Effect Of Controlled Cooling On Microstructure And Properties Of 20SiMn2MoV Bainitic Steel

The purpose of controlled cooling is to refine the microstructure of heat-treated parts,improve their mechanical properties during heat treatment,and reduce the residence time of cooling to prevent the high temperature oxidation and grain growth of steel at high temperature.Due to the slow rate of air cooling,the mechanical properties of bainitic steel parts with larger cross-sectional dimensions will be non-uniform in the cross-section of heat-treated parts.In order to improve the mechanical properties of bainitic steel components with larger sections,this paper reports 20SiMn2MoV high strength bainitic steel.The phase transition point was measured by the hardness-temperature method,and the bainitic steel with optimum properties is obtained by controlling heat treatment and cooling methods.The structure,phase,fracture morphology were measured by X-ray diffraction(XRD),metallographic microscope(OM)and scanning electron microscope(SEM).The mechanical properties were tested and analyzed.The effect of controlled cooling process on the microstructure and properties was studied.The conclusions are displayed as follows:(1)Through the hardness-temperature method,is known that the transformation point A_C1of 20SiMn2MoV bainitic steel is 720?,A_C3 is 880?and austenitizing heating temperature is 920?.(2)?53 mm bainitic steel is heated at 920?for 80 min,then air cooling,30 s water-air cooling and tempered at 250?.The microstructure of the different positions bar is mainly bainite ferrite and austenite,which is carbide-free bainite.Adding 30 s water cooling before air cooling can refine the bainite lath structure and increase the proportion of laths.Through heating at 920?and water cooling then tempered at 200?,the microstructure at different positions of the bar is tempered lath martensite and small amount of residual austenite.For the R/2 of bar sample,the mechanical properties of air cooling are shown as follows:the tensile strength(R_m)is 1314 MPa,impact value(A_KV)is 71.7 J,percentage elongation(A)is 14.3%and percentage reduction of area(Z)is 57.8%.The water-cooled mechanical properties R_m is1654 MPa,A_KV 55.1 J,A 17.5%,Z 57.8%.The mechanical properties of 30 s water cooling then air cooling bar are between air cooling and water cooling.(3)?53 mm bainitic steel is heated at 920?×80 min,water cooled at different times before air cooling and then tempered at 250?.With the increase of water cooling time before air cooling,the strength of bar exhibit an increase tendency,while the impact value grdually decrease.With the extension of water cooling time,the microstructure tends to be refined.The microstructure of bars with water cooling time less than 30 s is mainly carbide-free bainite,and that of bars with water cooling more than 50 s is tempered martensite,carbide-free bainite and small amount of retained austenite.Water cooled at different times before air cooling,the tensile strength(R_m)at the bar radius is bigger than 1250 MPa and the impact value(A_KV)is higher than 70 J,which meets the technical requirements of impact value of carbide-free bar.The impact value of the core and edge of the bar is high under the cooling condition of 10?20s water-air cooling,which meets the technical requirements.(4)?53 mm bainitic steel bar is heated at 920?×80 min,cooled by water for 10?20 s and then cooled by air,improving the mechanical properties of bainitic steel bar.This is a suitable cooling process,and could obtain combination of strength and toughness.?53mm bar is heat treated and cooled by water for 10?20 s before air cooling.The mechanical properties obtained at half radius of bar are achieved.The tensile strength(R_m)is 1330?1361 MPa,percentage elongation(A)is 15.5?17.5%,percentage reduction of area(Z)is 57.8?58.9%,impact value(A_KV)is 97.5?98.7 J.The mechanical properties meet the mechanical property indexes required by the bar.