Effects of K₂CO₃ Addition on Inclusions in High-Carbon Steel for Saw Wire

Sample preparation

A continuous casting billet of high-carbon steel for saw wire in a certain steel plant was sectioned into steel blocks and used as experimental raw material. The composition of the billet is shown in Table 1. Reagent grade CaO, SiO₂, MgO and Al₂O₃ powder was mixed evenly, oven-dried at 473 K for 10 h, then pressed into lumps and used as initial slag. The composition of the initial slag is shown in Table 2. Reagent grade K₂CO₃ was oven-dried at 473 K for 10 h, mixed with pure iron powder uniformly in predetermined proportions (shown in Tables 3 and 4), then pressed into lumps and wrapped with pure iron foil.

Experimental apparatus

All the smelting experiments were carried out using a graphite tube resistance furnace. The schematic diagram is shown in Figure 1. In order to prevent the oxidization of steel, argon gas (99.9 % purity) blew from the bottom to the top of the graphite tube in the furnace throughout the experiments. The graphite tube serves as heating unit. The Al₂O₃ tube is used to avoid carbon pick-up in steel caused by the graphite tube. Prior to the experiments, an empty magnesia crucible was heated to 1550 °C for temperature calibration. The temperature at point A was collected using a Pt-Rh thermocouple and compared with that at point B to eliminate the error caused by the limitation of temperature measurement position, since the temperature at point A is closer to practical temperature of molten steel while difficult to collect in the course of the experiment.

Figure 1:

Schematic diagram of graphite tube resistance furnace.

Experimental procedure

A magnesia crucible containing 100 g steel blocks and 20 g prepared slag was initially placed in a graphite tube resistance furnace at 1550 °C (Heats a through f). Once the slag and steel were completely melted, the prepared mixture of K₂CO₃ and Fe was inserted into the molten steel. After preserving heat for 25 min, the crucible was taken out and quenched in cold water after solidification of steel. Afterwards, the cooled ingot was placed in a muffle furnace at 1100 °C for 1 h, and then hot forged into wire rod (7 mm in diameter). Finally, the longitudinal section of the wire rod was mounted using resin, and then ground, mirror polished and gold-sprayed for the detection of inclusions.

In order to further explore the reaction mechanism, an additional experiment on the effect of reaction time between K₂CO₃ and steel was conducted. Firstly, 400 g steel blocks (without slag) were put in a magnesia crucible and melted at 1550 °C (Heat g). Secondly, the prepared mixture of 1.5 g K₂CO₃ and 1.5 g Fe powder was inserted into the molten steel. This moment was regarded as the starting point of the reaction between K₂CO₃ and steel (defined as 0 min). Steel samples were taken from the molten steel using a quartz tube coupled with rubber suction bulb at 0 min, 5 min, 20 min and 50 min respectively and then immediately quenched in water. After taking samples at 50 min, the remaining molten steel was cooled to room temperature in the furnace and a steel sample was cut from the cooled ingot. Finally, all the steel samples were mounted using resin, and then ground, mirror polished and gold-sprayed for the detection of inclusions.

Detection methods

The contents of conventional elements in steel including [Si], [Mn] and [Al] were determined by optical emission spectrometry, while [K] content in steel was determined by inductively coupled plasma mass spectrometry (ICP-MS) and [C] content in steel was determined by a combustion method. The compositions of slag were analyzed by using an X-ray fluorescence spectrometer (XRF). In addition, a scanning electron microscope (SEM) coupled to an energy dispersive X-ray spectrometry (EDS) microanalysis was employed to detect the morphology and compositions of inclusions. In each steel sample, thirty inclusions were characterized randomly.

Chemical compositions of slag and steel

The chemical compositions of slag after reaction are shown in Table 3. It can be seen that the slag in Heats b through e (with a mixture of K₂CO₃ and Fe powder added) contained considerably less K₂O compared with the slag in Heat f (with only K₂CO₃ added). The most probable cause is that Fe powder diluted K₂CO₃ and lowered the localized concentration of K₂CO₃ in molten steel at the beginning of K₂CO₃ addition [13], accordingly inhibiting the volatilization of K₂CO₃ and further the entrance of K₂CO₃ into slag. This can also be supported by the experimental phenomena that the splash of molten steel at the beginning of K₂CO₃ addition was weakened with the addition of Fe powder.

The chemical compositions of steel after reaction are shown in Table 4. Heats b through f (with K₂CO₃ added) had lower levels of [C] in steel than Heat a (with no K₂CO₃ added). Moreover, [C] content in steel decreased gradually with the increase of K₂CO₃ addition amount (Heats a, b, c and e) when Fe/K₂CO₃ was fixed at 1. Therefore, it can be indicated that [C] in steel reacted with K₂CO₃. As can be seen from Table 4, [K] in steel was detected but only in Heat e due to the low concentration of [K] in steel as well as the limitation of detection method. However, this is enough to prove that K₂CO₃ was reduced into [K] in steel. Based on the analysis above, it is not difficult to deduce the reaction K₂CO₃ + 2[C]=2[K]+3CO(g) between K₂CO₃/steel.

Chemical composition of inclusions

The effect of K₂CO₃ addition amount on the compositions of inclusions is shown in Figure 2. As K₂CO₃ addition amount increased (with Fe/K₂CO₃ fixed at 1), K₂O content increased obviously against the fall of MnO content while the other compositions in inclusions changed little. This implies the occurrence of the reaction 2[K]+(MnO)_inc=[Mn]+(K₂O)_inc.

Figure 2:

Effect of K₂CO₃ addition amount on compositions of inclusions.

It has been reported that Al₂O₃-SiO₂-CaO inclusions (with CaO>9.5 %) in tire cord steel mainly originated from collisions and coalescences between entrapped slag particles and endogenous inclusions [14]. Hence, the number of Al₂O₃-SiO₂-CaO inclusions can represent the degree of slag entrainment. The effects of Fe/K₂CO₃ on the types and compositions of inclusions are shown in Figure 3. It is evident that there were far more Al₂O₃-SiO₂-CaO inclusions in Heat f (with no Fe added) than in Heats c and d (with Fe added). This proved indirectly that Fe powder inhibited the volatilization of K₂CO₃, accordingly impairing slag entrainment. Due to the inhibition of Fe on the volatilization of K₂CO₃, the reaction K₂CO₃ + 2[C]=2[K]+3CO(g) was facilitated, which further accelerated the reaction 2[K]+(MnO)_inc=[Mn]+(K₂O)_inc. Therefore, Heats c and d (with Fe powder added) had a far higher level of K₂O in inclusions than Heat f (without Fe powder added).

Figure 3:

Effect of Fe/K₂CO₃ on the types and compositions of inclusions.

In order to verify the reaction between K₂CO₃/steel/inclusions, the average composition change of inclusions in Heat g (without slag) with reaction time after K₂CO₃ addition was investigated, as shown in Figure 4. No slag was added to prevent the interference from slag. At the initial stage, average K₂O content increased and MnO content declined, while the other compositions in inclusions changed little with the increase of reaction time. Therefore, the reactions between K₂CO₃/steel/inclusions were confirmed to be K₂CO₃+2[C]=2[K]+3CO(g) and 2[K]+(MnO)_inc=[Mn]+(K₂O)_inc when no slag was added. With the consumption of K₂CO₃, the reactions approached equilibrium, thus the average compositions of inclusions approached stable gradually.

Figure 4:

Average composition change of inclusions with reaction time in Heat g (without slag).

Reaction mechanism

It has been widely accepted that Reaction (1) will take place at slag/steel interface during the refining process of tire cord steel [15, 16, 17].

Since the composition of high-carbon steel for saw wire is similar to that of tire cord steel, it can be deduced Reaction (1) will also occur in high-carbon steel for saw wire. The reaction will definitely lead to the increase of [Al] content in steel when Al₂O₃ content in slag is relatively high (8 % in this study). The increased [Al] is very likely to reduce MnO in inclusions through Reaction (2) due to the strong reducibility of Al and strong oxidizability of MnO, leading to the increase of Al₂O₃ content in inclusions.

When the reaction time is the same, the degree of Reactions (1) and (2) will keep the same. Thus, the average Al₂O₃ content changed little with the increase of K₂CO₃ addition amount (see Figure 2) with the same reaction time. Because no slag was added in Heat g, Al₂O₃ content in inclusions remained a lower level (below 5 %) due to the absence of Reactions (1) and (2), as shown in Figure 4.

The reaction between 100 g steel (0.87C-0.17Si-0.52Mn-0.0005Al-98.44Fe) mass% and various amount of K₂CO₃ at 1550 °C was calculated by Equilib module of FactSage 7.0, as shown in Figure 5. It can be seen the generated CO(g) and K(g) increased while [C] content in steel decreased with the increase of K₂CO₃ addition amount. Though the content of [K] in steel was not obtained due to the limitation of the FactSage databases, it can still be deduced that K₂CO₃ will be reduced by [C] to generate CO(g) and K(g). The generated K(g) will inevitably dissolve into steel, causing the increase of [K] in steel. Therefore, Reaction (3) is very likely to occur after K₂CO₃ addition. This can be supported by the experimental results that [C] content in steel decreased gradually with the increase of K₂CO₃ addition amount when Fe/K₂CO₃ was fixed at 1 (Heats a, b, c and e) and that [K] was also detected in Heat e (see Table 4). In addition, no CO₂(g) is generated after K₂CO₃ addition, as shown in Figure 5. This also rules out the possibility of the decomposition reaction K₂CO₃(l)=K₂O(l)+CO₂(g).

Figure 5:

Composition changes of [C], CO(g) and K(g) with K₂CO₃ addition calculated by FactSage.

The dissolved [K] in steel caused by the Reaction (3) will definitely reduce MnO in inclusions to [Mn] in steel through the Reaction (4) due to the strong reducibility of K and strong oxidizability of MnO. This can be proved by the fact that average K₂O content increased sharply against the notable fall of MnO content in inclusions with the increase of K₂CO₃ addition amount (Fe/K₂CO₃ fixed at 1, see Figure 2).

Based on the analysis mentioned above, a reasonable reaction mechanism between K₂CO₃/slag/steel/inclusion can be deduced, as shown in Figure 6. Al₂O₃ in slag was reduced by [Si] in steel through Reaction (1), leading to the increase of [Al] against the fall of [Si] in steel. The increased [Al] further reduced MnO in inclusions through Reaction (2), causing the rise of Al₂O₃ against the drop of MnO in inclusions; K₂CO₃ was reduced by [C] in steel through Reaction (3), leading to the increase of [K] against the decrease of [C] in steel. The increased [K] further reduced MnO in inclusions through Reaction (4), resulting in the rise of K₂O against the decline of MnO in inclusions.

Figure 6:

Schematic diagram of the reaction mechanism between K₂CO₃/slag/steel/inclusion.

As shown in Figure 6, when there is slag, Reactions (2) and (4) will take place simultaneously between inclusions and steel, causing the rise of K₂O and Al₂O₃ as well as the decline of MnO in inclusions at the same time. This can be supported by the fact that Al₂O₃ increased while MnO declined with the increase of K₂O content in different inclusions (shown in Figure 7).

Figure 7:

Change of Al₂O₃ and MnO with the increase of K₂O in different inclusions (Heats b through e).

Melting temperature and deformability

All the phase diagrams in this article were calculated using Phase Diagram module of FactSage 7.0. By plotting the SiO₂-Al₂O₃-MnO phase diagram with different K₂O content, the relation between K₂O content and the proportion of low melting temperature (defined as melting temperature lower than 1400 °C) area in SiO₂-Al₂O₃-MnO-K₂O phase diagram can be obtained, as shown in Figure 8. It can be seen that the proportion of low melting temperature area in SiO₂-Al₂O₃-MnO-K₂O phase diagram increased at first and then decreased with the increase of K₂O. Proper content (0–30 %) of K₂O will promote the proportion of low melting temperature area in SiO₂-Al₂O₃-MnO-K₂O phase diagram, namely lower the melting temperature of SiO₂-Al₂O₃-MnO inclusions.

Figure 8:

Relation between K₂O content and the proportion of low melting temperature area in SiO₂-Al₂O₃-MnO-K₂O phase diagram.

The composition distributions of inclusions in phase diagrams are shown in Figure 9. The proportion of low melting point inclusions (PI) showed a notable rise from 16.7 % to 100 % with the increase of K₂O content in inclusions from 0 % (Heat a) to 9 % (Heat e). This agrees well with the conclusion mentioned above that proper content of K₂O could lower melting temperature of SiO₂-Al₂O₃-MnO inclusions.

Figure 9:

Composition distributions of inclusions in phase diagrams (PI – proportion of low melting point inclusions).

The effect of K₂O content on the deformability and melting temperature of inclusions is shown in Figure 10. The proportions of low melting temperature inclusions and plastic inclusions (length/width>3) increased at the same time with the increase of average K₂O content. This means that the proportion of low melting temperature inclusions was positively related to that of plastic inclusions. However, plastic inclusions had a far lower increasing amplitude in proportion than low melting temperature inclusions. This may be caused by the crystallization of inclusions containing K₂O, as explained in the following section ‘Crystallinity and deformability’.

Figure 10:

Effect of K₂O content on the deformability and melting temperature of inclusions.

Crystallinity and deformability

The backscattered electron images of typical inclusions in steel are shown in Figure 11, and chemical compositions of typical inclusions corresponding to Figure 11 are shown in Table 5. Inclusions in Heats a and f contained little or no K₂O and were mainly ellipsoidal (non-deformable) and amorphous (containing a single phrase). Their chemical compositions were mostly distributed outside the low melting temperature area of phase diagram (shown in Figures 9(a) and (f)). This may be the cause of their poor deformability. Inclusions in Heats b through e can be roughly classified into two subcategories according to their compositions, named K₂O-Al₂O₃-SiO₂-MnO inclusions (with K₂O<15 %, MnO>10 %) and K₂O-MgO-Al₂O₃-SiO₂ inclusions (with K₂O>15 %, MnO<10 %), as shown in Table 5. Both of the two types of inclusions were mostly distributed in the low melting temperature area of phase diagram (shown in Figures 9(b) through (e)) and thus should have had good deformability according to a popular belief that inclusions with low melting temperature have good deformability during hot rolling [8, 9, 10, 11]. However, K₂O-Al₂O₃-SiO₂-MnO inclusions (Inclusions b, c2, d2 and e2) were very likely to crystallize (form dual phase) (shown in Figure 11), which deteriorated the deformability of inclusions according to the previous reports [18, 19]. Thus, K₂O-Al₂O₃-SiO₂-MnO inclusions were nearly spherical (non-deformable), despite of their low melting temperature. K₂O-MgO-Al₂O₃-SiO₂ inclusions (Inclusions c1, d1 and e1) tended to remain amorphous (containing a single phase), and thus was stripe-like in shape (deformable), as shown in Figure 11.

Figure 11:

Backscattered electron images of typical inclusions in steel (cross-type dot – black phase, round-type dot – grey phase).

According to the research of Beskow et al., the measured compositions of tiny particles were likely to be effected by their neighboring phase when analyzing with an energy dispersive X-ray spectrometry (EDS) [20]. That is to say, the phases in K₂O-Al₂O₃-SiO₂-MnO inclusions were hard to determine by EDS due to their extremely tiny size. Therefore, in order to verify the type of the precipitated phase, Equilib module of FactSage 7.0 was employed to calculate the phase translation of K₂O-Al₂O₃-SiO₂-MnO inclusions during cooling process, as shown in Figure 12. The average composition of K₂O-Al₂O₃-SiO₂-MnO inclusions in Heats b through e (2.33MgO-9.46Al₂O₃-57.15SiO₂-6.94K₂O-0.86CaO-23.3MnO) mass% was employed for calculation. It can be seen that MnSiO₃ is prior to precipitate from the matrix of inclusions during cooling process. Moreover, it should also be noted that MnO content in MnSiO₃ (54.2 %) is far higher than average MnO content in inclusions (23.3 %). This is consistent with the experimental result that the grey phase contained much more MnO than black phase in K₂O-Al₂O₃-SiO₂-MnO inclusions (see Table 5). Therefore, the grey phase precipitated in the matrix (black phase) of K₂O-Al₂O₃-SiO₂-MnO inclusions was deduced to be MnSiO₃ phase.

Figure 12:

Phase translation of K₂O-Al₂O₃-SiO₂-MnO inclusions during cooling process (Heats b through e).

K₂O-Al₂O₃-SiO₂-MnO inclusions in Heat g were also found to crystallize (form dual phase) after cooling in furnace, as shown in Figure 13. This implies that the crystallization of inclusions may occur during cooling of molten steel. Given that K₂O-Al₂O₃-SiO₂-MnO inclusions are very likely to crystallize, which further deteriorates the deformability of inclusions, a new method should be developed in our future research to prevent the crystallization of K₂O-Al₂O₃-SiO₂-MnO inclusions.

Figure 13:

Backscattered electron image of typical K₂O-Al₂O₃-SiO₂-MnO inclusions in Heat g after cooling in furnace.