EP1709208A1 - Steel sheet for containers, and manufacturing method therefor - Google Patents

Abstract

Steel sheet and manufacturing method for manufacturing steel sheet for containers are provided. For example, the steel sheet (or at least a portion thereof) can include approximately, in terms of mass%, C: 0.0800% or less, N: 0.600% or less, Si: 2.0% or less, Mn: 2.0% or less, P: 0.10% or less, S: 0.05% or less, Al: 2.0% or less, and the remainder Fe. The sheet (or portion thereof) can be cold-rolled, and then the atmosphere, temperature, time, etc. of recrystallization annealing or subsequent heat treatment may be adjusted to control the change in N content in the steel, for example controlling the N content and hardness of the surface layer and mid-thickness layer to bring them within an appropriate range. The surface roughness can be 0.90 µm or less in terms of Ra, and PPI, which is the number of concavo-convex peaks per one inch of length, may be 250 or more.

Description

DESCRIPTION

STEEL SHEET FOR CONTAINERS, AND MANUFACTURING METHOD THEREFOR

TECHNICAL FIELD The present invention relates to steel sheet, including surface-treated steel sheet used in metal containers such as beverage cans, and to a manufacturing method therefor. Priority is claimed on Japanese Patent Application No. 2003-409918, filed December 9, 2003, the contents of which are incorporated herein by reference.

BACKGROUND ART Steel sheet for containers, typified by beverage cans, food cans and the like, has been made increasingly thinner in order to reduce the production cost for these kinds of various containers, with materials under 0.2 mm coming into use. Problems that emerge when containers are manufactured using such ultra-thin material include for instance reduction in color tone, adhesion of the surface coating and weldability due to the difficulty of controlling the surface condition. It is known that the surface condition of steel sheet greatly affects color tone, surface coating adhesion and weldability, as described, e.g., in Japanese Patent

Application Publications HI 1-197704, H8-3781 and H6-57488. Furthermore, a method for controlling surface roughness is described in Japanese Patent Application Publication H7-9005. In the above-referenced publications, a need exists to precisely control the manufacturing conditions in order to control the surface condition, but a deterioration in productivity is apparently unavoidable. Furthermore, the conventional control methods described in these publications do not always adequately improve the color tone, surface coating adhesion and weldability of containers manufactured with ultra-thin material, which is one of the objectives of the present invention. One of the objects of the present invention is to overcome the problems in containers manufactured using ultra-thin sheet materials by providing a specialized steel sheet and manufacturing method therefor, with respect to the color tone, surface coating adhesion and weldability of containers, which depend on the state of the steel sheet surface. Another object of the present invention is to provide improvements by certain state controlled in both of the surface and mid-thickness layer of the material through the application of nitriding, which can allow both of controlling the state of the steel sheet surface and avoiding to add any special processing that would obstruct productivity.

DISCLOSURE OF INVENTION Japanese Patent Application Nos. 2003-119381 and 2003-100720 describe techniques of nitriding steel sheet in a post-annealing process and appropriately controlling the nitriding condition in the thickness direction of the sheet. Such techniques are provided, e.g., for the purpose of significantly improving the deformation resistance of containers without overly degrading the ductility of the steel sheet. In the course of evaluating the weldability, etc. of these materials, it was ascertained that there exist conditions under which the surface condition of the steel sheet becomes favorable, making it possible to greatly improve the color tone, surface coating adhesion and weldability of containers. These characteristics depend on the state of the steel sheet surface, and have posed a problem in containers made from ultra-thin steel sheet material, even without applying cathodic electrolysis, using surfactants, finely controlling Cr oxides, performing special rolling with finely controlled roll precision, etc., as was conventionally implement for materials of this type. For example, when nitriding is performed after cold rolling to increase the content of nitrogen in the steel, simply creating a different surface hardness generally may not necessarily improve the color tone, surface coating adhesion or weldability of the can. However, exemplary embodiments of the present invention makes it possible to improve the color tone, surface coating adhesion and weldability of cans made from ultra-thin materials by appropriately controlling the steel components, nitriding conditions and the state of the steel sheet after nitriding. Furthermore, the exemplary embodiments of the steel sheets and corresponding methods therefore according to the present invention indicate the conditions preferable to achieve these exemplary benefits. One exemplary embodiment of a steel sheet for containers according to the present invention is the steel sheet for containers with a sheet thicknesses of 0.400 mm or less, distinguished in that it contains, in terms of mass%, C: 0.0800% or less, N: 0.600% or less, Si: 2.0% or less, Mn: 2.0% or less, P: 0.10% or less, S: 0.05%) or less and Al: 2.0% or less; (N content of 1/8 thickness surface layer) - (N content of 1/4 thickness mid-thickness layer) is 10 ppm or greater, (N content of 1/8 thickness surface layer) is 20000 ppm or less, the surface roughness is 0.90 μ or less in terms of Ra, and PPI, which is the number of concavo-convex peaks per one inch of length, is 250 or more. Another exemplary embodiment of a steel sheet for containers according to the present invention is the steel sheet for containers with a sheet thicknesses of 0.400 mm or less, distinguished in that it contains, in terms of mass%, C: 0.0800% or less, N: 0.600% or less, Si: 2.0% or less, Mn: 2.0% or less, P: 0.10% or less, S: 0.05% or less and Al: 2.0% or less; (steel sheet cross-sectional mean Vickers hardness of 1/8 thickness surface layer) - (steel sheet cross-sectional mean Vickers hardness of 1/4 thickness mid-thickness layer) > 10 points, or (steel sheet cross-sectional mean Vickers hardness of 1/8 thickness surface layer) - (steel sheet cross-sectional mean Vickers hardness of 1/4 thickness mid-thickness layer) > 20 points, the surface roughness is 0.90 μm or less in terms of Ra, and PPI, which is the number of concavo-convex peaks per one inch of length, is 250 or more. The exemplary embodiment(s) of the steel sheet according to the present invention may furthermore contain, in terms of mass%, one, two, or more of Ti: 0.05% or less, Nb: 0.05% or less, and B: 0.015% or less. In terms of the mean components of the

1/4 sheet thickness mid-thickness layer, the steel sheet may contain one, two, or more of

Ti: 4 C + 1.5 S + 3.4 N or more, Nb: 7.8 C + 6.6 N or more, and B: 0.8 x N or more. Furthermore, the steel sheet may contain, in terms of mass%, one, two, or more of Cr: 20%» or less, Ni: 10% or less, and Cu: 5% or less. The steel sheet may furthermore contain, in terms of mass%, a total of 0.1% or less of Sn, Sb, Mo, Ta, V, and . An exemplary embodiment of a manufacturing method for the steel sheet for containers according to the present invention is a manufacturing method for the steel sheet for containers with the sheet thickness of 0.400 mm or less, distinguished in that steel sheet including, in terms of mass%, C: 0.0800% or less, N: 0.0300% or less, Si: 2.0% or less, Mn: 2.0% or less, P: 0.10% or less, S: 0.05% or less, Al: 2.0% or less, and the remainder Fe and unavoidable impurities, is cold-rolled, and then, simultaneously with or after recrystallization annealing, nitriding is performed, whereby the (N average increase in content (mass %), all over the sheet thickness direction) is no more than 6000 ppm, and the (N increase in concentration of 1/8 thickness surface layer) is made 20000 ppm or less, the absolute value of (N increase in content (mass %) of 1/8 thickness surface layer) / (N increase in content (mass %) of 1/4 thickness mid-thickness layer) is made 2.0 or more, the surface roughness is made 0.90 μm or less in terms of Ra, and PPI, which is the number of concavo-convex peaks per one inch of length, is made 250 or more. Another exemplary embodiment of a manufacturing method for steel sheet for containers according to the present invention is the manufacturing method for the steel sheet for containers with the sheet thickness of 0.400 mm or less, distinguished in that steel sheet including, in terms of mass%, C: 0.0800% or less, N: 0.0300% or less, Si:

2.0% or less, Mn: 2.0% or less, P: 0.10% or less, S: 0.05% or less, Al: 2.0% or less, and the remainder Fe and unavoidable impurities, is cold-rolled, and then, simultaneously with or after recrystallization annealing, nitriding is performed, whereby the content increase in N content is no more than 6000 ppm mean across the thickness of the sheet, making the (steel sheet cross-sectional mean in Vickers hardness of 1/8 thickness surface layer) - (steel sheet cross-sectional mean Vickers hardness of 1/4 thickness mid-thickness layer) > 10 points, or (steel sheet cross-sectional mean Vickers hardness of 1/8 thickness surface layer) — (steel sheet cross-sectional mean Vickers hardness of 1/4 thickness mid-thickness layer) > 20 points, making the surface roughness 0.90 μm or less in terms of Ra, and making PPI, which is the number of concavo-convex peaks per one inch of length, 250 or more. Exemplary steel sheet components may furthermore be composed of, in terms of mass%, one, two, or more of Ti: 0.05% or less, Nb: 0.05% or less, and B: 0.015% or less. In terms of the mean components of the 1/4 sheet thickness mid-thickness layer, one, two, or more of Ti: 4 x C + 1.5 x S + 3.4 x N or more, Nb: 7.8 C + 6.6 * N or more, and B: 0.8 N or more may be included. Further exemplary steel sheet components may be composed of, in terms of mass%, one, two, or more of Cr: 20% or less, Ni: 10% or less, and Cu: 5% or less. A steel sheet components may also be composed of, in terms of mass%, a total of 0.1 % or less of Sn, Sb, Mo, Ta, V and W. To perform nitriding simultaneously with or after a recrystallization annealing, the steel sheet can be held for no less than 1 second and no more than 360 seconds in an atmosphere comprising of 0.02% or more ammonia gas with the sheet temperature being 550 to 800 °C, making the product of temperature (°C) and time (seconds) in the thermal history in the 550 °C or higher temperature range after nitriding 48000 or less, or else making the mean rate of cooling from 550 °C to 300 °C 10 °C/second or greater. A re-cold rolling reduction ratio after recrystallization annealing and before or after nitriding can be made 20% or less. The steel sheet for containers according to exemplary embodiments of the present invention and the manufacturing method therefor make it possible to improve the color tone, surface coating adhesion and weldability of containers while avoiding complicated processing after nitriding and obstruction of productivity due to this complicated processing. Therefore, it becomes possible to keep as high productivity as of conventional sheet and method with complicated processing for steel sheet for ultra-thin containers and provide an industrially-useful effect.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an exemplary profile view of a 1/4 thickness surface layer and 1/8 thickness surface layer of a steel sheet for containers according to an exemplary embodiment of the present invention. FIG. 2 is an illustration of the layers of FIG. 1 with Vickers hardness measurement positions provided on the steel sheet.

BEST MODE FOR CARRYING OUT THE INVENTION Steel material components used for exemplary embodiments of the present invention are described below. Steel material components are indicated in terms of mass%. A particular upper limit to the C content before annealing is preferable to avoid degradation of workability, and is set at C: 0.0800%. For example, the upper limit can be set to 0.0600% or less, or more preferably, 0.0400% or less. The steel according to an exemplary embodiment of the present invention, wherein the content of N, which has properties similar to C, can be increased by nitriding after annealing, can have a low content of C, which is preferable from the perspective of ensuring strength and the like. The strength can be ensured even with C: 0.0050% or less, with 0.0020% or less being permissible as well. At 0.0015% or less, it is possible to manufacture an ultra-soft material that would be outside the standard for normal container material, depending on the balance with the amount of nitriding. Similarly as with C, an upper limit to the N content before annealing is preferable to avoid degradation of workability, and is set at N: 0.0300%) or less. N: 0.0200%) or less can be preferable, N: 0.0150% or less can be more preferable, N:

0.0100%) is even more preferable, N: 0.0050% or less is yet more preferable, and N:

0.0030% is further preferable. It should be noted that, as will be described below, the N incorporated by nitriding after annealing is there to provide a beneficial effect with regard to the color tone, surface coating adhesion and weldability of the can, and has a different effect from the N present before annealing. Si can be added to adjust the strength of the container. Too much of Si may degrade the workability and coating characteristics, so it is preferably made 2.0% or less.

In the steel according to the present invention, Si may form nitrides with the N which penetrates into the steel due to nitriding at the crystal grain boundaries, causing brittle cracking and inhibiting the effect of the present invention, so it may be preferable to limit Si to 1.5% or less, or even 1.0% or less. Mn can be added to adjust strength. Too much of Mn will degrade workability, so it is set at 2.0% or less. P can be added to adjust strength. Too much of P may not only degrade workability, but also obstruct nitriding of the steel sheet, so it is preferably set P at 0.10% or less. S can degrade hot ductility, and may obstruct casting and hot rolling, so it is set at 0.05% or less. Al is an element which can be added for deoxidation. Too high content of Al makes being casted more difficult and causes damage such as increased surface flaws, so Al can be made 2.0% or less. The effects of elements other than the above basic elements, which are normally taken into consideration in steel sheet for containers, and the control thereof, will be described below. Ti can increase the recrystallization temperature of steel sheet, and degrade annealing pass-through of ultra-thin steel sheet, which is one of the objects of the present invention. Thus, Ti can be made 0.050% or less. In normal applications which do not require a high r value in particular, there is no need to add Ti, and it is preferably made 0.03% or less, and more preferably 0.02% or less. Nb has a similar effect to Ti, raising the recrystallization temperature of the steel sheet and markedly degrading annealing pass-through of ultra-thin steel sheet, which is one of the objects of the present invention. Thus, Nb is preferably made 0.050% or less. In normal applications which do not need a high r value in particular, there is not requisite, so much, to add Nb, and it is preferably made 0.03%) or less, more preferably 0.01% or less. When B is added to the steel sheet according to the exemplary embodiment of the present invention containing about 0.01% or more Ti and Nb, the recrystallization temperature of the steel sheet may be increased and the annealing pass-through of ultra-thin steel sheet, which is the object of the present invention, are markedly degraded. On the other hand, if the content of Ti and Nb is low, there is little adverse effect in this regard, and the recrystallization temperature can actually be lowered, thus enabling recrystallization annealing at a lower temperature. Furthermore, since B has the effect of improving annealing pass-through, it can even be actively added. However, with excessive addition, there is marked cracking of slab (including blooms, billets, etc.) during casting, so the upper limit is preferably set at 0.015%. In order to lower recrystallization temperature and improve annealing pass-through, it suffices to make the relationship to N content before nitriding B/N = 0.6 to 1.5. Furthermore, in order to increase the effect by leaving solid-dissolved Ti until before nitriding and causing Ti nitrides to be formed particularly in the steel sheet surface layer due to N which penetrates from the surface into the steel sheet during nitriding, which is one of the objects of the present invention, it is preferable to incorporate, in terms of mean components of 1/4 thickness mid-thickness layer, one or more of Ti: 4 x C + 1.5 S + 3.4 x N or more, Nb: 7.8 C + 6.6 x N or more, and B: 0.8 x N or more. In this manner, mean components of 1/4 thickness mid-thickness layer can be specified because the N content of the surface layer in the present invention changes greatly before and after nitriding, with the aforementioned values changing accordingly. According to an exemplary embodiment of the present invention, it is possible to specify the values in the final sheet after nitriding by using the mean components of the 1/4 thickness mid-thickness layer, where the change due to nitriding is small. Furthermore, to impart characteristics not specified by the exemplary embodiment of the present invention, such as increased corrosion resistance, adding Cr: 20% or less, Ni:

10% or less and Cu: 5% or less in no way detracts from the effect of the present invention. In particular, Cr solid-dissolved in the steel prior to nitriding binds with N which penetrates into the steel sheet due to nitriding, and has the effect of forming fine Cr nitrides in the steel, especially at the steel sheet surface, thus making it possible to utilize these nitrides to increase the effect of the present invention. To this end, it is preferable to add 0.01% or more of Cr. However, Cr also can increase the recrystallization temperature of the steel sheet, and excessive addition of it can markedly degrade the annealing pass-through of the ultra-thin steel sheet, which is one of the objects of the present invention. In order to avoid degradation of annealing pass-through due to increase in recrystallization temperature, it is preferable to limit the addition of Cr to 2.0% or less. At 0.6% or less, the rise in recrystallization temperature can be kept down to where there are likely no practical problems. Furthermore, a total of 0.1 % or less of Sn, Sb, Mo, Ta, V and W may be incorporated to impart characteristics not explicitly specified by exemplary embodiments of the present invention without in any way detracting from the effect of the present invention. Of the elements described above, P, B, Sn and Sb may, under certain conditions, lower the efficiency of nitriding, which is an important requirement for the present invention, thus it is preferable to consider to limit their maximum content in balance with the nitriding conditions. The partitioning in the thickness direction of the steel sheet which can be used to described the exemplary embodiments of the present invention are described below with reference to FIG. 1. For example, "1/8 thickness surface layer" represents the corresponding area in FIG. 1. Furthermore, "1/4 thickness mid-thickness layer" represents the corresponding area in FIG. 1. An area corresponding to the "1/8 thickness surface layer" can be present on both surfaces of the steel sheet, and according to the exemplary embodiment of the present invention may apply to any material, whereof at least one such surface falls within the scope of the present invention. The nitrogen distribution or hardness distribution between the top and bottom can be changed by the method of nitriding and by surface treatment before nitriding, as well as by various types of treatment after nitriding. The exemplary embodiments of the present invention also applies to such steel sheet with different top and bottom surface layers. This is because it is possible to achieve the color tone, surface adhesion and weldability that are some of the objectives of the present invention, e.g., just on one surface. N content of 1/8 thickness surface layer" can be determined by analysis after polishing the steel sheet to leave only the area of interest. Likewise, for the "N content of the 1/4 thickness mid-thickness layer," an analytical value is used which is obtained by analysis after polishing away both surfaces to leave only the area of interest. For the "steel sheet cross-sectional mean Vickers hardness of 1/8 thickness surface layer" and "steel sheet cross-sectional mean Vickers hardness of 1/4 thickness surface layer," Vickers hardness values can be used which are measured at positions in the thickness direction and with a load that leaves a sufficiently small impression to allow suitable evaluation of the hardness distribution in the thickness direction of the steel sheet cross-section. The measurement positions in the thickness direction can be set so as to obtain at least two measurement positions within 1/8 thickness and are equidistantly spaced in the thickness direction. The mean of the values measured in each area is then taken as the respective cross-sectional mean hardness. The distance between the indentations calls for care, but usually, for Vickers hardness determination, a suitable distance from the nearest impression can be provided according to the size of the indentations. To this end, displacing while leaving an appropriate distance along the direction of the sheet surface, as shown in FIG. 2, makes it possible to maintain an appropriate distance between the indentations. Furthermore, in the areas near the sheet surface, the influence of the sheet surface may become a problem; for such cases, measured values of cross-sectional hardness taken on steel sheet with equivalent stacked and tied thereto will be used. "Steel sheet cross-sectional maximum Vickers hardness of 1/8 thickness surface layer" and "steel sheet cross-sectional maximum Vickers hardness of 1/4 thickness mid-thickness layer" indicate the maximum hardness for each area in the hardness distribution obtained from the above-described "steel sheet cross-sectional mean Vickers hardness of 1/8 thickness surface layer" and "steel sheet cross-sectional mean Vickers hardness of 1/4 thickness surface layer." Analytical values and hardness distribution normally exhibit some errors and variations due to local segregation of component elements and structural non-uniformities, and can be determined through trials with suitable quantities sufficient to exclude outliers. The state of nitriding, which is an important condition for an exemplary embodiment of the present invention, including N increase due to nitriding and the content of N after annealing, is described below. The exemplary embodiment according to the present invention provides that a difference in N content may be created between the surface layer part and the mid-thickness layer part of steel sheet: This difference is specified as (N content of 1/8 thickness surface layer) - (N content of 1/4 thickness mid-thickness layer). Such value can be set at 100 ppm, preferably 200 ppm, more preferably 300 ppm, even more preferably 500 ppm, still more preferably 1000 ppm, yet more preferably 2000 ppm, and even more preferably 3000 ppm. If the difference is smaller than this, not only will the color tone, surface coating adhesion and weldability, which are some of the objectives of the present invention, not be achieved, but there may be a substantial variation in material quality due to variation in nitriding content, leading to substantial scattering in material quality within and between coils in actual production. Furthermore, the upper limit of (N content of 1/8 thickness surface layer) can be set at 20000 ppm. A mean of

20000 ppm can be specified for the 1/8 thickness surface layer because the N content of the outermost layer will become 20000 ppm or greater under normal conditions of the exemplary embodiment of the present invention, which can readily cause surface problems such as plating defects. Based on this situation, the upper limit of (N content of 1/8 thickness surface layer) is preferably set at 6000 ppm, more preferably, 3000 ppm. The resulting difference in hardness brought about between the surface layer and mid-thickness layer of the steel material is another distinguishing feature of the exemplary embodiment of the present invention. This difference can be specified as steel sheet cross-sectional mean Vickers hardness of 1/8 thickness surface layer - steel sheet cross-sectional mean Vickers hardness of 1/4 thickness surface layer, the value of which is made 10 points of greater, preferably 30 points or greater, even more preferably 90 points or greater. If the difference is smaller than such value the color tone, surface coating adhesion and weldability that are some of the objectives of the present invention cannot be obtained. Furthermore, the difference in hardness between the surface layer and mid-thickness layer of the steel material can be specified in terms of (steel sheet cross-sectional maximum Vickers hardness of 1/8 thickness surface layer) - (steel sheet cross-sectional maximum Vickers hardness of 1/4 thickness mid-thickness layer). In this case, the value can be made 20 points or greater, preferably 60 points or greater, more preferably 120 points or greater. To control the N content and hardness of the surface layer as compared to those of the mid-thickness layer as described above, an appropriate state before nitriding should also be provided. For example, the N content of the steel sheet before nitriding is preferably made 0.0300% or less, as described above. If a large content of N is already contained before nitriding, it becomes difficult to produce the effect of the present invention. Furthermore, in order to increase the N content by nitriding while avoiding degradation of workability, an upper limit to the N content after nitriding is needed, which is set at N: 0.600% or less. The N content is preferably made N: 0.300% or less, more preferably N: 0.150% or less, even more preferably N: 0.100% or less, still more preferably N: 0.050% or less, and even more preferably 0.030% or less. However, a higher N content is preferable not just in order to further harden the areas hardened by nitriding but also to stably obtain the effect of nitriding. Furthermore, according to the exemplary embodiment of the present invention, the N increase must not extend over the entire sheet thickness. For example, it is preferable to efficiently increase the N content of the surface layer part such that the absolute value of (N increase of 1/8 thickness surface layer) / (N increase of 1/4 thickness mid-thickness layer) becomes 2.0 or more. The reason for specifying an absolute value here is that the analytical value of the N content of the mid-thickness layer, in which the components hardly change, can in some cases become smaller than the value for the entire sheet thickness due to various types of errors and variations depending on the occasion of measurement. This coefficient is preferably made 3.0, more preferably 5.0 or more, even more preferably 10 or more. Next, control of the surface state, which is the greatest effect of the present invention, is described. There are various possibilities when it comes to describing the surface state, but in context of the exemplary embodiment of the present invention, it is described in terms of surface roughness Ra, and PPI, which indicates the number of concavo-convex peaks per one inch of length. There are no particular limitations as to the method of determination thereof, and conventional methods such as tracer and laser methods, two-dimensional and three-dimensional measurements and the like can be used. The exemplary embodiment of the present invention can be distinguished in that

Ra is 0.90 μm or less and PPI is 250 or more. If Ra is too high or PPI is too low, the properties of color tone, surface coating adhesion, weldability etc, which are the objective of the present invention, will degrade due to the concavo-convexity of the surface. Ra is preferably 0.80 μm or less, more preferably 0.70 μm or less, even more preferably 0.60 μm or less, still more preferably 0.50 μm or less. Furthermore, PPI is preferably 300 or more, more preferably 350 or more, even more preferably 400 or more, still more preferably 450 or more, and yet more preferably 500 or more. Qualitatively, it is preferable for concavo-convexities of uniform height to be present at high density. There is no particular lower limit specified for Ra, which can be controlled to a value appropriate to the purpose, based on nitriding conditions, temper rolling conditions, etc. However, the lower limit of Ra preferably does not include 0, and can be realistically 0.02 μm or greater. The upper limit of PPI is also not specified, and can be controlled based on nitriding conditions, temper rolling conditions, etc, depending on the purpose. Basically, the more N is segregated such that N concentration becomes higher closer to the surface, the lower the Ra and the higher the PPI will be. One exemplary method of segregating N toward the surface is to perform nitriding for a relatively short time in an ammonia atmosphere. The surface state can also be affected by the previously existing steel components and crystal grain diameter, the annealing temperature and cold rolling conditions, as well as the reduction ratio, number of passes and roll roughness during temper rolling after nitriding, the plating conditions when plating is performed, etc.

Therefore, while it is difficult to specify the surface state to a particular range, the basic control is the same as that conventionally performed, and can be achieved without problem by a person skilled in the art after several tests. Conventionally, to control roughness in this manner, it is possible to transfer the concavo-convexities of a roll during temper rolling after annealing, or perform morphological control based on surface coating, such as special electrolytic treatment or metal or other materials plating, as well as performing the morphological control of coating or the like precisely, since roughness also greatly depends on the state of adhesion of plating or the like to the steel sheet surface. However, the exemplary embodiment of the present invention is barely affected by such conditions, making it possible to be advantageous in manufacturing operations. For example, with regard to the concavo-convexities of the roll, conventionally, since the concavo-convexities of a roll are worn off by rolling, in order to keep the concavo-convexities of the steel sheet surface within a desirable range, it was necessary to frequently perform roll replacement or machining of concavo-convexities, which produced an excessive burden on productivity and labor, such as having to stop production for roll maintenance. In contrast, according to the present invention, the surface state of the steel sheet is hardly affected by the method of temper rolling, and there is not much need to manage the wearing of the concavo-convexities of the roll, making it possible to perform mass processing. Furthermore, with regard to the morphology of the metal plating, it becomes possible to evenly disperse a very fine metal plating coating of uniform shape without having to particularly finely control the plating conditions or the like. While the reasons why the roughness of the steel sheet surface is in this way hardly affected by the techniques and conditions which create that roughness are not clear, it is thought that the cause which produces roughness lies in the steel sheet itself. The mechanism of this technique is discussed below. For example, a large N concentration difference and a resulting hardness difference may be formed between the surface and mid-thickness layers of the steel according to the exemplary embodiment of the present invention. The N concentration of the outermost surface layer of the steel sheet in particular is thought to contain a high concentration of N, which generally cannot be attained in the steel obtained by conventional dissolution. The mid-thickness layer of the steel sheet is on the other hand as soft as conventional steel sheet. When such sheet is treated by rolling, many fine cracking are formed in the surface layer, which has poor ductility, and this is thought to directly and indirectly affect the concavo-convexity of the steel sheet surface, which is specified by another exemplary embodiment of the present invention and is necessary for steel sheet for containers. In this manner, it is possible to assume that the capability of the steel sheet of the present invention to form concavo-convexities produced by light processing, not dependent on processing conditions but inherent in the steel sheet itself, is due to differences in ductility between the surface layer part and mid-thickness layer part of the steel sheet, and indirectly to the difference in hardness. It would thus be preferable to properly control the hardness of the steel sheet in the thickness direction with the steel sheet according to an exemplary embodiment of the present invention. The thickness of the hardened layer in the surface layer, the material quality, especially the ductility, of the surface layer part, the ratio between the surface layer and mid-thickness layer parts, etc, may affect the concavo-convexity of the surface layer which is preferable according to the present invention. Therefore, if there are no extreme conditions deviating from the present invention, such as uniform nitriding on the entire sheet thickness, extreme increase of surface layer nitride concentration or formation of excessive Ti nitrides due to high Ti content of the steel sheet, and if the steel sheet is manufactured under conditions according to the present invention, the roughness of the steel sheet surface will be in a desirable range. Generally, since temper rolling is generally, to a large extent, performed after annealing, a desirable surface state can be obtained in the steel sheet according to the exemplary embodiment of the present invention without performing special control, but since bending of hearth rolls when passing the sheet through a conventional continuous annealing line causes fine cracking to form in the steel sheet surface, temper rolling is not considered essential. When the steel sheet itself has the capability of forming fine uniform concavo-convexities on the surface in this manner, coating may adhere finely and uniformly, and assume a desirable morphology and distribution according to the concavo-convexities of the steel sheet, even if fine control of the coating conditions or the like is not performed in the coating process. Next, the mtriding conditions are discussed in further detail. From the viewpoint of productivity, the nitriding process according to the exemplary embodiment of the present invention can be advantageously performed simultaneously with or after the recrystallization annealing that follows cold rolling, continuously with the recrystallization annealing, but it is not limited to this. With regards the annealing method, both batch and continuously annealing can be employed. However, from the perspective of productivity of the nitriding process and uniformity of material quality within the coil of the nitrided material, continuous annealing is advantageous. Furthermore, in order to obtain a large benefit by controlling the material quality of the sheet surface and mid-thickness layers as specified by the exemplary embodiment of the present invention, it may be disadvantageous for the nitriding time and the subsequent thermal history to become too long, and in this regard as well, it is preferable for at least the nitriding process to be conducted with continuous annealing equipment. In the absence of a special reason, the use of continuously annealed sheet is assumed. There are many merits in particular to conducting the continuous annealing process by controlling the atmosphere in the furnace in sections to perform recrystallization in the first half and nitriding in the latter half, such as productivity, uniformity of material quality, ease of controlling the nitriding state, etc. Furthermore, if nitriding is performed before recrystallization has been completed, recrystallization may be, suppressed and non-recrystallized sheet may remain, which can lead to marked deterioration of workability, so caution should be employed. This boundary is complexly determined by the steel components, nitriding conditions, recrystallization annealing conditions, etc, but it is easy for a person skilled in the art to find the conditions under which no non-recrystallized material remains after appropriate tests. The nitriding process should be determined by taking into consideration not only the increase in N content of the steel sheet due to nitriding, but also the steel sheet components and recrystallization annealing conditions, as well as the thermal history after nitriding and the like, and by looking at the diffusion of N from the steel sheet surface to the inner area and at the changes in hardness along the sheet cross-section. The color tone, surface coating adhesion and weldability that are the objective of the present invention cannot be obtained if one simply uses material quality determined by Rockwell hardness as an indicator. In practice, the conditions here need to be determined with reference to an appropriate number of tests, but the fundamental idea is as follows, and the exemplary embodiment of the present invention can thus be specified. For example, nitriding should be performed with the steel sheet temperature of 550 to 800 °C. This can be achieved by setting the nitriding atmosphere to this temperature as in conventional annealing, passing the steel sheet through this atmosphere to bring the sheet temperature into this range, and simultaneously performing nitriding.

Alternatively, the nitriding atmosphere can be set to a lower temperature, and nitriding can be carried out by inserting steel sheet heated to this temperature range into that atmosphere. When the nitriding atmosphere is raised to this temperature, the steel sheet nitriding efficiency may in some cases deterioration due to variation and breakdown of the atmosphere unrelated to nitriding of the steel sheet, so it may be specified as 550 to

750 °C. It is preferably made 600 to 700 °C, more preferably 630 to 680 °C. The nitriding atmosphere may contain, by volumetric ratio, 10% or more nitrogen gas, preferably 20% or more, more preferably 40% or more, even more preferably 60% or more, and as necessary, it may contain 90% or less hydrogen gas, preferably 80% or less, more preferably 60% or less, still more preferably 20% or less; it may also contain 0.02% or more ammonia gas as necessary. The rest can be oxygen gas, hydrogen gas, carbon dioxide gas, hydrogen carbide gas and various inert gases. Ammonia gas in particular is highly effective in raising the nitriding efficiency and makes it possible to obtain a specific amount of nitriding within a short period of time, thus preventing diffusion of N into the steel sheet mid-thickness and providing an effect favorable to the exemplary embodiment of the present invention. Even 0.02% or less is adequate to achieve this effect, but 0.1 % or more is preferable, 0.2%» or more being more preferable, 1.0% or more being even more preferable, and 5% or more being more preferable still. At 10% or more, an adequate effect can be obtained with less than 5 seconds of nitriding. Furthermore, with regard to the ratio of gases other than ammonia gas, in particular when nitrogen gas and hydrogen gas are the major gas components, making the volumetric ratio of (nitrogen gas) / (hydrogen gas) 1 or more is desirable from the viewpoint of nitriding efficiency, and making this ratio 2 or more allows even more efficient nitriding. Furthermore, while conventional annealing is performed under conditions such that nitriding generally does not take place in an atmosphere which mainly consists of nitrogen gas and hydrogen gas, a person skilled in the art would be able to modify the atmosphere to where nitriding took place after suitable trials, not only by admixture of ammonia gas as describe above, but also by modifying the dew point, by admixture of trace amounts of gases, modification of gas ratios, etc. The exemplary embodiment of the present invention covers atmospheres for which it can be detected based on modern analytical capabilities that nitriding took place due to heat treatment including annealing at least. The holding time in the nitriding atmosphere is generally not restricted, but in conjunction with the 550 °C or higher temperature conditions of the present invention, considering the steel sheet thickness of 0.400 mm maximum, in consideration of the fact that, if N, penetrating from the steel sheet surface due to nitriding by diffusion of N in the steel sheet, reaches the steel sheet mid-thickness layer while it is held in the nitriding atmosphere, it may not be possible to obtain the N distribution or hardness distribution which is the objective of the present invention, the upper limit is set at 360 seconds. Furthermore, even if nitriding efficiency is improved, 1 second is required to obtain the amount of nitriding and the nitrogen and hardness distribution in the thickness direction of the steel sheet that are required by the present invention. 2 to 120 seconds may be preferable, 3 to 60 seconds more preferable, 4 to 30 seconds even more preferable, and 5 to 15 seconds more preferable still. The nitriding efficiency should be increased by increasing the ammonia concentration or the like when controlling in a short time period. The thermal history of the steel sheet after nitriding is also important for controlling the nitrogen distribution in the thickness direction of the steel sheet. Considering the thickness of the steel sheets in question and the diffusion of nitrogen in the steel, holding for a long time at a high temperature may not be desirable. However, it is also possible to make the effect of the exemplary embodiment of the present invention more pronounced by making the nitrogen distribution appropriately gradual through heat treatment. To this end, the history in the 550 °C or higher temperature region is important, and the product of temperature (°C) and time (seconds) in this temperature region is preferably made 48000 or lower. This corresponds to 80 seconds at 600 °C or 60 seconds at 800 °C, but when the temperature is continuously changing, the effect thereof can be suitably evaluated by recording the temperature changes in approximately 5-second time slices and finding the sum of the products of temperature (°C) and time (seconds) for each time slice. It will be preferably 24000 or less, more preferably 12000 or less, and normally, the nitriding conditions will be preferably set such that the distribution of nitrogen in the steel is substantially determined once nitriding is completed. Along with the described thermal history, the rate of cooling after nitriding greatly can influence the effect of the present invention. For example, differences in the cross-sectional hardness distribution may be observed because the state of formation of nitrides changes greatly in the cooling process, even at low temperature and in a short period of time, when there is hardly any change in the nitrogen distribution. Making the mean rate of cooling from 550 ° to 300 °C 10 °C/s or greater will leave more solid-dissolved nitrogen, make the surface layer part relatively harder as compared to the mid-thickness layer, and improve the color tone, surface coating adhesion and weldability. Preferably, it is made 20 °C/s or more, with 50 °C/s or more being more preferable. However, it is preferable to control with regard to leaving excessive solid-dissolved nitrogen, as this can lead to ageing problems, depending on the application. In the manufacture of thin steel sheet for containers, re-cold rolling can be performed after recrystallization annealing for hardness adjustment and sheet thickness adjustment. The reduction ratios employed here range from 1 %, close to that of skin passing performed for shape adjustment, to 50% or more, the same as for cold rolling.

According to the exemplary embodiment of the present invention, the same sort of re-cold rolling can be employed as for conventional steel sheet. In cases where shape correction or the like is not required, it is possible to perform no re-cold rolling at all, while in cases where shape correction or the like is the objective, re-cold rolling is carried out in the range of about 0.5% to 2.5%). The steel sheet of the present invention may also be normally subjected to this extent of rolling. Particular operation or control must be applied with the steel sheet of the present invention in cases where high re-cold rolling ratios in excess of 2.5% are employed for achieving higher strength and thinness. For example, when re-cold rolling is applied to the steel of the present invention, which has a hard surface layer and a soft mid-thickness layer, the soft mid-thickness layer alone would preferentially undergo work hardening, and the preferential hardening of the surface layer alone, which is provided in the present invention to increase deformation resistance, would be lost, but the reality is the reverse. Namely, with the steel sheet according to the exemplary embodiment of the present invention, if the re-cold rolling ratio is of a normal extent, the re-cold rolling rather preferentially hardens the hard surface layer part with a high N content, making the hardness difference between the surface and mid-thickness layers formed in the steel sheet of the present invention more pronounced. This is because the surface layer is more susceptible to work hardening due to the large content of solid-dissolved N and nitrides, while the mid-thickness layer is constrained by the surface layer, so it cannot preferentially deform and does not selectively harden to greatly exceed the hardening of the surface layer. However, if the re-cold rolling ratio becomes remarkably high, the steel sheet itself will become sufficiently hard, making it possible to obtain adequate can strength even without controlling the material quality distribution in the sheet thickness direction as is done by the art of the present invention, but at the same time tending to reduce the effect of improved surface characteristics and weldability via control of surface roughness, which is a distinguishing feature of the present invention, so there is little meaning to increasing the re-cold rolling ratio beyond the normally applied range. Furthermore, since workability declines as the re-cold rolling ratio becomes higher, inadvertent application of high reduction is to be avoided. Based on the above, when applying re-cold rolling to the steel of the present invention, the reduction ratio is preferably made up to about 70%. To manufacture hard materials, if re-cold rolling is performed, higher re-cold rolling ratios are desirable, being preferably 6% or greater, more preferably 10% or greater, even more preferably 20% or greater, yet more preferably 30% or greatly, and more preferably still, 40% or greater. On the other hand, it goes without saying that lower reduction ratios of re-cold rolling are desirable from the viewpoint of ductility, with 50%) or less being preferable, 40% or less being more preferable, 30% or less being even more preferable, 20% or less being yet more preferable, 10%) or less being still more preferable, and 5% or less being even more preferable. Considering in particular the effect in the initial stage to middle stage of re-cold rolling, whereby the surface layer hardens preferentially and the deformation resistance markedly increases, the reduction ratio is preferably made 0.8 to 45%, with 4 to 35% being more preferable, 6 to 30% being even more preferable, and 8 to 25% being more preferable still. In a process wherein recrystallization annealing and nitriding are performed continuously, which is preferable from the viewpoint of productivity, the time of re-cold rolling would be after the nitriding, but if recrystallization annealing and nitriding are performed in separate processes, re-cold rolling can also be performed before nitriding. Furthermore, when the welding areas are considered, with conventional materials, the material may soften locally due to the welding heat, and machining strain may be concentrated during flange molding or the like, degrading the moldability, but in the steel sheet of the present invention, which contains a large amount of N in the surface layer part, softening due to welding heat is restricted, so benefits can be obtained with regard to the moldability of welding areas as well. In the process wherein recrystallization annealing and nitriding can be performed continuously, which is preferable from the viewpoint of productivity, the time of re-cold rolling would be after the nitriding, but if recrystallization annealing and nitriding are performed in separate processes, re-cold rolling can be performed before nitriding. The exemplary embodiment of the present invention can be applied to steel sheet with a sheet thickness of 0.400 mm or less. This is because with steel sheet of greater thickness, deformation of molded members is not likely to be a problem.

Furthermore, with a greater sheet thickness, the thickness of the surface layer hardening due to nitriding will be relatively small, so the effect of the present invention would not readily emerge. A steel sheet of preferably 0.300 mm or less, more preferably 0.240 mm or less is used, and a very marked effect can be obtained with steel sheet of 0.200 mm or less. One of the effects of the exemplary embodiment of the present invention does not depend on the thermal history after component adjustment and before annealing, or on the manufacturing history. The slab, bloom, or billet in cases where hot rolling is performed is not limited to any manufacturing method such as the ingot method or continuous casting method, and the effect of the present invention can be obtained with the slab reheating method, the CC-DR method, whereby hot rolling is performed directly without reheating the cast slab, or with thin slab casting whereby rough rolling and the like is omitted, since the effect of the present invention does not depend on the thermal history before annealing. The effect of the exemplary embodiment of the present invention furthermore does not have to depend on the hot rolling conditions and can be obtained with two-phase region rolling with an α + γ two-phase region finishing temperature, or with continuous hot rolling whereby rough bars are joined and rolled. Furthermore, when the steel sheet of the exemplary embodiment of the present invention is used as a material for containers with the welding area, softening of the heat affected zone is suppressed and the surface layer area with especially high N concentration rapidly cools and hardens, which has the effect of increasing the weld strength. This becomes more marked when elements such as B and Nb are added, which are conventionally used to control softening of heat affected zones. The steel sheet of the exemplary embodiment of the present invention includes cases where surface treatment of whatever sort is carried out. Namely, in steel sheets used by the customers after surface treatment, color tone and weldability are necessary for the steel sheet after surface treatment, and the favorable state of the steel sheet surface which is necessary for these characteristics is not damaged by surface treatment in the case of steel sheet manufactured as described above. Of course, the absolute value of PPI or Ra may change significantly due to the surface treatment, but the function which provides a favorable surface state for the steel sheet by controlling the hardness of the steel sheet in the thickness direction, i.e. the state wherein numerous low concavo-convexities are formed, can be adequately detected on the steel sheet even after surface treatment. This effect provides good color tone and weldability for the surface-treated steel sheet. With regard to adhesion of surface coatings such as metal plating, paint or organic film (laminate), the state of the steel sheet surface before surface treatment may be important. For this characteristic as well, creating a favorable steel sheet surface state by controlling the hardness of the steel sheet in the thickness direction as disclosed according to the exemplary embodiment of the present invention, e.g., creating a state wherein numerous low concavo-convexities are formed, may provide a good adhesion.

For the surface treatment, in the case of metal plating, conventionally used tin, chrome

(tin-free), Ni, zinc, aluminum or the like is applied. Not only the adhesion of these coatings, but also the color tone and weldability after formation of the coating are improved. Adhesion can be improved by the effect of the present invention also in the case of substrates for laminated steel sheet coated with an organic film, which has come into use in recent years, and in cases where the steel sheet is painted directly or after metal plating or the like. In terms of applications, the exemplary embodiment of the present invention can be used for containers in general, whether they be two-piece cans or three-piece cans, and it goes without saying that it can be used in cases where the problem to be solved is similar to that described above, for whatever application. [Examples] As an exemplary implementation of the present invention, an evaluation of color tone, surface coating adhesion and weldability was conducted using an Sn-plated steel sheet, which is one of the most commonly used types of steel sheet for containers. Regarding the adhesion, two sheets coated on both sides with 25 mg/m² epoxy phenol coating material, baked and dried, were thermal compression bonded with nylon adhesive to make test pieces, which were moistened with municipal water and subjected to a T-type peeling test to determine the peel strength. Obviously, higher peel strength was taken as indicating better adhesion in grading the samples. Peel strength also depends on conditions other than steel components and the manufacturing conditions for obtaining the present invention, and the required level differs depending on the application and the like, so determining acceptability solely on the basis of absolute values may in some respects not always conform to practical utility. Nonetheless, less than 1.5 kg/5 mm was taken as "improvement required," 1.5 to 2.5 kg/5 mm was taken as

"usable," 2.5 to 3.5 kg/5 mm was taken as "good," and 3.5 kg/5 mm was taken as "very good." Regarding the color tone, L values obtained using a spectrocolorimeter after applying 10 μm transparent polyester resin and drying were used as an indicator.

Higher L values indicate superior color tone, and these values were used to grade the samples. The L value also depends on conditions other than steel components and the manufacturing conditions for obtaining the present invention, and the required level differs depending on the application and the like, so determining suitability solely on the basis of absolute values may in some respects not always conform to practical utility. Nonetheless, less than 60 was taken as "improvement required," 60 to 75 was taken as "usable," 75 to 90 was taken as "good" and 90 or greater was taken as "very good." Regarding the weldability, for seam welding, which is conventionally used on three-piece cans, welding was performed while varying the welding current, the weldable current range was determined based on the occurrence of splash (occurrence of spatter) during welding, the weld strength based on a peel test (Hein test), and the welded surface damage due to arc current between the steel sheet surface and an electrode ring during welding, and the decision was made based on the width of that range and the lower limit value. The decision was made by taking a wider range as being desirable in terms of higher manufacturing stability, and taking a lower minimum as desirable in terms of being less prone to material quality change and peeling of plating due to temperature increase of the welding area. In this exemplary embodiment, with regard to the weldable current range, evaluations were made by finding the ratio to the median value of the welding current and taking cases where this ratio was high as preferable. This ratio also depends on conditions other than steel components and the manufacturing conditions for obtaining the exemplary embodiment of the present invention, and the required level differs depending on the application and the like, so determining acceptability solely on the basis of absolute values may in some respects not always conform to practical utility. Nonetheless, less than 1% was taken as "improvement required," 1 to 3% was taken as "usable," 3 to 6% was taken as "good" and 6% or greater was taken as "very good." Productivity was evaluated based on productivity during temper rolling. The "productivity" referred to here does not merely signify the volume of production per unit time, but also includes the ease of personnel and equipment management for maintaining the desired line operation. The reason for focusing on temper rolling is that surface control, which is a distinguishing feature of the steel of the present invention, is mainly performed in the state of the art through management of roll roughness and rolling conditions during temper rolling. In terms of categories, an investigation was conducted based primarily on the number of rolling passes and the management relating to roll roughness; basically, cases where rolling at low roughness is possible, roll roughness management tolerances are wide and the number of rolling passes is small are preferable. However, in actuality, the decision would be made by also considering the lubrication conditions for ensuring sheet shape and thickness precision after rolling and the ease of sheet temperature management, rolling speed control and tension control. The management tolerances for these parameters also depend on conditions other than steel components and the manufacturing conditions for obtaining the present invention, and the required levels differ depending on the application and the like, so there are difficulties to describing absolute criteria of acceptability. With respect to the components of the mid-thickness layer, since the steel sheet before nitriding is manufactured by conventional methods, the change in elements in the thickness direction before nitriding is extremely small, being of a negligible extent with regard to the effect of the exemplary embodiment of the present invention. Namely, the

N content of the 1/8 thickness surface layer and the N content of the 1/4 thickness mid-thickness layer were assumed to be the same in the steel sheet before nitriding. Hot rolling, cold rolling and recrystallization annealing were performed to manufacture steel sheets from steels with the components shown in Tables 1 through 4. The N quantities in Tables 1 through 4 are sheet thickness mean N quantities before nitriding. Some of the materials were nitrided by passing the sheet through under the conditions shown in Tables 1 through 4 while controlling the temperature, atmosphere, etc, of a nitriding furnace following the high temperature holding furnace for recrystallization annealing. All the nitriding was performed starting from the middle phase of the annealing, a condition under which recrystallization can be considered to have been completed before nitriding took place. Furthermore, temper rolling was performed to manufacture steel sheet. The rolling conditions, final sheet thickness, nitrogen content analysis results and property evaluation results for these steels are shown in Tables 5 through 8. It was possible to confirm that good color tone, surface coating adhesion and weldability could be obtained by controlling the state in the sheet thickness direction to be within the range of the present invention by the manufacturing method of the present invention. In some of the cases, an attempt was made to adjust the surface roughness by means of special temper rolling conditions on materials which were not nitrided, but efficient production was obstructed due to roll wear, the number of passes, etc. Furthermore, while this sort of special rolling was able to yield materials with evaluations of steel sheet roughness substantially equivalent to materials using steel of the present invention, the characteristics were not as favorable as those of the steel of the present invention. The reason for this are unclear, but it is thought that some sort of difference in the surface state may exist that cannot be detected by the roughness measurements of the exemplary present embodiment.

(Table 1)

(Table 2)

£ cT α> OJ > —

£ cr CD ■t-,

g cr CD \

a: ery goo , : oo , c: sa e, : mprovemen requre

£ < G\ "

D; (Sheet steel cross-sectional max mum ickers hardness of 1/8 thickness surface layer) — (Sheet steel cross-sectional mean Vickers hardness of 1/4 thickness core layer) a: Very good, b: Good, c: Usable, d; Improvement required

(Table 7)

.a .a a O :o

S 3ϋ* *!6

— u « ,<a _ -53 S &t g3 ra

& σ α> oo

: s ee ic ness mean ncrease C: (Sheet steel cross sectional mean Vickers hardness of 1/8 thickness surface layer) — (Sheet steel cross-sectional mean Vickers hardness of 1/4 thickness core layer) D: (Sheet steel cross-sectional maximum Vickers hardness of 1/8 thickness surface layer) — (Sheet steel cross sectioπal mean Vickers hardness of 1/4 thickness core layer) a: Very good, b: Good, c; Usable, d: Improvement required

The steel sheet for containers and manufacturing method thereof according to the exemplary embodiment of the present invention allows the color tone, surface coating adhesion and weldability of containers to be improved while avoiding complicated treatment after nitriding and hindrance of productivity due to such complicated treatment. Therefore, the productivity of ultra-thin steel sheet for containers can be improved, providing a remarkable industrially useful effect. The foregoing merely illustrates the principles of the invention. Various modifications and alterations to the described embodiments will be apparent to those skilled in the art in view of the teachings herein. It will thus be appreciated that those skilled in the art will be able to devise numerous variations of steel sheets and methods which, although not explicitly shown or described herein, embody the principles of the invention and are thus within the spirit and scope of the present invention. Various publications have been cited herein, the contents of which are hereby incorporated by reference in their entireties.

INDUSTRIAL APPLICABILITY The steel sheet for containers according to exemplary embodiments of the present invention and the manufacturing method therefor make it possible to improve the color tone, surface coating adhesion and weldability of containers while avoiding complicated processing after nitriding and obstruction of productivity due to this complicated processing. Therefore, it becomes possible to keep as high productivity as of conventional sheet and method with complicated processing for steel sheet for ultra-thin containers and provide an industrially-useful effect.

Claims

1. A steel sheet for at least one container, comprising: at least one portion having a sheet thicknesses of at most 0.400 mm, and including, in terms of mass%, approximately C: 0.0800% or less, N: 0.600% or less, Si: 2.0% or less, Mn: 2.0% or less, P: 0.10% or less, S: 0.05% or less, and Al: 2.0% or less, wherein the at least one portion includes a 1/8 thickness surface layer and 1/4 thickness surface layer, a first content of N of the 1/8 thickness surface layer minus a second N content of the 1/4 thickness mid-thickness layer is at least approximately 10 ppm, wherein the first content of N is at most approximately 20000 ppm, and a surface roughness of the at least one portion is approximately 0.90 μm or less in terms of Ra, and wherein a number of concavo-convex peaks per one inch of length of the at least one portion ("PPI") at least is approximately 250.

2. A steel sheet for at least one container, comprising: at least one portion having a sheet thicknesses of at most 0.400 mm, and including, in terms of mass%, approximately C: 0.0800% or less, N: 0.600% or less, Si: 2.0% or less, Mn: 2.0% or less, P: 0.10% or less, S: 0.05% or less, and Al: 2.0% or less, wherein the at least one portion includes a 1/8 thickness surface layer and 1/4 thickness mid-thickness surface layer, wherein one of a first steel sheet cross-sectional mean Vickers hardness of the

1/8 thickness surface layer minus a second steel sheet cross-sectional mean Vickers hardness of the 1/4 thickness mid-thickness layer is one of greater than 10 points and 20 points, a surface roughness of the at least one portion is 0.90 μm or less in terms of Ra, and wherein a number of concavo-convex peaks per one inch of length of the at least one portion ("PPI") is at least 250.

3. The steel sheet according to claim 1 or claim 2, wherein the at least one portion is further including, in terms of mass%, at least one of Ti: 0.05% or less, Nb: 0.05% or less, and B: 0.015% or less.

4. The steel sheet according to claim 2, wherein the mean components of the 1/4 sheet thickness mid-thickness layer are composed of at least one of Ti: 4 x C + 1.5 x S + 3.4 N or more, Nb: 7.8 C + 6.6 x N or more, and B: 0.8 x N or more.

5. The steel sheet according to claim 1 or claim 2, wherein the at least one portion is further including, in terms of mass%, at least one of Cr: 20% or less, Ni: 10% or less, and Cu: 5% or less.

6. The steel sheet according to claim 1 or claim 2, wherein the at least one portion is further including, in terms of mass%, a total of 0.1 % or less of Sn, Sb, Mo, Ta, V and W.

7. A method for manufacturing at least one steel sheet for at least one container, wherein the at least one steel sheet includes at least one portion having a sheet thickness of 0.400 mm or less, the at least one portion including, in terms of mass%, approximately C: 0.0800% or less, N: 0.0300% or less, Si: 2.0% or less, Mn: 2.0% or less, P: 0.10% or less, S: 0.05% or less, Al: 2.0% or less, and the remainder Fe and unavoidable impurities, the method comprising: a) cold-rolling the steel sheet; b) recrystallization annealing of the steel sheet; and c) during or after step (b), nitriding the steel sheet, wherein an average increase of content of N over a sheet thickness mean of the at least one portion is at most approximately 6000 ppm, a first concentration increase of N of a 1/8 thickness surface layer of the at least one portion is at most approximately 20000 ppm, an absolute value of the concentration increase of N of the 1/8 thickness surface layer divided by a second concentration increase of N of a 1/4 thickness mid-thickness layer is at most approximately 2.0, and wherein the surface roughness is at most approximately 0.90 μm in terms of Ra, and a number of concavo-convex peaks per one inch of length of the at least one portion ("PPI") is at least approximately 250.

8. A method for manufacturing at least one steel sheet for at least one container, wherein the at least one steel sheet includes at least one portion having a sheet thickness of 0.400 mm or less, the at least one portion including, in terms of mass%, approximately C: 0.0800% or less, N: 0.0300% or less, Si: 2.0% or less, Mn: 2.0% or less, P: 0.10% or less, S: 0.05% or less, Al: 2.0% or less, and the remainder Fe and unavoidable impurities, the method comprising: a) cold-rolling the steel sheet; b) recrystallization annealing of the steel sheet; and c) during or after step (b), nitriding the steel sheet, wherein an increase in N content is at most approximately 6000 ppm mean across a thickness of the at least one portion, wherein a first steel sheet cross-sectional mean in Vickers hardness of a 1/8 thickness surface layer minus a second steel sheet cross-sectional mean Vickers hardness of a 1/4 thickness mid-thickness layer is greater than approximately one of 10 points and 20 points, wherein a surface roughness of the at least one portion is at most approximately 0.90 μm in terms of Ra, and wherein a number of concavo-convex peaks per one inch of length of the at least one portion is at least 250.

9. The method according to claim 7 or claim 8, wherein the at least one portion further includes, in terms of mass%, one or more of Ti: 0.05% or less, Nb: 0.05% or less and B: 0.015% or less.

10. The method according to claim 8, wherein the at least one portion contains, in terms of the mean components of the 1/4 sheet thickness mid-thickness layer, one or more of Ti: 4 x C + 1.5 x S + 3.4 x N or more, Nb: 7.8 x C + 6.6 x N or more, and B: 0.8 N or more.

11. The method according to claim 7 or claim 8, wherein the at least one portion further includes, in terms of mass%, one or more of Cr: 20% or less, Ni: 10% or less, and Cu: 5% or less.

12. The method according to claim 7 or claim 8, wherein the at least one portion includes a total of 0.1% or less of Sn, Sb, Mo, Ta, V and W, in terms of mass%, as steel components.

13. The method according to claim 7 or claim 8, wherein, in order to perform step (c) simultaneously with or after step (b), the steel is at least one of: maintained for at least than 1 second and no more than 360 seconds in an atmosphere containing 0.02% or more ammonia gas and a sheet temperature of 550 to 800 °C, so as to make a product of temperature (°C) and time (seconds) in the thermal history in the 550 °C or higher temperature range after nitriding 48000 or less, and setting a mean rate of cooling from 550 °C to 300 °C at 10 °C/second or greater.

14. The method according to claim 7 or claim 8, wherein a re-cold rolling ratio after step (b) and before or after step (c) is 20% or less.