In Situ Observation of Acicular Ferrite Nucleation and Growth at Different Cooling Rate in Ti-Zr Deoxidized Steel
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ACICULAR ferrite (AF) had the chaotic arrangement[1] and played an important role in refining grain size and improving steel properties.[2] It was the most desirable microstructural feature in heat affectedzone (HAZ)[3] and high strength low alloy steel (HSLA).[4] Previous studies had shown AF nucleation and growth were not only related to inclusions characteristics,[5,6] but also related to austenite grain size[7]and cooling rate.[8] With regard to inclusion characteristics, researchers had found a large number of inclusion types could induce AF nucleation through thermal cycling simulation experiment,[3,9] deoxidization solidification experiment[10–12] and welding experiment,[13,14] ranging
YONGKUN YANG, DONGPING ZHAN, HONG LEI, and ZHOUHUA JIANG are with the School of Metallurgy, Northeastern University, NO. 3-11, Wenhua Road, Heping District, Shenyang, 110819, P.R. China. Contact email: [email protected] GUOXING QIU and YULU LI are with the School of Materials Science and Engineering, Northeastern University, Shenyang 110819, P.R. China. HUISHU ZHANG is with Metallurgical Engineering College, Liaoning Institute of Science and Technology, Benxi 117004, P.R. China. Manuscript submitted March 24, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
from single inclusions (as TiN,[15–17] Ti2O3,[18–20] CuS,[21] etc.) to complex non-metallic inclusions (as Zr-Ti Mg-Ti-O-MnS,[12] Al2O3-MgO-ZrO2,[22] oxides[3] [23] MgO-Al2O3-MnS, etc.). The currently recognized mechanisms of inclusion-induced AF nucleation were: (1) inclusion as effective interface reduced the interface energy of ferrite nucleation;[24] (2) Depletion of austenite stable elements (such as Mn,[15] C,[25] etc.) or enrichment of ferrite stable elements (such as P,[10] Si,[26] etc.) near inclusions increased the driving force of ferrite nucleation; (3) thermal strain at inclusions increased the driving force of ferrite nucleation;[27] and (4) low lattice misfit between inclusion and ferrite reduced the nucleation barrier.[28] However, with the development of exploratory research, researchers had found that a single theory could not fully explain the phenomenon of inclusion-induced AF nucleation, often two or more theories worked together.[10,14,23,26] Cooling rate, as one of factors to control the final microstructure, was significant for AF nucleation and growth.[29,30] At present, the research on the influence of cooling rate on the phase transition focused on the thermal dilatometry experiment.[29–32] Although the ferrite nucleation temperature could be determined by the thermal expansion curve,[33] the nucleation positions and ferrite types could not be accurately judged. Moreover, since the AF growth time and length were
Fig. 1—In situ observation of the coalescence of austenite grains at 1200 °C isothermal holding (a) 30 s and (b) 60 s.
Fig. 2—Relationship between austenite isothermal holding time and cooling time.
grain
sizes
with
the
unable exactly matched, the AF growth rate also cannot be obtained. Based on the these mentioned inability to observe f
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