發(fā)布時間：2018-06-06 23:58

  本文選題：超級貝氏體 + 相變動力學(xué)　； 參考：《北京科技大學(xué)》2016年博士論文

【摘要】：將成分為0.98C-1.59Si-1.94Mn-1.33Cr-0.30Mo-0.02Ni-0.11V的合金,在200-300℃等溫數(shù)十小時可以獲得超薄貝氏體鐵素體片與殘留奧氏體片層疊結(jié)構(gòu)(Slim Bainitic ferrite-Austenite,SBA)的超細(xì)貝氏體組織。SBA鋼以其超細(xì)(片厚100nm)的顯微結(jié)構(gòu)、低廉的原料成本、簡單易行的制備工藝和超高強(qiáng)韌性能(抗拉強(qiáng)度2.5GPa,硬度600HV,韌性30-40MPa·m1/2)受到學(xué)術(shù)界和工業(yè)界的廣泛關(guān)注。然而,數(shù)十小時的相變時間限制了其工業(yè)應(yīng)用。本文從合金優(yōu)化和預(yù)應(yīng)變加速相變兩方面入手,結(jié)合經(jīng)典熱力學(xué)計算和系統(tǒng)的實驗研究,成功將制備SBA組織(抗拉強(qiáng)度2154MPa,延伸率13%)的時間縮短至1-3小時,主要工作和結(jié)果如下。建立了貧碳區(qū)切變形核的熱力學(xué)模型,利用超組元模型修正后的KRC和LFG熱力學(xué)模型計算了低溫貝氏體相變形核自由能,確定了實驗鋼在300℃以下溫度的貝氏體形核驅(qū)動力為-1900J·mol-1到-2000J·mol-1,即在熱力學(xué)條件上相變形核可能以切變機(jī)制發(fā)生。結(jié)合MUCG83軟件(基于Russell的經(jīng)典形核理論計算孕育期)進(jìn)行了輔助成分設(shè)計與優(yōu)化。設(shè)計了三個成分體系的富硅(1.5～2.5 wt%)合金,包括不同C含量(0.6、0.8、0.9)的0.5Mn-0Cr系列合金,不同Cr、Mn含量的0.8C-1Mn-0Cr和0.8C-1Mn-1Cr合金以及對比合金0.9C-2Mn-1.5Cr-1.5Co-1Al系列,共八個成分。利用DIL805淬火相變膨脹儀測定了以上合金的等溫相變(TTT)曲線,精確測定了不同溫度下的貝氏體相變孕育期,實驗結(jié)果與理論分析一致。增加C元素含量能顯著降低Ms點(diǎn)溫度和Bs點(diǎn)溫度,并且能增大兩個相變點(diǎn)溫度區(qū)間,Mn元素是推遲相變開始和延長相變完成時間的主要元素,同時添加錳、鉻元素對貝氏體相變孕育期的推遲作遠(yuǎn)大于單一添加錳或鉻。利用Gleelbe熱模擬實驗機(jī)研究了預(yù)應(yīng)變作用下貝氏體相規(guī)律,結(jié)果表明,過冷奧氏體預(yù)應(yīng)變對后續(xù)等溫貝氏體相變加速效果顯著,且在合理控制預(yù)應(yīng)變溫度和應(yīng)變量的情況下,最終貝氏體轉(zhuǎn)變量略有增加。在大于600℃的較高溫度區(qū)間施加單道次50%預(yù)應(yīng)變,可明顯阻滯貝氏體相變導(dǎo)致最終轉(zhuǎn)變量的降低。在600℃進(jìn)行小應(yīng)變量(20%)單軸壓縮,雖不能顯著加速貝氏體相變,但能縮短孕育期,加速貝氏體形核。在低于600℃且高于馬氏體轉(zhuǎn)變溫度區(qū)間進(jìn)行預(yù)應(yīng)變可有效加速低溫貝氏體等溫相變,相變孕育期和完成時間隨著預(yù)應(yīng)變溫度的降低和應(yīng)變量的增加而減小。預(yù)應(yīng)變溫度為300℃,應(yīng)變量為20%時,過冷奧氏體在230℃的等溫貝氏體相變孕育期可由5小時縮短至30分鐘。在冷軋試驗機(jī)上完成了多步形變熱處理及快速制備SBA組織的控軋控冷工藝研究。利用改進(jìn)的多步形變熱處理工藝,在單次形變量不大于10%,總應(yīng)變量不大于30%的預(yù)應(yīng)變條件下,通過控制形變間隔時間,即控制形變奧氏體回復(fù)時間,使形變過冷奧氏體在2小時內(nèi)完成等溫貝氏體相變,并且不降低貝氏體最終轉(zhuǎn)變量。經(jīng)多步形變熱處理后的貝氏體鐵素體(BF)片層厚度約50-70nnm,抗拉強(qiáng)度2154MPa。殘留奧氏體片的厚度因形變奧氏體機(jī)械穩(wěn)定性而增加到100nnm,使該超細(xì)貝氏體組織在具備超高強(qiáng)度的同時,擁有高達(dá)13%的總延伸率。利用控制軋制控制冷卻工藝將0.8C-2.5Si-0.5Mn-1Al合金進(jìn)行溫軋后在空氣緩慢冷卻,可在不用等溫的情況下獲得SBA組織。控軋控冷SBA鋼抗拉強(qiáng)度在2600MPa時,延伸率為7%,強(qiáng)塑積18.2GPa·%,在2000MPa時,延伸率為13%,強(qiáng)塑積26GPa·%。闡明了SBA組織中塊狀殘留奧氏體的形成機(jī)理,提出了消除塊狀殘留奧氏體的解決方案。多道次小應(yīng)變量的預(yù)變形條件下,過冷奧氏體進(jìn)行單滑移系塑性應(yīng)變,通過塑性協(xié)調(diào)產(chǎn)生取向擇優(yōu),進(jìn)而減少單個奧氏體晶粒中可能出現(xiàn)的Packet(一系列互相平行的BF束包)數(shù)量,使得殘留奧氏體只能以片層狀存在。預(yù)應(yīng)變?yōu)锽F提供更多形核位置,從而細(xì)化高溫區(qū)形成的BF的厚度,在300℃等溫1小時形成的SBA組織強(qiáng)度與200℃等溫數(shù)十小時的強(qiáng)度相同。這一發(fā)現(xiàn)打破了抗拉強(qiáng)度超過2GPa的SBA組織僅能通過低溫(250℃)長時間等溫?zé)崽幚慝@得的傳統(tǒng)觀念,使工業(yè)推廣成為可能。
[Abstract]:The ultra-fine bainite ferrite sheet and residual austenite layer structure (Slim Bainitic ferrite-Austenite, SBA) of ultra-fine bainite microstructure of.SBA steel can be obtained from the alloy of 0.98C-1.59Si-1.94Mn-1.33Cr-0.30Mo-0.02Ni-0.11V at 200-300 degrees C for dozens of hours. The microstructure of ultra-fine (100nm).SBA steel with its ultra-fine (slice thickness 100nm) can be obtained. The simple and easy preparation process and super high strength and toughness (tensile strength 2.5GPa, hardness 600HV, ductile 30-40MPa. M1/2) are widely concerned in the academic and industrial circles. However, for decades, the phase transition time restricts its industrial application. This paper, starting with two aspects of alloy optimization and prestrain plus phase transition, combines classical thermodynamic calculation and In the experimental study of the system, the time of preparing SBA tissue (tensile strength 2154MPa, elongation 13%) is shortened to 1-3 hours. The main work and results are as follows. A thermodynamic model for the shear nucleation of the lean carbon region is established. The nuclear free energy of the low temperature bainite phase is calculated by using the modified KRC and LFG thermodynamic model. The driving force of the bainite nucleation drive at the temperature below 300 C is -1900J mol-1 to -2000J mol-1. That is, the phase deformation nucleus may occur in the shear mechanism on the thermodynamic condition. The auxiliary components are set up and optimized with the MUCG83 software (based on the classical nucleation theory calculation based on Russell). The silicon rich (1.) is designed. (1. 5 to 2.5 wt%) alloys, including 0.5Mn-0Cr series alloys with different C content (0.6,0.8,0.9), 0.8C-1Mn-0Cr and 0.8C-1Mn-1Cr alloys with different Cr and Mn content, and 0.9C-2Mn-1.5Cr-1.5Co-1Al series of contrast alloys, are eight components. The isothermal phase transition (TTT) curve of the upper alloy was measured by the DIL805 quenching phase change instrument, and the different temperatures were accurately measured. The experimental results are consistent with the theoretical analysis. The increase of C element content can significantly reduce the temperature of Ms point and Bs point, and can increase the temperature range of two phase transition points. The Mn element is the main element to postpone the start of phase transition and prolong the completion time of the phase transition, and the addition of manganese and chromium element to the bainite phase transition period. The postponement was far greater than a single addition of manganese or chromium. The bainite phase law under pre strain was studied by the Gleelbe thermal simulation test machine. The results showed that the pre strain pre strain of the supercooled austenite has a significant effect on the subsequent isothermal bainite transformation, and the final bainite transformation amount is slightly increased in the case of reasonable control of the pre strain temperature and the strain. The application of single pass 50% pre strain at a higher temperature range greater than 600 degrees C can obviously block the reduction of the final transformation of bainite phase transition. The small strain (20%) compression at 600 C can not significantly accelerate the bainite phase transition, but it can shorten the inoculation period and accelerate the bainite nucleation at a temperature of lower than 600 and higher than the martensite transition temperature zone. The pre strain can effectively accelerate the isothermal phase transition of low temperature bainite, and the phase transition period and completion time decrease with the decrease of pre strain temperature and the increase of the strain. The pre strain temperature is 300 C and the strain is 20%, the isothermal bainite phase transition period of supercooled austenite can be shortened from 5 hours to 30 minutes at 230. The process of controlled rolling and controlled cooling of SBA tissue with multi step deformation heat treatment and rapid preparation was studied on the machine. The modified multistep deformation heat treatment process was used to control the deformation interval time, that is, to control the deformation austenite recovery time, to make the deformation supercooled austenite under the pre strain condition with the single shape variable not greater than 10% and the total strain less than 30%. The isothermal bainite phase transition is completed within 2 hours, and the final transformation of bainite is not reduced. The thickness of the bainite ferrite (BF) lamellae after multistep heat treatment is about 50-70nnm, and the thickness of the retained austenite fragment of tensile strength 2154MPa. is added to 100nnm because of the mechanical stability of the deformation austenite, so that the ultrafine bainite structure is in the possession of superfine bainite. At the same time, it has a total elongation of up to 13%. Using controlled rolling control cooling process, the 0.8C-2.5Si-0.5Mn-1Al alloy is cooled slowly in the air, and SBA can be obtained without constant temperature. The tensile strength of the controlled rolling and controlled cold SBA steel is 7%, the strong plastic product 18.2GPa%, and the elongation at 2000MPa, when the tensile strength of the controlled rolling and controlled cold steel is at 2600MPa. The rate is 13%, the strong plastic product 26GPa%. Clarifies the formation mechanism of the massive retained austenite in the SBA tissue, and puts forward the solution to eliminate the massive retained austenite. Under the predeformation condition of the multipass small strain, the supercooled austenite is made by the plastic strain of the single slip system, and the orientation of the austenite is produced by the plastic coordination, and the single austenite grain is reduced. The possible presence of Packet (a series of parallel BF bundles) makes the retained austenite only in lamellar. The prestrain provides more nucleation positions to BF, thus refining the thickness of BF formed by the high temperature region. The strength of the SBA tissue formed at 300 degrees centigrade for 1 hours is the same as that of 200 hours at 200. The SBA structure with tensile strength exceeding 2GPa can only be obtained by the traditional idea of low temperature (250 degree) long time isothermal heat treatment, so that industrial extension can be made possible.
【學(xué)位授予單位】：北京科技大學(xué)
【學(xué)位級別】：博士
【學(xué)位授予年份】：2016
【分類號】：TG142.1

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