| Crack arrest steel is a class of High-strength Low-alloy Steel that is developed from low carbon steel.This steel has desirable crack resistance,strong plasticity and low temperature toughness,which can effectively prevent crack initiation and propagation to meet the demand of large container ships for wide and thick plate.It is well known that the properties of crack arrest steel mainly depend on the alloy elements and heat treatment technology.Therefore,this paper mainly focuses on the influence of these two factors on the microstructure and mechanical properties of crack arrest steel.Two components of high-strength and thick steel plate were designed and used to investigate the effects of alloy elements and heat treatment technology on the microstructure and mechanical properties by adjusting the content of Cu and Nb.This paper calculated the equilibrium phase diagram,CCT and TTT curves of the newly designed crack arrest steel using JMatPro software.We analyzed the effect of Nb and Cu alloy elements on the equilibrium phase diagram in order to verify rationality of the alloy composition and thermal mechanical control process,and chose an appropriate heat treatment temperature.The study found that the microstructure of 1-A550 steel was mainly composed of polygonal ferrite,and a small number of acicular ferrite,bainite and cementite after annealing.As the precipitation of Nb second phase and Cu rich phase,the effects of precipitation strengthening was very obvious,which led to a relatively good mechanical property during annealing process for sample 1#.The steel hardness,yield strength,tensile strength and elongation increased first and then decreased with extend of annealing time,and reached the maximum after 1h.The microstructure of 2-A550 steel was consisted of acicular ferrite,bainite and polygonal ferrite.The number of acicular ferrite was obviously more than 1-A550.The yield strength of 2-A550 increased first and then decreased,and the elongation rate was quite the contrary.1-A550-lh and 1-A550-10h were chose for low temperature impact test.As the sizes of Nb precipitated phase and Cu rich precipitates were relatively large after annealing for 1h,crack expansion could be accelerated which led to the decline of low temperature toughness.Hence,the ductile-brittle transition temperature of 1-A550-10h was lower than 1-A550-1h.After solid solution treatment,in order to obtain saturated and homogeneous solid solution,it was easier for the precipitate of fine grains and uniform distribution of carbides during tempering.In addition,in orer to make use of lattice distortion to improve the crack strength of steel,we studied the influence of different solid solution temperatures of 875 ?and 925 ? at the same time of solution conditions on microstructure and mechanical properties of both components of steel solution.The results showed that relatively excellent mechanical properties could be obtained at 875 ? for 2h and 925 ? for 1h solid solution.The low temperature impact toughness of 1-S925-1h was better than 2-S925-1h.The ductile-brittle transition temperature of 1-S925-lh was lower than-80?,and the ductile-brittle transition temperature of 2-S925-lh was between-60? and-80?.After quenching and tempering(solution treatment for 2h at 875? and then tempering at 550?),the microstructure distribution of both the samples exhibited stable mechanical properties which was attributed to the obvious effect of tempering softening and fine grain strengthening effect on mechanical properties;in addition,after solution treatment for 1h at 925 ? and then tempering at 550?,the low temperature impact toughness of sample 2#got better than 1#.The ductile-brittle transition temperature of sample 1#was between-60 ?and-80?,and the ductile-brittle transition temperature of sample 2#was lower than-80 ?. |
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