Prediction of hot deformation behaviour of Fe-25Mn-3Si-3Al TWIP steel

Hot deformation behaviour of Fe-25Mn-3Si-3Al twinning-induced plasticity (TWIP) steel was investigated by hot compression testing on Gleeble 3500 thermo-mechanical simulator in the temperature range from 800 to 1100 °C and at strain rate range from 0.01 to 5 s⁻¹, and the microstructural evolution was studied by metallographic observations. The results show that the true stress-true strain curves exhibit a single peak stress at certain strain, after which the flow stresses decrease monotonically until the end of deformation, showing a dynamic flow softening. The peak stress level decreases with increasing deformation temperature and decreasing strain rate, which can be predicted by the Zener-Hollomon (Z) parameter in the hyperbolic sine equation with the hot deformation activation energy Q of 405.95 kJ/mol. The peak and critical strains can also be predicted by Z parameter in power-law equations, and the ratio of critical strain to peak strain is about 0.7. The dynamic recrystallization (DRX) is the most important softening mechanism for the experimental steel during hot compression. Furthermore, DRX procedure is strongly affected by Z parameter, and the decreasing of Z value leads to more extensive DRX.     

退火温度对Fe-24.38Mn-0.44C TWIP钢组织性能的影响

以冷轧Fe-24.38Mn-0.44C钢为研究对象,通过光学显微镜(OM)、透射电镜(TEM)、室温拉伸等试验手段,研究了不同退火温度(部分再结晶退火、再结晶退火以及高温退火)下其微观组织及力学性能的演变。结果表明,随着退火温度降低,试验钢的微观组织由高温退火时粗大的无畸变等轴再结晶晶粒逐渐向纳米级变形孪晶和细小的再结晶晶粒混合组织转变,强化机制逐渐由孪生滑移为主向位错滑移为主纳米孪晶强化为辅的机制转变,导致试验钢屈服强度迅速提高,屈强比由0.36提高到0.49,伸长率有所降低。     

高锰奥氏体低温钢力学性能及Hall-Petch关系的研究

高锰奥氏体低温钢作为一种经济型低温钢材料,具有非常巨大的发展空间.为进一步研究晶粒尺寸对高锰奥氏体低温钢的影响,本文对其进行了不同工艺的固溶处理,测试了不同晶粒尺寸实验钢的室温拉伸及低温冲击性能, 通过扫描电镜(SEM)、背散射电子衍射(EBSD)等方法进行表征并计算其Hall-Petch关系式.-196 ℃冲击实验结果表明,高锰奥氏体低温钢拥有良好而稳定的超低温冲击性能,且其超低温冲击性能不随晶粒尺寸的增加而发生变化.经计算实验钢屈服强度所对应的Hall-Petch常数K_y为7.27 MPa·mm^0.5,明显小于其他被研究的奥氏体高锰钢.进一步计算拉伸过程中不同真应变ε对应的Hall-Petch常数K(ε),发现在拉伸过程中,受TWIP效应产生的变形孪晶影响,K(ε)值随应变量的增加而增加;当真应变达到0.3时,组织内变形孪晶不再继续生成,加工硬化过程不再进行,K(ε)也达到极大值.     

热轧变形对高碳TWIP钢组织缺陷和力学性能的影响

为解决高碳Fe-20Mn-3Cu-1.3C TWIP钢凝固组织中易形成显微疏松、损害合金的力学性能的问题,研究了在相同热轧温度下,改变轧制变形总量对合金微孔缺陷的消除及拉伸力学性能的影响.研究表明:通过热轧变形可以有效地减少Fe-20Mn-3Cu-1.3C TWIP钢的微孔缺陷,提高组织致密度;随着热轧变形量的增加,合金的综合力学性能显著提高,当热轧变形量达到91%时,该合金中的微孔面密度由固溶态的1.67%降低至0.71%,抗拉强度达到1223.7 MPa,延伸率达到86.8%,强塑积高达106217.2 MPa.%,比未热轧变形处理提高了78.3%,显示出优异的综合力学性能,表明消除微孔缺陷是充分发挥其高强韧性的关键.     

表面机械滚压处理对退火TWIP钢组织和力学性能的影响

孪晶诱发塑性变形(Twinning-induced-plasticity, TWIP)钢具有低屈服高塑性的特点,不利于其推广应用。为改善TWIP钢的综合力学性能,本文使用表面机械滚压处理(Surface mechanical rolling treatment, SMRT)在退火态的低层错能单相奥氏体结构TWIP钢表层引入一定厚度的强化层,通过激光共聚焦显微镜、光学显微镜和场发射扫描电镜对表面形貌,内部显微组织和断口形貌进行了对比分析,使用维氏硬度计和MTS万能试验机对样品的显微硬度分布和拉伸性能进行了对比测试。结果表明,经表面机械滚压处理后TWIP钢表层形成约1.4 mm厚的强化层,TWIP钢表面完整性较好。显微组织分析发现,表面强化层晶粒细化,位错和变形孪晶密度升高;力学性能测试发现,TWIP钢滚压后表层硬度达到647.5 HV0.1,相比退火态硬度提升了2倍左右,且硬度由表层逐渐递减至退火态的230 HV0.1。退火态TWIP钢屈服强度为270 MPa,抗拉强度为878 MPa,断裂总延伸率约为73%;经过滚压处理后,TWIP钢屈服强度提升至1008 MPa,提高至退火态的3.7倍左右,抗拉强度提升至1142 MPa,断裂总延伸率约为20.5%,表现出优异的综合力学性能。     

Microstructure and mechanical properties of laser welded austenitic high manganese steels

Abstract

The microstructure of laser welded austenitic twinning induced plasticity steel sheets joints was investigated by means of optical microscopy, SEM and electron backscattering diffraction in order to differentiate the fusion zone, heat affected zone and base material, as well as to establish present phases, grain size distribution and grain misorientation distribution caused by the welding process. Measurements of EDX were taken into account to evaluate the effect of Mn segregations. Microhardness measurements and tensile tests were performed to evaluate the mechanical properties of the joints. In addition, the twinning phenomena progress was assessed by investigating the texture evolution in the base material and fusion zone of samples plastically deformed by 5, 10 and 15%. Grain refinement was found in the fusion zone affecting substantially the mechanical properties of the welding, being the most resistant and harder region.