Abstract: This steel sheet has a predetermined chemical composition, and, on a surface parallel to a surface at a ¼ position of a sheet thickness in a sheet thickness direction from the surface, with respect to a total area of three types of oxides of MnO, Cr2O3 and Al2O3 having a major axis of more than 1.0 ?m, a total area ratio of the MnO and the Cr2O3 is 98.0% or more, and an area ratio of the Al2O3 is 2.0% or less.

Abstract: The invention provides a carbon steel sheet including C: 0.20% to 0.45% by mass, Si: 0.05% to 0.8% by mass, Mn: 0.85% to 2.0% by mass, P: 0.001% to 0.04% by mass, S: 0.0001% to 0.006% by mass, Al: 0.01% to 0.1% by mass, Ti: 0.005% to 0.3% by mass, B: 0.0005% to 0.01% by mass and N: 0.001% to 0.01% by mass, in which a K value that can be obtained from 3C+Mn+0.5Si is greater than or equal to 2.0; surface hardness is less than or equal to 77 on the Rockwell B Scale; and the average content of N in a zone from the surface to a depth of 100 ?m is less than or equal to 100 ppm. This carbon steel sheet is configured to be carburized in a carburization atmosphere with a carbon potential of 0.6 or less.

Abstract: This invention aims at obtaining a cold steel sheet having excellent cold formability irrespective of a high N content in order to re-utilize a scrap steel generated from scraps. A steel sheet comprising not greater than 0.1% of C, 60 to 150 ppm of N, not greater than 0.4% of Mn satisfying the relation Mn/S.gtoreq.7, not greater than 0.030% of S, not greater than 0.1% of Al and not greater than 0.08% of Ti satisfying the relation N (%)--Ti/3.42 (%).ltoreq.0.007, wherein the size of TiN in the weight at least 1/2 of the total TiN weight is 0.05 to 10 .mu.m, is obtained. When production is made by continuous annealing, Ti is added so as to satisfy the relation N (%)--Ti/3.42 (%).ltoreq.0.004% and furthermore, high temperature winding is effected in hot rolling or B is added.When production is made by box annealing, the Ti content is adjusted so as to satisfy the relation 0.002%.ltoreq.N (%)--Ti/3.42 (%).ltoreq.0.