Steel for heat treatment

Abstract

To provide a steel for heat treatment which exhibits high strength and high toughness even if heat treatment such as quenching and tempering ordinarily performed in the post-stage is performed. The steel for heat treatment has a composition comprising, by mass, 0.10 to 0.70% C, 0.1 to 3.0% Mn, 0.005 to 2.0% Al, <=0.050% P, <=0.50% S, <=0.0030% O and <=0.0200% N, further comprising at least one kind selected from the group consisting of <=0.30% Nb and <=0.30% Ti, and the balance Fe with inevitable impurities, and has a value TH calculated by Ti/48+Nb/93*104 of >=1.0, and a crystal grain size of <=10 [mu]m; wherein, Nb and Ti denote the contents (mass%) of Ti and Nb contained in precipitates of 5 to 100 nm, and denote the amounts measured by the respective extraction residues.

Description

technical field [0001] The present invention relates to steel for heat treatment used for manufacturing structural materials used for transportation machinery such as automobiles and industrial machinery after heat treatment such as quenching and tempering, and particularly relates to a steel having high strength and High toughness heat treated steel. Background technique [0002] Conventionally, the strength and toughness of structural materials used in transportation machinery such as automobiles and industrial machinery have been obtained by extruding heat-treated steel obtained from steel mills into a predetermined shape, followed by quenching and tempering. Fire and other heat treatment, thereby ensuring. Especially when high strength and high toughness are required, it is proposed to control the prior austenite (γ) particle size below 5 μm. [0003] For example, Patent Document 1 describes a high-strength steel member containing C: 0.25 to 0.35% by mass, Si: 0.5% by ...

AI Technical Summary

Problems solved by technology

[0014] However, the technology described in Patent Document 1 has the problems of high cost and complicated manufacturing process because the steel for heat treatment needs to increase the amount of alloying elements added.

[0015] The technology described in Patent Document 2 has a problem that the manufacturing process is complicated because processing such as wire drawing, cold rolling, cold forging, and quenching and tempering must be performed under specific conditions.

[0016] The technique described in Patent Document 3 has the problem of complicated manufacturing process because it needs to increase the temperature increase rate and shorten the retention time.

[0017] Moreover, the technologies described in Patent Documents 1 to 3 all require quenching and tempering under specific conditions after hot rolling is completed.

That is, it is necessary to carry out quenching and caulking under such specific conditions in the customer's factory after delivery to the customer, so there is a problem that it is difficult to manufacture high-strength and high-toughness steel for heat treatment

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