Steels for bearings

Manganese enrichment of austenite during prolonged intercritical annealing was used to produce a family of transformation-induced plasticity (TRIP) steels with varying retained austenite contents. Cold-rolled 0.1C-7.1Mn steel was annealed at incremental temperatures between 848 K and 948 K (575 °C and 675 °C) for 1 week to enrich austenite in manganese. The resulting microstructures are comprised of varying fractions of intercritical ferrite, martensite, and retained austenite. Tensile behavior is dependent on annealing temperature and ranged from a low strain-hardening “flat” curve to high strength and ductility conditions that display positive strain hardening over a range of strain levels. The mechanical stability of austenite was measured using in-situ neutron diffraction and was shown to depend significantly on annealing temperature. Variations in austenite stability between annealing conditions help explain the observed strain hardening behaviors.

Austenite Stability Effects on Tensile Behavior of Manganese-Enriched-Austenite Transformation-Induced Plasticity Steel

Thermomechanical processing of advanced high strength steels

Extensive research efforts are underway globally to develop new steel microstructure concepts for high-strength sheet products, driven largely by the need for lightweight automotive structures in support of designs to enhance occupant safety and energy efficiency. One promising approach, involving the quenching and partitioning (Q&P) process, was introduced in the predecessor to this paper series, Austenite Formation and Decomposition, 2003.[1] Development of the Q&P process has continued through to the present, and the current status is highlighted in this article, along with some alternative approaches that are also receiving attention. Special emphasis is placed on the synthesis and interpretation of the fundamental phase transformation responses, perspectives related to alloying and processing, and the resulting microstructure and properties. Key mechanistic issues are discussed, including carbide formation and suppression, migration of the martensite/austenite interface, carbon partitioning, and partitioning kinetics.

Analysis of Microstructure Evolution in Quenching and Partitioning Automotive Sheet Steel

THE only addition to this issue of “Steel and its Heat Treatment” is a chapter on nitriding, contributed by Dr. V. O. Homerberg. In the space of sixteen pages a brief outline is given of the nature and conduct of the nitriding process, of the steels suitable for nitriding, and of their properties after treatment. Otherwise the book remains the same, although eight years have passed since the preparation of the previous edition. There is still, therefore, no reference to the notched-bar impact tests to which British metallurgists rightly attach so much importance, to grain size control (although this originated in America), to the newer types of stainless steels, or to manganese -molybdenum steels.Steel and its Heat TreatmentD. K.BullensBy. Third edition, rewritten and reset. Pp. xiii + 580. (New York: John Wiley and Sons, Inc.; London: Chapman and Hall, Ltd., 1935.) 25s. net.

Steel and its heat treatment

The body design with light weight and enhanced safety is a key issue in the car industry. Corresponding to this trend, POSCO is developing various automotive steel products with advanced performance. Conventional advanced high strength steels such as DP and TRIP steels are now expanding their application since the steels exhibit higher strength and ductility than those of conventional solution and precipitation strengthened high strength steels. Efforts have been made to enhance the mechanical performance of these steels such as ductility, hole expansion ratio, deep drawability, etc. Current research is focused on development of extra- and ultra-AHSS. Extra-AHSS are designed to utilize nano-scale retained austenite embedded in fine bainite and martensite. Ultra-AHSS are designed to have austenite as the major phase, and the ductility is enhanced primarily by continuous strain hardening generated during forming. These steels including extra- and ultra-AHSS are believed to be the next generation automotive steels which will replace the existing high strength steels due to their extremely high strength and ductility combinations.

New Trends in Advanced High Strength Steel Developments For Automotive Application

Ultra-fine grained TRIP steels (UFG-TRIP) containing 6wt%Mn were produced by intercritical annealing. An ultra-fine grained microstructure with a grain size less than 1μm was obtained. The formation mechanism of the high volume fraction of retained austenite was investigated by dilatometry, XRD and magnetic saturation. The fraction of retained austenite was strongly dependent on the annealing temperature. The tensile properties were also found to be strongly influenced by the annealing temperature with poorer mechanical properties being observed at higher annealing temperatures. It was found that the stabilization of the retained austenite was both a composition and size-effect, made possible by the grain refinement due to the reversely transformed martensite.

Ultra Fine-grained 6wt% Manganese TRIP Steel

Enhanced mechanical property of Fe–Al alloy due to Mn insertion: ab initio study

Effect of volume fraction of undissolved cementite on the high cycle fatigue properties of high carbon steels

By applying a double cold rolling and annealing treatment, the evolution of the α and γ fiber components differed from the ones observed in conventional processing. This is attributed to the difference of the initial texture. An increased reduction of stored energy of the {111} component was found by monitoring the change of the stored energy during annealing, indicating that the onset of the nucleation stage of recrystallization by relaxation and annihilation of dislocations occurred mainly on the {111} component with its higher stored energy. The detailed texture evolution of the double cold rolled specimen during 2nd annealing is described by coupling the theory of oriented nucleation and orientation pinning, which is experimentally confirmed by OIM scan.

Kyoo Young Lee

Papers

New Trends in Advanced High Strength Steel Developments For Automotive Application

Ultra Fine-grained 6wt% Manganese TRIP Steel

Enhanced mechanical property of Fe–Al alloy due to Mn insertion: ab initio study

Effect of volume fraction of undissolved cementite on the high cycle fatigue properties of high carbon steels

Effect of Initial Texture on the Evolution of Texture and Stored Energy during Recrystallization of Interstitial Free Steel