JP6449642B2 - Manufacturing method of rust-proof steel sheet for automobiles - Google Patents

Description

  The present invention relates to a method for producing a rust-proof steel sheet for automobiles.

  Car bodies and parts for automobiles are required to have high rust prevention performance. As a rust-proof steel plate for automobiles, a Zn-based plated steel plate represented by a Zn-Fe alloy-plated steel plate or a Zn-Ni alloy-plated steel plate is mainly used (for example, see Patent Document 1). The rust-proof steel sheet has sufficient in-plane rust-proofing power. However, also in the rust-proof steel plate, the interface between the plating and the steel plate is exposed at the edge portion, and the steel plate exposed portion due to the melting of Zn by welding is generated at the weld portion, so There may be a portion where becomes low. Therefore, a method called post-plating is used in which plating is performed so as to cover the entire member after the member is processed.

JP-A 63-93691

  On the other hand, in sheet metal parts, in order to achieve both formability and strength, a BH steel plate (Bake Harding steel plate) has been used. However, when post-plating is performed using a BH steel sheet, there is a problem that cracks occur in the plating layer, and further repelling occurs on the painted surface when coating is performed.

  The present invention solves the above problems, and even when a BH steel sheet is used, it suppresses the occurrence of defective appearance such as cracking of the plating layer and repellency of the coated surface, and the appearance is good and prevented. It aims at providing the manufacturing method of the rust-proof steel plate for motor vehicles which the rust power improved.

In order to achieve the above object, a method for producing a rust-proof steel sheet according to the present invention is a method for producing a rust-proof steel sheet for automobiles using a BH steel sheet having a Zn layer on the surface thereof after the processing. the BH steel sheet after performs acid etching process, the post-plating and drying in this order, in the acid etching step, performed acid etching with proper conditions of Zn layer removal present in BH steel sheet surface after the machining, the In the post-plating step, plating is performed so that the thickness of the plating layer is in the range of 8 to 50 μm, and in the drying step, the drying temperature is 120 ° C. or less.

  According to the present invention, even when a BH steel sheet is used, the occurrence of poor appearance such as cracking of the plating layer and repellency of the coated surface is suppressed, and the appearance is good and the rust prevention power is improved. The manufacturing method of a rust-proof steel plate can be provided.

FIG. 1 is a schematic diagram for explaining the state of plating in the method for producing a rust-proof steel sheet for automobiles of the present invention. FIG. 2 is a diagram for explaining appropriate conditions in the acid etching process. FIG. 3 is a schematic diagram for explaining the state of plating of a BH steel sheet when post-plating is performed by a conventional process.

  Hereinafter, embodiments of the present invention will be described in detail. However, the present invention is not limited or limited to the following description.

  The manufacturing method of the antirust steel plate for motor vehicles of this invention is a process which can be used suitably when post-plating a BH steel plate. BH steel sheet is a steel sheet that is low in strength and good in formability, but that produces strain aging due to the heat of seizure coating after press forming and obtains a high yield point. Formability, strength, dent resistance It has the characteristic that it is excellent in property. Here, “dent” refers to a phenomenon in which a steel plate is dented due to a collision of a stone or the like that jumps while the automobile is running.

  FIG. 3 is a schematic diagram for explaining the state of plating of the BH steel sheet when post-plating is performed by a conventional process.

When post-plating is performed, an acid etching process mainly for the purpose of removing oxide films and surface adhesion residues is performed as a pretreatment. When acid etching treatment was performed on the BH steel plate 10 under the same conditions as other steel plates, as shown in FIG. 3A, the phenomenon that Fe grain boundaries on the surface of the BH steel plate 10 were dug occurred, and this occurred. The present inventor found that H ₂ and foreign matter 12 are fixed to the recess 11. When the plating process is performed after the acid etching process (FIG. 3B) and dried, as shown in FIG. 3C, the fixed H ₂ and the foreign matter 12 break out of the plating layer 20 and are ejected. At the same time, it was found that the foreign matter 12 was fixed on the surface of the plating layer 20.

  FIGS. 3D and 3E show a state during the coating process of the surface of the plating layer 20 to which the foreign material 12 is fixed. FIG. 3D shows a state in which an electrodeposition coating film 31 is formed by performing electrodeposition coating on the surface of the plating layer 20. Electrodeposition coating cannot be performed at the portion where the foreign matter 12 on the surface of the plating layer 20 is present, and the coating repellent 41 is generated. FIG. 3E shows a state in which an intermediate coating and a top coating are performed after the electrodeposition coating. In the portion where the paint repellent 41 is generated by the electrodeposition coating, the paint repellent 41 is also generated in the middle coating film 32 and the upper coating film 33.

  On the other hand, in FIG. 1, the schematic diagram explaining the state of the plating of the BH steel plate in the manufacturing method of the antirust steel plate for motor vehicles of this invention is shown.

  First, an acid etching process is performed on the processed BH steel sheet 10 as a pretreatment for post plating (acid etching process). At this time, acid etching is performed under appropriate conditions for removing the Zn layer present on the surface of the processed BH steel sheet. A Zn layer is formed on the surface of the BH steel plate before processing, but an exposed portion without the Zn layer exists at the end or joint of the BH steel plate after processing such as cutting or arc welding. By removing the remaining Zn layer before post-plating and coating on the processed BH steel sheet, the plating layer and the coating layer can be formed uniformly.

The appropriate conditions for removing the Zn layer refer to conditions under which the Zn layer present on the surface of the BH steel sheet is removed without excess or deficiency. As shown in FIG. 2, the appropriate condition is shown in a graph in which the etching processing time t is plotted on the horizontal axis, the temperature T of the etching processing solution is plotted on the vertical axis, and the conditions under which Zn having a predetermined thickness is removed are plotted with diagonal lines. It can be expressed by the conditions of the assigned area. This area refers to a condition in which the Zn layer is removed and the Fe grain boundary on the surface of the BH steel sheet is not dug even if the Zn layer to be removed has thickness unevenness. The acid etching step is performed by immersing the processed BH steel sheet in acid. In the acid etching step, for example, when the basis weight of the Zn layer is 45 g / m ² , the processed BH steel sheet is placed in an aqueous HCl solution (38% HCl) at a temperature of 32 ° C. or lower and a concentration of 300 to 450 mL / L. It can be performed under the condition of immersing within a range of 5 to 4.0 minutes. At this time, the acid concentration, the acid temperature, and the immersion time are combined so as to satisfy the above-mentioned appropriate conditions. For example, when the acid concentration is constant, the immersion time is shortened as the temperature increases.

When the acid etching step is performed under the appropriate conditions, as shown in FIG. 1A, the BH steel plate surface 10 becomes flatter than the conventional one (see FIG. 3A), and the H ₂ and foreign matter 12 Sticking is reduced.

  In the present invention, it is preferable to perform a degreasing step before the acid etching step. By performing the degreasing process, oil stains and the like adhering to the surface of the BH steel sheet can be removed by processing, so that an even layer can be formed in the subsequent process. The degreasing step is performed by immersing the processed BH steel sheet in an alkaline bath such as NaOH aqueous solution. The degreasing step can be performed, for example, under the condition that the processed BH steel sheet is immersed in a NaOH aqueous solution having a concentration of 50 to 60 g / L and a temperature of 64 to 65 ° C. within a range of 10 to 18 minutes. A part of the Zn layer to be removed is also removed in the degreasing step. For example, a Zn layer having an initial thickness of 8 μm has a thickness of about 6 μm after the degreasing step.

  After the acid etching step, a post plating step is performed (FIG. 1B). In the post-plating step, plating is performed so that the thickness of the plating layer 20 is in the range of 8 to 50 μm. The thickness of the plating layer 20 is preferably 15 μm or more, although the preferred thickness varies depending on the component to be plated. By making the plating layer 20 thick, the plating layer 20 becomes difficult to break.

A drying process is performed after a post-plating process (FIG.1 (c)). In the drying process, the drying temperature is 120 ° C. or lower, and the drying conditions are lower than the conventional drying temperature of 180 ° C. The drying temperature is preferably in the range of 50 ° C to 120 ° C. Thus, by lowering the drying temperature, the plating layer 20 becomes difficult to break. Even when acid etching is performed under the conditions of the acid etching step in the present invention, it is difficult to completely eliminate H ₂ and foreign matter 12 although the adhesion of H ₂ and foreign matter 12 is reduced. Therefore, in the drying process, the fixed H ₂ and the foreign matter 12 try to blow out through the plating layer 20 as in the case of the conventional conditions. However, in the present invention, since the thickness of the plating layer 20 is as thick as 8 to 50 μm and the drying conditions are gentle, H ₂ and foreign matter 12 stop in the middle of the plating layer and do not penetrate to the surface.

  FIG. 1 (d) shows the BH steel sheet 10 in a state where a coating process is performed after the drying process and a coating film 30 is formed. As shown in FIG.1 (d), the crack of the surface of the plating layer 20 and the coating repellency of the coating film 30 are not seen, and the rust-proof steel plate for motor vehicles which is favorable external appearance quality and improved rust prevention power is obtained.

  Thus, in the acid etching step, acid etching is performed under appropriate conditions for removing the Zn layer present on the surface of the processed BH steel sheet, and in the post plating step, the thickness of the plating layer is in the range of 8 to 50 μm. By performing plating and setting the drying temperature to 120 ° C. or lower in the drying step, post-plating and coating can be performed on the BH steel sheet. Therefore, it is possible to improve moldability and strength in parts and vehicle bodies that require rust prevention and coating. Moreover, by using a BH steel plate, cost reduction and weight reduction by gauge down are also possible.

DESCRIPTION OF SYMBOLS 10 ... BH steel plate 11 ... Concave part 12 ... Foreign material 20 ... Plating layer 30 ... Paint film 31 ... Electrodeposition film 32 ... Middle coating film 33 ... Top coating film 41 ... Paint repellent

Claims (1)

After processing, a method for producing a rust-proof steel sheet for automobiles using a BH steel sheet having a Zn layer on its surface ,
The BH steel sheet after the processing, performed acid etching process, the post-plating and drying in this order,
In the acid etching step, acid etching is performed under appropriate conditions for removing the Zn layer present on the surface of the processed BH steel sheet,
In the post-plating step, plating is performed so that the thickness of the plating layer is in the range of 8 to 50 μm,
In the drying step, the drying temperature is 120 ° C. or less,
A method for producing a rust-proof steel sheet.

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