發(fā)布時(shí)間：2018-04-10 12:03

  本文選題：TWIP鋼 + 疲勞行為　； 參考：《北京科技大學(xué)》2015年博士論文

【摘要】：TWIP(Twinning induced plasticity)鋼是國內(nèi)外目前正在積極研發(fā)中的一種新一代高強(qiáng)度交通用鋼,具有高強(qiáng)度、高塑性、高應(yīng)變硬化能力等顯著優(yōu)點(diǎn),并且具有良好的加工性能。在TWIP鋼從實(shí)驗(yàn)室推向?qū)嶋H生產(chǎn)的過程中,存在以下問題：一是與力學(xué)性能相比,TWIP鋼的材料應(yīng)用性能方面的研究還較少,其應(yīng)用特性還不明確,例如其耐腐蝕性能、疲勞性能、低溫韌性等；二是在生產(chǎn)和應(yīng)用中,遇到了難涂鍍、難焊接、延遲斷裂等技術(shù)問題,限制了這一新型鋼種的進(jìn)一步推廣與應(yīng)用。 本文在實(shí)驗(yàn)室條件下,綜合應(yīng)用掃描電鏡、透射電鏡、電化學(xué)充氫、有限元模擬等手段,設(shè)計(jì)并制備了不同稀土含量的TWIP鋼以研究稀土元素對(duì)TWIP鋼力學(xué)性能、疲勞性能和延遲開裂的影響,研究了TWIP鋼的低周疲勞行為特征、疲勞破壞機(jī)制和延遲斷裂機(jī)制,探索并提出了TWIP鋼延遲斷裂的控制手段。 稀土元素對(duì)TWIP鋼的主要影響包括：細(xì)化晶粒,改善夾雜物形態(tài),增加夾雜物總量,與鋼中的氫結(jié)合從而影響可擴(kuò)散氫含量等。從力學(xué)性能來看,TWIP鋼添加稀土元素后的力學(xué)性能和疲勞性能都有所降低,微量的稀土元素會(huì)使其延遲斷裂性能惡化,但合適的添加量則可以對(duì)延遲斷裂起到抑制作用。 TWIP鋼具有優(yōu)異的疲勞性能與強(qiáng)塑性的綜合性能,其低周疲勞壽命遠(yuǎn)高于一般800MPa級(jí)別高強(qiáng)鋼,又比具有相近疲勞性能的316L不銹鋼具有更高的強(qiáng)塑積和更好的加工性能。TWIP鋼在疲勞載荷下的變形機(jī)制是孿晶、滑移和駐留滑移帶的共同作用。破壞機(jī)制則是孿晶和滑移帶對(duì)晶界、夾雜物附近的相界面的碰撞,形成微孔洞,并連接成微裂紋,隨疲勞載荷擴(kuò)展。稀土元素引入的夾雜物由于形態(tài)圓滑,不會(huì)直接成為疲勞裂紋的萌生源,但增加的相界面增加了微裂紋的萌生的可能性。 TWIP鋼的延遲斷裂行為是在充分的沖壓變形量、殘余應(yīng)力及應(yīng)力梯度、較高的基體氫含量、強(qiáng)烈的缺口敏感性的共同作用下產(chǎn)生的。在充分的沖壓變形量下,由于TWIP鋼具有強(qiáng)加工硬化性能,會(huì)導(dǎo)致其接近抗拉強(qiáng)度的峰值殘余應(yīng)力。而充分的應(yīng)力梯度誘導(dǎo)的氫擴(kuò)散會(huì)在殘余應(yīng)力最大處產(chǎn)生氫富集,氫含量增大導(dǎo)致的氫致軟化使TWIP鋼容易萌生微裂紋。缺口敏感性和進(jìn)一步的應(yīng)力誘導(dǎo)氫擴(kuò)散使得微裂紋迅速擴(kuò)展,從而發(fā)生延遲斷裂。 通過合理添加稀土元素,以及控制試樣成型過程的條件,包括嚴(yán)格控制切邊質(zhì)量和深沖變形量,均能對(duì)TWIP鋼的延遲斷裂起到控制作用。適量的稀土元素的添加能有效降低奧氏體中的可擴(kuò)散氫含量,從而抑制延遲斷裂傾向。嚴(yán)格控制切邊質(zhì)量及限制沖壓件的深沖變形量,均可以有效的控制峰值殘余應(yīng)力與峰值氫濃度在安全范圍內(nèi),從而減小發(fā)生延遲斷裂的風(fēng)險(xiǎn)。
[Abstract]:TWIP(Twinning induced plasticity steel is a new generation of high strength transportation steel which is being developed at home and abroad. It has many remarkable advantages such as high strength, high plasticity, high strain hardening ability, and has good processability.In the process of TWIP steel from laboratory to actual production, there are the following problems: first, compared with mechanical properties, there is less research on the applied properties of TWIP steel, and its application characteristics are not clear, such as corrosion resistance, fatigue property, etc.Second, in production and application, some technical problems such as hard coating, difficult welding and delayed fracture are encountered, which limit the further popularization and application of this new steel.In this paper, using scanning electron microscope, transmission electron microscope, electrochemical hydrogen charging and finite element simulation, TWIP steels with different rare earth contents were designed and prepared to study the mechanical properties of rare earth elements on TWIP steel.The effects of fatigue properties and delayed cracking on the low cycle fatigue behavior, fatigue failure mechanism and delayed fracture mechanism of TWIP steel were studied. The control methods of delayed fracture of TWIP steel were explored and put forward.The effects of rare earth elements on TWIP steel include refining grain, improving the shape of inclusions, increasing the total amount of inclusions, and binding with hydrogen in steel to influence the content of diffusible hydrogen, etc.According to the mechanical properties of TWIP steel the mechanical properties and fatigue properties of TWIP steel are decreased after adding rare earth elements. The delayed fracture properties of TWIP steel can be deteriorated by trace rare earth elements but the delayed fracture can be inhibited by adding appropriate amount of rare earth elements.The low cycle fatigue life of TWIP steel is much higher than that of 800MPa grade high strength steel.Compared with 316L stainless steel with similar fatigue properties, the deformation mechanism of TWIP steel under fatigue load is the joint action of twin, slip and resident slip band, and the deformation mechanism of TWIP steel under fatigue load is higher than that of 316L stainless steel with similar fatigue properties.The failure mechanism is the collision of twin and slip band to the grain boundary and the phase interface near the inclusions to form micropores and to form microcracks which propagate with fatigue loading.Because of its smooth shape, the inclusion introduced by rare earth elements will not be a direct source of fatigue crack initiation, but the increase of phase interface increases the possibility of micro-crack initiation.The delayed fracture behavior of TWIP steel is caused by the combined action of sufficient stamping deformation, residual stress and stress gradient, high hydrogen content in matrix and strong notch sensitivity.Under sufficient stamping deformation, due to the strong working-hardening property of TWIP steel, it will be close to the peak residual stress of tensile strength.The hydrogen diffusion induced by sufficient stress gradient will lead to hydrogen enrichment at the maximum residual stress, and the hydrogen softening caused by the increase of hydrogen content will lead to the initiation of microcracks in TWIP steel.Notch sensitivity and further stress-induced hydrogen diffusion lead to rapid growth of microcracks, resulting in delayed fracture.The delay fracture of TWIP steel can be controlled by adding rare earth elements reasonably and controlling the conditions of specimen forming process, including strictly controlling the cutting quality and deep drawing deformation.The addition of rare earth elements can effectively reduce the diffusible hydrogen content in austenite and thus restrain the tendency of delayed fracture.Strictly controlling the cutting mass and limiting the deep drawing deformation of stamping parts can effectively control the peak residual stress and peak hydrogen concentration in a safe range, thus reducing the risk of delayed fracture.
【學(xué)位授予單位】：北京科技大學(xué)
【學(xué)位級(jí)別】：博士
【學(xué)位授予年份】：2015
【分類號(hào)】：TG142.1

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