WO2022202716A1 - Tôle d'acier galvanisé et élément et procédé de fabrication associé - Google Patents

Tôle d'acier galvanisé et élément et procédé de fabrication associé Download PDF

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WO2022202716A1
WO2022202716A1 PCT/JP2022/012855 JP2022012855W WO2022202716A1 WO 2022202716 A1 WO2022202716 A1 WO 2022202716A1 JP 2022012855 W JP2022012855 W JP 2022012855W WO 2022202716 A1 WO2022202716 A1 WO 2022202716A1
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steel sheet
area ratio
galvanized steel
galvanized
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PCT/JP2022/012855
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English (en)
Japanese (ja)
Inventor
翔 樋口
由康 川崎
達也 中垣内
友美 金澤
俊佑 山本
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Ｊｆｅスチール株式会社
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Application filed by Ｊｆｅスチール株式会社 filed Critical Ｊｆｅスチール株式会社
Priority to JP2022542316A priority Critical patent/JP7197063B1/ja
Priority to CN202280022963.4A priority patent/CN117062928A/zh
Priority to US18/546,428 priority patent/US20240124964A1/en
Priority to KR1020237028572A priority patent/KR20230135631A/ko
Priority to EP22775503.0A priority patent/EP4306672A1/fr
Priority to MX2023011170A priority patent/MX2023011170A/es
Publication of WO2022202716A1 publication Critical patent/WO2022202716A1/fr

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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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    • C21D2211/001—Austenite
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Definitions

  • the present invention relates to a galvanized steel sheet, a member made of the galvanized steel sheet, and a method for manufacturing the same.
  • Patent Document 1 describes steel sheets that are used as materials for such automotive members. "Expressed in mass %, C 0.04 to 0.22%, Si 1.0% or less, Mn 3.0% or less, P 0.05% or less, S 0.01% or less, It contains 0.01 to 0.1% of Al and 0.001 to 0.005% of N, and has a component composition consisting of the balance Fe and inevitable impurities, and has a ferrite phase as the main phase and a second phase.
  • a high-strength steel sheet having excellent stretch-flange formability and collision resistance, characterized by being composed of a certain martensite phase and having a maximum grain size of 2 ⁇ m or less and an area ratio of the martensite phase of 5% or more. is disclosed.
  • Patent Document 2 "A hot-dip galvanized layer on the surface of a cold-rolled steel sheet obtained by pre-plating 0.2 g/m 2 or more and 2.0 g/m 2 or less of Ni on a cold-rolled steel sheet from which the surface layer has been ground and removed to a thickness of 0.1 ⁇ m or more.
  • a galvanized steel sheet in % by mass, C: 0.05% or more, 0.4% or less, Si: 0.01% or more, 3.0% or less, Mn: 0.1% or more, 3.0% or less, P: 0.04% or less, S: 0.05% or less, N: 0.01% or less, Al: 0.01% or more, 2.0% or less, Si+Al>0.5%, containing, the remainder consisting of Fe and unavoidable impurities,
  • the microstructure contains 40% or more ferrite as the main phase in volume fraction, 8% or more retained austenite, and the following three types of martensite [1] [2] [3] martensite [3] and 1% or more of bainite and 0 to 10% of pearlite, and the three types of martensite [1] [2] [3] each have a volume fraction of Martensite [1]: 0% or more and 50% or less, Martensite [2]: 0% or more and less than 20%, Martensite [3]: 1% or more and
  • Patent Document 3 "In mass%, C: 0.15% or more and 0.25% or less, Si: 0.50% or more and 2.5% or less, Mn: 2.3% or more and 4.0% or less, P: 0.100% Below, S: 0.02% or less, Al: 0.01% or more and 2.5% or less, and the balance having a component composition consisting of Fe and unavoidable impurities, In terms of area ratio, tempered martensite phase: 30% to 73%, ferrite phase: 25% to 68%, retained austenite phase: 2% to 20%, other phases: 10% or less (including 0%) And, as the other phases, martensite phase: 3% or less (including 0%), bainitic ferrite phase: less than 5% (including 0%), and the average of the tempered martensite phase A high-strength hot-dip galvanized steel sheet having a steel sheet structure with a grain size of 8 ⁇ m or less and a C content in the retained austenite phase of less than 0.7% by mass. ” is disclosed.
  • YS yield stress
  • impact absorbed energy energy absorbed at the time of impact
  • TS and YS of a steel sheet generally reduces formability, particularly properties such as ductility, work hardening ability, and hole expansibility. These properties correlate with the resistance to cracking of members in bending crush tests and axial crush tests that simulate collision tests. Therefore, if such a steel sheet with increased TS and YS is applied to the impact energy absorption member of the above-mentioned automobile, not only is it difficult to form, but the member will crack in a test that simulates a crash test.
  • the impact energy absorbing member is limited to a steel plate with a TS of 590 MPa class. Work hardenability and hole expansibility correlate with stretchability and stretch flangeability, respectively.
  • the steel sheets disclosed in Patent Documents 1 to 3 cannot be said to have TS: 980 MPa or more, high YS, and excellent ductility, work hardening ability and hole expansibility.
  • the present invention has been developed in view of the above-mentioned current situation, and provides a galvanized steel sheet having a TS of 980 MPa or more, a high YS, and excellent ductility, work hardening ability and hole expansibility. It is an object to provide, together with an advantageous manufacturing method. Another object of the present invention is to provide a member made of the above galvanized steel sheet, and a method for manufacturing the same.
  • ⁇ YS measured in a tensile test in accordance with JIS Z 2241 satisfies the following formula according to TS measured in the tensile test
  • TS measured in the tensile test 980 MPa ⁇ TS ⁇ 1180 MPa, 550 MPa ⁇ YS
  • 1180 MPa ⁇ TS ⁇ 1310 MPa 700 MPa ⁇ YS 800 MPa ⁇ YS when 1310 MPa ⁇ TS
  • the total elongation (El) measured by a tensile test in accordance with JIS Z 2241 satisfies the following formula according to the TS measured by the tensile test
  • 980 MPa ⁇ TS ⁇ 1180 MPa 13.0% ⁇ El
  • 1180 MPa ⁇ TS ⁇ 1310 MPa 12.0% ⁇ El 10.0% ⁇ El when 1310 MPa ⁇ TS ⁇
  • the chemical composition of the base steel sheet of the galvanized steel sheet is appropriately adjusted, and the steel structure of the base steel sheet of the galvanized steel sheet is Ferrite area ratio: 65.0% or less (including 0%), Area ratio of bainitic ferrite: 5.0% or more and 40.0% or less, Area ratio of tempered martensite: 0.5% or more and 80.0% or less, Area ratio of retained austenite: 3.0% or more, Area ratio of fresh martensite: 20.0% or less (including 0%), S BF + STM + 2 x SMA : 65.0% or more, SMA1 / SMA : 0.80 or less and SMA2 / SMA : 0.20 or more, so that TS: 980 MPa or more, high YS, excellent ductility, work hardening ability and hole It was found that a galvanized steel sheet having spreadability can be obtained. The present invention has been completed based on the above findings and further studies.
  • the gist and configuration of the present invention are as follows. 1.
  • a galvanized steel sheet having a base steel sheet and a galvanized layer on the surface of the base steel sheet,
  • the base steel plate is in % by mass, C: 0.050% or more and 0.400% or less, Si: 0.20% or more and 3.00% or less, Mn: 1.00% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0200% or less, Al: 0.010% or more and 2.000% or less, N: 0.0100% or less, a carbon equivalent Ceq of 0.540% or more, and the balance being Fe and unavoidable impurities.
  • the base steel plate is Ferrite area ratio: 65.0% or less (including 0%), Area ratio of bainitic ferrite: 5.0% or more and 40.0% or less, Area ratio of tempered martensite: 0.5% or more and 80.0% or less, Area ratio of retained austenite: 3.0% or more, Area ratio of fresh martensite: 20.0% or less (including 0%), S BF + STM + 2 x SMA : 65.0% or more, having a steel structure in which SMA1/SMA: 0.80 or less and SMA2 / SMA : 0.20 or more; A galvanized steel sheet having a tensile strength of 980 MPa or more.
  • S BF Area ratio of the bainitic ferrite
  • STM Area ratio of the tempered martensite
  • SMA Area ratio of the hard second phase composed of the retained austenite and the fresh martensite
  • SMA1 The hard second phase
  • S MA2 the hard second phase Of the constituent island regions, 1% or more of the circumference is the total area ratio of the island regions in contact with the bainitic ferrite.
  • the chemical composition of the base steel plate is further, in mass%, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.100% or less, B: 0.0100% or less, Cu: 1.000% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 0.500% or less, Sb: 0.200% or less, Sn: 0.200% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.0200% or less, Ca: 0.0200% or less, Ce: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0200% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, 2.
  • a plating step of galvanizing the cold-rolled steel sheet to form a galvanized steel sheet a second cooling step of cooling the galvanized steel sheet to a second cooling stop temperature of 100° C. or more and less than 300° C.;
  • the galvanized steel sheet is reheated to a reheating temperature of (the second cooling stop temperature + 50 ° C.) or higher and 500 ° C. or lower, and the galvanized steel sheet is heated to a reheating temperature of (the second cooling stop temperature + 50 ° C.) or higher and 500 ° C. or lower.
  • a reheating step of holding in the temperature range for 10 seconds or more and 2000 seconds or less; has A method for producing a galvanized steel sheet, wherein the first cooling stop temperature and the temperature of the galvanizing bath in the galvanizing treatment satisfy the relationship of the following formula (1). ⁇ 150° C. ⁇ T 0 ⁇ T 1 ⁇ 50° C. (1)
  • T0 is the first cooling stop temperature (°C)
  • T1 is the temperature (°C) of the zinc plating bath in the zinc plating treatment.
  • a method for producing a member comprising the step of subjecting the galvanized steel sheet according to any one of the above 1 to 8 to at least one of forming and joining to form a member.
  • a galvanized steel sheet having a TS of 980 MPa or more, a high YS, and excellent ductility, work hardening ability and hole expansibility can be obtained.
  • members made of the galvanized steel sheet of the present invention have high strength and excellent impact resistance, they can be very advantageously applied to impact energy absorbing members of automobiles.
  • (A) is an example of a tissue image obtained by SEM used for tissue identification
  • (B) is the tissue image of (A) color-coded using Adobe Photoshop of Adobe Systems.
  • (A) is an example of a tissue image obtained by SEM used for determining the island-shaped regions of the hard second phase, in particular, an example of a tissue image including the island-shaped regions determined by MA1
  • (B) is an example of (A).
  • the tissue image of is color-coded using Adobe Photoshop of Adobe Systems.
  • (A) is an example of a tissue image obtained by SEM used for determining the island-shaped regions of the hard second phase, in particular, an example of a tissue image including the island-shaped regions determined by MA2;
  • the tissue image of is color-coded using Adobe Photoshop of Adobe Systems.
  • (A) is an example of a tissue image obtained by SEM used for determining the island-shaped regions of the hard second phase, particularly an example of a tissue image including the island-shaped regions determined by MA3, and (B) is an example of (A)
  • the tissue image of is color-coded using Adobe Photoshop of Adobe Systems.
  • (A) is a schematic diagram for explaining a method for evaluating resistance weld cracking resistance in a weld zone, and the upper diagram of (B) is a top view of a plate assembly after resistance spot welding used in the same evaluation, and (B ) is a sectional view taken along the line AA of the upper figure.
  • C 0.050% or more and 0.400% or less C generates appropriate amounts of fresh martensite, tempered martensite, bainitic ferrite, and retained austenite to ensure a TS of 980 MPa or more and a high YS. It is an effective element.
  • the C content is less than 0.050%, the ferrite area ratio increases, making it difficult to increase the TS to 980 MPa or more. In addition, a decrease in YS is also caused.
  • the C content exceeds 0.400%, the carbon concentration in retained austenite increases excessively. Therefore, when a steel plate is punched, the hardness of fresh martensite generated from retained austenite increases significantly.
  • the C content should be 0.050% or more and 0.400% or less.
  • the C content is preferably 0.100% or more. Also, the C content is preferably 0.300% or less.
  • Si 0.20% to 3.00% Si suppresses the formation of carbides during annealing and promotes the formation of retained austenite. That is, Si is an element that affects the area ratio of retained austenite and the carbon concentration in retained austenite.
  • Si content if the Si content is less than 0.20%, the area ratio of retained austenite decreases and the ductility decreases.
  • the Si content exceeds 3.00%, the ferrite area ratio increases excessively, making it difficult to increase the TS to 980 MPa or more. In addition, a decrease in YS is also caused.
  • the carbon concentration in retained austenite increases excessively.
  • the Si content should be 0.20% or more and 3.00% or less.
  • the Si content is preferably 0.40% or more.
  • the Si content exceeds 2.00%, the resistance weld cracking resistance may be lowered, so the Si content is preferably 2.00% or less.
  • Mn 1.00% or more and less than 3.50%
  • Mn is an element that adjusts the area ratio of bainitic ferrite, tempered martensite, and the like.
  • the Mn content is less than 1.00%, the ferrite area ratio increases excessively, making it difficult to achieve a TS of 980 MPa or more. In addition, a decrease in YS is also caused.
  • the Mn content is 3.50% or more, the area ratio of bainitic ferrite decreases and the area ratio of tempered martensite excessively increases. As a result, the desired ductility is not obtained. Therefore, the Mn content should be 1.00% or more and less than 3.50%.
  • the Mn content is preferably 1.80% or more. Also, the Mn content is preferably less than 3.20%.
  • P 0.001% or more and 0.100% or less
  • P is an element that has a solid-solution strengthening action and increases the strength of the steel sheet.
  • the P content is made 0.001% or more.
  • P segregates at the prior austenite grain boundaries and embrittles the grain boundaries. Therefore, when the steel sheet is punched, the amount of voids generated increases, leading to a decrease in hole expansibility. Therefore, the P content should be 0.001% or more and 0.100% or less.
  • the P content is preferably 0.030% or less.
  • S 0.0200% or less S exists as a sulfide in steel.
  • the S content exceeds 0.0200%, the ultimate deformability of the steel sheet is lowered. Therefore, when the steel sheet is punched, the amount of voids generated increases, leading to a decrease in hole expansibility. Therefore, the S content should be 0.0200% or less.
  • the S content is preferably 0.0080% or less. Although the lower limit of the S content is not specified, it is preferable that the S content is 0.0001% or more due to production technology restrictions.
  • Al 0.010% to 2.000%
  • Al suppresses the formation of carbide during annealing and promotes the formation of retained austenite. That is, Al is an element that affects the area ratio of retained austenite and the carbon concentration in retained austenite. In order to obtain such effects, the Al content is set to 0.010% or more. On the other hand, if the Al content exceeds 2.000%, the ferrite area ratio increases excessively, making it difficult to increase the TS to 980 MPa or more. In addition, a decrease in YS is also caused. Therefore, the content of Al is set to 0.010% or more and 2.000% or less. The Al content is preferably 0.015% or more. Also, the Al content is preferably 1.000% or less.
  • N 0.0100% or less N exists as a nitride in steel.
  • the N content exceeds 0.0100%, the ultimate deformability of the steel sheet is lowered. Therefore, when the steel sheet is punched, the amount of voids generated increases, leading to a decrease in hole expansibility. Therefore, the N content should be 0.0100% or less.
  • the N content is preferably 0.0050% or less.
  • the lower limit of the N content is not particularly specified, it is preferable that the N content is 0.0005% or more due to production technology restrictions.
  • Carbon Equivalent Ceq 0.540% or More Carbon equivalent Ceq affects TS. In particular, when the carbon equivalent Ceq is less than 0.540%, it becomes difficult to increase the TS to 980 MPa or more. Therefore, the carbon equivalent Ceq is set to 0.540% or more.
  • the basic components of the base steel sheet of the galvanized steel sheet according to one embodiment of the present invention have been described above.
  • the balance has a component composition containing Fe (iron) and unavoidable impurities.
  • the base steel sheet of the galvanized steel sheet according to one embodiment of the present invention preferably has a chemical composition containing the above-mentioned basic ingredients and the balance being Fe and unavoidable impurities.
  • the substrate steel sheet of the galvanized steel sheet according to one embodiment of the present invention may contain at least one selected from the following optional ingredients in addition to the basic ingredients described above. In addition, since the effect of this invention is acquired if it contains the optional component shown below below the upper limit shown below, a lower limit in particular is not set.
  • the optional components are included as unavoidable impurities.
  • Ti 0.200% or less Ti increases TS by forming fine carbides, nitrides or carbonitrides during hot rolling or annealing. In order to obtain such effects, it is preferable to set the Ti content to 0.001% or more. The Ti content is more preferably 0.005% or more. On the other hand, when the Ti content exceeds 0.200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, if diffusible hydrogen is present in the steel sheet, coarse precipitates and inclusions may become crack initiation points during the hole expanding test, that is, the hole expandability may be deteriorated. Therefore, when Ti is contained, the Ti content is preferably 0.200% or less. The Ti content is more preferably 0.060% or less.
  • Nb 0.200% or less
  • the 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 generated. In such a case, if diffusible hydrogen is present in the steel sheet, coarse precipitates and inclusions may become crack initiation points during the hole expanding test, that is, the hole expandability may be deteriorated. Therefore, when Nb is contained, the Nb content is preferably 0.200% or less.
  • the Nb content is more preferably 0.060% or less.
  • V 0.100% or less
  • V raises TS by forming fine carbides, nitrides or carbonitrides during hot rolling or annealing.
  • the 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 generated. In such a case, if diffusible hydrogen is present in the steel sheet, coarse precipitates and inclusions may become crack initiation points during the hole expanding test, that is, the hole expandability may be deteriorated. Therefore, when V is contained, the V content is preferably 0.100% or less.
  • the V content is more preferably 0.060% or less.
  • B 0.0100% or less
  • B is an element that increases the hardenability by segregating at the austenite grain boundary.
  • B is an element that suppresses the formation of ferrite and grain growth during cooling after annealing.
  • the B content is preferably 0.0001% or more.
  • the B content is more preferably 0.0002% or more.
  • the B content exceeds 0.0100%, cracks may occur inside the steel sheet during hot rolling, which may reduce the ultimate deformability of the steel sheet.
  • the B content is preferably 0.0100% or less.
  • the B content is more preferably 0.0050% or less.
  • Cu 1.000% or less
  • Cu is an element that enhances hardenability.
  • Cu is an element effective for adjusting the area ratio of hard fresh martensite and the like to a more suitable range, thereby adjusting TS to a more suitable range.
  • the Cu content is preferably 0.005% or more.
  • Cu content is more preferably 0.020% or more.
  • the area ratio of fresh martensite increases excessively, resulting in excessively high TS. Also, a large amount of coarse precipitates and inclusions may be generated.
  • the Cu content is preferably 1.000% or less.
  • the Cu content is more preferably 0.200% or less.
  • Cr 1.000% or less
  • Cr is an element that enhances hardenability
  • Cr is an element that is effective for generating retained austenite and fresh martensite.
  • the Cr content is preferably 0.0005% or more.
  • the Cr content is more preferably 0.010% or more from the viewpoint of making TS in a more suitable range.
  • the Cr content exceeds 1.000%, the area ratio of hard fresh martensite increases excessively, which may lead to a decrease in hole expansibility. Therefore, when Cr is contained, the Cr content is preferably 1.000% or less.
  • the Cr content is more preferably 0.250% or less, and still more preferably 0.100% or less.
  • Ni 1.000% or less
  • Ni is an element that enhances hardenability. Further, Ni is an element effective for adjusting the area ratio of retained austenite and fresh martensite to a more suitable range, thereby adjusting TS to a more suitable range.
  • the Ni content is preferably 0.005% or more.
  • the Ni content is more preferably 0.020% or more.
  • the Ni content exceeds 1.000%, the area ratio of fresh martensite may excessively increase, and ductility and dimensional accuracy during forming may deteriorate. Also, a large amount of coarse precipitates and inclusions may be generated.
  • the Ni content is preferably 1.000% or less.
  • the Ni content is more preferably 0.800% or less.
  • Mo 0.500% or less
  • Mo is an element that enhances hardenability. Moreover, Mo is an effective element for generating hard fresh martensite and the like. In order to obtain such effects, the Mo content is preferably 0.010% or more. 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 increases excessively, which may lead to a decrease in hole expansibility. Therefore, when Mo is contained, the Mo content is preferably 0.500% or less. The Mo content is more preferably 0.450% or less, still more preferably 0.400% or less.
  • Sb 0.200% or less
  • Sb is an element effective for suppressing the diffusion of C in the vicinity of the steel sheet surface during annealing and controlling the formation of a soft layer in the vicinity of the steel sheet surface.
  • the 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 0.200% or less
  • Sn is an element effective in suppressing the diffusion of C in the vicinity of the steel sheet surface during annealing and controlling the formation of a soft layer in the vicinity of the steel sheet surface.
  • the 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 0.100% or less Ta, like Ti, Nb and V, raises TS by forming fine carbides, nitrides or carbonitrides during hot rolling and annealing.
  • Ta partially dissolves in Nb carbides and Nb carbonitrides to form complex precipitates such as (Nb, Ta) (C, N). This suppresses coarsening of precipitates and stabilizes precipitation strengthening. This improves TS and also YS.
  • the Ta content is preferably 0.100% or less.
  • W 0.500% or less W is an effective element for improving hardenability and adjusting TS to a more suitable range.
  • the W content is preferably 0.001% or more.
  • the W content is more preferably 0.030% or more.
  • the W content is preferably 0.500% or less.
  • the W content is more preferably 0.450% or less, still more preferably 0.400% or less.
  • Mg 0.0200% or less
  • Mg is an element effective for making inclusions such as sulfides and oxides spherical and improving the ultimate deformability and hole expandability of the steel sheet.
  • the Mg content is preferably 0.0001% or more.
  • the Mg content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, if diffusible hydrogen exists in the steel sheet, coarse precipitates and inclusions may become crack initiation points during the hole expanding test, that is, the hole expandability may be deteriorated. Therefore, when Mg is contained, the Mg content is preferably 0.0200% or less.
  • Zn 0.0200% or less
  • Zn is an element effective for making the shape of inclusions spherical and improving the ultimate deformability and hole expandability of the steel sheet.
  • the Zn content is preferably 0.0010% or more.
  • the Zn content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, if diffusible hydrogen exists in the steel sheet, coarse precipitates and inclusions may become crack initiation points during the hole expanding test, that is, the hole expandability may be deteriorated. Therefore, when Zn is contained, the Zn content is preferably 0.0200% or less.
  • Co 0.0200% or less
  • Co is an element effective in making inclusions spherical and improving the ultimate deformability and hole expandability of the steel sheet.
  • the Co content is preferably 0.0010% or more.
  • the Co content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, if diffusible hydrogen is present in the steel sheet, coarse precipitates and inclusions may become crack initiation points during the hole expanding test, that is, the hole expandability may be deteriorated. Therefore, when Co is contained, the Co content is preferably 0.0200% or less.
  • Zr 0.0200% or less
  • Zr is an element effective in making inclusions spherical and improving the ultimate deformability and hole expandability of the steel sheet.
  • the Zr content is preferably 0.0010% or more.
  • the Zr content exceeds 0.0200%, a large amount of coarse precipitates and inclusions may be generated. In such a case, if diffusible hydrogen is present in the steel sheet, coarse precipitates and inclusions may become crack initiation points during the hole expanding test, that is, the hole expandability may be deteriorated. Therefore, when Zr is contained, the Zr content is preferably 0.0200% or less.
  • Ca 0.0200% or less
  • Ca exists as inclusions in steel.
  • the Ca content exceeds 0.0200%, a large amount of coarse inclusions may be generated. In such a case, if diffusible hydrogen exists in the steel sheet, the coarse inclusions may become crack initiation points during the hole expanding test, that is, the hole expandability may be deteriorated. Therefore, when Ca is contained, the Ca content is preferably 0.0200% or less.
  • the Ca content is preferably 0.0020% or less.
  • the lower limit of the Ca content is not particularly limited, the Ca content is preferably 0.0005% or more. Moreover, from the restrictions on production technology, 0.0010% or more of Ca content is more preferable.
  • the contents of Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi and REM are each preferably 0.0001% or more.
  • the contents of Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi and REM each exceed 0.0200%, a large amount of coarse precipitates and inclusions may be generated. be.
  • coarse precipitates and inclusions may become crack initiation points during the hole expanding test, that is, the hole expandability may be deteriorated. 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 one embodiment of the present invention is in % by mass, C: 0.050% or more and 0.400% or less, Si: 0.20% or more and 3.00% or less, Mn: 1.00% or more and less than 3.50%, P: 0.001% or more and 0.100% or less, S: 0.0200% or less, Al: 0.010% or more and 2.000% or less, N: 0.0100% or less, and a carbon equivalent Ceq of 0.540% or more, optionally, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.100% or less, B: 0.0100% or less, Cu: 1.000% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 0.500% or less, Sb: 0.200% or less, Sn: 0.200% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Mg
  • the steel structure of the base steel sheet of the galvanized steel sheet according to one embodiment of the present invention is Ferrite area ratio: 65.0% or less (including 0%), Area ratio of bainitic ferrite: 5.0% or more and 40.0% or less, Area ratio of tempered martensite: 0.5% or more and 80.0% or less, Area ratio of retained austenite: 3.0% or more, Area ratio of fresh martensite: 20.0% or less (including 0%), S BF + STM + 2 x SMA : 65.0% or more, A steel structure in which SMA1/SMA: 0.80 or less and SMA2 / SMA : 0.20 or more.
  • S BF Area ratio of the bainitic ferrite
  • STM Area ratio of the tempered martensite
  • SMA Area ratio of the hard second phase composed of the retained austenite and the fresh martensite
  • the hard second phase Among the constituent island regions, the total area ratio of the island regions having an equivalent circle diameter of 2.0 ⁇ m or more and having 20% or less of the circumference contacting the tempered martensite SMA2 : The hard second phase Of the constituent island regions, 1% or more of the circumference is the total area ratio of the island regions in contact with the bainitic ferrite.
  • the area ratio of ferrite is set to 65.0% or less.
  • the area ratio of ferrite is preferably 35.0% or less, more preferably 25.0% or less.
  • the 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.
  • Area ratio of bainitic ferrite 5.0% or more and 40.0% or less
  • Bainitic ferrite has intermediate hardness between soft ferrite and hard fresh martensite, etc., and ensures good hole expandability. is an important phase for Bainitic ferrite is also a useful phase for obtaining an appropriate amount of retained austenite by utilizing the diffusion of C from bainitic ferrite to untransformed austenite. Therefore, the area ratio of bainitic ferrite is set to 5.0% or more. Also, the area ratio of bainitic ferrite is preferably 10.0% or more. On the other hand, if the area ratio of bainitic ferrite is excessively increased, the hole expansibility is rather lowered. Therefore, the area ratio of bainitic ferrite is set to 40.0% or less. Also, the area ratio of bainitic ferrite is preferably 35.0% or less.
  • the area ratio of tempered martensite 0.5% or more and 80.0% or less important phase. Therefore, the area ratio of tempered martensite is set to 0.5% or more.
  • the area ratio of tempered martensite is preferably 40.0% or more.
  • the area ratio of tempered martensite is set to 80.0% or less. Further, the area ratio of tempered martensite is preferably 75.0% or less.
  • the area ratio of retained austenite is made 3.0% or more.
  • the area ratio of retained austenite is preferably 5.0% or more.
  • the 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.
  • Area ratio of fresh martensite 20.0% or less (including 0%)
  • the area ratio of fresh martensite is set to 20.0% or less from the viewpoint of ensuring good hole expandability.
  • the 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 the residual structure other than the above is preferably 10.0% or less.
  • the area ratio of the residual tissue is more preferably 5.0% or less.
  • the area ratio of the residual tissue may be 0%.
  • the residual structure is not particularly limited, and examples thereof include carbides such as lower bainite, pearlite, and cementite. The type of residual tissue can be confirmed, for example, by observation using a SEM (Scanning Electron Microscope).
  • the area ratios of ferrite, bainitic ferrite, tempered martensite, and hard second phase are measured at the 1/4 thickness position of the base steel sheet as follows. That is, a sample is cut out from the base steel plate so that the plate thickness cross-section parallel to the rolling direction of the base steel plate becomes the observation surface. Next, the observation surface of the sample is mirror-polished using diamond paste. Then, the observation surface of the sample was subjected to final polishing using colloidal silica, and then subjected to 3 vol. Etch with % nital to reveal the tissue.
  • Ferrite A black region with a blocky shape. In addition, it contains almost no iron-based carbides. However, when iron-based carbide is included, the area of ferrite is also included in the area of ferrite. The same applies to bainitic ferrite and tempered martensite, which will be described later.
  • Bainitic ferrite A black to dark gray region with a massive or irregular shape. In addition, it does not contain iron-based carbides or contains a relatively small number of iron-based carbides.
  • Tempered martensite A gray area with an amorphous shape. In addition, it contains a relatively large number of iron-based carbides.
  • Hard second phase restored austenite + fresh martensite
  • the austenite structure in the annealing process can be reproduced from these structures and confirmed. Such points serve as judgment materials for tissue identification.
  • the fact that the C concentration and Mn concentration differ depending on the tissue serves as a criterion for identifying the tissue.
  • the C concentration of ferrite and bainitic ferrite is lower than the C concentration of a region mainly composed of tempered martensite (including fine hard secondary phases, carbides, etc.).
  • the Mn concentration may be lower than in other tissues.
  • the difference in hardness depending on the structure is a criterion for identifying the structure.
  • ferrite has the lowest hardness and hard secondary phase has the highest hardness.
  • Bainitic ferrite and tempered martensite also exhibit a hardness between that of ferrite and that of hard secondary phases.
  • FIG. 1(A) is a part extracted from one visual field of the sample observation area (25.6 ⁇ m ⁇ 17.6 ⁇ m) for the above explanation.
  • the area ratio of retained austenite is measured as follows. That is, after mechanically grinding the substrate steel plate to the 1/4 position of the plate thickness in the plate thickness direction (depth direction), chemical polishing with oxalic acid is performed to obtain an observation surface. Then, the observation surface is observed by the X-ray diffraction method. CoK ⁇ rays were used as the incident X-rays, and the diffraction intensity of the (200), (211) and (220) planes of bcc iron (200), (220) and (311) planes of fcc iron (austenite) were compared.
  • the ratio of the diffraction intensity of each surface is obtained, and the volume fraction of retained austenite is calculated from the ratio of the diffraction intensity of each surface. Then, assuming that the retained austenite is three-dimensionally homogeneous, the volume ratio of the retained austenite is defined as the area ratio of the retained austenite.
  • the area ratio of fresh martensite is obtained by subtracting the area ratio of retained austenite from the area ratio of the hard second phase obtained as described above.
  • [Area ratio of fresh martensite (%)] [Area ratio of hard second phase (%)] - [Area ratio of retained austenite (%)]
  • the area ratio of the residual structure 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 the hard second phase obtained as described above from 100%.
  • Ask. [Area ratio of residual structure (%)] 100 - [Area ratio of ferrite (%)] - [Area ratio of bainitic ferrite (%)] - [Area ratio of tempered martensite (%)] - [Hard Second phase area ratio (%)]
  • S BF + STM + 2 x SMA 65.0% or more From the viewpoint of ensuring a TS of 980 MPa or more, S BF + STM + 2 x SMA is set to 65.0% or more.
  • the upper limit of S BF +S TM +2 ⁇ S MA is not particularly limited, but is preferably 130.0% or less.
  • SMA area ratio of a hard second phase composed of retained austenite and fresh martensite.
  • a hard second phase (hereinafter also referred to as MA ) composed of retained austenite and fresh martensite is composed of a plurality of island regions.
  • an island region (hereinafter also referred to as MA1) having an equivalent circle diameter of 2.0 ⁇ m or more and having a circumference of 20% or less in contact with tempered martensite (hereinafter also referred to as MA1) has a solid solution C Low concentration.
  • the stability of retained austenite contained in MA1 is low. Therefore, MA1 does not contribute to ensuring good ductility.
  • MA1 since MA1 has a high ratio of fresh martensite, MA1 reduces the hole expandability.
  • S MA1 /S MA which is the ratio of the area ratio of MA1 to the area ratio of the hard second phase, is set to 0.80 or less.
  • S MA1 /S MA is preferably 0.75 or less, more preferably 0.40 or less.
  • S MA1 /S MA is preferably 0.50 or less, more preferably 0.30 or less.
  • the lower limit of S MA1 /S MA is not particularly limited, and may be zero.
  • Each island-shaped region is separated from other island-shaped regions of the second hard phase by a phase other than the hard second phase (individual island-shaped regions are phase). Further, the specific shape of each island-shaped region is not particularly limited, and may be, for example, circular, elliptical, polygonal, or amoeba-shaped (a shape extending in a plurality of irregular directions).
  • SMA2 / SMA 0.20 or more Of the island-shaped regions constituting the hard second phase composed of retained austenite and fresh martensite, 1% or more of the island-shaped region in contact with the bainitic ferrite of the circumference (hereinafter referred to as Also called MA2.) has a high dissolved C concentration. In other words, the stability of retained austenite contained in MA2 is high. Therefore, MA2 plays a very important role in ensuring good work hardenability and ductility. That is, if bainitic ferrite is formed under appropriate conditions during cooling after annealing, solute C that diffuses from the bainitic ferrite into the surrounding untransformed austenite does not sufficiently diffuse into the untransformed austenite.
  • S MA2 /S MA which is the ratio of the area ratio of MA2 to the area ratio of the hard second phase, should be 0.20 or more.
  • SMA2 / SMA is preferably 0.25 or more, more preferably 0.30 or more.
  • the upper limit of S MA2 /S MA is not particularly limited, and may be one. Moreover, from the viewpoint of ensuring high YS and excellent hole expansibility , SMA2 /SMA is preferably 0.98 or less when 980 MPa ⁇ TS ⁇ 1180 MPa is required. Moreover, when 1180 MPa ⁇ TS is required, S MA2 /S MA is preferably 0.70 or less.
  • the steel structure of the base steel sheet of the galvanized steel sheet according to one embodiment of the present invention further has SMA3 / SMA of 0.05 or more.
  • SMA3 Of the island-shaped regions constituting the hard second phase, 1% or more of the peripheral length is in contact with bainitic ferrite, and more than 20% of the peripheral length is in contact with tempered martensite. is the area ratio of
  • SMA3 / SMA 0.05 or more Of the island-shaped regions that constitute the hard second phase composed of retained austenite and fresh martensite, 1% or more of the peripheral length is in contact with the bainitic ferrite, and 1% or more of the peripheral length
  • An island-shaped region (hereinafter also referred to as MA3) in which more than 20% is in contact with tempered martensite has a particularly high dissolved C concentration among MA2. That is, in MA3, in addition to bainitic ferrite, solid solution C diffuses from tempered martensite, so the solid solution C concentration is particularly high. Therefore, MA3 particularly effectively contributes to ensuring good work hardening ability and ductility.
  • S MA3 /S MA which is the ratio of the area ratio of MA3 to the area ratio of the hard second phase, is preferably 0.05 or more.
  • SMA3 / SMA is preferably 0.07 or more, more preferably 0.10 or more.
  • the upper limit of S MA3 /S MA is not particularly limited, and may be one. Also, S MA3 /S MA is preferably 0.70 or less.
  • S MA1 , S MA2 and S MA3 are each measured as follows. That is, in the manner described above, ferrite, bainitic ferrite, tempered martensite and hard second phase ( (retained austenite + fresh martensite). Next, after color-coding (quaternary imaging) using Adobe Photoshop from Adobe Systems, the island-shaped regions of the hard second phase were extracted, and the equivalent circle diameter of each island-shaped region was measured using the open source ImageJ. , the perimeter of each island region, and the length of contact of each island region with bainitic ferrite and tempered martensite. Note that the pixel density of the tissue image when determining the circumference is set to 30 pixels/ ⁇ m or more and 100 pixels/ ⁇ m or less.
  • each island-shaped area corresponds to MA1, MA2, or MA3, respectively, and color-coded using Adobe Photoshop of Adobe Systems (for example, FIG. 2 (B), FIG. 3 (B) , and FIG. 4(B)), each area is calculated. Then, the total area of each of the island-shaped regions identified as MA1, MA2, and MA3 was divided by the area of the observed region (25.6 ⁇ m ⁇ 17.6 ⁇ m) and multiplied by 100 to calculate the value (area ratio) for 5 fields of view. do. Then, the average values of the values (area ratios) for five fields of view for each of MA1, MA2 and MA3 are defined as S MA1 , S MA2 and S MA3 .
  • the amount of diffusible hydrogen is 0.50 ppm by mass or less.
  • Amount of diffusible hydrogen in substrate steel sheet 0.50 ppm by mass or less
  • the amount of diffusible hydrogen in the substrate steel sheet is preferably 0.50 ppm by mass or less.
  • the amount of diffusible hydrogen in the base steel sheet is more preferably 0.35 ppm by mass or less.
  • the lower limit of the amount of diffusible hydrogen in the base steel sheet is not particularly specified, and may be 0 ppm by mass.
  • the amount of diffusible hydrogen in the base steel sheet is more preferably 0.01 ppm by mass or more.
  • the amount of diffusible hydrogen in the base steel sheet is measured as follows. Specifically, a test piece having a length of 30 mm and a width of 5 mm is taken from a galvanized steel sheet, and the galvanized layer is removed with an alkali. Then, the amount of hydrogen released from the test piece is measured by thermal desorption spectroscopy. Specifically, the test piece is continuously heated from room temperature to 300° C. at a heating rate of 200° C./h, and then cooled to room temperature. At this time, the amount of hydrogen released from the test piece (accumulated amount of hydrogen) is measured in the temperature range from room temperature to 210° C. during the continuous heating. Then, the measured amount of hydrogen is divided by the mass of the test piece (the test piece after removing the galvanized layer and before continuous heating), and the value converted to mass ppm is taken as the diffusible hydrogen amount of the base steel sheet.
  • test pieces are cut out from the products placed in a general usage environment, and diffusible hydrogen is removed from the base steel sheet in the same manner as described above. If the amount is 0.50 mass ppm or less, the amount of diffusible hydrogen in the base steel sheet of the galvanized steel sheet at the raw material stage before forming or joining is also 0.50 mass ppm or less. can be regarded as
  • the galvanized steel sheet according to one embodiment of the present invention preferably has a decarburized layer.
  • the base steel sheet of the galvanized steel sheet according to one embodiment of the present invention has a decarburized layer.
  • Steel sheets containing Si, particularly plated steel sheets using a steel sheet with a high Si content as a base steel sheet may have a problem of cracking due to liquid metal embrittlement (LME) during resistance spot welding.
  • LME liquid metal embrittlement
  • the galvanized steel sheet has a decarburized layer particularly on the surface layer of the base steel sheet, resistance weld cracking resistance can be improved even when the Si content of the base steel sheet is high.
  • the thickness of the decarburized layer in other words, the depth in the sheet thickness direction from the surface of the base steel sheet is preferably 30 ⁇ m or more, more preferably 40 ⁇ m or more.
  • 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 analyzed from the surface of the base steel sheet in the plate thickness direction, and the C concentration is 80% or less of the C content in the chemical composition of the base steel sheet. is defined as the thickness of the region.
  • the thickness of the decarburized layer is determined by surface analysis or line analysis of the elemental distribution near the surface layer of the base steel sheet using an electron probe micro analyzer (EPMA) for the cross-sectionally processed sample.
  • EPMA electron probe micro analyzer
  • a resin-embedded galvanized steel sheet is polished to finish a cross section perpendicular to the rolling direction for observation, and then removed from the resin to obtain a sample for measurement.
  • the acceleration voltage is 7 kV
  • the irradiation current is 50 nA
  • the C strength is measured by performing area analysis or line analysis of the sample cross section in 1 ⁇ m steps in a range of 300 ⁇ 300 ⁇ m including the outermost layer (surface) of the base steel plate.
  • plasma cleaners are used to remove hydrocarbons from the surface and surroundings of the sample in two places, the measurement room and the sample preparation room, before starting the measurement.
  • the measurement is performed while the sample temperature is maintained at a maximum of 100° C. on the stage.
  • the C intensity is converted to the C concentration (% by mass) using a calibration curve separately prepared by measuring a standard sample. Confirm that the C detection limit is sufficiently lower than 0.10% by mass due to the effect of suppressing contamination.
  • the details of the equipment used and the method of contamination control are as described in reference 1 below.
  • a line profile in the plate thickness direction is extracted from the surface of the base steel plate, and it is averaged for 300 points in the direction parallel to the base steel plate surface to obtain a profile of the C concentration in the plate thickness direction.
  • the obtained C concentration profile in the plate thickness direction is smoothed by a simple moving average method. At this time, it is preferable to set the number of smoothing points to about 21 points.
  • the thickness of the decarburized layer is determined by specifying the range in the plate thickness direction where the C concentration is 80% or less of the C content in the chemical composition of the base steel plate.
  • the tensile strength of the galvanized steel sheet according to one embodiment of the present invention is 980 MPa or more.
  • the tensile strength of the galvanized steel sheet according to one embodiment of the present invention is preferably 1180 MPa or more.
  • TS tensile strength
  • Yield stress Yield stress
  • El total elongation
  • n value work hardening index/yield ratio
  • ⁇ limit hole expansion rate
  • the galvanized layer of the galvanized steel sheet according to one embodiment of the present invention may be provided only on one surface of the base steel sheet, or may be provided on both surfaces.
  • the galvanized layer refers to a galvanized layer containing Zn as a main component (Zn content is 50% or more), and examples thereof include a hot-dip galvanized layer and an alloyed hot-dip galvanized layer.
  • the hot-dip galvanized layer is preferably composed of, for example, Zn, 20% by mass or less of Fe, and 0.001% by mass or more and 1.0% by mass or less of Al.
  • the hot-dip galvanized layer optionally includes one selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi and REM.
  • a total of 0 mass % or more and 3.5 mass % or less of the seed or two or more elements may be contained.
  • the Fe content of the hot-dip galvanized layer is more preferably less than 7% by mass. The remainder other than the above elements is unavoidable impurities.
  • the alloyed hot-dip galvanized layer is preferably composed of, for example, 20% by mass or less of Fe and 0.001% by mass or more and 1.0% by mass or less of Al.
  • the alloyed hot-dip galvanized layer is optionally selected from the group consisting of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi and REM. 0% by mass or more and 3.5% by mass or less in total of one or two or more elements of
  • the Fe content of the alloyed hot-dip galvanized layer is more preferably 7% by mass or more, and still more preferably 8% by mass or more. Further, the Fe content of the alloyed hot-dip galvanized layer is more preferably 15% by mass or less, still more preferably 12% by mass or less. The remainder other than the above elements is unavoidable impurities.
  • the coating weight per side of the galvanized layer is not particularly limited, it is preferably 20 to 80 g/m 2 .
  • the plating adhesion amount of the galvanized layer is measured as follows. That is, a treatment liquid is prepared by adding 0.6 g of a corrosion inhibitor against Fe ("Ibit 700BK" (registered trademark) manufactured by Asahi Chemical Industry Co., Ltd.) to 1 L of a 10% by mass hydrochloric acid aqueous solution. Then, a galvanized steel sheet as a test material is immersed in the treatment liquid to dissolve the galvanized layer. Then, the mass reduction amount of the test material before and after melting was measured, and the value was divided by the surface area of the base steel plate (the surface area of the part coated with plating) to obtain the coating amount (g / m 2 ) is calculated.
  • a corrosion inhibitor against Fe (Ibit 700BK" (registered trademark) manufactured by Asahi Chemical Industry Co., Ltd.)
  • the galvanized steel sheet according to one embodiment of the present invention may have a metal plating layer other than the galvanized layer in at least one of the base steel sheet and the galvanized layer.
  • the metal plating layer contributes to improvement of resistance weld crack resistance. Further, the formation of the metal plating layer can suppress resistance weld cracking even when the base steel sheet has a large Si content.
  • the mechanism by which the resistance weld cracking resistance is improved by the metal plating layer is not necessarily clear, but the inventors have found that when the metal plating layer is provided between the base steel sheet and the galvanized layer, in other words, on the surface of the base steel sheet, It is thought that the metal plating layer acts as a barrier layer that prevents the zinc in the galvanizing layer from melting and penetrating into the base steel plate during resistance spot welding, making it difficult for resistance weld cracks to occur (zinc intrusion suppression effect).
  • the galvanized layer is provided on both sides of the base steel sheet, only one of the layers between the base steel sheet and the galvanized layer may have a metal plating layer. may have a metal plating layer on both.
  • the adhesion amount of the metal plating layer is preferably more than 0 g/m 2 , more preferably 2.0 g/m 2 or more.
  • the upper limit of the amount of the metal plating layer deposited on one side is not particularly limited, from the viewpoint of cost, the amount of the metal plating layer deposited is preferably 60 g/m 2 or less.
  • the adhesion amount of the metal plating layer is more preferably 50 g/m 2 or less, still more preferably 40 g/m 2 or less, and even more preferably 30 g/m 2 or less. It should be noted that the adhesion amount of the metal plating layer referred to here is per one side.
  • the adhesion amount of the metal plating layer is measured as follows. That is, a 10 ⁇ 15 mm size sample is taken from a galvanized steel sheet and embedded in resin to obtain a cross-sectional embedded sample. Any three points of the cross section of the same sample are observed using a scanning electron microscope (SEM) at an acceleration voltage of 15 kV at a magnification of 2000 to 10000 times depending on the thickness of the metal plating layer. The thickness of the metal plating layer is measured at various points, and the average value is calculated. Then, the calculated average value is multiplied by the specific gravity of the metal forming the metal plating layer to convert to the adhesion amount per one side of the metal plating layer.
  • SEM scanning electron microscope
  • a metal with a higher melting point than Zn is desirable, and for example, metals such as Fe and Ni can be used. Further, in addition to the effect of suppressing zinc penetration described above, the following effect of suppressing a decrease in toughness can be expected, so the Fe-based plating layer is desirable.
  • the Fe-based plating layer when the amount of Si in the vicinity of the surface of the base steel sheet is large, it is thought that the toughness of the weld zone is lowered and the resistance weld cracking resistance of the weld zone is deteriorated.
  • the Fe-based plating layer when the Fe-based plating layer is provided between the base steel sheet and the galvanized layer, that is, on the surface of the base steel sheet, the Fe-based plating layer acts as a solid solution Si depleted layer, and the amount of Si dissolved in the weld zone is reduced. do. It is believed that this suppresses a decrease in the toughness of the weld zone and improves the resistance weld cracking resistance of the weld zone (effect of suppressing a decrease in toughness).
  • the Fe-based plating layer functions as a soft layer and relieves the stress applied to the steel sheet surface during resistance spot welding. As a result, it is believed that the residual stress in the weld zone can be reduced and the resistance weld crack resistance can be improved (stress relaxation effect).
  • Fe-based plating layer in addition to a pure Fe plating layer, for example, Fe--B alloy, Fe--C alloy, Fe--P alloy, Fe--N alloy, Fe--O alloy, Fe--Ni alloy, Fe--Mn Alloy plating layers such as alloys, Fe--Mo alloys, and Fe--W alloys can be used.
  • the component composition of the Fe-based plating layer is not particularly limited as long as the Fe content is 50% by mass or more. Contains 10% by mass or less in total of one or more elements selected from the group consisting of Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co, and the balance is Fe and unavoidable impurities Component composition is preferred.
  • the total content of these elements is 10% by mass or less to prevent a decrease in electrolysis efficiency and to form an Fe-based plating layer, particularly an Fe-based electroplating, at a low cost. Layers can be formed.
  • the C content is preferably 0.08% by mass or less.
  • the galvanized steel sheet according to one embodiment of the present invention may have a metal 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 plating layer, ( It becomes the decarburized layer) of the surface layer of the base steel plate).
  • a metal plating layer the C concentration is analyzed in the sheet thickness direction from the surface of the metal plating layer or the interface between the galvanized layer and the cold-rolled steel sheet by the above method, and the thickness of the decarburized layer (underlying thickness direction depth from the surface of the steel sheet) may be evaluated.
  • the plate thickness of the galvanized steel sheet according to one embodiment of the present invention is not particularly limited, it is preferably 0.5 mm or more and 3.0 mm or less.
  • a member according to one embodiment of the present invention is a member (as a raw material) using the galvanized steel sheet described above.
  • a galvanized steel sheet which is a raw material, is subjected to at least one of molding and joining to form a member.
  • the galvanized steel sheet has a TS of 980 MPa or more, a high YS, and excellent ductility, work hardening ability and hole expandability. Therefore, the member according to one embodiment of the present invention has high strength and excellent impact resistance. Therefore, the member according to one embodiment of the present invention is particularly suitable for application as an impact energy absorbing member used in the automobile field.
  • a method for manufacturing a galvanized steel sheet comprises: A hot-rolling step of hot-rolling a steel slab having the above chemical composition to form a hot-rolled steel sheet; A cold rolling step of cold rolling the hot-rolled steel sheet to form a cold-rolled steel sheet; An annealing step of annealing the cold-rolled steel sheet at an annealing temperature of 760° C. or more and 900° C. or less and an annealing time of 20 seconds or more; a first cooling step of cooling the cold-rolled steel sheet to a first cooling stop temperature of 300° C. or higher and 550° C. or lower; a holding step of holding the cold-rolled steel sheet in a temperature range of 300° C.
  • the galvanized steel sheet is reheated to a reheating temperature of (the second cooling stop temperature + 50 ° C.) or higher and 500 ° C. or lower, and the galvanized steel sheet is heated to a reheating temperature of (the second cooling stop temperature + 50 ° C.) or higher and 500 ° C. or lower.
  • a reheating step of holding in the temperature range for 10 seconds or more and 2000 seconds or less; has The first cooling stop temperature and the temperature of the zinc plating bath in the zinc plating process satisfy the relationship of the following formula (1). ⁇ 150° C. ⁇ T 0 ⁇ T 1 ⁇ 50° C. (1)
  • T0 is the first cooling stop temperature (°C)
  • T1 is the temperature (°C) of the zinc plating bath in the zinc plating treatment. Note that each of the above temperatures means the surface temperature of the steel slab and steel plate, unless otherwise specified.
  • a steel slab having the above chemical composition For example, a steel material is melted to obtain molten steel having the above chemical composition.
  • the smelting method is not particularly limited, and known smelting methods such as converter smelting and electric furnace smelting can be used.
  • the resulting molten steel is then solidified into a steel slab.
  • the method of obtaining a steel slab from molten steel is not particularly limited, and for example, a continuous casting method, an ingot casting method, a thin slab casting method, or the like can be used.
  • a continuous casting method is preferable from the viewpoint of preventing macro segregation.
  • Hot rolling process Then, the steel slab is hot rolled to obtain a hot rolled steel sheet.
  • Hot rolling may be performed by applying an energy saving process.
  • Energy-saving processes include direct rolling (a method in which steel slabs are not cooled to room temperature, but are charged into a heating furnace and hot rolled) or direct rolling (a steel slab is slightly heat-retained). a method of rolling immediately afterwards).
  • the hot rolling conditions are not particularly limited, and the hot rolling can be performed under the following conditions, for example. That is, the steel slab is once cooled to room temperature, then reheated and then rolled.
  • the slab heating temperature (reheating temperature) is preferably 1100° C. or higher from the viewpoint of dissolving carbides and reducing the rolling load.
  • the slab heating temperature is preferably 1300° C. or less.
  • the slab heating temperature is based on the surface temperature of the steel slab.
  • the steel slab is subjected to rough rolling according to a conventional method to obtain a rough rolled plate (hereinafter also referred to as sheet bar).
  • the sheet bar is subjected to finish rolling to obtain a hot-rolled steel sheet.
  • the slab heating temperature is lowered, it is preferable to heat the sheet bar using a bar heater or the like before finish rolling from the viewpoint of preventing troubles during finish rolling.
  • the finish rolling temperature is preferably the Ar 3 transformation point or higher.
  • Ar 3 transformation point is obtained by the following formula.
  • the sheet bars may be joined together during hot rolling, and finish rolling may be performed continuously. Also, the sheet bar may be wound once before finishing rolling. In order to reduce the rolling load during hot rolling, part or all of the finish rolling may be lubricated rolling. Performing lubricating rolling is also effective from the viewpoint of homogenizing the shape of the steel sheet and homogenizing the quality of the steel sheet.
  • the coefficient of friction during lubricating rolling is preferably in the range of 0.10 or more and 0.25 or less.
  • hot rolling processes including rough rolling and finish rolling generally steel slabs are rough rolled into sheet bars and finish rolled into hot rolled steel sheets. However, depending on the mill capacity, there is no problem as long as the predetermined size is achieved regardless of such classification.
  • the finish rolling temperature is preferably in the range of 800°C or higher and 950°C or higher.
  • the winding temperature is preferably 450° C. or higher and 750° C. or lower.
  • the hot-rolled steel sheet after the hot-rolling process is pickled.
  • pickling oxides on the surface of the steel sheet can be removed, and good chemical conversion treatability and plating quality are ensured.
  • pickling may be performed only once, and may be divided into several times and may be performed.
  • the pickling conditions are not particularly limited, and conventional methods may be followed.
  • Cold rolling process Then, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet.
  • Cold rolling is performed, for example, by multi-pass rolling requiring two or more passes, such as tandem multi-stand rolling or reverse rolling.
  • the rolling reduction in cold rolling is not particularly limited, it is preferably 20% or more and 80% or less. If the rolling reduction in cold rolling is less than 20%, the steel structure tends to become coarse and non-uniform in the annealing process, and the strength and workability of the final product may deteriorate. On the other hand, if the rolling reduction in cold rolling exceeds 80%, the shape of the steel sheet tends to be defective, and the amount of galvanized coating may become non-uniform.
  • the cold-rolled steel sheet obtained after cold rolling may also be pickled.
  • At least one of the cold-rolled steel sheets obtained as described above is optionally added after the cold rolling step and before the annealing step described later.
  • a metal plating treatment may be applied to form a metal plating layer on the surface of the .
  • a cold-rolled steel sheet having a metal-plated layer on at least one surface before undergoing an annealing step, which will be described later may be hereinafter referred to as a metal-plated steel sheet.
  • the metal plating method is not particularly limited, but electroplating is preferable from the viewpoint of manufacturability.
  • the metal plating bath a sulfuric acid bath, a hydrochloric acid bath, or a mixed solution of both can be used.
  • the adhesion amount of the metal plating layer can be adjusted by the energization time or the like.
  • the metal-plated steel sheet means a steel sheet having a metal-plated layer on at least one surface of the cold-rolled steel sheet before undergoing the annealing process described later. It does not exclude the aspect in which the steel sheet is pre-annealed.
  • a metal used for metal plating a metal with a higher melting point than Zn is desirable, and for example, metals such as Fe and Ni can be used.
  • metals such as Fe and Ni can be used.
  • the plating bath for forming the Fe-based plating layer contains B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and One or two or more elements selected from the group consisting of Co can be contained.
  • the total content of these elements in the plating bath is preferably 10% by mass or less in the chemical composition of the metal plating layer of the metal plated steel sheet.
  • Metal elements may be contained as metal ions, and non-metal elements may be contained as part of boric acid, phosphoric acid, nitric acid, organic acids, and the like.
  • the iron sulfate plating solution may contain conductivity aids such as sodium sulfate and potassium sulfate, chelating agents, and pH buffers.
  • degreasing treatment and water washing for cleaning the surface of the cold-rolled steel sheet As a pretreatment before the metal plating treatment, optionally, degreasing treatment and water washing for cleaning the surface of the cold-rolled steel sheet, further pickling treatment for activating the surface of the cold-rolled steel sheet and It may be washed with water. Following these pretreatments, the metal plating treatment described above is performed.
  • Methods of degreasing treatment and washing with water are not particularly limited, and ordinary methods can be used.
  • Various acids such as sulfuric acid, hydrochloric acid, nitric acid and mixtures thereof can be used in the pickling process. Among them, sulfuric acid, hydrochloric acid or mixtures thereof are preferred.
  • the concentration of the acid is not particularly specified, it is preferably about 1 to 20% by mass in consideration of the ability to remove the oxide film and the prevention of surface roughness (surface defects) due to over-acid washing.
  • the pickling treatment liquid may contain an antifoaming agent, a pickling accelerator, a pickling inhibitor, and the like.
  • the cold-rolled steel sheets (including metal plated steel sheets) obtained as described above are annealed at an annealing temperature of 760° C. or more and 900° C. or less for an annealing time of 20 seconds or more.
  • the number of annealing times may be two or more, but one time is preferable from the viewpoint of energy efficiency.
  • Annealing temperature 760° C. or higher and 900° C. or lower If the annealing temperature is lower than 760° C., the rate of austenite formation during heating in the two-phase region of ferrite and austenite becomes insufficient. Therefore, the area ratio of ferrite excessively increases after annealing, and YS decreases. In addition, the hole expansibility is also lowered. Furthermore, it becomes difficult to make TS 980 MPa or more. On the other hand, if the annealing temperature exceeds 900° C., the grain growth of austenite occurs excessively, and the formation rate of bainitic ferrite in the post-process slows down. This makes it impossible to obtain appropriate amounts of bainitic ferrite and retained austenite area ratios.
  • the annealing temperature should be 760° C. or higher and 900° C. or lower.
  • the annealing temperature is preferably 780°C or higher, more preferably above 790°C.
  • the annealing temperature is preferably 880° C. or lower.
  • the annealing temperature is the highest temperature reached in the annealing process.
  • Annealing time 20 seconds or more If the annealing time is less than 20 seconds, the rate of austenite formation during heating in the two-phase region of ferrite and austenite becomes insufficient. Therefore, the area ratio of ferrite excessively increases after annealing, and YS decreases. In addition, the hole expansibility is also lowered. Furthermore, it becomes difficult to set TS to 980 MPa or more. Therefore, the annealing time should be 20 seconds or more. Although the upper limit of the annealing time is not particularly limited, it is preferably 900 seconds or less. The annealing time is the holding time in the temperature range of (annealing temperature -40°C) to the annealing temperature.
  • the annealing time includes, in addition to the holding time at the annealing temperature, the holding time in the temperature range above (annealing temperature -40°C) and below the annealing temperature in heating and cooling before and after reaching the annealing temperature.
  • the dew point of the annealing atmosphere in the annealing step is preferably over -30°C.
  • the dew point is preferably -20°C or higher, more preferably -5°C or higher.
  • the upper limit of the dew point is not particularly limited, but from the viewpoint of suitably preventing oxidation of the surface of the cold-rolled steel sheet or metal plating layer and improving the plating adhesion when providing the zinc plating layer, the dew point is preferably 30 ° C. or less. preferable.
  • First cooling stop temperature 300° C. or higher and 550° C. or lower
  • the area ratio of tempered martensite increases excessively, and the area ratio of appropriate amounts of bainitic ferrite and retained austenite decreases. will not be obtained.
  • untransformed austenite may decompose into pearlite and carbides in the subsequent galvanizing process. Therefore, SMA2 / SMA and SMA3 / SMA are decreased, and ductility and work hardening ability are decreased.
  • the first cooling stop temperature exceeds 550°C, the area ratio of bainitic ferrite decreases and the area ratio of tempered martensite excessively increases.
  • the first cooling stop temperature is set to 300°C or higher and 550°C or lower.
  • the first cooling stop temperature is preferably 350° C. or higher.
  • the first cooling stop temperature is preferably 510° C. or lower.
  • the cold-rolled steel sheet is held in a temperature range of 300° C. or higher and 550° C. or lower (hereinafter also referred to as a holding temperature range) for 3 seconds or longer and 600 seconds or shorter.
  • Holding time in the holding temperature range 3 seconds to 600 seconds
  • bainitic ferrite is generated, and C diffuses from the generated bainitic ferrite to untransformed austenite adjacent to the bainitic ferrite. occurs.
  • a predetermined amount of area ratio of retained austenite is secured, and SMA2 / SMA and further SMA3 / SMA are increased.
  • the holding time in the holding temperature range is less than 3 seconds
  • the area ratio of bainitic ferrite decreases and the area ratio of tempered martensite excessively increases.
  • SMA2 / SMA and further SMA3 / SMA decrease, and ductility and work hardening ability decrease.
  • the holding time in the holding temperature range exceeds 600 seconds, the area ratio of bainitic ferrite may excessively increase, resulting in a decrease in YS.
  • C may excessively diffuse from bainitic ferrite to untransformed austenite, resulting in an increase in SMA1 / SMA and a decrease in hole expansibility.
  • C diffuses excessively inside the untransformed austenite, and it becomes impossible to make only the untransformed austenite around the bainitic ferrite locally high in solid solution C amount.
  • S MA2 /S MA and further S MA3 /S MA may decrease and ductility may decrease. Therefore, the holding time in the holding temperature range is 3 seconds or more and 600 seconds or less.
  • the holding time in the holding temperature range is preferably 5 seconds or longer, more preferably 10 seconds or longer. Also, the holding time in the holding temperature range is preferably less than 200 seconds, more preferably less than 80 seconds.
  • the retention time in the retention temperature range includes the retention time in the temperature range until the first cooling stop temperature is reached in the first cooling step, and the cooling time until the start of zinc plating treatment in the plating step described later. It includes the residence time of the rolled steel sheet in the relevant temperature range (for example, the residence time in the relevant temperature range until the cold-rolled steel sheet is immersed in the galvanizing bath). However, the holding time in the holding temperature range does not include the residence time of the galvanized steel sheet after hot-dip galvanizing in the plating process.
  • the cold-rolled steel sheet is subjected to galvanizing treatment to obtain a galvanized steel sheet.
  • galvanizing include hot dip galvanizing and alloyed galvanizing.
  • the first cooling stop temperature in the first cooling step described above and the temperature of the zinc plating bath in the zinc plating treatment (hereinafter also referred to as the plating bath temperature) have the following relationship: It is necessary to satisfy ⁇ 150° C. ⁇ T 0 ⁇ T 1 ⁇ 50° C. (1)
  • T0 is the first cooling stop temperature (°C)
  • T1 is the temperature (°C) of the zinc plating bath in the zinc plating treatment.
  • T 0 ⁇ T 1 exceeds 50° C. or is less than ⁇ 150° C.
  • SMA2 /S MA and SMA3 /S MA decrease, and work hardening ability and ductility decrease.
  • T 0 ⁇ T 1 is preferably ⁇ 120° C. or higher, more preferably ⁇ 100° C. or higher.
  • T 0 ⁇ T 1 is preferably 45° C. or less, more preferably 40° C. or less.
  • Conditions other than the above are not particularly limited, and may be performed in accordance with conventional methods.
  • the plating bath temperature is 440° C. or higher and 500° C. or lower.
  • the zinc plating bath is not particularly limited as long as it has the composition of the zinc plating layer described above, but for example, the Al content is 0.10% by mass or more and 0.23% by mass or less, and the balance is It is preferable to use a plating bath with a composition consisting of Zn and unavoidable impurities.
  • the galvanized steel sheet it is preferable to heat the galvanized steel sheet to an alloying temperature of 450° C. or higher and 600° C. or lower after hot-dip galvanizing as described above. If the alloying temperature is lower than 450° C., the Zn—Fe alloying speed becomes slow and alloying may become difficult. On the other hand, if the alloying temperature exceeds 600° C., untransformed austenite may transform into pearlite, resulting in a decrease in TS and ductility.
  • the alloying temperature is more preferably 470° C. or higher. Also, the alloying temperature is more preferably 570° C. or lower.
  • the coating weight of both hot-dip galvanized steel sheet (GI) and alloyed hot-dip galvanized steel sheet (GA) is 20 to 80 g/m 2 per side.
  • the amount of plating deposited can be adjusted by gas wiping or the like.
  • the galvanized steel sheet is held in a temperature range of 300 ° C. or higher and 550 ° C. or lower (hereinafter also referred to as an additional holding temperature range) for 3 seconds or more.
  • An additional holding step of holding for 600 seconds or less may be performed.
  • the additional holding step is a step for obtaining the same effect as the holding step.
  • the additional holding step may be performed after or during the plating step as long as it is before the second cooling step described later.
  • the additional holding process may be performed during the plating process. That is, the plating process may serve as the additional holding process.
  • the total holding time of the holding step and the additional holding step is preferably 3 seconds or more and 600 seconds or less. More preferably, the total holding time of the holding step and the additional holding step is less than 200 seconds.
  • Second cooling stop temperature 100 ° C. or more and less than 300 ° C.
  • the second cooling step is performed in order to control the area ratio of tempered martensite and the area ratio of retained austenite generated in the reheating step, which is a subsequent step, within a predetermined range. This is a necessary process.
  • the second cooling stop temperature is less than 100°C, substantially all of the untransformed austenite present in the steel is transformed into martensite in the second cooling step. This excessively increases the area ratio of tempered martensite and decreases the area ratio of retained austenite. As a result, ductility and work hardenability are reduced.
  • the second cooling stop temperature is 300° C.
  • the second cooling stop temperature is set at 100°C or higher and lower than 300°C.
  • the second cooling stop temperature is preferably 120°C or higher.
  • the second cooling stop temperature is preferably 280° C. or less.
  • the galvanized steel sheet is reheated to a reheating temperature of (said second cooling stop temperature + 50°C) or more and 500°C or less, and the galvanized steel sheet is heated to (said second cooling stop temperature + 50°C) or more and 500°C or less. (hereinafter also referred to as reheating temperature range) for 10 seconds or more and 2000 seconds or less.
  • reheating temperature range 10 seconds or more and 2000 seconds or less.
  • Reheating temperature (second cooling stop temperature + 50 ° C) or more and 500 ° C or less
  • the reheating temperature is less than (cooling stop temperature + 50 ° C)
  • the martensite present in the steel at the end of the second cooling process is not transformed. Diffusion of C into austenite does not proceed sufficiently, and a predetermined amount of area ratio of retained austenite cannot be obtained. This reduces ductility. Moreover, fresh martensite increases. Furthermore, the release of hydrogen contained in the base steel plate to the outside becomes insufficient, and the amount of diffusible hydrogen in the base steel plate increases. As a result, the hole expansibility is lowered.
  • the reheating temperature should be (cooling stop temperature + 50°C) or more and 500°C or less.
  • the reheating temperature is preferably (cooling stop temperature + 70°C) or higher.
  • the reheating temperature is preferably 450° C. or lower. Note that the reheating temperature is the highest temperature reached in the reheating process.
  • Holding time in the reheating temperature range 10 seconds or more and 2000 seconds or less
  • the holding time in the reheating temperature range is less than 10 seconds
  • the martensite present in the steel at the end of the second cooling process transforms into untransformed austenite. Diffusion of C does not proceed sufficiently, and a predetermined amount of area ratio of retained austenite cannot be obtained. This reduces ductility.
  • the release of hydrogen contained in the base steel sheet to the outside becomes insufficient, and the amount of diffusible hydrogen in the base steel sheet increases. As a result, there is a possibility that the hole expansibility may be deteriorated.
  • the holding time in the reheating temperature range is set to 10 seconds or more and 2000 seconds or less.
  • the holding time in the reheating temperature range is preferably 15 seconds or longer. Further, the holding time in the reheating temperature range is preferably 1200 seconds or less.
  • the retention time in the reheating temperature range includes not only the retention time at the reheating temperature but also the retention time in the temperature range during heating and cooling before and after reaching the reheating temperature.
  • the cooling conditions after holding in the reheating temperature range are not particularly limited, and may be in accordance with the usual method.
  • Examples of cooling methods that can be applied include gas jet cooling, mist cooling, roll cooling, water cooling, and air cooling.
  • the average cooling rate in cooling after holding in the reheating temperature range is preferably 1° C./second or more and 50° C./second or less, for example.
  • the galvanized steel sheet obtained as described above may be further subjected to temper rolling. If the rolling reduction of the temper rolling exceeds 2.00%, the yield stress increases, and there is a risk that the dimensional accuracy when forming the galvanized steel sheet into a member decreases. Therefore, the rolling reduction of temper rolling is preferably 2.00% or less. Although the lower limit of the rolling reduction in temper rolling is not particularly limited, it is preferably 0.05% or more from the viewpoint of productivity.
  • the temper rolling may be performed on an apparatus continuous with the annealing apparatus for performing each process described above (online), or on an apparatus discontinuous from the annealing apparatus for performing each process (offline). you can go Also, the number of times of temper rolling may be one or two or more. Note that rolling by a leveler or the like may be used as long as the same elongation rate as that of temper rolling can be imparted.
  • a method for manufacturing a member according to an embodiment of the present invention includes subjecting the galvanized steel sheet (for example, a galvanized steel sheet manufactured by the method for manufacturing a galvanized steel sheet) to at least one of forming and joining. It has a step of forming a member.
  • the molding method is not particularly limited, and for example, a general processing method such as press working can be used.
  • the joining method is not particularly limited, and for example, general welding such as spot welding, laser welding, arc welding, riveting, caulking, or the like can be used.
  • the molding conditions and bonding conditions are not particularly limited, and conventional methods may be followed.
  • Example 1 A steel material having the composition shown in Table 1 (the balance being Fe and unavoidable impurities) was melted in a converter and made into a steel slab by continuous casting. The obtained steel slab was heated to 1250° C. After heating, the steel slab was subjected to hot rolling including rough rolling and finish rolling to obtain a hot rolled steel sheet. Then, the obtained hot-rolled steel sheets were pickled and cold-rolled (rolling reduction: 50%) to obtain cold-rolled steel sheets having thicknesses shown in Table 3.
  • the obtained cold-rolled steel sheets were subjected to an annealing process, a first cooling process, a holding process, a plating process, a second cooling process and a reheating process under the conditions shown in Table 2 to obtain galvanized steel sheets.
  • the dew point in the annealing process was -35°C to -30°C.
  • hot-dip galvanizing treatment or alloying galvanizing treatment was performed to obtain a hot-dip galvanized steel sheet (hereinafter also referred to as GI) or an alloyed hot-dip galvanized steel sheet (hereinafter also referred to as GA).
  • GI hot-dip galvanized steel sheet
  • GA alloyed hot-dip galvanized steel sheet
  • Table 2 the types of plating processes are also indicated as "GI” and "GA”. It should be noted that when the alloyed zinc plating treatment is performed, No. Except for Nos. 20, 27 and 28, the holding time in the holding temperature range and the holding time in the temperature range of 300° C. to 550° C. in the alloying treatment were set to be 3 seconds to 600 seconds in total.
  • GI zinc plating bath
  • GA a plating bath having a composition containing 0.20% by mass of Al and the balance being Zn and unavoidable impurities
  • GA a plating bath containing 0.14% by mass of Al with the balance being Zn and unavoidable impurities was used.
  • the amount of plating deposited was 45 to 72 g/m 2 per side when manufacturing GI, and 45 g/m 2 per side when manufacturing GA.
  • the composition of the galvanized layer of the galvanized steel sheet finally obtained is, in GI, Fe: 0.1 to 1.0% by mass, Al: 0.2 to 1.0% by mass, and the balance was Zn and unavoidable impurities.
  • GA contained 7 to 15% by mass of Fe, 0.1 to 1.0% by mass of Al, and the balance was Zn and unavoidable impurities. All galvanized layers were formed on both sides of the base steel plate.
  • a tensile test and a hole expansion test are performed according to the following procedures, and according to the following criteria, tensile strength (TS), yield stress (YS), total elongation (El), work hardening index (n value) / yield ratio (YR) and critical hole expansion ratio ( ⁇ ) were evaluated.
  • the hole expanding test was performed according to JIS Z 2256. That is, from the obtained galvanized steel sheet, a test piece of 100 mm x 100 mm was cut by shearing. A 10 mm diameter hole was punched in the specimen with a clearance of 12.5%. Then, using a die with an inner diameter of 75 mm, a wrinkle holding force of 9 tons (88.26 kN) is applied around the hole, and a conical punch with an apex angle of 60° is pushed into the hole to reach the crack initiation limit (crack initiation The diameter of the hole in the test piece was measured. Then, the limit hole expansion rate: ⁇ (%) was obtained from the following equation. ⁇ is an index for evaluating stretch flangeability.
  • the member obtained by molding or the member obtained by bonding has tensile strength (TS), yield stress (YS), total elongation (El), working It was found that both the hardening index (n value)/yield ratio (YR) and the critical hole expansion ratio ( ⁇ ) have the excellent properties characteristic of the present invention.
  • Example 2 A steel material having the composition shown in Table 1 (the balance being Fe and unavoidable impurities) was melted in a converter and made into a steel slab by continuous casting. The obtained steel slab was heated to 1250° C. After heating, the steel slab was subjected to hot rolling including rough rolling and finish rolling to obtain a hot rolled steel sheet. Then, the obtained hot-rolled steel sheet was pickled and cold-rolled (rolling reduction: 50%) to obtain a cold-rolled steel sheet having a thickness of 1.6 mm.
  • Table 1 the balance being Fe and unavoidable impurities
  • Fe-based electroplating was performed as metal plating treatment to form a metal plating layer (Fe-based plating layer) on the surface of the cold-rolled steel sheet.
  • the cold-rolled steel sheet was degreased with an alkali. Then, under the conditions shown below, the cold-rolled steel sheet was used as a cathode and electrolytic treatment was performed to form a metal plating layer on the surface of the cold-rolled steel sheet.
  • the obtained cold-rolled steel sheets (including metal-plated steel sheets in which a metal plating layer is formed on the surface of the cold-rolled steel sheet) are subjected to an annealing step, a first cooling step, a holding step, a plating step, A second cooling step and a reheating step were performed to obtain a galvanized steel sheet.
  • an alloyed galvanized steel sheet (GA) was obtained by performing an alloyed galvanized steel sheet.
  • the treatment conditions other than those described in Table 5 are the same as in Example 1. All galvanized layers were formed on both sides of the base steel plate.
  • the steel structure of the base steel sheet was identified, and the thickness of the decarburized layer, the amount of adhesion of the metal plating layer, and the amount of diffusible hydrogen were measured according to the procedures described above.
  • Table 6 shows the results.
  • F is ferrite
  • BF bainitic ferrite
  • TM tempered martensite
  • RA retained austenite
  • FM is fresh martensite
  • LB lower bainite
  • P pearlite
  • ⁇ cementite.
  • "-" for the thickness of the decarburized layer and the amount of the metal plating layer respectively means that the decarburized layer and the metal plating layer are not provided.
  • Example 7 tensile strength/yield ratio (YR) and critical hole expansion ratio ( ⁇ ) were evaluated. The results are also shown in Table 7.
  • the hot-dip alloyed galvannealed steel sheet 1 for testing had a coating amount of 50 g/m 2 per side of the galvannealed layer, and was cut into the same size as the test piece 2 .
  • the plate assembly consists of the evaluation target surface of the test piece 2 (when the galvanized layer and the metal coated layer are only on one side, the galvanized layer on that side) and the zinc of the alloyed hot-dip galvanized steel sheet 1 for test It was assembled so that the plated layers faced each other.
  • the plate assembly was fixed to a fixing table 4 via a spacer 3 having a thickness of 2.0 mm.
  • the spacer 3 is a pair of steel plates measuring 50 mm in the longitudinal direction, 45 mm in the transverse direction, and 2.0 mm in thickness. As shown in FIG. It was arranged so as to be aligned with both end faces. Therefore, the distance between the pair of steel plates is 60 mm.
  • the fixed base 8 is a single plate with a hole in the center.
  • the plate assembly is pressed with a pair of electrodes 5 (tip diameter: 6 mm) while bending the plate assembly. : 3.5 kN, hold time: 0.12 seconds, 0.18 seconds or 0.24 seconds, and welding time: 0.36 seconds, with a welding current that makes the nugget diameter r 5.9 mm Resistance spot welding was performed to form a plate assembly with a welded portion.
  • the pair of electrodes 5 pressurized the plate assembly from above and below in the vertical direction, and the lower electrode pressurized the test piece 2 through the hole of the fixing table 4 .
  • the lower electrode and the fixing table 4 are fixed so that the lower electrode of the pair of electrodes 5 is in contact with a plane extending the surface where the spacer 3 and the fixing table 4 are in contact, and the upper electrode is fixed.
  • the electrodes are movable.
  • the upper electrode was brought into contact with the central portion of the test alloyed hot-dip galvanized steel sheet 1 . Welding was performed while the plate assembly was tilted 5° to the longitudinal direction of the plate assembly with respect to the horizontal direction.
  • the hold time refers to the time from the end of the welding current to the start of opening the electrode.
  • the nugget diameter r means the distance between the ends of the nugget 6 in the longitudinal direction of the plate assembly.
  • the plate set with the welded portion is cut along the line AA in the upper diagram of FIG.
  • Observation was made with a microscope (200x magnification), and resistance weld crack resistance in the weld zone was evaluated according to the following criteria. If it is A+, A or B, it is judged that the resistance weld crack resistance in the weld is excellent. If it is C, it is judged that the resistance weld crack resistance in the weld is inferior.
  • the results are also shown in Table 7.
  • A+ No cracks having a length of 0.1 mm or more were observed at any of the hold times of 0.12 seconds, 0.18 seconds and 0.24 seconds.
  • A Cracks with a length of 0.1 mm or longer were observed at a hold time of 0.12 seconds, but cracks with a length of 0.1 mm or longer were observed at hold times of 0.18 seconds and 0.24 seconds. I didn't.
  • B Cracks with a length of 0.1 mm or longer were observed at hold times of 0.12 seconds and 0.18 seconds, but cracks with a length of 0.1 mm or longer were observed at a hold time of 0.24 seconds. I didn't.
  • C Cracks with a length of 0.1 mm or longer were observed at all hold times of 0.12 seconds, 0.18 seconds and 0.24 seconds.
  • the crack generated in the test piece 2 is schematically indicated by reference numeral 7.
  • the stress on the evaluation target steel sheet the steel sheets of each invention example and comparative example
  • an appropriate evaluation cannot be obtained. For this reason, data in which no cracks occurred in the mating steel plate were used as examples.
  • the resistance weld crack resistance in the weld zone was also excellent.
  • the member obtained by molding or the member obtained by bonding has tensile strength (TS), yield stress (YS), total elongation (El), working It was found that the hardening index (n value)/yield ratio (YR), limit hole expansion ratio ( ⁇ ), and resistance weld cracking resistance in the weld all have excellent properties that are characteristic of the present invention.

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Abstract

L'invention concerne une tôle d'acier galvanisé ayant une résistance à la traction de 980 MPa ou plus, une YS élevée et une excellente ductilité, une excellente propriété d'écrouissage et une excellente propriété d'élargissement de trou. Dans la présente invention : une tôle d'acier de base a une composition de composant prescrite ; ferrite, ferrite bainitique, martensite revenue, austénite retenue et martensite fraîche ont des rapports de surface prescrits dans la structure d'acier de la tôle d'acier de base ; le rapport SMA1/SMA de rapports de surface est de 0,80 ou moins ; et le rapport SMA2/SMA de rapports de surface est de 0,20 ou plus.　Ici : SMA est le rapport de surface d'une seconde phase dure constituée d'austénite retenue et de martensite fraîche ; SMA1 est le rapport de surface total des régions d'îlot, parmi des régions d'îlot constituant la seconde phase dure, dans laquelle le diamètre circulaire équivalent est de 2,0 µm ou plus et 20 % ou moins de la longueur circonférentielle est en contact avec la martensite revenue ; et SMA2 est le rapport de surface total des régions d'îlot, parmi les régions d'îlot constituant la seconde phase dure, dans laquelle au moins 1 % de la longueur circonférentielle étant en contact avec la ferrite bainitique.
PCT/JP2022/012855 2021-03-23 2022-03-18 Tôle d'acier galvanisé et élément et procédé de fabrication associé WO2022202716A1 (fr)

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CN202280022963.4A CN117062928A (zh) 2021-03-23 2022-03-18 镀锌钢板、部件以及它们的制造方法
US18/546,428 US20240124964A1 (en) 2021-03-23 2022-03-18 Galvanized steel sheet and member, and method for manufacturing same
KR1020237028572A KR20230135631A (ko) 2021-03-23 2022-03-18 아연 도금 강판 및 부재, 그리고, 그들의 제조 방법
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