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遼寧科技大學(xué)本科生畢業(yè)設(shè)計(jì) 第 9頁
在熱連軋中軋制條件對(duì)工作輥性能的影響
摘要
熱連軋中機(jī)械軋制條件是由許多變量決定的，這些變量可以直接從時(shí)間表讀?。ǚ蛛x力，扭矩，速度，帶鋼厚度） ，或者通過軋制安排的圖表（壓下量，輥直徑等）計(jì)算得出來的 。這些變量描述的機(jī)械軋制條件可以用在所有的粗加工和精加工工廠。這些變量應(yīng)輔以冶金軋制條件。然后，用他們的基本資料提供的條件來確定磨損（具體負(fù)荷，磨損速度）和消防裂縫（共同有效的熱滲透等） 。這是一個(gè)很好的機(jī)會(huì)，只要軋制條件類似就可以利用經(jīng)驗(yàn)與各種等級(jí)的軋機(jī)進(jìn)行模擬比較。這種方法是有限的，“不正常軋制條件”需要完全不同的軋輥等級(jí)，但如果它可以更好的來消除不正常的情況。
導(dǎo)言
在熱軋帶鋼軋機(jī)種中， 150至250毫米厚的鋼坯要軋制脫光1.5至12毫米厚。常規(guī)熱連軋機(jī)由粗精軋機(jī)組組成。粗軋機(jī)的構(gòu)造大相徑庭。一個(gè)機(jī)組有一個(gè)驅(qū)動(dòng)點(diǎn)和一個(gè)或兩個(gè)連續(xù)的粗軋點(diǎn)被稱為條件軋機(jī)，如果一個(gè)軋機(jī)有4至6個(gè)粗加共平臺(tái)被稱為連續(xù)軋機(jī)。
在3/4連續(xù)和連續(xù)軋機(jī)，第一站通常是兩個(gè)高看臺(tái)而其余4個(gè)高看臺(tái)。除了這些橫向來看，幾個(gè)立輥也使用。整理工廠至少有4站，但通常6至7站。
軋制條件在不同工廠，不同站，不同傳遞位置都是不同的。軋機(jī)結(jié)構(gòu)的設(shè)計(jì)想整體（厚度）減少，但是，每一個(gè)站臺(tái)被分離的最大力量，最大扭矩，風(fēng)險(xiǎn)滑移等所限制。
為了提供正確的軋輥給軋機(jī)，軋輥制造商需要詳細(xì)知道滾動(dòng)條件和任何特殊情況。然而，如何利用這一信息？如何比較通過第X架和第X + 架時(shí)的條件？有討論了多年，但比較了很少任何真正的結(jié)果。 例如，尋找粗軋機(jī)工作輥，有這么多等級(jí)的輥被用來在不同的工廠，顯而易見都是的最佳等級(jí)，收益率最高的質(zhì)量，但還沒有找到一個(gè)通用的。
迄今為止，沒有任何理論已被證明。事實(shí)上，在許多情況下，有著豐富的經(jīng)驗(yàn)和推出績(jī)效的工廠昨天和今天的理論是完全相反。
如果等級(jí)不行即使是最好的理論也是無助：
→ “帶狀圖”從未在粗加工中造成問題，但由于等級(jí)太多使移交的問題在整理階段
尚未解決。而且未來也沒有希望改變。
→ 粗軋機(jī)不存在單一的優(yōu)秀品質(zhì)的部分而且適應(yīng)所有其他素質(zhì)的各種應(yīng)用的要求。
這是因?yàn)闈L動(dòng)條件差別很大。
在該文件中試圖找出一些變數(shù)是獨(dú)立的工廠，這些變量是獨(dú)立于軋制和運(yùn)輸?shù)?，然后分析他們的軋制條件?；谶@些分析研究的實(shí)際軋制時(shí)間表，類似地帶層面和素質(zhì)從不同的工廠和經(jīng)驗(yàn)，推出在這些工廠不同的等級(jí)。我們必須確定不同的變量，每站和每一個(gè)傳輸點(diǎn)，然后嘗試找出不同等級(jí)輥這些變量之間的關(guān)系和業(yè)績(jī)數(shù)字。不考慮所有信息的特殊做法，工廠生產(chǎn)良好形象和平面條形地帶，這是非常重要的磨人，因?yàn)樗麄兛赡軟]有影響力的選擇正確的品位初步想法是答案所有的問題，解決所有的問題通過規(guī)則輥磨損和消防裂縫。我們很快發(fā)現(xiàn)，這是不可能的。即使有最先進(jìn)的方法，因?yàn)槲覀冎荒苎芯俊罢＼堉茥l件”和每一個(gè)往往是所謂的“不正常的情況”是每天都發(fā)生。只有簡(jiǎn)單的數(shù)字的滾動(dòng)計(jì)劃，可與任何實(shí)際的信息負(fù)載。扭矩或?qū)嶋H溫度分布地帶和卷 ，沒有改變總滾動(dòng)計(jì)劃（長(zhǎng)度，棺材形狀...）
因此，為消除“不正常的情況”，我們將設(shè)法制定規(guī)則來處理正常條件和其他人能解決的問題。我們已經(jīng)證明我們變量軋制條件有多好，以及他們?nèi)绾问堋安徽５那闆r”影響的。
軋制條件和理論背景
軋制條件直接關(guān)系到軋輥的構(gòu)造
a)軋機(jī)構(gòu)造包括：一些軋機(jī)、軋機(jī)的類型（ 二輥；四輥）和之間的差異、 最大分離力， 最大扭矩、速度范圍、 軋輥尺寸 、冷卻系統(tǒng)；
b ）軋制措施包括：板帶等級(jí)、板帶溫度、夾縫和通風(fēng)措施負(fù)荷的分配
這基本資料的限制，使每個(gè)工廠，每架軋機(jī)，都不能直接給予關(guān)于軋制條件的足夠信息。只有通過實(shí)際設(shè)計(jì)和每次咬入的實(shí)際軋制表來顯示發(fā)生的事情，因此，可以獲得軋制條件的基本信息。
軋制時(shí)間表經(jīng)常給出了通過每架軋輥時(shí)的實(shí)際函數(shù)而不是范圍。它使現(xiàn)實(shí)的號(hào)碼與每架軋機(jī)結(jié)合在一起并且通常接近板帶軋制過程中的軋制條件。
軋機(jī)的時(shí)間表往往不變的，只隨不同板帶等級(jí)和板帶尺寸發(fā)生很小的變化。精軋階段的時(shí)間表可能會(huì)因地方不同而更改。然而，這些變化通常是在相對(duì)狹小的范圍。
雖然很少做，但軋制表可用于計(jì)算每次通過的變量。這些變數(shù)可分為3類：
第一類
這些變量在軋制表中顯示了出來并可直接測(cè)量，圖1 ：
進(jìn)入前厚度H1，通過后厚度為H2 ；進(jìn)入速度V1，出來速度為V2；分離力p；
扭矩M；板帶溫度；帶鋼寬度b ，軋輥直徑D。
第2類
這些變量可以通過第一組的變量直接計(jì)算出：
-壓下量
-咬入角
-板帶與工作輥之間的接觸長(zhǎng)度??? =輥速度
-板帶在輥縫間的平均壓強(qiáng)
? ， （=板帶寬度）
-板帶和工作輥間的相對(duì)速度
第3類
第一類和第二類變量的結(jié)合：
-從板帶到工作輥的熱滲透系數(shù)
-輥縫誤差的的減小系數(shù)
實(shí)際機(jī)械軋制條件
為了了解熱連軋中的軋制條件，我們分析了來自不同熱工廠的軋制時(shí)間表。軋制時(shí)間表來自兩個(gè)連續(xù)軋機(jī)（特別是第4和第5架） ，一個(gè)連續(xù)軋機(jī)（一個(gè)二輥粗軋機(jī)和7個(gè)加工軋機(jī)，再加上兩個(gè)連續(xù)的粗加軋機(jī)） 和一個(gè)半條件軋機(jī)（四輥粗軋機(jī)和5個(gè)工作軋機(jī)） 。最后一道軋輥在這四個(gè)工廠各有7架。第1 ，第2和第3類的變量通過軋制時(shí)間表獲得或計(jì)算得到并對(duì)不同的工作站劃分成對(duì)。在第4至第10架粗軋機(jī)以某種方式均勻分布。
圖2顯示了分離力在粗軋機(jī)和首架精軋機(jī)之間不斷變化很大，但在后面的精軋平臺(tái)上不斷下降。最重要的是平均具體負(fù)荷在粗軋階段幾乎都是一樣的低，且在精軋階段迅速增加。這些變量是相反的。因?yàn)樵诰堧A段接觸長(zhǎng)度下降速度非常快。系數(shù)的工作減少顯示出的趨勢(shì)如扭矩圖3所示。
軋制速度V2在圖4中和相對(duì)滾動(dòng)速度V*在圖5中 ，V *是一個(gè)確定磨損變量。雖然分離力和扭矩表現(xiàn)出眾所周知的特點(diǎn)，更重要的是V* （圖5 ） ，具體載荷P和熱滲透系數(shù)。圖6 。
圖7顯示咬角和V2之間的關(guān)系 ，當(dāng)板或帶最初進(jìn)入通道的時(shí)侯，V2對(duì)咬角的影響是關(guān)鍵的關(guān)鍵;延誤后的軋輥咬角取決于V *圖8顯示了燒裂的大小與熱滲透系數(shù)之間的關(guān)系，這些數(shù)字直接顯示出一些對(duì)軋機(jī)非常重要的結(jié)果。很明顯，控制影響軋制條件的變量是有可能的。事實(shí)上，P，W和V *在整個(gè)軋機(jī)上大不相同。具體的載荷P幾乎在粗軋階段變化是在很窄的范圍之內(nèi)的而在精軋階段一直增加（通過分析四個(gè)不同軋輥的軋制表獲得） 。熱滲透系數(shù)W在通過每架粗軋機(jī)前都要遞減并且在每架軋機(jī)之間的差異是很大的。 W在精軋階段也是要減小的。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。磨損速度V *在粗加工階段和精加工階段的增加量比在有滑動(dòng)傾向的3/4連續(xù)粗軋機(jī)或半連續(xù)軋機(jī)增加的快。圖5和圖6表明了軋制條件的特點(diǎn)是：
-通過2-5架時(shí) ：低p高w-低V *
-通過6-10架時(shí)：低p-較低w –較高V *
-通過F1時(shí)：低p-較低w –較高V *
-通過F2 - F3時(shí)：較高p –更低的w –較高V *
-通過F4 –F7時(shí) ：很高p - W = 0 -最高V *.
熱滲透W在通過第一架粗軋機(jī)時(shí)影響最大，但在精軋階段到最后一架軋機(jī)逐步減小。具體負(fù)荷不斷地緩慢增加。在任何軋制條件時(shí)在最后一道粗加工和第一道精加工之間的變量都不存在明顯的差異。然而，在最后幾架精軋機(jī)的軋制條件與前面的完全不同。在標(biāo)準(zhǔn)的冷卻條件下，熱連軋機(jī)的燒裂可以與熱滲透w直接聯(lián)系起來如圖6 。然而，這種關(guān)系僅適合最好的軋輥?？磥?，一般軋輥軋制時(shí)還要受到其他變量的影響。它可能是冷卻條件差異太大，不僅是對(duì)軋輥的冷卻，還有對(duì)板帶的冷卻。好的軋輥與差一些的軋輥機(jī)械軋制條件可能都是相同的，但冶金條件是絕對(duì)不會(huì)相同的。
實(shí)際冶金軋制條件
本章某些方面引自D. Blazevic）為了描述冶金軋制條件比機(jī)械軋制條件更加復(fù)雜的和幾乎是不可能的。因此，我們只能作一般性發(fā)言。即使冶金條件與機(jī)械條件至少同樣重要?，F(xiàn)在的問題是，帶鋼溫度影響所有的變量和冶金地帶，溫度本身卻不能加以衡量。一旦離開了板坯爐，帶鋼溫度失去控制，時(shí)間和水除鱗和軋輥冷卻系統(tǒng)的工作地帶是表面上的。幾乎所有型式的輥除鱗和冷卻系統(tǒng)和計(jì)算機(jī)緊隨帶溫度某種程度上與“速度窗口”和/或“層冷卻系統(tǒng)”相同 ，并最終在達(dá)到卷取溫度達(dá)到時(shí)控制。但在整個(gè)軋制過程從加熱爐到卷取機(jī)之間實(shí)際上沒有任何的溫度控制。而且眾所周知，從帶的頭部到尾部，從中間到邊緣，從上方到下方溫度都是變化的（帶的上方一側(cè)20-40毫米厚的地方比下面溫度低達(dá)） 。
帶鋼溫度和帶鋼質(zhì)量（和時(shí)間，厚度的額外影響）決定了的可塑性和板帶上鱗片的種類。不同溫度下的板帶，因此創(chuàng)造出了工作輥上的不同的具體負(fù)荷和磨損等。板帶上鱗片的種類取決于帶鋼表面溫度。圖9 。高溫產(chǎn)生的鱗片是硬度第二的Fe 2O3，低溫產(chǎn)生的鱗片是最軟的FeO而過渡帶產(chǎn)生的溫度范圍是至。這個(gè)溫度是軋制熱軋帶鋼時(shí)的溫度。此外精軋階段的時(shí)間與軋制速度成反比。板帶上的鱗片應(yīng)該隨時(shí)清除，因?yàn)樗鼤?huì)增加軋輥的磨損和影響帶鋼質(zhì)量。總之，板帶上的鱗片總是以工作輥表面為基準(zhǔn)形成一個(gè)完整的層，這有助于保護(hù)輥面的磨損和降低從板帶傳熱到軋輥。然而，到現(xiàn)在為止的研究并沒有徹底查出板帶上的鱗片與軋輥的粘接強(qiáng)度或在一個(gè)軋制周期中板帶上鱗片厚度的增加量或軋制溫度和燒裂的樣式對(duì)粘接強(qiáng)度的影響或軋輥上氧化層上鱗片種類的變化之間的關(guān)系。這些問題的答案將有助于更好地了解缺陷產(chǎn)生的冶金條件。
除鱗和冷卻系統(tǒng)在所有熱連軋機(jī)中往往受到質(zhì)疑并且實(shí)現(xiàn)找到更好解決的辦法的目的。但是，一旦系統(tǒng)被修改，所有的冷卻參數(shù)通常是固定不變的而帶表面實(shí)際的溫度分布像它應(yīng)該的那樣是沒有統(tǒng)一的和持續(xù)的變化。冷卻系統(tǒng)的首要目標(biāo)是工作輥的冷卻。然而，這可能會(huì)造成溫度分布的地帶的問題，反之亦然會(huì)影響了工作輥表面。
軋制條件和軋輥表面的要求
在正常軋制條件下，在熱連軋廠我們往往會(huì)發(fā)現(xiàn)以下問題：
-粗軋機(jī)的磨損
-精軋初期的表面開裂
尤其是在F2軋機(jī)的軋輥：鱗片被壓入板帶中就會(huì)刮傷最后幾架精軋機(jī)，板帶表面的顆粒粘結(jié)在軋輥上繼續(xù)損壞板帶。這種現(xiàn)象在最后的精軋機(jī)中已經(jīng)被觀察到，特別是在所有精軋的特殊板帶等級(jí)（鐵素體不銹鋼）。
-磨損速度（圖5 ） ， -具體的負(fù)載（圖2 ， 6 ）
-滑動(dòng)長(zhǎng)度，-軋輥、帶鋼表面（氧化層！ ）
-含有腐蝕性和對(duì)零件有磨損的物質(zhì)的軋輥冷卻水
在粗軋階段，鱗片（高溫、低速等）造成了大部分輥磨損和高傳熱系數(shù)產(chǎn)生燒縫和高粗糙度。
但有時(shí)，過度磨損還與軋輥的滑移有關(guān)。滑移的原因是過低的摩擦。滑移是磨損速度、“具體負(fù)荷”和輥表面粗糙度產(chǎn)生的結(jié)果。
“條帶”是一個(gè)永遠(yuǎn)不會(huì)結(jié)束的故事。許多發(fā)表的論文都與此主題相關(guān)，有些人認(rèn)為，專利帽子這個(gè)問題根本沒有解決。各種斜紋人民有自己的經(jīng)驗(yàn)，但現(xiàn)在的問題是還沒有完全描述：有時(shí)真的造成問題，而有時(shí)卻不。有一些結(jié)論對(duì)大部分軋機(jī)是適用的：
-條帶不會(huì)在工作輥換后直接出現(xiàn)變化。但更常見的在今年下半年推出的標(biāo)準(zhǔn)軋制程序：
-經(jīng)發(fā)現(xiàn)條帶并不取決于輥制造商。特別的軋制等級(jí)，軋輥的熱處理，微觀結(jié)構(gòu)或其他性質(zhì)：
-條帶并不是任何特殊地級(jí)別或特殊地尺寸造成的。
看來這個(gè)問題不能通過對(duì)任何專門的軋輥等級(jí)來解決，只有通過研究軋制條件來解決。
擦傷往往是由堅(jiān)硬的高速帶鋼尾部對(duì)輥面的影響。硬度高的軋輥可以降低擦傷。但硬度只是一點(diǎn)-是另外一點(diǎn)。今天看來微觀結(jié)構(gòu)的推出是避免在以后精軋中出現(xiàn)刮傷的主要因素。老子他不銹鋼板帶中存在的問題早日精軋可以通過采用不同的材料來解決，這在過去到現(xiàn)在為止唯一一個(gè)單一的等級(jí)。
熱連軋機(jī)工作輥的質(zhì)量
用于熱連軋機(jī)工作輥中的軋輥等級(jí)種類是相當(dāng)多的，甚至令人混淆。此外，現(xiàn)在有必要用增加了若干軋制等級(jí)甚至噪音的最先進(jìn)的復(fù)合軋輥。
高耐磨材料用于關(guān)鍵處無法承受熱應(yīng)力、扭矩和彎曲載荷的工作層。復(fù)合工作輥材料的核心和關(guān)鍵通常是灰色鑄鐵或鋼。熱連軋機(jī)工作輥工作層所用的材料列于表1 。
表一包括一些特性像硬度、微觀結(jié)構(gòu)等。等級(jí)種類可通過不同的熱處理在這些軋制等級(jí)中增長(zhǎng)。圖10顯示出了表1 中的一些材料的典型微觀結(jié)構(gòu)。
表2顯示的是這些軋制技巧（表一）的典型應(yīng)用和先進(jìn)的技術(shù)。某些技巧已經(jīng)被成功應(yīng)用而另一些則沒有。使用性能指標(biāo)和軋制條件很容易比較不同軋輥和不同的工廠，并提高正常軋制條件下的總輥性能。
軋輥在正常和不正常滾動(dòng)條件的的性能
粗軋階段，經(jīng)過藥劑，所有技術(shù)都在使用。通常情況下，傳統(tǒng)的，特殊的經(jīng)驗(yàn)和極端軋制條件（適宜的負(fù)荷，滑移速度）需要特別注意。在第一架軋機(jī)中使用石墨鑄鋼。然后為了提供良好的性能和較低的風(fēng)險(xiǎn)在其他軋機(jī)中采用高鉻鑄鐵基本。高鉻鋼在許多工廠進(jìn)行了測(cè)試，并且應(yīng)用在聲波的性能是令人鼓舞的。即使在一些工廠的第一架軋機(jī)中表面出現(xiàn)了問題?？傊磥砀咪撦伮愿玫卣f明了不正常軋制條件時(shí)有發(fā)生。
在F1的軋制條件類似于通過粗軋階段最后粗軋機(jī)的條件。高鉻鑄鐵在這個(gè)位置做得很好。然而，高鉻鋼或石墨鑄鋼還應(yīng)工作好。
在精軋階段2-4架熱連軋機(jī)的高鉻鐵質(zhì)量滿足特別等級(jí)帶鋼的軋制如奧氏體或鐵素體鋼。以前經(jīng)常使用的是無限期可逆冷軋輥，但高鉻鐵取得了很大的改善，表現(xiàn)更好。在一些工廠的車間鋼輥軋機(jī)還在使用并取得了良好的效果。在高負(fù)荷工作下這些軋機(jī)中的等級(jí)趨向于粉碎并顯示出表面疲勞的問題。
在精軋過程的最后一架軋機(jī)有最高的負(fù)荷P和速度v，軋輥輥表面也不得不承受軋制沖擊。一個(gè)高硬度軋輥也需要“抵抗粘結(jié)”。軋輥質(zhì)量的唯一成功應(yīng)用并被多年來認(rèn)可的是無限期可逆冷軋輥。在不影響其他屬性的前提下提高耐磨性是必要的。熱抵抗是沒有問題的（非常低的熱滲透率） 。所有軋輥制造商正在開發(fā)和嘗試新的特點(diǎn)，但迄今還沒有成功。即使企圖利用有很高的硬度的高鉻鐵都沒有成功。硬度不能解決粘結(jié)和所有的表面問題。
要在正常軋制條件下獲得良好的性能指標(biāo)，在第三、四部分中的參數(shù)應(yīng)該是在正常軋制條件下的。通常情況下，所謂的異常軋制條件是正常的，參觀部門后這些異常情況應(yīng)該排除。
但是有特殊情況就有異常軋制條件：
-造成的損害源于粘結(jié)，制造差 ，帶鋼卡在軋機(jī)的缺口里等（i，e，w<10 ）當(dāng)軋輥表面較軟時(shí)會(huì)沒有那么嚴(yán)重。
-當(dāng)軋制材料硬度較低時(shí)燒裂的形式會(huì)變小很多
-因?yàn)檐垯C(jī)中存在較高的殘余壓應(yīng)力裂紋的擴(kuò)展就會(huì)減小或停止
-軋輥有較高的強(qiáng)度和核心材料有較低的殘余拉應(yīng)力可以減少軋輥的熱裂
-核心疲勞裂紋弧阻止以同樣的方式。
最好的解決辦法總是減少或消除不正常的滾動(dòng)條件。
結(jié)論
軋制條件可以通過翔實(shí)的軋制時(shí)間表來獲得信息來決定，而不是從工廠布局。軋制條件應(yīng)研究所有軋制程序的時(shí)間表以發(fā)現(xiàn)可能與具體的壓力、傳熱系數(shù)或板帶溫度有關(guān)的關(guān)鍵條件。例如，軋制條件、質(zhì)量和燒裂種類之間的相互關(guān)系。還有豐富的可見資料進(jìn)行各種軋制條件在進(jìn)入不同的軋機(jī)的比較：因此，基于這些相似的例子可以很容易地對(duì)軋制等級(jí)做出最適宜的決定。
異常軋制條件可能需要不同的軋輥等級(jí)不同的應(yīng)用。然而，這已超出了正常軋機(jī)的經(jīng)驗(yàn)。
一個(gè)軋輥的特別技術(shù)性能 “對(duì)軋機(jī)事故的免疫力”是必要的。并且這個(gè)“性能”是依賴于單獨(dú)軋機(jī)的異?；蛞馔獾臉?biāo)準(zhǔn)。
使用機(jī)械變量的第二次和第三次明確 。這能夠?qū)τ谲堉茥l件給予精確地信息并可以把這些經(jīng)驗(yàn)應(yīng)用到正常軋制條件下的各種其他的等級(jí)。
參考資料
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.
遼寧科技大學(xué)本科生畢業(yè)設(shè)計(jì) 第 14頁
Rolling conditions in hot strip mills and their influence on the performance of work rolls
Summary.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 distribution
This 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 1
These 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 3
There 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 Indefinite