WO2023162573A1 - 亜鉛めっき鋼板およびその製造方法 - Google Patents

亜鉛めっき鋼板およびその製造方法 Download PDF

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WO2023162573A1
WO2023162573A1 PCT/JP2023/002565 JP2023002565W WO2023162573A1 WO 2023162573 A1 WO2023162573 A1 WO 2023162573A1 JP 2023002565 W JP2023002565 W JP 2023002565W WO 2023162573 A1 WO2023162573 A1 WO 2023162573A1
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steel sheet
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layer
content
alloy layer
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PCT/JP2023/002565
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English (en)
French (fr)
Japanese (ja)
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祐介 常見
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日本製鉄株式会社
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Priority to EP23759579.8A priority Critical patent/EP4484590A4/en
Priority to US18/839,195 priority patent/US20250154633A1/en
Priority to CN202380023062.1A priority patent/CN118742664A/zh
Priority to JP2024502935A priority patent/JPWO2023162573A1/ja
Priority to KR1020247028757A priority patent/KR20240144265A/ko
Priority to MX2024010245A priority patent/MX2024010245A/es
Publication of WO2023162573A1 publication Critical patent/WO2023162573A1/ja

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    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
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    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
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    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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Definitions

  • the present invention relates to a galvanized steel sheet and its manufacturing method. This application claims priority based on Japanese Patent Application No. 2022-027919 filed in Japan on February 25, 2022, the content of which is incorporated herein.
  • High-strength steel sheets are used as steel sheets for automobiles in order to reduce the weight of automobiles, increase fuel efficiency, reduce carbon dioxide emissions, and ensure the safety of passengers.
  • high-strength alloyed hot-dip galvanized steel sheets have also been used as steel sheets for automobiles in order to sufficiently ensure the corrosion resistance of car bodies and parts (see, for example, Patent Document 1). ).
  • high-strength steel sheets used for automobile parts are required not only to have strength, but also to have properties (formability, elongation, bending resistance, etc.) necessary for forming parts, such as uniform elongation.
  • properties shapeability, elongation, bending resistance, etc.
  • DP steel plate and the like are known.
  • galvanized steel sheets can be spot welded together, or cold rolled steel sheets and galvanized steel sheets can be spot welded together.
  • LME liquid metal embrittlement
  • a high-strength TRIP steel sheet is a steel sheet that has higher C, Si, and Mn concentrations than ordinary high-strength steel sheets, and that contains retained austenite, thereby having excellent energy absorption capacity and press formability. Therefore, galvanized steel sheets, which are expected to be applied to automobile parts, are required to have high LME resistance.
  • Patent Document 2 discloses an internal oxide layer in which at least a part of the crystal grain boundary is covered with an oxide from the surface of the base material to a depth of 5.0 ⁇ m or more, A steel sheet, a hot-dip galvanized steel sheet, and a hot-dip galvanized steel sheet having excellent resistance to molten metal embrittlement cracking, wherein the grain boundary coverage of the oxide is 60% or more in a region from the surface of the base material to a depth of 5.0 ⁇ m, and An alloyed hot-dip galvanized steel sheet is disclosed.
  • Patent Literature 2 discloses that the generation of LME is suppressed by allowing a layer in which internal oxidation occurs to a predetermined depth and increasing the coverage of grain boundaries with oxide.
  • Patent Document 2 does not consider bending fatigue strength and bending resistance.
  • an object of the present invention is to provide a galvanized steel sheet that has high strength and is excellent in LME resistance and bending fatigue strength without lowering bending resistance.
  • the present inventors investigated a technique for improving the LME resistance and bending fatigue strength of high-strength galvanized steel sheets.
  • LME resistance can be obtained without reducing the bending resistance. It has been found that the performance can be improved.
  • bending fatigue strength is improved by forming a predetermined internal oxide layer on the surface of the base steel plate.
  • the control of the annealing process and the plating process is effective for the formation of such Fe--Al alloy layers and internal oxide layers.
  • a galvanized steel sheet according to an aspect of the present invention includes a base steel sheet, an Fe—Al alloy layer formed on at least part of the surface of the base steel sheet, and the base steel sheet or the Fe—Al a galvanized layer formed on the surface of the alloy layer, wherein the base material steel plate contains, in mass%, C: 0.10 to 0.40%, Si: 0.10 to 3.00%, Mn: 1.00-5.00%, sol.
  • the base steel plate has an internal oxide layer of 0.2 ⁇ m or more in the plate thickness direction from the surface of the base steel plate, and the average thickness of the Fe—Al alloy layer is 1 nm or more and less than 100 nm In the cross section in the
  • the chemical composition of the base steel sheet is, in mass%, Ti: 0.005 to 0.200%, B: 0.0005 to 0.0100% , Cr: 0.001 to 1.000%, Mo: 0.001 to 1.000%, Ni: 0.001 to 1.000%, Cu: 0.001 to 1.000%, Sn: 0.001 ⁇ 0.500%, Nb: 0.001-0.200%, V: 0.001-0.500%, W: 0.001-0.500%, Ca: 0.0001-0.0100%, Mg: 0.0001-0.0100%, Bi: 0.0001-0.0100%, Sb: 0.0001-0.1000%, Zr: 0.0001-0.0100%, and REM: 0.0001 ⁇ 0.1000%, may contain one or more selected from the group consisting of.
  • a method for producing a galvanized steel sheet according to another aspect of the present invention in mass%, C: 0.10 to 0.40%, Si: 0.10 to 3.00%, Mn: 1.00 ⁇ 5.00%, sol. Al: 0.001 to 1.500%, P: 0.0010 to 0.0300%, S: 0.0200% or less, N: 0.0100% or less, O: 0.0100% or less, Ti: 0 to 0.200%, B: 0-0.0100%, Cr: 0-1.000%, Mo: 0-1.000%, Ni: 0-1.000%, Cu: 0-1.000%, Sn: 0-0.500%, Nb: 0-0.200%, V: 0-0.500%, W: 0-0.500%, Ca: 0-0.0100%, Mg: 0-0 .0100%, Bi: 0 to 0.0100%, Sb: 0 to 0.1000%, Zr: 0 to 0.0100%, REM: 0 to 0.1000%, and the balance: chemical composition consisting of Fe and
  • the average heating rate in the first temperature range of 400 to 650 ° C. is 2.0 ° C./sec or more
  • the second temperature range from 650 ° C. to the annealing temperature.
  • the average heating rate in the second temperature range is 0.5 to 5.0 ° C./sec
  • the atmosphere (P(H2O)/P(H2)) is 0.05 to 2.00 in the second temperature range, and the plating In the process, the steel sheet is cooled to 440 to 550 ° C.
  • FIG. 1 is a schematic diagram showing an example of a cross section of a steel plate according to this embodiment.
  • the steel plate 1 according to the present embodiment includes a base steel plate 10 having a predetermined chemical composition, an Fe—Al alloy layer 20 formed on at least a part of the surface of the base steel plate, and the base steel plate 10 or Fe—Al and a galvanized layer 30 formed on the surface of the alloy layer 20 .
  • the base steel sheet 10 has an internal oxide layer 11 on the surface layer portion on the interface side with the Fe—Al alloy layer 20 or the galvanized layer 30 .
  • the Fe—Al alloy layer 20 When the Fe—Al alloy layer 20 is formed only on a part of the base steel plate 10, in the portion where the Fe—Al alloy layer 20 is formed on the surface of the base steel plate 10, the Fe—Al alloy layer The galvanized layer 30 is formed on the base steel plate 10 in the portion where the Fe—Al alloy layer 20 is not formed. Although the Fe--Al alloy layer and the galvanized layer are formed only on one side in FIG. 1, they may be formed on the other side as well.
  • the range shown between "-” basically includes the values at both ends as the lower limit and the upper limit. However, numerical values indicated as "more” and “less than” are not included in the range.
  • Base material steel plate First, the base material steel plate 10 included in the steel plate 1 according to this embodiment will be described.
  • the base steel plate 10 included in the steel plate 1 according to this embodiment contains the following elements.
  • % of the content of each element means % by mass.
  • C 0.10-0.40%
  • C (carbon) is an essential element for increasing the strength of steel sheets. If the C content is less than 0.10%, sufficient tensile strength cannot be obtained. Therefore, the C content is made 0.10% or more.
  • the C content is preferably 0.12% or more.
  • C is also an element that contributes to the formation of retained austenite. Retained austenite contributes to the improvement of elongation by the TRIP effect. To obtain this effect, the C content is preferably 0.16% or more.
  • the C content exceeds 0.40%, the weldability is remarkably deteriorated. Therefore, the C content is made 0.40% or less. From the viewpoint of suppressing deterioration of press formability and weldability, the C content is preferably 0.30% or less.
  • Si 0.10-3.00%
  • Si is a solid-solution strengthening element, and is an element effective in increasing the strength of a steel sheet. Si is also an element that contributes to the formation of retained austenite. In order to obtain these effects, the Si content is set to 0.10% or more. The Si content is preferably 0.30% or more. On the other hand, if Si is contained excessively, the chemical conversion treatability of the steel sheet and the wettability with hot-dip galvanization are remarkably deteriorated. Therefore, the Si content is set to 3.00% or less. The Si content is preferably 2.00% or less.
  • Mn 1.00-5.00%
  • Mn manganese
  • Mn is a strong austenite stabilizing element, and is an element effective in improving the hardenability of steel sheets.
  • the Mn content is set to 1.00% or more.
  • the Mn content is preferably 1.50% or more.
  • the Mn content is set to 5.00% or less. From the viewpoint of suppressing deterioration of weldability and low-temperature toughness, the Mn content is preferably 3.20% or less.
  • sol. Al 0.001-1.500%
  • Al aluminum
  • Al is an element that has a deoxidizing effect on steel.
  • Al is also an element that contributes to the formation of retained austenite.
  • sol. Al content is 0.001% or more.
  • sol. The Al content is preferably 0.005% or more.
  • sol. Al content is 1.500% or less.
  • the Al content is preferably 1.000% or less.
  • P 0.0010 to 0.0300%
  • P (phosphorus) is a solid-solution strengthening element, and is an element effective in increasing the strength of a steel sheet.
  • the P content is made 0.0010% or more.
  • the P content is preferably 0.0050% or more.
  • the P content is made 0.0300% or less.
  • the P content is preferably 0.0200% or less.
  • S 0.0200% or less
  • S sulfur
  • S is an element that causes hot shortness and is an element that impairs weldability and corrosion resistance. If the S content exceeds 0.0200%, the hot workability, weldability and corrosion resistance are remarkably lowered, so the S content is made 0.0200% or less.
  • the S content is preferably 0.0100% or less. The lower the S content is, the better, and it may be 0%, but if the S content is less than 0.0001%, the manufacturing cost increases significantly. Therefore, the S content may be 0.0001% or more.
  • the S content may be 0.0010% or more.
  • N 0.0100% or less
  • N nitrogen
  • the N content is preferably 0.0050% or less.
  • the N content is preferably as small as possible, and may even be 0%. good.
  • O 0.0100% or less
  • O oxygen
  • the O content is preferably 0.0070% or less.
  • the O content is preferably as small as possible, and may be 0%, but from the viewpoint of manufacturing costs, the O content may be 0.0001% or more.
  • the O content may be 0.0010% or more.
  • the steel sheet according to the present embodiment may contain the above elements and the balance may be Fe and impurities. However, for the purpose of improving various properties, it is further selected from the following Ti, B, Cr, Mo, Ni, Cu, Sn, Nb, V, W, Ca, Mg, Bi, Sb, Zr and REM It may contain one or more elements (optional elements). The lower limit is 0% because the optional element does not have to be contained.
  • Ti 0-0.200%
  • Ti titanium is an element that suppresses the formation of BN, which is a factor that reduces hardenability, by fixing N as TiN in steel.
  • Ti is an element that refines the austenite grain size during heating and improves toughness.
  • the Ti content is preferably 0.005% or more. More preferably, the Ti content is 0.010% or more.
  • the Ti content is set to 0.200% or less.
  • the Ti content is preferably 0.050% or less.
  • B 0 to 0.0100%
  • B is an element that segregates at the austenite grain boundary during welding, strengthens the grain boundary, and contributes to the improvement of molten metal embrittlement cracking resistance (LME resistance).
  • LME resistance molten metal embrittlement cracking resistance
  • the B content is preferably 0.0005% or more.
  • the B content is more preferably 0.0008% or more.
  • the content of B is set to 0.0100% or less.
  • the B content is preferably 0.0050% or less.
  • Cr 0 to 1.000% Mo: 0-1.000% Ni: 0 to 1.000% Cu: 0-1.000% Sn: 0-0.500% Cr (chromium), Mo (molybdenum), Ni (nickel), Cu (copper), and Sn (tin) are all effective elements for increasing the strength of steel sheets.
  • one or more selected from Cr, Mo, Ni, Cu and Sn is preferably contained in an amount of 0.001% or more, more preferably 0.010% or more.
  • the content is more preferably 0.050% or more.
  • excessive inclusion of these elements saturates the effect and increases the cost.
  • the contents of Cr, Mo, Ni and Cu are all set to 1.000% or less, and the Sn content is set to 0.500% or less.
  • the contents of Cr, Mo, Ni and Cu are all preferably 0.600% or less, and the Sn content is preferably 0.300% or less.
  • Nb 0-0.200%
  • V 0-0.500%
  • W 0-0.500%
  • Nb (niobium), V (vanadium) and W (tungsten) are carbide-forming elements and are effective elements for increasing the strength of steel sheets.
  • one or more selected from Nb, V and W is preferably contained in an amount of 0.001% or more, more preferably 0.005% or more, and 0.005% or more. It is more preferable to contain 0.10% or more.
  • the Nb content is set to 0.200% or less, and both the V content and the W content are set to 0.500% or less.
  • the Nb content is preferably 0.100% or less, and both the V content and the W content are preferably 0.300% or less.
  • Ca 0-0.0100% Mg: 0-0.0100% Bi: 0 to 0.0100% Sb: 0-0.1000% Zr: 0 to 0.0100% REM: 0-0.1000%
  • Ca (calcium), Mg (magnesium), Sb (antimony), Zr (zirconium), and REM (rare earth elements) are elements that contribute to the fine dispersion of inclusions in steel. It is an element that reduces the micro-segregation of substitutional alloy elements such as Si and Si. Each of these elements contributes to improving the bending resistance of the steel sheet. Therefore, it may be contained as necessary.
  • one or more selected from Ca, Mg, Bi, Sb, Zr and REM is preferably contained in an amount of 0.0001% or more, preferably 0.0010% or more. is more preferred.
  • Ca content, Mg content, Bi content, and Zr content are all set to 0.0100% or less.
  • the Sb content and the REM content are set to 0.1000% or less.
  • Ca content, Mg content, Bi content, and Zr content are all preferably 0.0080% or less, more preferably 0.0060% or less.
  • the Sb content and REM content are preferably 0.0800% or less, more preferably 0.0600% or less, and even more preferably 0.0200% or less.
  • REM refers to a total of 17 elements of Sc, Y and lanthanides, and REM content means the total content of these elements.
  • Lanthanides are added industrially in the form of misch metals.
  • the chemical composition of the base material steel sheet of the steel sheet according to this embodiment can be obtained by the following method.
  • the chemical composition of the base material steel plate may be measured by a general chemical composition. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry).
  • sol. Al can be measured by ICP-AES using the filtrate obtained by thermally decomposing the sample with acid.
  • C and S may be measured using the combustion-infrared absorption method, N using the inert gas fusion-thermal conductivity method, and O using the inert gas fusion-nondispersive infrared absorption method.
  • the coating layer is removed by mechanical grinding, and then the chemical composition can be analyzed.
  • the chemical composition of the base steel sheet of the steel sheet according to this embodiment is C, Si, Mn, sol. containing Al, P, S, O, N with the balance being Fe and impurities, or C, Si, Mn, sol.
  • the balance consists of Fe and impurities.
  • Impurities are elements that are mixed in raw materials or during the manufacturing process.
  • the total amount of impurities is preferably 0.5% or less, more preferably 0.1% or less.
  • the base steel plate of the steel plate according to the present embodiment is not limited in terms of metal structure, but when obtaining a tensile strength of 980 MPa or more, the center position is 1/4 of the plate thickness in the plate thickness direction from the surface of the base steel plate In the metal structure at the 1/4 thickness position, which is in the range of 1/8 to 3/8 of the plate thickness from the surface, the total volume fraction of fresh martensite and tempered martensite is 40% or more preferable. More preferably, it is over 50%, still more preferably 55% or more. When it is desired to obtain a higher tensile strength, the total volume fraction of fresh martensite and tempered martensite is preferably 80% or more. Other than fresh martensite and tempered martensite, it is, for example, one or more of ferrite, bainite, pearlite, cementite, and retained austenite.
  • the volume fractions of ferrite, bainite, martensite (tempered martensite and fresh martensite), pearlite, cementite, and retained austenite contained in the metal structure at the 1/4 thickness position can be measured using the method shown below.
  • a sample is taken with a cross section parallel to the rolling direction and thickness direction of the steel sheet as an observation surface, and the observation surface is polished and nital-etched.
  • magnification 5000 times, 1 field of view is 250 ⁇ m 2 or more, and a total of 5 fields of view are observed with a Field Emission Scanning Electron Microscope (FE-SEM).
  • the area ratios of ferrite, bainite, tempered martensite, fresh martensite, pearlite, cementite, and retained austenite are measured and regarded as the volume ratio.
  • a region having a substructure within grains and having carbides precipitated with a plurality of variants is determined to be tempered martensite.
  • a region in which cementite is precipitated in lamellar form is determined to be pearlite or cementite.
  • a region with low brightness and no substructure is judged to be ferrite.
  • a region with high brightness and in which the underlying structure is not revealed by etching is judged to be fresh martensite or retained austenite.
  • the remainder is determined to be bainite.
  • the volume ratio of each tissue is calculated by the point counting method.
  • the volume fraction of fresh martensite can be determined by subtracting the volume fraction of retained austenite determined by the EBSD method described below from the volume fraction of fresh martensite or retained austenite.
  • the volume fraction of retained austenite at the 1 ⁇ 4 thickness position is evaluated by high-resolution crystal structure analysis by the EBSD method (electron beam backscatter diffraction method). Specifically, a sample is collected using a cross section parallel to the rolling direction and thickness direction of the steel sheet as an observation surface, and the observation surface is polished to a mirror finish. Further, electrolytic polishing or mechanical polishing using colloidal silica is performed to remove the processed layer on the surface. Next, at the 1/4th thickness position of the steel plate, the crystal structure analysis is performed by the EBSD method for 5 fields of view with a magnification of 5000 times and a size of 1 field of view of 250 ⁇ m 2 or more.
  • the EBSD method electron beam backscatter diffraction method
  • the distance between scores (step) is set to 0.01 to 0.20 ⁇ m.
  • Data obtained by the EBSD method are analyzed using "OIM Analysys 6.0" manufactured by TSL. Based on the observation results at each position, the region judged to be FCC iron is judged to be retained austenite, and the volume ratio of retained austenite at each 1 ⁇ 4 thickness position is calculated.
  • the base steel plate 10 included in the steel plate 1 according to the present embodiment has a surface in the plate thickness direction (an interface with the Fe—Al alloy layer 20, or a galvanized layer for a portion where the Fe—Al alloy layer 20 is not formed. 30) has an internal oxide layer of 0.2 ⁇ m or more (the thickness of the internal oxide layer is 0.2 ⁇ m or more).
  • the internal oxide layer means that at least a part of the crystal grain boundary of the base material is covered with an oxide of an oxidizable element such as Si or Mn (when observing the cross section, an oxide layer on the crystal grain boundary is formed). layer in which objects are observed.
  • the grain boundary coverage of oxides in the internal oxide layer 11 is 60% or more.
  • the grain boundary coverage ratio is the ratio (%) of the length of the grain boundary covered with oxide to the total length of the grain boundary in the internal oxide layer 11 . Covering the crystal grain boundaries with oxides hinders dislocation movement and improves fatigue strength. If the thickness of the internal oxide layer 11 is less than 0.2 ⁇ m or the grain boundary coverage is less than 60%, the effect of improving the fatigue strength cannot be sufficiently obtained. There is no particular upper limit for the thickness of the internal oxide layer 11, but if it exceeds 3.0 ⁇ m, the effect of improving the fatigue strength is saturated, and the deformability and bendability may deteriorate.
  • the thickness of layer 11 is preferably 3.0 ⁇ m or less.
  • oxides are formed mainly on the grain boundaries in the internal oxide layer 11, so the oxides often exist in a mesh-like manner.
  • the thickness of the internal oxide layer (existing depth) and grain boundary coverage are determined by the following method.
  • a sample for microstructure observation is taken from the steel sheet so that the structure of the cross section in the plate thickness direction can be observed.
  • the surface parallel to the rolling direction and the plate thickness direction was subjected to wet polishing with emery paper, and further buffed using diamond abrasive grains having an average diameter of 1 ⁇ m. Mirror finish.
  • colloidal silica polishing is performed using a suspension containing alcohol as a solvent in order to remove the distortion introduced into the polished surface by the mechanical polishing described above. In colloidal silica polishing, if the load increases during polishing, strain may be further introduced, so the load is suppressed during polishing.
  • automatic polishing may be performed for 1 hour at a setting of 40% output using Vibromet 2 manufactured by BUEHLER.
  • electropolishing or chemical etching is applied in the process of removing the strain introduced by mechanical polishing, the oxide will dissolve, making it impossible to observe the actual state of the oxide existing on the grain boundary.
  • Similar precautions are required when performing polishing using water as a solvent. Water-soluble oxides are dissolved during polishing using water as a solvent, and internal oxides on grain boundaries cannot be observed. Therefore, in the polishing finishing process, a process that does not include the above procedure is adopted.
  • the surface layer of the observed surface of the sample prepared by the above procedure is observed by SEM and SEM-EBSD.
  • the observation magnification is selected from 1,000 to 9,000 times, and a magnification that includes 10 or more ferrite crystal grains in the microstructure is selected, for example, 3,000 times.
  • oxides existing at grain boundaries are confirmed in a backscattered electron image in a SEM. In the backscattered electron image, since the color tone changes depending on the atomic number (or mass), oxides and steel structures can be easily distinguished.
  • B.E. C. C. - Obtain the crystallographic orientation data of iron.
  • the magnification of the measurement is selected from 1000 to 9000, and may be the same magnification as the observation of the SEM-backscattered electron image described above.
  • the measurement interval (STEP) is 0.01 to 0.1 ⁇ m, and 0.05 ⁇ m may be selected.
  • the B.O.M. obtained under these measurement conditions.
  • C. C. - In the crystal orientation MAP data of iron, the boundary where the crystal orientation difference is 15° or more is defined as the grain boundary, except for the region where the confidence value (CI value) is less than 0.1.
  • the CI value is a numerical value that serves as an index of the reliability of crystal orientation determination shown in analysis software, and generally, if the value is less than 0.1, the reliability is considered to be low. If an oxide exists in the grain boundary of ferrite, B. C. C. - Since the crystal orientation data of iron cannot be obtained, there are many regions where the CI value is less than 0.1 between adjacent grains. In this case, although the crystal grain boundary cannot be clearly confirmed, at the boundary where the orientation difference of the adjacent ferrite crystal grains is 15° or more, the crystal grain boundary passes through the center of the region where the CI value is less than 0.1. is drawn on the MAP.
  • the grain boundary coverage (%) is calculated by dividing the obtained oxide coating length by the length of all grain boundaries.
  • Fe--Al alloy layer In the steel plate 1 according to the present embodiment, the Fe—Al alloy layer 20 having an average thickness of 1 nm or more is formed on the surface of the base steel plate 10 with a coverage of 40% or more.
  • the Fe—Al alloy layer 20 is formed between the base steel plate 10 and the galvanized layer 30 .
  • LME cracking is caused by penetration of molten zinc into grain boundaries during welding. Therefore, the presence of the Fe—Al alloy layer at the interface between the plated layer and the base material serves as a barrier to zinc penetration, improving the LME resistance.
  • the Fe—Al alloy layer has an average thickness of 1 nm or more and a coverage of 40% or more.
  • the average thickness of the Fe—Al alloy layer is preferably 5 nm or more, more preferably 10 nm or more, still more preferably 20 nm or more.
  • the coverage of the Fe—Al alloy layer is preferably 50% or more, more preferably 60% or more.
  • the average thickness of the Fe—Al alloy layer is 100 nm or more, the bending resistance is lowered. Therefore, the average thickness of the Fe—Al alloy layer is set to less than 100 nm. Moreover, although it is not necessary to limit the upper limit of the coverage, the cost is significantly increased to achieve 100%, so the coverage may be less than 100% or 98% or less.
  • the coverage rate is the ratio (%) of the length of the interface between the Fe—Al alloy layer and the base steel plate to the length of the surface of the base steel plate when viewed in a cross section in the thickness direction.
  • the average thickness and coverage of the Fe—Al alloy layer are obtained by the following methods. A sample is taken so that the cross section parallel to the rolling direction and the plate thickness direction of the steel plate will be the observation surface. Using an FE-SEM, this sample is photographed in the vicinity of the surface of the base steel sheet at a magnification of 50,000 times over a range of 1.5 ⁇ m 2 or more/1 field of view. Since the Fe--Al alloy layer is observed black in the backscattered electron image at the interface between the parent phase and the plating layer, the Fe--Al alloy layer is determined visually and its thickness is measured.
  • Photographing is performed for 5 fields of 5 points/1 field of view, and the average value of the thickness of the Fe-Al alloy layer in 5 fields of view (25 points) is the average thickness of the Fe-Al alloy layer of the steel sheet according to this embodiment. . Also, the length of the interface between the base steel plate and the Fe—Al alloy layer is measured in the length of the surface of the base steel plate in the observation field to determine the coverage. The measurement is performed for five fields of view, and the average of the coverage of each field of view is taken as the coverage of the steel sheet according to this embodiment.
  • the steel plate 1 has the base material steel plate 10 (the portion without the Fe—Al alloy layer) and / or the Fe—Al alloy layer 20 (the portion with the Fe—Al alloy layer on the base steel plate). , has a galvanized layer 30 .
  • the galvanized layer is, for example, a hot-dip galvanized layer.
  • the galvanized layer means a plated layer containing 80% by mass or more of Zn. Corrosion resistance is improved by the presence of the hot-dip galvanized layer on the surface.
  • the adhesion amount is preferably 100 g/m 2 or less, more preferably 80 g/m 2 or less.
  • the adhesion amount is preferably 10 g/m 2 or more.
  • the chemical composition of the galvanized layer is not limited, it preferably contains 0.1 to 2.0% by mass of Al, 5.0% or less of Fe, and the balance is Zn and impurities.
  • the total amount of impurities is preferably 0.1% by mass or less.
  • the adhesion amount and chemical composition of the galvanized layer are obtained by the following methods.
  • the plating layer is melted using inhibitor-containing hydrochloric acid, and the adhesion amount is determined by comparing the weights before and after melting.
  • the chemical composition of the plating layer is measured by quantitatively analyzing the solution obtained by melting with ICP.
  • the tensile strength is set to 980 MPa or more.
  • the tensile strength is preferably 1050 MPa or higher, more preferably 1100 MPa or higher.
  • the tensile strength is set to 2000 MPa or less.
  • the steel plate according to the present embodiment can obtain the effect as long as it has the above characteristics regardless of the manufacturing method.
  • the production conditions described below are preferable because stable production can be achieved.
  • the steel sheet according to the present embodiment can be produced by subjecting a steel sheet (hot-rolled steel sheet or cold-rolled steel sheet) to be a base material steel sheet to annealing and plating under predetermined conditions.
  • the manufacturing conditions for the steel sheet to be subjected to the annealing process are not limited.
  • a hot-rolled steel sheet can be produced by casting molten steel having the chemical composition described above to form a steel slab under normal conditions, and then subjecting the slab to hot rolling under normal conditions.
  • a cold-rolled steel sheet can be produced by subjecting the above hot-rolled steel sheet to cold rolling under normal conditions.
  • the annealing step includes a heating process of heating a steel plate having a predetermined chemical composition (the same chemical composition as the steel plate according to the present embodiment to be obtained) to an annealing temperature (maximum heating temperature) of 700 to 1000 ° C., and annealing the heated steel plate. and a holding step of holding at the temperature for 1 second or longer. From the viewpoint of productivity, annealing is preferably performed by passing the steel sheet through a continuous annealing line. If the annealing temperature is less than 700° C., the amount of austenite is insufficient, and a sufficient amount of hard structure cannot be secured by phase transformation during subsequent cooling, and sufficient tensile strength cannot be obtained.
  • the annealing temperature is set to 700° C. or higher.
  • the annealing temperature is preferably 720° C. or higher.
  • the annealing temperature is set to 1000° C. or lower.
  • the annealing temperature is preferably 900° C. or lower.
  • the steel sheet is heated to an annealing temperature (maximum heating temperature: 700 to 1000°C).
  • the average heating rate in the first temperature range of 400 to 650 ° C. is set to 2.0 ° C./sec or more, and the second temperature range from 650 ° C. to the annealing temperature is set.
  • the average heating rate is 0.5 to 5.0° C./sec, and the atmosphere (P(H2O)/P(H2)) is 0.05 to 2.00 in the second temperature range.
  • the average heating rate in the first temperature range of 400-650° C. recovery of dislocations mainly occurs during heating.
  • the average heating rate in this temperature range is 2.0° C./second or more, it is possible to suppress recovery of dislocations and leave many dislocations serving as recrystallization nuclei. In that case, recrystallization that occurs in the temperature range of 650° C. or higher can occur at many locations.
  • the upper limit of the average heating rate in the first temperature range is not limited, the average heating rate is preferably 20.0° C./sec or less from the viewpoint of cost.
  • the second temperature range from 650° C. to the annealing temperature is a temperature range in which recrystallization occurs, and is a temperature range in which an internal oxide layer is formed by controlling the atmosphere. If the average heating rate in this temperature range exceeds 5.0° C./sec, recrystallization of the steel sheet proceeds before oxides are formed on the surface layer, forming coarse ferrite grains. In this case, an internal oxide layer in which the grain boundaries are covered with oxide is not formed. On the other hand, if the average heating rate is less than 0.5° C./second, the decarburization reaction may proceed excessively and the tensile strength of the steel sheet may decrease.
  • (P(H2O)/P(H2)) exceeds 2.00, decarburization proceeds excessively, the thickness of the decarburized layer increases, and the tensile strength of the steel sheet decreases. Therefore, (P(H2O)/P(H2)) is set to 2.00 or less. (P(H2O)/P(H2)) is preferably 1.50 or less, more preferably 1.20 or less.
  • the average heating rate in the first temperature range should be higher than that in the second temperature range.
  • the average heating rate in the first temperature range is preferably faster than the average heating rate in the second temperature range by 2.0° C./second or more.
  • the steel sheet After heating to the annealing temperature in the manner described above, the steel sheet is held at a predetermined maximum heating temperature for 1 second or more. If the holding time is less than 1 second, it will not austenitize sufficiently. In this case, a sufficient amount of hard structure cannot be secured by phase transformation during subsequent cooling, and sufficient tensile strength cannot be obtained. There is no particular limitation on the upper limit of the retention time. However, if the holding time is too long, the manufacturability of the steel sheet is impaired, so the upper limit of the holding time is preferably 1000 seconds.
  • the steel sheet after the annealing process is cooled from the annealing temperature to 440 to 550 ° C. at an average cooling rate of 0.5 ° C./sec or more, and the steel plate is mainly composed of Zn and has an effective Al amount of 0.050. ⁇ 0.250% by mass, immersed in a plating bath, pulled up from the plating bath, cooled so that the time to reach 400 ° C. is within 10 seconds, followed by an average cooling rate of 400 to 350 ° C. is cooled to 350° C. or less so that the temperature is 1.0° C./sec or more and 5.0° C./sec or less.
  • a galvanized layer is formed on the surface of the steel sheet, and an Fe—Al alloy layer is formed on at least a portion of the interface between the steel sheet and the plating layer.
  • the average cooling rate from 440°C to 550°C is less than 0.5°C/sec, no hard structure is formed in the base steel sheet, resulting in reduced strength. Further, if the cooling stop temperature (steel sheet temperature when immersed in the plating bath) is less than 440°C, it is necessary to apply a large amount of heat to the plating bath in order to maintain the plating temperature, which increases the manufacturing cost. On the other hand, if the steel sheet temperature exceeds 550° C. when the steel sheet is immersed in the plating bath, equipment for removing a large amount of heat from the plating bath is required to maintain the plating bath temperature, which increases the manufacturing cost.
  • the composition of the plating bath in which the steel plate is immersed is mainly composed of Zn (for example, 80% by mass or more), and the effective Al amount (the value obtained by subtracting the total amount of Fe from the total amount of A in the plating bath) is 0.050 to 0.250. If it is mass%, it is not limited, and other elements such as Ag, B, Be, Bi, Ca, Cd, Co, Cr, Cs, Cu, Ge, Hf, I, K, La, Li , Mg, Mn, Mo, Na, Nb, Ni, Pb, Rb, S, Si, Sn, Sr, Ta, Ti, V, W, Zr and REM.
  • the effective amount of Al in the plating bath is preferably 0.065% by mass or more.
  • the effective amount of Al in the plating bath is preferably 0.180% by mass or less.
  • the plating bath temperature is not limited, it is preferably 450 to 490°C. If the plating bath temperature is lower than 450° C., the viscosity of the plating bath increases excessively, making it difficult to control the thickness of the plating layer, which may impair the appearance of the hot-dip galvanized steel sheet.
  • the plating bath temperature is preferably 455° C. or higher. On the other hand, if the plating bath temperature exceeds 490° C., a large amount of fumes may be generated, making safe plating operations difficult.
  • the plating bath temperature is preferably 480° C. or lower.
  • an Fe—Zn alloy layer is formed instead of an Fe—Al alloy layer, and a desired Fe—Al alloy layer cannot be obtained.
  • Fe- An Al alloy layer can be formed with a coverage of 40% or more.
  • the time at 400 to 350° C. exceeds 50.0 seconds, the thickness of the Fe—Al alloy layer becomes 100 nm or more. Therefore, the average cooling rate from 400 to 350° C. should be 1.0° C./second or more.
  • the plating bath temperature is over 450 ° C., the time to reach 400 ° C.
  • the average cooling rate up to 400°C is within 10 seconds (that is, the average cooling rate to 400 ° C. is over 5.0 ° C./sec), and then the average cooling of 400 to 350 ° C.
  • the average cooling rate up to 400°C is preferably higher than the average cooling rate from 400 to 350°C so that the rate is 5.0°C/sec or less.
  • an Fe—Al alloy layer is appropriately formed at the interface between the galvanized layer and the base steel sheet.
  • the Fe—Al alloy layer formed at the interface between the galvanized layer and the base steel sheet incorporates and integrates the internal oxides present at the grain boundaries of the surface layer of the steel sheet. As a result, the Fe--Al alloy layer is less likely to separate from the base steel sheet, and a higher bending fatigue strength can be obtained.
  • the average cooling rate up to 400°C is 6.0°C/sec or more.
  • the steel sheet after the plating process may be subjected to skin-pass rolling for the purpose of shape adjustment and the like.
  • skin pass rolling it is preferable to set the rolling rate to 0.5% or less.
  • the base material steel sheet was observed by the method described above, and the thickness of the internal oxide layer, the grain boundary coverage of the internal oxide layer, and the thickness at the 1/4 thickness position. observed the tissue.
  • the results are shown in Tables 3-1 and 3-2.
  • the coating weight of the galvanized layer was 10 to 80 g/m 2 .
  • TS tensile strength
  • LME resistance bending fatigue strength
  • bending resistance of the obtained galvanized steel sheets were evaluated in the following manner.
  • ⁇ Tensile strength> A JIS No. 5 tensile test piece was taken from a direction (width direction) perpendicular to the rolling direction and thickness direction of the galvanized steel sheet (rolling direction and thickness direction of the base steel sheet), and tensile according to JIS Z 2241: 2011. A test was performed to measure the tensile strength (TS). A tensile strength of 980 MPa or more was judged to be high strength.
  • ⁇ Bending fatigue strength> A plane bending fatigue test was performed according to JISZ2275:1978.
  • the test piece was a No. 1 test piece with a width of 30 mm and an R of 40 mm.
  • the fatigue strength after 106 cycles was evaluated as follows. 10 6 times fatigue strength > 0.35 ⁇ TS: Ex (especially excellent in bending fatigue strength) 10 6 times fatigue strength > 0.30 ⁇ TS: OK (excellent bending fatigue strength) 10 6 times fatigue strength ⁇ 0.30 ⁇ TS: NG
  • No. 1 which has a chemical composition and manufacturing method within the scope of the present invention.
  • Nos. 1 8, 11, 12, 17 to 46, preferable Fe—Al alloy layers and internal oxide layers were formed, and tensile strength, LME resistance, bending fatigue strength, and bending resistance were all excellent.
  • No. 47 to 56 at least one of tensile strength, LME resistance, bending fatigue strength, and bending resistance was inferior.
  • the manufacturing method of No. 1 is outside the scope of the present invention.
  • the tensile strength was low, or the internal oxide layer and Fe-Al alloy layer were not formed in a preferable state, so the LME resistance, bending fatigue strength, and bending resistance were inferior in one or more of the sexes.
  • a galvanized steel sheet that has high strength, sufficient bending resistance, and excellent LME resistance and bending fatigue strength.
  • Such a steel sheet is useful as a high-strength steel sheet for automobiles.

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PCT/JP2023/002565 2022-02-25 2023-01-27 亜鉛めっき鋼板およびその製造方法 WO2023162573A1 (ja)

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EP23759579.8A EP4484590A4 (en) 2022-02-25 2023-01-27 Galvanized steel sheet and method for producing same
US18/839,195 US20250154633A1 (en) 2022-02-25 2023-01-27 Zinc-plated steel sheet and method for producing same
CN202380023062.1A CN118742664A (zh) 2022-02-25 2023-01-27 镀锌钢板及其制造方法
JP2024502935A JPWO2023162573A1 (GUID-C5D7CC26-194C-43D0-91A1-9AE8C70A9BFF.html) 2022-02-25 2023-01-27
KR1020247028757A KR20240144265A (ko) 2022-02-25 2023-01-27 아연 도금 강판 및 그 제조 방법
MX2024010245A MX2024010245A (es) 2022-02-25 2023-01-27 Lamina de acero enchapada en zinc y metodo para producir la misma.

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WO2018043453A1 (ja) 2016-08-30 2018-03-08 Ｊｆｅスチール株式会社 薄鋼板およびその製造方法
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PL3378965T3 (pl) * 2016-02-25 2021-01-25 Nippon Steel Corporation Blacha stalowa cienka o dużej wytrzymałości cynkowana zanurzeniowo na gorąco o doskonałej odporności na łuszczenie przy uderzeniu oraz odporności na korozję obszaru poddawanego obróbce
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