φ950可逆式轧机压下系统的设计【说明书+CAD】
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辽宁科技大学本科生毕业设计 第 9页在热连轧中轧制条件对工作辊性能的影响摘要热连轧中机械轧制条件是由许多变量决定的，这些变量可以直接从时间表读取（分离力，扭矩，速度，带钢厚度） ，或者通过轧制安排的图表（压下量，辊直径等）计算得出来的 。这些变量描述的机械轧制条件可以用在所有的粗加工和精加工工厂。这些变量应辅以冶金轧制条件。然后，用他们的基本资料提供的条件来确定磨损（具体负荷，磨损速度）和消防裂缝（共同有效的热渗透等） 。这是一个很好的机会，只要轧制条件类似就可以利用经验与各种等级的轧机进行模拟比较。这种方法是有限的，“不正常轧制条件”需要完全不同的轧辊等级，但如果它可以更好的来消除不正常的情况。导言在热轧带钢轧机种中， 150至250毫米厚的钢坯要轧制脱光1.5至12毫米厚。常规热连轧机由粗精轧机组组成。粗轧机的构造大相径庭。一个机组有一个驱动点和一个或两个连续的粗轧点被称为条件轧机，如果一个轧机有4至6个粗加共平台被称为连续轧机。在3/4连续和连续轧机，第一站通常是两个高看台而其余4个高看台。除了这些横向来看，几个立辊也使用。整理工厂至少有4站，但通常6至7站。 轧制条件在不同工厂，不同站，不同传递位置都是不同的。轧机结构的设计想整体（厚度）减少，但是，每一个站台被分离的最大力量，最大扭矩，风险滑移等所限制。为了提供正确的轧辊给轧机，轧辊制造商需要详细知道滚动条件和任何特殊情况。然而，如何利用这一信息？如何比较通过第X架和第X + 架时的条件？有讨论了多年，但比较了很少任何真正的结果。 例如，寻找粗轧机工作辊，有这么多等级的辊被用来在不同的工厂，显而易见都是的最佳等级，收益率最高的质量，但还没有找到一个通用的。 迄今为止，没有任何理论已被证明。事实上，在许多情况下，有着丰富的经验和推出绩效的工厂昨天和今天的理论是完全相反。 如果等级不行即使是最好的理论也是无助： “带状图”从未在粗加工中造成问题，但由于等级太多使移交的问题在整理阶段 尚未解决。而且未来也没有希望改变。 粗轧机不存在单一的优秀品质的部分而且适应所有其他素质的各种应用的要求。 这是因为滚动条件差别很大。在该文件中试图找出一些变数是独立的工厂，这些变量是独立于轧制和运输的，然后分析他们的轧制条件。基于这些分析研究的实际轧制时间表，类似地带层面和素质从不同的工厂和经验，推出在这些工厂不同的等级。我们必须确定不同的变量，每站和每一个传输点，然后尝试找出不同等级辊这些变量之间的关系和业绩数字。不考虑所有信息的特殊做法，工厂生产良好形象和平面条形地带，这是非常重要的磨人，因为他们可能没有影响力的选择正确的品位初步想法是答案所有的问题，解决所有的问题通过规则辊磨损和消防裂缝。我们很快发现，这是不可能的。即使有最先进的方法，因为我们只能研究“正常轧制条件”和每一个往往是所谓的“不正常的情况”是每天都发生。只有简单的数字的滚动计划，可与任何实际的信息负载。扭矩或实际温度分布地带和卷 ，没有改变总滚动计划（长度，棺材形状.） 因此，为消除“不正常的情况”，我们将设法制定规则来处理正常条件和其他人能解决的问题。我们已经证明我们变量轧制条件有多好，以及他们如何受“不正常的情况”影响的。轧制条件和理论背景轧制条件直接关系到轧辊的构造a)轧机构造包括：一些轧机、轧机的类型（ 二辊；四辊）和之间的差异、 最大分离力， 最大扭矩、速度范围、 轧辊尺寸 、冷却系统；b ）轧制措施包括：板带等级、板带温度、夹缝和通风措施负荷的分配这基本资料的限制，使每个工厂，每架轧机，都不能直接给予关于轧制条件的足够信息。只有通过实际设计和每次咬入的实际轧制表来显示发生的事情，因此，可以获得轧制条件的基本信息。 轧制时间表经常给出了通过每架轧辊时的实际函数而不是范围。它使现实的号码与每架轧机结合在一起并且通常接近板带轧制过程中的轧制条件。 轧机的时间表往往不变的，只随不同板带等级和板带尺寸发生很小的变化。精轧阶段的时间表可能会因地方不同而更改。然而，这些变化通常是在相对狭小的范围。虽然很少做，但轧制表可用于计算每次通过的变量。这些变数可分为3类：第一类这些变量在轧制表中显示了出来并可直接测量，图1 ： 进入前厚度H1，通过后厚度为H2 ；进入速度V1，出来速度为V2；分离力p；扭矩M；板带温度；带钢宽度b ，轧辊直径D。第2类这些变量可以通过第一组的变量直接计算出： -压下量-咬入角-板带与工作辊之间的接触长度 =辊速度-板带在辊缝间的平均压强 ， （=板带宽度） -板带和工作辊间的相对速度第3类第一类和第二类变量的结合： -从板带到工作辊的热渗透系数 -辊缝误差的的减小系数 实际机械轧制条件为了了解热连轧中的轧制条件，我们分析了来自不同热工厂的轧制时间表。轧制时间表来自两个连续轧机（特别是第4和第5架） ，一个连续轧机（一个二辊粗轧机和7个加工轧机，再加上两个连续的粗加轧机） 和一个半条件轧机（四辊粗轧机和5个工作轧机） 。最后一道轧辊在这四个工厂各有7架。第1 ，第2和第3类的变量通过轧制时间表获得或计算得到并对不同的工作站划分成对。在第4至第10架粗轧机以某种方式均匀分布。图2显示了分离力在粗轧机和首架精轧机之间不断变化很大，但在后面的精轧平台上不断下降。最重要的是平均具体负荷在粗轧阶段几乎都是一样的低，且在精轧阶段迅速增加。这些变量是相反的。因为在精轧阶段接触长度下降速度非常快。系数的工作减少显示出的趋势如扭矩图3所示。轧制速度V2在图4中和相对滚动速度V*在图5中 ，V *是一个确定磨损变量。虽然分离力和扭矩表现出众所周知的特点，更重要的是V* （图5 ） ，具体载荷P和热渗透系数。图6 。图7显示咬角和V2之间的关系 ，当板或带最初进入通道的时侯，V2对咬角的影响是关键的关键;延误后的轧辊咬角取决于V *图8显示了烧裂的大小与热渗透系数之间的关系，这些数字直接显示出一些对轧机非常重要的结果。很明显，控制影响轧制条件的变量是有可能的。事实上，P，W和V *在整个轧机上大不相同。具体的载荷P几乎在粗轧阶段变化是在很窄的范围之内的而在精轧阶段一直增加（通过分析四个不同轧辊的轧制表获得） 。热渗透系数W在通过每架粗轧机前都要递减并且在每架轧机之间的差异是很大的。 W在精轧阶段也是要减小的。但在前四架是非常相似的，并且在第五，第六，第七架基本接近零。磨损速度V *在粗加工阶段和精加工阶段的增加量比在有滑动倾向的3/4连续粗轧机或半连续轧机增加的快。图5和图6表明了轧制条件的特点是：-通过2-5架时 ：低p高w-低V * -通过6-10架时：低p-较低w 较高V * -通过F1时：低p-较低w 较高V * -通过F2 - F3时：较高p 更低的w 较高V * -通过F4 F7时 ：很高p - W = 0 -最高V *.热渗透W在通过第一架粗轧机时影响最大，但在精轧阶段到最后一架轧机逐步减小。具体负荷不断地缓慢增加。在任何轧制条件时在最后一道粗加工和第一道精加工之间的变量都不存在明显的差异。然而，在最后几架精轧机的轧制条件与前面的完全不同。在标准的冷却条件下，热连轧机的烧裂可以与热渗透w直接联系起来如图6 。然而，这种关系仅适合最好的轧辊。看来，一般轧辊轧制时还要受到其他变量的影响。它可能是冷却条件差异太大，不仅是对轧辊的冷却，还有对板带的冷却。好的轧辊与差一些的轧辊机械轧制条件可能都是相同的，但冶金条件是绝对不会相同的。实际冶金轧制条件本章某些方面引自D. Blazevic）为了描述冶金轧制条件比机械轧制条件更加复杂的和几乎是不可能的。因此，我们只能作一般性发言。即使冶金条件与机械条件至少同样重要。现在的问题是，带钢温度影响所有的变量和冶金地带，温度本身却不能加以衡量。一旦离开了板坯炉，带钢温度失去控制，时间和水除鳞和轧辊冷却系统的工作地带是表面上的。几乎所有型式的辊除鳞和冷却系统和计算机紧随带温度某种程度上与“速度窗口”和/或“层冷却系统”相同 ，并最终在达到卷取温度达到时控制。但在整个轧制过程从加热炉到卷取机之间实际上没有任何的温度控制。而且众所周知，从带的头部到尾部，从中间到边缘，从上方到下方温度都是变化的（带的上方一侧20-40毫米厚的地方比下面温度低达） 。带钢温度和带钢质量（和时间，厚度的额外影响）决定了的可塑性和板带上鳞片的种类。不同温度下的板带，因此创造出了工作辊上的不同的具体负荷和磨损等。板带上鳞片的种类取决于带钢表面温度。图9 。高温产生的鳞片是硬度第二的Fe 2O3，低温产生的鳞片是最软的FeO而过渡带产生的温度范围是至。这个温度是轧制热轧带钢时的温度。此外精轧阶段的时间与轧制速度成反比。板带上的鳞片应该随时清除，因为它会增加轧辊的磨损和影响带钢质量。总之，板带上的鳞片总是以工作辊表面为基准形成一个完整的层，这有助于保护辊面的磨损和降低从板带传热到轧辊。然而，到现在为止的研究并没有彻底查出板带上的鳞片与轧辊的粘接强度或在一个轧制周期中板带上鳞片厚度的增加量或轧制温度和烧裂的样式对粘接强度的影响或轧辊上氧化层上鳞片种类的变化之间的关系。这些问题的答案将有助于更好地了解缺陷产生的冶金条件。除鳞和冷却系统在所有热连轧机中往往受到质疑并且实现找到更好解决的办法的目的。但是，一旦系统被修改，所有的冷却参数通常是固定不变的而带表面实际的温度分布像它应该的那样是没有统一的和持续的变化。冷却系统的首要目标是工作辊的冷却。然而，这可能会造成温度分布的地带的问题，反之亦然会影响了工作辊表面。轧制条件和轧辊表面的要求在正常轧制条件下，在热连轧厂我们往往会发现以下问题： -粗轧机的磨损 -精轧初期的表面开裂尤其是在F2轧机的轧辊：鳞片被压入板带中就会刮伤最后几架精轧机，板带表面的颗粒粘结在轧辊上继续损坏板带。这种现象在最后的精轧机中已经被观察到，特别是在所有精轧的特殊板带等级（铁素体不锈钢）。 -磨损速度（图5 ） ， -具体的负载（图2 ， 6 ） -滑动长度，-轧辊、带钢表面（氧化层！ ） -含有腐蚀性和对零件有磨损的物质的轧辊冷却水在粗轧阶段，鳞片（高温、低速等）造成了大部分辊磨损和高传热系数产生烧缝和高粗糙度。 但有时，过度磨损还与轧辊的滑移有关。滑移的原因是过低的摩擦。滑移是磨损速度、“具体负荷”和辊表面粗糙度产生的结果。 “条带”是一个永远不会结束的故事。许多发表的论文都与此主题相关，有些人认为，专利帽子这个问题根本没有解决。各种斜纹人民有自己的经验，但现在的问题是还没有完全描述：有时真的造成问题，而有时却不。有一些结论对大部分轧机是适用的：-条带不会在工作辊换后直接出现变化。但更常见的在今年下半年推出的标准轧制程序： -经发现条带并不取决于辊制造商。特别的轧制等级，轧辊的热处理，微观结构或其他性质：-条带并不是任何特殊地级别或特殊地尺寸造成的。看来这个问题不能通过对任何专门的轧辊等级来解决，只有通过研究轧制条件来解决。 擦伤往往是由坚硬的高速带钢尾部对辊面的影响。硬度高的轧辊可以降低擦伤。但硬度只是一点-是另外一点。今天看来微观结构的推出是避免在以后精轧中出现刮伤的主要因素。老子他不锈钢板带中存在的问题早日精轧可以通过采用不同的材料来解决，这在过去到现在为止唯一一个单一的等级。热连轧机工作辊的质量用于热连轧机工作辊中的轧辊等级种类是相当多的，甚至令人混淆。此外，现在有必要用增加了若干轧制等级甚至噪音的最先进的复合轧辊。高耐磨材料用于关键处无法承受热应力、扭矩和弯曲载荷的工作层。复合工作辊材料的核心和关键通常是灰色铸铁或钢。热连轧机工作辊工作层所用的材料列于表1 。 表一包括一些特性像硬度、微观结构等。等级种类可通过不同的热处理在这些轧制等级中增长。图10显示出了表1 中的一些材料的典型微观结构。表2显示的是这些轧制技巧（表一）的典型应用和先进的技术。某些技巧已经被成功应用而另一些则没有。使用性能指标和轧制条件很容易比较不同轧辊和不同的工厂，并提高正常轧制条件下的总辊性能。轧辊在正常和不正常滚动条件的的性能粗轧阶段，经过药剂，所有技术都在使用。通常情况下，传统的，特殊的经验和极端轧制条件（适宜的负荷，滑移速度）需要特别注意。在第一架轧机中使用石墨铸钢。然后为了提供良好的性能和较低的风险在其他轧机中采用高铬铸铁基本。高铬钢在许多工厂进行了测试，并且应用在声波的性能是令人鼓舞的。即使在一些工厂的第一架轧机中表面出现了问题。总之，看来高钢辊慢性更好地说明了不正常轧制条件时有发生。 在F1的轧制条件类似于通过粗轧阶段最后粗轧机的条件。高铬铸铁在这个位置做得很好。然而，高铬钢或石墨铸钢还应工作好。在精轧阶段2-4架热连轧机的高铬铁质量满足特别等级带钢的轧制如奥氏体或铁素体钢。以前经常使用的是无限期可逆冷轧辊，但高铬铁取得了很大的改善，表现更好。在一些工厂的车间钢辊轧机还在使用并取得了良好的效果。在高负荷工作下这些轧机中的等级趋向于粉碎并显示出表面疲劳的问题。在精轧过程的最后一架轧机有最高的负荷P和速度v，轧辊辊表面也不得不承受轧制冲击。一个高硬度轧辊也需要“抵抗粘结”。轧辊质量的唯一成功应用并被多年来认可的是无限期可逆冷轧辊。在不影响其他属性的前提下提高耐磨性是必要的。热抵抗是没有问题的（非常低的热渗透率） 。所有轧辊制造商正在开发和尝试新的特点，但迄今还没有成功。即使企图利用有很高的硬度的高铬铁都没有成功。硬度不能解决粘结和所有的表面问题。要在正常轧制条件下获得良好的性能指标，在第三、四部分中的参数应该是在正常轧制条件下的。通常情况下，所谓的异常轧制条件是正常的，参观部门后这些异常情况应该排除。但是有特殊情况就有异常轧制条件： -造成的损害源于粘结，制造差 ，带钢卡在轧机的缺口里等（i，e，w10 ）当轧辊表面较软时会没有那么严重。 -当轧制材料硬度较低时烧裂的形式会变小很多 -因为轧机中存在较高的残余压应力裂纹的扩展就会减小或停止-轧辊有较高的强度和核心材料有较低的残余拉应力可以减少轧辊的热裂-核心疲劳裂纹弧阻止以同样的方式。 最好的解决办法总是减少或消除不正常的滚动条件。结论轧制条件可以通过翔实的轧制时间表来获得信息来决定，而不是从工厂布局。轧制条件应研究所有轧制程序的时间表以发现可能与具体的压力、传热系数或板带温度有关的关键条件。例如，轧制条件、质量和烧裂种类之间的相互关系。还有丰富的可见资料进行各种轧制条件在进入不同的轧机的比较：因此，基于这些相似的例子可以很容易地对轧制等级做出最适宜的决定。 异常轧制条件可能需要不同的轧辊等级不同的应用。然而，这已超出了正常轧机的经验。 一个轧辊的特别技术性能 “对轧机事故的免疫力”是必要的。并且这个“性能”是依赖于单独轧机的异常或意外的标准。 使用机械变量的第二次和第三次明确 。这能够对于轧制条件给予精确地信息并可以把这些经验应用到正常轧制条件下的各种其他的等级。参考资料1)Bla/evic. David T.: Presented to: Ill Seminar on Rolling Mill Rolls. Instituto Latinoamericano del Fierro Acero. Monterrey. Nuevo Leon Meriko March. 6 .9. 1985. 2)Garber. S. Sturgeon, (3. M.: Scale on Wire Roil and Its Removal by Mechanical Means - The Wire Industry March. 1961 pages 257-259 and 295. 辽宁科技大学本科生毕业设计 第 14页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 thickness）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 rolling 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 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.Conventional 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 mill .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 from 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 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 roughing 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 perform 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 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 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 stand 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 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 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 change 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 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 torque- 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 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 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 from 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 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 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 penetration 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 respectively）,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 from 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 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 reduction 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 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 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 throughout 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 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-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*- 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 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 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 mechanical 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 more 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 variables 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 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 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 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 hardest. 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 speed. 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 .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 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 system 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 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 observed in the last finishing Stand, for special strip grades (ferritic stainless steel) in all finishing stands. Roll wear is a function of - wear speed (figure 5), - specific load (figures 2, 6). - sliding length, - roll-, strip surface (oxide layer!). - roll cooling water, containing corrosive and abrasive parts. In roughing stands, scale(high temperature. low speed etc.) causes most of the roll wear and a high coefficient of heat transfer creates fire cracks and a high roughness. Sometimes, however. excessive wear is also related to slippage in the mill .Slippage is a result of too low friction. mainly depending on the wear speed “the specific load” and the roll surface roughness. “Banding” is a never ending story. Some papers are published about this subject and some people believe in patents hat this problem is not solved at all. All twill people have their own experience but right now the problem is not even described completely: sometimes it really snakes problems, sometimes it does not . There are some observations which seem to be valid for most mills:- banding does not occur directly after work roll change .but more commonly in the second half of a standard rolling program m:- banding is not found depending on roll manufacturer. special roll grade ,heat-treatment of the rolls. roll micro structure or other roll property: - banding is not caused by any special strip grade or special strip dimension. It seems this problem cannot he solved by any special roll grade but only by research on rolling conditions.Bruises are often caused by hard .cold strip tails with high speed impact on roll surface. High roll hardness may reduce. bruises. But hardness is only one point - is the other. It seems today evident that the microstructure of the roll is the main factor to avoid sticking in the later finishing stands. I he stainless steel strip problem in the early finishing stands can be solved by different materials, that of the last stand up to now only by one single grade. Qualities for work rolls in hot strip mills. The variety of roll grades used for work rolls in hot strip mills is considerable and almost confusing. In addition, it is now necessary and state of the art to have compound rolls,which increases the number of roll grades even snore.High wear resistant materials used for the working layers are unable to withstand the thermal stress, torque and bending loads at the necks. The material of the Core and necks of compound work rolls is normally grey or nodular east iron, or steel. The materials used for the working layers of work rolls for hot strip mills are given in table 1. Table I includes some characteristic properties like hardness. microstructure etc. The variety of grades can be increased by varying the heat treatments within these roll grades. Figure 10 shows typical microstructures of materials from table 1. Table 2 shows typical applications of these roll grades (table I) and state of the art. Some grades are used successfully while others are not. Using performance figures and rolling conditions it is easy to compare different stands and different mills and to improve total roll performance under normal rolling conditions. Roll performance under normal and abnormal rolling conditions.In roughing stands, see tablet, all grades are in use. Frequently, tradition, special experiences and extreme rolling conditions (optimum of load, speed without slippage). require special attention. Using graphitic cast steel in the first passes. then high chrome iron in the other passes appears to give good performance with low risk. High chrome steel has been tested in many mills and in sonic applications the performance has been encouraging. even though there are surface problems in the first roughing stands in some mills. Anyway. it seems high chronic steel rolls give better results the more abnormal rolling conditions arc every days occurrence. Rolling conditions in F1 arc similar to the last passes of the roughing mills. High chrome iron is doing very well in this location. However, high chrome steel or graphitic cast steel should also work well.In finishing stands 2-4 of many hot strip mills high chronic iron is now standard. Qualities for rolling special strip grades such as austenitic or ferritic steels are available. Previously it was common to use ICDP rolls but the high chronic iron has been a great improvement performance-wise. In some mills Adamite steel rolls were (arc still) being used in these stands with good results. Under high loads these grades tend to shatter and show surface fatigue problems in the mill. In the last stand of the finishing mills where there are highest loads p and speeds v. the roll surface also has to withstand rolling impacts. A roll of high hardness as well as sticker resistance” is required. The only roll quality successfully used and available for many years has been the Indefinitely Chil
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