本文選題：冷軋 + TRIP鋼　； 參考：《北京科技大學(xué)》2015年博士論文

【摘要】：本文利用原位高能X射線衍射(HEXRD)技術(shù),并結(jié)合光學(xué)顯微鏡(OM)、掃描電鏡(SEM)、透射電鏡(TEM)等分析手段,對低合金冷軋C-Mn-Al-Si系TRIP鋼的組織-性能關(guān)系進(jìn)行了研究,重點探討了其微觀變形機理,并建立了能夠反映多相材料變形過程中各相間應(yīng)力應(yīng)變配分行為的本構(gòu)模型；同時利用Thermo-Calc和DICTRA等熱力學(xué)計算軟件對低合金冷軋TRIP鋼工藝-組織的對應(yīng)關(guān)系進(jìn)行了有益的探索。結(jié)果如下： 首先對兩種不同C含量的低合金冷軋C-Mn-Al-Si系TRIP鋼兩相區(qū)退火后,400℃保溫不同時間所得組織及其力學(xué)行為進(jìn)行了研究,結(jié)果顯示：相同的貝氏體區(qū)等溫時間下,與低C含量的鋼相比,C含量較高的鋼獲得了相對較少的貝氏體和較多的馬氏體,并且表現(xiàn)出較高的屈服強度和抗拉強度；與此同時,較高C含量的鋼組織中較高的殘余奧氏體含量及其C含量使該鋼的組織獲得了更高的延伸率,從而體現(xiàn)出了較好的綜合力學(xué)性能。隨著貝氏體區(qū)等溫時間的延長,兩種鋼中的貝氏體含量不斷增加而馬氏體含量不斷減少；屈服強度逐漸提高而抗拉強度逐漸下降,延伸率則總體呈現(xiàn)增加的趨勢。反映實驗鋼強度與塑性匹配情況的強塑積值線性依賴于組織中殘余奧氏體的含量與其C含量的乘積值。實驗鋼拉伸變形過程中應(yīng)變誘導(dǎo)馬氏體相的生成速率r與增量應(yīng)變硬化指數(shù)nincr.隨應(yīng)變的變化曲線具有很好的對應(yīng)關(guān)系,體現(xiàn)了TRIP效應(yīng)對實驗鋼變形過程中組織加工硬化能力的決定作用。 基于同步輻射的原位HEXRD實驗揭示了實驗鋼變形過程中各組成相之間存在的應(yīng)力配分行為,以及各相的屈服強度、卸載后的殘余應(yīng)力、殘余奧氏體的轉(zhuǎn)變動力學(xué)等結(jié)果。多相鋼彈性變形階段由于組織中各相彈性模量較為接近,應(yīng)力的配分不是十分明顯；塑性變形開始后由于組成相流變行為的差異,造成各相承擔(dān)的載荷發(fā)生了重新分配,鐵素體相作為較軟的相承擔(dān)了較小的載荷,但是表現(xiàn)出較大的塑性應(yīng)變；而貝氏體、殘余奧氏體、馬氏體相在變形過程中承擔(dān)了相對較大的應(yīng)力,但是它們對塑性的貢獻(xiàn)相對較小,扮演著“硬相”的角色。另外,研究發(fā)現(xiàn)組織中亞穩(wěn)的殘余奧氏體相在變形過程中的轉(zhuǎn)變動力學(xué)對各相間的協(xié)調(diào)變形行為或應(yīng)力配分具有顯著的影響,繼而影響了材料最終的力學(xué)行為。 基于Gladman型的混合定則以及原位HEXRD實驗結(jié)果,并結(jié)合描述單相材料力學(xué)行為的基于位錯演化理論的M-K模型和反映殘余奧氏體變形過程中應(yīng)變誘導(dǎo)馬氏體相變轉(zhuǎn)變動力學(xué)的O-C模型,建立了低合金TRIP鋼應(yīng)力-應(yīng)變關(guān)系的本構(gòu)方程。模型不僅考慮了TRIP效應(yīng),同時將對材料力學(xué)行為同樣具有重要影響的復(fù)合材料效應(yīng)也考慮在內(nèi),即各相間的協(xié)調(diào)變形行為對材料宏觀力學(xué)行為的影響。模型中各參量均具有明確的物理含義,特別是Gladman型混合定則中的指數(shù)n,其值變化的情況反映了多相材料變形過程中各組成相之間的協(xié)調(diào)變形行為。n隨應(yīng)變一般呈現(xiàn)先降后增的趨勢。變形初期,塑性變形主要集中在“軟相”上,造成組織中“硬相”和“軟相”之間應(yīng)變不相容程度逐漸增加,對應(yīng)了n值的下降；應(yīng)變繼續(xù)增加,“硬相”屈服并發(fā)生塑性變形,同時加之“軟相”中位錯動態(tài)回復(fù)的進(jìn)行,使得它們之間應(yīng)變不相容程度有所緩和,對應(yīng)了n值的上升。與此同時,發(fā)現(xiàn)n值還受到殘余奧氏體轉(zhuǎn)變動力學(xué)的顯著影響,也就是殘余奧氏體相的應(yīng)變誘導(dǎo)馬氏體相變將會對各相間應(yīng)力應(yīng)變的配分行為產(chǎn)生影響。鑒于此,建立了本研究所用成分的實驗鋼中殘余奧氏體相的應(yīng)變誘導(dǎo)馬氏體的生成速率r與指數(shù)n之間的定量關(guān)系,從而為模型的實際應(yīng)用奠定了基礎(chǔ)。 利用熱力學(xué)計算軟件對低合金冷軋TRIP鋼熱處理過程中的鐵素體／奧氏體臨界區(qū)退火和隨后的貝氏體區(qū)保溫的過程進(jìn)行了模擬計算,建立起了關(guān)于低合金冷軋TRIP鋼工藝-組織關(guān)系的預(yù)測方法,并通過實驗對此方法的可行性進(jìn)行了驗證。利用該方法可以對低合金冷軋TRIP鋼實際工業(yè)生產(chǎn)中工藝參數(shù)的確定提供有意義的指導(dǎo)。
[Abstract]:In this paper, in situ high energy X ray diffraction (HEXRD), optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and other analytical methods are used to study the relationship between microstructure and properties of low alloy cold rolled C-Mn-Al-Si system TRIP steel. The mechanism of micro deformation is discussed, and the deformation process of multiphase materials can be reflected in each process. At the same time, the constitutive model of stress and strain distribution in phase is made, and the corresponding relationship between process and structure of low alloy cold rolled TRIP steel is explored by using Thermo-Calc and DICTRA. The results are as follows:
At first, the microstructure and mechanical behavior of two low alloy cold rolled C-Mn-Al-Si TRIP steel TRIP steels at different time after annealing at different time were studied. The results showed that, under the same bainite isothermal time, compared with low C content steel, the steel with higher C content obtained relatively less bainite and more. At the same time, the higher residual austenite content and C content in the steel structure with high C content make the microstructure of the steel get higher elongation, which shows better comprehensive mechanical properties. With the prolongation of bainite isothermal time, two kinds of steel in the steel. The content of martensite is constantly increasing and the martensite content decreases continuously, the yield strength increases gradually and the tensile strength decreases gradually, and the elongation is generally increased. The strong plastic value reflecting the strength and plasticity of the experimental steel is linearly dependent on the product value of the content of retained austenite and the content of C in the tissue. The formation rate of strain induced martensite phase R and the increment strain hardening exponent nincr. have a good correspondence with the change curve of strain, which reflects the determining effect of TRIP effect on the working hardening ability of the experimental steel during the deformation process.
The in-situ HEXRD experiment based on synchrotron radiation reveals the stress distribution between the phases of the experimental steel, the yield strength of each phase, the residual stress after unloading, the transformation kinetics of the retained austenite, and so on. The elastic deformation phase of the multiphase steel is close to the elastic modulus of each phase and the distribution of the stress. It is not quite obvious; the load of each phase is redistributed due to the difference of the rheological behavior of the composition phase after the plastic deformation begins. The ferrite phase acts as a softer phase with a smaller load, but shows a large plastic strain, and the bainite, the remnant austenite and martensite phase bear relative in the process of deformation. Large stress, but their contribution to the plasticity is relatively small and plays a "hard phase" role. In addition, it is found that the transition kinetics of metastable retained austenite phase in the process of the deformation of the tissue has a significant effect on the coordinated deformation behavior or the stress distribution in each phase, and then influences the ultimate mechanical behavior of the material.
Based on the Gladman type mixed rule and in situ HEXRD experimental results, the constitutive equation of the stress strain relationship of the low alloy TRIP steel is established by combining the M-K model describing the mechanical behavior of the single-phase material based on the dislocation evolution theory and the O-C model reflecting the transformation kinetics of the strain induced martensitic transformation during the residual austenite deformation. It takes into account not only the TRIP effect but also the effect of composite material which has an important influence on the mechanical behavior of the material, that is, the influence of the coordinated deformation behavior between each phase on the macroscopic mechanical behavior of the material. All the parameters in the model have a clear physical meaning, especially the index n in the Gladman type mixing rule, and the change of its value. The situation reflects the coordinated deformation behavior between the components of various phases during the deformation process of multiphase material.N, the trend of the strain first descends and then increases with the strain. At the beginning of the deformation, the plastic deformation is mainly concentrated on the "soft phase", which causes the strain incompatibility between the "hard phase" and "soft phase" in the tissue to increase gradually, corresponding to the decrease of the n value; strain continues to continue. In addition, the yield and plastic deformation of "hard phase", and the dynamic recovery of dislocation in the "soft phase", make the strain incompatibility between them moderate and corresponding to the rise of the n value. At the same time, it is found that the n value is also influenced by the kinetics of the retained austenite transformation, which is the strain induction of the retained austenite phase. Martensitic transformation will affect the distribution of stress and strain between each phase. In view of this, the quantitative relationship between the formation rate of strain induced martensite R and the exponential n in the residual austenite phase of the experimental steel used in this study is established, thus laying a foundation for the practical application of the model.
The thermodynamic calculation software was used to simulate the annealing process of the ferrite / austenite critical zone and the subsequent bainite heat preservation during the heat treatment of low alloy cold rolled TRIP steel. The prediction method of the relationship between the process and the structure of the low alloy cold rolled TRIP steel was established, and the feasibility of this method was verified through the experiment. This method can provide meaningful guidance for the determination of technological parameters in the actual industrial production of low alloy cold rolled TRIP steel.
【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】：2015
【分類號】：TG142.1

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