Study On Regulation Mechanism For Microstructure And Mechanical Properties Of Ultra-low Carbon Nano-sized Cu Precipitation Strengthened HSLA Steel

At present,the biggest challenges of economy,resources and homeland security come from the ocean in our country.The establishment of large-scale ships can strengthen our economy and security.The steel materials with high properties are overriding physical basis during the construction of ships.A series of nano-sized Cu precipitation strengthened HSLA steels with high strength,high toughness and high welding property,which are produced by the United States,represent the highest level of hull structure steel in the world.The Ni-Cr-Mo alloy system is used for high strength hull structure steel in our country.There exist many problems,such as the strength is lower,the welding performance is worse and it is lag in application,et al.The development and application of nano-sized Cu precipitation strengthened HSLA steel can enhance the competitiveness of our hull structure steels in the international market.This paper took the ultra-low carbon nano-sized Cu precipitation strengthened HSLA steel as the research object.The isothermal phase transformation mechanism,the effect of Cu on continuous cooling phase transformation behavior of austenite,the relationship between morphology,crystal structure and composition during the nucleation and growth of Cu precipitates,the effect of cooling path on microstructure and properties,and the effect of heat treatment on microstructure and properties were studied in detail.The following main conclusions are drawn:(1)The isothermal phase transformation mechanism of an extremely low-carbon(<0.01wt%)Cu-bearing HSLA steel and the effect of Cu on continuous cooling phase transformation behavior of austenite were confirmed by adopting thermal simulation experiment.It was shown that with the decrease of isothermal temperature,the diffusion transformation,massive transformation and diffusion-shear integrated bainite transformation occurred in turn.Massive transformation showed distinct differences with diffusion transformation in terms of formation temperature,nucleation site,growth pattern,grain boundary feature and grain inner feature.The phase transformation starting temperature of austenite was lowered with the addition of Cu.The decomposition of austenite was delayed and the C-curve was moving to the right.Meanwhile,the nucleation and growth rate of ferrite were reduced.(2)The effects of finish cooling temperature,cooling rate and cooling time on Cu precipitation behavior were studied by adopting thermal simulation experiment.It was shown that with the increase of finish cooling temperature,the nucleation and growth of Cu precipitates occurred continuously,and the microhardness of ferrite increased in initial stage and then declined.During continuous cooling process,the formation of Cu precipitates and the phase transformation of ferrite occurred simultaneously.There existed two precipitation modes:random precipitation and interphase precipitation.The spacing and particle size of interphase precipitation were decreased gradually with the increase of cooling rate.The nano-hardness and the elasticity modulus were 2.76 GPa and 106.5 GPa for the matrix of Cu-free steel,respectively.While the nano-hardness and elasticity modulus were 3.37 GPa and 162.0 GPa for the matrix of Cu-2.0wt%steel,respectively.Therefore,it can be seen that the hardness of matrix was significantly enhanced with the formation of nano-sized precipitates.The microhardness of ferrite was increased continuously with the prolongation of cooling time.The nucleation and growth of Cu precipitates were enhanced with the prolongation of cooling time.(3)The evolution of morphology,crystal structure and composition for Cu precipitates were studied by using TEM and APT,and the relationship between the three was clarified.It was shown that the average diameter of Cu precipitates was increased from 7.6 nm to 19.7 nm,as the aging temperature was increased from 550 ℃ to 650 ℃.The number density was decreased and the microtopography was changed from spherical to ellipsoidal,and even rod-like.Cu precipitates have undergone a series transformation of crystal structure in accompany with the transformation of precipitation morphology,that is,bcc→9R→twinned fct/fcc→fcc.There were two structural styles for 9R Cu structure:multiple twinned 9R structure and untwinned 9R structure,with particle size within 10 nm.9R Cu structure was detwinned and then transformed to fct Cu structure when particle size was 14 nm.At last,the stable fcc Cu structure was formed when particle size was larger than 30 nm.Fe,Cu,Ni,Mn,Cr and Si atoms were enriched in Cu clusters at the nucleation stage.During the growth of Cu clusters,the enrichment amount of Cu atoms was increased and the enrichment amount of Fe atoms was decreased.Simultaneously,Ni atoms and Mn atoms were segregated obviously at the interface of Cu precipitates and matrix to form a core-shell structure,thus the growth of Cu precipitates was restrained.(4)The effects of cooling path on microstructure and mechanical properties of an extremely low carbon(<0.01wt%)Cu-bearing HSLA steel were studied by adopting laboratory hot rolling experiments.The strengthening and toughening mechanisms were deeply discussed and the strengthening mechanism was quantitatively analyzed by using the strength formulas.It was shown that the amount of Cu precipitation strengthening was 105 MPa and 252 MPa for furnace cooling + air cooling process and air cooling process,respectively.A good combination of mechanical properties can be obtained by adopting air cooling process:the yield strength was 612 MPa,the tensile strength was 692 MPa,the elongation was 25.2%and CVN impact energy was 143 J at-80 ℃.The yield strength was 603 MPa and the tensile strength was 691 MPa for Cu-2.0wt%steel with isothermal process.As compared with Cu-free steel,the yield strength and tensile strength was increased by 255 MPa and 290 MPa,respectively.The addition of Cu can provide a solid solution strengthening of 32 MPa,grain-boundary strengthening of 19 MPa and precipitation strengthening of 193 MPa.The strength was effectively improved with Cu addition,which inturn brought a small deterioration on ductility and toughness.(5)The effects of heat treatment process on microstructure and mechanical properties for a C-0.005wt%steel and a C-0.02wt%steel were studied.The strengthening and toughening mechanisms for the two steels were clarified.The composition design and the corrsponding process control method were proposed to produce steels with high strength.It was shown that ferrite structure can be obtained for C-0.005wt%steel with TMCP+T process.As the aging temperature was 500 ℃and aging time was 0.75 h,the steel showed a good combination of strength and toughness:the yield strength was 756 MPa,the tensile strength was 820 MPa,the elongation was 28.1%and CVN impact energy was 85 J at-80 ℃.The C-0.02wt%steel showed a feature of multiphase coexistence with QT process.As the aging temperature was 630 ℃,the excellent comprehensive mechanical properties can be obtained withr the coaction of the recovery of bainite/martenite matrix microstructure,reversed austenite and nano-sized precipitation strengthening.The yield strength was 862 MPa,the tensile strength was 880 MPa,the elongation was 23.0%and CVN impact energy was 143 J at-80 ℃.