JP3632182B2 - Method for producing DTR can compatible steel sheet with excellent side wall break resistance - Google Patents

Description

【００７８】
【表２】

【００７９】
実施例２
転炉溶製後、軽圧下連続鋳造によりスラブとし、１２００℃に加熱後、仕上温度８４０℃、巻取温度５４０℃で板厚１．６ｍｍの熱延鋼板とした。これら熱延鋼板を酸洗後、０．２００ｍｍまで冷間圧延し、表３に示す焼鈍条件で連続焼鈍した。その後、圧下率１０％のＤＲ圧延により０．１８０ｍｍに仕上げ、実施例１と同一条件でＴＦＳ、ＰＥＴラミネート鋼板とし、高速ドロービード試験に供した。
【００８０】
これらの鋼板の化学組成は表１のＡ〜Ｍの通りである。
得られた結果を焼鈍条件とともに表３に示す。本発明の方法で製造された鋼板は比較鋼板に比べ、優れたＤＴＲ成形性を有している。
【００８１】
【表３】

【００８２】
実施例３
転炉溶製後、軽圧下連続鋳造によりスラブとし、１２００℃に加熱後、仕上温度８４０℃、巻取温度６００℃で板厚１．６ｍｍの熱延鋼板とした。これら熱延鋼板を酸洗後、０．１８５ｍｍまで冷間圧延し、均熱温度７３０℃、過時効温度３５０℃の条件で連続焼鈍した。その後、調質圧延により０．１８０ｍｍに仕上げ、実施例１と同一条件でＴＦＳ、ＰＥＴラミネート鋼板とし、高速ドロービード試験に供した。
【００８３】
これらの鋼板の化学組成は表１のＡ〜Ｍの通りである。
得られた結果を表４に示す。調質圧延のみのＳＲ材とした場合にも、本発明鋼の優れたＤＴＲ適合性が認められる。
【００８４】
【表４】

【００８５】
【発明の効果】
本発明の鋼板の製造方法によれば、側壁破断することなく従来よりも側壁部薄肉化率の高いＤＴＲ缶を製造することが可能となり、ＤＴＲ缶の缶体軽量化を達成することができる。
【図面の簡単な説明】
【図１】ｅｔｈに及ぼす鋼板中のＮｂ添加効果、ｔｏｔａｌＯ量の影響をを示す図である。
【図２】ｅｔｈに及ぼす鋼板中のｔｏｔａｌＯ量、Ｓ量の影響を示す図である。
【図３】ｅｔｈに及ぼす鋼板中のＣ量、Ｎｂ量の影響を示す図である。
【図４】セメンタイトの析出形態を示す模式図である。
【図５】高速ドロービード試験により求めた限界薄肉化率ｅｔｈとＤＴＲシミュレーション試験により求めた限界薄肉化率Ｒｔｈの関係を示す図である。
【図６】高速ドロービード試験方法を示す図である。
【図７】ＤＴＲ成形後の缶側壁部の割れの断面の状態を示す断面ミクロ組織の図面代用写真である。（ＭｎＳと割れの対応も示す図である。）
【図８】ＤＴＲ成形後の缶側壁部の割れの断面の状態を示す断面ミクロ組織の図面代用写真である。（セメンタイトと割れの対応も示す図である。）[0001]
[Industrial application fields]
TECHNICAL FIELD The present invention relates to a method for manufacturing a steel plate for cans suitable for DTR (Draw and Thin Redraw) can applications, and the occurrence of a side wall break phenomenon which is manifested during tension drawing deep drawing of a can body performed in the can manufacturing process. It is an object of the present invention to provide a method for producing a steel sheet that is difficult to perform.
[0002]
[Prior art]
With a focus on beverage cans and the like, from the viewpoint of weight reduction, process omission, reduction of raw materials and manufacturing costs, the transition from 3-piece cans to 2-piece cans and further thinning of cans are being promoted. Today, the mainstream of two-piece cans for beverage cans is that a circular blank is deep drawn (Draw) into a cup shape and then the can body is ironed (Ironing) 2 to 3 times to form a side wall. This is a DI (Draw and Ironing) molding method that achieves a thin wall and a predetermined can height, but is generally limited to positive pressure can applications, and negative pressure such as retort cans (coffee cans, tea cans) that hot pack the contents Not used for cans.
[0003]
On the other hand, as a forming method not involving ironing, a DRD (Draw and ReDraw) forming method in which the drawing process is performed twice, and a high wrinkle pressing force is applied to the flange part during the second and subsequent drawing processes, and the side wall part from the flange. A DTR (Draw and Thin Redraw) molding method has been put into practical use in which the can body is thinned by performing a tension-added deep drawing molding that suppresses the flow into the side wall and positively applies tension to the side wall. The biggest feature of these methods is that no ironing is applied, so the use of pre-coated steel sheets and laminated steel sheets enables not only the process to be omitted, but also the production of beverage cans with excellent design and design. It is a point.
[0004]
In recent years, a technique for developing the above-mentioned DTR molding for beverage can applications has been developed and is in the stage of practical use. For this application, a steel plate made by laminating a polyester film (PET) on tin free steel (TFS) having a temper degree of about T5-CA to DR-9 is generally used as a raw material, and then cup-shaped. Tension-added deep drawing is performed. As a result, the wall thickness of the can can be reduced by 20% or more, and not only can the can be reduced in weight, but also by changing the thickness of the steel plate, the tempering degree of the steel plate and the dome shape of the bottom of the can, It is possible to adapt to both negative pressure cans. However, in the molding method, it is necessary to solve the following problems in designing the material.
1. Peeling of the laminate film by sliding with the die shoulder during tensioned drawing. 2. Since the can wall portion is not constrained between the punch and the die at the time of tension-added drawing and is subjected to tensile deformation in a free surface state, rough skin is likely to occur.
3. Since the tension-added drawing is performed at a high speed, side wall breakage is likely to occur starting from a portion (shock line) in contact with the punch shoulder during each cup molding.
[0005]
Conventionally, several patent techniques have been disclosed for the problems 2 and 3 related to the design of the base steel sheet in each of the above technical problems. For example, Japanese Patent Laid-Open No. 4-314535 discloses a technique for suppressing rough skin by reducing the grain size of a steel sheet to a predetermined size or less.
[0006]
In particular, with respect to sidewall fracture resistance, in JP-A-7-34192, JP-A-7-34193, and JP-A-7-34194, by defining the crystal grain size under a certain production method, workability, A technique for improving rough skin and corrosion resistance is disclosed. In these technologies, it is disclosed that solid solution C and N are reduced including a steel sheet added with Nb, and constriction and void connection are suppressed, thereby improving side wall fracture resistance. In particular, no reference is made to the part that becomes the starting point of the side wall breakage, and the side wall breakage cannot be fundamentally avoided.
[0007]
In JP-A-5-247669, a steel sheet to which B is added in order to improve hardenability is quenched from the ferrite-austenite two-phase region during continuous annealing, whereby the microstructure is divided into a ferrite phase and a low-temperature transformation phase. A technique for obtaining a phase structure and obtaining a sufficiently high strength with a single cold pressure is disclosed. Furthermore, Japanese Patent Laid-Open No. 4-337049 discloses a technique in which a microstructure is similarly made into a two-phase structure of a ferrite phase and a low-temperature transformation phase in a steel sheet added with Nb.
[0008]
However, in such a two-phase structure, voids are likely to be generated along with DTR processing at the interface between the ferrite phase and the hard low-temperature transformation phase, so that sufficient side wall fracture resistance cannot be obtained.
[0009]
Common to these techniques is an attempt to reduce the inclusions in the steel, which are the starting points of the refinement and cracking of the structure, based on the principle of metallographical structure.
[0010]
However, when examining each technology in detail, the absolute strength level required for the material, the essential mechanism of the side wall breaking phenomenon, and the optimum microstructure for the can side wall breaking problem during DTR molding. The optimal technique is not disclosed with respect to the specific process conditions for obtaining the optimal microstructure. For this reason, it is impossible to stably reduce the thickness of the side wall portion exceeding 25% within the scope of the conventional material design technique, and it is considered that ironing processing must be added after all.
[0011]
[Problems to be solved by the invention]
The present invention relates to a method for producing a steel sheet that remarkably improves the side wall fracture resistance among the above-mentioned problems of the prior art with respect to steel sheets laminated with various resins applied to DTR molding and steel sheets pre-coated with various paints. Is intended to provide.
[0012]
FIG. 7 and FIG. 8 are the results of observing the microstructure of the side wall portion of the beverage can formed by DTR according to the conventional technique by scanning electron microscope (SEM). The beverage can has performances required as a beverage can such as film adhesion, corrosion resistance, surface properties such as rough skin, pressure resistance, and paneling strength of side walls.
[0013]
However, in terms of microstructure, many micro cracks are observed at the interface between MnS and the matrix or between the carbide and the matrix. When such a material is subjected to more severe tensile deep drawing, it is expected that the side wall breaks starting from these minute cracks.
[0014]
The present invention provides a steel sheet that has no side wall fracture when the thickness is reduced by 20% or more, and hardly generates such minute cracks even when the thickness is reduced by 30% or more.
[0015]
[Means for Solving the Problems]
In order to achieve the above object, the gist of the present invention is as follows.
(1) By weight%, 0.02 <C ≦ 0.1%, Si <0.03%, 0.4 ≦ Mn ≦ 1.2%, P ≦ 0.03%, S ≦ 0.02%, 0.02 ≦ Sol.Al ≦ 0.1%, N ≦ 0.007%, Total-O ≦ 0.005%, (3.0 × 10^-Four) /C≦Nb≦0.03%, 10C + Mn ≧ 0.8%, S × O ≦ 4.0 × 10^-Five %WhenAfter hot rolling, pickling, and cold rolling a hot rolled material composed of the remainder Fe and inevitable impurities, the recrystallization temperature or higher, Ac_ThreeDTR can with excellent side wall fracture resistance, characterized by performing continuous annealing at a temperature below the transformation point and performing temper rolling with an elongation of 0.5 to 3% or DR rolling with a rolling reduction of 5 to 40% Manufacturing method of conforming steel sheet, (2) 0.02 <C ≦ 0.1%, Si <0.03%, 0.4 ≦ Mn ≦ 1.2%, P ≦ 0.03%, S ≦ 0.015%, 0.02 ≦ sol.Al ≦ 0.1% by weight% , N ≦ 0.007%, (3.0 × 10^-Four) /C≦Nb≦0.03%, Total-O ≦ 0.0025%, 10C + Mn ≧ 0.8%, S × O ≦ 2.5 × 10^-Five %WhenAfter hot rolling, pickling, and cold rolling a hot rolled material composed of the remainder Fe and inevitable impurities, the recrystallization temperature or higher, Ac_ThreeDTR can with excellent side wall fracture resistance, characterized by performing continuous annealing at a temperature below the transformation point and performing temper rolling with an elongation of 0.5 to 3% or DR rolling with a rolling reduction of 5 to 40% A method for producing a conformable steel sheet, and (3) In (1) to (2), a DTR can conformable steel sheet having excellent side wall fracture resistance, characterized by performing overaging at a temperature below 400 ° C. after continuous annealing. It is a manufacturing method of a steel plate.
[0016]
[Action]
The basic concept that led to the present invention, the present invention configured based on the basic concept, and the reasons for limitation will be described below.
[0017]
First, the present inventors pay attention to the microstructure of the current DTR molded product shown in FIG. 7 and FIG. 8, and the main causes of side wall breakage are steel-making oxide-based nonmetallic inclusions and steel plate manufacturing processes. MnS, AlN, cementite (Fe₃  The existence of C) and the amount, size, distribution density and the like of the precipitates were elucidated.
[0018]
That is, it has been found that in order to achieve a higher level of thinning of the side wall than ever before, it is extremely important to regulate the precipitation form of the precipitates and to optimize the microstructure of the steel sheet itself.
[0019]
That is, it has been found that minute cracks are generated at the interface between these precipitates and the parent phase, and the generated minute cracks are likely to propagate depending on the form of precipitation, leading to side wall fracture during DTR processing.
[0020]
In addition, as an evaluation method, the present inventors, as an index that can accurately and quantitatively evaluate a significant difference between materials with respect to the side wall fracture resistance, which has been only very vague and qualitative evaluation in the prior art. The critical thinning rate (еth) obtained by the high speed draw bead pull-out test method shown in FIG. 6 was used.
[0021]
eth = 100 (e₀-E) / e₀
However, e₀Is the initial plate thickness, and e is the plate thickness in the vicinity other than the fracture surface.
[0022]
A simulation test of DTR molding was also conducted in the laboratory. A round blank was punched out and a cup was formed by normal drawing using a crank press, and this cup was controlled using a double-acting high-speed hydraulic deep drawing tester with controlled tool conditions, wrinkle restraining force and forming temperature. DTR was formed into a 211 diameter size equivalent to a 350 cc can.
[0023]
FIG. 5 shows the relationship between the limit thinning rate eth obtained by the high speed draw bead test and the limit thinning rate Rth obtained by the DTR molding simulation test. As shown in the figure, it can be confirmed that the larger the eth is, the higher the limit thinning rate of the side wall of the can body is, and the side wall break resistance is excellent, and еth is the can body side wall that can be DTR-molded without side wall breaks. It was confirmed that there was a positive correlation with the limit thinning rate of and that the value of about (еth + 5%) and the limit thinning rate almost coincided.
[0024]
Using this high-speed draw bead pull-out test method, specific configuration requirements and a limited range were defined.
[0025]
In the steel sheet of the present invention, the microstructure is a substantially ferrite single phase structure composed of a ferrite phase and finely dispersed carbide. This is because when a phase (martensite, bainite, etc.) other than carbides (pearlite or cementite) inevitably present in low carbon steel is intentionally generated, the interface between such hard second phase and ferrite matrix phase is This is the starting point of micro cracks during DTR molding and further deteriorates the side wall fracture resistance.
[0026]
FIG. 1 shows the effect of Nb addition and the total O amount on the eth of TFS containing 0.07% C and adjusted to a tempering degree equivalent to DR9. It can be seen that by reducing the totalO amount, eth increases, and the decrease in the O amount is effective for the thinning rate up. However, the steel sheet with 0.01% Nb added has no Nb added regardless of the O amount. The eth is larger than that of the conventional steel, and the eth is 30% or more by making the O amount 0.005% or less. Further, when the amount of O is reduced to 0.0025% or less, eth is 35% or more.
[0027]
This is due to the fact that the ferrite crystal grain size of the TFS original plate was made finer by adding Nb, and that the precipitation form of the carbide was changed and the carbide was finely dispersed relatively uniformly. By adding Nb, the austenite grain size during hot rolling becomes finer, and the ferrite grain size after transformation, cold pressure, and ferrite grain size after annealing also become finer.
[0028]
In the Nb-added steel sheet, a part of C precipitates as a very fine Nb carbide (NbC) of about 10 to 20 nm as compared with the iron carbide. Therefore, the size of the iron carbide is small due to a synergistic effect with the ferrite refinement effect. Thus, the distribution density is increased and the average precipitation interval is made uniform. These effects are considered to improve the side wall fracture resistance.
[0029]
On the other hand, in the conventional steel sheet containing no Nb, C precipitates as cementite and partly pearlite at the grain boundaries and within the grains. Since such iron carbide is relatively large and has a sub-micron or micron-order size, the interface between the carbide and the matrix phase is likely to be the starting point of cracks, causing numerous micro-cracks during DTR molding and causing side wall fracture. It becomes.
[0030]
Al₂  O₃  Mainly based on MnO, MnO, CaO, SiO₂Since the oxide inclusions including the system become the starting point of cracking, the fewer the inclusions, the better the workability. The dramatic improvement of eth due to the addition of Nb accompanying the reduction of the totalO amount is due to the fact that the oxide inclusions have decreased, so the influence of carbide on eth is relatively large, and the effect of carbide refinement due to the addition of Nb is remarkable. It is because it came to appear. That is, in order to maximize the effect of Nb addition, it is extremely important to reduce the amount of O.
[0031]
Next, the effects of the totalO amount and the S amount on eth were examined. TFS eth containing 0.06 to 0.09% C and 0.008 to 0.015% Nb, having different totalO and S amounts, and adjusted to a tempering degree equivalent to DR9 The results of measuring are shown in FIG. Although the eth is improved by reducing the total O amount and the S amount, if the O amount is relatively high and exceeds 0.005%, a sufficient eth can be obtained even if the S is reduced to about 0.005%. Not.
[0032]
As is apparent from the figure, totalO ≦ 0.005%, S ≦ 0.02%, S × O ≦ 4.0 × 10^-5    %, Eth ≧ 30% is obtained, and further, totalO ≦ 0.0025%, S ≦ 0.015%, S × O ≦ 2.5 × 10^-5    In the case of%, eth ≧ 35% is obtained, and the side wall fracture resistance is remarkably improved. That is, by reducing O and S at the same time, excellent side wall fracture resistance can be obtained.
[0033]
This is due to the decrease in MnS that is the starting point of the occurrence of minute cracks as described above. However, when the oxide inclusions are relatively large, the oxide can be the starting point of cracking, so S is used. Even if it reduces, the effect is not fully exhibited. However, when the amount of oxide inclusions is small and the total O amount is low, the influence of MnS becomes relatively large, so that the effect of reducing MnS that can be the starting point of microcracking due to the reduction of S is remarkably exhibited. That is, it is important to reduce the amount of O in order to maximize the effect of improving the side wall fracture resistance by reducing S.
[0034]
Next, the reasons for limiting the claims of the present invention will be described.
Nb: Nb is an extremely important element in the present invention, and is added to make the ferrite structure finer, to reduce iron carbide that becomes the starting point of cracking, and to increase the strength of the steel sheet. In order to precipitate fine Nb carbides and to fully exhibit these effects, it is important to add Nb in a necessary minimum amount or more determined according to the amount of C. FIG. 3 shows the relationship between the amount of C, the amount of Nb, and eth.
[0035]
In relation to the amount of C in Nb, 3.0 × 10^-4    The above effect is exhibited by adding / C or more, and high eth can be obtained. Therefore, the lower limit of the addition amount is set to 3.0 × 10.^-4    / C (%). On the other hand, even if addition exceeding 0.03% is performed, the above effect is saturated and a remarkable improvement in characteristics cannot be expected. Therefore, the upper limit is defined as 0.03%.
[0036]
O: As described above, the oxide inclusions in the steel serve as a starting point for occurrence of cracks during DTR molding, and thus significantly impair the side wall fracture resistance. In the present invention, in order to improve the side wall fracture resistance, Nb is added to achieve fine dispersion of the carbide and reduce the S amount. However, as shown in FIGS. 1 and 2, the total O amount is reduced to 50 ppm or less. By doing so, these effects can be sufficiently exhibited.
[0037]
Therefore, in the present invention, the total O amount in the steel is limited to 50 ppm or less. Furthermore, in the relationship with the amount of S described above, S × O ≦ 4.0 × 10 is the upper limit for stably obtaining eth ≧ 30%.^-5    Limited to%. Moreover, totalO ≦ 0.0025%, S × O ≦ 2.5 × 10^-5By limiting to%, the effects of the present invention can be exhibited to the maximum, and even better sidewall fracture resistance can be obtained.
[0038]
S: S is an extremely important element in the present invention. S is present in the steel as MnS, and the expanded MnS tends to be the starting point of cracks that lead to side wall fracture during DTR molding. Accordingly, it is desirable that S is as small as possible, and is limited to 0.02% or less. Furthermore, by limiting the relationship between the amount of S and the amount of O in the steel to a specific range, good sidewall fracture resistance can be obtained.
[0039]
In the present invention, as shown in FIG. 2, the upper limit for stably obtaining eth ≧ 30% along with the amount of O described above is as follows: totalO ≦ 0.005%, S ≦ 0.02%, S × O ≦ 4.0 × 10^-5    Limited to%. Furthermore, totalO ≦ 0.0025%, S ≦ 0.015%, S × O ≦ 2.5 × 10^-5    By limiting to%, it is possible to obtain even better fast wall breaking resistance.
[0040]
C: C is an extremely important element in order to ensure the strength level required as a steel sheet compatible with DTR cans. In the present invention, the steel sheet is reinforced with C, Mn, and fine Nb carbides in order to ensure the strength of the can necessary for the DTR can. When C is 0.02% or less, it is difficult to obtain the required strength even if Mn and Nb are added. Therefore, in the present invention, it is essential to contain C exceeding 0.02%. And
[0041]
However, since a part of C precipitates as cementite in the ferrite grain boundaries or in the grains during pearlite or annealing, the interface between these iron carbides and the parent phase tends to be the starting point of cracking, and as shown in FIG. It causes a large number of micro cracks during molding. In the present invention, in order to alleviate these adverse effects due to iron carbide, Nb addition is essential, and a part of C is precipitated as fine Nb carbide at a higher temperature before iron carbide is formed.
[0042]
Further, by controlling the carbides other than fine Nb carbides, that is, the precipitation form of cementite, the side wall fracture resistance can be further improved. On the other hand, if the C content exceeds 0.1%, side wall breaks starting from pearlite will become apparent even if Nb is added, so the C content is limited to 0.1% or less.
[0043]
Si: Si is a substitutional solid solution element and has strengthening ability. However, in the present invention, Si is not actively added for the following reasons. Si is an element that remains in the steel as an impurity component and causes embrittlement of the steel plate even when not intentionally added. In addition, when used as a base steel plate of TFS, However, it is desirable that the content is small. When a large amount is added, SiO₂  Inclusions may remain in the steel, deteriorating the sidewall fracture resistance.
[0044]
Therefore, in the present invention, to avoid these adverse effects, the content is limited to less than 0.03%. Furthermore, in order to improve the local ductility of the steel sheet and obtain even better side wall fracture resistance, the content of Si as an impurity component is preferably regulated to 0.01% or less.
[0045]
Mn: Mn precipitates S in the steel as MnS, thereby preventing hot cracking of the hot rolled material of slabs and cast plates (slabs, etc.), and grains with MnS as the core in the overaging process after continuous annealing. It plays a role in promoting fine cementite precipitation inside. In the present invention, since Nb addition is essential, a part of C precipitates as Nb carbide, but the remaining C becomes iron carbide, so the above-described action of MnS is important. Further, Mn serves as a solid solution strengthening element to supplement the strengthening by C and the strengthening by Nb carbide.
[0046]
In the present invention, S is deposited and fixed, and further, as a lower limit for obtaining a can strength necessary for a DTR can, 0.4% or more and 10C + Mn is 0.8% or more in relation to the C content. It is essential to add Mn to.
[0047]
On the other hand, when a large amount of Mn is added, it is effective to increase the strength of the material, but on the other hand, the solubility product of MnS increases, and not only a relatively large MnS is formed in the slab stage, but also a band structure during hot rolling. This promotes the formation of C and promotes the non-uniform distribution of C in the steel. Both of these are factors that degrade the side wall fracture resistance during DTR molding, so the upper limit of Mn is 1.2%.
[0048]
P: P is a substitutional solid solution element similar to Si, and is an effective element for increasing the strength of a steel sheet with a greater strengthening ability than Si, but segregates at the ferrite grain boundary at the same time. Since it is also an element that embrittles the grain boundary, the content is preferably as small as possible from the viewpoint of resistance to side wall fracture.
[0049]
Therefore, the upper limit for avoiding an adverse effect on the practical side wall fracture resistance is limited to 0.03% or less. Furthermore, in order to obtain even more excellent side wall fracture resistance, it is effective to reduce the content to less than 0.01%.
[0050]
sol. Al: sol. Al precipitates N in the steel as AlN, thereby reducing the adverse effect of solid solution N, which reduces the local ductility of the steel sheet by dynamic strain aging. In addition, fine AlN is effective for refining ferrite grains and, like MnS, becomes the core of intra-grain precipitation of fine cementite during the continuous aging process of continuous annealing, so it is also effective for fine dispersion of carbides. Works.
[0051]
However, sol. When a large amount of Al is added to increase the amount of Al, minute Al₂  O₃Inclusions tend to remain and cause side wall breakage. Therefore, in the present invention, the lower limit is set to 0.02% in order to exert the above-mentioned effects, and the upper limit is set to 0.1% as a limit that practically adding more deteriorates the side wall fracture resistance. To do.
[0052]
N: N has the effect of being finely precipitated and dispersed as AlN, thereby making ferrite grains finer and becoming a fine cementite precipitation site. However, in order to exert such an action, it is not necessary to add it particularly positively, and the range contained in ordinary steel is sufficient. Excessive N addition makes it easier for solid solution N to remain even if Al is added, causing a reduction in local ductility and causing deterioration in side wall fracture resistance. Therefore, N is limited to 70 ppm or less.
[0053]
However, in the present invention, the addition of Nb is essential, so that the ferrite grains can be refined and the carbides can be refined by Nb carbide. Therefore, even if the above action by AlN is not actively used, The object of the invention can be achieved. In view of this, it is effective to completely avoid the adverse effects caused by the solute N remaining and to further improve the side wall fracture resistance, so that N is 30 ppm or less.
[0054]
In this invention, it is a manufacturing method for making the microstructure into a substantial ferrite single phase structure which consists of a ferrite phase and a fine dispersion carbide using the hot-rolling material which has said composition. This is intended to produce low-temperature transformation phase (martensite, bainite, etc.) other than iron carbide (pearlite or cementite) unavoidably present in low carbon steel and Nb carbide (NbC) precipitated by aggressive addition of Nb. This is because the interface between the hard second phase and the ferrite matrix phase becomes the starting point of microcracking during DTR molding, and the side wall fracture resistance is deteriorated.
[0055]
Since the steel sheet of the present invention contains Nb, it is strengthened by precipitation due to fine dispersion of Nb carbide, as well as refinement of ferrite crystal grains, refinement of carbide and iron carbide compared to conventional steel sheets without Nb addition. Is reduced and the side wall fracture resistance is improved.
[0056]
However, the total amount of C does not precipitate as Nb carbide, but partially precipitates as iron carbide. As described above, Nb carbide is very fine and does not serve as a starting point for cracking during DTR processing, but iron carbide can serve as a starting point for cracking. Therefore, by controlling the precipitation form of iron carbide, the side wall fracture resistance can be further improved.
[0057]
C that has not precipitated as Nb carbide in the hot rolling stage aggregates at the grain boundaries of the hot rolled steel sheet after winding, and partially precipitates as pearlite. These carbides are pulverized by cold rolling, and after recrystallization annealing, temper rolling or DR rolling, become a dense community of fine cementite that crosses the ferrite grain boundaries and continues into the grains of adjacent grains. Existing. Further, a part of 0.02% or less of C is dissolved in the ferrite during the recrystallization annealing, and in the cooling process after the annealing, the ferrite grain boundary and the inside of the crystal grain (for example, as fine cementite, FIG. This MFP precipitates as cementite in the average distance between cementite particles). The precipitation form of these cementites is schematically shown in FIG.
[0058]
Among these, a dense community of fine cementite that crosses the ferrite grain boundary and continues into the grains of the adjacent crystal grains is most likely to be the starting point of cracking. Therefore, the length Lc in the rolling direction (Lc Although the definition is not limited to the rolling direction, it is usually effective to further improve the side wall fracture resistance by shortening the length in the rolling direction. For this reason, in order to suppress the precipitation of pearlite at the hot rolling stage, the hot rolling coiling temperature is desirably low.
[0059]
Next, the manufacturing method of the steel plate of this invention is demonstrated.
After the converter melting, hot casting is performed after continuous casting into a slab having a normal thickness of about 200 to 280 mm or a thin slab having a thickness of about 30 mm. Since continuous casting reduces semi-macro and macrosegregation of P and S in steel, slab light reduction casting is preferable.
[0060]
The slab heating temperature, finishing temperature, and coiling temperature can be set to a temperature range of about 1100 to 1250 ° C., 800 to 890 ° C., and 500 to 700 ° C., respectively, but 1150 ° C. or higher from the viewpoint of uniform refinement of MnS. The high-temperature heating of the slab is preferably a low-temperature finish of 870 ° C. or less from the viewpoint of fine graining of the hot-rolled sheet structure, and a medium or low-temperature winding of 640 ° C. or less from the viewpoint of fine dispersion of pearlite.
[0061]
Furthermore, after pickling and cold rolling the hot-rolled steel sheet, recrystallization annealing is performed in a continuous annealing furnace. When the annealing temperature is lower than the recrystallization temperature, sufficient ductility cannot be obtained and the side wall fracture resistance deteriorates. Also, the annealing temperature is Ac₃When the transformation point is exceeded, an austenite phase is generated during soaking, and a hard low-temperature transformation phase is easily generated during the cooling process. Such an interface between the hard second phase and the ferrite matrix phase becomes a starting point of microcracking at the time of DTR molding, which causes the side wall fracture resistance to deteriorate. From these, the annealing temperature is higher than the recrystallization temperature, Ac₃Limited to the transformation point or lower.
[0062]
Although it is possible to manufacture without performing an overaging treatment, the side wall fracture resistance is further improved by adding the overaging treatment from the viewpoint of controlling the form of precipitation of iron carbide.
[0063]
As the overaging treatment, any of an inline overaging treatment (CAL-OA) in a continuous annealing furnace and a batch overaging treatment (CAL-BAF) by box annealing after continuous annealing may be used.
[0064]
In the case of CAL-OA, the overaging treatment is performed at a temperature lower than 300 to 400 ° C. in order to reduce residual solid solution C that deteriorates the local ductility of the steel sheet by dynamic strain aging and to finely disperse cementite in the crystal grains. In the case of CAL-BAF with a holding time of 20 seconds or more, it is effective to carry out in a temperature range of less than 400 ° C.
[0065]
In the overaging treatment at a temperature of 400 ° C. or higher, since the solid solubility limit of C is large, it is difficult to precipitate as fine cementite in the crystal grains. Therefore, in the present invention, the overaging temperature is limited to less than 400 ° C.
[0066]
The preferable ranges of Lc and MFP are 10 μm or less and 2 μm or less, respectively. The annealed steel plate is finished to a predetermined thickness by temper rolling or DR rolling. If the elongation rate of temper rolling is less than 0.5% or more than 3.0%, shape control is difficult, so it is limited to 0.5 to 3.0%. Further, if the rolling reduction rate of DR rolling is less than 5%, it is difficult to control the shape and surface properties, and if it exceeds 40%, ductility is lowered and side wall fracture resistance is lowered, so it is limited to 5 to 40%.
[0067]
After temper rolling or DR rolling, various surface treatments such as tin plating, ultra-thin tin plating, tin-nickel plating, nickel plating, and chromium plating are performed.
[0068]
In particular, when these surface-treated steel sheets are used as base steel sheets for film-laminated steel sheets and pre-coated steel sheets, an electrolytic chromic acid-treated steel sheet having a two-layer structure in which the lower layer is metallic chromium and the upper layer is chromium hydrated oxide, that is, TFS is processed. Most desirable from the viewpoint of adhesion. These surface-treated steel sheets are applied to DTR cans as steel sheets alone or as film-laminated steel sheets laminated with a resin film such as polyester or pre-coated steel sheets coated with a paint such as epoxy.
[0069]
The steel sheet of the present invention disclosed above can be applied to all steel sheets used in the so-called DTR can manufacturing method in which deep drawing is performed while applying tension, regardless of whether the can forming process is a wet process or a dry process. Technology.
[0070]
Further, the present invention can also be applied to the case where the thickness is reduced by further ironing (Ironing) after DTR processing.
[0071]
Hereinafter, the effects of the present invention will be described more specifically with reference to examples.
[0072]
【Example】
Example 1
After the steel was melted in the converter, it was made into a slab by continuous casting under light pressure. After heating these slabs to 1200 ° C., a hot rolled steel sheet having a finishing temperature of 850 ° C., a winding temperature of 600 ° C. and a thickness of 1.8 mm was obtained. After pickling, it was cold-rolled to 0.225 mm and recrystallized by continuous annealing.
[0073]
As overaging treatment conditions, three types of conditions were carried out: no overaging treatment, CAL-OA in CAL, and CAL-BAF in a box annealing furnace after continuous annealing.
[0074]
The annealed steel sheet was made 0.180 mm by DR rolling with a rolling reduction of 20.0%, and then subjected to electrolytic chromic acid treatment in a tin-free plating line to obtain TFS.
[0075]
The chemical composition of these steel sheets is as shown in Table 1.
A PET film was laminated on both sides of these TFSs in a laboratory, and the critical thinning ratios eth and Rth were evaluated by a high-speed draw bead test and a DTR simulation test. Furthermore, the can bottom pressure resistance of the DTR molded sample was evaluated using a buckling tester.
[0076]
The evaluation results are shown in Table 2 together with the annealing conditions. It can be seen that the steel sheet produced by the method of the present invention has high eth and Rth, excellent DTR formability, and sufficient pressure strength compared to the comparative steel sheet.
[0077]
[Table 1]

[0078]
[Table 2]

[0079]
Example 2
After melting the converter, it was made into a slab by continuous casting under light pressure, heated to 1200 ° C., and then a hot-rolled steel plate having a finishing temperature of 840 ° C., a winding temperature of 540 ° C., and a thickness of 1.6 mm. These hot-rolled steel sheets were pickled, cold-rolled to 0.200 mm, and continuously annealed under the annealing conditions shown in Table 3. Then, it was finished to 0.180 mm by DR rolling with a rolling reduction of 10%, and was made into a TFS / PET laminated steel plate under the same conditions as in Example 1 and subjected to a high speed draw bead test.
[0080]
The chemical compositions of these steel sheets are as shown in Tables 1 to M.
The obtained results are shown in Table 3 together with the annealing conditions. The steel plate produced by the method of the present invention has excellent DTR formability as compared with the comparative steel plate.
[0081]
[Table 3]

[0082]
Example 3
After melting the converter, it was made into a slab by continuous casting under light pressure, heated to 1200 ° C., and then a hot rolled steel plate having a finishing temperature of 840 ° C., a coiling temperature of 600 ° C. and a thickness of 1.6 mm. These hot-rolled steel sheets were pickled, cold-rolled to 0.185 mm, and continuously annealed under conditions of a soaking temperature of 730 ° C. and an overaging temperature of 350 ° C. Then, it finished to 0.180 mm by temper rolling, and it was set as the TFS and PET laminated steel plate on the same conditions as Example 1, and used for the high-speed draw bead test.
[0083]
The chemical compositions of these steel sheets are as shown in Tables 1 to M.
Table 4 shows the obtained results. The excellent DTR compatibility of the steel of the present invention is also observed when the SR material is only temper rolled.
[0084]
[Table 4]

[0085]
【The invention's effect】
According to the method for producing a steel sheet of the present invention, it is possible to produce a DTR can having a higher side wall portion thinning rate than before without breaking the side wall, and a reduction in the weight of the DTR can can be achieved.
[Brief description of the drawings]
FIG. 1 is a diagram showing the effects of Nb addition in a steel sheet and the total O amount on eth.
FIG. 2 is a diagram showing the influence of total O amount and S amount in a steel sheet on eth.
FIG. 3 is a diagram showing the influence of the amount of C and Nb in a steel sheet on eth.
FIG. 4 is a schematic view showing a precipitation form of cementite.
FIG. 5 is a diagram showing a relationship between a limit thinning rate eth obtained by a high-speed draw bead test and a limit thinning rate Rth obtained by a DTR simulation test.
FIG. 6 is a diagram showing a high-speed draw bead test method.
FIG. 7 is a drawing-substituting photograph of a cross-sectional microstructure showing a state of a cross-section of a crack in a can side wall after DTR molding. (It is also a diagram showing the correspondence between MnS and cracks.)
FIG. 8 is a drawing-substituting photograph of a cross-sectional microstructure showing a state of a cross-section of a crack in a can side wall after DTR molding. (It is also a diagram showing the correspondence between cementite and cracks.)

Claims (3)

0.02 <C ≦ 0.1% by weight, Si <0.03%, 0.4 ≦ Mn ≦ 1.2%, P ≦ 0.03%, S ≦ 0.02%, 0.02 ≦ Sol.Al ≦ 0.1%, N ≦ 0.007%, Total-O ≦ 0.005%, (3.0 × 10 ⁻⁴ ) /C≦Nb≦0.03%, 10C + Mn ≧ 0.8%, S × O ≦ 4.0 × 10 ⁻⁵ % , hot-rolled material consisting of the balance Fe and inevitable impurities , Hot rolling, pickling and cold rolling, followed by continuous annealing at a temperature not less than the recrystallization temperature and not more than the Ac ₃ transformation point, and further temper rolling with a stretch rate of 0.5 to 3% or a reduction rate of 5 to 40 % DR rolling, a method for producing a DTR can-compatible steel sheet having excellent sidewall fracture resistance. 0.02 <C ≦ 0.1%, Si <0.03%, 0.4 ≦ Mn ≦ 1.2%, P ≦ 0.03%, S ≦ 0.015%, 0.02 ≦ sol.Al ≦ 0.1%, N ≦ 0.007%, (3.0 × 10 ^-4 ) /C≦Nb≦0.03%, Total-O ≦ 0.0025%, 10C + Mn ≧ 0.8%, S × O ≦ 2.5 × 10 ^-5 %, and the hot-rolled material consisting of the balance Fe and inevitable impurities , Hot rolling, pickling and cold rolling, followed by continuous annealing at a temperature not less than the recrystallization temperature and not more than the Ac ₃ transformation point, and further temper rolling with a stretch rate of 0.5 to 3% or a reduction rate of 5 to 40 % DR rolling, a method for producing a DTR can-compatible steel sheet having excellent sidewall fracture resistance. 3. The method for producing a DTR can-compatible steel sheet having excellent side wall fracture resistance according to claim 1, wherein overaging is performed at a temperature lower than 400 ° C. after continuous annealing.

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