一(yi)、氮氣孔(kong)的形(xing)成(cheng)機理

  在21.5Cr5Mn1.5Ni0.25N含(han)氮(dan)(dan)雙相(xiang)鋼凝(ning)固(gu)(gu)過程(cheng)(cheng)中(zhong)，氮(dan)(dan)氣(qi)(qi)(qi)孔形(xing)(xing)(xing)成(cheng)和(he)凝(ning)固(gu)(gu)前(qian)沿(yan)處［％N]1iq隨距離變化的(de)(de)(de)(de)規律(lv)如(ru)(ru)(ru)圖(tu)(tu)2-55所示。由于(yu)糊(hu)狀(zhuang)(zhuang)區內大量枝晶(jing)(jing)網狀(zhuang)(zhuang)結(jie)構的(de)(de)(de)(de)形(xing)(xing)(xing)成(cheng)，液(ye)相(xiang)的(de)(de)(de)(de)對流只存在于(yu)一(yi)次(ci)枝晶(jing)(jing)尖端位(wei)置(zhi)附(fu)近(jin)。且枝晶(jing)(jing)間幾乎無液(ye)相(xiang)的(de)(de)(de)(de)流動。因此，枝晶(jing)(jing)間殘余(yu)液(ye)相(xiang)中(zhong)的(de)(de)(de)(de)氮(dan)(dan)傳質主要(yao)依靠氮(dan)(dan)的(de)(de)(de)(de)擴散行為，且糊(hu)狀(zhuang)(zhuang)區內氮(dan)(dan)傳質速(su)(su)率非常小(xiao)。初(chu)始(shi)(shi)相(xiang)貧氮(dan)(dan)鐵素(su)體相(xiang)8的(de)(de)(de)(de)氮(dan)(dan)溶解度(du)和(he)糊(hu)狀(zhuang)(zhuang)區的(de)(de)(de)(de)氮(dan)(dan)傳質速(su)(su)率較低，導致在貧氮(dan)(dan)鐵素(su)體相(xiang)枝晶(jing)(jing)附(fu)近(jin)的(de)(de)(de)(de)液(ye)相(xiang)中(zhong)出現氮(dan)(dan)富(fu)集，且［％N]iq迅(xun)速(su)(su)增(zeng)大，如(ru)(ru)(ru)圖(tu)(tu)2-55(a)所示。根據Yang和(he) Leel70]、Svyazhin 等、Ridolfi 和(he) Tassal的(de)(de)(de)(de)報道可知，當［％N]iq的(de)(de)(de)(de)最(zui)大值超(chao)過氮(dan)(dan)氣(qi)(qi)(qi)泡(pao)(pao)形(xing)(xing)(xing)成(cheng)的(de)(de)(de)(de)臨界氮(dan)(dan)質量分(fen)數(shu)（［％N]pore)時，該區域有氣(qi)(qi)(qi)泡(pao)(pao)形(xing)(xing)(xing)成(cheng)的(de)(de)(de)(de)可能性(xing)，如(ru)(ru)(ru)圖(tu)(tu)2-55(b)所示。在后續(xu)的(de)(de)(de)(de)凝(ning)固(gu)(gu)過程(cheng)(cheng)中(zhong)，隨著包晶(jing)(jing)反應的(de)(de)(de)(de)進(jin)行，富(fu)氮(dan)(dan)奧氏(shi)體相(xiang)γ以(yi)異質形(xing)(xing)(xing)核(he)的(de)(de)(de)(de)方式在鐵素(su)體相(xiang)8枝晶(jing)(jing)的(de)(de)(de)(de)表(biao)面(mian)(mian)開始(shi)(shi)形(xing)(xing)(xing)核(he)長(chang)(chang)大，逐漸包裹(guo)鐵素(su)體相(xiang)枝晶(jing)(jing)表(biao)面(mian)(mian)，并開始(shi)(shi)捕獲殘余(yu)液(ye)相(xiang)中(zhong)的(de)(de)(de)(de)氮(dan)(dan)氣(qi)(qi)(qi)泡(pao)(pao)，對比(bi)圖(tu)(tu)2-51和(he)圖(tu)(tu)2-56可知，此時枝晶(jing)(jing)間殘余(yu)［％N]1ig的(de)(de)(de)(de)增(zeng)長(chang)(chang)速(su)(su)率減小(xiao)。對平衡凝(ning)固(gu)(gu)而言，殘余(yu)液(ye)相(xiang)中(zhong)氮(dan)(dan)氣(qi)(qi)(qi)泡(pao)(pao)形(xing)(xing)(xing)成(cheng)以(yi)后，氮(dan)(dan)的(de)(de)(de)(de)富(fu)集程(cheng)(cheng)度(du)減弱，［％N]1iq增(zeng)長(chang)(chang)速(su)(su)率的(de)(de)(de)(de)減小(xiao)程(cheng)(cheng)度(du)明顯(xian)；相(xiang)比(bi)之下，Scheil凝(ning)固(gu)(gu)過程(cheng)(cheng)中(zhong)，氮(dan)(dan)氣(qi)(qi)(qi)泡(pao)(pao)形(xing)(xing)(xing)成(cheng)以(yi)后，殘余(yu)液(ye)相(xiang)中(zhong)氮(dan)(dan)富(fu)集狀(zhuang)(zhuang)態有所緩解，但幅度(du)很小(xiao)。隨著凝(ning)固(gu)(gu)界面(mian)(mian)的(de)(de)(de)(de)進(jin)一(yi)步推移，被捕獲的(de)(de)(de)(de)氮(dan)(dan)氣(qi)(qi)(qi)泡(pao)(pao)在奧氏(shi)體相(xiang)表(biao)面(mian)(mian)開始(shi)(shi)長(chang)(chang)大，并沿(yan)凝(ning)固(gu)(gu)方向拉(la)長(chang)(chang)，如(ru)(ru)(ru)圖(tu)(tu)2-55(c)所示。

  氮(dan)(dan)氣(qi)孔(kong)(kong)沿徑向生長，生長方向與凝固方向一(yi)致，那么氮(dan)(dan)氣(qi)孔(kong)(kong)初始(shi)形(xing)(xing)成(cheng)位(wei)置靠(kao)近鑄錠(ding)邊部(bu)，且氮(dan)(dan)氣(qi)泡初始(shi)位(wei)置邊緣全由奧(ao)氏體(ti)相γ構成(cheng)（圖2-57中I區），與圖2-55描述相符。隨著氮(dan)(dan)氣(qi)孔(kong)(kong)被拉長，鐵素(su)體(ti)相和奧(ao)氏體(ti)相以體(ti)積分數比約為0.92的(de)(de)關(guan)系交替在氮(dan)(dan)氣(qi)泡周圍形(xing)(xing)成(cheng)，直到氮(dan)(dan)氣(qi)孔(kong)(kong)閉合。凝固結(jie)束(shu)后，氮(dan)(dan)氣(qi)孔(kong)(kong)的(de)(de)宏(hong)觀形(xing)(xing)貌類(lei)似于橢(tuo)圓形(xing)(xing)，與Wei等的(de)(de)研究結(jie)果一(yi)致

二(er)、氮(dan)微觀偏析對氮(dan)氣孔(kong)的影響

  氮(dan)(dan)(dan)的(de)(de)(de)(de)(de)分配系數較小(xiao)，導致(zhi)液(ye)(ye)(ye)相(xiang)(xiang)(xiang)向固相(xiang)(xiang)(xiang)轉(zhuan)變(bian)的(de)(de)(de)(de)(de)過(guo)程(cheng)(cheng)中(zhong)(zhong)(zhong)，固相(xiang)(xiang)(xiang)會將多(duo)余的(de)(de)(de)(de)(de)氮(dan)(dan)(dan)轉(zhuan)移(yi)到(dao)殘(can)余液(ye)(ye)(ye)相(xiang)(xiang)(xiang)中(zhong)(zhong)(zhong)，形成氮(dan)(dan)(dan)偏(pian)析(xi)(xi)。在氮(dan)(dan)(dan)偏(pian)析(xi)(xi)程(cheng)(cheng)度(du)逐漸加重的(de)(de)(de)(de)(de)過(guo)程(cheng)(cheng)中(zhong)(zhong)(zhong)，當殘(can)余液(ye)(ye)(ye)相(xiang)(xiang)(xiang)中(zhong)(zhong)(zhong)氮(dan)(dan)(dan)質量(liang)分數超過(guo)其飽和度(du)時(shi)，極(ji)易(yi)形成氮(dan)(dan)(dan)氣(qi)(qi)(qi)泡。隨著凝(ning)固的(de)(de)(de)(de)(de)進行，若氣(qi)(qi)(qi)泡無(wu)法上浮而被捕獲，凝(ning)固結束后就會在鑄(zhu)錠(ding)內部形成氣(qi)(qi)(qi)孔(kong)(kong)。因此，凝(ning)固過(guo)程(cheng)(cheng)中(zhong)(zhong)(zhong)氮(dan)(dan)(dan)偏(pian)析(xi)(xi)和溶解度(du)對鑄(zhu)錠(ding)中(zhong)(zhong)(zhong)最終(zhong)氮(dan)(dan)(dan)氣(qi)(qi)(qi)孔(kong)(kong)的(de)(de)(de)(de)(de)形成有至關(guan)(guan)重要(yao)的(de)(de)(de)(de)(de)作用。氮(dan)(dan)(dan)氣(qi)(qi)(qi)孔(kong)(kong)多(duo)數情(qing)況下與疏(shu)松(song)縮(suo)(suo)孔(kong)(kong)共(gong)存，內壁凹(ao)凸不平呈(cheng)現裂紋狀(zhuang)，且(qie)整個氣(qi)(qi)(qi)孔(kong)(kong)形狀(zhuang)不規(gui)則，如圖(tu)2-58所示。此類氣(qi)(qi)(qi)孔(kong)(kong)不僅(jin)與鋼液(ye)(ye)(ye)中(zhong)(zhong)(zhong)氣(qi)(qi)(qi)泡的(de)(de)(de)(de)(de)形成有關(guan)(guan)，還受凝(ning)固收縮(suo)(suo)等因素的(de)(de)(de)(de)(de)影響，且(qie)多(duo)數分布于鑄(zhu)錠(ding)心(xin)部，尤其在中(zhong)(zhong)(zhong)心(xin)等軸晶(jing)區。這主要(yao)由(you)于中(zhong)(zhong)(zhong)心(xin)等軸晶(jing)區內枝晶(jing)生長較發達，容易(yi)形成復雜的(de)(de)(de)(de)(de)網狀(zhuang)結構，從而將液(ye)(ye)(ye)相(xiang)(xiang)(xiang)分割成無(wu)數個獨立的(de)(de)(de)(de)(de)液(ye)(ye)(ye)相(xiang)(xiang)(xiang)區域，當發生凝(ning)固收縮(suo)(suo)時(shi)，難以(yi)進行補縮(suo)(suo)，在形成疏(shu)松(song)縮(suo)(suo)孔(kong)(kong)的(de)(de)(de)(de)(de)同時(shi)，局部鋼液(ye)(ye)(ye)靜(jing)壓力降(jiang)低，促使氮(dan)(dan)(dan)從殘(can)余液(ye)(ye)(ye)相(xiang)(xiang)(xiang)中(zhong)(zhong)(zhong)析(xi)(xi)出，從而形成了氮(dan)(dan)(dan)氣(qi)(qi)(qi)孔(kong)(kong)和疏(shu)松(song)縮(suo)(suo)孔(kong)(kong)共(gong)存的(de)(de)(de)(de)(de)宏觀缺(que)陷。

  平衡凝固時(shi)，19Cr14Mn0.9N含氮(dan)奧氏(shi)(shi)體(ti)不銹鋼(gang)殘余液相(xiang)中(zhong)氮(dan)偏析(xi)與體(ti)系(xi)(xi)氮(dan)溶(rong)(rong)解度的(de)(de)差值如圖2-59所(suo)示。凝固初期鐵素體(ti)阱（ferrite trap)的(de)(de)形(xing)成，導致氮(dan)溶(rong)(rong)解度的(de)(de)降低(di)，進而使氮(dan)偏析(xi)與體(ti)系(xi)(xi)氮(dan)溶(rong)(rong)解度差值呈(cheng)現出略微增大的(de)(de)趨勢(shi)。但(dan)在(zai)后(hou)續凝固過程(cheng)中(zhong)，隨(sui)著鐵素體(ti)阱的(de)(de)消(xiao)失以及(ji)富氮(dan)奧氏(shi)(shi)體(ti)相(xiang)的(de)(de)不斷形(xing)成，差值減小；在(zai)整個凝固過程(cheng)中(zhong)差值始(shi)終較(jiao)小，且變化幅度較(jiao)窄。對于(yu)19Cr14Mn0.9N 含氮(dan)奧氏(shi)(shi)體(ti)不銹鋼(gang)，液相(xiang)中(zhong)氮(dan)氣(qi)泡的(de)(de)形(xing)成趨勢(shi)較(jiao)小，難以在(zai)鑄錠內形(xing)成獨立內壁光(guang)滑的(de)(de)規則氮(dan)氣(qi)孔。

  此外，目前有人對奧(ao)氏(shi)體(ti)(ti)鋼(gang)凝(ning)(ning)固過(guo)程中(zhong)(zhong)(zhong)氮(dan)(dan)(dan)氣孔(kong)(kong)的(de)形(xing)成(cheng)(cheng)進行了(le)(le)大(da)量研(yan)(yan)(yan)究，如Yang和(he)Leel901研(yan)(yan)(yan)究了(le)(le)奧(ao)氏(shi)體(ti)(ti)鋼(gang)16Cr3NixMn(x=9和(he)11)凝(ning)(ning)固過(guo)程中(zhong)(zhong)(zhong)壓力和(he)初始(shi)氮(dan)(dan)(dan)質量分數等因(yin)素對氮(dan)(dan)(dan)氣孔(kong)(kong)形(xing)成(cheng)(cheng)的(de)影(ying)(ying)響規(gui)(gui)律(lv)(lv)，并建立了(le)(le)相(xiang)(xiang)應(ying)的(de)預測模型。Ridolfi和(he)Tassal[84]分析了(le)(le)氮(dan)(dan)(dan)偏(pian)析、合金元素、冷卻速(su)率以(yi)及枝晶間(jian)距(ju)對奧(ao)氏(shi)體(ti)(ti)鋼(gang)中(zhong)(zhong)(zhong)氮(dan)(dan)(dan)氣孔(kong)(kong)的(de)影(ying)(ying)響規(gui)(gui)律(lv)(lv)，并揭(jie)示(shi)了(le)(le)奧(ao)氏(shi)體(ti)(ti)鋼(gang)中(zhong)(zhong)(zhong)氮(dan)(dan)(dan)氣孔(kong)(kong)形(xing)成(cheng)(cheng)機(ji)理。然而，目前對于雙(shuang)相(xiang)(xiang)鋼(gang)中(zhong)(zhong)(zhong)氮(dan)(dan)(dan)氣孔(kong)(kong)形(xing)成(cheng)(cheng)的(de)研(yan)(yan)(yan)究較(jiao)少，且(qie)主要集(ji)中(zhong)(zhong)(zhong)在合金元素、鑄造方式、冷卻速(su)率等因(yin)素對氮(dan)(dan)(dan)氣孔(kong)(kong)影(ying)(ying)響規(gui)(gui)律(lv)(lv)的(de)研(yan)(yan)(yan)究，鮮有對雙(shuang)相(xiang)(xiang)鋼(gang)中(zhong)(zhong)(zhong)氮(dan)(dan)(dan)氣孔(kong)(kong)形(xing)成(cheng)(cheng)機(ji)理的(de)報道。以(yi)21.5Cr5Mn1.5Ni0.25N含氮(dan)(dan)(dan)雙(shuang)相(xiang)(xiang)鋼(gang)為例，氮(dan)(dan)(dan)偏(pian)析與溶解度的(de)差值在整個凝(ning)(ning)固過(guo)程中(zhong)(zhong)(zhong)的(de)變化趨(qu)勢，如圖2-59所示(shi)。隨(sui)著(zhu)凝(ning)(ning)固的(de)進行，氮(dan)(dan)(dan)偏(pian)析始(shi)終大(da)于氮(dan)(dan)(dan)溶解度，且(qie)差值呈現出快速(su)增(zeng)大(da)的(de)趨(qu)勢。因(yin)此，在21.5Cr5Mn1.5Ni0.25N 含氮(dan)(dan)(dan)雙(shuang)相(xiang)(xiang)鋼(gang)凝(ning)(ning)固過(guo)程中(zhong)(zhong)(zhong)，氮(dan)(dan)(dan)偏(pian)析嚴(yan)重(zhong)，殘余液相(xiang)(xiang)內氮(dan)(dan)(dan)氣泡形(xing)成(cheng)(cheng)趨(qu)勢較(jiao)大(da)，明顯高(gao)于19Cr14Mn0.9N含氮(dan)(dan)(dan)奧(ao)氏(shi)體(ti)(ti)不銹(xiu)鋼(gang)。

  氮(dan)氣(qi)泡形成和長大具(ju)有重要的作用（圖2-60).其中(zhong)，σ為氣(qi)液界面(mian)的表面(mian)張(zhang)力(li)(li)，r為氣(qi)泡半徑(jing)。結合(he)經(jing)典(dian)形核理(li)論，氮(dan)氣(qi)泡在鋼(gang)液中(zhong)穩定存在的必要條件為氣(qi)泡內壓力(li)(li)大于作用于氣(qi)泡的所有壓力(li)(li)之和，即

  式中，Aso由(you)凝固過程中除氮(dan)(dan)(dan)以(yi)外其他合金元素的(de)(de)(de)微(wei)觀偏(pian)析(xi)進(jin)行計算，其值隨著枝(zhi)晶(jing)間(jian)(jian)(jian)(jian)殘(can)余(yu)(yu)液相中氮(dan)(dan)(dan)溶解度(du)的(de)(de)(de)增(zeng)加而減小，表征(zheng)了枝(zhi)晶(jing)間(jian)(jian)(jian)(jian)殘(can)余(yu)(yu)液相中氮(dan)(dan)(dan)溶解度(du)對氮(dan)(dan)(dan)氣泡形成的(de)(de)(de)影響程度(du)；Ase表征(zheng)了枝(zhi)晶(jing)間(jian)(jian)(jian)(jian)氮(dan)(dan)(dan)偏(pian)析(xi)對氮(dan)(dan)(dan)氣泡形成的(de)(de)(de)影響程度(du)，可由(you)凝固過程中枝(zhi)晶(jing)間(jian)(jian)(jian)(jian)殘(can)余(yu)(yu)液相中氮(dan)(dan)(dan)偏(pian)析(xi)計算獲得，其值隨著氮(dan)(dan)(dan)偏(pian)析(xi)的(de)(de)(de)增(zeng)大而增(zeng)大。此外，用于(yu)計算Aso和Ase時所需的(de)(de)(de)合金元素偏(pian)析(xi)均(jun)由(you)鋼凝固相變所致。

  氮氣(qi)(qi)泡(pao)的(de)形(xing)核和(he)長大(da)過(guo)(guo)程(cheng)復雜(za)，且影響(xiang)(xiang)因素眾多，包括(kuo)凝(ning)(ning)固(gu)(gu)(gu)收縮(suo)、冶煉環境以及坩堝材(cai)質等。因此，很難采(cai)用Pg值精確預(yu)測凝(ning)(ning)固(gu)(gu)(gu)過(guo)(guo)程(cheng)中氮氣(qi)(qi)泡(pao)的(de)形(xing)成(cheng)和(he)長大(da)。然而(er)基于Yang等的(de)實驗研究［70,77],在評估凝(ning)(ning)固(gu)(gu)(gu)壓(ya)力、合金(jin)成(cheng)分等因素對(dui)氮氣(qi)(qi)泡(pao)形(xing)成(cheng)的(de)影響(xiang)(xiang)程(cheng)度時，Pg起關(guan)鍵作用。實際凝(ning)(ning)固(gu)(gu)(gu)過(guo)(guo)程(cheng)介于平衡凝(ning)(ning)固(gu)(gu)(gu)（固(gu)(gu)(gu)／液相(xiang)中溶質完全擴散(san)(san)）和(he)Scheil凝(ning)(ning)固(gu)(gu)(gu)（固(gu)(gu)(gu)相(xiang)無溶質擴散(san)(san)，液相(xiang)中完全擴散(san)(san)）之間(jian)70].因此，可(ke)分別計算平衡凝(ning)(ning)固(gu)(gu)(gu)和(he)Scheil凝(ning)(ning)固(gu)(gu)(gu)過(guo)(guo)程(cheng)中的(de)Aso、Ase和(he)Pg,闡明實際凝(ning)(ning)固(gu)(gu)(gu)過(guo)(guo)程(cheng)中壓(ya)力等因素對(dui)氮氣(qi)(qi)泡(pao)形(xing)成(cheng)的(de)影響(xiang)(xiang)規律(lv)。

  現(xian)以21.5Cr5Mn1.5Ni0.25N含氮雙相(xiang)鋼(gang)D1鑄錠為例，對凝(ning)固(gu)過(guo)程中Aso、Ase和(he)P8的(de)(de)變化趨勢進行計算。圖2-61描述(shu)了ΔAso(=Asa-Aso,0)和(he)AAse(=Ase-Ase,o)隨固(gu)相(xiang)質量(liang)分(fen)數(shu)的(de)(de)變化趨勢（Aso,0和(he)Asc,0分(fen)別為D1鑄錠凝(ning)固(gu)時Aso和(he)Ase的(de)(de)初始(shi)值(zhi)）。

  在(zai)平衡凝(ning)(ning)固和(he)(he)Scheil凝(ning)(ning)固過(guo)程(cheng)中(zhong)(zhong)，ΔAso的最小值(zhi)分(fen)(fen)別為(wei)－0.145和(he)(he)－0.397,與(yu)此(ci)(ci)相對(dui)(dui)應(ying)的ΔAse值(zhi)最大(da)，分(fen)(fen)別為(wei)0.68和(he)(he)0.92.在(zai)整(zheng)個凝(ning)(ning)固過(guo)程(cheng)中(zhong)(zhong)，由于ΔAse與(yu)ΔAso之和(he)(he)始終(zhong)大(da)于零，因(yin)而(er)(er)枝(zhi)晶間殘余液(ye)相中(zhong)(zhong)氮(dan)(dan)偏析對(dui)(dui)D1 鑄錠凝(ning)(ning)固過(guo)程(cheng)中(zhong)(zhong)氮(dan)(dan)氣(qi)泡形成的影(ying)響大(da)于氮(dan)(dan)溶解度，起主導作用。此(ci)(ci)外，在(zai)整(zheng)個凝(ning)(ning)固過(guo)程(cheng)中(zhong)(zhong)，P8變(bian)化趨勢如圖2-62所示，其變(bian)化規律與(yu)Young等。的研究結果一致，Pg的最大(da)值(zhi)Pg與(yu)Ase+Aso的最大(da)值(zhi)相對(dui)(dui)應(ying)，且在(zai)平衡凝(ning)(ning)固和(he)(he) Scheil 凝(ning)(ning)固過(guo)程(cheng)中(zhong)(zhong)分(fen)(fen)別為(wei)0.63MPa和(he)(he)0.62MPa.此(ci)(ci)外，可通過(guo)對(dui)(dui)比(bi)不同鑄錠中(zhong)(zhong)的探討凝(ning)(ning)固壓力、初始氮(dan)(dan)質量分(fen)(fen)數以及(ji)合金(jin)元素（鉻和(he)(he)錳）等對(dui)(dui)液(ye)相中(zhong)(zhong)氮(dan)(dan)氣(qi)泡形成的影(ying)響，進而(er)(er)明晰各因(yin)素對(dui)(dui)氮(dan)(dan)氣(qi)孔形成的影(ying)響規律。