JP5655839B2 - Hot-rolled steel sheet used as a base material for steel sheet for can and manufacturing method thereof - Google Patents

Description

  The present invention relates to a steel plate for cans, a hot-rolled steel plate used as a base material thereof, and a method for producing the same, and more specifically, a steel plate for cans having high ductility, high strength, and small anisotropy (Δr). , And a hot-rolled steel sheet used for the base material, and a manufacturing method thereof.

In recent years, in order to expand the demand for steel cans, measures have been taken such as reducing can manufacturing costs and introducing new can types such as bottle cans and deformed cans to the market.
As a measure to reduce can manufacturing costs, the cost of materials can be reduced, and not only two-piece cans that are drawn, but also three-piece cans mainly made of simple cylindrical molding, the use of thinner steel sheets can be achieved. It is being advanced.
However, simply reducing the thickness of the steel sheet reduces the strength of the can, so it is recommended to use a steel sheet that has only been reduced in thickness where high-strength materials such as DRD cans and can bodies of welded cans are used. However, a high strength and extremely thin steel plate for cans has been desired.
Currently, ultra-thin and hard steel plates for cans are manufactured by the Duble Reduce method (hereinafter referred to as DR method) in which secondary cold rolling is performed after annealing. Steel sheets manufactured by the DR method are characterized by high strength and low yield elongation. In a DRD can application involving bottom processing, it is desirable that the yield elongation is as small as possible in order to prevent the occurrence of a threshold strain, and the DR method is effective in that respect. However, DRD cans are required to have low ear generation, but the DR method tends to increase anisotropy, so ears are likely to occur, and anisotropy (Δr) is reduced to prevent ear generation. There are challenges.
On the other hand, DR materials having poor ductility are difficult to apply to cans with cans having a high degree of processing, such as deformed cans that have recently been put on the market, because of poor workability. In addition, the DR material is expensive because the number of manufacturing steps is increased as compared with a steel plate that is pressure-regulated after normal annealing.

  In order to avoid the disadvantages of DR materials, secondary cold rolling is omitted, and high strength steel sheets are manufactured by the Single Reduce method (SR method), which controls the properties in the primary cold pressure and annealing processes using various strengthening methods. The following patents propose a method for manufacturing a steel plate with a low ear occurrence rate.

Patent Document 1 proposes that a steel plate for high strength can similar to DR can be obtained by adding a large amount of C and N and bake hardening. The yield stress after paint baking is as high as 550MPa or more, and the amount of N added and the hardness obtained by heat treatment are adjusted.
Also in Patent Document 2, as in Patent Document 1, the strength is increased by baking after coating.
Patent Document 3 proposes a steel sheet having a high strength-ductility balance by a combination of precipitation strengthening by Nb carbide and solid solution strengthening by P.
Patent Document 4 proposes a manufacturing method in which the yield elongation is 1.0% or less to prevent the occurrence of stretcher strain and to obtain a steel having a strength level equivalent to T6.
Patent Document 5 proposes an ultrathin steel plate that does not generate stretcher strain and has a low ear generation rate.
Patent Document 6 is an invention for obtaining a high-strength steel sheet using transformation strengthening, and hot rolling low carbon steel in an α + γ region, cooling at a high speed, and defining a heating rate for annealing, thereby providing a tensile strength of 600 MPa. A steel sheet having a total elongation of 30% or more has been proposed.

Japanese Patent Laid-Open No. 2001-107186 JP-A-11-199991 JP 2005-336610 A JP 59-129733 A Japanese Patent Laid-Open No. 11-222647 Japanese Patent Laid-Open No. 2003-34825

First, it is essential to secure strength in order to reduce the gauge. For example, to obtain the current can strength of steel with the same thickness (about 0.15-0.18 mm) as that of DR material, the yield strength must be 500 MPa or more. is there. Moreover, it is necessary to apply a highly ductile steel plate to a can body that performs high can body processing such as can expansion processing and a can body that performs high flange processing. When steel with a high ear generation rate is applied to a two-piece can such as a DRD can, the trim margin is increased and the yield is lowered. Therefore, a steel plate with small ear generation, that is, low anisotropy is desired.
In view of the above characteristics, in the above-described conventional technology, it is possible to manufacture a steel plate that satisfies any of strength, ductility, and anisotropy, but it is not possible to manufacture a steel plate that satisfies all of the requirements.
For example, the method of adding a large amount of C and N described in Patent Document 1 to increase the strength by bake hardenability is an effective method for increasing the strength. However, since the structure obtained in Patent Document 1 has an unrecrystallized structure that deteriorates anisotropy, Patent Document 1 cannot obtain the anisotropy targeted in the present invention.
Patent Document 2 mentions age hardening by baking treatment, but the yield strength of steel described in the examples is up to about 430 MPa, and the target 500 MPa or more cannot be obtained.
Patent Document 3 mentions that strengthening by precipitation strengthening and composite strengthening by solid solution strengthening, but generally steel using precipitation strengthening is inferior in anisotropy, especially hot rolling proposed in Patent Document 3. Under the conditions, the target anisotropy in the present invention cannot be obtained.
Although Patent Document 4 describes a T6 level steel with a yield elongation of almost 0, it is necessary to perform temper rolling at a rolling rate of 10% or more, and the manufacturing method is substantially the same as that of DR material. Yes, high cost. In addition, there is no description of producing steel exceeding T6. Further, although it is not described in the specification regarding ductility, it is expected that the ductility is inferior when rolling is performed at a rolling reduction of 10% or more.
Patent Document 5 discloses a method for producing a steel sheet that suppresses the generation of ears by controlling production conditions such as components and hot rolling conditions, but the yield strength of the steel described in the examples is about 380 MPa. However, the target of 500 MPa or higher is not reached.
The increase in strength by high-speed cooling proposed in Patent Document 6 is costly in operation.

  The present invention has been made in view of such circumstances, and for cans having a yield strength of 500 MPa or more, a yield ratio of 0.9 or more, a total elongation of 10% or more, and Δr of −0.50 to 0 after baking. An object of the present invention is to provide a steel sheet, a hot-rolled steel sheet as a base material thereof, and a production method thereof.

The inventors of the present invention have intensively studied to solve the above problems. As a result, the following knowledge was obtained.
Focusing on the combined combination of solid solution strengthening, precipitation strengthening, grain refinement strengthening, age hardening, solid solution strengthening using solid solution strengthening elements, solid solution strengthening by Nb, P, Mn, precipitation strengthening and crystal By strengthening the grain by strengthening the grain, it is possible to increase the strength while maintaining high elongation. Furthermore, by using the solid solution C and solid solution N in the steel, the strength can be increased by age hardening after paint baking. Plan. And by making the structure into a substantially single ferrite phase and defining the ferrite average crystal grain size, a high strength-ductility balance is maintained, and a yield strength of 500 MPa or more and a total elongation of 10% or more are obtained. In particular, in this patent, paying attention to the deterioration of anisotropy which becomes a problem when using precipitation strengthening, the anisotropy is improved by appropriately controlling the hot rolling conditions, and Δr is set to −0.50 to 0. Is possible.
In this invention, it came to complete the steel plate for high strength and high ductility cans, and its manufacturing method by managing a component and a manufacturing method in total based on the said knowledge.

The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] By mass%, C: 0.01 to 0.12%, Si: 0.005 to 0.5%, Mn: 0.3 to 1.5%, P: 0.005 to 0.2%, Al: 0.10% or less, N: 0.012% or less, Nb : 0.005 to 0.10%, with the balance consisting of iron and unavoidable impurities, and substantially having a ferrite single-phase structure, with a ferrite average crystal grain size of 7 μm or less, yield after coating baking Steel plate for cans having a strength of 500 MPa or more, a yield ratio of 0.9 or more, total elongation of 10% or more, and Δr of −0.50 to 0.
[2] When producing the steel plate for cans according to [1], in mass%, C: 0.01 to 0.12%, Si: 0.005 to 0.5%, Mn: 0.3 to 1.5%, P: 0.005 to 0.2 %, Al: 0.10% or less, N: 0.012% or less, Nb: 0.005 to 0.10%, with the balance being iron and unavoidable impurities, hot rolled at a finishing temperature of 870 ° C or higher and wound After cooling at an average cooling rate of 40 ° C./s or less, winding at a winding temperature of 620 ° C. or more, pickling, and then cold rolling at a reduction rate of 80% or more, then 650 to 750 ° C. A method for producing a steel plate for cans, comprising performing continuous annealing at a soaking temperature of 40 s or less and a soaking time of 40 s or less and temper rolling at a pressure regulation rate of 1.5% or less.
[3] By mass%, C: 0.01 to 0.12%, Si: 0.005 to 0.5%, Mn: 0.3 to 1.5%, P: 0.005 to 0.2%, Al: 0.10% or less, N: 0.012% or less, Nb: 0.005 Hot-rolling used for steel plate base materials for cans, containing ~ 0.10%, the balance being composed of iron and inevitable impurities, and having a substantially single-phase ferrite structure and an average ferrite grain size of 6 μm or more steel sheet.
[4] When manufacturing the hot-rolled steel sheet according to [3], C: 0.01 to 0.12%, Si: 0.005 to 0.5%, Mn: 0.3 to 1.5%, P: 0.005 to 0.2% in mass%. , Al: 0.10% or less, N: 0.012% or less, Nb: 0.005 to 0.10%, with the balance being iron and inevitable impurities hot-rolled at a finishing temperature of 870 ° C or higher until winding A method for producing a hot-rolled steel sheet used for a steel plate base material for cans, wherein the steel sheet is cooled at an average cooling rate of 40 ° C / s or less and wound at a winding temperature of 620 ° C or more.
In the present specification, “%” indicating the component of steel is “% by mass”. Further, in the present invention, the coating baking process is a process of performing a heat treatment at 210 ° C. for 20 minutes, which is equivalent to a coating baking.

According to the present invention, a high strength and high ductility steel plate for cans having a yield strength of 500 MPa or more, a yield ratio of 0.9 or more, a total elongation of 10% or more, and Δr of −0.50 to 0 can be obtained.
More specifically, the present invention strengthens a solid solution by using a solid solution strengthening element, and further strengthens the composite by strengthening the solution by Nb, P, Mn, precipitation strengthening and fine grain strengthening. Since the strength was increased while maintaining, the temper rolling after the annealing step can produce a steel sheet having a reduction rate of 1.5% or less and a yield strength of 500 MPa or more.
As a result, by increasing the strength of the original plate (steel plate), it becomes possible to ensure high strength of the can even if the welded can is made thinner. In addition, with respect to a positive pressure can application that requires the pressure resistance of the bottom portion, it is possible to obtain a high pressure resistance with the current gauge. Furthermore, by increasing the ductility, it becomes possible to perform high can body processing such as can expansion processing.
Further, in the case of a drawn can, it is necessary to prevent the occurrence of ears in order to reduce the trim margin of the can and increase the yield. In the present invention, the finishing temperature is 870 ° C. or higher, the cooling rate until winding is 40 ° C./s or lower, and the winding temperature is 620 ° C. or higher, so that Δr is suppressed to a range of −0.50 to 0 to prevent generation of ears. can do.

It is a figure which shows the relationship between anisotropy ((DELTA) r) and the ferrite average crystal grain diameter of a hot rolled material.

Hereinafter, the present invention will be described in detail.
The steel sheet for cans of the present invention is a steel sheet for high strength and high ductility cans having a yield strength of 500 MPa or more, a yield ratio of 0.9 or more, a total elongation of 10% or more, and Δr−0.50 to 0. Normally, a steel sheet that has been strengthened using the DR method exhibits only a few percent elongation. On the other hand, the present invention is characterized by increasing the strength while maintaining high elongation by manufacturing a steel plate that is solid solution strengthened, precipitation strengthened, refined strengthened by Nb, P, Mn by continuous annealing. To do. In addition, by leaving solid solution C and N in steel, age hardening of 30 MPa or more is caused by heat treatment essential in can manufacturing processes such as paint baking, and by increasing YP, It was possible to increase the pressure resistance of the bottom and the dent strength of the weld can. In addition, as a heat treatment for age hardening, a lamination process may be performed instead of a paint baking process. Furthermore, by setting the finishing temperature at the time of hot rolling to 870 ° C. or higher, the subsequent cooling rate to 40 ° C./s or lower, and the coiling temperature to 620 ° C. or higher, Δr has a value in the range of −0.50 to 0. These are features of the present invention and are the most important requirements. In this way, by optimizing the components, structures, and production conditions centering on solid solution strengthening elements, precipitation strengthening elements, and refinement strengthening elements, the yield strength is 500 MPa or more, the yield ratio is 0.9 or more, A steel plate for cans having an elongation of 10% or more and Δr of −0.50 to 0 is obtained.

Next, the component composition of the steel plate for cans of this invention is demonstrated.
C: 0.01-0.12%
In the steel plate for cans of the present invention, it is essential to achieve a strength of not less than a predetermined value (yield strength of 500 MPa or more) after continuous annealing and temper rolling, and at the same time to have a total elongation of 10% or more. The particle size needs to be 7 μm or less. In producing a steel sheet satisfying these characteristics, the amount of C added is important. In particular, since the amount and density of carbides are greatly related to the strength of the steel sheet and the average grain size of ferrite, it is necessary to secure the amount of carbon used for precipitation. Further, by precipitating carbides at the grain boundaries, the grain boundary segregation of P is relatively suppressed, and there is an effect that the solid solution strengthening of P can be utilized to the maximum. From the above, the lower limit of the C addition amount is limited to 0.01%. On the other hand, if the amount of C added exceeds 0.12%, subperitectic cracks occur during the cooling process during steel melting, so the upper limit is limited to 0.12%. Desirably, it is 0.04% or more and 0.10% or less.

Si: 0.005-0.5%
Si is an element that increases the strength of steel by solid solution strengthening, but if added in a large amount, corrosion resistance is significantly impaired. Therefore, the upper limit of Si addition amount is limited to 0.5%. On the other hand, in applications where high corrosion resistance is required, Si needs to be as low as possible, so the lower limit is limited to 0.005%.

Mn: 0.3-1.5%
Mn increases the strength of the steel by solid solution strengthening and also reduces the crystal grain size. The effect of reducing the crystal grain size is remarkably produced when the Mn addition amount is 0.3% or more, and at least 0.3% Mn addition amount is required to secure the target strength. Therefore, the lower limit of the Mn addition amount is limited to 0.3%. On the other hand, if Mn is contained in a large amount, the corrosion resistance is poor. Therefore, the upper limit is limited to 1.5%.

P: 0.005% to 0.2%
P is an element having a large solid solution strengthening ability, but if added in a large amount, the corrosion resistance is remarkably impaired. Therefore, the upper limit is limited to 0.2%. On the other hand, in applications where high corrosion resistance is required, it is necessary to reduce the P addition amount as much as possible, so the lower limit is limited to 0.005%.

Al: 0.10% or less An increase in the Al content results in an increase in the recrystallization temperature. Therefore, it is necessary to increase the annealing temperature. In the present invention, other elements added to increase the strength increase the recrystallization temperature and increase the annealing temperature. Therefore, it is best to avoid the increase of the recrystallization temperature due to Al as much as possible. Therefore, the upper limit of the Al content is limited to 0.10%.

N: 0.012% or less N is an element necessary for increasing age hardening. In order to exert the effect of age hardening, it is desirable to add 0.005% or more. On the other hand, when it is added in a large amount, hot ductility is deteriorated, and slab cracking is likely to occur in the straightening band during continuous casting. Therefore, the upper limit of N content is limited to 0.012%.

Nb: 0.005% to 0.10%
Nb is an important additive element in the present invention. Nb is an element having a high ability to generate carbides, and precipitates fine carbides to increase the strength. Moreover, strength is raised by making it fine. The particle size affects not only the strength but also the surface properties during drawing. If the average grain size of ferrite in the final product exceeds 7 μm, after the drawing process, a rough skin phenomenon will occur in part and the appearance of the surface will be lost. The strength and surface properties can be adjusted by the amount of Nb added, and this effect is produced when it exceeds 0.005%. Therefore, the lower limit is limited to 0.005%. On the other hand, Nb brings about an increase in recrystallization temperature. Therefore, if it is contained in an amount exceeding 0.10%, it is not regenerated by continuous annealing performed at a soaking temperature of 650 to 750 ° C. and a soaking time of 40 s or less described in the present invention. It becomes difficult to anneal, for example, some crystals remain. By increasing the annealing temperature, a recrystallized structure can be obtained, but the surface properties are inferior because the elements in the steel are concentrated on the surface layer. Therefore, the upper limit of Nb addition amount is limited to 0.10%.

  The balance is Fe and inevitable impurities.

Next, the structure of the steel plate for cans of the present invention will be described.
Ferrite single phase structure, ferrite average crystal grain size: 7 μm or less First, in the present invention, a ferrite single phase structure is substantially formed. Even when containing about 1% of cementite or the like, it is determined that the ferrite single phase structure is substantially obtained as long as the effects of the present invention are exhibited.
On the other hand, when the average ferrite grain size exceeds 7 μm, the beauty of the surface appearance after canning is lost. This is considered to correspond to an extreme change in surface roughness such as a rough skin phenomenon. In particular, this phenomenon is confirmed in the can body of a two-piece can and the can body of a three-piece can that performs canning processing. From the above, the ferrite average crystal grain size is set to 7 μm or less.
The ferrite crystal grain size is measured using a cutting method defined in JIS G0551.
Further, the ferrite average crystal grain size is controlled to a target value by the component, the cold rolling rate, and the annealing temperature.
Specifically, C: 0.01 to 0.12%, Si: 0.005 to 0.5%, Mn: 0.3 to 1.5%, P: 0.005 to 0.2%, Al: 0.10% or less, N: 0.012% or less, Nb: 0.005 Add ~ 0.10%, hot-roll at a finishing temperature of 870 ° C or higher, cool at a rate of 40 ° C / s or lower until winding, wind up on a coil in a temperature range of 620 ° C or higher, and then acid After performing cold rolling of 80% or more after washing, a crystal grain size of 7 μm or less is obtained by performing continuous annealing under conditions of a soaking temperature of 650 to 750 ° C. and a soaking time of 40 s or less.

Yield strength (hereinafter sometimes referred to as YP): 500 MPa or more Yield strength is an important factor in securing the dent strength of a welded can. Generally, the dent strength is expressed by a relational expression between the plate thickness and the yield strength. When the present invention is applied to an application in which a conventional DR material has been used, the yield strength is set to 500 MPa or more in order to ensure the dent strength with a plate thickness of the DR material of 0.15 to 0.17 mm.

Yield ratio: 0.9 or more (yield strength / tensile strength, hereinafter sometimes referred to as YR)
When the tensile strength is increased, the deformation resistance at the time of hot rolling or cold rolling increases, and the operability of rolling decreases. On the other hand, it is necessary to secure a yield strength of 500 MPa or more from the viewpoint of can strength. That is, it is necessary to increase the yield strength and decrease the tensile strength, and the yield ratio was set to 0.9 or more as a condition for obtaining the above characteristics without hindering operation.
YP and TS are controlled to target values based on the components, the cold rolling rate, and the annealing temperature.
Specifically, C: 0.01 to 0.12%, Si: 0.005 to 0.5%, Mn: 0.3 to 1.5%, P: 0.005 to 0.2%, Al: 0.10% or less, N: 0.012% or less, Nb: 0.005 Add ~ 0.10%, hot-roll at a finishing temperature of 870 ° C or higher, cool at a rate of 40 ° C / s or lower until winding, wind up on a coil in a temperature range of 620 ° C or higher, and then acid After performing cold rolling of 80% or more after washing, the temperature is controlled to a target value by performing continuous annealing under conditions of a soaking temperature of 650 to 750 ° C. and a soaking time of 40 seconds or less.

Total elongation: When the total elongation is 10% or more and less than 10%, for example, it becomes difficult to apply to cans with high can body processing such as can expansion processing. Therefore, the total elongation is 10% or more.

Δr: −0.50 to 0
In the present invention, Δr represented by the following formula is used as an anisotropy index.
Δr = (r0 + r90−2 × r45) / 4
r0 indicates the r value when the tensile test is performed in the rolling direction, r45 indicates the r value when the tensile test is performed in the rolling direction and the 45 ° direction, and r90 indicates the tensile value when the tensile test is performed in the rolling direction and the 90 ° direction.
For a steel sheet with an Δr of less than −0.50, for example, when it is processed into a DRD can, the generation of ears is large, so that the trim margin increases and the yield decreases. In order to suppress the ear generation amount from the viewpoint of yield, Δr needs to be in the range of −0.50 to 0. Further, when | Δr | is large, flange wrinkles are generated due to the plate thickness distribution in the circumferential direction at the flange portions of the DRD can and the weld can. Therefore, it is desirable to use steel of Δr (−0.45 to 0). Furthermore, when the roundness of the can is required, it is necessary to suppress the plate thickness distribution in the circumferential direction as much as possible.
In addition, (DELTA) r is controlled to a target value with the finishing temperature at the time of hot rolling, the cooling rate after finishing, and coiling temperature. Specifically, Δr is set to the target value by hot rolling at a finishing temperature of 870 ° C or higher, cooling at a rate of 40 ° C / s or lower until winding, and winding the coil in a temperature range of 620 ° C or higher. Control.

Hot-rolled steel sheet structure: ferrite single-phase structure, average crystal grain size of 6 μm or more In the present invention, the structure of the hot-rolled steel sheet is substantially a ferrite single-phase structure. Even when containing about 1% of cementite or the like, it is determined that the ferrite single phase structure is substantially obtained as long as the effects of the present invention are exhibited.
The anisotropy after continuous annealing and pressure adjustment is greatly affected by the ferrite grain size at the hot-rolled steel sheet stage. For example, Fig. 1 shows the anisotropy of cold-rolled steel sheets obtained by continuous annealing of Steel 1 shown in the Example with a cold rolling reduction of 90%, a soaking temperature of 710 ° C, and a soaking time of 30 s. And the average ferrite grain size at the hot-rolled steel sheet stage (hot-rolled material). According to FIG. 1, when the ferrite average crystal grain size of the hot-rolled material is less than 6 μm, Δr is less than −0.50, and a desired anisotropy value cannot be obtained. Therefore, it is preferable that the average grain size of ferrite in the hot-rolled material is 6 μm or more. When steel with Δr of −0.45 to 0 is to be used, the ferrite average crystal grain size in the hot rolled material is more preferably 7 μm or more.
The crystal grain size is controlled to a target value by the component, FT during hot rolling, the cooling rate to CT, and CT.
In addition, although plate | board thickness and an aging index | exponent are not specifically limited by a claim, the conditions desirable for implementing this patent are the ranges shown below.
Preferred thickness of steel plate for cans: 0.2 mm or less, preferred thickness of hot-rolled steel plate: 2 mm or less The present invention is mainly intended for application to gauge down of drawn cans and welded cans. Used at 0.2 mm or less. In order to reduce the thickness of the steel of the strength level proposed in the present invention to 0.2 mm or less without impairing the operability of cold rolling, it is desirable to roll at a rolling rate of 90% or less. The thickness of the material is preferably 2 mm or less.

Aging index: 30 MPa or more In order to reliably obtain a yield strength of 500 MPa after baking and laminating, it is desirable to set the aging index to 30 MPa or more. In the present invention, the aging index indicates the amount of age hardening when heat treatment is performed at 100 ° C. for 60 minutes after 8% pre-strain is applied.

Next, the manufacturing method of the steel plate for cans of this invention is demonstrated.
The molten steel adjusted to the above-described chemical composition is melted by a generally known melting method using a converter or the like, and then made into a rolled material by a commonly used casting method such as a continuous casting method.
Next, it is set as a hot-rolled sheet by hot rolling using the rolling raw material obtained by the above. At the start of rolling, the rolled material is preferably 1250 ° C. or higher. The finishing temperature is 870 ° C or higher. Moreover, it cools at a speed | rate of 40 degrees C / s or less until winding, and winds at the winding temperature of 620 degreeC or more. From the viewpoint of anisotropy, the ferrite average crystal grain size of the hot rolled material obtained here is 6 μm or more. Next, after pickling and cold rolling at a reduction rate of 80% or more, continuous annealing is performed under conditions of a soaking temperature of 650 to 750 ° C. and a soaking time of 40 s or less, and a pressure regulation of 1.5% or less is performed. Temper rolling at a rate.

Hot rolling finish temperature: 870 ° C. or higher The finish rolling temperature in hot rolling is an important item in controlling anisotropy. In order to secure Δr of −0.50 or more in the Nb-added steel, it is necessary to control the texture of the hot rolled material to have an average grain size of ferrite of 6 μm or more and to control the texture. In order to obtain this, the hot rolling finishing temperature is 870 ° C. or higher.

Average cooling rate from finish rolling to winding: 40 ° C./s or less Anisotropy is greatly affected by the average ferrite grain size of the hot rolled material. As described above, in order to make Δr in the range of −0.50 to 0, the ferrite average crystal grain size of the hot-rolled material needs to be 6 μm or more. In order to make the ferrite average crystal grain size of the hot-rolled material 6 μm or more, it is necessary to reduce the cooling rate after hot rolling, and the condition is that the average cooling rate after finishing is 40 ° C./s or less.
In order to reliably obtain a steel having Δr of −0.45 to 0 in the entire width direction, it is preferable that the ferrite average crystal grain size of the hot-rolled material is 7 μm or more. For this purpose, the average cooling rate is 30 ° C./s or less. It is necessary to.
In order to reliably obtain a steel having Δr of −0.30 to 0 in the entire width direction, it is preferable that the average grain size of ferrite of the hot-rolled material is 8 μm or more. For this purpose, the average cooling rate is 20 ° C. / s or less is required.

Winding temperature: 620 ° C. or higher In order to obtain a ferrite average crystal grain size of 6 μm or more in a hot-rolled material, it is necessary to increase the winding temperature. Moreover, in order to obtain steel with Δr of −0.30 to 0, the coiling temperature needs to be 700 ° C. or higher.

Cold rolling rate (rolling rate): 80% or more The rolling rate in cold rolling is one of the important conditions in the present invention. If the rolling reduction in cold rolling is less than 80%, it is difficult to produce a steel plate having a yield strength of 500 MPa or more. Furthermore, in order to obtain a plate thickness comparable to that of the DR material (about 0.17 mm), at a rolling reduction of less than 80%, at least the plate thickness of the hot-rolled plate needs to be 1 mm or less, which is difficult in operation. Therefore, the rolling reduction is 80% or more.

Annealing conditions: Soaking temperature: 650 ° C. to 750 ° C., soaking time: 40 s or less. 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 soaking is necessary at a temperature of 650 ° C. or more to make the structure more uniform. . On the other hand, in order to perform continuous annealing at a temperature exceeding 750 ° C., it is necessary to reduce the speed as much as possible in order to prevent the steel sheet from being broken, and productivity is lowered. 750 ° C. or less is a condition that does not reduce productivity. Since the productivity cannot be ensured at a speed at which the soaking time is 40 s or more, the soaking time is 40 s or less.

Pressure regulation rate: 1.5% or less When the pressure regulation rate is increased, the ductility is lowered due to an increase in strain introduced during processing, as in the case of DR material. In the present invention, since it is necessary to ensure a total elongation of 10% or more with an ultrathin material, the pressure regulation rate is 1.5% or less.

  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 at 1250 ° C., then hot-rolled at a finish rolling temperature of 880 ° C. to 900 ° C., cooled at an average cooling rate of 20-40 ° C./s until winding, and a winding temperature of 620 It wound up in the range of -700 degreeC. Then, after pickling, it was cold-rolled at a rolling reduction of 90% or more to produce a thin steel plate of 0.17 to 0.2 mm. The obtained thin steel sheet was made to reach 690 to 750 ° C. at a heating rate of 15 ° C./sec and subjected to continuous annealing at 690 ° C. to 750 ° C. for 20 seconds. Next, after cooling, temper rolling was performed so that the rolling reduction was 1.5% or less, and normal chrome plating was continuously applied to obtain tin-free steel. The soaking temperature was adjusted according to the amount of Nb added and was in the range of 690 ° C to 750 ° C.

  The plated steel sheet (tin-free steel) obtained as described above was subjected to a paint baking process at 210 ° C. for 20 minutes, and then a tensile test was performed to investigate the crystal structure and the average crystal grain size. The survey method is as follows.

In the tensile test, yield elongation, tensile strength, and elongation were measured using a JIS5 size tensile test piece to evaluate strength and ductility. The r value was measured using a JIS No. 5 half-size tensile test piece (width 12.5 mm, parallel part 35 mm, distance between gauge points 20 mm), and Δr was measured by the following method.
Δr = (r0 + r90-2r45) / 4
In addition, r0 indicates the r value when the tensile test is performed in the rolling direction, r45 indicates the tensile value when the tensile test is performed in the rolling direction and 45 ° direction, and r90 indicates the r value when the tensile test is performed in the rolling direction and 90 ° direction. .
The crystal structure was observed with an optical microscope after the sample was polished, the grain boundaries were corroded with nital. The average crystal grain size was measured using the cutting method of JIS G0551 for the crystal structure observed as described above.
The results obtained are shown in Table 2.

From Table 2, the steel plate for cans (No. 1-6) using the hot-rolled steel sheet of the present invention as the base material has an average grain size of ferrite of the annealed material (plated steel sheet) structure of 7 μm or less and includes a mixed grain structure. No uniform and fine ferrite single layer structure, and it is recognized that both the strength and ductility are excellent.
On the other hand, the comparative example (No7) has anisotropy, and the comparative example (No8) has insufficient strength.

  Steel containing the composition shown in Table 3 and the balance being Fe and inevitable impurities was melted in an actual converter to obtain a steel slab. The obtained steel slab is reheated at 1250 ° C, then hot rolled at a finish rolling temperature of 830-900 ° C, the average cooling rate until winding is cooled at 18-45 ° C / s, and the winding temperature is set to It wound up in the range of 580-700 degreeC. Subsequently, it cold-rolled with the rolling reduction of 90% or more, and manufactured the 0.15-0.18 mm thin steel plate. The obtained thin steel sheet was allowed to reach 710 to 730 ° C. at a heating rate of 20 ° C./sec, and subjected to continuous annealing at 710 to 730 ° C. for 20 to 30 seconds. Next, after cooling, temper rolling was performed so that the rolling reduction was 1.5% or less, and normal chrome plating was continuously applied to obtain tin-free steel. Detailed manufacturing conditions are shown in Table 4.

The plated steel sheet (tin-free steel) obtained as described above was subjected to a paint baking process at 210 ° C. for 20 minutes, and then a tensile test was performed to investigate the crystal structure and the average crystal grain size.
Each test and investigation method are the same as those in Example 1.
The results obtained are shown in Table 5.

From Table 5, in the inventive examples (Nos. 9 to 12), a high strength steel sheet having small anisotropy and high ductility was obtained by decreasing the cooling rate after finish rolling and increasing the coiling temperature.
On the other hand, in the comparative examples (No. 13 to 15), although the strength and ductility reach the target values, the finish rolling temperature is low, the coiling temperature is low, or the cooling rate after finish rolling is large, so It is a large steel plate.

  Further, when drawing these steel sheets, in the present invention examples (Nos. 9 to 12), the surface properties of the steel sheets are good, no rough skin is observed, and the amount of ears generated is small. On the other hand, in the comparative example in which Δr is −0.50 or less, the ear generation amount is large for the steel sheet.

  According to the present invention, a steel sheet having excellent strength, ductility, and anisotropy can be obtained. Therefore, pressure resistance is required like a three-piece can with a high degree of canning and a positive pressure can. It is most suitable as a steel plate for cans centering on 2-piece cans.

Claims (2)

By mass%, C: 0.02-0.12%, Si: 0.005-0.5%, Mn: 0.3-1.5%, P: 0.005-0.2%, Al: 0.10% or less, N: 0.012% or less, Nb: 0.005-0.10% And the balance consisting of iron and inevitable impurities, and substantially having a ferrite single-phase structure, the average grain size of ferrite is 6 μm or more , the yield strength after paint baking is 500 MPa or more, A hot-rolled steel sheet used for a steel plate base material for cans having a yield ratio of 0.9 or more, total elongation of 10% or more, and Δr of −0.50 to 0 . In producing the hot-rolled steel sheet according to claim 1,
C: 0.02 to 0.12% by mass, Si: 0.005 to 0.5%, Mn: 0.3 to 1.5%, P: 0.005 to 0.2%, Al: 0.10% or less, N: 0.012% or less, Nb: 0.005 to 0.10 Containing steel and the balance being iron and inevitable impurities,
Hot rolled at a finishing temperature of 870 ° C or higher,
Cooling at an average cooling rate of 40 ° C / s or less until winding,
For cans with a yield strength of 500 MPa or more, a yield ratio of 0.9 or more, a total elongation of 10% or more, and Δr of -0.50 to 0 after coating and baking, characterized by winding at a winding temperature of 620 ° C or higher A method for producing a hot-rolled steel sheet used for a steel sheet base material .

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