Hot deformation behavior and microstructural evolution of Nb-V-Ti microalloyed ultra-high strength steel
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chang Liub) State Key Lab of Hydraulic Engineering Simulation and Safety, School of Materials Science & Engineering, Tianjin University, Tianjin 300354, People’s Republic of China; and Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China (Received 9 May 2017; accepted 18 August 2017)
The hot deformation behavior of Nb–V–Ti microalloyed ultra-high strength steel was investigated by isothermal compression at 900–1200 °C with strain rates from 0.01 to 10 s1. The microstructure evolution and precipitation behavior were studied using an optical microscope and a transmission electron microscope Results indicate that the peak stress of experimental steel increases with increasing the strain rate and decreasing the deformation temperature. The constitutive equation of hot deformation was developed with the activation energy Q being about 407.29 kJ/mol. The processing maps were also obtained to identify the instable regions of the flow behavior and to evaluate the efficiency of hot deformation. The size of dynamically recrystallized grains increases gradually with a decrease in the strain rate. Three types of carbides were identified, namely M3C, rich-Ti MC, and rich-Nb MC. With the increase of the deformation rate, the amounts of carbides increase, and the average sizes of the carbides decrease gradually. I. INTRODUCTION
With the development of modern technology, the demand for oil and natural gas is increasing gradually. In the past several decades, the exploitation of the oil and natural gas has progressively extended to the deep sea area.1,2 To improve the transmission efficiency, higher delivery pressure and larger diameter pipelines are often adopted. For the sake of transportation safety, there is an urgent need to develop ultra-high strength pipeline steels with good toughness.3 The thermal deformation behavior and the microstructure evolution of steels during the hot working process are usually complicated. To obtain an excellent combination of strength and toughness, it is important to control various parameters of the hot deformation process strictly.4 In recent years, many scholars have done researches about the hot deformation behavior of ultrahigh strength pipeline steels. Shahrani et al.5 studied the effect of deformation temperature and strain rate on the dynamic recrystallisation (DRX) behavior of X70 pipeline steel. The examination of the microstructure shows that the DRX behavior of the investigated steel was dependent on the deformation temperature and strain rate, Contributing Editor: Jürgen Eckert Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2017.365
and the increasing deformation temperature and decreasing strain rate could lead to a more adequate proceeding of DRX. Qian et al.6 investigated the hot deformation behavior of the API X100 pipeline steel. Results show that the stress approached the steady state when the strain reached 0
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