Hot-dip galvanized steel sheet

Abstract

This hot-dip galvanized steel sheet comprises a steel sheet that has a prescribed chemical composition, and a plating layer that is present on the surface of the steel sheet and has a Zn content of 90.00% or more in terms of mass%. Wt, which is the plating adhesion weight per surface of the plating layer, is 30.0-120.0 g / m2. Wf, which is the Fe adhesion weight per surface of the plating layer, is 0.40-3.00 g / m2. Ia, which is the Al barrier index expressed by Ia = Wa-(Wt-Wf)×0.002, is 150 mg / m2 or more. In a cross section in the sheet thickness direction of the steel sheet including the steel sheet and the plating layer, if a line connecting points at which a Zn content of 90 mass% is obtained through EPMA analysis is defined as a 90% line, and a line connecting points at which a Zn content of 40 mass% is obtained is defined as a 40% line, then the number density d of regions in which the interval between the 90% line and the 40% line in the sheet thickness direction is 2.0 μm or more is 20 / mm or more.

Description

Hot-dip galvanized steel sheet

[0001] The present invention relates to a hot-dip galvanized steel sheet. This application claims priority based on Japanese Patent Application No. 2024-225949 filed in Japan on December 23, 2024, and incorporates its content herein by reference.

[0002] A hot-dip galvanized steel sheet is a steel sheet having a zinc plating layer on the surface of the steel sheet, obtained by immersing a steel sheet (plating base plate) in a hot-dip zinc plating bath. Since hot-dip galvanized steel sheets are excellent in corrosion resistance, they are used in a wide range of applications such as home appliances, building materials, and automobiles. When hot-dip galvanized steel sheets are used in these applications, they are processed into shapes suitable for each application and then used. Therefore, it is important that the plating does not peel off during processing, and high plating adhesion (adhesion between the plating layer and the steel sheet) is required for hot-dip galvanized steel sheets. On the other hand, when a high-strength steel sheet (high-tensile material) is used as the plating base plate, it is known that the plating adhesion tends to decrease. The reason for this is that Si (silicon), Mn (manganese), etc. added to improve the mechanical properties of high-strength steel sheets are easily oxidizable elements, so these elements are selectively oxidized on the surface layer of the steel sheet in the annealing process, and oxides such as Si and Mn are present in the outermost layer of the steel sheet, which is said to reduce the wettability (plating wettability) between the steel sheet and molten zinc. Also, B (boron, which may also be referred to as boron hereinafter) is an element that can greatly contribute to high strength by increasing the hardenability of the steel sheet even when contained in trace amounts. However, when a steel sheet containing boron is used as the plating base plate, the plating adhesion tends to decrease. Such problems can be avoided by not containing Si, etc. However, since steel sheets used in automobiles, etc. are required to have strength, it is not easy to avoid containing Si, Mn, and B. In response to such problems, technologies for improving the plating adhesion of hot-dip galvanized steel sheets containing Si have been developed (see Patent Document 1).

[0003] Japanese Patent Application Laid-Open 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 Fe deposition amount per 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 amount of Al deposited on one side of the plating layer, is 150 mg / m². ２In addition, in a cross-section of the steel sheet in the thickness direction, including the steel sheet and the plating layer, when the line connecting points where the Zn content obtained by EPMA analysis is 90% by mass is defined as the 90% line, and the line connecting points where the Zn content is 40% by mass is defined as the 40% line, the number density d of the region where the distance between the 90% line and the 40% line in the thickness direction is 2.0 μm or more is 20 particles / mm or more. Ia = Wa - (Wt - Wf) × 0.002 (1) [2] In the hot-dip galvanized steel sheet described in [1] above, d may be 50 particles / mm or more. [3] In the hot-dip galvanized steel sheet described in [1] or [2] above, the plating layer has a main plating layer and an Al barrier layer formed in at least a part 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 may be formed continuously. [4] In the hot-dip galvanized steel sheet described in [1] or [2] 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, the Al content of which is 1.50% by mass or more, and a portion of the Al barrier layer may penetrate into the main plating layer. [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: 0-0.100%, 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 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 No. 3 and No. 13 in the table.

[0008] The following describes embodiments for carrying out the invention. As shown in Figure 1, a hot-dip galvanized steel sheet 1 according to one embodiment of the present invention (hot-dip galvanized steel sheet according to this embodiment) has a plating layer 3 on the surface of a steel sheet 2.

[0009] The plating layer 3 applied to the surface of this steel plate 2 has a plating adhesion amount Wt of 30.0 to 120.0 g / m² per side. ２ The amount of Fe deposited on one side of the plating layer, Wf, is 0.40 to 3.00 g / m². ２ Furthermore, this hot-dip galvanized steel sheet 1 has an Al barrier index Ia expressed by the following formula (1) using the amount of Al deposited on one side of the plating layer Wa, the amount of plating deposited Wt, and the amount of Fe deposited Wf, which is 150 mg / m². ２That is all. Ia = Wa - (Wt - Wf) × 0.002 (1) Furthermore, in the cross-section of the hot-dip galvanized steel sheet including the steel sheet 2 and the plating layer 3 in the thickness direction, when the line connecting the points where the Zn content obtained by EPMA analysis is 90% by mass is defined as the 90% line, and the line connecting the points where the Zn content is 40% by mass is defined as the 40% line, the number density d of the region where the distance between the 90% line and the 40% line in the thickness direction is 2.0 μm or more is 20 particles / mm or more. EPMA analysis is an analysis using an electron probe microanalyzer (EPMA), and details will be described later.

[0010] Here, we will explain the basic aspects of the hot-dip galvanized steel sheet 1. Hot-dip galvanized steel sheets are manufactured in a continuous hot-dip galvanizing line, also known as a CGL (Continuous Galvanizing Line). The CGL is important for ensuring plating adhesion, and the process of preparing the steel sheets to pass through the CGL is equally important. In particular, for steel sheets containing easily oxidizable elements, which are the subject of this study, it may be possible to improve plating adhesion by controlling the formation of scale and oxides on the surface of the steel sheet during the hot-rolling process, and by controlling the scale removal and the progression of acid erosion on the surface of the steel sheet during the pickling process. Below, we will describe the hot-dip galvanized steel sheet according to this embodiment in more detail. In this specification, "~" indicating a numerical range means that the numbers written before and after it are included as the lower and upper limits, unless otherwise specified. However, if "greater than" or "less than" is attached to a number, that number is not included.

[0011] (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.

[0012] 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.

[0013] 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.

[0014] 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.

[0015] 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.

[0016] 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.

[0017] 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.

[0018] 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.

[0019] 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.

[0020] 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%.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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, may be measured by a general analysis method. For example, the chemical composition of the steel sheet may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) for the chips. Specifically, for example, (if necessary, after removing the plating layer on the surface by mechanical grinding), a test piece with a size of 35 mm square is obtained from the vicinity of the position at 1 / 4 of the plate thickness (1 / 4 thickness position) from the surface of the steel sheet 2, and it can be specified by measuring under the conditions based on the calibration curve prepared in advance using an ICPS-8100 or the like (measurement device) manufactured by Shimadzu Corporation. For C and S for which ICP-AES is difficult, the combustion-infrared absorption method may be used, for N, the inert gas fusion-thermal conductivity method may be used, and for O, the inert gas fusion-non-dispersive infrared absorption method may be used. When the molten steel analysis value, slab analysis value, or steel sheet analysis value of other steel sheets manufactured from the same molten steel can be confirmed, the analysis of the test piece sampled from the steel sheet may be omitted, and those analysis values may be regarded as the chemical composition of the steel sheet.

[0035] (Plating layer) <Adhesion amount> The plating layer 3 is usually applied to the entire surface of the steel sheet 2 in the same manner, but the plating adhesion amount Wt per side is set to 30.0 to 120.0 g / m ２ . If the adhesion amount Wt is less than 30.0 g / m ２ , there may be a problem with corrosion resistance, and if it exceeds 120.0 g / m ２ , it is uneconomical. From the perspective of corrosion resistance, the plating adhesion amount Wt per side may be 35.0 g / m ２ or more, 40.0 g / m ２ or more, 45.0 g / m ２ or more, 50.0 g / m ２ or more, or 55.0 g / m ２ or more. From the perspective of economy, the plating adhesion amount Wt per side is 110.0 g / m ２ or less, 105.0 g / m ２ or less, 100.0 g / m ２ or less, 95.0 g / m ２ or less, 90.0 g / m ２ or less, 85.0 g / m２ The following, or 75.0 g / m² ２ The following is also acceptable.

[0036] 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.

[0037] 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')

[0038] <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.

[0039] 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.

[0040] 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.

[0041] 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: 0-0.100%, 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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 0.200% or more. On the other hand, if the Ca content is excessive, 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.

[0046] 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: 0-0.100% 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 each, 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 0.100% or less, and may be, for example, 0.080% or less, 0.050% or less, 0.010% or less, 0.004% or less, or less than 0.002%. The B content may also be 0.0001% or more, 0.0002% or more, or 0.0003% or more. Furthermore, the B content of the plating layer 3 may be less than the B content of the steel plate 2.

[0047] 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 process, including raw materials and various other factors in the manufacturing process. The Zn content is 90.00% or more, and may be 93.00% or more, 95.00% or more, 96.00% or more, 97.00% or more, 98.00% or more, 99.00% or more, 99.80% or more, or 99.84% or more, as needed.

[0048] 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.

[0049] <Number Density of the Penetration Area into the Steel Plate> As a result of the inventors' investigation, it was found that if the plating layer 3 of the hot-dip galvanized steel plate 1 is properly penetrated into the steel plate 2, the plating layer will not peel off easily (the adhesion of the plating layer will be high), even if the steel plate 2 contains B (boron). Specifically, when EPMA analysis was performed on a cross-section in the thickness direction of the hot-dip galvanized steel plate 1, including the steel plate 2 and the plating layer 3, and the line connecting the points where the Zn content obtained by EPMA analysis is 90% by mass was defined as the 90% line, and the line connecting the points where the Zn content is 40% by mass was defined as the 40% line, it was found that the adhesion of the plating layer improves when the number density d of the area where the distance in the thickness direction between the 90% line and the 40% line is 2.0 μm or more is 20 pieces / mm or more. In other words, it was found that the degree of penetration of the plating layer 3 into the steel plate 2 can be evaluated using the above number density d as an indicator, and by setting this indicator to a range above a predetermined level, the adhesion of the plating layer improves. The number density d is preferably 30 pieces / mm or more, 40 pieces / mm or more, or 50 pieces / mm or more. There is no upper limit, but it may be 200 pieces / mm or less, 150 pieces / mm or less, or 100 pieces / mm or less.

[0050] The number density of the region in which the plating layer 3 penetrates the steel sheet 2 (number density d of the region where the distance between the 90% line and the 40% line in the thickness direction is 2.0 μm or more) can be confirmed using cross-sectional EPMA analysis. EPMA analysis is an analysis using an electron probe microanalyzer (EPMA), which is a method for obtaining information on element 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 penetration of the plating layer 3 into the steel sheet 2, 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 embedded in resin and mechanically polished. More specifically, a sample is taken from the hot-dip galvanized steel sheet 1, the sample is cut so that the cross section perpendicular to the rolling direction and parallel to the thickness direction (C section) is the observation section, and the sample is embedded in epoxy resin. Mechanical polishing is performed on the observation cross-section of the sample embedded in 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 (one field of view) encompassing the entire thickness direction of the plating layer and the steel plate. Specifically, EPMA analysis is performed on a 90 μm area of ​​the interface region between the plating layer 3 and the steel plate 2. The EPMA analysis is performed using, for example, 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. －８A. The measurement is performed under conditions such as a measurement time of 50 ms / point, a measurement area of ​​30 μm × 30 μm, and 200 measurement points in the thickness direction and 200 measurement 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, SEM (scanning electron microscope) observation is performed, and backscattered electrons, which can be observed with different contrasts depending on the atomic weight of the observed object, can be obtained to determine the area that includes the steel sheet 2 and the plating layer 3. SEM observation is performed using, for example, a JEOL JSM-7200F, with an observation magnification of 3000x, and conditions can be set to observe an area with a width of 30 μm or more. The results obtained from the measurement are output as a mapping image (contour map). From the mapping image, a line is drawn connecting the points where the Zn content is 90 mass%, and this is designated as the 90% line. Furthermore, a line is drawn connecting the points where the Zn content is 40% by mass, and this is designated as the 40% line. The number of regions in the entire field of view where the distance between the 90% line and the 40% line in the thickness direction is 2.0 μm or more is determined. This is done for three consecutive fields of view perpendicular to the thickness direction (i.e., a total area of ​​30 × 90 μm), and the number density (regions / mm) is calculated by dividing the number of regions where the distance between the 90% line and the 40% line in the thickness direction is 2.0 μm or more by the length of the target (90 μm).

[0051] To allow the plating layer 3 to penetrate the steel sheet 2, it is effective to increase the coiling temperature after hot rolling in the hot rolling process to increase the amount of internal oxides formed on the surface of the steel sheet 2, and to shorten the pickling time in the pickling process to leave an appropriate amount of area where the internal oxides on the surface of the steel sheet 2 formed in the hot rolling process have been acid-eroded.

[0052] <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.

[0053] 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.

[0054] 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.

[0055] Whether the Al barrier layer 5 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.

[0056] 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.

[0057] 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.

[0058] (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.

[0059] (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.

[0060] (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.

[0061] (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.

[0062] [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.

[0063] [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.

[0064] [Winding] After the completion of finish rolling, 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, internal oxidation of the hot-rolled steel sheet is suppressed, resulting in a decrease in the number density of particles in the region where the gap between the 90% line and the 40% line in the thickness direction is 2.0 μm or more. In addition, 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, internal oxidation is excessively promoted, which can lead to excessive alloying of the plating or concentration of alloying elements such as Mn in the cementite. This can delay the dissolution of cementite in the final annealing process, potentially causing a decrease in strength.

[0065] (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 process involves passing the sheet through an aqueous solution (pickling solution) containing 0.05 to 0.20% by mass of a pickling accelerator for stainless steel (specifically, Sunspeed PA-8 manufactured by Sugimura Chemical Co., Ltd.) at a temperature of 70 to 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 and less than 30 seconds. In this pickling process, by adding the pickling accelerator PA-8 for stainless steel to a hydrochloric acid-based pickling solution and controlling the pickling time to a short duration, a small amount of the internal oxide layer formed by hot rolling is left, and the reaction at the interface between the plating layer 3 and the steel sheet 2 is appropriately controlled. The HCl concentration in the pickling solution is less than 1.0 mol / L, or Fe ２＋If the concentration is 3.0 mol / L or higher, the temperature of the pickling solution is below 70°C, the average speed of the hot-rolled steel sheet is below 10 m / min, or the immersion time (pickling time) is 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 pickling time is 30 seconds or more, there is a concern that the internal oxide layer will be removed. On the other hand, if the HCl concentration in the pickling solution is above 5.0 mol / L, or the Fe3+ concentration in the pickling solution is above 0.10 mol / L, there is a concern that the pickling speed will increase and the variation in the pickling state due to concentration fluctuations will increase. When forming the Al barrier layer 5 continuously, it is preferable to set the pickling time to more than 20 seconds and 30 seconds or less, and when forming the Al barrier layer 5 so that a part of the Al barrier layer 5 penetrates into the main plating layer 4 side, it is preferable to set the pickling time to 15 to 20 seconds.

[0066] (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.

[0067] (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.

[0068] [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).

[0069] [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 ２The pH 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 plating defects caused by external oxidation of Si, Mn, Al, etc. can be suppressed. 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, and Al 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 deteriorate due to oxidation of Fe, depending on the heating temperature and annealing atmosphere.

[0070] [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 sufficiently austenitize the structure of the steel sheet, the steel sheet is heated to at least Ac1 + 30°C and soaked at that temperature (maximum heating temperature). If austenitization is insufficient, a large amount of ferrite may be formed in the final structure. If the maximum heating temperature is less than Ac1 + 30°C, or if 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 lead to deterioration of toughness due to the coarsening of the austenite grain size, but also to damage to the annealing equipment. For this reason, the maximum heating temperature should be 950°C or less, preferably 910°C or less. Furthermore, since excessively long holding times hinder productivity, it is preferable to keep the holding time below 1000 seconds, and more preferably below 600 seconds. During soaking, it is not necessary to keep the cold-rolled steel sheet at a constant temperature; it may fluctuate within a range that satisfies the above conditions. Here, the Ac1 temperature (°C) is the temperature calculated 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 sheet. Ac1 = 723 + 29.1 × Si - 10.7 × Mn + 16.9 × Cr + 16.9 × Ni + 6.38 × W + 290 × As

[0071] 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.

[0072] 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.

[0073] 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.

[0074] 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 at a completion temperature of 1090°C with a total reduction ratio of 70%, performing finish rolling at an entry temperature of 1030°C, an exit temperature of 930°C with a total reduction ratio of 85%, and winding at the winding temperature shown in Table 2 to obtain a hot-rolled steel sheet. This hot-rolled steel sheet was treated with 3.0 mol / L HCl and 1.5 mol / L Fe ２＋ and 0.02 mol / L Fe ３＋ The hot-rolled steel sheet was then cold-rolled at a reduction ratio of 45-60% to obtain a steel sheet with a thickness of 1.8-2.4 mm. The steel sheet was moved through this solution at an average speed of 20 m / min or more, and the immersion time in the solution was 16-44 seconds.

[0075] 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 process was set. ２ 0 / pH ２ The ratio was set to 0.2. 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. The time from removing the steel sheet from the plating bath until gas wiping was performed was 2.0 seconds, and the temperature of the steel sheet after gas wiping was set to 400°C. After that, it was cooled to room temperature to obtain a hot-dip galvanized steel sheet.

[0076] 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.

[0077] Furthermore, as an indicator of the degree of penetration of the plating layer into the steel sheet, the number density d of the region in the obtained hot-dip galvanized steel sheet where the distance between the 90% line and the 40% line in the thickness direction is 2.0 μm or more was determined using the method described above. The results are shown in Table 6.

[0078] 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.

[0079] (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.

[0080]

[0081]

[0082]

[0083]

[0084]

[0085]

[0086] As can be seen from the results in Tables 1 to 6, in a hot-dip galvanized steel sheet having a chemical composition within the range of the present invention including B, on which a plating layer is formed with an adhesion amount within the range of the present invention, Ia is 150 mg / m² ２In summary, when the number density d in the region where the gap between the 90% line and the 40% line in the plate thickness direction is 2.0 μm or more is 20 particles / mm or more, the plating adhesion was excellent.

[0087] 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.

[0088] 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: 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 Fe deposition amount per 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 amount 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, in a cross-section of the steel sheet in the thickness direction, including the steel sheet and the plating layer, when the line connecting points where the Zn content obtained by EPMA analysis is 90% by mass is defined as the 90% line and the line connecting points where the Zn content is 40% by mass is defined as the 40% line, the number density d of the region where the distance between the 90% line and the 40% line in the thickness direction is 2.0 μm or more is 20 particles / mm or more. Ia = Wa - (Wt - Wf) × 0.002 (1) 2. The hot-dip galvanized steel sheet according to claim 1, characterized in that the number of d is 50 or more per mm.

3. The hot-dip galvanized steel sheet according to claim 1 or 2, 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 the Al barrier layer is formed continuously.

4. The hot-dip galvanized steel sheet according to claim 1 or 2, 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 Al content being 1.50% by mass or more, and a portion of the Al barrier layer extends into the main plating layer.

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: 0-0.500%, La: 0-0.500%, Ce: 0-0.500%, B: 0-0.100%, Y: 0-0.500%, P: 0-0.500%, Sr: 0-0.500%, 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 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 5, characterized in that...

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