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The effect of final electromagnetic stirring parameters, with current intensity increasing from 300 A to 400 A and frequency increasing from 4 Hz to 12 Hz, on the electromagnetic forces and carbon concentration distribution of the central cross section of a 70 steel square billet have been studied. Along the center line of the liquid core zone, current intensity of 400 A and frequency of 8 Hz achieve the maximum electromagnetic force at the position 48 mm away from the billet edge among the 10 groups of stirring parameters. Nevertheless, along diagonal of the liquid core zone, the electromagnetic force near the diagonal center is the greatest and the current intensity of 280 A and frequency of 12 Hz obtain the maximum electromagnetic force. The optimal final electromagnetic stirring (F-EMS) parameter to uniform the central cross-sectional carbon concentration and minimize the center carbon segregation of 70 steel billet was obtained with a current intensity of 280 A and frequency of 12 Hz. Under this stirring parameter, the area ratios of carbon concentrations of 0.66 wt%, 0.70 wt% and 0.74 wt% in the middle of the billet cross section reached 28.5%, 56.9% and 10.9%, respectively. Moreover, the carbon segregation indexes for all sampling points were in the range of 0.92-1.05.

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Hot compression tests are performed on low‐carbon (LC) and medium‐carbon (MC) niobium microalloyed steels at temperatures of 900–1100 °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.