Effects of initial cooling temperature on elongation performance of 420 MPa grade marine engineering steels
This investigation focused on the development of 420 MPa grade steel for marine engineering applications,conducted within an in-dustrial production framework Three high strength marine engineering plates of 420 MPa with ferrite/bainite(F/B)dual-phase and varying volume fractions of fetriten and bainite were produced through controlled rolling,relaxing and accelerated cooling control processing.The effects of hard phase(bainite)content and soft phase(ferrite)grain size on the work-hardening behavior of the plate during deformation were studied using the modified Crussard-Jaoul(C-J)analysis and work-hardening rate curves,and the cooperatire deformation behavior of dual-phase microstructwe at different deformation stageswhich were validated by electron backscattered diffraction(EBSD)technique.At the same time,the effect of carbon diffusion during relaxation in two phase region on the dual-phase microstructure were also analyzed.The results il-lustrated that for the EH420 experimental plates,managing the controlled rolling followed by relaxation cooling temperatures within the range of 650-750 ℃ leads to an increase in the proportion of bainite as the cooling temperature decreases,along with a decrease in bainite dispersion.Simultaneously,a decline in cooling temperature leads to an increase in the average grain size of ferrite,with instances of partial mixed crystal phenomena emerging within the ferrite.This phenomenon is attributed to the lower cooling temperatures extending the duration of the plates within the Ar1 to Ar3 temperature intervals,thereby facilitating a more comprehensive diffusion of carbon atoms from the proeutectoid ferrite to the austenite.Consequently,this process enlarges the area of austenite regions enriched with carbon,which,upon rapid cooling,increases the proportion of transformed bainite,decreases its dispersion,and induces localized grain coarsening in ferrite.The study delineates that the plastic deformation of the plates is predominantly influenced by ferrite during the tensile deformation process.A higher proportion of ferrite,together with a decreased average grain size of ferrite,results in a reduced stress index mⅡ,prolongs the duration of themⅡphase,and fa-cilitates larger strain values at necking,thereby significantly enhancing the elongation properties of the plates.Consequently,improving fer-rite content and its grain size homogeneity and uniform distribution of bainite are identified as key strategies for enhancing the elongation per-formance of EH420 plates.
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