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WO2024150822A1 - Tôle d'acier et tôle d'acier plaquée - Google Patents

Tôle d'acier et tôle d'acier plaquée Download PDF

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Publication number
WO2024150822A1
WO2024150822A1 PCT/JP2024/000655 JP2024000655W WO2024150822A1 WO 2024150822 A1 WO2024150822 A1 WO 2024150822A1 JP 2024000655 W JP2024000655 W JP 2024000655W WO 2024150822 A1 WO2024150822 A1 WO 2024150822A1
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steel sheet
content
steel
steel plate
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PCT/JP2024/000655
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English (en)
Japanese (ja)
Inventor
卓哉 光延
敬之 古川
隆志 大毛
浩史 竹林
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日本製鉄株式会社
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Publication of WO2024150822A1 publication Critical patent/WO2024150822A1/fr

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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
    • 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
    • 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
    • C22C18/00—Alloys based on zinc
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C18/00—Alloys based on zinc
    • C22C18/04—Alloys based on zinc with aluminium as the next major constituent

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.
  • liquid metal embrittlement (LME) cracking can cause problems with reduced weldability, as described in Patent Document 1, for example.
  • LME cracking is thought to occur when the surface layer of the steel sheet transforms into austenite during welding, molten zinc penetrates the grain boundaries, embrittling the steel sheet, and tensile stress is applied to the steel sheet during welding.
  • 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 strain to 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 low cementite fraction is formed, which makes 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:
  • FIG. 2 is a diagram showing a layer formed on the surface of the steel plate of the present invention.
  • FIG. 1 is a diagram illustrating an evaluation of LME resistance in an example.
  • the present invention will be described below.
  • the present invention is not limited to the following embodiments. First, an outline of the configuration for improving LME resistance in the steel plate of the present invention will be described.
  • the plating melts and the surface layer of the steel sheet is heated, transforming the steel sheet structure into austenite. During this process, the molten plating penetrates into the steel sheet structure along the austenite grain boundaries, embrittling the grain boundaries. For this reason, LME cracking is likely to occur at the grain boundaries when stress is applied to the steel sheet. It is believed that LME is particularly likely to occur because tensile stress is applied to the steel sheet during welding.
  • the steel sheet of the present invention improves LME resistance through the structure formed in the surface layer of the steel sheet.
  • the surface layer of the steel sheet refers to the range from the outermost surface of the steel sheet to a depth of 100 ⁇ m.
  • LME cracking is likely to occur when carbon is present in the surface layer of a steel sheet
  • keeping the carbon concentration in the surface layer of the steel sheet low is effective in preventing LME cracking.
  • the carbon concentration in the surface layer of the steel sheet is unlikely to become low.
  • the depth from the surface of the steel sheet where the carbon concentration measured by GDS is 0.01% or less is 3 ⁇ m or more. This means that the concentration of carbon, an element that is likely to cause LME, is low in the surface layer of the steel sheet.
  • the layer in which the area ratio of cementite is 10% or less has a thickness of 5 ⁇ m or more in the depth direction from the steel sheet surface.
  • the steel sheet of the present invention has improved LME resistance by controlling the form of C in the surface layer.
  • the inventors have discovered that in order to reduce the area ratio of cementite in the surface layer, in addition to imparting strong strain to the surface layer of the steel sheet and annealing it, it is important to control the dew point during annealing.
  • by imparting strong strain to the surface layer without increasing the surface roughness of the steel sheet it is possible to promote the diffusion of oxygen into the steel sheet and lower the C concentration in the surface layer of the steel sheet.
  • the inventors have also discovered that by setting the C concentration and cementite area ratio in the surface layer of the steel sheet as described above, it is possible to improve LME resistance, and have thus developed the present invention.
  • % in relation to the chemical composition means “mass %”.
  • a numerical range expressed using “ ⁇ ” means a range that includes the numerical values written before and after " ⁇ " as the lower and upper limits.
  • C (C: 0.08-0.40%) C (carbon) is an element that ensures the strength of steel.
  • the C concentration in the surface layer of the steel plate is set in consideration of the balance with weldability.
  • the C content is set to 0.08 to 0.40% so that the amount of C does not become too high. If the C content is too high, the C concentration in the surface layer and the cementite fraction will decrease even with high dew point annealing, as described below.
  • the C content may be 0.10% or more, 0.12% or more, or 0.15% or more.
  • the C content may be 0.35% or less, 0.30% or less. , or 0.25% or less.
  • Silicon (Si) is an element that promotes ferrite stabilization and decarburization. By including silicon, decarburization progresses in the surface layer and the ferrite in the surface layer is stabilized by the pretreatment and heat treatment described below. By increasing the Si content, the LME resistance is improved. To obtain this effect, the Si content is set to 0.4 to 2.0%. If the Si content is too high, the steel will become brittle after high dew point annealing. However, external oxidation progresses and oxides (scale) are formed on the surface layer of the steel sheet, and conversely, decarburization on the outermost surface is suppressed, and the effect of improving LME resistance is reduced. %, 0.6% or more, 0.7% or more, or 0.8% or more. The Si content may be 1.8% or less, 1.6% or less, 1.4% or less, or It may be 1.2% or less.
  • 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 .
  • the term "acid-soluble Al” refers to Al that is soluble in acid, and is determined by subtracting the amount of Al measured from the insoluble residue on the filter paper that is generated during the analysis of Al.
  • the role of Al can also be achieved by including Si, so 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 become brittle after high dew point annealing.
  • the content of sol. Al is The content of sol. Al may be 0.1% or more, 0.2% or more, or 0.3% or more. The Al content may be 1.5% or less, 1.2% or less, or 1.0% or less.
  • the total content of Si and sol. Al is less than 1.8%.
  • the total content of Si and sol. Al may be less than 1.7%, or less than 1.6%.
  • Mn manganese
  • Mn manganese
  • Mn is an effective element for improving the strength of steel by obtaining a hard structure. Taking into consideration the balance between the strength of steel and the decrease in workability due to Mn segregation, the Mn content is 0.1 to 5.0%. The Mn content may be 0.5% or more, 1.0% or more, or 1.5% or more. The Mn content is 4.5% It may be 4.0% or less, or 3.5% 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 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 set to 0.0100% or less, 0.0050% or less, or 0.0020% or less. It is preferable that no S is contained, and the lower limit of the content of S is 0%. From the viewpoint of desulfurization costs, the content of S may be more than 0%, 0.0001% or more, or 0.0005% or more.
  • 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%. Manufacturing cost From this viewpoint, the N content may be more than 0%, 0.0001% or more, 0.0005% or more, or 0.0010% or more.
  • B (boron) is an element that improves hardenability, contributes to improving strength, and 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. The content of B may be 0.0002% or more, or 0.0003% or more. On the other hand, from the viewpoint of ensuring sufficient toughness, the content of B is set to 0.0100% or less. The content may be 0.0090% or less, 0.0080% or less, 0.0060% or less, 0.0040% or less, 0.0030% or less, or 0.0020% or less.
  • Ti titanium
  • 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.
  • the Ti content is 0.0002% or more.
  • Ti is contained in an excessive amount, coarse TiN is generated, which may impair toughness, so the Ti content is set to 0.1500% or less.
  • the Ti content is 0.1350% or less, 0.1200% or less, 0.0900% or less, 0.0600% or less, 0.0450% or less, 0.0300% or less, 0.0150% or less, 0. It may be 0.0050% or less, 0.0030% 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 0.001% or more. The Nb content may be 0.006% or more, or 0.007% or more. On the other hand, from the viewpoint of ensuring sufficient toughness, the Nb content is set to 0.150% or less. It may be 135% or less, 0.120% or less, 0.095% or less, 0.065% or less, 0.050% or less, 0.030% or less, or 0.020% or less.
  • 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. It may be 120% or less, 0.095% or less, 0.065% or less, 0.050% or less, 0.045% or less, 0.025% or less, or 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 0%. 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 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% or less. The Cr content is set to 1.80% or less, 1.60% or less, 1.25% or less, 0.85% or less, 0. It may be 65% or less, 0.50% or less, 0.30% 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 0.03% or more, 0.04% or more, or 0.05% or more.
  • the Ni content is set to 2.00% or less.
  • the Ni content is 1.80% or less, 1.60% or less, 1.25% or less, 0.85% or less, 0.65% or less, 0.40% or less, 0.25% or less, or 0 .15% 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 0%. Although this effect can be obtained even with a small amount of Cu, when contained, the Cu content is preferably 0.0001% or more. The Cu content is preferably 0.0002% or more, or On the other hand, from the viewpoint of suppressing deterioration in toughness and cracking of the slab after casting and deterioration in weldability, the Cu content is set to 2.0000% or less.
  • the Cu content is 1.8000% or less, 1.6000% or less, 1.2000% or less, 0.8000% or less, 0.6000% or less, 0.4000% or less, 0.2000% or less, 0. It may be 1000% or less, 0.0070% or less, 0.0050% or less, 0.0035% or less, 0.0020% or less, or 0.0015% or less.
  • Mo mobdenum
  • Mo mobdenum
  • Mo mobdenum
  • the Mo content is set to 1.00% or less.
  • the amount may be 0.90% or less, 0.80% or less, 0.65% or less, 0.45% or less, 0.35% or less, 0.30% 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 0%. 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. The W content is 0.002% or more, or The W content may be 0.003% 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. The W content is set to 0.900% or less, 0.800% or less. %, 0.600% or less, 0.400% or less, 0.300% or less, 0.200% or less, 0.100% or less, 0.050% or less, 0.025% or less, 0.015% or less , or 0.010% 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 content, but when Ca is contained, the Ca content is preferably 0.0001% or more. The Ca content may be 0.0002% or more, or 0.0003% or more. On the other hand, if the Ca content is excessive, deterioration of the surface properties may become apparent, so the Ca content is set to 0.1000% or less.
  • the Ca content is 0.0900% or less, 0.0800% or less, 0.0600% or less, 0.0400% or less, 0.0300% or less, 0.0200% or less, 0.0100% or less, 0. It may be 0.0050% or less, 0.0025% or less, 0.0015% 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 0.0002% or more, 0.0003% or more, or 0.0005% or more. On the other hand, if the Mg content is excessive, deterioration of the surface properties may become apparent.
  • the Mg content is 0.090% or less, 0.080% or less, 0.060% or less, 0.040% or less, 0.030% or less, 0.020% or less, 0.010% or less. % or less, 0.005% or less, 0.003% or less, or 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. On the other hand, if the content is excessive, deterioration of the surface properties may become evident.
  • the Zr content is 0.090% or less, 0.085% or less, 0.065% or less, 0.050% or less, 0.040% or less, or 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 0.0002% or more, 0.0003% or more, or 0.0005% or more. On the other hand, if the Hf content is excessive, deterioration of the surface properties may become apparent. % or less.
  • the Hf content is 0.090% or less, 0.080% or less, 0.060% or less, 0.040% or less, 0.030% or less, 0.020% or less, 0.010% or less, 0. It may be 0.005% or less, 0.003% or less, or 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, 0.0004% or more, or 0.0005% or more.
  • the percentage shall be 1000% or less.
  • the REM content is 0.0900% or less, 0.0800% or less, 0.0600% or less, 0.0400% or less, 0.0300% or less, 0.0200% or less, 0.0100% or less, 0.
  • the content of REM may be 0.0040% or less, 0.0025% or less, or 0.0015% or less.
  • REM is an abbreviation for Rare Earth Metal and refers to an element belonging to the lanthanide series. REM is usually added as misch metal. will be done.
  • the remainder other than the above chemical components consists 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 the steel plate is industrially manufactured, 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.
  • 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). However, C and S may be measured using the combustion-infrared absorption method, and N may be measured using the inert gas fusion-thermal conductivity method. These analyses may be performed on samples taken from the steel plate using methods in accordance with JIS G0417:1999.
  • ICP-MS inductively coupled plasma mass spectrometry
  • the depth at which the C concentration measured by GDS (glow discharge spectroscopy) is 0.01% or less in the depth direction from the surface of the steel sheet is 3 ⁇ m or more.
  • Such a surface structure can be obtained by setting the chemical composition of the steel sheet as described above and carrying out the pretreatment and heat treatment described below.
  • the effect of improving LME resistance can be obtained if the depth at which the C concentration is 0.01% or less is 3 ⁇ m or more, so there is no particular upper limit to the depth.
  • the depth at which the C concentration is 0.01% or less may be, for example, 50 ⁇ m or less, 40 ⁇ m or less, or 30 ⁇ m or less.
  • the depth at which the C concentration is 0.01% or less may be, for example, 5 ⁇ m or more, 7 ⁇ m or more, 10 ⁇ m or more, 15 ⁇ m or more, or 20 ⁇ m or more.
  • GDS measurements are performed five times in the sheet thickness direction, and the average of these measurements is taken as the C concentration. Measurement conditions are as follows. The starting point of "depth” is the surface of the steel sheet for unplated steel sheet, and the interface between the steel sheet and the plating layer for plated steel sheet. The interface between the steel sheet and the plating layer is taken as the position where the Fe concentration measured by GDS measurement is 93% of the Fe concentration at a depth of 150 ⁇ m.
  • FIG. 1 shows an example of a structural photograph taken by SEM at a magnification of 1000 times near the surface layer of the steel plate of the present invention.
  • FIG. 1 is a cross section parallel to the thickness direction of the steel plate, and the upper side of the drawing is the steel plate surface.
  • a low cementite layer 11 which is a layer having a low C concentration, mainly composed of ferrite, and having an area ratio of cementite of 10% or less.
  • the steel plate part in FIG. 1 is classified into a part having a relatively high brightness (bright) and a part having a relatively low brightness (dark), the part having a relatively high brightness can be judged as cementite.
  • the part having a relatively low brightness can be judged as ferrite.
  • the effect of improving LME resistance can be obtained if the low cementite layer is 5 ⁇ m or more in thickness, so there is no particular upper limit to its thickness.
  • the thickness of the low cementite layer may be, for example, 50 ⁇ m or less, 40 ⁇ m or less, or 30 ⁇ m or less.
  • the thickness of the low cementite layer may be 10 ⁇ m or more, or 20 ⁇ m or more.
  • the structure other than the cementite in the low cementite layer is not limited.
  • it can be one or more of martensite, bainite, and ferrite. Since ferrite has low LME susceptibility, a structure mainly composed of ferrite is preferable from the viewpoint of improving LME resistance.
  • the thickness of the low cementite layer is determined by nital etching the C-section of the steel plate (plate thickness section parallel to the rolling direction (L-direction)) and observing a 50 ⁇ m x 50 ⁇ m field of view including the surface layer of the steel plate at 1000x magnification using an SEM. From the structural morphology on the SEM image obtained by SEM observation, it is possible to distinguish between hard structures such as martensite and bainite, which contain relatively large amounts of cementite, and ferrite. The thickness of the low cementite layer is measured in a measurement range of 500 ⁇ m in the L-direction, and the measurement range is divided into five ranges spaced 1000 ⁇ m apart in the L-direction.
  • the average thickness of the low cementite layer in the plate thickness direction for the five measurement ranges is taken as the average value.
  • the area ratio of cementite refers to the area ratio determined by observation on the C-section.
  • the steel sheet of the present invention may have a plating layer as described below.
  • a plating layer When a plating layer is present, the depth at which the C concentration is 0.01% or less as measured by GDS and the thickness of the layer at which the cementite area ratio is 10% or less start from the interface between the steel sheet and the plating layer.
  • the steel sheet of the present invention has a surface roughness of 3.0 ⁇ m or less in terms of arithmetic mean height Ra defined by JIS B0601:2013.
  • Ra arithmetic mean height defined by JIS B0601:2013.
  • the present invention suppresses LME occurring in high-strength steel plates.
  • the high-strength steel plate is a steel plate having 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 may be measured in accordance with JIS Z 2241:2011 by taking a JIS No. 5 tensile test piece whose longitudinal direction is perpendicular to the rolling direction and the plate thickness direction.
  • the tensile strength may be 880 MPa or more, 980 MPa or more, 1080 MPa or more, or 1180 MPa or more.
  • the tensile strength may be 1900 MPa or less, or 1800 MPa or less.
  • the following method is adopted, for example, to identify the rolling direction of the steel plate.
  • the S concentration is measured using an electron probe microanalyzer (EPMA).
  • the measurement conditions are an acceleration voltage of 15 kV and a measurement pitch of 1 ⁇ m, and a distribution image is measured in a 500 ⁇ m square range in the center of the plate thickness.
  • the extended area with a high S concentration is determined to be an inclusion such as MnS.
  • observation may be performed in multiple fields of view.
  • a surface parallel to the surface rotated in 5° increments in the range of 0° to 180° around the plate thickness direction is observed by the above method.
  • the average value of the long axis length of the multiple inclusions in each obtained cross section is calculated for each cross section, and the cross section with the largest average long axis length of the inclusions is identified.
  • the direction parallel to the longitudinal axis of the inclusions in the cross section is determined to be the rolling direction.
  • the hardness (Vickers hardness) of the steel plate in the non-heat-affected zone which is 5 mm or more away from the outer edge of the nugget of the spot weld, can be measured as an alternative, and the tensile strength value can be estimated from the following correlation formula (Correlation between static strength parameters, Hasegawa Norihiko, Arai Junichi, Tanaka Michishichi, "Zairyo" Vol. 39 No. 442, pp. 859-863).
  • “heat-affected zone” refers to the part of the steel plate that has not melted and whose structure, metallurgical properties, mechanical properties, etc.
  • non-heat-affected zone refers to the part other than the heat-affected zone.
  • the part that is 5 mm or more away from the outer edge of the nugget of the spot weld can be determined to be the non-heat-affected zone.
  • Hv 0.301 ⁇ TS+5.701 (Here, Hv is Vickers hardness and TS is tensile strength (unit: MPa).)
  • the tensile strength can be considered to be 780 MPa or more.
  • 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.
  • the plating layer is formed on at least a part of the surface of the steel sheet, and may be formed on one side or both sides of the steel sheet.
  • 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.
  • the plating layer containing Zn may be, for example, Zn-0.2%Al (GI), Zn-(0.3-1.5)%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, or 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.
  • the acid solution containing the inhibitor may be, for example, a 10% by mass hydrochloric acid solution containing 0.06% by mass inhibitor (Ibit, manufactured by Asahi Chemical Industry Co., Ltd.).
  • 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/m 2 per side.
  • the coating weight of the plating layer is determined by dissolving the plating layer in an acid solution to which an inhibitor that suppresses corrosion of the steel sheet is added, and measuring the weight change before and after the plating layer is peeled off by pickling.
  • the thickness of the plating layer may be 5 ⁇ m or more, 10 ⁇ m or more, 15 ⁇ m or more, or 20 ⁇ m or more.
  • the thickness of the plating layer may be 40 ⁇ m or less, or 30 ⁇ m or less.
  • the coating weight of the plating layer may be 20 g/m 2 or more, 30 g/m 2 or more, 40 g/m 2 or more, or 50 g/m 2 or more per side.
  • the coating weight of the plating layer may be 150 g/m 2 or less, 130 g/m 2 or less, 120 g/m 2 or less, or 100 g/m 2 or less per side.
  • the roughness of the interface between the steel sheet and the plating layer is the surface roughness of the steel sheet described above, and is therefore 3.0 ⁇ m or less in terms of arithmetic mean roughness Ra. Considering the adhesion of the plating, Ra may be 2.5 ⁇ m or less, or 2.0 ⁇ m or less.
  • the roughness of the interface between the steel sheet and the plating layer may be the surface roughness of the steel sheet measured after dissolving and removing the plating.
  • 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 the spot welded portion comes into contact with molten zinc.
  • molten zinc occurs at the overlapping surface of the steel sheets during welding. For this reason, the molten zinc may come into contact with the surface of the non-plated steel sheet, causing LME cracking.
  • the zinc plating attached to the welding electrode may melt and come into contact with the surface of the other steel sheet, causing LME cracking. If the steel sheet of the present invention is used as a non-plated steel sheet, LME cracking can be suppressed even when molten zinc may come into contact with the surface of the other steel sheet as described above, because the surface layer has a low C concentration and the surface layer is a low cementite layer.
  • LME cracking in the outermost layer of a welded joint can be suppressed by using the steel plate of the present invention as the outermost steel plate.
  • LME cracking in the outermost layer of a welded joint can occur when the steel plate of the outermost layer is a high-strength steel plate with a high C concentration and has zinc plating on the surface side, or when molten zinc plating is attached to the welding electrode.
  • Examples of LME cracking in the outermost layer of a welded joint include cracking in the indentation part caused by the welding electrode (cracks directly below the welding electrode) and cracking in the inclined part (shoulder part) formed on the periphery of the indentation part (cracks in the weld shoulder part), and these can be suppressed by using the steel plate of the present invention as the outermost steel plate.
  • LME cracking can be suppressed by using the steel plate of the present invention as the high-strength steel plate.
  • Examples of LME cracking at such overlapping surfaces include cracks in the vicinity of the outer side of the portion where the steel plates are pressed together by spot welding (cracks directly outside the pressure-welded portion), and the like, which can be suppressed by using the steel plate of the present invention as the above-mentioned high-strength steel plate.
  • the thickness of the steel sheet and plated steel sheet of the present invention is not particularly limited. For example, it can be 0.6 to 3.2 mm.
  • the thickness may be 0.8 mm or more, or 1.0 mm or more.
  • the thickness may be 3.0 mm or less, 2.6 mm or less, 2.4 mm or less, 2.2 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 shot blasted, and an annealing process in which the pretreated cold rolled steel sheet is annealed.
  • the hot rolled steel sheet may not be coiled after the hot rolling process, but may be pickled and then cold rolled as is.
  • 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 reheating the cast steel slab directly or after cooling once, and then hot rolling 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 coiling, and then cold-rolled as described below.
  • the hot-rolled steel sheet After the hot-rolled steel sheet is subjected to pickling or the like, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet.
  • the rolling reduction of 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 shot blasting the surface of the cold-rolled steel sheet using a spherical abrasive.
  • the abrasive that can be used is not particularly limited, but for example, a steel ball having a central particle size of 40 to 450 ⁇ m, preferably 120 to 420 ⁇ m, more preferably 180 to 350 ⁇ m can be used.
  • TSH30 from WINOA IKK JAPAN can be mentioned.
  • the shot blasting amount is preferably 5 to 400 kg/m 2. This allows strain to be introduced without increasing the surface roughness. By performing such shot blasting, decarburization is promoted in the annealing process described later, and a structure with reduced cementite can be efficiently formed in the surface layer of the steel sheet.
  • the unit time/unit area at the level of a shot amount of 400 kg/m 2 is 4.0 ⁇ 10 ⁇ 4 kg/(mm 2 ⁇ min).
  • the surface roughness of the steel sheet after pretreatment is preferably 3.0 ⁇ m or less in arithmetic mean roughness Ra.
  • the roughness may be 2.5 ⁇ m or less, or 2.0 ⁇ m or less in terms of Ra.
  • the surface roughness of the steel sheet after pretreatment is maintained in the steel sheet and plated steel sheet according to the present invention after the annealing step and the plating step (including the alloying step) described below.
  • the cold-rolled steel sheet is annealed.
  • the 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, accelerating decarburization of the surface layer.
  • the holding temperature in the annealing process is 750 to 900°C.
  • the holding temperature may be 770 to 870°C. By keeping the temperature in this range, it is possible to promote decarburization, lower the C concentration in the surface layer, and reduce cementite.
  • the holding time at the holding temperature in the annealing process is 40 to 300 seconds.
  • the holding time may be 50 to 250 seconds.
  • the atmosphere in the annealing step has a dew point of -30 to 20°C.
  • the dew point may be -10 to 5°C.
  • the atmosphere may be, for example, N 2 -1 to 10 vol% H 2 or N 2 -2 to 4 vol% H 2.
  • decarburization can be promoted, the C concentration in the surface layer can be reduced, and cementite can be reduced.
  • the dew point is too high or too low, a phase containing oxides of Si, Mn, Al, etc. is formed outside the steel sheet, and decarburization cannot be promoted.
  • interdiffusion between the plating components and the steel components is inhibited, and plating properties may become insufficient.
  • the manufacturing method including the above-mentioned steps promotes decarburization in the surface layer of the steel plate, resulting in a steel plate with reduced cementite.
  • the plated steel sheet according to the present invention can be obtained by a production method including a plating process step of forming a plating layer on the steel sheet produced as described above.
  • the plating process may be performed according to a hot-dip plating method known to those skilled in the art.
  • the conditions of the plating process may be appropriately set in consideration of the chemical composition, thickness, and coating weight of the desired plating layer.
  • the steel sheet may be immersed in a hot-dip galvanizing bath at 420 to 480°C with adjusted chemical composition for 1 to 10 seconds, and after immersion, the steel sheet may be pulled out at 20 to 200 mm/sec, and the coating weight may be controlled by N2 wiping gas.
  • a known alloying process may be performed to obtain alloyed plating.
  • the alloying process may be performed at 500 to 550°C for 10 to 60 seconds, for example.
  • 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. It is particularly preferable for them to be used in the automobile field. Steel sheets and plated steel sheets used for automobiles are often spot welded, in which case LME cracking can be a significant problem. Therefore, when the steel sheet and plated steel sheet according to the present invention are used as automotive steel sheets, the effect of the present invention of having high LME resistance is suitably exerted.
  • Example 1 Molten steel adjusted to the chemical composition shown in No. 1 of Table 1 was melted in a blast furnace and cast by continuous casting to obtain a steel slab. 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 cold rolled steel sheet was subjected to a shot blasting treatment using TSH30 manufactured by WINOA IKK JAPAN as a blasting material at a blast rate of 5 kg/m 2 .
  • the surface roughness of the steel sheet was measured in accordance with JIS B 0601:2013. That is, 10 locations were randomly selected on the surface of the outer layer side, and the surface profile at each location was measured using a contact surface roughness meter. The arithmetic mean roughness Ra obtained by averaging the surface roughness at these locations was evaluated as follows:
  • 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
  • annealing was performed in a furnace with an oxygen concentration of 20 ppm or less, in an N2-4 % H2 gas atmosphere, with a dew point of 0°C, a holding temperature of 800°C, and a holding time of 40 seconds, to prepare steel sheet samples.
  • the heating rate during annealing was 6.0°C/sec up to 500°C, and 2.0°C/sec from 500°C to the holding temperature.
  • the annealing was performed under tension of 5.0 MPa.
  • a plating treatment was carried out to obtain a plated steel sheet.
  • the plating treatment consisted of immersion in a 450°C hot-dip galvanizing bath (Zn-0.14%Al) for 3 seconds. After immersion, the steel sheet was pulled out at 100 mm/sec, and the coating weight was controlled to 50 g/ m2 using N2 wiping gas. Following the plating treatment, an alloying treatment was carried out at 520°C for 30 seconds.
  • Examples 2 to 24 and Comparative Examples 25 to 37 Welded joints were manufactured in the same manner as in Example 1, except that the chemical components were those shown in Table 1, the conditions of the pretreatment process and the annealing process were those shown in Table 2, and the plating type was those shown in Table 3, and the LME resistance during manufacturing was evaluated. In addition, shot blasting was omitted in No. 32. In No. 33, a steel sheet with a large surface roughness due to skin pass was used. In No. 37, surface treatment was performed by grinding with a brush instead of shot blasting.
  • the resulting steel sheets were evaluated for surface structure, roughness of the steel sheet surface or steel sheet/plating interface, tensile strength, and LME resistance.
  • samples were cut into 25 mm x 15 mm plates, and nital etched.
  • the C-section (plate thickness section parallel to the rolling direction (L direction)) of each sample was observed with an SEM to measure the thickness of the low cementite layer, which is shown in "Cementite 10% or less thickness” in Table 3.
  • the starting point of "depth” is the surface of the steel sheet for unplated steel sheets, and the interface between the steel sheet and the plating layer for plated steel sheets.
  • the interface between the steel sheet and the plating layer was determined to be the position where the Fe concentration measured by GDS measurement was 93% of the Fe concentration at a depth of 150 ⁇ m.
  • 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 Grade B: Less than 780 MPa
  • LME resistance was evaluated based on the length of LME cracks (shoulder and off-shoulder cracks 23) that occurred at the shoulder and outside the shoulder of a weld 22 formed by overlapping two steel plates 21 and spot welding them together.
  • the shoulder refers to the sloping part of the edge of the depression created by spot welding, and the off-shoulder refers to the outside of the shoulder.
  • the evaluation was made as follows. In this example, if the evaluation was A or higher, it was determined that the LME resistance was excellent and the problem of the present invention was solved.
  • 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
  • No. 25 is a comparative example in which the steel plate has a high C content. It is believed that because the steel plate has a high C content, the C concentration in the surface layer of the steel plate did not decrease even after high dew point annealing. As a result, the depth where the C concentration is 0.01% or less and the thickness of the layer where the cementite area ratio is 10% or less became smaller. This resulted in poor LME resistance.
  • No. 26 is a comparative example in which the steel plate has a low Si content. It is believed that because the steel plate had a low Si content, decarburization did not progress in the surface layer even when annealed at a high dew point. As a result, the depth where the C concentration was 0.01% or less and the thickness of the layer where the cementite area ratio was 10% or less were small. This resulted in poor LME resistance.
  • No. 27 is a comparative example in which the steel plate has a high Si content. Because the steel plate had a high Si content, even when high dew point annealing was performed, external oxidation progressed, and oxides (scale) formed on the surface layer of the steel plate, suppressing decarburization at the outermost surface. As a result, the depth where the C concentration was 0.01% or less and the thickness of the layer where the cementite area ratio was 10% or less became smaller. As a result, the LME resistance was inferior.
  • No. 28 is a comparative example in which the sol. Al content of the steel plate is high. Because the sol. Al content of the steel plate was high, external oxidation proceeded, and oxides (scale) formed on the surface layer of the steel plate, even when high dew point annealing was performed, which is thought to have suppressed decarburization on the outermost surface. As a result, the thickness of the layer with a cementite area ratio of 10% or less became smaller. As a result, the LME resistance was inferior.
  • No. 29 was annealed at a low holding temperature, which is thought to be why decarburization was not sufficiently promoted during annealing. As a result, the thickness of the layer with a cementite area ratio of 10% or less was small. As a result, the LME resistance was poor.
  • No. 30 was held at a high temperature during annealing, which is thought to have prevented sufficient decarburization during annealing. As a result, the depth at which the C concentration was 0.01% or less was small. This resulted in poor LME resistance.
  • No. 33 used a steel sheet with a large surface roughness, which is thought to have led to a large roughness at the steel sheet/plating interface after annealing, making it easier for stress concentration to occur. As a result, the LME resistance was poor.
  • No. 35 had a low dew point during annealing, which is thought to have caused a phase containing oxides of Si, Mn, Al, etc. to form on the outside of the steel sheet, preventing decarburization.
  • the depth where the C concentration was 0.01% or less and the thickness of the layer where the cementite area ratio was 10% or less were small. This resulted in poor LME resistance.
  • No. 36 had a high dew point during annealing, which is thought to have led to the formation of a phase containing oxides of Si, Mn, Al, etc. on the outside of the steel sheet, preventing decarburization.
  • the depth where the C concentration was 0.01% or less and the thickness of the layer where the cementite area ratio was 10% or less were small. This resulted in poor LME resistance.
  • Nos. 1 to 24 are examples of the present invention and had high LME resistance. It was confirmed that examples with a depth where the C concentration was 0.01% or less and a large layer thickness where the cementite area ratio was 10% or less had particularly excellent LME resistance.
  • 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, and the like, particularly for automobiles. Therefore, the present invention has extremely high industrial applicability.

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Abstract

La présente invention aborde le problème de la fourniture d'une tôle d'acier et d'une tôle d'acier plaquée ayant chacune une résistance élevée à la fragilisation par métal liquide. La tôle d'acier et la tôle d'acier plaquée selon la présente invention sont chacune caractérisées en ce qu'elles ont des composants chimiques spécifiés, et sont également caractérisées en ce que la profondeur à laquelle la concentration en C mesurée par GDS est de 0,01 % ou moins est de 3 µm ou plus lorsqu'elle est observée dans la direction de la profondeur à partir de la surface de la tôle d'acier, l'épaisseur d'une couche dans laquelle le rapport de surface de la cémentite est de 10 % ou moins est de 5 µm ou plus lorsqu'elle est observée dans la direction de la profondeur à partir de la surface de la tôle d'acier, et la tôle d'acier a une rugosité de surface telle que la rugosité moyenne arithmétique Ra est de 3,0 µm ou moins.
PCT/JP2024/000655 2023-01-13 2024-01-12 Tôle d'acier et tôle d'acier plaquée WO2024150822A1 (fr)

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Cited By (1)

* Cited by examiner, † Cited by third party
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WO2025018302A1 (fr) * 2023-07-18 2025-01-23 日本製鉄株式会社 Feuille d'acier, feuille d'acier plaquée et élément d'automobile

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WO2021200579A1 (fr) * 2020-03-31 2021-10-07 Ｊｆｅスチール株式会社 Tôle d'acier, élément, et leur procédé de fabrication
WO2022210396A1 (fr) * 2021-03-31 2022-10-06 日本製鉄株式会社 Plaque d'acier, procédé de production de plaque d'acier, et procédé de production de plaque d'acier intermédiaire
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JP2002003948A (ja) * 2000-06-27 2002-01-09 Sony Corp 耐高温クリープ性に優れたテンションマスク用鋼板の製造方法
JP2010043323A (ja) * 2008-08-12 2010-02-25 Sumitomo Metal Ind Ltd 熱間プレス用熱延鋼板およびその製造方法ならびに熱間プレス鋼板部材の製造方法
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WO2022210396A1 (fr) * 2021-03-31 2022-10-06 日本製鉄株式会社 Plaque d'acier, procédé de production de plaque d'acier, et procédé de production de plaque d'acier intermédiaire
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