Effect of Ultra Fast Cooling on the Alloy Cost Reduction of the X80 Pipeline Steel

Herein, the microstructural evolution and mechanical properties of a low‐carbon Nb–Ti–Mo‐bearing steel subjected to hot rolling followed by four different processing schedules (air cooling, direct quenching, low‐temperature tempering, and high‐temperature tempering) are studied to elucidate the significance of structural refinement and nanoscale precipitates for obtaining the optimized combination of strength and toughness. The microstructure of air‐cooled steel composed of coarse polygonal ferrite and granular bainite, and minimum yield strength of 473 MPa and lowest impact energy at −40 °C of 40 J are obtained. The microstructure of steel direct quenched and tempered at 520 °C consists of sub‐micron tempered martensitic and bainitic laths with a high relative fraction of high misorientation boundaries and high density of 3–5 nm (Nb, Ti, Mo)C precipitates, and maximum yield strength of 726 MPa and highest impact energy at −40 °C of 213 J are obtained. The formation of nanoscale precipitates coupled with fine tempered martensitic and bainitic blocks contributes to 253 MPa improvement of yield strength and more than five times enhancement of impact energy. The small blocks of tempered martensite and bainite can deviate and finally arrest the crack propagation, whereas the coarse polygonal ferrite and bainite provide inadequate resistance for crack propagation.

Section: Introductionmentioning

confidence: 99%

Significant Improvement in Strength and Toughness of Nanoscale Precipitate–Strengthened Steel by Direct Quenching and Tempering Process

Liu

et al. 2020

“…That leads to the increase in the transformation nucleation sites through deformation of the prior austenite grain boundaries, causing formation of deformation bands and high‐density dislocations in the deformed austenite grains. Such effects accelerate the ferrite transformation, causing increase in the acicular ferrite content and minor microstructure refinement . The results of scanning electronic and digital microscopy (Figure , ) demonstrate smaller size and more elongated shape of MA constituents in the acicular ferrite compared to globular bainite.…”

Section: Discussionmentioning

confidence: 97%

Effect of hot Rolling and Cooling Conditions on the Microstructure, MA Constituent Formation, and Pipeline Steels Mechanical Properties

Kabanov

Korpała

Kawalla

et al. 2018

Two types of pipeline steels differentiated by the content of aluminum and silicon are produced during the pilot plant test with varying rolling temperature and accelerated cooling rate. Microstructures, MA constituent morphology, and mechanical properties of such materials are compared using digital and electron microscopy, tensile, and Charpy impact tests. Results show that low finishing rolling temperature stimulates MA constituent refinement. Cooling rate increase leads to MA constituent amount increase and refinement. Alloying with aluminum significantly stabilizes austenite and leads to MA constituent amount and size increase. Higher MA constituent amount has in most cases positive effect on plasticity without strength and impact toughness degradation.

“…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, Schmidtmann, Meyer, Kaspar, Streißelberger, DeArdo, Jonas, Sellars, Hodgson, Tamura und Tanaka, Saito, Ouchi, Militzer, and many others. The basic idea was to improve the strength and toughness behavior of structural steels by grain refining.…”

Section: A Brief Historical Reviewmentioning

confidence: 99%

Thermomechanical Treatment of Steels - A Real Disruptive Technology Since Decades

Buchmayr

2017

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...