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Korean Journal of Metals and Materials > Volume 56(3); 2018 > Article
Korean Journal of Metals and Materials 2018;56(3): 177-186. doi: https://doi.org/10.3365/KJMM.2018.56.3.177
자동차 구조용 열연강판의 고주기 피로 특성에 미치는 전변형 효과
하동욱, 정창열
동국대학교 원자력·에너지시스템공학과
Effect of Pre-Straining on the High-Cycle Fatigue Properties of Hot-Rolled Steel Sheets for Automotive Structural Uses
Dong-Woog Ha, Chang-Yeol Jeong
Dept. of Nuclear and Energy System Engineering, Dongguk University, Gyeongju 38066, Republic of Korea
Correspondence  Chang-Yeol Jeong ,Tel: +82-54-770-2858,, Email: jcy@dongguk.ac.kr
Received: 14 November 2017;  Accepted: 11 December 2017.  Published online: 6 March 2018.
ABSTRACT
The drive towards weight reduction in the automotive industry has led to the use of high strength steel sheets for structural applications. Pressing and stamping are important processes in automotive production and result in additional straining on the vehicle body and chassis sheet material. It is therefore important to have knowledge of the effect of pre-straining on the reliability and mechanical behavior of sheets such as tensile and fatigue strength. This work sought to evaluate the mechanical properties of hot-rolled steel sheets prepared with different levels of pre-straining, which can cause a change in the final thickness of the sheets. Hot-rolled steel sheets, SAPH440 and SPHC with a nominal initial thickness value of 2.4 mm were used to obtain pre-strained sheets with pre-straining levels of 5~20%. Final thickness with pre-strain was measured to verify the correlation between thickness reduction ratio and mechanical properties. Macro and microscopic analysis results showed there was a uniform relationship between the reduction ratio and pre-strain despite the increasing sheet straining levels. The mechanical test results showed that by increasing the pre-straining level up to uniform elongation, the tensile and fatigue strength also increased, because of the work hardening effects with deformation. When pre-straining exceeded the uniform elongation, the mechanical properties decreased drastically when the thickness was modified by the initial value before pre-straining. Based upon the relation between mechanical behaviors and pre-straining, one can precisely predict design reliability and material durability by measuring the thickness change of the sheet.
Keywords: hot-rolled steel sheet, automobile, pre-strain, tensile strength, fatigue limit
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