Study On Production Technology And Microstructure-properties Of Ultra-high Strength Steel Q1300 For Engineering Machinery

With the demanding for engineering machinery becoming bigger and lighter,higher requirement for ultra-high strength steel is proposed.Q960,Q1100 and Q1300 grade ultra-high strength steels have been developed recent years.However,Q1300 grade is monopolized by Sweden's SSAB,and then we have no access to their technology and product..In this research,some physical metallurgy behaviors of Q1300 were investigated,including effects of controlled rolling and cooling technology and heat treatment process parameters on the microstructure and properties.The main content is as follows:(1)The coarsening temperature and continuous cooling behavior of austenite were studied by thermal simulation..Results indicated that austenite grains grew up significantly because of solution of microalloy carbonitride when heated in the range of 1150-1200?,and the austenite coarsening seriously at 1250?.The microstructure of the test steel mainly consisted pearlite,pearlite+bainite,bainite,bainite+martensite,martensite in ooling rates.When the cooling rate is greater than 5?/s,the microstructure fully consisted of martensite,which indicated a good hardenability of this steels.(2)High temperature continuous rolling and two stages(recrystallization zone and not recrystallization zone)controlled rolling were conducted.Influence of air cooling to room-temperature,ultra-fast cooling(UFC)to about 600? followed by air cooling on the microstructure and properties were studied.Results showed that interrupt cooling could get better mechanical properties through off-line heat treatment.Mechanical properties of the test steel for high temperature continuous rolling and air-cooling was poor because of grain coarsening.(3)Effects of reheat quenching and tempering temperature on the microstructure and properties of the test steel were studied systemically.Results showed that at the quenched temperature of 820?,the excellent mechanical properties can be obtained.While,the yield strength will be destroyed for incomplete austenization or grain coarsening when it was beneath or beyond this temperature.The tensile strength reduced with tempering temperature increase,and at 190?,it reach to top.Discontinuous Precipitation of thin carbide between laths and along grain boundary result in temper brittleness happening at 300-450?.(4)Effect of direct-quenching and tempering process on the microstructure and properties of the test steel were investigated by using UFC technology.Results showed that yield strength and impact toughness can be effectively improved when tempered at-200?.Strength of the test steel reduced with the higher tempering temperature,when the temperature increased to 250-400?,temper brittleness will be happened.Furthermore,ductile brittle transition temperature of test steel is about-(45-55)?.(5)Strength can be improved by direct quenching and tempering process under UFC condition,which will be beneficial in reduction cost,saving energy and improvement of production efficiency,compared with reheat quenching and tempering process.