Steering system of vehicle

A steering system of a vehicle, comprising:

• a steering gear box configured to multiply steering force applied by a driver and to output the multiplied steering force by generating steering assist force using oil pressure,
• wherein the steering gear box comprises:
• an input shaft inserted into and coupled to a housing, the input shaft being configured to receive the steering force applied by the driver;
• a worm shaft installed in the housing, the worm shaft being rotatably supported by the housing;
• a cylinder into which the worm shaft is inserted and to which the worm shaft is screw-coupled, the cylinder moving forwards and backwards in an axial direction within the housing when the worm shaft rotates;
• a torsion bar interposed between the input shaft and the worm shaft to transmit rotational force of the input shaft to the worm shaft;
• a valve assembly configured to selectively apply or remove oil pressure to or from the housing to generate steering assist force;
• a sector shaft coupled to the cylinder, the sector shaft being configured to output steering force, to which the steering assist force is added, by being rotated by forward and backward movement of the cylinder;
• and
• a magneto-rheological (MR) assist device coupled to an end portion of the torsion bar, the MR assist device being configured to adjust rotation and torsional rigidity of the torsion bar using an MR fluid as a working fluid.