Free Access
Issue
Metall. Res. Technol.
Volume 117, Number 4, 2020
Article Number 409
Number of page(s) 6
DOI https://doi.org/10.1051/metal/2020042
Published online 29 July 2020

© EDP Sciences, 2020

1 Introduction

At present, the blast furnace still has an irreplaceable position in industrial ironmaking. Steel companies need to extend the life of their blast furnaces while maintaining high productivity to reduce production costs. So, the blast furnace longevity technology plays an important role in blast furnace ironmaking technology due to its irreplaceable position in industrial ironmaking, which allows the blast furnace operate in long life with high productivity. The longevity technology has greatly improved the service life of refractory materials above hearth, such as optimization of blast furnace type, copper cooling wall and soft water closed circulation cooling system [112]. However, the erosion of hearth area has become the limiting factor of blast furnace longevity, because the hearth lining has endured high temperature, high pressure, molten iron flow scouring and alkali erosion in all time, which make the hearth area keep in the severe erosion situation [1318]. And the limited on-line monitoring and control means in the blast furnace hearth area, the burn-through accidents of the blast furnace hearth and bottom occur occasionally. According to incomplete statistics of the partial blast furnace accidents in China from 2008 to 2017, the erosion of hearth account for about 30% of the blast furnace accidents, which cause a serious damage to personnel safety, environmental protection and enterprise economy. Table 1 shows the domestic accidents of blast furnaces with incomplete statistics in the past 10 years.

At present, the technology of the on-line control of the hearth is achieved by online adding titanium-containing materials in the blast furnace smelting process both inside and outside China [2527]. In the 1960s, this technology was widely applied in Japan to extend the blast furnace service life. In the 1970s, some industrial experience papers about using titanium-containing materials to control the temperature fluctuation of hearth wall and prolong the blast furnace service life were published by Japanese journals, which were widely recognized by international peers [27]. In the late 1980s, a large number of in-depth studies on blast furnace control by titanium ore were published. WISCO (Wuhan Iron and Steel Corporation) in China reported that the phenomenon of intense temperature fluctuation and hearth erosion disappear after continuous addition of vanadium titanium ore for nearly three months and the furnace temperature remained stable in the normal range after 10 months without vanadium titanium ore [28]. After the 1990s, the smelting of titanium materials to extend the blast furnace life has been recognized all over the world. Meanwhile, the control mechanism of adding titanium-containing minerals to protect lining has been extensively investigated [26,27,29,30]. But, the mechanism on-line control of hearth still has different opinions among researchers inside and outside China. The homogeneous nucleation theory of Ti(C, N) proposed by Frederiksson et al indicated that Ti(C, N) generated from the reaction between the dissolved supersaturated titanium in molten iron near the carbon liner surface and coke or carbon lining and, then, gather and grow on the surface of coke and carbon liner [31]. However, this theory cannot explain the formation of protective layer of Ti(C, N) on the lining surface, because the density of Ti(C, N) is between slag and iron, the phase of high melting point is floating between slag and iron instead of the surface of resistant material. The CPVS (Cooling, Precipitation, Viscous, Solidification) theory proposed by Zhao think that TiC, TiN and Ti(C, N) will form in the boundary layer when the solubility of titanium in the molten iron is larger than 0.04% or even form earlier outside of the boundary layer, which depend on the dissolved nitrogen content in the molten iron, as the temperature of molten iron in the boundary layer decreases [32]. With the precipitation and aggregation of Ti(C, N) particles, the viscosity of molten iron in the Ti(C, N) formation region increased, forming an iron protective layer riched in Ti(C, N). However, the blast furnace process using titanium-containing materials as the raw material will bring some problems, such as slag and iron sticky and foaming. Blast furnace anterograde must be concerned when the titanium-containing materials used in on-line control process.

In this study, based on the previous theoretical research, Ti(C, N) precipitation process was regulated online by using the self-built experimental platform of high melting point phase online precipitation, the mechanism of on-line control of hearth erosion by adding titanium-containing materials was further investigated.

Table 1

Partial blast furnace accidents in China from 2008 to 2017 [1924].

2 Experimental

In order to achieve the precipitation of high melting point phases and process control in high-temperature molten iron, the self-built platform of heat flow regulation was built for the mechanism study of on-line control of hearth erosion, which was shown in Figure 1. There are four parts in the platform, including the heating system of the silicon molybdenum furnace, the online monitoring cooling system, the program controller and the gas distribution system. The heating system of silicon molybdenum furnace is an independent research and development product of Chongqing University. This system can achieve accurate temperature control in various atmospheres, and precise platform lifting control. The system uses energized silicon-molybdenum rods as a heat source and is installed in refractory materials. The heat is transferred to the sample in the middle of the furnace through the refractory material, so that the sample is heated and reaches the target temperature. The thermocouple is installed at the bottom of the furnace and the temperature measuring end is inserted in the bottom of the sample (in order to the measured temperature is as close as possible to the sample temperature). Protective gas is introduced from the bottom of the system to ensure the samples keep in the atmosphere required during the experiment. The heating process of the system is controlled by the program controller. The chemical components of the raw materials were shown in Table 2. The purity of argon and nitrogen take from Chongqing Ruixin Gas Company limited were 99.999% and 99.999% respectively.

A 1 kg samples, which was mixed by Ti-Fe alloy and pig iron in titanium content of 0.3213% was placed into a crucible. The crucible was heated to 1773 K in furnace and kept for 4 hours. The whole process was flowed by argon and nitrogen in molar volume ratio 1:1 and the gas flow were 500 ml/min. When the samples were fully melted, the bottom of the cooling system, in which the water flow rate was set 160 L/h, was immersed in molten iron and the outlet temperature of cooling water was recorded in real time. When the temperature of cooling water does not change more than 1 K in 10 seconds, the cooling system was taken out from the crucible. After the precipitated sample was cooled, close the water inlet of the cooling system and take out the sample.

The solidification shell taken from the bottom and sides of the cooling system was polished by sandpaper. The morphology and chemical composition of samples were characterized by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) (TESCAN VEGA II with Oxford INCA Energy 350).

thumbnail Fig. 1

The experimental platform of the on-line control of the hearth erosion (1: the heating system; 2: the online monitoring cooling system; 3: the program controller; 4: the gas distribution system).

Table 2

Chemical components of the raw materials (%).

3 Thermodynamic analysis

The relationship between the solubility of titanium and the precipitation temperature of TiC in carbon-saturated molten iron was investigated by our groups and others, which was presented in Figure 2 [3337]. The results of the studies show that the solubility of titanium in carbon-saturated molten iron is less than 0.2% when the temperature is less than 1423 K. That is to say, according to the theoretical results of the solubility of titanium in carbon-saturated molten iron, during the cooling of molten iron, the amount of TiC precipitated from the molten iron due to the saturation of titanium is extremely small, relative to iron. And our group also investigated the precipitation process and morphology of TiC [37]. The precipitated phase particles were small and did not aggregate, which can be observed by confocal laser scanning microscope (CLSM) (VL2000DX-SVF17SP) in Figure 3. Figure 4 present SEM image and EDS analysis of the sample after CLMS analysis. From the SEM image and EDS analysis shown in Figure 4, the precipitation phase, composed of TiCxOy and TiC, has a regular shape and did not aggregate.

thumbnail Fig. 2

The relationship between the solubility of titanium and precipitation temperature of TiC in carbon-saturated molten iron.

thumbnail Fig. 3

The precipitation temperature of the high melting phase of molten iron with titanium content of 0.31%, 0.36% and 0.47%.

thumbnail Fig. 4

SEM image and EDS analysis of the sample after CLMS analysis.

4 Results and discussion

Figure 5 shows the solidification shell of the hearth erosion on-line control experiment. The solidified shell is wrapped around the cooling tube, and the thickness of the shell is around 2–3 mm. In order to further obtain the microstructure and composition of the solidified shell, the solidified shell was peel off from the cooling copper tube and grinded and polished. Figure 6 present the SEM images, processed SEM images and EDS analysis of samples at bottom and inner side wall. The SEM images and EDS analysis indicated that the solidification shell is mainly composed of iron and a small amount of TiC both in bottom and inner side wall of sample. The content of the TiC in the sample was obtained by calculating the ratio of TiC surface area to total image area in processed SEM images. The calculated results show that the content of the TiC in bottom and inner side wall of the sample is 0.89% and 0.69% (surface area %) respectively. Although erosion causes the thickness of lining to decrease, the thermal resistance of erode lining in blast furnace hearth still much higher than that of copper tube in the experiment of on-line control of hearth erosion. So, the content of TiC in solidification shell in blast furnace hearth would higher than that of in the experiment.

Base on theoretical and experimental results, the on-line control mechanism of blast furnace hearth erosion zone is proposed and the schematic diagram is shown in the Figure 7. The temperature decreases gradually from the center of the blast furnace hearth to the lining, which the blast furnace hearth can be divided into three regions: molten iron zone, erosion/protective layer zone and lining zone. The temperature gradient varies greatly at the erosion/protective layer zone due to cooling effect of the cooling system outside the blast furnace lining. From the center of the hearth to the bottom lining, the temperature gradually decreased from 1773 K to 1423 K, and the viscosity of the molten iron increased accordingly, as shown in Figure 7a. When the lining of blast furnace hearth was eroded, the thermal resistance in eroded zone decreased due to the lining becomes thinner. Hence, the temperature of the molten iron closed to the eroded lining decreased, which lead to the dissolved titanium supersaturated precipitation Ti (C, N) and dispersed in this region, as shown in Figure 7b. With the precipitation of Ti(C, N) particles in the eroded area, the viscosity of molten iron increases, decreasing the fluidity of molten iron in this area. The heat transfer enhancement in the erosion zone further reduces the temperature of molten iron with poor fluidity and, therewith, the molten iron solidified firstly adjacent to the erosion zone of blast furnace lining, as shown in Figure 7c. As the thickness of the Ti(C, N) contained solidified shell increases, the thermal resistance of the protective layer gradually increases. When heat transfer is insufficient to lower the temperature of molten iron outside the solidified shell below the solidification temperature, the solidified shell ceases to grow, as shown in Figure 7d. Finally, as shown in Figure 7e, a protective layer with carbon saturated pig iron as matrix and Ti(C, N) as solid phase fulcrum is formed, which can replace the eroded blast furnace lining. On the one hand, the precipitation of high melting point Ti(C, N) increases the viscosity of molten iron, making the solidified shell easier to form. On the other hand, high melting point Ti(C, N) particles play the role of solid fulcrum in the solidified shell, improving the strength of the protective layer.

thumbnail Fig. 5

The solidification shell of the experiment of on-line control of hearth erosion.

thumbnail Fig. 6

SEM images of the solidification shell, processed SEM images and EDS analysis of the same sample respectively (a: the bottom; b: inner side wall).

thumbnail Fig. 7

On-line control mechanism of blast furnace hearth erosion zone.

5 Conclusions

The heat flow control platform can be used for the on-line precipitation of high melting point phases in high temperature melts.

The experiment of on-line control of hearth erosion suggests that the Ti(C, N) particles were generated by the precipitation of supersaturated dissolved titanium in carbon-saturated molten iron. There are three steps for the on-line control of smelting titanium-containing materials in blast furnace. Firstly, Ti(C, N) precipitated from the molten iron due to the decrease of temperature in the erosion zone of the hearth. Secondly, the viscosity of the molten iron increased with the precipitation of Ti(C, N) in the erosional cryogenic zone. Finally, the protective layer, included pig iron matrix and Ti(C, N) solid fulcrum, formed because of the solidification of the molten iron.

High melting point Ti(C, N) increases the viscosity of molten iron, making the solidified shell easier to form and play the role of solid fulcrum in the solidified shell, improving the strength of the protective layer. Hence, the protective layer can replace the corroded lining of blast furnace to prevent its hearth from being eroded.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (grant No. 51674054).

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Cite this article as: Yan Li, Tingfang Jian, Tongxiang Ma, Meilong Hu, Leizhang Gao, Yu Yang, Effect of titanium on the skull formation of the blast furnace hearth, Metall. Res. Technol. 117, 409 (2020)

All Tables

Table 1

Partial blast furnace accidents in China from 2008 to 2017 [1924].

Table 2

Chemical components of the raw materials (%).

All Figures

thumbnail Fig. 1

The experimental platform of the on-line control of the hearth erosion (1: the heating system; 2: the online monitoring cooling system; 3: the program controller; 4: the gas distribution system).

In the text
thumbnail Fig. 2

The relationship between the solubility of titanium and precipitation temperature of TiC in carbon-saturated molten iron.

In the text
thumbnail Fig. 3

The precipitation temperature of the high melting phase of molten iron with titanium content of 0.31%, 0.36% and 0.47%.

In the text
thumbnail Fig. 4

SEM image and EDS analysis of the sample after CLMS analysis.

In the text
thumbnail Fig. 5

The solidification shell of the experiment of on-line control of hearth erosion.

In the text
thumbnail Fig. 6

SEM images of the solidification shell, processed SEM images and EDS analysis of the same sample respectively (a: the bottom; b: inner side wall).

In the text
thumbnail Fig. 7

On-line control mechanism of blast furnace hearth erosion zone.

In the text

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