Advanced High Strength Sheet Steels by Nina Fonstein

Author:Nina Fonstein
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

After intensive laboratory studies aiming to find the optimal composition to ensure sufficient weldability and moderate strength, on the one hand, and to match stamping capabilities of carmakers of that time, on the other, NSC reported very promising formability behavior of 0.11C–1.18Si–1.85Mn steel, which contained about 6 % of retained austenite and had very attractive balance of mechanical properties: YS = 399 MPa and TS = 614 MPa at TE = 35 % (Katayama et al. 1992).

Proposed steel composition and annealing cycle suitable for existing CAPL lines encouraged obtaining uncoated TRIP steels with very high balance of their mechanical properties. However, high silicon content complicated manufacturing of these steels as hot galvanized.

As a result, there was a period of intensive search for alternative steel designs with additions of P, low Si, Al, etc., which will be discussed later.

In particular, Thyssen Krupp chose and consistently pursued compositions based on high aluminum content pioneering TRIP steel production in Europe with TS of 600, 700, and 800 MPa (Engl et al. 1998).

Since it was practically impossible to achieve desired length of isothermal holding time for bainite reaction in continuous galvanizing lines, the thermal cycles were modified as to approach pseudo-overaging profile at temperatures near bainite reaction region.

By the beginning of millennium, a general consensus was reached that with equal strength, the TRIP steels offered higher ductility (formability) and strain hardening (energy absorption capability) than the DP steels (Cornette et al. 2001).

At that time, however, the strength of commercial TRIP steels, as well as that of DP steels, was restricted by deterioration of press formability with increasing strength. As shown by the NSC data (Sakuma 2004), up to 2001, the leading high strength grades (total dual-phase and TRIP steels) were steels with TS ~ 590 MPa.

Since then, in response to growing requirements to higher strength of auto parts driven by their weight reduction, considerable experience has been accumulated in cold pressing of HSS and in manufacturing of coated steels with elevated content of Al or/and Si. TRIP steels with tensile strength of 690 and 780 have been developed and commercialized worldwide. Recently, TRIP 980 or TRIP-assisted grade with TS > 980 MPa have been developed as well (POSCO catalog, 2014).

Combination of effects of complicated phase transformations, the chemical composition, and processing on mechanical behavior of TRIP steel evoked deep interest from academia and university researchers worldwide and still continues to be the subject of deep studies using the most modern techniques (Zaefferer et al. 2004; Galan et al. 2012; Barbe et al. 2002; Sakuma et al. 1992).

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