Steel sheet and method for manufacturing the same

Abstract

The present invention aims to provide a steel sheet having a lubricating coating film, which has a small sliding resistance at a fracture risk site during press forming, has excellent press formability at a site where surface pressure is high and sticking is assumed to occur, and has excellent press formability particularly for a wide range of surface roughness, and a method for manufacturing the same. A steel sheet which is a steel sheet having a coating film containing an organic resin and a wax formed on at least one surface, wherein the organic resin is at least any one of an acrylic resin, an epoxy resin, a urethane resin, a phenol resin, a vinyl acetate resin, and a polyester resin, the wax is a polyolefin wax having a melting point of 120°C or higher and 140°C or lower and an average particle diameter of 3.0 μm or less, the proportion of the wax in the coating film is 10% by mass or more, and the relationship between the attached amount W (g / m 2 ) of the coating film per one surface of the coating film and the arithmetic average roughness Ra (μm) of the steel sheet is within the range of the following formula (1). W ≥ 0.12 × Ra 2 + 0.2 … (1).

Description

Technical Field

[0001] This invention relates to a steel sheet with excellent sliding properties in stamping and a method for manufacturing the same. In particular, it relates to a steel sheet with a lubricating coating that exhibits excellent formability even during demanding deep drawing processes with large drawing ratios, and a method for manufacturing the same. Background Technology

[0002] Cold-rolled and hot-rolled steel sheets are widely used in a wide range of applications, primarily automotive bodywork, where they are typically produced through stamping. In recent years, the demand for more integrated and sophisticated designs to streamline processes has led to a need for more complex forming capabilities.

[0003] In cases where more complex stamping processes are required, there is a possibility that the steel sheet may not withstand the forming process and break, or that sticking may occur during continuous stamping, which could have a serious negative impact on automobile productivity.

[0004] One method to improve the stamping formability of cold-rolled and hot-rolled steel sheets is surface treatment of the dies. While this is a widely used method, it suffers from several drawbacks: the dies cannot be adjusted after surface treatment. Furthermore, it is costly. Therefore, there is a strong demand to improve the stamping formability of the steel sheets themselves.

[0005] One method to improve stamping formability without surface treatment of the mold is to use high-viscosity lubricating oil. However, in this method, poor degreasing sometimes occurs after stamping, which may degrade the paintability.

[0006] Therefore, various lubricated surface-treated steel sheets are being researched as a technique that enables stamping without the use of molds and with high-viscosity lubricating oil.

[0007] Patent document 1 describes a metal plate covered with a lubricating coating that allows solid lubricant to protrude 0.01 to 1.5 μm from the surface of a resin coating.

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

[0009] Patent document 3 describes a technique for forming an alkali-soluble organic coating on a steel plate, in which a lubricant is added to an epoxy resin.

[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] The problem that the invention aims to solve

[0016] However, in Patent Documents 1-3, although lubricity is demonstrated through the lubrication effect provided by the contained lubricants, the stamping formability may not be sufficient in complex forming processes. In particular, good stamping formability cannot be consistently obtained when the surface roughness of the steel sheet varies.

[0017] The present invention was made in view of the above circumstances, and aims to provide a steel sheet capable of performing complex forming that is difficult to form by stamping, which has low sliding resistance at the parts where it is prone to breakage during stamping, excellent stamping formability at the parts where the surface pressure is high and it is assumed that sticking will occur, and especially excellent stamping formability for steel sheets with a wide range of surface roughness, and a method thereof.

[0018] Furthermore, when used as automotive steel sheets, sufficient descaling properties are required during the alkaline degreasing process in the painting process. Therefore, a further objective is to provide steel sheets with good descaling properties for such applications and a method for manufacturing them.

[0019] Methods for solving problems

[0020] To solve the above-mentioned problems, the inventors conducted repeated and in-depth research. The results showed that, in order to dramatically improve stamping formability, an organic resin coating containing polyolefin wax with a melting point above 120°C and below 140°C and an average particle size of less than 3.0 μm is formed on the surface of the steel sheet. By controlling the surface roughness of the steel sheet and the amount of coating adhesion, the above-mentioned problems can be solved.

[0021] This invention is based on the above insights, and its main points are as follows.

[0022] [1] A steel plate having a coating comprising an organic resin and a wax formed on at least one side, wherein the organic resin is at least one of acrylic resin, epoxy resin, urethane resin, phenolic resin, vinyl acetate resin, and polyester resin, and the wax is a polyolefin wax with a melting point of 120°C or higher and 140°C or lower, and an average particle size of 3.0 μm or lower, wherein the proportion of wax in the coating is 10% by mass or higher, and the coating has an adhesion amount W (g / m²) on each side. 2The relationship between the roughness Ra (μm) of the steel plate and the arithmetic mean roughness Ra is within the range of the following formula (1).

[0023] W≥0.12×Ra 2 +0.2…(1)

[0024] [2] According to the steel plate described in [1], wherein the amount of coating applied to each single side of the above-mentioned coating is W (g / m²). 2 The relationship between the roughness Ra (μm) of the steel plate and the arithmetic mean roughness Ra is within the range of the following formula (2).

[0025] W≥0.25×Ra 2 +0.2…(2)

[0026] [3] According to the steel plate described in [1] or [2], wherein the amount of coating W applied to each single side of the above-mentioned coating is 2.0 g / m 2 the following.

[0027] [4] The steel plate according to any one of [1] to [3], wherein the arithmetic mean roughness Ra of the steel plate before the coating is formed is 0.4 μm or more and 2.5 μm or less.

[0028] [5] The steel plate according to any one of [1] to [4], wherein the average particle size of the wax is 0.01 μm or more and 0.5 μm or less.

[0029] [6] The steel plate according to any one of [1] to [5], wherein the proportion of wax in the above coating is less than 50% by mass.

[0030] [7] The steel plate according to any one of [1] to [6], wherein the amount of coating W applied to each single side of the above-mentioned coating is 0.9 g / m 2 the following.

[0031] [8] The steel plate according to any one of [1] to [7], wherein the organic resin is an alkali-soluble resin.

[0032] [9] The steel plate according to any one of [1] to [8], wherein the PPI of the steel plate before the coating is formed is 120 or more and 220 or less.

[0033]

[10] A method for manufacturing a steel plate, which is a method for manufacturing a steel plate according to any one of [1] to [9], wherein a coating comprising an organic resin and a wax described in any one of [1] to [9] is applied to at least one side of the steel plate and dried.

[0034]

[11] In the steel plate manufacturing method according to

[10] , the maximum temperature reached by the steel plate during drying is above 60°C and below 140°C.

[0035]

[12] In the steel plate manufacturing method according to

[10] or

[11] , the proportion of the above-mentioned organic resin and wax in the above-mentioned coating is more than 1% by mass and less than 25% by mass.

[0036]

[13] The method for manufacturing a steel plate according to any one of

[10] to

[12] , wherein, when the above formula (1) is not satisfied, the following step is performed before the above coating: changing the adhesion amount W (g / m 2 The arithmetic mean roughness Ra (μm) of the steel plate or the steel plate satisfies the above formula (1).

[0037] Invention Effects

[0038] According to the present invention, a steel sheet capable of undergoing complex forming processes that are difficult to achieve during stamping can be provided, exhibiting low sliding resistance at locations prone to breakage during stamping, excellent stamping formability at locations where surface pressure is high and sticking to the die is hypothetically likely to occur, and particularly excellent stamping formability for steel sheets with a wide range of surface roughness. A method for manufacturing the same is also provided. Furthermore, a steel sheet exhibiting good release properties when used as an automotive steel sheet can be provided, along with a method for manufacturing the same.

[0039] It should be noted that, in this invention, steel plate refers to both cold-rolled and hot-rolled steel plate. Furthermore, in this invention, high strength refers to an imaginary tensile strength (TS) of 440 MPa or higher, and low strength refers to an imaginary TS of less than 440 MPa. Attached Figure Description

[0040] Figure 1 This is a schematic front view showing the friction coefficient measuring device.

[0041] Figure 2 This illustrates Example 1. Figure 1 A rough three-dimensional diagram showing the shape and size of the reinforcing ribs.

[0042] Figure 3 This is a graph showing the relationship between the surface roughness of various steel plates and the coefficient of friction at various coating thicknesses, with regard to the coatings that are the subject of this invention.

[0043] Figure 4 The graph shows the surface roughness of various steel plates and the coefficient of friction for various coating thicknesses, for coatings other than those of the present invention.

[0044] Figure 5 This illustrates Example 2. Figure 1 A rough three-dimensional diagram showing the shape and size of the reinforcing ribs. Detailed Implementation

[0045] The embodiments of the present invention will be described below.

[0046] This invention relates to a steel plate having a coating comprising an organic resin and a wax formed on at least one side, characterized in that the wax is a polyolefin wax with a melting point of 120°C or higher and 140°C or lower and an average particle size of 3.0 μm or lower, and the proportion of wax in the coating is 10% by mass or higher. Furthermore, it is characterized in that the coating has an adhesion amount W (g / m²) on each side. 2 The relationship between the arithmetic mean roughness Ra (μm) of the steel plate before the coating is formed is within the range of the following formula (1).

[0047] W≥0.12×Ra 2 +0.2…(1)

[0048] The wax used in this invention is not limited to polyolefin waxes with a melting point of 120°C or higher and 140°C or lower and an average particle size of 3.0 μm or lower.

[0049] Polyolefin wax is used because it has low surface energy and self-lubricating properties, thus providing good lubrication. Furthermore, for polyolefins, the melting point can be relatively easily adjusted to above 120°C and below 140°C by controlling density and molecular weight.

[0050] When the melting point is above 120°C and below 140°C, in addition to the self-lubricating properties of the polyolefin wax itself, the wax becomes semi-molten due to the sliding during stamping. This allows the lubricating coating component, formed by mixing with organic resin, to coat the mold surface, inhibiting direct contact between the mold and the steel plate, thus achieving excellent lubrication. When the melting point is below 120°C, it completely melts due to the frictional heat generated during stamping, failing to achieve sufficient lubrication from the wax itself, nor the aforementioned mold coating effect. Furthermore, when the melting point exceeds 140°C, it does not melt during sliding, failing to achieve sufficient lubrication or the mold coating effect.

[0051] When the melting point of the wax is above 120°C and below 140°C, it is believed that the wax in the coating adheres efficiently to the mold under sliding conditions during stamping, resulting in a phenomenon that is not easy to fall off and achieving a high lubrication effect. When the melting point is below 120°C, even if the coating adheres to the mold, the adhesion is weak, and the coating is easy to fall off during sliding. When the melting point exceeds 140°C, the coating is difficult to adhere to the mold. The melting point of the wax is further preferably above 125°C. The melting point of the wax is further preferably below 135°C.

[0052] Here, the melting point of wax refers to the melting temperature determined based on JIS K 7121:1987 "Method for determination of the transition temperature of plastics".

[0053] When the average particle size of the wax exceeds 3.0 μm, it is difficult to mix with the organic resin during sliding, resulting in the failure to achieve the aforementioned coating effect on the mold and insufficient lubrication. The average particle size of the wax is preferably 1.5 μm or less. More preferably, it is 0.5 μm or less, and even more preferably, it is 0.3 μm or less.

[0054] The average particle size is preferably 0.01 μm or more. When the average particle size of the wax is less than 0.01 μm, it easily dissolves in the lubricating oil during sliding, sometimes failing to provide sufficient lubrication improvement. It also tends to aggregate in coatings used to form films, resulting in low coating stability. The average particle size of the wax is further preferably 0.03 μm or more. Considering the aforementioned miscibility with organic resins, the average particle size of the wax is preferably 0.01 μm or more and 0.5 μm or less.

[0055] The aforementioned average particle size refers to the median particle size of the volume average particle size, determined by laser diffraction / scattering. For example, it can be determined by measuring a sample diluted with pure water using a Partica (registered trademark) LA-960V2 laser diffraction / scattering particle size distribution measuring device (manufactured by Horiba Corporation).

[0056] Polyethylene wax provides the best lubrication effect when used in polyolefin waxes, therefore it is preferred to use polyethylene wax.

[0057] The wax content in the coating is set to 10% by mass or more. When it is less than 10% by mass, sufficient lubrication is not achieved. A particularly good lubrication effect can be obtained when the wax content in the coating is 15% by mass or more. Furthermore, the wax content in the coating is preferably less than 50% by mass. When it is 50% by mass or more, the wax may easily detach due to insufficient base resin components, resulting in poor adhesion to the steel sheet and inability to exist stably as a coating. Additionally, when used as automotive steel sheet, sufficient degreasing is sometimes not achieved during the alkaline degreasing process in the painting process. Moreover, even when using alkali-soluble organic resins, sometimes the coating cannot be fully removed during the alkaline degreasing process, leaving residual coating and deteriorating coatability. The wax content in the coating is further preferably 30% by mass or less.

[0058] Here, the mass ratio of wax in the coating refers to the ratio of the mass of the solid component of the wax in the coating to the total mass of the solid component of the organic resin in the coating and the solid component of the wax in the coating.

[0059] As a specific testing method, for organic resins and waxes, test pieces with known adhesion amounts on steel plates are prepared. Infrared absorption spectra are measured using an FT-IR measuring device, and calibration curves for the adhesion amounts of organic resins and waxes are constructed from the peak intensities of the organic resins and waxes, respectively. Next, the infrared absorption spectrum of the lubricated coating-coated steel plate, which is the test object, is measured. The adhesion amounts of resin and wax are determined from the calibration curves, thereby allowing the determination of the mass ratio of wax in the coating.

[0060] In this invention, the organic resin acts as a binder to retain the wax on the surface of the steel plate. When the organic resin is replaced with an inorganic binder, the sliding effect achieved by the mold coating of the wax-organic resin mixture formed during sliding is not realized due to its low affinity for polyolefins. As the organic resin, acrylic resins, epoxy resins, urethane resins, phenolic resins, vinyl acetate resins, and polyester resins can be used.

[0061] The acrylic resins used in this invention refer to polymers or copolymers composed of one or more of the following: unsaturated monocarboxylic acids having one carboxyl group in the molecule, such as acrylic acid and methacrylic acid; esters of the above-mentioned unsaturated monocarboxylic acids; and styrene; or their sodium salts, potassium salts, ammonium salts, amine salts, or other derivatives.

[0062] In the case of acrylic resins with two or more carboxyl groups in the molecule as monomers, the stability of the coating may be poor. Therefore, in this invention, acrylic resins with a fatty acid or its fatty acid ester having one carboxyl group in the molecule as monomers are used.

[0063] There are no particular limitations on epoxy resins; examples include bisphenol A type epoxy resin, bisphenol F type epoxy resin, and phenolic varnish type epoxy resin.

[0064] There are no particular limitations on urethane resins, but it is preferred that the molecule contains a carboxyl group.

[0065] There are no particular limitations on phenolic resins, but methyl phenolic resins that can be dissolved or dispersed in aqueous solvents are preferred.

[0066] There are no particular limitations on vinyl acetate resins, but polyvinyl acetate is preferred.

[0067] There are no particular limitations on polyester resins, but polyester resins containing monomers with carboxyl groups as constituent components are preferred.

[0068] Furthermore, these acrylic resins, epoxy resins, urethane resins, phenolic resins, vinyl acetate resins, and polyester resins can be mixed in combination with two or more. In addition, by using the alkali-soluble organic resins among these resins, the coating can be removed through alkaline degreasing during the coating process, resulting in improved coatability.

[0069] In this invention, in addition to organic resins and waxes, surface conditioners, defoamers, and dispersants may be included as components. Additionally, rust inhibitors that improve rust resistance may also be added.

[0070] In this invention, the inventors discovered that when forming the aforementioned coating on a steel sheet surface, for steel sheets with a wide range of surface roughness, forming a coating with a wide range of adhesion amounts, and evaluating stamping formability, good stamping formability is stably satisfied only in areas where a defined relationship between the steel sheet surface roughness and the coating adhesion amount is met. Generally, the greater the surface roughness of the steel sheet, the easier it is for the lubricating coating to become thinner on the protrusions of the steel sheet. During stamping, due to sliding with the die, the coating is chipped off, exposing the base steel sheet and making it difficult to achieve a lubricating effect. However, by using the technology of this invention, it is possible to promote the adhesion of the lubricating coating components to the die during sliding, and even with high steel sheet roughness, lubricity can be maintained by protecting the die side.

[0071] The coating adhesion range that demonstrates good stamping formability is described below.

[0072] The amount of film applied to each single side of the coating, W (g / m²). 2 The relationship between W and the arithmetic mean roughness Ra (μm) of the steel plate satisfies equation (1): W ≥ 0.12 × Ra 2 Within the range of +0.2, good stamping formability can be obtained. When W < 0.12 × Ra 2 At +0.2, the coating adhesion is insufficient, and the mold side cannot be adequately protected, resulting in poor stamping formability.

[0073] It should be noted that, in this invention, the arithmetic mean roughness Ra of the steel plate refers to the arithmetic mean roughness of the steel plate before the coating is formed.

[0074] Further optimization is to satisfy equation (2): W ≥ 0.25 × Ra 2 The range is +0.2.

[0075] In coating, the coating component that primarily contributes to sliding properties is present on the protruding parts of the steel plate that contact the die during stamping. It is assumed that the area of ​​the protruding parts of the steel plate in contact with the die is related to Ra. 2 The tendency to decrease proportionally. Therefore, it is believed that by making the amount of coating attached proportional to Ra... 2Proportionally increasing the amount of coating components ensures sufficient coverage for smoothness.

[0076] The preferred coating adhesion amount W is 2.0 g / m³. 2 Below. Exceeding 2.0g / m 2 Sometimes, the film release and weldability are poor. The film adhesion amount W is particularly preferably 0.9 g / m³. 2 The following is an example: The coating adhesion amount W is 0.9 g / m². 2 The following conditions result in particularly good desiccant properties.

[0077] The amount of coating adhesion can be determined by dividing the weight difference of the steel plate before and after coating by the area, or by completely removing the coating from the steel plate with an alkaline aqueous solution or organic solvent and then dividing the weight difference of the steel plate before and after coating removal by the area.

[0078] The arithmetic mean roughness Ra of the steel plate before coating is formed is preferably 0.4 μm or more and 2.5 μm or less.

[0079] When the roughness is less than 0.4 μm, minute damage that may occur during stamping can become noticeable, and sometimes sticking to the die can occur during stamping. When the arithmetic mean roughness Ra of the steel sheet before coating exceeds 2.5 μm, the required coating amount increases, sometimes increasing manufacturing costs or deteriorating the clarity after coating.

[0080] The arithmetic mean roughness Ra (μm) of steel plates can be determined according to JIS B 0633:2001 (ISO 4288:1996). For example, when Ra is greater than 0.1 and less than 2, the cutoff value and reference length are set to 0.8 mm, the evaluation length is set to 4 mm, and the arithmetic mean roughness Ra is obtained from the measured roughness curve. When Ra is greater than 2 and less than 10, the cutoff value and reference length are set to 2.5 mm, the evaluation length is set to 12.5 mm, and the roughness is obtained from the measured roughness curve.

[0081] As the steel sheet of the present invention, a steel sheet with a peak count PPI of 120 or more and 220 or less before coating formation is more preferably used. When the PPI is less than 120, sticking to the die may sometimes occur during stamping, and when it exceeds 220, the cleanability after coating may sometimes deteriorate. Here, the peak count PPI is the number of peaks with an unevenness of 0.635 μm or more from the average line in both positive and negative directions when the reference length of the roughness curve is 1 inch, and is determined according to the SAE J911 standard.

[0082] Next, the method for manufacturing the steel plate of the present invention will be described.

[0083] The steel sheet manufacturing method of the present invention refers to a method for manufacturing a steel sheet having an organic resin coating on its surface containing a polyolefin wax with a melting point of 120°C or higher and 140°C or lower and an average particle size of 3.0 μm or lower. Wax is added to an organic resin solution or emulsion in which the organic resin is dissolved or dispersed in a solvent, and the resulting coating is applied to the surface of the steel sheet and dried. Here, the coating is adjusted such that the mass (MA) of the solid component of the organic resin and the mass (MB) of the solid component of the wax are set to C = {MB / (MA+MB)} × 100, and the mass of C is 10% by mass or higher.

[0084] The preferred mass ratio of the coating components (organic resin and polyolefin wax) in the coating is 1% by mass or more and 25% by mass or less.

[0085] Water or organic solvents are used as solvents for coatings.

[0086] When the mass ratio of organic resin and wax as coating components in the coating is less than 1% or more than 25% by mass, uneven coating may sometimes occur. There are no particular limitations on the coating method; examples include using a roller coater or bar coater, or coating methods using spraying, dipping, or brushing. The steel sheet after coating can be dried using conventional methods. For example, methods using hot air drying, drying using an IH heater, or infrared heating can be used. The maximum temperature reached during drying is preferably 60°C or higher and 140°C or lower. When the maximum temperature reached during drying is below 60°C, drying takes longer, and sometimes rust prevention is poor. When the maximum temperature reaches above 140°C, sometimes the wax melts, clumps together, and the particle size becomes coarse, leading to deterioration of lubricity.

[0087] In the steel plate manufacturing method of the present invention, if the coating adhesion amount W (g / m²) per single side is... 2 The relationship between W and the arithmetic mean roughness Ra (μm) of the steel plate satisfies equation (1): W ≥ 0.12 × Ra 2 +0.2 enables the manufacture of steel sheets with good stamping formability.

[0088] As another embodiment of the manufacturing method, when formula (1) above is not satisfied, the adhesion amount W (g / m²) can be intentionally changed before coating. 2 The process satisfies at least one of the following: the arithmetic mean roughness Ra (μm) of the steel sheet or the steel sheet. This allows for more reliable management in manufacturing steel sheets with good stamping formability.

[0089] As an example of performing this process in an operation, one can cite a method in which the amount of coating W (g / m²) on each side is determined based on a target value or measured value of the arithmetic mean roughness Ra (μm) of the steel plate, satisfying the above formula (1). 2 In the coating process, the coating amount is adjusted to achieve a specific adhesion amount W. More specifically, the arithmetic mean roughness Ra (μm) of the steel plate is substituted into the above formula (1) to determine the adhesion amount W (g / m²) per side of the coating in a manner that satisfies the substituted formula. 2 Here, "substituting the value of the arithmetic mean roughness Ra (μm) of the steel plate into the above formula (1)" is not limited to strictly substituting into the same formula as above formula (1), but also includes substituting into inequalities that always satisfy these formulas. By managing in this way, for example, when the arithmetic mean roughness Ra (μm) of the steel plate changes significantly due to the switching of steel plates in continuous flow or the wear of the rolling rolls and does not satisfy the above formula, that is, needless to say, even when the formula is not actually satisfied, it is possible to manage and satisfy the formula.

[0090] When adjusting the arithmetic mean roughness Ra (μm) of the steel sheet in a manner that satisfies the above formula, known methods can be appropriately applied. Specifically, when the steel sheet is hot-rolled, the surface roughness of the steel sheet after removing the oxide scale by pickling can be adjusted by adjusting the oxide scale thickness during hot rolling using the amount of Si added to the steel sheet. Alternatively, the surface roughness of the steel sheet can also be adjusted by adjusting the intensity of the oxide scale removal process (using hydraulic crushing / removal of oxide scale) during hot rolling. When the steel sheet is cold-rolled, the surface roughness of the steel sheet can be adjusted by changing the roll load or roll surface roughness during surface finishing.

[0091] The above describes an example of an implementation method that manages the operation in a manner that satisfies equation (1). However, it is also possible to confirm the amount of film adhering to each single side of the coating (w / m²) before the start of the operation. 2 If the arithmetic mean roughness Ra (μm) of the steel plate satisfies the above formula, and if not, the amount of coating applied per side W (g / m²) can be changed beforehand. 2 The manufacturing conditions of the steel sheet are determined by means of either the roughness Ra (μm) of the steel sheet or the arithmetic mean roughness Ra (μm). This process of determining the manufacturing conditions can be performed as part of the steel sheet manufacturing method or as a separate process.

[0092] Example 1

[0093] The present invention will now be described through embodiments. It should be noted that the present invention is not limited to the following embodiments.

[0094] On one side of cold-rolled steel sheets (sheets No. A to C) with a thickness of 0.8 mm and a hot-rolled steel sheet (sheet No. D) with a thickness of 2.0 mm, using a bar coater, a coating with the composition shown in Table 2 was applied. The sheets were then dried using an IH heater to a maximum reaching temperature of 80°C, thus producing lubricated steel sheets, which were used as the test materials. It should be noted that steel sheets A to D are SPCD and SPHD with a tensile strength of 270 MPa.

[0095] The coating adhesion amount is calculated by removing the coating from the steel plate and dividing the weight difference (g) of the steel plate before and after coating removal by the area (m²) of the steel plate. 2 To find out.

[0096] (1) Evaluation method of stamping formability (sliding characteristics)

[0097] To evaluate stamping formability, the coefficient of friction of each test material was measured as follows.

[0098] Figure 1 This is a schematic front view of the friction coefficient measuring apparatus. As shown in the figure, a friction coefficient measuring sample 1, cut from the test material, is fixed on a sample stage 2, which is fixed to the upper surface of a horizontally movable sliding worktable 3. A vertically movable sliding worktable support 5, having a roller 4 in contact with the lower surface of the sliding worktable 3, is provided on the lower surface of the sliding worktable 3. By pushing the sliding worktable support 5 upward, a pressing load N is applied to the friction coefficient measuring sample 1 by the reinforcing rib 6. A first weighing sensor 7 for measuring the pressing load N is mounted on the sliding worktable support 5. A second weighing sensor 8 for measuring the sliding resistance F that causes the sliding worktable 3 to move horizontally under the applied pressing load N is mounted at one end of the sliding worktable 3. It should be noted that Preton (registered trademark) R352L, a pressure cleaning oil manufactured by Sugimura Chemical Industry Co., Ltd., is applied as a lubricant to the surface of the sample 1 before the test.

[0099] Figure 2 This is a schematic perspective view showing the shape and dimensions of the reinforcing rib used. The lower surface of the reinforcing rib 6 slides in a state where it is pressed against the surface of the sample 1. Regarding... Figure 2 The shape of the reinforcing rib 6 shown is 10 mm wide, the sliding length of the specimen is 59 mm, the lower part of both ends of the sliding direction is composed of a curved surface with a curvature of 4.5 mmR, and the lower surface of the reinforcing rib of the pressed specimen has a plane with a width of 10 mm and a sliding length of 50 mm.

[0100] Regarding the friction coefficient determination test, the following is used: Figure 2The reinforcing ribs shown were tested under the conditions of a compressive load N of 400 kgf and a sample pull-out speed (horizontal movement speed of the sliding worktable 3) of 20 cm / min. The coefficient of friction μ between the test material and the reinforcing ribs was calculated using the formula: μ = F / N.

[0101] A coefficient of friction of 0.119 or less is considered to have exceptionally good sliding performance and is rated as ◎; a coefficient of friction of more than 0.119 but less than 0.130 is considered to have good sliding performance and is rated as ○; and a coefficient of friction of more than 0.130 is considered to have insufficient sliding performance and is rated as ×.

[0102] It should be noted that in the evaluation of stamping formability (sliding characteristics), if it is ◎ or 〇, it can be judged that even in cases where stamping is difficult and complex, the sliding resistance at the parts where breakage is likely during stamping is small, and the stamping formability is excellent even in parts where surface pressure is high and sticking to the die is hypothetically expected.

[0103] (2) Evaluation method of desiccation property

[0104] Hypothetically, the decoating performance of the steel sheet of the present invention is evaluated when it is used in an automotive application. To determine the decoating performance of the steel sheet, each test piece was first degreased using an alkaline degreasing agent, FINE CLEANER (registered trademark) E6403 (manufactured by Parkerizing Co., Ltd., Japan). 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 surface carbon strength, along with pre-measured surface carbon strength values ​​for both the steel sheet before degreasing and the untreated steel sheet, the coating peeling rate was calculated using the following formula.

[0105] Coating peel rate (%) = [(Carbon strength before degreasing - Carbon strength after degreasing) / (Carbon strength before degreasing - Carbon strength of untreated steel sheet)] × 100

[0106] The peelability of the steel sheet is evaluated based on the immersion time in an alkaline degreasing solution with a peel rate of 98% or higher as defined above, according to the criteria shown below. Under the conditions ◎ and ○ below, the peelability is deemed good.

[0107] ◎(Exceptionally Good): Within 30 seconds

[0108] ○ (Good): More than 30 seconds but less than 60 seconds

[0109] Δ (Insufficient): More than 60 seconds but less than 120 seconds

[0110] × (Poor): Exceeded 120 seconds

[0111] [Table 1]

[0112] A 0.48 B 0.78 C 1.48 D 2.35

[0113] [Table 2]

[0114]

[0115] [Table 3]

[0116]

[0117] [Table 4]

[0118]

[0119] [Table 5]

[0120]

[0121] Set the cases in Tables 3-5 that satisfy Equation (1) and Equation (2) to 0, and set the cases that do not satisfy Equation (1) and Equation (2) to ×.

[0122] According to Tables 3-5, the steel sheets of the present invention all exhibit excellent stamping formability. In contrast, the steel sheets of the comparative examples, which do not possess the technical features of the present invention, all exhibit poor stamping formability.

[0123] Furthermore, it is evident that in the steel plate of the present invention, the adhesion amount W (g / m) of the lubricating coating on each single side is... 2 (W≥0.25×Ra) 2 +0.2. Steel sheets with a melting point of 125°C or higher and 135°C or lower and an average particle size of 0.01μm or higher and 0.5μm or lower have particularly good stamping formability.

[0124] Figure 3 , 4 This is a graph that extracts a portion of the results shown in Tables 3-5 and shows how the coefficient of friction of steel plates with different surface roughness varies with the amount of lubricating coating. Figure 3 This is a result concerning coatings, which are the subject of this invention. Figure 4 This refers to coatings other than those described in this invention.

[0125] like Figure 4 As shown, for steel plates coated with a paint that is not the subject of this invention, it was observed that the greater the surface roughness of the steel plate, the more it tends to exhibit a high coefficient of friction.

[0126] On the other hand, such as Figure 3As shown, the steel sheet coated with the coating object of the present invention and satisfying formula (1) consistently exhibits a low coefficient of friction even when the surface roughness of the steel sheet varies. Therefore, even with manufacturing deviations in the surface roughness of the steel sheet, good stamping formability can be consistently obtained.

[0127] In addition, regarding the peelability of the steel plate in the example of the present invention, besides the fact that the mass ratio of wax in the coating is 50% or more and / or the amount of lubricating coating adhering to each single side W exceeds 0.9 g / m, the peelability is further improved. 2 Except for the case of , it is marked as ◎ (especially good).

[0128] Example 2

[0129] Cold-rolled steel sheets (sheet No. E to H) with a thickness of 0.8 mm and an arithmetic mean roughness Ra and peak count PPI as shown in Table 6 were coated with the coating composition shown in Table 2 using a bar coater. The sheets were then dried using an IH heater to achieve a maximum reaching temperature of 80°C, thus producing lubricated steel sheets, which were used as the test materials. It should be noted that the steel sheets E to H are all SPCDs with a tensile strength of 270 MPa.

[0130] The coating adhesion amount is calculated by removing the coating from the steel plate and dividing the weight difference of the steel plate before and after coating removal by the area.

[0131] (1) Evaluation method of stamping formability (sliding characteristics)

[0132] The stamping formability was evaluated by measuring the coefficient of friction using the same method as in Example 1. A coefficient of friction of 0.119 or less was rated as particularly excellent sliding performance and marked with ◎. A coefficient of friction of more than 0.119 but less than 0.130 was rated as good sliding performance and marked with 0. A coefficient of friction of more than 0.130 was rated as insufficient and marked with ×.

[0133] (2) Evaluation method for stamping formability (anti-adhesion)

[0134] use Figure 1 The friction coefficient measuring device shown evaluates sliding characteristics by changing the shape of the reinforcing ribs and the pressing load, but otherwise the friction coefficient is measured in the same way. Figure 5 This is a schematic perspective view showing the shape and dimensions of the reinforcing rib used. The lower surface of the reinforcing rib 6 slides in a state where it is pressed against the surface of the sample 1. Regarding... Figure 5 The reinforcing rib 6 shown has a width of 10 mm and a sliding length of 5 mm. The lower parts at both ends of the sliding direction are formed by curved surfaces with a curvature of 1.0 mmR. The lower surface of the reinforcing rib pressing the specimen has a plane with a width of 10 mm and a sliding length of 3 mm. Regarding the adhesion test, use... Figure 5 The reinforcing ribs shown were subjected to a compressive load N of 800 kgf and a sample pull-out speed (horizontal movement speed of the sliding worktable 3) of 100 cm / min for measuring the coefficient of friction. The coefficient of friction μ between the test material and the reinforcing ribs was calculated using the formula: μ = F / N. The same coefficient of friction measurement was repeated using the same sample, and the resistance to adhesion was evaluated based on the number of repeated sliding operations until the coefficient of friction value exceeded 0.200. A value of 20 or more repeated sliding operations was considered excellent resistance to adhesion and was rated ◎, while a value of 19 or fewer repeated sliding operations was considered normal resistance to adhesion and was rated 0.

[0135] [Table 6]

[0136] E 0.78 110 F 0.78 120 G 0.78 170 H 0.78 220

[0137] [Table 7]

[0138]

[0139] Set the cases in Table 7 where Equation (1) and Equation (2) satisfy Equation (1) and Equation (2) respectively as ○, and set the cases where they do not satisfy Equation (2) as ×.

[0140] According to Table 7, the steel sheets of the present invention all exhibit excellent stamping formability. In contrast, the steel sheets of the comparative examples, which do not possess the technical features of the present invention, all exhibit poor stamping formability. Furthermore, the steel sheets of the present invention, when their PPI is 120 or higher, exhibit particularly excellent anti-adhesion properties.

[0141] Industrial availability

[0142] The steel sheet of the present invention has excellent stamping formability, and therefore can be applied to a wide range of fields, primarily automobile body applications.

[0143] Symbol Explanation

[0144] 1. Sample for determining the coefficient of friction

[0145] 2. Sample stage

[0146] 3. Sliding worktable

[0147] 4 rollers

[0148] 5. Sliding worktable support platform

[0149] 6 Reinforcing ribs

[0150] 7. First Weighing Sensor

[0151] 8. Second weighing sensor

[0152] 9 orbits

Claims

1. A method for manufacturing a steel plate, comprising forming a coating containing an organic resin and a wax on at least one side of the steel plate, wherein, The organic resin is at least one of acrylic resin, epoxy resin, urethane resin, phenolic resin, vinyl acetate resin, and polyester resin. The wax is a polyolefin wax with a melting point of 120°C or higher and 140°C or lower, and an average particle size of 3.0 μm or lower. The proportion of wax in the coating is 10% by mass or higher. The relationship between the amount of coating per single side W and the arithmetic mean roughness Ra of the steel plate is within the range of the following formula (1), wherein the unit of the amount of coating per single side W is g / m. 2 The arithmetic mean roughness Ra of the steel plate is expressed in μm. W≥0.12×Ra 2 +0.2…(1), The coating comprising the organic resin and wax is applied to at least one side of the steel plate and then dried. If equation (1) is not satisfied, the following step is performed before the coating is carried out: at least one of the adhesion amount W or the arithmetic mean roughness Ra of the steel plate is changed to satisfy equation (1).

2. The method for manufacturing a steel plate according to claim 1, wherein, The relationship between the amount of coating applied to each side of the steel plate and the arithmetic mean roughness Ra of the steel plate is within the range of the following formula (2). W≥0.25×Ra 2 +0.2…(2)。 3. The method for manufacturing the steel plate according to claim 1 or 2, wherein, The coating has an adhesion amount W of 2.0 g / m² on each single side. 2 the following.

4. The method for manufacturing the steel plate according to claim 1 or 2, wherein, The arithmetic mean roughness Ra of the steel plate before the coating is formed is greater than 0.4 μm and less than 2.5 μm.

5. The method for manufacturing the steel plate according to claim 1 or 2, wherein, The wax has an average particle size of 0.01 μm or more and 0.5 μm or less.

6. The method for manufacturing the steel plate according to claim 1 or 2, wherein, The proportion of wax in the coating is less than 50% by mass.

7. The method for manufacturing a steel plate according to claim 3, wherein, The coating has an adhesion amount W of 0.9 g / m² on each single side. 2 the following.

8. The method for manufacturing the steel plate according to claim 1 or 2, wherein, The organic resin is an alkali-soluble resin.

9. The method for manufacturing a steel plate according to claim 1 or 2, wherein, The PPI of the steel plate before the coating is formed is above 120 and below 220.

10. The method for manufacturing a steel plate according to claim 1, wherein, The maximum temperature reached by the steel plate during drying is above 60°C and below 140°C.

11. The method for manufacturing a steel plate according to claim 1 or 10, wherein, The proportion of organic resin and wax in the coating is more than 1% by mass and less than 25% by mass.

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