JP2021091966A - High-strength steel sheet for cans and method for producing the same - Google Patents

Abstract

To provide a steel sheet for cans that has high strength, excellent ductility, and excellent impact resistance, and a method for producing the same.SOLUTION: A high-strength steel sheet for cans has a component composition that comprises, in mass%, C: 0.02% or more and 0.15% or less, Si: 0.05% or less, Mn: 0.10% or more and 0.60% or less, P: 0.025% or less, S: 0.020% or less, Al: 0.060% or less, N: 0.0100% or more and 0.0160% or less, Nb: 0.005% or more and 0.040% or less, Mo: 0.005% or more and 0.050% or less with the balance being Fe and inevitable impurities, where N total-(N as AlN) of 0.0050 mass% or more and 0.0150 mass% or less. It has a metallographic structure that comprises, in area ratio, 85% or more of ferrite and 1.0% or more and less than 5.0% of martensite, with the martensite having an average particle size of 1.5 μm or more and 5.0 μm or less. It has a tensile strength of 500 MPa or more and 650 MPa or less.SELECTED DRAWING: None

Description

The present invention relates to a steel sheet for high-strength cans and a method for producing the same. The present invention relates to a high-strength steel sheet for cans having a tensile strength (TS) of 500 MPa or more, which is particularly suitable for use as a material for cans and has excellent ductility and impact resistance, and a method for producing the same.

From the viewpoint of reducing the environmental load and the cost of making cans, there is a demand for reducing the amount of steel sheets used in food cans and beverage cans, and the thickness of steel sheets is becoming thinner. The targets for thinning the steel sheet are the can body of a two-piece can formed by drawing, the can body of a three-piece can formed by cylindrical molding, and the can lid. Simply thinning the steel plate reduces the strength of the can body and lid, so high-strength steel sheets are used for parts such as re-squeezed cans (DRD (draw-redo) cans) and welded can bodies. is required. Further, in order to supplement the rigidity and strength lowered due to the thinning, for example, there is an increasing need for application of a deformed can having increased rigidity and strength by giving a beading or a geometric shape to a three-piece can body. High formability is required for steel sheets in bead processing and geometric shape processing. In addition, since the strength increase due to work hardening of the steel plate is small at the bottom of the can with a low degree of processing, the can is made using a thin steel plate, and the can filled with the contents is impacted, for example, when dropped during transportation. If a large dent is generated at the bottom of the can, the commercial value will decrease. Therefore, when the steel sheet is thinned, it is necessary to have excellent impact resistance.

In order to realize a steel plate having high strength and good formability, for example, in Patent Document 1, the steel structure is a composite structure of ferrite mainly made of ferrite and martensite, and the martensite fraction is 5% or more and 30%. A high-strength thin steel sheet for can making and a method for producing the same are disclosed, wherein the martensite particle size, the product plate thickness, the martensite hardness and the 30T hardness are defined as less than. Patent Document 2 describes a steel sheet for cans and a steel sheet for cans having a steel sheet structure having a ferrite phase as the main phase and a martensite phase and / or a retained austenite phase as the second phase in a total area fraction of 1.0% or more. The manufacturing method of is disclosed.

JP-A-2009-84687 International Publication No. 2016/078666

However, the above-mentioned prior art has the following problems.

In the invention described in Patent Document 1, the energy cost increases because the steel sheet is manufactured by cold rolling twice and annealing twice. In addition, the impact resistance of the steel sheet is not described at all, and the manufacturing method described in Patent Document 1 cannot obtain the steel sheet characteristics targeted by the present invention.

In the invention described in Patent Document 2, since quenching is required in the annealing step, temperature unevenness in the steel sheet tends to be large, and it is difficult to obtain stable and good formability. Further, the impact resistance of the steel sheet is not described at all, and the manufacturing method described in Patent Document 2 cannot obtain the steel sheet characteristics targeted by the present 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 having high strength, excellent ductility, and excellent impact resistance, and a method for producing the same.

The present invention has the following configurations.

[1] By mass%,
C: 0.02% or more and 0.15% or less,
Si: 0.05% or less,
Mn: 0.10% or more and 0.60% or less,
P: 0.025% or less,
S: 0.020% or less,
Al: 0.060% or less,
N: 0.0100% or more and 0.0160% or less,
Nb: 0.005% or more and 0.040% or less,
Mo: Contains 0.005% or more and 0.050% or less, and the balance has a component composition consisting of Fe and unavoidable impurities.
N total- (Na as AlN) is 0.0050% by mass or more and 0.0150% by mass or less.
It has a metallographic structure containing 85% or more ferrite and 1.0% or more and less than 5.0% martensite in terms of area ratio, and the average particle size of the martensite is 1.5 μm or more and 5.0 μm or less. And
A steel sheet for high-strength cans having a tensile strength of 500 MPa or more and 650 MPa or less.
However, the N total is the total amount of N, and the Na as AlN is the amount of N existing as AlN.
[2] The high-strength steel sheet for cans according to [1], wherein the total elongation is 15% or more and 35% or less.
[3] The method for producing a high-strength steel sheet for cans according to the above [1] or [2].
A hot rolling step in which a steel slab having the above-mentioned composition is heated at 1200 ° C. or higher, rolled under the condition of finish rolling end temperature: 880 ° C. or higher, and then wound in a temperature range of 450 to 650 ° C.
A cold rolling step of performing cold rolling of a rolling sheet after the hot rolling step with a rolling reduction ratio of 80% or more, and a cold rolling step.
The cold rolled sheet after the cold rolling step is held in a temperature range of 700 to 900 ° C. for 90 seconds or less, then cooled to a temperature range of 400 ° C. or lower at an average cooling rate of 50 ° C./s or more, and then 20. A method for producing a steel plate for high-strength cans, which comprises an annealing step of cooling to a temperature range of 150 ° C. or lower at an average cooling rate of ° C./s or higher.

According to the present invention, a steel sheet for cans having high strength, excellent ductility, and excellent impact resistance can be obtained.

According to the present invention, by increasing the strength of the steel sheet, it is possible to secure high strength of the can body even if the can is thinned when it is processed into a can. Further, the high-strength steel sheet for cans of the present invention is most suitable for strong can body processing and flange processing such as bead processing and can expansion processing used in welded cans due to its high ductility, and is suitable as a steel sheet for cans. Further, by using the high-strength steel sheet for cans of the present invention, it becomes possible to manufacture can products and can lid products having high impact resistance.

The high-strength can steel plate according to the present invention has a mass% of C: 0.02% or more and 0.15% or less, Si: 0.05% or less, Mn: 0.10% or more and 0.60% or less, P. : 0.025% or less, S: 0.020% or less, Al: 0.060% or less, N: 0.0100% or more and 0.0160% or less, Nb: 0.005% or more and 0.040% or less, Mo : Contains 0.005% or more and 0.050% or less, the balance has a component composition consisting of Fe and unavoidable impurities, and N total- (Na as AlN) is 0.0050% by mass or more and 0.0150% by mass. It contains 85% or more of ferrite and 1.0% or more and less than 5.0% of martensite in terms of area ratio, and the average particle size of the martensite is 1.5 μm or more and 5.0 μm or less. It has a metallic structure.

Hereinafter, the high-strength steel sheet for cans of the present invention will be described.

First, the composition of the high-strength steel sheet for cans of the present invention will be described. In the following description, the unit "%" of the content of each element means "mass%" unless otherwise specified.

C: 0.02% or more and 0.15% or less C increases the strength of steel by solid solution strengthening or formation of martensite. When the C content is less than 0.02%, the area ratio of martensite is less than 1.0% and the strength is lowered, and the average particle size of martensite is less than 1.5 μm and the impact resistance is lowered. Therefore, the C content needs to be 0.02% or more. When the C content exceeds 0.15%, the area ratio of martensite in the steel sheet becomes 5.0% or more, the steel sheet is excessively hardened and the ductility is lowered, and the average particle size of martensite is 5.0 μm. Since it becomes super, the impact resistance is lowered. Therefore, the upper limit of the C content is 0.15%. In order to achieve both steel sheet strength and ductility at a higher level, the C content is preferably 0.03% or more and 0.12% or less.

Si: 0.05% or less Si is an element that increases the strength of steel by solid solution strengthening. In order to obtain this effect, the Si content is preferably 0.01% or more. However, if the Si content exceeds 0.05%, the corrosion resistance is significantly impaired. Therefore, the Si content is set to 0.05% or less. In order to obtain better corrosion resistance, the Si content is preferably 0.03% or less.

Mn: 0.10% or more and 0.60% or less Mn increases the strength of steel by solid solution strengthening or formation of martensite. If the Mn content is less than 0.10%, the martensite after annealing becomes excessively small, and the impact resistance is lowered. The Mn content must be 0.10% or more to ensure the target tensile strength and impact resistance. Therefore, the lower limit of the Mn content is set to 0.10%. On the other hand, if the Mn content exceeds 0.60%, not only the surface characteristics are inferior, but also the impact resistance is lowered due to the excessive generation of martensite. Therefore, the upper limit of the Mn content is set to 0.60%. Preferably, the Mn content is 0.30% or more and 0.60% or less.

P: 0.025% or less When the P content exceeds 0.025%, the area ratio of martensite becomes 5.0% or more, the steel sheet is excessively hardened and the ductility is lowered, and the average grain size of martensite is reduced. Since the diameter is more than 5.0 μm, the impact resistance is lowered. Therefore, the P content is 0.025% or less. Preferably, the P content is 0.023% or less. On the other hand, when the P content is less than 0.005%, the dephosphorization time is significantly increased. Therefore, the P content is preferably 0.005% or more. More preferably, the P content is 0.008% or more.

S: 0.020% or less S produces MnS and reduces ductility. Therefore, the S content should be 0.020% or less. Preferably, the S content is 0.015% or less. On the other hand, if S is less than 0.005%, the cost of removing S becomes excessive, so the S content is preferably 0.005% or more.

Al: 0.060% or less Al is an element contained as an antacid, and forms N and AlN in steel to reduce solid solution N in steel. When Al is added excessively, the formation of AlN increases, the amount of N that contributes to the steel sheet strength and impact resistance as a solid solution N, which will be described later, decreases, and the steel sheet strength and impact resistance decrease. It shall be 0.060% or less. On the other hand, if the Al content is less than 0.001%, the effect as a deoxidizer is insufficient and coagulation defects occur. Therefore, the Al content is preferably 0.001% or more. The Al content is preferably 0.003% or more and 0.050% or less in order to allow Al to function sufficiently as a deoxidizer and to obtain the effect of increasing the strength by the solid solution N.

N: 0.0100% or more and 0.0160% or less N is one of the important additive elements in the present invention. N is an element necessary for increasing the solid solution N, which contributes to increasing the strength of the steel sheet as a solid solution strengthening, and for enhancing the impact resistance of the steel sheet. In order to exert the effects of solid solution strengthening and impact resistance improvement, it is necessary to set the N content to 0.0100% or more. On the other hand, if the N content is too high, slab cracks are likely to occur in the lower straightening band where the temperature during continuous casting decreases, and the solid solution N content increases, causing the steel sheet to excessively harden and reduce ductility. , N content is 0.0160% or less. Preferably, the N content is 0.0110% or more. Further, preferably, the N content is 0.0150% or less.

Nb: 0.005% or more and 0.040% or less Nb has an effect of promoting martensitic transformation and is one of the important additive elements in the present invention. In order to obtain this effect, the Nb content needs to be 0.005% or more. When the Nb content is less than 0.005%, the area ratio of martensite is less than 1.0% and the strength is lowered, and the average particle size of martensite is less than 1.5 μm and the impact resistance is lowered. On the other hand, when the Nb content exceeds 0.040%, the area ratio of martensite in the steel sheet becomes 5.0% or more, the steel sheet is excessively hardened and the ductility is lowered, and the average particle size of martensite is 5. Since it exceeds 0.0 μm, the impact resistance is lowered. Therefore, the Nb content is set to 0.040% or less. Preferably, the Nb content is 0.008% or more. Further, preferably, the Nb content is 0.030% or less.

Mo: 0.005% or more and 0.050% or less Mo is one of the important elements that increase the strength of steel by forming martensite. When the Mo content is less than 0.005%, the area ratio of martensite is less than 1.0% and the strength is lowered, and the average particle size of martensite is less than 1.5 μm and the impact resistance is lowered. Therefore, the Mo content needs to be 0.005% or more. On the other hand, when the Mo content exceeds 0.050%, the area ratio of martensite in the steel sheet becomes 5.0% or more, the steel sheet is excessively hardened and the ductility is lowered, and the average particle size of martensite is 5. Since it exceeds 0 μm, the impact resistance is lowered. Therefore, the Mo content is set to 0.050% or less. Preferably, the Mo content is 0.008% or more. Further, preferably, the Mo content is 0.030% or less.

The rest other than the above components are Fe and unavoidable impurities.

N total- (Na as AlN): 0.0050% by mass or more and 0.0150% by mass or less In the present invention, in order to set the tensile strength of the steel sheet in the rolling direction to 500 MPa or more and to improve the impact resistance, 0.0050 is required. A solid solution N amount of mass% or more is required. Here, since it is considered that N in the steel mainly exists as AlN in the steel composition of the present invention, the amount of N existing as AlN (Na as AlN) is subtracted from the total amount of N (N total) (N total- (N total-). N as AlN)) was regarded as the amount of solid solution N. On the other hand, if the amount of solid solution N exceeds 0.0150% by mass, the steel sheet is excessively hardened and the ductility is lowered, so the amount of solid solution N is set to 0.0150% by mass or less. Preferably, the amount of solid solution N is 0.0060% by mass or more. The amount of solid solution N is preferably 0.0135% by mass or less. The amount of N present as AlN can be confirmed by, for example, dissolving and extracting AlN using a 10 vol% Br-methanol solution and performing quantitative analysis of N existing as AlN by absorptiometry. it can.

Next, the metal structure of the high-strength steel sheet for cans according to the present invention will be described.

Area ratio of ferrite: 85% or more When the area ratio of ferrite is less than 85%, the ductility of the steel sheet decreases. Therefore, the area ratio of ferrite is set to 85% or more. In order to further improve ductility, the area ratio of ferrite is preferably 90% or more.

Area ratio of martensite: 1.0% or more and less than 5.0% When the area ratio of martensite is 5.0% or more, the strength increases excessively, the ductility decreases, and the impact resistance decreases. The area ratio of the site shall be less than 5.0%. On the other hand, if the area ratio of martensite is less than 1.0%, the desired strength cannot be obtained. Therefore, the area ratio of martensite is 1.0% or more and less than 5.0%. Preferably, the area ratio of martensite is 4.5% or less.

In the metal structure, the residual structure other than the ferrite and martensite need not be particularly limited. The residual tissue may contain, for example, one or more of retained austenite, cementite, pearlite, bainite and the like. The area ratio of the remaining tissue may be 0%. The area ratio of the remaining tissue is 10% or less as an example. The area ratio of the remaining structure is 1.0% or more as an example.

Average particle size of martensite: 1.5 μm or more and 5.0 μm or less Martensite contributes to the increase in the strength of the steel sheet and enhances the work hardening of the steel sheet. When an impact external force is applied to the steel sheet, if the work hardening of the steel sheet is high, the external force is dispersed and local deformation is suppressed, so that the amount of dents generated in the steel sheet is reduced. When the average particle size of martensite is less than 1.5 μm, when an external force is applied to the steel sheet, the external force is not effectively dispersed and the impact resistance is lowered. If the average particle size of martensite is more than 5.0 μm, it is not possible to obtain good impact resistance as in the case where the average particle size of martensite is small. Therefore, the average particle size of martensite is 1.5 μm or more and 5.0 μm or less.
The area ratio of each tissue and the average particle size of martensite can be measured by the method described in Examples.

Next, the mechanical properties of the high-strength steel sheet for cans according to the present invention will be described.

The tensile strength of the steel sheet shall be 500 MPa or more in order to secure the dent strength of the welded can of the 3-piece can and the compressive strength of the 2-piece can. On the other hand, in order to obtain a tensile strength exceeding 650 MPa, a large amount of element content is required. A large amount of elemental content may impair corrosion resistance and reduce ductility. Therefore, the tensile strength is set to 650 MPa or less. That is, the high-strength steel sheet for cans of the present invention has a tensile strength of 500 MPa or more and 650 MPa or less. Here, the tensile strength means the tensile strength in the rolling direction of the steel sheet.

The steel sheet for high-strength cans of the present invention preferably has a total elongation of 10% or more, and particularly preferably has a total elongation of 15% or more. When the total elongation is 10% or more, in the production of cans formed by can body processing such as bead processing and can expansion processing, problems such as cracks are unlikely to occur even when the degree of processing is large. In addition, the occurrence of cracks during flange processing of the can can be suppressed more effectively. The upper limit of the total elongation is not particularly limited, but if the total elongation is 35% or less, the dimensional accuracy of the can body can be further improved, and therefore 35% or less is preferable. As described in the examples, the tensile strength and the total elongation can be measured by the metal material tensile test method shown in "JIS Z 2241".

Further, high impact resistance can be ensured for the can body, bottom, etc. of the 3-piece can or 2-piece can when the amount of dent of the steel plate after the impact resistance test described later is 650 μm or less. Therefore, the steel sheet for high-strength cans of the present invention preferably has a dent amount of 650 μm or less after the impact resistance test described later.
The desired tensile strength, total elongation and impact resistance can be obtained by adjusting the component composition and manufacturing conditions (annealing temperature, annealing time, cooling rate, etc. in the annealing step).

Currently, steel sheets are being thinned for the purpose of reducing can manufacturing costs. However, there is a concern that the strength of the can body may decrease as the thickness of the steel sheet becomes thinner, that is, the thickness of the steel sheet decreases. On the other hand, the high-strength steel sheet for cans of the present invention can suppress a decrease in can body strength even when the plate thickness is thin. When the plate thickness is thin, the effect of the present invention of having both high ductility and high strength and also having excellent impact resistance can be remarkably enjoyed. From this point of view, the thickness of the steel plate for high-strength cans of the present invention is preferably 0.55 mm or less. The plate thickness may be 0.40 mm or less.

Next, a method for manufacturing a high-strength steel sheet for cans of the present invention will be described.

In the high-strength can steel plate of the present invention, a steel material (steel slab) having the above composition is heated at 1200 ° C. or higher, rolled under the condition that the finish rolling end temperature is 880 ° C. or higher, and then 450 to 450 to A hot rolling step of winding within a temperature range of 650 ° C., a cold rolling step of cold rolling on a hot rolled plate after the hot rolling step at a reduction ratio of 80% or more, and a cold rolling step after the cold rolling step. After holding the cold-rolled sheet in a temperature range of 700 to 900 ° C for 90 seconds or less, it is first cooled to a temperature range of 400 ° C or less at an average cooling rate of 50 ° C / s or more, and an average of 20 ° C / s or more. It is manufactured by performing an annealing step of secondary cooling to a temperature range of 150 ° C. or lower at a cooling rate.
In the following description, the temperature is specified based on the surface temperature of the steel sheet or the like. The average cooling rate is a value obtained by calculation based on the surface temperature. For example, the average cooling rate from the holding temperature to the cooling stop temperature in the temperature range of 400 ° C or lower is ((holding temperature- (cooling stop temperature in the temperature range of 400 ° C or lower))) / from the holding temperature to (temperature of 400 ° C or lower). It is expressed by the cooling time) up to the cooling stop temperature of the region.

When producing a steel sheet for high-strength cans according to the present invention, the molten steel is adjusted to the above-mentioned composition by a known method using a converter or the like, and then, for example, a steel slab is obtained by a continuous casting method.

Steel slab heating temperature: 1200 ° C or higher The heating temperature of the steel slab in the hot rolling process shall be 1200 ° C or higher. When the steel slab heating temperature is less than 1200 ° C., the amount of solid solution N required to secure the strength in the present invention is reduced, and the strength is lowered. In order to make the tensile strength of the steel sheet in the rolling direction 500 MPa or more and to improve the impact resistance, it is necessary that the amount of solid solution N is 0.0050 mass% or more, and the steel slab heating temperature is 1200 ° C. or more. By doing so, the amount of solid solution N can be secured. On the other hand, when the amount of solid solution N exceeds 0.0150% by mass, the steel sheet is excessively hardened and the ductility is lowered. Therefore, the amount of solid solution N is preferably 0.0150% by mass or less. A more preferable amount of solid solution N is 0.0060% by mass or more and 0.0150% by mass or less, and for that purpose, the steel slab heating temperature is preferably 1220 ° C. or more. The steel slab heating temperature is preferably 1300 ° C. or lower because the effect is saturated even if the temperature exceeds 1300 ° C.

Finish rolling end temperature: 880 ° C or higher When the finish rolling end temperature of the hot rolling process is less than 880 ° C, rolling is performed in a two-phase region in which ferrite and austenite are mixed, and coarse ferrite is generated on the surface layer of the steel sheet. Since it becomes difficult to obtain fine martensite by annealing, the average particle size of martensite becomes more than 5.0 μm, and the impact resistance is lowered, the finish rolling end temperature is set to 880 ° C. or higher. On the other hand, raising the finish rolling temperature more than necessary may make it difficult to manufacture thin steel sheets, and the metal structure in hot rolling becomes coarse, making it difficult to obtain fine martensite by subsequent annealing. The finish rolling end temperature is preferably 950 ° C. or lower, because the impact resistance may be lowered.

Winding temperature: 450-650 ° C
When the winding temperature in the hot rolling process is less than 450 ° C, the ferrite area ratio after continuous annealing decreases, the ductility decreases, and the martensite area ratio of the steel sheet becomes 5.0% or more, resulting in excess steel sheet. The martensite average particle size exceeds 5.0 μm, and the impact resistance is lowered. Therefore, the winding temperature is 450 ° C. or higher. On the other hand, when the winding temperature is higher than 650 ° C., the metal structure in hot rolling becomes coarse, and fine martensite cannot be obtained by subsequent annealing, and the impact resistance is lowered. Therefore, the winding temperature is 450 ° C. or higher and 650 ° C. or lower. The winding temperature is preferably 470 ° C. or higher. The winding temperature is preferably 600 ° C. or lower.

Pickling process (arbitrary process)
It is preferable to perform pickling after the hot rolling step. Pickling does not need to be particularly limited as long as the surface scale can be removed. Further, the scale may be removed by a method other than pickling.

Cold rolling reduction rate: 80% or more In the cold rolling process, cold rolling with a reduction rate of 80% or more is performed on the hot-rolled sheet after the hot rolling process. When the rolling reduction ratio of cold rolling is less than 80%, the strain applied to the steel sheet by cold rolling decreases, so that the martensite of the steel sheet after annealing becomes coarse, and fine martensite cannot be obtained, resulting in impact resistance. The sex is reduced. Therefore, the rolling reduction of cold rolling is set to 80% or more. The rolling reduction of cold rolling is preferably 85% or more. On the other hand, when the rolling reduction ratio of cold rolling exceeds 95%, the rolling load increases significantly and the load on the rolling mill increases. Therefore, the rolling reduction of cold rolling is preferably 95% or less. The rolling reduction of cold rolling is more preferably 94% or less.

After the hot rolling step and before the cold rolling step, other steps may be included as appropriate. Further, the cold rolling step may be performed after the hot rolling step without pickling.

In the annealing process after the cold rolling process, after holding in a temperature range of 700 to 900 ° C for 90 seconds or less, pre-stage cooling (primary cooling) in which the temperature is cooled to a temperature range of 400 ° C or less at an average cooling rate of 50 ° C / s or more. ), And then the subsequent cooling (secondary cooling), which cools to a temperature range of 150 ° C. or lower at an average cooling rate of 20 ° C./s or higher.

Holding in a temperature range of 700 to 900 ° C. for 90 seconds or less The holding temperature (equal heat temperature) in the annealing step shall be in the temperature range of 700 to 900 ° C. If the holding temperature is more than 900 ° C., plate troubles such as heat buckles are likely to occur in continuous annealing, which is not preferable. Further, the martensite of the steel sheet after annealing becomes coarse, and fine martensite cannot be obtained, so that the impact resistance is lowered. If the holding temperature is less than 700 ° C., the recrystallization of the ferrite grains is incomplete, the metal structure of the steel sheet after annealing becomes non-uniform, fine martensite cannot be obtained, and the impact resistance is lowered. Therefore, the holding temperature is set to the temperature range of 700 to 900 ° C. The holding temperature is preferably 720 ° C. or higher. The holding temperature is preferably 860 ° C. or lower.

The holding time in the temperature range of 700 to 900 ° C. is 90 seconds or less. If the holding time in the temperature range exceeds 90 seconds, the martensite of the steel sheet after annealing becomes coarse, fine martensite cannot be obtained, and the impact resistance is lowered. Therefore, the holding time in the temperature range of 700 to 900 ° C. is set to 90 seconds or less. Preferably, the holding time is 70 seconds or less. On the other hand, when the holding time is 5 seconds or more, more excellent ductility of the steel sheet can be obtained. Therefore, the holding time is preferably 5 seconds or more.

Pre-stage cooling (primary cooling): Cooling to a temperature range of 400 ° C or lower with an average cooling rate of 50 ° C / s or higher After holding at the above holding temperature, to a temperature range of 400 ° C or lower with an average cooling rate of 50 ° C / s or higher Cooling. That is, cooling is performed at an average cooling rate of 50 ° C./s or more from the holding temperature (primary cooling start temperature) in the temperature range of 700 to 900 ° C. to the primary cooling stop temperature in the temperature range of 400 ° C. or lower. When the average cooling rate is less than 50 ° C./s, the formation of martensite is suppressed during cooling, the area ratio of martensite is lowered, and the tensile strength of 500 MPa or more cannot be obtained and the strength is lowered. Furthermore, the martensite becomes finer and the impact resistance is lowered. The average cooling rate is preferably 65 ° C./s or higher. On the other hand, when the average cooling rate exceeds 180 ° C./s, not only the effect is saturated but also an excessive cost is generated in the cooling equipment. Therefore, the average cooling rate in the pre-stage cooling is preferably 180 ° C./s or less. Further, when the cooling stop temperature (primary cooling stop temperature) in the pre-stage cooling exceeds 400 ° C., the formation of martensite is suppressed, the area ratio of martensite decreases, and the tensile strength of 500 MPa or more cannot be obtained and the strength becomes high. descend. Furthermore, the martensite becomes finer and the impact resistance is lowered. Therefore, the primary cooling stop temperature is set to a temperature range of 400 ° C. or lower. Preferably, the primary cooling stop temperature is 380 ° C. or lower. The primary cooling stop temperature is preferably 250 ° C. or higher. The pre-stage cooling (primary cooling) can be performed by one or a combination of two or more types such as gas cooling, furnace cooling, mist cooling, roll cooling and water cooling.

Sub-stage cooling (secondary cooling): Cooling to a temperature range of 150 ° C or less at an average cooling rate of 20 ° C / s or more In the post-stage cooling (secondary cooling) after the pre-stage cooling (primary cooling), the average is 20 ° C / s or more. Cool to a temperature range of 150 ° C. or lower at a cooling rate. That is, cooling is performed at an average cooling rate of 20 ° C./s or more from the primary cooling stop temperature (secondary cooling start temperature) to the secondary cooling stop temperature in the temperature range of 150 ° C. or lower. When the average cooling rate is less than 20 ° C./s, the amount of solid solution N in the steel becomes less than 0.0050%, and the impact resistance is lowered. On the other hand, when the average cooling rate exceeds 40 ° C./s, not only the effect is saturated but also an excessive cost is generated in the cooling equipment. Therefore, the average cooling rate in the subsequent cooling is preferably 40 ° C./s or less. The average cooling rate in the subsequent cooling is more preferably 35 ° C./s or less. Further, in the subsequent cooling, the cooling stop temperature (secondary cooling stop temperature) is set to a temperature range of 150 ° C. or lower. When the secondary cooling stop temperature exceeds 150 ° C., the amount of solid solution N in the steel becomes less than 0.0050%, and the impact resistance is lowered. If the cooling stop temperature is less than 100 ° C., not only the effect is saturated but also an excessive cost is generated in the cooling equipment. Therefore, the cooling stop temperature is preferably 100 ° C. or higher. The cooling stop temperature is more preferably 120 ° C. or higher. The latter stage cooling (secondary cooling) can be performed by the same process as the first stage cooling.
The annealing is preferably continuous annealing, and in that case, a continuous annealing device is used. Further, after the cold rolling step and before the annealing step, other steps may be appropriately included, or the annealing step may be performed immediately after the cold rolling step.

From the above, the high-strength steel sheet for cans of the present invention can be obtained. In the present invention, various steps can be further performed after annealing. For example, the high-strength can steel sheet of the present invention may be subjected to plating treatment such as tin plating, chrome plating, nickel plating, etc. by electroplating to form a plating layer to form a plated steel sheet. Examples of the plating layer include a Sn plating layer, a Cr plating layer such as tin-free, a Ni plating layer, and a Sn—Ni plating layer. Further, a process such as a coating baking process and a film laminating process may be performed. Even when such a treatment is performed, the film thickness of the plating layer, the film laminate layer, etc. is sufficiently smaller than the thickness of the high-strength can steel sheet, so that the high-strength can steel sheet is used. The effect on mechanical properties is negligible.

A steel slab containing the composition shown in Table 1 and having the balance of Fe and unavoidable impurities was melted in a converter and continuously cast to obtain a steel slab. The steel slabs obtained here were hot-rolled at the steel slab heating temperature, finish rolling end temperature, and take-up temperature shown in Tables 2 and 3 (hot rolling step). After this hot rolling, pickling was performed. Next, cold rolling was performed at the rolling reduction rates shown in Tables 2 and 3 (cold rolling step), and continuous annealing was performed under the annealing conditions shown in Tables 2 and 3 (annealing step). The obtained steel sheet was continuously subjected to ordinary Sn plating to obtain a Sn-plated steel sheet (tinplate).

The steel sheet obtained as described above was subjected to a heat treatment corresponding to a coating baking treatment at 210 ° C. for 10 minutes, and then a tensile test was performed to measure the tensile strength and the total elongation. Furthermore, the impact resistance was evaluated. The measurement method and evaluation method are as follows.

The tensile test is carried out in accordance with the metal material tensile test method shown in "JIS Z 2241". That is, a JIS No. 5 tensile test piece (JIS Z 2201) having a rolling direction as a tensile direction is sampled, subjected to a coating baking equivalent treatment at 210 ° C. for 10 minutes, and then 50 mm (L) is applied to a parallel portion of the tensile test piece. A tensile test in accordance with JIS Z 2241 with a reference point is carried out at a tensile speed of 10 mm / min until the test piece breaks. Those with a tensile strength of 500 MPa or more and 650 MPa or less and a total elongation of 15% or more are "○" (high strength and particularly excellent ductility), and a tensile strength of 500 MPa or more and 650 MPa or less and total elongation Is 10% or more and less than 15% as "△" (high strength and excellent ductility), and other as "x" (high strength and excellent ductility cannot be compatible). ..

The area ratio of each structure to the entire structure was investigated by observing the surface at 1/2 of the plate thickness with Nital in the rolling direction cross section with a scanning electron microscope (SEM). Observations were performed in 10 randomly selected visual fields. Using a cross-sectional tissue photograph with a magnification of 2000 times, binarization processing is performed using image processing software (Photoshop, manufactured by Adobe), and occupancy of each tissue existing in an arbitrarily set 50 μm × 50 μm square area. The area was obtained, the average value in each of the above-mentioned visual fields was calculated, and this was used as the area ratio of each tissue.
In the SEM observation, the white region having a smooth surface and observed as a massive shape was regarded as martensite, and the area ratio thereof was taken as the area ratio of martensite. For the average particle size of martensite, the circle-equivalent diameter is calculated from the occupied area of martensite, the average value of the circle-equivalent diameter in each observation field is calculated, and the average value of five randomly selected observation fields is calculated as marten. The average particle size of the site was used.
As for the ferrite, in the SEM observation, the black region observed as a lump shape and containing no martensite inside was regarded as the ferrite, and the area ratio thereof was defined as the area ratio of the ferrite.

Impact resistance was evaluated using an impact resistance tester. A circular steel sheet having a diameter of 45 mm was cut out from the steel sheet and subjected to an impact resistance test. The shooting type had a diameter of 12.7 mm and a flat bottom, and the cradle and plate holder were provided with circular holes with a diameter of 13.5 mm. The positional relationship between the shooting type, the cradle, the plate holder, and the circular steel plate is set so that the holes in the shooting type, the cradle, the holes in the plate holder, and the center of the circular steel plate are aligned, and the bottom of the shooting type is 0 downward. It is possible to push in 5.5 mm. With the circular steel plate fixed by the plate holder so as not to move, a weight of 500 g was dropped onto the shooting mold from a height of 50 cm, and the circular steel plate was deformed by applying an impact. The dent depth of the deformed steel sheet was measured using a shape measuring device (Keyence 3D shape measuring machine VR-3000). At the measured location, the average value of the dent depths of the four cross sections of the deformed portion was evaluated as the dent depth of the steel plate. When the dent depth is 650 μm or less, the impact resistance is considered to be excellent. The recess depth is preferably 620 μm or less. Further, when the dent depth exceeds 650 μm, the impact resistance is inferior.

From Tables 2 and 3, in the example of the present invention, a steel sheet having high strength and excellent ductility and also excellent impact resistance was obtained.

According to the present invention, a high-strength steel sheet for cans having high strength, excellent ductility, and excellent impact resistance can be obtained. For example, since the steel plate has high strength, it is possible to secure high strength of the can body even if the can is thinned. In addition, due to its high ductility, it is possible to manufacture can products and can lid products that are ideal for strong can body processing such as bead processing and can expansion processing used in welded cans, and flange processing, and have high impact resistance. , Optimal as a steel plate for cans.

Claims (3)

By mass%
C: 0.02% or more and 0.15% or less,
Si: 0.05% or less,
Mn: 0.10% or more and 0.60% or less,
P: 0.025% or less,
S: 0.020% or less,
Al: 0.060% or less,
N: 0.0100% or more and 0.0160% or less,
Nb: 0.005% or more and 0.040% or less,
Mo: Contains 0.005% or more and 0.050% or less,
The balance has a component composition consisting of Fe and unavoidable impurities.
N total- (Na as AlN) is 0.0050% by mass or more and 0.0150% by mass or less.
It has a metallographic structure containing 85% or more ferrite and 1.0% or more and less than 5.0% martensite in terms of area ratio, and the average particle size of the martensite is 1.5 μm or more and 5.0 μm or less. And
A steel sheet for high-strength cans having a tensile strength of 500 MPa or more and 650 MPa or less.
However, the N total is the total amount of N, and the Na as AlN is the amount of N existing as AlN. The high-strength steel sheet for cans according to claim 1, wherein the total elongation is 15% or more and 35% or less. The method for manufacturing a high-strength steel sheet for a can according to claim 1 or 2.
A hot rolling step in which a steel slab having the above-mentioned composition is heated at 1200 ° C. or higher, rolled under the condition of finish rolling end temperature: 880 ° C. or higher, and then wound within a temperature range of 450 to 650 ° C.
A cold rolling step of performing cold rolling of a rolling sheet after the hot rolling step with a rolling reduction ratio of 80% or more, and a cold rolling step.
The cold-rolled sheet after the cold rolling step is held in a temperature range of 700 to 900 ° C. for 90 seconds or less, then cooled to a temperature range of 400 ° C. or lower at an average cooling rate of 50 ° C./s or more, and then 20. A method for producing a steel plate for high-strength cans, which comprises an annealing step of cooling to a temperature range of 150 ° C. or lower at an average cooling rate of ° C./s or higher.