WO2016031234A1

WO2016031234A1 - Steel sheet for cans and method for producing same

Info

Publication number: WO2016031234A1
Application number: PCT/JP2015/004269
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: 2014-08-29
Filing date: 2015-08-25
Publication date: 2016-03-03
Also published as: JP5939368B1; KR101923839B1; PH12017500200A1; MY177004A; PH12017500200B1; KR20170029635A; CN106605006A; TWI593811B; TW201610180A; JPWO2016031234A1; CN106605006B

Abstract

This invention provides: a highly workable, high-strength steel sheet for cans that has an upper yield strength of 450-600 MPa after baking the coating thereof, a total elongation characteristic of 13% or more, and good corrosion resistance to even highly corrosive contents; and a method for producing same. The highly workable, high-strength steel sheet for cans is characterized by having a component composition comprising, in mass %, C: over 0.020% and up to 0.130%, Si: 0.04% or less, Mn: 0.10-1.20%, P: 0.100% or less, S: 0.030% or less, Al: 0.10% or less, N: over 0.0120% and up to 0.020%, and Nb: 0.004-0.040%, the remainder comprising iron and inevitable impurities. The highly workable, high-strength steel sheet for cans is further characterized in that: the ratio of a precipitated Nb amount and a total Nb amount as expressed by precipitated Nb amount / total Nb amount ≥ 0.30; the average Nb precipitate particle size is 20 nm or less; the average ferrite grain size is 7.0 μm or less; the upper yield strength after baking the coating thereof is 450-630 MPa; and the total elongation is 13% or more.

Description

Steel plate for can and manufacturing method thereof

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 that requires pressure strength, and a method for manufacturing the same. Specifically, the present invention relates to a steel plate for cans having a large total elongation and excellent upper yield strength, and a method for producing the same.

In recent years, in order to increase 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 bottle cans and deformed cans.

Measures to reduce can manufacturing costs include cost reduction of materials. 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. Accordingly, it is not possible to use a steel plate that is simply thinned at a location where a high-strength material such as a DRD (drawing and redrawing) can or a can body of a welding 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 sheet manufactured using the DR method has a high strength but a small total elongation.

On the other hand, it is difficult from the viewpoint of workability to use a DR material with poor ductility as a material for a can formed by can body processing with a strong working degree such as a deformed can that has recently been put on the market. In addition, the DR material has a higher manufacturing cost because the number of manufacturing steps is increased as compared with a steel sheet that is subjected to temper rolling after normal annealing.

In order to avoid such defects of the DR material, secondary cold rolling is omitted, various strengthening methods are used, and properties are controlled in the primary cold rolling and annealing processes. The following patent document proposes a method for producing a high-strength steel sheet by light reduction at a sub-cold rolling reduction ratio of about 5% or less.

In Patent Document 1, in mass%, C: 0.02% or less, Si: 0.10% or less, Mn: 1.5% or less, P: 0.20% or less, S: 0.01% or less, Al: 0.01% or less, N: 0.0050 to 0.0250%, and (Solution C + Solution N) 0.0050% or more, with the balance being Fe and inevitable impurities , Having a structure having a recrystallization rate of 90% or more, bake hardening amount (BH amount): 100 MPa or more, increase amount of tensile strength by paint baking treatment ΔTS: 30 MPa or more, yield stress after painting / baking treatment: An ultra-thin cold-rolled steel sheet for high-strength cans having a sheet thickness of 0.3 mm or less, characterized by having 550 MPa or more is disclosed. Further, Patent Document 1 adjusts hot rolling conditions and cooling conditions, rapidly cools to a low temperature range after continuous annealing, and effectively uses the effect of solid solution C amount + solid solution N amount to use the age hardening phenomenon. By doing so, a technique for obtaining a steel plate for a high-strength can similar to a DR material has been proposed. 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.

In Patent Document 2, by weight ratio, C: 0.020 to 0.150%, Si: 0.05% or less, Mn: 1.00% or less, P: 0.050% or less, S: 0.010 %, N: 0.0100% or less, Al: 0.100% or less, Nb: 0.005 to 0.025%, and the balance consists of inevitable impurities and iron, and a substantial ferrite single phase structure The yield strength is 40 kgf / mm ² or more, the average crystal grain size is 10 μm or less, and the plate thickness is 0.300 mm or less. A steel plate for can making having excellent surface properties after can and having sufficient can strength is disclosed. Patent Document 2 proposes a steel plate having a balance between strength and ductility by combining precipitation strengthening with Nb carbide and refinement strengthening with Nb, Ti, and B carbonitrides.

In Patent Document 3, C: 0.001 to 0.010 wt%, Si: ≦ 0.05 wt%, Mn: ≦ 0.9 wt%, P: 0.131 to 0.200 wt%, S: ≦ 0.04 wt%, Al: 0. Disclosed is a steel plate for a thinned deep-drawn ironing can characterized by comprising a low-carbon steel plate containing 006 to 0.08 wt%, N: 0.0010 to 0.015 wt%, the balance Fe and inevitable impurities. A method of increasing the strength by using solid solution strengthening such as Mn, P, and N has been proposed.

JP 2001-107186 A JP-A-8-325670 JP 2004-183074 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 (total elongation), and corrosion resistance, but cannot produce a steel sheet that satisfies all of the above characteristics.

For example, although the method described in Patent Document 1 is an effective method for increasing the strength, since the amount of solute C and solute N in the steel is large, the yield elongation increases. Yield elongation occurs because solute C or solute N adheres to dislocations, thereby reducing movable dislocations. In a strain region where the yield elongation is large, a local yield phenomenon occurs, causing uneven deformation, and thus wrinkles called stretcher strains may occur, thereby impairing the appearance.

In Patent Document 2, high strength is realized by precipitation strengthening, and steel with a balance between strength and ductility is proposed, but yield elongation is not taken into consideration, and the manufacturing method described in Patent Document 2 is The yield elongation value targeted by the invention cannot be obtained.

Patent Document 3 proposes high strength by solid solution strengthening. In the technique described in this document, since P and Mn, which are generally known as elements that inhibit corrosion resistance, are added in excess, there is a high risk of inhibiting corrosion resistance.

The present invention has been made in view of such circumstances, and has an upper yield strength of 450 to 600 MPa after coating and baking, a total elongation of 13% or more, and good corrosion resistance even for highly corrosive contents. An object of the present invention is to provide a steel sheet for cans with high workability and high strength and a method for producing the same.

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 work strengthening, the balance between precipitation strengthening and solid solution strengthening can increase the strength without impairing the elongation.

Furthermore, the reduction rate in the secondary cold rolling is set to 1 to 19%, and the work strength at a reduction rate lower than that in the conventional secondary cold rolling can enhance the strength without reducing the total elongation.

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

The present invention has completed the high-workability and high-strength steel plate and its manufacturing method by comprehensively managing the components and the manufacturing method based on the above findings.

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

[1] By mass%, C: more than 0.020% and 0.130% or less, Si: 0.04% or less, Mn: 0.10 to 1.20%, P: 0.100% or less, S: 0 .030% or less, Al: 0.10% or less, N: 0.0120% to 0.020% or less, Nb: 0.004 to 0.040%, the balance being iron and inevitable impurities The ratio of the precipitated Nb amount to the total Nb amount is the ratio of precipitated Nb amount / total Nb amount ≧ 0.30, the Nb precipitate average particle size is 20 nm or less, and the ferrite average crystal particle size is 7. A high workability high strength steel sheet for cans, characterized by having an upper yield strength of 450 to 630 MPa and a total elongation of 13% or more.

[2] The volume ratio of Nb precipitates in the region from the surface to 1/8 depth position in the plate thickness direction, and the Nb precipitates in the region from the surface to 3/8 depth position to 4/8 depth position. The high workability steel plate for cans according to [1], wherein the ratio of volume ratios satisfies the following formula 1.
(Nb precipitate volume ratio of 3/8 to 4/8) / (Nb precipitate volume ratio of surface to 1/8) ≧ 1.10.
[3] A method for producing a steel plate for high workability and high strength can according to [1] or [2], wherein the steel is rolled under a condition that the finish rolling temperature is not less than Ar3 transformation point and not more than 990 ° C. A hot rolling step of winding at a temperature of 400 ° C. or higher and lower than 600 ° C., a primary cold rolling step of pickling after the hot rolling step and rolling at a rolling reduction of 80% or higher, After the primary cold rolling process, an annealing process in which the soaking temperature is 650 to 780 ° C. and the soaking time is 10 seconds to 55 seconds, and after the annealing process, the rolling reduction is 1 to 19%. A method for producing a steel sheet for high workability and high strength cans, comprising a secondary cold rolling step for rolling.

According to the present invention, a high workability high strength steel sheet having an upper yield strength of 450 to 630 MPa and a total elongation of 13% or more can be obtained. More specifically, in the present invention, precipitation strengthening by Nb, solid solution strengthening by N, and work strengthening by performing secondary cold rolling at a low pressure reduction rate of 1 to 19% after annealing are harmful to other characteristics. Strengthening the composite without increasing the strength. As a result, the final yield is 450 to 630 MPa in the final product while the total elongation is 13% or more.

Furthermore, according to the present invention, by increasing the strength of the original plate, it is possible to ensure high strength of the can even if the welded can is made thinner (thinned). Even if the high workability high strength steel sheet of the present invention is applied to a two-piece can application that requires the pressure resistance of the bottom portion, it is possible to obtain a high pressure resistance with the current gauge. In addition, by increasing 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 sheet for high workability and high strength can of the present invention is excellent in any of strength, workability 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 sheet for high workability and high strength can of the present invention has an excellent yield resistance with an upper yield strength (hereinafter sometimes referred to as U-YP) of 450 to 630 MPa and a total elongation of 13% or more. Moreover, in the high workability high strength steel plate for cans of the present invention, aging can be reduced.

In the present invention, Nb is added as a precipitation strengthening element, N is added as a solid solution strengthening element, and the upper yield strength is increased by work strengthening by performing secondary cold rolling with a rolling reduction of 1 to 19% after annealing. It can be made into a range. Furthermore, if the upper yield strength is increased by the above-described method in a specific component system, the total elongation is also high. Having excellent upper yield strength and high overall elongation is a feature of the present invention and is the most important requirement. Thus, by adding the precipitation strengthening element and the solid solution strengthening element and optimizing the component composition, structure, and production conditions so that the total elongation can be made high, the upper yield strength is 450 to 630 MPa, A steel with high workability and high strength cans having an elongation of 13% or more is obtained.

Next, the component composition of the steel plate for high workability and high strength can of the present invention (in this specification, the steel plate for high workability and high strength can may be referred to as a steel plate for cans) will be described. The steel sheet for high workability and high strength can of the present invention is in mass%, C: more than 0.020% and 0.130% or less, Si: 0.04% or less, Mn: 0.10 to 1.20%, P : 0.100% or less, S: 0.030% or less, Al: 0.10% or less, N: more than 0.0120% and 0.020% or less, Nb: 0.004 to 0.040%, The balance has a composition composed of iron and 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% to 0.130% or less In the steel sheet for cans according to the present invention, it is essential that the upper yield strength (450 to 630 MPa) is not less than a predetermined value after continuous annealing and at the same time the total elongation is 13% or more. It is. For that purpose, it is important to make the ferrite average crystal grain size 7.0 μm or less and to utilize precipitation strengthening by NbC generated by adding Nb. In order to adjust the ferrite average crystal grain size to the above range and utilize precipitation strengthening by NbC, the C content of the steel plate for cans is important. Specifically, the C content needs to 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. Moreover, in order to reduce this defect, it is necessary to make rolling speed small. 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%, subperitectic cracking occurs during the cooling process during steel melting. For this reason, 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 increases, and the deformation resistance during cold rolling tends to increase, so as to avoid surface defects after rolling. Since it may be necessary to reduce the rolling speed, the C content is preferably more than 0.020% to 0.040% from the viewpoint of ease of manufacture.

Si: 0.04% or less Si is an element that increases the strength of steel 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. In the present invention, since the upper yield strength is increased 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.

Mn: 0.10 to 1.20%
Mn increases the strength of the steel by solid solution strengthening and also reduces the ferrite average crystal 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%. On the other hand, if the Mn content exceeds 1.20%, the corrosion resistance and surface characteristics are inferior. Therefore, the upper limit of the Mn content is 1.20%.

P: 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.

S: 0.030% or less In addition, although the steel plate for high workability and high strength cans of the present invention may not contain S, it is preferable to set S to 0.030% or less when implementing this patent. . Since the steel plate for cans of the present invention has a high Nb, C, and N content, the slab edge tends to break in the straightening zone during continuous casting. From the viewpoint of preventing 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 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. Therefore, the Al content is set to 0.10% or less. 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. 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. On the other hand, in order to exert the effect of solid solution strengthening, the N content needs to be over 0.0120%.

Nb: 0.004 to 0.040%
Nb is an important additive element in the present invention. 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 and surface properties 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, Nb increases the recrystallization temperature. Therefore, if the Nb content exceeds 0.040%, non-recrystallization is not achieved in continuous annealing at an annealing temperature of 650 to 780 ° C. and a soaking time of 10 to 55 seconds. It becomes difficult to anneal, such as remaining part. For this reason, the upper limit of 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 during cold rolling.

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

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

Average ferrite grain size: 7.0 μm or less The structure of the steel sheet for cans of the present invention is a ferrite single-phase structure. The average ferrite grain size affects not only the upper yield strength but also the surface properties during drawing. If the ferrite average crystal grain size of the final product exceeds 7.0 μm, after the drawing process, a rough skin phenomenon occurs in part and the beauty of the surface appearance is lost. Therefore, the ferrite average crystal grain size is set to 7.0 μm or less. In order to reduce the average grain size of ferrite, the element that forms fine precipitates that suppress the grain growth of ferrite crystals by reducing the soaking temperature during continuous annealing or pin the grain boundary migration. The ferrite average crystal grain size is preferably 5.0 μm or more because the production cost is increased because it is necessary to add a large amount of. The ferrite average crystal grain size may be in the above range after coating baking, but since the ferrite average crystal grain size does not change before and after the coating baking process, it may be measured either before or after the coating baking process. In the present invention, the coating baking process is a process corresponding to heating during coating baking and laminating, and specifically refers to heat treatment in the range of 170 to 265 ° C. for 12 seconds to 30 minutes. In the examples described later, heat treatment is performed at 210 ° C. for 20 minutes as a standard condition.

Also, the ferrite average crystal grain size is controlled by the component composition, the cold rolling reduction ratio, and the annealing temperature. Specifically, the ferrite average crystal grain size of 7.0 μm or less can be obtained by adopting the above component composition and adopting the production conditions described later. Increasing the soaking temperature in continuous annealing increases the average ferrite grain size, and decreasing the soaking temperature decreases the average ferrite grain size.

Precipitation Nb amount / Total Nb amount ≧ 0.30
By setting the ratio of the precipitated Nb amount to the total Nb amount (precipitated Nb amount / total Nb amount) to be 0.30 or more, the target upper yield strength of 450 to 630 MPa can be realized while improving the total elongation and corrosion resistance. Further, the amount of precipitated Nb / total Nb amount is preferably 0.9 or less because the particle size of the precipitated Nb becomes coarse when the amount of precipitated Nb increases. The deposited Nb amount / total Nb amount may be within the above range after baking. Since the deposited Nb amount / total Nb amount does not change before and after the paint baking process, it may be measured before or after the paint baking process. Since the paint baking process is the same as described above, the description thereof is omitted.

As a method for adjusting so as to satisfy the amount of precipitated Nb / total Nb ≧ 0.30, for example, the amount of precipitated Nb can be increased by increasing the soaking temperature during continuous annealing.

Average particle size of Nb precipitates: 20 nm or less When the average particle size of Nb precipitates is larger than 20 nm, the effect of increasing strength due to dislocation pinning by precipitates cannot be expected. For this reason, in order to obtain a predetermined strength while improving the total elongation and corrosion resistance, the Nb precipitate average particle size is set to 20 nm or less. In addition, the value measured by the method as described in an Example is employ | adopted for Nb precipitate average particle diameter. Here, the Nb precipitate average particle diameter may be within the above range after the coating baking. Since the average particle size of Nb precipitates does not change before and after the paint baking process, it may be measured before or after the paint baking process. Since the paint baking process is the same as described above, the description thereof is omitted.

As a method for adjusting the average particle size of Nb precipitates to 20 nm or less, for example, when it is desired to reduce the average particle size of Nb precipitates, the soaking time of continuous annealing may be shortened to suppress the growth of Nb precipitates. .

(3/8 to 4/8 Nb precipitate volume fraction) / (surface to 1/8 Nb precipitate volume fraction) ≧ 1.10.
The volume fraction of Nb precipitates in the region from the surface to the 1/8 depth position in the plate thickness direction, and the volume fraction of Nb precipitates in the region from the surface to the 3/8 depth position to the 4/8 depth position. When the ratio is 1.10 or less, the density of 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 is increased in the central layer to increase the upper yield strength. Raise more. In the region from the surface to the 1 / 8th depth, the density of Nb precipitates is reduced to obtain a better overall elongation. Thus, by making a material difference in the plate thickness direction, both high workability and high strength can be achieved in an extremely excellent state. The ratio of the volume ratio means that the ratio of the volume ratio is in the above range after baking. Since the paint baking process is the same as described above, the description thereof is omitted.

The volume ratio of Nb precipitates in the region from the surface to the 1 / 8th depth position in the plate thickness direction can be obtained by, for example, lowering the temperature of the final finish rolling in hot rolling to roughen the surface layer, If adjusted by a method of promoting Nb precipitation, the value becomes large, and if the temperature of the final finish rolling is increased to refine the surface layer and suppress Nb precipitation in the crystal grains of the surface layer, the value becomes small.

The volume fraction of Nb precipitates in the region from the 3/8 depth position to the 4/8 depth position from the surface becomes large if, for example, the Nb precipitate is grown by increasing the coiling temperature of hot rolling. Thus, if the coiling temperature of hot rolling is lowered to suppress the growth of Nb precipitates, the value becomes small.

Upper yield strength: 450-630MPa
For the plate thickness of about 0.2 mm, the upper yield strength is set to 450 MPa or more in order to ensure the paneling strength and dent strength of the welded can and the pressure strength of the two-piece can. On the other hand, in order to obtain an upper yield strength exceeding 630 MPa, a large amount of element addition is required. Addition of a large amount of element has a risk of inhibiting the corrosion resistance of the steel sheet for cans of the present invention. Therefore, the upper yield strength is 630 MPa or less. The upper yield strength can be controlled to a target value by employing the above component composition and employing the manufacturing conditions described later. In the present invention, it means that the upper yield strength is in the above range after baking. Since the paint baking process is the same as described above, the description thereof is omitted.

Total elongation: 13% or more When the total elongation is less than 13%, for example, it becomes difficult to apply the steel plate for cans of the present invention to the production of cans formed by can barrel processing such as can expansion processing. On the other hand, if the total elongation is less than 13%, cracks are generated during the flange processing of the can, making it difficult to apply the steel plate for cans of the present invention to the production of the can. Therefore, the lower limit of total elongation is 13%. The total elongation is controlled to a target value by setting the component composition in a specific range and setting the rolling reduction ratio of secondary cold rolling after annealing in a specific range. In the present invention, it means that the total elongation after baking is in the above range. Since the paint baking process is the same as described above, the description thereof is omitted. In the present invention, the total elongation is usually 35% or less.

Next, an example of a production method capable of suitably producing the steel plate for cans of the present invention will be described. The steel plate for cans of this invention is manufactured by the 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 (for example, slab) is hot-rolled under conditions where the finishing temperature is Ar3 transformation point or higher and 990 ° C or lower, and the winding temperature is 400 ° C or higher and lower than 600 ° C. This is a winding process.

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

The hot rolled sheet is manufactured by subjecting the rolled material obtained as described above to hot rolling. At the start of hot rolling, the temperature of the rolled material is preferably set to 1230 ° C. or higher.

Also, the finishing temperature in the hot rolling is not less than the Ar3 transformation point. 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 is caused by two-phase hot rolling of γ + α, so that the upper yield strength is lowered and the pressure strength is insufficient. Therefore, the hot rolling finishing temperature is limited to the Ar3 transformation point or higher. When the finish rolling temperature is higher than 990 ° C., the total elongation is insufficient and the formability deteriorates. Also, from the viewpoint of preventing scale generation at high temperatures, the finish rolling temperature is 990 ° C. as the upper limit.

The coiling temperature in the hot rolling process is an important factor in controlling the upper yield strength and the total elongation, which are important in the present invention, to the target values. When the coiling temperature is set to 600 ° C. or higher, N added for solid solution strengthening is precipitated as AlN, so that the amount of solid solution N decreases, and as a result, the upper yield strength decreases. For this reason, the coiling temperature was set to less than 600 ° C. Further, when the winding temperature is less than 400 ° C., the total elongation is lowered and the moldability is deteriorated, so the winding temperature is set to 400 ° C. or more. In addition, when it cools rapidly in order to lower coiling temperature, since cooling will become non-uniform | heterogenous and plate shape will deteriorate, coiling temperature makes 400 degreeC a minimum from a viewpoint of manufacturing efficiency. Further, from the viewpoint of controlling Nb precipitates, the cooling rate after winding is preferably slow cooling, preferably cooling at 11.5 ° C./hour or less, and further cooling at 6.3 ° C./hour or less. Preferably, cooling at 1.7 ° C./hour or less is more preferable. After such cooling, it is preferable to perform the next step after the temperature reaches 200 ° C. or lower, more preferably 100 ° C. or lower, and further preferably 50 ° C. or lower.

Primary cold rolling step The primary cold rolling step is a step of pickling a steel plate (hot-rolled plate) after the hot rolling step and rolling under a condition where the rolling reduction is 80% or more.

Pickling is not particularly limited as long as the surface scale can be removed. Pickling can be performed by a commonly performed method.

The rolling reduction in primary cold rolling is one of the important conditions in the present invention. If the rolling reduction in 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. Further, when the reduction ratio in this step is less than 80%, at least the thickness of the hot-rolled plate needs to be 1 mm or less in order to obtain a plate thickness (about 0.17 mm) comparable to that of the DR material. However, in operation, it is difficult to set the thickness of the hot-rolled sheet to 1 mm or less. Therefore, the rolling reduction in this step is 80% or more.

Annealing Step An annealing step is a step in which a steel plate (cold rolled plate) is continuously annealed after the primary cold rolling step under conditions of a soaking temperature of 650 to 780 ° C. and a soaking time of 10 s to 55 s.

For annealing, continuous annealing is used. The soaking temperature needs to be equal to or higher than the recrystallization temperature of the steel sheet in order to ensure good workability, and in order to make the structure more uniform and ensure the total elongation, the soaking temperature is 650 ° C. or more. Limited to. On the other hand, when the soaking temperature exceeds 780 ° C., the ferrite crystal grain size is increased, the upper yield strength is lowered, and the pressure resistance is insufficient. Moreover, in order to carry out the continuous annealing on the conditions exceeding 780 degreeC, in order to prevent the fracture | rupture of a steel plate, it is necessary to reduce a conveyance speed as much as possible, and productivity falls. For this reason, the soaking temperature is set to a range of 650 to 780 ° C.

At a speed at which the soaking time exceeds 55 s, the Nb precipitate particle size becomes too large, the upper yield strength decreases, the pressure strength is insufficient, and the productivity cannot be ensured, so the soaking time is 55 s or less. And If the soaking time is less than 10 s, heating unevenness occurs during high-speed plate feeding, the Nb precipitation form does not become the desired form, the total elongation deteriorates, and the tension in the furnace becomes unstable and the plate may break. Therefore, the soaking time is 10 s or more.

Secondary cold rolling step The secondary cold rolling step is a step of rolling the steel plate (annealed plate) under the condition of a rolling reduction of 1 to 19% after the annealing step.

If the rolling reduction in the secondary cold rolling after annealing is made the same as the normal DR material manufacturing conditions, the strain introduced at the time of processing increases, so the total elongation decreases. In the present invention, since it is necessary to ensure a total elongation of 13% or more with an ultrathin material, the reduction ratio in the secondary cold rolling is set to 19% or less. Moreover, in order to adjust the surface roughness of the steel sheet, the rolling reduction of the secondary cold rolling needs to be 1% or more because the unevenness of the roll is transferred to the steel sheet. The rolling reduction in secondary cold rolling is preferably 4 to 12% from the viewpoint of preventing slippage of the steel sheet and roll during rolling and ensuring the total elongation.

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 steel plate temperature before pickling was in the range of 25 to 60 ° C. over the entire length of the coil. The obtained thin steel sheet was heated at a heating rate of 15 ° C./sec. Then, continuous annealing was performed. Next, after cooling, secondary cold rolling was performed, and normal Sn plating was continuously performed to obtain tinplate. Detailed production conditions are shown in Table 2. Note that the Ar3 transformation point was calculated by measuring the temperature at which the volume change was greatest due to the γ → α transformation during cooling.

The plated steel plate obtained above is subjected to a heat treatment corresponding to a coating baking process at 210 ° C. for 20 minutes, and then subjected to a tensile test to measure the upper yield strength and the total elongation. The texture and average crystal grain size were also investigated. The survey method is as follows.

The tensile test was performed using a JIS No. 5 size tensile test piece, and the upper yield strength (U-YP) and total elongation (El) were measured to evaluate the strength, ductility and aging. Aging was evaluated by the yield elongation that contributes to the generation of stretcher strain during processing. If the yield elongation is 4% or less, the occurrence of stretcher strain during processing can be suppressed. 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.

The ferrite average crystal grain size was measured using the cutting method of JIS G5503 for the crystal structure observed as described above. The obtained results are shown in Table 3.

The amount of precipitated Nb was 10% acetylacetone-1% tetramethylammonium chloride-methanol solution, electrolytically extracted and then dissolved in acid, and Nb was quantified by ICP measurement. The total Nb amount was measured by ICP after directly dissolving the sample in acid. Moreover, the Nb precipitate average particle diameter was measured by TEM. The obtained results are shown in Table 3.

The volume ratio of Nb precipitates in the region from the surface to 1/8 depth position in the plate thickness direction, and the volume ratio of Nb precipitates in the region from the surface to 3/8 depth position to 4/8 depth position, respectively. It measured by the method of measuring the particle size and number of precipitates from the photograph which observed each 10 visual field at depth position by TEM by 100,000 times, and converting into a volume ratio. The measurement results are shown in Table 4.

For the measurement of pressure strength, a 0.26mm thick plate (plated steel plate) is molded into a 63mmΦ lid, and then wrapped around a 63mmΦ welded can body, compressed air is introduced into the can, and the can lid is deformed. The pressure when measured was measured. The case where the can lid was not deformed even when the internal pressure was 0.20 MPa was designated as “◯”, and the case where the can lid was deformed at less than 0.20 MPa was designated as “x”. The results are shown in Table 4.

Formability was evaluated by a method specified in JIS Z 2247 using a test machine specified in JIS B 7729 using a sample having a thickness of 0.26 mm. The Erichsen value (molding height at the time of the occurrence of through cracking) was 6.5 mm or more as “◯”, and less than 6.5 mm as “x”. The results are shown in Table 4.

From Table 3, it can be seen that the example of the present invention has an average crystal grain size of 7.0 μm or less and a fine ferrite structure, and therefore has a high upper yield strength and is excellent in both strength and ductility. Moreover, in this invention, since it is adjusted to the component composition in Table 1, corrosion resistance is also excellent.

Moreover, in the comparative example, any of the conditions in the claims of the present application is out of order, so that the desired characteristics of the present application cannot be obtained.

According to the present invention, since a steel sheet having excellent strength, ductility, and corrosion resistance can be obtained, a three-piece can with a high degree of processing can body processing, and a two-piece can with a bottom portion of several percent processed. It is most suitable as a steel plate for cans.

Claims

C: 0.020% to 0.130% or less, Si: 0.04% or less, Mn: 0.10 to 1.20%, P: 0.100% or less, S: 0.030% In the following, Al: 0.10% or less, N: 0.0120% to 0.020% or less, Nb: 0.004 to 0.040%, with the balance being composed of iron and inevitable impurities And
The ratio of the amount of precipitated Nb to the amount of total Nb is the amount of precipitated Nb / the total amount of Nb ≧ 0.30.
Nb precipitate average particle size is 20 nm or less,
The ferrite average crystal grain size is 7.0 μm or less,
A steel plate for cans, characterized by having an upper yield strength of 450 to 630 MPa and a total elongation of 13% or more after paint baking.
The volume fraction of Nb precipitates in the region from the surface to the 1/8 depth position in the plate thickness direction, and the volume fraction of Nb precipitates in the region from the surface to the 3/8 depth position to the 4/8 depth position. The steel sheet for cans according to claim 1, wherein the ratio satisfies the following formula 1.
(Nb precipitate volume ratio of 3/8 to 4/8) / (Nb precipitate volume ratio of surface to 1/8) ≧ 1.10.
It is a manufacturing method of the steel plate for cans according to claim 1 or 2,
A hot rolling step in which the steel is rolled under a condition where the finish rolling temperature is not lower than the Ar3 transformation point and not higher than 990 ° C. and the winding temperature is not lower than 400 ° C. and lower than 600 ° C .;
After the hot rolling step, pickling and primary cold rolling step of rolling under a condition where the rolling reduction is 80% or more,
After the primary cold rolling step, an annealing step in which the soaking temperature is 650 to 780 ° C. and the soaking time is continuously annealed under the conditions of 10 s to 55 s;
A method for producing a steel plate for cans, comprising a secondary cold rolling step of rolling under a condition of a rolling reduction of 1 to 19% after the annealing step.