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

Effect of subsequent heating on the microstructure and mechanical properties of hot rolled niobium microalloyed steel was researched in this paper. Low carbon steel was microalloyed with 0.048 % niobium and it was heated at 1150 C followed by cooling in the air. Researches were performed on two types of samples: low carbon steel microalloyed with niobium and subsequently heated microalloyed steel in the rolling direction as well as in the direction perpendicular to the rolling direction. Mechanical properties of all samples were determined by the static tensile test at testing rate of 5 mm/min. After that their microstructure was observed by scanning electron microscope. Results have shown a significant effect of subsequent heating on the microstructure, i.e. grain size as well as on the mechanical properties.

Effect of subsequent heating on the microstructure and mechanical properties of Nb microalloyed 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.

C. S. Batista Castro

Papers

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