TiC precipitation induced effect on microstructure and mechanical properties in low carbon medium manganese steel
Highlights
► TiC mainly precipitates during the tempering and reheating-quenching process. ► TiC precipitation can induce the grain refinement after reheating-quenching process. ► Mechanical properties are determined by the reheating-quenching process. ► Dislocation strengthening is the dominant factor for the mechanical properties. ► TiC precipitation can induce the excellent toughness after reheating process.
Introduction
Microalloyed steels have been developed for many years and are widely used in modern industry. It is well known that microalloyed high-strength low-alloy steels are essentially low carbon low-alloy steels that contain small additions of alloying elements such as Nb, V, or Ti [1], [2], [3], [4]. These elements act as solution atoms or precipitation to suppress the recrystallization and grain growth of austenite. The obtained fine grain microstructure can enhance the mechanical properties of steels obviously. In addition, multimicroalloying can lead to the formation of carbide and nitride particles which can further influence the mechanical properties of the steels [5], [6], [7], [8].
Due to the high price of niobium and vanadium, the development of titanium microalloyed steels is attracted much more attention recently [9], [10], [11]. For example, the effect of composite precipitation on the microstructure and mechanical properties has been studied by some researchers [12], [13], [14], [15]. However, steel alloyed with titanium alone especially the formation mechanism of TiC precipitation during different processes and its effect are seldom studied in low carbon medium manganese steel.
It has been reported that the precipitation of carbonitride in austenite is of primary importance in controlling the microstructure and mechanical properties of steels [16], [17]. However, according to previous studies [18], precipitates are expected to be formed at different stages of processing of the steel on the basis of thermodynamic and kinetic considerations.
In the present study, the precipitation behavior of TiC in the different stages during the rolling and heat-treatments processes of low carbon medium manganese steel are investigated and the effect of TiC precipitation on microstructure and mechanical properties are also discussed.
Section snippets
Experimental scheme
Table 1 shows the chemical compositions of the designed steel used in the present work. Medium manganese is designed to enhance the strength and replace the interstitial solid solution carbon atoms which may be harmful to toughness. The steel was fabricated by vacuum induction furnace. The ingots were homogenized at 1250 °C for 2 h, and forged between 1200 °C and 850 °C into a plate of 60 mm × 200 mm × 100 mm. Thereafter the forged plates were heated to 1200 °C and kept for 2 h, then rolled to 12 mm by five
Microstructures of the steels
Fig. 2 shows the microstructures of the samples. It can be seen from Fig. 2a that the matrix of sample A experienced DQ (directly quenched) process is typical martensite structure having obvious lath martensite boundary. From Fig. 2b showing the steel after tempering at 575 °C AC (air cooled) for 5 h following a DQ process, one can observe many black holes appear along the grain boundary. Fig. 2c shows the microstructure of sample C. The elongated martensite grains formed during the hot rolling
Conclusion
The precipitation behavior of titanium treated by different processes and its influence on microstructure and mechanical properties has been investigated in low carbon medium manganese steel. It is found that TiC mainly precipitates during the tempering and reheating-quenching process. The size of them mainly ranges from 1 to 18 nm and 1 to 36 nm, respectively. And tempering treatment especially promotes a peak of TiC in diameter from 1 to 5 nm, but it also results in nucleating of parts of
Acknowledgements
This research is supported by National Basic Research Program of China (973 program) No. 2010CB630803 and National High-tech R&D Programs (863 programs) Nos. 2511 and 2009AA033401.
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