Abstract:Nb, V, and Ti were added to free-cutting steel to improve its mechanical properties by means of precipitation strengthening and fine grain strengthening. The process parameters during the hot deformation of Nb-V-Ti free-cutting steel were studied at strain rates of 0.01–10 s−1 and temperatures of 850–1250 °C. The isothermal compression test results showed that the temperature rise at low deformation temperature and high strain rate has a great influence on the softening of the steel. The processing maps at str… Show more
“…In the case of MnS precipitates (Figure 13), the simulation showed that these only appeared at 1100 °C, while at 1000 °C, their size was negligible, which, during cooling, may directly affect the formation of M23C6 particles, as they are affected by S content. The precipitation models [19,20] indicated that, at 1100 • C, the hot forging deformation produced dynamic recrystallization. As Figure 2 indicates, the upper zone of the steel bar suffered a larger strain difference from zero to~8% which, combined with a more pronounced quenching effect (the surface in direct contact with the quenching fluid), resulted in more pronounced differences in the hardness curve (Figure 3).…”
The role of alloying elements such as Cr, Mo and Mn on low-alloy 8620 steel during hot forging operations is not yet clear, as, during deformation in the 1000~1100 °C temperature range, the austenite grain size remains small, ensuring the capacity of the forged part to be subsequently modified by surface hardening procedures. This work analyzed a deformed bar considering hardness at different geometry zones, along with SEM and TEM microstructures of previous austenite grains and lamellar martensite spacing. Moreover, Thermocalc simulations of M7C3, M23C6 and MnS precipitation were combined with Design of Experiments (DOE) in order to detect the sensitivity and significant variables. The values of the alloying elements’ percentages were drastically modified, as nominal values did not produce precipitation, and segregation at the austenite matrix may have been responsible for short-term, nanometric precipitates producing grain growth inhibition.
“…In the case of MnS precipitates (Figure 13), the simulation showed that these only appeared at 1100 °C, while at 1000 °C, their size was negligible, which, during cooling, may directly affect the formation of M23C6 particles, as they are affected by S content. The precipitation models [19,20] indicated that, at 1100 • C, the hot forging deformation produced dynamic recrystallization. As Figure 2 indicates, the upper zone of the steel bar suffered a larger strain difference from zero to~8% which, combined with a more pronounced quenching effect (the surface in direct contact with the quenching fluid), resulted in more pronounced differences in the hardness curve (Figure 3).…”
The role of alloying elements such as Cr, Mo and Mn on low-alloy 8620 steel during hot forging operations is not yet clear, as, during deformation in the 1000~1100 °C temperature range, the austenite grain size remains small, ensuring the capacity of the forged part to be subsequently modified by surface hardening procedures. This work analyzed a deformed bar considering hardness at different geometry zones, along with SEM and TEM microstructures of previous austenite grains and lamellar martensite spacing. Moreover, Thermocalc simulations of M7C3, M23C6 and MnS precipitation were combined with Design of Experiments (DOE) in order to detect the sensitivity and significant variables. The values of the alloying elements’ percentages were drastically modified, as nominal values did not produce precipitation, and segregation at the austenite matrix may have been responsible for short-term, nanometric precipitates producing grain growth inhibition.
“…As VC forms at low PSTs, it can interact with DRX during hot working conditions. Several investigations have shown that V carbonitrides can effectively pin the grain boundaries of austenite and postpone or even inhibit DRX in microalloyed steels [23,24]. Figure 5. TTP diagram of the studied alloy developed by using JMatPro 7.…”
Hot compression tests were carried out on a quench and partitioning steel in the temperature range of 850-1050 °C and strain rates of 10 -3 -1 s -1. The single-peak flow curves were appeared at high strain rates and low temperatures and the multi-peaks at the opposite condition. It was found that the mechanism of dynamic recrystallization (DRX) changes by passing from 950 °C due to dynamic precipitation of carbide particles. The microstructural characterizations using optical microscopy and electron back scattered diffraction (EBSD) techniques revealed various mechanisms of DRX in different deformation conditions. EBSD results showed opposite variations of high and low angle grain boundaries with increasing deformation temperature and decreasing strain rate. By using the hyperbolic sine constitutive equation, the value of apparent activation energy at the typical strain of 0.5 was determined as 297.67 kJ/mol.
“…Advanced heavy steel plates with high strength and excellent cryogenic toughness are preferred as structural materials for ship hull. Especially for non-quenched and tempered ship plate steel, it is popular due to its economic applicability with fewer processes and low energy consumption [1][2][3]. In order to obtain excellent mechanical properties, non-quenched and tempered ship plate steel is often treated with microalloying or thermo-mechanical control process (TMCP) to compensate for the strength loss caused by the lack of quenching and tempering [4][5][6].…”
V-N microalloying treatment is an important way to improve the service performance of non-quenched and tempered ship plate steel. Herein, the influence of V(C, N) on the evolution of microstructure and improvement of mechanical properties was studied. In addition, the relationship between microstructure and mechanical properties of V-N microalloyed high strength ship plate steel was revealed. The results showed that the composite addition of V and N not only formed a fine dispersed precipitated phase, but more importantly, significantly refined the ferrite/pearlite microstructure, promoted the formation of intragranular acicular ferrite, increased the proportion of high angle grain boundaries, and decreased the kernel average misorientation value. The optimization of microstructure brought about by V-N microalloying achieved synchronous improvement of strength and cryogenic toughness. The impact energy of V-N microalloying ship plate steel increased from 97 J of V-N-free ship plate steel to 239 J at -40 ℃, and the impact fracture mode changed from brittle quasi-cleavage fracture to microvoid coalescence fracture with a large number of equiaxial dimples.
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