Hydrogen-based direct reduction of iron oxide at 700℃:Heterogeneity at pellet and microstructure scales

Steel production causes a third of all industrial CO_(2) emissions due to the use of carbon-based substances as reductants for iron ores,making it a key driver of global warming.Therefore,research efforts aim to replace these reductants with sustainably produced hydrogen.Hydrogen-based direct reduction(HyDR)is an attractive processing technology,given that direct reduction(DR)furnaces are routinely operated in the steel industry but with CH_(4) or CO as reductants.Hydrogen diffuses considerably faster through shaft-furnace pellet agglomerates than carbon-based reductants.However,the net reduction kinetics in HyDR remains extremely sluggish for high-quantity steel production,and the hydrogen consumption exceeds the stoichiometrically required amount substantially.Thus,the present study focused on the improved understanding of the influence of spatial gradients,morphology,and internal microstructures of ore pellets on reduction efficiency and metallization during HyDR.For this purpose,commercial DR pellets were investigated using synchrotron high-energy X-ray diffraction and electron microscopy in conjunction with electron backscatter diffraction and chemical probing.Revealing the interplay of different phases with internal interfaces,free surfaces,and associated nucleation and growth mechanisms provides a basis for developing tailored ore pellets that are highly suited for a fast and efficient HyDR.

Effect of volume fraction and mechanical stability of austenite on ductility of medium Mn steel

A hot-rolled medium Mn(0.2C5Mn)steel is annealed at 650℃ to produce an ultrafine-grained duplex microstructure with different austenite volume fractions by austenite reverted transformation(ART)annealing,and the orientation relationship strictly obeys K-S orientation relationship before deformation.Tensile tests are carried out in a temperature range from-196 to 400℃ to examine the effects of the austenite volume fraction and the deformation temperature on the tensile properties and the austenite stability.Microstructural observations reveal that the metastable austenite gradually transformed into a-martensite,which is controlled by the deformation strain,the temperature and the austenite volume fraction.Both strain hardening behavior and ductility of the studied steel are dependent on austenite volume fraction and deformation temperature significantly.The stress-strain curves of ART-annealed 0.2C5Mn steel assume an S shape and a very large work hardening rate of about 10 GPa is obtained at liquid nitrogen deformation temperature.Based on the experimental data,a quantitative relation is proposed to describe the ductility dependence on both the austenite volume fraction and its mechanical stability.

Current Challenges and Opportunities Toward Understanding Hydrogen Embrittlement Mechanisms in Advanced High-Strength Steels: A Review

Hydrogen embrittlement(HE)is one of the most dangerous yet most elusive embrittlement problems in metallic materials.Advanced high-strength steels(AHSS)are particularly prone to HE,as evidenced by the serious degradation of their load-bearing capacity with the presence of typically only a few parts-per-million H.This strongly impedes their further development and application and could set an abrupt halt for the weight reduction strategies pursued globally in the automotive industry.It is thus important to understand the HE mechanisms in this material class,in order to develop effective H-resistant strategies.Here,we review the related research in this field,with the purpose to highlight the recent progress,and more importantly,the current challenges toward understanding the fundamental HE mechanisms in modern AHSS.The review starts with a brief introduction of current HE models,followed by an overview of the state-of-the-art micromechanical testing techniques dedicated for HE study.Finally,the reported HE phenomena in different types of AHSS are critically reviewed.Focuses are particularly placed on two representative multiphase steels,i.e.,ferrite–martensite dual-phase steels and ferrite–austenite medium-Mn steels,with the aim to highlight the multiple dimensions of complexity of HE mechanisms in complex AHSS.Based on this,open scientific questions and the critical challenges in this field are discussed to guide future research efforts.

Segregation-assisted spinodal and transient spinodal phase separation at grain boundaries

Segregation to grain boundaries affects their cohesion,corrosion,and embrittlement and plays a critical role in heterogeneous nucleation.In order to quantitatively study segregation and low-dimensional phase separation at grain boundaries,here,we apply a density-based phase-field model.The current model describes the grain-boundary thermodynamic properties based on available bulk thermodynamic data,while the grain-boundary-density profile is obtained using atomistic simulations.To benchmark the performance of the model,Mn grain-boundary segregation in the Fe–Mn system is studied.3D simulation results are compared against atom probe tomography measurements conducted for three alloy compositions.We show that a continuous increase in the alloy composition results in a discontinuous jump in the segregation isotherm.The jump corresponds to a spinodal phase separation at grain boundary.For alloy compositions above the jump,we reveal an interfacial transient spinodal phase separation.The transient spinodal phenomenon opens opportunities for knowledge-based microstructure design through the chemical manipulation of grain boundaries.The proposed density-based model provides a powerful tool to study thermodynamics and kinetics of segregation and phase changes at grain boundaries.