Influence of aluminum as alloying element on the microstructure of stainless steels with predominantly austenitic matrices

在该项目的第一阶段，向不锈钢中添加质量分数为7%的铝，以获得奥氏体(六种合金)、双相(一种合金)和铁素体(一种合金)基组织的钢。对于含奥氏体钢，测定了拉伸性能的温度依赖性。Al的加入明显提高了奥氏体相的稳定性，表现为Mdγ→αʹ温度的降低。这一效应与Al的SFE升高效应有很好的相关性。分析奥氏体和双相合金拉伸性能和应变硬化行为的温度相关性，能够建立滑动平面度、应变硬化速率的线性和拉伸伸长率之间的相互关系。含7%Al的铁素体钢由于形成B2-(Ni，Fe)Al金属间化合物而发生脆化。时效处理还可以在双相合金中诱导出B2金属间化合物。该项目第二阶段的目标之一是设计在室温附近具有最大拉伸伸长率的铝合金钢。该项目第一阶段的见解将作为化学成分设计的指导方针。设计的基础是认识到C、Cr、Ni、Mn和Al降低了Mdγ→αʹ温度。据报道，除Al外，所有这些元素对马氏体开始温度(Ms)都有相似的影响。考虑到合金元素对Md、γ→αʹ和Ms温度影响的一致性，在不锈钢中添加Al有望降低Md、Ms值。因此，该项目第二阶段的目标之一是重新审查铝对ms温度的影响。第二阶段的另一个目标是研究含镍铝合金铁素体不锈钢中B2金属间化合物的形成。第一阶段介绍的基于膨胀的直接方法将被用来研究一系列同时含有Ni和Al的合金中的B2析出。这些知识有助于设计具有B2强化铁素体成分的双相不锈钢。淬火分配(Q&P)工艺是在马氏体-奥氏体钢中获得第三代先进高强度钢性能的有效方法。Q&P的成功应用要求抑制马氏体中渗碳体的析出，这是一种与奥氏体富碳竞争的过程。Al被认为是一种阻碍TRIP钢等温淬火过程中渗碳体析出的合金元素。本方案的最后一个目的是研究Al对马氏体-奥氏体不锈钢马氏体成分中准平衡渗碳体析出和奥氏体富碳的影响。

In the first phase of the project, Al in quantities up to 7 mass-% was added to stainless steels to obtain steels with austenitic (six alloys), duplex (one alloy), and ferritic (one alloy) matrix microstructures. For steels containing austenite, the temperature dependence of tensile properties was determined. Al addition was associated with a clear increase in the stability of austenite evidenced by a decrease in the Mdγ→αʹ temperature. This effect correlates well with the SFE-raising effect of Al. Analysis of the temperature dependence of tensile properties and strain hardening behavior for austenitic and duplex alloys enabled to establish interrelationships among glide planarity, linearity of the strain hardening rate, and tensile elongation. The ferritic steel containing 7 mass-%Al showed embrittlement caused by the formation of B2-(Ni,Fe)Al intermetallics. B2 intermetallics could also be induced in the duplex alloy by an aging treatment. One of the objectives of the second phase of the project is to design Al-alloyed steels exhibiting maximum tensile elongation near room temperature. Insights from the first phase of the project will serve as guidelines for the design of the chemical composition. The design is based on the recognition that C, Cr, Ni, Mn, and Al decrease the Mdγ→αʹ temperature. Reportedly, all of these elements except Al have a similar effect on martensite start (Ms) temperature. Given the consistency of the effect of alloying elements on Mdγ→αʹ and Ms temperatures, Al addition to stainless steels is expected to decrease the Ms temperature. Therefore, one of the objectives of the second phase of the project is to re-examine the reported influence of Al on Ms temperature. Another objective of the second phase is to study the formation of B2 intermetallics in Al-alloyed ferritic stainless steels containing Ni. The straightforward dilatometry-based method introduced in the first phase will be used to study B2 precipitation in a series of alloys containing both Ni and Al. This knowledge is an aid in the design of duplex stainless steels with B2-strengthened ferritic constituent. Quenching and Partitioning (Q&P) processing is an effective method of obtaining 3rd generation advanced high-strength steel properties in steels with martensitic-austenitic microstructures. Successful application of Q&P requires that the precipitation of cementite in martensite, a process that competes with the C enrichment of austenite, is inhibited. Al is regarded as an alloying element opposing the precipitation of cementite during austempering of TRIP-assisted steels. The last objective of the proposal is to study the influence of Al on the precipitation of paraequilibrium cementite in the martensitic constituent of martensitic-austenitic stainless steels and on the C enrichment of austenite.