Recrystallization and grain growth during plate rolling are prevented by Nb addition both with the solute drag and the Nb carbide precipitation. Although a fine microstructure is achieved in the base material, welding heat completely changes the microstructure in the heat affected zone (HAZ). In this study, laboratory simulation of the coarse grain HAZ (CGHAZ) thermal cycle of double submerged arc welded linepipe was carried out using low carbon steels containing different Nb contents. Extraction residue analysis of the simulated CGHAZ samples revealed that almost all the Nb remained in solid solution. To clarify the interaction of Nb carbide dissolution and grain growth on overall simulated HAZ microstructure evolution, additional weld HAZ thermal simulations were performed. It was found that Nb carbides remain undissolved at HAZ peak temperatures up to 1 200 & DEG;C and showed significant pinning effect to prevent austenite grain growth. Significant grain growth was seen after continuous fast heating to 1 350 & DEG;C peak temperature, while the higher Nb added steel showed a slower overall austenite grain growth rate, suggesting that grain growth in the HAZ at higher temperature was suppressed by the combined effects of slower coarse Nb carbide dissolution providing some pinning, and the solute drag effect of higher amounts of Nb in solid solution. A pronounced retardation of longer-term isothermal grain growth was identified at 1 350 & DEG;C at higher levels of solute Nb, confirming the influence of Nb solute drag on high temperature resistance to austenite grain coarsening.

Obtaining high levels of mechanical properties in steels is directly linked to the use of special mechanical forming processes and the addition of alloying elements during their manufacture. This work presents a study of a hot-rolled steel strip produced to achieve a yield strength above 600 MPa, using a niobium microalloyed HSLA steel with non-stoichiometric titanium (titanium/nitrogen ratio above 3.42), and rolled on a Steckel mill. A major challenge imposed by rolling on a Steckel mill is that the process is reversible, resulting in long interpass times, which facilitates recrystallization and grain growth kinetics. Rolling parameters whose aim was to obtain the maximum degree of microstructural refinement were determined by considering microstructural evolution simulations performed in MicroSim-SM (R) software and studying the alloy through physical simulations to obtain critical temperatures and determine the CCT diagram. Four ranges of coiling temperatures (525-550 degrees C/550-600 degrees C/600-650 degrees C/650-700 degrees C) were applied to evaluate their impact on microstructure, precipitation hardening, and mechanical properties, with the results showing a very refined microstructure, with the highest yield strength observed at coiling temperatures of 600-650 degrees C. This scenario is explained by the maximum precipitation of titanium carbide observed at this temperature, leading to a greater contribution of precipitation hardening provided by the presence of a large volume of small-sized precipitates. This paper shows that the combination of optimized industrial parameters based on metallurgical mechanisms and advanced modeling techniques opens up new possibilities for a robust production of high-strength steels using a Steckel mill. The microstructural base for a stable production of high-strength hot-rolled products relies on a consistent grain size refinement provided mainly by the effect of Nb together with appropriate rolling parameters, and the fine precipitation of TiC during cooling provides the additional increase to reach the requested yield strength values.

Intercritically deformed steels present combinations of different types of ferrite, such as deformed ferrite (DF) and non-deformed ferrite (NDF) grains, which are transformed during the final deformation passes and final cooling step. Recently, a grain identification and correlation technique based on EBSD has been employed together with a discretization methodology, enabling a distinction to be drawn between different ferrite populations (NDF and DF grains). This paper presents a combination of interrupted tensile tests with crystallographic characterization performed by means of Electron Backscatter Diffraction (EBSD), by analyzing the evolution of an intercritically deformed micro-alloyed steel. In addition to this, and using the nanoindentation technique, both ferrite families were characterized micromechanically and the nanohardness was quantified for each population. NDF grains are softer than DF ones, which is related to the presence of a lower fraction of low-angle grain boundaries. The interrupted tensile tests show the different behavior of low- and high-angle grain boundary evolution as well as the strain partitioning in each ferrite family. NDF population accommodates most of the deformation at initial strain intervals, since strain reaches 10%. For higher strains, NDF and DF grains behave similarly to the strain applied.

The effect of the initial microstructure and soft annealing temperature on cementite spheroidization and microstructure softening is studied on an AISI 5140 hot-rolled wire. In coarse pearlite microstructure (lambda: 0.27 mu m), the cementite spheroidization progresses slowly under subcritical treatment, and the microstructure does not achieve the minimum G2/L2 IFI rating defined in the ASTM F2282 to be used in cold forming operations under any of the annealing treatment studies. Fine pearlite (lambda: 0.10 mu m) and upper bainite microstructures are more prone to spheroidization, and the minimum G2/L2 IFI rating is achieved under subcritical annealing at 720 degrees C for 6 h. Independent of the initial microstructure, even in the case of martensite, low hardness values within 165-195 HV are attained after imposing a 10 h long treatment at 720 degrees C. Annealing treatments conducted at 660 degrees C and 600 degrees C on pearlitic microstructures give rise to very poor softening. The G2/L2 rating is not achieved in any of the treatments applied at these two temperatures in this study. In pearlitic microstructures, the spheroidization progresses according to a fault migration mechanism, enhanced by the presence of defects such as lamella terminations, holes, and kinks. In the upper bainite, the row-like disposition of the cementite along the ferrite lath interface provides necks where dissolution and consequent lamellae break-up take place quickly under annealing.

The mechanical properties of intercritically rolled microstructures have been scarcely reported in literature. Although the strengthening effect of intercritical rolling is generally recognized, there is no a clear opinion on its effect on toughness. Therefore, a greater knowledge of how different process parameters affect the mechanical properties during intercritical deformation is required. With the aim of evaluating the relationship between microstructure and mechanical properties on intercritically deformed low carbon steels, plane strain compression tests were carried out. Plane strain compression tests allow for both the characterization of the microstructural features and the evaluation of mechanical properties, via tensile and Charpy tests. Firstly, the intercritically deformed microstructures were characterized using the EBSD technique, and then a discretization methodology was used to distinguish both intercritically deformed and non-deformed ferrite populations. Next, strength and toughness properties were measured by means of tensile and Charpy tests. The results indicate that the reduction of the deformation temperature leads to an increment of yield strength for both steels, but at the same time toughness properties worsen. Deformed ferrite fractions higher than 25% result in a very pronounced loss of ductility. The yield strength was predicted by estimating the contribution of different strengthening mechanisms (solid solution, grain size refinement, dislocation density) corresponding to each ferrite population by considering a nonlinear law of mixtures. Similarly, the impact of different microstructural parameters (solid solution, grain size, microstructural heterogeneity, contribution of dislocation density and secondary phases) on toughness was evaluated and a new equation able to predict ductile to brittle transition temperature for intercritically deformed microstructures was developed.

Heavy gauge structural plates has been widely rolled in the austenite/ferrite two phase region, in order to meet the demanding market requirements regarding tensile properties. Even though strength levels can be increased by intercritical rolling, toughness properties may be impaired. Therefore, a greater knowledge of how different austenite-ferrite balances affect the microstructural evolution during intercritical deformation is required. With the aim of gaining a deep comprehension of the evolution of the microstructure during intercritical deformation, dilatometry tests were performed simulating intercritical rolling conditions. Different ferrite populations are identified in the resulting microstructures, composed of intercritically deformed ferrite and non-deformed ferrite transformed during final air cooling. In the deformed ferrite grains well defined substructure is clearly noticed, whereas the non-deformed grains formed during air cooling step do not show any evidence of substructure. In the current work, EBSD advanced characterization technique was used to develop a methodology that is able to differentiate the intercritically deformed ferrite from non-deformed ferrite for low carbon steels. Based on the Grain Orientation Spread (GOS) parameter, a threshold value of 2 degrees was defined to distinguish deformed and non deformed ferrite grains. The proposed procedure allows distinguishing both ferrite populations and quantifying microstructural parameters of each family. The effect of the addition of C and austenite-ferrite balance on the microstructural evolution of each ferrite type was analyzed.

Heavy gauge line pipe and structural steel plate materials are often rolled in the two-phase region for strength reasons. However, strength and toughness show opposite trends, and the exact effect of each rolling process parameter remains unclear. Even though intercritical rolling has been widely studied, the specific mechanisms that act when different microalloying elements are added remain unclear. To investigate this further, laboratory thermomechanical simulations reproducing intercritical rolling conditions were performed in plain low carbon and NbV-microalloyed steels. Based on a previously developed procedure using electron backscattered diffraction (EBSD), the discretization between intercritically deformed ferrite and new ferrite grains formed after deformation was extended to microalloyed steels. The austenite conditioning before intercritical deformation in the Nb-bearing steel affects the balance of final precipitates by modifying the size distributions and origin of the Nb (C, N). This fact could modify the substructure in the intercritically deformed grains. A simple transformation model is proposed to predict average grain sizes under intercritical deformation conditions.

Cost-effective advanced design concepts are becoming more common in the production of thick plates in order to meet demanding market requirements. Accordingly, precipitation strengthening mechanisms are extensively employed in thin strip products, because they enhance the final properties by using a coiling optimization strategy. Nevertheless, and specifically for thick plate production, the formation of effective precipitation during continuous cooling after hot rolling is more challenging. With the aim of gaining further knowledge about this strengthening mechanism, plate hot rolling conditions were reproduced in low carbon Ti-Mo microalloyed steel through laboratory simulation tests to generate different hot-rolled microstructures. Subsequently, a rapid heating process was applied in order to simulate induction heat treatment conditions. The results indicated that the nature of the matrix microstructure (i.e., ferrite, bainite) affects the achieved precipitation hardening, while the balance between strength and toughness depends on the hot-rolled microstructure.

Three main topics are covered in this paper regarding Nb-Mo interactions in microalloyed steels. First, the synergistic behavior of Nb and Mo in enhancing solute drag effects and modifying recrystallization and precipitation kinetics in austenite under hot working conditions is analyzed. Then, the effect of different microalloying additions on the final phase transformations is examined. In addition to composition and austenite conditioning effects on the phases formed and the corresponding CCT diagrams, a quantitative study using an EBSD technique is described which was performed in order to measure unit size distributions and homogeneity of complex microstructures. Finally, the contribution of different strengthening mechanisms to yield strength is evaluated for different coiling temperatures and compositions.

Hot rolling of steels involves not only geometrical and final shape requirements but also, in a significant number of cases, microstructural conditioning in order to achieve the final required mechanical properties. In those situations, a proper understanding, evaluation and control of the microstructural changes that occur at each step of a rolling schedule is necessary. The objective of this chapter is to describe the main metallurgical aspects that intervene during hot rolling, focusing on those that are relevant for achieving the proper final mechanical properties. In the first part of the chapter the microstructural changes that take place during hot rolling will be considered, taking into account mechanisms such as recovery, recrystallization, strain induced precipitation and the different interactions between them, concluding with the relationship between austenite microstructure, cooling conditions and final room temperature microstructure. In the second part, the relationship between room temperature mechanical properties and microstructure will be evaluated. Special emphasis will be given to the differences between mean attributes and local heterogeneities. Taking the concepts described in these two sections as a starting point, the next section will be devoted to the main aspects of the thermomechanical processes. The last part of the chapter will focus on considerations regarding the effect of several metallurgical factors on the mechanical properties.