Ridging Resistance and Formability in the AISI 430 Transformable Ferritic Stainless Steel

Abstract

Ferritic stainless steel sheets are commonly used in applications involving cold forming operations such as deep drawing to reach the final shape. Therefore, they are required to have good forming characteristics. Furthermore, they must maintain a good surface smoothness in order to remain visually appealing. Occurrence of a surface defect termed ridging during cold forming operations of ferritic stainless steel sheets is one of the factors restricting the use of this family of stainless steels in certain applications. Parts having undergone severe ridging need an extra polishing step which adds to their manufacturing cost. The present work primarily aimed at achieving an acceptable combination of ridging resistance and formability in the AISI 430 transformable ferritic stainless steel. For this, initially three different compositional variants of the AISI 430 were analyzed and differences in their microstructural evolution, which is of relevance to the ridging resistance, were studied. Hot rolled strips of the three compositions were subsequently cold rolled with and without the standard batch annealing step to see the influence of eliminating the batch annealing step on the ridging resistance and formability which is the subject of chapter 5. Cold rolling of unannealed hot strips which contained a small fraction of martensite was found to be associated with an enhanced ridging resistance but a lower r-value. The composition dependence of the ridging resistance was mainly justified based on the austenite fraction and the evolution of austenite fraction during hot rolling. Based on the observation of an enhanced ridging resistance in the presence of a small fraction of martensite during cold rolling (chapter 5), chapter 6 deals with the effect of deliberate introduction of approximately 15% and 35% fresh martensite in the hot rolled strip by annealing at two different intercritical annealing temperatures. The ridging resistance was found to increase with the martensite fraction so that the hot strip containing 35% martensite was almost ridging-free. Nevertheless, the r-value which can be used as a measure of formability was not satisfactory. Although the presence of martensite was found to be beneficial to the ridging resistance, it must eventually transform to ferrite to obtain fully ferritic sheets. Chapter 7 discusses the thermal restoration mechanisms (recrystallization and recovery) in the cold rolled sheets containing martensite.Due to the observation of a high ridging resistance but a low r-value in the single step cold rolled hot strips containing martensite, double step cold rolling with an intermediate annealing step of martensite-containing hot strips was performed in order to see how the ridging resistance and the formability are influenced. As reported in chapter 8, double step cold rolling improved the r-values of the martensite-containing microstructure by an amount proportional to the intermediate annealing time. Chapter 9 introduces a novel method to stabilize austenite in the AISI 430 grade, a concept which can be extended to a variety of other ferritic and martensitic grades as well. The quenching and partitioning (Q&P) processing, formerly applied to certain Advanced High Strength Steel (AHSS) grades only, is shown to be ideally suitable for the stainless steels. It is based on quenching the intercritical austenite to a temperature below the martensite start temperature in order to martensitically transform a controlled fraction of the high temperature austenite and obtain a smaller fraction of austenite to which the interstitial atoms C and N can diffuse from the supersaturated martensite. The austenite enrichment with C and N reduces the martensite start temperature of the remaining austenite, stabilizing it at room temperature. The presence of austenite can bring about the Transformation Induced Plasticity (TRIP) effect and have other possible implications for the mechanical properties improvement.