Process Development and Practical Research on Converter Smelting of Low-Phosphorus Clean Steel

This process uses a metallurgical equipment limited company to produce an LF refining furnace. The main equipment parameters are shown in Table 1 below.

The LF refining furnace clean steel smelting process is shown in Figure 1.

Figure 1.

LF refining furnace clean steel smelting process

Operation requirements: preoxygenation should be carried out in the process of iron water discharge in order to reduce the difficulty of subsequent deoxygenation; the alkali concentration should be adjusted according to the different types and cleanliness of clean steel; the heating process adopts strong electromagnetic stirring; stirring is required when carrying out degassing treatment to improve the degassing effect.

The ionic theory of slag structure allows for a greatly simplified description of the main slag-metal equilibries. Its advantage lies in the fact that it describes the physico-chemical behavior of alkaline slag with a much smaller number of flooded components than the molecular theory. The ionic theory is a more accurate depiction of the current state of alkaline steelmaking slag because it is consistent with the measured data on the physicochemical properties of the slag, such as conductivity and viscosity. In recent years, research on dephosphorization has mainly been based on ionic theory and analysis.

Considering the dephosphorization reaction formula should adopt the basic reaction equation (1), and then use the experimental data to determine the effect of changes in slag composition on the reactants and reactant activity. 12[P]+5[O]=(P2O5)$$2\left[ P \right] + 5\left[ O \right] = \left( {{P_2}{O_5}} \right)$$ 2KP=aP2O5a[O]5a[P]2=r(P2O5)⋅X(P2O5)a[O]5a[P]2$${K_P} = \frac{{{a_{{P_2}{O_5}}}}}{{a_{\left[ O \right]}^5a_{\left[ P \right]}^2}} = \frac{{{r_{\left( {{P_2}{O_5}} \right)}} \cdot {X_{\left( {{P_2}{O_5}} \right)}}}}{{{a_{\left[ O \right]}}^5a_{\left[ P \right]}^2}}$$ 3lgKP=36850/T−29.07$$\lg {K_P} = 36850/T - 29.07$$

Based on the ionic theory of slag, the following equations were derived using thermodynamic data on the activity and phosphate free energy of phosphorus in the Cao-P₂O₅ binary (0<(%CaO)<50)$$\left( {0 < (\% {\text{CaO}}) < 50} \right)$$ 4lg[%P][%P]=22350/T−16.0+0.08(%CaO)+2.5lg(T.Fe)$$\lg \frac{{\left[ {\% P} \right]}}{{\left[ {\% P} \right]}} = 22350/T - 16.0 + 0.08\left( {\% CaO} \right) + 2.5\lg \left( {T.Fe} \right)$$

Modern converter steelmaking, in addition to following the theory of steelmaking in alkaline flat furnaces, is saturated with slag MgO content in order to protect the furnace lining. Compared with the traditional steelmaking slag system, the MgO content of modern converter slag has undergone more important changes, and the MgO-saturated CaO − MgO − SiO₂ − FeO-slag system can be more accurately used as a generalization of the alkaline oxygen converter slag.The dephosphorization equilibrium of the MgO-saturated CaO − MgO − SiO₂ − FeO-slag system with the ferro-liquid was investigated by Suito et al. Using their own experimental data, they derived the formula for the phosphorus partition ratio by correcting the Healy formula to the Turkdogan ET rP2O5$${r_{{P_2}{O_5}}}$$ formula. 5lg(LT)=25log(%T,Fe) +0.072[(%CaO)+0.3(%MgO)]+11570T−10.554$$\begin{array}{l} \lg \left( {{L_T}} \right) = 25\log \left( {\% T,Fe} \right) \\ \qquad\quad\; + 0.072\left[ {\left( {\% CaO} \right) + 0.3\left( {\% MgO} \right)} \right] + \frac{{11570}}{T} - 10.554 \\ \end{array}$$ 6lgrP2O5=−1.01(23NCaO+17NMgO+8NFeO)−26300T+11.2$$\lg {r_{{P_2}{O_5}}} = - 1.01\left( {23{N_{CaO}} + 17{N_{MgO}} + 8{N_{FeO}}} \right) - 26300T + 11.2$$

Equation (5) is generally regarded as the equilibrium calculation formula for the converter smelting endpoint Lp.

The systematic study of MgO-saturated steelmaking slag system is more suitable for the current production of the actual conditions of the converter, which can be used as a standard for calculating the theoretical equilibrium of dephosphorization. The equilibrium experimental research in the laboratory for the equilibrium state of the slag and liquid steel, mainly considering the slag as a source of oxygen, while not considering the effect of molten pool carbon and silicon content on the equilibrium. The actual production of converter element oxidation by the slag oxygen level and molten pool oxygen level, respectively, the dephosphorization is accompanied by de-siliconization, carbon reaction, and the reaction has not reached equilibrium. To study the law of dephosphorization in the converter smelting process, the following dephosphorization oxygen and influencing factors are analyzed.

According to the suito formula, Lp between slag and steel can be calculated under the equilibrium state of dephosphorization reaction. In the equilibrium of smelting process, Lp is far away from the equilibrium in the pre-smelting period, and the end point of smelting is gradually close to the equilibrium.

As smelting proceeds, Lp gradually approaching equilibrium; smelting pre-smelting, Lp deviates from the equilibrium farther, mainly due to the smelting pre-smelting time is short, the equilibrium state is gradually changing, and kinetic stirring conditions are poor; the smelting process temperature is gradually increased, and by the decarbonization of the oxygen level of the influence of the Lp deviates from the equilibrium, but the degree of deviation than the previous weak; smelting end, the lowest level of the oxygen level of dephosphorus and slag oxygen levels close to the Lp gradually approaching the equilibrium. The Lp is gradually approaching the equilibrium.

According to the law of selective oxidation of elements, it can be seen that the smelting of pre- and post-smelting have the thermodynamic conditions of dephosphorization. Test dephosphorization kinetics and dephosphorization rate analysis shows that the smelting converter smelting process can only realize effective dephosphorization in the pre-blowing and blowing later.

Converter dephosphorization is determined by the melt pool thermodynamic (Lp) and kinetic conditions: improve Lp is conducive to reducing the equilibrium phosphorus content between slag and steel, improve the reaction rate. Strengthen the molten pool stirring, promote slag emulsification can improve the reaction speed of dephosphorization, inhibit decarburization; pre-dephosphorization thermodynamic conditions, but the reaction is far from equilibrium, improve the kinetic conditions is to improve the effect of dephosphorization of the technical key. Later dephosphorization reaction tends to thermodynamic equilibrium, improve the final slag conditions, catch high Lp is to improve the effect of dephosphorization technology key.

The study of Lp dephosphorization between converter slag and steel mainly follows the conclusion of the equilibrium state, while the actual converter smelting endpoint is limited by the reaction time. The smelting endpoint is a non-equilibrium state. The following analysis of the actual converter smelting endpoint apparent Lp.

According to the dephosphorization reaction formula (1), formula (2), (3), (4), the Lp benchmark formula: 7lg(%P)[%P]=2.5lga(FeO)−0.5lg(γ1P2O5) −a/T+lg(fp)+0.5lg(%P2O5)+m$$\begin{array}{c} \lg \frac{{\left( {\% P} \right)}}{{\left[ {\% P} \right]}} = 2.5\lg {a_{\left( {FeO} \right)}} - 0.5\lg \left( {{\gamma _{1{P_2}{O_5}}}} \right) \\ \qquad\qquad\qquad\quad- a/T + \lg \left( {{f_p}} \right) + 0.5\lg \left( {\% {P_2}{O_5}} \right) + m \\ \end{array}$$

where:

a is the temperature coefficient and m is a constant. 8lga(FeO)=lgγFeO+lg(XFeO)$$\lg {a_{\left( {FeO} \right)}} = \lg {\gamma _{FeO}} + \lg \left( {{X_{FeO}}} \right)$$ 9lg(γFeO5/γP2O5)=0.152{(%CaO)+0.3(%MgO)} +15350/T+4$$\begin{array}{l} \lg \left( {\gamma _{FeO}^5/{\gamma _{{P_2}{O_5}}}} \right) = 0.152\left\{ {\left( {\% CaO} \right) + 0.3\left( {\% MgO} \right)} \right\} \\ \qquad\qquad\qquad\quad+ 15350/T + 4 \\ \end{array}$$

x(FeO)=(%T.Fe)56⋅∑ni∑ni$${x_{\left( {FeO} \right)}} = \frac{{\left( {\% T.Fe} \right)}}{{56 \cdot \sum {{n_i}} }}\sum {{n_i}}$$ is the sum of the mole fractions of the elements in the slag (taken as 1.6).

The influence of dephosphorization slag alkalinity on the dephosphorization effect is shown in Figure 2. When the alkalinity of dephosphorization slag is 2.0 to 2.6, the dephosphorization rate gradually increases with the increase of alkalinity. The main alkaline oxides in the converter dephosphorization slag are CaO, MgO, MnO and FeO, of which CaO has the strongest dephosphorization ability. As the alkalinity of dephosphorization slag increases, the activity of lime also increases gradually, and CaO in the slag will combine with P₂O₅ to form stable calcium phosphate, which is conducive to further deep dephosphorization. Therefore, increasing the slag alkalinity can effectively improve the effect of dephosphorization. But the alkalinity is not easy to be too high, otherwise there will be a large number of CaO, MgO particles suspended in the liquid slag in the slag, reducing the mobility of the slag, making the slag sticky, not easy to dephosphorize. Comprehensive consideration of the end point of dephosphorization on the requirements of the phosphorus content of half-steel, should keep the slag alkalinity in 2.2 ~ 2.3.

Figure 2.

Effect of alkalinity of dephosphorized slag on dephosphorization rate

The effect of the pre-dephosphorization time on the dephosphorization rate is shown in Figure 3. The pre-dephosphorization time is from 225s to 375s, and with the prolongation of the dephosphorization time, the dephosphorization rate gradually increases. Through an in-depth understanding of the smelting process, the pre-slag pouring time should not be too early, otherwise the slag that has just been made is poured out, and this slag does not play its dephosphorization effect. Nanshan Iron and Steel re-blowing 120 t converter pre-phosphorus removal time control in 325s ~ 425s (average of 378s), to achieve a converter smelting pre-pour slag dephosphorization rate of 53.2% ~ 69.7% (average 62.2%). Comprehensive analysis of various influencing factors, according to the oxidation mechanism of phosphorus in steel, its reasonable slag pouring time should be controlled in the full range of 35% to 44% of the blowing time. For 100 t double blowing converter smelting, smelting high carbon steel clean steel for the whole oxygen supply time of 14 min, so the preliminary dephosphorization time control in the open blowing after 300s ~ 350s is appropriate.

Figure 3.

Effect of early dephosphorization time on dephosphorization rate

The effect of half steel temperature on the dephosphorization rate is shown in Figure 4. Semi-steel temperature in 1360 °C ~ 1450 °C, with the increase in temperature, the dephosphorization rate gradually decreased, but the overall change is small. Because the affinity of oxygen and silicon in iron water is stronger than phosphorus, at the lower temperature in the early stages of converter blowing, blowing oxygen to the molten pool and adding slagging agent dephosphorization, silicon and manganese are more preferentially oxidized than phosphorus, and phosphorus begins to oxidize in large quantities only when silicon is oxidized to trace amounts. With the dephosphorization reaction and melting pool temperature, carbon and phosphorus will also appear selective oxidation transition, that is, below a certain temperature in the liquid iron phosphorus preferential oxidation, and vice versa, carbon preferential oxidation, phosphorus oxidation is inhibited. Therefore, to convert the blowing pre-melting pool carbon and phosphorus selective oxidation temperature to a slagging operation control temperature. Theoretical calculations in the principle of iron and steel smelting indicate that the carbon and phosphorus selective oxidation temperature of 1332 °C. Li Jianxin et al analyzed the carbon and phosphorus selective oxidation temperature of 1320°C through the theoretical calculation of the double slag method of phosphorus removal. Liu Yue et al. will be an inverted furnace slagging of molten steel (or half-steel) temperature control at 1390 °C ~ 1420 °C, also obtained a high rate of dephosphorization. In addition, the furnace temperature is adjusted to ensure half-steel mobility and meet the requirements for rapid melting of slag. Semi-steel temperature is low, it is difficult to obtain high alkalinity, good fluidity of the uniform slag, the temperature can be increased to reduce the slag viscosity, accelerate the melting of lime, which is conducive to improving the kinetic conditions of phosphorus from the metal phase to the transfer of dephosphorization slag. According to the 100 t double blowing converter molten iron temperature, steel material structure and slag fluidity and other actual conditions, the half steel temperature control in 1390 °C ~ 1420 °C.

Figure 4.

Effect of semi-steel temperature on dephosphorization rate

Iron carbon mass fraction of 3.0% ~ 4.0%, converter blowing process carbon and phosphorus selective oxidation temperature of 1250 ~ 1400 °C. The higher the molten steel temperature, the more intense the carbon and oxygen reaction, the stronger the ability of the carbon in the molten iron to capture the oxygen required for the phosphorus reaction; however, the dephosphorization temperature is too low, the slag can not be penetrated, the slag mobility is poor, viscosity is high, and is also not conducive to dephosphorization. Should be selected according to the converter smelting conditions that are conducive to improving the dephosphorization rate, but also conducive to maintaining the slag temperature.

The relationship between the end temperature of the dephosphorization period and the dephosphorization rate is shown in Figure 5. Overall, the higher the temperature of the dephosphorization period, the lower the rate of dephosphorization, after a large number of furnace smelting tests and slag effect comparison, the dephosphorization period of the temperature control in 1300 ~ 1330 °C can maintain a higher rate of dephosphorization, but also conducive to the slag melting, to improve the stability of the dephosphorization control in the early stage.

The relationship between slag alkalinity and dephosphorization rate at the end of the dephosphorization period is shown in Figure 6. With the increase of slag alkalinity, the dephosphorization rate gradually increased. When the slag alkalinity is greater than 2.0, the dephosphorization rate reaches the highest point, and the slag alkalinity continues to increase, and the dephosphorization rate does not have a significant trend of improvement.

Too high alkalinity control in the dephosphorization period requires higher temperature slagging and higher FeO content, and high temperature and high FeO conditions will promote the premature occurrence of carbon and oxygen reactions, which is not conducive to the pre-dephosphorization reaction. Therefore, the control of slag alkalinity of about 1.5 to 2.0 can not only save white ash consumption, but also improve the rate of iron dephosphorization.

The relationship between carbon content and dephosphorization rate at the end of the dephosphorization period is shown in Figure 7. As the amount of iron decarbonization increases, the dephosphorization rate increases. With the increase of blowing time, the slag gradually melts and mixes with the molten steel, which is favorable for the dephosphorization reaction, so the dephosphorization rate of molten iron in the dephosphorization period increases with the decrease of the half-steel carbon content.

The end of the dephosphorization period half steel carbon content control, to consider for the decarburization period to retain sufficient heat source (molten steel carbon content), dephosphorization period half steel carbon mass fraction is not easy to control too low, should be not less than 2.6%. According to a comparison of many tests and operations, the carbon mass fraction control of the dephosphorization period is between 3.1% and 3.5%, with half-steel dephosphorization rate being higher.