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

淬火配分（Q&P）处理是一种稳定钢组织中奥氏体的方法，以提高钢的强度-塑性组合。采用简化的Q&P工艺，即淬火至室温（RT）和450℃分块，对含有13% Cr并添加Co、Si、Mn和n的高C不锈钢组织中的不同奥氏体组分进行了调整，并在20℃- 200℃温度范围内测定了拉伸性能。由于奥氏体的C含量不足，以及奥氏体对变形诱发马氏体转变的稳定性较低，在高温下拉伸伸长率很低。当温度升高到200℃时，奥氏体的稳定性增加，强度和延展性的良好组合（例如，添加si的钢的极限抗拉强度（UTS）为1520 MPa，均匀拉伸伸长率（UTE）为25%）。项目第二阶段的一个重要目标是设计与超过30,000 MPa的UTS x UTE产品相关的Q&P处理条件。这将通过对项目第一阶段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。由于马氏体提供分配所需的C，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分配程度的影响。马氏体渗碳体析出是与奥氏体C富集相竞争的过程之一。TEM研究表明，在添加硅的钢中不存在这种不良反应。然而，加硅钢中奥氏体的C富集程度并不高于无硅钢，而无硅钢中渗碳体析出明显。因此，将利用原子探针层析成像等高分辨率分析技术来研究加硅钢中C的状态。Q&P钢中奥氏体的热历史导致了分配阶段压应力的发展。由于奥氏体的C富集与膨胀有关，因此压应力的存在被认为是奥氏体中C富集的障碍。因此，将研究在分配步骤中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会影响马氏体转变的动力学。由于经简化Q&P工艺处理的钢中的奥氏体在高温下容易转变为马氏体，因此将研究在200°C - 400°C温度范围内预应变对高温下变形诱发马氏体转变动力学的影响。

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.