Formation And Effect Of Deformation Substructure In Low Carbon Hot-rolled Multiphase-steel

Advanced high strength multiphase steel with excellent plasticity,weldability,cold working formability and impact toughness is urgently needed by our country.However,with continuous improvement of strength,comprehensive mechanical properties of the steel continue to decrease,and many problems such as cracking,springback,and failure to meet dimension requirement of parts appear in the forming process.Under the background of low-carbon and green transformation and upgrading of iron and steel industry,this paper designed low carbon hot-rolled multiphase steel with composition of0.2C-1.74Mn-0.68Si-0.06Nb(wt.%)based on the principle of automobile lightweight and alloy reduction.The TMCP process design was conducted by means of thermal simulation and JMat Pro software.The microstructure and mechanical properties of the steel were characterized by OM,SEM,TEM,EBSD and room temperature tensile test.The mechanism of austenite grain refinement and substructure formation caused by UFC process after hot rolling was revealed.The influence of accumulated deformation substructure on the formation of multiphase structure of steel was explored.The factors of controlling metastable retained austenite in microstructure and its effect on properties were mastered.The main results are showed as follows.First,the hot-rolled DP process was proposed to obtain ultra-high-strength multiphase microstructure.Austenite with a large number of deformation substructures is obtained by two-stage controlled rolling.Isothermal forming of ferrite begins when the temperature cools to the two-phase region with ultra fast cooling rate.Ferrite inherits these deformation substructures and it is easy for ferrite to nucleate and grow at the austenite grain boundary,resulting in fine polygonal ferrite.After quenching to room temperature,martensitic transformation occurs.A large number of deformation substructures retained in undercooled austenite can effectively refine martensitic lath.Parallel arrangement of martensite laths has small orientation difference.This kind of deformation substructure can refine transformation structure and multi-phase coordinated deformation can effectively improve the strength,toughness and formability of the material.When the thickness of rolled steel is 3.8 mm,the finishing temperature is840 ? and the relaxation time is 9 s,the yield strength ratio is 0.55 and the n value is0.13 with ultra-high strength of 1258 MPa.Second,the hot-rolled Q&P are proposed to obtain high strength-plastic-product multiphase microstructure.Austenite with a large number of deformation substructures is obtained by two-stage controlled rolling,and then austenite transforms into dislocation lath martensite after ultra fast cooling to quenching temperature(QT).A large number of deformation substructures further divide the austenite into fine grains or subgrains during transformation,and promote the diffusion of carbon.It is beneficial for retained austenite to keep to room temperature.When the partition temperature is 400 ? and the partitioning time is 10 s,the maximum strength plastic product is 27.00 GPa·%(tensile strength is 982 MPa,elongation is 27.5%).Then,through two-stage controlled rolling,ultra fast cooling,relaxation and partitioning process treatment,the hot-rolled F-Q&P process can be used to obtain the multi-phase composite microstructure of small-size polygonal ferrite with deformation substructure,fine lath dislocation martensite and thin residual austenite,which can further improve the cold working formability of steel.When the relaxation time is 6 s,the partition temperature is 350 ? and the partition time is 30 s,the optimal comprehensive mechanical properties(tensile strength is 1004 MPa,strength plastic product is 20.58GPa·%,yield ratio is 0.55 and n value is 0.18)are obtained.Finally,through the composition design of low carbon and low cost,multiphase microstructure is refined by hot rolling,ultra fast cooling and phase transformation.And the mechanism of deformation substructure in steel is analyzed.The refined multiphase composite structure makes the high-strength steel have good mechanical properties and formability.Meanwhile,short technological process is conducive to improving efficiency and reducing cost,which is of great significance to realize sustainable and lightweight green manufacturing.