JP4380010B2 - Ultra-low carbon cold-rolled steel sheet excellent in homogeneity and surface appearance after forming and method for producing the same - Google Patents

Description

【００３２】
【表２】

【００３３】
【表３】

【００３４】
表３から明らかなように、この発明に従い得られたものはいずれも、機械的諸特性に優れるだけでなく、硬さ（HR30T)のバラツキが１以下小さく、また表面外観も良好であった。
さらに、試験片すべてについて、再結晶が終了していることが確認された。
【００３５】
【発明の効果】
かくして、この発明によれば、缶容器用鋼板としての諸特性に優れるのはいうまでもなく、鋼板長手方向にわたる各特性のバラツキが極めて小さく、しかも表面外観にも優れた極低炭素冷延鋼板を安定して得ることができる。
【図面の簡単な説明】
【図１】 ホットコイルの冷却速度を、先端部（LE）、中央部（Middle）および後端部（TE）で比較して示した図である。
【図２】 微細なNbＣが析出したコイル先後端部における「生焼け」の発生状況の説明図である。
【図３】 鋼中のＣおよびNb（Ｃ当量）が降伏点伸び（Y.el）に及ぼす影響を示したグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an ultra-low carbon cold-rolled steel sheet excellent in homogeneity and surface appearance after forming processing, suitable as a tin plate for can containers such as food cans and beverage cans, and a base plate for surface-treated steel sheets, and its It relates to a manufacturing method.
[0002]
[Prior art]
Steel plates for can containers such as food cans and beverage cans tend to be thinned to reduce can costs. However, if the plate thickness is reduced, the strength of the can decreases accordingly. Conventionally, for example, the strength has been improved by making it harder by reducing the crystal grain size.
[0003]
However, if the material is hardened, the workability during cold rolling or temper rolling and the deep drawability during canning are deteriorated.
Moreover, the workpiece of the can body part is subjected to flange processing for attaching a canopy after being formed on the can body, and so-called flange cracking may occur at this time.
For this reason, development of the material which does not cause the deterioration of workability as mentioned above was desired.
[0004]
As a steel plate for a can container having good workability as described above, for example, the inventors previously developed a steel plate for a two-piece can based on an Nb-added ultra-low carbon steel, and disclosed in JP-A-8-92695. Disclosed in.
The development of this steel sheet has made it possible to effectively compensate for the lack of can strength that accompanies the reduction in sheet thickness without incurring deterioration in workability during rolling or can manufacturing.
[0005]
However, when such an Nb-added ultra-low carbon steel material is actually manufactured on an industrial scale, the material variation in the coil longitudinal direction, specifically, the hardness at the tip and rear ends of the coil is greater than that at the center. Higher variations were often observed.
Such a portion having high hardness causes the above-described adverse effects, and therefore must be discarded, resulting in a significant decrease in yield. Moreover, in such a steel material, the appearance of the container after canning may become defective due to the elongation at yield of the material.
[0006]
[Problems to be solved by the invention]
The present invention advantageously solves the above-mentioned problems, has no variation in hardness in the longitudinal direction of the coil even during production on an industrial scale, and has an extremely low surface carbon appearance after molding. The object is to propose a cold-rolled steel sheet together with its advantageous production method.
[0007]
[Means for Solving the Problems]
In order to achieve the above object, the inventors first investigated the cause of the material variation, and obtained the following knowledge.
That is, as shown in FIG. 1, the front end portion (LE) and the rear end portion (TE) of the hot coil have a higher cooling rate after winding the hot rolled coil than the center portion (Middle). In the cooling process, NbC is not sufficiently precipitated and coarsened, and solid solution C also remains, so that the remaining C precipitates as fine NbC during annealing after cold rolling.
In this respect, the middle part of the hot coil has a slow cooling rate after winding the hot-rolled coil, and NbC is sufficiently precipitated and coarsened during the cooling process, so that almost no solid solution C remains. In addition, fine NbC is not newly deposited during annealing after cold rolling.
[0008]
As described above, since the recrystallization temperature rises at the coil front end and the rear end where fine NbC is deposited, the front end and the rear end are not recrystallized depending on the annealing conditions as shown in FIG. In other words, a so-called “burnt” state is obtained, and as a result, the front end portion and the rear end portion are harder than the central portion.
Also, even if the annealing temperature is higher than the temperature at which the recrystallization is completed at the coil front end and the rear end, the grain growth and the like are greatly different between the center portion of the coil having a low recrystallization end temperature and the front and rear end portions of the coil. , Material variation will increase.
[0009]
Then, as a result of intensive studies to solve the above problems, the inventors have obtained the following knowledge.
(1) In order to prevent the fine precipitation of NbC at the front and rear ends of the coil as described above, it is necessary to strictly control the component system, and particularly Nb must be limited to a narrow range of 0.010 to 0.012 mass%. is there.
[0010]
(2) When Nb is limited to the above-mentioned narrow range, the yield point elongation becomes large and the surface appearance after forming may be deteriorated. This can be solved by controlling to an appropriate range in view of the above.
Fig. 3 shows the effects of C and Nb (C equivalent) in steel on yield point elongation (Y.el) after temper rolling at 1 to 2% and after aging treatment (210 ° C, 20 minutes). As shown in the figure, the results of investigating the effects are shown. By limiting (C% -12 / 93 · Nb%) to 0.0005% or less, the yield point elongation after processing can be significantly reduced. it can. In addition, if the yield point elongation after the aging treatment is 1% or less, the surface appearance after the can manufacturing has no problem.
[0011]
(3) Furthermore, it is more effective for preventing fine precipitation of NbC to set the coiling temperature higher after the hot rolling during the manufacturing process.
The present invention is based on the above findings.
[0012]
That is, the present invention is, in mass percentage, C: 0.0010 to 0.0020%,
Si: 0.04% or less,
Mn: 0.1-0.5%
P: 0.02% or less,
S: 0.005 % or less,
N: 0.005% or less,
Al: 0.01-0.05%
Nb: 0.010 to 0.012%
And C and Nb are represented by the following formula (C% -12 / 93 · Nb%) ≦ 0.0005%
This is an ultra-low carbon cold-rolled steel sheet excellent in homogeneity and surface appearance after forming, characterized in that the balance is the composition of Fe and inevitable impurities.
[0013]
Moreover, this invention is the mass percentage C: 0.0010-0.0020%,
Si: 0.04% or less,
Mn: 0.1-0.5%
P: 0.02% or less,
S: 0.005 % or less,
N: 0.005% or less,
Al: 0.01-0.05%
Nb: 0.010 to 0.012%
And C and Nb are represented by the following formula (C% -12 / 93 · Nb%) ≦ 0.0005%
The steel slab with the composition of Fe and unavoidable impurities in the balance was rolled up at a temperature of 600-800 ° C after hot rolling, and then cold-rolled at a reduction rate of 80% or more. Then, a method for producing an ultra-low carbon cold-rolled steel sheet excellent in homogeneity and surface appearance after forming, characterized by performing continuous annealing at a temperature not lower than the recrystallization temperature and not higher than 800 ° C.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
The present invention will be specifically described below.
First, the reason why the composition of the steel sheet is limited to the above range in the present invention will be described. In addition,% display of the chemical composition shown below is all the mass percentage.
C: 0.0010-0.0020%
C is one of the important components in the present invention. As the amount of C increases, the amount of dissolved C also increases, and as shown in FIG. 3, the elongation at the yield point increases and surface roughness occurs. In order to prevent this problem, it is sufficient to increase the amount of Nb, which is an immobilized element of solute C. However, as will be described later, in the present invention, the amount of Nb is set to a predetermined narrow value from the viewpoint of preventing material variations. It must be limited to a range, and therefore the amount of C must be strictly controlled. Here, when the C content exceeds 0.0020%, surface roughness due to the above-described yield point elongation occurs, so the upper limit of the C content is set to 0.0020%. On the other hand, if the C content is less than 0.0010%, there is a concern about the adverse effect on the appearance of the product due to the coarsening of crystal grains, so the lower limit was set to 0.0010%.
[0015]
Si: 0.04% or less
If Si is contained in a large amount, not only does the steel harden significantly, but the amount of oxidation during surface treatment increases, and problems such as peeling of the plating layer due to prolonged heating occur. It was made to contain below.
[0016]
Mn: 0.1 to 0.5%
Mn is an element effective for preventing hot brittle cracking due to S, and needs to be contained according to the amount of S in the steel. However, if contained in a large amount, the steel sheet becomes hard and causes a significant decrease in ductility. Therefore, Mn is contained in the range of 0.1 to 0.5%. More preferably, it is 0.2 to 0.4%.
[0017]
P: 0.02% or less P not only hardens the steel and deteriorates deep drawability and flange workability during canning, but also significantly inhibits corrosion resistance, so it was limited to 0.02% or less.
[0018]
S: 0.005 % or less S is an element that reduces the ductility of the steel sheet and causes brittleness and deterioration of corrosion resistance. Therefore, its mixing is preferably reduced as much as possible, but 0.005 % or less is allowed.
[0019]
Al: 0.01-0.05%
Al is an element necessary for precipitating and fixing solute N as AlN. To improve the deep drawability and flange workability during canning, it is necessary to contain at least 0.01% or more. If excessively added, not only will the cost increase, but the flange workability will deteriorate during can making due to the increase in inclusions, so the Al content is limited to the range of 0.01 to 0.05%.
[0020]
N: 0.005% or less N, when present in a solid solution state, hardens the steel sheet and deteriorates deep drawability and flange workability during canning, so is limited to 0.005% or less.
[0021]
Nb: 0.010 to 0.012%, (C% -12 / 93 · Nb%) ≤ 0.0005%
Nb is an important element in the present invention whose content should be strictly controlled. That is, Nb is a useful element that improves the deep drawability (r value), which is an important characteristic of steel plates for cans, and the in-plane anisotropy of r value, which is particularly important for 2-piece can materials. As described above, when NbC is finely deposited on the rear end of the coil tip in continuous annealing after cold rolling, the material varies. Here, if the content is less than 0.010%, it is considered that the precipitation of NbC at the time of winding is delayed, but the variation of the material becomes large. On the other hand, if it exceeds 0.012%, there is a concern about the fine precipitation of NbC at the rear end of the coil tip, so Nb was limited to the range of 0.010 to 0.012%.
Nb also contributes effectively to the fixation of solute C, and as shown in FIG. 3, the two satisfy the following formula: (C% −12 / 93 · Nb%) ≦ 0.0005% By controlling within the range, it is possible to effectively prevent the occurrence of yield point elongation due to the solid solution C and the deterioration of the surface appearance after the forming process.
[0022]
In addition to the above-mentioned Nb, Ti, etc. can be considered as a fixed element of solute C, but when Ti is added, a relatively large inclusion is formed, and there is a concern about the occurrence of flange cracking during can manufacturing. Therefore, Ti is not contained in this invention.
[0023]
Next, the manufacturing method according to the present invention will be described.
Slab heating temperature: 1150 ~ 1300 ℃
If the slab heating temperature is less than 1150 ° C, it will be difficult to secure a sufficiently high finishing temperature in hot rolling. On the other hand, if it exceeds 1300 ° C, the surface properties of the steel will eventually deteriorate. Is preferably about 1150 to 1300 ° C.
[0024]
Finishing temperature during hot rolling (FDT): 880 ° C or more If the finishing temperature during hot rolling is less than 880 ° C, the hot rolling property may deteriorate, so the finishing temperature should be 880 ° C or more. It is preferable. More preferably, it is 930 ° C or higher.
[0025]
Winding temperature: 600-800 ° C
If the coiling temperature is too low, the shape of the hot-rolled sheet deteriorates, which not only hinders pickling and cold rolling in the next process, but also increases the cooling rate of the front and rear end portions of the hot-rolled coil, resulting in solid solution C As a result, fine NbC precipitates during annealing after cold rolling, and material variations occur especially at the coil tip rear end.
Therefore, in the present invention, winding is performed at a temperature as high as possible, that is, a temperature of 600 ° C. or higher. However, when the coiling temperature exceeds 800 ° C, a structure in which carbide aggregates is formed in the hot-rolled coil, which not only adversely affects the corrosion resistance of the steel sheet, but also increases the scale thickness that occurs on the steel sheet surface. The upper limit of the coiling temperature was set to 800 ° C because it would be difficult to remove the scale sufficiently in the process.
[0026]
Rolling ratio during cold rolling: 80% or more If the rolling ratio during cold rolling is less than 80%, sufficient deep drawability cannot be obtained, so the lower limit of the rolling ratio is set to 80%. In order to reduce the in-plane anisotropy of a steel plate made of extremely low carbon steel as in the present invention, the rolling reduction is preferably 85 to 95%.
[0027]
Annealing temperature: Recrystallization temperature or higher, 800 ° C or lower The annealing temperature during continuous annealing must be at least the recrystallization temperature or higher.
Normally, when steel made of Nb is used as a raw material, as described above, the recrystallization end temperature tends to be extremely high at the end in the longitudinal direction of the coil, but in this invention, the components are adjusted very strictly. Therefore, the recrystallization end temperature difference in the coil longitudinal direction is small, and it is easy to ensure an annealing temperature not less than the recrystallization temperature over the entire length of the coil.
However, if the recrystallization temperature becomes too high, the risk of causing defects such as heat buckles and plate breakage during continuous annealing not only increases, but surface treatment properties deteriorate due to increased surface concentration. Was 800 ° C. More preferably, it is 780 ° C. or lower.
[0028]
Temper rolling reduction ratio: 0.5 to 10.0%
After annealing, it is preferable to perform temper rolling of about 0.5 to 3.0% for the purpose of shape adjustment and the like. In addition, this type of steel sheet can be tempered according to each application by controlling the reduction ratio. However, if the reduction ratio exceeds 10.0%, the steel sheet becomes extremely hard, and sufficient canning workability is achieved. In addition, since the flange workability cannot be obtained, the rolling reduction in the temper rolling is preferably about 0.5 to 10.0%.
[0029]
【Example】
Steel slabs having various component compositions shown in Table 1 were heated to 1200 ° C., and then subjected to hot rolling, cold rolling, continuous annealing and temper rolling under the conditions shown in Table 2 to obtain products.
The specimens were collected from the coil tip of the ultra-low carbon steel plate thus obtained (position 5 m from the tip), the coil center and the coil rear end (position 5 m from the back), and observed for structure. Table 3 shows the results of examining the mechanical properties (hardness (HR30T), r value, Δr and material variation) and surface appearance.
[0030]
Here, the material variation means a difference between the maximum value and the minimum value of the values investigated at the coil front end portion, the central portion, and the rear end portion, and Table 3 shows the hardness variation.
Moreover, the structure | tissue observation of the plate | board thickness cross section of the extract | collected test piece was performed with the optical microscope, and it was determined whether recrystallization was complete | finished.
Furthermore, the r value and Δr are the in-plane anisotropy of the average r value and the r value defined by the following equations.
r value = (r _L + 2r _D + r _C ) / 4
Δr = (r _L −2r _D + r _C ) / 2
However, r _L , r _D , and r _C are r values in the rolling direction (r _L ) of the steel sheet, 45 ° direction (r _D ) with respect to the rolling direction, and 90 ° direction (r _C ) with respect to the rolling direction, respectively. (Rankford value).
In addition, since surface appearance defects after canning are due to yield point elongation (Y.el), they should be evaluated by Y.el. If this value is 1% or less after aging treatment, The surface appearance of can be said to be good. Yield point elongation was evaluated by performing a tensile test after processing a JIS No. 5 tensile test piece and performing an aging treatment (210 ° C., 20 minutes). The evaluation results are shown in Table 3 on behalf of the center portion of the plate width.
[0031]
[Table 1]

[0032]
[Table 2]

[0033]
[Table 3]

[0034]
As is apparent from Table 3, all of the products obtained in accordance with the present invention were not only excellent in mechanical properties, but also had a hardness (HR30T) variation of less than 1 and good surface appearance.
Further, it was confirmed that recrystallization was completed for all the test pieces.
[0035]
【The invention's effect】
Thus, according to the present invention, it is needless to say that the various characteristics as a steel plate for can containers are excellent, and the variation of each characteristic in the longitudinal direction of the steel plate is extremely small, and the ultra-low carbon cold-rolled steel plate is excellent in surface appearance. Can be obtained stably.
[Brief description of the drawings]
FIG. 1 is a diagram showing the cooling rate of a hot coil by comparing a front end portion (LE), a central portion (Middle), and a rear end portion (TE).
FIG. 2 is an explanatory view of the occurrence state of “burnt” at the coil front and rear end portions where fine NbC is deposited.
FIG. 3 is a graph showing the effect of C and Nb (C equivalent) in steel on yield point elongation (Y.el).

Claims (2)

In mass percentage C: 0.0010-0.0020%,
Si: 0.04% or less,
Mn: 0.1-0.5%
P: 0.02% or less,
S: 0.005 % or less,
N: 0.005% or less,
Al: 0.01-0.05%
Nb: 0.010 to 0.012%
And C and Nb are represented by the following formula (C% -12 / 93 · Nb%) ≦ 0.0005%
An ultra-low carbon cold-rolled steel sheet excellent in homogeneity and surface appearance after forming, characterized by satisfying the above relationship, with the balance being a composition of Fe and inevitable impurities. In mass percentage C: 0.0010-0.0020%,
Si: 0.04% or less,
Mn: 0.1-0.5%
P: 0.02% or less,
S: 0.005 % or less,
N: 0.005% or less,
Al: 0.01-0.05%
Nb: 0.010 to 0.012%
And C and Nb are represented by the following formula (C% -12 / 93 · Nb%) ≦ 0.0005%
The steel slab with the balance of Fe and the unavoidable impurities in the balance was rolled up at a temperature of 600-800 ° C after hot rolling, and then cold rolled at a reduction rate of 80% or more. Then, a method for producing an ultra-low carbon cold-rolled steel sheet excellent in homogeneity and surface appearance after forming, characterized by performing continuous annealing at a temperature not lower than the recrystallization temperature and not higher than 800 ° C.

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