Ultra-Low-Temperature Tensile Fracture Mechanism of 500 MPa Duplex Steel Bar

In the field of low-temperature-resistant steel bars in the liquefied natural gas (LNG) ultra-low-temperature environment, matching the strength and toughness of the material has become a key technical difficulty. In this paper, a duplex low-temperature-resistant steel bar was developed and designed, which adopts a continuous water-penetrating rolling process and a self-tempering process to effectively control the microstructure proportion of it at room temperature and effectively cope with ultra-low-temperature tensile failure at -163 °C. We studied the failure mechanism of 500 MPa steel grade low-temperature-resistant steel bars at tensile temperatures from 25 °C to -163 °C. We define a mixed microstructure of ferrite and pearlite (F + P) as the core of the material and tempered martensitic (TM) as the border of the material. It was found that the core and border microstructure had different response characteristics at different tensile temperatures. It is proved that, through the duplex microstructure design, it can meet the design requirements for the 500 MPa steel grade of low-temperature-resistant steel bars. By clarifying the effects of microstructure deformation, dislocation distribution, precipitated phase, and inclusions on the low-temperature resistance of steel bars under low-temperature tensile fracture, the deformation models of core and border microstructure under different tensile temperatures were constructed, and the methods for optimizing the production process of subsequent steel mills were given. After the optimization, the low-temperature toughness of the 500 MPa steel grade steel bar will be further guaranteed.