Composite steel plate

A rolled composite steel sheet having a crystalline grain structure that varies in a thickness direction of the sheet, between at least one region having a micro-grain or ultrafine-grain structure and a first nanocrystalline region having a nanocrystalline grain structure, wherein the nanocrystalline region has a mechanically-induced grain size of no more than 100 microns and a thickness in a range from 10 microns to 50 microns.

A rolled composite steel sheet having a crystalline grain structure that varies in a thickness direction of the composite steel sheet, between at least one microcrystalline region having a first grain size, and at least one nanocrystalline region having a mechanically-induced second grain size, wherein

• the second grain size is smaller than the first grain size,
• the mechanically-induced second grain size does not exceed 100 microns, and
• the nanocrystalline region has a thickness in a range from 10 microns to 50 microns.

A rolled composite steel sheet comprising a first surface region having a mechanically-induced first grain size, a second surface region, opposite the first surface region, the second surface region having a mechanically-induced second grain size, and a center region, located between the first and second surface regions, wherein

• the third grain size is larger than the first and second grain sizes,
• the first and second surface regions have a grain size not exceeding 100 microns, and a thickness in a range from 5 microns to 20 microns, and
• grain size varies smoothly from the first and second surface regions to the center region of the composite steel sheet.