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Herein, a novel dual-strengthening technology that combines the external second-phase nanoparticle addition method with the hot compression deformation tests was carried out to investigate the effects of surface-modified nanoparticles (NPs) and deformation conditions on the characteristics of inclusions and the evolution of microstructure in the micro-alloyed (MA) medium carbon steels. The results revealed that the inclusions in the steels were divided into single phase inclusions and composite phase inclusions. With the addition of NPs, Al2O3 inclusions were replaced by MgAl2O4 spinel, and the composite inclusions mainly consisted of TiN–VN, MgAl2O4–TiN and MgAl2O4–TiN–MnS inclusions, and the same types of inclusions became significantly smaller in size. According to the EDS results, some of the inclusions acted to induce ferrite nucleation, and the MgAl2O4–TiN–MnS composite inclusion was more favorable for the nucleation of acicular ferrite (AF). The deformation induced ferrite (DIF) started to form at the deformation temperature of 750 °C and the maximum stress peak in the NPs-bearing steel was 516 MPa, which was 28.4% higher than that in the original steel. The proportion of ferrite increased as the deformation temperature decreased. The dispersed NPs contributed to the ferrite nucleation and the grain refinement as the inclusion-modifying agent, which improved the material properties, and the relevant strengthening mechanisms were explained.

A hot compression experiment was conducted on the High Strength Low Alloy (HSLA) steel Q690E, using Gleeble-3500 thermal/mechanical simulator, with a temperature of 950 °C and maximum compression amount of 80%. The deformation induced ferrite (DIF) was obtained under different compression amounts and cooling conditions. The stress–strain diagram during compression was mapped and the micro-hardness of the thermal simulated specimens measured. Results show that a 40% compression effectively accelerates the formation of DIF and refines the grain size. The grain sizes after 80% deformation are in a concentrating distribution ranges from 1 μm to 3 μm, and the average grain size is 2.34 μm. The formation of DIF leads to an obvious drop on the stress–strain curve and makes it shows a hump-like distribution characteristic, and also leads to a decrease of microhardness. The mechanism of DIFT is analyzed. A mathematical model is established between the effective grain boundary area and the compression amount. And its calculations indicate that the increasing of the effective grain boundaries area caused by the compressive deformation is one of the essential reasons for DIFT. The ferrite phase transformation driving force can be increased by the dislocation energy, and the critical dislocation energy to drive DIFT in this experiment is calculated to be 17.56 J/mol. [ABSTRACT FROM AUTHOR]

Hot deformation of a continuously cast low alloyed steel is studied by means of hot compression and tensile tests by using a Gleeble® 1500 machine after austenitization in a wide range of strain rates and between 720-800 °C. The flow data are evaluated to obtain the strain rate sensitivity and the processing maps. A new calculation method is used, yielding on the instability parameter defined as κ which correlates well with the microstructural changes. The strain rate sensitivity does not predict any instability but all the others instability parameters do, including the new κ. Flow instability appears at high temperatures and low strain rates where pores are formed at the austenitic grain boundaries, causing a decay of ductility in the tensile test. During hot deformation more ferrite is formed than corresponding heat treatments without deformation. In these conditions, the deformation is concentrated in the softer ferrite phase. The deformation induced ferrite deforms by dynamic recovery forming new grains as revealed by metallography and is correlated with low coefficients of power dissipation. The sinh type constitutive equation represents the flow data well with a stress exponent n = 4.1 and an apparent activation energy Q = 218 kJ/mol. [ABSTRACT FROM AUTHOR]

The current study aims to estimate the influence of enhanced nitrogen on the hot ductility of medium carbon microalloyed steel. For this purpose, hot tensile tests were carried out at temperatures rangeing from 700°C-1000°C at a true strain rate of 0.001s-1. The fracture surfaces and their neighboring precipitates and matrix microstructures "frozen" in tensile temperatures were observed. The dependence of hot ductility on the fracture mode and in situ microstructural changes were discussed. The results indicate that raising the nitrogen content from 0.003% to 0.014% and 0.021% was found to deteriorate the ductility as the obtained ductility trough became deeper and wider. The trough deepening caused by the addition of nitrogen was due to the formation of film-like ferrite and fine VN precipitation along the austenite grain boundaries promoting low ductility intergranular failure. On the other hand, the retarded dynamic recrystallization, the promoted deformation induced ferrite formation and precipitation at higher temperatures by enhanced nitrogen were regarded as the possible reasons for a wider trough. In summary, the above results indicate the hot ductility of medium carbon microalloyed steels is weakened to some extent by enhanced nitrogen and their windows suitable for continuous casting should be schemed very carefully. [ABSTRACT FROM AUTHOR]

The hot ductility of four plain carbon steels containing different levels of V have been obtained after solution treating at 1330 °C, cooling to test temperatures in the range 700–1000 °C, and testing at a strain rate of 3×10−3 s−1. The obtained ductility troughs were compared with that obtained from Nb containing steel. Raising the V level from 0.009–0.1% (through 0.048 and 0.07%) was found to deteriorate the ductility as the obtained trough becomes wider and deeper with increasing V content, due to the increased amount of VN precipitation in the steel, but the lowest ductility obtained does not approach the low value exhibited by Nb containing steel. The recovery of ductility at high temperature side of the trough corresponds to dynamic recrystallization and the greater the degree of precipitation, the higher the recrystallization temperature. Ductility improved as well when the test temperature falls so that there is a sufficient transformation induced ferrite present to prevent strain concentration. [Copyright &y& Elsevier]