1. SONG J, IM Y: J. Mater. Process. Technol, Vol. 72 (1998), p.89.
2. Zhang Xueyin: Mechanical Engineer, n 4 (2004), p.77 (In Chinese).
3. Li Yanrong, Yuan Guangming: Forging & Stamping Technology, n 1 (2006), p.48 (In Chinese).
4. Park Jingee, Park Junyoung, Choi S, Na K & Kim Y: Journal of materials processing technology, Vol. 198 (2008), p.471.
5. Park Jingee, Park Junyoung, Choi S, YOON D, SEO S, KIM Y: Proceedings of the Spring Conference of Japan Society for Technology of Plasticity, Tokyo: 2004, p.53.