Surface coated steel

Abstract

DEPCT65 Surface-treated steel consists of: steel; and a plating layer provided on the surface of the steel. The surface coating layer consists of (in percentage by mass) Al: 5.00% to 35.00%. Percentage, Mg: 2.50% to 13.00%, Fe: 5.00% to 40.00%. Si: 0% to 2.00%, Ca: 0% to 2.00%, and the remainder is... Zn and impurities, and in the cross-section of the plating layer, the area ratio of the Fe2Al5 phase. A solution containing 5 percent or more of dissolved solid Zn is... 10 percent to 60 percent, and the area proportion of the MgZn2 phase is 10 percent to 60 percent. 90 percent -----------------------------------------------------------

Description

Plated steel

[0001] This invention relates to plated steel materials. This application claims priority based on Japanese Patent Application No. 2019-080289, filed in Japan on April 19, 2019, the contents of which are incorporated herein by reference.

[0002] In recent years, development of hot-dip Zn-Al-Mg plated steel sheets has been progressing, mainly in the building materials sector.

[0003] Patent Document 1 discloses a hot-dip Al-Zn plated steel sheet characterized by having a plating layer containing, by mass%, Al: 25 to 90% and Sn: 0.01 to 10%, and further containing a total of 0.01 to 10% of one or more elements selected from the group consisting of Mg, Ca, and Sr.

[0004] Patent Document 2 describes the [Al / Zn / Zn] on the outermost surface of the plating layer. ２ The base material is a molten Zn-Al-Mg alloy plated steel sheet in which the proportion of the Mg ternary eutectic structure is 60 area percent or more, and it contains at least one selected from Ni, Co, Fe, and Mn, with a total deposition amount of Ni, Co, and Fe of 0.05 to 5.0 mg / m². ２ The range is 0.05 to 30 mg / m², and the amount of Mn attached is 0.05 to 30 mg / m². ２ A chemical conversion treated steel sheet is disclosed, characterized in that the surface of the plating layer is covered with a precipitate layer in the range of , a phosphate coating consisting of phosphate crystals with an average particle size of 0.5 to 5.0 μm, and a chemical conversion coating in which the oxide or hydroxide of valve metal and the fluoride of valve metal coexist, the base of the phosphate crystals is embedded in the plating layer and stands upright from the plating layer, and the chemical conversion coating is an organic resin coating via an interfacial reaction layer formed at the interface between the exposed plating layer or precipitate layer and the phosphate crystals.

[0005] Patent Document 3 discloses a zinc alloy plated steel material characterized by having a zinc alloy plating layer on the surface of the steel material, the layer containing, by mass%, Mg: 1-10%, Al: 2-19%, Si: 0.01-2%, and Fe: 2-75%, with the remainder being Zn and unavoidable impurities.

[0006] However, in Patent Documents 1 to 3, the sacrificial corrosion resistance of the plated steel sheet, particularly the sacrificial corrosion resistance of the plated steel sheet when corrosion progresses under the coating film formed on the plated steel sheet (hereinafter sometimes collectively referred to as the coating film), was not sufficient. Against this background, there has been a demand for the development of a plated steel sheet having excellent sacrificial corrosion resistance (sacrificial corrosion resistance after coating film formation).

[0007] Japanese Patent Application Laid-Open No. 2015-214747, Japanese Patent No. 4579715, Japanese Patent Application Laid-Open No. 2009-120947

[0008] The present invention has been made in view of the above circumstances, and an object thereof is to provide a plated steel material having excellent sacrificial corrosion resistance.

[0009] In order to solve the above problems, the present invention employs the following configuration. (1) A plated steel material according to an aspect of the present invention includes a steel material and a plating layer provided on the surface of the steel material, and the plating layer contains, in mass %, Al: 5.00 to 35.00%, Mg: 2.50 to 13.00%, Fe: 5.00 to 40.00%, Si: 0 to 2.00%, and Ca: 0 to 2.00%, with the balance being composed of Zn and impurities. In the cross-section of the plating layer, the area fraction of the Zn solid solution Fe phase in which 5% or more of Zn is solid dissolved is 10 to 60%, and the area fraction of the MgZn phase is 10 to 90%. (2) In the plated steel material according to (1), the plating layer may contain Al: 10.00 to 30.00% in mass %. (3) In the plated steel material according to (1) or (2), the plating layer may contain Mg: 4.00 to 11.00% in mass %. (4) In the plated steel material according to any one of (1) to (3), the plating layer may contain Ca: 0.03 to 1.00% in mass %. (5) In the plated steel material according to any one of (1) to (4), in the cross-section of the plating layer, the area fraction of the Zn solid solution FeAl phase may be 20 to 60%. (6) In the plated steel material according to any one of (1) to (5), in the cross-section of the plating layer, the Zn non-solid solution FeAl phase in which the solid solution amount of Zn is less than 5% ２ Al ５ phase has an area fraction of 10 to 90%. (7) The plated steel material according to any one of (1) to (6) may further include a chemical conversion coating film provided on the plating layer. (8) In the plated steel material according to (7), the chemical conversion coating film may be a chromate film. (9) The plated steel material according to any one of (1) to (8) may further include an electrodeposited coating film provided on the chemical conversion coating film. (10) The plated steel material according to (9) may be used as an automotive panel. ２ Al ５ phase has an area fraction of 20 to 60%. (6) In the plated steel material according to any one of (1) to (5), in the cross-section of the plating layer, the Zn non-solid solution FeAl phase in which the solid solution amount of Zn is less than 5% ２ Al​​５ The area fraction of the phase may be 10% or less. (7) The plated steel material described in any one of (1) to (6) may have an area fraction of Al-Zn dendrites, mainly composed of Al phase and Zn phase, in the cross-section of the plating layer, of 5% or less. (8) The plated steel material described in any one of (1) to (7) may have a Zn / Al / MgZn area fraction in the cross-section of the plating layer. ２ The area fraction of the ternary eutectic structure may be 5% or less. (9) The plated steel sheet described in any one of (1) to (8) may have an area fraction of the FeAl phase in the cross-section of the plating layer of 10% or less. (10) The plated steel material described in any one of (1) to (9) may have an area fraction of the massive Zn phase in the cross-section of the plating layer of 10% or less. (11) The plated steel material described in any one of (1) to (10) may have an area fraction of the Mg ２ The area fraction of the Si phase may be 10% or less.

[0010] According to the above embodiment of the present invention, a plated steel material with excellent sacrificial corrosion resistance can be provided.

[0011] This is an SEM image showing the cross-sectional structure of the plated steel material according to this embodiment. This is an SEM image showing the cross-sectional structure of a plated steel sheet according to the prior art.

[0012] The following describes a plated steel material (a plated steel material according to this embodiment) and a preferred method for manufacturing the same, according to one embodiment of the present invention.

[0013] [Plated Steel Material] The plated steel material according to this embodiment comprises a steel material and a plating layer provided on the surface of the steel material, wherein the plating layer contains, by mass%, Al: 5.00 to 35.00%, Mg: 2.50 to 13.00%, Fe: 5.00 to 40.00%, Si: 0 to 2.00%, and Ca: 0 to 2.00%, with the remainder being Zn and impurities, and in the cross-section of the plating layer, Zn-solvent Fe containing 5% or more Zn solid solution ２ Al ５ The surface fraction of the phase is 10-60%, MgZn ２ The surface fraction of the phase is between 10% and 90%.

[0014] <Steel Material> The material of the steel material that serves as the base for the plated steel material is not particularly limited. General steel, Ni pre-plated steel, Al-killed steel, and some high-alloy steels can be used. The shape of the steel material is also not particularly limited. For example, the steel material is a steel sheet such as a hot-rolled steel sheet or a cold-rolled steel sheet. In this embodiment, the case in which the steel material is a steel sheet (a plated steel sheet according to this embodiment) will be described below.

[0015] <Plating layer> The plated steel material according to this embodiment has a plating layer on the surface of the steel material.

[0016] (Chemical Composition) Next, we will explain the chemical composition of the plating layer. In the following explanation of chemical composition, unless otherwise specified, "%" represents "mass%".

[0017] Al: 5.00-35.00% Al is an element necessary to include elements other than Zn in the plating layer. Originally, it is difficult to include other elements in a Zn plating layer (Zn layer), and for example, it is difficult to include Mg at a high concentration. However, by including Al in the plating layer (Zn-based plating layer), it is possible to manufacture a plating layer that contains Mg.

[0018] When the Al concentration (Al content) is less than 5.00%, it tends to be difficult to include alloying elements that impart performance to the plating layer, in addition to Mg. Also, because Al has a low density, compared to Zn, a large amount of Al phase is formed relative to the mass content. However, when the Al concentration is less than 5.00%, the majority of the plating layer tends to be the Zn phase. This leads to a significant decrease in sacrificial corrosion protection. In the plating layer, it is undesirable for the Zn phase to be the first phase. Furthermore, when the Al concentration is less than 5.00%, the plating layer contains MgZn, which has poor plastic deformability. ２ The phase tends to grow coarsely as it forms the primary crystal, significantly worsening the workability of the plating layer. Therefore, the Al concentration should be 5.00% or higher, preferably 10.00% or higher.

[0019] On the other hand, as the Al concentration increases, the proportion of the Al phase in the plating layer increases rapidly, and the Zn solid solution Fe necessary for imparting sacrificial corrosion protection... ２ Al５ Phase and MgZn ２ This is undesirable because it reduces the proportion of the phase. Therefore, the Al concentration should be 35.00% or less. Preferably, it should be 30.00% or less.

[0020] Mg: 2.50% to 13.00% Mg is an element necessary to provide sacrificial corrosion protection. When Mg is added to a Zn-based plating layer, Mg forms an intermetallic compound called MgZn. ２ This forms the sacrificial corrosion protection of the plating layer. The minimum Mg concentration required to sufficiently improve the sacrificial corrosion protection of the plating layer is 2.50%. Therefore, the Mg concentration should be 2.50% or higher. Preferably, it should be 3.00% or higher. Furthermore, from the viewpoint of resistance to red rust, it is preferable that the Mg concentration be 4.00% or higher. On the other hand, if the Mg concentration exceeds 13.00%, MgZn ２ The phase rapidly increases in volume, causing the plastic deformability of the plating layer to be lost and degrading its workability, which is undesirable. Therefore, the Mg concentration should be 13.00% or less. Preferably, it should be 11.00% or less.

[0021] Fe: 5.00–40.00% If the Fe concentration is less than 5.00%, the amount of Fe is insufficient, and the Fe that is formed... ２ Al ５ This is undesirable because it reduces the amount of phase. Also, if the Fe concentration is less than 5.00%, the area fraction of Al-Zn dendrites, which do not contribute to the improvement of sacrificial corrosion protection, may exceed 5%, which is undesirable. For this reason, the Fe concentration should be 5.00% or higher. Preferably, it should be 10.00% or higher, and more preferably 15.00% or higher. If the Fe concentration exceeds 40.00%, there is a high possibility that the desired metallic structure will not be formed in the plating layer according to this embodiment, and an increase in potential will occur due to the increase in the Fe component, which may prevent the maintenance of appropriate sacrificial corrosion protection for the steel material and may induce an increase in the corrosion rate, which is undesirable. For this reason, the Fe concentration should be 40.00% or lower. Preferably, it should be 30.00% or lower, and more preferably 25.00% or lower. Furthermore, it is preferable that the Fe / Al ratio is 0.9 to 1.2 with respect to the Al concentration. By setting the Fe / Al ratio within the above range, Fe ２ Al ５ Phase formation becomes easier. If Fe / Al is less than 0.9, Fe２ Al ５ It becomes difficult to generate a sufficient amount of [the substance], and dendrites, mainly composed of Al and Zn phases, may be generated in excess. Also, if the Fe / Al ratio is greater than 1.2, the Fe-Zn intermetallic compound phase is more likely to form, and in this case too, Fe ２ Al ５ Phase formation becomes more difficult.

[0022] Si: 0-2.00% Si is an effective element for improving adhesion between the steel material and the plating layer. Therefore, Si may be included in the plating layer. Since Si does not have to be included in the plating layer, the lower limit of Si concentration is 0%. The adhesion-improving effect of Si appears when the Si concentration in the plating layer is 0.03% or higher, so when Si is included in the plating layer, it is preferable to set the Si concentration to 0.03% or higher. On the other hand, even if the Si concentration in the plating layer exceeds 2.00%, the adhesion-improving effect of Si saturates. Therefore, even when Si is included in the plating layer, the Si concentration should be 2.00% or less. The Si concentration is preferably 1.00% or less.

[0023] Ca: 0-2.00% Ca is an effective element for improving the chemical conversion treatment properties of plated steel sheets. Therefore, Ca may be included in the plating layer for the purpose of improving the chemical conversion treatment layer of the plated steel sheet. Ca does not have a significant effect on improving sacrificial corrosion protection, so it is not necessary to include Ca in the plating layer. For this reason, the lower limit of the Ca concentration is 0%. The effect of Ca on improving sacrificial corrosion protection appears when the Ca concentration in the plating layer is 0.03% or higher, so it is preferable to have a Ca concentration of 0.03% or higher. More preferably, it is 0.05% or higher. On the other hand, even if the Ca concentration in the plating layer exceeds 2.00%, the effect of Ca on improving sacrificial corrosion protection saturates. Therefore, even when Ca is included in the plating layer, the Ca concentration should be 2.00% or less. The Ca concentration is preferably 1.00% or less.

[0024] Remainder: Zn and impurities. The remainder, excluding Al, Mg, Fe, Si, and Ca, consists of Zn and impurities. Here, impurities refer to elements that are inevitably mixed in during the plating process, and these impurities may make up about 3.00% in total. In other words, the impurity content in the plating layer may be 3.00% or less. Examples of elements that may be included as impurities and their concentrations include Sb: 0-0.50%, Pb: 0-0.50%, Cu: 0-1.00%, Sn: 0-1.00%, Ti: 0-1.00%, Sr: 0-0.50%, Ni: 0-1.00%, and Mn: 0-1.00%. If the concentration of impurity elements in the plating layer exceeds these levels, it may hinder the acquisition of desired properties and is therefore undesirable.

[0025] The chemical composition of the plating layer is measured by the following method. First, an acid solution is obtained by stripping and dissolving the plating layer with an acid containing an inhibitor that suppresses corrosion of the base metal (steel material). Next, the chemical composition of the plating layer can be obtained by measuring the obtained acid solution using ICP analysis. There are no particular restrictions on the type of acid, as long as it is an acid that can dissolve the plating layer. The chemical composition is measured as the average chemical composition.

[0026] (Organization) The plating layer of the plated steel sheet according to this embodiment is a Zn-solid-solution Fe in which 5% or more Zn is solid-solutioned in its cross-section (thickness direction cross-section) ２ Al ５ The surface fraction of the phase is 10-60%, MgZn ２ The surface fraction of the phase is between 10% and 90%.

[0027] Figure 1 is an SEM image showing the microstructure of the plated steel sheet 20 according to this embodiment. As shown in Figure 1, in the plated steel sheet 20 according to this embodiment, cross-sectional observation using SEM revealed that a plating layer 10 is formed on the surface of the steel material 5, and within the plating layer 10, Fe with Zn at a concentration of 5% or more is solid-dissolved. ２ Al ５ Phase 11 and MgZn ２ The existence of phase 12 is observed.

[0028] Figure 2 is an SEM image showing the microstructure of a conventional plated steel sheet 100. The conventional plated steel sheet 100 shown in Figure 2 is a conventional hot-dip Zn-Al-Mg plated steel sheet, in which a hot-dip Zn-Al-Mg plated layer 130 is formed on the surface of the steel material 5 by performing hot-dip Zn-Al-Mg plating on the steel material 5. As shown in Figure 2, in the hot-dip Zn-Al-Mg plated layer 130 of the conventional plated steel sheet 100, no alloying treatment is performed, so Zn / Al / MgZn ２ The ternary eutectic structure 131 and Al-Zn dendrites 133 make up the majority, and Fe ２ Al ５ In terms of, MgZn ２ No phases were observed. The microstructure of the plating layer of the plated steel sheet according to this embodiment will be described below.

[0029] Fe with 5% or more Zn dissolved in it ２ Al ５ phase (hereinafter referred to as Zn solid solution Fe ２ Al ５ Area fraction of the phase (sometimes referred to as the phase): 10-60% In the plated steel sheet according to this embodiment, as described later, an alloying process is performed after the hot-dip plating process, thereby adding Fe to the plated layer. ２ Al ５ A phase is formed, and this Fe ２ Al ５ The phase contains 5% or more Zn solid solution. The plating layer of the plated steel sheet according to this embodiment contains Zn solid solution Fe ２ Al ５ By having a phase of 10% or more, suitable sacrificial corrosion protection can be obtained. Therefore, Zn solid solution Fe in the plating layer ２ Al ５ The area fraction of the phase shall be 10% or more. Preferably it shall be 20% or more. On the other hand, Zn solid solution Fe in the plating layer ２ Al ５ When the area fraction of the phase exceeds 60%, the effect of improving sacrificial corrosion protection saturates, which is undesirable. Therefore, Zn solid solution Fe ２ Al ５ The area fraction of the phase is 60% or less, preferably 50% or less. Zn solid solution Fe ２ Al ５This phase is an important microstructure not only for sacrificial corrosion protection but also for effectively preventing liquid metal embrittlement cracking (LME) during spot welding (achieving excellent LME resistance).

[0030] Fe ２ Al ５ Because Zn is dissolved in the phase at a concentration of 5% or more, Fe ２ Al ５ This has the effect of greatly improving the sacrificial corrosion protection of the phase. Fe with a solid solution amount of Zn of less than 5% ２ Al ５ Phase (hereinafter referred to as Zn non-solid solution Fe) ２ Al ５ The phase (sometimes referred to as the "phase") does not have sufficient sacrificial corrosion protection, therefore, Zn non-solid solution Fe ２ Al ５ The area fraction of the phase is preferably small, and preferably 10% or less.

[0031] MgZn ２ Phase surface fraction: 10-90% To obtain suitable sacrificial corrosion protection, MgZn ２ The area fraction of the phase is 10% or more. Preferably it is 20% or more. On the other hand, MgZn ２ If the area fraction of the phase exceeds 90%, then Zn solid solution Fe ２ Al ５ The surface fraction of the phase becomes too low, making it difficult to obtain suitable sacrificial corrosion protection. Therefore, MgZn ２ The area fraction of the phase should be 90% or less. Preferably, it should be 80% or less.

[0032] Area fraction of the remainder: 20% or less. To obtain suitable sacrificial corrosion protection, Zn solid solution Fe ２ Al ５ Phase and MgZn ２ The area fraction of the remaining tissue other than the phase shall be 20% or less. Preferably, it shall be 15% or less, more preferably 10% or less. The tissue included in the remainder shall be the above-mentioned Zn non-solid-solution Fe ２ Al ５ Phase, Al-Zn dendrites, Zn / Al / MgZn (described later) ２ Ternary eutectic structure, FeAl phase, massive Zn phase, Mg ２ Examples include the Si phase. The structures contained in the remainder are described below. Zn non-solid solution Fe２ Al ５ The Al phase is as described above, so the description here is omitted.

[0033] Area fraction of dendrites (Al-Zn dendrites) mainly composed of Al phase and Zn phase: 5% or lessWhen forming the plating layer, in the process of cooling from the bath temperature after the molten plating process described later, first, primary Al crystals (α-(Zn,Al) phase crystallized as primary crystals) crystallize and grow in a dendritic shape (hereinafter referred to as Al-Zn dendrites). Then, by heating in the temperature range of 440°C to 480°C and performing an alloying treatment, most of the Al-Zn dendrites are replaced by another structure, but a part remains even after the alloying treatment. Since Al-Zn dendrites do not have a favorable influence on sacrificial corrosion resistance and LME resistance, it is preferable that their area fraction is lower. Therefore, in the plating layer of the plated steel sheet according to the present embodiment, it is preferable that the area fraction of Al-Zn dendrites is 5% or less. More preferably, it is 3% or less. "Mainly" means that the Al phase and the Zn phase in the dendrites contain about 91% or more, and as the remainder other than the Al phase and the Zn phase, 5% or less of Fe, 3% or less of Mg, and 1% or less of steel component elements (Ni, Mn) may be contained.

[0034] Zn / Al / MgZn ２ Area fraction of ternary eutectic structure: 5% or less Zn / Al / MgZn ２ The ternary eutectic structure is a layered structure of Zn layer, Al layer, and MgZn layer composed of the Zn phase, Al phase, and MgZn phase that finally solidify outside the primary Al crystal part by the Zn-Al-Mg eutectic reaction. Zn / Al / MgZn ２ phase. ２ Although the ternary eutectic structure also has an effect of improving sacrificial corrosion resistance, its improvement effect is inferior to that of the Fe ２ Al ２ phase and the MgZn ５ phase. Therefore, it is preferable that the area fraction of the Zn / Al / MgZn ２ ternary eutectic structure is lower. Therefore, in the plating layer according to the present embodiment, it is preferable that the area fraction of the Zn / Al / MgZn ２ ternary eutectic structure is 5% or less. More preferably, it is 3% or less. ２

[0035] FeAl phase: 10% or less. Since the FeAl phase has poor sacrificial corrosion resistance, it is preferably as little as possible. Therefore, the area fraction of the FeAl phase is preferably 10% or less. More preferably, it is 5% or less.

[0036] Massive Zn phase: 10% or less. The massive Zn phase is a structure that may form when the Mg content in the plating layer is low. When the massive Zn phase forms, the coating film swelling width tends to increase, so its area fraction is preferably low, and 10% or less is preferred. The massive Zn phase is a phase separate from the Zn phase contained in the Zn / Al / MgZn ２ ternary eutectic structure. The massive Zn phase has a dendritic shape and may be observed as circular in the cross-sectional structure.

[0037] Mg ２ Si phase and other intermetallic compound phases: 10% or less each. Mg ２ The Si phase is a phase formed when the Si addition amount is excessive, promotes the corrosion of the surroundings, and may result in a decrease in the rust resistance. In order to ensure sufficient rust resistance, its area fraction needs to be limited to 10% or less. Also, since other intermetallic compound phases do not favorably affect the chemical conversion treatment property, the area fraction is preferably 10% or less, more preferably 5% or less. Examples of other intermetallic compound phases include CaZn １１ phase, Al ２ CaSi ２ phase, Al ２ CaZn ２ phase, etc.

[0038] In this embodiment, the "area fraction" refers to the arithmetic mean value of the area fractions of the desired structure in the cross-section of the plating layer when calculated for five or more different samples randomly selected, unless otherwise specified. Substantially, this area fraction represents the volume fraction in the plating layer.

[0039] <Method for measuring area fraction> The area fraction of each structure in the plating layer is determined by the following method. First, the plated steel sheet to be measured is cut to 25 (C) × 15 (L) mm, embedded in resin, and polished. Then, a cross-sectional SEM image of the plating layer and an elemental distribution image by EDS are obtained. The constituent structure of the plating layer, i.e., Zn solid solution Fe ２ Al ５ Phase, MgZn ２ phase, Zn non-solid solution Fe ２ Al ５ Phase, (Al-Zn) dendrite, Zn / Al / MgZn ２ Ternary eutectic structure, FeAl phase, massive Zn phase, Mg ２ The area fractions of the Si phase and other intermetallic compound phases are determined by taking cross-sectional EDS mapping images of the plating layer from five different samples, one field of view from each sample (magnification 1500x: 60 μm × 45 μm / field of view), and measuring the area fraction of each structure through image analysis.

[0040] Fe ２ Al ５ The amount of Zn solid solution in the phase was determined using SEM-EDS and EPMA, and Fe ２ Al ５ It can be measured by measuring the concentration of the constituent elements that make up the phase.

[0041] <Characteristics> The plated steel sheet according to this embodiment has excellent sacrificial corrosion protection due to the steel material and plating layer having the above-described characteristics. Furthermore, the plated steel sheet according to this embodiment has excellent LME resistance due to the steel material and plating layer having the above-described characteristics.

[0042] [Method for Manufacturing Plated Steel Sheets] Next, a method for manufacturing plated steel sheets according to this embodiment will be described. The following description will focus on the case where the steel material is a steel sheet. The method for manufacturing plated steel sheets according to this embodiment includes a hot-dip plating step of immersing a base steel sheet in a plating bath containing at least Al, Mg, and Zn to perform hot-dip plating; an alloying step of heating the hot-dip plated base steel sheet to 480°C to 580°C for 1 to 15 seconds; and a cooling step of cooling the plated steel material to room temperature at an average cooling rate of 15°C / second or more.

[0043] <Hot-dip plating process> In the hot-dip plating process, the base steel plate is immersed in a plating bath containing at least Al, Mg, and Zn to perform hot-dip plating.

[0044] In the hot-dip galvanizing process, a plating bath is applied to the surface of the base steel sheet, and then the base steel sheet is removed from the plating bath to solidify the molten metal attached to the surface of the base steel sheet. This is a method known as hot-dip galvanizing.

[0045] (Plating bath) The composition of the plating bath only needs to contain at least Al, Mg, and Zn, and the raw materials should be mixed and dissolved to achieve the composition of the plating layer described above.

[0046] The temperature of the plating bath is preferably in the range of over 380°C and up to 600°C, and may also be in the range of 400 to 600°C.

[0047] Before immersing the base steel sheet in the plating bath, it is preferable to reduce the surface of the base steel sheet by heating it in a reducing atmosphere. For example, heat treatment is performed at 600°C or higher, preferably 750°C or higher, for 30 seconds or more in a mixed atmosphere of nitrogen and hydrogen. After the reduction treatment is complete, the base steel sheet is cooled to the temperature of the plating bath and then immersed in the plating bath. The immersion time can be, for example, 1 second or more. When removing the base steel sheet from the plating bath, the amount of plating deposited is adjusted by gas wiping. The amount of plating deposited is 10 to 300 g / m². ２ A range of 20 to 250 g / m² is preferred. ２ It can also be within that range.

[0048] <Alloying Process> The method for manufacturing plated steel sheets according to this embodiment includes an alloying process in which a base steel sheet that has been hot-dip plated is heated to 480°C to 580°C for 1 to 15 seconds.

[0049] In the alloying process, if the heating temperature (hereinafter referred to as the alloying temperature) is below 480°C, the alloying process is slow and the desired structure is not formed in the plating layer, which is undesirable. Therefore, the alloying temperature should be 480°C or higher. On the other hand, if the alloying temperature exceeds 580°C, the alloying process proceeds excessively in a short time, making it difficult to control the alloying process effectively, which is undesirable. Furthermore, if the alloying temperature exceeds 580°C, Fe forms an Fe-Zn intermetallic compound with Zn, which reduces resistance to red rust, which is undesirable. In addition, the formation of the Fe-Zn intermetallic compound reduces the amount of Zn dissolved in the Fe-Al phase, which is related to sacrificial corrosion protection, and the area fraction of Zn solid-solution Fe is reduced. ２ Al ５ This is undesirable because no phase is formed. Therefore, the alloying temperature should be 580°C or lower.

[0050] If the heating time in the alloying process (hereinafter referred to as the alloying time) is less than 1 second, the alloying process will not progress sufficiently when the molten plated base steel sheet is heated to a temperature range of 480°C to 580°C, which is undesirable. Therefore, the alloying time should be 1 second or more. On the other hand, if the alloying time exceeds 15 seconds, the alloying process will progress excessively, which is undesirable. Therefore, the alloying time should be 15 seconds or less.

[0051] In the alloying process, the heating method is not particularly limited, but examples include induction heating.

[0052] <Cooling Process> The method for manufacturing a plated steel sheet according to this embodiment preferably includes a step of removing the plated steel sheet from the plating bath, performing an alloying heat treatment, and then cooling it to 100°C or less (for example, around room temperature) at an average cooling rate of 15°C / second or more.

[0053] As described above, the plated steel sheet according to this embodiment can be manufactured. The plated steel sheet according to this embodiment has excellent sacrificial corrosion protection. Furthermore, the plated steel sheet according to this embodiment has excellent LME resistance.

[0054] "Example 1" <Base Steel Sheet> A cold-rolled steel sheet (0.2%C-1.5%Si-2.6%Mn) with a thickness of 1.6 mm was used as the base steel sheet to be plated.

[0055]

[0056]

[0057] <Hot-dip galvanizing process> After cutting the base steel plate to 100 mm x 200 mm, galvanizing was performed using a batch-type hot-dip galvanizing test apparatus. The plate temperature was measured using a thermocouple spot-welded to the center of the base steel plate. Before immersion in the galvanizing bath, N2 was used in a furnace with an oxygen concentration of 20 ppm or less. ２ -5%H ２ The surface of the base steel plate was heat-reduction treated at 860°C in a gas atmosphere with a dew point of 0°C. After that, N ２ After air cooling with gas to bring the immersion plate temperature to the bath temperature + 20°C, it was immersed for approximately 3 seconds in the plating bath at the bath temperatures shown in Tables 1A and 1B. After immersion in the plating bath, it was pulled up at a pulling speed of 100 to 500 mm / second. During withdrawal, N ２ The amount of plating deposited was controlled using a wiping gas.

[0058] <Alloying Process> After controlling the amount of plating adhesion with a wiping gas, the plated steel sheet was subjected to an alloying process according to the alloying temperature and alloying time conditions shown in Tables 1A and 1B. An induction heating device was used in the alloying process.

[0059] <Cooling Process> The plated steel sheet was cooled from the alloying temperature to room temperature by cooling it under the conditions shown in Tables 1A and 1B after the alloying process.

[0060] The composition of the plating layer was investigated using the method described above. The results are shown in Tables 1A and 1B.

[0061] <Microstructure Observation> To investigate the microstructure of the plated layer, the prepared sample was cut into 25 (C) × 15 (L) mm sections, embedded in resin, and polished. Cross-sectional SEM images and elemental distribution images by BDS were then obtained for the plated layer. The constituent structure of the plated layer, i.e., Zn solid-solution Fe ２ Al ５ Phase, MgZn ２ phase, Zn non-solid solution Fe ２ Al ５ Phase, (Al-Zn) dendrite, Zn / Al / MgZn ２ Ternary eutectic structure, FeAl phase, massive Zn phase, Mg ２The area fractions of the Si phase and other intermetallic compound phases were calculated by analyzing cross-sectional EDS mapping images of the plating layer, with one field of view taken from each of five different samples (magnification 1500x), for a total of five fields of view. The area fractions of each structure in each example and comparative example are shown in Tables 2A and 2B.

[0062] <Sacrificial Corrosion Protection (Base Metal Corrosion Protection)> The sacrificial corrosion protection (base metal corrosion protection) was evaluated for each example and comparative example using the following method. The plated steel sheets for each example and comparative example manufactured using the method described above were cut to a size of 50 mm x 100 mm and subjected to Zn phosphoric acid treatment (SD5350 system: standard manufactured by Nippon Paint Industrial Coating Co., Ltd.). Furthermore, electrodeposition coating with a film thickness of 20 μm (PN110 Powernix® Gray: standard manufactured by Nippon Paint Industrial Coating Co., Ltd.) was applied to the Zn phosphoric acid film under conditions of a baking temperature of 150°C and a baking time of 20 minutes. Next, cross-cut scratches (40 x √2 x 2) were made to reach the base metal of the manufactured coated plated steel sheet. The coated plated steel sheet with cross-cut scratches was subjected to a combined cycle corrosion test in accordance with JASO (M609-91). Sacrificial corrosion protection was evaluated by measuring the erosion depth of the base metal after testing using a micrometer and calculating the average value. At 240 cycles of the aforementioned JASO (M609-91), if the maximum base metal erosion depth in the 5 μm region near the cross-cut scratches was less than 0.1 mm, it was classified as "AA"; if it was 0.1 mm or more but less than 0.4 mm, it was classified as "A"; if it was 0.4 mm or more but less than 0.6 mm, it was classified as "B"; and if it was greater than 0.6 mm, it was classified as "C". For red rust, if no red rust was produced in the 5 μm region near the cross-cut scratches at 240 JASO cycles, it was classified as "AA"; if no red rust was produced at 180 cycles, it was classified as "A"; if red rust was produced between 120 and 60 cycles, it was classified as "B"; and if red rust was produced before 60 cycles, it was classified as "C".

[0063]

[0064]

[0065] In the examples prepared with a predetermined plating bath composition and appropriate cooling conditions, the predetermined structure was obtained, indicating that it possessed suitable sacrificial corrosion protection. On the other hand, at levels where proper alloying was not performed (Comparative Examples 7, 8, 14-19, 27) and levels where Al and Mg were insufficient (Comparative Examples 1, 2), a sufficient amount of Fe was obtained. ２ Al ５ The phase could not be formed, resulting in inferior performance. In particular, in Comparative Example 19, where the alloying temperature was high, the Fe-Zn phase was formed as other intermetallic compounds, and Zn solid solution Fe ２ Al ５ No phase was formed. As a result, the sacrificial corrosion protection was particularly low. Also, at levels where Ca and Si were excessively present (Comparative Examples 25 and 26), Mg reduced corrosion resistance in the plating layer. ２ Si, CaZn １１ Intermetallic compound phases such as the above were formed at a concentration of 10% or more, resulting in inferior corrosion resistance. At levels where the Mg content was excessive (Comparative Example 18), Zn solid solution Fe ２ Al ５ The phase is less than 10%, and MgZn ２ Because the phase was over 90%, the corrosion resistance was inferior. At a level where the Al content was excessive (Comparative Example 32), Zn solid solution Fe ２ Al ５ Over 60% of the phase was formed, resulting in inferior corrosion resistance.

[0066] "Example 2" In Example 2, the LME resistance was investigated for several of the examples used in Example 1. Specifically, the composition, structure, and manufacturing conditions of the plated steel sheets used in Example 2 are described in Tables 1A and 1B.

[0067] <LME Resistance> Several plated steel sheets used in Example 1 were cut to a size of 200 x 20 mm and subjected to a hot tensile test at a tensile speed of 5 mm / min and a chuck distance of 112.5 mm. The stress-strain curve at 800°C was measured. The amount of strain until the maximum stress was reached in the obtained stress-strain curve was measured. This amount of strain was compared to an unplated steel sheet sample, and a value of 80% or more was classified as "AA", and a value of 60% or less was classified as "A". The evaluation results of the LME resistance for each example are shown in Table 3. The measurement of the area fraction of each microstructure is described in Tables 2A and 2B, and is therefore not shown in Table 3.

[0068]

[0069] As shown in Table 3, LME resistance was also favorable in each embodiment. In particular, Zn solid solution Fe ２ Al ５ In cases where the area fraction of the phase was large, LME resistance was superior.

[0070] 20 Plated steel material (plated steel sheet) 5 Steel material 10 Plating layer 11 Fe with 5% or more Zn solid solution ２ Al ５ phase (Zn solid solution Fe ２ Al ５ Phase) 12 MgZn ２ Phase 100 Plated steel sheet (conventional steel) 130 Hot-dip Zn-Al-Mg plating layer 131 Zn / Al / MgZn ２ Ternary eutectic structure 133 Al-Zn dendrite

Claims

DEPCT651. Surface-treated steel comprising: steel; and a plating layer provided on the surface of the steel; where the plating layer consists of (in percentage by mass) Al: 5.00% to 35.00%, Mg: 2.50% to 13.00%, Fe: 5.00% to 40.00%, Si: 0% to 2.00%, Ca: 0% to 2.00%, and the remainder being Zn and impurities; and in the cross-section of the plating layer, the area proportion of the Fe2Al5 phase containing a solid solution of Zn with 5% or more of Zn dissolved in the ionic state.

1. Surface-treated steel according to Requisition 1 where the plating layer consists of (in percentage by mass) Al: 10.00% to 30.00%.

3. Surface-treated steel according to Requisition 1 or 2 where the plating layer consists of (in percentage by mass) Mg: 4.00% to 11.00%.

4. Surface-treated steel according to any one of Requisitions 1 to 3 where the plating layer consists of (in percentage by mass) Ca: 0.03% to 1.

5. Surface-treated steel according to any one of the claims 1 through 4 in which the cross-section of the plating layer consists of a surface area of ​​the Fe2Al5 phase with a solution of solid Zn of 20 percent to 60 percent.

6. Surface-treated steel according to any one of the claims 1 through 5 in which the cross-section of the plating layer consists of a surface area of ​​the Fe2Al5 phase without a solution of solid Zn of less than 5 percent of 10 percent or less.

7. Surface-treated steel according to any one of the claims 1 through 5 in which the cross-section of the plating layer consists of a surface area of ​​the Fe2Al5 phase without a solution of solid Zn of less than 5 percent of 10 percent or less.

7. Surface-treated steel according to any one of Requisitions 1 through 7 where, in the cross-section of the plating layer, the proportion of the area of ​​the Al-Zn dendrite primarily composed of Al and Zn phases is 5 percent or less.

8. Surface-treated steel according to any one of Requisitions 1 through 7 where, in the cross-section of the plating layer, the proportion of the area of ​​the ternary eutectic Zn / Al / MgZn2 structure is 5 percent or less.

9. Surface-treated steel according to any one of Requisitions 1 through 8 where, in the cross-section of the plating layer, the proportion of the area of ​​the FeAl phase is 10 percent or less. 10.

11. Surface-treated steel under any of the claims 1 through 9 where the cross-section of the plating layer contains a massive Zn phase area of ​​10 percent or less.

22. Surface-treated steel under any of the claims 1 through 10 where the cross-section of the plating layer contains a Mg2Si phase area of ​​10 percent or less.