Hot-dip plated steel material

Abstract

One embodiment of the present invention provides a hot-dip galvanized steel material comprising a base steel material and a hot-dip galvanized layer disposed on the surface of the base steel material. The chemical composition of the hot-dip galvanized layer, by mass%, includes: Al: 10.00–30.00%; Mg: 3.00–12.00%; Sn: 0–2.00%; Si: 0–2.50%; Ca: 0–3.00%; Ni: 0% or more and less than 0.25%; and Fe: 0–5.00%, with the remainder consisting of Zn and impurities. The metallographic structure of the hot-dip galvanized layer contains an α phase with a grain size of 0.5–2 μm at 5–45 area % and a MgZn2 phase at 15–70 area % . Among the α phases with a grain size of 0.5–2 μm, the α phase has a (111) relative to the adjacent MgZn2 phase. α / / (0001) MgZn2 The area fraction of the α phase with orientation relationship is 25%–100%.

Description

[0001] This application is a divisional application of the invention patent application filed on September 7, 2021, with application number 2021800463139 and invention title "Hot-dip galvanized steel". Technical Field

[0002] This invention relates to hot-dip galvanized steel. Background Technology

[0003] Steel with a hot-dip Zn coating containing Al and Mg (hot-dip Zn-Al-Mg steel) has excellent corrosion resistance. Therefore, hot-dip Zn-Al-Mg steel is widely used as a material for structural components requiring corrosion resistance, such as building materials.

[0004] For example, Patent Document 1 discloses a coated steel material having a steel material and a coating comprising a Zn-Al-Mg alloy layer disposed on the surface of the steel material. In this coated steel material, the coating has a specified chemical composition. After grinding the surface of the Zn-Al-Mg alloy layer to half its thickness, an electron reflection image of the Zn-Al-Mg alloy layer obtained by observing it at 100x magnification using a scanning electron microscope shows the presence of Al crystals. The average cumulative perimeter of the Al crystals is 88–195 mm / mm. 2 .

[0005] Patent Document 2 discloses a coated steel sheet having a steel sheet and a coating formed on at least a portion of the surface of the steel sheet. The chemical composition of the coating, by mass%, contains Al: more than 5.00% and less than 35.00%; Mg: 3.00 to 15.00%; Si: 0 to 2.00%; and Ca: 0 to 2.00%, with the remainder consisting of Zn and impurities. In the thickness direction section, the coating has a lamellar structure with a layered arrangement of (Al-Zn) phase and MgZn2 phase with an area ratio of 10 to 90%, the lamellar spacing of the lamellar structure being less than 2.5 μm, and the area ratio of (Al-Zn) dendrites being less than 35%.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: International Publication No. WO2019 / 221193

[0009] Patent Document 2: International Publication No. WO2020 / 213686. Summary of the Invention

[0010] The technical problem that the invention aims to solve

[0011] In recent years, resistance to water corrosion has become a requirement for hot-dip Zn-Al-Mg based steels. Water corrosion resistance refers to corrosion resistance when exposed to flowing water. In a flowing water environment, corrosion products adhering to the surface of the hot-dip coating are washed away, compromising the rust-preventive effect achieved by these corrosion products. Therefore, the water corrosion resistance of steel is evaluated using methods different from those for general corrosion resistance. For example, materials used for walls along water flow paths such as rainwater and industrial water require high resistance to water corrosion.

[0012] In existing technologies, there is almost no research on resistance to water corrosion. For example, in Patent Document 1, the corrosion resistance of the flat surface is evaluated based on JASO M609-91, which assumes a non-flowing corrosive solution. Therefore, Patent Document 1 does not include any research or evaluation on resistance to water corrosion. In Patent Document 2, the corrosion resistance after coating is evaluated in a non-flowing environment, without any research or evaluation on resistance to water corrosion.

[0013] Furthermore, the inventors of this invention, during their research, discovered that even the commonly used hot-dip Zn-Al-Mg steels with high corrosion resistance, as disclosed in Patent Documents 1 and 2, do not exhibit sufficient resistance to water corrosion. For example, in Patent Document 1, cooling was set to an average cooling rate of 12°C / s or less for the temperature range from the coating solidification start temperature to -30°C. In the hot-dip coating obtained under such cooling conditions, as described later, the inventors of this invention found that the interface between the α phase and the MgZn2 phase is prone to corrosion in a flowing water environment. Moreover, in Patent Document 2, the original coating plate after controlled cooling was stopped was cooled to below 335°C at an average cooling rate of 5°C / s or less; however, within the temperature range below 335°C, cooling control aimed at controlling the coating microstructure was not implemented. In the hot-dip coating obtained under such cooling conditions, the inventors of the present invention have discovered that the η phase crystallizes from the α phase, and corrosion easily occurs at the interface between the α phase and the η phase, and at the interface between the α phase and the MgZn2 phase, which impairs the resistance to water corrosion.

[0014] Furthermore, while adding a high concentration of Mg to the coating is effective in improving the corrosion resistance of hot-dip Zn-Al-Mg steel, such a high concentration of Mg can easily lead to powdering. Powdering refers to the phenomenon where the hot-dip coating peels off and becomes powdery during the pressure forming process of hot-dip Zn-Al-Mg steel. Therefore, resistance to powdering is also required for using hot-dip Zn-Al-Mg steel as a material for various structural components.

[0015] In view of the above, the object of the present invention is to provide a hot-dip galvanized steel with excellent resistance to powdering and water corrosion.

[0016] The purpose of this invention is as follows.

[0017] (1) A hot-dip galvanized steel product according to one aspect of the present invention comprises a base steel material and a hot-dip galvanized coating disposed on the surface of the base steel material. The chemical composition of the hot-dip galvanized coating, in mass %, contains: Al: 10.00-30.00%; Mg: 3.00-12.00%; Sn: 0-2.00%; Si: 0-2.50%; Ca: 0-3.00%; Ni: 0% or more and less than 0.25%; Cr: 0% or more and less than 0.25%; Ti: 0% or more and less than 0.25%; Co: 0% or more and less than 0.25%; V: 0% or more and less than 0.25%; Nb: 0% or more and less than 0.25%; Cu: 0% or more and less than 0.25%; Mn: 0% or more and less than 0.25%; Bi: 0% The content of the hot-dip coating is above and below 5.000%; In: above 0% and below 2.00%; Y: 0 to 0.50%; La: above 0% and below 0.50%; Ce: above 0% and below 0.50%; Fe: 0 to 5.00%; Sr: above 0% and below 0.50%; Sb: above 0% and below 0.50%; Pb: above 0% and below 0.50%; and B: above 0% and below 0.50%, with the remainder consisting of Zn and impurities. The metallographic structure of the hot-dip coating contains α phase with a particle size of 0.5 to 2 μm at 5 to 45 area %, and the metallographic structure of the hot-dip coating contains MgZn2 phase at 15 to 70 area %, wherein the α phase with a particle size of 0.5 to 2 μm has (111) for adjacent MgZn2 phases. α / / (0001) MgZn2 The area fraction of the α phase with orientation relationship is 25-100%.

[0018] (2) Alternatively, in the hot-dip galvanized steel described in (1) above, the α phase with a particle size of 0.5 to 2 μm has (111) for adjacent MgZn2 phases. α / / (0001) MgZn2 The area fraction of the α phase with the orientation relationship is 60-100%.

[0019] (3) Alternatively, in the hot-dip galvanized steel described in (1) or (2) above, the chemical composition of the hot-dip galvanized layer is, by mass%, Mg: 5.00-8.00% and Sn: 0.05-2.00%.

[0020] Invention Effects

[0021] According to the present invention, a hot-dip galvanized steel with excellent resistance to powdering and water corrosion can be provided. Attached Figure Description

[0022] Figure 1 This is a cross-sectional view of a hot-dip galvanized steel material according to one embodiment of the present invention.

[0023] Figure 2 This is a schematic diagram of an α phase with a dendritic crystal shape.

[0024] Figure 3 This is a schematic diagram of the cooling conditions during the manufacturing of hot-dip galvanized steel according to one aspect of the present invention. Detailed Implementation

[0025] The inventors of this invention have conducted repeated research on methods to improve the resistance of hot-dip galvanized steel to water corrosion. Furthermore, the inventors of this invention have focused on the crystal orientation relationship between the α phase and the MgZn2 phase on the surface of the hot-dip galvanized layer.

[0026] Hot-dip galvanized coatings composed of Zn-Al-Mg alloys include an α phase and a MgZn2 phase. The α phase is a solid solution with a face-centered cubic lattice structure, primarily composed of Al and Zn. It can contain elements other than Al and Zn, such as Mg, Ni, Fe, and Sn, in addition to these, with each element comprising less than 0.5% of the α phase. Because the α phase is predominantly Al, it can passivate, improving the corrosion resistance of the planar portion of the coating. Furthermore, its crystalline structure gives it plastic deformation capabilities, thus improving its resistance to powdering. The MgZn2 phase is an intermetallic compound phase primarily composed of Mg and Zn. The MgZn2 phase has a low potential in corrosive environments, thus providing corrosion resistance to the steel substrate. The insulating film formed by Mg-induced Zn-based corrosion products further improves the corrosion resistance and corrosion resistance of the planar portion of the hot-dip galvanized coating.

[0027] The inventors of this invention discovered that corrosion easily occurs at the interface between these α-phase and MgZn2 phases. The natural potential of the α-phase is higher than that of the MgZn2 phase. Therefore, dissimilar metal contact corrosion occurs at the interface between the α-phase and the MgZn2 phase.

[0028] In previous evaluations of the corrosion resistance of planar surfaces in non-flowing water environments, corrosion at the interface between the α-phase and the MgZn2 phase was not considered a problem. The reasoning was that, in non-flowing water environments, corrosion products generated at the α-phase / MgZn2 phase interface adhere to the surface of the hot-dip galvanized coating, providing rust protection. However, in flowing water environments where corrosion products are washed away from the surface of the hot-dip galvanized coating, this rust-protective effect is lost. Therefore, corrosion at the α-phase / MgZn2 phase interface is considered to impair resistance to flowing water corrosion.

[0029] Furthermore, the inventors of this invention have discovered that by setting the crystal orientation relationship between the α phase and the MgZn2 phase within a specific range, the corrosion resistance of the interface between the α phase and the MgZn2 phase can be improved. Moreover, the inventors of this invention have also discovered that, in addition to the previously concerned corrosion resistance of planar portions, improving the corrosion resistance of the interface between the α phase and the MgZn2 phase can improve the resistance of hot-dip galvanized steel to water corrosion.

[0030] Based on the above insights, a solution of the present invention, namely a hot-dip galvanized steel and its manufacturing method, will be described in detail below. Hereinafter, the "%" of the content of each element in the chemical composition indicates "mass %". The content of elements in the chemical composition is sometimes recorded as element concentration (e.g., Zn concentration, Mg concentration, etc.). "Flat surface corrosion resistance" refers to the property of the hot-dip galvanized coating (specifically, the Zn-Al-Mg alloy layer) itself being difficult to corrode. "Alternative corrosion resistance" refers to the property of suppressing corrosion of the base steel at exposed parts of the base steel (e.g., due to the cut ends of the coated steel, the fracture points of the hot-dip galvanized coating during processing, and the locations where the base steel is exposed due to the peeling of the hot-dip galvanized coating). "Water corrosion resistance" refers to the property of the hot-dip galvanized coating itself being difficult to corrode in a flowing water environment to the extent that corrosion products on the surface of the coated steel are washed away. "Hot-dip galvanized coating" refers to the coating film manufactured by the so-called hot-dip galvanizing process.

[0031] The hot-dip galvanized steel 1 of this embodiment has a base steel 11. The shape of the base steel is not particularly limited; one example is a steel plate. Furthermore, the base steel can be, for example, steel pipes, civil engineering materials (fences, corrugated pipes, drainage ditch covers, sand-proof boards, bolts, metal mesh, railings, waterstops, etc.), appliance parts (air conditioner outdoor unit housings, etc.), automotive parts (axle components, etc.), or other base steel that has undergone forming processes. Forming processes include various plastic forming methods such as stamping, roll forming, and bending.

[0032] The material of the base steel is not particularly limited. For example, the base steel can be general steel, pre-plated steel, Al-killed steel, very low carbon steel, high carbon steel, various high-tensile steels, and some high-alloy steels (steels containing strengthening elements such as Ni and Cr). The base steel can be hot-rolled steel plate, hot-rolled steel strip, cold-rolled steel plate, and cold-rolled steel strip as described in JIS G 3302:2010. There are also no particular restrictions on the manufacturing method (hot rolling method, pickling method, cold rolling method, etc.) and specific manufacturing conditions of the base steel plate.

[0033] The base steel can also be pre-plated steel. Pre-plated steel is obtained, for example, through electrolytic treatment or displacement plating. Electrolytic treatment is carried out by immersing the base steel in a sulfuric acid or chloride bath containing metal ions of various pre-plating components and then electrolyzing it. Displacement plating is carried out by immersing the base steel in an aqueous solution containing metal ions of various pre-plating components, with the pH adjusted by sulfuric acid, thereby displacing the metal. An example of pre-plated steel is pre-plated Ni steel.

[0034] The hot-dip galvanized steel 1 of this embodiment has a hot-dip galvanized layer 12 disposed on the surface of a base steel. Due to the chemical composition described later, the hot-dip galvanized layer of the hot-dip galvanized steel of this embodiment is mainly composed of a Zn-Al-Mg alloy layer. Furthermore, the hot-dip galvanized layer of the hot-dip galvanized steel of this embodiment may contain an Al-Fe alloy layer between the base steel and the Zn-Al-Mg alloy layer. That is, the hot-dip galvanized layer may be a single-layer structure of a Zn-Al-Mg alloy layer, or it may be a multilayer structure containing both a Zn-Al-Mg alloy layer and an Al-Fe alloy layer.

[0035] The chemical composition of the hot-dip galvanized coating in this embodiment is composed of Zn and other alloying elements. The chemical composition of the hot-dip galvanized coating will be explained in detail below. Furthermore, elements whose lower concentration limit is stated to be 0% are not necessary to solve the technical problems of the hot-dip galvanized steel in this embodiment, but are any elements permitted in the hot-dip galvanized coating for the purpose of improving properties, etc.

[0036] <AI: 10.00~30.00%>

[0037] Al forms the α phase of a solid solution with Zn, which helps improve the corrosion resistance, chemical corrosion resistance, water corrosion resistance, and processability of the planar portion. Therefore, the Al concentration is set to 10.00% or higher. Alternatively, the Al concentration can be set to 11.00%, 12.00%, or 15.00% or higher.

[0038] On the other hand, when Al is in excess, α crystallizes before MgZn2. Furthermore, α grows in a state where its crystallization orientation does not satisfy the α / MgZn2 phase. As a result, a sufficient amount of Al is not formed to satisfy the α / MgZn2 crystallization orientation relationship, thus reducing resistance to water corrosion. Therefore, the Al concentration is set to 30.00% or less. Alternatively, the Al concentration can be set to 28.00% or less, 25.00% or less, or 20.00% or less.

[0039] <Mg: 3.00~12.00%>

[0040] Mg is an essential element to ensure the corrosion resistance, chemical corrosion resistance, and resistance to water corrosion of the planar surface. Therefore, the Mg concentration is set to 3.00% or higher. The Mg concentration can be set to 4.00%, 5.00%, or 6.00% or higher.

[0041] On the other hand, when the Mg concentration is excessive, the processability, especially the powderability, deteriorates. Therefore, the Mg concentration is set to 12.00% or less. The Mg concentration can be set to 11.00% or less, 10.00% or less, 8.00% or less, less than 8.00%, or 6.00% or less.

[0042] <Sn: 0~2.00%>

[0043] The Sn concentration can also be 0%. On the other hand, Sn is an element that forms intermetallic compounds with Mg, improving the corrosion resistance of hot-dip galvanized coatings. Therefore, the Sn concentration can be set to 0.05% or more, 0.10% or more, 0.20% or more, or 0.50% or more.

[0044] However, if the Sn concentration is excessive, the surface corrosion resistance and water corrosion resistance will deteriorate. Therefore, the Sn concentration should be set below 2.00%. Alternatively, the Sn concentration can be set below 1.80%, 1.50%, or 1.20%.

[0045] <Si: 0%~2.50%>

[0046] The Si concentration can also be 0%. On the other hand, Si helps improve the surface corrosion resistance and water corrosion resistance. Therefore, the Si concentration can be set to 0.05% or more, 0.10% or more, 0.20% or more, or 0.50% or more.

[0047] On the other hand, when the Si concentration is excessive, the corrosion resistance, corrosion protection, and processability of the planar portion deteriorate. Therefore, the Si concentration can be set to 2.50% or less. Alternatively, the Si concentration can be set to 2.40% or less, 1.80% or less, or 1.20% or less.

[0048] <Ca: 0%~3.00%>

[0049] The Ca concentration can also be 0%. On the other hand, Ca is the element that can adjust the Mg dissolution rate to the optimal level for imparting corrosion resistance and water corrosion resistance to planar surfaces. Therefore, the Ca concentration can be 0.05% or more, 0.1% or more, or 0.5% or more.

[0050] On the other hand, when the Ca concentration is excessive, the corrosion resistance, water corrosion resistance, and processability of the planar parts deteriorate. Therefore, the Ca concentration is set to 3.00% or less. Alternatively, the Ca concentration can be set to 2.40% or less, 1.80% or less, or 1.20% or less.

[0051] <Ni, Cr, Ti, Co, V, Nb, Cu, and Mn: ≥0% and <0.25% respectively>

[0052] The concentrations of Ni, Cr, Ti, Co, V, Nb, Cu, and Mn can be 0%. On the other hand, they contribute to improving the corrosion resistance of the substitute. Therefore, the concentrations of Ni, Cr, Ti, Co, V, Nb, Cu, and Mn can also be set to 0.05% or more, 0.08% or more, or 0.1% or more.

[0053] On the other hand, if the concentrations of Ni, Cr, Ti, Co, V, Nb, Cu, and Mn are excessive, the surface corrosion resistance and resistance to flowing water corrosion will deteriorate. Therefore, the concentrations of each of Ni, Cr, Ti, Co, V, Nb, Cu, and Mn are set to less than 0.25%. Alternatively, the concentrations of each of Ni, Cr, Ti, Co, V, Nb, Cu, and Mn can be set to less than 0.22%, less than 0.20%, or less than 0.15%.

[0054] <Bi: 0% or more and less than 5.000%>

[0055] The concentration of Bi can be 0%. On the other hand, Bi helps to improve the corrosion resistance of the substitute. Therefore, the concentration of Bi can also be set to 0.100% or more, 1.000% or more, or 3.000% or more.

[0056] On the other hand, when the Bi concentration is excessive, the surface corrosion resistance and water corrosion resistance deteriorate. Therefore, the Bi concentration is set to less than 5.000%. Alternatively, the Bi concentration can be set to below 4.800%, 4.500%, or 4.000%.

[0057] <In: 0% or more and less than 2.00%>

[0058] The concentration of In can also be 0%. On the other hand, In helps to improve the corrosion resistance of chemical substitutes. Therefore, the concentration of In can also be 0.10% or more, 0.50% or more, or 1.00% or more.

[0059] On the other hand, when the In concentration is excessive, the surface corrosion resistance and water corrosion resistance deteriorate. Therefore, the In concentration is set to less than 2.00%. Alternatively, the In concentration can be set to less than 1.80%, less than 1.50%, or less than 1.00%.

[0060] <Y: 0%~0.50%>

[0061] The concentration of Y can also be 0%. On the other hand, Y helps to improve the corrosion resistance of the substitute. Therefore, the concentration of Y can also be set to 0.10% or more, 0.15% or more, or 0.20% or more.

[0062] On the other hand, when the γ concentration is excessive, the surface corrosion resistance and resistance to flowing water corrosion deteriorate. Therefore, the γ concentration is set to below 0.50%. The γ concentration can also be below 0.30%, below 0.25%, or below 0.22%.

[0063] <La and Ce: ≥0% and <0.50% respectively>

[0064] The concentrations of La and Ce can also be 0%. On the other hand, La and Ce help improve the corrosion resistance of the substitute. Therefore, the concentrations of La and Ce can also be set to 0.10% or more, 0.15% or more, or 0.20% or more, respectively.

[0065] On the other hand, when the La and Ce concentrations are excessive, the surface corrosion resistance and resistance to flowing water corrosion deteriorate. Therefore, the La and Ce concentrations are set to less than 0.50%. Alternatively, the La and Ce concentrations can be set to less than 0.40%, less than 0.30%, or less than 0.25%, respectively.

[0066] <Fe: 0%~5.00%>

[0067] The Fe concentration can also be 0%. Alternatively, Fe can be present in the hot-dip coating. It has been confirmed that if the Fe concentration is 5.00% or less, there is no adverse effect on the performance of the hot-dip coating. The Fe concentration can be set, for example, to 0.05% or more, 0.10% or more, 0.50% or more, or 1.00% or more. The Fe concentration can also be set, for example, to 4.00% or less, 3.50% or less, or 3.00% or less. Fe can also be introduced from the base steel sheet, therefore the Fe concentration can also be 0.05% or more.

[0068] <Sr, Sb, Pb, and B: ≥0% and <0.50% respectively>

[0069] The concentrations of Sr, Sb, Pb, and B can each be 0%. On the other hand, Sr, Sb, Pb, and B help improve the corrosion resistance of the substitute. Therefore, the concentrations of Sr, Sb, Pb, and B can also be set to 0.05% or more, 0.10% or more, or 0.15% or more.

[0070] On the other hand, if the concentrations of Sr, Sb, Pb, and B are excessive, the surface corrosion resistance and resistance to flowing water corrosion will deteriorate. Therefore, the concentrations of Sr, Sb, Pb, and B are each set to less than 0.50%. Alternatively, the concentrations of Sr, Sb, Pb, and B can be set to less than 0.40%, less than 0.30%, or less than 0.25%.

[0071] <Remaining portion: Zn and impurities>

[0072] The remaining components of the hot-dip galvanized coating in this embodiment are Zn and impurities. Zn is an element that gives the hot-dip galvanized coating its planar corrosion resistance and chemical corrosion protection. Impurities are components contained in the raw materials or mixed in during the manufacturing process; they refer to components that are not intentionally present. For example, in hot-dip galvanized coatings, due to the mutual atomic diffusion between the base steel and the electroplating bath, trace amounts of components other than Fe may sometimes be mixed in as impurities.

[0073] The chemical composition of the hot-dip galvanized coating was determined by the following method. First, an acid solution containing a corrosion inhibitor that inhibits corrosion of the base steel was used to obtain an acid solution that stripped and dissolved the hot-dip galvanized coating. Next, the obtained acid solution was subjected to ICP analysis. From this, the chemical composition of the hot-dip galvanized coating could be obtained. The type of acid is not particularly limited if it is an acid capable of dissolving the hot-dip galvanized coating. Furthermore, the chemical composition determined by the above method is the average chemical composition of the entire hot-dip galvanized coating.

[0074] Next, the metallographic structure of the hot-dip coating will be explained.

[0075] <α phase>

[0076] The metallographic structure of the hot-dip coating 12 contains α phase with a grain size of 0.5 to 2 μm at an area percentage of 5 to 45%. This area percentage is the area percentage of the α phase with a grain size of 0.5 to 2 μm relative to all the phases constituting the hot-dip coating 12.

[0077] Alpha phase with a particle size of 0.5–2 μm improves the corrosion resistance and powdering resistance of the planar portion of hot-dip galvanized coatings. However, these effects are not obtained when the amount of α phase with a particle size of 0.5–2 μm is less than 5% by area. Therefore, the amount of α phase with a particle size of 0.5–2 μm is set to 5% by area or more. Alternatively, the amount of α phase with a particle size of 0.5–2 μm can be set to 6% by area or more, 8% by area or more, or 10% by area or more.

[0078] On the other hand, when the amount of α phase with a particle size of 0.5–2 μm exceeds 45% by area, the amount of α phase is excessive relative to the MgZn2 phase. Therefore, α phase grows in a state where it is not adjacent to the MgZn2 phase. As a result, it is difficult to form a crystal orientation relationship at the interface between the α phase and the MgZn2 phase, leading to reduced resistance to water corrosion. Therefore, the amount of α phase with a particle size of 0.5–2 μm is set to 45% or less by area. Alternatively, the amount of α phase with a particle size of 0.5–2 μm can be set to 42% or less by area, 40% or less by area, or 35% or less by area.

[0079] As long as the area fraction of the α phase with a particle size of 0.5–2 μm is within the above range, the area fraction of the α phase with a particle size less than 0.5 μm and the α phase with a particle size greater than 2 μm are not particularly limited. When evaluating the microstructure of the hot-dip coating, the area fraction of the α phase with a particle size less than 0.5 μm and the α phase with a particle size greater than 2 μm are ignored.

[0080] <MgZn2 phase>

[0081] The MgZn2 phase improves the planar corrosion resistance, water corrosion resistance, and powdering resistance of the hot-dip galvanized coating 12. However, these effects are not achieved when the amount of the MgZn2 phase is less than 15% by area. Therefore, the amount of the MgZn2 phase is set to 15% by area or more. Alternatively, the amount of the MgZn2 phase can be set to 18% by area or more, 20% by area or more, or 25% by area or more.

[0082] On the other hand, when the amount of MgZn2 phase exceeds 70% by area, it impairs the powdering resistance of hot-dip galvanized steel. This is because the MgZn2 phase is relatively brittle. Therefore, the amount of MgZn2 phase is set to 70% by area or less. Alternatively, the amount of MgZn2 phase can be set to 65% by area or less, 60% by area or less, or 50% by area or less.

[0083] The hot-dip coating 12 may also contain phases other than the α phase and the MgZn2 phase. For example, the hot-dip coating with the above-mentioned chemical composition may contain the Mg2Sn phase, the α / η / MgZn2 ternary eutectic structure, the η-Zn phase, and the Al-Ca-Si phase. If the content of the α phase and the MgZn2 phase is within the above-mentioned range, resistance to water corrosion and resistance to powdering can be ensured. Therefore, the composition of phases other than the α phase and the MgZn2 phase is not particularly limited.

[0084] <Interface between α phase and MgZn2 phase>

[0085] The natural potential of the α phase is higher than that of the MgZn2 phase. Therefore, dissimilar metal contact corrosion occurs at the interface between the α and MgZn2 phases. This is particularly significant when the amount of the α phase (0.5–2 μm particle size) is above 5% by area. This is because as the amount of fine α phase particles smaller than 2 μm increases, the length of the α phase interface exposed in flowing water increases. To improve the resistance of hot-dip galvanized steel to flowing water corrosion, it is necessary to increase the corrosion resistance of the interface between the α and MgZn2 phases.

[0086] Based on the above reasons, in the hot-dip coating of the hot-dip galvanized steel in this embodiment, the α phase with a particle size of 0.5 to 2 μm has (111) relative to the adjacent MgZn2 phase. α / / (0001) MgZn2 The area fraction of the α phase with orientation relationship is set to 25% to 100%. In other words, the hot-dip coating of the hot-dip galvanized steel in this embodiment satisfies the following formula.

[0087] 0.25≦A2 / A1≦1.00

[0088] A1: Area of ​​α-phase with a particle size of 0.5–2 μm, measured in a cross-section of the hot-dip coated layer.

[0089] A2: Measured in the cross section of the hot-dip coated layer, with a particle size of 0.5–2 μm and a (111) relative to the adjacent MgZn2 phase. α / / (0001) MgZn2 Area of ​​α phase with orientation relation

[0090] Here, the term "relative to the adjacent MgZn2 phase has (111)" refers to this. α / / (0001) MgZn2 "Orientational relationship α phase" refers to the α phase that is adjacent to the MgZn2 phase and whose (111) plane is parallel to the (0001) plane of the adjacent MgZn2 phase.

[0091] There exists (111) between the α phase and the adjacent MgZn2 phase. α / / (0001) MgZn2 In the case of orientation relationships, the interfaces of these phases are chemically stable and exhibit high corrosion resistance. Therefore, by increasing the (111) orientation... α / / (0001) MgZn2The amount of α-phase with the correct orientation can suppress corrosion at the phase interface and improve resistance to water corrosion. For the above reasons, A2 / A1 is set to 0.25 or higher. A2 / A1 can also be set to 0.35 or higher, 0.50 or higher, or 0.60 or higher. A higher A2 / A1 is preferred, so A2 / A1 can also be 1.00. On the other hand, A2 / A1 can also be 0.95 or lower, 0.90 or lower, or 0.85 or lower.

[0092] The method for determining the area fraction of the α phase with a particle size of 0.5–2 μm is as follows. The surface of the coating, cut into 30 mm × 30 mm samples, was flattened by mechanical grinding. Next, the surface of the coating was chemically ground using colloidal grinding until it reached a mirror finish. The surface of the ground coating was then observed using SEM. Specifically, an elemental distribution image was captured using SEM-EDS at 5000x magnification. In this elemental distribution image, the phase where Al and Zn coexist was identified as the α phase. After identifying the α phase, the phase with a particle size of 0.5–2 μm was selected from the α phases included in the field of view. Subsequently, the area fraction of the α phase with a particle size of 0.5–2 μm was calculated using binarization with image analysis software. Furthermore, for α phases with a granular shape, the particle size of the α phase is the equivalent diameter of a circle. Furthermore, for α phases with dendritic growth, the particle size of the α phase is the short axis of the dendrite. The short axis of the dendrite is as follows: Figure 2 As shown, the maximum value of the dendrite width is measured in a direction orthogonal to the major axis direction of the dendrite.

[0093] The method for determining the area fraction of the MgZn2 phase is as follows. The surface of the coating, cut into 30mm × 30mm samples, was flattened by mechanical grinding. Next, the surface of the coating was chemically ground using colloidal grinding until it reached a mirror finish. The surface of the ground coating was then observed using SEM. Specifically, an elemental distribution image was captured using SEM-EDS at 5000x magnification. In this elemental distribution image, the phase in which Mg and Zn coexist was identified as the MgZn2 phase. After identifying the MgZn2 phase, the area fraction of the MgZn2 phase within the field of view was calculated using binarization with image analysis software.

[0094] Among the α phase with a particle size of 0.5–2 μm, it has (111) relative to the adjacent MgZn2 phase. α / / (0001) MgZn2 The method for determining the area fraction of the α phase with orientation relationship is described below.

[0095] First, the surface of the coating was mirror-polished, followed by chemical polishing as needed. Then, the polished surface was observed using SEM at 5000x magnification. Five α-phase particles with diameters of 0.5–2 μm were selected, representing more than 5% of the field of view based on area ratio. Crystal orientation analysis was performed on these fields using EBSD. The (111) pole diagram of the α-phase and the (0001) pole diagram of the MgZn2 phase were obtained. Comparing these pole diagrams, crystal orientations consistent with those of the α-phase and MgZn2 phase were selected.

[0096] Based on the steps above, the crystal orientation poles can be determined. Figure 1 The crystal orientation is consistent. From this crystal orientation, the α phase with a crystal orientation within ±10° in the measurement system is plotted in the IPF image. By binarizing the IPF image and providing it to image analysis, the area fraction of the α phase with a grain size of 0.5–2 μm, which has a crystal orientation consistent with the α phase and the MgZn2 phase within ±10°, can be calculated in the field of view.

[0097] As long as the area fraction of the α phase, the area fraction of the MgZn2 phase, and the interfacial state between the α phase and the MgZn2 phase are within the above-mentioned ranges, other components of the hot-dip coating are not particularly limited. The typical composition of hot-dip galvanized steel can be appropriately adopted in the hot-dip coating of the hot-dip galvanized steel of this embodiment. A preferred configuration of the hot-dip coating is shown below.

[0098] The adhesion amount of the hot-dip coating on each single side is set, for example, to be 20–150 g / m². 2 Within the specified range. This can be achieved by setting the adhesion amount per single side to 20g / m². 2 The above measures can further improve the water corrosion resistance of hot-dip galvanized steel. On the other hand, by setting the adhesion amount per single side to 150g / m²... 2 The following steps can further improve the workability of hot-dip galvanized steel.

[0099] The manufacturing method of the hot-dip galvanized steel in this embodiment is not particularly limited. For example, the hot-dip galvanized steel of this embodiment can be obtained according to the manufacturing conditions described below.

[0100] The method for manufacturing hot-dip galvanized steel in this embodiment includes:

[0101] The process of immersing a base steel material in a hot-dip galvanizing bath and then lifting it, thereby allowing a hot-dip galvanizing coating to adhere to the surface of the base steel material, and...

[0102] The process of cooling the hot-dip coating;

[0103] The cooling, such as Figure 3 Examples include:

[0104] The first cooling process involves rapidly cooling the hot-dip coated layer, after it has been lifted from the hot-dip plating bath, to a cooling stop temperature of 360°C to 520°C at an average cooling rate of 15°C / second or higher.

[0105] The second cooling process involves slowly cooling the hot-dip coating from the quenching stop temperature to 335°C at a cooling rate of less than 5°C / second.

[0106] The third cooling process involves rapidly cooling the hot-dip coating from 335°C to 70°C at a cooling rate of 70°C / second or higher.

[0107] First, the base steel is immersed in a hot-dip galvanizing bath. The chemical composition of the hot-dip galvanizing bath can be appropriately adjusted to obtain the chemical composition of the aforementioned hot-dip coating. Furthermore, the temperature of the hot-dip galvanizing bath is not particularly limited; a suitable temperature for hot-dip galvanizing can be selected. For example, the bath temperature can be set to a value approximately 20°C or higher than the melting point of the bath.

[0108] Next, the base steel is lifted from the hot-dip galvanizing bath. The amount of hot-dip galvanized coating can be controlled by adjusting the lifting speed of the base steel. Alternatively, the base steel with the hot-dip galvanized coating can be wiped as needed to control the amount of coating adhesion. The amount of hot-dip galvanized coating adhesion is not particularly limited; for example, it can be set within the range described above.

[0109] Then, the hot-dip coating is cooled. The cooling process consists of a first cooling, a second cooling, and a third cooling.

[0110] In the first cooling process, the molten metal (hot-dip coating) adhering to the surface of the base steel is rapidly cooled. Specifically, the molten metal is rapidly cooled to a quenching stop temperature (controlled cooling stop temperature) within a temperature range of 360°C to 520°C using accelerated cooling methods such as blowing a cooling medium. The quenching stop temperature is the temperature of the hot-dip coating at the moment when accelerated cooling stops. The average cooling rate in the first cooling process is set to be 15°C / second or higher. Furthermore, the average cooling rate in the first cooling process is a value obtained by dividing the difference between the temperature of the plating bath and the quenching stop temperature by the elapsed time from the moment the base steel is lifted from the plating bath to the moment when accelerated cooling stops.

[0111] In the second cooling process, the hot-dip coating is cooled slowly. Specifically, the average cooling rate over the temperature range from the aforementioned quenching stop temperature to 335°C is set to 5°C / second or less. The average cooling rate over the temperature range from the quenching stop temperature to 335°C is calculated by dividing the difference between the quenching stop temperature and 335°C by the time required for the temperature of the hot-dip coating to drop from the quenching stop temperature to 335°C. For example, this cooling rate can be achieved by placing the hot-dip coating in the atmosphere after stopping accelerated cooling. However, in manufacturing environments with extremely low temperatures, heat treatment may be necessary to reduce the rate of temperature drop of the hot-dip coating.

[0112] In the third cooling process, the hot-dip galvanized coating is rapidly cooled again. Specifically, the average cooling rate over the temperature range of 335°C to 70°C is set to at least 70°C / second. The average cooling rate over the 335°C to 70°C range is calculated by dividing the difference between 335°C and 70°C (265°C) by the time required for the temperature of the hot-dip galvanized coating to drop from 335°C to 70°C. For example, this cooling rate can be achieved by water cooling the hot-dip galvanized steel when the temperature of the coating drops to around 335°C.

[0113] By cooling the hot-dip coating in a manner that satisfies the above conditions, a coating with (111) can be formed. α / / (0001) MgZn2 The hot-dip coating has an α-phase content of 25% or more by area. The inventors of this invention presume this to be true for the following reasons.

[0114] In the first cooling process, the molten metal is rapidly cooled. As a result, both the α phase and the MgZn2 phase crystallize out from the molten metal.

[0115] In the subsequent second cooling process, the hot-dip coating of both the α phase and the MgZn2 phase is slowly cooled. This allows crystal growth while the α phase and the MgZn2 phase are in contact. As a result, the crystal orientation at the interface between the α phase and the MgZn2 phase can be integrated, achieving (111) α / / (0001) MgZn2 When the orientation relationship is established, the solidification of the molten metal ends.

[0116] In the third cooling process, a higher concentration of (111) will be added. α / / (0001) MgZn2 The hot-dip coating of the α phase, which has the established orientation relationship, is then rapidly cooled again. This suppresses the solid-phase phase transformation from the α phase to the η phase, preserving (111). α / / (0001) MgZn2 Orientation relationship.

[0117] Example

[0118] The effects of one aspect of the present invention will be further explained in detail with reference to the embodiments. The conditions in the embodiments are merely examples used to confirm the feasibility and effects of the present invention. The present invention is not limited to these specific examples. Various conditions may be used within the scope of the present invention without departing from its spirit and purpose.

[0119] Various hot-dip galvanized steel products are manufactured by immersing the base steel in various hot-dip galvanizing baths, lifting it out, thereby adhering a hot-dip galvanized coating to the surface of the base steel. The hot-dip galvanized coating is then cooled under various conditions. The chemical composition of the hot-dip galvanized coating is shown in Tables 1A and 1B. Furthermore, regarding the Fe content of the hot-dip galvanized coating, if it is less than 0.05%, it is marked with a "-" symbol in Tables 1A and 1B. The manufacturing conditions are described in Tables 2A and 2B. The metallographic structure of the coating was evaluated, and the results are shown in Tables 3A and 3B. The powdering resistance and water corrosion resistance of the hot-dip galvanized steel products were evaluated, and the results are shown in Tables 4A and 4B.

[0120] The chemical composition and metallographic structure of the hot-dip galvanized coating were evaluated using the methods described above. Furthermore, a portion of the base steel was pre-plated with Ni before hot-dip galvanizing. The composition of the pre-plated Ni is included in the chemical compositions of the hot-dip galvanized coatings disclosed in Tables 1 and 1B.

[0121] The powdering resistance was evaluated using the following method. Hot-dip galvanized steel was bent into a 90° V-shape using a metal mold with a bending radius (bending R) of 5 mm. A 24 mm wide celluloid tape was then pressed into the valley of the V-shape and pulled apart. The presence or absence of powdering was then visually evaluated. Tape with no powdery residue was rated "AA", slightly residue was rated "A", and residue with residue was rated "B". Hot-dip galvanized steel with an evaluation result of A or AA was judged to have excellent powdering resistance.

[0122] The evaluation of resistance to water corrosion was conducted using the following method. Hot-dip galvanized steel was cut to create test pieces measuring 200mm × 100mm × 0.8mm. Adhesive tape was applied to the opposite side of the evaluation surface and to a 5mm wide area on the evaluation surface from the cut end, ensuring it did not come into contact with the corrosive solution. The test pieces were then placed on a platform at a 60° angle relative to the horizontal plane. The process of exposing the test pieces to running water and then drying them was repeated alternately. During the water exposure process, a 0.5% NaCl solution was flowed at a rate of 100ml / min for 6 hours. During the drying process, the test pieces were placed for 18 hours. In both processes, the test environment was set to atmospheric temperature at 25°C. After 336 hours, the corrosion loss per unit area of ​​the coating was measured. The corrosion loss was defined as 30g / m².2 The following rating is "AA", with 60g / m 2 The following ratings are "A" and will exceed 60g / m 2 The rating is "B". Hot-dip galvanized steel with a rating of A or AA is considered to have excellent resistance to water corrosion. Furthermore, based on the above evaluation method, hot-dip galvanized steel with high resistance to water corrosion can be judged to have high corrosion resistance on its flat surfaces as well.

[0123] Table 1A

[0124]

[0125] Table 1B

[0126]

[0127] Table 2A

[0128]

[0129] Table 2B

[0130]

[0131] Table 3A

[0132] s

[0133] Table 3B

[0134]

[0135] Table 4A

[0136]

[0137] Table 4B

[0138]

[0139] In Comparative Example b1, the Al content in the hot-dip coating was insufficient. Therefore, the α phase was insufficient in Comparative Example b1. Furthermore, crystal growth occurred without contact between the α phase and the MgZn2 phase; therefore, the proportion of the α phase with an appropriate crystal orientation relationship with the MgZn2 phase was also insufficient in Comparative Example b1. Consequently, both powdering resistance and water corrosion resistance were insufficient in Comparative Example b1.

[0140] In Comparative Example b2, the amount of Mg in the hot-dip coating was insufficient. Therefore, the MgZn2 phase was insufficient in Comparative Example b2. As a result, the resistance to water corrosion was insufficient in Comparative Example b2.

[0141] In Comparative Example b3, the amount of Mg in the hot-dip coating is excessive. Therefore, in Comparative Example b3, the brittle MgZn2 phase is excessive, resulting in insufficient resistance to powdering and water corrosion.

[0142] In Comparative Example b4, the amount of Si in the hot-dip coating was excessive. Therefore, a large amount of brittle Si-based compounds were generated in the hot-dip coating of Comparative Example b3, resulting in insufficient resistance to powdering and water corrosion.

[0143] In Comparative Example b5, the Al content in the hot-dip coating was excessive. Therefore, in Comparative Example b5, the amount of α-phase growing without contact with the MgZn2 phase increased, while the proportion of α-phase with an appropriate crystal orientation relationship with the MgZn2 phase decreased. As a result, Comparative Example b5 exhibited insufficient resistance to water corrosion.

[0144] In Comparative Example b6, the amount of Ca in the hot-dip coating is excessive. Therefore, a large amount of brittle Ca-based compounds are formed in the hot-dip coating of Comparative Example b6, resulting in insufficient resistance to powdering and water corrosion.

[0145] In Comparative Examples b7 and b11, the average cooling rate during the first cooling process was insufficient. Therefore, in Comparative Examples b7 and b11, crystal growth occurred without contact between the α phase and the MgZn2 phase, resulting in an insufficient proportion of the α phase with an appropriate crystal orientation relationship to the MgZn2 phase. Consequently, in Comparative Examples b7 and b11, resistance to water corrosion was insufficient. Furthermore, in Comparative Example b7, resistance to powdering was also insufficient.

[0146] In Comparative Examples b8 and b12, the average cooling rate during the second cooling process was excessive. Therefore, in Comparative Examples b8 and b12, the α phase and MgZn2 phase, which are in contact with each other, could not grow sufficiently, and the proportion of the α phase with an appropriate crystal orientation relationship to the MgZn2 phase was insufficient. Consequently, in Comparative Examples b8 and b12, the resistance to water corrosion was insufficient.

[0147] In Comparative Example b9, the average cooling rate during the third cooling was insufficient. Therefore, in Comparative Example b9, during the third cooling, the α phase separated into an Al-rich α phase and a Zn-rich η phase, and the proportion of the α phase with an appropriate crystal orientation relationship with the MgZn2 phase was insufficient. As a result, in Comparative Example b9, the resistance to water corrosion was insufficient.

[0148] In Comparative Example b10, the amount of Sn in the hot-dip coating was excessive. Therefore, in Comparative Example b10, a Sn-based compound with low corrosion resistance was formed, and its resistance to water corrosion was insufficient.

[0149] On the other hand, embodiments of the present invention, with proper control of the chemical composition and metallographic structure of the hot-dip coating, exhibit excellent resistance to powdering and water corrosion.

[0150] Explanation of reference numerals in the attached figures

[0151] 1. Hot-dip galvanized steel

[0152] 11. Base steel

[0153] 12 Hot-dip coating

Claims

1. A hot-dip galvanized steel material, comprising: Base steel, and A hot-dip coating is applied to the surface of the base steel. The chemical composition of the hot-dip coating, by mass%, contains: Al: 10.00–20.00%; Mg: 4.00–8.00%; Sn: 0–2.00%; Si: 0–2.50%; Ca: 0–3.00%; Ni: 0% or more and less than 0.25%; Cr: 0% or more and less than 0.25%; Ti: 0% or more and less than 0.25%; Co: 0% or more and less than 0.25%; V: 0% or more and less than 0.25%; Nb: 0% or more and less than 0.25%; Cu: 0% or more and less than 0.25%; Mn: 0% or more and less than 0.25%; Bi: 0% or more and less than 5.000%; In: 0% or more and less than 2.00%; Y：0～0.50％； La: 0% or more and less than 0.50%; Ce: 0% or more and less than 0.50%; Fe: 0–5.00%; Sr: 0% or more and less than 0.50%; Sb: 0% or more and less than 0.50%; Pb: ≥0% and <0.50%; and B: 0% or more and less than 0.50%, The remaining portion consists of Zn and impurities. The metallographic structure of the hot-dip coating contains α phase with a grain size of 0.5-2 μm at 5-45% by area. The metallographic structure of the hot-dip coating contains MgZn2 phase at 15-70% by area. Among the α phases with a particle size of 0.5–2 μm, the adjacent MgZn2 phase has (111). α / / (0001) MgZn2 The area fraction of the α phase with orientation relationship is 25%–100%.

2. The hot-dip galvanized steel according to claim 1, characterized in that, Among the α phases with a particle size of 0.5–2 μm, the adjacent MgZn2 phase has (111). α / / (0001) MgZn2 The area fraction of the α phase with the orientation relationship is 60-100%.

3. The hot-dip galvanized steel according to claim 1 or 2, characterized in that, In the chemical composition of the hot-dip coating, by mass%, Mg: 5.00–8.00% Sn: 0.05–2.00%.

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