| With the upsizing and high-performance requirements of engineer equipments,and the mining environment of oil and gas exploration goes harsh,the demands for ultra high strength and high toughness steels became more and more urgent.Then,the developing of 1000MPa grade ultra high strength and high toughness steels with good weldability and low cost was of great significance to economy and social development.This study was intened to investigate the relationship among the component-process-microstucture-properties of 1 000MPa grade low carbon ultra high strength steel systematically,developing the components of 1 000MPa grade low carbon ultra high strength steel,revealing the principles of how to achieve ultra high strength and toughness,and making a fundamental base for the physical metallurgy of 1 000MPa grade low carbon ultra high strength steel in our country.Firstly,the effects of different carbon content(0.09-0.21 wt.%)and Mo content on the mechanical properties of quenching and tempering experimental steels were compared.The results showed that under the condition of high temperature tempering at 600℃,the influence of Mo content on the mechanical properties of experimental steels was greater than that of C content.When 0.36-0.7 wt.%Mo was added,the yield strength of experimental steels with different carbon content(0.090.21 wt.%)was between 1005-1145MPa after tempering at 600℃,and the tensile strength was between 1050-1185MPa,while the yield strength of the test steels without Mo was 958-984MPa and the tensile strength was 1011-1034MPa.However,when tempered at 450 and 500℃,the yield strength of Mo-free experimental steels could also reach 998-1106MPa,and the tensile strength was 1109-1179MPa.Secondly,the effects of tempering processes with different heating methods on the microstructure,carbide precipitation and properties of 1 000MPa grade low carbon ultra high strength steels were investigated and compared.The results showed that after quenching and induction tempering to 600℃,the yield strength,tensile strength and elongation of low carbon ultra high strength steels were 1050MPa,1092MPa and 19.2%,respectively,induction tempering was benefit to achieving high strength and toughness.TEM study showed that induction tempering could significantly refine cementite precipitations.Statistics showed that the spheroidization rate of induction tempering specimens reaches 84.4%,and the size distribution was concentrated at about 40nm.However,the spheroidization rate of cementite in the conventional tempering specimens was only 64.3%,and the size distribution was concentrated at about 90nm.The nano-refinement and spheroidization of cementite were the key to achieving high strength and high plasticity.Thirdly,for the development of 1000MPa(V140)grade ultra high strength and toughness oil tube steel,the effects of different carbon contents of 0.09 and 0.17 wt.%on the precipitation behavior of vanadium and its strengthening effect were comparatively studied.The results showed that the yield strength and tensile strength of 0.09 wt.%C steel were stable at 1000 and 1085MPa after tempering at 450-600℃,while the yield strength and tensile strength of 0.17 wt.%C steel with high crabon content reached 1198 and 1292MPa after tempering at 550℃,respectively,and the increase in strength after tempering reached 170MPa.This showed that the higher content carbon was more conducive to the precipitation of VC,thereby achieving excellent precipitation enhancement effect.Furthermore,using 0.16 wt.%low carbon,vanadium microalloying and Mo-free alloy design,a 1000MPa(V140)grade low cost ultra high strength and toughness oil tube steel was developed,and after induction tempering,the yield strength was 1014MPa,the tensile strength was 1057MPa,the elongation was 16.0%,and the impact energy at 0℃ was 139J.Finally,the abrasive wear behaviors of low carbon ultra high strength steels with carbon content of 0.12 wt.%and 0.16 wt.%after different tempering processes were studied.The results showed that the wear resistance of all experimental steels after quenching were worse than that after any tempering process,so it was necessary to carry appropriate heat treatment on the quenched specimens to improve the wear resistance.Comprehensively considering the strength,toughness and wear resistance,the steel with carbon content of 0.16 wt.%,Cr containing and induction tempered at 500℃ was selected,its Vickers hardness could reach 400,and the wear rate was only 4.65×10-2mm3/m,which was lower than the wear rate of commercial HARDOX450 of 6.49×10-2mm3/m. |
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