The Precipitation Behaviour And Roles Of Secondary Phases In Medium Carbon Low Alloy Ultra-high Strength Steel

Base on Fe-Nb-V-Ti-Mo alloy system of compound micro-alloycarbonitride mathematical model is established, the precipitation in the austeniteanalysed quantitatively. By optical microscope, scanning microscope, transmissionelectron microscopy, mechanical test, phase analysis and Gleeble thermal simulationtesting machine, studies the deformation, deformation temperature, cooling rate afterhot rolling on the microstructure and mechanical properties of carbon in low alloysteel, which provide theoretical basis and experimental basis to produce or furtherstudy the kind of ultra-high strength steel.Based on the multiple micro-alloy carbonitride precipitation compositehypothesis. In MATLAB development environment, a mathematical thermodynamicmodel which simulates complex precipitation behaviour quantitatively in austenite ofultra-high strength low alloy steel has been developed on the basis of Henry’s law ofdilute regular solution and thermodynamic models of Adrian. By the model,calculates the solid solution temperature of the multiple micro-alloy carbonitride, andthe estimation of the equilibrium compositions of the austenite and carbonitride, aswell as the mole fraction of each phase in Nb-V-Ti-Mo micro-alloy steels at differenttemperatures and different chemical compositions is simulated and analyzed. Thevariation of the mean size and the volume fraction with temperature and time can beobtained by model calculation. The results shows that Mo element can hardlyprecipitation in the austenite, but it improves micro-alloying elements in austeniticsolid solubility, and it reduces the activity of micro-alloying and interstitial elements,and then greatly delay the process of precipitation, reduces the volume fraction ofprecipitates, decreases mean radius of precipitate particles.After controlled rolling+water quenching+250℃tempering, MC type carbideparticles in low alloy steel are precipitated in the process of continuous precipitationand dispersed in the matrix, and to (Nb,Ti)(C,N). With the increase of deformation,the volume fraction of MC type carbide particles are increased, especially nanoscale(1～10nm) particles significantly increased, there is almost no large particles, theaverage particle size is reduced, wich refines the matrix grain significantly andcorresponding to martensite lath after quenching. In the end, the strength, plasticityand impact toughness of low alloy steel are improved obviously. And lowerdeformation temperature also increases the volume fraction of MC type carbideparticles, decreases the size of precipitation particles, refines austenite grain and themartensite lath after quenching, resulting in the strength and impact toughness of steelare increased. However, the M23C6phase at grain boundary is formed easily at lowdeformation temperature, which reduces the impact toughness of steel.Meantime, with the decrease of cooling rate after hot rolling, the coarsening phenomenon of MC type carbide particles have happened, which pins austenite-ferriteboundary and the dislocation. Its pinning effect gradually disappeare, the matrix grainbegins to grow, the dislocation density drastically reduce, the effect of MC typeparticles refine grain and dislocation strengthening significantly decreased, but thenumber of small size of10nm or less particles is obviously increased, and theproportion of Mo increases, the effect of precipitation strengthening of MC typepariticle is enhanced. However, with the decrease of cooling rate after hot rolling, thenumber of the large size particle increases in low alloy steel, resulting in the risk ofmicro-cracks propagation fracture increases in the vicinity of them. Meanwhile, the(Fe,Mo)23C6phase at the grain boundary also increases slightly, the mechanicalproperties rapidly worsens.