Link chain superior in low temperature toughness and heat treatment method of the same

Abstract

The present invention provides a link chain superior in low temperature toughness comprised of low carbon steel, said link chain characterized in that two-stage high frequency quenching-tempering is performed to give a link chain which has a martensite single phase structure, has an austenite grain size of 9 to 12, further contains in said structure a precipitate comprised of 40 nm or smaller TiC in 90% or more and 80 nm or smaller Ti(C,N) in 10% or less, contains AlN in not more than 0.0003%, and has a −40° C. Charpy impact value of at least 45 J.

1 . A heat treatment method of a link chain superior in low temperature toughness characterized by comprising the steps of:
 heating a link chain consisting of, by mass%, C:0.18 to 0.28%, Si:0.10 to 0.30%, Mn: 1.00 to 1.70%, Cu:0.10% or less, Ni:0.40% or less, Al:0.02 to 0.10%, Ti:0.015 to 0.10%, B:0.0003 to 0.0050%, and a balance of Fe and unavoidable impurities, to a temperatures of Ac 3  to Ac 3 +150° C.,   rapidly cooling the heated link chain for quenching,   heating the cooled link chain to a temperature of Ac 3  to Ac 3 +50° C., and   tempering the heated link chain at a temperature of Ac 1  or less,   wherein the link chain having a martensite single phase structure containing precipitates with a ration of number of precipitates of 40 nm or smaller TiC in 90% or more, and 80 nm or smaller Ti(C,N) in 10% or less, an austenite grain size of 9 to 12, and further containing AlN in not more than 0.0003 mass %, wherein said link chain has a −40° C. Charpy impact value of at least 45 J, and a tensile strength of more than 1000 N/mm 2 .