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 zinc-based plated steel sheets and their manufacturing method.
• delayed fracture refers to a phenomenon in which, when a formed part is placed in a hydrogen penetration environment, hydrogen penetrates into the steel plate that constitutes the part, reducing the interatomic bonding strength or causing localized deformation, resulting in microcracks, which then propagate and lead to the destruction of the steel plate.
• Patent Document 1 describes a plated steel sheet that has excellent mechanical properties, reduces the amount of hydrogen that penetrates during manufacturing, and has excellent hydrogen embrittlement resistance and plating adhesion, and a manufacturing method thereof.
• Patent Document 2 describes an ultra-high strength thin steel sheet that has excellent hydrogen embrittlement resistance by controlling the chemical composition and residual austenite of the steel sheet, and a manufacturing method thereof.
• Patent Document 1 and Patent Document 2 do not take into consideration the delayed fracture resistance of stretch flange processed parts, which are subjected to particularly severe processing, and the inventors' investigations have revealed that there is room for improvement. Furthermore, Patent Document 2 recommends that the carbon concentration in the retained austenite be 0.8 mass% or more, but the inventors' investigations have revealed that there is room for improvement in hole expandability.
• the present invention aims to provide a zinc-based plated steel sheet having a tensile strength (TS) of 1470 MPa or more, an elongation (El) of 9.0% or more, a hole expansion ratio ( ⁇ ) of 20% or more, and excellent delayed fracture resistance in stretch flanged areas, and a manufacturing method thereof.
• TS tensile strength
• El elongation
• ⁇ hole expansion ratio
• the present inventors have obtained the following findings. (1) At a position 1/4 of the thickness of the base steel sheet, the sum of the area ratios of tempered martensite and fresh martensite is 70.0% or more, and the sum of the area ratios of ferrite and bainitic ferrite is 10.0% or less, thereby achieving a TS of 1,470 MPa or more.
• the volume fraction of retained austenite is set to 6.0% or more and 20.0% or less, and the total coverage of retained austenite and fresh martensite at prior austenite grain boundaries is set to 20% or more and 45% or less, thereby achieving excellent workability with an elongation (El) of 9.0% or more and a hole expansion ratio ( ⁇ ) of 20% or more, and excellent resistance to delayed fracture of the stretch flange processed portion.
• a steel slab having a predetermined component composition is used, and a holding time or cooling time and cooling rate are controlled in the holding steps or each cooling step after the annealing step and the plating step, whereby a zinc-based plated steel sheet having a structure that satisfies the above (1) and (2) can be obtained.
• the gist of the present invention is as follows:
• the base steel sheet is In mass percent, C: 0.180% or more and 0.250% or less, Si: 0.800% or more and 1.550% or less, Mn: 2.400% or more and 3.200% or less, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, A composition comprising N: 0.0100% or less, and O: 0.0100% or less, with the balance being Fe and unavoidable impurities;
• the sum of the area ratios of tempered martensite and fresh martensite is 70.0% or more and 94.0% or less;
• the volume fraction of retained austenite is 6.0% or more and 20.0% or less,
• a structure in which the total area ratio of ferrite and bainitic ferrite is 10.0% or less, and the area ratio of
• the composition further includes, in mass%, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Zn: 0.100% or less, Pb: 0.100% or less, Te: 0.100% or less, Se: 0.020% or less, Ga: 0.020% or less, Ge: 0.020% or less, Sr: 0.020% or less, Hf: 0.10% or less; and Bi: 0.200% or less;
• the zinc-based plated steel sheet according to the above-mentioned [1],
• the composition further comprises, in mass%, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Zn: 0.100% or less, Pb: 0.100% or less, Te: 0.100% or less, Se: 0.020% or less, Ga: 0.020% or less, Ge: 0.020% or less, Sr: 0.020% or less, Hf: 0.10% or less; and Bi: 0.200% or less;
• 1 is a graph showing the relationship between temperature and time in a method for producing a galvannealed steel sheet according to an embodiment of the present invention.
• a zinc-based plated steel sheet according to an embodiment of the present invention has a base steel sheet and a zinc-based plated layer formed on the surface of the base steel sheet.
• the base steel sheet has a composition containing, in mass%, C: 0.180% to 0.250%, Si: 0.800% to 1.550%, Mn: 2.400% to 3.200%, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, and O: 0.0100% or less, with the balance being Fe and unavoidable impurities.
• % representing the content of a component element of the base steel sheet means “mass%” unless otherwise specified.
• C is one of the important basic components of the base steel sheet, and in particular in the present invention, it is an important element that affects the total area ratio of tempered martensite and fresh martensite, and the total area ratio of ferrite and bainitic ferrite. If the C content is less than 0.180%, the total area ratio of tempered martensite and fresh martensite decreases, and the total area ratio of ferrite and bainitic ferrite increases, making it difficult to achieve a TS of 1470 MPa or more. Therefore, the C content is 0.180% or more, preferably 0.200% or more, and more preferably 0.210% or more.
• the C content is 0.250% or less, and preferably 0.240% or less.
• Si is one of the important basic components of the base steel sheet and is an important element that affects the TS and the volume fraction of the retained austenite. If the Si content is less than 0.800%, the strength of the tempered martensite and the fresh martensite decreases, making it difficult to achieve a TS of 1470 MPa or more. Therefore, the Si content is 0.800% or more, preferably 0.850% or more, and more preferably 0.900% or more. On the other hand, if the Si content exceeds 1.550%, the retained austenite increases excessively and the hole expandability decreases. Therefore, the Si content is 1.550% or less, preferably 1.500% or less, and more preferably 1.400% or less.
• Mn is one of the important basic components of the base steel sheet, and is an important element that affects the total area ratio of tempered martensite and fresh martensite, and the total area ratio of ferrite and bainitic ferrite. If the Mn content is less than 2.400%, the total area ratio of tempered martensite and fresh martensite decreases, and the total area ratio of ferrite and bainitic ferrite increases, making it difficult to achieve a TS of 1470 MPa or more. Therefore, the Mn content is 2.400% or more, preferably 2.500% or more, and more preferably 2.600% or more.
• the Mn content is 3.200% or less, preferably 3.100% or less, and more preferably 3.000% or less.
• the P content is set to 0.100% or less, and preferably 0.070% or less.
• the P content is preferably set to 0.001% or more.
• the S content is set to 0.0200% or less, and preferably 0.0050% or less.
• the S content be 0.0001% or more due to constraints on production technology.
• Al 1.000% or less
• the Al content is set to 1.000% or less, and preferably 0.500% or less.
• the Al content is preferably set to 0.001% or more.
• N 0.0100% or less
• the N content is set to 0.0100% or less, and preferably 0.0050% or less.
• the N content is 0.0001% or more due to constraints on production technology.
• the O content is set to 0.0100% or less, and preferably 0.0050% or less.
• the O content be 0.0001% or more due to constraints on production technology.
• the base steel sheet further contains, by mass%, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.
• Ti 0.200% or less
• Nb 0.200% or less
• V 0.200% or less
• the contents of Ti, Nb, and V are 0.200% or less, large amounts of coarse precipitates or inclusions are not generated, and the ultimate deformability of the base steel sheet is not reduced, so that ⁇ and bendability are not reduced. Therefore, when any one or more of Ti, Nb, and V are contained, the contents are each set to 0.200% or less, and preferably 0.100% or less.
• these elements form fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing, thereby increasing the strength of the base steel sheet, so that the contents of Ti, Nb, and V are each preferably set to 0.001% or more.
• the lower limit of the content of Zr, Zn, Pb, and Te is not particularly specified, these elements spheroidize the shape of nitrides or sulfides, etc., and improve the ultimate deformability of the base steel sheet, so that the content of Zr, Zn, Pb, and Te is preferably 0.001% or more.
• Bi 0.200% or less
• the Bi content is 0.200% or less, the amount of coarse precipitates or inclusions will not increase, and the base steel sheet will not be embrittled, so the delayed fracture resistance of the stretch flanged portion will not be reduced. Therefore, when Bi is contained, its content is set to 0.200% or less, and preferably 0.100% or less.
• the Bi content is preferably set to 0.001% or more.
• the base steel sheet according to one embodiment of the present invention contains the above basic components, with the balance consisting of Fe (iron) and unavoidable impurities.
• the base steel sheet according to one embodiment of the present invention contains only the above basic components and the balance, with the balance consisting of Fe (iron) and unavoidable impurities.
• the base steel sheet has a structure in which, at a depth from the surface of the base steel sheet to a position of 1/4 of the sheet thickness, the total area ratio of tempered martensite and fresh martensite is 70.0% to 94.0%, the volume ratio of retained austenite is 6.0% to 20.0%, the total area ratio of ferrite and bainitic ferrite is 10.0% or less, and the area ratio of the remaining structure is 10.0% or less.
• Tempered martensite has a substructure (lath boundary, block boundary) and is a structure in which carbides are precipitated with multiple variants. Note that the volume ratio of retained austenite is almost equal to the area ratio, so in this invention it is treated as being equivalent to the area ratio.
• volume fraction of retained austenite 6.0% or more and 20.0% or less
• the volume fraction of the retained austenite is set to 6.0% or more, preferably 6.5% or more, and more preferably 7.0% or more.
• the volume fraction of the retained austenite is set to 20.0% or less, preferably 15.0% or less, and more preferably 13.0% or less.
• the area ratios of ferrite and bainitic ferrite can be determined as follows. After polishing the L-section of the base steel sheet, it is etched with 3 vol. % nital and a position at 1/4 of the sheet thickness is observed in 10 fields of view at a magnification of 3000 times using an SEM. In the observed structural images, the ferrite and bainitic ferrite are recessed and the inside of the structure is flat. The area ratios of ferrite and bainitic ferrite are determined in each field of view and the average of these values is taken as the total area ratio of ferrite and bainitic ferrite.
• Fresh martensite is formed by the transformation of untransformed austenite during final cooling. Therefore, the carbon concentration in the fresh martensite that covers the prior austenite grain boundaries is equal to or lower than the carbon concentration in the retained austenite.
• the hole expansion ratio ( ⁇ ) of the zinc-based plated steel sheet is 20% or more.
• the upper limit of the hole expansion ratio of the zinc-based plated steel sheet is not particularly limited, but is generally 50% or less.
• the hole expansion ratio ( ⁇ ) can be obtained as follows. A hole expansion test is performed in accordance with JIS Z 2256. After the test material is sheared to 100 mm x 100 mm, a hole with a diameter of 10 mm is punched with a clearance of 12.5%.
• ⁇ (%) ⁇ (D f - D 0 )/D 0 ⁇ 100 (5)
• Df is the hole diameter at the time of crack initiation (mm)
• D0 is the initial hole diameter (mm).
• the zinc-based plated steel sheet according to the present invention has excellent delayed fracture resistance properties of the stretch flanged portion.
• the delayed fracture resistance properties of the stretch flanged portion can be evaluated as follows. The above-mentioned hole expansion test is carried out within 30 days after the zinc-based plated steel sheet (test material) is produced. A photograph of the stretch flanged portion of the test material immediately after the hole expansion test is taken at a magnification of 20 times using a digital microscope (RH-2000: manufactured by Hirox). Thereafter, the test material is left to stand at room temperature (15 to 25 ° C.) for 24 hours, and the stretch flanged portion is observed again with the digital microscope.
• the photograph of the stretch flanged portion taken immediately after the hole expansion test is compared with the photograph of the stretch flanged portion after 24 hours, and it can be determined that the stretch flanged portion has excellent delayed fracture resistance properties when no increase or progression of cracks is observed.
• FIG. 1 shows a graph showing the relationship between temperature and time in the manufacturing method of a galvannealed steel sheet according to an embodiment of the present invention.
• the broken line in the graph shows the temperature change of the steel sheet from the annealing step to the tempering step.
• a first cooling step is performed in which the cold-rolled steel sheet is cooled to a temperature T1.
• a holding step is performed in which the cold-rolled steel sheet is held at T1 (°C), or a second cooling step is performed in which the cold-rolled steel sheet is cooled to a temperature T2.
• FIG. 1 shows a case in which the second cooling step is performed.
• a third cooling step is performed in which the cold-rolled steel sheet is cooled to a temperature T3.
• the cold-rolled steel sheet is subjected to a zinc-based plating process to obtain a plated steel sheet. Since the time for the hot-dip galvanizing process is short compared with the cooling process, etc., it is shown as a point at temperature T3 in Fig. 1.
• the plated steel sheet is heated and held at the alloying process temperature.
• a fourth cooling process is performed in which the plated steel sheet is cooled to a temperature T4.
• a fifth cooling process is performed in which the plated steel sheet is cooled to a cooling stop temperature T5.
• a tempering process is performed in which the plated steel sheet is held at a tempering temperature higher than T5 (°C) and not higher than 350°C.
• the obtained cold-rolled steel sheet is subjected to an annealing process.
• the annealing temperature is less than Ac3 (°C) defined by the following formula (1), the area ratio of tempered martensite and fresh martensite decreases, and the total area ratio of ferrite and bainitic ferrite increases, making it difficult to achieve a TS of 1470 MPa or more. Therefore, the annealing temperature is Ac3 (°C) or more, and Ac3 + 20 (°C) or more is preferable.
• the annealing temperature is 920°C or less, the energy efficiency does not decrease, so that the heating cost can be suitably prevented from increasing and the damage to the furnace body can be suitably prevented. Therefore, the annealing temperature is preferably 920°C or less.
• Ac3 (°C) 881-205.7 ⁇ [%C]+53.1 ⁇ [%Si]-15 ⁇ [%Mn]-27 ⁇ [%Cu]-20.1 ⁇ [%Ni]-0.7 ⁇ [%Cr]+41.1 ⁇ [%Mo]...(1)
• [% X] indicates the content (mass%) of element X in the composition, and is set to 0 when the composition does not contain element X.
• the holding time at the annealing temperature is set to 10 seconds or more, and 40 seconds or more is preferable.
• the holding time at the annealing temperature is 500 seconds or less, it is possible to effectively prevent an increase in heating costs and an increase in manufacturing time, that is, a decrease in productivity can be effectively prevented. Therefore, the holding time at the annealing temperature is preferably 500 seconds or less.
• First cooling step cooling to a temperature T1 of (3Bs-Ms)/3 (°C) or more and 0.95 x Bs (°C) or less at a cooling rate of 5°C/s or more]
• the cold-rolled steel sheet is subjected to a first cooling step. That is, the end of the annealing step is the start of the first cooling step.
• the first cooling step if the cooling rate is less than 5 ° C./s, the area ratio of tempered martensite and fresh martensite decreases, and the area ratio of ferrite and bainitic ferrite increases, making it difficult to achieve a TS of 1470 MPa or more.
• the cooling rate of the first cooling step is 5 ° C./s or more, and preferably 7 ° C./s or more.
• the cooling stop temperature can be suitably controlled. Therefore, the cooling rate of the first cooling step is preferably 20 ° C./s or less.
• T1 of the first cooling step is less than (3Bs-Ms)/3 (°C) using Bs and Ms defined by the following formulas (2) and (3), respectively. Therefore, cooling occurs at a position lower than the bainite nose. Therefore, the nucleation of ferrite and bainitic ferrite does not proceed, the total coverage of the retained austenite and fresh martensite at the old austenite grain boundary becomes excessive, and it becomes difficult to achieve a hole expansion ratio of 20% or more. Therefore, T1 is set to (3Bs-Ms)/3 (°C) or more. On the other hand, when T1 exceeds 0.95 x Bs (°C), cooling occurs at a position higher than the bainite nose.
• Cooling step or second cooling step holding at T1 (°C) or cooling at a cooling rate of 1.50°C/s or less for a time t (s) satisfying f of 0.010 or more and 0.200 or less]
• the cold-rolled steel sheet is subjected to a holding step in which the cold-rolled steel sheet is held at a temperature T1 (°C), or a second cooling step in which the cold-rolled steel sheet is cooled to a temperature T2.
• the holding temperature is set to T1 (°C), which suppresses the total coverage rate of the retained austenite and fresh martensite at the prior austenite grain boundaries from becoming excessive, and excellent hole expandability is obtained.
• the cooling rate of the second cooling step is 1.50°C/s or less.
• the cooling rate of the second cooling step is preferably 0.50°C/s or more.
• the holding time in the holding step or the cooling time in the second cooling step is a time t (s) in which f in the following formula (4) satisfies 0.010 or more and 0.200 or less.
• f is less than 0.010, nucleation of ferrite and bainitic ferrite does not occur at the old austenite grain boundary, and the coverage of the retained austenite and fresh martensite at the old austenite grain boundary increases excessively, making it difficult to achieve excellent hole expandability. Therefore, the time t (s) is a value in which f in formula (4) satisfies 0.010 or more.
• the series of processes from the annealing process to the plating process is not particularly limited, but from the viewpoint of productivity, it is preferable to carry out the processes in a continuous galvanizing line (CGL), which is a hot-dip galvanizing line.
• CGL continuous galvanizing line
• T4 of the fourth cooling step is less than Ms-200 (°C)
• the volume fraction of untransformed austenite will decrease, and the amount of retained austenite in the final structure will decrease, making it difficult to achieve excellent ductility. Therefore, T4 should be Ms-200 (°C) or higher.
• T4 exceeds Ms-80 (°C)
• the total area fraction of tempered martensite and fresh martensite will decrease, making it difficult to achieve a TS of 1,470 MPa or more. Therefore, T4 should be Ms-80 (°C) or lower.
• the plated steel sheet is subjected to a fifth cooling step. If the cooling rate in the fifth cooling step exceeds 3.0°C/s, the diffusion of carbon into untransformed austenite during the fifth cooling step does not proceed sufficiently, the volume fraction of retained austenite in the final structure decreases, and it becomes difficult to achieve excellent ductility. Therefore, the cooling rate in the fifth cooling step is set to 3.0°C/s or less, preferably 2.0°C/s or less, and more preferably 1.5°C/s or less. On the other hand, if the fifth cooling rate is 0.5°C/s or more, this is preferable because cooling can be performed without heating in a furnace. Therefore, the cooling rate in the fifth cooling step is preferably 0.5°C/s or more.
• T5 is set to 100°C or higher, preferably 110°C or higher, and more preferably 120°C or higher. Since the fifth cooling step is performed following the fourth cooling step, T5 will be less than T4 (°C). From the viewpoint of reducing fresh martensite, T5 is preferably Ms-100 (°C) or lower.
• the plated steel sheet is subjected to a tempering step.
• the plated steel sheet is reheated and tempered to stabilize the untransformed austenite. If the tempering temperature is T5 (°C) or lower, the desired amount of retained austenite is not obtained, and the ductility decreases. Therefore, the tempering temperature is set to be higher than T5 (°C), and preferably T5+50 (°C) or higher.
• the holding time at the tempering temperature in the tempering process is less than 5 seconds, the austenite will not be stabilized sufficiently, will transform into martensite during final cooling, and the volume fraction of the retained austenite in the final structure will decrease, making it difficult to achieve excellent ductility. Furthermore, since the tempering of the martensite is insufficient, it will be difficult to achieve excellent hole expandability. Therefore, the holding time at the tempering temperature should be 5 seconds or more, and 40 seconds or more is preferable. On the other hand, if the holding time at the tempering temperature exceeds 1000 seconds, the tempering will proceed excessively, reducing the strength and simultaneously causing the retained austenite to decompose, making it difficult to achieve an El of 9% or more. Therefore, the holding time at the tempering temperature should be 1000 seconds or less, and 800 seconds or less is preferable.
• the cooling rate of the second cooling step was expressed as 0.00 ° C. / s.
• the plating treatments shown in Table 2 are represented as "GI” for hot-dip galvanizing treatment, "GA” for alloyed hot-dip galvanizing treatment, and "EG” for electrolytic galvanizing treatment.
• the total area ratio of tempered martensite and fresh martensite, the volume ratio of retained austenite, the total area ratio of ferrite and bainitic ferrite, and the area ratio of the remaining structure were each determined by the above-mentioned method. Furthermore, at a position 1/4 of the way through the thickness of the test material, the total coverage ratio of retained austenite and fresh martensite at the prior austenite grain boundaries, and the carbon concentration in the retained austenite covering the prior austenite grain boundaries were each determined. The measurement results are shown in Table 3.
• the examples of the present invention have a tensile strength of 1470 MPa or more, an elongation of 9.0% or more, and a hole expansion ratio of 20% or more, and are excellent in the delayed fracture resistance of the stretch flange processed part.
• the comparative examples are inferior in one or more of the tensile strength, elongation, hole expansion ratio, and delayed fracture resistance of the stretch flange processed part.
• the present invention provides a zinc-based plated steel sheet and its manufacturing method that has a tensile strength of 1470 MPa or more, an elongation of 9.0% or more, a hole expansion ratio of 20% or more, and excellent delayed fracture resistance in the stretch flange processed portion.

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