Effect Of Manganese/Niobium And Hot Rolling Process On Microstructures And Mechanical Properties Of Enameled Steels

The yield strength of the water heater tanks,which is subjected to high water pressure,usually decreases after enamel fire annealing process(EFA)above 800°C,leading to deformation and failure in their service process.Therefore,the yield strength of the enameled steel used in the manufacture of the sealing tanks is required to be higher.In this thesis,the effects of hot rolling process(dynamic strain-induced ferrite transformation,DSIT),heat treatment process and Mn/Nb contents on the mechanical properties and microstructure of low carbon microalloyed enameled steels before and after heat treatment are studied using the combined approach of thermodynamic calculations,finite element simulation and experiments.The main results are as follows.The thermodynamic calculation results show that the austenite-ferrite transition temperature(A_e3)of 0.6Mn steel containing 0.02wt.%Nb is about 904°C and the total solid-solution temperature of precipitated Nb C is about 1108°C under equilibrium condition.The effects of different hot rolling processes on the microstructure and mechanical properties of low carbon microalloyed enameled steels are studied.It is found that the microstructure of as-rolled enameled steels mainly consists of polygonal ferrite and a small amount of lamellar pearlite;the average grain size is between 4-5?m.The yield strength of steels rolled at 1.2 m/s is higher than that of steels which is rolled at 1.0 m/s,and the difference of yield strength is about 20MP,while different rolling temperatures produce a similar level of yield strength.The results of finite element simulations show that during the 1^st-3^rd passes,different rolling speeds has little influence in the finishing rolling temperatures at the slab surface.During the latter three passes,with the increase of rolling speed,the finishing rolling temperature and the rolling force required for the slab deformation increases gradually;and the strain rate is faster.After heat treatment with the enameled temperatures between 705-871°C following by sand and air cooling,the yield strength and tensile strength of 0.6Mn steel decreases significantly by?40-60 MPa and the percentage elongation increases compared with the as-rolled steels.With the increase of heat treatment temperature,the yield strength decreases overall,and the grain size increases slightly(?4-7?m).After heat treatment,the yield strength of 1.2Mn steel decreases obviously first and then remains basically unchanged(?430-460 MPa)with the increment of heat treatment temperature.The grain size of 1.2Mn steel is about 4?m,which basically does not change with different heat treatment temperature.The strengthening mechanisms of 0.6Mn steel after heat treatment at 815-871°C for 5 min by air cooling are analyzed.It is found that when the heat treatment temperatures are 815-843°C,fine grain strengthening is the main strengthening mechanism,and the value of contribution is between 161-195 MPa.The next is the solution strengthening,which is about 85-87 MPa.The value of dislocation strengthening is between 63-83 MPa,and the precipitation strengthening is about 31.7 MPa.When the heat treatment temperature is 871°C,the value of fine grain strengthening to yield strength is the largest,and the value of dislocation strengthening is more than that of solid solution strengthening,becoming the second one,followed by solid solution strengthening and precipitation strengthening.With the increment of Nb content,the yield strength of the as-rolled 0.6Mn steel and 1.2Mn steel increases overall,and the grain size of the as-rolled 1.2Mn steel decreases gradually.The grain size of the as-rolled 1.2Mn steel does not change with Nb content basically.After heat treatment at 815°C,the grain size of heat-treated 0.6Mn steel increases slightly;with the increase of Nb content,the grain size gradually decreases,and the volume fraction of pearlite increases.The grain size of 1.2Mn steel before and after heat treatment is obviously smaller than that of 0.6Mn steel,and it does not change with Nb content basically.The yield strength of 1.2Mn steel after heat treatment is higher than that of 0.6Mn steel by?60-70 MPa.Compared with the as-rolled steels,the yield strength of 1.2Mn steel after heat treatment is basically unchanged or slightly decreased,and the yield ratio increases,while the yield strength of 0.6Mn steel decreases obviously,and the yield ratio decreases.