Abstract: Disclosed is a method for producing an annealed and pickled steel sheet in a continuous manner by continuously performing: subjecting a cold-rolled steel sheet to annealing to obtain an annealed steel sheet; subjecting the annealed steel sheet to pickling with a mixed acid solution containing a first acid that is oxidizing and a second acid that is non-oxidizing, by feeding the annealed steel sheet into a mixed acid tank holding the mixed acid solution; and subjecting the annealed steel sheet discharged from the mixed acid tank to repickling with an acid solution containing a third acid that is non-oxidizing. Fe concentration in the mixed acid solution in the mixed acid tank is measured, and a lower limit of ferrite fraction in a surface layer of the annealed steel sheet to be subjected to the pickling is set according to the Fe concentration measured.

Abstract: Provided is a low-cost cold rolled steel sheet for zirconium-based chemical conversion treatment that contains Si and Mn and has excellent zirconium-based chemical convertibility even when a relatively large amount of Si-based oxide and Si—Mn-based oxide exists on the steel sheet surface. A cold rolled steel sheet for zirconium-based chemical conversion treatment comprises a chemical composition containing, in mass %, C: 0.05% to 0.30%, Si: 0.01% to 1.4%, Mn: 0.14% to 3.2%, P: 0.10% or less, and S: 0.01% or less, with a balance consisting of Fe and inevitable impurities, wherein a Si/Mn mass ratio in steel is 0.10 to 0.7, and a surface coating ratio of steel sheet surface layer oxides having Si content of 10 mass % or more is 40% or less.

Abstract: An object is to provide a cold-rolled steel sheet having excellent phosphatability, even in the case where a phosphating solution has a low temperature, and a method for manufacturing the steel sheet, and a cold-rolled steel sheet for annealing. A cold-rolled steel sheet for annealing has a chemical composition containing C: 0.01 mass % to 0.30 mass %, Si: less than 0.10 mass % (including 0.0 mass %), Mn: 2.0 mass % to 3.5 mass %, P: 0.05 mass % or less, S: 0.01 mass % or less, and Al: 0.15 mass % or less, the balance being Fe and inevitable impurities, in which Si and/or Si oxide exists on a surface of the steel sheet with an average coating thickness of 1 nm to 20 nm.