JP7552576B2 - Steel plate and its manufacturing method - Google Patents

Description

The present invention relates to a steel sheet with excellent sliding properties in press forming and a manufacturing method thereof. In particular, the present invention relates to a steel sheet with excellent formability even during severe drawing and a lubricating coating that can be removed with alkali and a manufacturing method thereof.

Cold-rolled steel sheets and hot-rolled steel sheets are widely used in a wide range of fields, mainly for automobile body applications, and in such applications, they are subjected to press forming before use. In recent years, there has been a demand for the integration of parts to reduce processes and for improved design, making it necessary to enable more complex forming. If more complex press forming is attempted, there is a possibility that the steel sheet will not be able to withstand the forming and break, or that die galling will occur during continuous press forming, which could have a serious adverse effect on automobile productivity.
Therefore, it is necessary to improve the press formability of steel sheets.

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

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

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

Patent document 1 describes a technology for forming a lubricating film on a zinc-plated steel sheet, the lubricating film being an acrylic resin film containing synthetic resin powder.

Patent document 2 describes a technology in which lithium silicate is used as a coating component, to which wax and metal soap are added as lubricants to form a coating on a steel sheet.

Patent document 3 describes a lubricated surface-treated metal product with excellent press moldability, which is coated with a 0.5 to 5 μm film of polyurethane resin containing a lubricant.

Patent document 4 describes a technology for forming an alkali-soluble organic film on a steel sheet, in which a lubricant is added to an epoxy resin.

Japanese Patent Application Publication No. 9-170059 JP 2002-307613 A JP 2000-309747 A JP 2000-167981 A

However, in Patent Documents 1 to 4, although lubricity is achieved through the lubricating effect of the lubricant contained therein, press formability is not necessarily sufficient in complex molding.

The present invention was made in consideration of these circumstances, and aims to provide a steel sheet that undergoes complex forming that is difficult to press form, and that has low sliding resistance in areas at risk of cracking during press forming, and has excellent press formability in areas where the surface pressure is high and where die galling is expected to occur, and a method for manufacturing the same.

When used as steel sheet for automobiles, it is also required to have sufficient removability in the alkaline degreasing step of the painting process, and also to have excellent weldability. In such applications, it is an additional objective to have both good removability and weldability.

The inventors of the present invention have conducted extensive research to solve the above problems. As a result, they have discovered that the above problems can be dramatically improved by forming a phenolic resin film containing a specific amount of wax with a melting point of 100°C to 145°C on the surface of the steel sheet, or a base resin film consisting of a phenolic resin and a resin other than a phenolic resin, in a specific range of adhesion amount.

They also discovered that there is an optimal range for the type of wax and average particle size, and that the surface roughness of the steel plate to which the resin film is to be applied has an effect.

The present invention has been completed based on the above findings, and the gist of the present invention is as follows.
[1] A steel sheet having a lubricating coating on at least one side, the lubricating coating containing a phenolic resin and a wax, the wax having a melting point of 100°C or more and 145°C or less, and containing 10% by mass or more and 50% by mass or less of the wax relative to 100% by mass of the phenolic resin, in an amount of 0.2 g/m2 or ^more and 2.0 g/m2 or ^less per side after drying.
[2] A steel sheet having a lubricating coating on at least one side, the lubricating coating comprising a base resin composed of a phenolic resin and a resin other than a phenolic resin, and wax, the solid content of the phenolic resin being 30% by mass or more and less than 100% by mass of the total solid content of the base resin, the wax having a melting point of 100°C or more and 145°C or less, and the wax containing 10% by mass or more and 50% by mass or less of wax relative to 100% by mass of the total solid content of the base resin, in an amount of 0.2 g/m2 or ^more and 2.0 g/m2 or ^less per side after drying.
[3] The steel sheet according to [1] or [2], wherein the wax is a hydrocarbon-based wax.
[4] The steel sheet according to any one of [1] to [3], wherein the average particle size of the wax is 0.01 μm or more and 4 μm or less.
[5] The steel sheet according to any one of [1] to [4], characterized in that the surface roughness of the steel sheet before the application of the lubricating coating is 1.5 μm or less in arithmetic mean roughness Ra.
[6] A method for producing a steel sheet according to any one of [1] to [5], comprising applying a paint containing a mixture of wax and a base resin consisting of a phenolic resin or a phenolic resin and a resin other than the phenolic resin, using an aqueous solvent, onto the steel sheet, and drying by heating to form a lubricating film on the steel sheet in an amount of 0.2 g/m2 or ^more and 2.0 g/m2 or ^less per side after drying.

In the present invention, the term "steel sheet" refers to hot-rolled steel sheet and cold-rolled steel sheet.

The surface roughness of a steel sheet before a lubricating film is applied is the roughness of the base steel sheet before the lubricating film is applied, but after the lubricating film is applied, the lubricating film can be removed with an organic solvent or the like to expose the base steel sheet surface and the surface roughness can be measured.

According to the present invention, a steel sheet with excellent press formability can be obtained. The coefficient of friction between the steel sheet and the die, etc., is significantly reduced. As a result, it is possible to impart stable and excellent press formability to a relatively low-strength steel sheet that is subjected to complex forming. Furthermore, even in high-strength steel sheets where the surface pressure during press forming increases, a steel sheet with low sliding resistance in areas at risk of cracking during press forming and excellent press formability in areas where the surface pressure is high and die galling is expected to occur can be obtained. Furthermore, there is no impediment to subsequent processes such as welding, degreasing, chemical conversion treatment, and painting.

In the above, "high strength" refers to a tensile strength (TS) of 440 MPa or more, and "relatively low strength" refers to a TS of less than 440 MPa.

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

The following describes an embodiment of the present invention.

The present invention is characterized in that the steel sheet surface has a phenolic resin film and a lubricating film containing 10% by mass or more and 50% by mass or less of wax having a melting point of 100° C. or more and 145° C. or less, relative to 100% by mass of the phenolic resin, or a base resin film made of a phenolic resin and a resin other than the phenolic resin, and a lubricating film containing 10% by mass or more and 50% by mass or less of wax having a melting point of 100° C. or more and 145° C. or less, relative to 100% by mass of the total solids content of the base resin.
The reason for using a phenol-based resin is that it has better sliding properties than other resins.

The phenolic resin used in the present invention is not particularly limited. From the viewpoint of the coating environment, it is preferable to use a resol-based phenolic resin that is soluble or dispersible in an aqueous solvent. There are many unknowns regarding the mechanism by which the use of phenolic resins provides excellent sliding properties, but it is thought that the three-dimensional network structure obtained after thermal curing may contribute to protecting the steel sheet surface during sliding and reducing sliding resistance.

In the present invention, not only can a resin be used that is a phenolic resin, but also a base resin that is made of a phenolic resin and a resin other than a phenolic resin can be used. In such an embodiment, the solid content of the phenolic resin must be 30% by mass or more and less than 100% by mass of the total solid content of the base resin. If the solid content of the phenolic resin is less than 30% by mass of the total solid content of the base resin, the desired sliding properties cannot be obtained. Examples of resins other than phenolic resins include, but are not limited to, acrylic resins, urethane resins, epoxy resins, polyester resins, and vinyl resins.

The wax used in the present invention must have a melting point of 100°C or higher and 145°C or lower. From the viewpoint of cost and performance, a hydrocarbon wax is preferable. Hydrocarbon wax is a lubricant made of carbon and hydrogen, and includes natural waxes such as paraffin and microcrystalline wax, and synthetic waxes such as polyethylene and polypropylene. If the wax has a melting point of less than 100°C, it will melt due to the frictional heat caused by sliding during press molding, and sufficient lubricating effect will not be obtained. If the melting point exceeds 145°C, the lubricating effect will not be exerted during sliding.

The melting point of the wax here is the melting temperature measured based on JIS K 7121:1987 "Method for measuring transition temperature of plastics". The particle size of the wax is preferably 0.01 μm or more and 4 μm or less. If the particle size is 0.01 μm or less, the effect of improving sliding properties may not be achieved, and the paint stability may also be low. If the particle size exceeds 4 μm, the wax may not be mixed well into the film when applied, and may easily fall off from the coating when handling the steel sheet after application, resulting in a loss of sliding properties.

The particle size of the wax here refers to the median diameter of the volume average diameter, and is determined by the laser diffraction/scattering method. For example, it can be determined by measuring a sample diluted with pure water using a laser diffraction/scattering type particle size distribution measuring device Partica (registered trademark) LA-960V2 (manufactured by Horiba, Ltd.).

The wax content in the coating is 10% to 50% by mass relative to 100% by mass of the phenolic resin. If it is less than 10% by mass, sufficient lubrication effect cannot be obtained. If it exceeds 50% by mass, the wax is likely to fall off due to a lack of resin components, and it may not be possible for the coating to exist stably.

When a base resin consisting of a phenolic resin and a resin other than a phenolic resin is used as the resin, the wax content in the coating is 10% by mass or more and 50% by mass or less relative to 100% by mass of the total solid content of the base resin, for the reasons explained above.

Here, the mass ratio of wax in the coating is the ratio of the mass of the solid content of wax in the coating to the total mass of the solid content of the organic resin in the coating and the solid content of the wax in the coating. As a specific measurement method, a test piece with a known amount of organic resin and wax attached to the steel plate is prepared, infrared absorption spectrum is measured using an FT-IR measurement device, and a calibration curve of the amount of organic resin and wax attached is created from the peak intensity derived from each of the organic resin and wax. Next, the infrared absorption spectrum of the lubricating coating coated steel plate to be measured is measured, and the amount of resin and wax attached is calculated from the calibration curve, thereby making it possible to determine the mass ratio of wax in the coating.

In the present invention, components other than the phenolic resin and wax may include a surface conditioner for adjusting the surface tension, an antifoaming agent, and a dispersant. Also, a rust inhibitor for improving rust prevention may be added.

The surface roughness of the steel plate used in the present invention is preferably 1.5 μm or less in arithmetic mean roughness Ra. If Ra is 1.5 μm or less, the lubricating effect of the film can be stably obtained. If Ra exceeds 1.5 μm, the unevenness of the steel plate becomes large, so that the film in the recesses does not work effectively during sliding, and the lubricating effect of the film may be reduced. The arithmetic mean roughness Ra (μm) of the steel plate can be measured in accordance with JIS B 0633:2001 (ISO 4288:1996). For example, when Ra is greater than 0.1 and less than or equal to 2, the cutoff value and reference length are set to 0.8 mm, and the evaluation length is set to 4 mm, and the roughness is determined from the measured roughness curve. When Ra is greater than 2 and less than or equal to 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 roughness is determined from the measured roughness curve.

Next, we will explain the manufacturing method of the steel sheet of the present invention.

The method for producing a steel sheet of the present invention is a method for producing a steel sheet having a phenolic resin coating containing wax having a melting point of 100°C or more and 145°C or less on the surface of the steel sheet. A paint obtained by adding wax to a phenolic resin solution or emulsion in which a phenolic resin is dissolved or dispersed in a solvent is applied to the surface of the steel sheet and dried. The application method is not particularly limited, and examples include a method using a roll coater or a bar coater, and a coating method using a spray, immersion, or brush. The drying method for the steel sheet after application can be performed by a general method. For example, drying with hot air, drying with an IH heater, or a method using infrared heating can be mentioned. The maximum temperature reached by the steel sheet during drying is preferably 60°C or more and 150°C or less. If it is less than 60°C, it takes a long time to dry and the rust prevention properties may be poor. If it is 150°C or more, the film removal property may be deteriorated. The adhesion amount after drying is 0.2 g/m2 or ^more and 2.0 g/m2 ^or less. If it is less than 0.2 g/ ^m2 , sufficient press formability cannot be obtained, and if it exceeds 2.0 g/ ^m2 , weldability and removability may deteriorate. The coating weight can be determined by a method in which the difference in weight of the steel sheet before and after coating is divided by the area, or a method in which the coating of the steel sheet after coating is completely removed with an alkaline aqueous solution or an organic solvent, and the difference in weight of the steel sheet before and after coating removal is divided by the area.

The present invention will be described below with reference to examples. Note that the present invention is not limited to the following examples.

Cold-rolled steel sheets with a thickness of 0.8 mm (TS: 270 MPa) and a thickness of 1.0 mm (TS: 590 MPa class) shown in Table 1, with different surface roughness (arithmetic mean roughness Ra) as shown in A to D, were pretreated with Fine Cleaner (registered trademark) E6403 (Nihon Parkerizing Co., Ltd.) to degrease the surface and remove oil and dirt. After rinsing with tap water to confirm that the test material was 100% wetted with water, the test material was heated in an electric oven set at a heating temperature of 100°C to form a substrate on which a lubricating film was formed.

The types of resins used as test materials, the types of waxes, their mass percentages relative to the resin, melting points, and particle sizes are shown in Table 2. The resins used as test materials were phenolic resin, acrylic resin, and urethane resin.

The waxes used as test materials were polyethylene wax, polypropylene wax, and microcrystalline wax.

A paint containing the resin and wax shown in Table 2 was applied with a bar coater so that the amount of paint adhered to the steel plate after drying was the specified amount, and the steel plate was heated and dried in an electric oven set at 100°C until the surface temperature reached 80°C, forming a coating.

(1) Method for Evaluating Press Moldability (Sliding Characteristics) In order to evaluate press moldability, the friction coefficient of each test material was measured as follows.

Figure 1 is a schematic front view of a friction coefficient measuring device. As shown in the figure, a friction coefficient measuring sample 1 taken from a test material is fixed to a sample stage 2, which is fixed to the upper surface of a horizontally movable slide table 3. A vertically movable slide table support 5 having a roller 4 in contact with the slide table 3 is provided on the lower surface of the slide table 3, and a first load cell 7 is attached to the slide table support 5 to measure the pressing load N applied by the bead 6 to the friction coefficient measuring sample 1 by pushing it up. A second load cell 8 is attached to one end of the slide table 3 to measure the sliding resistance force F for moving the slide table 3 horizontally while the above pressing force is applied. The test was performed by applying press cleaning oil Pleton (registered trademark) R352L manufactured by Sugimura Chemical Industry Co., Ltd. to the surface of the sample 1 as a lubricant.

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

The friction coefficient measurement test was carried out using the bead shown in Figure 2, with a pressing load N of 400 kgf and a sample pull-out speed (horizontal movement speed of slide table 3) of 20 cm/min. The friction coefficient μ between the test material and the bead was calculated using the formula μ = F/N. A friction coefficient of 0.110 or less was evaluated as being particularly excellent slidability, with a rating of ◎; a friction coefficient of more than 0.110 and less than 0.130 was evaluated as being good slidability, with a rating of △ being slightly poorer than 0.130 and less than 0.150, and a friction coefficient of more than 0.150 was evaluated as being insufficient.

(2) Evaluation method of coating removal property To determine the coating removal property of the steel sheet, first, each test piece was degreased with an alkaline degreaser Fine Cleaner (registered trademark) E6403 (manufactured by Nippon Parkerizing Co., Ltd.). In this treatment, the test piece was immersed in a degreasing solution with a degreaser concentration of 20 g/L and a temperature of 40° C. for a predetermined period of time, and then washed with tap water. For the test piece after this treatment, the surface carbon intensity was measured using an X-ray fluorescence analyzer, and the coating peeling rate was calculated according to the following formula using this measured value, the previously measured surface carbon intensity before degreasing, and the measured value of the surface carbon intensity of the untreated steel sheet.

The removability of the coating of the steel sheet was evaluated according to the following criteria, based on the immersion time in the alkaline degreasing solution at which the coating peeling rate was 98% or more.
<Coating peeling rate>
Film peeling rate (%)=[(carbon strength before degreasing−carbon strength after degreasing)/(carbon strength before degreasing−carbon strength of untreated steel plate)]×100
<Evaluation criteria>
Good: Within 60 seconds Bad: Over 60 seconds

(3) Evaluation method of weldability For each test piece, a continuous weldability test was performed under the following conditions: electrode used: DR type Cr-Cu electrode, pressure: 150 kgf, current application time: 10 cycles/60 Hz, welding current: 7.5 kA, and the weldability was evaluated based on the number of continuous welds. The evaluation criteria were as follows:
<Evaluation criteria>
Good: 5,000 points or more. Poor: Less than 5,000 points. Table 3 shows the amount of the lubricating film adhered to the test materials and the performance evaluation results.

The test material of paint No. 1 did not have good sliding properties because the mass % of wax to the phenolic resin was less than 10 mass %. The test materials of paint No. 11, 12, and 13 did not have good sliding properties because the melting point of the wax was outside the range of 110 to 145°C. The test materials of paint No. 15, 16, 17, and 18 did not have good sliding properties because the resin was not a phenolic resin. Furthermore, the test materials of paint No. 17 and 18 had poor film removal properties because the resin was a urethane resin.
The test pieces No. 4 and No. 31 had a lubricating film amount of less than 0.2 g/ ^m2 , and therefore did not have good sliding properties. The test pieces No. 7 and No. 34 had a lubricating film amount of more than 2 g/ ^m2 , and therefore did not have good weldability.

In comparison, all of the examples of the present invention had good sliding properties, coating removal properties, and weldability.

Cold-rolled steel sheet (TS: 270 MPa) with a thickness of 0.8 mm and a surface roughness (arithmetic mean roughness Ra) shown in B of Table 1 was pretreated with Fine Cleaner (registered trademark) E6403 (manufactured by Nihon Parkerizing Co., Ltd.) to degrease the surface and remove oil and dirt. The test material was then rinsed with tap water to confirm that it was 100% wetted with water, and then heated in an electric oven with the heating temperature set to 100°C to form a substrate on which a lubricating film was formed.

Table 4 shows the type of base resin used in the test materials, the mass percentage of each base resin solid content relative to the total solid content of the resin, the type of wax, its mass percentage relative to the resin, melting point, and particle size. The base resin used in the test materials was a mixture of phenolic resin and acrylic resin. The wax used in the test materials was polyethylene wax.

A paint containing the base resin and wax shown in Table 4 was applied with a bar coater so that the amount of paint adhered to the steel plate after drying was the specified amount, and the steel plate was heated and dried in an electric oven set at 100°C until the surface temperature reached 80°C, forming a coating.

The press formability (sliding characteristics), coating removal property, and weldability were evaluated using the same method as in Example 1 described above.

Table 5 shows the amount of lubricating film attached to the test materials and the performance evaluation results.

The test material for paint No. 23 did not have good sliding properties because it did not contain phenolic resin in the film. In contrast, all of the inventive examples of the present invention had good sliding properties, film removal properties, and weldability. In addition, when the solid content of the phenolic resin relative to the total solid content of the resin was 30 mass% or more, the sliding properties were good, and when the solid content of the phenolic resin was 50 mass% or more, the sliding properties were particularly good.

The steel sheet of the present invention has excellent press formability, coating removal properties, and weldability, making it applicable in a wide range of fields, primarily in automobile body applications.

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

Claims (7)

A steel sheet having a lubricating coating on at least one side, the lubricating coating containing a resin consisting only of a phenol-based resin and a wax, the wax having a melting point of 100°C or more and 145°C or less, and containing 10 mass% or more and 50 mass% or less of wax relative to 100 mass% of the resin consisting only of the phenol-based resin, in an amount of 0.2 g/m2 ^{or more} and 2.0 g/m2 or ^less per side after drying. A steel sheet having a lubricating coating on at least one side, the lubricating coating comprising a base resin composed of a phenolic resin and a resin other than a phenolic resin, and wax, the solid content of the phenolic resin being 50% by mass or more and less than 100% by mass of the total solid content of the base resin, the wax having a melting point of 100°C or more and 145°C or less, and the wax containing 10% by mass or more and 50% by mass or less of wax relative to 100% by mass of the total solid content of the base resin, in an amount of 0.2 g/ ^m2 or more and 2.0 g/m2 or ^less per side after drying. The steel sheet according to claim 1 or 2, wherein the wax is a hydrocarbon wax. A steel sheet according to any one of claims 1 to 3, in which the average particle size of the wax is 0.01 μm or more and 4 μm or less. The steel sheet according to any one of claims 1 to 4, wherein the surface roughness of the steel sheet before the lubricating film is applied is 1.5 μm or less in arithmetic mean roughness Ra. The steel sheet according to claim 5, wherein the surface roughness of the steel sheet before the lubricating coating is applied is 0.5 μm or more in terms of arithmetic mean roughness Ra. A method for producing a steel sheet according to any one of claims 1 to 6, comprising the steps of applying a paint comprising a mixture of wax and a base resin consisting of only a phenolic resin or a phenolic resin and a resin other than the phenolic resin, using an aqueous solvent, onto a steel sheet, and drying by heating to form a lubricating film on the steel sheet with an adhesion amount of 0.2 g/ ^m2 to 2.0 g/ ^m2 per side after drying.

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