JP2001107186A - High strength steel sheet for can and its producing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength extra-thin cold rolled steel sheet excellent in baking hardenability in which baking hardening quantity(BH quantity) is >=100 MPa, and the quantity of tensile strength to be increased by coating/ baking treatment ΔTS is >=30 MPa, and to provide a method for producing the same. SOLUTION: A rolling stock having a composition containing, by weight, <=0.020% C, <=0.01% Al and 0.0050 to 0.0250% N is hot-rolled into a hot rolled sheet containing N by >=80% of the above N content, which is subjected to cold rolling, is therafter, in a continuous annealing stage, soaked at the recrystalization temperature or above to form into a structure having the recrystallization ratio of >=90%, which is subsequently subjected to rapid cooling treatment to a temperature region of <=250 deg.C at a cooling rate of >=150 deg.C/s and, within 60s after the completion of the soaking, cooling is executed to a room temperature. After the cooling, cold rolling at a draft of <=10% may be executed as well. Furthermore, it is preferable that water cooling is started within 30 min after coiling, and the average cooling rate is controlled to >=20 deg.C/h.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel plate for cans used for various kinds of containers, and more particularly to quenching and aging in a painting process, a baking process (printing process, a drying process) and the like which are performed before a can-making process. The present invention relates to a novel steel sheet capable of exhibiting sufficiently high strength by strain age hardening and capable of promoting thinning of a steel sheet, and a method for producing the same.

The object of the present invention is 0.3 mm or less,
This is a steel sheet for cans mainly in the category called ultra-thin steel sheet of 0.2 mm or less. These steel sheets may be finally coated with tin plating, nickel-tin plating, chromium plating (so-called tin-free plating), or even with an organic coating, so that they are applied to an extremely wide range of uses.

[0003]

2. Description of the Related Art A can container for storing various contents such as beverage cans and food cans has a two-piece can composed of a can body (a bottom cover and a body) and an upper cover, a can body, It is roughly divided into a three-piece can consisting of an upper lid and a bottom lid. In view of the growing demand for reduction in can-making costs in recent years, cost reduction of can-making materials has been urged, and in order to reduce material costs, not only two-piece cans that are formed by drawing but also three-pieces that mainly consist of simple cylindrical molding Even for cans, the thickness of steel sheets used is being reduced.

However, even if the steel sheet is thinned, it is necessary to maintain the strength as a can body as before, and therefore, the strength of the can body after forming and aging is excellent, and it is hard and extremely effective to contribute to thinning. A thin steel plate for cans has been desired. Currently, the mainstream method for producing hard and ultra-thin steel plates for cans is
So-called DR (Double cold rolling)
Reduced) method. In the DR method, cold rolling-annealing is followed by cold rolling of nearly 30% to harden the steel sheet by work hardening and reduce the thickness of the steel sheet.

However, the DR method requires a strong rolling mill in order to cold-roll the ultra-thin steel sheet after cold-rolling and annealing under a higher pressure, and also requires an in-line treatment after annealing. Has a disadvantage in equipment that the number of processes increases. Furthermore, in steel sheets manufactured by the DR method, surface flaws, surface dirt, and the like are chronically generated, and it is extremely difficult to completely eliminate them.

As described above, there has been a problem in manufacturing a hard and ultra-thin steel sheet for cans having sound surface properties stably and at low cost by the DR method. In addition, other than the DR method,
As another method for increasing the strength, for example, a method in which a large amount of alloying elements are added to solid-solution strengthen the steel, which is widely used in steel sheets for automobiles, can be considered. However, in the case of a steel sheet for cans, special corrosion resistance is required, so that the amount of alloying elements that can be added is limited. Actually, ASTM standard, JIS
According to the standard, additional elements are limited, and sufficient solid solution strengthening cannot be achieved. In addition, it has been difficult to apply any method in the steel sheet manufacturing process for cans, such as a method using transformation structure strengthening and a method using precipitation strengthening with carbonitride forming elements such as Nb, Ti and V.

On the other hand, Japanese Patent Application Laid-Open No. 5-345926 proposes a method for producing a high-strength steel sheet for cans that effectively uses C and N, which are solid solution strengthening elements usable for steel sheets for cans. In the technique described in Japanese Patent Application Laid-Open No. 5-345926, C: 0.01 wt.
% Or less, N: 0.04% by weight or less, and C + N is 0.008% by weight.
As described above, a slab containing Al: 0.005 wt% or less is subjected to hot rolling and cold rolling, and is continuously annealed at a temperature equal to or higher than a recrystallization temperature.
After that, temper rolling with a reduction rate of 5% or more is performed,
It is stated that four or more hard materials can be obtained.

Japanese Patent Application Laid-Open No. Hei 8-311609 discloses that carbon: 0.01 to 0.08 wt%, N: 0.01 wt% or less, acid-soluble Al: 0.
A steel sheet for a DI can containing less than 20 wt% and having a solid solution C content of 5 to 25 ppm and a yield strength in the rolling direction of 30 to 44 kgf / mm ² has been proposed. Japanese Patent Application Laid-Open No. Hei 10-72640 discloses that C:
0.0010 to 0.04 wt%, N: 0.0020 to 0.0150 wt%, Al: 0.00
A steel sheet for cans containing 5 to 0.060 wt%, containing 25% or more of the above-mentioned N amount, and containing 0.001 to 0.01 wt% of solute N has high age hardening properties and excellent material stability. .

[0009]

However, Japanese Patent Application Laid-Open Nos. Hei 5-345926, Hei 8-31609 and Hei 10
In the technique described in JP-A-72640, a hard material equivalent to T-5 is the limit, and in order to obtain a hard material higher than that, a DR method in which secondary cold rolling at a high pressure reduction after annealing is performed. Must be applied. However, in the DR method, there is a problem in producing hard and ultra-thin steel plates for cans having sound surface properties stably and at low cost. However, the strength in the rolling direction is low, that is, the in-plane anisotropy of the mechanical properties is large, which is disadvantageous from the viewpoint of increasing the strength of the can. For this reason, there has been a demand for a steel sheet for cans having a high strength stably in both the rolling direction and the direction perpendicular to the rolling direction.

The present invention solves the above-mentioned problems of the prior art, and proposes a high-strength ultrathin cold-rolled steel sheet having a hardness equivalent to DR8 or more and excellent in bake hardenability, and a method for producing the same. The purpose is to: The steel sheet of the present invention has a thickness of 0.3 mm or less and a yield stress after the baking treatment of 550 MPa.
Baking hardening, which has a high curing amount (bake hardening amount (BH amount)) in paint baking unique to steel plates for cans of 100 MPa or more, and an increase in tensile strength ΔTS of 30 MPa or more due to paint baking treatment. High strength ultra-thin cold-rolled steel sheet with excellent heat resistance. In the present invention, a high-strength ultra-thin cold-rolled steel sheet having a hardness equivalent to DR8 or more is simply applied by temper rolling of 10% or less after annealing without applying the so-called DR method. .

The bake hardening amount (BH amount) referred to in the present invention.
Is the difference in yield stress between before and after heat treatment equivalent to painting and baking, after applying a 2% tensile strain to the steel sheet and subjecting it to a heat treatment at 210 ° C for 20 minutes equivalent to painting and baking. Shall mean. The yield stress after the coating and baking treatment means the yield stress after applying a 2% tensile strain to the steel sheet and performing a heat treatment at 210 ° C. for 20 minutes corresponding to the coating and baking treatment. The amount of increase in tensile strength ΔTS due to the painting and baking treatment is 210 ° C. × 20 min, equivalent to the painting and baking treatment, after applying the tensile strength of the steel sheet before applying 2% tensile strain and 2% tensile strain. Means the difference from the tensile strength of the steel sheet subjected to the above heat treatment.

[0012]

Means for Solving the Problems In order to achieve the above objects, the present inventors have conducted intensive studies on steel components and production conditions. As a result, without applying the DR method, D
In order to obtain a hardening equivalent to or greater than R8 and a large amount of bake hardening, the steel composition should be a low-carbon steel or ultra-low-carbon steel with high ductility, steel composition, hot rolling conditions and hot rolling. Adjust the cooling conditions after rolling so that the amount of solute N in the base plate for cold rolling is 80% of the total amount of N.
It has been found that it is important to perform the above, to perform continuous annealing at a temperature higher than the recrystallization temperature, and to perform rapid cooling to a low temperature region after continuous annealing.

As a result, the decrease in the amount of solute N due to precipitation as AIN can be prevented, and the precipitation of solute C as carbide can be prevented. Amount (solid solution C + solid solution N) can be secured. By effectively utilizing a sufficient amount of (solid solution C + solid solution N), the strength of the steel sheet is remarkably increased (so-called strain aging, quenching aging), and stable high strength can be achieved. For this reason, it is not necessary to apply a strong reduction (reduction rate of about 30%) as in conventional secondary rolling to increase the strength after continuous annealing, and it is necessary to adjust the hardness, roughness, and shape of the steel sheet surface. It was found that only light reduction (reduction rate: 10% or less) was required.

The present invention has been made based on the above findings, and the gist thereof is as follows. That is, the first aspect of the present invention provides, in mass%, C:
0.02% or less, Si: 0.10% or less, Mn: 1.5% or less, P: 0.
20% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.00
50-0.0250% and (Solute C + Solute N) 0.0050
%, With the balance consisting of Fe and unavoidable impurities and a structure with a recrystallization rate of 90% or more, bake hardening amount (BH amount): 100 MPa or more, tensile strength by paint baking treatment Increase amount ΔTS: 30 MPa or more, yield stress after painting / baking treatment: 550 MPa or more
It is an ultra-thin cold-rolled steel sheet for high-strength cans having a thickness of 0.3 mm or less.
Group to Group 2 Group 1: Nb: 0.01% or less, Ti: 0.01% or less, B: 0.001
% Or less selected from the group consisting of not more than 0.2%: Cu: 0.01 to 0.2%, Ni: 0.01 to 0.2%, Cr: 0.1%.
It is preferable to contain one or two groups selected from one or more selected from 01 to 0.2% and Mo: 0.01 to 0.2%.

Further, the second invention is an ultrathin plated steel sheet for a high-strength can, characterized in that a plating layer is formed on the surface of the ultrathin cold-rolled steel sheet according to the first invention. In the third invention, C: 0.02% or less, Si: 0.10% or less, Mn:
1.5% or less, P: 0.20% or less, S: 0.01% or less, Al: 0.
Rolling is started at a temperature at which 90% or more of the N content becomes a solid solution state using a rolled material having a composition containing 01% or less and N: 0.0050 to 0.0250%, and the finish rolling temperature is set to (Ar ₃ transformation point). −
30 ° C) or higher, and after hot rolling,
Start forced cooling within 0.5 s, wind at a winding temperature of 600 ° C or less, and further cool with water after winding to obtain a hot rolled sheet containing preferably 80% or more of the total dissolved N, Then, after subjecting the hot-rolled sheet to cold rolling, in a continuous annealing step, after soaking at a temperature higher than the recrystallization temperature, at a cooling rate of 150 ° C./s or higher, 250
Baking hardening amount (BH amount): 100MPa characterized by quenching to a temperature range of less than or equal to ℃ or less, and then cooling within 40s after completion of soaking without stagnation for a long time.
As described above, the amount of increase in tensile strength due to the paint baking treatment ΔTS: 30
MPa or higher, yield stress after painting and baking treatment: 550 MPa or more Sheet thickness: 0.3 mm or less High-strength ultra-thin cold-rolled steel sheet manufacturing method, in the present invention, in order to obtain higher strength,
After the cooling, it is preferable to further perform cold rolling at a rolling reduction of 10% or less. In the present invention, the water cooling after winding starts water cooling within 30 minutes after winding, and the average cooling rate is reduced. It is preferably at least 20 ° C./h.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION The cold-rolled steel sheet of the present invention has a bake hardening amount of 100 MPa or more, an increase in tensile strength ΔTS of 30 MPa or more due to paint baking treatment, and a yield stress after painting and baking treatment of 55.
Ultra-thin cold-rolled steel sheet for high-strength cans having a sheet thickness of 0.3 mm or less having a pressure of 0 MPa or more. The cold rolled steel sheet of the present invention has a bake hardening amount of 100.
MPa or more, increase in tensile strength by paint baking treatment 30 MPa
With the above, the thinning of the steel sheet can be advantageously promoted.

The cold-rolled steel sheet of the present invention makes use of the effect of solid solution C + solid solution N effectively, so that during the coating baking process before press forming, the bake hardening amount (BH amount) is not less than 100 MPa and the age hardening is remarkable. A phenomenon occurs. In addition, a large amount of solid solution C,
Contains solid solution N and increases the tensile strength by baking treatment to 30 MPa or more, and can impart dent resistance (resistance to dents) to the can body after can making, especially to the can body. , Which results in a dramatic increase in can body strength.

First, the reasons for limiting the composition of the rolled material and cold-rolled steel sheet of the present invention will be described. In addition, mass% is simply%
It is written. C: 0.02% or less C is an effective element for increasing the strength of steel by solid solution strengthening, but on the other hand, it forms carbides and reduces the ductility of the steel sheet and, consequently, the workability. Therefore, in the present invention, C
By reducing the content, the amount of carbides in the steel is reduced, so that the ductility of the steel sheet and, consequently, the workability are stably secured at a high value. Further, since the amount of carbides precipitated in the steel is reduced, a large amount of solid solution C is left,
It is possible to increase the strength of the steel sheet. Such a desirable effect is remarkably recognized when the C content is 0.02% or less. For this reason, C is limited to 0.02% or less. In addition,
In order to further enhance the strength by solid solution C,
It is desirable that the content is in an extremely low carbon region of 0.010% or less. In order to obtain the target high bake hardenability (BH property), there is a lower limit of the C content, and the C content is preferably set to 0.002% or more.

Si: 0.10% or less Si is an element that increases the strength of steel by solid solution strengthening. However, if added in a large amount, it causes problems such as deterioration of surface treatment properties and corrosion resistance. Limited to the following. In addition,
When particularly excellent corrosion resistance is required, the content of Si is preferably set to 0.02% or less.

Mn: 1.5% or less Mn is an effective element for preventing hot cracking due to S.
In the present invention, it is added as appropriate according to the amount of S contained. Further, Mn has an effect of making crystal grains fine. Such an effect is remarkably recognized when the content is 0.1% or more.
Mn is preferably contained at 0.1% or more. On the other hand, when Mn is contained in a large amount, although high strength of the steel sheet can be achieved, the corrosion resistance tends to deteriorate, and further, the flange workability tends to deteriorate. For this reason, Mn was limited to 1.5% or less. In applications where better moldability is required, Mn is preferably set to 0.50% or less.

P: 0.20% or less P significantly hardens the steel, but deteriorates the flange workability and the neck workability and remarkably deteriorates the corrosion resistance. Therefore, in the present invention, P is limited to 0.20% or less. In applications where flange workability, neck workability, and corrosion resistance are particularly important, P is desirably 0.01% or less.

S: 0.01% or less S is an element existing as inclusions (sulfides) in steel, which reduces the ductility of the steel sheet and further deteriorates the corrosion resistance. In the present invention, it is desirable to reduce as much as possible. , 0.
Up to 01% is acceptable. Therefore, in the present invention, S is 0.01
% Or less. For applications requiring particularly good workability, S is preferably set to 0.007% or less.

Al: 0.01% or less Al combines with solute N to form AlN and has the effect of reducing the amount of solute N. Further, an increase in the Al content causes an increase in the recrystallization temperature, and it is necessary to increase the annealing temperature. High-temperature annealing inevitably increases the amount of AlN formed, so that the amount of dissolved N decreases, the age hardening decreases, and the strength of the steel sheet decreases. Such a phenomenon is remarkable when the Al content exceeds 0.01%. For these reasons, Al is limited to 0.01% or less. In addition,
From the viewpoint of stable operation in the steel melting process, Al is 0.00
It is desirable that the content be 1% or more. Further, from the viewpoint of further stabilizing the material, it is more desirable that the content of Al be 0.005% or less.

N: 0.0050 to 0.0250% N is an element that increases age hardening, and is positively contained in the present invention. A marked increase in age hardening is observed at a content of 0.0050% or more. On the other hand, if the content exceeds 0.0250%, the risk of generating cracking defects in the rolled material (slab) is significantly increased. Therefore, N is 0.00
Limited to 50-0.0250%. In addition, from the stability of the material, N
Is preferably 0.0070% or more.

(Solute C + Solute N) in cold rolled steel sheet: 0.0050
% Or more In order to enhance the age hardening of the product sheet (cold rolled steel sheet) and increase the strength of the product sheet, in the present invention, the amount of (solid solution C + solid solution N) in the cold rolled steel sheet is 0.0050% or more, preferably Should be 0.0070% or more. If the (Solute C + Solute N) amount is less than 0.0050%,
The increase in strength due to the paint baking process is small, and the desired bake hardening amount (100 MPa or more) and the increase in tensile strength (ΔTS 30 MPa or more) due to the paint baking process cannot be obtained. In addition, more preferable (solid solution C + solid solution N) is 0.0080 to 0.0150%.

In addition to the basic composition described above, optional elements can be contained as required. As the selected element, the following first group to
One or two groups selected from the second group can be selected as needed. Group 1: Nb: 0.01% or less, Ti: 0.01% or less, B: 0.001
% Or less, Nb, T
i and B are elements that increase the strength, and one or more of them can be selected as necessary.

Nb is an element effective for forming fine carbonitrides, refining crystal grains, and increasing the strength of steel after forming. In order to obtain this effect, Nb is preferably contained at 0.001% or more. However, if the content exceeds 0.01%, the recrystallization temperature of the steel rises remarkably, hindering the annealing process after cold rolling, and N in the solid solution state precipitates by high-temperature annealing, resulting in high strength of the steel. Losing function. Therefore, Nb is 0.
It is preferably at most 01%. From the viewpoint of annealing conditions, Nb is desirably 0.007% or less.

Ti is an element effective for forming fine carbonitrides like Nb, refining crystal grains, and increasing the strength of steel after forming. Such an effect is 0.003%
The above content becomes remarkable, and it is preferable that Ti contains 0.003% or more. On the other hand, if the content of Ti exceeds 0.01%,
The decrease in the amount of age hardening due to the decrease of the solid solution C and the solid solution N becomes remarkable. Therefore, Ti is preferably set to 0.01% or less.
From the viewpoint of material stabilization, Ti is 0.003 to 0.
More preferably, it is in the range of 007%.

B is an element that, despite having a tendency to form nitrides, is effective in increasing age hardening and increasing the strength of steel under the conditions of the present invention.
Since such an effect is remarkably observed at a content of 0.0002% or more, a content of 0.0002% or more is preferable. On the other hand, 0.0020
%, The effect is saturated, or
Conversely, age hardenability tends to decrease. For this reason, B
It is preferably set to 0.0020% or less. In addition, from the viewpoint of stabilization and uniformity of mechanical properties, B is 0.0003 to 0.0008%.
More preferably,

Second group: Cu: 0.01-0.2%, Ni: 0.01-0.
1% or more selected from 2%, Cr: 0.01 to 0.2%, Mo: 0.01 to 0.2% Cu, Ni, Cr, Mo can be used in the final It has the effect of increasing the strength and can be selected and contained as needed. Such effects are caused by Cu, Ni, Cr,
Each of Mo is recognized at a content of 0.01% or more. On the other hand, if each of Cu, Ni, Cr, and Mo exceeds 0.2%, the hot-rolled sheet is significantly hardened, and the risk of causing troubles in the cold rolling step increases. For this reason, C
u: 0.01 to 0.2%, Ni: 0.01 to 0.2%, Cr: 0.01 to 0.2
%, Mo: preferably limited to 0.01 to 0.2%. In addition,
The effects of Cu, Ni, Cr, and Mo are not canceled out even if added in combination. Therefore, Cu, Ni, Cr, and Mo can be added alone or in combination. It is more preferable that the total amount of Cu, Ni, Cr, and Mo be 0.2% or less.

The balance is Fe and inevitable impurities The balance other than the above components is Fe and inevitable impurities. The unavoidable impurities include, for example, Sn: 0.01
% Or less is acceptable. The cold rolled steel sheet of the present invention has the above composition and a structure having a recrystallization ratio of 90% or more.
The recrystallization rate in the present invention uses a value obtained by an area method by observing a cross-sectional structure using an optical microscope. If the recrystallization rate is less than 90%, workability, particularly ductility, is reduced. Therefore, in the present invention, the recrystallization rate is set to 90% or more.

Next, a method for producing the steel sheet of the present invention will be described. The molten steel having the above-described composition is smelted by a generally known smelting method using a converter or the like, and then a rolled material (slab) is formed by a generally known casting method such as a continuous casting method. Next, a hot rolled sheet is formed by hot rolling using these rolled materials. In the present invention, at the start of rolling, the rolling material is 90% of the total N content.
It is important that the temperature is equal to or higher than the temperature at which the solid solution is formed.

To make 90% or more of the total N amount into a solid solution state,
Although it depends on the components, in the normal reheating process, the material is heated to approximately 1150 ° C or higher, or in the case of the direct-feed rolling process in which the precipitation of AlN is delayed, the rolling material is cooled to a temperature below the transformation point after continuous casting. It is preferable to insert and heat in a heating furnace maintained at a temperature at which rolling is possible. In any case, in the present invention, at least at the start of hot rolling, 90% or more of the total N content in the rolled material should be in a solid solution state. Unless 90% or more is in a solid solution state, age hardening due to solid solution N is not sufficiently exhibited on a product plate. In order to make 90% or more of the total amount of N into a solid solution state at the start of hot rolling, the hot rolling start temperature is preferably 1100 ° C. or more.

Finish rolling temperature: (Ar ₃ transformation point −30 ° C.) or more In the present invention, in order to effectively suppress the precipitation of AlN, the finish rolling temperature in hot rolling is set to (Ar ₃ transformation point −30 ° C.) or more. And If the finish rolling temperature is less than (Ar ₃ transformation point −30 ° C.), precipitation of AlN becomes remarkable, and solute N decreases. It is more preferably (Ar ₃ transformation point −10 ° C.) or more.

Forced cooling: started within 0.5 seconds after completion of hot rolling Forced cooling is preferably performed by water cooling. The forced cooling is preferably started within 0.5 s after the end of the hot rolling. If the start of forced cooling exceeds 0.5 s from the end of rolling, precipitation of AlN cannot be suppressed. Winding temperature: 600 ° C or less Winding temperature is set at 600 ° C to prevent N from being fixed by Al.
C. or lower is preferred. When the winding temperature exceeds 600 ° C., the amount of AlN precipitated increases remarkably, and it is not possible to obtain the target sufficient bake hardenability. In order to stably obtain high bake hardenability, the winding temperature is more preferably 570 ° C. or lower. Further, by setting the winding temperature to 600 ° C. or lower, it becomes possible to disperse finer and more uniform carbides generated during cooling after winding.

After Winding: Water Cooling In the present invention, it is preferable to start water cooling within 30 minutes after winding and to cool at an average cooling rate of 20 ° C./h or more. Here, the cooling rate is the average cooling rate in the central part in the length direction of the steel strip and in the central part of the sheet width. As a result, precipitation of AlN can be prevented, and carbides precipitated during the cooling process after winding can be more finely and evenly dispersed. Start water cooling after winding
If it exceeds 30 min, the water cooling effect will be insufficient. If the cooling rate is less than 20 ° C./h, the effect of improving the distribution of carbides is not sufficient.

By the above-described hot rolling and cooling after rolling, a hot rolled sheet can be obtained in which at least 80% of the total N content is in a solid solution state. Solid solution N in hot-rolled sheet (base sheet for cold-rolled steel sheet): 80% or more of the total amount of N In order to secure a large age hardening property of the cold-rolled steel sheet which is a feature of the present invention, the hot-rolled sheet The amount of solid solution N in the base plate for the rolled steel sheet) must be 80% or more of the total amount of N. The cold-rolled steel sheet of the present invention is preferably produced by pickling a hot-rolled sheet, cold-rolling, and then performing continuous annealing for a short time.In this continuous annealing step, AIN tends to precipitate. . If the solute N content of the hot rolled sheet (base sheet for cold rolled steel sheet) is less than 80% of the total N amount,
The desired age hardening of the cold rolled steel sheet cannot be achieved. In the present invention, the amount of N in the form of AlN is determined by extraction analysis by dissolution using bromoester, which is usually performed (hereinafter referred to as N as AlN), and the value obtained by subtracting N as AlN from the total N amount is fixed. Let it be the amount of dissolved N.

Pickling and cold rolling are performed on such a hot-rolled sheet as a base sheet for a cold-rolled steel sheet to obtain a cold-rolled sheet. The pickling may be carried out in a conventional manner by removing the surface scale with an acid such as hydrochloric acid or sulfuric acid. The cold rolling reduction also follows the usual method, but as a matter of course, the higher the sheet thickness, the higher the reduction. Next, the cold rolled sheet is subjected to continuous annealing.

Soaking temperature in continuous annealing: not less than recrystallization temperature In the continuous annealing step, the structure is soaked at a temperature not lower than the recrystallization temperature in order to secure a certain or more workability as a structure having a recrystallization ratio of 90% or more. Is preferred. If the soaking temperature is lower than the recrystallization temperature, the structure does not have a recrystallization rate of 90% or more, and the uniformity of the material in the width direction and the length direction of the product coil is deteriorated. However, within this temperature range, it is not particularly necessary to maintain a constant temperature. From the stability of operation, it is sufficient if the soaking time is 10s or more.

Rapid quenching treatment after continuous annealing and soaking: at a cooling rate of 150 ° C./s or more to a temperature range of 250 ° C. or less The cooling condition after continuous annealing and soaking is one of the most important requirements in the present invention. . The purpose of this cooling is to form a structure in which fine carbides are finely dispersed after annealing, or to leave it as solid solution C, at least not to generate coarse carbides, which is sufficient to secure bake hardenability. The purpose is to secure the amounts of solid solution C and solid solution N. For this reason, it is preferable to rapidly cool at a cooling rate of 150 ° C./s or more from the soaking temperature. At a cooling rate lower than this cooling rate, the reduction of solid solution C becomes remarkable among solid solution N and solid solution C. In order to obtain the target characteristics more stably, 200
It is desirable to set the cooling rate to at least ° C / s.

The quenching treatment is preferably continued up to a temperature range of 250 ° C. or less. When the quenching treatment is stopped at a temperature higher than 250 ° C., the precipitation form of carbides becomes coarse, and at the same time, the amount of solute C is drastically reduced. In addition, more preferably, 200 ° C
It is desirable to perform the quenching treatment to the following temperature range. In order to stably perform such rapid cooling treatment, it is necessary to enhance the capacity of conventional gas jet cooling, apply hydrogen as a cooling gas, use continuous annealing capable of high-speed treatment, and the like. . Further, pickling treatment after annealing is required, but water cooling is also applicable.

Further, it is preferable to cool down to 40 ° C. within 60 seconds integrated from the end of the soaking (start of cooling). 40 ℃
If it takes more time to cool the steel sheet, a phenomenon similar to the so-called overaging phenomenon occurs, and the age hardening ability of the steel sheet is reduced. After continuous annealing and quenching, secondary rolling with a rolling reduction of 10% or less may be further performed. The secondary rolling of the present invention is not intended to increase the strength, but is intended to adjust the surface roughness, adjust the shape, and the like. Preferably, the rolling reduction is 1.0% or more. If the rolling reduction exceeds 10%, the ductility, particularly the uniform elongation, deteriorates. Since the rolling reduction is a light reduction of 10% or less, it is industrially useful from the viewpoint of reducing equipment load.

Through the above steps, a cold-rolled steel sheet is obtained. The cold-rolled steel sheet of the present invention is made of a hard material by painting and baking before can making (before pressing), and when applied to an ultra-thin steel sheet having a thickness of 0.3 mm or less, its superiority becomes more significant. Effectively demonstrated. The cold-rolled steel sheet manufactured by the above-described process has a (solid solution C + solid solution N) amount of 0.0050% or more, and has a bake hardening amount of 10 by coating and baking (210 ° C. × 20 min).
0MPa or more, yield stress after painting and baking treatment: 550MPa or more,
Ultra-thin cold-rolled steel sheet for high-strength cans having a difference in tensile strength before and after the coating and baking treatment ΔTS: 30 MPa or more. The steel sheet of the present invention can obtain an unprecedented large age hardening property (bake hardening property) by combining the actions of both the solid solution C and the solid solution N, and can obtain not only yield stress but also increase in tensile strength. And also has a remarkable effect of increasing the strength in both the rolling direction and the direction perpendicular to the rolling direction. Further, the cold-rolled steel sheet of the present invention increases the strength even after reflow treatment after plating by effectively utilizing the action of solid solution C + solid solution N, and also at the time of paint baking after press forming. A remarkable age hardening phenomenon of a bake hardening amount (BH amount) of 100 MPa or more occurs, and the load at the time of molding is almost the same level, resulting in a dramatic increase in can body strength.

In the present invention, an ultrathin cold-rolled steel sheet can be formed into a plating layer on the surface (at least one side) to obtain an ultrathin plated steel sheet. As the plating layer formed on the surface, any of those applied to steel plates for cans can be applied. Examples of the plating layer include tin plating, chromium plating, nickel plating, and nickel / chrome plating. In addition, there is no problem in applying a coating, an organic resin film or the like after these plating treatments.

[0045]

EXAMPLES The effects of the present invention will be described based on examples. (Example 1) Steel having the components shown in Table 1 was melted in a converter, and was made into a rolled material (slab) by a continuous casting method. Then, these rolled materials were subjected to hot rolling under the conditions shown in Table 2 to obtain a sheet thickness of 1.8
mm hot rolled sheet. In the production of these hot-rolled sheets, those that were subjected to water cooling after winding, 30 minutes after winding
Water cooling was started within. Thereafter, the hot-rolled sheet is subjected to descaling treatment by pickling, further subjected to cold rolling, and then subjected to continuous annealing and secondary rolling under the conditions shown in Table 2 to obtain an extremely thin sheet having a final finished sheet thickness of 0.15 mm. It was a rolled steel sheet. The soaking temperature during continuous annealing was equal to or higher than the recrystallization temperature.
Here, the cold-rolled steel sheet (steel sheet No. 12) in which the rolling reduction is increased to 20% by the secondary rolling is a conventional example (DR steel sheet).

The ultra-thin cold-rolled steel sheet thus obtained was subjected to the measurement of the amount of solute C and the amount of solute N, the measurement of the recrystallization ratio (structure examination), the tensile test, the hardness test, and the bake hardening test. Carried out. (I) Analysis of Solid Solution C and Solid Solution N The N amount in the steel sheet was analyzed by chemical analysis, and the N amount existing as AlN was determined by extraction analysis by dissolution using bromoester. The value of {(N amount in steel plate) − (N amount existing as AlN)} was used as the amount of solute N in the steel plate.

The amount of solid solution C was measured by internal wear.
C and N peaks were separated by a torsional pendulum type internal friction measuring device, and the C peak was adopted. It was confirmed that N was consistent with the chemical analysis. In addition, the amount of solid solution N at the time of slag heating measured the raw material rapidly cooled to room temperature heated to the slag heating temperature similarly. The measurement was also performed on a hot-rolled sheet (base material for a cold-rolled steel sheet). (Ii) Investigation of microstructure A test specimen was sampled from the center in the width direction of the cold-rolled steel sheet, and the cross-section microstructure of the cross-section parallel to the rolling direction, excluding the surface layer, was observed with an optical microscope, and the area was measured. By the method, the ratio of the recrystallized grains to the total area was calculated and defined as the recrystallization rate. (Iii) Tensile test JIS No. 13-B tensile test specimens were taken from the center of the width direction of these cold-rolled steel sheets in the rolling direction (L direction) and the direction perpendicular to the rolling direction (C direction), and the crosshead speed was 10 mm / s, a tensile test was performed, and the yield point YS, tensile strength TS, and elongation El were measured. The tensile test was performed within one day after commercialization.
The reason why the JIS No. 13-B test piece was used as the tensile test piece was to minimize the phenomenon of breaking outside the gauge. (Iv) Bake hardening test From the center in the width direction of these cold-rolled steel sheets, JIS
13B Tensile test specimens were collected, a 2% tensile prestrain was applied, then unloaded, subjected to a heat treatment equivalent to a paint baking treatment at 210 ° C x 20 min, and then subjected to a tensile test to determine the deformation stress (yield stress). The tensile strength was determined. Difference in yield stress before and after heat treatment equivalent to paint baking, (((yield stress after paint baking))-
(2% deformation stress before paint baking treatment)) was determined and defined as a bake hardening amount (BH amount).

The difference between the tensile strengths before and after the heat treatment equivalent to the paint baking treatment, ((tensile strength after the paint baking treatment) − (tensile strength before the paint baking treatment)) was determined, and the heat treatment equivalent to the paint baking treatment was performed. The change in tensile strength ΔTS was taken. (V) Hardness test The HR30T hardness of these cold-rolled steel sheets and the steel sheets obtained by subjecting these cold-rolled steel sheets to heat treatment equivalent to paint baking treatment were measured in accordance with the provisions of JIS Z 2245.

Table 3 shows the results.

[0050]

[Table 1]

[0051]

[Table 2]

[0052]

[Table 3]

In the example of the present invention, the change ΔTS in the tensile strength before and after the coating baking treatment was performed without increasing the rolling reduction of the secondary rolling (ie, without applying the DR method with a large rolling reduction).
It is a high-strength ultrathin cold-rolled steel sheet having a hardness of 30 MPa or more, a bake hardening amount of 100 MPa or more, a yield stress after paint baking treatment of 550 MPa or more, and a hardness equivalent to DR8. Further, in the present invention, the difference in intensity between the L direction and the C direction is small. On the other hand, Comparative Examples outside the scope of the present invention have a small amount of paint bake hardening and a small increase in tensile strength after the paint bake treatment. In the conventional example (DR material), the amount of bake hardening is small, and no increase in tensile strength is observed.

Further, these steel sheets (steel sheets No. 1 to No.
Using 7), a two-piece DRD can (can diameter 70 mm) was manufactured, and the resistance (dent resistance) to the dent of the can body, particularly the can bottom, was evaluated. The evaluation of the resistance to the dent of the can body, particularly the dent of the can bottom was performed by pressing an indenter having a tip radius of 15 mm against the can bottom and measuring a load that caused permanent deformation. As a result, the present invention example
On average, the dent load resistance was about 20% higher than that of a DR steel sheet (with a plating layer) of the same tempering degree (conventional example). (Example 2) With respect to steel A shown in Table 1, an ultrathin cold-rolled steel sheet was produced under the production conditions shown in Table 4. In this case, when performing coil water cooling, water cooling was started within 30 minutes after winding.
Further, the soaking temperature during annealing was equal to or higher than the recrystallization temperature. For these cold-rolled steel sheets, the measurement of the amount of solute C and the amount of solute N, the measurement of the recrystallization ratio (structure examination), the tensile test, the hardness test, and the bake hardening test were performed. The test method was the same as in Example 1. The surfaces of these steel sheets were subjected to Ni plating before annealing and further to continuous annealing to form Ni diffusion layers on the surfaces. Note that a cold-rolled steel sheet (steel No. 12) in which the rolling reduction was increased to 20% by the secondary rolling was used as a conventional example (DR steel sheet).

Table 5 shows the results.

[0056]

[Table 4]

[0057]

[Table 5]

In the cold-rolled steel sheet of the present invention (Example of the present invention), the increase in the tensile strength after the paint baking treatment without increasing the rolling reduction in the secondary rolling (that is, without applying the DR method) is reduced.
High strength ultra-thin steel sheet with 30MPa or more, bake hardening amount of 100MPa or more, yield strength after paint baking treatment of 550MPa or more, hardness equivalent to DR8, and unprecedented excellent properties. ing. Further, in the example of the present invention, the difference in strength between the L direction and the C direction is smaller than that of the DR material (conventional example). In contrast,
Comparative examples outside the scope of the present invention have a small amount of paint bake hardening, a small increase in tensile strength after the paint bake treatment, and have low strength.

Next, these steel sheets were subjected to a tin plating treatment (a plating weight of 25 g / m ² ) in an electric tin plating line, and further subjected to a reflow treatment in-line to obtain a tin-plated steel sheet having an island-like tin phase. And The properties of these tin-plated steel sheets were examined in the same manner as the cold-rolled steel sheets, but there were almost no changes from the properties of the cold-rolled steel sheets before plating.

A resin layer was painted and baked on each of these tin-plated steel sheets and formed into a three-piece can (can diameter: 60 mm), and the axial compressive strength and the resistance to dents in the can body were evaluated.
The resistance to the dent of the can body was evaluated in the same manner as in Example 1. As a result, in the example of the present invention, the average strength was improved by about 25% on average in comparison with the conventional example in which tin plating was performed using a DR steel sheet (conventional example) having the same tempering degree.

[0061]

According to the present invention, a high-hardness material, which has been obtained for the first time by performing secondary rolling at a high reduction rate after annealing, can be used as annealed or secondary rolling at a low reduction rate of 10% or less. And the bake hardening amount is 100MPa or more,
When the amount of increase in tensile strength by paint baking treatment is 30MPa or more,
Excellent bake hardenability, yield stress 55 after paint bake treatment
The use can be expanded as a steel sheet for cans having a pressure of 0 MPa or more, and a remarkable industrial effect is achieved. In addition, the steel sheet of the present invention has an effect that the strength increases in both the rolling direction and the direction perpendicular to the rolling direction and the anisotropy is small, so that it can more effectively contribute to the increase in the strength of the can body.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Yasuyuki Shono 2-3-2 Uchisaiwaicho, Chiyoda-ku, Tokyo Inside Kawasaki Steel Corporation (72) Inventor Osamu Furukuni 1 Kawasakicho, Chuo-ku, Chiba-shi, Chiba 4K037 EA01 EA02 EA04 EA05 EA11 EA13 EA15 EA17 EA18 EA19 EA20 EA23 EA25 EA27 EA31 EB02 EB03 EB06 FC07 FC08 FE01 FE02 FK03 FK08 FM01 GA05

Claims (6)

[Claims] 1. Mass%: C: 0.02% or less, Si: 0.10% or less, Mn: 1.5% or less, P: 0.20% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.0050 to 0.0250 % And (Solute C + Solute N)
With a composition of 0.0050% or more, with the balance being Fe and unavoidable impurities, and a structure with a recrystallization rate of 90% or more, bake hardening amount (BH amount): 100MPa or more, tensile strength by paint baking treatment Ultra-thin cold-rolled steel sheet for high-strength cans with a thickness of 0.3 mm or less, characterized by having an increase amount ΔTS of 30 MPa or more and a yield stress after painting and baking of 550 MPa or more. 2. The composition according to claim 1, further comprising one or two groups selected from the following first and second groups by mass% in addition to the composition. Ultra-thin cold rolled steel sheet for strength cans. 1st group: Nb: 0.01% or less, Ti: 0.01% or less, B: 0.001
% Or less selected from the group consisting of not more than 0.2%: Cu: 0.01 to 0.2%, Ni: 0.01 to 0.2%, Cr: 0.1%.
01-0.2%, Mo: One or more selected from among 0.01-0.2% 3. An ultrathin plated steel sheet for a high-strength can, wherein a plated layer is formed on the surface of the ultrathin cold-rolled steel sheet according to claim 1 or 2. 4. In mass%, C: 0.02% or less, Si: 0.10% or less, Mn: 1.5% or less, P: 0.20% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.0050 to 0.0250 % At a temperature at which 90% or more of the N amount is in a solid solution state, and the finish rolling temperature is (Ar ₃ transformation point −30 ° C.) or higher. Rolling is performed, after the completion of the hot rolling, forced cooling is started within 0.5 s, winding is performed at a winding temperature of 600 ° C. or less, and further winding is performed and water-cooled to obtain a hot-rolled sheet. After cold rolling, in a continuous annealing process, after soaking at a temperature above the recrystallization temperature, quenching to a temperature range of 250 ° C or less at a cooling rate of 150 ° C / s or more, and then soaking to 40 ° C Baking hardening amount (B) characterized by cooling within 60 seconds after completion
H): 100 MPa or more, increase in tensile strength due to paint baking treatment ΔTS: 30 MPa or more, yield stress after painting and baking treatment: 550 MPa or more Sheet thickness: 0.3 mm or less Ultra-thin cold for high-strength cans Manufacturing method of rolled steel sheet. 5. The method for producing an ultra-thin cold-rolled steel sheet for a high-strength can according to claim 4, wherein after the cooling, cold rolling is further performed at a draft of 10% or less. 6. The ultra-thin ultra-thin can for a high-strength can according to claim 4, wherein the water-cooling is started within 30 minutes after winding, and the average cooling rate is 20 ° C./h or more. Manufacturing method of cold rolled steel sheet.