Information technology and Juran'-'-s spiral of quality as a continuous process of improving the efficiency of section-rolling mill

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Экономика и экономические науки


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S. Yu. Sarancha, S.A. Levandovskiy, A.B. Moller, J.S. Statsenko
FSBEI HPE Nosov Magnitogorsk State Technical University
Abstract: In modern industry information technology and electronic system design, simulation, control systems, production flows, systems of control and accounting of production resources are playing a growing role. Metallurgical production isn 't an exception too. There is a task in the section and long product rolling companies associated with the shipment of finishing product by gage lengths. From the experience of production long product with a simple shape it is known that over a third of the cases of rejection of products is off-gage length. The application of mathematical modeling based on the use of modern information technologies contributes to the efficiency of solving such problems.
Keywords: off-gage length, not custom length, off-gage balance, measuring length, cut, varying cross-sectional area of the finished product, reducing metal losses, increasing efficiency of the rolling production, variation of the roll gap, information technologies, optimization technology of the cutting, long products rolling, section-rolling production, quality of the process, yield ratio, effectiveness.
In modern industry information technologies (IT) are playing a growing role and metallurgical industry isn’t exception. Electronic system design, control systems, production flows, systems of control and accounting of production resources, and automated control systems (management information system) and automated control systems of technological processes (APCS) — all of that is the norm for modern industry. For example, all of the modern continuous rolling mills are equipped with APCS that controls the speed of the mill at each stand to provide the desired tension in the stock, there are also cases where you can meet APCS that control roll gaps online.
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IT spreads due to the high efficiency, automatic operation without human’s i n-tervention and a high speed of response to the changing process conditions. On the basis of the conducted researches [1], we can conclude that a promising direction in the long product rolling is the development of such an APCS, which automatically selects the optimal roll gap in the last rolling stands of the mill based on the weight of the billet and on the rolls wear to reduce defects and to improve profitability of the long product rolling production, requiring a minimal human intervention.
Today the problems of efficiency are very actual, especially in the problem of an energy saving. The concept of quality should be understood not only as quality of material or production, but more extensive — the ISO 9000 standard includes in definition of this term also quality of process or management.
According to the research results we conclude [1−2] that there is a task of the correct material cutting in front of modern bar rolling production, and the solution depends not only the volume of the finished product and yield, but also profitability. The main task of cutting is to reduce the number and length of the bars that are shorter than customers need (hereinafter off-gage length). Those bars are generated in the production process at the stage of the gage length formation.
Analysis of the production defect types during long product rolling and its percentage distribution shows that most of them were caused by the wrong cutting, i.e. off-gage length (Fig. 1) [2].
On the whole, the full solution of the problem of correct cutting can reduce the number of defects for more than 40%.
Reducing the metal loss in the off-gauge length, and hence, increasing the yield ratio is an effective way to improve profitability.
The task of cutting has at least six solutions:
— The endless rolling: a — the technology of the continuous casting [3], b — billet welding before rolling [3−5]-
— Varying lengths: a — of billet, b — the technological trimmings within acceptable ranges-
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— The variation of cross-section: a — of billet, b — finished products within the tolerance range.
Defect type
Fig. 1. Manufacturing defects
Endless rolling of continuously cast billets fixes the problem of the off-gage length. The positive side is continuous production without & quot-off-gauge length& quot- defect. Negative — complexity and high cost. This technology is used since 2000 [3] in the framework of pilot production.
Endless rolling with the welded billets at the junction interface is a popular method of production. The positive side of this approach is the possibility of obtaining much longer billet. Negative — high cost, the need for purchasing expensive and complex welding units.
Installation of billet welding equipment requires almost complete reorganization of the rolling mill and has a high cost. The main technological problem of billetwelding is the difference between the quality of the welded joint, which limits the product range of the rolling mill, as the joint in the rolling process is rolled out at great length, and reduces uniformity of chemical composition, and the mechanical stability of the finished product.
Varying the length of the billet does not require any changes in the production technology, but requires the reorganization of production management and strategy of
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the enterprise as a whole, because in this approach the production of continuously cast billets must come from existing orders, adjusting the parameters of the casting under a specific profile. The billet length has to be chosen correctly to obtain shortest off-gage bars. The reverse side of this technology is a mandatory requirement of high production culture: maintenance work, strict adherence to technology, total control. In Russia the solution of this problem is extremely expensive and inefficient.
The method of varying the off-gage length is interesting because it actually does not require the purchase of new equipment. It needs the implementation of online trimmings control. But because the acceptable range of trimming is quite small, it is possible only to reduce the off-gage length and not eliminate it. It should also be noted that a significant complexity creates the current rolls wear, and the effectiveness of this method would be low without this.
The method of varying the cross-sectional area of the billet is quite interesting, but to change the cross-section on CCM we need to change the mold, which is costly and time-consuming. The technology of varying cross-section of the finished product within the tolerance range does not require any deep intervention in the production technology. Variation of the area goes on inside the tolerances. Varying the roll gap, and the cross-section the finished product, it is possible to minimize the off-gauge bar length. Increasing cross-section of the finished product is beneficial when company ships bundles at actual weight, and reducing it can increase the number of bars, which is beneficial when bundles are shipped by the theoretical mass and the bar quantity.
Analyzing these methods, comes to the conclusion that without IT the methods implementation impossible or difficult. Moreover, IT often allows to find a solution to a particular problem, opening a hidden reserve of modern production (an example is the technology of varying cross-section of the finished product within the tolerances). It’s not only difficult, but it’s waste of time to find the optimal solution without using IT.
Thus, there are at least 6 ways to solve this problem, but as shown by research and practice is most beneficial for long product rolling mill is the method of varying
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the cross-sectional area of the finished product within tolerances (hereinafter Method) as the most affordable, versatile and quickly implemented method [2].
Positive and negative tolerances, regulated by state standards (for example, GOST 2590−88 regulates hot-rolled round steel, and GOST 5781−82 — hot-rolled steel for reinforcement of concrete structures), allow to vary the cross-sectional area of the finished products in a wide range, while satisfying customer requirements (Fig. 2).
Fig. 2. The interval of variation of diameter
The weight of the billet and finished product are closely connected. In practice, it is necessary to consider head and tail cutting, scale, dividing when the stock enters the cooling bed and at the cold shear cutting into bars. By changing the crosssectional area of the finished product we can change the total length of the stock, which in turn changes the off-gage length because the total mass must remain relatively constant.
Thus, the goal of cutting is to find such a cross-section areas of the finished products, which gives the minimum off-gage length. The Method offers to change the cross-section of the finished product in the last stands of the rolling mill.
It is important to note that most manufacturers ship finished products by weight and not by the number of bars, so it’s better to vary the cross-section of the finished product towards positive tolerance, transferring metal from off-gage length to the finished products, increasing its mass. It will be most profitable to vary the cross-section of the finished product in negative tolerance, which should increase the number of bars for companies that ship long products by the theoretical mass.
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The ISO standard define not only quality of process, but also its efficiency. The ISO 9000: 2000 standard defines efficiency as a ratio of the reached results and the used resources. Actually it is about the coefficient of the output of the suitable material. Thus, the coefficient of the output of the suitable material is an indicator of efficiency of process of cutting, so it is the quality. For identification of the most meaning factors influencing coefficient of the output of the suitable material was constructed Ishikawa '-s diagram (Fig. 3).
Fig. 3. Simplified algorithm of the programme
As shown in diagram, the most significant factors are: efficiency of the algorithm of cutting, the mass of preparation, wear of rolls, the precision of exposure of an emphasis and precision of exposure of the gap. According to these requirements it was written the software for the computer which considers the all major factors [6].
Within R& amp-D [1] we developed software that allows quick finding the optimal cross-sectional area of the finished product in the workplace [6]. A simplified algorithm of the programme is presented in the diagram (Fig. 4).
There are the following stages of a spiral of quality that were offered by Juran [7] it includes 14 stages: 1 researches of the market, 2 development of the detailed design, 3 research and development, 4 drawing up TU, 5-technological preparation of production, 6 logistics, 7 production of the tool, adaptations, 8 production, 9 control
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of production, 10 control of finished goods, 11 test of production, 12 it is sold, 13 maintenance, 14 research of the market.
Fig. 4. Simplified algorithm of the programme
Nowadays this algorithm of the cutting section-rolling products passed all 14 stages: the market and demand for measured production which effective production is connected with a problem of optimum cutting was investigated. Research and development was carried out on OJSC «Magnitogorsk Iron and Steel Works» (OJSC «MMK») (Russian Federation). There were made the expert production and necessary measurements with the assessment of quality of production. As a result the production conformed to both requirements of the customer, and requirements of standards. Production was adjusted on agrees to the new technological scheme. Thus, the Method and ON could provide continuous improvement of quality and production efficiency.
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On the basis of the conducted research it can be concluded that the Method allows to reduce the off-gage length at the bar mill by 80−90% [4].
As shown by practice [1] and model [8] of rolling large bar, for example, the round 48, the off-gage length can be reduced from 11,3 meters up to half, thereby increasing the yield from 90,09% to 97,3% (billet weight 2000 kg- mill 370 OJSC ММК). Analyzing the research, it becomes clear that the greatest benefit from the application of the Method can be obtained in large profiles — for example, by rolling the round 30 mm at nominal size off-gage length is 8 meters, while using the Method, off-gage length was reduced to 0,5 m.
During the rolling the nominal size, the total length of the rolled stock is 143,4 meters or 11 separate bars. Off-gage length in this case is 11,3 meters, which cannot be shipped to the customer and is scrapped. Rolling in negative tolerance allows you to get the length of the stock to 144,8 meters or 12 separate bars, off-gage length in this case is half a meter. It is worth noting that the Method calculated off-gage length will usually be smaller less than in reality, because the software calculates optimal roll gaps and optimal bar length on the cooling bed, but in reality — without using the Method — dividing bars before the cooling bed is not optimal, which increases the output of crop and off-gage length (Fig. 5).
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Let’s take a closer look at the results: the weight of off-gage bar reduced from 158,02 kg to 7,19 kg and yield increases from 90,09% to 97,3%- money loss for one billet drops from 2918 to 132 rubles. This effect is achieved by reducing the roll gap from 2,00 to 1,63 mm. The method of varying the cross-sectional area of the finished product is one of the reserves for increasing the efficiency and profitability of production, while not requiring significant investment and time.
The attention should be payed to the accuracy of placing a gap — as shown by studies to ensure maximum efficiency of the Method and the required accuracy of changing the roll gaps in the last stands of the rolling mill at the level of 0,01 mm. Let’s have a look at situation at the long product rolling mills OJSC «MMK».
In the early 2000's OJSC & quot-MMK"- has installed three Danieli rolling mills 170, 370, and 450 [9]. The minimum step of placing a gap on the mill 370 is 0,05 mm, which according to R& amp-D «Cutting the rolling mills bars» [1] is not enough to obtain the maximum effect from the Method to reduce off-gage length. The reduction of the minimum step of placing a gap in the last stand from 0,05 mm to 0,01 not only reduces off-gage length at the bar mill, but it’s often the only way to do it for heavy section products. Let’s compare the minimum gap steps — 0,05 and 0,01 mm.
Table 1
Round 48 mm rolling results (the mill 370 of OJSC & quot-MMK"-)
Default Optimum with step 0,05 mm Optimum with step 0,01 mm
Weight of billet, kg 2000
Profile Circle
Diameter, mm 48
gap in the last stand, mm 2 No solutions 1,63
off-gage length, m 11,33 No solutions 0,52
Number of rods 11 No solutions 12
For round 48 mm step of 0,05 mm doesn’t allow to reduce the off-gage length using the method of varying the cross-sectional area of the finished product within tolerances. At the same time, improving the accuracy of setting the roll gap of up to
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0,01 mm will significantly reduce off-gage bar length of the round 48 mm (by 95%). Thus, for large profiles, improving the accuracy of setting the roll gap reduces the crop, whereas conventional precision placing of the gap is not enough.
Table 2
Round 20 mm rolling results (the mill 370 of OJSC & quot-MMK"-)
Default Optimum with step 0,05 mm Optimum with step 0,01 mm
Weight of billet, kg 2000
Profile Circle
Diameter, mm 20
gap in the last stand, mm 1 0,8 0,82
off-gage length, m 2,6 1,6 0,5
Number of rods 69 70 70
Table 3
Round 30 mm rolling results (the mill 370 of OJSC & quot-MMK"-)
Default Optimum with step 0,05 mm Optimum with step 0,01 mm
Weight of billet, kg 2000
Profile Circle
Diameter, mm 30
gap in the last stand, mm 1,5 1,2 1,23
off-gage length, m 8 1,1 0,6
Number of rods 30 31 31
Despite the fact that the number of bars remains generally the same, with variable balance is reduced, and this means that less metal goes into scrap — and, consequently, the weight of the finished product increases. Since delivery of the metal occurs, as a rule, by weight, not by number of bars, the reduction of off-gauge length -in other words, the transfer of metal from scrap into finished products — allows you to increase the profitability of the mill.
Generally, improving the accuracy of operation of the metallurgical equipment of the mills together with varying the cross-sectional area of the finished product in
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the tolerance range is one of the reserves for increasing the efficiency and profitability of long product rolling.
As has been written previously, the further development of the Method is the creation of APCS, which will automatically select the optimal parameters of the rolling mill based not only on the mass of the billet, but also the wear of the rolls. Detailed information on the wear of the rolls depending on the amount of rolled steel and profile shape can be obtained from the following sources: [9−12]. The principle of APCS: the operator of the rolling mill selects the desired profile and custom length. Software reads the mass of the billet, and then calculates the optimum parameters of rolling considering the wear of the rolls. After rolling on the quality control post the measurements of the off-gauge length are made and the system adjusts the coefficients (Fig. 6).
Fig. 6. Automated Control Systems of technological Processes (APCS)
It is proposed to insert the APCS in the process into several stages according to [13] to ensure the commissioning without stopping the production cycle for a long period. The first step is the implementation of a software system. All measurements and recordings in the database are made manually by the employees of the mill — in fact, we are talking about semi-automated system, but researches have shown [1] that this system allows to reduce off-gage length in production by more than 50%, requir-
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ing regular data input into the database by mill personnel. The main disadvantage of this system is the necessity of correction the burning coefficients in furnaces, thermal shrinkage and tolerances of shears based on empirical data to increase the accuracy of the Model. In the second stage of implementation proposes installation of automated weighing station before and after the reheating furnace, and automatic weighing of the off-gauge bar. Re-weighting of the billet after reheating furnace will let correct burn in scale factor in the database, and the off-gage bar weighing will correct internal index of the model describing the correspondence of theory with practice. The installation of this equipment will transfer the task of correction coefficients from employees to the APCS.
After the completion of implementation of such APCS the off-gauge length management at the rolling mill will be automated — optimal rolling schedules will be chosen based on the weight of the billet and wear of the rolls. The use of APCS will reduce the number of defects in production and will increase the automation level of the mill increasing the efficiency of the personnel, reducing the influence of human factor and reducing production costs.
The economic effect from implementation of the system is more than 50 million rubles per year on heavy-medium section mill with production capacity of about 500 000 tons per year [1]. A promising development of APCS is not only increasing the level of automation, but also the extension of the supported range, for example, the section profiles.
The next analysis of the market [1, 4, 14, 15−17] which revealed rather great demand on production with flange profiles of section was carried out to development of this direction. It is possible to draw a conclusion on need of adaptation of the offered technology to production of flange profiles.
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