TWIP钢的疲劳行为及延迟断裂研究

发布时间：2018-04-10 12:03

本文选题：TWIP钢 + 疲劳行为　；参考：《北京科技大学》2015年博士论文

【摘要】：TWIP(Twinning induced plasticity)钢是国内外目前正在积极研发中的一种新一代高强度交通用钢,具有高强度、高塑性、高应变硬化能力等显著优点,并且具有良好的加工性能。在TWIP钢从实验室推向实际生产的过程中,存在以下问题：一是与力学性能相比,TWIP钢的材料应用性能方面的研究还较少,其应用特性还不明确,例如其耐腐蚀性能、疲劳性能、低温韧性等；二是在生产和应用中,遇到了难涂镀、难焊接、延迟断裂等技术问题,限制了这一新型钢种的进一步推广与应用。本文在实验室条件下,综合应用扫描电镜、透射电镜、电化学充氢、有限元模拟等手段,设计并制备了不同稀土含量的TWIP钢以研究稀土元素对TWIP钢力学性能、疲劳性能和延迟开裂的影响,研究了TWIP钢的低周疲劳行为特征、疲劳破坏机制和延迟断裂机制,探索并提出了TWIP钢延迟断裂的控制手段。稀土元素对TWIP钢的主要影响包括：细化晶粒,改善夹杂物形态,增加夹杂物总量,与钢中的氢结合从而影响可扩散氢含量等。从力学性能来看,TWIP钢添加稀土元素后的力学性能和疲劳性能都有所降低,微量的稀土元素会使其延迟断裂性能恶化,但合适的添加量则可以对延迟断裂起到抑制作用。 TWIP钢具有优异的疲劳性能与强塑性的综合性能,其低周疲劳寿命远高于一般800MPa级别高强钢,又比具有相近疲劳性能的316L不锈钢具有更高的强塑积和更好的加工性能。TWIP钢在疲劳载荷下的变形机制是孪晶、滑移和驻留滑移带的共同作用。破坏机制则是孪晶和滑移带对晶界、夹杂物附近的相界面的碰撞,形成微孔洞,并连接成微裂纹,随疲劳载荷扩展。稀土元素引入的夹杂物由于形态圆滑,不会直接成为疲劳裂纹的萌生源,但增加的相界面增加了微裂纹的萌生的可能性。 TWIP钢的延迟断裂行为是在充分的冲压变形量、残余应力及应力梯度、较高的基体氢含量、强烈的缺口敏感性的共同作用下产生的。在充分的冲压变形量下,由于TWIP钢具有强加工硬化性能,会导致其接近抗拉强度的峰值残余应力。而充分的应力梯度诱导的氢扩散会在残余应力最大处产生氢富集,氢含量增大导致的氢致软化使TWIP钢容易萌生微裂纹。缺口敏感性和进一步的应力诱导氢扩散使得微裂纹迅速扩展,从而发生延迟断裂。通过合理添加稀土元素,以及控制试样成型过程的条件,包括严格控制切边质量和深冲变形量,均能对TWIP钢的延迟断裂起到控制作用。适量的稀土元素的添加能有效降低奥氏体中的可扩散氢含量,从而抑制延迟断裂倾向。严格控制切边质量及限制冲压件的深冲变形量,均可以有效的控制峰值残余应力与峰值氢浓度在安全范围内,从而减小发生延迟断裂的风险。
[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.
【学位授予单位】：北京科技大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG142.1

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