Method for manufacturing a steel sheet

A method for manufacturing steel sheet comprising the steps of:

• rough-rolling a continuously cast slab containing more than 0.8% C by weight to form a sheet bar;
• finish-rolling the sheet bar at finish temperatures of (Arcm transformation point −20° C.) or more to form a steel strip;
• primary-cooling the finish-rolled steel strip at cooling speeds of more than 120° C./sec to temperatures of from 500 to 800° C.;
• air-cooling the primary-cooled steel strip for a period of from 1 to 30 seconds;
• secondary-cooling the air-cooled steel strip at cooling speeds of 20° C./sec or more;
• and
• coiling the secondary-cooled steel strip at coiling temperatures of 650° C. or less.

A method for manufacturing steel sheet comprising the steps of:

• continuously casting a steel consisting essentially of 0.04 to 0.2% C, 0.25 to 2% Si, 0.5 to 2.5% Mn, and 0.1% or less Al, by weight, to form a slab;
• and
• applying hot-rolling by rough-rolling the slab to form a sheet bar and finish-rolling the sheet bar, the finish-rolling being carried out with the reductions in thickness at the final stand of less than 30%, and the finish-rolling being completed at temperature range of from Ar3 transformation point to (Ar3 transformation point +60° C.);
• primary-cooling the hot-rolled steel sheet starting the primary cooling within 1.0 seconds after the completion of hot-rolling and conducting the cooling at cooling speeds of higher than 200° C./sec down to the temperature range of from (Ar3 −30° C.) to Ar1 transformation point;
• applying slow cooling or air-cooling the primary-cooled steel sheet in a temperature range of from Ar3 transformation point to Ar1 transformation point at cooling speeds of 10° C./sec or less for 2 seconds or more;
• secondary-cooling the steel sheet after the slow cooling or the air-cooling at cooling speeds of 30° C./sec or more;
• and
• coiling the secondary-cooled steel sheet at temperatures of 300° C. or less.

A method for manufacturing steel sheet comprising the steps of:

• rough-rolling a steel consisting essentially of 0.04 to 0.2% C, 0.25 to 2% Si, 0.5 to 2.5% Mn, and 0.1% or less Al, by weight to form a a sheet bar;
• finish-rolling the sheet bar at rolling temperatures of 1,050° C. or less, cumulative reductions in thickness of 30% or more, and end temperatures of rolling of from Ar3 transformation point to (Ar3 transformation point +60° C.);
• primary-cooling the finish-rolled steel sheet within 1.0 second after completed the finish-rolling at cooling speeds of more than 200° C./sec through a cooling range where the difference between the temperature to start cooling and the end temperature of the cooling is in a range of from 100° C. and below 250° C.;
• applying slow cooling to the primary-cooled steel sheet at cooling speeds of 10° C./sec or less for a period of from 2 seconds to less than 20 seconds in a temperature range of from above 580° C. to 720° C.;
• secondary-cooling the steel sheet after completed the slow cooling at cooling speeds of 30° C./sec or more;
• and
• coiling the secondary-cooled steel sheet at temperatures of below 400° C.

A method for manufacturing steel sheet comprising the steps of:

• rough-rolling a steel consisting essentially of 0.04 to 0.12% C, 0.25 to 2% Si, 0.5 to 2.5% Mn, 0.1% or less Al, by weight, and balance of substantially Fe and inevitable impurities to form a sheet bar;
• finish-rolling the sheet bar at rolling end temperatures of Ar3 transformation point or more;
• primary-cooling the finish-rolled steel sheet within 1.0 second after completed the finish-rolling at cooling speeds of higher than 200° C./sec through a cooling range where the difference between the temperature to start cooling and the end temperature of the cooling is in a range of from 10° C. to below 250° C.;
• applying slow cooling to the primary-cooled steel at cooling speeds of 10° C./sec or less for a period of from 2 seconds to less than 20 seconds in a temperature range of from 580° C. to 720° C.;
• and
• coiling the slowly cooled steel sheet at temperatures of from 400° to below 540° C.