Disclosed is a grain-oriented electrical steel sheet capable of obtaining excellent magnetic properties stably over the entire coil length. A grain-oriented electrical steel sheet includes: a chemical composition containing, in mass %, C: 0.005% or less, Si: 2.0% to 4.5%, and Mn: 0.01% to 0.5%, and, in mass ppm, N: 20 ppm or less, each of Se, Te, and O: less than 50 ppm, S: less than 30 ppm, and acid-soluble Al: less than 40 ppm, and Ti: less than 30 ppm, of which 5 ppm or more and 25 ppm or less is acid-soluble Ti, with the balance being Fe and inevitable impurities; and precipitates containing Ti and N with a grain size of 200 nm or more at a frequency of 0.05 grains/mm.sup.2 or more.

The steel plate has a specific chemical composition, and a microstructure where a volume fraction of martensite at a depth of 1 mm from a surface of the steel plate is 95% or more, and at a depth of 1 mm from a surface of the steel plate, a Vickers hardness at 400° C. is 288 or more, and a Brinell hardness at 25° C. is 360 HBW10/3000 to 490 HBW10/3000.

This application discloses thin metal strips and methods of making thin metal strip. Particular embodiments of such methods include cooling the thin metal strip to a temperature equal to or less than a bainite or a martensite start transformation temperature B.sub.S or M.sub.S to thereby form bainite and/or martensite, respectively, within the thin metal strip, reheating the thin metal strip to a reheat temperature equal to or greater than transformation temperature Ac.sub.3 and holding the thin metal strip at the reheat temperature for at least 2 seconds and thereby forming austenite within the thin metal strip with at least 75% of austenite grains having a grain size equal to or less than 15 μm, and rapidly recooling the thin metal strip to a temperature equal to or less than the martensite start transformation temperature M.sub.S and thereby providing finer martensite within the thin metal strip from a finer prior austenite.

Disclosed is 500-MPa low-yield-ratio weather-resistant bridge steel and a manufacturing method therefor; the weather-resistant bridge steel includes the following components in percentage by mass: C: 0.04%-0.09%, Si: 0.15%-0.30%, Mn: 1.40%-1.50%, P: 0.009%-0.015%, S: ≤0.002%, Nb: 0.020%-0.050%, Ti: 0.010%-0.020%, V: 0.010%-0.030%, Cu: 0.30%-0.40%, Ni: 0.30%-0.45%, Cr: 0.45%-0.60%, Mo: 0.08%-0.15%, Alt: 0.02%-0.04%, and the balance Fe and inevitable impurities; through scientific component designing and a matched manufacturing method combining controlled rolling and cooling and tempering, the weather-resistant bridge steel has a low yield ratio, high low-temperature toughness and high elongation.

Disclosed is 500-MPa low-yield-ratio weather-resistant bridge steel and a manufacturing method therefor; the weather-resistant bridge steel includes the following components in percentage by mass: C: 0.04%-0.09%, Si: 0.15%-0.30%, Mn: 1.40%-1.50%, P: 0.009%-0.015%, S: ≤0.002%, Nb: 0.020%-0.050%, Ti: 0.010%-0.020%, V: 0.010%-0.030%, Cu: 0.30%-0.40%, Ni: 0.30%-0.45%, Cr: 0.45%-0.60%, Mo: 0.08%-0.15%, Alt: 0.02%-0.04%, and the balance Fe and inevitable impurities; through scientific component designing and a matched manufacturing method combining controlled rolling and cooling and tempering, the weather-resistant bridge steel has a low yield ratio, high low-temperature toughness and high elongation.

A method of heat treating a high strength cold rolled steel strip including a) soaking a cold rolled steel strip, b) cooling the soaked steel strip c) heat treating the cooled strip; d) cooling the heat treated steel strip to ambient temperature range;
such that the steel strip has a microstructure including various ferrites, retained austenite and martensite. The main components in the steel composition includes carbon, manganese, silicon and aluminium in addition to iron.

A method of heat treating a high strength cold rolled steel strip including a) soaking a cold rolled steel strip, b) cooling the soaked steel strip c) heat treating the cooled strip; d) cooling the heat treated steel strip to ambient temperature range;
such that the steel strip has a microstructure including various ferrites, retained austenite and martensite. The main components in the steel composition includes carbon, manganese, silicon and aluminium in addition to iron.

A heat treatment of a high strength cold rolled steel strip includes the steps of a) soaking a cold rolled steel strip, b) cooling the soaked steel strip c) heat treating the cooled strip; d) cooling the heat treated steel strip to ambient temperature range;
such that the steel strip has a microstructure including various ferrites, retained austenite and martensite. The main components in the steel composition include carbon, manganese, silicon and aluminium in addition to iron.

A heat treatment of a high strength cold rolled steel strip includes the steps of a) soaking a cold rolled steel strip, b) cooling the soaked steel strip c) heat treating the cooled strip; d) cooling the heat treated steel strip to ambient temperature range;
such that the steel strip has a microstructure including various ferrites, retained austenite and martensite. The main components in the steel composition include carbon, manganese, silicon and aluminium in addition to iron.

A method of heat treating a cold rolled steel strip includes soaking a cold rolled steel strip having a specific composition above (Ac3−60)° C. for a certain duration thereby obtaining a cold rolled strip having a partially austenitic microstructure; cooling of the resulting soaked steel strip to a temperature below Ms; heating and heat treating the cooled steel strip in the temperature range of Bs-Ms; and cooling the heat treated steel strip to ambient temperature.