Galvanized steel sheet for hot stamp and method for producing the same

Abstract

An galvanized steel sheet for hot stamp, in which a component composition of a base steel sheet satisfies: C: 0.15 to 0.50 mass%; Si: 0.02 to 2.5 mass%; Mn: 0.5 to 5 mass%; P: 0.03 mass% or less (including 0 mass%); S: 0.02 mass% or less (including 0 mass%); Al: 0.010 to 1 mass%; Ti: 0.005 to 0.080 mass%; and B: 0.0005 to 0.005 mass%, with the balance being Fe and inevitable impurities, when elemental analysis is performed in a thickness direction of a plated layer from a surface of the plated layer by glow discharge optical emission spectrometry (GD-OES), a carbon concentration [Cf] (mass%) at a position where a concentration of Zn constituting the plated layer is 1.0 mass% and a bulk carbon concentration [Cb] (mass%) satisfy the following formula (1): [Cf] ≤ 0.65 × [Cb].

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a galvanized steel sheet for hot stamp and a method for producing the same.BACKGROUND ART

[0002] Conventionally, improvement of safety of an occupant in a vehicle has been required, and strength of a material of a vehicle body has been improved for such a purpose. On the other hand, against the background of the growing global warming problem and the like, the movement for improving the fuel efficiency of automobiles is accelerating. It is known that weight reduction of a vehicle body is effective for improving fuel efficiency.

[0003] A steel sheet for hot stamp (for hot forming) can easily achieve both high strength and processability (shape freezability and the like) for the purpose of achieving weight reduction of a vehicle body, for example. A hot-dip galvanized steel sheet for hot stamp is applied particularly to a portion requiring corrosion resistance.

[0004] As the steel sheet for hot stamp (hot forming), the following steel sheet has been conventionally proposed. For example, Patent Document 1 discloses a steel sheet in which the relationship between Ti and N is defined as a steel sheet for hot forming, the steel sheet being excellent in the strength of a joint portion at the time of spot welding and can achieve favorable forming without causing breakage, cracking, or the like at the time of hot forming. In addition, Patent Document 2 discloses that as a steel sheet for hot stamp, excellent in both balance between strength and toughness and hardness stability, high strength is achieved by increasing the ratio of alloy elements, for example, by adjusting the balance of the contents of C, Si, Mn, and Cr.

[0005] However, the ratio of the alloy element of the galvanized steel sheet is increased, thereby easily causing cracking due to liquid metal embrittlement (LME) or liquid metal cracking (LMC) (LME cracking). In particular, hot stamp is performed using the galvanized steel sheet, a component is molded, then spot welding is performed for assembling a vehicle body, and then LME cracking occurs, which arises a problem of insufficient weld strength of the welded portion. For this problem, for example, Patent Document 3 proposes a technique of increasing the Fe concentration of an alloyed hot-dip galvanized coating in a alloyed hot-dip galvanized steel sheet for hot stamp to more than 8.0 mass% and reducing the coating weight (Zn amount: 15.0 to 40.0 g / m 2< ) in order to avoid taking time for Fe-Zn solid solution phase formation, in view of the fact that in a molded body obtained by hot-stamping an alloyed hot-dip galvanized steel sheet, it is necessary to set the heating time in a furnace (furnace time) to about 4 minutes or more in order to improve weldability and chemical convertibility and suppress LME, and that hot stamp is inferior in press productivity to cold pressing and thus it is required to shorten the furnace time.PRIOR ART DOCUMENTSPATENT DOCUMENTS

[0006] Patent Document 1: JP-A-2007-169679 Patent Document 2: JP-A-2019-173158 Patent Document 3: JP-A-2022-131411 NON-PATENT DOCUMENT

[0007] Non-patent Document 1: "Behavior of Corrosion Tip Portion in Under-Coating Corrosion of Zn-Fe Alloy-Plated Steel Sheet", Koryo Hayashi et al., Iron and Steel, 76 (1990), No. 9, p. 1496-1503SUMMARY OF THE INVENTIONPROBLEMS TO BE SOLVED BY THE INVENTION

[0008] In the technique of Patent Document 3, although it is possible to suppress LME, the coating weight is reduced, and thus the amount of effective zinc contributing to ensuring corrosion resistance of plating is reduced, and it is difficult to exhibit the effect of improving corrosion resistance, which is the original role of plating. Regarding the corrosion resistance of plating, as shown in Non-patent Document 1, it is known that particularly the Fe concentration in plating exceeds 65 mass%, thereby greatly deteriorating the corrosion resistance. Therefore, it has been desired to achieve both suppression of LME and securing of corrosion resistance while exhibiting high strength. The present disclosure has been made in view of the above circumstances, and an object of the present disclosure is to provide a galvanized steel sheet for hot stamp, the steel sheet exhibiting excellent LME resistance while ensuring corrosion resistance inherent in plating, and further exhibiting high strength (particularly, tensile strength of 1.5 GPa or more) after hot stamp, and a method for producing the same.SOLUTIONS TO THE PROBLEMS

[0009] A first aspect of the present invention is an galvanized steel sheet for hot stamp, in which a component composition of a base steel sheet satisfies: C: 0.15 to 0.50 mass%; Si: 0.02 to 2.5 mass%; Mn: 0.5 to 5 mass%; P: 0.03 mass% or less (including 0 mass%); S: 0.02 mass% or less (including 0 mass%); Al: 0.010 to 1 mass%; Ti: 0.005 to 0.080 mass%; and B: 0.0005 to 0.005 mass%, with the balance being Fe and inevitable impurities, when elemental analysis is performed in a thickness direction of a plating layer from a surface of the plated layer by glow discharge optical emission spectrometry (GD-OES), a carbon concentration [Cf] (mass%) at a position where a concentration of Zn constituting the plating layer is 1.0 mass% and a bulk carbon concentration [Cb] (mass%) satisfy a following formula (1).         [Cf]≤0.65×[Cb]...     (1)

[0010] The second aspect of the present invention is the galvanized steel sheet for hot stamp according to the first aspect, in which a component composition of the base steel sheet satisfies one or more of following (a) and (b). (a)Further, included are one or more elements selected from the group consisting of: Cr: more than 0 mass% and 1.2 mass% or less; Mo: more than 0 mass% and 1 mass% or less; and Ca: more than 0 mass% and 0.0040 mass% or less. (b)Further, included are one or more elements selected from the group consisting of: Nb: more than 0 mass% and 0.040 mass% or less; V: more than 0 mass% and 0.30 mass% or less; Cu: more than 0 mass% and 0.30 mass% or less; Ni: more than 0 mass% and 0.30 mass% or less; Mg: more than 0 mass% and 0.010 mass% or less; and REM: more than 0 mass% and 0.010 mass% or less.

[0011] A third aspect of the present invention is a method for producing a galvanized steel sheet for hot stamp, the method including: an annealing step comprising retaining a hot-rolled steel sheet or a cold-rolled steel sheet satisfying the component composition according to the first aspect or second aspect at 500 to 930°C for 90 to 1000 seconds in a reducing atmosphere having a dew point of -20°C to +10°C; and a subsequent galvanizing step.

[0012] A fourth aspect of the present invention is the method for producing a galvanized steel sheet for hot stamp according to the third aspect, wherein the galvanizing step is a hot-dip galvanizing step.EFFECTS OF THE INVENTION

[0013] The present disclosure can provide a high-strength galvanized steel sheet for hot stamp, the steel sheet excellent in LME resistance, and a method for producing the same.BRIEF DESCRIPTION OF THE DRAWINGS

[0014] [Fig. 1] Fig. 1 shows a view of a collection position of a steel sheet for evaluation in the Example. [Fig. 2] Fig. 2 shows a view of the carbon profile of a sample before hot stamp in the Example, where the left is the carbon profile of the Comparative Example and the right is the carbon profile of the Example of the present invention. [Fig. 3] Fig. 3 shows an SEM photograph of an example of an EDX analysis area of a sample before hot stamp in the Example. [Fig. 4] Fig. 4 shows a view of an EDX analysis result of a sample before hot stamp in the Example. [Fig. 5] Fig. 5 shows a view of a heat pattern of hot stamp in the Example. [Fig. 6] Fig. 6 shows a view of the carbon profile of a sample after hot stamp in Example, where the left is the carbon profile of the Comparative Example and the right is the carbon profile of the Example. [Fig. 7] Fig. 7 shows an SEM photograph of an example of an EDX analysis area of a sample after hot stamp in the Example. [Fig. 8] Fig. 8 shows a view of an EDX analysis result of a sample after hot stamp in the Example. [Fig. 9] Fig. 9 shows a view of conditions of the welding test in the Example. [Fig. 10] Fig. 10 shows a view of describing a collecting position of a sample for the welding test in the Example. [Fig. 11] Fig. 11 shows a view of describing a crack observation site in a sample sectional after the welding test in the Example. [Fig. 12] Fig. 12 shows a view of describing an internal crack and a void after the welding test in the Example. DETAILED DESCRIPTION

[0015] The present inventors have intensively studied to obtain a galvanized steel sheet for hot stamp, not only exhibiting high strength after hot stamp but also being excellent in LME resistance, on the premise of forming a galvanized layer for securing corrosion resistance. First, the present inventors have performed studies, and as a result, it has been found that in a steel sheet for hot stamp, unlike a general cold-rolled steel sheet, heating is performed at the time of hot stamp, thereby causing a phenomenon in which carbon (C) is concentrated at an interface between a plating layer and a base steel (hereinafter referred to as "plated layer / base steel interface") (as shown in the carbon profile on the left side of Fig. 6 described later, the carbon concentration [C] at the plated layer / base steel interface after hot stamp: 0.303 mass% is higher than the carbon concentration of the bulk: 0.220 mass%). Carbon is known as an element that deteriorates LME resistance, and it has been first found that a main factor of deterioration of LME resistance of a steel sheet for hot stamp is this carbon concentration phenomenon.

[0016] Then, it has been found that in order to suppress the phenomenon of concentration of carbon at the plated layer / base steel interface after hot stamp, it is important to suppress the carbon concentration at the plated layer / base steel interface of the galvanized steel sheet to be subjected to hot stamp, specifically, to satisfy the following formula (1).         [Cf]≤0.65×[Cb]...     (1)

[0017] In the formula (1), [Cf] is the carbon concentration (mass%) at a position where the concentration of Zn constituting the plated layer is 1.0 mass% when elemental analysis is performed in the thickness direction of the plated layer from the surface of the plated layer by glow discharge optical emission spectrometry (GD-OES), and [Cb] is the bulk carbon concentration (mass%).

[0018] In the above formula (1), the "position where the concentration of Zn constituting the plated layer is 1.0 mass% when elemental analysis is performed in the thickness direction of the plated layer from the surface of the plated layer by glow discharge optical emission spectrometry (GD-OES)" refers to a plated layer / base steel interface in the present embodiment. In other words, the galvanized layer refers to a region where the concentration of zinc (Zn) is 1 mass% or more from the surface of the plated layer. The elemental analysis performed in the thickness direction of the plated layer from the surface of the plated layer by the glow discharge optical emission spectrometry (GD-OES) is performed by a method shown in the Example.

[0019] In addition, the bulk carbon concentration [Cb] (mass%) refers to a carbon (C) concentration determined by analyzing a steel sheet for hot stamp having a total sheet thickness × 50 mm × 50 mm or more by a combustion-infrared absorption method. In determining the bulk carbon concentration, the content of each element in the bulk may be analyzed by a conventional method, and is performed by the following method. The amount of Si in the steel sheet produced in the Example described later was more than 0.7 mass%, and thus the amount of Si was determined by a weight method as described below, but when the amount of Si was 0.7 mass% or less, analysis by ICP was recommended.<Analysis method>

[0020] · Inductively coupled plasma atomic emission spectroscopy (ICP): Si (when 0.7 mass% or less), Mn, P, Cu, sol-Al, Ni, Cr, Mo, V, Nb, Ti, B, Ca · Flameless atomic absorption spectrophotometry: Sn · Weight method: Si (more than 0.7 mass%) · Combustion-infrared absorption method: C, S · Inert gas melting-TCD method [N], inert gas melting-infrared absorption method [O]: N, O

[0021] The present inventors have found that, as shown in the above formula (1), in the surface layer region of the galvanized steel sheet before hot stamp, for example, as shown in the carbon profile on the right side of Fig. 2 to be described later, the carbon concentration at the plated layer / base steel interface is reduced to [bulk carbon concentration × 0.65] or less, that is, by providing a surface decarburized layer, as shown in the carbon profile on the right side of Fig. 6 to be described later, the carbon concentration at the plated layer / base steel interface after hot stamp can be suppressed to 0.225 mass% that is almost the same as the bulk carbon concentration. As a result, as shown in the Example described later, LME cracking could be suppressed when welding was performed using the galvanized steel sheet. From this, it is considered that the carbon concentration phenomenon after hot stamp can be suppressed by reducing the carbon in the surface layer region of the galvanized steel sheet before hot stamp, and as a result, the effect of improving the LME resistance is exhibited. In the surface layer region of the galvanized steel sheet before hot stamp, the carbon concentration at the plated layer / base steel interface is preferably 0.60 or less of the bulk carbon concentration, and more preferably 0.50 or less of the bulk carbon concentration. From the viewpoint of enhancing the LME resistance, the ratio to the bulk carbon concentration is more preferably smaller. In consideration of producing conditions, mechanical properties of the steel sheet after hot stamp, and the like, the lower limit of the ratio of the carbon concentration at the plated layer / base steel interface to the bulk carbon concentration can be about 0.01.

[0022] The reduction of the carbon concentration at the plated layer / base steel interface in the galvanized steel sheet before hot stamp can be achieved by producing the galvanized steel sheet under the conditions described later.[Component composition]

[0023] Hereinafter, the component composition of the base steel sheet (the portion excluding the plated layer of the plated steel sheet) in the galvanized steel sheet for hot stamp of the present embodiment will be described. In the galvanized steel sheet for hot stamp according to the present embodiment, satisfying the following component composition allows strength of 1.5 GPa or more after hot stamp to be secured, and component productivity, plating property, and LME resistance to be improved.[C: 0.15 to 0.50 mass%]

[0024] C is an element effective for improving the strength of the steel sheet, and in order to achieve a strength of TS: 1470 MPa or more after hot stamp, the amount of C needs to be 0.15 mass% or more. The amount of C is preferably 0.18 mass% or more, more preferably 0.20 mass% or more. In contrast, when the amount of C exceeds 0.50 mass%, high strength is easily achieved, but leading to problems of excessive strength increase and deterioration of weldability of an original sheet such as a hot-rolled steel sheet. In addition, C is an element that adversely affects LME resistance. Therefore, the amount of C is 0.50 mass% or less, preferably 0.40 mass% or less, more preferably 0.38 mass% or less, and still more preferably 0.35 mass% or less.[Si: 0.02 to 2.5 mass%]

[0025] Si has an effect of suppressing self-tempering of martensite, and thus is an element effective for improving hardness stability and securing productivity at the time of producing a component. In addition, Si is included in a small amount, thereby exhibiting an effect of suppressing non-plating at the time of producing a steel sheet. In order to exhibit these effects, the amount of Si is set to 0.02 mass% or more. The amount of Si is preferably 0.1 mass% or more, more preferably 0.5 mass% or more, and still more preferably 0.7 mass% or more. In contrast, when the amount of Si exceeds 2.5 mass%, the Ac3 point increases, which leads to an increase in the hot stamp heating temperature during component production. Further, Si is an element that adversely affects LME resistance. The reason for this is not clear, but from the results of studies by the present inventors, it is considered that this is because the melting point of plating is lowered by solid solution in plating. From these viewpoints, the amount of Si is set to 2.5 mass% or less. The amount of Si is preferably 2.4 mass% or less, and more preferably 2.2 mass% or less.[Mn: 0.5 to 5 mass%]

[0026] Mn is an element necessary for ensuring hardenability and achieving a strength of TS: 1470 MPa or more after hot stamp. Therefore, the amount of Mn is set to 0.5 mass% or more. The amount of Mn is preferably 1.0 mass% or more, more preferably 1.5 mass% or more. In contrast, an excessive amount of Mn leads to an excessive strength of an original sheet such as a hot-rolled steel sheet. In addition, Mn is an element that deteriorates LME resistance. Therefore, the amount of Mn is set to 5 mass% or less. The amount of Mn is preferably 4.5 mass% or less, and more preferably 4.0 mass% or less.[P: 0.03 mass% or less (including 0 mass%)]

[0027] P is an element inevitably present as an impurity element. P adversely affects toughness and delayed fracture resistance. Therefore, P is preferably small, and the P content is 0.03 mass% or less, preferably 0.010 mass% or less. In the present specification, "including 0 mass%" means including an embodiment in which the impurities are not intentionally added, that is, a case in which the content is equal to or less than the inevitable impurity level (the case in which the impurities are intentionally added is not excluded).[S: 0.02 mass% or less (including 0 mass%)]

[0028] S is an element inevitably present as an impurity element. S adversely affects toughness and delayed fracture resistance. Therefore, the amount of S is preferably small, and the S content is 0.02 mass% or less, preferably 0.010 mass% or less, and more preferably 0.005 mass% or less.[Al: 0.010 to 1 mass%]

[0029] Al is an element that acts as a deoxidizer. In order to exhibit this effect, the Al content is set to 0.010 mass% or more. The Al content is preferably 0.015 mass% or more. In contrast, when Al is excessively included in the steel sheet, the hardness after mold cooling decreases. In addition, excessive generation of Al 2 O 3 deteriorates low-temperature toughness. Therefore, the Al content is set to 1 mass% or less. The Al content is preferably 0.8 mass% or less, and more preferably 0.1 mass% or less. The Al content herein means the content of Al (sol. Al) in a solid solution state.[Ti: 0.005 to 0.080 mass%]

[0030] Ti exerts the effect of B described later, and thus has an effect of precipitating N that adversely affects the solid solution of B with Ti and detoxifying the N. In addition, Ti has an effect of increasing the solid solution amount of B and improving resistance to LME cracking by being contained together with B. In order to exhibit these effects, the amount of Ti is 0.005 mass% or more. The amount of Ti is preferably 0.010 mass% or more, more preferably 0.015 mass% or more, and still more preferably 0.020 mass% or more. In contrast, when the amount of Ti is excessive, precipitation of carbide, grain refinement, and the like occur, the strength of the original sheet such as a hot-rolled steel sheet is increased more than necessary, and the processability is deteriorated, which adversely affects the economic efficiency. Therefore, the amount of Ti is 0.080 mass% or less. The amount of Ti is preferably 0.070 mass% or less, and more preferably 0.060 mass% or less.[B: 0.0005 to 0.005 mass%]

[0031] B is an element necessary for ensuring hardenability and achieving a strength of TS: 1470 MPa or more after hot stamp. In addition, there is an effect of improving resistance to LME cracking of the welded portion at the time of welding. The reason why this effect is exhibited is unknown, but it is considered from the study results of the present inventors that there is an effect of strengthening the old γ grain boundary that is a starting point of cracking. In order to exhibit these effects, the amount of B needs to be 0.0005 mass% or more. The amount of B is preferably 0.0010 mass% or more, and more preferably 0.0015 mass% or more. In contrast, when the amount of B is excessive, the strength of the original sheet such as a hot-rolled steel sheet is increased more than necessary, and the processability is deteriorated, which adversely affects the economic efficiency. Therefore, the amount of B is 0.005 mass% or less. The amount of B is preferably 0.004 mass% or less, and more preferably 0.003 mass% or less.

[0032] The galvanized steel sheet according to the embodiment of the present invention includes the component composition described above, and in one embodiment of the present invention, the balance is preferably iron and inevitable impurities. As the inevitable impurities, contamination of elements brought depending on the situation of raw materials, materials, production facilities, and the like is allowed. N is also an element inevitably present as an impurity element, and can be included, for example, in a range of 0.0100 mass% or less (including 0 mass%). For example, as in P and S, the content is preferably lower, and thus in spite of being an inevitable impurity, there are elements that are separately specified for the composition range as described above. Thus, in the present description, "inevitable impurities" constituting the balance mean the concept excluding elements whose composition ranges are defined.

[0033] The component composition of the steel sheet in the present embodiment may not include the elements described below. Any other element may be further included as long as desired properties can be maintained. Containing elements described below as necessary allows, for example, delayed fracture resistance to be improved and LME resistance to be further improved.

[0034] [(a) Further, one or more types of elements selected from the group consisting of: Cr: more than 0 mass% and 1.2 mass% or less; Mo: more than 0 mass% and 1 mass% or less; and Ca: more than 0 mass% and 0.0040 mass% or less.]

[0035] Cr is an element capable of further improving resistance to LME cracking while ensuring hardenability. In order to exhibit the effect, the Cr content is preferably more than 0 mass%, and more preferably 0.05 mass% or more. In contrast, when the amount of Cr is excessive, pickling property and the like required in the production step deteriorate. Furthermore, the strength of the original sheet such as a hot-rolled steel sheet is increased more than necessary. Therefore, the amount of Cr is preferably 1.2 mass% or less, and more preferably 1.0 mass% or less.

[0036] Mo has an effect of promoting diffusion of B and suppressing sensitivity to LME. From the viewpoint of exerting the effect, the Mo content is preferably more than 0 mass%, and more preferably 0.05 mass% or more. In contrast, when the Mo content is excessive, the strength of the original sheet such as a hot-rolled steel sheet is increased more than necessary, and the processability is deteriorated, which adversely affects the economic efficiency. Therefore, the Mo content is preferably 1 mass% or less.

[0037] Ca is an element that suppresses the generation of MnS that adversely affects delayed fracture resistance and improves delayed fracture resistance. From the viewpoint of exerting the effect, the Ca content is preferably more than 0 mass%, and more preferably 0.001 mass% or more. In contrast, when the amount of Ca is excessive, the effect is saturated, which adversely affects economic efficiency such as an increase in cost. Therefore, the Ca content is set to 0.0040 mass% or less.[(b) One or more types of elements selected from the group consisting of:

[0038] Nb: more than 0 mass% and 0.040 mass% or less; V: more than 0 mass% and 0.30 mass% or less; Cu: more than 0 mass% and 0.30 mass% or less; Ni: more than 0 mass% and 0.30 mass% or less; Mg: more than 0 mass% and 0.010 mass% or less; and REM: more than 0 mass% and 0.010 mass% or less]

[0039] Nb forms fine carbides, makes the structure of steel fine by a pinning effect, and contributes to strength improvement and toughness improvement. In order to exhibit the effect, the Nb content is preferably more than 0 mass%, and more preferably 0.0008 mass% or more. In contrast, when Nb is excessively included in the steel sheet, coarse carbides are formed, and this serves as a starting point of fracture, leading to deterioration of toughness. Therefore, the Nb content is preferably 0.040 mass% or less.

[0040] V forms fine carbides, makes the structure of steel fine by a pinning effect, and contributes to strength improvement and toughness improvement. In addition, precipitation occurs during tempering, and thus a secondary effect is exerted. In order to exhibit these effects, the V content is preferably more than 0 mass%, and more preferably 0.008 mass% or more. In contrast, when V is excessively included in the steel sheet, coarse carbides are formed, and this serves as a starting point of fracture, leading to deterioration of toughness. Therefore, the V content is preferably 0.30 mass% or less.

[0041] Cu and Ni are elements effective for improving the delayed fracture resistance of the member, and can be included in an amount of more than 0 mass% as necessary. In contrast, excessive Cu and Ni included in the steel sheet may cause flaws on the surface of the steel sheet, and eventually on the surface of the member. Therefore, for Cu and Ni, the respective content is preferably 0.30 mass% or less, and the total content is more preferably 0.50 mass% or less.

[0042] Mg and REM may be contained as necessary because of having a function of micronizing inclusions of the steel sheet and preventing cracks during hot forming due to the inclusions. When contained, the content of each element is preferably more than 0 mass%, and more preferably 0.0008 mass% or more. In contrast, when these elements are excessively included, the effect is saturated, leading to an increase in cost. Therefore, the content of any element is preferably 0.010 mass% or less, and more preferably 0.008 mass% or less. The REM means including lanthanoid elements (15 elements from La to Lu), Sc (scandium), and Y (yttrium).

[0043] The type of zinc-based plating in the galvanized steel sheet for hot stamp according to the present embodiment is not limited. Examples thereof include Fe-Zn plating and Al-Zn plating. The concentration of Fe in the plated layer of the galvanized steel sheet for hot stamp according to the present embodiment is preferably 40 mass% or less, and more preferably 30 mass% or less. Specific examples of the galvanized steel sheet include hot-dip galvanized steel sheets (GI), alloyed hot-dip galvanized steel sheets (GA), and electrogalvanized steel sheets (EG).

[0044] The coating amount of the zinc-based plating (in particular hot-dip galvanization, alloyed hot-dip galvanization) of the galvanized steel sheet for hot stamp according to the present embodiment is preferably 45 g / m 2< or more, more preferably 60 g / m 2< or more, still more preferably more than 65 g / m 2< from the viewpoint of securing corrosion resistance. In contrast, from the viewpoint of easily achieving the recommended Fe concentration in the plated layer, the coating weight of the galvanization is preferably small. Therefore, the coating weight of the zinc-based plating is preferably 190 g / m 2< or less, and more preferably 180 g / m 2< or less.

[0045] The concentration of Fe in the zinc-based plating of the galvanized steel sheet being subjected to hot stamp by a method described in the Example described later using the galvanized steel sheet for hot stamp according to the present embodiment is preferably 65 mass% or less, and more preferably 60 mass% or less. The Fe concentration may be 20 mass% or more from the viewpoint of increasing the Fe concentration of plating by hot stamp.

[0046] The present embodiment can improve the LME resistance by satisfying the predetermined component composition and providing the predetermined surface decarburized layer as described above without reducing the coating weight or increasing the Fe concentration in the hot-dip galvanized layer.[Method for producing galvanized steel sheet for hot stamp]

[0047] Then, a method for producing the galvanized steel sheet for hot stamp according to the present embodiment will be described.

[0048] The method for producing a galvanized steel sheet for hot stamp according to the present embodiment includes an annealing step including allowing a hot-rolled steel sheet or a cold-rolled steel sheet satisfying the component composition to be retained at 500 to 930°C for 90 to 1000 seconds in a reducing atmosphere having a dew point of - 20°C to +10°C, and a subsequent galvanizing step.

[0049] First, an annealing step and a subsequent galvanizing step, which are characteristics of the production method according to the present embodiment, will be described. Hereinafter, an aspect in which the annealing step according to the present embodiment and the subsequent galvanizing step (particularly, the hot-dip galvanizing step) are performed on a hot-dip galvanizing line of a reducing furnace system will be described as an example, but is not limited thereto. The method according to the present embodiment is not intended to be limited to the above aspect, and for example, the hot-dip galvanizing step can be performed by a continuous annealing line of a non-oxidizing furnace system.(Annealing step)

[0050] The annealing step of the hot-dip galvanizing line typically includes a reducing furnace and a cooling band. The present embodiment is characterized in that the annealing conditions in the reducing furnace, particularly the dew point of the reducing atmosphere, are appropriately controlled. The original sheet is charged into a reducing furnace. The original sheet to be charged into the reducing furnace may be subjected to a pretreatment step described later such as degreasing as necessary. In addition, the original sheet to be charged into the reducing furnace may be subjected to an oxidation treatment by being charged into an oxidizing furnace as necessary after undergoing a pretreatment step.

[0051] In the reducing furnace, the original sheet is subjected to heat treatment in a reducing atmosphere. The dew point of the reducing atmosphere is set to -20°C to +10°C. Setting the dew point within this range causes decarburization of the surface layer of the steel sheet, and a desired surface layer region can be obtained. The dew point of the reducing atmosphere is preferably -15°C or more, and more preferably - 10°C or more. In addition, the dew point of the reducing atmosphere is preferably +5°C or less, and more preferably 0°C or less.

[0052] The dew point can be controlled by, for example, a method in which a water vapor gas is charged and mixed with an atmospheric gas in a furnace, or a method in which an atmospheric gas is bubbled and water vapor is mixed. The reducing atmosphere is not particularly limited as long as it satisfies the above dew point and is reducing. The reducing atmosphere satisfies the above dew point, and for example, the H 2 concentration is preferably 1 to 30 vol% in the H 2 -N 2 mixed gas.

[0053] In addition, the annealing temperature is set to 500 to 930°C, and the residence time at the annealing temperature, that is, the annealing time is set to 90 to 1000 seconds. The annealing treatment in the above temperature range is also referred to as soaking treatment, and in this case, the annealing temperature is referred to as soaking temperature, and the annealing time is referred to as soaking time.

[0054] The annealing temperature is preferably 530°C or more, more preferably 560°C or more, and still more preferably 600°C or more. The annealing temperature is preferably 900°C or less, and more preferably 870°C or less. The annealing time is preferably 100 seconds or more, and more preferably 120 seconds or more. The annealing time is preferably 900 seconds or less, more preferably 700 seconds or less, still more preferably 500 seconds or less, still more preferably 400 seconds or less, and still more preferably 350 seconds or less. The annealing time can be controlled by the speed at which the original sheet passes through the reducing furnace (hereinafter, also referred to as "line speed" or "LS" for short). "To be retained at 500 to 930°C for 90 to 1000 seconds" means that it may be retained in the range of the annealing temperature of 500 to 930°C for 90 to 1000 seconds, and the temperature may be constant or may vary within the range of the annealing temperature.

[0055] According to the production method according to the present embodiment, a predetermined surface decarburized layer can be provided by particularly performing the annealing on a hot-rolled steel sheet or a cold-rolled steel sheet that is an original sheet having the above-described component composition.

[0056] A pretreatment step of the original sheet that may be performed before the annealing step will be described. The pretreatment is typically performed to remove oil (oil and fat) and dirt adhering to the surface of the original sheet, and is typically alkali degreasing. The alkali included in the degreasing liquid used for alkali degreasing is not particularly limited as long as, it is preferably, for example, caustic soda, a silicate, or a mixture thereof, and can remove oil and fat as a water-soluble soap. In addition, in order to improve the degreasing properties, electrolytic washing, scrubber treatment, and treatment of adding a surfactant or a chelating agent to the degreasing liquid can be performed. In the present embodiment, the method of the pretreatment is not limited as long as the surface of the original sheet is appropriately degreased, and the above-described treatments may be performed singly or in any combination.

[0057] The original sheet that has left the reducing furnace may be cooled in a cooling band. The cooling band may include a slow cooling band, a rapid cooling band, and an adjustment band. The adjustment band is also called a holding band. Cooling may be performed under conditions typically used so as not to cause non-plating, and examples thereof include a method of cooling the steel sheet by spraying a gas in a reducing atmosphere to the steel sheet.

[0058] The hot-dip galvanizing line of the reducing furnace system may be generally divided into a pretreatment step, an annealing step, and a plating step. In the plating step, an alloying treatment is also performed as necessary.

[0059] From the viewpoint of energy saving, the pretreated original sheet may be preheated in a preheating furnace in a reducing or oxidizing atmosphere using exhaust gas after the pretreating step and before entering the reducing furnace.(Galvanizing step)

[0060] After the annealing step (continuous annealing step), a galvanizing step is included. Hereinafter, a hot-dip galvanizing step will be described as an example of the galvanizing step. A hot-dip galvanized steel sheet (GI) is fabricated by a hot-dip galvanizing step. Alternatively, the GI may be alloyed to fabricate an alloyed hot-dip galvanized steel sheet (GA).

[0061] The hot-dip galvanizing step is not particularly limited, and a commonly used method can be adopted. For example, the temperature of the hot-dip galvanizing bath may be controlled to about 430 to 500°C. The coating weight of the hot-dip galvanized layer (the same as the coating weight of the following alloyed hot-dip galvanized layer) is preferably 45 g / m 2< or more, more preferably 60 g / m 2< or more, still more preferably more than 65 g / m 2< from the viewpoint of securing corrosion resistance. In contrast, from the viewpoint of easily achieving the recommended Fe concentration in the plated layer, the coating weight of the hot-dip galvanized layer (particularly, the alloyed hot-dip galvanized layer) is preferably small. Therefore, the coating weight of the hot-dip galvanized layer is preferably 190 g / m 2< or less, more preferably 180 g / m 2< or less.

[0062] The alloying treatment is also not particularly limited, and a commonly used method can be adopted. In the alloying treatment, when the Fe concentration in the plated layer is increased, the alloying temperature is controlled to, for example, about 400 to 700°C. The alloying temperature is further 430°C or more, further 440°C or more, and further 450°C or more. In contrast, when the alloying temperature is too high, the Fe concentration in the plated layer becomes too high, and thus the alloying temperature is preferably 680°C or less, and more preferably 650°C or less.

[0063] The step after the plating step is also not particularly limited, and a commonly used method can be adopted. Typically, skin pass treatment, tension leveler treatment, oil coating, and the like are performed, but these may be performed under the conditions typically used as necessary, and may not be performed as unnecessary. The galvanized steel sheet (GI or GA) thus obtained is suitably used as a steel sheet for hot stamp.

[0064] Electroplating may be performed instead of the hot-dip galvanizing. For example, the original sheet may be subjected to the annealing described above, and then electroplating may be performed to provide an electrogalvanized steel sheet having a plated layer / base steel interface defined in the present embodiment.(Other steps)

[0065] The production method according to the present embodiment may include the prescribed annealing step and the subsequent galvanizing step, and the other steps are not limited, and may include a step that is typically performed. Therefore, the method for producing the hot-rolled steel sheet or the cold-rolled steel sheet to be subjected to the annealing step is not limited, and for example, the hot-rolled steel sheet or the cold-rolled steel sheet can be produced as follows. First, a slab is produced. In the slab production step, steel is smelted according to a conventional method, and molten steel is poured into a mold and continuously cast to provide a slab. In this step, the component composition of steel is adjusted during melting so as to satisfy the above component range. A step of heating after casting and before hot rolling so as not to cause cracking by hot rolling may be provided (this step is different from the slab heating step in the following hot rolling step). The heating conditions are not particularly limited, and conditions usually used can be appropriately adopted, but it is desirable to perform heating at a temperature of about 1100°C to 1300°C.

[0066] Then, hot rolling is performed. In the hot rolling step, first, the slab is placed in a heating furnace, heated to a predetermined temperature (about 1100°C to 1300°C, for example 1200°C), and held at the heating temperature for a predetermined time (for example, 30 minutes).

[0067] Then, the slab in a heated state is placed upstream of the hot rolling line, and the slab is rolled into a steel sheet having a predetermined plate thickness by sequentially passing the slab between rolls of rolling stands of the rough rolling mill and the finish rolling mill in the downstream direction. The steel sheet after hot rolling is cooled to a predetermined temperature by a cooling device and then wound up by a coiler.

[0068] The hot-rolled steel sheet may be a hot-rolled and pickled steel sheet that is subsequently pickled in a pickling step. In the pickling step, at least the hot rolling scale may be removed by pickling. The hot-rolled and pickled steel sheet may be cold-rolled as necessary. In the cold rolling step, the hot-rolled steel sheet is further rolled so as to further reduce the sheet thickness. Specifically, the hot-rolled steel sheet after pickling is passed between rolls of a rolling stand, thereby further thinning the hot-rolled steel sheet. The cold-rolled steel sheet is particularly suitably used for automobile components intended for weight reduction of automobiles and the like. The base steel sheet constituting the galvanized steel sheet is desirably a cold-rolled steel sheet from the viewpoint of dimensional accuracy and flatness. The hot-rolled steel sheet (including a hot-rolled and pickled steel sheet) or the cold-rolled steel sheet (hereinafter, these are collectively referred to as "original sheets") may be subjected to the annealing step and a galvanizing step, for example, a reducing furnace type continuous plating step.[Example]

[0069] Hereinafter, the present disclosure will be described more specifically with reference to Examples. The present disclosure is not limited by the following examples, and can be implemented with appropriate modifications within the scope that can be consistent with the above-described and later-described gist, and all of them are included in the technical scope of the present disclosure.

[0070] Steel materials having component compositions shown in Table 1 were melted in a converter, and then slabs were produced by continuous casting. The obtained slab was heated at a temperature of 1100°C to 1300°C, and then hot-rolled under conditions of FDT: 890°C to 950°C and a coiling temperature of 500°C to 700°C, and then descaled in a pickling step, and then cold-rolled to provide a cold-rolled steel sheet. The cold rolling ratio at the time of cold rolling was 20% or more. The obtained cold-rolled steel sheet had a sheet thickness of 1.2 mm. In the present Example, a plated steel sheet was produced using the cold-rolled steel sheet as an original sheet. In Table 1, underlined numerical values indicate that they are out of the range defined in the present embodiment. The same applies to the following table. Steel type 2 in Table 1 is a steel type of Comparative Example because the carbon (C) is too low. [Table 1]Balance of component composition (mass%) is Fe and inevitable impurities.Steel typeCSiMnPSCuAlNiCrMoVNbTiBNCaMgREM10.221.22.20.010.001≦0.050.04≦0.05≦0.08≦0.05≦0.008≦0.0050.0310.0020.0025≦0.0015--20.0851.22.20.010.001≦0.050.04≦0.05≦0.08≦0.05≦0.008≦0.0050.0310.0020.0025≦0.0015--30.3550.20.80.010.0007<0.010.039<0.010.59<0.01<0.001<0.0040.0310.00420.0009---40.3570.810.50.010.001<0.010.041<0.010.58<0.01<0.001<0.0040.0290.00390.0013---*In the table, "-" indicates no intended addition.

[0071] The obtained cold-rolled steel sheet was subjected to reduction annealing in a hot-dip galvanizing annealing line under the conditions (soaking temperature (annealing temperature), soaking time (annealing time), dew point) described in Table 2. In Experiment Nos. 1 to 7, after immersion in a plating bath, alloying was performed under the conditions described in Table 2 (alloying temperature and alloying time) to provide an alloyed hot-dip galvanized steel sheet (GA steel sheet) having a width of about 1000 mm and having both surfaces of the steel sheet subjected to alloyed hot-dip galvanization. The GA steel sheet is also a steel sheet for hot stamp. The GA steel sheet may be referred to as a "steel sheet before hot stamp". In addition, in Experiment Nos. 8 to 10, plating (decarburization) annealing was performed using a small sample of 150 mm × 80 mm. In this small sample, it has been separately confirmed that the decarburized state, strength, and the like are equivalent to those of the steel sheet. In Experiment Nos. 8 to 10, the coating weight was not measured. The material annealed under the same conditions was separately evaluated to be 180 g / m 2< , and thus an equivalent coating weight (weight per unit area) is also assumed in Experiment Nos. 8 to 10, and is shown as 180 g / m 2< in Table 2. [Table 2]Experiment No.Steel typeSheet thickness (mm)Hot-dip galvanizing annealing stepCoating weight (g / m2)Soaking temperature (°C)Soaking time (s)Dew point (°C)Alloying temperature (°C)Alloying time (s)111.2720138-7500 to 65015120211.2635138-7500 to 65015120311.2700138-7500 to 65015120411.2652166-30500 to 65018113511.2652166-30500 to 65018113621.2675167-30500 to 6501769121.2666197-30500 to 6502387831.2870347-1055020180 * 1< 941.2870347-1055020180 * 1< 1041.2870347-4055020180 * 1< *1 Although no measurement was made, evaluation of a material annealed under the same conditions resulted in 180 g / m 2< , and thus 180 g / m 2< is described because an equivalent basis weight is assumed.

[0072] In Experiment Nos. 1 to 7, with respect to the GA steel sheet, steel sheets for evaluation having dimensions of 150 mmW × 70mmL or 220 mmW × 150 mmL were collected from each position of the central portion (range of W / 4 to 3W / 4 in the width direction) a and the end portion (region from the most end) b in Fig. 1. The collection position of the steel sheet for evaluation in each example is shown in Table 5. In addition, in Experiment Nos. 8 to 10, the obtained small sample (size: 150 mmW × 80 mmL) was used without being cut.

[0073] Using each steel sheet for evaluation, the decarburized state of the surface layer of the steel sheet before hot stamp and the Fe concentration in the plated layer were measured as follows.[Measurement of decarburized state of surface layer (before hot stamp) (measurement of carbon profile by GD-OES)]

[0074] As described below, the carbon profile was measured by glow discharge optical emission spectrometry (GD-OES), and the decarburization behavior was examined.(Preparation of sample)

[0075] Materials having a size of 50 mm × 40 mm × sheet thickness of 1.2 mm (total sheet thickness), 30 mm × 30 mm × sheet thickness of 1.2 mm (total sheet thickness), or 30 mm × 40 mm × sheet thickness of 1.2 mm (total sheet thickness) were collected. Thereafter, degreasing was performed according to a conventional method to prepare a sample. Then, using the sample, the concentration of mass% of each element was measured by GD-OES under the following conditions.(Measurement conditions)

[0076] Device used: Markus high-frequency glow discharge optical emission surface analyzer (rf-GD-OES) GD-Profiler2 manufactured by HORIBA, Ltd. Sputtering method: normal sputtering Measurement range: φ4 mm Gas type: Ar Element to be analyzed: B, C, O, Al, Si, Ti, Cr, Mn, Fe, Zn, P, S, and N (in this example, these elements were evaluated, but when elements other than the above elements are contained in, for example, a plated layer or a steel sheet, elements other than the above elements are also to be analyzed) (Measurement method)

[0077] The surface of the sample on which the plating was formed was subjected to GD-OES measurement until the depth reached 150 µm in the sheet thickness direction.(Analysis method)

[0078] The sputtering rate of the present device was substantially constant, and thus the sputter crater depth of the sample after the analysis was measured, and the horizontal axis was taken as the value (sputtering depth).

[0079] Details of the calibration curve method for converting the measured emission intensity of each element into a concentration will be described below.

[0080] The relationship between the sputtering weight W i (g / sec) per unit time of the element i and the emission intensity I i is represented by the following formula (I) using the slope a and the intercept b of the calibration curve. W i = aI i + b

[0081] The sputtering weight W i per unit time of the element i is determined by the following formula (II) using the sputtering area S (cm 2< ) in the reference sample in which the concentration C i (wt%), the density ρ (g / cm 3< ), and the sputtering rate Δd (cm / sec) are known. W i = C i × ρ × Δd × S

[0082] The emission intensity I i was measured using two or more types of reference samples in which W i was known, and the slope a and the intercept b of the above formula (I) were obtained to prepare a calibration curve in which the horizontal axis was the emission intensity and the vertical axis was the sputtering weight. The reference samples used are shown in Table 3 below. Using the prepared calibration curve, the sputtering weight was determined from the emission intensity of each target element, and the weight ratio was converted into the concentration. The calibration curve used for the conversion of the O concentration was corrected using SiO 2 such that the concentration ratio between Si and O was 1:2. [Table 3]Reference sample Main component B C O Al Si Ti Cr Mn Fe Zn BAS 113Fe0.00660.837-0.01510.9310.0391.2481.20794.99-BAS 114Fe0.00080.403-0.0780.2950.00960.1870.41696.47-MBH 13X NSD1Fe-0.046-0.0130.411-24.5123.5349.09-MBH 13X 8110LFe(1.09)0.792-0.0090.960.05512.330.77276.08-MBH 31X BIB3Cu---0.02980.061--0.2430.09932.46SPEX 185-CO2Cu--------0.0960.15JAPAN FINE CERAMICS CO., LTD. Al2O3O--47.07(52.93)------* The numerical value indicated in parentheses in the table was excluded from the reference value used for the concentration conversion.

[0083] From the above analysis results, zinc and carbon profiles, which are results of analysis on zinc and carbon, were obtained. Then, from the profiles of zinc and carbon, the carbon concentration [Cf] at the position where the concentration of Zn constituting the plated layer was 1.0 mass% was determined. The bulk C concentration [Cb] was determined by analysis as described above. Then, the value of [Cf] / [Cb] was obtained. These results are shown in Table 5.

[0084] In addition, as an example of the carbon profile, as a comparative example, a carbon profile of Experiment No. 4 that is a conventional steel is shown on the left of Fig. 2, and a carbon profile of Experiment No. 1 that is an example of the present invention is shown on the right of Fig. 2. From comparison of these carbon profiles, it is found that in the present invention example, the carbon concentration at the interface between the plated layer and the base steel sheet is sufficiently suppressed.

[0085] In Experiment No. 6, the C concentration at the base steel / plating interface was not evaluated. However, the component composition is the same as that in Experiment No. 7 and the reduction annealing conditions are similar to those in Experiment No. 7, and thus it is considered that the C concentration at the base steel / plating interface is similar to that in Experiment No. 7. Therefore, the C concentration at the base steel / plating interface of No. 6 is estimated to be approximately 0.070, which is the same as that in Experiment No. 7, and thus values are described in Table 5.[Measurement of Fe concentration in plated layer (before hot stamp)]

[0086] A sample of 20 mmW × 10 mmL × sheet thickness was cut and collected from the GA steel sheet (steel sheet before hot stamp). Degreasing was performed as necessary.<Preparation of observation sample>

[0087] A sectional in the L direction, that is, a surface formed by the sheet thickness and 10mmL as a side was embedded in a resin so as to be an observation surface, polished, and then subjected to gold deposition.<SEM observation>

[0088] SEM observation and EDX analysis were performed under the following conditions. In this example, the observation magnification is set to 1500 times, but the observation magnification depends on the coating weight of plating, and thus it is preferable to select a magnification appropriate for the measurement of the Fe concentration, for example, by decreasing the measurement magnification as the plated thickness increases.(SEM observation conditions)

[0089] · Apparatus: field emission scanning electron microscope (FE-SEM) Supra-35 manufactured by Carl Zeiss AG. · Observation image: reflected electron image · Observation point: near surface layer including plated layer · Observation magnification: 1500 times · Number of observation fields: one field of view representing sample / one sample <EDX analysis>

[0090] · Apparatus: energy dispersive X-ray (EDX) detector X-max80 manufactured by Oxford Instruments plc. · Analysis method: area analysis (qualitative semi-quantitative analysis) · Analysis position: entire plated layer, an example of an analysis area is shown in Fig. 3. · Number of analysis fields: one field of view representing sample / one sample · Analytical element: among elements detected by EDX, mass%, which is the ratio of Fe evaluated when elements other than C are taken as a population, was taken as the Fe concentration (%). As an example, the analysis result of Experiment No. 4 is shown in Fig. 4. In addition, as an example, the calculation results of the Fe concentration (12.0 mass%) of Experiment No. 4 are shown in Table 4. The Fe concentration in the plated layer of each example obtained by calculation in the same manner as in the following Table 4 is shown in Table 5. [Table 4] Elementmass%Mass% excluding CC8.70.0O1.161.3Al0.320.4Mn0.320.4Fe10.9312.0Zn78.5786.1Total100100.0Total mass% excluding C91.3 [Table 5] Experiment No.Position of steel sheet collectionState of plating before hot stampFe concentration in plated layerPlating typeC concentration at base steel / plating interface [Cf]Bulk C concentration [Cb][Cf] / [Cb](mass%)(mass%)(mass%)1Center8.8GA0.0550.220.252Center10.2GA0.0450.220.213Center9.9GA0.0560.220.264Center12.0GA0.1460.220.675End8.6GA0.2110.220.966Center17GA0.070 * 2< 0.0850.827Center16GA0.0700.0850.828Small sample8.8GA0.1110.3550.319Small sample13.7GA0.0930.3570.2610Small sample10.5GA0.3380.3570.95 *2 C concentration at base steel / plating interface estimated from Experiment No. 7 [Evaluation of steel sheet after hot stamp]

[0091] In order to evaluate the steel sheet after hot stamp, the steel sheet for evaluation was subjected to hot stamp as follows. First, degreasing was performed by a conventional method in order to suppress carburization during hot stamp heating. Then, hot stamp was performed according to the following conditions and the heat pattern in Fig. 5 to provide a sample after hot stamp.(Hot stamp conditions)

[0092] · Sample size used: 150 mmW × 70 mmL or 150 mmW × 220 mmL or 150 mmW × 80 mmL (small sample) · Mold used: mold for flat sheet <Heating condition>

[0093] · Atmospheric electric furnace set temperature: 910°C · Heating time: the sheet temperature reached 870°C, and then was held for 45 s (meanwhile, the sheet temperature was controlled to fall within a range of 870°C to 900°C). <Cooling conditions>

[0094] · Thereafter, natural cooling was performed, and when the sheet temperature reached 550°C or 700°C, both planar surfaces were pressed with a mold for flat sheet and cooled. · Press load: 0.5 MPa · Press speed: 20 spm · Pushing amount: 5 mm · Bottom dead center holding time: the bottom dead center was held until 50°C or less (in this case, 10 s).

[0095] Using the sample after hot stamp, tensile strength TS, LME resistance, and the like were evaluated as described in detail below.[Shot blasting]

[0096] For the sample after hot stamp, the resistance value of the steel sheet was measured in accordance with the method described in ISO-18594: 2007 (E) as to the surface resistance. Shot blasting was performed under the following resistance measurement conditions so as to be 2.0 mΩ or less. Each of the shot blasting conditions is shown in Table 6. In Experiment Nos. 8 to 10, the resistance value after shot blasting was not measured, but the average (Experiment Nos. 1 to 5) of the resistance values of the materials prepared under substantially the same conditions was 0.9 mΩ, and thus it is assumed to be substantially the same, and 0.9 (mΩ) is shown in Table 6 below.(Shot blasting conditions)

[0097] · Shot material: GH-3 (grid 0.3 mm) · Shot pressure: approximately 0.4 MPa · Time of shot per area of 150 mm x 120 mm (details of shot blasting time is described in Table 6) is controlled. · Shot blasting on both surfaces · Shot blasting was performed until the resistance value of the steel sheet became 2.0 mΩ or less.

[0098] (Resistance measurement conditions) · Three materials of 30 mm × 30 mm × sheet thickness were cut and collected . · Electrode material: Cu-Cr · Electrode diameter: Φ8 mm · Tip R: 40 mm · DC current value: 2 A · Pressure : 350 ± 17.5 × 10 N · The number of measurements for one sample was set to one, and a total of three measurements were performed for each experimental sample. · The average value of N3 was adopted as the value. · A plated surface of one steel sheet was sandwiched between electrodes and measured. · A value obtained by subtracting the setup resistance from the total resistance was defined as a resistance value. [Table 6] Hot stampShot blastingExperiment No.Heating temperature (°C)Heating time (s)Molding start temperature (°C)Shot blasting timeResistance value after shot blasting (mΩ)18704555030 s / one side0.728704555030 s / one side0.838704555030 s / one side1.048704555030 s / one side1.758704555030 s / one side0.5687045700120 s / one side1.7787045550120 s / one side0.888704570030 s / one side0.9 * 3< 98704570030 s / one side0.9 * 3< 108704570030 s / one side0.9 * 3< *3 Although no measurement was made, the average resistance value (Experiment Nos. 1 to 5) of the materials adjusted under almost the same conditions is 0.9 mΩ, and thus the value is adopted as approximately 0.9 mΩ. [Measurement of decarburized state of surface layer after hot stamp (measurement of carbon profile by GD-OES)]

[0099] As described below, the carbon profile was measured by GD-OES (Glow discharge optical emission spectrometry), and the decarburization behavior after hot stamp was examined.(Preparation of sample)

[0100] A material having a size of 30 mm × 90 mm × sheet thickness or 30 mm × 70 mm × sheet thickness was collected. Thereafter, degreasing was performed according to a conventional method to prepare a sample. Then, using the sample, the concentration of mass% of each element was measured by GD-OES under the following conditions.(Measurement conditions)

[0101] Device used: Markus high-frequency glow discharge optical emission surface analyzer (rf-GD-OES) GD-Profiler2 manufactured by HORIBA, Ltd. Sputtering method: normal sputtering Measurement range: φ4 mm Gas type: Ar Element to be analyzed: B, C, O, Al, Si, Ti, Cr, Mn, Fe, Zn (in this example, these elements were evaluated, but when elements other than the above elements are contained in, for example, a plated layer or a steel sheet, elements other than the above elements are also to be analyzed) (Measurement method)

[0102] The surface of the sample on which the plating was formed was subjected to GD-OES measurement until the depth reached 100 µm in the sheet thickness direction.(Analysis method)

[0103] Analysis was performed in the same manner as in the measurement of the decarburized state of the surface layer (before hot stamp) described above (measurement of the carbon profile by GD-OES).

[0104] Using the sample after hot stamp, the carbon profile was measured by GD-OES under the conditions described above, and the decarburization behavior was examined. As an example, as a comparative example, the carbon profile of Experiment No. 4 that is a conventional steel is shown on the left in Fig. 6, and the carbon profile of Experiment No. 1 that is an example of the present invention is shown on the right in Fig. 6. From the comparison of these carbon profiles, it is found that in the present invention example, the carbon concentration at the interface between the plated layer and the base steel sheet is sufficiently suppressed after hot stamp, as the difference in carbon concentration at the interface between the plated layer and the base steel sheet is indicated by a double-headed arrow in the vertical direction in Fig. 6. In Table 8, "-" in the C concentration at the base steel / plating interface indicates that the measurement was not performed.[Measurement of Fe concentration in plated layer after hot stamp]

[0105] The Fe concentration in the plated layer after hot stamp was measured in the same manner as in the measurement of the Fe concentration in the plated layer before hot stamp. Fig. 7 shows an example of an analysis area of EDX analysis in the measurement of the Fe concentration in the plated layer after hot stamp. As an example of the EDX analysis, the analysis result of Experiment No. 4 is shown in Fig. 8. In addition, as an example, the calculation results of the Fe concentration (53.4 mass%) of Experiment No. 4 are shown in Table 7. The Fe concentration in the plated layer of each example obtained by calculation in the same manner as in the following Table 7 is shown in Table 8. [Table 7]Elementmass%Mass% excluding CC7.060.0O1.71.8Al0.160.2Si0.660.7Fe49.6653.4Zn40.7643.9Total100100.0Total mass% excluding C92.94 [Tensile test]

[0106] Using the sample after hot stamp, a tensile test was performed under the following conditions, and the tensile strength (TS, unit: MPa) was measured.(Preparation of test piece)

[0107] For a material of 150 mmW × 70 mmL × sheet thickness or 150 mmW × 220 mmL × sheet thickness or 150 mmW × 80 mmL (small sample), a sample for collecting a test piece of 150 mmW × 30mmL was collected from the center of the sheet. Then, a JIS No. 5 test piece was prepared from a sample of 150 mmW × 30mmL for collecting a test piece.(Tensile test method)

[0108] Using the above JIS No. 5 test piece, the tensile strength (TS, unit: MPa) was measured by a method specified in JIS Z 2241 at a strain rate of 10 mm / min using AG-IS 250 kN autograph tensile tester manufactured by Shimadzu Corporation.[Welding test]

[0109] The sample after hot stamp was cut to prepare a plurality of samples for a welding test having a size of 30 mm × 30 mm × sheet thickness, and a welding test was performed under the following conditions and as shown in Fig. 9. In Fig. 9, X in "X kA" in the 2nd pulse (second energization) indicates 15 current conditions in which every 0.5 kA is changed between 5.0 kA and 12.0 kA to be described below.<Welding conditions>

[0110] · Pre-marked point: 20 welding points were performed at 6.5 kA on a pair of two mild steels such that the tip was well fitted before the main welding. Other conditions of the pre-marked point and conditions of main welding are as follows. Energization time: 10 cycles Pressure: 1.7 kN Holding time: 1 cycle · Main welding: Device name: servo pressurization spot welding device Manufacturer: Nastoa Welding Technologies Co., Ltd. Electrode: Cu-Cr with upper and lower dome-radius shape Electrode diameter: outer diameter Φ16 mm, tip diameter: Φ6 mm Marking angle: 0° Cooling water flow rate: approximately 2 liters / minute for upper and lower portions Pressurizing force: 500 kgf Initial pressing time: 60 cycles / 60 Hz Upslope: 1 cycle / 60 Hz First energization: energization time: 36 cycle / 60 Hz Current value: 4.5 kA Second energization: 18 cycle / 60 Hz Current value: 5.0 kA, 5.5 kA, 6.0 kA, 6.5 kA, 7.0 kA, 7.5 kA, 8.0 kA, 8.5 kA, 9.0 kA, 9.5 kA, 10.0 kA, 10.5 kA, 11.0 kA, 11.5 kA, 12.0 kA Holding time: 10 cycle / 60 Hz <Sheet assembly>

[0111] · Double sheet assembly, upper sheet and lower sheet are steel sheets of the same type. · A test piece for performing the second energization by nl for a total of 15 current conditions was prepared. [Evaluation of LME cracking]

[0112] From the sample (welding test piece) after the welding test, a test piece for evaluation of LME cracking was prepared as follows. Fig. 10 is a schematic top view of the sample 1 after the welding test in which the upper sheet and the lower sheet are welded. As the test piece 2 for evaluation of LME cracking, hatched portions (including a part of the nugget 3) in Fig. 10 were taken from the sample 1 after the welding test. In the collection, in order to be able to photograph the central portion of the nugget 3 (the diameter surface of the nugget), first, the nugget was cut at the cut surface 4 in consideration of a polishing margin, and then polished such that the central portion of the nugget (the diameter surface of the nugget) became the observation surface 5.

[0113] Then, the observation surface 5 was subjected to a corrosion treatment by a conventional method to such an extent that the HAZ portion and the nugget diameter were known. Then, in all the samples, the cross section was photographed (in the direction of the white arrow in Fig. 10) at a magnification that reached the HAZ portion. An example of the photograph is shown in Fig. 11. This photograph was taken at a magnification of 25 times. There are 15 current conditions for one experiment, and thus 15 samples for LME cracking evaluation were photographed for one experiment.

[0114] Cracking observation was performed at a total of 8 positions (1) to (8) shown in Fig. 11 for one current condition. That is, a total of cracks at 120 positions was confirmed at 15 current conditions × 8 positions. Then, the total number of positions where cracks having a length of 300 µm or more were confirmed at a total of 120 positions was obtained. In (1) to (6) of Fig. 11, cracks (surface cracks) in which cracks extend substantially in the sheet thickness direction from the surface were easily observed, and in (7) and (8) of Fig. 11, cracks (inner cracks) extending substantially in the nugget center direction were easily observed. The void Q shown in Fig. 12 is not a crack. An inner crack R extending from the void Q shown in Fig. 12 is counted.

[0115] These results are shown in Table 8. In the properties shown in Table 8, the case where the tensile strength TS after hot stamp was 1470 MPa or more and the crack evaluation result (total number of positions where LME cracking was confirmed) after the welding test was 3 or less was regarded as an invention example having high strength and excellent LME resistance. The tensile strength TS may be further 1500 MPa or more, further 1550 MPa or more, and further 1600 MPa or more. In contrast, a comparative example was regarded as a case where at least one of the strength and the crack evaluation result after the welding test did not satisfy the above evaluation criteria. In the present embodiment, the case where the Fe concentration in the plated layer was 65 mass% or less was evaluated as excellent in corrosion resistance. The measurement results of the Fe concentration in the plated layer are also shown in Table 8. [Table 8]Experiment No.State of plating after hot stampPropertiesC concentration at base steel / plating interface [Cf]Bulk C concentration [Cb]Tensile strength TSCracking evaluation result after welding test *Fe concentration in plated layerCorrosion resistance(mass%)(mass%)(MPa)(mass%)10.220.221618044○20.290.221618143○30.240.221611149○40.300.221647453○50.320.221642550○60.10 * 7< 0.085Approximately 1300 * 4< 060 * 6< ○70.100.08512220 * 5< 59○8-0.3551925344 or less * 8< ○9-0.3571875353 or less * 9< ○10-0.3571935653 or less * 9< ○* The total number of positions where cracks having a length of 300 µm or more were confirmed at a total of 120 positions. *4 Assumed strength estimated from Experiment No. 7 *5 Number of cracks estimated from Experiment No. 6 *6 Fe concentration in plated layer estimated from Experiment No. 7 *7 C concentration at base steel / plating interface estimated from Experiment No. 7 *8 Although no measurement was made, the Fe% after hot stamp is largely determined by the basis weight before hot stamp and the Fe% as long as the hot stamp conditions are the same. Therefore, Experiment No. 1 (basis weight: 120 g / m2 and Fe% before hot stamp: 8.8%) was referred to. Fe% before hot stamp in Experiment No. 8 is the same as in Experiment No. 1, but, because of a large basis weight, is assumed to be lower than the Fe% after hot stamp measured in Experiment No. 1. Therefore, 44% or less was set. *9 Although no measurement was made, Experiment No. 4 (basis weight: 113 g / m2 and Fe% before hot stamp: 12.0%) was referred to. In both Experiment Nos. 9 and 10, the Fe% was equivalent to that in Experiment No. 4, but, because of large basis weight, the Fe% after hot stamp was assumed to be less than 53% of that in Experiment No. 4. Therefore, 53% or less was set.

[0116] From the above results, the following is found. Experiment Nos. 1 to 3, 8, and 9 satisfy the component composition and the production conditions (reduction annealing conditions) specified in the present embodiment, and as a result, the tensile strength (TS) after hot stamp satisfies 1470 MPa or more. Further, the carbon concentration of the surface layer of the steel sheet before hot stamp satisfied the formula (1), and excellent LME resistance with three or less positions of LME cracking after welding was exhibited. In this example, the LME resistance was evaluated by the welding test after hot stamp, but the LME resistance is a property required also at the time of hot stamp at a high temperature. That is, LME resistance is also required for a hot-dip galvanized steel sheet before hot stamp. In Experiment Nos. 1 to 3, 8, and 9, hot stamp was performed favorably, and therefore it can be said that the hot-dip galvanized steel sheets for hot stamp in these examples are also excellent in LME resistance.

[0117] In contrast, Experiment Nos. 4 to 7 and 10 did not satisfy at least one of the component composition and the production conditions (reduction annealing conditions) specified in the present embodiment, and desired properties were not obtained.

[0118] Experiment Nos. 4 and 5 satisfied the component composition specified in the present embodiment, and the tensile strength after hot stamp was 1470 MPa or more. However, in these examples, the production conditions (reduction annealing conditions) were not satisfied, and the dew point at the time of reduction annealing was -20°C or less. As a result, the carbon concentration of the surface layer of the steel sheet before hot stamp did not satisfy the formula (1), and therefore the total number of cracks found in the evaluation of LME resistance was 4 or more, resulting in poor LME resistance.

[0119] In Experiment Nos. 6 and 7, the amount of C in the component composition was less than the range specified in the present embodiment, and thus the tensile strength (TS) after hot stamp was less than 1470 MPa. Experiment Nos. 6 and 7 did not satisfy the production conditions (reduction annealing conditions), and the carbon concentration of the surface layer of the steel sheet before hot stamp did not satisfy the formula (1), but the total number of positions of LME cracking was 3 or less in the evaluation of LME resistance. The reason is that the components of the bulk, in particular carbon, were rather low.

[0120] Experiment No. 10 satisfied the component composition specified in the present embodiment, and the tensile strength after hot stamp was 1470 MPa or more. However, the production conditions (reduction annealing conditions) were not satisfied, and the dew point at the time of reduction annealing was -20°C or less. As a result, the carbon concentration of the surface layer of the steel sheet before hot stamp did not satisfy the formula (1), and therefore the total number of cracks found in the evaluation of LME resistance was 4 or more, resulting in poor LME resistance.

[0121] In Table 8, some estimated values are included in Experiment Nos. 6 to 10, and thus this point will be described below. First, the C concentration at the base steel / plating interface after hot stamp in Experiment No. 6 was not measured. However, in Experiment No. 6, the component composition is the same as that in Experiment No. 7, and the reduction annealing conditions are similar. In addition, the hot stamp heating conditions are almost the same, and thus it is assumed that the C concentration at the base steel / plating interface is equivalent to that in Experiment No. 7. Therefore, the value is described assuming that the C concentration at the base steel / plating interface after hot stamp in Experiment No. 6 is substantially the same as that in Experiment No. 7.

[0122] In addition, in Experiment No. 6, the tensile strength after hot stamp is not measured. However, Experiment No. 6 has the same component composition as Experiment No. 7, and the reduction annealing conditions are also similar. Furthermore, the hot stamp heating condition and the shot blasting condition are almost the same. In contrast, the molding start temperature at the time of hot stamp in Experiment No. 6 is higher than that in Experiment No. 7, and thus it is assumed that the cooling rate after hot stamp is higher than that in Experiment No. 7. Therefore, it is assumed that the martensite microstructure formed is also hardened. In addition, assuming that the strength is increased by about 80 MPa, the strength (approximately 1300 MPa) estimated based on the value of Experiment No. 7 is described.

[0123] Further, in Experiment No. 6, the Fe concentration in the plated layer after hot stamp is not measured. However, Experiment No. 6 has the same component composition as Experiment No. 7 and the reduction annealing conditions are similar, assuming that the decarburized layer formed is the same. Further, the hot stamp heating condition and the shot blasting condition are the same. In contrast, the coating weight of Experiment No. 6 is smaller than that of Experiment No. 7, assuming that the Fe concentration in the plated layer is higher than that of Experiment No. 7 by approximately 1%. Therefore, the Fe concentration in the plated layer after hot stamp of Experiment No. 6 is assumed to be approximately 60% based on the value of Experiment No. 7, and the assumed value is described.

[0124] In Experiment No. 7, LME cracking evaluation was not performed. However, Experiment No. 7 has the same component composition as Experiment No. 6 and the reduction annealing conditions are similar, assuming that the decarburized layer formed is the same. Further, the hot stamp heating condition and the shot blasting condition are the same. In addition, it is assumed that there is no large difference in the assumed Fe concentration in the plated layer after hot stamp. Therefore, the LME crack of Experiment No. 7 is assumed to be 0 as in Experiment No. 6, and thus the assumed value is described.

[0125] In Experiment No. 8, the Fe concentration in the plated layer after hot stamp is not measured. However, the Fe concentration in the plated layer after hot stamp is largely determined by the coating weight (basis weight) before hot stamp and the Fe concentration in the plated layer as long as the hot stamp conditions are the same. Herein, Experiment No. 1 (coating weight (basis weight): 120 g / m 2< , Fe concentration in plated layer: 8.8 mass%) was referred to. The Fe concentration in the plated layer before hot stamp in Experiment No. 8 was equivalent to that in Experiment No. 1, whereas the coating weight (basis weight) in Experiment No. 8 was larger than that in Experiment No. 1. Therefore, it is assumed that the Fe concentration in the plated layer after hot stamp in Experiment No. 8 is lower than the value (44 mass%) measured in Experiment No. 1. Thus, as an assumed Fe concentration in the plated layer after hot stamp in Experiment No. 8, 44 mass% or less was described in Table 8.

[0126] In Experiment Nos. 9 and 10, the Fe concentration in the plated layer after hot stamp is not measured. However, as described above, the Fe concentration in the plated layer after hot stamp is largely determined by the coating weight (basis weight) before hot stamp and the Fe concentration in the plated layer as long as the hot stamp conditions are the same. Herein, Experiment No. 4 was referred to. The Fe concentration in the plated layer before hot stamp in Experiment Nos. 9 and 10 was equivalent to that in Experiment No. 4, whereas the coating weight (basis weight) in Experiment Nos. 9 and 10 was larger than that in Experiment No. 4. Therefore, it is assumed that the Fe concentration in the plated layer after hot stamp in Experiment Nos. 9 and 10 is lower than the value (53 mass%) measured in Experiment No. 4. Thus, as an assumed Fe concentration in the plated layer after hot stamp in Experiment Nos. 9 and 10, 53 mass% or less was described in Table 8.

[0127] In all of Experiments Nos. 1 to 3, 8, and 9, the coating weight was 45 g / m 2< or more and the Fe concentration in the plated layer after hot stamp was 65 mass% or less, and thus it is considered that excellent corrosion resistance was exhibited.

[0128] This application claims priority based on JP-2023-056200 filed on March 30, 2023 and JP-2024-022727 filed on February 19, 2024. JP-2023-056200 and JP-2024-022727 are incorporated herein by reference.REFERENCE SIGNS LIST

[0129] 1Sample after welding test (welding test piece) 2Test piece for LME cracking evaluation 3Nugget 4Cut surface 5Observation surface aCentral portion of steel sheet bEnd portion of steel sheet QVoid RInternal cracking

Claims

1. A galvanized steel sheet for hot stamp, wherein a component composition of a base steel sheet satisfies: C: 0.15 to 0.50 mass%; Si: 0.02 to 2.5 mass%; Mn: 0.5 to 5 mass%; P: 0.03 mass% or less (including 0 mass%); S: 0.02 mass% or less (including 0 mass%); Al: 0.010 to 1 mass%; Ti: 0.005 to 0.080 mass%; and B: 0.0005 to 0.005 mass%, and with the balance being Fe and inevitable impurities, when elemental analysis is performed in a thickness direction of a plated layer from a surface of the plated layer by glow discharge optical emission spectrometry (GD-OES), a carbon concentration [Cf] (mass%) at a position where a concentration of Zn constituting the plated layer is 1.0 mass% and a bulk carbon concentration [Cb] (mass%) satisfy a following formula (1):         [Cf] ≤ 0.65 × [Cb] ···     (1) 2. The galvanized steel sheet for hot stamp according to claim 1, wherein a component composition of the base steel sheet satisfies one or more of following (a) and (b): (a) further including one or more elements selected from the group consisting of Cr: more than 0 mass% and 1.2 mass% or less; Mo: more than 0 mass% and 1 mass% or less; and Ca: more than 0 mass% and 0.0040 mass% or less, and (b) further including one or more elements selected from the group consisting of Nb: more than 0 mass% and 0.040 mass% or less; V: more than 0 mass% and 0.30 mass% or less; Cu: more than 0 mass% and 0.30 mass% or less; Ni: more than 0 mass% and 0.30 mass% or less; Mg: more than 0 mass% and 0.010 mass% or less; and REM: more than 0 mass% and 0.010 mass% or less.

3. A method for producing a galvanized steel sheet for hot stamp, the method comprising: an annealing step comprising retaining a hot-rolled steel sheet or a cold-rolled steel sheet satisfying the component composition according to claim 1 or 2 at 500 to 930°C for 90 to 1000 seconds in a reducing atmosphere having a dew point of -20°C to +10°C; and a subsequent galvanizing step.

4. The method for producing the galvanized steel sheet for hot stamp according to claim 3, wherein the galvanizing step is a hot-dip galvanizing step.

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