NON-HEAT TREATED STEEL FOR HOT-FORGING, HAVING HIGH BENDING FATIGUE STRENGTH AND SMALL AMOUNT OF DEFORMATION DUE TO REPEATING STRESS, AND METHOD FOR MANUFACTURING PARTS OF THE SAME

摘要

PROBLEM TO BE SOLVED: To provide a non-heat treated steel for hot-forging, which has high bending fatigue strength and is difficult to cause the deformation when repeated stress is applied in a low strain zone, and to provide a method for manufacturing parts of the same.;SOLUTION: This steel has the composition composed of respective specific contents of C, Si, Mn, P, S, Cr, Mo, V, Al, Ti, N and if necessary, Cu or Ni wherein the formula (1) is satisfied and the balance Fe with inevitable impurities, and has a hot-forged coarse material structure, wherein a ferrite having volume fraction (F%) of 4-23, and a pearlite are contained, and the volume fraction of bainite is ≥5%, and the ferrite volume fraction (F%) and the ferrite average grain diameter (μm) and the content of V (mass%), satisfy the formula (2). The formula (1): 3.10≤2.7×Mn+4.6×Cr+V≤5.60, and the formula (2): 0.04≤ferrite volume fraction (F%)×V/ferrite average diameter (μm)≤0.18. The hot-forged rough material is heated to 1,200-1,300°C and then is subjected to hot-forging at the finished temperature of ≥1,050°C, and the forged steel is cooled at the cooling speed of ≤0.8°C/sec.;COPYRIGHT: (C)2011,JPO&INPIT