Study On Microstructure And Properties Of Grade690MPa Low Alloy High Strength Steel Based On The Trial-production

In order to develop the low alloy high strength steel with low cost, highstrength, good impact toughness and excellent welding properties for the market andproduction of the iron and steel enterprises, the composition and the process of thistest steel has been designed on base of the low cost, as well as improving the strongtoughness by the TMCP technology, subsequently the trial production of test steelplate has been completed on the production line in Pilot Plant of Steelmaking andRolling National Engineering Research Center of Advenced Steel Technology(PPL).This thesis analyses the influence of different controlled rolling and controlledcooling technology on microstructure and mechanical properties of the test steelthrough mechanical performance test, microstructure, precipitation analysis and othermethods, finally, and finalizes the appropriate rolling and cooling (after rolling)process for this test steel. The main conclusions are as follows:（1）The static CCT curve of this test steel is measured and drawn. This test steelproduced the pro-eutectoid ferrite and pearlite in high temperature change of phasetransition, produced the bainite in the middle temperature change, and produced themartensite in the low temperature change. With the rising of the cooling speed, thevolume fraction and hardness of the bainite increased in the middle temperaturechange.（2）Two-stage rolled trial-produce process route is formulated on theproduction line in the PPL. Process points are as follows: heating temperature at1280℃, the first pass in rough rolling at about l100℃, the first pass in finish rolling at900～950℃, the last pass at860℃～890℃. After rolling:(Ⅰ) The direct air coolingtechnology is designed;(Ⅱ)Firstly, the temperature of test steel is reduced to450℃by using the laminar cooling technology. After that, the temperature of test steel riseto680℃. At the last, the furnace cooling technology is adopted after keeping the teststeel at680℃in heating furnace for an hour.(Ⅲ) Firstly, the temperature of test steeldrops to650℃by using the laminar cooling technology. After that, the air coolingtechnology is adopted after keeping the test steel at650℃in heating furnace for anhour.(Ⅳ) Firstly, the temperature of test steel drops to650℃by using the air coolingtechnology. After that, the air cooling technology is adopted after keeping test steel at 650℃in heating furnace for an hour.（3）The metallographic structures of the1#and2#test steel in productionprocess Ⅰ are respectively ferrite+Widmanstaten+pearlite and ferrite+pearlite; themetallographic structures of the1#and2#test steel in production processⅡareferrite+bainite+globular pearlite and ferrite+nodular troostite+bainite; themetallographic structures of the1#and2#test steel in production process Ⅲ areferrite+globular，short clavite bainite+pearlite and ferrite+a little pearlite+a largenumber of densely distributed islands; the metallographic structures of the2-4teststeel in production process Ⅳ are ferrite+pearlite in fine grains.(4)The experimental results show that the1#and2#test steel in productionprocess Ⅱ all have high strength and good impact toughness. The main mechanicalproperties of1#test steel are as follow: the tensile strength is694MPa; the yieldstrength is554MPa; the percentage elongation after fracture is18%; the impactenergy are174J at24℃and46J at-40℃; the hardness is HV223. The mainmechanical properties of2#test steel are as follow: the tensile strength is778MPa;the yield strength is641MPa; the percentage elongation after fracture is21.5%; theimpact energy are135J at24℃and65J at-40℃; the hardness is HV249.(5) Compared with the production process Ⅱ, the production process Ⅲ issimpler and easier to realize in the hot rolling line. More importantly(Furthermore),the high strength low alloy steel has obtained the good mechanical properties inproduction process Ⅲ, and the mechanical properties of the test steel have no evidentdifferent between the production process Ⅱand Ⅲ.