| In recent years,Fe-Mn-Al-C alloy system has attracted extensive attention in the automotive industry.Fe-Mn-Al-C steel has the advantages of high strength,low density and low cost,which can be used as lightweight body structural material.However,the development of Fe-Mn-Al-C steel also faces the disciplinary problem of high strength steel for automobile: how to obtain the required plastic elongation ability while greatly improving the strength of steel.In this study,a kind of dual-phase steel with fine-grained ferrite dispersed in austenite matrix was prepared by reasonably adjusting Mn content and heat treatment process,which is beneficial to obtain excellent strength and ensure excellent ductility at the same time.Based on two kinds of fe-15/18Mn-8Al-0.8C(wt.%)light-weight high-strength steel with different Mn content,hot-rolled and cold-rolled high-strength light-weight steel with tensile strength greater than 1000 MPa and elongation greater than 30% were successfully developed by reasonable heat treatment process.The microstructure,mechanical properties,deformation mechanism and fracture behavior of the steel were analyzed by tensile test,micro hardness test,Charpy impact test,XRD,OM,TEM and fracture scanning.The main research contents are as follows:The hot-rolled 15 Mn and 18 Mn steel were treated at different temperatures.With the increase of solution temperature,the tensile strength of the experimental steel decreased and the elongation increased.The tensile strength of 15 Mn steel decreased from 1022 MPa to 815 MPa,and the elongation increased from 50% to 66%;the tensile strength of 18 Mn steel decreased from 988 MPa to 800 MPa,and the elongation increased from 44% to 58%.When the solution temperature is between 800℃ and 950℃,18 Mn steel has higher austenite volume fraction;When the solution temperature is increased to 1000℃,there is no obvious difference in microstructure among samples with different Mn content.The impact toughness of both steels at low temperature is affected by different Mn content.The impact toughness of 18 Mn steel after solution treatment is lower at low temperature,and the impact of low temperature on the toughness of 15 Mn material is obviously weaker than that of 18 Mn.The cold-rolled 15 Mn and 18 Mn steels were treated by solution treatment with different holding time.There were fine and dispersed ferrite grains in the microstructure of cold-rolled experimental steel with holding time of 5-20 min,and the austenite structure was relatively fine.This kind of structure characteristics made the yield strength of the experimental steel better than that of the hot-rolled steel,and the elongation did not decrease significantly.The yield strength of 15 Mn steel was increased from 726 MPa The yield strength of 18 Mn steel increases from 645 MPa to 725 MPa when it is increased to 770 MPa.After holding for 30 min,the grain size and volume fraction of austenite increase greatly,and the yield strength and tensile strength of the cold-rolled steel decrease significantly.The difference of Mn content makes the strain hardening rate of 18 Mn steel increase when holding time is 20 min.After aging treatment of cold rolled 15 Mn and 18 Mn steels,with the increase of holding time,κ-carbides precipitate from the matrix.The shape,distribution and volume fraction of κ-carbides are affected by the Mn content.The results show that the granular κ-carbides are distributed in austenite and ferrite grain boundaries in 15 Mn steel.However,the lamellar κ-carbides grow from austenite grain boundary to intragranular in 18 Mn steel;At the same aging time,the volume fraction of κ-carbide in 18 Mn steel is higher than that in 15 Mn steel;The strength and strain hardening rate of the experimental steel increase with the increasing of κ-carbides.However,the tensile strength of 18 Mn steel is lower than that of 15 Mn steel after aging for more than 60 min due to the decrease of intergranular bonding force caused by lamellar κ-carbides.After aging treatment,the ductility of the steel decreases and the fracture characteristics change from ductile fracture to quasi cleavage fracture. |
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