냉간성형용 비조질강의 기계적 성질에 미치는 열처리 및 합금원소의 영향

Abstract

The present study is aimed at developing the high strength steels for cold forming. The main emphasis has been placed on understanding the effects of heat treatments(austenitizing treatment and isothermal treatment) and alloying elements(Mn and C) on mechanical properties and microstructures of steels for cold forming. Five kinds of Fe-(0.15~0.25)C-0.25Si-(1.5~2.5)Mn steels were prepared, austenitized above AC3 temperature and isothermally heat treated around MS temperature. Microstructural observation, tensile tests, and impact tests were conducted. In order to construct time-temperature-transformation diagrams, the amounts of transformation products were measured by an image analyzer.

The results are summarized as follows :

1. It was confirmed that steels without carbide forming elements can get high strength, toughness, and yield ratio by proper heat treatment and alloy design.

2. Superior tensile properties were obtained, and heat treatment time could be shortened by the austenitizing treatment at A_C1+30~40℃ followed by isothermal treatment conducted around MS temperature to obtain microstructures of bainite or bainite and auto tempered martensite.

3. The addition of Mn was more effective than C addition to improve tensile properties, but in case of over 2% Mn, impact values were decreased dramatically.

4. 0.20C-2.0Mn steel has higher fatigue limit than S45C which is a conventional high strength bolting steel.