To investigate the evolution of inclusions in high‐Al steel with addition of La, a series of laboratory experiments and thermodynamic calculations are performed, considering the reaction time and amount of La added. The main inclusions in the high‐Al steel without the addition of La are Al2O3, MnS, and Al2O3–MnS. The La treatment can efficiently modify Al2O3 to La–Al–O or La–O–S inclusions. For La additions less than 0.0041 wt%, the evolution route for the inclusion in high‐Al steel is Al2O3 → LaAl11O18 → LaAlO3 with an increase in reaction time. For high La additions, the evolution route for the Al2O3 inclusion is Al2O3 → LaAl11O18 → LaAlO3 → La2O2S → La2S3. The experimental results correlate with those of the thermodynamic analysis. Notably, excess La in high‐Al molten steel may consume O and S to form La oxysulfide and sulfide, respectively, which prevents the precipitation of MnS inclusion and promotes the formation of AlN inclusion during solidification.

Herein, the effect of Ti addition on the formation and evolution of inclusions in high‐Al transformation‐induced plasticity (TRIP) steel is investigated by performing a series of laboratory experiments and thermodynamic calculations for different quantities and sequences of Ti addition. Before the addition of Al and Ti, the main inclusions are spherical Mn–Si–Al–O oxide particles. The addition of Al transforms the Mn–Si–Al–O inclusions to Al2O3 inclusion. After the subsequent addition of 0.02 wt% Ti, the main inclusions are Al2O3, AlN, Al2O3–AlN, Al2O3–TiN, and Al2O3–AlN–TiN. However, after the subsequent addition of 0.05 or 0.12 wt% Ti to molten steel, the main inclusions are Al2O3, TiN, and Al2O3–TiN, and almost no AlN is observed. Using different addition sequences of Al and Ti to high‐Al TRIP steel does not result in significant differences in the types of inclusions. Adding Ti to molten steel does not transform Al2O3 to Ti‐oxides, whereas Al addition causes Ti‐oxides to be transformed to Al2O3. Importantly, adding over 0.05 wt% Ti to TRIP steel prevents the formation of AlN inclusion.

To determine the effect of the CaO/Al 2 O 3 ratio on desulphurization and inclusion evolution in lowdensity steel, a series of laboratory-scale experiments are performed. The results show that violent reaction occurs at the interface between refining slag and molten steel. As the slag-steel reaction progresses, (CaO) and (MgO) are reduced to [Ca] and [Mg] by dissolved [Al]. The deoxidation product Al 2 O 3 dissolves into the slag and accumulates at the slag-steel interface. The evolution process of the sulphide inclusions is CaS → MgS → MnS. The sulphide outer layer wraps AlN and spinel inclusions, floats up and is absorbed by the top slag. During refining process, the accumulated Al 2 O 3 at the slag-steel interface easily forms MgO•Al 2 O 3 . Finally, a dense spinel layer can be formed at the slag-steel interface, which inhibits the desulphurization reaction. Reducing the C/A ratio can reduce the formation of MgO•Al 2 O 3 and has a positive effect on the desulphurization in low-density steel. The influence of the C/A ratio on desulphurization kinetics should be considered in the design of slag composition.

The formation and evolution of nonmetallic inclusions in pipeline steel were investigated by SEM, EDS and INCA Feature Analysis System, with the industrial process of electric arc furnace → ladle furnace (LF) refining → vacuum degassing → continuous casting. The composition, size and amount of inclusions during refining process were discussed systematically. The results show that inclusions at each refining step are mainly small-particle inclusions (below 5 µm), and the total number of inclusions has been reduced significantly due to the refining effect of slag during LF refining. The calcium (Ca) treatment increases the amount of small inclusions. The types of inclusion are mainly Al2O3 and MnO–SiO2–Al2O3 before LF, and they are transformed into CaO–Al2O3, MgO–Al2O3 and CaO–MgO–Al2O3 during LF process. After Ca treatment, inclusions are changed to CaO–Al2O3–(CaS) and CaO–MgO–Al2O3–(CaS). Typical inclusions are still mainly CaO–Al2O3 and CaO–MgO–Al2O3 in tundish, but the composition of those inclusions has been changed and located to the low melting point region in ternary phase diagram. Such inclusions will further be removed as continuous casting approaches.

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The effect of Ti and La complex treatment on the formation and evolution of inclusions in high-Al transformation-induced plasticity steel is investigated by laboratory experiments and thermodynamic calculations, with consideration given to the amount of La added and the sequence of the addition of Ti. The main inclusions are Al 2 O 3 and AlN inclusions before the Ti and La additions. The AlN inclusions are transformed into TiN inclusions with the addition of 0.1 wt% Ti. After the subsequent addition of 0.007 wt% La, the main inclusions are LaAlO 3 , La 2 O 2 S, and TiN. With the addition of 0.06 wt% La to molten steel, the main inclusions are La 2 O 2 S, La 2 S 3 , and TiN. La may prevent the precipitation of TiN inclusions. Different addition sequences of Ti and La result in the same types of inclusions, with higher La addition. In the case of La addition before Ti, Ti can restrain the formation of La-containing inclusions, and the number density of inclusions increases.

Herein, the relationships among nitrogen content, inclusion characteristics, microstructure, and mechanical properties of low-density steel are investigated. The austenite grain size is refined with nitrogen addition to the low-density steel, and the ultimate tensile strength, plasticity, fracture toughness, and impact toughness are distinctly enhanced. However, the yield strength decreases due to a decrease in the ferrite content with nitrogen addition. AlN is determined to be the main inclusion in the high-nitrogen low-density steel, and the number percentage of AlN that is less than 2 μm reaches 84%, which promotes the refinement of austenite grains. In addition, aggregated AlN inclusions in the dimples of the steel are observed, which are generated during the smelting process, and the aggregated AlN inclusions tend to cause crack initiation in the low-density steel.

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