Patent Application
20120321908
This disclosure relates to galvanized steel sheets that exhibit a low sliding friction during press forming and have excellent press-formability.
Galvanized steel sheets are used in various fields such as for automobile bodies. For galvanized steel sheets to be used in such an application, they are subjected to press forming. However, galvanized steel sheets are poor in press-formability compared to cold rolled steel sheets. This drawback arises from the fact that surface-treated steel sheets exhibit a higher sliding friction with respect to a press mold compared to cold rolled steel sheets.
That is, a high sliding friction is generated between a mold and a bead to prevent the surface-treated steel sheet from sliding smoothly into the press mold, often resulting in a fracture of the steel sheet. In particular, galvanized steel sheets coated with pure zinc exhibit a higher sliding friction due to the zinc coating having become attached to a mold (a phenomenon known as “galling”). As a result, cracks are generated in the middle of continuous press forming, thus severely adversely affecting the productivity of automobiles. Further, the recent tight restrictions on CO2 emissions have led to a trend in which the use rate of high-strength steel sheets has been increased for the purpose of automobile body weight reduction. When high-strength steel sheets are used, an increased contact pressure is encountered during press-forming and the attachment of a coating to a mold becomes a more serious problem.
A widely used method of improving press-formability of galvanized steel sheets is to apply a lubricant oil having a high viscosity. Because of the high viscosity of such a lubricant oil, however, paint defects occur during a painting step due to insufficient degreasing. Further, other problems are encountered such as a destabilized press performance caused by a lubricant being exhausted during pressing. Thus, there has been a strong demand for galvanized steel sheets themselves to be improved in terms of press-formability.
To solve these problems, Japanese Unexamined Patent Application Publication Nos. 53-60332 and 2-190483 disclose techniques in which the surface of a galvanized steel sheet is subjected to an electrolytic treatment, a dip treatment, a coating oxidation treatment or a heat treatment to form a zinc-based oxide film, thereby improving weldability and processability.
Japanese Unexamined Patent Application Publication No. 4-88196 discloses a technique in which a galvanized steel sheet is dipped into an aqueous solution containing sodium phosphate at 5 to 60 g/L and having a pH of 2 to 6, or is subjected to an electrolytic treatment or coated with the above aqueous solution to form an oxide film based on phosphorus oxide on the surface of the galvanized steel sheet, thereby improving press-formability and chemical conversion treatment properties.
Japanese Unexamined Patent Application Publication No. 3-191093 discloses a technique in which the surface of a galvanized steel sheet is subjected to an electrolytic treatment, a dip treatment, a coating oxidation treatment or a heat treatment to form nickel oxide, thereby improving press-formability and chemical conversion treatment properties.
The above conventional techniques are effective with respect to galvanized steel sheets with relatively low strength which are frequently used for automotive exterior panels. However, galvanized steel sheets with high strength which undergo an increased contact pressure during press forming cannot be necessarily improved in terms of press-formability.
It would therefore be helpful to provide galvanized steel sheets that exhibit excellent press-formability even when the galvanized steel sheets are hardly formable materials such as high-strength galvanized steel sheets exerting an increased contact pressure during press forming.
Conventional films achieve a decrease in frictional resistance by intermediating between the zinc coating and a mold. However, an increased amount of film is worn away under high contact pressure conditions which are encountered in the press forming of high-strength steel sheets. Thus, sufficient effects cannot be obtained after the slide distance has exceeded a certain level.
We discovered that a film of a crystalline layered substance which is formed so as to coat the surface of a galvanized steel sheet is effective to markedly improve sliding properties.
We thus provide:
Our galvanized steel sheets exhibit a low sliding friction at a portion of the steel sheet that is at risk of being fractured during press forming, even in the case where such a portion undergoes a high contact pressure during press forming, and which shows excellent press-formability even at a portion of the steel sheet that undergoes a high contact pressure and is expected to cause the attachment of coating onto a mold.
The term “galvanized steel sheet” is used as a collective term for steel sheets which have been coated with zinc by any of various processes such as hot dip coating, electrolytic coating, deposition coating and spray coating. The term “galvanized steel sheet” includes hot dip galvanized steel sheets which have not been subjected to any alloying treatment as well as galvannealed steel sheets which have been subjected to an alloying treatment.
Conventional films achieve a decrease in frictional resistance by intermediating between a zinc coating and a mold. However, an increased amount of film is worn away under contact pressure conditions which are encountered in the press forming of high-strength steel sheets. Thus, sufficient effects cannot be obtained after the slide distance has exceeded a certain level.
We found that sliding properties are markedly improved by coating the surface of a galvanized steel sheet with a crystalline layered substance. Further, we found that a crystalline layered substance can adhere to the surface firmly by being formed into a complex together with an organic resin. Thus, the surface of a galvanized steel sheet has an organic inorganic complex coating containing an organic resin and a crystalline layered substance.
The mechanism by which the crystalline layered substance contained in the organic inorganic complex coating produces lubricating effects is not clear, but can be explained as follows. During sliding, the adhesive force acting between a mold and a coating generates a shear stress on the surface of the coating. The crystalline layered substance intermediating between the coating and the mold is slip-deformed to absorb the shearing deforming stress generated on the surface. Even after the crystalline layered substance has been worn away from the surface layer of the galvanized steel sheet, it becomes attached to the mold and effectively works to reduce the frictional resistance. Thus, sufficient effects can be obtained even under high contact pressure conditions simulating the press forming of high-strength steel sheets.
Further, the configuration in which the organic inorganic complex coating contains an organic resin allows the crystalline layered substance to cover the surface of the steel sheet in a uniform thickness.
For the reasons described hereinabove, the galvanized steel sheet has an organic inorganic complex coating which contains an organic resin and a crystalline layered substance on the surface of the steel sheet. This configuration is the most important requirement for our steel sheets.
The organic inorganic complex coating which contains an organic resin and a crystalline layered substance (hereinafter, sometimes simply referred to as “organic inorganic complex coating”) has an average film thickness of 0.10 μm to 2.0 μm as measured from a cross section obtained by SEM. If the average film thickness is less than 0.10 μm, it is difficult to form such a coating on the surface of a steel sheet uniformly. If the average film thickness is in excess of 2.0 μm, there is a risk that spot weldability which is an important property for the production of automobiles may be lowered.
The thickness of the organic inorganic complex coating may be measured from a result obtained by ultralow-accelerating-voltage SEM with respect to an FIB-processed cross section. The identification of crystal structure for determining whether the crystalline layered substance is crystalline may be performed by thin-film X-ray diffractometry.
The crystalline layered substance is contained in a solid content of not less than 0.5 parts by weight with respect to 100 parts by weight of the solid content of the organic resin. Any content that is less than 0.5 parts by weight is not sufficiently high for the crystalline layered substance to exhibit desirable effects when in contact with a mold during sliding.
The term “crystalline layered substance” means a crystal in which unit crystal lattices formed of plate-shaped covalent crystals are stacked on top of one another with a relatively weak bond such as intermolecular force, hydrogen bond or electrostatic energy. Among such substances, a layered double hydroxide that has a structure represented by [M2+1-XM3+X(OH)2][An-]x/n.zH2O is a preferred crystalline layered substance because negatively charged anions bond to plate-shaped, positively charged divalent and trivalent metal hydroxides through electrostatic energy to neutralize electrical charge while these ions are stacked on top of one another to form a layered crystalline structure.
Such a layered double hydroxide represented by [M2+1-XM3+X(OH)2][An-]x/n.zH2O may be identified by X-ray diffractometry. It is known that a substance which can be represented by the above formula is a layered crystal.
M2+ is preferably one, or two or more selected from Mg2+, Ca2+, Fe2+, Ni2+ and Zn2+, Pb2+ and Sn2+. In particular, Mg2+, Ca2+, Fe2+, Ni2+ and Zn2+ have been confirmed to occur naturally or synthetically in layered double hydroxides, and are more preferable because such layered double hydroxide species give stable layered double hydroxides.
M3+ is preferably one, or two or more selected from Al3+, Fe3+, Cr3+, ¾Zr4+ and Mo3+. In particular, Al3+, Fe3+ and Cr3+ have been confirmed to occur naturally or synthetically in layered double hydroxides, and are more preferable because such layered double hydroxide species give stable layered double hydroxides.
An- is preferably one, or two or more selected from OH−, F−, CO32−, Cl−, Br−, (C2O4)2−, I−, (NO3)−, (SO4)2−, (BrO3)−, (IO3)−, (V10O28)6−, (Si2O5)2−, (ClO4)−, (CH3COO)−, [C6H4(CO2)2]2−, (C6H5COO)−, [C8H16(CO2)2]2−, n(C8H17SO4)−, n(C12H25SO4)−, n(C18H37SO4)− and SiO44−. These anions have been confirmed to form layered double hydroxides by being incorporated between layers of layered double hydroxide species. In particular, OH−, CO32−, Cl− and (SO4)2− may be used as interlayer anions more suitably because they can be incorporated between layers of layered double hydroxide species more easily compared to other kinds of anions and thus films can be formed on the surface of galvanized steel sheets in a short time.
Next, there will be described a method for forming the organic inorganic complex coating on the surface of a galvanized steel sheet.
First, a method of forming the crystalline layered substance is described. An exemplary method is illustrated in which a layered double hydroxide that is one kind of crystalline layered substance is prepared in the form of powder. For example, such a double hydroxide is formed by dropping an anion-containing solution into a cation-containing aqueous solution. An aqueous solution containing one or more kinds of inorganic anions or organic anions (An-) is dropped into an aqueous solution containing one or more kinds of divalent cations (M2+) and one or more kinds of trivalent cations (M3+). In this process, the pH of the reaction suspension liquid is adjusted to be 10±0.1 by dropping a 2.0 M NaOH solution.
In the reaction suspension liquid, the divalent cations and the trivalent cations form hydroxides and are present in the form of colloids. These hydroxides are precipitated as layered double hydroxides when one or more specific anions selected from OH−, F−, CO32−, Cl−, Br−, (C2O4)2−, I−, (NO3)−, (SO4)2−, (BrO3)−, (IO3)−, (V10O28)6−, (Si2O5)2−, (ClO4)−, (CH3COO)−, [C6H4(CO2)2]2−, (C6H5COO)−, [C8H16(CO2)2]2−, n(C8H17SO4)−, n(C12H25SO4)−, n(C18H37SO4)− and SiO44− and SiO44− are dropped into the liquid. Next, the precipitate is separated using a centrifugal separation apparatus and dried to give a powdery layered double hydroxide.
The obtained powdery substance may be identified as being a layered compound by X-ray diffractometry.
Next, the obtained powdery layered double hydroxide and an organic resin are appropriately mixed with each other and stirred to give a coating composition. The stirring may be carried out using, for example, a coating disperser (a sand grinder). The stirring time may be selected appropriately. The stirring time is preferably 30 minutes or more to make sure that the powdery layered double hydroxide is sufficiently dispersed in the organic solvent.
One or two or more kinds of organic resins may be appropriately selected from epoxy resins, modified epoxy resins, polyhydroxy polyether resins, polyalkylene glycol-modified epoxy resins, urethane-modified epoxy resins, resins obtained by further modifying these resins, polyester resins, urethane resins, silicon resins and acrylic resins. In particular, a preferred resin from the viewpoint of corrosion resistance is an epoxy-based resin whose molecular weight has been optimized to achieve improved processability or which has been partially modified with a urethane, a polyester or an amine.
Further, one, or two or more kinds of additives may be added as required, with examples including organic color pigments (such as condensed polycyclic organic pigments and phthalocyanine organic pigments), color dyes (such as water-soluble azo metal dyes), inorganic pigments (such as titanium oxide), conductive pigments (for example, powders of metals such as zinc, aluminum and nickel, as well as iron phosphide and antimony-doped tin oxide), coupling agents (such as titanium coupling agents) and melamine-cyanuric acid adducts.
Next, the coating composition is applied to the surface of a steel sheet and baked. The coating composition may be applied to the surface of a steel sheet by any means without limitation. A roll coater is suitably used.
The thermal drying (baking) treatment may be carried out using a dryer, a hot air furnace, a high frequency induction heating furnace, an infrared furnace or the like. From the viewpoint of corrosion resistance, a high frequency induction heating furnace is particularly preferable. The thermal treatment is desirably carried out at a reached sheet temperature in the range of 50 to 350° C., and preferably 80° C. to 250° C. If the heating temperature is below 50° C., a large amount of solvent remains in the film, thus resulting in insufficient corrosion resistance. Heating at a temperature exceeding 350° C. is not economical and can cause defects in the film, possibly resulting in a decrease in corrosion resistance.
By the method described hereinabove, a galvanized steel sheet may be obtained which has the organic inorganic complex coating containing the organic resin and the crystalline layered substance on the surface.
In the production of hot dip galvanized steel sheets or galvannealed steel sheets, it is necessary that Al be added to the plating bath. However, elements other than Al which may be added are not particularly limited. That is, the advantageous effects are not deteriorated even when the plating bath or the coating contains Al and other elements such as Pb, Sb, Si, Sn, Mg, Mn, Ni, Ti and Li.
Further, the advantageous effects are not deteriorated even when elements such as N, Pb, Na, Mn, Ba, Sr and Si are incorporated into the organic inorganic complex coating as a result of the contamination of treatment liquids used for the film production with such impurities.
Our steel sheets and methods will be described in greater detail by presenting examples below.
Layered double hydroxides were prepared by dropping an aqueous solution containing at least one kind of inorganic anion or organic anion (An-) (Composition of aqueous solution 2 in Table 1) to an aqueous solution shown in Table 1 which contained at least one kind of divalent cation (M2+) and at least one kind of trivalent cation (M3+) (Composition of aqueous solution 1 in Table 1). In this process, the pH of the reaction suspension liquid was adjusted to be 10±0.1 by dropping a 2.0 M NaOH solution. Next, each of the obtained precipitates was filtered and dried to give a powdery layered double hydroxide. The obtained powdery substances were identified as being layered double hydroxides by X-ray diffractometry.
According to the formulations described in Table 3, the layered double hydroxides prepared by the above process were appropriately mixed together with any of organic resin compositions shown in Table 2, and each mixture was stirred using a coating disperser (a sand grinder) for 45 minutes to give a coating composition for forming an organic inorganic complex coating on the surface of a galvanized steel sheet.
Cold rolled steel sheets having a sheet thickness of 0.7 mm were provided as base steel sheets. A galvannealed coating was formed on each steel sheet by a common method, and the coated steel sheet was temper rolled. Separately, a hot dip zinc coating or an electrolytic zinc coating was formed on similar steel sheets by a common method.
The surface of each of the various coated steel sheets obtained as described above was degreased with an alkali, washed with water and dried. Thereafter, any of the coating compositions was applied to the surface of the steel sheet with a roll coater and was baked (thermally dried) at a baking temperature shown in Table 3 (140° C.). The thickness of the organic inorganic complex coating was adjusted by controlling the solid content (the content of residues after heating) of the coating composition or application conditions (such as roll force or rotational speed).
The organic inorganic complex coatings formed on the surface of the galvannealed steel sheets, the hot dip galvanized steel sheets and the electrolytically galvanized steel sheets were analyzed to measure the average film thickness as well as to identify the layered double hydroxides. To evaluate press-formability, sliding properties were evaluated by measuring the friction coefficient and evaluating galling properties. The methods used for the measurements and the identification are described below.
The coating was sputtered at an angle of 45° using FIB and the cross section was observed by ultralow-accelerating-voltage SEM. The values of film thickness measured at 10 sites were averaged to determine the film thickness of the coating.
The presence of crystalline layered double hydroxide was confirmed by X-ray diffractometry. The peaks which were obtained by X-ray diffractometry using Cu—Kα radiation were verified against ICDD cards and the layered double hydroxide was identified. The cards which agreed with the obtained data are described below.
ICDD card reference code: 01-089-0460
[Mg0.667Al0.333(OH)2][CO32−]0.167.0.5H2O
ICDD card reference code: 00-048-1021
[Zn0.71Al0.29(OH)2][CO32−]0.145.H2O
iii) Iron Carbonate Hydroxide Hydrate
ICDD card reference code: 00-050-1380
[Fe0.67Fe0.33(OH)2][CO32−]0.145.0.33H2O
ICDD card reference code: 00-042-0573
[Ni0.75Fe0.25(OH)2][SO42−]0.125.0.5H2O
ICDD card reference code: 00-038-0478
[Mg0.75Al0.25(OH)2][OH−]0.25.0.5H2O
ICDD card reference code: 00-020-0500
[Mg0.75Fe0.25(OH)2][Cl−]0.25.0.5H2O
vii) Calcium Aluminum Hydroxide Chloride Hydrate
ICDD card reference code: 00-035-0105
[Ca0.67Al0.33(OH)2][Cl−]0.33.0.67H2O
viii) Magnesium Chromium Carbonate Hydroxide Hydrate
ICDD card reference code: 00-045-1475
[Mg0.67Cr0.33(OH)2][CO32−]0.157.0.5H2O
ICDD card reference code: 00-051-1527
[Fe0.67Al0.33(OH)2][CO32−]0.157.0.5H2O
ICDD card reference code: 00-015-0087
[Ni0.67Al0.33(OH)2][CO32−,OH−]0.157.0.5H2O
To evaluate press-formability (in particular, formability at a portion of steel sheet which was in contact with an object when the steel sheet would be drawn or slid), the dynamic friction coefficient of each test material was measured in the following manner.
The measurement of friction coefficient was carried out under any of 3 conditions in which the temperature was room temperature (25° C.) and the pressure load N was 400, 1200 or 1600 kgf so that the contact pressure would be a value expected in the press forming of high-strength steel sheets. Further, the pulling rate (the velocity of horizontal movement of the slide table 3) for the sample was 100 cm/min or 20 cm/min. The pressure load N and the sliding frictional force F were measured under any of these conditions. The friction coefficient μ between the test material and the bead was calculated from the equation: μ=F/N. The bead shape, the pressure load conditions and the pulling rate were used in the following combinations.
Condition 1: bead shape 1, pressure load 400 kgf and pulling rate 100 cm/min
Condition 2: bead shape 1, pressure load 1200 kgf and pulling rate 100 cm/min
Condition 3: bead shape 1, pressure load 1600 kgf and pulling rate 100 cm/min
Condition 4: bead shape 2, pressure load 400 kgf and pulling rate 20 cm/min
Galvanized steel sheets coated with pure zinc increase the sliding friction due to the coating having become attached to a mold after the coated portion has undergone a long sliding distance. Thus, galling tendency is an important property in addition to the dynamic friction coefficient. Using the friction coefficient measuring apparatus illustrated in
The results obtained by these tests as well as the test conditions are described in Table 3.
The test results described in Table 3 show the following.
Sample No. 1 using a hot dip galvanized steel sheet (GI) was a comparative example without any crystalline layered substance being contained. The friction coefficient was high and the galling properties were poor. Samples Nos. 2 to 27 were our examples containing the organic resin and the crystalline layered substance. In comparison with the results of Comparative Example No. 1, the friction coefficient was low and the galling properties were good.
Sample No. 28 using a galvannealed steel sheet (GA) was a comparative example without any crystalline layered substance being contained. The friction coefficient was high. Samples Nos. 29 to 38 were our examples containing the organic resin and the crystalline layered substance. In comparison with the results of Comparative Example No. 28, the friction coefficient was low.
Sample No. 39 using an electrolytically galvanized steel sheet (EG) was a comparative example without any crystalline layered substance being contained. The friction coefficient was high and the galling properties were poor. Samples Nos. 40 to 49 were our examples containing the organic resin and the crystalline layered substance. In comparison with the results of Comparative Example No. 39, the friction coefficient was low and the galling properties were good.
Our galvanized steel sheets are excellent in press-formability and can be used in various fields, in particular for automobile bodies which are necessarily manufactured from hardly formable materials.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009-268765 | Nov 2009 | JP | national |
This is a §371 of International Application No. PCT/JP2010/071190, with an inter-national filing date of Nov. 19, 2010, which is based on Japanese Patent Application No. 2009-268765, filed Nov. 26, 2009, the subject matter of which is incorporated by reference.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2010/071190 | 11/19/2010 | WO | 00 | 9/5/2012 |
