The mechanical properties and microstructure of as-cast Nb-Ti microalloyed steel at different temperatures
and cooling rates are investigated in this paper. The III brittle zone (700—900℃) is revealed. The
reduction of the hot ductility is due to the film-like proeutectoid ferrite or the Nb(C, N) precipitates along the
austenitic grain boundaries. In the temperature range of 850—1 000℃, with the increase of the cooling rate, the
hot ductility decreases. However, in the range of 650—850℃, the appearance of large volume fractions of ferrite on
austenite grain boundaries minimizes the effect of cooling rate on hot ductility. When the cooling rate is 10℃/s,
austenite transforms more quickly to ferrite and at a lower temperature a larger amount of ferrite nucleates and
precipitates in the grain, which leads to a sharper improvement in the hot ductility at 650℃.
ZHAO Su1* (赵素), WU Yu-juan2* (吴玉娟), HE Mei-feng3 (何美凤), ZHANG Li4 (张立)
. Effects of Cooling Rates on Microstructures and Mechanical Properties of Nb-Ti Microalloyed Steel[J]. Journal of Shanghai Jiaotong University(Science), 2012
, 17(6)
: 653
-657
.
DOI: 10.1007/s12204-012-1340-7
[1] Brimacombe J K, Sorimachi K. Crack formation in the continuous casting of steel [J]. Metallurgical Transactions B, 1977, 8(2): 489-505.
[2] Dobrovska J, Stransky K, Kavicka F, et al. Analysis of a transversal crack in a steel slab [J]. Materials Science Forum, 2008, 567-568: 105-108.
[3] Ma F J, Wen G H, Tang P, et al. Causes of transverse corner cracks in microalloyed steel in vertical bending continuous slab casters [J]. Ironmaking and Steelmaking, 2010, 37(1): 73-79.
[4] Mintz B, Yue S, Jonas J J. Hot ductility of steels and its relationship to the problem of transverse cracking during continuous casting [J]. International Materials Reviews, 1991, 36(5): 187-217.
[5] Han Chun-liang, Zhang Jun-fen, Kang Yi, et al. Optimization of secondary cooling system of square billet caster [J]. Hebei Metallurgy, 2008(1): 31-33 (in Chinese).
[6] Zhao Su, Zhang Li, Xu Guo-dong, et al. Thermodynamic calculations of the second-phase precipitation in Nb-Ti microalloyed steel [J]. Baosteel Technology,2012(2): 85-91 (in Chinese).
[7] Zarandi F, Yue S. The effect of Boron on hot ductility of Nb-microalloyed steel [J]. ISIJ International,2006, 46(10): 591-598.
[8] Lee U H, Park T E, Son K S, et al. Assessment of hot ductility with various thermal histories as an alternative method of in situ solidification [J]. ISIJ International,2010, 50(4): 540-545.
[9] Ma F J, Wen G H, Tang P, et al. In situ observation and investigation of effect of cooling rate on slab surface microstrucutre evolution in microalloyed steel [J].Ironmaking and Steelmaking, 2010, 37(3): 211-218.
[10] Mintz B, Lewis J, Jonas J J. Importance of deformation induced ferrite and factors which control its formation [J]. Materials Science Technology, 1997, 13(5):379-388.
[11] Luo H, Karjalainen P, Porter D A, et al. The influence of Ti on the hot ductility of Nb-bearing steels in simulated continuous casting process [J]. ISIJ International,2002, 42(3): 273-282.
[12] Cardoso G I S L, Mintz B, Yue S. Hot ductility of aluminium and titanium containing steels with and without cyclic temperature oscillations [J]. Ironmaking and Steelmaking, 1995, 22(5): 365-377.
[13] Carpenter K R, Dippenaar R, Killmore C R. Hot ductility of Nb- and Ti-bearing microalloyed steels and the influence of thermal history [J]. Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, 2009, 40(3): 573-580.
[14] Zarandi F, Yue S. Mechanism for loss of hot ductility due to deformation during solidification in continuous of steel [J]. ISIJ International, 2004, 44(10): 1705.
[15] Cho K C, Mun D J, Kim J Y, et al. Effect of boron precipitation behavior on the hot ductility of boron containing steel [J]. Metallurgical and Materials Transactions A, 2010, 41: 1421-1428.
