Development Of 1300 MPa Grade Ultra-high Strength Construction Machinery Steel And Study On The Weldability

With the rapid development of our national economy,resource and energy consumption continues to increase,environmental issues are getting worse,lightweight has became an inevitable trend for the development of equipment manufacturing industry.As one of the important methods to reduce the weight and consumption of construction machinery,the lightweight material has put forward higher requirements for steel used in construction machinery.Increasing strength is the key to achieve the lightweight of materials.At present,high strength structural steels for construction machinery with yield strength in the range of 690?960 MPa are widely used,the microstructure of these steel is usually hot rolled bainite or tempered sorbite.1300 MPa grade ultra-high strength structural steel plate is the highest grade structural steel for construction machinery in the world,and the domestic research is almost blank.Generally,the ductility and toughness of steel usually decrease with the increase in strength.The low plasticity and low toughness of ultra-high strength steel have limited their industrial application,and become one of the bottleneck problems.In addition,the poor weldability of ultra-high strength structural steel is another key problem that restricts its popularization and application,and relevant systematic research is insufficient.Therefore,it is urgent to develop structural steel for engineering machinery with high strength,high toughness,easy forming and good weldability.In this paper,effects of chemical composition,rolling,cooling condition and heat treatment on the strength and toughness of 1300 MPa grade ultra-high strength structural steel were systematically studied.Welding crack sensitivity of the experimental steel,influence of welding heat input and post-heating temperature on the microstructure and mechanical properties of the welded joints have been investigated by thermal simulation and real welding.The main research work and conclusions are as follows:(1)A Nb-V microalloy composition system based on the control of carbon content and carbon equivalent was proposed for the experimental steel.1300 MPa grade ultra-high strength construction machinery steel with good low-temperature impact toughenss and plastic formability was successfully developed and industrially produced by grain refinement and nanoprecipitaion based on the matrix of dislocation-type lath martensite.The yield strength(Rp0.2)of industrially produced steel plate reaches 1350 MPa,tensile strength(Rm)reaches 1580 MPa,percentage elongation after fracture(A50)is 11%.The impact absorbed energy of semi-size specimen at-40? is higher than 50 J,the ductile-brittle transition temperature is as low as-100?.Furthermore,there is no crack when the steel plate is bent under the condition that the bending radius is five times the thickness of the plate and the bending angle is 180°.(2)The effects of austenitizing temperature(AT)and cooling rate on the martensitic transformation kinetics were investigated.When the AT decreases,the martensite transformation point(Ms)decreases due to the increases of the strengthe of the austenite and the transformation activation energy.The martensitic transformation rate increases first and then decrease with the increase in transformation volume fraction,reaches the maximum value as the volume fraction is approximately 20%?30%(related to the austenite state and cooling rate).The effect of austenitizing temperature on the phase transformation rate is related to cooling rate,fine grain austenite has a higher martensitic peak transformation rate,high transformation rate of coarse grain austenite can be maintained for a relatively long time.When the cooling rate increases,the maximum transformation rate of fine grain austenite decreases,and the transformation rate at different austenitizing temperatures tends to be uniform.(3)The relationship between the austenite grain size of experimental steel and the heating temperature(940?1250?)was established:D2=7.1 ×1013 exp(-2.78×105/RT).The austenite grain will grow up sharply when the heating temperature is higher than 1150?.The effect of rolling and cooling processes on the reheated austenite grain size are analyzed.Increasing the rolling reduction is conducive to increase the number of precipitated particles,and improving the cooling rate after rolling can refine the particle size.The reheated austenite grain boundaries can be pinned by these fine particles is beneficial to obtain the fine grain.The relationship between the average size and uniformity of reheated austenite grain and the austenitizing temperature of rolled steel was studied.It is found that the variance coefficient of the austenite grain size decreases first and then increases with the increase of heating temperature.The reheated austenite average grain size of experimental steel can be reduced to 4.4 ?m by a whole process is integrated control of heating-rolling-cooling-heat treatment.(4)The cold crack sensitivity of experimental steel welded joints and the micro structure evolution of heat affected zone were studied.The welded joint of experimental steel has a certain tendency to cold crack according to the value of carbon equivalent(CEV=0.64%)and cold crack sensitivity coefficient(Pcm=0.35%).According to the results of the Y-groove cracking tests,there are no cold cracks in the welded joint when the preheating temperature is higher than 100?.M-A constituents is the main reason for the drop of impact toughness in the heat affected zone of experimental steel.For the single welding thermal cycle,when the heat input increases,the size of the M-A constituent in the CGHAZ gradually increases,and the hardness of the MA constituents in the ICHAZ increases.These changes may promote microcrack nucleation and reduce the critical stress of fracture.The near-connected coarse M-A constituents form along the prior austenite grain boundary is the main reason for the decline of the IC CGHAZ impact toughness.They not only weaken the grain boundary but also serve as crack initiation position.Increasing the peak temperature of the secondary thermal cycle of IC CGHAZ can improve the impact toughness promote due to the reversed austenite nucleated within the austenite crystal,and obtain the decomposed block M-A component.(5)The variation of the microstructure and inclusion size of the weld with the increase of heat input were revealed.The microstructure of the weld metal(WM)is mainly acicular ferrite,which is usually nucleated at the inclusions in the WM.The proportion of bainite acicular ferrite increases with the increase of the heat input.When the heat input is increased to 18.5 kJ/cm,the microstructure of WM changes from bainite acicular ferrite to Widmanst?tten acicular ferrite(WF-AF).The size of inclusion in WM increases with the increase of heat input.On the one hand,the large size inclusions are beneficial to the nucleation of acicular ferrite;on the other hand,it also may serve as crack initiation position.The relationship between the size of inclusions in the WM and the heat input was established:dv?0.24(2.13?E)1/3.(6)The effects of welding heat input and post-heating temperature on the microstructure and mechanical properties of welded joints were studied.The maximum hardness of the heataffected zone gradually decreases with the increase of welding heat input until the hardness value is similar to that of the base metal,the phenomenon of heat affected zone hardening disappears.At the same time,the degree of softening of the heat-affected zone increases,and the softened zone expands.The maximum drop of strength(113 MPa)occurred when the heat input is increased to 18.5 kJ/cm.The impact absorbed energy of the WM and HAZ increases first and then deceases with the increase of heat input.When the post-heating temperature is less than 300?,the microstructure of the WM has no obvious change,which is still composed of acicular ferrite with a small amount of lath bainite.Meanwhile,the martensite microstructure of CGHAZ and FGHAZ is tempered and ? carbide precipitates,which reduces the microstructure stress and thermal stress;the blocky martensite in ICHAZ and IC CGHAZ will decompose may reduce the stress concentration extent between the blocky martensite and the substrate,which is beneficial to improve to the strength and toughness of the welded joint.The experimental steel can obtain the welded joints with high strength and good toughness when the preheating temperature is 100?,welding heat input is in the range of 10.5?14.5 kJ/cm and the post-heating temperature is 300?.