The blocking effects of interphase precipitation on dislocations’ movement in Ti-bearing micro-alloyed steels
Introduction
Interphase precipitation can be formed during the advance of austenite/ferrite interface by the ledge, quasi-ledge or bowing mechanisms [1], and is recognized as an effective way to strengthening micro-alloyed steels with Ti, Nb, V, and Mo because these nano-sized precipitates in ferrite grains can provide sufficient precipitation strengthening effects [1], [2], [3], [4]. Titanium is a strong carbide forming element from supersaturated austenite and ferrite solid solutions to form random precipitation or during austenite to ferrite transformation to form interphase precipitation. These carbides, which exhibit the NaCl type crystal structure, have the Baker–Nutting (B–N) or Nishiyama–Wassermann (NW) orientation relationship with the ferrite [5]. Most of previous studies have estimated the precipitation strengthening effects by micro hardness testing [4], [6], in which the strengthening effects by grain boundaries had inevitably been included. Nano-indentation has been proved an effective method to precisely assessing mechanical behavior such as the hardening effects by grain boundaries and precipitates for metals, and new mechanical phenomenon can usually be discovered when it gets down to micro or nano-scales [7], [8], [9], [10], [11]. In order to further clarify the strengthening effects by interphase precipitation and understand the influence of precipitate׳s spatial arrangement on dislocations’ nucleation and movement, nano-indentations on ferrite grains with interphase precipitates and random dispersive precipitates were performed, and their nano-scale mechanical behaviors were investigated.
Section snippets
Experimental procedures
In present work, composition of the experimental steel has been chosen to be C-0.08, Mn-1.76, Si-0.21, Ti-0.11, Nb-0.058 and Fe-balance (wt%). Cylindrical specimens were prepared from hot rolled plates for thermo-mechanical simulation tests, during which the specimens were reheated to 1523 K with a heating rate of 10 K/s, held for 5 min in order to dissolve niobium and titanium carbonitrides, and cooled at 5 K/s to 1323 K and 1113 K to be hot deformed by the reduction of 30% at the strain rate of
Results and discussion
Fig. 1(a) and (b) show the typical metallographs under the coiling temperatures of 873 K and 913 K, which clearly indicate that the microstructure in the specimen coiled at 873 K is comprised of ferrite, bainite and small amount of pearlite, while the microstructure in the specimen coiled at 913 K is comprised of only ferrite and pearlite. High resolution EBSD scans of the microstructure at 873 K and 913 K where nano-indentation was performed are shown in Fig. 1(c) and (d). There are several
Conclusion
Interphase precipitation was found in ferrite grains coiled at 913 K while random dispersed precipitates were formed at 873 K. The nano-hardness values for ferrite grains with interphase precipitation and random dispersive precipitation were measured to be 4.19 GPa and 3.90 GPa, respectively. Plateaus can be observed at the initial stage of load–depth curves for the ferrite grains with interphase precipitation. The Ashby–Orowan models for random dispersive precipitates and interphase precipitates
Acknowledgements
This work was supported by the Major Innovative Research Project by Education Ministry of China under the contract of N120807001. One of the authors (Yang Xu) thanks the Graduate Research Innovation Program Funded Project (N110607006) for financial supports. The authors would like to thank Ms Wenying Xue and Dr. Yifeng Shen for the analyses of the nano-mechanical tests.
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