Architecture Of Heterogeneous Microstructure And Strengthening-Toughening Mechanism In High-Strength Low-Alloy Steel

To realize the sustained development of Chinese iron and steel industry it is important to improve the application of high-strength low-alloy(HSLA)steels vigorously.Therefore,achieving high performance has become an inevitable trend for the development of HSLA steels.The development of new-generation HSLA steels aims to greatly improve ductility and toughness,lower yield ratio and improve weldability whilst maintaining high strength,achieving high-strength,high-toughness and high-ductility low-alloy steels with a yield strength of 500-1000 MPa.In this study,an HSLA steel with heterogeneous microstructure and minimum yield strength of 690 MPa was developed by reasonable composition design and heat treatment control.Experimental and theoretical studies were conducted to figure out the mechanism of heterogeneous microstructure control and strengthening-toughening balance in HSLA steel,and thereby to develop the key technologies for heterogeneous microstructure architecture of HSLA steel.Based on the requirements of basic mechanical properties,an HSLA steel with low carbon,excellent weldability,high hardenability and good weather resistance was designed.The hardenability of the experimental steel was evaluated through Jominy test,by which the relationship between microstructure morphology,crystallographic features and hardness was revealed.The results show that the experimental steel exhibited good hardenability,and the microstructures at Jominy distance of 2,20 and 50 mm were typical lath martensite,lath bainite and granular bainite,respectively.The cooling rate was lowered with increasing Jominy distance,and the density of high angle grain boundaries(HAGBs)and the hardness were decreased,showing a near-linear relationship between the HAGBs density and hardness.The high cooling rate led to a large driving force and a low transformation temperature,which contributed to producing high density of HAGBs and hardness.The key to improve the hardenability is to ensure that the lath structure with high density of HAGBs can be formed at a low cooling rate by tailoring steel composition.To evade the trade-off between strength and toughness/ductility/yield ratio in HSLA steels,a heterogeneous microstructure and corresponding intercritical heat treatment were designed.The heterogeneous microstructure was compared with a full martensitic structure prepared by conventional quenching and tempering(Q&T)processing.It is found that a heterogeneous microstructure with soft intercritical ferrite and hard tempered martensite was obtained through introducing intercritical annealing between the Q&T processing.Compared to the steel with full martensitic microstructure,the intercritically heat-treated steel exhibited superior strength-toughness combination and lower yield ratio.The initial yielding and subsequent work hardening behavior of the annealed steel during tensile deformation were modified by the presence of soft intercritical ferrite,resulting in the decreased yield ratio.After tempering,the hardness difference between the soft intercritical ferrite and hard martensite was decreased due to the precipitation of nano-carbides and the recovery of dislocations.The reasonably small hardness difference between the soft and hard phases and the high density of HAGBs conduced to achieving an excellent low-temperature toughness in the intercritically heat-treated steel.The effect of annealing and tempering temperatures on the evolution of heterogeneous microstructure and resultant strength and toughness was elucidated to uncover the mechanism of heterogeneous microstructure control and strengthening-toughening balance in HSLA steel.The results indicate that the volume fraction of martensite in heterogeneous microstructure and the density of HAGBs increased with increasing annealing temperature,and the strength and toughness of the annealed steel were enhanced.The increase in volume fraction of martensite simultaneously enhanced the back stress and effective stress,and thus improved the strength-ductility balance.With the increase of annealing temperature,the enrichment of carbon and alloying elements in the hard phase was lowered,which led to the decreased hardness difference between the two phases.In addition,the globular reversed austenite preferentially grew into the adjacent microstructure that held none K-S orientation relationship with it,which effectively refined the coarse prior austenite grains(PAGs)and increased the density of HAGBs.The synergy of these two factors contributed to the enhanced low-temperature toughness.With the increase of tempering temperature,the recovery of heterogeneous microstructure was deepened,and the dislocation density in the matrix decreased,while a large number of carbides precipitated,and the proportion of HAGBs increased.In addition,the yield ratio and low-temperature toughness of the tempered steels increased.The tempering softening in the matrix increased with increasing tempering temperature.Hence,the hardness difference between the intercritical ferrite and tempered martensite decreased and the yield ratio increased.The crack propagation energy was increased with increasing tempering temperature,and thus the low-temperature toughness of the tempered steels was enhanced.The significance of prior austenite grain size(PAGS),initial microstructure and heating rate on the formation and refinement of heterogeneous microstructure was studied to clarify the refinement mechanism of heterogeneous microstructure.The results show that refining the PAGS of the matrix could increase the volume fraction of globular reversed austenite(?G),and thereby refine the coarse PAGs.In addition,the ?G transformed into fine martensite variants,which further refined the heterogeneous microstructure.Compared with granular bainite,a higher volume fraction of ?G and finer heterogeneous microstructure would be obtained in lath bainite after annealing.The formation of reversed austenite occurred at higher temperature with increasing heating rate,and the volume fraction of granular martensite and the density of HAGBs in the heterogeneous microstructure were increased.High heating rate contributed to the formation of more ?G and the refinement of heterogeneous microstructure.