WO2012073914A1 - Steel sheet for can, and process for producing same - Google Patents

Abstract

The present invention provides a steel sheet for cans which has high strength and high processability, characterized by containing, in terms of mass%, 0.070-0.080%, excluding 0.080%, C, 0.003-0.10% Si, 0.51-0.60% Mn, etc. and by having, in a rolling-direction cross-section thereof, an average crystal-grain diameter of 5 µm or more and a crystal-grain elongation rate of 2.0 or less. The steel sheet is further characterized in that the hardness difference obtained by subtracting the average Vickers hardness of a cross-section ranging from the surface to a depth of 1/8 the sheet thickness from the average Vickers hardness of a cross-section ranging from a depth of 3/8 the sheet thickness to a depth of 4/8 the sheet thickness is 10 points or more and/or the hardness difference obtained by subtracting the maximum Vickers hardness of the cross-section ranging from the surface to a depth of 1/8 the sheet thickness from the maximum Vickers hardness of the cross-section ranging from a depth of 3/8 the sheet thickness to a depth of 4/8 the sheet thickness is 20 points or more. The steel sheet is furthermore characterized by having a tensile strength of 500 MPa or higher and an elongation at fracture of 10% or higher. Also provided is a process for producing the steel sheet. This steel sheet for cans, which has high strength and high processability, is suitable for use as a material for easy-to-open cans.

Description

Steel plate for can and manufacturing method thereof

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

Among steel plates used for beverage cans and food cans, steel plates called DR (Double Reduce) materials may be used for lids, bottoms, three-piece can bodies, drawn cans, and the like. The DR material that undergoes cold rolling (secondary cold rolling) again after annealing is easier to reduce the plate thickness than SR (Single Reduce) material that performs only temper rolling with a small rolling rate, By using a thin steel plate, the can manufacturing cost can be reduced. The DR material is a thin and hard steel plate because work hardening occurs by performing cold rolling after annealing, but the DR material is inferior in workability compared to the SR material because the DR material is poor in ductility.

Also, EOE (Easy Open End) is widely used as a lid for beverage cans and food cans. When manufacturing an EOE, it is necessary to form a rivet for attaching a tab by stretching and drawing, and the ductility of the material required for this processing corresponds to an elongation of about 10% in a tensile test.

In addition, since the body of a three-piece beverage can is molded into a cylindrical shape and then flanged at both ends in order to tighten the lid and bottom, the can body end also has an elongation of about 10%. Required.

On the other hand, a steel plate as a can-making material is required to have a strength corresponding to the plate thickness, and in the case of a DR material, a tensile strength of about 500 MPa or more is required to ensure the strength of the can by making it thin.

Conventionally used DR materials are difficult to achieve both the above ductility and strength, and SR materials have been used for EOE and beverage can bodies. However, at present, from the viewpoint of cost reduction, there is an increasing demand for applying DR materials to EOE and beverage can bodies. Furthermore, this material can also be used as a raw material for steel plates for cans such as 2-piece can bodies, DI (Drawn and Ironed) cans, DRD (Draw-Redraw) cans, aerosol cans and bottom ends.

In view of these, Patent Document 1 discloses a method for producing a steel sheet having a high r value and excellent flange workability by producing a DR material of low carbon steel at a primary cold rolling rate of 85% or less. Has been.

Patent Document 2 discloses a method of manufacturing a DR material that achieves both hardness and workability by performing nitriding in a low carbon steel annealing process.

Patent Document 3 discloses a steel slab containing C: 0.01 to 0.08%, Mn: 0.05 to 0.50%, Al: 0.01 to 0.15%, and below the Ar ₃ transformation point. A thin steel plate having a thickness of less than 0.21 mm obtained by performing hot finish rolling, followed by cold rolling, followed by recrystallization annealing by continuous annealing, and then skin-passing at a rolling reduction of 5 to 10%. The manufacturing method of the lid | cover for easy open cans which performs score processing from which the ratio of score remaining thickness / steel plate thickness becomes 0.4 or less is disclosed.

In Patent Document 4, C: 0.04 to 0.08%, Si: 0.03% or less, Mn: 0.05 to 0.50%, P: 0.02% or less, S: 0.02% Hereinafter, Al: 0.02 to 0.10%, N: 0.008 to 0.015%, the amount of (N total-N as AlN) in the steel sheet is 0.007% or more, and rolling When the total elongation value in the direction is represented by X and the average value is represented by Y, excellent flange workability equal to or better than that of batch-annealed DR steel sheet when the relationship of X ≧ 10% and Y ≧ −0.05X + 1.4 is satisfied. A continuous annealed DR steel sheet for welding cans and a manufacturing method thereof are disclosed.

JP 63-7336 A JP 2004-323905 A JP 62-96618 A JP 2007-177315 A

However, each of the above conventional techniques has the following problems.

In the manufacturing method described in Patent Document 1, since it is necessary to reduce the primary cold rolling rate, an extremely thin steel sheet cannot be manufactured due to the limitation of the finish thickness of hot rolling. When the finish thickness of hot rolling is reduced, the finish rolling temperature is lowered, and it is difficult to maintain the predetermined temperature.

In the manufacturing method described in Patent Document 2, since it is necessary to perform nitriding after recrystallization is completed, even when nitriding is performed in the continuous annealing process, cost increases such as a decrease in line speed and an increase in the length of the heating furnace are caused. Inevitable.

In the manufacturing methods described in Patent Document 3 and Patent Document 4, the amount of Mn is suppressed to a low value of 0.05 to 0.50 wt%, and it is not possible to cope with an increase in strength for securing a pressure resistance strength by thinning. .

The present invention has been made in view of such circumstances, and is applicable to lids, bottoms, three-piece can bodies and two-piece can bodies, DI cans, DRD cans, aerosol cans, bottom ends, and the like, and in particular, materials for EOE. An object of the present invention is to provide a steel plate for a high-strength and highly workable can that is suitable as a manufacturing method and a method for producing the same.

The gist of the present invention is as follows.

1st invention is the mass%, C: 0.070% or more and less than 0.080%, Si: 0.003% or more and 0.10% or less, Mn: 0.51% or more and 0.60% or less, P : 0.001% to 0.100%, S: 0.001% to 0.020%, Al: 0.005% to 0.100%, N: 0.010% or less, the balance Is composed of Fe and unavoidable impurities, and has an average crystal grain size of 5 μm or more and a crystal grain elongation of 2.0 or less in the cross section in the rolling direction, and from a depth of 3/8 of the plate thickness to 4 of the plate thickness. The difference in hardness obtained by subtracting the average Vickers hardness of the cross section from the surface to the depth of 1/8 of the plate thickness from the average Vickers hardness of the cross section up to a depth of / 8 is 10 points or more and / or Maximum Vickers hardness of cross section between 3/8 depth and 4/8 depth of plate thickness The hardness difference obtained by subtracting the maximum Vickers hardness of the cross section from the surface to the depth of 1/8 of the plate thickness is 20 points or more, the tensile strength is 500 MPa or more, and the elongation at break is 10% or more. It is a steel plate for cans with high strength and high workability.

The second invention relates to the crystal grain size from an average crystal grain size between the surface and a depth of 1/8 of the plate thickness, from a depth of 3/8 of the plate thickness to a depth of 4/8 of the plate thickness. The average crystal grain size difference obtained by subtracting the average crystal grain size is 1 μm or more, and the high strength and high workability steel sheet for cans according to the first invention.

The third invention relates to the nitrogen amount from an average N amount between a depth of 3/8 of the plate thickness to a depth of 4/8 of the plate thickness, to a depth of 1/8 of the plate thickness from the surface. The difference in average N amount obtained by subtracting the average N amount between them is 10 ppm or more, and the steel sheet for high strength and high workability cans according to the first invention or the second invention.

The fourth invention relates to a nitride having a diameter of 1 μm or less and 0.02 μm or more, and has a depth of 1/4 of the plate thickness from the surface rather than the average nitride number density from the surface to a depth of 1/8 of the plate thickness. The steel sheet for a high-strength and high-workability can according to any one of the first to third inventions, wherein the average nitride number density is high.

The fifth invention relates to the nitride having a diameter of 1 μm or less and 0.02 μm or more, and the average nitride number density from the surface to a depth of 1/20 of the plate thickness is set to a depth of 1/4 of the plate thickness from the surface. The steel sheet for a high-strength and high-workability can according to any one of the first to fourth inventions, wherein a value divided by an average nitride number density is less than 1.5.

A sixth invention is a steel plate for a high-strength, high-workability can according to any one of the first to fifth inventions, wherein the amount of solid solution C in the steel is 51 ppm or more with respect to the carbon content. is there.

7th invention is mass%, C: 0.070% or more and less than 0.080%, Si: 0.003% or more and 0.10% or less, Mn: 0.51% or more and 0.60% or less, P : 0.001% to 0.100%, S: 0.001% to 0.020%, Al: 0.005% to 0.100%, N: 0.010% or less, the balance Is a steel made of Fe and unavoidable impurities made into a slab by continuous casting, and after hot rolling, it is wound at a temperature of less than 620 ° C., and then primary cold rolling at a primary cold rolling rate of 86% or more in total. Rolling with a cold rolling reduction rate of 30% or more in the final stand of rolling is performed, followed by annealing in an atmosphere where ammonia gas is less than 0.020 vol%, and then performing secondary cold rolling at a rolling rate of 20% or less. High strength and high workability steel for cans It is a method of manufacture.

According to the present invention, it is possible to obtain a high-strength and highly workable steel plate for cans having a tensile strength of 500 MPa or more and a breaking elongation of 10% or more. As a result, by improving the workability of the steel sheet, cracks do not occur during EOE rivet processing or flange processing of a three-piece can, making it possible to make cans with a thin DR material, greatly reducing the thickness of the steel sheet for cans. Achieved.

The inventors have conducted intensive research to solve the above problems and obtained the following knowledge.

In order to ensure ductility with a high-strength material, an appropriate amount of C is added to give strength, while the rolling rate of the final stand of primary cold rolling is improved and strain is introduced into the surface layer, and then annealing is performed. In order to coarsen the ferrite grains of the surface layer and suppress nitridation of the surface layer, the ammonia gas in the annealing atmosphere is suppressed to less than 0.020 vol%, and the secondary cold rolling rate is limited to an appropriate range, It is possible to achieve both strength and ductility by softening the surface layer of the steel sheet.

Also, if the coiling temperature after hot rolling is high, the cementite that precipitates becomes coarse and the local elongation decreases, so it is necessary to limit the coiling temperature to an appropriate temperature range.

In addition, in this specification,% which shows the component of steel is mass% altogether.
Further, the depth of 3/8 of the plate thickness indicates a position that is separated from the surface by a distance of 3/8 of the plate thickness in the center direction of the plate thickness. The same applies to the depth of 4/8 of the plate thickness, the depth of 1/8 of the plate thickness, the depth of 1/4 of the plate thickness, and the depth of 1/20 of the plate thickness.

Hereinafter, the present invention will be described in detail.
The steel plate for cans of the present invention is a steel plate for cans having a high strength and high workability having a tensile strength of 500 MPa or more and a breaking elongation of 10% or more. And such a steel plate uses the steel containing 0.070% or more and less than 0.080% C, and sets the coiling temperature after hot rolling and the secondary cold rolling rate to appropriate conditions. This makes it possible to manufacture.

The component composition of the steel plate for cans of the present invention will be described.

C: 0.070% or more and less than 0.080% In the steel plate for cans of the present invention, the secondary cold rolling rate is suppressed to ensure elongation, while high C content is exhibited by increasing the amount of C. If the C content is less than 0.070%, a tensile strength of 500 MPa necessary for obtaining a remarkable economic effect due to the thinning of the steel sheet cannot be obtained. Therefore, the C content is 0.070% or more. On the other hand, if the amount of C is 0.080% or more, it becomes excessively hard, and it becomes impossible to produce a thin steel plate by secondary cold rolling while ensuring workability. Therefore, the upper limit of the C amount is less than 0.080%.

Si: 0.003% or more and 0.10% or less If the amount of Si exceeds 0.10%, problems such as deterioration of surface treatment property and deterioration of corrosion resistance are caused, so the upper limit is made 0.10%. On the other hand, if it is less than 0.003%, the refining cost becomes excessive, so the lower limit is made 0.003%.

Mn: 0.51% or more and 0.60% or less Mn is an element necessary to prevent red heat embrittlement during hot rolling by S and to refine crystal grains and to secure a desirable material. . Further, in order to satisfy the can strength with the thinned material, it is necessary to increase the strength of the material. In order to cope with this increase in strength, it is necessary to add 0.51% or more of Mn. On the other hand, if Mn is added in a large amount, the corrosion resistance deteriorates and the steel plate becomes excessively hardened, so the upper limit is made 0.60%.

P: 0.001% or more and 0.100% or less P is a harmful element that hardens steel and deteriorates workability and at the same time deteriorates corrosion resistance. Therefore, the upper limit is made 0.100%. On the other hand, in order to make P less than 0.001%, the dephosphorization cost becomes excessive. Therefore, the lower limit is made 0.001%.

S: 0.001% or more and 0.020% or less S is a harmful element that exists as an inclusion in steel and causes deterioration in ductility and deterioration in corrosion resistance. Therefore, the upper limit is made 0.020%. On the other hand, desulfurization cost becomes excessive to make S less than 0.001%. Therefore, the lower limit is made 0.001%.

Al: 0.005% or more and 0.100% or less Al is an element necessary as a deoxidizer during steelmaking. When the addition amount is small, deoxidation becomes insufficient, inclusions increase, and workability deteriorates. If the content is 0.005% or more, it can be considered that deoxidation is sufficiently performed. On the other hand, when the content exceeds 0.100%, the frequency of occurrence of surface defects due to alumina clusters and the like increases. Therefore, the Al content is 0.005% or more and 0.100% or less.

N: 0.010% or less When N is added in a large amount, the hot ductility deteriorates and cracks of the slab occur in continuous casting. Therefore, the upper limit is made 0.010%. In addition, since the refining cost is excessive to make the N amount less than 0.001%, the N amount is preferably set to 0.001% or more.
The balance is Fe and inevitable impurities.

Next, the mechanical properties of the steel plate for cans of the present invention will be described.

The tensile strength is 500 MPa or more. If the tensile strength is less than 500 MPa, the steel plate cannot be made thin enough to obtain a remarkable economic effect in order to secure the strength of the steel plate as a can-making material. Therefore, the tensile strength is 500 MPa or more.

The elongation at break is 10% or more. If the elongation at break is less than 10%, cracking occurs during rivet processing when applied to EOE. Moreover, even when applied to a three-piece can body, cracking occurs during flange processing. Accordingly, the elongation at break is 10% or more.

In addition, the said tensile strength and the said breaking elongation can be measured by the metallic material tension test method shown by "JISZ2241".

Next, the crystal grains of the steel plate for cans of the present invention will be described.

The average grain size in the cross section in the rolling direction is 5 μm or more. The final mechanical properties of the steel plate for cans of the present invention are greatly influenced by the state of crystal grains. When the average crystal grain size in the cross section in the rolling direction is less than 5 μm, the steel sheet is insufficiently stretched and the workability is impaired.

Further, the elongation of the crystal grains in the cross section in the rolling direction is set to 2.0 or less. The degree of extension is a value that represents the degree to which ferrite crystal grains are extended by processing, as shown in “JISG0202.” When the elongation of the crystal grains in the cross section in the rolling direction exceeds 2.0, the elongation in the direction perpendicular to the rolling, which is important for flange workability and neck workability, is insufficient. Although the elongation increases with the rolling ratio of the secondary cold rolling, in order to suppress the above-described elongation at the secondary cold rolling ratio of up to about 20%, the steel should have a C content of 0.070% or more. Need to contain. That is, when C is less than 0.070%, the number of cementite grains precipitated after hot rolling decreases, and as a result, a large amount of solid solution C remains. Since solute C suppresses the grain growth during annealing, the shape of the crystal grains flattened by the primary cold rolling remains, and the degree of elongation increases.
The average crystal grain size in the rolling direction section and the degree of elongation of the crystal grains in the rolling direction section can be measured by a crystal grain size microscopic test method described in “JIS G 0551”.

Note that if there is no annotation, the front and back surfaces of the steel sheet are not distinguished.

Vickers hardness can be measured by the hardness test method shown in “JIS Z 2244”. A Vickers hardness test at a load of 10 gf is performed so that the hardness distribution in the plate thickness direction in the cross section of the steel plate can be appropriately evaluated. The measurement is performed at 10 locations, and the average value of the measured values is taken as the average cross-sectional hardness of each. Moreover, let the largest thing of Vickers hardness measurements be cross-section Vickers maximum hardness.

Hardness difference: 10 points or more, 20 points or more The strength increases when the surface layer is hardened, but since the soft central layer is sandwiched between the hard surface layers, the entire plate is constrained and the elongation decreases, and the necking is reduced. It tends to occur and processability decreases. When the surface layer is soft and the central layer is hard, only the central layer of the plate is constrained, so that the strength is high, and a high-strength and highly workable steel plate that does not cause reduction in elongation and constriction is obtained. When the difference in cross-sectional average hardness is less than 10 points and / or the maximum cross-section hardness is less than 20 points, the entire plate is homogeneous, so there is no difference from the current material, and a high strength and high workability steel sheet is obtained. I can't do that. When the difference in cross-sectional average hardness is 10 points or more and / or the maximum cross-section hardness is 20 points or more, the tensile strength can be 500 MPa or more and the elongation at break can be 10% or more.

Average crystal grain size difference: 1 μm or more In the steel sheet for high-strength workable cans of the present invention, the above-mentioned crystal grain size is determined from the average crystal grain size between the surface and a depth of 1/8 of the plate thickness. It is preferable that an average crystal grain size difference obtained by subtracting an average crystal grain size from a depth of 3/8 to a depth of 4/8 of the plate thickness is 1 μm or more. This is because a steel sheet having both excellent strength and ductility properties can be obtained when the average crystal grain size difference is 1 μm or more. This is because the crystal grain size is 1 / 8th of the plate thickness and softens due to the large crystal grain size, so that the elongation is improved, and the crystal grains having a depth of 3 / 8th of the plate thickness to 4 / 8th of the plate thickness. Since the diameter is small and it is hard and has high strength, high strength and ductility are compatible, and the tensile strength can be easily set to 500 MPa or more and the breaking elongation can be set to 10% or more.

The average amount of N between the depth of 3/8 of the plate thickness and the depth of 4/8 of the plate thickness is the amount of N using the combustion method for the samples subjected to the electropolishing to the depth of 3/8 of the plate thickness. It was measured. The average N amount from the surface to the depth of 1/8 of the plate thickness is obtained by tape-sealing one side of the sample and then chemically polishing from the surface to the depth of 1/8 of the plate thickness using oxalic acid. The amount of N was measured using a combustion method.

Average N amount difference: 10 ppm or more If the average N amount difference is less than 10 ppm, since the entire plate has a uniform N amount, large softening due to a decrease in the N amount of the surface layer cannot be expected, but the difference in the average N amount is By setting the surface layer to 10 ppm or more, the surface layer is softened by a small amount of solid solution N that contributes to solid solution strengthening because the amount of N is low, and the center layer is hard because the amount of N is high and hard, so both high strength and ductility are achieved. The tensile strength can easily be 500 MPa or more and the elongation at break can be 10% or more. Therefore, it is easy to obtain a high-strength and highly workable steel plate.

The number density of nitride is chemically polished to a predetermined position with oxalic acid, etc., electrolyzed with 10 μm using the SPEED method, an extraction replica is prepared, and the number of nitride per unit field of 1 μm square is measured using TEM. did. The nitride was identified by analysis using EDX.

Further, regarding nitrides having a diameter of 1 μm or less and 0.02 μm or more, the average nitride number density between the surface and the depth of 1/8 of the plate thickness is more than the depth between the surface and the depth of 1/4 of the plate thickness. A higher average nitride number density is preferred. This is because if the average nitride number density from the surface to the depth of 1/8 of the plate thickness is small, the number of fine precipitates is small, so that the softening occurs and the surface to the depth of 1/4 of the plate thickness. Large average nitride number density, because there are many fine precipitates, it is hard for precipitation strengthening, and it has both high strength and ductility, and can easily have a tensile strength of 500 MPa or more and an elongation at break of 10% or more. It is.

About Average Nitride Number Density Ratio: 1.5 or Less When the average nitride number density ratio is 1.5 or more, the nitride number density of the surface layer increases and precipitation strengthening due to nitride occurs, so it cannot be expected to be soft. However, by making the average nitride number density ratio smaller than 1.5, the tensile strength can be easily made 500 MPa or more and the breaking elongation can be made 10% or more. Therefore, it is easy to obtain a high-strength and highly workable steel plate.

The amount of solid solution C was calculated from the peak of internal friction. The internal friction is measured using a torsional vibration type internal friction measuring device manufactured by Vibran at a test piece shape of 1 mm × 1 mm × 80 mm, a measurement frequency of 0.001 to 10 Hz, and a temperature of 0 ° C. After removing the background, the peak value Q-1 was read and calculated from the calibration curve of Q-1 and the amount of dissolved C. If the amount of solute C in the steel is large, the strength is increased due to strengthening by the solute C, and the elongation is improved because the amount of carbide that becomes the starting point of fracture decreases.

Next, the manufacturing method of the steel plate for cans of this invention is demonstrated.
The steel sheet for a high-strength, high-workability can of the present invention uses a steel slab having the above composition produced by continuous casting, and after being hot-rolled, wound at a temperature of less than 620 ° C., and then 86% or more At the primary cold rolling rate, rolling is performed with a cold rolling rate of 30% or more in the final stand of the primary cold rolling, followed by annealing in an atmosphere where the ammonia gas is less than 0.020 vol%, and then 20% or less. It is created by performing secondary cold rolling at a rolling rate of.

Normally, it is difficult to achieve a thin plate thickness that can provide a remarkable economic effect by only one cold rolling. That is, in order to obtain a thin plate thickness by one cold rolling, the load on the rolling mill is excessive, and it is impossible depending on the equipment capacity. For example, when the final plate thickness is 0.15 mm, the primary cold rolling rate as large as 92.5% is required when the plate thickness after hot rolling is 2.0 mm.

In order to reduce the sheet thickness after cold rolling, it is conceivable that rolling is performed thinner than usual in the hot rolling stage, but if the rolling rate of hot rolling is increased, the temperature of the steel sheet during rolling is decreased. A predetermined finish rolling temperature cannot be obtained. Furthermore, if the plate thickness before annealing is reduced, when continuous annealing is performed, the possibility of troubles such as breakage and deformation of the steel plate during annealing increases. For these reasons, in the present invention, the second cold rolling is performed after annealing to obtain an extremely thin steel plate.

Winding temperature after hot rolling: less than 620 ° C. If the winding temperature after hot rolling is 620 ° C. or more, the pearlite structure to be formed becomes coarse, and this becomes the starting point of brittle fracture, so the local elongation decreases. Thus, the elongation at break of 10% or more cannot be obtained. Therefore, the coiling temperature after hot rolling is less than 620 ° C. More preferably, it is 560 ° C to 620 ° C.

Primary cold rolling rate: 86% or more When the primary cold rolling rate is small, it is necessary to increase the rolling rate of hot rolling and secondary cold rolling in order to finally obtain a very thin steel plate. Increasing the hot rolling rate is not preferable for the above-described reason, and the secondary cold rolling rate needs to be limited for the reason described later. For the above reasons, when the primary cold rolling rate is less than 86%, the production becomes difficult. Therefore, the primary cold rolling rate is 86% or more. More preferably, it is 90 to 92%.

Final stand rolling ratio of primary cold rolling: 30% or more In order to soften the surface layer of the steel sheet as coarse grains, increase the rolling rate of the final stand and introduce strain into the steel sheet surface layer, thereby annealing ferrite during annealing. It is necessary to promote grain growth. In order to increase the grain size of the surface layer by 1 μm compared to the center layer, it is necessary to set the final stand rolling ratio of primary cold rolling to 30% or more.

Annealing requires that the concentration of ammonia gas in the atmosphere be less than 0.020 vol% in order to suppress nitridation of the surface layer. Preferably it is 0.018 vol% or less, More preferably, it is 0.016 vol% or less. Moreover, it is necessary to complete recrystallization by annealing. The soaking temperature is preferably 600 to 750 ° C. from the viewpoint of operation efficiency and prevention of breakage during annealing of the thin steel sheet.

Secondary cold rolling rate: 20% or less The secondary cold rolling rate is 20% or less. If the secondary cold rolling rate exceeds 20%, work hardening by secondary cold rolling becomes excessive, and a breaking elongation of 10% or more cannot be obtained. Therefore, the secondary cold rolling rate is 20% or less. Preferably it is 15% or less, More preferably, it is 10% or less.

After the second cold rolling, the plating and other processes are performed as usual, and finished as a steel plate for cans.

A steel containing the component composition shown in Table 1 and the balance being Fe and inevitable impurities was made as a trial, and a steel slab was obtained by casting. After the obtained steel slab was reheated at 1250 ° C., hot rolling and primary cold rolling were performed under the conditions shown in Table 2. The finish rolling temperature of hot rolling is 890 ° C., and pickling is performed after rolling. Subsequently, after the primary cold rolling, the secondary cold rolling was performed under the conditions shown in Table 2 and continuous annealing at a soaking temperature of 630 ° C. and a soaking time of 25 seconds.
The steel plate obtained as described above was continuously subjected to Sn plating on both sides to obtain a tin plate having a single-side Sn adhesion amount of 2.8 g / m ² . The test results are shown in Tables 2 and 3. In Table 3, the crystal grain size, N content, and nitride number density mean the average crystal grain size, average N content, and average nitride number density, respectively.

The plated steel sheet (cover) obtained as described above was subjected to a heat treatment equivalent to baking at 210 ° C. for 10 minutes, and then subjected to a tensile test. In the tensile test, tensile strength (breaking strength) and elongation at break were measured at a tensile speed of 10 mm / min using a JIS No. 5 size tensile test piece.
Moreover, the sample of the plated steel plate was extract | collected and the average crystal grain diameter and the expansion degree of the crystal grain in the cross section of a rolling direction were measured. The average crystal grain size and the degree of crystal grain elongation in the cross section in the rolling direction are determined by grinding the vertical cross section of the steel sheet and revealing the grain boundary by night etching, and then cutting with the straight test line described in “JISG 0551” Measured by the method.

The pressure strength is measured by forming a sample with a thickness of 0.21 mm into a 63 mmΦ lid, then winding it around a 63 mmΦ weld can body, introducing compressed air into the can, and the pressure when the can lid is deformed. Was measured. The case where the can lid did not deform even when the internal pressure was 0.20 MPa, the can lid did not deform even when the internal pressure was increased to 0.19 MPa, and the can lid was deformed when the internal pressure was 0.20 MPa. When the can lid was deformed at ◯, 0.19 MPa or less, it was rated as x.

The moldability was tested by a method specified in JIS Z 2247 using a tester specified in JIS B 7729.
The Erichsen value (molding height at the time of the occurrence of through cracking) is 6.5 mm or more, 未 満, less than 6.5 mm, 6.0 mm or more is ◯, and less than 6.0 mm is x.

Tables 1 to 3 show examples of invention numbers. 6 ~ No. Nos. 12 and 18 are excellent in strength, and have achieved a tensile strength of 500 MPa or more necessary for an extremely thin steel plate for cans. Moreover, it is excellent in workability and has an elongation of 10% or more necessary for processing of a lid or a three-piece can body.

On the other hand, the comparative example No. No. 1 has insufficient tensile strength because the C content is too small. Moreover, No. of the comparative example. In No. 2, since the C content is too large, ductility is impaired by secondary cold rolling, and the elongation at break (denoted as “total elongation” in Table 2) is insufficient. Comparative Example No. Since No. 3 has too little Mn content, the tensile strength is insufficient. Comparative Example No. Since No. 4 has too much Mn content, ductility is impaired by secondary cold rolling and the elongation at break is insufficient. Moreover, No. of the comparative example. Since No. 5 has too much N content, ductility is impaired by secondary cold rolling and elongation at break is insufficient.

No of comparison example. In No. 13, since the coiling temperature is too high, the crystal grains become coarse and the strength is insufficient. Comparative Example No. No. 14, since the secondary cold rolling ratio of the final stand is too small, the average crystal grain size is large, the average crystal grain size of the central layer is large, and the strength is insufficient. Comparative Example No. No. 15, because the secondary cold rolling rate is too large, the ductility is impaired by secondary cold rolling, and the elongation at break is insufficient. Comparative Example No. 16, no. In No. 17, since the concentration of ammonia gas in the annealing atmosphere is too high, the surface layer is hardened so that the ductility is impaired and the elongation at break is insufficient.

According to the present invention, it is possible to obtain a steel plate for a can having a high strength such as a tensile strength of 500 MPa or more and a breaking elongation of 10% or more and high workability. As a result, due to the improvement of workability of the steel sheet, cracks do not occur even during rivet processing of EOE or flange processing of a three-piece can. Therefore, it is possible to make cans using a DR material having a thin plate thickness, and a significant reduction in the thickness of the steel plate for cans can be achieved.

Claims (7)

1. In mass%, C: 0.070% or more and less than 0.080%, Si: 0.003% or more and 0.10% or less, Mn: 0.51% or more and 0.60% or less, P: 0.001% or more 0.100% or less, S: 0.001% or more and 0.020% or less, Al: 0.005% or more and 0.100% or less, N: 0.010% or less, the balance being Fe and inevitable impurities In the cross section in the rolling direction, the average crystal grain size is 5 μm or more and the degree of elongation of the crystal grains is 2.0 or less, from a depth of 3/8 of the plate thickness to a depth of 4/8 of the plate thickness. The difference in hardness obtained by subtracting the average Vickers hardness of the cross section between the surface and the depth of 1/8 of the plate thickness from the average Vickers hardness of the cross section between 10 points and / or the depth of 3/8 of the plate thickness From the maximum Vickers hardness of the cross-section between 1 to 4/8 of the plate thickness, from the surface High strength and high workability characterized by a hardness difference of 20 points or more, a tensile strength of 500 MPa or more, and a breaking elongation of 10% or more, which is obtained by subtracting the maximum Vickers hardness of the cross section between 1 / 8th of the thickness. Steel plate for cans.
2. Regarding the crystal grain size, the average crystal grain size from the average crystal grain size between the surface and the depth of 1/8 of the plate thickness to the depth of 3/8 of the plate thickness to the depth of 4/8 of the plate thickness. The steel sheet for a high-strength, high-workability can according to claim 1, wherein a difference in average crystal grain size minus the grain size is 1 μm or more.
3. Regarding the nitrogen amount, the average N amount between the depth of 3/8 of the plate thickness and the depth of 4/8 of the plate thickness to the average N amount of 1/8 of the plate thickness from the surface. The steel sheet for a high-strength, high-workability can according to claim 1 or 2, wherein the drawn average N amount difference is 10 ppm or more.
4. For nitrides having a diameter of 1 μm or less and 0.02 μm or more, the average nitridation between the surface and the depth of 1/4 of the plate thickness is greater than the average nitride number density between the surface and the depth of 1/8 of the plate thickness The steel sheet for a high-strength, high-workability can according to any one of claims 1 to 3, wherein the object number density is large.
5. With respect to nitrides having a diameter of 1 μm or less and 0.02 μm or more, the average nitride number density from the surface to a depth of 1/20 of the plate thickness is defined as the average nitride number density from the surface to a depth of 1/4 of the plate thickness. The steel sheet for a high-strength and high-workability can according to any one of claims 1 to 4, wherein a value divided by the number density is smaller than 1.5.
6. The steel sheet for a high-strength, high-workability can according to any one of claims 1 to 5, wherein the amount of solid solution C in the steel is 51 ppm or more with respect to the carbon content.
7. In mass%, C: 0.070% or more and less than 0.080%, Si: 0.003% or more and 0.10% or less, Mn: 0.51% or more and 0.60% or less, P: 0.001% or more 0.100% or less, S: 0.001% or more and 0.020% or less, Al: 0.005% or more and 0.100% or less, N: 0.010% or less, the balance being Fe and inevitable impurities The steel comprising slab is made into a slab by continuous casting, and after hot rolling, it is wound at a temperature of less than 620 ° C., and then the primary cold rolling rate of 86% or more in total is used to cool the final stand of the primary cold rolling. The rolling is performed at a rolling rate of 30% or more, followed by annealing in an atmosphere where ammonia gas is less than 0.020 vol%, and then performing secondary cold rolling at a rolling rate of 20% or less. A method for producing steel sheets for high strength and high workability cans.