Hot-dip galvanized steel sheet

Abstract

This hot-dip galvanized steel sheet comprises: a steel sheet having a prescribed chemical composition; and a plating layer that is present on the surface of the steel sheet and whose Zn content is 90.00 mass% or greater. Wt, which is the plating adherence amount per one surface of the plating layer, is 30.0-120.0 g / m2. Wf, which is the Fe adherence amount per one surface in the plating layer, is 0.40-3.00 g / m2. Ia, which is an Al barrier index represented using Wa, which is the Al adherence amount per one surface in the plating layer, the Wt, and the Wf, is 150 mg / m2 or greater. When the hot-dip galvanized steel sheet is analyzed using glow discharge optical emission spectroscopy in the sheet thickness direction of the steel sheet from the surface of the plating layer, at least one of the relationships (tB – tAl) ≥ 10.0 and tB / tAl ≥ 1.50 is satisfied, where tB is the time in seconds from the start of the analysis until the detection of the maximum value of the emission intensity of B, and tAl is the time in seconds from the start of the analysis until the detection of the maximum value of the emission intensity of Al.

Description

Hot-dip galvanized steel sheet

[0001] This invention relates to a hot-dip galvanized steel sheet. This application claims priority based on Japanese Patent Application No. 2024-225948, filed in Japan on December 23, 2024, the contents of which are incorporated herein by reference.

[0002] Hot-dip galvanized steel sheets are steel sheets with a zinc plating layer on their surface, obtained by immersing a steel sheet (plating base sheet) in a hot-dip zinc plating bath. Because hot-dip galvanized steel sheets have excellent corrosion resistance, they are used in a wide range of applications such as home appliances, building materials, and automobiles. When used in these applications, hot-dip galvanized steel sheets are processed into shapes suitable for each application. Therefore, it is important that the plating does not peel off during processing, and hot-dip galvanized steel sheets require high plating adhesion (adhesion between the plating layer and the steel sheet). On the other hand, it is known that when high-strength steel sheets (high-tensile steel) are used as the plating base sheet, the plating adhesion tends to decrease. This is said to be because elements such as Si (silicon) and Mn (manganese), which are added to improve the mechanical properties of high-strength steel sheets, are easily oxidized. During the annealing process, these elements are selectively oxidized on the surface of the steel sheet, and oxides of Si, Mn, etc., are present on the outermost layer of the steel sheet, which reduces the wettability (plating wettability) between the steel sheet and the hot zinc. Furthermore, boron (B, sometimes referred to as boron below) is an element that greatly contributes to increasing the strength of steel sheets because even a small amount can improve their hardenability. However, when steel sheets containing boron are used as the base material for plating, there is a tendency for the adhesion of the plating to decrease. This problem can be avoided by not including Si, but since steel sheets used in automobiles and other applications require high strength, it is not easy to avoid including Si, Mn, and B. To address these challenges, technologies have been developed to improve the adhesion of plating on hot-dip galvanized steel sheets containing Si (see Patent Document 1).

[0003] Japanese Patent Application Publication No. 2001-323355

[0004] Patent Document 1 does not disclose the plating adhesion when the steel sheet contains boron. Therefore, the problem that the present invention aims to solve is to provide a hot-dip galvanized steel sheet that exhibits excellent plating adhesion even when the steel sheet contains boron.

[0005] The gist of the present invention for solving the above problems is as follows: [1] A hot-dip galvanized steel sheet according to one aspect of the present invention comprises a steel sheet and a plating layer present on the surface of the steel sheet, wherein the chemical composition of the steel sheet is, in mass%, C: 0.050 to 0.500%, Si: 0.010 to 2.000%, Mn: 0.10 to 2.50%, P: 0.030% or less, S: 0.010% or less, Al: 0 to 1.00%, N: 0 to 0.0100%, B: 0.0005 to 0.0050%, Ti: 0 to 0.20%, Cr: 0 to 1.50%, Mo: 0 to 1.00% The composition of the plating layer is as follows: Nb: 0-0.10%, Ni: 0-1.00%, V: 0-0.20%, Cu: 0-1.00%, W: 0-0.100%, REM: 0-0.100%, Ca: 0-0.100%, Sb: 0-0.050%, Sn: 0-0.050%, As: 0-0.050%, and the remainder is Fe and impurities, the Zn content of the plating layer is 90.00% or more by mass, and the amount of plating deposited on one side of the plating layer, Wt, is 30.0-120.0 g / m². ２ The amount of Fe deposited on one side of the plating layer, Wf, is 0.40 to 3.00 g / m². ２ The Al barrier index Ia, expressed by the following formula (1) using Wa, Wt, and Wf, which are the amounts of Al deposited on one side of the plating layer, is 150 mg / m². ２In addition, when an analysis is performed by glow discharge emission spectroscopy from the surface of the plating layer in the thickness direction of the steel sheet, the time in units of seconds from the start of the analysis until the maximum value of the emission intensity of B is detected, tB, and the time in units of seconds from the start of the analysis until the maximum value of the emission intensity of Al is detected, satisfy at least one of the following equations (2) and (3). Ia = Wa - (Wt - Wf) × 0.002 (1) tB - tAl ≥ 10.0 (2) tB / tAl ≥ 1.50 (3) [2] In the hot-dip galvanized steel sheet described in [1] above, equation (2) may also be satisfied. [3] In the hot-dip galvanized steel sheet described in [1] or [2] above, equation (3) may also be satisfied. [4] In the hot-dip galvanized steel sheet described in any of [1] to [3] above, when the time in units of seconds from the start of the glow discharge emission spectroscopy analysis until the maximum value of the emission intensity of Mn is detected is denoted as tMn, tB and tMn may satisfy the following equation (4): tB ≥ tMn (4) [5] In the hot-dip galvanized steel sheet described in any of [1] to [4] above, the chemical composition of the plating layer is, in mass%, Al: 0.10 to 1.00%, Fe: 0.33 to 5.00%, Mg: 0 to 0.500%, Si: 0 to 0.500%, Ni: 0 to 0.500%, Ca: 0 to 2.000%, Sb: 0 to 0.500%, Pb: 0 to 0.500%, Cu: 0 to 0.500%, Sn: 0 to 0.500%, Ti: 0 to 0.500%, Cr: 0 to 0.500%, Nb: 0 to 0.500%, Zr: 0 to 0.500%, Mn The composition is as follows: 0-0.500%, Mo: 0-0.500%, Ag: 0-0.500%, Li: 0-0.500%, La: 0-0.500%, Ce: 0-0.500%, B: 0-0.004%, Y: 0-0.500%, P: 0-0.500%, Sr: 0-0.500%, and the remainder is Zn and impurities. The total content of Mg, Si, Ni, Ca, Sb, Pb, Cu, Sn, Ti, Cr, Nb, Zr, Mn, Mo, Ag, Li, La, Ce, B, Y, P, and Sr may be 5.000% or less by mass.[6] In the hot-dip galvanized steel sheet described in any of [1] to [5] above, the plating layer comprises a main plating layer and an Al barrier layer formed in at least a portion between the main plating layer and the steel sheet, wherein the Al content is 1.50% by mass or more, and the Al barrier layer may be formed continuously. [7] In the hot-dip galvanized steel sheet described in any of [1] to [5] above, the plating layer comprises a main plating layer and an Al barrier layer formed in at least a portion between the main plating layer and the steel sheet, wherein the Al content is 1.50% by mass or more, and a portion of the Al barrier layer may penetrate into the main plating layer. [8] In the hot-dip galvanized steel sheet described in any of [1] to [7] above, the chemical composition of the steel sheet may contain, in mass%, one or more selected from the group consisting of Ti: 0.01 to 0.20%, Cr: 0.01 to 1.50%, Mo: 0.01 to 1.00%, Nb: 0.01 to 0.10%, Ni: 0.01 to 1.00%, V: 0.01 to 0.20%, Cu: 0.01 to 1.00%, W: 0.001 to 0.100%, REM: 0.001 to 0.100%, Ca: 0.001 to 0.100%, Sb: 0.001 to 0.050%, Sn: 0.001 to 0.050%, and As: 0.001 to 0.050%.

[0006] According to the above embodiment of the present invention, it is possible to provide a hot-dip galvanized steel sheet that exhibits excellent plating adhesion even when the steel sheet contains B (boron).

[0007] This is a conceptual diagram of a hot-dip galvanized steel sheet. The diagram shows examples of the results of EPMA analysis and SEM observation for Examples No. 1, No. 4, and No. 5. This is an example of the results of GD-OES analysis for Example No. 5. The arrows indicate the locations where the maximum emission intensity for each element is observed. This is an example of the results of GD-OES analysis for Example No. 3. The arrows indicate the locations where the maximum emission intensity for each element is observed.

[0008] The following shows embodiments for carrying out the invention. As shown in FIG. 1, a hot-dip galvanized steel sheet (the hot-dip galvanized steel sheet according to this embodiment) 1 according to one aspect of the present invention includes a plating layer 3 on the surface of a steel sheet 2. The plating layer 3 applied to the surface of this steel sheet 2 has a plating adhesion amount Wt per side of 30.0 to 120.0 g / m ２ , and the Fe adhesion amount Wf per side in the plating layer is 0.40 to 3.00 g / m ２ . Further, for this hot-dip galvanized steel sheet 1, an Al barrier index Ia represented by the following formula (1) using the Al adhesion amount Wa per side, the plating adhesion amount Wt, and the Fe adhesion amount Wf in the plating layer is 150 mg / m ２ or more. Ia = Wa - (Wt - Wf) × 0.002 (1) Also, when performing analysis by glow discharge optical emission spectrometry in the plate thickness direction of the steel sheet 2 from the surface of the plating layer 3 on the hot-dip galvanized steel sheet 1, the time tB in units of seconds from the start of the analysis until the maximum value is detected regarding the emission intensity of B, and the time tAl in units of seconds from the start of the analysis until the maximum value is detected regarding the emission intensity of Al satisfy at least one of the following formulas (2) and (3). tB - tAl ≧ 10.0 (2) tB / tAl ≧ 1.50 (3)

[0009] Here, basic matters of the hot-dip galvanized steel sheet 1 will be explained. The hot-dip galvanized steel sheet 1 is manufactured on a continuous hot-dip galvanizing line called CGL (Continuous Galvanizing Line). An annealing process carried out before immersion in the hot-dip galvanizing bath is an important process for ensuring plating adhesion. In the annealing process, in addition to obtaining necessary mechanical properties (strength, ductility, etc.) by creating a steel structure through appropriate heat treatment and cooling treatment, N ２ -H ２ atmosphere (N ２ and H ２By heat treatment in a mixed gas atmosphere, the wettability of the plating when immersed in the molten zinc plating bath is ensured. In this way, by controlling the surface condition of the steel sheet 2 before immersing it in the molten zinc plating bath, it is possible to improve the plating adhesion. The molten zinc plated steel sheet according to this embodiment will be described in more detail below. In this specification, unless otherwise specified, the "~" indicating a numerical range includes the numbers written before and after it as the lower limit and upper limit. However, if "greater than" or "less than" is attached to a number, that number is not included.

[0010] (Steel Sheet) <Chemical Composition> The steel sheet 2 included in the hot-dip galvanized steel sheet 1 according to this embodiment refers to a steel sheet having a chemical composition consisting of the following elements, among steel sheets that contain B (boron) in their chemical composition. Steel sheet 2 may be called a base steel sheet, base material steel sheet, or substrate steel sheet to distinguish it from the hot-dip galvanized steel sheet 1. Unless otherwise specified, the percentages for element content are in mass percent.

[0011] C: 0.050-0.500% The carbon (C) content in the steel sheet should be 0.050-0.500% by mass. C is an element that increases the strength of steel sheets, and if the C content is too low, it is difficult to manufacture high-strength steel sheets. However, if the C content is too high, the toughness of the steel sheet decreases. For this reason, the C content may be 0.450% or less, 0.400% or less, 0.350% or less, or 0.300% or less. The C content may be 0.080% or more, 0.100% or more, or 0.150% or more.

[0012] Si: 0.010 to 2.000% The silicon (Si) content in the steel sheet should be 0.010 to 2.000% by mass. Si is an element that increases the strength of steel through solid solution strengthening and microstructure strengthening. However, if the Si content is too high, oxides may form on the surface of the steel sheet, reducing the wettability of the plating. For this reason, the Si content may be 1.500% or less, 1.200% or less, 1.000% or less, or 0.800% or less. Alternatively, the Si content may be 0.100% or more, 0.200% or more, or 0.500% or more.

[0013] Mn: 0.10–2.50% The manganese (Mn) content in the steel sheet should be 0.10–2.50% by mass. Mn is an element that enhances the strength and hardenability of steel. If the Mn content is too low, it becomes difficult to manufacture high-strength steel sheets. On the other hand, if the Mn content is too high, the workability of the steel tends to deteriorate. For this reason, the Mn content may be 2.30% or less, 2.10% or less, 2.00% or less, or 1.80% or less. Alternatively, the Mn content may be 0.50% or more, 1.00% or more, or 1.50% or more.

[0014] P: 0.030% or less. Phosphorus (P) is an element that segregates at grain boundaries, reducing the toughness of steel and decreasing its resistance to delayed fracture. Therefore, the P content in steel sheets should be 0.030% or less. The P content may also be 0.020% or less, 0.015% or less, or 0.012% or less. A lower P content is preferable, and there is no need to limit the lower limit, but the lower limit may be 0%. From the viewpoint of suppressing increases in manufacturing costs, the P content may be 0.001% or more, 0.002% or more, or 0.005% or more.

[0015] S: 0.010% or less. S (sulfur) is an element that forms sulfides, reducing the toughness of steel and decreasing its resistance to delayed fracture. Therefore, the S content in steel sheets should be 0.010% or less. The S content may also be 0.008% or less, 0.006% or less, or 0.004% or less. A lower S content is preferable, and there is no need to limit the lower limit, but the lower limit may be 0%. From the viewpoint of suppressing increases in manufacturing costs, the S content may be 0.0001% or more, or 0.001% or more, 0.002% or more, or 0.005% or more.

[0016] Al: 0-1.00% While steel sheets do not necessarily need to contain aluminum (Al), it is commonly used for deoxidation of steel, in which case it may be present in the steel sheet. Its presence is not mandatory, and the lower limit of Al content is 0%. For deoxidation purposes, the Al content may be 0.005% or higher, or even 0.01% or higher. On the other hand, Al is easily oxidized, and higher Al content leads to an increase in inclusions, which can degrade the workability of the steel. Therefore, the Al content should be 1.00% or less. The Al content may be 0.50% or less, 0.30% or less, 0.20% or less, 0.10% or less, or 0.08% or less.

[0017] N: 0 to 0.0100% Nitrogen (N) is an element that forms nitrides and reduces the toughness of steel. Also, when boron is present, N combines with boron to reduce the amount of solid solution boron and reduces the hardenability of the steel sheet. For this reason, the N content should be 0.0100% or less. The N content may be 0.0080% or less, 0.0060% or less, or 0.0040% or less. A lower N content is preferable, and it may be 0%. From the viewpoint of suppressing increases in manufacturing costs, the N content may be 0.0001% or more, or 0.0010% or more.

[0018] B: 0.0005 to 0.0050% Boron (B) is an element that contributes to increasing the hardenability of steel and the strength of steel sheets. Therefore, the B content should be 0.0005% or more. The B content may be 0.0008% or more, or 0.0010% or more. On the other hand, if the B content is too high, its effect will saturate, and there is a concern that it will affect the oxidation behavior of the surface of the steel sheet during annealing heating, leading to a decrease in plating wettability. Therefore, the B content should be 0.0050% or less. The B content may be 0.0040% or less, 0.0030% or less, or 0.0020% or less.

[0019] The chemical composition of the steel sheet may contain the elements listed above, with the remainder being Fe (iron) and impurities. On the other hand, when using slabs manufactured by the electric furnace steelmaking method, which involves melting raw materials such as iron scrap in an electric furnace, the steel sheet tends to contain a large amount of elements. For this reason, in place of some of the Fe, one or more of the following elements may be included in the content range described later: Ti (titanium), Cr (chromium), Mo (molybdenum), Nb (niobium), Ni (nickel), V (vanadium), Cu (copper), W (tungsten), REM (rare earth elements), Ca (calcium), Sb (antimony), Sn (tin), and As (arsenic). That is, it may contain C, Si, Mn, P, S, Al, N, and B, and one or more of Ti, Cr, Mo, Nb, Ni, V, Cu, W, REM, Ca, Sb, Sn, and As, with the remainder being Fe and impurities. These optional elements may be included, not just intentionally, as long as they are within the content ranges described below. These optional elements may not be included at all, and the lower limit of their content is 0%.

[0020] Ti: 0-0.20% Titanium (Ti) is an element that combines with nitrogen to form nitrides, and it is an element that suppresses the bonding of boron and nitrogen, thereby contributing to the suppression of the decrease in hardenability due to the formation of boron nitrides. For this reason, it may be included as needed to obtain the above effect. In this case, the Ti content is preferably 0.01% or more, and more preferably 0.02% or more, or 0.05% or more. On the other hand, if the Ti content is too high, there is a concern that the toughness of the steel will decrease due to the excessive precipitation of titanium nitrides after the above effect has saturated. For this reason, the Ti content should be 0.20% or less. The Ti content may be 0.15% or less, 0.10% or less, 0.05% or less, or 0.03% or less.

[0021] Cr: 0-1.50% Cr (chromium) is an effective element for increasing the hardenability and strength of steel, and may be included as needed. This effect can be obtained even with trace amounts, but if included, the Cr content may be 0.01% or more, 0.05% or more, 0.07% or more, or 0.10% or more. On the other hand, if Cr is included in excess, a large amount of Cr carbide may be formed, which may conversely impair the hardenability. Therefore, if included, the Cr content should be 1.50% or less. The Cr content may be 1.20% or less, 0.80% or less, 0.60% or less, 0.50% or less, 0.30% or less, or 0.20% or less.

[0022] Mo: 0-1.00% Mo (molybdenum) is effective in increasing the hardenability and strength of steel, and may be included as needed. This effect can be obtained even with trace amounts, but if included, the Mo content may be 0.01% or more, 0.02% or more, 0.03% or more, 0.05% or more, or 0.06% or more. On the other hand, from the viewpoint of suppressing a decrease in toughness, if included, the Mo content should be 1.00% or less. The Mo content may be 0.80% or less, 0.60% or less, 0.40% or less, 0.30% or less, 0.20% or less, or 0.15% or less.

[0023] Nb: 0-0.10% Niobium (Nb) is an element that contributes to improving the strength of steel sheets through precipitation strengthening, fine grain strengthening by suppressing grain growth, and dislocation strengthening by suppressing recrystallization, and may be included as needed. This effect can be obtained even with trace amounts, but if Nb is included, the Nb content may be 0.01% or more, or 0.02% or more. On the other hand, from the viewpoint of ensuring toughness, if Nb is included, the Nb content should be 0.10% or less. The Nb content may be 0.06% or less, or 0.04% or less, or 0.03% or less.

[0024] Ni: 0-1.00% Nickel (Ni) is effective in increasing the hardenability and strength of steel, and may be included as needed. This effect can be obtained even with trace amounts, but if Ni is included, the Ni content may be 0.01% or more, 0.02% or more, 0.03% or more, 0.04% or more, or 0.05% or more. On the other hand, excessive addition of Ni increases costs, so if Ni is included, the Ni content should be 1.00% or less. The Ni content may be 0.80% or less, 0.60% or less, 0.45% or less, 0.30% or less, 0.20% or less, or 0.15% or less.

[0025] V: 0-0.20% Vanadium (V) is an element that contributes to improving the strength of steel sheets through precipitation strengthening, fine grain strengthening by suppressing grain growth, and dislocation strengthening through suppression of recrystallization, and may be included as needed. This effect can be obtained even with trace amounts, but if V is included, the V content may be 0.01% or more, 0.02% or more, 0.03% or more, or 0.04% or more. On the other hand, from the viewpoint of ensuring toughness, if V is included, the V content should be 0.20% or less. The V content may be 0.15% or less, 0.10% or less, 0.08% or less, 0.06% or less, or 0.05% or less.

[0026] Cu: 0-1.00% Cu (copper) is effective in increasing the hardenability and strength of steel, and may be included as needed. This effect can be obtained even with trace amounts, but if Cu is included, the Cu content may be 0.01% or more, 0.02% or more, 0.03% or more, 0.04% or more, or 0.05% or more. On the other hand, from the viewpoint of suppressing cracking of the slab after casting, if Cu is included, the Cu content should be 1.00% or less. The Cu content may be 0.80% or less, 0.60% or less, 0.40% or less, or 0.20% or less.

[0027] W: 0-0.100% W (tungsten) is effective in controlling the morphology of carbides and increasing the strength of steel, and may be included as needed. This effect can be obtained even with trace amounts, but if included, the W content may be 0.001% or more, 0.005% or more, 0.010% or more, or 0.020% or more. On the other hand, from the viewpoint of suppressing the decrease in toughness, if included, the W content should be 0.100% or less. The W content may be 0.080% or less, 0.060% or less, 0.040% or less, 0.030% or less, 0.020% or less, 0.015% or less, or 0.010% or less.

[0028] REM: 0-0.100% Rare earth elements (REM) contribute to inclusion control, particularly the fine dispersion of inclusions, and enhance toughness, so they may be included as needed. This effect can be obtained even with trace amounts, but if REM is included, the REM content may be 0.001% or more, 0.002% or more, 0.003% or more, or 0.004% or more. On the other hand, excessive REM content may lead to deterioration of surface properties, so if REM is included, the REM content should be 0.100% or less. The REM content may be 0.080% or less, 0.060% or less, 0.040% or less, 0.020% or less, 0.010% or less, 0.008% or less, or 0.006% or less. REM stands for Rare Earth Metal, and refers to 17 elements, which are the 15 elements belonging to the lanthanide series plus Sc (scandium) and Y (yttrium). REM content refers to the total amount of these 17 elements present.

[0029] Ca: 0-0.100% Calcium (Ca) is an element that contributes to inclusion control, particularly the fine dispersion of inclusions, and enhances toughness, so it may be included as needed. This effect can be obtained even with trace amounts, but if Ca is included, the Ca content may be 0.001% or more, 0.002% or more, 0.003% or more, or 0.004% or more. On the other hand, excessive Ca content may lead to deterioration of surface properties, so if Ca is included, the Ca content should be 0.100% or less. The Ca content may be 0.080% or less, 0.060% or less, 0.040% or less, 0.020% or less, 0.010% or less, or 0.006% or less.

[0030] Sb: 0-0.050% Sb (antimony) is an element that enhances the ductility of steel sheets by suppressing the formation of oxides that serve as the starting point for fracture. To reliably obtain this effect, it is preferable that the Sb content be 0.001% or more, 0.005% or more, or 0.010% or more. On the other hand, the above effect will saturate if a large amount of Sb is included, so if Sb is included, the Sb content should be 0.050% or less. Preferably, the Sb content is 0.020% or less, or 0.015% or less.

[0031] Sn: 0-0.050% Sn (tin) is an element that enhances the ductility of steel sheets by suppressing the formation of oxides that serve as the starting point for fracture. To more reliably obtain this effect, it is preferable that the Sn content be 0.001% or more, 0.005% or more, or 0.010% or more. On the other hand, the above effect saturates even if a large amount of Sn is included, so if Sn is included, the Sn content should be 0.050% or less. Preferably, the Sn content is 0.045% or less, or 0.040% or less.

[0032] As: 0-0.050% Arsenic (As) is an element that lowers the austenite single-phase temperature, thereby refining the prior austenite grains and increasing toughness. To more reliably obtain this effect, it is preferable that the As content be 0.001% or more, or 0.010% or more. On the other hand, the above effect saturates even if a large amount of As is included, so if As is included, the As content should be 0.050% or less. Preferably, the As content is 0.020% or less, or 0.015% or less.

[0033] The chemical composition of the steel sheet 2, which is the base steel sheet of the hot-dip galvanized steel sheet 1 according to this embodiment, can be measured by a general analytical method. For example, the chemical composition of the steel sheet can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) on chips. Specifically, for example, a 35 mm square test piece can be obtained from around 1 / 4 of the thickness (1 / 4 thickness position) from the surface of the steel sheet 2 (after removing the plating layer on the surface by mechanical grinding if necessary), and measured using a Shimadzu ICPS-8100 or similar (measuring device) under conditions based on a pre-created calibration curve. For C and S, which are difficult to measure with ICP-AES, the combustion-infrared absorption method can be used; for N, the inert gas fusion-thermal conductivity method can be used; and for O, the inert gas fusion-non-dispersive infrared absorption method can be used. If analysis values ​​for molten steel, slabs, or other steel sheets manufactured from the same molten steel can be confirmed, the analysis of test pieces taken from the steel sheet may be omitted, and those analysis values ​​may be considered as the chemical composition of the steel sheet.

[0034] (Plating layer) <Amount of adhesion> The plating layer 3 is usually applied similarly to the entire surface of the steel sheet 2, but the amount of plating Wt per side is 30.0 to 120.0 g / m ２ The adhesion amount Wt is 30.0 g / m². ２ If the value is less than 120.0 g / m², there is a risk of impairing corrosion resistance. ２ This is because exceeding this amount would be uneconomical. From the standpoint of corrosion resistance, the amount of plating Wt per side should be 35.0 g / m ２ Above, 40.0g / m ２ Above, 45.0g / m ２ Above, 50.0g / m ２ Above, or 55.0 g / m² ２ The above may also be used. From an economic standpoint, the amount of plating deposited per side, Wt, should be 110.0 g / m². ２ Below, 105.0g / m ２ Below, 100.0g / m ２ Below, 95.0g / m ２ Below, 90.0g / m ２ Below, 85.0g / m２ The following, or 75.0 g / m² ２ The following is also acceptable.

[0035] The plating layer 3 of the hot-dip galvanized steel sheet 1 according to this embodiment is mainly composed of Zn (zinc), but Zn (zinc) is not the only component present in the plating layer 3. For example, Fe is also contained in the plating. The amount of Fe Wf deposited on one side of the plating layer 3 is 0.40 to 3.00 g / m². ２ The Fe adhesion amount Wf is 0.40 g / m². ２ If the Fe deposition amount Wf is less than 3.00 g / m², there is concern about a decrease in the reactivity between the steel sheet 2 and the plating layer 3. On the other hand, if the Fe deposition amount Wf is 3.00 g / m², there is concern about a decrease in reactivity between the steel sheet 2 and the plating layer 3. ２ If this value is exceeded, there is a concern that corrosion resistance will decrease, such as the formation of red rust. From the viewpoint of the reactivity of the plating layer 3, the Fe deposition amount Wf per side should be 0.45 g / m ２ Above, 0.50g / m ２ Above, 0.60g / m ２ Above, 0.70g / m ２ Above, or 0.80 g / m ２ The above may also be used. From the viewpoint of corrosion resistance and economic efficiency, the amount of Fe adhering to each side, Wf, should be 2.80 g / m. ２ Below, 2.50g / m ２ Below, 2.20g / m ２ Below, 2.00g / m ２ Below, 1.80g / m ２ Below, 1.60g / m ２ The following, or 1.30 g / m ２ The following is also acceptable.

[0036] In this embodiment, the hot-dip galvanized steel sheet 1 has an Al barrier index Ia expressed by the following formula (1), using the amount of Al deposited per side of the plating layer Wa, the amount of plating deposited Wt, and the amount of Fe deposited per side of the plating layer Wf, which is 150 mg / m². ２ (0.150 g / m ２ ) or more. Ia = Wa - (Wt - Wf) × 0.002 (1) The Al barrier index Ia expressed by the above formula (1) is an empirical formula obtained by the inventors from the results of numerous tests. Al barrier index Ia is 150 mg / m ２The inventors have found that, under the above conditions, a sufficient Al barrier layer 5 is formed, the plating layer becomes less likely to peel off, and sufficient plating adhesion is obtained. The Al barrier index Ia is 160 mg / m². ２ Above, 170mg / m ２ Above, 180mg / m ２ Above, 190mg / m ２ Preferably, it is 200 mg / m² or more. ２ The above is more preferable. On the other hand, if the Al barrier index Ia becomes too high, there is a concern that aluminum oxide will form on the outermost layer of the plating layer 3, degrading the chemical conversion treatment performance. Therefore, the Al barrier index Ia should be 450 mg / m². ２ Preferably, it is 400 mg / m² ２ Below, 350mg / m ２ Below, 300mg / m ２ The following, or 250 mg / m² ２ The following is also acceptable. In order to calculate Ia using formula (1), it is necessary to standardize the units of Wa, Wt, and Wf. For example, all units of Wa, Wt, and Wf should be mg / m ２ Therefore, the unit of the calculated Ia is mg / m ２ Therefore, all units of Wa, Wt, and Wf are expressed as g / m ２ Therefore, the unit of the calculated Ia is g / m ２ The unit is mg / m ２ Multiplying the value of Ia by 1 / 1000 gives the unit g / m ２ It is converted to the value of Ia. The unit is g / m ２ Multiplying the value of Ia by 1000 gives the unit mg / m ２ It is converted to the value of Ia. The units of Wt and Wf are g / m ２ The unit of Wa is mg / m ２ In this case, by using equation (1') below instead of equation (1), the units of Wa, Wt, and Wf can be converted and made consistent (that is, the units of Wt and Wf can be converted to g / m). ２ Keep the unit Wa as is, mg / m ２ (as is), the unit is mg / m ２ The Ia can be calculated as follows: Ia = Wa - (Wt - Wf) × 2 (1')

[0037] <Chemical Composition> The plating layer 3 of the hot-dip galvanized steel sheet 1 according to this embodiment is a zinc plating layer with a Zn content of 90.00% or more by mass. A typical chemical composition of the plating layer 3 is exemplified below. Unless otherwise specified, the percentages for the content of elements included in the chemical composition of the plating layer are in mass percent.

[0038] Al: 0.10-1.00% Al is an effective element for suppressing excessive alloying reactions between the base metal and zinc in the molten zinc plating bath. To fully obtain this effect, Al is included in the plating bath, which also results in Al being included in the plating layer. To obtain the above effect, it is preferable that the Al content of the plating layer be 0.10% or more. The Al content may be 0.12% or more, 0.15% or more, 0.18% or more, 0.20% or more, 0.25% or more, 0.30% or more, 0.35% or more, or 0.40% or more. On the other hand, if Al is included in excess, an alumina film may be excessively formed on the surface of the plating layer, which may reduce the chemical conversion treatment properties of the plated steel sheet. Therefore, it is preferable that the Al content be 1.00% or less. The Al content may be 0.95% or less, 0.80% or less, 0.75% or less, 0.70% or less, 0.65% or less, or 0.60% or less. The amount of Al deposited on one side of the plating layer 3, Wa, is 210 to 900 mg / m². ２ This may also be done. If necessary, the amount of Al deposited on one side of the plating layer Wa may be set to 225 mg / m². ２ Above, or 250 mg / m² ２ The above is also acceptable, 800 mg / m² ２ Below, 700mg / m ２ Below, 600mg / m ２ Below, 500mg / m ２ The following, or 400 mg / m² ２ 0 or less, or 350 mg / m² ２ The following is also acceptable.

[0039] Fe: 0.33 to 5.00% Fe is often inevitably included in the plating layer as an alloying reaction between the base steel sheet and the plating layer progresses, for example, in the molten zinc plating bath and from the time the sheet is removed from the plating bath until it cools. For this reason, in the plated steel sheet according to this embodiment, the Fe content in the plating layer is often 0.33% or more. The Fe content may be 0.35% or more, 0.40% or more, 0.45% or more, 0.50% or more, 0.60% or more, 0.70% or more, 0.80% or more, or 0.90% or more. On the other hand, if the Fe content in the plating layer is too high, red rust caused by Fe is more likely to occur when exposed to a corrosive environment, and sufficient corrosion resistance may not be achieved. Therefore, it is preferable that the Fe content be 5.00% or less. The Fe content may be 4.00% or less, 3.00% or less, 2.50% or less, 2.00% or less, 1.80% or less, 1.50% or less, or 1.20% or less.

[0040] The plating layer is further composed of Mg: 0-0.500%, Si: 0-0.500%, Ni: 0-0.500%, Ca: 0-2.000%, Sb: 0-0.500%, Pb: 0-0.500%, Cu: 0-0.500%, Sn: 0-0.500%, Ti: 0-0.500%, Cr: 0-0.500%, Nb: 0-0.500%, and Zr It may contain at least one of the following elements: 0-0.500%, Mn: 0-0.500%, Mo: 0-0.500%, Ag: 0-0.500%, Li: 0-0.500%, La: 0-0.500%, Ce: 0-0.500%, B: 0-0.004%, Y: 0-0.500%, P: 0-0.500%, and Sr: 0-0.500%. Preferably, the total content of these elements, i.e., Mg, Si, Ni, Ca, Sb, Pb, Cu, Sn, Ti, Cr, Nb, Zr, Mn, Mo, Ag, Li, La, Ce, B, Y, P, and Sr, is 5.000% or less. The total content of these elements may be 4.500% or less, 4.000% or less, 3.500% or less, 3.000% or less, 2.500% or less, 2.000% or less, 1.500% or less, or 1.000% or less. These optional elements will be described in detail below.

[0041] Mg: 0-0.500% Mg is an effective element for improving the corrosion resistance of the plating layer. The Mg content may be 0%, but to obtain this effect, it is preferable that the Mg content be 0.001% or more. The Mg content may be 0.010% or more, 0.020% or more, or 0.030% or more. On the other hand, if the Mg content is excessive, a large amount of brittle MgZn-based compounds may be formed in the plating layer, which can cause a decrease in workability. Therefore, it is preferable that the Mg content be 0.500% or less. The Mg content may be 0.500% or less, 0.400% or less, 0.300% or less, 0.100% or less, or 0.200% or less.

[0042] Si: 0 to 0.500% Si is an effective element for improving the corrosion resistance of the plating layer. The Si content may be 0%, but if necessary, Si may be included in the plating layer at a content of 0.0001% or more, or 0.001% or more. On the other hand, if the Si content is excessive, the plating adhesion of the plating layer may decrease. Therefore, it is preferable that the Si content be 0.500% or less. The Si content may also be 0.400% or less, 0.300% or less, 0.100% or less, or 0.050% or less.

[0043] Ni: 0 to 0.500% Ni is an effective element for improving the corrosion resistance of the plating layer. The Ni content may be 0%, but to obtain this effect, it is preferable that the Ni content be 0.001% or more. The Ni content may be 0.005% or more, 0.010% or more, or 0.020% or more. On the other hand, if Ni is included in excess, many intermetallic compounds may be formed, which may reduce corrosion resistance. Therefore, it is preferable that the Ni content be 0.500% or less. The Ni content may be 0.400% or less, 0.300% or less, or 0.100% or less.

[0044] Ca: 0-2.000% Ca is an effective element for ensuring the wettability of the plating bath. The Ca content may be 0%, but to obtain this effect, it is preferable that the Ca content be 0.001% or more. The Ca content may be 0.010% or more, 0.100% or more, or 1.000% or more. On the other hand, if Ca is included in excess, a large amount of hard intermetallic compounds may be formed in the plating layer, making the plating layer brittle and reducing its adhesion to the steel sheet. Therefore, it is preferable that the Ca content be 2.000% or less. The Ca content may be 1.500% or less, 1.000% or less, 0.500% or less, 0.400% or less, 0.300% or less, 0.200% or less, or 0.100% or less.

[0045] Sb: 0-0.500% Pb: 0-0.500% Cu: 0-0.500% Sn: 0-0.500% Ti: 0-0.500% Cr: 0-0.500% Nb: 0-0.500% Zr: 0-0.500% Mn: 0-0.500% Mo: 0-0.500% Ag: 0-0.500% Li: 0-0.500% La: 0-0.500% Ce: 0-0.500% B: 0-0.004% Y: 0-0.500% P: 0-0.500% Sr: 0-0.500% Sb, Pb, Cu, Sn, Ti, Cr, Nb, Zr, Mn, Mo, Ag, Li, La, Ce, B, Y, P, and Sr do not necessarily have to be included in the plating layer, but they may be present in the plating layer in a content of 0.0001% or more or 0.001% or more. These elements do not adversely affect the performance of the plated steel sheet as long as they are within the predetermined content range. However, if the content of each element is excessive, it may reduce corrosion resistance. Therefore, the content of Sb, Pb, Cu, Sn, Ti, Cr, Nb, Zr, Mn, Mo, Ag, Li, La, Ce, Y, P, and Sr is preferably 0.500% or less, for example, 0.300% or less, 0.100% or less, or 0.050% or less. Similarly, the content of these elements may be 0.001% or more, 0.003% or more, or 0.005% or more. The B content is preferably 0.004% or less, and may be, for example, 0.003% or less, 0.002% or less, 0.001% or less, or less than 0.0005%. The B content may also be 0.0001% or more, 0.0002% or more, or 0.0003% or more. Furthermore, it is preferable that the B content of the plating layer 3 is less than the B content of the steel plate 2.

[0046] In the plating layer, the remainder of the elements other than those mentioned above consists of Zn and impurities. Impurities in the plating layer refer to components that are mixed in during the manufacturing of the plating layer due to various factors in the manufacturing process, including the raw materials. The Zn content is 90.00% or more, and may be 92.00% or more, 94.00% or more, 95.00% or more, 96.00% or more, 97.00% or more, 98.00% or more, 99.00% or more, or 99.57% or more, as needed.

[0047] In the plating layer 3, the amount of plating Wt, the amount of Al deposited per side of the plating layer Wa, the amount of Fe deposited per side of the plating layer Wf, and the content of the above elements can be measured in the following manner. For example, a 50 mm x 50 mm sample is taken from a hot-dip galvanized steel sheet. Next, the plating layer is peeled and dissolved from this sample using an acid solution containing an inhibitor (for example, Ibit 710K manufactured by Asahi Chemical Industries) that suppresses corrosion of the steel sheet (base steel sheet). The amount of plating Wt is measured from the change in mass of the sample before and after peeling and dissolution. The content of each element in the plating layer (mass %) is measured by quantitatively analyzing the content of each element in the acid solution using ICP (inductively coupled plasma) emission spectroscopy or the like. From these measurement results and the amount of plating, the amount of Al deposited Wa and the amount of Fe deposited Wf are calculated. The type of acid used in the acid solution is not particularly limited and any acid that can dissolve the plating layer may be used. For example, as an acid containing an inhibitor, an aqueous solution with a concentration of 0.05% by mass of Ibit 710K and a hydrochloric acid concentration of 5% by mass can be used.

[0048] <Depth position where the emission intensities of B and Al are at their maximum in glow discharge emission spectroscopy (GD-OES) analysis> As a result of the inventors' investigation, it was confirmed that in a hot-dip galvanized steel sheet obtained by hot-dip galvanizing a steel sheet containing B, the adhesion of the plating layer is improved if the region where B is present is located somewhat inward from the surface layer of the steel sheet (on the side opposite to the plating layer from the interface between the steel sheet and the plating layer). Specifically, when glow discharge emission spectroscopy (GD-OES) analysis is performed from the surface of the plating layer 3 in the thickness direction of the steel sheet 2, it was found that the adhesion of the plating layer is improved when the time tB per second from the start of the analysis until the maximum emission intensity of B is detected, and the time tAl per second from the start of the analysis until the maximum emission intensity of Al is detected, satisfy at least one of the following equations (2) and (3) (i.e., one or both). (2) tB - tAl ≥ 10.0 (3) tB / tAl ≥ 1.50

[0049] When formulas (2) and / or (3) are satisfied, the adhesion of the plating layer is improved because the Al barrier layer 5 is the position where the luminescence intensity of Al is at its maximum, and keeping B at a sufficiently deep position toward the center of the thickness of the steel sheet, i.e., inside the steel sheet 2, is effective for adhesion. The reason for this is not entirely clear, but it is thought that B greatly influences the oxidation behavior of the surface of the steel sheet 2, and suppressing the external oxidation of B and keeping it inside the steel sheet 2 is extremely effective in improving plating adhesion. When formulas (2) and / or (3) are satisfied, it is effective to promote the oxidation of easily oxidizable elements inside the steel sheet. It is sufficient to satisfy at least one of formulas (2) and (3), but it is also acceptable to satisfy both formulas (2) and (3). tB-tAl is 10.0 seconds or more, but it is preferable to be 15.0 seconds or more, 20.0 seconds or more, or 25.0 seconds or more. tB / tAl is 1.50 or higher, but preferably 1.70 or higher, 1.90 or higher, or 2.00 or higher. There is no need to set an upper limit for tB-tAl, but tB-tAl may be 50 seconds or less, 45 seconds or less, or 40 seconds or less. There is no need to set an upper limit for tB / tAl, but tB / tAl may be 2.50 or less, 2.30 or less, or 2.10 or less.

[0050] Furthermore, in glow discharge emission spectroscopy, when tMn is the measurement time in units of seconds from the start of the analysis until the maximum value of the emission intensity of Mn is detected, it is preferable that tB and tMn satisfy the following equation (4) (i.e., tB / tMn ≥ 1.00). That is, it is preferable that the depth position where B is concentrated is the same as or deeper than the depth where Mn is concentrated. tB ≥ tMn (4) In this case, the adhesion of the plating layer is further improved. It is more preferable that tB / tMn is greater than 1.00, 1.10 or more, or 1.20 or more. There is no need to set an upper limit for tB / tMn, but tB / tMn may be 1.50 or less or 1.40 or less.

[0051] The measurement time (analysis time) from the start of the analysis until the maximum emission intensity of the target element is detected serves as an indicator for evaluating the depth at which the target element is most abundant from the surface. By performing evaluations for multiple elements, it is possible to relatively evaluate the locations where each element is most abundant.

[0052] Regions containing high concentrations of B, Al, and Mn can be identified using glow discharge emission spectroscopy (GD-OES), which allows for the spectral measurement of atomic emission within an Ar plasma using sputtering in a glow discharge region.

[0053] Glow discharge optical emission spectroscopy (GD-OES) is a well-known technique, as specified in JIS K0144:2018, but a brief explanation will be provided to facilitate understanding. According to GD-OES, the abundance of an element can be measured by detecting the emission intensity corresponding to the abundance of the element being measured. Furthermore, since the analysis is performed by gradually drilling into the sample surface, the analysis is performed at positions that have progressed in the depth direction from the sample surface as time has passed since the start of the measurement. Therefore, for example, by comparing the measurement time at which the maximum emission intensity is detected (measurement time until the peak) from the start of the measurement for each element, it is possible to determine at what depth from the sample surface each element is concentrated.

[0054] Therefore, if the measurement time until the maximum emission intensity of a certain element is detected is longer than the measurement time until the maximum emission intensity of other elements is detected, it indicates that the element is concentrated at a greater distance (deeper position) from the surface of the sample than other elements. GD-OES is exemplified by using a GD-Profiler2 manufactured by Horiba, Ltd., with the conditions set to a discharge power of 35W, an Ar pressure of 600Pa, and a discharge diameter of 4mmφ. The measurement time can be set to 300 seconds, with a measurement interval of 0.5 seconds. However, for samples with a large amount of plating, the measurement time may be extended as needed. Furthermore, the measurement time at which the maximum emission intensity of each element is detected should be determined from 2.0 seconds after the start of measurement. This is because there is a concern that the elements being analyzed may be detected at high values ​​due to the influence of dirt and atmospheric oxidation on the outermost surface.

[0055] When a hot-dip galvanized steel sheet has a coating or chemical conversion coating on its surface, GD-OES analysis is performed on the surface of the hot-dip galvanized steel sheet. The time at which the Zn content first reaches 90.00 mass% is considered the start time of the GD-OES analysis, and the time from that time until the emission intensities of B, Al, and Mn reach their maximum values ​​is defined as tB, tAl, and tMn. Depending on the thickness of the coating, etc., in order to shorten the GD-OES analysis time, a portion of the coating may be removed by preliminary mechanical grinding or acid immersion before performing the GD-OES analysis, and the time at which the Zn content reaches 90.00 mass% may be considered the start time of the GD-OES analysis.

[0056] <Al Barrier Layer> The inventors investigated a method to further improve the adhesion of the plating layer 3 to a steel sheet 2 containing B (boron). As a result, they found that if an Al barrier layer 5 with a high Al concentration (Al concentration of 1.50% by mass or more) is appropriately formed between the main plating layer 4 and the steel sheet 2, the plating layer will not peel off easily (high adhesion) even if the steel sheet 2 contains boron.

[0057] The Al barrier layer 5 can be effective if it is formed in at least a portion of the area between the main plating layer 4 and the steel sheet 2, but it is preferable that it be formed continuously (in a state where continuous formation can be detected by cross-sectional EPMA analysis). This continuity can be confirmed using cross-sectional EPMA analysis, as will be described later. EPMA analysis is an analysis using an electron probe microanalyzer (EPMA), which is a method of obtaining information about elemental identification and quantitative values ​​by irradiating a solid sample with a finely focused electron beam in a vacuum and analyzing the generated characteristic X-rays. To analyze the formation state of the Al barrier layer 5, a cross-sectional EPMA analysis can be performed by irradiating the cross section of the hot-dip galvanized steel sheet 1 with an electron beam using a sample that has been mechanically polished after being embedded in resin. The specific measurement method will be described later.

[0058] Furthermore, through repeated experiments by the inventors, it was found that even when the Al barrier layer 5 is not continuously formed, if a portion of the Al barrier layer 5 penetrates into the main plating layer 4 (i.e., if it is detectable in cross-sectional EPMA analysis that a portion of the Al barrier layer 5 has penetrated into the main plating layer 4), the adhesion of the plating layer is enhanced. It is presumed that the interlocking becomes stronger when a portion of the Al barrier layer 5 penetrates into the main plating layer 4. Therefore, it is preferable that a portion of the Al barrier layer 5 penetrates into the main plating layer. Whether or not a portion of the Al barrier layer 5 has penetrated into the main plating layer 4 can be determined by cross-sectional EPMA analysis, as will be described later.

[0059] Whether the Al barrier layer is continuously formed is determined by the following procedure: A sample is taken from the hot-dip galvanized steel sheet 1, and the sample is cut so that, for example, a cross section perpendicular to the rolling direction and parallel to the thickness direction (C section) becomes the observation section, and embedded in epoxy resin or the like. Mechanical polishing is performed on the observation section of the sample embedded in the resin until it becomes mirror-like. After polishing, EPMA analysis is performed on three consecutive fields of view, with an arbitrary 30 μm × 30 μm area that includes the entire thickness direction of the plating layer 3 and the steel sheet 2 as the observation area. That is, EPMA analysis is performed on a 90 μm area of ​​the interface region between the plating layer 3 and the steel sheet 2. For example, EPMA analysis is performed using an EPMA1610 manufactured by Shimadzu Corporation, with an acceleration voltage of 15.0 kV, a beam diameter of 1 μm, and an irradiation current of 2.0 × 10⁻¹⁴. －８ A. The measurement is performed under conditions such that the measurement time is 50 ms / point, the measurement area is 30 μm × 30 μm, and the number of measurement points is 200 points in the thickness direction and 200 points in the width direction (0.15 μm pitch). When observing a 30 μm × 30 μm area that includes the plating layer 3 and the steel sheet 2, scanning electron microscope (SEM) observation is performed, and backscattered electrons, which can be observed with different contrasts depending on the atomic weight of the object being observed, can be obtained to determine the area that includes the steel sheet 2 and the plating layer 3. SEM observation can be performed using, for example, a JEOL JSM-7200F, with an observation magnification of 3000x, and conditions can be set so that an area with a width of 30 μm or more can be observed.

[0060] Based on the EPMA analysis results, regions with a Zn content (concentration) of 10.00 mass% or more are identified as the plating layer, and regions with a Zn content of less than 10.00 mass% are identified as the steel plate. Within the plating layer, regions with an Al content (concentration) of 1.50 mass% or more are identified as the Al barrier layer 5, and regions with an Al content of less than 1.50 mass% are identified as the main plating layer 4. Although the Al barrier layer 5 is formed between the main plating layer and the steel plate, if regions with an Al content of less than 1.50% (regions that are not the Al barrier layer 5) are continuously observed for 1.0 μm or more in a direction parallel to the interface (i.e., perpendicular to the thickness direction), it is determined that the Al barrier layer 5 is not continuously formed (discontinuous). If regions that are not the Al barrier layer 5 are not continuously observed for 1.0 μm or more, it is determined that the Al barrier layer 5 is continuously formed. Figure 2 shows an example of the results of EPMA analysis and SEM observation. Although not visible in the example shown in Figure 2, in reality, the contents shown in Figure 2 can be displayed in color. For example, a color bar (red to dark blue) with an upper limit of 4.00 mass% for Al content can be displayed, with the region of less than 1.50 mass% for Al content being shown as light blue to dark blue, and the region of 1.50 mass% or more being shown as red to yellow-green.

[0061] Whether a portion of the Al barrier layer has penetrated into the main plating layer is determined as follows: Perform EPMA analysis under the same conditions as for determining whether the Al barrier layer is continuously formed. Output the results obtained from the measurement as a mapping image (contour map). From the mapping image, identify the area with a Zn content of 10.00 mass% or more as the plating layer, and the area with a Zn content of less than 10.00 mass% as the steel plate, and identify the interface between the plating layer and the steel plate. Furthermore, identify the area within the plating layer with an Al content of 1.50 mass% or more as the Al barrier layer, and identify the area with an Al content of less than 1.50 mass% as the main plating layer, and identify the interface between the Al barrier layer and the steel plate. If the Al barrier layer is located at a distance of 2.0 μm or more from the interface between the plating layer and the steel plate on the main plating layer side in the thickness direction, it is determined that a portion of the Al barrier layer has penetrated into the main plating layer.

[0062] (Tensile Strength) The hot-dip galvanized steel sheet 1 according to this embodiment can have any appropriate tensile strength (TS). There is no particular need to limit it, but the tensile strength may be, for example, 690 MPa or more, 780 MPa or more, 980 MPa or more, 1080 MPa or more, or 1180 MPa or more. There is no particular need to limit its upper limit, but for example it may be 2300 MPa or less, 2000 MPa or less, 1800 MPa or less, or 1500 MPa or less. The tensile strength is measured, for example, by taking a No. 5 test piece of JIS Z 2241:2022 and performing a tensile test in accordance with JIS Z 2241:2022. It is preferable to take the test piece from a direction parallel to the direction perpendicular to the rolling direction of the hot-dip galvanized steel sheet 1.

[0063] (Thickness) The thickness of the hot-dip galvanized steel sheet 1 according to this embodiment is not particularly limited. For example, it can be 0.6 to 4.8 mm. The thickness may be 0.8 mm or more, or 1.0 mm or more. The thickness may be 4.2 mm or less, 3.6 mm or less, 3.0 mm or less, 2.8 mm or less, 2.6 mm or less, 2.4 mm or less, 2.2 mm or less, or 2.0 mm or less.

[0064] (Manufacturing Method) Next, a method for manufacturing a hot-dip galvanized steel sheet will be described. The following description is intended to illustrate a characteristic method for manufacturing a hot-dip galvanized steel sheet according to this embodiment, and is not intended to limit the hot-dip galvanized steel sheet to one manufactured by the manufacturing method described below. In other words, the effects of the hot-dip galvanized steel sheet according to this embodiment can be obtained regardless of the manufacturing method, as long as it has the above characteristics. The hot-dip galvanized steel sheet according to this embodiment is obtained by a manufacturing method including the following steps (A) to (D): (A) Hot rolling step (B) Pickling step (C) Cold rolling step (D) Plating step Preferred conditions will be described for each step. Known conditions can be applied to steps and conditions that are not described.

[0065] (A) Hot Rolling Process In the hot rolling process, a slab having the same chemical composition as the steel sheet is heated, and then rough rolling and finish rolling are performed as desired to produce a steel sheet (hot-rolled steel sheet), which is then wound up. The heating temperature of the slab is not particularly limited, but it is preferable to set it to 1150°C or higher in order to sufficiently dissolve borides, carbides, etc. The steel slab used is preferably cast by continuous casting from the viewpoint of manufacturability, but it may also be manufactured by ingot casting or thin slab casting.

[0066] [Rough Rolling] Rough rolling may be performed on the heated slab before finish rolling. The rough rolling conditions are not particularly limited, but it is preferable to perform it so that the completion temperature is 1050°C or higher and the total reduction ratio is 60% or higher. If the total reduction ratio is less than 60%, recrystallization during hot rolling will be insufficient, which may cause heterogeneity of the structure of the hot-rolled steel sheet. The above total reduction ratio may be, for example, 90% or less.

[0067] [Finish Rolling] Finish rolling is performed after rough rolling, or after heating if rough rolling is not performed. The conditions for finish rolling are not particularly limited, but it is preferable that the entry temperature for finish rolling is 950 to 1100°C, the exit temperature for finish rolling is 850 to 1000°C, and the total reduction ratio is 80 to 95%. If the entry temperature for finish rolling is below 950°C, the exit temperature for finish rolling is below 850°C, or the total reduction ratio is above 95%, the texture of the hot-rolled steel sheet will develop, which may result in anisotropy in the final product sheet. On the other hand, if the entry temperature for finish rolling is above 1100°C, the exit temperature for finish rolling is above 1000°C, or the total reduction ratio is below 80%, the grain size of the hot-rolled steel sheet will coarseen, which may cause coarsening of the structure of the final product sheet.

[0068] [Winding] After the finish rolling is complete, the steel sheet (hot-rolled steel sheet) is cooled to, for example, 630°C or below before being wound into a coil. The winding temperature is preferably 530 to 630°C. If the winding temperature is below 530°C, the strength of the steel sheet may become excessive, impairing its cold-rollability. On the other hand, if the winding temperature is above 630°C, alloying elements such as Mn become concentrated in the cementite, which may delay the dissolution of cementite in the final annealing process, leading to a decrease in strength.

[0069] (B) Pickling process In the pickling process, the steel sheet obtained in the hot rolling process is treated with HCl at 1.0 to 5.0 mol / L and Fe at less than 3.0 mol / L. ２＋ and Fe less than 0.10 mol / L ３＋ The pickling treatment is performed by passing the material through an aqueous solution (pickling solution) containing the material at a temperature of 70-90°C at an average speed of 10 m / min or more, while ensuring that the immersion time in the aqueous solution is 15 seconds or more. The HCl concentration in the pickling solution is less than 1.0 mol / L, or Fe ２＋ If the concentration exceeds 3.0 mol / L, the temperature of the pickling solution falls below 70°C, the average speed of the hot-rolled steel sheet falls below 10 m / min, or the immersion time (pickling time) falls below 15 seconds, the pickling will not proceed sufficiently, and the removal of scale mainly composed of iron oxides will be uneven. Also, if the HCl concentration in the pickling solution exceeds 5.0 mol / L, or if the Fe in the pickling solution... ３＋ When the concentration exceeds 0.10 mol / L, the pickling rate increases, raising concerns about greater variability in the pickling state due to concentration fluctuations. Furthermore, when the temperature of the pickling solution exceeds 90°C, water evaporates rapidly, making it difficult to control the liquid volume and concentration in the pickling tank.

[0070] (C) Cold Rolling Process In the cold rolling process, the steel sheet is cold-rolled after pickling. The reduction ratio (cumulative reduction ratio) of the cold rolling process is set to 30% or more in order to promote recrystallization and / or to smooth out the irregularities of the steel sheet after pickling. If the reduction ratio is less than 30%, there is a concern that the irregularities on the surface of the steel sheet cannot be sufficiently smoothed. The reduction ratio may be 40% or more. On the other hand, excessive reduction leads to an excessive rolling load and increases the load on the cold rolling mill, so it is preferable to set the reduction ratio to 75% or less or 70% or less.

[0071] (D) Plating process In the plating process, after heat treatment is performed on the cold-rolled steel sheet (cold-rolled steel sheet), the steel sheet is immersed in a molten zinc plating bath to form a plating layer on the surface.

[0072] [Heat Treatment] In the heat treatment performed prior to immersion in the plating bath, the steel plate is heated to a maximum heating temperature of Ac1 + 30°C to 950°C (heating process), and then held in the above temperature range for a predetermined time (soaking process).

[0073] [Heating Process] During the heating (temperature increase) process, the atmosphere surrounding the steel plate is expressed as log(pH), which is the logarithm of the ratio of the partial pressure of water vapor to the partial pressure of hydrogen. ２ 0 / pH ２ The pH value should be set to 0.0 to 0.5. By setting the atmosphere during the heating process within the above range, internal oxidation can be promoted even during heat treatment, and oxidation on the surface of B can be suppressed and retained inside the steel plate. log(pH) ２ 0 / pH ２ The pH value is also called the oxygen potential, and the larger this value, the more the internal oxidation of easily oxidizable elements such as Si, Mn, Al, and B present in the middle and surface layers of the steel progresses, and the oxidation of these elements on the surface of the steel sheet can be suppressed. ２ 0 / pH ２ If log(pH) is less than 0.0, sufficient effect cannot be obtained. ２ 0 / pH ２ If the ratio exceeds 0.5, there is a concern that the plating quality may decrease due to oxidation of Fe, depending on the heating temperature and annealing atmosphere.

[0074] [[Soaking Process]] In the soaking process, the steel sheet is held at a maximum heating temperature of Ac1 + 30°C to 950°C for 1 to 1000 seconds. In the soaking process, in order to fully austenitize the structure of the steel sheet, the steel sheet is heated to at least Ac1 + 30°C or higher, and soaking heat treatment is performed at this temperature (maximum heating temperature). If austenitization is not sufficient, a large amount of ferrite may be generated in the final structure. If the maximum heating temperature is less than Ac1 + 30°C or the holding time (soaking time) is less than 1 second, austenitization will not proceed sufficiently. The holding time is preferably 30 seconds or more or 60 seconds or more. On the other hand, if the maximum heating temperature is raised excessively, it will not only cause deterioration of toughness due to coarsening of austenite grain size but also lead to damage to the annealing equipment. Therefore, the maximum heating temperature is 950°C or lower, preferably 910°C or lower. Also, since a too long holding time will inhibit productivity, the holding time is preferably 1000 seconds or less, more preferably 600 seconds or less. During soaking, it is not necessary to always hold the cold-rolled steel sheet at a constant temperature, and it may vary within the range that satisfies the above conditions. Also, the log(pH ２ O / pH ２ ) represented by the logarithm of the ratio of the water vapor partial pressure to the hydrogen partial pressure in the soaking heating process is set to -1.5 to -0.5. If log(pH ２ O / pH ２ ) is less than -1.5, there is a concern that B will be oxidized on the steel sheet surface. Also, if log(pH ２ O / pH ２ ) exceeds -0.5, there is a concern that the plating property may deteriorate due to oxidation of Fe depending on the heating temperature and annealing atmosphere. When forming the Al barrier layer continuously, it is preferable that log(pH ２ O / pH ２ ) is -1.5 to -1.0. When forming the Al barrier layer such that a part of the Al barrier layer enters the plating main layer side, log(pH ２ O / pH ２It is preferable that the temperature is greater than -1.0 and less than or equal to -0.5. Here, the Ac1 temperature (°C) is the temperature determined by the following formula. The element symbols in the formula represent the mass percentage content of the element represented by the element symbol in the steel plate. Ac1 = 723 + 29.1 × Si - 10.7 × Mn + 16.9 × Cr + 16.9 × Ni + 6.38 × W + 290 × As

[0075] After heat treatment, the steel plate is cooled to a predetermined temperature and immersed in a plating bath (molten zinc plating bath). Next, it is removed from the plating bath and the amount of coating is adjusted by gas wiping. This forms a plating layer of a predetermined amount. After heat treatment and before immersion in the plating bath, it is preferable to cool the steel plate so that the average cooling rate in the temperature range of 550 to 700°C is 10 to 100°C / second. Also, if the difference between the steel plate temperature and the plating bath temperature is too large when immersing in the plating bath, the plating bath temperature may change, which may disrupt operations. Therefore, it is preferable that the steel plate temperature at the time of immersion (cooling stop temperature) be between the plating bath temperature - 20°C and the plating bath temperature + 20°C. Molten zinc plating can be carried out according to conventional methods. For example, the plating bath temperature can be 440 to 480°C and the immersion time can be 5 seconds or less. In order to form an Al barrier layer, it is preferable that the plating bath contains 0.15 to 0.40 mass% Al. The plating bath may also contain other impurities such as Fe, Si, Mg, Mn, Cr, Ti, and Pb. The time from immersion in the plating bath to gas wiping should be 5.0 seconds or less, and the steel plate temperature after gas wiping should be 440°C or less. If the time before gas wiping exceeds 5.0 seconds, or if the steel plate temperature after gas wiping exceeds 440°C, the Al-enriched layer will begin to break down, and a sufficient Al barrier layer will not be formed. The steel plate temperature after gas wiping may be 300°C or higher. The lower limit of the time before gas wiping is determined by the equipment configuration, but it is difficult to reduce it to less than 0.1 seconds in a typical hot-dip galvanizing line.

[0076] After the plating process, the product is cooled to room temperature to become the final product. The cooling conditions are not limited. In addition, after the plating process, temper rolling may be performed to straighten the steel sheet (adjust flatness) and adjust the surface roughness. In this case, it is preferable to keep the elongation rate at 2.0% or less to avoid deterioration of ductility.

[0077] The following describes some examples. The conditions in the examples are just one example of conditions adopted to confirm the feasibility and effectiveness of the Disclosure, and the Disclosure is not limited to this one example of conditions. The Disclosure may adopt various conditions as long as they do not depart from the gist of the Disclosure and achieve the objectives of the Disclosure.

[0078] The molten steel analysis results were obtained by heating a slab having the chemical composition shown in Table 1 to 1240°C, performing rough rolling with a completion temperature of 1090°C and a total reduction ratio of 70%, performing finish rolling with an entry temperature of 1030°C, an exit temperature of 930°C and a total reduction ratio of 85%, and winding at a winding temperature of 580°C to obtain a hot-rolled steel sheet. To this hot-rolled steel sheet, 3.0 mol / L HCl and 1.5 mol / L Fe were added. ２＋ and 0.02 mol / L Fe ３＋ The hot-rolled steel sheet was passed through an aqueous solution (pickling solution) containing the specified substance at a temperature of 80°C at an average speed of 10 m / min or more, and the pickling treatment was performed so that the immersion time in the aqueous solution was 25 seconds. After pickling, the hot-rolled steel sheet was cold-rolled with a reduction ratio of 45 to 60% to obtain a steel sheet (cold-rolled steel sheet) with a thickness of 1.8 to 2.4 mm.

[0079] After heat treatment, the obtained steel sheet was immersed in a hot-dip galvanizing bath to form a plating layer (hot-dip galvanized layer) on its surface. In the heat treatment, the maximum heating temperature was set to 910°C, which is higher than Ac1 + 30°C, and the holding time at the maximum heating temperature was set to 180 seconds. In each example of the heat treatment, the oxygen potential (log(pH)) of the atmosphere during the heating and soaking processes was set. ２ 0 / pH ２The values ​​were changed to match those shown in Table 2. After heat treatment, the steel sheet was cooled to a temperature of -20°C to +20°C, so that the average cooling rate in the temperature range of 550 to 700°C was 40°C / second, and the steel sheet was immersed in a 460°C plating bath for 2 seconds. After removing the sheet from the plating bath, the time until gas wiping was performed was 2.0 seconds, and the steel sheet temperature after gas wiping was set to 400°C. After that, it was cooled to room temperature to obtain a hot-dip galvanized steel sheet.

[0080] The amount of plating layer adhering to the obtained hot-dip galvanized steel sheet was measured. Specifically, a sample containing plating layer 3 was first taken from the obtained hot-dip galvanized steel sheet. Plating layer 3 was dissolved using an aqueous solution with an inhibitor concentration of 0.05 mass% (manufactured by Asahi Chemical Industry Co., Ltd., product name: Ibit 710K) and a hydrochloric acid concentration of 5 mass%. The completion of dissolution of plating layer 3 was determined from the way foaming occurred during the dissolution of plating layer 3. The amount of plating adhesion Wt was determined based on the sample mass before dissolution, the sample mass after dissolution, and the area where plating layer 3 was formed. In addition, the content of each element in the plating layer was determined by performing ICP analysis on the acid solution using ICPS-8100 manufactured by Tsu Seisakusho Co., Ltd., and the amount of Al adhering Wa and Fe adhering Wf in the plating layer were determined. The results are shown in Tables 3 to 5. In the plating layer, the remainder of the chemical composition shown in the table was impurities.

[0081] Furthermore, the obtained hot-dip galvanized steel sheet was analyzed using GD-OES in the same manner as described above, from the surface of the plating layer in the thickness direction of the steel sheet toward the steel sheet. The measurement time tB was determined as the time in units of seconds from the start of analysis until the maximum value of the emission intensity of B was detected, tAl was determined as the time in units of seconds from the start of analysis until the maximum value of the emission intensity of Al was detected, and tMn was determined as the time in units of seconds from the start of analysis until the maximum value of the emission intensity of Mn was detected. GD-OES was performed using a GD-Profiler2 manufactured by Horiba, Ltd., with the discharge power set to 35W, Ar pressure to 600Pa, and discharge diameter to 4mmφ. The measurement time was set to 300 seconds, and the measurement interval was set to 0.5 seconds. The measurement time at which the maximum value of the emission intensity of each element was detected was determined to be from 2.0 seconds after the start of measurement. The results are shown in Table 6.

[0082] Furthermore, using the procedure described above, SEM (scanning electron microscope) observation and EPMA analysis were performed to determine whether an Al barrier layer was present, and if so, whether it was continuous or discontinuous (not continuous). Also, using the procedure described above, SEM (scanning electron microscope) observation and EPMA analysis were performed to determine whether a portion of the Al barrier layer had penetrated into the main plating layer. The results are shown in Table 6.

[0083] (Plating Adhesion) The plating adhesion of the obtained hot-dip galvanized steel sheets was evaluated using a ball impact test. Specifically, a ball-tipped punch was dropped from a height corresponding to the thickness of the hot-dip galvanized steel sheet to be evaluated to create a recess in the steel sheet. After attaching tape to the convex side opposite the point where the punch was dropped and then peeling it off, the degree of plating peeling at that location was observed to evaluate the adhesion of the plating layer. The heights corresponding to the thickness of the hot-dip galvanized steel sheet were 350 mm for thicknesses less than 0.8 mm, 400 mm for thicknesses between 0.8 mm and 1.0 mm, 500 mm for thicknesses between 1.0 mm and 1.2 mm, 600 mm for thicknesses between 1.2 mm and 1.5 mm, 800 mm for thicknesses between 1.5 mm and 1.8 mm, and 950 mm for thicknesses of 1.8 mm or more. The punch used for the test weighed 25 kg, had a spherical tip with a diameter of φ = 12.7 mm, and a die hole diameter of 20 mm. Plating adhesion was evaluated by the area percentage of the plating layer attached (peeled off) to the tape. An area percentage of less than 1% was evaluated as Excellent, indicating very good plating adhesion. An area percentage of 1% or more but less than 5% was evaluated as Good, indicating good plating adhesion. An area percentage of 5% or more was evaluated as Not Good, indicating a problem with plating adhesion. The results are shown in Table 6.

[0084]

[0085]

[0086]

[0087]

[0088]

[0089]

[0090] As can be understood from these results, in a hot-dip galvanized steel sheet having a chemical composition within the scope of the present invention including B, on which a plating layer is formed with an adhesion amount within the scope of the present invention, Ia is 150 mg / m² ２The above conditions were met, and at least one of tB-tAl≧10.0 and tB / tAl≧1.50 was satisfied, indicating excellent plating adhesion. Figure 2 shows an example of results from EPMA analysis, etc.

[0091] Furthermore, Figures 3 and 4 show examples of the results of analysis using GD-OES. Figure 3 shows the analysis results for the inventive example, and Figure 4 shows the analysis results for the comparative example; it can be seen that there are differences in the distribution of elements.

[0092] Furthermore, when the plating adhesion receives an Excellent rating, it can be seen that in the GD-OES analysis, the measurement time at which the maximum value of the emission intensity of B is detected from the start of measurement is greater than or equal to the measurement time at which the maximum value of the emission intensity of Mn is detected (tB / tMn ≥ 1.00).

[0093] According to the present invention, it is possible to provide a hot-dip galvanized steel sheet that exhibits excellent plating adhesion even when the steel sheet contains boron. Therefore, it has high potential for industrial use.

[0094] 1. Hot-dip galvanized steel sheet 2. Steel sheet 3. Plating layer 4. Main plating layer 5. Al barrier layer

Claims

1. A steel plate and a plating layer present on the surface of the steel plate, wherein the chemical composition of the steel plate is, in mass%, C: 0.050 to 0.500%, Si: 0.010 to 2.000%, Mn: 0.10 to 2.50%, P: 0.030% or less, S: 0.010% or less, Al: 0 to 1.00%, N: 0 to 0.0100%, B: 0.0005 to 0.0050%, Ti: 0 to 0.20%, Cr: 0 to 1.50%, Mo: 0 to 1.00%, Nb: 0 to 0.10%, Ni: 0 to 1.00%, V: 0 to 0.20%, Cu: 0 to 1.00%, W: 0 to 0.100%, The composition is REM: 0-0.100%, Ca: 0-0.100%, Sb: 0-0.050%, Sn: 0-0.050%, As: 0-0.050%, and the remainder is Fe and impurities, the Zn content of the plating layer is 90.00% or more by mass%, and the plating adhesion amount Wt per side of the plating layer is 30.0-120.0 g / m². ２ The amount of Fe deposited on one side of the plating layer, Wf, is 0.40 to 3.00 g / m². ２ The Al barrier index Ia, expressed by the following formula (1) using Wa, Wt, and Wf, which are the amounts of Al deposited on one side of the plating layer, is 150 mg / m². ２ The hot-dip galvanized steel sheet is characterized in that, when glow discharge emission spectroscopy is performed from the surface of the plating layer in the thickness direction of the steel sheet, the time in units of seconds from the start of the analysis until the maximum value of the emission intensity of B is detected, tB, and the time in units of seconds from the start of the analysis until the maximum value of the emission intensity of Al is detected, satisfy at least one of the following equations (2) and (3). Ia = Wa - (Wt - Wf) × 0.002 (1) tB - tAl ≥ 10.0 (2) tB / tAl ≥ 1.50 (3) 2. The hot-dip galvanized steel sheet according to claim 1, characterized in that it satisfies formula (2) above.

3. The hot-dip galvanized steel sheet according to claim 1, characterized in that it satisfies formula (3) above.

4. The hot-dip galvanized steel sheet according to claim 1, characterized in that, in the glow discharge emission spectroscopy analysis, when tMn is the time in units of seconds from the start of the analysis until the maximum value of the emission intensity of Mn is detected, tB and tMn satisfy the following equation (4). tB ≥ tMn (4) 5. The chemical composition of the plating layer is, in mass%, Al: 0.10-1.00%, Fe: 0.33-5.00%, Mg: 0-0.500%, Si: 0-0.500%, Ni: 0-0.500%, Ca: 0-2.000%, Sb: 0-0.500%, Pb: 0-0.500%, Cu: 0-0.500%, Sn: 0-0.500%, Ti: 0-0.500%, Cr: 0-0.500%, Nb: 0-0.500%, Zr: 0-0.500%, Mn: 0-0.500%, Mo: 0-0.500%, Ag: 0-0.500%. A hot-dip galvanized steel sheet according to any one of claims 1 to 4, characterized in that the composition is Li: 0 to 0.500%, La: 0 to 0.500%, Ce: 0 to 0.500%, B: 0 to 0.004%, Y: 0 to 0.500%, P: 0 to 0.500%, Sr: 0 to 0.500%, and the remainder is Zn and impurities, and the total content of Mg, Si, Ni, Ca, Sb, Pb, Cu, Sn, Ti, Cr, Nb, Zr, Mn, Mo, Ag, Li, La, Ce, B, Y, P, and Sr is 5.000% or less by mass.

6. The hot-dip galvanized steel sheet according to any one of claims 1 to 5, wherein the plating layer comprises a main plating layer and an Al barrier layer formed in at least a portion of the area between the main plating layer and the steel sheet, and the Al content is 1.50% by mass or more, and the Al barrier layer is formed continuously.

7. The hot-dip galvanized steel sheet according to any one of claims 1 to 5, characterized in that the plating layer comprises a main plating layer and an Al barrier layer formed in at least a portion between the main plating layer and the steel sheet, the region having an Al content of 1.50% by mass or more, and a portion of the Al barrier layer penetrates into the main plating layer.

8. The chemical composition of the steel sheet contains, by mass%, one or more elements selected from the group consisting of: Ti: 0.01 to 0.20%, Cr: 0.01 to 1.50%, Mo: 0.01 to 1.00%, Nb: 0.01 to 0.10%, Ni: 0.01 to 1.00%, V: 0.01 to 0.20%, Cu: 0.01 to 1.00%, W: 0.001 to 0.100%, REM: 0.001 to 0.100%, Ca: 0.001 to 0.100%, Sb: 0.001 to 0.050%, Sn: 0.001 to 0.050%, As: 0.001 to 0.050%. A hot-dip galvanized steel sheet according to any one of claims 1 to 7, characterized in that...

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