| With the rapid development of modern shipbuilding industry, the main investigationsfocus on the large and lightweight ship. Higher demands for performance of ship plate steelare asked. Therefore, many countries aim to research and develop the microalloyed ship platesteel with high strength, good toughness, welding and machinability. At present,microalloying and Thermo-mechanical Control technology as the main methods are used forproduction of high strength ship plate steel. According to the design idea of adding a smallamount of alloy elements, Nb-Ti and Nb-V are compositely added on the basis of the16Mnsteel. Two ship plate steels with strength grade of D36and E40are obtained, and we arestudied continuous cooling transformation, microstructure evolution, toughness andweldability of Nb-Ti or Nb-V compound micro-alloyed steel, and determine the hightemperature deformation and cooling rate for influence of phase transformation,microstructure and properties. The main results are as follows:1. According to the mechanical properties of high strength ship plate steel andmicroalloying principles, using Mn/Si as main alloy elements, Nb and Ti or Nb and V asmicroalloying elements are compositely added. Through the melting and continuous casting,250mm thick slabs can be obtained. The slabs of250mm thick eventually become30mmthick steel plates after the rough milling and finish milling are performed. Purity of the steelplates is better, and strengths of Nb-Ti and Nb-V steel plates under hot rolled and normalizedconditions have reached the levels of D36and E40.2. The static and dynamic CCT curves of Nb-Ti and Nb-V microalloyed steels aredetermined by the static and dynamic thermal simulation test and metallographic method. Theresults show that the static CCT curves includes four transition regions: Aâ†'F transition(high temperature), Aâ†'P and Aâ†'B transition (intermediate temperature), and Aâ†'M(low temperature). Compared with the Nb-V microalloyed steel, beginning and finishingtemperature of the phase transformation for Nb-Ti microalloyed steel are higher. Thermaldeformation obviously improves the transformation temperature from austenite to eutectoidferrite and shortens the incubation period.3. The testes of the static and dynamic continuous cooling transformation of austenitemicrostructure analysis and hardness show:â‘ By controlling the deformation and coolingrate, we can get homogeneous and fine complex phase microstructure of Nb-Ti and Nb-Vmicro-alloyed steels.â‘¡Under the undeformed and lower cooling rates conditions, one canobtain acicular ferrite, polygonal ferrite and granular bainite; under the higher cooling rates,we can obtain bainite and martensite. But under the deformed conditions and lower coolingrates, one can be obtained polygonal ferrite and pearlite, higher cooling rates can be obtainedacicular ferrite, bainite and martensite.â‘¢Under the undeformed and deformation conditions, the growth rates of hardness values of the two steels shift from fast to slow. Compared with itunder the undeformed condition, the hardness value of deformed steel is higher under thesame cooling rate.4. The phase, impurity and precipitates of Nb-Ti and Nb-V microalloyed steels arecharacterized by XRD, EDS, SEM and TEM. The results show that, most of them are cubicferrite phase in microalloyed steel. By compositely adding Nb-Ti and Nb-V micro-alloyingelements, they can form the fine C-N compounds with C, N atoms of steel. They can inhibitaustenite grains growing and recrystallizing, which accuses the effect of precipitationstrengthen. There are a small amount of impurities in micro-alloy steel, and most of them aresulfides, the others are the compounds composted of Si, Mn and Ca elements.5. The tensile test result shows that, under the normalizing state, the tensile strength ofNb-Ti and Nb-V microalloyed steels respectively is over621MPa and721MPa. By watercooling and ice cooling, the tensile strength of two steel is above863MPa. The impact testresult shows that, the impact energy of the two steels of normalizing at room temperaturereach166J and169J, respectively. In addition, when the temperature decline to-20℃, theyshow good toughness (the impact energy are144J and140J respectively), and when thetemperature is-40℃, they show brittle fracture. The fracture of SEM analysis shows that thetoughness changed by temperature coincides with fracture morphology. The toughness ofNb-Ti and Nb-V microalloyed steels are better than16Mn steel by comparison. The researchalso shows that, after quenching of980℃, the toughness of two steels can be effectivelyincreased after tempering of450℃. The best match of strength and toughness can beobtained.6. Comparing with the16Mn steel, the microstructure grain of Nb-Ti and Nb-Vmicroalloyed steels is finer under the same cooling rate. It shows that refining effect ofmicro-alloying elements for the grain is very significant. The CCT curves of microalloyedsteels move left. The incubation period of Aâ†'F and Aâ†'P is shorten. In addition, addingmicro-alloying element is beneficial for the precipitation of F and B and expanded theforming area of B. But it hinders the transformation of P and narrow the forming area of P.7. The welding experimental result shows that Nb-Ti micro-alloy steel has goodweldability. the increase of welding current results less ferrite in the heat affected zone, moredense tissue and more component of martensite, making the welding area of Nb-Timicro-alloyed steel has good overall performance. |
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