JPH04325665A - Galvanized steel sheet excellent in press formability and its production - Google Patents

Abstract

PURPOSE:To improve the press formability of a galvanized steel sheet by regulating the surface roughness of a galvanized steel sheet to a specified value or below and thereafter forming an oxidized film essentially consisting of ZnO on the surface by oxidizing treatment. CONSTITUTION:The surface roughness Wca of a steel sheet subjected to galvanizing or galvannealing is regulated <=0.6mum, and after that, an oxide of 20 to 3000mg/m<2> (per face) essentially consisting of ZnO is formed on the surface of the plating by a method of electrolytic oxidation, immersing oxidation, coating oxidation or the like. The galvanized steel sheet having press formability not inferior to that of a cold rolled steel sheet can be manufactured.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a galvanized steel sheet with excellent press formability.

0002

BACKGROUND OF THE INVENTION It is already known that the press formability of galvanized steel sheets is inferior to that of cold rolled steel sheets. The reason for this is thought to be that the plating layer restrains the deformation of the steel base. Furthermore, as a means for improving the press formability of galvanized steel sheets, a method of applying various high-viscosity lubricating oils to the surface of galvanized steel sheets has been widely adopted. However, this method requires the work of applying lubricating oil before press molding and the work of degreasing after press molding. Therefore, there are problems such as not only increasing costs but also worsening the environment of the press workplace. As a method to solve this problem, as in JP-A-62-185883, the surface of a galvanized steel sheet with a soft plating layer is coated with chromium oxide or Fe-Zn to harden the surface. It is disclosed to avoid mold galling during press molding. Also, a method of improving press formability by specifying the regularity of the roughness pattern, as disclosed in Japanese Patent Application Laid-Open No. 63-33591. Japanese Patent Publication No. 02-2748
No. 53 discloses that press formability is improved by defining roughness parameters Ra and Wca to specific values. However, in order to avoid the press breakage phenomenon that represents press formability,
A sufficient effect cannot be expected only by providing a hard film or specifying the roughness. Various types of galvanized steel sheets are widely used as industrially the most effective means to strengthen the rust prevention of various press-formable parts, and there are high expectations for galvanized steel sheets that have press-formability comparable to cold-rolled steel sheets. .

0003

SUMMARY OF THE INVENTION The present invention has been made to advantageously solve these demands, and provides a galvanized steel sheet that can significantly improve press breakage during press forming.

0004

[Means for Solving the Problems] The features of the present invention are as follows: (1) Wca, one of the surface roughnesses of the galvanized steel sheet, is 0.6μ or less, and ZnO oxide is present on the surface of the plating layer.
Galvanized steel sheet with excellent press formability consisting of 0 to 3000 mg/m2 (per side). (2) The zinc plating with excellent press formability according to claim 1, characterized in that an oxide mainly consisting of zinc is produced on the surface of the plating layer of the galvanized steel sheet by electrolytic oxidation, immersion oxidation, or coating oxidation treatment. Method of manufacturing steel plates. (3) Claim 1 characterized in that the surface of the plating layer is in a solid phase state or that after the surface of the plating layer is alloyed, it is heated to form an oxide film mainly composed of ZnO.
The present invention provides a galvanized steel sheet with excellent press formability and a method for manufacturing the same. That is, breakage of the material that occurs during press forming occurs when the breakage strength of the material exceeds the inflow resistance force of the material. The inflow resistance force of the material is composed of the deformation resistance of the material and the sliding resistance force generated between the die of the mold and the wrinkle holding plate. Therefore, in order to reduce the material inflow resistance force, it is effective to reduce the sliding resistance force. This sliding resistance force occurs when there is relative slippage at the part where the mold and the material come into contact. The sliding resistance force is composed of the resistance force when the convex part on the material surface is crushed and the frictional force at the part where the mold and the material come into contact. It is known that the former surface deformation resistance can be reduced by smoothing or hardening the surface. In particular, regarding the surface morphology, it is said that a low surface Wca is effective, but as mentioned above, it is not possible to obtain a low sliding resistance equivalent to that of a cold-rolled steel sheet just by lowering the surface Wca. .

On the other hand, regarding the latter frictional force, it is well known that the presence of an oil film at the interface where the material and the mold come into contact can significantly reduce the frictional force. Usually, low-viscosity oil is used for press molding, which also serves as primary rust prevention. With low viscosity oil, in cases such as press molding where the contact surface pressure between the mold and the material is relatively high, the oil film is easily destroyed and direct contact between the mold and the material occurs, increasing frictional force. . In such a state, the ability to retain the oil film is important in order to suppress the increase in frictional force. From this point of view, detailed comparative experiments and research between cold-rolled steel sheets and galvanized steel sheets revealed that cold-rolled steel sheets have significantly superior oil film adhesion, while galvanized steel sheets have significantly better oil film adhesion. It was found that the adhesion was significantly poor. The cause of this is largely dependent on the amount of oxide film on the surface.In the case of cold-rolled steel sheets, the surface is sufficiently coated with iron oxide and the oil film is tightly adhered to it. In the case of galvanized steel sheets, oxides exist only discretely, and the adhesion of oil to surfaces where oxides are not present is significantly reduced.Therefore, the surface of galvanized steel sheets must be uniformly coated with oxides. It has been found that sliding resistance is significantly improved by increasing the surface area on which oil film adhesion is ensured. However, while the oxide formed on the surface of a galvanized steel sheet is Zn-based, in the case of a cold-rolled steel sheet, it is made of iron-based oxide. The difference in film components between the two appears as a difference in the adhesion of the oil film. Although the reason is not clear, Zn-based oxides have relatively low adhesion compared to iron-based oxides. Therefore, the sliding resistance equivalent to that of a cold-rolled steel sheet cannot be obtained only by applying oxides to the surface of a galvanized steel sheet. In a galvanized steel sheet coated with such a Zn-based oxide, the above-mentioned low Wc on the surface
It has been discovered that, for the first time, it is possible to achieve a sliding resistance comparable to that of a cold-rolled steel sheet when the effects of a-forming are combined.

[0006] Galvanized steel sheets can be manufactured by various methods such as hot-dip plating, electroplating, vapor deposition plating, and thermal spraying, and the plating compositions include pure Zn, Zn and Fe, and Zn. and Ni, Zn and Al, Zn
Zn, such as Mn and Mn, are the main components, and contain one or more alloying elements and impurity elements to improve various functions such as corrosion resistance, and also contain ceramic fine particles such as SiO2, Al2O3, oxides such as TiO2, and organic There are steel sheets in which polymers are dispersed in the plating layer, and there are those with a single composition in the thickness direction of the plating layer, and those with a composition that changes continuously or layered. Furthermore, in multi-layer plated steel sheets, the top layer may be Fe or Some materials include Ni as a main component and various alloying elements such as Zn and P. For example, hot-dip galvanized steel sheets, iron-zinc alloyed hot-dip galvanized steel sheets, hot-dip galvanized steel sheets made of alloys such as aluminum and iron containing mainly zinc, and alloyed hot-dip galvanized steel sheets in which only the lower layer is alloyed in the cross-sectional direction of the plating layer. Coated steel sheet (generally referred to as half-alloy), coated steel sheet consisting of an iron-zinc alloyed hot-dip galvanized layer on one side and a hot-dip galvanized layer on the other side, and zinc, iron, and nickel coated on these plating layers by electroplating, vapor deposition plating, etc. Or electrogalvanized steel sheets, electroplated steel sheets with alloys such as zinc, nickel, chromium, etc. Furthermore, single alloy layer or multilayer alloy electroplated steel sheets, and organic coatings on these plating layers. There are galvanized steel sheets coated with films, steel sheets coated with zinc and zinc-containing metals, etc. Others, SiO2
There is a dispersion-plated steel sheet in which ceramic fine particles such as , Al2O3, TiO2 oxide fine particles, organic polymers, etc. are dispersed in zinc or zinc alloy plating.

[0007] The present inventors have determined that regardless of the type of galvanized steel sheet, by setting the surface Wca to 0.6μ or less and forming an oxide film in a certain range on the surface of the galvanized steel sheet, press We have discovered that the resistance force against material inflow during forming can be reduced to that of cold-rolled steel sheets. The lower Wca, which is one of the factors governing the inflow resistance of the material, lowers the sliding resistance, so the lower limit is not particularly defined, and the upper limit of 0.6μ is based on the oxides listed below. This is an essential condition for achieving sliding resistance comparable to that of cold-rolled steel sheets. Next, as the oxide formed on the surface of such a galvanized steel sheet, in addition to ZnO, the oxide may contain, for example, components such as component elements contained in the plating layer or compounds such as their oxides. good. Further, in electrochemical processing such as anodic oxidation, a component or a compound contained in a processing liquid may be included. The amount of such oxide produced is required to be 20 to 3000 mg/m2 as the amount of ZnO in the oxide (per one side). If the amount of ZnO is less than 20 mg/m2, there is no effect, and if it exceeds 3000 mg/m2, chemical conversion treatment becomes difficult, which is not preferable. Such oxides are, for example, 5
% iodine in methanol solution, and the extraction residue is dissolved in a mixed melt (boric acid: 1: sodium carbonate: 3:3), then dissolved in hydrochloric acid, and the amount of zinc analyzed by ICP is converted to ZnO, or by glow discharge spectroscopy. Oxygen integrated intensity value for 1 second from the outermost surface measured by method (GDS) is 1.0 to 10
VS (per side) allows you to accurately understand. The ZnO film as described above may be formed only on one side of the galvanized steel sheet, but in order to effectively reduce sliding resistance, it is preferable to press-form with the treated side facing the die. In order to obtain further effects, it is necessary to form a film on both sides. Therefore, in order to obtain the above-mentioned low Wca on the surface, it is naturally necessary to reduce the Wca on the surface of the roll of the cold-rolled steel sheet or the cold-rolled roll and the skin-pass roll of the galvanized steel sheet, which serve as the plated base plate. Suitable methods for processing the rolls for this purpose include electric discharge dulling, laser dulling, and shot blasting using a specially manufactured grid.

[0008] Next, as a method for producing oxides, after plating, the outermost surface of the plating is in a solid state, or if the plating layer is alloyed, after the alloying has progressed and the surface roughening has been completed. (a state in which the outermost surface is a single layer of ζ or δ1 phase or a mixture of both, or a state in which η phase is partially present in these),
The reaction with water at high temperatures ensures the formation of ZnO-based oxides. For example, in the case of continuous hot-dip galvanizing, there is an alloying furnace in the line, and by heating in this alloying furnace with dew point adjustment, and after alloying. In the case of treatment, it can be reliably produced by performing the reaction at high temperature while spraying air and water, spraying water vapor, or in a high dew point atmosphere in the same manner after completion of alloying. As for the heat treatment conditions, when the dew point is 0°C, heating is required at a heating temperature of 480°C or higher for 1 second or more. Of course, if the dew point can be maintained even higher, a sufficient amount of oxide can be obtained for low temperature or short time processing. Furthermore, as an oxide formation method,
Effective methods include immersion oxidation in an alkaline or acidic solution after plating, oxidation by roll coating or spraying these solutions, and electrochemical oxidation in an alkaline, neutral or acidic solution.

[0009]

[Examples] Examples of the present invention are shown in Tables 1 and 2 together with comparative examples.

0010

[Table 1]

[0011]

[Table 2]

Note 1): The type of plated steel sheet is AS:
Alloyed hot-dip galvanized steel sheet (heat-treated after hot-dip galvanizing, iron content 7-13%, remaining zinc), Gi: Hot-dip galvanized steel sheet, EG: Electrogalvanized steel sheet, EL (Excelite): Lower layer iron 15 %, residual zinc (area weight 20g/m2
), upper layer iron 85%, residual zinc (area weight 3 g/m2), electroplated steel sheet, ZL (zincrite): 12% nickel
, electroplated steel sheet with residual zinc (area weight 20 g/m2), A
S-E: 85% iron in the upper layer of AS, residual zinc (basis weight 3g/
ZL-E: The upper layer of ZL was electroplated with 85% iron and residual zinc (basis weight 3 g/m2), and the steel plates were all ordinary steel with a thickness of 0.80 mm. Note 2): In oxide film formation method A, Gi is heated at 350°C in the air after the surface becomes a solid phase after hot-dip galvanizing.
Heated for 10 seconds. AS has a plate temperature of 5 after hot-dip galvanizing.
Heating was performed at 30° C. for 12 seconds, and immediately after the surface roughening was completed, heating was similarly performed at 400° C. for 3 seconds. In either case, the oxide film thickness can be adjusted by water vapor spraying at the dew point (0) on the plate surface.
~100°C). Note 3): Oxide film forming method AA is a method in which an air/water spray method is used as a method of adjusting the dew point (0 to 100° C.) of the plate surface in Method A. Note 4): In oxide film formation method B, an oxide film was formed by immersing the plate in an acidic solution after plating. Note 5): In oxide film formation method C, an oxide film was formed by performing cathodic treatment in a bath containing zinc ions and an oxidizing agent after plating. Note 6): The sliding resistance coefficient was quantified under the following conditions. After the material is ultrasonically cleaned with an acetone solution and thoroughly degreased, a low viscosity rust preventive oil (manufactured by Parker Kosan Co., Ltd.: NOX) is applied.
RUST-530F40/product name) 1g/side
m2 was applied, and the sliding resistance coefficient was determined by the method shown in FIG. The measurement conditions are as follows. 1) Contact: Diameter 10φ, material SKD11 hardened material 2) Applied load (P): 1 kgf 3) Contact moving speed: 2.8 mm/min4)
Traveling distance of the contact: 80 mm The sliding resistance force is measured via the load cell 1 attached to the contact as shown in Fig. 1. The tool 3 is configured to be able to move with a pressure load 2 applied to the workpiece 4 by the tool 3. An example of the measurement results is shown in FIG. The sliding resistance varies with the sliding distance as shown in the figure, depending on the friction condition of the contact surfaces. Here, the sliding resistance coefficient (R) was defined by the following formula by taking the average of the maximum and minimum sliding resistance force (F: kgf).
R=F/P Note 7) Measurement of ZnO in oxide film Dissolve only the plating layer with a 5% iodine methanol solution, melt the extraction residue with a mixed flux (boric acid: 1 part, sodium carbonate: 3 parts), and then dissolve it in hydrochloric acid. The amount of zinc analyzed by ICP was converted to ZnO. Note 8) Oxygen intensity measurement in oxide film The value of the integrated intensity of oxygen for 1 second from the outermost surface measured by glow discharge spectroscopy (GDS). Note 9) Surface waviness (Wca) is JIS B061
As defined in 6.

[0013]

[Effects of the invention] By doing so, it is possible to significantly reduce the resistance force of material inflow during press forming, and the phenomenon of defective breakage of molded products can be reduced to the same level as that of cold-rolled steel sheets, and productivity can be greatly improved. . Further, excellent effects such as the ability to significantly improve the yield of press-formed products can be obtained.

[Brief explanation of the drawing]

[Fig. 1] An explanatory diagram showing an example of sliding measurement,

FIG. 2 is an explanatory chart showing an example of sliding results.

[Explanation of symbols]

1 Load cell, 2　Pressure load, 3. Tools, 4 Work material, 5 Motor.

Claims (1)

[Scope of Claims] [Claim Steel 1] The surface roughness Wca of the galvanized steel sheet is 0.6 μ or less, and ZnO oxide is present on the surface of the plating layer.
Galvanized steel sheet with excellent press formability consisting of 0 to 3000 mg/m2 (per side). 2. The method according to claim 1, wherein an oxide mainly containing zinc is produced on the surface of the plating layer of the galvanized steel sheet by electrolytic oxidation, immersion oxidation or coating oxidation treatment. Method of manufacturing galvanized steel sheet. 3. The method according to claim 1, wherein the surface of the plating layer is in a solid state or the surface of the plating layer is alloyed and then heated to form an oxide film mainly composed of ZnO. A method for producing galvanized steel sheets.