Effects Of Manganese/Titanium Content And Heat Treatment On Microstructures And Mechanical Properties Of Enameled Steels

The yield strength of enameled steels,which are used in water heater tanks,usually decreases during enameling between 800 and 900 °C.However,the working conditions of the water heater tanks,which are certain temperature and pressure,require that the enameled steel has relatively higher yield strength.In this thesis,the influences of the heat treatment process on the microstructures and mechanical properties of the hot-rolled and cold-rolled enameled steels are studied by the combination of experiments and thermodynamic calculations.The extended Hall-Petch formula is used to analyze the mechanisms of the different yield strengths of the hot-rolled enameled steels with different Mn and Ti contents after heat treatment.The main results are as following.The results obtained from the thermodynamic calculations,expansion method and quenching method show that the ferrite-austenite transformation temperatures of the 0.6Mn steel are about 10 °C higher than those of the 0.8Mn steel.The microstructures of the hot-rolled 0.6Mn steel mainly consist of ferrite and pearlite for the cases of the as-rolled and after heat treatment with the annealing temperatures between 704 and 871 °C for 5 min following by air cooling or sand cooling.Regarding the cold-rolled 0.6Mn steel,the microstructure of the as-rolled sample consists of ferrite and cementite;after heat treatment at the temperatures higher than 732 °C,the microstructure mainly consists of ferrite and lamellar pearlite.The grain sizes of the hot-rolled and cold-rolled 0.6Mn steels are similar(6-7 ?m)before and after heat treatment.With the increase of annealing temperature,the yield strength of the hot-rolled and cold-rolled 0.6Mn steels first decreases significantly and then remains basically unchanged(?360-420 MPa).This is mainly due to the fact that the dislocation density gradually decreases with the increase of annealing temperature.The yield strength of air-cooled samples is slightly higher than that of sand-cooled samples by about 10 MPa,which is mainly due to the higher dislocation density in air-cooled samples.Under the conditions of the same heat treatment process,the microstructure and grain size(the contribution of grain refinement strengthening)of the hot-rolled 0.8Mn steel are similar to those of the hot-rolled 0.6Mn steel.For the samples of the as-rolled and after heat treatment(704-871 °C for 5 min,air cooling or sand cooling),the yield strength of the hot-rolled 0.8Mn steel is nearly unchanged,about370-380 MPa.This is because the element solid solubility,grain size,and dislocation density of the hot-rolled 0.8Mn steel do not change obviously before and after heat treatment.After heat treatment with the annealing temperatures below 760 °C following by air cooling,the yield strength of the hot-rolled 0.6Mn steel is higher than that of the hot-rolled0.8Mn steel,which is due to the higher dislocation density in the hot-rolled 0.6Mn steel.After heat treatment with the annealing temperatures above 787 °C,which is higher than the ferrite-austenite phase transformation temperature,following by air cooling,the yield strengths of the hot-rolled and 0.6Mn and 0.8Mn steels are similar.This is because the hot-rolled 0.8Mn steel has a slightly higher solid solution strengthening effect,while the hot-rolled 0.6Mn steel has a relatively higher dislocation strengthening effect.Compared with the hot-rolled 0.6Mn and 0.8Mn steels,the hot-rolled 1.2Mn steel contains Ti element and has a higher Mn content,which makes it have the higher effects of precipitation strengthening(?30 MPa),fine grain strengthening(?160 MPa)and solid solution strengthening(?105 MPa).Regarding the contributions of various strengthening mechanisms to the yield strength of the hot-rolled 0.6Mn,0.8Mn and 1.2Mn steels,the largest contribution is the grain refinement strengthening(?40%);the second one is the solution strengthening(?25%);and the third is the dislocation strengthening(?20%).