Thermomechanical Treatment of Steels - A Real Disruptive Technology Since Decades

Abstract

The basic concept of thermomechanical treatment (TMT) or thermomechanical controlled processing (TMCP) is responsible for the development of many advanced steel grades with improved mechanical properties during the last 50 years. A retroperspective view, an explanation of the most important influencing factors and a presentation of the enormous benefits for the costumer are given in this review. A sound knowledge on the synergism of recrystallization, precipitation, and transformation phenomena forms the basis to produce fine, homogeneous microstructures with improved properties. Starting from structural steels, improvements can be achieved with respect to higher strength and toughness values combined with better weldability and formability, mainly based on reduction of carbon content and finer grain sizes. The benefits in the application areas are described in detail. TMCP is not only used for flat products, but also for long products and forgings of different steel grades. Physical simulation and modeling contribute significantly to these developments, which also form the basis for computer-aided control or real-time online-models. In the next years, a complete shift to cyber physical systems (CPS) is predicted, which needs an aligned education effort. REVIEW A Brief Historical ReviewSince the mid-sixties, steel mills began to produce fine grained structural steels by lowering the final rolling temperature. Based on fundamental research in Germany, USA, UK, Germany, and Japan, the fundamental understanding was further developed in metallurgical laboratories. Pioneers at that time were Haneke, [10] Schmidtmann, [11,12] Meyer, [13,14] Kaspar, [15][16][17][18][19][20][21][22][23][24] Streißelberger, [9,15,[25][26][27] DeArdo, [28,29] Jonas, [30][31][32][33] Sellars, [34][35][36][37] Hodgson, [38][39][40][41][42] Tamura und Tanaka, [43] Saito, [44] Ouchi, [45] Militzer, [46,47] and many others. The basic idea was to improve the strength and toughness behavior of structural steels by grain refining. Compared to conventional hot rolling at high rolling temperatures, the new steels were rolled at lower final rolling temperature. It was found that repeated recrystallization of the austenite structures leads to a decrease of grain size, but there is a limit, which is difficult to overcome. Deformation at temperatures, where no recrystallization takes place was successful in the conditioning of austenite having a dense population of glide planes, high dislocation density, and a high intrinsic energy, which provided a high density of nucleation sites for the transformation products of austenite.At the beginning, mainly ferrite plus pearlite microstructures were considered and later the role of rapid cooling became an additional opportunity to increase the strength level.Higher cooling rates or higher undercooling increase the driving force and with a lower diffusivity a finer microstructure like bainite and martensite can be reached.A comparison of the contributions of the strengthening mechanism in a commercial hot rolle...

The Effect of Different Annealing Strategies on the Microstructure Development and Mechanical Response of Austempered Steels

“…Refinement of the parent austenite grain (PAG) size has been proved as an effective strategy towards fine-grained steel grades [2,[6][7][8]. Among the different methods for grain refinement, the addition of microalloying elements [9] and complex thermo-mechanical treatments [10] are well-known routes to achieve a fine distribution of PAGs and improved mechanical properties in high strength steels.…”

Effect of welding mode and filler metal on microstructure and mechanical properties of dissimilar joints of S900MC thermomechanical steel to St52 carbon steel welded by gas tungsten arc welding

“…The necessity to combine higher strength and better weldability with adequate toughness and ductility has resulted in the development of high-strength low alloy (HSLA) steels. 1,2 The carbon equivalent (CE) of HSLA steels is low, and their attributes are obtained through precise micro-alloy composition selection as well as optimization of thermomechanically controlled processing (TMCP) and accelerated cooling conditions. High-strength steels (HSS) and ultra-HSS (UHSS) are two subsets of HSLA steels that provide a cost-effective solution for material preservation.…”

Roles of Lanthanum and Cerium in Grain Refinement of Steels during Solidification

“…The problem associated with this method is that the carbides or carbonitrides could reduce ductility by enabling surface transverse cracking during hot working if the process was not well-controlled [5]. Advanced solid grain refinement of steels is achieved through thermomechanical processing [6,7], which involves deforming and refining the supercooled austenite prior to transformation, and therefore the fine microstructure is obtained at room temperature. This thermomechanical processing, however, is not obviously suitable for cast steels.…”