Hot-dip galvanized steel sheet and production method therefor

Abstract

The present invention provides a hot-dip galvanized steel sheet that is high-strength and that has a pleasing surface appearance without any missing plating and excellent delayed fracture resistance, and a production method therefor.　The hot-dip galvanized steel sheet comprises: a base steel sheet which has a component composition containing specific amounts of C, Si, Mn, P, S, sol. Al, N, and O, and further containing specific amounts of specific optional elements, with the remainder consisting of Fe and unavoidable impurities; and a zinc-based plating layer which is formed on a surface of the base steel sheet. The amount of inclusion groups satisfying specific conditions in the base steel sheet is 10 groups / mm2 or less, the base steel sheet has a hydrogen concentration HL of 0.30 mass ppm or less and a hydrogen concentration HH of 0.20 mass ppm or less, and the tensile strength of the hot-dip galvanized steel sheet is 780 MPa or more.

Description

Hot-dip galvanized steel sheet and its manufacturing method

[0001] The present invention relates to a hot-dip galvanized steel sheet and a method for producing the same.

[0002] In recent years, surface-treated steel sheets, which are steel sheets imparted with rust resistance, including hot-dip galvanized steel sheets (including galvannealed steel sheets) with excellent rust resistance, have been widely used in fields such as automobiles, home appliances, and building materials. Furthermore, from the viewpoint of improving automobile fuel efficiency and collision safety, the application of high-strength steel sheets to automobile body materials has been progressing in order to reduce the thickness of automobile bodies by increasing the strength of the body materials and to make the automobile bodies themselves lighter and stronger. Hot-dip galvanized steel sheets are generally produced by using hot-rolled steel sheets or cold-rolled steel sheets as base materials, recrystallizing the base steel sheets in a CGL annealing furnace, and then applying hot-dip galvanized coating. On the other hand, galvannealed steel sheets are produced by further alloying the hot-dip galvanized coating.

[0003] Annealing requires that a steel sheet be held in a reducing atmosphere containing hydrogen. During this process, hydrogen in the furnace penetrates the steel sheet. The steel sheet is then cooled and hot-dip galvanized, whereupon the hydrogen remains in the steel sheet as diffusible hydrogen in the steel. Because hydrogen is impermeable through plating, diffusible hydrogen in the steel is not released from the steel sheet after plating. A problem has arisen in that a large amount of diffusible hydrogen in the steel reduces delayed fracture resistance. In particular, in high-strength steel sheets with a tensile strength of 780 MPa or more, diffusible hydrogen in the steel tends to remain after annealing, significantly reducing delayed fracture resistance. This problem is due to the fact that high-strength steel sheets with a tensile strength of 780 MPa or more require the formation of hard structures such as martensite and bainite to achieve a desired strength. To achieve this, an austenite phase must be formed during the annealing process. However, the austenite phase is more likely to absorb large amounts of hydrogen than the ferrite phase, and because the hydrogen diffusion rate is slower, once hydrogen is absorbed during the annealing process, it is difficult to release it during the cooling process. In recent years, the standards for delayed fracture resistance have tended to become stricter.

[0004] Conventionally, the following proposals have been made for steel sheets with reduced hydrogen content. Patent Document 1 discloses a high-strength galvannealed steel sheet having excellent delayed fracture resistance and hole expandability, in which the amount of hydrogen released when the steel sheet is heated to 200°C is 0.35 mass ppm or less. Patent Document 2 discloses a high-strength galvannealed steel sheet having excellent galvanizability and bendability, in which the amount of hydrogen released when the steel sheet is heated to 210°C is 0.25 mass ppm or less. Patent Document 3 discloses a high-strength galvannealed steel sheet having excellent galvanizability and bendability, in which the amount of hydrogen released when the steel sheet is heated to 210°C is 0.25 mass ppm or less. Furthermore, the following proposals have been made, for example, as techniques for producing steel sheets with reduced hydrogen content. Patent Document 4 discloses a method for producing a steel sheet having reduced hydrogen content by reducing a hot-rolled steel sheet and then subjecting it to H ２ A technique is disclosed in which a dehydrogenation treatment is performed at 450 to 550°C in an atmosphere with a hydrogen concentration of 8 to 20%, followed by hot-dip galvanizing. Patent Document 5 also discloses a technique in which a hot-rolled steel sheet is subjected to reduction annealing in the range of 650 to 950°C and then hot-dip galvanizing, in which the amount of hydrogen in the steel sheet is reduced by controlling the relationship between the annealing temperature and the hydrogen concentration in the annealing furnace so as to satisfy the following formula (1): 1≦H≦−0.05×RT+57.5 (1) where H is the hydrogen concentration in the furnace, and RT is the annealing temperature.

[0005] Furthermore, Patent Document 6 discloses a technique for obtaining good surface quality in a method of performing hot-dip galvanizing after reduction annealing a steel sheet containing Si, Mn, and Al, by controlling the hydrogen concentration in the furnace during reduction annealing to be 10% by volume or more, and the relationship between the hydrogen partial pressure and the water vapor partial pressure in the furnace atmosphere gas at 650°C or higher and lower than 750°C to satisfy the following formula (2), and similarly controlling the relationship between the hydrogen partial pressure and the water vapor partial pressure in the furnace atmosphere gas at 750°C or higher and 950°C to satisfy the following formula (3): log(P Ｈ２Ｏ / P Ｈ２ )≦-1.55...(2) -0.91≦log(P Ｈ２Ｏ / P Ｈ２ ) ≦−0.635 ... (3)

[0006] International Publication No. 2019 / 189067 International Publication No. 2019 / 189849 International Publication No. 2019 / 189841 Japanese Patent Publication No. 54-130443 Japanese Patent No. 3266008 Japanese Patent No. 5811841

[0007] However, the techniques disclosed in Patent Documents 1, 2, and 3 have difficulty achieving both strict delayed fracture resistance and good coating appearance quality. Furthermore, the techniques disclosed in Patent Documents 4 and 5 are all intended to suppress blistering of hot-rolled steel sheets. To improve the delayed fracture resistance of high-strength steel sheets having an austenitic phase during the annealing process, it is necessary to further reduce the amount of hydrogen in the atmosphere to reduce the hydrogen content in the steel. However, further reducing the amount of hydrogen in the atmosphere promotes selective oxidation of easily oxidizable elements such as Si and Mn contained in the high-strength steel sheet, thereby impairing galvanization and preventing good surface quality from being obtained. Therefore, the methods disclosed in Patent Documents 4 and 5, which uniformly reduce hydrogen in a furnace, are difficult to achieve both good surface quality and delayed fracture resistance. Furthermore, the technique disclosed in Patent Document 6 aims to improve the galvanization of steels containing Si, Mn, and Al and obtain good surface quality by varying the ratio of water vapor partial pressure to hydrogen partial pressure depending on the annealing temperature. However, it is necessary to control the hydrogen concentration in the furnace to 10% or more, and no consideration is given to reducing the hydrogen concentration contained in the steel. As a result, it is difficult to improve the delayed fracture resistance of high-strength steel sheets.

[0008] Therefore, an object of the present invention is to solve the above-mentioned problems of the conventional art and to provide a manufacturing method by which a hot-dip galvanized steel sheet having high strength, a beautiful surface appearance without bare spots, and excellent delayed fracture resistance can be manufactured.

[0009] Here, "high strength" refers to a tensile strength (TS) of the steel sheet of 780 MPa or more when a tensile test is conducted in accordance with JIS Z2241 (2011). "A beautiful surface appearance without bare spots" refers to a condition in which the coating appearance of a hot-dip galvanized steel sheet is visually observed, and for those that show patterns or irregularities, SEM observation is conducted, and the steel sheet has fewer than three bare spots within a 1 m x 1 m area. "Excellent delayed fracture resistance" refers to a condition in which a rectangular test piece having a major axis length of 100 mm and a minor axis length of 20 mm is taken from a steel sheet, a punched hole with a diameter of 15 mm and a clearance of 14.0% is formed at the center of the major and minor axes of this test piece, and the test piece is subjected to a tensile test (tensile speed: 10 mm / min), with a maximum load time of 100 hours, and the maximum stress at which no cracks occur after 100 hours of load is defined as the critical stress, and the critical stress / yield stress ratio is 1.00 or more. The clearance was previously set at approximately 12.5%, but in light of the recent trend towards stricter quality standards regarding delayed fracture resistance, stricter conditions have been adopted than before.

[0010] The hydrogen concentration corresponding to the amount of hydrogen released at temperatures below 150°C when measured by temperature-programmed analysis is also the hydrogen concentration H Ｈ The inventors of the present invention have conducted extensive research to solve the above problem, and as a result, have found that the hydrogen concentration H Ｈ have newly revealed that hydrogen is difficult to release into the atmosphere when steel sheets are stored after production, but that when processed, especially by pressing, it transforms into diffusible hydrogen, which has a significant impact on delayed fracture resistance. Furthermore, the hydrogen concentration H Ｈ represents MnS and Al contained in the steel sheet. ２ O ３The inventors have found that, since the elements are trapped by a relatively weak force around groups of inclusions consisting of MnS composite precipitates with the elements being nuclei, by optimizing the annealing and cooling atmospheres in a manufacturing method for hot-dip galvanized steel sheet in which the group of inclusions and the shape and number of the inclusions, as well as the steel sheet (base steel sheet) thus produced, are annealed and cooled, and then hot-dip galvanized, a hot-dip galvanized steel sheet excellent in coating appearance and delayed fracture resistance can be manufactured.

[0011] The present invention was made based on such findings, and the gist of the present invention is as follows: [1] In mass%, C: 0.06% or more and 0.30% or less, Si: 0.01% or more and 3.00% or less, Mn: 1.5% or more and 3.5% or less, P: 0.1% or less (excluding 0%), S: 0.0030% or less (excluding 0%), sol. containing Al: 0.1% or less (0% or less), N: 0.0070% or less (0% or less), O: 0.0030% or less (0% or less), or further containing one or more elements selected from the following groups A, B, C and D: Group A: one or more elements selected from Nb: 0.05% or less (0% or less), Ti: 0.08% or less (0% or less), V: 0.2% or less (0% or less), W: 0.15% or less (0% or less), Zr: 0.15% or less (0% or less); Group B: one or more elements selected from Cr: 1% or less (0% or less), Ni: 1% or less (0% or less), Cu: 1% or less (0% or less), Mo: 1% or less (0% or less), Co: 1% or less (0% or less), B: 0.005% or less (0% or less); Group C: Group D: one or more selected from Ca: 0.005% or less (excluding 0%), Mg: 0.005% or less (excluding 0%), REM: 0.005% or less (excluding 0%), Group D: one or more selected from Sn: 0.2% or less (excluding 0%), Sb: 0.2% or less (excluding 0%), the balance being Fe and unavoidable impurities; and a zinc-based plating layer formed on the surface of the substrate steel sheet, wherein a group of inclusions contained in the substrate steel sheet satisfies the following conditions (i) and (ii), and the group of inclusions is 10 pieces / mm ２a hot-dip galvanized steel sheet according to [1], wherein the substrate steel sheet releases 0.30 mass ppm or less of hydrogen at temperatures from room temperature to 350°C and 0.20 mass ppm or less of hydrogen at temperatures from 150 to 350°C, the number of uncoated defects is less than three within a 1 m x 1 m area on the steel sheet surface, and the tensile strength is 780 MPa or more. (Conditions) (i) The length of the major axis of an inclusion group having one or more inclusion particles is 50 μm or more. (ii) When the inclusion group is composed of two or more of the inclusion particles, the length of the major axis of one inclusion particle is 0.3 μm or more and the shortest distance between the inclusion particles is 40 μm or less. [2] The hot-dip galvanized steel sheet according to [1], wherein the base steel sheet further contains, in mass %, one or more elements selected from the following groups E, F, G, and H as part of its chemical composition: Group E: Ta: 0.10% or less (not including 0%) Group F: One or more selected from Te: 0.10% or less (not including 0%), As: 0.10% or less (not including 0%), Hf: 0.10% or less (not including 0%) Group G: One or more selected from Bi: 0.20% or less (not including 0%), Pb: 0.20% or less (not including 0%) Group H: One or more selected from Zn: 0.10% or less (not including 0%), Ge: 0.10% or less (not including 0%), Sr: 0.10% or less (not including 0%), Cs: 0.10% or less (not including 0%) [3] The hot-dip galvanized steel sheet according to [1] or [2], wherein the amount of hydrogen released from the base steel sheet at a temperature from room temperature to 350°C is 0.20 ppm by mass or less, and the amount of hydrogen released from the base steel sheet at a temperature from 150°C to 350°C is 0.15 ppm by mass or less. [4] The hot-dip galvanized steel sheet according to any of [1] to [3], wherein the total area fraction of martensite, bainite, and retained austenite in the base steel sheet is 30% or more, and the tensile strength is 780 MPa or more. [5] The hot-dip galvanized steel sheet according to any of [1] to [3], wherein the total area fraction of martensite, bainite, and retained austenite in the base steel sheet is 50% or more, and the tensile strength is 980 MPa or more.[6] The hot-dip galvanized steel sheet according to any one of [1] to [3], wherein the base steel sheet has a total area fraction of martensite, bainite, and retained austenite of 70% or more and a tensile strength of 1180 MPa or more. [7] The hot-dip galvanized steel sheet according to any one of [1] to [3], wherein the base steel sheet has a total area fraction of martensite, bainite, and retained austenite of 85% or more and a tensile strength of 1310 MPa or more. [8] The hot-dip galvanized steel sheet according to any one of [1] to [3], wherein the base steel sheet has a total area fraction of martensite, bainite, and retained austenite of 90% or more and a tensile strength of 1470 MPa or more. [9] The hot-dip galvanized steel sheet according to any one of [1] to [8], wherein the zinc-based coating layer is a galvannealed coating layer.

[10] A method for manufacturing a hot-dip galvanized steel sheet, comprising annealing a base steel sheet and then hot-dip galvanizing the base steel sheet, wherein the base steel sheet contains, in mass %, C: 0.06% or more and 0.30% or less, Si: 0.01% or more and 3.00% or less, Mn: 1.5% or more and 3.5% or less, P: 0.1% or less (excluding 0%), S: 0.0030% or less (excluding 0%), sol. containing Al: 0.1% or less (0% or less), N: 0.0070% or less (0% or less), O: 0.0030% or less (0% or less), or further containing one or more elements selected from the following groups A, B, C and D: Group A: one or more elements selected from Nb: 0.05% or less (0% or less), Ti: 0.08% or less (0% or less), V: 0.2% or less (0% or less), W: 0.15% or less (0% or less), Zr: 0.15% or less (0% or less); Group B: one or more elements selected from Cr: 1% or less (0% or less), Ni: 1% or less (0% or less), Cu: 1% or less (0% or less), Mo: 1% or less (0% or less), Co: 1% or less (0% or less), B: 0.005% or less (0% or less); Group C: Group D: one or more selected from Ca: 0.005% or less (excluding 0%), Mg: 0.005% or less (excluding 0%), REM: 0.005% or less (excluding 0%), Group D: one or more selected from Sn: 0.2% or less (excluding 0%), Sb: 0.2% or less (excluding 0%), the balance being Fe and unavoidable impurities, and further, a group of inclusions contained in the base steel sheet satisfies the following conditions (i) and (ii), and the number of inclusions in the group is 10 / mm. ２a first annealing step in which the substrate steel sheet is held at a holding temperature of 650°C or higher and 950°C or lower for 20 seconds or higher and 150 seconds or lower in an atmosphere having a dew point of -55°C or higher and +30°C or lower and a hydrogen concentration of 8.0 volume % or higher and 25 volume % or lower; a second annealing step in which the substrate steel sheet that has undergone the first annealing step is held at a holding temperature of 700°C or higher and 950°C or lower for 30 seconds or higher and 300 seconds or lower in an atmosphere having a dew point of -50°C or higher and +30°C or lower and a hydrogen concentration of 0.2 volume % or higher and less than 8.0 volume %; and a cooling step in which the substrate steel sheet that has undergone the second annealing step is cooled from the holding temperature of 700°C or higher and 950°C or lower in the second annealing step to a temperature of 150°C or higher and 550°C or lower at an average cooling rate of 3 to 300°C / s in an atmosphere having a dew point of -20°C or lower and a hydrogen concentration of 0.2 volume % or higher and 25 volume % or lower. and a hot-dip galvanizing step of subjecting the substrate steel sheet after the cooling step to a hot-dip galvanizing treatment to form a zinc-based plating layer on the surface of the substrate steel sheet. (Conditions) (i) The length of the major axis of an inclusion group having one or more inclusion particles is 50 μm or more. (ii) When the inclusion group is composed of two or more of the inclusion particles, the length of the major axis of one inclusion particle is 0.3 μm or more and the shortest distance between the inclusion particles is 40 μm or less.

[11] The method for producing a hot-dip galvanized steel sheet according to

[10] , wherein the substrate steel sheet further contains, as the chemical composition, one or more elements selected from the group E, F, G and H below, in mass %. Group E Ta: 0.10% or less (excluding 0%) Group F One or more selected from Te: 0.10% or less (excluding 0%), As: 0.10% or less (excluding 0%), Hf: 0.10% or less (excluding 0%) Group G One or more selected from Bi: 0.20% or less (excluding 0%), Pb: 0.20% or less (excluding 0%) Group H One or more selected from Zn: 0.10% or less (excluding 0%), Ge: 0.10% or less (excluding 0%), Sr: 0.10% or less (excluding 0%), Cs: 0.10% or less (excluding 0%)

[12] The base steel sheet before the first annealing process contains O ２

[13] The method for producing a hot-dip galvanized steel sheet according to any one of

[10] to

[12] , comprising a reheating step of, after the hot-dip galvanizing treatment, holding the steel sheet at 200°C or higher and 450°C or lower for 5 seconds or longer and in an atmosphere having a hydrogen concentration of 15% by volume or lower, followed by cooling.

[14] The method for producing a hot-dip galvanized steel sheet according to any one of

[10] to

[13] , wherein the atmosphere in the second annealing step has a hydrogen concentration of less than 5.0% by volume.

[15] The method for producing a hot-dip galvanized steel sheet according to any one of

[10] to

[14] , wherein the cooling step comprises: a first cooling step in which the substrate steel sheet that has undergone the second annealing step is cooled in an atmosphere having a dew point of -20°C or less and a hydrogen concentration of 0.2 vol% or more and less than 8.0 vol% through a temperature range from a holding temperature of 700°C or more and 950°C or less in the second annealing step to a steel sheet temperature of 650°C or less over 10 s or more at an average cooling rate of 3°C / s or more and 25°C / s or less; and a second cooling step in which the substrate steel sheet that has undergone the first cooling step is cooled from 600°C to a temperature of 150°C or more and 550°C or less in an atmosphere having a dew point of -20°C or less and a hydrogen concentration of 8.0 vol% or more and 25 vol% or less at an average cooling rate of 10°C / s or more.

[16] The method for producing a hot-dip galvanized steel sheet according to any one of

[10] to

[15] , further comprising, after the hot-dip galvanizing step, an alloying treatment step of alloying the base steel sheet and the zinc-based plating layer.

[0012] According to the present invention, there are provided a hot-dip galvanized steel sheet having high strength, a beautiful surface appearance without bare areas, and excellent delayed fracture resistance, and a method for producing the same.

[0013] Hereinafter, as one embodiment of the hot-dip galvanized steel sheet and the method for manufacturing the same of the present invention, the method for manufacturing the hot-dip galvanized steel sheet and the hot-dip galvanized steel sheet will be described in that order.

[0014] <Method for manufacturing hot-dip galvanized steel sheet> The method for manufacturing hot-dip galvanized steel sheet of the present invention is a method for manufacturing hot-dip galvanized steel sheet by annealing a substrate steel sheet having a specific component composition described later and then hot-dip galvanizing the substrate steel sheet, wherein a group of inclusions contained in the substrate steel sheet satisfies the following conditions (i) and (ii), and the number of inclusions in the group of inclusions is 10 pieces / mm ２ a first annealing step in which the substrate steel sheet is held at a holding temperature of 650°C to 950°C for 20 seconds to 150 seconds in an atmosphere having a dew point of -55°C to +30°C and a hydrogen concentration of 8.0 volume % to 25 volume %, and a second annealing step in which the substrate steel sheet that has undergone the first annealing step is held at a holding temperature of 700°C to 950°C for 30 seconds to 300 seconds in an atmosphere having a dew point of -50°C to +30°C and a hydrogen concentration of 0.2 volume % to less than 8.0 volume %. The method includes a cooling step in which the substrate steel sheet that has undergone the second annealing step is cooled from the holding temperature of 700°C to 950°C in the second annealing step to a temperature of 150°C to 550°C at an average cooling rate of 3 to 300°C / s in an atmosphere with a dew point of -20°C or lower and a hydrogen concentration of 0.2 to 25% by volume, and a hot-dip galvanizing step in which the substrate steel sheet after the cooling step is subjected to a hot-dip galvanizing treatment to form a zinc-based plating layer on the surface of the substrate steel sheet. (Conditions) (i) The length of the major axis of an inclusion group having one or more inclusion particles is 50 μm or more. (ii) When an inclusion group is composed of two or more of the inclusion particles, the length of the major axis of one inclusion particle is 0.3 μm or more, and the shortest distance between the inclusion particles is 40 μm or less.

[0015] Refining Process: To ensure excellent delayed fracture resistance, it is necessary to reduce impurity elements such as S, N, and O, and to reduce large inclusions during the refining process of molten steel. The methods for reducing impurity elements and large inclusions are not particularly limited. For example, after oxygen blowing in a converter, the molten steel is tapped into a ladle and desulfurized. The molten steel is then vacuum degassed and refluxed using an RH vacuum degasser to reduce the impurity elements and float and separate large oxide-based inclusions containing Al and Ca (hereinafter also referred to as Al-Ca oxide-based). The molten steel is then left to stand for a certain period of time to float and separate a layer of large inclusions, after which the molten steel is poured into a tundish. From the viewpoint of reducing large Al-Ca oxide-based inclusions, which can particularly deteriorate delayed fracture resistance, it is preferable to reflux the molten steel in the RH vacuum degasser for more than 20 minutes. From the same viewpoint, it is also preferable to hold the molten steel for more than 10 minutes after the reflux process in the RH before pouring it into the tundish. By carrying out such treatment, it is possible to reduce large inclusions that deteriorate delayed fracture resistance.

[0016] A more detailed explanation is provided below. Raw ores, such as electrolytic manganese ores for manganese adjustment and ferrosilicon ores for silicon adjustment, contain a certain amount of impurity components that form inclusions with low melting points. The low-melting-point inclusions resulting from the addition of these ores to molten steel are elongated by hot rolling and crushed by cold rolling, resulting in a dotted-array distribution. These elongated inclusions or large inclusions distributed in dotted-array distribution have a large surface area relative to their volume and poor conformability with the steel sheet. Therefore, they are prone to occluding hydrogen in their surrounding areas during the annealing process prior to hot-dip galvanizing. Furthermore, the hydrogen occluded around these inclusions is relatively tightly trapped, and most of it can be released at temperatures as high as 150°C or higher, but is not easily diffused or released at room temperature. Therefore, hydrogen occluded around the inclusions is difficult to remove and tends to remain in the steel sheet. Furthermore, the hydrogen remaining around the inclusions can become harmful hydrogen that can cause cracks around the inclusions after press working. Furthermore, since cracks tend to originate around large inclusions where stress is likely to concentrate, cracks and fractures, especially delayed fractures, occur significantly after press working around large inclusions that trap hydrogen. To reduce this delayed fracture, it is desirable to reduce the distribution frequency of large inclusion groups with a specific distribution pattern that are likely to absorb hydrogen during annealing and cause stress concentration during press working. Here, a large inclusion group refers to a group of inclusions consisting of two or more inclusions that are close to each other.

[0017] The inclusion groups that have such a strong hydrogen absorption effect and are likely to cause delayed fracture after press working are as follows: the major axis length of each individual inclusion particle that makes up the inclusion group is 0.3 μm or more, when the inclusion group is made up of two or more inclusion particles, the shortest distance between the inclusion particles is 40 μm or less, and the major axis length of the inclusion group made up of individual inclusions is 50 μm or more. Inclusions that can form such inclusion groups include Al ２ O ３ , (Al, Ca)—O, MnS, Nb(C, N), (Nb, Ti)(C, N), Ti(C, S), etc., and in particular, coarse Al with a maximum thickness (width in the plate thickness direction) of 4 μm or more ２ O ３When inclusions including (Al, Ca)-O type inclusions are present, these inclusions significantly promote delayed fracture. ２ O ３ The presence of (Al, Ca)-O-based inclusions also makes it easier for MnS and (Nb, Ti)(C, N) to precipitate in combination, leading to the frequent formation of larger inclusions. Therefore, it is particularly important to reduce the inclusion group containing large oxide-based inclusions, and it is important to ensure a long reflux period in the above-mentioned RH vacuum degassing apparatus and a settling time before the molten steel is poured into the tundish. Other RH treatment conditions may be in accordance with established methods. For example, the gas pressure in the vessel of the RH vacuum degassing apparatus may be about 50 to 1000 Pa, and the reflux gas may be 0.02 to 0.1 Nm ３ Argon gas at a flow rate of 800 kg / s can be used. The processing amount of molten steel can be set to 80 to 350 tons, and the reflux rate can be set to 800 to 3500 kg / s. In addition, from the viewpoint of suppressing the formation of MnS and Ti (C, S), it is desirable to reduce the amount of S, and from the viewpoint of suppressing the formation of Nb, Ti-based carbonitrides, it is desirable to reduce the amount of N.

[0018] Slab Heating Process Methods for hot rolling a steel slab include a method in which the slab is heated and then rolled, a method in which the slab is directly rolled after continuous casting without reheating, and a method in which the slab is subjected to a short-term heat treatment and then rolled after continuous casting. In the manufacturing method of the present invention, the slab heating temperature is preferably set to exceed 1220°C. By setting the slab heating temperature (surface temperature) to exceed 1220°C, the solid solution of sulfides and carbonitrides is promoted and Mn segregation is reduced, thereby reducing the size and number of the above-mentioned inclusion groups. There is no particular upper limit for the slab heating temperature, but the slab heating temperature is preferably 1400°C or less. In addition, the average heating rate during slab heating may be set to 5 to 15°C / min.

[0019] It is preferable to maintain the slab surface temperature at a temperature exceeding 1220°C for a holding time (soaking time) of 100 min or more. Research by the present inventors has revealed that the main reason for the presence of inclusions is that even if the slab heating temperature reaches the inclusion dissolution temperature, if sufficient soaking time is not ensured, the dissolution state does not reach equilibrium. Ensuring a sufficient soaking time promotes the solid solution of sulfides and carbonitrides, improving delayed fracture resistance. If the holding time (soaking time) is less than 100 min, the solid solution of Nb- and Ti-based carbonitrides becomes insufficient, and they remain. Furthermore, MnS precipitates using these as nuclei, resulting in a deterioration of delayed fracture resistance. While the upper limit of the holding time (soaking time) is not particularly limited, the holding time (soaking time) is preferably 250 min or less, more preferably 200 min or less. The holding time (soaking time) is even more preferably 175 min or less.

[0020] Hot rolling process: Here, the ratio ε2 / ε1 of the elongation strain of the steel calculated from the rolling reduction: ε1 (the true strain assuming no change in the width direction) to the elongation strain of the MnS-based inclusions in the major axis as a group of inclusions: ε2 (the increase in the major axis, including the increase in the distance between inclusions due to crushing), is 0.55 to 0.60 in hot rolling and 0.65 to 0.70 in cold rolling. This means that the inclusions are elongated in the rolling direction in both processes. This means that, compared to the reduction in hot rolling, reducing the reduction in cold rolling is effective in reducing the elongation of the inclusions, but the effect of adjusting the reduction distribution is very small, and reducing the cumulative reduction is more effective in reducing the elongation of the inclusions. In other words, it is preferable to reduce the casting thickness of the slab relative to the thickness of the final product plate. However, in reality, since a thinner slab impairs productivity, the slab casting thickness is preferably set to a range of 100 to 250 mm, more preferably 150 to 200 mm. In addition, according to the standard method, the cumulative reduction in the temperature range above 950°C should be set to 70 to 98%, and the cumulative reduction at temperatures below 950°C, including cold rolling, should be set to 50 to 92%.

[0021] Finish rolling is preferably performed at a finish rolling temperature of 800 to 950°C, and then cooled to a temperature range of more than 400°C and not more than 630°C at a cooling rate of 10°C / s or more and 200°C / s or less, and then coiled. The finish rolling temperature (FT) is set to a range of 800 to 950°C from the viewpoint of promoting transformation, and Ar ３ It is preferable to set the temperature low within a range that does not fall below the transformation point. In the present invention, by setting the coiling temperature to 560°C or less, a fine ferrite + pearlite structure or a structure mainly composed of bainite can be obtained, and high strength can be ensured in the steel sheet after subsequent annealing, and strength stability within the coil can be improved, thereby suppressing thickness fluctuations of the steel sheet (coil). Furthermore, by setting the coiling temperature to more than 400°C, hydrogen absorbed in the steel during the refining process can be sufficiently released, making it possible to avoid cracking during cold rolling and contributing to a reduction in hydrogen in the steel during subsequent annealing. From the viewpoint of hydrogen release, the holding time of the coiled steel sheet at 350°C or higher is 3,000 seconds or more, and more preferably 5,000 seconds or more. The holding time at 350°C or higher is not particularly specified, but since special equipment or the like is required for long-term holding, it is preferably 180,000 seconds or less.

[0022] The wound coil is then removed from the coiler. If necessary, the coil may be water-cooled while rotating, but in this case, it is preferable to keep the water-cooling time as short as possible. It is more preferable not to perform water-cooling. After winding the coil in a temperature range of more than 400°C and not more than 630°C, the coil may be water-cooled as is, or the coil may be unwound and cooled with water or gas, from the viewpoint of suppressing subsequent surface oxidation and uneven transformation. Such a treatment is preferably performed after the steel has been held at 350°C or higher for a predetermined time to reduce the hydrogen in the steel.

[0023] It is also desirable to perform descaling to remove primary and secondary scale formed on the steel sheet surface. Descaling is preferably performed at a high impact pressure of 50 MPa or more. This reduces the remaining red scale and the thickness of the secondary scale, and reduces surface oxidation of the steel sheet caused by oxygen in the scale being taken into the steel sheet during coiling in hot rolling. As a result, the thickness of the surface oxide layer in the final product can be reduced, improving corrosion resistance. It is also preferable to thoroughly pickle the hot-rolled coil before cold rolling to reduce the remaining scale. Furthermore, from the viewpoint of reducing the load during cold rolling, hot-rolled sheet annealing may be performed as necessary.

[0024] Cold Rolling Process In cold rolling, the reduction rate (cumulative reduction rate) is set to 20 to 75%, and the thickness of the steel sheet after cold rolling is set to 0.5 to 3.2 mm, with other conditions following standard methods. The lower limit of the reduction rate is preferably 30% or more, more preferably 40% or more. The upper limit of the reduction rate is preferably 72% or less, more preferably 70% or less. The thickness of the steel sheet after cold rolling is preferably 0.6 mm or more, more preferably 0.8 mm or more, and even more preferably 1.0 mm or more. The thickness of the steel sheet after cold rolling is preferably 2.8 mm or less, more preferably 2.3 mm or less, and even more preferably 2.0 mm or less.

[0025] Oxidation treatment, annealing, cooling, hot-dip galvanizing, and reheating treatment The temperatures specified in the oxidation treatment, annealing, and cooling after annealing described below are all "steel sheet temperatures," more specifically, the surface temperatures of the steel sheet. Furthermore, a non-oxidizing atmosphere described below refers to an atmosphere in which iron does not oxidize, and is an atmosphere in which selective oxidation of easily oxidizable additive elements such as Si and Mn is permitted. Furthermore, a reducing atmosphere refers to an atmosphere in which iron oxide can be reduced to iron.

[0026] The type of hot-dip galvanized steel sheet to which the present invention is applicable is not particularly limited as long as it is a plated steel sheet having a plating layer containing zinc as a main component, and includes not only hot-dip galvanized steel sheets (hot-dip galvanized steel sheets (GI)) but also steel sheets that have been subjected to alloying treatment after hot-dip galvanizing (galvannealed steel sheets (GA)). The types of hot-dip galvanized steel sheets include hot-dip galvanized steel sheets (GI) and galvannealed steel sheets (GA), as well as hot-dip zinc-aluminum alloy-plated steel sheets, hot-dip zinc-aluminum-silicon alloy-plated steel sheets, and hot-dip zinc-aluminum-magnesium alloy-plated steel sheets, and the detailed coating composition of each is also not limited.

[0027] In the following description, the units of "%" used for the content of each element in the composition of a steel sheet (also referred to as a base steel sheet or a base steel sheet), the content of each element in the composition of a coating bath, and the degree of alloying of a coating layer are all "mass%," and the units of "%" used for the hydrogen concentration in the atmosphere during annealing and cooling are all "volume%." Furthermore, "high strength" steel sheet means that the tensile strength TS of the steel sheet measured in accordance with JIS Z2241 (2011) is 780 MPa or more.

[0028] The manufacturing method of the present invention is a method for manufacturing a hot-dip galvanized steel sheet, in which a base steel sheet having a predetermined steel composition and inclusion distribution pattern is annealed in a non-oxidizing atmosphere, and then the base steel sheet is hot-dip galvanized. The annealing includes a first annealing step and a second annealing step. The annealing can be performed in a non-oxidizing atmosphere. In the first annealing step, the steel sheet is annealed in a reducing atmosphere with a high hydrogen concentration and a predetermined dew point, thereby reducing natural iron oxide present in the surface layer of the steel sheet. In the subsequent second annealing step, the steel sheet is annealed in a non-oxidizing atmosphere with a low hydrogen concentration and a predetermined dew point, thereby releasing hydrogen dissolved in the steel from the steel sheet. The annealed steel sheet is cooled to a predetermined temperature and then immersed in a hot-dip galvanized coating bath to be hot-dip galvanized. The manufacturing method of the present invention also includes a case in which an alloying treatment is performed after hot-dip galvanizing to produce a hot-dip galvanized coated steel sheet. The hot-dip galvanized steel sheet (including alloyed hot-dip galvanized steel sheet) thus obtained has a hydrogen amount (hydrogen concentration H Ｌ ) is controlled to 0.30 mass ppm or less, and the amount of hydrogen released at 150 to 350°C (hydrogen concentration H Ｈ The total amount of CI 4, CI 5, CI 6, CI 7, CI 8, CI 9, CI 10, CI 11, CI 12, CI 13, CI 14, CI 15, CI 16, CI 17, CI 18, CI 19, CI 20, CI 21, CI 22, CI 23, CI 24, CI

[0029] Furthermore, an even more beautiful surface appearance can be obtained by performing an oxidation treatment in which iron oxide is generated on the surface layer of the steel sheet in a predetermined oxidizing atmosphere before annealing. Furthermore, by optimizing the atmosphere in which the annealed steel sheet is cooled or by reheating the plated steel sheet in an atmosphere having a predetermined hydrogen concentration, the delayed fracture resistance can be improved to an even higher level. The structure and chemical composition of the steel sheet (base steel sheet) that serves as the base material for the hot-dip galvanized steel sheet will be described in detail later. In the present invention, the oxidation treatment and the subsequent annealing in a non-oxidizing atmosphere are typically performed in a continuous annealing furnace having, from the entry side, an oxidation zone (a zone for the oxidation treatment), a reduction zone (a zone for the first annealing step of annealing), a soaking zone (a zone for the second annealing step of annealing), and a cooling zone. The oxidation treatment is not an essential step and can be performed as needed.

[0030] The manufacturing method of the present invention will be described below in the order of oxidation treatment, annealing (first annealing step, second annealing step), cooling after annealing (first cooling step, second cooling step), hot dip galvanizing, and reheating.

[0031] Oxidation treatment process In the oxidation treatment, O ２ The temperature of the steel sheet is controlled to 400°C or higher and 900°C or lower in an atmosphere containing 1000 volume ppm or more of O. ２ Surprisingly N ２ , CO, CO ２ , H ２ It may contain one or more of O and NOx. ２ is an inert gas, CO is a gas for adjusting oxidation and reduction, CO ２ is an inert gas, H ２ O can be contained as a gas for adjusting oxidation and reduction. ２ , H ２ O and NOx can be contained as fuel gas, gas derived from the components of the steel sheet being annealed, impurity gas in the atmosphere, or combustion gas of fuel.

[0032] In the present invention, the steel sheet is oxidized by this oxidation treatment and then reduced in the subsequent annealing (first annealing step) to form a reduced iron layer on the surface layer of the steel sheet, thereby preventing Si and Mn from diffusing into the surface layer of the steel sheet and being oxidized, thereby further improving the galvanizability. ２ The oxidation treatment in an atmosphere containing 1000 volume ppm or more of O achieves both excellent surface quality and excellent delayed fracture resistance to a high degree. Furthermore, the improvement effect is particularly remarkable in steels containing 0.1% or more of Si and 1.5% or more of Mn. ２ By setting the concentration to 1000 ppm by volume or more, oxidation of the steel sheet is promoted. ２ If the concentration is less than 1000 ppm by volume, the oxidation of the steel sheet will be insufficient, and oxides of Si and Mn will be formed, which may result in a decrease in plating properties. ２The upper limit of the concentration is not particularly limited, but in order to avoid problems such as pick-up on the transport roll due to peroxidation, it is preferably 50,000 ppm by volume or less, and more preferably 10,000 ppm by volume or less. ２ , CO, CO ２ , H ２ The oxidation treatment may contain O, NOx, etc., and the ratios thereof are not particularly limited. Although a more beautiful surface appearance can be obtained by the oxidation treatment, this step is not an essential requirement because it is possible to obtain a hot-dip galvanized steel sheet with excellent delayed fracture resistance even without the oxidation treatment.

[0033] In the oxidation treatment, the oxidation of the steel sheet is promoted by setting the steel sheet temperature to 400°C or higher. If the steel sheet temperature is below 400°C, the amount of oxidation will be insufficient, and oxides of Si and Mn will be formed, which may reduce the effect of improving platability. On the other hand, if the steel sheet temperature exceeds 900°C, the amount of oxidation of the steel sheet will be excessive, and reduction will not be completed in the subsequent reduction annealing (first annealing step), and the remaining iron oxide may inhibit platability. For this reason, the oxidation treatment is preferably carried out at 400°C or higher and 900°C or lower. Here, carrying out the oxidation treatment at 400°C or higher and 900°C or lower means that the oxidation treatment temperature is at least within the range of 400 to 900°C and does not exceed 900°C. That is, in the present invention, O ２ As long as the oxidation treatment is performed at a temperature of 400°C or higher and 900°C or lower in an atmosphere containing 1000 volume ppm or more of hydroxybenzoates, the oxidation treatment may also be performed at a temperature lower than 400°C (for example, when the oxidation treatment is performed during a temperature increase from 300°C to 700°C). This oxidation treatment is preferably performed for a treatment time in the range of 1 to 30 seconds. That is, from the viewpoint of ensuring a sufficient amount of oxidation and improving plating properties, the treatment time is preferably 1 second or longer, more preferably 2 seconds or longer, and even more preferably 3 seconds or longer. On the other hand, from the viewpoint of preventing excessive oxidation and suppressing pickup, the treatment time is preferably 30 seconds or shorter, more preferably 20 seconds or shorter, and even more preferably 15 seconds or shorter.

[0034] This oxidation treatment process can also include a step of heating the steel sheet (base steel sheet) to an annealing temperature. For example, the surface of the steel sheet can be oxidized by providing a soaking chamber capable of controlling the atmosphere during the steel sheet heating process and maintaining a constant temperature in a predetermined atmosphere. It is also possible to oxidize the surface of the steel sheet by controlling the atmosphere inside a direct-fire heating furnace equipped with a direct-fire burner while heating the steel sheet. Simultaneous heating and oxidation treatment can provide industrial advantages, such as a compact furnace and improved production speed. When oxidizing the surface of the steel sheet while heating it, a sufficient amount of oxidation can be achieved by providing an oxidizing atmosphere over a temperature range (temperature range) of 50°C or more after the steel sheet temperature reaches 400°C. When oxidizing the surface of the steel sheet while heating it, the heating rate within the oxidation temperature range is preferably 3 to 25°C / s in order to ensure an appropriate amount of oxidation.

[0035] Here, the direct-flame burner used for the oxidation treatment can be a burner that heats the steel sheet by directly applying the burner flame, which is a mixture of fuel such as coke oven gas (COG), a by-product gas from steel mills, and air, to the steel sheet surface. Heating using such a direct-flame burner has the advantage of increasing the temperature of the steel sheet faster than radiant heating methods, thereby shortening the furnace length and increasing the steel sheet transport speed. Furthermore, if the direct-flame burner has an air ratio of 0.95 or higher, i.e., a high ratio of air to fuel, unburned oxygen remains in the flame, which can promote the oxidation of the steel sheet. Therefore, adjusting the air ratio makes it possible to control the oxygen concentration in the atmosphere. In addition to COG, liquefied natural gas (LNG), ammonia gas, hydrogen gas, etc. can be used as fuel for the direct-flame burner.

[0036] Annealing - First Annealing Step In the first annealing step (first annealing step), the steel sheet is held at a temperature of 650°C to 950°C for 20 seconds to 150 seconds in an atmosphere with a dew point of -55°C to +30°C and a hydrogen concentration of 8.0% to 25% in which iron oxide is reduced. The natural iron oxide present in the surface layer of the steel sheet is reduced in a reducing atmosphere in the first annealing step to ensure galvanizability. Since the subsequent second annealing step in a low-hydrogen-concentration atmosphere hardly progresses in reduction, it is preferable to complete the reduction of iron oxide in the first annealing step. This first annealing step is essential for obtaining a good plating appearance. Furthermore, when oxidation treatment is performed, intentionally formed iron oxide is reduced in a reducing atmosphere in the first annealing step of this reduction annealing to form a reduced iron layer on the surface layer of the steel sheet. This prevents Si and Mn from diffusing into the surface layer of the steel sheet and oxidizing, thereby improving the appearance. Similarly, even if oxidation treatment is performed, reduction hardly progresses in the subsequent second annealing step in a low hydrogen concentration atmosphere, so it is preferable to reduce iron oxide in this first annealing step.

[0037] If the annealing temperature of the steel sheet in the first annealing step is less than 650°C, reduction is insufficient, resulting in roll pickup of iron oxide, which causes defects in the steel sheet. Furthermore, in the subsequent second annealing step, the iron oxide is not substantially reduced, resulting in non-galvanized areas. On the other hand, if the annealing temperature of the steel sheet exceeds 950°C, the furnace body life is significantly reduced. Therefore, the annealing temperature of the steel sheet is set to 650°C or higher and 950°C or lower. When the base steel sheet (base steel sheet) is a cold-rolled steel sheet, the annealing temperature is preferably set to 750°C or higher in order to ensure predetermined strength and ductility through recrystallization. Furthermore, to obtain a high-strength steel sheet with a tensile strength of 780 MPa or higher, it is necessary to ensure a predetermined total area ratio of martensite, bainite, and retained γ (retained austenite), and therefore the annealing temperature is preferably set to 780°C or higher. Higher annealing temperatures in the first annealing step promote selective oxidation of Si and Mn and increase the amount of hydrogen in the steel. In the present invention, by controlling the atmosphere and holding time in the first annealing step and the second annealing step, the effects of promoting selective oxidation and increasing the amount of hydrogen in the steel can be reduced, thereby providing excellent surface quality and excellent delayed fracture resistance. While a dew point of the atmosphere in the first annealing step below −55°C requires special equipment for lowering the dew point, which increases costs, a dew point of +30°C or less can reduce iron oxide in the surface layer of the steel sheet, and selective oxidation of Si and Mn can be suppressed within a specified annealing time range. If the dew point exceeds +30°C, the dew point distribution in the furnace becomes large, making dew point control difficult and raising concerns about the impact on the furnace body. Therefore, the dew point is set to −55°C or higher and +30°C or lower. The dew point is preferably −50°C or higher, more preferably −45°C or higher. The dew point is preferably 20°C or lower, more preferably 15°C or lower.

[0038] In the first annealing step, the higher the hydrogen concentration, the faster the reduction of iron oxide is completed and the more suppressed the selective oxidation of Si and Mn. However, the higher the hydrogen concentration, the more easily hydrogen dissolves in the steel, resulting in a decrease in delayed fracture resistance. In this regard, if the hydrogen concentration is less than 8.0%, reduction is insufficient. On the other hand, if the hydrogen concentration exceeds 25%, the reduction effect saturates and a large amount of hydrogen dissolves in the steel, making it difficult to sufficiently reduce the amount of hydrogen in the steel in the subsequent second annealing step. For this reason, the hydrogen concentration in the first annealing step is set to 8.0% or more and 25% or less. Furthermore, if oxidation treatment is performed, the hydrogen concentration is preferably 10% or more to ensure sufficient reduction. On the other hand, from the viewpoints of production cost and reducing hydrogen in the steel, the hydrogen concentration is preferably 22% or less, and more preferably 18% or less.

[0039] If the holding time in the first annealing step is less than 20 seconds at 650°C or higher and 950°C or lower, reduction is not sufficiently completed. On the other hand, if the holding time exceeds 150 seconds, the area ratio of martensite and bainite required to obtain high-strength steel with a tensile strength of 780 MPa or higher cannot be sufficiently secured. Furthermore, since reduction is sufficiently completed with a holding time of 150 seconds or shorter, a holding time of longer than 150 seconds unnecessarily reduces productivity. Furthermore, selective oxidation of Si and Mn progresses, deteriorating surface quality and coating adhesion. The amount of hydrogen in the steel saturates after a holding time of approximately 20 seconds, and the effect of the holding time is not significant. Therefore, the holding time in the first annealing step at 650°C or higher and 950°C or lower is set to 20 seconds or higher and 150 seconds or lower. The holding time is preferably 30 seconds or higher, more preferably 40 seconds or higher. The holding time is preferably 130 seconds or lower, more preferably 90 seconds or lower.

[0040] Second Annealing Step In the second annealing step, the steel sheet (base steel sheet) that has undergone the first annealing step is held at a holding temperature of 700°C to 950°C for 30 seconds to 300 seconds in an atmosphere with a dew point of -50°C to +30°C and a hydrogen concentration of 0.2% to less than 8.0%. In this second annealing step, the steel sheet that has been reduced in the first annealing step is maintained in a low-hydrogen atmosphere, thereby releasing hydrogen from the steel sheet. If the annealing temperature (holding temperature, soaking temperature) of the steel sheet in the second annealing step is less than 700°C, dehydrogenation is not promoted. On the other hand, if the annealing temperature exceeds 950°C, the furnace body is significantly affected. For this reason, the annealing temperature of the steel sheet is set to 700°C to 950°C. From the viewpoint of reducing the amount of hydrogen in the steel, the annealing temperature in the second annealing step is preferably set to 860°C or less, and more preferably to 830°C or less. Furthermore, in order to obtain a high-strength steel sheet having a tensile strength of 780 MPa or more, it is necessary to ensure a predetermined total area ratio of martensite, bainite, and retained γ, and the annealing temperature in the second annealing step is preferably 780°C or more.

[0041] In the second annealing step, the lower the dew point, the less the impact on the furnace body. However, to achieve a dew point of less than −50°C, special equipment for controlling the dew point is required, resulting in increased costs. On the other hand, if the dew point exceeds +30°C, the reduced Fe formed in the first annealing step may be reoxidized, impairing galvanization, and dew point control is difficult, raising concerns about the impact on the furnace body. For this reason, the dew point is set to −50°C or higher and +30°C or lower. From the viewpoint of controllability, the dew point is preferably +20°C or lower, more preferably +15°C or lower. The dew point is preferably −45°C or higher, more preferably −40°C or higher. In the second annealing step, the lower the hydrogen concentration, the more hydrogen dissolved in the steel sheet in the first annealing step is released. However, it is difficult to uniformly control the hydrogen concentration in the furnace to less than 0.2%, and there is a concern that the steel sheet may be reoxidized in areas with low hydrogen concentration. Therefore, the hydrogen concentration is set to 0.2% or higher. On the other hand, if the hydrogen concentration is 8.0% or more, the amount of hydrogen released at temperatures between 150°C and 350°C, as measured by temperature-programmed analysis, cannot be sufficiently reduced, so the hydrogen concentration is set to less than 8.0%. From the above viewpoints, the hydrogen concentration is preferably 1.0% or more, and more preferably 2.0% or more. Similarly, the hydrogen concentration is more preferably less than 5.0%.

[0042] If the holding time at 700°C or higher and 950°C or lower in the second annealing step is less than 30 seconds, hydrogen release is not fully completed. On the other hand, hydrogen release is fully completed with a holding time of 300 seconds or shorter, so a holding time of more than 300 seconds actually reduces productivity. Furthermore, selective oxidation of Si and Mn progresses, deteriorating surface quality and plating adhesion. For this reason, the holding time at 700°C or higher and 950°C or lower in the second annealing step is set to 30 seconds or longer and 300 seconds or shorter. In particular, from the viewpoint of fully releasing hydrogen released at 150°C or higher and 350°C or lower as measured by temperature ramp analysis, the holding time at 700°C or higher and 950°C or lower in the second annealing step is preferably 40 seconds or longer. The holding time is preferably 250 seconds or shorter, and more preferably 200 seconds or shorter. In the present invention, a high concentration of hydrogen is required in the first annealing step of annealing to reduce iron oxide that is naturally present on the steel sheet surface or iron oxide that has been produced by oxidation treatment. Since a large amount of hydrogen is dissolved in the steel accordingly, it is important to strike a balance between reduction and dehydrogenation. For this reason, it is necessary to optimize the conditions for the first annealing step and the second annealing step in annealing as described above.

[0043] Although the method for changing the hydrogen concentration in the first annealing step and the second annealing step is not particularly specified, it is possible to easily control the atmospheres in the first annealing step and the second annealing step individually by dividing the furnace, using furnaces connected via seal rolls, and controlling the hydrogen concentration and dew point of the gas introduced into each furnace. In the present invention, it is preferable to anneal the steel sheet using a continuous annealing furnace that is capable of controlling two or more separate different atmospheres.

[0044] Cooling Step: The base steel sheet that has undergone annealing (second annealing step) is cooled from the holding temperature of 700°C to 950°C (final holding temperature of the second annealing step) in the second annealing step to a temperature of 150°C to 550°C. Thereafter, after heating as necessary, the steel sheet is immersed in a hot-dip galvanized coating bath to perform hot-dip galvanized coating. Here, the final holding temperature in the second annealing step refers to the temperature at which a steel sheet that has been annealed within ranges that satisfy the annealing temperature, hydrogen concentration, dew point, and holding time requirements of the second annealing step falls outside of at least one of the requirements. In this cooling step, the average cooling rate from the holding temperature of 700°C to 950°C (final holding temperature of the second annealing step) in the second annealing step (final holding temperature of the second annealing step) to a temperature of 150°C to 550°C is set to 3°C / s or more and 300°C / s or less in an atmosphere with a dew point of -20°C or less and a hydrogen concentration of 0.2% to 25%. Here, the average cooling rate can be obtained by dividing the difference between the cooling start temperature (the final holding temperature of the second annealing step) (°C) and the cooling end temperature (150°C or higher and 550°C or lower) by the cooling time (s). In the cooling step, from the holding temperature of 700°C or higher and 950°C or lower (the final holding temperature of the second annealing step) to 550°C, the atmosphere has a dew point of -20°C or lower and a hydrogen concentration of 0.2% or higher and 25% or lower, and the average cooling rate in this temperature range is preferably 3°C / s or higher and 30°C / s or lower. Furthermore, even in the temperature range of 550°C or lower and 150°C or higher, the atmosphere has a dew point of -20°C or lower and a hydrogen concentration of 0.2% or higher and 25% or lower, and the average cooling rate is preferably 5°C / s or higher. Furthermore, the average cooling rate in this case is preferably 300°C / s or lower. Furthermore, when heating is performed after cooling, the heating rate is preferably 5°C / s or higher. This cooling step can also be performed in two separate cooling zones with different atmospheres, as described below, and will be referred to as a first cooling step and a second cooling step below. Here, the average cooling rate can be obtained by dividing the difference between the cooling start temperature (the final holding temperature of the second annealing step) (°C) and 550°C, and between 550°C and the cooling end temperature (150°C or higher and 550°C or lower), by the respective cooling times (s). The dew point in the cooling step refers to a dew point of -20°C or lower from the start to the end of cooling.The hydrogen concentration in the cooling step refers to a hydrogen concentration of 0.2% or more and 25% or less from the start of cooling to the end of cooling. The lower limit of the dew point in the cooling step is not particularly limited, but a dew point below −55° C. requires special equipment for lowering the dew point, which increases costs. Therefore, the dew point in the cooling step is preferably −55° C. or more.

[0045] First Cooling Step The cooling step includes a first cooling step, in which the base steel sheet that has undergone the annealing (second annealing step) is cooled in an atmosphere with a dew point of −20°C or lower and a hydrogen concentration of 0.2% by volume or higher but less than 8.0% by volume over a period of 10 seconds or longer at an average cooling rate of 3°C / s or higher and 25°C / s or lower through a temperature range from the holding temperature of 700°C or higher but not higher than 950°C in the second annealing step (the final holding temperature in the second annealing step) to a steel sheet temperature of 650°C or lower. By cooling at an average cooling rate of 3°C / s or higher through a temperature range from the final holding temperature in the second annealing step to a steel sheet temperature of 650°C or lower, the desired steel sheet strength can be obtained. The steel sheet temperature is preferably 600°C or higher. If the steel sheet temperature is lower than 600°C, the hydrogen reduction effect saturates and it becomes difficult to increase the cooling rate, making it difficult to ensure the desired microstructure and strength. If the average cooling rate is lower than 3°C / s, the steel sheet strength is likely to decrease. Therefore, the average cooling rate is set to 3°C / s or higher. The average cooling rate is preferably 5°C / s or more. On the other hand, if the average cooling rate exceeds 25°C / s in the temperature range up to a steel sheet temperature of 650°C or less, the holding time in the atmosphere with a hydrogen concentration of 0.2% or more but less than 8.0% in the first cooling step becomes short, and it becomes impossible to release the relatively stable quasi-diffusible hydrogen that was not released in the first annealing step. Therefore, the average cooling rate is set to 25°C / s or less. The average cooling rate is preferably 23°C / s or less, and more preferably 20°C / s or less. Furthermore, when the hydrogen concentration in the atmosphere is 8.0% or more, hydrogen that is released at a temperature between 150°C and 350°C as measured by temperature ramp analysis is likely to remain in the steel sheet. Here, the final holding temperature in the second annealing step refers to the temperature at which the steel sheet annealed within ranges satisfying the requirements of the annealing temperature, hydrogen concentration, dew point, and holding time in the second annealing step deviates from at least one of the requirements. The average cooling rate (°C / s) is obtained by dividing the difference between the cooling start temperature (final holding temperature in the second annealing step) (°C) and the cooling end temperature (steel sheet temperature of 650°C or less) by the cooling time (s). The cooling end temperature (steel sheet temperature of 650°C or less) refers to the temperature at which the hydrogen concentration transitions from less than 8.0% to 8.0% or more.

[0046] The cooling zone atmosphere will now be described. Because hydrogen has a high cooling capacity, the higher the hydrogen concentration in the atmosphere, the faster the cooling rate. However, if the hydrogen concentration is too high, hydrogen may penetrate into the steel sheet during cooling. Furthermore, hydrogen release, particularly at temperatures between 150°C and 350°C, may be insufficient, as measured by temperature ramp analysis. When cooling from a relatively high temperature, the cooling efficiency is high, so in the region of 650°C or higher, the amount of hydrogen required to achieve a cooling rate of 3°C / s or higher is less than in cooling at lower temperatures. In addition, the cooling rate required to achieve the required strength in the region of 650°C or higher is smaller than the cooling rate below 650°C, which has the greatest impact on steel sheet strength. Furthermore, by providing a cooling section in which the hydrogen concentration is less than 8.0% in the temperature range below the holding temperature of the second annealing temperature, the amount of hydrogen released at the temperature range of 150°C to 350°C can be further reduced. Therefore, in the first cooling step, a section in which the hydrogen concentration is 0.2% to less than 8.0% is provided within the temperature range from the final holding temperature of the second annealing step to a steel sheet temperature of 650°C or lower. If the hydrogen concentration is less than 0.2%, there is a risk that a sufficient cooling rate cannot be ensured, and the strength of the steel sheet is likely to decrease. On the other hand, if the hydrogen concentration is 8.0% or more, the cooling rate will be fast, but hydrogen may penetrate into the steel sheet during cooling, and the hydrogen that was not completely released in the first annealing step and is released at 150°C or more and 350°C or less as measured by temperature rise analysis will not be fully released. Therefore, the hydrogen concentration is set to 0.2% or more and less than 8.0%. The hydrogen concentration is preferably 0.5% or more, and more preferably 1.0% or more. Furthermore, the hydrogen concentration is preferably 4.0% or less, and more preferably 3.0% or less.

[0047] Furthermore, by setting the dew point at −20° C. or lower in the section where the atmosphere has a hydrogen concentration of 0.2% or higher but lower than 8.0%, it is possible to prevent the steel sheet from being reoxidized at low temperatures and resulting in a decrease in galvanizability. That is, when the dew point exceeds −20° C., the steel sheet is likely to be reoxidized at low temperatures and result in a decrease in galvanizability. In the section where the atmosphere has a dew point of −20° C. or lower and a hydrogen concentration of 0.2% or higher but lower than 8.0%, it is preferable to retain the steel sheet for 10 seconds or longer. Retaining the steel sheet for 10 seconds or longer can promote the release of hydrogen from the steel, thereby further reducing the amount of hydrogen released at the above-mentioned temperature range of 150° C. or higher but 350° C. or lower. Although there is no particular limitation on the lower limit, a dew point of less than −55° C. requires special equipment for lowering the dew point, which increases costs. Therefore, it is preferable to set the dew point in the first cooling step to −55° C. or higher.

[0048] The section in which the atmosphere has a dew point of −20° C. or less and a hydrogen concentration of 0.2% or more and less than 8.0% may be a section in which continuous cooling is performed, or a section in which cooling is stopped and the atmosphere is maintained at a substantially constant temperature. The section in which the atmosphere has a dew point of −20° C. or less and a hydrogen concentration of 0.2% or more and less than 8.0% may be set at least in a temperature range from the holding temperature of the second annealing temperature to 650° C. or less. For example, the section may be set in a range from the holding temperature of the second annealing temperature to 650° C., or may be set in a range from the holding temperature of the second annealing temperature to 620° C. As will be described later, in order to set the cooling start temperature of the second cooling step to 600° C., the section in which the atmosphere has a dew point of −20° C. or less and a hydrogen concentration of 0.2% or more and less than 8.0% is preferably set to a temperature above 600° C.

[0049] Second Cooling Step: The steel sheet (base steel sheet) that has undergone the first cooling step is cooled from the cooling start temperature (600°C) to a steel sheet temperature of 150°C to 550°C at an average cooling rate of 10°C / s or more in an atmosphere with a dew point of −20°C or less and a hydrogen concentration of 8.0% to 25%. Thereafter, after heating as necessary, the steel sheet is immersed in a hot-dip galvanized coating bath to perform hot-dip galvanized coating. By cooling at an average cooling rate of 10°C / s or more in the temperature range from 600°C to a steel sheet temperature of 150°C to 550°C at a temperature range of 10°C / s or more, the desired steel sheet strength can be obtained and the intrusion of hydrogen in the atmosphere into the steel sheet during cooling can be suppressed. In the temperature range of 600°C or less, the cooling rate has a significant effect on the steel sheet strength. Therefore, if the average cooling rate is less than 10°C / s, the steel sheet strength is likely to decrease and hydrogen in the atmosphere is likely to infiltrate the steel sheet, resulting in a decrease in delayed fracture resistance. Therefore, the average cooling rate is set to 10°C / s or more. The average cooling rate is preferably 13°C / s or more. The average cooling rate (°C / s) is obtained by dividing the difference between 600°C and the steel sheet temperature of 150°C or more and 550°C or less by the cooling time (s). Note that the temperature range of more than 600°C and less than 650°C is a transition zone for switching the hydrogen concentration, and the steel sheet may be cooled at a cooling rate of 3°C / s or more and 25°C / s or less with a dew point of -20°C or less and a hydrogen concentration of 0.2% or more and 25% or less.

[0050] The second cooling step has a lower cooling temperature than the first cooling step. Therefore, even if a higher concentration of hydrogen is present in the atmosphere than in the first cooling step, the hydrogen is less likely to penetrate into the steel sheet and is less likely to become relatively stable quasi-diffusible hydrogen. However, the second cooling step, which is performed at a lower temperature, has a greater impact on strength degradation when the cooling rate is reduced compared to the first cooling step. Therefore, the hydrogen concentration in the cooling zone atmosphere is set to 8.0% or more and 25% or less. If the hydrogen concentration is less than 8.0%, a sufficient cooling rate may not be ensured, which may lead to a decrease in steel sheet strength. On the other hand, if the hydrogen concentration exceeds 25%, the cooling effect saturates, hydrogen is more likely to penetrate into the steel sheet during cooling, and the amount of hydrogen released at temperatures between 150°C and 350°C as measured by temperature ramp analysis increases, which may lead to a decrease in delayed fracture resistance. The hydrogen concentration is preferably 10% or more, more preferably 12% or more. Furthermore, the hydrogen concentration is preferably 20% or less, more preferably 15% or less. Furthermore, by setting the dew point to -20°C or lower, it is possible to prevent the steel sheet from being reoxidized at low temperatures and thereby reducing platability. That is, if the dew point exceeds -20°C, the steel sheet is likely to be reoxidized at low temperatures and reduce platability. Although there are no particular restrictions on the lower limit, setting the dew point to less than -55°C requires special equipment for lowering the dew point, which increases costs. Therefore, it is preferable that the dew point in the second cooling step be -55°C or higher.

[0051] After the second cooling step, the base steel sheet is cooled to 150°C or higher and 550°C or lower, and then, before being immersed in the galvanizing bath, the steel sheet can be held at a temperature of 150°C or higher and 550°C or lower with a dew point of 0°C or lower and a hydrogen concentration of 15% or lower for 5 seconds or longer. Holding the steel sheet at 150°C or higher and 550°C or lower after the second cooling step can partially generate bainite and martensite. This treatment can increase the amount of retained γ and the C concentration in the retained γ in the final product. As a result, ductility, stretch flange formability, and bendability can be improved. Furthermore, ductility can be improved by generating bainitic ferrite and massive martensite, thereby reducing the YR. From the viewpoint of achieving these effects, the holding time is preferably 5 seconds or longer, more preferably 10 seconds or longer, and even more preferably 20 seconds or longer. Since a large amount of untransformed austenite remains in the steel during holding at 150°C or higher and 550°C or lower, the amount of hydrogen released from the steel is not large. However, it is possible to slightly reduce the amount of hydrogen in the steel by setting the hydrogen concentration in the atmosphere to 15% or lower. From the viewpoint of reducing hydrogen in the steel, a holding time of 20 seconds or longer is preferable. On the other hand, from the viewpoint of minimizing the decrease in the strength of the steel sheet, a holding time of 120 seconds or shorter is preferable, and 100 seconds or shorter is even more preferable.

[0052] Hot-dip galvanizing process: The conditions for the hot-dip galvanizing process are not particularly limited, and general conditions may be used. That is, preferably under the conditions described above, the substrate steel sheet is cooled to a temperature of 150°C to 550°C in the cooling process, and then, if necessary, heated to the bath temperature, and then immersed in the hot-dip galvanizing bath for coating. In the case of GA or GI, the coating bath contains Zn, Al, and unavoidable impurities. While the components are not particularly specified, the Al concentration in the bath is preferably 0.05% to 0.250%. If the Al concentration in the bath is less than 0.05%, the occurrence of bottom dross increases, and the dross tends to adhere to the steel sheet, resulting in defects. On the other hand, if the Al concentration exceeds 0.190%, the occurrence of top dross increases, and the dross also tends to adhere to the steel sheet, resulting in defects. The addition of Al also increases costs. The hot-dip galvanizing bath temperature is preferably 440 to 500°C.

[0053] Alloying Treatment Step: After the hot-dip galvanizing treatment in the hot-dip galvanizing step, an alloying treatment can be performed subsequently. The conditions for the alloying treatment are not particularly limited. The alloying treatment is preferably performed by heating the steel sheet having a zinc-based plating layer (hot-dip galvanized steel sheet) to an alloying temperature of 430°C or higher after the hot-dip galvanizing treatment. The alloying temperature is preferably 600°C or lower. If the alloying temperature is lower than 430°C, the Fe-Zn alloying rate may be slow, making alloying difficult. On the other hand, if the alloying temperature exceeds 600°C, excessive Fe-Zn alloying may occur, resulting in deterioration of the coating appearance and coating adhesion. Furthermore, untransformed austenite may transform to pearlite, making it impossible to obtain the desired tensile strength. The alloying temperature is more preferably 450°C or higher. The alloying temperature is more preferably 570°C or lower.

[0054] - Reheating Treatment Step Next, the steel sheet that has been subjected to the hot dip galvanizing step or further to the alloying step is cooled to room temperature to 450°C as needed, and further subjected to temper rolling and heat treatment as needed. After that, the steel sheet may be reheated for 5 seconds or more and 600 seconds or less at a temperature range of 200°C to 450°C in an atmosphere with a hydrogen concentration of 15% by volume or less, and then cooled. The cooling may be performed to room temperature. From the viewpoint of promoting the release of hydrogen that is released at 150°C to 350°C as measured by temperature rise analysis in steel during reheating, it is advantageous to lower the hydrogen concentration in the atmosphere. Therefore, the hydrogen concentration is preferably 15% or less, and more preferably 5% or less. Note that there is no particular limitation on the lower limit of the hydrogen concentration. However, hydrogen is unavoidably contained in the atmosphere. Therefore, the hydrogen concentration may be, for example, 0.00001% by volume or more. The hydrogen (H ２ The remaining gas other than the above is not particularly limited. For example, the remaining gas is N ２ , O ２ , H ２ O, CO ２、Ar and unavoidable impurities, as well as combinations thereof. The ratios of these are not particularly limited. Reheating may be performed in the atmosphere. To fully achieve the effect of reducing hydrogen in steel that is released at temperatures between 150°C and 350°C as measured by temperature-ramp analysis, the reheating temperature is preferably 200°C or higher. On the other hand, a reheating temperature above 450°C may remelt the coating layer, resulting in deterioration of the appearance. Therefore, the reheating temperature is preferably 200°C or higher and 450°C or lower. The reheating temperature is more preferably 250°C or higher. The reheating temperature is more preferably 400°C or lower. Here, the reheating temperature refers to the maximum temperature reached during reheating. The reheating temperature may be constant or variable within the range of 200°C or higher and 450°C or lower. If the reheating time is less than 5 seconds, the effect of reducing hydrogen released at temperatures between 150°C and 350°C as measured by temperature-ramp analysis may not be fully achieved. Therefore, a reheating time of 5 seconds or longer is preferred. The reheating time is more preferably 10 seconds or more, and even more preferably 30 seconds or more. On the other hand, from the viewpoint of productivity, the reheating time is preferably 600 seconds or less. The reheating time is a holding time in a temperature range of 200°C to 450°C at a temperature equal to or higher than the cooling stop temperature after the hot dip galvanizing treatment or further after the alloying treatment.

[0055] The temper rolling may be performed before and / or after the reheating treatment. When temper rolling is performed, it may be performed either before or after the reheating treatment, but it is preferable to achieve an elongation rate of 0.05% or more and 1.0% or less. A step of holding the steel sheet in an atmosphere at a temperature of room temperature to less than 200°C and with a hydrogen concentration of 15% or less may be added before or after the reheating treatment step. When the steel sheet is held in an atmosphere at a temperature of room temperature to less than 200°C and with a hydrogen concentration of 15% or less, the steel sheet temperature is held at 10 to 25°C for 4 days or more, at a temperature higher than 25°C and lower than 80°C for 1 day or more, or at a temperature higher than 80°C and lower than 200°C for 30 minutes or more, whereby the amount of hydrogen released at 25°C to 140°C as measured by temperature ramp analysis in the steel is reduced to 0.2 ppm or less.

[0056] The conditions other than those mentioned above are not particularly limited, and may be those according to conventional methods.

[0057] The hot-dip galvanized steel sheet of the present invention contains, in mass %, C: 0.06% to 0.30%, Si: 0.01% to 3.00%, Mn: 1.5% to 3.5%, P: 0.1% or less (excluding 0%), S: 0.0030% or less (excluding 0%), sol. containing Al: 0.1% or less (not including 0%), N: 0.0070% or less (not including 0%), O: 0.0030% or less (not including 0%), or further containing one or more elements selected from the following groups A, B, C, and D; Group A: one or more elements selected from Nb: 0.05% or less (not including 0%), Ti: 0.08% or less (not including 0%), V: 0.2% or less (not including 0%), W: 0.15% or less (not including 0%), Zr: 0.15% or less (not including 0%); Group B: one or more elements selected from Cr: 1% or less (not including 0%), Ni: 1% or less (not including 0%), Cu: 1% or less (not including 0%), Mo: 1% or less (not including 0%), Co: 1% or less (not including 0%), B: 0.005% or less (not including 0%); Group C: Group D: One or more selected from Sn: 0.2% or less (excluding 0%), Sb: 0.2% or less (excluding 0%); Group E: Ta: 0.10% or less (excluding 0%); Group F: One or more selected from Te: 0.10% or less (excluding 0%), As: 0.10% or less (excluding 0%), Hf: 0.10% or less (excluding 0%); Group G: One or more selected from Bi: 0.20% or less (excluding 0%), Pb: 0.20% or less (excluding 0%); Group H: Zn: 0.10% or less (excluding 0%), Ge: 0.10% or less (excluding 0%); A steel sheet having a chemical composition consisting of one or more elements selected from Sr: 0.10% or less (excluding 0%), Cs: 0.10% or less (excluding 0%), and the balance consisting of Fe and unavoidable impurities; and a zinc-based plating layer formed on the surface of the steel sheet,Furthermore, the inclusions contained in the base steel sheet satisfy the following conditions (i) and (ii), and the number of inclusions is 10 / mm ２ The amount of hydrogen released from the substrate steel sheet at a temperature from room temperature to 350°C (hydrogen concentration H Ｌ ) is 0.30 mass ppm or less, and the amount of hydrogen released at 150 to 350 ° C (hydrogen concentration H Ｈ ) is 0.20 mass ppm or less, the number of uncoated defects is less than 3 within a 1 m x 1 m area on the steel sheet surface, and the tensile strength is 780 MPa or more. (Conditions) (i) The length of the major axis of an inclusion group having one or more inclusion particles is 50 μm or more. (ii) When an inclusion group is composed of two or more of the above inclusion particles, the length of the major axis of one inclusion particle is 0.3 μm or more, and the shortest distance between the inclusion particles is 40 μm or less.

[0058] There is no particular limitation on the coating weight of the zinc-based coating layer on one side of the base steel sheet by hot dip galvanizing treatment, but it is preferably 20 to 120 g / m ２ The plating weight of the zinc-based plating layer per side may be 20 g / m ２ If the thickness is less than 120 g / m, not only is the corrosion resistance liable to decrease, but it is also difficult to control the coating weight. ２ If the alloying degree exceeds 7%, the coating adhesion is likely to decrease. There are no particular restrictions on the method for adjusting the coating weight, but gas wiping is generally used, and adjustment is made by adjusting the gas pressure of the gas wiping, the distance between the wiping nozzle and the steel sheet, etc. When alloying treatment is performed after hot-dip galvanizing treatment, there are no particular restrictions on the degree of alloying of the zinc coating layer after the alloying treatment, but an alloying degree of 7 to 15% is preferred. If the alloying degree is less than 7%, the η phase will remain and press formability will likely decrease, while if it exceeds 15%, coating adhesion will likely decrease.

[0059] The base steel sheet (base steel sheet) may be either a cold-rolled steel sheet or a hot-rolled steel sheet. Since delayed fracture resistance is a problematic property in high-strength steel sheets, the steel sheet is preferably a high-strength steel sheet having a tensile strength TS of 780 MPa or more, preferably 980 MPa or more, more preferably 1180 MPa or more, even more preferably 1310 MPa or more, and still more preferably 1470 MPa or more.

[0060] In the present invention, the inclusion group contained in the substrate steel sheet (base steel sheet) satisfies the following conditions (i) and (ii), and the inclusion group is 10 pieces / mm ２ (i) the length of the major axis of an inclusion group having one or more inclusion particles is 50 μm or more, and (ii) when an inclusion group is composed of two or more of the above inclusion particles, the length of the major axis of one inclusion particle is 0.3 μm or more and the shortest distance between the inclusion particles is 40 μm or less.

[0061] The steel contains MnS, Al ２ O ３ , (Al,Ca)-O, Nb(C,N), (Nb,Ti)(C,N), Ti(C,S) and inclusions consisting of complex precipitates of MnS with these as nuclei are present, and hydrogen trapped around these inclusions with a relatively weak force is detected as quasi-diffusible hydrogen, so the fewer these inclusions there are, the more the delayed fracture resistance can be improved. In order to improve delayed fracture resistance to a high level, the number of inclusions should be 10 / mm ２ The number of inclusions must be 5 or less per mm ２ The number of inclusions is preferably 0 pieces / mm or less. ２ It is more preferable that:

[0062] Furthermore, the length of the major axis of an inclusion group having one or more inclusion particles as specified in the present invention is 50 μm or more. An inclusion group having a major axis length of 50 μm or more adversely affects delayed fracture resistance. When an inclusion group is composed of two or more inclusion particles, the major axis length of the inclusion particles constituting the inclusion group is 0.3 μm or more. The reason for focusing on inclusion particles having a major axis length of 0.3 μm or more is that inclusion particles less than 0.3 μm in size, even when aggregated, have little adverse effect on delayed fracture resistance. This is because the frequency of inclusion groups is low, and cracks smaller in size are less likely to link and propagate. Furthermore, the length of the major axis refers to the length of the inclusion particle in the rolling direction. When an inclusion group is composed of two or more inclusion particles, the shortest distance between the inclusion particles is 40 μm or less. Here, the shortest distance between inclusion particles refers to the shortest distance between the two inclusion particles in question, and can also be called the shortest interparticle distance.

[0063] By defining the inclusion group in this way, the inclusion group that affects the delayed fracture resistance properties can be appropriately expressed for the inclusion group having one or more inclusion particles that satisfy the condition (i), and the steel sheet unit area (mm ２) can improve delayed fracture resistance. The shortest distance specified in condition (ii) applies to inclusion particles located in a sector-shaped region of ±10° relative to the rolling direction, with the longitudinal end of the inclusion particle as the center point (even if a part of the inclusion particle is included in the above region). The shortest distance between particles is the shortest distance between points on the periphery of each particle. There are no particular limitations on the shape or state of existence of the inclusion particles constituting the inclusion group, but in the present invention, they may be inclusion particles elongated in the rolling direction or inclusions distributed in a dotted row in the rolling direction. "Inclusion particles distributed in a dotted row in the rolling direction" refers to particles consisting of two or more inclusion particles distributed in a dotted row in the rolling direction. "Distributed in a dotted row in the rolling direction" refers to a distribution state similar to that in which inclusions elongated in the rolling direction are divided during cold rolling and distributed in a dotted row, for example. This is an explanation of the distribution state, and does not mean that it is limited to a distribution in a dotted pattern divided by cold rolling.

[0064] Furthermore, in the present invention, when measured by temperature-programmed analysis, the total amount of diffusible hydrogen released from room temperature to 350° C. (hydrogen concentration H Ｌ ) is 0.30 mass ppm or less, and the hydrogen concentration H Ｈ The total amount of hydrogen released from room temperature to 350°C (hydrogen concentration H Ｌ ) is the amount of hydrogen released at temperatures between 150°C and 350°C, and the hydrogen concentration H Ｈ The total amount of hydrogen (hydrogen concentration H Ｌ ), the diffusible hydrogen that is released at temperatures below 150°C when measured by temperature rise analysis is released into the atmosphere when the high-strength hot-dip galvanized steel sheet is left for about two weeks after production, and has a relatively small effect on the delayed fracture resistance. On the other hand, the amount of hydrogen that is released at temperatures between 150°C and 350°C (hydrogen concentration (quasi-diffusible hydrogen concentration) H Ｈ ) measured immediately after manufacture hardly decreases by leaving at room temperature, and it has a large effect on the delayed fracture resistance. Therefore, the amount of hydrogen released at 150°C or higher and 350°C or lower when measured by this temperature rise analysis (hydrogen concentration H ＨThe lower the value of (I) the better, and it is preferably 0.15 mass ppm or less, more preferably 0.10 mass ppm or less, and even more preferably 0.05 mass ppm or less. In the present invention, "room temperature" refers to 25°C.

[0065] Furthermore, the hot-dip galvanized steel sheet of the present invention has less than three uncoated defects in an area of ​​1 m × 1 m on the steel sheet surface, and has a beautiful surface appearance free of uncoated defects. Uncoated defects can be evaluated by visually observing the coating appearance of the hot-dip galvanized steel sheet, and the detailed method is as described in the examples below.

[0066] The thickness of the steel plate is not particularly limited, but is preferably 0.5 mm or more, and is preferably 3.2 mm or less.

[0067] - Elemental Composition A preferred elemental composition of the substrate steel sheet (base steel sheet) will be described below. The substrate steel sheet (base steel sheet) targeted in the present invention has an elemental composition of C: 0.06% or more and 0.30% or less, Si: 0.01% or more and 3.00% or less, Mn: 1.5% or more and 3.5% or less, P: 0.1% or less (not including 0%), S: 0.0030% or less (not including 0%), sol. Al: 0.1% or less (not including 0%), N: 0.0070% or less (not including 0%), and O: 0.0030% or less (not including 0%), and may further contain one or more elements selected from the following groups A, B, C, D, E, F, G, and H. Group A: One or more selected from Nb: 0.05% or less (not including 0%), Ti: 0.08% or less (not including 0%), V: 0.2% or less (not including 0%), W: 0.15% or less (not including 0%), Zr: 0.15% or less (not including 0%). Group B: One or more selected from Cr: 1% or less (not including 0%), Ni: 1% or less (not including 0%), Cu: 1% or less (not including 0%), Mo: 1% or less (not including 0%), B: 0.005% or less (not including 0%). Group C: One or more selected from Ca: 0.005% or less (not including 0%), Mg: 0.005% or less (not including 0%), REM: 0.005% or less (not including 0%). Group D: One or more selected from Sn: 0.2% or less (not including 0%), Sb: 0.2% or less (not including 0%). Group E: Ta: 0.10% or less (not including 0%) Group F: One or more selected from Te: 0.10% or less (not including 0%), As: 0.10% or less (not including 0%), Hf: 0.10% or less (not including 0%) Group G: One or more selected from Bi: 0.20% or less (not including 0%), Pb: 0.20% or less (not including 0%) Group H: One or more selected from Zn: 0.10% or less (not including 0%), Ge: 0.10% or less (not including 0%), Sr: 0.10% or less (not including 0%), Cs: 0.10% or less (not including 0%) The reasons for including each of the above elements are as follows.

[0068] C: 0.06% or more and 0.30% or less C forms martensite, bainite, residual γ, and the like as a steel structure, thereby having the effect of increasing the strength of the steel and improving its workability. In order to obtain good workability while achieving high strength, the C content is set to 0.06% or more. In order to obtain good workability while achieving high strength, the C content is more preferably set to 0.09% or more. On the other hand, in order to obtain good weldability, the C content is set to 0.30% or less. From the viewpoint of obtaining good weldability, the C content is preferably set to 0.25% or less.

[0069] Si: 0.01% or more and 3.00% or less Si is an effective element for achieving high strength in steel sheets because it has a significant effect of increasing the strength of steel through solid solution (solid solution strengthening ability) without significantly impairing workability. Therefore, the Si content is set to 0.01% or more. On the other hand, if the Si content exceeds 3.00%, the hot rolling property and cold rolling property are significantly reduced, adversely affecting productivity. In addition, there is a risk of reducing the ductility of the steel sheet. Therefore, the Si content is set to 3.00% or less. From this viewpoint, the Si content is preferably 2.50% or less, and more preferably 2.00% or less.

[0070] Mn: 1.5% or more and 3.5% or less Mn is an element that strengthens steel through solid solution strengthening, improves hardenability, and promotes the formation of retained γ, bainite, and martensite. These effects are achieved by including 1.5% or more of Mn. Therefore, the Mn content is set to 1.5% or more. The Mn content is preferably set to 1.8% or more. On the other hand, if the Mn content exceeds 3.5%, the amount of inclusions such as MnS formed increases, deteriorating the delayed fracture resistance. Therefore, the Mn content is set to 3.5% or less. The Mn content is preferably set to 3.3% or less.

[0071] P: 0.1% or less (excluding 0%). By suppressing the P content, it is possible to prevent a decrease in weldability, and furthermore, it is possible to prevent P from segregating at grain boundaries, thereby preventing deterioration of ductility, bendability, and toughness. Furthermore, if a large amount of P is contained, the ferrite transformation is promoted, which increases the grain size. For this reason, the P content is set to 0.1% or less. There is no particular lower limit for the P content, but due to constraints on production technology, it is preferable that the P content be 0.001% or more.

[0072] S: 0.0030% or less (excluding 0%). The S content must be strictly controlled because it has a significant adverse effect on delayed fracture resistance through the formation of MnS, TiS, Ti(C,S), etc. In particular, MnS is elongated during hot rolling and is crushed and elongated during cold rolling. This can result in the formation of inclusions reaching lengths of 80 to 400 μm in the final product. Furthermore, in Nb- and Ti-added steel, if Nb(C,N), (Nb,Ti)(C,N), etc. remain during slab heating, MnS forms around them in combination, which promotes the formation of inclusions with lengths of 20 μm or more but less than 100 μm. Furthermore, compared to Nb(C,N), etc., MnS covers the periphery, which reduces the peel strength with the matrix, potentially significantly worsening delayed fracture. In the case of the steel plate (thin steel plate) of the present invention, for example, if the thickness of the cast slab is approximately 180 to 250 mm and the thickness of the final product plate is 0.5 to 2.6 mm, the cumulative reduction amount will be approximately 99%. In thin steel plates, the elongation rate in the rolling direction is 5 to 10 times that of thick steel plates, making the adverse effects of MnS even greater. Furthermore, particularly at the sheared edge surface after punching, the entire thickness direction of the plate is exposed, resulting in the formation of large inclusions in the Mn-segregated area at the center of the plate thickness. As a result, combined with the Mn segregation, this has a particularly adverse effect on delayed fracture resistance. To mitigate the adverse effects of these inclusions, the S content must be 0.0030% or less. The S content is preferably 0.0020% or less, more preferably 0.0010% or less, and even more preferably 0.0003% or less. While there is no particular lower limit for the S content, a S content of 0.0001% or more is preferred due to production technology constraints.

[0073] Sol. Al: 0.1% or less (excluding 0%). A sol. Al content exceeding 0.1% increases costs. Therefore, the sol. Al content is set to 0.1% or less. While there is no particular lower limit for the Al content, removing impurity-level Al also increases costs, so the sol. Al content is preferably set to 0.001% or more. Furthermore, because Al is thermodynamically very susceptible to oxidation, it oxidizes prior to Si and Mn, suppressing the oxidation of Si and Mn in the outermost layer of the steel sheet and promoting the oxidation of Si and Mn inside the steel sheet. This effect is achieved when the sol. Al content is 0.01% or more. Therefore, the sol. Al content is preferably set to 0.01% or more.

[0074] N: 0.0070% or less (excluding 0%) N is an element that forms nitride and carbonitride inclusions such as TiN, (Nb, Ti)(C, N), and AlN in steel, and the formation of these inclusions degrades delayed fracture resistance. While these inclusions alone have little effect on delayed fracture, some of them become nuclei for MnS precipitation and, together with MnS, form a dotted array of large inclusions, thereby degrading delayed fracture resistance. To suppress the formation of such inclusions, the N content must be 0.0070% or less. The N content is preferably 0.005% or less, and more preferably 0.002% or less. While there is no particular lower limit for the N content, the N content is preferably 0.0005% or more due to production technology constraints.

[0075] O: 0.0030% or less (excluding 0%) O is the element that ２ O ３ , SiO ２ O is an element that forms oxide-based inclusions such as CaO, MgO, (Al,Ca)—O, and (Si,Mn)—O, and through the formation of these, it deteriorates delayed fracture resistance. In order to minimize such adverse effects on delayed fracture resistance, the O content is set to 0.0030% or less. From the viewpoint of improving delayed fracture resistance, the O content is more preferably set to less than 0.0020%. Note that the lower limit of the O content is not particularly limited, but considering the lower limit currently industrially feasible, the O content is preferably set to 0.0005% or more.

[0076] Group A [one or more elements selected from Nb: 0.05% or less (excluding 0%), Ti: 0.08% or less (excluding 0%), V: 0.2% or less (excluding 0%), W: 0.15% or less (excluding 0%), and Zr: 0.15% or less (excluding 0%)] Nb, Ti, V, W, and Zr are all effective elements for increasing the strength of the substrate steel sheet (base steel sheet) and can be contained as needed. Of these elements in Group A, Nb is an element that can obtain a fine structure even when contained in a small amount, and can achieve high strength without impairing toughness.

[0077] Nb: 0.05% or less (excluding 0%). Even a small amount of Nb contributes significantly to high strength and improved delayed fracture resistance through the refinement of prior γ grains, the resulting refinement of the internal structure of martensite and bainite, the formation of fine precipitates that serve as hydrogen trapping sites, and the formation of texture. From this perspective, it is preferable to add 0.002% or more of Nb. The Nb content is more preferably 0.004% or more, and even more preferably 0.010% or more. On the other hand, if a large amount of Nb is added, the amount of coarse Nb-based precipitates, such as NbN, Nb(C,N), and (Nb,Ti)(C,N), that remain in solid solution during slab heating in the hot rolling process increases, and MnS precipitates in combination with these precipitates, deteriorating delayed fracture resistance. Therefore, when Nb is added, the Nb content is set to 0.05% or less. The Nb content is preferably 0.04% or less, and more preferably 0.03% or less.

[0078] Ti: 0.08% or less (excluding 0%). Ti contributes significantly to high strength and improved delayed fracture resistance through refinement of prior γ grains, thereby refining the internal structure of martensite and bainite, forming fine precipitates that serve as hydrogen trapping sites, and forming texture, even in trace amounts. It also contributes to improved castability. From this perspective, Ti is preferably contained in an amount of 0.002% or more. The Ti content is more preferably 0.004% or more, and even more preferably 0.010% or more. On the other hand, if Ti is contained in a large amount, the amount of coarse Ti-based precipitates, such as Ti, N, Ti(C,N), Ti(C,S), and TiS, remaining in an undissolved state during slab heating in the hot rolling process increases, and MnS precipitates in combination with these precipitates, deteriorating delayed fracture resistance. Therefore, when Ti is contained, the Ti content is limited to 0.08% or less. The Ti content is preferably 0.05% or less, and more preferably 0.02% or less.

[0079] V: 0.2% or less (excluding 0%) When V is contained in an amount of 0.005% or more, the effect of improving strength can be obtained. However, from the viewpoint of preventing deterioration of delayed fracture resistance and an increase in costs, when V is contained, the V content is set to 0.2% or less.

[0080] W: 0.15% or less (excluding 0%) When W is contained in an amount of 0.005% or more, the effect of improving strength can be obtained. However, from the viewpoint of preventing a deterioration in delayed fracture resistance and an increase in costs, when W is contained, the W content is set to 0.15% or less.

[0081] Zr: 0.15% or less (excluding 0%) When Zr is contained in an amount of 0.0005% or more, the effect of improving strength can be obtained. However, from the viewpoint of preventing a deterioration in delayed fracture resistance and an increase in costs, when Zr is contained, the Zr content is set to 0.15% or less.

[0082] Group B [one or more elements selected from Cr: 1% or less (excluding 0%), Ni: 1% or less (excluding 0%), Cu: 1% or less (excluding 0%), Mo: 1% or less (excluding 0%), Co: 1% or less (excluding 0%), and B: 0.005% or less (excluding 0%)] Cr, Ni, Cu, Mo, Co, and B are all elements that improve the hardenability of the steel sheet.

[0083] Cr: 1% or less (excluding 0%). When 0.005% or more of Cr is contained, hardenability is improved and the balance between strength and ductility can be improved. However, from the viewpoint of preventing an increase in costs, if Cr is contained, the Cr content is set to 1% or less.

[0084] Ni: 1% or less (excluding 0%) Ni can promote the formation of residual gamma phase when contained in an amount of 0.005% or more. However, from the viewpoint of preventing an increase in costs, when Ni is contained, the Ni content is preferably 1% or less.

[0085] Cu: 1% or less (excluding 0%) Cu can promote the formation of residual gamma phase when contained in an amount of 0.005% or more. However, from the viewpoint of preventing an increase in costs, when Cu is contained, the Cu content is set to 1% or less.

[0086] Mo: 1% or less (excluding 0%) When Mo is contained in an amount of 0.005% or more, the strength adjusting effect is obtained, and this effect is particularly enhanced when the Mo content is 0.05% or more. However, from the viewpoint of preventing an increase in costs, when Mo is contained, the Mo content is set to 1.0% or less.

[0087] Co: 1% or less (excluding 0%). When Co is contained in an amount of 0.001% or more, the ultimate deformability of the steel sheet is improved, and the effect of improving stretch flangeability is obtained. In particular, when the Co content is 0.005% or more, this effect is enhanced. However, from the viewpoint of preventing an increase in costs, when Co is contained, the Co content is set to 1% or less.

[0088] B: 0.005% or less (excluding 0%) B is an element effective in improving the hardenability of steel. To improve hardenability, the B content is preferably 0.0003% or more, and more preferably 0.0005% or more. However, an excessive B content reduces formability, so if B is contained, the B content is set to 0.005% or less.

[0089] C group [one or more elements selected from Ca: 0.005% or less (excluding 0%), Mg: 0.005% or less (excluding 0%), and REM: 0.005% or less (excluding 0%)] Ca, Mg, and REM (rare earth elements) are all elements used to control the morphology of sulfides.

[0090] Ca: 0.005% or less (excluding 0%) When Ca is contained in an amount of 0.0005% or more, the morphology of sulfides can be controlled and ductility and toughness can be improved. However, from the viewpoint of obtaining good ductility, when Ca is contained, the Ca content is set to 0.005% or less.

[0091] Mg: 0.005% or less (excluding 0%) When Mg is contained in an amount of 0.0005% or more, the form of sulfides can be controlled and ductility and toughness can be improved. However, from the viewpoint of preventing an increase in costs, when Mg is contained, the Mg content is set to 0.005% or less.

[0092] REM: 0.005% or less (excluding 0%). By including 0.0005% or more of REM, the morphology of sulfides can be controlled and ductility and toughness can be improved. However, from the viewpoint of obtaining good toughness, when REM is included, the REM content is set to 0.005% or less. In the present invention, REM refers to scandium (Sc), which has atomic number 21, yttrium (Y), which has atomic number 39, and lanthanides ranging from lanthanum (La), which has atomic number 57, to lutetium (Lu), which has atomic number 71. In the present invention, the REM content refers to the total content of one or more elements selected from the above-mentioned REM. The REM is not particularly limited, but is preferably La and / or Ce.

[0093] Group D [one or more elements selected from Sn: 0.2% or less (excluding 0%), and Sb: 0.2% or less (excluding 0%)] Sb and Sn are elements that suppress decarburization, denitrification, deboronation, etc., and are effective in suppressing a decrease in the strength of the steel sheet. Therefore, when Sb and Sn are contained, their respective contents are set to exceed 0%.

[0094] Sn: 0.2% or less Sn is an element that is effective in suppressing denitrification, deboronation, etc., and thus suppressing a decrease in the strength of steel. To obtain this effect, a content of 0.002% or more is preferred. On the other hand, if Sn is contained to obtain good impact resistance, the Sn content is set to 0.2% or less.

[0095] Sb: 0.2% or less Sb can be added from the viewpoint of suppressing nitriding and oxidation of the steel sheet surface, or decarburization of the steel sheet surface in a region of several tens of microns caused by oxidation. By suppressing nitriding and oxidation of the steel sheet surface, Sb prevents a decrease in the amount of martensite formed on the steel sheet surface, improving the fatigue properties and surface quality of the steel sheet. To achieve this effect, the Sb content is preferably 0.001% or more. On the other hand, when Sb is added to obtain good toughness, the Sb content is set to 0.2% or less.

[0096] Group E [Ta: 0.10% or less (excluding 0%)] Ta is an element effective for increasing the strength of the base steel sheet, similar to the elements of Group A, and may be contained as needed. Ta has the effect of improving strength when contained in an amount of 0.005% or more, but from the viewpoint of preventing an increase in costs, when Ta is contained, the Ta content is set to 0.10% or less.

[0097] F group [one or more elements selected from Te: 0.10% or less (excluding 0%), As: 0.10% or less (excluding 0%), and Hf: 0.10% or less (excluding 0%)] Like the elements in C group, Te, As, and Hf are all elements used to control the morphology of sulfides.

[0098] Te: 0.10% or less (excluding 0%) When Te is contained in an amount of 0.001% or more, the form of sulfides can be controlled and ductility and toughness can be improved. However, from the viewpoint of preventing an increase in costs, when Te is contained, the Te content is set to 0.10% or less.

[0099] As: 0.10% or less (excluding 0%) When As is contained in an amount of 0.001% or more, the morphology of sulfides can be controlled and ductility and toughness can be improved. However, from the viewpoint of preventing an increase in costs, when As is contained, the As content is set to 0.10% or less.

[0100] Hf: 0.10% or less (excluding 0%) When Hf is contained in an amount of 0.01% or more, the form of sulfides can be controlled and ductility and toughness can be improved. However, from the viewpoint of preventing an increase in costs, when Hf is contained, the Hf content is set to 0.10% or less.

[0101] Group G [one or more elements selected from Bi: 0.20% or less (excluding 0%) and Pb: 0.20% or less (excluding 0%)] Bi and Pb are elements that suppress grain boundary segregation and improve ductility and toughness. When Bi and Pb are contained, their contents should each exceed 0%.

[0102] Bi: 0.20% or less (excluding 0%) When Bi is contained in an amount of 0.001% or more, grain boundary segregation can be suppressed and ductility and toughness can be improved. Bi also has the effect of improving machinability and improving the smoothness of cut edges, and has the effect of improving the delayed fracture resistance of cut edges. When Bi is contained, the Bi content is set to 0.10% or less from the viewpoint of preventing an increase in costs.

[0103] Pb: 0.20% or less (excluding 0%). When Pb is contained in an amount of 0.001% or more, it can suppress grain boundary segregation and improve ductility and toughness. Pb also has the effect of improving machinability and improving the smoothness of cut edges, and has the effect of improving the delayed fracture resistance of cut edges. When Pb is contained, the Pb content is set to 0.10% or less in order to prevent an increase in costs.

[0104] H group [one or more elements selected from Zn: 0.10% or less (excluding 0%), Ge: 0.10% or less (excluding 0%), Sr: 0.10% or less (excluding 0%), Cs: 0.10% or less (excluding 0%)] Zn, Ge, Sr, and Cs are elements that increase strength without significantly affecting mechanical properties or surface quality, and when Zn, Ge, Sr, or Cs is contained, each content should exceed 0%.

[0105] Zn: 0.10% or less (excluding 0%) Even if Zn is contained in an amount of 0.001% or more, it does not significantly affect the mechanical properties or surface quality. From the viewpoint of preventing an increase in costs, if Zn is contained, the Zn content is set to 0.10% or less.

[0106] Ge: 0.10% or less (excluding 0%) Even if Ge is contained in an amount of 0.001% or more, it does not significantly affect the mechanical properties or surface quality. From the viewpoint of preventing an increase in costs, if Ge is contained, the Ge content is set to 0.10% or less.

[0107] Sr: 0.10% or less (excluding 0%) Even if the Sr content is 0.001% or more, it does not significantly affect the mechanical properties or surface quality. From the viewpoint of preventing an increase in costs, if Sr is contained, the Sr content is set to 0.10% or less.

[0108] Cs: 0.10% or less (excluding 0%) Even if Cs is contained in an amount of 0.001% or more, it does not have a significant effect on mechanical properties or surface quality. From the viewpoint of preventing an increase in costs, if Cs is contained, the Cs content is set to 0.10% or less.

[0109] Steel Sheet (Base Steel Sheet) The remainder of the steel sheet other than the above-mentioned component composition is Fe and unavoidable impurities.

[0110] Steel Structure The structure of the base steel sheet (base steel sheet) is not particularly limited either, but in order to ensure a tensile strength of 780 MPa or more, it is preferable to have the following steel sheet structure (steel structure in the range of ⅛ thickness to ⅜ thickness, centered at a position of ¼ thickness from the surface of the base steel sheet): That is, it is preferable to set the total area ratio of martensite, bainite, and retained austenite (retained γ) to 30% or more, thereby obtaining a base steel sheet having a tensile strength of 780 MPa or more.

[0111] Furthermore, by making the total area ratio of martensite, bainite, and retained γ 50% or more, a base steel plate having a tensile strength of 980 MPa or more can be obtained, and by making the total area ratio of martensite, bainite, and retained γ 70% or more, a base steel plate having a tensile strength of 1180 MPa or more can be obtained.

[0112] Furthermore, by setting the total area ratio of martensite, bainite, and retained γ to 85% or more, a base steel sheet having a tensile strength of 1310 MPa or more can be obtained, and by setting the total area ratio of martensite, bainite, and retained γ to 90% or more, a base steel sheet having a tensile strength of 1470 MPa or more can be obtained. Furthermore, in steels having a total area ratio of martensite, bainite, and retained γ of 50% or more, particularly 90% or more, the hydrogen diffusion rate is significantly reduced and hydrogen tends to remain in the steel, but by employing the manufacturing method of the present invention described above, it is possible to reduce quasi-diffusible hydrogen in the base steel sheet of the present invention.

[0113] In order to ensure a desired tensile strength TS and to obtain the total area ratio of martensite, bainite, and residual γ, it is particularly desirable to appropriately control the C content, Si content, Mn content, and annealing temperature. For example, to achieve a total area ratio of martensite, bainite, and residual γ of 90% or more, it is preferable that the C content be 0.17% or more, the Si content be 0.1% or more, the Mn content be 2.3% or more, and the annealing temperature (particularly the annealing temperature (holding temperature) in the second annealing step) be 820°C or more.

[0114] Here, the size and abundance of each structure are not particularly limited, but the following sizes and abundances can be exemplified as one embodiment of the present invention. The aspect ratio refers to the ratio of the length of the major axis to the length of the minor axis perpendicular to the major axis, the thickness refers to the length of the minor axis, and the circle equivalent diameter refers to the diameter when the area of ​​each structure is taken as the area of ​​a circle. Martensite exists as both tempered martensite and fresh martensite. Tempered martensite has the effect of improving stretch flangeability, while fresh martensite has the effect of improving ductility.

[0115] ・Tempered martensite Aspect ratio ≦8, equivalent circle diameter ≦30 μm Distribution density of carbides inside the structure: 0.10 to 12 pieces / μm ２ Fresh martensite and residual γ Lumpy: Aspect ratio ≦8, equivalent circle diameter: 3 to 30 μm Granular: Aspect ratio ≦8, equivalent circle diameter: 0.40 μm or more, less than 3 μm Plate or film: Aspect ratio greater than 8, Thickness: 0.10 to 8 μm Bainite Film or plate: Aspect ratio greater than 8, Thickness ≦8 μm Lumpy: Aspect ratio ≦8, equivalent circle diameter ≦30 μm Distribution density of carbides within the structure: 0.10 to 6 particles / μm in all forms ２ Carbide Granular: Aspect ratio ≦ 8, Equivalent circle diameter: 0.01 μm or more, less than 0.40 μm Film-like: Aspect ratio greater than 8, Equivalent circle diameter: 0.01 μm or more, less than 0.10 μm

[0116] The hot-dip galvanized steel sheet and the hot-dip galvanized steel sheet have been described above as one embodiment of the hot-dip galvanized steel sheet and the hot-dip galvanized steel sheet according to the present invention. ２ O ３In a manufacturing method of hot-dip galvanized steel sheet, the shape and number of inclusions composed of MnS, (Al, Ca)-O, Nb(C, N), (Nb, Ti)(C, N), Ti(C, S), and the like, which are composite precipitates with MnS as a nucleus, are controlled, and the steel sheet is annealed and cooled, and then hot-dip galvanized is performed. By optimizing the annealing and cooling atmosphere, it is possible to obtain a beautiful surface appearance without bare spots, and to reduce the amount of hydrogen released from room temperature to 350°C (hydrogen concentration H L ) is suppressed to 0.30 mass ppm or less, and the amount of hydrogen released at 150°C or higher and 350°C or lower when measured by temperature-programmed analysis (hydrogen concentration H Ｈ In the present invention, by performing an oxidation treatment before the annealing, and further performing the annealing under more limited conditions, and by performing a reheat treatment in a specific atmosphere after the galvanizing, it is possible to produce a hot-dip galvanized steel sheet having even higher levels of excellent coating appearance and excellent delayed fracture resistance.

[0117] Steels having the chemical compositions shown in Table 1 were refined under the conditions shown in Tables 2 to 5 to obtain 200 mm thick slabs, which were then reheated and hot-rolled under the conditions shown in Tables 2 to 6. The hot rolling was adjusted so that the cumulative reduction in the temperature range above 950°C was 95%, and the final thickness was 2.8 mm. The finish rolling temperature in the hot rolling was 850°C, and the steel was cooled to 500°C at a cooling rate of 50°C / s, and then wound into a coil. The coil was then water-cooled for 10 seconds and allowed to cool to room temperature. The produced hot-rolled steel sheets were pickled and then cold-rolled to a thickness of 1.4 mm at a reduction of 50%. The cumulative reduction below 950°C, including cold rolling, was 86%. The cold-rolled steel sheets produced as described above were annealed under the conditions shown in Tables 2 to 4 and 6 in a continuous hot-dip galvanizing line (CGL) equipped with an all-radiant tube (ART) type annealing furnace, and then hot-dip galvanized (coating composition: Zn-0.2 mass% Al) and gas-wiped to a coating weight per side of 30 to 70 g / m. ２ Then, all the samples except for No. 89 were subjected to an alloying treatment.

[0118]

[0119] Tables 3 and 6 show examples in which the cooling step was divided into two steps (first cooling step and second cooling step). Similarly, Table 4 shows examples in which reheating was performed after plating and alloying. The primary cooling end temperature (final holding temperature (steel sheet temperature) in the first cooling step) was 650°C for Nos. 100 to 128, and varied from 600 to 640°C for Nos. 176 to 178 shown in Table 6.

[0120] Separately from the above-mentioned examples, in a CGL having a direct-fire annealing furnace (DFF-type annealing furnace) with a direct flame burner, steel sheets were oxidized and annealed under the conditions shown in Table 5, and then hot-dip galvanized (coating composition: Zn-0.2 mass% Al) was applied, and the coating weight per side was adjusted to 30 to 70 g / m by gas wiping. ２ The temperature was adjusted to 100°C / s, and then alloying treatment was performed. Note that No. 152 (Table 5) is an example in which oxidation treatment was performed at a constant temperature, and the holding time (treatment time) of the oxidation treatment was 8 seconds. In the other examples (Nos. 150, 151, 153 to 175), oxidation treatment was performed during temperature increase, and the temperature increase rate of this oxidation treatment was in the range of 5 to 20°C / s.

[0121] The hot-dip galvanized steel sheets thus obtained were subjected to measurement of the amount of diffusible hydrogen in the steel sheets and evaluation of the coating appearance and delayed fracture resistance using the following measurement and evaluation methods. The results, along with the manufacturing conditions, are shown in Tables 2 to 6. In the oxidation treatment of the examples in Table 5, the "oxidation start temperature" refers to the sheet temperature at the entry side of the oxidation zone in the heating zone of a DFF-type annealing furnace, the "oxidation end temperature" refers to the sheet temperature at the exit side of the oxidation zone, and the oxygen concentration refers to the oxygen concentration in the oxidation zone. Therefore, the range from the oxidation start temperature to the oxidation end temperature is the oxidation treatment temperature. Furthermore, the "oxidation temperature range" refers to the temperature range over which the steel sheet rises in the oxidation zone (the temperature range from the oxidation start temperature to the oxidation end temperature), and the "maximum steel sheet temperature" refers to the maximum temperature in the heating zone of a DFF-type annealing furnace. Therefore, when the "maximum steel sheet temperature" is higher than the "oxidation end temperature," the steel sheet is further heated in a non-oxidizing atmosphere in the zone next to the oxidation zone (a zone that is not an oxidation zone), and the maximum temperature in the zone next to the oxidation zone is the "maximum steel sheet temperature."

[0122]

[0123]

[0124]

[0125]

[0126]

[0127]

[0128] Measurement of the amount of hydrogen in steel sheet (hydrogen analysis method) A strip-shaped test piece with a major axis length of 30 mm and a minor axis length of 5 mm was taken from the width center of a hot-dip galvanized steel sheet, the zinc plating layer of the test piece was removed with a router, and immediately subjected to hydrogen analysis using a thermal desorption analyzer under the conditions of an analysis start temperature of 25°C, an analysis end temperature of 400°C, and a temperature rise rate of 200°C / hour, and the amount of released hydrogen (mass ppm / min), which is the amount of hydrogen released from the surface of the test piece at each temperature, was measured. The total amount of released hydrogen from room temperature (25°C) to 350°C was taken as the hydrogen concentration H Ｌ The total amount of released hydrogen from 150°C to 350°C is taken as the hydrogen concentration H Ｈ It was calculated as:

[0129] Here, the hydrogen concentration H in the steel Ｌ When the hydrogen concentration in the steel was 0.05 mass ppm or less, it was evaluated as excellent "◎+", when it was more than 0.05 mass ppm and less than 0.10 mass ppm, it was evaluated as excellent "◎", when it was more than 0.10 mass ppm and less than 0.20 mass ppm, it was evaluated as good "◯", and when it was more than 0.20 mass ppm and less than 0.30 mass ppm, it was evaluated as fair "△". Ｌ When the content exceeds 0.30 ppm by mass, the delayed fracture resistance is often reduced, and therefore, when the content exceeds 0.30 ppm by mass, the steel is rated as poor (x).

[0130] In addition, the hydrogen concentration H Ｈ When the hydrogen concentration in the steel was 0.05 mass ppm or less, it was evaluated as excellent "◎+", when it was more than 0.05 mass ppm and less than 0.10 mass ppm, it was evaluated as excellent "◎", when it was more than 0.10 mass ppm and less than 0.15 mass ppm, it was evaluated as good "◯", and when it was more than 0.15 mass ppm and less than 0.20 mass ppm, it was evaluated as fair "△". Ｈ When the content exceeds 0.15 ppm by mass, the delayed fracture resistance is often reduced, and therefore, those exceeding 0.15 ppm by mass were rated as poor "x".

[0131] Evaluation of plating appearance The plating appearance of hot-dip galvanized steel sheets was visually observed, and those without any visible patterns or irregularities were rated as excellent (◎). For those with visible patterns or irregularities, SEM observation of the relevant areas at 50x magnification revealed that areas on the steel sheet that were not covered with zinc plating over an area of ​​1.0 mm × 1.0 mm or more were judged to be unplated defects. 3D shape measurement at 10x magnification revealed that areas on the steel sheet that were 0.5 mm × 0.5 mm or more in area and had a maximum irregularity of 5 μm or more relative to the average height of the surrounding area were judged to be dents. More specifically, unplated defects refer to areas of 1.0 mm × 1.0 mm or more in area where the zinc plating was not coated, which were observed with a darker contrast than the zinc plating-coated areas when backscattered electron images were observed using an SEM at an accelerating voltage of 15 kV and a magnification of 50x. Even if patterns or irregularities were observed, those without any unplated defects were rated as good "○+", and those with three or more unplated defects within a 1 m × 1 m area on the steel sheet surface were rated as poor "×". Furthermore, those without any unplated defects but with indentations caused by picking up onto the rolls or a scale pattern in the shape of a V mark in the sheet running direction as a sign of such defects, or those with fewer than three unplated defects within a 1 m × 1 m area on the steel sheet surface were rated as passing "○" even though they were not good (○+).

[0132] Tensile Test A JIS Z2241 No. 5 test piece was taken from the hot-dip galvanized steel sheet in the direction perpendicular to the rolling direction (so that the sheet width direction was the tensile direction), and a tensile test in accordance with JIS Z2241 (2011) was performed on this test piece to measure the tensile strength (TS).

[0133] Observation and Measurement of Base Steel Sheet Structure (Base Steel Sheet Structure) The total area ratio of martensite, bainite, and retained austenite (residual γ) in the base steel sheet structure (base steel sheet structure) was measured as follows. A sample was cut out so that the cross section (L cross section) parallel to the rolling direction of the steel sheet and parallel to the sheet thickness direction served as the observation surface. The observation surface of this sample was polished with diamond paste and then finish-polished with alumina. Next, the observation surface of the sample was etched with 3 vol% nital to reveal the structure. The observation position on this sample observation surface was set to 1 / 4 of the sheet thickness, and five fields of view were observed using an SEM at a magnification of 3000x. This observation was considered to have observed the steel structure in the range of 1 / 8 to 3 / 8 thickness, centered at the 1 / 4 thickness position from the surface of the base steel sheet. The total area of ​​martensite, bainite, and residual γ was calculated from the obtained structural images. The area ratio was calculated by dividing this total area by the measured area for five fields of view. The average of these values ​​was used as the total area ratio of martensite, bainite, and residual γ. The distinction between martensite, bainite, residual γ, and other microstructures was performed as follows: - Martensite: There are two types of martensite: tempered martensite and fresh martensite. - Tempered martensite: Tempered martensite is the gray or dark gray area close to black in the SEM photograph. Tempered martensite exhibits a blocky morphology with boundaries at the prior γ grain boundaries and interfaces with other structures such as ferrite. However, tempered martensite may contain other structures such as bainite within, resulting in a concave shape. Tempered martensite contains many carbides, but the amount of carbides may be small depending on the plane orientation. - Fresh martensite: Fresh martensite is the gray or white area in the SEM photograph. Fresh martensite is in the form of blocks, granules, plates, or films, and does not contain carbides. Bainite Bainite is the dark gray area in the SEM photograph. Bainite takes the form of either a film, a plate, or a block formed by some or all of these adjacent regions being connected, and contains a small amount of carbides inside. Bainite also includes bainite that has been tempered after formation, resulting in coarsened carbides.Retained austenite (retained γ) Retained γ is a region that has the same color and shape as the above-mentioned fresh martensite. Note that retained γ and fresh martensite cannot be distinguished using an SEM.

[0134] To ensure the strength of steel plates, the total area ratio of martensite, bainite, and residual γ must be controlled. However, the remainder may include the following structures, but is not limited to these. Ferrite: Ferrite is the black area in SEM images. Ferrite has a massive morphology and contains almost no carbides. Bainitic ferrite contains almost no carbides internally and has similar mechanical properties to ferrite, so it belongs to the ferrite group. Ferrite may contain either or both granular and massive fresh martensite, or either or both granular and massive residual γ. Fresh martensite and residual γ contained within ferrite are not included in the area ratio of ferrite, but are treated as the area ratio of fresh martensite or residual γ. Carbide: Carbide is the white area in SEM images. Carbide has a granular or film-like morphology. Carbides are mainly formed finely inside ferrite, martensite, and bainite, but their area ratios are small and negligible, so the area ratio of carbides is not excluded from the area ratio of each structure, but is included in the area ratio of each structure. Structures other than those mentioned above In addition to the above, nitrides such as TiN, carbonitrides such as (Nb,Ti)(C,N), sulfides such as MnS and CaS, Al ２ O ３ , SiO ２ The total area percentage of oxides such as these may also be a few percent. Since these area percentages are small and can be ignored, these area percentages are included in the area percentage of each structure that contains them. In some cases, pearlite may also be included. In the case of pearlite, the area percentage of pearlite is calculated separately.

[0135] Observation and measurement of inclusions and inclusion groups In the base steel sheet, the number of inclusion groups per unit area that satisfy the following conditions (i) and (ii) was determined by the following measurement method. (Conditions) (i) The length of the major axis of an inclusion group having one or more inclusion particles is 50 μm or more. (ii) When an inclusion group is composed of two or more of the above inclusion particles, the length of the major axis of one inclusion particle is 0.3 μm or more and the shortest distance between the inclusion particles is 40 μm or less. Within 1 mm of the inclusion group ２ The number density per 1 mm was measured in the area from t / 5 to 4t / 5 (t is the thickness of the steel sheet) from the surface of the steel sheet without corrosion after polishing the L cross section (vertical cross section parallel to the rolling direction), that is, in the area from the 1 / 5 thickness position of the sheet thickness from the surface to the 1 / 5 thickness position on the back surface side, sandwiching the center of the sheet thickness. ２ Approximately 100 fields of view of the area were photographed consecutively with an SEM, and the number of such inclusion groups was counted from the photographed SEM photographs. The reason for measuring in the area from 1 / 5t to 4 / 5t from the surface of the steel plate is that inclusion groups are less frequent in the surface layer of the steel plate and are particularly more prevalent near the center of the plate thickness, and as a result, initial cracks initiate near the center of the plate thickness. The photographs were measured by enlarging them 1000 times. Since the inclusion groups are formed in a thin film-like form, they are difficult to identify when observed on a surface parallel to the rolled surface or with an optical microscope, and it is also difficult to accurately measure their length.

[0136] Here, the SEM image was a backscattered electron image. The magnification for taking the image was preferably 1000x. However, when it was difficult to accurately determine the size of the inclusion particles or the distance between the inclusion particles, the individual inclusion particles were appropriately magnified to 5000x, and a visual field equivalent to that of the 1000x visual field was measured, and the above-mentioned inclusion group was evaluated by image.

[0137] As mentioned above, the shortest distance between inclusion particles is the surface-to-surface distance, and means the surface-to-surface distance that can be determined from this image. Furthermore, as mentioned above, the measurement direction of the shortest distance is limited to the rolling direction or within a range of ±10 degrees from the rolling direction. Furthermore, when an inclusion group is composed of two or more inclusion particles, the total length (length of the major axis) of the inclusion group in the rolling direction is the length in the rolling direction between the outer ends of the inclusion particles located at both ends of the inclusion group in the rolling direction. Furthermore, when an inclusion group is composed of a single inclusion particle, the total length of the inclusion group in the rolling direction is the length of this inclusion particle in the rolling direction.

[0138] Furthermore, the individual inclusion particles that make up this group of inclusions are mainly sulfides of Mn, Ti, Zr, Ca, and REM, oxides of Al, Ca, Mg, Si, and Na, nitrides of Ti, Zr, Nb, and Al, and carbides of Ti, Nb, Zr, and Mo. Most of these inclusions are formed in the casting process and existed in an undissolved state when the slab was subsequently heated, while the remainder were re-precipitated in combination with or adjacent to the former during the subsequent hot rolling and coiling.

[0139] Evaluation of Delayed Fracture Resistance: A rectangular test specimen with a major axis length of 100 mm and a minor axis length of 20 mm was cut from the hot-dip galvanized steel sheet in the direction perpendicular to the rolling direction. A punched hole with a diameter of 15 mm and a clearance of 14.0% was formed at the center of the major and minor axes of the test specimen. Conventionally, the clearance was set to approximately 12.5%, but in light of the recent trend toward stricter quality standards for delayed fracture resistance, a more stringent condition was adopted. The test specimen was subjected to a tensile test, and the delayed fracture resistance was evaluated based on the occurrence of delayed fracture through the punched hole. To prevent the release of diffusible hydrogen in the steel due to aging, the time from cutting the rectangular test specimen from the hot-dip galvanized steel sheet to starting the delayed fracture tensile test (tensile speed: 10 mm / min) was set to within 10 minutes. The maximum loading time in the tensile test was 100 hours, and the maximum stress at which no cracks (here, "cracks" means fractures when tensile stress is applied) occurred after 100 hours of loading was defined as the critical stress, and the delayed fracture resistance was evaluated by the ratio of the critical stress to the yield stress. The evaluation criteria for delayed fracture resistance were as follows: when the critical stress / yield stress was 1.10 or more, it was given an excellent "◎", when it was less than 1.10 and 1.05 or more, it was given a good "◯", when it was less than 1.05 and 1.00 or more, it was given a pass "△" (not good (◯)), and when it was less than 1.00, it was given a poor "×". Note that the delayed fracture resistance evaluated in the delayed fracture test is generally lower (disadvantageous) for steel sheets with higher strength.

[0140] According to Tables 1 to 6, the hot dip galvanized steel sheets of the present invention have high strength, a beautiful surface appearance with no unplated areas, and excellent delayed fracture resistance.

Claims

1. In mass percent, C: 0.06% or more and 0.30% or less, Si: 0.01% or more and 3.00% or less, Mn: 1.5% or more and 3.5% or less, P: 0.1% or less (excluding 0%), S: 0.0030% or less (excluding 0%), sol. containing Al: 0.1% or less (not including 0%), N: 0.0070% or less (not including 0%), O: 0.0030% or less (not including 0%), or further containing one or more selected from the following groups A, B, C, and D; Group A: one or more selected from Nb: 0.05% or less (not including 0%), Ti: 0.08% or less (not including 0%), V: 0.2% or less (not including 0%), W: 0.15% or less (not including 0%), Zr: 0.15% or less (not including 0%); Group B: one or more selected from Cr: 1% or less (not including 0%), Ni: 1% or less (not including 0%), Cu: 1% or less (not including 0%), Mo: 1% or less (not including 0%), Co: 1% or less (not including 0%), and B: 0.005% or less (not including 0%); Group C: Group D: one or more selected from the group consisting of Sn: 0.2% or less (excluding 0%), Sb: 0.2% or less (excluding 0%), the balance being Fe and unavoidable impurities; and a zinc-based plating layer formed on a surface of the substrate steel sheet, wherein a group of inclusions contained in the substrate steel sheet satisfies the following conditions (i) and (ii), and the group of inclusions is 10 pieces / mm ２ A hot-dip galvanized steel sheet, in which the amount of hydrogen released from room temperature to 350°C is 0.30 ppm by mass or less and the amount of hydrogen released from 150 to 350°C is 0.20 ppm by mass or less, there are less than three uncoated defects in an area of ​​1 m x 1 m on the steel sheet surface, and the tensile strength is 780 MPa or more. (Conditions) (i) The length of the major axis of an inclusion group having one or more inclusion particles is 50 μm or more. (ii) When the inclusion group is composed of two or more of the inclusion particles, the length of the major axis of one inclusion particle is 0.3 μm or more and the shortest distance between the inclusion particles is 40 μm or less.

2. The hot-dip galvanized steel sheet according to claim 1, wherein the base steel sheet further contains, as the chemical composition, one or more elements selected from the following groups E, F, G and H, in terms of mass %: Group E: Ta: 0.10% or less (excluding 0%) Group F: One or more elements selected from Te: 0.10% or less (excluding 0%), As: 0.10% or less (excluding 0%), Hf: 0.10% or less (excluding 0%) Group G: One or more elements selected from Bi: 0.20% or less (excluding 0%), Pb: 0.20% or less (excluding 0%) Group H: One or more elements selected from Zn: 0.10% or less (excluding 0%), Ge: 0.10% or less (excluding 0%), Sr: 0.10% or less (excluding 0%), Cs: 0.10% or less (excluding 0%) 3. The hot-dip galvanized steel sheet according to claim 1 or 2, wherein the amount of hydrogen released from the base steel sheet at from room temperature to 350°C is 0.20 mass ppm or less, and the amount of hydrogen released at 150 to 350°C is 0.15 mass ppm or less.

4. The hot-dip galvanized steel sheet according to any one of claims 1 to 3, wherein the combined area ratio of martensite, bainite and retained austenite in the base steel sheet is 30% or more, and the tensile strength is 780 MPa or more.

5. The hot-dip galvanized steel sheet according to any one of claims 1 to 3, wherein the total area ratio of martensite, bainite and retained austenite in the base steel sheet is 50% or more, and the tensile strength is 980 MPa or more.

6. The hot-dip galvanized steel sheet according to any one of claims 1 to 3, wherein the combined area ratio of martensite, bainite and retained austenite in the base steel sheet is 70% or more, and the tensile strength is 1180 MPa or more.

7. The hot-dip galvanized steel sheet according to any one of claims 1 to 3, wherein the combined area ratio of martensite, bainite and retained austenite in the base steel sheet is 85% or more, and the tensile strength is 1,310 MPa or more.

8. The hot-dip galvanized steel sheet according to any one of claims 1 to 3, wherein the total area ratio of martensite, bainite and retained austenite in the base steel sheet is 90% or more, and the tensile strength is 1470 MPa or more.

9. The hot-dip galvanized steel sheet according to any one of claims 1 to 8, wherein the zinc-based plating layer is a galvannealed layer.

10. A method for manufacturing a hot-dip galvanized steel sheet, comprising annealing a base steel sheet and then applying hot-dip galvanizing, wherein the base steel sheet has the following mass percentages: C: 0.06% or more and 0.30% or less, Si: 0.01% or more and 3.00% or less, Mn: 1.5% or more and 3.5% or less, P: 0.1% or less (excluding 0%), S: 0.0030% or less (excluding 0%), sol. containing Al: 0.1% or less (not including 0%), N: 0.0070% or less (not including 0%), O: 0.0030% or less (not including 0%), or further containing one or more selected from the following groups A, B, C, and D; Group A: one or more selected from Nb: 0.05% or less (not including 0%), Ti: 0.08% or less (not including 0%), V: 0.2% or less (not including 0%), W: 0.15% or less (not including 0%), Zr: 0.15% or less (not including 0%); Group B: one or more selected from Cr: 1% or less (not including 0%), Ni: 1% or less (not including 0%), Cu: 1% or less (not including 0%), Mo: 1% or less (not including 0%), Co: 1% or less (not including 0%), and B: 0.005% or less (not including 0%); Group C: Group D: One or more selected from the group consisting of Sn: 0.2% or less (excluding 0%), Sb: 0.2% or less (excluding 0%), and the balance consisting of Fe and unavoidable impurities, and further, a group of inclusions contained in the base steel sheet satisfies the following conditions (i) and (ii), and the group of inclusions is 10 pieces / mm ２ a first annealing step in which the base steel sheet is held at a holding temperature of 650°C to 950°C for 20 s to 150 s in an atmosphere with a dew point of -55°C to +30°C and a hydrogen concentration of 8.0 vol% to 25 vol%; a second annealing step in which the base steel sheet that has undergone the first annealing step is held at a holding temperature of 700°C to 950°C for 30 s to 300 s in an atmosphere with a dew point of -50°C to +30°C and a hydrogen concentration of 0.2 vol% to less than 8.0 vol%; and a cooling step in which the base steel sheet that has undergone the second annealing step is cooled from the holding temperature of 700°C to 950°C in the second annealing step to a temperature of 150°C to 550°C at an average cooling rate of 3 to 300°C / s in an atmosphere with a dew point of -20°C or less and a hydrogen concentration of 0.2 vol% to 25 vol%. and a hot-dip galvanizing step of performing a hot-dip galvanizing treatment on the base steel sheet after the cooling step to form a zinc-based plating layer on the surface of the base steel sheet. (Conditions) (i) The length of the major axis of an inclusion group having one or more inclusion particles is 50 μm or more. (ii) When the inclusion group is composed of two or more of the inclusion particles, the length of the major axis of one inclusion particle is 0.3 μm or more and the shortest distance between the inclusion particles is 40 μm or less.

11. The method for producing a hot-dip galvanized steel sheet according to claim 10, wherein the base steel sheet further contains, as the chemical composition, one or more elements selected from the following groups E, F, G and H, in terms of mass %: Group E: Ta: 0.10% or less (excluding 0%) Group F: One or more elements selected from Te: 0.10% or less (excluding 0%), As: 0.10% or less (excluding 0%), Hf: 0.10% or less (excluding 0%) Group G: One or more elements selected from Bi: 0.20% or less (excluding 0%), Pb: 0.20% or less (excluding 0%) Group H: One or more elements selected from Zn: 0.10% or less (excluding 0%), Ge: 0.10% or less (excluding 0%), Sr: 0.10% or less (excluding 0%), Cs: 0.10% or less (excluding 0%) 12. The base steel sheet before the first annealing process is ２ The method for producing a hot-dip galvanized steel sheet according to claim 10 or 11, comprising an oxidation treatment step of performing an oxidation treatment at a temperature of 400° C. or higher and 900° C. or lower in an atmosphere containing 1000 ppm by volume or more of 13. A method for producing a hot-dip galvanized steel sheet according to any one of claims 10 to 12, comprising a reheating treatment step of holding the steel sheet at a temperature of 200°C or higher and 450°C or lower in an atmosphere with a hydrogen concentration of 15% by volume or lower for a time of 5 s or higher and 600 s or lower after the hot-dip galvanizing treatment, and then cooling the steel sheet.

14. The method for producing a hot-dip galvanized steel sheet according to any one of claims 10 to 13, wherein the atmosphere in the second annealing step has a hydrogen concentration of less than 5.0 volume %.

15. A method for producing a hot-dip galvanized steel sheet according to any of claims 10 to 14, wherein the cooling step comprises: a first cooling step in which the base steel sheet after the second annealing step is cooled in an atmosphere with a dew point of -20°C or less and a hydrogen concentration of 0.2 vol.% or more and less than 8.0 vol.% over a period of 10 s or more through a temperature range from the holding temperature of 700°C or more and 950°C or less in the second annealing step to a steel sheet temperature of 650°C or less at an average cooling rate of 3°C / s or more and 25°C / s or less; and a second cooling step in which the base steel sheet after the first cooling step is cooled from 600°C to a temperature of 150°C or more and 550°C or less at an average cooling rate of 10°C / s or more in an atmosphere with a dew point of -20°C or less and a hydrogen concentration of 8.0 vol.% or more and 25 vol.%.

16. A method for producing a hot-dip galvanized steel sheet according to any one of claims 10 to 15, comprising, after the hot-dip galvanizing step, an alloying treatment step of alloying the base steel sheet and the zinc-based plating layer.

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