Effect of κ-carbide on Rolling Cracking Phenomenon and Mechanical Properties in Ferritic Lightweight Steel

Abstract

Many efforts to reduce the weight of steel plates in automotive industries have been conducted in order to increase fuel efficiency and to decrease CO2 emissions. In addition to light weight needs, automotive steels require excellent strength to sustain automotive structures and to reduce the impact or shock in cases of accidents. Thus, highly deformable steel plates such as transformation induced plasticity (TRIP) steels and twinning induced plasticity (TWIP) steels have been actively developed. Recently, a large amount of Mn and Al has been added to automotive steels to achieve the lightweight effect as well as excellent strength and ductility, and their total amounts are generally larger than 15 wt.%. This addition leads to about 10% weight reduction in comparison with TRIP or TWIP steels, and often offers excellent properties such as strengths of over 780 MPa and elongation of over 30%. Lightweight steels containing a large amount of Mn have posed problems of the increased manufacturing cost or the deteriorated productivity due to the temperature decrease of the melt during the steel-making process. Thus, efforts to reduce the Mn content have been made. When the Mn and Al addition is lower than 10 wt.%, however, the productivity decreases because cracks often occur on hot-rolled and cold-rolled steel plates. Since the lightweight steels containing about 10 wt.% of Mn and Al are ones of new advanced steels under recent development, the detailed formation process of microstructures or the deformation and fracture mechanisms are hardly known. Furthermore, very few studies have been conducted in order to systematically explain the deformability and cracking phenomenon of rolled steel plates in terms of microstructural parameters, except some studies related with the textures and the segregation of precipitates.Alloying elements used for lightweight steels mainly include Mn, Al, and C because the decreased specific gravity due to substitutional or interstitial atoms works to reduce the weight. Mn raises the volume fraction of austenite at high temperatures as an austenite stabilizer, but the increased Mn content often leads to the formation of a large amount of ferrite during cooling. It also poses problems such as increased manufacturing costs and deteriorated productivity because the temperature of the steel melt can be lowered during the steel-making process. Thus, efforts to reduce the Mn content in the lightweight steels have been made. Al, a ferrite stabilizer, helps form a dual phase structure of ferrite and austenite at high temperatures, and promotes the precipitation during cooling. When the steels contain hardenability elements such as C, precipitates such as carbides or nitrides are well formed, and the amount of precipitates varies with contents of Mn and Al as well as C. According to these precipitates, the steels often expose to the cracking occurring during cold rolling.Several recent investigations revealed that the Al addition up to ~10 wt.% into the high Mn austenitic steels is beneficial for achieving not only remarkable weight savings but also mechanical properties comparable to or better than those of TWIP steels. These improvements mainly result from the SFE increase with increasing the Al content in the high Mn austenitic steels and the corresponding change of the deformation mode from TWIP to dislocation slip. It is also reported that the C addition into the high Mn–Al austenitic steels provides further strengthening by κ-carbides precipitation. As compared to austenite-base lightweight steels, however, the ferrite-base lightweight steels have not been received attention at all. Considering that the properties of steels are largely dependent on the types of constituent phases, it is of scientific and technological interests to study the deformation and fracture behavior, and mechanical properties of lightweight steels with various matrix and second phases.Lightweight steels can be divided into austenite- and ferrite-based ones according to their matrix microstructures. Typical austenitic lightweight steels are transformation-induced plasticity (TRIP) steels or twinning-induced plasticity (TWIP) steels, in which the austenite is stabilized at room temperature. There have been many researches which intend to increase the formability by adding Al, which raises the stacking fault energy and stabilizes the ferrite, and by controlling deformation mechanisms. Ferritic lightweight steels have advantages of lightweight effects because of the higher content of Al rather than Mn. They have only the ferrite as a matrix at room temperature, although both the austenite and ferrite exist at high temperatures. The development of ferritic lightweight steels started in late 1990’s as Baligidad et al., suggested Fe-Al-C alloying systems. Controlling Al and C contents in 8.5~11 wt.% and 0.5~1.0 wt.%, respectively, both the strength and ductility were improved by homogeneously distributing large amounts of κ-carbides, which have the composition of (Fe,Mn)3-Al-C and perovskite structure. In 1980’s when researches on κ-carbides just began, it was thought that κ-carbides had a harmful effect on ductility because of its hardness, and the formation of κ-carbides was inhibited by adding precipitation-hardening elements such as B, Ti, and Nb. Recently, κ-carbides have been used for increasing mechanical properties by controlling their size, fraction, and distribution. However, the formation processes of κ-carbides have not yet been fully understood, and it still has been reported that κ-carbides induced the cracking by forming band structures during hot- or cold-rolling of ferritic lightweight steels. Therefore, for the successful development of ferritic lightweight steels, the verification of formation mechanisms of κ-carbides, which can vary with rolling or heat-treatment conditions, is essentially needed. From these understandings, it is possible to fabricate lightweight steels without any defects and to improve their mechanical properties simultaneously.