JP5672907B2 - Steel sheet for high strength and high workability can and method for producing - Google Patents

Description

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

  Among steel plates used for beverage cans and food cans, steel plates called DR (Double Reduce) materials may be used for lids, bottoms, 3-piece can bodies, drawn cans, and the like. DR material, which is cold-rolled again after annealing, is easier to reduce the plate thickness than SR (Single Reduce) material, which only performs temper rolling with a small rolling rate. The can cost can be reduced.

  In the DR method for producing a DR material, work hardening occurs by performing cold rolling after annealing, so that a thin and hard steel plate can be produced. However, on the other hand, the DR material manufactured by the DR method has poor ductility, so that the workability is inferior to that of the SR material.

  The body of food cans and beverage cans composed of three pieces is molded into a cylindrical shape, and then flanged at both ends in order to tighten the lid and bottom. Therefore, good elongation is required at the end of the can body.

  On the other hand, a steel plate as a can-making material is required to have a strength corresponding to the plate thickness, and in the case of a DR material, a tensile strength of about 550 MPa or more is required in order to ensure an economic effect by reducing the thickness.

  Conventionally used DR materials are difficult to achieve both the above-described ductility and strength, and SR materials have been mainly used for the body of food cans and beverage cans. However, at present, in order to reduce the plate thickness from the viewpoint of cost reduction, it is desired to use DR material for the body of food cans and beverage cans, and the demand for expanding the application of DR material is increasing. ing.

In response to these, Patent Document 1 manufactures a steel sheet having a high r value and excellent flange workability by applying the DR method at a primary cold rolling rate of 85% or less to low carbon steel. A method is disclosed.
Patent Document 2 discloses a method for producing a DR material that achieves both hardness and workability by performing nitriding in a low carbon steel annealing step.

JP 63-7336 JP JP 2004-323905 A

  However, all of the above conventional techniques have problems.

  In the manufacturing method described in Patent Document 1, since it is necessary to reduce the primary cold rolling rate, an extremely thin steel sheet cannot be manufactured due to the restriction of the finished thickness of hot rolling. When the finish thickness of hot rolling is reduced, the finish rolling temperature is lowered, and it is difficult to maintain the predetermined temperature.

  In the manufacturing method described in Patent Document 2, since it is necessary to perform nitriding after recrystallization is completed, an increase in costs such as a reduction in line speed and an increase in the length of the heating furnace cannot be avoided in the continuous annealing process.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a steel plate for a high-strength, high-workability can that is suitable as a material for a lid, a bottom, a three-piece can body, and the like, and a method for manufacturing the same.

  The inventors of the present invention have intensively studied to solve the above problems. As a result, the following knowledge was obtained.

  In order to achieve both ductility and strength, it is effective to secure ductility by limiting the secondary cold rolling rate after annealing to an appropriate range while adding strength by adding an appropriate amount of N. is there.

  Further, if the slab reheating temperature before hot rolling is low, the AlN deposited after casting is not sufficiently remelted, and if the coiling temperature after hot rolling is high, the precipitated AlN becomes excessive. In any case, since the solute N responsible for the strength is insufficient, the slab reheating temperature and the winding temperature must be limited to an appropriate temperature range.

The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] By mass%, C: 0.001% to 0.040%, Si: 0.003% to 0.100%, Mn: 0.10% to 0.80%, P: 0.001% to 0.100%, S: 0.001% to 0.020% In the following, Al: 0.005% or more and 0.100% or less, N: 0.015% to 0.020% or less, B: 0.0002% or more and 0.0050% or less, and the balance is composed of Fe and unavoidable impurities, and further, N which exists as AlN A steel plate for a high-strength, high-workability can, characterized by having a content of 0.0060% or less, an average crystal grain size of 5.00 μm or more, and a crystal grain elongation of 2.50 or less in a cross section in the rolling direction.
[2] By mass%, C: 0.001% to 0.040%, Si: 0.003% to 0.100%, Mn: 0.10% to 0.80%, P: 0.001% to 0.100%, S: 0.001% to 0.020% In the following, Al: 0.005% or more and 0.100% or less, N: 0.015% to 0.020% or less, B: 0.0002% or more and 0.0050% or less, the balance being Fe and unavoidable impurities, steel is made into a slab by continuous casting, After performing hot rolling with a slab reheating temperature of 1200 ° C. or higher, winding at a temperature of less than 700 ° C., then performing primary cold rolling at a rolling rate of greater than 85%, subsequently performing continuous annealing, A method for producing a steel plate for a high-strength, high-workability can, characterized by performing secondary cold rolling at a rolling rate of 20% or less.
In addition, in this specification,% which shows the component of steel is mass% altogether.

According to the present invention, it is possible to obtain a high-strength and highly workable steel sheet for cans having a tensile strength of 550 MPa or more and a breaking elongation of 7% or more.
As a result, by improving the workability of the original plate (steel plate), it is possible to make cans using DR material with a thin plate thickness, without causing cracks during flange processing of 3-piece cans. .

Hereinafter, the present invention will be described in detail.
The steel plate for cans of the present invention is a high-strength, high-workability steel plate for cans having a tensile strength of 550 MPa or more and a breaking elongation of 7% or more. And such a steel plate uses steel containing N exceeding 0.015%, slab reheating temperature before hot rolling, coiling temperature after hot rolling, and secondary cold rolling rate under appropriate conditions It becomes possible to manufacture by setting to.

The component composition of the steel plate for cans of this invention is demonstrated.
C: 0.001% to 0.040%
If the C content exceeds 0.040%, the ductility deteriorates. In addition, the cold rollability also decreases. For this reason, the C content is 0.040% or less. On the other hand, when the amount of C is less than 0.001%, crystal grain size increases significantly, and the risk of causing rough skin defects in the processed portion increases. Therefore, the C content is 0.001% or more and 0.040% or less.

Si: 0.003% to 0.100%
If the amount of Si exceeds 0.100%, problems such as deterioration of surface treatment properties and deterioration of corrosion resistance are caused, so the upper limit is made 0.100%. On the other hand, if it is less than 0.003%, the refining cost becomes excessive, so the lower limit is made 0.003%.

Mn: 0.10% to 0.80%
Mn has an effect of preventing red heat embrittlement during hot rolling due to S and refines crystal grains, and is an element necessary for securing a desirable material. In order to exert these effects, it is necessary to add at least 0.10%. On the other hand, if too much Mn is added, the corrosion resistance deteriorates and the steel sheet becomes excessively hard, so the upper limit is made 0.80%.

P: 0.001% to 0.100%
P is a harmful element that hardens steel and deteriorates workability and at the same time deteriorates corrosion resistance. Therefore, the upper limit is made 0.100%. On the other hand, the P removal cost is excessive to make P less than 0.001%. Therefore, the lower limit is made 0.001%.

S: 0.001% to 0.020%
S exists as an inclusion in steel, and is a harmful element that lowers ductility and deteriorates corrosion resistance. Therefore, the upper limit is made 0.020%. On the other hand, in order to make S less than 0.001%, the cost of removing S becomes excessive. Therefore, the lower limit is made 0.001%.

Al: 0.005% to 0.100%
Al is an element necessary as a deoxidizer during steelmaking. When the addition amount is small, deoxidation becomes insufficient, inclusions increase, and workability deteriorates. If the content is 0.005% or more, it can be considered that deoxidation is sufficiently performed. On the other hand, when the content exceeds 0.100%, the frequency of occurrence of surface defects due to alumina clusters and the like increases. Therefore, the Al content is 0.005% or more and 0.100% or less.

N: 0.015% to 0.020% or less In the steel sheet for cans of the present invention, the secondary cold rolling rate is suppressed to ensure elongation, while increasing the N content contributes to high strength. If the N content is 0.015% or less, the tensile strength of 550 MPa necessary for obtaining a remarkable economic effect due to the thinning of the steel sheet cannot be obtained. Therefore, the N content is over 0.015%. On the other hand, if the N content exceeds 0.020%, it becomes excessively hard, and it becomes difficult to produce a thin steel plate by secondary cold rolling while ensuring workability. Therefore, the N content is 0.020% or less.

B: 0.0002% to 0.0050%
B has the effect of suppressing grain growth in the heat-affected zone near the weld and preventing cracking during flange processing due to local strength reduction. In addition, cracking at the welded portion is likely to occur due to thinning. In order to sufficiently obtain the effect of preventing such cracking, the amount of B needs to be 0.0002% or more. On the other hand, if it exceeds 0.0050%, no further effect can be expected, resulting in high costs. Therefore, the B content is 0.0002% or more and 0.0050% or less.

N present as AlN: 0.0060% or less The steel sheet for cans of the present invention ensures strength by containing N at a high level, but since the deposited nitride has a small effect of improving strength, most of the added N is It must be in the state of solid solution N. If the amount of N present as AlN exceeds 0.0060%, the amount of solute N becomes insufficient, and the required tensile strength of 550 MPa cannot be obtained. Therefore, the amount of N present as AlN is 0.0060% or less.
The content of N present as AlN can be confirmed, for example, by performing dissolution extraction of AlN using a 10% Br-methanol solution and performing quantitative analysis of N present as AlN by absorptiometry.
Further, in order to make the amount of N present as AlN 0.0060% or less, the slab reheating temperature before hot rolling is set to 1200 ° C. or more, and the winding temperature after hot rolling is set to less than 700 ° C.

  The balance is Fe and inevitable impurities.

  Next, the crystal grain of the steel plate for cans of this invention is demonstrated.

  The average crystal grain size in the cross section in the rolling direction is 5.00 μm or more. The final mechanical properties of the steel plate for cans of the present invention are greatly influenced by the state of crystal grains. If the average grain size in the cross section in the rolling direction is less than 5.00 μm, the elongation of the steel sheet is insufficient, and the workability is impaired.

  Further, the elongation of the crystal grains in the cross section in the rolling direction is 2.50 or less. The degree of extension is a value representing the degree to which ferrite crystal grains are extended by processing, as shown in the document “JIS G 0202”. If the elongation of crystal grains in the cross section in the rolling direction exceeds 2.50, the elongation in the direction perpendicular to the rolling, which is important for flange workability and neck workability, is insufficient. The elongation increases with the rolling ratio of secondary cold rolling, but when the secondary cold rolling ratio is 20% or less, in order to keep the elongation to 2.50 or less, the content of N present as AlN The amount should be 0.0060% or less. In other words, since AlN works as pinning particles against grain boundary movement, the presence of AlN exceeding 0.0060% in the N content remains the shape of the flattened crystal grains after primary cold rolling even after annealing. Becomes larger. AlN hardly precipitates in the continuous annealing process, so it is considered that the amount precipitated at the time of winding after hot rolling is almost brought to the final product.

  The average grain size in the cross section in the rolling direction and the degree of elongation of the crystal grains in the cross section in the rolling direction can be measured by a microscopic test method for crystal grain size described in the document “JIS G 0551”. In addition, according to the steel composition / manufacturing method of the present invention, the cementite and pearlite to be formed are much smaller than the ferrite grains, so the measurement of the crystal grain size and elongation is performed only on the ferrite crystal grains.

Next, the manufacturing method of the steel plate for cans of this invention is demonstrated.
The steel sheet for high strength and high workability cans of the present invention uses a steel slab having the above composition produced by continuous casting, and the slab reheating temperature before hot rolling is 1200 ° C. or more, and after hot rolling Manufactured by winding at a temperature of less than 700 ° C, followed by primary cold rolling at a rolling rate of over 85%, followed by annealing, followed by secondary cold rolling at a rolling rate of 20% or less Is done.

  Usually, it is difficult to obtain a thin plate thickness that can provide a remarkable economic effect by only one cold rolling. That is, in order to obtain a thin plate thickness by a single cold rolling, the load on the rolling mill is excessive, which is impossible depending on the equipment capacity. For example, when the final plate thickness is 0.15 mm, the primary cold rolling rate as large as 92.5% is required when the plate thickness after hot rolling is 2.0 mm. In order to reduce the sheet thickness after cold rolling, it is conceivable that rolling is performed thinner than usual in the hot rolling stage. A predetermined finish rolling temperature cannot be obtained. Furthermore, if the plate thickness before annealing is reduced, when continuous annealing is performed, the possibility of troubles such as breakage and deformation of the steel plate during annealing increases. For these reasons, in the present invention, the second cold rolling is performed after annealing to obtain an extremely thin steel plate.

Slab reheating temperature before hot rolling: 1200 ° C or higher If the slab reheating temperature before hot rolling is lower than 1200 ° C, the AlN deposited after casting will not be sufficiently remelted and AlN will remain excessively . Therefore, the slab reheating temperature before hot rolling is set to 1200 ° C. or higher. Preferably, it is 1200-1300 degreeC.

Winding temperature after hot rolling: less than 700 ° C. When the winding temperature after hot rolling is 700 ° C. or more, AlN is excessively precipitated, and the amount of solute N responsible for strength becomes insufficient. Therefore, the coiling temperature after hot rolling is less than 700 ° C. Preferably, it is 600-680 degreeC.

Primary cold rolling rate: When the primary cold rolling rate is over 85% and the primary cold rolling rate is small, it is necessary to increase the rolling rate of hot rolling and secondary cold rolling in order to finally obtain a very thin steel plate. Increasing the hot rolling rate is not preferable for the reasons described above. The secondary cold rolling rate needs to be limited for reasons described later. That is, when the primary cold rolling rate is 85% or less, the production becomes difficult. Therefore, the primary cold rolling reduction is over 85%. Preferably, it is 90 to 92%.

The annealing conditions are not particularly limited, but the recrystallization needs to be completed by annealing. The continuous annealing method is preferably used from the viewpoint of production cost, and the soaking temperature is preferably 600 to 750 ° C from the viewpoint of operation efficiency and prevention of breakage during annealing of the thin steel sheet.

Secondary cold rolling rate: 20% or less The secondary cold rolling rate is 20% or less. When the secondary cold rolling rate exceeds 20%, work hardening by secondary cold rolling becomes excessive, and a breaking elongation of 7% or more cannot be obtained. Therefore, the secondary cold rolling rate is 20% or less. Preferably, it is 10% or more and 20% or less.
After the secondary cold rolling, steps such as plating are performed as usual, and finished as a steel plate for cans.

Steel containing the composition shown in Table 1 and the balance being Fe and inevitable impurities was melted in an actual converter, and a steel slab was obtained by a continuous casting method. The obtained steel slab was subjected to hot rolling and primary cold rolling under the conditions shown in Table 2. The finish rolling temperature of hot rolling is 890 ° C., and pickling is performed after hot rolling. Next, after the primary cold rolling, continuous annealing was performed at a soaking temperature of 700 ° C. and a soaking time of 20 seconds, and then secondary cold rolling was performed under the conditions shown in Table 2.
The steel plate obtained as described above was continuously subjected to Sn plating on both sides to obtain a tinplate with a single-side Sn adhesion amount of 2.8 g / m ² .

  The plated steel sheet (blink) obtained as described above was subjected to a heat treatment equivalent to paint baking at 210 ° C. for 20 minutes, and then subjected to a tensile test. In the tensile test, tensile strength (breaking strength) and elongation at break in the direction perpendicular to rolling were measured using tensile test pieces of JIS No. 5 size.

  Also, a can body with an outer diameter of 52.8mm is formed by seam welding using a steel plate that has been heat-treated equivalent to paint baking, and the end is necked in to an outer diameter of 50.4mm and then flanged to an outer diameter of 55.4mm. The presence or absence of flange cracking was evaluated. The can body was formed into a 190 g beverage can size, and welding was performed along the rolling direction of the steel sheet. Neck-in processing was performed by a die neck method, and flange processing was performed by a spin flange method. The case where cracking occurred at the flange processed part was evaluated as x, and the case where cracking did not occur was evaluated as ◯.

  Further, AlN was dissolved and extracted using a 10% Br-methanol solution, and quantitative analysis of N present as AlN was performed by absorptiometry.

Moreover, the sample of the plated steel plate was extract | collected and the average crystal grain diameter and the expansion degree of the crystal grain in the cross section of a rolling direction were measured. The average crystal grain size and the elongation of crystal grains in the cross section in the rolling direction were determined by polishing the vertical cross section of the steel sheet and revealing the grain boundary by night etching, and then testing the linear test line described in the document “JIS G 0551” It was measured by the cutting method by
The results obtained are shown in Table 3.

  From Table 3, Nos. 1 to 7, which are examples of the present invention, are excellent in strength and have achieved a tensile strength of 550 MPa or more required as an ultrathin steel plate for cans. It is also excellent in workability and has a break elongation of 7% or more necessary for processing of lids and 3-piece can bodies.

  On the other hand, No. 8 of the comparative example has too much C content, so the ductility is impaired by secondary cold rolling and the elongation at break is insufficient. Since No. 9 of the comparative example does not contain B, the weld heat affected zone becomes extremely soft, and cracking occurs in the flange processing. No. 10 of the comparative example is too low for the slab reheating temperature, and No. 11 of the comparative example is too high for the coiling temperature, so there is too much N amount present as AlN and the tensile strength is insufficient. ing. No. 12 in the comparative example has insufficient tensile strength because the N content is too small. In Comparative Example No. 13, the secondary cold rolling rate is too large, so the average crystal grain size is small, the degree of elongation is large, and the elongation at break is insufficient.

  The steel plate for cans of the present invention has a tensile strength of 550 MPa or more, a breaking elongation of 7% or more, and can be obtained with a thin plate thickness. Therefore, it is optimal as a material for manufacturing can lids, can bottoms, 3-piece can bodies, etc. at low cost.

Claims (2)

In mass%, C: 0.001% to 0.040%, Si: 0.003% to 0.100%, Mn: 0.10% to 0.80%, P: 0.001% or more 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: more than 0.015% and 0.020% or less, B: 0.0002 % And 0.0050% or less, the balance consists of Fe and inevitable impurities,
Furthermore, the content of N present as AlN is 0.0060% or less,
A steel plate for a high-strength, high-workability can characterized by having an average crystal grain size of 5.00 μm or more and a crystal grain elongation of 2.50 or less in a cross section in the rolling direction. In mass%, C: 0.001% to 0.040%, Si: 0.003% to 0.100%, Mn: 0.10% to 0.80%, P: 0.001% or more 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: more than 0.015% and 0.020% or less, B: 0.0002 The steel is made of Fe and unavoidable impurities, and the balance is made into slab by continuous casting, and the slab reheating temperature is set to 1200 ° C. or higher, and after hot rolling, the temperature is less than 700 ° C. As AlN , characterized in that it is wound at, and then primary cold rolling is performed at a rolling rate exceeding 85%, followed by annealing, and then secondary cold rolling at a rolling rate of 20% or less. Existing N content is 0.0060% A lower, in the rolling direction cross-section, an average crystal grain size of more than 5.00, the method of producing a high strength and high formability steel sheet for cans which crystal grains of elongation rate is 2.50 or less.