Papers by Author: J.M. Rodriguez-Ibabe

Abstract: The aim of this work is to investigate the dissolution behavior of Nb in hot charging hot rolling configurations. To do so, an indirect experimental procedure is used to quantify the amount of Nb present in solution before rolling. The method is based on the effect of dissolved Nb on static recrystallization kinetics due to its solute drag effect. After different thermal cycles, simulating cold and hot charging conditions, double hit torsion tests have been performed with a 0.23%C steel microalloyed with 0.03% Nb. By means of these tests, the static softening behavior has been determined. Comparison of the recrystallization times allows indirect evaluation of the amount of Nb in solid solution after each treatment. The results have been correlated with the precipitation state of the samples.

Abstract: While the role of Nb in flat rolling of low carbon steels has been investigated in many works, the information about the use of Nb in rebar rolling of higher carbon grades is more limited. Rebar rolling presents differences relative to flat rolling that can affect the role of Nb, such as the application of higher number of rolling passes, higher strain rates, lower interpass times, and, consequently, enhanced adiabatic heating. Increasing the number of passes can contribute to austenite grain refinement. However, the high finishing temperatures in rebar rolling can lead also to significant austenite grain growth and microstructural heterogeneity development before phase transformation. This phenomenon will directly influence the final grain size and can also lead to the appearance of second hard phases in the final product. One of the options to avoid austenite grain growth is to add microalloying elements that retard grain growth kinetics, either in solid solution or as precipitates. This can open new roles for the application of Nb in rebar rolling. To analyze this, in this work laboratory torsion tests were performed with two 0.2%C steels microalloyed with two different Nb contents (0.029% and 0.015%). Soaking temperatures from 1100°C to 1250°C were applied to obtain different amounts of Nb in solid solution before grain growth study. The study shows that not only finish rolling temperature and cooling time, but also reheating temperature and the amount of Nb remaining in the form of undissolved precipitates are important factors controlling austenite grain growth.

Abstract: Nb is a classical microalloying element in the design of thermomechanical treatments in low carbon steels for flat products applications. However, its use in medium-high carbon grades, as occurs in hot rolling of bars, is less common. This is, in part, because of the diversity of characteristics required to those grades of steels and the less knowledge about the function of Nb in these cases. Consequently, less information is reported concerning thermo-mechanical processing of Nb microalloyed steels in long products applications. In this case, it is necessary to consider the singularities related to these processes, such as the short interpass times and the wide range of chemical compositions usually applied on these products. Short interpass times result in high strain rate values that can lead to metallurgical changes on the mechanisms occurring during the hot rolling must be considered. Moreover, the high Carbon contents applied in long products, usually between 0.20–0.40%, can influence the Nb solubility and precipitation in each stage of the process: prior to hot rolling on the reheating furnace, during the process and after hot rolling, depending on the cooling strategy adopted and on the post-rolling heat treatments that can be applied. This paper analyses different singularities associated with the use of Nb microalloying for long products. Several aspects, such as the partial or complete dissolution of the Nb prior to hot rolling, its role in the control of austenite microstructure and its incidence in the final microstructure and mechanical properties, will be considered.

Abstract: Niobium in steels can be used as substitutional solid solute or as precipitates. In solution, Nb exerts a solute drag effect delaying but usually not interrupting static recrystallization during hot rolling and increasing hardenability during post rolling cooling. Fine precipitates generated during rolling/cooling can interrupt recrystallization in finishing and precipitate in the ferrite matrix increasing strength. As a relatively fine precipitate Nb can also inhibit austenite grain growth during reheating.This paper highlights the idea that micro-additions of Nb, up to 0.02%, to ordinary commodity C-Mn structural steels can improve their strength. Industry trial results are presented giving evidence that mechanical properties can be improved, and a leaner/optimized chemistry may be used by adding these micro-quantities of Nb to otherwise ordinary commodity C-Mn steels.Microstructural analysis of a C-Mn vs. a leaner/optimized C-Mn-micro Nb steel along with austenite evolution modeling using MicroSim-PM^© helped identifying which type of metallurgical mechanisms are in-play resulting in higher strengths. This alternative composition has led to lower costs, lower CE, improved microstructure and a more stable process.

Abstract: During hot rolling, austenite recrystallization determines the grain size evolution and the extent of strain accumulation, and therefore, it can be used to tailor the microstructure and mechanical properties of the final product. However, at the moment, models describing the recrystallization kinetics of high-Mn steels are scarce and they do not take into account the effect of the alloying elements present in these steels. The aim of this work is to provide a quantitative model for the determination of the static recrystallization kinetics valid for a wide range of high-Mn steel compositions. Softening data determined for steels with different Mn (20 to 30%), Al (0 to 1.5%) and C (0.2 to 1%) levels at different strain, strain-rate and temperature conditions were analyzed. Static recrystallization of the investigated high-Mn steels follow Avrami’s law, with n Avrami exponents which are temperature dependent and lower than those determined for low C steels. A dependence of the t0.5 (time for 50% fractional softening) on the carbon content has been also observed and it was incorporated into an equation for the calculation of this parameter.

Abstract: Thermomechanical processing is a well-defined route to achieve adequate combinations of strength/toughness for a wide range of applications. Inside this family, that based on accumulated strain in austenite followed by proper accelerated cooling strategies is probably one of the most selected. Usually this has been achieved with steels with Nb additions and other microalloying combinations. Recently, it has been observed an increase in the relevance of Ti as microalloying element. In addition to the classical approaches based on its availability to avoid grain growth, Ti provides, mainly in near-net-shape technologies, additional possibilities in relation to austenite conditioning and precipitation hardening. Multiple factors intervene simultaneously (solidification rates, Ti amount, Ti/N ratio, interaction with Nb...) leading nowadays, to important difficulties to properly predict its behavior. This manuscript focuses on some of these issues.

Abstract: The microstructural refinement induced when the holding time between last deformation pass and accelerated cooling is reduced, affects the mechanical properties in low carbon Nb and Nb-Mo microalloyed steels. Plane strain compression tests were performed and mechanical property samples machined in order to quantify this effect using tensile and Charpy impact tests. A complete microstructural characterization was carried out using electron backscattered diffraction (EBSD) measuring unit size distributions and homogeneity of complex microstructures. The synergetic combination of Nb and Mo elements modifies the final microstructures and, therefore, affects the contribution of different strengthening mechanisms, such as substructure, precipitation hardening and dislocation density. Even though strength is not clearly affected by the reduction of the holding time after the last deformation pass, Charpy properties are considerably improved in the case of the Nb steel. The presence of MA islands in the Nb-Mo steel limits the beneficial effect of the microstructural refinement and toughness remains unmodified.

Abstract: Free cutting steels belong to a family associated with hot workability problems. This study has focused on the analysis of microstructural features located near the surface of the billet before and after reheating and how they can affect the nucleation/propagation of damage during initial hot working operations. The work has been done with free cutting steels containing sulfur contents ranging from 0.32 to 0.42%. The relevance of clustering of inclusions, their location at grain boundaries or triple points and the influence of softening mechanisms (as recrystallization) have been considered with the help of several parameters determined by image analysis techniques.

Abstract: Thin slab direct rolling (TSDR) technology has some important metallurgical differences compared to conventional routes. One of these differences deals with the combination of coarse as-cast austenite grains and the possibility of limited solubility of microalloyed addition at the entry of the rolling mill. The industrial tendency of increasing the strength and toughness of steel grades produced by TSDR route requires increased amounts of Nb addition to achieve a proper austenite conditioning prior to transformation. Nevertheless, an increase in the Nb content rises the risk of premature Nb(C,N) precipitation during early rolling stages. In order to avoid this, one possibility is to add Mo and limit the amount of Nb. This study analyzes the influence of different Mo contents on static recrystallization of Nb microalloyed steels. The analysis includes the evaluation of the incidence of the combined effect of Nb and Mo solute drag retardation in both heterogeneous recrystallization evolution and grain size distribution for the case of initial coarse austenite grain sizes. To do that, partially recrystallized microstructures have been analyzed and microstructural parameters of interest quantified.

Abstract: The softening processes that take place after hot deformation in two high Mn steels, one of them microalloyed with 0.1%Nb, have been investigated. Double hit torsion tests were carried out in order to determine the mechanical softening at temperatures ranging from 900 to 1100°C. In addition, the applicability of different parameters obtained by means of the Electron Back Scattering Diffraction (EBSD) technique to estimate the recrystallized fraction was investigated. It has been found that the Grain Orientation Spread (GOS) is the parameter that best allows quantifying the recrystallized fraction under the conditions investigated. The correlation between the mechanical softening and the recrystallized fraction measured by EBSD depends on the material and deformation conditions. A good correlation between both values is observed for the base steel at all the temperatures. However, for the Nb microalloyed steel, although at high temperatures a good correlation is also observed, at low temperatures the mechanical softening fraction tends to be larger than the recrystallized fraction denoting that recovery has an important contribution.