| With the development of production technology and the increasingly strict regulations by governments,the requirements for safety,low emissions,low cost and fuel efficiency are increasing in the automotive industry.High strength steels are cost-effective materials that improve safety performance while ensuring lightweight of automotive manufacturing,which further improve fuel efficiency and reduce emissions.In recent years,due to its excellent performance,cost reduction,energy saving and environmental protection,high strength non-quenched and tempered steels have gradually replaced a considerable amount of quenched and tempered steels used in automobile parts,such as crankshafts and connecting rods.The traditional ferrite-pearlite non-quenched and tempered steel has a lower upper limit of strength and a higher cost of strength improvement,and it has been difficult to meet the demand for higher strength.Bainitic non-quenched and tempered steel has higher strength while maintaining toughness.Particularly,carbide-free bainite microstructure can improve the match between strength and toughness further.In this work,five experimental steels with different Mo and Nb contents are designed to combine carbide-free bainite microstructure and non-quenched and tempered steel production process.Simultaneously,the effects of controlled rolling parameters on alloy composition design,microstructural evolution and mechanical properties have been systematically analyzed.Thermo-Calc software was used to calculate the equilibrium phases and their composition content change versus temperature in the studied steels.The results showed that there are many kinds of equilibrium precipitated phases,such as ferrite,austenite,Fe3C,Ti(C,N),(Nb,Ti)C,B2M,M3B2,M6C and M7C3 and so on.With the increase in Mo content,the precipitataion temperature range of Fe3C narrowed,and the precipitation was suppressed.The precititation of other Mo-containing phases was slightly affected.With the increase in Nb content,the maximum precipitataion amount of(Nb,Ti)C phase increased significantly and the maximum precipitataion amount of Ti(C,N)phase decreased slightly.Temperature-Time-Transformation curves calculated by JMatPro and MUCG83 software and the experimental results obtained from isothermal heat treatment at different temperatures showed that microstructure was manily composed of lath carbide-free bainite at temperatures ranging between 350? and 450?.When isothermal temperature reached 500?,granular bainite would appear.When isothermal temperature reached 550?,lath carbide-free bainite would disappear,microstructure consisted of typical granular bainite.As the isothermal time increased and the isothermal temperature decreased,the amount of bainite microstructure increased,the degree of incomplete bainite transformation decreased,the carbide-free bainitic ferrite laths were refined,and the amout of retained austenite increased.The increase in the contents of Mo and Nb reduced the amount of bainite transformation under the same isothermal condition,and this trend increased as the isothermal temperature rised,which means the degree of incomplete bainite transformation further increased.Furthermore,when the isothermal temperature was 400? and the isothermal time was 30min,the experimental steel has best match of strength and toughness,which provided the basis for the establishment of the heat treatment after hot rolling.The results of dynamic Continuous-Cooling-Transformation obtained by Gleeble thermal simulator showed that the content of alloying element and cooling rate after hot rolling have significant effects on obtaining the desired microstructure and ideal amount of retained austenite film.In order to ensure the strength and toughness of the experimental steels,the cooling rate after hot rolling should be kept within the range of 0.3?/s?1?/s,especially at 0.5?/s.At the cooling rate after hot rolling of 0.5?/s,the microstructure containd more lath carbide-free bainte and more retained austenite film accompanied by a large ratio of high angle grain boundary.As the cooling rate increased,the microstructure changed from granular bainite and a very small amount of massive proeutectoid ferrite to lath carbide-free bainite and martensite,accompanied by an increase in dislocation density and decrease in average particle size of the precipitated phase particles and content of retained austenite.The granular bainite were composed of massive bainitic ferrite with high density of dislocations and M/A islands distributed on the bainitic ferrite.The lath carbide-free bainite were composed of parallel bainite ferrite laths with high density of dislocations and retained austenite film uniformly distributed between the bainitic ferrite laths.Besides,the increase of the Mo and Nb contents delayed the high-temperature diffusion transformation,promoted the bainite transformation,leading to the increased microhardness and the decreased content of retained austenite.The synergy of fine grain strengthening,solid solution strengthening,dislocation strengthening and precipitation strengthening of secondary phase particles promoted the hardness of the experimental steel.With the finish rolling deformation increasing and the finish rolling temperature decreasing,the area fractions of lath bainite and martensite increased,the width of bainitic ferrite lath decreased,the dislocation density and the number fraction of high-angle grain boundaries were raised.Meanwhile,the larger deformation and lower temperature reduced the amount of retained austenite and refined the precipitated particle.The refinement strengthening,dislocation strengthening and precipitation strengthening synergistically contributed to the increase in hardness with increasing deformation and decreasing temperature.In summary,when the final rolling temperature was 850?,the final rolling deformation was 30%,and the cooling rate after hot rolling was controlled at about 0.5?/s,the strength and toughness of carbide-free baninite would be the best match,which was related to the more area fraction of lath bainite,better amount of film-like retained austenite,and smaller precipitated phase particles.Under pilotscale experimental condition,the mechanical properties of the experimental steels with various Mo and Nb contents under different finish rolling conditions can meet or even exceed the performance requirements of non-quenched and tempered steels in the national standard.With the decrease of finishing rolling temperature,the increase of finishing rolling deformation and the increases of Mo and Nb contents,the strength and toughness of the experimental steel have been improved,which indicated it is feasible to obtain high-strength,high-toughness carbide-free bainitic non-quenched and tempered steel through reasonable alloying design and controlled rolling and cooling process.When the Mo content was 0.51wt%and the Nb content was 0.032wt%,the experimental steel can achieve excellent strength and toughness matching only through the current controlled rolling and controlled cooling process,whose tensile strength was 1328MPa,and yield strength was 873MPa and the Charpy impact energy(V-notch)was 42J.Besides,for the experimental steel with poor matching of strength and toughness,it can significantly improve toughness while ensuring strength through tempering at 400? for 30 minutes after hot rolling,which was conducive to actual production and application by broadening the content range of the added elements in the alloy system with less increase in production processes. |
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