DEVELOPMENT OF “SLAG-REMAINING+DOUBLE-SLAG” BOF STEELMAKING TECHNOLOGY IN SHOUGANG CO.
Haibo Li1, Yanchun Lu1, Guosen Zhu1, Xinhua Wang2
1Shougang Research Institute of Technology; Shijingshan District, Beijing, 100041, China
2University of Science and Technology Beijing; 30 Xueyuan Road, Beijing, 100083, China
Keywords: BOF Steelmaking, Less Slag, Lime Consumption, Dephosphorization, Slag Abstract
The “Slag-Remaining+Double-Slag” BOF steelmaking process has been developed and applied in Shougang Corporation, Ltd., by which consumption of lime and volume of slag in BOF steelmaking are remarkably decreased. In this paper, three important technologies taken in application of the new steelmaking process are introduced: 1. To solve the two most serious problems, i.e. difficult to make fast and enough amount of deslagging and decrease the metallic Fe droplets in the slag, low basicity slag is used in the dephosphorization stage. 2. Hard blow pattern is adopted to utilize the top blown O2 jet to strengthen agitation of the bath in the dephosphorization stage, through which good dephosphorization has been obtained. (3) By speeding up the steelmaking operations and particularly better matching the BOF, secondary refining and continuous casting productions, productivity has not been reduced though the BOF tap to tap time has been increased by about 4min after using the new process.
1. Introduction
The Chinese steel industry which has developed rapidly in the past two decades is facing serious challenges in saving natural resources, energy conversation and reducing the solid wastes generated in steel production. Taking the BOF steelmaking in China for instance, it produced about 700 million tons of steels last year and consumed about 35 million tons of burnt limes, 10 million tons of dolomites and generated about 70 million tons of slags.
In 2001, Nippon Steel Corporation [1] reported the idea of “Slag-Remaining+Double-Slag”
steelmaking process and the experiment result using an 8t converter. The new steelmaking process was named as MURC, in which, the blowing is divided into two stages, i.e. the dephosphorization stage and decarburization stage. When the blowing has been completed, the slag is kept in the furnace after tapping the steel and reused in the dephosphorization stage of the next heat. It was reported [2-6] that MURC has been applied in Oita, Yawata, Muroran and Kimitsu Steelworks of Nippon Steel Corporation.
In 2009, Shougang Corporation started the investigation on developing the “Slag-Remaining +Double-Slag” steelmaking process. Now, the new steelmaking process has been applied on a large scale in its Qianan Steelworks which has five 210t combined blowing converters and Shouqin Steelworks which has three 100t combined blowing converters. After using the new process, consumptions of the burnt lime and dolomites have been decreased by more than 47%
Advances in Molten Slags, Fluxes, and Salts: Proceedings of The 10th International Conference on Molten Slags, Fluxes and Salts (MOLTEN16) Edited by: Ramana G. Reddy, Pinakin Chaubal, P. Chris Pistorius, and Uday Pal TMS (The Minerals, Metals & Materials Society), 2016
and 55% and, the volume of slag generated in BOF steelmaking has been decreased by 30%, respectively.
2. The “Slag-Remaining+Double-Slag” Steelmaking Process developed in Shougang Co.
As illustrated in Figure 1, the “Slag-Remaining+Double-Slag” steelmaking process developed mainly consists of the following operations: (1) keeping the molten slag in the furnace after tapping of steel, (2) blowing in N2 with the top lance to splash part of the molten slag on the lining wall, (3) adding in some lime and dolomite to fully solidify the slag, (4) charging scrapes and hot metal, (5) dephosphorization stage blowing, (6) deslagging off 50-60% slag from the furnace, (7) decarburization stage blowing, and (8) tapping while remaining the slag for next heat.
Figure 1. Illustration of the “Slag-Remaining+Double-Slag” process applied in Shougang Co
The dephosphorization stage lasts about 3.5-4 minutes, in which about 27% of the total O2 is blown and 35% of the total lime is added. The temperature of the metal bath at end of the dephosphorization stage is between 1320-1370 C. In decarburization blowing stage, O2 is blown for 9.5-10 minutes and rest of flux is added in. The total lime consumed is about 22kg/t in Qianan Steelworks and 35kg/t in Shouqin Steelworks.
Taking use of the influence of the temperature change on dephosphorization is the main principle of the new steelmaking process. Taking the equilibrium constant of the dephosphorization reaction expressed by Equation (1) for instance, the equilibrium constant (Kp) calculated with the thermodynamic data given by Equation (2) and (3) [7,8] is about 1.68×10-11 at the temperature around 1680 C. However, at the temperature around 1350 C, Kp is about 5.59×
10-7, more than 30,000 times larger than the Kp at 1680 C.
2[P]+5[O]=(P2O5) (1)
△G0= -832384+632.65T (2) log Kp=log (aP2O5/a[P]2
/a[O]5
) = 43443/T-33.02 (3)
In “Slag-Remaining+Double-Slag” steelmaking, the slag at the blowing end has almost no dephosphorization ability due to the high temperature. However, if the slag is used in the early blowing period of the next heat, i.e. the dephosphorization stage, it can regain the dephosphorizaton power because of the temperature decrease. But, the intermediate deslagging operation must be made to pour off most of the reused slag between the dephosphorization stage and the decarburization stage. Otherwise, rephosphorization from the slag will occur owing to the temperature increase in later blowing period.
Since the slag of the previous heat can be reused, consumptions of the burnt lime and dolomite and volume of the slag can be remarkably decreased. Yield of steel has also increased due to the decrease of the exhausted slag which contains 14-25 mass% FetO. In addition, as most of the exhausted slag in the new steelmaking process is the low basicity slag formed in the dephosphorization stage, treatment of the exhausted slag can be simplified.
3. Key Technologies of the New Steelmaking Process 3.1 Fast and Enough Deslagging after Dephosphorization Stage
In the “Slag-Remaining+Double-Slag” steelmaking process, whether or not fast and enough amount of deslagging can be made after the dephosphorization stage is of great importance. If the deslagging amount is not enough, the slag in the furnace will accumulate heat by heat. The basicity of the slag will increase and it makes more difficult to pour off the slag. Consequently, the normal circulation of the process will be stopped. Furthermore, if enough deslagging cannot be made, large amount of iron droplets will be wrapped in the slag and be poured off the furnace during deslagging. In addition, the process control can also be affected because of the variation of the slag volume in the furnace.
Figure 2. The phase diagram and iso-viscosity curve of the CaO-SiO2-FeO system To make the slag with good fluidity for fast and enough deslagging, the slag should be fully melted, i.e. no undissolved lime, MgO, 2CaOSiO2, etc. in the slag. It can be seen in Figure 2(a) [8] which shows the phase diagram of the CaO-SiO2-FeO system that, for making fully melted slag at the end of the dephosphorization stage, the basicity of the slag (mass% CaO)/(mass%
SiO2) should be less than 1.3. As the slag also contains some Al2O3, MnO, etc. which enlarges the liquid phase region, the slag basicity should not be more than 1.5.
Figure 2(b) [8] shows the iso-viscosity curves of the CaO-SiO2-FeO system at 1400 C. It can be seen that lower viscosity slag (0.2-0.4 Ns/m2) can be obtained when the slag basicity is between 0.8-1.3. In case the basicity is above 1.3, the iso-viscosity curves become denser, i.e. the viscosity of the slag increases quickly with increasing the basicity and the fluidity of the slag will be worsen.
The relationships between the deslagging amount and the slag basicity and MgO content after the dephosphorization stage are shown Figure 3(a) and 3(b), respectively. It is seen that the deslagging amount increases with decreasing the basicity. By controlling the slag basicity within the range of 1.3-1.5, the deslagging amount each heat can be increased to more than 8.0t in Qianan Steelworks and 5.0t in Shouqin Steelworks, respectively.
Figure 3. Relationship between deslagging amount with (a) slag basicity and (b) MgO content
As can be seen in Figure 3(b), the deslagging amount increases with decreasing the MgO content of the slag. For making fast and enough deslagging, MgO in the slag should be controlled no more than 7 mass%. This MgO control limit is less than that in conventional BOF steelmaking.
But, no influence has been found on the lining life of the furnace after the new steelmaking process was applied.
By adopting the new slag basicity and MgO control strategy, the two most serious problems, i.e.
difficult to make fast and enough amount of deslagging and large amount metallic iron droplets in the slag, are solved have been solved. Now, the deslagging amount each heat is 6.5~12.5t in Qianan Steelworks and 4.0~8.0t in Shouqin Steelworks, respectively, varied mainly due to the Si contents of the hot metal. The deslagging time has been decreased to 4-5min in Qianan Steelworks and 3-4.5min in Shouqin Steelworks, respectively.
3.2 High Efficient Dephosphorization in Dephosphorization Stage
Compared with conventional BOF steelmaking, dephosphorization in the “Slag-Remaining+
Double-Slag” process is more difficult because the reused slag of the previous heat already
contained about 1.5 mass% P2O5 and, as above mentioned, the slag basicity in the dephosphori- zation stage must be low to meet the demand of enough deslagging. If good dephosphorization cannot be made in the dephosphorization stage, dephosphorization burden in decarburization stage will be increased, which may result in re-blow or after-blow at the end of blowing.
In BOF steelmaking, the dephosphorization reaction may take place inside the metal bath, at the slag-metal interface and inside the foaming slag. In dephosphorization stage of the“Slag- Remaining+Double-Slag” steelmaking process, activity of the oxygen dissolved in the metal bath is governed mainly by the dissolved carbon which is usually between 3.2 mass% to 4.5 mass%.
With the standard free energy change (G) of the decarburization reaction and the activity interaction coefficient given by Equation (5) and (6) [8], the activity of the dissolved oxygen (a[O]) is between 0.00010 to 0.00018 at the temperature around 1350 C.
[C]+[O]=CO (4)
△G0=-21244-38.91T (5)
log f[C]=0.14[%C] (6)
log f[P]=(105.1/T+0.0723)[%C] (7)
There is no basic slag inside the hot metal bath. If dephosphorization could take place inside the metal bath, the activity of the reaction product P2O5 would be one. With the data of the standard free energy change (G) of the dephosphorization reaction given by Equation (2) and the activity coefficient f[P] in high carbon molten iron given by Equation (7) [9], the free energy change (G) of the dephosphorization reaction inside the metal bath in the dephosphorization stage at the temperature around 1350 C is between 820 kJ/mol to 865 kJ/mol. It indicates that it is almost impossible that the dephosphorization takes place inside the metal bath. However, as top blowing O2 is used, the FeO content in the slag can be controlled at a relatively higher level. The dephosphorization reaction taking place at the slag-metal interface or inside the foaming slag between the metal droplets and the slag may can occur as expressed by Equation (8). With the data of P2O5 in slag [7], f[P] in high carbon molten iron [9] and using the usual composition of the slag in the dephosphorization stag ((CaO): 40 mass%, (MgO): 7 mass%, (FeO): 10 mass%, (P2O5): 3 mass%), the free energy change (G) of the dephosphorization reaction expressed by Equation (8) is between -27.0 kJ/mol to -38.0 kJ/mol. In the calculation, aFeO in slag is 0.2 referring to the iso-activity curve of FeO in CaO-SiO2-FeO system [8]. This calculation result indicates that dephosphorization can take place at the slag-metal interface or inside the foaming slag. 2[P]+5(FeO)=(P2O5)+5Fe (8)
△G0=-283634+403.0T (9)
log P2O5=-9.84-0.142((%CaO)+0.3(%MgO)) (10)
It is known from the above discussion that, for making good dephosphorization in the dephosphorization stage, strong agitation of the metal bath is of great importance to accelerate the transfer of [P] to the slag-metal interface where dephosphorization can take place. However, most BOF steelmaking plants in China use relatively milder bottom blowing for prolonging lives of the bottom blowing units. For instance, the bottom blowing rates in Qianan Steelworks and Shouqin Steelworks of Shougang Co. are only 0.03-0.06 Nm3/min/t, which cannot provide strong enough agitation for making good dephosphorization.
So, in the new steelmaking process, hard blow pattern is adopted so as to utilize the top blown O2
jet to strengthen the agitation of the bath in the dephosphorization stage. The oxygen lance position is 100-200mm lower than the position used in conventional BOF steelmaking and the O2
flowrate is higher than 3.0Nm3/min/t. In addition, iron ore addition is also increased in the dephosphorization stage to compensate the FeO decrease in the slag because of the hard blowing.
Figure 4. Relation between the phosphorus contents of the metal and slag FeO contents at end of dephosphorization stage
Figure 4 shows the relation between [P] contents after the dephosphorization stage blowing and the slag FeO contents. It can be seen that better dephosphorization has been achieved after the hard blow pattern has been applied, in spite of that the slag FeO contents are decreased. With the above mentioned techniques, phosphorus content at the end of dephosphorization stage is lowered to 0.029 mass% on average and, the [P] contents at the end of decarburization stage lowered to 0.0096 mass% on average, which can meet the specifications of most steel grades produced in Qianan Steelworks and Shouqin Steelworks.
3.3 Solidification of the Remained Molten Slag
In the “Slag-Remaining+Double-Slag” steelmaking process, the molten slag of the previous heat remained in the furnace must be fully solidified before charging hot metal so as to avoid hot
metal eruption during the charging. In Qianan Steelworks and Shouqin Steelworks, slag splashing technology is adopted to solidify the remained slag.
In the new steelmaking process, after tapping of the liquid steel of the previous heat, the top lance is lowered into the furnace to blow N2 for 4-5 minutes to splash part of the remained slag on the lining wall of the furnace. In addition, owing to the cooling effect of the N2 jet, temperature of the un-solidified slag pool decreases rapidly and some high melting temperature solid phases such as 2CaOSiO2, 3CaOSiO2, MgO, etc. precipitate from the molten slag.
Figure 5. EDS mapping analysis result of the slag sample taken after slag splashing
As can be seen in Figure 5, after slag splashing, the un-solidified slag pool at bottom of the furnace changes to two-phase mixtures, i.e. the solidified phase of high melting temperature precipitates and the liquid “RO” phase constituted by FeO, MnO, CaO, etc. In order to fully solidify the slag, certain amount of lime and dolomite are added in. Then, the furnace is rotated forward and backward to mix the added lime, dolomite and the slag. With the above mentioned techniques, the time for slag solidification has been decreased to less than 5.5 minutes and no hot metal eruption has occurred after the new steelmaking process has been adopted in Shougang Corporation.
3.4 Fast Production Technology
The tap to tap time in the “Slag-Remaining+Double-Slag” steelmaking process is longer than the conventional BOF steelmaking due to the added operations of intermediate deslagging, solidification of the remained slag, etc. In order to minimize the influence on the productivity and particularly on the synchronization of the BOF steelmaking, secondary refining and continuous casting, production speed in the new steelmaking process must be raised.
Some measures are taken to speed up the production, which includes (1) higher oxygen blowing rate in dephosphorization stage, (2) low basicity slag to shorten the deslagging time, (3) using flux of SiO2-C system to suppress the emulsification of the slag in slag pot during the deslagging, (4) better synchronizing the BOF steelmaking, secondary refining and continuous casting with the computer added production scheduling system, etc.
There are both three converters in the No.1 Steelmaking Plant of Qianan Steelworks and Shouqin Steelworks, which supply liquid steels to two secondary refining units and two slab casters.
Since the production rate of the three BOF converters is higher than the secondary refining and continuous casting, the productivity has not been decreased after adopting the new process.
In No.2 Steelmaking Plant of Qianan Steelworks, there are two BOF converters which supply liquid steels to two secondary units and two slab casters. As the No.2 Steelmaking Plant mainly produces narrower slabs for grades of electric steels, cold rolled sheet of LCAK steels, etc., the BOF steelmaking still can meet the demand of the two casters, although the BOF operation time is prolonged after adopting the new steelmaking process.
4. Conclusions
The “Slag-Remaining+Double-Slag” BOF steelmaking technology has been developed and applied on a large scale in Qianan Steelworks and Shouqin Steelworks of Shougang Corporation, by which consumptions of lime and dolomite and volume of slag generated in BOF steelmaking have been decreased by more than 47%, 55% and 30%, respectively.
Low basicity slag (CaO/SiO2: 1.3~1.5) is used in the dephosphorization stage, by which the two most serious problems in the new steelmaking process, i.e. difficult to make fast and enough amount of deslagging and large amount metallic iron droplets in the slag, are solved.
Hard blow pattern is adopted to utilize the top blown O2 jet to strengthen agitation of the bath in the dephosphorization stage, through which good dephosphorization has been obtained.
By speeding up the steelmaking operations and better matching the BOF, secondary refining and continuous casting productions, productivities of the steelworks have not been reduced though the BOF tap to tap time has been increased after using the new technology.
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