1、 辽宁科技大学本科生毕业设计 第 22页外文原文：Rolling conditions in hot strip mills and their influence on the performance of work rollsSummary.The mechanical rolling conditions in hot strip mills are precisely defined by variables,which are taken directly from the rolling scheduleseperation force,torque,speed,strip thi
2、ckness）or calculated from figures of the rolling schedule and dimensions of the mill(strip reduction,roll diameter etc).These variables allow to describle the mechanical rolling conditions of all passes in roughing and finishing mills . These variables should be supplemented by the metallurgical rol
3、ling conditions .They then give basic information on the conditions which determine wear(specific load ,wear speed)and fire crazing(co-efficient of heat penetration etc).There is a good chance to use the experiences of other mills with various roll grades by analog comparison-as long as the rolling
4、conditions are similar.This method is limited by “abnormal rolling conditions”, which require totally different roll grades,although if it would be much better to eliminate the abnormal conditions.Introduction.in hot strip mills,slabs of 150 to 250 mm thick are rolled to strip1.5 to 12 mm thick.Conv
5、entional hot strip mills consist of roughing and finishing stands.The configuration of the roughing mills varies widely .A mill with one reversing stand and one or two continuous roughing stands is called a % conditions mill and a mill with 4 to 6 conditions roughing stands is called a continuous mi
6、ll .In 3/4 continuous and continuous mills ,the first stands are usually two high stands while the remainder are 4 high stands.In addition to these horizontal stands,several edgers are also used .The finishing mills have a minimum of 4 stands but normally have 6 to 7 stands.Rolling conditions vary f
7、rom mill to mill,stand to stand and pass to pass.Mill configurations are designed for a desired total stip(thickness)reduction,however,each stand is limited in strip reduction by the maximum separation force, maximum torque ,risk of slippage etc.In order to supply the correct roll for each mill,roll
8、 makers ask for details of the rolling conditions and any special circumstances.However,how to use this information?How to compare the conditions of pass No.X and No.X +?There have many discussions over the years but rarely any really good results with these comparisons. For example,looking at rough
9、ing mill work rolls,there are so many roll grades being used in different mills that it is evident that the optimum grade to yield the maximum quality for all applications has not yet been found.To date,no theories have been proved. In fact,in many instances the combination of experience and roll pe
10、rform ance in the mills is totally contrary to the theories of yesterday and today.Even the finest theory does not help if a roll grade fails：“Banding”in roughing stands never created problems,but the handing problem in finishing mills has not been solved by any roll grade. And there is little hope
11、for change in future.There is not single outstanding quality for roughing mills which out-performs all other qualities in every application.This is because rolling conditions vary widely. In this paper an attempt is made to identify some variables which are independent on the mill and the passes in
12、they mill，and then to analyse tile“rolling conditions”.The bases for these analytical studies are actual rolling schedules for similar strip dimensions and qualities from different mills and experience with different roll grades in these mills .We have to identify the different variables for every s
13、tand and every pass and then try to find the relationship between these variables and the performance figure for different roll grades. All information about special practices in the mills for producing good strip profile and flat strip,which are of high importance for mill people,are not considered
14、 because they probably have no influence on the choice of the correct roll grade.The initial idea was to answer all questions，to solve all problems by having rules for roll wear and fire crazing .We very quickly found that this was impossible .Even with the most sophisticated methods,because we can
15、only study“normal rolling conditions”and every often the so called“abnormal conditions”are every day occurrences .And only the simple figures from the rolling schedules are available and no actual information on loads .torque or the real temperature distribution on strip and rolls，nothing about chan
16、ge of the total rolling program（length,coffin shape.） Therefore we will try to define the rules for normal conditions and the other problem,to eliminate the“abnormal conditions”，is up to the mill people .We have to prove how good our variables for rolling conditions are and how they are affected by
17、tile“abnormal conditions”. Rolling conditions and theoretical background. The rolling conditions are directly related to tile configuration of tile mill.a) The mill configuration consists of:- number of stands- type of stands（two；four-high）and for each stand- maximum separation force，- maximum torqu
18、e- speed ranges- roll dimensions and - cooling system；b) The rolling practices consist of:- strip grade- slab and strip dimensions- gap tulle and - draughting practices load distributionThis basic information gives the limits for each mill and each stand,but does not directly give enough information
19、 about the rolling conditions .Only the actual pass design and the real rolling schedules show what happens in the bite of each pass and therefore basic information of the rolling conditions is obtained.The rolling schedule used gives the actual figures for each pass and stand but not the ranges.It
20、gives realistic numbers for each pass which fit together and normally close to the rolling conditions in the mill rolling slab to strip.The schedule for rolling mills are often constant,varying little for different strip grades and strip dimensions.The schedules for the finishing mill may change fro
21、m strip to strip.However,these variations are normally within relatively narrow limits.Although rarely done,the rolling schedules can be used to calculate the variables for each pass.These variables can be divided into 3 categories: Category 1These variables are shown in the rolling schedule itself
22、and can be directly measured,figure 1:- strip thickness H1 before and H2 after pass- speed of strip V1 before and V2 after pass- separation force P- torque M- strip temperature- strip width b,roll diameter D. Category 2 The variables are directly calculated from the firest group of variables:- strip
23、 reduction - bite angle - contact length between strip and work roll =roll speed- average specific load on strip in the gap , (=strip width)- relative speed between the strip and work roll Category 3There are a combination of the first and second Categories of variables：- Coefficient of heat penetra
24、tion from strip to work roll - Coefficient of work for reduction in the gap Actual mechanical rolling conditions. To understand rolling conditions in hot strip mills,rolling schedules from different hot mills were analysed.The schedules were taken from two continuous mill（4 and 5 roughing stands res
25、pectively）,one continuous mill（one two high rougher with 7 passes,plus two continuous roughing stands）,and one semi-conditions mill （four high rougher with 5 passes）.The finishing mills in these four mills each had 7 stands.Variables of the 1st,2nd and 3rd categories were obtained and calculated fro
26、m the rolling schedules and then plotted versus the different passes.The four to nine passes of the different roughing mills were somehow equally distributed.Figure 2 shows the separation force P, varying on a high level in the roughing mill and the first finishing stands but decreasing in the later
27、 stands of the finishing mill .The important average specific load is low and almost the same in all analysed roughing mills and increases rapidly in the finishing mills .These variables are inverse because the contact length decreases very fast in the finishing mill .The Coefficient of work for red
28、uction it shows the trend as the torque M. figure 3.Rolling speed V2, is given in figure 4 and the relative rolling speed V* in figure 5, V* is one of the variables determining wear. While separation force and torque show the well known characteristics, far more important are V* (figure 5), specific
29、 load p and coefficient of heat penetration W. figure 6. Figure 7 shows the relationship between the bite angle and V2, V2 is critical only for the critical bite angle at the moment when the slab or strip initially enters the pass; afterwards the slippage in the roll bite angle depends on V*. Figure
30、 8 plots the size of fire crack pattern versus the coefficient of heat penetration W. These figures show some direct results which are important for rolling mills. It is evident that it is possible to control the variables which influence the rolling conditions. In fact p, Wand V* differ widely thro
31、ughout the mill. The specific load p is within a marrow range - almost constant in the roughing mill and increasing in the finishing mill (for the four analysed rolling schedules from different mills). The coefficient of heat penetration W decreases in the roughing mill front pass to pass and there
32、are significant differences between the mills. W decreases also in the finishing mill. but is very similar for the first four stands and is close to zero for stands 5. 6 and 7. The wear speed V* increases in roughing and finishing mills and is higher in continuous roughing mills than in 3/4 or semi-
33、continuous mills - where there is a tendency for slippage. Figures 5 and 6 show that the rolling conditions are characterized as: - passes 2-5: low p high W - low V* - passes 6-10: low p - lower W - higher V* - pass F1; low p - lower W - higher V*- passes F2-F3; higher p - even lower W - higher V*-
34、passes F4-7; very high p - W = Zero - highest V*. The heat penetration W is dominant in the first passes of a roughing mill but progressively decreases in the finishing mill down to the last stand. Specific load increases slowly but continuously. There is no significant difference in any of the roll
35、 condition variables between the last roughing passes and the 1st finishing stand. However, the rolling conditions of the last stands of finishing mills are totally different from the early stands. With standard cooling conditions in hot strip mills the fire crack pattern can he related directly to
36、the heat penetration W, figure 6. however this is only valid for the top rolls. It appears that the pattern on the bottom rolls is influenced by other variables. It might be that the cooling conditions vary widely, not only for the cooling conditions of the rolls, but also for the strip. The mechani
37、cal rolling conditions are the same for top and bottom work rolls in the same stand, but the metallurgical conditions are definitely not the same. Actual metallurgical rolling conditions. Some aspects of this Chapter are related to D. Blazevic). To describe the metallurgical rolling conditions is mo
38、re complicated than the mechanical rolling conditions and almost impossible. We can therefore only make general statements. even though the metallurgical conditions are at least of the same importance as the mechanical. The problem is that the strip temperature is influencing all metallurgical varia
39、bles and strip temperature itself cannot be measured. As soon as the slab has left the furnace, strip temperature is out of control and time and water from descaling and roll cooling systems work on the strip surface. Almost everything varies in the mill besides the descaling and cooling system and
40、the computer follows the strip temperature somehow with speed ups and/or lamellar cooling systems and finally the right coiling temperature is reached and controlled. But all the way down through the whole mill between furnace and coiler there is actually no temperature control. And it is well known
41、 that the strip temperature varies from head to tail. from the middle to the edges. from top to bottom side (the upper side of strip 20-40 mm thick may be up to 100 C cooler than the bottom side). Strip temperature and strip quality determine plasticity and the type (and with additional influence of
42、 time the thickness) of scale on the strip. Different temperatures of the strip consequently create different specific loads on the work rolls and different wear etc. The type of scale which grows on the strip depends on strip surface temperature. figure 9. High temperature scale Fe 2O3, is 2 the ha
43、rdest. low temperature scale FeO is the softest and the transition from one to the other is in the temperature range between 900 and 1100 C. which is the main range of temperature for rolling in hot strip mills. Additionally. the time between the stands of the finishing mill is inverse to rolling sp
44、eed. Scale on the strip should he always removed because it could increase roll wear and influence strip quality. Anyway, scale on the strip is always found on work roll surfaces as a complete layer and this helps to protect the roll surface against wear and reduces heat transfer from strip to roll
45、However, up to now research did not thoroughly investigate the adhesive strength of scale on the strip and roll or the growth of thickness of scale on the roll during a rolling period or the influence of roll temperature and fire crack pattern on the adhesive strength or the influence of change of
46、scale type on the oxide layer on the roll. Answers to these questions would help to understand the metallurgical conditions in the gap much better. Descaling and cooling systems in all hot strip mills are often subject to trials and change with the aim of achieving a better solution. But once the sy
47、stem is modified, all cooling parameters usually remain fixed and actual temperature distribution on the Strip surface is not uniform and constant as it should he. The primary aim of roll cooling systems is cooling the work rolls - however, this may create problems on strip temperature distribution
48、which vice versa influences the work roll surface .Rolling conditions and requirements on roll surface. In hot mills, under normal rolling conditions, we very often find the following problems: - wear in roughing mills. - surface breakdown in early finishing stands. Especially in F2 bottom roll: scale rolled in the strip, bruises in the very last finishing stands, strip surface particles sticking to the roll and hack to strip again. This phenomenon is observ
