Hot-dip galvanized steel sheet, frame, and method for manufacturing hot-dip galvanized steel sheet

Abstract

This hot-dipped steel sheet comprises a steel sheet and a plating layer provided on the steel sheet. The steel sheet has a prescribed first chemical composition. The tensile strength of the steel sheet is not less than 740 MPa. In a region from a surface of the steel sheet to 3 μm in the sheet thickness direction, the area ratio of SiO2 is less than 0.4%. The plating layer has a prescribed second chemical composition. The plating layer includes an Al-Fe interface alloy layer that is in contact with the steel sheet surface. The thickness of the Al-Fe interface alloy layer is 1.0-3.0 μm.

Description

[Technical Field]

[0001] The present invention relates to a hot-dip galvanized steel sheet, a frame, and a method for manufacturing the hot-dip galvanized steel sheet. This application claims priority based on Japanese Patent Application No. 2023-055729, filed on March 30, 2023, the contents of which are incorporated herein by reference. [Background technology]

[0002] Steel sheets with a hot-dip Zn-coated layer containing Al and Mg formed on the surface (hot-dip Zn-Al-Mg-coated steel sheets) have excellent corrosion resistance, and are therefore widely used as materials for structural components that require corrosion resistance, such as building materials.

[0003] For example, Patent Document 1 discloses a Si-containing high-strength hot-dip galvanized steel sheet having good coating adhesion and corrosion resistance after painting, in which a steel layer having an SiO2 internal oxide content of 0.4 to 2.0 mass% is formed as a first layer of 3 μm or less on the surface of a high-strength steel sheet having an Si content of 0.4 to 2.0 mass%, and a hot-dip galvanized layer consisting of Al: 0.2 to 10 mass%, the balance being Zn and unavoidable impurities is formed thereon. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2001-323355 Summary of the Invention [Problem to be solved by the invention]

[0005] There is a demand for even higher strength in hot-dip Zn-Al-Mg-plated steel sheets. However, because high-strength steel sheets have a high Si content and low plating wettability, conventional plating methods often result in poor plating, poor appearance, and peeling of the coating during processing. For example, the technology disclosed in Patent Document 1 uses continuous hot-dip plating equipment with an oxidation zone for plating, which oxidizes Si, Mn, and Fe in the surface layer together in the oxidation zone and reduces only Fe in the reduction zone. While this improves plating wettability, it also creates problems such as the formation of Si oxides in the surface layer of the steel sheet, which reduces the carbon concentration in the surface layer of the steel sheet and alters the surface layer of the steel sheet.

[0006] The present invention has been made in consideration of the above-mentioned problems, and aims to provide a hot-dip galvanized steel sheet, a mounting frame, and a method for manufacturing a hot-dip galvanized steel sheet that has excellent workability and appearance and suppresses the formation of SiO2 oxide. [Means for solving the problem]

[0007] In order to solve the above problems, the present invention proposes the following means. <1> The hot-dip galvanized steel sheet according to aspect 1 of the present invention is Steel plate and a plating layer provided on the steel sheet; Equipped with The first chemical composition, which is the chemical composition of the steel plate, is, in mass%, C: 0.05%~0.20%, Mn: 1.00% to 3.00%, Si: 0.40% to 2.00%, P: 0.001%~0.100%, S: 0.0001% to 0.0100%, and Al: 0.001% to 1.500% and the balance being Fe and impurities, The tensile strength of the steel plate is 740 MPa or more, In a region up to 3 μm from the surface of the steel plate in the plate thickness direction, The area ratio of SiO2 is less than 0.4%, A second chemical composition, which is a chemical composition of the plating layer, is, in mass%, Al: 10% to 13%, Mg: 3% to 5%, and Si: 0.5% or less and the remainder being Zn and impurities, the plating layer includes an Al-Fe-based interfacial alloy layer in contact with the surface of the steel sheet, The thickness of the Al-Fe-based interface alloy layer is 1.0 to 3.0 μm. <2> A second aspect of the present invention is the hot-dip galvanized steel sheet of the first aspect, the first chemical composition, in mass %, further comprising: Ti: 0.001% to 0.150%, Nb: 0.001%~0.100%, V: 0.001%~0.300%, It may contain one or more selected from the following. <3> A third aspect of the present invention is the hot-dip galvanized steel sheet of the first or second aspect, the first chemical composition, in mass %, further comprising: Cr: 0.01%~2.00%, Ni: 0.01%~2.00%, Cu: 0.01%~2.00%, Mo: 0.01% to 2.00%, B: 0.0001%~0.0100%, W: 0.01% to 2.00%, It may contain one or more selected from the following. <4> A fourth aspect of the present invention is a hot-dip galvanized steel sheet according to any one of the first to third aspects, The aforementioned The area ratio of SiO2 may be 0.1% or more. <5> A fifth aspect of the present invention is a hot-dip galvanized steel sheet according to any one of the first to fourth aspects, the first chemical composition, in mass %, further comprising: One or more of Ca, Mg, Zr and REM may be contained in a total amount of 0.0001% to 0.0100%. <6> A method for producing a hot-dip galvanized steel sheet according to a sixth aspect of the present invention comprises: a continuous coating process of coating a base steel sheet having a first chemical composition using an all-radiant tube type continuous hot-dip galvanizing facility, In the continuous plating process, reduction annealing the base steel sheet under reducing conditions in an atmosphere of 2 to 10 vol% hydrogen and 0.002 vol% to 0.05 vol% water vapor, with the maximum surface temperature of the base steel sheet being 775°C or higher and a residence time of 30 seconds or more at 750°C or higher; The base steel sheet after the reduction annealing Plating bath temperature 470℃~520℃ and entry material temperature 470℃~520℃, or Plating bath temperature: 450℃ to less than 470℃ and entry material temperature: 490℃ to 520℃ Immerse in the plating bath for at least 3 seconds. The first chemical composition is, in mass %, C: 0.05%~0.20%, Mn: 1.00% to 3.00%, Si: 0.40% to 2.00%, P: 0.001%~0.100%, S: 0.0001% to 0.0100%, and Al: 0.001% to 1.500% and the balance being Fe and impurities, The chemical composition of the plating bath is, in mass %, Al: 10% to 13%, Mg: 3% to 5%, and Si: 0.5% or less The remainder consists of Zn and impurities. <7> The frame of the seventh aspect of the present invention uses the hot-dip galvanized steel sheet of the first aspect. [Effects of the Invention]

[0008] According to the above aspects of the present invention, it is possible to provide a hot-dip galvanized steel sheet, a frame, and a method for manufacturing a hot-dip galvanized steel sheet that are excellent in workability and appearance and in which the formation of SiO2 oxide is suppressed. DETAILED DESCRIPTION OF THE INVENTION

[0009] The present inventors conducted extensive research into hot-dip galvanized steel sheets that exhibit excellent workability and appearance while suppressing the formation of SiO2 oxides. They discovered that the use of an all-radiant tube continuous hot-dip galvanizing system that does not have an oxidation zone can suppress the formation of Si oxides on the surface of the steel sheet. Furthermore, they discovered that hot-dip galvanized steel sheets with excellent workability and appearance can be obtained by coating the steel sheet with suppressed Si oxide formation on the surface while controlling the temperature of the coating bath and the temperature of the steel sheet entering the coating bath within a predetermined range. This finding led to the completion of the present invention. In this specification, the workability of hot-dip galvanized steel sheets refers to the adhesion between the steel sheet and the coating layer at the bent portion of the hot-dip galvanized steel sheet, which is evaluated after bending the hot-dip galvanized steel sheet. Furthermore, the appearance refers to the coverage of the coating layer.

[0010] Hereinafter, a hot-dip galvanized steel sheet according to an embodiment of the present invention will be described. The hot-dip galvanized steel sheet according to this embodiment includes a steel sheet and a coating layer provided on the steel sheet, and a first chemical composition of the steel sheet contains, in mass %, C: 0.05% to 0.20%, Mn: 1.00% to 3.00%, Si: 0.40% to 2.00%, P: 0.001% to 0.100%, S: 0.0001 to 0.0100%, and Al: 0.001 to 1.500%, with the balance being Fe and impurities. The tensile strength of the steel sheet is 740 MPa or more, an area ratio of SiO2 is less than 0.4% in a region from the surface of the steel sheet to 3 μm in the sheet thickness direction, a second chemical composition that is the chemical composition of the plating layer contains, in mass %, 10% to 13% Al, 3% to 5% Mg, and 0.5% or less Si, with the balance being Zn and impurities, the plating layer includes an Al-Fe-based interfacial alloy layer in contact with the steel sheet surface, and the thickness of the Al-Fe-based interfacial alloy layer is 1.0 to 3.0 μm.

[0011] In the following description, the "%" used to indicate the content of each element in the chemical composition means "% by mass." The content of an element in the chemical composition may be expressed as an element concentration (e.g., Zn concentration, Mg concentration, etc.). In this specification, the term "plated layer" refers to a plated film produced by a so-called hot-dip galvanizing process.

[0012] (steel plate) The first chemical composition of the steel sheet contains, by mass%, C: 0.05% to 0.20%, Mn: 1.00% to 3.00%, Si: 0.40% to 2.00%, P: 0.001% to 0.100%, S: 0.0001% to 0.0100%, and Al: 0.001% to 1.500%, with the balance being Fe and impurities. Each element will be described below.

[0013] [C: 0.05% to 0.20%] C is an element added to increase the strength of steel sheets. However, if the C content exceeds 0.20%, weldability deteriorates, so the C content is set to 0.20% or less. From the viewpoint of weldability, the C content is preferably 0.15% or less. On the other hand, if the C content is less than 0.05%, the strength decreases and it becomes difficult to ensure sufficient tensile strength, so the C content is set to 0.05% or more. In order to further increase the strength, the C content is preferably 0.06% or more, and more preferably 0.07% or more.

[0014] [Mn: 1.00% to 3.00%] Mn is added to improve the hardenability of steel sheets and thereby increase their strength. However, if the Mn content exceeds 3.00%, coarse Mn-enriched areas form in the center of the steel sheet's thickness, making it more susceptible to embrittlement and problems such as cracking of the cast slab. Therefore, the Mn content is set to 3.00% or less. In addition, an increase in Mn content also deteriorates weldability. For this reason, the Mn content is preferably set to 2.80% or less, more preferably 2.70% or less. On the other hand, if the Mn content is less than 1.00%, a large amount of soft tissue is formed during cooling after annealing, making it difficult to ensure sufficiently high tensile strength. Therefore, the Mn content is set to 1.00% or more. To further increase strength, the Mn content is preferably set to 1.20% or more, more preferably 1.40% or more.

[0015] [Si: 0.40% to 2.00%] Si is an element that suppresses the formation of iron-based carbides in steel sheets and improves their strength and formability. However, Si also embrittles steel sheets, and if its content exceeds 2.00%, problems such as cracking of cast slabs become more likely to occur. For this reason, the Si content is set to 2.00% or less. Furthermore, Si forms oxides on the surface of the base steel sheet during the annealing process, significantly impairing the adhesion of the coating. From this perspective, the Si content is preferably set to 1.90% or less, and more preferably 1.60% or less. On the other hand, if the Si content is less than 0.40%, a large amount of coarse iron-based carbides is formed during the coating process of hot-dip galvanized steel sheets, resulting in deterioration of their strength and formability. For this reason, the Si content is set to 0.40% or more. From the perspective of suppressing the formation of iron-based carbides, the Si content is preferably set to 0.50% or more, and more preferably 0.60% or more.

[0016] [P: 0.001% to 0.100%] P is an element that embrittles steel sheets, and if the P content exceeds 0.100%, problems such as cracking of cast slabs become more likely to occur. For this reason, the P content is set to 0.100% or less. P is also an element that embrittles the molten zone created by spot welding, so in order to obtain sufficient welded joint strength, the P content is preferably set to 0.040% or less, and more preferably set to 0.020% or less. On the other hand, setting the P content to less than 0.001% results in a significant increase in manufacturing costs. For this reason, the P content is set to 0.001% or more, and preferably 0.010% or more.

[0017] [S: 0.0001% to 0.0100%] S is an element that combines with Mn to form coarse MnS, reducing formability. For this reason, the S content is set to 0.0100% or less. S is also an element that deteriorates spot weldability. For this reason, the S content is preferably set to 0.0060% or less, and more preferably set to 0.0035% or less. On the other hand, setting the S content to less than 0.0001% results in a significant increase in manufacturing costs. For this reason, the S content is set to 0.0001% or more, preferably 0.0005% or more, and more preferably 0.0010% or more.

[0018] [Al: 0.001% to 1.500%] Al is an element that embrittles steel sheets. If the Al content exceeds 1.500%, problems such as cracking of the cast slabs become more likely to occur, so the Al content is set to 1.500% or less. Furthermore, since an increase in the Al content deteriorates weldability, the Al content is preferably set to 1.200% or less, and more preferably 1.000% or less. Al is an impurity present in trace amounts in raw materials, and since reducing its content to less than 0.001% would result in a significant increase in manufacturing costs, the Al content is set to 0.001% or more. Furthermore, Al is also an effective element as a deoxidizer, but to obtain the deoxidizing effect more fully, the Al content is preferably set to 0.010% or more.

[0019] The steel sheet according to this embodiment may contain the above elements, with the balance being Fe and impurities. Here, the impurities are elements that are mixed in during industrial steel production due to raw materials such as ore and scrap, and various factors in the manufacturing process, and whose presence is permitted to the extent that they do not impair the properties of the steel sheet according to this embodiment. They also include elements that are not intentionally added to the steel sheet.

[0020] Furthermore, the hot-dip galvanized steel sheet according to an embodiment of the present invention may contain the following elements as needed. Specifically, in addition to the above chemical components, the steel sheet may contain one or more elements selected from Ti: 0.001% to 0.150%, Nb: 0.001% to 0.100%, and V: 0.001% to 0.300%. These elements do not necessarily have to be contained, so the lower limit of the content of each element is 0%.

[0021] [Ti: 0.001% to 0.150%] Ti is an element that contributes to increasing the strength of steel sheets by precipitation strengthening, grain refinement by inhibiting ferrite grain growth, and dislocation strengthening by inhibiting recrystallization. However, if the Ti content exceeds 0.150%, the precipitation of carbonitrides increases, deteriorating formability. Therefore, the Ti content is preferably 0.150% or less. From the viewpoint of formability, the Ti content is more preferably 0.080% or less. In order to fully obtain the strength-increasing effect of Ti addition, the Ti content is preferably 0.001% or more. In order to further increase the strength of steel sheets, the Ti content is more preferably 0.010% or more.

[0022] [Nb: 0.001% to 0.100%] Nb is an element that contributes to increasing the strength of steel sheets by strengthening precipitates, strengthening fine grains by inhibiting ferrite grain growth, and strengthening dislocations by inhibiting recrystallization. However, if the Nb content exceeds 0.100%, the precipitation of carbonitrides increases, deteriorating formability. Therefore, the Nb content is preferably 0.100% or less. From the viewpoint of formability, the Nb content is more preferably 0.060% or less. To fully obtain the strength-increasing effect of Nb addition, the Nb content is preferably 0.001% or more. To further increase the strength of steel sheets, the Nb content is more preferably 0.005% or more.

[0023] [V: 0.001% to 0.300%] V is an element that contributes to increasing the strength of steel sheets by strengthening precipitates, strengthening fine grains by inhibiting ferrite grain growth, and strengthening dislocations by inhibiting recrystallization. However, if the V content exceeds 0.300%, the precipitation of carbonitrides increases, deteriorating formability. Therefore, the V content is preferably 0.300% or less, and more preferably 0.200% or less. To fully obtain the strength-increasing effect of V addition, the V content is preferably 0.001% or more, and more preferably 0.010% or more.

[0024] Furthermore, the steel sheet according to this embodiment may further contain one or more elements selected from Cr: 0.01% to 2.00%, Ni: 0.01% to 2.00%, Cu: 0.01% to 2.00%, Mo: 0.01% to 2.00%, B: 0.0001% to 0.0100%, and W: 0.01% to 2.00%. These elements do not necessarily have to be contained, and the lower limit of the content of each element is 0%.

[0025] [Cr: 0.01% to 2.00%] Cr is an element that suppresses phase transformation at high temperatures and is effective in increasing strength, and may be added. However, if the Cr content exceeds 2.00%, hot workability is impaired, resulting in a decrease in productivity. For this reason, the Cr content is preferably 2.00% or less, and more preferably 1.20% or less. To fully obtain the effect of increasing strength due to the addition of Cr, the Cr content is preferably 0.01% or more, and more preferably 0.10% or more.

[0026] [Ni: 0.01% to 2.00%] Ni is an element that suppresses phase transformation at high temperatures and is effective in increasing strength, and may be added. However, if the Ni content exceeds 2.00%, weldability is impaired. For this reason, the Ni content is preferably 2.00% or less, and more preferably 1.20% or less. To fully obtain the effect of increasing strength due to the addition of Ni, the Ni content is preferably 0.01% or more, and more preferably 0.10% or more.

[0027] [Cu: 0.01% to 2.00%] Cu is an element that increases strength by being present in steel as fine particles and can be added. However, if the Cu content exceeds 2.00%, weldability is impaired. It can also cause cracking due to surface red shortness during hot rolling. For these reasons, the Cu content is preferably 2.00% or less, and more preferably 1.20% or less. To fully obtain the high strength effect of Cu addition, the Cu content is preferably 0.01% or more, and more preferably 0.10% or more.

[0028] [Mo: 0.01% to 2.00%] Mo is an element that suppresses phase transformation at high temperatures and is effective in increasing strength, and may be added. However, if the Mo content exceeds 2.00%, hot workability is impaired, resulting in a decrease in productivity. For this reason, the Mo content is preferably 2.00% or less, and more preferably 1.20% or less. To fully obtain the effect of increasing strength due to the addition of Mo, the Mo content is preferably 0.01% or more, and more preferably 0.05% or more.

[0029] [B: 0.0001% to 0.0100%] B is an element that suppresses phase transformation at high temperatures and is effective in increasing strength, and may be added. However, if the B content exceeds 0.0100%, hot workability is impaired and productivity decreases, so the B content is preferably 0.0100% or less. From the viewpoint of productivity, the B content is more preferably 0.0050% or less. To fully obtain the effect of increasing strength by adding B, the B content is preferably 0.0001% or more. To further increase strength, the B content is more preferably 0.0005% or more.

[0030] [W: 0.01% to 2.00%] W is an element that suppresses phase transformation at high temperatures and is effective in increasing strength, and may be added. However, if the W content exceeds 2.00%, hot workability is impaired, resulting in a decrease in productivity. For this reason, the W content is preferably 2.00% or less, and more preferably 1.20% or less. In order to fully obtain the high strength provided by W, the W content is preferably 0.01% or more, and more preferably 0.10% or more.

[0031] Furthermore, the steel sheet of this embodiment may contain one or more of Ca, Mg, Zr, and REM in a total amount of 0.0001% to 0.0100% as other elements. The reasons for adding these elements are as follows. REM is an abbreviation for Rare Earth Metal, and in this case refers to elements belonging to the lanthanide series.

[0032] Ca, Mg, Zr, and REM are elements effective in improving formability, and one or more of them can be added. However, if the total content of one or more of Ca, Mg, Zr, and REM exceeds 0.0100%, ductility may be impaired. Therefore, the total content of each element is preferably 0.0100% or less, and more preferably 0.0070% or less. On the other hand, the effects of the present invention can be achieved even if the lower limit of the content of one or more of Ca, Mg, Zr, and REM is not particularly specified. In order to fully obtain the effect of improving the formability of the steel sheet, the total content of these elements is preferably 0.0001% or more. From the viewpoint of formability, the total content of one or more of Ca, Mg, Zr, and REM is more preferably 0.0010% or more.

[0033] The chemical composition of the steel sheet according to this embodiment can be determined by the following method. For example, the chemical composition (first chemical composition) of the steel sheet can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S can be measured using a combustion-infrared absorption method, N can be measured using an inert gas fusion-thermal conductivity method, and O can be measured using an inert gas fusion-non-dispersive infrared absorption method. The chemical composition of the steel sheet can be measured after removing the coating layer by mechanical grinding.

[0034] (The tensile strength of the steel plate is 740 MPa or more) The steel plate according to this embodiment has a tensile strength (TS) of 740 MPa or more. The strength of the steel plate is preferably 780 MPa or more, and more preferably 1000 MPa or more. The tensile strength is preferably 1250 MPa or less. Tensile strength (TS) can be determined by taking a JIS No. 5 tensile test piece from the steel sheet in the direction perpendicular to the rolling direction and thickness direction, and conducting a tensile test in accordance with JIS Z 2241:2011. The tensile strength is measured after peeling off the plating.

[0035] (The area ratio of SiO2 is less than 0.4% in the region from the surface of the steel plate to 3 μm in the plate thickness direction) In the steel sheet according to this embodiment, the area ratio of SiO2 is less than 0.4% in the region from the surface of the steel sheet to 3 μm in the thickness direction. If the area ratio of SiO2 is 0.4% or more in the region from the surface of the steel sheet to 3 μm in the thickness direction, SiO2 oxide is formed in the surface layer, making the coating more likely to peel when the high-strength steel sheet is processed. More preferably, the area ratio of SiO2 is 0.2% or less in the region from the surface of the steel sheet to 3 μm in the thickness direction. It is preferable that the area ratio of SiO2 is 0.1% or more in the region from the surface of the steel sheet to 3 μm in the thickness direction.

[0036] (plating layer) The second chemical composition of the coating layer contains, by mass, 10% to 13% Al, 3% to 5% Mg, and 0.5% or less Si, with the remainder consisting of Zn and impurities. The coating layer includes an Al-Fe-based interfacial alloy layer in contact with the surface of the steel sheet. The portion of the coating layer excluding the Al-Fe-based interfacial alloy layer is a Zn-Al-Mg alloy layer. That is, the coating layer includes a Zn-Al-Mg alloy layer and an Al-Fe-based interfacial alloy layer. Each element in the coating layer will be described below.

[0037] [Al: 10% to 13%] Al is an element that improves corrosion resistance and suppresses Mg oxide dross in the plating bath, contributing to bath stability. Therefore, the Al content is set to 10% or more. The Al content may be set to 11% or more. On the other hand, if Al is excessive, the Mg content and Zn content may decrease relatively, resulting in a deterioration of corrosion resistance. Therefore, the Al content is set to 13% or less. The Al content may be set to 12% or less.

[0038] [Mg: 3% to 5%] Mg is an essential element for ensuring corrosion resistance. Therefore, the Mg content is set to 3% or more. The Mg content may be set to 4% or more. On the other hand, if the Mg content is excessive, workability and corrosion resistance may be deteriorated. Therefore, the Mg content is set to 5% or less. The Mg content may be set to 4% or less.

[0039] [Si:0.5% or less] The Si content may be 0%. On the other hand, Si contributes to improving corrosion resistance. It also suppresses the excessive growth of a brittle Al-Fe alloy layer at the interface between the coating and the base material, improving plating workability. Therefore, the Si content may be 0.05% or more. On the other hand, if the Si content is excessive, corrosion resistance may deteriorate. Therefore, the Si content is set to 0.5% or less. The Si content may also be set to 0.4% or less. The Si content is preferably 0.10% or more.

[0040] [balance: Zn and impurities] The remainder of the chemical composition (second chemical composition) of the plating layer according to this embodiment is Zn and impurities. Zn is an element that provides corrosion resistance to the plating layer. Impurities refer to components contained in raw materials or components mixed in during the manufacturing process, but not intentionally added. For example, trace amounts of components other than Fe may be mixed into the plating layer as impurities due to mutual atomic diffusion between the base steel sheet and the plating bath.

[0041] The chemical composition of the plating layer according to this embodiment is measured by the following method. First, an acid containing an inhibitor that suppresses corrosion of the steel sheet is used to remove and dissolve the plating layer to obtain an acid solution. Next, the obtained acid solution is subjected to ICP analysis. This allows the chemical composition (second chemical composition) of the plating layer to be obtained. The acid species is not particularly limited as long as it is an acid that can dissolve the plating layer. Note that the chemical composition measured by the above-mentioned method is the average chemical composition of the entire plating layer. The formation reaction of the Al-Fe-based interfacial alloy layer is completed in the plating bath, and the thickness of the Al-Fe-based interfacial alloy layer is also sufficiently small compared to the thickness of the plating layer. Therefore, unless special heat treatment such as a thermal alloying treatment is performed after plating, the average chemical composition of the entire plating layer is substantially equal to the chemical composition of the Zn-Al-Mg alloy layer, and components such as the Al-Fe-based interfacial alloy layer can be ignored.

[0042] [Al-Fe interfacial alloy layer] The plating layer according to this embodiment includes an Al-Fe-based interfacial alloy layer in contact with the surface of the steel sheet. The Al-Fe-based interfacial alloy layer is formed on the surface of the steel sheet, specifically between the steel sheet and the plating layer, and is a layer whose structure contains an Al-Fe intermetallic compound as a main component. For example, the Al-Fe-based interfacial alloy layer is a layer whose main phase is the Al5Fe2 phase. Here, the term "main phase is the Al5Fe2 phase" means that 80% or more of the area of ​​the Al-Fe-based interfacial alloy layer is the Al5Fe2 phase.

[0043] The Al-Fe-based interfacial alloy layer is formed by mutual atomic diffusion between the steel sheet and the coating bath. In this embodiment, the coating bath contains a certain concentration of Al, so the Al5Fe2 phase is formed most frequently. However, because atomic diffusion takes time, the Fe concentration in the Al-Fe-based interfacial alloy layer is not uniform, and the Fe concentration may be higher in the area closer to the steel sheet. Therefore, the Al-Fe-based interfacial alloy layer may partially contain small amounts of AlFe phase, Al3Fe phase, etc. Furthermore, because the coating bath also contains a certain concentration of Zn, the Al-Fe-based interfacial alloy layer may also contain small amounts of Zn or Si, which tends to accumulate at the interface.

[0044] When Si is contained in the coating layer, it may be incorporated into the Al-Fe-based interfacial alloy layer and form an Al-Fe-Si intermetallic compound phase. The identified intermetallic compound phase is the AlFeSi phase, and isomers include α-, β-, q1-, and q2-AlFeSi phases. Therefore, these AlFeSi phases may be detected in the Al-Fe-based interfacial alloy layer.

[0045] The Al-Fe-based interfacial alloy layer ensures adhesion between the plating layer and the steel sheet during processing of the hot-dip galvanized steel sheet and affects processability. In this embodiment, the thickness of the Al-Fe-based interfacial alloy layer is 1.0 to 3.0 μm. If the Al-Fe-based interfacial alloy layer exceeds 3.0 μm, cracks may occur during processing. If the Al-Fe-based interfacial alloy layer is less than 1.0 μm, sufficient adhesion between the plating layer and the steel sheet may not be obtained.

[0046] The thickness of each layer of the plating layer according to this embodiment and the area ratio of SiO2 in the region from the surface of the steel sheet to 3 μm in the sheet thickness direction can be evaluated by the following method.

[0047] First, a test specimen was cut using a focused ion beam (FIB) so that the cutting direction was parallel to the thickness direction and perpendicular to the width direction. The cross-sectional structure of this cut surface was observed using a scanning electron microscope with an electron probe microanalyzer (SEM-EPMA) at a magnification that allowed each layer to fit within the observation field. If the Zn-Al-Mg alloy layer, the Al-Fe interfacial alloy layer, and the region extending from the surface of the steel plate to 3 μm in the thickness direction are not included within the observation field, the cross-sectional structure was observed using multiple consecutive fields of view. For example, a field of view of 1 μm x 1 μm or larger is sufficient. The width of the observation area in the thickness and width directions is confirmed by referring to the scale bar used during SEM imaging. An accelerating voltage of 20 kV is also recommended. Because it is difficult to obtain average information for the steel plate when observing only one field of view, it is sufficient to observe 10 randomly selected, mutually separated locations within the field of view.

[0048] In order to identify each layer in the cross-sectional structure of the steel sheet according to this embodiment, a line analysis is performed along the sheet thickness direction using an electron probe microanalyzer (EPMA), and the chemical composition of each layer is quantitatively analyzed. The elements to be quantitatively analyzed are six elements: Fe, Si, O, Mg, Zn, and Al.

[0049] Based on the results of the SEM-EPMA quantitative analysis described above, the layered region located deepest in the thickness direction and having an Fe content of 70 atomic % or more and an O content of less than 30 atomic % (excluding measurement noise) was determined to be steel sheet, and the region with an Fe content of 10 atomic % or more but less than 70 atomic % and an O content of less than 30 atomic % was determined to be the Al-Fe-based interfacial alloy layer. The region with an Fe content of less than 10 atomic % was determined to be the Zn-Al-Mg alloy layer. The surface of the steel sheet was determined to be the interface between the steel sheet and the Al-Fe-based interfacial alloy layer. The thickness of the Al-Fe-based interfacial alloy layer was determined to be the length along the thickness direction from the interface between the Zn-Al-Mg alloy layer and the Al-Fe-based interfacial alloy layer to the interface between the Al-Fe-based interfacial alloy layer and the steel sheet.

[0050] For example, SiO2 can be identified by performing a line analysis of the precipitates observed in the steel sheet along the thickness direction of the steel sheet identified by the above method using an electron probe microanalyzer (EPMA) and performing a quantitative analysis of the chemical composition. The six elements that are quantitatively analyzed are Fe, Si, O, Mg, Zn, and Al. From the results of the quantitative analysis by EPMA described above, excluding measurement noise, regions where the Fe content is 15 atomic % or less, the Si content is 30 atomic % or more, and the O content is 55 atomic % or more are judged to be internally oxidized SiO2. The area ratio of SiO2 can be calculated from the total area of ​​the region extending from the surface of the steel sheet to 3 μm in the thickness direction and the area of ​​the obtained SiO2 (area of ​​SiO2 / total area of ​​the region extending from the surface of the steel sheet to 3 μm in the thickness direction).

[0051] (area ratio of plating layer) In the hot-dip plated steel sheet according to this embodiment, the area ratio (coverage) of the plating layer to the steel sheet surface is preferably 99% or more. The area ratio of the plating layer can be evaluated using image analysis software on a surface observation image of the hot-dip plated layer.

[0052] (Plating layer adhesion amount) The coating weight per side of the plating layer is, for example, 20 to 150 g / m 2 The amount of adhesion per side should be within the range of 20 g / m 2 By setting the coating weight per side to 150 g / m or more, the corrosion resistance of the hot-dip galvanized steel sheet can be further improved. 2 By setting the following, the workability of the hot-dip galvanized steel sheet can be further improved.

[0053] The coating weight of the coating layer according to this embodiment is measured by the following method. First, the area of ​​the coating layer of the hot-dip galvanized steel sheet and the weight of the hot-dip galvanized steel sheet are measured before the coating layer is peeled off. Next, the coating layer is peeled off using an acid containing an inhibitor that suppresses corrosion of the steel sheet. The weight of the hot-dip galvanized steel sheet after peeling is measured and calculated to determine the coating weight of the coating layer (g / m 2 ) is obtained.

[0054] <Method of manufacturing hot-dip galvanized steel sheets> Next, a method for producing the hot-dip galvanized steel sheet according to the present embodiment will be described, but the method for producing the hot-dip galvanized steel sheet according to the present embodiment is not particularly limited. For example, the hot-dip galvanized steel sheet according to the present embodiment can be obtained under the production conditions described below.

[0055] The method for producing a hot-dip galvanized steel sheet according to this embodiment includes a continuous plating step of plating a base steel sheet having the above-mentioned first chemical composition using an all-radiant tube type continuous hot-dip galvanizing facility, and in the continuous plating step, the base steel sheet is reduction-annealed in an atmosphere of 2 to 10 vol% hydrogen and 0.002 vol% to 0.05 vol% steam under reducing conditions in which the maximum surface temperature of the base steel sheet is 775°C or higher and the residence time at 750°C or higher is 30 seconds or longer, and the base steel sheet after reduction annealing is immersed in a plating bath having a second chemical composition at a plating bath temperature of 470°C to 520°C and an entry temperature of 470°C to 520°C, or at a plating bath temperature of 450°C or higher but lower than 470°C and an entry temperature of 490°C to 520°C for 3 seconds or longer. Specifically, the chemical composition (second chemical composition) of the plating bath contains, in mass %, 10% to 13% Al, 3% to 5% Mg, and 0.5% or less Si, with the remainder being Zn and impurities.

[0056] (Base material steel plate) As the base steel sheet, a steel sheet having the above-mentioned first chemical composition and a tensile strength (for example, the tensile strength of the steel sheet after the above-mentioned reduction annealing) of 740 MPa or more is used. The base steel sheet is not particularly limited as long as it has the above-mentioned first chemical composition and, for example, a tensile strength of 740 MPa or more. As the base steel sheet, for example, a hot-rolled steel sheet obtained by steelmaking a slab having the above-mentioned first chemical composition and hot-rolling it may be used. Alternatively, a cold-rolled steel sheet obtained by cold-rolling the obtained hot-rolled steel sheet may be used.

[0057] (All-radiant tube continuous hot-dip galvanizing equipment) The all-radiant tube (ART) continuous hot-dip galvanizing equipment according to this embodiment does not have an oxidation zone in the continuous furnace. Therefore, by appropriately controlling the annealing dew point, it is possible to suppress the formation of internal Si oxides in the surface layer of the steel sheet. This makes it possible to keep the area ratio of SiO2 below 0.4% in the region from the surface of the steel sheet to a depth of 3 μm in the sheet thickness direction.

[0058] In the continuous plating process according to this embodiment, reduction annealing is performed in an atmosphere containing 2 to 10 vol% hydrogen and 0.002 to 0.05 vol% water vapor. During this process, the maximum surface temperature of the base steel sheet is set to 775°C or higher, and the residence time at 750°C or higher is set to 30 seconds or longer. By reducing the base steel sheet under these reducing conditions, the area ratio of SiO2 can be reduced to less than 0.4% in a region extending from the surface of the steel sheet to 3 μm in the thickness direction. The maximum temperature may be 900°C or lower, and the residence time may be 150 seconds or shorter. By setting the maximum temperature to 775°C or higher and the residence time at 750°C or higher to 30 seconds or longer, the tensile strength of the steel sheet can be increased to 740 MPa or higher. A water vapor concentration of more than 0.05 vol% improves plating wettability but undesirably reduces workability. To obtain the hot-dip plated steel sheet according to this embodiment, the water vapor concentration is set to 0.002 vol% or higher.

[0059] The dew point during reduction annealing is preferably −30° C. or lower. By setting the dew point to −30° C. or lower, it becomes easier to make the area ratio of SiO less than 0.4% in the region from the surface of the steel sheet to a depth of 3 μm in the sheet thickness direction.

[0060] During reduction annealing, log(PH2O / PH2) is preferably -2.20 to -1.00. Here, PH2O represents the partial pressure of water vapor, and PH2 represents the partial pressure of hydrogen. When log(PH2O / PH2) during reduction annealing is -2.20 to -1.00, the area ratio of SiO2 can be easily made less than 0.4% in the region from the surface of the steel sheet to a depth of 3 μm in the sheet thickness direction.

[0061] The base steel sheet after reduction annealing is immersed for 3 seconds or more in a coating bath having the same composition as the second chemical composition under the following condition A or condition B. The immersion time may be 10 seconds or less. By immersing the base steel sheet having the first chemical composition under condition A or condition B for 3 seconds or more, the coating wettability can be improved. Fe diffuses into the coating bath and reacts with the coating bath, forming an Al-Fe-based interfacial alloy layer at the interface between the coating layer and the steel sheet. Furthermore, the Al-Fe-based interfacial alloy layer can be 1 to 3 μm thick. If the coating bath temperature exceeds 520°C, the thickness of the Al-Fe-based interfacial alloy layer will exceed 3 μm, which will reduce the workability of the hot-dip coated steel sheet according to this embodiment. Condition A: Plating bath temperature 470℃~520℃ and entry material temperature 470℃~520℃ Condition B: Plating bath temperature 450°C to less than 470°C and entry material temperature 490°C to 520°C

[0062] The immersion time in the plating bath is 3 seconds or more. If the immersion time in the plating bath is less than 3 seconds, the plating area rate will not be 99% or more.

[0063] Next, the base steel sheet with the molten metal attached thereto is removed from the coating bath. The steel sheet with the molten metal attached thereto is subjected to gas wiping to control the coating weight of the coating layer. The coating weight of the coating layer is not particularly limited and can be, for example, within the above-mentioned range.

[0064] Next, after controlling the coating weight of the coating layer, the adhered molten metal is cooled to solidify. Cooling is performed continuously on the steel sheet with the molten metal attached immediately after it is pulled up from the coating bath until the temperature of the molten metal drops from the bath temperature to 300°C. The cooling method may be to spray nitrogen, air, or a hydrogen-helium mixed gas, or mist cooling. The cooling conditions below 300°C are not particularly limited, and subsequent mist cooling may be performed, or the sheet may be allowed to cool naturally. The average cooling rate between the bath temperature and 300°C is, for example, 10°C / second or more.

[0065] The hot-dip galvanized steel sheet of the present disclosure has excellent workability and appearance, and suppresses the formation of SiO2 oxide, making it suitable for use in solar cell mounting frames, etc. Because the steel sheet has excellent coating adhesion, it can be processed more heavily than conventional steel sheets, allowing for a reduction in the number of parts, and reducing the cost of manufacturing parts and the cost of assembling parts on-site. [Example]

[0066] Examples of the present invention will be described below. However, the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention. The present invention is not limited to these examples. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0067] The base steel sheet used had the chemical composition shown in Table 1. The base steel sheet was then subjected to reduction annealing under the conditions shown in Tables 2A and 2B. The base steel sheet after reduction annealing was then galvanized under the conditions shown in Tables 3A and 3B. The base steel sheet was then removed from the galvanizing bath and gas wiped to adjust the coating weight, and cooled with nitrogen gas. All radiant tube continuous hot-dip galvanizing equipment without an oxidation zone was used for all plating except for Experiment No. 26. No. 26 was plated using continuous hot-dip galvanizing equipment with an oxidation zone.

[0068] (Evaluation of the chemical composition of the coating layer, the thickness of the Al-Fe-based interfacial alloy layer, and the area ratio of SiO2 in the region from the surface of the steel sheet to 3 μm in the thickness direction) The chemical composition of the resulting coating layer, the thickness of the Al-Fe-based interfacial alloy layer, and the area ratio of SiO2 in the region from the surface of the steel sheet to a depth of 3 μm in the sheet thickness direction were evaluated by the methods described above. The results are shown in Tables 4A, 4B, 4C, and 4D.

[0069] (tensile strength) Tensile strength (TS) was evaluated on the steel sheets after the coating layer was peeled off from the obtained hot-dip coated steel sheets. Peeling was performed using the same method as used for analyzing the chemical composition of the coating layer described above. JIS No. 5 tensile test specimens were taken perpendicular to the rolling direction of the steel sheets and subjected to tensile tests in accordance with JIS Z 2241:2011. The results are shown in Tables 4A, 4B, 4C, and 4D. Similarly, the tensile strength was evaluated on the base steel sheets after reduction annealing. The results are shown in Table 1.

[0070] (Plating evaluation) The plating evaluation was carried out with a rating of ◯ when the area ratio of the plating layer was 99% or more, △ when the area ratio of the plating layer was 67% or more but less than 99%, and × when the area ratio of the plating layer was less than 67%. An area ratio of the plating layer of 99% or more was considered pass. The results are shown in Tables 4A, 4B, 4C, and 4D.

[0071] (Evaluation of workability) Formability was evaluated based on the adhesion of the plating layer. Plates with a plating layer area ratio of 99% or more were evaluated. Each steel sheet was bent 180 degrees in accordance with JIS G 3323:2019. The inner spacing between the bent steel sheets was 3T (number of sheets). Tape was applied to the bent steel sheets, and the remaining plating layer area ratio (remaining area ratio) after the tape was peeled off was evaluated. A plated layer area ratio of 99% or more was evaluated as ◯, a plated layer area ratio of 67% to less than 99% was evaluated as △, and a plated layer area ratio of less than 67% was evaluated as ×. Plated layer area ratios of 99% or more were evaluated as excellent formability and passed. Plated sheets with bare areas that were not evaluated were evaluated as "-". The results are shown in Tables 4A, 4B, 4C, and 4D.

[0072] As shown in Tables 4A, 4B, 4C, and 4D, the chemical composition of the steel sheet, the chemical composition of the coating layer, the thickness of the Al-Fe-based interfacial alloy layer, and the area ratio of SiO2 in the region from the surface of the steel sheet to 3 μm in the thickness direction were appropriately controlled. Examples 4 to 7, 10 to 12, 16 to 19, 29, 32, 35, 38, 41, 44, 47, 50, 53, 56, 59, 62, 65, 67, and 68 according to the present invention were excellent in both appearance and workability. Furthermore, because the coating was performed using an all-radiant tube continuous hot-dip galvanizing facility, the area ratio of SiO2 in the region from the surface of the steel sheet to 3 μm in the thickness direction was low, and the formation of SiO2 oxide was suppressed. The coating weight per side of the coating layer in the examples was, for example, 20 to 150 g / m 2 In Comparative Example 27, since the Si content in the steel sheet was low, even though the plating bath temperature and sheet temperature were outside the ranges, the area ratio of the plating layer was 99% or more, but the tensile strength was low.

[0073] [Table 1]

[0074] [Table 2A]

[0075] [Table 2B]

[0076] [Table 3A]

[0077] [Table 3B]

[0078] [Table 4A]

[0079] [Table 4B]

[0080] [Table 4C]

[0081] [Table 4D] [Industrial Applicability]

[0082] The hot-dip galvanized steel sheet of the present disclosure has excellent workability and appearance, and the formation of SiO2 oxide is suppressed, so it has high industrial applicability.

Claims

1. Steel plate and a plating layer provided on the steel sheet; Equipped with The first chemical composition, which is the chemical composition of the steel plate, is, in mass%, C: 0.05% to 0.20%, Mn: 1.00% to 3.00%, Si: 0.40% to 2.00%, P: 0.001% to 0.100%, S: 0.0001% to 0.0100%, and Al: 0.001% to 1.500% and the balance being Fe and impurities, The tensile strength of the steel plate is 740 MPa or more, In a region up to 3 μm from the surface of the steel plate in the plate thickness direction, SiO ２ The area ratio of is less than 0.4%, A second chemical composition, which is a chemical composition of the plating layer, is, in mass%, Al: 10% to 13%, Mg: 3% to 5%, and Si: 0.5% or less and the remainder being Zn and impurities, the plating layer includes an Al-Fe-based interfacial alloy layer in contact with the surface of the steel sheet, The hot-dip plated steel sheet has an Al-Fe-based interface alloy layer having a thickness of 1.0 to 3.0 μm.

2. the first chemical composition, in mass percent, further comprising: Ti: 0.001% to 0.150%, Nb: 0.001% to 0.100%, V: 0.001% to 0.300%, The hot-dip galvanized steel sheet according to claim 1, comprising one or more selected from the group consisting of:

3. the first chemical composition, in mass percent, further comprising: Cr: 0.01% to 2.00%, Ni: 0.01% to 2.00%, Cu: 0.01% to 2.00%, Mo: 0.01% to 2.00%, B: 0.0001% to 0.0100%, W: 0.01% to 2.00%, The hot-dip galvanized steel sheet according to claim 1, comprising one or more selected from the group consisting of:

4. The SiO ２ The hot-dip galvanized steel sheet according to claim 1, wherein the area ratio of

5. the first chemical composition, in mass percent, further comprising:

2. The hot-dip galvanized steel sheet according to claim 1, containing one or more of Ca, Mg, Zr and REM in a total amount of 0.0001% to 0.0100%.

6. a continuous coating process of coating a base steel sheet having a first chemical composition using an all-radiant tube type continuous hot-dip galvanizing facility, In the continuous plating process, reduction annealing the base steel sheet under reducing conditions in an atmosphere of 2 to 10 vol% hydrogen and 0.002 vol% to 0.05 vol% water vapor, with the maximum surface temperature of the base steel sheet being 775°C or higher and a residence time of 30 seconds or longer at 750°C or higher; The base steel sheet after the reduction annealing A plating bath temperature of 470°C to 520°C and an entry material temperature of 470°C to 520°C, or Plating bath temperature: 450°C to less than 470°C and entry material temperature: 490°C to 520°C and immersing it in the plating bath for at least 3 seconds. The first chemical composition, in mass %, C: 0.05% to 0.20%, Mn: 1.00% to 3.00%, Si: 0.40% to 2.00%, P: 0.001% to 0.100%, S: 0.0001% to 0.0100%, and Al: 0.001% to 1.500% and the balance being Fe and impurities, The chemical composition of the plating bath is, in mass %, Al: 10% to 13%, Mg: 3% to 5%, and Si: 0.5% or less and the balance consisting of Zn and impurities.

7. A stand using the hot-dip galvanized steel sheet according to claim 1.

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