JP4677914B2 - Steel plate for soft can and method for producing the same - Google Patents

Description

  The present invention relates to a steel sheet for a soft can made of a slab produced by a continuous casting method and produced by a continuous annealing method, and in particular, there is no occurrence of slab surface cracks during continuous casting, and the box annealing method. The present invention relates to a steel sheet for soft cans of T1 to T4 class having a tempering resistance equivalent to or higher than that of a steel sheet for soft cans manufactured in, and a manufacturing method thereof.

  Steel plates for cans such as tinplate specified in JIS G3303 and tin free steel (TFS) specified in JIS G3315 used for food cans and beverage cans, etc. The degree of tempering is divided into T1 to T6 depending on the Rockwell T hardness, and the hardness (HR30T) is T1: 49 ± 3, T2: 53 ± 3, T2.5: 55 ± 3, T3: 57. ± 3, T4: 61 ± 3, T5: 65 ± 3, and T6: 70 ± 3.

  Among them, the soft material having a tempering degree of T1 to T3 is made of a slab manufactured by a continuous casting method, and the slab is reheated, hot-rolled, pickled, cold-rolled, and then In general, it is manufactured by batch annealing (box annealing), temper rolling, and subsequent plating treatment. However, the box annealing method is inferior in productivity because the annealing time is longer than that in the continuous annealing method, and tends to be inferior in terms of uniformity of the shape and mechanical properties of the steel plate.

  Therefore, conventionally, it has been studied to manufacture a steel sheet for a soft can by a continuous annealing method instead of the box annealing method. Furthermore, in recent years, with the advancement of refining technology, it has been studied to produce a soft material having a refining degree of T1 to T4 class by a continuous annealing method using a continuously cast slab made of extremely low carbon steel as a raw material. The ultra-low carbon steel slab is added with carbonitride-forming elements such as B, Al, and Nb in order to satisfy various required characteristics for steel plates for cans such as strength, workability, aging, and weldability. .

  By the way, in a continuous casting machine used for manufacturing a slab which is a material of a steel plate for cans, due to its structure, the cast slab is subjected to bending and unbending deformation during solidification and after completion of solidification. Therefore, the slab containing carbonitride-forming elements such as B, Al, and Nb has a problem that cracks are likely to occur along the austenite grain boundaries when undergoing the above deformation. When a slab crack occurs, a process for surface grinding or slab cutting of the slab is required, which is a factor that greatly hinders productivity.

  As a technique related to this problem, for example, Patent Document 1 specifies a component composition of carbon steel containing N, Nb, Ti and satisfies a specific relational expression between N, Nb, Ti, or Furthermore, by satisfying a specific relational expression between Nb and N, there has been proposed a technique for preventing slab surface cracks caused by grain boundary cracks regardless of the cooling conditions of continuous casting and the conditions for imparting strain. . However, since the slab of this technique contains 0.004 to 0.1 mass% of Ti, Ti concentrates on the surface of the obtained steel sheet and deteriorates plating properties (corrosion resistance and surface appearance). It cannot be used for steel plates.

  Patent Document 2 discloses a high toughness continuous cast slab with improved resistance to surface cracking by adding Ti to Ti-added steel to refine TiN and suppressing γ grain growth during slab heating. A manufacturing method has been proposed. However, addition of Ti is not preferable from the viewpoint of plating properties as described above, and Mg is an active element and is not practical because it is difficult to add stably to molten steel.

  In addition, in Patent Document 3, when the carbon steel slab continuously cast is subjected to width reduction, by controlling the temperature at the completion of the width reduction so as to satisfy a specific relational expression, the amount of precipitates precipitated in the slab, Methods have been proposed for producing crack free slabs. However, this technique is intended for cracks that occur during width rolling after continuous casting, and is not applicable to slab cracks caused by bending deformation or unbending deformation in a curved portion or a straightening portion of a continuous casting machine.

Further, Patent Document 4 proposes a continuous casting method in which the surface crack of the slab does not occur even if the boron-added steel is continuously cast at a high speed. However, this method controls the casting conditions according to the components and suppresses boron precipitation on the austenite grain boundary on the slab surface, that is, the casting is completed in a solid solution state of boron and nitrogen. When the slab is slow or when the cooling rate of the slab is slow, the amount of BN deposited increases, and B and N are not in a solid solution state, so slab cracking occurs. Therefore, slab cracking cannot be avoided even with this technique.
As described above, in the conventional technique, it is difficult to say that a technique for preventing surface cracking of a continuously cast slab to which carbonitride elements such as B, Al, and Nb are added is established.

On the other hand, the steel sheet for soft cans is naturally excellent in corrosion resistance, but is also required to have excellent can processability, aging resistance, and weldability.
For example, steel sheets for cans used in 2-piece cans are required to have excellent deep drawability because they are drawn. In addition, steel sheets for cans used in 3-piece cans are often subjected to paint baking before the can making process, but if they are easy to age, they may cause frying, stretcher strain, etc. Therefore, it is necessary to have excellent aging resistance. In addition, can manufacturers make various types of welding, such as seam welding, spot welding, and projection welding, depending on the type of can, and after welding, expand processing and bead processing are performed. Often it is done. For this reason, the welded part, particularly the welded heat affected part, can be separated into a welded part and other parts even if the expanded process and the bead process are performed in addition to the sufficient strength. It is required that there is no difference in height.

As described above, various characteristics are required for a steel sheet for a soft can, but it is difficult to say that a steel sheet for a soft can manufactured using a conventional continuous annealing method satisfies these requirements.
For example, Patent Document 5 proposes a method of manufacturing a steel sheet for soft surface treatment having excellent fluting resistance by annealing a low carbon steel sheet in a continuous annealing line provided with an overaging treatment zone. Yes. This technique rapidly cools after soaking in continuous annealing to make the solid solution C supersaturated, and then precipitates most of the solid solution C when passing through the overaging treatment zone, thereby softening and non-aging. It is to become. However, in this method, C cannot be completely precipitated, and some solid solution C remains, so that it cannot be said that the obtained steel sheet has sufficient aging resistance. For this reason, there is no problem in applications that do not receive heating prior to can manufacturing, but when used in applications that undergo heating processes such as paint baking before can manufacturing, aging is promoted and stretch can strain is applied in can manufacturing. And defects such as fluting may occur.

  Patent Document 6 proposes a method of manufacturing a steel plate for cans having a tempering degree of T1 to T3 by using a continuous cast slab obtained by adding Nb to ultra-low carbon steel as a raw material. In this technique, since Nb having a strong affinity for C is added and all C is fixed as NbC, complete non-aging is achieved. However, when used for applications where welding is performed, cracks may occur in the weld heat affected zone (HAZ portion) depending on the processing conditions after welding. This is considered to be because the hardenability is inferior due to the extremely low carbon steel and the strength of the HAZ part is insufficient. In addition, steel plates for cans made of Nb-added ultra-low carbon steel have a high rank ford value (r value), which indicates deep drawability. Therefore, after welding, the can body is subjected to expansion processing or bead processing. When applied, there is a problem that the reduction in can height is large.

  Patent Document 7 discloses a steel plate for cans that is easy to manufacture and has excellent weld strength by continuously annealing a steel plate obtained by adding B to ultra-low carbon steel, without adding expensive Nb and Ti. The manufacturing method of this is proposed. However, in this method, solid solution N is precipitated as BN, but solid solution C cannot be fixed. Therefore, the obtained steel plate is not sufficiently resistant to aging, and when heat treatment such as paint baking is performed in the can making process, aging is promoted, and the subsequent can making process includes stretcher strain and frying. May cause molding defects.

  Furthermore, Patent Document 8 discloses that a steel sheet obtained by adding one or more of Nb, Ti, and B to ultra-low carbon steel is continuously annealed so that the degree of tempering is equal to or lower than T3 and equal to or higher than that of the box annealed material. The manufacturing method of the steel plate for soft cans which has the non-aging property of this is proposed. However, in this method, when a large amount of Nb, B, N is added, it is impossible to prevent slab cracking caused by deformation that is received by the lower straightening band during continuous casting.

Further, in Patent Document 9, various additions required for steel sheets for cans, such as workability, weldability, and corrosion resistance, are made by adding Nb and B to an ultra-low carbon steel in combination and controlling the crystal grain size within an optimum range. A technique for satisfying the required characteristics is disclosed. However, since the composition range is very wide such as Nb: 0.001 to 0.1 mass% and B: 0.0001 to 0.005 mass%, it cannot be said that the optimum composition range was found for each required characteristic. It was.
Japanese Patent Laid-Open No. 2003-166038 JP 2004-315881 A JP 2002-346602 A JP 2002-020836 A Japanese Examined Patent Publication No. 63-010213 Japanese Patent Publication No. 01-052450 JP 09-227947 A JP 05-263143 A Japanese Patent Application Laid-Open No. 06-041683

  As explained above, the conventional technology for manufacturing steel sheets for soft cans cannot completely prevent cracking of continuously cast slabs added with carbonitride forming elements such as B, Al, Nb, etc. Care has greatly hampered productivity. Moreover, the continuous annealing method could not stably produce a steel plate for a soft can having can manufacturing processability, aging resistance and weldability equivalent to or higher than those of a steel plate for cans produced by a box annealing method. Therefore, the use of the steel sheet for soft cans manufactured by the continuous annealing method is limited, and the steel sheet manufactured by the box annealing method has been used for the use of most steel sheets for soft cans.

  Therefore, an object of the present invention is to continuously cast slabs that do not require slab maintenance by optimizing the composition of steel plates for cans to which carbonitride-forming elements such as B, Al, and Nb are added to reduce slab cracking susceptibility. Can be manufactured with high productivity, and the workability, aging resistance, and weldability are equal to or higher than those of steel plates for cans manufactured by the box annealing method using the slab as a raw material. The purpose is to propose a technique for producing steel sheets for soft cans having a tempering degree of T1 to T4 class.

  In order to establish a technology for manufacturing a steel sheet for soft cans having a tempering degree of T1 to T4 using a continuous casting slab and using a continuous annealing method, the inventors first set B, Al, Nb, N The cause of cracking of continuously cast slab containing slab was investigated. As a result, in steel to which B, Al, Nb, and N are added, nitrides and carbonitrides such as BN, AlN, and Nb (N, C) are present when the solidified steel structure is transformed from γ to α. Brittleness is caused by precipitation in large amounts at the austenite grain boundaries, and slab cracks are generated due to bending and unbending deformation that the continuously cast slab receives at the lower part of the casting machine. In addition to materials and carbonitrides, it has been found that MnS also causes slab surface cracking when precipitated in large amounts at the austenite grain boundaries. And in order to prevent this slab crack, it became clear that content of B, Al, Nb, N and Mn, S needs to be optimized in each relationship.

  Next, the inventors use a continuous casting slab to produce a steel sheet for a soft can having the same or better characteristics as a steel sheet produced by a box annealing method with workability, aging resistance and weldability by a continuous annealing method. The manufacturing method was examined. As a result, the steel composition is controlled within the proper range, and the cold rolling reduction ratio and continuous annealing conditions are further optimized, and the rank ford value (r value) and the ferrite crystal grain size are controlled within the proper range. It was found that this is effective, and the present invention has been completed.

That is, the present invention includes C: 0.0014 mass% or less, Si: 0.01 to 0.1 mass%, Mn: 12.50 × S to 0.8 mass%, S: 0.034 mass% or less, Al: 0.00. 01 to 0.10 mass%, N: 0.0015 to 0.0070 mass%, Nb: 2.3 × C to 15.48 × Cmass%, B: (0.769 × N−0.00304) mass% or more and A refining degree containing 0.0003 mass% or more and 0.60 × Nmass% or less, the balance being Fe and inevitable impurities, and the Nb content in Nb carbonitride precipitated in steel being 0.0080 mass% or less Is a steel plate for soft cans having T1 to T4.

Further, the steel sheet for soft cans of the present invention has an average r value (r _ave ) of 1.3 to 1.8, and r values (r ₀ , 90 °, 45 ° with respect to the rolling direction). r ₉₀ , r ₄₅ ) satisfy one or more of the following relational expressions: r ₄₅ −r ₀ > 0.2, r ₄₅ −r ₉₀ > 0.2 and | r ₀ −r ₉₀ |> 0.3 It is characterized by.

In the steel sheet for soft cans of the present invention, the ratio (L _s-ave / L _c-ave ) of the average length in the rolling direction of the ferrite crystal grains in the surface layer portion and the plate thickness center portion of the steel plate is less than 0.90. The ratio (L _s-max / L _c-max ) of the maximum length in the rolling direction of the ferrite crystal grains in the surface layer portion and the thickness center portion of the steel plate is less than 0.80.

  Moreover, the steel sheet for soft cans of the present invention is characterized in that the area ratio of non-recrystallized grains in the cross section in the rolling direction of the steel sheet is 0.5 to 5%.

In the present invention, C: 0.0014 mass% or less, Si: 0.01 to 0.1 mass%, Mn: 12.50 × S to 0.8 mass%, S: 0.034 mass% or less, Al: 0.0. 01 to 0.10 mass%, N: 0.0015 to 0.0070 mass%, Nb: 2.3 × C to 15.48 × Cmass%, B: (0.769 × N−0.00304) mass% or more and A steel slab containing 0.0003 mass% or more and 0.60 × Nmass% or less, the balance being Fe and inevitable impurities, hot-rolled, and then cold-rolled with a reduction ratio of 70 to 90%, The present invention proposes a method for producing a steel sheet for soft cans, characterized in that a steel sheet for cans having a refining degree of T1 to T4 is obtained by performing continuous annealing satisfying the following conditions.
Storage temperature T: 700-780 ° C
Soaking time t: 20 to 90 seconds 770 ≦ t / 3 + T−14.8 × log (Nb) −32 × B / N ≦ 840

  According to the present invention, by optimizing the composition of the steel, the cracking sensitivity of the continuously cast slab can be reduced and the occurrence of surface cracking of the slab can be prevented, thereby greatly reducing the load required for surface maintenance of the slab. be able to. Further, according to the present invention, a steel plate for a soft can having a workability, aging resistance and weldability equal to or higher than those of a steel plate for cans manufactured by a box annealing method can be manufactured by a continuous annealing method. Therefore, it greatly contributes to the improvement of productivity.

Inventors investigated the hot ductility of a slab in order to investigate the cause of surface cracking of a continuously cast slab.
Steel with various contents of B, Al, Nb and N was melted and made into an ingot, and from this ingot, a round bar test piece having a diameter of 8 mmφ × 15 mm in parallel was collected. It used for the high temperature tensile test. In the high-temperature tensile test, using a high-frequency induction hot-working reproducibility tester, rapidly heated to 1420 ° C., held for 60 seconds, cooled, held at 950 ° C. for 60 seconds, and then strain rate 2 × A tensile test was performed at 10 ⁻³ , and the drawing ratio (%) of the fracture surface of the test piece was measured. The reason for focusing on 950 ° C. is that the temperature of the lower straightening zone of the continuous casting machine where slab cracking is about 950 ° C. As a result, slab cracking occurs when nitride (BN, AlN) and carbonitride Nb (C, N) precipitate in large amounts at the austenite grain boundaries when the solidified structure transforms from γ to α. It was found to occur by embrittlement.

  Next, the inventors determine the amount of precipitates at the fracture portion of the test piece after the high-temperature tensile test by wet analysis using a bromine methanol method, and based on this result, to prevent a decrease in hot ductility. For this purpose, the range of precipitation amounts of nitrides (BN, AlN) and carbonitrides (Nb (C, N)) should be controlled. The results will be described below.

<N amount precipitated as BN>
According to the inventors' investigation, when the mass ratio of B and N is B / N <0.77, that is, when the atomic ratio of B to N is as small as less than 1, B is precipitated as a total amount of BN. Therefore, in this case, the amount of N deposited as BN in the slab (hereinafter also referred to as “N (BN)”) is:
N (BN) = 1.30 × B (1)
Can be expressed as Here, the coefficient 1.30 is (N atomic weight) / (B atomic weight).

<N amount precipitated as AlN>
The proportion of Al precipitated as AlN in the slab is unexpectedly low, and the amount of precipitation is about 2 mass% of the total Al amount. According to the results of the inventors' investigation, the amount of N precipitated as AlN (hereinafter referred to as AlN) , Also referred to as “N (AlN)”)
N (AlN) = 0.00938 × Al−0.000688 (2)
Can be expressed as

<Nb amount precipitated as Nb (C, N)>
Nitride and carbonitride precipitated in the slab containing B, Al, and Nb are precipitated in the order of BN, AlN, and Nb (C, N) as the temperature decreases, that is, Nb carbonitride. Precipitates at a lower temperature than nitrides of B and Al. Therefore, the precipitation amount of Nb (C, N) may be calculated with respect to the surplus N amount that is not used as BN or AlN in the total N amount. Here, N (excess) which is the surplus N amount is assumed that the total N amount is N (total).
N (excess) = N (total) −N (BN) −N (AlN)
= N (total)-1.30 x B-0.00938 x Al + 0.000688
... (3)
Therefore, the amount of Nb precipitated as Nb carbonitride (hereinafter also referred to as “Nb (Nb (C, N))”) is
Nb (Nb (C, N)) = 6.63 × N (excess) × 0.29 (4)
Can be expressed as Here, the coefficient 6.63 is (Nb atomic weight) / (N atomic weight), and the coefficient 0.29 is the ratio of the precipitated Nb amount to the total Nb amount. The amount of precipitated Nb was 10 mass% acetylacetone-1 mass% tetramethylammonium chloride-methano- after cooling the sample imparted with strain at a temperature of 950 ° C. at a strain rate of 2.0 × 10 ⁻³ / s to room temperature. The solution was obtained by conducting constant current electrolysis using an electrolyte solution and analyzing the residue.

Subsequently, the inventors investigated the relationship between the amount of Nb precipitated as the Nb carbonitride described above and slab cracking. As a result, slab cracking occurs when the amount of Nb (Nb (Nb (C, N))) precipitated as Nb carbonitride (Nb (C, N)) in the slab exceeds 0.0080 mass%. That is, when Nb (Nb (C, N)) satisfy | fills following (5) Formula, it discovered that a slab crack did not occur.
Nb (Nb (C, N)) ≦ 0.0080 mass% (5)
Therefore, from the above equations (3) to (5),
Nb (Nb (C, N)) = 6.63 × (N (total) −1.30 × B−0.00938 × Al + 0.000688) × 0.29 ≦ 0.0080 mass%
... (6)
Is obtained.
Here, since the precipitation amount of AlN does not change greatly in the range of Al: 0.01 to 0.10 mass%, assuming that Al: 0.05 mass%, the addition amount of B for preventing slab cracking Is
B ≧ 0.769 × N−0.00304 (7)
Can guide you. That is, in order not to cause slab cracking, it is necessary to add B in an amount satisfying the above expression (7).

<MnS precipitation without slab cracking>
As a result of further investigation on precipitates that cause slab cracking, the inventors have found that MnS is also greatly involved in slab cracking. That is, MnS precipitates at a higher temperature than nitrides and carbonitrides, and also serves as a precipitation nucleus for BN, AlN, and Nb (C, N). Therefore, it was found that by controlling the precipitation of MnS, specifically, by suppressing the precipitation amount of MnS, the hot ductility of the slab can be improved and slab cracking can be avoided. And for that purpose, it discovered that Mn needs to satisfy | fill following (8) Formula with respect to S content.
Mn ≧ 12.50 × S (8)
The reason is that by increasing the ratio of Mn / S, the amount of MnS precipitation can be suppressed, so that slab cracking is avoided. From the viewpoint of more reliably preventing cracks caused by MnS, it is preferable that Mn ≧ 23.40 × Smass%.

Next, the reason for prescribing the component composition of the steel sheet for soft cans of the present invention within the above range will be described.
C: 0.0014 mass% or less C is an element that greatly affects aging. In particular, when it exists as solid solution C in steel, aging is promoted in the coating baking process at the can-making machine, and defects such as stretcher strain and fritting occur in the subsequent can-making process. In the present invention, Nb is added to form Nb (C, N) to reduce the solid solution C. However, when the amount of C exceeds 0.0014 mass%, the amount of Nb added increases. Not only does the raw material cost increase, but the steel sheet is excessively hardened by the strengthening action of the precipitated Nb (C, N). Therefore, the C content is 0.0014 mass% or less.

Si: 0.01-0.1 mass%
As described above, when the C content is 0.0014 mass% or less, an increase in steel plate strength due to C cannot be expected. Si is an element that is added to secure the strength of the steel sheet in place of C, and is an essential element in particular for obtaining a strength of T3 to T4 class. The increase in steel sheet strength can be obtained by adding 0.01 mass% or more. Si may exist as solute Si or may exist as fine Si oxide. In particular, since the oxide of Si precipitates at a higher temperature than BN or AlN, it is already uniformly distributed within the grains and does not precipitate at the austenite grain boundaries during transformation from γ grains to α grains. Therefore, there is an effect of increasing the steel sheet strength without adversely affecting the slab cracking. However, if the Si content exceeds 0.1 mass%, the steel sheet is excessively hardened, and not only can processability is deteriorated, but also the corrosion resistance of the steel sheet is lowered. Therefore, the Si content is in the range of 0.01 to 0.1 mass%. Preferably, it is the range of 0.03-0.1 mass%.

Mn: 12.50 × S to 0.8 mass%
Mn is an element that increases the strength of the steel sheet. However, if the Mn content is less than 12.50 × Smass%, the precipitation of MnS is promoted, the hot ductility of the slab is lowered, and slab cracking occurs, so Mn is 12.50 × Smass% or more. It is necessary to add. On the other hand, if the Mn content exceeds 0.8 mass%, the steel sheet is excessively cured. Therefore, the Mn content is in the range of 12.50 × S to 0.8 mass%. In addition, from the viewpoint of reliably preventing slab cracking, a range of 23.40 × S to 0.8 mass% is preferable.

S: 0.034 mass% or less S is a harmful element that combines with Mn to form MnS and lowers the hot ductility of the slab. In particular, when the S content is greater than 0.034 mass%, a large amount of MnS precipitates, and BN, AlN, Nb (C, N), which are nitrides and carbonitrides, are precipitated in large amounts from this. , Greatly reduce the hot ductility. Therefore, the S content is 0.034 mass% or less. In order to improve hot ductility, the S content is preferably 0.020 mass% or less. More preferably, it is 0.010 mass% or less.

Al: 0.01-0.10 mass%
Al is an element added as a deoxidizer. Further, by forming N and AlN, there is an effect of reducing solid solution N in the steel. However, if the Al content is less than 0.01 mass%, a sufficient deoxidizing effect and a solid solution N reducing effect cannot be obtained. On the other hand, if it exceeds 0.10 mass%, not only is the above effect saturated, but also inclusions such as alumina increase, such being undesirable. Therefore, the Al content is in the range of 0.01 to 0.10 mass%.

N: 0.0015 to 0.0070 mass%
N is preferably as small as possible because it combines with Al, B, Nb and the like to form nitrides and carbonitrides and harms the hot ductility of the slab. However, it is difficult to stably make N less than 0.0015 mass%, and the refining cost also increases. Further, as will be described later, in the present invention, the ratio of B and N is important, but if the amount of N is small, it becomes difficult to control the B amount in order to keep the ratio of B and N within a certain range. . On the other hand, if the content of N exceeds 0.0070 mass%, the amount of nitride deposited increases and slab cracking increases, making it difficult to produce a slab by the continuous casting method. Moreover, as a result of increasing the addition amount of B necessary for ensuring weldability, the amount of BN precipitated in the crystal grains increases, and the steel sheet may be excessively hardened. Therefore, the N content is in the range of 0.0015 to 0.0070 mass%.

Nb: 2.3 × C to 15.48 × Cmass%
Nb forms NbC or Nb (C, N), has a function of reducing solid solution C in the steel, and is an element necessary for ensuring aging resistance. In order to exhibit the effect, addition of 2.3 × Cmass% or more is necessary. In order to further enhance the effect and achieve complete non-aging, addition of 7.0 × Cmass% or more is preferable. On the other hand, if the amount of Nb added is too large, not only does the effect of reducing the solid solution C saturate, but also the recrystallization temperature increases and the raw material cost increases. Therefore, the amount of Nb added needs to be 15.48 × Cmass% or less.

Nb amount precipitated as Nb carbonitride As described above, Nb carbonitride (Nb (C, N)) causes slab cracking when precipitated in a large amount in the slab during continuous casting. The amount is preferably as small as possible. The amount of Nb carbonitride for preventing slab cracking varies depending on the surplus N amount and the precipitation rate, but it is preferable to keep the amount of Nb precipitated as Nb (C, N) to 0.0080 mass% or less.

B: (0.769 × N−0.00304) mass% or more or 0.0003 to 0.60 × N mass%
B is a very important element because it greatly affects various properties of the steel plate for cans of the present invention as shown below.
The first is the effect on the surface cracking of the continuously cast slab. Continuously cast slabs are usually subjected to back bending deformation in the lower straightening zone of the continuous casting machine. Therefore, if the amount of Nb (C, N) deposited is large, slab cracking is likely to occur. In order to prevent this cracking, it is effective to suppress the precipitation amount of Nb (C, N) by adding B and precipitating N as BN. In order to obtain the effect, it is necessary to add B (0.769 × N−0.00304) mass% or more as described above.
Second, there is an effect on weldability. When a steel plate for cans is welded, the HAZ part may cause abnormal grain growth and soften, and the HAZ part may be cracked in subsequent processing. B segregates at the grain boundaries during welding and has the effect of suppressing abnormal grain growth and improving weldability. In order to obtain the effect, addition of 0.0003 mass% or more is necessary.
Third is the effect on the can height reduction. Since the Nb-added ultra-low carbon steel sheet exhibits an extremely high rank ford value (r value), the amount of reduction in can height tends to increase. However, by adding B thereto, the r value can be lowered, and in particular, the r value in the rolling direction and the width direction of the steel sheet can be lowered appropriately, so that the reduction in can height can be reduced. This effect can be obtained by adding 0.0003 mass% or more.
Fourth, there is an effect on the recrystallization temperature. B has the effect of increasing the recrystallization temperature. If it exceeds 0.60 × Nmass%, the recrystallization temperature increases too much, and the non-recrystallized portion is made 1% or less under the continuous annealing conditions described later. Can not be. Therefore, the addition amount of B is set to a range of 0.0003 to 0.60 × Nmass%. Preferably, the addition amount of B is in the range of 0.0003 to 0.46 × Nmass%.
Fifth, it is an influence on the ferrite crystal grains of the steel sheet. According to the inventors' investigation, it was found that when an appropriate amount of B is added, the ferrite crystal grains in the center portion of the plate thickness are larger than the ferrite crystal grains in the surface layer portion of the steel sheet. As described above, when the crystal grain at the center of the plate thickness becomes larger than the surface layer portion, the yield stress can be lowered even with the same tempering degree, so that the workability can be improved. This phenomenon is observed only when the amount of B added is in the range of 0.0003 to 0.60 × N (mass%), and it is not recognized when the amount is less than or greater than that. Therefore, in order to obtain good can manufacturing processability, B is added in a range of 0.0003 to 0.60 × N (mass%). Preferably, it is the range of 0.0003-0.46 * N (mass%). The cause of such a phenomenon has not yet been fully clarified, but the effect of suppressing the grain growth of B is different between the surface layer part and the center part of the plate thickness, and there is a difference in crystal grain size. Presumed.
As described above, in order to effectively exhibit the first to fifth effects, the amount of B added is (0.769N−0.00305) mass% or more, 0.0003 mass% or more, 0.60. XNmass% or less is required, and preferably (0.769N−0.00305) mass% or more, 0.0003 mass% or more, and 0.46 × Nmass% or less.

  In the steel plate for cans of the present invention, the balance other than the above components is Fe and inevitable impurities. However, components such as P, Cu, Cr, Mo, V, and Ni may be included beyond the range of unavoidable impurities as long as the effects of the present invention are not impaired.

Next, the rank ford value (r value) that the steel sheet for soft cans of the present invention should have will be described.
Average r value (r _ave ): 1.3 to 1.8
In order to manufacture a two-piece can by deep drawing a steel sheet, it is advantageous that the rank ford value (r value) is higher. If the average r value (r _ave ) is less than 1.3, processing troubles such as fracture may be caused during deep drawing. On the other hand, the steel plate for cans may be used not only for 2-piece cans but also for 3-piece cans. The can body of a three-piece can is formed into a cylindrical shape by seam welding or the like, and is often subjected to can body processing such as expansion processing or bead processing. Elongation strain is imparted. At this time, if the average r value is small, the plate thickness is likely to decrease, so the reduction amount of the can height is small. However, if the average r value is large, the plate thickness does not decrease, the can height decreases, and a step occurs between the weld and the can height that hardly changes. According to the investigation results of the inventors, when the average r value (r _ave ) exceeds 1.8, the reduction in can height becomes significant. Therefore, the average r value (r _ave ) needs to be in the range of 1.3 to 1.8.

Any one or more of r ₄₅ -r ₀ > 0.2, r ₄₅ -r ₉₀ > 0.2 and | r ₀ -r ₉₀ |> 0.3 Further, the can body of the 3 piece can is: In order to suppress the reduction in can height and reduce the level difference, it is preferable that the rank forward value (r value) in the circumferential direction of the can body is small. In general, steel plates for cans used in the cans of three-piece cans are chamfered so that the circumferential direction is the rolling direction or the width direction of the steel plates. Is desirable to be small. For that purpose, r ₄₅ -r ₀ > 0.2, r ₄₅ -r when the rank forward values in the rolling direction, coil width direction, and 45 degree direction of the steel sheet are expressed as r ₀ , r ₉₀ , and r ₄₅ , respectively. It is preferable to satisfy any one or more of ₉₀ > 0.2 and | r ₀ -r ₉₀ |> 0.3.

Next, the ferrite crystal grains of the steel sheet for soft cans of the present invention will be described.
(L _s-ave / L _c-ave ): less than 0.90 and (L _s-max / L _c-max ): less than 0.80 Generally, a steel sheet for soft cans obtained by batch annealing is annealed for a long time. As a result, the crystal grains grow sufficiently and almost no solid solution C exists, so that a steel sheet having a small yield ratio (YR), which is the ratio of the yield stress to the tensile strength, can be obtained. On the other hand, the steel sheet for cans obtained by the conventional continuous annealing method has a very short annealing time, and therefore YR tends to increase. The refining degree of steel plate for cans is classified by Rockwell hardness (HR30T), and this Rockwell hardness has a good correlation with the average value of tensile strength and yield stress. Continuous annealing materials) are disadvantageous in terms of canning workability corresponding to yield stress because yield stress is higher even with the same tempering degree compared to box-annealed steel plate (box annealing material). there were.

  In order to solve the above problems, the inventors studied to improve workability without changing the tempering degree of the continuously annealed material. As a result, it has been found that it is effective to make the size of the ferrite crystal grains in the central portion of the steel plate thicker than that in the surface layer portion. This is because the Rockwell hardness of the steel sheet is affected by the size of crystal grains on the surface of the steel sheet because it is determined from the difference in penetration depth between the standard load when the diamond indenter is pushed into the steel sheet surface and the test load. . On the other hand, the yield stress of a steel sheet related to can manufacturing processability is influenced by the size of crystal grains in the entire steel sheet. Therefore, in order to reduce only the yield stress without changing the tempering degree of the continuous annealing material, that is, in order to obtain the same level of canning workability as the box annealing material plate, It is effective to enlarge the ferrite crystal at the center of the plate thickness while maintaining the size of the crystal grains.

Specifically, the average lengths in the rolling direction of the ferrite crystal grains in the surface layer portion and the plate thickness center portion of the continuously annealed steel plate are L _s-ave , L _c-ave , and the ferrite in the surface layer portion and plate thickness center portion of the steel plate, respectively. When the maximum length of the crystal grains in the rolling direction is L _s-max and L _c-max , the ratio of the average length in the L direction of the ferrite crystal grains in the surface layer portion and the center portion of the plate thickness (L _s-ave). / L _c-ave ) is less than 0.90, and the ratio (L _s-max / L _c-max ) of the maximum length in the L direction of the ferrite crystal grains in the surface layer portion and the plate thickness center portion of the steel plate is 0.80. It was found that when it is less than the range, can processability equivalent to that of a batch annealed material having the same refining degree can be obtained. More preferably, (L _s-ave / L _c-ave ) is less than 0.80, and (L _s-max / L _c-max ) is less than 0.70. In addition, the said surface layer part means the part from the surface layer of a steel plate to 1/4 depth of full thickness, and plate | board thickness center part means the part of plate | board thickness 1/2 part +/- 10%.

Next, the residual ratio of non-recrystallized grains in the steel sheet for soft cans of the present invention will be described.
The inventors examined the influence of annealing conditions on the mechanical properties of the steel sheet for cans of the present invention. As a result, in the steel sheet of the present invention, even if unrecrystallized grains remain, if the amount thereof is small, the steel sheet strength is increased, but it is clear that there is no significant effect on can manufacturing processability. It was. This means that non-recrystallized grains can be used effectively in order to increase the steel sheet strength (increase the tempering degree) without reducing the workability. In order to express the above steel plate strength increasing effect, it is preferable that non-recrystallized grains exist in an area ratio of 0.5% or more in the cross section in the rolling direction. However, when the area ratio of non-recrystallized grains exceeds 5%, the steel sheet strength is excessively increased, and adverse effects such as deterioration of can-making processability occur. Therefore, in the case where unrecrystallized grains are left, it is necessary to set the area ratio in the rolling direction cross section in the range of 0.5 to 5%.

Next, the manufacturing method of the steel plate for soft cans which concerns on this invention is demonstrated.
The steel making process may be any method as long as the steel having the above-mentioned composition defined by the present invention can be melted. However, the slab is manufactured by a continuous casting method with little component segregation. However, the conditions for continuous casting need not be particularly limited because the composition of the steel is optimized and the slab cracking sensitivity is reduced.

  The reheating of the slab prior to hot rolling is not particularly limited, but if the heating temperature is too high, defects may occur on the surface of the product or the energy cost will increase. It becomes difficult to secure the finishing temperature. Therefore, the reheating temperature is preferably in the range of 1050 to 1300 ° C.

  The hot rolling step does not particularly define conditions, but the finish rolling finish temperature is 860 to 950 ° C. from the viewpoint of the uniformity of crystal grains and precipitate distribution of the hot rolled steel sheet, surface properties, mechanical properties, and production costs. The winding temperature is preferably in the range of 550 to 720 ° C. Subsequent pickling is not particularly limited as long as the scale on the surface is removed.

  In cold rolling, in order to obtain an appropriate ferrite grain size (crystal grain size in the rolling direction) and an appropriate rank ford value (r value) specified by the present invention, the rolling reduction is set to a range of 70 to 90%. There is a need to. If the rolling reduction is less than 70%, it is necessary to reduce the thickness of the hot-rolled sheet in order to obtain a cold-rolled sheet having the same thickness. Therefore, it is possible to ensure the target finish rolling end temperature by hot rolling. On the other hand, if it exceeds 90%, the load in cold rolling increases and rolling becomes difficult. Preferably, it is 80 to 90% of range.

The continuous annealing is a step of recrystallizing a cold-rolled steel plate (cold rolled plate) to give desired strength and workability, and is a particularly important step in the present invention. The recrystallization behavior of a cold-rolled sheet varies depending on the steel plate components, particularly the contents of Nb, B, and N, in addition to the soaking temperature and soaking time. The inventors melted steels having various component compositions and investigated the effects of the component composition and annealing conditions on the recrystallization behavior. As a result, the recrystallization behavior of the cold-rolled sheet is expressed by the following formula according to the content of Nb, B, N, the soaking temperature and the soaking time
A = t / 3 + T-14.8 × log (Nb) −32 × B / N (8)
Here, Nb: Nb content (mass%), B: B content (mass%), N: N content (mass%), T: soaking temperature (° C.), t: soaking time (seconds)
It was found that there is a good correlation with parameter A defined by

  FIG. 1 shows the relationship between the parameter A and the area ratio (%) of unrecrystallized grains in the cross section in the rolling direction of the steel sheet after continuous annealing. When the value of A is less than 770, The area ratio of non-recrystallized grains in the cross section in the rolling direction exceeds 5%, the steel sheet strength is increased, and can manufacturing processability is deteriorated.

Further, it has been found that the parameter A affects the rank fore value (r value) and the crystal grain growth in the steel sheet surface layer part and the plate thickness center part through the recrystallization behavior. FIG. 2 shows the relationship between the parameter A and the average r value (r _ave ). When the value of A exceeds 840 and becomes too large, grain growth after completion of recrystallization is promoted. As a result, it can be seen that the average r value exceeds 1.8. 3 and 4 show the parameter A, the ratio (L _s-ave / L _c-ave ) of the average length in the rolling direction of the ferrite crystal grains in the surface layer portion and the plate thickness center portion of the steel plate, and The relationship between the ratio of the maximum length in the rolling direction of the ferrite crystal grains (L _s-max / L _c-max ) in the surface layer portion and the center portion of the plate thickness is shown, but when the value of A exceeds 840, As a result of the grain growth of the steel sheet surface layer portion being promoted to the same level as the center portion of the plate thickness, L _s-ave / L _c-ave <0.9 and L _s-max / L _c-max <0.8 are not satisfied. .
From the above results, from the viewpoint of controlling the area ratio of the non-recrystallized grains, the rank ford value, and the crystal grains of the steel sheet, the value of the parameter A is controlled in the range of 770 to 840 and continuous annealing is performed. It turns out that it is necessary. Preferably, it is the range of 780-830.

  In addition, the soaking annealing conditions in the continuous annealing include the soaking temperature: 700 to 780 ° C. and the soaking time: 20 to 90 seconds, in addition to controlling the value of the parameter A in the range of 770 to 840. It is necessary to satisfy. If the soaking temperature is less than 700 ° C., the target steel sheet structure may not be obtained even if the above parameter A condition is satisfied. On the other hand, if it exceeds 780 ° C., an ultrathin material such as a steel plate for cans. Then, operational troubles such as in-furnace breakage and shape defects are likely to occur. Also, if the soaking time is less than 20 seconds, the target steel sheet structure may not be obtained even if the condition of the above parameter A is satisfied. This is because the speed is lowered and the productivity is hindered.

  Moreover, in order to reduce the solid solution C and to further improve the aging resistance, an overaging treatment may be performed after the soaking. The conditions for the overaging treatment are not particularly specified, but in order to sufficiently reduce the solid solution C, it is desirable to hold at a temperature of 350 to 450 ° C. for 30 to 90 seconds.

  After continuous annealing, temper rolling with a rolling reduction of 0.5 to 5% is preferably performed for the purpose of shape correction, adjustment of surface roughness, and improvement of mechanical properties. If the rolling reduction is too low, it is difficult to correct the shape and adjust the surface roughness. On the other hand, if the rolling reduction exceeds 5%, the steel sheet is work-hardened, and the anisotropy of mechanical properties is increased, so that the can-making processability is impaired.

  When corrosion resistance is required, the steel plate for cans of the present invention is tinplate or tin-free steel by subjecting the steel plate produced as described above to tin plating or electrolytic chromic acid treatment. If necessary, a polyester film may be further laminated to form a film.

Steels of steel symbols B to AA having various composition shown in Table 1 are melted in a converter and continuously cast using a vertical bending type continuous casting machine (vertical length: 3.5 m, bending radius: 10 m). Thus, a slab having a thickness of 230 mm and a width of 1000 mm was manufactured. These slabs were evaluated as follows.
<Evaluation of slab surface crack resistance>
The presence or absence of cracks on the surface of the slab was visually observed, and cracks with no cracks were observed with good slab surface crack resistance (◎), and cracks of 100 mm or less were confirmed at the corners of the slab. Slab surface crack resistance is slightly good (○), which can be dealt with by surface welding, etc., and slab resistance that has been forced to slab cut due to cracks occurring over 100 mm or more on the long side of the slab The surface cracking property was evaluated as inferior (x).
<Evaluation of hot ductility>
Surface cracks in the slab mainly occur near the temperature at which the steel transforms from γ to α (about 850 ° C. to 1000 ° C.). Therefore, from the 300 mm position of the slab, in the width direction, a round bar test piece having a diameter of 8 mmφ × 15 mm in length was collected, and the temperature history and tensile stress at the time of continuous casting were simulated. A hot tensile test was performed to evaluate the hot ductility of each slab. In the high-temperature tensile test, using a high-frequency induction type hot working reproduction tester, after heating to a temperature of 1420 ° C. at a heating rate of 10 ° C./s in a vacuum, the temperature is soaked for 60 seconds, and then the test temperature is 950. After quenching to 5 ° C. at a rate of 5 ° C./s and holding for 60 seconds, a tensile test was performed at a strain rate of 2 × 10 ⁻³ . For hot ductility, the drawing rate (cross-sectional reduction rate) is obtained from the fracture surface of the test piece after the tensile test. When the drawing rate is 35% or more, high-temperature ductility is good (良), and the drawing rate is 10% or more and less than 35%. Were evaluated as having high hot ductility (good) and those having a drawing value of less than 10% as poor hot ductility (x). In addition, it means that it is excellent in hot ductility, and a slab surface crack does not occur easily so that a drawing rate is large. The steel sheet for soft cans of the present invention is characterized in that the Nb content in Nb carbonitride precipitated in the steel is 0.0080 mass% or less.

  The evaluation results of the slab are shown together in Table 1. From Table 1, it can be seen that steel satisfying the conditions of the component composition and Mn / S and precipitation Nb defined by the present invention has good hot ductility at 950 ° C. and no or small slab surface cracks.

  The slab is then reheated to a temperature of 1250 ° C., and then hot-rolled to a finish rolling finish temperature of 890 ° C. and a coiling temperature of 620 ° C. to obtain a hot-rolled sheet having a sheet thickness of 2.3 mm. Then, pickling with hydrochloric acid, and further cold-rolling under the conditions shown in Table 2 to obtain a cold-rolled sheet having a thickness of 0.35 to 0.81 mm, continuous annealing, temper rolling, and finally Then, an electrolytic chromic acid treatment was performed to obtain a steel plate for cans (tin-free steel). Thereafter, an aging heat treatment was performed at 210 ° C. for 10 minutes in consideration of the fact that the test piece was collected from the steel plate for cans, subjected to paint baking at a can-maker and then processed into a can.

The test piece of the steel plate for cans obtained as described above was subjected to the following test.
<Hardness measurement>
Based on the Rockwell hardness test method of JIS Z2245, the Rockwell 30T hardness (HR30T) in the position prescribed | regulated to JISG3315 was measured, and the refining degree was determined.
<Measurement of rank forward value (r value)>
From the test piece, JIS No. 5 tensile test pieces were taken in the 0, 45, and 90 degree directions with respect to the rolling direction, and the r value in each direction when 15% tensile deformation was performed was measured. Moreover, the average r value (r _ave ) was determined from the r value in each direction using the following formula.
r _ave = (r ₀ + r ₉₀ + 2 × r ₄₅ ) / 4
<Measurement of non-recrystallization ratio>
About the above test piece, the ferrite structure of the rolling direction cross section is etched to appear, and a 200 × photograph taken using an optical microscope is subjected to image processing to distinguish an unrecrystallized portion and a recrystallized completion portion, and recrystallization The area ratio of the crystal grains that had not been calculated was calculated.
<Measurement of rolling length of ferrite crystal grains>
About a part of the test piece which took out the said ferrite structure, the rolling direction length (particle diameter) of the ferrite crystal grain in the surface layer part and plate | board thickness center part of the steel plate was further measured. The measurement is performed at the center of the plate thickness at 1/2 the plate thickness and the surface layer at the position of 15 μm from the surface, and the number of ferrite grain boundaries crossing the 300 μm long line drawn in the rolling direction is measured. 300 μm was divided by the number of grain boundaries, and the average rolling direction length (L _s-ave , L _c-ave ) of the ferrite crystal grains at each position was determined, and L _s-ave / L _c-ave was calculated. Further, the interval between the longest grain boundaries recognized within the range of 300 μm is defined as the maximum length in the rolling direction of the ferrite crystal grains (L _s-max , L _c-max ), and from this (L _s-max / L _{c -Max} ) was calculated.
<Measurement of springback>
Moreover, the spring back was further measured about the steel plate which measured the rolling direction length of the said ferrite crystal grain. The springback is measured by winding a test piece around a mandrel with a diameter of 1 inch (25.4 mm), applying a 180 ° bend, and measuring the bending angle of the steel sheet after removing the pressure. When the spring back angle of batch annealed material with the same thickness and tempering degree is 1.00, less than 1.03 times that is good spring back (◎), 1.03 times or more and less than 1.05 times The thing was evaluated as a springback slightly good (◯) and a thing of 1.05 times or more as a springback inferior (x).

Furthermore, in order to evaluate the can manufacturing processability of the steel plate for cans, the steel plate for cans was formed into a can body of a 3 piece can. In the can body forming of 3 piece cans, the steel plate for cans is blanked into a rectangle of length direction 400 × width direction 850 mm, and the width direction is the circumferential direction and the winding width (wrapping amount at both ends). Roll forming and forming a cylindrical shape under conditions such that the thickness is 0 to 3 mm, and then seam welding is performed with an upper limit welding current that does not generate dust and a lap amount of 0.8 mm, and a diameter of about 270 mm. The can body was obtained. Subsequently, the can body was subjected to an extra band processing with a maximum diameter increase rate of about 6%, a bead processing with a bead height of 6 to 8 mm, and finally a flange width of 6 mm. Flanged processing was applied.
The following evaluation was performed on the can body of the 3-piece can thus obtained.
<Evaluation of aging resistance>
The aging resistance was determined by visually observing the state of occurrence of fluting when the above roll forming was performed, and when no fluting was observed at all. In the case where tinging was observed, there was no problem in practical use. The aging resistance was evaluated as slightly good (◯), and the case where fluting was severe was evaluated as inferior aging resistance (×).
<Evaluation of weldability>
The weldability is determined by observing the polished surface of the sample taken from the flanged welded portion under a microscope, and determining the occurrence rate of cracks in the HAZ portion. When the crack occurrence rate was over 0.5% and 1% or less, the weldability was evaluated as slightly good (◯), and when the crack generation rate exceeded 1%, the weldability was evaluated as poor (x).
<Evaluation of can height change>
The change in can height is determined by the difference in can height between the welded and non-welded parts after banding and bead processing. (A) More than 1 mm and 1.5 mm or less were evaluated as slightly changeable in can height (◯), and those exceeding 1.5 mm were evaluated as inferior in can height change (×).

Furthermore, the steel plate for cans was formed into a two-piece can, and can manufacturing processability was evaluated. For forming a two-piece can, a circular blank having a diameter of 100 mmφ is punched from a test piece of a steel plate for a can, and after drawing with a drawing ratio of about 0.6, redrawing with a drawing ratio of about 0.75 is performed. A two-piece can body having a diameter of 45 mmφ was obtained.
The following evaluation was performed on the two-piece can thus obtained.
<Evaluation of aging resistance>
With regard to aging resistance, the occurrence of stretcher strain in the region from the bottom of the can body to the bottom of the can was visually or microscopically observed, and no stretcher strain was observed at all. Although aging was observed, there was no problem in practical use, and the aging resistance was evaluated as slightly good (◯), and the generation of stretcher strain was evaluated as poor aging resistance (×).
<Evaluation of deep drawability>
Deep drawability is evaluated by the rate of occurrence of cans that have been ruptured by drawing and redrawing, and those with a rupture rate of 0.3% or less have good deep drawability (◎), and the rupture rate is 0. More than 3% and less than 0.5% were evaluated as slightly deep drawability (◯), and those with a fracture occurrence rate exceeding 0.5% were evaluated as poor deep drawability (×).

  The results of the evaluation are shown in Table 2 and Table 3. From these, the steel plate for cans manufactured using the continuous cast slab satisfying the composition of the present invention as a raw material under the conditions satisfying the present invention (cold rolling reduction ratio, continuous annealing conditions) can be a two-piece can or three-piece can. -It turns out that it has the characteristic suitable for a can.

It is a graph which shows the relationship between the parameter A (= t / 3 + T-14.8 * log (Nb) -32 * B / N) and the area ratio of a non-recrystallized grain. It is a graph which shows the relationship between the parameter A (= t / 3 + T-14.8 * log (Nb) -32 * B / N) and a rank ford value (r value). Parameter A (= t / 3 + T-14.8 × log (Nb) −32 × B / N) and the average rolling direction length ratio of the ferrite crystal grains of the steel sheet surface layer portion and the plate thickness center portion (L _s− It is a graph which shows the relationship with ( _ave / _{Lc -ave} ). Parameter A (= t / 3 + T-14.8 × log (Nb) −32 × B / N) and the maximum length ratio in the rolling direction of the ferrite crystal grains in the steel plate surface layer portion and the plate thickness center portion (L _s− It is a graph which shows the relationship with ( _max / _Lc-max ).

Claims (5)

C: 0.0014 mass% or less,
Si: 0.01-0.1 mass%,
Mn: 12.50 × S to 0.8 mass%,
S: 0.034 mass% or less,
Al: 0.01-0.10 mass%,
N: 0.0015 to 0.0070 mass%,
Nb: 2.3 × C to 15.48 × Cmass%,
B: (0.769 × N−0.00304) mass% or more and 0.0003 mass% or more and 0.60 × N (mass%) or less, with the balance consisting of Fe and inevitable impurities , precipitated in steel A steel plate for soft cans having a tempering degree of T1 to T4 in which the amount of Nb in the Nb carbonitride is 0.0080 mass% or less . The average r value (r _ave ) is 1.3 to 1.8, and the r values (r ₀ , r ₉₀ , r ₄₅ ) in the 0 degree, 90 degree, and 45 degree directions with respect to the rolling direction are r ₄₅ − 2. The soft material according to claim 1, wherein at least one of r ₀ > 0.2, r ₄₅ −r ₉₀ > 0.2 and | r ₀ −r ₉₀ |> 0.3 is satisfied. Steel plate for cans. The ratio (L _s-ave / L _c-ave ) of the average length in the rolling direction of the ferrite crystal grains in the surface layer portion of the steel plate and the center portion of the plate thickness is less than 0.90, and in the surface layer portion of the steel plate and the center portion of the plate thickness soft steel sheet for cans according to claim 1 or 2, wherein the ratio of the rolling direction maximum length of ferrite crystal grains _{(L s-max / L c} -max) is less than 0.80. The steel sheet for soft cans according to any one of claims 1 to 3 , wherein an area ratio of non-recrystallized grains in a cross section in the rolling direction of the steel sheet is 0.5 to 5%. C: 0.0014 mass% or less,
Si: 0.01-0.1 mass%,
Mn: 12.50 × S to 0.8 mass%,
S: 0.034 mass% or less,
Al: 0.01-0.10 mass%,
N: 0.0015 to 0.0070 mass%,
Nb: 2.3 × C to 15.48 × Cmass%,
B: A steel slab containing (0.769 × N−0.00304) mass% and 0.0003 mass% to 0.60 × N mass% with the balance being Fe and unavoidable impurities is hot-rolled. Then, after cold rolling with a rolling reduction of 70 to 90%, by performing continuous annealing satisfying the following conditions, a steel sheet for soft cans having a refining degree of T1 to T4 is obtained. Manufacturing method.
Storage temperature T: 700-780 ° C
Soaking time t: 20 to 90 seconds 770 ≦ t / 3 + T-14.8 × log (Nb) −32 × B / N ≦ 840

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