JPH04299133A - Lightweight steel sheet for molding and manufacture of lightweight steel sheet molded product - Google Patents

Abstract

PURPOSE:To provide a lightweight steel sheet (sandwich steel sheet comprized of a resin layer and steel sheets placing the resin layer between them) superior in adhesion and processability and a method improving dent-resistance of a molded product to be obtained by processing the lightweight steel sheet. CONSTITUTION:As a method improving adhesion and processability, crystallinity of polyester resin to be placed between steel sheets (sandwich) is made not exceeding 20%. When the crystallinity of the resin is low, since adhesion between the resin and steel is favorable and processabilty of the resin itself is favorable, when the steel sheet is made a lightweight steel, the processability and moldability are improved drastically. However, in the case where the crystallinity of the resin layer is low, it is inferior in dent resistance (it is easily dented when force is applied to it) in respect of a molded product obtained by performing molding processing of a lightweight steel sheet. As an improving method of the dent resistance, the lightweight steel sheet is heat-treated after the molding processing and the crystallinity of the thermoplastic polyester resin is improved to 30% or higher. The lightweight steel sheet superior in the moldability and a lightweight steel sheet molded product having the favorable dent resistance are obtained.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to lightweight steel plates for forming used in automobiles, home appliances, etc., and steel plates used in the building materials sector.

0002

2. Description of the Related Art A large number of sandwich steel plates have been proposed and put into practical use for the purpose of reducing the weight of steel plates, and have greatly contributed to the reduction in weight of steel plates. Lightweight steel plates made from polyester resin are already being manufactured and sold. However, when using polyester resin, a biaxially stretched film is often used, making it difficult to obtain adhesion between the film and the steel plate, or requiring the use of adhesives.
The workability of biaxially stretched film is poor, which causes the workability of lightweight steel plates to be poor, limiting the parts that can be used. In addition, the steel plate sandwiched with resin has better dent resistance than a normal single plate (steel plate of the same thickness) (dent refers to damage caused by pressing the surface of the steel plate with your finger or by being hit by foreign objects such as pebbles in the case of cars). This refers to the phenomenon in which a steel plate dents when the steel plate is bent, and this becomes a problem when the thickness of the steel plate used is made thinner or the steel plate is made softer.

0003

[Problems to be Solved by the Invention] The product of the present invention is intended to improve the adhesion between the above-mentioned steel plate and resin as well as the workability of sandwich steel plates. Furthermore, by improving the dent resistance of the molded product obtained by molding this sandwich steel sheet, we aim to prevent dents (dents) in products using this sandwich steel sheet.

0004

[Means for Solving the Problems] A feature of the present invention is to provide a lightweight steel sheet with excellent workability in a state where the degree of crystallinity is 20% or less even if the resin is crystallized; It is characterized by increasing the hardness of the resin and improving dent resistance by heating the molded parts after processing them into lightweight steel plates or products to achieve a crystallinity of 30% or more. do. That is, the gist of the present invention is the adhesion with the steel plate,
Another object of the present invention is to provide a sandwich steel sheet in which the polyester resin in the intermediate layer has a crystallinity of 20% or less in order to ensure workability of the sandwich steel sheet, thereby ensuring good adhesion and workability. In addition, this lightweight steel plate is processed into a steel product for the purpose, and the steel product is heated after processing to increase the crystallinity of the intermediate polyester resin to 30% or more, resulting in a lightweight steel plate with excellent dent resistance. The present invention provides a method for manufacturing a molded article.

[0005] The contents of the present invention will be explained in detail below. First, FIG. 1 shows the structure of the sandwich steel plate of the present invention. Reference numeral 1 shown in FIG. 1 is a steel plate that is a skin material. 2 is a polyester resin with a crystallinity of 20% or less. of the skin material,
The thickness of the steel plate is not particularly limited, but generally in the case of a lightweight steel plate, it is 0.1 to 1.0 mm. 0.1
If it is less than mm, the manufacturing cost of the skin material will be very high,
This is because the advantages of a lightweight steel plate cannot be utilized.
Moreover, if it exceeds 1.0 mm, the total thickness (in the case of sandwich steel sheets, double sheet thickness) will be 2.0 mm or more, and the purpose of reducing the weight of the steel sheet will be diminished. Next, regarding the surface treatment of the skin material steel plate, this patent does not particularly limit this either. That is, for the purpose of improving the corrosion resistance of steel plates, many surface-treated steel plates are used which are coated with various types of plating, and all of these surface-treated steel plates are applicable to the present invention. That is, the application of all surface-treated steel sheets such as electrogalvanized steel sheets, electrolytic tin-plated steel sheets, hot-dip galvanized steel sheets, hot-dip aluminized steel sheets, hot-dip zinc-aluminum alloy-plated steel sheets, electrolytic zinc-iron alloy-plated steel sheets, vapor deposited galvanized steel sheets, etc. is possible. Rather, in the case of sandwich steel plates, corrosion at the interface between the resin and the steel plate often becomes a problem, and it is preferable to use a surface-treated steel plate such as a galvanized steel plate. In addition, these surface-treated steel sheets are generally subjected to chromate treatment, phosphate treatment, organic resin treatment, etc. for the purpose of further improving corrosion resistance and adhesion with paint, but the present invention In this case, the surface-treated steel plate, surface treatment method, etc. may be selected as necessary.

Next, regarding the resin layer, thermoplastic polyester resins used as the resin layer in the present invention include, for example, terephthalic acid, isophthalic acid, phthalic acid, naphthalene dicarboxylic acid, and sebacic acid. , aromatic or aliphatic dicarboxylic acids such as adipic acid as the dibasic acid component, ethylene glycol, diethylene glycol, propylene glycol, 1-4 butanediol, 1-
6 hexanediol, 1-10 decanediol and 1-
Polyester or copolymerized polyester containing an aliphatic or alicyclic glycol such as 4-hexane diol as a diol component, specifically polyethylene terephthalate, polybutylene terephthalate, polypropylene terephthalate, polyethylene isophthalate, polyethylene terephthalate.・Isophthalate etc. can be mentioned. Among these thermoplastic polyester resins, a dicarboxylic acid component containing 50 mol% or more of terephthalic acid or isophthalic acid, diethylene glycol, or It is particularly preferable to use a polyester or a copolymerized polyester comprising a diol component containing 50% or more of -4-butanediol. In addition, resins other than polyester, such as polyethylene, polypropylene, etc., may be added to the thermoplastic polyester resin used as the resin layer in the present invention for the purpose of improving the adhesion with the steel plate or the mechanical properties of the resin. It is also possible to add additives such as calcium carbonate and zinc oxide powder in an amount of 30% or less based on the thermoplastic polyester. Next, although this patent does not particularly limit the thickness of the thermoplastic polyester resin layer, in general, the thickness of the resin layer is
It is preferably 0.1 mm or more and less than 3.0 mm. 0.1m
This is because the meaning of weight reduction becomes small below 3 m.
．． The reason why the thickness is set to be less than 0 mm is that a lightweight steel plate having a thickness larger than this is difficult to process and is impractical.

Next, the content of the present invention will be explained in more detail. Sandwich steel plates, in which a resin such as polyethylene, polypropylene, nylon, or polyester is sandwiched between two steel plates, are used for the purpose of reducing the weight of the steel plates. However, conventional sandwich steel sheets have poor workability, making it difficult to deep draw steel sheets for use in automobiles and home appliances. If a resin with improved workability (for example, elongation properties of the resin) is used for the purpose of improving deep drawability, there are problems such as deterioration of dent resistance of the sandwich steel plate. The object of the present invention is to use a resin that has excellent processability when processing a sandwich steel plate, and to use a resin that has dent resistance after processing a sandwich steel plate. That is, a sandwich steel plate is prepared in a state with a low crystallinity of 20% or less, which has excellent workability as a resin, and in this state, the sandwich steel plate is formed into a desired shape by press working or the like. Thereafter, it is heat-treated at 120-240°C for 30-60 minutes or more, and by this heat treatment, a part of the polyester resin is crystallized, thereby improving the dent resistance. In the case of steel plates, this heat treatment is carried out in a commonly applied paint baking process, so that dent resistance can be improved without requiring a special crystallization process. Another purpose of using a polyester resin with a low crystallinity of 20% or less in the present invention is to improve the adhesion between the steel plate and the resin. Generally, as a method of increasing the strength of a film, polyester resin is stretched and oriented and then heat set at a temperature below its melting point to crystallize it. Although the polyester resin produced in this way has high strength, it has poor adhesion to steel plates, so it was necessary to use a resin with excellent adhesive properties to bond both the polyester resin and the steel plate, which is called an adhesive resin. . However, when the resin has a low crystallinity of 20% or less, as used in the present invention, the adhesion to the steel plate is generally good, and as described above, it can be bonded without using an adhesive resin. It is possible to give gender. Even if the adhesion to the steel plate is insufficient, it is possible to apply chromate treatment to the surface of the steel plate (a common steel plate surface treatment method that involves applying a solution containing chromic acid or electrolytically treating it in an aqueous solution containing chromic acid). Since sufficient adhesion can be ensured by this method alone, the above-mentioned adhesive is not required.

[0008] The crystallinity as used in the present invention is a value measured by the following procedure. (1) The X-ray diffraction intensity for the resin layer is 2θ=5 to 4.
Measure in the range of 0. (2) The X-ray diffraction intensity curves at 2θ=10 and 2θ=35 are connected with a straight line to form a baseline. (3) After melting a polyester resin having the same composition as the resin layer, it is poured into liquid nitrogen and rapidly cooled to form a resin that is considered to be almost completely amorphous. Regarding this, under the same conditions as (1),
Measure the line diffraction intensity. (4) Connect the bases of the crystal diffraction peaks of the diffraction intensity lines obtained in (1) with a smooth curve, and make the shape of the curve similar to the diffraction intensity curve of the amorphous sample measured in (3). Make it. (5) Let the area of the part surrounded by the baseline of (2) and the curve of (4) be Ia, and the area of the part surrounded by the diffraction intensity curve of (1) and the curve of (4) be Ic. (6) Let {Ic/Ia+Ic}×100 be the crystallinity. Next, a method of manufacturing a sandwich steel plate according to the present invention will be described. Although the present invention does not particularly limit the manufacturing method, the most efficient method for manufacturing the steel sheet is melt extrusion. That is, after heating two steel strips fed from the input side to 150°C or higher, laminating resin melted and extruded from a T-die on the surface of one of the steel strips, the other steel strip is placed on the laminated surface. After that, the two steel strips are sandwiched between heating rolls and the steel strips are heated to 200° C. or higher. It is then cooled to room temperature within 30 seconds. By changing the cooling rate, the crystallinity of the polyester resin can be arbitrarily changed from amorphous (0% crystallinity) to 20% crystallinity. In addition, a sandwich steel plate can be obtained by forming a polyester resin into a film in advance, sandwiching this film between two steel plates, heating it from 150°C to 240°C, and then cooling it, similar to the extrusion melting method. Hereinafter, the content of the present invention will be explained in detail based on Examples.

[0009]

[Example] Electrogalvanized steel sheet with a thickness of 0.3 mm (zinc plating amount 20 g/m2, chromate treatment Cr amount 60 m
g/m2), sandwich the polyester resin shown in the column of the present invention in Table 1, and then use hot rolls to lower the temperature of the steel plate to 2.
The temperature is 00°C or higher, and the layers are laminated at a speed of 5 m/min. After lamination,
It was cooled to below 50°C within 30 seconds. As a comparative example,
A product was shown in which the polyester resin shown in the Comparative Example column of Table 1 was sandwiched and made into a sandwich steel plate using the same method. For each sandwich steel plate, the resin crystallinity, adhesion, and workability (evaluated by the limit drawing ratio) were investigated. Further, the dent resistance was evaluated by the amount of dents after heat treating the sandwich steel plate in a flat plate state at 180°C. In addition, after processing the sandwich steel plate into a cup with a diameter of 50 mm and a height of 30 mm, 1
After heating at 80° C. for 10 to 20 minutes (same heating conditions as those used to evaluate dentability), the degree of crystallinity was measured by the method shown in the specification. These evaluation measurement results are also listed in Table 1.

0010

[Table 1]

1) Crystallinity before heating After manufacturing the sandwich steel plate and before pressing, the crystallinity of the resin was measured by the method shown in the specification. 2) Crystallinity after heating After processing the sandwich steel plate into a cup with a diameter of 50 mm and a height of 30 mm, it was heated at 180°C for 15 to 20 minutes (under the same heat treatment conditions as those used to measure dentability in Table 1). Thereafter, the degree of crystallinity was measured by the method shown in the specification. 3) Adhesion A sandwich steel plate is cut out to a width of 25 mm x length of 150 mm, one end in the length direction is peeled off, each steel plate is pulled in a 180° direction, and the force required for peeling is measured. (T-peel strength) 4) Limit drawing ratio A cup was drawn to a diameter of 40 mm, and expressed as a ratio to the blank diameter that was formed to the final cup. That is, a blank with a diameter Do is 40 mmφ
DoMAX/40 is the limit drawing ratio when the cup is squeezed.
shall be. 5) Dent amount After pressing an indenter with a load of 25 kg on the steel plate (sample) using the method shown in Figure 2,
The maximum amount of concavity on the steel plate side was measured. In view of the purpose of the present invention, it is preferable to measure the dent resistance of a processed molded product, but it is difficult to quantitatively measure the dent resistance of a molded product, so evaluation using a flat plate was used instead. Note that there is a good correlation between the dent resistance after processing and the dent resistance in a flat plate. From Comparative Examples 2, 3, 5, and 6 in Table 1, the crystallinity of the resin layer was
Sandwich steel sheets with a thickness exceeding 20% have poor adhesion and workability (
On the other hand, from Comparative Examples 1, 2, or 4, molded products with a crystallinity of less than 30% have poor dent resistance;
On the other hand, from Examples 1 to 13, sandwich steel sheets with a crystallinity of 20% or less have excellent adhesion and workability, and molded products with a resin crystallinity of 30% or more after molding and heat treatment have excellent dent resistance. That is clear.

0012

[Effects of the Invention] The lightweight steel plate obtained by the present invention has excellent workability. Moreover, by heat treating this lightweight steel plate after forming, the dent resistance is significantly improved.

[Brief explanation of drawings]

[Fig. 1] An explanatory diagram of the structure of a sandwich steel plate,

FIG. 2 is an explanatory diagram of a dent resistance evaluation method.

[Explanation of symbols]

1 Steel plate 2 Steel plate 3 Thermoplastic polyester resin.

Claims (2)

[Claims] 1. A lightweight steel plate for forming, comprising a thermoplastic polyester resin layer with a crystallinity of 20% or less and steel plates sandwiching this resin layer from both sides. 2. A lightweight steel plate characterized in that the crystallinity of the thermoplastic polyester resin layer is made 30% or more by forming the lightweight steel plate for forming according to claim 1 and then heat-treating the molded product. Method of manufacturing molded products.