WO2025018302A1 - 鋼板、めっき鋼板及び自動車部材 - Google Patents

鋼板、めっき鋼板及び自動車部材 Download PDF

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Publication number
WO2025018302A1
WO2025018302A1 PCT/JP2024/025302 JP2024025302W WO2025018302A1 WO 2025018302 A1 WO2025018302 A1 WO 2025018302A1 JP 2024025302 W JP2024025302 W JP 2024025302W WO 2025018302 A1 WO2025018302 A1 WO 2025018302A1
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Prior art keywords
steel sheet
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content
steel
steel plate
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PCT/JP2024/025302
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English (en)
French (fr)
Japanese (ja)
Inventor
卓哉 光延
敬之 古川
将明 浦中
浩史 竹林
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Nippon Steel Corp
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Nippon Steel Corp
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Priority to JP2025534035A priority Critical patent/JP7773119B2/ja
Priority to KR1020267000362A priority patent/KR20260021034A/ko
Priority to CN202480046662.4A priority patent/CN121488060A/zh
Publication of WO2025018302A1 publication Critical patent/WO2025018302A1/ja
Priority to MX2026000529A priority patent/MX2026000529A/es
Anticipated expiration legal-status Critical
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—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
    • 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
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K11/00—Resistance welding; Severing by resistance heating
    • B23K11/16—Resistance welding; Severing by resistance heating taking account of the properties of the material to be welded
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite

Definitions

  • the present invention relates to steel sheets and plated steel sheets. More specifically, the present invention relates to steel sheets and plated steel sheets having high LME resistance.
  • Patent Document 2 discloses a steel sheet having an improved weldability by suppressing LME cracking, in which Si oxide particles having a particle size of 20 nm or more are present in a surface layer of the steel sheet at a number density of 3,000 to 6,000 particles/ mm2 with an appropriate particle size distribution.
  • the present invention aims to provide steel sheets and plated steel sheets with high LME resistance.
  • the inventors have thoroughly investigated means for solving the above problems. As a result, they have found that by applying distortion to a steel sheet before annealing using a projectile under appropriate conditions to create an appropriate surface condition, and then performing high dew point annealing, the surface layer of the steel sheet is decarburized and a layer with a high ferrite fraction in which the ferrite is randomly oriented is formed, making it possible to suppress LME.
  • the present invention was developed based on the above findings and through further investigation, and its gist is as follows:
  • Steel plate having a tensile strength of 780 MPa or more and having chemical compositions, in mass%, of C: 0.05-0.40%, Si: 0.5-3.0%, Mn: 0.1-5.0%, sol. and the balance being Fe and impurities.
  • the steel sheet has a surface roughness Ra of 3.0 ⁇ m or less.
  • An automotive component comprising any one of the steel sheets (1) to (5) above or the plated steel sheet (6) above.
  • FIG. 1 is a diagram showing an example of the results of grazing incidence X-ray diffraction measurement when the ferrite phase is randomly oriented and when it is not randomly oriented.
  • FIG. 2 is a diagram for explaining the positions of cracks targeted in the LME resistance evaluation in the examples.
  • LME cracking is caused by, for example, when the metal structure of a steel sheet is heated and transformed into austenite during spot welding, and molten zinc produced by melting the plating penetrates into the grain boundaries of the austenite in the surface layer of the steel sheet.
  • the molten zinc that penetrates into the austenite grain boundaries embrittles the steel sheet, and is further caused by the tensile stress applied to the steel sheet during welding.
  • the inventors of the present invention came up with the idea of utilizing the metal structure of the surface layer of the steel sheet as a method for improving LME resistance.
  • the surface layer of the steel sheet has a metal structure mainly composed of a ferrite phase with a low C concentration and low LME sensitivity, and the occurrence of LME is suppressed by randomly orienting the ferrite phase.
  • LME resistance means a property in which LME cracking is suppressed in a steel sheet
  • LME sensitivity means a property in which LME cracking is likely to occur in a steel sheet.
  • the random orientation of the ferrite phase means that the characteristics of the ferrite grain boundaries are averaged as a whole. In other words, it means that the crystal orientation of each ferrite particle in the ferrite phase is randomly oriented.
  • the random orientation of the crystal orientation of the ferrite particles prevents grain boundaries oriented in a specific direction from being unevenly distributed and connected continuously or intermittently. It is believed that LME cracking occurs when Zn from the plating invades grain boundaries where the grain boundary energy is locally low. In other words, if there are continuous grain boundaries where the grain boundary energy is locally low, Zn from the plating will concentrate there, making LME cracking more likely to occur. By averaging the characteristics of the grain boundaries as a whole, grain boundaries where the grain boundary energy is locally low will no longer be connected continuously, and local concentration of Zn from the plating will be suppressed, which is believed to result in improved LME resistance.
  • the random orientation of the ferrite phase is expressed by the following conditional formula.
  • a steel sheet that satisfies the following conditional formula means that the ferrite phase is randomly oriented.
  • the diffraction intensity corresponding to the (110) plane is I(110)
  • the diffraction intensity corresponding to the (200) plane is I(200)
  • the diffraction intensity corresponding to the (211) plane is I(211), 0.45 ⁇ I(110)/(I(110)+I(200)+I(211)) ⁇ 0.90
  • the present invention when manufacturing the steel sheet, strain is imparted to the cold-rolled steel sheet, and then annealing is performed at a high dew point. This promotes decarburization and makes it easier to form a ferrite phase on the steel sheet surface. Furthermore, the present invention was made based on the discovery that the orientation of the ferrite phase can be randomized by controlling the temperature at which humidification begins. The present invention will be described in detail below.
  • the steel plate according to the present invention has a tensile strength of 780 MPa or more, i.e., it is a high-strength steel plate.
  • the present invention suppresses LME occurring in high-strength steel plates.
  • the steel plate according to the present invention specifically has a tensile strength of 780 MPa or more.
  • the upper limit of the tensile strength is not particularly limited, but may be, for example, 2000 MPa or less from the viewpoint of ensuring toughness.
  • the tensile strength is measured by taking a JIS No. 5 tensile test piece whose longitudinal direction is perpendicular to the rolling direction and the plate thickness direction, and performing the measurement in accordance with JIS Z 2241:2011.
  • the tensile strength may be 980 MPa or more, 1180 MPa or more.
  • a JIS No. 5 test piece whose longitudinal direction is any direction on the surface of the steel plate may be taken when measuring the tensile strength.
  • C 0.05-0.40%
  • C (carbon) is an element that ensures the strength of steel.
  • the C content is set to 0.05% or more.
  • the C content is set to 0.40% or less.
  • the C content is set to 0.08% or more, 0.10% or more, 0.08 ...
  • the C content may be 0.37% or less, 0.35% or less, or 0.30% or less.
  • Silicon (Si) is an element that promotes ferrite stabilization and decarburization. By including silicon, decarburization advances in the surface layer and the ferrite in the surface layer is stabilized by the pretreatment and heat treatment described below. By annealing the steel sheet at a high dew point, the LME resistance is improved. To obtain this effect, the Si content is set to 0.5% or more. If the Si content is too high, the external Oxidation progresses and oxides (scale) are formed on the surface layer of the steel sheet, which in turn suppresses decarburization on the outermost surface, reducing the effect of improving LME resistance. The Si content is 0.6% or more, 0.7% or more, or 0.8% or more. The Si content is 2.5% or less, 2.0 % or less, or 1.5% or less.
  • Mn manganese
  • Mn manganese
  • the Mn content is set to 0.1% or more.
  • the Mn content is set to 5.0% or less.
  • the Mn content is set to 0.5% or more, 1.0% or more, or 1.5% or more.
  • the Mn content may be 4.5% or less, 4.0% or less, or 3.5% or less.
  • sol.Al 0-3.0%
  • Al (aluminum) is an element that, like Si, promotes ferrite stabilization and decarburization by dissolving in steel.
  • Sol. Al is not in the form of oxides such as Al2O3 .
  • sol. Al means acid-soluble Al that is soluble in acid, and is calculated as the amount of Al measured after excluding the insoluble residue on the filter paper that is generated during the analysis of Al. Since the role of Al can be achieved by including Si, sol. Al is not essential, and the lower limit of the content of sol. Al is 0%. If the content of sol. Al is too high, the steel sheet may be deteriorated due to high dew point annealing.
  • the content of sol. Al is 3.0% or less.
  • the content of sol. Al may be 0.1% or more, 0.3% or more, or 0.5% or more.
  • the Al content may be 2.0% or less, 1.5% or less, or 1.0% or less.
  • Si and sol. Al are elements that reduce LME resistance when added in excess, so the total content of Si and sol. Al is preferably 1.8% or less.
  • the total content of Si and sol. Al may be 1.7% or less, 1.6% or less, or 1.5% or less.
  • P 0.0300% or less
  • P (phosphorus) is an impurity generally contained in steel. If the P content exceeds 0.0300%, there is a risk of reduced weldability. Therefore, the P content is set to 0.0300% or less.
  • the P content may be 0.0200% or less, 0.0100% or less, or 0.0050% or less. It is preferable that no P is contained, and the lower limit of the P content is 0%. From the viewpoint of phosphorus cost, the P content may be more than 0%, 0.0001% or more, or 0.0005% or more.
  • S sulfur
  • S is an impurity generally contained in steel. If the S content exceeds 0.0300%, the weldability decreases, and further, the amount of MnS precipitated increases, which reduces workability such as bendability. Therefore, the S content is set to 0.0300% or less.
  • the S content may be 0.0100% or less, 0.0050% or less, or 0.0020% or less. It is preferable that S is not contained, and the lower limit of the S content is 0%. From the viewpoint of desulfurization costs, the S content may be more than 0%, 0.0001% or more, or 0.0005% or more. good.
  • N nitrogen
  • nitrogen is an impurity generally contained in steel. If the N content exceeds 0.0100%, there is a risk of reduced weldability. Therefore, the N content is set to 0.0100% or less.
  • the N content may be 0.0080% or less, 0.0050% or less, or 0.0030% or less. It is preferable that N is not contained, and the lower limit of the N content is 0%. From the viewpoint of cost, the N content may be more than 0%, 0.0005% or more, or 0.0010% or more.
  • O oxygen
  • oxygen is an element that forms oxides and reduces the workability of steel sheets. If the O content is too high, oxides are generated in excess, and the workability of the steel sheet is reduced. Therefore, the O content is set to 0.0030% or less.
  • the O content may be 0.0026% or less, 0.0024% or less, 0.0020% or less, or 0.0018% or less. It is preferable that O is not contained, and the lower limit of the O content is 0%. From the viewpoint of production cost, the O content may be more than 0%, 0.0005% or more, or 0.0010% or more. good.
  • B (boron) is an element that improves hardenability and contributes to improving strength, and also segregates at grain boundaries to strengthen the grain boundaries and improve toughness, so it may be contained as necessary. Since B is not an essential element, the lower limit of the B content is 0%. This effect can be obtained even with a small amount of B, but if B is contained, the B content is preferably 0.0001% or more. In order to ensure sufficient toughness, the B content is set to 0.0100% or less. The B content is set to 0.0002% or more, 0.0003% or more, or 0.0005% or more. The B content may be 0.0080% or less, 0.0060% or less, 0.0040% or less, or 0.0020% or less.
  • Ti titanium
  • Ti titanium
  • Ti titanium
  • the Ti content is 0%. This effect can be obtained even with a small amount of Ti, but when Ti is contained, the Ti content is preferably 0.0001% or more. However, if Ti is contained in an excessive amount, coarse TiN may be generated, which may impair toughness, so the Ti content is set to 0.1500% or less. It may be 0.1000% or less, 0.0500% or less, 0.0050% or less, or 0.0020% or less.
  • Niobium (Nb) is an element that contributes to improving strength by improving hardenability, and may be contained as necessary. Since it is not an essential element, the lower limit of the Nb content is 0%. This effect can be obtained even with a small amount of Nb, but when Nb is contained, the Nb content is preferably 0.0001% or more. On the other hand, from the viewpoint of ensuring sufficient toughness, the Nb content is set to 0.1500% or less. The Nb content is set to 0.1000% or less, 0.0600% or less, 0.0200% or less. It may be the following:
  • V Vanadium
  • V is an element that contributes to improving strength by improving hardenability, and therefore may be contained as necessary. Since it is not an essential element, the lower limit of the V content is 0%. This effect can be obtained even with a small amount of V, but when V is contained, the V content is preferably 0.001% or more. On the other hand, from the viewpoint of ensuring sufficient toughness, the V content is set to 0.150% or less. % or less, and may be 0.020% or less.
  • Cr 0-2.00% Cr (chromium) is effective in increasing the hardenability of steel and increasing the strength of steel, so it may be contained as necessary. Since it is not an essential element, the lower limit of the Cr content is This effect can be obtained even with a small amount of Cr, but when Cr is contained, the Cr content is preferably 0.001% or more. However, if the Cr content is excessive, a large amount of Cr carbide is formed, which may adversely affect the hardenability. Therefore, the Cr content is set to 2.00%. The Cr content may be 1.80% or less, 1.50% or less, 0.50% or less, or 0.20% or less.
  • Ni 0-2.00%
  • Ni nickel
  • the lower limit of the Ni content is This effect can be obtained even with a small amount of Ni, but when Ni is contained, the Ni content is preferably 0.001% or more.
  • the Ni content may be 0.02% or more, or 0.05% or more.
  • the Ni content is set to 2.00% or less.
  • the Ni content is 1.80%.
  • the content may be 1.50% or less, 0.50% or less, or 0.20% or less.
  • Cu (copper) is effective in increasing the hardenability of steel and increasing the strength of steel, so it may be contained as necessary. Since it is not an essential element, the lower limit of the Cu content is This effect can be obtained even with a small amount of Cu, but when Cu is contained, the Cu content is preferably 0.0001% or more. On the other hand, from the viewpoint of suppressing deterioration of toughness and cracking of the slab after casting, the Cu content is set to 2.0000% or less. The Cu content is set to 1.8000% or less, 1.0005% or more. . 5000% or less, 0.0050% or less, or 0.0020% or less.
  • Mo mobdenum
  • Mo mobdenum
  • Mo mobdenum
  • the Mo content is preferably 0.001% or more.
  • the Mo content may be 0.02% or more, or 0.03% or more.
  • the Mo content is 1.00% or less.
  • the Mo content is 0.80% or less, It may be 0.60% or less, or 0.20% or less.
  • W 0-1.000%) W (tungsten) is effective in increasing the hardenability of steel and increasing the strength of steel, so it may be contained as necessary. Since it is not an essential element, the lower limit of the W content is This effect can be obtained even with a small amount of W, but when W is contained, the W content is preferably 0.001% or more. On the other hand, from the viewpoint of suppressing a decrease in toughness, the W content is set to 1.000% or less. . 300% or less, 0.100% or less, or 0.020% or less.
  • Ca (Ca: 0-0.1000%)
  • Ca (calcium) is an element that contributes to inclusion control, particularly to finely dispersing inclusions, and has the effect of increasing toughness, so it may be contained as necessary. Since it is not an essential element, Ca The lower limit of the Ca content is 0%. This effect can be obtained even with a small amount of Ca included, but when Ca is included, the Ca content is preferably 0.0001% or more. However, if the Ca content is excessive, deterioration of the surface properties may become evident, so the Ca content is set to 0.1000% or less. may be 0.0800% or less, 0.0500% or less, 0.0300% or less, 0.0100% or less, or 0.0010% or less.
  • Mg manganesium
  • Mg is an element that contributes to inclusion control, particularly to finely dispersing inclusions, and has the effect of increasing toughness, so it may be contained as necessary. Since it is not an essential element, Mg The lower limit of the Mg content is 0%. This effect can be obtained even with a small amount of Mg, but when Mg is contained, the Mg content is preferably 0.0001% or more. The Mg content may be 0.0005% or more, or 0.0008% or more. On the other hand, if the Mg content is excessive, deterioration of the surface properties may become evident, so the Mg content is set to 0.100% or less. The amount may be 0.090% or less, 0.080% or less, 0.030% or less, 0.010% or less, 0.002% or less.
  • Zr zirconium
  • Zr zirconium
  • the lower limit of the Zr content is 0%. This effect can be obtained even with a small amount of Zr, but when Zr is contained, the Zr content is preferably 0.001% or more. However, if the Zr content is excessive, deterioration of the surface properties may become evident, so the Zr content is set to 0.100% or less. may be 0.050% or less, 0.030% or less.
  • Hf (hafnium) is an element that contributes to inclusion control, particularly to finely dispersing inclusions, and has the effect of increasing toughness, so it may be contained as necessary.
  • the lower limit of the Hf content is 0%. This effect can be obtained even with a small amount of Hf content, but when Hf is contained, the Hf content is preferably 0.0001% or more.
  • the Hf content may be 0.0003% or more, or 0.0005% or more.
  • the Hf content is set to 0.100% or less.
  • the amount may be 0.050% or less, 0.030% or less, 0.010% or less, 0.005% or less, 0.002% or less.
  • REM 0-0.1000%
  • REM rare earth elements
  • the lower limit of the REM content is 0%. This effect can be obtained even with a small amount of REM content, but when REM is contained, the REM content is preferably 0.0001% or more.
  • the REM content may be 0.0003% or more, or 0.0005% or more.
  • the REM content is set to 0.1000% or less.
  • the content may be 0.0500% or less, 0.0300% or less, 0.0100% or less, 0.0050% or less, or 0.0020% or less.
  • REM is an abbreviation for Rare Earth Metal. REM refers to elements that belong to the lanthanide series. REM are usually added as misch metals.
  • the remainder other than the above chemical components consists of Fe and impurities.
  • the remainder may be Fe and impurities, that is, the remainder may consist only of Fe and impurities.
  • impurities refer to components that are mixed in due to various factors in the manufacturing process, including raw materials such as ores and scraps, when industrially manufacturing steel plate, and do not adversely affect the LME resistance of the steel plate according to the present invention, that is, impurities are contained in a range that allows the steel plate according to the present invention to obtain the LME resistance required for the steel plate.
  • Analysis of the chemical components of steel plate may be performed using elemental analysis methods known to those skilled in the art, for example, inductively coupled plasma mass spectrometry (ICP-MS).
  • ICP-MS inductively coupled plasma mass spectrometry
  • C and S may be measured using the combustion-infrared absorption method
  • N may be measured using the inert gas fusion-thermal conductivity method
  • O may be measured using the inert gas fusion-infrared absorption method.
  • the surface layer of the steel sheet means a layered region having a depth from the surface of the steel sheet (sheet surface of the steel sheet) to a specified distance in the sheet thickness direction of the steel sheet.
  • the surface of the steel sheet means the surface of the steel sheet (sheet surface) excluding the plating.
  • the specified distance in the sheet thickness direction can be the longer (deeper) length (depth) of the "depth at which the C concentration of the surface layer is 0.02% or less" or the "depth at which the area ratio of the ferrite phase is 90% or more" described below.
  • the steel sheet of the present invention has a surface roughness Ra of 3.0 ⁇ m or less.
  • the surface roughness Ra is the arithmetic mean roughness Ra defined in JIS B0601: 2013.
  • the surface roughness Ra is the surface roughness of the interface between the steel sheet and the plating layer, excluding the plating.
  • the surface roughness Ra when measuring the surface roughness Ra, 10 measurement points are randomly selected on the surface of the steel plate in accordance with JIS B 0601:2013 so that the distance between each measurement point is 1 mm or more.
  • the surface profile is measured using a laser microscope (for example, Keyence's "VK-X3000"). Specifically, an image is taken at a magnification of 20 times using a laser microscope, and the arithmetic mean roughness (Ra) at each measurement point is calculated using a reference length of 2000 ⁇ m in the image taken.
  • the arithmetic mean value of the arithmetic mean roughness (Ra) at each measurement point is defined as the "surface roughness Ra".
  • the depth at which the C concentration measured by GDS (glow discharge spectroscopy) is 0.02% or less in the sheet thickness direction from the surface of the steel sheet is 3 ⁇ m or more.
  • Such a surface structure (metal structure in the surface layer of the steel plate) can be obtained as a decarburized layer produced by setting the chemical composition of the steel plate as described above and carrying out the pretreatment process and annealing process described below.
  • the depth where the C concentration is 0.02% or less may be, for example, 50 ⁇ m or less, 40 ⁇ m or less, or 30 ⁇ m or less.
  • the depth where the C concentration is 0.02% or less is preferably 5 ⁇ m or more, more preferably 7 ⁇ m or more, and even more preferably 10 ⁇ m or more, 15 ⁇ m or more, or 20 ⁇ m or more.
  • GDS measurements are performed at five measurement points in the plate thickness direction, and the arithmetic mean value of the depth of the area where the C concentration is 0.02% or less at each measurement point is taken as the depth where the C concentration in the surface layer is 0.02% or less.
  • the five measurement points are randomly selected so that there is an interval of 5 mm or more between each measurement point on the steel plate surface.
  • the measurement conditions are as follows. Naturally, measurement results can be obtained even if the measurement device, etc., are not exactly as follows, but if there is a difference in the measurement results, the steel plate according to the present invention is identified by the measurement results according to the following conditions.
  • a layer having an area ratio of ferrite phase of 90% or more has a thickness of 3 ⁇ m or more in the thickness direction from the surface of the steel sheet.
  • a high ferrite layer having a thickness of 3 ⁇ m or more contributes to improved LME resistance, so there is no particular upper limit to its thickness.
  • the thickness of the high ferrite layer may be, for example, 100 ⁇ m or less, 80 ⁇ m or less, 60 ⁇ m or less, or 40 ⁇ m or less.
  • the thickness of the high ferrite layer is preferably 5 ⁇ m or more, more preferably 8 ⁇ m or more, and even more preferably 10 ⁇ m or more, or 20 ⁇ m or more.
  • the structure other than ferrite in the high ferrite layer is not limited.
  • it can be one or more of martensite, bainite, and cementite.
  • the thickness of the high ferrite layer is measured by analyzing the secondary electron image by SEM observation of the observation cross section, which is mechanically polished to a mirror finish and then etched with nital.
  • a field emission scanning electron microscope e.g., JSM 7000F manufactured by JEOL Ltd., acceleration voltage: 15 kV
  • the observation field ranges from the surface of the steel sheet (sheet surface) to a depth of 500 ⁇ m in the sheet thickness direction (vertical direction of the observation cross section) and a width range of 600 ⁇ m in the direction perpendicular to the sheet thickness direction (horizontal direction of the observation cross section).
  • observation cross section Five observation fields are observed with an SEM so that the observation fields are spaced 1000 ⁇ m or more apart in the direction perpendicular to the sheet thickness direction (horizontal direction of the observation cross section), and a secondary electron image is obtained.
  • the observation resolution is 1280 ⁇ 960 pixels.
  • the surface of the steel sheet (sheet surface) is the surface of the steel sheet excluding the plating.
  • the ferrite fraction is calculated for the five secondary electron images obtained using the point counting method. More specifically, a grid with equal spacing is first drawn on the secondary electron image. Next, the number of grid points at each grid point where the structure is ferrite is found and divided by the total number of grid points to measure the ferrite fraction. The greater the total number of grid points, the more accurately the area ratio can be calculated. In this embodiment, the grid spacing is 2 ⁇ m ⁇ 2 ⁇ m, and the total number of grid points is 1,500 points.
  • an area with relatively low brightness and no substructure can be determined as ferrite.
  • substructure means a transformed structure formed inside the old austenite phase such as laths or blocks.
  • ferrite is observed as an area with relatively low brightness and a relatively monotonous spread of brightness and color tone.
  • there is no need to particularly distinguish metal structures other than ferrite but the discrimination criteria in the secondary electron image of tempered martensite, pearlite, ferrite, fresh martensite or retained austenite, or bainite are shown below.
  • An area that has a substructure (lath boundary, block boundary) within the grain and where carbides are precipitated with multiple variants is determined as tempered martensite.
  • an area where cementite is precipitated in a lamellar form is determined as pearlite.
  • An area with high brightness and where the substructure is not revealed by etching is determined as fresh martensite or retained austenite.
  • An area that does not fall into any of the above categories is determined as bainite.
  • Ferrite has low LME susceptibility.
  • the surface structure of the steel sheet is mainly composed of ferrite.
  • Such a surface structure can be obtained by setting the chemical composition of the steel sheet as described above and carrying out the pretreatment process and annealing process described below.
  • the value of the middle part of the conditional formula, "I(110)/(I(110)+I(200)+I(211))" is preferably 0.85 or less, more preferably 0.80 or less, and even more preferably 0.75 or less.
  • the conditional formula means that the ferrite phase is randomly oriented. If the ferrite phase is completely randomly oriented, the value of the middle part is 0.67.
  • grazing incidence X-ray diffraction also called grazing incidence XRD, low angle incidence XRD, or tilted XRD
  • grazing incidence X-ray diffraction is a measurement technique in which the angle of incidence of the incident X-rays is set small, and the detector is scanned (the detection angle is changed) while maintaining the angle of incidence.
  • the angle of incidence of the X-rays is fixed at 1° to detect the orientation of ferrite in the surface layer of the steel plate.
  • the angle of incidence is the angle between the surface of the sample (steel plate) and the direction of incidence of the incident X-rays.
  • Figure 1 shows examples of the results of oblique incidence XRD analysis when the ferrite phase is randomized (b) and not (a).
  • (a) is the result of oblique incidence XRD analysis of a normal (conventional) steel sheet, and it can be seen that it is oriented in the (110) direction. Therefore, the value of the middle part of the conditional formula "I(110)/(I(110)+I(200)+I(211))" is relatively large at 0.91.
  • (b) is the result of oblique incidence XRD analysis of the steel sheet of the present invention, and the orientation in the (110) direction is smaller than in (a). Therefore, the value of the middle part of the conditional formula "I(110)/(I(110)+I(200)+I(211))" is relatively small at 0.58.
  • the steel sheet of the present invention may have a plating layer as described below.
  • the depth at which the C concentration is 0.02% or less in GDS measurement and the thickness of the layer at which the area ratio of the ferrite phase is 90% or more start from the interface between the steel sheet and the plating layer.
  • the interface between the steel sheet and the plating layer is defined as follows. First, the Fe content in the thickness direction of the plated steel sheet is measured by GDS measurement. The value with the highest Fe content is defined as the Fe content of the steel sheet. The point at which the Fe content is 93% of the Fe content of the steel sheet is defined as the "interface between the steel sheet and the plating layer".
  • the plated steel sheet according to the present invention has a Zn-containing plating layer on the above-mentioned steel sheet according to the present invention.
  • This plating layer may be formed on one or both sides of the sheet surface of the steel sheet. Also, it may be formed only on a part of the surface.
  • the plating layer may be one that has been subjected to an alloying treatment.
  • the chemical composition of the plating layer is not limited as long as it contains Zn.
  • Examples of the plating layer containing Zn include Zn-0.2%Al (GI), Zn-(0.3 to 1.5)%Al, Zn-4.5%Al, Zn-0.09%Al-10%Fe (GA), Zn-1.5%Al-1.5%Mg, Zn-11%Al-3%Mg-0.2%Si, Zn-11%Ni, and Zn-15%Mg.
  • the chemical composition of the plating layer can be determined by dissolving the plating layer in an acid solution containing an inhibitor that suppresses corrosion of the steel sheet, and measuring the resulting solution using ICP (inductively coupled plasma) emission spectroscopy.
  • an acid solution containing an inhibitor for dissolving the plating layer for example, a 10% by mass hydrochloric acid solution containing 0.06% by mass inhibitor (Ibit 710K, manufactured by Asahi Chemical Industry Co., Ltd.) can be used.
  • the thickness of the plating layer may be, for example, 3 to 50 ⁇ m.
  • the coating weight of the plating layer is not particularly limited, but may be, for example, 10 to 170 g/ m2 per side.
  • the coating weight of the plating layer is determined by dissolving the plating layer in an acid solution containing an inhibitor that suppresses corrosion of the steel sheet, and measuring the change in weight before and after the plating layer is peeled off by pickling.
  • the roughness (surface roughness Ra) of the interface between the steel sheet and the plating layer corresponds to the roughness of the surface of the steel sheet described above, and the surface roughness Ra is 3.0 ⁇ m or less. Considering the adhesion of the plating, the surface roughness Ra is preferably 2.0 ⁇ m or less.
  • the roughness of the interface (surface roughness Ra) may be the surface roughness of the steel sheet measured after removing the plating layer. The plating layer is removed by pickling by dissolving the plating layer in an acid solution containing an inhibitor.
  • the steel sheet of the present invention can exhibit the effect of improving LME resistance even if it is not zinc-plated.
  • LME cracking does not occur unless there is a situation in which the steel sheets come into contact with molten zinc near the spot weld.
  • molten zinc is generated at the overlapping surface of the steel sheets during welding. As a result, the molten zinc may come into contact with the surface of the non-galvanized steel sheet, causing LME cracking.
  • the thickness of the steel sheet and plated steel sheet of the present invention is not particularly limited. For example, it can be 0.1 to 3.2 mm.
  • the thickness may be 0.2 mm or more, 0.4 mm or more, or 0.6 mm or more.
  • the thickness may be 3.0 mm or less, 2.5 mm or less, 2.0 mm or less, or 1.8 mm or less.
  • the steel sheet according to the present invention can be obtained by a manufacturing method including, for example, a casting process in which molten steel with adjusted chemical composition is cast to form a steel slab, a hot rolling process in which the steel slab is hot rolled to obtain a hot rolled steel sheet, a coiling process in which the hot rolled steel sheet is coiled, a cold rolling process in which the coiled hot rolled steel sheet is cold rolled to obtain a cold rolled steel sheet, a pretreatment process in which the cold rolled steel sheet is pretreated (shot blasted), and an annealing process in which the pretreated cold rolled steel sheet is annealed.
  • the hot rolled steel sheet may be pickled and cold rolled as it is without being coiled after the hot rolling process.
  • the conditions for the casting process are not particularly limited. For example, after melting in a blast furnace or an electric furnace, various secondary smelting processes may be carried out, and then casting may be carried out by a method such as ordinary continuous casting or casting by an ingot method.
  • the steel slab obtained by casting can be hot-rolled to obtain a hot-rolled steel sheet.
  • the hot rolling step is performed by hot-rolling the cast steel slab directly or after cooling it once and then reheating it.
  • the heating temperature of the steel slab may be, for example, 1100 to 1250°C.
  • rough rolling and finish rolling are usually performed.
  • the temperature and reduction rate of each rolling may be appropriately changed depending on the desired metal structure and plate thickness.
  • the finishing temperature of the finish rolling may be 900 to 1050°C, and the reduction rate of the finish rolling may be 10 to 50%.
  • the hot-rolled steel sheet can be coiled at a predetermined temperature.
  • the coiling temperature may be appropriately changed depending on the desired metal structure, etc., and may be, for example, 500 to 800°C.
  • the hot-rolled steel sheet may be subjected to a predetermined heat treatment by recoiling before or after coiling. Alternatively, the hot-rolled steel sheet may be pickled after the hot rolling process without performing the coiling process, and then subjected to the cold rolling process described later.
  • the hot-rolled steel sheet After pickling or the like is performed on the hot-rolled steel sheet, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet.
  • the rolling reduction in the cold rolling may be appropriately changed depending on the desired metal structure and sheet thickness, and may be, for example, 20 to 80%.
  • the steel sheet After the cold rolling process, the steel sheet may be cooled to room temperature, for example, by air cooling.
  • the pretreatment includes performing shot blasting treatment in which a spherical abrasive is projected onto the surface of the cold-rolled steel sheet.
  • the abrasive that can be used is not particularly limited, but for example, steel balls (shots) with a central particle size of 40 to 450 ⁇ m can be used. Examples of such steel balls (shots) include TSH30 manufactured by WINOA IKK JAPAN.
  • the amount of shot projection is preferably 5 to 400 kg/m 2. This allows strain to be introduced into the surface layer of the steel sheet without increasing the surface roughness Ra of the steel sheet.
  • decarburization is promoted in the annealing process described later, and a structure with stable ferrite can be efficiently formed in the surface layer of the steel sheet.
  • the projection amount per unit time and unit area at a projection amount of 400 kg/m 2 is 4.0 ⁇ 10 ⁇ 4 kg/(mm 2 ⁇ min).
  • the steel sheet to which strain is imparted by shot blasting is subjected to an annealing process including holding the steel sheet at a predetermined holding temperature and a high dew point.
  • the heating rate to the predetermined holding temperature is not particularly limited, and may be 1 to 10°C/sec.
  • the dew point is controlled by humidification control from 300°C or higher, preferably humidification control from 450 to 550°C. That is, the dew point (humidification) control start temperature is 300°C or higher and lower than 600°C, preferably within the range of 450 to 550°C.
  • the dew point without dew point (humidification) control is usually lower than -30°C.
  • the dew point (high dew point) during annealing is -30 to 20°C in order to promote decarburization.
  • the dew point (high dew point) during annealing is preferably -10°C or higher.
  • the dew point during annealing is preferably 5°C or lower.
  • the predetermined holding temperature (maximum heating temperature) in the annealing step is 750 to 900°C, preferably 770 to 870°C, in order to promote decarburization.
  • the holding time at the holding temperature (maximum heating temperature) in the annealing step is 20 to 300 seconds, preferably 50 to 200 seconds.
  • the atmosphere is preferably a non-oxidizing atmosphere, and can be, for example, N2 -1 to 10 vol% H2 or N2 -2 to 4 vol% H2 .
  • dew point, holding temperature, and holding time it is possible to promote decarburization, reduce the C concentration in the surface layer, and appropriately control the ferrite phase fraction. Furthermore, by setting the dew point (humidification) control start temperature in the above range, decarburization of the surface layer of the steel plate is promoted. At the same time, internal oxidation of Si and Mn progresses rapidly, and internal oxides are rapidly formed. The internal oxides formed function as nucleation sites, resulting in randomization of the orientation of the ferrite phase. If the dew point (humidification) control start temperature is too low, external oxidation progresses and internal oxidation of Si and Mn does not progress, making it difficult to randomize the orientation of the ferrite phase.
  • Annealing is performed under tension of 1 to 20 MPa. Applying tension during annealing makes it possible to introduce strain into the steel sheet more effectively, promoting decarburization of the surface layer.
  • the plated steel sheet according to the present invention can be obtained by carrying out a plating treatment step of forming a plating layer on the surface of the steel sheet produced as described above.
  • the plating process may be performed according to a method known to those skilled in the art.
  • the plating process may be performed, for example, by hot-dip plating or by electroplating.
  • the plating process is performed by hot-dip plating.
  • the plating process conditions may be appropriately set in consideration of the chemical components, thickness, and deposition amount of the desired plating layer.
  • a known alloying process may be performed to form alloy plating.
  • the steel sheet and plated steel sheet according to the present invention have high strength and high LME resistance, and therefore can be suitably used in a wide range of fields, such as automobiles, home appliances, and building materials. They can be particularly suitably used in the automobile field.
  • Steel sheets and plated steel sheets used for automobiles are often spot welded, in which case LME cracking is likely to occur. Therefore, when the steel sheet and plated steel sheet according to the present invention are used as automotive steel sheets for automobile components, the effect of the present invention, that is, high LME resistance, is suitably exerted.
  • the obtained steel slab was heated to 1200°C and hot-rolled with a finish rolling end temperature of 950°C and a finish rolling reduction of 30% to obtain a hot-rolled steel sheet.
  • the obtained hot-rolled steel sheet was coiled at a coiling temperature of 650°C, pickled, and then cold-rolled with a reduction of 50% to obtain a cold-rolled steel sheet.
  • the cold-rolled steel sheet had a thickness of 1.6 mm.
  • the surface of the obtained cold-rolled steel sheet was subjected to a shot blasting treatment in which a blasting material TSH30 manufactured by WINOA IKK JAPAN was blasted at a blasting amount of 5 kg/m 2.
  • the surface roughness Ra of the cold-rolled steel sheet after the shot blasting treatment was 2.9 ⁇ m.
  • the cold-rolled steel sheet that had been subjected to the shot blasting treatment was annealed in a furnace with an oxygen concentration of 20 ppm or less in an N 2 -4% H 2 gas atmosphere by increasing the temperature to 500°C at a rate of 6.0°C/sec, and then increasing the temperature to 800°C at a rate of 2.0°C/sec and holding for 40 seconds.
  • dew point control was started so that the dew point would be 0°C from 300°C.
  • the annealing treatment was performed with a tension of 5.0 MPa applied to the steel sheet.
  • the annealed steel sheet was immersed in a hot-dip galvanizing bath (Zn-0.14%Al) at 450°C for 3 seconds, and then pulled out at 100 mm/sec, and the coating weight was controlled to 50 g/ m2 with N2 wiping gas. Thereafter, an alloying treatment was performed at 520°C for 30 seconds to obtain a galvannealed steel sheet.
  • a hot-dip galvanizing bath Zn-0.14%Al
  • Grade AA 2.0 ⁇ m or less Grade A: More than 2.0 ⁇ m, 3.0 ⁇ m or less Grade B: More than 3.0 ⁇ m
  • the surface roughness Ra of the steel sheet surface was measured for unplated steel sheets, and for plated steel sheets, the surface roughness Ra of the steel sheet surface exposed by removing the plating was measured.
  • the plating layer was removed by dissolving the plating layer in a 10 mass% hydrochloric acid solution containing 0.06 mass% inhibitor (Ibit 710K, manufactured by Asahi Chemical Industry Co., Ltd.) that inhibits corrosion of the base steel sheet.
  • Step surface structure A sample of 30 mm x 30 mm was cut from the steel plate and subjected to GDS measurement, measurement of the thickness of the high ferrite layer, and oblique incidence XRD analysis by the above-mentioned method. The obtained results are shown in the columns of "C ⁇ 0.02% depth”, “High ferrite layer thickness”, and “Oblique incidence XRD (value of the middle side of the conditional formula)" in Table 4, respectively.
  • Grade AAA 1180 MPa or more Grade AA: 980 MPa or more, less than 1180 MPa Grade A: 780 MPa or more, less than 980 MPa
  • non-plated same type indicates that the same type of steel plate as the steel plate of the test number (Test No.) was used as the mating steel plate, but was not plated
  • GA same type indicates that the same type of steel plate as the steel plate of the test number (Test No.) was used as the mating steel plate, which was zinc alloyed.
  • GI270IF indicates that a commercially available hot-dip galvanized steel sheet having a tensile strength of 270 MPa was used as the mating steel sheet
  • GA590 indicates that a commercially available alloyed hot-dip galvanized steel sheet having a tensile strength of 590 MPa was used as the mating steel sheet.
  • the method of evaluating LME resistance will be described with reference to FIG. 2.
  • the LME resistance was evaluated by the length of the LME crack (crack 11 immediately outside the pressure weld) that occurred immediately outside the pressure welded portion 2 formed by overlapping two steel sheets 1 and spot welding.
  • the two steel sheets 1 are the steel sheets with each test number (Test No.) and their mating steel sheets.
  • the immediately outside the pressure welded portion of the welded portion refers to the outer part of the pressure welded portion 3, which is the part that is pressure welded by spot welding on the overlapping surface of the two steel sheets, and refers to the position near the pressure welded portion 3 (within a range of about 1 mm from the end of the pressure welded portion 3 to the outside).
  • the length of the crack 11 immediately outside the pressure welded portion was evaluated. Note that the spot welding test was performed three times, and the crack 11 immediately outside the pressure welded portion with the longest length was evaluated.
  • the evaluation criteria were as follows. In this example, if the grade was A or higher (i.e., grades A, AA, and AAA), it was determined that the LME resistance was excellent.
  • Grade AA More than 0 ⁇ m, less than 60 ⁇ m Grade A: 60 ⁇ m or more, less than 120 ⁇ m Grade B: 120 ⁇ m or more
  • Test Nos. 5-1 to 5-7 were performed under the same conditions as Test No. 5 shown in Tables 1, 3, and 4, except that the dew point in the annealing process shown in Table 5, the holding temperature was 860° C., and the mating material was the same type. The results are shown in Table 5.
  • Tests No. 3-1 to 3-7 were performed under the same conditions as Test No. 3 shown in Tables 1, 3, and 4, except that the dew point control start temperature and the holding temperature in the annealing process shown in Table 6 were 860° C. The results are shown in Table 6.
  • Steel plates Nos. 1 to 25, 37 to 55, 3-2 to 3-6, and 5-1 to 5-7 are examples of the present invention and had high LME resistance.
  • the dew point control start temperature in the annealing process was low, which is thought to have caused external oxidation to progress, preventing decarburization and resulting in a shallow depth at which the C concentration was 0.02% or less in the GDS measurement. It is also thought that internal oxidation of Si and Mn did not progress, preventing randomization of the orientation of the ferrite phase. As a result, the LME resistance was poor.
  • the present invention makes it possible to provide high-strength steel sheets and plated steel sheets with high LME resistance, and the steel sheets and plated steel sheets can be suitably used for automobiles, home appliances, building materials, etc., particularly for automobiles. Therefore, the present invention has extremely high industrial applicability.

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WO2019116531A1 (ja) 2017-12-15 2019-06-20 日本製鉄株式会社 鋼板、溶融亜鉛めっき鋼板および合金化溶融亜鉛めっき鋼板
WO2020218575A1 (ja) 2019-04-24 2020-10-29 日本製鉄株式会社 鋼板
JP2022514847A (ja) * 2018-12-19 2022-02-16 ポスコ 電気抵抗スポット溶接性に優れた高強度亜鉛めっき鋼板及びその製造方法
WO2022131671A1 (ko) * 2020-12-18 2022-06-23 주식회사 포스코 도금성이 우수한 고강도 용융아연도금강판 및 그 제조방법
WO2022149505A1 (ja) * 2021-01-08 2022-07-14 日本製鉄株式会社 溶接継手及び自動車部品
WO2024053663A1 (ja) * 2022-09-06 2024-03-14 日本製鉄株式会社 めっき鋼板
WO2024150822A1 (ja) * 2023-01-13 2024-07-18 日本製鉄株式会社 鋼板及びめっき鋼板

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WO2019116531A1 (ja) 2017-12-15 2019-06-20 日本製鉄株式会社 鋼板、溶融亜鉛めっき鋼板および合金化溶融亜鉛めっき鋼板
JP2022514847A (ja) * 2018-12-19 2022-02-16 ポスコ 電気抵抗スポット溶接性に優れた高強度亜鉛めっき鋼板及びその製造方法
WO2020218575A1 (ja) 2019-04-24 2020-10-29 日本製鉄株式会社 鋼板
WO2022131671A1 (ko) * 2020-12-18 2022-06-23 주식회사 포스코 도금성이 우수한 고강도 용융아연도금강판 및 그 제조방법
WO2022149505A1 (ja) * 2021-01-08 2022-07-14 日本製鉄株式会社 溶接継手及び自動車部品
WO2024053663A1 (ja) * 2022-09-06 2024-03-14 日本製鉄株式会社 めっき鋼板
WO2024150822A1 (ja) * 2023-01-13 2024-07-18 日本製鉄株式会社 鋼板及びめっき鋼板

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