A Comparative Study on Hot Deformation Behaviors of Niobium Microalloyed Low-Carbon and Medium-Carbon Steels by Physical Constitutive Analysis

Hot compression tests are performed on low-carbon (LC) and medium-carbon (MC) niobium microalloyed steels at temperatures of 900-1100 degrees C and strain rates of 0.01-10 s(-1). The constitutive equations are studied by a physical method based on creep theory considering the relationship between the self-diffusion coefficient, Young's modulus, and temperature. It is found that carbon addition in niobium microalloyed steels shows an obvious softening effect. The physical constitutive analysis indicates that the deformation mechanism of MC steel is the slide and climb of dislocation; however, other deformation mechanisms may occur in LC steel. The accuracy of the physical constitutive equations is quantified by employing correlation coefficient (R) and average absolute relative error (AARE). For LC steel, the R value of the equation containing exponent 5 and exponent n is 0.98 and 0.99; the AARE value is 9.06% and 4.15%, respectively, and the accuracy of the latter equation is significantly higher. For MC steel, the R value of the two equations is the same and equal to 0.99, the AARE value is 5.96% and 4.91%, respectively, the accuracy of the equations is quite close to each other. The accuracy analysis is also in reasonable agreement with the speculation of the deformation mechanism.