JPS63227328A - Precoated steel plate for automobile - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to prepainted steel sheets for automobiles, more specifically,
Ti used for strength direction F and weight reduction of automobile bodies
The resin layer is an unfoamed resin layer that is easily sheared and press-molded in the shearing and pressing process, and the paint is coated on the surface of the membrane surface fat layer. The tensile rigidity of the car body is characterized by a heat-foamable resin layer that foams and becomes thick under the heating conditions of the baking furnace.
This invention relates to a pre-coated steel sheet for automobiles that improves sound and vibration characteristics and enhances the characteristics of high-strength steel sheets.

BACKGROUND OF THE INVENTION Recently, various high-strength steel plates have been developed to reduce the weight of automobile bodies to reduce fuel consumption, and the thickness of steel plates has been reduced.

However, as the thickness of the steel plate is reduced, new problems have arisen, such as a decrease in the tensile rigidity of the outer plate and a decrease in vibration characteristics from 0.75 mm or less. As a means to solve this problem, various methods of reinforcing the outer panel using lightweight resin materials have been studied.

For example, there is a method of reinforcing the back of a steel plate by attaching felt made of plant fibers or foam made of synthetic resin to the back of a steel plate. Since it is difficult to attach reinforcing material to the steel plate, lightweight reinforcing material is attached to the required position 1 of the press-formed steel plate, and then the J vertical is formed to achieve the purpose.

However, in the car body manufacturing process, after parts are assembled, processes such as degreasing, cleaning, chemical treatment, washing, and electrodeposition painting are carried out. There is a method of covering the surface of the sheet with a hot melt material to prevent these degreasing and cleaning liquids from seeping into the surface (Japanese Patent Application Laid-open No. 105285/1985), or a method of covering the surface of the sheet with a thick multi-layer structure on the back surface of the press-formed steel sheet. A method has been proposed in which a reinforcing material is applied and a part of the reinforcing material is heated and foamed at the same time during the paint baking process to form a bead-like swelling part to provide reinforcement (Japanese Unexamined Patent Publication No. 59-48152). .

However, these methods require the work of attaching reinforcing materials to predetermined positions on the back side of press-formed steel plates in accordance with the shape of the parts before assembling the parts, which significantly impedes the automated process of automobile production. In addition, dirt on the surface of the steel plate after pressing tends to cause poor adhesion, and even if there is good adhesion, if the adhesion to the steel plate decreases, corrosion will occur at the interface between the reinforcing material and the steel plate during actual vehicle use.

”It becomes easier to let go.

In addition, when using a foam sheet as a reinforcing material attached to the steel plate after press forming in the former case, the sheet should be 5 mm or more to obtain tension rigidity of the car body, and in the latter case, a part of the multilayer body is foamed at the baking temperature of the paint. In this case, the total resin thickness before foaming is 1
The shoes must be at least m long.

However, these are applied to steel plates after press forming, and are supplied as raw materials in the same way as ordinary steel plates for automobiles, and when used, they are first subjected to shearing and press processing. In this case, if the resin layer becomes thick, it is necessary to leave a large gap between the steel plate and the press die during pressing and drawing, which often makes it difficult to form the resin layer through the press operation.

Furthermore, there has been no attempt to use a pre-coated steel sheet whose surface is coated with a heat-foamable resin layer for automobile manufacturing. It wasn't clear.

Therefore, the present inventors conducted various studies on the use of such pre-coated steel sheets for automobile manufacturing, and found that it is necessary to use pre-coated steel sheets whose surfaces are coated with a heat-foamable resin layer for automobile manufacturing. It has been found that the resin layer must have excellent quality characteristics that can meet at least all of the following processing conditions in the pressing process and car body painting process.

The conditions are as follows.

First, it should be easy to shear, press, and draw, meaning that it should be a resin layer that can be molded into all parts of an automobile that require reinforcement.

Second, there should be no hindrance to welding when assembling parts. Third, the resin layer should not change even if the degreasing, chemical conversion treatment, electrodeposition coating, etc. Fourthly, it foams to a sufficient thickness to improve the tensile rigidity, sound vibration characteristics, etc. of the car body under the paint baking conditions of the car body painting process. Fifth, the adhesiveness of the foamed resin layer to the steel plate and anti-corrosion properties do not deteriorate even after long-term actual vehicle driving.

Problems to be Solved by the Invention The present invention solves these problems of the prior art: (1) After press molding, it is necessary to attach a reinforcing material that matches the shape of the part. (2) The purpose is to solve the problem of poor adhesion and easy peeling of reinforcing materials, which are applied to the dirty steel plate surface after press forming, and a thin resin layer is coated on the surface. The present invention provides a pre-painted steel sheet for automobiles that can improve the productivity of automobiles, save labor for workers, and reduce the cost of reinforcing materials by using the pre-painted steel sheet for automobile manufacturing.

One step to solve the problem, that is, the present invention is a pre-coated steel sheet for automatic illumination, the surface of which is coated with a thin resin layer in order to enhance the properties of the high-strength cold-rolled steel sheet, and the resin layer is In the press process of the car body manufacturing process, it is an unfoamed resin layer that can be easily press-formed, and in the car body painting process, the resin layer is foamed and thickened under the heating conditions of the paint baking oven to improve the tension rigidity and sound vibration characteristics of the car body. This is a pre-painted steel sheet for automobiles, which is characterized by being suitable for the following purposes.

Function Press molding can be easily performed in the automobile manufacturing process, and the film can be foamed and thickened at the baking temperature of the paint baking oven in the car body painting process, so there is no need for complicated work or special equipment, According to the flow of the production process, 9 items were obtained.

It has many advantages such as improved productivity, labor saving, and cost reduction.

The present invention will be further explained in detail below.

The plate thickness of an automobile body is roughly determined by five characteristics: tensile rigidity, Punto resistance (resistance to minute permanent deformation), durability (fatigue) strength, single member rigidity, and large deformation impact strength. These properties are correlated with the properties of the steel sheet material used, and high-strength cold-rolled steel sheets with excellent properties have been developed.For example, Nippon Steel M(7) high-strength cold-rolled steel plate 5ANC,
5AFC-R, 5AFC-D, 5AFG-E, 5AFC
- Examples include the RB series.

Among the various characteristics of high-strength cold-rolled steel sheets, the present invention aims to improve the tensile rigidity, sound vibration characteristics, etc. of the car body even when the sheet thickness is reduced to make it lighter and heavier, and to enhance the characteristics of high-strength steel sheets. This is a pre-painted steel sheet for automobiles that has been coated in advance.

The pre-coated steel sheet has a thin resin layer with good workability, so it can be easily sheared and press-formed in the automobile manufacturing process.The S-resin layer foams at the baking temperature of the car body paint and becomes a thick film, resulting in high strength. This creates a reinforcing layer that supports the steel plate. Therefore, there is no need to apply reinforcing material to fit the shape of the part after press forming, which is the case with conventional technology, and there are many advantages such as significantly improving automobile productivity, saving labor for work personnel, and reducing the cost of reinforcing materials. is obtained.

In addition, we use continuous coating equipment to clean the surface of high-strength steel plates.
After applying anti-corrosion treatment, a thin resin layer is applied, so unlike the conventional method of applying reinforcing material to the surface of a dirty steel plate after pressing, it has excellent adhesion and can withstand long-term use in actual vehicles. Also, no peeling occurs.

Next, a method for manufacturing a prepainted steel sheet according to the present invention will be explained.

For pre-coated steel sheets, if the resin layer becomes thick, it is necessary to create a large gap between the steel sheet and the press mold during pressing and drawing, and it is often difficult to form the sheet by pressing. A certain resin layer is better, and it is desirable that the resin layer is an M film resin layer with a thickness of at least 1 mm or less.A thin film is also preferable from the viewpoint of product cost and transportation and handling of the steel material.

The elongation rate of the resin layer is required to be 15% or more.

The press formability and drawing workability of high-strength cold-rolled steel sheets are determined by applying a lubricating oil or solid lubricant (metal soap such as sodium stearate, pentaerythritol, etc.) to the surface of the thin resin layer.
It will further improve if you apply it.

This is because a sliding phenomenon occurs at the interface between the lubricating material and the resin layer during processing.

However, the thicker the reinforcing material is due to its heavy weight reinforcement effect, and the greater the mechanical strength of the resin layer, the better the characteristics of the high-strength steel sheet. In this case, it is desirable that the resin layer is highly foamed, thick, and has sufficient strength after foaming.The higher the foaming ratio of the resin layer, the thicker the initial unfoamed resin layer becomes. Although it is easy to form a thin film, if the expansion ratio is increased, the strength of the resin layer after foaming will decrease, so the preferred range of expansion ratio is 10 to 40 times.

Many thermoplastic resins and thermosetting resins can be used as foamable resins, including polyolefin resins, polystyrene resins, and polyurethane resins that are usually used as foams, but pre-coated steel sheets can be assembled after press forming. As it undergoes processes such as degreasing, chemical conversion treatment, and electrodeposition coating, it has a high elongation rate and has excellent chemical resistance and electrical insulation properties.
Polyolefin resins are the most suitable.

Also, among polymer compounds, it is a resin with a low specific gravity, so it is suitable as a lightweight reinforcing material. Since the viscosity of polyolefin resins rapidly decreases as the temperature rises, in order to expand the foaming temperature range and achieve stable foaming, it is preferable to improve the viscoelasticity of the resin when it is melted by chemical crosslinking. Therefore, it is recommended that the polyolefin resin layer coated on the steel plate contain a chemical crosslinking agent in advance. Examples of crosslinking agents include dicumyl peroxide, 2,5-dimethyl-2,5-di
t-butylperoxy)hexane, 2,5-dimethyl-
All of the conventional crosslinking agents can be used, such as 2,5-di(tart-butyl-oxy)hexyl-3 and 1,3-bis(tert-butylperoxyisoprobyl)benzene. The amount added is preferably 0.1 to 20 parts by weight per 100 parts by weight of the resin.

Similarly, the blowing agents that are pre-contained in the resin layer include azodicarbonamide and isobutyronitrile.

Many commercially available blowing agents can be used, such as diazoaminobenzene, N,N'-dinitrosopentamethylenetetramine, p,p'-oxybisbenzenesulfonyl hydrazide.

The amount added varies depending on the expansion ratio, but the resin 10
It is usually 2 to 30 parts by weight per 0 parts by weight.

It is necessary to select a foaming agent that decomposes and foams at the paint baking temperature after car body painting, and the baking temperature and baking time after electrodeposition coating are usually 160 to 200°C for 20 to 30 minutes, but in the future From an energy-saving perspective, paints that cure at low temperatures close to 100°C have evolved.

It is possible to carry out the present invention by selecting an inorganic low-temperature decomposition type blowing agent such as sodium bicarbonate or ammonium carbonate in accordance with this.

When polyolefin resin is foamed at a high ratio, the strength of the resin layer tends to decrease, so in order to improve the reinforcing effect on bending rigidity, it is better to further increase the degree of crosslinking. If the degree of crosslinking is increased too much prior to foaming, stable foaming will be suppressed. Therefore, chemical crosslinking is first performed using a crosslinking agent with a low decomposition temperature, and the fluidity when the resin is melted is reduced, resulting in a certain level of shape retention. At the point where it comes out, the foaming agent decomposes and foaming begins. Even when a complete foamed resin layer is obtained, chemical crosslinking is continued using a crosslinking agent having a higher decomposition temperature to produce a foamed resin layer with a higher degree of crosslinking.

As another method for improving the reinforcing effect, it is also possible to mix a resin with a low elongation rate and high mechanical strength with the polyolefin resin within a range that does not impair press moldability. Also, one way to increase the reinforcing effect is to mix glass ta fibers or the like into the resin layer and coat it.

Alternatively, the reinforcing effect can be improved by multilayering resin layers without mixing resins with different mechanical properties. In other words, the top layer is a hard resin layer mixed with glass fiber, the middle layer is a soft resin layer made of highly foamed material, and the bottom layer is a resin layer with good adhesion to steel plates and corrosion resistance. If the resin layer is coated as a thin film resin layer with a total thickness of 1 inch or less, good results in press moldability can be obtained. The coating of the foamable resin layer on the high-strength steel plate can be carried out using continuous coating equipment for strip steel plates.

Foaming agents, chemical crosslinking agents, and other additives (flame retardants, fillers, stabilizers, antistatic agents, etc.) are uniformly mixed with the raw material polymer resin using a mixer, and the mixture is extruded. The resin mixture is melted and kneaded under temperature conditions such that the blowing agent does not decompose, and extruded into a film using a T-die, which can be continuously coated onto a preheated steel strip.

Adhesion between the foamable resin layer and the steel strip can be achieved by applying an adhesive to a clean rust-proofed steel strip in advance.

Alternatively, the adhesive resin can be melted in an extruder in the same way as the foamable resin, and the adhesive layer and the foamable resin layer are co-extruded using a two-layer T-die to form a film, which is laminated onto a preheated steel strip. . When continuously coating a steel strip with a resin layer, the width of the welded area at both ends of the steel strip is 50 mm in advance in the L direction (lengthwise direction of the steel strip) to facilitate welding in order to form one assembly. It is best to leave approximately 75 mm continuous uncoated.In the C direction (width direction of the steel strip), for each blanking dimension (blanking size length) of the parts to be assembled, It is convenient to leave them uncoated at regular intervals (100-150°C).

In addition, the C direction is coated continuously as an alternative method.
When shearing the wound coil to blanking size,
At the same time, it is also possible to use a traveling grinder to remove the resin layer at the sheared portion by polishing the width of the welded portion.

As a rust prevention treatment method before coating steel strips, ordinary chromate salts (approximately 30 to 500 mg/rn' of deposited Cr) are used.
, chemical conversion treatment such as phosphate (approximately 0.5 to 1.5 g/rn' coating amount), and thin film coating of brimer paint containing antirust pigment (j! thickness approximately 0.1 to 8 p). Furthermore, if a metal plating layer is applied to the high-strength steel sheet prior to these anti-rust treatments, even better corrosion resistance can be obtained.

EXAMPLES The present invention will be explained in more detail below with reference to Examples.

EXAMPLE As a specific example of the present invention, a pre-coated steel sheet coated with foamable resin for automobiles was prototyped in a continuous coating facility for strip steel sheets, and in the automobile manufacturing process, it was press-formed into a door outer panel, welded to an inner panel, and then assembled into a door assembly. An example will be described in detail in which the product was made into a three-dimensional product, assembled into a white body, passed through degreasing, chemical conversion treatment, electric halide equipment, etc., and carried out in the normal automobile manufacturing process.

Steel strips (Nippon Steel ■) are used in continuous coating equipment.

High strength cold rolled steel plate for drawing 5AFC35R), plate thickness 0.
A 7cm x 1350mm board width was degreased, washed and dried, and a rust preventive treatment agent containing chromate (200mg/rn' as adhesion amount of Cr) was applied to one side (the surface coated with foamable resin).

The temperature of the copper strip was raised to 120°C in a dry baking oven, and on the exit side of the oven, 1 part by weight of a chemical crosslinking agent was added to 100 parts by weight of polyethylene resin.
．． 1-bis(tert-butylperoxy) 3°3.5
-) Limethyl-cyclohexane (decomposition temperature 153°C)
2 parts by weight and 2,5-dimethyl-2,5-di(tert
-butylperoxy)hexane (decomposition temperature 179°C) 2
Parts by weight of the blowing agent p,p'-oxybisbenzenesulfonyl hydrazide (decomposition temperature 130"C) and Qi parts by weight and 9 parts by weight of azodicarbonamide (decomposition temperature 175°C) were mixed, and the mixture was heated to 120°C using an extruder. melted and kneaded with
A modified polyethylene resin into which a carboxyl group has been introduced as an adhesive layer to the steel strip is melted at 130°C using an extruder and coextruded using a two-layer T die.The film is then heated to 120°C with the adhesive layer on the bottom. The coating was applied to a preheated steel strip. The thickness of the foamable resin layer was 700 mm, and the thickness of the adhesive layer was 70 mm.

When coating the steel strip with foamable resin, a width of 75s+s at both ends of the steel strip was left unpainted to facilitate welding to the assembly. Also, for every 1570cm of shear length for door outer panels, 15
An unpainted part with a width of about 0 mm was made and covered with a film-like resin layer. The coated steel strip was cooled to room temperature, antirust lubricant was applied to both sides of the steel strip including the foamable resin layer, and the steel strip was wound into a coil.

Next, this steel band plate was sheared into a punching size of 1350 layers x 1570 mm in length for press forming into a door outer panel. In other words, a steel plate blanked to 1350 x 1570 i*m has an unpainted part of 75 layers around the welding area width.

The blanked steel plate was press-formed into the door outer panel.
Due to the synergistic effect of the stretchable polyethylene resin and the lubricating oil applied thereon, no peeling or damage to the resin layer was observed after pressing.

Next, it is welded to the inner plate to form an assembly, and assembled into the White Pode after degreasing, cleaning, chemical treatment, and cleaning.

It was dipped in a cationic electrodeposition paint for electrodeposition coating, washed with water and dried, and then kept at 190°C. The electrodeposited coating was baked in a furnace for 20 minutes. As the temperature of the door assembly rose, crosslinking of the polyethylene began with a chemical crosslinking agent with a low decomposition temperature, and at the same time, the two types of blowing agents began to thermally decompose one after another as the temperature rose. A chemical crosslinking agent having a high decomposition temperature further promoted crosslinking to produce a completely crosslinked foamed polyethylene reinforcing layer with a degree of crosslinking of approximately 70% in terms of gel fraction.

Furthermore, after completing the painting process by applying intermediate coating, baking, topcoating, and baking to the surface of the door, the door assembly is cut and the interior is inspected, and processes such as degreasing, chemical conversion treatment, and electrodeposition coating are applied. Since it is made of polyethylene resin, which has excellent chemical resistance and electrical insulation properties, there was no change in appearance at all, and it adhered completely to the back of the door's outer panel. The foamed resin layer was about 21 thick reinforcing layers, and in some parts of the door assembly, it was completely filled between the foam and the inner panel of the door.

A salt water spray test (JIS 22371) was conducted for 3,000 hours on the inside of the door outer panel, which has a foamed resin layer that adheres tightly, but there was no progress of corrosion from the periphery of the resin layer to the interface between the resin layer and the steel plate, and there was no deterioration in adhesion. , no peeling was observed.

In addition, in order to examine the reinforcing effect on the door outer panel, the bending rigidity of the steel plate was investigated by applying a bending load of 40 kg, but the displacement was 1
At 1 or less, no change in shape was observed, and the effect was more than twice that of pasting a conventional reinforcing material. This is thought to be due to the thick foamed resin layer being reinforced over a wide area, and in some cases it may be possible to omit the bead portion for reinforcing the outer skin of the vehicle body.

In addition, in order to examine the damping effect, vibration energy was applied to the test material using an electromagnet and the damping characteristics were measured.

It has a damping capacity of 20X10-2 or more, which is much higher than asphalt, which is normally used as a damping material for car body floors, and it can greatly contribute to reducing the radiated sound caused by vibrations received from the engine and the road surface. confirmed.

In addition, the sound absorption coefficient of the test material was measured according to JIS A1409. 30% in the 200 Hz low frequency range, 3 K) lz
It has been confirmed that it is an excellent sound-absorbing material that exhibits a sound absorption coefficient of 70% in the high-frequency sound range, reduces reflected sound inside the vehicle interior and engine room, and reduces internal and external noise.

In addition, since almost the entire back surface of the door outer panel is covered with a foamed resin layer, the thermal conductivity of the foamed resin layer, which has been found to have excellent heat insulation properties as a secondary effect, was 0.0.
029 kcal/m, · h · °C, and even if the surface temperature of the door outer panel (assembly) is maintained at 80 °C for a long time, the temperature of the back surface of the door inner panel remains below 23 °C, which helps save energy in the car air conditioner. It shows excellent heat insulation properties.

DETAILED DESCRIPTION OF THE INVENTION As described above, the foamable resin-coated precoated steel sheet for automobiles of the present invention improves the press formability of high-strength cold-rolled steel sheets through the synergistic effect of the thin film resin layer with good workability and the lubricating oil on its surface. Not only is the assembly improved after welding, but the processes such as degreasing, chemical conversion treatment, and electrodeposition coating are resistant to chemicals, so there is no erosion of the paint film, and the electrical insulation property eliminates wasteful electrodeposition. This reduces the cost of electrodeposition paint, and also eliminates foaming during the paint baking process.
When the resin layer becomes thicker and stronger, the tensile rigidity, sound vibration characteristics, heat insulation characteristics, etc. of the high-strength cold-rolled steel sheet are significantly improved compared to the conventional method of applying reinforcing materials.

Furthermore, in addition to these improvements in quality, the material can be used directly in the automobile manufacturing process, resulting in many advantages such as improved automobile productivity and labor savings for working personnel.

Therefore, many parts of the car body that require these features,
It is widely used as a pre-painted steel sheet for automobiles for doors, roofs, dashes, floors, etc., and the effects of the invention are extremely large.

Claims (7)

[Claims] (1) A pre-coated steel sheet for automobiles whose surface is coated with a thin resin layer to enhance the characteristics of a high-strength cold-rolled steel sheet,
Moreover, the resin layer is an unfoamed resin layer that can be easily press-molded in the press process of the car body manufacturing process, and in the car body painting process, the resin layer is foamed and thickened under the heating conditions of the paint baking oven to increase the tension of the car body. A pre-coated steel sheet for automobiles that is characterized by improved rigidity, sound and vibration characteristics, etc. (2) The precoated steel sheet for automobiles according to claim (1), wherein the thin resin layer has a thickness of less than 1 mm in view of press formability. (3) The thin resin layer has a lubricant applied to the surface to further improve the press formability of the high-strength cold-rolled steel sheet.
) The prepainted steel sheet for automobiles described in item 2. (4) The thin film resin layer contains a foaming agent that is unfoamed at temperatures below 100°C during blanking and pressing processes, and decomposes and foams at paint baking temperatures of 100 to 240°C during car body painting processes. The prepainted steel sheet for automobiles according to any one of claims (1) to (3). (5) The automobile according to any one of claims (1) to (4), characterized in that the thin resin layer contains glass fiber mixed therein in order to increase the car body reinforcement effect. Pre-painted steel plate. (6) The thin resin layer consists of three layers: the top layer made of a hard resin mixed with glass fiber, the middle layer made of a soft resin forming a highly foamed body, and the bottom layer made of a resin with good adhesion to steel plates and corrosion resistance. The pre-coated steel sheet for automobiles according to any one of claims (1) to (5), characterized in that it consists of a layer. (7) Claims (1) to (6) characterized in that the high-strength steel sheet has a metal plating layer, a chemical conversion treatment layer, an anticorrosive primer layer, and/or an adhesive layer.
) The prepainted steel sheet for automobiles according to any one of the above items.