【摘要】：隨著汽車工業(yè)的發(fā)展,減輕車重、降低能耗、減少環(huán)境污染已成為現(xiàn)代汽車的發(fā)展趨勢(shì)。提高汽車用鋼板的強(qiáng)度是降低鋼板厚度、減輕車重從而降低能耗的有效途徑。新型高錳奧氏體TWIP鋼兼具高強(qiáng)度、高塑性,被認(rèn)為是最具發(fā)展?jié)摿Φ钠嚱Y(jié)構(gòu)用鋼。作為結(jié)構(gòu)材料,TWIP鋼在實(shí)際服役過程中必然會(huì)受到交變載荷的作用,導(dǎo)致其疲勞失效。因此,研究高錳奧氏體TWIP鋼的疲勞行為不僅具有實(shí)用價(jià)值,也可以為結(jié)構(gòu)件的抗疲勞設(shè)計(jì)和安全使用提供可靠的理論依據(jù)。本文以高錳奧氏體TWIP鋼為研究對(duì)象,在室溫下研究了它的低周疲勞行為,分析了應(yīng)變范圍和應(yīng)變速率對(duì)高錳奧氏體TWIP鋼低周疲勞行為的影響;研究了它的疲勞裂紋擴(kuò)展行為,分析了晶粒尺寸、應(yīng)力比和合金元素Al添加對(duì)TWIP鋼疲勞裂紋擴(kuò)展行為的影響規(guī)律。得到以下主要結(jié)果:低周疲勞實(shí)驗(yàn)結(jié)果表明,高錳奧氏體0Al鋼在循環(huán)變形過程中出現(xiàn)了動(dòng)態(tài)應(yīng)變時(shí)效,且此動(dòng)態(tài)應(yīng)變時(shí)效與應(yīng)變速率有關(guān)。在固定應(yīng)變范圍為1.4%時(shí),在低應(yīng)變速率(8×10-4 s~(-1)和2×10-3 s~(-1))下,應(yīng)力-應(yīng)變滯后回線上無鋸齒波動(dòng)現(xiàn)象。而在中等應(yīng)變速率(8×10-3 s~(-1))下,出現(xiàn)了鋸齒波動(dòng)現(xiàn)象,但隨著循環(huán)周次的增加,鋸齒波動(dòng)幅度減弱,并逐漸消失。在較高應(yīng)變速率(2×10-2 s~(-1))下,鋸齒波動(dòng)幅度增大,且同樣隨著循環(huán)周次的增加而減弱,但并沒有消失,它伴隨整個(gè)疲勞循環(huán)過程。高錳奧氏體0Al鋼在低周疲勞循環(huán)變形過程中出現(xiàn)循環(huán)硬化/軟化現(xiàn)象,此循環(huán)硬化/軟化行為的程度與外加應(yīng)變范圍和應(yīng)變速率密切相關(guān)。在應(yīng)變速率為8×10-3 s~(-1)、且在較高應(yīng)變范圍(1.0%~1.4%)下,高錳奧氏體0Al鋼表現(xiàn)為初始階段的循環(huán)硬化和循環(huán)飽和之后、循環(huán)軟化直至失效。而在低應(yīng)變范圍(0.8%)下,高錳奧氏體0Al鋼在經(jīng)過循環(huán)硬化達(dá)到飽和后,只表現(xiàn)出輕微的循環(huán)軟化,而且在循環(huán)失效前還經(jīng)歷了一個(gè)循環(huán)穩(wěn)定階段。此外,在恒定應(yīng)變范圍為1.4%、不同應(yīng)變速率下,高錳奧氏體0Al鋼的循環(huán)應(yīng)力響應(yīng)均表現(xiàn)為初始循環(huán)硬化、循環(huán)飽和、循環(huán)軟化直至失效。在恒定應(yīng)變速率(8×10-3 s~(-1))下,高錳奧氏體0Al鋼的低周疲勞壽命隨著應(yīng)變范圍的增大而降低,且遵循Coffin-Manson關(guān)系。因此,利用文中獲得的疲勞性能參數(shù),可以較為準(zhǔn)確地預(yù)測(cè)高錳奧氏體0Al鋼的低周疲勞壽命。在恒定應(yīng)變范圍(1.4%)時(shí),低周疲勞壽命隨著應(yīng)變速率的增大而降低。這是因?yàn)?在高應(yīng)變速率下,低周疲勞循環(huán)過程中每一個(gè)循環(huán)所累積的塑性損傷高于低應(yīng)變速率。此外,在較高應(yīng)變速率時(shí)出現(xiàn)的動(dòng)態(tài)應(yīng)變時(shí)效會(huì)使材料在循環(huán)變形過程中產(chǎn)生較多的不均勻變形和駐留滑移帶。這些局部不均勻變形和駐留滑移帶有利于裂紋形核、促進(jìn)疲勞裂紋擴(kuò)展,從而降低其疲勞壽命。無論是粗晶粒還是細(xì)晶粒高錳奧氏體0Al鋼,其疲勞裂紋擴(kuò)展抗力隨著循環(huán)應(yīng)力比的降低而增大。在高、低應(yīng)力比(0.6和0.1)下,粗晶粒高錳奧氏體0Al鋼的疲勞裂紋擴(kuò)展門檻值ΔKth均高于細(xì)晶粒鋼的裂紋擴(kuò)展門檻值。在Paris區(qū),隨著應(yīng)力強(qiáng)度因子范圍ΔK的逐漸增大,晶粒尺寸對(duì)疲勞裂紋擴(kuò)展速率的影響減弱。晶粒尺寸對(duì)疲勞裂紋擴(kuò)展門檻值的影響,主要是由裂紋閉合和平面滑移性的不同引起的。此外,在粗晶粒高錳奧氏體0Al鋼的疲勞裂紋擴(kuò)展過程中產(chǎn)生的裂紋分叉也會(huì)消耗很多的能量,從而降低了裂紋尖端的有效驅(qū)動(dòng)力,有利于疲勞裂紋擴(kuò)展門檻值的升高�；趥鹘y(tǒng)二維觀察方法和三維同步輻射CT技術(shù)研究了高錳奧氏體0Al和3Al鋼在應(yīng)力比為0.1時(shí)的疲勞裂紋擴(kuò)展行為。結(jié)果表明,高錳奧氏體0Al鋼的疲勞門檻值高于3Al鋼。這是因?yàn)樵诮T檻區(qū),疲勞裂紋閉合對(duì)高錳奧氏體0Al鋼的影響高于3Al鋼。而且在3Al鋼中,Al的加入增加了其層錯(cuò)能,從而降低了材料的平面滑移性,導(dǎo)致3Al鋼的循環(huán)滑移可逆性降低。此外,裂紋在擴(kuò)展過程中,兩兩裂紋片之間的相交也會(huì)降低裂紋尖端的有效驅(qū)動(dòng)力,從而降低裂紋擴(kuò)展速率。
[Abstract]:With the development of automotive industry, reducing vehicle weight, energy consumption and environmental pollution has become the development trend of modern automobiles. Increasing the strength of automotive steel sheet is an effective way to reduce the thickness of steel sheet, reduce vehicle weight and reduce energy consumption. As a structural material, TWIP steel will inevitably be subjected to alternating load in the actual service process, resulting in fatigue failure. Therefore, the study of fatigue behavior of high manganese austenitic TWIP steel is not only of practical value, but also provides a reliable theoretical basis for fatigue design and safe use of structural parts. The low cycle fatigue behavior of austenitic TWIP steel was studied at room temperature, and the effects of strain range and strain rate on the low cycle fatigue behavior of high manganese austenitic TWIP steel were analyzed. The main results are as follows: The results of low cycle fatigue test show that dynamic strain aging occurs during cyclic deformation of high manganese austenitic 0Al steel, and this dynamic strain aging is related to strain rate. There is no sawtooth fluctuation on the line, but the sawtooth fluctuation appears at moderate strain rate (8 x 10-3 s-1). However, with the increase of cycle number, the sawtooth fluctuation amplitude decreases and gradually disappears. At higher strain rate (2 x 10-2 s-1), the sawtooth fluctuation amplitude increases and decreases with the increase of cycle number, but it does not disappear. The cyclic hardening/softening behavior of high manganese austenitic 0Al steel during low cycle fatigue cyclic deformation is closely related to the applied strain range and strain rate. In addition, in the low strain range (0.8%), the high manganese austenitic 0Al steel shows only slight cyclic softening after cyclic hardening and saturation, and it also undergoes a cyclic stable stage before cyclic failure. The cyclic stress response of high manganese austenitic 0Al steel is shown as initial cyclic hardening, cyclic saturation, cyclic softening until failure at different strain rates. The low cycle fatigue life of high manganese austenitic 0Al steel decreases with the increase of strain range at constant strain rate (8 *10-3 s~(-1)), and follows the Coffin-Manson relationship. Therefore, the low cycle fatigue life of high manganese austenitic 0Al steel can be predicted more accurately by using the fatigue performance parameters obtained in this paper. The low cycle fatigue life decreases with the increase of strain rate in the constant strain range (1.4%). This is because the plastic damage accumulated in each cycle during the low cycle fatigue cycle at high strain rate. The damage is higher than the low strain rate. In addition, the dynamic strain aging at higher strain rate will result in more inhomogeneous deformation and resident slip bands during cyclic deformation. These local inhomogeneous deformation and resident slip bands are beneficial to crack nucleation, promote fatigue crack propagation and reduce fatigue life. The fatigue crack growth resistance of coarse grain or fine grain high manganese austenitic 0Al steel increases with the decrease of cyclic stress ratio. At high and low stress ratios (0.6 and 0.1), the fatigue crack growth threshold value Kth of coarse grain high manganese austenitic 0Al steel is higher than that of fine grain steel. The influence of grain size on the threshold value of fatigue crack growth is mainly caused by the difference of crack closure and plane slip. In addition, the bifurcation of cracks produced in the process of fatigue crack growth of coarse grain high manganese austenitic 0Al steel will consume a lot. Based on the traditional two-dimensional observation method and three-dimensional synchrotron radiation CT technique, the fatigue crack propagation behavior of high manganese austenitic 0Al and 3Al steels at the stress ratio of 0.1 was studied. This is because the effect of fatigue crack closure on high manganese austenitic 0Al steel is higher than that of 3Al steel in the near threshold region. Moreover, the addition of Al increases the stacking fault energy of 3Al steel, which decreases the planar slip of the material and results in a decrease in the reversibility of cyclic slip of 3Al steel. Intersection will also reduce the effective driving force at the crack tip, thus reducing the crack growth rate.
【學(xué)位授予單位】：燕山大學(xué)
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2016
【分類號(hào)】：TG142.1

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本文編號(hào)：2217552