JP4318633B2 - Hot-dip hot-dip galvanized steel sheet - Google Patents

Description

  The present invention relates to a hot-pressed hot-dip Zn-plated steel sheet that is heated to a high strength and high tension after being heated in an austenite region, and is particularly useful for manufacturing automobile chassis, undercarriage parts, reinforcing parts, and the like. The present invention relates to a hot-dip hot-dip galvanized steel sheet. The “hot-dip Zn-plated steel sheet” of the present invention means both non-alloyed hot-dip Zn-plated steel sheets and alloyed hot-dip Zn-plated steel sheets.

  In recent years, there has been an active movement to protect the global environment by reducing exhaust gas and reducing fuel consumption by reducing the weight of automobiles. However, since the collision safety cannot be ensured only by thinning the steel plate to be used, the use of a high-strength, high-tensile steel plate (high-tensile material) having high strength (590 MPa or more) is progressing even if it is thin. Recently, in order to improve the strength of the vehicle body at the time of a side collision, high-strength and high-strength steel sheets of 1000 MPa and 1500 MPa class (ultra-high tensile materials) are being used, but such ultra-high tensile materials have poor workability, It cannot be pressed into a desired shape, and the shape freezing property such as deformation after pressing is not sufficient, and there is a concern about the problem of delayed fracture.

  For this reason, the induction hardening technique and the hot press method are considered as a technique for obtaining a high-strength processed member without using a high-strength high-tensile steel plate. The induction hardening technique is a method of increasing the hardness by heating a part of the part after heating, but there is a tendency that the temperature distribution is formed in the part, the structure is changed, and the corrosion resistance is deteriorated.

  On the other hand, there are currently two types of hot-press (hot stamping) technology: a method in which a steel sheet is heated to an austenite region and then rapidly cooled while being processed with a mold, and after cold rolling, it is heated and cooled with a mold. The method to do is adopted. In other words, the steel sheet is heated and softened so that it can be easily processed, and by cooling and quenching during the subsequent processing, an attempt is made to obtain a processed member that has excellent shape freezing properties and exhibits high strength and high tension. Technology.

  As conventional steel plates for hot pressing, Al-based plated steel plates have been often used (for example, Patent Document 1). However, in the case of an Al-based plating layer, Fe rapidly diffuses into the plating layer when heated in the austenite region, and an alloy layer of Al and Fe is formed. This Al-Fe alloy layer is hard and brittle, It sometimes peels off, reducing the processing productivity and shortening the mold life. Furthermore, the presence of an Al—Fe alloy layer inferior in adhesion to the substrate has a problem that the adhesion with the top coat film (adhesion after coating) is remarkably lowered and the corrosion resistance is also deteriorated.

On the other hand, there is a problem in applying a Zn-plated steel sheet on which Zn plating has been applied to a hot press. That is, in the hot press, the steel base material must be heated to the Ac ₃ point or higher, but since this temperature range is near the boiling point of Zn (907 ° C. under atmospheric pressure), Zn evaporates and the plating layer deteriorates. It will be done. Moreover, since an oxide (ZnO) having poor adhesion is formed on the surface of the plating layer, there is a problem similar to the case of the Al—Fe alloy layer. In Patent Document 2, it is introduced that a Zn oxide layer is formed in advance to prevent evaporation of Zn during hot pressing, but according to the inventors of the present application, ZnO is formed thick. It was found that the adhesion after painting was considerably reduced.

On the other hand, Patent Document 3 discloses a technique of further providing a plating layer containing Ni, Cu, Cr, and Sn having a melting point higher than that of Zn on the surface of the Zn plating layer in order to prevent evaporation of Zn. It is complicated and disadvantageous in terms of cost. In Patent Document 4, an element that is more easily oxidized than Zn is contained in the Zn plating layer, and these oxide layers are formed on the surface of the Zn plating layer during hot pressing to prevent Zn evaporation. Although the technology has been disclosed, according to the study by the inventors of the present application, the formation of ZnO cannot be suppressed only by containing an easily oxidizable element in the plating layer, and the adhesion with the top coat film is poor. It was confirmed that this would be sufficient.
JP 2003-82436 A JP 2003-73774 A JP 2004-124207 A JP 2004-270029 A

  As described above, in the prior art, even if a hot-dip zinc-plated steel sheet having excellent corrosion resistance is used in the hot press process, Zn evaporation cannot be sufficiently prevented, or ZnO or the like peels off from the molten Zn-plated layer. A high-strength, high-tensile steel processed member with sufficient adhesion after coating (corrosion resistance after coating) cannot be obtained. Therefore, in the present invention, it is an object to provide a hot-pressed hot-dip Zn-plated steel sheet capable of improving such problems and providing a high-strength, high-tensile steel processed member having good adhesion after coating. It is said.

The present invention is a hot-pressed hot-dip Zn-plated steel sheet that is processed after being heated to a temperature equal to or higher than the Ac ₃ point of the steel base material, and is Al powder and / or Al-based alloy powder 40 that can be melted during the heating. It is characterized in that a film composed of ˜98 mass% and 2 to 60 mass% of organic resin is formed on the surface of the hot-dip Zn plating layer with a thickness of 3 to 15 μm. More preferably, the coating contains 60 to 95% by mass of Al powder.

In the case where the hot-dip Zn plating layer is an unalloyed hot-dip Zn plating layer, the adhesion amount is preferably 50 g / m ² or more, and in the case of an alloying hot-dip Zn plating layer, the adhesion amount is 30 g / m. It is preferably ² or more.

Even if it is heated to a temperature of Ac ₃ or higher in the hot pressing step by a low-cost means by simply forming an organic resin film having Al (or Al-based alloy) powder on the hot-dip Zn plating layer, the hot-dip Zn plating layer Evaporation could be suppressed, and adhesion to the substrate and the top coat could also be improved. Therefore, the hot-dip hot dip galvanized steel sheet which can make the product excellent in corrosion resistance was able to be provided.

The greatest feature of the hot-pressed hot-dip hot-dip galvanized steel sheet of the present invention is that a coating comprising 40 to 98% by mass of Al powder and / or Al-based alloy powder and 2 to 60% by mass of organic resin is formed on the Zn plating layer. It is in the place where it is formed with a thickness of 3 to 15 μm. In hot pressing, the steel must be heated to the Ac ₃ point or higher of the base metal, but since this temperature range is near the boiling point of Zn (907 ° C. under atmospheric pressure), the Zn is evaporated and the plating layer deteriorates. End up. However, in the present invention, since the coating made of the Al powder and / or Al-based alloy powder and the organic resin is formed on the Zn plating layer, when heated during hot pressing, the Al powder and / or Al-based As the alloy powder melts, the organic resin burns and disappears, so that a layer derived from Al and / or an Al-based alloy is formed on the surface of the Zn plating layer, thereby preventing evaporation of Zn. Moreover, the oxidation of the Zn plating layer could be stopped on the outermost surface. Furthermore, the layer formed from the said membrane | film | coat turned out that adhesiveness with a plating layer and the top coat film formed after hot press is favorable. For example, after heating to Ac ₃ point or more, even if an attempt is made to peel the formed layer with a tape, no change is observed in the appearance in the example of the present invention. When it is formed on one surface and is peeled off with a tape, this ZnO layer is peeled off immediately and the lower plating layer appears.

The Al and / or Al-based alloy-derived layer formed by heating in the present invention is formed in close contact with the Zn plating layer, and includes ZnO, Al ₂ O ₃ , Zn-Al alloy (Al-based alloy). It was found that the layer was a mixture of other alloy components. That is, each of ZnO, Al ₂ O ₃ , and Zn—Al alloy forms a discontinuous layer, and these are present on the surface layer in close contact with the Zn plating layer. And Al is not added to the plating layer, but is present in the upper layer of the plating layer from the beginning, that Zn and Al are easy to make an alloy, Al is more oxidizable than Zn, As a result of the combined effects of the ZnO layer being formed discontinuously, the Al and Al-based alloy-derived layer being formed in close contact with the plated layer, etc., this Al and Al-based alloy-derived layer Presence of Zn is considered to suppress the evaporation of Zn and improve the adhesion between the layer and the Zn plating layer or the top coat film formed after hot pressing.

  In order to sufficiently obtain such an adhesion improving effect, it is necessary to adjust the amount of the Al powder, the Al-based alloy powder and the organic resin, and the film thickness. The Al powder and / or the Al-based alloy powder is 40-98 mass%, and organic resin shall be 2-60 mass%. When the Al powder and / or Al-based alloy powder is less than 40% by mass, it is difficult to suppress the evaporation of Zn. A more preferred lower limit is 60% by mass. When the Al powder and / or Al-based alloy powder exceeds 98% by mass, the amount of the organic resin is too small and the powder is easily detached from the film, which is not preferable. A more preferred upper limit is 95% by mass. The film thickness is 3 to 15 μm. If it is 3 μm or less, the amount of Al powder and / or Al-based alloy powder may be insufficient, and the coating workability is inferior. A thicker film is preferable from the viewpoint of suppressing the evaporation of Zn and suppressing the formation of ZnO. However, when the thickness exceeds 15 μm, these effects are almost saturated and the cost is increased, so the upper limit of the film thickness is set to 15 μm. A more preferable film thickness is 3 to 8 μm.

  If the average particle size of the Al powder and / or the Al-based alloy powder is too large compared to the film thickness, it is a cause of generation of powder, and therefore it is preferably 20 μm or less, more preferably 15 μm or less. The lower limit is not particularly limited, but the cost increases as the particle size decreases, so it is preferably 1 μm or more, more preferably 3 μm or more. The shape is not particularly limited, and is not particularly limited to a spherical shape, a pulverized indefinite shape, a scale shape, or the like.

An Al-based alloy powder can be used in place of (or in addition to) Al (melting point: 660 ° C.), but Al-Si, Al-Mg, Al-Mn, Al-Fe can be used as the Al-based alloy powder. Can be used. However, since these Al-based alloys must be melted at the heating temperature at the time of hot pressing (Ac ₃ points or more of the steel base material: usually 800 to 1000 ° C.), Mg having a melting point lower than that of Al (648.8 ° C.). ) Is not particularly limited, but Si (mp: 1410 ° C.) should be 40% by mass or less, Mn (mp: 1240 ° C.) should be 25% by mass or less, and Fe (mp: 1540 ° C.) should be 10% by mass or less. There is.

  The film contains an organic resin. The organic resin is not particularly limited because it burns and disappears by heating during hot pressing. For example, acrylic resin, polyester resin, melamine resin, phenol resin, epoxy resin, urethane resin, alkyd resin Any resin can be used as long as it is a known resin as a binder resin for paints, such as polyolefin resin. In view of easiness of disappearance, those not containing a benzene ring or N atom are preferable, and inexpensive acrylic resins, polyolefin resins and the like are preferable.

  In order to form a film composed of Al powder and / or Al-based alloy powder and an organic resin on a Zn plating layer, both are mixed well to form a paint, and coating is performed using a known coating means. That's fine. In this case, the form of the paint may be any of a solution type using an organic solvent as a medium, an emulsion type of an aqueous medium, and a solventless type, but an emulsion type or a solventless type is desirable environmentally. After coating, drying may be performed according to the medium.

The hot-dip Zn-plated layer in the hot-dip Zn-plated steel sheet of the present invention may be either a normal, ie, non-alloyed (non-alloyed) hot-dip Zn-plated layer or an alloyed hot-dip Zn-plated layer. However, considering the corrosion resistance, it is preferable to secure an adhesion amount of 50 g / m ² or more for the non-alloyed hot-dip Zn plating layer and 30 g / m ² or more for the alloyed hot-dip Zn plating layer. The upper limit of the adhesion amount is not particularly limited, but is preferably 150 g / m ² or less in any case in consideration of cost and appearance. The conditions for hot-dip Zn plating are not particularly limited, and may be performed under known conditions using a normal Zn-Al plating bath. The conditions for alloying with Fe are not particularly limited, and for example, alloying treatment may be performed under known conditions using a gas heating furnace or an induction heating furnace.

  As the base steel base material, a well-known hardened steel which can become high strength and high tension by heating and rapid cooling during hot pressing is suitable. For example, C: 0.1 to 0.4 mass%, Mn: 0.3-2% by mass, Si: 1.0% by mass or less, Al: 0.2% by mass or less, P: 0.03% by mass or less, S: 0.03% by mass or less, Ti: 0.03% by mass %, Steels that may contain other known elements and other inevitable impurities that are actively added as hardened steels are suitable.

The hot-dip hot-dip galvanized steel sheet of the present invention is processed while being rapidly cooled in a mold after being heated to a temperature not lower than the Ac ₃ point of the steel base material. After the processing, usually, a known base treatment such as a phosphate treatment is performed, and a top coat film is formed by an electrodeposition coating method or the like to be commercialized.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. It is also possible to implement, and they are all included in the technical scope of the present invention. In the following examples, “%” and “part” mean “% by mass” and “part by mass” unless otherwise specified.

Experimental Example 1 (Nos. 1 to 17)
Steel plate with a thickness of 0.8 mm (C: 0.20%, Mn: 1.2%, Si: 0.1%, Al; 0.03%, S: 0.01%, P: 0.01%, The surface of Ti: 0.01%, the balance Fe) was subjected to Zn plating with an adhesion amount described in Table 1 by a hot dipping method. These plating layers contain Al as shown in Table 1. GI remains as hot-dip Zn plating, and GA is alloyed at 600 ° C. for 30 seconds.

  Next, the amount of Al powder having an average particle size of 10 μm shown in Table 2 (granular; atomized powder made of Yamaishi Metal) is well used as an acrylic resin emulsion (“Bind Serum (registered trademark)” manufactured by Mitsui Chemicals). The mixture was mixed to prepare an Al powder-containing acrylic resin emulsion, which was coated on the molten Zn plating layer so as to have a dry film thickness shown in Table 1 by the bar coating method, and dried at 90 ° C. for 60 seconds.

Subsequently, atmospheric heating was performed at 900 ° C. for 1 minute. Here, the first tape peel resistance test was performed. This test is a test for evaluating the adhesion between the layer formed by heating at Ac ₃ points or more and the plating layer. Specifically, after the above heating, a 24 mm wide Nichiban tape (product number: “CT405A-24”) was manually affixed to the surface of the sample that had been air-cooled to room temperature (25 ° C.) and immediately removed. The degree of change in appearance was visually observed. If there is a yellowish ZnO layer on the surface, and this ZnO layer is peeled off due to tape peeling and a silver gray layer (Zn plating layer) is exposed, the appearance changes, and no matter what the tape is peeled off. The case where there was no change in the appearance and color tone was marked as ◯.

  Next, the zinc-plated steel sheet heated to the atmosphere at 900 ° C. for 1 minute is subjected to ordinary zinc phosphate treatment using “Palbond L3020” manufactured by Nihon Parkerizing Co., Ltd. System electrodeposition paint “Powernicks Excel 1100” (manufactured by Nippon Paint Co., Ltd.) was electrodeposited under 200V energization and baked at 150 ° C. for 20 minutes to form a top coat film having a thickness of 20 μm.

  A second tape peel resistance test was performed on this sample in order to evaluate adhesion and corrosion resistance with the top coat film. In this test, a crosscut was applied with a load of 500 g from the top coat film side, immersed in a 5% sodium chloride aqueous solution at 55 ° C. for 10 days, and then the Nichiban tape was applied to the crosscut by hand and immediately removed. The case where the peeling width of the coating film from the cross cut exceeded 4 mm was evaluated as x, 3-4 mm as Δ, and less than 3 mm as ○. The evaluation results are shown in Table 1.

  As is clear from Table 1, even when Al is contained in the Zn plating layer as in the prior art described in Patent Document 4, the steel plate having only the Zn plating layer (No. 1, 3, 5, 11) Then, after heating to 900 ° C., yellowish ZnO was generated on the entire surface of the plating layer, resulting in poor tape peel resistance. Of course, the tape peel resistance after immersion in salt water was also poor. On the other hand, among the examples in which the resin film of the present invention was laminated, No. 1 was used. 2 or No. No. 4, because the amount of plating adhesion is less than the preferred range, the tape peel resistance after immersion in salt water is slightly inferior, but the tape peel resistance after heating is good, and the formation of a ZnO layer that is easy to peel off can be suppressed. I understand that. No. In No. 6, the amount of Al was insufficient. In No. 12, since the film thickness was small, the effect of Al was not fully exhibited. Others are examples in which a resin film that satisfies the specified requirements of the present invention was formed, so that a film that can withstand tape peeling resistance after heating is formed, and adhesion to the top coat film and corrosion resistance are improved. It was confirmed that it was excellent.

  In FIG. 16 (invention example), the SEM image of the sample cross section after heating at 900 ° C. for 1 minute, FIGS. 2 to 4, Zn by EPMA (“JXA-8100” manufactured by JEOL Ltd.) of the same part as FIG. Mapping images of O and Al are shown. In FIG. 11 (Comparative Example) SEM images of the sample cross section after heating at 900 ° C. for 1 minute, and FIGS. 6 to 8 show mapping images of Zn, O, and Al by EPMA in the same part as FIG. The magnification is 1000 times. The lower side of the figure is a base steel plate.

  2 and 3, it can be seen that the high concentration portions of Zn and O are discontinuous. Moreover, it turns out that it is continuous from a base steel plate to a surface layer, a space | gap etc. do not exist, and the surface layer formed after the heating is closely_contact | adhered with the lower plating layer. However, no. In Comparative Example 11, since Zn and O are present in a high concentration in the surface layer, it can be seen that the ZnO layer is formed on the entire surface. Further, on the lower side of this ZnO layer, voids were observed in some places between the plated layer and the poor adhesion between the ZnO layer and the plated layer could be confirmed.

No. 16 is an SEM image of a sample cross section after heating at 900 ° C. for 1 minute at 16 (invention example). It is Zn mapping image by EPMA of the same part as FIG. It is O mapping image by EPMA of the same part as FIG. It is Al mapping image by EPMA of the same part as FIG. No. 11 is a SEM image of a sample cross section after heating at 900 ° C. for 1 minute at 11 (Comparative Example). It is Zn mapping image by EPMA of the same part as FIG. It is O mapping image by EPMA of the same part as FIG. It is Al mapping image by EPMA of the same part as FIG.

Claims (3)

A hot-pressed molten Zn-plated steel sheet to be processed after being heated to a temperature of Ac ₃ point or higher of the steel base material, and Al powder and / or Al-based alloy powder 40-98 mass% that can be melted during the heating. And a hot-pressed hot-dip Zn-plated steel sheet, wherein a film comprising 2 to 60% by mass of an organic resin is formed on the surface of the hot-dip Zn plating layer in a thickness of 3 to 15 μm. The hot-dip hot-dip galvanized steel sheet according to claim 1, wherein the hot-dip Zn plated layer is a non-alloyed hot-dip Zn plated layer and has an adhesion amount of 50 g / m ² or more. The hot-pressed hot-dip Zn-plated steel sheet according to claim 1, wherein the hot-dip Zn-plated layer is an alloyed hot-dip Zn-plated layer and has an adhesion amount of 30 g / m ² or more.

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