High-strength steel product excelling in fatigue strength and process for producing the same

Abstract

A high-strength and high-fatigue-strength steel having a base metal strength of 1000 MPa or more and a rotating bending fatigue strength of 550 MPa or more is provided. The steel contains 0.3-0.8 percent by mass of C, 0.01-0.9 percent by mass of Si, 0.01-2.0 percent by mass of Mn, and Fe and unavoidable impurities as the remainder. The steel has a ferrite-cementite structure having a grain size of 7 mum or less or a ferrite-cementite-pearlite structure having a grain size of 7 mum or less. A surface metal of the steel after high-frequency induction quenching has a martensite structure having a prior austenite grain size of 12 mum or less. Alternatively, a surface metal of the steel after nitriding has a fine structure having a ferrite grain size of 10 mum or less.

Description

technical field

[0001] The present invention relates to a high-strength steel material with excellent fatigue strength suitable for automobile parts using bar steel, such as constant velocity joints, transmission rods, crankshafts, connecting rods, wheel axles, and the like, and a manufacturing method thereof. Background technique

[0002] Products such as connecting rods and axles are manufactured by high-temperature forging or roll forming, followed by cutting. Products such as constant velocity universal joints, transmission rods, crank shafts, and wheel shafts are subjected to high-temperature forging or roll forming after annealing or spheroidizing annealing to improve machinability, and then high-frequency quenching is performed partially or entirely , or manufactured by nitriding treatment. In order to reduce the weight of the vehicle body, high strength and long fatigue life are required for products for such applications.

[0003] Conventionally, it has been known...