Microstrcture And Property Regulation Of Nb Micro-alloyed High Strength Ship Plate Steel With Ultra Fast Cooling Technology

With shipbuilding industry develops towards maximization,light-duty and environmental protection,the ship plate steel with high strength,toughness and deformability are used widely.It is an effective way in ship plate steel production to obtain a higher mechanical property by adding microalloying elements and heavy reduction at low temperature.But the production cost is raised because of production equipments and alloying elements addition.So it is important to develop a new-type process route for ship plate steel production.In oder to overcome the limitations of conventional TMCP technology,the new generation TMCP with the key technology of ultra fast cooling has been widely used to tap the potential of steel materials,which can fully mobilize all means of strengthening mechanisms in improving the comprehensive mechanical properties of steel while effectively reducing the amount of micro-alloying elements to achieve saving-type and reduction manufacturing.So it is important to study the microstructure and mechanical properties under new generation TMCP and realize industrialization.Based on this background,this paper took Nb Micro-alloyed high strength ship plate steel as research oblect,and studied the high temperature deformation behavior of austenite,transformation behavior,precipitation behavior of micro-alloy carbonitride by using of thermal simulation experiment,and aslo studied the influence of TMCP parameters on mechanical properties of the tested steel.Major researches and innovative achievements in this paper are as follows:(1)The high temperature deformation behavior and microstructure evolution of austenite were studied and the mathematical model of Tnr was established.Because of the higher finishing temperature of the new generation TMCP,the deformation behavior of autenite needs to be studied to obtain obtain work-hardened austenite as much as possible.The results show that,the austenite recrystallize at the temperature of 1000? with a 20%reduction.At the temperature Of 90?,the autenite can not recrustallize when the reduction is less than 30%,and partial recrystallization occurrs when the reduction is between 40%?50%.At the temperature of 850?,even 50%reduction can not make the austenite recrystallized.The softening rate platform appears because of the precipitation of the micro-alloy carbonitride in the static recrystallization proress,and the temperature of the softening rate platform decreases with the raise of perstrain.The value of T,r decreases the increase of pass strain,strain rate and interpass time.The pass strain and interpass time have a larger effect on the Tnr than the strain rate does.The model of Tnr was established and the calculated results agreed well with the actual experiment value.(2)The austenite transformation behavior were conducted by thermal simulation experiments.The influence of deformation schedule and cooling rate on the transformation behavior were investigated.The results show that the transformation behavior is determined by austenite state.Rcrystallized austenite is gained when deformed at temperature of 1050 ?,while unrecrstallized austenite is gained when deformed at the temperature of 910 and 850 ?.The transformation behavior for deformation temperature of 850? and 910? are almost the same due to the similar deformed austenite state.The plastic deformation in the non-recrystallization region of austenite accelerates the onset of transformation from austenite to ferrite,while the progress of ferrite transformation is retarded.When the recrystallized austenite cooled to the isothermal temperature of 680?,the higher cooling rate retards the diffusion of carbon duiring the cooling progress,and inhibite the ferrite transformation,so the amount of ferrite under lower cooling rate is more than that of higher cooling rate.When the unrecrystallized austenite cooled to the isothermal temperature of 680?,more defomation energy and nucleolinus are reserved to the ferrite region,so the the amount of ferrite under higher cooling rate is more than that of lower cooling rate after 100s.(3)The effects of deformation and cooling parameters on precipitation behavior were investigated,and the role of ultra fast cooling in controlling strain-induced precipitation was explained.The nose temperature of Nb(C,N)precipitation at austenite region is between 910??930?.The maximum rate of homogeneous nucleation and nucleation at dislocation are 620? and 660? respectively.The nose temperature of Nb(C,N)precipitation at ferrite region are 7000C and 740 ? for homogeneous nucleation and nucleation at dislocation respectively.The thermal simulation experiment results show that the precipitation start time is between 10s and 30s when holding at 910? and when holding at 850? the precipitation start time is between 30s and 100s.Under the continuous cooling condition,the precipitation start temperature is lowered as the cooling rate is increased,and the precipitation behavior is inhibited completely when the cooling rate reaches 10?/s.The precipitation of Nb(C,N)at ferrite region is promoted by the plastic deformation in austenite region,and the amount of precipitates increases with the increase of holding time.The diffusion of Nb is retarded when the isothermal temperature decreases from 680? to 600?,so the prcipitates are refined.(4)The effects of cooling path on the microstructures and mechanical property of ship plate steel were studied by means of laboratory hot rolling experiments adopting higher finishing temperature +different cooling path.The microstructures and mechanical property are determined by the cooling path.The results show that the feriite/bainite dual phase structure have a good deformation coordination,whitch can provid higher strength and good elongation.The plastic deformation of microstrcutrue can absorb more impact deformation energy and stop crack propagation.The toughness of tested steel inceases with the increase of percentage of large angle grain bounderies and the derease of the grain size.With the "ultra fast cooling?690?+accelerated cooling?500?”cooling path,the mechanical property of the tested steel can meet the requirements of FH500 grade ship plate steel.(5)Different grade ship plate ship steels were produced in one steel factory with different progresses."High finishing temperature+UFC" progress can accelerate the rolling rhythm,improve the production efficiency,reduce the microalloy additon and also can improve the surface quality of the ship plate steel.With the "high finishing temperature+ultra fast cooling+accelerated cooling" progress,low Nb microalloyed(0.01%)AH32 ship plate steel and AH32 ship plate steel updated toFH420 ship plate steel industrial production are conducted.With the "low finishing temperature+ultra fast cooling+accelerated cooling" progress,low Nb microalloyed(0.01%)FH36 ship plate steel and low Nb-V microalloyed(0.01%Nb)EH420 ship plate steel industrial production are conducted.The mechanical properties all meet the requirements of each target grade ship plate steel,and the production cost is significantly reduced.