| The rapid development of the automotive industry has raised higher requirements for automotive steel.Fe-Mn-Al-C steel have become the focus of research in recent years due to their outstanding mechanical properties,simple alloy composition and lightweight.This paper mainly starts from the design of metastable austenite,which is the key structure affecting the mechanical properties of medium manganese steel,combined with the annealing process in the two-phase zone,an ultra-high strength Fe-Mn-Al-C series low-carbon medium-manganese steel was prepared with a certain metastable austenite content is designed.In this paper,the effects of rolling process,critical annealing process and strain rate on the stability of metastable austenite and the amount of martensite transformation during deformation were systematically studied.The microstructure of ultra-high strength Fe-Mn-Al-C low carbon medium manganese steel was controlled and the mechanism of its strengthening and toughening was studied.This article draws the following conclusions:(1)In order to simplify the annealing process of low carbon medium manganese steel,the medium temperature rolling in the temperature range of two phase zone after hot rolling is adopted.The tensile strength and elongation of the hot-rolled test steel are 757.9MPa and 13.1%respectively.The tensile strength and elongation of the test steel after warm rolling are1313.2MPa and 35.8%.Compared with the hot-rolled sample,its strength is increased by 555.3MPa,the elongation is increased by 22.7%.Several cracks propagate in the fracture of the hotrolled specimen,and two kinds of fracture characteristics,namely quasi cleavage fracture and dimple,coexist near the crack,accompanied by microcracks,The warm-rolled samples show delaminated cracking,and the crack width is larger than that of the hot-rolled samples.The large number of dimples are mainly distributed near the cracks,showing good fracture toughness.(2)Through the annealing process of different annealing temperature and holding time,the results show that the experimental steel can reach the best mechanical properties after annealing at 600? for 40 minutes after warm rolling,the tensile strength of the experimental steel is 1367.6MPa,the elongation is 53%.SEM and XRD tests were carried out on the samples before and after the deformation of different annealing processes.It was found that the content of the residual austenite was different under different annealing processes,and the transformation amount of the residual austenite was also different after the deformation.After annealing at 600?for 40 min,the amount of transformation of residual austenite into martensite is the largest.The fracture was characterized by SEM.It was observed that the size and depth of dimples of the specimens held at 600? for 40 min were larger than those held at 550? for40 min and those held at 650? for 40 min,and the tearing edge was obvious.(3)The maximum tensile strength is 1385.1MPa at a low strain rate(10-1s-1)after testing the samples at different strain rates.Under high strain rate,the best elongation is 60%.The increase of strain rate leads to the decrease of tensile strength and the increase of elongation.At low strain rate,44% of the retained austenite can be completely transformed into martensite,while at high strain rate,only 32% of the retained austenite can undergo transformation.At low strain rate,the fracture is stepped and the dimples are shallow and small.At high strain rate,the dimples are flat and larger and deeper than those at low strain rate. |
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