Numerical Simulation and Experimental Investigation of Nitrogen Transfer Mechanism from Gas to Liquid Steel During Press
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OVER the past decades, nitrogen has become increasingly attractive as an inexpensive alloying element for enhancing the mechanical properties, heat resistance, corrosion resistance, and oxidation resistance.[1–4] Meanwhile, nitrogen as an austenite stabilizing element partially replaces nickel to produce high-alloyed steels cost-effectively.[5,6] In particular, Europe has been very active in the research and development of high-nitrogen steels. However, the nitrogen content is limited by the solubility at an atmosphere pressure. Nitrogen beyond the solubility limit could be added by high-pressure melting method.[7]
JIA YU, FUBIN LIU, HUABING LI, ZHOUHUA JIANG, YANG LI, CONGPENG KANG, AO WANG, WENCHAO ZHANG, and HAO FENG are with the School of Metallurgy, Northeastern University, Shenyang 110819, China. Contact e-mail: [email protected] Manuscript submitted June 9, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
Pressurized electroslag remelting (PESR) has emerged as the most robust method for producing high-nitrogen steels.[8] Since common electroslag remelting (ESR) slags have virtually no solubility for nitrogen and the dwelling time of metal droplets in the slag is extremely short, the nitrogen pickup via a nitrogen-rich atmosphere is long considered to be negligible during pressurized electroslag remelting process.[6,9] The nitrogen is therefore supplied continuously during remelting in the form of solid nitrogen-bearing additives.[3,8,10–12] But some researches have shown that the nitrogen content in ingot could have an evident increase during remelting only via a nitrogen-rich atmosphere. Carosi et al.[13] found that the nitrogen content in ingot increased from 0.13 to 0.2 pct when the electrode of Cronidur30 was remelted at the nitrogen pressure of 4.0 MPa without any addition of nitrides. In addition, the similar phenomena have also observed in ESR with an open-air atmosphere. Mitchell et al.[14] pointed that low (~ 20 ppm) initial nitrogen containing electrodes of IN 718 alloy would experience nitrogen pickup during ESR in air, and the final nitrogen content was around 60 ppm.
Fig. 1—The transfer mechanism of nitrogen in a gas–slag–metal system.
Hou[15] melted electrodes of stainless steel with ESR in air, and found that the nitrogen content increased from 0.019 to 0.033 pct in ingot. Figure 1 shows the transfer mechanism of nitrogen in a gas–slag–metal system. Gaseous nitrogen firstly dissolves in molten slag by the ionic gas–slag reaction, and then the free nitride N3- in molten slag enter liquid metal by the slag–metal reaction.[16–22] The oxygen potential gradient between gas and metal acts as a driving force of the nitrogen transfer from gas to liquid metal through slag.[13] It is worthwhile mentioning that a portion of CO or H2 is introduced to ensure a strong reducing condition in the majority of studies about the nitrogen solubility in slag. However, the atmosphere in PESR could not satisfy with the strong reducing condition, which is discussed in Section III–B in detail. Although the
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