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The hot tensile behavior of a low-alloyed ultra-high strength (LUHS) steel is studied by performing the isothermal tensile tests under different tensile processing parameters (strain rates and tensile temperatures). The effects of tensile processing parameters on the hot tensile behaviors and fracture characteristics are comprehensively discussed by analyzing the hot tensile data and microstructure observation. For reproducing the hot tensile behaviors, the dislocation density based constitutive model is constructed and further improved by considering plastic damage. It is found that the hot tensile curves always show the high stress under low tensile temperatures or high strain rates. The ductile fracture characterized by dimples is the dominant fracture type. Meanwhile, the elongation to fracture roughly follows a decreasing trend under low strain rates or high tensile temperatures, and the variation of the reduction in cross-sectional area is opposite to that of elongation to fracture. This is because that the multiple coalescence of microvoids easily takes place with the progress of necking under low strain rates or high tensile temperatures. The relatively narrow error band (controlled in ±5.518 MPa), low mean absolute relative error (equals to 2.695%) and high correlation coefficient (higher than 0.998) indicate that the improved dislocation density based constitutive model is preferred for reconstructing the hot tensile behaviors of the studied steel.