Steering rack comprising steel bar with rack teeth
A steel bar for a steering rack that contains 0.50 to 0.60% by mass of C, 0.05 to 0.5% by mass of Si, 0.2 to 1.5% by mass of Mn, 0.0005 to 0.003% by mass of B, 0.005 to 0.05% by mass of Ti, 0.0005 to 0.1% by mass of Al, and 0.002 to 0.02% by mass of N is provided. Given D as a diameter of the steel bar, then the steel bar is adjusted in such a manner that quenched and tempered structures in a portion of the steel bar at a depth of D/4 from a surface satisfy conditions I), II), and III) as follows: I) a sum of a tempered bainitic structure and a tempered martensitic structure accounts for 30 to 100% in area percentage; II) a regenerated perlite structure accounts for 0 to 50% in area percentage; and III) a sum of the tempered bainitic structure, the tempered martensitic structure, and the regenerated perlite structure accounts for 50 to 100% in area percentage.
1. A steering rack comprising:
a steel bar that contains 0.50 to 0.60% by mass of C, 0.05 to 0.5% by mass of Si, 0.2 to 1.5% by mass of Mn, 0.0005 to 0.003% by mass of B, 0.005 to 0.05% by mass of Ti, 0.0005 to 0.1% by mass of Al, and 0.002 to 0.02% by mass of N;
a rack teeth forming portion formed in the steel bar and including plural rack teeth, a hardening layer having undergone induction quenching and tempering, and a surface hardness of 680 to 800 HV in Vickers hardness; and
a main body including the rack teeth forming portion on a peripheral surface thereof, a portion that opposes the rack teeth forming portion in a radius direction, and a portion present at a depth of (¾)D from a surface of the portion that opposes the rack teeth forming portion in the radius direction, where D is a diameter of the main body,
wherein:
the steel bar and the steering rack are adjusted such that quenched and tempered structures in a portion of the steel bar at a depth of D/4 from a surface and quenched and tempered structures in the portion of the main body of the steering rack at the depth of (¾)D both satisfy conditions I), II), and III) as follows:
I) a sum of a tempered bainitic structure and a tempered martensitic structure accounts for 30 to 100% in area percentage,
II) a regenerated perlite structure accounts for 0 to 50% in area percentage, and
III) a sum of the tempered bainitic structure, the tempered martensitic structure, and the regenerated perlite structure accounts for 50 to 100% in area percentage.
2. A steering rack according to claim 1 , wherein:
the rack teeth forming portion includes a teeth bottom portion; and
an effective case hardened depth in the teeth bottom portion is 0.1 to 1.5 mm from a surface of the teeth bottom portion.
3. A steering rack according to claim 1 , wherein:
the rack teeth forming portion includes a teeth bottom portion; and
an effective case hardened depth in the teeth bottom portion is 0.3 to 1.2 mm from a surface of the teeth bottom portion.
4. A steering rack according to claim 1 , wherein:
the rack teeth forming portion includes a teeth bottom portion; and
no residual ferrite is contained in the teeth bottom portion in a 0.1 mm deep region from a surface thereof.