Hot-dip galvanized steel sheet

Abstract

This hot-dip galvanized steel sheet comprises a steel sheet having a predetermined chemical composition and a plating layer on the surface of the steel sheet having a Zn content of 90.00% or more by mass, and the amount of plating deposited on one side of the plating layer, Wt, is 30.0 to 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 Therefore, the Al barrier index Ia, expressed as Ia = Wa - (Wt - Wf) × 0.002, is 150 mg / m². 2 In addition, in a cross-section of the steel plate in the thickness direction, including the steel plate 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.

Description

Technical Field

[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.

Background Art

[0002] A hot-dip galvanized steel sheet is a steel sheet having a zinc plating layer on the surface of the steel sheet, which is obtained by immersing the steel sheet (plating base plate) in a hot-dip zinc plating bath. Since the hot-dip galvanized steel sheet has excellent corrosion resistance, it is used in a wide range of applications such as home appliances, building materials, and automobiles. When the hot-dip galvanized steel sheet is used in these applications, it is processed into a shape 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 the hot-dip galvanized steel sheet. 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 the high-strength steel sheet 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 lower the wettability (plating wettability) between the steel sheet and molten zinc. In addition, 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 a trace amount. 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 or the like, but since the steel sheets used in automobiles and the like are required to have strength, it is not easy to avoid containing Si, Mn, or 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).

Prior Art Documents

Patent Document

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

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

Means for Solving the Problems

[0005] The gist of the present invention for solving the above problems is as follows. [1] The hot-dip galvanized steel sheet according to one aspect of the present invention has a steel sheet and a plating layer present on the surface of the steel sheet, and 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%, 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%, REM: 0 to 0.100%, Ca: 0 to 0.100%, Sb: 0 to 0.050%, Sn: 0 to 0.050%, As: 0 to 0.050%, and the balance: Fe and impurities. The Zn content of the plating layer is 90.00% or more in mass%, the Wt which is the plating adhesion amount per one side of the plating layer is 30.0 to 120.0 g / m 2 and the Wf which is the Fe adhesion amount per one side in the plating layer is 0.40 to 3.00 g / m 2The 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 In addition, in a cross-section of the steel plate in the thickness direction, including the steel plate 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, the number of d may be 50 or more per mm. [3] 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, wherein 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%. [Effects of the Invention]

[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). [Brief explanation of the drawing]

[0007] [Figure 1] This is a conceptual diagram of a hot-dip galvanized steel sheet. [Figure 2]The figure shows examples of the results obtained by performing EPMA analysis and SEM observation on items No. 3 and No. 13 in the table. [Modes for carrying out the invention]

[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 aspect of the present invention (hot-dip galvanized steel sheet according to this embodiment) has a plating layer 3 on the surface of the 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. 2 That's 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, when the line connecting points where the Zn content is 90% by mass, obtained by EPMA analysis, 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. 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 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 discussion, 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. The hot-dip galvanized 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 before and after it as the lower and upper limits. However, if "greater than" or "less than" is appended to a number, that number is not included.

[0011] (steel plate) <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 whose chemical composition contains B (boron). To distinguish it from the hot-dip galvanized steel sheet 1, the steel sheet 2 may be called a base steel sheet, base material steel sheet, or substrate steel sheet. Unless otherwise specified, percentages for elemental content refer to mass percentages.

[0012] C: 0.050~0.500% The carbon (C) content in the steel sheet should be 0.050 to 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: 0.010~2.000% The silicon (Si) content in the steel sheet should be between 0.010% and 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 between 0.10% and 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 set to 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 sulfur content in steel sheets should be 0.010% or less. The sulfur content may also be 0.008% or less, 0.006% or less, or 0.004% or less. A lower sulfur 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 sulfur 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), aluminum 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 for Al content is 0%. When used for deoxidation, the Al content may be 0.005% or higher, or even 0.01% or higher. On the other hand, Al is easily oxidized, and as its content increases, the number of inclusions increases, which can easily 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: 0~0.0100% Nitrogen (N) is an element that forms nitrides, thereby reducing the toughness of steel. Furthermore, when boron is present, N combines with boron to reduce the amount of boron in solid solution, thus decreasing the hardenability of the steel sheet. Therefore, the N content should be 0.0100% or less. The N content may also be 0.0080% or less, 0.0060% or less, or 0.0040% or less. A lower N content is preferable, and it may even be 0%. From the perspective of suppressing increases in manufacturing costs, the N content may be 0.0001% or more, or 0.0010% or more.

[0019] B: 0.0005~0.0050% Boron (B) is an element that contributes to improving the hardenability of steel and increasing the strength of steel sheets. Therefore, the B content should be 0.0005% or more. The B content may also 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 steel sheet surface during annealing heating, leading to a decrease in plating wettability. Therefore, the B content should be 0.0050% or less. The B content may also 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 consisting of 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 within the content ranges described below: 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 contributes to suppressing the reduction in hardenability caused by the formation of boron nitrides by inhibiting the bonding of boron and nitrogen. Therefore, 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 excessive precipitation of titanium nitrides after the above effect has saturated. Therefore, 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% Chromium (Cr) 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 Cr is 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, excessive Cr content can lead to the formation of large amounts of Cr carbides, which may conversely impair hardenability. Therefore, if Cr is 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% Molybdenum (Mo) 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 inhibiting grain growth, and dislocation strengthening by inhibiting 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 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 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 inhibiting grain growth, and dislocation strengthening by inhibiting recrystallization, and may be included as needed. This effect can be obtained even with trace amounts, but if 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 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% Copper (Cu) 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 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 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% Tungsten (W) may be included as needed, as it is effective in controlling the morphology of carbides and increasing the strength of steel. 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 (REMs) contribute to inclusion control, particularly the fine dispersion of inclusions, and enhance toughness; therefore, they may be included as needed. This effect can be obtained even with trace amounts, but if REMs are 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; therefore, if REMs are 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; therefore, 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; therefore, 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% Antimony (Sb) is an element that enhances the ductility of steel sheets by suppressing the formation of oxides that act as fracture initiators. 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 saturates even 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% Tin (Sn) is an element that enhances the ductility of steel sheets by suppressing the formation of oxides that act as fracture initiators. 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% As (arsenic) is an element that enhances toughness by reducing the austenite single-phase transformation temperature and refining the prior austenite grains. To more reliably obtain this effect, the As content is preferably 0.001% or more or 0.010% or more. On the other hand, even if a large amount of As is contained, the above effect saturates. Therefore, when containing As, the As content should be 0.050% or less. The As content is preferably 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 swarf. Specifically, for example, (after removing the plating layer on the surface by mechanical grinding if necessary), a test piece with a size of 35 mm square is obtained from near 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 etc. manufactured by Shimadzu Corporation (measurement device). 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-nondispersive infrared absorption method may be used for measurement. 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) <coating weight> The plating layer 3 is usually applied uniformly to the entire surface of the steel sheet 2, but the plating coating weight Wt per side is 30.0 to 120.0 g / m 2 shall be set. If the coating weight 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 coating weight Wt per side is 35.0 g / m2 More than 40.0g / m 2 More than 45.0g / m 2 More than 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 / m 2 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 deposited on one side of the plating layer 3, Wf, is 0.40 to 3.00 g / m². 2 The Fe deposition amount Wf is 0.40 g / m². 2 If the amount of Fe Wf is less than 3.00 g / m², there is a concern that the reactivity between the steel sheet 2 and the plating layer 3 will decrease. On the other hand, if the amount of Fe adhering Wf is 3.00 g / m², there is a concern that the reactivity between the steel sheet 2 and the plating layer 3 will decrease. 2 If this value is exceeded, there is a concern that corrosion resistance will decrease, such as the formation of red rust. From the perspective of the reactivity of the plating layer 3, the amount of Fe deposited on one side, Wf, should be 0.45 g / m². 2 More than 0.50g / m 2 More than 0.60g / m 2 More than 0.70g / m 2 Above, or 0.80 g / m² 2 The above may also be used. From the viewpoint of corrosion resistance and economic efficiency, the amount of Fe deposited on one side, Wf, should be 2.80 g / m². 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 (0.150g / m 2 That's all. Ia = Wa - (Wt - Wf) × 0.002 (1) The Al barrier index Ia, represented by formula (1) above, is an empirical formula obtained by the inventors from numerous test results. Al barrier index Ia = 150 mg / m² 2 The inventors have found that, under these 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 More than 170mg / m 2 More than 180mg / m 2 More than 190mg / m 2 Preferably, it should be 200 mg / m² or more. 2 It is more preferable that the above conditions are met. 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. To calculate Ia using equation (1), the units of Wa, Wt, and Wf must be consistent. For example, all units of Wa, Wt, and Wf must be in 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 2Multiplying the value of Ia by 1000 gives the unit mg / m 2 This is converted to the value of Ia. The units 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 Ia can be calculated. 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 elemental content 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 210-900 mg / m². 2This 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: 0.33~5.00% Fe is often inevitably included in the plating layer, for example, due to the alloying reaction between the base steel sheet and the plating layer that progresses during 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: 0~0.500% 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 in 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, the Si content is preferably 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: 0~0.500% 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 the Ni content is excessive, 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 a predetermined content range. However, excessive amounts of each element 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 0.100% 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%. Alternatively, the B content may 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 sheet 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 introduced during the manufacturing process, including raw materials, due to various factors in the manufacturing process. The Zn content is 90.00% or higher, and may be 93.00% or higher, 95.00% or higher, 96.00% or higher, 97.00% or higher, 98.00% or higher, 99.00% or higher, 99.80% or higher, or 99.84% or higher, 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. A sample of, for example, 50 mm x 50 mm 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 resulting acid solution is quantitatively analyzed for the content of each element in the acid solution using ICP (inductively coupled plasma) emission spectroscopy, thereby measuring the content (mass %) of each element in the plating layer. From these measurement results and the amount of plating deposited, 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 capable of dissolving 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 areas entering 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 sheet 1 is properly penetrated into the steel sheet 2, the plating layer will not peel off easily (the adhesion of the plating layer will be high), even if the steel sheet 2 contains B (boron). Specifically, EPMA analysis was performed on a cross-section of a hot-dip galvanized steel sheet 1, including a steel sheet 2 and a plating layer 3, in the thickness direction. When the line connecting points where the Zn content is 90% by mass, obtained by EPMA analysis, 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, it was found that the adhesion of the plating layer improves when the number density d in the region where the distance in the thickness direction between the 90% line and the 40% line is 2.0 μm or more is 20 particles / mm or more. In other words, the degree of penetration of the plating layer 3 into the steel sheet 2 can be evaluated using the above number density d as an indicator, and it was found that the adhesion of the plating layer improves by setting this indicator to a range above a predetermined level. 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 method that uses an electron probe microanalyzer (EPMA), in which a finely focused electron beam is irradiated onto a solid sample in a vacuum, and the resulting characteristic X-rays are analyzed to obtain insights into elemental identification and quantitative values. 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 then mechanically polished. More specifically, a sample is taken from a hot-dip galvanized steel sheet 1, and the sample is cut so that the cross section (C section) perpendicular to the rolling direction and parallel to the thickness direction becomes the observation cross section, and then embedded in epoxy resin. The observation cross section of the sample embedded in the resin is mechanically polished until it becomes mirror-like. After polishing, an arbitrary 30 μm × 30 μm area encompassing the entire thickness direction of the plating layer and the steel plate of the observed cross-section is designated as the observation area (1 field of view), and EPMA analysis is performed on three consecutive fields of view. In other words, EPMA analysis is performed on a 90 μm area of ​​the interface region between the plating layer 3 and the steel plate 2. EPMA analysis was performed using, for example, the 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 the following conditions: measurement time of 50 ms / point, measurement area of ​​30 μm × 30 μm, and number of measurement points of 200 in the thickness direction and 200 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 microscopy (SEM) observation is performed. By obtaining backscattered electrons, which exhibit different contrasts depending on the atomic weight of the observed object, the area containing the steel sheet 2 and the plating layer 3 can be determined. SEM observation can be 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 plot). From the mapping image, lines are drawn connecting points where the Zn content is 90% by mass, and these are designated as the 90% lines. Similarly, lines are drawn connecting points where the Zn content is 40% by mass, and these are designated as the 40% lines. 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] In order to make the plating layer 3 penetrate into 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, or to shorten the pickling time in the pickling process to appropriately leave the region where the internal oxides on the surface of the steel sheet 2 generated in the hot rolling process are acid-etched.

[0052] <Al barrier layer> The inventors of the present invention studied a method for further improving the adhesion of the plating layer 3 in the steel sheet 2 containing B (boron). As a result, it was found that if an Al barrier layer 5 with a high Al concentration (Al concentration of 1.50 mass% or more) is appropriately formed between the main plating layer 4 and the steel sheet 2, even if the steel sheet 2 contains boron, the plating layer is difficult to peel off (has high adhesion).

[0053] The Al barrier layer 5 can obtain its effect if it is formed at least partially between the main plating layer 4 and the steel sheet 2, but it is preferable that it is continuously formed (in a state where it can be detected by cross-sectional EPMA analysis that it is continuously formed). This continuity can be confirmed by using cross-sectional EPMA analysis as described later. EPMA analysis is an analysis using an Electron Probe Micro Analyzer (EPMA). It is a technique for obtaining knowledge about the identification and quantitative values of elements by irradiating a solid sample with an electron beam thinned in a vacuum and analyzing the characteristic X-rays generated. In order to analyze the formation state of the Al barrier layer 5, cross-sectional EPMA analysis can be performed by irradiating an electron beam on the cross-section of the hot-dip galvanized steel sheet 1 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, the inventors 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., 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 described later.

[0055] Whether the Al barrier layer 5 is formed continuously can be determined by the following procedure. A sample is taken from a hot-dip galvanized steel sheet 1, and the sample is cut so that, for example, the cross section perpendicular to the rolling direction and parallel to the thickness direction (C section) becomes the observation cross section, and then embedded in epoxy resin or the like. The observation cross section of the sample embedded in the resin is mechanically polished 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 encompassing the entire thickness of the plating layer 3 and the steel plate 2 of the observed cross-section. Specifically, EPMA analysis is performed on a 90 μm area of ​​the interface region between the plating layer 3 and the steel plate 2. EPMA analysis was performed using, for example, the 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 the following conditions: measurement time of 50 ms / point, measurement area of ​​30 μm × 30 μm, and number of measurement points of 200 in the thickness direction and 200 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 microscopy (SEM) observation is performed. By obtaining backscattered electrons, which exhibit different contrasts depending on the atomic weight of the observed object, the area containing the steel sheet 2 and the plating layer 3 can be determined. SEM observation can be 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.

[0056] Based on the EPMA analysis results, the region with a Zn content (concentration) of 10.00 mass% or more is identified as the plating layer, and the region with a Zn content of less than 10.00 mass% is identified as the steel plate. Within the plating layer, the region with an Al content (concentration) of 1.50 mass% or more is identified as the Al barrier layer 5, and the region with an Al content of less than 1.50 mass% is identified as the main plating layer 4. Although an Al barrier layer 5 is formed between the main plating layer and the steel sheet, if a region with an Al content of less than 1.50% (a region that is not the Al barrier layer 5) is observed continuously for 1.0 μm or more in a direction parallel to the interface (i.e., a direction perpendicular to the thickness direction), it is determined that the Al barrier layer 5 is not continuously formed (discontinuous). If no region that is not the Al barrier layer 5 is observed continuously 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 obtained from EPMA analysis and SEM observation. Although not apparent in the example shown in Figure 2, the content shown in Figure 2 can actually 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 Al content less than 1.50 mass% being displayed in light blue to dark blue, and the region of Al content 1.50 mass% or more being displayed in red to yellow-green.

[0057] Whether a portion of the Al barrier layer has penetrated into the main plating layer can be determined as follows. EPMA analysis is performed under the same conditions as for determining whether the Al barrier layer is continuously formed. The results obtained from the measurement are output as a mapping image (contour map). From the mapping image, regions with a Zn content 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, and the interface between the plating layer and the steel plate is identified. Furthermore, within the plating layer, regions with an Al content of 1.50 mass% or more are identified as the Al barrier layer, and regions with an Al content of less than 1.50 mass% are identified as the main plating layer, and the interface between the Al barrier layer and the steel plate is identified. In the thickness direction, if an Al barrier layer is present at a distance of 2.0 μm or more from the interface between the plating layer and the steel sheet on the side of the main plating layer, 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 suitable tensile strength (TS). While not particularly limited, the tensile strength may be, for example, 690 MPa or higher, 780 MPa or higher, 980 MPa or higher, 1080 MPa or higher, or 1180 MPa or higher. The upper limit is also not particularly limited, but may be, for example, 2300 MPa or lower, 2000 MPa or lower, 1800 MPa or lower, or 1500 MPa or lower. The tensile strength is measured, for example, by taking a No. 5 test specimen according to JIS Z 2241:2022 and performing a tensile test in accordance with JIS Z 2241:2022. It is preferable that the longitudinal direction of the test specimen be parallel to the direction perpendicular to the rolling direction of the hot-dip galvanized steel sheet 1.

[0059] (plate 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 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-described characteristics. The hot-dip galvanized steel sheet according to this embodiment is obtained by a manufacturing method that includes the following steps (A) to (D). (A) Hot rolling process (B) Pickling process (C) Cold rolling process (D) Plating process For each process, the preferred conditions will be described. For processes and conditions not described, publicly known conditions may be applied.

[0061] (A) Hot rolling process In the hot rolling process, a slab having the same chemical composition as a steel sheet is heated, and then subjected to rough rolling and finish rolling 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] A heated slab may be subjected to rough rolling before finish rolling. While the rough rolling conditions are not particularly limited, it is preferable to perform the rough rolling 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 microstructure of the hot-rolled steel sheet. The above total reduction ratio may be, for example, 90% or less.

[0063] [Finishing Rolling] After rough rolling, or after heating if rough rolling is not performed, finish rolling is carried out. The finish rolling conditions are not particularly limited, but it is preferable that the finish rolling is carried out within a range that satisfies the following conditions: the entry temperature of the finish rolling is 950 to 1100°C, the exit temperature of the finish rolling is 850 to 1000°C, and the total reduction ratio is 80 to 95%. If the entry temperature for finish rolling falls below 950°C, the exit temperature for finish rolling falls below 850°C, or the total reduction ratio exceeds 95%, the texture of the hot-rolled steel sheet develops, which may result in anisotropy becoming apparent in the final product sheet. On the other hand, if the entry temperature for finish rolling exceeds 1100°C, the exit temperature for finish rolling exceeds 1000°C, or the total reduction ratio falls below 80%, the grain size of the hot-rolled steel sheet may coarseen, leading to coarsening of the final product sheet structure.

[0064] [Rewind] 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 coiling temperature falls 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. This can also lead to excessive strength of the steel sheet, impairing its cold-rollability. On the other hand, if the coiling temperature exceeds 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 a concentration of 1.0 to 5.0 mol / L and Fe at a concentration of 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 to which 0.05-0.20% by mass of a pickling accelerator for stainless steel (specifically, Sunspeed PA-8 manufactured by Sugimura Chemical Co., Ltd.) has been added, 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, a pickling accelerator PA-8 for stainless steel is added to a hydrochloric acid-based pickling solution, and the pickling time is controlled to be relatively short. This leaves a small amount of the internal oxide layer formed during hot rolling, and appropriately controls the reaction at the interface between the plating layer 3 and the steel sheet 2. If 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) is less than 15 seconds, the pickling will not proceed sufficiently, resulting in uneven removal of scale mainly composed of iron oxides. Furthermore, if the pickling time exceeds 30 seconds, there is a concern that the internal oxide layer may be removed. On the other hand, if the HCl concentration in the pickling solution exceeds 5.0 mol / L, or if the Fe3+ concentration in the pickling solution exceeds 0.10 mol / L, there is a concern that the pickling rate will increase and the variation in the pickling state due to concentration fluctuations will become greater. 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. When forming the Al barrier layer 5 such that a portion of the Al barrier layer 5 penetrates 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) in cold rolling should be 30% or more in order to promote recrystallization and / or to smooth out the irregularities on 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 sheet 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 set to a value of log(pH2O / pH2), which is the logarithm of the ratio of the partial pressure of water vapor to the partial pressure of hydrogen, between 0.0 and 0.5. By maintaining the atmosphere during the heating process within the above range, internal oxidation can be promoted even during heat treatment, suppressing plating defects caused by external oxidation of Si, Mn, Al, etc. log(pH2O / pH2) 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, suppressing the oxidation of these elements on the surface of the steel sheet. If log(pH2O / pH2) is less than 0.0, sufficient effects cannot be obtained. However, if log(pH2O / pH2) exceeds 0.5, there is a concern that the plating quality may decrease due to the oxidation of Fe depending on the heating temperature and annealing atmosphere.

[0070] [[Soaking process]] During the soaking process, the steel plate is held at the maximum heating temperature of Ac1 + 30°C to 950°C for 1 to 1000 seconds. In the soaking process, the steel sheet is heated to at least Ac1 + 30°C to ensure sufficient austenitization of the steel's structure, and the soaking treatment is performed at this 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, excessively raising the maximum heating temperature can lead to a deterioration of toughness due to the coarsening of the austenite grain size, as well as damage to the annealing equipment. Therefore, the maximum heating temperature should be 950°C or lower, preferably 910°C or lower. Also, since holding time that is too long hinders productivity, the holding time should preferably be 1000 seconds or less, and more preferably 600 seconds or less. 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 plate. 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 plating is adjusted by gas wiping. This forms a plating layer of the predetermined amount. After heat treatment, it is preferable to cool the material so that the average cooling rate in the temperature range of 550 to 700°C is 10 to 100°C / second before immersion in the plating bath. Furthermore, if the difference between the steel plate temperature and the plating bath temperature is too large when immersing the steel plate in the plating bath, the plating bath temperature may change, potentially disrupting operations. Therefore, it is preferable that the steel plate temperature during immersion (cooling stop temperature) be between the plating bath temperature -20°C and the plating bath temperature +20°C. Hot-dip galvanizing can be carried out according to conventional methods. For example, the plating bath temperature can be 440-480°C and the immersion time can be 5 seconds or less. To form an Al barrier layer, the plating bath preferably contains 0.15-0.40% by mass of Al. The plating bath may also contain other impurities such as Fe, Si, Mg, Mn, Cr, Ti, Pb, etc. The time between immersion in the plating bath and gas wiping shall be 5.0 seconds or less, and the steel plate temperature after gas wiping shall be 440°C or less. If the time until gas wiping exceeds 5.0 seconds, or if the steel plate temperature after gas wiping exceeds 440°C, the Al-enriched layer begins to break down, and a sufficient Al barrier layer is not formed. The steel plate temperature after gas wiping may be 300°C or higher. The lower limit of the time until 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. Cooling conditions are not limited. Furthermore, 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. [Examples]

[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 example. 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 the 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 the winding temperature shown in Table 2 to obtain hot-rolled steel sheets. This hot-rolled steel sheet contains 3.0 mol / L HCl and 1.5 mol / L Fe 2+ and 0.02 mol / L Fe 3+ The material was passed through an aqueous solution (pickling solution) containing the material at a temperature of 80°C, to which a pickling accelerator (Sunspeed PA-7 or PA-8 manufactured by Sugimura Chemical Co., Ltd.) in the amounts shown in Table 2 was added. The material was moved through the aqueous solution at an average speed of 20 m / min or more, and the pickling treatment was performed so that the immersion time in the aqueous solution was 16 to 44 seconds. Cold-rolled steel sheets with a thickness of 1.8 to 2.4 mm were obtained by cold-rolling hot-rolled steel sheets with a reduction ratio of 45 to 60% after pickling. The chemical composition of the steel sheets was the same as that of the slab used.

[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, 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 of the atmosphere during the heating process (log(pH2O / pH2)) was set to 0.2. After heat treatment, the plate was cooled to a temperature of -20°C to +20°C above the plating bath temperature, so that the average cooling rate in the temperature range of 550-700°C was 40°C / second. The steel plate was then immersed in a 460°C plating bath for 2 seconds. The time between removing the plate from the plating bath and performing gas wiping was set to 2.0 seconds, and the steel plate temperature after gas wiping was set to 400°C. The material was then 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 the dissolution of plating layer 3 was determined by the way foaming occurred during the dissolution of plating layer 3. The amount of plating Wt was determined based on the sample mass before dissolution, the sample mass after dissolution, and the area on which plating layer 3 was formed. Furthermore, by performing ICP analysis on the acid solution using the ICPS-8100 manufactured by Tsu Seisakusho Co., Ltd., the content of each element in the plating layer was determined, and the amount of Al (Wa) and Fe (Wf) deposited in the plating layer was calculated. The results are shown in Tables 3 to 5. In the plating layer, the remainder of the chemical composition shown in the tables 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, we performed SEM (scanning electron microscope) observation and EPMA analysis to determine whether an Al barrier layer was present, and if so, whether it was continuous or discontinuous (not continuous). Furthermore, using the procedure described above, we performed SEM (scanning electron microscope) observation and EPMA analysis 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) Furthermore, the plating adhesion of the obtained hot-dip galvanized steel sheets was evaluated using a ball impact test. Specifically, a spherical punch was dropped onto the hot-dip galvanized steel sheet to be evaluated from a height corresponding to the sheet's thickness to create a recess in the sheet. Tape was then applied to the convex side of the sheet opposite to where the punch was dropped, and after removing the tape, the degree of plating peeling at that location was observed to evaluate the adhesion of the plating layer. The height of hot-dip galvanized steel sheets, depending on the sheet thickness, is as follows: 350mm for sheets less than 0.8mm thick, 400mm for sheets between 0.8mm and 1.0mm thick, 500mm for sheets between 1.0mm and 1.2mm thick, 600mm for sheets between 1.2mm and 1.5mm thick, 800mm for sheets between 1.5mm and 1.8mm thick, and 950mm for sheets 1.8mm or thicker. The punch that was dropped weighed 25 kg, had a spherical tip with a diameter of φ=12.7 mm, and a die hole diameter of 20 mmφ. Plating adhesion was evaluated by the area percentage of the plating layer attached to (or peeled off) 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 problems with plating adhesion. The results are shown in Table 6.

[0080] [Table 1]

[0081] [Table 2]

[0082] [Table 3]

[0083] [Table 4]

[0084] [Table 5]

[0085] [Table 6]

[0086] As can be seen from the results in Tables 1-6, 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 2 In 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. [Industrial applicability]

[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. [Explanation of Symbols]

[0088] 1. Hot-dip galvanized steel sheet 2 steel plate 3 Plating layer 4. Main plating layer 5. Al barrier layer

Claims

1. Steel plate and, The plating layer present on the surface of the steel plate, It has, The chemical composition of the steel plate is, in mass%, C: 0.050-0.500%, Si: 0.010-2.000%, Mn: 0.10 to 2.50%, P: 0.030% or less, S: 0.010% or less, Al: 0-1.00%, N: 0 to 0.0100%, B: 0.0005-0.0050%, Ti: 0 to 0.20%, Cr: 0-1.50%, Mo: 0-1.00%, Nb: 0 to 0.10%, Ni: 0 to 1.00%, V: 0-0.20%, Cu: 0 to 1.00%, W: 0-0.100%, REM: 0-0.100%, Ca: 0-0.100%, Sb: 0 to 0.050%, Sn: 0 to 0.050%, As: 0-0.050%, and, Remainder: Fe and impurities, And, The Zn content of the aforementioned plating layer is 90.00% or more by mass. The amount of plating deposited on one side of the aforementioned plating layer, Wt, is 30.0 to 120.0 g / m². ２ The Fe deposition amount Wf per side of the plating layer is 0.40 to 3.00 g / m². ２ And, 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 That's all. In a cross-section of the steel plate in the thickness direction, including the steel plate and the plating layer, When the line connecting points with a Zn content of 90% by mass obtained by EPMA analysis is defined as the 90% line, and the line connecting points with a Zn content of 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 plate thickness direction is 2.0 μm or more is 20 particles / mm or more. A hot-dip galvanized steel sheet characterized by the following features. Ia = Wa - (Wt - Wf) × 0.002 (1)

2. The above d is 50 pieces / mm or more. The hot-dip galvanized steel sheet according to claim 1, characterized in that

3. The aforementioned plating layer is The main plating layer, The plated main layer and the steel sheet are formed in at least a portion of the area between them, and the Al barrier layer is a region in which the Al content is 1.50% by mass or more. The Al barrier layer is formed continuously, A hot-dip galvanized steel sheet according to claim 1 or 2, characterized in that...

4. The aforementioned plating layer is The main plating layer, The plated main layer and the steel sheet are formed in at least a portion of the area between them, and the Al barrier layer is a region in which the Al content is 1.50% by mass or more. A portion of the Al barrier layer penetrates into the main plating layer. A hot-dip galvanized steel sheet according to claim 1 or 2, characterized in that...

5. The chemical composition of the aforementioned plating layer is, in mass%, Al: 0.10-1.00%, Fe: 0.33-5.00%, Mg: 0 to 0.500%, Si: 0 to 0.500%, Ni: 0 to 0.500%, Ca: 0-2.000%, Sb: 0 to 0.500%, Pb: 0 to 0.500%, Cu: 0-0.500%, Sn: 0-0.500%, Ti: 0 to 0.500%, Cr: 0-0.500%, Nb: 0 to 0.500%, Zr: 0 to 0.500%, Mn: 0 to 0.500%, Mo: 0-0.500%, Ag: 0-0.500%, Li: 0 to 0.500%, La: 0 to 0.500%, Ce: 0-0.500%, B: 0 to 0.100%, Y: 0 to 0.500%, P: 0 to 0.500%, Sr: 0-0.500%, and, The remainder consists of Zn and impurities. The hot-dip galvanized steel sheet according to claim 1 or 2, characterized in that 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 plate is, in mass%, Ti: 0.01-0.20%, Cr: 0.01-1.50%, Mo: 0.01-1.00%, Nb: 0.01 to 0.10%, Ni: 0.01-1.00%, V: 0.01-0.20%, Cu: 0.01 to 1.00%, W: 0.001-0.100%, REM: 0.001-0.100%, Ca: 0.001-0.100%, Sb: 0.001 to 0.050%, Sn: 0.001 to 0.050%, As: 0.001 to 0.050%, It contains one or more selected from the group consisting of, A hot-dip galvanized steel sheet according to claim 1 or 2, characterized in that...

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