1. Field of the Invention
The present invention relates to a steel sheet for use in applications requiring electric conduction, such as grounding, supply of electricity or electric welding, which is endowed with both an electrical conductivity as one feature of a steel sheet and an excellent corrosion resistance, and specifically to a precoated steel sheet which is used as the casings of electric or electronic appliances and office automation appliances, and which is free of blocking in piling (shearing of plates to the required dimensions) or coiling and has a corrosion resistance, an electrical conductivity and an electromagnetic wave shielding effect.
2. Description of the Related Art
Steel sheets have many features and are used in a wide range of applications. Among such many features, electrical conductivity is one of the important features. Thus, steel sheets have many fields of utilization in grounding, supply of electricity, electric welding, etc. However, they always involve a problem of rusting.
The use of a steel sheet without any treatment for the purpose of securing electrical conductivity does not meet the requirement of corrosion resistance. The method of using a conductive coating (see, for example, Japanese Patent Laid-Open No. 189,843/1982) involves insufficiency of electric conductivity and high cost due to an expensive conductive coating. In a method of using other metal sheets such as an aluminum sheet instead of a steel sheet, the electrical conductivity of such a metal sheet is considerably poor as compared with that of a steel sheet, and other properties such as strength are also inferior.
There has recently arisen a problem that electromagnetic waves generated in an electric or electronic appliance or an office automation appliance bring about malfunction or noise generation of other electric or electronic appliance or office automation appliance (this phenomenon is called electromagnetic interference, hereinafter referred to briefly as EMI). This problem can be solved if the appliance is wholly covered with a conductive substance to ground the same. However, plastics as insulating substances and precoated steel sheets having insulating coatings formed on both sides thereof have recently been increasingly used particularly in casings of appliances, so that there has been an increasing demand for a countermeasure against the problem of EMI.
As for plastics, there has been proposed various methods as the EMI countermeasures including spray coating of a metal, vacuum evaporation and deposition of a metal, coating of the surface of a plastic with a paint containing a conductive pigment (see, for example, Japanese Patent Laid-Open No. 207,938/1984), and incorporation of a conductive substance into a plastic (see, for example, Japanese Patent Laid-Open No. 102,953/1984). However, any of these methods has disadvantages that the electrical conductivity is insufficient, a technical difficulty is involved, and the cost is increased.
As for precoated steel sheets, there have been proposed no particular EMI countermeasures as yet. Thus, a countermeasure is taken by leaving one side of a steel sheet untreated or subjecting the same to only chemical treatment or conversion coating, or by shaving off part of a coating from a precoated steel sheet. However, these methods involve a problem of a decrease in corrosion resistance in the exposed portion of the steel sheet. Particularly in the method of leaving one side of a steel sheet untreated or subjecting the same to only chemical treatment, there occurs, blocking, that is, injury of a decorative side (coated side) of a steel sheet by an untreated or chemically treated side thereof in piling or coiling. The method of shaving off part of a coating has a defect of an increase in the number of steps of manufacturing.
As the method of imparting an electrical conductivity to a precoated steel sheet, there has been proposed one in which a steel sheet is coated with a coating containing a metallic powder incorporated thereinto for imparting an electrical conductivity as described above. Also in this case, blocking is caused by the protruded portions of metal particles incorporated into the coating in piling or coiling just like the method of leaving one side of a steel sheet untreated or subjecting the same to only chemical treatment. Further, the electrical conductivity is insufficient for the EMI countermeasure.