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
  • 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
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • 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 sheets and strips with excellent mechanical properties and stability of mechanical properties, which are used in a variety of applications such as automobiles and home appliances and contribute to improving yields in the manufacture of structural components, as well as methods for manufacturing them.
• Patent Document 1 discloses a method for producing high-strength cold-rolled steel sheet with a composite structure primarily composed of ferrite and tempered martensite in a continuous annealing line, in which the continuous annealing line sequentially undergoes a heating process, a slow cooling process to a quenching start temperature Tq, a quenching process by rapid cooling, and a tempering process by reheating.
• the ferrite fraction Vf of the steel sheet is measured using magnetic properties immediately after the quenching process, and compared with a predetermined target ferrite fraction Vf0 necessary to achieve the target mechanical properties of the product steel sheet.
• the present invention was developed in light of the above-mentioned current situation, and aims to provide steel sheets and steel strips having a TS of 590 MPa or more and less than 1180 MPa, excellent mechanical properties (YS, TS, El, ⁇ ), and small variations in the mechanical properties (YS, TS, El, ⁇ ), i.e., excellent stability of mechanical properties, as well as methods for manufacturing the same.
• steel strip refers to a product wound into a coil with a mass of 5 tons or more and a width of 500 mm or more.
• the longitudinal length of the steel strip is more than 10 m.
• steel plate refers to a portion extracted from the steel strip, and refers to a product with a width of 500 mm or more and a longitudinal length of 10 m or less, preferably less than 1 m.
• being excellent in mechanical properties means that the yield stress (YS) and total elongation (El) measured in a tensile test in accordance with JIS Z 2241 (2011), and the hole expansion ratio ( ⁇ ) measured in a hole expansion test in accordance with the Japan Iron and Steel Federation Standard JFST 1001 satisfy the following formulas 1 to 3 according to the tensile strength (TS) measured in the tensile test.
• Formula 4 When 590 MPa ⁇ TS ⁇ 780 MPa, ⁇ TS ⁇ 40 MPa When 780 MPa ⁇ TS ⁇ 980 MPa, ⁇ TS ⁇ 50 MPa When 980 MPa ⁇ TS ⁇ 1180 MPa, ⁇ TS ⁇ 60 MPa
• Formula 5 When 590 MPa ⁇ TS ⁇ 780 MPa, ⁇ TS ⁇ 20 MPa When 780 MPa ⁇ TS ⁇ 980 MPa, ⁇ TS ⁇ 25 MPa When 980 MPa ⁇ TS ⁇ 1180 MPa, ⁇ TS ⁇ 30 MPa
• Formula 6 When 590 MPa ⁇ TS ⁇ 780 MPa, ⁇ YS ⁇ 40 MPa When 780 MPa ⁇ TS ⁇ 980 MPa, ⁇ YS ⁇ 50 MPa When 980 MPa ⁇ TS ⁇ 1180 MPa, ⁇ YS ⁇ 60 MPa
• Formula 7 When 590 MPa ⁇ TS ⁇ 780 MPa, ⁇ YS
• the present inventors have conducted extensive research to achieve the above-mentioned object, and as a result have obtained the following findings.
• (1) With a specified composition by controlling the area ratio of the soft phase (one or more selected from unrecrystallized ferrite, recrystallized ferrite, transformed ferrite, epitaxial ferrite, and bainitic ferrite) to 90.0% or less and the area ratio of the hard phase (one or more selected from fresh martensite and tempered martensite) to 10.0% or more, it is possible to ensure a TS of 590 MPa or more.
• the area ratio of the soft phase (one or more selected from unrecrystallized ferrite, recrystallized ferrite, transformed ferrite, epitaxial ferrite, and bainitic ferrite) is controlled to 30.0% or more, and the area ratio of the hard phase (one or more selected from fresh martensite and tempered martensite) is controlled to 70.0% or less, thereby achieving a TS of less than 1180 MPa.
• the following formulas 1 to 3 can be realized.
• Formula 1 When 590 MPa ⁇ TS ⁇ 780 MPa, 360 MPa ⁇ YS When 780 MPa ⁇ TS ⁇ 980 MPa, 460 MPa ⁇ YS If 980 MPa ⁇ TS ⁇ 1180 MPa, then 580 MPa ⁇ YS Formula 2: When 590 MPa ⁇ TS ⁇ 780 MPa, 23.0% ⁇ El When 780 MPa ⁇ TS ⁇ 980 MPa, 17.0% ⁇ El When 980 MPa ⁇ TS ⁇ 1180 MPa, 11.0% ⁇ El Formula 3: When 590 MPa ⁇ TS ⁇ 780 MPa, 45% ⁇ ⁇ When 780 MPa ⁇ TS ⁇ 980 MPa, 30% ⁇ ⁇ When 980 MPa ⁇ TS ⁇ 1180 MPa, 20% ⁇ ⁇ (4) Furthermore, by controlling the area ratio of the hard phases in which [C content (mass%) in the hard phase]/[C content (mass%) in the steel] is 1.50 or
• annealing/soaking process a structure containing ferrite (unrecrystallized ferrite and recrystallized ferrite) and austenite is formed, and then the steel is rapidly heated from the annealing/soaking temperature to a temperature above 10°C (annealing/soaking temperature + 10°C) at an average heating rate of 10°C/sec or more.
• the amount of diffusion of the diffusion-substitutional element Mn from the ferrite to austenite is small, so a large amount of austenite is generated and grown, mainly due to the diffusion of the interstitial element C.
• the region where this austenite is generated and grown, mainly due to the diffusion of C has a high-C, low-Mn composition.
• the final structure formed after the subsequent cooling process becomes a hard phase with a high-C, low-Mn composition.
• This hard phase with a high-C, low-Mn composition is a new structure realized through the annealing/soaking process and the rapid heating process. By generating an appropriate amount of this hard phase with a high-C, low-Mn composition and controlling the structure fraction of the hard phase and the soft phase, it is possible to manufacture a steel sheet with excellent stability of mechanical properties.
• a plating layer is provided on the steel strip surface, The steel strip according to [4] or [5], wherein the plating layer is any one of a hot-dip galvanized layer, a galvannealed hot-dip galvanized layer, an electrogalvanized layer, and a hot-dip aluminum plating layer.
• FIG. 1a is an SEM image showing the soft phases of recrystallized ferrite, unrecrystallized ferrite, and bainitic ferrite, and the hard phase of fresh martensite.
• FIG. 1b is an SEM image showing the soft phase of recrystallized ferrite and the hard phase of fresh martensite.
• FIG. 1c is an SEM image showing the soft phases of recrystallized ferrite, transformed ferrite, and epitaxial ferrite, and the hard phase of fresh martensite.
• FIG. 1d is an SEM image showing the soft phase bainitic ferrite and the hard phases fresh martensite and tempered martensite.
• Figure 2(a) shows the structure (1) of Inventive Example No.
• FIG. 5 shows an SEM image ((1), backscattered electron image) of Invention Example No. 2 (Steel B) at the measurement position indicated by reference numeral 8 in FIG. 5 , as well as the results of line analysis (quantitative) of C ((2)) and line analysis (quantitative) of Mn ((3)) obtained by EPMA measurement.
• FIG. 3b shows an SEM image ((1), backscattered electron image) of Comparative Example No. 39 (Steel B) at the measurement position indicated by reference numeral 8 in FIG. 5 , as well as the results of line analysis (quantitative) of C (2) and line analysis (quantitative) of Mn (3) obtained by EPMA measurement.
• FIG. 2 is a schematic diagram of a steel plate viewed from above, illustrating measurement positions of YS, TS, and El of the steel plate.
• FIG. 1 is a schematic diagram of a steel strip viewed from above to explain the measurement positions of YS, TS, and El within the steel strip.
• FIG. 6a is a schematic diagram showing an example of a heat treatment pattern including an annealing and soaking step (A), a rapid heating step (B), a cooling step (C), and a reheating and holding step (E).
• FIG. 6b is a schematic diagram showing an example of a heat treatment pattern including an annealing and soaking step (A), a rapid heating step (B), a cooling step (C), a hot-dip galvanizing treatment step (D1), and a reheating and holding step (E).
• FIG. 6c is a schematic diagram showing an example of a heat treatment pattern including an annealing/soaking step (A), a rapid heating step (B), a cooling step (C), a hot-dip galvanizing treatment step (D1), an alloying treatment step (D11), and a reheating/holding step (E).
• FIG. 6d is a schematic diagram showing an example of a heat treatment pattern including an annealing/soaking step (A), a rapid heating step (B), a cooling step (C), an electrogalvanizing treatment step (D2), and a reheating/holding step (E).
• Steel plate and steel strip A steel plate according to one embodiment of the present invention contains, in mass%, C: 0.030% or more and 0.250% or less, Si: 0.01% or more and 2.50% or less, Mn: 1.30% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0200% or less, Al: 0.010% or more and 2.000% or less, N: 0.0100% or less, with the balance being Fe and unavoidable impurities.
• the steel plate has a component composition in which the structure at a 1/4 position of the plate thickness has an area ratio of a soft phase of: The hard phase has an area ratio of 30.0% to 90.0% and an area ratio of the hard phase is 10.0% to 70.0%.
• the area ratio of the hard phase is 65% or more, and the ratio of [C content (mass%) in the hard phase]/[C content (mass%) in the steel sheet] is 1.50 or more.
• the area ratio of the hard phase is 10% or more, and the ratio of [Mn content (mass%) in the hard phase]/[Mn content (mass%) in the steel sheet] is 1.30 or less.
• a steel strip according to one embodiment of the present invention has a composition containing, in mass %, C: 0.030% or more and 0.250% or less, Si: 0.01% or more and 2.50% or less, Mn: 1.30% or more but less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0200% or less, Al: 0.010% or more and 2.000% or less, N: 0.0100% or less, with the balance consisting of Fe and unavoidable impurities, and the structure at the 1/4 position of the steel strip thickness has an area ratio of soft phase: 30.0% or more and 90.0% or less, and an area ratio of hard phase: 10.
• the area ratio of the hard phases in which [C content (mass%) in the hard phase]/[C content (mass%) in the steel strip] is 1.50 or more is 65% or more, and the area ratio of the hard phases in which [Mn content (mass%) in the hard phase]/[Mn content (mass%) in the steel strip] is 1.30 or less is 10% or more, and further, when the tensile strength is TS, the yield stress is YS, the total elongation is El, and the hole expansion ratio is ⁇ , the longitudinal direction of the steel strip is 1/4 position, 1/2 position, and 3/4 position in the width direction of the steel strip.
• Formula 1 When 590 MPa ⁇ TS ⁇ 780 MPa, 360 MPa ⁇ YS When 780 MPa ⁇ TS ⁇ 980 MPa, 460 MPa ⁇ YS If 980 MPa ⁇ TS ⁇ 1180 MPa, then 580 MPa ⁇ YS Formula 2: When 590 MPa ⁇ TS ⁇ 780 MPa, 23.0% ⁇ El When 780 MPa ⁇ TS ⁇ 980 MPa, 17.0% ⁇ El When 980 MPa ⁇ TS ⁇ 1180 MPa, 11.0% ⁇ El Formula 3: When 590 MPa ⁇ TS ⁇ 780 MPa, 45% ⁇ ⁇ When 780 MPa ⁇ TS ⁇ 980 MPa, 30% ⁇ ⁇ When 980 MPa ⁇ TS ⁇ 1180 MPa, 20% ⁇ ⁇ Formula 4: When 590 MPa ⁇ TS ⁇ 780 MPa, ⁇ TS ⁇ 40 MPa When 780 MPa ⁇ TS ⁇ 980 MPa,
• the chemical composition of the steel strip is the same as that of the steel sheet.
• the following explanation of the chemical composition of the steel sheet can be applied to the chemical composition of the steel strip. Note that the unit of chemical composition is always “mass%”, but hereinafter, unless otherwise specified, it will be simply expressed as "%".
• C 0.030% or more and 0.250% or less C is an element effective for generating appropriate amounts of fresh martensite, tempered martensite, and retained austenite to ensure a TS of 590 MPa or more.
• the C content is less than 0.030%, the area ratios of the soft phases of unrecrystallized ferrite, recrystallized ferrite, transformed ferrite, epitaxial ferrite, and bainitic ferrite increase excessively, making it difficult to achieve a TS of 590 MPa or more.
• the C content exceeds 0.250%, the area ratio of the hard phases of fresh martensite and tempered martensite increases excessively, resulting in a decrease in El. Therefore, the C content is set to 0.030% or more and 0.250% or less.
• the C content is preferably 0.050% or more.
• the C content is preferably 0.130% or less.
• Si 0.01% or more and 2.50% or less Si promotes the formation of unrecrystallized ferrite and recrystallized ferrite in the soft phase during the temperature rise to the annealing soaking temperature and in the annealing soaking step. That is, Si is an element that affects the area ratio of the soft phase. Here, if the Si content is less than 0.01%, the area ratio of the soft phase decreases, and El decreases. On the other hand, if the Si content exceeds 2.50%, it becomes difficult to generate and grow a large amount of austenite in the rapid heating process, in which rapid heating is performed from the annealing soaking temperature to (annealing soaking temperature + 10°C) or higher at a heating rate of 10°C/sec or higher.
• the Si content is set to 0.01% or more and 2.50% or less.
• the Si content is preferably 0.10% or more.
• the Si content is also preferably 1.80% or less.
• the Mn content is set to 1.30% or more and less than 3.50%.
• the Mn content is preferably 1.60% or more.
• the Mn content is preferably 3.00% or less.
• S 0.0200% or less S exists as sulfides in steel.
• the S content is set to 0.0200% or less.
• the S content is preferably 0.0080% or less.
• the S content is preferably 0.0001% or more.
• Al 0.010% or more and 2.000% or less Al promotes the formation of soft phase unrecrystallized ferrite and recrystallized ferrite during the temperature rise to the annealing soaking temperature and in the annealing soaking step. That is, Al is an element that affects the area ratio of the soft phase. Here, if the Al content is less than 0.010%, the area ratio of the soft phase decreases, and El decreases. On the other hand, if the Al content exceeds 2.000%, it becomes difficult to generate and grow a large amount of austenite in the rapid heating step, in which rapid heating is performed from the annealing soaking temperature to (annealing soaking temperature + 10°C) or higher at a heating rate of 10°C/sec or higher.
• the Al content is set to 0.010% or more and 2.000% or less.
• the Al content is preferably 0.015% or more.
• the Al content is preferably 1.000% or less.
• N 0.0100% or less N exists as nitrides in steel.
• the N content is set to 0.0100% or less.
• the N content is preferably 0.0050% or less.
• the N content is preferably 0.0005% or more.
• the steel sheet according to one embodiment of the present invention has a chemical composition containing the basic chemical components, with the balance other than the basic chemical components including Fe (iron) and unavoidable impurities.
• the base steel sheet of a high-strength steel strip according to one embodiment of the present invention contains the basic chemical components, with the balance consisting of Fe and unavoidable impurities.
• the steel sheet according to one embodiment of the present invention may contain, in addition to the basic chemical components, at least one selected from the optional chemical components listed below.
• Nb 0.200% or less
• Ti 0.200% or less
• V 0.200% or less
• B 0.0100% or less
• Cr 1.000% or less
• Ni 1.000% or less
• Mo 1.000% or less
• Sb 0.200% or less
• Sn: 0.200% or less Cu: 1.000% or less, Ta: 0.200% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200 %, Zr: 0.1000% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and at least one selected from REM: 0.0200% or less
• Nb 0.200% or less Nb forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing TS and YS.
• the Nb content is preferably 0.001% or more, and more preferably 0.005% or more.
• the Nb content exceeds 0.200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the coarse precipitates and inclusions may become the starting points for voids and cracks during tensile testing, which may prevent good local elongation and the desired El from being obtained. Therefore, when Nb is contained, the Nb content is preferably 0.200% or less. The Nb content is more preferably 0.060% or less.
• Ti 0.200% or less Like Nb, Ti forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing TS and YS. To achieve this effect, the Ti content is preferably 0.001% or more. The Ti content is more preferably 0.005% or more. On the other hand, if the Ti content exceeds 0.200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the coarse precipitates and inclusions may become the starting points for voids and cracks during tensile testing, which may prevent good local elongation and the desired El from being obtained. Therefore, when Ti is contained, the Ti content is preferably 0.200% or less. The Ti content is more preferably 0.060% or less.
• V 0.200% or less Like Nb and Ti, V forms fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing TS and YS. To achieve this effect, the V content is preferably 0.001% or more. The V content is more preferably 0.005% or more. On the other hand, if the V content exceeds 0.200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the coarse precipitates and inclusions may become the starting points for voids and cracks during tensile testing, which may prevent good local elongation and the desired El from being obtained. Therefore, when V is contained, the V content is preferably 0.200% or less. The V content is more preferably 0.060% or less.
• B 0.0100% or less
• B is an element that segregates at austenite grain boundaries to improve hardenability.
• B also controls the formation of soft phase transformed ferrite and epitaxial ferrite during cooling after annealing.
• the B content is preferably 0.0001% or more.
• the B content is more preferably 0.0002% or more.
• the B content is preferably 0.0100% or less.
• the B content is more preferably 0.0050% or less.
• Cr 1.000% or less Cr is an element that improves hardenability.
• the addition of Cr increases TS and YS by generating appropriate amounts of hard phases, fresh martensite and tempered martensite.
• the Cr content is preferably 0.0005% or more.
• the Cr content is more preferably 0.010% or more.
• the Cr content is preferably 1.000% or less.
• the Cr content is more preferably 0.800% or less.
• Ni 1.000% or less
• Ni is an element that improves hardenability, and the addition of Ni generates appropriate amounts of hard phases, fresh martensite and tempered martensite, thereby increasing TS and YS.
• the Ni content is preferably 0.005% or more.
• the Ni content is more preferably 0.020% or more.
• the Ni content is preferably 1.000% or less.
• the Ni content is more preferably 0.800% or less.
• Mo 1.000% or less
• Mo is an element that improves hardenability, and the addition of Mo generates appropriate amounts of hard phases, fresh martensite and tempered martensite, thereby increasing TS and YS.
• the Mo content is preferably 0.010% or more.
• the Mo content is more preferably 0.030% or more.
• the Mo content exceeds 1.000%, the area ratio of the hard phase increases, and during tensile testing, voids may be generated and cracks may propagate from the fresh martensite of the hard phase, which may prevent good local elongation and the desired El from being obtained. Therefore, when Mo is contained, the Mo content is preferably 1.000% or less.
• the Mo content is more preferably 0.500% or less, even more preferably 0.450% or less, and even more preferably 0.400% or less.
• Sb 0.200% or less
• Sb is an element that suppresses the diffusion of C near the steel sheet surface during annealing and is effective in controlling the formation of a soft layer near the steel sheet surface. If the soft layer increases excessively near the steel sheet surface, it may be difficult to achieve a TS of 590 MPa or more. Therefore, the Sb content is preferably 0.002% or more. The Sb content is more preferably 0.005% or more. On the other hand, if the Sb content exceeds 0.200%, a soft layer is not formed near the steel sheet surface, the surface layer hardness varies within the steel sheet, and there is a risk of large variations in TS and YS. Therefore, when Sb is contained, the Sb content is preferably 0.200% or less. The Sb content is more preferably 0.020% or less.
• Sn 0.200% or less
• Sn is an element that suppresses the diffusion of C near the steel sheet surface during annealing and is effective in controlling the formation of a soft layer near the steel sheet surface. If the soft layer increases excessively near the steel sheet surface, it may be difficult to achieve a TS of 590 MPa or more. Therefore, the Sn content is preferably 0.002% or more. The Sn content is more preferably 0.005% or more. On the other hand, if the Sn content exceeds 0.200%, a soft layer is not formed near the surface of the steel sheet, the hardness of the surface layer varies within the steel sheet, and there is a risk of large variations in TS and YS. Therefore, when Sn is contained, the Sn content is preferably 0.200% or less. The Sn content is more preferably 0.030% or less.
• Cu 1.000% or less
• Cu is an element that improves hardenability.
• the addition of Cu generates appropriate amounts of hard phases, fresh martensite and tempered martensite, thereby increasing TS and YS.
• the Cu content is preferably 0.005% or more.
• the Cu content is more preferably 0.020% or more.
• the area ratio of the hard phase may increase excessively.
• a large amount of coarse precipitates and inclusions may be formed. In such cases, the excessively formed fresh martensite hard phase and the coarse precipitates and inclusions may cause void formation and crack propagation starting from the fresh martensite hard phase during a tensile test, which may prevent good local elongation and the desired El from being obtained. Therefore, when Cu is contained, the Cu content is preferably 1.000% or less. The Cu content is more preferably 0.300% or less.
• Ta 0.200% or less Like Ti, Nb, and V, Ta increases TS and YS by forming fine carbides, nitrides, or carbonitrides during hot rolling and annealing. In addition, Ta partially dissolves in Nb carbides and Nb carbonitrides to form complex precipitates such as (Nb, Ta)(C, N). This suppresses the coarsening of precipitates and stabilizes precipitation strengthening. To achieve this effect, the Ta content is preferably 0.001% or more. On the other hand, if the Ta content exceeds 0.200%, a large amount of coarse precipitates and inclusions may be generated.
• the Ta content is preferably 0.200% or less.
• the Ta content is more preferably 0.100% or less.
• W 0.500% or less
• W is an element that improves hardenability, and the addition of W increases the formation of large amounts of hard phases of fresh martensite and tempered martensite, thereby increasing TS and YS.
• the W content is preferably 0.001% or more.
• the W content is more preferably 0.010% or more.
• the W content exceeds 0.500%, the area ratio of the hard phase increases, and during a tensile test, voids may be generated and cracks may grow from the fresh martensite of the hard phase, which may prevent good local elongation and the desired El from being obtained. Therefore, when W is contained, the W content is preferably 0.500% or less.
• the W content is more preferably 0.450% or less, and even more preferably 0.400% or less.
• Mg 0.0200% or less
• Mg is an element that is effective in spheroidizing inclusions such as sulfides and oxides and increasing the local elongation of the steel sheet.
• the Mg content is preferably 0.0001% or more.
• the Mg content exceeds 0.0200%, large amounts of coarse precipitates and inclusions may be formed. In such cases, the excessively coarse precipitates and inclusions may cause void formation and crack propagation starting from the fresh martensite hard phase during a tensile test, which may prevent good local elongation and the desired El from being obtained. Therefore, when Mg is added, the Mg content is preferably 0.0200% or less.
• Co 0.0200% or less
• Co is an element that is effective in spheroidizing inclusions and increasing the local elongation of the steel sheet.
• the Co content is preferably 0.0010% or more.
• the Co content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be formed. In such cases, the excessively coarse precipitates and inclusions may cause void formation and crack propagation starting from the fresh martensite hard phase during a tensile test, which may prevent good local elongation and the desired El from being obtained. Therefore, when Co is contained, the Co content is preferably 0.0200% or less.
• Ca 0.0200% or less Ca exists as inclusions in steel.
• the Ca content is preferably 0.0200% or less.
• the Ca content is preferably 0.0040% or less.
• the Ca content is preferably 0.0005% or more due to constraints on production technology, and more preferably 0.0010% or more due to constraints on production technology.
• Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are all effective elements for increasing the local elongation of steel sheet. To achieve this effect, the contents of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are each preferably 0.0001% or more.
• the contents of Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are each 0.0200% or less, and the content of As is 0.0500% or less.
• REM refers to scandium (Sc), which has atomic number 21, yttrium (Y), which has atomic number 39, and the lanthanides from lanthanum (La), which has atomic number 57, to lutetium (Lu), which has atomic number 71.
• the REM content in the present invention refers to the total content of one or more elements selected from the above-mentioned REM. There are no particular limitations on the REM, but La and/or Ce are preferred.
• Impurities are impurities that are inevitably mixed in from raw materials, manufacturing processes, manufacturing equipment, etc., and are permitted to be included to the extent that they do not impair the objectives of the present invention.
• Raw materials include iron ore, reduced iron, scrap, etc.
• Impurities include, for example, O.
• the soft phase referred to here refers to one or more types selected from unrecrystallized ferrite, recrystallized ferrite, transformed ferrite, epitaxial ferrite, and bainitic ferrite.
• the soft phase (one or more types selected from unrecrystallized ferrite, recrystallized ferrite, transformed ferrite, epitaxial ferrite, and bainitic ferrite) is a phase (structure) that improves El.
• the area ratio of the soft phase is set to 30.0% or more.
• the area ratio of the soft phase is preferably 35.0% or more.
• the area ratio of the soft phase is set to 90.0% or less.
• the area ratio of the soft phase is preferably 80.0% or less.
• both the unrecrystallized ferrite and the recrystallized ferrite are ferrites that are generated during the temperature rise up to the annealing soaking temperature and during the annealing soaking.
• the unrecrystallized ferrite is ferrite that has a crystal orientation similar to that of the processed ferrite grains in the structure before heating and includes subgrain boundaries within the crystal grains.
• the recrystallized ferrite is ferrite that has a crystal orientation different from that of the processed ferrite grains in the structure before heating and does not contain subgrain boundaries within the crystal grains.
• Both transformed ferrite and epitaxial ferrite are ferrites that are formed during cooling down to 550°C after the rapid heating step.
• Transformed ferrite and epitaxial ferrite are relatively harder than recrystallized ferrite, and bainitic ferrite is the ferrite that forms during cooling and holding below 550°C after the rapid heating step.
• Bainitic ferrite is ferrite transformed from austenite produced in the annealing soaking process and the rapid heating process, and has a relatively high dislocation density within the crystal grains, and contains fresh martensite, retained austenite, cementite, or carbides such as cementite within the crystal grains.
• the hard phase referred to here refers to one or more selected from fresh martensite and tempered martensite.
• the hard phase fresh martensite and tempered martensite
• the hard phase is a phase (structure) that improves TS and YS.
• the area fraction of the hard phase is set to 10.0% or more.
• the area fraction of the hard phase is preferably 20.0% or more.
• the area ratio of the hard phase is set to 70.0% or less.
• the area ratio of the soft phase is preferably set to 65.0% or less.
• a sample is cut out so that the observation surface is the thickness cross section (L cross section) parallel to the rolling direction of the steel plate.
• the observation surface of the sample is then polished with diamond paste, and then finish-polished using alumina.
• the observation surface of the sample is then etched with 3 vol. % nital to reveal the structure.
• the observation position is then set at 1/4 of the steel plate's thickness, and five fields of view, each measuring 25 ⁇ m x 35 ⁇ m, are observed using an SEM at a magnification of 3000x. From the obtained structure images, Adobe Photoshop from Adobe Systems Inc.
• Recrystallized ferrite An example of recrystallized ferrite is shown in Figures 1a, 1b, and 1c (see symbol F2). Recrystallized ferrite is a black region with a blocky morphology. Recrystallized ferrite contains almost no subgrain boundaries or carbides. Unrecrystallized ferrite: An example of unrecrystallized ferrite is shown in Figure 1a (see symbol F1). Unrecrystallized ferrite is a black region with a blocky shape.
• Unrecrystallized ferrite also contains many subgrain boundaries and may also contain carbides.
• Transformed ferrite An example of transformed ferrite is shown in Figure 1c (see symbol F3). The transformed ferrite is a black region adjacent to fresh martensite and retained austenite.
• Epitaxial ferrite An example of epitaxial ferrite is shown in Figure 1c (see symbol F4). Epitaxial ferrite is a black region that is adjacent to either unrecrystallized ferrite or recrystallized ferrite.
• Bainitic ferrite An example of bainitic ferrite is shown in Figures 1a and 1d (see symbol F5).
• Bainitic ferrite is a region that is black to dark gray and has a blocky or amorphous morphology. Bainitic ferrite also contains carbides, and the amount is relatively small. Tempered martensite: An example of tempered martensite is shown in Figure 1d (see symbol TM). Tempered martensite is a gray region with an amorphous shape. Tempered martensite also contains a relatively large number of carbides. Fresh martensite + retained austenite: Examples of fresh martensite and retained austenite are shown in Figures 1a, 1b, 1c, and 1d (see symbols FM). Fresh martensite and retained austenite are white to light gray regions with amorphous morphology.
• Cementite Cementite is a white region that appears as dots or lines. Cementite is included in tempered martensite (see symbol TM in FIG. 1d).
• Remaining structure In addition to the above-mentioned retained austenite and cementite (carbide), the remaining structure may include pearlite, other precipitates, oxides, etc., and the forms thereof are as known.
• the base steel plate is mechanically ground in the thickness direction (depth direction) to a position 1/4 of the way through the plate thickness, and then chemically polished with oxalic acid to create the observation surface.
• the observation surface is then observed using X-ray diffraction.
• MoK ⁇ rays are used as the incident X-rays, and the ratio of the diffraction intensity of the (200), (220), and (311) planes of fcc iron (austenite) to the diffraction intensity of the (200), (211), and (220) planes of bcc iron is determined, and the volume fraction of retained austenite is calculated from the ratio of the diffraction intensity of each plane.
• the retained austenite is then considered to be three-dimensionally homogeneous, and the volume fraction of retained austenite is taken as the area fraction of retained austenite.
• the area ratio of fresh martensite is determined by subtracting the area ratio of retained austenite from the area ratio of fresh martensite + retained austenite determined as described above.
• [Area ratio of fresh martensite (%)] [Area ratio of fresh martensite + retained austenite (%)] - [Area ratio of retained austenite (%)]
• the area ratio of the remaining structure is determined by subtracting the area ratios of the soft phases (unrecrystallized ferrite, recrystallized ferrite, transformed ferrite, epitaxial ferrite, and bainitic ferrite) and the hard phases (fresh martensite and tempered martensite) determined as described above from 100.0%.
• [Area ratio (%) of remaining structure] 100.0 - [Area ratio (%) of soft phase] - [Area ratio (%) of hard phase]
• the ratio of the hard phases in which the ratio of [C content (mass%) in the hard phase]/[C content (mass%) in the steel sheet (steel strip)] is 1.50 or more is less than 65%, that is, when there are few hard phases with sufficient hardness, TS decreases. Therefore, the area ratio of hard phases having a ratio of [C content (mass%) in hard phases]/[C content (mass%) in steel plate (steel strip)] of 1.50 or more is set to 65% or more of the total hard phases.
• Mn content in the hard phase (hereinafter also simply referred to as Mn content) is low, that is, when austenite with a low Mn content is generated in the rapid heating process, a hard phase with a high C content and a low Mn content is generated in the subsequent cooling process.
• Mn content when austenite with a low Mn content is generated in the rapid heating process, a hard phase with a high C content and a low Mn content is generated in the subsequent cooling process.
• the hardness difference between the structures is large, so voids are likely to occur between the structures, leading to a decrease in the hole expansion ratio and variation in mechanical properties.
• the high-C, low-Mn hard phase is relatively softer than the typical high-C, high-Mn hard phase, the hardness difference between the structures is small, the occurrence of voids is suppressed, and this contributes to an improvement in the hole expansion ratio and a reduction in the variation in mechanical properties.
• the hard phase having a ratio of [Mn content (mass%) in hard phase]/[Mn content (mass%) in steel plate (steel strip)] of 1.30 or less is 10% or more, the above-mentioned effects can be obtained.
• a sample is cut out so that the observation surface is the thickness cross section (L cross section) parallel to the rolling direction of the steel plate.
• the observation surface of the sample is polished with diamond paste, and then finish polished using alumina.
• the characteristic X-ray intensity and concentration of C and Mn are measured at a position 1/4 of the way through the steel plate's thickness.
• the characteristic X-ray intensity and concentration of C and Mn are obtained by mapping analysis (qualitative values) and line analysis (quantitative values) using a field-emission electron probe microanalyzer (FE-EPMA).
• Mapping analysis is performed under the following conditions: a measurement range of 16.6 ⁇ m x 16.6 ⁇ m, 256 x 256 measurement points, an acceleration voltage of 9 kV, and an acquisition time of 200 ms, and the characteristic X-ray intensity (counts: qualitative values) of C and Mn is obtained.
• Linear analysis was performed under conditions of a measurement length of 16.6 ⁇ m, 256 measurement points, an acceleration voltage of 9 kV, and an acquisition time of 2 s, and the concentrations of C and Mn (mass %: quantitative values) were obtained.
• FIG. 2 shows the structure (1) of Inventive Example No. 2 (Steel B) at the measurement position indicated by reference numeral 8 in Fig. 5, as well as the C map (2) and Mn map (3) obtained by EPMA measurement. Also, in Fig. 2, (b) shows the structure (1) of Comparative Example No. 39 (Steel B) at the measurement position indicated by reference numeral 8 in Fig. 5, as well as the C map (2) and Mn map (3) obtained by EPMA measurement. 3a shows an SEM image ((1), backscattered electron image) of Invention Example No. 2 (Steel B) at the measurement position 8 in FIG.
• 3b shows an SEM image ((1), backscattered electron image) of Comparative Example No. 39 (Steel B) at the measurement position indicated by reference numeral 8 in FIG. 5, and the results of line analysis (quantitative) of C (2) and line analysis (quantitative) of Mn (3) obtained by EPMA measurement.
• the soft phase is the white region in (1) of Figure 2(a) and (1) of Figure 2(b).
• the soft layer is the region with a low C content (no peak) in (2) of Figure 3(a) and (2) of Figure 3(b).
• the hard phase with a high C and low Mn composition is the gray region in (1) of Fig. 2(a). Also, the hard phase with a high C and low Mn composition is the region with a high C content and a low Mn content (where a peak of C content exists and no peak of Mn content exists) in (2) and (3) of Fig. 3(a).
• the hard phase with a high C/Mn composition is the region shown in black in (1) of Figure 2(a) and (1) of Figure 2(b). It also refers to the region where both the C content and the Mn content are high (where peaks exist) in (2) and (3) of Figure 3(a) and (2) and (3) of Figure 3(b).
• yield stress (YS) and total elongation (El) of the steel plate and steel strip according to one embodiment of the present invention satisfy the following formulas 1 and 2. This makes it possible to obtain desired collision energy absorption performance and press formability.
• hole expansion ratio ( ⁇ ) of the steel plate and steel strip according to one embodiment of the present invention satisfies the following formula 3.
• Formula 1 When 590 MPa ⁇ TS ⁇ 780 MPa, 360 MPa ⁇ YS When 780 MPa ⁇ TS ⁇ 980 MPa, 460 MPa ⁇ YS If 980 MPa ⁇ TS ⁇ 1180 MPa, then 580 MPa ⁇ YS Formula 2: When 590 MPa ⁇ TS ⁇ 780 MPa, 23.0% ⁇ El When 780 MPa ⁇ TS ⁇ 980 MPa, 17.0% ⁇ El When 980 MPa ⁇ TS ⁇ 1180 MPa, 11.0% ⁇ El Formula 3: When 590 MPa ⁇ TS ⁇ 780 MPa, 45% ⁇ ⁇ When 780 MPa ⁇ TS ⁇ 980 MPa, 30% ⁇ ⁇ When 980 MPa ⁇ TS ⁇ 1180 MPa, 20% ⁇ ⁇
• the tensile strength (TS), yield stress (YS) and total elongation (El) are measured by a tensile test in accordance with JIS Z 2241 (2011) described later in the examples.
• the hole expansion ratio ( ⁇ ) is measured by a hole expansion test in accordance with the Japan Iron and Steel Federation standard JFST 1001.
• the measurement positions of TS, YS, El, and ⁇ on the steel plate are shown in FIG. 4, and the measurement positions of TS, YS, El, and ⁇ in the steel strip (coil) are shown in FIG.
• Formula 4 When 590 MPa ⁇ TS ⁇ 780 MPa, ⁇ TS ⁇ 40 MPa When 780 MPa ⁇ TS ⁇ 980 MPa, ⁇ TS ⁇ 50 MPa When 980 MPa ⁇ TS ⁇ 1180 MPa, ⁇ TS ⁇ 60 MPa
• Formula 5 When 590 MPa ⁇ TS ⁇ 780 MPa, ⁇ TS ⁇ 20 MPa When 780 MPa ⁇ TS ⁇ 980 MPa, ⁇ TS ⁇ 25 MPa When 980 MPa ⁇ TS ⁇ 1180 MPa, ⁇ TS ⁇ 30 MPa
• Formula 6 When 590 MPa ⁇ TS ⁇ 780 MPa, ⁇ YS ⁇ 40 MPa When 780 MPa ⁇ TS ⁇ 980 MPa, ⁇ YS ⁇ 50 MPa When 980 MPa ⁇ TS ⁇ 1180 MPa, ⁇ YS ⁇ 60 MPa
• Formula 7 When 590 MPa ⁇ TS ⁇ 780 MPa, ⁇ YS
• ⁇ TS is the difference between the maximum and minimum values of TS (maximum value of TS ⁇ minimum value of TS) at each of the 1/4 position, 1/2 position, and 3/4 position (see position a with reference numeral 31, position b with reference numeral 32, and position c with reference numeral 33, respectively, in FIG. 4) in the width direction of the steel plate (see reference numeral 21 in FIG. 4, a direction perpendicular to the longitudinal direction 22 of the steel plate).
• ⁇ YS is the difference between the maximum and minimum values of YS (maximum value of YS ⁇ minimum value of YS) at each of the 1/4, 1/2, and 3/4 positions in the width direction of the steel plate.
• ⁇ El is the difference between the maximum and minimum values of El (maximum value of El ⁇ minimum value of El) at each of the 1/4 position, 1/2 position, and 3/4 position in the width direction of the steel sheet.
• ⁇ is the difference between the maximum and minimum values of ⁇ (maximum value of ⁇ minimum value of ⁇ ) at each of the 1/4, 1/2, and 3/4 positions in the width direction of the steel sheet.
• ⁇ TS is the standard deviation of TS at each of the 1/4, 1/2, and 3/4 positions in the width direction of the steel sheet.
• ⁇ YS is the standard deviation of YS at each of the 1/4, 1/2, and 3/4 positions in the width direction of the steel sheet.
• ⁇ El is the standard deviation of El at each of the 1/4 position, 1/2 position, and 3/4 position in the width direction of the steel sheet.
• ⁇ is the standard deviation of ⁇ at each of the 1/4, 1/2, and 3/4 positions in the width direction of the steel sheet.
• ⁇ TS is measured at 1/4, 1/2 and 3/4 positions (refer to reference numerals 31, 32 and 33, respectively, in FIG. 5) in the width direction of the steel strip (see reference numeral 21 in FIG. 4, a direction perpendicular to the longitudinal direction 22 of the steel plate), from the leading end position 23 in the longitudinal direction of the steel strip to the tail end position 24 in the longitudinal direction of the steel plate, at positions 10 m from the leading end position in the longitudinal direction of the steel strip (see reference numerals 1, 6 and 11 at position 41 in FIG. 5).
• the difference between the maximum and minimum values of TS (maximum value of TS - minimum value of TS) at each of the following positions: the 10m position at the tail end (see symbols 5, 10, and 15 at position 45 in Figure 4), the 1/4 position (see symbols 2, 7, and 12 at position 42 in Figure 5), the 1/2 position (see symbols 3, 8, and 13 at position 43 in Figure 4), and the 3/4 position (see symbols 4, 9, and 14 at position 44 in Figure 5).
• ⁇ is the difference between the maximum and minimum values of ⁇ (maximum value of ⁇ - minimum value of ⁇ ) at the 1/4, 1/2 and 3/4 positions in the width direction of the steel strip, and at the leading edge 10 m position, the tail edge 10 m position, the 1/4 position, the 1/2 position and the 3/4 position in the longitudinal direction of the steel strip.
• ⁇ TS is the standard deviation of TS at the 1/4, 1/2 and 3/4 positions in the width direction of the steel strip, and at each of the positions of the leading edge 10 m, the trailing edge 10 m, the 1/4, 1/2 and 3/4 positions in the longitudinal direction of the steel strip.
• ⁇ is the standard deviation of ⁇ at the 1/4, 1/2 and 3/4 positions in the width direction of the steel strip, and at each of the positions of the leading edge 10 m, the trailing edge 10 m, the 1/4, 1/2 and 3/4 positions in the longitudinal direction of the steel strip.
• the steel sheet and steel strip may have a plating layer on their surface.
• Possible types of plating layer include a hot-dip galvanized layer, a galvannealed hot-dip galvanized layer, an electrogalvanized layer, and a hot-dip aluminum plated layer.
• the plating layer may be provided on only one surface of the steel sheet or steel strip, or on both surfaces.
• the hot-dip galvanized layer be composed of, for example, Zn, 20.0 mass% or less Fe, and 0.001 mass% to 1.0 mass% Al.
• the hot-dip galvanized layer may also optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0.0 mass% to 3.5 mass%.
• the Fe content of the hot-dip galvanized layer is more preferably less than 7.0 mass%. The remainder other than the above elements is unavoidable impurities.
• the galvannealed layer is preferably composed of, for example, 20% by mass or less of Fe and 0.001% by mass or more and 1.0% by mass or less of Al.
• the galvannealed layer may optionally contain one or more elements selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0% by mass or more and 3.5% by mass or less.
• the Fe content of the galvannealed layer is more preferably 7.0% by mass or more, and even more preferably 8.0% by mass or more.
• the Fe content of the galvannealed layer is more preferably 15.0% by mass or less, and even more preferably 12.0% by mass or less.
• the electrogalvanized layer is preferably composed primarily of Zn (Zn content of 70.0% or more).
• the electrogalvanized layer may optionally contain one or more elements selected from the group consisting of Fe, Ni, Co, Mn, Cr, Mg, Si, Zr, V, Cu, Pb, Sb, Sn, Ca, Li, Ti, Be, Bi, and REM in a total amount of 0.0% by mass or more but less than 30.0% by mass.
• the remainder other than the above elements is unavoidable impurities.
• the plating weight of the zinc plating layer per side is not particularly limited, but is preferably 20 g/m 2 or more and 80 g/m 2 or less.
• the type of aluminum plating layer is not particularly limited, but a typical example is one that is preferably composed of Al and less than 50.0 mass% Fe.
• the hot-dip aluminum plating layer may optionally contain one or more elements selected from the group consisting of Zn, Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0.0 mass% to 25.0 mass%.
• the Fe content of the hot-dip aluminum plating layer is more preferably less than 25.0 mass%.
• the remainder other than the above elements is unavoidable impurities.
• the plating weight of the aluminum plating layer per side is not particularly limited, but is preferably 20 g/m 2 or more and 120 g/m 2 or less.
• a method for producing a steel strip includes the steps of: a hot rolling step in which hot rolling is performed under conditions where the coiling temperature after finish rolling is 350°C or higher and 650°C or lower to obtain a hot-rolled steel strip; an annealing and soaking step in which the hot-rolled steel strip is heated and annealed and soaked under conditions of an annealing and soaking temperature of 720°C or higher and 860°C or lower and a holding time of 20 seconds or longer; and a rapid heating step in which the hot-rolled steel strip is rapidly heated from the annealing and soaking temperature to (annealing and soaking temperature + 10°C) or higher under conditions of an average heating rate of 10°C/second or higher, and further in which the reached temperatures ⁇ T (maximum reached temperature - minimum reached temperature) at each of the 1 ⁇ 4, 1 ⁇ 2 and 3 ⁇ 4 positions of the width of the steel strip after the rapid heating are 25°C or lower; a cooling step of cooling the steel
• 6e is a schematic diagram showing an example of a heat treatment pattern including an annealing/soaking step (A), a rapid heating step (B), a cooling step (C), a hot-dip aluminum plating treatment step (D3), and a reheating/holding step (E).
• 6a to 6e show two patterns for each of the reheating and holding steps (E).
• the manufacturing method for steel strips is the same as that for steel plates.
• the temperature in the manufacturing method is the surface temperature of the steel plate or steel strip.
• the temperature in the manufacturing method is the average temperature at 1/4, 1/2, and 3/4 positions of the width of the steel plate or steel strip.
• the temperatures in the hot rolling process, annealing and soaking process, rapid heating process and cooling process refer to the temperatures at the leading 10 m position, the tail 10 m position, the 1/4 position, the 1/2 position and the 3/4 position in the longitudinal direction of the steel strip.
• the temperatures in processes other than the hot rolling process, annealing and soaking process, rapid heating process, and cooling process refer to the temperatures at a position halfway along the longitudinal direction of the steel strip.
• the slab heating temperature When heating the slab, it is preferable to set the slab heating temperature to 1100°C or higher from the perspective of dissolving carbides and reducing the rolling load. Furthermore, it is preferable to set the slab heating temperature to 1300°C or lower to prevent increased scale loss.
• the slab heating temperature is the temperature of the slab surface. Furthermore, slabs are made into sheet bars by rough rolling under normal conditions, but if the heating temperature is low, it is preferable to heat the sheet bars using a bar heater or the like before finish rolling in order to prevent problems during hot rolling.
• the finish rolling temperature of the hot rolling is preferably 820° C. or higher. If the finish rolling temperature of the hot rolling is less than 820° C., the rolling load increases, and the reduction ratio in the non-recrystallized state of austenite increases, which may result in the development of an abnormal structure elongated in the rolling direction, resulting in a decrease in El of the final material.
• the coiling temperature after finish rolling is set to 650°C or lower.
• the coiling temperature after finish rolling is preferably 600°C or lower.
• the rough-rolled sheets may be joined together and continuous finish rolling may be performed. Furthermore, in order to reduce the rolling load during hot rolling, some or all of the finish rolling may be performed as lubricated rolling. Performing lubricated rolling is also effective from the perspective of uniforming the shape and material properties of the steel sheet. Furthermore, it is preferable that the coefficient of friction during lubricated rolling be in the range of 0.10 or more and 0.25 or less.
• Annealing soaking temperature 720°C or higher and 860°C or lower If the annealing soaking temperature is lower than 720°C, the proportion of austenite generated during soaking in the two-phase region of ferrite and austenite will be insufficient. As a result, austenite with little Mn enrichment (close to the Mn content of the steel) will account for the majority of the annealing soaking step, and after the subsequent rapid heating step, this will transform into transformed ferrite and epitaxial ferrite in the cooling step. As a result, the desired hard phases (fresh martensite and tempered martensite) will not be generated, and a TS of 590 MPa or higher will not be obtained.
• the holding time includes not only the holding time at the annealing soaking temperature but also the residence time in a temperature range of (annealing soaking temperature - 30°C) or more and (annealing soaking temperature) or less during heating before reaching the annealing soaking temperature.
• a radiant tube furnace is preferably used as the heat treatment furnace for the annealing and soaking process.
• Radiant tube furnaces heat the steel plate with radiant heat from the radiant tubes and furnace walls, so the heat source has a very large volume and high thermal inertia. This makes it difficult to quickly follow changes in the set temperature, and responsiveness to temperature control commands is low.
• the annealing soaking temperature is obtained using a thermometer that measures the surface temperature of the steel sheet.
• a thermometer that measures the surface temperature of the steel sheet.
• a suitable example is a radiation thermometer that measures the temperature by sensing infrared rays emitted by the steel sheet.
• a cover may be provided between the measurement part of the radiation thermometer and the detection part of the steel sheet.
• a multiple reflection measurement method that utilizes the wedge-shaped space between the transport roll in the furnace and the steel sheet may also be used.
• Rapid heating process After the annealing and soaking step (A), in the rapid heating step (B) shown in Figs. 6a to 6e, rapid heating is performed from the annealing and soaking temperature to a temperature equal to or higher than (annealing and soaking temperature + 10°C) under conditions of an average heating rate of 10°C/sec or more, and among the temperatures reached at each of the 1/4 position, 1/2 position and 3/4 position in the width direction of the steel sheet after the rapid heating, (maximum reached temperature - minimum reached temperature) is 25°C or less.
• Rapid heating to (annealing soaking temperature + 10°C) or higher at an average heating rate of 10°C/sec or higher.
• the steel sheet is rapidly heated from the annealing soaking temperature to an ultimate temperature of (annealing soaking temperature + 10°C) or higher.
• the ultimate temperature is preferably (annealing soaking temperature + 20°C) or higher.
• the ultimate temperature is preferably (annealing soaking temperature + 150°C) or lower. If the average heating rate is less than 10°C/s, austenite grains will become coarse and Mn will diffuse into austenite due to ferrite, making it difficult to control the mechanical properties.
• the average heating rate is set to 10°C/s or more.
• the average heating rate is preferably 20°C/s or more, more preferably 30°C/s or more.
• the average heating rate is 30° C./sec or more, the diffusion of Mn from ferrite to austenite is suppressed, and only the diffusion of C from ferrite to austenite is likely to occur.
• the line length can be further shortened.
• the average heating rate is preferably 200°C/sec or less.
• the average heating rate is more preferably 170°C/sec or less, and even more preferably 150°C/sec or less.
• the average heating rate is preferably 120°C/sec or less. If the average heating rate is 170°C/sec or less, the risk of buckling deformation of the steel sheet due to thermal stress can be further reduced.
• the average heating rate is preferably 300°C/sec or less, and more preferably 150°C/sec or less, from the viewpoint of reducing the risk of buckling deformation of the steel sheet due to thermal stress.
• the average heating rate (°C/sec) is obtained by dividing the difference (°C) between the annealing soaking temperature (heating start temperature) and the final temperature in rapid heating by the heating time (sec) from the annealing soaking temperature to the final temperature.
• the annealing soaking temperature (heating start temperature) and ultimate temperature for determining the average heating rate refer to the average temperatures at the 1/4, 1/2, and 3/4 positions in the width direction of the steel sheet.
• the annealing soaking temperature (heating start temperature) and ultimate temperature for determining the average heating rate refer to the average temperatures at the 1/2 position in the longitudinal direction of the steel strip and the 1/4, 1/2, and 3/4 positions in the width direction of the steel strip.
• ⁇ T of the temperature reached maximum value of the temperature reached - minimum value of the temperature reached
• ⁇ T exceeds 25°C
• the ratio of ferrite and austenite generated during heating in the two-phase region of ferrite and austenite becomes non-uniform in the width direction of the steel sheet, and in the subsequent cooling process, variations occur in the ratios of soft phase and hard phase generated, making it impossible to keep all of ⁇ TS, ⁇ YS, ⁇ El, ⁇ , ⁇ TS, ⁇ YS, ⁇ El, and ⁇ within the desired ranges.
• the suitable temperature range of the temperature to be reached is predicted in advance by measurement, calculation, or simulation, and set in consideration of the temperature of the steel sheet in the annealing and soaking step.
• a method for determining the temperature to be reached in the rapid heating step there is a method for determining the temperature to be reached based on the information on the component composition of the steel sheet and the information on the coiling temperature in the hot rolling step.
• 6a to 6e one example is a method in which the ultimate temperature is determined based on information on the composition of the steel sheet and information F0 on the coiling temperature after finish rolling in the hot rolling process in a feedforward control process F.
• the information F0 may include information on the thickness and width of the steel sheet and the reduction rate in cold rolling (cumulative reduction rate), in addition to information on the composition of the steel sheet and information on the coiling temperature after finish rolling in the hot rolling process.
• Another method for determining the temperature to be reached in the rapid heating step is to determine the temperature to be reached based on information about the austenite fraction of the steel sheet measured with a transformation rate meter or the like in a temperature range of 150°C or higher and 600°C or lower after the rapid heating step.
• the ultimate temperature is determined based on information G0 on the austenite fraction of the steel sheet measured with a transformation rate meter or the like in a temperature range of 150°C or higher and 600°C or lower.
• the temperature to be reached in the rapid heating process there is also a method for determining the temperature to be reached based on the information on the chemical composition of the steel sheet described above, information on the coiling temperature in the hot rolling process, and information on the austenite fraction of the steel sheet measured with a transformation rate meter or the like in the temperature range of 150°C to 600°C after the rapid heating process.
• the temperature to be reached may be determined based on both the processing in the feedforward control process (F) and the processing in the feedback control process (G).
• thermometer that measures the surface temperature of the steel plate.
• a suitable example is a radiation thermometer that measures the temperature by sensing infrared rays emitted by the steel plate.
• a cover may be provided between the measurement part of the radiation thermometer and the detection part of the steel plate.
• a multi-reflection measurement method that utilizes the wedge-shaped space between the furnace transport roll and the steel plate may also be used.
• a phase fraction prediction model is constructed, and performance data is collected offline.
• the performance data includes thermal history and phase fractions for a data set of operating conditions with various changes to the operating conditions, and multiple training data sets based on the performance data are prepared in a storage device or the above database.
• the machine learning model for utilizing the above-mentioned performance data is not limited to any particular generation method, as long as it can achieve predictions with the required level of accuracy for practical use.
• commonly used methods such as neural networks (including deep learning), decision tree learning, random forests, and support vector regression may be used.
• An ensemble model combining multiple methods may also be used.
• a machine learning model with binarized output may be used as the phase fraction prediction model, determining whether the phase fraction of the steel sheet is within a predetermined acceptable range (pass or fail) rather than calculating the phase fraction.
• a classification model such as k-nearest neighbor or logistic regression may be used.
• Operating conditions are adjusted based on the phase fraction during annealing obtained by the material quality prediction model and the phase fraction prediction model to obtain steel sheet with excellent mechanical property stability.
• the annealing temperature of the steel sheet can be adjusted, enabling the production of products with stable mechanical properties.
• the material quality prediction model used may be a physical model obtained through offline laboratory experiments or numerical analysis, or a machine learning model obtained from accumulated manufacturing experience.
• examples of zinc plating processes include hot-dip galvanizing, galvannealed hot-dip galvanizing, and electrolytic zinc plating.
• At least one of the following properties was insufficient in the steel strips and steel plates (CR, GI, GA, EG, Al): tensile strength (TS), yield stress (YS), total elongation (El), hole expansion ratio ( ⁇ ), difference between maximum and minimum TS values ( ⁇ TS), difference between maximum and minimum YS values ( ⁇ YS), difference between maximum and minimum El values ( ⁇ El), difference between maximum and minimum ⁇ values ( ⁇ ), standard deviation of TS ( ⁇ TS), standard deviation of YS ( ⁇ YS), standard deviation of El ( ⁇ El), and standard deviation of ⁇ ( ⁇ ).
• the present invention makes it possible to manufacture steel plates and steel strips with a TS of 590 MPa or more but less than 1180 MPa and small variations in mechanical properties (YS, TS, El, ⁇ ), i.e., with excellent stability of mechanical properties.
• Feedforward control process F0 Information on the steel sheet composition, sheet thickness, sheet width, reduction rate in cold rolling, and coiling temperature after finish rolling in hot rolling G Feedback control process G0 Information on austenite fraction a 1/4 position in the width direction of the steel sheet b 1/2 position in the width direction of the steel sheet c 3/4 position in the width direction of the steel sheet 1 At a position 10 m from the tip of the steel strip and at a position 1/4 in the width direction 2 1/4 position in the longitudinal direction and 1/4 position in the width direction of the steel strip 3 1/2 position in the longitudinal direction and 1/4 position in the

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  • 2025-03-26 JP JP2025546396A patent/JPWO2025205979A1/ja active Pending
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