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熱軋工藝
該軋機(jī)的主要功能是將半成品鋼重新加熱到接近其熔點(diǎn)， 然后通過(guò)由共計(jì)7.7萬(wàn)的大功率連續(xù)滾動(dòng)驅(qū)動(dòng)馬力發(fā)動(dòng)機(jī)帶動(dòng)的12道連續(xù)軋制使鋼板變得更薄更長(zhǎng)，最后卷曲被拉長(zhǎng)的鋼板以運(yùn)輸?shù)较乱坏拦ば颉?
熱軋板卷的重量高達(dá)30噸至30”和74”。 將8至9英寸厚、36英尺長(zhǎng)的鋼板被卷成薄如16英寸和1 / 2英里長(zhǎng)的帶鋼。線圈由兩個(gè)內(nèi)徑（'眼睛'）為30”卷取機(jī)，外徑上限分別為72”和74” 分別與850和1000磅每英寸寬（PIW）相對(duì)應(yīng)的兩個(gè)卷曲機(jī)產(chǎn)生。該廠為每一個(gè)CSI提供售后業(yè)務(wù)，以及負(fù)責(zé)運(yùn)輸成品給CSI顧客。
大部分材料是由一種自動(dòng)線圈處理系統(tǒng)運(yùn)出軋機(jī)，通過(guò)運(yùn)輸線被分批運(yùn)往軋機(jī)的東側(cè)，直到它被冷卻到足以載入鐵路車輛。
加熱爐
對(duì)于熱軋工藝至關(guān)重要的是它的步進(jìn)梁加熱爐，國(guó)家的最先進(jìn)的設(shè)備，現(xiàn)在優(yōu)于三老一輩（推車）式爐。其額定生產(chǎn)速度為每小時(shí)270噸，效率和與板溫一致的方面得到改善以使生產(chǎn)率能比計(jì)劃提高25﹪。把這些鋼從室溫加熱到2200～2400攝氏度需每天消耗約10立方米的天然氣。
就像板材是按訂單分配，日程安排是規(guī)定的，材料被熱軋廠最西端的板廠的鐵路小車和起重機(jī)分批運(yùn)輸。在一條軋制線上，每一放一個(gè)軋板。因?yàn)檐埌灞环胖迷谀蟼?cè)加熱爐的控制門的前邊，所以其規(guī)模和重量是確定的。當(dāng)爐內(nèi)的空間足夠，大型電鍍機(jī)械推拉臂能夠?qū)宀囊频綘t內(nèi)。
一旦進(jìn)入內(nèi)部，板材由大約8英尺長(zhǎng)的爐板支撐，它是通過(guò)冷卻水的耐火涂層管也被稱為滑軌。為了降低鋼板殘留的冰點(diǎn)（滑板標(biāo)記），滑板間距變化大約為熔爐內(nèi)部空間的三分之二。兩個(gè)獨(dú)立的滑軌裝置，一個(gè)固定，一個(gè)運(yùn)動(dòng)，輪流支撐鋼板當(dāng)它在爐內(nèi)運(yùn)動(dòng)時(shí)經(jīng)過(guò)一個(gè)由一對(duì)大型液壓缸提供能量的機(jī)架。
該爐內(nèi)部的寬是38'9”，從地面到天花板有15英尺，142’長(zhǎng)。它分為管制區(qū)內(nèi)的溫度：預(yù)熱，頂面和底，加熱，頂部和底，浸泡，頂和底，東和西。預(yù)熱和加熱區(qū)燃燒一種天然氣的混合物，同時(shí)通過(guò)在熔爐側(cè)邊的大量燃燒器預(yù)燃空氣，加熱鋼板的上部和底部到接近其排氣溫度。
大部分鋼的預(yù)熱是通過(guò)熱廢氣經(jīng)過(guò)鋼板直達(dá)控制門的方式達(dá)到的。留在廢氣中熱量在這些大規(guī)模的熱交換器中將引入的空氣預(yù)熱到1000℉。相反地，在加熱區(qū)，鋼板主要是通過(guò)加熱爐壁加熱。在浸泡區(qū)，很多小型燃燒器尋求保持各區(qū)域間溫度的統(tǒng)一，平衡鋼板冷點(diǎn)。耐火分隔有助于身體區(qū)分區(qū)域，全爐內(nèi)的熱電偶溫度傳感器與自動(dòng)燃燒控制系統(tǒng)相互作用，以保持每個(gè)區(qū)域的目標(biāo)溫度。
復(fù)雜的計(jì)算機(jī)模型計(jì)算出目標(biāo)粗軋機(jī)出口溫度，以獲得一個(gè)爐排放（'排氣'）的目標(biāo)溫度。在連續(xù)的基礎(chǔ)上通過(guò)爐內(nèi)的每片鋼板的厚度來(lái)估計(jì)剖面溫度，計(jì)算機(jī)幫助操作者選擇產(chǎn)品等級(jí)和能最大限度地將鋼板加熱到盡可能接近目標(biāo)溫度的生產(chǎn)區(qū)設(shè)定點(diǎn)。軋制過(guò)程開始后，當(dāng)鋼板離開粗軋機(jī)時(shí)它的溫度會(huì)反饋給熔爐，更新計(jì)算機(jī)模型并通知加熱器達(dá)到溫度的均勻性。
當(dāng)板坯到達(dá)熔爐盡頭的’卸載門’，并且該計(jì)算機(jī)已經(jīng)確定該鋼板已經(jīng)被充分加熱，門打開了，大量的'提取手臂'到達(dá)鋼板的的下方，把它舉起使之遠(yuǎn)離滑軌的支持，并把它拉出熔爐。東部和西部的提取器能夠一次獨(dú)立移除兩控鋼板，同時(shí)提取較長(zhǎng)的鋼板。具有強(qiáng)熱的鋼板被放置在滾轉(zhuǎn)機(jī)上被帶到粗加工工序。
除鱗
在退出加熱爐之后，鋼板經(jīng)過(guò)一個(gè)除鱗單元，外殼采用了兩對(duì)噴霧頭，能夠用1500的壓力沖擊具有強(qiáng)熱的鋼板以消除1/8英尺厚的在富含氧氣的加熱爐內(nèi)在鋼板表面形成的氧化鐵層。除垢后不久，一個(gè)（相對(duì)）小2-hi軋機(jī)稱為能減少板坯大約1英寸的厚度,來(lái)打破遺鋼板上遺留下來(lái)的鱗片。在進(jìn)入下一個(gè)工序前，’掃除噴霧器’將附著在鋼板表面的松動(dòng)的鱗片進(jìn)行清理。轉(zhuǎn)移條多在粗軋過(guò)程中除鱗兩次，前夕的第三次或是最后一次軋制操作，以消除其又重新長(zhǎng)出的鱗片在超過(guò)三分鐘的時(shí)間里或以便能花費(fèi)在粗軋過(guò)程中。
粗加工
粗軋機(jī)是有六個(gè)獨(dú)立的粗軋機(jī)機(jī)架組成，其中最后的四個(gè)包含了體積較小的稱為修邊機(jī)的立式軋機(jī)。在加熱爐內(nèi)加熱直到煥發(fā)明亮的橙黃色的鋼板被軋制，一次經(jīng)過(guò)一個(gè)站來(lái)生產(chǎn)所謂的轉(zhuǎn)移條以適應(yīng)精軋。高壓水射流能清理沿途經(jīng)過(guò)的鋼板表面的氧化鐵層或是鱗片。
在轉(zhuǎn)移條從粗軋機(jī)最后一站出來(lái)的時(shí)候，其主要邊緣的厚度被估計(jì)。同樣的，有高溫計(jì)測(cè)量板條從頭到腳的剖面溫度，并且一種特殊的照相機(jī)可以給板條的兩端拍照。依據(jù)被軋制產(chǎn)品所要達(dá)到的長(zhǎng)度、寬度和等級(jí)，板條在離開最后一道軋制線的平均溫度一般會(huì)達(dá)到1900～2100℃。此數(shù)據(jù)期望在精軋過(guò)程中被收集。計(jì)算機(jī)能立即計(jì)算穿過(guò)這6個(gè)連續(xù)軋制鋼板的線程的速度和差距。
重型粗軋機(jī)有135”寬的輥?zhàn)?，以便能卷曲較寬的一面（作為第一次軋制時(shí)的稱謂）使鋼板更寬。一個(gè)5000馬力的電動(dòng)機(jī)驅(qū)動(dòng)直徑為42”的工作輥通過(guò)28：1的齒輪來(lái)減少鋼板的厚度高達(dá)”。
最后的四個(gè)軋制裝置每個(gè)都包含一個(gè)修邊裝置來(lái)控制鋼板的寬度，并將其軋制為5到6英尺厚，根據(jù)客戶所要求的寬度、尺寸和鋼種其厚度可在此基礎(chǔ)上上下浮動(dòng)公分。如前所述，第三和第五粗軋機(jī)組都有操作壓力為1500的高壓除鱗裝置。這些獨(dú)立的粗軋機(jī)組放置的間距逐漸變大以適應(yīng)轉(zhuǎn)移條在被軋制的越來(lái)越薄時(shí)的長(zhǎng)度。
裁剪
因?yàn)橐粋€(gè)方頭段對(duì)于能正確的經(jīng)過(guò)精軋工序是至關(guān)重要的，并且一個(gè)不平整的尾部會(huì)挫傷工作輥表面或是導(dǎo)致今后的生產(chǎn)過(guò)程的線程問(wèn)題。幾乎每一個(gè)轉(zhuǎn)移條的頭部和尾部末端都會(huì)通過(guò)一對(duì)具有大剪刀片的鋼鼓被裁剪并增加其長(zhǎng)度。在鋼板以100的速度在輥?zhàn)由匣瑒?dòng)時(shí)，傳感器可檢測(cè)其位置和速度以便測(cè)定裁剪的時(shí)間來(lái)優(yōu)化裁切的數(shù)量；由于轉(zhuǎn)移條的厚度超過(guò)一英寸，裁剪長(zhǎng)度每增加一英寸將廢棄15～30磅。
精加工
規(guī)格的熱軋機(jī)包含六個(gè)能夠降低轉(zhuǎn)移條的厚度到根據(jù)顧客或是下一道工序的要求所得規(guī)定尺寸的精軋機(jī)組。軋制速度被設(shè)定為能允許在最后一道工序以1500～1600℃的精軋溫度完成最后的壓下量，并達(dá)到特定的機(jī)械性能。
到現(xiàn)在為止，鋼板被軋制成長(zhǎng)達(dá)200英尺的平板。與粗軋相比，精軋是將轉(zhuǎn)移條進(jìn)行連續(xù)壓軋，這就意味著每一個(gè)板條將一次性通過(guò)六個(gè)軋制機(jī)組。熱鋼在精軋過(guò)程中被拉伸的時(shí)候是相當(dāng)脆弱的，所以必須及時(shí)用一個(gè)低標(biāo)準(zhǔn)來(lái)控制為了避免將鋼板被撕裂。
在精軋操作之前，轉(zhuǎn)移條的頭部和尾部將被剪裁成方形，以幫助確保正確的線程。最后兩步的除鱗操作是為了清除在粗軋過(guò)程中新生成的鱗片。一旦轉(zhuǎn)移條穿梭在每對(duì)連續(xù)的輥?zhàn)由?，在電控樞軸上的自由轉(zhuǎn)動(dòng)的輥?zhàn)幽苁刮摪宓牡撞縼?lái)檢測(cè)各個(gè)精軋機(jī)組間的張力。
按需要作出調(diào)整，以確保鋼板能夠正確通過(guò)每個(gè)輥?zhàn)硬粫?huì)通過(guò)不正常循環(huán)、拉伸和折疊過(guò)度或撕裂。每個(gè)輥?zhàn)拥奈恢媚芊答伒骄垯C(jī)的先進(jìn)自動(dòng)化系統(tǒng)，以及能檢測(cè)輥?zhàn)铀芰?，通過(guò)X射線測(cè)量帶鋼厚度的信息，順利工作以調(diào)整滾動(dòng)速度和差距，保持穩(wěn)定的帶鋼軋制必要的厚度盡管每一個(gè)轉(zhuǎn)移條所呈現(xiàn)的溫度不同。
層流冷卻
對(duì)于冶金學(xué)上的熱軋鋼材性能至關(guān)重要的是卷曲溫度，線圈會(huì)從這個(gè)溫度冷卻到室溫，而這個(gè)過(guò)程的完成需要三天的時(shí)間。這基本上相當(dāng)于一個(gè)熱處理退火， 在從9英寸厚減少到規(guī)定尺寸的過(guò)程中所施加在鋼板上的壓力使線圈得以冷卻來(lái)緩解自己。雖然鋼是在重結(jié)晶過(guò)程中不斷熱軋，削減厚度有時(shí)超過(guò)99％，并采取相當(dāng)于鋼的不到10分鐘的壓力;卷取溫度由產(chǎn)品的冶金性能來(lái)指定以利用操縱這些應(yīng)力學(xué)性能獲得最佳的機(jī)械性能。
由客戶進(jìn)行激光切割的售出的經(jīng)過(guò)熱軋工序、熱軋浸泡和熱軋涂油的產(chǎn)品是在相對(duì)較高的溫度下進(jìn)行連續(xù)松弛以便使從線圈上切割下來(lái)的部分能夠平坦放置，即使是在殘余應(yīng)力已經(jīng)自行消退的情況下。相反的，在相對(duì)涼爽的溫度下卷曲鋼板能使鋼種的物理性能保持一個(gè)較高的內(nèi)應(yīng)力水平，限制了由自身和相互晶體之間形成的晶體和碳化物的尺寸；這些因素有助于使鋼板在熱軋的精軋階段獲得較高的應(yīng)力。
400℉的冷鋼在以2700的速度經(jīng)過(guò)該系統(tǒng)的時(shí)候需要大量的水，所以總共有152個(gè)由自動(dòng)化系統(tǒng)控制的噴霧頭以層流的形式來(lái)噴灑在鋼板的頂部和底部。
計(jì)算機(jī)以板帶行進(jìn)的素的和所要達(dá)到的目標(biāo)軋制溫度為基礎(chǔ)來(lái)估計(jì)需要多少水來(lái)冷卻鋼板，并且估計(jì)的準(zhǔn)確性是由在后卷曲前面的高溫計(jì)來(lái)確定的。為了調(diào)整所需要的噴霧器的數(shù)量，計(jì)算機(jī)可以控制噴霧器的開和閉來(lái)調(diào)整線程的長(zhǎng)度來(lái)滿足目標(biāo)溫度。由于要加快完成軋制過(guò)程，一旦后卷曲使鋼板繼續(xù)維持最終溫度，當(dāng)鋼板被軋制時(shí)為了彌補(bǔ)它在運(yùn)行出平臺(tái)所減少的時(shí)間越來(lái)越多的噴霧器被激活。
整個(gè)連軋機(jī)泵每分鐘輸送高達(dá)75000加侖的水來(lái)冷卻冷軋帶鋼、加熱爐滑軌、卷取單元和除鱗裝置。所有的水是在除鱗系統(tǒng)和污泥收集系統(tǒng)中循環(huán)的，通過(guò)層流冷卻系統(tǒng)，然后回到兩個(gè)專門的冷卻塔中的其中之一。
卷取
CSI的熱連軋機(jī)目前的配置依賴于兩個(gè)卷取機(jī)。兩個(gè)卷取機(jī)之間存在細(xì)小的差異，但是兩個(gè)可操作卷取機(jī)都是以一對(duì)能抓住鋼板頭部和尾部并在行進(jìn)和返回軋機(jī)的過(guò)程中施加張力的夾送輥開始。它的頭部在出入口到與卷取機(jī)相連的30”心軸處有一個(gè)傾斜，并引導(dǎo)與輸送帶相連的驅(qū)動(dòng)輥芯軸周圍的氣動(dòng)。
一旦鋼板的頭部一直纏繞在心軸上，板卷開始在心軸上建立并迫使遠(yuǎn)離包裝卷。一旦鋼板的頭部被’抓住’或者是摩擦力和防止打滑的張力組織鋼卷相對(duì)于心軸滑動(dòng)，驅(qū)動(dòng)軸會(huì)使日益增長(zhǎng)的鋼卷松開。在鋼板的尾部完成最終軋制后，夾送輥繼續(xù)保持后張力以防止鋼卷松開；在鋼板的尾部被拉出夾送輥前，驅(qū)動(dòng)輥一直在運(yùn)作。具有液壓線圈的小車運(yùn)動(dòng)到板卷的下方，然后上升到支持線圈的體積，將板卷從心軸上拿走并把它放置在能運(yùn)輸?shù)綐?biāo)記和自動(dòng)打捆機(jī)程序的位置。
板卷處理
板圈被具有液壓線圈的小車從卷取機(jī)上取下放置在’洞’內(nèi)的平臺(tái)上，在這兩個(gè)能來(lái)回運(yùn)動(dòng)的’步進(jìn)梁’中的其中之一會(huì)將板卷放置在能被鑒定和捆扎的位置。因?yàn)楫a(chǎn)品仍然很熱以至于適用于使用能在整個(gè)工廠辨別板卷的紙質(zhì)標(biāo)簽，一對(duì)’鍍錫薄鐵皮’采用激光刻錄鑒別信息在不銹鋼上，被親切的稱為’牌照’。在板帶被應(yīng)用前這些都是準(zhǔn)確焊接在鋼卷外部的。鋼板以前是以供應(yīng)商的連鑄機(jī)的壓力和剪切系數(shù)來(lái)區(qū)分，現(xiàn)在被一具有六位字母數(shù)字代碼的特殊輥?zhàn)訔l碼來(lái)區(qū)分。
第二輛具有液壓線圈的小車將板卷從打捆機(jī)中取出送往軋機(jī)外的旋轉(zhuǎn)體中，這個(gè)旋轉(zhuǎn)體能夠緩慢旋轉(zhuǎn)板卷90°，另一輛小車能夠?qū)寰硭屯詣?dòng)升降機(jī)。兩個(gè)以上（高速）的線圈車和另一輛升降機(jī)能夠攜帶板卷完成它的行程，要么送往能把它送往熱軋最終工序的輸送帶，要么送往冷卻池?？傊?，自動(dòng)化系統(tǒng)采用10個(gè)獨(dú)特的電動(dòng)液壓裝置，每一個(gè)依賴于多個(gè)傳感器來(lái)運(yùn)輸90﹪的熱軋帶鋼產(chǎn)品到下一場(chǎng)地，在那里下一個(gè)操作將被執(zhí)行。
附件2
The Hot Rolling Process
The primary function of the Hot Strip Mill is to reheat semi-finished steel slabs of steel nearly to their melting point, then roll them thinner and longer through 12 successive rolling mill stands driven by motors totaling 77,000 hp, and finally coiling up the lengthened steel sheet for transport to the next process.
The Hot Mill rolls slabs weighing up to 30 tons between 30” and 74”. Steel slab 8 to 9 inches thick and up to 36。feet long is rolled into strip as thin as 1/16 inches and up to a half-mile in length. Coils are produced with a 30” inside diameter ('eye') on one of two coilers, with outside diameter limitations of 72” and 74”, corresponding to850 and 1000 pounds-per-inch-width (PIW), respectively. The mill supplies coil for each of CSI's remaining operations, as well as a finished product for shipment directly to CSI's customers.
Most material is transported out of the mill area by an automated coil handling system, though some skelp for the Pipe Mill is staged toward the east end of the mill bay until it is cool enough to load onto rail cars.
Reheating
Critical to the Hot Strip Mill is its walking-beam reheat furnace, state-of-the art equipment that replaced and now outperforms three older- (pusher-) style furnaces. Nominally rated to produce 270 tons-per-hour, improvements in efficiency and some sacrifice in slab temperature uniformity enable extended production。rates 25% above design. Heating this much steel from room temperature to 2200-2400 degrees Fahrenheit consumes around 10 million cubic feet of natural gas each day.
As slabs are assigned to orders, schedules are written and material is staged with rail-cars and overhead cranes in the slab yard at the west end of the Hot Strip Mill. Slabs are placed, one at a time, on a roll line. The slab's dimensions and weight are confirmed as it is positioned in front of the charge door on the south side of the furnace. When space is available in the furnace, large electro-mechanical ' pusher armsengage to move the slabs into the furnace.
Once inside, the slabs are supported about eight feet off of the furnace floor by water-cooled, refractory-coated pipes called 'skids'. To minimize the cold spots ('skid marks') left in the slab, the skid spacing changes approximately two-thirds of the way through the furnace Two independent sets of skids, one fixed, one walking, take turns supporting the slab as it is walked through the furnace by a massive sub-frame energized by a pair of large hydraulic cylinders.
The interior of the furnace is 38'9” wide, fifteen feet from floor to ceiling, and 142' long. 8 It is divided into eight zones for temperature control: preheat, top-and-bottom; heating, top-and-bottom; and soak, top-and-bottom, east-and-west. The preheat and heating zones combust a mixture of natural gas and preheated combustion air with massive burners on the side walls of the furnace, both above and below the skids, to heat the slab nearly to its discharge temperature.
Much of the preheating of the steel is achieved by the hot exhaust gases rushing past the slabs on the way to the recuperators' above the charge door. Whatever heat is left in the exhaust gases preheats the incoming combustion air to over 1000° F in these massive heat-exchangers. Conversely, in the heating zone the steel is primarily heated by the glowing-hot furnace walls. In the soak zone, numerous smaller burners seek to maintain a uniform temperature within the zones to equilibrate any cold spots in the slabs. Refractory dividers help to physically distinguish the zones, and thermocouple temperature sensors throughout the furnace interact with the automatic burner control systems to maintain the target temperatures in each zone.
Complex computer models calculate the targeted roughing mill exit temperature to obtain a furnace discharge ('drop-out') aim temperature. Estimating the temperature profile through the thickness of each slab in the furnace on an ongoing basis, the computer aids the operator in selecting the production rate and zone set-points that will maximize production of steel slabs uniformly heated to as close to the target temperature as possible. After the rolling process begins, as the steel exits the roughing mill, its temperature is fed back to the furnace,
updating the computer models and informing the Heater as to the temperature uniformity.
When the slab reaches the 'discharge door' at the exit end of the furnace, and the computer has determined that the slab has been sufficiently heated, the door opens and massive 'extractor arms' reach beneath the slab, lift it off of the skid supports, and draw it out of the furnace. The east and west extractors can act independently of one another to remove double-charged slabs one-at-a-time, or in conjunction to extract longer slabs. The intensely hot slab is placed on a roller table which carries it into the roughing mill.
Descaling
After exiting the reheat furnace, the slab passes through a descaling unit, an enclosure employing two pairs of spray headers that blast the intensely hot slab with 1,500 psi pressurized water to remove the 1/8-inch thick layer of oxidized iron that forms at the surface of the slab in the oxygen-rich atmosphere of the reheat furnace. Shortly after descaling, a (relatively) small 2-hi rolling mill called a scalebreaker reduces the slab's thickness by about one inch to break up any scale that remains. Just before the next reduction pass is taken, 'sweep sprays' clean away any loosened scale that remains on the slab surfaces. The transfer bar will be descaled twice more during roughing, immediately prior to the third and to the last rolling operation, to remove the scale that has grown back over the three minutes or so that it spends in the roughing mill.
Roughing
The roughing mill is made up of six independent rolling mill stands, the last four of which incorporate small vertical rolling mills called edgers. Slabs heated in the furnace until they glow bright orange-yellow are rolled through one stand at a time to produce so-called transfer bars suitable for finish rolling. High-pressure water-jet nozzles clean the oxidized iron, or scale, from the surface along the way.
As the transfer bar exits the last roughing mill stand, the thickness of the leading edge of the bar is estimated. Similarly, a pyrometer measures the temperature profile of the bar from head to tail and a special camera photographs both ends. Depending on the gauge, width, and grade of the product to be rolled, the average temperature of the bar as it exits the last roughing mill normally ranges from 1900° to 2100° Fahrenheit. This data is collected in anticipation of finish rolling. Computers immediately begin calculating the speeds and gaps for threading the six finishing mills, which will roll the steel in tandem with one another.
The workhorse roughing mill has 135” wide rolls for rolling 'broadside' (as the first roughing mill is commonly called) to make a slab wider. A 5,000 hp motor drives 42”-diameter work-rolls through 28:1 gears to reduce the slab's thickness by as much as 2-?”.
The last four roughing mills each incorporate edgers for width control and roll the bar from five to six inches thick incrementally down to around an inch and a quarter, depending on the customer's ordered width, gauge, and steel grade. As mentioned previously, the third and fifth roughing mills each have high-pressure descaling headers operating at 1,500 psi. The individual roughing mills are spaced increasingly further apart to accommodate the lengthening of the transfer bars as they are rolled thinner and thinner.
Cropping
Because a square head-end is critical to properly threading the finish mills and the downcoilers, and because an uneven tail can bruise work-roll surfaces or cause threading problems for the next production process, the head- and tail-ends of nearly every transfer bar are cropped by a pair of large steel drums each with a shear- blade extending along its length. With the bar crawling along the roller table at around 100 fpm, sensors detect its position and speed in order to time the crop shear drums to optimize the amount cropped; since transfer bars are over an inch thick, each extra inch of crop-length scraps another 15-30 lbs.
Finishing
CSI's Hot Strip Mill includes six finishing mills, which reduce the thickness of the transfer bar down to the gauge required by the customer or the next process. The rolling speed is set to allow the last stand to perform the final reduction at the finishing temperature, between 1500° to 1650°F, specified to reach certain mechanical properties.
By now, the steel has been rolled into a flat bar as long as 200 feet. In contrast to the roughing mills, the finishing mills roll the transfer bar in tandem, meaning each bar will be rolled through all six stands at once. The hot steel is quite fragile as it is rolled and tension between the finishing mill stands must be closely controlled at very low levels in order to avoid stretching or tearing the strip.
Prior to the finish rolling operation, the head- and tail-ends of the transfer bar will be sheared to square them up, helping to ensure proper threading and tail-out. A final two-stage descaling operation is performed to clean off the scale that has grown on the bar during roughing. Once the bar is threaded between each successive pair of mills, a free-turning roll on an electro-mechanical pivot called a looper roll engages the bottom of the strip to monitor the tension between the stands.
Adjustments are made as necessary to ensure the strip threads properly through each of the mills without looping up and folding over or stretching and tearing apart. The position of each roll is fed back to the finishing mill's sophisticated automation system which, along with information from the load cells that monitor rolling force and from the X-ray gauge measuring final strip thickness, work to smoothly adjust the roll gaps and speeds to maintain stable rolling of strip to the necessary thickness in spite of the temperature variations present in every bar.
Laminar Cooling
Metallurgically critical to the properties of hot-rolled steel is the coiling temperature, as the coil will cool from this temperature to ambient over the course of three days. Essentially a heat treatment comparable to annealing, the stresses imparted to the steel during reduction from nine inches thick down to ordered gauge are given the opportunity as the coil cools to relieve themselves. Though the steel is continually recrystallizing during hot- rolling, reductions in thickness sometimes in excess of 99% and taking place in less than ten minutes stress the steel considerably; coiling temperature is specified by product metallurgists to harness and manipulate those stress levels in search of optimal mechanical properties.
Product sold as hot rolled and hot rolled pickled and oiled to be laser cut by a customer is coiled at relatively high temperatures to try to relax the steel as much as possible so that parts cut from the coil will lie flat even after residual stresses have resolved themselves around the part's configuration。Conversely, coiling at a relatively cool temperature allows physical quality steel grades to retain higher internal stress levels and limits the size of the individual crystals and of the carbides that form within and between the crystals; each of these factors contributes to higher strength levels in the finished hot-rolled strip.
Cooling steel 400°F as it rushes past at speeds up to 2700 fpm requires，tremendous amounts of water, so a total of 152 spray headers, individually valved and controlled by the automation system, drench the steel from the top and bottom with curtains of water. The computer estimates, based on the thread speed of the strip and target finishing temperature, how much water will be needed to cool the head-end, and the accuracy of this estimate is confirmed by a pyrometer in front of the downcoilers. As adjustment to the number of sprays in use is needed, the computer will turn sprays on and off to meet the targeted temperature through the length of the coil。Since the finishing mills will accelerate once the downcoiler is threaded to continue to make finishing temperature, increasingly more sprays are activated as the steel is rolled in order to compensate for the reduced time it spends on the run-out table.
Up to 75,000 gallons of water are pumped each minute throughout the Hot Strip Mill to cool finish-rolled strip, furnace skids, mill rolls, and coiler components, and to descale transfer bars. All water is recycled through a system of scale/sludge collection pits, through the laminar cooling system, and back to one of the two dedicated cooling towers.
Coiling
CSI's Hot Strip Mill's present configuration relies on two Coilers. Minor differences exist between the two, but both operable coilers begin with a pair of pinch rolls that catch the strip head-end and establish tension across the run-out table and back to the finishing mills. The head-end is deflected by a gate down to the 30” mandrel associated with the coiler and is guided around the mandrel by pneumatically-actuated wrapper rolls linked by aprons.
Once the head-end is all the way around the mandrel, laps begin to build around the mandrel, forcing away the wrapper rolls Once the head-end is'cinched' 'cinched' and friction and tension prevent the wraps of steel from slipping relative to the mandrel, the wrapper rolls disengage from the growing coil After the strip tails out of the finishing mill, the pinch rolls continue to hold back-tension to prevent the coil from unraveling; before the strip tail is pulled through the pinch rolls, the wrapper rolls are reengaged. A hydraulic coil car moves into place beneath the coil, and, after rising up to support the coil's bulk, strips the coil from the mandrel and places it in position for transport to the tagging and automatic bander procedures.
Coil Handling
Coils are removed from each coiler by hydraulic 'coil cars' that set the product down on the platform in the 'hole' where one of the two 'walking beams' cycle back and forth to move coils into position to receive identification and banding. Since the product is still too hot to apply the paper tickets that identify coils throughout the rest of the plant, a pair of 'taggers' employ lasers to burn identifying information onto stainless steel tags affectionately referred to as 'license plates'. These are spot welded to the outside wrap of steel before the band is applied. Slabs that previously were identified by Heat and Cut numbers from the supplier's caster are re-identified as a specific roll item with a six-digit alpha-numeric code.
A second coil car takes the coil from the bander to a rotator outside of the mill building which slowly spins the coil 90° so yet another coil car can take the coil east to '。the 'lift-and-carry'. Two more (high-speed) coil cars and another lift-and-carry complete the coil's journey either to the conveyor that will take it into Hot Strip Finishing or to the Cooling Pond. In all, the automated system employs ten distinct electro-hydraulic devices each depending on multiple sensors to transport 90% of the Hot Strip Mill's production to the building where the next operation will be performed.
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