Materials Science Forum Vol. 1105

Abstract: Every year digitalization is taking a bigger role in the steel industry. Models for predicting metallurgical phenomena, roll forces and microstructure have been commonly used in development of novel steel grades. These individual models may predict certain phenomena thoroughly, but input values are usually based on an assumption or on a “good guess”. To produce reliable boundary conditions for these models of individual phenomena, a virtual rolling model is developed. This model computes the whole process of the hot strip mill from roughing to accelerated water cooling on a run-out table. Strip location and temperature evolution is calculated continuously. Thermal and thermo-mechanical (rolling stands) boundary conditions are according to process layout. Input data for the model is automatically read from raw process data. Rolling parameters are calculated using a coupled ARCPRESS model, which is developed by authors, and calculates normal and frictional shear stress distributions in the roll gap to predict roll forces and displacements of the work roll surface. Recrystallization is considered when calculating the flow stress of the rolled strip. Phase fractions during water cooling are calculated as well. The virtual rolling model minimizes the need for parameter speculation as all parameters are calculated throughout the process. All the input values are read from actual process data and the metallurgical and mechanical state of the strip are computed throughout the whole process. As required by the state-of-art virtual rolling model, this model is based on generally accepted theories and experimentally studied metallurgical and physical phenomena along with the thermo-mechanical response of the actual rolling process.

Abstract: For saving mass, to improve the payloads and the durability of commercial vehicles, it is necessary to increase the thickness range of PHS - Press Hardening Steel, replacing low strength steel grades on structural and saving parts. This work presents the industrial development of Heavy-Gaged PHS-Press hardening Steel (4-8mm) starting from the alloy design concept, especially adding niobium to promote a fine and homogeneous microstructure. The development included hot forming simulations, in order to achieve martensitic microstructure through the thickness, correlating with cooling rates and mechanical characteristics, after quenching, as well as welding qualification and fatigue evaluation. Furthermore, were presented the characterization of 22MnB5(Nb+Mo) steel after industrial hot stamping that were evaluated mechanical properties, using the Vickers 10Kgf and the relationship between the microstructure and the thermodynamic aspects.

Abstract: The effect of Ti/Al ratio (mass%) on the evolution of the microstructures after casting and hot rolling of Ca treated 441 dual stabilized ferritic stainless steel (FSS) was investigated in order to understand its effects on grain refinement mechanism. Industrially cast and lab simulated hot rolled samples, were subjected to similar processing conditions but with different Ti/Al ratios of 2.4 and 7.8. The microstructures and inclusions were analysed by the OM, SEM-EDS, SEM-EBSD and AzTecFeature software. The results showed that the steel with higher Ti/Al ratio exhibited finer grains after continuous casting and hot rolling, i.e., the initial finer as-cast structure resulted in finer grains and less substructure after hot rolling. The steel with higher Ti/Al ratio contained more Ti-rich complex inclusions and precipitates (especially TiN), which led to more heterogeneous nucleation of the 𝜹-ferrite and grain refinement during solidification. On the contrary, the steel with low Ti/Al ratio exhibited coarser as-cast grain structure, less recrystallization and higher volume fraction of substructure after hot rolling. Therefore, it was deduced that the Ti/Al ratio is one of the essential parameters to achieving grain refinement in Ca treated 441 FSS during continuous casting.

Abstract: Back-annealing during the hot dip galvanising (HDG) process was applied to heavily cold-worked low C-microalloyed VN steels in an attempt to achieve a yield strength of 700 MPa and a minimum A₈₀ elongation of 10 percent. HDG simulations were performed to compare recovery and recrystallisation kinetics in VN steel with that experienced in plain low C and Nb-Ti grades. Based on these results, an industrial melt was subjected to conventional hot rolling, cold rolling and back-annealing cycles during HDG. Precipitation in both VN and Nb-Ti steels extended the recovery window by about 75 degrees Celsius when compared to that found in low CMn steel. A temperature-time parameter M was used to compare the rates of softening. The M_rs (recrystallisation start) in both VN and Nb-Ti steels was 20.1 compared to 18 in a low CMn grade. The above properties were achieved by subjecting low C-microalloyed VN steel to low finishing and low coiling temperatures followed by back-annealing heavily cold-worked strip to a maximum temperature to prevent full recrystallisation. The softening rate during annealing is higher in both microalloyed steels than the CMn grade, with recrystallisation being completed more rapidly in the VN steel. Laboratory results implied insensitivity of restoration behaviour to hot rolling parameters whilst industrial results suggest that they are effective.

Abstract: The quench and partition process is a means to develop third-generation high-strength steels using many possible process variants. In this work, two variants of quench and partitioning heat treatments, one-step and two-step, were carried out for high Si and high Al steel alloys. The kinetics of isothermal transformation occurring during the one-step quench and partition process were analysed using dilatometry. Experimental analysis revealed the swing-back phenomenon in high Si steel, and the transformation characteristics above and below the M_s temperature differed. The high Al alloy resulted in higher retained austenite (19%) compared to high Si steel (17%) during the one-step quench and partition process. Aluminium addition favoured bainite formation more than silicon addition. A comparison of two heat treatment variants shows the two-step quench and partition heat treatment seemed preferable as it produced more retained austenite (22%) in the high Si steel.

Abstract: Austenite restoration during thermomechanical (TM) rolling of typical vanadium-microalloyed structural steels was studied to optimize strength in the as-rolled and air-cooled condition. Multi-pass plate rolling simulations were performed on V-N microalloyed and CMn steels to compare recrystallisation behaviour in various temperature regions. Included were a conventional schedule ending at high temperature and two TM schedules with mill exit temperatures in the intermediate and low austenite regions. Increasing delay periods after roughing enhance the suppression of recrystallisation after the start of finishing thereby increasing both nucleation site density and nucleation rate for ferrite formation and refinement in grain size. Good agreement was found between microstructures after industrial TM rolling and those obtained from laboratory simulations. Although precipitation of vanadium carbonitrides is an effective strengthening mechanism, appreciable gains in yield strength due to grain refinement can be achieved by rolling in the lower austenite region. Low nitrogen contents in V steels produce coarser final ferrite grain sizes and lower strengths probably due to a larger precipitate size. V-N steels display similar flow behaviour to CMn grades down to approximately 825°C at low to intermediate strain rates but may experience alternate regions of work hardening and dynamic softening at lower temperatures in austenite.

Abstract: The influence of moderate thermomechanical rolling and heat treatment time on the microstructure and mechanical properties of normalised Ni-containing Nb-Ti-V microalloyed plate steels was investigated. Conventional rolling (CR) and low austenitic temperature rolling (TM) schedules were applied to 25 and 55mm thick plates. Predicted plate thermal profiles were used to simulate normalising of 25 and 55mm thick plates at the mid-thickness. As expected, the microstructure after TM rolling was finer due to austenite sub-structure development below the no-recrystallisation temperature which increases the amount of ferrite nucleation sites. The tensile strength of the CR plates was generally higher due to a larger pearlite volume fraction resulting from slow cooling from a relatively coarse austenite. In 25mm plate, the presence of acicular ferrite after CR resulted in a higher yield strength than that obtained after TM rolling. Furnace residence times of 0.75min/mm and longer allowed for plates to be completely normalised – characterised by continuous, well-defined polygonal ferrite grain boundaries. Short furnace residence times restricted the plate temperature to below the end of the ferrite-to-austenite transformation (Ac₃), where untransformed ferrite is retained from the as-rolled microstructure and is characterized by poorly defined grain boundaries. Rolling practice and normalising residence time had little to no effect on impact toughness, but significantly influenced the normalised yield strength of thicker plate.

Abstract: The Fe-0.44%C-1.8%Si-1.3%Mn-0.82%Cr-0.28%Mo steel treated by the quenching-partitioning process showed a product of strength and elongation of 30 GPa×% with the yield stress of 1150 MPa. The influence of the partitioning time on the structure and mechanical properties is discussed. It is shown that the volume fraction of retained austenite depends on carbon content in the solid solution of primary martensite. The formation of bainite leads to no deterioration of mechanical properties.

Abstract: The effect of tempforming on strength and fracture toughness of a 0.4%C-2%Si-1%Cr- 1%Mo-VNb steel was examined. Tempering at a temperature of 600°C followed by plate rolling at the same temperature results in the formation of lamellar structure with a spacing of 72 nm between longitudinal boundaries and a lattice dislocation density of ~10¹⁵ m⁻² that enhances fracture toughness in normal direction of tempformed plate. The increase in the absorbed impact energy is attributed to delamination, which occurs in plains intersecting the propagation path of main crack that blunts crack tip.

Abstract: Effects of pre-strain and tempering on mechanical properties in high-strength martensitic steels were investigated. In this study, strain tempering (ST) and quenching and tempering (QT) martensitic steels were prepared, and their mechanical properties were studied. In the tensile tests at the deformation temperatures between 296 and 573 K, the ST sample increased both of tensile strength (TS) and uniform elongation (U.El) from 473 to 523 K whereas the QT sample increased U.El with little change of TS. From the in situ neutron diffraction experiments, stress partitioning to the bcc phase increased with an increase in the deformation temperature from 296 to 523 K. The difference of phase stress between the bcc and cementite phases decreased with increasing the temperature because of a decrease in the cementite strength. In the ST sample, Pre-straining of 0.5% increased YS at 296 K with slight work hardening. The initial value of dislocation density (ρ) decreased at 523 K but ρ increased significantly after yielding, leading to better combination of TS and U.El. The combinations of pre-strain, tempering, and deformation temperatures have changed ρ before deformation and the increase of ρ after yielding of the martensitic steels.