JP4374281B2 - Hot-dip galvanized steel with excellent corrosion resistance - Google Patents

Description

  The present invention relates to a plated steel material, and more particularly to a plated steel material that has excellent processed portion corrosion resistance and can be applied as a steel sheet for various uses such as home appliances, automobiles, and building materials.

  Zinc-based plated steel sheets are the most used as plated steel materials with good corrosion resistance. These plated steel sheets are used in various manufacturing industries such as automobiles, home appliances, and building materials.

  In particular, the use of Al-added plating has increased in recent years due to its high corrosion resistance.

  In order to improve the corrosion resistance of such a galvanized steel sheet, the present inventors have proposed a hot-dip Zn—Al—Mg—Si plated steel sheet (see, for example, Patent Document 1).

  Moreover, for the purpose of improving the smoothness of the surface, the present inventors have made a plated steel sheet to which a high melting point intermetallic compound is added (for example, see Patent Document 2), and a plated steel sheet to which an Al-based intermetallic compound is added ( For example, Patent Document 3) has been proposed.

Japanese Patent No. 3179446 JP 2003-293108 A JP 2003-328100 A

  However, in the above-mentioned and other plated steel sheets disclosed so far, it cannot be said that the corrosion resistance of the processed part is sufficiently ensured.

In the zinc-based plated steel sheet to which Mg is added, the MgZn ₂ phase is crystallized during plating. Since this MgZn ₂ phase is hard and brittle, when severe processing such as T-bending is performed on a zinc-based plated steel sheet to which Mg has been added, cracks occur in the plating, and corrosion resistance deterioration after processing is likely to occur due to this. Has the problem.

The present invention has been made in view of the above problems, and aims to improve the processing portion corrosion resistance of a galvanized steel material containing MgZn ₂ phase in the plating.

The present inventors have made intensive studies on the development of the plated steel sheet processed portion corrosion resistance was excellent, in the matrix of [Al / Zn / MgZn ₂ ternary eutectic structure] and [Mg ₂ Si phase] [ The present inventors have found a new finding that the corrosion resistance of a processed part is improved by having a plating layer containing a Zn-Mg intermetallic compound on the surface in the [Al phase] of the plating layer in which the [Al phase] is mixed. It has come to be completed.

  That is, the gist of the present invention is as follows.

(1) (1) Al: 4-10 mass%, Mg: 2-10 mass%, and the plating layer of the plating steel material which has Zn alloy plating layer which consists of Zn and an unavoidable impurity on the surface [Al / Zn / MgZn ₂ ternary eutectic structure] has a metal structure containing [Al phase], and one of the lattice directions constituting the lattice plane of the Bravay lattice in this [Al phase] lattice spacing than 2.57Å 3.15Å or less, the other surface interval containing intermetallic compound and the Zn-Mg intermetallic compound having a grating surface is less than 4.46Å than 3.64A, further [Al phase A hot-dip galvanized steel material having excellent corrosion resistance in the processed portion , wherein the dendrite is an equiaxed crystal grown in the [110] direction .

(2) A plating layer of a plated steel material containing Al: 4 to 22% by mass, Mg: 2 to 10% by mass, Si: 2% by mass or less and the balance being Zn alloy plating layer composed of Zn and inevitable impurities. The [Al / Zn / MgZn ₂ ternary eutectic structure] has a metallic structure in which [Mg ₂ Si phase] and [Al phase] are mixed, and the [Al phase] has a Brabey lattice An intermetallic compound having a lattice plane in which one plane spacing in the lattice direction constituting the lattice plane is 2.57 mm to 3.15 mm and the other plane spacing is 3.64 mm to 4.46 mm, and a Zn-Mg system A hot-dip galvanized steel material excellent in corrosion resistance of a processed part, characterized in that it contains an intermetallic compound and the [Al phase] dendrite is an equiaxed crystal grown in the [110] direction .

(3) A hot-dip galvanized steel material excellent in corrosion resistance of a processed part, wherein the Zn-Mg intermetallic compound according to (1) or (2) is either MgZn _{2 or} Mg ₂ Zn ₁₁ .

( 4 ) The processed part, wherein the crystal system of the intermetallic compound according to (1) or (2) is any one of cubic, tetragonal, orthorhombic, monoclinic and hexagonal Hot-dip plated steel with excellent corrosion resistance.

(5) above (1), (2), is either the intermetallic compound according to any one of _{TiAl 3, ZrAl 3, HfAl 3} , SrAl 4, CaAl 4, NiAl 3, TiB 2 (4) A hot-dip galvanized steel material with excellent corrosion resistance in the machined part.

According to the present invention, in a zinc-based plated steel material in which an MgZn ₂ phase is crystallized during plating, it is possible to produce a hot-dip plated steel material having excellent processed portion corrosion resistance, and an extremely excellent industrial effect can be achieved.

  The present invention is described in detail below.

The hot-dip galvanized steel material of the present invention contains Al: 4 to 10% by mass, Mg: 2 to 10% by mass, the balance being Zn and inevitable impurities, or Al: 4 to 22% by mass, Mg: 2 10% by mass, Si: 2% by mass or less, and the plated layer of the plated steel sheet having any of the plated layers consisting of Zn and inevitable impurities is a base of [Al / Zn / MgZn ₂ ternary eutectic structure] [Al phase] is contained therein, and in some cases, [Mg ₂ Si phase] has a mixed metal structure, and [Al phase] contains a Zn—Mg-based intermetallic compound. It is a plated steel material.

  In the Zn-Al-Mg-based plating layer, the reason for limiting the Al content to 4 to 10% by mass is that if the Al content exceeds 10% by mass, a decrease in plating adhesion is observed, so Si is added. This is because the content of Al in the plating layer that is not necessary needs to be 10% by mass or less. In addition, when the amount is less than 4% by mass, the Al phase does not crystallize as the primary crystal, and thus the effect of improving the corrosion resistance of the processed part due to the Zn—Mg intermetallic compound is not observed.

  Therefore, in the hot-dip plated steel material according to the present invention, it is essential to add Si to the plating layer in order to ensure plating adhesion, particularly when the Al concentration exceeds 10% by mass. is there.

  On the other hand, in the Zn-Al-Mg-Si-based plating layer, the reason why the Al content is limited to 4 to 22% by mass is that when less than 4% by mass, the Al phase does not crystallize as a primary crystal. This is because the effect of improving the corrosion resistance of the processed part due to the intermetallic compound is not seen, and when it exceeds 22% by mass, the effect of improving the corrosion resistance is saturated.

  The reason for limiting the Si content to 2% by mass or less (excluding 0% by mass) is that Si has an effect of improving adhesion, but exceeds 2% by mass in order to dissolve in the plating bath. This is because the bath temperature needs to be considerably high and cannot be established industrially. Desirably, it is 0.00001-1 mass%, More desirably, it is 0.0001-0.5 mass%.

The addition of Si is essential for plating layers with an Al content of more than 10% by mass. However, even in plating layers with an Al content of 10% or less, the effect of improving plating adhesion is great, so the parts are severely processed. It is effective to add Si when high plating adhesion is required. Moreover, [Mg ₂ Si phase] crystallizes in the solidified structure of the plating layer by addition of Si. Since this [Mg ₂ Si phase] is effective in improving corrosion resistance, it is more desirable to increase the amount of Si added and to produce a metal structure in which [Mg ₂ Si phase] is mixed in the solidified structure of the plating layer.

  The reason why the content of Mg is limited to 2 to 10% by mass is that if it is less than 2% by mass, the Zn-Mg intermetallic compound cannot be precipitated in the Al phase, and the effect of improving the corrosion resistance of the processed part is insufficient. This is because if the amount exceeds 10% by mass, the plating layer becomes brittle and the adhesiveness decreases. Desirably, it is 2-5 mass%.

This plating layer has at least one of [Zn phase], [Al phase], [MgZn ₂ phase], and [Mg ₂ Si phase] in the [Al / Zn / MgZn ₂ ternary eutectic structure] substrate. A metallographic structure can be formed.

Here, [Al / Zn / MgZn ₂ ternary eutectic structure] is an ternary eutectic structure of an Al phase, a Zn phase, and an intermetallic compound MgZn ₂ phase. The formed Al phase corresponds to, for example, an "Al" phase "(Al solid solution that dissolves the Zn phase in a solid solution and contains a small amount of Mg) in an Al-Zn-Mg ternary equilibrium diagram. Is. The Al ″ phase at high temperature usually appears separated into a fine Al phase and a fine Zn phase at room temperature. The Zn phase in the ternary eutectic structure dissolves a small amount of Al, and in some cases Is a Zn solid solution in which a small amount of Mg is dissolved, and the MgZn ₂ phase in the ternary eutectic structure is a metal present in the vicinity of Zn: about 84% by mass in the Zn-Mg binary equilibrium diagram. As far as the phase diagram shows, Si and other additive elements are not dissolved in each phase, or even if they are dissolved, the amount is considered to be very small, but the amount is normal analysis. In this specification, the ternary eutectic structure consisting of these three phases is expressed as [Al / Zn / MgZn ₂ ternary eutectic structure].

  In addition, the [Al phase] is a phase that looks like an island with a clear boundary in the ternary eutectic structure, which is, for example, at a high temperature in an Al—Zn—Mg ternary equilibrium diagram. "Al" phase "(Al solid solution in which Zn phase is dissolved, and contains a small amount of Mg). The Al ″ phase at this high temperature differs in the amount of Zn and Mg dissolved depending on the Al and Mg concentrations in the plating bath. The Al ″ phase at this high temperature is usually fine Al phase and fine Zn at room temperature. Although it is separated into phases, it can be seen that the island-like shape seen at room temperature retains the shape of the Al ″ phase at high temperature. As far as the phase diagram shows, this phase contains Si and other additive elements. Although it is considered that it is not solid solution or is in very small amount even if it is in solid solution, it cannot be clearly distinguished by ordinary analysis. In this specification, the phase holding the structure is referred to as [Al phase], which can be clearly distinguished from the Al phase forming the ternary eutectic structure by microscopic observation.

  In addition, the [Zn phase] is a phase that looks like an island with a clear boundary in the ternary eutectic structure, and actually contains a small amount of Al and a small amount of Mg as a solid solution. There is also. As far as the phase diagram is concerned, it is considered that Si and other additive elements are not dissolved in this phase, or even if they are dissolved. This [Zn phase] can be clearly distinguished from the Zn phase forming the ternary eutectic structure by microscopic observation. The plating layer of the present invention may contain [Zn phase] depending on the production conditions, but since the experiment hardly showed any influence on the corrosion resistance improvement of the processed part, the plating layer contained [Zn phase]. There is no particular problem.

[MgZn ₂ phase] is a phase that looks like an island with a clear boundary in the ternary eutectic structure, and a small amount of Al may actually be dissolved. As far as the phase diagram is concerned, it is considered that Si and other additive elements are not dissolved in this phase, or even if they are dissolved. This [MgZn ₂ phase] can be clearly distinguished from the MgZn ₂ phase forming the ternary eutectic structure by microscopic observation. The plating layer of the present invention may not contain [MgZn ₂ phase] depending on the production conditions, but is contained in the plating layer under most production conditions.

The [Mg ₂ Si phase] is a phase that looks like an island with a clear boundary in the solidified structure of the plating layer to which Si is added. As far as the phase diagram is concerned, it is considered that Zn, Al, and other additive elements are not dissolved, or even if they are dissolved. This [Mg ₂ Si phase] can be clearly distinguished by microscopic observation during plating.

The hot-dip plated steel of the present invention was subjected to severe processing such as T-bending because MgZn ₂ phase crystallized during plating and MgZn _{2 in} the ternary eutectic structure of Al / Zn / MgZn ₂ are hard and brittle. In this case, cracks are generated in the plating, and corrosion resistance deterioration after processing is likely to occur due to this.

  In order to improve the post-processing corrosion resistance, it is effective to deposit a Zn—Mg intermetallic compound in the [Al phase].

In the plated layer that has not been processed, the [Al / Zn / MgZn ₂ ternary eutectic structure] corrodes preferentially, and the corrosion product is stable due to the effect of MgZn _{2 in} the ternary eutectic structure. Basic zinc chloride, basic zinc aluminum chloride, basic zinc aluminum carbonate and the like. These stable corrosion products are considered to suppress the progress of subsequent corrosion of the plating layer by acting as a strong protective film. On the other hand, when a crack is generated by processing, the Al phase is exposed, and the corrosion of the Al phase proceeds simultaneously with the corrosion of [Al / Zn / MgZn ₂ ternary eutectic structure]. The Al phase is separated into a fine Al phase and a fine Zn phase at room temperature. Since the fine Zn phase in the Al phase does not become a stable corrosion product, the corrosion resistance after processing is reduced to the corrosion resistance before processing. It is thought that it is inferior compared.

  Accordingly, the reason why the post-processing corrosion resistance is improved by precipitating the Zn—Mg intermetallic compound in the Al phase is that the Zn—Mg intermetallic compound is precipitated in the Al phase, so that the fineness in the Al phase is reduced. When the Zn phase corrodes, the Zn-Mg intermetallic compound converts the corrosion products into stable basic zinc chloride, basic zinc aluminum chloride, basic zinc aluminum carbonate, etc. This is considered to be for suppressing the progress.

  Moreover, it is thought that it is contributing to stabilizing a corrosion product that Mg-concentrated so high concentration that it becomes possible to precipitate a Zn-Mg type intermetallic compound in an Al layer.

  In order to precipitate a Zn—Mg intermetallic compound in the Al phase, it is necessary to make Mg super-saturated in the Al phase that crystallizes at a high temperature. Specifically, it is effective to use a plating bath with a high Mg concentration and to bring the Al phase closer to an equiaxed crystal from a dendrite structure having a large secondary arm in order to suppress a decrease in Mg concentration in the Al phase. . As a method of bringing the solidified structure closer to the equiaxed crystal, a method of increasing the cooling rate and increasing the degree of systematic supercooling is general, but since there is a limit to the cooling rate that can be industrially produced, Al It is effective to add a substance that becomes a crystallization nucleus to reduce the nucleation energy of the Al phase.

  Specifically, a metal having a lattice plane in which the lattice spacing of one of the lattice directions constituting the lattice plane of the Bravais lattice is 2.57 mm to 3.15 mm and the other surface spacing is 3.64 mm to 4.46 mm. An intermetallic compound is added to the plating layer. Thereby, it is considered that the reason why the crystal of the Al phase becomes a fine and uniform equiaxed crystal is that this lattice plane has good consistency with the {110} plane of Al. Since Al has a crystal structure of FCC, the {110} plane is most likely to grow. By adding an intermetallic compound having a lattice plane having good consistency with the Al {110} plane, it functions as a nucleation site for the Al {110} plane which is easy to grow, thereby reducing the nucleation energy of the Al phase. Therefore, it is considered that the solidified structure can be brought closer to an equiaxed crystal.

  The reason for limiting the distance between one surface in the lattice direction constituting the lattice surface of the Bravais lattice to 2.57 mm or more and 3.15 mm or less is that it matches the Al {110} plane when less than 2.57 mm or more than 3.15 mm. This is because the corrosion resistance of the processed part is deteriorated and the corrosion resistance of the processed part is lowered, and the reason why the other surface interval is limited to 3.64 mm or more and 4.46 mm or less is less than 3.64 mm or exceeds 4.46 mm. This is because the compatibility with the {110} plane is deteriorated and the corrosion resistance of the processed portion is lowered.

  In addition, since the crystal system of Al is cubic, the crystal system of the intermetallic compound may be any one of cubic, tetragonal, orthorhombic, monoclinic, and hexagonal with a right angle to the axis angle. desirable.

  The intermetallic compound is effective when added in a small amount, and when the added amount increases, appearance defects such as a rough appearance after plating occur. Therefore, the upper limit is desirably 1% by mass.

  As a result of the inventors investigating a large number of Al phases in plating, intermetallic compounds having a size of several μm were observed from the center of most Al phase dendrites. Further, when the crystal orientations of the intermetallic compound and the Al phase were identified by using the EBSP method, one interplanar spacing in the lattice direction of the intermetallic compound was 2.57 mm to 3.15 mm, and the other interplanar spacing was 3.64 mm to 4 It was confirmed that the lattice plane of .46 cm or less and the Al phase {110} plane were parallel, and the Al phase dendrite grew in the [110] direction.

  As an example of the Zn-Mg intermetallic compound existing in the Al phase, between the Zn-Mg based metals present in the Al phase in the Al-Zn-Mg-Si based plating to which the Al-Ti intermetallic compound is added. The compounds are shown in FIGS.

(A) in the upper part of FIG. 1 is a micrograph (magnification 5000 times) of the plating layer of the plated steel material in the present invention, and the lower part (b) illustrates the distribution state of each structure in the photograph. . 1, [Al phase] 1, have a [Al / Zn / MgZn ₂ ternary eutectic structure] 2, and [containing Zn-Mg intermetallic compound phase] 3 is present tissue, As can be seen from this figure, the Al phase can be clearly identified by the micrograph of the plated layer of the plated steel material in the present invention.

Similarly, FIG. 2 shows a transmission electron micrograph (magnification 28,000 times) of an Al phase containing a Zn—Mg intermetallic compound. The Al phase 1 precipitates a fine Al phase and a fine Zn phase 4 at room temperature. As a result of confirmation by EDX analysis with a transmission electron microscope, the Al phase and Zn phase coexist in the portion where Mg is supersaturated. It was found that precipitation 5 and MgZn ₂ phase 6 were precipitated. In addition, (Al _0.8 Si _0.2 ) ₃ intermetallic compound 7 was present.

  As the base steel material of the present invention, not only a steel plate but also various steel materials such as a wire, a shape steel, a steel bar, and a steel pipe can be used. As the steel sheet, both hot-rolled steel sheets and cold-rolled steel sheets can be used, and the steel grades are extremely low carbon steel sheets to which Al killed steel, Ti, Nb, etc. are added, and high strength elements such as P, Si, Mn, etc. added to these. Various materials such as strength steel and stainless steel can be applied. There are no particular limitations on the method for producing the product of the present invention, and various methods such as continuous plating of steel plates and dip plating of steel materials and wires can be applied. Even when Ni pre-plating is applied as the lower layer, a conventional pre-plating method may be applied.

There are no particular restrictions on the amount of plating deposited, but it is preferably 10 g / m ² or more from the viewpoint of corrosion resistance and 350 g / m ² or less from the viewpoint of workability.

  Hereinafter, the present invention will be described specifically by way of examples.

First, a cold-rolled steel sheet having a thickness of 1 mm was prepared, and hot-plated for 3 seconds in a 450 ° C. Zn—Mg—Al plating bath and Zn—Mg—Al—Si plating bath to which various metals or intermetallic compounds were added. The amount of plating adhesion was adjusted to 80 g / m ² on one side by N ₂ wiping. The cooling rate was changed in the range of 5 to 20 ° C./s. Table 2 shows the plating composition of the obtained plated steel sheet. Regarding the Zn—Mg intermetallic compound in the Al phase, the prepared sample was polished, the Al phase was observed using SEM and EDX, and the presence or absence was investigated.

  The intermetallic compounds were analyzed for elements and composition using EDX. Table 1 shows the surface index of the surface close to the Al {110} surface of each intermetallic compound, the direction index in the lattice direction constituting the surface, and the surface spacing.

  Although some Al-based intermetallic compounds were dissolved in the plating bath and recrystallized, some of the Al was considered to be replaced by Si, but there were significant changes in crystal orientation and interplanar spacing. In the examples, it was expressed as an Al-based intermetallic compound not substituted with Si.

The corrosion resistance of the processed part was evaluated by bending the prepared plated steel sheet at 180 ° C., and evaluating the corrosion resistance of the bent part in a salt water spray exposure test. In the salt spray exposure test, samples exposed outdoors were sprayed 3 times / week with a 5% NaCl solution. Evaluation examined the test period until red rust generate | occur | produces in a bending part, and made ○ pass by the following evaluation.
○: The test period until red rust occurs is 150 weeks or more
Δ: Test period until red rust occurs 100 weeks or more and less than 150 weeks ×: Test period until red rust occurs less than 100 weeks

The results are shown in Table 1 . Nos. 8, 9, 10, 18, 19, and 20 failed in corrosion resistance because there was no Zn—Mg intermetallic compound in the Al phase.

  The products of the present invention other than these were plated steel sheets having excellent corrosion resistance of the processed parts.

First, prepare the cold-rolled steel plate having a thickness of 1 mm, this for 3 seconds dip plating with Zn-Mg-Al-Si plating bath 450 ° C. with the addition of Ti-Al system intermetallic intermetallic compound, in N ₂ wiping The amount of plating adhesion was adjusted to 80 g / m ² on one side. The cooling rate was 10 ° C./s. The composition in the plating layer of the obtained plated steel sheet was Mg 3%, Al 11%, Si 0.18%, and Ti 0.001%. After polishing the prepared sample, the Al phase was observed with an SEM, the place containing the Zn—Mg intermetallic compound in the Al phase was searched, and after sampling using FIB, it was observed with a transmission electron microscope. As a result of analyzing the Al-phase Zn—Mg intermetallic compound by EDX, the mass% of Zn was about 84 mass%. As a result of identification of an electron-ray diffraction results, Zn-Mg intermetallic compound showed a diffraction pattern of the MgZn _2.

  As described above, according to the present invention, it is possible to produce a plated steel material having excellent processed portion corrosion resistance in a Zn-Al-Mg-based plated steel sheet. By expanding the use of high corrosion-resistant steel sheets to members that could not be used due to a decrease in the corrosion resistance of processed parts so far, it becomes possible to greatly contribute to improving the durability of these processed products.

It is a figure which shows an example of the Al phase containing a Zn-Mg type | system | group intermetallic compound, (a) is a drawing substitute micrograph of the plating layer of a plated steel plate (3000 times), (b) is distribution of each structure | tissue in a photograph It is the figure which showed the state. It is a drawing-substitution transmission electron micrograph of an Al phase containing a Zn—Mg-based intermetallic compound.

DESCRIPTION OF SYMBOLS 1 Al phase 2 Al / Zn / MgZn2 ternary eutectic structure 3 Zn-Mg phase containing intermetallic compound 4 Zn phase 5 Al-Zn eutectoid 6 MgZn ₂ phase 7 Ti (Al _0.8 Si _0.2 ) ₃ Intermetallic compounds

Claims (5)

The plating layer of the plated steel material containing Al: 4 to 10% by mass, Mg: 2 to 10% by mass, the balance being Zn and an unavoidable impurity is formed on the surface [Al / Zn / MgZn ₂ ternary The [eutectic structure] substrate has a metal structure containing [Al phase], and the spacing between one surface in the lattice direction constituting the lattice plane of the Bravey lattice is 2.57 mm in this [Al phase]. It contains an intermetallic compound having a lattice plane of 3.15 mm or less and an interval between the other surfaces of 3.64 mm or more and 4.46 mm or less and a Zn—Mg intermetallic compound, and [Al phase] dendrite is [110]. A hot-dip galvanized steel material excellent in corrosion resistance of the processed part, characterized by being equiaxed crystals grown in the direction . A plating layer of a plated steel material containing Al: 4 to 22% by mass, Mg: 2 to 10% by mass, Si: 2% by mass or less, with the balance being Zn and an inevitable impurity Zn alloy plating layer on the surface [Al / has a Zn / MgZn ₂ ternary eutectic structure] metallographic structure in the matrix and [Mg ₂ Si phase] is [Al phase] were mixed, and the grating surface of the Bravais lattice in the [Al phase] And an intermetallic compound having a lattice plane in which the distance between one plane in the lattice direction constituting the layer is 2.57 mm to 3.15 mm and the other plane distance is 3.64 mm to 4.46 mm, and a Zn-Mg intermetallic compound Further, the [Al phase] dendrite is an equiaxed crystal grown in the [110] direction . A hot-dip galvanized steel material excellent in corrosion resistance of a processed part, wherein the Zn-Mg intermetallic compound according to claim 1 or 2 is either MgZn ₂ or Mg ₂ Zn ₁₁ . The melt having excellent corrosion resistance in the processed part, characterized in that the crystal system of the intermetallic compound according to claim 1 or 2 is any one of cubic, tetragonal, orthorhombic, monoclinic and hexagonal. Plated steel. The intermetallic compound according to any one of claims 1, 2, and 4 is any one of TiAl ₃ , ZrAl ₃ , HfAl ₃ , SrAl ₄ , CaAl ₄ , NiAl ₃ , and TiB _2. Hot-dip galvanized steel with excellent corrosion resistance.

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