Compared with niobium and vanadium, titanium has been regarded as a relative minor element in microalloyed (MA) steels. More recently, titanium compounds in MA steels have been recognised as having...

/pdf/titanium-microalloyed-steels-1qrgeqimnv.pdf

Titanium microalloyed steels

A kinetic model was developed using FactSage Macro Processing to simulate the re-oxidation of ultra-low carbon steel via different oxidising slags. The calculated results show good agreement with experimental laboratory thermal simulation data. Therefore, the model can be used to predict the change behaviour of slag-metal-inclusion in the re-oxidation reaction of liquid steel. It can provide prediction and guidance for an accurate secondary oxidation control process. During the slag re-oxidation process, when the oxygen in the steel is supersaturated and the slag is low in oxidation, it can easily form stick-like and dendritic shape inclusions of Al2O3 in steel. As the (FeO) content increases in slag, the oxygen transfer from slag to steel is evident, and the inclusion size increases, showing clusters and spherical shapes. In addition, supersaturated oxygen in steel easily forms unstable Al2O3-TiOx inclusions with [Ti]. As the components of liquid steel tend to be uniform, the Al2O3-TiOx inclusions will decompose and disappear, forming stable Al2O3 and TiO2 inclusions. The number of inclusions can be reduced by increasing the basicity and the ratio of CaO to Al2O3 in the initial slag.

/pdf/prediction-of-re-oxidation-behaviour-of-ultra-low-carbon-2p71sep2hf.pdf

Prediction of re-oxidation behaviour of ultra-low carbon steel by different slag series.

Full-scale water modelling studies combined with nail-dipping industrial trials at Shougang Jingtang (SGJT) Works were carried out in order to investigate the parameters that influence flow patterns in the continuous slab casting mould. The effects of casting speed/mould width combinations, argon bubbling flow rates, submerged entry nozzle (SEN) immersion depths and SEN geometries (port angle and bottom shape) on the flow patterns were examined. A CMI factor defined as casting speed/mould width ratio index is put forward to describe the combined effect of these parameters on the fluid flow patterns in the mould. The results show that the CMI and argon injection rates are the dominant factors that determine the flow pattern. On the basis of the results from this study, critical argon flow rates for different casting combinations have been evaluated. These findings provide operating guidelines for generating optimal flow patterns based on double roll flow with appropriate surface velocities in continuous sl...

Flow pattern control in continuous slab casting moulds: physical modelling and plant trials

Al2O3 and TiN inclusions in interstitial-free (IF) steel deteriorate the properties of the steel. To decrease the defects of cold-rolled sheet, it is important to clearly distinguish between the degrees of damage caused by these two inclusions on the surface quality of the steel. In this study, a nanoindenter was used to test the mechanical properties of the inclusions, and the distribution and size of the inclusions were obtained by scanning electron microscopy (SEM). It was found that when only mechanical properties are considered, TiN inclusions are more likely to cause defects than Al2O3 inclusions of the same size during the rolling process. However, Al2O3 inclusions are generally more inclined to cause defects in the rolling process than TiN inclusions because of their distribution characteristic in the thickness direction. The precipitation of Al2O3 and TiN was obtained through thermodynamical calculations. The growth laws of inclusions at different cooling rates were calculated by solidification and segregation models. The results show that the precipitation regularity is closely related to the distribution law of the inclusions in IF slabs along the thickness direction.

Comparison between the surface defects caused by Al 2 O 3 and TiN inclusions in interstitial-free steel auto sheets

The evolution of macro inclusions during continuous casting was investigated by large-area inclusion characterization using ASPEX and analysis of nozzle blockage deposits. Six kinds of inclusions over 5 μm were observed in samples taken from the tundish and the slabs: single alumina particles, alumina dendrites, refractory-related alumina, alumina associated with bubbles, alumina clusters and Al–Ti complex oxides. By examining the morphology of the nozzle blockage deposit, it was concluded that the refractory-related alumina in the slabs came from the decarburization layer washed away by the steel stream. Some of the alumina clusters that came from the nozzle blockage deposit dislodged by the steel flow, were formed by the agglomeration and sintering of 5–20 μm inclusions that were carried over from the tundish. Two kinds of Al–Ti oxides were found in the tundish, and their evolution mechanisms during the casting process were proposed.

Formation and evolution of macro inclusions in IF steels during continuous casting

Subsurface macro-inclusions and hooks are detrimental to the surface quality of deep-drawing steel sheets. However, little is known about the relationship between macro-inclusions and hooks. Thus, in this work, two ultralow carbon (ULC) steel slabs and two low carbon (LC) aluminum-killed steel slabs were sampled to study the relationship between hooks and subsurface macro-inclusions, which were detected on the cross-sections of steel samples with an area of 56058 mm2 using an automated scanning electron microscopy/energy-dispersive X-ray spectroscopy system. Results show that subsurface inclusions larger than 200 μm were almost entrapped by hook structures, whereas the location of other inclusions smaller than 200 μm had no obvious dependence on the location of solidified hooks. Furthermore, the number density (ND) of subsurface inclusions larger than 200 μm decreased from 0.02 to 0 cm−2 in ULC steel as the mean hook depth decreased from 1.57 to 1.01 mm. Similar trends were also observed in LC steel. In addition, the detected inclusions larger than 200 μm were concentrated in the region near the slab center (3/8 width-5/8 width), where hook depths were also larger than those at any other locations. Therefore, minimizing the hook depth is an effective way to reduce inclusion-induced sliver defects in deep-drawing steels.

Subsurface macro-inclusions and solidified hook character in aluminum-killed deep-drawing steel slabs

The effect of oxidizing slag on cleanliness of IF steel produced by ShouGang during ladle holding process was investigated by comparing the differences of slag composition, steel composition and inclusion morphology between two processes of different slag oxidability(Process-A < Process-B). The results showed that, the reoxidation of slag to steel was observed during holding process, and Process-A with the lower slag oxidability was more helpful to improve the cleanliness of molten steel during holding process by comparing the T.O of the two processes. Due to the slag reoxidation during holding process, the large petal shaped and dendritic Al2O3 inclusions generated for Process-A, and both the numbers of Al2O3 and Al2O3-TixO inclusions increased significantly for Process-B. In order to slow down reoxidation rate and improve the ability of slag to absorb inclusions, (FeO) content should be reduced and (CaO)/(Al2O3) should be increased in the case of liquid slag. In conclusion, whether prolonging ladle holding time should depend on (FeO) content, (CaO)/(Al2O3) etc. parameters of slag and initial T.O of molten steel.

Effect of Oxidizing Slag on Cleanliness of IF Steel during Ladle Holding Process

Macro-inclusions in low carbon, aluminium-killed steel slabs were characterised by step-machining within a 10 mm zone from the slab surface using an ASPEX automatic inclusion analyzer. Dendritic structures within the cross-section of slabs were examined. The results show that alumina clusters and alumina associated with bubbles are the dominant macro-inclusions. Along the slab width direction, macro-inclusions were mostly found at the slab centre because of the deeper hooks and freezing meniscus surrounding the submerged entry nozzle. In terms of slab thickness, inclusions were mainly concentrated within the zone 3.5–6 mm from the top of the slab surface, where the columnar dendrites showed a relatively small inclination angle, indicating small cross-flow velocities at the solidification front. The number density of macro-inclusions were strongly dependent on the washing effect produced by the flow velocity. High speed casting promotes this behaviour and improves the surface quality of the slabs.

Distribution of macro-inclusions in low carbon aluminium-killed steel slabs

The refinement of coarse grained austenite in a Nb–V–Ti microalloyed steel during roughing rolling of a medium thin slab casting and direct rolling process was investigated by considering the austenite grain size distribution. The evolution of grain size distribution was analysed by taking into account dynamic recrystallisation (DRX), static recrystallisation (SRX) and metadynamic recrystallisation (MDRX), strain induced precipitation and retained strain. The results indicated that the initial coarse austenite was significantly refined during roughing rolling. Static recrystallisation is the main post-dynamic softening mechanism during all interpass intervals, but SRX+MDRX may also take place at certain times. The microstructural homogeneity of microalloyed steel produced in this work is better than that in steel produced by thin slab and direct rolling (TSDR) process. This is associated with a higher reheating temperature, a higher entry temperature and a larger reduction due to increased slab thickness ...

Linping Li

Papers

Subsurface macro-inclusions and solidified hook character in aluminum-killed deep-drawing steel slabs

Flow pattern control in continuous slab casting moulds: physical modelling and plant trials

Effect of Oxidizing Slag on Cleanliness of IF Steel during Ladle Holding Process

Distribution of macro-inclusions in low carbon aluminium-killed steel slabs

Refinement of coarse grained austenite in Nb–V–Ti microalloyed steel during roughing rolling