JP2010248541A - ORGANIC COMPOSITE Mg BASED PLATED STEEL SHEET - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an organic composite Mg based plated steel sheet solving a corrosion problem under coating film of a crack part conventionally existing in a plated steel sheet for automobile. <P>SOLUTION: The organic composite Mg based plated steel sheet is provided, wherein a Mg based plated film containing 30 to 85 atom% Mg, 0.03 to 10 atom% Ca and the rest constituted of Zn and/or Al and inevitable impurities is formed on at least one surface of a steel sheet; a Zn phosphate film of 0.3 to 10 g/m<SP>2</SP>is formed on the surface of the Mg based plated film and an organic film is formed on at least one surface of the Zn phosphate film; the Zn phosphate film contains 20 to 250 mg/m<SP>2</SP>Mg; and Mg/P (mass ratio) in the Zn phosphate film is 0.15 to 0.35. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a coated plated steel sheet, and more particularly to an organic composite Mg-based plated steel sheet having a Zn phosphate coating.

  The performance requirements for corrosion resistance of automobile bodies are becoming stricter year by year. In automobiles, the corrosion resistance of the chipping part is regarded as important. In order to prevent rust in automobiles, the amount of Zn-based plated steel sheets is increasing. When these Zn-plated steel sheets or alloyed Zn-plated steel sheets are used in automobile bodies, they are subjected to press forming, welding, and assembly processes, followed by phosphating, primer coating by electrodeposition coating, and further coating on them. A top coat of the layer is applied.

The corrosion pattern of the plated material used in such a manner is different from the corrosion pattern of the bare plated material, and it is necessary to consider the corrosion mechanism in an alkaline environment. When a buttock reaching the iron core is formed in the coated steel material by chipping or the like, the anode reaction in which the plating metal such as Zn or Fe or the iron iron dissolves in the heel and under the coating film near the heel. a pair of water and oxygen combine to OH ^- cathode reaction occurs to form a. That is, under the coating film, the pH rises due to the generation of OH ⁻ , resulting in an alkaline environment, so that the plating metal and the phosphate film are dissolved, and further, the corrosion proceeds so that the coating film rises due to the corrosion products. For example, coating film peeling occurs. In particular, in the case of a plated steel sheet that is not so high in sacrificial anticorrosive ability, such as an alloyed Zn-plated steel sheet, the swelling width of the coating film itself is small, but corrosion products accumulate due to the progress of corrosion of the buttocks, resulting in a scab shape. There was also a problem in the appearance after corrosion because the ridges were prominent and the buttock was conspicuous.

  For such corrosion, means for improving corrosion resistance in an alkaline environment by adding a Mg component to the plating layer or the phosphoric acid film is known.

  For example, Patent Document 1 discloses a Zn—Al—Mg-based plated steel sheet in which Al and Mg are contained in the plating layer to improve the under-coating corrosion resistance of the plating layer. However, the main component of this plating layer is composed of Zn, and since the Mg content is low, the main component of the corrosion product is a Zn-based corrosion product, and the effect of improving corrosion resistance under coating by Mg is maintained. I can't. Therefore, a big change cannot be obtained in the form of undercoat corrosion.

  Patent Document 2 discloses an organic composite Zn-based plated steel sheet used for an automobile body, and a plated steel sheet having improved corrosion resistance and coating film adhesion by containing Mg in a phosphoric acid film is disclosed. . The present invention is a Zn phosphate treatment bath containing Mg nitrate, in which Mg is contained in the phosphate film, but this method has a limit on the concentration of Mg nitrate dissolved in the Zn phosphate treatment bath. Therefore, only a small amount of Mg can be contained in the film. For this reason, the amount of Mg contained in the Zn phosphate coating is not sufficient for the amount of zinc coating and the amount of Zn phosphate coating, so the effect of improving the corrosion resistance of Mg cannot be obtained and sufficient corrosion resistance is obtained. I can't get it. Therefore, it is not possible to improve the corrosion form in which corrosion products accumulate on the buttock and swell up and proceed in a scab shape.

  Patent Document 3 discloses a method in which Mg compound such as Mg hydroxide and Mg oxide is contained in the phosphoric acid treatment solution and Mg is contained in the phosphoric acid Zn film. In this case as well, the Mg compound is disclosed. It is difficult to control the pH change due to dissolution of metal, and, similarly to Patent Document 2, the amount of Mg contained in the Zn phosphate film is less than the amount of plating adhesion and the amount of Zn phosphate film. It cannot produce change.

  Thus, although the effect of Mg content in the plating layer and Zn phosphate film has been studied for corrosion under the coating film so far, the amount of Mg contained is small, and the plating that can fully exhibit the effect of Mg Since no layer or Zn phosphate coating has been developed, there has been no means to solve the problem of undercoat corrosion of the buttocks that has been present in galvanized steel sheets for automobiles.

JP 2001-329383 A Japanese Patent Laid-Open No. 2001-131763 Japanese Patent Laid-Open No. 2001-152355

  The problem to be solved by the present invention is to develop a plated steel sheet capable of suppressing the progress of undercoat corrosion, a scab-like corrosion product, which is a problem with coated Zn-based plated steel sheets. For the purpose of solving the problem, organic composite Mg with excellent corrosion resistance in an alkaline environment, characterized in that the steel material is plated with Mg as a matrix and the Zn phosphate film contains Mg-based Zn phosphate. A steel-plated steel sheet was studied.

  Measures to improve corrosion resistance by the inclusion of Mg in the plating layer and phosphate coating have been studied so far, but the structure of the plated steel sheet has been shown so as to dramatically improve the corrosion resistance of undercoat corrosion. I did not come.

  Therefore, in the present invention, a plating layer structure capable of stopping the progress of undercoat corrosion is shown, and it is possible to prevent the progress of undercoat corrosion that could not be solved by an organic composite Zn-based plated steel sheet until now. The object is to provide an organic composite Mg-based plated steel sheet.

  In order to prevent the progress of corrosion under the coating film, it is effective that the corrosion product has a two-layer structure. The two-layer structure of the corrosion product optimized by the present inventors is a dense Mg-based oxide, an upper layer mainly composed of a carbonic acid compound, etc., and basic Zn chloride or Al · Mg-based hydroxide. It is the structure which consists of the lower layer which has as a main component. In other words, in the structure under the coating film, having the structure of the base iron, the plating layer, the Zn-based corrosion product, the Mg-based corrosion product, and the coating film in order is the most effective in preventing the progress of corrosion under the coating film. I found that there is a good means. Fig. 1 shows the structure of the corroded portion that is effective in preventing the progress of the undercoat corrosion.

  In conventional Zn plating, the corrosion products formed under the coating film are thermodynamically unstable in an alkaline environment, so they disappear immediately and can serve as an anticorrosion function for the plating layer and the underlying steel plate. It cannot be done, and red rust is generated from the coating film in a short time. However, as in the present invention, the presence of an upper-layer corrosion product mainly composed of an Mg-based oxide, a carbonate compound, etc., stable in an alkaline environment, on the Zn-based corrosion product. The corrosion product is retained on the plating layer for a long time, and the corrosion protection effect of the plating by the Zn-based corrosion product is maintained. The present inventors consider that this is due to the stabilization of pH caused by Mg-based corrosion products. That is, in the aqueous solution around the Mg-based corrosion product, the pH is maintained at around 10, and an excessive increase in pH under the coating film is suppressed. At this pH, the disappearance of the Zn-based corrosion product is suppressed.

In addition, Mg-based corrosion products have the effect of inhibiting Fe corrosion in addition to stabilizing Zn-based corrosion products by stabilizing the pH, and Fe ₂ O ₃ (red rust It was found that the problem of scab-like corrosion appearance due to the accumulation of corrosion products can be solved by preventing the formation of) and the like. The present inventors consider that the formation of a passive film due to Fe (OH) ₂ is involved because the pH is fixed at around 10.

  These Mg-based corrosion products can be formed by including Mg of a certain concentration or more in the plating layer.

  Further, in the undercoat corrosion, it is important to ensure the adhesion of the coating film at the same time. Ensuring coating film adhesion can be further improved by subjecting the plating containing Mg at a certain concentration or more to chemical conversion treatment. So far, the formation of Mg-based Zn phosphate has mainly been the method of depositing Zn phosphate by adding Mg nitrate and hydroxide to the chemical conversion treatment solution of Zn phosphate. In the case of such a technique, in order to maintain the stability of the bath, there is an upper limit on the amount added, and there is also a limit on the amount of Mg contained. However, in the present invention, a sufficient amount of dissolved Mg ions and Zn ions in the chemical conversion solution are precipitated as Zn and Mg phosphate Zn from a plating layer containing Mg of a certain concentration or more, and a chemical conversion film is formed. Therefore, Zn phosphate can be formed without impairing the stability of the bath, and an organic composite Mg-based plated steel sheet having excellent coating film adhesion can be produced.

  Furthermore, since the generation rate of wrinkles due to chipping of automobile steel plates decreases with an increase in the hardness of the plating layer, it is important to increase the hardness of the plating layer in order to prevent wrinkles generated in the plating layer. In the case of Mg-based plating, which is a constituent requirement of the present application, the plating layer hardness can be increased by including a hard amorphous phase, the amount of wrinkles reaching the base iron can be reduced, and the chipping wrinkle resistance is excellent. It is possible to provide steel sheets.

  The present invention has been made based on such findings, and the gist thereof is as follows.

(1) On at least one surface of the steel sheet, Mg is contained in an amount of 30 atomic% or more and 85 atomic% or less, Ca is contained in an amount of 0.03 atomic% or more and 10 atomic% or less, and the balance is Zn and / or Al and inevitable impurities. An Mg-based plating film is formed, a Zn phosphate film of 0.3 g / m ² or more and 10 g / m ² or less is formed on the surface of the Mg-based plating film, and at least the Zn phosphate film An organic composite Mg-based plated steel sheet in which an organic film is sequentially formed on one side,
The Zn phosphate coating contains 20 mg / m ² or more and 250 mg / m ² or less of Mg, and Mg / P (mass ratio) in the Zn phosphate coating is 0.15 or more and 0.35 or less. Organic composite Mg-plated steel sheet.

(2) Mg ₂ PO ₄ (OH) is included in the phosphoric acid Zn film in the total diffraction peak intensities of all phosphate components appearing at 0.2 to 0.4 nm in terms of diffraction plane spacing. ₂ The organic composite Mg-based plated steel sheet according to (1) above, which contains 5% or more of the diffraction peak intensity of PO ₄ (OH).

(3) The organic composite Mg-based plating as described in (1) or (2) above, wherein the Mg-based plating layer contains 10 atomic% or less of Al and 5% or more of amorphous by volume. steel sheet.

(4) The organic composite Mg-based plated steel sheet according to any one of (1) to (3) above, wherein the mass of the organic film is 0.2 to 3 g / m ² .

  The organic composite Mg-based plated steel material of the present invention is excellent in versatility and economy because it can be manufactured by a normal hot dipping process, chemical conversion treatment process, and electrodeposition coating process. In addition, it is superior in corrosion resistance under the paint film compared to the widely used paint zinc-based plated steel material, so it is widely used even in fields where paint zinc-plated steel material could not be used due to the problem of corrosion resistance under the paint film. Is possible. In addition, it is possible to contribute to the development of the industry by extending the life of steel materials and reducing maintenance labor.

The schematic diagram of the anticorrosion structure by formation of Mg corrosion product is shown.

The XDR identification result of the phosphoric acid Zn film | membrane in the Example of this invention is shown.

The external appearance at the time of 28 cycles, 56 cycles, and 84 cycles after the CCT test in the Example and comparative example of this invention is shown.

  Hereinafter, the organic composite Mg-based plated steel material according to the present invention will be described in detail. In order to increase the corrosion resistance of corrosion under the coating film, the present inventors examined the application of an Mg component that is stable in a high pH alkaline environment and has high corrosion resistance.

The basic undercoat corrosion of Zn-plated steel materials starts from the coating film so that it progresses under the coating film, and the corrosion mechanism is considered as follows. First, ZnCl ₂ · 4Zn (OH) ₂ which is basic Zn chloride is formed at the initial stage of corrosion, and finally changes to ZnO. Subsequent to the formation of ZnCl ₂ · 4Zn (OH) ₂ , the base iron begins to corrode, and the corrosion product finally changes to porous Fe ₃ O ₄ and FeOOH mainly composed of Fe. In this corrosion mechanism, the formation of ZnO is suppressed and ZnCl ₂ · 4Zn (OH) ₂ is maintained for a long period of time.

  The present invention also provides good corrosion resistance by this mechanism. That is, the improvement in corrosion resistance by Mg is due to the effect of stabilizing the formation of basic Zn chloride. Although the mechanism by which Mg stabilizes basic Zn chloride is not known in detail, the present inventors, first of all, believe that when Mg is formed as a corrosion product, the pH is around 10 This stabilizes basic Zn chloride, and secondly, Mg, C, and O bind preferentially, thereby inhibiting (inhibiting) the reaction between Zn, C, and O, and binding to Cl. As a result, we believe that the formation of basic Zn chloride may be stabilized.

  Referring to FIG. 1, Mg corrosion products are formed in the outer layer of Zn-based corrosion products. The main corrosion products of Mg are carbonate compounds and oxides. That is, the collar portion 7 has the structure of the coating film 6, the Mg-based corrosion product 5, the Zn-based corrosion product 3, and the ground iron 1 from the outside. This two-layer structure is formed by corrosion of the plating layer 2 containing Mg and Zn at the same time (particularly when Mg is in the range of 30 atomic% to 85 atomic%). The mechanism of formation of Mg-based corrosion products in the outer layer and Zn-based corrosion products in the inner layer has not been elucidated in detail, but it is related to the ease of binding of Mg ions, Zn ions to O, C, Cl. It is estimated that.

  The Mg-based corrosion product 5 formed in the outer layer has a denser structure than the Zn-based corrosion product 3, and the Mg-based corrosion product 5 having this dense structure covers the flange 7, so that ions from the outside world are present. It becomes a barrier that blocks the diffusion of, and has the effect of greatly suppressing the progress of corrosion.

  Therefore, by taking this two-layer corrosion product structure, since basic Zn chloride is protected for a long period of time, corrosion of the base iron is suppressed, and therefore volume expansion due to the formation of Fe-based oxides can also be suppressed. Corrosion resistance under coating film is greatly improved, such as suppression of scab-like wrinkles.

As will be described later, there are cases where Al is contained in the plating layer for the purpose of improving the corrosion resistance, but in this case as well, the lower layer is formed of corrosion such as Mg ₂ Al (OH) ₇ and Mg ₄ Al ₂ (OH) _14. The anticorrosion mechanism is the same except that it is composed of objects. However, the stability of corrosion products in an alkaline environment is higher for Zn-based corrosion products than for Al-based corrosion products, so the constituents of the underlying corrosion products are Zn-based corrosion products. Is preferred. For this reason, the plating layer has a tendency to prevent corrosion of the base iron for a long time when the Zn concentration is higher than the Al concentration.

  Mg-based corrosion products are rapidly formed at the same time as the soot formation and close the soot. For this reason, in the very initial stage of corrosion, the swelling around the buttock may be larger than the existing organic composite Zn-plated steel sheet, but in the long term, the organic composite Mg-based steel sheet Is also excellent in appearance.

  In order to rapidly form Mg-based corrosion products in the buttock, the Mg concentration in the plating layer needs to be a certain concentration or higher. By setting Mg in the plating layer to 30 atomic% or more (hereinafter, “%” unless otherwise specified means atomic%), Mg-based corrosion products are rapidly formed in the heel. It was found that a two-layered corrosion product formed. The upper limit concentration of Mg in the plating layer is 85%. This is because Mg has poor reactivity with Fe, and when the Mg concentration exceeds 85%, it is difficult to adhere to the steel sheet as a plating layer.

  Ca is an indispensable element for industrially performing Mg-based plating. Mg in a molten state may be rapidly oxidized and ignited, but Mg can be prevented from igniting by adding Ca which is more easily oxidized than Mg. If plating is performed in an inert atmosphere, addition of Ca is not necessary, but in the case of atmospheric operation, addition of Ca is essential. Addition of 0.03% or more of Ca has the effect of preventing the ignition of Mg, so this is the lower limit concentration. If Ca is added up to 10%, it can be added while lowering the melting point of the plating bath. However, Ca is a metal that is easier to oxidize than Mg, so if it exceeds 10%, the oxidation of Ca becomes too intense, Ca sudden oxidation and ignition occur, and the plating bath viscosity becomes high, Since the plating operation itself becomes difficult, the upper limit of Ca addition is 10%.

  The plating remainder other than Mg and Ca is preferably Zn. A preferable Zn concentration range from the viewpoint of corrosion resistance of the plating layer, plating bath temperature, dross generation, and the like is 20% or more and 40% or less. The plating remainder may be Al other than Zn. The preferable concentration range of Al is the same as that of Zn, and is 20% or more and 40% or less. Both Zn and Al are suitable for plating production because the melting point of the plating bath can be kept low in this concentration range. Further, from the viewpoint of improving corrosion resistance, it is preferable that both Zn and Al are contained. However, if the Al concentration is high, the corrosion resistance of the plating layer itself is improved, but a lot of Al-based corrosion products are contained in the corrosion products. Corrosion product stability is higher for Zn-based corrosion products than for Al-based corrosion products, so when considering the anticorrosion period of the base iron, the remainder of the plating other than Mg and Ca is mainly Zn. Preferably there is.

  The Mg-based plating of the present invention contains elements such as Ni, Cu, Ag, Si, Co, Mn, Nb, Mo, Ti, Cr, etc., in an amount of about 0.1 to 5% for the purpose of imparting corrosion resistance. It is possible. For example, by adding these metals, the electrochemical properties change, the metal dissolution reaction (anode reaction) is suppressed, the corrosion current density is reduced, and the corrosion resistance is improved. The present inventors presume that these effects of improving corrosion resistance are caused by the dispersion of electrochemically noble elements in the plating layer.

  In the corrosion under the coating film, it is important for the organic composite plated steel material to improve the coating film adhesion. Ensuring film adhesion can be improved by including Mg in the chemical conversion coating. When Mg is contained in the chemical conversion film, the structure of the Zn phosphate crystal changes. Zn phosphate crystals that do not contain Mg are phosphonated, and crystals with a size of 5-10 μm are formed, whereas Zn phosphate crystals containing Mg are columnar, fine crystals of 2 to 3 μm. Form.

The chemical conversion treatment used for the paint base is generally a phosphorylation chemical treatment. For example, Zn phosphate, Fe (II) phosphate, Mn phosphate, Zn-Ca phosphate, and Ca phosphate treatment are known. These can also be applied to Mg-based plating. Among these, the application of Zn phosphate is common for plated steel sheets for automobiles. The treatment bath used to form this Zn phosphate film contains phosphate ions, Mn ions, Zn ions, Fe ions, etc., and further contains reaction accelerators (F ions, SiF ₆ ions, Ni ions), etc. May be contained.

When Zn treatment is performed on Mg plating using such a treatment bath, Mg is dissolved from the plating layer in the treatment solution, and Mg is contained at a high concentration in the phosphoric acid Zn film. In the case of the Mg-based plating of the present invention, Mg of 20 mg / m ² or more and 250 mg / m ² or less as Mg adhesion amount and 0.15 or more and 0.35 or less as Mg / P (mass ratio) are contained in the Zn phosphate coating. . In the present invention, Mg in the plating layer serves as a source of Mg in the Zn phosphate coating, so Mg compounds such as Mg nitrate and Mg hydroxide are added to the treatment solution for the purpose of containing Mg in the Zn phosphate coating. do not have to. However, when it is desired to contain Mg in the film at a higher concentration, a treatment solution in which the Mg compound is dissolved may be used.

The mass of the Zn phosphate coating is preferably 0.3 g / m ² or more, and if it is less than this mass, the coating film adhesion tends to deteriorate and the coating swelling width tends to increase. On the other hand, if it exceeds 10 g / m ² , it is difficult to obtain the effect of refining Zn phosphate crystals, the coating film adhesion tends to be poor, and the coating swelling width tends to increase. ² In addition, the upper limit value and the lower limit value of the Mg amount and the Mg / P ratio indicate average values of a film that can be formed when the Mg-based plating is treated with Zn phosphate. Therefore, the amount of Mg depends on the amount of Zn phosphate coating deposited and the Mg content of the Mg-based plating. When the Mg-based plating specified in the present invention is treated with Zn phosphate, the amount of Mg contained is A Zn phosphate film having a concentration of 20 mg / m ² or more and 250 mg / m ² or less and an Mg / P (mass ratio) of 0.15 or more and 0.35 or less is usually formed.

  The present application exhibits excellent corrosion resistance by further coating an organic film on the upper surface of the Zn phosphate film on at least one side.

The organic film to be coated may be an organic resin alone, but considering the corrosion resistance, the organic resin and SiO ₂ , Al ₂ O ₃ , MgO, Fe ₂ O ₃ , Fe ₃ O ₄ , ZrO ₂ , TiO ₂ Desirably, a powder selected from SnO ₂ or a composite film combined with one or more colloids. In addition, in this organic film, a wax component, a color pigment, a rust preventive agent, etc. can be contained, and when a wax component is added, it can be processed without oil depending on molding processing conditions. Become. There are no particular restrictions on the type of organic resin, but when considering automotive steel sheet applications, epoxy resin (such as bisphenol A type epoxy resin) is used as the current electrodeposition coating. From the viewpoint of ensuring compatibility and adhesion, an epoxy resin is preferred.

In the case of a thin film organic composite Mg-plated steel sheet premised on welding after painting, the amount of organic coating attached is preferably 0.2 to 3 g / m ² . If it is less than 0.2 g / m ^2, it is difficult to produce a uniform organic thin film with a roll coater, and if it exceeds 3 g / m ² , the weldability tends to deteriorate.

  In addition to Mg, it is also preferable to add one or more of Ni, Mn, Co, Fe, Cu, Al, and Ca to the phosphoric acid Zn film. It is done.

Furthermore, when Mg ₂ PO ₄ (OH) is formed in the coating by containing Mg of a certain concentration or more in the Zn phosphate coating, the adhesion between the plating layer and the coating is improved, and the swollen width of the coating is increased. Improve. The present inventors estimate that the Mg amount necessary for precipitation of Mg ₂ PO ₄ (OH) is Mg / P ratio of 0.25 or more.

Detection of Mg ₂ PO ₄ (OH) in the plating layer can be performed by a general X-ray diffraction method. For example, the X-ray diffractometer using Kα line of Cu, the diffraction pattern is measured, to determine the presence or absence of Mg ₂ PO ₄ (OH) diffraction peak, Mg ₂ PO ₄ by X-ray diffraction pattern of (OH) For the identification, it is preferable to use a diffraction peak of 0.20280 to 0.37220 nm with a diffraction plane interval. This is because the amount of Mg ₂ PO ₄ (OH) is small and the diffraction peaks at 0.485 nm and 0.4417 nm overlap with the strongest line of the hopate (Zn ₃ (PO ₄ ) ₂ .4H ₂ O) diffraction peak. Further, discrimination by TEM-EDX is also possible, and Mg ₂ PO ₄ (OH) may be identified from a characteristic X-ray spectrum obtained from a specific crystal phase.

Intensity ratio of Mg ₂ PO ₄ (OH) to hopate (Mg ₂ PO ₄ (OH) diffraction peak intensity appearing at 0.2 to 0.4 nm with diffraction plane spacing / {all phosphoric acid appearing at 0.2 to 0.4 nm with diffraction plane spacing) If the total of the diffraction peak intensities of the salt components (Mg ₂ PO ₄ (OH) strength + haupate strength)}) is included at 5% or more, the adhesion between the plating layer and the coating film is improved, and the coating film Bulge width etc. improve.

  Moreover, undercoat corrosion is corrosion that occurs when plating or wrinkles reaching the ground iron are formed. Therefore, if it is possible to prevent wrinkles reaching the ground iron, it is not necessary to sacrificial corrosion protection of Fe, and therefore the progress of corrosion can be suppressed. In order to prevent wrinkles reaching the base iron, it is only necessary to improve the hardness of the plating layer. In the Mg-based plating adopted in this application, the hardness of the plating layer can be increased by including an amorphous phase in the plating layer. Is possible. Specifically, corrosion under the coating film can be suppressed by setting the Al concentration in the plating layer to 10 atomic% or less and further containing amorphous in a volume content of 5% or more. The volume fraction of the amorphous phase necessary to make the average hardness of the plating layer composition described in the present application about 100 Hv or more is 5%.

  The amorphous phase is a hard phase obtained when the molten metal is rapidly cooled, and the cooling conditions necessary for the formation of the amorphous phase vary depending on the alloy composition, but in the composition range disclosed as the Mg-based plating of the present invention. If Al is 10% or less, it can be obtained by rapidly cooling the plated layer in a molten state after plating at a cooling rate of about 1000 ° C./sec or more. The upper limit of the Al concentration is because Al is an element that reduces the amorphous forming ability.

  Confirmation of the formation of the amorphous phase can be confirmed by obtaining a halo pattern in the X-ray diffraction image of the plating layer. If it is a single amorphous phase, it can be confirmed only by a halo pattern (in some cases, an Fe diffraction peak of a steel material may be detected).

  When the amorphous phase and the crystalline phase are mixed and the amorphous volume fraction is low, the plating layer is detected by detecting the exothermic peak when the amorphous phase crystallizes during the temperature rising process using a differential thermal analyzer. The presence of the amorphous phase can be confirmed.

  In order to obtain the amorphous volume fraction, the plated steel material is cut, its cross section, polished and etched, and the cross section of the plated layer on the steel material surface is observed with an optical microscope (hereinafter, light microscope). No structure is observed in the amorphous part even by etching, but in the part where the crystal phase remains, the structure due to the crystal grain boundaries, sub-grain boundaries, precipitates, and the like is observed. Thereby, since the area | region of an amorphous part and a crystal | crystallization part is distinguished clearly, it can convert into a volume ratio by the line segment method or image analysis.

  When the structure is too fine and measurement with a light microscope is difficult, a thin piece is produced from the cross section of the plating layer and observed with a transmission electron microscope, so that the measurement can be performed in the same manner. In the case of a transmission electron microscope, it is also possible to confirm an amorphous structure by a halo pattern of an electron beam diffraction image in a region where no tissue is observed. Therefore, in the light microscope observation, if the structure is not observed on the entire surface, or if there is a part where the structure is not observed even if it is suspected to be a coarse and undistorted crystal grain, a thin slice for an electron microscope is further collected. Thus, it is desirable to confirm that the phase is an amorphous phase by observing a halo pattern without a diffraction spot in the electron diffraction pattern.

  In both the light microscope and the electron microscope, it is desirable to obtain the area ratio by image processing using a computer for 10 or more different fields of view and average them to obtain the volume ratio.

  A plating bath having the components shown in Tables 1 and 2 was prepared, and a surface-treated steel material was prepared using a cold-rolled steel plate having a thickness of 0.8 mm as a base material. Cold-rolled steel sheets (thickness 0.8 mm) were cut into 200 mm × 100 mm, and then plated with a batch-type hot dipping test apparatus manufactured in-house. The bath temperature of the plating bath was maintained at 480 to 670 ° C. and 50 ° C. higher than the melting point of the plating bath. The basis weight was adjusted by air wiping. The plating thicknesses produced were all about 7 μm. Before being subjected to each test, the plated steel sheet was cut into 50 × 100 mm.

  As the phosphoric acid Zn treatment, the surface was adjusted and the phosphoric acid Zn film was formed in accordance with the standards of Nippon Paint Co., Ltd. and Surfdyne SD5350 system. If the Mg concentration in the plating layer is low and the Mg content in the phosphoric acid film is low, Mg nitrate hexahydrate is added to the Zn phosphate treatment solution as necessary, and the Mg ion concentration is 20 g / l. Those added with Mg nitrate are shown as “Mg” in Table 4. The amount of adhesion was adjusted by changing the immersion time. In addition, as a comparative material, when forming a film with a smaller amount of Mg than the Mg / P ratio in the Zn phosphate film normally formed, Zn nitrate hexahydrate is added to the Zn phosphate treatment solution as necessary. Was added to increase the Zn ion concentration. In Table 4, the material to which Zn nitrate was added was represented as “Zn”.

The Zn phosphate coating of the sample after the Zn phosphate treatment was dissolved in chromic acid (chromium oxide (VI) (5% solution, immersed for 3 minutes at 20 ° C). The solution was analyzed by ICP, and the Zn phosphate coating was analyzed. The components were analyzed, the Mg amount and the Mg / P ratio were calculated by mass ratio, and the formation phase of the Zn phosphate coating was identified by X-ray diffraction. _{(Zn 3 (PO 4) 2} · 4H 2 O) takes the intensity sum of the _{Mg 2 (PO4) (OH)} , the 0.2~0.4nm intensity ratio (diffraction spacings _{Mg 2 (PO 4) (OH} ) Mg ₂ PO ₄ (OH) Diffraction Peak Intensity Appearing / {The sum of diffraction peak intensities of all phosphate components appearing at 0.2 to 0.4 nm with diffraction plane spacing (Mg ₂ PO ₄ (OH) intensity + Hopeite intensity .)}) Was determined to be “◯” for those containing 5% or more, and “×” for those not exceeding 5%.

  When electrodeposition coating was used as the organic coating, Powernicks 110 gray paint made by Nihon Paint Kogyo Co., Ltd. was applied as a coating treatment with a film thickness of 20 μm. The baking heat treatment conditions were 150 ° C. and 20 minutes.

For other thin film organic coatings, vinyl-modified epoxy ester resin is used and blended with blocked isocyanate curing agent, modified polyethylene wax, and condensed azo red pigment (each solid content ratio is 100: 10: 5: 3) Based on water-based resin, a coating with colloidal silica added to a solid content mass ratio of 16% is used. The dry film mass is 0.2 to 3.5 g / m ² (per side) while controlling the number of revolutions with a roll coater. Then, the coating was applied to both sides, and then baked to reach 150 ° C. at the ultimate plate temperature and cooled with water.

  At the center of the sample, make a cut bowl (70 mm in length) that reaches the base iron with a cutter knife from the surface, then CCT test (salt spray (0.5% NaCl, 35 ° C) for 6 hours → dry (50 ° C, 45% RH) 3 hours → wet (50 ° C., 95% RH) 14 hours → drying (50 ° C., 45% RH) 1 hour) was performed 140 cycles. The outer appearance of the buttock after 140 cycles and the swollen width of the coating film were measured. In Tables 3 and 4, when the scab-like corrosion product was visually detected in the heel portion, “X” was indicated, and when it was not detected, “◯” was indicated. By visual inspection, the swollen width of the coating film (average of 4 points around the cut ridge) was 2 mm or less as “◎”, 2 to 3 mm as “◯”, 3 to 3.5 mm as “△”, and 3.5 mm or more as “x”. . A thin film organic coating with a roll coater was evaluated as “-” because it was not evaluated.

  In order to observe the cross section of the buttock at the time when 140 cycles had elapsed, the rust structure of the buttock was observed with EPMA after being embedded in the resin and polished. In Tables 3 and 4, “O” indicates that the rust structure of Mg oxide / carbonic acid compound and basic Zn chloride, or Mg, Al hydroxide compound is detected, and “X” indicates that the rust structure is not detected. did.

  For coating adhesion, the sample was left to stand overnight and then immersed in warm water at 50 ° C., taken out after 10 days, put in a grid cut with 1 mm intervals, and peeled off with cello tape (registered trademark). In Tables 3 and 4, the peeled area rate 0% was “◎”, the peeled area rate 5% was “◯”, the peeled area rate 5-50% was “Δ”, and 50% or more was “x”. A thin film organic coating with a roll coater was evaluated as “-” because it was not evaluated.

  A sample containing an amorphous phase employs a water cooling method as a cooling method after plating. The amorphous volume fraction of the plating layer was determined by taking two pieces of transmission electron microscope slices each at a position where the thickness of the plating layer of the test piece was equally divided into five pieces, and analyzing the image by using a computer. The area ratio of the amorphous region was measured, and the average value of the area ratio of the amorphous region in the entire visual field was defined as the amorphous volume fraction. In Tables 3 and 4, when the plating layer contained an amorphous phase in a volume fraction of 5% or more, “◯” was less than 5% and “x”. The Vickers hardness was measured with an average of 10 points to measure the hardness. In Tables 3 and 4, the Vickers hardness, 200Hv or more is “◯”, 200 to 100Hv is “Δ”, and less than 100 Hv is “×”.

  For the evaluation of weldability, a Cu-Cr CF-type electrode tip (5 mmφ) was used, and an appropriate current range was measured with a pressure of 1961 N (200 kgf) and an energization time of 13 cycles. In Tables 3 and 4, over 1.5 kA is “◎”, 1.0-1.5 kA is “◯”, 0.3-1.0 kA is “△”, less than 0.3 kA is “×”, and evaluation is not performed. “−”.

FIG. 2 shows the XRD identification results of No. 9 Zn phosphate film in Table 1. In addition to Hopeto, Mg ₂ (PO ₄ ) (OH) was detected.

  Figure 3 shows the appearance of No. 9 (Mg-plated steel sheet) in Table 1 and No. 70 (alloyed Zn-plated steel sheet) in Table 2 after 28, 56 and 84 cycles after CCT test. It is shown. In alloyed Zn-plated steel sheet, scab-like red rust is generated in 56 cycles. On the other hand, the organic composite Mg-based plated steel sheet does not generate scab red rust.

  The organic composite Mg-based plated steel material of the present invention is excellent in versatility and economy because it can be manufactured by a normal hot dipping process, chemical conversion treatment process, and electrodeposition coating process. In addition, it is superior in corrosion resistance under the paint film compared to the widely used paint zinc-based plated steel material, so it is widely used even in fields where paint zinc-plated steel material could not be used due to the problem of corrosion resistance under the paint film. Is possible. In addition, it is possible to contribute to the development of the industry by extending the life of steel materials and reducing maintenance labor.

1 Fe
2 Plating layer 3 Zn-based corrosion product 4 Phosphoric acid Zn coating 5 Mg-based corrosion product 6 Coating 7 Cut collar

Claims (4)

On at least one surface of the steel sheet, Mg is contained in an amount of 30 atomic% or more and 85 atomic% or less, Ca is contained in an amount of 0.03 atomic% or more and 10 atomic% or less, and the balance is composed of Zn and / or Al and inevitable impurities. An Mg-based plating layer is formed, a Zn phosphate film of 0.3 g / m ² or more and 10 g / m ² or less is formed on the surface of the Mg-based plating layer, and an organic coating is formed on at least one surface of the Zn phosphate film. An organic composite Mg-based plated steel sheet in which a film is sequentially formed,
The Zn phosphate coating contains 20 mg / m ² or more and 250 mg / m ² or less of Mg, and Mg / P (mass ratio) in the Zn phosphate coating is 0.15 or more and 0.35 or less. Organic composite Mg-plated steel sheet. During the phosphate Zn _film, Mg ₂ PO 4 and (OH), occupying in the total sum of the diffraction peak intensities of all the phosphate component appearing in 0.2~0.4nm the diffraction plane spacing, _Mg 2 PO ₄ 2. The organic composite Mg-based plated steel sheet according to claim 1, wherein the organic composite Mg-based plated steel sheet is contained at a diffraction peak intensity ratio of (OH) of 5% or more.   3. The organic composite Mg-based plated steel sheet according to claim 1, wherein the Mg-based plated layer contains Al at 10 atomic% or less and amorphous at a volume content of 5% or more. Organic composite Mg plated steel sheet according to claim 1, wherein the mass of the organic film is 0.2 to 3 g / m ^2.