JP2005226121A - Galvanized steel sheet having excellent press formability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a galvanized steel sheet having excellent press formability. <P>SOLUTION: Regarding the galvanized steel sheet in which the surface of galvanizing is provided with a phosphate film essentially consisting of zinc phosphate, the difference between the number of peaks PPI (0.080) in the case the cutoff in the peak count PPI of the surface roughness in the galvanized steel sheet is controlled to 0.080 μm, and the number of peaks PPI (0.318) in the case the cutoff in the peak count PPI is controlled to 0.318 μm is ≥40. The surface roughness Ra of the galvanized steel sheet satisfies 0.7 μm≤Ra≤1.7 μm. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a zinc-based plated steel sheet excellent in press formability used for applications such as automobile bodies.

  Conventionally, when press forming a steel sheet, press oil or lubricating oil containing wax or the like and having a relatively high viscosity is widely used. In addition, in order to enhance the rust prevention property, the use of a steel plate having a zinc-containing metal plated surface (hereinafter referred to as a zinc-based plated steel plate) formed by pressing or the like is increasing.

  However, the zinc-based plated steel sheet has a property that zinc contained in the plating layer is relatively soft and has a low melting point, so that the zinc easily adheres to the mold during press molding. With lubrication, it is difficult to prevent galling between the plating on the steel sheet and the mold.

  In recent years, in a manufacturing process of an automobile body or the like, press processing is performed at a higher speed than ever before in order to improve productivity and reduce manufacturing costs. In addition, the need to simplify the assembly process and reduce manufacturing costs by molding parts with complex shapes with an integrated press increases, and the above-mentioned conventional lubrication methods such as press oil and lubricating oil are increasingly used. It is difficult to respond to press molding.

  On the other hand, the use ratio of galvanized steel sheets to automobile bodies is increasing more and more for the purpose of improving rust prevention of automobile bodies, etc., and the above-mentioned complicated shape parts (hereinafter difficult to form parts) etc. Improvement of press formability is important. As an improvement method thereof, a technique for forming a phosphate film mainly composed of zinc phosphate on galvanizing is known. However, Patent Document 1 discloses that a zinc phosphate film, iron, cobalt, nickel, By using a zinc phosphate composite film containing at least one of calcium, magnesium, and manganese, the endothermic peak temperature due to dehydration of crystal water is shifted to the high temperature side to improve the heat resistance of the zinc phosphate composite film. Thus, it is disclosed that a zinc-containing plated steel sheet composite having excellent high-speed press formability can be obtained.

  Patent Document 2 discloses a technique for improving a bead sliding property under a high surface pressure of a phosphate-treated zinc-based plated steel sheet and providing a phosphate-treated zinc-based plated steel sheet having excellent workability. .

  Further, in Patent Document 3, the surface roughness Ra of the zinc-based plated steel sheet having an amorphous phosphate film, the adhesion amount Cp of the phosphate film, and the relationship between the two are specified in a specific range, thereby providing a flash plating apparatus. A technology for providing a galvanized steel sheet that can be manufactured without using special equipment such as the above and exhibits excellent lubricity is disclosed.

  However, in the zinc phosphate composite coating disclosed in Patent Document 1, the zinc phosphate composite coating (phosphate) is caused by strong shear deformation by a mold under high surface pressure and unbending deformation conditions in press molding of difficult-to-mold parts. In many cases, the film characteristics are not exhibited.

  Since the phosphate-treated zinc-based plated steel sheet disclosed in Patent Document 2 is characterized in that the phosphate has granular crystals by adding Mg, the region where Ra is low (0.2 μm ≦ Ra ≦ 0 .7 μm) is considered to have good press formability. In the case of such a steel plate, when the bead diameter is large and the sliding length is long, the soft zinc layer is deformed by sliding, and the surface is flattened at an early stage. As a result, Ra becomes low in the later stage of sliding, the micropool is not formed, the amount of oil retained is reduced, and micro adhesion between the mold and the steel sheet occurs, and galling is likely to occur.

  Furthermore, when the galvanized steel sheet disclosed in Patent Document 3 is used for difficult-to-form parts such as double beads, excellent pressability cannot always be obtained.

The prior art document information is described below.
Japanese Patent Laid-Open No. 07-138764 JP 2003-119568 A JP-A-11-334581

  An object of the present invention is to provide a zinc-based plated steel sheet excellent in press formability, in particular, a zinc-based plated steel sheet excellent in press formability that can be used for press forming on difficult-to-form parts.

  As a result of intensive investigations such as the surface shape of a zinc-based plated steel sheet, the inventors of the present invention have solved the problems described above. It discovered that a system plating steel plate was obtained. The present invention is based on this finding.

  Means of the present invention for solving the above problems are as follows.

  (1) In a zinc-based plated steel sheet having a phosphate coating mainly composed of zinc phosphate on the zinc-based plated surface, when the cutoff of the peak count PPI of the surface roughness of the zinc-based plated steel sheet is 0.080 μm The difference between the number of peaks PPI (0.080) and the number of peaks PPI (0.318) when the cutoff of the peak count PPI is 0.318 μm is 40 or more. Excellent galvanized steel sheet.

  (2) In the above (1), the surface roughness Ra of the zinc-based plated steel sheet is 0.7 μm ≦ Ra ≦ 1.7 μm.

(3) In the above (1) or (2), the amount of the phosphate coating deposited is 0.2 to 3 g / m ² .

  In the present invention, the peak count PPI indicates the number of peaks per inch (peaks per inch) in the roughness curve of the surface roughness defined by the US SAE standard. Figure 1 shows the roughness of the surface roughness related to the definition of PPI defined by the SAE J911-JUN 86 “SURFACE TEXTURE MEASUREMENT OF COLD ROLLED SHEET STEEL” for the SAE standard. The height curve is shown. That is, in FIG. 1, a constant reference level H is provided in both the positive and negative directions from the average line of the roughness curve, and after exceeding the negative reference level, 1 count is counted when the positive reference level is exceeded. This count is repeated until the evaluation length Ln is reached, and what is displayed by the counted number is defined as PPI. In the present invention, Ln = 1 inch (= 25.4 mm), and H (peak count level: the width between the average line of the roughness curve and the reference level) is 0.318 μm and 0.080 μm. Are described as PPI (0.318) and PPI (0.080), respectively.

  The surface roughness (arithmetic average roughness) Ra in the present invention is based on JIS B 0601-1994.

  The galvanized steel sheet according to the present invention has a predetermined roughness, so that even if it is press-molding with high surface pressure and bending back deformation, even if the film becomes flat, a large number of microscopic parts are formed on the flat part. The presence of the pool in a dispersed manner improves the oil holding capacity, and oil is supplied between the mold and the surface of the plated steel sheet during sliding, so that excellent press formability is exhibited.

  The galvanized steel sheet according to the present invention can be applied to press forming applications that cannot be handled by conventional press oil, lubricating oil, and phosphate coating technologies. It can be applied to difficult-to-mold parts such as molding, and it is possible to obtain high-quality press-molded products and improve productivity while reducing the cost of press molding. .

  Moreover, since the zinc-based plated steel sheet according to the present invention has good coating quality, it can also be used for coating applications after press molding or without press molding.

  The present invention will be described in detail below.

  The lubricating action of the phosphate coating itself is known. By forming a simple zinc phosphate film consisting of phosphoric acid and zinc on a galvanized steel sheet, the zinc phosphate film is lubricated in a deep drawing test for forming a simple shape that evaluates normal press formability. The press formability that can withstand practical use can be expressed by the action.

  However, in the conventional zinc phosphate coating, galling occurs in the bead portion of the difficult-to-mold part, which has a particularly high surface pressure, and the flow of the material is hindered, which may cause press cracks. The reason is considered as follows.

  When a phosphate film is formed on a galvanized steel sheet, for example, an electrogalvanized steel sheet, and press molding is performed at a high surface pressure, a strong shearing force is generated and the phosphate film is destroyed. This not only impairs the original lubricity of the phosphate coating, but also exposes a pure zinc layer at the damaged portion of the phosphate coating, causing zinc adhesion due to direct contact between the press die and zinc. It is thought that galling occurs due to this, and eventually press cracks occur.

  In particular, in press molding of difficult-to-mold parts, there are cases where the bead portion has not only a high surface pressure but also a mold having double bead portions called double beads. In a double bead die press, the same part is slid twice during press molding, so the coating is destroyed by the first bead passage, and the die and zinc are directly passed by the second bead passage. Zinc adhesion due to contact occurs, and press cracking is likely to occur.

  In the present invention, a zinc-based plated steel sheet having a phosphate coating mainly composed of zinc phosphate on the zinc-based plated surface is a steel sheet in which only fine irregularities are increased on the surface of the plated steel sheet. Even if the coating is deformed by passing the bead, it is not only flattened, but a large number of micropools derived from the fine irregularities are dispersed and present on the entire surface of the steel sheet, so that the oil retention amount is increased. Adhesion between the mold and zinc on the steel sheet surface can be suppressed during the passage of the beads. Further, even when the bead diameter is large and the sliding distance is long, the above-described micropool is formed and the oil retention amount is kept large, so that micro adhesion between the mold and the steel plate can be made difficult to occur.

  With reference to FIG. 2, the mechanism in which the zinc-based plated steel sheet of the present invention exhibits excellent press formability will be described. 2, each of (a) a zinc-based plated steel sheet of a comparative example (hereinafter also referred to as a comparative steel sheet) and (b) a zinc-based plated steel sheet of the present invention example (hereinafter also referred to as an inventive example steel sheet). Each of (1) roughness profile before sliding, (2) roughness profile after sliding, and (3) surface micropit distribution state after sliding are schematically shown. In addition, since the fine unevenness | corrugation which the phosphate film itself has is further finer than the fine unevenness | corrugation provided on the above-mentioned plated steel plate surface, in FIG. 2, the fine unevenness | corrugation which the phosphate film itself has is abbreviate | omitted. Yes.

  FIG. 2- (1) shows the roughness profile before sliding. In the figure, the straight lines indicated by H = 0.635, H = 0.318, and H = 0.080 are the peak count levels H = 0.635 μm, H provided in both positive and negative directions from the average line of the roughness curve. = 0.318 μm and H = 0.080 μm. The normal PPI count number is the same for both the comparative steel plate of (a) and the inventive steel plate of (b) at 25, but the parameters defined in the present invention, namely PPI (0.080) and PPI (0. 318), PPI (0.080) -PPI (0.318), the comparative steel plate of (a) is 0, and the inventive steel plate of (b) has many fine irregularities on the surface. , 60. The normal PPI is the number of peaks exceeding the peak count level H = 0.635 μm. In this specification, it is described as PPI (0.635). In the following description, PPI that does not describe H (peak level) is a count of all peaks that exceed H (peak level): 0.635 μm.

  Assuming that the zinc-based plated steel sheet having such a surface profile is slid by pressing, the surface is subjected to processing, so that the comparative example steel sheet (a) and the present invention example steel sheet (b) were subjected to equivalent deformation, Here, assuming that the convex portion of the surface protruding from the average line of the roughness curve has been deformed to the average line of the roughness curve, as shown in FIG. Since the zinc layer, which is the lower layer of the phosphate film, is soft, the surface is deformed and the surface layer portion is flattened. In the comparative example steel plate of (a), the distance between the flat portions after sliding is longer, but the distance between the flat portions becomes shorter because the present invention example steel plate of (b) has fine irregularities. Yes. The surface after sliding at this time is schematically represented as shown in FIG.

  In FIG. 2-(3), the comparative example steel plate of (a) has scattered micropits that are parts capable of holding oil, whereas the surface of the steel plate of the present invention example of (b) has fine irregularities. Since it remains after flattening, micropits derived from it remain dispersed over the entire surface. As a result, since the steel sheet of the present invention (b) is constantly supplied with oil between the mold and the surface of the plated steel sheet during sliding, even if the phosphate film is detached and the zinc layer is exposed, Adhesion between mold and zinc is suppressed. As a result, it is presumed that a zinc-based plated steel sheet having a phosphate film excellent in press formability that can withstand high surface pressure forming that could not be achieved by the prior art can be obtained.

  Normally, when PPI is measured, the number of PPI is H (peak count level) shown in FIG. 1: all peaks exceeding 0.635 μm are counted. In the case of severe press conditions, for example, when repeatedly subjected to sliding, the present invention improves the press formability by making fine irregularities present after being deformed, as shown by the mechanism described above. Focusing on the fact that it is possible to achieve the above problems, we measured the coefficient of friction in the state after flattening the surface by beading with a bead, and examined the peak count level that can express the optimal fine irregularities . As a result, the difference between the PPI (0.080) and the PPI (0.318): the distribution of the recesses after the flattening of the surface after the PPI (0.080) -PPI (0.318) is subjected to sliding. I concluded that I could express it properly. The optimum value of this value is 40 or more, more preferably 45 or more. If the difference between PPI (0.080) and PPI (0.318) is less than 40, the micro pool that can hold the oil is reduced, so that excellent press formability cannot be obtained.

  The surface roughness Ra of the galvanized steel sheet of the present invention is preferably 0.7 μm ≦ Ra ≦ 1.7 μm. When Ra is smaller than 0.7 μm, the oil storage capacity of the rust preventive oil becomes low, and a sufficient lubricity improving effect is not recognized. When Ra is larger than 1.7 μm, the degree of processing in temper rolling increases, and the workability of the original sheet decreases due to work hardening.

  The zinc-based plated steel sheet in the present invention is made of zinc or an alloy of at least one metal selected from zinc and other metals such as nickel, iron, aluminum, manganese, chromium, lead, and antimony, and unavoidable impurities. Selected from plated steel sheet.

  The phosphate film in the present invention may contain iron, cobalt, nickel, calcium, magnesium, manganese, etc. as metals other than zinc.

Although there is no restriction | limiting in particular in the adhesion amount of the said phosphate membrane | film | coat, It is preferable to form on the said galvanized steel plate with the adhesion amount of 0.2-3 g / m < ² >. If the adhesion amount of the phosphate film is less than 0.2 g / m ² , the press formability may be insufficient, and if the adhesion amount exceeds 3 g / m ² , phosphorus destroyed in the press die during press molding. Since the acid salt film is deposited and pushed into the surface of the galvanized steel sheet as a foreign substance, press scratches may easily occur.

  In addition, if the amount of phosphate coating is within the above range, coating quality such as coating adhesion, coating clarity, and post-coating corrosion resistance will not be impaired, and coating properties will be excellent. Also suitable for use in.

  The method for producing a galvanized steel sheet having the roughness specified in the present invention is not particularly limited. In the case of an electrogalvanized steel sheet, the roughness of the original plate (plating original plate) before plating and the roughness after plating. Since there is a certain correspondence relationship, pressure regulation conditions of the original plate before electrogalvanizing are controlled, and then electrogalvanizing and phosphating are performed to give the required roughness to the electrogalvanized steel sheet. It is preferable to do.

  For example, it is possible to control the pressure adjusting conditions of the plating original plate by temper rolling the plating original plate twice. In the first temper rolling, rolling is performed using a rolling roll having a large peak count PPI, and fine irregularities are imparted to the steel sheet surface. The second temper rolling is performed using a rolling roll having a surface roughness Ra larger and a smaller peak count PPI than the rolling roll used in the first temper rolling. The second temper rolling not only obtains the target surface roughness Ra of the steel sheet, but also the fine irregularities obtained by the first temper rolling remain in the non-contact portion between the roll and the steel sheet. Thus, a roughness profile having a large number of fine irregularities on the surface as shown in FIG.

  In order to make the peak count PPI within the range specified by the present invention after forming the electrogalvanized and phosphate coatings, the first temper rolling has a surface roughness Ra of 1.0 to 2.9 μm and a peak count PPI of The 225 to 600 electric discharge machining rolls and the second temper rolling use Ra 2.9 to 5.0 μm and the PPIs 100 to 225, respectively, and the elongation ratio at the end of the second temper rolling ( The temper rolling may be performed so that the total elongation ratio is 0.4 to 0.7%. The PPI is a count of all peaks exceeding H (peak level): 0.635 μm.

  When the roughness is imparted to the rolling roll, the surface of the roll is imparted with a roughness having a roughness profile corresponding to the roughness profile shown in FIG. You may use and give required roughness by one temper rolling.

After forming the phosphate film mainly composed of zinc phosphate on the surface of the zinc-based plated steel sheet, it is preferable to further form a lubricating oil layer thereon. It is preferable to apply and form the lubricant with an application amount of 0.2 to 3 g / m ² . The type of lubricating oil to be applied is not particularly limited, but is preferably selected from mineral oils having a relatively low melting point, natural fats and oils, synthetic ester oils, and waxes. These lubricating oils can be used containing various additives such as rust preventive additives and extreme pressure additives. If the application amount of the lubricating oil is less than 0.2 g / m ² , the press molding performance may be insufficient. If it exceeds 3 g / m ² , the press molding performance may reach saturation, which may be economically disadvantageous. .

  The present invention is specifically described by the following examples, but the scope of the present invention is not particularly limited by these examples.

1. Substrate (cold rolled steel sheet)
By controlling the pressure regulation conditions of the original plate before electrogalvanization, a steel plate having the surface roughness that is a feature of the present invention was obtained. In the present invention example, a predetermined roughness was obtained by adjusting the pressure of the original plate before plating twice. Specifically, the first temper rolling is performed using an electric discharge roll having a surface roughness Ra of 2.3 μm and a peak count PPI of 270, and the second temper rolling is performed with a surface roughness Ra. Is 3.4 and 4.0, and rolling is performed using an electric discharge machining roll having a peak count PPI of 180 to 220, and the elongation ratio (total elongation ratio) at the end of the second temper rolling is 0.4 to 0.00. The temper rolling was performed so as to be 7%. Further, for the comparative steel sheet, one temper rolling was performed using either the rolling roll used in the first temper rolling of the invention example or the roll used in the second temper rolling. Then, rolling was performed so that the elongation ratio was 0.5 to 0.7% (comparative example). Thus, the sample manufactured by changing the roughness by temper rolling was used as a substrate. The thickness of the substrate after the temper rolling is 0.8 mm, and the material is soft ultra-low carbon steel.

2. Electro-galvanizing 2-1 pretreatment After alkaline degreasing by electrolysis, it was pickled in an aqueous solution of 70 g / liter sulfuric acid for 5 seconds.
2-2 Electro-galvanization Using a liquid circulation type plating experimental apparatus, energization during plating was turned on and off on the assumption that plating was performed in a plurality of electrolytic cells on an actual line. The current density was 60 A / dm ² , the on-time of 1 second was repeated 15 times, and the plating amount was 50 g / m ² . The plating solution was prepared by adjusting a sulfuric acid acidic plating bath at 50 ° C. with zinc sulfate heptahydrate 400 g / liter and pH 1.4 with sulfuric acid. It was set to 1.5 m / s.

3. 3. Phosphate film formation treatment 3-1. Surface tone treatment Surface tone treatment solution preparen ZN (manufactured by Nihon Parkerizing Co., Ltd.), a commercially available titanium colloid, is diluted with water to a concentration of 3 g / liter, treatment temperature is 40 ° C., treatment time is 10 seconds, spray pressure is 1 kgf. The spray treatment was performed under the conditions of / cm ² .
3-2. Phosphate film forming treatment Each substrate subjected to the surface tone treatment was diluted with commercially available PB-PP100 (manufactured by Nippon Parkerizing Co., Ltd.) with water to a concentration of 50 g / liter, treatment temperature 57 ° C., treatment time 30 seconds. The phosphate film formation process was performed by the spray method.

  By performing electrogalvanization and phosphate film formation treatment according to the above, zinc-based plated steel sheets having phosphate films with different roughnesses were prepared. Table 1 shows the roughness of the galvanized steel sheet.

4). Evaluation Test Method The phosphate film weight, press formability, and coating quality of the zinc-based plated steel sheet were evaluated as follows.
4-1. Phosphate film weight The phosphate film weight was determined by the method described below.
(1) The weight (W1: g) of the test piece was measured in advance with a precision balance, and the test piece was placed in a deionized aqueous solution containing 20 g / liter of ammonium dichromate and 490 g / liter of 25% aqueous ammonia at room temperature for 15 minutes. Immersion and dissolved the phosphate crystals.
(2) This was washed with water to remove the ammonium dichromate aqueous solution remaining on the test piece and dried.
(3) The weight (W2: g) of the test piece was again measured with a precision balance, and the coating weight per unit area was calculated from the weight difference (W1-W2).
4-2. Roughness measurement Arithmetic mean roughness: Ra was measured based on JIS B 0601-1994. PPI was measured according to a method defined in the US SAE standard.

4-3. Press Formability The press formability of the treated galvanized steel sheet was evaluated by performing tests under the following conditions, which are tests simulating press forming with high surface pressure and bending back deformation.
4-3-1. Draw bead test A 30 mm x 250 mm test piece was coated with 1.5 g / m ² of Fuchs anticorrit PL3802 as a lubricating oil, a bead tip radius of 5 mm, a bead height of 3 mm, a pressing force of 2000 kgf, and a draw speed of 200 mm / min. A test was conducted to determine the pulling resistance (R). The results are shown in Table 1 and FIG.
4-3-2. Slidability after processing The test was performed under the same conditions except that the draw bead test was changed to a pressing force of 1000 kgf, and a flat sliding test (bead : 3 × 10 mm, pressing pressure: 130.4 Mpa, pulling speed 1.0 m / min), the pulling force (F) at that time is measured, and the coefficient of friction (μ = F / N) (F: pulling force, N: pressing force). The results are shown in Table 1 and FIG.

  As is apparent from Table 1, FIG. 3 and FIG. 4, the invention examples (Nos. 1 to 7) to which the roughness specified in the present invention is applied, both the drawing resistance R and the draw bead friction coefficient μ are small, The press moldability was good. On the other hand, in the comparative examples (Nos. 8 to 14) to which the roughness specified in the present invention is not given, both the drawing resistance R and the coefficient of friction after draw bead μ are large, and the evaluation results of press formability are higher than those of the inventive examples. It was quite inferior.

  As for the coating quality of the inventive example, after 20 μm of cationic electrodeposition coating for automobiles, (i) cut 1 mm grid and evaluate paint adhesion by peeling tape, and (ii) measure the swollen width of paint film by cross-cut salt spray When the corrosion resistance after coating was evaluated, the quality was equivalent to that of a galvanized steel sheet on which a normal phosphate film was formed.

  The zinc-based plated steel sheet according to the present invention can be used as a zinc-based plated steel sheet used for applications such as automobile bodies that require excellent press formability.

  The galvanized steel sheet according to the present invention can be used as a galvanized steel sheet used for difficult-to-mold parts that are integrally formed and press-formed using a double bead mold.

  Since the zinc-based plated steel sheet according to the present invention has good coating quality, it can be used as a zinc-based plated steel sheet used for coating purposes after performing press molding or without performing press molding. it can.

It is a figure explaining the definition of PPI in the roughness curve of surface roughness. It is a figure explaining the mechanism in which the zinc-based plated steel sheet of the present invention exhibits excellent press formability, (a) a zinc-based plated steel sheet of a comparative example, (b) a zinc-based plated steel sheet of the present invention example (each of Each of (1) roughness profile before sliding, (2) roughness profile after sliding, and (3) surface micropit distribution state after sliding is schematically shown. It is a figure explaining the relationship between PPI (0.080) -PPI (0.318) and the extraction resistance R. FIG. It is a figure explaining the relationship between PPI (0.080) -PPI (0.318) and the friction coefficient μ after a draw bead.

Claims (3)

In a zinc-based plated steel sheet having a phosphate coating mainly composed of zinc phosphate on the zinc-based plated surface, the peak of the peak count PPI of the surface roughness of the zinc-based plated steel sheet is 0.080 μm. The difference between the number PPI (0.080) and the peak number PPI (0.318) when the cutoff of the peak count PPI is 0.318 μm is 40 or more, and the press formability is excellent. Galvanized steel sheet. 2. The zinc-based plated steel sheet having excellent press formability according to claim 1, wherein the surface roughness Ra of the zinc-based plated steel sheet is 0.7 μm ≦ Ra ≦ 1.7 μm. Coverages good galvanized steel sheet in press formability according to claim 1 or 2, characterized in that a 0.2 to 3 g / m ² of the phosphate film.

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