Processing of thin sheet multiphase steel grades

Abstract

Despite the extensive research and development work done in the field of high strength thin sheet multiphase steel grades, in the past, there are still two main challenges to be tackled, particularly for steel grades with tensile strength levels above 780 MPa. First a challenging optimisation and improvement of the overall processing in the steel plant is still necessary. This includes modified and improved layouts of processing lines. Further, due to the limited application and lacking feedback an overall customer oriented development and optimization is still necessary. Therefore, in a first step, this work reports on the alloy concepts and phase transformations in the annealing lines required for the adjustment of the microstructure of DP and TRIP grades. Then, the most critical processing steps are mentioned and in particular the demanded processing parameters in the annealing lines and the resulting layouts are highlighted. In a further part, three selected items of a costumer oriented development and optimisation of DP and TRIP steel grades are discussed. These include the improvement of the flange‐ability and bendability and efforts oriented towards the reduction of the banded structure in DP and TRIP steel grades. Finally, the improvement of the fracture appearance of spot welded joints of TRIP steels is highlighted and alloy concepts with a balance between weldability and formability are introduced.

Formation of Medium Carbon TRIP Steel Microstructure During Annealing in the Intercritical Temperature Range

“…In addition, a possibility of a mechanical destabilisation of retained austenite and its transformation to martensite causes that during forming process they are becoming strengthened [4,5]. Thus, in consequence, after finishing the technological process, elements produced of the TRIP steel can preserve high strength at a reduction of their cross-section [6][7][8][9][10].…”

Segregation Behaviour of Third Generation Advanced High-Strength Mn-Al Steels

“…TRIP steels contain also Si and Al for hampering of carbide precipitation and more recently Mo addition and Nb, Ti or V microadditions are used for further strength improvement [3][4][5][6]. Unfortunately, Mn alloying results in some problems, e.g.…”

“…Unfortunately, Mn alloying results in some problems, e.g. ability to microsegregation, localized deformation and hot-working [3][4][5][6][7]. A banded microstructure of ferrite / bainite-martensite-austenite is typical for TRIP steels [6 and 7].…”

“…A banded microstructure of ferrite / bainite-martensite-austenite is typical for TRIP steels [6 and 7]. The metallurgical reason for banded structures is segregation of Mn and Al during continuous casting [6]. Pichler et al [6] noticed that the most significant hard bainite-martensite-austenite bands are occurred in the centre of the strips.…”

“…The metallurgical reason for banded structures is segregation of Mn and Al during continuous casting [6]. Pichler et al [6] noticed that the most significant hard bainite-martensite-austenite bands are occurred in the centre of the strips. Severe plastic deformation can lead to some damaged zones in these bands.…”