Electroslag Refining of CrNiMoWMnV Ultrahigh-Strength Steel

Mohammed Ali,Hoda El Faramawy,Taha Mattar,David Porter,Jukka Kömi,Mamdouh Eissa,M. F. El-Shahat

doi:10.4236/jmmce.2017.56032

Mohammed Ali, Hoda El Faramawy + Show 5 more

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https://doi.org/10.4236/jmmce.2017.56032

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Abstract

Increasing demands for ultrahigh-strength steels in commercial as well as military applications have raised interest in finding alternatives to the high-cost high-alloyed steel and super-alloys currently used, e.g. the use of economic low-alloy compositions processed via low-cost air induction melting and electroslag refining (ESR). In this work the yield of alloying elements and the removal of the impurities nitrogen, sulphur and phosphorus as a result of electroslag refining (ESR) in a newly developed CrNiMoWMnV ultrahigh-strength steel (UHSS) have been studied in relation to their activities in the molten metal pool. Six experimental heats of CrNiMoWMnV UHSS with different chemical compositions were designed, melted in an induction furnace (IF) and refined using ESR. This was followed by hot forging of the ingots at 1100°C to 950°C. ESR using a CaF2-CaO-Al2O3 slag system led to a high yield in Cr, Ni, Mo, W, Mn and V, while the yield of Si is low. The desulphurization of all six UHSS grades was pronounced with most of the sulphur removed either to the slag or by gas reactions. The degree of dephosphorization was only 5% irrespective of the steel composition. On the other hand, denitrification (removal of nitrogen) was achieved. It ranged from 8% to 63% depending on the steel composition. The yield of the alloying elements and removal of impurities from the steel during ESR depends on the chemical and physical properties of the ESR slag and the activity of the elements in the molten state, taking into account elemental interactions.

Highlights

There is no generally accepted definition of what constitutes ultrahigh-strength in structural steels as it depends on the field of application
Mo is increased as a percentage in steels C, D, E and F due to its extremely low interaction coefficient with oxygen and its less negative free energy of oxidation combined with the fact that the impurities S, N and P and other alloying elements like Si, Mn, V and Cr are removed
The effects of the activities of the alloying elements C, Cr, Ni, Mo, W, Mn and V and the impurities S, P, N and O on the yield of alloying elements and the degree of purification caused by electroslag remelting with a prefused CaF2 based slag has been studied for six potential ultrahigh-strength steel compositions

Summary

Introduction

There is no generally accepted definition of what constitutes ultrahigh-strength in structural steels as it depends on the field of application. According to Philip [1] [2] it implies a yield strength exceeding 200 ksi, i.e. 1380 MPa, but more recently the term has been applied to steels with a yield strength of about 900 MPa, and above [3]. Common to all ultrahigh-strength steels is the need to have a low level of impurities, such as sulfur, phosphorous and non-metallic inclusions (NMI), as well as freedom from defects like voids and cracks, and homogeneous compositions within narrow tolerances [4]. The electroslag refining (ESR) process is one of the most important secondary refining processes owing to its relatively low production cost, low capital investment and ability to produce high quality steel with desired chemistry and homogenous structure. ESR has been shown to enhance strength as a result of the grain refinement resulting from the addition of some inoculants during the ESR process [4] [5] [6]

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