To create MS steels, the austenite that exists during hot-rolling or annealing is transformed almost entirely to martensite during quenching on the run-out table or in the cooling section of the continuous annealing line. The MS steels are characterized by a martensitic matrix containing small amounts of ferrite and/or bainite (note Figure 2-12). Within the group of multiphase steels, MS steels show the highest tensile strength level. This structure also can be developed with post-forming heat treatment. MS steels provide the highest strengths, up to 1700 MPa ultimate tensile strength. MS steels are often subjected to post-quench tempering to improve ductility, and can provide adequate formability even at extremely high strengths.
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Figure 2-12: Microstructure for MS 950/1200 |
Engineering and true stress-strain curves for MS steel grades are located in Figure 2-13 in the Advanced High-Strength Steels Application Guidelines.
Adding carbon to MS steels increases hardenability and strengthens the martensite. Manganese, silicon, chromium, molybdenum, boron, vanadium, and nickel are also used in various combinations to increase hardenability. MS steels are produced from the austenite phase by rapid quenching to transform most of the austenite to martensite.
Current production grades of MS steels and example automotive applications:
MS 950/1200 Cross-members, side intrusion beams, bumper beams, bumper reinforcements
MS1150/1400 Rocker outer, side intrusion beams, bumper beams, bumper reinforcements
MS1250/1500 Side intrusion beams, bumper beams, bumper reinforcements