KR20020046581A - A method for manufacturing ultra-thin steel sheet for can with excellent welding workability - Google Patents

Abstract

PURPOSE: A method for manufacturing a thin steel plate for tube is provided to prevent the badness of welding caused by the concentration of stress at a shape discontinuous portion generated during a welding process. CONSTITUTION: In the method for manufacturing a thin steel plate for tube, a steel slab is composed of C:0.0020-0.010 wt percent; Si:less than 0.05 wt percent, Mn:0.3-0.6 wt percent; P: less than 0.04 wt percent; S: less than 0.04 wt percent, Al:0.020-0.10 wt percent, N: less than 0.0040 wt percent, Cr:0.08-0.15 wt percent, B:0.0005-0.0050 wt percent(the content of B is determined in relation to C), remainder Fe and other inevitable impurities, and all processes of the method are appropriately controlled to manufacture the thin steel plate for tube having a thickness of less than 0.21mm, HR30T, and a hardness of more than 61.

Description

Manufacturing method of ultra-thin steel sheet excellent in weldability {A METHOD FOR MANUFACTURING ULTRA-THIN STEEL SHEET FOR CAN WITH EXCELLENT WELDING WORKABILITY}

The present invention relates to a method for producing an ultrathin tubular steel sheet applied to a food pipe or a beverage tube, and more particularly, to a method for producing an ultrathin tubular steel sheet having a thickness of 0.21 mm or less having excellent flange workability near a welded portion.

Conventional steel sheets, such as tin plated steel sheets tin-plated on the surface of steel sheets or chromium steel sheets (TFS; Tin Free Steel) subjected to electrochromic acid treatment, are widely used in food or beverage tubes. These food pipes and beverage pipes are classified into three-piece pipes and two-piece pipes according to the difference in the manufacturing method thereof, and the three-piece pipes are classified according to the difference in the method of joining the pipe body. Although it is classified into a welded pipe, a welded pipe, a soldered pipe, and the like, the welded pipe is mainstream because of the small joining part (Lap) of the joining part and the strong joining strength, which is suitable for high-speed manufacturing.

In recent years, such a steel plate for welded pipes is required to further gauge down the steel sheet itself as a raw material for the purpose of lightening the tube and reducing the cost.

However, when the steel sheet is made extremely thin by thinning the sheet thickness of the steel sheet, it is easy to cause a decrease in tube strength such as paneling strength and flange cracks are easily generated. Steel sheet is required.

In particular, in welded tubes, the plate thickness of the tube body joint becomes thicker than that of the joint except for the stress concentration in the vicinity of the weld.

When welding, the part where the structure and mechanical properties changed due to heat input and pressure (junction and its vicinity), that is, the boundary between the heat-affected part and the part (base material) which is not substantially affected by the welding, is stressed in shape. This part is substantially disadvantageous to necking and flange processing because it is often almost the same position as the site, and flange cracks, which are a problem in the weld pipe making process, are often generated near the weld.

In addition, flange cracks frequently occur in the welded portion and the vicinity thereof due to the change in the structure of the welded portion due to the heat-affected portion at the time of welding, and the unevenness in workability of the base material portion and the welded portion other than the welded portion.

Therefore, the manufacturing method of the ultra-thin conventional steel sheet excellent in weldability is conventionally proposed. For example, Japanese Patent Application Laid-Open No. 2-118028 discloses an Nb-added ultra low carbon steel of C: ≤0.004% and Nb: ≤0.01% at continuous annealing temperature of 750-800 ° C, and subjected to work hardening at temper rolling to give a roughness T4. The above method of manufacturing the ultra-thin steel sheet for welded pipes is proposed. However, in this technique, since the addition of Nb is essential, the recrystallization temperature is high and the high temperature annealing must be performed at 750 ° C. or higher. In addition, in the production of ultra-thin conventional steel sheet having a plate thickness of 0.20 mm or less, the cold-rolled sheet thickness is also thinner than usual, so when such high temperature annealing is performed, it is easy to cause meandering and the like. This leads to an increase in manufacturing cost. Therefore, it is difficult to achieve the original purpose of achieving cost down of the pipe by gauge down of the material.

In Japanese Patent Laid-Open No. 3-294432 and Japanese Patent Laid-Open No. 7-109527, secondary cold rolling is performed on ultra low carbon steels of C: ≤0.0060% and N:> 0.0060%, and the plate thickness is 0.15mm or less and HR30T Disclosed is a method for manufacturing at least 62 ultra-thin steel sheets for welded pipes. However, these techniques have a problem of poor aging resistance because they contain a large amount of N exceeding 0.0060%. That is, there is a large deterioration of aging due to reflow treatment after tin plating, heat history during coating place, film lamination, or long time standing at room temperature up to pipe making, and by roll forming before welding. There was a problem that the fluctuation (Flutting) occurs in the body molding and the molding shape is unstable and the deviation occurs.

As another technique, Japanese Patent Laid-Open Publication Nos. 5-263143, Japanese Laid-Open Patent Publication No. 5-271755, Japanese Laid-Open Patent Publication No. 5-295427, and Japanese Laid-Open Publication No. 6-192744, which disclose a conventional steel sheet in which B is added to ultra low carbon steel. Although these are all soft steels of T3 or less, and are simply added boron for the purpose of preventing soft nitration or nitrogen aging, and no consideration is given to the effect of boron on flange workability near the welds. No consideration is given to the flange workability itself in the vicinity.

Therefore, the above-mentioned conventional techniques cannot produce ultra-thin steel sheets for hard welded pipes having a plate thickness of 0.21 mm or less and HR30T of 61 or more, which are excellent in weldability near the welds, which are recently required.

Accordingly, the inventors of the present invention have repeatedly conducted research and experiments to solve the above problems, and propose the present invention based on the results. The present invention derives the content of steel B from the relationship with C and manufactures the process. It is an object of the present invention to provide a method for producing an ultrathin tubular steel sheet having a thickness of 0.21 mm or less and an HR30T hardness of 61 or more, which can significantly improve the weldability defect due to the stress concentration at the shape discontinuous portion by welding.

1 is a graph showing the relationship between the B content and the flange crack incidence according to the carbon content

The present invention for achieving the above object,

By weight%, C: 0.0020 to 0.010%, Si: 0.05% or less, Mn: 0.3 to 0.6%, P: 0.04% or less, S: 0.04% or less, Al: 0.020 to 0.10%, N: 0.0040% or less, Cr : 0.08 ~ 0.15% is contained, B is 0.0005 ~ 0.0050% and added to satisfy 5 × 10 ^-6 / [C%] ~ 1 × 10 ^-5 / [C%]%, balance Fe and other unavoidable impurities The present invention relates to a method for producing an ultra-thin steel sheet having excellent weldability, which includes rolling after slab reheating at a temperature of at least ₃ Ar, pickling and cold rolling, followed by continuous annealing at above a recrystallization temperature, and temper rolling. .

In addition, the present invention relates to a method for producing a conventional steel sheet having a thickness of 0.21 mm or less and HR30T of 61 or more.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

The inventors of the present invention have investigated in detail the cause of flange cracking in the vicinity of the weld in detail, and researches and experiments on a method for resolving the flange, the weld is thicker than the other parts, it is easy to cause stress concentration Therefore, it has been found that the occurrence of flange cracks can be suppressed by increasing the tissue uniformity of the base material portion and the weld heat affected zone.

The weld heat affected zone is roughly divided into a nugget that causes tissue change with transformation in the tissue change behavior and a peripheral portion that causes changes such as recrystallization without transformation. In the case of seam welding in the proper current range, the temperature of the material is the highest in the nagette part of the welding center, so the structure of the material is transformed into austenite and becomes single phase of austenite or two phases of ferrite and austenite, and then cooled to room temperature. Transform into a stable phase.

On the other hand, the post-weld structure of the nagget portion contributes to the material conditions (particle size, size of processing deformation, precipitate size, etc.) and welding conditions (heating rate, maximum attainment temperature, cooling rate, etc.) Depends mainly on the material composition, especially the carbon content. That is, when there is too much carbon amount, in this part, many low temperature transformation phases will precipitate and it will harden excessively. Therefore, in order to suppress the structure of the junction center, it is necessary to make the addition amount of carbon into an appropriate range.

The inventors of the present invention have studied from this point of view, and found that when the amount of C exceeds 0.01%, precipitates of low temperature transformation rapidly increase, resulting in poor tissue uniformity. As a result of examining the method of suppressing the grain coarsening of the heat-affected portion in the C range of 0.01% or less where the nagget portion is not excessively hardened, it was found that adding an appropriate amount of B is extremely effective. In addition, by reducing the amount of carbon to the ultra-low carbon region, it avoids the formation of a low temperature transformation phase that is vulnerable to hardening due to the heat effect during welding, and at the same time, by adding an appropriate amount of B, the welding part accompanying ultra-low carbonization and its assembly It was found that suppressing fire is effective for improving weldability.

As a result of this experiment, in the case of the steel without B, the weld crack incidence is lowered by the amount of carbon up to 0.01%, but the weld crack incidence is rapidly increased when the amount is less than 0.0050%. there was. This means that in the case where the carbon content is relatively high, the flange workability is deteriorated because the low temperature transformation phase is generated in the welded portion, and when the carbon amount decreases, the welded portion and the vicinity thereof are assembled, so that local shrinkage occurs and flange cracking is likely to occur.

Accordingly, it has been found that the control of the carbon amount alone does not provide sufficient weldability, and in particular, the effect of improving the weldability due to the addition of B becomes apparent in an extremely low carbon region having a carbon content of 0.01% or less.

This is because the uniformity of the structure of the base material and the welded part is improved by the addition of B. B is segregated at the ferrite grain boundary to suppress the grain growth of the heat affected zone and to increase the recrystallization temperature to suppress the recrystallization of the heat affected zone. A composite effect, such as making austenite grains refine | miniaturize in the site | part which reached high temperature above transformation point in a weld part, is exhibited.

However, it can be seen that there is an optimum content range because the content of B is too high or at least the weldability tends to deteriorate.

In addition, the content of B is also related to the content of C. When the content of C is small, it is necessary to add a relatively large amount of B in order to suppress the assembly of the vicinity of the welded part and to improve the flange workability. In many cases, the effect can be achieved even with the addition of a minimum amount of B.

Hereinafter, a steel component and a manufacturing process are demonstrated.

C is an element that suppresses the structure near the weld and improves the flange workability near the weld. If the content is less than 0.0020%, the structure of the weld is easy to assemble, and the addition of B suppresses the nonuniformity of the weld and the base metal. It is difficult to obtain good weldability. In addition, in the case where the content of C is less than 0.0020%, in order to make the HR30T ≥ 61 hardness required as the ultra-thin steel sheet for welding pipes of 0.21 mm or less, excessive reinforcing elements such as Mn and P must be added and secondary rolling with high pressure reduction rate It is difficult to obtain the required paneling strength as a weld tube without deteriorating the weldability.

On the other hand, if the content of C is 0.01% or more, it is difficult to form low-temperature transformation phases in the heat affected zone during welding, so that the weld uniformity and the structure uniformity with the base material portion are degraded. In addition, a large amount of hard phases are precipitated by heat input during welding, which adversely affects the neck workability.

Therefore, the content of C is preferably set to 0.0020 to 0.010%.

Since Si is an element which remains in steel as an impurity to embrittle steel sheet and degrades corrosion resistance, it is preferable to limit the content range to 0.05% or less in order to avoid such an adverse effect.

Mn is an element that prevents hot cracking of the slab by depositing sulfur in steel as MnS, and at the same time secures the strength of the tube as a solid solution strengthening element. Thus, in order to precipitate and fix the sulfur to secure the steel sheet strength and hardness, 0.3% or more should be added. However, when excessively added, it is advantageous to improve the strength of the steel sheet, but the hardenability increases, so that the welded part becomes brittle, resulting in deterioration of workability. Therefore, the upper limit is preferably limited to 0.6%.

P is a substituted solid solution element similar to Mn, and has a large reinforcing ability of Mn or more, which is effective for achieving high strength of the steel sheet, but segregates at the ferrite grain boundary to embrittle the grain boundary, resulting in a decrease in flange formability. Moreover, since corrosion resistance is also reduced, it is preferable to limit the content to 0.04% or less.

S should be controlled below 0.04% to prevent hot cracking of the slab. In addition, when the content of S exceeds 0.04%, it is easy to cause hot brittleness.

Al is an element added to precipitate nitrogen in the steel as AlN, and when the content is less than 0.02%, most of the added B forms BN, so that the effect of improving the flange workability due to the addition of B is weakened. In addition, since the Al inclusion easily occurs when the content of Al exceeds 0.1%, it is preferable to add it at 0.02 to 0.10%.

N is so preferable that N is sufficient in order to fully exhibit the addition effect of B which suppresses granulation of the weld part vicinity and improves flange workability. That is, when there is much N in steel, even if it optimizes content of Al and B, BN will be easy to form and the effect of B addition will become weak and the improvement effect of weldability may not be fully exhibited. In addition, when a large amount of N is present, fluctuation may occur during roll forming due to nitrogen aging, and the shape may be unstable, causing variations. Therefore, the content of N is preferably limited to 0.0040% or less.

Cr is an element added for reinforcement. If the content is less than 0.08%, it is difficult to obtain a reinforcing effect corresponding to HRT30T hardness of 61 or more. If the content exceeds 0.15%, the reinforcing effect of the steel sheet is increased, but the amount of chromium is increased. Since it raises the manufacturing cost, it is preferable to limit the component range to 0.08 to 0.15%.

B is the most important element in the present invention, B segregated at the ferrite grain boundary suppresses coarsening in the vicinity of the welded part and increases the uniformity of structure and workability of the welded part and the base metal part, thereby reducing the occurrence of flange cracks near the welded part. However, the content range is preferably set in consideration of the content of C. In the present invention, a number of experiments were repeated about the relationship between the content of B and the content of C, and the results as shown in FIG. 1 were obtained. That is, Figure 1 is a graph showing the flange crack incidence rate according to the content of B and setting the content of C in the range of 0.0020 ~ 0.010%, the inventors of the present invention from this result, the optimum amount of B corresponding to the amount of C It was derived as in the following relation (1).

[Relationship 1]

B (%): 5 × 10 ^-6 / [C%] to 1 × 10 ^-5 / [C%]

When the content of B is less than 5 × 10 ⁻⁶ / [C%], the tube making property of the product is deteriorated, and in order to prevent this, the winding temperature of hot rolling must be increased, which is not preferable. On the other hand, when the content of B exceeds 1 × 10 ⁻⁵ / [C%], formation of boron oxide serving as a crack origination and formation of low temperature transformation phase are promoted, and weldability is deteriorated. In addition, there is a problem that the recrystallization temperature is significantly increased and the ferroalloy cost also increases.

On the other hand, in the present invention, while satisfying the content of the above relation (1), B is preferably added within the range of 0.0005 to 0.0050%. The reason is that if the B content is less than 0.0005%, the tube manufacturing characteristics of the product are deteriorated, so that the hot-rolling coiling temperature needs to be increased during the providing process. If the content of B is more than 0.005%, boron oxide, which is the starting point of cracking, is formed and the low temperature transformation This is because the production is accelerated and the weldability decreases.

After reheating the steel slab formed as described above, the rough rolling is carried out or omitted, and directly inserted into a hot rolling mill to perform hot rolling. At this time, when the hot rolling temperature is less than ₃ Ar, the grain size of the surface layer of the hot rolled steel sheet may be coarsened in response to the coiling temperature, and it is easy to obtain a mixed structure such that a processing structure exists in the center of the plate thickness. In addition, since the structure of the final product tends to be mixed, as a result, the material deviation increases and the flange workability deteriorates. Therefore, the finishing rolling temperature is preferably set to Ar ₃ or more. On the other hand, the slab heating is preferably carried out at 1100 ~ 1250 ℃ which is a normal temperature range.

Thereafter, the hot rolled sheet is wound in a conventional manner, followed by pickling and cold rolling, continuous annealing and then temper rolling. At this time, a preferable winding temperature is 500-700 degreeC, and the reduction ratio of the said cold rolling and temper rolling does not need to specify especially, According to a conventional method according to hot-thin plate | board thickness and a product plate | board thickness, it implements. On the other hand, it is preferable to perform annealing after cold rolling by continuous annealing from a viewpoint of productivity and manufacturing cost, and it is preferable to set annealing temperature more than recrystallization temperature. The reason is that when the annealing temperature is lower than the recrystallization temperature, the microcrystalline structure is partially present, the uniformity of the material and the structure is lowered, and the weldability is deteriorated.

The cold rolled steel sheet produced as mentioned above becomes a thin steel sheet whose thickness is 0.21 mm or less, and HR30T shows 61 or more.

Thereafter, the steel sheet is subjected to various surface treatments such as ultrathin tin plating or tin-nickel plating and then chemical conversion. In the present invention, the kind of such surface treatment is not particularly limited, and any effect of the surface treatment is sufficiently exhibited.

Hereinafter, the present invention will be described in more detail with reference to Examples.

(Example)

The steel slab, as shown in Table 1 below, was heated to 1,180 ° C, hot rolled at 870 ° C, and wound up at 710 ° C. Thereafter, pickling and cold rolling were performed to prepare steel sheets having a thickness of 0.17 to 0.21 mm, respectively, ultrathin tin plating and chemical conversion treatment were performed, and then HR30T hardness was measured. As a result, if HR30T ≥ 61, which is a level that can secure the tube strength, it is expressed as O, and HR30T <61, ×.

On the other hand, the steel sheet manufactured as described above was subjected to core welding of the tube body after blanking and blanking, and processed into a 250CC beverage tube. Then, flange processing was performed after the necking process of the pipe edge part.

In order to clearly detect the difference in workability between materials, flange processing was performed more severely than usual. That is, flange work whose circumferential elongation of a pipe edge part corresponds to 25% was performed, and the flange workability by the flange crack incidence rate (flange crack irrigation / flange work irrigation) was evaluated. As a result, if the flange cracking rate is 3% or less, the customer can perform the flange processing without problems during the manufacturing process, so 3% or less was evaluated as good. In Table 2 below, 1% or less and 3% or less were evaluated. More than 3% was marked with x.

The weldability was evaluated by visual observation of the weld using a commercially available welder to test the fracture of the weld in a tearing test on the weld at the maximum welding current at which no splash was observed. As a result, after the tearing test (Tearing Test) if the weld break is generated by ×, and if good without breaking was indicated as ㅇ.

The results of the evaluations are shown in Table 2 below.

division Steel component (% by weight) C Si Mn P S N Al B Cr Inventive Example 1 0.0025 0.01 0.35 0.021 0.021 0.0019 0.043 0.0035 0.09 Inventive Example 2 0.0028 0.01 0.39 0.018 0.016 0.0021 0.035 0.0027 0.10 Inventive Example 3 0.0030 0.01 0.36 0.010 0.013 0.0021 0.031 0.0025 0.12 Inventive Example 4 0.0036 0.02 0.41 0.009 0.008 0.0020 0.038 0.0020 0.12 Inventive Example 5 0.0041 0.01 0.32 0.017 0.009 0.0018 0.041 0.0020 0.11 Inventive Example 6 0.0054 0.04 0.45 0.021 0.011 0.0013 0.034 0.0013 0.12 Inventive Example 7 0.0057 0.02 0.56 0.009 0.013 0.0015 0.041 0.0012 0.10 Inventive Example 8 0.0061 0.02 0.35 0.013 0.017 0.0017 0.051 0.0012 0.08 Inventive Example 9 0.0079 0.01 0.39 0.031 0.017 0.0011 0.051 0.0007 0.11 Inventive Example 10 0.0091 0.02 0.38 0.018 0.016 0.0011 0.035 0.0007 0.09 Comparative Example 1 0.0011 0.01 0.35 0.015 0.018 0.0019 0.028 0.0048 0.08 Comparative Example 2 0.0026 0.03 0.37 0.021 0.025 0.0024 0.015 － 0.11 Comparative Example 3 0.0129 0.01 0.32 0.014 0.013 0.0018 0.022 0.0005 0.10 Comparative Example 4 0.0035 0.02 0.35 0.012 0.015 0.0054 0.028 0.0023 0.11 Comparative Example 5 0.0040 0.02 0.39 0.017 0.019 0.0024 0.029 0.0008 0.09 Comparative Example 6 0.0042 0.01 0.34 0.012 0.025 0.0018 0.037 0.0013 － Comparative Example 7 0.0049 0.02 0.68 0.020 0.017 0.0025 0.038 0.0015 0.10 Comparative Example 8 0.0050 0.02 0.32 0.016 0.019 0.0019 0.041 0.0025 0.11 Comparative Example 9 0.0062 0.03 0.35 0.015 0.022 0.0015 0.035 0.0005 0.10 Comparative Example 10 0.0069 0.02 0.34 0.014 0.021 0.0017 0.051 0.0019 0.12

division Plate thickness (mm) Hardness (HR30T) Flange Machinability Weldability Inventive Example 1 0.21 O ◎ O Inventive Example 2 0.20 Inventive Example 3 0.19 Inventive Example 4 0.19 O Inventive Example 5 0.17 ◎ Inventive Example 6 0.17 Inventive Example 7 0.18 O Inventive Example 8 0.20 ◎ Inventive Example 9 0.17 Inventive Example 10 0.21 Comparative Example 1 0.17 X X X Comparative Example 2 0.18 O Comparative Example 3 0.21 Comparative Example 4 0.18 Comparative Example 5 0.17 Comparative Example 6 0.21 X O O Comparative Example 7 0.19 O X X Comparative Example 8 0.20 Comparative Example 9 0.21 Comparative Example 10 0.19 * HR30T: Displayed as 'O' if 61 or more, 'X' if less than 61 * Flange cracking rate (flange workability): '◎' if 1% or less, 'O' if 1% or less, and 3% or more Is displayed as 'X' * Weldability: 'O' if good, 'X' if bad

As shown in Table 2, Examples (1) to (10) of the present invention are all hardness (HR30T) of 61 or more, it can be seen that the flange workability and weldability is also excellent.

However, Comparative Example (1) having a low content of C had a HR30T hardness of 57, which was low in hardness, and the welded structure was degraded, resulting in poor weldability and flange workability due to nonuniformity in the welded portion and the base metal. In addition, the comparative example (3) having a high C content exhibited an appropriate hardness as HR30T 64, but the weldability and flange workability deteriorated due to the brittleness of the welded portion and the uneven structure of the base material portion.

Comparative Examples (2) without B added and Comparative Examples (5), (8), (9), and (10) in which the amount of B did not satisfy the above relational expression (1), had the uniformity of the structure of the welded part and the base material part. Low flange workability and weldability were confirmed.

In the comparative example (4) which contained N excessively, boron which improves weldability turns into BN, the boron addition effect fell, and weldability and workability deteriorated.

Comparative Example (6), in which Cr was not added as a hardness strengthening element, had a HR30T hardness of 54, making it impossible to secure tube strength.

Moreover, the comparative example (7) which added Mn excessively increased hardenability and deteriorated workability by the brittleness of a weld part.

According to the present invention as described above, by setting the optimum amount of B corresponding to the amount of C to control the structure of the welded portion and the heat affected zone of the welded tube, it is possible to efficiently manufacture the ultra-thin conventional steel sheet which greatly improves the weldability defects. It can be effective.

Claims (2)

By weight%, C: 0.0020 to 0.010%, Si: 0.05% or less, Mn: 0.3 to 0.6%, P: 0.04% or less, S: 0.04% or less, Al: 0.020 to 0.10%, N: 0.0040% or less, Cr : 0.08 ~ 0.15% is contained, B is 0.0005 ~ 0.0050% and added to satisfy 5 × 10 ^-6 / [C%] ~ 1 × 10 ^-5 / [C%]%, balance Fe and other unavoidable impurities Method for producing ultra-thin steel sheet with excellent weldability, including re-heating steel slab, which is composed of hot-rolled at a temperature of at least ₃ Ar, pickling and cold rolling, followed by continuous annealing at above recrystallization temperature and temper rolling. The method of claim 1, wherein the conventional steel sheet has a thickness of 0.21 mm or less and HR30T of 61 or more.