KR20170029635A - Steel sheet for cans and method for producing same - Google Patents

Abstract

Provided is a steel sheet for high-strength, high-strength can having excellent properties such as an upper yield strength of 450 to 600 MPa and a pre-elongation of 13% or more after coating baking, and a corrosion resistance even for corrosive contents, and a production method thereof.
C: not more than 0.030%, Si: not more than 0.04%, Mn: 0.10 to 1.20%, P: not more than 0.100%, S: not more than 0.030%, Al: not more than 0.10% %, Nb: 0.004 to 0.040%, and the balance of iron and inevitable impurities, wherein the ratio of the amount of precipitated Nb to the total amount of Nb, the amount of precipitated Nb / the total amount of Nb ≧ 0.30, Wherein the steel has a grain size of 20 nm or less, an average grain size of ferrite of 7.0 mu m or less, an upper yield strength after coating baking treatment of 450 to 630 MPa, and a total elongation of 13% or more.

Description

STEEL SHEET FOR CANS AND METHOD FOR PRODUCING SAME

The present invention relates to a can steel plate used as a material such as a three-piece can formed by machining a can body at a high cost and a two-piece can requiring a high-strength steel, and a method of manufacturing the same. More specifically, the present invention relates to a steel sheet for can having a large overall elongation and excellent superior yield strength and a method for producing the same.

In recent years, in order to increase the demand of the steel can, a method of reducing the manufacturing cost of the can, and a method of injecting the steel can into a new can type such as a bottle can or a deformed can.

As a book for reducing the can manufacturing cost, the cost of the material is reduced. Thinning of a steel sheet to be used is progressing not only in a two-piece can formed by drawing processing, but also in a three-piece can, which is mainly a simple cylindrical molding.

However, if the steel sheet is simply thinned, the strength of the can body is lowered. Therefore, it is not possible to use a steel plate which is merely thinned at a point where a high strength material such as a DRD (drawing and redrawing) can or a can body of a welding can is used. Therefore, there is a demand for a can steel plate having a high strength and a very thin thickness.

At present, a very thin, hard can steel plate is manufactured by a double reduction method (hereinafter referred to as DR method) in which a secondary reduction step is carried out with a reduction rate of 20% or more after annealing. The steel sheet produced by the DR method has a high strength, but is characterized by a small total elongation.

On the other hand, it is difficult from the viewpoint of workability to use a DR material that is poor in ductility as a material of a can which is formed by can body processing with a strong degree of processing, such as a mold releasing can currently being put into the market. In addition, the DR material has a higher manufacturing cost than conventional steel sheets after annealing because the manufacturing process is increased as compared with the steel sheet subjected to temper rolling.

In order to avoid the drawbacks of such a DR material, a single reduction method (SR method) in which the secondary cold rolling is omitted and various strengthening methods are used and the characteristics are controlled in the primary cold rolling and annealing steps, or the secondary cold rolling reduction A method for producing a high strength steel sheet under a light pressing of about 5% or less has been proposed in the following patent documents.

Patent Document 1 discloses a steel sheet which comprises 0.02% or less C, 0.10% or less Si, 1.5% or less Mn, 0.20% or less P, 0.01% or less S, 0.01% or less Al, (BH): 100%, and further contains a composition containing at least 0.0050% of (solid C + solid N) and the balance of Fe and inevitable impurities and a structure having a recrystallization ratio of at least 90% A thin cold-rolled steel sheet for a high strength can with a thickness of 0.3 mm or less has been disclosed, which has a tensile strength increase? TS of 30 MPa or more by coating baking treatment, and a yield stress after coating / baking treatment of 550 MPa or more have. In Patent Document 1, hot rolling and cooling conditions are adjusted to quench to a low temperature region after continuous annealing, effectively utilizing the action of solute C amount + solids N amount, and by using age hardening phenomenon, A technique of obtaining a high strength can steel plate has been proposed. The can steel strip described in Patent Document 1 has a yield stress as high as 550 MPa or more after the coating baking treatment.

Patent Document 2 discloses a ferritic stainless steel which contains 0.020 to 0.150% of C, 0.05% or less of Si, 1.00% or less of Mn, 0.050% or less of P, 0.010% or less of S, 0.0100% or less of N and 0.100% , And Nb: 0.005 to 0.025%, the balance being inevitable impurities and iron and having a substantially ferrite single phase structure, a yield strength of 40 kgf / mm 2 or more, an average crystal grain size of 10 μm or less, And has a thickness of 0.300 mm or less, which has excellent deep drawability and flange processability at the time of manufacturing a can and surface properties after can manufacture, and has sufficient can strength. Patent Document 2 proposes a steel sheet having a balance of strength and ductility by combining precipitation strengthening by Nb carbide and refinement strengthening by Nb, Ti and B carbonitride.

Patent Document 3 discloses a steel sheet comprising 0.001 to 0.010 wt% of C, 0.05 wt% of Si, 0.9 wt% of Mn, 0.131 to 0.200 wt% of P, 0.04 wt% of S, 0.006 to 0.08 wt% of Al, And a low carbon steel sheet containing N: 0.0010 to 0.015% by weight, the balance being Fe and inevitable impurities. A steel sheet for a deep drawn ironing can with a thinner shape is disclosed, Is proposed.

Japanese Patent Application Laid-Open No. 2001-107186 Japanese Patent Application Laid-Open No. 8-325670 Japanese Laid-Open Patent Publication No. 2004-183074

First, it is necessary to secure strength in order to become thinner (thinner). On the other hand, when a steel plate is used for a can body formed by can body processing such as can extension processing or for a can body formed by flange processing, it is necessary to apply high ductility steel.

For example, in the case of can body trimming and flange working at the time of manufacturing a three-piece can represented by bottom processing and can extension processing at the time of manufacturing a two-piece can, a steel sheet having a large pre- Need to use.

In consideration of resistance to corrosive contents, it is necessary to use a steel sheet having good corrosion resistance. Therefore, excessive element addition that inhibits corrosion resistance can not be performed.

With respect to the above characteristics, it is possible to produce a steel sheet satisfying any of strength, ductility (pre-stretch) and corrosion resistance by the above-mentioned conventional techniques, but a steel sheet satisfying all of them can not be produced.

For example, although the method described in Patent Document 1 is an effective method for increasing the strength, yield and elongation are increased because the amount of solid solution C and solid solution N in the steel is large. Yield elongation is caused by the decrease of the movable potential by fixing the dislocations of solid solution C or solid solution N. In the deformation region where the yield elongation is large, a local yielding phenomenon occurs and non-uniform deformation occurs, resulting in a wrinkle called stretch strain, which may damage the appearance.

Patent Document 2 proposes a steel in which strength is strengthened by precipitation strengthening and balance of strength and ductility is proposed. However, no consideration is given to yield elongation. In the manufacturing method described in Patent Document 2, The value of yield elongation is not obtained.

Patent Document 3 proposes to increase the strength by strengthening employment. In the technique described in this document, since P and Mn, which are generally known as elements inhibiting corrosion resistance, are added in excess, there is a high possibility that the corrosion resistance is inhibited.

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and an object of the present invention is to provide a steel sheet for a high-strength, high-strength can which has a superior yield strength of 450 to 600 MPa and a full elongation of 13% or more after coating baking, And a method for producing the same.

The present inventors have conducted intensive studies in order to solve the above problems. As a result, the following findings were obtained.

Attention is focused on a complex combination of precipitation strengthening, solid solution strengthening and processing enhancement, thereby making it possible to balance the precipitation strengthening and the solid solution strengthening, thereby enhancing the strength without deteriorating the elongation.

The reduction in the secondary cold rolling can be set to 1 to 19%, and the strength can be increased without lowering the overall drawability by strengthening the work at a reduction ratio lower than that in the conventional secondary cold rolling.

Further, by designing the component of the disc with the addition amount of the element in the range that does not interfere with the corrosion resistance, it shows good corrosion resistance even for contents having high corrosiveness.

The present invention has accomplished the steel sheet for high-strength, high-strength can, and the manufacturing method thereof by totally managing the components and the manufacturing method based on the above knowledge.

The present invention has been made based on the above findings, and its gist of the invention is as follows.

The steel according to any one of the above items [1] to [20], wherein the steel sheet comprises: C: not less than 0.020% but not more than 0.130%, Si: not more than 0.04%, Mn: 0.10 to 1.20%, P: not more than 0.100%, S: not more than 0.030% And the total amount of precipitated Nb and total Nb is not more than 0.020% and not more than 0.020%, Nb: 0.004 to 0.040%, the balance being iron and inevitable impurities, Wherein the Nb precipitate has an average grain size of 20 nm or less, an average ferrite crystal grain size of 7.0 m or less, an upper yield strength after coating baking treatment of 450 to 630 MPa, and a total elongation of 13% or more.

[2] The volume ratio of the Nb precipitates in the region from the surface to the 1/8 depth position in the plate thickness direction and the volume ratio of the Nb precipitates in the region from 3/8 depth position to 4/8 depth position from the surface (1) according to claim 1, wherein the ratio of the specific surface area to the surface area ratio satisfies the following formula (1).

(Nb precipitate volume ratio of 3/8 to 4/8) / (volume fraction of Nb precipitate surface of 1/8) ≥ 1.10 (Equation 1)

[3] A method of producing a high strength steel can for high strength can according to [1] or [2], wherein the steel is rolled under conditions of a finish rolling temperature of Ar 3 transformation point to 990 캜, , A primary cold rolling step of rolling under conditions of pickling and picking up at a reduction ratio of 80% or more, and a secondary cold rolling step in which after the primary cold rolling step, And a second cold rolling step of rolling under a condition of a reduction rate of 1 to 19% after the annealing step, characterized in that the step (METHOD FOR MANUFACTURING STEEL SHEET FOR SAME).

According to the present invention, it is possible to obtain a high-strength, high-strength steel sheet having an upper yield strength of 450 to 630 MPa and a total elongation of 13% or more. Specifically, in the present invention, by strengthening precipitation by Nb, solid solution strengthening by N, and annealing, by strengthening the processing by performing secondary cold rolling at a reduction ratio of 1 to 19% It does not go crazy, but it strengthens strength by compound strengthening. As a result, the total elongation is 13% or more, and the final yield strength is 450 to 630 MPa.

Further, in the present invention, high strength of the original plate makes it possible to ensure a high can body strength even if the welding can is thinned (thinned). Even when the high-performance high-strength steel sheet of the present invention is applied to a two-piece can requiring a pressure resistance strength of the bottom portion, it is possible to obtain a high pressure resistance strength as it is in the current gauge state. In addition, by increasing the ductility, it is possible to perform strong can body trimming and flanging such as can extension work used in welding cans.

In the present invention, the composition of the components is set so that the corrosion resistance does not occur. As a result, the steel sheet for high-strength, high strength can according to the present invention has excellent strength, workability, and corrosion resistance.

Hereinafter, an embodiment of the present invention will be described. The present invention is not limited to the following embodiments.

The steel sheet for high strength, high strength can according to the present invention has an excellent yield strength of 450 to 630 MPa (hereinafter sometimes referred to as U-YP) and a total elongation of 13% or more, and has excellent corrosion resistance. Further, in the steel sheet for high-strength, high strength can according to the present invention, the aging property can be reduced.

In the present invention, the Nb is added as the precipitation strengthening element, N is added as the solid solution strengthening element logo, and the second round cold rolling with a reduction ratio of 1 to 19% is carried out after the annealing, . Further, when the upper yield strength is increased by the above-described method with a specific component system, the leading edge elongation becomes high. It is a feature of the present invention that the excellent elongation at break and the high total elongation are the most important requirements. As described above, by appropriately adjusting the component composition, the structure and the manufacturing conditions so that the pre-stretching can be made high while adding the precipitation strengthening element and the solid solution strengthening element, the high yield strength of 450 to 630 MPa and the pre- A steel sheet for a sintered high strength can is obtained.

Next, the composition of the steel sheet for high-strength high strength can according to the present invention (in the present specification, the steel sheet for high-strength high-strength can is sometimes referred to as steel sheet for can) will be described. The steel sheet for a high strength, high strength can according to the present invention contains, by mass%, C: not less than 0.020% but not more than 0.130%, Si: not more than 0.04%, Mn: 0.10 to 1.20% 0.10% or less, N: 0.0120% or more and 0.020% or less, Nb: 0.004 to 0.040%, and the balance of iron and inevitable impurities. Each component will be described below. In the present specification, "%" in the description of the component composition means "% by mass".

C: not more than 0.020% and not more than 0.130%

In the steel sheet for a can according to the present invention, after continuous annealing, it is necessary to achieve a predetermined upper yield strength (450 to 630 MPa) and to have a pre-stretch of 13% or more. For this purpose, it is important to use ferrite average crystal grain size of not more than 7.0 mu m and precipitation strengthening by NbC produced by Nb addition. The C content of the steel sheet for a can becomes important in order to adjust the ferrite average crystal grain size to the above range and utilize precipitation strengthening by NbC. Concretely, it is necessary to make the C content exceed 0.020%. If the C content exceeds 0.040%, the strength of the hot rolled sheet increases, and the deformation resistance during cold rolling increases, so that surface defects may easily occur after rolling. Further, in order to reduce the defects, it is necessary to reduce the rolling speed. However, when the upper yield strength is 600 MPa or more, the C content is preferably 0.070% or more. On the other hand, if the C content exceeds 0.130%, cracking of the low pores occurs in the cooling process of the steel solvent. Therefore, the upper limit of the C content is 0.130%. As described above, when the C content exceeds 0.040%, the strength of the hot-rolled sheet rises, and the deformation resistance during cold rolling tends to increase. In order to avoid surface defects after rolling, it is necessary to reduce the rolling speed It is preferable that the C content is more than 0.020% to 0.040% from the viewpoint of easiness of production.

Si: not more than 0.04%

Si is an element that strengthens the steel by solid solution strengthening. However, when the Si content exceeds 0.04%, the corrosion resistance is remarkably damaged. Therefore, the Si content is 0.04% or less. In addition, in the present invention, since the yield strength is increased by adjusting the elements other than Si and the manufacturing conditions, it is not necessary to use solid solution strengthening by Si. Therefore, Si may not be contained in the present invention.

Mn: 0.10 to 1.20%

Mn increases the strength of steel by solid solution strengthening, and also reduces the mean ferrite grain size. The Mn content is 0.10% or more when the ferrite average crystal grain size is remarkably reduced. In order to secure a target upper yield strength, it is necessary to set the Mn content to 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%, corrosion resistance and surface characteristics are deteriorated. Therefore, the upper limit of the Mn content is set to 1.20%.

P: not more than 0.100%

P is a large element of employment enhancement. However, when the content of P exceeds 0.100%, the corrosion resistance is deteriorated. Therefore, the P content is set to 0.100% or less.

S: not more than 0.030%

The high-strength steel sheet for high strength can of the present invention may not contain S, but in carrying out this patent, it is preferable that S is 0.030% or less. Since the can steel strip of the present invention has a high content of Nb, C and N, the slab edge is liable to be cracked in the calibration zone during continuous casting. In order to prevent slab cracking, the S content is preferably 0.030% or less. Preferably, the S content is 0.020% or less. More preferably, the S content is 0.010% or less.

Al: 0.10% or less

When the Al content is increased, the recrystallization temperature is increased. Therefore, it is necessary to set the annealing temperature as high as the increase of the Al content. In the present invention, the recrystallization temperature must rise due to the influence of other elements added to increase the yield strength, and the annealing temperature must be set high. Therefore, it is necessary to avoid the increase of the recrystallization temperature by Al as much as possible. Therefore, the Al content is set to 0.10% or less. In addition, Al is preferably added as a deoxidizer, and in order to obtain this effect, the Al content is preferably 0.010% or more.

N: not more than 0.0120% and not more than 0.020%

N is a necessary element to increase employment intensification. On the other hand, if the N content is too large, slab cracking easily occurs in the lower calibrating table where the temperature at the time of continuous casting is lowered. Therefore, the N content should be 0.020% or less. On the other hand, in order to exhibit the effect of strengthening employment, it is necessary to make the N content exceed 0.0120%.

Nb: 0.004 to 0.040%

Nb is an important additive element in the present invention. Nb is a high-carbide element, and precipitates fine carbides. As a result, the upper yield strength is increased. In the present invention, the upper yield strength and the surface property can be adjusted by the Nb content. Since this effect occurs when the Nb content is 0.004% or more, the lower limit of the Nb content is limited to 0.004%. On the other hand, since Nb causes an increase in the recrystallization temperature, when the Nb content exceeds 0.040%, annealing temperature of 650 to 780 ° C and continuous annealing of 10 s or more and 55 s or less, , And it becomes difficult to anneal. Therefore, the upper limit of the Nb content is limited to 0.040%. The Nb content is preferably 0.004 to 0.020% from the viewpoint of suppressing an increase in deformation resistance at the time of cold rolling.

The remainder other than the above essential components and optional components are Fe and inevitable impurities.

Next, the structure of the can steel plate of the present invention will be described.

Ferrite average crystal grain size: 7.0 탆 or less

The structure of the steel sheet for a can according to the present invention is a ferrite single phase structure. The ferrite average crystal grain size influences not only the upper yield strength but also the surface property at the time of drawing processing. If the average crystal grain size of the ferrite of the final product is more than 7.0 mu m, roughness of the surface is generated in part after the drawing, and the appearance of the surface appearance is lost. For this reason, the ferrite average crystal grain size was set to 7.0 μm or less. Further, in order to reduce the average crystal grain size of ferrite, it is necessary to reduce the cracking temperature at the time of continuous annealing so as to suppress the grain growth of the ferrite crystal or to add a large amount of elements forming fine precipitates for pinning the grain boundary migration, It is preferable that the average crystal grain size of ferrite is 5.0 占 퐉 or more because the cost is increased. The ferrite average crystal grain size may be measured at any point before or after the coating baking treatment, since the ferrite average crystal grain size does not change before and after the coating baking treatment. In the present invention, the coating baking treatment refers to a treatment equivalent to heating during baking and laminating, specifically, a heat treatment at 170 to 265 ° C for 12 seconds to 30 minutes. In the later-described embodiments, heat treatment is performed at 210 캜 for 20 minutes as a standard condition.

The ferrite average crystal grain size is controlled by the composition of the components, the reduction ratio of the cold rolling, and the annealing temperature. Concretely, ferrite mean crystal grain size of 7.0 탆 or less is obtained by adopting the above-mentioned component composition and employing the manufacturing conditions to be described later. When the cracking temperature in the continuous annealing is increased, the ferrite average crystal grain size becomes larger. When the cracking temperature is lowered, the ferrite average crystal grain size becomes smaller.

The amount of precipitated Nb / total Nb amount > = 0.30

By setting the ratio of the amount of precipitated Nb to the total amount of Nb (the amount of precipitated Nb / the total amount of Nb) to be not less than 0.30, it is possible to achieve the target upper yield strength of 450 to 630 MPa while improving the creep resistance and corrosion resistance. Further, when the amount of precipitated Nb is increased, it is preferable that the amount of precipitated Nb / total amount of Nb is 0.9 or less because the particle size of precipitated Nb is coarse. The amount of precipitated Nb / total amount of Nb may be in the above range after the coating baking. Since the amount of precipitated Nb / total amount of Nb does not change before and after the coating baking treatment, it may be measured at any point before or after the coating baking treatment. Since the paint baking process is the same as the above, the description is omitted.

As a method of adjusting the amount of precipitated Nb / total amount of Nb to satisfy 0.30, for example, it is possible to increase the Nb precipitation amount by increasing the cracking temperature at the time of continuous annealing.

Nb precipitate Average particle diameter: 20 nm or less

If the average grain size of the Nb precipitate is larger than 20 nm, the effect of increasing the strength by pinning of the dislocation by the precipitate can not be expected. For this reason, the average grain size of the Nb precipitates is set to 20 nm or less in order to obtain a predetermined strength while improving the wear resistance and corrosion resistance. The average particle diameter of the Nb precipitate is a value measured by the method described in the embodiment. Here, the average grain size of the Nb precipitates may be within the above-mentioned range after the coating baking. Since the average particle diameter of the Nb precipitate does not change before and after the coating baking treatment, it may be measured at any point before or after the coating baking treatment. Since the paint baking process is the same as the above, the description is omitted.

As a method for adjusting the average grain size of the Nb precipitates to 20 nm or less, for example, when it is desired to lower the average grain size of the Nb precipitates, the cracking time of the continuous annealing can be shortened to suppress the growth of the Nb precipitates.

(Nb precipitate volume ratio of 3/8 to 4/8) / (volume fraction of Nb precipitate of surface to 1/8) ≥ 1.10

The ratio of the volume ratio of the Nb precipitate in the region from the surface to the depth position to the depth position in the plate thickness direction and the ratio of the volume ratio of the Nb precipitate in the region from the surface position to 3/8 depth position to the 4/8 depth position from the surface 1.10 or less, the density of the Nb precipitates in the region from the 3/8 depth position to the 4/8 depth position from the surface is increased, and the amount of precipitation strengthening in the center layer is increased to further raise the upper yield strength. Further, in the region from the surface to the 1/8 depth position, the density of the Nb precipitate is reduced to obtain better pre-stretching. By forming the material difference in the plate thickness direction in this manner, high porosity and high strength can be made compatible in a very excellent state. The ratio of the volume ratio means that the ratio of the volume ratio is in the above range after the coating baking. Since the paint baking process is the same as the above, the description is omitted.

The volume ratio of the Nb precipitates in the region from the surface to the 1/8 depth position in the sheet thickness direction can be adjusted by, for example, lowering the temperature of the final finish rolling of hot rolling to granulate the surface layer to precipitate Nb If the temperature is adjusted by increasing the temperature of the final finishing rolling, the surface layer is made fine and the Nb precipitation in the surface layer is suppressed to a small value.

The volume ratio of the Nb precipitates in the region from the 3/8 depth position to the 4/8 depth position from the surface becomes large when the Nb precipitates are grown with the hot-rolled coiling temperature increased, for example, If the temperature is lowered and the growth of Nb precipitates is suppressed, the value becomes small.

Over yield strength: 450 ~ 630 MPa

With respect to the plate thickness of about 0.2 mm, the upper yield strength is set to 450 MPa or more in order to secure the panel ring strength, the dent strength and the withstand pressure strength of the two-piece can of the welded can. On the other hand, in order to obtain an upper yield strength exceeding 630 MPa, a large amount of element addition is required. The addition of a large amount of element may deteriorate the corrosion resistance of the steel sheet for a can according to the present invention. Therefore, the upper yield strength is set to 630 MPa or less. The upper yield strength can be controlled to a target value by adopting the above-described composition and employing the production conditions described later. In the present invention, it means that the upper yield strength after the coating baking is in the above range. Since the paint baking process is the same as the above, the description is omitted.

Prior stretch: 13% or more

If the total elongation is less than 13%, it becomes difficult to apply the steel sheet for can according to the present invention to the production of a can formed by can body processing such as can extension processing, for example. When the total elongation is less than 13%, cracks are generated during flanging of the can, which makes it difficult to apply the steel sheet for can according to the present invention to the production of a can. Therefore, the lower limit of the leading edge should be 13%. In addition, the leading edge is controlled to a target value by setting the composition of the component to a specific range and reducing the reduction ratio of the secondary cold rolling after annealing to a specified range. In the present invention, it means that the pre-stretching after the baking is in the above range. Since the paint baking process is the same as the above, the description is omitted. In the present invention, the total elongation is usually 35% or less.

Next, an example of a manufacturing method capable of preferably producing the steel plate for a can according to the present invention will be described. The steel sheet for a can according to the present invention is manufactured by a method including a hot rolling step, a primary cold rolling step, an annealing step and a secondary cold rolling step. Hereinafter, each manufacturing process will be described.

Hot rolling process

The hot rolling step is a step of hot rolling a steel (for example, a slab) at a finishing temperature of not lower than the Ar3 transformation point and not higher than 990 캜, and winding the steel at a coiling temperature of not less than 400 캜 and less than 600 캜.

The raw steel will be described. The steel is obtained by melting the molten steel adjusted to the above-mentioned component composition by a commonly known solvent method using a converter or the like, and then casting it by a commonly used casting method such as a continuous casting method. Hereinafter, the rolled material means a raw material.

The rolled material obtained by the above process is subjected to hot rolling to produce a hot rolled sheet. At the start of rolling of the hot rolling, it is preferable that the temperature of the rolling material is 1230 캜 or higher.

The finishing temperature in the hot rolling is set to the Ar3 transformation point or more. The finish rolling temperature in hot rolling is an important factor in securing the upper yield strength. When the finishing temperature is lower than the Ar3 transformation point, grain growth occurs due to the biaxial hot rolling of gamma + alpha, so that the upper yield strength is lowered and the withstand pressure strength is insufficient. Therefore, the hot rolling finishing temperature was limited to the Ar3 transformation point or higher. When the finishing rolling temperature is higher than 990 DEG C, the pre-drawing is insufficient and the formability is deteriorated. From the viewpoint of preventing scale generation at a high temperature, the finish rolling temperature is set at 990 DEG C as the upper limit.

The coiling temperature in the hot rolling process is an important factor in controlling the target upper yield strength and the pre-stretching as targets, which are important in the present invention. When the coiling temperature is set to 600 占 폚 or more, N added for solid solution strengthening becomes AlN and precipitates, and the amount of dissolved N is lowered, resulting in lowered yield strength. For this reason, the coiling temperature was set to less than 600 캜. When the coiling temperature is lower than 400 占 폚, the coiling temperature is 400 占 폚 or higher because the pre-stretching deteriorates and the formability deteriorates. Further, in the case of quenching in order to lower the coiling temperature, since the cooling becomes uneven and the plate shape is deteriorated, the coiling temperature is set to the lower limit of 400 占 폚 in view of production efficiency. From the viewpoint of controlling the Nb precipitate, it is preferable that the cooling rate after the winding is a sliver. The cooling is preferably performed at 11.5 ° C / hour or less, more preferably at 6.3 ° C / hour or less, The cooling below is more preferable. After such cooling, it is preferable to carry out the treatment of the next step after the temperature is lowered to 200 ° C or lower, more preferably 100 ° C or lower, and further preferably 50 ° C or lower.

Primary cold rolling process

The primary cold rolling step is a step of pickling a steel sheet (hot rolled steel sheet) after the hot rolling step and rolling the steel sheet at a reduction ratio of 80% or more.

The pickling is only required if the surface scale can be removed, and conditions are not specially specified. And can be pickled by a conventional method.

The reduction ratio in the primary cold rolling is one of the important conditions in the present invention. When the reduction ratio in the primary cold rolling is less than 80%, it is difficult to produce a steel sheet having an upper yield strength of 450 MPa or more. In addition, in order to obtain a plate thickness (about 0.17 mm) equal to the DR material when the reduction rate is less than 80% in this step, at least the plate thickness of the hot-rolled plate needs to be 1 mm or less. However, it is difficult to reduce the plate thickness of the hot rolled plate to 1 mm or less in operation. Therefore, the reduction rate in this step is 80% or more.

Annealing process

The annealing step is a step of continuously annealing a steel sheet (cold rolled sheet) at a temperature of 650 to 780 deg. C and a cracking time of 10 s or more and 55 s or less after the primary cold rolling step.

Annealing uses continuous annealing. The cracking temperature is required to be not less than the recrystallization temperature of the steel sheet in order to ensure good workability and the cracking temperature is limited to not less than 650 DEG C in order to make the structure more uniform and to secure the leading edge. On the other hand, when the crack temperature is higher than 780 DEG C, the ferrite crystal grain size becomes larger, the upper yield strength decreases, and the breakdown strength is insufficient. Further, in order to perform continuous annealing at a temperature exceeding 780 DEG C, it is necessary to reduce the conveying speed as much as possible in order to prevent the steel sheet from being broken, and the productivity is lowered. Therefore, the cracking temperature is set in the range of 650 to 780 ° C.

At a rate at which the cracking time exceeds 55 s, the Nb precipitate particle size becomes excessively large, the yield strength is lowered, the pressure resistance strength is insufficient, and the productivity can not be ensured. When the cracking time is less than 10 s, heating unevenness occurs at the time of high-speed delivery, the Nb precipitation form does not become the desired shape, the pre-stretching deteriorates, and the tension in the furnace becomes unstable, Should be more than 10 s.

Secondary cold rolling process

The secondary cold rolling step is a step of rolling the steel sheet (annealing sheet) under the condition that the reduction rate is 1 to 19% after the annealing step.

If the reduction rate in the secondary cold rolling after annealing is made to be the same as that in ordinary DR production conditions, the deformation introduced during processing is increased, and the pre-stretching is lowered. In the present invention, since it is necessary to secure 13% or more of the total elongation with a superfine material, the reduction rate in the secondary cold rolling is set to 19% or less. Further, since the unevenness of the roll is transferred to the steel sheet in order to adjust the surface roughness of the steel sheet, the reduction ratio of the secondary cold rolling needs to be 1% or more. The reduction ratio of the secondary cold rolling is preferably 4 to 12% from the viewpoint of slip prevention of the steel sheet and roll at the time of rolling and securing of the pre-stretching.

Example

A steel slab was obtained by dissolving a steel containing the composition shown in Table 1 and the balance consisting of Fe and inevitable impurities by a practical machine. The obtained steel slab was reheated, hot rolled, and wound. Subsequently, the steel sheet was subjected to primary cold rolling to prepare a thin steel sheet. The temperature of the steel sheet before pickling was in the range of 25 to 60 占 폚 in the entire length of the coil. The obtained thin steel sheet was heated at a heating rate of 15 DEG C / sec. Thereafter, continuous annealing was performed. Subsequently, after cooling, secondary cold rolling was performed, and ordinary Sn plating was continuously carried out to obtain raw iron. Table 2 shows the detailed production conditions. Further, the Ar3 transformation point was calculated by measuring the temperature at which the volume change was the greatest at the time of cooling by? -? Transformation.

The plated steel sheet (raw iron) obtained above was subjected to a heat treatment corresponding to a coating baking treatment at 210 캜 for 20 minutes and then subjected to a tensile test to measure the yield strength and elongation, The particle size was also investigated. The survey method is as follows.

The tensile test was carried out using a tensile test piece having a size of JIS No. 5, and the upper yield strength (U-YP) and the elongation (El) were measured to evaluate the strength, ductility and aging property. Aging resistance was evaluated by yield stretching which contributes to generation of stretch strain during processing. When the yield elongation is 4% or less, generation of stretch strain at the time of processing can be suppressed. The obtained results are shown in Table 3.

The crystal structure was polished and the crystal grain boundaries were etched away and observed with an optical microscope.

The ferrite average crystal grain size was measured using the cutting method of JIS G 5503 with respect to the crystal structure observed as described above. The obtained results are shown in Table 3.

The amount of precipitated Nb was electrolytically extracted using a 10% acetylacetone-1% tetramethylammonium chloride-methanol solution, acid-dissolved, and Nb was quantitatively determined by ICP measurement. For the total Nb content, samples were directly acid-dissolved and measured as ICP. The average particle size of the Nb precipitate was measured by TEM. The obtained results are shown in Table 3.

The volume ratio of the Nb precipitates in the region from the surface to the 1/8 depth position in the plate thickness direction, the volume ratio of the Nb precipitates in the region from the 3/8 depth position to the 4/8 depth position from the surface, And the particle diameters and the number of the precipitates were measured from the photographs obtained by observing each of the 10 fields of view at a magnification of 100,000 times with a TEM to measure the volume ratio. The measurement results are shown in Table 4.

The pressure-resistant strength was measured by forming a sample (plated steel plate) having a plate thickness of 0.26 mm into a lid of 63 mmΦ and then machining it by winding it on a 63 mmΦ welded can body, To measure the pressure when the can lid was deformed. The case where the can lid was not deformed even when the internal pressure was 0.20 MPa was evaluated as "? &Quot;, and the case where the can lid was deformed at less than 0.20 MPa was evaluated as " x ". The results are shown in Table 4.

The formability was evaluated by the method specified in JIS Z 2247 using a test machine specified in JIS B 7729 using a sample having a plate thickness of 0.26 mm. The erichen value (molding height at the time of occurrence of the penetration crack) of 6.5 mm or more was defined as "? &Quot;, and less than 6.5 mm was defined as " x ". The results are shown in Table 4.

It can be seen from Table 3 that in the present invention, since the structure has an average crystal grain diameter of 7.0 mu m or less and has a fine ferrite structure, the superior yield strength is large and both strength and ductility are excellent. Further, in the present invention, since the composition is adjusted to the composition shown in Table 1, corrosion resistance is also excellent.

Further, in the comparative example, the desired characteristics of the present invention can not be obtained because the condition of any one of claims is out of the scope of the present invention.

Industrial availability

According to the present invention, a steel sheet excellent in all properties of strength, ductility and corrosion resistance can be obtained. Therefore, it is possible to provide a three-piece can with a can body work at a high cost and a two- to be.

Claims (3)

C: not more than 0.030%, Si: not more than 0.04%, Mn: 0.10 to 1.20%, P: not more than 0.100%, S: not more than 0.030%, Al: not more than 0.10% % Or less, Nb: 0.004 to 0.040%, and the balance of iron and inevitable impurities,
The ratio of the amount of precipitated Nb to the total amount of Nb is such that the amount of precipitated Nb / total Nb amount? 0.30,
The average particle size of the Nb precipitate is 20 nm or less,
The ferrite average crystal grain size is not more than 7.0 mu m,
Wherein an upper yield strength after coating baking treatment is from 450 to 630 MPa, and a total elongation is from 13% or more. The method according to claim 1,
The ratio of the volume ratio of the Nb precipitate in the region from the surface to the depth position to the depth position in the plate thickness direction and the ratio of the volume ratio of the Nb precipitate in the region from the surface position to 3/8 depth position to the 4/8 depth position from the surface , And the following formula (1) is satisfied.
(Nb precipitate volume ratio of 3/8 to 4/8) / (volume fraction of Nb precipitate surface of 1/8) ≥ 1.10 (Equation 1) A manufacturing method of a steel plate for a can according to any one of claims 1 to 3,
A hot rolling step in which the steel is rolled under the condition that the finishing rolling temperature is from Ar 3 transformation point to 990 ° C and the coiling temperature is from 400 ° C to less than 600 ° C;
A primary cold rolling step of pickling after the hot rolling step and rolling under a condition of a reduction rate of 80%
The annealing process for continuous annealing under the conditions of a cracking temperature of 650 to 780 캜 and a cracking time of 10 s or more and 55 s or less after the primary cold rolling process,
And a second cold rolling step of rolling the steel sheet at a reduction ratio of 1 to 19% after the annealing step.