Broadly speaking this work will focus on the effects of microstructure on the mechanical properties and formability of steels having ultimate tensile strengths of 1200 MPa and higher. Microstructurally, there are two main components to the work, In the first, PIs will evaluate the effects of microstructure on the mechanical behavior and formability of complex microstructures consisting of 0%, 25% or 50 % ferrite combined with 100%, 75% or 50% of either Martensite or Bainite. At each ferrite level the Martensite and Bainite will have similar strength levels. In all of these structures PIs can vary the amount and mechanical stability of the retained austenite. Thus PIs will address the effects of the amount of ferrite, of retained austenite characteristics and of a Bainitic structure as opposed to a Martensitic structure on work hardening behavior, ductility, toughness and formability. In the second, they will examine the effects of the resistance of inclusions to void nucleation on formability. In the broader impact, accomplishing the above will provide useful information on how to achieve high formability in high strength sheet steels which would be of benefit to those actually involved in the production and use of such materials.
One objective of this work has been the development of new steels and heat treatments which result in microstructures characterized by high strength, high uniform strains and total elongations to fracture which would allow good formability at much higher strength levels than steels currently used in automotive sheet applications. For this work four compositions were selected. One composition was the base steel and the other three compositions were the base steel modified by additions of aluminum and silicon. The composition of the base steel, designated MP1, was, in wt. %, 0.2C-4Mn. The steel MP2 was the base steel modified by 1.5 wt. % aluminum. The steel MP3 was the base steel modified by the addition of 1.5 wt. % silicon. The steel MP4 was the base steel modified by the addition of 3 wt. % silicon. The heat treatments selected were designed to obtain complex microstructures consisting of ferrite, martensite and retained austenite. All steels were annealed over a wide range of temperatures followed by an oil quench to room temperature and a temper at 200°C. The annealing temperatures ranged from high temperatures where the alloys would be completely austenitic and subsequent quenching would yield a martensitic structure. As the annealing temperature is lowered the annealing temperatures would be in the two phase a+g field and quenching would result in a mixture of ferrite, martensite and retained austenite. At the lowest annealing temperatures used the structures were essentially tempered martensite and contained no retained austenite. It was found for all four steels that for annealing temperatures in the two phase a+g field there were very high uniform strains, indicative of high working hardening, and high total % elongations in a tensile test. It was found that these high uniform strains were associated with large amounts of retained austenite. For example the steel containing 1.5 wt. % aluminum had a uniform strain of about 0.32 and a retained austenite content of about 31 volume % after annealing at 680°C. All steels except the base steel which contained no aluminum or silicon were characterized by a range of annealing temperatures where the uniform strains and the ultimate tensile strengths were both sufficiently high that the steels represented mechanical properties which were superior to sheet steels currently used for automotive sheet applications. The steel containing aluminum on annealing at 687°C had an ultimate tensile strength of about 735 MPa and a uniform strain of about 34%. After annealing at 720°C the steel containing 3 wt. % silicon had an ultimate tensile strength of about 1360 MPa, and a uniform strain of strain of about 0.20. The second objective of the work was to examine the effect of inclusion void nucleation resistance on the toughness of steels having strengths, which might be used in automotive sheet applications. Two heats were made having compositions in wt. % of 0.15C-3Ni-2Si. In one of these heats the sulfide inclusions were particles of calcium sulfide and these inclusions were not resistant to void nucleation. In the second of these heats the sulfur was gettered as particles of titanium carbosulfide. It is known that titanium carbosulfide inclusions in steel are resistant to void nucleation. Both steels were austenitized and oil quenched and then tempered at 200°. The heat in which the sulfur was gettered as calcium sulfide had a yield strength of 1086 MPa, an ultimate tensile strength of 1401 MPa, a true strain to fracture of 0.99 and a Charpy impact energy of 124 J. The heat in which the sulfur was gettered as titanium carbosulfide had a yield strength of 1138 MPa, an ultimate tensile strength of 1441 MPa, a true strain to fracture of 1.16 and a Charpy impact energy of 176 J. Thus the heat in which the sulfur was gettered as particles resistant to void nucleation had a much higher toughness. It is felt that steels in which the sulfur is gettered as particles of titanium carbosulfide could be used in automotive applications where energy absorbed during impact would be important for safety.
