Magnetostrictive coat forming method, magnetostrictive torque sensor manufacturing method, and electric power steering apparatus employing the sensor

An electric power steering apparatus, comprising:

• a motor for providing steering assist torque to a steering system;
• a steering torque sensor for detecting a steering torque in the steering system;
• and
• control means for controlling an operation of the motor on the basis of at least a steering torque signal output from the steering torque sensor,
• wherein the steering torque sensor is a magnetostrictive torque sensor including a magnetostrictive coat provided on a rotational shaft,
• the magnetostrictive coat having a magnetic anisotropy imparted thereto by subjecting the magnetostrictive coat to a heating process with a predetermined twisting torque kept applied to the rotational shaft and then removing the twisting torque from the rotational shaft,
• the magnetostrictive coat provided on the rotational shaft being then subjected to a reheating process at a temperature higher than a normal use temperature at which the magnetostrictive torque sensor is actually used for torque detection.