Hot-dip galvanized steel sheet and production method therefor

Abstract

The present invention provides a high-strength hot-dip galvanized steel sheet having 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 material steel sheet which has a component composition containing, in mass%, 0.06% to 0.30% C, 0.01% to less than 3.00% Si, 1.5% to 3.5% Mn, 0.1% or less (not including 0%) P, 0.003% or less (not including 0%) S, 0.1% or less (not including 0%) sol. Al, 0.007% or less (not including 0%) N, and 0.003% or less (not including 0%) O, with the remainder consisting of Fe and unavoidable impurities; and a zinc-based plating layer which is formed on the surface of the base material steel sheet and in which the plating coating mass per surface is 20-120 g / m2. The total hydrogen released at 200-350°C is 0.20 mass ppm or less when measured using dynamic temperature analysis.

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 been encountered in steel sheets with a high amount of diffusible hydrogen in the steel, resulting in a deterioration in delayed fracture resistance. In particular, in high-strength steel sheets with a tensile strength of 780 MPa or greater, diffusible hydrogen in the steel tends to remain after annealing, significantly reducing delayed fracture resistance. This is due to the fact that high-strength steel sheets with a tensile strength of 780 MPa or greater 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 the hydrogen diffusion rate is slower. Therefore, 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 galvanization. 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 galvanization, 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 (°C).

[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, which require the generation of an austenite phase, particularly during the annealing process, to meet the stricter standards for delayed fracture resistance, have not been able to sufficiently improve the delayed fracture resistance of high-strength steel sheets. 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 that have an austenite 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, impairing galvanization and preventing good surface quality. Therefore, the methods disclosed in Patent Documents 4 and 5, which uniformly reduce hydrogen in a furnace, are unlikely to achieve good surface quality and delayed fracture resistance. Furthermore, the technique disclosed in Patent Document 6 attempts to improve the galvanization of Si-, Mn-, and Al-containing steels and thereby 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]

[0005] As a result of extensive investigations aimed at solving the above-mentioned problems, the present inventors have newly discovered that hydrogen released when the temperature of a steel sheet is raised to less than 200°C is released into the atmosphere when the steel sheet is held after production, whereas hydrogen released when the temperature of a steel sheet is raised to between 200°C and 350°C (hereinafter referred to as quasi-diffusible hydrogen) is not released into the atmosphere when the steel sheet is held after production, and is transformed into diffusible hydrogen when the steel sheet is worked, particularly by press working, which significantly affects delayed fracture resistance. Furthermore, the present inventors have discovered that in a manufacturing method of a hot-dip galvanized steel sheet in which a steel sheet (base steel sheet) is 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 by optimizing the annealing and cooling atmospheres.

[0011] The present invention was made based on these findings, and its gist is as follows: [1] A base steel sheet having a chemical composition containing, by mass%, C: 0.06% or more and 0.30% or less, Si: 0.01% or more but less than 3.00%, Mn: 1.5% or more and 3.5% or less, P: 0.1% or less (excluding 0%), S: 0.003% or less (excluding 0%), sol. Al: 0.1% or less (excluding 0%), N: 0.007% or less (excluding 0%), O: 0.003% or less (excluding 0%), with the balance consisting of Fe and unavoidable impurities; and a coating weight of 20 g / m per side formed on the surface of the base steel sheet. ２ 120g / m or more ２and a zinc-based plating layer having a hydrogen content of 0.20 mass ppm or less at a temperature of 200°C or higher and 350°C or lower, wherein the base steel sheet has a total hydrogen release of 0.20 mass ppm or less at a temperature of 200°C or higher and 350°C or lower when measured by temperature ramp analysis, has less than three uncoated defects within a 1 m x 1 m area on the steel sheet surface, and has a tensile strength of 780 MPa or higher. [2] The hot-dip galvanized steel sheet according to [1], wherein the base steel sheet further contains, as the chemical composition, one or more elements selected from the following groups A to D, in mass %: Group A: One or more 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%), Zr: 0.15% or less (excluding 0%); Group B: One or more 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%), B: 0.005% or less (excluding 0%); Group C: 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: Sn: 0.2% or less (excluding 0%); [3] The hot-dip galvanized steel sheet according to [1] or [2], wherein the base steel sheet further contains, in mass %, one or more elements selected from the following groups E to H as the 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%) [4] The hot-dip galvanized steel sheet according to any one of [1] to [3], wherein a steel structure in a range from ⅛ thickness to ⅜ thickness centered at a position ¼ thickness from the surface of the base steel sheet has a total area fraction, in area %, of martensite, bainite, and retained austenite of 30% or more, and a tensile strength of 780 MPa or more. [5] The hot-dip galvanized steel sheet according to any one of [1] to [3], wherein a steel structure in a range from ⅛ thickness to ⅜ thickness centered at a position ¼ thickness from the surface of the base steel sheet has a total area fraction, in area %, of martensite, bainite, and retained austenite of 50% or more, and a tensile strength of 980 MPa or more. [6] The hot-dip galvanized steel sheet according to any one of [1] to [3], wherein a steel structure in a range from ⅛ thickness to ⅜ thickness centered at a position ¼ thickness from the surface of the base steel sheet has a total area fraction, in area %, 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 a steel structure in a range from ⅛ thickness to ⅜ thickness centered at a position ¼ thickness from the surface of the base steel sheet has a total area fraction, in area %, 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 a steel structure in a range from ⅛ thickness to ⅜ thickness centered at a position ¼ thickness from the surface of the base steel sheet has a total area fraction, in area %, 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 plating layer is a galvannealed layer.

[10] A method for producing a hot-dip galvanized steel sheet, comprising: a first annealing step of holding the base steel sheet at a holding temperature of 650°C to 950°C in an atmosphere having a dew point of -55°C to +30°C and a hydrogen concentration of 5.0% by volume to 25% by volume for a period of 20 seconds to 150 seconds; and a second annealing step of holding the base steel sheet that has been subjected to the first annealing step at a holding temperature of 700°C to 950°C in an atmosphere having a dew point of -50°C to +30°C and a hydrogen concentration of 0.2% by volume to less than 5.0% by volume for a period of 30 seconds to 300 seconds. a first cooling step of cooling the base steel sheet that has undergone the second annealing step 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 in an atmosphere having a dew point of -20°C or less and a hydrogen concentration of 0.2% by volume or more and less than 5.0% by volume at an average cooling rate of 5°C / s or more, a second cooling step of cooling the base steel sheet that has undergone the first cooling step from a steel sheet temperature of 650°C or less at the completion of the cooling in the first cooling step to a steel sheet temperature of 550°C or less in an atmosphere having a dew point of -20°C or less and a hydrogen concentration of 5.0% by volume or more and 25% by volume at an average cooling rate of 10°C / s or more, and a hot dip galvanizing step of performing a hot dip galvanizing treatment on the base steel sheet that has undergone the second cooling step to form a zinc-based coating layer on the surface of the base steel sheet. ２

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

[10] , comprising an oxidation treatment step of performing oxidation treatment at a temperature of 400°C or higher and 900°C or lower in an atmosphere containing 1000 volume ppm or more of hydrogen.

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

[10] or

[11] , comprising a reheat treatment step of, after the hot-dip galvanizing coating, holding the steel sheet at a temperature of 200°C or higher and 450°C or lower in an atmosphere having a hydrogen concentration of 15 volume % or lower for 5 seconds or longer and 600 seconds or shorter, followed by cooling.

[13] The method for producing a hot-dip galvanized steel sheet according to any of [9] to

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

[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 of manufacturing hot-dip galvanized steel sheet> The method of manufacturing a hot-dip galvanized steel sheet of the present invention is a method of manufacturing a hot-dip galvanized steel sheet by performing hot-dip galvanizing treatment on the surface of a base steel sheet having a specific chemical composition described later, and includes a first annealing step of holding the base steel sheet at a holding temperature of 650°C or higher and 950°C or lower for 20 s or higher and 150 s or lower in an atmosphere having a dew point of -55°C or higher and +30°C or lower and a hydrogen concentration of 5.0 vol% or higher and 25 vol% or lower, a second annealing step of holding the base steel sheet that has been subjected to the first annealing step at a holding temperature of 700°C or higher and 950°C or lower for 30 s or higher and 150 s 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 vol% or higher and lower than 5.0 vol%, and a second annealing step of holding the base steel sheet that has been subjected to the second annealing step at a holding temperature of 700°C or higher and 950°C or lower for 30 s or higher and 150 s 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 vol% or higher and lower than 5.0 vol%. the first cooling step of cooling the base steel sheet from the final holding temperature of the first cooling step (a holding temperature of 700°C or more and 950°C or less) to a steel sheet temperature of 650°C or less in an atmosphere with a hydrogen concentration of 0.2% by volume or more and less than 5.0% by volume at an average cooling rate of 5°C / s or more; the second cooling step of cooling the base steel sheet that has undergone the first cooling step from the final holding temperature of the first cooling step (a steel sheet at 650°C or less at the end of cooling) to a steel sheet temperature of 550°C or less in an atmosphere with a dew point of -20°C or less and a hydrogen concentration of 5.0% by volume or more and 25% by volume at an average cooling rate of 10°C / s or more; and the hot-dip galvanizing step of performing a hot-dip galvanizing treatment on the base steel sheet that has undergone the second cooling step to form a zinc-based plating layer on the surface of the base steel sheet.

[0015] 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.

[0016] 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.

[0017] 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.

[0018] The manufacturing method of the present invention is a method for manufacturing a hot-dip galvanized steel sheet, in which a steel sheet is annealed, cooled, and then hot-dip galvanized. The annealing can be performed in a non-oxidizing atmosphere. The annealing method includes a first annealing step and a second annealing step. In the first annealing step, the steel sheet is annealed in a reducing atmosphere with a high hydrogen concentration and a predetermined dew point to reduce 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 to release hydrogen dissolved in the steel from the steel sheet. Furthermore, cooling can also be performed in a non-oxidizing atmosphere. The cooling method also includes a first cooling step and a second cooling step. In the first cooling step, the cooling temperature is relatively higher than in the second cooling step described below, the steel sheet is annealed in an atmosphere with low hydrogen and a predetermined dew point to further release hydrogen dissolved in the steel. In the second cooling step, the cooling temperature is relatively lower than in the first cooling step, and oxidation of the steel sheet is suppressed by cooling in an atmosphere with high hydrogen and a predetermined dew point. After being cooled to a predetermined temperature, the steel sheet is immersed in a hot-dip galvanized bath to be subjected to hot-dip galvanized plating. The manufacturing method of the present invention also includes a case in which an alloying treatment is performed after hot-dip galvanized plating to produce a galvannealed hot-dip galvanized steel sheet. The hot-dip galvanized steel sheet (including the galvannealed hot-dip galvanized steel sheet) obtained in this manner can control the total amount of hydrogen released at 200°C or higher and 350°C or lower, as measured by temperature ramp analysis, to 0.20 mass% or less, and has a beautiful surface appearance and good delayed fracture resistance.

[0019] Furthermore, an even more beautiful surface appearance can be obtained by performing an oxidation treatment to generate iron oxide on the surface layer of the steel sheet in a predetermined oxidizing atmosphere before annealing. Furthermore, by reheating the coated steel sheet in an atmosphere having a predetermined hydrogen concentration, the delayed fracture resistance can be further improved. 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 inlet side, an oxidation zone (a zone for the oxidation treatment), a reducing zone (a zone for the first annealing step), a soaking zone (a zone for the second annealing step), a cooling zone 1 (a zone for the first cooling step), and a cooling zone 2 (a zone for the second cooling step). The oxidation treatment is not an essential step and can be performed as needed.

[0020] 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 (first cooling step, second cooling step), hot dip galvanizing, and reheating.

[0021] 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.

[0022] 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 By performing oxidation treatment in an atmosphere containing 1000 volume ppm or more of O, excellent surface quality and excellent delayed fracture resistance are both highly improved. Furthermore, this 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. ２ The 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 a hot-dip galvanized steel sheet having excellent delayed fracture resistance can be obtained without the oxidation treatment.

[0023] 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.

[0024] 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. Alternatively, the surface of the steel sheet can be oxidized 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. If the temperature range in which the steel sheet is exposed to the oxidizing atmosphere is less than 50°C, the amount of oxidation will be insufficient, resulting in the formation of oxides of Si and Mn, which may reduce the effect of improving galvanizability. When the surface of the steel sheet is oxidized while the temperature is being increased, the rate of temperature increase within the oxidation temperature range is preferably 3 to 25° C. / s in order to ensure an appropriate amount of oxidation.

[0025] 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 increases the temperature of the steel sheet faster than radiant heating methods, thereby offering the advantage of shortening the furnace length and increasing the steel sheet transport speed. Furthermore, when 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.

[0026] First Annealing Step In the first annealing step, the steel sheet (base 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 5.0% 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 coating 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.

[0027] 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 is a cold-rolled steel sheet, the annealing temperature is preferably set to 750°C or higher from the viewpoint of ensuring 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 15°C or lower, more preferably 5°C or lower.

[0028] 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 5.0%, 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 5.0% or more and 25% or less. Furthermore, if oxidation treatment is performed, the hydrogen concentration is preferably 8.0% 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.

[0029] If the holding time at 650°C or higher and 950°C or lower in the first annealing step is less than 20 seconds, 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 a high-strength steel sheet with a tensile strength of 780 MPa or higher may not 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 at 650°C or higher and 950°C or lower in the first annealing step is set to 20 seconds or longer and 150 seconds or shorter. The holding time is preferably 30 seconds or longer, more preferably 40 seconds or longer. The holding time is preferably 130 seconds or shorter, more preferably 90 seconds or shorter.

[0030] 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 5.0%. 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.

[0031] In the second annealing step, the lower the dew point, the smaller 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 +10°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 5.0% or more, the amount of hydrogen in the steel cannot be sufficiently reduced, so the hydrogen concentration is set to less than 5.0%. From the above viewpoint, the hydrogen concentration is preferably 1.0% or more, more preferably 2.0% or more. Similarly, the hydrogen concentration is more preferably 4.0% or less.

[0032] 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. In addition, 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. From the viewpoint of fully releasing hydrogen from the steel, the holding time at 700°C or higher and 950°C or lower in the second annealing step is preferably set to 50 seconds or longer. The holding time is preferably set to 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, and therefore it is necessary to optimize the conditions for the first annealing step and the second annealing step in which annealing is performed, as described above.

[0033] 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.

[0034] First Cooling Step: The steel sheet (base steel sheet) that has undergone 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% or higher but less than 5.0% at an average cooling rate of 5° C. / s or higher in the temperature range from the holding temperature at the end of the second annealing step, i.e., the final holding temperature of the second annealing step (holding temperature of 700° C. or higher but 950° C. or lower), to a steel sheet temperature of 650° C. or lower. By cooling at an average cooling rate of 5° C. / s or higher in the temperature range from the final holding temperature of the second annealing step to a steel sheet temperature of 650° C. or lower, the desired steel sheet strength can be obtained, and the penetration of hydrogen in the atmosphere into the steel sheet during cooling can be suppressed, while the relatively stable quasi-diffusible hydrogen that was not fully released in the first annealing step can be released. If the average cooling rate is less than 5° C. / s, the steel sheet strength is likely to decrease. Therefore, the average cooling rate is set to 5° C. / s or higher. The average cooling rate is preferably 8° C. / s or higher. Because special equipment is required to achieve a high cooling rate, the average cooling rate is preferably 40°C / s or less, and more preferably 30°C / s or less. The steel sheet temperature (cooling end temperature of the first cooling step) is preferably 600°C or higher. If the steel sheet temperature is below 600°C, the hydrogen reduction effect saturates, making it difficult to increase the cooling rate, and it may be difficult to ensure the desired microstructure and strength. Furthermore, if the hydrogen concentration in the atmosphere exceeds 5.0%, relatively stable quasi-diffusible hydrogen tends to remain in the steel sheet. Here, the final holding temperature in the second annealing step refers to the temperature at which a steel sheet annealed within the ranges satisfying the annealing temperature, hydrogen concentration, dew point, and holding time requirements of 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 of the first cooling step, i.e., the final holding temperature (°C) of the second annealing step, and the cooling end temperature (650°C or less) of the first cooling step, 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 changes from less than 5.0% to 5.0% or more.

[0035] The atmosphere of the cooling zone in the first cooling step will be described. Hydrogen has a high cooling capacity, so 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, and the release of quasi-diffusible hydrogen, which is released at temperatures between 200°C and 350°C during temperature rise analysis, may be insufficient. When cooling from a relatively high temperature, the cooling efficiency is high, so the amount of hydrogen required to achieve a cooling rate of 5°C / s or more in the 650°C or higher region is smaller than that required for cooling at lower temperatures. In addition, the cooling rate required to achieve the required strength in the 650°C or higher region is smaller than the cooling rate below 650°C, which has the greatest impact on steel sheet strength. Therefore, the hydrogen concentration is set to 0.2% or more and less than 5.0%. If the hydrogen concentration is less than 0.2%, the cooling rate may not be sufficient, and the steel sheet strength is likely to decrease. On the other hand, if the hydrogen concentration is 5.0% or higher, the cooling rate increases, but hydrogen may penetrate the steel sheet during cooling, making it impossible to fully release the relatively stable semi-diffusible hydrogen that was not released in the first annealing step. The hydrogen concentration is preferably 0.5% or higher, more preferably 1.0% or higher. The hydrogen concentration is preferably 4.0% or lower, more preferably 3.0% or lower. Furthermore, by setting the dew point to −20°C or lower, it is possible to prevent the steel sheet from reoxidizing at low temperatures and thereby reducing its galvanic properties. That is, if the dew point exceeds −20°C, the steel sheet is likely to reoxidize at low temperatures and thereby reduce its galvanic properties. Therefore, the dew point in the first cooling step is set to −20°C or lower. While the lower limit is not particularly limited, setting the dew point to less than −55°C requires special equipment to lower the dew point, which increases costs. Therefore, the dew point in the first cooling step is preferably set to −55°C or higher.

[0036] Second Cooling Step: The steel sheet (base steel sheet) that has undergone the first cooling step is cooled in an atmosphere with a dew point of −20° C. or lower and a hydrogen concentration of 5.0% to 25% from a steel sheet temperature of 650° C. or lower at the end of the first cooling step (cooling end temperature (650° C. or lower)) to a steel sheet temperature of 550° C. or lower at an average cooling rate of 10° C. / s or higher. It may also be cooled to a temperature of 150° C. or higher and 550° C. or lower. Thereafter, the steel sheet is heated as necessary, and then 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 higher in the temperature range from the cooling end temperature in the first cooling step to a steel sheet temperature of 550° C. or lower, 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 below 650° C., 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. Because special equipment is required to achieve a high cooling rate, the average cooling rate is preferably set to 40°C / s or less, and more preferably 30°C / s or less. Here, the cooling end temperature in the first cooling step refers to the temperature at which a steel sheet annealed within ranges satisfying the requirements of the steel sheet temperature, hydrogen concentration, dew point, and holding time in the first cooling 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 in the second cooling step (the cooling end temperature in the first cooling step) (°C) and the cooling end temperature in the second cooling step (550°C or less) by the cooling time (s).

[0037] The second cooling process has a lower cooling temperature than the first cooling process. Therefore, even if a higher concentration of hydrogen is present in the atmosphere than in the first cooling process, the hydrogen is less likely to penetrate into the steel sheet and to become relatively stable quasi-diffusible hydrogen. On the other hand, the second cooling process, 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 process. Therefore, the hydrogen concentration in the cooling zone atmosphere is set to 5.0% or more and 25% or less. A hydrogen concentration of less than 5.0% may not ensure a sufficient cooling rate, resulting in a decrease in steel sheet strength. On the other hand, a hydrogen concentration exceeding 25% saturates the cooling effect, and hydrogen easily penetrates into the steel sheet during cooling, increasing the amount of relatively stable quasi-diffusible hydrogen, which can lead to a decrease in delayed fracture resistance. The hydrogen concentration is preferably 8.0% or more, more preferably 10% or more. The hydrogen concentration is preferably 20% or less, more preferably 15% or less. Furthermore, by setting the dew point at -20°C or less, it is possible to prevent the steel sheet from reoxidizing at low temperatures and thereby reducing its galvanizability. That is, if the dew point exceeds −20° C., the steel sheet is likely to reoxidize at low temperatures, resulting in a decrease in platability. Therefore, the dew point in the second cooling step is set to −20° C. or lower. Although there are no particular restrictions on the lower limit, setting the dew point below −55° C. requires special equipment for lowering the dew point, which increases costs. Therefore, the dew point in the second cooling step is preferably set to −55° C. or higher.

[0038] 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, the base steel sheet is cooled in the second cooling process under the conditions described above, and then, if necessary, the base steel sheet is heated to approximately the bath temperature and immersed in a hot-dip galvanizing bath for coating. In the case of GA or GI, the coating bath contains Zn, Al, and unavoidable impurities. Although the components are not particularly specified, the Al concentration in the bath is preferably 0.05% or more and 0.250% or less. If the Al concentration in the bath is less than 0.05%, the occurrence of bottom dross increases, and the dross easily adheres 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 easily adheres 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.

[0039] 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.

[0040] - Reheating Treatment Step The steel sheet (hot-dip galvanized steel sheet) that has been subjected to treatment in the hot-dip galvanizing step or further treatment in the alloying step and then cooled as necessary may be further subjected to temper rolling and heat treatment as necessary, and then reheated under the following conditions: atmosphere: hydrogen concentration 15% by volume or less, reheating temperature: 200°C to 450°C, reheating time: 5 seconds to 600 seconds. From the viewpoint of promoting the release of diffusible hydrogen in the steel during reheating, it is advantageous to lower the hydrogen concentration in the atmosphere. Therefore, the hydrogen concentration is preferably 15% or less, more preferably 5% or less. Note that there is no particular restriction 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 quasi-diffusible hydrogen in the steel, the reheating temperature is preferably 200°C or higher. On the other hand, if the reheating temperature exceeds 450°C, the coating layer may remelt, 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 the hydrogen concentration in the steel may not be sufficient. Therefore, the reheating time is preferably 5 seconds or longer. The reheating time is more preferably 10 seconds or longer, and even more preferably 30 seconds or longer. On the other hand, from the viewpoint of productivity, the reheating time is preferably 600 s 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.

[0041] After reheating under the conditions of an atmosphere with a hydrogen concentration of 15% by volume or less, a reheating temperature of 200°C or more and 450°C or less, and a reheating time of 5 seconds or more and 600 seconds or less, the hot-dip galvanized steel sheet can be cooled. The above-mentioned temper rolling may be carried out before and / or after the reheat treatment. When temper rolling is carried out, it may be carried out either before or after the reheat treatment step, or both, but it is preferable to achieve an elongation rate of 0.05% or more and 1.0% or less.

[0042] Conditions other than those mentioned above are not particularly limited, and may be carried out in accordance with conventional methods.

[0043] <Hot-dip galvanized steel sheet> Next, a high-strength hot-dip galvanized steel sheet will be described. The hot-dip galvanized steel sheet of the present invention comprises a base steel sheet having a composition containing, by mass%, C: 0.06% or more and 0.30% or less, Si: 0.01% or more but less than 3.00%, Mn: 1.5% or more and 3.5% or less, P: 0.1% or less (not including 0%), S: 0.003% or less (not including 0%), sol. Al: 0.1% or less (not including 0%), N: 0.007% or less (not including 0%), O: 0.003% or less (not including 0%), with the balance being Fe and unavoidable impurities; and a coating weight of 20 g / m2 per side formed on the surface of the base steel sheet. ２ 120g / m or more ２ and a zinc-based plating layer having a thickness of 0.20 mass ppm or less, wherein the base steel sheet has a total hydrogen release amount of 0.20 mass ppm or less at a temperature of 200°C or higher and 350°C or lower when measured by temperature rise analysis, has fewer than three unplated defects within a 1 m x 1 m area on the steel sheet surface, and has a tensile strength of 780 MPa or higher.

[0044] The coating weight of the zinc-based coating layer on one side of the base steel sheet by hot dip galvanizing treatment is 20 g / m ２ 120g / m or more ２ The plating weight of the zinc-based plating layer per side may be 20 g / m or less. 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 Therefore, the zinc-based coating layer formed on the surface of the base steel sheet should be such that the coating weight per side is 20 g / m ２ 120g / m or more ２ It is preferable that the coating weight of the zinc-based coating layer is not more than 7%. There are no particular restrictions on the method for adjusting the coating weight of the zinc-based coating layer, 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-based coating layer after alloying treatment, but it is preferable that the degree of alloying is 7 to 15%. If the degree of alloying is less than 7%, the η phase will remain and press formability will tend to deteriorate, while if it exceeds 15%, coating adhesion will tend to deteriorate.

[0045] The base steel sheet may be either a cold-rolled steel sheet or a hot-rolled steel sheet. Delayed fracture resistance is a problematic property in high-strength steel sheets, and 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.

[0046] The total amount of hydrogen (quasi-diffusible hydrogen) released at temperatures between 200°C and 350°C (when measured by temperature-programmed analysis) contained in the base steel sheet must be 0.20 mass ppm or less. This is because, when measured by temperature-programmed analysis, diffusible hydrogen released at temperatures below 200°C is released into the atmosphere and rendered harmless to delayed fracture resistance by leaving the high-strength hot-dip galvanized steel sheet for about two weeks after production, whereas the value of quasi-diffusible hydrogen released at temperatures between 200°C and 350°C (when measured immediately after production) is little reduced by leaving the steel sheet at room temperature, and therefore significantly affects delayed fracture resistance. The lower the value of this quasi-diffusible hydrogen, the better, and it is preferably 0.15 mass ppm or less, and more preferably 0.10 mass ppm or less.

[0047] 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 also 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.

[0048] 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.

[0049] - Elemental Composition A preferred elemental composition of the base steel sheet will be described below. The base steel sheet targeted by the present invention contains, as its elemental composition, C: 0.06% or more and 0.30% or less, Si: 0.01% or more but less than 3.00%, Mn: 1.5% or more and 3.5% or less, P: 0.1% or less (not including 0%), S: 0.003% or less (not including 0%), sol. Al: 0.1% or less (not including 0%), N: 0.007% or less (not including 0%), and O: 0.003% 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. The reasons for including each of the above elements are as follows. 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%), 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: Group E: One or more selected from Sn: 0.2% or less (not including 0%), Sb: 0.2% 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%)

[0050] C: 0.06% or more and 0.30% or less C has the effect of improving workability by forming martensite or the like as a steel structure, but in order to obtain good weldability, the C content is preferably 0.30% or less, more preferably 0.25% or less. On the other hand, in order to obtain good workability, the C content is preferably 0.06% or more, and more preferably 0.09% or more.

[0051] Si: 0.01% or more and less than 3.00% 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. On the other hand, Si is also an element that adversely affects the resistance weld cracking resistance characteristics of welds. To increase the strength of steel sheets, the Si content is set to 0.01% or more. On the other hand, if the Si content is 3.00% or more, the hot rolling and cold rolling properties are significantly reduced, which may adversely affect productivity or lead to a decrease in the ductility of the steel sheets themselves. Therefore, the Si content is set to less than 3.00%. From the above viewpoints, the Si content is preferably 2.50% or less, and more preferably 2.00% or less.

[0052] 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, and preferably 1.8% or more. On the other hand, if the Mn content is 3.5% or less, the above effects can be achieved without increasing costs. Therefore, the Mn content is set to 3.5% or less, and preferably 3.3% or less.

[0053] 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.

[0054] S: 0.003% or less (excluding 0%) It is preferable to reduce the S content as much as possible. By suppressing the S content, deterioration in weldability can be prevented, and deterioration in ductility during hot rolling can be prevented, thereby suppressing hot cracking and significantly improving surface properties. Furthermore, by suppressing the S content, deterioration in delayed fracture resistance, ductility, bendability, and stretch flangeability of the steel sheet due to the formation of coarse sulfides as an impurity element can be prevented. Since the problems caused by S become significant when the S content exceeds 0.003%, the S content is set to 0.003% or less, and preferably 0.002% or less. From the viewpoint of improving delayed fracture resistance, the S content is preferably set to 0.001% or less, and more preferably 0.0003% or less. Although the lower limit of the S content is not particularly limited, the S content is preferably set to 0.0001% or more due to constraints on production technology.

[0055] 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. On the other hand, although there is no particular lower limit for the sol. Al content, removing impurity-level Al also increases costs, so the sol. Al content is preferably set to 0.001% or less. 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.

[0056] N: 0.007% or less (excluding 0%) By setting the N content to 0.007% or less, N forms coarse nitrides with Ti, Nb, and V at high temperatures. This prevents the effect of increasing the strength of the steel sheet by adding Ti, Nb, and V from being impaired. Furthermore, by setting the N content to 0.007% or less, a decrease in toughness can also be prevented. Furthermore, by setting the N content to 0.007% or less, the occurrence of slab cracks and surface defects during hot rolling can be prevented. Therefore, the N content is set to 0.007% or less, preferably 0.005% or less, more preferably 0.003% or less, and even more preferably 0.002% or less. There is no particular lower limit for the N content, but due to production technology constraints, it is preferably set to 0.0005% or more.

[0057] O: 0.003% or less (excluding 0%) O is the element that reduces Al in steel. 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 0.003% or less. From the viewpoint of improving delayed fracture resistance, the O content is more preferably less than 0.0030%. Note that there is no particular restriction on the lower limit of the O content, but in consideration of the industrially feasible lower limit, the O content is preferably set to 0.0005% or more.

[0058] 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 base steel plate and can be contained as needed. Of these elements in Group A, Nb is an element that can obtain a fine structure even with a small amount of Nb, and can achieve high strength without impairing toughness.

[0059] Nb: 0.05% or less (excluding 0%) When Nb is contained in an amount of 0.005% or more, the effect of improving strength can be obtained. However, from the viewpoint of preventing an increase in costs, when Nb is contained, the Nb content is set to 0.05% or less.

[0060] Ti: 0.08% or less (excluding 0%) Ti is an element effective for precipitation strengthening of steel. There is no particular lower limit for Ti, but in order to obtain the effect of adjusting strength, it is preferable to set it to 0.005% or more. However, if Ti is added in excess, the hard phase becomes excessively large and formability decreases, so when Ti is contained, the Ti content is preferably 0.08% or less, and is set to 0.05% or less.

[0061] 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 an increase in costs, when V is contained, the V content is set to 0.2% or less.

[0062] 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 an increase in costs, when W is contained, the W content is set to 0.15% or less.

[0063] 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 an increase in costs, when Zr is contained, the Zr content is set to 0.15% or less.

[0064] 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.

[0065] 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.

[0066] 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 set to 1% or less.

[0067] 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.

[0068] 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% or less.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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%.

[0076] Sn: 0.2% or less (excluding 0%) 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.

[0077] Sb: 0.2% or less (excluding 0%) 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, to obtain good toughness, the Sb content is 0.2% or less.

[0078] 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.

[0079] 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.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 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%.

[0084] 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.

[0085] 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.

[0086] 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%.

[0087] 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.

[0088] 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.

[0089] 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.

[0090] 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.

[0091] The remainder of the steel sheet (base steel sheet) other than the above-mentioned component composition is Fe and unavoidable impurities.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] 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.

[0096] 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.

[0097] ・Tempered martensite Aspect ratio ≦8, equivalent circle diameter ≦30 μm Distribution density of carbides inside the structure: 0.10 to 12 pieces / μm ２Fresh 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

[0098] The above describes a method for manufacturing a hot-dip galvanized steel sheet and a hot-dip galvanized steel sheet as one embodiment of the hot-dip galvanized steel sheet and its manufacturing method of the present invention. According to the present invention, in a method for manufacturing a hot-dip galvanized steel sheet in which a steel sheet is annealed and cooled, and then hot-dip galvanized, the annealing is performed in a first annealing step with a high hydrogen concentration and a second annealing step with a low hydrogen concentration, and the cooling is performed in a first step with a low hydrogen concentration and a second step with a high hydrogen concentration, each under specific conditions. This allows the production of a hot-dip galvanized steel sheet that has a beautiful surface appearance free of bare spots and suppresses the amount of quasi-diffusible hydrogen released at 200°C to 350°C during temperature rise analysis to 0.20 mass ppm or less, thereby achieving a higher level of delayed fracture resistance. Furthermore, in the present invention, by performing an oxidation treatment before the annealing, further performing the annealing under more specific conditions, and performing a pressure reheating treatment in a specific atmosphere after galvanizing, it is possible to produce a hot-dip galvanized steel sheet that has even higher levels of coating appearance and delayed fracture resistance.

[0099] Steels having the chemical compositions shown in Table 1 were melted and cast slabs were obtained, which were then hot-rolled, pickled, and cold-rolled to form cold-rolled steel sheets having a thickness of 1.2 mm. These cold-rolled steel sheets were used as base steel sheets for hot-dip galvanized steel sheets.

[0100]

[0101] In a continuous hot-dip galvanizing line (CGL) equipped with an all-radiant tube (ART) type annealing furnace, the steel sheets were annealed under the conditions shown in Table 2 (Table 2-1, Table 2-2), Table 3, and Table 5, and then hot-dip galvanized (coating composition: Zn-0.2 mass% Al), and the coating weight per side was adjusted to 30 to 70 g / m by gas wiping. ２ The steel sheets were then adjusted to the desired value, and then, except for No. 95, they were subjected to an alloying treatment. Table 3 shows examples in which reheating treatment was carried out after the plating and alloying treatment. Separately from the above examples, in a CGL facility equipped with a direct-fire annealing furnace (DFF-type annealing furnace) equipped with a direct flame burner, steel sheets were oxidized and annealed under the conditions shown in Table 4, 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 The temperature was adjusted to 100°C, and then alloying treatment was performed. Note that No. 125 (Table 4) 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 examples other than No. 125 in Table 4 (Nos. 123, 124, 126 to 148), 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.

[0102] 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 5. Here, in the oxidation treatment of the examples in Table 4, 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 heats up 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." The primary cooling end temperature (final holding temperature (steel sheet temperature) in the first cooling step) was 650°C for Nos. 1 to 148, and was changed between 600 and 640°C for Nos. 149 to 151 shown in Table 5.

[0103]

[0104]

[0105]

[0106]

[0107]

[0108] Measurement of the amount of quasi-diffusible hydrogen in steel sheet (hydrogen analysis method) A rectangular 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 galvanized layer of the test piece was removed with a router. Immediately, hydrogen analysis was performed 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 heating rate of 200°C / hour. 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 200°C to 350°C was then calculated as the amount of diffusible hydrogen in the steel. Here, the steel having a diffusible hydrogen content of 0.05 mass ppm or less was rated as excellent "◎+", more than 0.05 mass ppm and less than 0.10 mass ppm was rated as excellent "◎", more than 0.10 mass ppm and less than 0.15 mass ppm was rated as good "◯", and more than 0.15 mass ppm and less than 0.20 mass ppm was rated as fair "△". From experience, when the diffusible hydrogen content in the steel exceeds 0.20 mass ppm, the delayed fracture resistance often deteriorates, so that steel having a diffusible hydrogen content of more than 0.20 mass ppm was rated as poor "X".

[0109] 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 (○+).

[0110] 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).

[0111] Observation and Measurement of Base Steel Sheet Microstructure The total area ratio of martensite, bainite, and retained austenite (residual γ) in the base steel sheet microstructure was measured as follows. A sample was cut out so that the cross section (L cross section) parallel to the rolling direction and the thickness direction of the steel sheet 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 microstructure. The observation position on this sample observation surface was set to ¼ of the sheet thickness, and five fields of view were observed using an SEM at a magnification of 3000x. This observation confirmed that the steel microstructure was observed in the range of ⅛ to ⅜ thickness, centered at ¼ thickness from the surface of the base steel sheet. The total area of ​​martensite, bainite, and retained γ was determined from the obtained microstructure image, and the area ratio was calculated by dividing this total area by the measured area for the five fields of view. The average of these values ​​was used as the total area ratio of martensite, bainite, and retained 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 has 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 it, resulting in a concave shape. Tempered martensite contains many carbides, but depending on the plane orientation, the amount of carbides may be small. ・Fresh martensite Fresh martensite is the gray or white area in the SEM photograph. Fresh martensite is blocky, granular, plate-like, or film-like, and does not contain carbides. ・Bainite Bainite is the dark gray area in the SEM photograph. Bainite is in the form of a film, a plate, or a mass formed by connecting some or all of the adjacent regions of these, and contains a small amount of carbides. Bainite also includes bainite that has been tempered after formation to coarsen the 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.

[0112] 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.

[0113] 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.

[0114] According to Tables 1 to 5, 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. A base steel sheet having a composition containing, by mass%, C: 0.06% or more and 0.30% or less, Si: 0.01% or more and less than 3.00%, Mn: 1.5% or more and 3.5% or less, P: 0.1% or less (excluding 0%), S: 0.003% or less (excluding 0%), sol. Al: 0.1% or less (excluding 0%), N: 0.007% or less (excluding 0%), O: 0.003% or less (excluding 0%), with the balance being Fe and unavoidable impurities; and a coating weight per side of 20 g / m formed on the surface of the base steel sheet. ２ 120g / m or more ２ and a zinc-based plating layer having a hydrogen content of 0.20 mass ppm or less at 200°C or higher and 350°C or lower, wherein the base steel sheet has, when measured by temperature rise analysis, a total amount of hydrogen released at 200°C or higher and 350°C or lower of 0.20 mass ppm or less, has less than 3 uncoated defects within an area of ​​1 m x 1 m on the steel sheet surface, and has a tensile strength of 780 MPa or more.

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 A to D, in mass %: Group A 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%), Zr: 0.15% or less (excluding 0%), one or more selected from Group B 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%), B: 0.005% or less (excluding 0%), one or more selected from Group C Ca: 0.005% or less (excluding 0%), Mg: 0.005% or less (excluding 0%), REM: 0.005% or less (excluding 0%), one or more selected from Group D Sn: 0.2% or less (excluding 0%), Sb: 0.2% or less (excluding 0%), one or more selected from the following:

3. The hot-dip galvanized steel sheet according to claim 1 or 2, wherein the base steel sheet further contains, as the chemical composition, in mass%, one or more elements selected from the following groups E to H. 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%) 4. The hot-dip galvanized steel sheet according to any one of claims 1 to 3, wherein the steel structure in the range of 1 / 8 thickness to 3 / 8 thickness centered at a position of 1 / 4 thickness from the surface of the base steel sheet has a total area ratio, in area percentage, of martensite, bainite and retained austenite of 30% or more, and has a tensile strength of 780 MPa or more.

5. The hot-dip galvanized steel sheet according to any one of claims 1 to 3, wherein the steel structure in a range of 1 / 8 thickness to 3 / 8 thickness centered at a position of 1 / 4 thickness from the surface of the base steel sheet has a total area ratio, in area percentage, of martensite, bainite and retained austenite of 50% or more, and has a tensile strength of 980 MPa or more.

6. The hot-dip galvanized steel sheet according to any one of claims 1 to 3, wherein the steel structure in a range of 1 / 8 thickness to 3 / 8 thickness centered on a position of 1 / 4 thickness from the surface of the base steel sheet has a total area ratio, in area %, of martensite, bainite and retained austenite of 70% or more, and has a tensile strength of 1180 MPa or more.

7. The hot-dip galvanized steel sheet according to any one of claims 1 to 3, wherein the steel structure in a range of 1 / 8 thickness to 3 / 8 thickness centered at a position of 1 / 4 thickness from the surface of the base steel sheet has a total area ratio, in area percentage, of martensite, bainite and retained austenite of 85% or more, and has a tensile strength of 1310 MPa or more.

8. The hot-dip galvanized steel sheet according to any one of claims 1 to 3, wherein the steel structure in a range of 1 / 8 thickness to 3 / 8 thickness centered at a position of 1 / 4 thickness from the surface of the base steel sheet has a total area ratio, in area percentage, of martensite, bainite and retained austenite of 90% or more, and has a tensile strength of 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 producing a hot-dip galvanized steel sheet by annealing and hot-dip galvanizing the surface of a base steel sheet having a chemical composition as defined in any one of claims 1 to 3, comprising: a first annealing step in which the base steel sheet is held in an atmosphere having a dew point of -55°C or higher and +30°C or lower and a hydrogen concentration of 5.0% by volume or higher and 25% by volume or lower at a holding temperature of 650°C or higher and 950°C or lower for a period of 20 s to 150 s; and a second annealing step in which the base steel sheet that has been subjected to the first annealing step is held in an atmosphere having a dew point of -50°C or higher and +30°C or lower and a hydrogen concentration of 0.2% by volume or higher and less than 5.0% by volume at a holding temperature of 700°C or higher and 950°C or lower for a period of 30 s to 300 s. a first cooling step of cooling the base steel sheet after the second annealing step 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 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 5.0 vol% at an average cooling rate of 5°C / s or more; a second cooling step of cooling the base steel sheet after the first cooling step from a steel sheet temperature of 650°C or less at the completion of the cooling in the first cooling step to a steel sheet temperature of 550°C or less in an atmosphere having a dew point of -20°C or less and a hydrogen concentration of 5.0 vol% or more and 25 vol% at an average cooling rate of 10°C / s or more; and a hot-dip galvanizing step of performing a hot-dip galvanizing treatment on the base steel sheet after the second cooling step to form a zinc-based plating layer on a surface of the base steel sheet.

11. The base steel sheet before the first annealing process is subjected to O ２ The method for producing a hot-dip galvanized steel sheet according to claim 10, further 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 12. A method for producing a hot-dip galvanized steel sheet according to claim 10 or 11, comprising a reheating treatment step of holding the steel sheet at a temperature of 200°C or higher and 450°C or lower for a time of 5 s or higher and 600 s or lower in an atmosphere with a hydrogen concentration of 15 vol% or lower, followed by cooling.

13. A method for producing a hot-dip galvanized steel sheet according to any one of claims 9 to 12, further comprising, after the hot-dip galvanizing step, an alloying treatment step of alloying the base steel sheet and the zinc-based plating layer.

Images