Studies On Strengthening And Toughening Mechanisms And Weldability Of A Copper-rich Nano-phase-strengthened Steel

Copper-rich nano-precipitation strengthened steel is a new kind of high strength steel based on nano-precipitation strengthening mechanism.For traditional high strength steel,the yield strength is mainly improved by obtaining high carbon microstructure,but it sacrifices the toughness and welding properties of materials as well.This severely limits its application range.Therefore,exploring methods to improve the comprehensive mechanical properties and weldability for various applications attracts more and more attention.In this study,copper-rich nano-precipitation strengthened steels were investigated.Firstly,to improve the yield strength of materials,the nucleation and growth process of nanophases was controlled by optimizing alloy composition and heat treatment processes.Meanwhile the strengthening factors and corresponding mechanisms of copper-rich nano-precipitation strengthened steel were also studied.Secondly,thermal mechanical processing and solid solution processing were optimized to obtain the fine lath martensite structure.Microscopic characterization and mechanical property evaluations were carried out in detail to further analyze the effects of various toughening methods,and systematically study the toughening mechanism of copper-rich nano-precipitation strengthened steel.Finally,through welding experiments,the cold crack sensitivity and the mechanical properties after welding were comprehensively analyzed,and the mechanism of nanophase precipitation during the welding thermal-cycle was further studied.This study provides scientific basis for promoting the applications of copper-rich nano-precipitation strengthened steels and understanding the strengthening and toughening mechanisms of similar alloy systems.The microstructure of granular ferrite,polygonal ferrite and lath martensite can be obtained by controlling thermomechanical treatment and solid solution treatment.After rolling with a finishing-rolling temperature of 800～850℃,the microstructure changes from granular ferrite to polygonal ferrite when increasing the solid solution temperature from 800℃to 900℃.The yield strength increased obviously accordingly,while the elongation and low temperature toughness decreased.At the same time,small-angle neutron scattering results show that the same aging treatment（550℃,1 h）on granular ferrite and polygonal ferrite matrix can obtain nanometer precipitates with the same size and quantity density.At the same aging temperature,with the extension of aging time,the size of nanophase gradually grows and the number density decreases.Copper-rich nano-precipitation strengthened steel with lath martensitic structure exhibit better low temperature impact toughness than the polygonal ferrite steel.With the yield strength increased by almost 90 MPa,the ductile brittle transition temperature decreased by about 60°C.This is mainly due to the expansion of lath martensite structure with crack stable stage,which can inhibit the occurrence of crack instability propagation.While for the polygonal ferrite structure,once the cracks initiate,they rapidly spread in an unstable propagation way.As a result,despite of their similar crack initiation work,the lath martensite structure showed higher crack propagation work when compared to the polygonal ferrite.Meanwhile,lath martensite structure contains a higher content of high angle grain boundaries and smaller effective grain size,where the high angle grain boundary can effectively deflect crack propagation angle and consume crack extension energy,thus significantly preventing crack propagation.Moreover,the effective grain size refinement could improve the cleavage of fracture stress,thus reducing the ductile to brittle transition temperature.Within the range from room temperature to-80°C,the impact energy is kept above 300 J without obvious ductile brittle transformation.Layered structure shows unique crack blocking mechanism.It can effectively passivate the three-way stress state at the crack tip,significantly increasing the propagation resistance of the crack along the vertical lamellar direction.And it can also deflect the crack to extend to the parallel lamellar direction,so as to significantly improve the low-temperature toughness of copper-rich nano-precipitation strengthened steel.Meanwhile,after aging treatment,fine and evenly distributed nano-precipitated phases in layered ferrite matrix can be obtained,which significantly improve the yield strength of layered Cu-rich nanometer phase reinforced steel by about 220 MPa.A low carbon equivalent（0.75%）along with a high yield strength of 1000-MPa grade was obtained through design by copper rich nanometer phase precipitation-strengthening.Implant crack test shows that,without preheating,the cold crack sensitivity index is about 25%and the critical fracture stress is 1350 MPa.When preheated at 120℃,the sensitivity index of cold crack significantly decreased to 13%,and the critical fracture stress increased to 1560MPa.After welding thermal-cycle,due to its high peak temperature and high temperature residence time for the coarse-grained heat affected zone（CGHAZ）,the nanoscale precipitates are all dissolved in the matrix,forming the thick lath martensite structure.As a result,its yield strength decreased significantly compared with the base material,while its low-temperature impact toughness was improved.The peak temperature within the fine-grained heat affected zone（FGHAZ）was low（950°C）,and the thin lath martensite structure was formed after welding.With a long cooling time(t_8/3)of 180 s,nucleation sites and growth time of the nanophase are more abundant than CGHAZ,indicating that the dynamic re-precipitation can occur in FGHAZ.Compared with the base material,size of the new precipitated nanophase increased and number density reduced.Therefore,the yield strength of FGHAZ is significantly higher than that of CGHAZ,and little bit lower than base material.Since FGHAZ has a smaller effective grain size than the parent metal,its low temperature impact toughness is also significantly improved.