Analysis of metallurgical properties of refining slag for bearing steel

1 Introduction

High-quality bearing steels require high purity and uniform organization, ie, less impurity and non-metallic inclusions, fine carbides and uniform distribution. The refining slag has the functions of deoxidation, desulfurization and de-intercalation, and its properties directly affect the metallurgical effect of the LF refining process. When the alkaline reducing slag is in close contact with the molten steel, the actual values ​​of oxygen and sulfur in the molten steel are greater than the values ​​of oxygen and sulfur in the equilibrium of the same slag, so that oxygen and sulfur in the molten steel are diffused into the slag; CaO in the refined slag The components such as Al2O3 can be combined with the deoxidation products of Si, Al, Mn, etc. to form a compound having a low melting point, thereby reducing the activity of the deoxidation product and strengthening the deoxidation reaction; since the refining slag is composed of an oxide, the interfacial tension between the oxides Small, easy to combine into a low melting point compound, and the interfacial tension between the molten steel and the deoxidation product is greater than the interfacial tension between the slag and the deoxidation product, and the refining slag can absorb the deoxidation product, so that the deoxidation product is easily removed from the molten steel. In addition, the molten slag melts to form foam slag, and the slag layer covers the molten steel, which can effectively prevent gas inhalation, and is beneficial to the submerged arc operation, reducing the damage of the arc to the ladle lining and the ladle cover, and improving the thermal efficiency. Therefore, studying the composition of refined slag and its influence on the cleanliness of steel is of great significance for the full play of LF refining.

To control the inclusions in the bearing steel, the type and shape of the inclusions in the steel must be qualitatively analyzed. According to the refining process, there may be A-type sulfide inclusions, B-type alumina inclusions, C-type calcium aluminate composite inclusions, and magnesium-aluminum spinel and titanium nitride inclusions. Due to the precipitation deoxidation process in the whole process, the adsorption of deoxidation products (mainly alumina) by slag is particularly important. The kinetic conditions of inclusions can be improved by weak agitation of argon, but if the slag itself absorbs the inclusions. Not good, so that the inclusions can not be completely separated from the molten steel, will deteriorate the mechanical properties of the bearing steel. Therefore, the composition and properties of the refining slag directly affect the performance of the bearing steel. This study systematically discusses the influence of the slag forming process and composition on the slag forming process, and systematically discusses the alkalinity and desulfurization effect of the refining slag, and obtains the refining slag which can effectively remove the sulfur and oxide slag in the steel. system.

2. Influence of production process on refining into slag

2.1, refining slag composition

The traditional bearing steel refining slag system is mainly based on CaO-Al2O3 and CaO-SiO2-Al2O3 high alkalinity refining slag system. It can be seen from the binary phase diagram of CaO-Al2O3 that there is a low melting point compound 12CaO.7Al2O3 in the slag, which can reduce the melting point of the slag by adjusting the Al2O3 content in the refining slag, and improve the kinetic conditions of the synthetic slag refining. SiO2 is an acidic oxide, which is not conducive to desulfurization of refined slag, but SiO2 has a great influence on the foaming performance of slag. It can be seen from the surface tension diagrams of the CaO-Al2O3 binary system and the CaO-SiO2-Al2O3 ternary system that SiO2 is a surface active material, and its content increases the surface tension, promotes foaming, and increases the elasticity and strength of the slag film.

2.2, the impact of production process on slag composition

The GCr15 refining process for the production of bearing steel adopts the ternary refining slag system, which is produced by two processes: LD+LF+CC and LD+LF+VD+CC. In this study, the three main components in the refining slag system were studied, and the changes in the refining process were discussed. Figure 1 shows the variation of slag composition during the slag-forming process from steelmaking to refining. Figure 2 shows the variation of the binary alkalinity in the refining process of different slag forming routes.

From the two process routes, the process of slag formation is basically the same. Mainly divided into two phases:

In the first stage, before the tapping of the converter to the refining station, the converter smelting adopts a high-pull-filling operation, and the alkalinity of the end slag is controlled at about 3 to 3.5. When tapping, the slag plug is used to block the slag tapping, the deoxidation agent is deoxidized and alloyed in the tapping process, and the binary synthetic slag is added to the slag after the furnace to prevent the phosphorus from being returned. Due to the addition of binary slag, the CaO content in the slag is slightly increased; the aluminum oxide slag is not used in the converter blowing process, and the Al2O3 content of the tapping slag is not high; the steel is stabilized by aluminum, and the Al2O3 in the slag is increased. The alloying with ferrosilicon ferromanganese increases the SiO2 content in the slag.

The second stage is the slag forming process of the ladle refining furnace. In this process, synthetic slag, submerged arc slag and slag deoxidizer aluminum and calcium carbide are added. During this process, the content of SiO2 and Al2O3 in the slag will be significantly reduced. On the one hand, the aluminum deoxidizer added in the refining process will reduce the SiO2 in the slag, and the molten steel will have different degrees of silicon increase. On the other hand, the slag is mainly added during the refining process, and the SiO2 and Al2O3 in the slag are diluted. concentration.

The white slag is formed within 10 minutes before the slag making process. After the refining, the final slag basicity is 4.5~5.0. The steel slag with high alkalinity, good fluidity and low oxidization is beneficial to the steel slag interface reaction and the maximum removal of oxygen in the steel. The oxide inclusions in the steel are floated up and absorbed by the refining slag to improve the purity of the molten steel.

The slag-forming route of bearing steel from the tapping of the converter to the end of refining is shown in Figure 1. It can be seen from Fig. 1 that the content of CaO is basically unchanged in this process; the content of SiO2 is reduced, which is due to the fact that there is more acid-soluble aluminum in the molten steel; and the Al2O3 content in the slag is slightly increased. With the prolonged processing time of the ladle, the large-particle inclusions have sufficient time to float. On the other hand, after the refining, the molten steel is weakly stirred, so that the fine inclusions can collide with each other, and after being floated, they are adsorbed by the refining slag, and finally Calcium treatment of molten steel makes the Al2O3 inclusions denatured, effectively avoiding the occurrence of water shutoff and ensuring stable and smooth production.

2.3. Change law of alkalinity in refining process

The refining slag should have the ability to absorb the deoxidation product Al2O3 inclusions in order to minimize the amount of oxide inclusions during the argon agitation of the molten steel. The high alkalinity (R=4.5) slag refining bearing steel represented by the Japanese Sanyo process has reduced the sulfur and Al2O3 inclusion content to a very low level.

The change of alkalinity in the process of refining of bearing steel, taking the binary alkalinity as an example, the LD+LF+CC process, the alkalinity of reaching the refining station is about 3, the alkalinity of the refining slag rising rapidly, and the alkalinity after refining Maintained between 4 and 5; and the alkalinity of the LD+LF+VD+CC process refining process can reach 6~7. After vacuum degassing, the binary alkalinity decreases slightly, and finally it is about 4 .

3. Desulfurization effect of refining slag

The synthetic slag is used together with the submerged arc slag, and the submerged arc effect is good in the slag forming process, which greatly improves the thermal efficiency of the LF furnace. After 10 minutes of slag formation, the slag system with w(FeO+MnO)<1% is basically formed, and the alkalinity is compared. High, meets the thermodynamic conditions of desulfurization, and obtains a good desulfurization effect. It can be seen from Table 2 that under the condition that the average ton steel consumption is 9.31kg synthetic slag and 2.79kg submerged arc slag, the desulfurization rate reaches about 80%, especially within 10min from the beginning of slagging, the desulfurization effect is obvious, and the desulfurization rate is up to About 75%.

Note: S1, S2, S3, and S4 indicate the sulfur content of the converter during tapping, before refining, after refining for 10 min, and when refining is completed; η12 and η14 respectively indicate the desulfurization rate from steel to refining and from tapping to refining.

4. Oxygen content in bearing steel

Oxygen content is an important indicator to measure the quality level of bearing steel. For aluminum deoxidized bearing steel, the dissolved oxygen content in molten steel is required to be <2×10-6. Therefore, it is especially effective to remove oxide inclusions in molten steel. important.

The total oxygen in the bearing steel includes dissolved oxygen and oxygen in the inclusions, so the removal of oxygen is discussed in two aspects. On the one hand, oxygen in the molten steel enters the refining slag by diffusion, so it is necessary to ensure that the steel slag has low oxidizing property, and only a certain oxygen potential difference between the steel slag can continuously diffuse oxygen into the slag. The oxygen in the refining slag mainly exists in the form of FeO and MnO. In the different periods of the refining process of the bearing steel, the slag sample is taken, and the content of each component is analyzed by chemical analysis. The variation of the relationship between FeO and MnO is shown in Fig. 3. Show. The results show that during the slagging process, the oxygen in the slag drops sharply due to the addition of the deoxidizer. After the reduction slag is formed, the oxygen in the slag is stable at about 1%. Figure 3: FeO+MnO content in refining slag of different production processes

On the other hand, through the above process, it can be known that the refining process forms a high-alkalinity white slag with good fluidity as early as possible, and absorbs non-metallic inclusions (mainly Al2O3) in the molten steel, and in the production process, as far as possible to ensure the refining later Sufficient argon blowing to remove the non-metallic inclusions operating time, so that the total oxygen content of the bearing steel rolled material is lower, improving the fatigue life of the bearing steel.

After the smelting of the modified steel, the total oxygen content of the steel is <12×10-6, and the lowest total oxygen content is 5×10-6, which can fully meet the production requirements of bearing steel.

5,

5.1. According to the conditions of refining equipment, formulate two refining process routes (LD+LF+CC and LD+LF+VD+CC) in accordance with the production of bearing steel; the refining process uses CaO-SiO2-Al2O3 ternary refining slag system, and gives The composition range of the refining slag system is obtained; the variation of each component in the refining process is that the CaO content is basically unchanged, the content of SiO2 is slightly decreased, and the content of Al2O3 is slightly increased; LD-LF-CC and LD are adopted. The -LF-VD-CC process produces qualified products.

5.2. During the refining process, the alkalinity gradually increases. After the refining, the final slag alkalinity is maintained at 4.5~5.0. Under the condition of 9.31kg of synthetic slag and 2.79kg of submerged arc slag, the desulfurization rate of refining slag reaches 80. The %.5.3CaO-SiO2-Al2O3 refining slag system can remove free oxygen and oxide inclusions in the steel, and the total oxygen content in the steel is lower than 12×10-6.