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Lutsenko A.N., Rumyantsev M.I., Tulupov O.N., Moller A.B., Novitskiy R.V.
TECHNOLOGICAL RESERVES: REASONABLE IMPLEMENTATION OF SIMPLE SOLUTIONS TO IMPROVE HOT ROLLING TECHNOLOGY
Abstract. This article presents results of solving the multifactorial problem of finding and using process-reserves in flat and long product rolling.
Keywords: hot rolling technology, mechanical properties, quality, technological reserves.
First. To reduce the number of products with nonconforming mechanical properties we improve the technology: realign rolling schedule to minimize the variation of the properties.
Technology improving in order to reduce the amount of low-quality rolling in terms of mechanical properties should be done on account of searching for rolling modes that enable minimum actual variation of product properties. We suggest using properties hereinafter referred to as conformance aspects to estimate process performance in terms of products quality [1]. If the quality is established with maximum tolerable values (top estimate of conformance),
(1)
qPL ~~
3s
3s
(2)
qpk = min (qpu ; ).
(3)
In (1)-(2) Ausl = USL - x and ALSL = x - LSL are tolerable intervals of quality property variation (Fig. 1), and 3s is the share of its actual variability, falling within tolerable interval. Standard deviation
to calculate Ppk. Average value (as actual variation centre) and standard deviation (as actual variation property) for sampling from one lot of products or several lots of the same products, but produced from the same melt are
used to calculate qpk . Due to such similarity of conformance aspects to process indexes, «excellent», «good» and «satisfactory» process performance estimates that correspond to the following values of aspects (1)-(3): more than 1.67; 1.33-1.67 and 1.00-1.33 can be used.
and, if it is established with minimum tolerable value (low estimate of conformance).
If maximum and minimum tolerable values are established (conformance estimate with regard to average position)
I .
s = 1/ (n -1)X(x - x)2 and sampling mean x =1
\ 1=1 i=1
are used as the property of actual variability and scattering centre appropriate to it, where x i is single measurement result of quality property for a certain product unit (i = 1,...,n ).
Conformance aspect qpk is similar to process performance index Ppk, known in SPC [2], as they compare
tolerable variation of quality property with regard to position of centre of actual variation (numerator) and actual variation (denominator). Though aggregate data presented in sampling made in the course of production of many lots of products of certain types from melts with different chemical compositions for a long period of time are used
Fig. 1. For calculation of conformance aspect at bilateral restiction of quality property (T is tolerance field middle)
As one of the examples of suggested approach application we provide results of estimating quality of hot rolled steel of current production to requirements of GOST 16523 to mechanical properties of cold rolled steel of strength groups K270B and OK300B. Actual variation of aspects analyzed in comparison with tolerable variation is demonstrated with data in Table 1. If we compare sampling means x with limits of rated intervals of variation LSL and USL, estimated hot rolled stripes can be deemed good to replace cold rolled steel profile in terms of strength group 0K300B, as well as K270B group (in both cases LSL < x < USL). Though comparison of actual variation of temporary resistance with tolerable variation of group K270B shows that x is shifted towards top limit of tolerance, and aspect range (6s = 6 • 9,9 = 59,4 MPa) makes 42% of tolerable vari-
ation (USL — LSL =410-270=140 MPa). Thus, at excellent low estimate of conformance (qpL =3.85) top estimate, as well as final differential estimates of temporary resistance are unsatisfactory (q pk = qpu =0.89).
Table 1
Estimate of conformance of mechanical properties of hot rolled stripes of 2 mm in thickness from steel 08ps to requirements of GOST 16523-97 to cold rolled steel [3]
Quality aspects Tolerable variation Actual variability Differential estimates
LSL USL x s qpL qpu qPk
strength grou p K270B
U b, MPa 270 410 383.8 9,9 3.85 0.89 0.89
S , % 25 - 34.38 1.31 2.39 - -
strength group 0K300B
a b, MPa 300 480 383.8 9.9 2.83 3.25 2.83
S % 25 - 34.38 1.31 2.39 - -
If we compare actual variability of properties of hot rolled stripes with requirements for strength group 0K300B it can be seen that centre of resistance variation almost coincides with the middle of rated variation range, and scatter does not exceed 33% of rated range value. Therefore, in this case we see excellent differential estimates of mechanical properties, certifying that these lots metal can be used for deliveries instead of cold rolled steel.
Based on the obtained results production method of hot rolled steel to replace cold rolled steel of normal, deep and extremely deep drawing has been developed to include hot rolling of steel bars with set temperatures of end and reeling, and its difference lies in application of steel containing 0.09...0.11% of carbon and 0.25...0.56% of manganese with carbon equivalent C * = C + Mn/6 + Si/3 = = 0.12. 0.18, and temperature of rolling end and reeling
is set with regard to the following ratios (°C):
tKn = Ar3 + 300C* - 40 and tCM = Ar1 -(100...110) [4]. Whereas temperature values of the beginning (Ar3) and end (Ar1) of phase transformation are calculated with regard to actual chemical composition of a certain melt by formulas [5]
Ar3 = 913,7 - 207,13C - 46,6Mn +
3 (4)
+110,54Cr +108,1#; Ar = 741,7 - 7,13C -14,09 Mn +
1 (5)
+16,26Si + 11,54Cr - 49,69Ni.
Dependencies application (4)-(5) enables prompt changing of temperature t and t for real-scale compensation of natural deviations of steel chemical composition, thus ensuring specified properties of hot rolled stripes designed to replace cold rolled steel of general purpose.
' X
4,0-----------------
3,5
0,0 I n ' ' -1 ^ ' ' ' -If
810 815 820 825 830 835
\nliy 1
Fig. 2. Influence of rolling end temperature in the field of non-recrystallizing austenite on conformance aspects for yield strength (1) and ultimate strength (2)
The temperature mode improvement of 5,000 heavy-duty sheets production of 16-20 mm in thickness on plate mills for ship-building from steel grade D32 can be used as example of application of conformance aspects to improve the technology. Sheets are produced by controlled rolling (CR) technology followed by accelerated cooling. Where deformation process is divided in two stages - rolling in the field of recrystallizing austenite (stage I) and rolling in the field of inhibited recrystallizing (stage II). In this relation, the following reference properties of temperature mode can be defined as: tHn(j) and tM„(II) is the temperature of stages I and II rolling beginning; tK„(I) and tK„(II) is the temperature of stages I and II rolling end; tHyo and t^o is the temperature of accelerated cooling beginning and end. The application of conformances aspects to ensure required mechanical properties of rolled steel at minimum variation is as follows.
The higher qpk the better applied technology ensures conformance of actual values of product quality to requirements specified in regulatory documentation (i.e. the closer actual quality values are to the middle of interval of tolerated variation and the smaller their actual variability is). Therefore values of references aspects of technology process that produce maximum values qpk [1] are preferable. Conformance aspect value dependencies for yield strength and ultimate strength on temperature at the end of rolling in the field of non-recrystallizing austenite are shown as an example in Fig. 2. If conformance aspect is more than 2 for the whole yield strength range tK„(I/), qpk <
1 for ultimate strength at tKn^II) = 825°C and more. It certifies insufficient performance of the process in terms of ultimate strength. To ensure good performance in terms of ultimate strength (qpk > 1.33), tKn(II) <820°C should be ensured. Similar analysis of performance in terms of other aspects enabled recommendations given in Table 2.
The specific feature of suggested temperature mode is the following. For stage I temperature of the rolling beginning should be at least 1050°C, and temperature of the rolling end should be at least 1020°C, that is higher than under the current technology (1020 and 980°C, respectively). At the same time temperature of the rolling end
should be reduced at the second stage (810-820°C instead of 810-850°C under current technology). It has been also suggested to set requirements to temperature of accelerated cooling in the range of 790-800°C (not specified before testing) and reduce the temperature of accelerated cooling end to 610-620°C instead of 600-650°C currently used.
Second. Identifying technological reserves in analyzing the effectiveness of various roll pass designs of structural shapes (e.g. channels) through the assessment of valid power parameters.
Matrix approach helps to solve a wide range of technological tasks for various processes of long product rolling [7, 8]. Decision was taken to improve and use it for the mentioned problem.
The demand for rolled steel with improved performance properties is constantly increasing. It is very difficult to get high mechanical properties of channel beams using low-alloy steel, in particular channel beams from 09G2S steel grade according to GOST 19281-89, at continuous high-speed mills.
Differentiated cooling of shaped profiles and modification of heating and metal rolling temperatures can be used as possible resources of improving strength properties. Implementation of the cooling idea is related to equipment and associated technology costs. Thus, modification of temperature mode of rolling is a more flexible and cost-effective tool to achieve required complex of mechanical properties.
Reduced temperature of billet heating at continuous mills results in reduced heat loss of metal in roughing mill stands. That certifies about reasonable reduction of temperature at heating. Whereas increased power consumption for billet deformation at roughing stands is greatly compensated by reduced fuel consumption for billet heating, and total energy saving makes at least 15%. Whereas, metal loss in furnace is reduced, i.e. good metal output is increased.
Reduced temperature of metal in the mill will result in metal yield strength increase and equipment increased loads. To ensure rolling at reduced temperature energy reserve is required to be used at rolling with reduced temperature. Such reserve is established on account of deformation distribution that is mainly caused by pass design scheme at rolling of channel beams. Therefore, search for the ways of purposeful redistribution of metal deformation at rolling of channel beams is a vital task.
The main tasks of estimating performance of channel beam pass design in terms of structural-matrix approach are the following:
- to establish a method to improve roll pass design with reasonable distribution of deformation [9] enabling of rolling at reduced temperature in order to improve operation properties of channel beams;
- to improve roll pass design of channel beam at continuous rolling mill.
Availability of a single centre describing profile of complex shape (Fig. 3) enables applying estimates of deformation efficiency with the use of previous experience of making use of performance indexes and non-uniform deformation of simple profiles [10, 11].
New roll pass design developed within the framework of this study covers finishing milling group. Shapes of the second and third passes opposite to rolling direction have been changed in suggested calibration. For identification purposes suggested calibration is called KZ and is compared with two current roll pass designs: AVD and DAN.
Fig. 3. Suggested way of description with one centre
Developed roll pass design enables reducing rolling force in stands 13, 14 and 15. DAN roll pass design uses more traditional approach and it thickens the wall during rolling. As far as such method stipulates wall thickening for 2-3 mm for every run, it requires a certain “stock” of metal to ensure wall thickness increase, as far as natural thickening of the wall does not give such growth. Thus, upstream rolled width is smaller than that of pass width and the stripe is free at least in one direction to be shifted in the pass. In other words, the pass is unstable holding rolled metal. Rolled metal is centered in the pass due to holding wings. Results of roll pass design of rolling force, drawing index and non-uniform deformation index are given below (Table 3).
The main differences of developed roll pass design versus existing one:
- wall bending radius increased;
- wall draft is increased for 4% in 13 cage;
- wall draft is reduced for 3% in 14 cage;
- changed wall bending radius resulted in reduced angle between wings. Calibration scheme is closer to traditional one, designed to increase wall;
- increased wing height resulted in improved entry of rolled metal to control pass (in cage with critical load No.14).
Having reached optimum deformation distribution we can say, that a «reserve» on rolling effort is established. Thus, we can assume metal temperature reduction, which will result in increased rolling forces, but such forces will not be excessive and hazardous due to application of established «reserve».
Table 2
Recommendations how to improve temperature mode of heavy-duty steel sheets production for ship building from steel grade D32 at plate mill 5000 [6]
Rating Temperature, °C
t Hn (I ) t Kn (I ) t Hn (II ) t Kn (II ) t Hyo t Kyo
Current 1,020-1,100 980-1,060 790-830 810-850 Not rated 600-650
Recom- mended 1,050-1,100 1,020-1,060 820-830 810-820 790-800 600-620
Table 3
Comparative analysis of rated values of rolling aspects for various roll pass design options of finishing mill group
Type of pass design Stand No.
9 11 12 13 14 15
Force, kN
AVD 1,796.51 1,448.75 1,574.61 1,233.75 881.31 1,020.55
DAN 2,103.27 1,688.30 1,367.27 1,046.25 1,233.30 920.61
KZ 1,796.51 1,448.75 1,574.61 1,030.01 922.31 961.18
Elongation
AVD 1.51 1.40 1.45 1.29 1.17 1.02
DAN 1.63 1.51 1.35 1.29 1.20 1.15
KZ 1.51 1.40 1.45 1.23 1.16 1.08
Non-uniformity index
AVD 0.221 0.242 0.287 0.195 0.146 0.399
DAN 0.221 0.242 0.287 0.150 0.135 0.171
KZ 0.200 0.218 0.241 0.168 0.143 0.223
Such solution to control channel beam rolling quality was required by lack of possibility to produce rolled channel beams with uniform conformance of requirements for specified strength class of 345 N/mm2 of finished products (channel beams No. 16 from steel 09G2S) according to GOST 19281-89.
Temperature in the last stand and temperature of the metal rolling beginning can be taken as function of cast sections rolling temperature in heating furnace of the mill.
Observations for rolling of channel beam No. 16 have been provided to estimate influence of rolling temperature in the last cage on strength properties. Temperature at the end of rolling was recorded.
Yield strength was estimated on 57 sample lots with regard to temperature mode in the last cage (Table 4).
If we connect the temperature at the end of trial samples rolling with received values of yield strength in a linear equation, we can use it to calculate the required temperature at the end of rolling to get strength class 345. Equation of connection is as follows:
OY =-1,10781 +1424,6.
To ensure strength class 345 (yield no less than 345 N/mm2) the temperature at the end of rolling not more than 970°C is required, thus enabling production of respective strength class and higher operation properties.
Third. Identifying technological reserves to improve roughing process: the influence of square billet’s rhomboidity on power parameters and wear in roughing stands.
Billet rhomboidity is a serious problem that influences stability of the rolling process and our ability to ensure the required geometry and quality of the product.
To study specific features of rerolling of continuously cast 150x100 mm billet with rhomboidity up to 30 mm, two boundary conditions of billet feed to the first stand have been considered: locations of big and small diagonals of billet and diamond pass coincides (a), i.e. small diagonal of billet is in vertical plane, and big one is in horizontal and do not coincide (b) (Fig. 4).
Drafts of continuously cast bloom of 150 mm with rhomboidity aspect (with increment 0, 1, 2, 3, 4,
5, 6, 7, 8, 9, 10, 15, 20, 25, 30) for the above ways of feeding to the first cage, as well as drafts of the first cage of three passes (“old”, modified and with hold-down), applicable/applied at mill 350 of «Severstal» OJSC for rerolling of 150 mm bloom in 100 mm bloom, have been used to calculate deformation uniformity coefficient (DUC), breakdown pass performance coefficient (Krp) and rolling forces. Results of rolling DUC calculation are given in Table 5. Calculations of the rest aspects have been entered to similar tables.
a b
Fig. 4. Locations of big and small diagonals of billet and diamond pass
Calculation of forces was given for St 3 at heating temperature 1200°C.
Based on comparison of two ways of billet feeding into stands and three options of roll pass design has been performed and process options have been estimated with regard to feeding conditions, rolling forces, contact areas between metal and rolls, surface defects and descaling quality. Dependence diagram of force/rhomboidity aspect of the billet for the first way of stripe feeding to cage (alignment of planes) is progressive, whereas it is decreasing for the second way of stripe feeding to cage (misalignment of planes).
Thus, targeted application of new, simple, and effective models and methods enables to find and use various technological reserves in rolling processes.
Table 4
Experimental data on influence of temperature at the end of yield strength rollingin finished rolled steel
Temperature after finishing stand, °C 945 950 955 960 965 970 975 980 985 990 995 1,000 1,010
Yield strength of trial samples, N/mm2 375 376 364 363 359 351 341 339 334 325 323 311 311
DUC calculation data
Pass design DUC
Rhomboidity, mm
0 1 2 3 4 5 6 7 8 9 10 15 20 25 30
Alignment of planes of big and small diagonals of billet and diamond pass
With hold-down 0.123 0.121 0.120 0.118 0.116 0.114 0.113 0.112 0.110 0.109 0.107 0.099 0.091 0.083 0.075
Old 0.115 0.114 0.113 0.111 0.109 0.107 0.106 0.104 0.103 0.102 0.100 0.092 0.084 0.076 0.069
Modified 0.141 0.139 0.138 0.136 0.134 0.132 0.131 0.129 0.127 0.126 0.124 0.116 0.107 0.099 0.091
Misalignment of planes of big and small diagonals of billet and diamond pass
With hold-down 0.123 0.124 0.125 0.127 0.129 0.131 0.132 0.134 0.135 0.136 0.138 0.146 0.154 0.162 0.169
Old 0.115 0.117 0.118 0.120 0.122 0.124 0.125 0.126 0.128 0.129 0.131 0.139 0.147 0.155 0.163
Modified 0.141 0.143 0.144 0.146 0.148 0.149 0.151 0.152 0.154 0.156 0.157 0.166 0.174 0.182 0.191
Conclusion
Targeted application of new, simple, and effective models and methods enables to find and use various technological reserves in rolling processes.
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Drobny O.F., Cherchintsev V.D.
DEVELOPMENT AND IMPLEMENTATION OF MEASURES TO IMPROVE ENVIRONMENTAL SITUATION WITHIN MAGNITOGORSK INDUSTRIAL HUB
Abstract. The article presents the results of implementation of long-term environmental program at OJSC «Magnitogorsk Iron and Steel Works», which includes measures aimed at reducing harmful environment impact of metallurgical production thus improving the production efficiency and ecological characteristics.
Keywords: environmental, environmental protection, pollutants, industrial wastes, ecological characteristics.
The program of technological, technical, organiza- technological processes with environmental protection
tional and socio-economic activities as the basis of envi- installations based on the best available technologies and
ronmental policy at OJSC «MMK» (Magnitogorsk Iron removing obsolete facilities.
and Steel Works) has been developed and now is being The implementation of the technical revamping pro-
successfully implemented complying with the Russian gram has radically changed not only the manufacturing
Federation state policy in the field of environment pro- structure at OJSC «MMK» but considerably reduced the
tection. environment impact. In 2012 the overall discharge of pol-
The OJSC «MMK» strategy, aimed at negative envi- lutants into the atmosphere was reduced by 3.7 and the
ronmental impact minimizing, consists in using creative emissivity by 3.5 times as compared with 1989.
