JP5400455B2 - Resin-coated galvanized steel sheet with excellent spot weldability - Google Patents

Description

  The present invention relates to a resin-coated galvanized steel sheet in which a resin film is formed on the surface of a galvanized steel sheet, and more particularly to a resin-coated galvanized steel sheet having excellent spot weldability despite the presence of the resin film. Is.

  Conventionally, galvanized steel sheets have been widely used in fields such as home appliances, automobiles, and building materials. In order to improve corrosion resistance, fingerprint resistance, coating film adhesion, etc., galvanized steel sheets used for these applications are coated with a resin-based film with a film thickness of around 1 μm after the chromate treatment is applied to the surface of the galvanized layer. It has been.

  In recent years, there has been a movement to reduce the use of hexavalent chromium from the viewpoint of reducing environmental impact, and a resin-based film with a film thickness of around 1 μm is applied directly to the surface of a galvanized steel sheet, or through a chromate-free substrate. Chromate-free processing to paint has been developed.

  On the other hand, in office automation equipment in the field of home appliances, spot welding may be performed in the chassis assembly process, and stable continuous spotting is required to ensure high productivity. However, since the resin-based film is an insulator, the conductivity of the surface of the resin-coated galvanized steel sheet is hindered, which may cause a conduction failure during spot welding.

  Moreover, when assembling the chassis by welding, for example, the end portion 7-8 mm of the resin-coated galvanized steel sheet molded into a U-shape is bent at a right angle as a welding allowance, and the same resin-coated galvanized steel sheet Although fixed by spot welding, the nugget [the part where the weld metal (steel plate) is melted and solidified in the shape of a meteorite is melted and solidified in a welded portion] does not protrude from the welding allowance and maintains the minimum size that can ensure the strength. Uniformity of the size is required for continuous hit points.

  Considering the melting point (Fe: about 1500 ° C., Zn: about 420 ° C.) and boiling point (Fe: about 2900 ° C., Zn: about 900 ° C.) of each element, resin and galvanization on the steel sheet surface are excluded during the nugget formation process. The nugget is composed only of the components of the steel sheet as the base, but if the resin or galvanization is not smoothly removed, the nugget formation becomes unstable.

  Various proposals have been made so far for techniques for improving the electrical conductivity of a resin-coated galvanized steel sheet. For example, Patent Document 1 proposes a resin-coated steel sheet that secures a conduction point by including Ni powder in an organic resin film. Further, Patent Document 2 proposes an organic resin-coated steel sheet that ensures electrical conductivity by forming a Zn-based plating and an organic resin film having a specific thickness on a steel sheet having a constant peak count RPVC on the surface. . Furthermore, Patent Document 3 discloses a surface-treated metal material that defines the height, density, inclination, and coating rate of the projections on the surface of the plated steel sheet.

  However, in these technologies, although improvement of the electrical conductivity is disclosed, the weldability is only evaluated by the number of continuous hit points up to the standard nugget diameter, and the stability of the nugget diameter Is not considered.

  On the other hand, Patent Document 4 discloses a composite coated steel sheet that defines the decomposition temperature of the resin, the amount of adhesion, the maximum roughness of the steel sheet surface, and the like. Patent Document 5 discloses a coated steel sheet that defines the relationship between Ra (arithmetic mean roughness) of the steel sheet surface and the organic resin film thickness, and the number of peaks PPI per inch. In these techniques, it is shown that an energization path can be secured and the resin can be decomposed or removed by heat generation.

  However, these technologies also disclose improvement in electrical conductivity and removal of resin, but as for weldability, the number of continuous hit points up to the standard nugget diameter, the number of occurrences of sparks and light welding, etc. It is only evaluated in the welding current range and the like, and the stability of the nugget diameter is not disclosed.

  As a technique considering the elimination of galvanization at the time of welding, a galvanized steel sheet (Patent Document 6) that defines Ra of the steel sheet after plating removal, a galvanized steel sheet that defines Ra on the surface of the galvanized steel sheet and the steel sheet surface ( Patent Document 7) and the like have been proposed.

  However, these techniques do not disclose the case where resin coating is applied, and the technique of Patent Document 6 is basically different in welding method, and the weldability in Patent Document 7 is a standard nugget diameter. It is only evaluated by the number of consecutive hit points up to, and the stability of the diameter is not disclosed.

Japanese Patent Laid-Open No. 4-71840 JP 2002-363766 A JP 2005-139551 A JP-A-9-41157 Japanese Patent Laid-Open No. 3-79341 Japanese Unexamined Patent Publication No. 1-123091 JP-A-2005-240109

  The present invention has been made paying attention to the circumstances as described above, and its purpose is to achieve good electrical conductivity and stability of the nugget diameter at continuous hit points when assembling OA equipment and the like by spot welding. It is to provide a chromate-free resin-coated galvanized steel sheet that can be used.

  The resin-coated galvanized steel sheet of the present invention that has solved the above problems is a resin-coated galvanized steel sheet in which a galvanized film is formed on the surface of a cold-rolled steel sheet and a resin film is further formed on the galvanized film. The kurtosis (Rku) in the roughness curve of the surface of the cold-rolled steel sheet is 3.0 or less, and the number of peaks per 2.54 cm (PPI, peak count level 2H = 0.5 μm) in the roughness curve. Has a gist that it is 300 or more. The phrase “on the galvanized film” means to include both the case where the resin film is directly applied to the surface of the galvanized steel sheet and the case where the resin film is applied via a chromate-free base.

  In the resin-coated galvanized steel sheet of the present invention, the average film thickness of the resin film is 0.8 μm or less, and in the roughness curve of the resin-coated galvanized steel sheet, the number of peaks per 2.54 cm (PPI, peak count) Level 2H = 2.54 μm) is preferably 50 or more. Moreover, it is also an embodiment that the 0.2% yield strength σ of the resin-coated galvanized steel sheet is 250 MPa or less.

  The resin-coated galvanized steel sheet of the present invention has an appropriate kurtosis (Rku) and the number of peaks per 2.54 cm (PPI, peak count level 2H = 0.5 μm) in the roughness curve of the surface of the cold-rolled steel sheet used as the base steel sheet. By specifying, the stability of the nugget diameter at the continuous hitting point can be achieved, and the resin-coated galvanized steel sheet exhibiting such characteristics is useful as a constituent material for OA equipment and the like.

It is a figure for demonstrating the concept of PPI. It is a figure for demonstrating the concept of Rku.

  The present inventor has studied the resin-coated galvanized steel sheet on which the resin film is formed from various angles in order to achieve good spot weldability, particularly stability of the nugget diameter at the continuous hitting point. As a result, the resin film of the resin-coated galvanized steel sheet and the numerical value of fine irregularities of the steel sheet (cold rolled steel sheet) from which galvanization has been removed [peak count level 2H = 0.5 μm (20 μin.) Number of peaks per inch: PPI ], And found that good spot weldability can be obtained by appropriately controlling the degree of sharpness (Cultosis: Rku) that characterizes the shape of the unevenness, and completed the present invention.

  Here, the PPI is a value measured according to a method defined in SAE J911-1986 (American Automotive Technical Standards) and serves as an index of surface properties. As shown in FIG. 1, this PPI provides a constant level H in both the positive (+) and negative (−) directions from the average line of the roughness curve Z (x) (the width of the reference level between positive and negative = 2H), when the negative reference level (−H, valley portion, a in FIG. 1) is exceeded and the positive reference level (+ H, peak portion, b in FIG. 1) is exceeded, “1 count” This refers to the number of counts (number of peaks-valley counts) per 2.54 cm (1 inch). Here, the width (2H) of the reference level between positive and negative is called a peak count level, and 2H = 0.5 μm (20 μin.) Or 2.54 μm (100 μin.) Is adopted in the present invention.

  On the other hand, the kurtosis (Rku) is a value measured in accordance with the provisions of JIS B0601 (ISO 4287: 1997), and the roughness curve (the height of the mountain) Z (x) at the reference length lr. This is a value calculated by dividing the root mean square by the fourth power of the root mean square roughness Rq of the roughness curve (the following formula (II)) (the following formula (I)).

  This kurtosis (Rku) value represents the kurtosis of the distribution of the probability density function in the roughness curve, and when Rku is 3, it indicates a normal distribution. Moreover, it shows that the peak (mountain or valley) forming the roughness curve has a sharper shape as the Rku value is larger, and the smaller the Rku, the smoother the peak and the higher the height. (See FIG. 2).

  The principle of improving the spot weldability (especially the stability of the nugget diameter at the continuous striking point) by appropriately controlling the PPI and Rku on the surface of the cold-rolled steel sheet has not been clarified. I was able to think as follows.

  Spot welding is performed by stacking two resin-coated galvanized steel sheets, sandwiching them from outside with a welding electrode, energizing them, and heating the energized parts to melt and weld the steel sheets. At this time, the resin film and zinc plating existing between the steel plates must be liquefied and further gasified, and must be removed from between the steel plates before the nugget is formed. Cause variations in nugget diameter.

  When electricity flows to the energization point (convex part) between resin-coated galvanized steel sheets and heat generation begins, it is presumed that the resin is first altered (carbonized) and gasified, and then zinc liquefaction and gasification occur. However, they are discharged out of the nugget forming portion through a fine gap between the steel plates by the pressure and volume expansion between the steel plates by the electrodes. Subsequently, the melting of the Fe of the convex portion starts and the interval between the steel plates narrows, and then the convex portions with the lower height come into contact with each other to melt, and then the contact and melting occur in sequence, the interval between the steel plates narrows, and the nugget formation It is thought that it leads to.

  At this time, if only the high convex portion that contributes to the initial energization and the lower convex portion is insufficient, a new energization point is not formed even if the interval between the steel plates is narrowed, and heat is generated at the same convex portion for a while. Will continue, and the temperature rise of the concave portion on the steel plate surface will be delayed from the inside of the steel plate already energized from the convex portion. Therefore, it seems that the resin and zinc liquefaction and gasification will gradually progress with the surrounding heat even in the recess, but since it lags behind the current-carrying part, the exhaust of gas to the outside becomes insufficient and finally the nugget It seems that it causes a nugget formation defect.

  On the other hand, when there are many fine protrusions on the surface of the cold-rolled steel sheet, when the initial energized part is melted and the interval between the steel sheets is narrowed, resin transformation and gasification are progressing due to the surrounding heat, The energization points are secured one after another, and the heat generation on the surface of the cold rolled steel sheet proceeds smoothly in the surface direction. Therefore, liquefaction and gasification of resin and zinc proceed quickly, and in addition to this, fine irregularities serve as the discharge path, and gas is quickly discharged out of the nugget formation part under the pressing pressure between the steel plates. . For these reasons, it is considered that a large number of fine irregularities (2H = 0.5 μm (20 μin.) Peak / valley count PPI per inch) on the surface of the cold-rolled steel sheet are required.

  In order to exert the above effect, the PPI value on the surface of the cold rolled steel sheet needs to be 300 or more, preferably 320 or more. The upper limit of the PPI value is not particularly limited, but is preferably 400 or less from the viewpoints of productivity, mechanical properties of the steel plate, and the like.

  On the other hand, when kurtosis (Rku) increases, the number of recesses with sharp notches increases, and zinc and resin carbides are likely to remain, which is likely to cause poor nugget formation. From such a viewpoint, the kurtosis (Rku) on the surface of the cold-rolled steel sheet needs to be 3.0 or less.

  When spot-welding resin-coated galvanized steel sheets, it is necessary to energize the steel sheets, and as a requirement for improving spot weldability, at least good electrical conductivity is also required. Regarding the requirements for improving the electrical conductivity, the present inventors have examined (a) that the average film thickness of the resin film is 0.8 μm or less, and (b) in the roughness curve of the resin-coated galvanized steel sheet, It has been found effective to satisfy the requirements such as the number of peaks per 2.54 cm (PPI, peak count level 2H = 2.54 μm) being 50 or more.

  The resin film in the resin-coated galvanized steel sheet of the present invention preferably has an average film thickness of 0.8 μm or less from the above viewpoint. More preferably, it is 0.7 μm or less. However, when the resin film is too thin, it may be difficult to obtain the effects (corrosion resistance, decorativeness, fingerprint resistance) by providing the resin film, and therefore it is preferably 0.3 μm or more.

  The electrical conductivity of the resin-coated galvanized steel sheet correlates with the number of peaks per 2.54 cm (PPI, peak count level 2H = 2.54 μm) in the roughness curve, and ensures PPI of a predetermined value or more (50 or more). It is effective. This is because the resin film is an insulator, and when it is coated on the galvanized surface, the electrical conductivity deteriorates. However, the resin film on the projections on the surface of the galvanized steel sheet becomes thin, and the electrode for spot welding and galvanized resin coating are used. When the steel plates are pressed and brought into contact with each other, the insulating properties of the convex portions are destroyed and the conductivity is ensured.

  Although little attention has been paid to the peak count level in the PPI of the resin film so far, according to the study of the present inventors, in the resin-coated galvanized steel sheet having a resin film thickness of 0.8 μm or less, the peak count level is It has been found that good electrical conductivity can be secured by controlling the PPI at 2H = 2.54 μm as an index.

  The reason why setting the peak count level 2H = 2.54 μm to give good results in the electrical conductivity could be considered as follows. When coating a resin film on a galvanized steel sheet, generally, a liquid is applied onto the galvanized steel sheet to evaporate water and solvent, but most of the dry film is roughened to the roughness of the underlying galvanized steel sheet. Although part of the galvanized steel sheet is leveled during the drying process, the unevenness of the underlying galvanized steel sheet is slightly relaxed.

  As a result of an experiment conducted by the present inventors, when the resin film is 1 μm or less, the number of PPIs before and after coating is almost the same as the peak count level 2H after coating. It has been confirmed that 2H is about 10% larger before coating. Accordingly, the unevenness of the peak count level 2H = 2.54 μm on the surface of the resin-coated galvanized steel sheet is presumed to be about 2H = 2.8 μm before coating, and the unevenness on the surface of the galvanized steel sheet is assumed to be a triangular wave, The average film thickness at which the resin is completely leveled and all the dents are filled with the resin is theoretically calculated to be 1.4 μm. When the thickness of the resin film is about 1 μm, the peak count level 2H is calculated to be about 2.0 μm. However, as described above, since the resin film follows the roughness of the ground, the peak count level 2H is higher than the calculated value. In the present invention, it was judged that it was appropriate to adopt a peak count level of 2H = 2.54 μm (100 μin.) In the galvanized steel sheet after coating, that is, 2H = 2.8 μm before coating. .

  As a method for controlling the surface roughness characteristics of the resin-coated galvanized steel sheet and cold-rolled steel sheet of the present invention as described above, a roll that has been subjected to discharge dull processing so as to have a predetermined PPI is used, and is annealed after cold rolling. The surface of the cold-rolled steel sheet is preferably temper-rolled. The electric discharge dull processing roll can impart a large number of fine irregularities on the surface of the steel sheet, and can appropriately control the Rku on the surface of the cold-rolled steel sheet.

  Specifically, it is preferable to use a discharge dull processing roll having Ra ≧ 1 μm, PPI (peak count level 2H = 0.5 μm) ≧ 300, and PPI (peak count level 2H = 2.54 μm) ≧ 170. . In addition, when producing a steel sheet to be tempered by cold rolling, Ra ≧ 1 μm, PPI (peak count level 2H = 0.5 μm) ≧ 150, PPI (peak count level) in the final cold rolling stand. 2H = 2.54 μm) It is preferable to perform cold rolling with a rolling reduction of 1.5% or more using a roll of ≧ 100.

  This is because if a certain amount of PPI is not applied to the surface of the annealed steel sheet before temper rolling, the predetermined PPI cannot be applied even by temper rolling using a discharge dull processing roll. Moreover, in order to achieve a predetermined PPI only by temper rolling, in order to transfer the roll surface to the steel sheet surface excessively, it is necessary to increase the rolling reduction, and the proof stress may become too high.

The above rolling reduction is calculated by the following formula by marking a steel sheet before rolling with a certain length, measuring the length between the marks again after rolling.
Reduction ratio = [(length after rolling) − (length before rolling)] / (length before rolling) × 100 (%)

The cold-rolled steel sheet, which has been temper-rolled as described above and whose surface properties have been adjusted, is galvanized on its surface. It is preferable that the electrogalvanizing be formed along the line. For example, a horizontal electroplating apparatus is provided with a current-carrying part consisting of a set of metal conductor rolls and rubber backup rolls placed above and below the horizontal conveying steel plate, alkaline scrubber degreasing, electrolytic degreasing, water washing, After being subjected to sulfuric acid pickling, electrogalvanizing is formed by cathodic electrolytic treatment and water washing in a zinc plating bath. At this time, the amount of galvanized adhesion is, for example, preferably 40 g / m ² or less, and more preferably 30 g / m ² or less. In particular, in the case of an electrogalvanized steel sheet, it is generally 20 g / m ² . But not limited coating weight of the lower limit is particularly from the viewpoint of corrosion resistance, is preferably from 5 g / m ^2, and more preferably 10 g / m ^2.

  In the galvanized steel sheet obtained as described above, a resin film is formed on the galvanized steel. As the resin film used at this time, any of those conventionally used as a resin film of a resin-coated metal plate can be used. Can also be used. Specifically, the base resin which is the main component constituting the resin film according to the present invention includes acrylic resin, melamine resin, phenol resin, epoxy resin, urethane resin, polyester resin, polyamide resin. Alkyd resins, polyolefin resins, silicone resins, fluorine resins, aminoplast resins, vinyl chloride resins, polycarbonate resins and the like. These may be used alone or in combination of two or more.

  As the resin film according to the present invention, among the above base resins, those formed from an emulsion composition of polyolefin resin are preferable. As a preferred polyolefin resin emulsion composition, an ethylene-unsaturated carboxylic acid copolymer (including a neutralized state) is the main component, and 0 per mole of carboxyl groups of the ethylene-unsaturated carboxylic acid copolymer. 0.02 to 0.4 mol with respect to 1 mol of the carboxyl group of the amine having an boiling point of 100 ° C. or less corresponding to 2 to 0.8 mol (20 to 80 mol%) and the ethylene-unsaturated carboxylic acid copolymer. And a cross-linking agent having two or more functional groups capable of reacting with a carboxyl group, in addition to a monovalent metal compound corresponding to 1 mol (2 to 40 mol%) of the emulsion composition. 0.5 to 20% by mass of amine and ammonia having a boiling point of more than 100 ° C. are preferably substantially free.

  Monovalent metal ions are also used for preparing the emulsion composition. Effective for improving solvent resistance and film hardness. The monovalent metal compound preferably contains one or more metals selected from sodium, potassium, and lithium, and a hydroxide, carbonate, or oxide of these metals is preferable. Among these, NaOH, KOH, LiOH and the like are preferable, and NaOH has the best performance and is preferable.

  The amount of the monovalent metal compound is in the range of 0.02 to 0.4 mol (2 to 40 mol%) with respect to 1 mol of the carboxyl group in the ethylene-unsaturated carboxylic acid copolymer. When the amount of the metal compound is less than 0.02 mol, the emulsion stability becomes insufficient. However, when the amount exceeds 0.4 mol, the hygroscopicity (particularly with respect to the alkaline solution) of the resulting resin film increases, and degreasing Since the corrosion resistance after a process may deteriorate, it is not preferable. The lower limit of the more preferable amount of the metal compound is 0.03 mol, and the more preferable lower limit is 0.1 mol. The upper limit of the more preferable amount of the metal compound is 0.3 mol, and the more preferable upper limit is 0.2 mol.

  The preferred ranges of the amounts used of the amines and the monovalent metal compound are as described above, and these are all emulsified by neutralizing carboxyl groups in the ethylene-unsaturated carboxylic acid copolymer. Used to do. Therefore, if the total amount (neutralization amount) is too large, the viscosity of the emulsion composition may rapidly increase and solidify, and excessive alkali will cause corrosion resistance deterioration, and thus volatilize. For this reason, a large amount of energy is required, which is not preferable. However, if the neutralization amount is too small, the emulsifiability is inferior, which is not preferable. Therefore, the total amount of the amines and the monovalent metal compound used should be in the range of 0.3 to 1.0 mol with respect to 1 mol of the carboxyl group in the ethylene-unsaturated carboxylic acid copolymer. preferable.

  In the neutralization step (emulsification step) of the ethylene-unsaturated carboxylic acid copolymer with the above-described amines and monovalent metal ions, an amine having a boiling point of 100 ° C. or less and a monovalent metal compound are almost simultaneously copolymerized. It is desirable that the amine having a boiling point of 100 ° C. or lower is added first. The reason is not clear, but if an amine having a boiling point of 100 ° C. or lower is added later, the effect of improving the corrosion resistance may be insufficient.

  The emulsion composition is blended with a crosslinking agent having two or more functional groups capable of reacting with a carboxyl group. This is because the ethylene-unsaturated carboxylic acid copolymer is chemically crosslinked to improve the film strength. The amount of the crosslinking agent is preferably 1 to 20% by mass (more preferably 5 to 10% by mass) out of 100% by mass of the solid content in the emulsion composition. If it is less than 1% by mass, the effect of crosslinking by chemical bonding becomes insufficient, and the effect of improving corrosion resistance is difficult to be exhibited. On the other hand, when it exceeds 20% by mass, the crosslinking density of the resin film becomes excessively high and the hardness is increased, and cracks occur because it becomes impossible to follow the deformation of the metal plate when performing press working, As a result, corrosion resistance and paintability may be lowered, which is not preferable. In addition, as a ratio of the amount of the crosslinking agent to the ethylene-unsaturated carboxylic acid copolymer, it is desired to appropriately change the amount of the crosslinking agent according to the amount of the carboxyl group in the copolymer. It is preferable to make a crosslinking agent into 0.5-50 mass parts (more preferably 5-20 mass parts) with respect to mass parts.

  Although it does not specifically limit as a crosslinking agent which has two or more functional groups which can react with a carboxyl group in 1 molecule, Sorbitol polyglycidyl ether, (poly) glycerol polyglycidyl ether, pentaerythritol polyglycidyl ether, trimethylolpropane polyglycidyl Glycidyl group-containing crosslinking agents such as ether, neopentyl glycol diglycidyl ether, (poly) ethylene glycol diglycidyl ether, and polyglycidyl amines; 4,4′-bis (ethyleneiminecarbonylamino) diphenylmethane N, N′-hexamethylene-1,6-bis (1-aziridinecarboxamide), N, N′-diphenylmethane-4,4′-bis (1-aziridinecarboxamide), toluenebisua Bifunctional aziridine compounds such as lysine carboxamide; tri-1-aziridinylphosphine oxide, tris [1- (2-methyl) aziridinyl] phosphine oxide, trimethylolpropane tris (β-aziridinylpropionate), tris An aziridinyl group-containing crosslinking agent such as a tri- or higher functional aziridine compound such as 2,4,6- (1-aziridinyl) -1,3,5-triazine, tetramethylpropanetetraaziridinylpropionate, or a derivative thereof. Are mentioned as preferred examples, and one or more of them can be used. Of these, aziridinyl group-containing crosslinking agents are preferred. In addition, you may use together polyfunctional aziridine and monofunctional aziridine (ethyleneimine etc.).

  In addition, the resin film containing inorganic fine particles (organic-inorganic composite film) is a preferred embodiment of the present invention. In general, an electrode for spot welding is mainly made of Cu, and is said to deteriorate by alloying with Zn plating at a continuous hit point. However, it is considered that such deterioration can be suppressed by containing inorganic fine particles.

  Preferred inorganic fine particles used above include silica (silicon dioxide), Ca ion exchange silica, oxides / hydroxides such as Al, Ti, Ce, Sb, Zr, Fe, Sn, Mg, Ca, and Zn, and phosphoric acid. And metal salts such as Al, Mn, Mg, Ca, and Ni, such as sulfuric acid, nitric acid, and carbonic acid, molybdate, tungstate, vanadate, and phosphomolybdate. These inorganic fine particles preferably have a 50% volume average particle diameter of 1 to 100 nm as measured by a laser diffraction method (scattering method). More preferably, it is 2-20 nm. The amount of the inorganic fine particles is preferably 5 to 80% by mass in the resin film. If the amount of inorganic fine particles is small, the effect of adding inorganic fine particles may be difficult to obtain. On the other hand, if the amount of inorganic fine particles is too large, the amount of resin in the film will be relatively reduced, and the film will easily crack. Tend to be. More preferably, it is 10-75 mass%, More preferably, it is 20-70 mass%.

  The method for applying the raw material composition to the metal plate is not particularly limited, and a bar coater method, a roll coater method, a spray method, a curtain flow coater method, or the like can be employed. Although drying is performed after coating, when a crosslinking agent is added, it is preferable to perform drying by heating at a temperature at which the crosslinking agent can react. Specifically, heat drying is preferably performed at 40 to 250 ° C. for about 1 to 60 seconds.

  By spot welding using the resin-coated galvanized steel sheet as described above, the spot weldability is improved (good electrical conductivity is obtained as a prerequisite), but in order to further enhance the effect For this, it is preferable that the proof stress (0.2% proof stress) of the resin-coated galvanized steel sheet is kept below a certain level. That is, at the time of spot welding, it is necessary to sufficiently press the resin-coated galvanized steel sheets with welding application electrodes in order to ensure conductivity, liquefy, gasify resin, discharge of zinc, and fusion of Fe. However, since the steel plate surfaces are not always flat, when sandwiched by welding electrodes, if the proof stress is large, the repulsive force becomes large and the steel plates may not be sufficiently in contact with each other. From the viewpoint of preventing this, the 0.2% yield strength σ of the resin-coated galvanized steel sheet is preferably 250 MPa or less.

  In order to control the yield strength as described above, it is preferable to select an appropriate chemical component composition for the cold-rolled steel sheet to be the base steel sheet. From such a viewpoint, the chemical component composition of the cold-rolled steel sheet used in the present invention is C: 0.030 to 0.060% (meaning mass%, the same applies to the chemical component composition below), Si: 0.03% Or less (excluding 0%), Mn: 0.15 to 0.25%, P: 0.025% or less (not including 0%), S: 0.020% or less (not including 0%), sol. Al: 0.030 to 0.060%, Cr: 0.04 to 0.10%, N: 0.0040 to 0.0080% (remainder: iron and inevitable impurities) are preferable.

  Hereinafter, the present invention will be described in more detail 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.

(Experimental example 1)
Table 1 below shows low-carbon Al-killed steels having chemical composition compositions that are cold-rolled to a thickness of 0.5 mm, degreased, washed, and batch-annealed at 630-670 ° C. The temper rolling was performed under each condition. Table 2 also shows the PPI (peak count level 2H = 0.5 μm and 2.54 μm) and roll processing conditions of the rolling roll used at this time. In addition, rolling at the final stand of cold rolling was performed under the three types (AC) shown in Table 3 below.

  Next, the steel sheet after temper rolling was passed through a horizontal electroplating apparatus, and electrogalvanized in the following steps. The amount of galvanized adhesion was measured by the method described later, and the results are shown in Table 2.

(Electrogalvanizing)
(I) Alkaline scrubber degreasing step: The surface of the steel sheet was degreased at 60 ° C. using a 3% sodium orthosilicate aqueous solution.
(Ii) Electrolytic degreasing and rinsing step: Using a 3% sodium orthosilicate aqueous solution, electrolytic degreasing of the steel sheet surface was performed at 60 ° C. and 20 A / dm ² , followed by washing with water.
(Iii) Pickling and rinsing steps: The steel sheet was pickled at room temperature using a 5% sulfuric acid aqueous solution and then washed with water.
(Iv) Electrogalvanization and water washing process: Electrogalvanization was performed according to the following plating solution composition and conditions, and then washed with water.
Plating solution composition: ZnSO ₄ .7H ₂ O 300 to 400 g / L, Na ₂ SO ₄ 50 to 100 g / L, H ₂ SO ₄ 25 to 35 g / L
Current density: 50 to 200 A / dm ²
Plating solution temperature: 60 ° C
Plating solution flow rate: 0.8 to 2.4 m / sec

  Subsequently, an organic / inorganic composite film treatment solution was applied to the surface of the obtained galvanized steel sheet by a roll coating method, and then water was evaporated. The average film thickness after drying was measured by the following method. The measurement results are also shown in Table 2 below. The adjustment and drying conditions of the organic / inorganic composite coating solution used at this time are shown below.

(Preparation of organic / inorganic composite coating solution)
626 parts by mass of water (hereinafter simply referred to as “parts”) and 160 parts of an ethylene-acrylic acid copolymer (acrylic acid 20% by mass, melt index (MI) 300) are added, and the ethylene-acrylic acid copolymer is added. An emulsion of an ethylene-acrylic acid copolymer containing 40 mol% triethylamine and 15 mol% NaOH was obtained with respect to 1 mol of carboxyl groups in the coalescence. Next, 5% by mass (emulsion composition) of 4,4′-bis (ethyleneiminocarbonylamino) diphenylmethane (“Chemite (registered trademark) DZ-22E”; manufactured by Nippon Shokubai Co., Ltd.) as a cross-linking agent was added to this emulsion. The solid content of the product is 100% by mass, the same shall apply hereinafter) and a glycidyl group-containing compound ("Denacol (registered trademark) EX-321L" (hereinafter abbreviated as "EX-321L")); manufactured by Nagase ChemteX Corporation ) With a solid content of 5% by mass and silica particles having a particle size of 10 to 20 nm (“Snowtex 40”; manufactured by Nissan Chemical Industries Ltd.) with a solid content of 30% by mass, a softening point of 120 ° C. and an average particle size of 1 μm. Spherical polyethylene wax (“Chemipearl (registered trademark) W-700”; manufactured by Mitsui Chemicals, Inc.) was blended to a solid content of 5% by mass and stirred. Rujon composition (treatment solution B) was prepared. The solid content of the obtained emulsion composition was 15% by mass.

  Drying conditions: The obtained emulsion composition was applied to the steel sheet after the electrogalvanizing and rinsing steps by a roll coating method, and then dried under conditions of an air temperature of 200 ° C., an air speed of 53 m / sec, and a drying time of 1 to 2 seconds. Then, an organic-inorganic composite film was formed on the steel plate.

  The obtained resin-coated galvanized steel sheets were evaluated according to the following evaluation methods, and the results are shown in Table 2. The surface properties (PPI, Rku) of the cold rolled steel sheet were also evaluated.

[Evaluation method]
(1) PPI
The PPI was measured according to SAE (Society of Automotive Engineers) J911-1986. The peak count level was 2H = 0.5 μm or 2.54 μm (in the case of a cold-rolled steel plate, 2H = 0.5 μm). For the measurement, a surface roughness shape measuring instrument (“Surfcom 1400A-3DF”; manufactured by Tokyo Seimitsu Co., Ltd.) was used. In addition, the small surface roughness measuring device ("Surf Test SJ-301"; Mitutoyo Corporation) was used for PPI measurement of the roll surface used at the time of temper rolling. Measurement is performed with a cut-off value of 0.8 mm, a stylus tip radius R of 2 μm (the stylus part is regarded as a sphere), a measurement length of 25.4 mm (in the case of a roll, the measurement length is 4 mm) The value was converted to a value when the measurement length was 25.4 mm.). In addition, the measurement place is two places in the same direction of the resin-coated galvanized steel sheet surface and the cold-rolled steel sheet, respectively, and two places in the direction perpendicular to the direction (in the case of a roll, only in the width direction), and the average value is calculated. did.

(2) Kurtosis (Rku)
Cultosis (Rku) was measured in accordance with JIS B0601 (ISO 4287: 1997). As the measurement apparatus, a surface roughness shape measuring machine (“Surfcom 1400-3DF”, manufactured by Tokyo Seimitsu Co., Ltd.) was used as in the PPI measurement. The measurement conditions were a cut-off value of 0.8 mm, a stylus tip radius R: 2 μm (the stylus part is regarded as a sphere), and a measurement length: 25.4 mm. Moreover, the measurement place was two places in the same direction of the cold-rolled steel sheet, and two places in the direction perpendicular | vertical to the said direction, the average value was computed and it was set as Rku on the surface of a cold-rolled steel sheet.

(3) Zinc plating adhesion amount Using a fluorescent X-ray analyzer ("MIF-2100"; manufactured by Shimadzu Corporation), the galvanization adhesion amount on the cold rolled steel sheet was measured. In measuring the galvanized adhesion amount, a calibration curve representing the relationship between the zinc amount and the fluorescent X-ray intensity was prepared in advance, and the galvanized adhesion amount was determined based on this calibration curve.

(4) Average thickness of resin film The amount of Si deposited from silica particles (silicon dioxide) contained in the film was measured by fluorescent X-ray analysis. As the X-ray fluorescence analyzer, “MIF-2100” manufactured by Shimadzu Corporation was used. In measuring the Si adhesion amount, a calibration curve representing the relationship between the Si amount and the fluorescent X-ray intensity was prepared in advance, and the Si adhesion amount in the film was determined based on the calibration curve. Based on the obtained Si content (fluorescence X-ray value), the specific gravity was converted to calculate the mass of the resin film, and the average thickness t (μm) was determined. The specific conversion method is as follows.

[Si / SiO ₂ ] = 28/60
SiO ₂ mass ratio = 0.3

  The specific gravity of the resin film was determined by dividing the film mass per unit area by the film thickness by the following method. First, after applying an organic-inorganic composite film treatment liquid to SUS304-BA and drying, a 20 mm × 20 mm resin-coated SUS board is cut out, Au is evaporated on the film surface, and the cut surface of the resin film-coated SUS board is exposed. The measurement sample was prepared by embedding in a resin and polishing the cut surface of the resin-coated SUS plate. Next, an SEM (scanning electron microscope) photograph of “sample surface layer cross section” at an acceleration voltage of 20 kV and a magnification of 5000 times was taken, and the film thickness of the resin film was measured from the photograph. Photographing at this time was performed at three arbitrary locations, and the film thickness was measured at three arbitrary locations for one photograph (that is, a total of nine locations), and the average value calculated from these was taken as the film thickness. On the other hand, a 100 mm × 100 mm resin-coated SUS plate was cut out, and the resin film was peeled off with a film peeling material (“Neoriver-SP-751” manufactured by Sansai Kako Co., Ltd.). From the mass difference before and after peeling, 100 mm × 100 mm The film thickness per hit was determined.

(5) Evaluation of spot weldability Two sheets of the same kind of resin-coated galvanized steel sheet shown in Table 2 below are stacked, and an upper F (convex) type (original diameter: 16 mm, tip diameter: 8 mm), lower DR type (original Using a Cu-1% Cr electrode (spot welding electrode) with a diameter of 18 mm and a tip diameter of 8 mm, the pressure is 1960 N, the energization time is 12 cycles (60 Hz), and the welding current is 7 kA. A spot was made, and the case where the nugget diameter was smaller than 3.54 mm was defined as the limit of the weld spot. In addition, the nugget diameter measured the minimum diameter and the maximum diameter every 100 striking points, and adopted the average value. Moreover, the standard deviation of the nugget diameter measured for every 100 shots was calculated | required, and spot weldability was evaluated on the basis shown in the following Table 4 as spot weldability.

  From Tables 2-4, it can consider as follows. First, test no. It can be seen that the resin-coated galvanized steel sheets 1 to 7 all satisfy the requirements defined in the present invention and exhibit good spot weldability. In particular, test no. 1 to 5 satisfy the preferable requirements of the present invention, and it can be seen that the spot weldability is very good.

  In contrast, no. In the 8-10 resin-coated galvanized steel sheet, Rku exceeds 3.0, and the recesses on the surface of the steel sheet become too sharp, so that the discharge of resin and Zn is hindered and good spot weldability is obtained. It is thought that it is not. No. In Example 11, the Rku value is satisfied, but the number of convex portions is small (PPI value is less than 300), so that the resin and Zn are not sufficiently discharged, and good spot weldability is not obtained. It is done.

(Example 2)
After cold rolling the low carbon Al killed steel having chemical composition shown in Table 5 below to a thickness of 0.5 mm, degreasing, washing and annealing, and performing batch annealing under the conditions listed in Table 5 below. It was. Next, temper rolling was performed under the conditions shown in Table 6 below. Note that rolling (cold working conditions) at the final stand of cold rolling is the same as in Table 3 above.

  About each cold-rolled steel plate obtained above, it carried out similarly to Example 1, formed electrogalvanization, and also formed the resin film on it, and obtained the resin-coated galvanized steel plate.

  For each of the obtained resin-coated galvanized steel sheets, the proof stress (0.2% proof stress σ) when the permanent elongation reached 0.2% was measured according to JIS Z 2241, and the galvanized adhesion amount, PPI, film The thickness and spot weldability were measured or evaluated in the same manner as in Example 1. The results are also shown in Table 6 below.

  As is clear from this result, it can be seen that setting the 0.2% proof stress of the resin-coated galvanized steel sheet to 250 MPa or less is effective for obtaining excellent spot weldability.

Claims (2)

A resin-coated galvanized steel sheet in which a galvanized film is formed on the surface of a cold-rolled steel sheet and a resin film is further formed on the galvanized film, and the kurtosis (Rku) in the roughness curve of the surface of the cold-rolled steel sheet is 3 0.0 or less, and in the roughness curve, the number of peaks per 2.54 cm (PPI, peak count level 2H = 0.5 μm) is 300 or more, and the average film thickness of the resin film is 0.8 μm. In the roughness curve of the resin-coated galvanized steel sheet, the number of peaks per 2.54 cm (PPI, peak count level 2H = 2.54 μm) is 50 or more. Excellent resin-coated galvanized steel sheet. The resin-coated galvanized steel sheet according to claim 1 , wherein the 0.2% proof stress σ of the resin-coated galvanized steel sheet is 250 MPa or less.