JP7533427B2 - cold-rolled steel plate - Google Patents

Description

The present invention relates to a cold-rolled steel sheet that has excellent sliding properties when press-forming. In particular, it relates to a cold-rolled steel sheet that has a lubricating film that has excellent formability even during severe drawing.

Cold-rolled steel sheets are widely used in a wide range of fields, mainly for automobile body applications, and in such applications, they are generally subjected to press forming before use. In recent years, in such press forming, there has been a demand for integration of parts to reduce processes and improvement of design, and it is necessary to enable more complex forming.
However, if more complex press forming is attempted, the steel sheet may not be able to withstand the forming and break, or die galling may occur during continuous press forming, which could have a serious impact on automobile productivity.

One method for improving the press formability of cold-rolled steel sheets is to perform a surface treatment on the die. This method is widely used to improve press formability, but it does not allow adjustment of the die after the surface treatment. It also has problems such as high costs. Therefore, there is a strong demand for improving the press formability of the steel sheet itself.

One way to improve press formability without surface treatment of the mold is to use a high viscosity lubricant. However, this method can lead to poor degreasing after press forming, which can lead to poor paintability.

In light of the above, various types of lubricant surface-treated steel sheets are being investigated as a technology that allows press forming without the need for surface treatment of dies or high-viscosity lubricants.

For example, Patent Document 1 describes a metal plate coated with a lubricating film in which a solid lubricant protrudes 0.01 to 1.5 μm from the surface of the resin film.

Patent document 2 describes a lubricated surface-treated metal product with excellent press formability, which is coated with a 0.5 to 5 μm film of polyurethane resin containing a lubricant.

Patent document 3 describes a technology for forming an alkali-soluble organic film on a steel sheet, in which a lubricant is added to an epoxy resin.

Japanese Patent Application Publication No. 10-52881 JP 2000-309747 A JP 2000-167981 A

However, while the techniques described in Patent Documents 1 to 3 provide a certain degree of lubricity through the lubricating effect of the lubricants contained therein, they do not necessarily provide sufficient press formability for the complex forming processes of recent years. In particular, when the surface roughness of the steel sheet changes, it is not possible to obtain stable and good press formability.

The present invention was made in consideration of these circumstances, and aims to impart excellent press formability to steel sheets that are difficult to press and that undergo complex forming, even in areas where high contact pressures are expected to cause die galling, by reducing the sliding resistance in areas where cracks are likely to occur during press forming, and in particular to impart excellent press formability to steel sheets with a wide range of surface roughness.

In addition, when used as steel sheets for automobiles, sufficient removability is also required in the alkaline degreasing step of the painting process. Therefore, for such applications, a further objective is to provide a coated steel sheet that has good removability in addition to the above-mentioned press formability.

The inventors conducted extensive research to find a solution to the problems of the conventional technology. As a result, they discovered that the above problems can be solved by forming a coating that contains a binder that satisfies specific conditions and a wax that satisfies specific conditions, and that contains the wax in a specific mass ratio, and further by controlling the surface roughness of the base steel surface and the amount of adhesion of the coating.

The present invention has been completed based on the above findings, and the gist of the present invention is as follows.
1. A cold-rolled steel sheet having a coating containing an organic resin and a wax on at least one side,
the organic resin is an alkali-soluble resin, and the alkali-soluble resin is at least one selected from the group consisting of a copolymer of styrene and maleic acid, a copolymer of styrene and maleic anhydride, and a salt of a copolymer of styrene and maleic acid; the wax is a polyolefin wax having a melting point of 120°C or more and 140°C or less and an average particle size of 0.01 μm or more and 3.00 μm or less; a proportion C of the wax to the total amount of the organic resin and the wax, as defined by the following formula (1), is 10 mass% or more; and the relationship between the adhesion weight W (g/ ^m2 ) per side of the coating and the arithmetic mean roughness Ra (μm) of the surface of the base steel of the cold-rolled steel sheet satisfies the following formula (2):
C={M _B /(M _A + M _B )}×100...Formula (1)
Where, M _A : the mass of the organic resin converted into an acid anhydride
M _B : Mass of the polyolefin wax W≧0.25×Ra ² +0.2 Formula (2)

2. The cold-rolled steel sheet according to 1, wherein the proportion C of wax in the total amount of the organic resin and wax is 50 mass% or less.

3. The cold-rolled steel sheet according to 1 or 2, wherein the coating contains the fixed resin and the wax in a total amount of 70 mass% or more.

4. The cold-rolled steel sheet according to any one of 1 to 3, wherein the coating weight W is 2.0 g/ ^m2 or less.

5. The cold-rolled steel sheet according to any one of 1 to 4, wherein the arithmetic mean roughness Ra is 0.40 μm or more and 2.50 μm or less.

According to the present invention, the coefficient of friction between the steel sheet and the die or the like can be significantly reduced, and therefore a cold-rolled steel sheet with excellent press formability can be obtained. Therefore, it is possible to provide stable and excellent press formability to a steel sheet with a relatively low strength that is subjected to complex forming. Furthermore, even in a high-strength steel sheet in which the surface pressure during press forming increases, the sliding resistance at the portion at risk of cracking during press forming is reduced, and therefore excellent press formability can be exhibited, particularly in the portion where the surface pressure is high and where die galling is expected to occur.
In the above, high strength is assumed to have a tensile strength (TS) of 440 MPa or more, and relatively low strength is assumed to have a TS of less than 440 MPa.
Furthermore, by further adding specific conditions to the organic resin and wax, it is possible to impart excellent alkaline removability, making it possible to provide a cold-rolled steel sheet that is optimal for use as a steel sheet for automobiles.

FIG. 2 is a schematic front view showing a friction coefficient measuring device. FIG. 2 is a schematic perspective view showing the shape and dimensions of the bead in FIG. 1 .

Hereinafter, an embodiment of the present invention will be described.
The present invention relates to a cold-rolled steel sheet having a coating containing an organic resin and a wax formed on at least one side thereof. Hereinafter, when simply referring to a steel sheet, it means a cold-rolled steel sheet having no coating formed thereon.
In the present invention, the organic resin is an alkali-soluble resin, which is at least one selected from the group consisting of a copolymer of styrene and maleic acid, a copolymer of styrene and maleic anhydride, and a salt of a copolymer of styrene and maleic acid.Furthermore, the wax is a polyolefin wax having a melting point of 120° C. or more and 140° C. or less and an average particle size of 0.01 μm or more and 3.00 μm or less.
Furthermore, in the steel sheet of the present invention, the proportion C of wax in the total amount of organic resin and wax defined by the following formula (1) is 10 mass % or more, and the relationship between the coating weight W (g/ ^m2 ) per side and the arithmetic mean roughness Ra (μm) of the base steel surface of the coated steel sheet satisfies the following formula (2):
C={M _B /(M _A + M _B )}×100...Formula (1)
Where, M _A : the mass of the organic resin converted into an acid anhydride
M _B : Mass of the polyolefin wax W≧0.25×Ra ² +0.2 Formula (2)
When the organic resin (A) is a copolymer of styrene and maleic anhydride, the actual mass of the copolymer is taken as M _A. When the organic resin (A) is a copolymer of styrene and maleic acid or a salt of a copolymer of styrene and maleic acid, the mass converted into an acid anhydride (the mass of maleic acid or maleate converted into maleic anhydride) is taken as M _A.

The wax used in the present invention is not particularly limited as long as it is a polyolefin wax having a melting point of 120° C. or more and 140° C. or less and an average particle size of 0.01 μm or more and 3.00 μm or less.
The reason why polyolefin wax is used as the wax in the present invention is that it has low surface energy and self-lubricating properties, and therefore provides good lubrication. In addition, it is relatively easy to adjust the melting point of polyolefin wax to 120° C. or more and 140° C. or less by controlling the density and molecular weight.

When the melting point of the polyolefin wax is 120°C or higher and 140°C or lower, in addition to the self-lubricating properties of the polyolefin wax itself, the wax becomes semi-molten due to sliding during press molding, allowing the lubricating film components mixed with the organic resin to cover the surface of the mold, and direct contact between the mold and the steel sheet can be suppressed, resulting in an excellent lubricating effect. In addition, the wax in the film efficiently adheres to the mold in the sliding state during press molding, making it difficult to fall off, and it is thought that a high lubricating effect can be obtained.

That is, if the melting point is less than 120° C., the wax will completely melt due to frictional heat caused by sliding during press molding, and the wax itself will not provide a sufficient lubricating effect, and the above-mentioned effect of covering the mold will not be obtained. On the other hand, if the melting point exceeds 140° C., the wax will not melt during sliding, and will not provide a sufficient lubricating effect, will not provide a covering effect for the mold, and even if the coating adheres to the mold, the adhesion will be weak, and the coating will easily fall off when slid.
The lower limit of the melting point is preferably about 125° C., while the upper limit of the melting point is preferably about 135° C.
The melting point of the wax in the present invention is the melting temperature measured based on JIS K 712:1987 "Method of measuring transition temperature of plastics".

If the wax has an average particle size of more than 3.00 μm, it will be difficult to mix with the organic resin during sliding, and the aforementioned mold coating effect will not be obtained, resulting in insufficient lubrication. It is preferably 1.50 μm or less. More preferably, it is 0.50 μm or less, and even more preferably, it is 0.30 μm or less. On the other hand, it is essential that the average particle size of such wax is 0.01 μm or more. If it is less than 0.01 μm, it will be easily dissolved in the lubricating oil during sliding, and may not exhibit a sufficient lubrication improvement effect, and it will be easily aggregated in the paint, resulting in low paint stability. It is preferably 0.03 μm or more.

The average particle size is the median diameter based on volume, and is determined by a laser diffraction/scattering method. For example, it can be determined by measuring a sample diluted with pure water using a laser diffraction/scattering particle size distribution measuring device Partica LA-960V2 (manufactured by Horiba, Ltd.). Among polyolefin waxes, polyethylene wax is preferred because it provides the greatest lubricating effect.

In the present invention, the mass proportion of the wax in the total amount of the organic resin and the wax (C calculated using the above formula (1)) is 10 mass% or more. If it is less than 10 mass%, a sufficient lubricating effect cannot be obtained. On the other hand, the mass proportion of the wax in the film is preferably 50 mass% or less. If it exceeds 50 mass%, the wax is likely to fall off due to a lack of base resin components, adhesion to the steel sheet is poor, and the film may not be able to exist stably.

The mass percentage C of wax in the coating is more preferably 30 mass% or less. By making the mass percentage of wax in the coating 30% or less, it is possible to ensure degreasing in the alkaline degreasing step of the painting process when the coating is used as an automotive steel sheet, and to prevent deterioration of paintability due to residual coating.

Here, the proportion C of the wax in the total amount of the organic resin and the wax means the proportion by mass of the solid content of the wax in the coating to the sum of the mass of the solid content of the organic resin in the coating and the mass of the solid content of the wax in the coating, and can be calculated by the above-mentioned formula (1).
As a specific method for measuring such mass, for the organic resin and wax, test pieces with known amounts of the organic resin and wax attached on a coated steel plate are prepared, infrared absorption spectra are measured using an FT-IR measurement device, and calibration curves for the amounts of the organic resin and wax attached are created from the peak intensities originating from the organic resin and the wax. In this case, the intensity of the infrared absorption spectrum originating from the binder may be measured using as indicators the spectra characteristic of the aromatic group of the styrene-derived structural unit, the ester group of the maleic acid-derived structural unit, and the lactone ring portion of the maleic anhydride-derived structural unit, while also taking into consideration the ratio of each structural unit constituting the copolymer. In addition, the intensity of the infrared absorption spectrum originating from the wax may be measured using as an indicator the spectrum characteristic of the methylene group.
Next, the infrared absorption spectrum of the coated steel sheet to be measured is measured, and the amounts of resin and wax attached are determined from the calibration curve, thereby making it possible to determine the mass proportion of wax in the coating.

In the present invention, the organic resin acts as a binder to hold the wax on the steel sheet surface. The sliding effect caused by the mold coating of the mixture of wax and organic resin formed during sliding described above is not exhibited by inorganic binders because they have low affinity with polyolefins.

As the binder, at least one selected from the group consisting of a copolymer of styrene and maleic acid, a copolymer of styrene and maleic anhydride, and a salt of a copolymer of styrene and maleic acid is used.
That is, styrene was selected as the binder component because it contributes to improving press moldability. Maleic acid was selected because it improves removability with alkali. The reason for selecting maleic anhydride is the same as that for maleic acid.
Therefore, by using as the binder a copolymer of styrene and maleic acid, a copolymer of styrene and maleic anhydride, or a salt of a copolymer of styrene and maleic acid, a coating can be formed that has an excellent balance between press formability and alkali removability, compared to other acrylic resins, epoxy resins, urethane resins, phenol resins, vinyl acetate resins, and polyester resins.

Furthermore, in the present invention, the same effect can be obtained whether a copolymer of styrene and maleic acid, a copolymer of styrene and maleic anhydride, or a salt of a copolymer of styrene and maleic acid is selected. The binder only needs to hold the wax particles and have excellent removability of the coating by alkaline degreasing, and is not affected by the type of salt or whether maleic acid is anhydrous or not.

The copolymer in the present invention can be obtained by polymerizing styrene and maleic acid, and styrene and maleic anhydride, and the reaction may be a method using a generally known copolymerization reaction. The salt of the copolymer in the present invention can be obtained by neutralizing the polymer, and the method may be a generally known method.
In the present invention, the salt of the copolymer is not particularly limited as long as it is a salt of the copolymer with a known cation, but a salt with ammonium is most preferred.

The mass average molecular weight of the copolymer is preferably in the range of 4,000 to 400,000. The lower limit is more preferably 6,000, and even more preferably 9,000. On the other hand, the upper limit is more preferably 100,000, and even more preferably 50,000. When the mass average molecular weight of the copolymer is in the range of 4,000 to 400,000, it is advantageous in that better press moldability and alkali removability can be obtained.
Here, the mass average molecular weight can be measured by GPC measurement (gel permeation chromatography) using a high-speed GPC apparatus HLC-8320GPC (manufactured by Tosoh Corporation), a TSKgel-G column, tetrahydrofuran as an eluent, and polystyrene as a standard sample.

The arrangement of styrene and maleic acid or styrene and maleic anhydride in the present invention may be any of a random copolymer, an alternating copolymer, a block copolymer, and a graft copolymer, but in consideration of the production cost, a random copolymer is preferred.
In the present invention, the random copolymer means a copolymer in which styrene and maleic acid or styrene and maleic anhydride are randomly arranged.

In the present invention, in forming the copolymer of styrene and maleic acid or styrene and maleic anhydride, the ratio of the constituent units derived from each monomer of styrene and maleic acid or styrene and maleic anhydride (styrene/maleic acid or styrene/maleic anhydride) is not particularly limited, but the lower limit of the molar ratio is preferably 1/9, more preferably 2/8, and even more preferably 5/5. On the other hand, the upper limit of the molar ratio is preferably 9/1, and more preferably 8/2.
If the molar ratio is less than 1/9, press moldability may be poor. In other words, the hardness of the coating is insufficient and the coating may be easily damaged during pressing, which is not preferable. On the other hand, if the molar ratio exceeds 9/1, the coating removability (removability by alkaline degreasing) may remain at the same level as that of the conventional technology.

The coating in the present invention preferably contains the inorganic resin and the wax in a total amount of 70% by mass or more. If the total amount is less than 70% by mass, the adhesion between the base steel and the coating may be poor, the coating may be prone to peeling off from the base steel, and sufficient lubrication may not be achieved.
In addition to the organic resin and wax, the composition may contain surface conditioners, defoamers, dispersants, etc. that are generally added to paints. In addition, rust inhibitors and pigments that improve rust resistance may also be added.
In other words, components other than the organic resin and wax are preferably permitted up to about 30% by mass.

Furthermore, when forming the above-mentioned coating on the surface of a steel sheet, the inventors used steel sheets with a wide range of surface roughness, formed coatings with a wide range of adhesion amounts, and evaluated press formability. As a result, they found that stable and good press formability is achieved only in a region where the surface roughness of the steel sheet and the adhesion amount of the coating satisfy a certain limited relationship.

The reason why stable and good press formability is achieved only in the region that satisfies such a limited relational expression is not clear.
However, the inventors believe that as the surface roughness of the steel plate increases, the lubricating film tends to become thinner at the convex parts of the coated steel plate, and when the plate is press-formed, the film tends to be scraped off by sliding against the die, exposing the underlying steel plate, making it difficult to obtain a lubricating effect.
In the present invention, adhesion of the lubricating film components to the die during sliding is promoted, so that even if the roughness of the steel sheet is greater than in the past, the die side is protected and the lubricity is not impaired.

Specifically, the range of coating weight that exhibits good press formability is as follows:
That is, the relationship between the coating weight W (g/ ^m2 ) per side and the arithmetic mean roughness Ra (μm) of the base steel surface of the coated steel sheet is set to a range that satisfies W≧0.25× ^Ra2 +0.2. If W<0.25× ^Ra2 +0.2, the coating weight is insufficient, the protective effect on the die side is insufficient, and good press formability is not obtained.

Of the coating, the main contributors to the sliding properties are the coating components present in the convex parts of the coated steel sheet that come into contact with the die during press working. It is believed that the area of the convex parts of the coated steel sheet that come into contact with the die tends to decrease in proportion to the value of ^Ra2 . In other words, by increasing the coating weight in proportion to the value of ^Ra2 , it is possible to ensure a sufficient amount of the coating components that contribute to the sliding properties.

In the present invention, the coating weight W is preferably 2.0 g/ ^m2 or less. If it exceeds 2.0 g/ ^m2 , the coating removal property and weldability may be deteriorated. The coating weight W is more preferably 0.9 g/ ^m2 or less. If the coating weight W is 0.9 g/ ^m2 or less, the coating removal property is particularly good.
On the other hand, the coating amount W is preferably 0.2 g/m ² or more in order to obtain the effects of the present invention.

The coating weight can be determined by dividing the difference in mass of the steel sheet before and after coating by its area, or by completely removing the coating from the coated steel sheet after coating with an alkaline aqueous solution or organic solvent, and then dividing the difference in mass of the steel sheet before and after removing the coating by its area.

The arithmetic mean roughness Ra according to the present invention is preferably 0.40 μm or more and 2.50 μm or less. If the Ra is less than 0.40 μm, fine scratches that may occur during press molding may be easily noticeable, and galling may occur during press molding. On the other hand, if the Ra exceeds 2.50 μm, the required coating weight may become large, which may increase manufacturing costs and may deteriorate the sharpness (blur) after painting.

The Ra (μm) can be measured according to JIS B0633:2001 (ISO 4288:1996). For example, when the Ra is greater than 0.1 and equal to or less than 2, the cutoff value and reference length are set to 0.8 mm, and the evaluation length is set to 4 mm, and the roughness curve is determined. On the other hand, when the Ra is greater than 2 and equal to or less than 10, the Ra can be determined by determining the cutoff value and reference length from the roughness curve, set to 2.5 mm, and the evaluation length is set to 12.5 mm.

Next, a method for producing a cold rolled steel sheet according to the present invention will be described.
The method for producing a cold-rolled steel sheet according to the present invention is a method for producing a steel sheet having an organic resin coating on the surface of the steel sheet, the organic resin coating containing the polyolefin wax having a melting point of 120°C or more and 140°C or less and an average particle size of 0.01 μm or more and 3.00 μm or less, and includes at least a step of applying to the surface of the steel sheet a paint comprising an organic resin solution or emulsion in which an organic resin is dissolved or dispersed in a solvent and wax added thereto, and drying the paint.

The mass ratio of the coating components (organic resin and polyolefin wax) in the paint is preferably 1% by mass or more and 25% by mass or less. If it is less than 1% by mass or more than 25% by mass, there is a risk of unevenness occurring during application.

The method for applying the paint is not particularly limited, but examples include a method using a roll coater or a bar coater, or a method using a spray, immersion, or brush. The coated steel sheet after application can be dried by a general method. For example, drying with hot air, drying with an induction heater, or infrared heating can be used.

The maximum temperature that the coated steel sheet reaches during drying is preferably 60°C or higher and 140°C or lower. If the temperature is lower than 60°C, it will take a long time to dry and the rust prevention properties may be poor. On the other hand, if the temperature exceeds 140°C, the wax will melt and coalesce, causing the particle size to become coarse, which may result in a deterioration in lubricity.

The present invention will be described below with reference to examples, but the present invention is not limited to the following examples.
Cold-rolled steel plates (steel plates No. A to D) having a thickness of 0.8 mm and an arithmetic mean roughness Ra shown in Table 1 were used, and coating materials having compositions shown in Table 2 were applied with a bar coater, followed by drying with an IH heater so that the maximum temperature of the steel plate reached was 80°C, to obtain coated steel plates for use in this example. Note that, in addition to organic resin and wax, coating material No. 22 contained silica as a pigment in an amount of 30 mass% in the coating. Steel plates No. A to D are all SPCD (JIS G 3141) having a tensile strength of 270 MPa.

(1) Method for Evaluating Press Moldability (Sliding Characteristics) In order to evaluate press moldability, the friction coefficient of each test material was measured as follows.
FIG. 1 is a schematic front view showing a friction coefficient measuring device.
As shown in Figure 1, a friction coefficient measurement sample 1 taken from a test material is fixed to a sample stage 2, and the sample stage 2 is fixed to the upper surface of a horizontally movable slide table 3. A vertically movable slide table support stage 5 having a roller 4 in contact with the slide table 3 is provided on the lower surface of the slide table 3, and a pressing load N is applied from a bead 6 to the friction coefficient measurement sample 1 by pushing up the support stage 5. A first load cell 7 for measuring the pressing load N is attached to the slide table support stage 5.
In addition, the sliding resistance force F for moving the slide table 3 in the horizontal direction while the pressing force of the pressing load N is applied is measured. A second load cell 8 for such measurement is attached to one end of the slide table 3. The slide table 3 moves horizontally via a rail 9.
In the above embodiment, the test was performed by applying press cleaning oil Plethon R352L manufactured by Sugimura Chemical Industry Co., Ltd. to the surface of the sample 1 as the lubricating oil.

Figure 2 is a schematic perspective view showing the shape and dimensions of the bead 6 used. The underside of the bead 6 slides while being pressed against the surface of the sample 1. The shape of the bead 6 shown in Figure 2 is 10 mm wide, 59 mm long in the sliding direction of the sample, and the lower parts of both ends in the sliding direction are curved surfaces with a curvature of 4.5 mmR, and the underside of the bead against which the sample is pressed has a flat surface with a width of 10 mm and a length of 50 mm in the sliding direction.

The friction coefficient measurement test was performed by applying the bead shown in Fig. 2 to the friction coefficient measurement device shown in Fig. 1, with a pressing load N of 400 kgf and a sample pull-out speed (horizontal movement speed of the slide table 3) of 20 cm/min. The friction coefficient μ between the test material and the bead was calculated by the formula: μ = F/N.
When the friction coefficient was 0.119 or less, the sliding property was evaluated as being particularly excellent and was marked with an ⊚, when it was more than 0.119 and 0.130 or less, the sliding property was evaluated as being good and was marked with an ◯, and when it exceeded 0.130, the sliding property was evaluated as being insufficient and was marked with an X.

(2) Method for Evaluating Film Removability Assuming that the coated steel sheet according to the present invention is used in automotive applications, the film removability during degreasing was evaluated.
To determine the removability of the coated steel sheets, first, each test piece was degreased with an alkaline degreaser Fine Cleaner E6403 (manufactured by Nippon Parkerizing Co., Ltd.). This treatment consisted of immersing the test piece in a degreasing solution with a degreaser concentration of 20 g/L and a temperature of 40° C. for a prescribed period of time, and then rinsing with tap water. After this treatment, the surface carbon intensity of the test piece was measured using an X-ray fluorescence analyzer, and the coating peeling rate was calculated according to the following formula using this measured value, the previously measured surface carbon intensity before degreasing, and the measured value of the surface carbon intensity of the untreated steel sheet.
Film peeling rate (%)=[(carbon strength before degreasing−carbon strength after degreasing)/(carbon strength before degreasing−carbon strength of untreated steel plate)]×100
The removability of the coated steel sheet was evaluated according to the following criteria based on the immersion time in the alkaline degreasing solution until the coating peeling rate reached 98% or more. The following cases of ⊚ and ◯ were judged to be good removability.
◎ (particularly good): within 30 seconds ○ (good): more than 30 seconds, within 60 seconds △ (poor): more than 60 seconds, within 120 seconds × (poor): more than 120 seconds

In the binder types in Table 2, the meanings of each indication are as follows:
R1: Styrene and maleic acid copolymer (acid anhydride equivalent)
R2: copolymer of styrene and maleic acid R3: ammonium salt of copolymer of styrene and maleic acid R4: maleic acid R5: styrene

In Tables 3 to 5, the formula (2) was judged as ◯ when the formula (2) was satisfied, and × when it was not satisfied.
As shown in Tables 3 to 5, all of the steel sheets according to the present invention have excellent press formability. In contrast, all of the steel sheets of the comparative examples that do not satisfy the technical features of the present invention have poor press formability.
Furthermore, among the coated steel sheets of the present invention, those in which the coating weight W (g/ ^m2 ) of the lubricating film per side is W≧0.25× ^Ra2 +0.2, the melting point of the wax in the lubricating film is 125°C or more and 135°C or less, and the average particle size of the wax in the lubricating film is 0.01 μm or more and 0.50 μm or less, clearly have better press formability.

Since the steel sheet according to the present invention exhibits a stable low friction coefficient even when the roughness of the steel sheet changes, it is considered that stable and good press formability can be obtained even when the roughness of the steel sheet varies during production.
The removability of the coated steel sheets in the invention examples was rated as ⊚ (particularly good) except for cases where the mass ratio of wax in the coating was 50% or more or the coating weight W of the lubricating coating per side exceeded 0.9 g/ ^m2 .

The coated steel sheet of the present invention has excellent press formability and can be applied to a wide range of fields, primarily in automobile body applications.

1 Friction coefficient measurement sample 2 Sample stage 3 Slide table 4 Roller 5 Slide table support 6 Bead 7 First load cell 8 Second load cell 9 Rail

Claims (5)

A cold-rolled steel sheet having a coating containing an organic resin and a wax on at least one surface thereof,
the organic resin is an alkali-soluble resin, and the alkali-soluble resin is at least one selected from the group consisting of a copolymer of styrene and maleic acid, a copolymer of styrene and maleic anhydride, and a salt of a copolymer of styrene and maleic acid;
The wax is a polyolefin wax having a melting point of 120° C. or more and 140° C. or less and an average particle size of 0.01 μm or more and 3.00 μm or less, and a proportion C of the wax in the total amount of the organic resin and the wax defined by the following formula (1) is 10 mass% or more,
A cold-rolled steel sheet in which the relationship between the coating weight W (g/m ² ) per one side of the steel sheet and the arithmetic mean roughness Ra (μm) of the base steel surface of the cold-rolled steel sheet satisfies the following formula (2):
C={M _B /(M _A + M _B )}×100...Formula (1)
Where, M _A : the mass of the organic resin converted into an acid anhydride
M _B : Mass of the polyolefin wax W≧0.25×Ra ² +0.2 Formula (2) The cold-rolled steel sheet according to claim 1, wherein the proportion C of wax in the total amount of the organic resin and wax is 50 mass% or less. The cold-rolled steel sheet according to claim 1 or 2, wherein the coating contains the organic resin and the wax in a total amount of 70 mass% or more. The cold rolled steel sheet according to any one of claims 1 to 3, wherein the coating weight W is 2.0 g/ ^m2 or less. The cold-rolled steel sheet according to any one of claims 1 to 4, wherein the arithmetic mean roughness Ra is 0.40 μm or more and 2.50 μm or less.

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