Method for manufacturing hot-dip galvanized steel sheet

Abstract

A hot-dip galvanized steel sheet having an aesthetically pleasing surface appearance free from coating bare spots and having excellent delayed fracture resistance is manufactured. A method for manufacturing a hot-dip galvanized steel sheet includes subjecting a steel sheet to annealing in a non-oxidizing atmosphere and subsequently subjecting the steel sheet to hot-dip galvanizing. The annealing in a non-oxidizing atmosphere includes a first step and a second step. The first step includes subjecting the steel sheet to annealing for a predetermined period in a reducing atmosphere having a high hydrogen concentration, a predetermined dew point, and a predetermined temperature, to reduce oxidized Fe present in a surface layer of the steel sheet. The subsequent second step includes subjecting the steel sheet to annealing for a predetermined period in a non-oxidizing atmosphere having a low hydrogen concentration, a predetermined dew point, and a predetermined temperature, to release hydrogen dissolved in the steel from the steel sheet. Optionally, an oxidation treatment for forming oxidized Fe in the surface layer of the steel sheet is performed in a predetermined oxidizing atmosphere before the annealing is performed.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a hot-dip galvanized steel sheet.Background Art

[0002] Today, surface-treated steel sheets having corrosion protection properties imparted to their base steel sheets are widely used in fields of, for example, automobiles, home appliances, and construction materials. Among such surface-treated steel sheets are hot-dip galvanized steel sheets (including hot-dip galvannealed steel sheets), which have excellent corrosion protection properties. To improve the fuel efficiency and crash safety of automobiles, efforts are being made to reduce the weight and increase the strength of vehicle bodies by increasing the strength of materials for vehicle bodies to reduce their thickness. Accordingly, high strength steel sheets are being increasingly used as materials for vehicle bodies.

[0003] Typically, hot-dip galvanized steel sheets are manufactured by using a hot rolled steel sheet or a cold rolled steel sheet as a base material and subjecting the base steel sheet to recrystallization annealing in an annealing furnace of a CGL and subsequently to hot-dip galvanizing. Galvannealed steel sheets are manufactured by additionally performing an alloying treatment after the hot-dip galvanizing.

[0004] The annealing requires holding the steel sheet in a reducing atmosphere containing hydrogen. In this process, the hydrogen in the furnace enters the steel sheet, and the subsequent process of cooling and hot-dip galvanizing of the steel sheet causes the hydrogen to remain in the steel sheet as diffusible hydrogen in steel. Since the coating is not permeable to hydrogen, the diffusible hydrogen in steel is not released from the steel sheet after galvanizing, and, therefore, problems exist in that if the amount of diffusible hydrogen in steel is large, delayed fracture resistance decreases. In particular, in the case of high strength steel sheets designed to have a tensile strength of 780 MPa or greater, a problem exists in that the hydrogen in steel is likely to remain after annealing, and that, consequently, delayed fracture resistance significantly decreases. The reason for this is that achieving the predetermined strength in high strength steel sheets designed to have a tensile strength of 780 MPa or greater requires the formation of hard microstructures, such as martensite and bainite, and this requires the formation of an austenite phase in the annealing step; austenite phases have properties in which they easily absorb large amounts of hydrogen compared to ferrite phases and in which the diffusion rate of the hydrogen is low, with the result that once hydrogen is absorbed during the annealing step, the hydrogen is unlikely to be released in the cooling process.

[0005] In the related art, technologies for decreasing the amount of hydrogen in steel sheets have been proposed. Examples of such technologies are as follows.

[0006] Patent Literature 1 discloses a technology in which after a hot rolled steel sheet is subjected to a reduction treatment, the steel sheet is subjected to a dehydrogenation treatment at a temperature of 450 to 550°C in an atmosphere having a H 2 concentration of 8 to 20% and subsequently subjected to hot-dip galvanizing.

[0007] Patent Literature 2 discloses a technology that uses a method in which after a hot rolled steel sheet is subjected to reduction-annealing within a range of 650 to 950°C, the steel sheet is subjected to hot-dip galvanizing; this method performs control such that a relationship between an annealing temperature and a hydrogen concentration in an annealing furnace satisfies inequality (1) below, thereby decreasing the amount of hydrogen in the steel sheet. 1 ≤ H ≤ − 0.05 × RT + 57.5

[0008] In the inequality, H is the hydrogen concentration in the furnace, and RT is the annealing temperature.

[0009] Patent Literature 3 discloses a technology that uses a method in which after a steel sheet containing Si, Mn, and Al is subjected to reduction-annealing, the steel sheet is subjected to hot-dip galvanizing; in this method, a hydrogen concentration in a furnace during the reduction-annealing is 10 vol.% or greater, and control is performed such that a relationship between a hydrogen partial pressure and a water vapor partial pressure in a furnace atmosphere gas having a temperature of 650°C or greater and less than 750°C satisfies inequality (2) below and also that the relationship between the hydrogen partial pressure and the water vapor partial pressure in a furnace atmosphere gas having a temperature of 750°C or greater and 950°C or less satisfies inequality (3) below, to achieve a good surface quality. log P H 2 O / P H 2 ≤ − 1.55 − 0.91 ≤ log P H 2 O / P H 2 ≤ − 0.635Citation ListPatent Literature

[0010] PTL 1: Japanese Unexamined Patent Application Publication No. 54-130443 PTL 2: Japanese Patent No. 3266008 PTL 3: Japanese Patent No. 5811841 Summary of InventionTechnical Problem

[0011] Unfortunately, the technologies disclosed in Patent Literature 1 and 2 are both designed to inhibit blistering (swelling of a coating) of hot rolled steel sheets; thus, regarding the improvement of the delayed fracture resistance of a high strength steel sheet including an austenite phase, the technologies have a need to decrease hydrogen in steel by further decreasing the amount of hydrogen in the atmosphere. If the amount of hydrogen in the atmosphere is further reduced, however, selective oxidation of easily oxidizable elements present in the high strength steel sheet, such as Si and Mn, is promoted, which results in impairment of coating properties, and, consequently, it is impossible to achieve a good surface quality. Accordingly, the methods disclosed in Patent Literature 1 and 2, in which hydrogen is decreased in the furnace in a uniform manner, present difficulties in achieving a good surface quality and improving delayed fracture resistance.

[0012] The technology disclosed in Patent Literature 3 is designed to achieve a good surface quality by improving the coating properties of a steel containing Si, Mn, and Al by varying the ratio between the water vapor partial pressure and the hydrogen partial pressure for each of the annealing temperatures; the technology requires control for achieving a hydrogen concentration of 10% or greater in the furnace, that is, the technology does not address lowering the hydrogen concentration in the steel and, therefore, presents difficulties in improving the delayed fracture resistance of high strength steel sheets.

[0013] Accordingly, an object of the present invention is to provide a manufacturing method that can solve the problems associated with the technologies of the related art, such as those described above, and which enables the manufacture of a hot-dip galvanized steel sheet having an aesthetically pleasing surface appearance free from coating bare spots and having excellent delayed fracture resistance.Solution to Problem

[0014] The present inventors conducted studies to solve the problems described above and found that in a method for manufacturing a hot-dip galvanized steel sheet including subjecting a steel sheet to annealing in a non-oxidizing atmosphere and subsequently to hot-dip galvanizing, by optimizing conditions for the annealing in a non-oxidizing atmosphere, it is possible to manufacture a hot-dip galvanized steel sheet having an excellent coating appearance and excellent delayed fracture resistance.

[0015] The present invention was made based on the above-described findings, and a summary of the present invention is as follows. [1] A method for manufacturing a hot-dip galvanized steel sheet, the method including subjecting a steel sheet to annealing in a non-oxidizing atmosphere and subsequently subjecting the steel sheet to hot-dip galvanizing, in a continuous annealing furnace, the method optionally including subjecting the steel sheet to an alloying treatment after the hot-dip galvanizing, wherein the annealing includes a first step and a second step, the first step includes holding the steel sheet at a temperature of 650°C or greater and 950°C or less for a period of 20 s or more and 150 s or less in an atmosphere having a dew point of -55°C or greater and +20°C or less and a hydrogen concentration of 5 vol.% or greater and 25 vol.% or less, and the second step includes holding, after the steel sheet undergoes the first step, the steel sheet at a temperature of 700°C or greater and 950°C or less for a period of 30 s or more and 300 s or less in an atmosphere having a dew point of -50°C or greater and +20°C or less and a hydrogen concentration of 0.2 vol.% or greater and less than 5.0 vol.%. [2] A method for manufacturing a hot-dip galvanized steel sheet according to the manufacturing method of [1], further including subjecting, before the annealing, the steel sheet to an oxidation treatment at a temperature of 400°C or greater and 900°C or less in an atmosphere containing O 2 in an amount of 1000 vol-ppm or greater. [3] A method for manufacturing a hot-dip galvanized steel sheet according to the manufacturing method of [2], wherein the oxidation treatment is performed in a process in which the steel sheet is heated for the annealing. [4] A method for manufacturing a hot-dip galvanized steel sheet according to the manufacturing method of [3], wherein the oxidation treatment is performed over a heating temperature span of 50°C or greater in the process in which the steel sheet is heated for the annealing. [5] A method for manufacturing a hot-dip galvanized steel sheet according to the manufacturing method of any one of [1] to [4], wherein the hydrogen concentration of the atmosphere for the first step of the annealing is 8 vol.% or greater. [6] A method for manufacturing a hot-dip galvanized steel sheet according to the manufacturing method of any one of [1] to [5], wherein the hydrogen concentration of the atmosphere for the second step of the annealing is 2.0 vol.% or greater. [7] A method for manufacturing a hot-dip galvanized steel sheet according to the manufacturing method of any one of [1] to [6], wherein a substrate steel sheet of the hot-dip galvanized steel sheet that is manufactured has a hydrogen concentration of 0.30 mass-ppm or less, where the hydrogen concentration is an amount of diffusible hydrogen. [8] A method for manufacturing a hot-dip galvanized steel sheet according to the manufacturing method of any one of [1] to [7], wherein a substrate steel sheet has a Si content of 0.1 mass% or greater. [9] A method for manufacturing a hot-dip galvanized steel sheet according to the manufacturing method of any one of [1] to [8], wherein a substrate steel sheet has a total area fraction of martensite, bainite, and retained γ of 30% or greater and a tensile strength of 780 MPa or greater.

[10] A method for manufacturing a hot-dip galvanized steel sheet according to the manufacturing method of any one of [1] to [8], wherein a substrate steel sheet has a total area fraction of martensite, bainite, and retained γ of 50% or greater and a tensile strength of 980 MPa or greater.

[11] A method for manufacturing a hot-dip galvanized steel sheet according to the manufacturing method of any one of [1] to

[10] , further including cooling, after the steel sheet undergoes the annealing, the steel sheet in an atmosphere having a dew point of -20°C or less and a hydrogen concentration of 5 vol.% or greater and 25 vol.% or less, the cooling including cooling the steel sheet at an average cooling rate of 5°C / s or greater over a temperature range of a final holding temperature of the annealing to 600°C and then further cooling the steel sheet to a temperature of 150°C or greater and less than 600°C, the cooling being optionally followed by heating of the steel sheet, before the steel sheet is immersed in a hot-dip galvanizing bath to be hot-dip galvanized. Advantageous Effects of Invention

[0016] The present invention provides a method for manufacturing a hot-dip galvanized steel sheet including subjecting a steel sheet to annealing and subsequently subjecting the steel sheet to hot-dip galvanizing; in this method, the annealing is performed in a first step that uses a high hydrogen concentration and a second step that uses a low hydrogen concentration, each under specific conditions, and, consequently, a hot-dip galvanized steel sheet having an aesthetically pleasing surface appearance free from coating bare spots and having excellent delayed fracture resistance can be manufactured. Furthermore, in the present invention, an oxidation treatment may be performed prior to the annealing, and the annealing may be performed under more limited conditions; in this case, a hot-dip galvanized steel sheet having higher levels of coating appearance properties and delayed fracture resistance can be manufactured.Description of Embodiments

[0017] In the present invention, the temperatures specified for an oxidation treatment, annealing, and cooling after the annealing are all temperatures of the steel sheet. In the present invention, a "non-oxidizing atmosphere" is an atmosphere in which iron does not become oxidized but in which easily oxidizable additive elements, such as Si and Mn, may become selectively oxidized. In the present invention, a "reducing atmosphere" is an atmosphere in which iron oxide can be reduced to iron.

[0018] The type of the hot-dip galvanized steel sheet to which the present invention is applicable is not particularly limited as long as the steel sheet is a coated steel sheet that includes a coated layer formed primarily of zinc. Such steel sheets include hot-dip galvanized steel sheets (GI) and hot-dip galvannealed steel sheets (GA) and further include hot-dip zinc-aluminum alloy coated steel sheets, hot-dip zinc-aluminum-silicon alloy coated steel sheets, and hot-dip zinc-aluminum-magnesium alloy coated steel sheets, and a specific composition of each of these coatings is not limited.

[0019] In the described below, the unit "%" used to describe the contents of elements in the chemical composition of a steel sheet (also referred to as a "substrate steel sheet" or a "base steel sheet"), the contents of elements in the chemical composition of a galvanizing bath, and a degree of alloying of a coated layer is "mass%" in all cases, and the unit "%" used to describe the hydrogen concentration of atmospheres for annealing and cooling is "vol.%" in all cases. Regarding the steel sheet, "high strength" means that the steel sheet has a tensile strength TS of 590 MPa or greater as measured in accordance with JIS Z 2241 (2011).

[0020] A manufacturing method of the present invention is a method for manufacturing a hot-dip galvanized steel sheet including subjecting a steel sheet to annealing in a non-oxidizing atmosphere and subsequently subjecting the steel sheet to hot-dip galvanizing. The annealing in a non-oxidizing atmosphere includes a first step and a second step. The first step includes subjecting the steel sheet to annealing in a reducing atmosphere having a high hydrogen concentration and a predetermined dew point, to reduce naturally oxidized Fe present in a surface layer of the steel sheet. The subsequent second step includes subjecting the steel sheet to annealing in a non-oxidizing atmosphere having a low hydrogen concentration and a predetermined dew point, to release hydrogen dissolved in the steel from the steel sheet. The annealed steel sheet is cooled to a predetermined temperature and subsequently immersed in a hot-dip galvanizing bath to be hot-dip galvanized. The manufacturing method of the present invention may include performing an alloying treatment after the steel sheet is subjected to the hot-dip galvanizing, to manufacture a hot-dip galvannealed steel sheet.

[0021] In addition, an oxidation treatment for forming oxidized Fe in the surface layer of the steel sheet may be performed in a predetermined oxidizing atmosphere before the annealing is performed. In this case, a more aesthetically pleasing surface appearance can be obtained.

[0022] The steel sheet serving as the base material for the hot-dip galvanized steel sheet will be described in detail later, regarding its microstructures and chemical composition.

[0023] In the present invention, the oxidation treatment and the annealing in a non-oxidizing atmosphere that follows are typically performed in a continuous annealing furnace that includes an oxidation zone (a zone for the oxidation treatment), a reduction zone (a zone for the first step of the annealing), a soaking zone (a zone for the second step of the annealing, and a cooling zone, which are disposed in this order, starting from an entry side.

[0024] Note that the oxidation treatment is not an essential step and may be appropriately performed as needed.

[0025] Now, regarding the manufacturing method of the present invention, the oxidation treatment, the annealing (first step, second step, and cooling after annealing), and the hot-dip galvanizing will be described in this order.Oxidation Treatment

[0026] In the oxidation treatment, the temperature of the steel sheet is controlled to fall within a range of 400°C or greater and 900°C or less in an atmosphere containing O 2 in an amount of 1000 vol-ppm or greater, to form oxidized Fe in the surface layer of the steel sheet. The atmosphere for the oxidation treatment may include one or more of N 2 , CO, CO 2 , H 2 O, and NOx, in addition to O 2 . N 2 may be included as an inert gas, CO may be included as a gas for adjusting oxidation and reduction, CO 2 may be included as an inert gas, and H 2 O may be included as a gas for adjusting oxidation and reduction. CO, CO 2 , H 2 O, and NOx may be included as fuel gases, gases derived from a component of the steel sheet that is annealed, impurity gases in air, or gases resulting from the combustion of a fuel.

[0027] In the present invention, the steel sheet is oxidized by this oxidation treatment and then reduced by the subsequent annealing (first step) to form a reduced iron layer in 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 therein; consequently, coating properties can be further improved. In the present invention, in which the second step of the annealing uses a low-hydrogen atmosphere, the oxidation treatment performed in an atmosphere containing O 2 in an amount of 1000 vol-ppm or greater is a very important step in terms of improving the surface quality and delayed fracture resistance and achieving higher levels of both of these properties. The improvement effect is particularly notable in steels containing Si in an amount of 0.1% or greater and Mn in an amount of 1.5% or greater.

[0028] The O 2 concentration of 1000 vol-ppm or greater in the atmosphere for the oxidation treatment promotes the oxidation of the steel sheet. If the O 2 concentration is less than 1000 vol-ppm, the steel sheet is insufficiently oxidized, and, consequently, oxides of Si and Mn are formed, which may degrade coating properties.

[0029] The atmosphere for the oxidation treatment may include N 2 , CO, CO 2 , H 2 O, NOx, and the like, depending on the gases used. The ratio between them is not particularly limited. While the oxidation treatment contributes to obtaining a more aesthetically pleasing surface appearance, a hot-dip galvanized steel sheet having excellent delayed fracture resistance can be obtained even without the oxidation treatment, and, therefore, this step is not an essential requirement.

[0030] The oxidation treatment uses a temperature of the steel sheet of 400°C or greater, which promotes the oxidation of the steel sheet. If the temperature of the steel sheet is less than 400°C, an amount of oxidation that results may be insufficient, which may lead to the formation of oxides of Si and Mn, and consequently, the effect of improving coating properties may be decreased. On the other hand, if the temperature of the steel sheet is greater than 900°C, the amount of oxidation of the steel sheet may become excessively large, and, therefore, reduction may not be completed in the subsequent reduction-annealing (first step); consequently, remaining iron oxide may impair coating properties. Accordingly, it is preferable that the oxidation treatment be performed at a temperature of 400°C or greater and 900°C or less. "The oxidation treatment is performed at a temperature of 400°C or greater and 900°C or less" means the temperature for the oxidation treatment is at least within the range of 400 to 900°C and is not greater than 900°C. Accordingly, under this condition, a portion of the oxidation treatment may be performed at a temperature less than 400°C (e.g., in some cases, the oxidation treatment may be performed in a process of heating from 300°C to 700°C).

[0031] Preferably, the oxidation treatment is performed for a treatment period within a range of 1 to 30 s. Specifically, it is preferable, from the standpoint of ensuring a sufficient amount of oxidation to improve coating properties, that the treatment period be 1 s or more. The treatment period is more preferably 2 s or more and even more preferably 3 s or more. On the other hand, it is preferable, from the standpoint of preventing excessive oxidation to inhibit pickup, that the treatment period be 30 s or less. The treatment period is more preferably 20 s or less and even more preferably 15 s or less.

[0032] The oxidation treatment may utilize a step of heating the steel sheet to a temperature at which the annealing is performed. For example, the oxidation of the surface of the steel sheet can be accomplished in an atmosphere-controllable soaking chamber during the step of heating the steel sheet, by holding the steel sheet therein at a given temperature in a predetermined atmosphere. Furthermore, the oxidation of the surface of the steel sheet can be accomplished in a direct-firing-type furnace equipped with a direct firing burner, by controlling the atmosphere in the furnace while increasing the temperature. Performing the heating and the oxidation treatment simultaneously enables a size reduction of a furnace while improving a production speed and, thus, provides industrial advantages. In the instance where the oxidation of the surface is performed while the steel sheet is heated, a sufficient amount of oxidation can be achieved by using the oxidizing atmosphere over a heating temperature span (a temperature range over which heating is performed) of 50°C or greater after the temperature of the steel sheet reaches 400°C. If the heating temperature span over which the steel sheet is exposed to the oxidizing atmosphere is less than 50°C, the amount of oxidation that results is insufficient, which leads to the formation of oxides of Si and Mn, and consequently, the effect of improving coating properties is decreased. In the instance where the oxidation of the surface is performed while the steel sheet is heated, the heating rate over the temperature range in which the oxidation is performed may be 3 to 25°C / s; this is preferable from the standpoint of ensuring an appropriate amount of oxidation.

[0033] The direct firing burner used to perform the oxidation treatment may be a burner for heating a steel sheet by applying a burner flame directly to the surface of the steel sheet; the burner flame is obtained by mixing a fuel, such as coke oven gas (COG), which is a by-product gas from a steelworks, with air and combusting the mixture. The heating with a direct firing burner provides a higher heating rate for the steel sheet than that of a radiant-type heating means and, therefore, has advantages such as being able to shorten the length of the furnace and being able to increase a line speed. In addition, regarding the direct firing burner, in instances where an air ratio of 0.95 or greater is used to increase the proportion of air with respect to the proportion of fuel, unburned oxygen remains in the flame, and, consequently, the oxidation of the steel sheet can be promoted by the oxygen. Accordingly, by adjusting the air ratio, the oxygen concentration in the atmosphere can be controlled. Fuels that can be used for the direct firing burner include COGs and further include liquefied natural gases (LNGs), ammonia gases, and hydrogen gases.First Step of Annealing

[0034] The first step of the annealing includes holding the steel sheet at a temperature of 650°C or greater and 950°C or less for a period of 20 s or more and 150 s or less in an atmosphere that has a dew point of -55°C or greater and +20°C or less and a hydrogen concentration of 5% or greater and 25% or less and in which oxidized Fe becomes reduced.

[0035] In the first step of the annealing, naturally oxidized Fe present in the surface layer of the steel sheet is reduced in a reducing atmosphere, to ensure coating properties. Since reduction does not substantially proceed in the subsequent second step, which uses an atmosphere having a low hydrogen concentration, it is necessary that the reduction of oxidized Fe be completed in this first step. This first step is essential for obtaining a good coating appearance.

[0036] In the instance where the oxidation treatment is performed, the oxidized Fe that is intentionally formed is reduced in a reducing atmosphere in this first step of reduction-annealing, to form a reduced iron layer in 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 therein, to achieve an aesthetically more pleasing appearance. Likewise, since reduction does not substantially proceed in the subsequent second step, which uses an atmosphere having a low hydrogen concentration, it is necessary that the reduction of oxidized Fe be completed in this first step.

[0037] If the annealing temperature of the steel sheet in the first step is less than 650°C, the reduction is not sufficiently carried out, which causes oxidized Fe to form roll pickup and to become a cause of defects of the steel sheet, and in addition, in the subsequent second step, the oxidized Fe is not substantially reduced and, therefore, becomes a cause of coating bare spots. On the other hand, if the annealing temperature of the steel sheet is greater than 950°C, the temperature significantly degrades the life of the furnace body. Accordingly, the annealing temperature of the steel sheet is specified to be 650°C or greater and 950°C or less. In instances where the base material is a cold rolled steel sheet, it is preferable, from the standpoint of ensuring a predetermined strength and ductility by recrystallizing the steel sheet, that the annealing temperature be 750°C or greater. Furthermore, producing a high strength steel sheet having a tensile strength of 780 MPa or greater requires ensuring that a predetermined amount of martensite, bainite, and retained γ (retained austenite) in terms of a total area fraction is present, and, accordingly, it is preferable that the annealing temperature be 780°C or greater. Using a high annealing temperature in the first step results in the promotion of selective oxidation of Si and Mn and an increase in the amount of hydrogen in steel; however, in the present invention, since the atmosphere and the holding time in the first step and the second step are controlled, an excellent surface quality and excellent delayed fracture resistance can be achieved.

[0038] Regarding the dew point of the atmosphere of the first step, the dew point of +20°C or less is sufficient to reduce oxidized Fe present in the surface layer of the steel sheet and to inhibit the selective oxidation of Si and Mn, provided that the annealing period is within the predetermined range. Using a dew point of less than -55°C requires special equipment for lowering the dew point and, therefore, increases cost. On the other hand, if the dew point is greater than +20°C, a dew point distribution in the furnace broadens, which makes it difficult to control the dew point and also raises a concern about an influence on the furnace body. Accordingly, the dew point is specified to be -55°C or greater and +20°C or less.

[0039] In the first step, the higher the hydrogen concentration, the earlier the reduction of oxidized Fe is completed, and the greater the degree to which the selective oxidation of Si and Mn is inhibited; however, the higher the hydrogen concentration, the more likely it is that hydrogen dissolves into the steel, and, consequently, delayed fracture resistance decreases. If the hydrogen concentration is less than 5%, the reduction is not sufficiently carried out. On the other hand, if the hydrogen concentration is greater than 25%, the effect of the reduction no longer increases, in addition, large amounts of hydrogen dissolve into the steel, and, in the subsequent second step, it becomes difficult to sufficiently decrease the amount of hydrogen present in steel. Accordingly, the hydrogen concentration for the first step is specified to be 5% or greater and 25% or less. In the instance where the oxidation treatment is performed, the hydrogen concentration is preferably 8% or greater so that the reduction can be sufficiently carried out. On the other hand, it is preferable, from the standpoint of running cost and decreasing the amount of hydrogen in steel, that the hydrogen concentration be 22% or less. More preferably, the hydrogen concentration is 18% or less.

[0040] Regarding the first step, if the holding time associated with the temperature of 650°C or greater and 950°C or less is less than 20 s, the reduction is not sufficiently completed. Furthermore, a sufficient area fraction of martensite and bainite, which is necessary to obtain a high strength steel having a tensile strength of 780 MPa or greater, cannot be ensured. On the other hand, since the reduction is sufficiently completed with a holding time of 150 s or less, if the holding time is greater than 150 s, productivity unnecessarily decreases. In addition, the selective oxidation of Si and Mn proceeds, which degrades the surface quality and coating adhesion. The amount of hydrogen present in the steel no longer increases after the holding time reaches approximately 20 s and is, therefore, not significantly affected by the holding time. Accordingly, the holding time associated with the temperature of 650°C or greater and 950°C or less in the first step is specified to be 20 s or more and 150 s or less.Second Step of Annealing

[0041] The second step of the annealing includes holding, after the steel sheet undergoes the first step, the steel sheet at a temperature of 700°C or greater and 950°C or less for a period of 30 s or more and 300 s or less in an atmosphere that has a dew point of -50°C or greater and +20°C or less and a hydrogen concentration of 0.2% or greater and less than 5.0%.

[0042] This second step is performed to release hydrogen from the steel sheet, the reduction of which has been completed in the first step, by holding the steel sheet in a low-hydrogen atmosphere.

[0043] If the annealing temperature of the steel sheet in the second step is less than 700°C, dehydrogenation is not promoted. On the other hand, if the annealing temperature is greater than 950°C, the temperature has a significant influence on the furnace body. Accordingly, the annealing temperature of the steel sheet is specified to be 700°C or greater and 950°C or less. It is preferable, from the standpoint of decreasing the amount of hydrogen present in the steel, that the annealing temperature of the second step be 860°C or less. More preferably, the annealing temperature is 830°C or less. Furthermore, producing a high strength steel sheet having a tensile strength of 780 MPa or greater requires ensuring that a predetermined amount of martensite, bainite, and retained γ in terms of a total area fraction are present, and, accordingly, it is preferable that the annealing temperature for the second step be 780°C or greater.

[0044] Regarding the second step, the lower the dew point, the less influence the dew point has on the furnace body. However, using a dew point of less than -50°C requires special equipment for controlling the dew point and, therefore, increases cost. On the other hand, if the dew point is greater than +20°C, the reduced Fe formed in the first step may be reoxidized and impair coating properties, and in addition, the control of the dew point is difficult, which raises a concern about an influence on the furnace body. Accordingly, the dew point is specified to be -50°C or greater and +20°C or less. From the standpoint of controllability, it is preferable that the dew point be +10°C or less. More preferably, the dew point is +5°C or less.

[0045] Regarding the second step, the lower the hydrogen concentration, the larger amounts of the hydrogen dissolved into the steel sheet in the first step is released; however, it is difficult to perform control to achieve a uniform hydrogen concentration of less than 0.2% in the furnace, and there is a concern that the steel sheet may be reoxidized at a portion having a low hydrogen concentration. Accordingly, the hydrogen concentration is specified to be 0.2% or greater. On the other hand, if the hydrogen concentration is 5.0% or greater, it is impossible to sufficiently decrease the amount of hydrogen present in steel, and, therefore, the hydrogen concentration is specified to be less than 5.0%. From this standpoint, it is preferable that the hydrogen concentration be 1.0% or greater. More preferably, the hydrogen concentration is 2.0% or greater. Also, the hydrogen concentration is more preferably 4.0% or less.

[0046] Regarding the second step, if the holding time associated with the temperature of 700°C or greater and 950°C or less is less than 30 s, the release of hydrogen is not sufficiently completed. On the other hand, since the release of hydrogen is sufficiently completed with a holding time of 300 s or less, if the holding time is greater than 300 s, productivity actually decreases. In addition, the selective oxidation of Si and Mn proceeds, which degrades the surface quality and coating adhesion. Accordingly, the holding time associated with the temperature of 700°C or greater and 950°C or less in the second step is specified to be 30 s or more and 300 s or less. From the standpoint of sufficiently releasing the hydrogen in the steel, it is preferable that the holding time associated with the temperature of 700°C or greater and 950°C or less in the second step be 50 s or more.

[0047] In the present invention, high-concentration hydrogen is required for reducing the oxidized Fe that naturally exists on the surface of the steel sheet or the oxidized Fe formed by the oxidation treatment in the first step of the annealing, and, therefore, large amounts of hydrogen dissolve into the steel. Accordingly, a balance between the reduction and the dehydrogenation is important. Because of this, it is necessary to optimize the conditions for the first step and the second step of the annealing, as described above.

[0048] The use of different hydrogen concentrations in the first step and the second step of the annealing may be accomplished, without limitation, as follows: the furnace to be used is composed of divided sections that are connected to each other via seal rolls, and the hydrogen concentration and the dew point of the gases that are introduced to the respective divided sections are controlled; in this manner, the atmospheres for the first step and the second step can be separately controlled with ease. In the present invention, it is preferable that the annealing of the steel sheet be carried out in a continuous annealing furnace configured to control two or more different atmospheres that are separated from each other.Cooling After Annealing

[0049] Preferably, after the steel sheet has completed the annealing (second step), the steel sheet is cooled at an average cooling rate of 5°C / s or greater over a temperature range of a final holding temperature of the annealing to 600°C and then further cooled to a temperature of 150°C or greater and less than 600°C, in an atmosphere having a dew point of -20°C or less and a hydrogen concentration of 5% or greater and 25% or less. Subsequently, the steel sheet is optionally heated, before the steel sheet is immersed in a hot-dip galvanizing bath to be hot-dip galvanized.

[0050] The cooling at an average cooling rate of 5°C / s or greater over a temperature range of a final holding temperature after the annealing to 600°C makes it possible to achieve a desired strength of the steel sheet and to inhibit hydrogen present in the atmosphere from entering the steel sheet during the cooling. If the average cooling rate is less than 5°C / s, the strength of the steel sheet is likely to decrease, and hydrogen present in the atmosphere is likely to enter the steel sheet and cause a decrease in delayed fracture resistance. Regarding the second step of the annealing, the final holding temperature is a temperature of the steel sheet at the time the steel sheet, which has undergone the annealing within a range that satisfies the requirements of the second step of the annealing, fails to satisfy at least one of the requirements, where the requirements are the annealing temperature, hydrogen concentration, dew point, and holding time.

[0051] The average cooling rate (°C / s) can be determined by dividing the difference between a cooling start temperature (the final holding temperature) (°C) and a cooling end temperature (600°C) by the cooling period (s).

[0052] Regarding the atmosphere in the cooling zone, since hydrogen has a high cooling ability, the higher the hydrogen concentration in the atmosphere, the greater the degree to which the cooling rate can be increased; however, if the hydrogen concentration is excessively high, hydrogen may enter the steel sheet during the cooling, and, therefore, it is preferable that the hydrogen concentration be 5% or greater and 25% or less. If the hydrogen concentration is less than 5%, it may be impossible to ensure a sufficient cooling rate; consequently, the strength of the steel sheet is likely to decrease, and in addition, a reduced cooling rate makes it likely that hydrogen enters the steel sheet during the cooling and that, therefore, delayed fracture resistance decreases. On the other hand, if the hydrogen concentration is greater than 25%, the effect no longer increases, and, even with a high cooling rate, hydrogen is likely to enter the steel sheet during the cooling, which makes it likely that delayed fracture resistance decreases.

[0053] Furthermore, the use of a dew point of -20°C or less makes it possible to inhibit degradation in coating properties due to reoxidation of the steel sheet that may occur at a low temperature. That is, if the dew point is greater than -20°C, the steel sheet is likely to be reoxidized at a low temperature, and, therefore, coating properties are likely to be degraded.Hot-Dip Galvanizing

[0054] The hot-dip galvanizing may be carried out under any conditions that are typically used. Specifically, the steel sheet is preferably cooled to a temperature of 150°C or greater and less than 600°C under the conditions described above, subsequently, if necessary, the steel sheet is heated to a temperature similar to the temperature of a galvanizing bath, and thereafter, the steel sheet is galvanized by being immersed in the hot-dip galvanizing bath. Typically, in the case of GA and GI, the galvanizing bath is composed of Zn, Al, and incidental impurities; the contents are not particularly specified, and, in general, an Al concentration in the bath is approximately 0.05% or greater and 0.190% or less. If the Al concentration in the bath is less than 0.05%, generation of bottom dross increases, which is likely to cause adhesion of dross to the steel sheet and, thus, cause a defect. On the other hand, if the Al concentration in the bath is greater than 0.190%, top dross increases, which is also likely to cause adhesion of dross to the steel sheet and, thus, cause a defect, and furthermore, the addition of Al increases cost. The temperature of the hot-dip galvanizing bath is a typical temperature, which is approximately 440 to 500°C.

[0055] Regarding the hot-dip galvanizing, a coating weight per side is typically, without limitation, controlled to be a coating weight of approximately 25 to 80 g / m 2< . If the coating weight per side is less than 25 g / m 2< , corrosion resistance is likely to decrease, and in addition, the control of the coating weight is not easy. On the other hand, if the coating weight per side is greater than 80 g / m 2< , coating adhesion is likely to decrease. The adjustment of the coating weight may be carried out by any method, typically by gas wiping. Specifically, the adjustment is made by using a gas pressure, a distance between a wiping nozzle and the steel sheet, and the like of gas wiping.

[0056] In the instance where an alloying treatment is performed after the hot-dip galvanizing, a degree of alloying in the coated layer after the alloying treatment is not particularly limited. Typically, the degree of alloying is preferably approximately 7 to 15%. If the degree of alloying is less than 7%, an η phase is likely to remain, which likely decreases press formability. On the other hand, if the degree of alloying is greater than 15%, coating adhesion is likely to decrease.

[0057] Now, the base steel sheet of the hot-dip galvanized steel sheet will be described.

[0058] The base steel sheet may be a cold rolled steel sheet or a hot rolled steel sheet. Furthermore, since delayed fracture resistance is a property of concern in the case of high strength steel sheets, the steel sheet is preferably a high strength steel sheet with a tensile strength TS of 590 MPa or greater, preferably 780 MPa or greater, and more preferably 980 MPa or greater.

[0059] The components of the base steel sheet are not particularly limited, and it is sufficient that they be within a compositional range of a typical cold rolled steel sheet or hot rolled steel sheet. Preferably, the base steel sheet has the following chemical composition.

[0060] The steel sheet may have any thickness and typically has a thickness of approximately 0.5 to 3.2 mm.

[0061] Now, a preferred chemical composition of the base steel sheet will be described.C: 0.8% or less (excluding 0%)

[0062] C contributes to the formation of steel microstructures such as martensite and, therefore, has an effect of improving formability. It is preferable, however, that a C content be 0.8% or less so that good weldability can be achieved. More preferably, the C content is 0.3% or less. While the lower limit of the C content is not particularly specified, it is preferable, in terms of achieving good formability, that the C content be 0.03% or greater. More preferably, the C content is 0.05% or greater.Si: 3.0% or less (excluding 0%)

[0063] Si has a large effect in increasing the strength of steel by dissolving into the steel (solid solution strengthening ability) while avoiding significantly compromising formability. Accordingly, Si is an element effective for achieving an increased strength of the steel sheet. On the other hand, Si is an element that has a negative influence on the resistance to resistance-welding cracking in a weld. In instances where Si is contained to achieve an increased strength of the steel sheet, it is preferable that the Si be contained in an amount of 0.1% or greater. On the other hand, if the Si content is greater than 3.0%, hot rollability and cold rollability significantly decrease, which may adversely affect productivity and cause a decrease in the ductility of the steel sheet itself. Accordingly, it is preferable that Si be contained within a range of 3.0% or less. For a similar reason, the Si content is more preferably 2.5% or less and particularly preferably 2.0% or less.Mn: 1.3% or greater and 3.5% or less

[0064] Mn is an element that has effects of strengthening steel through solid solution strengthening, thereby achieving an increased strength of the steel, and of increasing hardenability, thereby promoting the formation of retained γ, bainite, and martensite. These effects are produced when Mn is contained in an amount of 1.3% or greater. Accordingly, the Mn content is preferably 1.3% or greater and more preferably 1.8% or greater. On the other hand, when the Mn content is 3.5% or less, the above-described effects can be produced without causing an increase in cost. Accordingly, the Mn content is preferably 3.5% or less and more preferably 3.3% or less.P: 0.1% or less (excluding 0%)

[0065] Limiting a P content can prevent a decrease in weldability and, in addition, can prevent segregation of P at grain boundaries, thereby preventing degradation in ductility, bendability, and toughness. If P is contained in large amounts, ferrite transformation is promoted, which results in an increased grain size. Accordingly, the P content is preferably 0.1% or less. The lower limit of the P content is not particularly limited, and typically, because of restrictions on the production technology, it is preferable that the lower limit be 0.001% or greater.S: 0.03% or less (excluding 0%)

[0066] Preferably, a S content is reduced as much as possible. Limiting the S content can prevent a decrease in weldability and can also prevent a decrease in ductility that may occur during hot rolling; consequently, hot cracking can be inhibited, and surface properties can be significantly improved. Furthermore, limiting the S content can prevent a decrease in the delayed fracture resistance, ductility, bendability, and stretch flangeability of the steel sheet that may be caused if S, which is an impurity element, forms a coarse sulfide. Since problems associated with S become prominent if the S content is greater than 0.03%, the S content is preferably 0.03% or less and more preferably 0.02% or less. From the standpoint of improving delayed fracture resistance, it is preferable that the S content be 0.01% or less. More preferably, the S content is 0.003% or less. The lower limit of the S content is not particularly limited, and typically, because of restrictions on the production technology, it is preferable that the lower limit be 0.0001% or greater.N: 0.010% or less (excluding 0%)

[0067] When the N content is 0.010% or less, it is possible to prevent N from forming coarse nitrides with Ti, Nb, and / or V at a high temperature and, therefore, prevent impairment of an effect of increasing the strength of the steel sheet due to the addition of Ti, Nb, and / or V. Furthermore, when the N content is 0.010% or less, it is also possible to prevent a decrease in toughness. In addition, when the N content is 0.010% or less, it is possible to prevent slab cracking and surface defects from occurring during hot rolling. Accordingly, the N content is preferably 0.010% or less, more preferably 0.005% or less, even more preferably 0.003% or less, and particularly preferably 0.002% or less. The lower limit of the N content is not particularly limited, and typically, because of restrictions on the production technology, it is preferable that the lower limit be 0.0005% or greater.Al: 0.1% or less

[0068] Since Al is thermodynamically most likely to be oxidized, Al is oxidized before Si and Mn are oxidized. Accordingly, Al has an effect of inhibiting Si and Mn from being oxidized in the outermost layer of the steel sheet, thereby promoting the oxidation of Si and Mn in an inner portion of the steel sheet. This effect can be produced when an Al content is 0.01% or greater. On the other hand, if the Al content is greater than 0.1%, cost increases. Accordingly, in instances where Al is contained, it is preferable that the Al content be 0.1% or less. The lower limit of the Al content is not particularly limited; however, removing Al present in a level similar to that of impurities also increases cost, and, therefore, it is preferable that the lower limit be 0.001% or greater. Preferably, the Al content is 0.01% or greater, as mentioned above.

[0069] If necessary, the steel sheet may further contain one or more selected from B: 0.005% or less, Ti: 0.2% or less, Cr: 1.0% or less, Nb:0.20% or less, Mo: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Sb: 0.20% or less, V: 0.5% or less, Ta: 0.1% or less, W: 0.5% or less, Zr: 0.1% or less, Sn: 0.20% or less, Ca: 0.005% or less, Mg: 0.005% or less, and REMs: 0.005% or less.B: 0.005% or less

[0070] B is an element effective for improving the hardenability of steel. It is preferable, in terms of improving hardenability, that a B content be 0.0003% or greater. More preferably, the B content is 0.0005% or greater. However, when an excessive amount of B is contained, formability is reduced, and, therefore, it is preferable that the B content be 0.005% or less.Ti: 0.2% or less

[0071] Ti is an element effective for the precipitation strengthening of steel. The lower limit of a Ti content is not particularly limited, and it is preferable, in terms of producing an effect of adjusting strength, that Ti be contained in an amount of 0.005% or greater. If an excessive amount of Ti is added, however, an excessive amount of a hard phase is formed, which reduces formability. Accordingly, in instances where Ti is contained, it is preferable that the Ti content be 0.2% or less. More preferably, the Ti content is 0.05% or less.Cr: 1.0% or less

[0072] When Cr is contained in an amount of 0.005% or greater, hardenability is improved, which can improve a balance between strength and ductility; however, from the standpoint of preventing an increase in cost, it is preferable that the Cr content be 1.0% or less.Nb: 0.20% or less

[0073] When Nb is contained in an amount of 0.005% or greater, an effect of improving strength can be produced; however, from the standpoint of preventing an increase in cost, it is preferable that the Nb content be 0.20% or less.Mo: 1.0% or less

[0074] When Mo is contained in an amount of 0.005% or greater, an effect of adjusting strength can be produced, and this effect particularly increases when the Mo content is 0.05% or greater; however, from the standpoint of preventing an increase in cost, it is preferable that the Mo content be 1.0% or less.Cu: 1.0% or less

[0075] When Cu is contained in an amount of 0.005% or greater, the formation of a retained γ phase can be promoted; however, from the standpoint of preventing an increase in cost, it is preferable that the Cu content be 1.0% or less in instances where Cu is contained.Ni: 1.0% or less

[0076] When Ni is contained in an amount of 0.005% or greater, the formation of a retained γ phase can be promoted; however, from the standpoint of preventing an increase in cost, it is preferable that the Ni content be 1.0% or less in instances where Ni is contained.Sb: 0.20% or less

[0077] Sb may be contained to inhibit nitriding and oxidation of a surface of the steel sheet and decarburization in a region extending several tens of microns from the surface of the steel sheet caused by oxidation. By inhibiting the nitriding and oxidation of the surface of the steel sheet, Sb prevents a decrease in the amount of martensite formed on the surface of the steel sheet, thereby improving the fatigue properties and surface quality of the steel sheet. It is preferable, in terms of producing these effects, that an Sb content be 0.001% or greater. On the other hand, in terms of achieving good toughness, it is preferable that the Sb content be 0.20% or less.V: 0.5% or less

[0078] When V is contained in an amount of 0.005% or greater, an effect of improving strength can be produced; however, from the standpoint of preventing an increase in cost, it is preferable that the V content be 0.5% or less in instances where V is contained.Ta: 0.1% or less

[0079] When Ta is contained in an amount of 0.001% or greater, an effect of improving strength can be produced; however, from the standpoint of preventing an increase in cost, it is preferable that the Ta content be 0.1% or less in instances where Ta is contained.W: 0.5% or less

[0080] When W is contained in an amount of 0.005% or greater, an effect of improving strength can be produced; however, from the standpoint of preventing an increase in cost, it is preferable that the W content be 0.5% or less in instances where W is contained.Zr: 0.1% or less

[0081] When Zr is contained in an amount of 0.0005% or greater, an effect of improving strength can be produced; however, from the standpoint of preventing an increase in cost, it is preferable that the Zr content be 0.1% or less in instances where Zr is contained.Sn: 0.20% or less

[0082] Sn inhibits denitrification, deboronation, and the like and is, therefore, an element effective for inhibiting a decrease in the strength of steel. It is preferable, in terms of producing this effect, that Sn be contained in an amount of 0.002% or greater. On the other hand, in terms of achieving good impact resistance, it is preferable that the Sn content be 0.20% or less in instances where Sn is contained.Ca: 0.005% or less

[0083] In cases where Ca is contained in an amount of 0.0005% or greater, a morphology of sulfides can be controlled to improve ductility and toughness; however, from the standpoint of achieving good ductility, it is preferable that the Ca content be 0.005% or less in instances where Ca is contained.Mg: 0.005% or less

[0084] In cases where Mg is contained in an amount of 0.0005% or greater, a morphology of sulfides can be controlled to improve ductility and toughness; however, from the standpoint of preventing an increase in cost, it is preferable that the Mg content be 0.005% or less in instances where Mg is contained.REMs: 0.005% or less

[0085] In cases where one or more REMs are contained in an amount of 0.0005% or greater, a morphology of sulfides can be controlled to improve ductility and toughness; however, from the standpoint of achieving good toughness, it is preferable that the REM content be 0.005% or less in instances where one or more REMs are contained.

[0086] In the steel sheet, the balance, other than the chemical composition described above, is Fe and incidental impurities.

[0087] The base steel sheet (substrate steel sheet) may have any microstructures. In terms of ensuring a tensile strength of 780 MPa or greater, it is preferable that the base steel sheet have the following microstructures.

[0088] Specifically, the base steel sheet preferably includes martensite, bainite, and retained γ (retained austenite) in a total area fraction of 30% or greater. In this case, the base steel sheet can have a tensile strength of 780 MPa or greater. When the total area fraction of martensite, bainite, and retained γ is 50% or greater, the base steel sheet can have a tensile strength of 980 MPa or greater.

[0089] The hot-dip galvanized steel sheet manufactured in accordance with the present invention has a low hydrogen concentration in the substrate steel sheet and, therefore, has excellent delayed fracture resistance. In particular, the hydrogen concentration (amount of diffusible hydrogen) in the substrate steel sheet is preferably 0.30 mass-ppm or less and particularly preferably 0.25 mass-ppm or less. The amount of diffusible hydrogen is an amount of hydrogen in a steel sheet measured by a method described below in the Examples section.EXAMPLES

[0090] Slabs were produced from steels each having the chemical composition shown in Table 1 and were subjected to hot rolling. Subsequently, the resulting steel sheets were subjected to pickling and cold rolling to form cold rolled steel sheets having a thickness of 1.2 mm. These cold rolled steel sheets were used as base steel sheets of hot-dip galvanized steel sheets.

[0091] In a CGL including an all radiant tube (ART) annealing furnace, the steel sheets were annealed under the conditions shown in Tables 2 and 3; subsequently, the steel sheets were subjected to hot-dip galvanizing (composition of the coating: Zn-0.2 mass% Al), in which the coating weight per side was adjusted to be approximately 50 g / m 2< by gas wiping; and subsequently, in some Examples, an alloying treatment was performed.

[0092] In addition to the Examples described above, other Examples were carried out. Specifically, in a CGL including a DFF annealing furnace, steel sheets were subjected to an oxidation treatment and annealing under the conditions shown in Tables 4 to 9; subsequently, the steel sheets were subjected to hot-dip galvanizing (composition of the coating: Zn-0.2 mass% Al), in which the coating weight per side was adjusted to be approximately 50 g / m 2< by gas wiping; and subsequently, in some Examples, an alloying treatment was performed. No. 60 (Tables 4 and 5) is an Example in which the oxidation treatment was performed at a constant temperature and in which a holding time (treatment time) for the oxidation treatment was 8 s. In the other Examples, the oxidation treatment was carried out during the heating, and the heating rate for the oxidation treatment was within a range of 5 to 20°C / s.

[0093] The hot-dip galvanized steel sheets, obtained as described above, were measured for the amount of diffusible hydrogen in the steel sheet and evaluated for coating appearance and delayed fracture resistance, with a measurement method and evaluation methods described below. The results and the manufacturing conditions are shown in Tables 2 to 9.

[0094] Regarding the oxidation treatment performed in the Examples shown in Tables 4 to 9, "Oxidation Start Temperature" is the temperature of the steel sheet at an entry side of an oxidation zone of a heating zone in the DFF annealing furnace, "Oxidation End Temperature" is the temperature of the steel sheet at an exit side of the oxidation zone, and the oxygen concentration is an oxygen concentration of the oxidation zone; accordingly the range of the oxidation start temperature to the oxidation end temperature is the oxidation treatment temperature. In addition, "Oxidation Temperature Range" is a temperature range over which the steel sheet is heated in the oxidation zone (temperature range from the oxidation start temperature to the oxidation end temperature), and "Reached Maximum Temperature of Steel Sheet" is a reached maximum temperature in the heating zone of the DFF annealing furnace. Accordingly, if the "Reached Maximum Temperature of Steel Sheet" was greater than the "Oxidation End Temperature", it indicates that the steel sheet was further heated in a non-oxidizing atmosphere in a zone next to the oxidation zone (a zone that is not an oxidation zone).Measurement of Amount of Diffusible Hydrogen in Steel Sheet (method for hydrogen analysis)

[0095] A strip-shaped test specimen with a long axis length of 30 mm and a short axis length of 5 mm was cut from a widthwise middle portion of each of the hot-dip galvanized steel sheets, and the coated layer of the test specimen was removed with a Leutor. Immediately thereafter, hydrogen analysis was performed with a thermal desorption spectrometer under the conditions of an analysis start temperature of 25°C, an analysis end temperature of 300°C, and a heating rate of 200°C / hour. A hydrogen release amount (mass-ppm / min), which is an amount of hydrogen released from a surface of the test specimen, was measured at various temperatures. An amount of diffusible hydrogen in steel was calculated as the sum of the hydrogen release amounts over the range of the analysis start temperature to 300°C. When the amount of diffusible hydrogen in steel was 0.25 mass-ppm or less, a rating of "⊚" (excellent) was given, and when the amount was greater than 0.25 mass-ppm and 0.30 mass-ppm or less, a rating of "o" (good) was given. It is experimentally known that when the amount of diffusible hydrogen in steel is greater than 0.30 mass-ppm, delayed fracture resistance decreases in many cases. Accordingly, when the amount was greater than 0.30 mass-ppm, a rating of "×" (poor) was given.Evaluation of Coating Appearance

[0096] The coating appearance of each of the hot-dip galvanized steel sheets was visually observed. In instances where no patterns or irregularities were observed, a rating of "⊚" (excellent) was given; in instances where no coating bare spot defects or indentation flaws due to roll pickup were observed although patterns or irregularities were observed, a rating of "o+" (good) was given; and in instances where coating bare spot defects or indentation flaws due to roll pickup were observed, a rating of "×" (poor) was given. In addition, in instances where no coating bare spot defects or indentation flaws due to roll pickup were observed, but scale patterns in a V shape with respect to a steel sheet passing direction were formed as a sign of the mentioned defects, a rating of "o" (pass) was given while the rating of "o+" (good) was not given.Tensile Test

[0097] A test specimen was cut from each of the hot-dip galvanized steel sheets in a direction perpendicular to a rolling direction of the steel sheet (such that a sheet width direction could become a tensile direction). A tensile test in accordance with JIS Z 2241 (2011) was conducted on the test specimen to measure the tensile strength (TS).Observation and Measurement of Microstructures of Base Steel Sheet

[0098] The total area fraction of martensite, bainite, and retained γ in the microstructures of the base steel sheet was measured in the following manner. A sample was cut such that a thickness cross section thereof parallel to the rolling direction (L-cross section) of the steel sheet could serve as an observation surface. The observation surface of the sample was polished with a diamond paste and subsequently finish-polished with alumina. Next, the observation surface of the sample was etched with 3 vol.% nital to reveal the microstructures. In the observation surface of the sample, a 1 / 4 thickness position was used as an observation position and was observed in five fields of view with an SEM at a magnification of 3000×. The total area of martensite, bainite, and retained γ was determined from the obtained images of microstructures. An area fraction, which was the result of dividing the total area by the area that was measured, was calculated for each of the five fields of view, and an average of these values was determined to be used as the total area fraction of martensite, bainite, and retained γ. The martensite, the bainite, and the retained γ, and the other microstructures were distinguished from one another in the following manner.Martensite

[0099] There are two types of martensite: tempered martensite and fresh martensite.Tempered Martensite

[0100] Tempered martensite is a region that is gray or nearly black dark gray in SEM images. Tempered martensite has a blocky morphology with its boundaries being prior γ grain boundaries or interfaces with other microstructures, such as ferrite. Note that, in some cases, tempered martensite may contain a different microstructure, such as bainite, in its interior, and, therefore, have a recessed shape. Tempered martensite contains large amounts of carbides in its interior; in some cases, however, the amount of carbides may be small, depending on a plane orientation.Fresh Martensite

[0101] Fresh martensite is a region that is gray or white in SEM images. Fresh martensite has a blocky, particulate, plate-shaped, or film-shaped morphology and does not contain carbides.Bainite

[0102] Bainite is a region that is dark gray in SEM images. Bainite has a film-shaped or plate-shaped morphology or a blocky morphology in which portions or the whole of neighboring regions of these are joined together. Bainite contains a slight amount of carbides. Bainite may be one that has been tempered after its formation and thus in which coarsened carbides are present.Retained γ

[0103] Retained γ is a region that has the same color and morphology as fresh martensite, described above. Note that retained γ cannot be distinguished from fresh martensite with an SEM.

[0104] While it is necessary to control the total area fraction of martensite, bainite, and retained γ to ensure a strength of the steel sheet, it is possible to include, as the remainder, the microstructures described below, without limitation.Ferrite

[0105] Ferrite is a region that is black in SEM images. Ferrite has a blocky morphology and contains few or no carbides. Bainitic ferrite contains few or no carbides in its interior and has mechanical properties similar to those of ferrite. Accordingly, bainitic ferrite is classified as ferrite. In some cases, ferrite contains, in its interior, one or both of particulate or blocky fresh martensite and particulate or blocky retained γ.Carbides

[0106] Carbides are regions that are white in SEM images. Carbides have a particulate or film-shaped morphology. Carbides are finely formed mainly in the interiors of ferrite, martensite, and bainite. Accordingly, the area fraction of carbides is not excluded from the area fractions of the corresponding microstructures but is included in the area fractions of the corresponding respective microstructures.Microstructures Other Than Foregoing

[0107] Microstructures other than the foregoing may also be present in a total area fraction of approximately several percent. Examples thereof include nitrides, such as TiN, carbonitrides, such as (Nb,Ti) (C,N), sulfides, such as MnS and CaS, and oxides, such as Al 2 O 3 and SiO 2 . Since the area fractions of these microstructures are small, their area fractions are included in the area fractions of the corresponding respective microstructures. Furthermore, pearlite may be present. The area fraction of pearlite is to be calculated independently.

[0108] A size and an abundance of the various microstructures are not particularly limited. In an embodiment of the present invention, for example, the size and the abundance may be as described below. Note that an aspect ratio is a ratio of a length of a long axis to a length of a short axis, which is an axis perpendicular to the long axis; a thickness is the length of the short axis; and an equivalent circular diameter is a diameter of a circle having an area equivalent to the area of a corresponding microstructure.Tempered Martensite

[0109] aspect ratio: ≤ 8, equivalent circular diameter: ≤ 30 µm, distribution density of carbides in microstructure: 0.10 to 12 pieces / µm 2< Fresh Martensite and Retained γ Blocky: aspect ratio: ≤ 8, equivalent circular diameter: 3 to 30 µm Particulate: aspect ratio: ≤ 8, equivalent circular diameter: 0.40 µm or greater and less than 3 µm Plate-shaped or film-shaped: aspect ratio: greater than 8, thickness: 0.10 to 8 µm Bainite

[0110] Film-shaped or plate-shaped: aspect ratio: greater than 8, thickness: ≤ 8 µm Blocky: aspect ratio: ≤ 8, equivalent circular diameter: ≤ 30 µm, distribution density of carbides in microstructure: 0.10 to 6 pieces / µm 2< for all morphologies Carbides

[0111] Particulate: aspect ratio: ≤ 8, equivalent circular diameter: 0.01 µm or greater and less than 0.40 µm Film-shaped: aspect ratio: greater than 8, equivalent circular diameter: 0.01 µm or greater and less than 0.10 µm Evaluation of Delayed Fracture Resistance

[0112] A strip-shaped test specimen with a long axis length of 100 mm and a short axis length of 20 mm was cut from each of the hot-dip galvanized steel sheets in a direction perpendicular to the rolling direction thereof. A punched hole having a diameter of 15 mm with a clearance of 12.5% was formed in a middle location of the test specimen with respect to the long axis and the short axis. The test specimen was subjected to a tensile test, and delayed fracture resistance was evaluated by determining whether a delayed fracture was initiated from the punched hole. The time from when the strip-shaped test specimen was cut from the hot-dip galvanized steel sheet to the time when the tensile test (crosshead speed: 10 mm / min) for delayed fracture was started was limited to 10 minutes or less so that release of diffusible hydrogen present in the steel due to changes over time could be prevented. A loading time for the tensile test was up to 100 hours. After the 100-hour loading, delayed fracture resistance was evaluated by using a ratio between a critical stress and a yield stress, where the critical stress was the maximum stress withstood before a fracture occurred (the "fracture" refers to a breakage due to the loading of a tensile stress). The evaluation criteria for the delayed fracture resistance were as follows. When the ratio between the critical stress and the yield stress was 1.10 or greater, a rating of "⊚" (excellent) was given, when the ratio was less than 1.10 and 1.05 or greater, a rating of "∘" (good) was given, when the ratio was less than 1.05 and 1.00 or greater, a rating of "△" (pass) was given while the rating of "∘" (good) was not given, and when the ratio was less than 1.00, a rating of "×" (poor) was given. Note that delayed fracture resistance evaluated by a delayed fracture test is typically lower (disadvantageous) in steel sheets having high strength.

[0113] As indicated by Tables 2 to 9, the hot-dip galvanized steel sheets of Invention Examples had an aesthetically pleasing surface appearance free from coating bare spots and had excellent delayed fracture resistance. [Table 4]No.Steel TypeOxidation Treatment (optionally including additional heating in non-oxidizing atmosphere performed after oxidation treatment)AnnealingCooling StepAlloying TreatmentClassificationFirst StepSecond StepOxygen ConcentrationOxidation Start TemperatureOxidation End TemperatureOxidation Temperature RangeReached Maximum Temperature of Steel SheetAnnealing TemperatureHeat Retention TimeDew PointHydrogen ConcentrationAnnealing TemperatureHolding TimeDew PointHydrogen ConcentrationCooling RateDew PointHydrogen Concentration(vol-ppm)(°C)(°C)(°C)(°C)(°C)(s)(°C)(vol.%)(°C)(s)(°C)(vol.%)(°C / s) *1(°C)(vol.%)58F300040268228068279846-121180466-132.212-249YesInvention Example59F30004044413768380047-141480065-152.513-2510YesInvention Example60F3000601601068179945-161580166-142.312-2410YesInvention Example61F300049160511467879545-151279864-162.511-2512YesInvention Example62F300050268017868079844-131279867-152.512-2610YesInvention Example63F30006296805168080346-151380067-152.314-2411YesInvention Example64F9006036827968279547-131280065-142.512-2510YesInvention Example65F10006016797867980045-151178765-142.412-269YesInvention Example66F15006026787667880345-161280464-152.212-2712YesInvention Example67F50006056807568079847-151479864-122.311-2410YesInvention Example68F100006046837968380546-121379865-142.413-2512YesInvention Example69F30003023959339580144-121380165-152.412-259YesInvention Example70F30004024126046279545-131280167-152.214-2710YesInvention Example71F30004515318053180347-161180067-132.512-2611YesInvention Example72F30006037029974679846-121279865-152.313-2411YesInvention Example73F300070588812988880144-121280064-142.512-259YesInvention Example74F300070391020791094044-151180264-162.313-2712YesInvention Example75F30006016787767864345-151378765-152.214-2712YesComparative Example76F30006026838168365544-151280167-132.314-2510YesInvention Example77F30006006808069089545-151480166-152.513-2510YesInvention Example78F30005986858768580518-141280065-152.512-2612YesComparative Example79F30006056807568080122-121280365-122.513-2711YesInvention Example80F30006036787567879545-151380067-152.413-2611YesInvention Example81F300060168079680803120-161280165-142.512-2512YesInvention Example82F300060068080680804150-151380464-152.313-2510YesInvention Example83F30006046837968380544-551380065-132.512-2610YesInvention Example*1 Average cooling rate over a temperature range of a final holding temperature of annealing to 600°C [Table 5] No.TS (MPa)Total Area Fraction of Martensite, Bainite, and Retained γ (%)Coating AppearanceAmount of Diffusible Hydrogen in Steel SheetDelayed Fracture ResistanceClassification(mass-ppm)Evaluation58131682⊚0.15⊚⊚Invention Example59130080○+0.14⊚⊚Invention Example60130481○+0.13⊚⊚Invention Example61129279⊚0.12⊚⊚Invention Example62129279⊚0.16⊚⊚Invention Example63131282⊚0.15⊚⊚Invention Example64130080○+0.14⊚⊚Invention Example65130080⊚0.13⊚⊚Invention Example66131682⊚0.18⊚⊚Invention Example67129279⊚0.19⊚⊚Invention Example68132083⊚0.12⊚○Invention Example69130481○+0.11⊚⊚Invention Example70130481⊚0.15⊚⊚Invention Example71131282⊚0.14⊚⊚Invention Example72129279⊚0.15⊚⊚Invention Example73130481⊚0.18⊚⊚Invention Example741440100○+0.15⊚○Invention Example75124872×0.08⊚⊚Comparative Example76130481⊚0.11⊚⊚Invention Example771470100⊚0.15⊚○Invention Example78132083×0.14⊚○Comparative Example79131282⊚0.13⊚⊚Invention Example80130080⊚0.16⊚⊚Invention Example81131282⊚0.20⊚⊚Invention Example82131682⊚0.22⊚⊚Invention Example83132083⊚0.15⊚○Invention Example [Table 6] No.Steel TypeOxidation Treatment (optionally including additional heating in non-oxidizing atmosphere performed after oxidation treatment)AnnealingCooling StepAlloying TreatmentClassificationFirst StepSecond StepOxygen ConcentrationOxidation Start TemperatureOxidation End TemperatureOxidation Temperature RangeReached Maximum Temperature of Steel SheetAnnealing TemperatureHeat Retention TimeDew PointHydrogen ConcentrationAnnealing TemperatureHolding TimeDew PointHydrogen ConcentrationCooling RateDew PointHydrogen Concentration(vol-ppm)(°C)(°C)(°C)(°C)(°C)(s)(°C)(vol.%)(°C)(s)(°C)(vol.%)(°C / s) *1(°C)(vol.%)84F30005976828568280145-301280364-132.411-279YesInvention Example85F3000599680816808004511280065-152.312-2510YesInvention Example86F30006026858368580544201380064-142.213-2612YesInvention Example87F30006006808068080344231278765-162.512-2511YesComparative Example88F30006016818068180044-15480065-142.413-2511YesComparative Example89F30006036787567880146-15680467-152.512-2412YesInvention Example90F30006046827868279545-14880366-152.514-2510YesInvention Example91F30006026838168380045-152480365-122.312-2512YesInvention Example92F30006086807268080044-122680064-152.413-2610YesComparative Example93F30006026838168379844-151260341-152.414-279YesComparative Example94F30006006818168180346-151271565-142.512-2511YesInvention Example95F30005986808268080045-151380766-152.213-2610YesInvention Example96F30005996828368582045-131289566-152.512-2511YesInvention Example97F30006006808068080147-121480528-152.312-2510YesComparative Example98F30006026787667880547-151478733-142.412-2410YesInvention Example99F30006036838068380045-1211805120-152.513-269YesInvention Example100F30006056858068579546-1512795295-152.412-2510YesInvention Example101F30006036807768080044-121280065-502.513-2512YesInvention Example102F30006026838168380047-151378764-352.514-2611YesInvention Example103F30006006818168179845-12128056522.313-2710YesInvention Example104F30005976788167880544-151279564192.412-2711YesInvention Example105F30006056827768280047-151280067212.512-2510YesComparative Example106F30006046807668080345-161480365-150.113-2510YesComparative Example107F30006026838168380544-151480364-130.212-2411YesInvention Example108F30006006808068080147-151180465-122.211-269YesInvention Example109F30006036817868179546-151280567-154.812-2412YesInvention Example *1 Average cooling rate over a temperature range of a final holding temperature of annealing to 600°C [Table 7] No.TS (MPa)Total Area Fraction of Martensite, Bainite, and Retained γ (%)Coating AppearanceAmount of Diffusible Hydrogen in Steel SheetDelayed Fracture ResistanceClassification(mass-ppm)Evaluation84131282⊚0.14⊚⊚Invention Example85130080⊚0.13⊚⊚Invention Example86132083⊚0.13⊚○Invention Example87131282×0.14⊚⊚Comparative Example88130080×0.05⊚⊚Comparative Example89131682○+0.07⊚⊚Invention Example90131282⊚0.10⊚⊚Invention Example91131282⊚0.28○△Invention Example92130080⊚0.33××Comparative Example93129279⊚0.34××Comparative Example94131282⊚0.23⊚⊚Invention Example95132884⊚0.13⊚○Invention Example961450100⊚0.30○△Invention Example97132083⊚0.31××Comparative Example98132083⊚0.24⊚○Invention Example99132083⊚0.14⊚○Invention Example100128077⊚0.08⊚⊚Invention Example101130080⊚0.13⊚⊚Invention Example102130080⊚0.15⊚⊚Invention Example103132083⊚0.10⊚⊚Invention Example104132083⊚0.08⊚⊚Invention Example105130080×0.09⊚⊚Comparative Example106131282×0.05⊚⊚Comparative Example107132083○+0.08⊚⊚Invention Example108131682⊚0.13⊚⊚Invention Example109132083⊚0.26○△Invention Example [Table 8] No.Steel TypeOxidation Treatment (optionally including additional heating in non-oxidizing atmosphere performed after oxidation treatment)AnnealingCooling StepAlloying TreatmentClassificationFirst StepSecond StepOxygen ConcentrationOxidation Start TemperatureOxidation End TemperatureOxidation Temperature RangeReached Maximum Temperature of Steel SheetAnnealing TemperatureHeat Retention TimeDew PointHydrogen ConcentrationAnnealing TemperatureHolding TimeDew PointHydrogen ConcentrationCooling RateDew PointHydrogen Concentration(vol-ppm)(°C)(°C)(°C)(°C)(°C)(s)(°C)(vol.%)(°C)(s)(°C)(vol.%)(°C / s) *1(°C)(vol.%)110F30005956788367880045-141280065-125.214-2511YesComparative Example111F30005986808268080547-151479565-152.59-2610YesInvention Example112F30006006828268279844-151479565-152.36-2710YesInvention Example113F30006056807568080045-151280364-132.34-2410YesInvention Example114F30006026838168380344-121180565-162.512-509YesInvention Example115F30006056817668180046-161480564-142.312-3511YesInvention Example116F30006066817568179545-151380364-152.412-2110YesInvention Example117F30006106837368380044-141480467-152.413-199YesInvention Example118F30006026828068280145-151279567-152.24-254YesInvention Example119F30006056787367880544-151279565-152.514-255YesInvention Example120F30006016807968080045-121280565-162.513-2624YesInvention Example121F30006086807268079846-161280364-142.213-2626YesInvention Example122F30006026828068279547-151278766-122.512-2410NoInvention Example123F30006026828068285847-151278550-121.018-2410YesInvention Example124A30006036838068380045-151180565-152.312-2710YesInvention Example125B30006036858268580144-151279567-142.212-2711YesInvention Example126C30006026787667880347-121380065-152.013-2410YesInvention Example127C30006036807768084047-121380065-151.013-2410YesInvention Example128D30006006808068079845-151280567-132.312-249YesInvention Example129E30005986808268080046-141380367-162.213-2510YesInvention Example130G30005996838468379544-151380565-152.312-269YesInvention Example131H30006006858568579847-151180467-152.512-2710YesInvention Example132I30006006828268282645-151280065-152.412-2410YesInvention Example133J30006056807568080246-161380668-162.513-2510YesInvention Example134J30006036797667982544-141480363-142.314-2610NoInvention Example135F30005986788067880046-151280565-151.212-2510YesInvention Example *1 Average cooling rate over a temperature range of a final holding temperature of annealing to 600°C [Table 9] No.TS (MPa)Total Area Fraction of Martensite, Bainite, and Retained γ (%)Coating AppearanceAmount of Diffusible Hydrogen in Steel SheetDelayed Fracture ResistanceClassification(mass-ppm)Evaluation110130080⊚0.33××Comparative Example111132083⊚0.15⊚○Invention Example112129279⊚0.19⊚⊚Invention Example113131282⊚0.28○△Invention Example114132083⊚0.11⊚⊚Invention Example115132083⊚0.13⊚⊚Invention Example116131282⊚0.14⊚⊚Invention Example117131682○+0.16⊚⊚Invention Example118130481○+0.26○△Invention Example119132083⊚0.11⊚○Invention Example120132083⊚0.23⊚○Invention Example121131282⊚0.27○△Invention Example122128077⊚0.12⊚⊚Invention Example1231512100⊚0.04⊚○Invention Example1246057⊚0.13⊚⊚Invention Example12594248⊚0.13⊚⊚Invention Example126133085⊚0.10⊚○Invention Example1271478100⊚0.06⊚○Invention Example12887338⊚0.11⊚⊚Invention Example129101155⊚0.16⊚⊚Invention Example13084235⊚0.14⊚⊚Invention Example13198050⊚0.12⊚⊚Invention Example132110469⊚0.13⊚⊚Invention Example133109868⊚0.15⊚⊚Invention Example134110368⊚0.16⊚⊚Invention Example135132584⊚0.08⊚○Invention Example

Claims

1. A method for manufacturing a hot-dip galvanized steel sheet, the method comprising subjecting a steel sheet to annealing in a non-oxidizing atmosphere and subsequently subjecting the steel sheet to hot-dip galvanizing, in a continuous annealing furnace, the method optionally comprising subjecting the steel sheet to an alloying treatment after the hot-dip galvanizing, wherein the annealing comprises a first step and a second step, the first step comprises holding the steel sheet at a temperature of 650°C or greater and 950°C or less for a period of 20 s or more and 150 s or less in an atmosphere having a dew point of -55°C or greater and +20°C or less and a hydrogen concentration of 5 vol.% or greater and 25 vol.% or less, and the second step comprises holding, after the steel sheet undergoes the first step, the steel sheet at a temperature of 700°C or greater and 950°C or less for a period of 30 s or more and 300 s or less in an atmosphere having a dew point of -50°C or greater and +20°C or less and a hydrogen concentration of 0.2 vol.% or greater and less than 5.0 vol.%.

2. The method for manufacturing a hot-dip galvanized steel sheet according to Claim 1, further comprising subjecting, before the annealing, the steel sheet to an oxidation treatment at a temperature of 400°C or greater and 900°C or less in an atmosphere containing O2 in an amount of 1000 vol-ppm or greater.

3. The method for manufacturing a hot-dip galvanized steel sheet according to Claim 2, wherein the oxidation treatment is performed in a process in which the steel sheet is heated for the annealing.

4. The method for manufacturing a hot-dip galvanized steel sheet according to Claim 3, wherein the oxidation treatment is performed over a heating temperature span of 50°C or greater in the process in which the steel sheet is heated for the annealing.

5. The method for manufacturing a hot-dip galvanized steel sheet according to any one of Claims 1 to 4, wherein the hydrogen concentration of the atmosphere for the first step of the annealing is 8 vol.% or greater.

6. The method for manufacturing a hot-dip galvanized steel sheet according to any one of Claims 1 to 5, wherein the hydrogen concentration of the atmosphere for the second step of the annealing is 2.0 vol.% or greater.

7. The method for manufacturing a hot-dip galvanized steel sheet according to any one of Claims 1 to 6, wherein a substrate steel sheet of the hot-dip galvanized steel sheet that is manufactured has a hydrogen concentration of 0.30 mass-ppm or less, where the hydrogen concentration is an amount of diffusible hydrogen.

8. The method for manufacturing a hot-dip galvanized steel sheet according to any one of Claims 1 to 7, wherein a substrate steel sheet has a Si content of 0.1 mass% or greater.

9. The method for manufacturing a hot-dip galvanized steel sheet according to any one of Claims 1 to 8, wherein a substrate steel sheet has a total area fraction of martensite, bainite, and retained γ of 30% or greater and a tensile strength of 780 MPa or greater.

10. The method for manufacturing a hot-dip galvanized steel sheet according to any one of Claims 1 to 8, wherein a substrate steel sheet has a total area fraction of martensite, bainite, and retained γ of 50% or greater and a tensile strength of 980 MPa or greater.

11. The method for manufacturing a hot-dip galvanized steel sheet according to any one of Claims 1 to 10, further comprising cooling, after the steel sheet undergoes the annealing, the steel sheet in an atmosphere having a dew point of -20°C or less and a hydrogen concentration of 5 vol.% or greater and 25 vol.% or less, the cooling comprising cooling the steel sheet at an average cooling rate of 5°C / s or greater over a temperature range of a final holding temperature of the annealing to 600°C and then further cooling the steel sheet to a temperature of 150°C or greater and less than 600°C, the cooling being optionally followed by heating of the steel sheet, before the steel sheet is immersed in a hot-dip galvanizing bath to be hot-dip galvanized.

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