US20230093068A1 - Hot-dip zinc-plated steel sheet - Google Patents

Hot-dip zinc-plated steel sheet Download PDF

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US20230093068A1
US20230093068A1 US17/909,231 US202117909231A US2023093068A1 US 20230093068 A1 US20230093068 A1 US 20230093068A1 US 202117909231 A US202117909231 A US 202117909231A US 2023093068 A1 US2023093068 A1 US 2023093068A1
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hot
steel sheet
less
dip zinc
plated
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Shota Kikuchi
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Nippon Steel Corp
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
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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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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
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    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • C—CHEMISTRY; METALLURGY
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • C—CHEMISTRY; METALLURGY
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/06—Zinc or cadmium or alloys based thereon

Definitions

  • Hot stamping is a technique for pressing a blank that is heated to a temperature at which the single-phase region of austenite is formed (Ac 3 point) or more (for example, heated to about 900° C.) and then rapidly cooling the blank in a die at the same time as forming to perform quenching. According to this technique, it is possible to manufacture a press-formed product having high shape fixability and high strength.
  • Electrode sticking a phenomenon in which a copper electrode and plating provided on the surface of the formed product are melted and adhered to each other
  • Electrode sticking is not preferable because it could cause a poor weld or it will inevitably cause manufacturing downtime to replace the electrode. Electrode sticking during spot welding is not considered in Patent Document 1.
  • the present inventor investigated the cause of electrode sticking during spot welding. As a result, the present inventor found that electrode sticking during spot welding is further suppressed as the number of voids present in a zinc-plated layer is smaller since electrode sticking during spot welding is greatly affected by voids (vacancy) present in the zinc-plated layer (a hot-dip zinc-plated layer obtained after hot stamping) of a hot-stamping formed body. The present inventor thought that an overcurrent occurs by a narrow electric current path caused by the voids in the zinc-plated layer, and the overcurrent causes overheating which makes electrode sticking between an electrode and zinc plating.
  • the present inventor thought that voids formed in the hot-stamping formed body are caused by a difference in thermal contraction between a base metal and a hot-dip zinc-plated layer and a difference in thermal contraction between different phases present in a plating layer during hot stamping forming.
  • the present inventor investigated a method of reducing a difference in thermal contraction during hot stamping forming.
  • Si is an element that improves the fatigue property of the hot-stamping formed body. Further, Si is also an element that improves a hot-dip galvanizing property, particularly plating wettability, by forming a stable oxide film on the surface of the steel sheet during recrystallization annealing.
  • the Si content is set to 0.10% or more.
  • the Si content is preferably 0.15% or more or 0.18% or more.
  • Si contained in steel is diffused during heating at the time of hot stamping and forms oxide on the surface of the steel sheet. The oxide formed on the surface of the steel sheet deteriorates a phosphate treatment property.
  • Si is also an element that raises the Ac 3 point of the hot-dip zinc-plated steel sheet.
  • a heating temperature during hot stamping needs to be raised in order to sufficiently austenitize the steel sheet and a heating temperature during hot stamping exceeds the evaporation temperature of the hot-dip zinc-plated layer.
  • the Si content is set to 1.50% or less.
  • the Si content is preferably 1.40% or less, 1.20% or less, or 1.00% or less.
  • Mn is an element that improves the hardenability of steel.
  • the Mn content is set to 0.5% or more to improve hardenability and obtain the desired strength of the hot-stamping formed body.
  • the Mn content is preferably 1.0% or more or 1.5% or more.
  • the Mn content is set to 2.5% or less.
  • the Mn content is preferably 2.1% or less or 2.0% or less.
  • Al is an element that deoxidizes molten steel to suppress the formation of oxide serving as the origin of fracture. Further, Al is also an element that has an action of suppressing an alloying reaction between Zn and Fe and an action of improving the corrosion resistance of the hot-stamping formed body.
  • the sol.Al content is set to 0.001% or more.
  • the sol.Al content is preferably 0.005% or more.
  • the sol.Al content is set to 0.100% or less.
  • the sol.Al content is preferably 0.090% or less, 0.070% or less, or 0.050% or less.
  • sol.Al means acid-soluble Al, and indicates solute Al that is present in steel in the state of a solid solution.
  • Ti is an element that increases oxidation resistance after hot-dip galvanizing. Further, Ti is also an element that improves the hardenability of the steel sheet by combining with N present in steel to form nitride (TiN) and suppressing the formation of nitride (BN) from B. In order to obtain these effects, the Ti content is set to 0.010% or more. The Ti content is preferably 0.020% or more. On the other hand, in a case where the Ti content is excessive, the Ac 3 point is raised and a heating temperature during hot stamping is raised. For this reason, productivity may deteriorate, and it may be difficult to secure a F phase since formation into a Fe—Zn solid solution may be facilitated.
  • the S content is an element that is contained as an impurity and is an element that forms sulfide in steel to cause the deterioration of the toughness of the hot-stamping formed body and to deteriorate a delayed fracture resistance property. For this reason, the S content is set to 0.0100% or less.
  • the S content is preferably 0.0050% or less. It is preferable that the S content is 0%.
  • the S content may be set to 0.0001% or more.
  • the P is an element that is included as an impurity, and is an element that segregates at a grain boundary to deteriorate the toughness and delayed fracture resistance property of steel. For this reason, the P content is set to 0.100% or less.
  • the P content is preferably 0.050% or less. It is preferable that the P content is 0%. However, since cost required to remove P is increased in a case where the P content is to be excessively reduced, the P content may be set to 0.001% or more.
  • the remainder of the chemical composition of the steel sheet of the hot-dip zinc-plated steel sheet according to this embodiment is Fe and impurities. Elements, which are unavoidably mixed from a steel raw material or scrap and/or during the manufacture of steel and are allowed in a range where the properties of the hot-stamping formed body obtained from the hot stamping of the hot-dip zinc-plated steel sheet according to this embodiment do not deteriorate, are exemplified as the impurities.
  • the steel sheet of the hot-dip zinc-plated steel sheet according to this embodiment may contain the following elements as arbitrary elements instead of a part of Fe.
  • the contents of the following arbitrary elements in a case where the following arbitrary elements are not contained are 0%.
  • Nb has an action of forming carbide to refine crystal grains during hot stamping. In a case where crystal grains are refined, the toughness of steel is increased. In order to reliably obtain this effect, it is preferable that the Nb content is set to 0.02% or more. However, in a case where the Nb content is excessively high, the above-mentioned effect may be saturated and the hardenability of steel may deteriorate. Accordingly, the Nb content is set to 0.05% or less.
  • V 0% to 0.50%
  • V is an element that finely forms carbonitride in steel to improve strength. In order to reliably obtain this effect, it is preferable that the V content is set to 0.005% or more. On the other hand, in a case where the V content exceeds 0.50%, the toughness of steel deteriorates during spot welding and cracks are likely to occur. For this reason, the V content is set to 0.50% or less.
  • Cr is an element that improves the hardenability of steel. In order to reliably obtain this effect, it is preferable that the Cr content is set to 0.10% or more. On the other hand, in a case where the Cr content exceeds 0.50%, Cr carbide is formed in steel and it is difficult for Cr carbide to be dissolved during heating of hot stamping, so that hardenability deteriorates. For this reason, the Cr content is set to 0.50% or less.
  • Ni is an element that has an effect of improving the toughness of steel, an effect of suppressing the embrittlement of steel caused by liquid Zn during heating of hot stamping, and an effect of improving the hardenability of steel.
  • the Ni content is set to 0.01% or more.
  • the Ni content is set to 2.00% or less.
  • the chemical composition of the steel sheet described above may be measured by a general analysis method.
  • the chemical composition of the steel sheet described above may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES).
  • C and S may be measured using a combustion-infrared absorption method and N may be measured using an inert gas fusion-thermal conductivity method.
  • sol.Al may be measured by ICP-AES using a filtrate that is obtained in a case where a sample is decomposed with an acid by heating.
  • the chemical composition may be analyzed after the hot-dip zinc-plated layer provided on the surface of the hot-dip zinc-plated steel sheet is removed by mechanical grinding.
  • the steel sheet of the hot-dip zinc-plated steel sheet according to this embodiment has the above-mentioned chemical composition, and has an average grain size of 4.0 ⁇ m or less and a standard deviation of grain sizes of 2.0 ⁇ m or less in a surface layer region thereof.
  • the surface layer region of the steel sheet of the hot-dip zinc-plated steel sheet according to this embodiment will be described below.
  • Surface layer region an average grain size of 4.0 ⁇ m or less and a standard deviation of grain sizes of 2.0 ⁇ m or less
  • the surface layer region refers to a region between the surface of the steel sheet and a position that is away from the surface of the steel sheet in a depth direction by a distance of 25 ⁇ m.
  • an average grain size in this surface layer region exceeds 4.0 ⁇ m or the standard deviation of grain sizes exceeds 2.0 ⁇ m, it is not possible to suppress the evaporation of zinc present in the hot-dip zinc-plated layer in the heating during hot stamping. Accordingly, a lot of voids are formed in the hot-stamping formed body. As a result, desired spot weldability cannot be obtained in the hot-stamping formed body.
  • an average grain size is set to 4.0 ⁇ m or less and the standard deviation of grain sizes is set to 2.0 ⁇ m or less. Since a smaller average grain size in the surface layer region of the steel sheet is more preferable, an average grain size in the surface layer region of the steel sheet may be set to 3.5 ⁇ m or less or 3.0 ⁇ m or less. Further, since a smaller standard deviation of grain sizes in the surface layer region of the steel sheet is more preferable, the standard deviation of grain sizes in the surface layer region of the steel sheet may be set to 1.8 ⁇ m or less or 1.5 ⁇ m or less.
  • the lower limit of an average grain size in the surface layer region of the steel sheet does not need to be particularly limited, but may be set to 1.5 ⁇ m. Further, the lower limit of the standard deviation of grain sizes in the surface layer region of the steel sheet does not need to be particularly limited, but may be set to 1.0 ⁇ m.
  • EBSP-OIM electron back scatter diffraction pattern-orientation image microscopy
  • an analysis is made in at least 5 visual fields in a region having a size of 40 ⁇ m ⁇ 30 ⁇ m with a magnification of 1200.
  • a spot where an angle difference between adjacent measurement points is 5° or more is defined as a grain boundary, and the equivalent circle diameters of crystal grains are calculated and are regarded as grain sizes.
  • the average value of the obtained grain sizes of crystal grains is calculated, so that an average grain size in the surface layer region is obtained.
  • a standard deviation is calculated from the obtained grain sizes of crystal grains, so that the standard deviation of grain sizes in the surface layer region is obtained.
  • the steel sheet, the boundary layer, and the hot-dip zinc-plated layer may be specified using a method to be described later, and the above-mentioned measurement may be performed for the steel sheet and the surface layer region of the specified region.
  • the concentrations (mass %) of Fe, Zn, and Al are measured using glow discharge optical emission spectrometry (GDS) up to a depth of 50 ⁇ m from the surface of the hot-dip zinc-plated steel sheet in the depth direction (sheet thickness direction).
  • GDS glow discharge optical emission spectrometry
  • a GDS profile shown in FIG. 1 can be obtained.
  • a depth range in which the Fe concentration is 85 mass % or more is defined as the steel sheet and a depth range in which the Zn concentration is 90 mass % or more is defined as the hot-dip zinc-plated layer.
  • a depth range between the steel sheet and the hot-dip zinc-plated layer is defined as the boundary layer.
  • the metallographic structure of the steel sheet is not particularly limited as long as desired strength and desired spot weldability can be obtained after hot stamping.
  • the metallographic structure of the steel sheet may consist of, by area %, 20% to 90% of ferrite, 0% to 100% of bainite and martensite, 10% to 80% of pearlite, and 0% to 5% of residual austenite.
  • the metallographic structure of the steel sheet may be measured using the following methods.
  • a region which has a length of 50 ⁇ m and is present between a depth corresponding to 1 ⁇ 8 of the sheet thickness from the surface and a depth corresponding to 3 ⁇ 8 of the sheet thickness from the surface, is measured at a measurement interval of 0.1 ⁇ m with an electron backscatter diffraction method at an arbitrary position in a longitudinal direction on the cross section of the sample so that a region having a depth corresponding to 1 ⁇ 4 of the sheet thickness from the surface can be analyzed. As a result, crystal orientation information is obtained.
  • a device which includes a schottky emission scanning electron microscope (JSM-7001F manufactured by JEOL Ltd.) and an EBSP detector (DVC5 detector manufactured by TSL Solutions), is used for the measurement.
  • the degree of vacuum in the device is set to 9.6 ⁇ 10 ⁇ 5 Pa or less, an accelerating voltage is set to 15 kV, an irradiation current level is set to 13, and the irradiation level of an electron beam is set to 62. Further, a reflected electron image is taken in the same visual field.
  • crystal grains in which ferrite and cementite are precipitated in layers are specified from the reflected electron image and the area ratio of the crystal grains is calculated, so that the area ratio of pearlite is obtained.
  • a region where a grain average misorientation value is 1.0° or less is determined as ferrite from the obtained crystal orientation information using “Grain Average Misorientation” function provided in software “OIM Analysis (registered trademark)” incorporated in the EBSP analysis device.
  • the area ratio of the region determined as ferrite is obtained, so that the area ratio of ferrite is obtained.
  • the area ratio of residual austenite is measured using an electron backscatter diffraction image (EBSP). Analysis using EBSP is performed for a region, which is present between a depth corresponding to 1 ⁇ 8 of the sheet thickness from the surface and a depth corresponding to 3 ⁇ 8 of the sheet thickness from the surface, using a sample taken at the same sampling position as that in a case where the volume percentage of ferrite is measured so that a region having a depth corresponding to 1 ⁇ 4 of the sheet thickness from the surface of a hot-rolled steel sheet can be analyzed.
  • EBSP electron backscatter diffraction image
  • an accelerating voltage is set in a range of 15 to 25 kV, the measurement is performed at intervals of at least 0.25 ⁇ m or less, and crystal orientation information about each measurement point in a range of 150 ⁇ m or more in the sheet thickness direction and 250 ⁇ m or more in the rolling direction is obtained.
  • a measurement point at which a crystal structure is fcc is determined as residual austenite using “PhaseMap” function provided in software “OIM Analysis (registered trademark)” incorporated in the EBSP analysis device.
  • a ratio of the measurement points, which are determined as residual austenite, is obtained, so that the area ratio of residual austenite is obtained.
  • the sum of the area ratios of bainite and martensite in this embodiment is a value that is obtained in a case where the sum of the area ratios of ferrite and pearlite and the volume percentage of residual austenite measured using the above-mentioned method is subtracted from 100%.
  • the hot-dip zinc-plated steel sheet according to this embodiment includes the above-mentioned steel sheet, the boundary layer provided on the steel sheet, and the hot-dip zinc-plated layer provided on the boundary layer.
  • the boundary layer and the hot-dip zinc-plated layer will be described below.
  • Boundary Layer the Maximum Al Concentration is 0.30 Mass % or More
  • the boundary layer refers to a layer that is present between the above-mentioned steel sheet and the hot-dip zinc-plated layer to be described later.
  • the maximum Al concentration of the boundary layer of the hot-dip zinc-plated steel sheet according to this embodiment is 0.30 mass % or more. In a case where the maximum Al concentration of the boundary layer is less than 0.30 mass %, desired spot weldability cannot be obtained in the hot-stamping formed body. For this reason, the maximum Al concentration of the boundary layer is set to 0.30 mass % or more.
  • the maximum Al concentration of the boundary layer is preferably 0.35 mass % or more or 0.40 mass % or more. Since higher maximum Al concentration of the boundary layer is more preferable, the upper limit thereof does not need to be particularly specified but may be set to 1.00 mass %.
  • the maximum Al concentration of the boundary layer is measured using the following method. At arbitrary five points on the hot-dip zinc-plated steel sheet, the concentrations (mass %) of Fe, Zn, and Al are measured using glow discharge optical emission spectrometry (GDS) up to a depth of 50 ⁇ m from the surface in the depth direction (sheet thickness direction). In a case where a depth range in which the Fe concentration is 85 mass % or more is defined as the steel sheet, a depth range in which the Zn concentration is 90 mass % or more is defined as the hot-dip zinc-plated layer, and a depth range between the steel sheet and the hot-dip zinc-plated layer is defined as the boundary layer, the maximum Al concentration (mass %) of the boundary layer is obtained at each measurement point. The average value of the maximum Al concentrations of the boundary layer at the respective measurement points is calculated, so that the maximum Al concentration of the boundary layer is obtained.
  • GDS glow discharge optical emission spectrometry
  • a cumulative rolling reduction during the cold rolling was set in a range of 30% to 90%.
  • a hot-dip zinc-plated layer was formed on the obtained steel sheets by a continuous hot-dip galvanizing line, so that hot-dip zinc-plated steel sheets shown in Tables 2-1 and 2-2 were obtained.
  • the adhesion amount of the hot-dip zinc-plated layer was set in a range of 5 g/m 2 to 150 g/m 2 per side.
  • test pieces having a size of 100 mm ⁇ 30 mm were taken from a position excluding a region within 10 mm from an end surface, these test piece overlapped with each other, and spot welding was performed while current was changed under the following conditions.
  • tensile strength is in a range of 1500 MPa to 2500 MPa and the cross-sectional area ratio of voids in the hot-stamping formed body was reduced to 15.0% or less.
  • Examples of the present invention were excellent in spot weldability.
  • the cross-sectional area ratio of voids in the hot-stamping formed body was reduced to 13.0% or less and spot weldability was better.

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