| 其他摘要 | Grain refinement not only can enhace the strength of steel, but also can improve the toughness. The deformation induced ferrite transformation (DIFT) has attracted considerable interests and great efforts because of its effective grain refinement and low costs. Unfortunately, many works are focused on the DIFT below austenite-ferrite equilibrium transformation temperature Ae3 recently, less above Ae3 temperautre (to distinguish it from the DIFT below Ae3 temperature，we call the DIFT that takes place above Ae3 temperature as high temperature DIFT). When the DIFT temperature is above Ae3, the mechanism of DIFT can be investigated clearly by excluding the influence of supercooling. The aim of this work is to systemically investigate the effect of deformation on the high temperature DIFT in a low carbon steel Q235. The main research of this work is listed as following:
1. A series of unidirectional compression tests of a low carbon steel Q235 were performed on a Gleeble 3500 thermal simulator, and the influence of strain, strain rate and deformation temperature (above the austenite-ferrite equilibrium transformation temperature Ae3) on DIFT has been examined. The results show that the DIFT can take place above Ae3 temperature, the higher the strain rate and the strain, the more favorable for DIFT. When and s-1, the upper limit temperature of DIFT can be elevated to 945℃ (Ae3+98℃). An important phenomenon is found that when deformation is loaded between 870 to 920℃, the total stress is decreased with deformation temperature increasing. However, when deformation is loaded between 830 to 870℃, the volume fraction of deformation induced ferrite (DIF) is increased for DIFT and ferrite dynamic recystalliation. The total stress is increased with deformation temperature increasing.
2. The microstructure, carbon concentration, nanoindentation hardness and the elastic modulus of DIF and proeutectoid ferrtie were determined by optical microscope, scanning electron microscope (SEM), X-ray diffraction (XRD) and transmission electron microscope (TEM). Compared with the diffraction peak of proeutectoid ferrite, the carbon concentration was significantly supersaturated in the deformation induced ferrite grains，the diffraction peak of DIF was shift to a low angle in XRD analysis, and the nanoindentation hardness and elastic modulus of DIF is much higher than that of proeutectoid ferrite for containing nanometer martensite flakes. After annealing at 700℃ for 1h, fine carbides precipitate due to the decomposition of the nanometer martensite flakes.
3. The thermal stability of high temperature DIF was studied systematically through the post-treatments such as isothermally hot holding above Ae3 temperature, different cooling rates after deformation and tempering temperatures. The results show that the DIF volume fraction decreases during isothermally hot holding and finally disappears. The DIF grains were shown rapid growing at both low tempering temperatures (＜200℃) and high tempering temperatures(500～700℃) and relatively stable at middle tempering temperatures(200～500℃). Compared with the microstructure of DIF with martensite in quenched specimens, the proeutectoid ferrite and pearlite appear in controlled cooling specimens at low cooling rates (i.e. 10℃/s and 1℃/s). It revealed that the high temperature DIF is not stable and hardly conserved in hot mills.
4. A series of multipass compression tests for the low carbon steel were performed, and the influence of compressive deformation on DIF was examined. It shows that the DIF grains can be refined by multipass deformation with large reduction. At the same strain, these factors, such as less passes, short multipass deformation, low deformation temperature and the strain close to critical value ( ) in the first pass deformation, are beneficial for refining the DIF grains. |
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