Estimation of the Temperature-Dependent Nitrogen Solubility in Stainless Fe-Cr-Mn-Ni-Si-C Steel Melts During Processing
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TRODUCTION
THE knowledge about the maximum N-content dissolvable in the liquid steel bath is crucial to guarantee a safe process control and a fast production of nitrogen alloyed steels. The solubility of N in liquid iron alloys is based on the chemical composition, temperature, and the nitrogen partial pressure (pN2 ) applied to the melt under the condition of thermodynamic equilibrium. At 1873 K (1600 C) and pN2 = 1 atm (1 atm = 1013 mbar), the maximum N-solubility in liquid pure iron is 0.044 wt pct and thus very limited.[1–3] Hence, the N-dissolving ability of the melt can be influenced by alloying elements. For instance, Cr- or Mn-additions sharply increase the N-solubility of the liquid steel bath. Additionally, a rising nitrogen partial pressure supports the dissolution of N in the melt too.[4] A further increase in the N-solubility can be observed when the temperature of the liquid steel bath was reduced.[5] The maximum N-concentration in the solid state is strongly related to the crystal structure formed during solidification of the steel. Consequently, a significant reduced N-solubility in the bcc phase during
primary ferritic solidification was obtained in comparison to the liquid state or a primary austenitic solidification.[3] Moreover, a d-ferrite to austenite solid phase transformation during cooling significantly increases the N-solubility.[1] In this study, the N-solubility of various Fe-Cr-Mn-Ni-Si-C stainless steel grades, namely Fe-14Cr-(0.17-7.77)Mn-6Ni-0.5Si-0.03C [wt pct], Fe-15Cr-3Mn-4Ni-0.5Si-0.1C [wt pct], and Fe-19Cr-3Mn-4Ni-0.5Si-0.15C [wt pct], was investigated in the melt. The influence of the chemical composition on the maximum attainable N-content in the liquid steel bath has been discussed. Further, the temperature-dependent N-solubility was calculated and compared with the measured N-values during processing of the steels. With the modification of the calculation approach by Wada and Pehlke,[5] which is applicable up to nitrogen partial pressures of about 1 atm, a precise estimation of the temperature-dependent nitrogen solubility in the liquid steel bath becomes available.
II.
EXPERIMENTAL PROCEDURE
A. Processing of Cast Steels MARCO WENDLER, MICHAEL HAUSER, ECKARD FRANK SANDIG, and OLENA VOLKOVA are with the Institute of Iron and Steel Technology at TU Bergakademie Freiberg, 09599 Freiberg, Germany. Contact e-mail: [email protected] Manuscript submitted August 9, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
The stainless steels used in this study have been fabricated in a vacuum induction melting and casting plant VIM12 from ALD Vacuum Technologies GmbH shown in Figure 1(a). Initially, the feedstock, consisting of X6Cr17 (1.4016) and precalculated main alloying
Fig. 1—Fabrication of the cast steels in the plant VIM 12: (a) general view of the vacuum induction melting and casting plant; (b) exemplary arrangement of the components inside the furnace chamber; (c) schematic view of the ingate system with pouring sprue and processed ingots (A and B). The arrows in
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