STEEL FOR HIGH STRENGTH BOLT EXCELLENT IN DELAYED FRACTURE RESISTANCE AND COLD WORKABILITY AND METHOD OF MANUFACTURING HIGH STRENGTH BOLT USING THE SAME

摘要

PROBLEM TO BE SOLVED: To provide a steel for a high strength bolt excellent in delayed fracture resistance and cold workability materialized by general hardening and tempering.;SOLUTION: The steel for a high strength bolt is provided that contains by mass%, C:0.35-0.55%, Si:1.50-2.60%, Mn:0.15-0.85%, P:0.015% or less, S:0.015% or less, Cr:0.25-1.15%, Mo:0.35-1.25% and the balance Fe with inevitable impurities and satisfies formulae (1) to (3). 2.202[C]+0.996[Mo]-0.041 Wp≥1.50... Formula (1). 4.957C+3.541Si+1.372Cr+2.473Mo≤15.2... Formula (2). 19.304[C]+13.169[Mo]+3.434[Si]+1.781[Cr]≥25.0... Formula (3), where [C], [Mo], [Si] and [Cr] are solid solution amount of C, Mo, Si and Cr during hardening (mass%) and Wp is amount of carbide not entered into solid solution during hardening (mass%).;SELECTED DRAWING: Figure 1;COPYRIGHT: (C)2017,JPO&INPIT