Effects of Heat Treatment on the Microstructure and Mechanical Properties of Low-Carbon Steel with Magnesium-Based Inclu
- PDF / 5,606,478 Bytes
- 9 Pages / 593.972 x 792 pts Page_size
- 92 Downloads / 261 Views
NTRODUCTION
ACICULAR ferrite (AF) is a microstructure of ferrite in steel that is characterized by needle-shaped crystallites and chaotic ordering when viewed in two dimensions. This microstructure is considered to be one of the most desirable microstructures in low-carbon steels because its presence directly correlates with improved toughness.[1–3] AF plates radiate from the point of nucleation sites, resulting in randomly oriented short ferrite needles with a ‘‘basket-weave structure,’’[4] which is characterized by high-angle boundaries between the ferrite grains that reduce the chances of cleavage. It can thus be expected that the steel’s
JIAN ZHANG, Assistant Research Professor, and WENG-SING HWANG, Professor, are with the Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan, and also with the Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan 701, Taiwan. Contact e-mail: [email protected] PEI-HSIEN FENG and YAN-CHI PAN, Masters of Science, are with the Department of Materials Science and Engineering, National Cheng Kung University. YEN-HAO SU, Research Fellow, and MUH-JUNG LU, Senior Research Fellow, are with the Steel and Aluminium Research and Development Department, China Steel Corporation, Kaohsiung 812, Taiwan. Jian Zhang, Pei-Hsien Feng, and Yan-Chi Pan contributed equally to this work. Manuscript submitted April 14, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
toughness increases with an increasing amount of AF in the steel.[5,6] Upon cooling of the low-carbon steels from temperatures above the A3 temperature, the austenite, having a closely packed face-centered cubic (FCC) structure, will transform into ferrite, having a body-centered cubic (BCC) structure. When different cooling rates are used, different morphologies of BCC structure will form, such as the Widmansta¨tten, bainite, pearlite, sorbite, and troostite morphologies. Martensite, with a body-centered tetragonal (BCT) crystal structure, will also form at some lower temperatures with a high cooling rate.[7] Under certain conditions, small nonmetallic inclusions (0.5 to 3 lm) can have beneficial effects and serve as the heterogeneous nucleation sites for AF, although the nucleation potential of AF varies according to the chemical composition and crystal structure of the inclusions.[8,9] Other factors that might affect the formation of AF include austenite grain size,[10,11] the type and size of inclusions,[7,12,13] cooling rate from 1073 K to 773 K (800 C to 500 C),[14–16] and the hardenability of the steels.[1] Moreover, previous research has provided evidence that Ti-oxides are effective nuclei for AF.[17] The transformation of AF, which is believed to be intragranularly nucleated bainite, exhibits an incomplete reaction phenomenon.[10,18] Much evidence in support of this statement has been reported, such as the AF microstructure exhibiting surface relief phenomena[19] and AF plates being crystallographically related to the parent austenite grain, by
Data Loading...
