Report on Phosphorus Transfer Kinetics of Bearing Steel During Remelting

Jul 16, 2022

Abstract:  G20CrNi2MoA is a common material for manufacturing medium-sized carburized steel bearings. Its corresponding ASTM A534 steel grade is SAE4320H. Excellent performance FV uses  G20CrNi2MoA to manufacture rolling mill bearings, which are widely used in hot/cold mill work rolls and intermediate rolls. After Carbonitriding on the surface of this material, the bearing rings and rollers show excellent performance in the working environment of high load, high speed and high temperature. Phosphorus is undesirable in steel for it greatly decreases ductility and causes embrittlement in most cases. The kinetic behavior of phosphorus transfer was investigated during electroslag remelting (ESR) of G20CrNi2MoA  bearing steel. Four heat treatments were carried out using an industrial furnace with a capacity to refine 2400 kg ingot. It was found the P content in the four ingots were all higher than that in the electrodes, indicating rephosphorization occurs during ESR. A kinetic model based on film and penetration theory was developed to elucidate the variation of phosphorus from metal film to droplet and metal pool. The model indicates that the rate-determining step of phosphorus transfer is at the slag side. Rephosphorization mainly occurs in the metal film and falling droplet. In addition, the effect of P in the slag and electrode, as well as the temperature of the slag pool on the P content in the metal pool were discussed. In order to achieve a low-P ingot of no more than 0.015%, the corresponding maximum P content in slag under the condition of a certain P content in the electrode was proposed.

Keywords: Case hardened steel; bearing steel; electroslag remelting; slag-metal reaction; phosphorus transfer; kinetics; SAE4320H

1. Introduction

G20CrNi2Mo is one of the typical carburized bearing steels, widely used in the bearings of heavy-duty locomotives in China due to its good surface hardness, high contact fatigue strength, and mechanical properties [1]. With the railway speed-up, a higher requirement for superior quality of G20CrNi2Mo has been required in recent years. During traditional secondary refining, large-size inclusions are occasionally formed because of the technology instability [2], which exerts a detrimental effect on the rolling contact fatigue life. Based on this, electroslag remelting (ESR) is utilized to produce G20CrNi2Mo in China, due to its excellent abilities of inclusion removal and solidification structure improvement [3–5].

Phosphorus is undesirable in steel for it greatly decreases ductility and causes embrittlement [6,7]. Lowering phosphorus content has become a critical topic for steelmakers. Removal of phosphorus is thermodynamically favored at relatively low temperature, high oxygen potential, and high slag basicity [8]. However, these conditions are not applicable during electroslag remelting of bearing steel. On one hand, the oxygen potential of slag should be controlled as low as possible in order to achieve a low-oxygen ingot; on the other hand, the temperature of slag pool is usually very high, even more than 2073 K (1800 ◦C). In this case, dephosphorization becomes rather difficult. Sometimes, rephosphorization occurs. Since ESR is the last step in controlling the steel cleanliness, a thorough understanding of phosphorus behavior is of great necessity for producing low-phosphorus steel. Thermodynamics of dephosphorization have been of interest to researchers and there are numerous studies to determine phosphorus partition ratios and phosphate capacity [9–17]. However, it has been observed that dephosphorization may not be able to reach complete equilibrium in a basic oxygen furnace or electric arc furnace, mainly due to liquid slag formation, and time constraint.

Therefore, kinetic analyses are necessary to elucidate the phosphorus behavior. Several studies have been conducted on dephosphorization kinetics assuming equilibrium of the reactions at the slag-metal interface [18–22]. It has been well established that dephosphorization is controlled by mass transfer in the metal, slag, or both simultaneously. Monaghan et al. [20] proposed that the rate-determining step is the mass transfer in slag, while Mori et al. [21] indicated that the rate of dephosphorization is under mixed control by mass transport in both the slag and metal phase. More recently, Manning [22] showed that the mass transfer parameter (A*ko) decreased as the reaction proceeded and appeared to be a function of interfacial tension. However, most studies have mentioned the above focus on the kinetic behavior of phosphorus during basic oxygen steelmaking. The various slag-metal interfaces during ESR make the mass transfer behavior of phosphorus become more complicated. To the best of the authors’ knowledge, related kinetic studies have been seldom reported. The current work is focused on investigating the phosphorus behavior during ESR from kinetics. Four industrial heats were conducted using an industrial ESR furnace. Then, a kinetic model on the basis of mass transfer theory was developed to elucidate the variation of phosphorus from electrode tip to droplet and metal pool. This practice is expected to provide some guidance for commercial ESR production of low-phosphorus bearing steel. 

2. Methodology

2.1. Experimental Procedure

Carburized bearing steel G20CrNi2Mo with Fe-0.20C-0.32Si-0.60Mn-0.50 Cr-1.80Ni-0.25Mo in mass percent was produced by the process of 70-ton electric arc furnace (EAF) melting → 70-ton ladle furnace (LF) refining → 70-ton vacuum degassing (VD) refining → continuous casting. Four different heat treatments, i.e., A, B, C, and D, were carried out using a furnace with a capacity to refine 2400 kg ingot, as schematically shown in Figure 1.

The water-cooled copper mold had a height of 2000 mm and an inner diameter of 470 mm. Due to the limited plant height, two 2200 × 250 × 280 mm3 billets were utilized as the consumable electrodes for each heat treatment. Before the experiments, the iron oxide scale on the electrode surface was removed mechanically. About 75 kg slag was prepared by mixing CaF2, Al2O3, CaO, and MgO. High-purity argon with a flow of 100–140 L/min was introduced at the start into the mold to isolate the air from outside before arcing. After the mixed slag was added into the mold, a graphite electrode was used for arcing while the slagging process lasted about 30 min. Thereafter, the consumable electrode was switched into the mold. Thirty minutes later, one slag sample was taken from the slag pool for each heat treatment.

A portable infrared thermometer (Produced by B&S instrument corporation, Canada) with the range of 473–2673 K (200–2400 ◦C) was used to measure the surface temperature of the slag pool. Three measurements were taken and the highest temperature taken. In addition, a voltmeter was used to detect the secondary voltage of the furnace mouth. Table 1 shows the related refining parameters at the beginning of the four heat treatments. It can be seen that within a specified time interval, the parameters do not vary much.

After the experiments, one 20-mm-high slice was cut away 70 mm from the bottom of each ingot. Then, three steel cuttings samples were cut from the center, 1/2 the radius, and the edge of the slice for chemical analysis.

2.2. Chemical Analysis

Chemical analysis was performed at the National Analysis Center for Iron and Steel (NACIS), Beijing, China. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was used for the detection of Als, Si, Mn, and P in steel.

The contents of CaO, MgO, MnO, SiO2, Al2O3, and P in the slag were determined by wavelength dispersion X-ray fluorescence spectrometry while FeO was measured by the potassium dichromate volumetric method. The EDTA method was used for the determination of calcium fluoride.

3. Kinetic Analysis of Phosphorus Transfer

Table 2 lists the chemical compositions of slag at the beginning of refining. It can be seen that the FeO content in slag D is the highest, mainly due to the fact that the permeability of oxygen through molten slag obviously increases as the CaF2 content increases [23], thus enhancing the accumulation of iron oxide in the slag [24]. In addition, the contents of P in the four slags are all less than 0.020%.

Table 3 shows the chemical compositions of the electrode and ingots, from which the contents of P in ingots are all shown to be higher than that in the electrodes. Besides that, a certain extent of soluble Al (Als)-, Si- and Mn-oxidation occurs during ESR. Combining Tables 2 and 3, it can be seen that the extent of Al-oxidation increases with increased FeO.

In order to better understand the rephosphorization mechanism, a kinetic model was developed based on the film and penetration theory.

3.1. Model Assumptions

(1) The temperature gradients within the slag-metal phases are assumed to be negligible.

(2) Three reaction sites, i.e., metal film-, droplet-, and metal pool-slag interface are considered in this model.

(3) The concentrations of components in the metal and slag are evenly distributed. The concentration difference only appears near the slag-metal interface based on film theory.

(4) The rephosphorization reaction occurs very quickly, and reaches thermodynamic equilibrium at the interface. The rate-determining step is both controlled by the diffusion of reactants and products through the boundary layer.

3.2. Mass Transfer Kinetics

The kinetic analysis is based on mass control across the slag-metal interface at which equilibrium is maintained. The rephosphorization reaction occurs, as given in Equation (1).

To maintain electrical neutrality, the following equation of oxygen mass balance should be kept. The left-hand side of Equation (26) is the oxygen consumption and the right side hand is the oxygen supply rate. Mi is the molecular weight of the species in the steel. With Equation (26), the interface activity aO* can be obtained. Further, we can get the equilibrium phosphorus partition ratio LP with Equation (6), 

For more information, please check the following file:

Kinetics_of_Phosphorus_Transfer_during_Industrial_2022