Title of Invention : Galvanized steel
Technical field
[0001]
The present invention relates to plated steel materials.
Background technology
[0002]
Hot stamping (hot press) is known as a technology for press forming materials that are difficult to form, such as high-strength steel sheets. Hot stamping is a hot forming technique in which the material to be formed is heated and then formed. In this technique, since the material is heated and then formed, the steel material is soft and has good formability during forming. Therefore, even high-strength steel can be formed into complex shapes with high accuracy. In addition, since quenching is performed at the same time as forming with a press die, the steel after forming can have sufficient strength. Are known.
[0003]
In Patent Document 1, the steel plate surface has an Al-Zn alloy plating layer containing 20 to 95% by mass of Al, 0.01 to 10% by mass of Ca + Mg, and Si. A plated steel sheet is described. Further, in Patent Document 1, in such a plated steel sheet, oxides of Ca and Mg are formed on the surface of the Al—Zn-based alloy plating layer, so that the plating does not adhere to the mold during hot pressing. It is stated that it is possible to prevent
[0004]
In relation to Al-Zn alloy plating, in Patent Document 2, the plating layer contains Al: 2 to 75% and Fe: 2 to 75% in mass%, and the balance is 2% or more. of Zn and unavoidable impurities. Further, in Patent Document 2, from the viewpoint of improving corrosion resistance, as components in the plating layer, Mg: 0.02 to 10%, Ca: 0.01 to 2%, Si: 0.02 to 3%, etc. It is taught that inclusion is effective.
[0005]
Further, in relation to Al—Zn alloy plating, Patent Document 3 discloses a plated steel material having a steel material and a plating layer including a Zn—Al—Mg alloy layer disposed on the surface of the steel material, In the cross section of the Zn-Al-Mg alloy layer, the area fraction of the MgZn 2 phase is 45 to 75%, the total area fraction of the MgZn 2 phase and the Al phase is 70% or more, and the Zn-Al-MgZn 2 ternary The area fraction of the eutectic structure is 0 to 5%, and the plating layer contains, in mass%, Zn: more than 44.90% to less than 79.90%, Al: more than 15% to less than 35%, Mg: A plated steel material containing more than 5% to less than 20%, Ca: 0.1% to less than 3.0%, Si: 0% to 1.0%, etc. is described.
[0006]
Similarly, in Patent Document 4, a plated steel material having a steel material and a plating layer including a Zn-Al-Mg alloy layer disposed on the surface of the steel material, wherein the Zn-Al-Mg alloy layer is a Zn phase and contains an Mg—Sn intermetallic compound phase in the Zn phase, and the plating layer contains, by mass%, Zn: more than 65.0% and Al: more than 5.0% to 25.0% Mg: more than 3.0% to less than 12.5%, Ca: 0% to 3.00%, Si: 0% to less than 2.5%, etc. are described.
prior art documents
patent literature
[0007]
Patent Document 1: Japanese Patent Application Laid-Open No. 2012-112010
Patent Document 2: JP-A-2009-120948
Patent Document 3: International Publication No. 2018/139620
Patent Document 4: International Publication No. 2018/139619
SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
[0008]
For example, when Zn-based plated steel is used in hot stamping, the Zn is processed in a molten state, so the molten Zn may enter the steel and cause cracks inside the steel. Such a phenomenon is called liquid metal embrittlement (LME), and it is known that the fatigue properties of steel materials deteriorate due to the LME.
[0009]
On the other hand, when a plated steel containing Al as a component in the plating layer is used in hot stamping, for example, hydrogen generated during heating in the hot stamping penetrates into the steel and causes hydrogen embrittlement cracking. It is known that sometimes
[0010]
However, conventional Al-Zn plated steel materials used in hot stamping have not been sufficiently studied from the viewpoint of suppressing LME and hydrogen embrittlement cracking.
[0011]
Therefore, an object of the present invention is to provide a plated steel material with improved LME resistance and hydrogen penetration resistance, as well as excellent corrosion resistance after hot stamping.
Means to solve problems
[0012]
The present invention that achieves the above objects is as follows.
(1) A plated steel material comprising a steel base material and a plating layer formed on the surface of the steel base material, wherein the chemical composition of the plating layer is, in mass%,
Al: 25.00-75.00%,
　Mg: 7.00 to 20.00%,
Si: 0.10 to 5.00%,
Ca: 0.05 to 5.00%,
Sb: 0 to 0.50%,
Pb: 0 to 0.50%,
Cu: 0 to 1.00%,
Sn: 0 to 1.00%,
Ti: 0 to 1.00%,
Sr: 0 to 0.50%,
Cr: 0 to 1.00%,
Ni: 0 to 1.00%,
Mn: 0 to 1.00%, and
The balance: Zn and impurities,
A plated steel material in which the surface structure of the plating layer has an area ratio of 2.0% or more of an acicular Al-Zn-Si-Ca phase.
(2) The surface texture of the plating layer is, in terms of area ratio,
Acicular Al-Zn-Si-Ca phase: 2.0-20.0%,
α phase: 5.0 to 80.0%,
α/τ eutectic phase: 20.0 to 90.0%,
Other residual structure: The plated steel material according to (1) above, which is less than 10.0%.
(3) The chemical composition of the plating layer is mass%,
Al: 35.00 to 50.00%, and
The plated steel material according to (1) or (2) above, containing Mg: 9.00 to 15.00%.
(4) The plated steel material according to any one of (1) to (3) above, wherein the area ratio of the acicular Al-Zn-Si-Ca phase in the surface texture is 8.0% or more.
Effect of the invention
[0013]
According to the present invention, it is possible to provide a plated steel material with improved LME resistance and hydrogen penetration resistance, as well as excellent corrosion resistance after hot stamping.
Brief description of the drawing
[0014]
FIG. 1 shows a scanning electron microscope (SEM) backscattered electron image (BSE image) of the surface of a plating layer in a conventional Al—Zn—Mg-based plated steel.
2 shows a scanning electron microscope (SEM) backscattered electron image (BSE image) of the plating layer surface of the plated steel material (Example 10) according to the present invention. FIG.
3 shows a SEM BSE image of the plating layer surface of the plated steel material (Example 7) according to the present invention. FIG.
4 is a graph showing the relationship between a cooling rate change point when cooling a coating layer and the formation of an acicular Al--Zn--Si--Ca phase. FIG.
MODE FOR CARRYING OUT THE INVENTION
[0015]

A plated steel material according to an embodiment of the present invention includes a steel base material and a plating layer formed on the surface of the steel base material, and the chemical composition of the plating layer is, in mass%,
Al: 25.00-75.00%,
　Mg: 7.00 to 20.00%,
Si: 0.10 to 5.00%,
Ca: 0.05 to 5.00%,
Sb: 0 to 0.50%,
Pb: 0 to 0.50%,
Cu: 0 to 1.00%,
Sn: 0 to 1.00%,
Ti: 0 to 1.00%,
Sr: 0 to 0.50%,
Cr: 0 to 1.00%,
Ni: 0 to 1.00%,
Mn: 0 to 1.00%, and
The balance: Zn and impurities,
The surface texture of the plating layer is characterized by having an acicular Al-Zn-Si-Ca phase with an area ratio of 2.0% or more.
[0016]
For example, when conventional Zn-based plated steel or Al--Zn-based plated steel is used in hot stamping, the plated steel is generally heated to a temperature of about 900° C. or higher during hot stamping. Since Zn has a relatively low boiling point of about 907° C., Zn in the coating layer evaporates or melts at such high temperatures to partially generate a high-concentration Zn liquid phase in the coating layer. Penetration of liquid Zn into grain boundaries in steel can cause liquid metal embrittlement (LME) cracking.
[0017]
On the other hand, in conventional Al-plated steel materials that do not contain Zn, although LME cracking caused by Zn does not occur, during heating in hot stamping, water vapor in the atmosphere is reduced by Al in the coating layer to generate hydrogen. I have something to do. As a result, the generated hydrogen may penetrate into the steel material and cause hydrogen embrittlement cracking. Also, in the Al-Zn plated steel material, Zn has a relatively low boiling point as described above, so part of it evaporates during hot stamping at a high temperature of 900 ° C or higher, and May react with water vapor to generate hydrogen. In such a case, hydrogen embrittlement cracking may occur due to hydrogen penetration into the steel due to not only Al but also Zn. In addition, from the viewpoint of improving corrosion resistance, elements such as Mg added to Zn-based plated steel or Al--Zn-based plated steel are partly evaporated during heating in hot stamping at high temperatures, As in the case of Zn, hydrogen may be generated to cause hydrogen embrittlement cracking.
[0018]
Also, during hot stamping at high temperatures, if Zn and/or Mg, which are elements that have an effect of improving corrosion resistance, evaporates and some of these elements disappear, naturally, the molded body after hot stamping A problem arises that sufficient corrosion resistance cannot be maintained. Furthermore, when Zn and/or Mg in the plating layer evaporates and disappears, in the plating layer after hot stamping, Fe diffused from the base iron and Al and/or Zn in the plating layer Al--Fe intermetallic compounds and/or Zn--Fe intermetallic compounds are formed in relatively large amounts, and these intermetallic compounds cause red rust in corrosive environments.
[0019]
Therefore, the present inventors studied the corrosion resistance, LME resistance, and hydrogen penetration resistance of a plated steel material for use in hot stamping, which includes an Al-Zn-Mg-based plating layer. As a result, the present inventors found that by allowing a predetermined amount of acicular Al-Zn-Si-Ca phase to exist in the surface structure of an Al-Zn-Mg-based plating layer having a predetermined chemical composition, LME and steel materials It has been found that the penetration of hydrogen can be remarkably reduced or suppressed, and sufficient corrosion resistance can be achieved even in molded articles after hot stamping. A more detailed description will be given below with reference to the drawings.
[0020]
Fig. 1 shows a scanning electron microscope (SEM) backscattered electron image (BSE image) of the coating layer surface of a conventional Al-Zn-Mg-based plated steel. Referring to FIG. 1, the surface structure of the plating layer is mainly composed of a large black structure α phase 1 and a small black structure in the matrix phase τ phase, more specifically, a small rod-shaped black structure α phase. It can be seen that it is composed of a dispersed α/τ eutectic phase 2 and a massive τ phase 3 that does not form such an eutectic phase with the α phase. The α-phase is a structure mainly composed of Al and Zn, while the τ-phase is a structure mainly composed of Mg, Zn and Al. In the present invention, the term "main component" means that the total content of elements constituting the main component exceeds 50%. In the conventional Al-Zn-Mg-based plated steel material as shown in FIG. Zn and Mg evaporate during heating in forming, hydrogen penetrates into the LME and steel material, and furthermore, the corrosion resistance after hot stamping is greatly reduced due to the disappearance of these elements due to the evaporation of Zn and Mg. put away.
[0021]
FIG. 2 shows a scanning electron microscope (SEM) backscattered electron image (BSE image) of the plating layer surface of the plated steel material (Example 10) according to the present invention. Referring to FIG. 2, in contrast to the case of FIG. 1, in the surface structure of the plating layer, in addition to the α phase 1 (dendritic structure in FIG. 2) and the α/τ eutectic phase 2, acicular Al It can be seen that the -Zn-Si-Ca phase 4 is present in a relatively large amount. In addition, FIG. 3 shows a SEM BSE image of the plating layer surface in the plated steel material (Example 7) according to the present invention.ing. Referring to FIG. 3, in addition to α phase 1, α/τ eutectic phase 2, and acicular Al—Zn—Si—Ca phase 4, MgZn 2 phase 5 is also present in the surface texture of the plating layer. I know there is a case. In any case, in the plated steel material according to the present invention, the acicular Al-Zn-Si-Ca phase 4 as shown in FIGS. 2 and 3 has an area ratio of 2.0% or more, so that LME occurs And it is possible to remarkably reduce or suppress the penetration of hydrogen into the steel material, and to achieve sufficient corrosion resistance even in the compact after hot stamping.
[0022]
Although not intending to be bound by any particular theory, the presence of the acicular Al-Zn-Si-Ca phase 4 in the surface texture of the plating layer causes needle It is thought that Ca dissolved from the Al--Zn--Si--Ca phase 4 is preferentially oxidized by oxygen in the atmosphere, forming a dense Ca-based oxide film on the outermost surface of the plating layer. In other words, the acicular Al--Zn--Si--Ca phase 4 is believed to function as a Ca supply source for forming a Ca-based oxide film during high-temperature heating in hot stamping. Such a Ca-based oxide film, more specifically a Ca- and Mg-containing oxide film, functions as a barrier layer to reduce the evaporation of Zn and Mg in the plating layer to the outside and the entry of hydrogen from the outside. Furthermore, it is thought that the decrease in corrosion resistance due to the evaporation of Zn and Mg to the outside can be remarkably suppressed. As a result, according to the present invention, it is possible to provide a plated steel material with improved LME resistance and hydrogen penetration resistance, as well as excellent corrosion resistance after hot stamping.
[0023]
The plated steel material according to the embodiment of the present invention will be described in detail below. In the following description, "%" regarding the content of each component means "% by mass" unless otherwise specified.
[0024]
[Steel base material]
The steel base material according to embodiments of the present invention may be a material having any thickness and composition, and is not particularly limited, for example, a material having a thickness and composition suitable for applying hot stamping. Preferably. Such a steel base material is known, for example, has a thickness of 0.3 to 2.3 mm, and, in mass%, C: 0.05 to 0.40%, Si: 0.50 % or less, Mn: 0.50 to 2.50%, P: 0.03% or less, S: 0.010% or less, sol. Al: 0.10% or less, N: 0.010% or less, balance: Fe and steel sheets that are impurities (for example, cold-rolled steel sheets). Hereinafter, each component contained in the above steel base material, which is preferably applied in the present invention, will be described in detail.
[0025]
[C: 0.05 to 0.40%]
Carbon (C) is an effective element for increasing the strength of hot stamped compacts. However, if the C content is too high, the toughness of the hot-stamped product may decrease. Therefore, the C content should be 0.05 to 0.40%. The C content is preferably 0.10% or more, more preferably 0.13% or more. The C content is preferably 0.35% or less.
[0026]
[Si: 0 to 0.50%]
Silicon (Si) is an effective element for deoxidizing steel. However, if the Si content is too high, Si in the steel diffuses during hot stamping heating to form oxides on the surface of the steel material, which may reduce the efficiency of phosphating. Also, Si is an element that raises the Ac 3 point of steel. For this reason, the heating temperature for hot stamping must be Ac 3 points or more, so if the amount of Si is excessive, the heating temperature for hot stamping steel must be high. That is, steel with a large amount of Si is heated to a higher temperature during hot stamping, and as a result, evaporation of Zn and the like in the coating layer cannot be avoided. In order to avoid such a situation, the Si content should be 0.50% or less. The Si content is preferably 0.30% or less, more preferably 0.20% or less. The Si content may be 0%, but in order to obtain an effect such as deoxidation, the lower limit of the Si content is generally 0.05%, although it varies depending on the desired deoxidization level. is.
[0027]
[Mn: 0.50 to 2.50%]
Manganese (Mn) increases hardenability and increases the strength of the hot stamped product. On the other hand, even if Mn is contained excessively, the effect is saturated. Therefore, the Mn content should be 0.50 to 2.50%. The Mn content is preferably 0.60% or more, more preferably 0.70% or more. The Mn content is preferably 2.40% or less, more preferably 2.30% or less.
[0028]
[P: 0.03% or less]
Phosphorus (P) is an impurity contained in steel. P segregates at grain boundaries to lower the toughness of the steel, thereby lowering the resistance to delayed fracture. Therefore, the P content should be 0.03% or less. It is preferable to reduce the P content as much as possible, preferably 0.02% or less. However, excessive reduction of the P content leads to an increase in cost, so it is preferable to set the P content to 0.0001% or more. Since the content of P is not essential, the lower limit of the P content is 0%.
[0029]
[S: 0.010% or less]
Sulfur (S) is an impurity contained in steel. S forms sulfides to lower the toughness of the steel and reduce the resistance to delayed fracture. Therefore, the S content should be 0.010% or less. It is preferable to reduce the S content as much as possible, preferably 0.005% or less. However, excessive reduction of the S content leads to an increase in cost, so the S content is preferably 0.0001% or more. Since the content of S is not essential, the lower limit of the S content is 0%.
[0030]
[sol. Al: 0 to 0.10%]
　Aluminum (Al) is effective in deoxidizing steel. However, the excessive content of Al raises the Ac 3 point of the steel material, so that the heating temperature of hot stamping becomes high, and evaporation of Zn and the like in the coating layer cannot be avoided. Therefore, the Al content should be 0.10% or less, preferably 0.05% or less. Although the Al content may be 0%, the Al content may be 0.01% or more in order to obtain effects such as deoxidation. In this specification, the Al content means the content of so-called acid-soluble Al (sol. Al).
[0031]
[N: 0.010% or less]
Nitrogen (N) is an impurity that is unavoidably contained in steel. N forms nitrides and lowers the toughness of steel. When boron (B) is further contained in the steel, N combines with B to reduce the amount of solid solution B and lower the hardenability. Therefore, the N content should be 0.010% or less. It is preferable to reduce the N content as much as possible, preferably 0.005% or less. However, excessive reduction of the N content causes a cost increase, so it is preferable to set the N content to 0.0001% or more. Since the N content is not essential, the lower limit of the N content is 0%.
[0032]
The basic chemical composition of the steel base material suitable for use in the embodiments of the present invention is as described above. Furthermore, the above steel base material optionally contains: B: 0-0.005%, Ti: 0-0.10%, Cr: 0-0.50%, Mo: 0-0.50%, Nb: One or more of 0 to 0.10% and Ni: 0 to 1.00% may be contained. These elements will be described in detail below. The content of each of these elements is not essential, and the lower limit of the content of each element is 0%.
[0033]
[B: 0 to 0.005%]
Boron (B) increases the hardenability of steel and increases the strength of the steel material after hot stamping, so it may be contained in the steel base material. However, even if B is contained excessively, the effect is saturated. Therefore, the B content should be 0 to 0.005%. The B content may be 0.0001% or more.
[0034]
[Ti: 0 to 0.10%]
Titanium (Ti) combines with nitrogen (N) to form nitrides, and can suppress deterioration of hardenability due to BN formation. In addition, Ti can refine the grain size of austenite during heating by hot stamping due to its pinning effect, and can increase the toughness of steel materials. However, even if Ti is contained excessively, the above effect is saturated, and if Ti nitrides precipitate excessively, the toughness of the steel may decrease. Therefore, the Ti content should be 0 to 0.10%. The Ti content may be 0.01% or more.
[0035]
[Cr: 0 to 0.50%]
Chromium (Cr) is effective in increasing the hardenability of steel and increasing the strength of hot-stamped products. However, if the Cr content is excessive and a large amount of Cr carbides that are difficult to dissolve during hot stamping are formed, the austenitization of the steel is difficult to progress, and conversely the hardenability is lowered. Therefore, the Cr content should be 0 to 0.50%. The Cr content may be 0.10% or more.
[0036]
[Mo: 0 to 0.50%]
Molybdenum (Mo) increases the hardenability of steel. However, even if Mo is contained excessively, the above effect is saturated. Therefore, the Mo content should be 0 to 0.50%. Mo content may be 0.05% or more.
[0037]
[Nb: 0 to 0.10%]
Niobium (Nb) is an element that forms carbides, refines grains during hot stamping, and increases the toughness of steel. However, if Nb is contained excessively, the above effect is saturated and the hardenability is further lowered. Therefore, the Nb content should be 0 to 0.10%. The Nb content may be 0.02% or more.
[0038]
[Ni: 0 to 1.00%]
Nickel (Ni) is an element that can suppress embrittlement caused by molten Zn during hot stamping heating. However, even if Ni is contained excessively, the above effect is saturated. Therefore, the Ni content should be 0 to 1.00%. The Ni content may be 0.10% or more.
[0039]
In the steel base material according to the embodiment of the present invention, the balance other than the above components consists of Fe and impurities. Impurities in the steel base material are components that are mixed due to various factors in the manufacturing process, including raw materials such as ores and scraps, when industrially manufacturing the plated steel material according to the embodiment of the present invention. means a component that is not intentionally added to the plated steel.
[0040]
[Plating layer]
According to an embodiment of the present invention, a plating layer is formed on the surface of the steel base material. For example, when the steel base material is a steel plate, a plating layer is formed on at least one side of the steel plate, that is, on one or both sides of the steel plate. be done. The plating layer has the following average composition.
[0041]
[Al: 25.00 to 75.00%]
Al is an element necessary for forming acicular Al-Zn-Si-Ca phase, α phase, and α/τ eutectic phase in the plating layer. If the Al content is less than 25.00%, it becomes difficult to generate a sufficient amount of the acicular Al--Zn--Si--Ca phase, so the Al content is made 25.00% or more. It is necessary for the plated steel sheet to have cold workability that can withstand bending and the like before it is subjected to hot stamping. It is preferably 30.00% or more or 35.00% or more. Furthermore, the Al content is preferably 40.00% or more in order to ensure the generation of the τ phase. On the other hand, when the Al content exceeds 75.00%, intermetallic compounds such as Al 4 Ca are preferentially generated, and as a result, it becomes difficult to generate acicular Al--Zn--Si--Ca phases. Therefore, the Al content is 75.00% or less, preferably 65.00% or less, more preferably 50.00% or less, and most preferably 45.00% or less.
[0042]
[Mg: 7.00 to 20.00%]
　Mg is an element that is effective in improving the corrosion resistance of the plating layer and improving coating film blistering. On the other hand, if the Mg content is too low, the equilibrium balance will be lost, making it difficult to generate a sufficient amount of the acicular Al--Zn--Si--Ca phase. In addition, the MgZn 2 phase is easily formed, Workability is also reduced. Therefore, the Mg content should be 7.00% or more. From the viewpoint of corrosion resistance, the Mg content is preferably 9.00% or more. On the other hand, if the Mg content is too high, excessive sacrificial anti-corrosion action tends to rapidly increase blistering of the paint film and flow rust. Therefore, the Mg content should be 20.00% or less, preferably 15.00 or less.
[0043]
[Si: 0.10 to 5.00%]
Si is an essential element for forming the acicular Al-Zn-Si-Ca phase. In order to form a sufficient amount of acicular Al--Zn--Si--Ca phase, the Si content should be 0.10% or more, preferably 0.40% or more. On the other hand, when the Si content is excessive, a Mg 2 Si phase is formed at the interface between the steel base material and the plating layer, greatly deteriorating the corrosion resistance. Moreover, when the Si content is excessive, the Mg 2 Si phase is preferentially formed, and as a result, it becomes difficult to generate a sufficient amount of the acicular Al--Zn--Si--Ca phase. Therefore, the Si content is 5.00% or less, preferably 1.50% or less, and more preferably 1.00% or less.
[0044]
[Ca: 0.05 to 5.00%]
Ca is an essential element for forming the acicular Al-Zn-Si-Ca phase. Furthermore, Ca can also suppress the formation of top dross formed on the plating bath during manufacturing. In order to generate a sufficient amount of acicular Al--Zn--Si--Ca phase, the Ca content should be 0.05% or more, preferably 0.40% or more. On the other hand, when the Ca content is excessive, intermetallic compounds such as Al4Ca are preferentially generated, and as a result, it is difficult to generate a sufficient amount of acicular Al-Zn-Si-Ca phase. Become. Therefore, the Ca content is 5.00% or less, preferably 3.00% or less, and more preferably 1.50% or less.
[0045]
The chemical composition of the plating layer is as above. Furthermore, the plating layer is optionally Sb: 0 to 0.50%, Pb: 0 to 0.50%, Cu: 0 to 1.00%, Sn: 0 to 1.00%, Ti: 0 to 1 .00%, Sr: 0 to 0.50%, Cr: 0 to 1.00%, Ni: 0 to 1.00%, and Mn: 0 to 1.00% containing one or more You may Although not particularly limited, the total content of these elements is preferably 5.00% or less, and 2.00% or less, from the viewpoint of sufficiently exhibiting the actions and functions of the above basic components constituting the plating layer. is more preferable. These elements will be described in detail below.
[0046]
[Sb: 0-0.50%, Pb: 0-0.50%, Cu: 0-1.00%, Sn: 0-1.00%, Ti: 0-1.00%]
Sb, Pb, Cu, Sn, and Ti can be contained in the MgZn 2 phase or τ phase present in the plating layer, but within a predetermined content range, it does not adversely affect the performance as a plated steel material. . However, if the content of each element is excessive, oxides of these elements may form on the outermost surface of the plating layer during heating in hot stamping. In such a case, the phosphating treatment becomes unsatisfactory and the corrosion resistance after painting deteriorates. Furthermore, when the Pb and Sn contents are excessive, the LME resistance tends to decrease. Therefore, the content of Sb and Pb is 0.50% or less, preferably 0.20% or less, and the content of Cu, Sn and Ti is 1.00% or less, preferably 0.80% or less, more preferably is 0.50% or less. On the other hand, the content of each element may be 0.01% or more. The content of these elements is not essential, and the lower limit of the content of each element is 0%.
[0047]
[Sr: 0 to 0.50%]
By including Sr in the plating bath when manufacturing the plating layer, it is possible to suppress the formation of top dross formed on the plating bath. In addition, since Sr tends to suppress atmospheric oxidation during heating for hot stamping, it is possible to suppress color change in the molded body after hot stamping. Since these effects are exhibited even in a small amount, the Sr content may be 0.01% or more. Also, when Sr is added, Sr may be contained in acicular Al--Zn--Si--Ca phases. Even if a small amount of Sr is contained in the acicular Al--Zn--Si--Ca phase, it does not significantly affect the performance after hot stamping. However, when the Sr content increases, the corrosion resistance after hot stamping tends to decrease. Therefore, the Sr content is 0.50% or less, preferably 0.10% or less.
[0048]
[Cr: 0 to 1.00%, Ni: 0 to 1.00%, Mn: 0 to 1.00%]
Cr, Ni, and Mn are concentrated near the interface between the plating layer and the steel base material, and have the effect of eliminating spangles on the surface of the plating layer. In order to obtain such effects, the contents of Cr, Ni and Mn are each preferably 0.01% or more. On the other hand, these elements are contained in the α phase and α/τ eutectic phase in the plating layer, but when the content of these elements is excessive, the occurrence of paint film swelling and flowing rust increases. , the corrosion resistance tends to deteriorate. Therefore, the contents of Cr, Ni and Mn are each set to 1.00% or less, preferably 0.50% or less, and more preferably 0.10% or less.
[0049]
[Remainder: Zn and impurities]
　In the plating layer, the balance other than the above components consists of Zn and impurities. Zn contains needle-like Al-Zn-Si-Ca phase, α phase mainly composed of Al and Zn, and α/τ eutectic phase (the above α phase and Mg, Zn and Al as main components) in the plating layer. It exists as a eutectic phase of the τ phase). Therefore, Zn is an essential element for suppressing the generation of LME and the penetration of hydrogen into steel in the hot stamping process, and also for maintaining sufficient corrosion resistance in the compact after hot stamping. If the Zn content is less than 10.00%, it may not be possible to form a sufficient amount of acicular Al--Zn--Si--Ca phases in the surface texture of the plating layer. As a result, Zn and Mg evaporate during heating in hot stamping, causing hydrogen penetration into LME and steel materials, and furthermore, after hot stamping due to the disappearance of these elements due to the evaporation of Zn and Mg. There is a possibility that the corrosion resistance may be greatly reduced. Therefore, the Zn content is preferably 10.00% or more. The lower limit of Zn content may be 20.00%, 30.00%, 40.00% or 50.00%. Although there is no particular upper limit for the Zn content, it may be 65.00%, 60.00% or 55.00%. The sum of the Al content and the Zn content does not have to be specified, but the sum may be 70.00% or more. If necessary, the total may be 75.00% or more, 78.00% or more, 80.00% or more, 83.00% or more, or 85.00% or more.
[0050]
In addition, impurities in the plating layer are components that are mixed due to various factors in the manufacturing process, including raw materials, when manufacturing the plating layer, and are not intentionally added to the plating layer. means For example, impurities in the plating layer include elements such as Fe dissolved from the steel base material and the like into the plating bath, and the content of such Fe is generally 0 to 5.00%. More specifically, it is 1.00% or more and 3.00% or less or 2.50% or less. The plating layer may contain, as impurities, a small amount of elements other than the elements described above as long as the effects of the present invention are not hindered.
[0051]
In the present invention, the chemical composition of the plating layer is basically the same as the chemical composition in the plating bath for forming the plating layer, except for impurities mixed in when forming the plating layer. can be done.
[0052]
The thickness of the plating layer may be, for example, 3-50 μm. Moreover, when the steel base material is a steel plate, the plating layer may be provided on both sides of the steel plate, or may be provided only on one side. The coating weight of the plated layer is not particularly limited, but may be, for example, 10 to 170 g/m 2 per side. The lower limit may be 20 or 30 g/m 2 and the upper limit may be 150 or 130 g/m 2 . In the present invention, the adhesion amount of the plating layer is determined by dissolving the plating layer in an acid solution to which an inhibitor for suppressing corrosion of the base iron is added, and from the change in weight before and after pickling.
[0053]
[Surface structure of plating layer]
In the surface structure of the plating layer, there is an acicular Al-Zn-Si-Ca phase with an area ratio of 2.0% or more.
[0054]
[Acicular Al-Zn-Si-Ca phase]
The acicular Al-Zn-Si-Ca phase is an intermetallic compound whose main components are Al, Zn, Si and Ca. An acicular Al-Zn-Si-Ca phase is present in the surface texture of the plating layer, and as described above, Ca dissolved from the acicular Al-Zn-Si-Ca phase during heating in hot stamping. is preferentially oxidized with oxygen in the atmosphere to form an oxide film on the outermost surface of the plating layer. Then, the oxide film functions as a barrier layer, so that it is possible to reduce or suppress the evaporation of Zn and Mg in the plating layer to the outside and the entry of hydrogen from the outside. And it is thought that the decrease in corrosion resistance due to the evaporation of Mg to the outside is remarkably suppressed. As a result, it is considered that the LME resistance and hydrogen penetration resistance are improved.
[0055]
Such a Ca-based oxide film, for example, a Ca- and Mg-containing oxide film, functions as a barrier layer to reduce or suppress the evaporation of Zn and Mg in the plating layer to the outside and the entry of hydrogen from the outside. Furthermore, it is thought that the decrease in corrosion resistance due to the evaporation of Zn and Mg to the outside can be remarkably suppressed. As a result, according to the present invention, it is possible to provide a plated steel material with improved LME resistance and hydrogen penetration resistance, as well as excellent corrosion resistance after hot stamping. In order to obtain such an effect, the area ratio of the acicular Al--Zn--Si--Ca phase in the surface texture of the plating layer must be 2.0% or more. The greater the area ratio of the acicular Al--Zn--Si--Ca phase, the greater the effect of reducing or suppressing the evaporation of Zn and Mg in the coating layer to the outside and the entry of hydrogen from the outside. Therefore, the area ratio of the acicular Al--Zn--Si--Ca phase is preferably 4.0% or more or 6.0% or more, more preferably 8.0% or more or 10.0% or more. In particular, by setting the area ratio of the acicular Al-Zn-Si-Ca phase to 8.0% or more, a dense Ca-based oxide film, for example, a Ca- and Mg-containing oxide film can be formed in a sufficient amount. As a result, corrosion resistance, particularly long-term corrosion resistance, can be remarkably improved. The upper limit of the area ratio of the acicular Al-Zn-Si-Ca phase is not particularly limited, but is generally 20.0% or less, even if it is 18.0% or less or 15.0% or less. good.
[0056]
The acicular Al-Zn-Si-Ca phase can be easily identified from the characteristic needle-like shape and the density of the structure in the SEM BSE images shown in FIGS. 1, 2 and 3. , from which the area ratio can be measured. The chemical composition of the acicular Al-Zn-Si-Ca phase is, in mass%, Al content of 36.0 to 50.0%, Zn content of 20.0 to 80.0%, and Si content of 1 0 to 10.0%, the Ca content is 5.0 to 25.0%, and the total content of other elements as the balance is 5.0% or less. For this reason, if you cannot identify the acicular Al-Zn-Si-Ca phase from the SEM-BSE image, or if you want to confirm that it is an acicular Al-Zn-Si-Ca phase, SEM-EDS (Energy Dispersive X -Ray Spectroscopy) or EPMA to analyze the chemical composition of the structure, and if the resulting chemical composition is within the above range, the structure is considered to be an acicular Al-Zn-Si-Ca phase. can judge. In addition, the chemical composition analysis point during SEM-EDS mapping may be one point in the tissue to be investigated, but in order to improve the analysis accuracy, the average value of the chemical composition of at least three points in the tissue is used. is preferred.
[0057]
The surface structure of the plating layer according to the embodiment of the present invention has an acicular Al-Zn-Si-Ca phase with an area ratio of 2.0% or more, and other phases such as an α phase and an α/τ phase described later. The area ratio of each phase is within the normal surface texture of the plating layer having the above chemical composition. Therefore, in the present invention, it is not necessary to specify the surface structure other than the acicular Al--Zn--Si--Ca phase. For reference, the organization is described below.
[0058]
[α phase]
The α-phase is a structure mainly composed of Al and Zn. A plated steel material may be subjected to cold working such as bending before it is subjected to hot stamping. The α phase is a solid solution and has ductility. Therefore, the α phase can function to suppress the peeling of the plating layer from the steel base material during such cold working. In order to ensure cold workability, the area ratio of the α phase in the surface texture of the plating layer is preferably 5.0% or more, more preferably 10.0% or more, or 15.0% or more, or 20.0% or more. On the other hand, if the area ratio of the α phase exceeds 80.0%, a sufficient area ratio of the acicular Al—Zn—Si—Ca phase cannot be secured, and Zn and Mg during heating in hot stamping In some cases, it becomes difficult to suppress the evaporation of the hydrogen and the penetration of hydrogen. Therefore, the area ratio of the α phase is preferably 80.0% or less, more preferably 70.0% or less or 60.0% or less.
[0059]
The α-phase can be easily identified from the density of the structure and its characteristic shape in the SEM BSE images shown in FIGS. 1, 2 and 3, and the area ratio can be measured from the results. can be done. The chemical composition of the α phase is 20.0 to 89.9% Al content, 0.1 to 70.0% Zn content, 0 to 5.0% Mg content, Al content and Zn content. The total amount is 90.0% or more, and the total content of other elements as the balance is 1.00% or less. Therefore, if the α phase cannot be identified from the SEM-BSE image, or if it is desired to confirm that it is the α phase, the chemical composition of the structure is analyzed by SEM-EDS or EPMA, and the resulting chemical composition is within the above range, the tissue can be determined to be in the α phase. In addition, the chemical composition analysis point during SEM-EDS mapping may be one point in the tissue to be investigated, but in order to improve the analysis accuracy, the average value of the chemical composition of at least three points in the tissue is used. is preferred.
[0060]
[α/τ eutectic phase]
The α/τ eutectic phase is composed of the α phase and the τ phase, which is a structure mainly composed of Mg, Al and Zn. The stoichiometric composition of the τ phase is Mg 32 (Zn, Al) 49 and there is some degree of freedom in the ratio of Al and Zn. The α/τ eutectic phase has a lamellar structure in which small rod-like α phases are dispersed in the τ phase, which is the matrix phase, as indicated by reference numeral 2 in FIG. Since the α/τ eutectic phase contains both Mg and Zn, which are elements having the effect of improving corrosion resistance, it is a structure useful for ensuring corrosion resistance after hot stamping. In order to ensure the effect of improving corrosion resistance, the area ratio of the α/τ eutectic phase in the surface texture of the plating layer is preferably 20.0% or more, more preferably 25.0% or more. . On the other hand, when the area ratio of the α/τ eutectic phase exceeds 90.0%, the cold workability may deteriorate. Therefore, the area ratio of the α/τ eutectic phase is preferably 90.0% or less, more preferably 80.0% or less. The α/τ eutectic phase is a characteristic structure of a lamellar structure in which small rod-like α phases are dispersed in the τ phase, which is the matrix phase. The α/τ eutectic phase can be easily identified without analyzing the composition. Therefore, the plated steel material according to the present invention does not require analysis of the chemical composition of the α/τ eutectic phase.
[0061]
[other phases]
In the surface structure of the plating layer, there may be other phases other than the above three phases as a residual structure. Examples of the other phases include, but are not limited to, massive τ phase, MgZ 2 phase, and phases composed of other compounds (such as Al 4 Ca and Mg 2 Si). However, if the area ratio of the other phase in the surface structure of the plating layer becomes too large, it is not possible to secure a sufficient area ratio of the acicular Al—Zn—Si—Ca phase, and heating in hot stamping is difficult. It may be difficult to suppress the evaporation of Zn and Mg and the penetration of hydrogen during the process. Therefore, the total area ratio of the other phases as the residual structure is preferably less than 10.0%, more preferably 5.0% or less or 4.0% or less, and most preferably 3.0% or less. be. The presence of these phases (including compounds; the same shall apply hereinafter) is not essential, and the lower limits of the area ratios of these phases are all 0%. The residual structure is within the range of the surface structure of the plated layer of the known plated steel. Therefore, although it is not necessary to specify the area ratio of these residual structures, the chemical composition and area ratio of each phase constituting the residual structure, etc., they are described below for reference.
[0062]
(Lumpy τ phase)
The massive τ phase is mainly composed of Mg, Zn and Al (however, the total of Mg content, Zn content and Al content is 90% or more), and is a phase that may be formed in non-equilibrium solidification. The massive τ phase tends to form more easily as the Mg content of the plating bath increases, and is often formed adjacent to the α/τ eutectic phase. The difference from the τ phase in the α/τ eutectic phase is that it does not form a lamellar structure with the α phase. Therefore, when there is a region of the τ phase that is surrounded by the α phase and does not form a lamellar structure with the α phase, the longest diameter (major diameter) of the τ phase region and the diameter perpendicular to it When the longest diameter (minor axis) is measured and the minor axis is at least 3 times the interval (lamellar interval) of the α phase constituting the α/τ eutectic phase, such a τ phase is α/ It is defined as a massive τ phase separately from the τ eutectic phase. The massive τ phase is essentially brittle, and unlike the α/τ eutectic phase, it does not have a mixed phase structure with an α phase having plastic deformability, and thus causes deterioration of the cold workability of the coating layer. If the area ratio of the massive τ phase in the surface texture of the plating layer is 10.0% or more, the amount of active Mg--Zn intermetallic compounds increases and the cold workability may deteriorate. Therefore, the area ratio of the massive τ phase in the surface texture of the plating layer is preferably less than 10.0%, more preferably 5.0% or less, and most preferably 3.0% or less or 2.0% or less. .
[0063]
(MgZn 2 phase)
The MgZn2 phase is a phase in which Mg and Zn are the main components (however, the total content of Mg and Zn is 90% or more) and may be formed in non-equilibrium solidification. The MgZn 2 phase tends to be formed more easily as the Mg content of the plating bath is lower. Since the MgZn2 phase contains Mg and Zn, it may contribute to the improvement of corrosion resistance after hot stamping. It may lead to deterioration of workability. Therefore, the area ratio of the MgZn2 phase in the surface texture of the plating layer is preferably less than 10.0%, more preferably 5.0% or less, most preferably 3.0% or less or 2.0% or less .
[0064]
(other compounds)
Other compounds include intermetallic compounds such as Al4Ca and Mg2Si. Al 4 Ca tends to be formed more easily as the Si content of the plating bath is lower, and similarly Mg 2 Si tends to be more easily formed as the Ca content of the plating bath is lower. All of these intermetallic compounds are brittle, so if the content is 5.0% or more, cold workability may be deteriorated. Therefore, the area ratio of each compound of Al 4 Ca and Mg 2 Si in the surface texture of the plating layer is preferably less than 5.0%, more preferably 3.0% or less or 2.0% or less. Incidentally, Si may be contained in Al 4 Ca.
[0065]
In the present invention, the area ratio of each phase in the surface texture of the plating layer is determined as follows. First, the prepared sample was cut into a size of 25 mm × 15 mm, and the area ratio of each phase was determined from the backscattered electron image (BSE image) of a scanning electron microscope (SEM) taken at a magnification of 1500 times the plating layer surface. Measured by computer image processing, the average of these measured values ​​in any 5 fields of view (however, the measurement area of ​​​​each field of view is 400 μm 2 or more) is acicular Al-Zn-Si-Ca phase, α phase, α / τ is determined as the area fraction of the eutectic phase, as well as other phases and compounds. In particular, the area ratio of the massive τ phase is determined by measuring the area ratio of the region surrounded by the boundary defined by the α phase present around the massive τ phase by computer image processing.
[0066]
In the present invention, it is not necessary to analyze the chemical compositions of the acicular Al-Zn-Si-Ca phase and the α phase, but it cannot be identified as the acicular Al-Zn-Si-Ca phase or the α phase from the SEM-BSE image. If you want to confirm that it is an acicular Al-Zn-Si-Ca phase or an α phase, the chemical composition of the structure is analyzed by SEM-EDS or EPMA, and the resulting chemical composition is the above. within the range of, it can be determined that the tissue is in the α phase. In addition, the chemical composition analysis point during SEM-EDS mapping may be one point in the tissue to be investigated, but in order to improve the analysis accuracy, the average value of the chemical composition of at least three points in the tissue is used. is preferred.
[0067]

Next, a preferred method for manufacturing the plated steel material according to the embodiment of the present invention will be described. The following description is intended to illustrate a characteristic method for manufacturing the plated steel material according to the embodiment of the present invention, and the plated steel material is manufactured by the manufacturing method described below. is not intended to be limited to
[0068]
The above manufacturing method includes a step of forming a steel base material and a step of forming a plating layer on the steel base material. Each step will be described in detail below.
[0069]
[Formation process of steel base material]
In the process of forming the steel base material, for example, first, molten steel having the same chemical composition as described above for the steel base material is produced, and the produced molten steel is used to produce a slab by casting. Alternatively, an ingot may be manufactured by an ingot casting method using the manufactured molten steel. The slab or ingot is then hot rolled to produce a steel base material (hot rolled steel plate). If necessary, the hot-rolled steel sheet may be pickled, then the hot-rolled steel sheet may be cold-rolled, and the obtained cold-rolled steel sheet may be used as the steel base material.
[0070]
[Step of forming plating layer]
Next, in the plating layer forming step, a plating layer having the chemical composition described above is formed on at least one side, preferably both sides, of the steel base material.
[0071]
More specifically, first, the above steel base material is heat-reduced in a N 2 -H 2 mixed gas atmosphere at a predetermined temperature and time, for example, at a temperature of 750 to 850 ° C., and then treated with an inert gas such as a nitrogen atmosphere. Cool to near the plating bath temperature in an atmosphere. Then, after the steel base material is immersed in a plating bath having the same chemical composition as the chemical composition of the plating layer described above for 0.1 to 60 seconds, it is pulled out and immediately blown with N 2 gas or air by a gas wiping method. By doing so, the adhesion amount of the plating layer is adjusted within a predetermined range.
[0072]
The chemical composition of the plating bath preferably has the chemical composition described above for the plating layer and satisfies the following formula (1).
Zn/(Mg + 3 x Ca) ≤ 6.5 (1)
　In formula (1), Zn, Mg and Ca are the contents (mass%) of each element.
By satisfying the formula (1), it is possible to more reliably allow the acicular Al-Zn-Si-Ca phase to exist in the surface structure of the plating layer at an area ratio of 2.0% or more. Therefore, it is possible to remarkably reduce or suppress the penetration of hydrogen into the LME and the steel material, and achieve sufficient corrosion resistance even in the compact after hot stamping.
[0073]
In addition, it is preferable that the coating weight of the plating layer is 10 to 170 g/m 2 per side. In this process, pre-plating such as Ni pre-plating and Sn pre-plating can be applied to assist plating adhesion.0057]
The surface structure of the plating layer according to the embodiment of the present invention has an acicular Al-Zn-Si-Ca phase with an area ratio of 2.0% or more, and other phases such as an α phase and an α/τ phase described later. The area ratio of each phase is within the normal surface texture of the plating layer having the above chemical composition. Therefore, in the present invention, it is not necessary to specify the surface structure other than the acicular Al--Zn--Si--Ca phase. For reference, the organization is described below.
[0058]
[α phase]
The α-phase is a structure mainly composed of Al and Zn. A plated steel material may be subjected to cold working such as bending before it is subjected to hot stamping. The α phase is a solid solution and has ductility. Therefore, the α phase can function to suppress the peeling of the plating layer from the steel base material during such cold working. In order to ensure cold workability, the area ratio of the α phase in the surface texture of the plating layer is preferably 5.0% or more, more preferably 10.0% or more, or 15.0% or more, or 20.0% or more. On the other hand, if the area ratio of the α phase exceeds 80.0%, a sufficient area ratio of the acicular Al—Zn—Si—Ca phase cannot be secured, and Zn and Mg during heating in hot stamping In some cases, it becomes difficult to suppress the evaporation of the hydrogen and the penetration of hydrogen. Therefore, the area ratio of the α phase is preferably 80.0% or less, more preferably 70.0% or less or 60.0% or less.
[0059]
The α-phase can be easily identified from the density of the structure and its characteristic shape in the SEM BSE images shown in FIGS. 1, 2 and 3, and the area ratio can be measured from the results. can be done. The chemical composition of the α phase is 20.0 to 89.9% Al content, 0.1 to 70.0% Zn content, 0 to 5.0% Mg content, Al content and Zn content. The total amount is 90.0% or more, and the total content of other elements as the balance is 1.00% or less. Therefore, if the α phase cannot be identified from the SEM-BSE image, or if it is desired to confirm that it is the α phase, the chemical composition of the structure is analyzed by SEM-EDS or EPMA, and the resulting chemical composition is within the above range, the tissue can be determined to be in the α phase. In addition, the chemical composition analysis point during SEM-EDS mapping may be one point in the tissue to be investigated, but in order to improve the analysis accuracy, the average value of the chemical composition of at least three points in the tissue is used. is preferred.
[0060]
[α/τ eutectic phase]
The α/τ eutectic phase is composed of the α phase and the τ phase, which is a structure mainly composed of Mg, Al and Zn. The stoichiometric composition of the τ phase is Mg 32 (Zn, Al) 49 and there is some degree of freedom in the ratio of Al and Zn. The α/τ eutectic phase has a lamellar structure in which small rod-like α phases are dispersed in the τ phase, which is the matrix phase, as indicated by reference numeral 2 in FIG. Since the α/τ eutectic phase contains both Mg and Zn, which are elements having the effect of improving corrosion resistance, it is a structure useful for ensuring corrosion resistance after hot stamping. In order to ensure the effect of improving corrosion resistance, the area ratio of the α/τ eutectic phase in the surface texture of the plating layer is preferably 20.0% or more, more preferably 25.0% or more. . On the other hand, when the area ratio of the α/τ eutectic phase exceeds 90.0%, the cold workability may deteriorate. Therefore, the area ratio of the α/τ eutectic phase is preferably 90.0% or less, more preferably 80.0% or less. The α/τ eutectic phase is a characteristic structure of a lamellar structure in which small rod-like α phases are dispersed in the τ phase, which is the matrix phase. The α/τ eutectic phase can be easily identified without analyzing the composition. Therefore, the plated steel material according to the present invention does not require analysis of the chemical composition of the α/τ eutectic phase.
[0061]
[other phases]
In the surface structure of the plating layer, there may be other phases other than the above three phases as a residual structure. Examples of the other phases include, but are not limited to, massive τ phase, MgZ 2 phase, and phases composed of other compounds (such as Al 4 Ca and Mg 2 Si). However, if the area ratio of the other phase in the surface structure of the plating layer becomes too large, it is not possible to secure a sufficient area ratio of the acicular Al—Zn—Si—Ca phase, and heating in hot stamping is difficult. It may be difficult to suppress the evaporation of Zn and Mg and the penetration of hydrogen during the process. Therefore, the total area ratio of the other phases as the residual structure is preferably less than 10.0%, more preferably 5.0% or less or 4.0% or less, and most preferably 3.0% or less. be. The presence of these phases (including compounds; the same shall apply hereinafter) is not essential, and the lower limits of the area ratios of these phases are all 0%. The residual structure is within the range of the surface structure of the plated layer of the known plated steel. Therefore, although it is not necessary to specify the area ratio of these residual structures, the chemical composition and area ratio of each phase constituting the residual structure, etc., they are described below for reference.
[0062]
(Lumpy τ phase)
The massive τ phase is mainly composed of Mg, Zn and Al (however, the total of Mg content, Zn content and Al content is 90% or more), and is a phase that may be formed in non-equilibrium solidification. The massive τ phase tends to form more easily as the Mg content of the plating bath increases, and is often formed adjacent to the α/τ eutectic phase. The difference from the τ phase in the α/τ eutectic phase is that it does not form a lamellar structure with the α phase. Therefore, when there is a region of the τ phase that is surrounded by the α phase and does not form a lamellar structure with the α phase, the longest diameter (major diameter) of the τ phase region and the diameter perpendicular to it When the longest diameter (minor axis) is measured and the minor axis is at least 3 times the interval (lamellar interval) of the α phase constituting the α/τ eutectic phase, such a τ phase is α/ It is defined as a massive τ phase separately from the τ eutectic phase. The massive τ phase is essentially brittle, and unlike the α/τ eutectic phase, it does not have a mixed phase structure with an α phase having plastic deformability, and thus causes deterioration of the cold workability of the coating layer. If the area ratio of the massive τ phase in the surface texture of the plating layer is 10.0% or more, the amount of active Mg--Zn intermetallic compounds increases and the cold workability may deteriorate. Therefore, the area ratio of the massive τ phase in the surface texture of the plating layer is preferably less than 10.0%, more preferably 5.0% or less, and most preferably 3.0% or less or 2.0% or less. .
[0063]
(MgZn 2 phase)
The MgZn2 phase is a phase in which Mg and Zn are the main components (however, the total content of Mg and Zn is 90% or more) and may be formed in non-equilibrium solidification. The MgZn 2 phase tends to be formed more easily as the Mg content of the plating bath is lower. Since the MgZn2 phase contains Mg and Zn, it may contribute to the improvement of corrosion resistance after hot stamping. It may lead to deterioration of workability. Therefore, the area ratio of the MgZn2 phase in the surface texture of the plating layer is preferably less than 10.0%, more preferably 5.0% or less, most preferably 3.0% or less or 2.0% or less .
[0064]
(other compounds)
Other compounds include intermetallic compounds such as Al4Ca and Mg2Si. Al 4 Ca tends to be formed more easily as the Si content of the plating bath is lower, and similarly Mg 2 Si tends to be more easily formed as the Ca content of the plating bath is lower. All of these intermetallic compounds are brittle, so if the content is 5.0% or more, cold workability may be deteriorated. Therefore, the area ratio of each compound of Al 4 Ca and Mg 2 Si in the surface texture of the plating layer is preferably less than 5.0%, more preferably 3.0% or less or 2.0% or less. Incidentally, Si may be contained in Al 4 Ca.
[0065]
In the present invention, the area ratio of each phase in the surface texture of the plating layer is determined as follows. First, the prepared sample was cut into a size of 25 mm × 15 mm, and the area ratio of each phase was determined from the backscattered electron image (BSE image) of a scanning electron microscope (SEM) taken at a magnification of 1500 times the plating layer surface. Measured by computer image processing, the average of these measured values ​​in any 5 fields of view (however, the measurement area of ​​​​each field of view is 400 μm 2 or more) is acicular Al-Zn-Si-Ca phase, α phase, α / τ is determined as the area fraction of the eutectic phase, as well as other phases and compounds. In particular, the area ratio of the massive τ phase is determined by measuring the area ratio of the region surrounded by the boundary defined by the α phase present around the massive τ phase by computer image processing.
[0066]
In the present invention, it is not necessary to analyze the chemical compositions of the acicular Al-Zn-Si-Ca phase and the α phase, but it cannot be identified as the acicular Al-Zn-Si-Ca phase or the α phase from the SEM-BSE image. If you want to confirm that it is an acicular Al-Zn-Si-Ca phase or an α phase, the chemical composition of the structure is analyzed by SEM-EDS or EPMA, and the resulting chemical composition is the above. within the range of, it can be determined that the tissue is in the α phase. In addition, the chemical composition analysis point during SEM-EDS mapping may be one point in the tissue to be investigated, but in order to improve the analysis accuracy, the average value of the chemical composition of at least three points in the tissue is used. is preferred.
[0067]

Next, a preferred method for manufacturing the plated steel material according to the embodiment of the present invention will be described. The following description is intended to illustrate a characteristic method for manufacturing the plated steel material according to the embodiment of the present invention, and the plated steel material is manufactured by the manufacturing method described below. is not intended to be limited to
[0068]
The above manufacturing method includes a step of forming a steel base material and a step of forming a plating layer on the steel base material. Each step will be described in detail below.
[0069]
[Formation process of steel base material]
In the process of forming the steel base material, for example, first, molten steel having the same chemical composition as described above for the steel base material is produced, and the produced molten steel is used to produce a slab by casting. Alternatively, an ingot may be manufactured by an ingot casting method using the manufactured molten steel. The slab or ingot is then hot rolled to produce a steel base material (hot rolled steel plate). If necessary, the hot-rolled steel sheet may be pickled, then the hot-rolled steel sheet may be cold-rolled, and the obtained cold-rolled steel sheet may be used as the steel base material.
[0070]
[Step of forming plating layer]
Next, in the plating layer forming step, a plating layer having the chemical composition described above is formed on at least one side, preferably both sides, of the steel base material.
[0071]
More specifically, first, the above steel base material is heat-reduced in a N 2 -H 2 mixed gas atmosphere at a predetermined temperature and time, for example, at a temperature of 750 to 850 ° C., and then treated with an inert gas such as a nitrogen atmosphere. Cool to near the plating bath temperature in an atmosphere. Then, after the steel base material is immersed in a plating bath having the same chemical composition as the chemical composition of the plating layer described above for 0.1 to 60 seconds, it is pulled out and immediately blown with N 2 gas or air by a gas wiping method. By doing so, the adhesion amount of the plating layer is adjusted within a predetermined range.
[0072]
The chemical composition of the plating bath preferably has the chemical composition described above for the plating layer and satisfies the following formula (1).
Zn/(Mg + 3 x Ca) ≤ 6.5 (1)
　In formula (1), Zn, Mg and Ca are the contents (mass%) of each element.
By satisfying the formula (1), it is possible to more reliably allow the acicular Al-Zn-Si-Ca phase to exist in the surface structure of the plating layer at an area ratio of 2.0% or more. Therefore, it is possible to remarkably reduce or suppress the penetration of hydrogen into the LME and the steel material, and achieve sufficient corrosion resistance even in the compact after hot stamping.
[0073]
In addition, it is preferable that the coating weight of the plating layer is 10 to 170 g/m 2 per side. In this process, pre-plating such as Ni pre-plating and Sn pre-plating can be applied to assist plating adhesion.Therefore, the equilibrium balance is disturbed, and the acicular Al-Zn-Si-Ca phase is not sufficiently formed in the surface structure of the coating layer, resulting in poor evaluation of LME resistance, hydrogen penetration resistance and corrosion resistance. rice field. In addition, a large amount of MgZn 2 phase was generated, and evaluation of cold workability was also unsatisfactory. In Comparative Example 4, the Mg content in the plating layer was high, and the excessive sacrificial anti-corrosion action reduced the corrosion resistance. . Furthermore, since a relatively large amount of massive τ phase containing Mg, Zn and Al as main components was generated, evaluation of cold workability was also unsatisfactory. In Comparative Example 11, since the value of Zn/(Mg+3×Ca) in the plating layer exceeded 6.5, the equilibrium balance was lost, and the surface structure of the plating layer had sufficient needle-like Al—Zn—Si—Ca phases. As a result, the evaluation of LME resistance, hydrogen penetration resistance and corrosion resistance was unsatisfactory. In Comparative Examples 12 and 13, since the cooling of the plating layer did not satisfy the predetermined two-stage cooling conditions, the acicular Al-Zn-Si-Ca phase was not sufficiently formed in the surface structure of the plating layer, resulting in As a result, the evaluation of LME resistance, hydrogen penetration resistance and corrosion resistance was unsatisfactory. In Comparative Example 14, since Si was not contained in the plating layer, an acicular Al-Zn-Si-Ca phase was not formed in the surface texture of the plating layer, resulting in LME resistance and hydrogen penetration resistance. And the evaluation of corrosion resistance was unsatisfactory. In addition, in Comparative Example 14, since Si was not contained in the plating layer, 5.0% or more of Al 4 Ca was formed, and the evaluation of cold workability was also unsatisfactory. In Comparative Example 19, since Ca was not contained in the plating layer, an acicular Al-Zn-Si-Ca phase was not formed in the surface structure of the plating layer, resulting in LME resistance and hydrogen penetration resistance. And the evaluation of corrosion resistance was unsatisfactory. In addition, in Comparative Example 19, since Ca was not contained in the plating layer, 5.0% or more of Mg 2 Si was formed, and the evaluation of cold workability was also unsatisfactory. In Comparative Examples 20 and 30, the Ca content or Al content in the plating layer was too high, so Al 4 Ca was preferentially formed in the plating layer, and the acicular Al-Zn-Si-Ca phase was sufficiently formed. As a result, the evaluation of LME resistance, hydrogen penetration resistance and corrosion resistance was unsatisfactory. Furthermore, since Al 4 Ca was generated in an amount of 5.0% or more, the evaluation of cold workability was also unsatisfactory. In Comparative Example 26, since the Si content in the plating layer was too high, the Mg 2 Si phase was preferentially formed in the plating layer, and the acicular Al-Zn-Si-Ca phase was not sufficiently formed. Evaluation of LME resistance, hydrogen penetration resistance and corrosion resistance was unsatisfactory. Furthermore, since the Mg 2 Si phase was formed in an amount of 5.0% or more, the evaluation of cold workability was also unsatisfactory. In Comparative Example 31 using a conventional galvannealed steel sheet, the resistance to hydrogen penetration was excellent, but the evaluation of the LME resistance and corrosion resistance was unsatisfactory. In Comparative Example 32 using a conventional hot-dip aluminized steel sheet, although the LME resistance was excellent, the hydrogen penetration resistance and corrosion resistance were evaluated as unsatisfactory.
[0091]
In contrast, in all the examples according to the present invention, the chemical composition of the plating layer and the area ratio of the acicular Al-Zn-Si-Ca phase contained in the surface texture of the plating layer were appropriately controlled. By doing so, it was possible to obtain a plated steel material with improved LME resistance and hydrogen penetration resistance, as well as excellent corrosion resistance after hot stamping. In particular, referring to Tables 1 and 2, by controlling the Al content in the plating layer to 35.00 to 50.00%, the LME resistance is significantly improved, and similarly, the Mg content in the plating layer is It can be seen that the corrosion resistance is remarkably improved by controlling to 9.00 to 15.00%. In addition, in Examples 3, 5, 15 and 18 in which the area ratio of the acicular Al-Zn-Si-Ca phase is 2.0% or more and less than 8.0%, corrosion resistance evaluation after 150 cycles in the combined cycle corrosion test is B, and the corrosion resistance evaluation after 360 cycles is somewhat lower than the corrosion resistance evaluation after 150 cycles, while the area ratio of the acicular Al-Zn-Si-Ca phase is 8.0%. In the above Examples 6 to 10, 16, 17, 21 to 25, and 27 to 29, the corrosion resistance evaluation after 150 cycles is A, and the corrosion resistance evaluation after 360 cycles is the corrosion resistance evaluation after 150 cycles. were the same and therefore showed high corrosion resistance and long-term corrosion resistance.
[0092]
[Example B]
In this example, we examined the two-stage cooling conditions for the plating layer. First, plating layers were formed on both sides of the steel base material in the same manner as in Example A except that a plating bath having the chemical composition shown in Table 3 was used and the plating layers were formed under the conditions shown in Table 3. A plated steel material was obtained. The surface structure of the plated layer in the obtained plated steel and the properties of the plated steel after hot stamping were examined in the same manner as in Example A. Table 4 shows the results.
[0093]
[Table 3]

[0094]
[Table 4]

[0095]
Referring to Tables 3 and 4, in Comparative Example 41, in which the average cooling rate in the first stage of the plating layer was 10 ° C./sec, the average cooling rate was somewhat low, so acicular Al -Zn-Si-Ca phase was not sufficiently formed, resulting in poor evaluation of LME resistance, hydrogen penetration resistance and corrosion resistance. In addition, in Comparative Examples 42 and 43, in which the average cooling rate of the second stage of the plating layer was 7° C./sec, the average cooling rate was somewhat high. The Zn--Si--Ca phase was not sufficiently formed, and as a result, the evaluation of LME resistance, hydrogen penetration resistance and corrosion resistance was unsatisfactory. From the results in Tables 1 to 4, in order to more reliably form the acicular Al-Zn-Si-Ca phase at an area ratio of 2.0% or more, It has been found preferable to cool from the bath temperature to 450° C. at an average cooling rate and then from 450° C. to 350° C. at an average cooling rate of 5.5° C./s or less, or 5° C./s or less.
[0096]
[Example C]
In this example, we investigated the cooling rate changes between rapid cooling and slow cooling in the two-stage cooling of the plating layer. First, the plating bath (bath temperature 600 ° C.), and the cooling rate change points were changed to 375 ° C., 400 ° C., 425 ° C., 450 ° C., 475 ° C. and 500 ° C., the average cooling rate in the first stage was 15 ° C./s and in the second stage A plated steel material having a plated layer formed on both sides of the steel base material was obtained in the same manner as in Example A except that the average cooling rate was 5°C/sec. The area ratio of the acicular Al--Zn--Si--Ca phase in the surface structure of the plating layer in the obtained plated steel material was investigated. The results are shown in FIG.
[0097]
Referring to FIG. 4, when the cooling rate change point was 400 ° C., the area ratio of the acicular Al-Zn-Si-Ca phase was 1.9%, and although 2.0% or more could not be secured. , When the cooling rate change point is 425 ° C., 450 ° C. and 475 ° C., 2.0% or more of the acicular Al-Zn-Si-Ca phase can be formed, especially when the cooling rate change point is 450 ° C. In this case, the highest acicular Al-Zn-Si-Ca phase area fraction could be achieved.
Code explanation
[0098]
1 α phase
2 α/τ eutectic phase
3 Massive τ phase
4 Acicular Al-Zn-Si-Ca phase
5 MgZn 2-phase
The scope of the claims
[Claim 1]
A plated steel material comprising a steel base material and a plating layer formed on the surface of the steel base material, wherein the chemical composition of the plating layer is, in mass%,
Al: 25.00-75.00%,
　Mg: 7.00 to 20.00%,
Si: 0.10 to 5.00%,
Ca: 0.05 to 5.00%,
Sb: 0 to 0.50%,
Pb: 0 to 0.50%,
Cu: 0 to 1.00%,
Sn: 0 to 1.00%,
Ti: 0 to 1.00%,
Sr: 0 to 0.50%,
Cr: 0 to 1.00%,
Ni: 0 to 1.00%,
Mn: 0 to 1.00%, and
The balance: Zn and impurities,
A plated steel material in which the surface structure of the plating layer has an area ratio of 2.0% or more of an acicular Al-Zn-Si-Ca phase.
[Claim 2]
The surface texture of the plating layer is, in terms of area ratio,
Acicular Al-Zn-Si-Ca phase: 2.0-20.0%,
α phase: 5.0 to 80.0%,
α/τ eutectic phase: 20.0 to 90.0%,
The plated steel material according to claim 1, wherein other residual structures: less than 10.0%.
[Claim 3]
The chemical composition of the plating layer is, in mass%,
Al: 35.00 to 50.00%, and
The plated steel material according to claim 1 or 2, containing Mg: 9.00 to 15.00%.
[Claim 4]
The plated steel material according to any one of claims 1 to 3, wherein the area ratio of the acicular Al-Zn-Si-Ca phase in the surface structure is 8.0% or more.