WO2016157878A1

WO2016157878A1 - Steel sheet for cans and method for manufacturing steel sheet for cans

Info

Publication number: WO2016157878A1
Application number: PCT/JP2016/001774
Authority: WO
Inventors: 多田　雅毅; 克己小島; 裕樹中丸
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2015-03-31
Filing date: 2016-03-28
Publication date: 2016-10-06
Also published as: MY173780A; KR101994914B1; TW201702404A; TWI617677B; CN107429360A; CO2017009718A2; JPWO2016157878A1; CN107429360B; KR20170121277A; JP6028884B1

Abstract

The present invention provides a steel sheet for cans and a method for manufacturing the steel sheet for cans. The steel sheet for cans has a component composition containing, on a percentage by mass basis: 0.020-0.130% of C; not more than 0.04% of Si; 0.10-1.2% of Mn; 0.007-0.100% of P; not more than 0.03% of S; 0.0010-0.10% of Al, 0.0120-0.020% of N; one or more selected from 0.010-0.050% of Nb, 0.010-0.050% of Ti and 0.0010-0.010% of B; the balance being iron and unavoidable impurities. The steel sheet for cans has a structure including a ferrite phase where the area ratio of the ferrite phase is no less than 50%. The steel sheet for cans has upper yield strength of 480 to 700 MPa after a heating treatment at 210ºC for 20 minutes, and has a total elongation of no less than 12%. The ratio between the solute N content in a region extending from the surface to a depth of 1/8 in the thickness direction and the solute N content in a region extending from a depth of 3/8 from the surface to a depth of 4/8 satisfies formula 1. (The amount of solute N in region extending from surface to depth of 1/8 in thickness direction)/(the amount of solute N in region extending from depth of 3/8 from surface to depth of 4/8) ≤ 0.96 (formula 1)

Description

Steel plate for cans and method for producing steel plate for cans

The present invention relates to a steel plate for cans and a method for producing a steel plate for cans. In particular, the present invention relates to a steel plate for a can used as a raw material for a three-piece can formed by high-working can body processing, a two-piece can requiring pressure strength, and a method for manufacturing the same.

In recent years, in order to expand the demand for steel cans, measures have been taken to reduce can manufacturing costs and to introduce steel cans into new can types such as deformed cans.

Measures to reduce the can manufacturing cost include reducing the cost of the material. In addition to 2-piece cans formed by drawing, even 3-piece cans mainly made of simple cylindrical molding are being used to reduce the thickness of the steel sheets used.

However, simply reducing the thickness of the steel sheet decreases the strength of the can. Therefore, it is not possible to use a steel plate that is simply thinned at a location where a high-strength material such as a redrawn can (DRD can) or a can body of a welded can is used. Therefore, a high strength and extremely thin steel plate for cans is desired.

At present, ultra-thin and hard steel plates for cans are manufactured by the Double Reduce method (hereinafter referred to as DR method) in which secondary cold rolling with a reduction rate of 20% or more is performed after annealing. A steel plate manufactured by using the DR method (hereinafter also referred to as a DR material) has a high strength, but has a feature that the total elongation is small.

Also, it is difficult from the viewpoint of workability to use a DR material having poor ductility as a can material formed by can body processing with a strong working degree, such as a deformed can.

In order to avoid the drawbacks of the DR material, methods for producing high-strength steel sheets using various strengthening methods have been proposed in the following patent documents.

Patent Document 1 proposes a technique for obtaining a steel sheet for high-strength cans by adding a large amount of C and N and baking and hardening. The steel sheet for cans described in Patent Document 1 has a high yield stress of 550 MPa or more after the paint baking process. Moreover, in the steel plate for cans of patent document 1, it is supposed that hardness can be adjusted with the addition amount of N and heat processing.

In Patent Document 2, as in Patent Document 1, high strength of about +50 MPa is realized by post-coating baking treatment.

Patent Document 3 proposes a steel plate that balances strength and ductility by combining precipitation strengthening with Nb carbide and refinement strengthening with Nb, Ti, and B carbonitrides.

Patent Document 4 proposes a method for increasing the strength by using solid solution strengthening such as Mn, P, and N.

In Patent Document 5, the tensile strength is less than 540 MPa using precipitation strengthening by Nb, Ti, and B carbonitrides, and the deformability due to inclusions and precipitates is controlled by controlling the particle diameter of oxide inclusions. Steel plates for cans that prevent deterioration and improve the formability of welds have been proposed.

JP 2001-107186 A Japanese Patent Laid-Open No. 11-199991 JP-A-8-325670 JP 2004-183074 A JP 2001-89828 A

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

For example, it is necessary to use a steel plate with a large total elongation as a raw material so that cracking of the steel plate does not occur in the can body processing and flange processing at the time of manufacturing the three-piece can represented by bottom processing and can expansion processing at the time of manufacturing the two-piece can There is.

Furthermore, considering the resistance to highly corrosive contents, it is necessary to use a steel sheet with good corrosion resistance. Therefore, it is impossible to add an excessive element that inhibits the corrosion resistance.

With regard to the above characteristics, the above-described conventional technology can produce a steel sheet that satisfies any of the strength, ductility, and corrosion resistance, but cannot produce a steel sheet that satisfies all of the requirements.

For example, the method of adding a large amount of C and N described in Patent Documents 1 and 2 and increasing the strength by bake hardenability is an effective method for increasing the strength, but the solid solution C and N amount in steel Since there are many, yield elongation becomes large. And since yield elongation becomes large, the surface appearance is spoiled by generating wrinkles called stretcher strain during processing. Therefore, there is room for improvement in the techniques described in Patent Documents 1 and 2.

Patent Document 3 realizes high strength by precipitation strengthening and proposes a steel with a balance between strength and ductility, but does not describe the yield elongation that impairs the surface appearance. The yield elongation targeted by the invention cannot be obtained.

Patent Document 4 proposes an increase in strength by solid solution strengthening, but P and Mn, which are generally known as elements that inhibit corrosion resistance, are excessively added, so that there is a high risk of inhibiting corrosion resistance.

In Patent Document 5, target strength is obtained by using precipitation and refinement strengthening of Nb, Ti and the like. However, from the viewpoint of the formability and surface properties of the welded portion, Patent Document 5 requires the addition of not only Ti but also Ca and REM. Furthermore, under the invention described in Patent Document 5, there is a problem that the yield of the Ti alloy is poor as compared with the conventional method of deoxidizing with Al.

The present invention has been made in view of such circumstances, and provides a steel plate for cans having high strength, excellent ductility, and good corrosion resistance even for highly corrosive contents and a method for producing the same. With the goal.

The present inventors have conducted intensive research to solve the above problems. As a result, the following knowledge was obtained.

Focusing on the combined combination of precipitation strengthening, solid solution strengthening, and process strengthening, the strength can be increased without inferior in ductility by achieving solid solution strengthening with N and precipitation strengthening with Nb, Ti, and B.

Also, by making a difference in the amount of solute N between the surface side and the center side in the thickness direction of the steel sheet, excellent ductility and high strength can be arranged side by side.

In addition, by designing the composition of the original plate with the addition amount of elements within the range that does not affect the corrosion resistance, it shows good corrosion resistance even for highly corrosive contents.

Furthermore, in the manufacturing method, the coiling temperature in the hot rolling process and the cooling rate after coiling can be adjusted appropriately to increase the strength without reducing the total elongation.

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

[1] By mass%, C: 0.020% or more and 0.130% or less, Si: 0.04% or less, Mn: 0.10% or more and 1.2% or less, P: 0.007% or more, and 0.0. 100% or less, S: 0.03% or less, Al: 0.0010% or more and 0.10% or less, N: 0.0120% to 0.020% or less, and Nb: 0.010% or more and 0 .050% or less, Ti: 0.010% or more and 0.050% or less, B: One or two or more selected from 0.0010% or more and 0.010% or less, with the balance being iron and inevitable impurities Having a component composition, the structure has a ferrite phase, the area ratio of the ferrite phase is 50% or more,
The upper yield strength after heat treatment at 210 ° C. for 20 minutes is 480 to 700 MPa, and the total elongation is 12% or more.
The ratio of the amount of solute N in the region from the surface to 1/8 depth position in the sheet thickness direction and the amount of solute N in the region from the surface to 3/8 depth position to 4/8 depth position is as follows. A steel plate for cans that satisfies Equation 1 below.
(Solution N amount in region from surface to 1/8 depth position in plate thickness direction) / (Solution N in region from surface to 3/8 depth position to 4/8 depth position in plate thickness direction) Amount) ≦ 0.96 (Formula 1)
[2] The steel plate for cans according to [1], wherein the ferrite phase has a recrystallized structure.

[3] The steel plate for cans according to [1] or [2], wherein the area ratio of the ferrite phase is 70% or more.

[4] A method for producing a steel plate for cans according to any one of [1] to [3],
A hot rolling step in which the steel is rolled at a finish rolling temperature of Ar3 transformation point or higher, wound at a winding temperature of 500 to 620 ° C., and cooled at a cooling rate of 10 ° C./hr or lower after winding;
After the hot rolling step, a primary cold rolling step of rolling at a rolling reduction of 80% or more,
After the primary cold rolling step, an annealing step of continuous annealing at a soaking temperature of 660 to 800 ° C. and a soaking time of 55 s or less;
And a secondary cold rolling step of rolling at a rolling reduction of 1 to 19% after the annealing step.

According to the present invention, it is possible to obtain a steel plate for cans having high strength, excellent ductility, and good corrosion resistance even for highly corrosive contents.

Furthermore, according to the present invention, by increasing the strength of the steel sheet, it is possible to ensure a high strength of the can even if the welded can is made thinner. Further, due to the excellent ductility, it is possible to perform strong can barrel processing and flange processing such as can expansion processing used in welded cans.

Furthermore, in the present invention, the component composition is set so that the corrosion resistance is not hindered. As a result, the steel plate for cans of the present invention is excellent in any of strength, ductility, and corrosion resistance.

Hereinafter, embodiments of the present invention will be described. In addition, this invention is not limited to the following embodiment.

The steel plate for cans according to the present invention has an excellent yield strength (hereinafter sometimes referred to as U-YP) after heat treatment at 210 ° C. for 20 minutes and a total elongation of 12% or more. Has corrosion resistance. Moreover, in the steel plate for cans of this invention, aging can also be made small.

In the present invention, by containing the precipitation strengthening element and the solid solution strengthening element, by optimizing the component composition, structure, etc., the upper yield strength is 480 to 700 MPa as described above, the total elongation is 12% or more, and A steel plate for cans having excellent corrosion resistance is obtained.

Next, the component composition of the steel plate for cans of the present invention will be described. The steel plate for cans of the present invention is, by mass%, C: 0.020% or more and 0.130% or less, Si: 0.04% or less, Mn: 0.10% or more and 1.2% or less, P: 0.00. 007% or more and 0.100% or less, S: 0.03% or less, Al: 0.0010% or more and 0.10% or less, N: 0.0120% to 0.020% or less, and Nb: 0 0.010% or more and 0.050% or less, Ti: 0.010% or more and 0.050% or less, B: 0.0010% or more and 0.010% or less selected from one or two or more, with the balance being iron and It has a component composition consisting of inevitable impurities. Hereinafter, each component will be described. In the present specification, “%” in the description of the component composition means “% by mass”.

C: 0.020% or more and 0.130% or less In the steel sheet for cans of the present invention, it is essential that the upper yield strength (480 to 700 MPa) is achieved at a predetermined level or more and the total elongation is 12% or more. For that purpose, it is important to use precipitation strengthening by NbC produced by addition of Nb, precipitation strengthening by TiC produced by addition of Ti, and precipitation strengthening by BN produced by addition of B. In order to utilize precipitation strengthening by NbC and TiC, the C content of the steel plate for cans is important. Specifically, it is necessary to set the lower limit of the C content to 0.020%. Preferably, the lower limit for the C content is 0.030%. On the other hand, if the C content exceeds 0.130%, subperitectic cracking occurs during the cooling process during steel melting. For this reason, the upper limit of the C content is 0.130%. Preferably, the upper limit of the C content is 0.080%.

Si: 0.04% or less Si is an element that increases the strength of a steel sheet by solid solution strengthening. 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, the Si content is 0.02% or less. In the present invention, since it is possible to increase the upper yield strength by adjusting elements other than Si and manufacturing conditions, it is not necessary to use solid solution strengthening by Si. For this reason, in this invention, it is not necessary to contain Si. If a preferable example on the lower limit side is given about Si content, it is 0.001% or more.

Mn: 0.10% or more and 1.2% or less Mn increases the strength of the steel sheet by solid solution strengthening and also reduces the average ferrite grain size. The effect of reducing the average ferrite grain size is noticeably produced when the Mn content is 0.10% or more. Moreover, in order to ensure the target upper yield strength, the Mn content must be 0.10% or more. Therefore, the lower limit of the Mn content is 0.10%. Preferably, the lower limit of the Mn content is 0.20%. On the other hand, if the Mn content exceeds 1.2%, the corrosion resistance and surface properties are inferior. Therefore, the upper limit of the Mn content is 1.2%. Preferably, the upper limit of Mn content is 0.80%.

P: 0.007% or more and 0.100% or less P is an element having a large solid solution strengthening ability. However, if the P content exceeds 0.100%, the corrosion resistance is poor. For this reason, the P content is 0.100% or less. The P content is preferably 0.080% or less, more preferably 0.030% or less. Moreover, in order to make P content less than 0.007%, dephosphorization time rises significantly. For this reason, the P content is set to 0.007% or more.

S: 0.03% or less The steel plate for cans of the present invention has a high C and N content, and includes one or more selected from Nb, Ti, and B that form precipitates that cause slab cracking. For this reason, the slab edge tends to break in the straightening zone during continuous casting. In view of preventing slab cracking, the S content is set to 0.03% or less. Preferably, the S content is 0.02% or less. More preferably, the S content is 0.01% or less.

Al: 0.0010% or more and 0.10% or less Increasing the Al content results in an increase in the recrystallization temperature. Therefore, it is necessary to set the annealing temperature as high as the increase in the Al content. In the present invention, the recrystallization temperature rises due to the influence of other elements added to increase the upper yield strength, and the annealing temperature must be set high. Therefore, it is necessary to avoid the increase in the recrystallization temperature due to Al as much as possible, and the Al content is set to 0.10% or less. The Al content is preferably 0.070% or less. On the other hand, since it is difficult to completely remove the solid solution N, the Al content is set to 0.0010% or more from the viewpoint of inclusion control. Al is preferably added as a deoxidizer, and in order to obtain this effect, the Al content is preferably 0.010% or more.

N: 0.0120% to 0.020% or less N is an element necessary for increasing solid solution strengthening. In order to exert the effect of solid solution strengthening, the N content needs to be over 0.0120%. On the other hand, when there is too much N content, it will become easy to produce a slab crack in the lower correction zone where the temperature at the time of continuous casting falls. Therefore, the N content is 0.020% or less.

Nb: 0.010% or more and 0.050% or less Nb is an element having a high carbide generating ability and precipitates fine carbides. As a result, 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 0.010% or more, the lower limit of the Nb content is limited to 0.010%. Preferably, the lower limit is 0.015%. On the other hand, Nb brings about an increase in recrystallization temperature. Therefore, if the Nb content exceeds 0.050%, a large amount of unrecrystallized structure is caused by continuous annealing at an annealing temperature of 660 to 800 ° C. and a soaking time of 55 s or less. It remains difficult to anneal. For this reason, the upper limit of Nb content is limited to 0.050%.

Ti: 0.010% or more and 0.050% or less Ti is also added for the purpose of obtaining upper yield strength and yield elongation for the same reason as Nb. Since this effect occurs when the content is 0.010% or more, the lower limit is made 0.010%. Preferably the lower limit is 0.015%. The upper limit is also set to 0.050% from the viewpoint of the recrystallization temperature, similarly to Nb. Preferably the upper limit is 0.030%.

B: 0.0010% or more and 0.010% or less B has the effect of reducing yield elongation because it promotes cementite precipitation using B-based precipitates in ferrite grains as nuclei. Since this effect occurs when the content is 0.0010% or more, the lower limit is made 0.0010%. Preferably the lower limit is 0.0012%. The upper limit is made 0.010% from the viewpoint of the recrystallization temperature. Preferably the upper limit is 0.0050%.

The remainder other than the above components is Fe and inevitable impurities.

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

The structure has a ferrite phase, and the area ratio of the ferrite phase is 50% or more. The steel plate for cans of the present invention has a ferrite phase. From the viewpoint of securing strength and ductility, the steel plate for cans of the present invention has an area ratio of ferrite phase of 50% or more. The area ratio of the ferrite phase is preferably 70% or more, and more preferably 100%. The area ratio of the ferrite phase is determined from the structure photograph taken by grinding a cross section parallel to the rolling direction and then corroding with a nital liquid, in the field of view at a depth of 4/8 from the steel sheet surface in the thickness direction. It is determined by dividing and dividing the area of the ferrite phase by the total area.

From the viewpoint of setting the total elongation to 12% or more, the ferrite phase of the present invention preferably has a recrystallized structure. In addition to the recrystallized structure, the present invention may include a rolled structure that is a high-strength non-recrystallized structure. In the microstructure photograph (observed with an optical microscope) taken by corrosion with a night liquid, the rolled structure, which is an unrecrystallized structure, appears to be black due to corrosion because the crystal grains are crushed by rolling, and the recrystallized structure is ferrite. In the phase, since the crystal grains are grown by recrystallization, the crystal grains look white without being corroded.

(Solution N amount in region from surface to 1/8 depth position in plate thickness direction) / (Solution N in region from surface to 3/8 depth position to 4/8 depth position in plate thickness direction) Amount) ≤ 0.96
The upper yield strength can be further increased by increasing the amount of solute N in the region from the surface to the 3/8 depth position to the 4/8 depth position in the thickness direction. On the other hand, in the region from the surface to the 1 / 8th depth position in the plate thickness direction, the amount of dissolved N can be reduced to make it soft and to obtain good total elongation. From these, the solid solution N amount in the region from the surface to 1/8 depth position in the plate thickness direction and the solid solution in the region from the surface 3/8 depth position to 4/8 depth position in the plate thickness direction. The ratio of N amount is set to 0.96 or less. It is considered that by making the material difference in the plate thickness direction, it is possible to achieve both excellent ductility and strength while maintaining good corrosion resistance. The greater the material difference, the better the balance between ductility and strength, and both high strength and high ductility can be achieved. Therefore, the amount of dissolved N in the region from the surface to 1/8 depth position in the plate thickness direction and the solid solution in the region from the surface to 3/8 depth position to 4/8 depth position in the plate thickness direction. The ratio of N amount is preferably 0.93 or less, more preferably 0.91 or less, and even more preferably 0.89 or less. The amount of solute N in the region from the surface to the depth of 1/8 in the plate thickness direction increases as the hot rolling coiling temperature decreases, and decreases as the hot rolling coiling temperature increases. Become. Further, if the cooling rate after winding is reduced, the amount of solute N in the region from the surface to the 1/8 depth position in the plate thickness direction becomes a small value.

The amount of solute N in the region from the surface to the 1/8 depth position in the plate thickness direction is preferably 0.0114 to 0.0190 mass%. The amount of solute N in the region from the surface to the 3/8 depth position to the 4/8 depth position in the thickness direction is preferably 0.0118 to 0.0198 mass%.

The amount of solute N between the surface in the plate thickness direction and the depth of 1/8 of the plate thickness is extracted with 10% Br methanol to a depth of 1/8 of the plate thickness, and precipitated as AlN, BN, etc. The amount of N present is analyzed, and then the amount of N deposited as AlN, BN, etc. is subtracted from the total amount of N.

The amount of solute N between the depth position of 3/8 and the depth position of 4/8 from the surface in the plate thickness direction is obtained after oxalic acid polishing to the depth position of 3/8 of the plate thickness. Extracted and washed, extracted with 10% Br methanol, analyzed the amount of N deposited as AlN, BN, etc., and then calculated by subtracting the amount of N deposited as AlN, BN, etc. from the total N amount To do. The total N amount is expressed by mass%, and the sample is continuously included from the surface to the depth position of 4/8 that is the center in the plate thickness direction, and 4/8 that is the center in the plate thickness direction from the surface. The average N mass% up to the depth position was calculated.

In the present invention, the upper yield strength and the total elongation after heat treatment at 210 ° C. for 20 minutes are defined.
Upper yield strength: 480 to 700 MPa
In order to ensure the dent strength of the welding can and the pressure strength of the two-piece can, the upper yield strength is set to 480 MPa or more for the plate thickness of about 0.19 mm. The upper yield strength is preferably 500 MPa or more. On the other hand, in order to obtain an upper yield strength exceeding 700 MPa, a large amount of element needs to be added. Addition of a large amount of element may hinder the corrosion resistance of the steel plate for cans of the present invention. Therefore, the upper yield strength is 700 MPa or less. The upper yield strength of the steel plate for cans can be controlled to 480 to 700 MPa by employing the above component composition and, for example, the production conditions described later.

Total elongation: 12% or more If the total elongation of the steel plate for cans is less than 12%, there is a risk that defects such as cracks may occur in the production of cans formed by can body processing such as can expansion processing. There is. On the other hand, if the total elongation is less than 12%, cracks may occur during flange processing of the can. Therefore, the lower limit of total elongation is 12%. The total elongation is preferably 13% or more, more preferably 14% or more. For example, the total elongation can be controlled to 12% or more by setting the amount of ferrite phase as a recrystallized structure in a specific range and then setting the rolling reduction ratio of secondary cold rolling after annealing in a specific range. The total elongation obtained when producing by controlling the reduction ratio of secondary cold rolling is preferably 35% or less, more preferably 25% or less.

The plate thickness of the steel plate for cans of the present invention is not particularly limited, but may be 0.4 mm or less, 0.3 mm or less, or 0.2 mm or less.

The steel plate for cans of the present invention may further be provided with a plating layer. Examples of the plating layer include an Sn plating layer, a Cr plating layer such as tin-free, an Ni plating layer, and an Sn—Ni plating layer.

Next, the manufacturing method of the steel plate for cans of the present invention will be described. It is preferable that the steel plate for cans of this invention is manufactured with the manufacturing method which has a hot rolling process, a primary cold rolling process, an annealing process, and a secondary cold rolling process. Hereinafter, each manufacturing process will be described.

Hot rolling process The hot rolling process is a process in which steel is rolled at a finish rolling temperature of Ar3 transformation point or higher, wound at a winding temperature of 500 to 620 ° C, and cooled at a cooling rate of 10 ° C / hr or lower after winding. It is the process of cooling.

The steel used as a rolling material will be explained. Steel is obtained by melting molten steel adjusted to the above-described component composition by a known melting method using a converter or the like, and then forming a rolled material by a commonly used casting method such as a continuous casting method. It is done.

鋼 Hot rolled steel sheet is manufactured by hot rolling the steel obtained as described above. At the start of hot rolling, the temperature of the steel is preferably 1200 ° C. or higher.

Also, the finish rolling temperature in hot rolling is set to the Ar3 transformation point or higher. In the present invention, the Ar3 transformation point is determined at a temperature at which the volume of the sample expands due to the γ → α transformation in the process of heating the sample to 1200 ° C. and then slowly cooling it with a processing for master. The finish rolling temperature in the hot rolling is an important condition for securing the upper yield strength. When the finish rolling temperature is lower than the Ar3 transformation point, the grain yields by the two-phase hot rolling of γ + α, and the crystal grains after cold rolling and annealing are coarsened, so that the upper yield strength is lowered. Therefore, the finish rolling temperature in the hot rolling is limited to the Ar3 transformation point or higher. The finish rolling temperature in hot rolling (finish rolling finishing temperature) is preferably in the range of Ar3 transformation point to Ar3 transformation point + 20 ° C. In addition, although the upper limit of finish rolling temperature is not specifically limited, It is preferable to make 980 degreeC into an upper limit for the reason of suppressing scale generation.

The coiling temperature in the hot rolling process is an important condition for controlling the upper yield strength and the total elongation, which are important in the present invention. When the coiling temperature is less than 500 ° C., the surface layer is cooled quickly, so the amount of AlN in the surface layer decreases and the amount of solid solution N in the surface layer increases. For this reason, the minimum of coiling temperature shall be 500 degreeC. Preferably, the lower limit of the winding temperature is 550 ° C. On the other hand, when the coiling temperature exceeds 620 ° C., N added for solid solution strengthening becomes AlN and precipitates in the central layer, so that the amount of solid solution N decreases, and as a result, the upper yield strength decreases. . For this reason, the upper limit of coiling temperature shall be 620 degreeC. Preferably, the upper limit of the coiling temperature is 600 ° C.

A cooling rate of 10 ° C./hr or less after winding in the hot rolling process is an important condition. When the cooling rate after winding exceeds 10 ° C./hr, the surface layer is rapidly cooled, so that the precipitation of AlN on the surface layer decreases, the amount of solute N increases, and the total elongation decreases. On the other hand, the lower limit of the cooling rate is not particularly limited, but is preferably 2 ° C./hr or more from the viewpoint of the production efficiency of the steel sheet.

Primary cold rolling step The primary cold rolling step is a step of cold rolling at a rolling reduction of 80% or more after the hot rolling step. In addition, another process may be suitably included after the hot rolling process and before the primary cold rolling process, or the primary cold rolling process may be performed immediately after the hot rolling process.

For example, it is preferable to remove the surface layer scale formed in the hot rolling process. The method for removing the surface scale is not particularly limited, and various methods such as chemical removal such as pickling and physical removal can be applied.

The rolling reduction in the primary cold rolling process is one of the important conditions in the present invention. If the rolling reduction in the primary cold rolling process is less than 80%, it is difficult to produce a steel sheet having an upper yield strength of 480 MPa or more. Furthermore, when the reduction ratio in this process is less than 80%, in order to obtain a plate thickness (about 0.17 mm) similar to that of a conventional DR material in which the reduction ratio in the secondary cold rolling process is 20% or more At least the thickness of the hot rolled sheet needs to be 0.9 mm or less. However, in operation, it is difficult to set the thickness of the hot rolled sheet to 0.9 mm or less. Therefore, the rolling reduction in this step is 80% or more. The upper limit of the rolling reduction in the primary cold rolling step is not particularly limited, but a rolling reduction of 95% or less is preferable from the viewpoint of suppressing surface defects.

Annealing Step An annealing step is a step of performing continuous annealing after the primary cold rolling step at a soaking temperature of 660 to 800 ° C. and a soaking time of 55 seconds or less. Here, the unit “s” means “second”. In addition, another process may be appropriately included before the annealing process after the primary cold rolling process, or the annealing process may be performed immediately after the primary cold rolling process.

A continuous annealing device is used for annealing. In order to make the structure of the steel plate more uniform, the soaking temperature is set to 660 ° C. or higher. On the other hand, in order to perform continuous annealing under conditions where the soaking temperature exceeds 800 ° C., it is necessary to reduce the conveying speed as much as possible in order to prevent breakage of the steel sheet, and productivity is lowered. From the above points, the soaking temperature is set in the range of 660 to 800 ° C. The soaking temperature is preferably 660 to 710 ° C, more preferably 660 to 705 ° C.

Since the productivity cannot be secured at a speed where the soaking time exceeds 55 s, the soaking time is 55 s or less. The soaking time is preferably 40 s or less. The lower limit of the soaking time is not particularly limited, but in order to shorten the soaking time, it is necessary to increase the transport speed, so that it is difficult to stably transport without meandering. It is preferable that

Secondary cold rolling step The secondary cold rolling step is a step of cold rolling at a rolling reduction of 1 to 19% after the annealing step. In addition, another process may be appropriately included before the secondary cold rolling process after the annealing process, or the secondary cold rolling process may be performed immediately after the annealing process.

If the reduction ratio in the secondary cold rolling after annealing is the same as the normal DR material production conditions (20% or more), the strain introduced during processing increases, so the total elongation decreases. In the present invention, since it is necessary to ensure a total elongation of 12% or more with an ultrathin material, the reduction ratio in the secondary cold rolling is set to 19% or less. In addition, secondary cold rolling has a role of imparting surface roughness of the steel sheet, and in order to uniformly impart surface roughness to the steel sheet, the reduction ratio of secondary cold rolling needs to be 1% or more. The rolling reduction in the secondary cold rolling process may be 8 to 19%.

After the secondary cold rolling process In the production method of the present invention, various processes can be performed even after the secondary cold rolling process. For example, you may perform processes, such as a plating process, a paint baking process, and a film lamination.

Steel containing the composition shown in Table 1 and the balance being Fe and inevitable impurities was melted in an actual converter to obtain a steel slab. The obtained steel slab was reheated, then hot rolled and wound up. Then, after pickling, primary cold rolling was performed to produce a thin steel plate. The obtained thin steel plate was heated at a heating rate of 15 ° C./sec and subjected to continuous annealing under the soaking conditions described in Table 2. Then, after cooling, secondary cold rolling was performed, and normal Sn plating was continuously performed to obtain a tinplate. Detailed production conditions are shown in Table 2. “Final plate thickness” in Table 2 is a thickness not including the Sn plating layer.

For the Sn-plated steel sheet (cover) obtained as described above, after performing a heat treatment corresponding to a coating baking process at 210 ° C. for 20 minutes, a tensile test is performed to measure the upper yield strength and the total elongation. The crystal structure of the ferrite phase was also investigated. The measurement method and survey method are as follows.

The tensile test was performed using a JIS No. 5 size tensile test piece, the upper yield strength (U-YP) was measured according to JIS Z 2241, and the total elongation (El) was measured according to JIS Z 2241. The obtained results are shown in Table 3.

The crystal structure was observed with an optical microscope by polishing the sample, corroding the crystal grain boundary with nital. When the crystal structure was observed, the steel plate for cans of the invention example had an area ratio of the ferrite phase of 50% or more. The ferrite phase had a recrystallized structure.

The solute N amount in the region from the surface to 1/8 depth position in the plate thickness direction, and the solute N amount in the region from the surface 3/8 depth position to 4/8 depth position are nitrided from the total N amount It measured by the method of reducing N amount of a thing. The measurement results are shown in Table 4.

Compressive strength: After roll form, welding, neck forming, and flange forming using steel plates, a lid was wound and a blank can sample was prepared, placed in a chamber, and the pressure at which the sample was buckled after being pressurized with compressed air was measured. A buckling pressure of 0.2 MPa or more was rated as ◎, 0.14 to 0.13 MPa as ◯, and less than 0.13 MPa as x (failed).

Formability: Wrinkles at the time of neck forming when roll forming, welding, and neck forming using a steel plate were observed. The case where there was no visual wrinkle was rated as 、, the case where one fine wrinkle was seen visually was marked as ○, and the case where two or more fine wrinkles were seen visually was marked as x (failed).

Corrosion resistance: It was evaluated using an alloy tin couple (ATC) test facility used for evaluating the corrosion resistance of electroplated tin. What ATC value is less than 0.05μA / cm ² ◎, those 0.05 ~ 0.12μA / cm ² ○, and a × (fail) those exceeding 0.12μA / cm ^2.

According to the present invention, it is possible to obtain a steel plate for cans having high strength, excellent ductility, and good corrosion resistance even for highly corrosive contents. The present invention is most suitable as a steel plate for cans centering on a three-piece can with a high degree of can body processing and a two-piece can whose bottom portion is processed by several percent.

Claims

In mass%, C: 0.020% or more and 0.130% or less, Si: 0.04% or less, Mn: 0.10% or more and 1.2% or less, P: 0.007% or more and 0.100% or less S: 0.03% or less, Al: 0.0010% or more and 0.10% or less, N: 0.0120% or more and 0.020% or less, and Nb: 0.010% or more and 0.050% Hereinafter, a component composition comprising Ti: 0.010% or more and 0.050% or less, B: one or more selected from 0.0010% or more and 0.010% or less, with the balance being iron and inevitable impurities. Have
The structure has a ferrite phase, the area ratio of the ferrite phase is 50% or more,
The upper yield strength after heat treatment at 210 ° C. for 20 minutes is 480 to 700 MPa, and the total elongation is 12% or more.
The ratio of the amount of solute N in the region from the surface to 1/8 depth position in the sheet thickness direction and the amount of solute N in the region from the surface to 3/8 depth position to 4/8 depth position is as follows. A steel plate for cans that satisfies Equation 1 below.
(Solution N amount in region from surface to 1/8 depth position in plate thickness direction) / (Solution N in region from surface to 3/8 depth position to 4/8 depth position in plate thickness direction) Amount) ≦ 0.96 (Formula 1)
The steel plate for cans according to claim 1, wherein the ferrite phase has a recrystallized structure.
The steel plate for cans according to claim 1 or 2, wherein the area ratio of the ferrite phase is 70% or more.
A method for producing a steel plate for cans according to any one of claims 1 to 3,
A hot rolling step in which the steel is rolled at a finish rolling temperature of Ar3 transformation point or higher, wound at a winding temperature of 500 to 620 ° C., and cooled at a cooling rate of 10 ° C./hr or lower after winding;
After the hot rolling step, a primary cold rolling step of rolling at a rolling reduction of 80% or more,
After the primary cold rolling step, an annealing step of continuous annealing at a soaking temperature of 660 to 800 ° C. and a soaking time of 55 s or less;
And a secondary cold rolling step of rolling at a rolling reduction of 1 to 19% after the annealing step.