Technical field
[0001]The present invention relates to a surface treated steel sheet.
BACKGROUND
[0002]Structural member (molded article) used in an automobile or the like, to enhance both the strength and dimensional accuracy, it may be produced by hot stamping (hot press). In producing a molded body by a hot stamp, a steel sheet Ac 3 heated above points, quenched with pressing in a mold. That is, in the manufacture, perform the press working and quenching at the same time. According to hot stamping, high dimensional accuracy, and it is possible to produce shaped bodies of high strength.
[0003]
　On the other hand, moldings produced by hot stamping, because it is processed at high temperatures, the scale is formed on the surface. Accordingly, by using the plated steel plate as a steel sheet for hot stamping (coated steel sheets), (refer to Patent Documents 1 to 3) to suppress the formation of scale, yet the technique for improving the corrosion resistance has been proposed.
[0004]
　For example, Patent Document 1, hot press steel sheet Zn plating layer is formed is disclosed. Further, Patent Document 2, high strength automobile parts Aluminum plated steel sheet Al plating layer is formed is disclosed. Further, Patent Document 3, various elements hot Zn-plated steel for press, which is the addition of Mn or the like in the plating layer of the Zn-plated steel sheet is disclosed.
CITATION
Patent Document
[0005]
Patent Document 1: JP 2003-73774 JP
Patent Document 2: JP 2003-49256 JP
Patent Document 3: JP 2005-113233 JP
Summary of the Invention
Problems that the Invention is to Solve
[0006]
　In Patent Document 1 technique, for Zn after hot stamping remains in steel surface layer, it can be expected high sacrificial protection effect. However, since Zn is steel sheet is processed in a molten state, the molten Zn to penetrate the steel sheet, there is a possibility that cracking occurs inside the steel. This cracking is referred to as the liquid metal embrittlement cracking (Liquid Metal Embrittlement, hereinafter also referred to as "LME".). Then, due to the LME, fatigue properties of the molded body is deteriorated.
[0007]
　In the present situation, in order to avoid the occurrence of LME, it is necessary to appropriately control the heating conditions during steel processing. Specifically, all of the molten Zn is diffused in the steel sheet, and a method of heating until Fe-Zn solid solution is employed. However, for these methods, it is necessary to prolonged heating, resulting in a problem that productivity is lowered.
[0008]
　Further, in the technique of Patent Document 2, since Al is used a higher melting point than the Zn in the plating layer, it may enter the molten metal is steel as in Patent Document 1 is low. Accordingly, to obtain excellent resistance to LME resistance is expected to be superior fatigue properties of the molded body after thus hot stamping. However, the steel material Al plating layer is formed, during the phosphating carried out before application of automotive parts, there is a problem that it becomes difficult to form a phosphate film. In other words, phosphating property by the steel is not sufficiently obtained, there is a concern that corrosion resistance after painting is lowered.
[0009]
　Further, in the technique of Patent Document 3, by reforming the surface layer (oxide film) after the hot stamping, while improving the spot weldability, by the addition to elemental, hot stamp still LME occurs there is a possibility that the fatigue properties of the steel material can not be obtained sufficiently. Further, by the addition to elemental, not only fatigue characteristics of the steel material, which may lower the phosphating properties.
[0010]
　The present invention is to solve the above problems, fatigue characteristics, spot weldability, and aims to provide a suitable surface treated steel sheet as a material of the molded body having excellent corrosion resistance after painting.
Means for Solving the Problems
[0011]
　The present invention has been made to solve the above problems, and the gist of the surface treated steel sheet below.
[0012]
　(1) A surface treated steel sheet and a base material and the base material plated layer formed on the surface of,
　the average composition of the plating layer, by mass%,
　Mg: 0.5 ~ 2.0%, contain, and
　satisfies the following (i) ~ (iii) wherein
　the surface treated steel sheet.
　≦ Zn + 75.0 Al ≦ 98.5 · · ·
　(i) 0.4 ≦ Zn / Al ≦ 1.5 · · ·
　(ii) Zn / Al × Mg ≦ 1.6 · · · (iii)
　where the element symbol in the formulas represents the content of each element contained in the plating layer (mass%).
[0013]
　(2) the average composition of the plating layer, in further
　mass%, Si: 0% to greater than 15.0% or less, containing,
　surface-treated steel sheet according to the above (1).
[0014]
　(3) the average composition of the plating layer further satisfies the following (iv) wherein
　the surface treated steel sheet according to (1) or (2).
　Mg + Ca + Ti + Sr + Cr ≦ 2.0 ··· (iv)
　where each element symbol in the above formula represents the content of each element contained in the plating layer (mass%).
[0015]
　(4) The plating layer has a Fe diffusion layer on the base material side of the plating layer,
　the ratio of the thickness of the Fe diffusion layer to the total thickness of the plating layer is 15 to 50%
　surface treated steel sheet according to any one of (1) to (3).
[0016]
　(5) the average composition of the plating layer, in further
　mass%, Fe: 5.0 ~ 25.0%, containing,
　surface-treated steel sheet according to (4).
[0017]
　(6) The chemical composition of the base material, in
　mass%, C: 0.05
　~ 0.4%, Si: 0.5% or less, and
　Mn: 0.5 ~ 2.5%, containing,
　surface treated steel sheet according to any one of (1) to (5).
[0018]
　(7) is for hot stamping,
　the surface treated steel sheet according to any one of (1) to (6).
The invention's effect
[0019]
　By performing the hot stamping to the surface-treated steel sheet according to the present invention, fatigue properties, it is possible to obtain a spot weldability, and excellent molded body corrosion resistance after painting.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020]
[1] is an example of an image of the cross section of the surface-treated steel sheet according to an embodiment was observed by SEM of the present invention.
DESCRIPTION OF THE INVENTION
[0021]
　The present inventors have found that excellent resistance to LME resistance during hot stamping, and was examined the structure of the preferred surface-treated steel plate as a material of the molded article which is excellent in spot weldability and corrosion resistance after painting after hot stamping.
[0022]
　First, the present inventors have examined a method of improving the corrosion resistance after painting of the molded body. As a result, by containing Mg in the plating layer of the surface treated steel sheet has been found to be able to improve the corrosion resistance of the molded body after the hot stamping. However, when the hot stamping to the surface treated steel sheet containing Mg in the plating layer, LME is likely to occur, fatigue characteristics has been found to deteriorate. Further, when the Mg content in the coating layer is excessive, thereby also reduced spot weldability of the moldings produced.
[0023]
　Therefore, the present inventors have found that without degrading the resistance to LME resistance and spot weldability was conducted extensive studies on how to improve the corrosion resistance. As a result, by appropriately managing the Mg content in the plating layer of the surface treated steel sheet, it was found that can secure good balance all the characteristics described above.
[0024]
　The present invention has been made based on the above findings. It will be described in detail below each requirement of the present invention.
[0025]
　(A) overall structure
　surface-treated steel sheet according to an embodiment of the present invention is provided with a plating layer formed on the surface of the base material and the base material. For each, it will be described in detail below.
[0026]
　(B) the base material
　fatigue properties after hot stamping is a problem according to the present embodiment, spot weldability, and improving the corrosion resistance after painting is realized by the configuration of the plating layer of the surface treated steel sheet. Accordingly, the base material of the surface-treated steel sheet according to the present embodiment is not particularly limited. However, if the components of the base material is in the range described below, fatigue properties, spot weldability, and in addition to the corrosion resistance after painting, the molded body having suitable mechanical properties.
[0027]
　Reasons for limiting each element is as follows. Incidentally, "%" for the content in the following description means "mass%".
[0028]
　C: 0.05 ~ 0.4%
　carbon (C) is an element to increase the strength of the molded body after the hot stamping. When C content is too small, not the effect. On the other hand, the C content is excessive, toughness of the steel sheet is lowered. Therefore, C content is 0.05 to 0.4%. C content is more preferably in is 0.10% or more, more preferably 0.13% or more. Also, C content is preferably not more than 0.35%.
[0029]
　Si: 0.5% or less
　Silicon (Si) is inevitably contained, which is an element having an effect of deoxidizing steel. However, when the Si content is excessive, Si in the steel diffuses during heating of hot stamping, and oxide is formed on the surface of the steel sheet, reducing the phosphating properties. Si further, Ac of the steel plate 3 is an element to increase the points, Ac 3 the points is increased, there is a possibility that the heating temperature during hot stamping may exceed the evaporation temperature of the Zn plating. Therefore, Si content is 0.5% or less. Si content is preferably 0.3% or less, and more preferably 0.2% or less. From the viewpoint of the product performance is not constrained lower limit of Si content, to be used for the purpose of deoxidation to above, substantially the lower value exists. Depending on the deoxidation level required, usually 0.05%.
[0030]
　Mn: 0.5 ~ 2.5%
　manganese (Mn) increases the hardenability, is an element to increase the strength of the molded body after the hot stamping. When Mn content is too small, not the effect is obtained. On the other hand, when the Mn content is excessive, this effect is saturated. Therefore, Mn content is 0.5 to 2.5%. Mn content is preferably 0.6% or more, more preferably 0.7% or more. Further, it is preferable Mn content is less 2.4%, and more preferably not more than 2.3%.
[0031]
　P: 0.03% or less
　phosphorus (P) is an impurity contained in steel. P reduces the toughness of the steel segregates on grain boundaries, reducing the resistance to delayed fracture. Accordingly, P content is 0.03% or less. P content is preferably as small as possible.
[0032]
　S: 0.01% or less
　Sulfur (S) is an impurity contained in steel. S is to form a sulfide to lower the toughness of the steel, reduce the resistance to delayed fracture. Thus, S content is 0.01% or less. S content is preferably as small as possible.
[0033]
　sol. Al: 0.1% or less
　of aluminum (Al) are generally used in deoxidation purposes of steel, is an element inevitably contained. However, the Al content is excessive, deoxidation is being performed sufficiently, Ac of the steel plate 3 was elevated points, there is a possibility that the heating temperature during hot stamping exceeds the vaporization temperature of the Zn plating. Therefore, Al content is 0.1% or less. Al content is preferably 0.05% or less. To obtain the above effect, Al content is preferably 0.01% or more. In the present specification, Al content, sol. It means the content of Al (acid soluble Al).
[0034]
　N: 0.01% or less
　Nitrogen (N) is an impurity which is inevitably contained in steel. N reduces the toughness of the steel by forming nitrides. N Further, when B is contained in steel, B binding to reduce solid solute B amount and reduces the turn hardenability. Therefore, N content is 0.01% or less. N content is preferably as small as possible.
[0035]
　B: 0 ~ 0.005%
　boron (B) increases the hardenability of steel, because it has an effect of increasing the strength of the molded body after the hot stamping, it may be contained as needed. However, when the B content is excessive, this effect is saturated. Therefore, B content is 0.005% or less. To obtain the above-mentioned effects, B content is preferably 0.0001% or more.
[0036]
　Ti: 0 ~ 0.1%
　of titanium (Ti) combines with N to form a nitride. If this is the Ti and N to bind to, is suppressed binding of B and N, the decrease in hardenability by BN formation can be suppressed. Therefore, it may be contained as necessary Ti. However, the effect is saturated and the Ti content is excessive, further, Ti nitrides excessively precipitated lowers the toughness of the steel. Therefore, Ti content is 0.1% or less. Incidentally, Ti by its pinning effect, to refine the austenite grain size during hot stamping heating, thereby enhancing the toughness and the like of the molded article. To obtain the above-mentioned effects, Ti content is preferably 0.01% or more.
[0037]
　Cr: 0 ~ 0.5%
　Chromium (Cr), since it has an effect to enhance the hardenability of steel, may be contained as needed. However, the Cr content is excessive, Cr carbide is formed. The Cr carbide, since the hardly soluble when heated hot stamp, it austenitizing hardly proceeds, hardenability is reduced. Therefore, Cr content is 0.5% or less. To obtain the above effect, Cr content is preferably 0.1% or more.
[0038]
　Mo: 0 ~ 0.5%
　of molybdenum (Mo), in order to have the effect of increasing the hardenability of steel, may be contained as needed. However, when the Mo content is excessive, the effect is saturated. Therefore, Mo content is 0.5% or less. To obtain the above-mentioned effects, Mo content is preferably 0.05% or more.
[0039]
　Nb: 0 ~ 0.1%
　niobium (Nb) is to form a carbide, the grain refining during hot stamping, because it has an effect of enhancing the toughness of the steel, may be contained as needed. However, when the Nb content is excessive, not only the effect is saturated, the hardenability decreases. Therefore, Nb content is 0.1% or less. To obtain the above effect, Nb content is preferably 0.02% or more.
[0040]
　Ni: 0 ~ 1.0%
　nickel (Ni) has the effect of increasing the toughness of the steel. Ni is further upon heating in a hot stamp, suppressing embrittlement due to the presence of the molten Zn. Therefore, it may be contained if necessary Ni. However, the Ni content is excessive, these effects are saturated. Therefore, Ni content is 1.0% or less. To obtain the above-mentioned effects, Ni content is preferably 0.1% or more.
[0041]
　In the chemical composition of the base material constituting the surface-treated steel sheet of the present embodiment, the balance being Fe and impurities. Here, the impurities, in producing the steel sheet industrially, ore or scrap that can be incorporated into component as a raw material, or a component that may be mixed due to the manufacturing environment, intentionally added means are non component.
[0042]
　(C) plating layer
　plated layer in the present invention is mainly composed of Zn and Al. That is, the average composition of the plating layer satisfies the following formula (i). By plating layer of the surface treated steel sheet satisfies the following conditions, it is possible to improve the fatigue properties of the molded body after the hot stamping, spot weldability, and corrosion resistance after painting.
　75.0 ≦ Zn + Al ≦ 98.5 ··· (i)
　where the element symbol in the above formula represents the content of each element contained in the plating layer (mass%).
[0043]
　The ratio of Zn and Al is also important. Therefore, the average composition of the plating layer of the present invention satisfies the following (ii) expression. When the value of Zn / Al is less than 0.4, without being able to secure a phosphate treatment properties, corrosion resistance after coating is deteriorated. If the value of Zn / Al is more than 1.5, can not be suppressed LME, fatigue characteristics deteriorate. Preferably the value of Zn / Al is 1.2 or less, more preferably 1.0 or less, even more preferably 0.8 or less.
　0.4 ≦ Zn / Al ≦ 1.5 ··· (ii)
[0044]
　Further in the present invention, the average composition of the plating layer, by mass%, Mg: containing 0.5 to 2.0 percent. In less than 0.5% Mg content in the coating layer, the effect of improving the corrosion resistance of the molded body after the hot stamping becomes insufficient. On the other hand, when the Mg content exceeds 2.0%, the risk of LME during hot stamping occurs increases. Further, Mg because easily oxidized, thickening as oxides on the surface layer of the molded body after the hot stamping. Because oxides of Mg has a high electrical resistance, the excessive thickening, weldability of the molded article is deteriorated. Mg content in the coating layer is preferably 0.6% or more, and more preferably 0.8% or more. Further, it is preferable Mg content is 1.8% or less, and more preferably 1.5% or less.
[0045]
　Further, Mg content in the coating layer, it is necessary also to adjust the relationship between the content of Zn and Al, in particular, it is necessary to satisfy the (iii) below formula. When the value of Zn / Al × Mg exceeds 1.6, it is impossible to suppress the LME, fatigue characteristics deteriorate. The value of Zn / Al × Mg is preferably at 1.4 or less, more preferably 1.2 or less, even more preferably 1.0 or less.
　Zn / Al × Mg ≦ 1.6 ··· (iii)
[0046]
　The average composition of the plating layer, by mass%, Si: 0% Beyond may further contain 15.0% or less. The plating layer that contains Si, it is possible to improve the adhesion between the base material and the plated layer. On the other hand, when the Si content of the plating layer is more than 15.0%, it may become impossible to secure the performance of corrosion resistance and weldability and the like of the molded body after the hot stamping. Si content is preferably at least 0.1%, more preferably not less than 0.3%.
[0047]
　Also, the Si content of the plating layer is increased, the formation of which will be described later Fe diffusion layer is suppressed. Therefore, if you want to promote the formation of the Fe diffusion layer, Si content is preferably at most 10.0%, more preferably 5.0% or less.
[0048]
　Furthermore, Cr in the plating layer, Ca, Sr, may be included such as Ti. However, these elements, and is easily oxidized as with Mg, thickening as oxides on the surface layer of the molded body after the hot stamping. Since these oxides are high electrical resistance, the excessive thickening, weldability of the molded article is deteriorated. Therefore, if it contains these elements in the plating layer, the average composition of the plating layer, in relation to the Mg content, it is preferable to satisfy the following (iv) expression.
　Mg + Ca + Ti + Sr + Cr ≦ 2.0 ··· (iv)
[0049]
　In the present invention, for the average composition of the plating layer, and request the following manner. First, dissolve the surface-treated steel sheet comprising plating layer with aqueous 10% HCl. At this time, in order to dissolve only the plating layer is added to suppress inhibitor dissolution of Fe in the base material in hydrochloric acid. Then, each element contained in the lysate is measured by inductively coupled plasma emission spectrometry (ICP-OES).
[0050]
　Plating layer in the present invention preferably has a Fe diffusion layer on the base material side of the plating layer. Fe diffusion layer is composed of structure mainly of Fe-Al-Zn phase. The Fe-Al-Zn phase is mainly the total area ratio of the Fe-Al-Zn phase means that 90% or more. The total area ratio of the Fe-Al-Zn phase, more preferably 95% or more, further preferably 99% or more. The Fe-Al-Zn phase of the present invention, Fe (Al, Zn) 2 , Fe 2 (Al, Zn) 5 or Fe (Al, Zn) 3 is a general term for. In particular, Fe content in the Fe diffusion layer is in the range of 20 to 55 wt%. Note that the above-mentioned Fe-Al-Zn phase may also contain Si.
[0051]
　If the surface treated steel sheet is subjected to cold working, the starting point of cracking when the Fe diffusion layer is present. Therefore, there is a normal, Fe diffusion layer is better not to minimize formation preferable. However, when the surface treated steel sheet is subjected to hot stamping, the Fe diffusion layer made mainly of Fe-Al-Zn phase in coating layer is present, alloying of Zn and Al in the plating layer during hot stamping There is promoted quickly becomes Fe-Al alloy is formed. Formation of Fe-Al alloys, in particular because it is promoted in the vicinity of the interface between the base material, it exhibits an effect of suppressing LME. In the present invention, FeAl alloys, alpha iron, Fe 3 is a general term for Al and FeAl.
[0052]
　For obtaining the above effect is the ratio of the thickness of the Fe diffusion layer to the total thickness of the plating layer of the present invention, it is preferable that 15 to 50%. The proportion of the less than 15%, the effect of suppressing LME is not sufficiently obtained. On the other hand, when the proportion of the more than 50%, there is a possibility that cracks may occur in winding the steel sheet into a coil. The ratio of the thickness of the Fe diffusion layer to the total thickness of the plating layer is preferably 20% or more, more preferably 25% or more. The ratio of the thickness of the Fe diffusion layer is preferably not more than 45%, and more preferably 40% or less.
[0053]
　Figure 1 is an example of an image that the cross-section of the surface-treated steel sheet according to an embodiment of the present invention was observed by SEM. Incidentally, FIG. 1 (a) is an example of performing a plating treatment in conditions to positively form the Fe diffusion layer. Meanwhile, FIG. 1 (b) is an example in which the plating process under normal conditions. From Figure 1, the boundary between the Fe diffusion layer and the other layers in the coating layer is understood that it is possible to clearly observe.
[0054]
　Further, from the results of EPMA analysis of the plating layer, Fe content of Fe diffusion layer is 20% or more, confirmed that the structure mainly of Fe-Al-Zn phase serving as a range of 20 to 55 mass% did it. Further, in the other layers, it was less than 20%. Accordingly, in the present invention, the thickness of the overall thickness and Fe diffusion layer of the plated layer, and measuring the results of EPMA analysis and SEM observation. In the present invention, plating on observed by SEM from a cross-sectional, thickness of the total thickness and the Fe diffusion layer of the plating layer was measured at arbitrary 12 points, measurements at 10 points excluding the maximum and minimum and adopting the mean value of as respective thicknesses.
[0055]
　Incidentally, without providing any restriction for the entire thickness of the plating layer of the present invention, for example, be a 5 ~ 40 [mu] m. The total thickness of the plating layer is preferably at 10μm or more, preferably 30μm or less. Although not provided any special restriction on the thickness of the Fe diffusion layer, when it is desired to obtain the effect of suppressing the LME is preferably a 3μm or more. Meanwhile, since there is a possibility that cracks when winding the steel plate whose thickness is to be excessive in a coil shape is generated, it is preferable to 10μm or less.
[0056]
　Furthermore, the Fe diffusion layer sufficiently formed, when it is desired to obtain the effect of suppressing the LME, the average composition of the plating layer, by mass%, Fe: 5.0 further contain ~ 25.0% preferable.
[0057]
　(D) the production method
　in the process of manufacturing a surface treated steel sheet of the present embodiment includes a step of producing a preform, a step of forming a plating layer on the surface of the base material. Hereinafter, each step will be described in detail.
[0058]
[Base material manufacturing process]
　In the preform manufacturing process, to produce a base material of a surface treated steel sheet. For example, to produce a molten steel having the chemical composition described above, by using the molten steel, or to produce a slab by casting, or, to produce an ingot by ingot-making method. Then, by a slab or ingot is hot rolled, the base material of the coated steel sheets (hot-rolled sheet) is obtained. Note that performs pickling process on the hot-rolled plate, cold rolled steel sheet obtained by performing cold rolling may be the base material of the surface-treated steel sheet with respect to hot-rolled sheet after pickling.
[0059]
[Plating process]
　The plating process, to form a Al-Zn-Mg plating layer on the surface of the base material, producing a surface treated steel sheet. Method of forming the plating layer may be a hot-dip plating process, spray coating process, such as vapor plating, may be any other process. To increase the adhesion between the base material and the plated layer, it is preferred to incorporate Si in the plating layer.
[0060]
　For example, formation example of Al-Zn-Mg plating layer by hot dipping process is as follows. That is, the base material, Al, Zn, and immersed in a molten plating bath consisting of Mg and impurities, depositing a plating layer on the surface of the base material. Then, raise the preform plating layer is deposited from a plating bath.
[0061]
　In this step, the pulling speed of the steel sheet from the plating bath, by appropriately adjusting the flow rate of wiping gases, it is possible to adjust the thickness of the plating layer. As described above, it is preferable to adjust so that the whole thickness of the plating layer is 5 ~ 40 [mu] m.
[0062]
　Incidentally, when it is desired to form a Fe diffusion layer described above in the plating layer, the plating process, Si content in the plating bath, to control the cooling rate after dipping time and dipping is important. More specifically, in order to promote the formation of Fe diffusion layer, as described above, Si content in the plating bath should be low.
[0063]
　Further, immersed least 5s in the plating bath, further, after pulled up from the plating bath, subjected to thermal insulation or heating, by suppressing the average cooling rate below 30 ° C. / s, as the diffusion of Fe proceeds sufficiently Become. However, when the thickness of the Fe diffusion layer is excessive, there is a possibility that cracks may occur in winding the steel sheet into a coil, the immersion time in the plating bath is less 15s, the average cooling rate after soaking is 5 ° C. it is preferable that the / s or more.
[0064]
　Therefore, to form a Fe diffusion layer in the plating layer, and the ratio of the thickness of the Fe diffusion layer to the total thickness of the plating layer, if it is desired to adjust the range of 15 to 50%, dipping in the plating bath the time and 5 ~ 15s, it is preferable that the average cooling rate after immersion than 5 ~ 30 ℃ / s.
[0065]
　(E) hot stamp condition
　by applying a hot stamp to a surface treated steel sheet of the present invention, it is possible to obtain the fatigue characteristics, spot weldability, and excellent molded body corrosion resistance after painting. By performing hot stamping under conditions described below, it is possible to obtain more reliably moldings excellent in the characteristics. Note that before the hot stamping, if necessary, may be subjected to rust-preventive oil film formation processing and blanking processing.
[0066]
Hot stamping process]
　normal hot stamp, heat the steel sheet to hot stamping temperature range (hot working temperature range), then carried out by hot working, further cooling. According to conventional hot stamping techniques, in order to shorten the manufacturing time, it is better able to as large as possible a heating rate of the steel sheet. Further, since the alloying of the plating layer when heating the steel sheet to hot stamping temperature range progresses sufficiently, conventional hot stamping techniques, not important to control the heating conditions of the steel sheet.
[0067]
　However, in order to obtain excellent more reliably the characteristic molded body, when raising the temperature of the surface treated steel sheet to hot stamping temperature, it is preferable to perform alloying heat treatment for a predetermined time held in a predetermined temperature range . Then, after performing alloying heat treatment is heated to hot stamping temperature (quenching heating temperature) to hot-working and cooling.
[0068]
　Specifically, first, charged surface treated steel sheet heating furnace (gas furnace, an electric furnace, infrared oven, etc.). In a heating furnace, a surface treated steel sheet was heated to a temperature range of 500 ~ 750 ° C., performing an alloying heat treatment of holding 10 ~ 450s within this temperature range. By performing the alloying heat treatment, Fe of the base material is diffused in the plating layer, the alloying proceeds. This alloying, it is possible to suppress the LME. Incidentally, the alloying heating temperature need not be constant, may vary within a range of 500 ~ 750 ° C..
[0069]
　After alloying heat treatment is finished, the surface treated steel sheet Ac 3 was heated to a temperature range of point ~ 950 ° C., followed by a hot working. At this time, the temperature of the surface treated steel sheet is Ac 3 to limit the time within a temperature range of point ~ 950 ° C. (oxidation temperature range) below 60s. When the temperature of the surface treated steel sheet is within the oxidation temperature range, oxide film of the surface layer of the plating layer is grown. When the temperature of the surface treated steel sheet is the time within the oxidation temperature range exceeds 60s, oxide film is excessively grown, a decrease in weldability of the shaped body is concerned. On the other hand, since the formation rate of the oxide film is very fast, the lower limit of the time the temperature of the surface treated steel sheet is within the oxidation temperature range is 0s greater. However, when the heating of the surface treated steel sheet is performed in a non-oxidizing atmosphere such as a 100% nitrogen atmosphere, since the oxide film is not formed, the heating is performed in an oxidizing atmosphere such as air atmosphere.
[0070]
　As long as the time the temperature of the surface treated steel sheet is within the oxidation temperature range is less than 60s, conditions such as heating rate and maximum heating temperature is not particularly defined, it is possible to select various conditions that may perform hot stamp.
[0071]
　Next, the surface treated steel sheet taken out from the heating furnace to press molding using a mold. In this step, simultaneously with this pressing, quenching the steel plate by the mold. The the mold and the cooling medium (e.g. water) is circulated, mold encourage the heat removal surface treated steel sheet, hardening is performed. Through the above steps, it is possible to produce a compact.
[0072]
　Incidentally, a method of heating the surface-treated steel sheet by using a heating furnace has been described as an example, it may be heated by electrical heating. Even in this case, the steel sheet was heated for a predetermined time by electrical heating, performing press forming of the steel using a mold.
[0073]
[Rustproof oil film forming step]
　rustproof oil film forming step, after the plating treatment step, and before the hot stamping process, the surface of the surface treated steel sheet is coated with a rust preventive oil is intended to form a rustproof oil film, it may be optionally included in the production process. If a long time from the surface treated steel sheet is manufactured until the hot stamping is carried out, there is a possibility that the surface of the surface treated steel sheet is oxidized. However, since the surface is hard to oxidation of the surface treated steel sheet antirust oil film is formed by rustproof oil film forming step, rustproof oil film forming step, can suppress the formation of scale of the molded article. In addition, the method of forming the anti-corrosion oil film can also be used any known technique.
[0074]
[Blanking processing step]
　This step, after the rust preventive oil film forming step, and before the hot stamping process, carried out shearing and / or punching to the surface treated steel sheet, forming the steel sheet into a particular shape it is a step for. Shear plane of the steel sheet after blanking working is easily oxidized. However, if it is pre-rust oil film is formed on the steel sheet surface, rust-preventive oil spreads to some extent on the shear plane. This makes it possible to suppress oxidation of the steel sheet after blanking working.
[0075]
　Having described an embodiment of the present invention, the above-described embodiments are merely illustrative of the present invention. Accordingly, the present invention is not limited to the embodiment described above, in a range not departing from its spirit, it is possible to appropriately design changed.
[0076]
　The following examples illustrate the present invention more specifically, the present invention is not limited to these examples.
Example 1
[0077]
　First, prepare the base material. That is, using a molten steel having a chemical composition shown in Table 1, were produced slabs by continuous casting. Then the slab was hot-rolled to produce a hot-rolled steel sheet, after further pickled hot-rolled steel sheet to produce a cold-rolled steel sheet by performing cold rolling. Then was the cold-rolled steel sheet base material of the surface-treated steel sheet (thickness 1.4 mm).
[0078]
[Table 1]

[0079]
　Next, using the thus produced preform performs plating treatment according to the conditions shown in Table 2, were produced surface-treated steel sheet of each test example.
[0080]
[Table 2]

[0081]
　Measurement of the average composition of the plating layer of the obtained surface-treated steel sheet was subjected. In the measurement, first, the surface treated steel sheet comprising plating layer was dissolved in aqueous 10% HCl. At this time, in order to dissolve only the plating layer and the suppressing inhibitor dissolution of Fe in the base material was added to the hydrochloric acid. Then, each element contained in the lysate was measured by ICP-OES.
[0082]
　Also, cut out a section of the surface-treated steel sheet, by performing SEM observation, to measure the thickness of the overall thickness and Fe diffusion layer of the plated layer. The measurement results are shown in Table 3.
[0083]
[table 3]

[0084]
　Thereafter, the surface-treated steel sheet of each test example, as shown below, hot V bending test was carried out spot weldability evaluation test and corrosion resistance after coating evaluation test.
[0085]
[Hot V bending test]
　to the surface treated steel sheet of each test example, after performing an alloying heat treatment of 120s held at 700 ° C., and 30s heated at 900 ° C., immediately using three types of hand press was molded body subjected to V bending hot. The shape of the mold, V bending machining end bent outer portion of the bending radius due to processing, 10% respectively, and the shape as extended 15% and 20%.
[0086]
　Thereafter, the cross section in the thickness direction of the V bending portion of the molded body, by observing the reflected electron image by SEM and the reflection electron detector to observe the occurrence of LME. Then, the case where cracking to the base material (Fe concentration of 98% or more places) is progressing to the LME occur. In the evaluation of the resistance to LME resistance by hot V bending test, excellent what was no cracking at 20% elongation (1), although cracks occurred at 20% elongation, good what was no cracks in the 15% elongation (2), although cracks occurred at 15% elongation, allowed what was no cracking at 10% elongation (3), cracks were evaluated as bad (4) those that occurred at 10% elongation.
[0087]
　Incidentally, if the determination of the end position of the cracks is difficult in the above observation, using an energy dispersive X-ray microanalyzer, the surrounding area of ​​the crack end position, to perform energy dispersive X-ray analysis of (EDS) in, and determines whether the crack until the base material extends. In this case, Al, the total content of Zn is the plating layer areas that exceed 0.5%, the inner region of the steel was identified as the base material than that.
[0088]
[Spot Weldability Evaluation Test]
　to the surface treated steel sheet of each test example, after performing an alloying heat treatment of 120s held at 700 ° C., and 30s heated at 900 ° C., immediately flat mold having a water cooling jacket It was prepared a plate-shaped body by sandwiching the steel sheet. Even at a cooling rate slower part during hot stamping, to the extent martensitic transformation start point (410 ° C.), was hardened so that the 50 ° C. / s or more cooling rate.
[0089]
　For these moldings, using a DC power supply, it was carried out spot welding by pressure 350 kgf. Perform tests at various welding current, a nugget diameter of a weld is a lower limit value exceeding 4.7 mm, will increase the value of appropriate welding current, and the upper limit dust generated value during welding . Then, it sets the value between the upper and lower limit values ​​and the proper current range was the difference between the upper limit value and the lower limit value as an index of spot weldability. In the evaluation of the spot weldability, excellent what the value is more than 1.5A (1), the good ones less than 1.0A 1.5A (2), those less than 0.5A 1.0A Yes (3), it was evaluated as bad (4) of less than 0.5A.
[0090]
[After painting corrosion resistance evaluation test]
　to the surface treated steel sheet of each test example, after performing an alloying heat treatment of 120s held at 700 ° C., and 30s heated at 900 ° C., immediately flat mold having a water cooling jacket It was prepared a plate-shaped body by sandwiching the steel sheet. Even at a cooling rate slower part during hot stamping, to the extent martensitic transformation start point (410 ° C.), was hardened so that the 50 ° C. / s or more cooling rate.
[0091]
　Furthermore, for each shaped body, Nippon Parkerizing surface conditioning treatment agent Co., Ltd. (trade name: Prepalene X) with the surface conditioning was performed 20s at room temperature. Then, Japan Parkerizing zinc phosphate treatment solution Co., Ltd. (trade name: PALBOND 3020) was performed using a phosphoric acid salt treatment. Specifically, the temperature of the processing solution and 43 ° C., and 120s immersed molded body to the treatment liquid. Thus, Phosphate coated surface of the steel material is formed.
[0092]
　After performing the phosphating described above, for each shaped body, in the Nippon Paint Co., Ltd. of cationic electrodeposition paint, and electrodeposition coating in the slope energizing voltage 160 V, further baking temperature 170 ° C. 20 minutes to baking. Thickness control of the coating after electrodeposition coating at the surface treated steel sheet before hot stamping, electrodeposition coating was carried out under the conditions to be 15 [mu] m.
[0093]
　On the molding body after the electrodeposition coating, crosscut to reach the steel base material were carried out composite corrosion test (JASO M610 cycle). Evaluate the corrosion resistance at paint blistering width, 180 cycles Yu those composite corrosion test coating after performing a blister width of 2.0mm below (1), 2.0mm ultra 3.0mm a good following ones ( 2), 3.0mm ultra-4.0mm allowed the following things (3), was evaluated as bad (4) those of 4.0mm greater.
[0094]
[Evaluation Results]
　In the present invention, the fatigue characteristics (resistance to LME resistance), and its object is to provide a suitable surface treated steel sheet spot weldability, and as a material of a molded article which is excellent with good balance in all of corrosion resistance after painting. Therefore, taking into account the results of evaluation Overall, any also as acceptable overall evaluation B at least impossible was not in overall evaluation A one and test were also excellent or good in the test, either those of the overall evaluation C there is impossible in the test was a failure. The results are shown in Table 4.
[0095]
[Table 4]

[0096]
　Table 4 As is apparent from, the surface-treated steel sheet according to the present invention as a material, the balance by hot stamping under appropriate conditions, the fatigue characteristics (resistance to LME resistance), spot weldability, and in all corrosion resistance after painting often the superior moldings obtained were confirmed.
Industrial Applicability
[0097]
　By performing the hot stamping to the surface-treated steel sheet according to the present invention, fatigue properties, it is possible to obtain a spot weldability, and excellent molded body corrosion resistance after painting. Therefore, the molded body of the surface-treated steel sheet according to the present invention as a material can be suitably used as a structural member or the like used in an automobile or the like.

The scope of the claims

[Requested item 1]A surface treated steel sheet and a base material and the base material plated layer formed on the surface of,
　the average composition of the plating layer, by mass%,
　Mg: 0.5 ~ 2.0%, containing and
　satisfies the following (i) ~ (iii) wherein
　the surface treated steel sheet.
　≦ Zn + 75.0 Al ≦ 98.5 · · ·
　(i) 0.4 ≦ Zn / Al ≦ 1.5 · · ·
　(ii) Zn / Al × Mg ≦ 1.6 · · · (iii)
　where the element symbol in the formulas represents the content of each element contained in the plating layer (mass%).
[Requested item 2]
　The average composition of the plating layer, in further mass%,
　Si: 0% to greater than 15.0% or less, containing,
　surface-treated steel sheet according to claim 1.
[Requested item 3]
　The average composition of the plating layer further satisfies the following (iv) wherein
　the surface treated steel sheet according to claim 1 or claim 2.
　Mg + Ca + Ti + Sr + Cr ≦ 2.0 ··· (iv)
　where each element symbol in the above formula represents the content of each element contained in the plating layer (mass%).
[Requested item 4]
　The plating layer has a Fe diffusion layer on the base material side of the plating layer,
　the ratio of the thickness of the Fe diffusion layer to the total thickness of the plating layer is 15 to 50%
　claim 1 surface treated steel sheet as claimed in any one of up to claim 3.
[Requested item 5]
　The average composition of the plating layer, in further
　mass%, Fe: 5.0 ~ 25.0%, containing,
　surface-treated steel sheet according to claim 4.
[Requested item 6]
　Chemical composition of the base material, in
　mass%, C: 0.05
　~ 0.4%, Si: 0.5% or less, and
　Mn: 0.5 ~ 2.5%, containing,
　according to claim 1 surface treated steel sheet as claimed in any one of up to claim 5.
[Requested item 7]
　Is for hot stamping,
　the surface treated steel sheet according to any one of claims 1 to 6.