Design,Mechanical Behavior And Microstructure Of Novel Ultra-high Strength Steel

Now,the highest strength bainitic steel is the high-carbon and carbide-free ultra-high strength bainitic steel.The tensile strength of the steel can reach as high as 2500MPa?true stress?,which results from the ultra-fine plates of bainitic ferrite.However,high carbon conent results in poor toughness and plasticity of the steel.Although the ultra-high strength martensitic steels,such as Aermet100,M250 and G99,are with comprehensive mechanical properties,large content of Ni and Co elements in the steels leads to high cost.Therefore,a novel ultra-high strength steel?G33?was developed.The design of G33 steel combined with the design concept of ultra-high strength bainitic steel and martensitic steel.G33 steel with lower carbon content and cost had good match of ultra-high strength,high toughness and plasticity,which can satisfy the requirement of armor and projectile warhead.The main research is as follows:JMat Pro software was employed to design a novel ultra-high strength steel?G33?,and the patent for the invention had been authorized.The chemical compositions of the steel are as follows:C:0.25⁰.45;Cr:2.0³.5;W:2.0⁴.5;Ni:2.5⁴.0;Si:0.40¹.2;Mn:0.30¹.5;Mo:0.40⁰.80;V:0.10⁰.40?wt%?.And the true transformation points of the steel are as follows:A_C3=800?,A_C1=644?,Ms=250?.A series of baintic G33 steels were gained by the rational heat treatment designs.The acicular and lower bainite ferrite and the retained anstenite?films and blocks?were obtained after the heat treatment of 1100? austenization+265? isothermal quenching.The quasi-static ultimate compressive strength of 2500MPa?true stress?as well as good matching of the toughness and plasticity of bainitic G33 steel were achieved,when the propotion of retained anstenite films,retained anstenite blocks and bainite ferrite was 1:3:16.The microstructure and its effect on mechanical properties of martensitic G33 steel were studied systematically after the heat treatment of 900?/1100? quenching+cold treatment+200⁷00? temper.The study results showed that the yield strength range of1630¹695MPa?engineering stress?,and good toughness and plasticity were achieved when the martensitic steels were quenched from 900? and tempered at no higher than 482?.However,the yield strength of the martensitic G33 steels quenched from 1100? was lower than 1550MPa?engineering stress?because of the bigger grains.Moreover,a martensitic G33steel with fine grains was designed by a method of high temperature?1100??and thermo-mechanical treatment?TMT?.The study results showed that the ultimate tensile strength and compressive strength of the TMT specimens could reach as high as 2345MPa(engineering stress,10^-3s^-1)and 2770MPa(true stress,10^-3s^-1),respectively.And the yield strength range of 1630¹750MPa?engineering stress?,and better toughness and plasticity were achieved when TMT specimens tempered at no higher than 482? after cold treatment.The split Hopkinson pressure bar was introduced to conduct the dynamic tests of bainitic and martensitic G33 steels.The test results showed that the bainitic steels exhibited higher true strain of dynamic critical fracture than martensitic G33 steels.And the bainitic steels of1100? quenching+265?/10h,20h isothermal quenching showed better dynamic mechanical properties,the dynamic ultimate compressive strengths of the two steels were2516 MPa?true stress?and 2245MPa?true stress?,the true strain of dynamic critical fracture were 0.33 and 0.40(4000s^-1),and the absorbed-in-fracture energy were 830 J/mm³ and 885J/mm³.The strength of all specimens of the martensitic G33 steels under dynamic loading was higher than 2500MPa?true stress?.The dynamic ultimate compressive strength and the true strain of dynamic critical fracture(4500s^-1)of raw TMT specimens were 2830MPa?true stress?and 0.295,respectively,and the absorbed-in-fracture energy was 815 J/mm³,which were highest of all test specimens of martensitic G33 steels.After dynamic test,the microstructure of baintic and martensitic G33 steels showed that the formation of adiabatic shear bands?ASB?obeyed the sub-grains rotation dynamic recrystallization mechanism.The nucleation of cracks occurred mainly in the interior or at the edge of ASB.The simulation projectiles test showed that the martensitic G33 steels of different heat treatments all recycled intact projectiles,and there was no damage in the interior of the recycled intact projectiles,which suggested that the martensitic G33 steels had good high-speed penetration performance.