一、氮(dan)氣孔的形(xing)成(cheng)機理

  在(zai)21.5Cr5Mn1.5Ni0.25N含氮(dan)(dan)(dan)雙相(xiang)(xiang)鋼(gang)凝(ning)(ning)固(gu)(gu)過程(cheng)(cheng)中，氮(dan)(dan)(dan)氣(qi)孔形(xing)成(cheng)(cheng)(cheng)(cheng)和凝(ning)(ning)固(gu)(gu)前沿處(chu)［％N]1iq隨距離變(bian)化的(de)(de)(de)規(gui)律如(ru)(ru)(ru)圖2-55所(suo)(suo)示(shi)。由(you)于(yu)糊(hu)狀(zhuang)(zhuang)區內大量枝(zhi)晶(jing)網狀(zhuang)(zhuang)結構的(de)(de)(de)形(xing)成(cheng)(cheng)(cheng)(cheng)，液(ye)(ye)(ye)相(xiang)(xiang)的(de)(de)(de)對(dui)流(liu)只存在(zai)于(yu)一(yi)次(ci)枝(zhi)晶(jing)尖端(duan)位置附近。且枝(zhi)晶(jing)間(jian)幾乎無液(ye)(ye)(ye)相(xiang)(xiang)的(de)(de)(de)流(liu)動。因此(ci)，枝(zhi)晶(jing)間(jian)殘余液(ye)(ye)(ye)相(xiang)(xiang)中的(de)(de)(de)氮(dan)(dan)(dan)傳(chuan)質(zhi)(zhi)主要依靠氮(dan)(dan)(dan)的(de)(de)(de)擴散(san)行(xing)為，且糊(hu)狀(zhuang)(zhuang)區內氮(dan)(dan)(dan)傳(chuan)質(zhi)(zhi)速(su)率非常小(xiao)。初始(shi)相(xiang)(xiang)貧(pin)氮(dan)(dan)(dan)鐵素體相(xiang)(xiang)8的(de)(de)(de)氮(dan)(dan)(dan)溶解度和糊(hu)狀(zhuang)(zhuang)區的(de)(de)(de)氮(dan)(dan)(dan)傳(chuan)質(zhi)(zhi)速(su)率較(jiao)低，導致(zhi)在(zai)貧(pin)氮(dan)(dan)(dan)鐵素體相(xiang)(xiang)枝(zhi)晶(jing)附近的(de)(de)(de)液(ye)(ye)(ye)相(xiang)(xiang)中出(chu)現氮(dan)(dan)(dan)富(fu)(fu)集，且［％N]iq迅速(su)增(zeng)大，如(ru)(ru)(ru)圖2-55(a)所(suo)(suo)示(shi)。根據Yang和 Leel70]、Svyazhin 等、Ridolfi 和 Tassal的(de)(de)(de)報道(dao)可(ke)知，當(dang)［％N]iq的(de)(de)(de)最大值超(chao)過氮(dan)(dan)(dan)氣(qi)泡(pao)形(xing)成(cheng)(cheng)(cheng)(cheng)的(de)(de)(de)臨界氮(dan)(dan)(dan)質(zhi)(zhi)量分數（［％N]pore)時，該區域有(you)氣(qi)泡(pao)形(xing)成(cheng)(cheng)(cheng)(cheng)的(de)(de)(de)可(ke)能(neng)性(xing)，如(ru)(ru)(ru)圖2-55(b)所(suo)(suo)示(shi)。在(zai)后續(xu)的(de)(de)(de)凝(ning)(ning)固(gu)(gu)過程(cheng)(cheng)中，隨著包晶(jing)反應的(de)(de)(de)進行(xing)，富(fu)(fu)氮(dan)(dan)(dan)奧氏體相(xiang)(xiang)γ以異質(zhi)(zhi)形(xing)核(he)的(de)(de)(de)方式在(zai)鐵素體相(xiang)(xiang)8枝(zhi)晶(jing)的(de)(de)(de)表面(mian)(mian)(mian)開(kai)始(shi)形(xing)核(he)長(chang)大，逐漸包裹鐵素體相(xiang)(xiang)枝(zhi)晶(jing)表面(mian)(mian)(mian)，并開(kai)始(shi)捕獲殘余液(ye)(ye)(ye)相(xiang)(xiang)中的(de)(de)(de)氮(dan)(dan)(dan)氣(qi)泡(pao)，對(dui)比圖2-51和圖2-56可(ke)知，此(ci)時枝(zhi)晶(jing)間(jian)殘余［％N]1ig的(de)(de)(de)增(zeng)長(chang)速(su)率減(jian)小(xiao)。對(dui)平衡凝(ning)(ning)固(gu)(gu)而言(yan)，殘余液(ye)(ye)(ye)相(xiang)(xiang)中氮(dan)(dan)(dan)氣(qi)泡(pao)形(xing)成(cheng)(cheng)(cheng)(cheng)以后，氮(dan)(dan)(dan)的(de)(de)(de)富(fu)(fu)集程(cheng)(cheng)度減(jian)弱，［％N]1iq增(zeng)長(chang)速(su)率的(de)(de)(de)減(jian)小(xiao)程(cheng)(cheng)度明顯；相(xiang)(xiang)比之下，Scheil凝(ning)(ning)固(gu)(gu)過程(cheng)(cheng)中，氮(dan)(dan)(dan)氣(qi)泡(pao)形(xing)成(cheng)(cheng)(cheng)(cheng)以后，殘余液(ye)(ye)(ye)相(xiang)(xiang)中氮(dan)(dan)(dan)富(fu)(fu)集狀(zhuang)(zhuang)態有(you)所(suo)(suo)緩(huan)解，但幅度很小(xiao)。隨著凝(ning)(ning)固(gu)(gu)界面(mian)(mian)(mian)的(de)(de)(de)進一(yi)步推(tui)移，被捕獲的(de)(de)(de)氮(dan)(dan)(dan)氣(qi)泡(pao)在(zai)奧氏體相(xiang)(xiang)表面(mian)(mian)(mian)開(kai)始(shi)長(chang)大，并沿凝(ning)(ning)固(gu)(gu)方向拉長(chang)，如(ru)(ru)(ru)圖2-55(c)所(suo)(suo)示(shi)。

  氮(dan)(dan)氣孔(kong)沿徑(jing)向生(sheng)長，生(sheng)長方向與凝固方向一(yi)致，那么(me)氮(dan)(dan)氣孔(kong)初始形成位置靠近鑄錠(ding)邊部，且氮(dan)(dan)氣泡初始位置邊緣全由(you)奧氏體(ti)相(xiang)(xiang)γ構成（圖2-57中(zhong)I區(qu)），與圖2-55描述(shu)相(xiang)(xiang)符。隨著(zhu)氮(dan)(dan)氣孔(kong)被拉長，鐵素(su)體(ti)相(xiang)(xiang)和奧氏體(ti)相(xiang)(xiang)以體(ti)積分數比約為0.92的關系交(jiao)替(ti)在氮(dan)(dan)氣泡周(zhou)圍(wei)形成，直(zhi)到氮(dan)(dan)氣孔(kong)閉合(he)。凝固結束后(hou)，氮(dan)(dan)氣孔(kong)的宏觀形貌類似于橢圓形，與Wei等的研究結果一(yi)致

二、氮(dan)微觀(guan)偏析對氮(dan)氣孔(kong)的(de)影響

  氮(dan)(dan)(dan)(dan)(dan)的(de)分配系數(shu)(shu)較(jiao)小，導致(zhi)液相(xiang)(xiang)向固(gu)相(xiang)(xiang)轉變(bian)的(de)過程(cheng)(cheng)中(zhong)(zhong)(zhong)，固(gu)相(xiang)(xiang)會將(jiang)多(duo)余(yu)的(de)氮(dan)(dan)(dan)(dan)(dan)轉移到殘(can)余(yu)液相(xiang)(xiang)中(zhong)(zhong)(zhong)，形(xing)(xing)成氮(dan)(dan)(dan)(dan)(dan)偏析(xi)。在(zai)氮(dan)(dan)(dan)(dan)(dan)偏析(xi)程(cheng)(cheng)度逐漸加重的(de)過程(cheng)(cheng)中(zhong)(zhong)(zhong)，當(dang)殘(can)余(yu)液相(xiang)(xiang)中(zhong)(zhong)(zhong)氮(dan)(dan)(dan)(dan)(dan)質量分數(shu)(shu)超過其飽和(he)度時，極易形(xing)(xing)成氮(dan)(dan)(dan)(dan)(dan)氣(qi)(qi)(qi)泡。隨著凝固(gu)的(de)進(jin)行，若氣(qi)(qi)(qi)泡無(wu)法上浮而被捕獲，凝固(gu)結(jie)束(shu)后就會在(zai)鑄錠內部(bu)(bu)形(xing)(xing)成氣(qi)(qi)(qi)孔(kong)(kong)(kong)。因此，凝固(gu)過程(cheng)(cheng)中(zhong)(zhong)(zhong)氮(dan)(dan)(dan)(dan)(dan)偏析(xi)和(he)溶解度對(dui)鑄錠中(zhong)(zhong)(zhong)最(zui)終氮(dan)(dan)(dan)(dan)(dan)氣(qi)(qi)(qi)孔(kong)(kong)(kong)的(de)形(xing)(xing)成有至關重要的(de)作(zuo)用(yong)。氮(dan)(dan)(dan)(dan)(dan)氣(qi)(qi)(qi)孔(kong)(kong)(kong)多(duo)數(shu)(shu)情況下與(yu)疏(shu)松縮(suo)孔(kong)(kong)(kong)共(gong)存(cun)，內壁凹凸不平呈現裂紋狀，且整個(ge)氣(qi)(qi)(qi)孔(kong)(kong)(kong)形(xing)(xing)狀不規則(ze)，如圖2-58所示。此類氣(qi)(qi)(qi)孔(kong)(kong)(kong)不僅與(yu)鋼液中(zhong)(zhong)(zhong)氣(qi)(qi)(qi)泡的(de)形(xing)(xing)成有關，還受(shou)凝固(gu)收縮(suo)等因素的(de)影響，且多(duo)數(shu)(shu)分布于鑄錠心部(bu)(bu)，尤其在(zai)中(zhong)(zhong)(zhong)心等軸晶區(qu)(qu)。這主要由于中(zhong)(zhong)(zhong)心等軸晶區(qu)(qu)內枝(zhi)晶生長較(jiao)發達，容易形(xing)(xing)成復雜的(de)網(wang)狀結(jie)構，從而將(jiang)液相(xiang)(xiang)分割成無(wu)數(shu)(shu)個(ge)獨立的(de)液相(xiang)(xiang)區(qu)(qu)域，當(dang)發生凝固(gu)收縮(suo)時，難(nan)以進(jin)行補(bu)縮(suo)，在(zai)形(xing)(xing)成疏(shu)松縮(suo)孔(kong)(kong)(kong)的(de)同時，局部(bu)(bu)鋼液靜壓力降低，促使(shi)氮(dan)(dan)(dan)(dan)(dan)從殘(can)余(yu)液相(xiang)(xiang)中(zhong)(zhong)(zhong)析(xi)出，從而形(xing)(xing)成了(le)氮(dan)(dan)(dan)(dan)(dan)氣(qi)(qi)(qi)孔(kong)(kong)(kong)和(he)疏(shu)松縮(suo)孔(kong)(kong)(kong)共(gong)存(cun)的(de)宏觀缺陷。

  平(ping)衡凝(ning)(ning)固(gu)時，19Cr14Mn0.9N含氮奧氏(shi)體(ti)不銹(xiu)鋼殘余液相(xiang)中氮偏析(xi)與體(ti)系氮溶解度(du)的(de)(de)差(cha)(cha)值如圖2-59所(suo)示。凝(ning)(ning)固(gu)初期鐵素(su)體(ti)阱（ferrite trap)的(de)(de)形(xing)成，導致氮溶解度(du)的(de)(de)降低，進而使氮偏析(xi)與體(ti)系氮溶解度(du)差(cha)(cha)值呈現出略微增(zeng)大(da)的(de)(de)趨(qu)(qu)勢(shi)。但在(zai)后續凝(ning)(ning)固(gu)過程中，隨(sui)著鐵素(su)體(ti)阱的(de)(de)消(xiao)失(shi)以及富氮奧氏(shi)體(ti)相(xiang)的(de)(de)不斷形(xing)成，差(cha)(cha)值減小；在(zai)整個凝(ning)(ning)固(gu)過程中差(cha)(cha)值始終較(jiao)小，且變化幅度(du)較(jiao)窄(zhai)。對于19Cr14Mn0.9N 含氮奧氏(shi)體(ti)不銹(xiu)鋼，液相(xiang)中氮氣泡的(de)(de)形(xing)成趨(qu)(qu)勢(shi)較(jiao)小，難以在(zai)鑄錠內(nei)形(xing)成獨立內(nei)壁(bi)光滑的(de)(de)規則氮氣孔。

  此(ci)外(wai)，目前有(you)人(ren)對(dui)奧氏(shi)體(ti)(ti)鋼(gang)(gang)凝固(gu)過程(cheng)(cheng)中(zhong)(zhong)氮(dan)(dan)(dan)(dan)(dan)氣孔(kong)(kong)的(de)形(xing)成進(jin)行了大量(liang)研(yan)(yan)究(jiu)(jiu)，如Yang和(he)(he)Leel901研(yan)(yan)究(jiu)(jiu)了奧氏(shi)體(ti)(ti)鋼(gang)(gang)16Cr3NixMn(x=9和(he)(he)11)凝固(gu)過程(cheng)(cheng)中(zhong)(zhong)壓(ya)力和(he)(he)初(chu)始氮(dan)(dan)(dan)(dan)(dan)質量(liang)分數(shu)等因素(su)對(dui)氮(dan)(dan)(dan)(dan)(dan)氣孔(kong)(kong)形(xing)成的(de)影(ying)(ying)響規(gui)律，并建立了相應的(de)預測模型(xing)。Ridolfi和(he)(he)Tassal[84]分析了氮(dan)(dan)(dan)(dan)(dan)偏析、合金元素(su)、冷(leng)卻速率以及枝晶間(jian)距對(dui)奧氏(shi)體(ti)(ti)鋼(gang)(gang)中(zhong)(zhong)氮(dan)(dan)(dan)(dan)(dan)氣孔(kong)(kong)的(de)影(ying)(ying)響規(gui)律，并揭示(shi)了奧氏(shi)體(ti)(ti)鋼(gang)(gang)中(zhong)(zhong)氮(dan)(dan)(dan)(dan)(dan)氣孔(kong)(kong)形(xing)成機理(li)。然而(er)，目前對(dui)于雙(shuang)相鋼(gang)(gang)中(zhong)(zhong)氮(dan)(dan)(dan)(dan)(dan)氣孔(kong)(kong)形(xing)成的(de)研(yan)(yan)究(jiu)(jiu)較(jiao)(jiao)少，且(qie)主(zhu)要集(ji)中(zhong)(zhong)在(zai)合金元素(su)、鑄造方(fang)式、冷(leng)卻速率等因素(su)對(dui)氮(dan)(dan)(dan)(dan)(dan)氣孔(kong)(kong)影(ying)(ying)響規(gui)律的(de)研(yan)(yan)究(jiu)(jiu)，鮮有(you)對(dui)雙(shuang)相鋼(gang)(gang)中(zhong)(zhong)氮(dan)(dan)(dan)(dan)(dan)氣孔(kong)(kong)形(xing)成機理(li)的(de)報道。以21.5Cr5Mn1.5Ni0.25N含(han)氮(dan)(dan)(dan)(dan)(dan)雙(shuang)相鋼(gang)(gang)為例，氮(dan)(dan)(dan)(dan)(dan)偏析與溶解(jie)度的(de)差值在(zai)整個凝固(gu)過程(cheng)(cheng)中(zhong)(zhong)的(de)變化趨勢(shi)，如圖2-59所示(shi)。隨著凝固(gu)的(de)進(jin)行，氮(dan)(dan)(dan)(dan)(dan)偏析始終大于氮(dan)(dan)(dan)(dan)(dan)溶解(jie)度，且(qie)差值呈(cheng)現(xian)出快速增大的(de)趨勢(shi)。因此(ci)，在(zai)21.5Cr5Mn1.5Ni0.25N 含(han)氮(dan)(dan)(dan)(dan)(dan)雙(shuang)相鋼(gang)(gang)凝固(gu)過程(cheng)(cheng)中(zhong)(zhong)，氮(dan)(dan)(dan)(dan)(dan)偏析嚴(yan)重，殘余液相內氮(dan)(dan)(dan)(dan)(dan)氣泡形(xing)成趨勢(shi)較(jiao)(jiao)大，明顯高于19Cr14Mn0.9N含(han)氮(dan)(dan)(dan)(dan)(dan)奧氏(shi)體(ti)(ti)不(bu)銹鋼(gang)(gang)。

  氮(dan)氣(qi)泡形(xing)成和長大(da)具有(you)重要的(de)作用（圖2-60).其中，σ為氣(qi)液(ye)界(jie)面(mian)的(de)表(biao)面(mian)張力，r為氣(qi)泡半徑。結合經典形(xing)核理論，氮(dan)氣(qi)泡在鋼(gang)液(ye)中穩定存在的(de)必要條(tiao)件為氣(qi)泡內壓力大(da)于作用于氣(qi)泡的(de)所有(you)壓力之(zhi)和，即

  式中(zhong)(zhong)，Aso由凝(ning)固過程中(zhong)(zhong)除氮(dan)(dan)(dan)以(yi)外其(qi)他合金元素的微觀(guan)偏(pian)析(xi)進行計算，其(qi)值隨著枝晶間殘余液(ye)相(xiang)(xiang)中(zhong)(zhong)氮(dan)(dan)(dan)溶解度(du)的增加而減小，表征(zheng)(zheng)了枝晶間殘余液(ye)相(xiang)(xiang)中(zhong)(zhong)氮(dan)(dan)(dan)溶解度(du)對(dui)氮(dan)(dan)(dan)氣泡形成的影響程度(du)；Ase表征(zheng)(zheng)了枝晶間氮(dan)(dan)(dan)偏(pian)析(xi)對(dui)氮(dan)(dan)(dan)氣泡形成的影響程度(du)，可由凝(ning)固過程中(zhong)(zhong)枝晶間殘余液(ye)相(xiang)(xiang)中(zhong)(zhong)氮(dan)(dan)(dan)偏(pian)析(xi)計算獲得，其(qi)值隨著氮(dan)(dan)(dan)偏(pian)析(xi)的增大而增大。此外，用于計算Aso和Ase時所需(xu)的合金元素偏(pian)析(xi)均(jun)由鋼凝(ning)固相(xiang)(xiang)變所致。

  氮(dan)氣(qi)(qi)(qi)泡(pao)的形(xing)核和長(chang)大過程(cheng)(cheng)復雜，且(qie)影(ying)響(xiang)因(yin)素(su)眾多，包括凝(ning)(ning)(ning)(ning)固(gu)(gu)收縮、冶(ye)煉環境以及坩堝材質等(deng)。因(yin)此(ci)(ci)，很難采用Pg值精確(que)預測凝(ning)(ning)(ning)(ning)固(gu)(gu)過程(cheng)(cheng)中氮(dan)氣(qi)(qi)(qi)泡(pao)的形(xing)成(cheng)和長(chang)大。然而(er)基于Yang等(deng)的實驗研究［70,77],在評估(gu)凝(ning)(ning)(ning)(ning)固(gu)(gu)壓力、合金成(cheng)分等(deng)因(yin)素(su)對氮(dan)氣(qi)(qi)(qi)泡(pao)形(xing)成(cheng)的影(ying)響(xiang)程(cheng)(cheng)度時，Pg起關鍵(jian)作(zuo)用。實際凝(ning)(ning)(ning)(ning)固(gu)(gu)過程(cheng)(cheng)介(jie)于平衡凝(ning)(ning)(ning)(ning)固(gu)(gu)（固(gu)(gu)／液(ye)相(xiang)中溶質完(wan)全擴(kuo)散）和Scheil凝(ning)(ning)(ning)(ning)固(gu)(gu)（固(gu)(gu)相(xiang)無溶質擴(kuo)散，液(ye)相(xiang)中完(wan)全擴(kuo)散）之間70].因(yin)此(ci)(ci)，可分別計算平衡凝(ning)(ning)(ning)(ning)固(gu)(gu)和Scheil凝(ning)(ning)(ning)(ning)固(gu)(gu)過程(cheng)(cheng)中的Aso、Ase和Pg,闡明實際凝(ning)(ning)(ning)(ning)固(gu)(gu)過程(cheng)(cheng)中壓力等(deng)因(yin)素(su)對氮(dan)氣(qi)(qi)(qi)泡(pao)形(xing)成(cheng)的影(ying)響(xiang)規(gui)律。

  現以21.5Cr5Mn1.5Ni0.25N含氮雙相(xiang)鋼D1鑄(zhu)錠(ding)為例，對(dui)凝固過(guo)程(cheng)中(zhong)Aso、Ase和P8的變化趨勢進行計算。圖2-61描(miao)述(shu)了(le)ΔAso(=Asa-Aso,0)和AAse(=Ase-Ase,o)隨(sui)固相(xiang)質量分數的變化趨勢（Aso,0和Asc,0分別(bie)為D1鑄(zhu)錠(ding)凝固時Aso和Ase的初(chu)始值）。

  在平衡凝(ning)固(gu)和(he)Scheil凝(ning)固(gu)過(guo)程(cheng)中(zhong)，ΔAso的(de)(de)最(zui)小值(zhi)分(fen)別(bie)為(wei)－0.145和(he)－0.397,與(yu)(yu)此相對(dui)(dui)應(ying)的(de)(de)ΔAse值(zhi)最(zui)大(da)，分(fen)別(bie)為(wei)0.68和(he)0.92.在整(zheng)個凝(ning)固(gu)過(guo)程(cheng)中(zhong)，由(you)于ΔAse與(yu)(yu)ΔAso之和(he)始(shi)終大(da)于零(ling)，因而枝晶間殘余液相中(zhong)氮(dan)偏(pian)析(xi)對(dui)(dui)D1 鑄錠凝(ning)固(gu)過(guo)程(cheng)中(zhong)氮(dan)氣(qi)泡形(xing)成的(de)(de)影響大(da)于氮(dan)溶(rong)解度，起主導作(zuo)用。此外(wai)，在整(zheng)個凝(ning)固(gu)過(guo)程(cheng)中(zhong)，P8變(bian)化(hua)趨勢如圖2-62所示，其變(bian)化(hua)規(gui)律(lv)與(yu)(yu)Young等。的(de)(de)研(yan)究結果一(yi)致，Pg的(de)(de)最(zui)大(da)值(zhi)Pg與(yu)(yu)Ase+Aso的(de)(de)最(zui)大(da)值(zhi)相對(dui)(dui)應(ying)，且在平衡凝(ning)固(gu)和(he) Scheil 凝(ning)固(gu)過(guo)程(cheng)中(zhong)分(fen)別(bie)為(wei)0.63MPa和(he)0.62MPa.此外(wai)，可(ke)通過(guo)對(dui)(dui)比不同鑄錠中(zhong)的(de)(de)探討(tao)凝(ning)固(gu)壓力、初始(shi)氮(dan)質(zhi)量分(fen)數以及合金(jin)元素(su)(su)（鉻(ge)和(he)錳(meng)）等對(dui)(dui)液相中(zhong)氮(dan)氣(qi)泡形(xing)成的(de)(de)影響，進而明(ming)晰各因素(su)(su)對(dui)(dui)氮(dan)氣(qi)孔(kong)形(xing)成的(de)(de)影響規(gui)律(lv)。