In general, a method of protecting can contents by providing organic coatings on the inner surfaces of cans after can making has been used for general two-piece cans, like DRD (Drawing and Redrawing) cans and DI (Drawing and Wall Ironing) cans. On the other hand, laminated steel sheets including metal sheets previously coated with organic resin films before forming have recently attracted attention in view of global environment conservation. Since the laminated steel sheets do not require lubricating oil, which is generally required for deep drawing and ironing, because the films have lubricity, a step of washing off the lubricating oil is omitted, thereby causing the advantage of no discharge of cleaning waste water. Further, the need for a coating step and a baking step for the inner surfaces of cans in order to protect the contents and surfaces of steel sheets is eliminated, and thus there occurs the advantage of no occurrence of carbon dioxide which is discharged as green house gas in the baking step.
As described above, the can making method using the laminated steel sheets can greatly contribute to global environment conservation and the demand therefore is considered to be expanded in future. However, this method causes the new problem of degrading corrosion resistance due to exfoliation of coating films from the steel sheets, which are used as base materials, after can making.
Therefore, factors for the steel sheets used as base materials include high formability enough to resist a high rate of working such as deep drawing and ironing and the surface properties enough to prevent surface roughness in order to keep the good adhesion to the films after can making.
In addition, a coolant for cooling dies which are heated by working heat during can making of the steel sheets and lubricating oil are not used, and thus the working head is likely to adversely affect the productivity of can making. As a countermeasure against this, it is also a factor that the steel sheets are soft and cause little working heat in addition to the resistance to surface roughening.
In view of the above, Patent Literature 1 proposes a method for producing a steel sheet in which Nb is added to ultra-low-carbon steel containing about 0.001 to 0.005% by mass of C, and the average crystal gain diameter is adjusted to 6 μm or less by shortening the time required from the end of finish hot-rolling to the start of strip quenching, appropriately determining a hot-rolling coiling temperature, and the effect of addition of Mn, thereby preventing surface roughness. The method of Patent Literature 1 realizes refinement of crystal grains by controlling NbC precipitation during hot rolling while maintaining high workability by chemical component design using the ultra-low-carbon steel as a base. However, 0.4 to 1.0% by mass of Mn which is a typical solid-solution hardening element is added for achieving refinement of crystal grains, and thus the working heat of a steel sheet during can making cannot be sufficiently suppressed.
Patent Literature 2 proposes a method for producing a steel sheet using steel containing 0.0050% by mass or less of C, 0.0200% or less of N, and one or two selected from Nb and Ti, in which grain refinement of a hot-rolled sheet is achieved by controlling the sheet thickness after hot rolling to less than 1.8 mm and increasing the cooling rate after finish hot rolling, and surface roughness is suppressed by a high reduction rate of cold rolling and continuous annealing for a short time, so that performances such as a balance between excellent strength and ductility, a high average r value, and good planar anisotropy are satisfied. The method of Patent Literature 2 is capable of producing a steel sheet having excellent quality, but hot ductility may be decreased by positively adding N, and water cooling equipment is required to be installed near the outlet side of a rolling machine because water cooling is started within a short time after the end of finish rolling after hot rolling. This is accompanied by the need for removing a thermometer and a sheet thickness meter which are generally installed. Therefore, there occurs the problem of modification of equipment and an operational problem, such as the need for a higher degree of rolling control capability.
Patent Literature 3 proposes a technique for achieving grain refinement of ultra-low-carbon steel containing Nb and Ti and preventing film hair during DI can working. In addition, with 0.007 to 0.01% by mass of C, softening is achieved by overaging during annealing. However, Ti has the possibility of impairing plating performance by a linear defect such as a Ti mark according to the amount of Ti added, and Ti is preferably added in as a small mount as possible from the viewpoint of attaching importance to corrosion resistance and appearance.
Patent Literature 4 proposes a can making method using as a raw material a steel sheet containing 0.0005 to 0.0050% by mass of C, 0.20% by mass or less of Si, 0.05 to 1.00% by mass of Mn, 0.005 to 0.100% by mass of Al, 0.003 to 0.020% by mass of Nb, 0.100% by mass or less of P, 0.010% by mass or less of S, and 0.0050% by mass or less of N, the steel sheet having excellent formability and being controlled to an average r value of 1.5 or more and an absolute Δr value of 0.30 or less, in which a drawing ratio during cupping for DI can making is controlled to 1.80 or more so that work hardening is performed by applying elongation strain to a bottom portion, thereby increasing the compression strength of the bottom portion. However, this method requires a change in a drawing/ironing schedule and thus possibly influences the can making rate.
Patent Literature 5 proposes a steel sheet having excellent burr resistance and a method for producing the same, in which steel containing 0.004 to 0.01% by mass of C, 0.05% by mass or less of P, 0.02% by mass or less of S, 0.01 to 0.1% by mass of sol. Al, 0.004% by mass or less of N, 0.03% by mass or less of Ti, and Nb added to satisfy 1≦(93/12)×(Nb/C)≦2.5 is subjected to finish final two passes of hot rolling under high pressure to finely and uniformly disperse Nb-based precipitates. It is essential to perform the finish final two passes under high pressure, thereby causing the problem of increasing an operating load of hot-rolling.
Patent Literature 6 proposes a thin steel sheet for pressing, in which one of Nb-based and Ti-based precipitates is precipitated in a ferrite phase so that the ferrite grain size is 10 or more and a low-density precipitate region is provided near a ferrite grain boundary. The low-density precipitate region extends a degree of forming allowance of pressing.
Patent Literature 7 proposes a steel sheet with excellent press formability, characterized in that the steel sheet is composed of steel containing 0.0040 to 0.015% by mass of C, 0.05% by mass or less of Si, 1.5 to 3.0% by mass of Mn, 0.01 to 0.1% by mass of P, 0.02% by mass or less of S, 0.01 to 0.1% by mass of sol. Al, 0.004% by mass or less of N, and 0.04 to 0.25% by mass of Nb and satisfying the expression 1.5≦Nb/(7.75×C) defined by a C amount and a Nb amount being 1.5-2.5, and has a region near a ferrite grain boundary at a lower Nb-based precipitate density than inside a grain.
Patent Literature 8 proposes a high-strength cold-rolled steel sheet characterized in that the steel sheet includes ferrite grains having an average grain diameter of 10 μm or less, the average number of Nb(C,N) grains having a diameter of 50 nm or more per unit area is 7.0×10−2/μm, and a region with a width of 0.2 to 2.4 μm is formed along the grain boundary of a ferrite grain, the average area density of NbC in the region being 60% or less of the average area density of NbC precipitated in a central portion of the ferrite grain. Patent Literature 8 provides a high-strength cold-rolled steel sheet excellent in resistance to planar strain and in punch stretchability by decreasing YS to 270 MPa.
However, in Patent Literatures 6 to 8, a region where NbC is coarsely distributed is formed near a ferrite grain boundary by controlling NbC precipitation, thereby decreasing YS and improving formability. However, lower YS is undesirable for two-piece cans in view of maintaining the compression strength of bottom portions which have a relatively low working ratio.
Patent Literature 9 proposes a cold-rolled steel sheet with excellent dent resistance, characterized in that the steel sheet contains 0.0040 to 0.02% by mass of C, 1.5% by mass or less of Si, 0.5 to 3.0% by mass of Mn, 0.01 to 0.1% by mass of P, 0.02% by mass or less of S, 0.15 to 1.5% by mass of sol. Al, 0.001 to 0.005% by mass of N, and 0.04 to 0.2% by mass of Nb and has C and Nb contents satisfying 1.0≦(12/93)×(Nb/C)≦2.2 and Al and N contents satisfying 26≦(14/27)×(Al/N)≦400, and the average grain diameters of Nb carbide and Al nitride are 10 to 200 nm and 50 to 500 nm, respectively.