Abstract: A high-strength cold rolled steel sheet having mechanical characteristics having a tensile strength of not less than 780 MPa, a yield ratio of not more than 70%, and a small in-plane anisotropy of a tensile characteristicis obtained by hot rolling a steel slab comprising by mass % C: 0.07 to 0.12%, Si: not more than 0.7%, Mn: 2.2 to 2.8% and Ti and Nb: 0.02 to 0.08% in total, and cold rolling the sheet, followed by continuous annealing to form a steel texture comprised of ferrite having an area ratio of 40 to 80% with respect to the whole texture, and a second phase constituted by tempered martensite, fresh martensite and bainite, wherein the total area ratio of the bainite and the tempered martensite to the second phase is 50 to 80%, and the aspect ratio of the fresh martensite is in the range of 1.0 to 1.5.

Abstract: A cold rolled steel sheet having a high strength, an aging resistance, a high yield ratio and a small anisotropy of tensile strength is obtained by hot rolling and cold rolling a steel material containing in percent by mass C: 0.06-0.14%, Si: less than 0.50%, Mn: 1.6-2.5%, Nb: not more than 0.080% (including 0%), Ti: not more than 0.080% (including 0%), provided that Nb and Ti are contained in an amount of 0.020-0.080% in total, subjecting a cold rolled steel sheet continuous annealing including steps of soaking-annealing at a temperature of 840-940° C. for a holding time of 30-120 seconds, cooling from the soaking temperature to 600° C. at a rate of not less than 5° C./s, retaining in a temperature range of 600-500° C. for 30-300 seconds and then conducting a secondary cooling to apply such a steel structure that martensite is finely dispersed into ferrite base.

Abstract: A high-strength steel sheet having a high yield ratio and a reduced difference in strength between the center and edges of the steel sheet in the width direction. The high-strength steel sheet includes a composition containing, by mass, C: 0.02% or more and less than 0.10%, Si: less than 0.10%, Mn: less than 1.0%, P: 0.10% or less, S: 0.020% or less, Al: 0.01% or more and 0.10% or less, N: 0.010% or less, Nb: 0.005% or more and less than 0.070%, and the balance being Fe and inevitable impurities. Additionally, the steel sheet has a microstructure including, by area, ferrite: 90% or more, pearlite: 0% to 10%, and the total of martensite, retained austenite, and cementite: 0% to 3%. The average crystal grain diameter dC of the ferrite at the center of the steel sheet in the width direction is 15.0 ?m or less.

Abstract: A high-strength steel sheet having high yield ratio, excellent stretch flange formability, and resistance to secondary working embrittlement. The steel sheet has a composition containing C: 0.02% to less than 0.10%, Si: less than 0.3%, Mn: less than 1.0%, P: 0.10% or less, S: 0.020% or less, Al: 0.01% to 0.10%, N: 0.010% or less, and Nb: 0.003% to less than 0.070% on a mass basis, the remainder being Fe and inevitable impurities. A steel microstructure of the steel sheet contains ferrite: 90% or more and a total of pearlite, martensite, retained austenite, and cementite: 0% to 10% on an area fraction basis, in which the average grain size of the ferrite is 15.0 ?m or less, and in which the average aspect ratio of the ferrite is 1.2 or more; and a tensile strength of 500 MPa or less.

Abstract: A high-strength steel sheet having a high yield ratio and excellent stretch flangeability, and a method for producing the high-strength steel sheet. The high-strength steel sheet has a composition containing, by mass: C: 0.02% or more and less than 0.10%, Si: less than 0.10%, Mn: less than 1.0%, P: 0.10% or less, S: 0.020% or less, Al: 0.01% or more and 0.10% or less, N: 0.010% or less, Nb: 0.005% or more and less than 0.070%, and the balance being Fe and inevitable impurities; and a microstructure including, by area: ferrite: 85% or more, pearlite: 0% to 15%, and a total of martensite, retained austenite and cementite: 0% to 3%. The average crystal grain diameter of the ferrite is 15.0 ?m or less. The grain diameter of a Nb carbide is 5 to 50 nm. The amount of Nb carbide precipitate is 0.005% to 0.050% in terms of volume fraction.

Abstract: A high-strength steel sheet having a high yield ratio and a reduced difference in strength between the center and edges of the steel sheet in the width direction. The high-strength steel sheet includes a composition containing, by mass, C: 0.02% or more and less than 0.10%, Si: less than 0.10%, Mn: less than 1.0%, P: 0.10% or less, S: 0.020% or less, Al: 0.01% or more and 0.10% or less, N: 0.010% or less, Nb: 0.005% or more and less than 0.070%, and the balance being Fe and inevitable impurities. Additionally, the steel sheet has a microstructure including, by area, ferrite: 90% or more, pearlite: 0% to 10%, and the total of martensite, retained austenite, and cementite: 0% to 3%. The average crystal grain diameter dC of the ferrite at the center of the steel sheet in the width direction is 15.0 ?m or less.