ZL50裝載機總體及工作裝置設計
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連鑄
連鑄的發(fā)展
二戰(zhàn)之后，連鑄發(fā)展非常迅速＿今天鋼鐵生產(chǎn)者普遍相信連鑄至少和模鑄一樣在經(jīng)濟上是合理的，并且能與大部分高質(zhì)量鋼的生產(chǎn)系列相匹配。這項技術(shù)不斷開發(fā)的目的在于改善鋼的性能，這促使生產(chǎn)特殊高級鋼時企業(yè)對其生產(chǎn)工藝過程不斷進行調(diào)整。使用連鑄系統(tǒng)的理由有：
(l）和初軋機組（小型車間）相比，降低投資費用；
(2）和傳統(tǒng)的鑄錠相比，提高10%的生產(chǎn)能力；
(3）在整個鑄坯長度上鋼的成分較均勻；中心質(zhì)量比較好，尤其是板坯；高的內(nèi)表面質(zhì)量，比其他需要昂貴的清理表而的工序節(jié)??；
(4)高度的自動化；
(5)益于保護環(huán)境；
(6)較好的工作條件設備類型
首臺連鑄機是立式連鑄機，可是，由于橫斷面的增大，注流長度的增加，而且主要是隨著澆注速度的增加，這種設備迫使廠房建筑高度增加。這些因素也導致了具有冶金影響的液相長度的大大增加。連鑄坯的液相長度由下式?jīng)Q定：
L=D2/4x2Vc
這里，D=鑄壞厚度(mm）
x=凝固特征系數(shù)(mm/min1/2）
對于全部的冷卻長度這些值達到26-33。
Vc=拉坯速度( m／分）
為了減少廠房高度，首先研制出將鋼水倒人立式結(jié)晶器中，并且在彎曲之前讓鋼水完全凝固的連鑄系統(tǒng)，或彎曲時鑄坯仍處在液相，這種系統(tǒng)隨后發(fā)展為弧形結(jié)晶器，這是目前最常用的方法。立式連鑄機和那些鑄坯在完全凝固時被彎曲的連鑄機都有一個長直的液相，這大大增加了成本。
然而從維修的角度看，這些系統(tǒng)有冶金學優(yōu)點。鑄坯內(nèi)部仍為液相就進行彎曲的連鑄機比完全凝固后再彎曲的立式連鑄機更好，它不需要修建與立式連鑄機一樣高的廠房。然而，液相彎曲系統(tǒng)要求更高的初期投資和更大的維護費用?；⌒芜B鑄機是考慮了投資費用和維護費用的折衷產(chǎn)物，而且可以在冶金上實現(xiàn)。
連鑄適合于生產(chǎn)任何橫斷而的產(chǎn)品：正方形的、長方形的、多邊形的、圓形的、橢圓形斷面都可以。也有些基本斷面的例子，如管坯、板坯、大型坯、方坯。斷面寬厚比大于1.6的鑄壞通常稱為板坯。方坯鑄機生產(chǎn)正方形或近于方形、圓形或多邊形斷面，斷面尺寸達到160mm的產(chǎn)品。較大斷面或那些寬厚比小于1.6的產(chǎn)品用大型坯鑄機生產(chǎn)。現(xiàn)在80 x 80到300 x 300mm的方坯以及50-350mm厚300-2,500mm寬的板坯都用這種方式生產(chǎn)。
連鑄比已經(jīng)迅速增長，尤其在最近幾年。這本質(zhì)上是鑄坯寬度和拉坯速度的增加。每個注流每分鐘生產(chǎn)出的產(chǎn)品已超出了如下數(shù)據(jù)：
板坯 5噸
大型坯 1噸
方坯 350kg
最后，我們應該提到水平連鑄機，它已經(jīng)應用于有色金屬和鑄鐵的生產(chǎn)，而且可以進一步開發(fā)用于鋼的生產(chǎn)。R. Thielmann和R. Steffen針對用水平連鑄機生產(chǎn)非合金鋼和合金鋼方坯的發(fā)展狀況提出了一份綜合報告。水平連鑄機比傳統(tǒng)連鑄機有如下三個顯著優(yōu)勢：
(1)低的建筑高度和建設費用：
(2)防止鋼水二次氧化的方法簡單；
(3)由于鋼水靜壓力非常低，沒有鑄壞變形。
澆鑄技術(shù)
鋼水由澆注大包注入中間包后流人敞口的水冷銅結(jié)晶器。首先結(jié)晶器底部用一個引錠桿塞住，然后由它將熱的鑄坯從結(jié)晶器拉出進人連續(xù)拉輥。鑄坯從結(jié)晶器開始凝固,然后經(jīng)過冷卻系統(tǒng)，到達拉環(huán)輥，在拉輥中繼續(xù)傳送。引錠桿在進入切割裝置之前或之后與鑄環(huán)分離。切割裝置可能是火焰切割機或熱切割機，其行進速度與熱鑄坯相同，它將熱鑄坯切割成所需長度。
使用中間包的目的是將確定的鋼水量分流到一個或多個結(jié)晶器。這可以通過使用塞棒、滑動水口或其他力法控制的水口來實現(xiàn)。中間包的初始狀態(tài)根據(jù)其耐火襯材料的不同可以是冷的、溫的或熱的。對于要求嚴格的鋼使用浸入盒保護鋼液注流以防止其氧化。結(jié)晶器不僅形成鑄坯斷面而且吸收一定的熱量，使鑄坯到達結(jié)晶器出口時坯殼有足夠的運送強度。依據(jù)所鑄鑄坯的尺寸和形狀，結(jié)晶器可以用銅管或硬質(zhì)銅合金制成。按慣例，管狀結(jié)晶器用于較小斷面。結(jié)晶器的內(nèi)表面可以使用鉻或鉬鍍層以減少磨損并且適合于澆鑄過程中從合金傳熱。結(jié)晶器的錐度是為了與鋼的收縮、拉速和鋼種匹配?，F(xiàn)在使用的結(jié)晶器長度約為400 - 1,200mm,但通常在700 - 800mm之間。結(jié)晶器壁粘鋼問題通常通過按正弦規(guī)律振動結(jié)晶器和加入潤滑劑（油或連鑄保護渣）以消除結(jié)晶器與鋼水之間的摩擦加以解決。潤滑劑，尤其是連鑄保護渣，有一個附加的冶金功能。潤滑劑的選擇取決于所要求的質(zhì)量和連鑄條件；尤為重要的是所選擇的連鑄保護渣必須嚴格與質(zhì)量工藝匹配。
結(jié)晶器內(nèi)鋼液面可以人工控制或進行自動控制，兩者中任何一種都可用于保持液面穩(wěn)定或滿足輸人的鋼水量，如與拉坯速度的變化相適應。人工控制通過調(diào)整中間包塞棒或流出速率實現(xiàn)。自動控制系統(tǒng)則可以通過放射測位儀或紅外線放射儀或用安裝在結(jié)晶器壁的測溫探針測溫來確定鋼液面，并且通過操作水口塞棒機構(gòu)（對于穩(wěn)定流速）或控制拉輥速度（變化拉速）來補償任何鋼液面的變化。
連鑄中使用的引錠桿類型取決于連鑄機的類型。立式連鑄機可以使用剛性引錠桿，而組合式的或靈活式的引錠桿必須用于弧形連鑄機。引錠桿與鑄坯可以采用不同方式連接，一種是用連接部件（平板、螺釘、碎條鋼）將鋼液與引錠桿焊接在一起；另一種是在引錠桿頭部鑄造一個特殊連接頭，它能使引錠桿像打開扣環(huán)那樣進行脫錠。
鑄坯離開結(jié)晶器時的坯殼厚度首先取決于鋼液與結(jié)品器的接觸長度，它也依賴于結(jié)晶器的具體導熱系數(shù)和鋼水進人結(jié)晶器時的過熱度。它可以由下面的拋物線公式進行精確計算：
C=xt
式中：C-坯殼厚度(mm)
x—凝固特性（mm/min1/2)
t-凝固時間(min)
鑄坯在結(jié)晶器內(nèi)或附近的凝固特性是20到26，它取決于操作條件；二冷區(qū)是29到33.離開結(jié)晶器時鑄坯坯殼厚度約為鑄坯厚度的8-10%，它取決于拉坯速度。結(jié)晶器下面的二冷區(qū)加速了鑄坯的凝固過程。通常使用水進行冷卻，但有時也用水和空氣的混合物或壓縮空氣。為了適應冷卻劑的流速，二冷區(qū)被分成很多部分。通過噴嘴將需要的水量噴到整個鑄壞上。與鑄坯斷面和拉速有關(guān)的鋼水靜壓力可能會太高，以至于鑄壞不得不被支撐以防止鼓肚。在生產(chǎn)大型坯尤其是板坯的工廠，這種裝置是很昂貴的。
工藝控制
由于生產(chǎn)率和質(zhì)量的原因，在現(xiàn)代鋼鐵生產(chǎn)中，有一種轉(zhuǎn)移費時操作的趨勢，例如，將溫度調(diào)整、脫氧和合金從熔化爐轉(zhuǎn)到鋼包處理站進行。這些操作在連鑄過程中尤為重要，因為在這個過程中要嚴格控制溫度和成分。
連鑄過程中進入結(jié)晶器的鋼水溫度控制要比常規(guī)鑄造中的溫度控制更精確。太高的過熱度能導致拉漏或一種柱狀結(jié)構(gòu)，帶來較差的內(nèi)部質(zhì)量。另一方面太低的溫度會導致水口堵塞造成澆鑄困難和產(chǎn)生不潔凈鋼。板坯連鑄中間包溫度通常在液相線以上5到20度，而方坯或大型坯則為5到50℃。這種不同取決于鋼的等級，例如，小熔化爐中不銹鋼板坯連鑄過熱度為45℃。
在整個澆鑄過程中，為使鋼水溫度保持在上面所說的范圍之內(nèi)，在鋼包中溫度的均勻性是最重要的。在澆鑄以前為了保持鋼包內(nèi)鋼液溫度的均勻，需要攪拌，有時也進行清洗氮氣或氬氣可以帶走熱量，它們由鋼包底部的多孔塞噴入或在獨立的清洗站通過一個中空的塞棒噴入。
在真空或清洗處理期間可以進行化學成份控制。在鋼液均勻后，進行取樣分析或用電動勢法測量氧活度，在此基礎上可以計算切屑的加入量以保證脫氧。加入切屑脫氧劑的最好方式是在攪拌熔池的同時保證高的速率（用惰性氣體進行噴粉、喂線或噴丸）。通過小心除去鋼包中的爐渣來減少對合金的需要。真空處理是實現(xiàn)良好鋼包冶金的一種靈活、有效的手段，而低壓處理是在澆鑄前將氫或碳脫到很低的惟一方法。
結(jié)晶器液面控制
一臺連鑄機最重要的控制部分是保證鑄坯拉出和部分冷卻鋼坯的生產(chǎn)能保持結(jié)晶器內(nèi)鋼液面的穩(wěn)定（在幾個厘米范圍內(nèi)波動）。這可以通過兩種方式來完成：
(1)稱量中間包，從鋼包到中間包的鋼液流動速度自動變化以保持整個中間包重量不變。通過這個方式，從中間包流出的鋼液速度是不變的。
(2)要控制部分冷卻鋼坯的拉出速度以保持在結(jié)晶器中鋼的液面大致不變。
在連鑄初期，通過操作者觀察液面，并相應地調(diào)整中間包的塞棒來保持鑄機中鋼的上液面不變?，F(xiàn)在，通常用測量設備測量并自動地調(diào)整液面。下表中列出了幾種測量液面的方法。在此對r射線（放射線）和紅外法這兩種方法作詳細描述.
類型
制造者
備注
r射線
Distingtan Engineering(UK)
應用廣泛，可靠
渦流
NKK(日本）
熱敏電阻
United States Steel
僅用在USS機器
紅外
Sert. Danielli
廣泛用在歐洲大陸
電磁線圈
Concast
由這個表可以看出各操作方法的特點。為避免較強放射性同位素的應用，開發(fā)了紅外線設備。這種探測器檢測金屬液面與結(jié)晶器后壁的連接處。當金屬液面上升到觀測范圍內(nèi)時，單個光電元件會收到更多的反射信號，此時輸出增加。檢測中斷時可以采取特殊的措施來補償。光電元件單元收到紅外反射，輸出一個電信號給控制單元，這個控制單元接著與操作者和連鑄機相聯(lián)系。操作者能夠選擇自動或手動控制，并且接收某個信號燈發(fā)出的操作指示。從液面返回的信號穿過夾縫罩，被圓柱面透鏡聚焦到光電檢測器。通過過濾除去波長1mm以下的反射光，以減少環(huán)境光和油焰的干擾。
整個系統(tǒng)在冒口有兩個探測儀，兩個固定的光束檢查鋼液流股的每一邊。通過改變夾縫間隔，可以調(diào)整光電元件檢測到的兩個區(qū)域的間距。
在每一通道都裝有三個光電檢測器：第一個用上面描述的光束測定金屬液面；第二個不收到光束而能進行溫度漂移補償；第三個通過夾縫觀察正常鋼液面上方位于主光束與金屬流之間的一個小區(qū)域。它的作用是當金屬流偏離中心位置，可能會干涉主光束時對其進行探測。兩個主光束與液流界面探測間的平衡能夠通過安裝在單元后部的小電位計來調(diào)整。
溫度補償以后，每一通道探測到的液面信號被輸進一個選擇最大信號的簡單電路。因此這個單元總是控制兩個液面信號中較高一個。如果液流傳感光電元件發(fā)現(xiàn)液流向檢測束移動時，它切斷信號，并且元件轉(zhuǎn)向進行控制另一個通道面。還有另一個特征，如果兩個通道都被切斷，例如被一個扇形金屬流切斷，單元轉(zhuǎn)向記憶單元，相當于快速檢查金屬液面，防止突然控制失靈。在記憶單元釋放時，液面逐漸下降，使操作者有足夠的時間來調(diào)整。
如果拉坯速度有一個大的跳躍，通過阻止自動運行，單元將給出一個從人工控制到自動控制的平滑轉(zhuǎn)換。在從自功到手動的改變時，不提供無振動轉(zhuǎn)換。在電纜有故障時，也有一個防止變到自動狀態(tài)的保護。
控制系統(tǒng)收到被選擇過的液面水平信號，伴隨比例和積分作用，直接給拉坯驅(qū)動單元輸出一個電壓信號。驅(qū)動單元產(chǎn)生與電壓信號相對應的拉坯速度。
連鑄的益處操作步驟
在連鑄發(fā)展之前，只有鋼錠為熱加工成型的鋼鐵產(chǎn)品提供了初始原料。從煉鋼爐到軋機的典型操作步驟是：
（1）將鋼水澆入鋼錠模；
（2）鋼包運到澆鑄平臺，將鋼水注人鋼錠模；
（3）將鑄后錠模運到脫錠區(qū)脫錠；
（4）運送鋼錠到均熱爐，加熱到軋制溫度；
（5）從均熱爐取出加熱的鋼錠，運送到初軋機軋制成半成品形狀；
（6）運送半成品鋼到軋機。
用連鑄，只需如下更短的步驟：
(1)從煉鋼爐出鋼到鋼包；
(2)鋼包運到澆注平臺，連續(xù)把鋼水澆成半成品形狀。
(3)運送半成品鋼到軋機
從較短的操作步驟獲得的利益是人們采用連鑄的主要原因；連鑄增加了產(chǎn)量；提高了產(chǎn)品質(zhì)量；節(jié)約能源；減少污染和降低了成本。
產(chǎn)量 從鋼包中鋼水到軋成半成品形狀的產(chǎn)量提高在于三個方面金屬廢料的減少：初軋機；澆注；鋼錠加熱。對產(chǎn)量增加貢獻最大的是無需初軋時鋼錠切頭、切尾。與澆鑄操作有關(guān)的產(chǎn)量損失的減少，包括短錠，鑄錠殘頭和一般的廢鋼的減少。由鋼錠在均熱爐中加熱引起的氧化皮燒損也被避免了。
質(zhì)量 冶金質(zhì)量的提高包括在化學成分和凝固特征上變化小。除了在鑄坯橫切面上，改善碳、硫和合金元素偏析特性以外，沿著鑄坯長度方向也沒有什么變化（當將一爐鋼水進行模鑄時，每一支鋼錠都有垂直偏析和組織變化，而連鑄坯不僅是一塊鋼錠而且垂直方向上沒有什么變化）。在現(xiàn)代連鑄過程中，鑄坯表面的質(zhì)量要高于軋制半成品質(zhì)量，軋制半成品的表面有例如結(jié)疤和疤痕等表面缺陷，因此，對鑄錠的精整和產(chǎn)量的損失均降到最低程度。大多數(shù)連鑄鋼坯均無需經(jīng)過任何修整就可進一步加工。因此能得到有較少的內(nèi)部和表面缺陷、性能得到改善、更均勻的最終產(chǎn)品。
能量 連鑄能夠節(jié)約能量，因為連鑄過程減少了在模鑄過程中的能量消耗。這些包括在均熱爐中的燃料消耗和初軋機的電能消耗。能量也可以通過產(chǎn)量增加來間接節(jié)省，因為它能減少用于生產(chǎn)大量半成品的原料鋼的消耗。除此之外，人們正在關(guān)注將熱的連鑄坯直接熱送到精軋機加熱爐的實踐，因此連鑄壞的顯熱被節(jié)約了。
污染 連鑄過程通過省略模鑄工藝設備如均熱爐減少了污染。
成本 連鑄的資金和運行成本與模鑄工藝相比均減少了。資金節(jié)約歸功于省掉了模鑄工藝所需要的設備。運行成本節(jié)約主要是較少的勞動力投人和較高的產(chǎn)量。
煉鋼
連鑄的煉鋼操作與用電爐或堿性氧氣轉(zhuǎn)爐生產(chǎn)鋼錠的煉鋼操作相似，僅有某些不同，主要有兩個：
(1)溫度控制；
(2)脫氧實踐。
溫度控制 溫度控制比模鑄生產(chǎn)更關(guān)鍵。出鋼溫度通常更高，以補償因運送到鑄機的時間增加引起的熱量損失，出鋼溫度要維持在一個較小的范圍內(nèi)，以避免溫度太高時拉漏和溫度太低時中間包水口過早凝固。澆鑄溫度也能影響鑄坯的晶體結(jié)構(gòu)。在整個澆注過程中采用均一且低的過熱度可獲得鑄坯最佳晶體結(jié)構(gòu)。為了達到此目的，必須進行使鋼液溫度均勻的操作。廣泛使用的一種方法是利用鋼包底部的多孔塞吹入少量氬氣或?qū)姌尣迦虽摪好嫦麓禋鍞嚢桎撘骸?
脫氧 連鑄鋼必須完全脫氧（鎮(zhèn)靜）以防止在鑄坯表面或接近表面的皮下形成氣泡或氣孔，氣泡和氣孔會導致隨后軋制過程中產(chǎn)生裂紋。根據(jù)鋼的等級和用途，采用如下兩種方法脫氧：
(1)對于粗晶粒鋼加人少量鋁，用硅進行脫氧；
(2)對于細晶粒鋼進行鋁脫氧。硅鎮(zhèn)靜鋼比鋁鎮(zhèn)靜鋼更容易澆鑄，因為避免了氧化鋁沉淀帶來的中間包水口堵塞問題。為了生產(chǎn)高質(zhì)量的產(chǎn)品，在連鑄之前，進行鋼包精煉正成為一種很普遍的操作。
Continuous Casting
出處From the Making, Shaping and Treating of Steel by William，McGraw—Hill Companies, Inc., 2002
The Development of Continuous Casting
Continuous casting was developed very rapidly after the Second World War. Steel-producers arc today generally convinced that continuous casting is at least as economical as ingot production and can match the quality of the latter across much of the production spectrum for high-quality steels. Continual development of the technique aimed at improved steel characteristics is leading to increasing adoption of the process in works producing special high-grade steels. The reasons for continuous-casting systems are:
(1) lower investment outlay compared with that for a blooming train (mini-steelworks);
(2) about 10% more productivity than with conventional ingot-casting;
(3) high degree of consistency of steel composition along the whole length of the strand; better core quality, especially with flat strands; high inherent surface quality, leading to savings on an otherwise expensive surfacing process;
(4) high degree of automation;
(5) friendlier to the environment;
(6) better working conditions.
Types of Installation
The first continuous-casting plants were aligned vertically; however, with larger cross-sections, increasing strand-length, and, above all, with increasing pouring-rates this type of construction leads to unreasonable building-heights. These factors also lead to a considerable increase in the length of the liquid phase which has metallurgical effects. The length of the liquid phase in a continuously-cast strand is determined by the following formula:
L=D2/4x2Vc
Where D =strand thickness (mm)
x = solidification characteristic (mm / min1/2)
These values amount to 26~33 for the whole cooling length.
Vc = casting rate (m /min)
Efforts to reduce building-height first led to continuous-casting systems in which molten metal passed into a vertical mould and solidified completely before being bent or where the strand has been in the liquid phase and later to the bow-type installation which has a curved mould and is the system most used today. Vertical systems and those in which the strand is bent when completely solidified have long straight liquid phases and can lead to unacceptably high capital outlay.
However, these systems have metallurgical advantages from the point of view of maintenance. A vertical system in which the strand is bent while still in the liquid phase has the advantage that the building need not be as tall as when the strand is bent after solidification; however, the liquid-phase bending system requires higher initial outlay and greater maintenance costs. The bow-type system represents a compromise between the costs of capital outlay and of maintenance and what can be achieved metallurgic ally.
Continuous-casting is suitable for the production of almost any cross-section imaginable; square, rectangular, polygonal, round, and oval sections are all available. There are also some instances of preliminary sections for tubes and slabs, blooms, and billets. Sections with a breadth /thickness ratio greater than 1.6 are normally described as slabs. Billet-machines produce square or nearly-square, round, or polygonal cross-sections up to 160mm across. Larger sections and those with a breadth /thickness ratio less than 1.6 are cast in bloom-machines. Billets nowadays normally produced in this way range from 80 x80 to 300 x300 mm, and slabs are 50 - 350mm thick and 300 - 2500 mm wide.
Continuous-casting output-rates have risen sharply, especially in the last few years. This is essentially because of increase in the breadth of the strand and in casting rate. The following outputs have been exceeded per section per minute:
slabs 5 tones
blooms 1 tones
billets 350 kg
Finally, we should mention horizontal continuous-casting systems which are already used for non-ferrous metals and cast iron and which are being further developed for steel. R. Thieimann and R. Steffen have produced a comprehensive report about the state of development of horizontal continuous-casting systems for producing billets from unalloyed and alloy steels. Horizontal continuous-casting systems have three important advantages over conventional continuous-casting system:
(1) low height and cost of building;
(2) simple means of protecting the melt against reoxidatioin;
(3) no strand deformation because the ferrostatic pressure is much lower.
Casting Technique
Molten steel is poured from a casting ladle via a tundish into an open water-cooled copper mould. At first the bottom of the mould is closed off by a starting-bar, which then leads transport of the hot strand from the mould into the continuous withdrawing rolls. The strand, which starts to solidify in the mould, passes through a cooling system before it finally reaches the withdrawing rolls, whereupon the hot strand takes over transport. The starting-bar is separated from the hot strand before or after it reaches the parting device. The latter, which may either be a flame-cutter or hot shears, moves at the same rate as the hot strand and cuts it into the lengths required.
The purpose of the tundish is to feed a defined quantity of molten steel into one or more moulds. This can be done by using nozzles controlled by stoppers, slide-gates, or other means. The tundish may initially be cold, warm, or hot according to the nature of its refractory lining. Where difficult steels are processed the pouring stream is protected against oxidation between the submerged boxes. The mould not only forms the strand section but also extracts a defined quantity of heat, so that the strand shell is strong enough for transport by the time it reaches the mould-outlet. The mould may be made from copper tube or hard enable copper alloy, depending on the shape and size of the strand to be cast. As a rule, tubular moulds tire used for smaller sections. The interior surface of the mould may be coated with chronic or molybdenum to reduce wear and to suit heat-transfer from the alloy being cast. The mould is tapered to match steel-shrinkage and casting-rate and the type of steel concerned. Moulds used today range from 400 to 1200 mm in length overall, but their usual length is between 700 and 800 mm. The problem of steel adhering to the mould-sides is usually countered by oscillating the mould sinusoidally relative to the strand and by adding lubricant (oil or casting flux} in an attempt to cut friction between the mould and the steel. The lubricant, particularly casting-flux, has an additional metallurgical function. The choice of lubricant depends on the qualities required and the casting conditions; it is particularly important that casting-flux should be chosen to match the quality-programme precisely.
The level of steel in the mould may be controlled manually or by an automatic system. Either method may be used to keep the level constant or to match the incoming molten steel, i. e. to accommodate variations in casting rate. Manual control is affected via the stopper in the tundish or by varying the output rate. An automatic control system may meter radioactivity or infrared radiation or measure temperature via a probe in the mould wall to determine the steel-level and compensate any changes by actuating the stopper-mechanism (for constant pouring rate) or controlling the speed of the withdrawing rolls (varying casting rate).
The type of starting-bar used for continuous-casting depends on the type of installation. Rigid starting-bars can be used in vertical systems, while articulated dummy bars or flexible strip have to be used in bowed installations. The starting bar can be connected to the hot strand in different ways, one is by welding the fluid steel using a jointing element (flat slab, screw, or fragment of rail) which is soluble in the starting-bar; another is by casting the connector in a specially shaped head in the dummy bar in a way that enables it to be released by unlatching.
The thickness of the solidified strand shell on leaving the mould depends first of all on how long the steel is in contact with the mould, but it also depends on the specific thermal conductivity of the mould and on the amount of superheat that steel has when it enters the mould. It can be determined with fair accuracy using the following parabolic formula:
C=x. T
where C is the thickness of the strand shell (mm)
x is the solidification characteristic (mm/min1/2)
t is the solidification time (min)
The solidification characteristic in and near the mould lies between 20 and 26, depending on the operating conditions; for the secondary cooling-area the figure is 29 -33. The thickness of the solidified strand shell on leaving the mould is about 8 10% of the strand-thickness, depending on casting rate. A secondary cooling-area under the mould speeds up completion of the solidification process. The coolant usually is water but a water / air mixture or compressed air is also sometimes used. The secondary cooling area is divided into several zones to suit coolant flow rates. The necessary quantity of water is sprayed over the entire strand by spray-bars. The ferrostatic pressure may be so high in relation to the strand cross-section and the casting rate that the strand has to be supported to prevent buckling. The equipment for this is expensive in plants producing blooms and especially slabs.
Process Control
For productivity and quality reasons there is a trend in modern steelmaking to transfer time-consuming operations, such as temperature adjustment, deoxidation and alloying, from the furnace to the ladle treatment stations. These treatments are particularly important where the continuous casting process is involved because temperature and composition must closely be controlled.
The temperature control of molten steel as it enters the mould needs to be more accurate in the continuous casting process than in conventional casting. Too high a superheat can cause breakouts or a dendritic structure, which is often associated with poor internal quality. On the other hand, too low a temperature may cause casting difficulties due to nozzle clogging and result in dirty steel. The steel temperature in the tundish normally lies between 5 and 20℃ above the liquids for slab casting and between 5 and 50℃ for billet or bloom casting. This differential depends on steel grade and, for example, is about 45t for stainless steel slab casting from small furnaces.
In order to keep the steel temperature within the prescribed limits during the whole cast, temperature uniformity in the ladle is of paramount importance. Stirring is required before casting in order to destroy any temperature variations in the ladle, and rinsing is sometimes used. The heat is flushed with either nitrogen or argon, injected by means of a porous plug at the bottom of the ladle or through a hollow stopper rod at a separate rinsing station.
Control of chemical composition can be performed during vacuum or rinsing treatments. On the basis of the analysis of a sample or of an electromotive force oxygen activity measurement made after homogeneity of the metal is attained, trimming additions can be calculated to ensure correct deoxidation. The best way to introduce trim deoxidants is at a high velocity (powder injection with inert gas, wire feeding or bullet shooting) while stirring the bath. Decreasing the need for alloys by careful exclusion of furnace slag from the ladle simplifies trimming. Vacuum treatment is versatile and useful to achieve for good ladle metallurgy. Low-pressure treatment, however, is the only way to remove hydrogen before casting or to decarburize to extremely low levels.
Mould-level control
The most vital part of the control of a continuous casting machine is to ensure that the withdrawal of the cast and the partially-cooled billet is such as to keep the liquid level in the mould constant (within a few centimeters). This is done in two ways.
(1) The tundish is weighed and the rate of feed to the tundish from the ladle varied automatically to keep the total tundish weight constant. In this way the rate of feed from the tundish is constant.
(2) The rate of withdrawal of the partially cooled billet is controlled so as to keep the level of liquid steel in the mould roughly constant.
In the early days of continuous casting the level of the top of the liquid steel in the caster was maintained constant by an operator viewing it and adjusting the tundish stopper accordingly. It is now normal to have a means of finding the level using a measuring instrument and automatically adjusting the level. The table below lists several ways in which the level is detected. Two of them, the gamma-ray (radioactive) and the infrared methods will be described in detail.
The operation is self-evident from this diagram. The infrared device was developed in order to avoid the use of powerful radioactive isotopes. The detector views the junction of the metal level with the back wall of the mould. As the metal level rises within the field of view more radiation is received by the single photocell and an increased output is obtained. Special provisions are made to compensate for interruption of the view of the metal. The photocell unit receives the infrared radiation and provides an electrical signal to the control unit, which is in turn connected to the operator's unit and the casting-machine drives. The operator can select automatic or manual control and he receives indication of the operating rod from signal lamps. The radiation emitted from the liquid steel is collimated through a slotted mask and then focused on to a photo detector by a cylindrical lens. The light is filtered to eliminate radiation below a wavelength of 1 mm, so reducing interference from ambient light and oil flames.
The entire system is duplicated within the had with two detectors and two fit beams normally arranged to view either side