JP3496333B2 - DTR can-adaptive steel plate with excellent side wall break resistance - Google Patents

Description

Detailed Description of the Invention

[0001]

The present invention relates to a DTR (Draw and Thin Red)
The present invention relates to a steel sheet for cans which is suitable for use in can manufacturing, and provides a steel sheet which is less likely to cause a side wall rupture phenomenon which becomes apparent during tension-added deep drawing of a can body performed in the can manufacturing process.

[0002]

2. Description of the Related Art Focusing on beverage cans and the like, from the viewpoints of weight reduction, process omission, material and manufacturing cost reduction, the shift from 3 piece cans to 2 piece cans and further reduction of the thickness of the can body are being promoted. . The mainstream of two-piece cans for beverages today is the deep drawing of circular blanks into cups (Draw).
After that, the can body is ironed by ironing 2-3 times to obtain a thin wall and obtain a predetermined can height DI (Draw an
d Ironing) molding method, but is generally limited to positive pressure can applications and is not used for negative pressure cans such as retort cans (coffee cans, tea cans) that hot-pack contents.

On the other hand, as a forming method which does not involve ironing, a DRD (Draw and ReDraw) forming method in which drawing is performed twice and a flange with a high wrinkle holding force applied to the flange portion during the second and subsequent drawing DTR (Draw and Draw and thin) to reduce the thickness of the can body by suppressing the flow from the wall to the side wall and applying tension deep drawing to positively apply tension to the side wall.
Thin Redraw) molding method has been put to practical use.

[0004] The greatest feature of these methods is that since ironing is not applied, the use of pre-coated steel sheets or laminated steel sheets makes it possible not only to omit the steps but also to produce beverage cans excellent in design and design. Is a point that can be.

In recent years, a technique for developing the above-mentioned DTR molding for use in beverage cans has been developed and is in the stage of practical application. For this application, tin-free steel (TFS), which generally has a temper degree from T5-CA to DR-9, is used.
After forming a cup from a steel sheet laminated with a polyester film (PET) as a raw material, two-stage tension-added deep drawing is performed. As a result, the thickness of the can side wall has been reduced by more than 20%, and the weight of the can has been reduced.By changing the material thickness and the dome shape of the bottom of the can, it can be used for both positive pressure and negative pressure cans. It is possible to adapt. However, in the molding method, it is necessary to solve the following problems in designing the material. 1. Peeling of the laminated film by sliding with the shoulder of the die during tension drawing. 2. During the tension drawing, the wall of the can is not constrained between the punch and the die and is subjected to tensile deformation in a free surface state, so that rough skin is likely to occur. 3. Since tension drawing is performed at high speed, side wall breakage is likely to occur at the point of contact with the punch shoulder (shock line) at the time of forming each cup.

Regarding the problems 2 and 3 related to the design of the base steel sheet among the above-mentioned technical problems, some patented technologies have been conventionally disclosed. For example, in Japanese Patent Laid-Open No. 4-314535,
A technique is disclosed in which the grain size of a steel sheet is reduced to a predetermined size or less to suppress rough skin.

Particularly, regarding the resistance to side wall breakage, JP-A-7-
JP 34192-34194 discloses a technique for improving workability, skin roughness, and corrosion resistance by defining the crystal grain size under a certain manufacturing method. These techniques disclose that the solid solution C and N are reduced, the occurrence of necking and the connection of voids are suppressed, and the side wall fracture resistance is thereby enhanced. It is considered that it is not possible to fundamentally avoid side wall rupture, since it has not been mentioned.

Further, in Japanese Unexamined Patent Publication (Kokai) No.5-247669, a steel sheet added with B for improving hardenability is rapidly cooled from a ferrite-austenite two-phase region during continuous annealing to transform a microstructure into a ferrite phase and a low temperature transformation. A technique is disclosed in which a two-phase structure of phases is obtained and a sufficiently high strength is obtained by cold pressure once. However, in such a two-phase structure, voids are likely to occur at the interface between the ferrite phase and the hard low-temperature transformation phase due to the DTR processing, and thus it is difficult to obtain sufficient side wall fracture resistance.

[0009] On the other hand, as a general technique related to fracture during drawing, a technique for restricting only steel-making inclusions to reduce working defects (Japanese Patent Laid-Open No. 58-16026), cementite precipitated in steel Technology for improving the corrosion resistance and workability by defining the average particle size of (Fe3C) (JP-A-60-149743, JP-A-60-215739), Mn produced by solid-phase reaction
Japanese Patent Publication No. 4-78714 and Japanese Patent Publication No. 6-76618 disclose techniques for improving the workability by simultaneously defining the size and crystal grain size of non-metallic inclusions such as S and AlN.

Common to these techniques is to reduce the inclusions in the steel, which are the origins of the refinement of the structure and cracks, based on the metallographic principle.

However, a detailed study of each technology shows that the absolute strength level required for the material, the essential mechanism of the side wall breakage phenomenon, and the optimum mechanism against the problem of the can side wall breakage during DTR molding. No optimum technology has been disclosed regarding the ideal microstructure. For this reason,
It is impossible to stably realize the wall thickness reduction of more than 25% within the range of the conventional material design technique, and it is considered that ironing is inevitably added after all.

[0012]

SUMMARY OF THE INVENTION An object of the present invention is to significantly improve the side wall rupture resistance among the above-mentioned problems of the prior art for a steel sheet laminated with various resins applied to DTR forming. It is what

FIGS. 7 and 8 show the results of scanning electron microscope (SEM) observation of the microstructure of the side wall of a beverage can DTR-molded by the prior art. The beverage can has film adhesion, corrosion resistance, surface properties such as rough skin, pressure resistance, sidewall paneling strength, sidewall buckling strength, and other properties required for a beverage can.

However, in terms of microstructure, many small cracks are observed at the interface between MnS and the matrix or between the cementite and the matrix. When such a material is subjected to further intense deep drawing, it is expected that these minute cracks will cause the side walls to fracture.

According to the present invention, when the wall thickness is reduced to 20% or more, there is no side wall fracture, and even if the wall thickness is reduced to 30% or more,
The present invention provides a steel sheet in which such minute cracks hardly occur.

[0016]

Means for Solving the Problems The subject matter of the present invention in order to achieve the above object is as follows. (1) The first aspect of the invention is mass % (hereinafter the same), 0.02 <
C ≦ 0.06%, Si ≦ 0.03%, 0.5 ≦ Mn ≦ 1.2%, P ≦ 0.03
%, S ≤ 0.008%, 0.02 ≤ Sol.Al ≤ 0.1%, 0.002 ≤ N ≤
Ferrite single-phase structure consisting of 0.007%, Total-O ≤ 0.005%, log [Mn] [S] ≤ -2.22 and a steel composition consisting of balance Fe and inevitable impurities, and consisting essentially of ferrite phase and finely dispersed cementite. Among the cementites , the cementite that precipitates in the cooling process of annealing is 2 μm or less in average grain-to-particle distance (MFP) in the crystal grains of the ferrite phase.
Pearlite with a hot-rolled structure is cold
Dense cementite formed by crushing in the rolling process
Length Lc of the communities is excellent DTR cans adapted steel耐側wall breakable, characterized in that at 10μm or less, (2) second
Advantageously, in the invention, among the steel composition, and in particular Mn
Total- For O, 0.3 ≦ Mn ≦ 1.2% , Total-O ≦ 0.0
DTR with excellent side wall rupture resistance characterized by 025%
It is a compatible steel plate.

[0017]

The basic concept that led to the present invention, the present invention constructed on the basis of the basic idea, and the reasons for limitation thereof will be described below.

In the current DTR steel sheet, side wall rupture is caused by the probability that there is a deliberate oxide-based non-metallic inclusion present in the steel sheet, which coincides with the shock line portion formed on the punch shoulder portion during DTR forming. It is said that
It is primarily important to reduce the steel-making oxide-based non-metallic inclusions in order to improve the side wall fracture resistance.
However, in order to achieve a higher level of thinning,
It is extremely important to optimize the microstructure itself of the steel sheet.

First, the inventors of the present invention focused on the microstructure of the current DTR molded product shown in FIGS. 7 and 8, and the main causes of side wall rupture were steelmaking oxide nonmetallic inclusions and steel plates. It was clarified that MnS, AlN, and cementite (Fe ₃ C), which are precipitated by solid-state reaction in the manufacturing process, exist and their precipitation morphology.

Focusing on MnS, as shown in FIG. 7, MnS spread in a film shape in the rolling direction was recognized among the minute cracks generated by the DTR processing. MnS is
S, which causes hot brittleness as an impurity in the steel sheet, is generated as a result of being fixed as MnS by the addition of Mn.
The existence cannot be completely eliminated. However, in order to improve the side wall rupture resistance, it is necessary to suppress the precipitation of MnS as much as possible and to avoid the presence of MnS spread in a film shape in the rolling direction microscopically.

Next, the inventors of the present invention observed many cross sections by paying attention to the precipitation morphology of finely dispersed carbide in the cross sectional structure of the current DTR molded product, and as a result, as shown in FIG. Found that fine cementite was present, and found that the interface between cementite and the matrix was the origin of cracking.

According to the present invention, minute cracks are generated at the interface between the above-described precipitate such as MnS and cementite and the mother phase, and the generated minute cracks easily propagate depending on the precipitation form.
This was made based on the knowledge that side wall fracture is likely to occur during R processing.

Further, regarding the evaluation method, the present inventors accurately and quantitatively determine the significant difference between the materials with respect to the side wall fracture resistance, which has been extremely vague and qualitatively evaluated by the conventional techniques. As an index that can be evaluated, the limiting thinning rate (еth) obtained by the high-speed draw bead drawing test method shown in FIG. 6 was used.

Eth = 100 (e ₀ −e) / e ₀ where e ₀ is the initial plate thickness and e is the plate thickness in the vicinity other than the fracture surface.

According to the DTR simulation, еth has a positive correlation with the limit thinning rate of the can body side wall that can be DTR-molded without side wall rupture, and a value of approximately (еth + 5%) and a limit thinning rate I confirmed that they match.

By using this limiting thinning rate (еth), concrete constituent requirements and limiting ranges were determined. The reasons for each limitation of the present invention will be specifically described below.

S: S is an extremely important element in the present invention. S is present in the steel sheet as MnS and expanded MnS
Tends to be the starting point of cracks that lead to side wall fracture during DTR forming.
Therefore, it is preferable that S is as small as possible. Conventionally, there is a case where S is added from the viewpoint of corrosion resistance in the steel sheet for cans, but in the laminated steel sheet applied to DTR, the excellent corrosion resistance is ensured by the laminate layer.
There is no need to increase the amount.

In the present invention, the upper limit of S is set to 0.008% from the result of FIG. 1 in order to obtain sufficient side wall fracture resistance of eth> 30%.

Mn: Mn not only prevents hot cracking of the slab by precipitating S in the steel sheet as MnS, but also, in the overaging process of continuous annealing, the Mn: Mn in the ferrite crystal grains with MnS as the core It plays a role in promoting the precipitation of fine cementite. It also plays a role of supplementing the strengthening by C as a solid solution strengthening element. Therefore, the lower limit is 0.5%, which is sufficient to precipitate and fix S. However, the second invention ( Total- O ≤ 0.00
25 %), the lower limit of Mn is 0.3 %.

On the other hand, addition of a large amount of Mn is effective for increasing the strength of the material, but on the other hand, the solubility product of MnS increases, so that a relatively large Mn is produced at the casting plate (eg slab) stage.
Not only is S formed, but it also promotes the formation of a band structure during hot rolling and promotes a microscopic non-uniform distribution of C in the steel sheet. All of these generate a disadvantageous structure against side wall fracture during DTR. Therefore, the upper limit of Mn is limited to 1.2%.

As described above, in order to improve the side wall rupture resistance, it is necessary to suppress the precipitation of MnS as much as possible and to avoid the presence of MnS spread in a film form in the rolling direction in terms of microstructure. Therefore, it is necessary to limit Mn and S in association with each other.

FIG. 1 shows S and Mn which affect the critical thinning rate eth of a steel sheet adjusted to a temper degree equivalent to DR-9.
It shows the effect of the content.

As shown in FIG. 1, as the S and Mn contents are reduced, eth tends to be improved, and particularly in the region where the Mn content is relatively low, the effect of reducing the S content is large. Eth> 30% is achieved with ≤0.008%.

In the region where Mn is relatively high, eth is dominated by the solubility product [Mn] [S], and eth> 30% is achieved with log [Mn] [S] ≦ −2.22. That is, by reducing the absolute amount of S, MnS, which is a microstructural starting point of fracture, is reduced, and eth is improved. This is because as log [Mn] [S] increases, MnS tends to precipitate from the initial stage of slab soaking, and since it grows throughout the furnace, it tends to become coarse MnS. Is estimated.

Since such coarse MnS is expanded into a film shape in the rolling direction by hot rolling and tends to become a starting point of fracture in terms of microstructure, lowering log [Mn] [S] causes MnS to decrease. Coarseness is suppressed and eth is improved.

That is, in order to obtain a microstructure that a steel sheet suitable for DTR processing in which side wall fracture is unlikely to occur, S should be reduced and log [Mn] [S] should be based on the results shown in FIG. Limit the upper limit to -2.22.

O: Oxide inclusions significantly impede side wall fracture resistance. Generally, a small amount of Al ₂ O ₃ system becomes a problem, and CaO and MnO system inclusions may remain. As a countermeasure, it is effective to reduce the total oxygen concentration (Total-O) in the steel sheet. Is. Figure 2 shows the eths of various steel sheets organized by Total-O.

These steel sheets obtain almost eth> 30% by reducing S and limiting log [Mn] [S], but Total-O
By setting ≤ 0.005%, eth will improve, and Total-O ≤ 0.0025
% Improves even further. Therefore, in the present invention, Total-O is limited to 0.005% or less, more preferably 0.0025% or less, depending on the required level of eth.

C: C is an extremely important element for ensuring the strength level required as a DTR compliant steel sheet. Also,
It is also important in controlling the precipitation morphology of the carbide defined in the present invention. If it is 0.02% or less, the strength level required for a DTR steel sheet cannot be secured.

On the other hand, if it exceeds 0.06%, the workability may be deteriorated due to excessively high strength, and coarse pearlite may be formed to form in the rolling direction as described later (FIG. 5).
The formed cementite community is formed , and the length of adjacent fine cementite fine particles in the rolling direction increases, so that side wall fracture becomes apparent. Therefore, the range of C is limited to 0.02 <C ≦ 0.06%.

Si: Si is an element that causes the steel sheet to become brittle, and the smaller the amount, the better. In addition, the steel sheet according to the present invention has a TFS
In terms of surface treatment such as, the smaller amount is preferable. In the present invention, it is set to 0.03% or less, which is not a practical problem.

P: P is an element that segregates at the ferrite crystal grain boundaries and embrittles the grain boundaries, so it is preferably as small as possible. In the present invention, the upper limit that practically does not affect the sidewall breakage resistance is limited to 0.03% or less.

Sol.Al: Sol.Al is a solid solution N that reduces the local ductility of the steel sheet by a dynamic strain aging phenomenon (similar to solid solution C) by precipitating N in the steel sheet as AlN.
Reduce the harmful effects of.

Further, fine AlN is effective for refining ferrite crystal grains, and like MnS, it becomes a nucleus for precipitating fine cementite in ferrite crystal grains during the overaging process of continuous annealing. However, if a large amount of Al is added to increase the amount of Sol.Al, minute Al ₂ O ₃ inclusions are likely to remain, which causes sidewall breakage.

Therefore, in the present invention, 0.02% is set as the lower limit at which the above-mentioned effects are exhibited, and 0.1% is set as the upper limit as the limit at which addition of more than that practically impairs sidewall fracture resistance.

N: N is finely dispersed as AlN and utilized as a fine grain of ferrite crystal grains and as a precipitation nucleus of fine cementite, and its range is 20 to 70 ppm from the economical viewpoint.
And

Further, the steel sheet of the present invention has a microstructure of a substantially ferrite single-phase structure composed of a ferrite phase and finely dispersed cementite. 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 because it becomes a starting point of minute cracks during DTR molding and further deteriorates side wall fracture resistance.

In the present invention, more stable side wall rupture resistance is obtained by defining the precipitation form of the above-mentioned finely dispersed cementite.

Some cementites in the steel sheet are those which are precipitated in the hot rolling step and those which are solute C are precipitated after the annealing step. These cementites have an influence on the side wall fracture resistance depending on the precipitation form.

In other words, in order to secure the strength that should be provided as a steel sheet for DTR cans, a C content of more than 0.02% and not more than 0.06% is necessary.
In the steel sheet having the content, pearlite transformation occurs in the cooling process of the hot rolling process, and pearlite (fine mixed structure of ferrite and cementite) is formed at the grain boundary of the hot rolled structure. This pearlite is crushed in the rolling direction in the cold rolling process, and as a result, it is shaped in the rolling direction as shown in FIG.
The formed cementite community is formed.

Such community-like cementite locally increases the interface between cementite and the parent phase, which have a relatively weak bonding force, and causes the origin of minute cracks during DTR molding as shown in FIG. Will be given. Therefore, it is desirable to avoid the occurrence of such cementite.
In order to secure the strength that should be provided as a steel sheet for R cans,
As mentioned above, the proper C content exceeds 0.02% 0.06
% Or less is necessary, and the presence of cementite is unavoidable in steel with such a C content.

However, in order to reduce the interface between cementite and the mother phase per unit volume, it is effective to limit the size. That is, when the length Lc in the rolling direction shown in FIG. 5 (Lc of the present invention does not limit the direction of the canopy, but usually the longest is the rolling direction), The limit thinning rate eth tends to improve.

FIG. 3 shows the effect of Lc on eth in steel sheets with various cementite precipitation morphologies varied and adjusted to a temper degree equivalent to DR-9.

As shown in FIG. 3, the critical thinning rate eth
Depends on Lc, and eth> 30% can be secured at Lc of 10 μm or less. Therefore, in the present invention, pearlite is cold rolled.
The length Lc of the cementite community formed by crushing in the rolling direction is specified to be 10 μm or less.

In the present invention, among the above-mentioned finely dispersed cementite, 0.02% or less which is dissolved and precipitated during annealing is used.
Stable sidewall fracture resistance is obtained by defining the precipitation state of C.

That is, in the annealing step, C in the ferrite phase is
Solubility increases, and about 0.02% of C dissolves in the ferrite phase by annealing at about 700 ℃. During the subsequent cooling process, this C precipitates as cementite (Fe ₃ C) in the ferrite grain boundaries or in the ferrite crystal grains.

Especially when the cooling rate is low, most of C is deposited in a film form on the ferrite crystal grain boundaries. This is DTR
It gives the starting point of microcracks during molding. To this end, a certain amount of C should be finely precipitated in the ferrite crystal grains to finely disperse the cementite below the critical size that induces microcracks.

Then, paying attention to the mean inter-grain distance (MFP) of the fine cementite precipitated in the ferrite crystal grains shown in FIG. 5, the steel sheet with the C content changed as shown in FIG.
The effect of MFP on the was investigated.

The level of eth changes depending on the C in the steel, but when looking at the MFP, it becomes possible to obtain a stable eth for each steel sheet by limiting it to approximately 2 μm. This is because by precipitating fine cementite in the ferrite crystal grains, the amount of film-like cementite precipitated at the ferrite crystal grain boundaries can be reduced, and the occurrence of cracks from the crystal grain boundaries can be suppressed.

Therefore, in the present invention, as a microstructure condition for maximizing the effect of the invention, the cooling process of annealing is performed.
For cementite precipitated in, the average inter-particle distance MF
Fine cementite with P ≦ 2 μm is uniformly dispersed in ferrite crystal grains.

The steel sheet of the present invention disclosed above is a so-called DTR for performing deep drawing while applying tension regardless of whether the can forming process is the Wet process or the Dry process.
This is a technology that can be applied to all steel sheets used in the can manufacturing method.

Further, after DTR processing, further ironing (Ir
It can also be applied when thinning is performed by carrying out oning).

The characteristics are not impaired not only in TFS but also in tin-plated steel sheet, ultra-thin tin plating, tin-nickel plating, nickel plating, chromium plating, Ni flash plating steel sheet and the like.

These surface-treated steel plates are applied to DTR cans as the steel plates alone, as film-laminated steel plates laminated with a resin film such as polyester, or as precoated steel plates coated with a paint such as epoxy.

In particular, when these surface-treated steel sheets are used as base steel sheets for film-laminated steel sheets and pre-coated steel sheets, electrolytic chromic acid-treated steel sheet or TFS having a two-layer structure in which the lower layer is metallic chromium and the upper layer is chromium hydrate oxide.
However, it is most desirable from the viewpoint of processing adhesion.

[0066]

EXAMPLES The present invention will be described below based on examples.

Example 1 Steel was smelted and continuously light-pressure-casted into a slab (slab or the like) having the components shown in Table 1. The slab was heated to 1200 ° C.,
A hot rolled sheet having a finishing temperature of 840 ° C and a coiling temperature of 600 ° C was obtained by hot rolling into a 1.8 mm hot rolled sheet.

In the present invention, slab light pressure casting of continuous casting intended to reduce semi-macro and macro-segregation of P and S in steel, high temperature heating of slab at 1150 ° C. or higher, hot rolled sheet from the viewpoint of uniform miniaturization of MnS From the viewpoint of fine structure, 87
Low temperature finish below 0 ℃ , cementite community shown in Fig. 5
From the viewpoint of finely dispersing the pearlite forming the pearlite, low-temperature winding at 620 ° C or lower is preferable.

This hot-rolled steel sheet was pickled, cold-rolled to 0.24 mm, and then recrystallized in a continuous annealing furnace (CA).

In the continuous annealing, 400 ° C. is set in order to reduce the residual solid solution C that deteriorates the local ductility of the steel sheet by dynamic strain aging and to control the precipitation form to be finely dispersed in the ferrite crystal grains. Overaging treatment with a retention time of 20 seconds or more at 300 to 400 ° C within the temperature range that does not exceed (C
A + OA) was carried out.

2 up to a plate thickness of 0.18 mm for the annealed steel plate
Next rolling was performed to obtain a DR (Double Reduced) steel plate. For some, only temper rolling is applied to the annealed steel sheet and SR
(Single Reduced) Steel plate.

These steel sheets were used as TFS steel sheets having a multi-layered coating composed of metal Cr and Cr hydrated oxide on a tin-free plating line. PET films were laminated on the front and back surfaces of this steel sheet by a heat fusion method in a laboratory, and eth was evaluated by a high speed draw bead pull-out test method.

The evaluation results are shown in Table 2. By chemical composition
There are some differences in eth levels, but the composition and MF
By using the steel sheet (I) of the present invention in which P and Lc are specified, a higher eth than that of the comparative steel sheet (C) can be obtained, and it has DTR compatibility excellent in side wall rupture resistance.

[0074]

[Table 1]

[0075]

[Table 2]

Example 2 Steel was melted by a conventional method, continuously light-pressure-cast, and cast pieces of the components of steel Nos. 6, 14, 21, 23, 25, 27, and 34 shown in Table 1 (slabs, etc.). Was manufactured. This slab is 1200 ℃
After heating to hot rolling, finishing temperature 840 ℃, winding temperature 560 ~ 6
It was a hot rolled sheet of 1.8 mm at 80 ° C.

This hot-rolled steel sheet was pickled, cold-rolled to 0.24 mm, and then continuously annealed (CA) or box annealed (B
Recrystallization annealing was performed in A).

In continuous annealing, 400 ° C. is set in order to reduce the residual solid solution C that deteriorates the local ductility of the steel sheet by dynamic strain aging and to control the precipitation form to be finely dispersed in the ferrite crystal grains. Overaging treatment with a retention time of 20 seconds or more at 300 to 400 ° C within the temperature range that does not exceed (C
A + OA) was also conducted.

Further, in this embodiment, there is also a method (CA + BA) of carrying out overaging treatment in a box annealing furnace within a temperature range not exceeding 400 ° C. after annealing in a continuous annealing furnace not provided with an overaging treatment zone. went.

2 up to a plate thickness of 0.18 mm for the annealed steel plate
Next rolling was performed to obtain a DR (Double Reduced) steel plate.

These steel sheets were used as TFS steel sheets having a multi-layered coating composed of metal Cr and Cr hydrated oxide on a tin-free plating line. PET films were laminated on the front and back surfaces of this steel sheet by a heat fusion method in a laboratory, and eth was evaluated by a high speed draw bead pull-out test.

The evaluation results are shown in Tables 3 and 4. The steel sheet (I) of the present invention has DTR compatibility that is more excellent in side wall fracture resistance by optimizing the components, Lc, and MFP.

[0083]

[Table 3]

[0084]

[Table 4]

[0085]

EFFECTS OF THE INVENTION The present invention prevents side wall rupture of a steel sheet in DTR forming which undergoes intense tensile deep drawing by preventing minute cracks at the interface between MnS and the matrix of the microstructure or the interface between the carbide and the matrix. Remarkably improved the sex.

Therefore, when the wall thickness is reduced to 20% or more, there is no side wall fracture, and even if the wall thickness is reduced to 30% or more, such minute cracks hardly occur. Can be provided.

[Brief description of drawings]

FIG. 1 is a diagram showing an influence of S and Mn contents in a steel sheet on еth.

FIG. 2 is a diagram showing the influence of Total-O in a steel sheet on еth.

FIG. 3 is a diagram showing the influence of Lc in the steel sheet on еth.

FIG. 4 is a diagram showing an influence of MFP in a steel sheet on еth.

Figure 5: Morphology of finely dispersed cementite, Lc and MFP
It is a figure which shows the definition of.

FIG. 6 is a schematic diagram of a high speed draw bead pull-out 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 side wall of a can after DTR molding. (MnS
It is a figure which also shows correspondence of a crack. )

FIG. 8 is a drawing-substituting photograph of a cross-sectional microstructure showing a cross-sectional state of a crack in a side wall of a can after DTR molding. (It is also a diagram showing correspondence between cementite and cracks.)

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yukio Kawase 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Japan Steel Pipe Co., Ltd. (72) Hideki Nishihara 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Nippon Steel Pipe Incorporated (56) References Japanese Patent Laid-Open No. 2-301519 (JP, A) Japanese Patent Laid-Open No. 4-337049 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00 -38/60

Claims (2)

(57) [Claims] 1. In mass %, 0.02 <C ≦ 0.06%, Si ≦ 0.03
%, 0.5 ≦ Mn ≦ 1.2%, P ≦ 0.03%, S ≦ 0.008%, 0.02
≦ Sol.Al ≦ 0.1%, 0.002 ≦ N ≦ 0.007%, Total-O ≦ 0.
005%, log [Mn] [ S] ≦ which has a steel composition comprising the balance Fe and unavoidable impurities at -2.22, ferrite single consisting essentially of ferrite phase and finely dispersed cementite
It consists of phase structure, among the cementite, cooling over the annealing
Fine cementite that precipitates within the crystal grains of the ferrite phase
Doo is dispersed precipitates in the distance between the following average particle 2μm (MFP), and pearlite is disrupted by cold rolling step of hot-rolled tissue
The length Lc of the dense cementite community formed by
DTR can-compatible steel sheet with excellent side wall rupture resistance, which is characterized by being 10 μm or less. 2. A DTR excellent in side wall rupture resistance according to claim 1.
Among the steel compositions of can-compatible steel sheets, Mn and Total- O
Te is, 0.3 ≦ Mn ≦ 1.2%, Total-O ≦ 0.0025% and was superior in this <br/> and耐側wall rupturable characterized was the DTR cans adapted steel.