Dual-phase hot-rolled steel strip

Claim 1

A dual-phase hot-rolled steel strip characterized as having a microstructure in the as-rolled condition comprised of a substantially continuous matrix comprised predominantly of polygonal ferrite having interspersed therethrough discrete islands of martensite, said steel containing as its essential alloying constituents from about 0.05% to about 0.11% carbon, about 0.6% to about 1.8% manganese, about 0.7% to about 1.2% silicon, about 0.2% to about 0.4% molybdenum, about 0.3% to about 0.9% chromium, up to about 0.1% vanadium, and the balance consisting essentially of iron along with the conventional impurities and residuals present in the usual amounts.

Claim 7

The method of making a dual-phase hot-rolled steel strip characterized as having a microstructure in the as-rolled condition comprised of a matrix of polygonal ferrite having interspersed therethrough discrete islands of martensite which comprises the steps of forming a solidified mass of an alloy consisting essentially of about 0.05% to about 0.11% carbon, about 0.6% to about 1.8% manganese, about 0.7% to about 1.2% silicon, about 0.2% to about 0.4% molybdenum, about 0.3% to about 0.9% chromium, up to about 0.1% vanadium, and the balance consisting essentially of iron along with the usual impurities and residuals present in conventional amounts; heating said mass to an elevated temperature for a period of time sufficient to convert the microstructure of said mass to the austenitic form, deforming said mass in the temperature range of about 2150° F to about 1600° F, followed by cooling at a controlled cooling rate through a transformation range whereby the predominant portion of austenite is transformed to polygonal ferrite in a manner to avoid the formation of appreciable amounts of pearlite, coiling the deformed said mass into a coil at a coiling temperature ranging from about 1000° F to about 1200° F and thereafter permitting said coil to further air cool and effect a transformation of the predominant remaining portion of austenite to martensite in the form of discrete islands interspersed through the substantially continuous polygonal ferrite matrix.

Claim 8

The method of making a dual-phase hot-rolled steel strip characterized as having a microstructure in the as-rolled condition comprised of a matrix of polygonal ferrite having interspersed therethrough discrete islands of martensite which comprises the steps of forming a solidified mass of an alloy consisting essentially of about 0.05% to about 0.11% carbon, about 0.6% to about 1.8% manganese, about 0.7% to about 1.2% silicon, about 0.2% to about 0.4% molybdenum, about 0.3% to about 0.9% chromium, up to about 0.1% vanadium, and the balance consisting essentially of iron along with the usual impurities and residuals present in conventional amounts; heating said mass to an elevated temperature for a period of time sufficient to convert the microstructure of said mass to the austenitic form, deforming said mass in the temperature range of about 2150° F to about 1600° F, followed by cooling at a controlled cooling rate through a transformation range whereby the predominant portion of austenite is transformed to polygonal ferrite in a manner to avoid the formation of appreciable amounts of pearlite, coiling the deformed said mass into a coil at a coiling temperature ranging from about 1000° F to about 1200° F and thereafter permitting said coil to further air cool and effect a transformation of the predominant remaining portion of austenite to martensite in the form of discrete islands interspersed through the substantially continuous polygonal ferrite matrix.