一、氮氣孔(kong)的形成機理

  在21.5Cr5Mn1.5Ni0.25N含氮(dan)(dan)(dan)(dan)(dan)(dan)雙相(xiang)鋼凝(ning)固過(guo)(guo)程(cheng)中(zhong)(zhong)，氮(dan)(dan)(dan)(dan)(dan)(dan)氣(qi)孔(kong)形(xing)(xing)成(cheng)和凝(ning)固前沿(yan)處［％N]1iq隨(sui)距(ju)離變化的(de)(de)(de)(de)規律如圖2-55所(suo)示。由于(yu)糊(hu)狀(zhuang)區(qu)內大量枝(zhi)晶(jing)(jing)(jing)網狀(zhuang)結構的(de)(de)(de)(de)形(xing)(xing)成(cheng)，液(ye)(ye)相(xiang)的(de)(de)(de)(de)對(dui)流只存在于(yu)一次枝(zhi)晶(jing)(jing)(jing)尖端位置附(fu)近。且枝(zhi)晶(jing)(jing)(jing)間幾乎無液(ye)(ye)相(xiang)的(de)(de)(de)(de)流動。因此，枝(zhi)晶(jing)(jing)(jing)間殘余液(ye)(ye)相(xiang)中(zhong)(zhong)的(de)(de)(de)(de)氮(dan)(dan)(dan)(dan)(dan)(dan)傳(chuan)質(zhi)主要依靠氮(dan)(dan)(dan)(dan)(dan)(dan)的(de)(de)(de)(de)擴(kuo)散行(xing)為，且糊(hu)狀(zhuang)區(qu)內氮(dan)(dan)(dan)(dan)(dan)(dan)傳(chuan)質(zhi)速率(lv)(lv)非常小(xiao)。初(chu)始(shi)(shi)相(xiang)貧(pin)氮(dan)(dan)(dan)(dan)(dan)(dan)鐵素(su)體相(xiang)8的(de)(de)(de)(de)氮(dan)(dan)(dan)(dan)(dan)(dan)溶解度(du)(du)和糊(hu)狀(zhuang)區(qu)的(de)(de)(de)(de)氮(dan)(dan)(dan)(dan)(dan)(dan)傳(chuan)質(zhi)速率(lv)(lv)較低(di)，導致在貧(pin)氮(dan)(dan)(dan)(dan)(dan)(dan)鐵素(su)體相(xiang)枝(zhi)晶(jing)(jing)(jing)附(fu)近的(de)(de)(de)(de)液(ye)(ye)相(xiang)中(zhong)(zhong)出現氮(dan)(dan)(dan)(dan)(dan)(dan)富(fu)集，且［％N]iq迅速增大，如圖2-55(a)所(suo)示。根據Yang和 Leel70]、Svyazhin 等、Ridolfi 和 Tassal的(de)(de)(de)(de)報道可(ke)知，當［％N]iq的(de)(de)(de)(de)最大值超過(guo)(guo)氮(dan)(dan)(dan)(dan)(dan)(dan)氣(qi)泡(pao)形(xing)(xing)成(cheng)的(de)(de)(de)(de)臨界(jie)氮(dan)(dan)(dan)(dan)(dan)(dan)質(zhi)量分(fen)數（［％N]pore)時，該區(qu)域有氣(qi)泡(pao)形(xing)(xing)成(cheng)的(de)(de)(de)(de)可(ke)能性，如圖2-55(b)所(suo)示。在后(hou)續的(de)(de)(de)(de)凝(ning)固過(guo)(guo)程(cheng)中(zhong)(zhong)，隨(sui)著(zhu)包晶(jing)(jing)(jing)反(fan)應的(de)(de)(de)(de)進行(xing)，富(fu)氮(dan)(dan)(dan)(dan)(dan)(dan)奧(ao)氏體相(xiang)γ以(yi)異質(zhi)形(xing)(xing)核的(de)(de)(de)(de)方式在鐵素(su)體相(xiang)8枝(zhi)晶(jing)(jing)(jing)的(de)(de)(de)(de)表(biao)(biao)面(mian)開(kai)始(shi)(shi)形(xing)(xing)核長大，逐(zhu)漸包裹鐵素(su)體相(xiang)枝(zhi)晶(jing)(jing)(jing)表(biao)(biao)面(mian)，并開(kai)始(shi)(shi)捕(bu)獲(huo)殘余液(ye)(ye)相(xiang)中(zhong)(zhong)的(de)(de)(de)(de)氮(dan)(dan)(dan)(dan)(dan)(dan)氣(qi)泡(pao)，對(dui)比(bi)圖2-51和圖2-56可(ke)知，此時枝(zhi)晶(jing)(jing)(jing)間殘余［％N]1ig的(de)(de)(de)(de)增長速率(lv)(lv)減小(xiao)。對(dui)平衡凝(ning)固而言，殘余液(ye)(ye)相(xiang)中(zhong)(zhong)氮(dan)(dan)(dan)(dan)(dan)(dan)氣(qi)泡(pao)形(xing)(xing)成(cheng)以(yi)后(hou)，氮(dan)(dan)(dan)(dan)(dan)(dan)的(de)(de)(de)(de)富(fu)集程(cheng)度(du)(du)減弱(ruo)，［％N]1iq增長速率(lv)(lv)的(de)(de)(de)(de)減小(xiao)程(cheng)度(du)(du)明顯；相(xiang)比(bi)之下，Scheil凝(ning)固過(guo)(guo)程(cheng)中(zhong)(zhong)，氮(dan)(dan)(dan)(dan)(dan)(dan)氣(qi)泡(pao)形(xing)(xing)成(cheng)以(yi)后(hou)，殘余液(ye)(ye)相(xiang)中(zhong)(zhong)氮(dan)(dan)(dan)(dan)(dan)(dan)富(fu)集狀(zhuang)態有所(suo)緩解，但(dan)幅度(du)(du)很(hen)小(xiao)。隨(sui)著(zhu)凝(ning)固界(jie)面(mian)的(de)(de)(de)(de)進一步推移，被捕(bu)獲(huo)的(de)(de)(de)(de)氮(dan)(dan)(dan)(dan)(dan)(dan)氣(qi)泡(pao)在奧(ao)氏體相(xiang)表(biao)(biao)面(mian)開(kai)始(shi)(shi)長大，并沿(yan)凝(ning)固方向(xiang)拉長，如圖2-55(c)所(suo)示。

  氮氣孔(kong)沿徑向生長，生長方向與凝固方向一致，那么氮氣孔(kong)初始(shi)形成(cheng)(cheng)位置(zhi)靠近(jin)鑄錠邊部，且氮氣泡初始(shi)位置(zhi)邊緣(yuan)全由(you)奧氏體(ti)相(xiang)γ構成(cheng)(cheng)（圖2-57中I區），與圖2-55描述(shu)相(xiang)符。隨著氮氣孔(kong)被拉長，鐵素體(ti)相(xiang)和奧氏體(ti)相(xiang)以體(ti)積分數比約(yue)為0.92的(de)關系(xi)交替在氮氣泡周(zhou)圍形成(cheng)(cheng)，直到(dao)氮氣孔(kong)閉合。凝固結束(shu)后，氮氣孔(kong)的(de)宏(hong)觀形貌類似(si)于橢圓形，與Wei等的(de)研究結果(guo)一致

二、氮微(wei)觀偏析對氮氣孔的影響(xiang)

  氮(dan)的(de)(de)分(fen)(fen)配系數(shu)較(jiao)小，導致液(ye)(ye)相(xiang)(xiang)向(xiang)固(gu)相(xiang)(xiang)轉變的(de)(de)過程(cheng)中(zhong)，固(gu)相(xiang)(xiang)會將多(duo)余(yu)(yu)的(de)(de)氮(dan)轉移到殘余(yu)(yu)液(ye)(ye)相(xiang)(xiang)中(zhong)，形(xing)(xing)成氮(dan)偏(pian)析(xi)(xi)。在(zai)(zai)氮(dan)偏(pian)析(xi)(xi)程(cheng)度逐漸加重的(de)(de)過程(cheng)中(zhong)，當(dang)殘余(yu)(yu)液(ye)(ye)相(xiang)(xiang)中(zhong)氮(dan)質量(liang)分(fen)(fen)數(shu)超過其飽(bao)和(he)(he)度時，極易(yi)形(xing)(xing)成氮(dan)氣(qi)(qi)(qi)(qi)泡(pao)(pao)。隨著(zhu)凝(ning)(ning)固(gu)的(de)(de)進(jin)(jin)行，若氣(qi)(qi)(qi)(qi)泡(pao)(pao)無(wu)法(fa)上浮而被捕獲，凝(ning)(ning)固(gu)結束后就會在(zai)(zai)鑄(zhu)錠內(nei)(nei)部(bu)形(xing)(xing)成氣(qi)(qi)(qi)(qi)孔(kong)(kong)。因(yin)此(ci)(ci)，凝(ning)(ning)固(gu)過程(cheng)中(zhong)氮(dan)偏(pian)析(xi)(xi)和(he)(he)溶解度對鑄(zhu)錠中(zhong)最終(zhong)氮(dan)氣(qi)(qi)(qi)(qi)孔(kong)(kong)的(de)(de)形(xing)(xing)成有至關重要的(de)(de)作用。氮(dan)氣(qi)(qi)(qi)(qi)孔(kong)(kong)多(duo)數(shu)情況(kuang)下與疏(shu)松縮(suo)孔(kong)(kong)共存，內(nei)(nei)壁凹凸不平(ping)呈現裂(lie)紋狀(zhuang)，且(qie)整個氣(qi)(qi)(qi)(qi)孔(kong)(kong)形(xing)(xing)狀(zhuang)不規則，如圖2-58所示(shi)。此(ci)(ci)類氣(qi)(qi)(qi)(qi)孔(kong)(kong)不僅(jin)與鋼(gang)液(ye)(ye)中(zhong)氣(qi)(qi)(qi)(qi)泡(pao)(pao)的(de)(de)形(xing)(xing)成有關，還受凝(ning)(ning)固(gu)收縮(suo)等因(yin)素(su)的(de)(de)影響(xiang)，且(qie)多(duo)數(shu)分(fen)(fen)布于鑄(zhu)錠心部(bu)，尤其在(zai)(zai)中(zhong)心等軸晶區(qu)。這(zhe)主要由(you)于中(zhong)心等軸晶區(qu)內(nei)(nei)枝晶生(sheng)長較(jiao)發達(da)，容易(yi)形(xing)(xing)成復雜(za)的(de)(de)網(wang)狀(zhuang)結構，從而將液(ye)(ye)相(xiang)(xiang)分(fen)(fen)割成無(wu)數(shu)個獨(du)立的(de)(de)液(ye)(ye)相(xiang)(xiang)區(qu)域，當(dang)發生(sheng)凝(ning)(ning)固(gu)收縮(suo)時，難以進(jin)(jin)行補縮(suo)，在(zai)(zai)形(xing)(xing)成疏(shu)松縮(suo)孔(kong)(kong)的(de)(de)同時，局(ju)部(bu)鋼(gang)液(ye)(ye)靜壓力降(jiang)低，促使(shi)氮(dan)從殘余(yu)(yu)液(ye)(ye)相(xiang)(xiang)中(zhong)析(xi)(xi)出，從而形(xing)(xing)成了氮(dan)氣(qi)(qi)(qi)(qi)孔(kong)(kong)和(he)(he)疏(shu)松縮(suo)孔(kong)(kong)共存的(de)(de)宏觀(guan)缺陷。

  平衡凝固時(shi)，19Cr14Mn0.9N含氮(dan)(dan)(dan)奧氏體(ti)(ti)不銹(xiu)鋼殘余(yu)液相中(zhong)(zhong)氮(dan)(dan)(dan)偏析與(yu)體(ti)(ti)系氮(dan)(dan)(dan)溶解度(du)的(de)(de)(de)差(cha)值如(ru)圖2-59所示。凝固初期鐵素(su)體(ti)(ti)阱（ferrite trap)的(de)(de)(de)形(xing)成(cheng)，導致氮(dan)(dan)(dan)溶解度(du)的(de)(de)(de)降低，進(jin)而使氮(dan)(dan)(dan)偏析與(yu)體(ti)(ti)系氮(dan)(dan)(dan)溶解度(du)差(cha)值呈現出略微增大(da)的(de)(de)(de)趨(qu)勢。但在(zai)(zai)后(hou)續凝固過(guo)程中(zhong)(zhong)，隨(sui)著鐵素(su)體(ti)(ti)阱的(de)(de)(de)消失以及富氮(dan)(dan)(dan)奧氏體(ti)(ti)相的(de)(de)(de)不斷形(xing)成(cheng)，差(cha)值減小(xiao)；在(zai)(zai)整(zheng)個凝固過(guo)程中(zhong)(zhong)差(cha)值始(shi)終較(jiao)(jiao)小(xiao)，且變化幅(fu)度(du)較(jiao)(jiao)窄。對于19Cr14Mn0.9N 含氮(dan)(dan)(dan)奧氏體(ti)(ti)不銹(xiu)鋼，液相中(zhong)(zhong)氮(dan)(dan)(dan)氣(qi)泡的(de)(de)(de)形(xing)成(cheng)趨(qu)勢較(jiao)(jiao)小(xiao)，難以在(zai)(zai)鑄錠內形(xing)成(cheng)獨立內壁光滑的(de)(de)(de)規則(ze)氮(dan)(dan)(dan)氣(qi)孔(kong)。

  此外，目(mu)前有人對(dui)(dui)奧氏(shi)(shi)體(ti)(ti)(ti)鋼(gang)(gang)凝固(gu)(gu)過(guo)程(cheng)(cheng)中(zhong)(zhong)(zhong)氮(dan)(dan)(dan)(dan)氣(qi)孔的(de)形成(cheng)進(jin)行了(le)大量(liang)研(yan)究，如Yang和(he)Leel901研(yan)究了(le)奧氏(shi)(shi)體(ti)(ti)(ti)鋼(gang)(gang)16Cr3NixMn(x=9和(he)11)凝固(gu)(gu)過(guo)程(cheng)(cheng)中(zhong)(zhong)(zhong)壓力(li)和(he)初始(shi)氮(dan)(dan)(dan)(dan)質量(liang)分(fen)數等因素(su)(su)(su)對(dui)(dui)氮(dan)(dan)(dan)(dan)氣(qi)孔形成(cheng)的(de)影響(xiang)規律，并建立了(le)相應的(de)預測模型。Ridolfi和(he)Tassal[84]分(fen)析(xi)了(le)氮(dan)(dan)(dan)(dan)偏析(xi)、合金元素(su)(su)(su)、冷卻速率以及枝晶間距對(dui)(dui)奧氏(shi)(shi)體(ti)(ti)(ti)鋼(gang)(gang)中(zhong)(zhong)(zhong)氮(dan)(dan)(dan)(dan)氣(qi)孔的(de)影響(xiang)規律，并揭示(shi)了(le)奧氏(shi)(shi)體(ti)(ti)(ti)鋼(gang)(gang)中(zhong)(zhong)(zhong)氮(dan)(dan)(dan)(dan)氣(qi)孔形成(cheng)機(ji)理(li)。然而，目(mu)前對(dui)(dui)于雙(shuang)相鋼(gang)(gang)中(zhong)(zhong)(zhong)氮(dan)(dan)(dan)(dan)氣(qi)孔形成(cheng)的(de)研(yan)究較少，且主要集中(zhong)(zhong)(zhong)在(zai)合金元素(su)(su)(su)、鑄造(zao)方式(shi)、冷卻速率等因素(su)(su)(su)對(dui)(dui)氮(dan)(dan)(dan)(dan)氣(qi)孔影響(xiang)規律的(de)研(yan)究，鮮有對(dui)(dui)雙(shuang)相鋼(gang)(gang)中(zhong)(zhong)(zhong)氮(dan)(dan)(dan)(dan)氣(qi)孔形成(cheng)機(ji)理(li)的(de)報道。以21.5Cr5Mn1.5Ni0.25N含(han)氮(dan)(dan)(dan)(dan)雙(shuang)相鋼(gang)(gang)為例，氮(dan)(dan)(dan)(dan)偏析(xi)與(yu)溶解度(du)的(de)差值在(zai)整個(ge)凝固(gu)(gu)過(guo)程(cheng)(cheng)中(zhong)(zhong)(zhong)的(de)變化趨勢，如圖2-59所(suo)示(shi)。隨著凝固(gu)(gu)的(de)進(jin)行，氮(dan)(dan)(dan)(dan)偏析(xi)始(shi)終大于氮(dan)(dan)(dan)(dan)溶解度(du)，且差值呈現出快速增(zeng)大的(de)趨勢。因此，在(zai)21.5Cr5Mn1.5Ni0.25N 含(han)氮(dan)(dan)(dan)(dan)雙(shuang)相鋼(gang)(gang)凝固(gu)(gu)過(guo)程(cheng)(cheng)中(zhong)(zhong)(zhong)，氮(dan)(dan)(dan)(dan)偏析(xi)嚴重，殘余液相內氮(dan)(dan)(dan)(dan)氣(qi)泡形成(cheng)趨勢較大，明(ming)顯(xian)高于19Cr14Mn0.9N含(han)氮(dan)(dan)(dan)(dan)奧氏(shi)(shi)體(ti)(ti)(ti)不(bu)銹鋼(gang)(gang)。

  氮氣(qi)(qi)(qi)泡形(xing)成和(he)長大(da)具(ju)有重(zhong)要的作(zuo)用(yong)（圖2-60).其(qi)中(zhong)，σ為氣(qi)(qi)(qi)液界面(mian)的表面(mian)張(zhang)力(li)，r為氣(qi)(qi)(qi)泡半徑。結合經(jing)典形(xing)核(he)理論，氮氣(qi)(qi)(qi)泡在(zai)鋼液中(zhong)穩定存(cun)在(zai)的必(bi)要條件(jian)為氣(qi)(qi)(qi)泡內(nei)壓力(li)大(da)于(yu)作(zuo)用(yong)于(yu)氣(qi)(qi)(qi)泡的所有壓力(li)之和(he)，即(ji)

  式中，Aso由(you)(you)凝(ning)固(gu)過程中除氮(dan)(dan)(dan)以外(wai)(wai)其(qi)他合(he)金元素(su)的微(wei)觀偏析(xi)進行(xing)計算(suan)，其(qi)值隨(sui)著枝(zhi)晶(jing)(jing)間殘余(yu)液(ye)相(xiang)(xiang)(xiang)中氮(dan)(dan)(dan)溶(rong)(rong)解(jie)度的增(zeng)加而減(jian)小，表(biao)征(zheng)了(le)枝(zhi)晶(jing)(jing)間殘余(yu)液(ye)相(xiang)(xiang)(xiang)中氮(dan)(dan)(dan)溶(rong)(rong)解(jie)度對(dui)氮(dan)(dan)(dan)氣泡(pao)形(xing)成的影(ying)響程度；Ase表(biao)征(zheng)了(le)枝(zhi)晶(jing)(jing)間氮(dan)(dan)(dan)偏析(xi)對(dui)氮(dan)(dan)(dan)氣泡(pao)形(xing)成的影(ying)響程度，可由(you)(you)凝(ning)固(gu)過程中枝(zhi)晶(jing)(jing)間殘余(yu)液(ye)相(xiang)(xiang)(xiang)中氮(dan)(dan)(dan)偏析(xi)計算(suan)獲得，其(qi)值隨(sui)著氮(dan)(dan)(dan)偏析(xi)的增(zeng)大(da)而增(zeng)大(da)。此外(wai)(wai)，用于(yu)計算(suan)Aso和Ase時所需(xu)的合(he)金元素(su)偏析(xi)均由(you)(you)鋼凝(ning)固(gu)相(xiang)(xiang)(xiang)變所致。

  氮(dan)(dan)氣(qi)泡(pao)的(de)形(xing)(xing)核和(he)長(chang)大(da)過程復雜，且影響因(yin)素眾多，包括(kuo)凝(ning)(ning)固收(shou)縮、冶煉(lian)環境以(yi)及坩堝材質(zhi)等(deng)。因(yin)此(ci)，很難采(cai)用(yong)Pg值精確(que)預測凝(ning)(ning)固過程中(zhong)氮(dan)(dan)氣(qi)泡(pao)的(de)形(xing)(xing)成(cheng)和(he)長(chang)大(da)。然而基于Yang等(deng)的(de)實(shi)驗研究(jiu)［70,77],在評估凝(ning)(ning)固壓力、合金成(cheng)分(fen)等(deng)因(yin)素對氮(dan)(dan)氣(qi)泡(pao)形(xing)(xing)成(cheng)的(de)影響程度時，Pg起關鍵作(zuo)用(yong)。實(shi)際(ji)凝(ning)(ning)固過程介于平衡(heng)凝(ning)(ning)固（固／液相中(zhong)溶質(zhi)完(wan)全擴散(san)）和(he)Scheil凝(ning)(ning)固（固相無溶質(zhi)擴散(san)，液相中(zhong)完(wan)全擴散(san)）之間(jian)70].因(yin)此(ci)，可分(fen)別計算(suan)平衡(heng)凝(ning)(ning)固和(he)Scheil凝(ning)(ning)固過程中(zhong)的(de)Aso、Ase和(he)Pg,闡明實(shi)際(ji)凝(ning)(ning)固過程中(zhong)壓力等(deng)因(yin)素對氮(dan)(dan)氣(qi)泡(pao)形(xing)(xing)成(cheng)的(de)影響規(gui)律。

  現以21.5Cr5Mn1.5Ni0.25N含氮雙相鋼(gang)D1鑄(zhu)錠(ding)為例，對凝固(gu)過程(cheng)中(zhong)Aso、Ase和P8的(de)(de)變化趨(qu)勢(shi)進(jin)行計算(suan)。圖2-61描述(shu)了(le)ΔAso(=Asa-Aso,0)和AAse(=Ase-Ase,o)隨固(gu)相質量分(fen)數的(de)(de)變化趨(qu)勢(shi)（Aso,0和Asc,0分(fen)別為D1鑄(zhu)錠(ding)凝固(gu)時Aso和Ase的(de)(de)初始值(zhi)）。

  在平衡凝(ning)(ning)固(gu)和(he)(he)Scheil凝(ning)(ning)固(gu)過(guo)(guo)程(cheng)(cheng)中(zhong)(zhong)(zhong)(zhong)(zhong)，ΔAso的(de)(de)最(zui)小(xiao)值(zhi)(zhi)分(fen)別(bie)為(wei)－0.145和(he)(he)－0.397,與此相對應(ying)的(de)(de)ΔAse值(zhi)(zhi)最(zui)大，分(fen)別(bie)為(wei)0.68和(he)(he)0.92.在整(zheng)(zheng)個凝(ning)(ning)固(gu)過(guo)(guo)程(cheng)(cheng)中(zhong)(zhong)(zhong)(zhong)(zhong)，由于ΔAse與ΔAso之和(he)(he)始終(zhong)大于零，因而枝(zhi)晶間殘余液(ye)相中(zhong)(zhong)(zhong)(zhong)(zhong)氮(dan)偏析對D1 鑄錠凝(ning)(ning)固(gu)過(guo)(guo)程(cheng)(cheng)中(zhong)(zhong)(zhong)(zhong)(zhong)氮(dan)氣(qi)泡(pao)形(xing)成的(de)(de)影響(xiang)大于氮(dan)溶解度，起主(zhu)導作用(yong)。此外(wai)，在整(zheng)(zheng)個凝(ning)(ning)固(gu)過(guo)(guo)程(cheng)(cheng)中(zhong)(zhong)(zhong)(zhong)(zhong)，P8變化趨勢(shi)如圖(tu)2-62所(suo)示，其變化規律與Young等。的(de)(de)研(yan)究結果一致，Pg的(de)(de)最(zui)大值(zhi)(zhi)Pg與Ase+Aso的(de)(de)最(zui)大值(zhi)(zhi)相對應(ying)，且在平衡凝(ning)(ning)固(gu)和(he)(he) Scheil 凝(ning)(ning)固(gu)過(guo)(guo)程(cheng)(cheng)中(zhong)(zhong)(zhong)(zhong)(zhong)分(fen)別(bie)為(wei)0.63MPa和(he)(he)0.62MPa.此外(wai)，可通過(guo)(guo)對比不同鑄錠中(zhong)(zhong)(zhong)(zhong)(zhong)的(de)(de)探討凝(ning)(ning)固(gu)壓(ya)力、初始氮(dan)質量分(fen)數以及合金元素（鉻和(he)(he)錳）等對液(ye)相中(zhong)(zhong)(zhong)(zhong)(zhong)氮(dan)氣(qi)泡(pao)形(xing)成的(de)(de)影響(xiang)，進而明晰各因素對氮(dan)氣(qi)孔形(xing)成的(de)(de)影響(xiang)規律。