Component for machine structural use and method for making the same

A component for machine structural use comprising a steel material at least tart of which is subjected to hardening, wherein the hardened structure has an average diameter of former austenitic grains of 12 μm or less and a maximum grain diameter not exceeding four times the average grain diameter, the steel material comprising, a composition containing in terms of percent by mass:

• C:
• 0.3% to 1.5%;
• Si:
• 0.05% to 3.0%;
• Mn:
• 0.2% to 2.0%;
• at least one element selected from the group consisting of
• Cr:
• 0.03% to 2.5%,
• Mo:
• 0.05% to 1.0%,
• Cu:
• 0.03% to 1.0%,
• Ni:
• 0.05% to 2.5%,
• Co:
• 0.01% to 1.0%,
• V:
• 0.01% to 0.5% and
• W:
• 0.005% to 1.0%,
• and satisfying a formula (2):
• C1/2(1+0.7Si)(1+3Mn)(1+2.1Cr)(1+3.0 Mo)(1+0.4 Cu)(1+0.3Ni)(1+5.0V)(1+0.5W)>2.0 (2)
• wherein the balance is Fe and inevitable impurities.

A component for machine structural use comprising a steel material at least part of which is subjected to hardening, wherein the hardened structure has an average diameter of former austenitic grains of 12 μm or less and a maximum grain diameter not exceeding four times the average grain diameter, the steel material comprising, a composition containing, in terms of percent by mass:

• C:
• 0.3% to 1.5%;
• Si:
• 0.05% to 3.0%;
• Mn:
• 0.2% to 2.0%;
• at least one element selected from the group consisting of
• Cr:
• 0,03% to 2.5%,
• Mo:
• 0.05% to 1.0%,
• Cu:
• 0.03 to 1.0%,
• Ni:
• 0.05% to 2.5%,
• Co:
• 0.01% to 1.0%,
• V:
• 0.01% to 0.5% and
• W:
• 0.005% to 1.0%;
• at least one element selected from the group consisting of
• Ti:
• 0.005% to 0.1%,
• Nb:
• 0.005% to 0.1%,
• Zr:
• 0.005% to 0.1%,
• B:
• 0.0003% to 0.01%,
• Ta:
• 0.02% to 0.5%,
• Hf:
• 0.02% to 0.5% and
• Sb:
• 0.005% to 0.015%,
• and satisfying a formula (3):
• C1/2(1+0.7Si)(1+3Mn)(1+2.1Cr)(1+3.0 Mo)(1+0.4 Cu)(1+0.3Ni)(1+5.0V)(1+1000B)(1+0.5W)>2.0 (3)
• wherein the balance is Fe and inevitable impurities.