SynopsisBecause of improved strength-ductility combination over HSLA steels, dual phase steels have recently become of commercial importance to both the sheet users and producers. These steels possess good properties by virtue of their microstructure which consists, typically, of about 15'20 % martensite uniformly distributed in a soft matrix of ferrite. Although, the desired microstructural features of a dual phase steel can be obtained by various process routes, the most economical method is the production of this steel in as hot rolled condition. The sucessful production of dual phase steels in the hot strip mill, however, requires a careful control of process parameters particularly the finishing temperature, the cooling of the sheet on the runout table, the coiling temperature and the subsequent cooling of the coils.As a development effort some commercial heats of dual phase steel in C-Mn-Si-Cr-Mo chemistry have been produced in as hot rolled condition at Rourkela Steel Plant. The effect of coiling temperature and cooling rate on the final structure and properties of the steel has been discussed in detail. All the coils coiled at lower temperatures of about 470° C or less showed dual phase structure with uniform properties where as coiling at about 500 °C or above did not yield the desired microstructure and properties. Possible reasons have been given to explain the effect of coiling temperature on microstructure. Acceleration of the cooling rate of the coil, after coiling, has been found to improve the tensile strength without significantly affecting the ductility of the material.

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Although a great deal of mechanical properties data on titanium alloys have appeared in the literature, no attempt has been made to relate the fine structures in beta titanium alloys to strength and fracture toughness. Therefore, strength and fracture toughness properties of B-120VCA—obtained through a variety of thermal and thermomechanical treatments—were related to fine structures by transmission electron microscopy, electron fractography, etc. Effects of (1) prior solution-treating temperatures, (2) cold working after solution treating, and (3) cold working both before and after solution treating were evaluated in terms of strength and fracture toughness, and parallel fine structure studies. The double cold-work treatment, without aging, produced σt with a fracture toughness of 115 ksi√in. and may be promising for certain high-strength applications. Also short aging times produce unique combinations of strength and fracture toughness (σt ≈ 190 ksi, Kc ≈ 70 ksi√in. The kinetics of β→ α transformation were characterized and related to fracture toughness and strength properties. Precipitation of omega was suggested in the solution-annealed material during cooling to room temperature; further growth of this phase upon prolonged aging at 600 F was established. At 900 F, omega growth was limited, being replaced by alpha precipitation first in a discontinuous and later in a general dispersion.

A new replica technique for electron microscopy of fracture surfaces is described, which can reveal the relationship between the microstructural features of metals and alloys and the detailed topography of their fracture surfaces. This etch-shadow extraction replica technique combines in one replica the salient features of shadowed-surface replicas and precipitate-extraction replicas. Thus a bridge is provided between the fracture topography, which is governed mainly by the effects of fracture-velocity changes, and the significant microstructural features such as precipitates, grain boundaries, etc.

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