This invention provides a method for quenching a steel product such as a high-strength boron steel, which method can prevent the formation of a slack hardened structure of a steel product surface layer after quenching to provide an even structure and thus to improve mechanical properties of the steel product. In quenching of a high-strength boron steel, a vapor film breaking time, which is an elapse time from a cooling start point to a treating material temperature sharp drop point, is regarded as a control item for determining the properties of a quenching oil, and quenching is carried out in a quenching oil which has been regulated so thatthe vapor film breaking time is a predetermined time or less (2.1 sec or less as determined by a cooling property testing method specified in JIS K 2242). The vapor film breaking time is regulated by preparing a quenching oil comprising a mineral oil as a base oil and various polymers with a high molecular weight mixed, into the base oil, as an additive for vapor film breaking time shortening purposes.
C10M 159/04 - Petroleum fractions, e.g. tars, solvents
C10M 171/00 - Lubricating compositions characterised by purely physical criteria, e.g. containing as base-material, thickener or additive, ingredients which are characterised exclusively by their numerically specified physical properties, i.e. containing ingredients which are physically well defined but for which the chemical nature is either unspecified or only very vaguely indicated
C21D 1/18 - HardeningQuenching with or without subsequent tempering
C21D 9/00 - Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articlesFurnaces therefor
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