With a recent increase in requirement for achieving high efficiency of various electrical apparatuses, a non-oriented electrical steel sheet to be used as an iron core is required to achieve high magnetic flux density and achieve low core loss. A low-Si steel is advantageous for manufacturing a steel sheet having a particularly high magnetic flux density, which inevitably results in using a steel in a range of a chemical composition having α-γ transformation. In a low-Si non-oriented electrical steel sheet, there have been proposed many methods of improving a magnetic property.
For example, in Patent Literature 1, there has been proposed a method of finishing hot rolling at an Ar3 transformation point or higher and slowly cooling a temperature region of the Ar3 transformation point to an Ar1 transformation point at 5° C./sec or less. However, it is difficult to perform this cooling rate in hot rolling in an actual machine.
Further, in Patent Literature 2, there has been proposed a method of adding Sn to a steel and controlling a finishing temperature of hot rolling according to the concentration of Sn, thereby obtaining a high magnetic flux density. However, in this method, the concentration of Si is limited to 0.4% or less, which is not enough to obtain a low core loss.
Further, in Patent Literature 3, there has been proposed a steel sheet having a high magnetic flux density and having an excellent grain growth property at the time of stress relieving annealing by limiting a heating temperature and a finishing temperature at the time of hot rolling. This method does not include a process of self annealing or the like in place of hot rolling annealing, so that it has been impossible to obtain a high magnetic flux density.
Further, Patent Literature 4 has proposed to, in hot rolling, heat a rough bar before finish rolling on line, set a finishing temperature of the hot rolling to Ar1+20° C. or higher, and set a winding temperature to 640 to 750° C. However, this method aims to make precipitates harmless, resulting in that a high magnetic flux density has not been obtained.
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