WO2020045449A1

WO2020045449A1 - Steel sheet for can, and method for producing same

Info

Publication number: WO2020045449A1
Application number: PCT/JP2019/033548
Authority: WO
Inventors: 房亮假屋; 芳恵椎森; 克己小島; 大介大谷; 貴文神宮
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2018-08-30
Filing date: 2019-08-27
Publication date: 2020-03-05
Also published as: MY196226A; EP3845678A4; JPWO2020045449A1; MX2021002226A; EP3845678A1; JP6809619B2; PH12021550410A1

Abstract

Provided is a steel sheet for a can, which has high strength and excellent ductility, and also exhibits uniform deformability and excellent workability. This steel sheet has a constituent composition that contains, in terms of mass%, 0.020-0.130% of C, not more than 0.04% of Si, 0.10-1.20% of Mn, 0.007-0.100% of P, not more than 0.030% of S, 0.001-0.100% of Al, more than 0.0120% and not more than 0.0200% of N, 0.0060-0.0300% of Nb and not more than 0.040% of Cr, with the remainder comprising Fe and unavoidable impurities. The ratio of the content of Nb in precipitates having diameters of less than 20 nm relative to the content of Nb in all precipitates is at least 40%. The average interval between all precipitates is not more than 30 nm. The upper yield strength is 500-640 MPa and the total elongation is at least 10% following a heat treatment at 210ºC for 10 minutes.

Description

Steel plate for can and method of manufacturing the same

The present invention relates to a high-strength steel plate for cans and a method for producing the same.

缶 Steel cans, beverage cans, and other cans use a steel plate for their body and lid. In these cans, there is a demand for a reduction in manufacturing costs, and reduction in costs required for can materials has been promoted by reducing the thickness of steel plates provided for cans. The objects of thinning the steel plate are mainly a two-piece can can body formed by drawing, a three-piece can body formed by cylindrical forming, and these can lids. Simply reducing the thickness of the steel plate for cans reduces the strength of the body and lid of the cans, and especially in areas such as the can body of redrawable cans (DRD (draw-redraw) cans) and welded cans. It is desired to apply a high-strength and ultra-thin steel sheet for cans.

Such a high-strength steel sheet for ultra-thin cans is manufactured by a Double Reduce method (hereinafter, referred to as a DR method) in which, after annealing, a secondary cold rolling is performed at a rolling reduction of 20% or more. A steel sheet manufactured by using the DR method (hereinafter, also referred to as a DR material) has a high strength, but has a property of being inferior in workability due to low total elongation and poor ductility.

ＤＲ The use of DR materials is increasing for can bodies having straight shapes. On the other hand, a can body having a bead or a can lid (EOE (Easy Open End)) of a food can that is opened with a stainless steel tub has a complicated shape. Often occur, or a highly accurate processed shape cannot be obtained. Specifically, the can lid (EOE) is manufactured by sequentially performing blanking, shell processing, curling processing, and rivet processing on a steel sheet by press forming. In particular, since the flange portion of the can body and the curled portion of the can lid are tightly wound to ensure the sealability of the can, high precision is required for the processed shape of the can lid curled portion. DR materials, which are generally used as high-strength ultra-thin steel sheets, are often difficult to apply to can lids with poor ductility and complicated shapes from the viewpoint of workability. Through the product. Furthermore, in the DR material, the steel sheet is strengthened by work hardening by the secondary cold rolling, so that depending on the accuracy of the secondary cold rolling, the work hardening is unevenly introduced into the steel sheet, so that the DR material is processed. In some cases, local deformation may occur. Local deformation is a phenomenon that should be avoided because the dimensional accuracy of the curled portion of the can lid decreases.

In order to avoid such drawbacks of the DR material, methods for manufacturing a high-strength steel sheet using various strengthening methods have been proposed.
For example, Patent Document 1 proposes a steel sheet in which strength and ductility are balanced by complexly combining precipitation strengthening with Nb carbide and miniaturization strengthening with carbonitrides of Nb, Ti, and B. Patent Literature 2 proposes a method of increasing strength by using solid solution strengthening of Mn, P, N, and the like. Patent Document 3 discloses that the tensile strength is less than 540 MPa using precipitation strengthening by carbonitrides of Nb, Ti, and B, and that the formability of a weld is improved by controlling the particle size of oxide-based inclusions. A steel plate for cans has been proposed. Patent Literature 4 discloses that high strength is achieved by increasing the amount of N to increase the strength by solid solution N and controlling the dislocation density in the thickness direction of the steel sheet so that the tensile strength is 400 MPa or more and the breaking elongation is 10% or more. A steel plate for a container has been proposed.

JP-A-8-325670 JP 2004-183074 A JP 2001-89828 A Japanese Patent No. 5858208

強度 As described above, it is necessary to ensure strength to reduce the thickness. On the other hand, when a steel plate is used as a material for a can lid (for example, EOE) having a high degree of processing, the steel plate needs to be a highly ductile steel plate. Furthermore, in order to improve the dimensional accuracy of the curled portion of the can lid, local deformation during processing of the steel sheet is suppressed, that is, the deformation during processing is uniform (hereinafter, having uniform deformability, It is necessary). Therefore, it is required that the steel sheet for cans used as described above simultaneously satisfy high strength, high ductility (total elongation), and uniform deformability (dimensional accuracy of the curled portion).

However, Patent Literature 1 does not mention local deformation of a steel sheet, and it is desired to provide the steel sheet with uniform deformability.

Patent Document 2 proposes strengthening by solid solution strengthening. However, increasing the strength of a steel sheet by adding P excessively tends to cause local deformation, and does not provide uniform deformability.

Patent Document 3 uses precipitation of Nb, Ti, and the like and strengthening of grain refinement. However, from the viewpoint of formability and surface properties of a welded portion, addition of not only Ti but also Ca and REM is indispensable. There is a problem of deterioration. Moreover, there is no description about local deformation of the steel sheet, and it is desired to impart uniform deformability to the steel sheet.

Patent Document 4 also does not describe the shape of the curled portion of the can lid at all, and there is no mention of local deformation of the steel plate. Therefore, it is desired to provide the steel plate with uniform deformability. .

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a steel sheet for cans having uniform deformability and excellent workability in addition to high strength and excellent ductility, and a method for producing the same.

[1] In mass%,
C: 0.020% to 0.130%,
Si: 0.04% or less,
Mn: 0.10% to 1.20%,
P: 0.007% or more and 0.100% or less,
S: 0.030% or less,
Al: 0.001% to 0.100%,
N: more than 0.0120% and not more than 0.0200%,
Nb: 0.0060% or more and 0.0300% or less and Cr: 0.040% or less, and the balance has a component composition of Fe and unavoidable impurities, and a diameter of 20 nm in the Nb content in all precipitates. Is less than 40%, the average interval of all the precipitates is 30 nm or less, the upper yield strength after heat treatment at 210 ° C. for 10 minutes is 500 MPa or more and 640 MPa or less, and the total elongation is Steel plate for cans that is 10% or more.

[2] In mass%,
C: 0.020% to 0.130%,
Si: 0.04% or less,
Mn: 0.10% to 1.20%,
P: 0.007% or more and 0.100% or less,
S: 0.030% or less,
Al: 0.001% to 0.100%,
N: more than 0.0120% and not more than 0.0200%,
A steel material containing Nb: 0.0060% or more and 0.0300% or less and Cr: 0.040% or less and the balance being Fe and an unavoidable impurity is heated at 1200 ° C. or more, and the finishing temperature is increased. A hot rolling step of performing hot rolling at a temperature of 850 ° C. or more and a rolling reduction of the final stand of 8% or more, and winding in a temperature range of 640 ° C. or more and 780 ° C. or less;
A primary cold rolling step of performing cold rolling with a reduction ratio of 86% or more after the hot rolling step;
After the primary cold rolling step, the temperature is soaked in the temperature range of 660 ° C. or more and 800 ° C. or less, and the primary cooling is performed to the temperature range of 600 ° C. or more and 650 ° C. or less at an average cooling rate of 3 ° C./s or more and less than 10 ° C./s. And an annealing step of performing secondary cooling to a temperature range of 150 ° C. or less at an average cooling rate of 10 ° C./s or more,
A secondary cold rolling step of performing cold rolling with a rolling reduction of 0.1% or more and 3.0% or less;
A method for producing a steel sheet for cans having:

According to the present invention, a highly ductile and high-strength steel sheet for cans having uniform deformability without impairing corrosion resistance even for highly corrosive contents can be obtained. By increasing the strength of the steel sheet, a high strength of the can body can be ensured even when the thickness of the can is reduced. Further, since the steel sheet for cans of the present invention has high ductility, it is an optimal material for can body processing with a high processing rate such as bead processing and can expanding processing used for welding cans and for flange processing. That is, in the processing, since the steel sheet has uniform deformability, can products and can lid products can be manufactured with good workability and high dimensional accuracy.

Hereinafter, the steel sheet for cans of the present invention will be described in detail.
First, the component composition of the steel sheet for cans according to the present invention will be described. The unit “%” in the content of each component is “% by mass” unless otherwise specified.
C: 0.020% or more and 0.130% or less In the steel sheet for cans of the present invention, it is important to have an upper yield strength of 500 MPa or more and a total elongation of 10% or more. For that purpose, it is important to utilize precipitation strengthening by NbC generated by containing Nb. In order to utilize precipitation strengthening by NbC, the C content of the steel sheet for cans is important. Specifically, it is necessary to set the lower limit of the C content to 0.020%. That is, when the C content is less than 0.020%, the ratio of the Nb content in precipitates having a diameter of less than 20 nm to the Nb content in all precipitates is less than 40%, and the uniform deformability or the can lid The dimensional accuracy of the curl height deteriorates. On the other hand, if the C content exceeds 0.130%, subperitectic cracking may occur in the cooling process during melting of steel. Further, the ratio of precipitates having a precipitate diameter of 20 nm or more increases, and the ratio of precipitates having a precipitate diameter of less than 20 nm to all the precipitates becomes less than 40%, and the uniform deformability decreases. Furthermore, since the steel sheet is excessively hardened, ductility decreases. For this reason, the upper limit of the C content is set to 0.130%.
When the C content is 0.040% or less, an increase in deformation resistance during cold rolling is further suppressed, and thus it is not necessary to reduce the rolling speed to avoid surface defects after rolling. Furthermore, the ratio of the Nb content in precipitates having a diameter of less than 20 nm to the Nb content in all precipitates becomes more uniform. Therefore, from the viewpoint of manufacturability, the C content is preferably set to 0.040% or less.

Si: 0.04% or less Si is an element that increases the strength of steel by solid solution strengthening. To obtain this effect, the Si content is preferably set to 0.01% or more. However, if the Si content exceeds 0.04%, the corrosion resistance is significantly impaired. Therefore, the Si content is set to 0.04% or less. Preferably it is 0.03% or less.

Mn: 0.10% or more and 1.20% or less Mn increases the strength of steel by solid solution strengthening. To secure the target upper yield strength, the Mn content needs to be 0.10% or more. Therefore, the lower limit of the Mn content is set to 0.10%. On the other hand, when the Mn content exceeds 1.20%, the corrosion resistance and the surface characteristics are poor. Furthermore, the ratio of the Nb content in precipitates having a diameter of less than 20 nm to the Nb content in all the precipitates is less than 40%, causing local deformation and lowering the uniform deformability. Therefore, the upper limit of the Mn content is set to 1.20%. Preferably, it is 0.20% or more and 0.60% or less.

P: 0.007% or more and 0.100% or less P is an element having a large solid solution strengthening ability. To obtain such an effect, the content of 0.007% or more is necessary. On the other hand, if the P content is less than 0.007%, a long time is required for dephosphorization, and the production cost is greatly increased. Therefore, the P content is set to 0.007% or more. However, when the P content exceeds 0.100%, the ratio of the Nb content in precipitates having a diameter of less than 20 nm to the Nb content in all precipitates becomes less than 40%, and local deformation occurs. And the uniform deformability decreases. Further, the corrosion resistance is poor. Therefore, the P content is set to 0.100% or less. Preferably, it is 0.008% or more and 0.015% or less.

S: 0.030% or less The steel sheet for cans of the present invention has a high content of C and N and contains Nb which forms a precipitate that causes slab cracking. It is easy to break. In order to prevent this slab crack, the S content is set to 0.030% or less. Preferably, the S content is 0.020% or less. On the other hand, if the content of S is less than 0.005%, the cost of removing S becomes excessive, so the S content is preferably set to 0.005% or more.

Al: 0.001% or more and 0.100% or less Al is an element to be contained as a deoxidizing agent, and also forms N and AlN in steel to reduce solid solution N in steel. If Al is added excessively, the formation of AlN increases, and the amount of N that contributes to the strength of the steel sheet as solid solution N, which will be described later, decreases, and the strength of the steel sheet decreases. Therefore, the Al content is set to 0.100% or less. On the other hand, when the Al content is less than 0.001%, the effect as a deoxidizing agent becomes insufficient, which causes the occurrence of solidification defects and increases steelmaking costs. Therefore, the Al content is 0.001% or more. I do. In order to make Al sufficiently function as a deoxidizing agent and obtain the effect of increasing the strength by solid solution N, the Al content is preferably set to 0.010% or more and 0.060% or less.

N: more than 0.0120% and 0.0200% or less N is an element necessary for increasing the strength of a steel sheet by solid solution strengthening. In order to exert the effect of solid solution strengthening, the N content needs to be more than 0.0120%. On the other hand, if the N content is too large, slab cracking in the lower straightening zone where the slab temperature decreases in continuous casting is likely to occur. Further, the ratio of the Nb content in precipitates having a diameter of less than 20 nm to the Nb content in all precipitates is less than 40%, local deformation occurs, and uniform deformability is reduced. Therefore, the N content is set to 0.0200% or less. Preferably, it is 0.0130% or more and 0.0185% or less.

Nb: 0.0060% or more and 0.0300% or less Nb is an element having a high carbide-forming ability, and precipitates fine carbides. Thereby, the upper yield strength increases. In the present invention, the upper yield strength can be adjusted by the Nb content. Since this effect occurs when the Nb content is 0.0060% or more, the lower limit of the Nb content is set to 0.0060%. On the other hand, Nb raises the recrystallization temperature, and when the Nb content exceeds 0.0300%, a large amount of unrecrystallized structure remains in the annealing at a soaking temperature of 660 ° C. or more and 800 ° C. or less, which will be described later. Will be. When a large amount of unrecrystallized crystal remains, strain is unevenly applied to the steel sheet when the steel sheet is deformed, and the total elongation is reduced. Further, the ratio of the Nb content in precipitates having a diameter of less than 20 nm to the Nb content in all precipitates is less than 40%, local deformation occurs, and uniform deformability is reduced. For this reason, the upper limit of the Nb content is limited to 0.0300%. Preferably, it is 0.0080% or more and 0.0200% or less.

Cr: 0.040% or less Cr is an element that affects the composition of fine carbides and the average spacing between precipitates. That is, when the Cr content exceeds 0.040%, the ratio of the Nb content in precipitates having a diameter of less than 20 nm to the Nb content in all precipitates is less than 40%. In addition, the average interval of all the precipitates exceeds 30 nm, local deformation occurs, and uniform deformability decreases. In particular, the dimensional accuracy of the height of the curled portion of the can lid formed through a plurality of processes is significantly impaired. Therefore, the Cr content is set to 0.040% or less. Preferably it is 0.037% or less. In addition, since steelmaking cost becomes excessive to make Cr less than 0.001%, the content of Cr is preferably made 0.001% or more.
The balance other than the components described above contains Fe and inevitable impurities.

Next, the metal structure of the steel sheet for cans according to the present invention will be described. As the metallographic structure, it is important that the ratio of the Nb content in the precipitates having a diameter of less than 20 nm to the Nb content in all the precipitates is 40% or more and the average interval of all the precipitates is 30 nm or less. It is.
[Ratio of Nb content in precipitates having a diameter of less than 20 nm to Nb content in all precipitates: 40% or more]
The steel sheet for cans of the present invention is mainly composed of a ferrite structure, and the precipitate has a structure of Nb-based carbide. Regarding the Nb content of the precipitate, the ratio of the Nb content in the precipitate having a diameter of less than 20 nm to the Nb content in all the precipitates (hereinafter, also referred to as the Nb content fraction of the precipitate having a diameter of less than 20 nm) ) Is 40% or more.

If the Nb content fraction of the precipitate having a diameter of less than 20 nm is less than 40%, it becomes difficult to ensure uniform deformability or dimensional accuracy of the height of the curled portion of the can lid. Although the mechanism is not clear, when the Nb content fraction of the precipitates having a diameter of less than 20 nm is less than 40%, the precipitates having a large diameter increase, the local change in the strength of the steel sheet becomes large, and the dimension of the curl processing increases. It is presumed that the accuracy decreases. Therefore, the Nb content fraction of precipitates having a diameter of less than 20 nm is set to 40% or more. Preferably, it is at least 45%.
In addition, it is preferable that the Nb content fraction of the precipitate having a diameter of less than 20 nm is 70% or less. That is, if it is 70% or less, the effect is not saturated, excessive precipitation strengthening of the steel sheet is suppressed, and the total elongation is further improved.

Here, the Nb content in the precipitate having a diameter of less than 20 nm can be measured by the following method.
That is, after a sample is electrolyzed in a predetermined amount in an electrolytic solution, a sample piece is taken out from the electrolytic solution and immersed in a dispersible solution. Next, the precipitate contained in this solution is filtered using a filter having a pore size of 20 nm. The precipitate having passed through the filter having a pore size of 20 nm together with the filtrate is a precipitate having a diameter of less than 20 nm. Next, the amount of Nb in the residue on the filter after filtration and the filtrate is analyzed, and the content of Nb in the precipitate having a diameter of 20 nm or more and the precipitate having a diameter of less than 20 nm is determined. For the analysis of the Nb amount, an analysis method appropriately selected from inductively coupled plasma (ICP) emission spectrometry, ICP mass spectrometry, atomic absorption spectrometry and the like can be used. The amount of the precipitate having a diameter of 20 nm or more and the amount obtained by adding the both are defined as the total amount of the precipitates, and the ratio of the Nb content in the precipitates having a precipitate diameter of less than 20 nm to the Nb content in the total precipitates is calculated.

[Average spacing between all precipitates: 30 nm or less]
If the average distance between all the precipitates exceeds 30 nm, local deformation occurs in can lid processing to be described later, so that the height of the curled portion becomes uneven and the dimensional accuracy of the curled portion is reduced. Therefore, the average interval is set to 30 nm or less. Preferably, the thickness is 25 nm or less.

Here, the dimensional accuracy of the curled portion is evaluated as follows. First, a circular blank having a diameter of 67 mm is collected from a steel plate, and shell processing and curling processing are sequentially press-formed to produce a can lid. The height of the curled portion of the prepared can lid is measured by a height gauge at eight locations in the circumferential direction, and the standard deviation σH of the curled portion height is determined. If the σH is 0.07 mm or less, the dimensional accuracy of the curled portion is excellent. And
The mechanism by which the average spacing between all precipitates affects the dimensional accuracy of the curl height is not clear, but processing that involves the interaction between dislocations and precipitates by suppressing the average spacing between all precipitates It is assumed that the curing behavior is stabilized.

On the other hand, if the average interval is 10 nm or more, the effect is not saturated, excessive precipitation strengthening of the steel sheet is suppressed, and the ductility is further improved. Therefore, the average distance between all the precipitates is preferably 10 nm or more.

In measuring the average interval of all the precipitates, the precipitates are observed using a transmission electron microscope (TEM). The observation sample was produced by the extraction replica method after polishing the surface layer of the steel sheet by electrolytic polishing. The observation was performed with a bright-field image at an accelerating voltage of 200 kV and an observation magnification of 300,000, and three images were taken of each sample. The photographed image was subjected to image analysis with image analysis software (software “particle analysis” manufactured by Nippon Steel & Sumikin Technology Co., Ltd.) to determine the equivalent circle diameter and area ratio of the precipitate. The precipitate spacing d was determined by the following equation, using the circle equivalent diameter as the precipitate diameter and the area ratio as the precipitate volume integral ratio. Then, the average of the precipitate intervals d obtained for the three captured images was defined as the average interval of all the precipitates.

The steel sheet for cans having the above component composition and structure can have the following mechanical properties. It is customary that the steel sheet for cans is formed into a can shape and then subjected to paint baking to obtain a product can. The steel sheet for cans of the present invention is a heat treatment at 210 ° C. for 10 minutes corresponding to the paint baking treatment. Later mechanical properties satisfy the following requirements.
[Upper yield strength: 500 MPa or more and 640 MPa or less]
In order to ensure the dent strength of the welding can and the pressure resistance of the two-piece can, it is necessary that the upper yield strength is 500 MPa or more. On the other hand, in order to obtain an upper yield strength exceeding 640 MPa, a large amount of strengthening elements must be contained. If a large amount of reinforcing element is contained, the ductility may be reduced in addition to the possibility of inhibiting the corrosion resistance. Therefore, the upper yield strength is 640 MPa or less. Preferably, it is 520 MPa or more and 630 MPa or less.

[Total elongation: 10% or more]
The total elongation needs to be 10% or more. When the total elongation is less than 10%, for example, in the production of cans formed by can body processing such as bead processing or can expanding processing, there is a possibility that a defect such as cracking may occur. On the other hand, if the total elongation is less than 10%, cracks may occur during flange processing of the can. Therefore, the lower limit of the total elongation is set to 10%. It is preferably at least 11%. In addition, when the total elongation is 30% or less, the dimensional accuracy of the can body becomes higher, which is preferable.
In addition, the yield strength and the total elongation can be measured by a metal material tensile test method shown in “JIS Z2241”.

The desired yield strength and total elongation can be obtained by adjusting the component composition and adjusting the cooling rate in the continuous annealing step. In order to obtain a yield strength of 500 MPa or more, the above-mentioned component composition is used, and after the soaking in the continuous annealing step, primary cooling is performed in a temperature range of 600 ° C. or more at an average cooling rate of less than 10 ° C./s. Then, secondary cooling is performed at an average cooling rate of 10 ° C./s or more to a temperature range of 150 ° C. or less, and the rolling reduction in the secondary cold rolling step may be 3.0% or less.

The tensile test is performed in accordance with the metal material tensile test method specified in “JIS Z2241”. That is, a JIS No. 5 tensile test piece (JIS Z # 2201) whose tensile direction is perpendicular to the rolling direction is sampled and subjected to a coating baking treatment at 210 ° C. for 10 minutes. Thereafter, a 50 mm (L) mark is given to the parallel portion of the tensile test piece such that the center in the length direction of the parallel portion is a center point on a straight line connecting the mark, and a tensile test in accordance with JIS Z2241 is provided. At a tensile speed of 10 mm / min until breaking. When the fracture position after the completion of the tensile test is -1 / 2L to 1 / 2L with the center point of L as a zero point, it is regarded as having excellent uniform deformability (no local deformation occurs). The mechanism by which the ratio of the amount of Nb-containing precipitates having a precipitate diameter of less than 20 nm to the total amount of Nb-containing precipitates affects the uniform deformability is not clear, but controls the particle size distribution of precipitates contributing to the increase in strength of the steel sheet. It is inferred that the work hardening behavior involving the interaction between dislocations and precipitates is thereby stabilized.

In addition, it is preferable that the steel plate for cans of the present invention has a thickness of 0.4 mm or less.
At present, thinning of steel sheets is being promoted for the purpose of reducing can-making costs. However, as the thickness of the steel sheet is reduced, that is, the thickness of the steel sheet is reduced, there is a concern that the strength of the can body may be reduced. On the other hand, the steel sheet for cans of the present invention does not reduce the strength of the can body even when the sheet thickness is small. When the sheet thickness is small, the effect of the present invention of high ductility and high strength is remarkably exhibited. From this point, the plate thickness is preferably set to 0.4 mm or less. It may be 0.3 mm or less, or may be 0.2 mm or less.

Next, a method for producing the steel sheet for cans of the present invention will be described.
The steel sheet of the present invention is prepared by heating a steel material having the above-mentioned composition at 1200 ° C. or higher, hot rolling at a finishing temperature of 850 ° C. or higher, and a final stand rolling reduction of 8% or higher, at 640 ° C. A hot rolling step of winding in a temperature range of 780 ° C. or lower,
A primary cold rolling step of performing cold rolling with a reduction ratio of 86% or more after the hot rolling step;
After the primary cold rolling step, the temperature is soaked in the temperature range of 660 ° C. or more and 800 ° C. or less, and the primary cooling is performed to the temperature range of 600 ° C. or more and 650 ° C. or less at an average cooling rate of 3 ° C./s or more and less than 10 ° C./s. And a continuous annealing step of secondary cooling to a temperature range of 150 ° C. or less at an average cooling rate of 10 ° C./s or more,
A secondary cold rolling step of performing cold rolling with a rolling reduction of 0.1% or more and 3.0% or less;
Can be manufactured.

温度 In the following description, the temperature is defined based on the surface temperature of the steel sheet. The average cooling rate is a value obtained by calculating based on the surface temperature. For example, the average cooling rate from the soaking temperature to the temperature range of 600 ° C. or more is ((soaking temperature− (temperature range of 600 ° C. or more)) / cooling time from the soaking temperature to (temperature range of 600 ° C. or more). ).

First, as the above-mentioned steel material, a molten steel is adjusted to the above-mentioned chemical composition by a known method using a converter or the like, and then a slab obtained by, for example, a continuous casting method is used.

[Steel material heating temperature: 1200 ° C or more]
The heating temperature of the steel material in the hot rolling step is 1200 ° C. or higher. When the heating temperature is lower than 1200 ° C., the amount of dissolved N required for securing the strength in the present invention is reduced, and the strength is reduced. In the steel composition of the present invention, since N in steel is considered to exist mainly as AlN, the amount of N present as AlN (NasAlN) is subtracted from the total amount of N (Ntotal) to obtain (Ntotal- (NasAlN)). Consider the dissolved N amount. Then, in order to make the upper yield strength in the rolling direction 500 MPa or more, the amount of solute N is preferably 0.0121% or more. In order to ensure this solid solution N amount, the steel material heating temperature is set to 1200 ° C. or higher. The more preferable amount of solute N is 0.0130% or more, and for that purpose, the steel material heating temperature is preferably set to 1220 ° C. or more. Even if the heating temperature of the steel material exceeds 1350 ° C., the effect is saturated, so that it is preferably 1350 ° C. or less.

[Finishing temperature in hot rolling process: 850 ° C or higher]
When the finishing temperature in the hot rolling step is less than 850 ° C., the Nb content fraction of precipitates having a diameter of less than 20 nm becomes less than 40%, and local deformation occurs in a tensile test, so that the temperature is 850 ° C. or more. Preferably it is 855 ° C or higher. On the other hand, increasing the finishing temperature in the hot rolling step more than necessary may make the production of thin steel sheets difficult. For example, when the finishing temperature is high, the generation of scale on the surface of the steel sheet becomes remarkable, and the surface properties are impaired. Specifically, the finishing temperature is preferably 950 ° C. or less. More preferably, it is 945 ° C. or lower.

[Drop rate of final stand: 8% or more]
The rolling reduction of the final stand in the hot rolling step is 8% or more. When the rolling reduction of the final stand is less than 8%, the average distance between all the precipitates is more than 30 nm, the standard deviation of the height of the curled portion of the can lid is more than 0.07 mm, and the dimension of the curled portion height of the can lid Accuracy deteriorates. Therefore, the rolling reduction of the final stand is set to 8% or more. In order to reduce the standard deviation of the height of the curled portion of the can lid, the rolling reduction of the final stand is preferably 10% or more. The upper limit of the rolling reduction of the final stand is preferably 15% or less from the viewpoint of rolling load.

[Winding temperature: 640 ° C or higher and 780 ° C or lower]
When the winding temperature in the hot rolling step is lower than 640 ° C., the Nb content fraction of precipitates having a diameter of less than 20 nm becomes lower than 40% and local deformation occurs in a tensile test. Is 640 ° C. or higher. On the other hand, when the winding temperature is higher than 780 ° C., part of the ferrite of the steel sheet after continuous annealing is coarsened, the steel sheet is softened, and the upper yield strength is less than 500 MPa, so that the winding temperature is 780 ° C. or less. . Preferably it is 660 ° C or more and 760 ° C or less.

[Pickling]
Thereafter, it is preferable to perform pickling as needed. The pickling may be performed as long as the surface scale of the steel sheet can be removed, and there is no particular limitation on the conditions. The scale may be removed by a method other than pickling.

Next, cold rolling is performed by dividing into two times including annealing.
[Primary cold rolling reduction: 86% or more]
First, the rolling reduction in the primary cold rolling step is set to 86% or more. If the rolling reduction in the primary cold rolling step is less than 86%, the strain imparted to the steel sheet by the cold rolling decreases, so that it is difficult to set the upper yield strength of the steel sheet after continuous annealing to 500 MPa or more. Therefore, the rolling reduction in the primary cold rolling step is set to 86% or more. Preferably it is 87% or more and 94% or less.
In addition, before the primary cold rolling process after the hot rolling process, another process may be included as appropriate. Further, the primary cold rolling step may be performed immediately after the hot rolling step without performing the pickling.

(4) In the annealing step after the primary cold rolling, primary cooling is performed in which the temperature is soaked in a temperature range of 660 ° C. or more and 800 ° C. or less, and cooled to a temperature range of 600 ° C. or more at an average cooling rate of less than 10 ° C./s. Next, secondary cooling is performed to cool to a temperature range of 150 ° C. or less at an average cooling rate of 10 ° C./s or more.

[Soaking temperature: 660 ° C or higher and 800 ° C or lower]
The soaking heat treatment in the annealing step is performed at a temperature of 660 ° C. or more and 800 ° C. or less. When the soaking temperature is higher than 800 ° C., a trouble in passing a sheet such as a heat buckle easily occurs during annealing. Further, the ferrite grain size of the steel sheet is partially coarsened, the steel sheet is softened, and the upper yield strength becomes less than 500 MPa. If the annealing temperature is lower than 660 ° C., recrystallization of ferrite grains becomes incomplete, and unrecrystallized remains. When unrecrystallized remains, when the steel sheet is deformed, strain is unevenly applied to the steel sheet, local deformation occurs, and total elongation is reduced. Therefore, the soaking is performed at a temperature of 660 ° C. or more and 800 ° C. or less. Preferably, it is performed at a temperature of 680 ° C. or more and 760 ° C. or less.

If the holding time at a soaking temperature of 660 ° C. or more and 800 ° C. or less is 60 seconds or less, segregation of C contained in the steel sheet at the ferrite grain boundary is further suppressed, and carbon is precipitated as a carbide in the cooling process of the annealing process. Can be prevented. Therefore, the amount of solid solution C that contributes to the strength of the steel sheet can be maintained, and accordingly, the upper yield strength can be stably secured. Therefore, the holding time at the soaking temperature of 660 ° C. or more and 800 ° C. or less is preferably 60 seconds or less. If the holding time is 5 seconds or more, the soaking temperature becomes more stable when the steel sheet passes through the roll in the soaking zone. Therefore, the holding time is preferably 5 seconds or more.

[Primary cooling: cooling to a temperature range of 600 ° C to 650 ° C at an average cooling rate of 3 ° C / s or more and less than 10 ° C / s]
After the soaking, it is cooled to a temperature range of 600 ° C. or more and 650 ° C. or less at an average cooling rate of less than 10 ° C./s. When the average cooling rate is 10 ° C./s or more, precipitation of carbides is promoted during cooling, the amount of solute C contributing to the strength of the steel sheet is reduced, and the upper yield strength is reduced. On the other hand, when the average cooling rate is less than 3 ° C./s, the Nb content fraction of precipitates having a diameter of less than 20 nm becomes less than 40%, and the dimensional accuracy of the curled portion height of the can lid is reduced. The speed is 3 ° C./s or more. Further, if the cooling stop temperature in the primary cooling after soaking is less than 600 ° C., carbide precipitation is promoted after the primary cooling, the amount of solute C contributing to the strength of the steel sheet is reduced, and the upper yield strength is reduced. Therefore, the cooling stop temperature is set to 600 ° C. or higher. More preferably, the cooling stop temperature in the primary cooling after soaking is 620 ° C. or higher. When the cooling stop temperature in the primary cooling after soaking exceeds 650 ° C., the Nb content fraction of precipitates having a diameter of less than 20 nm becomes less than 40%, and the dimensional accuracy of the curl height of the can lid decreases. The cooling stop temperature is 650 ° C. or less.

[Secondary cooling: Cooling to a temperature range of 150 ° C or less at an average cooling rate of 10 ° C / s or more]
In the secondary cooling after the primary cooling, cooling is performed to a temperature range of 150 ° C. or less at an average cooling rate of 10 ° C./s or more. When the average cooling rate is less than 10 ° C./s, the Nb content fraction of the precipitate having a diameter of less than 20 nm becomes less than 40%, and local deformation occurs in a tensile test. It is preferably at least 12 ° C./s. On the other hand, if the average cooling rate exceeds 30 ° C./s, not only the obtained effect is saturated, but also excessive cost is generated in the cooling equipment, the average cooling rate in the secondary cooling is preferably 30 ° C./s or less. . More preferably, it is 25 ° C./s or less. In the secondary cooling, the temperature is cooled to 150 ° C. or less. If the temperature exceeds 150 ° C., the amount of solute C contributing to the strength of the steel sheet decreases, and the upper yield strength decreases. Preferably it is 145 ° C or lower. On the other hand, if the cooling stop temperature is lower than 100 ° C., not only the effect is saturated, but also excessive cost is generated in the cooling equipment, so that the temperature is preferably 100 ° C. or higher. More preferably, it is 120 ° C. or higher.
In addition, it is preferable to use a continuous annealing apparatus for annealing. Further, other steps may be included as appropriate before the annealing step after the primary cold rolling step, or the annealing step may be performed immediately after the primary cold rolling step.

[Secondary cold rolling reduction: 0.1% or more and 3.0% or less]
The steel sheet of the present invention is required to secure a total elongation of 10% or more with an extremely thin material. In the present invention, if the secondary cold rolling after annealing is performed at the same rolling reduction (20% or more) as the DR material manufacturing conditions that are usually performed, the strain introduced at the time of processing increases and the total elongation decreases. In addition, in the secondary cold rolling, since the work hardening of the steel sheet is introduced non-uniformly, if the rolling reduction is excessive, local deformation occurs when deforming the manufactured steel sheet, and sufficient uniform deformability is obtained. I can't get it. For these reasons, the rolling reduction in the secondary cold rolling is set to 3.0% or less. In order to increase the uniform deformability of the steel sheet, it is desirable that the secondary cold rolling reduction is low, and the rolling reduction in the secondary cold rolling is preferably less than 1.0%. On the other hand, the secondary cold rolling has a role of imparting the surface roughness of the steel sheet, and the rolling reduction of the secondary cold rolling needs to be 0.1% or more in order to uniformly impart the surface roughness to the steel sheet. is there. Preferably, it is 0.2% or more and less than 1.0%.

As described above, the steel sheet for a can of the present invention is obtained. In the present invention, various other steps can be performed after the secondary cold rolling step. For example, a plating layer may be further formed on the surface of the steel sheet for cans of the present invention. Examples of the plating layer include a Sn plating layer, a Cr plating layer such as tin-free, a Ni plating layer, and a Sn—Ni plating layer. Further, a process such as a paint baking treatment or a film lamination may be performed.
In addition, since the film thickness of the plating or the laminate film is sufficiently small with respect to the plate thickness, the influence on the mechanical properties of the steel plate for cans can be ignored.

A steel slab was obtained by melting a steel having the component composition shown in Table 1 and the balance consisting of Fe and inevitable impurities in a converter and continuously casting the steel. The steel slab obtained here was subjected to hot rolling at a steel material heating temperature, a finish rolling temperature, a final stand draft, and a winding temperature shown in Tables 2 and 3. After this hot rolling, pickling was performed. Next, primary cold rolling was performed at rolling reductions shown in Tables 2 and 3, continuous annealing was performed under continuous annealing conditions shown in Tables 2 and 3, and then secondary cold rolling was performed at rolling reductions shown in Tables 2 and 3. gave. The obtained steel sheet was subjected to normal Sn plating to obtain an Sn-plated steel sheet (tinplate).
After subjecting the steel sheet obtained as described above to a heat treatment corresponding to a paint baking treatment at 210 ° C. for 10 minutes, a tensile test was performed to measure the upper yield strength and the total elongation. In addition to examining corrosion resistance and precipitates, can lid processing was performed, and the curl height of the can lid was measured. The measurement method and investigation method are as follows.

The tensile test was carried out in accordance with the metal material tensile test method shown in “JIS Z2241”. That is, a JIS No. 5 tensile test piece (JIS Z2201) having a tensile direction perpendicular to the rolling direction was sampled from the Sn-plated steel sheet and subjected to a coating baking treatment at 210 ° C. for 10 minutes. Thereafter, a 50 mm (L) mark is applied to the parallel portion of the tensile test piece such that the center in the length direction of the parallel portion is the center point on a straight line connecting the mark, and a tensile test in accordance with JIS Z2241 is provided. The test was performed at a tensile speed of 10 mm / min until breaking. In the evaluation of the uniform deformability, those where the broken position is -1 / 2L to 1 / 2L with the center point of L as the zero point pass (〇), and those where the broken position is-／ L to 1 / 4L Was passed (◎), and those in which the broken position was outside the mark were rejected (×).

The ratio of the Nb content of the precipitates having a precipitate diameter of less than 20 nm to the Nb content in all the precipitates. About 0.2 g of the test piece was subjected to constant current electrolysis at a current density of 20 mA / cm ² in (10 vol% acetylacetone-1 mass% tetramethylammonium chloride-methanol). After electrolysis, a sample piece having a precipitate attached to its surface is taken out of the electrolytic solution, immersed in an aqueous sodium hexametaphosphate solution (500 mg / l, hereinafter referred to as an aqueous SHMP solution), and subjected to ultrasonic vibration to give a precipitate. Was peeled off from the sample and extracted into an aqueous SHMP solution. Next, the precipitate contained in this solution was filtered using a filter having a pore size of 20 nm. The residue on the filter after filtration and the filtrate were analyzed using inductively coupled plasma (ICP) emission spectroscopy, and the absolute amounts of Nb in the residue on the filter and in the filtrate were measured. The measured value for the residue on the filter indicates the amount of precipitate having a size of 20 nm or more, and the measured value for the filtrate indicates the amount of precipitate having a size of less than 20 nm. The ratio of the Nb content in precipitates having a precipitate diameter of less than 20 nm to the Nb content in all precipitates was calculated by taking the amount obtained by adding both as the total precipitate amount.

Corrosion resistance The amount of Sn plating on one side of the Sn steel plate was set to 11.2 g / m ^2, and the number of portions where the Sn plating was thinned and observed as holes was measured. Observation was performed with a measuring area of 2.7 mm ² with an optical microscope (× 50). The case where the number was 20 or less was evaluated as ○, and the case where the number was 21 or more was evaluated as ×.

Can lid processing The can lid was produced by collecting a circular blank having a diameter of 67 mm from the above-mentioned Sn-plated steel sheet, and sequentially processing shell processing and curling processing. The height of the curled portion of the prepared can lid was measured at eight locations in the circumferential direction using a height gauge, and the standard deviation σH of the curled portion height was determined. Those with σH of 0.07 mm or less were judged as acceptable (合格), and those with σH of more than 0.07 mm were judged as unacceptable (x).
Tables 2 and 3 show the evaluation results obtained as described above.

より From Tables 2 and 3, in the examples of the present invention, a highly ductile and high-strength steel sheet for cans having excellent uniform deformability was obtained. Furthermore, the dimensional accuracy of the corrosion resistance and the height of the curled portion of the can lid was also excellent.

According to the present invention, a steel sheet for cans having high strength, excellent ductility, and excellent uniform deformability can be obtained. Further, a steel sheet for cans having good corrosion resistance even for highly corrosive contents can be obtained. Therefore, the present invention is most suitable as a steel plate for cans mainly used for three-piece cans with can body processing with a high degree of processing, two-piece cans whose bottoms are processed by several percent, and can lids.

Claims

In mass%,
C: 0.020% to 0.130%,
Si: 0.04% or less,
Mn: 0.10% to 1.20%,
P: 0.007% or more and 0.100% or less,
S: 0.030% or less,
Al: 0.001% to 0.100%,
N: more than 0.0120% and not more than 0.0200%,
Nb: not less than 0.0060% and not more than 0.0300% and Cr: not more than 0.040%, the balance having a component composition of Fe and unavoidable impurities, and having a diameter of 20 nm in the Nb content in all precipitates. Is less than 40%, the average spacing of all the precipitates is 30 nm or less, the upper yield strength after heat treatment at 210 ° C. for 10 minutes is 500 MPa or more and 640 MPa or less, and the total elongation is Steel plate for cans that is 10% or more.
In mass%,
C: 0.020% to 0.130%,
Si: 0.04% or less,
Mn: 0.10% to 1.20%,
P: 0.007% or more and 0.100% or less,
S: 0.030% or less,
Al: 0.001% to 0.100%,
N: more than 0.0120% and not more than 0.0200%,
A steel material containing Nb: 0.0060% or more and 0.0300% or less and Cr: 0.040% or less and the balance being Fe and an unavoidable impurity is heated at 1200 ° C. or more, and the finishing temperature is increased. A hot rolling step of performing hot rolling at a temperature of 850 ° C. or more and a rolling reduction of the final stand of 8% or more, and winding in a temperature range of 640 ° C. or more and 780 ° C. or less;
A primary cold rolling step of performing cold rolling with a reduction ratio of 86% or more after the hot rolling step;
After the primary cold rolling step, the temperature is soaked in the temperature range of 660 ° C. or more and 800 ° C. or less, and the primary cooling is performed to the temperature range of 600 ° C. or more and 650 ° C. or less at an average cooling rate of 3 ° C./s or more and less than 10 ° C./s. And an annealing step of performing secondary cooling to a temperature range of 150 ° C. or less at an average cooling rate of 10 ° C./s or more,
A secondary cold rolling step of performing cold rolling with a rolling reduction of 0.1% or more and 3.0% or less;
A method for producing a steel sheet for cans having: