A study of five laboratory steels was conducted to determine the relative effects of ferrite grain refinement, dislocation substructure, and niobium (carbon, nitrogen) [Nb(C,N)] precipitation strengthening on the as-rolled strength and toughness properties of 19-mm (0.75-in.) plates finish rolled in the two-phase region, moderately below the Ar₃ temperature. The steels were molybdenum-niobium (Mo-Nb) pearlite-reduced steels having a nominal composition range of 0.08C, 1.4Mn, 0.1Si, 0.2/0.4Mo and 0/0.6Nb. The amount of ferrite formed prior to the last pass and the percent reduction imparted on the last pass were varied while the finishing temperature was held constant at 745°C (1375°F).

It was found that the introduction of deformed ferrite into the structure brought two strengthening effects: dislocation substructure and, in the niobium-containing steels, strain-enhanced precipitation. The combined effect of these mechanisms reached a maximum in plates containing approximately 30 to 40 percent deformed ferrite that had experienced a 20 percent reduction on the final pass. The impact toughness was not impaired by this method of upgrading strength.

Regarding grain refinement, the 0.2Mo-0.035Nb steel experienced increases in yield strength of up to 85 MPa (12.4 ksi) due to the additional grain refinement produced by rolling below the Ar₃ temperature instead of finishing just above the Ar₃. This type of strengthening was smaller in the 0.2Mo-0.06Nb steel and absent in the 0.4Mo-0.06Nb steel, because the initial grain sizes of these steels were finer, and lowering the finish-rolling temperature produced little or no further refinement.