| Considering the safety of ships,the requirements of comprehensive properties such as strength,toughness and weldability of steel for hull are constantly increasing.The original composition design,process and properties of FH36 shipboard steel can’t meet the market demand.In this paper,the effect of rolling process on microstructure and properties of FH36 were studies.The results show that reducing the carbon content,using TMCP process instead of traditional normalizing heat treatment process are beneficial to improve the low temperature toughness,plasticity,weldability and anti-lamellar tearing property.The developed new type low carbon bainite steel shipboard steel has excellent comprehensive properties.At the same time,this project has laid foundation for the development of TMCP high strength construction steel,low yield ratio building structural steel and so on.The main contents and results of this paper are as follows:(1)The general idea of composition design is as follows:The steels is a low alloy steel with low carbon,silicon and manganese.Solid solution strengthening and phase transformation strengthening are the main strengthening modes of the steel.Grain refinement strengthening and precipitation strengthening are secondary strengthening mechanisms.The content of C is 0.06%～0.08%,which is much lower than the requirement of national standard(≤0.16%).The content of Si is 0.30%-0.40%.The content of Mn is 1.45%-1.55%.The content of Ni is 0.10%-0.20%.The content of Nb is 0.015%～0.025%.The content of Ti is 0.010%～0.020%.(2)The continuous cooling transformation curves were drawn and cooling rates affecting on microstructure of the steel were studied by Gleeble 3800 hot-simulation testing machine.The results show that the optimum cooling rate is 5-15℃/s.According to the results of laboratory research,the parameters of controlled rolling and controlled cooling in industrial production were studied.When the finish rolling temperature is about 860℃,relaxation temperature is within 750℃～700℃,final cooling temperature is about 580℃ and cooling rate is 8～12℃/s,the plate has the best properties.(3)According the trial production,the key process parameters affecting the microstructure and properties were determined.The compression ratio by rolling with large billet was increased.The optimum final rolling temperature is between 830 and 890℃according to the the thickness and the temperature drop of the plate.In the accelerated cooling process,ladder water volume control method,head and tail water shielding technology were adopted to ensure the final cooling temperature of the plate is steadily between 520 and 590℃.In the hot roller leveling process,the reduction in the first pass is zero.In the second pass,the plate is reverse levelled.The trumpet-shaped roller are set to ensure the flatness of the plate can meets the customer’s requirements.(4)The properties of the trail products were studied by statistical analysis method.The results show that the tensile properties,the impact properties and the Z-direction tensile properties of the plates are far higher than the requirement of the classification societies’ rules.The microstructure of the plates is excellent ferrite and bainite dual phase structure.(5)Cold formability and properties after welding of the FH36 plate were studied.The cold bending properties of the plate are qualified.The properties of the plate with the flame straightening temperature below 800℃ can meet the requirements of the classification societies’ rules. |
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