Effect Of Rolling Process On Mechanical Properties Of High-manganese Low Density Steel At Intermediate Temperature

In recent years,high-speed aircraft and automobiles have put forward higher requirements for lightweight and high-strength alloys in medium-temperature service conditions.Fe-Mn-Al-C low-density high-manganese steels developed by the intercritical rolling process have better strength-plasticity combination and lower specific gravity.Therefore,how to improve their medium-temperature strength and maintain good plasticity has become an important development direction to expand their application fields.Consequently,this paper added the intense carbide-forming elements of Mo and V to the traditional Fe-Mn-Al-C high-manganese steel.The structure evolution,medium-temperature mechanical properties,and microfracture behavior of the experimental steel under the influence of rolling temperature,rolling reduction,and deformation temperature were systematically investigated.The effects of rolling temperature,rolling reduction,and deformation temperature on the microstructure and mechanical properties of the experimental steels were emphatically analyzed,and the microscopic fracture mechanism was investigated.The main experimental results obtained in this paper are as follows:The results of the microstructure evolution and mechanical properties of the experimental steel after rolling treatment at different temperatures showed that the rolling temperature significantly influenced its microstructure evolution.The experimental steel was a duplex heterogeneous structure mainly consisting of equiaxed austenite and banded ferrite after hot rolling(HR)at 1150℃,and a triplex lamellar heterogeneous structure mainly composed of lamellar austenite and ferrite and granularκ-carbide at the phase boundary after warm rolling(WR)at 750℃,Mo and V-rich precipitates were precipitated in both experimental steels.Rolling at the intercritical temperature significantly refined austenite and ferrite promoted the evolution of equiaxed structure to lamellar structure.Also,it promoted the κ-carbide nucleation at the phase boundary and precipitation from the matrix.Meanwhile,many low-angle grain boundaries were formed after the warm rolling treatment,which together led to a significant increase in the medium-temperature tensile strength of the WR steel,obtaining a tensile strength of 1040 MPa at 500℃,significantly higher than that of the HR steel of 695 MPa.The elongation of the experimental steel decreased from 45.5%to 20.0% as the rolling temperature decreased.The microscopic fracture mechanisms of the experimental steels after rolling treatment at different temperatures showed that the differences in the microstructure of HR and WR steels caused significant differences in their plastic deformation and fracture mechanisms.The high ductility of HR steel is mainly derived from the work hardening brought by the generation and refinement of slip bands during deformation.In contrast,the refinement and intersection of slip bands led to the sprouting and extension of cracks within the austenite.The deformation of the soft phase mainly caused the ductility of the WR experimental steels in the lamellar structure,and the pinning of the κ-carbide to the lamellar ferrite grain boundaries led to the sprouting of cracks and their extension within the ferrite.The precipitates fractured during plastic deformation of both experimental steels and rapidly sprouted cracks producing holes.The results of microstructure evolution and mechanical properties of the experimental steels with different warm rolling reductions showed that the structure of the experimental steels with 73% ～ 83% warm rolling reduction was lamellar,and the experimental steels with 88% warm rolling reduction were lamellar mixed with equiaxed grains.The grain size decreased with the increase of the warm rolling reduction,while the volume fraction of ferrite and the proportion of recrystallized grains increased.The tensile properties testing of warm-rolled steel at 500℃ with different rolling reductions showed that the tensile strength of the experimental steel increased and then decreased with the increase of rolling reduction.In contrast,the elongation showed a decrease and then increased.The tensile strength of warm-rolled steel increased from 1027 MPa to 1121 MPa,and elongation decreased from 22.9% to17.7% when the warm rolling reduction was increased from 73% to 83%.The strength decreased to 1040 MPa,and elongation increased to 19% when the warm rolling reduction increased to 88%.The results of the microstructure evolution and mechanical properties of the experimental steel at different deformation temperatures showed that the lamellar structure of the experimental steel with 83% warm-rolled depression would not change after holding at 500℃.Maintaining at 600℃ would cause κ-carbide at the phase boundary to partially dissolve,and recrystallization to generate part of equiaxed grains would occur when holding at 700℃.When the deformation temperature rose from500℃ to 600℃,the experimental steel would undergo dynamic recrystallization during plastic deformation,and this behavior was significantly intensified when the deformation temperature rose to 700℃.The strength of the experimental steel decreased rapidly with the increase of temperature,and the tensile strength decreased from 1121 MPa to 365 MPa when the tensile temperature increased from 500℃ to700℃;while the elongation increased rapidly with the increase of temperature,and the elongation of the experimental steel was 108.8% at 700℃,which was much higher than that of 17.7% at 500℃.