Study On The Calculation Of Precipitates And Cleanliness Of 14MnNb Pipeline Steel During The Production Process

In order to provide the reference for the development of new products via improving the properties of pipeline steel and the control level during the production process furtherly.We analyzed the thermodynamic precipitation conditions of Nb,Ti,Al as well as MnS in this work based on the regular solution and equilibrium reaction.The conclusions are listed as follows:(1)No precipitation formed in the liquid phase region(T>1791K)because the thermodynamic conditions cannot satisfy the precipitation requirements of carbide,nitride,and MnS in 14 MnNb pipeline steel.(2)Molten steel enters into the solidification stage(temperature from 1736 K to 1791K)and TiN precipitate in 1780 K following the decrease of temperature,but other precipitates can not as the precipitation conditions can not be satisfied.(3)Molten steel solidified(below 1736K)and transformed into austenite when the temperature decreased continually.Most kinds of precipitate have been seperated out at this stage in the order of MnS > NbC > TiC > AlN > NbN,the corresponding precipitation temperature are 1670 K,1444K,1357 K,1313K,and 1308 K,respectively.In addition,we concluded the conclusions based on the analysis for the cleanliness of 14 MnNb pipeline steel under the production process of EAF-LF-VD-CC by the spectral analysis,infrared analysis,bulk sample electrolysis,image analyzer analysis,scanning electron microscopy,and energy spectrum analysis methods.(1)Roughly,the average amount of oxygen in molten steel exhibits a decrease tendency.Most of the oxygen in molten steel can be removed in the LF and VD refining process.The oxygen content in 14 MnNb steel increased 2.2×10-6 from the end of VD refining to the tundish process,which indicates that the reoxidation occurrs in the transportation and casting stages.The amount of nitrogen in the molten steel shows an increasing trend because the vacuum treatment in VD furnace is beneficial to the precipitation of dissolved nitrogen in steel.The average nitrogen content decreased by 11.67% after the VD refining process.(2)The number of inclusions in 14 MnNb pipeline steel was reduced significantly in the whole production process.The inclusions in molten steel can be removed effectively during the LF and VD refining processes.CaO·CaS·Al2O3 and MnS is the main inclusion and the secondary inclusion comprise CaO·MgO·Al2O3 and CaO·Al2O3.In the slab,large inclusions are mainly composed of CaO·Al2O3·SiO2,CaO·MgO·Al2O3,and CaO·TiO·SiO2·Al2O3,as well as little TiN,K2O·Al2O3·SiO2,Na2O·CaO·Al2O3·SiO2,and so forth.The presence of K and Na suggest that the mould fluxes can be drawn into the molten steel in casting process.