| Considering the insufficiency in the recent developed V-N microalloyed ferrite-pearlite hot rolled high-strength low-alloy steel (HSLA) H-shape steel, Nb-V-Ti-Mo microalloyed acicular ferrite HSLA pipeline steel, and Nb-Ti-Cr-Mo microalloyed bainite HSLA abrasion resistance steel, the compositions and plastic deformation mechanisms, the prediction of the microstructure and property, the repairing and remanufactuering were investigated, respectively. The primary conclusions are shown following:(1) The highest strength and lowest plasticity can be obtained in the V-N microsalloyed steel with 3.64 of V/N ratio in weight percentage. The better overall properies can be received in the steel with 0.19 wt.%C and V/N ratio being 7.14 in the range of 0.06-0.19 wt. %C.(2) The plastic deformation behavior under high temperatures of the high strength low alloy steel with ferrite-pearlite microstructure microalloyed by V and N elements is power-law creep. The plastic deformation mechanism at temperatures of 900-1150℃and shear strain rates of 0.001-15 s"1 is power-law creep by dislocation core diffusion and the plastic instabilities cannot be obtained. Under the above deformation conditions, the plastic deformation mechanism of the observed steel can be studied by plastic deformation mechanism map built based on the strain rate constitutive equations for the Fe-C alloys. The dynamic recrystallization (DRX) microstructure can be obtained at a low strain rate and temperatures of 900-1033℃. Increasing temperature to 1033℃, the DRX microstructure can be obtained at 0.1-100 s-1.(3) During the hot deformation of the high strength low alloy steel with ferrite-pearlite microstructure microalloyed by V and N elements, the relationship between a and V(C,N) being(010)V(C,N)//(011)α,αandγbeing [110]γ//[111]αcan be obtained. The IGF can be easily obtained by nucleating the ferrite on the V(C, N) particles. The contents of IGF increase with decreasing the phase transformation temperature, increasing the deformation degree, decreasing the soaking temperature or increasing the soaking time, and the peak values can be obtained when the temperatures are 650℃. The cooling rate and the equivalent strain of the hot rolled H-beam steel decrease gradually from the edge of the flange to the boundary between the flange and the wave (R). The obvious difference in the grain size of the ferrite in the flange 1/4 and 1/2 cannot be found, where the grain size of the ferrite is lower than that of the R. Little difference about the contents of pearlite in the different sections of the H-beam can be obtained, though the lamellar distance of pearlite increases from flange 1/4 to R. The V(C, N) particles in the morphology of spherical or block distributes on the ferrite. The content of V(C, N) particles increases from flange 1/4 to R. The proportion of the high-angle boundary in flange 1/4 and 1/2 is almost the same, where the proportion of the high-angle boundary is higher than R by 15%. Using on-line controlled cooling technique, the decreasing of the temperature difference and the grain size fluctuating on the different cross-section of the hot rolled H-beam steel is obtained successfully. Under such condition, the highest temperature difference by 80% and the grain size fluctuating by 64% can be received. Besides, the yield strength fluctuating and the residual stress decrease significantly. And the waves on the web can be removed.(4) The results about the prediction of the microstructure and properties of the submarine pipe steel with acicular ferrite microstructure microalloyed by Nb, Ti and Mo elements invesitigated by numerical simulation companied by finite element simulation show that the maximum difference of the yield strength and the yield strength between the predication and the measurement values are about 8.72% and 9.43%, respectively. Nevertheless, for the 80% sample, the maxiumum difference is lower than 6.80%. Therefore, the production process of submarine pipe line steel can be improved.(5) The nickel-based alloy coating deposited by plasma surfacing is consisted of y (Ni, Fe),(Fe, Cr)7C3 and (Fe, Cr)2B phases. The substructure is mainly dislocation. The hypoeutectic microstructure and the obvious component segregation can be seen in the deposited coating. Adding 30 wt.%Cr3C2 leads to increase the relative contents of (Fe, Cr)-rich phases and the occurrence of Cr3C2 and Ni4B3 phases. The hypereutectic microstructure can be obtained in the Cr3C2-strengthen nickel-based alloy coating. Adding 0.8 wt.% nanometerα-Al2O3 with rhombohedral lattice structure does not change the hypoeutectic microstructure and the relative contents between y (Ni, Fe) and (Fe, Cr)-rich phases in the nickel-based alloy coating, but decreases the relative content of (Fe, Cr)7C3 phase and increases that of (Fe, Cr)2B phase. Besides, adding 0.8 wt.% namometerα-Al2O3 with rhombohedral lattice structure leads to the occurrence of nanometerγ-Al2O3 with tetragonal lattice structure and promote the formation of the stacking fault. Adding Cr3C2 and nano-Al2O3 can decrease the component segregation in the nickel-based alloy coating. The impact abrasion resistance of the nickel-based alloy,30 wt.% Cr3C2 and 0.8 wt.% nano-Al2O3 strengthen nickel-based alloy coatings is 1.3,2.2 and 6 times to that of the Nb-Ti-Cr-Mo microalloyed NM450 high strength low alloy abrasion resistance steel, respectively. Considerating the economic fator further, it is suitable to use 0.8% nano-Al2O3-strengthen nickel-based alloy coating deposited by plasma surfacing to repair NM450 steel to reach the purpose to remanufacture.
|
PDF Full Text Request