A method of making thin steel strip having a thickness less than 5 mm comprising: providing a pair of counter-rotatable casting rolls having casting surfaces laterally positioned to form a gap at a nip between the casting rolls through which a thin steel strip having a thickness of less than 5 mm can be cast, providing a steel delivery system adapted to deliver molten steel above the nip to form a casting pool, the casting pool being supported on the casting surfaces of the pair of counter-rotatable casting rolls and confined at the ends of the casting rolls, delivering a molten steel to the delivery system to produce a thin steel strip comprising the following composition: by weight, between 0.20% and 0.35% carbon, less than 1.0% chromium, less than 1.0% nickel, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than 0.08% niobium, less than 0.08% vanadium, less than 0.5% molybdenum, silicon killed with less than 0.01% aluminum; delivering the molten steel from the delivery system above the nip to form the casting pool; counter rotating the pair of counter-rotatable casting rolls to form steel shells on the casting surfaces of the casting rolls that are brought together at the nip to deliver the thin steel strip downwardly, the thin steel strip having a thickness less than 5 mm, cooling the thin steel strip to a temperature equal to or less than a bainite or a martensite start transformation temperature B S or M S to thereby form bainite and/or martensite, respectively, within the thin steel strip, reheating the thin steel strip to a reheat temperature equal to or greater than transformation temperature Ac 3 and holding the thin steel strip at the reheat temperature for at least 2 seconds and forming austenite within the thin steel strip with at least 75% of austenite grains having a fine grain size equal to or less than 15 μm, and rapidly recooling the thin steel strip at a rate equal to or greater than 100 °C/s to a temperature equal to or less than the martensite start transformation temperature M S and providing finer martensite within the thin steel strip from a finer grain size austenite, where at least 75% of the finer prior austenite grains have a grain size equal to or less than 15 μm. providing a pair of counter-rotatable casting rolls having casting surfaces laterally positioned to form a gap at a nip between the casting rolls through which a thin steel strip having a thickness of less than 5 mm can be cast, providing a steel delivery system adapted to deliver molten steel above the nip to form a casting pool, the casting pool being supported on the casting surfaces of the pair of counter-rotatable casting rolls and confined at the ends of the casting rolls, delivering a molten steel to the delivery system to produce a thin steel strip comprising the following composition: by weight, between 0.20% and 0.35% carbon, less than 1.0% chromium, less than 1.0% nickel, between 0.7% and 2.0% manganese, between 0.10% and 0.50% silicon, between 0.1% and 1.0% copper, less than 0.08% niobium, less than 0.08% vanadium, less than 0.5% molybdenum, silicon killed with less than 0.01% aluminum; delivering the molten steel from the delivery system above the nip to form the casting pool; counter rotating the pair of counter-rotatable casting rolls to form steel shells on the casting surfaces of the casting rolls that are brought together at the nip to deliver the thin steel strip downwardly, the thin steel strip having a thickness less than 5 mm, cooling the thin steel strip to a temperature equal to or less than a bainite or a martensite start transformation temperature B S or M S to thereby form bainite and/or martensite, respectively, within the thin steel strip, reheating the thin steel strip to a reheat temperature equal to or greater than transformation temperature Ac 3 and holding the thin steel strip at the reheat temperature for at least 2 seconds and forming austenite within the thin steel strip with at least 75% of austenite grains having a fine grain size equal to or less than 15 μm, and rapidly recooling the thin steel strip at a rate equal to or greater than 100 °C/s to a temperature equal to or less than the martensite start transformation temperature M S and providing finer martensite within the thin steel strip from a finer grain size austenite, where at least 75% of the finer prior austenite grains have a grain size equal to or less than 15 μm.