Dilatometric determination of four critical temperatures and phase transition fraction for austenite decomposition in hy
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This work was aimed to use the peak separation method to directly measure the critical temperatures and phase transition fractions of austenite decomposition products based on experimental dilatometric curves in hypo-eutectoid steels. The results indicated that pearlite transformation start temperature and ferrite transformation finish temperature could be clearly obtained through peak separation processing, which were generally hidden in the overlapped peaks of the linear thermal expansion coefficient curve. Moreover, four critical temperatures of austenite decomposition were retarded to lower temperature with cooling rate increasing. The phase transition fraction for austenite decomposition was quantitated by measuring the area of the corresponding phase transformation peak. The final ferrite phase fraction after austenite decomposition decreased with cooling rate increasing. On the contrary, the final pearlite phase fraction increased with cooling rate increasing. Compared with the lever rule, the calculation result using peak area method can accurately reflect the actual phase fraction change versus the temperature during austenite decomposition.
I. INTRODUCTION
In the production process of the steels, the stable and efficient continuous casting is an essential way to solidify large volumes of molten metal with simple shapes, and it is prepared for subsequent thermomechanical control process. Therefore, the property and quality of casting slabs play a fundamental role for subsequent control rolling and control cooling schemes, as well as the quality of final steel products.1,2 Currently, microalloyed hypoeutectoid steels with a superior combination in mechanical properties, such as high strength, high toughness, and weldability, have been considered as promising engineering materials.3–5 The solidification of hypo-eutectoid steel undergoes generally a series of phase transformation process during continuous casting, such as L ! d ! c ! a and c ! p. The occurrence of austenite decomposition in hypo-eutectoid steel is inevitable due to frequent temperature fluctuations in the secondary cooling zone of continuous casting.6–8 Since the lattice structure difference between austenite (c, face-centered cubic, FCC) and ferrite (a, body-centered cubic, BCC), austenite decomposition is in principle accompanied by Contributing Editor: Jürgen Eckert Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2017.484
a noteworthy expand in specific volume.9 The volume expansion can cause simultaneously thermal stresses on both the surface and inside of a strand.10 The formation of film-like ferrite along austenite grain boundary destroys the continuity of the austenite matrix, and it also results in the embrittlement of matrix microstructures.11,12 Under a combination of excessive thermal and mechanical stresses, many microcracks will initiate potentially in the embrittlement film and then propagate along grain boundaries to form various surface and internal cracks.13–15
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