Temperature dependence of mechanical properties in martensitic-austenitic stainless steels obtained by quenching and partitioning (Q&P) processing

淬火和配分（Q&P）处理是稳定钢的显微组织中的奥氏体以提高强度-延展性组合的方法。借助于由淬火至室温（RT）和在450 °C下配分组成的简化Q&P处理，在含有13%Cr和添加Co、Si、Mn和N的高碳不锈钢的显微组织中调节不同的奥氏体分数。随后在20 °C - 200 °C的温度范围内测定拉伸性能。由于奥氏体的C富集不足和奥氏体抵抗变形诱导的马氏体转变的低稳定性，在RT下拉伸伸长率非常低。随着温度升高到200 °C并且奥氏体稳定性增加，获得了良好的强度和延展性组合（例如，对于添加Si的钢，极限拉伸强度（UTS）为1520 MPa，均匀拉伸伸长率（UTE）为25%）。该项目第二阶段的一个重要目标是设计与UTS x UTE产品相关的Q&P处理条件，超过30，000 MPa。%这将通过对项目第一阶段中使用的钢施加零度以下的淬火温度来实现，以增加马氏体的比例。由于马氏体提供了配分所需的C，因此其分数的增加将增强奥氏体的C富集。随后将通过X射线衍射估计奥氏体的C富集来研究马氏体分数对配分程度的影响。渗碳体在马氏体中的析出是Q&P钢中与奥氏体富碳竞争的过程之一。TEM研究表明，这种不希望的反应是不存在的Si-添加钢。然而，加Si钢中奥氏体的C富集并不高于无Si钢中的C富集，在无Si钢中明显发生了铁素体沉淀。因此，碳在加硅钢中的状态将通过高分辨率的分析技术，如原子探针层析成像进行研究。Q&P钢中奥氏体的热历史导致在配分步骤中压应力的发展。由于奥氏体的C富集与膨胀有关，因此奥氏体中存在的压应力被认为是阻止奥氏体C富集的障碍。因此，将检查在分配步骤期间施加外部拉伸应力是否有助于C的分配。最后，已知奥氏体的预应变影响马氏体转变的动力学。由于根据简化的Q&P工艺处理的钢中的奥氏体在RT下容易转变为马氏体，因此将研究在200 °C - 400 °C的温度范围内的预应变对在RT下变形诱导的马氏体转变的动力学的影响。

Quenching and partitioning (Q&P) processing is a method of stabilizing austenite in the microstructure of steels in order to enhance the strength-ductility combination. With the aid of a simplified Q&P processing consisting of quenching to room temperature (RT) and partitioning at 450 °C, different austenite fractions were adjusted in the microstructure of high C stainless steels containing 13 % Cr and additions of Co, Si, Mn, and N. Tensile properties were subsequently determined in the temperature range of 20 °C - 200 °C. Due to the insufficient C enrichment of austenite and the low stability of austenite against the deformation-induced martensitic transformation, tensile elongations were very low at RT. As the temperature was raised to 200 °C and the austenite stability increased, favorable combinations of strength and ductility were achieved (e.g. an ultimate tensile strength (UTS) of 1520 MPa and a uniform tensile elongation (UTE) of 25 % for the Si-added steel). An important objective of the second phase of the project is to design Q&P processing conditions associated with UTS x UTE products exceeding 30,000 MPa.% at RT. This will be done by applying subzero quench temperatures to the steels used in the first phase of the project so as to increase the fraction of martensite. Since martensite supplies the C required for the partitioning, an increase in its fraction will enhance the C enrichment of austenite. The effect of martensite fraction on the extent of partitioning will be subsequently studied by estimating the C enrichment of austenite by means of X-ray diffraction. Cementite precipitation in the martensite is one of the processes competing with the C enrichment of austenite in the Q&P steels. TEM investigations indicated that this undesirable reaction was absent in the Si-added steel. Nevertheless, the C enrichment of austenite in the Si-added steel was not higher than that in the Si-free steel where cementite precipitation had clearly occurred. Therefore, the status of C in the Si-added steel will be studied by high-resolution analytical techniques such as the atom probe tomography. The thermal history of austenite in Q&P steels results in the development of compressive stresses in the partitioning step. Since the C enrichment of austenite is associated with an expansion, the presence of compressive stresses in the austenite is considered to be a barrier against the C enrichment of austenite. Accordingly, it will be examined if the application of external tensile stresses during the partitioning step aids with the partitioning of C. Finally, the pre-straining of austenite is known to influence the kinetics of martensitic transformation. Since the austenite in the steels processed according to the simplified Q&P processing readily transformed to martensite at RT, the influence of pre-straining in the temperature range of 200 °C - 400 °C on the kinetics of deformation-induced martensitic transformation at RT will be investigated.