Effects Of Hot-rolling Process And Mn/Si Contents On Microstructure And Mechanical Properties Of Enameling Steels

Enameling steels used for manufacturing water heater tanks are studied in this thesis.It is required that enameling steels should have a high yield strength,since water tanks are subject to a certain pressure during service.Due to ferrite-austenite phase transformations occurring during the enamel-firing process,the mechanical properties of enameling steels might change after enamel-firing.The final mechanical properties of enameling steels are mainly determined by hot-rolling process,alloy composition and heat treatment,so it is of great significance to study the effects of these factors on the microstructure and mechanical properties of the enameling steels before and after firing.In this thesis,experiments combined with Gleeble physical simulation and finite-element simulation are adopted to investigate the enameling steels with different compositions and produced by different hot-rolling processes.The influence of dynamic strain induced transformation?DSIT?process with different start rolling temperatures,as well as Mn and Si contents on the microstructure and mechanical properties before and after heat treatment is studied.The main results are summarized as follows.Gleeble hot compression experiments show that when the sample is compressed at A_r3+10°C,a large amount of fine DSIT ferrite grains are produced,but the grain size is non-uniform.After compressed at A_r3+50°C,the average ferrite grain size becomes a bit larger,but the uniformity is better.With the increase of strain rate,more fine DSIT ferrite grains are produced.The finite element simulation of hot compression shows that the center of the sample has a higher stress and temperature increase caused by deformation,compared to the sample surface.Based on the Gleeble experiment results,the start rolling temperatures are taken to be 860°C and 900°C for the fourth and fifth passes of the hot-rolling process,respectively.The results show that the average grain size?⁵.5?m?and yield strength??420MPa?of samples with different start rolling temperatures are similar.After heat treated at815-871°C followed by sand cooling,the average grain size of the sample with the lower start rolling temperatures is⁸?m,and the yield strength?³40 MPa?,is about 60-70 MPa lower than the as-rolled samples.On the other hand,the average grain size of the sample with a high start rolling temperatures is⁶.5?m,and the yield strength reaches³70 MPa.According to the finite element simulation for the hot-rolling process,the possible reason for the above phenomena can be explained as follows.The sample with a low start rolling temperature is subject to a higher rolling force and effective stress,resulting in a higher stored strain energy in the as-rolled sample.During heat treatment,the release of the strain energy promotes the growth of the ferrite grains,which weakens the grain refinement strengthening and thus leads to the lower yield strength.The effect of different Mn and Si contents on microstructure and mechanical properties of the enameling steels is studied.For as-rolled samples,the sample with a low Mn content has obvious yield platforms and the yield strengths reach⁴20 MPa.The samples with a high Mn content contain white lump phase that transformed from the austenite,and have no obvious yield phenomenon and exhibit low yield strength below 400 MPa.After heat treatment,yield strengths of the samples with a low Mn content decrease due to the growth of ferrite grains.For the samples with a high Mn content,the lump phase decomposes and the stress-strain curves exhibit obvious yield platform with increasing yield strengths.In addition,for both samples with the low and high Mn contents,the yield strengths of as-rolled and heat treated samples increase with increasing Si content,due to the solid solution strengthening of Si element.