferrite may be 4.5 ⁇ m or less, 4.2 ⁇ m or less, 4.0 ⁇ m or less, 3.8 ⁇ m or less, 3.5 ⁇ m or less, 3.2 ⁇ m or less, 3.0 ⁇ m or less, or 2.8 ⁇ m or less.
• the lower limit is not particularly limited, but the average grain size of ferrite may be, for example, 0.5 ⁇ m or more, 1.0 ⁇ m or more, 1.5 ⁇ m or more, or 2.0 ⁇ m or more.
• the average grain size of ferrite in the steel sheet according to the embodiment of the present invention is determined by analyzing the ferrite separated by the point counting method performed in the identification of the metallographic structure and calculation of the area ratio described above using an SEM-EBSD device (for example, JSM-7001F manufactured by JEOL Ltd.). Specifically, the average grain size of ferrite is determined by the following formula (1) by analyzing the grain size of the BCC phase defined by high-angle grain boundaries with a tilt angle of 15° or more using an SEM-EBSD device for ferrite separated by the point counting method after taking five photographs in connection with the identification of the metallographic structure and calculation of the area ratio.
• an SEM-EBSD device for example, JSM-7001F manufactured by JEOL Ltd.
• d represents the average grain size of ferrite
• di represents the circle equivalent diameter of the i-th grain
• N represents the number of grains included in the evaluation area of the average grain size of ferrite.
• the steel sheet according to the embodiment of the present invention is not particularly limited, but generally has a thickness of 1.0 to 8.0 mm.
• the thickness may be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more, and/or 7.0 mm or less, 6.0 mm or less, 5.0 mm or less, or 4.0 mm or less.
• the steel sheet according to the embodiment of the present invention encompasses various steel sheets, and may be, for example, a hot-rolled steel sheet, a cold-rolled steel sheet, or a hot-rolled steel sheet or a cold-rolled steel sheet that has been subjected to a surface treatment such as plating.
• the steel sheet according to the embodiments of the present invention despite its high strength, is able to achieve a high yield ratio and particularly excellent hole expandability. Therefore, the steel sheet according to the embodiments of the present invention reliably achieves a high level of the contradictory properties of high strength and excellent formability, while also achieving excellent impact resistance. Therefore, the steel sheet according to the embodiments of the present invention is useful for use in parts in technical fields where these properties are required, and is particularly useful in the automotive field.
• an automotive part particularly an automotive suspension part, is provided that includes the steel sheet according to the embodiments of the present invention. Examples of automotive suspension parts include lower arms and trailing arms.
• automotive parts particularly automotive suspension parts
• the metallographic characteristics do not change significantly before and after forming.
• Steel sheets having the above chemical composition and metallographic structure can achieve high tensile strength, specifically, tensile strength of 780 MPa or more.
• the tensile strength is preferably 800 MPa or more, 820 MPa or more, or 840 MPa or more.
• steel sheets according to embodiments of the present invention can achieve improved hole expandability and a high yield ratio due to the specific combination of chemical composition and metallographic structure described above.
• the upper limit of tensile strength is not particularly limited, and the tensile strength of the steel sheet may be, for example, 1180 MPa or less, 980 MPa or less, 940 MPa or less, 900 MPa or less, or 860 MPa or less.
• Tensile strength is measured by taking a JIS No. 5 test piece in a direction in which the longitudinal direction of the test piece is preferably parallel to the rolling direction of the steel sheet (C direction) and conducting a tensile test in accordance with JIS Z 2241:2022. If the rolling direction of the steel sheet cannot be identified, the JIS No. 5 test piece may be taken from any direction within the steel sheet plane.
• Yield ratio: YR Steel sheets having the above chemical composition and metallographic structure can achieve not only high tensile strength but also an increased yield ratio; more specifically, a yield ratio of 0.70 or higher.
• the yield ratio is preferably 0.75 or higher, more preferably 0.80 or higher.
• the upper limit is not particularly limited, but the yield ratio may be, for example, 0.90 or lower or 0.85 or lower.
• ⁇ Steel sheet manufacturing method> a preferred method for manufacturing a steel sheet according to an embodiment of the present invention will be described.
• the following description is intended to exemplify a characteristic method for manufacturing a steel sheet according to an embodiment of the present invention, and is not intended to limit the steel sheet to one manufactured by the manufacturing method described below. More specifically, although the following specifically describes the manufacture of a hot-rolled steel sheet, the steel sheet according to an embodiment of the present invention encompasses any steel sheet having the chemical composition and metallographic structure described above, i.e., not only a hot-rolled steel sheet, but also a cold-rolled steel sheet, a plated steel sheet, and the like. Therefore, the following description merely describes one example of a preferred manufacturing method when the steel sheet according to an embodiment of the present invention is a hot-rolled steel sheet.
• a method for producing a steel plate according to an embodiment of the present invention includes a hot rolling step that includes heating a slab having the chemical composition described above in relation to the steel plate and then finish rolling the slab, and satisfies the following conditions (a) to (e): (a) The heating temperature of the slab is 1200 to 1300°C; (b) the holding time in the temperature range of 1200 to 1300°C is 1000 to 4000 seconds; (c) The finish rolling is performed using a tandem rolling mill consisting of five or more rolling stands, and the total reduction rate in the rolling passes in the front stages other than the rear three stages is 60 to 90%; (d) the total reduction in the latter three rolling passes is more than 50%, and (e) the end temperature of finish rolling is 900 to 1000°C.
• the method is characterized by including an intermediate air-cooling step in which the finish-rolled steel sheet is primarily cooled to an intermediate air-cooling temperature of 705 to 750°C at an average cooling rate of 50 to 200°C/sec and then intermediate air-cooled for 3 to 10 seconds, and a cooling step in which the intermediate air-cooled steel sheet is secondarily cooled at an average cooling rate of 50 to 200°C/sec and then coiled at a coiling temperature of 20 to 300°C.
• an intermediate air-cooling step in which the finish-rolled steel sheet is primarily cooled to an intermediate air-cooling temperature of 705 to 750°C at an average cooling rate of 50 to 200°C/sec and then intermediate air-cooled for 3 to 10 seconds
• a cooling step in which the intermediate air-cooled steel sheet is secondarily cooled at an average cooling rate of 50 to 200°C/sec and then coiled at a coiling temperature of 20 to 300°C.
• a slab having the chemical composition described above in relation to the steel plate is heated.
• the slab used is preferably cast by a continuous casting method, but may also be produced by an ingot casting method or a thin slab casting method.
• the slab used contains a relatively large amount of alloying elements in order to obtain a high-strength steel plate. Therefore, the slab must be heated before being subjected to hot rolling to dissolve the alloying elements in the slab.
• the heating temperature is preferably 1200°C or higher.
• the upper limit of the heating temperature is not particularly limited, but is preferably 1300°C or lower from the viewpoints of the capacity of the heating equipment and productivity.
• the upper limit of the holding time is not particularly limited, but is preferably 4000 seconds or lower from the viewpoints of productivity, etc.
• the heated slab may be subjected to rough rolling before finish rolling in order to adjust the plate thickness, etc.
• the conditions for rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured.
• Refining the metal structure through such recrystallization is highly advantageous in forming a desired metal structure and improving properties such as hole expandability and yield ratio. If the total reduction in the preceding rolling passes is less than 60%, the desired metal structure containing ferrite, bainite, and martensite in specific proportions cannot be obtained, and properties such as hole expandability and/or yield ratio may be reduced. Therefore, the total reduction in the preceding rolling passes is set to 60% or more, and preferably 70% or more. On the other hand, if the total reduction in the preceding rolling passes is too high, the rolling load becomes excessive, increasing the load on the rolling mill. For this reason, the total reduction in the preceding rolling passes is set to 90% or less.
• the total reduction ratio in the last three rolling passes is controlled to exceed 50%.
• recrystallization can be further promoted and austenite grains can be refined.
• the number of austenite grain boundaries can be increased, thereby increasing the number of ferrite nucleation sites.
• the growth rate of ferrite can be increased in the subsequent intermediate air-cooling step, allowing ferrite to be rapidly generated in a relatively short period of time.
• the intermediate air-cooling step not only ferrite but also TiC precipitates are generated, as will be described in detail later.
• the total reduction rate in the last three rolling passes of finish rolling is preferably 55% or more, or 60% or more. There is no particular upper limit, but for example, the total reduction rate in the last three rolling passes of finish rolling may be 90% or less, or 85% or less.
• finish rolling end temperature is also important for controlling the metallographic structure of the steel sheet. If the finish rolling end temperature is low, the metallographic structure may become nonuniform, resulting in reduced strength and/or the hardness variations of ferrite and bainite may not be reduced to within the desired range, resulting in reduced hole expandability. Therefore, the finish rolling end temperature is set to 900°C or higher. Preferably, the finish rolling end temperature is set to 920°C or higher.
• the finish rolling end temperature is set to 1000°C or lower.
• the finish rolling end temperature is set to 980°C or lower.
• the finish-rolled steel sheet is primarily cooled on a run-out table (ROT) at an average cooling rate of 50 to 200°C/s to an intermediate cooling temperature of 705 to 750°C, and then intermediate cooling is performed for 3 to 10 seconds.
• ROT run-out table
• Primary cooling to an intermediate cooling temperature of 705 to 750°C at an average cooling rate of 50 to 200°C/s suppresses excessive ferrite formation and/or ferrite coarsening, while the subsequent intermediate cooling at high temperatures promotes the precipitation of TiC precipitates.
• the ferrite is sufficiently precipitation-strengthened, thereby reducing the difference in hardness between ferrite and bainite and improving the hole expandability and yield ratio.
• the intermediate cooling temperature in order to promote the formation and grain growth of TiC precipitates during intermediate cooling and thereby sufficiently precipitation-strengthen the ferrite, the intermediate cooling temperature must be set to a relatively high temperature range, i.e., a temperature range of 705 to 750°C.
• a relatively high temperature range i.e., a temperature range of 705 to 750°C.
• the average cooling rate in the primary cooling from finish rolling to the intermediate air-cooling temperature is set to 50°C/s or more to suppress excessive ferrite generation and ensure sufficient precipitation of TiC precipitates by the subsequent intermediate air-cooling at high temperatures.
• the average cooling rate in the primary cooling is set to 200°C/s or less, preferably 160°C/s or less.
• the intermediate cooling temperature exceeds 750°C or the intermediate cooling time exceeds 10 seconds, excessive ferrite may form or TiC precipitates may become coarse. If excessive ferrite forms, it becomes impossible to form the desired metal structure containing ferrite, bainite, and martensite in a specific ratio in the final steel sheet. Furthermore, if the TiC precipitates become coarse, the number density of the TiC precipitates may also decrease significantly. In such cases, the effect of precipitation strengthening on improving the hardness of ferrite may not be fully achieved. If the intermediate cooling temperature is too high, exceeding 800°C, the intermediate cooling temperature may be higher than the ferrite transformation point, making it difficult for ferrite to form.
• ferrite formation may be excessively suppressed, which would make it impossible to form the desired metal structure containing ferrite, bainite, and martensite in specific proportions in the final steel sheet.
• the suppression of TiC precipitate formation and grain growth, along with the excessive suppression of ferrite formation may prevent the desired amount of ferrite from being properly precipitation strengthened. In such cases, it becomes impossible to reduce the variation in hardness of ferrite and bainite to within the desired range.
• the intermediate air-cooling step primary cooling is performed at an average cooling rate of 50 to 200°C/s, preferably 50 to 160°C/s, to an intermediate air-cooling temperature of 705 to 750°C, followed by intermediate air-cooling for 3 to 10 seconds, preferably 4 to 9 seconds, thereby precipitating ferrite in the desired proportion and generating TiC precipitates in the ferrite, which then undergo appropriate grain growth to ultimately produce TiC precipitates having a diameter of 2.0 to 8.0 nm at a number density of 1.0 x 1016 / cm3 or more.
• the steel sheet after intermediate air cooling is secondarily cooled at an average cooling rate of 50 to 200°C/s, and then coiled at a coiling temperature (secondary cooling stop temperature) of 20 to 300°C. Coiling is performed immediately after secondary cooling.
• bainite and martensite can be appropriately precipitated, making it possible to form a metallographic structure containing ferrite, bainite, and martensite in specific proportions in the final steel sheet.
• the average cooling rate in secondary cooling is set to 50°C/s or more, preferably 70°C/s or more.
• the average cooling rate in the secondary cooling is set to 200°C/sec or less, and preferably 180°C/sec or less or 150°C/sec or less.
• the coiling temperature should be 20°C or higher. Note that although this process is called the cooling process for convenience, as mentioned above, it is a process that includes secondary cooling and coiling (excluding primary cooling).
• Steel sheets manufactured by the above manufacturing method have a metal structure containing ferrite, bainite, and martensite in specific proportions, which allows for improved hole expandability and yield ratio while maintaining a relatively high level of strength. Furthermore, TiC precipitates with diameters of 2.0 to 8.0 nm are present in the ferrite at a density of 1.0 ⁇ 10 16 precipitates/cm 3 or more. This not only contributes to improving the overall strength of the steel sheet through precipitation strengthening, but also sufficiently reduces the difference in hardness between the relatively abundant bainite phase in the hard structure and the softest ferrite phase in the three-phase structure, thereby further improving the hole expandability and yield ratio of the steel sheet.
• the standard deviation in the hardness of ferrite and bainite is controlled to 0.40 GPa or less, in combination with the reduction in the difference in hardness between bainite and ferrite due to the use of TiC precipitates, not only can the difference in hardness be reduced throughout the metal structure, but the increase in local hardness differences within the metal structure can also be reliably reduced, resulting in a more significant improvement in the hole expandability of the steel sheet. Therefore, the steel sheet manufactured by the above manufacturing method is particularly useful in the automotive field, as it can be effectively used in components that require both the contradictory properties of high strength and excellent workability, and furthermore, impact resistance.
• steel sheets according to embodiments of the present invention were manufactured under various conditions, and the tensile strength (TS), hole expansion ratio ( ⁇ ), and yield ratio (YR) of the resulting steel sheets were investigated.
• the properties of the obtained steel sheets were measured and evaluated using the following methods.
• the diameter and number density of TiC precipitates were calculated using the three-dimensional atom probe measurement method described in detail herein, with the device-specific atomic detection rate ⁇ set to 0.35.
• the detection rate ⁇ is calculated by dividing the number of detected atoms by the total number of original atoms, where the "total number of original atoms" corresponds to the number of atoms in a standard sample (the number of atoms of which is known in advance).
• the detection rate ⁇ of the device can be determined by dividing the number of atoms detected from the standard sample using the actual device by the total number of original atoms in the standard sample.
• Tensile strength (TS) and yield ratio (YR) Tensile strength (TS) was measured by taking a JIS No. 5 test piece from a direction in which the longitudinal direction of the test piece was parallel to the rolling direction of the steel plate (C direction) and conducting a tensile test in accordance with JIS Z 2241:2022. The gauge length was 50 mm. More specifically, the test was conducted at room temperature in the range of 10 to 35°C, and a tensile test force was applied to the test piece, and strain was applied until fracture.
• Comparative Example 13 the intermediate air-cooling temperature was low, which suppressed the formation and grain growth of TiC precipitates, preventing the desired number density of the TiC precipitates from being obtained.
• the formation of ferrite was excessively suppressed, preventing the desired amount of ferrite from being adequately precipitation strengthened, and preventing the variation in the hardness of ferrite and bainite from being reduced within the desired range. As a result, the effect of improving the hardness of ferrite by precipitation strengthening could not be fully achieved, and ⁇ decreased.
• Comparative Example 14 the end temperature of finish rolling was low, resulting in a non-uniform metal structure, and as a result, the standard deviation in the hardness of ferrite and bainite exceeded 0.40 GPa, and ⁇ decreased.
• Comparative Example 15 it is believed that the average cooling rate in the primary cooling to the intermediate air-cooling temperature was slow, causing the ferrite to coarsen. As a result, the standard deviation in the hardness of ferrite and bainite exceeded 0.40 GPa, and ⁇ decreased.
• Comparative Example 16 the intermediate air-cooling time was short, which suppressed the formation and grain growth of TiC precipitates, making it impossible to obtain the desired number density.
• Comparative Example 19 the high end temperature of the finish rolling caused the austenite grains after recrystallization to coarsen, reducing the number of austenite grain boundaries and the number of ferrite nucleation sites.
• the desired ferrite area ratio could not be achieved even by the subsequent intermediate air-cooling step, and the standard deviation of the hardness of ferrite and bainite in the finally obtained metal structure could not be controlled to 0.40 GPa or less, resulting in reduced ⁇ and YR.
• the average cooling rate in the secondary cooling after the intermediate air-cooling was slow, preventing sufficient precipitation of bainite and martensite, resulting in reduced TS.
• Comparative Example 21 the high coiling temperature resulted in the area ratio of martensite being less than 5%, resulting in reduced TS.
• Comparative Example 22 the low C content prevented TiC precipitates from precipitating at a sufficient number density. As a result, TS, ⁇ , and YR were reduced.
• Comparative Example 23 the low Ti content similarly prevented TiC precipitates from precipitating at a sufficient number density. As a result, TS, ⁇ , and YR decreased.
• the Si content was low, so cementite precipitation could not be sufficiently suppressed, and it is believed that C in the steel was consumed in the formation of cementite.
• all of the steel sheets according to the examples of the present invention had a predetermined chemical composition, and by appropriately controlling the conditions in the manufacturing method, it was possible to obtain a steel sheet having a metallographic structure containing, by area ratio, 40 to 80% ferrite, 15 to 55% bainite, and 5 to 20% martensite, in which TiC precipitates with a diameter of 2.0 to 8.0 nm are present in the ferrite at a number density of 1.0 x 1016 precipitates/ cm3 or more, and in which the standard deviation in hardness of ferrite, bainite, and martensite is 0.40 GPa or less.
• the strength of the steel sheet was improved due to precipitation strengthening of ferrite by TiC precipitates, and the precipitation strengthening reduced the difference in hardness between ferrite and bainite, and further reduced the variation in hardness of ferrite and bainite, thereby achieving a high tensile strength of 780 MPa or more, while significantly improving the hole expandability and yield ratio.
• the residual structure when a residual structure was present, the residual structure was at least one of pearlite and retained austenite.

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