Study On The Evolution Of Nano Cu Precipitates In High Strength Low Alloy Steel With Multiphase Structure And Its Effect On Strength And Plasticity

There is a strong demand for developing high performance steels with high strength,high ductility and high toughness for structural applications,to save energy and increase safety.The design and development of low alloy steel with M3 microstructure consisting of Multi-phase structure,Meta-stable austenite and Multi-scale nano-precipitation can further play an important role of steels in national economic construction.The emphasis of this paper is the modulation of multi-phase structures with the coexistence of reverted austenite and nano precipitates in low carbon low alloy steels.Nano precipitates can improve the yield strength of steel,while reverted austenite can increase the plasticity of steel.The combination effect of them can simutaniously improve both strength and ductility of steel.The two-step inter-critical heat treatment can obtain multi-phase structure containing inter-critical ferrite,tempered martensite and reverted austenite in low carbon and low alloy system.Most reverted austenite was long rod-like shape and distributed along lath boundaries.The volume fraction of reverted austenite can be regulated by controlling the time of inter-critical tempering.It is convenient to adjust the strength and plastility of steel.The yield strength of all experimental steels exceeds 700 MPa.Meanwhile,the uniform elongation of experimental steels reaches in the range of 16.4-16.9%,and the total elongation was 29.5-31%.Especially after inter-critical tempering for a short time,nano B2 FeCu clusters and nano reverted austenite were present in the steel,which showed an excellent mechanical property.The yield strength reached 984 MPa,the tensile strength reached 1013 MPa,the uniform elongation was 9.56%,and the total elongation was 22.5%.The formation of reverted austenite and the state of solute elements in heat treatment process were comprehensively studied by 3 DAP.The experimental steel is a very complicated alloy system,because it contains eight different solute elements of C,Mn,Ni,Cu,Ni,Mo,Al and Si.C played a role in promoting the nucleation of reverted austenite,and Mn,Ni and Cu increased the size of reverted austenite.C shows diversity in the two-step inter-critical heat treatment,and the three states of segregation,partition and precipitation coexist and compete with each other.C is first taken away by Nb element to form Nb carbides.Subsequently,it formed Mo-C co-segregation layer,nano Mo-C clusterw and MoC precipitates with Mo at interfaces.The rest of C was partitioning between reverted austenite and inter-critical ferritin/fresh martensite.In the two-step inter-critical heat treatment,Mo shows diversity,and the three states of segregation,partition and precipitation coexist and compete with each other.Mo first participates in the precipitation process of Nb to form Nb-Mo precipitates.Then the two processes of partition and segregation were carried out simultaneously.The Mo content in BCC Fe was higher than that in FCC Fe.At the interface,Mo segregated to form Mo-C co-segregation layer,nano Mo-C clusters,nano Mo clusters,and MoC precipitates.Cu has only partition in the first step of inter-critical annealing process.In the second step of inter-critical tempering,Cu exists two states of precipitation and partition,which are competing with each other.The precipitation state of Cu is stronger than the partition state,and a large number of Cu elements are continuously precipitated from the matrix to form the precipitation phase.Cu precipitates not only exist in the inter-critical ferrite but also in the reverted austenite.In the two-step inter-critical heat treatment process,Mn,Ni and Si elements are mainly partitioning.Mn and Ni atoms continued to diffuse and enrich towards the reverted austenite,while the partition of Si was completed in the first step of inter-critical annealing.Nb only has a single state,and forms(NbxMo1-x)C precipitates with C and Mo.The evolution of Cu precipitates during inter-critical tempering was studied in detail.Inter-critical tempering at 680℃ for 5 min,nano ordered Cu clusters were obtained in the matrix.9R Cu appeared in the matrix of inter-critical tempering for 15 to 30 min.With the increase of the inter-critical tempering time,the number of twin variants in 9R Cu increased from 2 to 7,and the(1 1-4)9R twin surface developed from a straight surface to a stepped surface or even an arched surface.The orientation relation between 9R Cu and matrix is(1 1-4)9R//(0 1 1)α,[-1 1 0]9R//[1-1 1]α.At the inter-critical tempering for 60 min,the detwinned 9R Cu appeared in the matrix.It consists of two 9R parts and a movable(1 1-4)9R plane.Loss of twin relationship between two parts.The orientation relation between the Cu crystal on the wider side of the detwinned 9R Cu and the matrix is(1 1-4)9R//(0 1 1)α,[-1 0 0]9R//[1-1 1]α,while there is no orientation relation between the Cu crystal on the narrower side and the matrix.FCC Cu was obtained after inter-critical tempering for 180 min.It consists of Cu crystals with two FCC structures and a micro-twin region.The growth direction of elliptic FCC Cu is parallel to the micro-twin planes.The orientation relationship between FCC Cu and matrix is(1-1 1)FCC//(0 1 1)α,and[1 1 0]FCC//[1-1 1]α.At 680℃,the crystal structure evolution sequence of Cu precipitates at different inter-critical tempering time is as follows:Cu-rich nano ordered cluster(B2 FeCu and BCC Cu)→9R Cu(single crystal,double-twinning variants and multiple-twinning variants)→detwinned 9R Cu→transition Cu precipitates(complex structure of 9R+FCC)→FCC Cu.