JPH08295986A - Extra thin steel sheet for welded can, excellent in necking formability - Google Patents

Abstract

PURPOSE: To equalize the hardness of a base material of can after welding with that of a nugget part, to narrow a softened part affected by welding heat, and to provide excellent necking fomability by adding trace amounts of Cr to a low carbon steel and controlling carbon equivalent and the hardness of a steel sheet after cold rolling, respectively. CONSTITUTION: This steel sheet has a composition consisting of, by weight ratio, 0.03-0.08% C, 0.10-0.60% Mn, 0.035-0.100% Cr, <0.05% Si, <0.04% P, 0.01-0.04% S, 0.0010-0.010% N, 0.05-0.100% sol. Al, and the balance Fe with inevitable impurities. Further, the carbon equivalent represented by equation is regulated so that it satisfied 62.3<=HR30T-88.4Ceq<=68.2. At this time, HR30T means the Rockwell hardness (HR30T) of the steel sheet after cold working, and the hardness is controlled to 70-85. Moreover, rolling, tin plating, etc., of this steel sheet can be done by the ordinary methods.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultra-thin steel sheet used for welding cans such as beverage cans, and particularly in a step of performing flange processing after neck-in processing after welding of cans by resistance seam welding. Relates to an ultra-thin steel plate for welded cans having excellent neck formability.

[0002]

2. Description of the Related Art Conventionally, methods of soldering, resin bonding, and resistance seam welding have been used for joining cans. Among them, the welding method that can reduce the joint margin to improve the yield of steel sheets is the mainstream.
Starting with various beverage cans such as fruit juice or coffee,
Widely used in food cans, aerosol cans, etc.

Such a welded can is a can in which the side seam portion of the can body is joined by resistance seam welding. Since the lap portion is thinner than the solder can and the adhesive can, the tightening property is improved, and the side seam portion. Has a number of advantages such as strong resistance, excellent printing effect because printing can be narrowed, and no solder jump.

By the way, in this welding can, there is a step of forming a flange portion extending diametrically outward at both ends of the can body in order to attach a lid to the welded can body. This is called flanging. ing. When performing this flange processing,
The contents of the can may leak due to defects such as flange cracks. Therefore, it is required that flange cracking is unlikely to occur in this flanging, that is, flanging is good.

Further, recently, in order to reduce the amount of the lid material used, a lid having a small diameter is often used. In this case, there is a step of narrowing both ends of the can body inward in the diameter direction, which is called neck-in processing. When using a lid with a diameter smaller than the body diameter of the can, repeat neck-in processing several times to form a stepped neck part, or perform neck-in processing with a high degree of processing once and then perform flange processing. To do. Partial buckling is unlikely to occur in such neck-in processing, and it is required that flange cracking is unlikely to occur in subsequent flange processing, but such performance is collectively referred to as neck formability here, Distinguish from the previous flange formability. Flange formability after neck-in is
Local distortion is added in the neck-in process, and the welding becomes severer than usual flanging, especially at the welded part and its vicinity.

[0006] Causes of these flange cracks include defective can body joining, poor workability of steel plates, and inclusions of steel plates. On the other hand, from the viewpoint of resource saving and the demand for cost reduction of cans, the plate thickness of the material for cans tends to be thin. Since the thinning of the material substantially lowers the strength of the can, the hardness of the steel plate is made higher than in the past to deal with it. Therefore, it is necessary to consider the deterioration of the flange formability due to the reduced plate thickness.

In recent years, as a steel sheet in consideration of such a situation, a steel sheet manufactured by a so-called two-time cold rolling method in which a hot-rolled steel sheet is cold-rolled, annealed, and cold-rolled again (double rolling) Juice material, hereinafter abbreviated as DR material),
Even in simple flanging without neck-in after welding, flange cracks often occur.

As a method for solving these problems, it is conceivable to reduce the carbon content and the inclusion content in the steel sheet as much as possible, which is a conventional method of improving the workability of the steel sheet. As measures against the above, measures such as low carbonization and careful control of inclusion content are taken.

[0009] However, treatments for lowering carbon and cleaning for reducing inclusions (for example, vacuum degassing treatment) bring about disadvantages such as an increase in steelmaking cost, and lowering carbon is essentially a soft material. This is a disadvantage and is disadvantageous to the above-mentioned technical requirement of "hardening to obtain sufficient strength while being extremely thin".

As a technique considering both hardness and workability of steel, a method of improving flange formability while suppressing softening of a heat-affected zone of a weld by refining cementite has been conventionally used (Japanese Patent Publication No. 5-8264). Japanese Patent Laid-Open Publication No. 62-156) or a method of improving the flange formability by suppressing the softening of the weld heat affected zone by coarsening the grain size of the steel sheet structure.
No. 10, JP-A-60-24327, JP-A-6
3-310922), or carbon equivalent (Ce
q) and a method of controlling the secondary cold rolling rate to suppress crystal grain coarsening after welding and improve flange formability (Japanese Patent Laid-Open No. 59-25934).

However, these techniques do not consider the neck-in processing prior to the flange processing. Further, among the above-mentioned technologies, Japanese Examined Patent Publication No. 62-15610
Japanese Patent Publication No. 5-8264, Japanese Patent Laid-Open No. 59-25
Although the technique disclosed in Japanese Patent No. 934 is effective in suppressing the softening of the heat-affected zone, since it contains a large amount of C, it becomes the highest temperature due to the heat input during welding and causes a structural change accompanied by austenite transformation. There is a problem that the portion (so-called nugget portion) that occurs is hardened and the flange formability is impaired, and the flange formability is insufficient.

[0012]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and in welding cans,
An object of the present invention is to provide a steel plate for an ultra-thin welded can, which does not cause flange cracks in the flanging process after the neck-in process and has a particularly thin plate thickness and high strength.

[0013]

Means and Actions for Solving the Problems As a result of a detailed investigation of the cause of cracking during flanging in order to solve the above problems, the present inventors have found that in order to suppress flange cracking, the material components It has been found that controlling the carbon equivalent Ceq and hardness within a specific range is particularly effective.

The welded portion of the welding can is thicker than the other portions, and the boundary portion is a portion where stress concentration is geometrically generated. A portion where the structure and mechanical properties are changed by heat input and pressure during welding (joint and its vicinity, hereinafter referred to as heat affected zone) and a portion which is not substantially affected by welding (hereinafter referred to as base metal In many cases, the boundary part of the welding can manufacturing process is essentially disadvantageous for neck-in and flanging. In many cases, the flange crack, which is a problem with, occurs near the weld.

The inventors of the present invention have conducted detailed research on this region, and as a result, control the hardness of the heat-affected zone to be equal to the hardness of the base material to alleviate the stress concentration and to improve the above-mentioned neck formability. It has been found that the above can be improved, and for that purpose, it is effective to control the material components, particularly the carbon equivalent Ceq and the hardness within a specific range.

The heat-affected zone is roughly classified into a so-called nugget portion that causes a change in structure accompanied by transformation and a peripheral portion that causes a change such as recrystallization without transformation, based on the behavior of the change in structure. When seam welding is performed in the proper current range, the nugget at the center of the joint has the highest temperature, so the structure of the material is austenite transformed into a single phase of austenite or two phases of ferrite and austenite, and then cooled at room temperature. It transforms into a stable phase. The structure of the nugget part after welding is mainly composed of material components, especially carbon equivalent (Ceq = [%
C] +1/6 [% Mn] +1/5 [% Cr]). Other conditions of the material (grain size, size of processing strain, precipitate size, etc.) and welding conditions (temperature rising rate, maximum temperature reached, cooling rate, etc.) also contribute to the structural change. In the condition range, the degree of contribution of Ceq is small as compared with the degree of contribution of Ceq.

Ceq is used as an index of hardenability in the cooling process after welding, and the structure after welding can be inferred from this. The contributions (coefficients) of Mn and Cr in Ceq are general values, and are reasonable values in the present invention. If the Ceq is high, a hard low-temperature transformation phase such as martensite or bainite is likely to precipitate, so that the nugget portion tends to be hard. On the other hand, when Ceq is low, soft nuclei tend to be softened because soft ferrite is precipitated and further grain growth occurs to form coarse grains.

When high strength is required, a DR material utilizing work hardening is often used, but the effect of work strain is lost in the nugget portion due to transformation of the structure. In order to make the hardness of the base material equal to the hardness of the nugget portion and satisfy the neck formability, it is necessary to supplement the strength by transformation strengthening, that is, by precipitating a hard low temperature transformation phase.

As described above, in order to use a steel plate having a desired material hardness as a material having advantageous properties with respect to neck formability, the hardness of the nugget portion after welding and the hardness of the base material which change depending on the magnitude of Ceq. Must be carefully considered from both perspectives.

On the other hand, since recrystallization and grain growth occur in the heat affected zone around the nugget, if the material is a DR material, the effect of processing strain is lost and this portion is generally softened. C
Even if the base material hardness and the nugget part hardness can be made equal by controlling eq, if this part existing between the base material and the nugget is softened, stress concentration occurs and the neck formability is improved. Adversely affect. Make this softened part narrow enough,
Recrystallization and grain growth must be suppressed in order to make the material substantially advantageous for neck moldability.

As a result of the investigation by the present inventors of means for obtaining the effect, it was found that the addition of Cr is effective. However, the amount added should be carefully controlled within the optimum range of Ceq described above and so as to exert the effect of suppressing recrystallization and grain growth.

The present invention is to improve the mechanical properties of the welded heat-affected zone of the raw steel sheet, regardless of the direction of lower carbonization of the raw material, which was one of the conventional wisdom for improving neck formability. With such a proper composition of components, it has been possible to significantly improve the local deformability of the welded portion and the vicinity of the welded portion in the neck-in processing of the welding can body and the subsequent flange processing.

That is, the present invention was made on the basis of the above-mentioned findings, and in% by weight, C: 0.03 to.
0.08%, Mn: 0.10 to 0.60%, Cr: 0.
035 to 0.100%, Si: less than 0.05%, P:
Less than 0.04%, S: 0.01 to 0.04%, N: 0.
0010 to 0.010%, sol. Al: 0.005-
0.100%, the balance consisting of Fe and unavoidable impurities, the Rockwell hardness (HR30T) is 70 or more and 85 or less, and the carbon equivalent Ceq is Ceq = [% C] +1/6 [% Mn] +1. / 5 [% Cr], an ultra-thin steel plate for a weld can having excellent neck formability, which satisfies the relationship of 62.3 ≤ HR30T-88.4Ceq ≤ 68.2. Is.

According to the present invention, not only excellent workability in necking, but also from the viewpoint of cost reduction and resource saving of a can, a high-strength ultra-thin steel plate capable of satisfying a demand for reduction in plate thickness of a material for a can body. By changing the balance between the content of C, Mn, and Cr and the hardness, it is possible to provide steel plates for various cans having different strength levels without impairing neck formability. it can.

Hereinafter, the present invention will be described in detail. First, the reasons for limiting each additive element will be described. C is one of the most important elements in the present invention, and is added to secure the strength of the raw material and the characteristics of the weld zone. However, if the content is less than 0.03%, the required strength cannot be secured, and conversely, the content is increased to 0.08%.
When it exceeds, the heat input during welding causes excessive precipitation of hard phase, which adversely affects the neck workability. Therefore, the content of C is set to the range of 0.03 to 0.08%.

Like C, Mn is an important element for securing the strength of the raw material and the weld properties, but if the content is less than 0.10%, the desired effect cannot be obtained. On the contrary, if the content increases and exceeds 0.60%, it has a disadvantageous effect on the neck workability as with C. Therefore, the Mn content is set to a range of 0.10 to 0.60%.

Cr, like C and Mn, is effective for controlling the microstructural change associated with transformation in welding and securing the characteristics, and also suppresses recrystallization and grain growth, and thus the peripheral portion of the weld nugget is suppressed. It is an element necessary for suppressing softening. However, if the content is less than 0.035%, the effect of suppressing recrystallization and grain growth cannot be obtained. Conversely, if the content exceeds 0.100%, not only the cost increases but also in the annealing step. It has an adverse effect such as non-recrystallization, impairing the workability of the material. Therefore, the content of Cr is set to the range of 0.035 to 0.100%.

When the content of Si is 0.05% or more, the plating adhesion is deteriorated and the corrosion resistance is deteriorated.
Its content is specified to be less than 0.05%. When the content of P increases, the steel plate excessively hardens to deteriorate the workability and also deteriorates the corrosion resistance. Therefore, the content of P is specified to be less than 0.04%, which does not cause such a fear.

When S content is low, S is likely to cause pitting corrosion, so S is required to be at least 0.01%. If the content exceeds 0.04%, hot brittleness is likely to occur. Therefore, the content of S is set to the range of 0.01 to 0.04%.

When the content of N is less than 0.0010%, the steel sheet becomes soft and the required strength cannot be secured even in the case of the DR material. If it exceeds 0100%, the steel sheet is excessively hardened and the workability is impaired. Therefore, the content of N is 0.0010
The range is 0.0100%.

Sol. Al acts as a deoxidizing agent, but when the content thereof is less than 0.005%, deoxidation becomes insufficient, the amount of inclusions increases, and conversely, when it exceeds 0.100%, the workability is adversely affected. . Therefore, sol. A
The content of 1 is set to 0.005 to 0.100%.

Next, the reason for limiting the hardness (HR30T) will be described. In order to meet the demands of can manufacturers for thinning can materials in recent years, and to provide a material having a thin plate thickness and sufficient strength, it is necessary that HR30T is 70 or more. However, if HR30T exceeds 85, it becomes difficult to secure the neck workability required for can making. Therefore, the hardness is set to the range of 70 to 85 at HR30T.

Next, the reason why the carbon equivalent Ceq is limited to the range of the present invention in relation to the hardness will be explained. Ceq
As mentioned above, is an index suitable for evaluating a change (substantially change in hardness) accompanied by transformation of the structure due to heat input during welding. FIG. 1 shows the relationship between Ceq and hardness of various steel plates. From the evaluation of the neck workability shown in this figure, a steel plate satisfying the component range of the present invention is to have both required strength and good neck workability,
It is clear that Ceq needs to be within the range that satisfies 62.3 ≦ HR30T-88.4Ceq ≦ 68.2.

In order to clarify the reason why good workability is obtained in this range, the present inventors conducted a detailed inspection of the welded portion of the can body where the flange crack had occurred and its vicinity. As a result, of the ranges outside the above range, the flange cracks generated in the range of HR30T-88.4Ceq <62.3 (referred to as range A), and the range of 68.2 <HR30T-88.4Ceq (range B and It was found that the flange cracks that occurred in step 1) occurred due to the mechanism described below.

That is, in the range A, the Ceq of the material is large and the contents of C and Mn are large. Therefore, in order to control the hardness within a range that does not impair the workability, the secondary cold rolling rate should be lowered. Therefore, the strengthening by work hardening needs to be in an appropriately small range. When seam welding such steel plates,
Since the Ceq is large, the hardness of the nugget portion is relatively large, which is excessive with respect to the hardness of the base material having a small degree of strengthening due to work hardening, so that stress concentration cannot be relaxed,
It was a crack.

On the other hand, in the range B, the Ceq of the material is small and the C and Mn contents are small. Therefore, in order to obtain the required hardness, the secondary cold rolling rate is increased to give a large work hardening. There is a need. When such a steel plate is seam-welded, the hardness of the nugget portion is relatively small due to the small Ceq, which is insufficient for the hardness of the base material reinforced by work hardening, so that stress concentration is relaxed. It was impossible to do so, and it resulted in cracking.

On the other hand, 62.3 ≦ HR30T-8
In the range satisfying 8.4 Ceq ≦ 68.2, the difference between the hardness of the base material strengthened by work hardening and the hardness of the nugget part determined depending on Ceq is sufficiently small so that the neck workability is not adversely affected. Therefore, it is considered that good neck workability could be secured.

From the above knowledge, in the present invention, in the relationship between Ceq and hardness expressed by HR30T, the range of 62.3≤HR30T-88.4Ceq≤68.2 is satisfied. Further, when the above-mentioned component range of the present invention is satisfied, Ceq is 0.054 ≦ Ceq ≦ 0.2.
The range of 00 is substantially satisfied.

[0039]

Next, an embodiment of the present invention will be described. A slab having the components shown in Table 1 was prepared as shown in Table 2.
After hot rolling to a thickness of 8 to 2.0 mm, it was pickled and rolled to 0.188 to 0.266 mm by a tandem cold rolling mill. After annealing this steel plate, 0.142 to 0.17
It was cold-rolled again to 9 mm and tin-plated.

The steel sheet thus obtained was paint-baked except for the varnishing part for welding and cut into a can body size. Then, this steel plate was applied to a pseudoronic welding machine FBB-
Welding using 5600, then performing neck-in processing using a can body die necker, then flange processing,
The workability was evaluated.

The workability was evaluated according to the following criteria based on the rate of occurrence of flange cracks when flange processing was performed after neck-in. Evaluation Flange crack ○ 0ppm △ 1-100ppm × 101ppm or more The results are shown in Table 2. The hardness values are also shown in Table 2.

As shown in Table 2, it was confirmed that all the steels of the present invention satisfying the range of the present invention have excellent neck formability after welding. On the other hand, it was confirmed that the comparative steels out of the scope of the present invention were inferior in neck formability or could not obtain sufficient material hardness.

[0043]

[Table 1]

[0044]

[Table 2]

[0045]

As described above, according to the present invention,
Provided is a steel plate for an ultra-thin welded can, which has excellent neck formability in a welded can and has high strength, and therefore can be applied to thinning a steel plate. The present invention has an extremely high economic value in terms of resource saving and energy saving such as the fact that the plate thickness of the steel plate can be reduced in this way.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between Ceq and HR30T.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshinori Yomura 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Shinsuke Watanabe 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Nippon Steel Tube Co., Ltd.

Claims (1)

[Claims] 1. C: 0.03 to 0.08% by weight,
Mn: 0.10 to 0.60%, Cr: 0.035 to 0.
100%, Si: less than 0.05%, P: less than 0.04%, S: 0.01 to 0.04%, N: 0.0010.
0.010%, sol. Al: 0.005 to 0.100
%, The balance consisting of Fe and unavoidable impurities, the Rockwell hardness (HR30T) is 70 or more and 85 or less, and the carbon equivalent Ceq is Ceq = [% C] +1/6 [% Mn] +1/5 [ % Cr], an ultra-thin steel plate for a welded can excellent in neck formability, which satisfies a relationship of 62.3 ≦ HR30T-88.4Ceq ≦ 68.2.