Plated steel sheet for automotive structural components

Abstract

The plated steel sheet for automotive structural members has: a steel sheet; a plating layer formed on at least a part of the surface of the steel sheet; and an oxide layer formed on at least a part of the surface of the plating layer. The plating layer has a prescribed chemical composition. When measured by XPS at a position 5.0 nm in the thickness direction from the surface of the oxide layer, I Mg / I MgOx is 0.00 or more and 1.20 or less.

Description

Technical Field

[0001] The present invention relates to a plated steel sheet for automotive structural components.

[0002] This application claims priority based on Japanese Patent Application No. 2021-004012 filed on January 14, 2021 in Japan, and incorporates its content herein. Background Art

[0003] In recent years, from the viewpoint of rust prevention, plated steel sheets are used in automotive structural components, and hot-dip galvanized steel sheets such as alloyed hot-dip galvanized steel sheets are mainly applied in the domestic Japanese market. The alloyed hot-dip galvanized steel sheet is a plated steel sheet that improves weldability and corrosion resistance after coating by performing alloying heat treatment after hot-dip galvanizing the steel sheet, so that Fe diffuses from the steel sheet (base steel sheet) into the plating layer. However, further improvement in corrosion resistance such as corrosion resistance after coating and red rust resistance is required for hot-dip galvanized steel sheets.

[0004] As a method for improving the corrosion resistance of hot-dip galvanized steel sheets, adding Al or the like to the plating layer containing Zn can be cited. For example, in the building materials field, hot-dip Al-Zn alloy plated steel sheets are widely put into practical use as highly corrosion-resistant plated steel sheets. The plating layer of such hot-dip Al-Zn alloy plating is formed of the following structure: dendritic α-(Zn, Al) phase (Al primary crystal part: In the Al-Zn binary phase diagram, etc., the α-(Zn, Al) phase that crystallizes as a primary crystal. It is not necessarily a phase rich in Al, but crystallizes as a solid solution of Zn and Al.) that first crystallizes from the molten state; and a structure composed of a Zn phase and an Al phase formed in the gaps of the dendritic Al primary crystal part (Zn / Al mixed phase structure). The Al primary crystal part is passivated, and the Zn / Al mixed phase structure has a higher Zn concentration than the Al primary crystal part. Therefore, corrosion concentrates on the Zn / Al mixed phase structure. As a result, since corrosion proceeds in a worm-eaten shape in the Zn / Al mixed phase structure and the corrosion progress path becomes complicated, it is difficult for corrosion to easily reach the base steel sheet. Thus, the hot-dip Al-Zn alloy plated steel sheet has excellent corrosion resistance compared to a hot-dip galvanized steel sheet having the same plating layer thickness. In addition, for the purpose of improving corrosion resistance, Zn-Al-Mg alloy plated steel sheets in which elements such as Mg are further added to the Al-Zn alloy plating are also being studied.

[0005] On the other hand, workability is required for plated steel sheets applied to automotive structural components. In particular, stamping is performed in an uncoated state, so lubricity between the die and the plated steel sheet is emphasized. Therefore, excellent lubricity is required for steel sheets applied to automotive structural components for stamping.

[0006] In view of such technical problems, Patent Document 1 discloses a lubricating plated steel sheet with excellent workability, which is characterized in that a Zn alloy plating layer containing Al: 4 to 22% by mass, Mg: 1 to 5% by mass, Ti: 0.1% by mass or less, Si: 0.5% by mass or less, with the balance being composed of Zn and inevitable impurities is formed on the surface of the steel sheet. A substrate treatment layer formed of a chromate film, a phosphate film, or a resin-based film containing an aqueous resin is formed on the plating layer. Further, a film is formed on the substrate treatment layer at an adhesion amount of 0.2 to 5 g / m 2 and the film is obtained by coating an aqueous lubricating paint containing 5 to 50% by mass of silica particles (b) and 1 to 40% by mass of a solid lubricant (c) based on 100% by mass of the solid content of the aqueous resin (a) and drying it.

[0007] In addition, Patent Document 2 discloses a hot-dip galvanized steel sheet with excellent formability, which is a hot-dip galvanized steel sheet having a galvanized layer containing Al: 0.05 to 10% by mass, and optionally containing Mg: 0.01 to 5% by mass, with the balance being composed of Zn and inevitable impurities. It is characterized in that the average centerline roughness Ra on the surface of the plated steel sheet is 0.5 to 1.5 μm, the PPI (the number of peaks with a size of 1.27 μm or more per 1 inch (2.54 cm)) is 150 to 300, and Pc (the number of peaks with a size of 0.5 μm or more per 1 cm) is Pc ≥ PPI / 2.54 + 10. Patent Document 2 shows that the sliding property is improved by making Pc ≥ PPI / 2.54 + 10.

[0008] Prior Art Documents

[0009] Patent Documents

[0010] Patent Document 1: Japanese Patent Application Laid-Open No. 2004-338397

[0011] Patent Document 2: Japanese Patent Application Laid-Open No. 2003-13192 Summary of the Invention

[0012] Technical Problems to be Solved by the Invention

[0013] However, in Patent Document 1, as a post-treatment after the plating process, it is necessary to coat and dry an aqueous lubricating paint, which has the technical problem of high manufacturing cost. In addition, in Patent Document 2, there is a problem that sufficient sliding property cannot be obtained only by improving the roughness.

[0014] The present invention has been completed in view of the above technical problems. An object of the present invention is to provide a plated steel sheet for automotive structural members having excellent lubricity, which is a Zn - Al - Mg - based plated steel sheet (a steel sheet having a plating containing Zn, Al, and Mg) with excellent corrosion resistance.

[0015] Technical means for solving the technical problems

[0016] The inventors of the present invention have studied methods for improving the lubricity of Zn - Al - Mg - based plated steel sheets. As a result, it has been found that by increasing the proportion of Mg contained in the plating layer present as an oxide [MgO] or a hydroxide [Mg(OH)₂], the lubricity of the plating layer is improved.

[0017] In addition, it has been found that by reducing the proportion of Al contained in the plating layer present as an oxide [Al₂O₃] or a hydroxide [Al(OH)₃], the chemical conversion treatment property can also be improved.

[0018] The present invention has been completed based on the above - mentioned insights. The gist of the present invention is as follows.

[0019] [1] A plated steel sheet for automotive structural members according to one aspect of the present invention has: a steel sheet; a plating layer formed on at least a part of the surface of the steel sheet; and an oxide layer formed on at least a part of the surface of the plating layer. The plating layer has the following chemical composition, in mass%: Al: 0.5 to 35.0%, Mg: 0.5 to 15.0%, Si: 0 to 2.0%, Ca: 0 to 2.0%, Fe: 0 to 2.0%, La + Ce: a total of 0 to 0.5%, Sb: 0 to 0.5%, Pb: 0 to 0.5%, Sr: 0 to 0.5%, Sn: 0 to 1.0%, Cu: 0 to 1.0%, Ti: 0 to 1.0%, Ni: 0 to 1.0%, Mn: 0 to 1.0%, Cr: 0 to 1.0%, Nb: 0 to 1.0%, Zr: 0 to 1.0%, Mo: 0 to 1.0%, Li: 0 to 1.0%, Ag: 0 to 1.0%, B: 0 to 0.5%, Y: 0 to 0.5%, and P: 0 to 0.5%, with the balance being composed of Zn and impurities. When measured by XPS at a position 5.0 nm in the thickness direction from the surface of the oxide layer, I, which is the ratio of the maximum detection intensity of Mg to the maximum detection intensity of the oxide or hydroxide of Mg Mg / I MgOx is 0.00 or more and 1.20 or less.

[0020] [2] In the plated steel sheet for automotive structural members described in the above [1], the chemical composition of the plating layer may also contain, in mass%, Al: 6.0 to 30.0% and Mg: 3.0 to 11.0%.

[0021] [3] The plated steel sheet for automotive structural components as described in [1] or [2] above, when measured by XPS at a position 5.0 nm in the thickness direction from the surface of the oxide layer, I, which is the ratio of the maximum detection intensity of Al to the maximum detection intensity of the oxide or hydroxide of Al Al / I AlOx may also be 0.77 or more.

[0022] [4] The plated steel sheet for automotive structural components as described in any one of [1] to [3] above, the I Mg / I MgOx may also be 0.00 or more and 0.80 or less.

[0023] Advantages of the Invention

[0024] According to the above solution of the present invention, it is possible to provide a plated steel sheet for automotive structural components having excellent lubricity, which is a steel sheet having a plating containing Zn, Al, and Mg (Zn-Al-Mg based plated steel sheet). In addition, according to a preferred solution of the present invention, it is possible to provide a plated steel sheet for automotive structural components having excellent chemical conversion treatment properties in addition to lubricity. Detailed Embodiments

[0025] The plated steel sheet for automotive structural components according to one aspect of the present invention (the plated steel sheet of the present embodiment) has: a steel sheet; a plating layer formed on at least a part of the surface of the steel sheet; and an oxide layer formed on at least a part of the surface of the plating layer. In addition, in the plated steel sheet of the present embodiment, the chemical composition of the plating layer contains, by mass%: Al: 0.5 to 35.0% (preferably 6.0 to 30.0%), Mg: 0.5 to 15.0% (preferably 3.0 to 11.0%), Si: 0 to 2.0%, Ca: 0 to 2.0%, Fe: 0 to 2.0%, La + Ce: a total of 0 to 0.5%, Sb: 0 to 0.5%, Pb: 0 to 0.5%, Sr: 0 to 0.5%, Sn: 0 to 1.0%, Cu: 0 to 1.0%, Ti: 0 to 1.0%, Ni: 0 to 1.0%, Mn: 0 to 1.0%, Cr: 0 to 1.0%, Nb: 0 to 1.0%, Zr: 0 to 1.0%, Mo: 0 to 1.0%, Li: 0 to 1.0%, Ag: 0 to 1.0%, B: 0 to 0.5%, Y: 0 to 0.5%, and P: 0 to 0.5%, and the balance consists of Zn and impurities.

[0026] In addition, in the plated steel sheet of the present embodiment, when measurement is performed by XPS on the surface layer portion of the oxide layer (for example, at a position 5.0 nm from the surface in the thickness direction), I, which is the ratio of the maximum detection intensity of Mg to the maximum detection intensity of the oxide or hydroxide of Mg Mg / I MgOx is 0.00 or more and 1.20 or less (preferably 0.00 or more and 0.80 or less).

[0027] In addition, the plated steel sheet of the present embodiment is preferably such that when measurement is performed by XPS at a position 5.0 nm from the surface in the thickness direction of the oxide layer, I, which is the ratio of the maximum detection intensity of Al (I Al ) to the maximum detection intensity of the oxide or hydroxide of Al (I AlOx ) Al / I AlOx is 0.77 or more.

[0028] <Steel sheet>

[0029] The plating layer and the oxide layer of the plated steel sheet of the present embodiment are important. Therefore, the type of the steel sheet (base steel sheet) is not particularly limited and may be determined according to the product to be applied or the required strength, plate thickness, etc. For example, hot-rolled steel sheets described in JIS G3193:2008 and cold-rolled steel sheets described in JIS G3141:2017 can be used.

[0030] <Plating layer>

[0031] The plated steel sheet of the present embodiment has a plating layer on at least a part of the surface of the steel sheet. The plating layer may be formed on one side of the steel sheet or on both sides.

[0032] The adhesion amount of the plating layer is preferably 15 to 250 g / m per side 2 .

[0033] [Chemical composition]

[0034] The chemical composition of the plating layer of the plated steel sheet of the present embodiment will be described. Hereinafter, all % related to the chemical composition are mass %.

[0035] Al: 0.5 to 35.0%

[0036] Al is an element effective for ensuring corrosion resistance after coating in a plating layer containing aluminum (Al), zinc (Zn), and magnesium (Mg). In order to sufficiently obtain the above effects, the Al content is made 0.5% or more. The Al content is preferably 1.0% or more, more preferably 6.0% or more.

[0037] On the other hand, if the Al content exceeds 35.0%, the corrosion resistance after coating and the corrosion resistance of the cut end face of the plating layer decrease. In addition, more Al oxides are formed, and the formation of MgO and Mg(OH)2 in the oxide layer is hindered. Therefore, the Al content is 35.0% or less. The Al content is preferably 30.0% or less.

[0038] Mg: 0.5 - 15.0%

[0039] Mg is an element that has the effect of improving the corrosion resistance of the plating layer. To fully obtain the above effect, the Mg content is 0.5% or more. The Mg content is preferably 1.0% or more, and more preferably 3.0% or more.

[0040] On the other hand, if the Mg content exceeds 15.0%, the corrosion resistance after coating decreases, and the workability of the plating layer decreases. In addition, since Mg compounds are formed not near the surface but inside the plating layer, the formation of MgO and Mg(OH)2 (oxides and hydroxides) near the surface is hindered. In addition, manufacturing problems such as an increase in the amount of scum generated in the plating bath occur. Therefore, the Mg content is 15.0% or less. The Mg content is preferably 11.0% or less.

[0041] Si: 0 - 2.0%

[0042] Si is an element that forms a compound with Mg and contributes to the improvement of the corrosion resistance after coating of the plating layer. In addition, Si is also an element that has the effect of suppressing the formation of an alloy layer formed between the steel sheet and the plating layer from becoming too thick when forming the plating layer on the steel sheet, and improving the adhesion between the steel sheet and the plating layer. Therefore, it can also be contained. Si does not necessarily need to be contained, and the lower limit is 0%, but in the case of obtaining the above effect, it is preferably that the Si content is 0.1% or more.

[0043] On the other hand, if the Si content exceeds 2.0%, the corrosion resistance after coating decreases due to the crystallization of excess Si in the plating layer or the inability to form a layered structure sufficiently, etc. In addition, the workability of the plating layer decreases. Therefore, the Si content is 2.0% or less. The Si content is more preferably 1.5% or less.

[0044] Ca: 0 - 2.0%

[0045] If Ca is contained in the plating layer, the amount of scum that is easily formed during the plating operation will decrease as the Mg content increases, and the plating manufacturability is improved. Therefore, Ca can also be contained. Ca does not necessarily need to be contained, and the lower limit is 0%, but in the case of obtaining the above effect, it is preferably that the Ca content is 0.1% or more.

[0046] On the other hand, if the Ca content is excessive, the corrosion resistance after coating decreases. In addition, the corrosion resistance after coating of the flat part with the plating layer itself tends to deteriorate, and sometimes the corrosion resistance around the welded part also deteriorates. Therefore, the Ca content is 2.0% or less. The Ca content is preferably 1.0% or less.

[0047] Fe: 0 to 2.0%

[0048] When manufacturing the plating layer, Fe is mixed into the plating layer as an impurity from a steel sheet or the like serving as a plating substrate. Sometimes it contains up to about 2.0%, but if it is within this range, the adverse effect on the characteristics of the plated steel sheet of the present embodiment is small. Therefore, it is preferable to make the Fe content 2.0% or less. The Fe content is more preferably 1.5% or less, and still more preferably 1.0% or less.

[0049] On the other hand, as described above, Fe is mixed into the plating layer as an impurity. To completely prevent the mixing of Fe, it will cost a significant amount of cost, so the Fe content can also be 0.1% or more.

[0050] The chemical composition of the plating layer of the plated steel sheet of the present embodiment is based on having the above chemical composition, with the remainder being Zn and impurities.

[0051] However, the plating layer of the plated steel sheet of the present embodiment may further contain La, Ce, Sb, Pb, Cu, Sn, Ti, Sr, Ni, Mn, Cr, Nb, Zr, Mo, Li, Ag, B, Y, P (whether intentionally added or contained as an impurity) instead of a part of Zn within the following ranges. Since these elements do not necessarily have to be contained, the lower limit of the content is 0%.

[0052] The total content of impurities is preferably 5.0% or less, and more preferably 3.0% or less.

[0053] La + Ce: The total is 0 to 0.5%

[0054] La and Ce are elements that contribute to improving the corrosion resistance of the plating layer. Therefore, one or both of La and Ce may be contained. It is not necessary to contain La and / or Ce, and the lower limit is 0%, but when the above effects are obtained, the total content of La and Ce is preferably 0.01% or more.

[0055] On the other hand, if the total content of La and Ce exceeds 0.5%, the viscosity of the plating bath increases, and it is mostly difficult to form the plating bath itself, and a plated steel sheet with good plating properties cannot be manufactured. Therefore, the total content of La and Ce is made 0.5% or less.

[0056] Sb: 0 to 0.5%

[0057] Sr: 0 to 0.5%

[0058] Pb: 0 to 0.5%

[0059] If the plating layer contains Sr, Sb, and Pb, the appearance of the plating layer changes, zinc flowers are formed, and an increase in metallic luster is confirmed. Therefore, they can also be contained. However, if the content of these elements exceeds 0.5%, various intermetallic compound phases are formed, and the workability and corrosion resistance deteriorate. In addition, if the content of these elements is excessive, the viscosity of the plating bath increases, and it becomes difficult to form the plating bath itself in most cases, and a plated steel sheet with good plating properties cannot be manufactured. Therefore, the Sr content is made 0.5% or less, the Sb content is made 0.5% or less, and the Pb content is made 0.5% or less.

[0060] Sn: 0 to 1.0%

[0061] Sn is an element that increases the dissolution rate of Mg in a plating layer containing Zn, Al, and Mg. If the dissolution rate of Mg increases, the corrosion resistance of the flat part deteriorates. Therefore, the Sn content is made 1.0% or less.

[0062] Cu: 0 to 1.0%

[0063] Ti: 0 to 1.0%

[0064] Ni: 0 to 1.0%

[0065] Mn: 0 to 1.0%

[0066] Cr: 0 to 1.0%

[0067] Nb: 0 to 1.0%

[0068] Zr: 0 to 1.0%

[0069] Mo: 0 to 1.0%

[0070] Li: 0 to 1.0%

[0071] Ag: 0 to 1.0%

[0072] B: 0 to 0.5%

[0073] Y: 0 to 0.5%

[0074] P: 0 to 0.5%

[0075] These elements are elements that contribute to the improvement of corrosion resistance. Therefore, they can also be contained. On the other hand, if the content of these elements is excessive, the viscosity of the plating bath increases, and it becomes difficult to form the plating bath itself in most cases, and a plated steel sheet with good plating properties cannot be manufactured. Therefore, the content of each element is made 1.0% or less.

[0076] The chemical composition of the plating layer is determined by the following method.

[0077] First, an acidic solution is obtained by stripping and dissolving the plating layer with an acid containing an inhibitor that inhibits the corrosion of the steel base (steel plate). Next, by analyzing the obtained acidic solution using ICP, the chemical composition of the plating layer can be obtained (in the case where an alloy layer is formed between the plating layer and the steel plate, it is the combined chemical composition of the plating layer and the alloy layer, but since the alloy layer is thin, the influence is small). The type of acid is not particularly limited as long as it can dissolve the plating layer. The chemical composition is measured as the average chemical composition.

[0078] [Structure]

[0079] In the plated steel sheet of the present embodiment, the plating structure is not particularly limited. Depending on the chemical composition of the plating layer of the plated steel sheet of the present embodiment, the plating structure includes, for example: (Al-Zn) dendrites, a layered structure of (Al-Zn) phase / MgZn2 phase, a layered structure of Zn phase / MgZn2 phase, a ternary eutectic structure of Zn / Al / MgZn2, MgZn2 phase, dendritic or amorphous Zn phase, Mg2Si phase, and / or other intermetallic compound phases.

[0080] [Oxide layer]

[0081] The plated steel sheet of the present embodiment has an oxide layer on at least a part of the surface of the plating layer. The oxide layer can be formed on one side or on both sides.

[0082] The inventors of the present invention have studied a method for improving lubricity in a Zn-Al-Mg based plated steel sheet. As a result, it was found that by increasing the proportion of Mg contained in the plating layer present as an oxide or hydroxide, the lubricity of the plating layer is improved. Specifically, it was found that when measuring by XPS (X-ray photoelectron spectroscopy) at a position 5.0 nm in the thickness direction from the surface of the oxide layer, when the maximum detection intensity (I Mg ) of Mg (in the metallic state) relative to the maximum detection intensity (I MgOx ) of the oxide or hydroxide of Mg, the ratio of I Mg / I MgOx is 0.00 or more and 1.20 or less, the lubricity is improved.

[0083] The reason is not yet clear, and it is considered that in the surface layer part of the oxide layer in contact with a mold or the like, by I Mg / I MgOxIt becomes lower, that is, the proportion of the presence of MgO (oxide of Mg) or Mg(OH)₂ (hydroxide of Mg) becomes higher, so that MgO or Mg(OH)₂ functions as a lubricating material and the lubricity is improved. In terms of lubricity, either MgO or Mg(OH)₂ can achieve the effect, but in the plated steel sheet of this embodiment, it is considered that mainly MgO.

[0084] In the past, if it was the surface (or, the range significantly thinner than 5 nm from the surface), MgO or Mg(OH)₂ was sometimes formed in a relatively large amount. However, as a result of the research by the present inventors, it was found that it is important that the proportion of the presence of MgO (oxide of Mg) or Mg(OH)₂ (hydroxide of Mg) becomes higher at the position 5 nm in the thickness direction from the surface (the position where the surface of the plating layer is shaved off by 5.0 nm in the depth direction as described below).

[0085] On the other hand, in the oxide layer, if Al₂O₃ (Al oxide) or Al(OH)₃ (Al hydroxide) is formed in excess, the chemical conversion treatment property sometimes decreases. In order to ensure excellent chemical conversion treatment property, when measuring by XPS at the position 5.0 nm in the thickness direction from the surface, as the maximum detection intensity (I Al ) of Al (in the metallic state) relative to the maximum detection intensity (I AlOx ) of the oxide or hydroxide of Al, the ratio of I Al / I AlOx is preferably 0.77 or more. The upper limit is not limited, but it is considered that the case exceeding 2.00 is less.

[0086] The I Mg / I MgOx and I Al / I AlOx at the position 5.0 nm in the thickness direction from the surface of the oxide layer are measured using XPS.

[0087] Specifically, the surface of the plating layer is shaved off by 5.0 nm in the depth direction by argon sputtering or the like (as long as it is in the range of 4.0 to 6.0 nm, it is allowed), and XPS measurement is performed at this position (the position at a depth of 5.0 nm (the range of 4.0 to 6.0 nm in depth is sufficient)). When performing XPS measurement, for example, a Quantera SXM type XPS analyzer manufactured by ULVAC-PHI or an equivalent device is used, and the following conditions are adopted.

[0088] X-ray source: mono-Al Kα (1486.6 eV)

[0089] Vacuum degree: 9×10 -10 torr

[0090] Ion species: Ar +

[0091] Accelerating voltage: 4 kV

[0092] Rate: 22.7 nm / min (for SiO2)

[0093] As a result of XPS measurement, the peak in the energy range of 304 - 309 eV was regarded as the peak obtained from Mg oxide or Mg hydroxide, and the peak in the range of 300 - 303 eV was regarded as the peak obtained from metallic Mg. The maximum detection intensity of each peak was measured, and I Mg / I MgOx .

[0094] Regarding I Al / I AlOx , the peak in the energy range of 73.5 - 76.5 eV was regarded as the peak obtained from Al oxide or Al hydroxide, and the peak in the range of 72.0 - 73.4 eV was regarded as the peak obtained from metallic Al. I Al / I AlOx was calculated based on the maximum intensity of each peak.

[0095] The thickness of the oxide layer is not particularly limited, for example, it is more than 5.0 nm and 50.0 nm or less.

[0096] The thickness of the oxide layer was measured by the following method. XPS measurement was performed from the surface of the plated steel sheet at intervals of 1 - 3 nm in the depth direction, and the depth until the maximum intensity of oxygen became 1 / 20 of the maximum intensity at the outermost surface was defined as the thickness of the oxide layer.

[0097] <Manufacturing method>

[0098] A preferred manufacturing method for the plated steel sheet of the present embodiment will be described. The plated steel sheet of the present embodiment is not dependent on the manufacturing method, and as long as it has the above characteristics, its effects can be obtained. However, it can be stably manufactured by the following method, so it is preferred.

[0099] Specifically, the steel sheet of the present embodiment can be manufactured by a manufacturing method including the following steps (I) - (III).

[0100] (I) Annealing step of subjecting the steel sheet to reduction annealing

[0101] (II) Plating step of immersing the steel sheet in a plating bath containing Al, Mg, and Zn to form a plated base sheet

[0102] (III) Controlled Cooling Process: For the plated base plate, it is cooled in an inert gas atmosphere with a dew point of -10°C or higher at an average cooling rate of 10.0°C / second or less in the temperature range of bath temperature to 380°C, and in an atmosphere with a dew point of -20°C or lower at an average cooling rate of 15°C / second or more in the temperature range of 380 to 100°C.

[0103] Hereinafter, the preferred conditions for each process will be described.

[0104] [Annealing Process]

[0105] In the annealing process, before the plating process, reduction annealing is performed on a steel sheet (hot-rolled steel sheet or cold-rolled steel sheet) obtained by a known method. The annealing conditions can be known conditions, for example, heating to 750 to 900°C in a 5% H2-N2 gas atmosphere with a dew point of -10°C or higher and holding for 30 to 240 seconds.

[0106] [Plating Process]

[0107] In the plating process, the steel sheet is immersed in a plating bath to form a plated base plate. The plating bath preferably contains Al: 0.5 to 35.0%, Mg: 0.5 to 15.0%, Si: 0 to 2.0%, Ca: 0 to 2.0%, and the balance consists of Zn and impurities. The plating bath may further contain La, Ce, Fe, Sb, Pb, Cu, Sn, Ti, Sr, Ni, Mn, Cr, Nb, Li, Ag as required. Since the composition of the formed plating layer can be inferred from the composition of the plating bath, the composition of the plating bath can be adjusted according to the desired chemical composition of the plating layer.

[0108] [Controlled Cooling Process]

[0109] In the controlled cooling process, after the plating process (lifted from the plating bath), the plated base plate is adjusted for the plating adhesion amount with a wiping gas such as N2 and then cooled. During cooling, the steel sheet lifted from the plating bath (at the same temperature as the plating bath temperature) is cooled to 100°C. At this time, the average cooling rate from the bath temperature to 380°C is set to 10.0°C / second or less, and the atmosphere during cooling from the bath temperature to 380°C is set to an inert gas atmosphere with a dew point of -10°C or higher (first cooling). In addition, the average cooling rate from 380 to 100°C is set to 15°C or more, and the dew point of the atmosphere is set to -20°C or lower (second cooling).

[0110] If the average cooling rate until 380°C exceeds 10.0°C / second, oxidation becomes insufficient, and I Mg / I MgOx becomes larger.

[0111] In addition, if the dew point of the atmosphere is less than -10°C, the oxide of Al is formed more preferentially than MgO or Mg(OH)2, and I Mg / I MgOx increases in the oxide layer. Even if the oxide of Al is formed, it is not helpful for improving lubricity. In addition, when increasing the ratio of the maximum detection intensity of Al to the maximum detection intensity of the oxide or hydroxide of Al to improve chemical conversion treatability, it is preferable to set the dew point of the atmosphere to 0°C or higher.

[0112] Regarding the atmosphere, the mechanism is not necessarily clear, but in the atmosphere, even if the dew point is -10°C or higher, a specified oxide layer cannot be obtained. Therefore, the atmosphere is set to an inert gas atmosphere. For example, it is an N2 atmosphere, an Ar atmosphere, or a He atmosphere. However, since the dew point of the inert gas alone is low (less than -20°C or higher), the dew point is controlled by introducing H2O.

[0113] If the average cooling rate from 380 to 100°C is less than 15°C, oxidation proceeds excessively, oxides such as ZnO grow, and I Mg / I MgOx decreases in the oxide layer.

[0114] In addition, if the dew point of the atmosphere during cooling in this temperature range exceeds -20°C, oxidation proceeds excessively, oxides such as ZnO grow, and I Mg / I MgOx decreases in the oxide layer.

[0115] The cooling start temperature of the second cooling is preferably 380°C (preferably switching the cooling rate immediately after the first cooling is completed), but if the average cooling rate until 100°C is less than 15°C / second, the start temperature of the second cooling can also be in the range of 380 to 330°C.

[0116] According to the above manufacturing method, the plated steel sheet of the present embodiment can be obtained.

[0117] Examples

[0118] Hereinafter, the present invention will be described more specifically by way of examples, but the present invention is not limited to these examples.

[0119] As the steel sheet to be annealed and plated, a cold-rolled steel sheet (0.2% C - 2.0% Si - 2.3% Mn) with a thickness of 1.6 mm was prepared.

[0120] After cutting the steel sheet into a size of 100 mm × 200 mm (× thickness), annealing and hot dip plating were continuously performed using an intermittent hot dip plating test apparatus.

[0121] During annealing, in a furnace with an oxygen concentration of 20 ppm or less, in an atmosphere composed of a gas containing 5% H2 gas and the remainder consisting of N2 gas, with a dew point of 0 °C, annealing is carried out at 860 °C for 120 seconds.

[0122] After annealing, the steel plate is air-cooled with N2 gas. When the temperature of the steel plate reaches the bath temperature + 20 °C, it is immersed in the plating bath at the bath temperature shown in Table 1A for about 3 seconds.

[0123] The plated substrate with the plating layer formed thereon is cooled to room temperature in such a manner that the average cooling rate and the atmosphere (atmosphere gas, dew point) from the bath temperature to 380 °C and from 380 to 100 °C are the conditions shown in Table 1B. The temperature of the steel plate is measured using a thermocouple spot-welded at the center of the plated substrate.

[0124] The composition of the formed plating layer is as shown in Table 1.

[0125] For the obtained plated steel plate, using XPS and by the above method, the thickness of the oxide layer, the I at a position 5 nm from the surface of the oxide layer Mg / I MgOx and I Al / I AlOx .

[0126] The results are shown in Table 2.

[0127] [Table 1A]

[0128]

[0129] [Table 1B]

[0130]

[0131] [Table 2]

[0132]

[0133] [Lubricity]

[0134] In addition, for the obtained plated steel plate, a ball-on-disk test is carried out according to the following procedure to evaluate the lubricity. In this test, the load P is set to 30 N, a 5 mmφ SUS ball is pressed against the sample, and the sample is rotated at a rotation radius of 10 mm and a rotation speed of 1 rpm while maintaining the load of the load P. The load F in the direction perpendicular to the SUS ball is measured, and the dynamic friction coefficient is obtained by dividing F by P when the sliding distance is 200 mm. Based on the dynamic friction coefficient, the evaluation is carried out as follows. If it is AA or A, it is judged that the lubricity is excellent.

[0135] AA: Dynamic friction coefficient 0.2 or less

[0136] A: The coefficient of kinetic friction exceeds 0.2 to 0.4

[0137] B: The coefficient of kinetic friction exceeds 0.4

[0138] [Chemical conversion treatability]

[0139] In addition, for the obtained plated steel sheet, the chemical conversion treatability was evaluated according to the following procedure.

[0140] A sample of 50×100 mm (× sheet thickness) was collected from the obtained plated steel sheet, and zinc phosphate treatment was performed on the sample according to (SD5350 system: standard manufactured by Nippon Paint-Industries Co., Ltd.) to form a chemical conversion treatment film. By observing the surface of the plated steel sheet having the chemical conversion treatment film by SEM, the uncovered ratio (area %) of the chemical conversion treatment film was measured. At this time, in the SEM observation field of view, the area ratio of the exposed area of the steel sheet was defined as the uncovered ratio. Based on the uncovered ratio, the chemical conversion treatability was evaluated as follows.

[0141] AA: No uncovered area

[0142] A: Uncovered area is 5% or less

[0143] As can be seen from Table 1A, Table 1B, and Table 2, excellent lubricity was obtained in Invention Examples No. 2 to 8, 10 to 13, 19 to 22, 24, 26 to 30. In addition, among these, in No. 3 to 8, 10 to 13, 19 to 22, 24, 26, 28, 29 where Al / Al Ox is relatively large, the chemical conversion treatability is also excellent.

[0144] In contrast, in Comparative Examples No. 1, 9, 14 to 18, 23, 25, 31, 32, at least one of the chemical composition of the plating layer or the cooling conditions of the bath temperature of 380°C to 380°C, 380°C to 100°C deviated from the preferred range. Therefore, I Mg / I MgOx becomes large and the lubricity decreases.

[0145] Industrial applicability

[0146] According to the present invention, it is possible to provide a plated steel sheet having excellent lubricity, which is a Zn-Al-Mg-based plated steel sheet. Since the stamping formability of this steel sheet is improved, it can be suitably applied to automotive structural components.

Claims

1. A plated steel sheet for an automotive structural member, comprising: a steel sheet; a plating layer formed on at least a part of the surface of the steel sheet; and an oxide layer formed on at least a part of the surface of the plating layer; the plating layer has the following chemical composition, in mass %, containing: Al：0.5～35.0％、 Mg: 0.5 to 15.0%, Si: 0 to 2.0%, Ca: 0 to 2.0%, Fe: 0.1 to 0.3%, La + Ce: a total of 0 to 0.5%, Sb: 0 to 0.5%, Pb: 0 to 0.5%, Sr：0～0.5％、 Sn: 0 to 1.0%, Cu: 0 to 1.0%, Ti: 0 to 1.0%, Ni: 0 to 1.0%, Mn: 0 to 1.0%, Cr：0～1.0％、 Nb: 0 to 1.0%, Zr：0～1.0％、 Mo: 0 to 1.0%, Li: 0 to 1.0%, Ag: 0 to 1.0%, B：0～0.5％、 Y: 0 to 0.5%, and P：0～0.5％， the balance consists of Zn and impurities; When measured by XPS at a position 5.0 nm in the thickness direction from the surface of the oxide layer, I, which is the ratio of the maximum detection intensity of Mg to the maximum detection intensity of the oxide or hydroxide of Mg Mg / I MgOx is 0.00 or more and 1.20 or less.

2. The plated steel sheet for an automotive structural member according to claim 1, the chemical composition of the plating layer, in mass %, contains: Al：6.0～30.0％、 Mg: 3.0 to 11.0%.

3. The plated steel sheet for an automotive structural member according to claim 1 or 2, When measured by XPS at a position 5.0 nm in the thickness direction from the surface of the oxide layer, I, which is the ratio of the maximum detection intensity of Al to the maximum detection intensity of the oxide or hydroxide of Al Al / I AlOx is 0.77 or more.

4. The plated steel sheet for an automotive structural member according to claim 1 or 2, The said I Mg / I MgOx is 0.00 or more and 0.80 or less.

5. The plated steel sheet for an automotive structural member according to claim 3, The said I Mg / I MgOx is 0.00 or more and 0.80 or less.

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