本文選題：中錳鋼 + 組織演變�。� 參考：《北京科技大學》2015年博士論文

【摘要】：相變誘導塑性(transformation induced plasticity, TRIP)鋼是利用亞穩(wěn)態(tài)的殘留奧氏體在應力應變作用下發(fā)生相變誘導塑性效應而研發(fā)的一種先進高強鋼,具有優(yōu)良的綜合力學性能。傳統(tǒng)TRIP鋼是一種多相鋼,其微觀組織由鐵素體、貝氏體和一定數(shù)量的殘留奧氏體(5%-15%)組成。而當Mn元素含量適當增加后,淬透性提高,可得到由鐵素體和殘留奧氏體組成的兩相組織,并且殘留奧氏體的含量可以達到20%～30%。為了獲得高強度和高塑性,不僅要控制各相所占的比例,得到較高的殘留奧氏體體積分數(shù),還必須控制各相的晶粒尺寸、形貌和分布,細化晶粒是相變誘導塑性鋼組織控制的熱點之一。在殘留奧氏體增多及晶粒細化后,相變誘導塑性行為將表現(xiàn)出獨有的特點,因此本文采用綜合細化晶粒技術(shù)制備了超細晶中錳鋼,通過定量拉伸實驗深入分析了其相變誘導塑性行為,并利用ABAQUS有限元模擬軟件建立了中錳鋼變形過程的計算模型,研究了殘留奧氏體的相變規(guī)律。結(jié)果表明： 采用中錳鋼合金成分體系,控制Mn元素含量為5%-7%,增加Mn元素含量能降低Ac1和Ac3溫度,實現(xiàn)低溫臨界區(qū)退火。設計了預淬火+退火的兩段式退火工藝,即在常規(guī)退火工藝前進行一次預淬火處理,隨后進行兩相區(qū)退火工藝,控制回復和再結(jié)晶過程獲得超細晶組織,同時得到較多的殘留奧氏體。對10Mn7鋼(0.1C-7Mn-0.04Nb)退火后的晶粒尺寸進行統(tǒng)計發(fā)現(xiàn)鐵素體基體的晶粒尺寸基本在1μm以下,而殘留奧氏體的晶粒尺寸約0.5μm,殘留奧氏體的體積分數(shù)最高可達40.29%。此工藝還能夠明顯縮短最優(yōu)退火時間,提高實驗鋼的力學性能。10Mn7鋼在625℃保溫4h后即可達到最佳力學性能,抗拉強度為1177MPa,延伸率為30.92%,強塑積為36.39GPa·%。 分析了退火過程中微觀組織的演變規(guī)律,發(fā)現(xiàn)與常規(guī)的中錳鋼退火工藝相比,加入預淬火工藝后能夠明顯消除實驗鋼微觀組織中Mn元素的顯微偏析,并在退火后得到兩種形貌的殘留奧氏體——長條狀和塊狀。對退火過程中奧氏體的演變規(guī)律進行分析發(fā)現(xiàn),第一階段預淬火后獲得馬氏體組織,并在馬氏體板條間有細小的殘留奧氏體。在第二階段低溫臨界區(qū)退火時,馬氏體板條間細小的殘留奧氏體將沿板條界長大,成為長條狀奧氏體：同時碳化物沿馬氏體板條界或原奧氏體晶界析出,成為塊狀奧氏體的核心。隨后冷卻過程中,由于奧氏體中C、Mn元素較多,兩種形貌的奧氏體均被保留至室溫。由FCC相和BCC相的反極圖可發(fā)現(xiàn),在同一原奧氏體晶粒內(nèi)的長條狀殘留奧氏體具有相同的取向,并且與周圍基體保持K-S關系。而在富碳區(qū)形成的塊狀殘留奧氏體則與基體沒有固定的位向關系。并且兩種形貌的殘留奧氏體的形成方式不同,導致其化學成分也存在差異,長條狀殘留奧氏體內(nèi)的Mn元素含量高于塊狀殘留奧氏體。 研究了熱軋后的鋼板在加熱和保溫過程中碳化物的析出和分布規(guī)律,發(fā)現(xiàn)在500℃保溫時碳化物尺寸最細小、分布最彌散,并且在500℃變形時,動態(tài)回復和再結(jié)晶等軟化作用與加工硬化作用相當,變形抗力較低。因此優(yōu)化了冷軋工藝,采用溫軋工藝。首先將熱軋酸洗后的鋼板加熱至500℃保溫1h后進行軋制,每道次軋制后放入爐中保溫5min再進下一道次的軋制,如此循環(huán),總壓下率為55%左右。采用溫軋工藝后15Mn7鋼(0.15C-7Mn-0.04Nb)的最終抗拉強度由1021MPa上升至1135MPa,延伸率由31.16%上升至35.30%,強塑積由31.81GPa·%上升至40.06GPa·%。并且采用溫軋工藝后,實驗鋼的最終顯微組織中長條狀殘留奧氏體所占比重上升,殘留奧氏體的體積分數(shù)由39.86%增加至44.68%。 對中錳鋼的塑性變形規(guī)律進行了研究,發(fā)現(xiàn)10Mn7鋼在不同溫度退火后的工程應力-應變曲線分為兩種情況：在580℃和600℃退火溫度較低時,由于實驗鋼中的晶粒尺寸小且殘留奧氏體的含量少,加工硬化作用較弱,塑性變形主要依靠Liiders應變。而退火溫度升高至625℃和650℃后,鋼中的殘留奧氏體體積分數(shù)增加,變形時有較多的馬氏體生成,加工硬化作用明顯,瞬時n值較大。 采用定量拉伸的方法研究了10Mn7鋼625℃退火后實驗鋼內(nèi)TRIP效應的發(fā)生過程和影響。發(fā)現(xiàn)在變形初期即有大量的殘留奧氏體發(fā)生馬氏體相變,轉(zhuǎn)變機制可歸結(jié)為應力誘導馬氏體相變：當應變量在0.03～0.12之間時處于Luders變形階段,應力值和殘留奧氏體的體積分數(shù)基本保持不變,變形主要依靠鐵素體基體的Luders應變;應變量大于0.12后,進入具有明顯的加工硬化階段,殘留奧氏體的體積分數(shù)隨應變量的增加逐漸減少。與常規(guī)退火工藝相比經(jīng)預淬火+退火工藝后,殘留奧氏體在變形時能夠?qū)崿F(xiàn)漸進式的轉(zhuǎn)變,在應變量較大時仍能產(chǎn)生TRIP效應。采用TEM和EBSD技術(shù)對不同應變量變形后實驗鋼的顯微組織進行觀察,發(fā)現(xiàn)大角度晶界附近的殘留奧氏體具有更高的穩(wěn)定性,小角度晶界處的殘留奧氏體首先發(fā)生轉(zhuǎn)變：長條狀殘留奧氏體比塊狀殘留奧氏體穩(wěn)定性更強,塊狀奧氏體會優(yōu)先發(fā)生相變；但由于馬氏體相的生成對周圍產(chǎn)生“保護”作用,同時馬氏體相變時會引起體積膨脹使靜水壓力增強等原因會使塊狀殘留奧氏體的中心區(qū)域穩(wěn)定性增加。 利用ABAQUS有限元軟件對中錳鋼的變形過程進行了模擬計算,發(fā)現(xiàn)在單向拉伸條件下馬氏體相變首先發(fā)生在殘留奧氏體內(nèi)的尖角或兩相交界處,并在新生馬氏體周圍出現(xiàn)應力集中。隨著變形量的增加,相變和應力集中主要沿著拉伸方向逐漸擴展,較不容易發(fā)生在橫向,應變量首先在鐵素體晶粒內(nèi)增大并與拉伸方向成45°。變形量較大時首先在個別新生馬氏體相的尖角位置出現(xiàn)局部應變量過大,繼續(xù)變形時此位置將成為微孔的形核區(qū)域。殘留奧氏體的穩(wěn)定性降低后屈服強度和抗拉強度變化不明顯,但會降低材料的延伸率。無論是升高馬氏體的強度,還是降低鐵素體的強度,材料的塑性都大大降低。在雙軸拉伸條件下相變發(fā)生早、擴展迅速,應力主要集中在新生馬氏體相內(nèi),應變主要集中在軟相鐵素體內(nèi)或與硬相的邊界處。最終斷裂時,在水平和垂直方向出現(xiàn)兩條斷裂帶。
[Abstract]:The phase transition induced plasticity (transformation induced plasticity, TRIP) steel is a kind of advanced high strength steel developed by the residual austenite under the stress-strain effect of metastable austenite. It has excellent comprehensive mechanical properties. The traditional TRIP steel is a kind of multiphase steel, and its microstructure is composed of ferrite, bainite and certain. The amount of residual austenite (5%-15%) is made up, and when the content of Mn is increased properly, the hardenability is improved, and the two phase structure composed of ferrite and retained austenite can be obtained. The content of retained austenite can reach 20% ~ 30%. to obtain high strength and high plasticity, and not only control the proportion of each phase, but also obtain higher residual austenite. The volume fraction of the body must also be controlled by the grain size, morphology and distribution of each phase. Grain refinement is one of the hot spots in the microstructure control of phase transition induced plastic steel. After the increase of retained austenite and grain refinement, the ductile behavior of phase transition will show unique characteristics. Therefore, this paper uses a comprehensive grain refinement technique to prepare the manganese in ultrafine crystals. Steel, through the quantitative tensile test, the phase transition induced plastic behavior was analyzed, and the calculation model of the deformation process of middle manganese steel was established by using the ABAQUS finite element simulation software. The phase transformation law of the retained austenite was studied. The results showed that the phase transformation of the retained austenite was studied.
Using the alloy composition system of middle manganese steel, the content of Mn element is 5%-7%, the content of Mn element can be increased, the temperature of Ac1 and Ac3 can be reduced and the temperature of the critical region is annealed at low temperature. The two stage annealing process of pre quenching + annealing is designed, that is, a pre quenching treatment is carried out before the conventional annealing process, and the two phase annealing process is carried out after the annealing process, and the recovery and recrystallization are controlled. The grain size of 10Mn7 steel (0.1C-7Mn-0.04Nb) annealed at the same time was obtained. The grain size of the annealed 10Mn7 steel (0.1C-7Mn-0.04Nb) was calculated. The grain size of the ferrite matrix was below 1 u m, while the grain size of the retained austenite was about 0.5 mu, and the volume fraction of retained austenite was up to 40.29%.. The optimum annealing time is shortened and the mechanical properties of the experimental steel are improved. The best mechanical properties of.10Mn7 steel can be reached at 625 C for 4H. The tensile strength is 1177MPa, the elongation is 30.92%, and the strong plastic product is 36.39GPa%.
The evolution of microstructure in the annealing process is analyzed. It is found that compared with the conventional annealing process of medium manganese steel, the microsegregation of Mn elements in the microstructure of the experimental steel can be eliminated obviously after adding the pre quenching process. After annealing, the retained austenite, the long strip and the lump of two kinds of morphologies, is obtained after annealing. It is found that martensitic structure is obtained after the first stage of quenching, and there are small retained austenite between martensitic plates. In the second stage, the small retained austenite between martensitic plates will grow up and become long strip austenite during the second stage low temperature critical zone. The austenite grain boundary precipitates and becomes the core of the massive austenite. In the process of cooling, the austenite of two forms of austenite in the austenite is retained to the room temperature because of the more C and Mn elements in the austenite. It is found that the long retained austenite in the same original austenite grain has the same orientation and is preserved with the surrounding matrix. The bulk retained austenite formed in the rich carbon region has no fixed relationship with the matrix, and the formation of retained austenite in the two morphologies is different, which leads to the difference in the chemical composition, and the content of the Mn element in the oblong retained austenite is higher than that of the massive retained austenite.
The carbide precipitation and distribution of hot rolled steel plate during heating and heat preservation are studied. It is found that the size of carbide is the finest and the distribution is most dispersed at 500 C, and the softening effect of dynamic recovery and recrystallization is equal to the working hardening, and the deformation resistance is low at 500 C. So the cold rolling process is optimized. Hot rolling process. First, the hot rolled steel plate was heated to 500 C for 1H and then rolled. After each rolling, it was put into the furnace and then kept in the furnace for 5min and then into the next rolling. The total pressing rate was about 55%. The ultimate tensile strength of 15Mn7 steel (0.15C-7Mn-0.04Nb) was increased from 1021MPa to 1135MPa after the rolling process, and the elongation rate was 3. 1.16% up to 35.30%, the strong plastic product rises from 31.81GPa% to 40.06GPa%. And after the warm rolling process, the proportion of the retained austenite in the final microstructure of the experimental steel increases and the volume fraction of the retained austenite increases from 39.86% to 44.68%..
The plastic deformation law of medium manganese steel is studied. It is found that the stress strain curve of 10Mn7 steel is divided into two kinds of stress-strain curves at different temperatures. When the annealing temperature is low at 580 and 600, the grain size in the experimental steel is small and the residual austenite content is few, the work hardening is weak, and the plastic deformation depends mainly on the Liiders. After the annealing temperature rises to 625 and 650 C, the residual austenite volume fraction in steel increases, and more martensite is formed when the deformation is deformed, and the working hardening effect is obvious, and the instantaneous n value is larger.
The process and influence of TRIP effect in 10Mn7 steel annealed at 625 C were studied by quantitative tensile method. It was found that a large number of retained austenite occurred martensitic transformation at the early stage of deformation, and the transformation mechanism could be attributed to stress induced martensitic transformation: when the strain was between 0.03 and 0.12, the strain was in the phase of Luders deformation and stress. The volume fraction of the value and residual austenite remains basically the same, and the deformation depends mainly on the Luders strain of the ferrite matrix. After the strain is greater than 0.12, the entry has an obvious stage of processing hardening. The volume fraction of retained austenite decreases with the increase of the strain. The progressive transformation can be achieved when the body is deformed, and the TRIP effect can still be produced when the strain is large. The microstructure of the experimental steel after the deformation of different variables is observed by TEM and EBSD. It is found that the retained austenite near the large angle grain boundary is more stable, and the retained austenite at the small angle grain boundary is first turned. The long strip retained austenite is more stable than the massive retained austenite, and the block like austenite takes precedence of phase transition. However, the formation of martensite phase produces a "protection" effect on the surrounding area, while the martensitic phase change causes volume expansion to increase the static water pressure and other reasons for the stability of the central region of the massive retained austenite. Increase.
The deformation process of middle manganese steel is simulated by ABAQUS finite element software. It is found that the martensitic transformation occurs at the sharp angle or two phase boundary in the residual austenite under uniaxial tension, and the stress concentration around the new martensite. With the increase of the deformation amount, the phase transition and stress concentration are mainly along the tensile direction. Gradually expanding, it is not easy to occur in the transverse direction. The strain increases first in the ferrite grain and is 45 degrees in the direction of the tensile. First, the local strain is too large in the sharp angle position of the new martensite phase when the deformation amount is large, and this position will become the nucleation area of the micropore when the deformation is continued. The stability of the retained austenite is reduced. The change of strength and tensile strength is not obvious, but it can reduce the elongation of the material. Whether it is the strength of martensite or the strength of the ferrite, the plasticity of the material is greatly reduced. In the condition of biaxial tension, the phase transition occurs early, the expansion is rapid, the stress is mainly concentrated in the new martensite phase, and the strain is mainly concentrated in the soft ferrite. At the end of the fracture, there are two fracture zones in the horizontal and vertical directions.
【學位授予單位】：北京科技大學
【學位級別】：博士
【學位授予年份】：2015
【分類號】：TG142.1

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