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

This paper aims at the phase identification and quantification in transformation induced plasticity duplex stainless steel (TDSS) base and weld metal containing ferrite, austenite, and martensite. Light optical microscopy (LOM) and electron backscatter diffraction (EBSD) analysis were employed to analyze phases. Samples were either mechanically or electrolytically polished to study the effect of the preparation technique. Mechanical polishing produced up to 26% strain-induced martensite. Electrolytic polishing with 150 g citric acid, 300 g distilled water, 600 mL H3PO4, and 450 mL H2SO4 resulted in martensite free surfaces, providing high-quality samples for EBSD analysis. Martensite identification was challenging both with LOM, due to the similar etching response of ferrite and martensite, and with EBSD, due to the similar lattice structures of ferrite and martensite. An optimized Beraha color etching procedure was developed that etched martensite distinctively. A novel step-by-step EBSD methodology was also introduced considering grain size and orientation, which successfully identified and quantified martensite as well as ferrite and austenite in the studied TDSS. Although here applied to a TDSS, the presented EBSD methodology is general and can, in combination with knowledge of the metallurgy of the specific material and with suitable adaption, be applied to a multitude of multiphase materials. It is also general in the sense that it can be used for base material and weld metals as well as additive manufactured materials.

Identification and quantification of martensite in ferritic-austenitic stainless steels and welds

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

Phase identification and quantification in multiphase steels has been a major challenge due to the preparation effects and the specificities of the analysis technique. This work aimed to identify and quantify the retained austenite fraction in 15XX class steel, in addition to verifying the best metallographic preparation methodology. X-ray diffraction (XRD) and electron backscatter diffraction (EBSD) methods were used to identify and quantify retained austenite. It was possible to observe the difference between the methods of analysis adopted and to conclude that the XRD quantification results are less affected by surface preparation. Under the conditions reported in this study, electrolytic polishing was found to be the best methodology of surface preparation for EBSD.

D. Pérez Escobar

Papers

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

Influence of surface preparation method on retained austenite quantification