Research On Phase Transformation Behavior And Intercritical Heat Treatment Of High Strength Low Alloy Steel

In the recent years, high strength low alloy (HSLA) steels have been widelydeveloped to protect environment, reduce structure weight and cut the cost.1.15Ni-0.65Cu-Mo-Nb steel is a Ni-Mo-Cu HSLA steel. However, relatively higheryield tensile ratio and lower toughness are the disadvantages of this steel. Therefore,the development of microstructures and phase transformation behaviors during bothcontinuous cooling and isothermal transformation processes were systematicallystudied in HSLA steel by means of thermal simulation testing machine, opticalmicroscopy (OM), scanning electron microscopy (SEM), electron probemicro-analyzer (EPMA), transmission electron microscopy (TEM) and mechanicalproperties testing. Based on it, we brought forward different intercritical heattreatments in order to improve the mechanical properties of HSLA steel. The primaryconclusions were obtained as follows:(1) The variation of microstructures and transformation behaviors of the presentsteel in continuous cooling were investigated. At the cooling rates of0.1~1.5°C/s, themicrostructure varied as ferrite/mixture (pearlite and lower bainite)â†'ferrite/lowerbainiteâ†'ferrite/acicular ferrite, and their banded structures were distinct. At3°C/s,the homogeneous morphology of acicular ferrite was formed. At4~50°C/s, thespecimens showed a mixture of AF and martensite in both segregated bands. At100°C/s, the homogeneous morphology of martensite was formed.(2) The development of microstructures and transformation behaviors of thepresent steel in isothermal transformation process were investigated. At the isothermaltemperatures of700~620°C, the banded microstructures consisted of polygonal ferriteand martensite. At620~470°C, the banded microstructures consisted acicular ferriteand martensite. When the isothermal temperature was below470°C, themicrostructure contained wholly acicular ferrite.(3) The effect of different intercritical quenching and intercritical normalizingtreatments on the development of microstructures and transformation behaviors wasstudied in the present steel. The microstructure of SQ specimen consisted of bandedpolygonal ferrite and martensite, and the microstructure of IQ specimen containedferrite and martensite with lath-type morphology. Under SN treatment (the coolingrate is3°C/s), the microstructure consisted of polygonal ferrite, acicular ferrite and martensite. Under IN treatment (the cooling rate is3°C/s), the microstructureconsisted of ferrite and acicular ferrite with lath-type morphology.(4) The effect of different intercritical quenching+tempering treatments onmechanical properties of the present steel was studied, and the roles of austenitizingtemperature and intercritical temperature were discussed. Under SQT treatment, themicrostructure consisted of polygonal ferrite and tempered lath martensite. Under IQTtreatment, the microstructure contained fibrous ferrite and martensite. In order toobtain the low yield tensile ratio and good toughness, the austenitizing temperatureshould be900~1000°C and the intercritical temperature should be760~780°C.(5) The effect of different intercritical normalizing (air cooling)+temperingtreatments on mechanical properties of the present steel was studied. Under SNTtreatment, the microstructure consisted of polygonal ferrite and tempered acicularferrite. Under INT treatment, the microstructure contained fibrous ferrite andtempered acicular ferrite. In comparison with traditional NT treatment, SNT and INTtreatments reduced the yield strength, ultimate tensile strength and yield tensile ratiovalues. However, only INT treatment improved the toughness, and SNT treatmentresulted in the decreased the toughness.