Cold-rolled steel sheet

Abstract

The present invention provides a cold-rolled steel sheet in which the coefficient of friction against a die or the like is significantly reduced and the press formability is excellent. The organic resin of the coating film of the cold-rolled steel sheet 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, and the wax of the coating film is a polyolefin wax having a melting point of 120°C to 140°C and an average particle diameter of 0.01 μm to 3.00 μm, the proportion of the wax in the total amount of the organic resin and the wax is set to a prescribed range, and the relationship between the adhesion amount W (g / m 2 ) of the coating film per single side of the steel sheet and the arithmetic average roughness Ra (μm) of the base iron surface of the cold-rolled steel sheet satisfies a prescribed range.

Description

Technical Field

[0001] This invention relates to a cold-rolled steel sheet with excellent sliding properties during stamping. In particular, it relates to a cold-rolled steel sheet with a lubricating film that exhibits excellent formability even during harsh drawing processes. Background Technology

[0002] Cold-rolled steel sheets are widely used in a wide range of applications, primarily in automotive bodies, where they are typically produced through stamping. In recent years, the demand for integrated parts that eliminates manufacturing processes, improved aesthetics, and the ability to perform more complex forming processes have increased in such stamping operations.

[0003] Here, in the case of more complex stamping, there is a possibility that the steel sheet may not be able to withstand the forming and break, or that the die may wear out during continuous stamping, which may seriously affect the productivity of automobiles.

[0004] One method to improve the stamping formability of cold-rolled steel sheets is surface treatment of the dies. This surface treatment method is widely used to improve stamping formability, but it prevents die adjustments after the surface treatment is performed. Furthermore, it is costly. Therefore, there is a strong demand for improvements in the stamping formability of steel sheets themselves.

[0005] One method to improve stamping formability without surface treatment of the mold is by using high-viscosity lubricating oil. However, this method can lead to poor degreasing after stamping, potentially resulting in deterioration of paintability.

[0006] Based on the above situation, various studies were conducted on lubricated surface-treated steel sheets as a surface treatment technology that enables stamping without the use of molds and high-viscosity lubricating oil.

[0007] For example, Patent Document 1 describes a metal plate coated with a lubricating film that allows solid lubricant to protrude 0.01 to 1.5 μm from the surface of a resin film.

[0008] Patent document 2 describes a lubricated surface-treated metal product with excellent stamping properties, which has a 0.5-5 μm lubricant-containing film coated on a polyurethane resin.

[0009] Patent document 3 describes a technique for forming an alkali-soluble organic film by adding a lubricant to an epoxy resin on a steel plate.

[0010] Existing technical documents

[0011] Patent documents

[0012] Patent Document 1: Japanese Patent Application Publication No. 10-52881

[0013] Patent Document 2: Japanese Patent Application Publication No. 2000-309747

[0014] Patent Document 3: Japanese Patent Application Publication No. 2000-167981 Summary of the Invention

[0015] However, while the technologies described in Patent Documents 1-3 achieve a degree of lubrication due to the lubricating effect of the contained lubricants, they may not provide sufficient stamping formability in complex forming processes in recent years. In particular, it is difficult to consistently obtain good stamping formability when the surface roughness of the steel sheet changes.

[0016] The present invention was made in view of the above circumstances, and its purpose is to reduce the sliding resistance of the crack-prone parts during stamping of steel sheets that are difficult to stamp and form in complex processes, thereby giving excellent stamping formability even in parts where high surface pressure is expected to cause die wear, and in particular, giving excellent stamping formability to steel sheets with a wide range of surface roughness.

[0017] Furthermore, when used as automotive steel sheet, sufficient release properties are required during the alkaline degreasing process in the painting process. Therefore, in this application, the aim is to provide a coated steel sheet that, in addition to the aforementioned stamping formability, also possesses good release properties.

[0018] The inventors conducted in-depth research to solve the problems of the prior art, and found that by including an adhesive that meets specific conditions and a wax that meets specific conditions, and by making a film containing the wax in a specific mass ratio, and by controlling the surface roughness of the iron substrate and the amount of the film, the above problems can be solved.

[0019] The present invention was made based on the above circumstances, and its main purpose is as follows.

[0020] 1. A cold-rolled steel sheet having a coating comprising organic resin and wax on at least one side.

[0021] The aforementioned organic resin is an alkali-soluble resin, which is at least one 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 aforementioned wax is a polyolefin wax with a melting point of 120°C to 140°C and an average particle size of 0.01 μm to 3.00 μm. The proportion C of wax in the total amount of organic resin and wax as defined by the following formula (1) is 10% by mass or more. The amount of film adhering to each single side of the aforementioned film is W (g / m²). 2 The relationship between the arithmetic mean roughness Ra (μm) of the base iron surface of the cold-rolled steel sheet and the roughness of the base iron surface satisfies the following equation (2).

[0022] C = {M B / (MA +M B )}×100 …… Equation (1)

[0023] Here, M A Convert the above organic resin to the mass of acid anhydride.

[0024] M B The quality of the above-mentioned polyolefin wax

[0025] W≥0.25×Ra 2 +0.2……Equation (2)

[0026] 2. The cold-rolled steel sheet according to 1 above, wherein the proportion C of wax in the total amount of the organic resin and wax is less than 50% by mass.

[0027] 3. The cold-rolled steel sheet according to 1 or 2 above, wherein the coating comprises at least 70% by mass of the organic resin and the wax.

[0028] 4. The cold-rolled steel sheet according to any one of claims 1 to 3 above, wherein the aforementioned adhesion amount W is 2.0 g / m 2 the following.

[0029] 5. The cold-rolled steel sheet according to any one of 1 to 4 above, wherein the arithmetic mean roughness Ra is 0.40 μm to 2.50 μm.

[0030] According to the present invention, the coefficient of friction between the steel sheet and the die can be significantly reduced, thus obtaining a cold-rolled steel sheet with excellent stamping formability. Therefore, stable and excellent stamping formability can be imparted to low-strength steel sheets subjected to complex forming processes. Furthermore, even with high-strength steel sheets where the surface pressure increases during stamping, the sliding resistance at crack-prone areas during stamping is reduced, thus exhibiting excellent stamping formability, particularly in areas with high surface pressure and where die wear is anticipated.

[0031] It should be noted that, in the above, "high strength" assumes a tensile strength (TS) of 440 MPa or higher, and "relatively low strength" assumes a TS of less than 440 MPa.

[0032] Furthermore, by applying specific conditions to the organic resin and wax, it is possible to provide a cold-rolled steel sheet that exhibits excellent alkali-free properties and is best suited for automotive applications. Attached Figure Description

[0033] Figure 1 This is a simplified front view of the friction coefficient measuring device.

[0034] Figure 2 It means Figure 1 A simplified three-dimensional diagram showing the shape and dimensions of the flange. Detailed Implementation

[0035] Cold-rolled steel sheet

[0036] The embodiments of the present invention will be described below.

[0037] This invention relates to cold-rolled steel sheets with a coating comprising organic resin and wax formed on at least one side. Hereinafter, "steel sheet" refers to cold-rolled steel sheets without a coating.

[0038] In this invention, the aforementioned organic resin is an alkali-soluble resin. This alkali-soluble resin is selected from at least one 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 aforementioned wax is a polyolefin wax with a melting point of 120°C to 140°C and an average particle size of 0.01 μm to 3.00 μm.

[0039] Furthermore, in the cold-rolled steel sheet of the present invention, the proportion C of the total amount of organic resin and wax defined by the following formula (1) in the wax is 10% by mass or more, and the adhesion amount W (g / m) of the above-mentioned coating per single side is... 2 The relationship between the arithmetic mean roughness Ra (μm) of the substrate iron surface of the coated steel plate and the coating satisfies the following equation (2).

[0040] C = {M B / (M A +M B )}×100 …… Equation (1)

[0041] Here, M A Convert the above organic resin to the mass of acid anhydride.

[0042] M B The quality of the above-mentioned polyolefin wax

[0043] W≥0.25×Ra 2 +0.2……Equation (2)

[0044] It should be noted that when the aforementioned organic resin (A) is a copolymer of styrene and maleic anhydride, the actual mass of the copolymer is set as M. A Furthermore, when the aforementioned organic resin (A) is a copolymer of styrene and maleic acid or a salt of a copolymer of styrene and maleic acid, the mass of these resins converted to anhydrides (converting the portion of maleic acid or maleate salt to the mass of maleic anhydride) is set to M. A .

[0045] <Wax>

[0046] There are no particular limitations on the type of wax used in this invention, provided that it is a polyolefin wax with a melting point of 120°C to 140°C and an average particle size of 0.01 μm to 3.00 μm.

[0047] Polyolefin wax is used as the wax in this invention because it has low surface energy and self-lubricating properties, thus providing good lubricity. Furthermore, polyolefin waxes are relatively easy to adjust to a melting point of 120°C–140°C by controlling their density and molecular weight.

[0048] When the melting point of the aforementioned polyolefin wax is 120℃~140℃, in addition to the self-lubricating properties of the polyolefin wax itself, due to the sliding during stamping, the wax becomes semi-molten. Therefore, the lubricating film component mixed with the organic resin can coat the mold surface, further inhibiting direct contact between the mold and the steel plate, resulting in excellent lubrication. Furthermore, it is believed that the wax in the film adheres efficiently to the mold under the sliding state during stamping, resulting in a phenomenon where it is not easily detached, thus achieving a high lubrication effect.

[0049] In other words, if the melting point is less than 120°C, the wax will completely melt due to the frictional heat generated during the sliding process of stamping, resulting in insufficient lubrication from the wax itself and also failing to achieve the coating effect of the mold. On the other hand, if the melting point exceeds 140°C, melting will not occur during sliding, resulting in insufficient lubrication and also failing to achieve the coating effect of the mold. Even if the coating adheres to the mold, the adhesion will be weak, and the coating will easily fall off during sliding.

[0050] It should be noted that the lower limit of the above-mentioned melting point is preferably around 125°C. On the other hand, the upper limit of the above-mentioned melting point is preferably around 135°C.

[0051] Here, the melting point of the wax of the present invention is the melting temperature determined in accordance with JIS K 712:1987 "Method for determination of the transition temperature of plastics".

[0052] If the average particle size of the wax exceeds 3.00 μm, it is difficult to mix with the organic resin during sliding, resulting in an inability to achieve the aforementioned coating effect on the mold and insufficient lubricity. Preferably, it is 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 important that the average particle size of this wax is 0.01 μm or more. When it is less than 0.01 μm, it easily dissolves in the lubricating oil during sliding, sometimes failing to provide sufficient lubricity to improve the effect. Even in coatings, it easily solidifies, resulting in low coating stability. Preferably, it is 0.03 μm or more.

[0053] The average particle size mentioned above is the median particle size on a volume basis, obtained by laser diffraction / scattering. For example, it can be obtained by measuring a sample diluted with pure water using a laser diffraction / scattering particle size distribution measuring device, such as Partica LA-960V2 (manufactured by Horiba Corporation). Polyethylene wax provides the best lubrication effect when used in polyolefin waxes; therefore, polyethylene wax is preferred.

[0054] In this invention, the mass percentage of wax in the total amount of organic resin and wax (C calculated using the above formula (1)) is set to 10% by mass or more. Insufficient lubrication effect cannot be obtained when it is less than 10% by mass. On the other hand, the mass percentage of wax in the coating is preferably 50% by mass or less. When it exceeds 50% by mass, the wax is prone to detachment due to insufficient matrix resin content, resulting in poor adhesion to the steel plate and sometimes preventing it from stably existing as a coating.

[0055] The mass percentage C of wax in the aforementioned coating is more preferably 30% by mass or less. By setting the mass percentage of wax in the coating to 30% or less, the degreasing properties of the alkaline degreasing process in the coating process can be ensured when the coating is used as automotive steel sheet, and the deterioration of coatability caused by residual coating can be suppressed.

[0056] Here, the proportion C of wax in the total amount of organic resin and wax refers to the proportion of the solid component of wax in the film relative to the total mass of the solid component of organic resin in the film and the solid component of wax in the film, which can be obtained by the above formula (1).

[0057] As a specific method for determining the aforementioned quality, for organic resins and waxes, test pieces with known amounts of organic resin and wax adhering to a coated steel plate are prepared. Infrared absorption spectra are measured using an FT-IR measuring device, and calibration curves for the respective amounts of organic resin and wax are constructed based on the peak intensities originating from the organic resin and wax, respectively. In this case, the intensity of the infrared absorption spectrum originating from the adhesive is determined using the characteristic spectra of the aromatic groups from the styrene structural unit, the ester groups from the maleic acid structural unit, and the lactone rings from the maleic anhydride structural unit, taking into account the ratio of each structural unit constituting the copolymer. Furthermore, the intensity of the infrared absorption spectrum originating from the wax can be determined using the characteristic spectrum of the methylene group as an indicator.

[0058] Next, the infrared absorption spectrum of the coated steel plate of the object was measured, and the mass ratio of wax in the coating was determined by calculating the amount of resin and wax adhering from the above calibration curve.

[0059] <Organic Resins>

[0060] In this invention, the organic resin acts as a binder to retain the wax on the surface of the steel plate. The sliding effect of the mold coated with the wax-organic resin mixture formed during sliding is not achieved due to the low affinity of the inorganic binder for polyolefins.

[0061] As the above-mentioned adhesive, at least one selected from the copolymer of styrene and maleic acid, the copolymer of styrene and maleic anhydride, and the salt of the copolymer of styrene and maleic acid is used.

[0062] Choosing styrene as the monomer component of the adhesive helps improve stamping formability. Additionally, maleic acid was chosen because its removability is improved through alkali treatment. It should be noted that the reasons for choosing maleic anhydride are the same as those for maleic acid.

[0063] Therefore, by using any one of the copolymers of styrene and maleic acid, copolymers of styrene and maleic anhydride, or salts of copolymers of styrene and maleic acid as binders, a film with excellent balance between stamping properties and alkali removal properties can be formed with other acrylic resins, epoxy resins, polyurethane resins, phenolic resins, vinyl acetate resins, and polyester resins.

[0064] Furthermore, in this invention, the same effect can be obtained by selecting any of the following: 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. The adhesive only needs to retain the wax particles, and the removal of the film based on alkali degreasing is excellent, regardless of the type of salt or whether the maleic acid is anhydrous.

[0065] It should be noted that the copolymers of the present invention can be obtained by polymerizing styrene with maleic acid and styrene with maleic anhydride, and this reaction can be a commonly known copolymerization method. Furthermore, the salts of the copolymers of the present invention can be obtained by neutralizing the aforementioned polymers, and this method can be a commonly known method.

[0066] In this invention, the salt of the above-mentioned copolymer is a known salt of a cationic copolymer, and is not particularly limited, but is most preferably a salt of ammonium.

[0067] The mass-average molecular weight of the above-mentioned copolymer is preferably in the range of 4,000 to 400,000. Furthermore, 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. It should be noted that if the mass-average molecular weight of the above-mentioned copolymer is set in the range of 4,000 to 400,000, better stamping formability and alkali removal properties can be obtained.

[0068] Here, the mass-average molecular weight mentioned above can be determined using a high-speed GPC apparatus, HLC-8320GPC (manufactured by Tosoh Corporation), with a TSKgel-G column, tetrahydrofuran as the eluent, and polystyrene as the standard sample, and is determined by GPC (gel permeation chromatography).

[0069] The monomer arrangement of styrene and maleic acid or styrene and maleic anhydride in this invention can be any of random copolymers, alternating copolymers, block copolymers or graft copolymers, with random copolymers preferred if manufacturing costs are taken into consideration.

[0070] It should be noted that the random copolymer of the present invention refers to a copolymer of styrene and maleic acid or styrene and maleic anhydride arranged irregularly.

[0071] In this invention, when forming the aforementioned copolymers of styrene and maleic acid or styrene and maleic anhydride, the content ratio (styrene / maleic acid or styrene / maleic anhydride) of the structural units from each monomer of the aforementioned styrene and maleic acid or styrene and 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, more preferably 8 / 2.

[0072] When the molar ratio is less than 1 / 9, the stamping formability may be poor. That is, the film hardness is insufficient and the film is prone to breakage under pressure, so it is not preferred. On the other hand, when the molar ratio exceeds 9 / 1, the film release property (the ability to remove alkali degreasing) may remain at the same level as the prior art.

[0073] <Membrane>

[0074] The coating of the present invention preferably contains at least 70% by mass of the above-mentioned organic resin and the above-mentioned wax. When the content is less than 70% by mass, the adhesion between the substrate iron and the coating may deteriorate, the coating may easily peel off from the substrate iron, and sometimes it may fail to provide sufficient lubrication.

[0075] It should be noted that, in addition to organic resins and waxes, the components typically include surface conditioners, defoamers, dispersants, etc., added to the coating. Additionally, rust inhibitors and pigments that enhance rust resistance may also be added.

[0076] That is, the components other than the above-mentioned organic resins and waxes are preferably allowed to be about 30% by mass.

[0077] Furthermore, the inventors discovered that when forming the aforementioned coating on the surface of a steel sheet, using a steel sheet with a wide range of surface roughness results in a coating with a wide range of adhesion amounts, and the stamping formability was evaluated. As a result, the surface roughness of the steel sheet and the amount of coating adhesion stably maintain good stamping formability only within a region that satisfies a certain defined relationship.

[0078] The reason why good stamping formability is consistently maintained only within the region that satisfies such a defined relation is unclear.

[0079] However, the inventors considered that the greater the surface roughness of the steel plate, the easier it is for the lubricating film on the protrusion of the coated steel plate to become thinner. In the case of stamping, the film is shaved off due to sliding with the mold, making it easy for the base steel plate to be exposed and difficult to obtain a lubricating effect.

[0080] In this invention, since the lubricating film components are promoted to adhere to the mold during sliding, the mold side is protected even when the roughness of the steel plate is greater than that of the past, and the lubricity is not compromised.

[0081] Specifically, the film adhesion range that demonstrates good stamping properties is as follows.

[0082] The amount of film adhering to each single side, W (g / m²). 2 The relationship between the arithmetic mean roughness Ra (μm) of the substrate iron surface of the coated steel plate and the roughness of the coated steel plate satisfies W ≥ 0.25 × Ra 2 The range is +0.2. This applies when W < 0.25 × Ra. 2 At +0.2, the amount of film adhesion is insufficient, and the mold side cannot be adequately protected, resulting in poor stamping formability.

[0083] In the coating, the coating component that primarily contributes to slip resistance is present on the coated steel plate protrusions that come into contact with the die during stamping. It is assumed that the area of ​​the coated steel plate protrusions in contact with the die is related to Ra. 2 The value of decreases proportionally. Therefore, by comparing with Ra 2 The value increases the amount of film adhesion proportionally, which can adequately ensure the amount of film components that contribute to lubrication.

[0084] In this invention, the adhesion amount W is preferably 2.0 g / m³. 2 The following. If it exceeds 2.0g / m 2 If this is not done, issues such as deterioration in film release and weldability may occur. It should be noted that the adhesion amount W is more preferably 0.9 g / m². 2 The following applies if the adhesion amount W is 0.9 g / m. 2 The following have particularly good desiccant properties.

[0085] On the other hand, in order to achieve the effects of the present invention, the above-mentioned adhesion amount W is preferably 0.2 g / m 2 above.

[0086] The amount of coating can be obtained by dividing the mass difference of the steel plate before and after coating formation by the area, or by completely removing the coating from the coated steel plate after coating formation using alkaline aqueous solution or organic solvent, and then dividing the mass difference of the steel plate before and after coating removal by the area.

[0087] The arithmetic mean roughness Ra of the present invention is preferably 0.40 μm to 2.50 μm. When Ra is less than 0.40 μm, micro-scratches that may occur during stamping are easily noticeable, and wear may sometimes occur during stamping. On the other hand, if Ra exceeds 2.50 μm, the required amount of film adhesion increases, resulting in increased manufacturing costs and deterioration of the clarity (blurriness) after coating.

[0088] The Ra (μm) mentioned above can be measured according to JIS B 0633:2001 (ISO 4288:1996). For example, if the Ra is greater than 0.1 and less than 2, the cutoff value and reference length are 0.8 mm, and the evaluation length is 4 mm, and the value is obtained from the measured roughness curve. On the other hand, if the Ra is greater than 2 and less than 10, the cutoff value and reference length are set to 2.5 mm, and the evaluation length is set to 12.5 mm, and the value is obtained from the measured roughness curve, thereby determining the Ra value.

[0089] [Manufacturing method of cold-rolled steel sheet]

[0090] Next, the method for manufacturing the cold-rolled steel sheet of the present invention will be described.

[0091] The method for manufacturing cold-rolled steel sheet of the present invention is a method for manufacturing steel sheet having an organic resin coating containing polyolefin wax with a melting point of 120°C to 140°C and an average particle size of 0.01 μm to 3.00 μm on the surface of the steel sheet. It also includes at least the step of applying an organic resin solution in which the organic resin is dissolved or dispersed in a solvent or a coating in which the wax is added to an emulsion to the surface of the steel sheet and then drying it.

[0092] The preferred mass percentage of the film-coating components (organic resin and polyolefin wax) in the above-mentioned coating is 1% to 25% by mass. If the mass percentage is less than 1% or more than 25% by mass, uneven coating may occur.

[0093] There are no particular limitations on the coating methods described above. Examples include roller coating, bar coating, spraying, dipping, and brushing. The drying of the coated steel sheet can be carried out using conventional methods. Examples include hot air drying, IH heater drying, or infrared heating.

[0094] The maximum temperature that the coated steel sheet can reach during drying is preferably 60°C to 140°C. Below 60°C, drying is time-consuming and rust prevention deteriorates. On the other hand, above 140°C, the wax melts and coalesces, resulting in coarser particle size and deterioration of lubricity.

[0095] Example

[0096] The present invention will now be described using examples. It should be noted that the present invention is not limited to the following examples.

[0097] Cold-rolled steel sheets (steel sheet No. A to D) with a thickness of 0.8 mm and an arithmetic mean roughness Ra as shown in Table 1 were coated with a paint with the composition shown in Table 2 using a bar coater. The sheets were then dried using an IH heater to bring the maximum reach temperature of the steel sheet to 80°C, thereby producing the coated steel sheet supplied in this embodiment. It should be noted that in paint No. 22, in addition to organic resin and wax, 30% by mass of silica was added as a pigment to the coating. Furthermore, the steel sheets No. A to D are all SPCD (JIS G 3141) with a tensile strength of 270 MPa.

[0098] (1) Evaluation method of stamping formability (sliding characteristics)

[0099] To evaluate stamping formability, the coefficient of friction of each test material was measured as follows.

[0100] Figure 1 This is a simplified front view of the friction coefficient measuring device.

[0101] like Figure 1 As shown, a friction coefficient measuring sample 1, taken from the test material, is fixed to a sample stage 2, which is fixed to the upper surface of a horizontally sliding platform 3. A sliding platform support 5, having a roller 4 in contact with it and capable of vertical movement, is provided on the lower surface of the sliding platform 3. By pushing the support 5 upward, a pressing load N is applied from the flange 6 to the friction coefficient measuring sample 1. A first weighing sensor 7 for measuring the pressing load N is mounted on the sliding platform support 5.

[0102] Furthermore, the sliding resistance F used to move the sliding table 3 horizontally is measured under the pressing force of the aforementioned pressing load N. A second weighing sensor 8 used for this measurement is mounted at one end of the sliding table 3. The sliding table 3 moves horizontally via guide rail 9.

[0103] It should be noted that in the above embodiments, Preton R352L, a pressure cleaning oil manufactured by Sugimura Chemical Industry Co., Ltd., was applied as a lubricant to the surface of sample 1 for testing.

[0104] Figure 2 This is a simplified perspective view showing the shape and dimensions of the flange 6 used. The lower surface of the flange 6 slides while being pressed against the surface of the sample 1. Figure 2 The flange 6 shown has a width of 10 mm, a sliding length of 59 mm, and the lower part at both ends of the sliding direction is composed of a curved surface with a curvature of 4.5 mmR. The lower surface of the flange pressing the sample has a plane with a width of 10 mm and a sliding length of 50 mm.

[0105] The friction coefficient determination test is to... Figure 2 The flange shown is applied to Figure 1 The friction coefficient measuring device shown was used with a pressing load N of 400 kgf and a sample traction speed (horizontal movement speed of sliding table 3) of 20 cm / min. The friction coefficient μ between the test material and the flange was calculated by the following formula: μ = F / N.

[0106] A friction coefficient of 0.119 or less is considered exceptionally good sliding performance and is rated as "◎"; a coefficient of friction greater than 0.119 but less than 0.130 is considered good sliding performance and is rated as "〇"; and a coefficient of friction greater than 0.130 is considered insufficient sliding performance and is rated as "×".

[0107] (2) Evaluation methods for desiccant properties

[0108] Assuming the coated steel sheet of the present invention is used in automotive applications, the decoction properties during degreasing are evaluated.

[0109] To determine the decoction properties of coated steel plates, the test pieces were first degreased using E6403 (manufactured by Nihon Parkerizing Co., Ltd.), a fine cleaning agent derived from alkaline degreasing. This treatment involved immersing the test pieces in a degreasing solution with a concentration of 20 g / L and a temperature of 40°C for a specified time, followed by rinsing with tap water. The surface carbon strength of the treated test pieces was measured using a fluorescence X-ray analysis device. Using this measured value, along with pre-measured surface carbon strength values ​​of the steel plate before degreasing and the untreated steel plate, the coating peeling rate was calculated using the following formula.

[0110] Coating peel rate (%) = [(Carbon strength before degreasing - Carbon strength after degreasing) / (Carbon strength before degreasing - Carbon strength of untreated steel plate)] × 100

[0111] The peelability of the coated steel sheet was evaluated by the immersion time in an alkaline degreasing solution where the coating peeling rate reached 98% or higher, according to the criteria shown below. It should be noted that good peelability is determined by the presence of "◎" and "○" below.

[0112] ◎(Exceptionally Good): Within 30 seconds

[0113] ○ (Good): More than 30 seconds but less than 60 seconds

[0114] △ (Insufficient): More than 60 seconds but less than 120 seconds

[0115] × (Poor): Exceeded 120 seconds

[0116] [Table 1]

[0117] Table 1

[0118] Steel plate No. Ra(μm) A 0.48 B 0.78 C 1.48 D 2.35

[0119]

[0120]

[0121] The meanings of each type of adhesive in Table 2 are as follows.

[0122] R1: A copolymer of styrene and maleic acid (equivalent to an anhydride)

[0123] R2: Copolymer of styrene and maleic acid

[0124] R3: Ammonium salt of a copolymer of styrene and maleic acid

[0125] R4: Maleic acid

[0126] R5: Styrene

[0127] In Table 2, "PTFE" indicates polytetrafluoroethylene wax.

[0128]

[0129]

[0130]

[0131]

[0132]

[0133] In the determination of equation (2) in Tables 3-5, the case that satisfies the above equation (2) is determined as "0", and the case that does not satisfy it is set as "×".

[0134] As shown in Tables 3-5, the coated steel sheets of the present invention all exhibit excellent stamping formability. In contrast, the steel sheets of the comparative examples that do not satisfy the technical features of the present invention, or the coated steel sheets, all have poor stamping formability.

[0135] Furthermore, it is known that in the coated steel plate of the present invention, the adhesion amount W (g / m) of the lubricating coating on each single side is... 2 ) is W≥0.25×Ra 2 +0.2. The melting point of the wax coating is 125℃~135℃, and the average particle size of the wax coating is 0.01μm~0.50μm. Steel plates with coatings have better stamping formability.

[0136] The coated steel sheet of the present invention consistently exhibits a low coefficient of friction even when the roughness of the steel sheet varies, thus it is believed that even if there are deviations in the roughness of the steel sheet during manufacturing, good stamping formability can be consistently obtained.

[0137] In addition, the peelability of the coated steel sheet in the invention example is that the mass ratio of wax in the coating is more than 50% or the amount of lubricating coating adhering to each single side, W, exceeds 0.9 g / m. 2 All cases are ◎ (Excellent).

[0138] Industrial availability

[0139] The coated steel sheet of the present invention has excellent stamping formability, and therefore can be applied to a wide range of fields, with automotive body applications as the main focus.

[0140] Symbol Explanation

[0141] 1. Sample for determining the coefficient of friction

[0142] 2. Sample stage

[0143] 3 Sliding platform

[0144] 4 rollers

[0145] 5. Sliding table support platform

[0146] 6 flanges

[0147] 7 First Weighing Sensor

[0148] 8 Second Weighing Sensor

[0149] 9 guide rails

Claims

1. A cold-rolled steel sheet having a coating comprising organic resin and wax on at least one side. The organic resin is an alkali-soluble resin, and the alkali-soluble resin is selected from at least one of the following: copolymers of styrene and maleic acid, copolymers of styrene and maleic anhydride, and salts of copolymers of styrene and maleic acid. The wax is a polyolefin wax with a melting point of 120℃~140℃ and an average particle size of 0.01μm~3.00μm, wherein the proportion C of the wax in the total amount of organic resin and wax as defined by the following formula (1) is 10% by mass or more. The coating thickness W (g / m²) on each side of the steel plate is [not specified]. 2 The relationship between the arithmetic mean roughness Ra (μm) of the base iron surface of the cold-rolled steel sheet and the roughness of the base iron surface satisfies the following equation (2). C = {M B / (M) A +M B )}×100 …… Equation (1) Here, M A The organic resin is converted to the mass of acid anhydride. M B The mass of the polyolefin wax. W≥0.25×Ra 2 +0.2……Equation (2).

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 less than 50% by mass.

3. The cold-rolled steel sheet according to claim 1 or 2, wherein, The coating comprises the organic resin and the wax in an aggregate weight of 70% or more.

4. The cold-rolled steel sheet according to claim 1 or 2, wherein, The adhesion amount W is 2.0 g / m 2 the following.

5. The cold-rolled steel sheet according to claim 3, wherein, The adhesion amount W is 2.0 g / m 2 the following.

6. The cold-rolled steel sheet according to claim 1 or 2, wherein, The arithmetic mean roughness Ra is 0.40 μm to 2.50 μm.

7. The cold-rolled steel sheet according to claim 3, wherein, The arithmetic mean roughness Ra is 0.40 μm to 2.50 μm.

8. The cold-rolled steel sheet according to claim 4, wherein, The arithmetic mean roughness Ra is 0.40 μm to 2.50 μm.

9. The cold-rolled steel sheet according to claim 5, wherein, The arithmetic mean roughness Ra is 0.40 μm to 2.50 μm.

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