US20240124964A1 - Galvanized steel sheet and member, and method for manufacturing same - Google Patents

Galvanized steel sheet and member, and method for manufacturing same Download PDF

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Publication number
US20240124964A1
US20240124964A1 US18/546,428 US202218546428A US2024124964A1 US 20240124964 A1 US20240124964 A1 US 20240124964A1 US 202218546428 A US202218546428 A US 202218546428A US 2024124964 A1 US2024124964 A1 US 2024124964A1
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steel sheet
galvanized
area ratio
galvanized steel
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Inventor
Sho HIGUCHI
Yoshiyasu Kawasaki
Tatsuya Nakagaito
Tomomi KANAZAWA
Shunsuke Yamamoto
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JFE Steel Corp
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JFE Steel Corp
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Assigned to JFE STEEL CORPORATION reassignment JFE STEEL CORPORATION ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: HIGUCHI, Sho, KANAZAWA, Tomomi, KAWASAKI, YOSHIYASU, NAKAGAITO, TATSUYA, YAMAMOTO, SHUNSUKE
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    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
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    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
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    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
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Definitions

  • the present disclosure relates to a galvanized steel sheet, a member made from the galvanized steel sheet, and a method of producing same.
  • anti-crash properties in addition to excellent anti-crash properties in the event of a collision while a vehicle is in motion.
  • steel sheets used as material of automotive components are often galvanized. Therefore, developing galvanized steel sheets that have high strength as well as excellent anti-crash properties is desirable.
  • JP 3887235 B (PTL 1) describes:
  • JP 5953693 B (PTL 2) describes:
  • JP 6052472 B (PTL 3) describes:
  • YS yield stress
  • TS and YS of a steel sheet generally reduces formability, and particularly ductility, strain hardenability, and hole expansion formability. These properties correlate with a member's resistance to cracking in bending crush tests and axial crush tests that simulate crash tests. Therefore, when steel sheets having higher TS and YS are applied to the impact energy absorbing members of automobiles, the steel sheets are not only more difficult to form, but also crack in tests that simulate crash tests. In other words, actual impact absorption energy is not as high as might be assumed from the YS value. Therefore, the impact energy absorbing members are currently limited to steel sheets having a TS of 590 MPa grade. Strain hardenability and hole expansion formability are correlated with stretch formability and stretch flangeability, respectively.
  • the steel sheets described in PTL 1-3 have TS: 980 MPa or more, and cannot be said to have high YS, excellent ductility, strain hardenability, and hole expansion formability.
  • the present disclosure was developed in view of the current situation mentioned above, and it would be helpful to provide a galvanized steel sheet having TS: 980 MPa or more, high YS, excellent ductility, strain hardenability, and hole expansion formability, and an advantageous method of producing same.
  • the inventors conducted intensive studies to solve the technical problem outlined above.
  • a galvanized steel sheet comprising a base steel sheet and a galvanized layer on a surface of the base steel sheet, wherein the base steel sheet comprises:
  • a method of producing a galvanized steel sheet comprising:
  • a method of producing a member wherein the galvanized steel sheet of any one of aspects 1 to 8 is subjected to at least one of a forming process and a joining process to make the member.
  • the galvanized steel sheet having a TS of 980 MPa or more, high YS, excellent ductility, strain hardenability, and hole expansion formability is obtainable. Further, the member made from the galvanized steel sheet of the present disclosure has high strength and excellent anti-crash properties, and is therefore extremely advantageous for use in impact energy absorbing members of automobiles.
  • FIG. 1 A illustrates an example of a microstructure image from a scanning electron microscope (SEM) used for microstructure identification
  • FIG. 1 B is a color-coded microstructure image of FIG. 1 A made using Adobe Photoshop by Adobe Systems Inc.;
  • FIG. 2 A illustrates an example of a microstructure image from a SEM used to identify island regions in a hard secondary phase, in particular including island regions identified as MA1, and FIG. 2 B is a color-coded microstructure image of FIG. 2 A made using Adobe Photoshop by Adobe Systems Inc.;
  • FIG. 3 A illustrates an example of a microstructure image from a SEM used to identify island regions in the hard secondary phase, in particular including island regions identified as MA2, and FIG. 3 B is a color-coded microstructure image of FIG. 3 A made using Adobe Photoshop by Adobe Systems Inc.;
  • FIG. 4 A illustrates an example of a microstructure image from a SEM used to identify island regions in the hard secondary phase, in particular including island regions identified as MA3, and FIG. 4 B is a color-coded microstructure image of FIG. 4 A made using Adobe Photoshop by Adobe Systems Inc.; and
  • FIG. 5 A is a schematic diagram illustrating a method of evaluating resistance weld crack resistance at a welded portion
  • an upper part of FIG. 5 B illustrates a top view of a sheet combination after the resistance spot welding of the evaluation
  • a lower part of FIG. 5 B illustrates a cross-section taken along the line A-A in the upper part.
  • C is an element effective for securing TS of 980 MPa or more and high YS by generating appropriate amounts of fresh martensite, tempered martensite, bainitic ferrite and retained austenite.
  • the C content is less than 0.050%, an area ratio of ferrite increases, making achieving a TS of 980 MPa or more difficult. This also leads to a decrease in YS.
  • the C content exceeds 0.400%, carbon concentration in the retained austenite increases excessively. Therefore, when a steel sheet is punched, hardness of fresh martensite formed from retained austenite increases significantly. As a result, crack propagation during hole expansion is accelerated in the steel sheet after the punching process (that is, leading to a reduction in hole expansion formability).
  • the C content is 0.050% or more and 0.400% or less.
  • the C content is preferably 0.100% or more. Further, the C content is preferably 0.300% or less.
  • Si 0.20% or More and 3.00% or Less
  • Si suppresses carbide formation during annealing and promotes formation of retained austenite.
  • Si is an element that affects an area ratio of retained austenite and carbon concentration in the retained austenite.
  • Si content is less than 0.20%, an area ratio of retained austenite decreases and ductility is reduced.
  • the Si content exceeds 3.00%, the area ratio of ferrite increases excessively and achieving a TS of 980 MPa or more becomes difficult. This also leads to a decrease in YS.
  • the carbon concentration in the retained austenite increases excessively. Therefore, when a steel sheet is punched, hardness of fresh martensite formed from retained austenite increases significantly. As a result, crack propagation during hole expansion is accelerated in the steel sheet after the punching process (that is, leading to a reduction in hole expansion formability).
  • the Si content is 0.20% or more and 3.00% or less.
  • the Si content is preferably 0.40% or more.
  • the Si content is preferably 2.00% or less, since there is concern about a decrease in resistance weld crack resistance when the Si content exceeds 2.00%.
  • Mn 1.00% or more and less than 3.50%
  • Mn is an element that adjusts area ratios such as those of bainitic ferrite and tempered martensite.
  • Mn content is less than 1.00%
  • the area ratio of ferrite increases excessively, making achieving a TS of 980 MPa or more difficult. This also leads to a decrease in YS.
  • the Mn content is 3.50% or more, the area ratio of bainitic ferrite decreases and the area ratio of tempered martensite increases excessively. As a result, desired ductility is not achieved.
  • the Mn content is 1.00% or more and less than 3.50%.
  • the Mn content is preferably 1.80% or more. Further, the Mn content is preferably less than 3.20%.
  • P is an element that acts as a solid solution strengthener and increases steel sheet strength. To achieve this effect, P content is 0.001% or more. On the other hand, when the P content exceeds 0.100%, P segregates to a prior austenite grain boundary and embrittles the grain boundary. Therefore, when steel sheets are punched, an amount of void formation increases, leading to a decrease in hole expansion formability.
  • the P content is 0.001% or more and 0.100% or less.
  • the P content is preferably 0.030% or less.
  • S is present in steel as sulfide.
  • S content exceeds 0.0200%, steel sheet ultimate deformability is reduced. Therefore, when steel sheets are punched, an amount of void formation increases, leading to a decrease in hole expansion formability.
  • the S content is therefore 0.0200% or less.
  • the S content is preferably 0.0080% or less.
  • a lower limit of S content is not particularly specified, the S content is preferably 0.0001% or more in view of production technology constraints.
  • Al suppresses carbide formation during annealing and promotes formation of retained austenite.
  • Al is an element that affects the area ratio of retained austenite and the carbon concentration in the retained austenite.
  • the Al content is 0.010% or more.
  • the Al content exceeds 2.000%, the area ratio of ferrite increases excessively, making achieving a TS of 980 MPa or more difficult. This also leads to a decrease in YS.
  • the Al content is 0.010% or more and 2.000% or less.
  • the Al content is preferably 0.015% or more. Further, the Al content is preferably 1.000% or less.
  • N is present in steel as nitride.
  • N content exceeds 0.0100%, steel sheet ultimate deformability is reduced. Therefore, when steel sheets are punched, an amount of void formation increases, leading to a decrease in hole expansion formability.
  • the N content is therefore 0.0100% or less. Further, the N content is preferably 0.0050% or less. Although a lower limit of N content is not particularly specified, the N content is preferably 0.0005% or more in view of production technology constraints.
  • the carbon equivalent Ceq affects TS.
  • the carbon equivalent Ceq is less than 0.540%, achieving a TS of 980 MPa or more becomes difficult.
  • the carbon equivalent Ceq is therefore 0.540% or more.
  • the [element symbol %] in the above formula represents the content (mass %) of the element in the chemical composition of the base steel sheet. Elements not included in the chemical composition of the base steel sheet are calculated as 0.
  • the base steel sheet of the galvanized steel sheet according to an embodiment of the present disclosure includes the basic composition above, with the balance being Fe (iron) and inevitable impurity.
  • the base steel sheet of the galvanized steel sheet according to an embodiment of the present disclosure preferably has a chemical composition including the basic composition above, with the balance being Fe and inevitable impurity.
  • the base steel sheet of the galvanized steel sheet according to an embodiment of the present disclosure may contain at least one of the components selected from the group listed below.
  • the components listed below do not have a lower limit because the effect of the present disclosure is obtainable whenever content is equal to or less than the upper limit indicated below. When any of the following components are included below an appropriate lower limit described below, such a component is included as an inevitable impurity.
  • Ti causes TS to increase due to formation of fine carbides, nitrides, and carbonitrides during hot rolling and annealing.
  • Ti content is preferably 0.001% or more.
  • the Ti content is more preferably 0.005% or more.
  • the Ti content exceeds 0.200%, a large amount of coarse precipitates and inclusions may be formed. In such cases, when diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions may become initiation points of cracks during hole expanding tests, that is, hole expansion formability may decrease. Therefore, when Ti is included, the Ti content is preferably 0.200% or less.
  • the Ti content is more preferably 0.060% or less.
  • Nb like Ti, causes TS to increase due to formation of fine carbides, nitrides, and carbonitrides during hot rolling and annealing.
  • Nb content is preferably 0.001% or more.
  • the Nb content is more preferably 0.005% or more.
  • the Nb content exceeds 0.200%, a large amount of coarse precipitates and inclusions may be formed. In such cases, when diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions may become initiation points of cracks during hole expanding tests, that is, hole expansion formability may decrease. Therefore, when Nb is included, the Nb content is preferably 0.200% or less.
  • the Nb content is more preferably 0.060% or less.
  • V like Ti and Nb, causes TS to increase due to formation of fine carbides, nitrides, and carbonitrides during hot rolling and annealing.
  • V content is preferably 0.001% or more.
  • the V content is more preferably 0.005% or more.
  • the V content exceeds 0.100%, a large amount of coarse precipitates and inclusions may be formed. In such cases, when diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions may become initiation points of cracks during hole expanding tests, that is, hole expansion formability may decrease. Therefore, when V is included, the V content is preferably 0.100% or less.
  • the V content is more preferably 0.060% or less.
  • B is an element that increases hardenability by segregating at an austenite grain boundary. Further, B is an element that suppresses ferrite formation and grain growth during cooling after annealing. To obtain this effect, B content is preferably 0.0001% or more. The B content is more preferably 0.0002% or more. On the other hand, when the B content exceeds 0.0100%, cracking may occur inside the steel sheet during hot rolling, which may reduce the ultimate deformability of the steel sheet. Further, the reduction in the ultimate deformability of the steel sheet increases an amount of voids generated when the steel sheet is punched, leading to a reduction in hole expansion formability. Therefore, when B is included, the B content is preferably 0.0100% or less. The B content is more preferably 0.0050% or less.
  • Cu is an element that increases hardenability.
  • Cu is an element that is effective for adjusting the area ratio of hard fresh martensite and the like to a more suitable range, and thereby adjusting TS to a more suitable range.
  • Cu content is preferably 0.005% or more.
  • the Cu content is more preferably 0.020% or more.
  • the area ratio of fresh martensite increases excessively and TS becomes excessively high.
  • a large amount of coarse precipitates and inclusions may be formed. In such cases, when diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions may become initiation points of cracks during tensile testing, that is, hole expansion formability may decrease. Therefore, when Cu is included, the Cu content is preferably 1.000% or less.
  • the Cu content is more preferably 0.200% or less.
  • the Cr content is preferably 0.0005% or more.
  • the Cr content is more preferably 0.010% or more.
  • the Cr content is preferably 0.250% or less.
  • the Cr content is even more preferably 0.100% or less.
  • Ni is an element that increases hardenability. Further, Ni is an element that is effective for adjusting the area ratio of retained austenite and fresh martensite to a more suitable range, and thereby adjusting TS to a more suitable range. To obtain this effect, Ni content is preferably 0.005% or more. The Ni content is more preferably 0.020% or more. On the other hand, when the Ni content exceeds 1.000%, the area ratio of fresh martensite may increase excessively, and may cause reduced ductility and dimensional accuracy during forming. Further, a large amount of coarse precipitates and inclusions may be formed. In such cases, when diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions may become initiation points of cracks during hole expanding tests, that is, hole expansion formability may decrease. Therefore, when Ni is included, the Ni content is preferably 1.000% or less. The Ni content is more preferably 0.800% or less.
  • Mo is an element that increases hardenability. Further, Mo is an element effective for formation of hard fresh martensite and the like. To obtain this effect, Mo content is preferably 0.010% or more. The Mo content is more preferably 0.030% or more. On the other hand, when the Mo content exceeds 0.500%, the area ratio of fresh martensite may increase excessively, leading to a decrease in hole expansion formability. Therefore, when Mo is included, the Mo content is preferably 0.500% or less. The Mo content is more preferably 0.450% or less. The Mo content is even more preferably 0.400% or less.
  • Sb is an element effective for suppressing diffusion of C in the vicinity of the steel sheet surface during annealing and for controlling the formation of a soft layer in the vicinity of the steel sheet surface.
  • Sb content is preferably 0.002% or more.
  • the Sb content is more preferably 0.005% or more.
  • the Sb content is preferably 0.200% or less.
  • the Sb content is more preferably 0.020% or less.
  • Sn is an element effective for suppressing the diffusion of C in the vicinity of the steel sheet surface during annealing and for controlling the formation of a soft layer in the vicinity of the steel sheet surface.
  • Sn content is preferably 0.002% or more.
  • the Sn content is more preferably 0.005% or more.
  • the Sn content is preferably 0.200% or less.
  • the Sn content is more preferably 0.020% or less.
  • Ta like Ti, Nb, and V, causes TS to increase due to formation of fine carbides, nitrides, and carbonitrides during hot rolling and annealing. Further, Ta is partially solid-soluble in Nb carbides and Nb carbonitrides to form composite precipitates such as (Nb, Ta) (C, N). This suppresses coarsening of precipitates and stabilizes strengthening by precipitation. This improves TS and YS. To obtain this effect, Ta content is preferably 0.001% or more. On the other hand, when the Ta content exceeds 0.100%, a large amount of coarse precipitates and inclusions may be formed.
  • the Ta content is preferably 0.100% or less.
  • W is an element effective for increasing hardenability and adjusting TS to a more suitable range.
  • W content is preferably 0.001% or more.
  • the W content is more preferably 0.030% or more.
  • the W content is more preferably 0.450% or less.
  • the W content is even more preferably 0.400% or less.
  • Mg is an element effective for sphericalizing the shape of inclusions such as sulfides and oxides to improve steel sheet ultimate deformability and hole expansion formability.
  • Mg content is preferably 0.0001% or more.
  • Mg content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be formed. In such cases, when diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions may become initiation points of cracks during hole expanding tests, that is, hole expansion formability may decrease. Therefore, when Mg is included, the Mg content is preferably 0.0200% or less.
  • Zn is an element effective for sphericalizing the shape of inclusions to improve steel sheet ultimate deformability and hole expansion formability.
  • Zn content is preferably 0.0010% or more.
  • the Zn content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be formed. In such cases, when diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions may become initiation points of cracks during hole expanding tests, that is, hole expansion formability may decrease. Therefore, when Zn is included, the Zn content is preferably 0.0200% or less.
  • Co is an element effective for sphericalizing the shape of inclusions to improve steel sheet ultimate deformability and hole expansion formability.
  • 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, when diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions may become initiation points of cracks during hole expanding tests, that is, hole expansion formability may decrease. Therefore, when Co is included, the Co content is preferably 0.0200% or less.
  • Zr like Zn and Co, is an element effective for sphericalizing the shape of inclusions to improve steel sheet ultimate deformability and hole expansion formability.
  • Zr content is preferably 0.0010% or more.
  • the Zr content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be formed. In such cases, when diffusible hydrogen is present in the steel sheet, the coarse precipitates and inclusions may become initiation points of cracks during hole expanding tests, that is, hole expansion formability may decrease. Therefore, when Zr is included, the Zr content is preferably 0.0200% or less.
  • Ca is present in steel as inclusions.
  • the Ca content is preferably 0.0200% or less.
  • the Ca content is more preferably 0.0020% or less.
  • a lower limit of Ca content is not particularly limited, the Ca content is preferably 0.0005% or more. Further, the Ca content is more preferably 0.0010% or more, in view of production technology constraints.
  • Ce 0.0200% or Less
  • Se 0.0200% or Less
  • Te 0.0200% or Less
  • Ge 0.0200% 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
  • REM 0.0200% or Less
  • Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are all elements effective for improving steel sheet ultimate deformability and hole expansion formability.
  • content of each of Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM is preferably 0.0001% or more.
  • the content of any one of Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM exceeds 0.0200%, a large amount of coarse precipitates and inclusions may form.
  • the coarse precipitates and inclusions may become initiation points of cracks during hole expanding tests, that is, hole expansion formability may decrease. Therefore, when at least one of Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM is included, the content thereof is preferably 0.0200% or less.
  • the base steel sheet of the galvanized steel sheet according to an embodiment of the present disclosure comprises:
  • Soft ferrite is a phase that improves ductility and strain hardenability.
  • the area ratio of ferrite is preferably 65.0% or less.
  • the area ratio of ferrite is preferably 35.0% or less.
  • the area ratio of ferrite is more preferably 25.0% or less.
  • a lower limit of the area ratio of ferrite is not particularly limited, and may be 0%.
  • the area ratio of ferrite is preferably 5.0% or more.
  • Bainitic ferrite has an intermediate hardness between soft ferrite and hard fresh martensite and the like, and is an important phase for securing good hole expansion formability. Further, bainitic ferrite is a useful phase for obtaining an appropriate amount of retained austenite by utilizing C diffusion from bainitic ferrite to untransformed austenite. Therefore, the area ratio of bainitic ferrite is 5.0% or more. Further, the area ratio of bainitic ferrite is preferably 10.0% or more. On the other hand, an excessive increase in the area ratio of bainitic ferrite decreases hole expansion formability. Therefore, the area ratio of bainitic ferrite is 40.0% or less. Further, the area ratio of bainitic ferrite is preferably 35.0% or less.
  • Tempered martensite has an intermediate hardness between soft ferrite and hard fresh martensite and the like, and is an important phase for securing good hole expansion formability. Therefore, the area ratio of tempered martensite is 0.5% or more.
  • the area ratio of tempered martensite is preferably 40.0% or more.
  • the area ratio of tempered martensite is 80.0% or less. Further, the area ratio of tempered martensite is preferably 75.0% or less.
  • the area ratio of retained austenite is 3.0% or more.
  • the area ratio of retained austenite is preferably 5.0% or more.
  • an upper limit of the area ratio of retained austenite is not particularly limited, the area ratio of retained austenite is preferably 20.0% or less.
  • the area ratio of fresh martensite is 20.0% or less.
  • a lower limit of the area ratio of fresh martensite is not particularly limited, and may be 0%.
  • the area ratio of fresh martensite is preferably 3.0% or more.
  • Fresh martensite is martensite as quenched (not tempered).
  • the area ratio of residual microstructure other than the above is preferably 10.0% or less.
  • the area ratio of the residual microstructure is more preferably 5.0% or less. Further, the area ratio of the residual microstructure may be 0%.
  • the residual microstructure is not particularly limited, and may include carbides such as lower bainite, pearlite, cementite, and the like.
  • the type of residual microstructure can be confirmed, for example, by observation with a scanning electron microscope (SEM).
  • the area ratio of ferrite, bainitic ferrite, tempered martensite, and hard secondary phase is measured at a 1 ⁇ 4 position of the thickness of the base steel sheet, as follows.
  • a sample is cut from the base steel plate such that a thickness cross-section parallel to a rolling direction of the base steel plate becomes an observation plane.
  • the observation plane of the sample is then mirror-polished using diamond paste.
  • the observation plane of the sample is then polished using colloidal silica and etched with 3 vol % nital to reveal the microstructure.
  • ferrite, bainitic ferrite, tempered martensite, and hard secondary phase are identified as follows.
  • Ferrite a black-colored area, blocky in morphology. Further, almost no iron-based carbides are encapsulated. However, when iron-based carbides are encapsulated, the area of ferrite also includes the area of the iron-based carbides. The same is also true for bainitic ferrite and tempered martensite, which are described below.
  • Bainitic ferrite black to dark gray area, which may be blocky or irregular in morphology. Further, no, or relatively few iron-based carbides are encapsulated.
  • Tempered martensite gray area, irregular in morphology. Further, a relatively large number of iron-based carbides are encapsulated.
  • Hard secondary phase (retained austenite+fresh martensite): white to light gray area, irregular in morphology. Further, iron-based carbides are not encapsulated. When size is relatively large, the color gradually darkens farther away from an interface with other microstructure, and an interior may be dark gray.
  • Residual microstructure lower bainite, pearlite, cementite, and other carbides mentioned above have morphology and the like as known in the art.
  • ferrite in EBSD analysis, ferrite has no substructure (no substructure is observed).
  • bainitic ferrite, tempered martensite, and fresh martensite have a substructure and a specific crystal orientation in relation to retained austenite.
  • austenite microstructure in an annealing process may be reproduced from these microstructures and confirmed, for example. Such points may be a factor in judging microstructure identification.
  • C concentration and Mn concentration differing depending on microstructure may be a factor in judging tissue identification.
  • the C concentration of ferrite and bainitic ferrite is lower than that of an area that is primarily tempered martensite (including fine hard secondary phase, carbides, and the like).
  • Mn concentration may be lower than in other microstructures.
  • hardness differing according to microstructure is a factor in judging tissue identification.
  • ferrite is the least hard and hard secondary phase is the hardest.
  • bainitic ferrite and tempered martensite exhibit hardness between that of ferrite and hard secondary phase.
  • FIG. 1 A is a partial image extracted from one field of view of an observation area (25.6 ⁇ m ⁇ 17.6 ⁇ m) of a sample, as a reference for the above description.
  • the area ratio of retained austenite is measured as follows.
  • the base steel sheet is machine ground in the thickness direction (depth direction) to a 1 ⁇ 4 position of the sheet thickness, and then chemically polished with oxalic acid to prepare the observation plane.
  • the observation plane is then observed by X-ray diffraction.
  • CoK ⁇ radiation is used for incident X-rays to determine a ratio of diffraction intensity of the (200), (220) and (311) planes of fcc iron (austenite) to diffraction intensity of the (200), (211) and (220) planes of bcc iron, and a volume fraction of retained austenite is calculated from the ratio of diffraction intensity of each plane. Then, assuming that the retained austenite is homogeneous in three dimensions, the volume fraction of the retained austenite is taken as the area ratio of retained austenite.
  • the area ratio of fresh martensite is obtained by subtracting the area ratio of retained austenite from the area ratio of hard secondary phase obtained as described above.
  • the area ratio of residual microstructure is obtained by subtracting the area ratio of ferrite, the area ratio of bainitic ferrite, the area ratio of tempered martensite, and the area ratio of hard secondary phase as determined above from 100%.
  • S BF +S TM +2 ⁇ S MA is 65.0% or more.
  • An upper limit of S BF +S TM +2 ⁇ S MA is not particularly limited, but is preferably 130.0% or less.
  • the hard secondary phase consisting of retained austenite and fresh martensite (hereinafter also referred to as MA) includes multiple island regions.
  • an island region having an equivalent circular diameter of 2.0 ⁇ m or more where 20% or less of the circumference is in contact with tempered martensite (hereafter also referred to as MA1) has a low concentration of solute C.
  • stability of the retained austenite in MA1 is low. Therefore, MA1 does not contribute to securing good ductility.
  • the ratio of fresh martensite in MA1 is high, and therefore MA1 reduces hole expansion formability. Therefore, S MA1 /S MA , the ratio of the area ratio of MA1 to the area ratio of the hard secondary phase, is 0.80 or less.
  • S MA1 /S MA is preferably 0.75 or less.
  • S MA1 /S MA is more preferably 0.40 or less.
  • S MA1 /S MA is preferably 0.50 or less.
  • S MA1 /S MA is more preferably 0.30 or less.
  • a lower limit of S MA1 /S MA is not particularly limited and may be 0.
  • Each island region is separated from other island regions of the hard secondary phase by a phase other than the hard secondary phase (each island region is in contact with a phase other than the hard secondary phase around an entire circumference of the island region).
  • specific shape of each island region is not particularly limited, and may be any of circular, elliptical, polygonal, ameboid (a shape extending in a plurality of irregular directions), and the like.
  • an island region where 1% or more of the circumference is in contact with bainitic ferrite (hereinafter also referred to as MA2) has a high concentration of solute C.
  • stability of the retained austenite in MA2 is high. Therefore, MA2 plays a crucial role in securing good strain hardenability and ductility.
  • S MA2 /S MA the ratio of the area ratio of MA2 to the area ratio of the hard secondary phase, is 0.20 or more.
  • S MA2 /S MA is preferably 0.25 or more.
  • S MA2 /S MA is more preferably 0.30 or more.
  • An upper limit of S MA2 /S MA is not particularly limited and may be 1. Further, from the viewpoint of securing high YS and excellent hole expansion formability, when 980 MPa ⁇ TS ⁇ 1,180 MPa is required, S MA2 /S MA is preferably 0.98 or less. Further, when 1,180 MPa ⁇ TS is required, S MA2 /S MA is preferably 0.70 or less.
  • S MA3 /S MA is preferably 0.05 or more.
  • an island region where 1% or more of the circumference is in contact with bainitic ferrite and more than 20% of the circumference is in contact with tempered martensite (hereinafter also referred to as MA3) has an even higher concentration of solute C than MA2.
  • MA3 has a particularly high concentration of solute C because solute C diffuses from tempered martensite as well as bainitic ferrite. Therefore, MA3 contributes particularly effectively to securing good strain hardenability and ductility.
  • S MA3 /S MA the ratio of the area ratio of MA3 to the area ratio of the hard secondary phase, is preferably 0.05 or more.
  • S MA3 /S MA is more preferably 0.07 or more.
  • S MA3 /S MA is more preferably 0.10 or more.
  • An upper limit of S MA3 /S MA is not particularly limited and may be 1. Further, S MA3 /S MA is preferably 0.70 or less.
  • S MA1 , S MA2 , and S MA3 are respectively measured as follows.
  • Ferrite, bainitic ferrite, tempered martensite, and hard secondary phase are identified in a microstructure image (see, for example, FIG. 2 A , FIG. 3 A , and FIG. 4 A ), as described above. Then, after color-coding (conversion to a 4-value image) using Adobe Photoshop by Adobe Systems Inc., the island regions of the hard secondary phase are extracted, and the equivalent circular diameter of each island region, the circumference of each island region, and the length of each island region in contact with bainitic ferrite and tempered martensite are determined using ImageJ, which is open source. Pixel density of the microstructure image when determining the circumference is 30 pixels/ ⁇ m or more and 100 pixels/ ⁇ m or less.
  • each island region is identified as corresponding to MA1, MA2, and MA3, respectively, color-coded using Adobe Photoshop by Adobe Systems Inc. (see, for example, FIG. 2 B , FIG. 3 B , and FIG. 4 B ), and respective areas are calculated.
  • a total area for island regions identified as either MA1, MA2, or MA3 is divided by the area of the observation area (25.6 ⁇ m ⁇ 17.6 ⁇ m), and multiplied by 100 (area ratio) for each of five fields of view.
  • the average of the values (area ratio) for the five fields of view for each of MA1, MA2 and MA3 is then used as S MA1 , S MA2 , and S MA3 .
  • FIG. 2 A , FIG. 3 A , and FIG. 4 A are each partial images extracted from one field of view of an observation area (25.6 ⁇ m ⁇ 17.6 ⁇ m) of a sample, as a reference for the above description.
  • an amount of diffusible hydrogen is preferably 0.50 mass ppm or less.
  • an amount of diffusible hydrogen of the base steel sheet is preferably 0.50 mass ppm or less. Further, the amount of diffusible hydrogen of the base steel sheet is more preferably 0.35 mass ppm or less. A lower limit of the amount of diffusible hydrogen of the base steel sheet is not particularly specified and may be 0 mass ppm. Further, the amount of diffusible hydrogen of the base steel sheet is preferably 0.01 mass ppm or more, in view of production technology constraints.
  • the amount of diffusible hydrogen of the base steel sheet is measured as follows.
  • a test piece 30 mm long and 5 mm wide is taken from a galvanized steel sheet, and the galvanized layer is removed with alkali. Then, an amount of hydrogen released from the test piece is measured by a thermal desorption analysis method. Specifically, the test piece is continuously heated from room temperature to 300° C. at a rate of 200° C./h, and then cooled to room temperature. At this time, the amount of hydrogen released from the test piece in the temperature range from room temperature to 210° C. during the continuous heating is measured (cumulative hydrogen amount). The measured hydrogen amount is then divided by the mass of the test piece (after removal of the galvanized layer and before continuous heating), and a value converted to mass ppm units is the amount of diffusible hydrogen of the base steel sheet.
  • a test piece is cut from the product under a general operating environment and the amount of diffusible hydrogen of the base steel sheet is measured as described above.
  • the value is 0.50 mass ppm or less
  • the amount of diffusible hydrogen of the base steel sheet of the galvanized steel sheet at a material stage before the forming or joining may also be considered to be 0.50 mass ppm or less.
  • the galvanized steel sheet according to an embodiment of the present disclosure preferably has a decarburized layer.
  • the base steel sheet of the galvanized steel sheet according to an embodiment of the present disclosure preferably has a decarburized layer.
  • Cracking due to liquid metal embrittlement (LME) during resistance spot welding may be a problem for steel sheets containing Si, especially for coated or plated steel sheets in which the base steel sheet has a high Si content.
  • LME liquid metal embrittlement
  • resistance weld crack resistance may be improved even when the base steel sheet has a high Si content.
  • Thickness of the decarburized layer in other words, depth in the thickness direction from the surface of the base steel sheet, is preferably 30 ⁇ m or more. Thickness of the decarburized layer is more preferably 40 ⁇ m or more. Although an upper limit of the thickness of the decarburized layer is not particularly limited, the thickness of the decarburized layer is preferably 130 ⁇ m or less in order to keep the tensile strength within a good range.
  • the decarburized layer is defined as a region where the C concentration of the base steel sheet is 80% or less of that of the C content of the chemical composition of the base steel sheet according to analysis of the C concentration of the base steel sheet in the thickness direction from the surface of the base steel sheet. The thickness of the decarburized layer is defined as the thickness of the region.
  • the thickness of the decarburized layer is measured by surface or line analysis of element distribution in the vicinity of the surface layer of the base steel sheet using an electron probe microanalyzer (EPMA) on a cross-sectioned sample.
  • EPMA electron probe microanalyzer
  • a resin-embedded galvanized steel sheet is polished and a vertical section in the rolling direction is finished for observation, and then removed from the resin to be used as a sample for measurement.
  • the accelerating voltage is 7 kV and the irradiation current is 50 nA.
  • Surface analysis or line analysis of the sample cross-section is performed in 1 ⁇ m steps over a 300 ⁇ m ⁇ 300 ⁇ m area including a topmost surface layer of the base steel sheet to measure C intensity.
  • hydrocarbons on and around the sample surface are removed by a plasma cleaner in the measurement room and sample preparation room before the start of measurement.
  • the measurement is performed while the sample is heated to and held at a maximum sample temperature of 100° C. on the stage.
  • the C intensity is converted to a C concentration (in mass %) using a calibration curve prepared by performing measurements on a standard sample separately.
  • the next step is to confirm that the lower limit of C detection is sufficiently lower than 0.10 mass % due to the effect of contamination control. Details of equipment used and the method of contamination control are described in Reference 1 below.
  • the above configuration is not necessarily required because the necessity of contamination countermeasures during measurement depends on the machine model used and conditions.
  • the measurement conditions are only required to confirm that sufficient accuracy has been obtained, and the measurement conditions are not intrinsically related to the effect of the present disclosure.
  • a line profile in the thickness direction is extracted from the surface of the base steel sheet and averaged over 300 points in the direction parallel to the base steel sheet surface to obtain a profile of C concentration in the thickness direction.
  • the obtained profile of C concentration in the thickness direction is smoothed by a simple moving average method. In this case, the number of smoothing points is preferably about 21.
  • the thickness of the decarburized layer is then determined by identifying the range in the thickness direction where the C concentration in the intensity profile after the smoothing treatment is 80% or less of that of the C content of the chemical composition of the base steel sheet.
  • the tensile strength of the galvanized steel sheet according to an embodiment of the present disclosure is 980 MPa or more.
  • the tensile strength of the galvanized steel sheet according to an embodiment of the present disclosure is preferably 1,180 MPa or more.
  • the yield stress (YS), total elongation (El), strain hardening index (n-value)/yield ratio (YR), and maximum hole expansion ratio ( ⁇ ) of the galvanized steel sheet according to an embodiment of the present disclosure are as described above.
  • tensile strength (TS), yield stress (YS), total elongation (El), and strain hardening index (n value)/yield ratio (YR) are measured by tensile testing in accordance with JIS Z 2241, as described later in reference to Examples.
  • the maximum hole expansion ratio ( ⁇ ) is measured by a hole expanding test in accordance with JIS Z 2256, as described later in reference to Examples.
  • the galvanized layer of the galvanized steel sheet according to an embodiment of the present disclosure may be provided on only one surface of the base steel sheet, and may be provided on both surfaces.
  • the galvanized layer here refers to a coating layer in which Zn is the main component (Zn content of 50% or more), for example, a hot-dip galvanized layer or a galvannealed layer.
  • the hot-dip galvanized layer being composed of Zn, Fe: 20 mass % or less, and Al: 0.001 mass % or more and 1.0 mass % or less is appropriate.
  • the hot-dip galvanized 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, totaling 0 mass % or more and 3.5 mass % or less.
  • the Fe content of the hot-dip galvanized layer is more preferably less than 7 mass %. Other than the above elements, the balance is inevitable impurity.
  • the galvannealed layer being composed of Fe: 20 mass % or less and Al: 0.001 mass % or more and 1.0 mass % or less is appropriate.
  • 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, totaling 0 mass % or more and 3.5 mass % or less.
  • the Fe content of the galvannealed layer is more preferably 7 mass % or more.
  • the Fe content of the galvannealed layer is even more preferably 8 mass % or more.
  • the Fe content of the galvannealed layer is more preferably 15 mass % or less.
  • the Fe content of the galvannealed layer is even more preferably 12 mass % or less.
  • the balance is inevitable impurity.
  • a coating weight per side of the galvanized layer is not particularly limited, but is preferably 20 g/m 2 to 80 g/m 2 .
  • the coating weight of the galvanized layer is measured as follows.
  • a coating solution is prepared by adding 0.6 g of a corrosion inhibitor for Fe (“IBIT 700BK” (IBIT is a registered trademark in Japan, other countries, or both) manufactured by Asahi Chemical Co., Ltd.) to 1 L of a 10 mass % hydrochloric acid aqueous solution. Then, the galvanized steel sheet to be the test piece is immersed in the coating solution to dissolve the galvanized layer. Mass loss of the test piece before and after dissolving is measured, and the value is divided by the surface area of the base steel sheet (surface area of a coated portion) to calculate the coating weight (g/m 2 ).
  • a corrosion inhibitor for Fe (“IBIT 700BK” (IBIT is a registered trademark in Japan, other countries, or both) manufactured by Asahi Chemical Co., Ltd.)
  • the galvanized steel sheet according to an embodiment of the present disclosure may have a metal coating or plating layer other than the galvanized layer at least on one side between the base steel sheet and the galvanized layer.
  • the metal coating or plating layer contributes to an improvement in resistance weld crack resistance. Formation of the metal coating or plating layer suppresses resistance weld cracking even when the Si content of the base steel sheet is high.
  • the metal coating or plating layer improves resistance weld crack resistance
  • the inventors understand that when the metal coating or plating layer is between the base steel sheet and the galvanized layer, or in other words on the surface of the base steel sheet, the metal coating or plating layer acts as a barrier layer that hinders the zinc in the galvanized layer from melting and penetrating into the base steel sheet during resistance spot welding, thereby making resistance weld cracking less likely to occur (zinc penetration suppression effect).
  • the metal coating or plating layer may be between the base steel sheet and the galvanized layer on only one side, and may be between the base steel sheet and the galvanized layer on both sides.
  • the coating weight of the metal coating or plating layer is preferably more than 0 g/m 2 .
  • the coating weight of the metal coating or plating layer is more preferably 2.0 g/m 2 or more.
  • an upper limit of the coating weight of the metal coating or plating layer per side is not particularly limited, the coating weight of metal coating or plating layer is preferably 60 g/m 2 or less, in view of cost.
  • the coating weight of the metal coating or plating layer is more preferably 50 g/m 2 or less.
  • the coating weight of the metal coating or plating layer is even more preferably 40 g/m 2 or less.
  • the coating weight of the metal coating or plating layer is even more preferably 30 g/m 2 or less.
  • the coating weight of the metal coating or plating layer is per side.
  • the coating weight of the metal coating or plating layer is measured as follows. A 10 mm ⁇ 15 mm size sample is taken from the galvanized steel sheet and embedded in resin to make a cross-sectional embedded sample. The thickness of the metal coating or plating layer is measured at three arbitrary locations on the cross section of the sample using a scanning electron microscope (SEM) at an accelerating voltage of 15 kV and a magnification of 2,000 times to 10,000 times, depending on the thickness of the metal coating or plating layer, and the average value of the three locations is calculated. The calculated average value is then multiplied by the relative density of the metal constituting the metal coating or plating layer to convert to the coating weight per side of the metal coating or plating layer.
  • SEM scanning electron microscope
  • a metal used for the metal coating or plating layer a metal having a higher melting point than Zn is desirable, for example, Fe, Ni, and the like may be used. Further, in addition to the zinc penetration suppression effect, the metal coating or plating layer is preferably Fe-based, because a toughness reduction suppression effect described below may be expected.
  • the Fe-based coating or plating layer acts as a solute Si deficient layer, and the amount of solute Si in a welded portion decreases. This may suppress the decrease in toughness of the welded portion and improve the resistance weld crack resistance in the welded portion (toughness reduction suppression effect). Further, the Fe-based coating or plating layer also functions as a soft layer and relaxes stress applied to the steel sheet surface during resistance spot welding. This may reduce residual stress in the welded portion and improve the resistance weld crack resistance (stress relaxation effect).
  • the Fe-based coating or plating layer may be an Fe (pure Fe) coating or plating layer, and may be an alloy coating or plating layer such as one formed from Fe—B alloy, Fe—C alloy, Fe—P alloy, Fe—N alloy, Fe—O alloy, Fe—Ni alloy, Fe—Mn alloy, Fe—Mo alloy, Fe—W alloy, and the like.
  • the chemical composition of the Fe-based coating or plating layer is not particularly limited as long as the Fe content is 50 mass % or more, but in particular, a chemical composition consisting of Fe and inevitable impurity, and a chemical composition containing one or more elements selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co totaling 10 mass % or less, with the balance being Fe and inevitable impurity, are preferable.
  • the total content of the one or more elements is preferably 10 mass % or less to help prevent a decrease in electrolytic efficiency and to form an Fe-based coating or plating layer, in particular an Fe-based electroplating layer, at low cost.
  • the C content is preferably 0.08 mass % or less.
  • the galvanized steel sheet according to an embodiment of the present disclosure may have a metal coating or plating layer and a decarburized layer at the same time (that is, from the surface of the galvanized steel sheet, the galvanized layer, the metal coating or plating layer, and the decarburized layer (surface layer of the base steel sheet) in that order). This may further improve the resistance weld crack resistance.
  • the thickness of the decarburized layer depth in the thickness direction from the surface of the base steel sheet
  • the thickness of the decarburized layer depth in the thickness direction from the surface of the base steel sheet
  • the method described above may be evaluated by analyzing the C concentration from the surface of the metal coating or plating layer or an interface between the galvanized layer and the cold-rolled steel sheet in the direction of the sheet thickness by the method described above.
  • the thickness of the galvanized steel sheet according to an embodiment of the present disclosure is not particularly limited, but is preferably 0.5 mm or more.
  • the thickness of the galvanized steel sheet is preferably 3.0 mm or less.
  • the member according to an embodiment of the present disclosure is a member made using the galvanized steel sheet described above as a material.
  • the material, the galvanized steel sheet is subjected to at least one of a forming process and a joining process to make the member.
  • the galvanized steel sheet has a TS of 980 MPa or more, and has high YS, excellent ductility, strain hardenability and hole expansion formability. Therefore, the member according to an embodiment of the present disclosure has high strength and excellent anti-crash properties. Therefore, the member according to an embodiment of the present disclosure is particularly suitable for application as an impact energy absorbing member for use in the automotive field.
  • T 0 is the first cooling stop temperature (° C.) and T 1 is the temperature of the galvanizing bath in the galvanizing treatment (° C.).
  • each of temperatures above refers to a surface temperature of the steel slab and the steel sheet.
  • a steel slab having the chemical composition described above is prepared.
  • steel raw material is melted to produce molten steel having the chemical composition described above.
  • the steelmaking method is not particularly limited, and any known steelmaking method may be used, such as using a converter, electric furnace, and the like. Obtained molten steel is then solidified into a steel slab.
  • the method of obtaining a steel slab from molten steel is not particularly limited. For example, continuous casting, ingot making, and thin slab casting methods may be used. A continuous casting method is preferred from the viewpoint of hindering macro-segregation.
  • the steel slab is then hot rolled to obtain a hot-rolled steel sheet.
  • An energy saving process may be applied to the hot rolling process.
  • Energy saving processes include hot charge rolling (where a steel slab is charged into a furnace as a warm slab without cooling to room temperature and then hot rolled) and direct rolling (where a steel slab is hot rolled immediately after being subjected to heat retaining for a short period).
  • hot rolling conditions there are no particular limitations on hot rolling conditions.
  • hot rolling may be performed under the following conditions.
  • the steel slab is temporarily cooled to room temperature and then reheated before rolling.
  • the slab heating temperature (reheating temperature) is preferably 1,100° C. or more in view of carbide dissolution and reduced rolling load.
  • the slab heating temperature is preferably 1,300° C. or less, in order to prevent increased scale loss.
  • the slab heating temperature is based on the temperature of the steel slab surface.
  • the steel slab is then subjected to rough rolling according to a conventional method to produce a rough-rolled sheet (hereinafter also referred to as a sheet bar).
  • the sheet bar is then subjected to finish rolling to produce a hot-rolled steel sheet.
  • the rolling finish temperature is preferably the Ar 3 transformation temperature or more, in order to reduce rolling load.
  • the rolling finish temperature is preferably the Ar 3 transformation temperature or more because a high rolling reduction rate in an unrecrystallized state of austenite may result in the development of an abnormal microstructure elongated in the rolling direction, which may reduce the workability of the annealed sheet.
  • the Ar 3 transformation temperature is determined by the following formula.
  • Ar 3 (° C.) 868 ⁇ 396 ⁇ [C %]+25 ⁇ [Si %] ⁇ 68[Mn %]
  • the [element symbol %] in the above formula represents the content (mass %) of the element in the chemical composition of the base steel sheet.
  • Sheet bars may be joined together during hot rolling, and finish rolling may be performed continuously. Further, the sheet bar may be rolled once before finish rolling. Further, at least part of finish rolling may be conducted as lubrication rolling to reduce rolling load in the hot rolling. Conducting lubrication rolling in such a manner is effective from the perspective of making the shape and material properties of the steel sheet uniform. In lubrication rolling, the coefficient of friction is preferably 0.10 or more. The coefficient of friction is preferably 0.25 or less.
  • the rolling finish temperature is preferably 800° C. or more and 950° C. or less.
  • the rolling finish temperature being 800° C. or more makes the steel microstructure at the hot-rolled steel sheet stage and, consequently, the steel microstructure of the final product more likely to be uniform. Uneven steel microstructure tends to reduce bendability.
  • the rolling finish temperature is more than 950° C., the amount of oxide (scale) formation increases. As a result, an interface between the steel substrate and oxide may be roughened, and the surface quality of the steel sheet after pickling and cold rolling may deteriorate. Further, coarse crystal grains may also cause a reduction in the strength and bendability of a steel sheet.
  • the hot-rolled steel sheet is coiled.
  • the coiling temperature is preferably 450° C. or more.
  • the coiling temperature is preferably 750° C. or less.
  • the hot-rolled steel sheet is optionally pickled.
  • Pickling may remove oxides from the steel sheet surface, securing good chemical convertibility and coating or plating quality. Pickling may be performed in one or more batches. Pickling conditions are not particularly limited, and a conventional method may be followed.
  • Cold rolling is performed by multi-pass rolling that requires two or more passes, for example, tandem-type multi-stand rolling, reverse rolling, and the like.
  • the rolling reduction of the cold rolling is not particularly limited.
  • the rolling reduction of the cold rolling is preferably 20% or more.
  • the rolling reduction of the cold rolling is preferably 80% or less.
  • the rolling reduction of the cold rolling is less than 20%, coarsening and non-uniformity of the steel microstructure is more likely to occur during the annealing process, which may result in reduced strength and workability in the final product.
  • the rolling reduction of the cold rolling exceeds 80%, the steel sheet may be prone to shape defects and the coating weight of the galvanized coating may become uneven.
  • the cold-rolled steel sheet obtained after cold rolling may be subjected to pickling.
  • a metal coating or plating treatment to form a metal coating or plating layer on at least one surface of the cold-rolled steel sheet obtained as described above may optionally be applied after the cold rolling process and before the annealing process described below.
  • a cold-rolled steel sheet that has a metal coating or plating layer on at least one surface before undergoing the annealing process described below may hereinafter be referred to as a metal coated or plated steel sheet.
  • the metal coating or plating treatment method is not particularly limited, but electroplating is preferred from the viewpoint of manufacturability.
  • a sulfuric acid bath, a hydrochloric acid bath, a mixed solution thereof, and the like may be used as a metal plating bath.
  • a metal coated or plated steel sheet means a steel sheet that has a metal coating or plating layer on at least one surface of the cold-rolled steel sheet before the annealing process described below, and does not exclude a cold-rolled steel sheet that is pre-annealed before the metal coating or plating treatment.
  • a metal used in the metal coating or plating treatment a metal having a higher melting point than Zn is desirable, for example, Fe, Ni, and the like may be used. Further, formation of the Fe-based coating or plating layer is a preferred result of the metal coating or plating treatment, as an improved effect of resistance weld crack resistance may be expected.
  • the coating or plating bath to form the Fe-based coating or plating layer may contain one or more elements selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co.
  • the total content of these elements in the coating or plating bath is preferably 10 mass % or less of the chemical composition of the metal coating or plating layer of the metal coated or plated steel sheet.
  • Metallic elements may be included as metal ions, while non-metallic elements may be included as part of, for example, boric acid, phosphoric acid, nitric acid, or organic acid.
  • an iron sulfate plating solution may also contain conductivity aids such as sodium sulfate and potassium sulfate, chelating agents, and pH buffers.
  • degreasing and water washing may be performed to clean the surface of the cold-rolled steel sheet, and pickling and water washing may also be performed to activate the surface of the cold-rolled steel sheet.
  • the metal coating or plating process described above is performed.
  • the methods of degreasing treatment and water washing are not particularly limited, and a conventional method may be used.
  • Various acids such as sulfuric acid, hydrochloric acid, nitric acid, and mixtures of these acids may be used in the pickling. Among these, sulfuric acid, hydrochloric acid, or a mixture of sulfuric acid and hydrochloric acid are preferred.
  • the acid concentration is not particularly limited, approximately 1 mass % to 20 mass % is preferable, in consideration of the ability to remove oxide coating and the prevention of rough skin (surface defects) due to excessive pickling.
  • the pickling solution may also contain, for example, a defoamer, a pickling promoter, a pickling inhibitor, and the like.
  • the cold-rolled steel sheet obtained as described above (including the case of the metal coated or plated steel sheet) is annealed at an annealing temperature of 760° C. or more and 900° C. or less, for an annealing time of 20 s or more.
  • the number of annealing cycles may be two or more, but one is preferred from the viewpoint of energy efficiency.
  • Annealing Temperature 760° C. or More and 900° C. or Less
  • the annealing temperature is less than 760° C.
  • the ratio of austenite formation during heating in the two-phase region of ferrite and austenite is insufficient.
  • an excessive increase occurs in the area ratio of ferrite after annealing, leading to a decrease in YS. Hole expansion formability is also reduced. Further, achieving a TS of 980 MPa or more becomes difficult.
  • the annealing temperature exceeds 900° C., excessive austenite grain growth occurs and a subsequent formation rate of bainitic ferrite slows down. As a result, an appropriate amount of bainitic ferrite and retained austenite area ratio are not obtained.
  • the annealing temperature is 760° C. or more and 900° C. or less.
  • the annealing temperature is preferably 780° C. or more.
  • the annealing temperature is more preferably more than 790° C.
  • the annealing temperature is preferably 880° C. or less.
  • the annealing temperature is the maximum arrival temperature during the annealing process.
  • the annealing time is 20 s or more.
  • An upper limit of the annealing time is not particularly limited, but is preferably 900 s or less.
  • the annealing time is the holding time in a temperature range from (annealing temperature ⁇ 40° C.) or more to the annealing temperature or less.
  • the annealing time includes not only the holding time at the annealing temperature, but also the time in the temperature range from (annealing temperature ⁇ 40° C.) or more to the annealing temperature or less during heating and cooling before and after reaching the annealing temperature.
  • Dew Point More than ⁇ 30° C.
  • the dew point of the annealing atmosphere in the annealing process is preferably more than ⁇ 30° C.
  • the dew point being more than ⁇ 30° C. means the decarburization reaction is promoted and the C concentration in the surface layer of the cold-rolled steel sheet (base steel sheet) is reduced to form a decarburized layer.
  • the dew point of the annealing atmosphere is preferably ⁇ 20° C. or more.
  • the dew point of the annealing atmosphere is more preferably ⁇ 5° C. or more.
  • the dew point of ⁇ 5° C. or more may further enhance the resistance weld crack resistance in the welded portion.
  • the dew point is preferably 30° C. or less from the viewpoint of suitably hindering oxidation on the surface of the cold-rolled steel sheet or metal coating or plating layer and securing good adhesion when a galvanized layer is applied.
  • the cold-rolled steel sheet annealed as described above is cooled to the first cooling stop temperature of 300° C. or more and 550° C. or less.
  • the first cooling stop temperature is 300° C. or more and 550° C. or less.
  • the first cooling stop temperature is preferably 350° C. or more.
  • the first cooling stop temperature is preferably 510° C. or less.
  • the cold-rolled steel sheet is held at the temperature range from 300° C. or more to 550° C. or less (hereinafter also referred to as the holding temperature range) for 3 s or more to 600 s or less.
  • bainitic ferrite is formed and C diffusion from the formed bainitic ferrite to the untransformed austenite adjacent to the bainitic ferrite occurs.
  • the area ratio of a defined amount of retained austenite is secured and S MA2 /S MA and S MA3 /S MA increase.
  • the holding time in the holding temperature range is less than 3 s, the area ratio of bainitic ferrite decreases and the area ratio of tempered martensite increases excessively. Further, S MA2 /S MA and S MA3 /S MA decrease, resulting in lower ductility and strain hardenability.
  • the holding time in the holding temperature range exceeds 600 s, the area ratio of bainitic ferrite may increase excessively and YS may decrease. Further, excessive C diffusion from bainitic ferrite to untransformed austenite may cause an increase in S MA1 /S MA and reduced hole expansion formability.
  • the holding time in the holding temperature range is 3 s or more and 600 s or less.
  • the holding time in the holding temperature range is preferably 5 s or more.
  • the holding time in the holding temperature range is more preferably 10 s or more.
  • the holding time in the holding temperature range is preferably less than 200 s.
  • the holding time in the holding temperature range is more preferably less than 80 s.
  • the holding time in the holding temperature range includes the time the cold-rolled steel sheet remains in the temperature range until reaching the first cooling stop temperature in the first cooling process, and the time the cold-rolled steel sheet remains in the temperature range until the start of galvanizing in the coating process described below (for example, the time the cold-rolled steel sheet remains in the temperature range until it is dipped into the galvanizing bath).
  • the holding time in the holding temperature range does not include the time in the temperature range of the galvanized steel sheet after hot-dip galvanizing treatment in the coating process.
  • the cold-rolled steel sheet is then subjected to a galvanizing treatment to produce the galvanized steel sheet.
  • the galvanizing treatment include hot-dip galvanizing treatment and galvannealing treatment.
  • coating bath temperature the relationship between the first cooling stop temperature in the first cooling process and the temperature of the galvanizing bath in the galvanizing treatment (hereinafter also referred to as coating bath temperature) must satisfy the following Formula (1):
  • T 0 is the first cooling stop temperature (° C.) and T 1 is the temperature of the galvanizing bath in the galvanizing treatment (° C.).
  • T 0 ⁇ T 1 is more than 50° C. or less than ⁇ 150° C.
  • S MA2 /S MA and S MA3 /S MA decrease, and strain hardenability and ductility decrease.
  • T 0 ⁇ T 1 is preferably ⁇ 120° C. or more.
  • T 0 ⁇ T 1 is more preferably ⁇ 100° C. or more.
  • T 0 ⁇ T 1 is preferably 45° C. or less.
  • T 0 ⁇ T 1 is more preferably 40° C. or less.
  • Conditions other than the above are not particularly limited, and a conventional method may be used.
  • the coating bath temperature is 440° C. or more and 500° C. or less.
  • a galvanizing bath there is no particular limitation as long as the composition of the galvanized layer is as described above, but, for example, a galvanizing bath having an Al content of 0.10 mass % or more and 0.23 mass % or less with the balance being Zn and inevitable impurity is preferable.
  • the galvanized steel sheet is preferably heated to an alloying temperature of 450° C. or more and 600° C. or less and subjected to an alloying treatment.
  • the alloying temperature is less than 450° C., the Zn—Fe alloying rate becomes slow and alloying may be difficult.
  • the alloying temperature exceeds 600° C., untransformed austenite may transform to pearlite, resulting in a decrease in TS and ductility.
  • the alloying temperature is more preferably 470° C. or more.
  • the alloying temperature is more preferably 570° C. or less.
  • the coating weight for both a hot-dip galvanized steel sheet (GI) and a galvannealed steel sheet (GA) is preferably 20 g/m 2 to 80 g/m 2 per side.
  • the coating weight may be adjusted by gas wiping and the like.
  • an additional holding process may be performed in which the galvanized steel sheet is held at a temperature range from 300° C. or more to 550° C. or less (hereinafter also referred to as the additional holding temperature range) for 3 s or more to 600 s or less.
  • the additional holding process is a process that has a similar effect to the holding process. Further, the additional holding process may be performed after and/or during the coating process, as long as the additional holding process is performed before the second cooling process described below. Further, when the coating process is a galvannealing treatment, the additional holding process may be performed during the coating process. In other words, the coating process may also serve as the additional holding process.
  • the total holding time for the holding process and the additional holding process is preferably 3 s or more.
  • the total holding time for the holding process and the additional holding process is preferably 600 s or less.
  • the total holding time for the holding process and the additional holding process is more preferably less than 200 s.
  • the galvanized steel sheet is then cooled to the second cooling stop temperature of 100° C. or more and less than 300° C.
  • Second cooling stop temperature 100° C. or more and less than 300° C.
  • the second cooling process is necessary to control the area ratio of tempered martensite and retained austenite generated in the subsequent reheating process within a defined range.
  • the second cooling stop temperature is less than 100° C.
  • almost all of the untransformed austenite in the steel is transformed to martensite in the second cooling process.
  • ductility and strain hardenability are reduced.
  • the second cooling stop temperature is 300° C. or more, the area ratio of tempered martensite decreases and that of fresh martensite increases.
  • the second cooling stop temperature is 100° C. or more and less than 300° C.
  • the second cooling stop temperature is preferably 120° C. or more.
  • the second cooling stop temperature is preferably 280° C. or less.
  • the galvanized steel sheet is reheated to a reheating temperature of (the second cooling stop temperature+50° C.) or more and 500° C. or less, and the galvanized steel sheet is held at the temperature range of (the second cooling stop temperature+50° C.) or more and 500° C. or less (hereinafter also referred to as the reheating temperature range) for 10 s or more and 2,000 s or less.
  • Reheating Temperature (the Second Cooling Stop Temperature+50° C.) or More and 500° C. or Less
  • the reheating temperature is (the second cooling stop temperature+50° C.) or more and 500° C. or less.
  • the reheating temperature is preferably (the second cooling stop temperature+70° C.) or more.
  • the reheating temperature is preferably 450° C. or less.
  • the reheating temperature is the maximum arrival temperature in the reheating process.
  • the holding time in the reheating temperature range is 10 s or more and 2,000 s or less.
  • the holding time in the reheating temperature range is preferably 15 s or more.
  • the holding time in the reheating temperature range is preferably 1,200 s or less.
  • the holding time in the reheating temperature range includes not only the holding time at the reheating temperature, but also the time in the reheating temperature range during heating and cooling before and after reaching the reheating temperature.
  • Cooling conditions after holding in the reheating temperature range are not particularly limited, and a conventional method may be followed.
  • gas jet cooling, mist cooling, roll cooling, water cooling, air cooling, and the like may be applied as cooling methods.
  • cooling down to 50° C. or less is preferable. Cooling to about room temperature is more preferable.
  • an average cooling rate of 1° C./s or more and 50° C./s or less in cooling after holding in the reheating temperature range is suitable.
  • the galvanized steel sheet obtained as described above may be further subjected to temper rolling.
  • the reduction ratio of the temper rolling exceeds 2.00%, yield stress may increase and dimensional accuracy may decrease when forming the galvanized steel sheet into a member. Therefore, the reduction ratio of the temper rolling is preferably 2.00% or less.
  • a lower limit of the reduction ratio of the temper rolling is not particularly limited, but is preferably 0.05% or more from the viewpoint of productivity.
  • the temper rolling may be performed on equipment that is continuous (on-line) with the annealing equipment used to perform each of the aforementioned processes, and may be performed on equipment that is discontinuous (off-line) with the annealing equipment used to perform each of the processes.
  • the number of rolling cycles for the temper rolling may be one, two, or more. Rolling by a leveler or the like is also acceptable, as long as the same elongation rate as temper rolling is provided.
  • Conditions other than those described above are not particularly limited, and a conventional method may be used.
  • the method of producing a member according to an embodiment of the present disclosure includes a process of forming or joining at least one of the galvanized steel sheet (for example, the galvanized steel sheet produced by the method of producing the galvanized steel sheet) into a member by applying at least one of a forming process and a joining process.
  • the galvanized steel sheet for example, the galvanized steel sheet produced by the method of producing the galvanized steel sheet
  • the method of the forming process is not particularly limited, and a general processing method such as press working may be used, for example.
  • the method of the joining process is also not particularly limited, and for example, general welding such as spot welding, laser welding, arc welding, and the like, rivet joining, swaging joining, and the like may be used.
  • Forming and joining conditions are not particularly limited and may follow a conventional method.
  • hot-dip galvanizing treatment or galvannealing treatment was performed to obtain hot-dip galvanized steel sheets (hereinafter also referred to as GI) or galvannealed steel sheets (hereinafter also referred to as GA).
  • GI hot-dip galvanized steel sheets
  • GA galvannealed steel sheets
  • the type of coating process is also indicated as “GI” and “GA”.
  • the total of the holding time in the holding temperature range and the holding time in the temperature range of 300° C. or more and 550° C. or less in the alloying treatment was 3 s or more and 600 s or less.
  • a galvanizing bath was used that had a composition of Al: 0.20 mass % with the balance being Zn and inevitable impurity.
  • a galvanizing bath was used that had a composition of Al: 0.14 mass % with the balance being Zn and inevitable impurity.
  • the coating weight was 45 g/m 2 to 72 g/m 2 per side when producing GI and 45 g/m 2 per side when producing GA.
  • the composition of the galvanized layer of the final galvanized steel sheet in the case of GI was Fe: 0.1 mass % to 1.0 mass %, Al: 0.2 mass % to 1.0 mass %, with the balance being Zn and inevitable impurity.
  • the composition was Fe: 7 mass % to 15 mass %, Al: 0.1 mass % to 1.0 mass %, with the balance being Zn and inevitable impurity.
  • Galvanized layers were formed on both sides of the base steel sheet.
  • Tensile strength (TS), yield stress (YS), total elongation (El), strain hardening index (n-value)/yield ratio (YR), and maximum hole expansion ratio ( ⁇ ) were evaluated according to the following criteria by conducting tensile tests and hole expanding tests according to the following procedures.
  • Tensile testing was performed in accordance with JIS Z 2241. That is, a JIS No. 5 test piece was taken from the obtained galvanized steel sheet such that the longitudinal direction was perpendicular to the rolling direction of the base steel sheet. The collected test piece was subjected to tensile testing at a crosshead velocity of 10 mm/min to measure TS, YS, El, and n value.
  • the n-value was calculated from elongation and strength at 0.4 times and 0.8 times uniform elongation (U-El).
  • the n value/YR represents the strain hardenability, which is a comprehensive evaluation index of the formability and anti-crash property of the steel sheet. The results are listed in Table 3.
  • the hole expanding test was performed in accordance with JIS Z 2256. That is, a 100 mm ⁇ 100 mm test piece was taken from the obtained galvanized steel sheet by shearing. A 10 mm diameter hole was punched through the test piece with a clearance of 12.5%. Then, a blank holding force of 9 tons (88.26 kN) was applied around the hole using a die having an inside diameter of 75 mm, and a conical punch having an apex angle of 60° was pressed into the hole to measure the diameter of the hole in the test piece at the crack initiation limit (when cracks occurred).
  • the maximum hole expansion ratio: ⁇ (%) was obtained by the following formula. Note that ⁇ is an index for evaluating stretch flangeability. The results are listed in Table 3.
  • tensile strength (TS), yield stress (YS), total elongation (El), strain hardening index (n value)/yield ratio (YR), and maximum hole expansion ratio ( ⁇ ) were all judged to pass for all of the Examples.
  • tensile strength (TS), yield stress (YS), total elongation (El), strain hardening index (n value)/yield ratio (YR), and maximum hole expansion ratio ( ⁇ ).
  • members obtained by a forming process or joining process using the steel sheet of the present disclosure have excellent properties in terms of tensile strength (TS), yield stress (YS), total elongation (El), strain hardening index (n value)/yield ratio (YR), and maximum hole expansion ratio ( ⁇ ).
  • the cold-rolled steel sheets obtained No. 8 to No. 10 were subjected to Fe-based electroplating as a metal plating process to form a metal plating layer (Fe-based plating layer) on the surfaces of the cold-rolled steel sheet.
  • the cold-rolled steel sheets were first degreased with alkali.
  • the cold-rolled steel sheets were then subjected to electrolytic treatment using the cold-rolled steel sheet as the cathode under the following conditions to form a metal plating layer on the surfaces of the cold-rolled steel sheets.
  • the coating weight of the metal plating layer was controlled by the energizing time.
  • the obtained cold-rolled steel sheets (including metal plated steel sheets having a metal plating layer formed on the surfaces of the cold-rolled steel sheets) were then subjected to the annealing process, the first cooling process, the holding process, the coating process, the second cooling process, and the reheating process under the conditions listed in Table 5 to obtain galvanized steel sheets.
  • galvannealing treatment was applied to obtain galvannealed steel sheets (GA).
  • Treatment conditions other than those listed in Table 5 were the same as in Example 1.
  • Galvanized layers were formed on both sides of the base steel sheet.
  • resistance weld crack resistance of the welded portions was evaluated according to the following procedure.
  • test piece 2 that was cut from the obtained galvanized steel sheet into 150 mm (long direction) ⁇ 50 mm (short direction) with the transverse direction (direction orthogonal to the rolling direction) (TD) as the long direction and the rolling direction as the short direction, was overlapped with overlapped with a test galvannealed steel sheet 1 (thickness: 1.6 mm, TS: 980 MPa grade).
  • the test galvannealed steel sheet 1 had a coating weight of 50 g/m 2 per side of the galvannealed layer and was cut to the same size as the test piece 2 .
  • the sheet combination was assembled so that the surface to be evaluated of the test piece 2 (in a case where the galvanized layer and the metal plating layer is on only one side, the galvanized layer on the one side) and the galvanized layer of the test galvannealed steel sheet 1 faced each other.
  • the sheet combination was fixed to a fixing stand 4 via spacers 3 having a thickness of 2.0 mm.
  • the spacers 3 were a pair of steel sheets, each measuring 50 mm (long direction) ⁇ 45 mm (short direction) ⁇ 2.0 mm thick. As illustrated in FIG. 5 A , the long direction end faces of the pair of steel sheets are aligned with the short direction end faces of the sheet combination. Thus, the distance between the pair of steel sheets was 60 mm.
  • the fixing stand 4 was a single plate with a hole in the center.
  • the sheet combination was subjected to resistance spot welding at a welding current that resulted in a nugget diameter r of 5.9 mm while being deflected by applying pressure with a pair of electrodes 5 (tip diameter: 6 mm) under the conditions of an electrode force of 3.5 kN, a holding time of 0.12 s, 0.18 s, or 0.24 s, and a welding time of 0.36 s, to form a sheet combination with a welded portion.
  • the pair of electrodes 5 applied pressure to the sheet combination from above and below in the vertical direction, with the lower electrode applying pressure to the test piece 2 through the hole in the fixing stand 4 .
  • the lower electrode of the pair of electrodes 5 and the fixing stand 4 were fixed so that the lower electrode was in contact with a plane that was an extension of a plane where the spacers 3 were in contact with the fixing stand 4 , and the upper electrode was movable. Further, the upper electrode was in contact with the center of the test galvannealed steel sheet 1 .
  • the sheet combination was welded with an inclination of 5° in the long direction of the sheet combination with respect to the horizontal direction.
  • the holding time refers to the time between the end of passage of the welding current and the beginning of electrode release.
  • the nugget diameter r means the distance between the ends of a nugget 6 in the long direction of the sheet combination.
  • each sheet combination with a welded portion was cut along the A-A line indicated in the upper part of FIG. 5 B to include the center of the welded portion including the nugget 6 , and the cross-section of the welded portion was observed under an optical microscope (200 ⁇ ) to evaluate the resistance weld crack resistance at the welded portion using the following criteria.
  • the result was A+, A, or B
  • the sheet combination was judged to have satisfactory resistance weld crack resistance at the welded portion.
  • the result was C
  • the sheet combination was judged to have poor resistance weld crack resistance at the welded portion.
  • Table 7 The results are listed in Table 7.
  • a crack in the test piece 2 is schematically illustrated in the lower part of FIG. 5 B , as indicated by reference numeral 7 .
  • the stress in the steel sheet to be evaluated any of the steel sheets in the Examples and Comparative Examples
  • an appropriate evaluation is not obtained. For this reason, the data in which no cracking occurred in the counterpart steel sheet was used as Examples.
  • tensile strength (TS), yield stress (YS), total elongation (El), strain hardening index (n value)/yield ratio (YR), and maximum hole expansion ratio ( ⁇ ) were all judged to pass for all of the Examples. Resistance weld crack resistance at the welded portion was also excellent.
  • steel sheets of the present disclosure have excellent properties in terms of tensile strength (TS), yield stress (YS), total elongation (El), strain hardening index (n value)/yield ratio (YR), maximum hole expansion ratio ( ⁇ ), and resistance weld crack resistance at the welded portion.

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