Abstract: A steel sheet has a microstructure including ferrite phase: 40% to 60%, bainite phase: 10% to 30%, tempered martensite phase: 20% to 40%, and retained austenite phase: 5% to 20% by volume fraction, and satisfying a condition that a ratio of tempered martensite phase having major axis length ?5 ?m to a total volume fraction of the tempered martensite phase is 80% to 100%.

Abstract: This invention provides methods and apparatuses which can manufacture Si-containing cold rolled steel sheets exhibiting excellent chemical conversion properties even if a chemical conversion treatment solution is used at a lower temperature while minimally suppressing the generation of sludge as well as reducing running costs. The method includes steps of cold rolling a steel containing 0.5 to 3.0 mass % Si, continuously annealing the cold rolled steel sheet, pickling the surface of the continuously annealed cold rolled steel sheet, and repickling the surface of the pickled steel sheet with a non-oxidative acid. The repickling is performed such that a repickling solution is sampled continuously or periodically, an acid concentration in the sampled solution is measured, and the acid concentration in the repickling solution is regularly controlled within a prescribed concentration range.

Abstract: A steel sheet has a microstructure including ferrite phase: 40% to 60%, bainite phase: 10% to 30%, tempered martensite phase: 20% to 40%, and retained austenite phase: 5% to 20% by volume fraction, and satisfying a condition that a ratio of tempered martensite phase having major axis length ?5 ?m to a total volume fraction of the tempered martensite phase is 80% to 100%.

Abstract: In a method for prevention of yellowing on a surface of a steel sheet subjected to re-pickling, washing with water and drying after a surface of a continuously annealed steel sheet is pickled to remove Si-containing oxide layer from a surface layer of the steel sheet, the surface of the steel sheet is held at a wet state between the pickling and the re-pickling and between the re-pickling and the washing, and more preferably the washing is carried out with water having an iron ion concentration decreased to not more than 20 g/L, whereby the yellowing on the surface of the steel sheet after the pickling is prevented. Thus, cold rolled steel sheets being excellent in not only the appearance quality but also the phosphatability and corrosion resistance after painting are manufactured stably.

Abstract: This invention provides methods and apparatuses which can manufacture Si-containing cold rolled steel sheets exhibiting excellent chemical conversion properties even if a chemical conversion treatment solution is used at a lower temperature while minimally suppressing the generation of sludge as well as reducing running costs. The method includes steps of cold rolling a steel containing 0.5 to 3.0 mass % Si, continuously annealing the cold rolled steel sheet, pickling the surface of the continuously annealed cold rolled steel sheet, and repickling the surface of the pickled steel sheet with a non-oxidative acid. The repickling is performed such that a repickling solution is sampled continuously or periodically, an acid concentration in the sampled solution is measured, and the acid concentration in the repickling solution is regularly controlled within a prescribed concentration range.