| Cryogenic steels used for ships under extreme cold conditions require high strength,excellent low-temperature toughness and good crack arresting ability.The paper adopts a test rolling steels EH40 and EH47 produced by a steel plant and focus on the factors which affect the low temperature toughness of steel,the crack arresting mechanism is also analyzed in detail here.The main work are as follows.(1)The continuous cooling transformation(CCT)curve after no deformation and 30%deformation of austenite at 900? is determined by MMS-200 thermal simulation machine.Results show that when there is no deformation,the micro structure is dominated by polygonal ferrite,ferrite and very small amount of pearlite at lower cooling rates.At the same time,Granular bainite occurs in the range of 2?5?/s and when the cooling rate is 10?30?/s,the granular bainite and bainitic ferrite structure are obtained.But after 30%deformation of austenite at 900?,bainite can be obtained during all cooling rates.The bainite species did not change with the increase of cooling rate and all of them were mainly granular bainite.However the distribution of island M/A component was more uniform and small.Results also show that the deformation treatment at 900? can accelerate the transformation of ferrite and restrain the transformation of bainitic.Comared with the static transformation,dynamic CCT curve mobiles to the left corner.Also the shape of two CCT curves are slightly different,but the transformation product corresponds roughly,The deformation induced transformation can make the transition region left 20?/s.(2)Through the high temperature quenching test,we studied the austenite grain growth behavior of EH40 steel.The results show that the austenite coarsening temperature is over 1250?,which means that is appropriate to heat the billet up to 1200?1250? during production under the condition that the microalloying elements are completely solubled.(3)Single pass compression test display that the experimental steel has undergone two processes of work hardening and recovery softening under various temperature conditions and the dynamic recrystallization occurs at temperatures above 1100?.The deformation resistance model of test steel is established by Origin through mathematical regression.The establishment of the model will have high valus for the steel EH40 during actual prediction of deformation resistance in controlled rolling process.(4)Double channel compression test shows that the static nonrecrystallization critical temperature Tnr of the test steel is between 900 and 950?,and that the activation energy of the static recrystallization is 2.115×105J/mol.(5)The micro structure properties of EH40 and EH47 steel with thickness of 86 mm were examined.And the low temperature impact toughness was analyzed,and the mechanism of low temperature strength and toughness of ship plate steel was determined by comparison of organization and performance.The test results show that the two kinds of test steel are acicular ferrite and quasi-polygonal ferrite composite structure and this composite organization is the key to the good performance of the test steel.The interface between the acicular ferrite strips and the two tissues is a large angle grain boundary,which has a good effect on the crack propagation and increases the crack propagation.Acicular ferrite and partially quasi-polygonal ferrite have high density dislocations,and partial dislocations are pinned by fine precipitates,all of which are key to the excellent low temperature toughness of the test steel. |
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