JP2001107187A - High strength steel sheet for can and its producing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a high strength extra-thin cold rolled steel sheet with a sheet thickness of <=0.3 mm, excellent in baking hardenability in which the baking hardening quantity is >=50 MPa, and yield stress after coating/baking treatment is >=550 MPa and to provide a method for producing the same. SOLUTION: A hot rolled steel sheet having a composition containing, by mass, <=0.02% C, <=0.10% Si, <=1.5% Mn, <=0.20% P, <=0.01% S, <=0.01% Al and 0.0050 to 0.0250% N and containing solid solution N by >=80% of the N content is cold-rolled, is thereafter, in a continuous annealing stage, soaked in the temperature range of 500 deg.C to the one at which the recrystallization ratio reaches <90%, is subsequently subjected to rapid cooling treatment at a cooling rate of >=150 deg.C/s to the temperature region of <=250 deg.C and, within 5 min after the completion of the soaking to 40 deg.C, is cooled. After the cooling, cold rolling at a draft of <=10% may be executed.

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 containers, and more particularly to a quenching aging and strain aging hardening in a printing process, a drying process, etc., which are performed before a can-making process. The present invention relates to a novel steel sheet which exhibits high strength and which can be made to be superior in thinning a steel sheet, and a method for producing the same.
The object of the present invention is 0.3 mm or less, mainly 0.2 mm
It is a steel sheet for cans in the category called ultra-thin steel sheet of mm or less. These steel sheets are coated with tin plating, nickel-tin plating, chromium plating (so-called tin-free plating), or further with an organic coating, and are applicable to an extremely wide range of applications.

[0002]

2. Description of the Related Art A can container for storing various contents such as a beverage can, a food can, a pail can, an 18 liter can, and the like has a two-piece can consisting of a can body including a bottom surface and an upper lid due to its component structure. , A three-piece can consisting of a can body, top lid, and 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.

[0003] However, even if the steel sheet is thinned, it is necessary to maintain the strength as a can body as before, so that 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 requires a strong rolling mill. Has a disadvantage in equipment that the number of processes increases. Further, it is extremely difficult to completely eliminate chronically generated surface flaws, surface stains, and the like in steel plates manufactured by the DR method.

[0005] Thus, there has been a problem in manufacturing a hard and ultrathin steel sheet for cans having sound surface properties stably and at low cost by the DR method. As a method of increasing the strength other than the DR method, for example, a method in which a large amount of alloying elements are added to solid-solution strengthen the steel as in a wide range of steel plates for automobiles is considered. However, steel plates for cans require special corrosion resistance,
The amount of alloying elements that can be added is limited. Actually, in the ASTM standard and the JIS standard, additional elements are limited, and sufficient solid solution strengthening cannot be achieved.

[0006] In addition, the method using the transformation structure strengthening, Nb, Ti
And methods using precipitation strengthening by carbonitride-forming elements such as V, etc., were difficult to apply in the steel plate manufacturing process for cans. 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 that can be used in steel sheets for cans. In the technology described in JP-A-5-345926,
C: 0.01 wt% or less, N: 0.04 wt% or less, and C + N
Hot rolling and cold rolling of a slab containing 0.008 wt% or more and Al: 0.005 wt% or less, continuous annealing at a temperature higher than the recrystallization temperature, and then temper rolling at a draft of 5% or more It is described that a hard material of T-4 or more 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. .

[0008]

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 limited, and in order to obtain a hard material larger than that, application of the DR method of performing high-pressure reduction secondary rolling after annealing is applied. Is required. However, the DR method has a problem in producing a hard and ultrathin steel sheet for cans having sound surface properties stably and at low cost.

On the other hand, when paint baking is performed before can-making, such as a DRD can, even in a three-piece can or a two-piece can, an increase in strength can be expected due to aging in the paint baking step. The present invention solves the problems of the prior art described above, and
It is an object of the present invention to propose a high-strength ultrathin cold-rolled steel sheet having a hardness of R8 or more, and having excellent bake hardenability and high cost competitiveness, and a method for producing the same. The steel sheet of the present invention has a sheet thickness of 0.3 mm or less, a high yield strength after baking treatment of 550 MPa or more, and a hardening amount (baking hardening amount) in baking specific to can steel plates of 50 MPa. Thus, it is a high-strength ultra-thin cold-rolled steel sheet excellent in bake hardenability. In the present invention,
A high-strength ultra-thin cold-rolled steel sheet having a hardness of DR8 or more is obtained by applying a temper rolling of 10% or less after annealing without applying the so-called DR method. The bake hardening amount (BH amount) in the present invention is determined by applying a 2% tensile strain to a steel sheet and then applying a 21% equivalent to a coating and baking treatment.
It means the difference in yield stress between before and after heat treatment equivalent to painting and baking when conducting a tensile test after heat treatment at 0 ° C x 20 min. The yield stress after the coating and baking treatment means the yield stress after applying a 2% tensile strain to the cold-rolled steel sheet and performing a heat treatment at 210 ° C. for 20 minutes corresponding to the coating and baking treatment.

[0010]

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, hot rolling conditions, and cooling conditions after hot rolling are adjusted to adjust the solid solution N in the cold rolling base plate. 80% of the total N amount
The above, continuous annealing should be performed in a so-called partial recrystallization state, not at a high temperature at which complete recrystallization is completed as in the past, rapid cooling to a low temperature region after continuous annealing, steel composition with high ductility It has been found that it is important to use a low-carbon steel composition or a very low-carbon steel composition.

As a result, it is possible to prevent a decrease in the amount of solute N due to precipitation as AIN, and further to prevent the precipitation of solute C as carbide, which is sufficient for strengthening a product plate. The amount of solid solution C and N can be secured. In addition, although a part is precipitated, the precipitation is at a sufficiently low temperature, and a large increase in strength is brought about. By effectively utilizing solid solution C and solid solution N, and utilizing so-called strain aging or quenching aging, the strength of the steel sheet can be significantly increased. For this reason, after continuous annealing, in order to increase the strength, a strong reduction (reduction rate of 30) as in conventional secondary rolling is performed.
%) To adjust the hardness of the steel sheet surface,
It has been found that it is only necessary to apply a light reduction (reduction rate: 10% or less) for roughness adjustment and shape correction.

The present invention has been made based on the above findings. That is, the first aspect of the present 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: 0.01% or less, N: 0.0050 to 0.0250%, and N 80% of quantity
A high-strength cold-rolled steel base plate comprising the above-mentioned solid solution N and having a balance of Fe and unavoidable impurities, and in the present invention, in addition to the composition, %, First group to second group, first group: Nb: 0.001 to 0.040%, Ti: 0.001 to 0.040
%, B: one or more selected from 0.0002 to 0.0020%, provided that Nb satisfies the following formula (1) and Ti satisfies the following formula (2).

(12/93) × (Nb / C) ≦ 0.8 (1) (12/48) × (Ti ^* / C) ≦ 0.8 (2) where Ti ^* = Ti− (48 / 32) S- (48/14) N Second group: Cu: 0.01 to 0.2%, Ni: 0.01 to 0.2%, Cr: 0.
It is preferable to contain one or two selected from one or more selected from 01 to 0.2% and Mo: 0.01 to 0.2%.

[0014] The second present invention relates to a method of the present invention, wherein C: 0.
02% or less, Si: 0.10% or less, Mn: 1.5% or less, P: 0.20
%, S: 0.01% or less, Al: 0.01% or less, N: 0.0050
-0.0250%, and 0.0050% of (Solute C + Solute N)
The composition contains at least the following, with the balance being Fe and unavoidable impurities, a structure having a recrystallization rate of 60% or more and less than 90%, a bake hardening amount of 50 MPa or more, and a yield stress after painting and baking treatment: 550
It is an ultra-thin cold-rolled steel sheet for high-strength cans having a thickness of 0.3 mm or less, characterized by having a pressure of not less than 1 MPa. 2nd group 1st group: Nb: 0.001 to 0.040%, Ti: 0.001 to 0.040
%, B: one or more selected from 0.0002 to 0.0020%, provided that Nb satisfies the following formula (1) and Ti satisfies the following formula (2).

(12/93) × (Nb / C) ≦ 0.8 (1) (12/48) × (Ti ^* / C) ≦ 0.8 (2) where Ti ^* = Ti− (48 / 32) S- (48/14) N Second group: Cu: 0.01 to 0.2%, Ni: 0.01 to 0.2%, Cr: 0.
It is preferable to contain one or two selected from one or more selected from 01 to 0.2% and Mo: 0.01 to 0.2%.

According to a third aspect of the present invention, there is provided the ultra-thin cold-rolled steel sheet according to the second aspect, wherein a plating layer is formed on at least one surface of the ultra-thin cold-rolled steel sheet. It is a steel plate. In the fourth invention, C: 0.02% by mass%.
Below, 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: 0.0050-0.
0250%, or more gradually, Group 1 to Group 2 Group 1: Nb: 0.001 to 0.040%, Ti: 0.001 to 0.040
%, B: one or more selected from 0.0002 to 0.0020%, provided that Nb satisfies the following formula (1) and Ti satisfies the following formula (2).

(12/93) × (Nb / C) ≦ 0.8 (1) (12/48) × (Ti ^* / C) ≦ 0.8 (2) where Ti ^* = Ti− (48 / 32) S- (48/14) N Second group: Cu: 0.01 to 0.2%, Ni: 0.01 to 0.2%, Cr: 0.
01-0.2%, Mo: One or two or more selected from at least 0.01-0.2% selected from the group consisting of two or more selected from the group consisting of a balance of Fe and unavoidable impurities. Using a material, rolling is started at a temperature at which 90% or more of the N amount is in a solid solution state, and hot rolling is performed at a finishing rolling temperature of (Ar ₃ transformation point −30 ° C.) or higher. After rolling is completed, forced cooling is started within 0.5 s, winding is performed at a winding temperature of 600 ° C or less, and further cooling is performed, followed by water cooling, preferably for 30 minutes.
Start water cooling within 20 ℃ / h
A method for producing a base plate for a high-strength ultra-thin cold-rolled steel sheet, wherein the hot-rolled sheet contains solute N of 80% or more of the N amount.

Further, the fifth invention is characterized in that C: 0.
02% or less, Si: 0.10% or less, Mn: 1.5% or less, P: 0.20
%, S: 0.01% or less, Al: 0.01% or less, N: 0.0050
0.00.0250%, or more gradually, the first group to the second group, the first group: Nb: 0.001 to 0.040%, Ti: 0.001 to 0.040
%, B: one or more selected from 0.0002 to 0.0020%, provided that Nb satisfies the following formula (1) and Ti satisfies the following formula (2).

(12/93) × (Nb / C) ≦ 0.8 (1) (12/48) × (Ti ^* / C) ≦ 0.8 (2) where Ti ^* = Ti− (48 / 32) S- (48/14) N Second group: Cu: 0.01 to 0.2%, Ni: 0.01 to 0.2%, Cr: 0.
01-0.2%, Mo: One or two or more selected from at least 0.01-0.2% selected from the group consisting of two or more selected from the group consisting of a balance of Fe and unavoidable impurities. Using a material, rolling is started at a temperature at which 90% or more of the N amount is in a solid solution state, and hot rolling is performed at a finishing rolling temperature of (Ar ₃ transformation point −30 ° C.) or higher. After rolling is completed, forced cooling is started within 0.5 s, winding is performed at a winding temperature of 600 ° C or less, and further cooling is performed, followed by water cooling, preferably for 30 minutes.
Start water cooling within 20 ℃ / h
A hot rolled sheet containing solute N of 80% or more of the above-mentioned amount of N is then cold-rolled. Then, in a continuous annealing step, the temperature is 500 ° C. or more and the recrystallization rate is less than 90%. After soaking in the temperature range, baking is performed at a cooling rate of 150 ° C / s or more, followed by quenching to a temperature range of 250 ° C or less, then cooling to 40 ° C within 5 minutes after completion of soaking. Curing amount:
This is a method for producing an ultra-thin cold-rolled steel sheet for high-strength cans having a yield stress after painting and baking treatment of at least 50 MPa and having a thickness of at least 550 MPa, which is 0.3 mm or less. It is preferable to perform cold rolling at a rate of 10% or less.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION The cold rolled steel sheet of the present invention has a bake hardening amount of 50 MPa or more and a yield stress after painting and baking treatment of 550 MP.
a Ultra-thin cold-rolled steel sheet for high-strength cans with a thickness of 0.3 mm or less, having a thickness of at least 0.3 mm. The cold-rolled steel sheet of the present invention has a yield stress after the coating and baking treatment of 550 MPa or more, and can promote the thinning of the steel sheet. The cold-rolled steel sheet of the present invention has a solid solution C
+ By effectively utilizing the effect of solid solution N, the paint bake hardening amount (BH amount) 50 during the paint baking process before press molding
There is a remarkable age hardening phenomenon of MPa or more, and furthermore, it contains a large amount of solid solution C and solid solution N and imparts dent resistance (resistance to dents) to the can body after can making, especially the can body. Can result in a dramatic increase in can strength.

First, the reasons for limiting the composition of the base plate for cold-rolled steel sheet and the cold-rolled steel sheet of the present invention will be described. Hereinafter, mass% is simply described as%. 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, and the ductility of the steel sheet and, consequently, the workability are stably secured at a high value. Further, by reducing the amount of carbides present in the steel where the solid solution C precipitates, a large amount of solid solution C is consequently left, and the strength of the steel sheet can be increased. Such a desirable effect is recognized by making the C content 0.02% or less. Therefore, C is 0.02%
Limited to the following. In order to further enhance the strength by solid solution C, it is desirable that the C content is in an extremely low carbon region of 0.010% or less. On the other hand, in order to obtain the target high bake hardenability (BH property), the C content is 0.002%.
It is preferable to make the above.

Si: 0.10% or less Si is an element which increases the strength of steel by solid solution strengthening. However, since addition of a large amount causes problems such as deterioration of surface treatment properties and deterioration of corrosion resistance, Si is 0.10% or less. 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 also significantly 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 present as an inclusion (sulfide) 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 0.005% 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. In addition, an increase in the Al content causes an increase in the recrystallization temperature, and it is necessary to increase the annealing temperature.
In high-temperature annealing, the amount of solute N is reduced due to the formation of AlN, the amount of age hardening is reduced, and thus the strength of the steel sheet is reduced. This phenomenon is remarkable because the Al content is 0.01%.
%. From this, Al is 0.01
% Or less. From the viewpoint of stable operation in the steel smelting process, the content of Al is desirably 0.001% or more. Also, from the viewpoint of further stabilizing the material, Al
Is more preferably not more than 0.005%.

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.

Solid solution N in base plate for cold rolled steel sheet: 80% of total N content
As described above, in order to secure a large age hardening property of the cold-rolled steel sheet, which is a feature of the present invention, the amount of solid solution N in the base plate (hot-rolled sheet) for the cold-rolled steel sheet needs to be 80% or more of the total N amount. There is. 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 after the usual dissolution treatment with bromoester (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.

(Solute C + Solute N) in cold rolled steel sheet: 0.0050
% Or more In order to increase the age hardening property 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 made 0.0050% or more. . In addition to the basic composition described above, a selective element can be contained as needed. As the selected element, one selected from the following first and second groups:
A group or two groups can be selected as needed.

First group: Nb: 0.001 to 0.040%, Ti: 0.00
1 to 0.040%, B: one or more selected from 0.0002 to 0.0020%, Nb, Ti, and B are elements that increase the strength, and one or more as needed. Can be selected. Nb is an element effective for forming fine carbonitrides, refining crystal grains, and increasing the strength of steel after forming. Such an effect is recognized when the content of Nb is 0.001% or more. However, if the content exceeds 0.040%,
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, losing the function of strengthening the steel. For this reason, Nb is preferably set to 0.040% or less. From the viewpoint of annealing conditions, Nb
Is desirably 0.007% or less. When Nb is contained, the following formula (1) is further satisfied: (12/93) × (Nb / C) ≦ 0.8 (1) If the formula (1) is not satisfied, there is an inconvenience that the amount of solid solution C is significantly reduced.

Ti is an element effective for forming fine carbonitrides like Nb, refining crystal grains, and increasing the strength of steel after forming. Such effects are 0.001%
The above content is recognized, and becomes significant when the content is 0.003% or more. Therefore, the content of Ti is 0.001% or more, more preferably 0.001%.
It is preferable to contain 3% or more. On the other hand, Ti
%, The decrease in the amount of age hardening due to the decrease in solid solution C and N becomes remarkable. For this reason, Ti is preferably set to 0.040% or less. From the viewpoint of stabilizing the material, the content of Ti is more preferably in the range of 0.003 to 0.007%. When Ti is contained, the following equation (2) is used: (12/48) × (Ti ^{* /} C) ≦ 0.8 (2) where Ti ^* = Ti− (48/32) S− (48) / 14) N shall be satisfied. If the expression (2) is not satisfied, there is a disadvantage that the amount of solute C and the amount of solute N are significantly reduced.

B is an element effective in stabilizing the age hardenability and stably ensuring the strength of the steel under the conditions of the present invention, despite the tendency to form nitrides. Such an effect is remarkably observed at a content of 0.0002% or more, and is preferably contained at a content of 0.0002% or more. On the other hand, 0.00
If the content exceeds 20%, the effect tends to be saturated or, conversely, age hardenability tends to decrease. For this reason,
B 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 0.00.
More preferably, it is set to 08%.

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 base plate (hot-rolled plate) for cold-rolled steel sheet hardens remarkably, and there is a risk of causing troubles in the cold rolling process. Sex is increased. Therefore, Cu: 0.01-0.2%, Ni: 0.01-0.2%
%, Cr: 0.01-0.2%, Mo: 0.01-0.2%. Note that the effects of Cu, Ni, Cr, and Mo are not canceled out even if they are added in combination. Therefore, Cu, Ni, Cr,
Mo can be added alone or in combination. In addition,
More preferably, the total amount of Cu, Ni, Cr and Mo should be 0.2% or less.

The balance is Fe and unavoidable impurities. The balance other than the above components is Fe and unavoidable 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 60% or more and less than 90%. In addition, the recrystallization rate in the present invention refers to a value obtained by an area method based on observation with an optical microscope. If the recrystallization ratio is less than 60%, the workability, particularly the elongation properties, will be insufficient, while if it exceeds 90%, the strength and hardness will decrease. For this reason, in the present invention, the recrystallization rate is limited to 60% or more and less than 90%.

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% or more is in a solid solution state.

Rolling start temperature: at least the temperature at which 90% or more of the total N content is in a solid solution state. To make 90% or more of the total N content into a solid solution state, although it depends on the components, it is generally 1150 ° C or more. Or Al
In the case of the direct-feed rolling process in which the precipitation of N is delayed, it is preferable that the rolled material be inserted into a heating furnace maintained at a rollable temperature and heated without cooling to a transformation point or lower after continuous casting. In any case, in the present invention, at the start of hot rolling,
It suffices that 90% or more of the total N amount in the rolled material is in a solid solution state. Therefore, the hot rolling start temperature is preferably set to 1100 ° C. or higher. Unless 90% or more of the total amount of N is in a solid solution state at the start of hot rolling, age hardening due to solid solution N is not sufficiently exhibited on a product sheet.

Finish rolling temperature: (Ar_ThreeTransformation point -30 ℃)
In the present invention, in order to effectively suppress the precipitation of AlN,
The finishing rolling temperature in rolling_ThreeTransformation point -30 ℃)
Preferably it is above. Finish rolling temperature is (Ar _ThreeTransformation point
Below −30 ° C.), precipitation of AlN becomes remarkable, and
Reduce. In addition, more preferably (Ar_ThreeTransformation point -10 ℃)
That is all.

Forced cooling: started within 0.5 s after completion of hot rolling Forced cooling is preferably water-cooled from the viewpoint of cooling capacity. 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. If the winding temperature exceeds 600 ° C., the amount of AlN precipitated increases remarkably and the amount of solute N decreases, so that the target age hardening property cannot be obtained. In order to stably obtain high age hardening properties, the winding temperature is more preferably set to 570 ° C. or lower. Further, by setting the winding temperature to 600 ° C. or lower, more preferably 570 ° C. or lower, it becomes possible to more finely and uniformly disperse carbides generated during cooling after winding, thereby reducing the steel sheet product state. The amount of solid solution C can be increased.

After Winding: Water Cooling In the present invention, it is preferable to start water cooling within 30 minutes in a coil state after winding, and to cool at an average cooling rate of 20 ° C./h or more. Here, the cooling rate refers to the average cooling rate at the central portion in the longitudinal direction of the steel strip and at the central portion of the plate 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. If the start of water cooling after winding 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. Water cooling after winding is performed in a coil state, but there is no problem even if it is rewound and cooled again.

By the above-mentioned hot rolling and cooling after rolling, a hot rolled sheet in which 80% or more of the total N content is in a solid solution state can be obtained. 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 the higher the sheet thickness, the higher the reduction.

Next, the cold-rolled sheet is subjected to continuous annealing. Soaking temperature for continuous annealing: 500 ° C or higher and recrystallization rate of 90%
Temperature range of less than In the continuous annealing step, the temperature is soaked in a temperature range of 500 ° C. or more and a recrystallization rate of less than 90%. If the soaking temperature is less than 500 ° C, the progress of recrystallization is slow, and the recrystallization rate is less than 60% in continuous annealing. Since the processing strain introduced by cold rolling remains, the ductility is extremely low. Not suitable. on the other hand,
When the temperature exceeds the temperature at which the recrystallization ratio is less than 90%, the ductility is sufficiently increased, but the strength is remarkably reduced, which is not preferable.
For this reason, in the present invention, the recrystallization rate is higher than 500 ° C.
% Is preferably carried out in a temperature range of less than 10%. As a result, sufficient workability can be ensured in normal use, and a target high strength can be obtained. The temperature at which the recrystallization rate of 90% is obtained is preferably determined by observing the structure with an optical microscope.
The hardness in the fully annealed state may be set at 100%, and may be lower than the temperature at which the hardness reduction of 90% is achieved. Also, it can be determined by an X-ray diffraction technique. This is a technique based on the fact that the width of the peak of the diffraction line corresponds to the remaining strain amount, that is, the recrystallization rate.

It is not necessary to maintain a constant temperature within this temperature range. 10s from operational stability
The above soaking time is sufficient. In addition, from the viewpoint of workability, the recrystallization rate is set to 60% or more. Rapid cooling after continuous annealing and soaking: Cooling rate of 150 ° C / s or more to a temperature range of 250 ° C or less Cooling conditions after continuous annealing and soaking are one of the most important requirements in the present invention. The purpose of this cooling is to obtain a structure in which extremely fine carbides are finely dispersed after annealing, and thereby to secure a sufficient amount of solid solution C and solid solution N to secure age hardenability. . 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 among solid solution N and solid solution C becomes remarkable. In order to obtain the target characteristics more stably, it is desirable to set the cooling rate to 200 ° C./s or more.

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 the amount of solute C is drastically reduced. It is more preferable to perform the quenching treatment to a temperature range of 200 ° C. or less. In order to stably perform such quenching processing, 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 sheet passing processing, and use these methods. A combination of technologies is required.

Further, it is preferable to cool to 40 ° C. or less within 5 minutes integrated from the end of soaking (start of cooling). If it takes more time to cool down to 40 ° C. or less, a phenomenon similar to the so-called overaging phenomenon occurs, and solid solution C
The amount decreases and the age hardening ability of the steel sheet decreases. Continuous annealing,
After the quenching treatment, secondary rolling with a draft of 10% or less may be further performed. The secondary rolling in the present invention is not mainly intended to increase the strength, but is mainly intended to adjust the surface roughness, shape, etc., and it is preferable that the rolling reduction is 1.0% or more. If the rolling reduction exceeds 10%, the ductility decreases, and the rolling operation becomes difficult due to the increase in deformation resistance. 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 can be used for coating and coating before can-making (before pressing).
It becomes a hard material by baking, and when applied to ultra-thin steel sheets with a thickness of 0.3 mm or less, its superiority is more effectively exhibited. The cold-rolled steel sheet manufactured by the above process has a (solid solution C + solid solution N) amount of 0.0050% or more, a bake hardening amount of 50 MPa or more, and a yield stress after painting and baking treatment: 55
Ultra-thin cold-rolled steel sheet for high-strength cans having 0 MPa or more. The steel sheet of the present invention obtains an unprecedented large age hardening property by combining both actions of solid solution C and solid solution N. Therefore, the cold-rolled steel sheet of the present invention has a yield stress of 55 after painting and baking.
Since it has a pressure of 0 MPa or more, it is possible to advance thinning of steel sheets. 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 50 MPa or more occurs, which can bring about a dramatic increase in can strength.

In the present invention, an ultrathin cold-rolled steel sheet can be formed into an ultrathin plated steel sheet by forming a plating layer (at least one side) on the surface. 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.

[0047]

EXAMPLES (Example 1) Steels having the components shown in Table 1 were melted in a converter and made into slabs by a continuous casting method. Next, these slabs were subjected to hot rolling under the conditions shown in Table 2 to obtain a hot-rolled sheet having a thickness of 1.8 mm. In the production of these steel sheets, when performing coil water cooling, water cooling was started within 30 minutes after winding. 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 cold sheet having a final finished sheet thickness of 0.15 mm. It was a rolled steel sheet. Note that a cold-rolled steel sheet (steel No. 12) in which the rolling reduction was increased to 30% by the secondary rolling was used as 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 the amount of solute C and the amount of solute N The amount of N in the cold-rolled steel sheet was analyzed by chemical analysis, and the amount of N existing as AlN was determined by extraction analysis after dissolution treatment with bromoester. The amount of solute N in the cold rolled steel sheet is
｛(N content in cold rolled steel sheet)-(N existing as AlN
The value of｝) was used.

The amount of solute 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 result of the chemical analysis. (Ii) Structural investigation A test specimen was sampled from the center of the cold-rolled steel sheet in the width direction, and the cross-section in the sheet thickness direction excluding the surface layer was observed by an optical microscope, and the area of the recrystallized grains was observed. Was measured, and the ratio of the area of the recrystallized grains to the total area was calculated, and the result was defined as the recrystallization rate. (Iii) Tensile test From the center in the width direction of these cold-rolled steel sheets, JIS
No. 13-B Tensile test piece was sampled and the strain rate crosshead speed:
Tensile test was conducted at 10mm / s, yield point YS, tensile strength T
S and elongation El were measured. In addition, the tensile test is 1
Carried out within days. The reason why the tensile test piece was a JIS No. 13-B 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
A No. 13-B tensile test specimen was sampled, subjected to a 2% tensile prestrain, unloaded, subjected to a heat treatment equivalent to a paint baking treatment at 210 ° C for 20 minutes, and then subjected to a tensile test to determine the deformation stress (yield). Stress). Difference in yield stress before and after heat treatment equivalent to paint bake treatment, ((Yield stress after heat treatment equivalent to paint bake treatment)
− (2% deformation stress before heat treatment corresponding to paint baking treatment)) was obtained and defined as a bake hardening amount (BH amount). (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.

[0051]

[Table 1]

[0052]

[Table 2]

[0053]

[Table 3]

In the present invention, the rolling reduction of the secondary rolling is not increased (without applying the DR method with a large rolling reduction).
It is a high-strength ultrathin cold-rolled steel sheet having a yield strength after paint baking treatment of 550 MPa or more, a bake hardening amount of 50 MPa or more, and a hardness equivalent to DR8. On the other hand, Comparative Examples outside the scope of the present invention have a small amount of paint bake hardening. Further, using these steel plates, two-piece DRD cans (can diameter: 70 mm) were manufactured, and the resistance (dent resistance) to dents in the can body, particularly the can body, was evaluated. The evaluation of the resistance to the dent of the body is a test in which a can body having a shape equivalent to that of a closed lid is statically pushed and deformed with an indenter to measure and evaluate a load that causes permanent deformation.

As a result, according to the present invention, the D
The load causing dent was about 20% higher than that of the R steel plate (conventional example), and the effect of increasing the strength of the can by strain aging of this steel plate was confirmed. (Example 2) An ultrathin cold-rolled steel sheet having a thickness of 0.14 mm was prepared from steel A shown in Table 1 under the manufacturing conditions shown in Table 4. In the production of these steel sheets, when performing coil water cooling, water cooling was started within 30 minutes after winding. 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 plates were subjected to Ni plating before annealing, and a Ni diffusion layer was formed on the surfaces by continuous annealing. A cold-rolled steel sheet (steel No. 2-11) in which the rolling reduction was increased to 30% by secondary rolling was used as a conventional example (DR steel sheet).

Table 5 shows the results.

[0057]

[Table 4]

[0058]

[Table 5]

The cold-rolled steel sheet of the present invention (Example of the present invention) has a yield strength after painting and baking of 550 MPa or more without increasing the rolling reduction in the secondary rolling (without applying the DR method) and baking. With a hardening amount of 50MPa or more, and having a hardness equivalent to DR8,
It is a high-strength ultra-thin cold-rolled steel sheet that has unprecedented excellent properties. On the other hand, Comparative Examples outside the scope of the present invention have a low amount of paint bake hardening and a low strength of less than 550 MPa in yield strength after paint bake treatment.

Next, these cold-rolled steel sheets are subjected to a tin plating treatment (a plating weight of 25 g / m ² ) in an electric tin plating line, and then subjected to a reflow treatment in-line to obtain tin having an island-like tin phase. It was a plated steel sheet. 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. Also,
Using these high-strength ultra-thin plated steel sheets, they were formed into three-piece cans, and their axial compressive strength and resistance to dents in the can body (dent resistance) were evaluated. The dent resistance of the can body is
Evaluation was performed in the same manner as in Example 1. As a result, in the example of the present invention, the dent resistance was improved by about 25% as compared 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 after the secondary rolling at a high reduction rate after annealing, can be used as it is or as a low reduction rate of 10% or less. It can be obtained only by performing secondary rolling, and has a remarkable industrial effect. In the steel sheet of the present invention, the yield stress is greatly increased by the coating baking treatment after forming, and accordingly the strength of the can body is greatly increased, so that it can greatly contribute to thinning of the steel sheet.

 ──────────────────────────────────────────────────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 FJ04 FK03 FK08

Claims (8)

[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 a composition comprising 80% or more of the solute N of the N amount and a balance of Fe and unavoidable impurities. 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. Base plate for high strength cold rolled steel sheet. 1st group: Nb: 0.001 to 0.040%, Ti: 0.001 to 0.040
%, B: one or more selected from 0.0002 to 0.0020%, provided that Nb satisfies the following formula (1) and Ti satisfies the following formula (2). (12/93) × (Nb / C) ≦ 0.8 (1) (12/48) × (Ti ^* / C) ≦ 0.8 (2) where Ti ^* = Ti− (48/32) S -(48/14) N Second group: Cu: 0.01 to 0.2%, Ni: 0.01 to 0.2%, Cr: 0.
01-0.2%, Mo: One or more selected from among 0.01-0.2% 3. 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 % And (Solute C + Solute N)
Of 0.0050% or more, with the balance being Fe and unavoidable impurities and a structure with a recrystallization rate of 60% or more and less than 90%, bake hardening amount: 50 MPa or more, yield stress after painting and baking treatment: Sheet thickness characterized by having 550 MPa or more: 0.
Ultra-thin cold rolled steel sheet for high strength cans of 3 mm or less. 4. The composition according to claim 3, further comprising one or two groups selected from the following first group and second group in mass% in addition to the composition. Ultra-thin cold rolled steel sheet for strength cans. 1st group: Nb: 0.001 to 0.040%, Ti: 0.001 to 0.040
%, B: one or more selected from 0.0002 to 0.0020%, provided that Nb satisfies the following formula (1) and Ti satisfies the following formula (2). (12/93) × (Nb / C) ≦ 0.8 (1) (12/48) × (Ti ^* / C) ≦ 0.8 (2) where Ti ^* = Ti− (48/32) S -(48/14) N Second group: Cu: 0.01 to 0.2%, Ni: 0.01 to 0.2%, Cr: 0.
01-0.2%, Mo: One or more selected from among 0.01-0.2% 5. 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 3 or 4. 6. 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. 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 lower, and further cooling is performed, followed by water cooling to remove solid solution N of 80% or more of the N amount. A method for producing a mother plate for a high-strength ultra-thin cold-rolled steel sheet, comprising: 7. 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. After 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 cooling is performed, followed by water cooling to form a hot rolled sheet. After cold-rolling, the steel sheet is soaked in a temperature range of 500 ° C or higher and a recrystallization rate of less than 90% in a continuous annealing process,
A quenching treatment is performed at a cooling rate of 0 ° C / s or more to a temperature range of 250 ° C or less, and then cooling within 5 minutes after the completion of soaking to 40 ° C. Baking hardening amount: 50MPa or more, coating Production method of ultra-thin cold rolled steel sheet for high-strength cans having a yield stress after baking treatment of 550 MPa or more and a thickness of 0.3 mm or less. 8. The method for producing an ultrathin cold-rolled steel sheet for a high-strength can according to claim 7, wherein after the cooling, cold rolling is further performed at a rolling reduction of 10% or less.