Method of manufacturing magnetostrictive torque sensor shaft

A method for manufacturing a magnetostrictive torque sensor including a sensor portion, the method comprising:

• a heat treatment step of subjecting an iron-based shaft member to a carburizing, quenching, and tempering process;
• and
• a shot peening step of performing shot peening using a boron free shot media having a Vickers hardness at least equal to 1100 and at most equal to 1300 and being free of boron, at least in a position on the shaft member, after the heat treatment step, to which the sensor portion is to be attached,
• an attaching step of attaching the sensor portion, comprising coils, to the shaft member for detecting a change in magnetic permeability at angles +45° and −45° at the position to which the sensor portion is attached,
• wherein the shot media comprising boron-free zirconia is used,
• wherein the shot media having a particle diameter of not less than 0.6 mm is used, and an injection pressure is not less than 0.4 MPa and not more than 0.55 MPa in the shot peening step,
• wherein the method further comprises a grinding step of grinding a surface of the shaft member, in such a manner that a grinding amount at the position to which the sensor portion is to be attached, is even in a circumferential direction, and
• wherein the surface of the shaft member, after shot peening, has a total error, including hysteresis error and angle error, of not more than 3%.