Effects Of Mn Content And Heat Treatment On Microstructure And Mechanical Properties Of Enamelling Steels

When a enameling steel used for producing water heater tanks is fired at 800-900℃,the phase transformations between ferrite and austenite occur,which changes the final microstructure and corresponding mechanical properties.Since water heater tanks are subject to a certain temperature and pressure during service,a high yield strength is required.In this thesis,the impact of heat treatment temperature and cooling rate on the microstructure and yield strength of cold-rolled（Sample 28#）and hot-rolled（Sample 29#）steels with the composition of Fe-0.07C-0.8Mn-0.5Si–0.046P-0.005N（wt.%）were studied,through experiments combined with thermodynamic calculations.The microstructure and mechanical properties of cold-rolled steels with 0.8 wt.%Mn（Sample 28#）and 1.2 wt.%Mn（Sample 12#）were compared and analyzed.The extended Hall-Petch empirical formula was used to quantitatively analyze the contributions of different strengthening mechanisms to yield strength.The main results are as follows:The A_c1（⁷45℃）and A_c3（⁹32℃）temperatures obtained by thermomechanical analysis（TMA）and quenching experiments are higher than the A_e1（705℃）and A_e3（887℃）temperatures calculated using the thermodynamic calculation software Pandat.The microstructure of the as-received sample for Sample 28#contains a small amount of cementite dispersed in the ferrite matrix.Cementite disappears after heat treated at 760-871℃ and cooled by either air cooling or sand cooling,while pearlite appears at the ferrite/ferrite grain boundaries.The average grain size of the air cooled sample is slightly smaller than that of the sand cooled sample.The yield strengths of both the air cooled and sand cooled samples are in the range of370-400MPa,and decrease with increasing heat treatment temperature.Regarding Sample 29,the microstructures of the as-received sample and samples heat treated at 760-871℃ consist of ferrite and a small amount of pearlite distributed at ferrite/ferrite grain boundaries.The air cooled and sand cooled samples have a similar grain size;their yield strengths are in the range of380-410MPa,and decrease with increasing heat treatment temperature.After heat treated at 760℃,Sample 28#shows a higher yield strength（404 MPa）compared to the samples heated treated at other temperatures,while Sample 12#exhibits the lowest yield strength（323 MPa）among all the heat treatment temperatures.The reason can be attributed to the formation of martensite enriched with C at ferrite/ferrite grain boundaries in Sample 12#when heat treated at 760℃,which weakens the solid solution strengthening of C in the ferrite matrix.For Sample 28#with a lower Mn content,pearlite instead of martensite forms after heat treated at different temperatures,and thus no yield strength dip is observed.After heat treated at 871℃,the second phase at ferrite/ferrite grain boundaries in Sample 12#changes to pearlite,and the yield strength was increased（399 MPa）,even higher than Sample 28#（376 MPa）heat treated at the same temperature.The air cooled samples heat treated at 760℃ were additionally tempered at 400℃ for 5 min.The microstructure of Sample 28#changes little after tempering.While for Sample 12#,martensite decomposes and the yield strength is significantly improved（382 MPa）after tempering.The calculated yield strengths of Sample 28#using the extended Hall-Petch empirical formula agrees reasonably with experimental values for different heat treatments.With respect to Sample 12#,the calculated yield strength of the as-received sample agrees reasonably with the experimental value.However,for the sample heat treated at 760℃,the calculated yield strength is higher than the experimental value,which results from the increase of dislocation density due to the formation of martensite,as analyzed by XRD data.The calculation of yield strength using the extended Hall-Petch empirical formula indicates that grain refinement strengthening is the main strengthening mechanism,followed by dislocation strengthening and solid solution strengthening.The increase of average grain size and the decrease of dislocation density are main reasons for the reduction in yield strength of samples after heat treatment.