Non-metallic inclusions (NMIs) play a key role in many important properties of steel, influencing both processing and application of steel products. In this work, the current understanding of the origin and classification of NMIs is reviewed, highlighting the dramatic development of the last decades. This includes the discussion of the thermodynamics of inclusion formation and the current knowledge on the effects of melt shop processing variable on NMIs composition, amount and size distribution. The development of inclusion engineering – tailoring the process to obtain the desired NMIs is highlighted and the development in selected areas – tire cord, springs and bearing steels as well as prevention of nozzle clogging – is used to illustrate this development. The promising field of “oxide metallurgy” is also discussed in the context of inclusion engineering. Finally, the difficulties in meaningfully characterizing and quantifying NMIs are briefly commented. In summary it is concluded that inclusion control in steels has evolved significantly in the last decades. This is due to the progress in understanding the interplay between thermodynamics, steel and slag chemical composition as well as melt shop processing. This made possible the tailoring of non-metallic inclusions via processing, to optimize steel properties. Nonetheless, some important problems remain and must still be solved to improve inclusion control and optimization.

Non-metallic inclusions in steels – origin and control

Most High Strength Low-Alloy (HSLA) steels rely on the control of the carbonitrides of Ti, Nb and V to achieve their properties. To properly control the distribution and size of precipitates the knowledge of the thermodynamics of the Fe–C–N-(Ti,Nb,V) systems is essential. Furthermore, understanding the kinetics of dissolution and precipitation of the carbonitrides is fundamental to properly design these steels and their processing. In this work, we emphasize Ti and Nb behavior in HSLA steels. First, the thermodynamic information concerning the stability of their carbonitrides in steel is reviewed, highlighting the difficulties associated with the variable C/N ratio of the carbonitrides and with the fact that there is also miscibility between Ti, Nb and V in the carbonitrides of the “NaCl” structure. These difficulties make the use of the compound energy formalism (CEF) and of computational thermodynamics extremely recommended. To demonstrate this, some frequently used solubility products are compared to results obtained with current computational thermodynamics databases. Currently, many efforts have been made to model the kinetics of dissolution and precipitation of these particles in steel. We focus on the potential advantages of using computational thermodynamics methods both for diffusion and for nucleation, growth and coalescence, in special using software based on Kampmann-Wagner numerical (KWN) theory. The potential of these methods is demonstrated in some selected applications. We emphasize the importance of interfacial energy in these models. Frequently the interfacial energy is used as an adjustable parameter in these models. Even when this is done, the models can be very useful for steel and process design, as long as the user is aware of the limitations associated to the adjustment.

Challenges and opportunities in thermodynamic and kinetic modeling microalloyed HSLA steels using computational thermodynamics

The mechanism of the heterogeneous nucleation of NbC on TiC precipitates was investigated systematically in this paper. The interfacial properties of NbC (100)/TiC (100), NbC (110)/TiC (110), and NbC (111)/TiC (111) interfaces were studied by first-principles calculations. The results showed that the NbC (111)/TiC (111) interface with the Nb–C bond is the most stable one, and the stability of interfaces with the C–Ti, Nb–Ti, and C–C bonds decreases in turn. The interface of the Nb/C-terminated and Third Layer (TL) stacking sequence (NCTL) has the largest adhesion work (10.15 J/m2) and the smallest equilibrium interface spacing (1.290 A). In the range of low niobium (Nb) chemical potential and high carbon (C) chemical potential, the nucleation of NbC on TiC precipitates takes precedence over the epitaxy growth in the coherent relationship of [ 1 1 ¯ 0 ] ( 111 ) NbC / / [ 1 1 ¯ 0 ] ( 111 ) TiC , while the nucleation of NbC on TiC precipitates is prior to the epitaxy growth in the coherent relationship of [ 001 ] ( 100 ) NbC / / [ 001 ] ( 100 ) TiC in the range of high niobium (Nb) chemical potential and low carbon (C) chemical potential. Besides, the characteristics of heterogeneous nucleation precipitates in Nb–Ti microalloyed steels were analyzed by transmission electron microscopy (TEM). The orientation relationship between the (Ti, Nb) C and (Nb, Ti) C precipitates follows [ 1 1 ¯ 0 ] ( 111 ) ( Nb , TiC ) / / [ 1 1 ¯ 0 ] ( 111 ) ( Ti , Nb ) C , which is consistent with the calculated result.

https://www.mdpi.com/2075-4701/9/12/1265/pdf

Investigation of NbC/TiC Heterogeneous Nucleation Interface by First-Principles and Experimental Methods

Steelmaking processes have reached a level of composition control in which variations of composition lower than 0.01% mass are easily achieved for most alloying elements in liquid ladles as large as 350 t. However, solidification processing in industrial conditions introduces inevitable segregation of solutes resulting in product heterogeneity. To mitigate this problem, alloy composition and steel processing steps are carefully evaluated and designed. The understanding of phase equilibria and diffusion processes is essential to understanding solidification segregation, in special in the micro segregation range. Computational thermodynamics (the CALPHAD approach) is very useful for that purpose. Macrosegregation is, furthermore, influenced by fluid flow during solidification. While external causes of fluid flow must be understood and controlled using other analytical tools, the current information about density (molar volume) variations incorporated in the CALPHAD approach can give important indications concerning buoyance forces. In this work, we present examples of the application of the CALPHAD method to the understanding and control of segregation in steel products. Furthermore, we point current limitations and desired improvements for industrial applications.

Using Computational Thermodynamics to Understand the Evolution of Solidification Segregation during Steel Processing

Analysis of niobium-rich phases in the submerged arc welds of high strength low alloy steel

The solidification path of a high-temperature processed (HTP) X65 sour service steel with 0.039 wt pct C, 0.09 wt pct Nb, and 0.54 wt pct Mn and its effect on the segregation, microstructure, and precipitation distribution of Ti,Nb(C,N) was studied using optical and confocal microscopy, scanning electron microscopy (SEM), and computational simulation (Thermo-Calc and DICTRA). The results were compared with those obtained for another commercial microalloyed steel, containing 0.09 wt pct C, 0.04 wt pct Nb, and 0.97 wt pct Mn. The results indicate that the main parameter that influences microsegregation is the C content, which has a large influence on the solidification path. The difference in segregation between different positions in industrial continuous cast slabs of the steels was also observed, as expected. The larger solidification interval (TL-TS) of the commercial microalloyed steel indicates the formation of a solidification front that has higher solute concentration than the X65 HTP sour service steel, which concurs with the higher macro- and microsegregation observed.

Correlation of the Solidification Path with As-Cast Microstructure and Precipitation of Ti,Nb(C,N) on a High-Temperature Processed Steel

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EFEITO DO CAMINHO DE SOLIDIFICAÇÃO NA MICROESTRUTURA E NA PRECIPITAÇÃO DE (Ti,Nb)(C,N) EM UM AÇO HTP COM BAIXO Mn-ALTO Nb

This paper investigated the formation of inclusions in a steel HTP (High Temperature Processing) API X65, sour service, submitted to the Ca injection process and continuously cast with dynamic soft reduction. The results obtained on the slabs were compared with a liquid sample taken from the tundish. Inclusion engineering was supported by scanning electron microscope mapping and specific methodology. There was a difference in the clean steel level (NL) and types of inclusions / particles between the samples, and the cause-effect relationships were analyzed and discussed. Most inclusions and particles detected do not favor hydrogen-induced cracking (HIC).

Ed Juarez Mendes Taiss

Papers

Correlation of the Solidification Path with As-Cast Microstructure and Precipitation of Ti,Nb(C,N) on a High-Temperature Processed Steel

EFEITO DO CAMINHO DE SOLIDIFICAÇÃO NA MICROESTRUTURA E NA PRECIPITAÇÃO DE (Ti,Nb)(C,N) EM UM AÇO HTP COM BAIXO Mn-ALTO Nb

Estudo das inclusões não-metálicas no processo de solidificação de um aço api x65 sour service htp