A method of manufacturing a high-strength hot rolled steel sheet by hot rolling a steel having a chemical composition including, by mass %: C: more than 0.010% and not more than 0.06%, Si: not more than 0.3%, Mn: not more than 0.8%, P: not more than 0.03%, S: not more than 0.02%, Al: not more than 0.1%, N: not more than 0.01% and Ti: 0.05 to 0.10%, the balance including Fe and inevitable impurities, including: after the steel is heated to an austenite single phase region, the steel is finish rolled at a finishing delivery temperature of 860 C. to 1050 C., the steel sheet is cooled at an average cooling rate of not less than 30 C/s in a temperature range of from a temperature after the completion of the finish rolling to 750 C., and the steel sheet is coiled into a coil at a coiling temperature of 580 C. to 700 C.

A high-strength hot rolled steel sheets with excellent stretch flangeability has small variations in mechanical properties in individual coils. Variations in strength from place to place in a coil are decreased by minimally reducing the Si and Mn contents to suppress the occurrence of problems such as segregation. Further, the microstructure of the steel sheets is configured such that a ferrite phase represents an area ratio of not less than 95%, the ferrite crystal grains have an average grain size of not less than 1 m, and the ferrite crystal grains contain TiC with an average particle size of not more than 7 nm dispersed in the crystal grains.

A cold-rolled and annealed ferritic steel sheet is provided. The steel has a composition comprising, expressed by weight: 0.001C0.15%; Mn1%; Si1.5%; 7.5%AI10%; 0.020%Ti0.5%; S0.050%; and P0.1%.

A balance of the composition includes iron and inevitable impurities resulting from the smelting. The structure includes kappa () precipitates and equiaxed ferrite, an average grain size d of the equiaxed ferrite is less than 50 microns, and a linear fraction f of intergranular precipitates is less than 30%. The linear fraction f is defined by

[00001]$f=\frac{{}_{\left(A\right)}.Math.\mathrm{di}}{{}_{\left(A\right)}.Math.\mathrm{Li}}.$

.sub.(A)di denotes the total length of grain boundaries containing precipitates relative to an area (A) and .sub.(A)Li denotes the total length of the grain boundaries relative to the area (A). A content of carbon in solid solution is less than 0.005% by weight, and the cold-rolled and annealed ferritic steel sheet has a thickness between 0.6 mm and 1.5 mm. A skin part or structural part for the automotive field is also provided.

The invention relates to a hot-rolled ferritic steel sheet, the composition of the steel of which comprises, the contents being expressed by weight: 0.001C0.15%, Mn1%, Si1.5%, 6%Al10%, 0.020%Ti0.5%, S0.050%, P0.1%, and, optionally, one or more elements chosen from: Cr1%, Mo1%, Ni1%, Nb0.1%, V0.2%, B0.010%, the balance of the composition consisting of iron and inevitable impurities resulting from the smelting, the average ferrite grain size d.sub.IV measured on a surface perpendicular to the transverse direction with respect to the rolling being less than 100 microns.