Dual-phase steel sheet

A micro-duplex steel sheet comprises a ferrite matrix having a secondary phase of martensite or martensite and retained austenite dispersed therein. The above microstructure occupies at least 90% of the steel. The secondary phase has an average grain size of 3 microns or less, occupies 10-60% of the steel and is present in ferrite grains and at grain boundaries with the proportion of the secondary phase present in the ferrite grains is 50% or more. The steel comprises (by weight): 0.02-0.3 % C, 0.01-3 % Si, 0.5-3% Mn, 0.0001-0.005% B, 0.01-1.5 % Al, with the balance being iron and impurities and is processed by cooling from at least the A3 temperature or 600-1000{C to a temperature of Ms or lower at a rate of 0.2-20 {Cs . 0.03-1 % Mo, 0.01-0.1 % total Nb, Ti and V, 0-0.5 % Ni, 0-0.5 % Cu, 0-1.5 % Cr, 0-0.003 % Ca and/or 0-0.003 % REM may also be present.