WO2003069010A1

WO2003069010A1 - Steel sheet for container excellent in formability and properties at weld, and method for producing the same

Info

Publication number: WO2003069010A1
Application number: PCT/JP2003/000561
Authority: WO
Inventors: Hidekuni Murakami; Seiichi Tanaka; Keiichiro Torisu
Original assignee: Nippon Steel Corporation
Priority date: 2002-02-13
Filing date: 2003-01-22
Publication date: 2003-08-21
Also published as: KR20040077966A; CN1322159C; JP4234932B2; JP2003231948A; KR100623681B1; WO2003069010A8; CN1630733A

Abstract

The present invention is a steel sheet characterized by: containing, in mass, C: 0.0030% or less, S: 0.020% or more, N: 0.0080% or less, and Al: 0.040% or less; and satisfying, B/N: 0.40 to 2.70, and Al/B: 30 or less; and,if occasion demands, satisfying the following expression:(amount of N existing as AlN) / (amount of N existing as BN) < 0.40. By the present invention, good workability can be secured, fracture resulting from stress concentration caused by poor weldability can be decreased and, further, as the properties are good even when an annealing temperature is low, highly efficient production can be realized while heat buckling is prevented from occurring.

Description

DESCRIPTION

STEEL SHEET FOR CONTAINER EXCELLENT IN FORMABILITY AND PROPERTIES AT WELD, AND METHOD FOR PRODUCING THE SAME

Technical Field

The present invention relates to a steel sheet for a can, the steel sheet being used for a metal can such as a food can or a beverage can, and a method for producing the same. More specifically, the present invention provides an ultra-thin steel sheet for a container, the steel sheet being produced with high productivity and being excellent in formability and properties at a weld during the use, and a method for producing the same, in the field of steel sheet production, in particular, in the field of can manufacturing.

Background Art

It goes without saying that a steel sheet having a good formability is generally preferred when a product is produced by forming the steel sheet. In the field of manufacturing a food can, a beverage can, or the like, from a steel sheet, namely in the field of can manufacturing, the steel sheet itself is required, as a matter of course, to have a good workability and to not cause any problem during working such as drawing, ironing, punch-stretching, expanding, and moreover flanging which is applied to a can drum for expanding the opening thereof so that a can bottom and a can top may be attached to the can drum, and further, in the field of manufacturing a so-called 3-piece can which requires the forming of a weld, a steel sheet is required to have a good formability at the weld. In addition, in the case of a large container or the like, welding is often adopted when a metal handgrip is attached to the container and sometimes the strength, particularly the fatigue strength, at the weld, is a problem. In the meantime, the thickness of a steel sheet for a container has become thinner from the viewpoint of cost reduction and, in this case, the ductility and fatigue property of the steel sheet tend to deteriorate. Therefore, even an ultra-thin steel sheet is required to have a good formability and a high strength. In addition, as an ultra-thin steel sheet is apt to generate buckling, called a heat-buckle, in a continuous annealing process employed in the production of the steel sheet, the strip threading performance is very poor and, for this reason, the productivity is lowered remarkably.

To solve the above problems, Japanese Unexamined Patent Publications No. H3-257123 and No. H2-118026 and other publications disclose a DR material produced by the so-called DR method (double reduction method), wherein a steel sheet having a thickness thicker than the final product is processed in annealing and, after the annealing, the thickness of the final product is obtained by secondary cold-rolling. However, as the ductility of the material is drastically deteriorated by the secondary cold-rolling, a very high reduction ratio cannot be applied at the secondary cold-rolling and, as a result, a satisfactorily thin steel sheet cannot be obtained. What is more, in the steel sheet excessively hardened by the application of a high reduction ratio at the secondary cold-rolling, the softening of the material at a weld occurs conspicuously caused by the recovery and recrystallization of the material due to the heat generation during welding and, thus, the stress concentration to the vicinity of the weld is increased and the formability and fatigue property are deteriorated.

In addition, Japanese Unexamined Patent Publication No. H6-41683 discloses a technology of suppressing cracking at a heat affected zone (HAZ) during welding and improving drawing formability by adding Nb and B to an ultra-low-carbon steel and controlling the crystal grain size. However, as the proposed technology does not take into consideration the strip threading performance of an ultra-thin material during annealing and the influence of precipitates such as nitrides and sulfides on the softening of the HAZ, the recrystallization temperature is high and, thus, the annealing temperature cannot be lowered sufficiently. Therefore the strip threading performance is poor in the annealing process, and also the improvement of the properties at the weld is insufficient.

The present invention makes it possible to produce an ultra-thin material used for a container with high productivity but without the deterioration of the strip threading performance during annealing, and provides a steel sheet for a container, wherein the formability during can manufacturing and the formability at a weld are improved and the cracking caused by fatigue at the weld, which is a problem during use, is decreased, and a method for producing the steel sheet.

The present invention is a technology for improving formability at working such as drawing, ironing, expanding and punch-stretching, flange formability at a weld during can forming and fatigue strength at the weld during the use by appropriately specifying the contents of S and other elements in the base material so that not only the material quality of the base material but also that of the weld, which is apt to generate stress concentration during the flange forming or the use, may be suitable for the object. That is, the present invention is a technology of, in a B added ultra-low- carbon steel, improving the properties by controlling the shapes, kinds and amounts of nitrides and sulfides within respective appropriate ranges.

Disclosure of the Invention

The present invention is, therefore, composed of the following items.

(1) A steel sheet for a container, the steel sheet being excellent in formability and properties at a weld, characterized by: containing, in mass, C: 0.0030% or less, S: 0.020% or more, N: 0.0080% or less, and Al: 0.040% or less; and satisfying, B/N: 0.40 to 2.70, and

Al/B: 30 or less.

(2) A steel sheet for a container, the steel sheet being excellent in formability and properties at a weld, according to the item (1), characterized by satisfying the following expression:

(amount of N existing as AlN) / (amount of N existing as BN) < 0.40.

(3) A steel sheet for a container, the steel sheet being excellent in formability and properties at a weld, according to the item (1) or (2), characterized by: further containing, in mass, Mn: 0.2 to 2.0%; and, with respect to the sulfides in the steel, satisfying the following expression:

(amount of S existing as Cu sulfides) / (amount of S existing as Mn sulfides) < 0.10.

(4) A steel sheet for a container, the steel sheet being excellent in formability and properties at a weld, according to any one of the items (1) to (3), characterized by further containing, in mass, Si: 0.015 to 2.00% and P: 0.005 to 0.080%.

(5) A method for producing a steel sheet for a container, the steel sheet being excellent in formability and properties at a weld, according to any one of the items (1) to (4), characterized by controlling the temperature of annealing after cold-rolling to 690 °C or lower .

Brief Description of the Drawings

Fig. 1 is an illustration showing the method for evaluating the workability of a weld. Fig. 2 is an illustration showing the method for measuring the strength of a weld.

Fig. 3 is an illustration showing the method for measuring the fatigue strength of a weld.

Best Mode for Carrying out the invention

The present invention is hereunder explained in detail. In the first place, the chemical components are explained hereunder. In the explanation, the "amount of each chemical component is expressed in terms of mass %. C content is generally preferred to be as low as possible from the viewpoint of workability, and therefore, the upper limit thereof is set at 0.0030%. In particular, when good ductility with a low aging property is required, the property can be improved significantly by reducing C content to 0.0015% or less. However, because an excessive reduction of C content causes not only the cost to increase but also a steel sheet to soften and hence the strength of a can to deteriorate, the lower limit is set at 0.0003%. N is an important element for controlling the formation of nitrides, which is an important requirement of the present invention. Because an excessive content of N causes excessive formation of nitrides and hence the object of the present invention cannot be achieved, the upper limit thereof is set at 0.0080%. When the addition amount of B is comparatively small, as will be explained later, a problem with an aging property caused by the residue of solute N may arise, and therefore it is preferable to control N content to 0.0030% or less in order to reduce the aging effect. Further, if the N content is controlled to 0.0020% or less by applying a vacuum degassing treatment sufficiently, the formation of nitrides is suppressed and, in particular, formability is improved. When the amount of nitrides is too small, the properties of a weld are deteriorated and, therefore, the lower limit is preferred to be set at 0.0008%. B is added as an indispensable element in the preset invention, because B affects the form of nitrides, changes material properties at the heat affected zone of a weld, lowers the recrystallization temperature of a steel sheet when it is added properly, hence making it possible to anneal the steel sheet at a lower temperature, and, as a result, improves the strip threading performance during annealing. However, an excessive addition of B causes a weld to harden excessively, workability to deteriorate, and a recrystallization temperature to rise. Therefore, the annealing temperature must be raised and, as a result, heat-buckles tend to occur easily. An important point is the ratio of B to N, and the ratio B/N is set at 0.40 to 2.70, preferably 0.60 to 2.00. An important requirement in the present invention is to control the kind and amount of nitrides, and the ratio of the amount of N existing as AlN to the amount of N existing as BN in a B added ultra-low-carbon steel is desirable to be less than 0.40, preferably 0.20 or less. Here, the amount of N existing as AlN is a value obtained by analyzing the Al amount contained in a residue when a steel sheet is dissolved in an iodine alcohol solution and then calculating the N amount ■ regarding the whole Al amount as a constituent of AlN. Likewise, the amount of N existing as BN is a value obtained by analyzing the B amount contained in a residue when a steel sheet is dissolved in an iodine alcohol solution and then calculating the N amount regarding the whole B amount as a constituent of BN.

As mentioned above, in order to control nitrides, the addition amounts of Al and B, the ratio between them, the oxides which act as the precipitation nuclei of nitrides, namely 0 (oxygen) content in a steel, and heat history throughout the all production processes are important factors. By controlling Al/B to 30 or less, preferably 20 or less, and Al to 0.040% or less, preferably 0.020% or less, solute N existing excessively in a steel combines with B more preferentially than with Al when nitrides precipitate and, by so doing, the kind and amount of nitrides can be controlled desirably.

In the present invention, besides the control of nitrides, the control of the form of sulfides is important especially for improving the properties of a weld. Here, the main form of sulfides is MnS. For the above end, the content of S is controlled to 0.020% or more, preferably 0.030% or more, and yet preferably 0.035% or more. When the content of S is lower than the above figures, the amount of sulfides decreases and, in addition, the sulfides tend to be unstable and easily take an undesirable form by the influence of the heat during welding, and the properties of the weld are deteriorated as a result. The present invention does not specify an upper limit of the S content but, in consideration of the hot workability of a steel sheet during the production processes and other factors, the upper limit is usually somewhere around 0.10% at the highest.

The content of Mn is determined to be 0.2 to 2.0%. If its content is less than 0.2%, sulfides become unstable and they take an undesirable form by the influence of the heat during welding. if the content exceeds 2.0%, on the other hand, solute Mn increases, thus the base material and the heat affected zone of a weld become too hard and, therefore, the workability is deteriorated.

Further, with regard to sulfides in a steel, suppressing the formation of Cu sulfides is also important. In general, S in a steel must be fixed as sulfides in relation to the hot-rolling performance. Therefore, it is important to fix S as MnS in a steel according to the present invention. In the present invention, it is desirable that the ratio of (S existing as Cu sulfides) to (S existing as MnS) is set at less than 0.10. The reason is that Cu sulfides not only precipitate finely and cause the recrystallization temperature of a steel sheet to rise but they also cause the complex-precipitates with B and Al nitrides to form and thus the form of nitrides to be undesirable. Here, (S existing as Cu sulfides) is a value obtained by quantitatively measuring Cu amount in a residue obtained by the electrolytic extraction of a steel sheet and then converting the Cu amount into S amount using the expression, Cu/S = 2/1, and (S existing as Mn sulfides) is a value obtained by quantitatively measuring Mn amount in a residue obtained by the electrolytic extraction of a steel sheet and then converting the Mn amount into S amount using the expression, Mn/S = 1/1. While no specific limitation is set regarding O (oxygen) content in the present invention, 0 (oxygen) exists in a steel in the form of oxides containing Si, Al, Mn, Fe and, further, elements such as Ca, .Mg, etc., acts effectively as the precipitation nuclei of nitrides when the oxides exist in an adequate amount, and thus has a positive effect on the control of nitrides. On the other hand, however, an excessive amount of 0 (oxygen) in a steel coarsens oxides, acts as the origin of cracks during working, and hence markedly deteriorates the product quality. For this reason, the desirable range of the 0 (oxygen) content is from 0.0010 to 0.0070%.

In order to desirably control the form of oxides as mentioned above or to improve workability and fatigue strength by adjusting the strength of a base steel sheet and thus mitigating stress concentration to the vicinity of a weld, Si, P, etc. may be added. In this case, it is desirable that the addition amounts are set at Si: 0.005 to 2.00% and P: 0.005 to 0.080%, respectively. When the addition amounts miss the ranges, not only a base material hardens excessively caused by solid solution hardening and workability deteriorates, but also the form of oxides changes or a weld softens or hardens unusually, and therefore desired properties of the weld cannot be obtained.

In the present invention, it is of vital importance to minimize the amounts of Ti and Nb, which are generally added in order to improve drawing formability when a steel material is to be subjected to drawing or another type of forming or in order to making a crystal structure fine for a specific purpose. Therefore, Ti and Nb are not added in principle and their contents must be limited to those inevitably included in a steel from iron ore, steel scrap mixed in a steelmaking process and dust, residues and the like unavoidably included during the production. In general, a desirable content of Ti or Nb is 0.006% or less. Their addition in excess of this value is not desirable, because, if they are added excessively, the recrystallization temperature of a steel sheet rises, the strip threading performance in an annealing process deteriorates conspicuously, and, in addition, a crystal structure may abnormally coarsen and soften, by the influence of heat in the vicinity of a weld, accelerating the stress concentration at the portion and, in such a case, the formability and fatigue strength of a product may fluctuate significantly.

It is not necessary to specify a thermal history or the like in the production processes. However, the slab reheating temperature (SRT) and the coiling temperature during hot-rolling and the temperature of annealing after cold-rolling slightly affect the material properties, and therefore the workability and fatigue strength of a weld can be improved by controlling the slab reheating temperature at hot-rolling to 1,100°C or higher, the coiling temperature at hot-rolling to 730 °C or lower and the temperature of annealing after cold-rolling to 700 °C or lower. The reason for this is not clear, but it is thought that the above defined conditions affect the form of nitrides or that of precipitates other than nitrides and, as a result, the excessive coarsening of the nitrides and the precipitates is suppressed and their forms are controlled adequately. By restricting the temperature of annealing after cold-rolling to 700 °C or lower, the occurrence of heat-buckles can be suppressed and thus the strip threading performance in an annealing process can be improved.

Though the mechanism of improving workability and fatigue strength at a weld by controlling the forms of nitrides and sulfides as mentioned above is not particularly clarified, as a phenomenon, it is thought that the hardness of a material at a weld and at a heat affected zone in the vicinity thereof is properly adjusted and, by so doing, the stress concentration to the portions is mitigated and a desirable hardness can be obtained. At a weld and in the vicinity thereof, nitrides and sulfides are dissolved by the temperature rise during welding and solute N, B, S and Mn increase, and, at the same time, the hardness is determined by fine nitrides and sulfides remaining without fully dissolved, fine nitrides and sulfides precipitating again during cooling and the like. Therefore, it is estimated that, in order to obtain the preferable forms of solute N, solute B and nitrides, it is necessary to control, in advance, the form of nitrides in a steel before welding as specified in the present invention.

In the production of a thin steel sheet for a container, there is a case where a steel sheet which is subjected to 2CR (second cold-rolling after annealing) rolling and hardened by the work hardening after annealing for securing the strength of a container is used. In such a steel sheet too, the effects of improving workability and fatigue strength at a weld can be provided by the present invention. However, as a work-hardened material is likely to be softened by the influence of heat as described above, it is desirable to control the extent of the work hardening to a low level. An appropriate 2CR ratio is 10% or less.

Further, even in the case of adding elements for improving corrosion resistance and other properties, the effects of the present invention are not lost. Even in the case of adding Sn, W, Mo, Ca, Cr, Ni, V, Sb, etc. for improving not only the drawability of a steel sheet and the properties of a weld but also workability at secondary working or the like, corrosion resistance, strip threading performance at various processes and other properties, the effects of the present invention are not lost in the least. However, as these elements generally raise the recrystallization temperature and deteriorate the strip threading performance during annealing, their addition must be limited to the ranges where an adverse effect does not appear. A steel sheet according to the present invention is generally used as the substrate of a surface treated steel sheet and, in that case too, the effects of the present invention are not spoiled at all by the surface treatment. As a surface treatment for a can, a treatment by tin, chromium (tin-free), nickel, lead, aluminum or the like is adopted. Further, as the substrate of a laminated steel sheet covered by an organic film, which has come to be used recently, a steel sheet according to the present invention can be adopted without spoiling the effects of the present invention.

Example In the first place, the method for evaluating the workability of the steel sheets in the following examples is explained. The workability was evaluated through a tensile test using JIS No. 5 tensile test pieces and in terms of the total elongation in the direction of rolling in the production of the steel sheets and the average value of the Lankford values (r-values) in the directions forming the angles of 0°, 45° and 90° with the rolling direction, which was calculated according to the following expression:

{(r-value at the angle of 0°) + (r-value at the angle of 90°) + 2 x (r-value at the angle of 45°)} x 1/4.

The workability of a weld was evaluated, as shown in Figure 1, by welding a quadrangular steel sheet with seam welding and forming it into a cylindrical shape, as in the case of manufacturing the can drum of a regular three-piece beverage can, expanding an opening by thrusting a conical die into the opening, and calculating the amount of deformation until cracking occurred at the opening end using the following expression:

{(Diameter at crack generation) - (Initial diameter )}/( Initial diameter).

The strength of a weld was evaluated, as shown in Figure 2, by welding two quadrangular steel sheets with spot welding at a welding current just lower than the current at which welding expulsions or surface flushes occurred, and measuring the maximum load at a tensile test. The fatigue strength of a weld was evaluated, as shown in Figure 3, by cutting out a strap 20 mm in width having a weld in the center from a welded cylindrical can drum formed as shown in Figure 1, subjecting the strap to a fatigue test under pulsating tension, and measuring the maximum load withstanding 10 million cycles of pulsation.

As the workability and the properties of a weld fluctuate depending on the respective values of the conditions specified in the present invention even though the conditions are within the respective ranges defined in the present invention and they are influenced also by chemical components and production conditions not particularly specified in the present invention, it is inappropriate to judge the effects of the present invention by the absolute values thereof. For this reason, in the examples described below, it was determined to evaluate the effects of the present invention by relatively comparing the specimens whose chemical components and production conditions, not particularly specified in the present invention, were substantially identical. Based on that, in the substantially identical examples, specific properties were judged by relative comparison, and the results were shown by the marks as follows: ® very good; O good; Δ same as conventional products; and x poor.

Heat buckling was judged by whether or not heat buckling occurred when cold-rolled coils having identical thickness and width were passed through an identical continuous annealing line at the temperature of the recrystallization temperature + 40 °C, and the results were expressed by the marks as follows: O no occurrence; Δ few occurrence; and x frequent occurrence.

The effects of the present invention were evaluated by synthetically judging the above-mentioned four evaluation items and the results were expressed by the marks as follows: ©very good (invented steels); O good (invented steels); Δ good in some of the evaluation items (invented steels); and x same as conventional products (comparative steels).

(Example 1)

The steels having the basic chemical components of C of 0.002%, Si of 0.1%, Mn of 0.5% and P of 0.01% and containing, in addition, other components as shown in Table 1, with the balance substantially consisting of Fe, were cast into slabs 250 mm in thickness, then hot-rolled sheets 2.0 mm in thickness were produced at the slab reheating temperature of 1,150°C and the coiling temperature of 650 °C, and then steel sheets 0.16 mm in thickness were produced through the processes of pickling, cold-rolling at the reduction ratio of 92%, annealing at 670°C for 1 min., and then skin-pass rolling at the reduction ratio of 2%, and the produced steel sheets were evaluated. As it is clear from Table 2, the steel sheets produced within the ranges specified in the present invention have good properties in all the evaluation items such as the workability of the steel sheets, the properties of the welds and the heat buckling resistance.

[Table 2]

(Example 2)

The steels having the basic chemical components of C of 0.002%, Si of 0.01%, Mn of 0.9%, P of 0.02%, Al of 0.02%, N of 0.002% and B of 0.002%, having the value of B/N of 0.9 and the value of Al/B of 8 to 12 and containing, in addition, other components as shown in Table 3, with the balance substantially consisting of Fe, were cast into slabs 250 mm in thickness, then hot-rolled sheets 2.2 mm in thickness were produced at the slab reheating temperatures shown in Table 3 and the coiling temperature of 620 °C, and then steel sheets 0.15 mm in thickness were produced through the processes of pickling, cold-rolling at the reduction ratio of 93%, annealing at 670°C for 1 min., and then skin-pass rolling at the reduction ratio of 3%, and the produced steel sheets were evaluated. As it is clear from Table 4, the steel sheets produced within the ranges specified in the present invention have good properties in all the evaluation items such as the workability of the steel sheets, the properties of the welds and the heat buckling resistance.

[Tables 3]

SRT: Slab reheating temperature (°C) *1: (amount of N existing as AlN) /(amount of N existing as BN)

[Table 4]

(Example 3)

The steels having the basic chemical components of C of 0.002%, Si of 0.02%, P of 0.01%, Al of 0.01%, N of 0.002% and B of 0.0022%, having the value of B/N of 0.9 and the value of Al/B of 8 to 12 and containing, in addition, other components as shown in Table 5, with the balance substantially consisting of Fe, were evaluated. The production conditions were the same as in Example 1. As it is clear from Table 6, the steel sheets produced within the ranges specified in the present invention have good properties in all the evaluation items such as the workability of the steel sheets, the properties of the welds and the heat buckling resistance.

[Table 5]

*2: (amount of S existing as Cus)/ (amount of S existing as MnS)

[Table 6]

As explained above, the present invention makes it possible to decrease the forming failure and the fracture during use which are caused by the welding of a container formed through working such as drawing, punch-stretching, expanding and welding. In addition, the present invention makes it possible to produce an ultra-thin steel sheet for a container with high efficiency while heat buckling is prevented from occurring because the steel sheet according to the present invention demonstrates better properties than those of a conventional steel sheet even when the annealing temperature is low.

Claims

1. A steel sheet for a container, the steel sheet being excellent in formability and properties at a weld, characterized by: containing, in mass, C: 0.0030% or less,

S: 0.020% or more, N: 0.0080% or less, and Al: 0.040% or less; and satisfying, B/N: 0.40 to 2.70, and Al/B: 30 or less.

2. A steel sheet for a container, the steel sheet being excellent in formability and properties at a weld, according to claim 1, characterized by satisfying the following expression:

(amount of N existing as AlN) / (amount of N existing as BN) < 0.40.

3. A steel sheet for a container, the steel sheet being excellent in formability and properties at a weld, according to claim 1 or 2 , characterized by: further containing, in mass,

Mn: 0.2 to 2.0%; and, with respect to the sulfides in the steel, satisfying the following expression: (amount of S existing as Cu sulfides) / (amount of S existing as Mn sulfides) < 0.10.

4. A steel sheet for a container, the steel sheet being excellent in formability and properties at a weld, according to any one of claims 1 to 3 , characterized by further containing, in mass, Si: 0.015 to 2.00% and P: 0.005 to 0.080%.

5. A method for producing a steel sheet for a container, the steel sheet being excellent in formability and properties at a weld, according to any one of claims 1 to 4 , characterized by controlling the temperature of annealing after cold-rolling to 690 °C or lower.