Hot billet charging saves millions of energy costs and helps to increase rolling mill productivity considerably.

Transcription

1 Hot billet charging saves millions of energy costs and helps to increase rolling mill productivity considerably. Authors: Volker Preiß - Billet Yard Operation Manager BSW Sebastian Ritter Project Engineer Rolling Mill BSW Klaus Zehnle - Senior Consultant Logistic BSE A hot charging rate of 85 % in the wire rod mill RM2 of BSW sounds good and is compulsory for high productivity. Its implementation, however, means an enormous logistical effort. New tools like for example simulations are employed to continuously improve the process. In the bar mill RM1 the hot charging rate could be continuously increased from 25 to 60 %. Introduction Energy prices for gas and oil are increasing continuously while prices and sales volumes for rolled products are still stagnating. This also influences the focus of the logistics consulting services with regard to material flow optimisation in the steel plant and the rolling mill. In the past the primary task in the case of production increases was to eliminate internal transportation bottlenecks or to prevent them when planning new plants. Nowadays, the focus is increasingly put on cost reduction, which is also due to the fact that bottlenecks are often no longer existent with reduced quantities. The billet yard, which is the link between the steel plant and the rolling mill and has often been a neglected interface in the entire material flow in the past, offers an interesting approach for a reduction of the energy consumption cost of the pusher or the walking beam furnace at minimum investment efforts and often with an extremely short payback time and a constant potential for optimisation, all of this against the background of continuously increasing energy costs. BSW can revert to an experience with hot charging of nearly 30 years and is therefore an excellent show-case for hot charging optimisation. In the course of the last 10 years, the gas Page1/10

2 consumption of the pusher furnace at the bar mill RM1 at BSW could be reduced by 90 to 100 kwh/t to 240 kwh/t (Figure 1). In the wire rod mill RM2, which mainly focuses on hot charging, the average hot charging rate was kept for the third time on a constant level of 85 %. This corresponds to an average natural gas consumption of less than 200 kwh/t for both rolling mills at a total hot charging rate of 76 %. Figure 1: Hot charging Development at BSW bar mill RM1 and wire rod mill RM2 What does hot charging mean? Hot charging means the direct billet handling from the end of the continuous caster to the rolling mill pusher furnace without any intermediate storage. This can be done by crane as it is the case at BSW or by means of a roller table. Modern billet transportation units are equipped with an additional heating box in order to reduce the cooling of those billets which are not charged immediately. In case of a short-term intermediate storage on the billet yard or if further handling to the rolling mill via transport carriages is required, we talk about warm charging (as of approx. 600 C). As it Page2/10

3 is never possible to realise hot charging over a longer period of time the process practiced at BSW is called warm charging. Conditions for optimum hot charging All factors listed in Figure 2 are basic prerequisites for achieving high hot charging rates. Suitable Layout Suitable Equipment Level 2 Reporting System Production Planning/ Control Production Automation Reheating Furnace Level 2 High Quality Accuracy Figure 2: Conditions for hot charging Suitable layout and suitable equipment This is of course a decisive factor for the quality of the warm or hot charging practice, respectively. New steel plants are therefore designed with a direct roller table connection from the discharging roller table or the cooling bed to the charging grid of the reheating furnace, and in existing installations like BSW such considerations are also frequently discussed. The advantages of a crane unit should, however, not be underestimated, as it permits to react to any fluctuation in billet availability on the caster side or charging in the rolling mill in a fast and flexible way. Especially if no heating boxes are available as additional bypass, such fluctuations negatively influence the hot charging practice. In addition, the crane must at least be temporarily manned for storage or removal activities Page3/10

4 Level 2 reporting system automation In order to be able to evaluate the hot charging practice objectively and to continuously optimise the process, the respective technical data must be recorded and entered into the production data acquisition system. Shift-specific data about the specific kwh consumption at the furnace are essential for the daily report, at the best with the respective average charging temperature and the percentage of hot charging. They are then included in the monthly and annual reports as average values. For this, a level 2 automation of the reheating furnace is indispensable, which is however not investigated in detail in this paper. Production planning and production control This is based on a comprehensive and detailed production planning and control, as high hot charging rates and charging temperatures can only be achieved with a close cooperation between the steel plant and the rolling mill. Hot charging therefore starts in principle with production planning, and the dispatcher at the billet yard plays a key role as he acts as an interface between the two plants (Figure 3). The disciplinary assignment of the billet yard personnel including crane drivers to the rolling mill proved to be very advantageous in this context. Figure Page4/10

5 Automation reheating furnace Level 2 A Level 2 automation system is an absolute must, as it will not be possible to achieve an optimum furnace operation with regard to cost and consumption data with only a Level 1 system or without any automated burner control at all. The charging temperature of each billet is measured directly before charging by means of a pyrometer. The burners are then regulated in the different heating zones depending on the current rolling performance. A manual intervention is only required during planned shutdowns or major delays. This also includes the corresponding furnace visualisation (Figure 4). The first automation system at the BSW pusher furnace no. 2 was realised in Currently, the old Level 1 automation of pusher furnace no. 1 is being replaced by a modern level 2 automation system. Figure 4: Pusher furnace no. 2 Template: billet temperature / furnace condition High quality accuracy At the beginning of 2001 the software Chardis was implemented (Figure 5) to support the dispatchers at the billet yard. Based on the analytical results of the final ladle sample Chardis makes it possible to decide about the metallurgical possibility of direct charging of the heat Page5/10

6 Figure 5: Chardis template heat analysis All quality regulations and boundary conditions for charging in the rolling mills are stored in Chardis. In case of deviations in the analyses Chardis gives answers about the further application and measures in a matter of seconds, thus avoiding an interruption of the hot charging process under certain boundary conditions. In addition, Chardis is a self-learning system. The results of all trial rolling campaigns are integrated in the Chardis database and can support the decision-making process in case of deviations in the analyses. Especially at night or during weekends, when the quality department is not manned, heats had to be deposited even in case of only slight deviations of the analysis from the standard settings. With Chardis BSW now has a software at its disposal which gives a detailed view on the heat analysis and considers the exact rolling conditions and the experience gained from previous trials for decision-making. In 2010, deviations of analyses were detected for t; for t thereof direct hot charging was possible. Restrictions of hot charging Independent of the respective specific plant layout there are further restrictions minimising the possible hot charging rate, leading to warm charging or in the worst case even ruling out hot charging (Figure 6). The most important factors are: General restrictions with regard to the steel grade Problems with billet tracking, e.g. only possibility of heat-wise charging Charging delays caused by waiting for analytical results Discrepancies between steel plant and rolling mill performance Page6/10

7 System failures in the steel plant or the rolling mill Fluctuations with regard to tap-to-tap time or casting time, respectively Short sequence lengths Differences between current steel quality of the casting unit and the required quality in the rolling mill Quality changes within a casting sequence Staggered repair times in the steel plant and the rolling mill Many of these restrictions can be considerably reduced or even completely eliminated by a continuous optimisation of the processes. One basic obstacle which has to be eliminated is the island view which can often be seen in the steel plant and the rolling mill. Steel plant and rolling mill staffs have to grow together more closely. The steel plant operator must learn to see himself as internal supplier of the rolling mill which has to be supplied at the right time with the requested quantity in the corresponding quality. Figure 6: Reasons for limitations Page7/10

8 Energy savings by hot charging The higher the billet temperature during charging into the reheating furnace, the higher are the energy savings. Billet cooling follows an exponential function, which means that especially at the beginning every minute of handling time between continuous caster and reheating furnace is essential in order to minimise energy losses. As the cooling curve for billets which are not directly charged is getting flatter, warm charging at approx. 400 C is still possible after several hours. In addition, the cooling rate is further reduced when storing the billets in boxes or even under cover hoods. The billet cross section is much less relevant, especially for hot charging, as shown in the comparison between the red curve (130 mm 2 BSW) and the green curve (160 mm 2 ). At BSW charging temperatures of about 840 C are achieved during hot charging (Figure 7). The average value achieved over an 8-hour-shift (with 100 % hot charging) amounts to approx. 750 C. C 1, C C 700 Ø 642 C 600 Average cooling rate from 130 mm up to 160 mm billet dimension max. Input temp. RM 2 = 11 min total handling time from CCM to RM Record Ø shift input temp. BSW RM2 in 2008 Total Ø input temp. BSW RM 2 in 2008 F 1,832 1,652 1,472 1,292 1, Y= 1000 * EXP^(-0.035* X ^0.71) BSW cooling curve of 130 mm² billet Ø 35 min min Remark: billet cooling curves based on BSW surface measurement 130 ² mm² 160 ² mm² Source S. Ritter BSW 2009 with open air cooling (average billet temp. of the middle billet of a billet package) Figure 7: Billet cooling curve starting at the end of the CCM The evaluation of the gas consumption values of the pusher furnace no. 2 at BSW (cold performance 180 t/h) showed a nearly linear consumption curve for the billet heating (similar to previous investigations at the old 105-t/h-furnace) (Figure 8) Page8/10

9 As a yearly average 2008 BSW achieved an average charging temperature of 642 C as an average value of hot charging and the billets which are intermediately stored in boxes and thus charged more or less cold depending on their storage time. The resulting average total consumption amounts to 174 kwh/t. The energy consumption varies between the optimum value of 125 kwh/t and max. 360 kwh/t for cold charged billets. BSW thus reduced 50 % of their natural gas energy consumption costs. With the current energy prices this means savings of more than 10 per ton good billets. It must be noted that the cold charged billets from the storage box still have a temperature of more than 200 C even after some days. Energy consumption pusher reheating furnace depending on billet temperature KWh / t ,022 1,112 1,202 1,292 1,382 F 1,472 MJ / t 450 1, , Y= x KWh/ t 1,260 1, KWh/t 150 average energy consumption BSW RM C C Remark: average furnace energy consumption based on 130 mm² billet surface temperature during entering pusher furnace BSW RM 2 Trend BSW Source K. Zehnle BSE 2009 Figure 8: Energy consumption at the BSW pusher furnace RM2 Productivity increase by hot charging Pusher furnace II presents a bottleneck at BSW for all dimensions > 7 mm. Maximum capacity can only be achieved by hot charging. Besides the high energy cost savings productivity increase is the main incentive for hot charging at BSW. Depending on the diameter capacity increases of up to 50 % are possible with hot charging. But even with warm charging productivity increases of about 20 % are feasible (see Figure 9) Page9/10

10 Figure 9: Good coils performance BSW wire rod mill RM II Conclusion The implementation of hot charging is a very complex issue with different key aspects and problems. The first step is therefore always a detailed investigation in order to determine the potentials as well as the resulting cost-benefit factor. Based on this investigation, further steps can be defined and the respective specific solutions can be developed (Figure 10). In the course of the last 10 years, BSE has performed in close cooperation with BSW consulting services with regard to hot charging with an emphasis on various topics. The following graph shows the procedure and possible key aspects of a hot charging study. Initial study to define the potentials investment cost and benefit valuation showing of main requirements Layout and billet handling study Equipment recommendation if required with billet handling simulation Production planning study Planning optimization adjustment between melt shop and rolling mill reporting system requirements Reheating furnace study Adaptation furnace design (level 2) Energy optimized Productivity driven Production control study Developing hot charging decision tool Chardis Further developing possible hot charging stock management tool Orchestrate Figure 10: Approach and fields of a BSE Hot Charging Study Page10/10