Typical properties required of steel sheets for use in domestic electrical appliances, automobiles, furniture, building and the like include strength and resistance to rusting. Parts of television cathode-ray tubes, such as explosion-proof bands and support frames, should shield the influence of geomagnetism so that electron beams, when passed through a space constituted thereby, are not deflected. Improved geomagnetic shielding properties referred to herein mean that the relative permeability in a d.c. magnetic field around 0.3 Oe corresponding to geomagnetism is large. Use of steel sheets satisfying this property requirement even in automobiles, which have more and more become electronically controlled, creates a possibility of the prevention of erroneous actuation of instruments.
In general, improved geomagnetic shielding properties can easily be realized by using non-oriented magnetic steel sheets such as specified in JIS C 2552. In this case, what is required is only to increase the relative permeability in a d.c. magnetic field around 0.3 Oe corresponding to geomagnetism, and, unlike rotary machines, properties in high magnetic field are not required. If such steel sheets could be produced in the same equipment as used for the production of steel sheets for press working, the thickness range of producible sheets could be broadened and, in addition, the production cost could be reduced.
Reducing fine precipitates present in steel or coarsening ferrite grains to facilitate the movement of domain walls is known to be effective in increasing the relative permeability in a d.c. magnetic field around 0.3 Oe corresponding to geomagnetism. For example, Japanese Patent Laid-Open Publication No. 61330/1991 discloses a method wherein a low carbon aluminum killed steel is subjected to open coil decarburization annealing to coarsen grains. Further, Japanese Patent Publication No. 6134/1996 and Japanese Patent Laid-Open Publication No. 27520/1996 discloses a method wherein a steel having a carbon content reduced to not more than 0.01% with reduced impurities is continuously annealed to coarsen grains. For steel sheets produced by these methods, however, the yield point is estimated to be about 250 MPa at the highest.
On the other hand, when reducing the amount of steel products used is contemplated from the viewpoints of a reduction in weight and life cycle assessment (LCA), a high yield point of, for example, 250 to 300 MPa or more, is required. This necessitates enhancing the yield point through utilization of one of or a combination of two or more of solid solution strengthening, fine grain strengthening, precipitation strengthening, and work strengthening. In any case, however, geomagnetic shielding properties rapidly deteriorate with increasing the yield point. Further, when the silicon content is increased, plates are likely to be broken at the time of rolling, leading to lowered productivity and yield. This makes it impossible to attain the object.
Accordingly, it is an object of the present invention to solve the above problems of the prior art and to provide high strength cold rolled steel sheets and high strength plated steel sheets having improved geomagnetic shielding properties, that is, high relative permeability in a d.c. magnetic field around 0.3 Oe, and a process for producing the same. It is another object of the present invention to provide explosion-proof bands for television cathode-ray tubes or outer magnetic shielding materials using these high strength cold rolled steel sheets and high strength plated steel sheets. Cold rolled steel sheets and plated steel sheets refer to cold rolled steel sheets not subjected to surface treatment in the narrow sense which are used in domestic electrical appliances, including explosion-proof bands and support frames for television cathode-ray tubes, automobiles, furniture, building and other applications, and surface treated steel sheets, for example, plated steel sheets subjected to plating for rust preventive purposes, for example, electroplated steel sheets subjected to plating with zinc or zinc-nickel, galvanized steel sheets, and alloyed galvanized steel sheets, and plated steel sheets subjected to treatment for further improving press forming properties and rusting resistance, such as plated steel sheets subjected to alloying of the plating and plated steel sheets with an organic coating formed as an upper layer.