JP2004323905A - Extra thin steel sheet for container having remarkably excellent can characteristic, and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve both deformation resistance and can formability of a container manufactured using an extra thin steel sheet for a container, without sacrificing either of the two. <P>SOLUTION: Steel having a composition consisting of, by mass, ≤0.0800% C, ≤0.600% N, ≤2.0% Si, ≤2.0% Mn, ≤0.10% P, ≤0.05% S, ≤2.0% Al and the balance mainly Fe is cold rolled. Then, atmosphere, temperature, time, etc., in recrystallization annealing or subsequent heat treatment are regulated and also proper surface treatment is carried out prior to the heat treatment, by which changes in N content in the steel, particularly the size and number density of nitrides with respect to a surface layer part and a central layer part, are controlled to values within proper different ranges, respectively. By this method, both the deformation resistance and can formability of the container manufactured using the extra thin steel sheet of ≤0.400 mm sheet thickness can be improved without sacrificing either of them. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【表２】

【００３７】
【発明の効果】
以上述べたごとく本発明によれば、容器の耐変形性、缶成形性の一方を犠牲にすることなく両立して著しく向上できる極薄容器用鋼板を高生産性にて得ることが可能となる。
【図面の簡単な説明】
【図１】鋼板の厚み方向の位置を示す図である。
【図２】変形試験における金型押し込み量と押し込み荷重の関係を示す図である。
【図３】窒化時間と缶強度の関係を示す図である。
【図４】窒化前後の窒化物数の比と缶強度の関係を示す図である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a steel plate used for a metal container such as a beverage can and a method for producing the same.
[0002]
[Prior art]
As for the steel plate for containers such as beverage cans and food cans, the thickness of the steel plate has been reduced in order to reduce the cost of the container, and materials having a thickness of 0.2 mm or less have been applied. One of the problems that has become apparent when a container is manufactured from such an extremely thin material is deformation of the container.
This is due not only to the action of external forces that occur during the handling of containers in the general manufacturing process after the container is manufactured or after the contents are filled, but also to increase or decrease the internal pressure of the containers, that is, to increase the pressure during heating of the contents and the contents. This is a pressure reduction treatment for holding the object, or a pressure increase which is essential depending on the contents such as carbonated beverages, and a deformation of the container due to a temperature change during distribution or holding.
In order to improve the deformation resistance, it is necessary to use not only a container design but also a harder material. However, generally, a hard material has low ductility, and causes problems such as material breakage during can molding.
[0003]
In addition, since ultra-thin materials break at a relatively lower strain than thick materials, ultra-thin materials require materials having better ductility than thick materials. Further, in the case forming, the welded portion may be further formed after the steel sheet is welded. In such a case, the deformation tends to concentrate on a specific portion, and good ductility is also required from this point.
As a method of increasing the strength without significantly impairing the ductility in the steps after annealing, techniques of nitriding during annealing are disclosed in JP-A-08-170122, JP-A-08-176788, JP-A-2001-107148, and the like. It has been disclosed.
However, these techniques lack the viewpoint of optimally controlling the hardness of the inner surface layer especially for the ultra-thin material in consideration of the steel sheet composition and the nitriding conditions, and the above-described technology produces cans based on the ultra-thin material. In this case, the moldability of the material and the deformation resistance of the can were not always satisfactory.
[0004]
[Patent Document 1] JP-A-08-170122
[Patent Document 2] Japanese Patent Application Laid-Open No. 08-176788
[Patent Document 3] JP-A-2001-107148
[Patent Document 4] JP-A-2002-012948
[0005]
[Problems to be solved by the invention]
The present invention solves the problems of the prior art as described above, and applies a nitriding method to the material of the surface layer and the inner layer of the material for the deformation that is a problem in a container manufactured using an ultrathin material. It is an object of the present invention to provide a steel sheet which can be controlled and changed greatly and has good ductility even when hard, and a method for manufacturing the same.
[0006]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, the present inventors have studied the components of a steel plate having a thickness of 0.4 mm or less and the relationship between the nitriding conditions and the materials, particularly, the components, particularly the N, By limiting the amount to a specific range and further adjusting the nitriding conditions optimally, it is possible to preferably control the nitride morphology of the surface layer portion and the inner layer portion of the material. It has been found that the problematic deformation can be greatly suppressed.
That is, according to the present invention, the deformation resistance of the can is not so much improved only by separately producing the surface hardness by performing the nitriding treatment after the cold rolling and increasing the amount of nitrogen in the steel, and the ultra-thin material. The present inventors have found that there is a nitriding condition necessary for improving the deformation resistance of the can and a method for controlling the nitriding condition, and the gist is as follows as described in the claims.
[0007]
(1) In 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 0.05% or less, Al: 2.0% or less,
Regarding nitride having a diameter of 1 μm or less and 0.02 μm or more, a number density of 0.2 / μm within 1/8 thickness of the surface layer of the steel sheet ³ A steel sheet for an ultra-thin container having a region existing as described above and satisfying the following formula (A) and having remarkably good can properties with a sheet thickness of 0.400 mm or less.
(Number density at 1/8 thickness position of steel sheet)> (Number density at 1/4 thickness position of steel sheet) (A)
(2) In 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 0.05% or less, Al: 2.0% or less,
An ultra-thin container steel sheet having a can thickness of 0.400 mm or less, characterized by satisfying the following formula (B) with respect to a nitride having a diameter of 1 μm or less and 0.02 μm or more.
(Number density at 1/20 position of steel plate thickness) / (Number density at 1/4 position of steel plate thickness)
> 1.5 (B)
(3) The number density of nitrides having a diameter of not more than 1 μm and not less than 0.02 μm at a position of 1/4 thickness of the steel sheet is 10 pieces / μm ³ The steel sheet for an ultra-thin container according to (1) or (2), which has the following remarkable characteristics:
(4) As a steel component, one or more of Ti: 0.08% or less, Nb: 0.08% or less, B: 0.015% or less, and Cr: 2.0% or less in mass%. (1) to (3), wherein the can properties are extremely good.
(5) The can according to any one of (1) to (4), further comprising, as a steel component, 0.1% or less in total of Sn, Sb, Mo, Ta, V, and W in mass%. Ultra-thin steel sheet with extremely good properties.
(6) The ultra-thin container steel sheet according to (1) to (5), wherein the balance of the steel component is Fe and inevitable impurities.
[0008]
(7) In producing the steel sheet according to (1) to (6), in mass%, C: 0.0800% or less, N: 0.0300% or less, Si: 2.0% or less, Mn: 2 0.0% or less, P: 0.10% or less, S: 0.05% or less, Al: 2.0% or less after cold rolling, simultaneously with recrystallization annealing, or after recrystallization annealing Nitriding is performed, and nitrides having a diameter of 1 μm or less and 0.02 μm or more have a number density of 0.2 / μm within 1/8 thickness of the surface layer of the steel sheet. ³ An extremely thin container steel sheet having a can thickness of 0.400 mm or less, characterized by forming a region existing as described above and setting the N in the steel sheet to 0.600% or less by mass%. Production method.
(8) In producing the steel sheet according to (1) to (6), in mass%, C: 0.0800% or less, N: 0.0300% or less, Si: 2.0% or less, Mn: 2 0.0% or less, P: 0.10% or less, S: 0.05% or less, Al: 2.0% or less after cold rolling, simultaneously with recrystallization annealing, or after recrystallization annealing Nitriding treatment is performed to satisfy the following formula (B) for nitrides having a diameter of 1 μm or less and 0.02 μm or more within 1/8 thickness of the surface layer of the steel sheet, and N in the steel sheet is 0.600 by mass%. % Or less, and a method for producing an extremely thin steel sheet for a container having a remarkably good can property with a sheet thickness of 0.400 mm or less.
(Number density at 1/20 position of steel plate thickness) / (Number density at 1/4 position of steel plate thickness)
> 1.5 (B)
(9) In producing the steel sheet according to (1) to (6), in mass%, C: 0.0800% or less, N: 0.0300% or less, Si: 2.0% or less, Mn: 2 0.0% or less, P: 0.10% or less, S: 0.05% or less, Al: 2.0% or less after cold rolling, simultaneously with recrystallization annealing, or after recrystallization annealing A thickness of a nitride having a diameter of 1 μm or less and 0.02 μm or more satisfying the following formula (C) and N in the steel sheet being set to 0.600% or less by mass% in a nitriding treatment. A method for producing an ultra-thin steel sheet for containers having extremely good can properties of 0.400 mm or less.
(Number density of steel sheet after nitriding treatment at 1/20 position) / (number density of steel sheet before nitriding treatment at 1/20 position)> 1.5 (C)
(10) In 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 A steel containing 0.05% or less, Al: 2.0% or less, and the balance consisting of Fe and unavoidable impurities is subjected to nitriding after cold rolling and simultaneously with recrystallization annealing or after recrystallization annealing. The number density of nitride having a diameter of 1 μm or less and 0.02 μm or more at a plate thickness 1/4 position is 10 / μm ³ The method for producing a steel sheet for an ultra-thin container according to any one of (7) to (9), wherein the N is not more than 0.600% by mass%.
(11) Simultaneously with the recrystallization annealing, or when performing the nitriding treatment after the recrystallization annealing, at a plate temperature of 550 to 800 ° C., in an atmosphere containing 0.02% or more of ammonia gas for 0.1 second or more. After the nitriding treatment, the product of the temperature and the time is set to 48000 (° C. · s) or less in the temperature range of 550 ° C. or more, or the average cooling rate from 550 ° C. to 300 ° C. is set to 10 ° C. / (7) The method for producing a steel sheet for an ultra-thin container according to (7) to (10), wherein the can property is remarkably good.
(12) After the recrystallization annealing, before or after the nitriding treatment, re-rolling is performed at a rolling reduction of 20% or less, and the can characteristics according to (7) to (11) are remarkably good. Manufacturing method of steel sheet for ultra-thin containers.
[0009]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail.
First, the steel component in the present invention will be described. All components are% by weight. The upper limit of the C amount is necessary to avoid deterioration of workability, and is set to 0.0800% or less. Preferably it is 0.0600% or less, more preferably 0.040% or less.
In the steel of the present invention in which N having the same properties as C is increased by nitriding, the C content required from the viewpoint of securing strength may be low. C: The required strength can be ensured even at 0.0050% or less, and may be 0.0020% or less. If it is 0.0015% or less, there is a balance with the amount of nitriding, but extremely soft material can be produced. In order to improve the r value and maintain high drawability, C is preferably lower.
The upper limit of the amount of N before nitriding is also necessary to avoid deterioration of workability, and is set to N: 0.0300% or less. Preferably, N: 0.0200% or less, more preferably N: 0.0150% or less, further preferably N: 0.0100% or less, more preferably N: 0.0100% or less, and further preferably N: 0. 0.0050% or less, more preferably N: 0.0030% or less. In order to improve the r value and keep the drawability high, the N content before nitriding is preferably low. It should be noted that, as described later, N contained by nitriding exists in different amounts depending on the thickness position of the steel sheet in order to impart the deformation resistance effect of the can, etc., and exists before nitriding. The effect is somewhat different from N.
[0010]
The upper limit of the amount of N after nitriding is necessary not only to avoid deterioration in workability but also to avoid deterioration in surface treatment properties such as plating, and is set to 0.600% or less. Preferably, N: 0.300% or less, more preferably, N: 0.150% or less, further preferably, N: 0.100% or less, more preferably, N: 0.050% or less, and further preferably, N: 0. 030% or less. However, it goes without saying that a higher N content is preferable in order to harden the hardened portion by nitriding.
Si is added for the purpose of adjusting the strength, but if it is too much, the workability is deteriorated, so that the content is set to 2.0% or less. In the steel of the present invention, nitrides formed in the steel at the crystal grain boundaries by nitridation form N and nitrides, which not only cause brittle cracking but also may impair the effects of the present invention. In some cases, it may be necessary to reduce the amount to 1.0% or less. In particular, in order to keep the formability high, the amount of Si is preferably low, and the formability is improved by setting it to 0.5% or less, and further to 0.1% or less.
[0011]
Mn is added for adjusting the strength, but if it is too much, the workability is deteriorated, so that Mn is made 2.0% or less. In order to keep the moldability high, the Mn content is preferably low, and the moldability is improved by setting it to 0.6% or less, and more preferably 0.2% or less.
P is added for adjusting the strength, but if too much, the workability is deteriorated, so P is set to 0.10% or less. In order to keep the moldability high, the P content is preferably low, and the moldability is improved by setting the P content to 0.05% or less, further 0.01% or less.
S deteriorates hot ductility and becomes a hindrance factor of casting and hot rolling. Therefore, S is set to 0.05% or less. In order to maintain the formability, the S content is preferably low, and the formability is improved by setting the S content to 0.02% or less, and further to 0.01% or less.
[0012]
Al is an element added for deoxidation, but if it is high, casting becomes difficult. Since there is harm such as an increase in surface flaws, the content is set to 2.0% or less. In addition, when the Al content is as high as 0.2% or more, there is an effect that nitriding is combined with N infiltrated into the steel sheet to form a large amount of AlN in the steel and harden the nitrided portion. In order to maintain high formability in the center of the steel sheet thickness where the degree of nitriding is low, the Al content is preferably low. Sex is improved.
The effects of elements considered in ordinary steel plates for containers other than the above basic elements and their control will be described below.
Ti raises the recrystallization temperature of the steel sheet and significantly deteriorates the annealing passability of the ultra-thin steel sheet targeted by the present invention. For this reason, it is set to 0.080% or less. In ordinary applications where a particularly high r value is not required, it is not necessary to add Ti, and the content is set to 0.04% or less, more preferably 0.01% or less. Further, Ti which is dissolved in the steel before nitriding is combined with N which has infiltrated the steel plate by nitriding to form fine TiN in the steel and has a strong effect of hardening the nitrided portion. For this reason, even in the case of a steel sheet having a low degree of nitriding, the hardening of the material may appear more than necessary even in the center layer, and if it is necessary to obtain a soft steel sheet, the lower the Ti content, the better. % Or less, more preferably 0.003% or less, it is possible to suppress inadvertent hardening of the steel sheet.
[0013]
Nb also has the same effect as Ti, raises the recrystallization temperature, and significantly deteriorates the passability of the ultra-thin steel sheet targeted by the present invention. Therefore, the content is set to 0.08% or less. In a normal use where a particularly high r value is not required, in a normal use where a particularly high r value is not required, it is not necessary to add Nb, and the Nb content is set to 0.04% or less, more preferably 0.01% or less. Further, Nb dissolved in the steel before nitriding is combined with N infiltrated into the steel sheet by nitriding to form fine NbN in the steel and has a strong effect of hardening the nitrided portion. For this reason, even in the case of a steel sheet having a low degree of nitriding, the hardening of the material may appear more than necessary even in the central layer, and if it is necessary to obtain a soft steel sheet, the Nb content is preferably as low as 0.005. % Or less, more preferably 0.003% or less, it is possible to suppress inadvertent hardening of the steel sheet.
When B is added to a steel sheet containing about 0.01% or more of Ti and Nb, B raises the recrystallization temperature of the steel sheet and significantly deteriorates the annealing passability of the ultra-thin steel sheet targeted by the present invention. When the content of Nb is small, the adverse effect on this point is small, but rather the recrystallization temperature is lowered, so that recrystallization annealing can be performed at a low temperature and the effect of improving the annealing passability is also provided. It is possible. However, excessive addition causes significant cracking of the slab during casting, so the upper limit is made 0.015%. For the purpose of lowering the recrystallization temperature and improving the annealing passability, it is sufficient to set B / N = 0.6 to 1.5 in relation to the N content before nitriding. In addition, B, which forms a solid solution in the steel before nitriding, has a strong effect of forming fine BN in the steel by combining with N infiltrated into the steel sheet by nitriding to harden the nitrided portion. When utilizing the surface layer hardening by BN, it is preferable to set the ratio of the B content and the N content before nitriding to B / N> 0.8. When this ratio is 1.5 or more, and more preferably 2.5 or more, the hardening due to the formation of BN becomes remarkable. On the other hand, care must be taken because the formation of BN may cause the material to harden more than necessary and deteriorate the formability. If the hardening due to BN formation is not particularly utilized in the steel of the present invention, the ratio between the B content and the N content before nitriding is set to B / N <0.8, and more strictly, B / N <0.1. do it.
[0014]
The Cr dissolved in the steel before nitriding combines with N infiltrated into the steel sheet by nitriding to form fine Cr nitrides in the steel and has an effect of hardening the nitrided portion. For this reason, the hardening of the material may appear more than necessary. On the contrary, it is also possible to effectively increase the hardness of the nitrided portion by using this nitride. For this purpose, it is preferable to add 0.01% or more of Cr. However, on the other hand, if Cr increases the recrystallization temperature of the steel sheet and is excessively added, it may significantly deteriorate the annealing passability of the ultra-thin steel sheet targeted by the present invention. In order to avoid a decrease in annealing passability due to an increase in the recrystallization temperature, the content is preferably set to 2.0% or less, and if it is 0.6% or less, the increase in the recrystallization temperature is practically acceptable. Can be suppressed.
It is possible to add Cr, Ni, Cu, etc. to impart properties not specified in the present invention, such as to increase corrosion resistance, but excessive addition lowers the nitriding ability essential for the steel of the present invention. Therefore, it is preferable that the content of Cr is 30% or less, the content of Ni is 15% or less, and the content of Cu is 5% or less. More preferably, the content of Cr is 15% or less, the content of Ni is 5% or less, and the content of Cu is 2% or less. Should.
[0015]
Further, Sn, Sb, Mo, Ta, V, and W are summed up to give characteristics not specified in the present invention.
Although it is possible to contain 0.1% or less, it should be noted that excessive addition may lower the nitriding ability essential for the steel of the present invention. In particular, the inclusion of Sn and Sb may lower the nitriding efficiency, so care must be taken when controlling the nitride by applying nitriding. In order to prevent Sn and Sb from notably impairing the nitriding efficiency, the respective contents are set to 0.06% or less, preferably 0.02% or less.
Here, the division of the parts in the thickness direction of the steel sheet used in this specification will be described with reference to FIG.
The “surface layer 1/8 thickness” indicates the corresponding region in FIG. In addition, although the area | region corresponding to "1/8 thickness of a surface layer" exists in both surfaces of a steel plate, in this invention, the thing which corresponds to the limited range of this invention about any one of them. It is relatively easy to change the nitride distribution on the front and back by nitriding method and surface treatment before nitriding, and even some treatment after nitriding, but in the present invention, such a steel sheet with different front and back surface layers set to target. This is because the deformation resistance targeted by the present invention can be obtained even with only one surface.
[0016]
The “plate thickness 1/8 position” indicates the corresponding position in FIG. The “plate thickness 1/4 position” indicates the corresponding position in FIG. In addition, the positions corresponding to these exist on both surfaces of the steel sheet, but the present invention is intended for any one of the surfaces that falls within the limited range of the present invention.
It is relatively easy to change the nitride distribution on the front and back by nitriding method and surface treatment before nitriding, and even some treatment after nitriding, but in the present invention, such a steel sheet with different front and back surface layers set to target. This is because the deformation resistance targeted by the present invention can be obtained even with only one surface.
Although not shown in the drawing, the “sheet thickness 1/20 position” indicates a position at a depth of 1/20 of the sheet thickness from the steel sheet surface similarly to the “sheet thickness 1/8 position”.
[0017]
In the present invention, the size and the number density of the nitride present at a specific position or a specific layer in the thickness direction of the steel sheet are specified. The existing precipitate can be identified by a diffraction pattern such as an electron microscope or an attached X-ray analyzer. Of course, other methods such as chemical analysis
Can be identified. The average diameter of the nitride targeted in the present invention is 1.0 μm or less.
Above this, not only does the efficiency of high strength remarkably decrease, but it also becomes a starting point of cracking during processing and deteriorates ductility, and when coarse nitride is exposed on the steel sheet surface, it adversely affects surface treatment such as plating . In light of these characteristics, the average diameter is preferably equal to or less than 0.40 μm, more preferably equal to or less than 0.20 μm, and still more preferably equal to or less than 0.10 μm. These diameters and the number density described below can be quantified by, for example, observation with an electron microscope.
[0018]
Control of the nitride size and number density is very important from the viewpoint of achieving both high strength and good workability. This is because not only do they affect the strength and the workability, but also when they are changed, the behavior in which the strength or the workability changes is different. That is, it is necessary to control the region to have a high strength increasing effect and a low workability deterioration efficiency. To this end, it is effective to appropriately control the temperature and time in the above-mentioned temperature range of 450 to 700 ° C. and the cooling rate immediately before entering this temperature range. Similar to the formation of a precipitate.
That is, the higher the cooling rate and the lower the temperature, the finer and denser the nitride size, and the longer the size, the coarser the size.
It should be noted that not only nitrides but also oxides, carbides, sulfides, and the like, as well as composite precipitates, are included. When complex precipitates are formed, it is difficult to specify the type of one precipitate and the size of each compound, but when one precipitate is clearly divided into a nitride part and other parts Is determined as one nitride except for.
[0019]
Basically, in the present invention, the extracted replica obtained by the SPEED method is observed with an electron microscope equipped with an EDX in the present invention, but when the nitride is considered to be very fine and the extraction is not good, it passes through the thin film. You may observe with an electron microscope. The composition is determined by EDX analysis, and when the observed nonmetallic element is N, it is regarded as sulfide. Further, since the characteristic spectrum of N is not clear because of its small size, Fe, Ti, Nb, B, Cr and the like are detected, and no clear spectrum such as O and S is observed, and it can be specified as nitride. In the present invention, a precipitate which can be almost determined to be a nitride based on a morphological comparison with other precipitates is also considered as a nitride. In addition, an electron diffraction pattern or the like may be used for the qualification of the precipitate. The identification of the nitride is not based on a technique such as EDX or an electron diffraction pattern, and any analytical instrument whose performance is remarkably improved at present may be used. In short, it suffices if the type, size and number density of the precipitate can be determined by a method recognized as appropriate. It is considered that it may be difficult to determine whether the precipitate is a carbide or a nitride depending on the precipitate. However, those whose type cannot be properly determined by ordinary analytical instruments are excluded from the present invention. Those which are very small in size and cannot be qualitatively determined by the EDX spectrum or ordinary analytical instruments are excluded from the nitrides to be considered in the present invention. Since the minimum size is about 0.02 μm in an analytical instrument usually used by the inventor at the time of filing the present invention, the lower limit of the present invention is 0.02 μm. Naturally, the number density will increase if more advanced analytical instruments are used and finer nitrides are considered.
[0020]
In addition, when the arrangement of individual atoms is clearly specified by a device that the inventor has not used before, the problem of determining how much a superfine atomic union of N and metal atoms is determined to be a nitride is included. It is considered important to clearly specify the lower limit of the nitride size.
The diameter and the number of nitrides are measured in a field of view where there is no deviation. In the present invention, the magnification is set such that the number of nitrides of the target diameter is about 500 in one visual field, and 10 visual fields are selected at random. The field diameter was divided by the electrolytic thickness by the SPEED method, and the average diameter was obtained by dividing the total of individual nitride diameters by the number. Here, it is needless to say that all the target sulfides in the visual field need to be measured. It should be noted that the number and diameter of nitrides can also be obtained by using image analysis or the like.
[0021]
In some cases, the elongated shape may be seen, but for the non-isotropic shape, the average of the major axis and the minor axis is defined as the diameter of the precipitate.
The number density of the precipitates is determined by the fact that the total electric charge applied to the sample surface in the electrolysis step in the replica making process is a divalent ion of Fe (Fe ²⁺ ) Was calculated assuming that the steel sheet was consumed to be electrolyzed and that any precipitate remaining as a residue during electrolysis was captured on the replica. For example, in replica production, 50 C (coulomb) / cm in sample surface area ² When the electrolysis is performed with the amount of electricity, a precipitate within a thickness of 18 μm from the sample surface is observed on the replica. However, when the steel plate to be measured is extremely thin, for example, if precipitates within a thickness of 18 μm are collectively observed, it becomes unclear which position of the plate thickness corresponds to the observation position. Since the meaning of the definition such as "1/8 thickness" or "1/4 position", "1/8 position", "1/20 position" specified by the above becomes ambiguous, the electrolytic thickness in the SPEED method is Is not limited to 18 μm. Ideally, a precipitate existing on a plane having a thickness of 0 should be observed, but this may cause an increase in measurement error. Although it depends on the plate thickness, the electrolytic thickness should be about 5 to 20 μm, and the polishing should be performed so that the target plate thickness position is the center of the thickness of the electrolytic portion.
In addition, electrolysis is performed not from the plate surface to the plate thickness direction but from the plate thickness cross section to the direction in the plate surface, and a replica containing information on the plate thickness direction is created. It is also possible to determine the number density of nitride at a specific thickness position from this distribution.
[0022]
Hereinafter, the state of nitriding, which is an important requirement of the present invention, will be described.
The technology to which the present invention is applied is basically applied to an ultra-thin steel sheet for a container having excellent can characteristics in which the components and materials of the surface layer and the central layer, which the present inventor has filed in Japanese Patent Application No. 2002-337647, are appropriately controlled. However, the effect is extremely excellent, but the present invention is not limited to this. However, in the description of the present invention, the nitride in the region of “surface 1/8 thickness” and at “plate thickness 1/20 position”, “plate thickness 1/8 position”, “plate thickness 1/4 position” The main effect of the present invention is to mainly use the state and control the state of the nitride at the plate thickness position differently according to the state. By controlling the state of the nitride in this way, Japanese Patent Application No. 2002-337647. Can be more preferably obtained. This is in line with the knowledge that the state of the surface layer is very important in terms of the characteristics of use as cans in ultra-thin materials for containers, and is used to express the distribution of precipitates in steel sheets that have characteristic fluctuations in the thickness direction. And the size and number density of the nitride at the plate thickness position are used. The present invention mainly disperses the nitride in the surface layer in a larger amount and finely as compared with the central part, and is considered from the general nitriding method assumed as one of the manufacturing methods by the present invention. Basically, it is assumed that the surface of the steel sheet is preferentially nitrided, and the amount of nitride generated along with nitriding should increase as compared with the central layer. In addition, the nitride formed at that time is rather unpreferable if it is coarse for the purpose of the present invention, and it is preferable to disperse the heat history after nitriding, particularly finely by cooling conditions and the like. Therefore, in the present invention, control of fine nitrides is performed.
[0023]
As described above, one of the features of the present invention is to provide a difference in the state of the nitride at the thickness position of the steel sheet. This difference is due to the fact that the nitride having a number density of 0.2 pieces / μm in the (surface layer 1/8 thickness) of the steel sheet for the nitride of the present invention ³ It has a region that exists as described above, and is limited by (number density of steel sheet at (plate thickness 1/8 position))> (number density of steel sheet at (plate thickness 1/4 position)). Although the number density of the nitride is limited to a range that can be taken depending on the relationship between the N content and the size of the nitride, the number density is 0.2 pieces / μm. ³ More preferably, it is more preferably 2 pieces / μm ³ 20 pieces / μm ³ More than 200 pieces / μm ³ More than 1000 pieces / μm ³ This is very effective in terms of hardening.
It can also be defined by (number density of steel sheet at (plate thickness 1/20 position)) / (number density of steel sheet at (plate thickness 1/4 position)), and this ratio exceeds 1.5, preferably 3 or more. And more preferably 6 or more, more preferably 10 or more, further preferably 30 or more, and still more preferably 100 or more. If this ratio is small, the effect of the present invention will be small, and the desired steel sheet cannot be obtained. When nitriding is applied as a method for increasing the number density of nitrides in the surface layer in this manner, (number density of steel sheet after nitriding at (sheet thickness 1/20 position)) / (number of steel sheet before nitriding treatment) It can also be defined by the number density at the plate thickness 1/20 position), and in this case as well, this ratio exceeds 1.5, preferably 3 or more, more preferably 6 or more, and still more preferably 10 or more. , More preferably 30 or more, more preferably 100 or more. Needless to say, the effect of the present invention basically increases as the ratio increases.
In addition, it is clear from the fact that the main control object of the present invention is to disperse a large amount of fine nitrides in the surface layer of the steel sheet as compared with the central layer of the steel sheet. It is not preferable from the viewpoint of more preferably enjoying the effect of (1). In order to make the effect of the present invention remarkable, the number density of nitrides having a diameter of 1 μm or less and 0.02 μm or more at a steel plate (at a plate thickness of 1/4 position) is set to 10 / μm. ³ It is preferable to set the following.
[0024]
Next, the nitriding conditions will be described. It is convenient from the viewpoint of productivity that the nitridation treatment of the present invention is performed simultaneously with or after recrystallization annealing after cold rolling and subsequently with recrystallization annealing, but it is not particularly limited. The method of annealing can be applied regardless of batch type or continuous annealing.
However, the continuous annealing method is much more advantageous from the viewpoint of the productivity of the nitriding treatment and the uniformity of the coil material of the nitride material. In addition, since it is disadvantageous that the nitriding time and the subsequent heat history are prolonged in order to obtain a large effect by controlling the material of the inner layer as specified by the present invention, at least the nitriding treatment is performed by a continuous annealing facility. It is preferably performed. If there is no special reason, continuous annealing shall be applied. In particular, the step of partially controlling the atmosphere in the furnace in the continuous annealing step, recrystallization in the first half, and nitriding in the second half has many advantages such as productivity, uniformity of materials, and easy control of the nitriding state.
[0025]
If the nitriding treatment is performed before the recrystallization is completed, recrystallization is remarkably suppressed, an unrecrystallized structure remains, and remarkable deterioration in workability may occur. This limit is complicatedly determined by the steel composition, nitriding conditions, recrystallization annealing conditions, etc., but it is easy for those skilled in the art to find conditions after which unrecrystallized structures do not remain after appropriate trials. . The nitriding treatment considers not only the amount of N increase in the steel sheet due to nitriding, but also the steel composition, recrystallization annealing conditions, heat history after nitriding, etc., and diffusion of N from the steel sheet surface to the inside and nitriding in the sheet thickness cross section. It is necessary to decide in consideration of changes in things. Simply using only the material determined by Rockwell hardness, tensile test, or the like as an index, it is not possible to obtain the desired deformation resistance intended by the present invention. In actual operation, these conditions need to be determined with reference to an appropriate number of trials. The basic concept is as follows, and the present invention is defined based on the following. That is, nitriding needs to be performed at a plate temperature of 550 to 800 ° C. This is because it is possible to set the plate temperature to this range and simultaneously perform nitriding by setting the nitriding atmosphere to this temperature as in normal annealing and passing the steel sheet through the atmosphere, and the nitriding atmosphere is set to a lower temperature. Alternatively, the nitriding may be advanced by infiltrating a steel sheet heated to a temperature in this range into the steel sheet. When the temperature of the nitriding atmosphere is raised to this temperature, the nitriding efficiency of the steel sheet may decrease due to the alteration and decomposition of the atmosphere unrelated to the nitriding of the steel sheet. Preferably it is 600-700 degreeC, More preferably, it is 630-680 degreeC. The nitriding atmosphere contains nitrogen gas at a volume ratio of 10% or more, more preferably 20% or more, further preferably 40% or more, further preferably 60% or more, and if necessary, hydrogen gas at 90% or less, more preferably 80% or more. % Or less, more preferably 60% or less, more preferably 20% or less, and if necessary, 0.02% or more of ammonia gas, and the balance is oxygen gas, hydrogen gas, carbon dioxide gas, hydrocarbon Gas or various inert gases.
[0026]
In particular, ammonia gas is highly effective in increasing the nitriding efficiency, and it is possible to obtain a predetermined amount of nitriding in a short time, so that diffusion of N to the center of the steel sheet can be suppressed, and a favorable effect for the present invention can be obtained. . This effect is sufficient even if 0.02% or less, but preferably 0.1% or more, more preferably 0.2% or more, further preferably 1.0% or more, more preferably 5% or more, and 10% or more. In this case, a sufficient effect can be obtained even with a nitriding treatment of 5 seconds or less, and if it is 20% or more, and even 40% or more, depending on the nitriding temperature and the plate thickness, it can be as short as 1 second or less. A clear effect can be obtained. In addition, when the ratio other than ammonia gas, particularly nitrogen gas and hydrogen gas are the main gas components, it is preferable to make (nitrogen gas) / (hydrogen gas) 1 or more by volume from the viewpoint of nitriding efficiency. By setting the ratio to 2 or more, more efficient nitriding becomes possible. In addition, in normal annealing, annealing is performed under conditions that do not cause nitridation in an atmosphere mainly composed of nitrogen gas and hydrogen gas. However, those skilled in the art are not limited to the mixing of ammonia gas described above, and the dew point It is also possible to change to conditions under which nitriding occurs by a change, a slight mixing of a small amount of gas, a change in the gas ratio, etc. after an appropriate trial. An object of the present invention is to detect at least the nitridation by heat treatment including annealing by the current analysis capability.
[0027]
The holding time in the nitriding atmosphere is not particularly limited. However, in view of the steel sheet thickness of 0.400 mm at maximum in view of the temperature condition of the present invention of 550 ° C. or more, the nitriding due to the diffusion of N in the steel being held. It is preferable that the upper limit is set to 360 seconds in view of the fact that N penetrating from the steel sheet surface reaches the steel sheet central layer due to the above and the N distribution or the nitride distribution targeted by the present invention cannot be obtained. Further, even if the nitriding efficiency is improved, it takes 0.1 seconds to obtain the amount of nitriding and the distribution of nitrogen and hardness in the thickness direction of the steel sheet required by the present invention. It is preferably 1 to 60 seconds, more preferably 2 to 20 seconds, and still more preferably 3 to 10 seconds.
[0028]
In order to control the distribution of nitrides in the thickness direction of the steel sheet, the thermal history of the steel sheet after nitriding is also important. Considering the thickness of the target steel sheet and the diffusion of nitrogen in the steel and the formation and growth of nitrides, long-term holding at a high temperature is not preferable. However, the effect of the present invention can be made more remarkable by appropriately smoothing the nitrogen distribution by this heat treatment. For this purpose, the history in a temperature range of 550 ° C. or more is important, and the product of the temperature and time in this temperature range is preferably 48000 or less. This corresponds to 80 seconds at 600 ° C. and 60 seconds at 800 ° C. When the temperature changes continuously, the temperature change is divided into time regions of about 5 seconds so that the effect is appropriately evaluated. Evaluation is also possible by recording and obtaining the sum of the product of temperature and time for each area.
Of course, this may be evaluated by dividing into temperature regions having a certain temperature range. Preferably, the nitriding condition is set so as to be 24,000 or less, more preferably 12,000 or less, and still more preferably 6000 or less, and usually the distribution of nitrogen in the steel is almost determined at the end of nitriding. Is preferably controlled. The nitridation targeted by the present invention is mainly performed in a state where a large amount of N is dissolved, and since a large amount of nitride is generated with a decrease in temperature thereafter, control of a cooling process after nitriding is important.
[0029]
In connection with the heat history in the cooling step, the cooling rate after nitriding greatly affects the effect of the invention. In other words, the state of nitride formation during the cooling process may change significantly even at a low temperature for a short time when the nitrogen distribution hardly changes. By setting the average cooling rate from 550 ° C. to 300 ° C. to be 10 ° C./s or more, a large number of fine nitrides are generated particularly in the surface layer portion having a relatively high N concentration and a high cooling rate compared to the central layer. Becomes possible. It is preferably at least 20 ° C / s, more preferably at least 50 ° C / s. However, it should be noted that if the cooling rate is too high, solute nitrogen may remain excessively and aging may become a problem depending on the application.
[0030]
In the production of thin steel sheets for containers, re-rolling may be performed after recrystallization annealing for hardness adjustment and sheet thickness adjustment. The rolling reduction has been put to practical use from a few percent close to the skin pass performed for shape adjustment to 50% or more similar to that of cold rolling. When the re-cold rolling method is applied to the present invention, the rolling reduction is limited to 20% or less. When the rolling reduction is more than this, not only does the material difference between the surface layer and the inner layer characterized by the present invention become small and the effect of the invention disappears, but also the steel sheet itself becomes hard and it is necessary to impart deformation resistance by the present invention. Disappears. In addition, an increase in the re-rolling rolling reduction deteriorates the workability of the steel sheet, and is not an inherently preferable method if it is limited to the purpose of imparting can strength. In consideration of welding, it is preferable to reduce the rolling reduction because the work hardened portion is easily softened and the strength of the welded portion is easily reduced. It is preferably at most 15%, more preferably at most 10%, preferably at most 5%, preferably at most 3%. The re-rolling is performed after the nitriding treatment in the step of continuously performing the recrystallization annealing and the nitriding treatment, which is preferable from the viewpoint of productivity. However, when the recrystallization annealing and the nitriding treatment are performed in different steps, the nitriding treatment is performed. It is also possible to do before.
[0031]
The present invention is applied to a steel plate having a thickness of 0.400 mm or less. This is because deformation of the formed member is less likely to be a problem with a steel plate having a larger thickness. In addition, when the plate thickness is large, the thickness of the hardened surface layer due to nitriding becomes relatively small, so that the effect of the invention is hardly exhibited. For a steel sheet preferably 0.300 mm or less, more preferably 0.240 mm or less, a very remarkable effect can be obtained with a steel sheet of 0.190 mm or less, more preferably 0.160 mm or less.
In this way, by mainly controlling the state of nitride after nitriding by distinguishing the surface layer and the central layer and considering the distribution in the thickness direction, the purpose is simply to separate N-containing steel and surface hardness. The mechanism by which the nitrided steel has a material unique to the steel of the present invention that is not present is not clear, but it is considered that the nitride effectively increases the resistance to bending deformation of the surface layer of the steel sheet accompanying deformation of the can. This effect is conscious of the external force when the sheet thickness or deformation of the target material occurs, the stress state involving conditions such as the inner profile and the shape of the container, and the difference between the surface layer and the central layer, which is combined with the nitriding conditions specified in the present invention. It is presumed that the deformation and the density of the nitride are very effective in expressing the deformation resistance.
[0032]
The effect of the present invention does not depend on the heat history and the manufacturing history before the annealing after the component adjustment. The slab for hot rolling is not limited to manufacturing methods such as ingot method and continuous casting method, and it does not depend on the heat history up to hot rolling, so slab reheating method, reheating the cast slab The effect of the present invention can also be obtained by the CC-DR method in which hot rolling is performed directly without performing, and also by thin slab casting in which rough rolling and the like are omitted. In addition, the effects of the present invention can be obtained by two-phase zone rolling in which the finishing temperature is a two-phase zone of α + γ or continuous hot rolling in which a rough bar is joined and rolled, regardless of the hot rolling conditions.
Further, when the steel of the present invention is used as a material for a container having a welded portion, the softening of the heat-affected zone is suppressed, and particularly, the surface layer having a large amount of nitride is rapidly heated and rapidly cooled to dissolve the nitride, and furthermore. It reprecipitates as fine nitrides and partially remains as solid solution N and hardens, so that it also has the effect of improving the strength of the weld. This becomes even more remarkable when an element that suppresses the softening of the heat-affected zone, such as B and Nb, is added. On the other hand, in a so-called two-piece can manufactured through drawing or ironing, the surface of the plate is hardened so that the coefficient of friction with the molding die is reduced and the moldability is improved. Further, since the surface layer is hardened and the resistance to bending deformation is increased, bending buckling of the steel sheet during forming hardly occurs, that is, an effect of suppressing generation of wrinkles also appears.
Usually, the steel sheet of the present invention is used as an original sheet for a surface-treated steel sheet, but the surface treatment does not impair the effects of the present invention at all. As the surface treatment for cans, nickel, tin, chromium (tin-free) or the like is usually applied. Further, it can also be used as a raw sheet for a laminated steel sheet coated with an organic film, which has recently been used without impairing the effects of the present invention.
[0033]
<Example 1>
In a three-piece can in which the can body was formed by welding, a three-piece can body was manufactured from a steel sheet in which nitriding conditions were changed and nitrides were controlled. The deformation resistance when the body of the can was pushed in with a cylindrical mold having a diameter of 10 mm and a length of 40 mm was measured, and the end of the can was flange-formed in the same manner as when a normal lid was wound.
In the deformation test, the correlation between the amount of pressing of the mold and the pressing load is as shown in FIG. 2, and an inflection point occurs at a certain load. The load serving as the inflection point was used as an index of deformation resistance. The higher the value, the smaller the deformation due to external force and the better the deformation resistance. Further, in the flange forming, the length of the flange until a crack was generated in the flange portion was measured. The longer the length is, the better the flange formability is, and the more difficult it is for defects to occur when the lid is tightened.
[0034]
The steel of each component shown in Table 1 was subjected to skin rolling or re-cold rolling after annealing involving hot rolling, cold rolling, and nitriding, and a steel sheet was manufactured, and deformation resistance and flange formability were evaluated. Table 1 shows hot rolling, cold rolling, annealing, nitriding conditions and the like. All the nitriding is performed after the middle stage of the annealing, and the conditions are considered to be that the recrystallization was completed before the nitriding occurred. The N amount in Table 1 is the average N thickness before the nitriding. Since the steel sheet is manufactured by an ordinary method, changes in elements in the sheet thickness direction and changes in the state of nitride before nitriding are negligible and negligible for the effects of the present invention. That is, regarding the components, nitride size and number density of the steel sheet before nitriding, the numerical values of the surface layer 1/20 thickness, the surface layer 1/8 thickness and the central layer 1/4 thickness are the same.
Table 2 shows the materials for these steels. It can be confirmed that the production method of the present invention achieves both good deformation resistance and flange formability.
[0035]
<Example 2>
In mass%, C: 0.02%, Si: 0.02%, Mn: 0.2%, P: 0.01%, S: 0.01%, Al: 0.04%, N: 0. A slab having a thickness of 250 mm containing 002% was produced by continuous casting, and was formed into a 2.0 mm hot-rolled sheet at a slab heating temperature of 1100 ° C., a finishing temperature of 880 ° C., and a winding temperature of 600 ° C. It was pickled, cold rolled to 0.17 mm, and recrystallized and annealed at 650 ° C. for 30 seconds in a continuous annealing line. Some of the materials were passed through a nitriding furnace filled with a continuous ammonia-containing atmosphere in an annealing furnace of a continuous annealing line to perform a nitriding treatment. No heating equipment was installed in the nitriding furnace, and nitriding was performed by injecting the plate heated in the recrystallization annealing furnace at 650 ° C. into the nitriding furnace. Since the atmosphere in the nitriding furnace is heated by bringing in heat from the steel sheet, the drop of the sheet temperature during the nitriding treatment is not so large, and the temperature of the sheet coming out of the nitriding treatment furnace depends on the nitriding treatment time, but is 600 ° C. It was about.
[0036]
After the steel plate thus manufactured was subjected to a 1.5% skin pass, normal electrical Sn plating was applied to manufacture a tinplate steel plate. Using these, a three-piece can was manufactured in the same manner as that performed by a normal can maker, and the can strength was evaluated in the same manner as in Example 1. No problems such as welding and lid tightening occurred in all the canned materials. FIG. 3 shows the obtained can strengths arranged by the ammonia concentration in the nitriding atmosphere, the cooling rate after the nitriding treatment, and the nitriding time.
In FIG. 3, A is an ammonia concentration of 4%, a cooling rate after nitriding is 20 ° C./sec, B is an ammonia concentration of 4%, a cooling rate after nitriding is 120 ° C./sec, C is an ammonia concentration of 10%, and a cooling rate is 20 after nitriding. ° C / sec, D indicates the case where the ammonia concentration was 20% and the cooling rate after nitriding was 20 ° C / sec, and the cooling rate after nitriding was an average cooling rate from 550 ° C to 300 ° C. In addition, the can strength was arranged by (number density at (surface layer 1/20 thickness) of steel sheet after nitriding treatment) / (number density at (surface layer 1/20 thickness) of steel sheet before nitriding treatment). Is shown in FIG. According to the present invention, can strength can be significantly increased. The figure also shows the can strength of materials with different sheet thicknesses manufactured by changing only the cold rolling ratio before recrystallization annealing using steel of the same composition. It can be seen that the present invention makes it possible to make the material thinner while maintaining the desired can strength.
[Table 1]

[Table 2]

[0037]
【The invention's effect】
As described above, according to the present invention, it is possible to obtain, with high productivity, an ultra-thin container steel sheet which can be significantly improved without sacrificing one of the container's deformation resistance and can formability. .
[Brief description of the drawings]
FIG. 1 is a diagram showing a position in a thickness direction of a steel sheet.
FIG. 2 is a diagram showing a relationship between a mold pushing amount and a pushing load in a deformation test.
FIG. 3 is a diagram showing the relationship between nitriding time and can strength.
FIG. 4 is a diagram showing the relationship between the ratio of the number of nitrides before and after nitriding and the strength of the can.

Claims (12)

In 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% Hereinafter, containing Al: 2.0% or less,
Regarding the nitride having a diameter of 1 μm or less and 0.02 μm or more, it has a region having a number density of 0.2 / μm ³ or more within the 1/8 thickness of the surface layer of the steel sheet and satisfies the following formula (A). An ultra-thin steel sheet for containers having a remarkably good can property with a sheet thickness of 0.400 mm or less.
(Number density at 1/8 thickness position of steel sheet)> (Number density at 1/4 thickness position of steel sheet) (A) In 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% Hereinafter, containing Al: 2.0% or less,
An ultra-thin container steel sheet having a can thickness of 0.400 mm or less, characterized by satisfying the following formula (B) with respect to a nitride having a diameter of 1 μm or less and 0.02 μm or more.
(Number density at 1/20 position of steel plate thickness) / (Number density at 1/4 position of steel plate thickness)> 1.5 (B) ^3. The can characteristic according to claim 1, wherein the number density of nitrides having a diameter of 1 μm or less and 0.02 μm or more at a quarter thickness position of the steel sheet is 10 or less / μm ³ or less. 4. Good steel sheet for ultra-thin containers. As a steel component, one or more of Ti: 0.08% or less, Nb: 0.08% or less, B: 0.015% or less, and Cr: 2.0% or less are further contained in mass%. 4. The steel sheet for an ultra-thin container according to claim 1, wherein the can properties are remarkably good. 5. The can characteristics according to claim 1, wherein the steel component further contains 0.1% or less by mass in total of Sn, Sb, Mo, Ta, V and W. Good steel sheet for ultra-thin containers. 6. The steel sheet for an ultra-thin container according to claim 1, wherein the balance of the steel component is Fe and inevitable impurities. In producing the steel sheet according to any one of claims 1 to 6, in mass%, C: 0.0800% or less, N: 0.0300% or less, Si: 2.0% or less, Mn: 2.0% Hereinafter, a steel containing P: 0.10% or less, S: 0.05% or less, and Al: 2.0% or less is subjected to nitriding treatment after cold rolling and simultaneously with recrystallization annealing or after recrystallization annealing. Then, a region in which nitrides having a diameter of 1 μm or less and 0.02 μm or more exist at a number density of 0.2 / μm ³ or more within the surface 1/8 thickness of the steel sheet, and N in the steel sheet is measured by mass %. A method for producing a steel sheet for an ultra-thin container, which has remarkably good can properties with a sheet thickness of 0.400 mm or less, characterized by being 0.600% or less. In producing the steel sheet according to any one of claims 1 to 6, in mass%, C: 0.0800% or less, N: 0.0300% or less, Si: 2.0% or less, Mn: 2.0% Hereinafter, a steel containing P: 0.10% or less, S: 0.05% or less, and Al: 2.0% or less is subjected to a nitriding treatment after cold rolling and simultaneously with recrystallization annealing or after recrystallization annealing. Performing, within the surface layer 1/8 thickness of the steel sheet, for the nitride having a diameter of 1 μm or less and 0.02 μm or more, satisfy the following expression (B), and set the N in the steel sheet to 0.600% or less by mass%. A method for producing a steel sheet for an ultra-thin container having a remarkably good can property with a sheet thickness of 0.400 mm or less.
(Number density at 1/20 position of steel plate thickness) / (Number density at 1/4 position of steel plate thickness)> 1.5 (B) In producing the steel sheet according to any one of claims 1 to 6, in mass%, C: 0.0800% or less, N: 0.0300% or less, Si: 2.0% or less, Mn: 2.0% Hereinafter, a steel containing P: 0.10% or less, S: 0.05% or less, and Al: 2.0% or less is subjected to nitriding treatment after cold rolling and simultaneously with recrystallization annealing or after recrystallization annealing. For a nitride having a diameter of 1 μm or less and 0.02 μm or more, the following formula (C) is satisfied and N in the steel sheet is set to 0.600% or less by mass%. A method for producing a steel sheet for an ultra-thin container having extremely good can properties as follows.
(Number density of steel sheet after nitriding treatment at 1/20 position) / (number density of steel sheet before nitriding treatment at 1/20 position)> 1.5 (C) In 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% Hereinafter, a steel containing 2.0% or less of Al and the balance of Fe and unavoidable impurities is subjected to nitriding after cold rolling and simultaneously with recrystallization annealing or after recrystallization annealing. The number density of the nitride having a diameter of 1 μm or less and 0.02 μm or more at the / 4 position is 10 pieces / μm ³ or less, and the N of the steel sheet is 0.600% or less by mass%. The method for producing a steel sheet for an ultra-thin container according to any one of claims 7 to 9, which has remarkably good can properties. At the same time as the recrystallization annealing or when performing the nitriding treatment after the recrystallization annealing, at a plate temperature of 550 to 800 ° C., in an atmosphere containing 0.02% or more of ammonia gas for 0.1 to 360 seconds. After the nitriding treatment, the product of temperature and time is set to 48000 (° C. · s) or less in a temperature range of 550 ° C. or more after the nitriding treatment, or the average cooling rate from 550 ° C. to 300 ° C. is set to 10 ° C./s or more. The method for producing a steel sheet for an ultra-thin container according to claim 7, wherein the properties of the can are remarkably good. The ultra-thin container having remarkably good can characteristics according to claim 7, wherein re-rolling is performed at a rolling reduction of 20% or less after the recrystallization annealing, before the nitriding treatment, or after the nitriding treatment. Manufacturing method for steel sheet.

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