Steel sheet and method for manufacturing the same

Abstract

The present invention aims to provide a steel sheet having a lubricating coating film, which is a steel sheet to be subjected to complex forming that is difficult to press-form, the steel sheet having small sliding resistance at a cracking-prone site at the time of press-forming, and excellent press-formability at a site where surface pressure is high and sticking is expected to occur. A steel sheet according to the present invention has a lubricating coating film on the surface thereof, the lubricating coating film having an attached amount W of 0.3 g / m 2 The above and 2.5 g / m 2 The following range forms an organic resin coating film containing an acrylic resin having a glass transition temperature (Tg) of 100°C or higher and a ratio R = acid value (mg-KOH / g) / Tg (°C) of the acid value of 1.50 or more, and 5% by mass or more of a polyolefin wax having a melting point of 100°C or higher and 145°C or lower and an average particle diameter of 3.0 μm or less.

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 harsh deep drawing processes, 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 formed by stamping. In recent years, there has been a demand for integrated components that eliminate manufacturing processes, improved design flexibility, and the ability to perform more complex forming operations.

[0003] In cases where more complex stamping processes are required, there is a possibility that the steel sheet may not be able to withstand the forming process and break, or that sticking to the die may occur during continuous stamping, which could seriously negatively impact automobile productivity.

[0004] One method to improve the stamping formability of cold-rolled and hot-rolled steel sheets is surface treatment of the metal dies. While surface treatment of metal dies is a widely used method, it prevents die adjustment after treatment. Furthermore, it is costly. Therefore, there is a strong demand to improve the stamping formability of the steel sheet itself.

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

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

[0007] Patent document 1 describes a technique for forming a lubricating coating on a galvanized steel sheet, in which an acrylic resin coating contains synthetic resin powder.

[0008] Patent document 2 describes a metal plate covered with a lubricating coating that allows solid lubricant to protrude from the surface of a resin coating by 0.01 μm to 1.5 μm.

[0009] Patent document 3 describes a lubricated surface-treated metal product with excellent stamping formability, which is coated with a film of 0.5μm to 5μm, obtained by containing lubricant in polyurethane resin.

[0010] Patent document 4 describes a technique for forming an alkali-soluble organic coating on a steel plate by adding a lubricant to an epoxy resin.

[0011] Existing technical documents

[0012] Patent documents

[0013] Patent Document 1: Japanese Patent Application Publication No. 9-170059

[0014] Patent Document 2: Japanese Patent Application Publication No. 10-52881

[0015] Patent Document 3: Japanese Patent Application Publication No. 2000-309747

[0016] Patent Document 4: Japanese Patent Application Publication No. 2000-167981 Summary of the Invention

[0017] The problem that the invention aims to solve

[0018] However, in Patent Documents 1 to 4, although lubricity is exhibited due to the lubrication effect produced by the lubricant contained therein, the stamping formability may not be sufficient in complex forming processes.

[0019] The present invention was made in view of the above circumstances, and its object is to provide a steel sheet that is difficult to stamp and form, and a method thereof, which has low sliding resistance at easily cracked parts during stamping and excellent stamping formability at parts with high surface pressure where sticking to the die is expected.

[0020] In addition, rust resistance is required when the steel sheet is stored in coil form. Furthermore, when the steel sheet is used in automobile bodies, it needs to have sufficient release properties during the alkaline degreasing process in the painting process, and also needs excellent adhesion and weldability during the assembly process.

[0021] Methods for solving problems

[0022] To solve the aforementioned problems, the inventors conducted repeated and in-depth research. The results showed that by setting the adhesion amount W on the steel plate surface to 0.3 g / m² per single side... 2 Above and 2.5g / m 2 The above-mentioned problems can be solved by forming an organic resin coating within the following range, wherein the organic resin coating contains an acrylic resin with a glass transition temperature (Tg) of 100°C or higher and an acid value ratio R = acid value (mg-KOH / g) / Tg (°C) of 1.50 or higher, and 5% by mass of a polyolefin wax with a melting point of 100°C or higher and 145°C or lower and an average particle size of 3.0 μm or lower.

[0023] This invention is based on the above insights, and its main points are as follows.

[0024] [1] A steel sheet having a coating containing an acrylic resin and a wax formed on at least one side, wherein the acrylic resin has a glass transition temperature (Tg) of 100°C or higher, the ratio of acid value to glass transition temperature R = acid value (mg-KOH / g) / Tg (°C) is 1.50 or higher, the wax is a polyolefin wax with a melting point of 100°C or higher and 145°C or lower and an average particle size of 3.0 μm or lower, the proportion of wax in the coating is 5% by mass or higher, and the amount of coating W applied per side is 0.3 g / m 2 Above and 2.5g / m 2 the following.

[0025] [2] The steel plate according to [1], wherein the acid value of the acrylic resin is above 180 mg-KOH / g and below 350 mg-KOH / g.

[0026] [3] The steel plate according to [1] or [2], wherein the ratio R of the acid value to the glass transition temperature of the acrylic resin is 2.05 or less.

[0027] [4] The steel plate according to any one of [1] to [3], wherein the coating contains more than 30% by mass of the acrylic resin and the wax is less than 50% by mass.

[0028] [5] The steel plate according to any one of [1] to [4], wherein the mass-average molecular weight of the acrylic resin is 5,000 or more and 30,000 or less.

[0029] [6] The steel plate according to any one of [1] to [5], wherein the acrylic resin is a styrene acrylic resin.

[0030] [7] The steel plate according to any one of [1] to [6], 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.

[0031] [8] The steel plate according to any one of [1] to [7], wherein the coating contains 5% by mass or more and 30% by mass or less of a rust inhibitor.

[0032] [9] The steel plate according to any one of [1] to [8], wherein the rust inhibitor is at least one selected from the group consisting of aluminum salts of phosphate, zinc salts and zinc oxide.

[0033]

[10] The steel plate according to any one of [1] to [9], wherein the average particle size of the wax is 0.01 μm or more and 0.5 μm or less.

[0034]

[11] The steel plate according to any one of [1] to

[10] , wherein the coating contains more than 1% by mass and less than 10% by mass of silicon dioxide.

[0035]

[12] A method for manufacturing a steel plate, which is the method for manufacturing a steel plate according to any one of [1] to

[11] , wherein a coating containing an acrylic resin and a wax as described in any one of [1] to

[11] is applied to at least one side of the steel plate and dried.

[0036]

[13] According to the steel plate manufacturing method of

[12] , the maximum temperature reached by the steel plate during drying is above 60°C and below the melting point of the wax.

[0037]

[14] The method for manufacturing steel plate according to

[12] or

[13] , wherein the proportion of all solid components in the coating is more than 1% by mass and less than 30% by mass.

[0038] In this invention, the steel plate is a cold-rolled steel plate or a hot-rolled steel plate.

[0039] Invention Effects

[0040] According to the present invention, steel sheets with significantly reduced coefficient of friction with metal molds and the like, and excellent stamping formability, can be obtained. Therefore, for steel sheets requiring complex forming, excellent stamping formability is consistently achieved. Furthermore, the coated steel sheets also exhibit good rust resistance. Moreover, due to good adhesion, adhesives can be used in the same manner as conventional steel sheets, and the excellent release properties based on alkaline degreasing do not hinder the coating process. Attached Figure Description

[0041] Figure 1 This is a schematic front view showing the friction coefficient measuring device.

[0042] Figure 2 It is shown Figure 1 A rough perspective view of the shape and dimensions of the bead in the design. Detailed Implementation

[0043] The embodiments of the present invention will be described below.

[0044] The steel plate of the present invention is a steel plate having a coating containing acrylic resin and wax formed on at least one side, characterized in that the acrylic resin has a glass transition temperature (Tg) of 100°C or higher, the ratio of acid value to glass transition temperature R = acid value (mg-KOH / g) / Tg (°C) is 1.50 or higher, the wax is a polyolefin wax with a melting point of 100°C or higher and 145°C or lower and an average particle size of 3.0 μm or lower, the proportion of wax in the coating is 5% by mass or higher, and the amount of organic resin coating adhering to each single side, W, is 0.3 g / m². 2 Above and 2.5g / m 2 the following.

[0045] Hereinafter, the ratio of acid value to glass transition temperature R = acid value (mg-KOH / g) / Tg (°C) will be recorded as R = acid value / Tg.

[0046] The glass transition temperature of the acrylic resin used in the coating of this invention is set to 100°C or higher to obtain good lubricity. When the glass transition temperature is below 100°C, the resin softens during sliding, the wax retention force decreases, and the ability to prevent direct contact between the metal plate and the metal mold is reduced, thus resulting in poor sliding performance. The glass transition temperature is preferably 110°C or higher and 150°C or lower. Conversely, when the glass transition temperature exceeds 150°C, the resin becomes hard and brittle during sliding, sometimes resulting in poor lubricity.

[0047] Here, the glass transition temperature refers to the intermediate glass transition temperature determined based on JIS K 7121 "Method for determination of transition temperature of plastics".

[0048] The ratio of acid value to glass transition temperature (R = acid value / Tg) for acrylic resins is set to 1.50 or higher. Even with a glass transition temperature above 100°C, excellent lubricity cannot be achieved at low acid values ​​(R < 1.50). Although the reason is unclear, it is believed that the carboxyl groups in acrylic resins have a high affinity for metal molds, enabling the transfer of polyolefin wax from the coating to the metal mold during sliding. By transferring the acrylic resin component containing polyolefin wax to the metal mold during sliding, the surface of the metal mold is protected by the polyolefin wax, improving the effect of preventing direct contact with the steel plate and thus improving sliding performance. Therefore, at low acid values ​​(R < 1.50), the sliding performance is poor due to insufficient carboxyl groups. As the glass transition temperature of acrylic resins increases, the resin is less likely to soften due to sliding, making it difficult to transfer and adhere to the metal mold. Therefore, to achieve excellent sliding performance at higher glass transition temperatures, the acid value also needs to be increased. That is, the ratio of acid value to glass transition temperature, R = acid value / Tg, needs to be 1.50 or higher. R is preferably 1.80 or higher. It should be noted that there is no particular upper limit to R, but it is preferably 2.05 or lower. The reason is that when R is greater than 2.05, the rust prevention performance sometimes deteriorates.

[0049] Furthermore, the acid value of acrylic resins is preferably 180 mg-KOH / g or higher and 350 mg-KOH / g or lower. When the acid value is less than 180 mg-KOH / g, the release properties are sometimes poor due to the alkali, and sometimes the adhesive strength produced by the adhesive cannot be sufficiently obtained. When the acid value is greater than 350 mg-KOH / g, the rust prevention properties sometimes deteriorate.

[0050] Here, acid value refers to the number of mg of potassium hydroxide required to neutralize the carboxyl groups contained in 1g of resin, and can be determined based on JIS K0070 "Test methods for acid value, saponification value, ester value, iodine value, hydroxyl value and unsaponifiable matter of chemical products". In this invention, the unit is expressed as mg-KOH / g.

[0051] The wax used in this invention can be any polyolefin wax with a melting point of 100°C or higher and 145°C or lower and an average particle size of 3.0 μm or lower.

[0052] Polyolefin waxes are used because they have low surface energy and self-lubricating properties, thus providing good lubrication. Furthermore, polyolefins are relatively easy to control by adjusting their melting point to above 100°C and below 145°C through control of density and molecular weight.

[0053] When the melting point of the polyolefin wax is above 100°C and below 145°C, in addition to the self-lubricating properties of the polyolefin wax itself, it also becomes semi-molten due to the sliding during stamping. This allows the lubricating coating component obtained by mixing with organic resin to coat the surface of the metal mold. Therefore, by suppressing direct contact between the metal mold and the steel plate, excellent lubrication can be achieved. When the melting point of the polyolefin wax is below 100°C, it completely melts due to the frictional heat generated during stamping, failing to achieve sufficient lubrication from the polyolefin wax itself, nor the aforementioned coating effect on the metal mold. Furthermore, when the melting point of the polyolefin wax exceeds 145°C, it does not melt during sliding, failing to achieve sufficient lubrication or the coating effect on the metal mold. Preferably, the melting point of the polyolefin wax is above 120°C and below 140°C.

[0054] Here, the melting point of polyolefin wax is the melting temperature determined based on JIS K 7121 "Method for determination of the transition temperature of plastics".

[0055] When the average particle size of the polyolefin wax is greater than 3.0 μm, it is difficult to mix with the organic resin during sliding, and the aforementioned coating effect on the metal mold cannot be obtained, nor can sufficient lubrication be achieved. The average particle size of the polyolefin wax is preferably 0.5 μm or less, and more preferably 0.3 μm or less.

[0056] The average particle size of the polyolefin wax is preferably 0.01 μm or more. When the average particle size of the polyolefin 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 polyolefin wax is further preferably 0.03 μm or more. Considering the miscibility with the aforementioned acrylic resins, the average particle size of the polyolefin wax is preferably 0.01 μm or more and 0.5 μm or less.

[0057] The aforementioned average particle size refers to the median particle size of the volume average diameter, determined by laser diffraction / scattering. 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 Corporation).

[0058] Among polyolefin waxes, polyethylene wax provides the best lubrication effect, therefore it is preferred to use polyethylene wax.

[0059] The mass percentage of polyolefin wax in the coating is set to 5% by mass or more. When the mass percentage of polyolefin wax in the coating is less than 5% by mass, sufficient lubrication cannot be achieved. When the mass percentage of polyolefin wax in the coating is 10% by mass or more, a particularly good lubrication effect can be obtained. Furthermore, the mass percentage of polyolefin wax in the coating is preferably 50% by mass or less. When the mass percentage of polyolefin wax in the coating is greater than 50% by mass, due to insufficient matrix resin content, the polyolefin wax is prone to detachment, resulting in poor adhesion to the steel sheet, and sometimes it cannot exist stably in the coating form, leading to poor adhesion. Additionally, when used in automotive bodies, sometimes sufficient degreasing cannot be achieved in the alkaline degreasing process during painting, and sometimes the coating cannot be completely removed in the alkaline degreasing process, leaving residual coating, which deteriorates the coatability. The mass percentage of polyolefin wax in the coating is further preferably 30% by mass or less.

[0060] Here, the mass ratio of polyolefin wax in the coating refers to the ratio of the mass of the solid component of the polyolefin wax to the mass of the total solid component in the coating.

[0061] The coating of the present invention preferably contains 30% by mass or more of the aforementioned acrylic resin. When the mass percentage of acrylic resin in the coating is 30% by mass or more, the properties influenced by the physical properties of the acrylic resin component, such as improved lubricity, release properties, and adhesion, resulting from the transfer and adhesion to the metal mold during sliding, can be sufficiently obtained. When the mass percentage of acrylic resin in the coating is less than 30% by mass, the influence of other components becomes greater, and sometimes the target performance cannot be obtained.

[0062] The mass-average molecular weight of the aforementioned acrylic resins is preferably 5,000 or higher and 30,000 or lower. When the mass-average molecular weight of the acrylic resin is less than 5,000, its rust resistance is sometimes poor; when it is greater than 30,000, its adhesion is sometimes deteriorated.

[0063] Here, mass-average molecular weight refers to the mass-average molecular weight determined based on JIS K 7252 "Plastics - Determination of average molecular weight and molecular weight distribution of polymers based on size exclusion chromatography".

[0064] Furthermore, the aforementioned acrylic resin is preferably a styrene-based acrylic resin. By including styrene in the resin monomers, water resistance is improved, thus resulting in good rust prevention. In addition, compared to the case without styrene, it also exhibits good lubrication properties.

[0065] The coating of the present invention preferably contains 5% to 30% by mass of a rust inhibitor. While rust will not occur under normal storage conditions even without the presence of a rust inhibitor, rust may occur under adverse storage conditions when the rust inhibitor content is less than 5% by mass. Particularly when steel strips are stored in an overlapped roll, they may absorb moisture and rust. When the rust inhibitor ratio is greater than 30% by mass, adhesion may deteriorate, and the rust inhibitor may precipitate and the coating stability may deteriorate in the coated state. As a rust inhibitor, at least one selected from the group consisting of aluminum salts, zinc salts, and zinc oxide of phosphoric acid is preferred. Here, phosphoric acid includes not only orthophosphoric acid but also condensed phosphoric acid such as pyrophosphoric acid, tripolyphosphoric acid, tetrapolyphosphoric acid, and metaphosphoric acid. Using these rust inhibitors provides sufficient rust prevention and minimizes the deterioration of coating stability.

[0066] Furthermore, the coating of the present invention preferably contains 1% by mass or more and 10% by mass less silica. The presence of silica improves the water repellency and rust resistance of the coating. Moreover, the presence of silica inhibits the precipitation of rust inhibitors, thus improving coating stability. However, when the silica content is less than 1% by mass, the above-mentioned effects are difficult to obtain, and when it is greater than 10% by mass, adhesion sometimes deteriorates. When the coating of the present invention contains silica, colloidal silica with a particle size of 5 nm or more and 200 nm or less is preferably used.

[0067] In this invention, in addition to acrylic resins, waxes, rust inhibitors, and silica, the components may include surface conditioners, defoamers, dispersants, etc., which are usually added to coatings.

[0068] The surface roughness of the steel sheet used in this invention before coating formation is preferably 0.4 μm or more and 2.5 μm or less, expressed as an arithmetic mean roughness Ra. When Ra is 2.5 μm or less, a stable lubrication effect from the coating can be obtained. When Ra is less than 0.4 μm, fine damage that may occur during stamping can easily become noticeable, and sometimes sticking to the die can occur during stamping. When Ra is greater than 2.5 μm, the unevenness of the steel sheet increases, so the coating in the recesses is difficult to function effectively during sliding, and sometimes the lubrication effect from the coating decreases. The arithmetic mean roughness Ra (μm) of the steel sheet can be measured according to JIS B0633:2001 (ISO 4288:1996). For example, when Ra is greater than 0.1 and less than 2, it can be obtained from a roughness curve obtained by setting the cutoff value and reference length to 0.8 mm and the evaluation length to 4 mm. When Ra is greater than 2 and less than 10, the roughness curve is obtained by setting the cutoff value and reference length to 2.5 mm and the evaluation length to 12.5 mm.

[0069] Next, the method for manufacturing the steel plate of the present invention will be described.

[0070] The method for manufacturing the steel plate of the present invention is a method for manufacturing a steel plate having an organic resin coating. The steel plate is a steel plate having a coating containing an acrylic resin and a wax formed on at least one side. The organic resin coating contains an acrylic resin with a glass transition temperature (Tg) of 100°C or higher and an acid value to glass transition temperature ratio R = acid value / Tg of 1.50 or higher, and at least 5% by mass of a polyolefin wax with a melting point of 100°C or higher and 145°C or lower, and an average particle size of 3.0 μm or lower. A coating obtained by adding wax to an acrylic resin solution or emulsion in which the acrylic resin is dissolved or dispersed in a solvent is applied to the surface of the steel plate and dried. Water or an organic solvent can be used as the solvent for the coating, but water is preferred. The concentration of all solid components in the coating is preferably 1% by mass or higher and 30% by mass or lower. When the concentration of all solid components in the coating is less than 1% by mass or greater than 30% by mass, uneven coating may sometimes occur. There are no particular limitations on the coating method. Examples include using a roller coater, a doctor blade coater, or coating methods using spraying, dipping, or brushing. The steel sheet can be dried using conventional methods. For example, methods using hot air drying, IH heater drying, or infrared heating can be used. The maximum temperature reached during drying is preferably above 60°C and below the melting point of the wax used. When the maximum temperature of the steel sheet is below 60°C, drying is time-consuming, and sometimes rust prevention is poor. When the maximum temperature of the steel sheet is above the melting point of the wax, the wax melts and coalesces, resulting in coarser particle size, which sometimes deteriorates lubricity. Furthermore, it is preferable to achieve a coating adhesion amount of 0.3–2.5 g / m² per single side of the steel sheet based on the dried mass. 2 The coating is applied using the method described above. The content is less than 0.3 g / m³. 2 Sometimes, sufficient sliding property cannot be obtained, exceeding 2.5 g / m. 2 Sometimes, the weldability, release properties, and adhesion are deteriorated due to the presence of alkali.

[0071] Example

[0072] The present invention will now be described through embodiments. It should be noted that the present invention is not limited to the following embodiments.

[0073] Cold-rolled steel sheets with a thickness of 0.8 mm (steel sheet No. A to C) and hot-rolled steel sheets with a thickness of 2.0 mm (steel sheet No. D) with an arithmetic mean roughness Ra as shown in Table 1 were coated with the coating composition shown in Table 2 using a doctor blade coater. The sheets were then dried using an IH heater at a maximum reaching temperature of 80°C to produce lubricated steel sheets. It should be noted that steel sheets A to D are SPCD (JIS G 3141) and SPHD (JIS G 3131) with a tensile strength of 270 MPa. Furthermore, colloidal silica with a volume average particle size of 9 nm was used as the silica.

[0074] The coating adhesion amount is calculated by removing the coating from the steel plate and dividing the difference in mass of the steel plate before and after coating removal by the area.

[0075] [Table 1]

[0076] A 0.48 B 0.78 C 1.48 D 2.35

[0077]

[0078] (1) Evaluation method of stamping formability (sliding characteristics)

[0079] To evaluate stamping formability, the coefficient of friction of each test material was measured as follows.

[0080] Figure 1 This is a schematic front view of the friction coefficient measuring apparatus. As shown in the figure, a friction coefficient measuring specimen 1, cut from the test material, is fixed on a specimen stage 2, which is fixed to the upper surface of a horizontally movable slide 3. A vertically movable slide support 5 with rollers 4 in contact with it is provided on the lower surface of the slide 3. A first force sensor 7 is mounted on the slide support 5, which measures the pressing load N of the pressure rib 6 on the friction coefficient measuring specimen 1 due to the upward pushing of the slide support 5. A second force sensor 8 is mounted at one end of the slide 3, which measures the sliding resistance F that causes the slide 3 to move horizontally under the aforementioned pressing force. It should be noted that PRETON (registered trademark) R352L, a stamping cleaning oil manufactured by Sugimura Chemical Industry Co., Ltd., is applied to the surface of the specimen 1 for testing.

[0081] Figure 2 This is a schematic perspective view showing the shape and dimensions of the blank holder used. The blank holder 6 is slid in a position where its lower surface is pressed against the surface of the sample 1. Regarding... Figure 2The shape of the pressing rib 6 shown is 10 mm wide, the sliding length of the sample 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 pressing rib of the sample has a plane with a width of 10 mm and a sliding length of 50 mm.

[0082] Friction coefficient determination test used Figure 2 The pressure bar shown was tested under the following conditions: a compressive load N of 400 kgf and a sample pull-out speed (horizontal movement speed of slide 3) of 20 cm / min. The coefficient of friction μ between the test material and the pressure bar was calculated using the formula: μ = F / N.

[0083] Regarding the evaluation of sliding characteristics, a friction coefficient below 0.119 is evaluated as having excellent sliding properties and is marked with ◎; a friction coefficient greater than 0.119 but below 0.130 is evaluated as having good sliding properties and is marked with ○; and a friction coefficient greater than 0.130 is evaluated as having insufficient sliding properties and is marked with ×.

[0084] (2) Evaluation methods for weldability

[0085] For each test piece, continuous spot welding tests were conducted under the following conditions: electrode: DR type Cr-Cu electrode; pressure: 150 kgf; energizing time: 10 cycles / 60 Hz; welding current: 7.5 kA. The number of consecutive spot welds was used for evaluation. A number of consecutive spot welds of 5000 or more was considered good weldability and marked as ○, while a number of consecutive spot welds of less than 5000 was considered insufficient weldability and marked as ×.

[0086] (3) Evaluation methods for desiccation properties

[0087] The degreasing properties of the steel sheet of this invention were evaluated in the context of its use in automotive applications. To determine the degreasing properties of the coating, each test piece was first degreased using Fine Cleaner (registered trademark) E6403 (manufactured by Japan Parking Co., Ltd.) as an alkaline degreasing agent. The degreasing process involved immersing the test pieces in a degreasing solution with a degreasing agent concentration of 20 g / L and a temperature of 40°C for a specified time, followed by rinsing with tap water. For the degreased test pieces, the surface carbon strength was measured using a fluorescence X-ray analysis device. Using the measured values, along with pre-measured surface carbon strength values ​​before degreasing and those of the untreated metal sheet, the coating peeling rate was calculated using the following formula.

[0088] Coating peel rate (%) = [(Carbon strength before degreasing - Carbon strength after degreasing) / (Carbon strength before degreasing - Carbon strength of untreated steel sheet)] × 100

[0089] The peelability of the coating is evaluated according to the following criteria based on the immersion time in an alkaline degreasing solution with a coating peeling rate of 98% or higher: a peeling time of less than 120 seconds is evaluated as good peelability and is marked as ○, and a peeling time of more than 120 seconds is evaluated as insufficient peelability and is marked as △.

[0090] (4) Evaluation method of rust resistance

[0091] Imagine storing the steel sheet of this invention in coil form as steel strip, and evaluate its rust resistance under overlapping conditions. Each test piece was machined to a size of 150mm × 70mm, and the rust-preventive oil was applied at a rate of 1.0 g / m² per single side. 2 The coating was applied to both sides using a method that involved overlapping two test pieces and applying a surface pressure of 0.02 kgf / mm². 2 The test was conducted under load applied in a manner that was maintained at 50°C and 95% RH. For the evaluation of rust resistance, the overlapping inner surfaces were checked every 7 days, and the number of days until rusting was assessed. A rust resistance of 56 days or more until rusting was considered exceptionally good and marked ◎; a rust resistance of 21 days or more until rusting was considered good and marked ○; and a rust resistance of less than 21 days until rusting was considered insufficient and marked △.

[0092] (5) Methods for evaluating adhesiveness

[0093] Two test pieces were processed to a size of 100 × 25.4 mm and immersed in rust-preventive oil. After being held vertically for 24 hours to remove excess oil, a 0.2 mm thick layer of epoxy adhesive was evenly applied to the 25.4 mm × 13 mm portion of each test piece. These pieces were then overlapped and clamped together and baked at 180°C for 20 minutes to allow them to dry and cure. After cooling, a shear tensile test was performed using an autograph tester to determine the shear bond strength. Regarding adhesion, a bond strength of 20 MPa or higher was considered good and denoted as ○, while a bond strength less than 20 MPa was considered insufficient and denoted as △.

[0094]

[0095]

[0096]

[0097] According to Tables 3-1 to 3-3, 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.

[0098] Industrial availability

[0099] The steel sheet of this invention exhibits excellent properties in terms of sliding during stamping, weldability, alkali-based release properties, rust resistance, and adhesion. Due to these superior characteristics, it can be applied to a wide range of fields, primarily automotive body applications.

[0100] Symbol Explanation

[0101] 1. Sample for friction coefficient determination

[0102] 2 Sample Stage

[0103] 3 slides

[0104] 4 rollers

[0105] 5 slide support platform

[0106] 6 edge reinforcement bars

[0107] 7 First force sensor

[0108] 8. Second force sensor

[0109] 9 guide rails

Claims

1. A steel sheet having a coating containing an acrylic resin and a wax formed on at least one side, wherein, The acrylic resin has a glass transition temperature (Tg) of 100°C or higher, and the ratio of acid value to glass transition temperature (R = acid value / Tg) is 1.50 or higher and 2.05 or lower. Here, the unit of acid value is mg-KOH / g, and the unit of Tg is °C. The wax is a polyolefin wax with a melting point of 100°C or higher and 145°C or lower, and an average particle size of 3.0 μm or lower. The proportion of wax in the coating is 5% by mass or higher, and the adhesion amount W per single side of the coating is 0.3 g / m. 2 Above and 2.5g / m 2 the following.

2. The steel plate according to claim 1, wherein, The acrylic resin has an acid value of 180 mg-KOH / g or higher and 350 mg-KOH / g or lower.

3. The steel plate according to claim 1 or 2, wherein, The coating contains more than 30% by mass of the acrylic resin, and the proportion of the wax is less than 50% by mass.

4. The steel plate according to claim 1 or 2, wherein, The quality average molecular weight of the acrylic resin is above 5000 and below 30000.

5. The steel plate according to claim 1 or 2, wherein, The acrylic resin is a styrene acrylic resin.

6. The steel plate according to claim 1 or 2, wherein, The arithmetic mean roughness Ra of the steel plate before coating formation is above 0.4 μm and below 2.5 μm.

7. The steel plate according to claim 1 or 2, wherein, The coating contains 5% to 30% by mass of rust inhibitor.

8. The steel plate according to claim 7, wherein, The rust inhibitor is at least one selected from the group consisting of aluminum salts, zinc salts, and zinc oxide composed of phosphates.

9. 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.

10. The steel plate according to claim 1 or 2, wherein, The coating contains more than 1% by mass and less than 10% by mass of silicon dioxide.

11. The steel plate according to claim 9, wherein, The coating contains more than 1% by mass and less than 10% by mass of silicon dioxide.

12. A method for manufacturing a steel plate, comprising the method for manufacturing a steel plate according to any one of claims 1 to 11, wherein, A coating containing an acrylic resin and wax as described in any one of claims 1 to 11 is applied to at least one side of a steel plate and dried.

13. The method for manufacturing a steel plate according to claim 12, wherein, The maximum temperature reached by the steel plate during drying is above 60°C and below the melting point of the wax.

14. The method for manufacturing a steel plate according to claim 12 or 13, wherein, The proportion of all solid components in the coating is more than 1% by mass and less than 30% by mass.

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