JP5316036B2 - Mother board for high-strength ultrathin cold-rolled steel sheet and manufacturing method thereof - Google Patents

Description

  The present invention relates to a base plate for a high-strength ultrathin cold-rolled steel plate and a method for producing the same, and the high-strength ultrathin cold-rolled steel plate from the high-strength ultrathin cold-rolled steel plate is used for various containers. In particular, it is a new steel that can be hardened and strain-aged in the printing process and drying process, etc., before the can-making process, exhibit sufficiently high strength, and can promote the thinning of the steel sheet. Steel plate. The target of the high-strength ultra-thin cold-rolled steel plate from the high-strength ultra-thin cold-rolled steel plate is a steel plate for cans in a category called ultra-thin steel plate of 0.3 mm or less, mainly 0.2 mm or less. These steel plates are tin-plated, nickel-tin-plated, chrome-plated (so-called tin-free plating), or further organically coated, and can be applied to a very wide range of applications.

  Cans that contain various contents such as beverage cans, food cans, pail cans, 18 liter cans, etc., are two-piece cans consisting of a can body including a bottom surface and an upper cover, and a can body and an upper cover. It is roughly divided into a three-piece can consisting of a bottom lid. In view of the increasing demand for reduction in can manufacturing costs in recent years, the cost reduction of can manufacturing materials has been urged, and in order to reduce material costs, not only two-piece cans that are drawn, but also simple three-piece molding Even if it is a can, the thinning of the steel plate to be used is advanced.

  However, even if the steel sheet is thinned, it is necessary to maintain the strength of the can as before, so that it is excellent in can body strength after molding and aging, and it is a hard and extremely thin can that can contribute effectively to thinning Steel plates were desired. The so-called DR (Double cold Reduced) method, in which secondary cold rolling is performed after annealing, is the mainstream method for producing hard and extremely thin steel plates for cans. In the DR method, after cold rolling-annealing, cold rolling is further applied to close to 30% to harden the steel sheet by work hardening and to reduce the plate thickness.

However, in the DR method, in order to cold-roll the ultra-thin steel sheet after cold rolling and annealing under high pressure, a strong rolling mill is necessary, and when in-line processing after annealing is not possible, There is an equipment demerit that the number of processes increases. Furthermore, it is extremely difficult to completely eliminate surface wrinkles and surface contamination that occur chronically in a steel plate manufactured by the DR method.
For this reason, there has been a problem in producing a hard and extremely thin steel plate for cans having sound surface properties stably and at low cost by the DR method. Further, as a method for increasing the strength other than the DR method, for example, a method of adding a large amount of alloying elements and strengthening the steel by solid solution strengthening as in a wide range of steel plates for automobiles can be considered. However, since special corrosion resistance is required in the case of steel plates for cans, the amount of alloy elements that can be added is limited. Actually, in the ASTM standard and the JIS standard, the additive elements are limited, and sufficient solid solution strengthening cannot be achieved.

In addition, the method using transformation structure strengthening and the method using precipitation strengthening by carbonitride-forming elements such as Nb, Ti and V are all difficult to apply in the steel plate manufacturing process for cans.
On the other hand, Patent Document 1 proposes a method for producing a steel plate for high-strength cans that effectively uses C and N, which are solid solution strengthening elements that can be used in steel plates for cans. In the technique described in Patent Document 1, C: 0.01 wt% or less, N: 0.04 wt% or less, and C + N containing 0.008 wt% or more and Al: 0.005 wt% or less are hot-rolled and cold-rolled. It is said that a hard material of T-4 or higher can be obtained by rolling, performing continuous annealing at a temperature higher than the recrystallization temperature, and then performing temper rolling with a rolling reduction of 5% or higher.

Patent Document 2 includes carbon: 0.01 to 0.08 wt%, N: 0.01 wt% or less, acid-soluble Al: 0.20 wt% or less, and the amount of solid solution C is 5 to 25 ppm. Steel sheets for DI cans having a strength of 30 to 44 kgf / mm ² have been proposed.
Patent Document 3 includes C: 0.0010 to 0.04 wt%, N: 0.0020 to 0.0150 wt%, Al: 0.005 to 0.060 wt%, 25% or more of the above N amount, and 0.001 to 0.01 wt%. A steel plate for cans containing a solid solution N and having high age-hardening properties and excellent material stability has been proposed.

JP-A-5-345926 JP-A-8-311609 Japanese Patent Laid-Open No. 10-72640

However, in the techniques described in Patent Document 1, Patent Document 2, and Patent Document 3, the limit is to a hard material equivalent to T-5. It is necessary to apply the DR method for performing secondary rolling. However, the DR method has a problem in stably producing a hard and extremely thin steel plate for cans having a healthy surface property at a low cost.
On the other hand, when paint baking is performed before can-making even in the case of 3-piece cans or 2-piece cans as in DRD cans, an increase in strength can be expected due to aging in the paint baking process.

The present invention solves the above-described problems of the prior art, has a hardness equivalent to or higher than DR8, and has excellent bake hardenability and high cost competitiveness, and is used as a material for a high-strength ultrathin cold-rolled steel sheet. That is, it aims at proposing the mother board for high intensity | strength ultra-thin cold-rolled steel plates, and its manufacturing method.
The steel plate from the base plate for high-strength ultra-thin cold-rolled steel sheet according to the present invention has a plate thickness of 0.3 mm or less, and has a high yield strength after paint baking of 550 MPa or more. It is a high-strength ultrathin cold-rolled steel sheet with excellent bake hardenability, with a bake hardening amount (baking hardening amount) of 50 MPa or more. In the present invention, a high-strength ultrathin cold-rolled steel sheet having a hardness equivalent to or higher than DR8 is obtained by applying temper rolling of 10% or less after annealing without applying the so-called DR method. . The bake hardening amount (BH amount) referred to in the present invention is a value obtained when a tensile test is performed after applying a heat treatment of 210 ° C. × 20 min corresponding to a coating / baking process after applying a tensile strain of 2% to the steel sheet. It shall mean the difference in yield stress before and after heat treatment equivalent to painting and baking treatment. The yield stress after painting / baking treatment refers to the yield stress after applying a 2% tensile strain to the cold-rolled steel sheet and applying a heat treatment of 210 ° C. × 20 min equivalent to the painting / baking treatment.

In order to achieve the above-described problems, the present inventors have conducted intensive research on steel components and production conditions.
As a result, in order to obtain a hardened amount equivalent to or higher than DR8 and a large bake hardening amount without applying the DR method,
<1> Adjust the steel composition, hot rolling conditions and cooling conditions after hot rolling so that the amount of solute N in the base plate for cold rolling is 80% or more of the total N amount,
<2> Continuous annealing in a so-called partially recrystallized state, not at a high temperature at which recrystallization is completely completed as in the prior art,
<3> Rapid cooling to a low temperature range after continuous annealing,
<4> The steel composition should be a low-carbon steel composition with a high ductility or an ultra-low carbon steel composition.
I learned that is important.

  By ensuring <1> to <4>, it is possible to prevent a decrease in the amount of solute N due to precipitation as AlN, and further to prevent precipitation of solute C as carbides. A sufficient amount of solute C and N can be secured. Moreover, although a part is precipitated, it is precipitation at a sufficiently low temperature, resulting in a large increase in strength. By effectively using solid solution C and solid solution N and utilizing so-called strain aging or quenching aging, the strength of the steel sheet can be remarkably increased. For this reason, it is no longer necessary to apply a strong reduction (a reduction ratio of about 30%) as in conventional secondary rolling in order to increase the strength after continuous annealing, and to adjust the hardness, roughness, and shape correction of the steel sheet surface. It was found that only light reduction (reduction rate: 10% or less) is required.

The present invention is configured based on the above findings.
That is, according to the first aspect of the present invention, in mass%, C: 0.02% or less, Si: 0.02% or less, Mn: 1.5% or less, P: 0.20% or less, S: 0.01% or less, Al: 0.009% or less, N : 0.0050 to 0.0250% and 80% or more of the N content of solid solution N, and further containing one or two groups selected from the first group to the second group, the balance being Fe and thickness, characterized in that a hot-rolled steel sheet having a composition consisting of incidental impurities: 0.3 mm or less of a high strength ultra-thin cold-rolled steel sheet for the base plate (invention motherboard) Ru der.
<< First group: Nb: 0.001 to 0.040%, Ti: 0.001 to 0.040%, B: One or more selected from 0.0002 to 0.0020%, where Nb is the following formula (1), Ti is The following equation (2) is satisfied.

(12/93) x (Nb / C) ≤ 0.8 (1)
(12/48) × (Ti ^{* /} C) ≦ 0.8 (2)
In addition, Ti ^* = Ti- (48/32) S- (48/14) N
Second group: Cu: 0.01 to 0.2%, Ni: 0.01 to 0.2%, Cr: 0.01 to 0.2%, Mo: 0.01 to 0.2% or one or more selected from >>

The second aspect of the present invention is, by mass%, C: 0.02% or less, Si: 0.02% or less, Mn: 1.5% or less, P: 0.20% or less, S: 0.01% or less, Al: 0.009% or less, N: 0.0050 using a rolling stock having including composition of ～0.0250%, the 90% of the N content starts rolling at a temperature which is a solid solution state, the finish rolling temperature (Ar ₃ transformation point -30 ° C.) or higher and to subjected to hot rolling, after the heat-rolling finished, take-up by forced cooling was initiated 600 ° C. or less of the winding temperature within 0.5 s, further water cooling started within the take after up 30min, at an average cooling rate It is cooled at a cooling rate of 20 ° C / h or more, and is a hot-rolled sheet containing solid solution N of 80% or more of the N amount. It is a manufacturing method of a mother board.

  According to the present invention, a high-hardness material that has been obtained for the first time after performing secondary rolling at a high pressure reduction after annealing is subjected to secondary rolling at a low reduction rate of 10% or less as it is annealed. It can be obtained only by this, and has a remarkable industrial effect. The steel sheet produced from the mother board of the present invention can greatly contribute to the thinning of the steel sheet because the yield stress is greatly increased and the strength of the can body is greatly increased by the coating baking process after forming.

  The cold-rolled steel sheet of the reference invention is an ultra-thin cold-rolled steel sheet for high-strength cans having a bake hardening amount of 50 MPa or more, a yield stress after coating and baking treatment of 550 MPa or more, and a thickness of 0.3 mm or less. The cold-rolled steel sheet of the reference invention has a yield stress after painting and baking treatment of 550 MPa or more, and can promote the thinning of the steel sheet. In addition, the cold rolled steel sheet of the reference invention effectively uses the action of solute C + solute N, so that it can be markedly age hardened with a paint bake hardening amount (BH amount) of 50 MPa or more during the paint baking process before press forming. In addition, it contains a large amount of solute C and solute N, and can give dent resistance (resistance to dents) to the can body, especially the can body part after canning, and can strength Bring about a dramatic increase.

First, the reasons for limiting the composition of the cold-rolled steel sheet of the present invention and the cold-rolled steel sheet of the reference invention will be described. Hereinafter, mass% is simply referred to as%.
C: 0.02% or less C is an effective element that increases the strength of steel by solid solution strengthening, but on the other hand, it forms carbides and lowers the ductility and consequently the workability of the steel sheet. For this reason, in this invention, the amount of carbide | carbonized_material in steel is reduced by reducing C content, and the ductility of a steel plate and by extension, workability are ensured stably at a high value. Further, by reducing the amount of carbides present in the steel where the solid solution C precipitates, a large amount of the solid solution C remains as a result, and the strength of the steel sheet can be increased. Such a desirable effect is recognized by setting the C content to 0.02% or less. For this reason, C was limited to 0.02% or less. In order to further promote the increase in strength due to solute C, it is desirable to set the C content to an extremely low carbon range of 0.010% or less. On the other hand, in order to obtain the target high bake hardenability (BH property), the C content is preferably 0.002% or more.

Si: 0.02 % or less
If Si is an element that increases the strength of steel by solid solution strengthening, addition of a large amount of the surface treatment of the degradation, to produce a corrosion resistance degradation problems such as, country excellent corrosion resistance is required and is Si is preferably 0.02% or less.
Mn: 1.5% or less
Mn is an effective element for preventing hot cracking due to S. In the present invention and the reference invention, Mn is appropriately added according to the amount of S contained. Further, Mn has an effect of refining crystal grains. Such an effect is remarkably recognized when the content is 0.1% or more, and Mn is preferably contained at 0.1% or more. On the other hand, when a large amount of Mn is contained, the strength of the steel sheet can be increased, but the corrosion resistance tends to deteriorate, and further, the tendency to deteriorate flange workability becomes remarkable. For this reason, Mn was limited to 1.5% or less. In applications where better moldability is required, Mn is preferably 0.50% or less.

P: 0.20% or less P makes the steel remarkably hard, but deteriorates flange workability and neck workability, and remarkably deteriorates corrosion resistance. For this reason, in the present invention and the reference invention, P is limited to 0.20% or less. In applications where flange workability, neck workability and corrosion resistance are particularly important, P is preferably 0.01% or less.

S: 0.01% or less S is an element that exists as an inclusion (sulfide) in steel, reduces the ductility of the steel sheet, and further deteriorates the corrosion resistance. In the present invention and the reference invention, S is preferably reduced as much as possible. , Up to 0.01% is acceptable. Therefore, in the present invention and the reference invention, S is limited to 0.01% or less. In addition, in the case of an application that requires particularly good workability, S is preferably 0.005% or less.

Al: 0.009 % or less
Al combines with solid solution N to form AlN and has the effect of reducing the amount of solid solution N. Further, an increase in the Al content causes an increase in the recrystallization temperature, and it is necessary to increase the annealing temperature. In high temperature annealing, the amount of solute N is reduced due to the formation of AlN, the amount of age hardening is reduced, and thus the strength of the steel sheet is lowered. Such a phenomenon becomes remarkable when the Al content exceeds 0.01%. For these reasons, Al is limited to 0.009 % or less. From the viewpoint of stable operation in the steel melting process, it is desirable that Al is 0.001% or more. Further, from the viewpoint of further stabilization of the material, Al is more preferably 0.005% or less.

N: 0.0050-0.0250%
N is an element that increases age hardenability and is positively contained in the present invention and the reference invention. A significant increase in age-hardening is observed at a content of 0.0050% or more. On the other hand, if the content exceeds 0.0250%, the risk of generating cracking defects in the rolled material (slab) is significantly increased. Therefore, N is limited to 0.0050-0.0250%. In view of the stability of the material, N is preferably 0.0070% or more.

Solid solution N in cold rolled steel sheet base plate: 80% or more of the total N amount In order to ensure the large age hardening of the cold rolled steel sheet, which is a feature of the reference invention, = The solid solution N amount in the present invention mother board) needs to be 80% or more of the total N amount. The cold-rolled steel sheet of the reference invention is preferably manufactured by pickling the hot-rolled sheet, then cold rolling, and then performing continuous annealing for a short time, but AlN tends to precipitate in this continuous annealing process. . If the solute N amount of the hot-rolled sheet (base plate for cold-rolled steel sheet) is less than 80% of the total N amount, the desired age-hardening property of the cold-rolled steel sheet cannot be achieved. In the present invention and the reference invention, the amount of N that is AlN is determined by extraction analysis after dissolution treatment with a bromoester that is usually carried out (hereinafter referred to as N as AlN), and N as AlN is subtracted from the total N amount. The value is defined as the amount of solute N.

(Solution C + Solution N) in cold-rolled steel sheet: 0.0050% or more In order to increase the age hardening of the product plate (cold-rolled steel plate) and increase the strength of the product plate, in the reference invention, The amount of (solid solution C + solid solution N) is 0.0050% or more.
In addition to the basic composition described above, and the selected択元element contains one group or two groups selected from among the first group-second group follows.

First group: Nb: 0.001 to 0.040%, Ti: 0.001 to 0.040%, B: one or more selected from 0.0002 to 0.0020%,
Nb, Ti, and B are all elements that increase the strength, and one or more can be selected as necessary.
Nb is an effective element for forming fine carbonitrides, refining crystal grains, and increasing the strength of steel after forming. Such an effect is recognized when the Nb content is 0.001% or more. However, if the content exceeds 0.040%, the recrystallization temperature of the steel rises remarkably, hinders the annealing process after cold rolling, and solid solution N precipitates during high temperature annealing, resulting in high strength of the steel. Loses its function. For this reason, Nb is preferably 0.040% or less. From the viewpoint of annealing conditions, Nb is preferably 0.007% or less. When Nb is contained, the following formula (1)
(12/93) x (Nb / C) ≤ 0.8 (1)
Shall be satisfied. When the formula (1) is not satisfied, there is an inconvenience that the amount of dissolved C is significantly reduced.

Ti, like Nb, is an effective element for forming fine carbonitrides, refining crystal grains, and increasing the strength of steel after forming. Such an effect is recognized when the content is 0.001% or more, and becomes remarkable when the content is 0.003% or more. Therefore, Ti is preferably contained in an amount of 0.001% or more, more preferably 0.003% or more. On the other hand, when Ti is contained in excess of 0.040%, the amount of age-hardening due to the decrease in solid solution C and N becomes significant. For this reason, Ti is preferably 0.040% or less. From the viewpoint of stabilizing the material, Ti is more preferably in the range of 0.003 to 0.007%. When Ti is contained, the following formula (2)
(12/48) x (Ti ^{* /} C) ≦ 0.8 (2)
In addition, Ti ^* = Ti- (48/32) S- (48/14) N
Shall be satisfied. When the formula (2) is not satisfied, there is a disadvantage that the amount of dissolved C and the amount of dissolved N are significantly reduced.

  In spite of the tendency to form nitrides, B is an element effective for stabilizing the age-hardening property and ensuring the strength of the steel stably under the conditions of the present invention and the reference invention. Such an effect is noticeable when the content is 0.0002% or more, and preferably 0.0002% or more. On the other hand, even if the content exceeds 0.0020%, the effect is saturated, or conversely, age-hardening tends to be lowered. For this reason, B is preferably 0.0020% or less. In view of stabilization and homogenization of mechanical properties, B is more preferably 0.0003 to 0.0008%.

  Second group: Cu: 0.01-0.2%, Ni: 0.01-0.2%, Cr: 0.01-0.2%, Mo: 0.01-0.2% or more selected from Cu, Ni, Cr, Mo All have the effect of increasing the final strength without impairing age hardening, and can be selected and contained as necessary. Such an effect is recognized when Cu, Ni, Cr, and Mo are each contained at 0.01% or more. On the other hand, if any of Cu, Ni, Cr, and Mo exceeds 0.2%, the base plate for hot-rolled steel sheet (hot-rolled sheet) becomes noticeably hard and may cause problems in the cold rolling process. Increase. For this reason, it is preferable to limit to Cu: 0.01-0.2%, Ni: 0.01-0.2%, Cr: 0.01-0.2%, Mo: 0.01-0.2%. Note that the effects of Cu, Ni, Cr, and Mo are not offset even if they are added in combination. Therefore, Cu, Ni, Cr, and Mo can be added alone or in combination. It is more desirable that the total amount of Cu, Ni, Cr and Mo is 0.2% or less.

The balance is Fe and unavoidable impurities The balance other than the above-described components is Fe and unavoidable impurities. As an inevitable impurity, for example, Sn: 0.01% or less is acceptable. The cold-rolled steel sheet of the reference invention has the above composition and a structure having a recrystallization rate of 60% or more and less than 90%. In addition, the recrystallization rate as referred to in the reference invention refers to a value obtained by an area method through observation with an optical microscope. If the recrystallization rate is less than 60%, the workability, particularly the elongation property, is insufficient, while if it exceeds 90%, the strength and hardness are lowered. For this reason, in the reference invention, the recrystallization rate is limited to 60% or more and less than 90%.

Below, the manufacturing method of this invention mother board and a reference invention steel plate is demonstrated.
The molten steel having the above composition is melted by a generally known melting method using a converter or the like, and then rolled into a rolled material (slab) by a generally known casting method such as a continuous casting method.
Subsequently, these rolled materials are used to form hot rolled sheets by hot rolling.
In the present invention, it is important that the rolling material is at a temperature at which 90% or more of the total N amount is in a solid solution state at the start of rolling.

Rolling start temperature: Over 90% of the total N amount is in a solid solution state. Over 90% of the total N amount is in a solid solution state. Alternatively, in the case of a direct feed rolling process in which precipitation of AlN is delayed, it is preferable to heat the rolled material by inserting it into a heating furnace maintained at a rollable temperature without cooling it below the transformation point after continuous casting. In any case, in the present invention, at the start of hot rolling, 90% or more of the total N amount in the rolled material only needs to be in a solid solution state. For this reason, the hot rolling start temperature is preferably 1100 ° C. or higher. When 90% or more of the total N amount is not in a solid solution state at the start of hot rolling, age hardening due to the solid solution N is not sufficiently exhibited on the product plate.

Finish rolling temperature: (Ar ₃ transformation point −30 ° C.) or higher In the present invention, in order to effectively suppress precipitation of AlN, the finish rolling temperature in hot rolling is set to (Ar ₃ transformation point −30 ° C.) or higher. Is preferred. When the finish rolling temperature is lower than (Ar ₃ transformation point −30 ° C.), precipitation of AlN becomes remarkable and solid solution N is reduced. More preferably, it is (Ar ₃ transformation point −10 ° C.) or more.

Forced cooling: Start within 0.5 s after completion of hot rolling Forced cooling is preferably water cooling from the viewpoint of cooling capacity. The forced cooling is preferably started within 0.5 s after completion of hot rolling. If the start of forced cooling exceeds 0.5 s from the end of rolling, the precipitation of AlN cannot be suppressed.
Winding temperature: 600 ° C. or lower The winding temperature is preferably 600 ° C. or lower in order to suppress the fixation of N by Al. When the coiling temperature exceeds 600 ° C., the precipitation amount of AlN is remarkably increased and the solid solution N is decreased. As a result, the target age hardening cannot be obtained. In order to stably obtain high age-curing property, the winding temperature is more preferably 570 ° C. or lower. Further, by setting the coiling temperature to 600 ° C. or less, more preferably 570 ° C. or less, it becomes possible to finely and uniformly disperse the carbides generated during cooling after winding, and thereby in the state of steel sheet products. The amount of solute C can be increased.

After winding: water cooling In the present invention, it is preferable to start water cooling within 30 minutes in a coil state after winding and cool at an average cooling rate of 20 ° C / h or more. In addition, a cooling rate means the average cooling rate of the center part of the longitudinal direction of a steel strip, and a plate width center part here. Thereby, precipitation of AlN can be prevented, and carbides precipitated in the cooling process after winding can be dispersed more finely and more uniformly. If the start of water cooling after winding exceeds 30 min, the water cooling effect will be insufficient. Further, when the cooling rate is less than 20 ° C./h, the effect of improving the distribution of carbides is not sufficient. Although water cooling after winding is performed in a coil state, there is no problem even if it is rewound and cooled again.

By the above-described hot rolling and cooling after rolling, a hot rolled sheet in which 80% or more of the total N amount is in a solid solution state can be obtained.
Such a hot-rolled sheet is used as a cold-rolled steel sheet base plate, and pickling and cold rolling are performed to obtain a cold-rolled sheet. For pickling, the surface scale may be removed with an acid such as hydrochloric acid or sulfuric acid according to a conventional method. The cold rolling reduction follows the usual method, but the higher the plate thickness, the higher.

Next, the cold-rolled sheet is subjected to continuous annealing.
Soaking temperature of continuous annealing: Temperature range over 500 ° C and recrystallization rate less than 90% In the continuous annealing process, soaking is performed in a temperature range over 500 ° C and recrystallization rate less than 90%. If the soaking temperature is less than 500 ° C, the recrystallization progresses slowly and the continuous annealing results in a recrystallization rate of less than 60%, and the work strain introduced by cold rolling remains, so the ductility is extremely low and it is difficult to press work. Not suitable. On the other hand, when the temperature exceeds the temperature at which the recrystallization rate is less than 90%, the ductility is sufficiently increased, but the strength is remarkably lowered, which is not preferable. For this reason, in the reference invention, it is preferable to perform soaking in a temperature range of 500 ° C. or higher and a recrystallization rate of less than 90%. Thereby, sufficient workability is ensured in a normal application, and a target high strength can be obtained. The temperature at which the recrystallization rate is 90% is preferably determined by observing the structure with an optical microscope. However, the hardness in the cold-rolled state is 0% and the hardness in the completely annealed state is 100%. It may be less than the temperature at which the hardness reduction is achieved. It can also be determined by X-ray diffraction techniques. This is a technique based on the fact that the peak width of the diffraction line corresponds to the remaining strain, that is, the recrystallization rate.

Further, it is not necessary to keep the temperature constant within this temperature range. A soaking time equivalent to 10s or more is sufficient for the stability of operation. From the viewpoint of workability, the recrystallization rate is set to 60% or more.
Rapid cooling treatment after continuous annealing: Cooling conditions after continuous annealing are one of the most important requirements in the reference invention at a cooling rate of 150 ° C./s or higher to a temperature range of 250 ° C. or lower. The purpose of this cooling is to make a structure in which very fine carbides are finely dispersed after annealing, and thereby to secure a sufficient amount of solid solution C and solid solution N to ensure age hardening. . For this reason, it is preferable to perform rapid cooling from the soaking temperature at a cooling rate of 150 ° C./s or more. At a cooling rate lower than this cooling rate, among the solid solution N and the solid solution C, the decrease in the solid solution C is particularly significant. In order to obtain the target characteristics more stably, it is desirable to set the cooling rate to 200 ° C./s or more.

  Further, this rapid cooling treatment is preferably continued up to a temperature range of 250 ° C. or lower. When the quenching process is stopped at a temperature higher than 250 ° C., the carbide precipitation form becomes coarse, and the amount of solute C is drastically reduced. More preferably, it is desirable to perform the rapid cooling treatment to a temperature range of 200 ° C. or lower. In order to stably perform such quenching treatment, it is necessary to enhance the ability of conventional gas jet cooling, to apply hydrogen as a cooling gas, to use continuous annealing capable of high-speed plate treatment, and to A combination of technologies is required.

Furthermore, it is preferable to cool to 40 ° C. or less within 5 minutes after completion of soaking (start of cooling). If it takes more time to cool to 40 ° C. or lower, a phenomenon similar to the so-called overaging phenomenon occurs, and the amount of solute C decreases and the age hardening capacity of the steel sheet decreases.
After continuous annealing and rapid cooling treatment, secondary rolling with a rolling reduction of 10% or less may be further performed. The secondary rolling in the reference invention is not mainly intended to increase the strength, but is mainly intended to adjust the surface roughness, shape, etc., and the rolling reduction is preferably 1.0% or more. When the rolling reduction exceeds 10%, the ductility is lowered and the rolling operation becomes difficult as the deformation resistance increases. Since the rolling reduction is as light as 10% or less, it is industrially useful from the viewpoint of reducing the equipment load.

A cold-rolled steel sheet is obtained through the above-described steps. Cold-rolled steel plate (reference invention steel plate) from the base plate of the present invention is a hard material by painting and baking before canning (before press processing), and is applied to ultra-thin steel plates with a thickness of 0.3 mm or less If so, the advantage is more effectively exhibited.
The cold-rolled steel sheet manufactured by the above-described process has a (solid C + solid solution N) amount of 0.0050% or more, a bake hardening amount: 50 MPa or more, and a yield stress after painting / baking treatment: 550 MPa or more. It becomes an ultra-thin cold-rolled steel sheet for high-strength cans. The reference invention steel sheet obtains a large age-hardening property that has never been obtained by combining the actions of both solid solution C and solid solution N. For this reason, the reference invention steel sheet has a yield stress after painting and baking treatment of 550 MPa or more, and can promote the thinning of the steel sheet. In addition, the steel of the reference invention increases the strength even after the reflow treatment after plating by effectively utilizing the action of solute C + solute N, and the amount of bake hardening also during the paint baking process after press forming. A remarkable age hardening phenomenon (BH amount) of 50 MPa or more occurs, which can bring about a dramatic increase in can strength.

  In the reference invention, a plating layer can be formed on the surface of the ultra-thin cold-rolled steel sheet (at least one side) to obtain an ultra-thin plated steel sheet. Any plating layer applied to the steel plate for cans can be applied to the plating layer formed on the surface. Examples of the plating layer include tin plating, chromium plating, nickel plating, and nickel / chromium plating. Moreover, there is no problem in applying a coating, an organic resin film, etc. after these plating treatments.

Example 1
Steels having the components shown in Table 1 were melted in a converter and made into slabs by a continuous casting method. Subsequently, these slabs were hot-rolled under the conditions shown in Table 2 to obtain hot-rolled sheets having a plate thickness of 1.8 mm. In the production of these steel plates, when coil water cooling was performed, water cooling was started within 30 minutes after winding. After that, these hot-rolled sheets were descaled by pickling, further cold-rolled, then subjected to continuous annealing and secondary rolling under the conditions shown in Table 2, and the final finished sheet thickness: 0.15 mm ultra-thin cooling A rolled steel sheet was used. A cold rolled steel plate (steel plate No. 12) whose rolling reduction was increased to 30% by secondary rolling was used as a conventional example (DR steel plate).

The ultrathin cold-rolled steel sheet thus obtained was subjected to solid solution C, solid N content measurement, recrystallization rate measurement (structure investigation), tensile test, hardness test, and bake hardening test.
(I) Analysis of solid solution C and solid solution N amount The amount of N in the cold-rolled steel sheet was analyzed by chemical analysis, and the amount of N present as AlN was determined by extraction analysis after dissolution treatment with bromoester. As the amount of solute N in the cold-rolled steel sheet, a value of {(N amount in cold-rolled steel sheet) − (N amount present as AlN)} was used.

Moreover, the amount of solid solution C was measured by internal wear. The C and N peaks were separated by a torsion pendulum type internal friction measuring device, and the C peak was adopted. About N, it confirmed that it was consistent with the result of chemical analysis.
(Ii) Microstructure investigation A specimen was taken from the center in the width direction of the cold-rolled steel sheet, and the cross-section parallel to the rolling direction and the cross-section in the plate thickness direction excluding the surface layer portion were observed by an optical microscope, and the area of recrystallized grains Was measured, and the ratio of the recrystallized grain area to the total area was calculated and used as the recrystallization rate.
(Iii) Tensile test JIS No. 13-B tensile test specimens were collected from the center in the width direction of these cold-rolled steel sheets in the rolling direction, and subjected to a tensile test at a strain rate crosshead speed of 10 mm / s, yield point. YS, tensile strength TS, and elongation El were measured. The tensile test was carried out within one day after commercialization. The reason why the tensile test piece is a JIS 13-B test piece is to reduce as much as possible the phenomenon of breaking outside the gauge.
(Iv) Bake hardenability test JIS 13-B tensile test specimens were sampled in the rolling direction from the center in the width direction of these cold-rolled steel sheets, unloaded once after 2% tensile pre-strain was added, and 210 A heat treatment equivalent to a baking process at ℃ × 20 min was performed, and then a tensile test was performed to determine the deformation stress (yield stress). The difference in yield stress before and after heat treatment equivalent to paint baking treatment, ((yield stress after heat treatment equivalent to paint baking treatment)-(2% deformation stress before heat treatment equivalent to paint baking treatment)) was obtained, and the bake hardening amount (BH amount) It was.
(V) Hardness test HR30T hardness was measured in accordance with the provisions of JIS Z 2245 for these cold-rolled steel sheets and the steel sheets that had been subjected to a heat treatment equivalent to paint baking treatment.

  These results are shown in Table 3.

  The present invention example (cold rolled steel plate from the present invention base plate) has a yield strength after coating baking treatment of 550 MPa without increasing the rolling reduction of secondary rolling (without applying the DR method with a large rolling reduction). As described above, the bake hardening amount is 50 MPa or more, and it is a high-strength ultrathin cold-rolled steel sheet (reference invention steel sheet) having a hardness equivalent to DR8. On the other hand, the comparative example which remove | deviates from the scope of the present invention has a small amount of paint baking.

Furthermore, using these steel plates, a two-piece DRD can (can diameter 70 mm) was manufactured, and the resistance (dent resistance) to the can body, particularly the can body dent, was evaluated. The evaluation of the resistance to the dent of the trunk is a test in which a can body having a shape equivalent to that of a lid being squeezed is statically pushed and deformed with an indenter, and a load causing permanent set is measured and evaluated.
As a result, the example of the present invention shows a value about 20% higher than the DR steel sheet of the same tempering degree (conventional example), and the effect of increasing the strength of the can due to the strain aging of the steel sheet is confirmed. It was.
(Example 2)
Steel A shown in Table 1 was an ultrathin cold-rolled steel sheet having a thickness of 0.14 mm under the manufacturing conditions shown in Table 4. When manufacturing these steel sheets, when coil water cooling was performed, water cooling was started within 30 minutes after winding. These cold-rolled steel sheets were subjected to measurement of solid solution C, solid solution N amount, recrystallization rate measurement (structure investigation), tensile test, hardness test, and bake hardening test. The test method was the same as in Example 1. The steel sheet surfaces were plated with Ni before annealing, and a Ni diffusion layer was formed on the surface by continuous annealing. A cold-rolled steel plate (steel plate No. 2-11) whose rolling reduction was increased to 30% by secondary rolling was used as a conventional example (DR steel plate).

  These results are shown in Table 5.

  Cold-rolled steel sheets (examples of the present invention) from the base metal of the present invention have a yield strength after paint baking of 550 MPa or more without increasing the rolling reduction of secondary rolling (without applying the DR method) and baking It is a high-strength ultrathin cold-rolled steel sheet (reference invention steel sheet) having a hardened amount of 50 MPa or more and having a hardness equivalent to DR8 and having unprecedented excellent characteristics. On the other hand, the comparative example which deviates from the scope of the present invention has a low coating bake hardening amount and a low yield strength after the coating baking treatment of less than 550 MPa.

Then, these cold-rolled steel sheets were subjected to tin plating treatment (plating basis weight 25g / m ² ) in an electric tin plating line, followed by in-line reflow treatment, and a tin-plated steel sheet having an island-like tin phase. did. Although the characteristics of these tin-plated steel sheets were investigated in the same manner as the cold-rolled steel sheets, there was almost no change from the characteristics of the cold-rolled steel sheets before plating.
Further, these high-strength ultra-thin plated steel sheets were formed into a three-piece can, and the axial compression strength and resistance (dent resistance) to the dents in the can body were evaluated. The dent resistance of the can body was evaluated in the same manner as in Example 1. As a result, the dent resistance of the example of the present invention was improved by about 25% compared to the conventional example in which the tin plate was plated using the same tempered DR steel plate (conventional example).

Claims (2)

In mass%, C: 0.02% or less, Si: 0.02% or less, Mn: 1.5% or less, P: 0.20% or less, S: 0.01% or less, Al: 0.009% or less, N: 0.0050 to 0.0250%, and It contains solid solution N of 80% or more of the N amount, and further contains one or two groups selected from the following first group to second group, with the balance being composed of Fe and inevitable impurities. Thickness of hot-rolled steel sheet: Base plate for high-strength ultra-thin cold-rolled steel sheet of 0.3 mm or less.
Record
First group: Nb: 0.001 to 0.040%, Ti: 0.001 to 0.040%, B: One or more selected from 0.0002 to 0.0020%, where Nb is the following formula (1) and Ti is the following The expression (2) is satisfied.
(12/93) x (Nb / C) ≤ 0.8 (1)
(12/48) × (Ti ^{* /} C) ≦ 0.8 (2)
In addition, Ti ^* = Ti- (48/32) S- (48/14) N
Second group: Cu: 0.01-0.2%, Ni: 0.01-0.2%, Cr: 0.01-0.2%, Mo: 0.01-0.2% or one or more selected from 0.01-0.2% A composition containing, in mass%, C: 0.02% or less, Si: 0.02% or less, Mn: 1.5% or less, P: 0.20% or less, S: 0.01% or less, Al: 0.009% or less, N: 0.0050 to 0.0250% Rolling is started at a temperature at which 90% or more of the N amount is in a solid solution state, and hot rolling is performed at a finish rolling temperature of (Ar ₃ transformation point −30 ° C.) or higher. After the hot rolling is completed, forced cooling is started within 0.5 s, winding is performed at a winding temperature of 600 ° C. or less, water cooling is started within 30 minutes after winding, and an average cooling rate of 20 ° C./h or more is started. A method for producing a base plate for a high-strength ultra-thin cold-rolled steel sheet having a thickness of 0.3 mm or less, wherein the hot-rolled sheet contains a solid solution N containing 80% or more of the N amount.