Study On Preparation Of Ultra-Fine Microstructure And Mechanical Property Of Mild And Low Alloy High Strength Steel

In this dissertation, the high strength steel with micron/submicron grade ultra-finegrain was producted by different temperatures and time annealing for15CrMnMoVmartensite microstructure steel after warm rolling process. After microhardness, tensileand notched impact test for rolling sample, the mechanical properties of the experimentalsteel after rolling were determined, analyzed the microstructure and fracture morphologyfor the steel after different treatments by means of OM, TEM, SEM.Experimental results show that: the high-strength steel plate can be obtained by warmrolling in500°C with75%and80%reduction and subsequent annealing process, thetensile strength range is from1296MPa to1423MPa, the yield strength range is from1294MPa to1421MPa, the elongation range is from8%to9.2%, it carried out thesecondary heating in the amount of50%reduction; Steel plate showed better stability oforganization during the annealing process, annealing at650°C or less, the recrystallizationprocess is relatively slow, the recrystallization activation energy about this experimentalsteel after rolling is338kJ/mol in its recrystallization annealing process.The high-strength steel plate can be obtained with having a high yield and a relativelyhigh toughness by warm rolling in600°C and650°C with60%reduction and subsequentannealing process, the tensile intensity range is from1305MPa to1544MPa, the yieldstrength range is from1303MPa to1498MPa, the elongation range is from8.1%to11.6%, the average grain size range is from0.1μm to0.25μm, the impact toughnessrange is from42J/cm2to75J/cm2.Room temperature tensile tests showed that tensile fracture in various process for15CrMnMoVA steel are always stratification fracture significant, between layers are alarge number of small pancake-shaped dimple-like organization, the rupture situation istypical of ductile fracture, the rationale cracking occurred with parallelling to the directionof the tensile axis; impact fracture at room temperature is also significant stratificationfracture and the stratification effects effectively impede crack propagation, the specialstructure make the experimental steel having a high strength, while still having a certaindegree of toughness.Cr, Mo, V and other alloying elements and their contents have a greater impact forthe ultra-fine organization tissue stability in the15CrMnMoV steel, in the annealingprocess, the effect of dispersion strengthening about alloying elements and special carbide precipitation result in ultrafine grain structure with a low the annealing temperaturesensitivity within a certain temperature range, the sub-micron ultra-fine grain structure canbe maintain in the higher temperature range.