The aim of the present study was to investigate the possibilities of reaching yield strengths beyond 600 MPa for low carbon bainitic hot strip steels by vanadium microalloying together with suitable base alloying. The processing conditions and levels of carbon and nitrogen chosen in this laboratory investigation correspond to those of a typical 8 mm hot strip steel containing 0.04 mass% carbon and 0.010 mass% nitrogen from electric arc furnace practice processed in conventional or compact strip mills. It was found that a base alloying corresponding to 1.4 mass% Mn, 1.0 mass% Cr and 0.25 mass% Mo is required to form a fully bainitic structure after coiling at 400°C. The decisive factors determining the strength of bainitic hot strip steels are firstly the bainite transformation temperature and secondly the extent to which recovery of the densely dislocated bainitic ferrite can be prevented. The results of this study demonstrate that vanadium microalloying effectively prevents the recovery of the bainitic ferrite and leads to retention of the strength of the virgin bainite after coiling. This is primarily due to retardation of recovery by fine vanadium carbonitrides precipitates on dislocations and only to a lesser extent to true precipitation strengthening. With 0.08 mass% V together with 0.010-0.020 mass% N the yield strength lies in the range of 750-790 MPa compared to 680 MPa for a similar reference steel without vanadium. By raising the chromium content to 2 %, yield strengths in the range of 840-880 MPa have been reached. This is attributed to a lowering of the bainite transformation temperature resulting from the higher base alloying.KEY WORDS: high strength steel; bainite; alloying; hot rolling; cooling; microstructure; mechanical properties.* 1 This paper is dedicated to Michael Korchynsky, who recently retired from Stratcor Inc. after 60 years service to steel and vanadium microalloying. The authors of this paper, as well as former colleagues in Stockholm, are indebted to him for his profound engagement in our research and for countless rewarding discussions over the years. * 2 All steel compositions in this paper are expressed in mass%. Alloy Design and Experimental SteelsOut of the normal alloying elements, manganese, molybdenum and chromium are the most effective for raising the hardenability of steel, defined as their ability to prevent formation of ferrite and pearlite during hardening. Their hardenabilities expressed as the Grossman coefficient 6) are all high but decline somewhat in the order Mn, Mo and Cr, viz. 4.10, 3.14 and 2.83. In the present case, the retardation of bainite formation is particularly significant. Here, the coefficient differs more between these three elements, Mn 4.10, Cr 1.16 and Mo 0. 7) Hence, to stimulate bainite formation and at the same time prevent formation of ferrite/pearlite, molybdenum is most effective, followed by Cr and then Mn. In accordance with this, 1.4% Mn, 1.0% Cr and 0.25% Mo were chosen as the standard composition to create an adequate base hardenabilit...
The availability of low-cost and powerful mobile devices and 3D modeling and visualization tools provides new opportunities for bringing innovation into mathematics education. This paper reports on the findings from a mobile geometry project pilot for middle school students designed together with teachers that consisted of outdoors and indoors activities. The aim of the project has been to design and implement a prototype that combines mobile and 3D technologies that allow students to collaboratively, explore, and discuss geometrical concepts. The focus of this paper is to present and reflect on the lessons we have learned after experimenting with novel pedagogical activities that rely on mobile applications, 3D modeling and visualization to support learning in the field of geometry. Finding a balance between team goals and expectations while focusing on the learning activity flow can lead to more innovative solutions. Mobile Learning, Augmented Reality, Design-Based Research, CSCL
Copper in recycled steel made from scrap is well known to create problems of hot shortness during rolling. The present study was carried out to ascertain how the situation could be improved when using strip casting in conjunction with direct hot rolling. This has included steels having copper contents up to 2.5 wt.% and in some cases also tin levels up to 0.1 wt.%. Laboratory simulations have been carried out to simulate the process conditions from the outlet of the strip caster through hot rolling, and the resulting materials have been examined with regard to their hot cracking behaviour and microstructural condition. Mechanical properties have also been measured on samples having different simulated coiling temperatures after hot deformation. Conditions can be established to avoid hot shortness even in steel containing high copper contents, due to the short time that is available for oxidation. Depending on the steel composition and coiling temperature, it is possible to achieve very significant strengthening due to precipitation of copper‐rich particles.
Mobile technology opens up opportunities for collaborative learning in otherwise remote contexts outside the classroom. A successful realization of these opportunities relies, however, on mobile learning activities providing adequate collaboration structures. This article presents an empirical study aimed at examining the role played by mobile devices, teachers and task structures as a means for collaborative learning in geometry. The study focused on the analysis of the nature of collaboration that unfolded when students measured areas outdoors in the field. The analysis of the mobile learning activity was conducted from an Activity theory perspective. The findings obtained indicate that the collaboration observed may be impaired if: 1) the functionalities needed for collaborative problem-solving are asymmetrically distributed on a number of mobile devices; 2) task-related information is not accessible to all learners; 3) the task structure is not sufficiently complex; 4) teacher scaffolding is too readily available; and 5) necessary collaborative skills are not developed.
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