| With the development of marine industry,large thickness steel plate,which combines with high strength and toughness,good weldability has become the future development trend of ship hull steels.Compared with the traditional Ni-Cr-Mo-V steel,Cu-age-hardening hull ship steel possesses the lower hardenability due to the design idea of low-carbon + Cu microalloyed,which makes it difficult to guarantee the homogenization of section performance for large thickness plates.As a result,the combination of high strength and good toughness is a core technique and main difficulty of heavy steels.In this paper,the effects of quenching process on the cross-section microstructure and properties of Cu bearing plates with the strength grade of 590MPa and 35mm in thickness were studied.Meanwhile,microstructural evolution during quenching and tempering was analyzed in detail,the relationships between heatment process,microstructure and mechanical properties were discussed.In addition,the differences between microstructural evolution,strengthening and toughening mechanisms of NiCrMoV and Cu bearing heavy steel plate with the same strength grade and the same thickness were studied in this paper.The conclusion of the present study can be drawn as follows:The quenching cooling rate has a significant effect on the microstructure and mechanical properties at the near surface about 8mm thickness of Cu bearing steel plate.With the increase of quenching cooling rate,the content of lath structure,the density of dislocation and low misorientation angle boundaries significantly increased,and the width of lath was markedly fined,the size and volume fraction of MA constituent decreased,leading the obviously increase of the hardness and strength in the near surface of plates.Bainite was mainly obtained in the core of steel plates.There is no significant effect of the quenching cooling rate on microstructure and properties from the quarter to the core.After tempering,the cross-section structure and properties were homogenized,tensile strength reduced and low temperature toughness significantly improved.There is no significant effect of the quenching cooling rate on the size of overaged copper particles.The precipitation strengthening effect of copper particles is favorable to the increase of yield strength.Since the degree of dislocation density after tempering is positively correlated with the degree of quench hardening,the quenching cooling rate has little effect on the strength of tempered steel plates.The increase of quenching cooling rate is beneficial to the effective grain size refinement,which is helpful to improve the low temperature impact toughness of tempered steel plates.There are significant differences in properties between NiCrMoV steel and Cu bearing steel in the same condition of heat treatment process.The hardenability of NiCrMoV steel is suiperior to Cu bearing steel,but the ability against temper softening is reversed.After quenching and tempering,the combination of high strength and good toughness was obtained in NiCrMoV and Cu bearing steel heavy plates,with the yield strength grade of 650MPa,585MPa in the core respectively,and with-84?Charpy absorbed energy of 250J.The difference of strengthening mechanisms between NiCrMoV and Cu bearing steel stems from two aspects,one at structure strengthening due to the hardenability,and a second at precipitation strengthening caused by VC and Cu respectively.Predominantly tempered lath martensite in the cross section was obtained in the center for NiCrMoV steel after quenching and tempering as a result of a high carbon content,possessing the finer substructure,which contribute to its high strength and good toughness.Cu bearing steel obtained bainitic ferrite and polygonal ferrite in the cross section,having a larger substructure due to a lower hardenability.A major strengthening comes from Cu particles in Cu bearing steel,and the good toughness is contributed to not only the lower dislocation density but also the lower carbon content. |
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