From the viewpoint of the weight reduction of automobiles, there have recently been increasing demands for high strength cold rolled steel sheets having a tensile strength of not less than 590 MPa. Automotive cold rolled steel sheets are subjected to a chemical conversion treatment called phosphatization as a pretreatment prior to painting. Chemical conversion treatment of cold rolled steel sheets is one of the important treatments in order to ensure corrosion resistance after painting.
The addition of Si is effective for increasing the strength of cold rolled steel sheets. During continuous annealing, however, silicon is oxidized even under an atmospheric gas composition including reducing N2 and H2 which does not induce the oxidation of Fe (which reduces Fe oxides). As a result, a thin film of Si oxide (SiO2) is formed on the outermost surface of a steel sheet. This Si oxide inhibits a reaction forming a chemical conversion layer during chemical conversion treatment, thereby resulting in micro areas (non-covered areas) where any chemical conversion layer is not formed. Thus, chemical conversion properties are lowered.
Among conventional technologies addressing the improvement of the chemical conversion properties of high-Si cold rolled steel sheets, Patent Literature 1 describes a method in which the steel sheet temperature is brought to 350 to 650° C. in an oxidizing atmospheric gas so as to form an oxide film on the surface of steel sheet, and thereafter the steel sheet is heated to a recrystallization temperature in a reducing atmospheric gas and subsequently cooled.
Further, Patent Literature 2 describes a method for manufacturing cold rolled steel sheets containing, in terms of mass %, Si at not less than 0.1% and/or Mn at not less than 1.0%, which includes forming an oxide film on the surface of steel sheet at a steel sheet temperature of not less than 400° C. under an iron oxidizing atmospheric gas composition, and thereafter reducing the oxide film on the surface of steel sheet under an iron reducing atmospheric gas.
Furthermore, Patent Literature 3 describes a high strength cold rolled steel sheet which contains Si at not less than 0.1 wt % and not more than 3.0 wt % and has a superficial layer that contains an oxide effective for improving properties such as chemical conversion properties in crystal grain boundaries and/or within crystal grains. Patent Literature 4 describes a steel sheet with excellent phosphatability in which when a cross section in a direction perpendicular to the surface of steel sheet is observed with an electron microscope at a magnification of 50000× or more, the proportion of a Si-containing oxide occupying the cross section over a 10 μm length of the surface of steel sheet is not more than 80% in terms of an average of randomly selected 5 points. Patent Literature 5 describes a high strength cold rolled steel sheet with excellent chemical conversion properties which contains, in terms of mass %, C at more than 0.1% and Si at not less than 0.4%, and has a Si content (mass %)/Mn content (mass %) ratio of not less than 0.4 and a tensile strength of not less than 700 MPa, and in which a Si-based oxide containing Si as a main component covers the surface of steel sheet at a surface coverage ratio of not more than 20% by area, and the largest circle inscribed in the region covered by the Si-based oxide has a diameter of not more than 5 μm. Patent Literature 6 describes a high strength steel sheet with excellent chemical conversion properties containing, in terms of mass %, C at 0.01 to 0.3%, Si at 0.2 to 3.0%, Mn at 0.1 to 3.0% and Al at 0.01 to 2.0% and having a tensile strength of not less than 500 MPa, wherein crystal grains of the surface of steel sheet have an average grain diameter of not more than 0.5 μm, further wherein when an observation region of the surface of steel sheet that is not less than 10 μm in width is sliced into a thin piece for cross sectional TEM observation and when the thin piece sample is observed by TEM under such conditions that oxides 10 nm or less in size are observable, an oxide species including one or two of silicon oxide and manganese silicate at a total content of not less than 70 mass % is present at not more than 30% relative to the surface of a grain boundary region viewed with respect to the cross section, and wherein the oxide species that is present in a region found at a depth of 0.1 to 1.0 μm from the surface of steel sheet has a grain diameter of not more than 0.1 μm.