TWI460029B - High tensile strength and high formability steel sheet for can and its production method - Google Patents

Description

Steel plate for high strength and high processability and manufacturing method thereof

The present invention relates to a steel sheet for cans having high strength and high workability and a method for producing the same.

In the steel sheet used for the beverage can or the food can, there is a case where a steel sheet called a secondary cold rolling (DR) material is used for the can lid or the bottom of the can, the main body of the three-piece can, and the press can. The DR material which is cold-rolled again after annealing is easier to make the sheet thickness thinner than the one-time cold rolling (SR, Single Reduce) material which is only subjected to the temper rolling in which the rolling reduction is small, and by using a steel sheet. Can reduce costs.

The DR method for producing a DR material is work hardened by performing cold rolling after annealing, so that a thin and hard steel sheet can be produced. On the other hand, however, the DR material produced by the DR method lacks ductility and is therefore inferior in workability compared with the SR material.

After the main body material of the three-piece food can or beverage can is formed into a cylindrical shape, flange processing is performed at both ends in order to wind up the can lid or the can bottom. Therefore, a good elongation is required for the end of the can body.

On the other hand, the steel sheet as the material for the can is required to have a strength corresponding to the thickness of the sheet. In the case of the DR material, in order to ensure the economic effect of thinning, it is necessary to have a tensile strength of SR or more (about 520 MPa or more). ).

In the DR materials used in the past, it is difficult to balance the processability as described above. Both the strength and the strength, the SR material is mainly used for the main material of the food can or the beverage can. However, from the viewpoint of cost reduction, in order to make the thickness of the sheet thinner, it is also desired to use the DR material for the main material of the food can or the beverage can, and the application for expanding the DR material is increased.

According to such a case, Patent Document 1 discloses flange workability obtained by setting the amount of solid solution N in the low carbon steel to a fixed amount or more and defining a total elongation value and a Rankford value. Excellent DR material.

Patent Document 2 discloses a DR material which is excellent in flange workability obtained by specifying the amount of solid solution N and the amount of solid solution C in the low carbon steel.

[Previous Technical Literature] [Patent Literature]

Patent Document 1: Japanese Patent Laid-Open Publication No. 2007-177315

Patent Document 2: Japanese Patent Laid-Open Publication No. 2002-294399

However, the above conventional techniques all have problems.

Patent Document 1 discloses that when the total elongation value in the rolling direction is represented by X and the average Rankford value is represented by Y, X≧10% and Y≧0.9 or X<10% and Y are satisfied. DR steel plate with a relationship of -0.05X+1.4, but due to welding conditions, HAZ (Heat-Affected Zone softening) is generated, resulting in flange fracture.

In the manufacturing method described in Patent Document 2, it is necessary in the continuous annealing step. The aging treatment is required, so the manufacturing cost becomes too large.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a steel sheet for high-strength, high-workability cans which is suitable as a material for a can lid, a can bottom, and a three-piece can body, and a method for producing the same.

The present inventors conducted intensive studies to solve the above problems. As a result, the following findings were obtained.

In order to achieve both workability and strength, it is effective to add an appropriate amount of N to impart strength, and to limit the secondary cold rolling rate after annealing to an appropriate range to ensure workability.

Further, if the reheating temperature of the slab before hot rolling is low, re-dissolution of AlN precipitated after casting cannot be sufficiently performed, and if the coiling temperature after hot rolling is high, the precipitated AlN becomes excessive. In either case, the solid solution N of the strength is insufficient, so the reheating temperature or the coiling temperature of the billet must be limited to the appropriate temperature range.

Further, by limiting the annealing temperature and the annealing time to an appropriate range, a balance between strength and workability can be achieved.

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

[1] A steel sheet for high-strength and high-processability, which contains C: 0.001% or more and 0.080% or less, Si: 0.003% or more and 0.100% or less, and Mn: 0.10% or more and 0.80% by mass%. Hereinafter, P: 0.001% or more and 0.100% or less, S: 0.001% or more and 0.020% or less, and Al: 0.005% The above is 0.100% or less, N: 0.0050% or more and 0.0150% or less, B: 0.0002% or more and 0.0050% or less, and the remainder contains Fe and unavoidable impurities, and further, the area ratio is 0.01 in the cross section in the rolling direction. ~1.00% of crystal grains having a crystal grain stretch of 5.0 or more.

[2] A method for producing a steel sheet for high-strength and high-processability, characterized in that the steel is slab by continuous casting, and the slab reheating temperature is 1200 ° C or higher and hot rolling is performed at a temperature of less than 650 ° C. Winding, followed by a cold rolling, followed by continuous annealing at a soaking temperature of 680 to 760 ° C and a soaking time of 10 to 20 seconds, followed by secondary cold rolling at a rolling reduction of 20% or less; The mass% includes C: 0.001% or more and 0.080% or less, Si: 0.003% or more and 0.100% or less, Mn: 0.10% or more and 0.80% or less, P: 0.001% or more and 0.100% or less, and S: 0.001% or more. 0.020% or less, Al: 0.005% or more and 0.100% or less, N: 0.0050% or more and 0.0150% or less, and B: 0.0002% or more and 0.0050% or less, and the balance contains Fe and unavoidable impurities.

In addition, in this specification, the % which shows the component of steel is a mass %.

According to the present invention, a steel sheet for high strength and high workability cans having a tensile strength of 520 MPa or more and an elongation at break of 7% or more can be obtained.

As a result, the workability of the original plate (steel plate) is improved, so that no chipping occurs during the flange processing of the three-piece can, and the DR material having a small thickness can be used for canning, and the steel plate for the can is large. Thinning.

Hereinafter, the present invention will be described in detail.

The steel sheet for cans of the present invention is a steel sheet for high strength and high workability cans having a tensile strength of 520 MPa or more and an elongation at break of 7% or more. Moreover, the steel sheet can be reheated by hot rolling, the coiling temperature after hot rolling, the annealing temperature, the annealing time, and the secondary cold rolling by using a steel containing 0.0050% or more and 0.0150% or less of N. The rate is set to the appropriate conditions for manufacture.

The composition of the steel sheet for cans of the present invention will be described.

C: 0.001% or more and 0.080% or less

When the amount of C exceeds 0.080%, workability is deteriorated and cold rolling properties are also lowered. Further, sub-cracking is likely to occur during casting, and there is a possibility that the cost of billet processing increases. Therefore, the amount of C is set to 0.080% or less. On the other hand, when the amount of C is less than 0.001%, the coarsening of crystal grains becomes conspicuous, and the risk of occurrence of surface roughness defects in the processed portion increases. Therefore, the amount of C is set to be 0.001% or more and 0.080% or less.

Si: 0.003% or more and 0.100% or less

When the amount of Si exceeds 0.100%, problems such as a decrease in surface treatment property and deterioration in corrosion resistance occur, so the upper limit is made 0.100%. On the other hand, if it is less than 0.003%, the refining cost becomes too large, so the lower limit is made 0.003%.

Mn: 0.10% or more and 0.80% or less

Mn has an effect of preventing red hot brittleness in hot rolling caused by S and refining crystal grains, and is an essential element for securing a preferable material. in order to To achieve these effects, it is necessary to add at least 0.10% or more. On the other hand, when Mn is excessively added, the corrosion resistance is deteriorated and the steel sheet is excessively hardened, so the upper limit is made 0.80%.

P: 0.001% or more and 0.100% or less

P is a harmful element that hardens steel and deteriorates workability, and also deteriorates corrosion resistance. Therefore, the upper limit is set to 0.100%. On the other hand, if P is set to less than 0.001%, the cost of removing P becomes excessive. Therefore, the lower limit is set to 0.001%.

S: 0.001% or more and 0.020% or less

The S system is present as an intermediary in steel and is a harmful element that causes deterioration in workability and deterioration in corrosion resistance. Therefore, the upper limit is set to 0.020%. On the other hand, if S is set to less than 0.001%, the cost of the S is excessively increased. Therefore, the lower limit is set to 0.001%.

Al: 0.005% or more and 0.100% or less

Al is an essential element for deacidification at the time of steel making. When the amount added is small, the deacidification is insufficient, the intermediary is increased, and the workability is deteriorated. When the content is 0.005% or more, it can be considered that the deacidification is sufficiently performed. On the other hand, if the content exceeds 0.100%, the frequency of occurrence of surface defects caused by alumina clusters or the like increases. Therefore, the amount of Al is set to be 0.005% or more and 0.100% or less.

N: 0.0050% or more and 0.0150% or less

In the steel sheet for cans of the present invention, the secondary cold rolling ratio is suppressed to ensure elongation, and on the other hand, the amount of N is increased to contribute to high strength. If the amount of N is not full At 0.0050%, the tensile strength required to obtain the significant economic effect of the flaking of the steel sheet is 520 MPa. Therefore, the amount of N is set to 0.0050% or more. On the other hand, when the amount of N exceeds 0.0150%, it is excessively hard, and it is difficult to produce a thin steel sheet by secondary cold rolling while ensuring workability. Therefore, the amount of N is set to 0.0150% or less.

B: 0.0002% or more and 0.0050% or less

B has an effect of suppressing grain growth of the heat-affected zone in the vicinity of the welded portion and preventing chipping at the time of flange processing due to a decrease in local strength. In order to sufficiently obtain the effect of preventing chipping, the amount of B must be 0.0002% or more. On the other hand, even if it exceeds 0.0050%, further effects cannot be expected, and the cost becomes high. Therefore, the amount of B is set to be 0.0002% or more and 0.0050% or less.

The rest is set to Fe and unavoidable impurities, but may also contain constituent elements which are usually contained in steel sheets for welding cans. For example, it may contain Cr: 0.10% or less, Cu: 0.20% or less, Ni: 0.15% or less, Mo: 0.05% or less, Ti: 0.3% or less, Nb: 0.3% or less, and Zr: 0.3% or less, V: Component elements such as 0.3% or less and Ca: 0.01% or less (each elemental component range includes 0%).

Next, the crystal grains of the steel sheet for high strength and high workability cans of the present invention will be described.

In the cross section in the rolling direction, it is necessary to include crystal grains having an area ratio of 0.01 to 1.00% and an elongation of 5.0 or more. In general, when a DR material is produced using the N amount of steel as described above, the degree of stretching of the crystal grains in the cross section in the rolling direction is less than 3.0. However, by limiting the annealing temperature and the annealing time to an appropriate range, the elongation of a part of the crystal grains appears to be large. Further, although the mechanism is not clear, the workability is improved when the crystal grains having an elongation of 5.0 or more are present at an area ratio of 0.01% or more. If the area ratio exceeds 1.00%, it will hinder the workability. From the above, it is understood that the area ratio of the crystal grains having an elongation of 5.0 or more is set to 0.01 to 1.00%. On the other hand, since the effect of improving the workability of the crystal grains having an elongation of more than 50.0 is small, it is more preferable that the area ratio of the crystal grains having an elongation of 5.0 or more and 50.0 or less is 0.01 to 1.00%.

Further, the elongation of the crystal grains in the cross-section of the rolling direction can be determined by the microscopic test method of the crystal grain size shown in the literature "JIS G 0551", and the length of the rolling direction and the length direction of the rolling are measured for each crystal grain. The length of the right angle is calculated and the ratio is calculated. Moreover, according to the steel composition and the manufacturing method of the present invention, the formed swarf carbon iron and the stellite iron are very small compared with the ferrite iron particles, and therefore the crystal grain size and the elongation are determined only by the ferrite iron crystal grains. It is carried out as an object.

The area ratio of the crystal grains can be measured by a dot algorithm shown in the document "JIS G 0555 Attachment 1". The purpose is to determine the area ratio of the non-metallic interpolymer in the steel, but it can also be used for the area ratio measurement of the crystal grains of the specific shape as described above. Further, the area ratio can be measured using a microscope photograph and an arbitrary image analysis device.

Next, a method of producing the steel sheet for a can according to the present invention will be described.

The steel sheet for high strength and high workability of the present invention is produced by the following method The steel slab having the above composition produced by continuous casting is used, and the slab reheating temperature before hot rolling is set to 1200 ° C or higher, and after hot rolling, it is taken up at a temperature of less than 650 ° C, and then carried out. One cold rolling, followed by continuous annealing at a soaking temperature of 680 to 760 ° C and a soaking time of 10 to 20 seconds, followed by secondary cold rolling at a rolling reduction of 20% or less.

In general, it is difficult to form a thinner sheet thickness such as to obtain a significant economic effect by only one cold rolling. That is, in order to obtain a thinner plate thickness by one cold rolling, the load on the rolling mill becomes too large, which cannot be satisfied in terms of equipment capability. For example, when the final thickness is 0.15 mm, if the thickness after hot rolling is 2.0 mm, a larger primary cold rolling ratio of 92.5% is required. Further, although it is considered that the thickness of the steel sheet after cold rolling is reduced and the rolling is performed at a hot rolling stage, if the rolling reduction ratio of the hot rolling is increased, the temperature of the steel sheet during rolling is lowered. It becomes larger and it is impossible to obtain a predetermined finishing temperature. Further, when the thickness of the sheet before annealing is made small, when continuous annealing is performed, there is a possibility that failure such as cracking or deformation of the steel sheet during annealing increases. For these reasons, in the present invention, the second cold rolling is performed after annealing to obtain an extremely thin steel sheet.

Reheating temperature of billet before hot rolling: above 1200 °C

If the reheating temperature of the slab before hot rolling is less than 1200 ° C, the re-dissolution of AlN precipitated after casting cannot be sufficiently performed, and the amount of solid solution N which is subjected to strength becomes insufficient. Therefore, the reheating temperature of the slab before hot rolling is set to 1200 ° C or higher. On the other hand, excessive heating is related to an increase in energy cost, so it is preferably 1200 to 1300 °C.

Coiling temperature after hot rolling: less than 650 ° C

When the coiling temperature after hot rolling is 650 ° C or more, AlN is excessively precipitated, and the amount of solid solution N which is subjected to strength becomes insufficient. Therefore, the coiling temperature after hot rolling is set to be less than 650 °C. On the other hand, the coiling temperature which is set too low may cause a cost increase such as a decrease in the rolling speed, and therefore it is preferably 580 to 620 °C.

One cold rolling

The primary cold rolling rate is not particularly limited, but in order to finally obtain an extremely thin steel sheet, the rolling reduction rate of the primary cold rolling must be increased to some extent. That is, for the above reasons, the case where the hot rolling ratio is increased is not preferable, and the secondary cold rolling ratio is necessarily limited by the following reasons. Therefore, the primary cold rolling rate is preferably more than 85%. On the other hand, if the primary cold rolling ratio exceeds 92%, the load on the rolling mill becomes too large, and therefore, it is more preferably 89 to 92%.

annealing

Annealing is carried out by continuous annealing, the soaking temperature is set to 680 to 760 ° C, and the soaking time is set to 10 to 20 seconds. When the soaking temperature is less than 680 ° C or the soaking time is less than 10 seconds, the area ratio of the crystal grains having a degree of stretching of 5.0 or more in the rolling direction section exceeds 1.00%, and workability is insufficient. In addition, when the soaking temperature exceeds 760 ° C or the soaking time exceeds 20 seconds, the area ratio of the crystal grains having a degree of stretching of 5.0 or more in the rolling direction section is less than 0.01%, and the effect of improving workability cannot be obtained.

Secondary cold rolling rate: 20% or less

The secondary cold rolling ratio is set to 20% or less. If the secondary cold rolling rate exceeds 20%, then The work hardening becomes too large, and the elongation at break of 7% or more cannot be obtained. Therefore, the secondary cold rolling ratio is set to 20% or less. On the other hand, when the secondary cold rolling ratio is less than 10%, the economic effect of reducing the thickness of the sheet by the DR method is small, and therefore it is preferably 10% or more and 20% or less.

After the secondary cold rolling, the steps such as plating are carried out in accordance with a conventional method, and finally processed into a steel sheet for cans.

[Examples]

A steel containing the composition shown in Table 1 and containing the Fe and the unavoidable impurities in the remainder was melted in a converter, and a slab was obtained by a continuous casting method. The obtained slab was reheated under the conditions shown in Table 2, and hot rolling and primary cold rolling were carried out under the conditions shown in Table 2. The finish rolling temperature of hot rolling was set to 890 ° C, and pickling was performed after hot rolling. Then, after one cold rolling, continuous annealing was carried out under the conditions shown in Table 2, and then secondary cold rolling was carried out under the conditions shown in Table 2.

Sn plating was continuously performed on both surfaces of the steel sheet obtained above, and a tin-plated iron sheet having a Sn adhesion amount of 2.8 g/m ² on one side was obtained.

The plated steel sheet (tin-plated iron sheet) obtained above was subjected to a tensile test at 210 ° C for 20 minutes after heat treatment equivalent to coating and baking. In the tensile test, a tensile test piece of JIS No. 5 size was used, and the tensile strength and elongation at break in the direction perpendicular to the rolling were measured.

Further, a steel plate having a heat treatment equivalent to that of the coating and baking is used, and a can body having an outer diameter of 52.8 mm is formed by seam welding, and the end portion is subjected to Neck Forming until the outer diameter is 50.4. After mm, the flange processing was performed until the outer diameter was 55.4 mm, and the presence or absence of cracking of the flange was evaluated. The can body was formed into a 190 g beverage can size and welded along the direction in which the steel sheet was rolled. The necking process is performed by Die-neck Processing, and the flange processing is performed by a spin-flange process. The case where the flanged portion was broken was evaluated as ×, and the case where no chipping occurred was evaluated as ○.

Further, a sample of a plated steel sheet was taken, and the area ratio of the crystal grains having a degree of extension in the rolling direction section of 5.0 or more was measured. The elongation of the crystal grains in the rolling direction section is obtained by grinding the vertical section of the steel sheet and etching the grain boundary by nitric acid etching solution, and then applying the cutting method of the straight line test line described in the document "JIS G 0551". Each crystal grain was measured.

The results obtained are shown in Table 3.

As is clear from Table 3, the strength of Nos. 1 to 5 of the present invention is excellent, and the tensile strength required for the extremely thin steel sheet for cans is 520 MPa or more. Moreover, it is excellent in workability, and has an elongation at break of 7% or more which is required for processing the can lid or the three-piece can body.

On the other hand, in No. 6 of the comparative example, since the C content was too large, workability was impaired by secondary cold rolling, and the elongation at break was insufficient. In the comparative example No. 7, since the B was not contained, the welded heat-affected zone was extremely softened, and chipping occurred during the flange processing. In Comparative Example No. 8, since the reheating temperature of the slab was too low, AlN precipitated after casting was not sufficiently dissolved, and in Comparative Example No. 9, since the coiling temperature was too high, AlN was excessively precipitated, so that either In the case where all of N is present as AlN, the tensile strength is insufficient. In Comparative Example No. 10, since the N content was too small, the tensile strength was insufficient. In Comparative Example No. 11, since the soaking temperature of continuous annealing is too low, the area ratio of crystal grains having a degree of extension of 5.0 or more becomes Too large, the elongation at break is insufficient. In Comparative Example No. 12, the soaking temperature of the continuous annealing was too high, and in Comparative Example No. 13, the soaking time of the continuous annealing was too long, so that the area ratio of the crystal grains having the elongation of 5.0 or more was It becomes too small and the elongation at break is insufficient. In the comparative example No. 14, since the secondary cold rolling ratio was too large, the work hardening became too large, and the elongation at break was insufficient.

(industrial availability)

The steel sheet for cans of the present invention has a tensile strength of 520 MPa or more and an elongation at break of 7% or more, and can be obtained from a relatively thin plate thickness. Therefore, it is preferable as a material for manufacturing a can lid, a can bottom, a three-piece can body or the like at low cost.

Claims (2)

A high-strength and high-performance steel sheet for cans containing C: 0.001% or more and 0.080% or less, Si: 0.003% or more and 0.100% or less, and Mn: 0.10% or more and 0.80% or less, and P. 0.001% or more and 0.100% or less, S: 0.001% or more and 0.020% or less, Al: 0.005% or more and 0.100% or less, N: 0.0050% or more and 0.0150% or less, B: 0.0002% or more and 0.0050% or less, and the rest The part contains Fe and unavoidable impurities, and further contains crystal grains having an area ratio of 0.01 to 1.00% and a crystal grain extension of 5.0 or more in the roll direction cross section. A method for producing a steel sheet for high-strength and high-processability, characterized in that steel is continuously slab, steel is reheated at a temperature of 1200 ° C or higher, and hot rolled at a temperature of less than 650 ° C. And then performing a cold rolling, followed by continuous annealing at a soaking temperature of 680 to 760 ° C and a soaking time of 10 to 20 seconds, followed by secondary cold rolling at a rolling reduction of 20% or less; C: 0.001% or more and 0.080% or less, Si: 0.003% or more and 0.100% or less, Mn: 0.10% or more and 0.80% or less, P: 0.001% or more and 0.100% or less, and S: 0.001% or more and 0.020% or less Al: 0.005% or more and 0.100% or less, N: 0.0050% or more and 0.0150% or less, B: 0.0002% or more and 0.0050% or less, and the balance contains Fe and unavoidable impurities.