JPH0435334B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to pre-coated steel sheets for automobiles, more specifically, high-strength steel sheets coated with a thin resin layer on the surface, which are used to improve the strength and reduce the weight of automobile bodies. In the steel plate, the resin layer is an unfoamed resin layer that can be easily sheared and press-formed in the shearing and pressing process, and in the car body painting process, it is a heated foaming layer that foams and thickens under the heating conditions of the paint baking oven. The present invention relates to a pre-coated steel sheet for automobiles, which is characterized by a resin layer and which improves the tensile rigidity, sound vibration characteristics, etc. of a vehicle body, and enhances the characteristics of a high-strength steel sheet.

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 this has contributed to reducing the thickness of steel plates.

However, as the steel plate thickness was reduced, new problems arose, including a decrease in the tensile rigidity of the outer plate and a decrease in sound and vibration characteristics from below 0.75 mm. 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 with felt made of plant fibers or foam made of synthetic resin, but when the outer and inner panels of a car door are welded together,
Since it is difficult to attach these reinforcing materials to the box-shaped assembly, lightweight reinforcing materials are attached to the required positions of the press-formed steel plate, and then the assembly is formed to achieve the purpose. .

However, the car body manufacturing process involves steps such as degreasing, washing, chemical treatment, washing, and electrodeposition painting after parts are assembled, so when attaching a lightweight foam sheet to the back of a press-formed steel plate with adhesive, A method of covering the surface of the foam sheet with a hot melt material to prevent these degreasing and cleaning solutions from seeping into the surface (Japanese Patent Application Laid-open No. 105265/1983), or a method of covering the surface of the foam sheet with a thick multilayer structure at localized locations on the back side of the press-formed steel sheet. A method has been proposed in which a reinforcing material is pasted and a part of the reinforcing material is simultaneously heated and foamed during the paint baking process to form a bead-shaped bulge for 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, poor adhesion tends to occur due to dirt on the surface of the steel plate after pressing, 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, making it easy to peel off.

In addition, when using a foam sheet as a reinforcing material attached to the steel plate after press forming, the sheet should be at least 5 mm in order 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 mm or more.

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 mold during pressing and drawing, which often makes it difficult to form the resin layer by pressing.

Furthermore, there has been no attempt to use pre-coated steel sheets whose surfaces are coated with a heat-foamable resin layer for automobile manufacturing, and it is unclear what kind of quality characteristics pre-coated steel sheets have that can be used for automobile manufacturing. It wasn't.

Therefore, the present inventors have conducted various studies on the use of such pre-coated steel sheets for automobile manufacturing, and have found that in order 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 withstand at least all of the processing conditions shown below in the pressing process and car body painting process.

The conditions are as follows.

Firstly, it must be easy to shear, press, and draw, meaning that the resin layer can be molded into all parts of the car that require reinforcement.Secondly, it must not interfere with welding when assembling parts. , Thirdly, the resin layer does not change even after undergoing processes such as degreasing, chemical conversion treatment, and electrodeposition painting, that is, it has excellent chemical resistance and electrical insulation. Fourthly, the paint used in the car body painting process The foam should be sufficiently thick to improve the tension rigidity and sound vibration characteristics of the car body under baking conditions, and the foam should be foamed to the required thickness even if the paint baking conditions change depending on the manufacturing equipment. Fifth, long-term actual vehicle running. The adhesion of the foamed resin layer to the steel plate and anti-corrosion properties do not deteriorate even if the resin layer is heated.

Problems to be Solved by the Invention The present invention solves these problems in the prior art. The aim is to solve the problem of poor adhesion and easy peeling due to reinforcing material being pasted onto the surface of a dirty steel sheet.The purpose is to use pre-coated steel sheets whose surfaces are coated with a thin resin layer for use in automobile manufacturing. By using
The objective is to provide prepainted steel sheets for automobiles that can improve automobile productivity, save labor for workers, and reduce the cost of reinforcing materials.

Means for Solving the Problems That is, the present invention provides a pre-coated steel sheet for automobiles whose surface is coated with a thin resin layer in order to enhance the properties of a high-strength cold-rolled steel sheet, and furthermore, the resin layer is coated in a vehicle body manufacturing process. In the press process, it is an unfoamed resin layer that can be easily press-molded, 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, sound vibration characteristics, etc. of the car body. This is a pre-painted steel sheet for automobiles that is characterized by improved properties.

Function It can be easily press-formed in the automobile manufacturing process, and it can foam and form a thick film at the baking temperature of the paint baking oven in the car body painting process.
The desired product can be obtained in accordance with the flow of the automobile production process without requiring complicated work or special equipment, and has many advantages such as improved productivity, labor saving, and cost reduction.

The present invention will be further explained in detail below.

The thickness of an automobile body is roughly determined by five characteristics: tensile rigidity, dent resistance (resistance to minute permanent deformation), durability (fatigue) strength, member rigidity, and large deformation impact strength. These properties are correlated with the property values of the steel sheet material used, and high-strength cold-rolled steel sheets with excellent property values have been developed. For example, Nippon Steel Corporation's high-strength cold-rolled steel sheets SANC, SAFC-R,
Examples include the SAFC-D, SAFC-E, and SAFC-RB series.

Among the various properties of these high-strength cold-rolled steel sheets, the present invention has been developed in advance for the purpose of improving the tensile rigidity, sound vibration characteristics, etc. of the vehicle body even when the plate thickness is reduced to reduce weight, and to enhance the characteristics of high-strength steel sheets. This is a pre-painted steel plate for automobiles.

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 thin resin layer foams at the baking temperature of the car body paint and becomes a thick film, forming a high-strength steel sheet. This creates a reinforcing layer that helps. 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, resulting in significant improvements in automobile productivity, labor savings for workers, and cost reductions for reinforcing materials, among other benefits. is obtained.

In addition, the surface of the high-strength steel plate is cleaned using continuous coating equipment, subjected to anti-rust treatment, and then coated with a thin resin layer, making it possible to apply reinforcing material to the surface of the steel plate that is dirty after conventional pressing. Unlike other cases, it has excellent adhesion and does not peel off even after long-term driving.

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

In a pre-coated steel sheet, if the resin layer becomes thick, a large gap must be created between the steel sheet and the press die during pressing and drawing, and it is often difficult to form the sheet by pressing. Therefore, the thinner and more stretchable the resin layer is, the better, and the resin layer preferably has 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. Note that the press formability and drawability of the high-strength cold-rolled steel sheet can be further improved by applying lubricating oil or solid lubricant (metal soap such as sodium stearate, pentaerythritol, etc.) to the surface of the thin film resin layer. 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 from the car body reinforcement effect, and the higher the mechanical strength of the resin layer, the better the characteristics of the high-strength steel plate. In this case, it is desirable that the resin layer be highly foamed and thick, and that the resin layer after foaming has sufficient strength. The higher the foaming ratio of the resin layer, the thinner the initially unfoamed resin layer can be, making it easier to press mold, but considering that increasing the foaming ratio will reduce the strength of the resin layer after foaming. The preferred range of magnification is 10 to 40 times.

Foamable resins include polyolefin resins, polystyrene resins, and polystyrene resins that are usually used as foams.
Many thermoplastic resins and thermosetting resins, including polyurethane resins, can be used, but precoated steel sheets undergo processes such as degreasing, chemical conversion treatment, and electrodeposition painting after press forming, so they have a high elongation rate. Polyolefin resin is most suitable because it has excellent chemical resistance and electrical insulation properties.

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. As a crosslinking agent, dicumyl peroxide, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, 2,5-
All conventional crosslinking agents can be used, such as dimethyl-2,5-di(tert-butylperoxy)hexyne-3 and 1,3-bis(tert-butylperoxyisopropyl)benzene. The amount added is preferably 0.1 to 20 parts by weight per 100 parts by weight of the resin.

Similarly, many commercially available blowing agents that may be pre-contained in the resin layer include azodicarbonamide, isobutyronitrile, diazoaminobenzene, N,N'-dinitrosopentamethylenetetramine, and PP'-oxybisbenzenesulfonyl hydrazide. Foaming agents can be used.

The amount added varies depending on the foaming ratio, but
The amount is usually 2 to 30 parts by weight per 100 parts by weight of the resin.
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 160-200℃ and 20-30℃.
100 minutes, but from the point of view of energy saving in the future,
Even if paints that cure at low temperatures close to 0.degree. C. are developed, it is still possible to carry out the present invention by selecting an appropriate inorganic low-temperature decomposition blowing agent such as sodium bicarbonate or ammonium carbonate.

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 to reduce the fluidity when the resin is melted and maintain a certain degree of shape retention. The foaming agent decomposes at the point where it appears, 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 high degree of crosslinking.

Further, as an alternative method for improving the reinforcing effect, a resin having a low elongation rate and high mechanical strength can be mixed with the polyolefin resin to the extent that press moldability is not impaired. Also, one way to increase the reinforcing effect is to mix glass fiber 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 mm 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 that do not decompose the blowing agent.
It is possible to continuously coat a preheated steel strip by extruding it into a film using a T-die.

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.

In addition, the adhesive resin is melted in an extruder in the same way as the foamed resin, and the adhesive layer and the foamed resin layer are coextruded using a two-layer T-die to form a film, which is then laminated onto a preheated steel strip. It can be implemented. When coating a steel strip with a resin layer continuously, in order to facilitate welding to assemble the steel strip, in the L direction (lengthwise direction of the steel strip) both ends of the steel strip should be pre-welded with a width of 50 mm. It is best to leave about 75mm continuous uncoated. In the C direction (width direction of the steel strip plate), for each blanking dimension (blanking size length) of the parts to be assembled, the welding part width is set at a constant interval (100~
It is convenient to leave a gap (approximately 150 mm) uncoated. Alternatively, for the C direction, when shearing the continuously coated and wound coil to blanking size, it is also possible to simultaneously use a traveling grinder to remove the resin layer at the sheared area by the width of the welding area. It is.

As a rust prevention treatment method before coating steel strips, ordinary chromate (30 to 500 mg/ ^m2 of Cr deposited) is used.
chemical conversion treatment with phosphate (adhesive amount: 0.5 to 1.5 g/ ^m2 ), or coating of a thin film of primer paint containing anti-corrosion pigment (film thickness: approximately 0.1 to 8 μm). Even better corrosion resistance can be obtained by applying a metal plating layer to a high-strength steel sheet prior to rust treatment.

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 foamable resin-coated pre-coated steel sheet for automobiles was prototyped in a continuous coating facility for strip steel sheets, and it was press-formed into a door outer panel in the automobile manufacturing process, and then welded to an inner panel to assemble the door. Three-dimensional, assembled on a white body, degreased, chemically treated,
An example in which the film was passed through electrodeposition coating and carried out in a normal automobile manufacturing process will be described in detail.

In continuous coating equipment, steel strips (Nippon Steel Corporation,
High-strength cold-rolled steel plate SAFC35R) for drawing processing, plate thickness 0.7
mm x board width 1350 mm was degreased, washed, and dried, and one side (foaming resin coated side) was coated with a rust preventive treatment agent containing chromate (200 mg/m ² as adhesion amount of Cr). The temperature of the steel strip is raised to 120℃ in a dry baking furnace,
On the exit side of the furnace, 2 parts by weight of chemical crosslinking agent 1,1-bis(tert-butylperoxy) 3,3,5-trimethyl-cyclohexane (decomposition temperature 153°C) and 2,5- 2 parts by weight of dimethyl-2,5-di(tert-butylperoxy)hexane (decomposition temperature 179°C) and blowing agents P, P'-
Add 6 parts by weight of oxybisbenzenesulfonyl hydrazide (decomposition temperature 130°C) and 9 parts by weight azodicarbonamide (decomposition temperature 175°C), mix and melt-knead at 120°C in an extruder, and adhere to the steel strip. Modified polyethylene resin into which a carboxyl group has been introduced as a layer is melted at 130℃ using an extruder and co-extruded using a two-layer T-die to form a film, with the adhesive layer on the bottom surface and a steel strip preheated to 120℃. It was repainted. The thickness of the foamable resin layer is 700 μm.
The thickness of the adhesive layer was 10 μm.

When coating the steel strip with foamable resin, 75 mm wide areas at each end of the steel strip were left unpainted to facilitate welding to the assembly. In addition, the shear length for door outer panels is 1570 mm.
An unpainted area approximately 150mm wide was created for each area and covered with a film-like resin layer. The coated steel strip was cooled to room temperature, anti-rust 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 punching dimensions of width 1350 mm x length 1570 mm for press forming into a door outer panel. In other words, a blanked steel plate of 1350 mm x 1570 mm has an unpainted part around it with a weld width of about 75 mm.

The blanked steel plate was press-formed into the outer panel of the door. 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, the inner plate is welded to form an assembly, assembled on a white body, degreased, washed, chemically treated, washed, dipped in cationic electrodeposition paint, electrocoated, washed with water, dried, It was kept at ℃. 20 in the furnace
Baking of the electrodeposition coating was carried out for minutes.

As the temperature of the door assembly increases, crosslinking of polyethylene begins with a chemical crosslinking agent with a low decomposition temperature, and at the same time, as the temperature rises, the two types of blowing agents sequentially thermally decompose to produce foamed polyethylene, and then the chemical crosslinking agent with a high decomposition temperature begins to crosslink. The crosslinking agent further promoted crosslinking, and a completely crosslinked foamed polyethylene reinforcing layer with a degree of crosslinking of about 70% in terms of gel fraction was produced.

Furthermore, after completing the painting process by applying intermediate coating, baking, top coating, and baking to the body surface,
When we cut the door assembly and inspected the inside, we found that even after going through processes such as degreasing, chemical conversion treatment, and electrocoating, there was no change in the appearance of the door because it is a polyethylene resin with excellent chemical resistance and electrical insulation properties. It was completely attached to the back of the outer panel. The thickness of the foamed resin layer is
The reinforcing layer is about 21mm thick, and in some parts of the door assembly, it completely filled the space between it and the inner panel of the door.

A salt spray test (JIS Z2371) was conducted for 3,000 hours on the inside of the door outer panel, which has a tightly adhered foamed resin layer, 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, to check the reinforcing effect on the door outer panel, 40kg
The bending rigidity of the steel plate was investigated by applying a bending load of 1 mm, but the displacement was less than 1 mm and no change in shape was observed.
The effect was more than twice that of pasting conventional reinforcing material. This covers a large area,
This is thought to be due to the reinforcement with a thick foamed resin layer, and in some cases it is thought that the bead section for reinforcing the outer skin of the vehicle body could also be omitted.

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 20×10 ^-2 or more, which is better compared to asphalt, which is normally used as a damping material for car body floors.
It has been confirmed that it exhibits a much greater damping capacity and greatly contributes to reducing radiated sound caused by vibrations received from the engine and road surface.

The sound absorption coefficient of the test material was also measured according to JIS A1409. 30% in 200Hz low frequency range, 3KHz high frequency range
It has been confirmed that it is an excellent sound-absorbing material that exhibits a sound absorption rate of 70%, reducing reflected sound inside the vehicle interior and engine compartment, and reducing noise inside and outside the vehicle.

It was also found that because almost the entire back surface of the door outer panel is covered with a foamed resin layer, it has excellent heat insulation properties as a secondary effect. When the thermal conductivity of the foamed resin layer was measured, it was as low as 0.029kcal/m・h・℃, and even if the surface temperature of the door outer panel (assembly) was kept at 80℃ for a long time, the temperature of the back surface of the door inner panel remained low. It exhibits excellent insulation properties that save energy in car air conditioners at temperatures below 23℃.

Effects of the Invention As explained 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 lubricant on its surface. Not only is it improved, but even if it goes through processes such as degreasing, chemical conversion treatment, and electrodeposition painting as an assembly after welding, it is chemical resistant so there is no erosion of the coating film, and its electrical insulation property eliminates unnecessary electrodeposition. This reduces the cost of electrodeposited paint, and the foaming, thickening, and strong resin layer created during the paint baking process improves the tensile rigidity and sound vibration characteristics of high-strength cold-rolled steel sheets compared to the conventional method of attaching reinforcing materials. , significantly improves insulation properties, etc. 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, the invention is widely used as a prepainted steel sheet for automobiles in many parts of the car body that require these features, such as doors, roofs, darts, floors, etc., and the effects of the invention are extremely large.

Claims (1)

[Scope of Claims] 1. A pre-coated steel sheet for automobiles whose surface is coated with a thin resin layer in order to enhance the characteristics of a high-strength cold-rolled steel sheet, and furthermore, the resin layer is not press-formed in the pressing process of the car body manufacturing process. It is an unfoamed resin layer that can be easily used, and is characterized in that the resin layer is foamed under the heating conditions of a paint baking oven during the car body painting process to thicken the film and improve the tension rigidity, sound vibration characteristics, etc. of the car body. Prepainted steel sheet for automobiles. 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 automobile according to claim 1 or 2, wherein the thin resin layer is coated with a lubricant on the surface to further improve the press formability of the high-strength cold-rolled steel sheet. Prepainted steel plate. 4. The thin 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 the car body painting process. A prepainted steel sheet for automobiles according to any one of claims 1 to 3. 5. The precoated steel sheet for automobiles according to any one of claims 1 to 4, wherein the thin resin layer contains glass fiber mixed therein in order to enhance the car body reinforcement effect. 6 The thin film 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 the steel plate and corrosion resistance. The prepainted steel sheet for automobiles according to any one of claims 1 to 5, characterized in that: 7 Claim 1, characterized in that the high-strength steel plate has a metal plating layer, a chemical conversion treatment layer, an anticorrosion primer layer, and/or an adhesive layer.
The prepainted steel sheet for automobiles according to any one of items 6 to 6.