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The steels of the current study contain 0.3% carbon with different amounts of Cr, and Mo, in addition to W. The steels were cast as Y-blocks. Cast slabs with 150x100x35 mm dimensions were then subjected to 4-steps directional hot forging into 15 mm diameter round bars in the temperature range 1200-830 oC. Austenite grain refinement was taken place by the 4-steps cross-sectional reductions, where it secures fine lath martensite combined with crammed nano needle-like Cr23W2C6 carbides between the martensite laths. A quenching tempering cycle was carried on the forged bars at 250 oC for 25 min. and then air cooled to modify the martensite laths and reorganize the nano needle-like carbides to modify the mechanical properties. The optical microscopic investigation confirms extensive grain refining of the martensite laths. However, scanning electron microscope (SEM) photos at high magnifications reveal. The structure composed mainly of fine laths martensite with nano needle-like structured carbides with 50 nm diameter crammed between the martensite laths. Little amounts of retained austenite (RA) phase was located tightening on the boundaries of martensite laths as white nano films. A blend of -fine laths martensite and nano films of retained austenite (RA) phases in addition to the crammed nano needle-like structured carbides are working cooperatively for ultra-high strength creation. The tensile test specimens possessed continuous yielding, as the predominant microstructure contains a martensite phase. Strength increases continuously due to tungsten (W) addition up to 1.6%. During tempering, some of the nano needle-like Cr21W2C6 carbides, which were crammed between the martensite laths have a chance to migrate out from the martensite laths. The migrated nano needle carbides favor to coagulate and stick to each other to form 3D nano bulk-size spheroidal shaped carbides sizing 200 nm and precipitating on the martensite laths surface. [ABSTRACT FROM AUTHOR]

Five quenching temperatures were designed to investigate the relationship between volume fraction of retained austenite and microstructure, mechanical properties of a medium carbon quenching and partitioning (Q&P) steel. It is normally accepted that the optimal mechanical property of Q&P steels is obtained by quenching at optimum temperature, which corresponds to the maximum amount of retained austenite. However, the present work clarified that maximum volume fraction of retained austenite in Q&P steels is indeed unrelated to the optimum mechanical property of steels. The microstructure consisting of coarse martensite laths, dispersive bainite and thin retained austenite films was obtained by quenching at fine martensite start temperature (FMs), resulting in the optimum comprehensive mechanical property of Q&P steel with the better elongation at the expense of slightly smaller yield strength and tensile strength. [ABSTRACT FROM AUTHOR]

Thermo-mechanical controlled process (TMCP) of 30MnCr22 oil well pipes was investigated by experimental simulation. The results showed that controlled rolling of TMCP effectively exerted the comprehensive effects of grains refinement by recrystallization in piercing and continuous rolling and deformation strengthening by strains accumulation in the non-recrystallization zone of reducing deformation. And the key point of TMCP is that quenching realized the microstructural heritability of fine and strengthened austenite, which achieved the best refinement of martensite laths to be 170 nm, nanoscale carbides with the average size of 101 nm and dislocations with the high-density of 8.38 × 10 ^9 mm ^−2 , presenting a kind of complicated multi-layer microstructural characteristics. Finally, the synergistic strengthening effects on fine grain strengthening, laths refinement strengthening, dislocation strengthening and precipitation strengthening were realized by TMCP. Further, the synergistic strengthening mechanisms are explored to be that quenching enforced the interaction between martensite laths and dislocations, as well as the entanglement between θ -(Fe, Cr) _3 C and dislocations.

The microstructure of Nb-Ti micro-alloy hot stamping steels was observed and investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The toughness and tear properties were investigated by Kahn tear test. The thermodynamic software Thermo-Calc was used to calculate and analyze the behavior and composition of precipitates. The results show that Nb-Ti micro-alloy hot stamping steels with mass fraction 0.13% C are composed of martensite. Compared with traditional hot stamping steel (22MnB5), the Nb-Ti micro-alloy steel has finer prior austenite grains, martensite packets, and martensite laths. The tensile strength is more than 1500 MPa, tear strength is 1878 MPa, and the unit propagation energy is 436 kN·m-1. Nearly all Nb——Ti elements exist as precipitates during austenization at 950℃ and they can hinder the growth of austenite grains effectively. Two types of precipitates of different sizes are dispersed in the martensite matrix. The sizes of Ti(C, N) precipitates range from 100 nm to 200 nm, and their quantity is low. The nanometer-sized precipitates are carbides or nitrides of Ti and Nb, which can strengthen the martensite matrix and improve the intensity.

The flow deformation behavior and microstructure evolution of a novel tempered martensite ferritic steel G115 were systematically investigated under temperatures ranging from 625 to 675 °C and stain rates of 5.2 × 10−5–5.2 × 10−3 s−1. The strain dependent constitutive model was established to reflect the flow deformation behavior of G115 steel. The corresponding material constants and the activation energies for this steel were determined under the different strains (0.005–0.07). The flow stresses predicted by the exponential equation show a good agreement with the experimental data, which suggests the stress-strain relationship of G115 steel can be accurately described using the exponential equation. Fine M 23 C 6 carbides and Cu-rich phase particles have been characterized for G115 steel under different deformation conditions. The size of M 23 C 6 carbide increases by a factor of 1.6 while that of Cu-rich phase particles increases in 2.4 times when the temperature increases from 625 °C to 675 °C. In addition, the number density of Cu-rich phase particles decreased drastically at 675 °C. Regular martensite lath structure still existed at 625 and 650 °C, but breakup of the martensite laths occurred at 675 °C. This was mainly strongly related to the evolution of M 23 C 6 carbides and Cu-rich phase particles. The fine Cu-rich phase particles with a high number density contributed to slowing down the degradation of microstructure in G115 steel. [ABSTRACT FROM AUTHOR]