一、氮氣孔的形成(cheng)機理


  在(zai)(zai)21.5Cr5Mn1.5Ni0.25N含氮(dan)(dan)雙相(xiang)(xiang)鋼凝固(gu)(gu)過(guo)(guo)程(cheng)(cheng)中(zhong),氮(dan)(dan)氣(qi)孔形(xing)(xing)成和(he)(he)凝固(gu)(gu)前(qian)沿處(chu)[%N]1iq隨(sui)距離變化的(de)(de)規律如圖(tu)2-55所(suo)(suo)示。由于(yu)糊狀(zhuang)區(qu)內大(da)(da)量(liang)枝(zhi)(zhi)晶網狀(zhuang)結構(gou)的(de)(de)形(xing)(xing)成,液(ye)相(xiang)(xiang)的(de)(de)對流只存在(zai)(zai)于(yu)一次(ci)枝(zhi)(zhi)晶尖端位置附近。且(qie)枝(zhi)(zhi)晶間(jian)幾乎無液(ye)相(xiang)(xiang)的(de)(de)流動。因此,枝(zhi)(zhi)晶間(jian)殘(can)余(yu)(yu)(yu)液(ye)相(xiang)(xiang)中(zhong)的(de)(de)氮(dan)(dan)傳質(zhi)主要依(yi)靠氮(dan)(dan)的(de)(de)擴(kuo)散行為,且(qie)糊狀(zhuang)區(qu)內氮(dan)(dan)傳質(zhi)速(su)(su)率非(fei)常小(xiao)。初始相(xiang)(xiang)貧氮(dan)(dan)鐵(tie)(tie)素(su)(su)(su)體(ti)(ti)相(xiang)(xiang)8的(de)(de)氮(dan)(dan)溶解(jie)度和(he)(he)糊狀(zhuang)區(qu)的(de)(de)氮(dan)(dan)傳質(zhi)速(su)(su)率較(jiao)低,導致在(zai)(zai)貧氮(dan)(dan)鐵(tie)(tie)素(su)(su)(su)體(ti)(ti)相(xiang)(xiang)枝(zhi)(zhi)晶附近的(de)(de)液(ye)相(xiang)(xiang)中(zhong)出現氮(dan)(dan)富集(ji),且(qie)[%N]iq迅速(su)(su)增大(da)(da),如圖(tu)2-55(a)所(suo)(suo)示。根據(ju)Yang和(he)(he) Leel70]、Svyazhin 等、Ridolfi 和(he)(he) Tassal的(de)(de)報(bao)道可知,當[%N]iq的(de)(de)最大(da)(da)值超過(guo)(guo)氮(dan)(dan)氣(qi)泡形(xing)(xing)成的(de)(de)臨界氮(dan)(dan)質(zhi)量(liang)分(fen)數([%N]pore)時,該區(qu)域有氣(qi)泡形(xing)(xing)成的(de)(de)可能性(xing),如圖(tu)2-55(b)所(suo)(suo)示。在(zai)(zai)后(hou)續的(de)(de)凝固(gu)(gu)過(guo)(guo)程(cheng)(cheng)中(zhong),隨(sui)著包(bao)晶反應的(de)(de)進行,富氮(dan)(dan)奧(ao)氏體(ti)(ti)相(xiang)(xiang)γ以異(yi)質(zhi)形(xing)(xing)核的(de)(de)方式(shi)在(zai)(zai)鐵(tie)(tie)素(su)(su)(su)體(ti)(ti)相(xiang)(xiang)8枝(zhi)(zhi)晶的(de)(de)表面開始形(xing)(xing)核長大(da)(da),逐漸包(bao)裹鐵(tie)(tie)素(su)(su)(su)體(ti)(ti)相(xiang)(xiang)枝(zhi)(zhi)晶表面,并開始捕(bu)(bu)獲(huo)殘(can)余(yu)(yu)(yu)液(ye)相(xiang)(xiang)中(zhong)的(de)(de)氮(dan)(dan)氣(qi)泡,對比圖(tu)2-51和(he)(he)圖(tu)2-56可知,此時枝(zhi)(zhi)晶間(jian)殘(can)余(yu)(yu)(yu)[%N]1ig的(de)(de)增長速(su)(su)率減小(xiao)。對平衡凝固(gu)(gu)而言,殘(can)余(yu)(yu)(yu)液(ye)相(xiang)(xiang)中(zhong)氮(dan)(dan)氣(qi)泡形(xing)(xing)成以后(hou),氮(dan)(dan)的(de)(de)富集(ji)程(cheng)(cheng)度減弱(ruo),[%N]1iq增長速(su)(su)率的(de)(de)減小(xiao)程(cheng)(cheng)度明顯;相(xiang)(xiang)比之下,Scheil凝固(gu)(gu)過(guo)(guo)程(cheng)(cheng)中(zhong),氮(dan)(dan)氣(qi)泡形(xing)(xing)成以后(hou),殘(can)余(yu)(yu)(yu)液(ye)相(xiang)(xiang)中(zhong)氮(dan)(dan)富集(ji)狀(zhuang)態有所(suo)(suo)緩解(jie),但幅度很小(xiao)。隨(sui)著凝固(gu)(gu)界面的(de)(de)進一步推移,被捕(bu)(bu)獲(huo)的(de)(de)氮(dan)(dan)氣(qi)泡在(zai)(zai)奧(ao)氏體(ti)(ti)相(xiang)(xiang)表面開始長大(da)(da),并沿凝固(gu)(gu)方向拉長,如圖(tu)2-55(c)所(suo)(suo)示。



  氮氣(qi)(qi)(qi)孔(kong)沿徑向(xiang)生長,生長方向(xiang)與凝(ning)固方向(xiang)一(yi)致(zhi),那么氮氣(qi)(qi)(qi)孔(kong)初始形(xing)(xing)成(cheng)位置(zhi)靠近(jin)鑄錠邊(bian)部,且氮氣(qi)(qi)(qi)泡初始位置(zhi)邊(bian)緣全由奧(ao)氏體(ti)相(xiang)γ構成(cheng)(圖(tu)2-57中(zhong)I區),與圖(tu)2-55描述(shu)相(xiang)符。隨著氮氣(qi)(qi)(qi)孔(kong)被(bei)拉長,鐵(tie)素體(ti)相(xiang)和(he)奧(ao)氏體(ti)相(xiang)以體(ti)積分數比約(yue)為(wei)0.92的(de)關(guan)系交(jiao)替在氮氣(qi)(qi)(qi)泡周圍形(xing)(xing)成(cheng),直到氮氣(qi)(qi)(qi)孔(kong)閉合。凝(ning)固結束后,氮氣(qi)(qi)(qi)孔(kong)的(de)宏觀(guan)形(xing)(xing)貌類似于(yu)橢圓形(xing)(xing),與Wei等的(de)研究結果一(yi)致(zhi)



二、氮(dan)微觀(guan)偏析(xi)對(dui)氮(dan)氣孔的(de)影響


  氮(dan)的(de)(de)(de)(de)分配系數(shu)(shu)較小,導致液相(xiang)向(xiang)固(gu)(gu)相(xiang)轉變的(de)(de)(de)(de)過程中(zhong)(zhong),固(gu)(gu)相(xiang)會將(jiang)多余(yu)的(de)(de)(de)(de)氮(dan)轉移到殘(can)余(yu)液相(xiang)中(zhong)(zhong),形(xing)成(cheng)氮(dan)偏析。在氮(dan)偏析程度(du)逐(zhu)漸加重(zhong)的(de)(de)(de)(de)過程中(zhong)(zhong),當(dang)殘(can)余(yu)液相(xiang)中(zhong)(zhong)氮(dan)質(zhi)量分數(shu)(shu)超過其(qi)飽和(he)度(du)時(shi),極易形(xing)成(cheng)氮(dan)氣(qi)(qi)(qi)泡。隨著凝固(gu)(gu)的(de)(de)(de)(de)進行(xing),若氣(qi)(qi)(qi)泡無(wu)(wu)法上浮(fu)而被捕獲,凝固(gu)(gu)結束(shu)后(hou)就會在鑄錠(ding)內(nei)部(bu)形(xing)成(cheng)氣(qi)(qi)(qi)孔(kong)(kong)。因(yin)此,凝固(gu)(gu)過程中(zhong)(zhong)氮(dan)偏析和(he)溶解度(du)對鑄錠(ding)中(zhong)(zhong)最終氮(dan)氣(qi)(qi)(qi)孔(kong)(kong)的(de)(de)(de)(de)形(xing)成(cheng)有(you)至關重(zhong)要的(de)(de)(de)(de)作用(yong)。氮(dan)氣(qi)(qi)(qi)孔(kong)(kong)多數(shu)(shu)情況下與疏(shu)(shu)松(song)縮(suo)(suo)孔(kong)(kong)共(gong)存,內(nei)壁凹凸(tu)不平(ping)呈現裂紋狀(zhuang),且整個氣(qi)(qi)(qi)孔(kong)(kong)形(xing)狀(zhuang)不規則,如圖2-58所(suo)示。此類氣(qi)(qi)(qi)孔(kong)(kong)不僅與鋼液中(zhong)(zhong)氣(qi)(qi)(qi)泡的(de)(de)(de)(de)形(xing)成(cheng)有(you)關,還受(shou)凝固(gu)(gu)收(shou)縮(suo)(suo)等因(yin)素的(de)(de)(de)(de)影響,且多數(shu)(shu)分布于鑄錠(ding)心(xin)部(bu),尤其(qi)在中(zhong)(zhong)心(xin)等軸晶區(qu)。這主要由(you)于中(zhong)(zhong)心(xin)等軸晶區(qu)內(nei)枝晶生(sheng)長較發(fa)(fa)達,容易形(xing)成(cheng)復雜(za)的(de)(de)(de)(de)網(wang)狀(zhuang)結構,從而將(jiang)液相(xiang)分割成(cheng)無(wu)(wu)數(shu)(shu)個獨立的(de)(de)(de)(de)液相(xiang)區(qu)域,當(dang)發(fa)(fa)生(sheng)凝固(gu)(gu)收(shou)縮(suo)(suo)時(shi),難以進行(xing)補縮(suo)(suo),在形(xing)成(cheng)疏(shu)(shu)松(song)縮(suo)(suo)孔(kong)(kong)的(de)(de)(de)(de)同時(shi),局(ju)部(bu)鋼液靜壓力降低,促使氮(dan)從殘(can)余(yu)液相(xiang)中(zhong)(zhong)析出(chu),從而形(xing)成(cheng)了氮(dan)氣(qi)(qi)(qi)孔(kong)(kong)和(he)疏(shu)(shu)松(song)縮(suo)(suo)孔(kong)(kong)共(gong)存的(de)(de)(de)(de)宏觀(guan)缺陷。


圖 58.jpg



  平衡(heng)凝固(gu)(gu)時,19Cr14Mn0.9N含氮(dan)(dan)(dan)奧氏體(ti)(ti)不銹鋼殘余液(ye)相(xiang)(xiang)中(zhong)氮(dan)(dan)(dan)偏(pian)(pian)析與體(ti)(ti)系氮(dan)(dan)(dan)溶(rong)(rong)解度(du)的(de)(de)(de)差(cha)(cha)值(zhi)(zhi)如圖2-59所(suo)示。凝固(gu)(gu)初期(qi)鐵(tie)素(su)體(ti)(ti)阱(ferrite trap)的(de)(de)(de)形(xing)成,導致氮(dan)(dan)(dan)溶(rong)(rong)解度(du)的(de)(de)(de)降低,進而(er)使氮(dan)(dan)(dan)偏(pian)(pian)析與體(ti)(ti)系氮(dan)(dan)(dan)溶(rong)(rong)解度(du)差(cha)(cha)值(zhi)(zhi)呈(cheng)現出(chu)略微增(zeng)大的(de)(de)(de)趨勢。但在(zai)后(hou)續凝固(gu)(gu)過程中(zhong),隨著鐵(tie)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(xiang)(xiang)的(de)(de)(de)不斷(duan)形(xing)成,差(cha)(cha)值(zhi)(zhi)減(jian)小;在(zai)整個(ge)凝固(gu)(gu)過程中(zhong)差(cha)(cha)值(zhi)(zhi)始終較(jiao)(jiao)小,且變(bian)化幅度(du)較(jiao)(jiao)窄。對于19Cr14Mn0.9N 含氮(dan)(dan)(dan)奧氏體(ti)(ti)不銹鋼,液(ye)相(xiang)(xiang)中(zhong)氮(dan)(dan)(dan)氣泡的(de)(de)(de)形(xing)成趨勢較(jiao)(jiao)小,難以在(zai)鑄錠內形(xing)成獨(du)立(li)內壁(bi)光滑的(de)(de)(de)規則氮(dan)(dan)(dan)氣孔。


  此外,目前(qian)有(you)(you)人(ren)對(dui)奧(ao)氏(shi)體(ti)鋼凝固(gu)過程中(zhong)(zhong)氮(dan)(dan)(dan)氣(qi)孔(kong)(kong)的(de)(de)(de)形成(cheng)進(jin)行了(le)大量研(yan)究,如Yang和Leel901研(yan)究了(le)奧(ao)氏(shi)體(ti)鋼16Cr3NixMn(x=9和11)凝固(gu)過程中(zhong)(zhong)壓力和初始(shi)氮(dan)(dan)(dan)質量分(fen)數(shu)等(deng)因素(su)(su)對(dui)氮(dan)(dan)(dan)氣(qi)孔(kong)(kong)形成(cheng)的(de)(de)(de)影(ying)響(xiang)規(gui)律,并(bing)建(jian)立了(le)相(xiang)應的(de)(de)(de)預(yu)測(ce)模(mo)型。Ridolfi和Tassal[84]分(fen)析(xi)(xi)了(le)氮(dan)(dan)(dan)偏析(xi)(xi)、合金(jin)元素(su)(su)、冷(leng)卻(que)速率(lv)以及枝晶間距對(dui)奧(ao)氏(shi)體(ti)鋼中(zhong)(zhong)氮(dan)(dan)(dan)氣(qi)孔(kong)(kong)的(de)(de)(de)影(ying)響(xiang)規(gui)律,并(bing)揭示(shi)了(le)奧(ao)氏(shi)體(ti)鋼中(zhong)(zhong)氮(dan)(dan)(dan)氣(qi)孔(kong)(kong)形成(cheng)機理。然而(er),目前(qian)對(dui)于雙相(xiang)鋼中(zhong)(zhong)氮(dan)(dan)(dan)氣(qi)孔(kong)(kong)形成(cheng)的(de)(de)(de)研(yan)究較少,且主要集(ji)中(zhong)(zhong)在合金(jin)元素(su)(su)、鑄造方(fang)式、冷(leng)卻(que)速率(lv)等(deng)因素(su)(su)對(dui)氮(dan)(dan)(dan)氣(qi)孔(kong)(kong)影(ying)響(xiang)規(gui)律的(de)(de)(de)研(yan)究,鮮有(you)(you)對(dui)雙相(xiang)鋼中(zhong)(zhong)氮(dan)(dan)(dan)氣(qi)孔(kong)(kong)形成(cheng)機理的(de)(de)(de)報(bao)道。以21.5Cr5Mn1.5Ni0.25N含氮(dan)(dan)(dan)雙相(xiang)鋼為例,氮(dan)(dan)(dan)偏析(xi)(xi)與溶解(jie)度的(de)(de)(de)差值(zhi)在整個凝固(gu)過程中(zhong)(zhong)的(de)(de)(de)變化趨勢,如圖2-59所示(shi)。隨著凝固(gu)的(de)(de)(de)進(jin)行,氮(dan)(dan)(dan)偏析(xi)(xi)始(shi)終(zhong)大于氮(dan)(dan)(dan)溶解(jie)度,且差值(zhi)呈現出快速增(zeng)大的(de)(de)(de)趨勢。因此,在21.5Cr5Mn1.5Ni0.25N 含氮(dan)(dan)(dan)雙相(xiang)鋼凝固(gu)過程中(zhong)(zhong),氮(dan)(dan)(dan)偏析(xi)(xi)嚴重,殘余液相(xiang)內氮(dan)(dan)(dan)氣(qi)泡形成(cheng)趨勢較大,明(ming)顯高于19Cr14Mn0.9N含氮(dan)(dan)(dan)奧(ao)氏(shi)體(ti)不銹鋼。


圖 59.jpg

  氮氣(qi)(qi)泡形成(cheng)和(he)長大具有重要的作用(圖2-60).其中,σ為(wei)氣(qi)(qi)液界面的表面張(zhang)力(li),r為(wei)氣(qi)(qi)泡半(ban)徑。結(jie)合經典(dian)形核理論(lun),氮氣(qi)(qi)泡在鋼液中穩定(ding)存在的必(bi)要條(tiao)件為(wei)氣(qi)(qi)泡內壓力(li)大于作用于氣(qi)(qi)泡的所有壓力(li)之(zhi)和(he),即


圖 60.jpg


  式(shi)中(zhong)(zhong),Aso由(you)凝固(gu)過(guo)程(cheng)中(zhong)(zhong)除氮(dan)(dan)以外其(qi)(qi)他合金元素(su)的(de)(de)(de)(de)微觀偏(pian)析(xi)進行計算,其(qi)(qi)值(zhi)隨著枝(zhi)晶(jing)間殘(can)余(yu)(yu)液相中(zhong)(zhong)氮(dan)(dan)溶(rong)解(jie)度(du)的(de)(de)(de)(de)增(zeng)加而(er)減小,表(biao)(biao)征了枝(zhi)晶(jing)間殘(can)余(yu)(yu)液相中(zhong)(zhong)氮(dan)(dan)溶(rong)解(jie)度(du)對氮(dan)(dan)氣泡形成(cheng)的(de)(de)(de)(de)影(ying)響程(cheng)度(du);Ase表(biao)(biao)征了枝(zhi)晶(jing)間氮(dan)(dan)偏(pian)析(xi)對氮(dan)(dan)氣泡形成(cheng)的(de)(de)(de)(de)影(ying)響程(cheng)度(du),可由(you)凝固(gu)過(guo)程(cheng)中(zhong)(zhong)枝(zhi)晶(jing)間殘(can)余(yu)(yu)液相中(zhong)(zhong)氮(dan)(dan)偏(pian)析(xi)計算獲得,其(qi)(qi)值(zhi)隨著氮(dan)(dan)偏(pian)析(xi)的(de)(de)(de)(de)增(zeng)大而(er)增(zeng)大。此外,用(yong)于(yu)計算Aso和Ase時(shi)所需的(de)(de)(de)(de)合金元素(su)偏(pian)析(xi)均由(you)鋼凝固(gu)相變所致(zhi)。


  氮(dan)(dan)氣泡(pao)的形核和(he)長(chang)大過(guo)程(cheng)復雜,且影(ying)響(xiang)因(yin)(yin)(yin)素眾多,包括凝(ning)固(gu)(gu)收縮(suo)、冶煉環(huan)境以及(ji)坩堝材質等。因(yin)(yin)(yin)此,很(hen)難(nan)采(cai)用Pg值精確(que)預測凝(ning)固(gu)(gu)過(guo)程(cheng)中氮(dan)(dan)氣泡(pao)的形成和(he)長(chang)大。然而基于Yang等的實(shi)驗研究(jiu)[70,77],在(zai)評(ping)估凝(ning)固(gu)(gu)壓力(li)、合金成分(fen)等因(yin)(yin)(yin)素對氮(dan)(dan)氣泡(pao)形成的影(ying)響(xiang)程(cheng)度(du)時,Pg起(qi)關鍵作用。實(shi)際(ji)凝(ning)固(gu)(gu)過(guo)程(cheng)介于平衡凝(ning)固(gu)(gu)(固(gu)(gu)/液(ye)相(xiang)中溶質完全擴(kuo)散)和(he)Scheil凝(ning)固(gu)(gu)(固(gu)(gu)相(xiang)無(wu)溶質擴(kuo)散,液(ye)相(xiang)中完全擴(kuo)散)之間70].因(yin)(yin)(yin)此,可(ke)分(fen)別計(ji)算平衡凝(ning)固(gu)(gu)和(he)Scheil凝(ning)固(gu)(gu)過(guo)程(cheng)中的Aso、Ase和(he)Pg,闡明實(shi)際(ji)凝(ning)固(gu)(gu)過(guo)程(cheng)中壓力(li)等因(yin)(yin)(yin)素對氮(dan)(dan)氣泡(pao)形成的影(ying)響(xiang)規(gui)律。


  現以21.5Cr5Mn1.5Ni0.25N含(han)氮雙相(xiang)(xiang)鋼D1鑄(zhu)錠(ding)為例(li),對凝固過程中Aso、Ase和(he)P8的變化(hua)趨勢進行(xing)計算。圖2-61描述了ΔAso(=Asa-Aso,0)和(he)AAse(=Ase-Ase,o)隨固相(xiang)(xiang)質量分數的變化(hua)趨勢(Aso,0和(he)Asc,0分別(bie)為D1鑄(zhu)錠(ding)凝固時Aso和(he)Ase的初始值)。


  在(zai)(zai)平衡凝固(gu)和(he)(he)(he)Scheil凝固(gu)過(guo)程(cheng)中(zhong)(zhong),ΔAso的最小(xiao)值分(fen)別(bie)為(wei)-0.145和(he)(he)(he)-0.397,與(yu)此相(xiang)對(dui)應的ΔAse值最大(da)(da),分(fen)別(bie)為(wei)0.68和(he)(he)(he)0.92.在(zai)(zai)整(zheng)個凝固(gu)過(guo)程(cheng)中(zhong)(zhong),由(you)于ΔAse與(yu)ΔAso之(zhi)和(he)(he)(he)始終(zhong)大(da)(da)于零,因而枝晶間殘余(yu)液(ye)相(xiang)中(zhong)(zhong)氮(dan)(dan)偏析對(dui)D1 鑄錠凝固(gu)過(guo)程(cheng)中(zhong)(zhong)氮(dan)(dan)氣(qi)(qi)泡(pao)形成(cheng)的影(ying)響(xiang)(xiang)大(da)(da)于氮(dan)(dan)溶解度,起主(zhu)導作用。此外(wai),在(zai)(zai)整(zheng)個凝固(gu)過(guo)程(cheng)中(zhong)(zhong),P8變化(hua)趨勢如圖2-62所示,其變化(hua)規律與(yu)Young等。的研究結果一致,Pg的最大(da)(da)值Pg與(yu)Ase+Aso的最大(da)(da)值相(xiang)對(dui)應,且在(zai)(zai)平衡凝固(gu)和(he)(he)(he) Scheil 凝固(gu)過(guo)程(cheng)中(zhong)(zhong)分(fen)別(bie)為(wei)0.63MPa和(he)(he)(he)0.62MPa.此外(wai),可通過(guo)對(dui)比不同鑄錠中(zhong)(zhong)的探(tan)討(tao)凝固(gu)壓力、初始氮(dan)(dan)質量分(fen)數以(yi)及(ji)合金元素(鉻(ge)和(he)(he)(he)錳(meng))等對(dui)液(ye)相(xiang)中(zhong)(zhong)氮(dan)(dan)氣(qi)(qi)泡(pao)形成(cheng)的影(ying)響(xiang)(xiang),進而明晰各因素對(dui)氮(dan)(dan)氣(qi)(qi)孔形成(cheng)的影(ying)響(xiang)(xiang)規律。


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