Galvanized steel sheet for hot stamping and method for producing same

Abstract

A galvanized steel sheet for hot stamping in which the component composition of a base steel sheet satisfies the following conditions: 0.15-0.50% by mass of C, 0.02-2.5% by mass of Si, 0.5-5% by mass of Mn, 0.03% by mass or less (including 0% by mass) of P, 0.02% by mass or less (including 0% by mass) of S, 0.010-1% by mass of Al, 0.005-0.080% by mass of Ti, and 0.0005-0.005% by mass of B, with the remainder being Fe and unavoidable impurities, and when elemental analysis is performed from the surface of a plating layer in the thickness direction of the plating layer by glow discharge emission spectroscopy (GD-OES), the content of Si in the plating layer in the thickness direction of the plating layer is less than 1% by mass. The carbon concentration [Cf] (mass%) at a position where the Zn concentration constituting the plating layer is 1.0 mass% and the bulk carbon concentration [Cb] (mass%) satisfy formula (1): [Cf] < = 0.65 [Cb].

Description

TECHNICAL FIELD

[0001] The present application relates to a galvannealed steel sheet for hot stamping and a method for manufacturing the same. BACKGROUND

[0002] Hitherto, passenger safety in a vehicle has been required to be improved, and the strength of a vehicle body material has been improved for this purpose. On the other hand, in the background of the exacerbation of global warming and the like, the trend toward improvement in fuel efficiency of an automobile is accelerating. It is known that the light weight of a vehicle body is effective for the improvement in fuel efficiency.

[0003] A steel sheet for hot stamping (hot forming), for example, can easily achieve the coexistence of high strength and workability (shape freezing property and the like) for the purpose of achieving the light weight of a vehicle body. In particular, at a portion where corrosion resistance is required, a molten galvannealed steel sheet for hot stamping is applied.

[0004] As the above-described steel sheet for hot stamping (hot forming), various steel sheets have been proposed so far. For example, a steel sheet in which the relationship between Ti and N is defined is disclosed in Patent Literature 1 as a hot forming steel sheet in which the strength of a joint portion at the time of spot welding is excellent, and good forming can be achieved without occurrence of breakage and cracking and the like at the time of forming in a hot state. In addition, a hot stamping steel sheet in which both the balance between strength and toughness and the stability of hardness are excellent is disclosed in Patent Literature 2, and the balance of the contents of C, Si, Mn, and Cr and the like is adjusted to achieve high strength by increasing the proportion of alloying elements.

[0005] However, if the proportion of alloying elements of a galvannealed steel sheet is increased, cracking (LME cracking) due to liquid metal embrittlement (LME) is likely to occur. In particular, when hot stamping is performed using the galvannealed steel sheet, and a part is formed, and spot welding is performed for vehicle body assembly, if LME cracking occurs, there is a problem that the joint strength of a welded portion is insufficient. In order to address this problem, for example, a technology is proposed in Patent Literature 3 in which, in a formed body obtained by hot stamping an alloyed molten galvannealed steel sheet, in order to improve weldability and formation treatment properties and in order to suppress LME, the heating time in a furnace (furnace time) needs to be about 4 minutes or more, the press production rate is poor in hot stamping compared to cold stamping, and thus it is necessary to shorten the furnace time, and in view of this, in order to increase the Fe concentration of an alloyed molten galvannealed film of an alloyed molten galvannealed steel sheet to more than 8.0 mass%, avoid Fe-Zn solid solution, and take time, the attachment amount of a plated layer (the amount of Zn is 15.0 to 40.0 g / m 2 ), is reduced to achieve an increase in the melting point of the plated layer.

[0006] PRIOR ART DOCUMENTS

[0007] Patent Literature

[0008] Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No. 2007-169679

[0009] Patent Literature 2: Japanese Patent Application Laid-Open (JP-A) No. 2019-173158

[0010] Patent Literature 3: Japanese Patent Application Laid-Open (JP-A) No. 2022-131411

[0011] Non-Patent Literature

[0012] Non-Patent Literature 1: K. K. Hayashi et al., "Behavior of Corrosion Front in Underfilm Corrosion of Zn-Fe Alloy Coated Steel Sheet", Tetsu-to-Hagane, Vol. 76 (1990), No. 9, pp. 1496-1503 SUMMARY

[0013] PROBLEMS TO BE SOLVED BY THE INVENTION

[0014] In the technology of Patent Literature 3, although LME can be suppressed, the attached amount of the plated layer is reduced, and thus the effective zinc amount that contributes to ensuring the corrosion resistance of the plated layer decreases, and the corrosion resistance improving effect that the plated layer inherently functions is difficult to exert. With regard to the corrosion resistance of the plated layer, as shown in Non-Patent Literature 1, it is known that particularly if the Fe concentration in the plated layer is higher than 65 mass%, the corrosion resistance is seriously deteriorated. Therefore, it is desired to realize suppression of LME and ensuring of corrosion resistance together while exhibiting high strength. The present invention is made in view of the above circumstances, and aims to provide a hot press-formed zinc-plated steel sheet for hot press forming that exhibits high strength (particularly tensile strength of 1.5 G or more) after hot press forming, while ensuring the corrosion resistance that the plated layer inherently has, and exhibiting excellent LME resistance, and a method for manufacturing the same.

[0015] MEANS FOR SOLVING PROBLEMS

[0016] Mode 1 of the present invention is a hot press-formed zinc-plated steel sheet, in which

[0017] The component composition of the base steel sheet satisfies:

[0018] C: 0.15 to 0.50 mass%,

[0019] Si: 0.02 to 2.5 mass%,

[0020] Mn: 0.5 to 5 mass%,

[0021] P: 0.03 mass% or less (including 0 mass%),

[0022] S: 0.02 mass% or less (including 0 mass%),

[0023] Al: 0.010 to 1 mass%,

[0024] Ti: 0.005 to 0.080 mass% and

[0025] B: 0.0005 to 0.005 mass%,

[0026] the balance being Fe and unavoidable impurities,

[0027] When the element analysis is performed from the surface of the plated layer in the thickness direction of the plated layer by a glow discharge optical emission spectrometry (GD-OES), the carbon concentration [Cf] (mass%) at the position where the concentration of Zn constituting the plated layer is 1.0 mass% and the bulk carbon concentration [Cb] (mass%) satisfy the following formula (1).

[0028] [Cf] ≤ 0.65 x [Cb]... (1)

[0029] In Mode 2 of the present application, the galvannealed steel sheet for hot stamping according to Mode 1, wherein the composition of the base steel sheet satisfies one or more of the following (a) and (b).

[0030] (a) further contains 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.

[0031] (b) further contains 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.

[0032] Mode 3 of the present application is a method for producing a galvannealed steel sheet for hot stamping, which includes an annealing step and a subsequent zinc-based plating step, the annealing step including the step of holding a hot-rolled steel sheet or a cold-rolled steel sheet satisfying the composition according to Mode 1 or 2 at 500 to 930°C for 90 to 1000 seconds in a reducing atmosphere having a dew point of -20 to +10°C.

[0033] Mode 4 of the present application is the method for producing a galvannealed steel sheet for hot stamping according to Mode 3, wherein the zinc-based plating step is a molten galvanizing step.

[0034] Effects of the Invention

[0035] According to the present application, a high-strength galvannealed steel sheet for hot stamping having excellent LME resistance and a method for producing the same can be provided. Attached Figure Description

[0036] Figure 1 This is a diagram showing the extraction location of the evaluation steel plate in an embodiment.

[0037] Figure 2 This is a graph showing the carbon distribution of the sample before hot stamping of the embodiment. The left side shows the carbon distribution of the comparative example, and the right side shows the carbon distribution of the present invention example.

[0038] Figure 3 This is an example of an EDX analysis region of a sample before hot stamping, illustrating an embodiment.

[0039] Figure 4 This is a graph showing the EDX analysis results of the sample before hot stamping in the embodiment.

[0040] Figure 5 This is a diagram illustrating the heating mode of hot stamping in an embodiment.

[0041] Figure 6 This is a graph showing the carbon distribution of the samples after hot stamping according to the embodiments. The left side shows the carbon distribution of the comparative example, and the right side shows the carbon distribution of the present invention example.

[0042] Figure 7 This is an example of an EDX analysis region of a sample after hot stamping, illustrating an embodiment.

[0043] Figure 8 This is a graph showing the EDX analysis results of the sample after hot stamping in the embodiment.

[0044] Figure 9 This is a diagram illustrating the conditions of the welding test in the embodiment.

[0045] Figure 10 This is a diagram illustrating the extraction location of the sample used in the welding test of the embodiment.

[0046] Figure 11 This is a diagram illustrating the location of cracks in the cross-section of the sample after the welding test in the embodiment.

[0047] Figure 12 This is a diagram illustrating the internal cracks and pores after the welding test of the embodiment. Detailed Implementation

[0048] The present inventors have conducted intensive research in order to obtain a hot press-formed zinc-plated steel sheet that exhibits high strength after hot press forming and also has excellent LME resistance, with the aim of forming a zinc plating layer for ensuring corrosion resistance. First, the present inventors found that, unlike a general cold-rolled steel sheet, a hot press-formed steel sheet is heated during hot press forming, and as a result, a carbon (C) enrichment phenomenon occurs at the interface between the plating layer and the base material (hereinafter referred to as the "plating layer-base material interface") (as shown by the left carbon distribution in FIG. 1, the carbon concentration [C] at the plating layer-base material interface after hot press forming: 0.303 mass%, which is higher than the carbon concentration 0.220 mass% in the bulk phase). It is known that carbon is an element that deteriorates LME resistance, and the primary cause of deterioration of LME resistance in a hot press-formed steel sheet was found to be this carbon enrichment phenomenon. Figure 6

[0049] It was thus found that, in order to suppress the carbon enrichment phenomenon at the plating layer-base material interface after hot press forming, it is important to suppress the carbon concentration at the plating layer-base material interface of the hot press-formed zinc-plated steel sheet, and specifically, it is important to satisfy the following formula (1).

[0050] [Cf] ≤ 0.65 x [Cb]... (1)

[0051] In formula (1),

[0052] [Cf] is the carbon concentration (mass%) at a position where the concentration of Zn constituting the plating layer is 1.0 mass% when performing element analysis from the surface of the plating layer in the thickness direction of the plating layer by glow discharge optical emission spectrometry (GD-OES), and [Cb] is the bulk phase carbon concentration (mass%).

[0053] In the above formula (1), the "position where the concentration of Zn constituting the plating layer is 1.0 mass% when performing element analysis from the surface of the plating layer in the thickness direction of the plating layer by glow discharge optical emission spectrometry (GD-OES)" refers to the plating layer-base material interface in the present embodiment. In other words, it refers to a region in the zinc plating layer where the concentration of zinc (Zn) is 1 mass% or more from the surface of the plating layer. The element analysis from the surface of the plating layer in the thickness direction of the plating layer by the above glow discharge optical emission spectrometry (GD-OES) is performed by the method shown in the examples.

[0054] In addition, the bulk phase carbon concentration [Cb] (mass%) refers to the carbon (C) concentration obtained by analyzing a hot press-formed steel sheet having a total sheet thickness of 50 mm x 50 mm or more by combustion-infrared absorption analysis. When obtaining the bulk phase carbon concentration, the analysis of the content of each element in the bulk phase can be performed by a conventional method, and is performed by the following method. Also, in the steel sheets produced in the examples described below, since the Si amount is higher than 0.7 mass%, the Si amount is obtained by gravimetric analysis as shown below, but when the Si amount is 0.7 mass% or less, it is recommended to perform ICP analysis. ​

[0055] <Analysis method>

[0056] • Inductively coupled plasma emission spectrometry (ICP): Si (0.7 mass% or less), Mn, P, Cu, sol-Al, Ni, Cr, Mo, V, Nb, Ti, B, Ca

[0057] • Non-flame atomic absorption spectrophotometry: Sn

[0058] • Gravimetry: Si (more than 0.7 mass%)

[0059] • Combustion-infrared absorption method: C, S

[0060] • Inert gas fusion-TCD method [N], inert gas fusion-infrared absorption method [O]: N, O

[0061] The present inventors have found that, as shown in the above formula (1), in the surface layer region of the galvannealed steel sheet before hot stamping, for example, as in the right side carbon distribution of the following Figure 2 , by reducing the carbon concentration at the interface between the plated layer and the base material to [bulk carbon concentration x 0.65] or less, that is, by providing a surface decarburized layer, it is possible to suppress the carbon concentration at the interface between the plated layer and the base material after hot stamping to 0.225 mass%, which is approximately the same as the bulk carbon concentration, as in the right side carbon distribution of the following Figure 6 . As shown in the following examples, even when this plated steel sheet is used for welding, it is possible to suppress LME cracking. It is thus considered that, by reducing the carbon in the surface layer region of the galvannealed steel sheet before hot stamping, it is possible to suppress the carbon densification phenomenon after hot stamping, as a result of which the improvement effect on LME resistance is exhibited. In the surface layer region of the galvannealed steel sheet before hot stamping, the carbon concentration at the interface between the plated layer and the base material 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 improving LME resistance, the smaller the ratio is, the better. Also, if manufacturing conditions, the mechanical properties of the steel sheet after hot stamping, and the like are taken into consideration, the lower limit of the ratio of the carbon concentration at the interface between the plated layer and the base material to the bulk carbon concentration can be around 0.01.

[0062] The reduction of the carbon concentration at the interface between the plated layer and the base material in the above galvannealed steel sheet before hot stamping can be achieved by manufacturing under the following conditions.

[0063] [Component composition]

[0064] The following describes the composition of the base steel sheet (the portion excluding the plated layer of the plated steel sheet) in the hot press forming galvannealed steel sheet of the present embodiment. The hot press forming galvannealed steel sheet of the present embodiment can ensure a strength of 1.5 GPa or more after hot press forming by satisfying the following composition, and can also achieve improvements in part productivity and platability, and improvements in LME resistance.

[0065] [C: 0.15 to 0.50 mass%]

[0066] C is an element effective for increasing the strength of the steel sheet, and in order to achieve a strength of TS: 1470 MPa or more after hot press forming, the C amount needs to be 0.15 mass% or more. The C amount is preferably 0.18 mass% or more, and more preferably 0.20 mass% or more. On the other hand, if the C amount is higher than 0.50 mass%, the strength of the original sheet such as a hot-rolled steel sheet tends to increase more than necessary, and the weldability deteriorates. C is also an element that adversely affects LME resistance. Therefore, the C amount is 0.50 mass% or less, preferably 0.40 mass% or less, more preferably 0.38 mass% or less, and further preferably 0.35 mass% or less.

[0067] [S1: 0.02 to 2.5 mass%]

[0068] Si has an effect of suppressing the self-tempering of martensite, and is therefore an element effective for increasing the hardness stability and further ensuring the productivity during part production. In addition, by containing Si in a small amount, an effect of suppressing non-plating during steel sheet production can be exerted. In order to exert this effect, the Si amount is made 0.02 mass% or more. The Si amount is preferably 0.1 mass% or more, more preferably 0.5 mass% or more, and further preferably 0.7 mass% or more. On the other hand, if the Si amount is higher than 2.5 mass%, the Ac3 point rises, and therefore the hot press forming heating temperature during part production rises. In addition, Si is an element that adversely affects LME resistance. The reason for this is not clear, but according to the results of research by the present inventors and others, it is believed to be because it lowers the melting point of the plated layer by dissolving in the plated layer. From these viewpoints, the Si amount is made 2.5 mass% or less. The Si amount is preferably 2.4 mass% or less, and more preferably 2.2 mass% or less.

[0069] [Mn: 0.5 to 5 mass%]

[0070] Mn is an element necessary to ensure quenchability and achieve a strength of TS: 1470 MPa or more after hot stamping. Therefore, the amount of Mn is 0.5 mass% or more. The amount of Mn is preferably 1.0 mass% or more, and more preferably 1.5 mass% or more. On the other hand, if the amount of Mn is excessive, the strength of the original plate such as a hot-rolled steel sheet increases more than necessary. In addition, Mn is also an element that deteriorates LME resistance. Therefore, the amount of Mn is 5 mass% or less. The amount of Mn is preferably 4.5 mass% or less, and more preferably 4.0 mass% or less.

[0071] [P: 0.03 mass% or less (including 0 mass%)]

[0072] P is an element that is inevitably present as an impurity element. P adversely affects toughness and delayed fracture resistance. Therefore, P is preferably as little as possible, and the content of P is 0.03 mass% or less, and preferably 0.010 mass% or less. Also, in the present specification, the phrase "0 mass%" is meant to include the case where the content is below the level of an unavoidable impurity (not excluding the case where it is intentionally added) without the intention of adding it.

[0073] [S: 0.02 mass% or less (including 0 mass%)]

[0074] S is an element that is inevitably present as an impurity element. S adversely affects toughness and delayed fracture resistance. Therefore, S is preferably as little as possible, and the content of S is 0.02 mass% or less, and preferably 0.010 mass% or less, and more preferably 0.005 mass% or less.

[0075] [Al: 0.010 to 1 mass%]

[0076] Al is an element that functions as a deoxidizer. In order to exert this effect, the content of Al is 0.010 mass% or more. The content of Al is preferably 0.015 mass% or more. On the other hand, if Al is excessively contained in the steel sheet, the hardness after die cooling decreases. In addition, Al2O3 is excessively generated to deteriorate low-temperature toughness. Therefore, the content of Al is 1 mass% or less. The content of Al is preferably 0.8 mass% or less, and more preferably 0.1 mass% or less. Also, the content of Al here means the content of Al in a solid solution state (sol. Al).

[0077] [Ti: 0.005 to 0.080 mass%]

[0078] Ti has an effect of precipitating N, which adversely affects solid solution of B, to be harmless in order to exhibit the effect of B described later. In addition, Ti is contained together with B, which increases the solid solution amount of B, and has an effect of improving the resistance to LME cracks. 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 further preferably 0.020 mass% or more. On the other hand, if the amount of Ti is excessive, precipitation of carbides, grain refinement, and the like occur, the strength of the original plate of the hot-rolled steel sheet and the like is unnecessarily increased, the workability is poor, and the like, which adversely affect the economy. 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.

[0079] [B: 0.0005 to 0.005 mass%]

[0080] B is an element necessary to ensure quenchability and achieve a strength of TS: 1470 MPa or more after hot stamping. In addition, it has an effect of improving the resistance to LME cracks of the welded portion at the time of welding. The factor that this effect is exhibited is not clear, but according to the research results of the present inventors and the like, it is considered that it is because it has an effect of strengthening the original γ grain boundary that is a crack initiation point. In order to exhibit this effect, 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. On the other hand, if the amount of B is excessive, the strength of the original plate of the hot-rolled steel sheet and the like is unnecessarily increased, the workability is poor, and the like, which adversely affect the economy. 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.

[0081] The galvannealed steel sheet of the embodiment of the present application contains the above-described composition, and in one embodiment of the present application, it is preferable that the balance be iron and inevitable impurities. As the inevitable impurities, the mixing of elements due to the conditions of raw materials, materials, manufacturing equipment, and the like is allowed. N is also an element that is inevitably present as an impurity element, and for example, can be contained in a range of 0.0100 mass% or less (including 0 mass%). Also, for example, P and S are elements that are generally better the less the content, and thus are inevitable impurities, but there are elements for which the composition range is separately specified as described above. Therefore, in the present specification, the "inevitable impurities" that constitute the balance are a concept that excludes elements for which the composition range is separately specified.

[0082] The component composition of the steel sheet of the present embodiment can not include the elements described below. Any other element can be further included as long as the desired properties are maintained. The elements described below can be included as needed to achieve, for example, an improvement in delay fracture resistance, a further improvement in LME resistance, and the like.

[0083] [(a) further includes 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]

[0084] Cr is an element that ensures quenchability and enables further improvement in resistance to LME cracks. To exert this effect, the Cr content is preferably more than 0 mass%, and more preferably 0.05 mass% or more. On the other hand, if the amount of Cr is excessive, pickling properties and the like required in the manufacturing process are deteriorated. In addition, the strength of the original plate such as a hot-rolled steel sheet is undesirably increased. Therefore, the amount of Cr is preferably 1.2 mass% or less, and more preferably 1.0 mass% or less.

[0085] Mo has an effect of promoting the diffusion of B and suppressing the susceptibility to LME. From the viewpoint of exerting this effect, the Mo content is preferably more than 0 mass%, and more preferably 0.05 mass% or more. On the other hand, if the Mo content is excessive, the strength of the original plate such as a hot-rolled steel sheet is undesirably increased beyond necessity, and the workability is poor, which adversely affects the economy. Therefore, the Mo content is preferably 1 mass% or less.

[0086] Ca is an element that suppresses the generation of MnS, which adversely affects the delay fracture resistance, and improves the delay fracture resistance. From the viewpoint of exerting this effect, the Ca content is preferably more than 0 mass%, and more preferably 0.001 mass% or more. On the other hand, even if the amount of Ca is excessive, the effect is saturated, and the cost increases, which adversely affects the economy. Therefore, the Ca content is 0.0040 mass% or less.

[0087] [(b) further includes 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]

[0088] Nb forms fine carbides, and makes the structure of the steel fine by pinning effect, which contributes to the increase in strength and toughness. In order to exert this effect, the content of Nb is preferably higher than 0 mass%, and more preferably 0.0008 mass% or more. On the other hand, if Nb is excessively contained in the steel sheet, coarse carbides are formed, which become a starting point of damage and cause deterioration of toughness. Therefore, the content of Nb is preferably 0.040 mass% or less.

[0089] V forms fine carbides, and makes the structure of the steel fine by pinning effect, which contributes to the increase in strength and toughness. In addition, by precipitating at the time of tempering, it also has a secondary effect. In order to exert this effect, the content of V is preferably higher than 0 mass%, and more preferably 0.008 mass% or more. On the other hand, if V is excessively contained in the steel sheet, coarse carbides are formed, which become a starting point of damage and cause deterioration of toughness. Therefore, the content of V is preferably 0.30 mass% or less.

[0090] Cu and Ni are effective elements for improving the resistance to delayed fracture of the member, and can be contained as necessary in an amount higher than 0 mass%. On the other hand, if Cu and Ni are excessively contained in the steel sheet, they can become the cause of flaws on the surface of the steel sheet, and finally on the surface of the member. Therefore, the content of Cu and Ni alone is preferably 0.30 mass% or less, and more preferably the total content is 0.50 mass% or less.

[0091] Mg and REM make the inclusions in the steel sheet fine, and have an effect of preventing cracks in hot forming caused by inclusions, and therefore can be contained as necessary. When contained, the content of each element is preferably higher than 0 mass%, and more preferably 0.0008 mass% or more. On the other hand, these elements are saturated in effect when excessively contained, and cause an increase in cost. Therefore, the content of any of the elements is preferably 0.010 mass% or less, and more preferably 0.008 mass% or less. Also, the REM means lanthanoid elements (15 elements from La to Lu), Sc (scandium), and Y (yttrium).

[0092] The type of the galvannealed steel sheet of the hot-stamped galvannealed steel sheet of the present embodiment is not limited. Examples include Fe-Zn plated steel, Al-Zn plated steel, and the like. The Fe concentration in the plated layer of the hot-stamped galvannealed steel sheet of the present embodiment is preferably 40 mass% or less, and more preferably 30 mass% or less. As the galvannealed steel sheet, specifically, examples include molten galvanized steel sheet (GI), alloyed molten galvanized steel sheet (GA), electro-galvanized steel sheet (EG), and the like.

[0093] The attached amount of the galvannealing (particularly, molten galvanizing, alloyed molten galvanizing) of the hot-stamped galvannealed steel sheet of the present embodiment is preferably 45 g / m 2 More preferably, it is 60 g / m2 Further preferably, the Fe concentration in the zinc-based plated layer is 65 g / m2 or less. 2 On the other hand, from the viewpoint of easily achieving the recommended Fe concentration in the plated layer, the amount of zinc plating is preferably small. Therefore, the amount of zinc plating is preferably 190 g / m2 or less. 2 More preferably, the amount of zinc plating is 180 g / m2 or less. 2 Further preferably, the amount of zinc plating is 170 g / m2 or less.

[0094] The Fe concentration in the zinc-based plated layer of the hot-stamped zinc-based plated steel sheet of the present embodiment is preferably 65 mass% or less, more preferably 60 mass% or less, after hot stamping using the method described in the Examples described later. Also, from the viewpoint that the Fe concentration in the plated layer increases by hot stamping, the Fe concentration can be 20 mass% or more.

[0095] According to the present embodiment, without reducing the amount of plated layer or increasing the Fe concentration in the molten zinc plated layer, by satisfying the prescribed composition and providing the prescribed surface decarburized layer, the LME resistance can be improved as described above.

[0096] [Method for manufacturing hot-stamped zinc-based plated steel sheet]

[0097] Next, the method for manufacturing the hot-stamped zinc-based plated steel sheet of the present embodiment will be described.

[0098] The method for manufacturing the hot-stamped zinc-based plated steel sheet of the present embodiment includes an annealing step and a subsequent zinc-based plating step, the annealing step including the step of holding a hot-rolled steel sheet or a cold-rolled steel sheet satisfying the prescribed composition in a reducing atmosphere having a dew point of -20°C to +10°C at 500 to 930°C for 90 to 1000 seconds.

[0099] First, the features of the manufacturing method of the present embodiment, i.e., the annealing step and the subsequent zinc-based plating step will be described. Hereinafter, as an example, the method of the annealing step and the subsequent zinc-based plating step (particularly, the molten zinc plating step) of the present embodiment will be described for a molten zinc plating line in a reducing furnace system, but is not limited thereto. The method of the present embodiment is not intended to be limited to the above-described system, and for example, the above-described molten zinc plating line can also be performed on a continuous annealing line in an oxidizing furnace system.

[0100] (Annealing step)

[0101] The annealing process of a galvannealing line is generally composed of a reduction furnace and a cooling belt. In the present embodiment, it is characterized by appropriately controlling the annealing conditions in the reduction furnace, particularly the dew point of the reducing atmosphere. The original sheet is introduced into the reduction furnace. The original sheet introduced into the reduction furnace can also be subjected to a pretreatment process described later, such as degreasing, as needed. The original sheet introduced into the reduction furnace can also be subjected to oxidation treatment by being introduced into an oxidation furnace after the pretreatment process, as needed.

[0102] In the reduction 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. By setting the dew point within this range, decarburization of the surface layer of the steel sheet occurs, and a desired surface layer region can be obtained. The dew point of the reducing atmosphere is preferably -15°C or higher, and more preferably -10°C or higher. In addition, the dew point of the reducing atmosphere is preferably +5°C or lower, and more preferably 0°C or lower.

[0103] The control of the dew point described above can be performed, for example, by a method in which water vapor gas is introduced into the furnace to mix with the atmosphere gas, a method in which the atmosphere gas is made to bubble to mix in water vapor, or the like. The reducing atmosphere is not particularly limited as long as it satisfies the dew point described above and is reducing. For example, in an H2-N2 mixed gas, the H2 concentration is preferably set to 1 to 30% by volume, so that the dew point described above is satisfied.

[0104] In addition, the annealing temperature is set to 500 to 930°C, and the residence time at this annealing temperature, that is, the annealing time is set to 90 to 1000 seconds. The annealing treatment within the temperature range described above is also referred to as soaking treatment, in which case the annealing temperature is referred to as the soaking temperature, and the annealing time is referred to as the soaking time.

[0105] The annealing temperature is preferably 530°C or higher, more preferably 560°C or higher, and further preferably 600°C or higher. The annealing temperature is preferably 900°C or lower, and more preferably 870°C or lower. The annealing time is preferably 100 seconds or longer, and more preferably 120 seconds or longer. The annealing time is preferably 900 seconds or shorter, and more preferably 700 seconds or shorter, further preferably 500 seconds or shorter, more further preferably 400 seconds or shorter, and more further preferably 350 seconds or shorter. The annealing time can be controlled by the speed at which the original sheet passes through the reduction furnace (hereinafter also referred to as "line speed" or simply "LS"). By "residence for 90 to 1000 seconds at 500 to 930°C", it means that the residence is for 90 to 1000 seconds at the annealing temperature within the range of 500 to 930°C, and the temperature can be constant or can vary within the range of the annealing temperature described above.

[0106] According to the production method of the present embodiment, by subjecting a hot-rolled steel sheet or a cold-rolled steel sheet, which is an original sheet having the composition described above, particularly to the annealing described above, a prescribed surface decarburized layer can be provided.

[0107] The pretreatment of the original sheet before the annealing process will be described. In order to remove oil (grease) and dirt adhering to the surface of the original sheet, a pretreatment is usually performed, and an example of the pretreatment is alkaline degreasing. The degreasing solution used for the alkaline degreasing contains an alkali, and sodium hydroxide, silicate, or a mixture thereof is preferably used, but is not particularly limited as long as it can remove grease and the like as a water-soluble soap. In addition, in order to improve the degreasing property, electrolytic cleaning, washer treatment, and treatment of a surfactant and a chelating agent added to the degreasing solution 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 properly degreased, and the above treatments can be performed alone or in any combination.

[0108] The original sheet of the reduction furnace can be cooled in a cooling belt. The cooling belt can be composed of a slow cooling belt, a rapid cooling belt, and a conditioning belt. The conditioning belt is also called a holding belt. The cooling is performed in a manner to avoid non-plating, and can be performed under conditions usually employed, for example, a method of cooling by blowing a gas of a reducing atmosphere to the steel sheet.

[0109] On a reduction furnace type molten zinc plating line, in general, a pretreatment process, an annealing process, and a plating process can be distinguished. In the plating process, an alloying treatment can be performed as needed.

[0110] Further, from the viewpoint of energy saving, the pretreated original sheet can be preheated in a preheating furnace using a reducing or oxidizing atmosphere of exhaust gas before being introduced into the reduction furnace after the pretreatment process.

[0111] (Zinc-based plating process)

[0112] The above annealing process (continuous annealing process) is followed by a zinc-based plating process. Hereinafter, as an example of the zinc-based plating process, a molten zinc plating process will be described. A molten zinc plated steel sheet (GI) is produced by the molten zinc plating process. Alternatively, the above GI can be alloyed to produce an alloyed molten zinc plated steel sheet (GA).

[0113] The above molten zinc plating process is not particularly limited, and a method usually employed can be employed. For example, the temperature of the molten zinc plating bath can be controlled to about 430 to 500°C. The deposition amount of the molten zinc plating layer (the same as the deposition amount of the alloyed molten zinc plating layer described below), from the viewpoint of ensuring corrosion resistance, is preferably 45 g / m 2 More preferably, it is 60 g / m 2 Further preferably, it is higher than 65 g / m 2 On the other hand, from the viewpoint of easily achieving the recommended Fe concentration in the plating layer, the deposition amount of the molten zinc plating layer (particularly, the alloyed molten zinc plating layer) is preferably small. Therefore, the deposition amount of the molten zinc plating layer is preferably 190 g / m 2 More preferably, it is 180 g / m 2 Further preferably, it is 180 g / m

[0114] The alloying treatment is not particularly limited, and a method commonly used can be employed. In the alloying treatment, in the case where the Fe concentration in the plated layer is increased, the alloying temperature can be controlled to, for example, about 400 to 700°C. The alloying temperature is further 430°C or higher, further 440°C or higher, and still further 450°C or higher. On the other hand, if 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 lower, and more preferably 650°C or lower.

[0115] The process after the plating process is not particularly limited, and a method commonly used can be employed. Generally, a finishing treatment, a temper-rolling treatment, oiling, or the like can be performed, and these treatments can be performed with conditions commonly used as needed, or can not be performed if not needed. The zinc-plated steel sheet (GI or GA) thus obtained is suitable for use as a hot-stamping steel sheet.

[0116] Instead of the above-described molten zinc plating, electroplating can also be performed. For example, after the above-described annealing is performed on a raw sheet, electroplating is performed, and a zinc-plated steel sheet having a plated layer-substrate interface defined in the present embodiment can be obtained.

[0117] (Other Processes)

[0118] The production method of the present embodiment includes the defined annealing process and the subsequent zinc-based plating process, and other processes are not limited and can include processes commonly performed. Thus, regardless of the production method of the hot-rolled steel sheet or the cold-rolled steel sheet used in the annealing process, for example, the hot-rolled steel sheet or the cold-rolled steel sheet can be produced in the following manner. First, a slab is produced. In the slab production process, a steel is melted following a conventional method, and the molten steel is poured into a mold to be continuously cast, thereby obtaining a slab. In this process, the composition of the steel is adjusted at the time of melting in a manner to satisfy the above-described composition range. After the casting and before the hot-rolling, a heating process can also be provided to avoid occurrence of cracks in the hot-rolling (this process is different from the slab heating process in the hot-rolling process described below). The conditions of the above-described heating are not particularly limited, and conditions commonly used can be appropriately employed, but it is generally desired to be performed at a temperature of 1100°C to 1300°C.

[0119] Next, hot-rolling is performed. In the hot-rolling process, first, the slab is arranged in a heating furnace to be heated to a predetermined temperature (approximately 1100°C to 1300°C, for example, 1200°C), and maintained at the heating temperature for a predetermined time (for example, 30 minutes).

[0120] Next, the slab in the heated state is placed upstream of a hot rolling line, and is caused to travel in the downstream direction while passing successively between the rolls of the rolling stands of the roughing mill and the finishing mill, whereby the slab is rolled to a prescribed sheet thickness. The hot-rolled steel sheet is cooled to a prescribed temperature by a cooling device, and is coiled by a coiler after the hot rolling.

[0121] The hot-rolled steel sheet can also be a hot-rolled pickled steel sheet that is subjected to pickling by a pickling process after the hot rolling. In the pickling process, at least the hot-rolled scale is removed by pickling. The hot-rolled pickled steel sheet can also be cold-rolled as needed. In the cold rolling process, the hot-rolled steel sheet is further subjected to rolling to further reduce the sheet thickness. Specifically, the hot-rolled pickled steel sheet is caused to pass between the rolls of a rolling stand, whereby the hot-rolled pickled steel sheet is further thinned. The cold-rolled steel sheet is particularly suitable for automobile parts for the purpose of weight reduction of automobiles and the like. The base steel sheet that constitutes the galvanized steel sheet is desirably a cold-rolled steel sheet from the viewpoint of dimensional accuracy and flatness. The above-described hot-rolled steel sheet (including the hot-rolled pickled steel sheet) or the cold-rolled steel sheet (hereinafter, these will be collectively referred to as "original sheet") is subjected to the above-described annealing process and zinc-based plating process, for example, a continuous plating process by a reducing furnace.

[0122] Example

[0123] Hereinafter, embodiments of the present application will be described more specifically by citing examples. The embodiments of the present application are not limited by the following examples, and can be modified as appropriate within a range that can achieve the aforementioned and hereinafter described objects, and these are included in the technical scope of the embodiments of the present application.

[0124] After a steel material having the composition shown in Table 1 was smelted by a converter, a slab was manufactured 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 coiling temperature: 500°C to 700°C, and subjected to scale removal by a pickling process, and then cold-rolled to obtain a cold-rolled steel sheet. The cold-rolling rate 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 this example, the cold-rolled steel sheet was used as an original sheet to manufacture a plated steel sheet. Also in Table 1, the values underlined indicate a departure from the range prescribed by the embodiments. The same applies to the following tables. Steel No. 2 in Table 1 is a comparative example steel because the carbon (C) is too low.

[0125] [Table 1]

[0126]

[0127] The obtained cold-rolled steel sheets were subjected to reduction annealing on a melt-galvanizing annealing line under the conditions (soaking temperature (annealing temperature), soaking time (annealing time), dew point) described in Table 2. In Experiments No. 1-7, after bath immersion, alloying was performed under the conditions (alloying temperature, alloying time) described in Table 2, resulting in alloyed melt-galvanized steel sheets (GA steel sheets) with an approximate width of 1000 mm and alloyed melt-galvanized coating on both sides. These GA steel sheets are also used for hot stamping. Sometimes, these GA steel sheets are referred to as "steel sheets before hot stamping." Furthermore, in Experiments No. 8-10, a 150 mm × 80 mm sample was used for coating (decarburization) annealing. If this sample was used, its decarburization state and strength were confirmed to be comparable to the aforementioned steel sheets. Also, in Experiments No. 8-10, the coating adhesion was not measured. This is because the material annealed under the same conditions was evaluated separately and found to be 180 g / m². 2 Therefore, in experiments No. 8 to 10, it was also assumed that the same amount of coating (mass per unit area) was attached, which is shown in Table 2 as 180 g / m². 2 .

[0128] Table 2

[0129]

[0130] In experiments No. 1 to 7, for the aforementioned GA steel plates, from Figure 1 Evaluation steel plates with dimensions of 150mmW×70mmL or 220mmW×150mmL were extracted from the central part (width range of W / 4 to 3W / 4) a and the end (region from the outermost edge) b, respectively. The extraction locations of the evaluation steel plates for each example are shown in Table 5. In addition, in experiments No. 8 to 10, the small sample (size: 150mmW×80mmL) obtained above was cut.

[0131] Using various evaluation steel plates, the decarburization state of the surface layer and the Fe concentration in the coating of the steel plate before hot stamping were measured in the following manner.

[0132] [Measurement of the decarburization state of the surface layer (before hot stamping) (carbon distribution measured by GD-OES)]

[0133] As described below, carbon distribution was measured using GD-OES (Glow discharge optical emission spectrometry) to investigate decarbonization behavior.

[0134] (Sample preparation)

[0135] A material having a size of 50 mm x 40 mm x plate thickness 1.2 mm (total plate thickness) or 30 mm x 30 mm x plate thickness 1.2 mm (total plate thickness) or 30 mm x 40 mm x plate thickness 1.2 mm (total plate thickness) was extracted. Thereafter, the sample was prepared by degreasing as a conventional method. Then, using the sample, the concentration measurement of mass % of each element was performed by GD-OES under the following conditions.

[0136] (Measurement conditions)

[0137] Apparatus used: High-frequency glow discharge optical emission surface analyzer (rf-GD-OES) GD-Profiler 2 manufactured by Horiba, Ltd.

[0138] Sputtering method: Ordinary sputtering

[0139] Measurement range: φ 4 mm

[0140] Kind of gas: Ar

[0141] Elements to be analyzed: B, C, O, Al, Si, Ti, Cr, Mn, Fe, Zn, P, S, N (In this example, these elements were evaluated as the objects, but when elements other than the above are contained in the plated layer and the steel sheet, for example, the elements other than the above are also analyzed as the objects)

[0142] (Measurement method)

[0143] The GD-OES measurement was performed for the face of the sample on which the plated layer was formed, in the plate thickness direction until the depth reached 150 μm.

[0144] (Analysis method)

[0145] Since the sputtering rate of the apparatus was substantially constant, the sputtering pit depth of the sample after the measurement analysis was completed was taken as the value (sputtering depth) of the horizontal axis.

[0146] Details of the calibration curve method for converting the emission intensity of each element measured into the concentration will be shown below.

[0147] The relationship between the sputtering weight W of element i per unit time i (g / sec) and the emission intensity I i is represented by the slope a and the intercept b of the calibration curve, by the following formula (I).

[0148] W i = aI i + b... Formula (I)

[0149] The sputtering weight W of element i per unit time i , the concentration C i (wt. %), the density p (g / cm3 ), the sputtering speed Δd (cm / sec) is known, and the sputtering area S (cm 2 ) is known, the sputtering weight W (g) is obtained from the following equation (II).

[0150] W i = C i × p × Δd × S... Equation (II)

[0151] The sputtering weight W i is obtained from the sputtering intensity I i of the known two or more reference samples. The slope a and the intercept b of the above equation (I) are obtained, and a calibration curve is prepared in which the horizontal axis is the sputtering intensity and the vertical axis is the sputtering weight. The reference samples used are shown in Table 3 below. Using the calibration curve prepared, the sputtering weight is calculated from the sputtering intensity of each element as an object, and the concentration is converted from the weight ratio. Also, the calibration curve for the conversion of the O concentration is corrected so that the concentration ratio of Si to O is 1:2 using SiO2.

[0152] [Table 3]

[0153]

[0154] From the above analysis results, the analysis results of zinc and carbon, i.e., the distribution of zinc and carbon, are obtained. Then, from the distribution of zinc and carbon, the carbon concentration [Cf] at the position where the concentration of Zn constituting the plated layer is 1.0 mass% is obtained. Also, the C concentration [Cb] of the bulk phase is obtained from the above analysis. Then, the value of [Cf] / [Cb] is obtained. These results are shown in Table 5.

[0155] In addition, as an example of the carbon distribution, the carbon distribution of Experiment No. 4 of the existing steel as a comparative example is shown on the left side of Figure 2 , and the carbon distribution of Experiment No. 1 as an example of the present application is shown on the right side of Figure 2 . From the comparison of these carbon distributions, it is understood that the carbon concentration at the interface of the plated layer and the base steel sheet is sufficiently suppressed in the example of the present application.

[0156] Also, in Experiment No. 6, the evaluation of the C concentration at the interface of the base material plated layer was not performed. However, since the composition is the same as that of Experiment No. 7 and the reduction annealing conditions are also similar, it is considered that the C concentration at the interface of the base material plated layer is similar to that of Experiment No. 7. Therefore, the C concentration at the interface of the base material plated layer of No. 6 is estimated to be about 0.070, which is the same as that of Experiment No. 7, and this value is entered in Table 5.

[0157] [Measurement of the Fe concentration in the plated layer (before hot stamping)]

[0158] From the above GA steel sheet (steel sheet before hot stamping), a sample of 20 mm W x 10 mm L x sheet thickness was cut out and extracted. Degreasing was performed as necessary.

[0159] Preparation of Observation Sample

[0160] The L-direction cross section, that is, the surface constituted by the sheet thickness and 10 mm L as sides, was embedded in resin as an observation surface, and after polishing, gold evaporation was performed.

[0161] SEM Observation

[0162] SEM observation and EDX analysis were performed under the following conditions. Also, in the present example, the observation magnification was made 1500 times, but the observation magnification depends on the amount of plating layer attached, and therefore, for example, if the plating layer thickness is thick, the measurement magnification can be lowered, and the like, and a magnification suitable for Fe concentration measurement is selected.

[0163] (SEM Observation Conditions)

[0164] • Apparatus: Field emission type scanning electron microscope (FE-SEM) Supra-35 manufactured by Carl Zeiss Co.

[0165] • Observation image: Reflection electron image

[0166] • Observation position: Near surface layer including plating layer

[0167] • Observation magnification: 1500 times

[0168] • Number of observation fields: 1 field per sample representing the sample

[0169] EDX Analysis

[0170] • Apparatus: Energy dispersion type X-ray (EDX) detector X-max80 manufactured by Oxford Instruments Co.

[0171] • Analysis method: Surface analysis (qualitative semi-quantitative analysis)

[0172] • Analysis position: Entire plating layer, an example of analysis area is shown in Figure 3

[0173] • Number of analysis fields: 1 field per sample representing the sample

[0174] • Analysis elements: From among the elements detected by the EDX detector, the mass % of Fe ratio evaluated as Fe concentration (%) when C is excluded as a parent group. As an example, the analysis results of Experimental No. 4 are shown in Figure 4 ​The calculated results of the Fe concentration (12.0 mass%) of Example No. 4 are shown in Table 4. The Fe concentration in the plated layer of each example calculated in the same manner as Table 4 below is shown in Table 5.

[0175] [Table 4]

[0176]

[0177] [Table 5]

[0178]

[0179] [Evaluation of Steel Sheet after Hot Stamping]

[0180] In order to perform the evaluation of the steel sheet after hot stamping, hot stamping was performed on the above-described evaluation steel sheet in the following manner. First, in order to suppress carburization during hot stamping, degreasing was performed in a conventional manner. Next, hot stamping was performed in the following conditions and Figure 5 The heating mode of the following was performed to obtain the sample after hot stamping.

[0181] (Hot Stamping Conditions)

[0182] • Sample size used: 150 mm W x 70 mm L or 150 mm W x 220 mm L or 150 mm W x 80 mm L (small sample)

[0183] • Molds used: flat plate mold

[0184] <Heating Conditions>

[0185] • Atmospheric electric furnace set temperature: 910°C

[0186] • Heating time: 45 s after the plate temperature reached 870°C (during which, the plate temperature was managed so as to be between 870°C and 900°C).

[0187] <Cooling Conditions>

[0188] • After that, the sample was naturally cooled, and at the plate temperature of 550°C or 700°C, the mold was pressed flat on both sides with a flat plate mold and cooled.

[0189] • Stamping load: 0.5 MPa

[0190] • Stamping speed: 20 spm

[0191] • Pressing amount: 5 mm

[0192] • Bottom dead center holding time: held until 50°C or lower (in this case, 10 s).

[0193] Using the sample after hot stamping, the evaluation of the tensile strength TS, LME resistance, and the like was performed in the following detailed manner.

[0194] [Shot peening]

[0195] For the sample after hot stamping, the resistance value of the steel sheet was measured for the surface resistance in accordance with the method described in ISO-18594:2007(E). The shot peening was performed under the following resistance measurement conditions to achieve 2.0 mΩ or less. The respective shot peening conditions are shown in Table 6. Also, the resistance values after shot peening were not measured for Experiment Nos. 8 to 10, but since the average of the resistance values of the materials prepared under the same conditions (Experiment Nos. 1 to 5) was 0.9 mΩ, it was assumed to be substantially the same, and 0.9 (mΩ) is shown in the following Table 6.

[0196] [Shot peening conditions]

[0197] • Shot peening material: GH-3 (grid 0.3 mm)

[0198] • Shot peening pressure: about 0.4 MPa

[0199] • The shot time was managed every 150 mm x 120 mm (the details of the shot peening time are recorded in Table 6)

[0200] • Shot peening was performed on both sides

[0201] • Shot peening was performed until the resistance value of the steel sheet was 2.0 mΩ or less.

[0202] [Resistance measurement conditions]

[0203] • Three 30 mm x 30 mm x sheet thickness materials were cut and extracted.

[0204] • Electrode material: Cu-Cr

[0205] • Electrode diameter: 8 mm

[0206] • Tip R: 40 mm

[0207] • DC current value: 2 A

[0208] • Pressing force: 350 ± 17.5 x 10 N

[0209] • However, the number of measurements for one test piece was one, and a total of three measurements were performed for each test piece.

[0210] • The average value of N3 was used as the extracted value.

[0211] • For one steel sheet, the plated side was measured with the electrode clamped.

[0212] • The total resistance minus the set resistance value was used as the resistance value.

[0213] [Table 6]

[0214]

[0215] Measurement of decarburization state of surface layer after hot stamping (measurement of carbon distribution by GD-OES)

[0216] The measurement of carbon distribution by GD-OES (Glow discharge optical emission spectrometry) was performed as follows, and the decarburization behavior after hot stamping was investigated.

[0217] (Preparation of test material)

[0218] A material having a size of 30 mm x 90 mm x plate thickness or 30 mm x 70 mm x plate thickness was extracted. Thereafter, the test material was prepared by performing degreasing as a conventional method. Then, the test material was used to perform measurement of the concentration of each element in mass % by GD-OES under the following conditions.

[0219] (Measurement conditions)

[0220] Apparatus used: GD-Profiler 2 (rf-GD-OES) manufactured by HORIBA, Ltd.

[0221] Sputtering method: ordinary sputtering

[0222] Measurement range: 4 mm in diameter

[0223] Type of gas: Ar

[0224] Elements to be analyzed: B, C, O, Al, Si, Ti, Cr, Mn, Fe, Zn (in this example, these elements were evaluated as the objects, but when elements other than the above are contained in a plated layer and a steel sheet, for example, the elements other than the above are also analyzed as the objects)

[0225] (Measurement method)

[0226] The GD-OES measurement was performed in the plate thickness direction until the depth reached 100 μm for the surface on which the plated layer was formed of the test material.

[0227] (Analysis method)

[0228] The analysis was performed by the same method as the measurement of the decarburization state of the surface layer before hot stamping (measurement of carbon distribution by GD-OES) described above.

[0229] The measurement of carbon distribution by GD-OES was performed under the conditions described above using the sample after hot stamping, and the decarburization behavior was investigated. As one example, the carbon distribution of Experimental No. 4 of the existing steel as a comparative example is shown inFigure 6 of the present application, Experiment No. 1, is shown on the left side of Figure 6 of the present application. From the comparison of these carbon profiles, in Figure 6 , as the vertical double arrow shows the difference in carbon concentration at the interface between the plated layer and the base steel sheet, in the present application example, even after hot stamping, the carbon concentration at the interface between the plated layer and the base steel sheet is sufficiently suppressed. Also in Table 8, "-" in the C concentration at the base material plated layer interface means that it was not measured.

[0230] [Measurement of Fe concentration in the plated layer after hot stamping]

[0231] The measurement of Fe concentration in the plated layer after hot stamping was performed in the same manner as the measurement of Fe concentration in the plated layer before hot stamping. Also, an example of the analysis area for EDX analysis in the measurement of Fe concentration in the plated layer after hot stamping is shown in Figure 7 . As an example of the EDX analysis, the analysis results of Experiment No. 4 are shown in Figure 8 . Also, as an example, the calculation results of the Fe concentration (53.4 mass%) of Experiment No. 4 are shown in Table 7. The Fe concentrations in the plated layer of each example obtained by the same calculation as Table 7 below are shown in Table 8.

[0232] [Table 7]

[0233]

[0234] [Tensile test]

[0235] Using the sample after hot stamping, a tensile test was performed under the following conditions, and the tensile strength (TS, unit: MPa) was measured.

[0236] (Preparation of test piece)

[0237] From a material of 150 mm W x 70 mm L x sheet thickness or 150 mm W x 220 mm L x sheet thickness or 150 mm W x 80 mm L (small sample), a test piece of 150 mm W x 30 mm L was extracted from the center of the sheet, and a JIS No. 5 test piece was prepared from the test piece of 150 mm W x 30 mm L.

[0238] (Tensile test method)

[0239] Using the above JIS No. 5 test piece, the tensile strength (TS, unit: MPa) was measured by the method prescribed in JIS Z 2241 using an AG-IS 250 kN Autograph tensile testing machine manufactured by Shimadzu Corporation at a strain rate of 10 mm / min.

[0240] [Welding test]

[0241] The hot-stamped sample was cut, and a plurality of samples having a size of 30 mm x 30 mm x plate thickness were prepared for the welding test, and the welding test was performed under the conditions shown below. Figure 9 In addition, in the welding test, the welding conditions were changed as shown in Table 1. Figure 9 In Table 1, X of "XkA" in the 2nd pulse indicates 15 kinds of current conditions changed every 0.5 kA between 5.0 kA and 12.0 kA as shown below.

[0242] Welding Conditions

[0243] • Pre-spot:

[0244] Before the present welding, 20 spottings were performed at 6.5 kA for two pieces of group soft steel to make the matching of the welding head good. The other conditions of the pre-spotting and the conditions of the present welding are as follows.

[0245] Current application time: 10 cycles

[0246] Pressing force: 1.7 kN

[0247] Holding time: 1 cycle

[0248] • Main welding:

[0249] Device name: Servo pressing type spot welding device

[0250] Manufacturer: NASTOA Welding Technology Co., Ltd.

[0251] Electrode: Upper and lower symmetrical spherical radius type Cu-Cr

[0252] Electrode diameter: outer diameter φ 16 mm, tip diameter: φ 6 mm

[0253] Spot angle: 0°

[0254] Cooling water flow rate: about 2 liters / minute

[0255] Pressing force: 500 kgf

[0256] Initial pressing time: 60 cycles / 60 Hz

[0257] Ramp-up: 1 cycle / 60 Hz

[0258] First current application: current application time: 36 cycles / 60 Hz

[0259] Current value: 4.5 kA

[0260] Second current application: 18 cycles / 60 Hz

[0261] Current value: 5.0kA, 5.5kA, 6.0kA, 6.5kA, 7.0kA, 7.5kA, 8.0kA, 8.5kA, 9.0kA, 9.5kA, 10.0kA, 10.5kA, 11.0kA, 11.5kA, 12.0kA

[0262] Hold time: 10 cycles / 60Hz

[0263] <Plate Set>

[0264] • The upper and lower plates of the two-piece assembly are made of the same type of steel plate.

[0265] • Prepare test pieces for n1 second energizations under a total of 15 different current conditions.

[0266] [Evaluation of LME cracks]

[0267] From the samples (welding test pieces) after the above welding tests, LME crack evaluation test pieces are prepared in the following manner. Figure 10 This is a top view of sample 1 after a welding test, showing the upper and lower plates welded together. Test piece 2, used for LME crack evaluation, was extracted from sample 1 after the welding test. Figure 10 The oblique section (including a part of the melt core 3). During extraction, in a manner that allows for photographing the center part (diameter surface of the melt core) of the melt core 3, first, taking into account the grinding allowance, cut along the cutting surface 4, and then grind, so that the center part (diameter surface of the melt core) of the melt core becomes the observation surface 5.

[0268] Next, observation surface 5 was etched using conventional methods until the diameter of the HAZ and the melt nugget was distinguishable. Then, cross-sectional images were taken of all samples at magnification reaching the HAZ. Figure 10 (Photo taken along the direction of the hollow arrow). An example of the photo is shown. Figure 11 The photo was taken at 25x magnification. Because one experiment corresponds to 15 different current conditions, 15 samples were photographed for LME crack evaluation in one experiment.

[0269] Regarding one type of current condition, Figure 11 A total of 8 cracks were observed as shown in (1) to (8). In other words, 120 cracks were identified under 15 different current conditions × 8 locations. Then, the total number of locations with cracks longer than 300 μm was determined from these 120 locations. Furthermore, Figure 11 In (1) to (6), it is easy to observe cracks that extend from the surface roughly along the thickness direction of the plate (surface cracks). Figure 11 Cracks (7) and (8) that extend roughly toward the center of the melt nucleus (internal cracks) are easily observed. Furthermore, Figure 12 The hole Q shown is not a crack. FromFigure 12 The inner crack R extending from the hole Q shown is the statistical object.

[0270] These results are shown in Table 8. In the characteristics shown in Table 8, the tensile strength TS after hot stamping is 1470 MPa or more, and the crack evaluation result (the total number of places where LME cracks are confirmed) after the welding test is 3 or less, and in this case, the present application is excellent in high strength and LME resistance. The tensile strength TS can be further 1500 MPa or more, even 1550 MPa or more, and still further 1600 MPa or more. On the other hand, at least either one of the strength and the crack evaluation result after the welding test does not satisfy the above evaluation criteria, and in this case, it is a comparative example. Also in the present embodiment, in the case where the Fe concentration in the plated layer is 65 mass% or less, it is evaluated that the corrosion resistance is excellent. The measurement result of the Fe concentration in the plated layer is also shown in Table 8.

[0271] [Table 8]

[0272]

[0273] From the above results, the following can be known. In Experimental Nos. 1 to 3, 8 and 9, the composition and the manufacturing conditions (reduction annealing conditions) prescribed in the present embodiment are satisfied, and as a result, the tensile strength (TS) after hot stamping is 1470 MPa or more. In addition, the carbon concentration of the surface layer of the steel sheet before hot stamping satisfies formula (1), and excellent LME resistance in which the number of places where LME cracks are confirmed after welding is 3 or less is shown. Also in the present embodiment, the LME resistance is evaluated by the welding test after hot stamping, but the LME resistance is a characteristic required also at the time of hot stamping processing at a high temperature. That is, the molten galvanized steel sheet before hot stamping is also required to have LME resistance. In Experimental Nos. 1 to 3, 8 and 9 described above, since the hot stamping processing can be favorably performed, the hot stamping molten galvanized steel sheet of these examples can also be said to have excellent LME resistance.

[0274] On the other hand, in Experimental Nos. 4 to 7 and 10, at least either one of the composition and the manufacturing conditions (reduction annealing conditions) prescribed in the present embodiment is not satisfied, and the desired characteristics are not obtained.

[0275] In Experimental Nos. 4 and 5, the composition prescribed in the present embodiment is satisfied, and the tensile strength after hot stamping is 1470 MPa or more. However, in these examples, the manufacturing conditions (reduction annealing conditions) are not satisfied, and the dew point at the time of reduction annealing is -20°C or less. As a result, the carbon concentration of the surface layer of the steel sheet before hot stamping does not satisfy formula (1), and therefore, in the LME resistance evaluation, the total number of places where cracks are confirmed is 4 or more, and the result is poor LME resistance.

[0276] In Experimental Nos. 6 and 7, the C amount in the composition is lower than the range prescribed in the present embodiment, and therefore the tensile strength (TS) after hot stamping is lower than 1470 MPa. Also, in Experimental Nos. 6 and 7, the manufacturing conditions (reduction annealing conditions) are not satisfied, and the carbon concentration in the surface layer of the steel sheet before hot stamping does not satisfy Equation (1), but the total number of LME crack confirmation locations in the LME resistance evaluation is 3 or less. The reason for this can be attributed to the composition of the bulk phase, and particularly the carbon being relatively low.

[0277] Experimental No. 10 satisfies the composition prescribed in the present embodiment, and the tensile strength after hot stamping is 1470 MPa or more. However, the manufacturing conditions (reduction annealing conditions) are not satisfied, and the dew point at the time of reduction annealing is -20°C or less. As a result, the carbon concentration in the surface layer of the steel sheet before hot stamping does not satisfy Equation (1), and therefore the total number of crack confirmation locations in the LME resistance evaluation is 4 or more, which is a result of poor LME resistance.

[0278] In Table 8, a portion of the estimated values are included in Experimental Nos. 6 to 10, and therefore the following points are described. First, the C concentration at the base material coating layer interface after hot stamping in Experimental No. 6 was not measured. However, the composition of Experimental No. 6 is the same as that of Experimental No. 7, and the reduction annealing conditions are also similar. In addition, the hot stamping heating conditions are also substantially the same, and therefore it is assumed that the C concentration at the base material coating layer interface is the same as that of Experimental No. 7. Therefore, the C concentration at the base material coating layer interface after hot stamping in Experimental No. 6 is described as a value that is substantially the same as that of Experimental No. 7.

[0279] In addition, the tensile strength after hot stamping was not measured in Experimental No. 6. However, the composition of Experimental No. 6 is the same as that of Experimental No. 7, and the reduction annealing conditions are also similar. In addition, the hot stamping heating conditions and the shot blasting conditions are also substantially the same. On the other hand, the forming start temperature at the time of hot stamping in Experimental No. 6 is higher than that of Experimental No. 7, and therefore it is assumed that the cooling speed after hot stamping is also faster than that of Experimental No. 7. Therefore, it is assumed that the martensite structure formed is also hardened. In addition, it is assumed that the strength is increased by about 80 MPa or so, and the estimated strength (about 1300 MPa) is described based on the value of Experimental No. 7.

[0280] In addition, in Experimental No. 6, the Fe concentration in the coating after hot stamping was not measured. However, the composition of Experimental No. 6 is the same as that of Experimental No. 7, and the reduction annealing conditions are also similar, and it is assumed that the decarburized layer formed is the same. In addition, the hot stamping heating conditions and the shot blasting conditions are also the same. On the other hand, the coating adhesion amount in Experimental No. 6 is less than that of Experimental No. 7, and it is assumed that the Fe concentration in the coating is increased by about 1% or so compared to Experimental No. 7. Therefore, the Fe concentration in the coating after hot stamping in Experimental No. 6 is assumed to be about 60% or so based on the value of Experimental No. 7, and this assumed value is described.

[0281] In Experiment No. 7, the LME cracking evaluation was not performed. However, Experiment No. 7 has the same composition as Experiment No. 6, and the reduction annealing conditions are also similar, so the decarburized layer is assumed to be the same. In addition, the hot stamping heating conditions and the shot blasting conditions are also the same. In addition, the Fe concentration in the plated layer after hot stamping is also assumed to have no significant difference. Therefore, the LME cracking of Experiment No. 7 is assumed to be the same as that of No. 6, and is therefore recorded as the assumed value.

[0282] In Experiment No. 8, the Fe concentration in the plated layer after hot stamping was not measured. However, if the hot stamping conditions are the same, the Fe concentration in the plated layer after hot stamping is generally determined by the plated layer adhesion amount (mass per unit area) before hot stamping and the Fe concentration in the plated layer. Here, reference is made to Experiment No. 1 (plated layer adhesion amount (mass per unit area) 120 g / m 2 , Fe concentration in the plated layer 8.8 mass%). The Fe concentration in the plated layer before hot stamping of Experiment No. 8 is the same as that of Experiment No. 1, and in relation thereto, the plated layer adhesion amount (mass per unit area) of Experiment No. 8 is greater than that of Experiment No. 1. Therefore, the Fe concentration in the plated layer after hot stamping of Experiment No. 8 is assumed to be lower than the measured value (44 mass%) of Experiment No. 1. Therefore, the assumed Fe concentration in the plated layer after hot stamping of Experiment No. 8 is 44 mass% or less, and is shown in Table 8.

[0283] In Experiment Nos. 9 and 10, the Fe concentration in the plated layer after hot stamping was not measured. However, as described above, if the hot stamping conditions are the same, the Fe concentration in the plated layer after hot stamping is generally determined by the plated layer adhesion amount (mass per unit area) before hot stamping and the Fe concentration in the plated layer. Here, reference is made to Experiment No. 4. The Fe concentration in the plated layer before hot stamping of Experiment Nos. 9 and 10 is the same as that of Experiment No. 4, and in relation thereto, the plated layer adhesion amount (mass per unit area) of Experiment Nos. 9 and 10 is greater than that of Experiment No. 4. Therefore, the Fe concentration in the plated layer after hot stamping of Experiment Nos. 9 and 10 is assumed to be lower than the measured value (53 mass%) of Experiment No. 4. Therefore, the assumed Fe concentration in the plated layer after hot stamping of Experiment Nos. 9 and 10 is 53 mass% or less, and is shown in Table 8.

[0284] Further, in Experiment Nos. 1 to 3, 8, and 9, the plated layer adhesion amount was 45 g / m 2 As described above, the Fe concentration in the plated layer after hot stamping is 65 mass% or less, and therefore it is considered that excellent corrosion resistance is exhibited.

[0285] This application claims priority based on Japanese Patent Application No. 2023-056200 filed on March 30, 2023, and No. 2024-022727 filed on February 19, 2024. Japanese Patent Application No. 2023-056200 and No. 2024-022727 are incorporated herein by reference.

[0286] Explanation of Reference Signs

[0287] 1 Sample after welding test (welding test piece)

[0288] 2 Test piece for LME crack evaluation

[0289] 3 Weld nugget

[0290] 4 Cut surface

[0291] 5 Observation surface

[0292] a Central portion of steel sheet

[0293] b End portion of steel sheet

[0294] Q Hole

[0295] R Internal crack

Claims

1. A galvanized steel sheet for hot stamping, wherein, The composition of the base steel plate satisfies: C: 0.15–0.50% by mass Si: 0.02–2.5% by mass Mn: 0.5–5% by mass P: less than 0.03% by mass and including 0% by mass S: less than 0.02% by mass and including 0% by mass Al: 0.010–1% by mass Ti: 0.005–0.080% by mass, and B: 0.0005~0.005% by mass The balance consists of Fe and unavoidable impurities. When elemental analysis is performed on the surface of the coating along the thickness direction of the coating by glow discharge emission spectroscopy (GD-OES), the carbon concentration [Cf] at the position where the Zn concentration of the coating is 1.0% by mass and the bulk carbon concentration [Cb] by mass satisfy the following equation (1). [Cf]≤0.65×[Cb]…(1).

2. The galvanized steel sheet for hot stamping according to claim 1, wherein, The composition of the base steel plate satisfies one or more of the following (a) and (b): (a) It also contains one or more elements selected from the group consisting of Cr: more than 0% by mass and less than 1.2% by mass, Mo: more than 0% by mass and less than 1% by mass, and Ca: more than 0% by mass and less than 0.0040% by mass; (b) It also contains one or more elements selected from the group consisting of Nb: more than 0 mass% and less than 0.040 mass%, V: more than 0 mass% and less than 0.30 mass%, Cu: more than 0 mass% and less than 0.30 mass%, Ni: more than 0 mass% and less than 0.30 mass%, Mg: more than 0 mass% and less than 0.010 mass%, and REM: more than 0 mass% and less than 0.010 mass%.

3. A method for manufacturing galvanized steel sheet for hot stamping, wherein, The process includes an annealing process and a subsequent zinc-based plating process, wherein the annealing process involves holding a hot-rolled or cold-rolled steel sheet that meets the composition of claim 1 or 2 at 500 to 930°C for 90 to 1000 seconds in a reducing atmosphere with a dew point of -20°C to +10°C.

4. The method for manufacturing galvanized steel sheet for hot stamping according to claim 3, wherein, The zinc plating process is a hot-dip zinc plating process.

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