Effect Of Heat Treatment On Microstructures And Mechanical Properties Of Enameling Steels

After firing in the ferrite-austenite two-phase region,the yield strength of enameling steels used for water heater tank will decreases.The loss of yield strength requires thicker base metal to resist deformation from stress imposed during service,which results in increased costs.In this thesis,experiments and thermodynamic calculations are carried out to study the effects of chemical composition,firing temperature,cooling rate and tempering process on the microstructures and mechanical properties of the cold rolled and hot rolled enameling steels,to figure out the main reasons for yield strength reduction due to the firing process.The results are used to modify compositions and firing process to improve the final mechanical properties of enameling steels.The main results are summarized as follows.According to the thermodynamic calculations and experiments,the samples with high P composition will form P-rich precipitates at low temperature,leading to a high yield strength.After heat treatment at 815?871?.the yield strength and grain size of the samples with high P level nearly keep unchanged.When the temperature is around 800?1000?,the majority of C and Si exists in austenite.After holding at 815?871? and cooling with a cooling rate higher than 3.0?/s,,samples with high Si composition have a low yield strength while Si composition has few effects on the yield strength when the cooling rate is lower than 1.5?/s.This can be attributed to the inhibition of C diffusion from austenite to ferrite caused by element Si,which weakens the solid-solution strengthen of C.According to the variations of microstructures and mechanical properties of cold rolled and hot rolled steels before and after firing,it is found that the yield strength of samples decreases at first and then increases with increasing firing temperature when the cooling rate is higher than 3.0?/s,indicating a yield strength dip between 760?815?.This may be caused by a kind of carbon-rich lump phase appearing along the grain boundaries after heated at 760?815? and cooled with a high cooling rate,which weakens the solid-solution strengthen and leads to a decreased yield strength.While the cooling rate is lower than 1.5?/s,the yield strength of samples stay at a high level since little lump phase could be observed with a low cooling rate.Therefore,due to the fact that the firing process in industry is holding at 800?900? and then cooling with 3.0?4.0?/s,it is understandable that there is a reduction in after-fire yield strength because of the formation of lump phase.In order to eliminate the lump phase,after firing at 815?and 8710C and then cooled by air cooling,samples are tempered at 300?600?.It is noted that the lump phase almost transforms into pearlite and the yield strength increase by 50?70MPa after tempered at 400?600?.In addition,tempering time has few effects on yield strength when tempered at 400?500?.Thus tempering process can be adopted to improve the after-fire strength.The heat treatment that direct heating at 500? for 5 min after air cooling from the firing temperature(815?871?)to 500? can also increase the yield strength.Obviously this firing process will not impact the original productivity.